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Energy Storage Research in Switzerland –The SCCER Heat & Electricity Storage

CHIMIA2015,VOLUM

E69,NUM

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Novartis Celebrates Frontiers of ChemistryNovartis Chemistry Lecture DayThursday, February 4, 2016Horburg Auditorium, WKL-430.3.20Müllheimerstrasse 1954057 Basel

Program

Morning Session: Total Synthesis of Natural ProductsChair: Dr. Peter von Matt, NIBR GDC ATI08:30 h – 08:45 h Opening Remarks08:45 h – 09:45 h Prof. Mohammad Movassaghi

Massachusetts Institute of Technology, Cambridge, MA, USA09:45 h – 10:15 h Coffee Break10:15 h – 11:15 h Prof. Tohru Fukuyama

Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Japan11:15 h – 12:15 h Prof. Brian M. Stoltz

California Institute of Technology, Pasadena, California, USA

Afternoon Session: Late Stage FunctionalizationChair: Dr. Fabrice Gallou, TRD CHAD14:00 h – 15:00 h Prof. Varinder K. Aggarwal

University of Bristol, Cantock's Close, Bristol, UK15:00 h – 16:00 h Prof. Cristina Nevado

University of Zürich, Zürich, Switzerland16:00 h – 16:30 h Coffee Break16:30 h – 17:30 h Prof. Eric N. Jacobsen

Harvard University, Cambridge, MA, USA17:30 h Apéro

Symposium organized by NIBR GDC and TRD CHAD

energy storage research in switzerland – the sccer heat & electricity storage CHIMIA 2015, 69, No. 12 721

Energy Storage Research in Switzerland –The SCCER Heat & Electricity Storage

The declared aim of the Swiss Energy Strategy 2050 is the transition from nuclear power to a highlyefficient energy system with power supply based on renewable sources, to meet the CO2 emissiontargets. For a smooth transition, an important factor is to expand and strengthen the knowledge inthe energy field through the increase of personnel resources, e.g. scientists, engineers, techniciansalongside with the development of new technologies. Besides other actions taken by the SwissGovernment, in 2014 eight Swiss Competence Centers for Energy Research (SCCER) have beenestablished, receiving their funding through the Commission for Technology and Innovation Switzer-land (CTI). The Centers cover different action fields in energy research, viz. Mobility,[1] Efficiency,[2]Supply of Electricity,[3] Grids,[4] Biomass,[5] Economy & Environment,[6] as well as Heat and ElectricityStorage.[7]

The Centers are organized as virtual consortia of industrial and academic institutions (CantonalUniversities, Universities of Applied Sciences, Federal Institutes of Technology and Research Insti-tutions, respectively) with the intention to maximize the technological outcome by combining thestrongest competencies in each area of expertise.

Energy Storage is a key element within the Federal Energy Strategy since energy, sourced fromrenewables like wind and solar energy, is only available on an intermittent, stochastic basis. Storingexcess energy during times of low energy demand and releasing it in times of high energy demandis not only useful from an energetic perspective, it also may create an economic value within the en-ergy market. With an increasing contribution of the aforementioned renewable energy sources to theelectricity mix, the significance of energy storage increases. This is clearly demonstrated by countrieshaving installed large capacities of wind and photovoltaic power, e.g. Germany and Denmark. Largeintermittent discrepancies between electricity production and demand are being observed with theconsequence of a strongly fluctuating electricity price causing also challenges to the stability of thepower supply system. In order to stabilize the grid, an increase in short-term electricity storage ca-pacity (hrs) with high response time is needed within the next years. In the long run, seasonal storagebecomes important to ensure constant electricity supply without conventional fossil based powergeneration.

Heat, aside from electricity, is one of the most required types of energy today. About 50% of theprimary energy carriers are transformed to heat by modern industrialized societies required for spaceheating, hot water and process heat. Thus, it becomes obvious that a sensible use of energy mustnot neglect the questions related to heat storage.

The research and development within the SCCER Heat and Electricity Storage concentrates onfive different topics with the involvement of more than 20 research groups from eleven public institu-tions as well as from the private sector. In detail, R&D is performed on direct electricity storage inbatteries, electricity storage in hydrogen and synthetic fuels, short-term and seasonal heat storageas well as the interaction and integration of different storage technologies.

Within this special issue of CHIMIA entitled Energy Storage Research in Switzerland we havethe opportunity to present 15 papers from the participating groups of the SCCER Heat & ElectricityStorage highlighting their research in this exciting, interdisciplinary field of Energy Storage forming aperfect place to find an overview of the Swiss activities.

Prof. Dr. Thomas J. Schmidt Dr. Jörg RothDirector SCCER Heat and Electricity Storage Manager SCCER Heat and Electricity Storage

SCCER Heat and Electricity Storagec/o Paul Scherrer InstitutCH-5232 Villigen [email protected]

[1] SCCER Mobility, www.sccer-mobility.ch, accessed Sept. 09, 2015.[2] SCCER FEEB&D, www.sccer-feebd.ch; SCCER EiP, www.sccer-eip.ch, both accessed Sept. 09, 2015.[3] SCCER SoE, www.sccer-soe.ch, accessed Sept. 09, 2015.[4] SCCER Furies, www.sccer-furies.epfl.ch, accessed Sept. 09, 2015.[5] SCCER Crest, www.sccer-crest.ch, accessed Sept. 09, 2015.[6] SCCER Biosweet, www.sccer-biosweet.ch, accessed Sept. 09, 2015.[7] SCCER Heat & Electricity Storage, www.sccer-hae.ch, accessed Sept. 09, 2015.

The Editorial Board of CHIMIA is very grateful to the guest editors Prof. Dr. Thomas J. Schmidtand Dr. Jörg Roth for the successful realization of this special issue on Energy Storage Research inSwitzerland - The SCCER Heat & Electricity Storage; providing readers with an excellent overview ofan interdisciplinary project with great significance for the future.

722 CHIMIA 2015, 69, No. 12 contents inhalt sommaire

Energy Storage Research in Switzerland– The SCCER Heat & Electricity Storage

editorial

721 Energy Storage Research in Switzerland - The SCCER Heat& Electricity StorageT. J. Schmidt, J. Roth

724 Evaluation of Metal Phosphide Nanocrystals as Anode Materials for Na-ion BatteriesM. Walter, M. I. Bodnarchuk, K. V. Kravchyk, M. V. Kovalenko*

729 Electrode Engineering of Conversion-based Negative Electrodes for Na-ion BatteriesL. O. Vogt, C. Marino, C. Villevieille*

734 Nanomaterials Meet Li-ion BatteriesN. H. Kwon*, J.-P. Brog, S. Maharajan, A. Crochet, K. M. Fromm*

737 Stress-induced Ageing of Lithium-Ion BatteriesM. Held*, U. Sennhauser

741 Storing Renewable Energy in the Hydrogen CycleA. Züttel*, E. Callini, S. Kato, Z. Ö. Kocabas Atakli

746 Hydrogen Storage in the Carbon Dioxide – Formic Acid CycleC. Fink, M. Montandon-Clerc, G. Laurenczy*

753 Redox Flow Batteries, Hydrogen and Distributed StorageC. R. Dennison, H. Vrubel, V. Amstutz, P. Peljo, K. E. Toghill, H. H. Girault*

759 CO2 Hydrogenation: Supported Nanoparticles vs. Immobilized CatalystsS. Tada, I. Thiel, H.-K. Lo, C. Copéret*

765 Soft Approaches to CO2 ActivationS. Das, F. D. Bobbink, A. Gopakumar, P. J. Dyson*

769 Electrochemical CO2 Reduction – A Critical View on Fundamentals, Materials and ApplicationsJ. Durst*, A. Rudnev*, A. Dutta, Y. Fu, J. Herranz, V. Kaliginedi, A. Kuzume, A. A. Permyakova,Y. Paratcha, P. Broekmann, T. J. Schmidt

777 Storage of Heat, Cold and ElectricityA. Stamatiou, A. Ammann, A. Abdon, L. J. Fischer, D. Gwerder, J. Worlitschek*

780 Phase Change Material Systems for High Temperature Heat StorageD. Y. S. Perraudin, S. R. Binder, E. Rezaei, A. Ortona, S. Haussener*

784 Seasonal Solar Thermal Absorption Energy Storage DevelopmentX. Daguenet-Frick*, P. Gantenbein, M. Rommel, B. Fumey, R. Weber, K. Goonesekera, T. Williamson

789 Challenges towards Economic Fuel Generation from Renewable Electricity:The Need for Efficient Electro-CatalysisF. Le Formal, W. S. Bourée, M. S. Prévot, K. Sivula*

799 Experimental and Numerical Investigation of Combined Sensible/Latent Thermal Energy Storagefor High-Temperature ApplicationsL. Geissbühler, S. Zavattoni, M. Barbato, G. Zanganeh, A. Haselbacher*, A. Steinfeld

contents inhalt sommaire CHIMIA 2015, 69, No. 12 723

corrigendum

804 CHIMIA 2015, 69, 52–56

columns

805 swiss science concentrates

Prepared by C. D. Bösch, M. Probst, Y. Vyborna, M. Vybornyi, S. M. Langenegger, R. Häner*

806 highlights of analytical sciences in switzerland

Deep UV-LED Based Absorbance Detectors for Narrow-Bore HPLC and Capillary ElectrophoresisD. A. Bui, P. C. Hauser*

universities of applied sciences

807 Effect of Experimental Parameters on Water Splitting Using a Hematite PhotoanodeO. Vorlet*, F. Giordano, T. Chappuis, C. Ellert

Biotechnet switzerland

809 Platform for a Technological Leap in AntibioticsE. Heinzelmann

812 scnatChemical Landmark 2015 –Designation of the Former Institute of Chemistry of the University of FribourgL. Merz

swiss chemical society

814 News

814 A warm welcome to our new members!

815 6th EuCheMS Chemistry Congress

816 SCS Prize Winners 2016

817 Division of Analytical Sciences: Weiterbildung Analytik

information

818 Conferences in Switzerland

819 Lectures

euchems

821 EuCheMS Newsletter

chimia report/company news

Markt, Apparate, Chemikalien und Dienstleistungen

724 CHIMIA 2015, 69, No. 12 energy storage research in switzerland – the sccer heat & electricity storage

doi:10.2533/chimia.2015.724 Chimia 69 (2015) 724–728 © Schweizerische Chemische Gesellschaft

*Correspondence: Prof. Dr. M. Kovalenkoab

E-mail: [email protected];[email protected] ZürichLaboratory of Inorganic ChemistryDepartment of Chemistry and Applied BiosciencesHCI H 139Vladimir-Prelog-Weg 1CH-8093 ZurichbEMPA - Swiss Federal Laboratories forMaterials Science and TechnologyLaboratory for thin films and photovoltaicsÜberlandstrasse 129CH-8600 Dübendorf#These authors contributed equally to this work

Evaluation of Metal PhosphideNanocrystals as Anode Materials forNa-ion Batteries

Marc Walterab#, Maryna I. Bodnarchukab#, Kostiantyn V. Kravchykab, and Maksym V. Kovalenko*ab

Abstract: Sodium-ion batteries (SIBs) are potential low-cost alternatives to lithium-ion batteries (LIBs) becauseof the much greater natural abundance of sodium salts. However, developing high-performance electrode ma-terials for SIBs is a challenging task, especially due to the ~50% larger ionic radius of the Na+ ion comparedto Li+, leading to vastly different electrochemical behavior. Metal phosphides such as FeP, CoP, NiP2, and CuP2

remain unexplored as electrode materials for SIBs, despite their high theoretical charge storage capacities of900–1300 mAh g–1. Here we report on the synthesis of metal phosphide nanocrystals (NCs) and discuss theirelectrochemical properties as anode materials for SIBs, as well as for LIBs. We also compare the electro-chemical characteristics of phosphides with their corresponding sulfides, using the environmentally benign ironcompounds, FeP and FeS2, as a case study. We show that despite the appealing initial charge storage capaci-ties of up to 1200 mAh g–1, enabled by effective nanosizing of the active electrode materials, further work to-ward optimization of the electrode/electrolyte pair is needed to improve the electrochemical performance uponcycling.

Keywords: Anode materials · Li-ion batteries · Na-ion batteries · Nanocrystals · Synthesis

1. Introduction

Lithium-ion batteries (LIBs) have be-come the battery technology of choice forapplications demanding high energy andpower densities, such as portable electron-ics and electric vehicles, and also showgreat promise for the large-scale grid stor-age of electricity. Yet, the irregular geo-graphic distribution and relatively lownatural abundance of lithium salts raisedoubts as to the future security and cost ofsupply. In this regard, conceptually iden-tical sodium-ion batteries (SIBs) are a fa-vorable alternative due to the much greaterabundance (by a factor of 103) and there-fore lower price of sodium salts.[1] How-ever, the seemingly simple replacement

of the Li+ ion with its 50% larger group Ineighbor has drastic consequences for theresulting electrochemistry. For instance,both silicon and graphite, which are well-known anode materials with outstandinglithium ion storage properties, show negli-gible capacities for sodium ions.[2] Exten-sive research toward new electrode materi-als is needed to advance the developmentof high-performance SIBs.

Of all possible anode materials forSIBs, red phosphorus (P) is probably themost appealing candidate due to its lowcost, nontoxicity and, most importantly,extremely high sodium capacity (2596mAh g–1 for P↔Na

3P, the highest Na+

capacity known) at a low desodiationpotential (~0.6 V vs. Na+/Na). However,similarly to other alloying/conversion typematerials, P suffers from massive volumechanges during sodiation/desodiation (∆V= 291%, by molar volume) leading to themechanical disintegration of the electrodesand therefore rapid capacity fading due toloss of electrical contact. The other maindisadvantage of P is its relatively low elec-tronic conductivity, causing slow reactionkinetics. Although noticeable progresshas been demonstrated for P-based SIBanodes,[3] typically very large amounts ofconductive carbons are used to provide suf-ficient conductivity as well as mechanicalstability of the electrodes, and often highcapacities with good cycling stability canonly be achieved at low charge/dischargecurrents of ~100 mA g–1 (~0.05 C).

In this study, we were intrigued by thepossibility of addressing the aforemen-tioned issues facing P-based SIB anodesby using metal phosphide nanocrystals(NCs) as the active material. Generally,nanostructured materials often show im-proved electrochemical performance overtheir bulk counterparts due to mitigation ofthe effects caused by volumetric changesand improved ionic and electronic con-ductivities upon homogeneous mixingwith conductive carbon additives.[4] More-over, metallic inclusions, which form insitu upon electrochemical conversion ofthe transition metal phosphide to alkalimetal phosphide, are also expected to im-prove the electronic connectivity withinthe electrode. Despite the additional massof the transition metal, theoretical specificcharge-storage capacities of metal phos-phides are still extremely high (900–1300mAh g–1), surpassing all of the main alter-natives to P such as Sn (847 mAh g–1) andSb (660 mAh g–1). Herein, we present thesodium and also lithium storage propertiesof highly uniform FeP, CoP, NiP

2and CuP

2NCs prepared via colloidal synthesis meth-ods. It should be noted that, with the excep-tion of a recent study on FeP,[5] this is thefirst report on the electrochemical perfor-mance of such metal phosphides in SIBs.All of the phosphide NCs investigated inthis work show high charge-storage ca-pacities, close to the theoretically expectedvalues. In comparison to the correspond-ing metal sulphide NCs, the phosphides


and dimethyl carbonate (DMC) with 3%FEC. FEC was added to the electrolyte inboth the Li and Na coin cells to improvecapacity retention.[3c,7]All electrochemicaltests were carried out at room temperatureand the capacities were reported relative tothe mass of the metal phosphide NCs.

3. Results and Discussion

3.1 Synthesis and Characterizationof Metal Phosphide Nanocrystals

FeP NWs were synthesized accordingto the procedure reported by Qian et al.[8]In order to obtain NiP

2NCs, a two-step

procedure was developed. First, Ni2P NCs

were synthesized according to a knownprotocol reported by Popczun et al.[9] Wethen added a second step: conversion of theas-prepared Ni

2P NCs into NiP

2NCs by

adding red P to the reaction mixture, fol-lowed by heating at 330 °C for 22 h. Anal-ogously, this two-step approach was alsoapplied in the synthesis of CuP

2and CoP

NCs, simply by replacing nickel(ii) acety-lacetonate with the respective copper orcobalt salt (for details, see the Experimen-tal section). Fig. 1 summarizes the char-acterization of the metal phosphide NCsobtained by these methods. FeP NCs wereon average ~300 nm in length and ~7 nm inwidth. CoP, NiP

2and CuP

2NCs exhibited

diameters of 25, 10 and 60 nm, respective-ly. All materials showed phase-pure XRDpatterns, indexed according to the standardICSD files for these compounds.

3.2 Electrochemical Performanceof Metal Phosphide Nanocrystals

Fig. 2 shows the electrochemical per-formance of the metal phosphide NCs inNa-ion and Li-ion half-cells. Na-ion cellswere cycled at a current rate of 100 mA g–1

in the potential range of 0.02–2.5V. For Li-ion cells, current rates of 300 mA g–1 anda potential range of 0.02–2.0 V were used.Assuming the formation of Na

3P or Li

3P

via the general conversion reaction

MP× + 3×e– + 3×A+ ↔ ×A3P + M

(M = Fe, Co, Ni, Cu; A = Li, Na),

the metal phosphides FeP, CoP, NiP2and

CuP2have theoretical capacities of 926,

894, 1333 and 1282 mAh g–1, respectively.In close agreement, CuP

2indeed showed

the highest capacity in the first cycle.However, the capacities of all studied ma-terials rapidly faded during cycling. Thecompounds with higher P content, NiP

2and CuP

2, showed higher initial capacities

but poorer capacity retention. Namely, forCuP

2NCs the charge capacity decreased

from 1140 mAh g–1 to 570 mAh g–1 withinthe first 16 cycles. For the FeP, CoP and

Then, 2 mL of TOP were added to theflask under Ar and the reaction mixturewas heated to 320 °C for 65 min. The flaskwas cooled to 200 °C by flowing air andthen 105 mg (3.4 mmol) of red phospho-rous were added. Then the reaction mix-ture was heated again to 330 °C and heldat this temperature for 22 h. CoP NCs wereisolated and purified identically to the NiP

2NCs above.

2.1.4 CuP2 NCsIn a typical experiment, 4.5 mL ODE,

6.4 mL OLA and 0.262 g (1 mmol)copper(ii) acetylacetonate (≥97%, Sigma-Aldrich) were dried at 110 °C under vacu-um for 1 h to removewater and low-boilingpoint impurities. Then, 2 mL of TOP wereadded to the flask underAr atmosphere andthe reaction mixture was heated to 320 °Cfor 75 min. The flask was cooled to 200 °Cby flowing air and then 200 mg (6.4 mmol)of red phosphorous were added. The re-action mixture was then heated again to330 °C and was held at this temperaturefor 22 h. CuP

2NCs were isolated and puri-

fied identically to the NiP2NCs above.

2.2 Characterization of MetalPhosphide Nanocrystals

Transmission electron microscopy(TEM) was performed using a JEOL JEM-2200FS instrument operated at 200 kV,using carbon-coated Cu grids as sub-strates (Ted-Pella). Powder X-ray diffrac-tion (XRD) was measured using a STOESTADI P diffractometer (with Cu-Kα

1ir-

radiation, λ = 1.540598 Å).

2.3 Electrode Preparation, CellAssembly and ElectrochemicalMeasurements

In order to evaluate the electrochemi-cal properties of FeP, CoP, NiP

2and CuP

2NCs, Na-ion and Li-ion half-cells were as-sembled. Prior to electrode preparation, or-ganic ligands were removed from the sur-face of the NCs by stirring them in a 1 Msolution of hydrazine in acetonitrile for 2 hat room temperature, as is commonly per-formed for colloidal quantum dots.[6] Elec-trodes were prepared bymixing the respec-tive metal phosphide NCs (63.75 wt%)with carbon black (21.25 wt%, TIMCAL),carboxymethylcellulose (CMC, 15 wt%)and water as a solvent using a planetaryball-mill at 500 rpm for 1 h. The aqueousslurries were coated onto Cu current col-lectors, which were dried at 80 °C undervacuum overnight prior to cell assembly.For electrochemical testing, coin cells withelemental Na or Li were assembled in anAr-filled glovebox (O

2< 0.1 ppm, H

2O <

0.1 ppm) using either 1 M NaClO4in pro-

pylene carbonate (PC) with 10% fluoro-ethylene carbonate (FEC) or 1 M LiPF

6in

a 1:1 mixture of ethylene carbonate (EC)

exhibit lower desodiation potentials andare hence better suited as SIB anode ma-terials, but suffer from very fast capacityloss upon cycling. Further work on the op-timized formulation of the electrodes andthe selection of suitable electrolytes andelectrolyte additives is needed to improvelong-term cycling stability.

2. Experimental

2.1 Synthesis of Metal PhosphideNanocrystals

2.1.1 FeP Nanowires (NWs)In a typical experiment, 2.5 g tri-n-

octylphosphine oxide (TOPO, 99%,Strem) and 3 mL tri-n-octylphosphine(TOP, ≥97%, Strem), previously dried at100 °C under vacuum for 1 h, were heatedto 300 °C under Ar. At 300 °C, 0.5 mL ofFe stock solution, prepared by mixing 1mL TOP and 0.25 mL Fe(CO)

5(99.99%,

Strem), was injected into the TOP/TOPOmixture. After 30 min, a second injectionof 0.5mL of stock solution was carried out.The reaction was stopped after an addition-al 30 min. FeP NWs were precipitated byadding hexane and ethanol, separated bycentrifugation, and re-dispersed in chloro-form containing 1 wt% oleic acid. Thesecond precipitation was induced by add-ing ethanol. After centrifugation, the FePnanowires were re-dispersed in chloroformand stored under ambient conditions.

2.1.2 NiP2 NCsIn a typical experiment, 4.5 mL oc-

tadecene (ODE, 90%, Sigma-Aldrich),6.4 mL oleylamine (OLA, 95%, Strem)and 0.25 g (1 mmol) nickel(ii) acetylace-tonate (≥98%,Merck) were dried at 110 °Cunder vacuum for 1 h to remove water andlow-boiling point impurities. Then, 2 mLof TOP were added to the flask under Arand the reaction mixture was heated to320 °C and held at this temperature for1 h. The flask was cooled to 200 °C byflowing air and then 105 mg (3.4 mmol) ofred phosphorous (≥97%, Sigma-Aldrich)were added. The reaction mixture wasthen heated again to 330 °C and held atthis temperature for 22 h. NiP

2NCs were

precipitated twice by adding chloroformand ethanol, separated by centrifugation,and re-dispersed again in chloroform. Af-ter centrifugation, the NiP

2NPs were re-

dispersed in chloroform and stored underambient conditions.

2.1.3 CoP NCsIn a typical experiment, 4.5 mL ODE,

6.4mLOLA and 0.25 g (1mmol) cobalt(ii)acetylacetonate (≥98%, Merck) were driedat 110 °C under vacuum for 1 h to removewater and low-boiling point impurities.


tion plateau can be identified in all casesat ~0.6 V vs. Na+/Na, which is at the samepotential as reported for the electrochemi-cal reaction of red P with Na.[3e] This im-plies that metal phosphides rather convertinto elemental P and that cycling proceedsmainly by the reaction P + 3e– + 3Na+ ↔Na

3P, as has been suggested for FeP.[5] In

Li-ion half-cells, the majority of delithia-tion occurs at a potential of more than 1.0V showing that metal phosphides are gen-erally better suited as SIB anode materialsdue to lower voltages of desodiation.

3.3 Comparison of FeP and FeS2NCs as Anode Materials for Na-ionBatteries

Clearly, from the prospects of lowcost and low toxicity, iron-based sodiumstorage electrode materials are the mostinteresting candidates, in particular whenthe other chemical constituents of thecompound comprise equally abundantelements such as phosphorus and sulfur.Hence, iron sulfides can be seen as a mainalternative to phosphides. Similar difficul-ties with capacity fading might occur forFeS

2(pyrite) due to its large (~280%) vol-

ume expansion upon Na2S formation.[10] In

order to compare the electrochemical per-formance of Fe phosphides and sulfides,we synthesized pyrite FeS

2NCs with sizes

from 50–100 nm and tested them underthe same conditions as the FeP NCs. Thesynthesis, characterization and electro-chemical properties of FeS

2NCs have been

detailed in our recent report.[11] Assumingthe formation of Na

2S, FeS

2NCs possess a

theoretical maximum capacity of 894 mAhg–1, similar to the value for FeP (926 mAhg–1). However, as can be seen in Fig. 3, theelectrochemical performance of FeS

2and

FeP NCs is in fact very different. WhereasFeP NCs show rapid capacity fading uponcycling, FeS

2NCs exhibit stable capaci-

ties of ≥800 mAh g–1 (near the theoreti-cal value), clearly demonstrating that theidentity of the anion in a conversion-typeelectrode material plays a critical role indetermining its electrochemical properties.The only relevant previous investigation ofFeP as a SIB anode material is the recentreport by Li et al.;[5] in that work, anodesprepared by ball-milling FeP showedmuchfaster capacity fading, from 460mAh g–1 to~200 mAh g–1 within 40 cycles at a currentof 50 mA g–1. Compared to FeP NCs, theonly obvious drawback of FeS

2NCs is the

higher desodiation potential (Figs 3b and3c), that is, however, well compensated bygood capacity retention.

4. Conclusion

In conclusion, we have prepared NCsof FeP, CoP, NiP

2and CuP

2using colloi-

interface (SEI) (see Figs 2c and 2d). No-tably, rather poor coulombic efficienciesof 92–95% for Na-ion and 96–98% for Li-ion cells were obtained during subsequentcycles, indicating continuous deteriorationand reformation of the SEI caused by pul-verization of the electrode material.

Figs 2e and 2f show the galvanostaticcharge and discharge voltage profiles forthe first cycle for all testedmetal phosphideNCs. For sodium ion storage, a desodia-

NiP2NCs, the capacities fell to below 600

mAh g–1 after just the first 10 cycles, andfaded to less than 400 mAh g–1 during sub-sequent cycling. Similar observations weremade when testing the metal phosphideNCs in Li-ion half-cells (Fig. 2b). Dueto the high surface area of the nanosizedmaterials, low coulombic efficiencies (20–70%) were obtained for the first cycle dueto the irreversible decomposition of theelectrolyte forming the solid electrolyte

Fig. 1. Characterization of metal phosphide NCs. Transmission electron microscopy (TEM) imag-es, X-ray diffraction (XRD) patterns and schematic representations of the crystal structures (fromleft to right) of FeP, CoP, NiP2 and CuP2 NCs (a–d). The XRD patterns are indexed according to theICSD database: to orthorhombic FeP (PDF No.: 00-071-2262, space group Pna21 (33), a = 5.193Å, b = 5.792 Å, c = 3.099 Å), orthorhombic CoP (PDF No.: 00-029-0497, space group Pnma (62),a = 5.077 Å, b = 3.281 Å, c = 5.587 Å), cubic NiP2 (PDF No.: 00-073-0436, space group Pa3 (205),a = 5.4706 Å) and monoclinic CuP2 (PDF No.: 00-076-1190, space group P21/c (14), a = 5.8004 Å,b = 4.8063 Å, c = 7.5263 Å, β = 112.7°).


AcknowledgementsThis work was financially supported by

the Swiss Federal Commission for Technologyand Innovation (CTI, Project Nr. 14698.2PFIW-IW), CTI Swiss Competence Centersfor Energy Research (SCCER, ‘Heat andElectricity Storage’), SNF Ambizione Energygrant (PZENP2_154287) and ETH Zurich(Grant Nr. ETH-56 12-2). Electron micros-copy was performed at the Empa ElectronMicroscopy Center. We thank Dr. NicholasStadie for reading the manuscript.

Received: August 21, 2015

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that metal phosphides are inherently lessstable than their corresponding sulfidesand this instability is a combined effect oflarge volumetric changes and phosphorus-specific processes, such as the reactivityof Na

3P towards the electrolyte.[3c] Fur-

ther progress towards high performancephosphide-based electrodes is expected toresult from smart electrode engineering bydesigning secondary structures in whichthe metal phosphide NPs are encapsulatedinto conductive carbons,[12] thereby elimi-nating the direct large-area contact with theelectrolyte. Furthermore, a sensible choiceof the electrolyte and electrolyte additivesmight enable the higher stability of the SEIlayer in future studies.

dal synthesis methods. Motivated by theirhigh theoretical capacities, the low electro-chemical potentials needed for the anodicside of the battery, and the high naturalabundance of their constituting elements,we explored the potential of these nano-materials as SIB anode materials. We alsocompared their sodium storage proper-ties with that of lithium. We find that withconventionally formulated electrodes (bymixing with amorphous carbon and water-soluble binders), metal phosphide NCsdeliver high sodium capacities but exhibitlow capacity retention upon cycling. Thedirect comparison of FeP and FeS

2as SIB

anode materials indicates the much bettercyclability of the latter. We thus conclude

Fig. 2. Electrochemical performance of metal phosphide nanocrystals. Galvanostatic cycling ofmetal phosphide NCs in Na-ion (a) and Li-ion half-cells (b) with the respective coulombic effi-ciency plots (c, d). Galvanostatic charge and discharge curves for the first cycle for Na-ion (e) andLi-ion (f) half-cells. Electrodes were composed of 63.75% metal phosphide NCs, 21.25% CB and15% CMC. 1 M NaClO4 in PC with 10% FEC served as the electrolyte for Na-ion and 1 M LiPF6

in EC:DMC (1:1) for Li-ion half-cells. Galvanostatic cycling tests were carried out with a current of100 mA g–1 in the potential range of 0.02–2.5 V for Na-ion and 300 mA g–1 in the potential range of0.02–2.0 V for Li-ion half-cells.


Vogt, M. El Kazzi, E. Jämstorp Berg, S. PérezVillar, P. Novák, C. Villevieille, Chem. Mater.2015, 27, 1210.

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[10] F. Klein, B. Jache, A. Bhide, P. Adelhelm, Phys.Chem. Chem. Phys. 2013, 15, 15876.

[11] M. Walter, T. Zund, M. Kovalenko, Nanoscale2015, 7, 9158.

[12] a) X. Ji, K. T. Lee, L. F. Nazar,Nat. Mater. 2009,8, 500; b) X. W. Lou, C. M. Li, L. A. Archer,Adv. Mater. 2009, 21, 2536; c) H. Zhang, X.Yu,P. V. Braun, Nat. Nanotechnol. 2011, 6, 277; d)Y.Yu, L. Gu, C.Wang,A. Dhanabalan, P.A. vanAken, J. Maier, Angew. Chem. Int. Ed. 2009,48, 6485; e) N. Liu, H. Wu, M. T. McDowell,Y. Yao, C. Wang, Y. Cui, Nano Lett. 2012, 12,3315.

Fig. 3. Comparison of the electrochemical performance of FeP and FeS2 NCs. Galvanostaticcycling of FeS2 and FeP in Na-ion half-cells (a) and the respective charge/discharge curves (b, c).Galvanostatic cycling tests were carried out with a current of 100 mA g–1 in the potential range of0.02–2.5 V.

energy storage research in switzerland – the sccer heat & electricity storage CHIMIA 2015, 69, No. 12 729doi:10.2533/chimia.2015.729 Chimia 69 (2015) 729–733 © Schweizerische Chemische Gesellschaft

*Correspondence: Dr. C. VillevieillePaul Scherrer InstitutElectrochemistry LaboratoryCH-5232 Villigen PSIE-mail: [email protected]

Electrode Engineering of Conversion-based Negative Electrodes for Na-ionBatteries

Leonie O. Vogt, Cyril Marino, and Claire Villevieille*

Abstract: Due to lower costs and higher abundance of sodium, Na-ion battery technology can offer a goodalternative to Li-ion batteries. Much research is focusing on developing new cathode and anode materials butthe importance of the electrode engineering on the electrochemical performance is often neglected. The elec-trode composition is especially crucial for conversion reaction-based materials where the composite electrode(active material, conducting additive and binder) has to buffer the huge volume change occurring upon cycling.This work highlights the differences observed on Sn-CMC electrode performance by using different Sn par-ticle sizes (micro- and nanoparticles) and evaluating the role of the conductive additive in the electrode. Car-bon fibers (VGCF) demonstrate a good ability to surround micrometer particles but not especially nanometerparticles leading to an improvement in the performance of microparticles but not of nanoparticles. For a highloading electrode suitable for full cell applications (>3.5 mg/cm2 of active material), nanometer particles showlimited performance for long-term cycling. The combination of VGCF with micrometer particles seems to bethe most promising composition to obtain good performances for conversion reaction based-materials.

Keywords: Electrode engineering · Na-ion batteries · Negative electrode · Sn particle · VGCF

Li-ion batteries are starting to reachtheir limits in terms of energy density, cost,and abundance, and progress is slower thanexpected. Thus, researchers are currentlyre-investigating other alkali metals as Lisubstitutes, mainly focusing on Na. Thissystem has been considered to be purelyacademic, and no real applications or pro-totypes have been developed to investigateits viability or possible commercialization,the only exception being the high tempera-ture Na-S system, which was commercial-ized in the 1960s.[1]Recently however, theamount of research and number of papersdevoted to the development of active ma-terials for Na-ion batteries has increasedexponentially, leading the community toconsider the commercialization of Na-ionbatteries in the near future.[2–6] Electro-chemically, specifically voltage and theo-retical specificcharge, there isnoadvantageto replacing Li with Na. However, when

considering additional parameters such asabundance and cost, Na-ion batteries maybe competitive with the currently availableLi-ion batteries. Another significant ad-vantage in terms of both cost and weightis that no alloy exists between sodium andaluminum. This means that Al can be usedas a cheaper current collector for both thepositive and negative electrodes, loweringthe total weight and cost of the cell pack.Furthermore, the electrolyte in Na-ion bat-teries has a higher conductivity comparedto that of Li-ion batteries. The larger ionicradius of Na compared with Li results ina low de-solvation energy, which stronglyinfluences the kinetics and allows highpower Na-ion batteries to be envisaged.To achieve commercialization, suitableanodes[2,4,7–11] and cathodes[12–21] must bedeveloped and studied in depth as the Na-system is often not analogous to the Li-sys-tem.A couple of years ago, the observationthat pure commercial elements, such as Snor Sb can electrochemically react with Na,leading to sustainable reversible capaci-ties as high as 500 mAh/g over more than100 cycles (twice as much as hard carbon),surprised the scientific community.[7,22]These results were especially interestingconsidering the immense volume changeupon cycling, which exceeds 400%. How-ever, while the search for new materials isprogressing rapidly, the engineering of theelectrodes is quite often neglected. Manygroups in the literature reported as an ex-ample that polyvinylidene fluoride binder(PVDF) works poorly in Na-ion batter-

ies and that other type of binders shouldbe used instead.[23–25] We were one of thefirst groups to demonstrate that the know-how acquired over the past few decades onLi-ion battery technology cannot be trans-ferred one-to-one to the emerging Na-ionbatteries.[26] In fact, we demonstrated thatthe most commonly used binder in Li-ionbatteries (PVDF) is not suitable for Na-ionbatteries as it decomposes to create NaFduring cycling and thus the electrode losesits integrity and cannot accommodate thevolume change of the conversion-basedelectrode materials anymore.[26] Besidesthe binder, carbon-conductive agents (car-bon fibers, super P, acetylene black, car-bon nanotubes, etc.) are another source ofvariation from group to group, where manydifferent types are used but it is difficultto establish which of them leads to thebest cycling performance with conversion-based electrode materials for Na-ion bat-teries.[27–29]

The influence of the particle size (ofthe active material) is another often ne-glected factor in literature, since articleseither focus on nanoparticles or they focuson solid-state bulk particles (tens of μmrange). The comparison between both par-ticle sizes (nano and micro range) is not of-ten reported and it is difficult to assess theadvantages and drawbacks of each sincethe loading of the electrodes (using eithernano or micro range) can differ by a fac-tor of 5 to 10.[9,30–33] Based on the commonapproach of using very low loading fornanoparticle active materials (unsuitable


the volume change upon sodiation. Thisphenomenon would lead to a decrease ofthe cell performance. The n-Sn electrodeshows a narrow particle size distributionsince the Sn particles are below 500 nm,with most of them around 100 nm diam-eter. At low magnification (micrometerrange SEM picture, Fig. 1, bottom), the Snnanoparticles seem to be well dispersedand embedded in the carbon/binder ma-trix unlike the m-Sn electrode. Thus, uponcycling versus Na the nanoparticle sys-tem is expected to accommodate the vol-ume expansion better than the micropar-ticle system. However, the picture takenat higher magnification (Fig. 1, bottom)reveals some n-Sn agglomerates, indicat-ing that the electrode engineering could beoptimized further. This agglomeration ofnanoparticles may be a result of the highnanoparticle loading used here, as com-pared to the low loading generally report-ed in literature. In both cases, the imagesalso indicate a good porosity such that thewettability of the electrode should not be acause of fading.

Fig. 2 depicts the cycling performanceof m-Sn and n-Sn in electrode composi-tion 1. The specific charge of the m-Sn isconstantly decreasing along cycling. Thespecific charge starts at the first cycle witha value close to 800 mAh/g and dropsdown to reach only 300 mAh/g after 20 cy-cles. Additionally, it is noteworthy that thedifference between the charge and the dis-charge is very low, indicating a good Cou-lombic efficiency. For the n-Sn, the resultsare different with a similar specific chargefor the first cycle at around 750mAh/g, andstabilization over six cycles at a specificcharge of 650 mAh/g. After six cycles, thespecific charge collapses dramatically toreach only 50 mAh/g after 20 cycles. The

cell performance was measured in galva-nostatic mode at 25 °C between 5 mV and1.0 V at a C/30 rate and monitored by anASTROL cycling device. An additionalpotentiostatic step of 2h was added at theend of each discharge and charge to ensureproper sodiation and desodiation. All thepotentials mentioned in the manuscript aregiven versus Na+/Na.

Scanning Electron Microscopy(SEM)

SEM measurements were performedin a Carl Zeiss Ultra55 scanning electronmicroscope using the secondary electronmode.

Results and Discussion

Influence of Particle SizeThe SEM pictures of m-Sn and n-Sn

electrodes are displayed in Fig. 1 and showthe impact of the particle size on the elec-trode engineering. The m-Sn electrode re-veals a broad range of particle sizes fromca. 1 μm to 15 μm. Some Sn particles areclose to each other and in other areas thereare big gaps, indicating a certain inho-mogeneity at the micrometer range in thedispersion of the particles in the electrode.The images also show some Sn particlesthat are only partially covered by the car-bon Super C and the CMCbinder and somecracks around the particles. Those fracturesoccurring in the electrode can be problem-atic considering the huge volume changeoccurring during the sodiation of Sn par-ticles (especially for the m-Sn electrode).Particles that are only partially connectedto the carbon/binder/current collector arelikely to be totally disconnected during thecycling due to the strong strain caused by

for battery application) it is not surprisingthat nanoparticles are often presented to bebetter in terms of cycling stability, long-term cycling and power performance, asfor example reported recently by Nam etal.[34] and others.[35–39]

In this article, we establish an engi-neering guideline for conversion-basednegative electrodes used in Na-ion batter-ies. Sn was selected as a model conversion-based material that undergoes high volumechanges during cycling. Due to the insta-bility of the PVDF binder in Na-ion bat-teries, shown previously, it was decided touse the promising binder Na-CMC. Herewe compare the impact of different particlesizes on the electrochemical performancein electrodes of comparable loading. Ad-ditionally, we investigate the impact of thecarbon conductive agent in both the micro-particle and the nanoparticle systems,looking specifically at the use and impactof carbon Super C and carbon fiber. Thebest combination of binder/conductive ad-ditive/particle size for improved cyclingperformance of conversion-based negativeelectrodes in Na-ion batteries is then elu-cidated.

Experimental Section

Electrode PreparationFor the electrode preparation we used

two different tin powders: Sn-micro (AB-CR, 325 mesh) (hereafter called m-Sn)and Sn-nano (ABCR, ca. 100 nm, hereaf-ter called n-Sn). For the conductive agent,two different types were used: carbonblack SuperC65 (CB, Imerys) and vapor-grown carbon fibers (VGCF, Showa Den-ko). Electrodes without VGCF were pre-pared by casting a suspension of 70 wt%Sn-micro (or Sn-nano), 18 wt% CB and12 wt% carboxymethyl cellulose sodiumsalt (CMC, Alfa Aesar) in deionized wateronto aluminum foil, used as current collec-tor (described in the article as composition1). For electrodes containing VGCF, theratio between the elements wasmodified to70 wt% Sn-micro (or Sn-nano), 9 wt%CB,9 wt%VCGF and 12 wt%CMC (describedin the article as composition 2). Once driedin air, 13 mm diameter electrodes werepunched out and were dried under vacuumat 120 °C for a few hours. The loading ofactive material on the electrodes was be-tween 2.8 mg/cm2 and 3.5 mg/cm2.

ElectrochemistryElectrochemical cells were assembled

in an Ar-filled glove box using the elec-trode, a glass-fiber sheet as the separa-tor and metallic sodium (Sigma-Aldrich,75 μm) as counter electrode. A mixture of1 M NaClO

4dissolved in propylene car-

bonate (PC) was used as electrolyte. The Fig. 1. SEM images of m-Sn-micro (top) and n-Sn electrodes (bottom) in electrode composition 1.


distinguishable for both m-Sn and n-Snallowing some comparison to the first cy-cle. For m-Sn the two potential plateaus at0.27 V and 0.54 V are the most prominentfeatures but have shortened in comparisonto the 1st cycle. For n-Sn a new, small po-tential plateau is seen at 0.22 V followedby one at 0.28 V. Then the potential in-creases rapidly with a small bend at 0.55 Vand another at 0.81 V.

So far we demonstrated that whenthe loading of the electrodes is the same,nanoparticles do not outperform micropar-ticles after a few cycles. Instead, n-Snshows a poor Coulombic efficiency and adramatic specific charge fading. Nanopar-ticles can only competewithmicroparticlesif the loading is low. However, due to thelimited storage capacity such low loadingis not commercially viable. A similar com-parative approach was used to investigatethe most suitable conductive agent and op-timal electrode engineering conditions forthe best electrochemical performances.

Influence of Conductive AdditiveSEM pictures of the electrode designed

with VGCF are shown in Fig. 4 and can becomparedwith the one having only SuperCas a conductive additive (Fig. 1). TheVGCF, a long rod shape of around 100 nmdiameter, can easily be distinguished incomposition 2. For both particle sizes,the observations made previously aboutthe dispersion of Sn particles in Super C-based electrodes are also valid for VGCF-based electrodes: the micrometer particlesare not homogeneously distributed in theelectrode and the nanometer particles ag-glomerate slightly. The new electrodeconstituent, VGCF, shows a homogeneousdispersion throughout the electrode. Form-Sn VGCF electrodes, the Sn particlesare covered by the VGCF (Fig. 4), suchthat electrode fractures around the mi-cro Sn particles are bridged by the fibers.This is expected to lead to better electri-cal contact in the electrode during cycling.For the n-Sn, the situation is slightly dif-ferent since the diameter of the VGCF isroughly the same as the one of the primary

For the rest of the first sodiation, wecan see the three characteristic potentialplateaus of Na-Sn alloys at 0.2 V, 0.07 Vand 0.01 V. For the n-Sn electrode, thosepotential plateaus are slightly lower in po-tential, indicating a higher polarization.This higher polarization probably arisesfrom the larger SEI formation which hin-ders the lowest sodiation plateau to be ac-cessed since with the additional overpoten-tial it lies below the 5 mV cut-off voltage.On desodiation, a first potential plateau ap-pears in the m-Sn electrode at 0.16 V butno such potential plateau is seen for then-Sn electrode. Thereafter the desodiationplateaus of the n-Sn electrode (0.27 V and0.54 V) are analogous to those of the m-Sn electrode (0.26 V and 0.53 V), with theslight shift in potential being attributable tothe higher polarization in the n-Sn system(thicker SEI).

At the 10th cycle (Fig. 3, right), thepolarization has grown in both electrodessuch that the lowest potential plateau isno longer accessible for both systems. Ingeneral the different potential plateaus onsodiation are no longer distinguishable form-Sn electrode and for the n-Sn electrodeonly two vague potential plateaus are vis-ible at ca. 0.3 V and ca. 40 mV. In contrast,on desodiation potential plateaus are still

Coulombic efficiency in the case of n-Sn isrelatively low compared to the m-Sn.

The difference observed between thetwo electrodes m-Sn and n-Sn can be ex-plained by the difference in particle size.The nanoparticles have such high surfacearea that when cycling in a potential win-dow where the electrolyte is reduced, moreelectrolyte decomposition occurs and con-sequently more solid electrolyte interphase(SEI) forms, leading to a low Coulombicefficiency. With every cycle the SEI layeris thickened, its isolating character leadsto an increased resistance which finallyhinders cyclability. For the m-Sn the fad-ing is mainly caused by the particle frac-ture which occurs during sodiation. In factwhen fully sodiated the particles expand420%, leading to cracking which can causeloss of electrical contact of some particles.Those parts become dead materials thatcannot cycle anymore, leading to a gradualfading of specific charge. These results arein accordance with the SEM observationsand predictions, in that m-Sn was seen tointegrate less well in the carbon/binder ma-trix than n-Sn (Fig. 1) and consequently itis more likely to suffer from pulverizationand electrical disconnection of particles,leading to gradual specific charge fadingas seen in Fig. 2. However compared to then-Sn electrode, the m-Sn electrode has abetter specific charge after 20 cycles, com-ing from the fact that during cycling mostprobably after each fracture of the particle,a core-shell process is occurring and thus‘new-fresh’ Sn can be revealed and can becycled, which helps to maintain the spe-cific charge higher than the isolating n-Sn.

The galvanostatic curves presented inFig. 3 validate our hypothesis about theSEI formation in the n-Sn system. At ca.0.4 V during the first sodiation a long po-tential plateau ascribable to the SEI forma-tion is visible in the n-Sn sample whereasalmost nothing appears in the same voltagerange for the m-Sn electrode.

Fig. 3. Galvanostatic curves for the 1st (left) and 10th (right) cycle for the m-Sn and n-Sn elec-trodes.

Fig. 2. Cycling per-formance obtainedfor m-Sn and n-Sn incomposition 1.


potential plateaus are still distinguishable.With composition 1, the lowest potentialplateau at 0.16 V is no longer visible,however, with composition 2 an activ-ity is still present at this potential. At ca.0.28 V both compositions show a poten-tial plateau, however with composition 2,

corresponds to the lowest potential plateauin sodiation, is longer.

At the 10th cycle, features observed onsodiation have merged into sloping curvesfor electrode composition 1 and electrodecomposition 2. As already observed withcomposition 1, on desodiation the different

particles. The interaction between the n-Snparticles and the VGCF is therefore lim-ited and VGCF mainly serves to connectaggregates.

Looking now at composition 2 (withVGCF, Fig. 5), we can see that the trendbetween m-Sn and n-Sn is roughly thesame as the one observed for composi-tion 1. While the composition 2 does notseem to affect cycling for n-Sn, for m-Snit increases and helps to maintain the spe-cific charge. As already stated before form-Sn, when particle fracture occurs uponsodiation, this can lead to some particlesbecoming disconnected leading to uncy-clable material. Due to their length andhigh conductivity carbon fibers can com-bat this problem, allowing more Sn parti-cles to stay connected to the carbon/bindermatrix and current collector, which leadsto a higher specific charge retention (ca.400 mAh/g after 20 cycles). In n-Sn elec-trodes where the particles are well integrat-ed and disconnection is not a problem butinstead a thick SEI layer causes trouble,the addition of VGCF does not improvecycling performance.

This demonstrates that it is crucial tounderstand the cause of capacity fading anduse the appropriate electrode engineeringto combat the drawbacks of the active ma-terial. The VGCF improves the long-termcycling performance of the m-Sn electrodedue to better electrical contact. However,there is a small cost in Coulombic effi-ciency due to SEI formation on the surfaceof the carbon fibers. For n-Sn, where thickSEI formation leads to dramatic specificcharge fading, VGCF cannot help but in-stead, a coating of the Sn particles mayhelp to reduce electrolyte decompositionto improve long-term performance. Thegalvanostatic curves of m-Sn for the 1st

and 10th cycle in composition 1 and com-position 2 are shown in Fig. 6. In the firstcycle, a new feature at ca. 0.42V for thecomposition 2 electrode can be seen. Thisextra ‘bump’ is most probably due to theSEI formation at the surface of the VGCFleading to a slightly reduced Coulombicefficiency as previously mentioned. Wenotice a similar plateau in the n-Sn galva-nostatic curve for the composition 1. Theaddition of VGCF to the electrode compo-sition reduces the overpotential comparedto the standard Super C electrode, which isseen by the different Na-Sn sodiation po-tential plateaus all lying at slightly higherpotential.

This means that the lowest of the threecharacteristic tin sodiation potential pla-teaus at 0.01 V is fully accessible whichleads to a higher specific charge for com-position 2. On desodiation the curves ob-tained with composition 2 are analogousto the ones of composition 1 except thatthe potential plateau at ca. 0.16 V, which

Fig. 4. SEM images of electrode containing VGCF as conductive agent (composition 2) of m-Sn(top) and n-Sn (bottom).

Fig. 5. CyclingPerformance ob-tained with the com-position 2.

Fig. 6. Galvanostatic curves for the 1st (left) and 10th (right) cycle for m-Sn electrodes using eitherthe composition 1 or the composition 2.


it is much longer. At higher potentials, thelast potential plateau at 0.55 V matches forboth compositions. These differences in‘length’ (i.e. specific charge) indicate thatVGCF is of great help to maintain the co-hesion of m-Sn electrodes during the highvolume changes occurring while cycling.

The same analysis was conductedfor the n-Sn electrodes, and the galvano-static curves are plotted and presented inFig. 7. For the first sodiation the curvesmatch almost perfectly between the twocompositions. Only a tiny difference atca. 0.45 V and at 10 mV where the lastpotential plateau is slightly shorter can beseen. On desodiation we notice that thecapacity retention is better for composi-tion 1 than for composition 2 (though only20–30 mAh/g). After 10 cycles, the specif-ic charge fades dramatically compared tothe first cycles. The curves turn smoothercompared to the first cycle which makes itdifficult to see the different potential pla-teaux characteristic of the Na-Sn reaction.In both sodiation and desodiation we cansee an increase of the polarization for theelectrode with composition 2, which canbe attributed to a thicker SEI resulting inhigher resistance. A thicker SEI can be at-tributed to the nanoparticles themselves,since their specific area is high and leads tomore electrolyte decomposition and whenVGCF is present additional SEI formationoccurs on its surface. On sodiation, we cannotice a small feature at around 0.4 V at-tributed most probably to the SEI, and thena potential plateau at around 50 mV attrib-uted to the Na-Sn reaction. On desodiation,the different potential plateaus are morevisible and look the same for the two sys-tems. Once again the specific charge of theelectrode cycled with the composition 2 islower than the one cycled with the compo-sition 1 due to the observed overpotential.

Conclusion

Contrary to the frequent reports ofhigh-performance nanoparticle batteries,we find that nanoparticles offer no long-term advantage compared to micro-sizedparticles once the electrode loading is thesame. Regardless of any electrode engi-neering, the drawback of the high surfacearea of nanoparticles and the consequentelectrolyte decomposition on this surfacecannot be mitigated. For electrodes con-taining the m-Sn, adding carbon fibersleads to stabilization of the specific chargecompared to electrodes without such fi-bers. This improvement can be traced tothe conductive fiber maintaining electricalcontact between the active material andthe conductive carbon/binder matrix dur-ing the volume changes occurring upon cy-cling. A composition of 70% active mate-

rial, 12% carboxymethyl cellulose binder,9% Super C65 and 9% VGCF shows thebest electrochemical performance.We thusstress the need for careful electrode engi-neering already at the stage of fundamen-tal research, to be able to make conclusivestatements about how materials and theirperformances compare.

AcknowledgementThe Swiss National Science Foundation

is thanked for financial support (Project200021_156597). This work was performedwithin the Swiss Competence Center of EnergyResearch Heat and Storage (SCCER) frame-work.


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Fig. 7. Galvanostatic curves for the 1st (left) and 10th (right) cycle for n-Sn electrodes using eitherthe composition 1 or the composition 2.



*Correspondence: Prof. Dr. K. Fromm, Dr. N. H. Kwon,Department of ChemistryUniversity of FribourgChemin du Musée 9, CH-1700 FribourgE-mail: [email protected], [email protected]

Nanomaterials Meet Li-ion Batteries

Nam Hee Kwon*, Jean-Pierre Brog, Sivarajakumar Maharajan, Aurélien Crochet,and Katharina M. Fromm*

Abstract: Li-ion batteries are used in many applications in everyday life: cell phones, laser pointers, laptops,cordless drillers or saws, bikes and even cars. Yet, there is room for improvement in order to make the batteriessmaller and last longer. The Fromm group contributes to this research focusing mainly on nanoscale lithium ioncathode materials. This contribution gives an overview over our current activities in the field of batteries. Afteran introduction on the nano-materials of LiCoO2 and LiMnPO4, the studies of our cathode composition andpreparation will be presented.

Keywords: Cathode · Composite structure · Li-ion batteries · Nano-LiCoO2 · Nano-LiMnPO4

Nano-LiCoO2

Today’s commercial Li-ion batteriesare typically based on the layered struc-ture of the high-temperature (HT) phaseof LiCoO

2as active material in the form

of a micron-sized powder, using a carbonadditive as well as a binder to increasethe electronic conductivity and to processthe so-obtained paste into a cathode.[1]Commercial HT-LiCoO

2is produced via a

solid-state synthesis operating under oxy-gen at very high temperature (600–900 °C)and over long times (2–3 days).[2,3] Usingthe precursor method, the Fromm groupwas able to simplify the reaction con-ditions and to reduce i) the productiontime, ii) the reaction temperature as wellas iii) the grain size. Indeed, by reactingdry CoCl

2with different ratios of LiOR in

dry THF (R = Ph, tBu, Et, Me), we wereable to obtain Li-Co alkoxides and arylox-ides which combined the two metal ions.Compounds such as [(thf)

2Li(μ-OR)

2Co(μ-

OR)2Li(thf)

2] (for R = Ph) and [(thf)

2Li(μ-

OR)2Co(μ-OR’)

2Co(μ-OR)

2Li(thf)

2] (R =

Ph, R’ = tBu) could be identified.[4] Whilethe second compound has the correct stoi-chiometric ratio between the metal ionsand solely yields HT-LiCoO

2as combus-

tion product, the first compound yields alsoLi

2CO

3upon heating in air. This byproduct

can be eliminated by a short washing stepwith water.[4] Optimizing the alkoxide andaryloxide ligands, it was possible to gener-ate the desired high-temperature phase ofLiCoO

2at as low as 350 °C and within two

hours. The so-obtained nano-powders ofdifferent sizes, depending on the precursorand the temperature gradients used duringcombustion, are now being studied by usfor their electrochemical performance inSwagelok and coin cells. In comparisonwith the commercial micron-scale mate-rial, the nano-sized LiCoO

2is expected to

possess a higher Li-ion diffusivity as thegrains are smaller and the Li-ions shouldbe able to migrate in and out of the grainsmuch easier. Therefore, the reversible ca-pacities of nano-sized LiCoO

2are higher

at high C-rates (>1 C) compared to the mi-cron-sized LiCoO

2.[5,6]We will soon report

on these results.

Nano-LiMnPO4

In parallel to this research on LiCoO2,

we are also developing the technologyaround nano-scale LiMnPO

4, an olivine-

type material with high structural stabil-ity.[7] Compared to the commercially usedLiFePO

4, the Mn-analogue has a 20%

higher energy density based on its higherpotential versus Li+/Li. On the other hand,it has a lower ionic and electronic con-ductivity resulting in a poor capacity andrate capability.[8] One way to overcomethese drawbacks is to generate nano-scaleLiMnPO

4and to add conductive carbon

additives.[9–13] Indeed, smaller particles re-duce the Li-ion diffusion pathway and im-prove hence the ionic conductivity, whilecarbonaceous material such as graphite orcarbon black improves the electronic con-ductivity (Scheme 1).[12]

We therefore studied i) different syn-theses to obtain nanoscale LiMnPO

4in

order to gain control over grain size andshape, ii) the influence of the particle size,and iii) the amount of carbon additive nec-essary to obtain full capacity.

In a previous publication,[12] it wasshown that for sample sizes ranging from410 nm to 140 nm, the smaller the parti-cle size, the better the reversible capacity(Fig. 1).

We therefore fine-tuned and improvedthe synthesis pathway using the polyol di-rect precipitation method (Scheme 2) inorder to obtain yet smaller particles withdifferent shapes.

In particular, we added a ligand ex-change step to eliminate remaining sur-factant from the surface of the particles,reducing thus the amount of remaining car-bon residue after the final heating step.[15]

Scheme 1. a) Comparison of the maximumLi-ion (blue) diffusion pathway (red) as a func-tion of particle size of active material (valid forLiCoO2 as well as LiMnPO4); b) ideal mixing ofactive material LiMnPO4 (olive green) with car-bon black (black) as conductive additive for theimprovement of electronic conductivity.


Carbon Additives and CathodePreparation Methods

Since many different carbon additivesare available on the market, and since thegeneration of homogeneous nanocompos-ites is non-trivial, we decided to study bothaspects. In a first test, four different com-mercial carbon additives, two graphitic andtwo carbon black materials, were testedunder similar conditions with commercialmicron-scale LiCoO

2.[16] The platelet-

shaped graphite material has a lower spe-cific surface area compared to the nano-sized carbon black with spherical particles.We expected that carbon black would thusperform better than graphite, as it wouldmix better with the LiCoO

2-particles. In

parallel, we also investigated the influenceof the ball-milling on the formation of thecomposites.[17] For this study, we expecteda longer ball-milling to lead to a more ho-

Indeed, we succeeded in controllingthe reaction conditions such that we couldreduce the particle sizes below 30 nm ob-taining different shapes, from spherical torod-like (Fig. 2), and, more importantly, invery high yield.[15]

The nanoscale LiMnPO4shown in

Fig. 2 (c) was used to study the best com-position of a cathode in terms of carbonadditive content. The higher percentageof active material versus carbon addi-tive required to improve the electronicconductivity, the better is the capacityper weight of an electrode. While in theliterature, amounts of 20–40% of carbonare reported, we could show that as littleas 10% of carbon black could give rise tothe full theoretical capacity of 170 mAh/g

(b) (c)

Scheme 2. a) A schematic of the process of LiMnPO4 surface modification from oleic acid to citricacid. Chloroform (bottom) and water (above) solution before b) and after c) ligand exchange.[15]

Reprinted from ref. [15] with permission from Elsevier.

(a)

at a charging rate of C/40 (Fig. 3).[15]Thisbrings LiMnPO

4into the game as potential

commercial product for cathode materialsof future Li-ion batteries.

Fig. 3. a) The charge and discharge curves of 10 wt% carbon added nano-LiMnPO4 cathode. b) The rate capability of the same cathode.

(a) (b)

Fig. 2. TEM morphologies of LiMnPO4 controlled by the concentration of oleic acid, the reaction temperature and time. a) and b) are 9:1 and 3:1 inthe molar ratio of oleic acid and precursors at 280 °C, respectively. c) is obtained at the reaction temperature of 265 °C. d) is obtained by the reac-tion time of 1 h. The molar ratio of 3:1 = oleic acid and precursors in c) and d). The scale bar is 100 nm.

(a) (b) c) d)

140120100806040200

Reversiblecapacity(mAh/g)

987654321Number of cycle

140 nm156 nm200 nm270 nm410 nm

C/10

Fig. 1. The revers-ible capacities ofLiMnPO4 consistingof different particlesizes. The smallestparticle size showedthe highest reversiblecapacity.[12,14]


Broekmann group of WP4 of the SCCER,and we have worked here on the synthesisand electrochemical analysis of pseudo-capacitor multilayer thin films.[22]

AcknowledgementThe authors thank the Swiss National

Science Foundation for generous funding(NRP-64, NRP-70), and the Swiss Center forCompetence in Energy Research (SCCER)Heat and Electricity Storage in collaborationwith the CTI and the University of Fribourg aswell as the Fribourg Center for Nanomaterials,FriMat, for their motivating support. We alsothank EKZ (Elektrizitätswerke des KantonsZurich) for the innovation grant and PolytypeSA for the collaboration.


[1] K. Mizushima, P. C. Jones, P. J. Wiseman, J. B.Goodenough, Mater. Res. Bull. 1980, 15, 783.

[2] M. Yoshio, H. Tanaka, K. Tominaga, H.Noguchi, J. Power Sources 1992, 40, 347.

[3] J. N. Reimers, J. R. Dahn, J. Electrochem. Soc.1992, 139, 2091.

[4] A. Crochet, J.-P. Brog, K. M. Fromm, WO2012000123, 2012.

[5] M. G. Kim, J. Cho, Adv. Funct. Mater. 2009, 19,1497.

[6] A. S. Aricò, P. Bruce, B. Scrosati, J.-M.Tarascon, W. V. Schalkwijk, Nat. Mater. 2005,4, 366.

[7] A. K. Padhi, K. S. Nanjundaswamy, J. B.Goodenough, J. Electrochem. Soc. 1997, 144,1188.

[8] M. Yonemura, A. Yamada, Y. Takei, N.Sonoyama, R. Kanno, J. Electrochem. Soc.2004, 151, A1352.

[9] D. Rangappa, K. Sone, Y. Zhou, T. Kudo, I.Honma, J. Mater. Chem. 2011, 21, 15813.

[10] P. Barpanda, K. Djellab, N. Recham, M.Armand, J.-M. Tarascon, J. Mater. Chem. 2011,21, 10143.

[11] S.-M. Oh, S.-W. Oh, C.-S.Yoon, B. Scrosati, K.Amine, Y.-K. Sun, Adv. Funct. Mater. 2010, 20,3260.

[12] N.-H. Kwon, T. Drezen, I. Exnar, I. Teerlinck,M. Isono, M. Graetzel, Elecrochem. Solid StateLett. 2006, 9, A277.

[13] C. Delacourt, P. Poizot, M. Morcrette, J. M.Tarascon, C. Masquelier, Chem. Mater. 2004,16, 93.

[14] T. Drezen, N.-H. Kwon, P. Bowen, I. Teerlinck,M. Isono, I. Exnar, J. Power Sources 2007, 174,949.

[15] N.-H. Kwon, K. M. Fromm, Electrochim. Acta2012, 69, 38.

[16] N. H. Kwon, Solid State Sci. 2013, 21, 59.[17] N. H. Kwon, H.Yin, P. Brodard, C. Sugnaux, K.

M. Fromm, Elecrochim. Acta 2014, 134, 215.[18] M. Priebe, K. M. Fromm, Chem. Eur. J. 2015,

21, 3854.[19] M. Priebe, K. M. Fromm, Part. Part. Syst.

Charact. 2014, 31, 645.[20] J. Gagnon, P. Weber, K. M. Fromm, Europ.

Cells & Materials 2011, 21, 49.[21] J. Gagnon, M. J. D. Clift, D. Vanhecke, D.

A. Kuhn, P. Weber, A. Petri-Fink, B. Rothen-Ruthishauser, K. M. Fromm, J. Mat. Chem. B2015, 3, 1760.

[22] V. Kaliginedi, H. Ozawa, H. Kuzume, S.Maharajan, I. V. Pobelov, N. H. Kwon, M.Mohos, P. Broekmann, K. M. Fromm, M. Haga,T. Wandlowski, Nanoscale 2015, 7, 17685.

determine the best conditions for high-quality electrodes.

Conclusions, Ongoing Work, andOutlook

We are now able to provide nano-scaleLiCoO

2and LiMnPO

4in a multi-gram

scale to partners who wish to test batterypacks. Furthermore, the Fromm grouphas considerable expertise in the encap-sulation of nanoparticles into porous in-organic shells.[18–21] We have thus startedto investigate the use of encapsulated Sn-nanoparticles as anode material for Li-ionbatteries. Within the framework of theNRP-70 of the Swiss National ScienceFoundation, research on the next genera-tion batteries, such as Li–air and Li–waterare ongoing in order to provide new mem-branes in collaboration with the group ofH.G. Park at ETHZ, as well as electrolytes,which can also serve in classical Li-ionbatteries.

Within the framework of the SCCER‘Heat and Electricity Storage’, we contrib-ute to the work package WP1 by provid-ing our nanoscale cathode materials, as-sembling half cells against Li-metal andfull cells against anode material providedby e.g. the group of M. Kovalenko fromETHZ. In situ and in operando studies ofour composite materials are ongoing incollaboration with the group of P. Novakand C. Villevieille at the PSI. The Frommgroup also collaborates with the Peter

mogenous composite and hence to betterelectrochemical properties.

For the generation of the compos-ite, SEM and TEM analyses surprisinglyshowed that the platelet-shaped graph-ite assembles homogeneously with mi-cron-sized LiCoO

2using the ball-milling

method. Furthermore, a 5-min ball-millingprocess provided better results and smallerparticles than a 30-min or 60-min ball mill-ing or no ball-milling at all. Graphite thusturned out to provide a superior quality ofthe composite cathode material with a stillhigh specific surface area. It also gives bet-ter quality cyclic voltammograms (Fig. 4,top) and leads to higher specific capacities(Fig. 4, bottom) of the LiCoO

2electrodes

compared to nano-sized spherical carbonblack.

The nano-sized carbon black on theother hand aggregates during the ball-milling process and forms isolated clus-ters for which the specific surface area isdramatically reduced. Furthermore, thethick coating of nano-carbon black on theLiCoO

2particles renders the contact with

the liquid electrolyte difficult. We furtherassume that a long ball-milling processleads to a heating of the material and henceformation of aggregates and particle/crys-tallite growth. Our two studies thus leadto the conclusion that a 5-min ball millingprocess of micron-scale LiCoO

2with gra-

phitic carbon provides the best compositecathodes. We are currently studying theseeffects also for our above-mentioned na-noscale LiCoO

2and LiMnPO

4in order to

(d)

0

50

100

150

200

250

0 1 2 3 4 5 6 7 8 9Dischargecapacity(mAhg-1)

Number of cycle

LiCoO2/Ket_0 minLiCoO2/Ket_5 minLiCoO2/Ket_30 minLiCoO2/Ket_60 min

Fig. 4. Top: Cyclic voltammograms of a) SFG/LiCoO2 and b) Ket/LiCoO2 composite electrodes.The composites were prepared by ball milling for various milling time.[17] Bottom: Specific capaci-ties of LiCoO2 electrodes containing the composites with either (c) SFG or (d) Ket by various ballmilling times.[17] Reprinted from ref. [17] with permission from Elsevier.

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

2.5 3 3.5 4 4.5

Current(mAcm

-2)

Potential (V vs. Li+/Li)

0 min5 min30 min60 min

0.1 mV/s

(a)

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

2.5 3 3.5 4 4.5

Current(mAcm

-2)

Potential (V vs. Li+/Li)

0 min5 min30 min60 min

0.1 mV/s

(b)

(c)

0

50

100

150

200

250

0 1 2 3 4 5 6 7 8 9

Dischargecapacity(mAhg-1)

Number of cycle

LiCoO2/SFG_0 minLiCoO2/SFG_5 minLiCoO2/SFG_30 minLiCoO2/SFG_60 min


*Correspondence: M. HeldReliability Science and Technology LaboratoryEmpaÜberlandstrasse 129CH-8600 DübendorfE-mail: [email protected]

Stress-induced Ageing of Lithium-IonBatteries

Marcel Held* and Urs Sennhauser

Abstract: Lithium-ion batteries are well established for use in portable consumer products and are increas-ingly used in high power electro-mobility and photovoltaic storage applications. In hybrid and plug-in electricvehicles degradation and useful lifetime at standard operation conditions are critical parameters in addition toperformance and safety. Here stress-induced ageing of commercially available high power battery cells of thetype A123 AHR32113M1 Ultra-B, consisting of a LiFePO4 cathode and a graphite anode have been investigat-ed. A usually accepted capacity loss for electric vehicles of 20% was reached after 8560 stress profiles cor-responding to a driving distance of almost 200’000 km. Cycling with a stress profile applying constant powercorresponding to the average power and energy of a full stress profile and starting at 60% state of chargeshowed a much faster capacity loss. Electric impedance measurements show the dependence of the capacityloss and constant phase element at low frequency, indicating Li-ion diffusion blocking in the cathode. Micro-scopic analysis of anode, separator, and cathode, shows defect formation in bulk material and at interfaces.

Keywords: Electrochemical impedance spectroscopy · Li-ion batteries · Microscopy · Stress cycles

1. Introduction

Lithium-ion rechargeable batteries pro-vide high volumetric and gravimetric en-ergy and power density, have no memoryeffect and show very little self-dischargewhen not in use. Despite some early safetyand lifetime concerns they are now wellestablished for use in portable consumerelectronic products and they are increas-ingly used in electro-mobility, energystorage, aerospace, and medical implants.Li-ion batteries, including cells and man-agement electronics, are optimized forthe various applications concerning per-formance, environmental conditions, loadprofile, useful lifetime, biocompatibility,and system reliability and costs. Despiteextensive investigations in industrial and

academic work[1–4] to identify degradationmechanisms of connectors, electrodes,electrolytes, separators, and various inter-faces, standardized procedures for differ-ent applications have yet to be established.

In this study the focus lies on capac-ity loss for different stress cycles, state ofcharge (SOC) ranges, and storage periods.Electrochemical impedance spectros-copy and microscopy studies have beenperformed to investigate capacity lossand degradation processes. Adapting thebattery power cycling to the worldwideharmonized light vehicles test procedure(WLTC) to electric vehicles allows an es-timation of their total cruising range until20% of the nominal capacity is lost.

2. Test and Electrical MeasurementProcedures

In thisworkcommercialhighpowerbat-tery cells of the type A123 AHR32113M1Ultra-B, consisting of a LiFePO

4cathode

and a graphite anode were investigated.New battery cells exhibit a nominal volt-age of 3.3 V, a capacity of 4.4 Ah, and amaximum discharge power of 550 W. Thistype of battery has been designed for hy-brid power trains of light vehicles.[5] Thefull battery consists of 70 single cells in se-ries to provide a total capacity of 1.04 kWhand a maximum power of 20 kW.

In order to study the capacity loss, re-chargeable Li-ion cells are usually cycledby a complete charge and discharge pro-cess.Forautomotiveapplications, so-calledworldwide harmonized light vehicles testprocedures (WLTP) were developed to

measure the real power consumption dur-ing drive cycles (WLTC).[6] In this work theWLTC driving cycle class 3 – comprisinglow, medium, high, and extra high speedparts – was used to derive the power stressprofile of a single cell of the battery pack(Fig. 1). The maximum power is 284.5 Wfor charging and 356.8 W for dischargingand the average power 80.56 W for charg-ing and 55.52 W for discharging. The totaltime of the stress profile is 1800 secondswhereas 996 seconds are charging and 804seconds discharging. In order to optimizethe overall energy consumption of the hy-brid power train and to keep the battery ina defined state the state of charge (SOC) ofthe cell is 60% (2.64 Ah) at beginning andend of the power stress cycle.

Cell 1 was aged with WLTC powerstress cycles. During each cycle the cellfluctuates between 42% and 100% SOC.Intermediate characterization measure-ments were performed every 51 cycles.

For cell 2 the average power values oftheWLTC procedure were used to first dis-charge the cell for 804 s and then chargeit for 996 s. With this procedure the cellwas completely discharged and charged,i.e. SOC fluctuated between 0 and 100%.

The average power values of the powerstress profile were also taken for cell 3 andcell 4, but with modified time and SOCspans. Cell 3 was discharged and chargedwith 0.88 Ah between 40% and 60% SOC,whereas cell 4 was discharged and chargedwith 1.76 Ah between 20% and 60%SOC. Cells 3 and 4 had been stored for10 months at 25 °C ambient temperatureand were (over-) discharged. However theywere revitalized by slow charging showing


charged state. This shows that the absolutecell capacity is not always a conclusiveindicator for its state of health and over-discharged periods have to be consideredwhen evaluating stress induced ageing andestimating residual lifetime. In conclusion,theWLTC profile was gentler compared topure charge-discharge cycles with compa-rable power stresses, as performed withcells 2 to 4 (Table 1).

3.2 Impedance SpectraImpedance spectra of cells 1 to 4 taken

at 60% SOC are presented in Fig. 3. Spec-tra of cell 1 have been already published byCuervo Reyes et al.[8]At medium frequen-cies an increase of the resistance Re(Z)with ageing is observed. At the initial statecells 1 and 2 show a typical RC-semicircle,which is attributed to the charge transferresistance at the cathode-electrolyte inter-face and the current collector interfaces.Their shift to the right and up at the endof test is an indication of increased imped-ance of electrode contacts and electrolyte.Cells 3 and 4 show a different impedancecharacteristic already at the initial state be-cause they were stored for long periods ina discharged state, which leads to decom-position of electrolyte and probably to dis-solution of copper from the anode contact.

The low frequency end of the imped-ance is dominated by the concentration-driven Li-ionmass transport in the cathode,which for ideal diffusion should appear asa straight line with slope one.[9,10]A changeof this slope with ageing has been reportedand discussed within the constant phaseelement model[8] in which the increase ofthe slope of impedance is explained bysub-diffusive ion transport. The constantphase behavior has the form Z = (iω)(α/2–1)for concentration-driven transport, and Z= (iω)(α–1) for voltage-driven electronictransport with α = 0 for perfect insulatorsand α = 1 for perfect diffusive transport.

3. Results and Discussion

3.1 Discharge CapacityFor standard applications end-of-life of

batteries is usually defined to be reachedwhen the capacity is reduced to 80% oftheir nominal discharge capacity.[7] Withcycling according to the WLTC procedurecell 1 was fluctuating on each cycle be-tween 42% and 100%SOC. It reached 80%of the nominal capacity after 8560 WLTCstress profiles (Fig. 2), corresponding toa vehicle-driven distance of 195’779 km.Cell 2 was cycled between 0% and 100%SOC with the average power of the WLTCprocedure. End-of-life capacity of 80%was reached after 1175 cycles (Fig. 2).

The total energy throughput to reachthe 80% nominal capacity was 284.2 kWhfor cell 1 and 38.1 kWh for cell 2, respec-tively. This demonstrates that the totalcapacity throughput until end-of-life isreached strongly depends on stress pro-files and is application specific. During thestorage period at room temperature beforestress cycling cells 3 and 4 lost about 15%of their nominal capacity. The comparablyinferior performance of cells 3 and 4 canbe explained by their storage time in a dis-

a lower discharge capacity of 3.79Ah (cell3) and 3.87 Ah (cell 4).

Cell 5 served as unstressed referencefor microscopic analysis.

Capacity measurements were per-formed between power stress cycles in or-der to determine the discharge capacity ofthe cells. The characterization procedurestarted with a complete discharge of thecell to the discharge end voltage of 1.6 V.Then the cell was charged using the con-stant current–constant voltage procedure(1 C: voltage limit of 3.8 V, terminationcurrent of 0.22 A equivalent to 0.05 C) todetermine the charge capacity. In orderto obtain the discharge capacity the cellwas discharged with a current of 0.88 A(0.2 C) to the end voltage of 1.6 V. Imped-ance spectroscopy (IS) was conducted oncell 1 in a frequency range from 1 mHz to10 kHz. During the cycling experiments,the IS at 183 different aging stages wasmeasured, one every 51 stress profiles. Ateach stage four spectra were recorded: at20, 40, 60, and 80% SOC. All stress testsand characterization measurements wereperformed in a temperature-controlledsafety chamber at 23 °C using the MaccorSeries 4000 with a FRA 0355 setup.

Fig. 2. Discharge capacity of the investigated cells as a function of the number of stress cycles or applied WLTC profiles, respectively, (a) absolutecapacity, (b) normalized to 100% capacity at the beginning of stress cycles.

3

3.2

3.4

3.6

3.8

4

4.2

4.4

4.6

4.8

0 2000 4000 6000 8000 10000 12000

discha

rgecapa

city

[Ah]

stress cycles

Cell 1 SOC 42-100%

Cell 2 SOC 0-100%

Cell 3 SOC 40-60%

Cell 4 SOC 20-60%

80% of nom. cap.

60

65

70

75

80

85

90

95

100

0 2000 4000 6000 8000 10000 12000

%of

initial

discha

rgeca

pacity

stress cycles

Cell 1 SOC 42-100%Cell 2 SOC 0-100%Cell 3 SOC 40-60%Cell 4 SOC 20-60%

a) b)

-300

-200

-100

0

100

200

300

400

0 300 600 900 1200 1500 1800

cellpower

[W]

time [s]

Fig. 1. Derived WLTCcell power profileversus time: posi-tive values representcharging, negativevalues discharging.[6]


with a size of about 10 nm at the surface ofthe carbon grains and the copper interfacecan be observed. They are also observed oncathode and separator surfaces. They maybe explained by salt precipitation from theelectrolyte and may have grown during thedrying process.

In the LiFePO4cathode of cell 4 re-

gions of enhanced density with a size ofseveral microns are observed (Fig. 8). EDXanalysis (Fig. 9) reveals that they containan order of magnitude more carbon thanon average, presumably added by the cellmanufacturer to increase the conductivityof otherwise poorly conductive LiFePO

4

imaging and material analysis, an electronbeam with secondary electron detection(SE) and EDX were used. High resolutionion beam imaging was done with a ZeissOrion He-FIBwith a specified spatial reso-lution of 0.35 nm.

An overview of the polished cross sec-tion showing a stack with anode, separa-tor, cathode, and again separator is givenin Fig. 5. To reduce material re-depositionduring the FIB polishing process on theapproximately 160 μm thick cross section,separate regions were polished from twoopposite faces and combined in the splitimage of Fig. 5 for better visualization oflayers and interfaces. Starting from the topof the polished cross section the carbonanode with interlaced copper, a separatorwith three layers (high-low-high density),the LiFePO

4anode with interlaced alumi-

num, and again a separator with three lay-ers at the bottom starting the next stack ofthe rolled topology of the cell can be iden-tified. The copper and aluminum sheets ofanode and cathode, respectively, are con-nected to the outside of the battery cell.In Fig. 6 an expanded view of the three-layered separator is given.

The carbon anode is irregularly struc-tured with surface corrugated grains andvoids with size of order 1 to 10 μm. At theinterface of carbon and copper conduc-tor (Fig. 7) strings of residual binder areshown. In He-ion images (expanded insertin upper-right corner of Fig. 7) particles

Considering that Li-ion cells are complexsystems composed of inhomogeneous ma-terials with local deviations of ionic andelectronic conductivity one cannot excludefrom impedance spectra alone that elec-tronic transport in the cathode can becomethe limiting factor at low frequencies, too.

3.3 Microscopic AnalysisTo perform microscopic analysis cells

4 and 5 were opened, unrolled (Fig. 4), andsamples of about 1.5 × 2.5 cm at selectedpositions have been extracted. The end fac-es of the samples were properly cut with apicosecond laser and afterwards polishedwith the Ga-beam of about 20 nA and 30keV of a FEI Helios NanoLab 660 G3 UCDualBeam Ga-FIB/SEM microscope. For

Table 1. Summary of stress cycle parameters until cells reached 3.52 Ah discharge capacity

Charged

[Ah]

Discharged

[Ah]

Total

[Ah]

Stresstime[h]

Total Energythroughput[kWh]

# of cycles to 80%of nominal capacity(80% of 4.4Ah)

Cell 1 42426 42328 84754 4579 284.2 8560

Cell 2 5784 5780 11564 690 38.1 1175

Cell 3 2816 2816 5632 282 18.8 3200

Cell 4 5808 5808 11616 580 38.5 3300

0

0.005

0.01

0.015

0.02

0.025

0 0.005 0.01 0.015 0.02 0.025

-Im(Z)[Ω

]

Re(Z) [Ω]

Cell 1 initial

Cell 1 end of test

Cell 2 initial

Cell 2 end of test

Cell 3 initial

Cell 3 end of test

Cell 4 initial

Cell 4 end of test

0.E+00

2.E-04

4.E-04

6.E-04

8.E-04

1.E-03

0.002 0.003 0.004 0.005 0.006 0.007

-Im(Z)[Ω]

Re(Z) [Ω]

Fig. 3. Nyquist plotof the electric impe-dance of cells 1 to4 at initial state andend of tests, taken at60% SOC and in thefrequency range from1 mHz to 400 Hz. Theinsert in the upperleft corner shows anexpanded view ofthe lower left corner(medium frequencybehavior).

Fig. 4. Opened cell 5 showing from outsideto inside separator, cathode (LiFePO4 – Al -LiFePO4 stack), separator, anode (C – Cu - Cstack).

Fig. 6. SEM image of cross section of the3-layered structure of the separator of cell 4.

C

Cu

C

Sep.

LiFePO4

LiFePO4

Al

Sep.

Fig. 5. SEM image of cross section of elec-trodes and separator of cell 4 prepared bypicosecond laser ablation and polished byGa-FIB. The vertical waterfall structure is anartefact of polishing.


finite element calculations may contributeto resolve some ambiguous interpretationsof impedance spectroscopy data.

AcknowledgmentsWe thank D. Adams, R. Brönnimann, E.

Cuervo-Reyes, M. Stiefel, C. Pecnik, and E.Stilp for their contributions to sample prepara-tions, measurements, and discussions.


[1] R. Hausbrand, G. Cherkashinin, H. Ehrenberg,M. Gröting, K. Albe, C. Hess, W. Jaegermann,J. Mater. Sci. Eng. B 2015, 192, 3.

[2] J. Vetter, P. Novák, M.R. Wagner, C. Veit, K.-C. Möller, J.O. Besenhard, M. Winter, M.Wohlfahrt-Mehrens, C. Vogler, A. Hammouche,J. Power Sources 2005, 147, 269.

[3] A. Barré, B. Deguilhem, S. Grolleau,M. Gérard,F. Suard, D. Riu, J. Power Sources 2013, 241,680.

[4] E. M. Fellberg, Ph.D. Thesis Univ. Munster No.10156, 2012.

[5] T. Ott, C. Onder, L. Guzzella, Energies 2013, 6,3571.

[6] Transport Division/World Forum forHarmonization of Vehicle Regulations (UN/ECE/WP29).

[7] IEC 62660-1, Secondary Lithium-Ion Cells forthe Propulsion of Electric RoadVehicles, Part 1:Performance Testing, 2010.

[8] E. Cuervo-Reyes, C. P. Scheller, M. Held, U.Sennhauser, J. Electrochem. Soc. 2015, 162.8,A1585.

[9] U. Tröltzsch, O. Kanoun, H.-R. Tränkler,Electrochim. Acta 2006, 51.8, 1664.

[10] J.R. Macdonald, E., Barsoukov ‘Impedancespectroscopy: theory, experiment, andapplications’, Wiley, Hoboken, 2005.

In conclusion the strong dependence ofuseful lifetime of LiFePO

4batteries from

stress profile, state of charge, and envi-ronmental conditions require quantitative

estimations of the influence of the differ-ent material degradation processes of thecomplex battery system for efficient opti-mization. Microscopic analysis of stressedand unstressed cells combined with elec-tronic and ionic transport modeling and

material. To estimate the influence of thisinhomogeneous carbon distribution oncathode efficiency, further investigationsare required.

At the interface in the cathode ofLiFePO

4and Al (Fig. 10) the formation of

cavities at the few micrometer scale canbe observed. In the expanded insert in theupper-right corner it is observed that for alarge part of the cross section the crackingdid not occur at the interface itself, but afew tens of nanometers into the LiFePO

4cathode layer.

Fig. 7. SEM image of anode material at thecopper interface of cell 4 showing the cor-rugated surface of the carbon particles andstrings of binder. In the expanded view of abinder string taken with a He-FIB and dis-played in the top-right corner the depositionof nanoparticles of about 10 nm diameter canbe observed. Similar particle deposition is alsoobserved at inner surfaces of separator andcathode material.

Fig. 8. He-FIB image of a compact area of thecathode.

Fig. 10 He-FiB image of cathode aluminum in-terface of cell 4 showing cavity formation nearthe interface. The expanded view in the upperright corner demonstrates that cracking pref-erentially occurs not at the surface layer but inthe cathode material.

All Elements

/ CKseries/ 0 Kseries

P K series/ Fe Kseries

1 2 3 4 5 6 7

Fig. 9. EDX analysis of cell 4 along the indicated line (upper figure) showing high C content ofcompact areas of cathode material.


*Correspondence: Prof. Dr. A. Züttelab

E-mail: [email protected] of Materials for Renewable Energy(LMER)Institute of Chemical Sciences and Engineering (ISIC)Basic Science Faculty (SB)École polytechnique fédérale de Lausanne (EPFL)Valais/WallisEnergypolis, Rue de l’Industrie 17, CP 440,CH-1951 Sion, SwitzerlandbEMPA Materials Science & Technology, Dübendorf,Switzerland

Storing Renewable Energy in theHydrogen Cycle

Andreas Züttel*ab, Elsa Calliniab, Shunsuke Katoab, and Züleyha Özlem Kocabas Ataklib

Abstract: An energy economy based on renewable energy requires massive energy storage, approx. half of theannual energy consumption. Therefore, the production of a synthetic energy carrier, e.g. hydrogen, is neces-sary. The hydrogen cycle, i.e. production of hydrogen from water by renewable energy, storage and use ofhydrogen in fuel cells, combustion engines or turbines is a closed cycle. Electrolysis splits water into hydrogenand oxygen and represents a mature technology in the power range up to 100 kW. However, the major tech-nological challenge is to build electrolyzers in the power range of several MW producing high purity hydrogenwith a high efficiency. After the production of hydrogen, large scale and safe hydrogen storage is required.Hydrogen is stored either as a molecule or as an atom in the case of hydrides. The maximum volumetrichydrogen density of a molecular hydrogen storage is limited to the density of liquid hydrogen. In a complexhydride the hydrogen density is limited to 20 mass% and 150 kg/m3 which corresponds to twice the densityof liquid hydrogen. Current research focuses on the investigation of new storage materials based on combina-tions of complex hydrides with amides and the understanding of the hydrogen sorption mechanism in order tobetter control the reaction for the hydrogen storage applications.

Keywords: Electrolysis · Energy storage · Hydrides · Hydrogen · Synthetic hydrocarbons

Introduction

The energy turnaround requires thestorage of large amounts of renewable en-ergy, including seasonal storage. In centralEurope for the complete coverage of ourenergy demands a storage capacity thatcorresponds to 2000 kg of oil per capitawould be required. The transition from anenergy economy based on mining resourc-es, i.e. materials and fossil fuels, to a soci-ety based on renewable energy and closedmaterials cycles is essential for the globaldevelopment towards a sustainable andprosperous economy. The growth of theworld population depends on wealth dis-tribution, and according to the analysis ofHansRosling theworld population is goingto increase to approx. 11 billion people in2100. The extrapolation is based on the as-sumption that global wealth will continueto grow and the birth rate will accordinglydecrease worldwide. The global energy de-

mand is expected to increase from todayto 2050 by a factor of 3. This will only bepossible if the materials cycles are closed,especially the materials used as energy car-riers. Despite the plans to reduce energyconsumption in the western industrializedcountries in the future the political strat-egies are to grow economically, which isonly possible with growing consumption.Physically, wealth is the availability ofmaterials and energy, therefore, increasingwealth requires an increase in energy andmaterial consumption. More than 80% ofthe energy demand today is covered by fos-sil fuels, i.e. hydrocarbons that release CO

2and H

2O upon combustion with air. Unlike

water CO2does not precipitate out of the

atmosphere. Due to the limited resourcesof fossil fuels and materials, and the ef-fect of the increasing CO

2concentration

in the atmosphere on the climate, a largepart of the increasing energy demand mustbe covered by renewable energy sources.Therefore, the hydrogen cycle, i.e. theproduction of hydrogen from renewableenergy and water, the storage of hydrogenand the combustion of hydrogen in a fuelcell, internal combustion engine or a tur-bine offers a technical feasible solution toproduce an energy carrier directly from re-newable energy in a naturally closed cycle.

In this article we describe the basis andcurrent level of technological developmentof water electrolysis and hydrogen storagein hydrides and describe the recent devel-opments and achievements connected tothe Swiss Competence Center in EnergyResearch (SCCER).

Hydrogen Production byElectrolysis

The Swiss company Lonza SA wasfounded in Gampel (VS) in 1897 andused electricity to produce calcium car-bide and acetylene. In the beginning ofthe 20th century the products expandedto synthetic fertilizers from nitrogen andammonia and, therefore, the need for hy-drogen grew. Around 1940 Lonza decidedto produce the hydrogen on site. Ewald A.Zdansky was mandated to develop a gasgenerator able to deliver the hydrogen re-quirements for the chemical production.Due to the hydroelectric power availablein the region Zdansky worked on the con-struction of an industrial electrolyzer incollaboration with the Giovanola FrèresSA (GFSA), located in Monthey, thathad the large manufacturing tools to pro-duce the first prototype electrolyzer.[1] In1950 the development led to a patent ofthe high-pressure industrial electrolyzer(Zdansky-system) manufactured at GFSAand commissioned at Lonza.[2] The mar-ket demand for electrolyzers was growingand Lonza sold the intellectual property ofthe high pressure electrolyzer to LURGI(‘Metallurgische Gesellschaft’) in Butz-bach, Germany. LURGI commercializedthe electrolyzers and further improved thedesign of the electrodes and the mechani-cal assembly in collaboration with GFSAin Monthey, where Jurgen Borchardt (aLURGI engineer) managed the researchand development. More than 100 alkalinehigh-pressure electrolyzers with a power


exhibit a significantly higher efficiencycompared to PEM electrolyzers of compa-rable power. There are only a few suppli-ers of electrolyzers on the western market.Very little is known about the develop-ments in Asia, where certainly companiesin China and Japan are working on thedevelopment of large-scale electrolyzers.Hydrogenics[7] delivers alkaline electro-lyzers (250 kW) and Proton OnSite (Dia-mond Lite SA) delivers PEM electrolyzerswith a power up to 200 kW. Recent devel-opment for large-scale PEM electrolyzersby Proton OnSite[8] and SIEMENS[9] willsoon make systems >1MW available.

Hydrogen Storage

The critical point of hydrogen is at atemperature of 32 K,[11] therefore, hydro-gen does not exist as a liquid at ambienttemperature.[12] The volume of the hydro-gen is reduced by compression, liquefac-tion at 20 K, or interaction of hydrogenwith materials by physisorption, chemi-sorption, intercalation and chemical reac-tion. 1 kg of hydrogen at ambient tempera-ture and atmospheric pressure takes a vol-ume of 11.2 m3, while the volume of 1 kgliquid hydrogen is 0.01413 m3 (density ofliquid para-hydrogen at 20.217 K is 70.78kg·m–3).

Compression of hydrogen allows in-creasing the density of the gas up to ap-proximately half of the density of liquidhydrogen at ambient conditions (35 kg/m3)and requires an isothermal compressionwork corresponding to 1 kWh/kg per pres-sure decade or less than 3% of the heatingvalue. Modern high-pressure compositecylinders[13] allow to store up to 4 mass%of compressed hydrogen at pressures upto 800 bar. The mechanical stability[14] ofthe composite over many hundred pressurecycles and the hydrogen diffusion acrossthe material are beside the safety concernsthe major challenges of the developmentof high pressure hydrogen storage systems.

Liquid hydrogen[16] storage at 20 K isa non-equilibrium storage method and suf-

is in the order of 6 S/m and water is pro-vided on the oxygen side (anode). There-fore, high purity hydrogen is produced onthe cathode. The challenges in the furthertechnical development of PEM electrolyz-ers are the increase of the conductivity ofthe polymer membrane, the chemical andmechanical stability of the polymer and thedissipation of the heat produced during theelectrolysis process.

The alkaline electrolyzer consists ofa microporous membrane (ZrO

2in poly-

phenylene sulphide, Zirfon PERL®,[3]NiO) filled with electrolyte that providesthe OH– ion conductivity in the order of120 S/m at 80 °C (Fig. 1b). The 25wt%KOH in water electrolyte is pumped on theanode and cathode side, while the water isconsumed on the cathode (H

2) electrode

and half of it appears on the anode (O2)

electrode. Therefore, the electrolyte con-centrates at the cathode and dilutes on theanode, the electrolyte from the two sideshas to be mixed in order to compensate forthe concentration difference. The evolvedgases hydrogen and oxygen are extractedfrom the circulation electrolyte in gasseparation units. A heat exchanger allowsto maintain the electrolyte at the operationtemperature.

The solid oxide electrolyzer (SOEC)[4]transports O2– ions in a solid oxide (ZrO

2+

8 mol% Y2O

3, La

0.8Sr

0.2Ga

0.8Mg

0.2O

3) with

a conductivity of 1 S/m at 500–850 °C (Fig.1c). Water vapor is provided on the cath-ode and therefore the evolved hydrogencontains water and is dried subsequently,while the oxygen on the anode is pure.[5]The major advantage of high-temperaturewater electrolysis is the possibility to splitwater partially by heat provided by thesteam in installations where lots of high-temperature heat is available.

The LURGI high-pressure electrolyz-ers are still the most efficient and worldlargest electrolyzers today, followed bythe ambient pressure alkaline electrolyzersfrom NEL[6] (former Norsk Hydro) (Fig.2). Currently mainly alkaline systems arefound at a production rate above 30 Nm3/hhydrogen. These large scale electrolyzers

of up to 4 MW were installed worldwide.In 1996, the intellectual property as wellas the customer database were acquired byGFSA and LURGI discontinued the elec-trolyzer development and manufacture andclosed the electrolyzer section. However,GFSA faced financial shortages in 2001and the daughter company, GTec SA,created in 2002 based on the electrolysisactivities, also went out of business soonafter. Finally, the technology of the high-pressure electrolysis was transferred to anew company, IHT Industrie Haute Tech-nologie SA (IHT) in 2003 with the goal tofurther develop the large-scale electrolyzerunits and replace the asbestos membraneswith new materials keeping the particularproperties of very long lifetime (>30 years)and high energy efficiency (>80%).

Electrolysis is based on the splittingof water by means of an electrical poten-tial. Hydrogen is evolved on the cathode(–) and oxygen on the anode (+). Betweenthe electrodes is an electrolyte, which actsas an electrical insulator and ionic con-ductor. The ions transferred between theelectrodes are H+, OH– or O2– and the cor-responding electrolyzers are called acidic(PEM), alkaline or solid oxide. Betweenthe electrodes a membrane separates theevolved gases H

2and O

2.

The membrane has to fulfill several re-quirements, e.g. stability under operatingconditions, separation of the gases, me-chanical separation of the electrodes, ionconduction and mechanical support forpressure differences between the two sidesin the cell. Electrolysis requires catalyticelectrode materials for a low over potentialof the electron transfer and an electrolyte,which provides high conductivity for ionsbut high resistance for electrons and sepa-rates the two gases hydrogen and oxygen.A technical electrolyzer is a compromisebetween the ion conductivity of the elec-trolyte and themechanical stability and gasseparation of the membrane.

The polymer electrolyte membrane(PEM) electrolyzer transports H+ ions in asolid polymer (Nafion®) at around 60 °C(Fig. 1a). The ion conductivity of the PEM

a) b) c)

Fig. 1. The three types of electrolyzers: a) acidic (PEM: polymer electrolyte membrane), b) alkaline (AEL) and c) solid oxide (SOEC) electrolyzer cells.


the stability of the hydride formed. Thethermodynamics of the hydride formationis described by the lattice gas model[23]and leads to the Van’t Hoff equation[24]R·ln(p/p

0) = ∆H/T – ∆S. At low concen-

tration (up to 0.1 H/M) hydrogen forms asolid solution followed by a phase transi-tion from solid solution into the hydridephase (1 H/M). The entropy change uponhydrogen adsorption ∆S corresponds formany systems to the standard entropy ofhydrogen ∆S = 130 J/(mol·K) and an equi-librium pressure of 1 bar at 20 °C is foundfor a hydride with∆H=T·∆S = –38 kJ/molH

2. Hydrogen occupies sites with a radius

greater than 0.037 nm[25] and a distance be-tween intercalated hydrogen atoms greaterthan 0.21 nm.[26] Intercalated hydrogen canreach more than twice the density of liq-uid hydrogen, e.g. in metal hydrides andcomplex hydrides. The highest volumetricdensity was found in metal hydrides[27] tobe 150 kg·m–3. Metal hydrides have manyapplications, e.g battery electrode materi-als, catalysts, sensors, stationary or marinehydrogen storage and hydrogen purifiers,selective separators and compressors. Thegravimetric hydrogen density of metallichydrides is less than 3 mass%.

The elements Al and B form complexhydrides with hydrogen, e.g. alanates Na-AlH

4and borohydrides LiBH

4. With the

discovery of the Ti-catalyzed hydrogendesorption from NaAlH

4in 1996[28] a

wide research field on a new class of solidstorage materials was opened. Few yearslater the alanates were complemented bythe borohydrides.[29,30] The stability of thecomplex hydrides is determined by the lo-calization of the charge[31] on the centralatom (Al, B) and, therefore, is proportionalto the electronegativity of the cation-form-ing element. The enthalpy of formation ofa series of borohydrides was computed byDFT calculation and a linear correlationbetween the enthalpy of formation andthe electronegativity of the cation-formingelement was found.[32] ∆H [kJ/mol BH

4]=

247.4·EN – 421.2 where EN is the Paul-ing electronegativity[33] of M in M(BH

4)x.

A similar equation can be derived for ala-nates ∆H [kJ/mol AlH

4]= 308·EN – 411.

Therefore, by applying the Pauling electro-negativity of B (2.04) andAl (1.61), a gen-eral equation for the enthalpy of formationis derived ∆H [kJ/mol ZH

4]= 143·EN(B,

Al)·EN – 224·EN(B, Al).[34] The forma-tion of the complex hydrides requires wetchemical synthesis; only a few productshave been successfully synthesized fromthe elements. The formation reaction isoften different from the hydrogen desorp-tion reaction. Furthermore, the enthalpy offormation of the complex hydride is muchlarger than the enthalpy of the hydrogendesorption reaction.[35] Alanates tend todesorb via an intermediate hexahydride

portional to the surface area of approx.1.5 mass% of hydrogen on a material witha specific surface area of 1000m2/g.[18]Theadsorption of hydrogen on all kind of na-no-structured or porous materials,[19] e.g.graphite, carbon nanotubes, zeolites, met-al organic frameworks, is described by themodel of Brunauer-Emmnet and Teller.[20]Due to the rather weak interaction, sig-nificant adsorption of hydrogen requireslow temperatures around 100 K. The ma-jor technical and scientific challenges ofhydrogen storage by physisorption is theincrease of the interaction energy and thesearch for materials with large specific sur-face areas.

Hydrogen dissociates and chemisorbson many metals, intermetallic compoundsand alloys (Fig. 3).[21] The hydrogen atomsdiffuse on the subsurface layer and inter-calate on interstitial sites. The electrondensity on the interstitial sites of the metallattice determines the binding energy ofthe hydrogen.[22] Partial substitution of themetal elements allows the electron densityto be modified and therefore influences

fers from continuous loss of hydrogen dueto evaporation at ambient temperature. Thestorage systems are open or semi-open inorder to limit the pressure increase in thestorage tank. The energy demand for theliquefaction process is theoretically 3.92kWh/kg, technically around 10 kWh/kg.The hydrogen density in the storage systemdepends on the size of the storage. Liquidhydrogen storage is the method of choicefor air and space applications, where largeamounts of hydrogen are consumed in ashort time and the energy cost for liquefac-tion is not an economic concern. Themajorchallenges of the liquid hydrogen storagetechnology are the thermal insulation ofthe storage tank and the safe release of theevaporated hydrogen.

Materials with a large specific surfacearea physisorb hydrogen by the Van derWaals interaction. This rather weak inter-action, isosteric heat of adsorption,[17] var-ies between 4 and 8 kJ/mol H

2, allows to

absorb at maximum one molecular mono-layer of (liquid) hydrogen on the substratematerial. This leads to an adsorption pro-

100%

90%80%

70%

60%

50%

40%

Efficiency

PEM

PEM stackAEL stack

AEL

NEL Lurgi

Fig. 2. Specific en-ergy vs. the power ofthe stack for polymerelectrolyte membraneelectrolyzer (PEM)and for alkalineelectrolyzer (AEL),adapted from NOW-Studie.[10]

Fig. 3. Lennard-Jones potential[15] ofa hydrogen moleculeapproaching the sur-face of a solid metal.Compressed gas(molecular H2); liquidhydrogen (molecularH2); physisorption(molecular H2) onmaterials, e.g. car-bon with a very largespecific surface area;hydrogen (atomic H)intercalation in hostmetals, metallic hy-drides working at RTare fully reversible;complex compounds([AlH4]

– or [BH4]–), and

hydrogen chemicallybound in hydrocar-bons.


CO2and, therefore, provide sites for the

catalytic reduction of CO2to hydrocar-

bons. The reaction mechanism of the CO2

reduction will be investigated in detailwith the goal to identify the parametersdetermining the C–H and C–C bonds. Theunderstanding and control of the CO

2re-

duction reaction bridges between the purehydrogen storage and the storage of hydro-gen in synthetic hydrocarbons.

AcknowledgementsThe financial support of KTI/CTI for the

Swiss Competence Center Energy Research(SCCER) ‘Heat & Electricity Storage’ is ac-knowledged.


[1] E. Burkhalter, Hydrogen Report SwitzerlandHRS 13/14, 2013, www.hydropole.ch.

[2] E. A. Zdansky, ‘Pressure electrolyzers’, PatentUS 2717872 A, Priority date Aug 12, 1950(DE879543C).

[3] http://www.agfa.com/sp/global/en/binaries/Zirfon%20Perl%20UTP%20500_v8_tcm611-56748.pdf.

[4] M. Ni, M. K. H. Leung, D. Y. C. Leung, Int. J.Hydrogen Ener. 2008, 33, 2337.

[5] M. S. Sohal, J. E. O’Brien, C. M. Stoots, M.G. McKellar, E. A. Harvego, J. S. Herring,‘Challenges in Generating Hydrogen by HighTemperature Electrolysis Using Solid OxideCells’, NHA 08 Idaho National Laboratory,March 2008.

[6] http://wpstatic.idium.no/www.nel-hydrogen.com/2015/03/Efficient_Electrolysers_for_Hydrogen_Production.pdf.

[7] http:/ /www.hydrogenics.com/hydrogen-products-solutions/industrial-hydrogen-generators-by-electrolysis.

[8] http://protononsite.com/products/m/.[9] http://www.industry.siemens.com/topics/global/

en/pem-electrolyzer/silyzer/pages/silyzer.aspx.[10] T. Smolinka, M. Gunther, J. Garche, NOW-

Studie „Stand und Entwicklungspotenzial derWasserelektrolyse zur Herstellung von Wasser-stoff aus regenerativen Energien“, Kurzfassungdes Abschlussberichts, Redaktionsstand:22.12.2010, Revision 1 vom 05.07.2011.

[11] ‘Ullmann’s Encyclopedia of IndustrialChemistry’, ‘Hydrogen’, Eds. P. Häussinger, R.Lohmuller, A. M. Watson, published online: 15June 2000, DOI: 10.1002/14356007.a13_297.

[12] W. B. Leung, N. H. March, H. Motz, Phys. Lett.1976, 56A, 425.

[13] Dynetek Europe GmbH, Breitscheider Weg117a, D-40885 Ratingen, URL: http://www.dynetek.de.

[14] W. Matek, D. Muhs, H. Wittel, M. Becker,‘Roloff/Matek Maschinenelemente’, ViewegsFachbucher der Technik, 1994, 690 pp, ISBN:3-528-74028-0.

[15] J. E. Lennard-Jones, Trans. Faraday Soc. 1932,28, 333.

[16] J. Stanga, P. Neksåb, E. Brendengc, ‘On thedesign of an effient hydrogen liquefactionprocess’, WHEC 16 / 13-16 June 2006 – LyonFrance, http://www.cder.dz/A2H2/Medias/Download/Proc%20PDF/posters/[GV]%20Hydrides/480.pdf.

[17] B. Schmitz, U.Muller, N.Trukhan,M. Schubert,G. Férey, M. Hirscher, ChemPhysChem 2008,9, 2181.

[18] A. Zuttel, P. Sudan, P. Mauron, T. Kiyobayashi,C. Emmenegger, L. Schlapbach, Int. J.Hydrogen Ener. 2002, 27, 203.

of a hydrogen storage system is limited toless than half of the energy density of hy-drocarbon fuels, e.g. diesel (12.8 kWh/kg,10 kWh/l).

Outlook

The potential of complex hydrides forlarge-scale hydrogen storage is huge, butrequires a better understanding and im-proved control of the hydrogen sorptionreaction. The reaction pathway in complexhydrides exhibits various intermediatesand transition states depending on the typeof the central atom, Al, B, or N, and on thereaction conditions. The knowledge of thereaction pathway and the identification ofthe transient states is of fundamental im-portance in order to control the hydrogenabsorption and desorption processes. Fur-thermore, development of new materialsrequiresmethods to synthesize the complexhydrides as well as overcome the activa-tion barriers by an appropriate catalyst. Inother cases it may be important to developmethods to stabilize a certain hydride or toavoid the formation of undesired interme-diates, e.g. diborane or ammonia. Finally,the knowledge of the reaction mechanismrepresents an added value concerning allsafety issues of complex hydrides. Besidethe pure hydrides, composites, e.g. borohy-drides and amides, represent a wide fieldof new storage materials with interestingproperties.While the complex hydrides arein a very early stage of development to thehydrogen storage application, metal hy-drides are on a technology readiness levelthat allows the construction of large-scalehydrogen storage systems.

Metal hydrides represent surfaces,[36]which offer atomic hydrogen and absorb

and finally form the elemental hydride,while borohydrides do not exhibit a hexa-hydride. The hydrogen desorption reactionfrom borohydrides is accompanied by thedesorption of diborane, a product of a sidereaction of the hydrogen desorption. Thisside reaction depends on the stability andmobility of B

2H

6and BH

3, respectively,

therefore, above a temperatures of 200 °Cno diborane is observed. The alanates andborohydrides have the potential to store upto 7 mass% and 20 mass% of hydrogen,respectively. The volumetric hydrogendensity reaches, similar to metal hydrides,around 150 kg/m3. The scientific and tech-nical challenges are the synthesis of thecomplex hydrides preferably directly fromthe elements, the control of the desorptionreaction including the development of thecatalysts. New complex hydrides, whichare liquid at room temperature and desorbhydrogen close to ambient temperature areunder investigation. With the complex hy-drides, the gravimetric hydrogen storagedensity in hydrides was increased by anorder of magnitude from around 2 mass%to 20 mass% in the last 20 years.

With the complex hydrides the gravi-metric and volumetric hydrogen density iseven greater than that of the hydrocarbonsand has reached the maximum densitiespossible based on the current knowledgeof hydrides (Fig. 4). Comparing the avail-able energy from the combustion of hydro-carbons and hydrides, the hydrocarbonsdeliver twice as much energy because thehydrogen and the carbon is oxidized, whilein case of the complex hydride only thehydrogen is oxidized and the host materialremains unreacted. The hydrogen storagedensity of 20 mass% corresponds to an en-ergy density of 7.8 kWh/kg. Therefore, thegravimetric and volumetric energy density

Coal

OilMetalhydride

Complexhydride

Butane

Alcohol

→

WoodBa8ery

Fig. 4. Volumetric vs. gravimetric energy density in the various fuels and energy carriers togetherwith the typical application.


[19] M. G. Nijkamp, J. Raaymakers, A. J. VanDillen, K. P. De Jong, Appl. Phys. A 2001, 72,619.

[20] S. Brunauer, P. H. Emmett, E. Teller, J. Am.Chem. Soc. 1938, 60, 309.

[21] L. Schlapbach, in ‘Intermetallic CompoundsI’, Chap. 1, Ed. L. Schlapbach, Springer SeriesTopics in Applied Physics, Vol. 63, Springer–Verlag, 1988, p. 10.

[22] J. K. Norskov, F. Besenbacher, J. Less-CommonMetals 1987, 130, 475.

[23] H. Hemmes, E. Salomons, R. Griessen, P.Sänger, A. Driessen, Phys. Rev. B Condens.Matter. 1989, 39, 10606.

[24] P. Atkins, J. De Paula, ‘Physical Chemistry’, 8th

ed., 2006, W.H. Freeman and Company, p. 212.ISBN 0-7167-8759-8.

[25] D. G. Westlake, J. Less Common Metals 1983,91, 275.

[26] A. C. Switendick, Z. Phys. Chem. N.F. 1979,117, 89.

[27] A. Zuttel, Naturwissenschaften 2004, 91, 157.[28] B. Bogdanovic, M. Schwickardi, J. Alloys

Comp. 1997, 253, 1.[29] A. Zuttel, P. Wenger, S. Rentsch, P. Sudan, P.

Mauron, C. Emmenegger, J. Power Sources2003, 118, 1.

[30] S. Orimo, Y. Nakamori, G. Kitahara, K. Miwa,N. Ohba, S. Towata, A. Zuttel, J. Alloys Comp.2005, 404, 427.

[31] P. Jena, Virginia Commonwealth University,Richmond, VA, to be published.

[32] Y. Nakamori, K. Miwa, A. Ninomiya, H. Li, N.Ohba, S.-I. Towata, A. Zuttel, S. Orimo, Phys.Rev. B 2006, 74, 1.

[33] L. Pauling, J. Am. Chem. Soc. 1929, 51, 1010.[34] A. Zuttel, P. Mauron, S. Kato, E. Callini, M.

Holzer, J. Huanga, CHIMIA 2015, 69, 264.[35] Z. Ö. Kocabas Atakli, E. Callini, S.

Kato, P. Mauron, S. Orimo, A. Zuttel,PhysChemChemPhys 2015, 17, 20932.

[36] S. Kato, A. Borgschulte, D. Ferri, M. Bielmann,J-C. Crivello, D. Wiedenmann, M. Parlinska-Wojtan, P. Rossbach, Y. Lu, A. Remhof, A.Zuttel, PhysChemChemPhys 2012, 14, 5518.



*Correspondence: Prof. Dr. G. LaurenczyInstitut des Sciences et Ingénierie ChimiquesEcole polytechnique Fédérale de Lausanne (EPFL)CH-1015 Lausanne, SwitzerlandE-mail: [email protected]

Hydrogen Storage in the Carbon Dioxide –Formic Acid Cycle

Cornel Fink, Mickael Montandon-Clerc, and Gabor Laurenczy*

Abstract: This year Mankind will release about 39 Gt carbon dioxide into the earth’s atmosphere, where it actsas a greenhouse gas. The chemical transformation of carbon dioxide into useful products becomes increas-ingly important, as the CO2 concentration in the atmosphere has reached 400 ppm. One approach to contrib-ute to the decrease of this hazardous emission is to recycle CO2, for example reducing it to formic acid. Thehydrogenation of CO2 can be achieved with a series of catalysts under basic and acidic conditions, in widevariety of solvents. To realize a hydrogen-based charge-discharge device (‘hydrogen battery’), one also needsefficient catalysts for the reverse reaction, the dehydrogenation of formic acid. Despite of the fact that theoverwhelming majority of these reactions are carried out using precious metals-based catalysts (mainly Ru),we review here developments for catalytic hydrogen evolution from formic acid with iron-based complexes.

Keywords: Aqueous solution · Carbon dioxide · Fe · Formic acid · Homogeneous catalysis · Hydrogen storage· Phosphine ligands · Ru

CO2/HCO3–/CO3

2– hydrogenation

Atmospheric carbondioxide (CO2) is an

almost infinite source of carbon and if uti-lized as a C

1building block, countless feed

stock chemicals and compounds could besynthesized.[1] Despite its obviously hugepotential for the chemical industry, it is nota widespread exploited resource. Severalreasons account for this situation such asdifficulties to capture CO

2with economic

efficiency from air (approximately 400ppm). A currently more promising methodseems to be to capture carbon dioxide atthe source (e.g. power plants) and processthe off-gas for further applications.[2] Thehigh thermodynamic stability of the CO

2(∆H° = –393.5 kJ/mol)[3] is another chal-lenge.[4] Nature managed to process car-bon dioxide as the sole carbon source forall plant life by mastering sunlight-drivenphotosynthesis,[5] a brilliant concept whichinspires scientists to mimic the procedurefor hydrogen production and storage.[6]

An alternative approach for the reduc-tion of carbon dioxide, besides electro-chemical or photochemical reduction, isvia a catalytic reaction, to hydrogenateCO

2to form formic acid (HCOOH), meth-

anol (H3COH) or methane (CH

4). Various

metals, ranging from precious rare ele-ments such as ruthenium, rhodium, palla-dium, iridium or osmium down to abun-dant bulk metals (iron, cobalt, copper),in combination with countless differentligands have been screened towards theirability to hydrogenate carbon dioxide. Al-though Ru(ii) complexes with phosphineligands are predominant for this task, thenumber of other successfully tested metalcomplexes such as iron-based Fe(ii)-tris[2-diphenylphosphino)-ethyl]phosphine(PP

3) is steadily increasing.[7] The ap-

plied reaction conditions are as diverse asthe catalysts. Nonetheless, in all – exceptone[8] – cases, the reductions were carriedout under basic conditions, which ratherframes bicarbonate (HCO

3–) or carbon-

ate (CO32–) the substrate than CO

2.[9] High

hydrogen- and carbon dioxide pressuresshift the equilibria towards formic acidformation, while increased temperaturesaccelerate the reaction rate, but exhibit andetrimental effect on the absolute yieldsince the hydrogenation of bicarbonate isan exothermic process (∆G° = –35.4 kJ/mol; ∆H°

298= –59.8 kJ/mol; ∆S°298 = –81

J/mol*K).[10]The reduction of carbon dioxide with

heterogeneous and homogeneous catalystshas been under investigation for many de-cades. A milestone in CO

2fixation with

heterogeneous catalysts was achieved bythe Nobel Prize laureate Sabatier in the1910s by reducing CO

2directly to methane

(Sabatier process). For practical reasons,the focus in the further course of this re-view article will be on homogeneous catal-ysis, bearing the advantage that homoge-neous catalytic processes can be studied ona molecular level more straightforwardly,which is relevant for mechanistic studiesand the fine tuning of the catalysts allowsbetter selectivity. Successful homogeneouscatalytic hydrogenations of bicarbonatewith homogenous catalysts have been re-ported as early as from the beginning ofthe last century.[11] More recent publica-tions from the 1970s describe alreadymore advanced systems, which producedesters (HCOOEt),[12] and upon hydrolysisHCOOH, whereas others formed formatedirectly in the presence of triethylamine(basic conditions) and catalytically activetransition metal complexes (p(H

2)/p(CO

2)

= 1:1, 50 bar, RT).[13] Leitner et al. reportedon a homogeneous catalyst, formed in situfrom [Rh(cod)Cl

2] and Ph

2P(CH

2)4PPh

2,

which produced up to 1150 moles formateper mole rhodium.[14] In 2010, Nozaki pre-sented an Ir-trihydride-pincer complex,which achieved an astonishing TON (turnover number) up to 3 500 000 (after 48 h re-action time) and TOF (turnover frequency)of 150 000 h–1.[15] The latest developmentsin the field are neatly summarized in sev-eral recent reviews.[16]

Important research on the formation ofmethanol from CO

2and the reverse reac-

tion, liberation of hydrogen, was done bythe Milstein group and numerous high-impact publications give proof of their ex-cellent work.[17] Another approach, basedon a three-step cascade synthesis, was re-ported by the Sanford group.[18]To producemethanol, three different homogeneous


Later, a series of imidazolium-tetheredruthenium(ii)-arene complexes was syn-thesized and their application for catalysiswas subsequently assessed.[25] DimericRu(ii) salts with the general structure[RuCl(μ2-Cl)(η6-arene)

2] were treated

with phosphine ligands (PPh3, PCy

3) which

lead to catalyst precursors. These complex-es were active in aqueous solution on thereduction of bicarbonate and carbonate.High-pressure NMR measurements al-lowed the identification of a bicarbonate-hydride intermediate.

A similar group of compounds wasscrutinized in 2007.[26] There, the water-soluble Ru(ii) complexes [Cp’RuX(PTA)

2]

Y and [CpRuCl(PPh3)(mPTA)]OTf (Cp’ =

Cp, Cp* - (1,2,3,4,5-pentamethylcyclo-pentadienyl), X = Cl and Y = nil; or X =MeCN and Y = PF

6; mPTA = 1-methyl-

1,3,5-triaza-7-phosphaadamantane) actedas precatalysts in the hydrogenation ofHCO

3– and CO

2in amine- and additive-

free aqueous solution under reasonableconditions (30–80 °C, p(H

2) = 100 bar).

unreached high TOFs of 9600 h–1 for wa-ter-soluble Ru(ii) phosphine complexes inaqueous solution were measured.

Another class of catalytically activecomplexes are the Ru(ii) arene compoundswith the general formula [(η6-arene)Ru(ii)Cl

2(PTA)].[24] These complexes can ex-

change one or both Cl– for hydrides, form-ing [(η6-arene)Ru(ii)H(PTA)Cl] and [(η6-arene)Ru(ii)H

2(PTA)]. Besides the ability

to hydrogenate bicarbonate, an interestingdynamic behavior of the compounds wasnoticed. During prolonged hydrogena-tion at elevated temperatures, an excess ofPTA leads to the loss of the arene group,and the resultant complexes show cata-lytic activity for hydrogenation of HCO

3–.

Identified species were [RuH(PTA)4Cl],

[RuH(PTA)4H

2O]+, [RuH

2(PTA)

4] and

[RuH(PTA)5]+. At the end of the experi-

ment, the in situ formed catalyst reachedalmost full conversion of bicarbonate. In-terestingly, no initial induction period wasobserved as described for the direct appli-cation of [RuCl

2(PTA)

4] catalysts.[20]

catalysts transform carbon dioxide succes-sively to methanol via reduction of CO

2to

formic acid, then esterification (formateester) and finally hydrogenation of theester to obtain free methanol (Fig. 1).[18]While the first two steps are well presentin literature, the final hydrogenation is aninnovative feature of their work.

Formic acid has an advantage overmethanol and methane in terms of hydro-gen storage efficiency since no water asco-product is formed during the reduc-tion process (starting from CO

2), which

consumes valuable hydrogen equivalents(Fig. 2).Accordingly, formic acid (or moreprecisely the formate salts) is a promisingcandidate for constructing a ‘hydrogenbattery’, where the energy is stored as re-versibly bound hydrogen.[19]

In 2000, we reported on the water-soluble tertiary phosphine ruthenium(ii)complex, [RuCl

2(PTA)

4] (PTA = 1,3,5-tri-

aza-7-phosphaadamantane), as a pre-catalyst which is capable of hydrogenat-ing bicarbonate (HCO

3–) in aqueous so-

lution and does not depend on amines orother additives.[20] The hydride species,which was observed at 60 bar H

2in acidic

aqueous solution, is [RuH2(PTA)

4] (pH =

2.0) while [RuH(PTA)4Cl] was detected

in basic media (pH = 12). Moreover, theturnover frequency of the catalyst dependsstrongly on the pH. An initial TOF of800 h–1 was measured in a 9:1 CO

2/HCO

3–

mixture (pH = 5.86), whereas a reducedreaction rate was observed in very basicsolutions (substrate Na

2CO

3). More de-

tailed investigations on the active speciessuggested that HCO

3– is the primary sub-

strate, which was confirmed later.[21] Theobserved induction period at the beginningof the catalytic cycle could originate fromthe slow formation of the catalyticallyactive species. Furthermore, studies onwater-soluble rhodium(i) complexes withmeta-monosulfonated triphenylphosphine(mTPPMS) ligands confirmed the pH de-pendency of hydride species. It was shownthat the pH change caused by CO

2treat-

ment in aqueous solution affects the dis-tribution of catalytically relevant hydridospecies.[22] mTPPMS was further exam-ined in combination with Ru(ii), where thedimeric [RuCl(μ2-Cl)(mTPPMS)

2] com-

plex was identified as a suitable precatalystfor bicarbonate hydrogenation, yieldingHCOONa under mild conditions (50 °C,P(H

2) = 10 bar; Fig. 3).[23] The reaction did

not require amine additives, nonethelessthe reaction rate was considerably higherafter the addition of quinolone. The pro-posed reaction mechanism involves Ru(ii)-hydride species with the generalized for-mula [RuHX(mTPPMS)

4] and HCO

3– as

substrate, where X represents H–, HCO3–

or the product HCOO–. Under harsherconditions, at 80 °C and 95 bar, previously

Fig. 1. Proposed cas-cade for the conver-sion of carbon diox-ide and hydrogen tomethanol and water.Reprinted from ref.[18] with permissionfrom Journal of theAmerican ChemicalSociety.

Fig. 3. Time course ofHCO3

– reduction by[RuCl2(mTPPMS)22]and mTPPMS; finalconcentration ofsodium formate didnot exceed the initialbicarbonate concen-tration; no other prod-ucts were detected.Reprinted from ref.[23] with permis-sion from AppliedCatalysis A: General.

Fig. 2. CO2 hydrogenation in respect to consumed H2 equivalents.

Energy content

[MJ/kg]

Storage efficiency

[%]

5.22 100

15.2 66.7

30.15 50


by multi-nuclear NMR spectroscopy andan X-ray crystal structure of the initial[FeH(PP

3)] was obtained.

The catalyst was tested on other sub-strates, verifying that amides and estersare also accessible by hydrogenating CO

2.

Methyl formate was produced in goodyield (56%) and a maximum TON of 585was accomplished – two times higher thanthe best previously reported iron cata-lysts. In addition, dimethylformamide wasformed in high yield (75%) with a TONof 727, which was previously only knownfrom precious metal systems (Ru, Ir, orRh), and N-formylpiperidine was obtainedin 41% yield (TON = 373). Ethyl or pro-pyl formate esters were formed as well, theyields and TONs were lower compared tothose of methanol (MeOH) based systems.It was known from earlier publications thatthe presence of base is crucial for favorablethermodynamics.[29]

Later in 2012, a new generation of iron-based catalysts was presented.[30] The ad-dition of fluorotris-2-(diphenylphosphino)phenyl)phosphine iron(ii) tetrafluorobo-rate to a methanolic bicarbonate solutionafforded high TONs

20h> 7500 and TOFs

5h> 750 (100 °C; p(H

2) = 60 bar; 0.005 mmol

[RuCl2(PTA)

4], which afforded 0.2 M FA

aqueous solutions at 40 °C and 200 bar(p(H

2):p(CO

2) = 3:1), however the same

compound achieved excellent 1.9 M for-mic acid in DMSO (p(H

2)/p(CO

2) = 1:1,

100 bar, 50 °C (Fig. 6), in D2O, H-D ex-

change in formic acid).[28] The high stabil-ity of the catalyst allowed multiple recy-cling without detectable decreases in activ-ity in both reaction media. Moreover, thecatalyst exclusively produces formic acidand the final pH was measured as 2.70,proving the robustness of the catalyst in anacidic environment.

It was only in 2010 when, in collabora-tion with Beller’s group, a homogeneousiron catalyst for bicarbonate hydrogenationwas discovered.[7] Different iron-contain-ing precursors and numerous phosphine-and nitrogen-containing ligands were stud-ied. An excellent catalyst, Fe(BF

4)2/PP

3(PP

3= tris[2(diphenylphosphino)ethyl]

phosphine), which forms in situ, was iden-tified, hydrogenations proceed smoothlywith low catalyst loadings of 0.14 mol%at 80 °C. NaOOCH was produced in 88%yield with a turnover number of 610, thesuggested catalytic cycle is shown in Fig.7.[7] The active species were identified

The activities are described as moderatefor bicarbonate hydrogenation.

A NMR study confirmed in 2007 theexistence of a previously proposed inter-mediate in the catalytic hydrogenation ofcarbon dioxide/bicarbonate in aqueousme-dia.[21]The water soluble Ru(ii) precatalystwith the structure [RuCl

2(PTA)([9]aneS

3)]

([9]aneS3= 1,4,7-trithiacyclononane) has

low catalytic activity for the hydrogenationof bicarbonate but allowed the identifica-tion of an important intermediate by 1H,13C, and 31P NMR spectroscopy, where ahydride and a bicarbonate are coordinatedto the Ru center. Accordingly, the in situobserved catalytically active species canbe described as [Ru(H)(CO

3H)(PTA)([9]

aneS3)] (Fig. 4).[21] The reduction of car-

bon dioxide takes place via bicarbonatehydrogenation and the rate-limiting stepseems to be the intramolecular transfer ofhydrides on the substrate.

Beside ruthenium(ii) complexes,iridium(iii) complexes showed compa-rable catalytic activity in aqueous phasecarbon dioxide (HCO

3–) hydrogenation.[27]

We investigated two water-soluble irid-ium complexes, [Cp*Ir(PTA)Cl

2] and

[Cp*Ir(PTA)2Cl]Cl, as catalyst precursors.

The monophosphine compound performedpoorly while the bisphosphine precatalystdemonstrated moderate activity for bicar-bonate hydrogenation. Furthermore, thecatalysts were fully characterized (solutionand solid state, Fig. 5)[27] and the catalyti-cally active species [Cp*IrH(PTA)]+ wasidentified by multi-nuclear NMR studiesand independent synthesis. Optimal con-ditions for the hydrogenation were foundat higher temperatures and a slightly basicpH of 9.

Fig. 4. Catalytic Ru(ii) complex with coordi-nated substrate (HCO3

–) and hydride. Reprintedfrom ref. [21] with permission from InorganicChemistry Communications.

We patented in 2013 and publishedin 2014 the first and currently unique di-rect hydrogenation of CO

2to formic acid

without any additives or amines, workingin acidic aqueous solution.[8] There, wereported on the highly active precatalyst

Fig. 5. ORTEP diagram of [Cp*Ir(PTA)Cl2] and [Cp*Ir(PTA)2Cl]Cl with solvates; anions omittedfor clarity; shown with 50% probability ellipsoids. Reprinted from ref. [27] with permission fromEuropean Journal of Inorganic Chemistry.

Fig. 6. 13C NMR sig-nals of DCOOD inthe hydrogenation ofCO2 into formic acidin D2O. Reprintedfrom ref. [8] with per-mission from NatureCommunications.


tion, producing water and CO (HCOOH→H

2O + CO). As one of the goals is to use

the produced hydrogen in PEM fuel cellsto generate electricity, the later reactionshould be avoided as CO is a poison forthe membrane of such cells.

Back to 2006, our group was the firstone to report the use of ruthenium com-plexes for selective formic acid cleavagewith the idea of producing hydrogen.[33]The decomposition of formic acid wascarried out in aqueous solutions usinghydrophilic ruthenium-based homoge-neous catalysts, generated from the highlywater-soluble ligand m-trisulfonated tri-phenylphosphine (mTPPTS, Fig. 8) witheither [Ru(H

2O)

6]2+ or the commercially

available RuCl3·3H

2O.

Using sodium formate to activate thecatalyst, 100% conversion of formic acidwas reported, with generated H

2and CO

2pressure from 1 to 800 bar with no inhi-bition of the catalytic activity. Moreover,no traces of CO could be seen using FT-IRtechniques (detection limit: 3 ppm). Usingthese catalysts, a continuous system wasdeveloped, allowing constant high-pres-sure hydrogen generation (Table 1).

In 2009, we detailed the mechanism ofthe aforementioned dehydrogenation.[34]Using multinuclear NMR techniques, in-termediate structures were elucidated anda complete reaction mechanism was pro-posed. It consists of two competitive cy-cles, explaining the catalytic behavior (fastactivation period followed by high catalystactivity).

Later attempts were made to combinethe advantages of heterogeneous systemswith homogeneous catalysts. Immobiliza-tion of the highly active Ru(ii)-mTPPTScatalyst on ion exchange resins, polymersand zeolites was carried out.[35] Using ionexchange to bind the catalyst, the activitywas the same as for the first catalytic cy-cle in the homogeneous catalytic system.However, recycling led to a decrease inthe reaction rate, although the conversionstayed the same. Polymerized phosphinecatalyst precursors were not effective,having a decreased reaction rate and yield.Regarding the immobilization on zeolites,there is a dependence on the type of the ap-plied zeolite. Zeolites with low absorbingability resulted in low reaction rates whilestronger absorbents gave similar results to

The homogeneous catalyst [RuCl2

(benzene)]2/dppe(dppe=1,2-bis(diphenyl-phosphino)ethane) is active in both the hy-drogenation of carbon dioxide in basic me-dia and the dehydrogenation of formic ac-id-triethylamine adducts to hydrogen andCO

2.[32] The direction of the reaction can

be reversed by changing the partial pres-sures of the corresponding gases. A lowoverall pressure (atmospheric pressure)facilitates the dehydrogenation of formicacid (discharging), whereas high hydrogenand CO

2pressures lead to the hydrogena-

tion to formate (charging). The successfulcombination of both pathways is a funda-mental requirement for operational hydro-gen charge/discharge devices.

Formic Acid Dehydrogenation

Formic acid and its decomposition intoH

2and CO

2is a sustainable way to store

and produce hydrogen, alongside CO2

(HCOOH → H2+ CO

2), if formic acid

is synthetized by using ‘green’ energy/hydrogen (see above). One liter of for-mic acid contains 53 g of H

2, which cor-

responds to 4.4 wt%, making it an elegantand interesting liquid as organic hydrogencarrier. However, formic acid exhibits asecond decomposition pathway, dehydra-

catalyst load) for hydrogenation. A cata-lytically crucial iron-hydrido-dihydrogenspecies was identified by high-pressureNMR studies. Furthermore, a tetradentatephosphorus ligand, which is easy obtain-able in a one-pot reaction, was essentialfor successful catalysis. The synthesizedcomplexes are stable at high temperatures(>100 °C) and under air.

A novel synthesis route for formatesalts, an important industrial precur-sor, was discovered by Beller’s group in2014.[31] In their study, a series of ruthe-nium pincer complexes was examinedtowards simultaneous methanol oxidationand bicarbonate reduction (hydrogena-tion), a green process without involvingtoxic gases (CO) to synthesize formatesalts with excellent TOF >1300 h–1, TONup to 18000, and total conversions over90%. Above all, this is the first reportabout combined catalytic dehydrogenationof methanol and reduction of HCO

3– to a

formate salt. The application of a commer-cially available pincer complex, HPNPph/Ru, resulted in a TON over 18000 (36 h)for the formation of KOOCH.

Table 1. Formic acid consumption rates, hydrogen production rates, and turnover frequencies ofthe continuous system at 150 bar constant pressure.

T [°C] HCOOH input [mL · min–1] H2outflow [mL · min–1] TOF [h–1]

100 0.21 ±0.01 140 ±10 230 ±5

120 0.42 ±0.01 290 ±10 460 ±24

50 mL reactor, 12 mL initial solution of HCOOH/HCOONa (9:1, 4M), [Ru(H2O)6](tos)2 (125 mmol),mTPPTS (250 mmol).

Fig. 7. Catalyticscheme for thehydrogenation ofcarbon dioxidewith Fe(BF4)2/PP3.Reprinted from ref. [7]with permission fromAngewandte ChemieInternational Edition.

Fig. 8. Water-soluble m-triphenylphosphinetrisulfonated ligand.


the aqueous homogeneous systems, evenafter 92 recycling experiments. However,according to the XRF spectra, when thesesolid catalysts were washed with water, theRu–mTPPTS complex could be removedgradually, showing that the zeolites herewere mainly acting as physical adsorbents.The catalyst immobilization on mesopo-rous silica was successful and led to a pat-ent application.[36]

In 2010, the effect of the water-solublesulfonated phosphine ligands on rutheni-um-catalyzed generation of hydrogen fromHCOOH was investigated.[37] Differentphosphines were synthetized by changingsubstituents, thus varying the bulkiness,basicity, hydrophobicity. It was shown thatthe best ligands were mTPPTS and mTP-PDS (triphenylphosphine, m-disulfonated)that offer a good compromise between ste-ric effects and phosphine basicity, along-side with a good stability and an excellentsolubility in water.

So far, viable results in our group wereobtained using ruthenium-based catalysts,but the need for precious metal has inher-ent drawbacks. Due to its scarcity, it is rela-tively expensive; the large scale and indus-trial use could have limitations. With theidea of using a non-precious metal-basedcatalyst, a collaboration between our groupand Beller’s resulted in the developmentof an outstanding system, with TOF up to9425 h–1 and TONs up to 92’000, based ona [Fe(BF

4)2]·6H

2O metal precursor with

PP3(Fig. 9) as ligand capable of selective

formic acid dehydrogenation in propylenecarbonate solution (Table 2).[38]

Fig. 9. Tris[2-(diphenylphosphino)ethyl]phos-phine ligand.

The catalytic activity was tested fordifferent precursors and with already ac-tivated catalyst.

It is to be noticed that the use of twoequivalents of PP

3greatly enhances the

catalytic activity. The chloride ion hasa poisoning effect on the catalyst, as[FeCl(PP

3)]BF

4complex showed no ac-

tivity in the dehydrogenation reaction.The same effect could be seen when add-ing excess Cl– into the reaction mixture.The use of four equivalents of ligand at80 °C allowed continuous production ofhydrogen with a TOF of 5400 h–1 over 16 h.The activation energy, E

A, was determined

to give additional kinetic information, itwas found to be equal to 77 kJ·mol–1 in pro-pylene carbonate and 82 kJ·mol–1 in THF.

In 2011, the reaction of formate dehy-drogenation was combined with the hydro-genation of bicarbonate (Eqn. (1)) to obtaina viable hydrogen storage charge/dischargedevice (‘chemical hydrogen cylinder’), ca-pable of storing and releasing hydrogen ondemand.[39] This process can be controlledby modifying the equilibrium position viathe temperature and the hydrogen pressurechange. By increasing the H

2pressure,

bicarbonate will undergo hydrogenationand form formate; and, by releasing theH

2pressure, formate will decompose and

produce hydrogen. Both reactions are cata-lyzed by the same [RuCl

2-(mTPPMS)

22]

+ mTPPMS complex, without the need ofisolating either the formate or bicarbonateto start a new cycle, in aqueous solution.

HCO3– + H

2 HCOO– + H2O (1)

The reversible hydrogen storage hasbeen achieved using different conditionsand catalysts.[19] In order to come closerto the realization of a practical H

2stor-

age–discharge device, the equilibriumposition of formic acid/amine–CO

2sys-

tems has been examined as a function ofpressure and temperature under isochoricconditions.[32] It appears that high yieldsof formic acid dehydrogenation into H

2and CO

2are favored by low gas pressures

and/or high temperatures and H2uptake is

possible at elevated H2/CO

2pressures. The

development of systems capable of charg-ing/discharging is of great interest, as itcould be used for small and portable ap-plications.

Recently we have evaluated and sum-marized the potential of the formic acidfor hydrogen storage and delivery.[40] Thereview widely explains the trends usingboth homogeneous and heterogeneouscatalysts, explaining the advantages andthe disadvantages of each method. Obvi-ously heterogeneous catalysts are easier torecycle, but in general, homogeneous cata-

lysts tend to be more active and selective,although the gap is closing. The alterna-tive hydrogen storage possibilities werealso reviewed, focusing on the differentapproaches, including both chemically orphysically bound H

2, comprising the use

of formic acid.[41]For a long period, only neutral and an-

ionic ligands were used as catalyst precur-sors for the liberation of hydrogen fromHCOOH. In 2013 we published the use ofcationic phosphine precatalysts for formicacid dehydrogenation.[42] To obtain suchphosphines, ammoniomethyl substituentswere introduced into the triarylphosphines.Several similar ligands were tested, vary-ing in the charge (from 1+ to 6+) and inbulkiness. The best results were achievedwith a 2+ ligand, charged twice on thesame aryl group. Comparing the dehydro-genation reaction rates with these phos-phines to the mTPPTS under similar ex-perimental conditions, the cationic ligandsare more efficient. This could be due to theoverall positive charges, which renders theenvironment cationic around the centralmetal atom and, therefore, leads to a fastercoordination/migration of the negativelycharged species (HCOO–, HCO

3– and H–).

However, it is more difficult to synthesizesuch cationic ligands, and they are less ro-bust, being sensitive to oxygen.

In the case of pressurized reaction sys-tems it is not easy to determine the con-centration of dissolved species such asHCOOH, HCOONa, CO

2, Na

2CO

3and

NaHCO3, as well as the pH, in situ, under

H2and/or CO

2pressure.[43] It can be done

by multinuclear NMR spectroscopy, as thechemical shift for 13C and 1H NMR are de-pendent on the pH of the solution and thenwith a calibration curve it is then possibleto relate the pH to the chemical shift of theH and C atoms of the formic acid.

For the solute concentration, it wasfound that the integrals of formic acid, for-mate, carbonate, bicarbonate and carbondioxide NMR signals are proportional totheir concentrations if appropriate long re-laxation delay times (D1) were chosen inthe experiments. This tool has been provenvaluable to investigate reaction kinetics

Table 2. Selective iron-catalyzed hydrogen evolution from formic acid using iron precursors.Reprinted fron ref. [38b] with permission from Science.

Entry Catalyst V2h[mL] V

3h[mL] TON

2hTON

3h

1 [Fe(BF4)2]·6H

2O/PP

3146 215 562 825

2 [Fe(BF4)2]·6H

2O/2PP

3333 505 1279 1942

3 [FeH(PP3)]BF

4194 295 745 1135

4 [FeH(PP3)]BF

4/PP

3319 500 1227 1923

5 [FeH(H2)(PP

3)]BF

4189 294 727 1129

6 [FeH(H2)(PP

3)]BPh

4174 264 670 1015

7 [FeCl(PP3)]BF

40.4 0.4 – –



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action conditions and identified as the keyactive species for the catalysis. The deac-tivation of the catalyst by chloride couldalso be monitored easily by in situ IR spec-troscopy and correlated with the evolutionof H

2gas. Computational studies were also

carried out to support the proposed reac-tion pathways.

AcknowledgementsThe authors are grateful to the Swiss

Competence Center for Energy Research(SCCER), the Swiss Commission forTechnology and Innovation (CTI) and EPFLfor financial support.

and mechanisms in H2storage/delivery

with the carbon dioxide–formic acid sys-tems under H

2and CO

2pressures (Fig.

10).[43]In the meantime, research on mTPPTS

was still going on and the mechanismof the first ‘fast’ cycle for formic aciddehydrogenation catalyzed by mTPPTSruthenium complex was elucidated.[34,44]Using NMR techniques and time-resolvedmanometry, the dehydrogenation reac-tion was intensively studied and some keycatalytic intermediates were identified.With those data, a rational cycle was pro-

posed, explaining the transition to the slowcycle.

The iron(ii) based catalytic precur-sors were further investigated (Fig. 11).[44]Optimization of the conditions (solvent,concentration, temperature) led to highlyefficient catalyst systems with comparableactivity to most known noble-metal cata-lysts used for this transformation. Spectro-scopic investigations (IR, Raman, UV/Vis,XAS) revealed the presence of differentiron formate and hydride complexes. Theiron κ2-formate [Fe(κ2-OOCH)(PP

3)] was

observed for the first time under in situ re-

Fig. 10. Chemical shifts of the HCOO−/HCOOH 13C NMR doublet (ctotal = 0.1 M) (blue squares, leftaxis) and 1H NMR singlet (c = 0.1 M) (red triangles, right axis) as a function of pH. Reprinted fromref. [42] with permission from Dalton Transactions.

Fig. 11. Summary of the activation and deactivation pathways of the Fe(BF4 )2·6H2O/PP3 catalyst system as well as proposed species formed basedon the results of spectroscopic analyses. Reprinted from ref. [45] with permission from Chemistry - A European Journal.


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*Correspondence: Prof. Dr. H. H. Giraulta

E-mail: [email protected] d’Electrochimie Physique et Analytique(LEPA)École Polytechnique Fédérale de Lausanne (EPFL) –Valais WallisRue de l’Industrie 17Case Postale 440CH-1951 SionbDepartment of ChemistryLancaster UniversityLancasterLA1 4YB, United Kingdom

Redox Flow Batteries, Hydrogen andDistributed Storage

C. R. Dennisona, Heron Vrubela, Véronique Amstutza, Pekka Peljoa, Kathryn E. Toghillb,and Hubert H. Girault*a

Abstract: Social, economic, and political pressures are causing a shift in the global energy mix, with a prefer-ence toward renewable energy sources. In order to realize widespread implementation of these resources,large-scale storage of renewable energy is needed. Among the proposed energy storage technologies, redoxflow batteries offer many unique advantages. The primary limitation of these systems, however, is their limitedenergy density which necessitates very large installations. In order to enhance the energy storage capacity ofthese systems, we have developed a unique dual-circuit architecture which enables two levels of energy stor-age; first in the conventional electrolyte, and then through the formation of hydrogen. Moreover, we have be-gun a pilot-scale demonstration project to investigate the scalability and technical readiness of this approach.This combination of conventional energy storage and hydrogen production is well aligned with the current tra-jectory of modern energy and mobility infrastructure. The combination of these two means of energy storageenables the possibility of an energy economy dominated by renewable resources.

Keywords: Electrical energy storage · Hydrogen · Redox flow batteries

1. Introduction

1.1 Growing Challenges for theElectrical Grid

Around the world, concerns about en-ergy security, sustainability, and the en-vironment have prompted a re-evaluationof the ways in which we produce andconsume (or more precisely, convert) en-ergy. As a result, there is a growing effortto transition the global energy mix fromconventional sources, such as fossil fuelsand nuclear energy, to more sustainablesources such as hydro, wind and solar. In-deed, from 2002 to 2012, the net renewableelectricity generation worldwide increasedby 62.5%.[1]As of 2013, 25.4% of Europe-an electricity, and 13.6% of overall energy(including transport, electricity, and heat-ing/cooling) was derived from renewable

resources.[2] Moreover, these values are setto grow significantly in the coming years.By 2020, the European Union is targeting20% reliance on renewables for its overallenergy mix.[3] These targets are primarilybeing met through the installation of wind,photovoltaic, and concentrated solar gen-eration facilities. However, the growingpenetration of electric vehicles also playsa key role in shifting the energy mix byreducing the need for petroleum.

While these changes to the energy mixrepresent significant progress toward so-cial, environmental, and political goals,they also represent a growing challengefor the world’s electrical grids. Currently,most electrical grids are designed to pro-duce electricity ‘just in time’ – as addi-tional load is added to the grid, generatingstations must simultaneously ramp up tomeet the demand and keep the grid volt-age and frequency stable. As consumerdemand is inherently unpredictable, some‘reserve’ generation capacity needs to beavailable at all times to cope with largeincreases in demand. This reserve capac-ity has traditionally taken the form of re-dundant generating stations which are idle,but synchronized with the grid so that theycan react immediately (so called ‘spinningreserve’). These resources are both ineffi-cient and costly, as they are spinning (i.e.consuming fuel), but primarily operating atzero-load. Nonetheless, this model for theelectrical grid is quite satisfactory for cop-ing with highly variable demand for elec-tricity, provided that the supply of electric-ity is reliable. However, with the growingimplementation of renewable resources

such as wind and solar, this assumption israpidly losing its validity.[4]

For grids with a large penetration of re-newable energy, both supply and demandbecome unpredictable. Solar irradiationcan vary significantly from minute to min-ute depending on cloud formations andother atmospheric conditions. A passingcloud can causemegawatts of solar genera-tion to suddenly disappear from the grid,necessitating other generating stationsto rapidly ramp up their output to main-tain the stability of the grid. As the cloudpasses, the solar generation becomes avail-able again, requiring the other generatingstations to suddenly curtail their outputto compensate. The availability of windis equally unpredictable. If these fluctua-tions are not compensated for, the powerquality (i.e. line voltage and frequency) onthe grid can deteriorate, eventually causinglocalized or even cascading power outages.As the energy mix continues to evolve, fa-vouring renewable resources, these fluctu-ations will become increasingly disruptive,pushing beyond the ramping limitations ofconventional power stations.A larger shareof ‘spinning reserves’, primarily gas-firedgenerators, will need to be allocated to sta-bilize the system. In effect, this means thatit becomes increasingly necessary to burnfossil fuels simply to utilize renewable en-ergy sources, and of course this largely un-dermines the goal of using renewable en-ergy in the first place (e.g. reduced depen-dence on fossil fuels, reduced atmosphericemissions, etc.).

The growing use of electric vehiclescan create similar problems of grid insta-


value of these systems lies in their energycapacity.

Several technologies are being con-sidered to address the growing technicalneeds and market opportunities. Indeed,from a technological standpoint large-scale electrical energy storage is not a newconcept. Large pumped hydroelectric sta-tions have long been used to store energyby pumping water from a low elevation toa higher elevation. To recover this energy,the water is allowed to flow downhill underthe influence of gravity, passing through aturbine along the way. Unfortunately, thisapproach is highly geographically con-strained, as it typically requires existingnatural features such as mountain lakes tobe practical.[5,8] In a similar approach, airis compressed into a reservoir to store en-ergy, and allowed to expand through a tur-bine to recover the energy. These systemstend to be quite large, and have a relativelylow efficiency due to heat rejection duringthe compression step.[5,8] Instead, it is de-sirable to have an efficient, scalable, flex-ible technology that can be deployed to avariety of locations.

Various other technologies, such ashigh energy flywheels, semiconductingmagnetic energy storage, and electrochem-ical capacitors have been proposed as well.While these systems are relatively modularand suitable for power conditioning appli-cations, they do not provide the energy ca-pacity needed to time-shift the energy pro-duced by renewable resources.[5,8] Instead,electrochemical systems appear to be themost promising for these applications.

Several battery technologies are cur-rently being considered and demonstratedfor grid-scale deployment.[9] High-temper-ature sodium sulfur batteries provide highenergy density and relatively long lifetime.However, these systems must operate athigh temperatures (250–350 °C) in orderto maintain conductivity. Large, container-ized arrays of lithium ion batteries have al-so been proposed and demonstrated. Thesesystems are very sensitive to temperature,and thus require good auxiliary thermalmanagement systems. Moreover, verycomplex battery management systemsare required to keep all of the individualcells balanced and prevent overcharging.Perhaps the largest concerns, however, aresafety and cost. Even the recently releasedTesla Powerwall costs between 350 and430 USD/kWh,[10] falling short of the U.S.Department of Energy target of 100 USD/kWh.[11] Moreover, the serious fire hazardpresented by these batteries is well knownat the small scale.[12] For a grid-scale in-stallation, the results could be catastrophic.For these reasons, one of the most promis-ing candidates for grid-scale energy stor-age is the redox flow battery.

production by essentially decoupling ener-gy production from consumption. Instead,such a grid operates on an ‘as available’ or‘on demand’ basis – energy is stored when-ever it is available from the generation in-frastructure, and supplied to consumersaccording to demand. In effect, the energystorage infrastructure acts as a buffer be-tween generation and consumption.[5] Thisbuffering is critical for the next evolutionof the electrical grid for two reasons; it pro-tects consumers from variability in genera-tion (e.g. clouds passing over a solar sta-tion), and it protects generators from largevariability in demand (e.g. electric vehiclecharging). Thus, grid-scale energy storageis necessary to enable the widespread im-plementation of renewable energy sourcesand electric vehicles.

However, there arenumerousadditionalbenefits to electrical energy storage. Suchsystems can be used to offset the ‘peaks’in electrical demand. In such a regime,energy is stored when demand is low, andthen time-shifted to periods when demandis high. Large electrical consumers (e.g.steel mills) may engage in this practice toreduce their peak demand (known as ‘peakshaving’), and the accompanying demandcharges. Meanwhile, electrical generatorsmay do the same in order to reduce theirperceived load (known as ‘load levelling’),thereby avoiding the need to start up costlypeak generating units. Additionally, smallindependent power producers may utilizeenergy storage assets to capitalize on fluc-tuations in the price of electricity (a prac-tice known as ‘energy arbitrage’). Grid op-erators may use electrical energy storageto help regulate voltage and frequency onthe grid. Moreover, energy storage instal-lations may be placed downstream of con-gested transmission lines, and used to alle-viate that congestion in a manner similar toload levelling. In some cases, this strategyallows grid operators to defer transmissionline upgrades which would otherwise benecessary to serve peak demand for only afew hours per day.[4]

All of these applications give rise toa growing market, which promises to bequite large in the coming years. Accord-ing to Deutsche Bank Research, in the nexttwenty years approximately 30 billionEurowill be invested in electrical energy storageinfrastructure within Germany alone.[6] By2040, they estimate that 40 TWh of storagewill be required to cope with the expectedsurpluses. At the global scale, Citi Groupestimates that the global market for energystorage will reach 240 GW, amounting toa market size of more than 400 billionUSD.[7] It is interesting to note the differentorders of magnitude between the projectedpower (GW) and energy (TWh) require-ments for future energy storage systems. Itis clear that from a market perspective, the

bility. Owners of these vehicles expect tobe able to recharge in a timeframe whichis reasonably similar to conventional liq-uid-fuelled vehicles. These quick chargingstations can easily consume over 100 kWeach, and charging may take up to an houror more. Moreover, drivers will expectto be able to charge whenever necessary,creating a large magnitude, unpredictableload on the grid. Finally, each driver whoreplaces a petroleum-fuelled vehicle withan electric vehicle is essentially shiftingload from the petroleum infrastructure tothe electrical grid. This will increase thetotal load on the electrical grid, which willnot only require additional generators, butwill also tax the electrical transmission anddistribution infrastructure. As the infra-structure reaches its limits, blackouts willbecome increasingly common.[4]

Hydrogen-fuelled vehicles have beenconsidered as an alternative to fossil-fuelled vehicles for years. Unfortunately,however, hydrogen is commonly obtainedby reforming fossil fuels, resulting in hy-drogen which is not truly ‘clean’ (in thesense of being carbon neutral). Electroly-sers may be used to obtain hydrogen bywater splitting. Currently, however, theelectricity used to drive the electrolysiscomes from fossil fuel and nuclear re-sources, again undermining the advantag-es of this approach. Moreover, commonalkaline electrolysers are not very tolerantof highly variable, intermittent operation,making them somewhat incompatible withmost renewable resources. Polymer elec-trolyte membrane (PEM) electrolysers aremore capable of coping with an intermit-tent power supply, but they require largeamounts of preciousmetal catalysts (e.g. Ptand Ir), making them very costly. Finally,the lack of a hydrogen distribution infra-structure has largely undermined effortsto transition to a ‘clean’ hydrogen energyeconomy.

The current grid architecture is ableto tolerate the existing level of unpredict-ability on the demand side. However, thepresent trajectory of energy production andconsumption will significantly stress thesystem in the coming years. Fundamen-tally, the notion of ‘just-in-time’ electric-ity production is incompatible with unpre-dictable energy sources, such as wind andsolar, and large, unpredictable loads, suchas electric vehicles. In order to achieve theenergy goals of this decade, a more flexiblegrid infrastructure is needed, and the keyto achieving this flexibility will be large-scale electrical energy storage.[4]

1.2 The Role of Energy Storage inTomorrow’s Electrical Grid

An electrical grid with a large amountof energy storage capacity moves awayfrom the paradigm of ‘just-in-time’ energy


3. Dual-Circuit Redox Flow Battery

3.1 Two Levels of Energy StorageThe dual-circuit redox flow bat-

tery was developed in order to by-pass the energy density limitations ofconventional RFBs, by adding a sec-ond ‘level’ of energy storage.[28,29]Energy may be stored conventionally, inthe liquid electrolyte, or converted to hy-drogen gas through indirect electrolysis ofwater. This design allows energy exceed-ing the conventional capacity of the batteryto still be captured and stored in the formof hydrogen. The energy stored within thehydrogen may be returned to the electricalgrid using stationary fuel cells, or used topower fuel cell vehicles, to enrich naturalgas, or in other industrial processes. In thisway, excess energy is never wasted. Thisapproach enables continuous storage ofsurplus renewable energy, even when theconventional storage capacity of the bat-tery has been reached. Moreover, this ap-proach eliminates the need to oversize thebattery.

In order to accomplish this, a secondaryflow circuit was added to each half of theredox flow battery architecture (Fig. 2). Onthe negative half-cell, the secondary flowcircuit contains a catalytic reactor wherethe negative electrolyte is chemically dis-charged, producing hydrogen gas accord-ing to Eqn. (3):

2H+ + 2D– → H2+ 2D (3)

where D– is an electron donor. The produc-tion of hydrogen consumes protons in theelectrolyte solution. It is critical to main-tain a relatively constant concentration ofprotons in order to ensure good electrolyteconductivity and maintain the solubilityof the active species throughout the cycle.

Protons conduct the current throughthe membrane; upon charge a number ofprotons equal to the number of transferredelectrons are transferred from the positiveelectrolyte into the negative electrolyte.Upon discharge, the opposite reactions oc-cur.The standard cell potential is ca.1.25V.The active vanadium species are typicallydissolved in an aqueous supporting electro-lyte containing sulfuric acid, although oth-er acids, such as methanesulfonic acid andhydrochloric acid have also been used.[19]Since both half-cells are based on vana-dium active species, cross-contaminationbetween the two half-cells is not a majorconcern. In effect, unintentional mixingof the two electrolyte streams results in achemical discharge and subsequent loss ofstored energy, but does not otherwise de-grade the solutions.[13–17]

Regardless of the composition of theelectrolyte, the maximum energy densityof the system is typically determined bythe limits of solubility of the active species.In practice, the concentration of active spe-cies is limited from 1 to 2 M, yielding anachievable energy density of approximate-ly 25 to 40Wh per litre of electrolyte.[13–17]This is the main technical limitation ofmodern flow battery systems – the energydensity is intrinsically limited by the solu-bility of the active species.

Significant effort has been placed onenhancing the solubility of the variousactive species in solution. In particular,previous work has focused on improvingsolubility by changing the composition ofthe supporting electrolyte[19–22] and intro-ducing various additives.[23–27] Althoughsignificant progress has been made, theseapproaches are unlikely to yield disruptiveimprovements in energy density. More-over, they are highly dependent on the spe-cific chemistry, and thus cannot be readilygeneralized to other redox chemistries.

2. Redox Flow Batteries

A redox flow battery (RFB) is a type ofsecondary battery system in which chargeis stored and released by oxidizing or re-ducing active species in a flowing solu-tion. The active species remain solubilizedin the surrounding electrolyte during op-eration. During charging, the electrolyte ispumped through an electrochemical cell,consisting of two electrodes separatedby an ion-exchange membrane (Fig. 1).Current is supplied to the electrodes viaconductive current collectors, polarizingthe electrodes. As a result of polarization,redox reactions occur at the surfaces ofthe electrodes, resulting in the oxidation(positive half-cell) and reduction (negativehalf-cell) of the active species dissolvedin electrolytes. These charged electrolytesthen pass out of the electrochemical celland into storage reservoirs. To dischargethe system and recover the stored energy,the electrochemical process is reversed.[13]

The unique aspect of RFBs is theirsystem architecture consisting of a centralelectrochemical cell/stack and externaltanks. In conventional batteries, the activematerial is stored within the cell itself; ineffect coupling the power output and en-ergy capacity. By storing the electrolytesexternally, the energy capacity can bescaled arbitrarily by increasing the volumeof stored electrolytes. Similarly, the poweroutput of RFB systems is determined bythe cell area. This decoupling of power andenergy makes RFBs highly scalable andflexible, which in turn makes them unique-ly suited for grid-scale applications.[13]

The specific redox chemistry is anoth-er key aspect of these systems. The redoxchemistry determines the maximum en-ergy density (according to the solubility ofthe active species) and the typical operat-ing voltage. Numerous redox chemistrieshave been proposed and demonstrated.Among the most common are: Fe/Cr, Zn/Br, and all-vanadium (V/V). Other chem-istries involving two-phase processes alsoexist: all-iron (Fe/Fe), all-copper (Cu/Cu),H/Br, V/air, etc.[13–18]

Among the numerous redox chemis-tries that can be utilized in an RFB archi-tecture, the all-vanadium chemistry is per-haps the most prototypical. We consider ithere, as an illustrative example. In a vana-dium redox flow battery (VRFB), V(iv) isoxidized to V(v) in the positive half-cellduring charging, while V(iii) is reduced toV(ii) on the negative half-cell:

V(iv)V(v) + e–, E° = 0.991 V (1)

V(iii) + e– V(ii), E° = –0.255 V (2)

Load

Pump Pump

Positive

Electrolyte

Tank

Negative

Electrolyte

Tank

Fig. 1. System archi-tecture for conven-tional flow batteriesbased on liquid elec-trolytes.


controlled hydrogen evolution inside thetank. For these reasons, we decided to usea supported catalyst. A spray-coating pro-cess was developed to synthesize Mo

2C

on the surface of inert, spherical supports.The spherical supports provide reasonablesurface area while maintaining large inter-stitial voids for electrolyte and gas to flowthrough. Initially, porous alumina sup-ports were used, however the mechanicalstability of the catalytic coating on thesespheres was not satisfactory. After severaliterations, 3 mm ceramic beads (Denstone2000, Saint Gobain, Germany) were foundto provide good adhesion and mechanicalstrength for the catalytic coating.

Having developed a suitable catalyst,the next step was to design the catalyticreactor. Initially, a horizontal fixed bedreactor design was used. Charged electro-lyte was injected at one end of the reac-tor, flowing through the catalytic bed andgenerating hydrogen. Hydrogen could becollected from the headspace above thecatalytic bed. Baffles were installed alongthe length of the reactor to help promotemixing of the electrolyte throughout thereactor. Unfortunately, this reactor designhad problematic mass transport charac-teristics. The hydrogen evolution reactioncaused the electrolyte in the catalytic bedto foam. This foam had sufficient capillar-ity to block many of the interstitial voidsbetween particles, severely limiting theutilization of the catalyst.

To address this limitation, a new reac-tor was recently designed and built (Fig.3c). This reactor is a vertical, multi-stagefixed bed reactor. In each stage, electrolyteis injected at the bottom of the catalyticbed. The liquid then flows up, through thecatalytic bed, until reaching the raised out-let and flowing into the next stage (Figs3d,e). The hydrogen gas is collected fromthe headspace in each stage. In total, thereactor contains eight stages, each contain-ing ca. 2 g of Mo

2C.

This reactor design has several advan-tages over the previous reactor design. Thelarge number of stages increases the resi-dence time of the reactor, allowing greaterconversion of the electrolyte in a singlepass. Moreover, the vertical flow of theelectrolyte helps to facilitate the sheddingof hydrogen bubbles from the catalyticbed. This helps to prevent blocking of thecatalyst, and increases the amount of con-version achieved in each stage.

We have recently started characterizingthe performance of this new reactor design.Our initial results indicate that the reactor isvery well suited for hydrogen production,and is able to provide a higher conversionrate as a result of the various improvementsintroduced. It appears that full conversionof the electrolyte may be achieved for flowrates up to 1 L/minute.At this flow rate, the

strated the proof-of-concept at the labora-tory scale.[29] Specifically, indirect hydro-gen evolution and water oxidation weredemonstrated at the surface of Mo

2C and

RuO2, respectively, using the fully charged

V and Ce electrolytes. Moreover, the con-version efficiency and apparent reactionrate for both reactions were determined.The hydrogen evolution reaction, in partic-ular, was found to have nearly 100% con-version efficiency.[29] Based on the successof this preliminary work, it was decidedto begin scaling up the process to a moreindustrially-relevant scale.

3.2 Scale-up and DemonstrationIn order to further validate the dual-cir-

cuit approach and determine its technicalreadiness, a demonstration project was un-dertaken. The basic platform for this devel-opment effort was a CellCube 10 kW/40kWh all-vanadium redox flow battery pro-duced by Gildemeister Energy Solutions(Fig. 3b). The battery was installed at a sitein Martigny, Valais, Switzerland (Fig. 3a).After installation and commissioning, webegan the process of retrofitting the batterywith the necessary secondary circuit andcatalytic reactor for hydrogen generation.As the commercial RFB is based on the all-vanadium redox chemistry, the water oxi-dation reaction is not possible on the posi-tive half. Accordingly, we have focusedour efforts on demonstrating the hydrogenevolution reaction.

Oneof thekeysteps in this effortwas thedevelopment of a catalyst which could bepractically employed at this scale. A fixedbed of the microparticulate Mo

2C catalyst

used in our proof-of-concept work wouldcause too much pressure drop in the sec-ondary hydraulic circuit. Moreover, suchsmall particles could become entrained inthe flow of the electrolyte and travel intothe electrolyte storage tank, causing un-

Thus, a source of protons is also needed.These protons are liberated in the positivecatalytic reactor via the water oxidationreaction:

2H2O + 4A+ → O

2+ 4H+ + 4A (4)

where A+ is an electron acceptor. The pro-tons liberated by this reaction pass from thepositive half-cell to the negative half-cell,via the membrane, providing additionalreactant for the hydrogen evolution reac-tion. Thus, if water is added stoichiometri-cally, both of these reactions may proceedindefinitely, assuming a constant supply ofcharged electrolytes.[29]

In order to drive these reactions, elec-trolytes with appropriate redox potentialsmust be utilized. For the hydrogen evolu-tion reaction (Eqn. (3)), the V(ii)/V(iii) re-dox couple (Eqn. (2)) has a thermodynami-cally favourable redox potential. Unfortu-nately, the V(iv)/V(v) couple (Eqn. (1))does not have a sufficient redox potentialto drive water oxidation (Eqn. (4)). For thisreason, the Ce(iii)/Ce(iv) couple was uti-lized for our proof-of-concept work:

Ce(iv) + e– Ce(iii), E° = 1.72 V (5)

Despite sufficient thermodynamic po-tentials to drive the indirect water electrol-ysis reactions, catalysts were still needed toenhance the kinetics of the reactions. RuO

2was utilized to drive the water oxidation re-action, due to its suitability in acidic condi-tions. For the hydrogen evolution reaction,Mo

2C was utilized. These catalysts lower

the overpotential sufficiently to allow bothreactions to proceed spontaneously in thepresence of the charged electrolytes.[29]

Using the V/Ce redox chemistry, andthe catalysts mentioned above, we demon-

Load

H2

O2

Valve

ValveValve

Valve

Pump Pump

Positive

Electrolyte

Tank

Negative

Electrolyte

Tank

Fig. 2. Dual-circuit redox flow battery architecture. The valves allow electrolyte to pass throughthe external reactors, where the electrolyte may be chemically discharged ‘on demand’ to pro-duce hydrogen and oxygen on the negative and positive halves, respectively.


battery can be completely chemically dis-charged in 16.7 hours, which is equivalentto a conventional discharge at ca. 2.4 kW.If a greater equivalent discharge power isneeded, several reactors may be placed inparallel.

4. Looking Forward

The dual-circuit redox flow batterydiscussed here provides a unique approachfor enhancing the energy capacity of redoxflow battery systems. Since this approachdoes not alter the fundamental RFB ar-chitecture, existing systems can be easilyretrofitted to enable the production of hy-drogen. Moreover, besides indirect waterelectrolysis, other industrial processes canpotentially be accomplished using this sys-tem architecture, such as the oxidation ofH

2S and SO

2to produce sulfuric acid.[30]

More generally, the concept of indirectelectrolysis could be extended to otherelectrosynthetic processes, for example forthe on-site production of chlorine (chlorinetransport through inner cities such as Laus-anne being a hazardous process) or for theproduction of hydrogen peroxide, which isan environmentally friendly oxidant.

In a broader context, this system is rep-resentative of the growing importance ofdiverse energy storage infrastructure. Theinterplay between hydrogen and electric-ity will become increasingly important inthe coming years. It is clear that large-scaleenergy storage is needed to facilitate thewidespread implementation of renewableenergy sources. Growing reliance on elec-tric vehicleswill further reinforce this need.For mobile applications, hydrogen energystorage is particularly beneficial due toits high energy density, and the ability toinstantly refuel vehicles. However, wide-spread hydrogen infrastructure is lackingand remains a significant barrier to the ac-ceptance of hydrogen fuelled vehicles.

Nonetheless, we can imagine a futurewhich capitalizes on the synergies betweenelectricity and hydrogen. Energy storagetechnologies, such as redox flow batteries,may be installed at renewable energy gen-eration sites to buffer supply and demand.Additional energy storage sites may beinstalled at nodes throughout the distribu-tion system in order to flatten the load onthe network and provide greater reliability.As more storage capacity comes online,this will enable further implementation ofrenewable resources, starting a cycle of

constructive feedback for both technolo-gies. Moreover, the overall energy storagecapacity available in the grid will make itmore tolerant to the large amplitude pertur-bations caused by the charging of batteryelectric vehicles. Some of these distributedenergy storage sites may also function asfuelling sites for hydrogen fuelled vehi-cles. Hydrogen can be produced directlyon-site by electrolysis of water, avoidingthe need for a hydrogen distribution in-frastructure. In some cases, indirect wa-ter electrolysis, as seen in the dual-circuitflow battery system presented here, may beutilized. However, conventional RFBs canalso be used as a buffer between renewableresources and conventional electrolysers,allowing the electrolyser to operate at aconstant load. In both cases, the electricityused to drive the electrolysis will be de-rived primarily from renewable resources,rather than fossil fuel or nuclear resources,producing truly ‘clean’ hydrogen.

A future grid powered solely by renew-able energy sources is rapidly becomingpossible with ongoing advances in tech-nology. The dual-circuit redox flow batterysystem can play a key role as an energymanagement platform, directly connectingthe producer’s needs to the consumer’s re-

Fig. 3. a) Dual-circuit RFB demonstrator site in Martigny, Valais, Switzerland. The demonstrator is based around b) a 10 kW/40 kWh all-vanadiumflow battery which has been retrofitted with c) an auxiliary hydraulic circuit for producing hydrogen. d) The reactor contains eight identical stages,e) where electrolyte is injected at the bottom of the catalytic bed and flows upward through the catalyst before reaching the outlet. Hydrogen is re-moved from the headspace above each stage.


[19] M.Vijayakumar,W.Wei, N. Zimin,V. Sprenkle,H. JianZhi, J. Power Sources 2013, 241, 173.

[20] Z. Shudi, Z. Yuchun, Appl. Mech. Mater. 2013,281, 461.

[21] Y. H. Wen, Y. Xu, J. Cheng, G. P. Cao, Y. S.Yang, Electrochim. Acta 2013, 96, 268.

[22] X.W.Wu, J. Liu, X. J. Xiang, J. Zhang, J. P. Hu,Y. P. Wu, Pure Appl. Chem. 2014, 86, 661.

[23] M. Skyllas‐Kazacos, C. Peng, M. Cheng,Electrochem. Solid-State Lett. 1999, 2, 121.

[24] J. Zhang, L. Li, Z. Nie, B. Chen, M.Vijayakumar, S. Kim, W. Wang, B. Schwenzer,J. Liu, Z. Yang, J. Appl. Electrochem. 2011, 41,1215.

[25] G.Wang, J. Chen, X.Wang, J. Tian, H. Kang, X.Zhu,Y. Zhang, X. Liu, R. Wang, J. Electroanal.Chem. 2013, 709, 31.

[26] Y. Lei, S. Q. Liu, C. Gao, X. X. Liang, Z. X. He,Y. H. Deng, Z. He, J. Electrochem. Soc. 2013,160, A722.

[27] G. Wang, J. Chen, X. Wang, J. Tian, H. Kang,X. Zhu, Y. Zhang, X. Liu, R. Wang, J. Ener.Chem. 2014, 23, 73.

[28] V.Amstutz, K. E. Toghill, C. Comninellis, H. H.Girault, WO Patent WO2013131838, 2013.

[29] V. Amstutz, K. E. Toghill, F. Powlesland, H.Vrubel, C. Comninellis, X. Hu, H. H. Girault,Ener. Environ. Sci. 2014, 7, 2350.

[30] P. Peljo, H. Vrubel, V. Amstutz, J. Pandard,J. Morgado, A. Santasalo-Aarnio, D. Lloyd,F. Gurny, C. R. Dennison, K. E. Toghill, H.H. Girault, Green Chem. 2015, doi:10.1039/C5GC02196K.

[8] United States Department of Energy, ‘GridEnergy Storage’, http://www.sandia.gov/ess/docs/other/Grid_Energy_Storage_Dec_2013.pdf, 2013.

[9] B. Dunn, H. Kamath, J.-M. Tarascon, Science2011, 334, 928.

[10] Tesla Powerwall, http://www.teslamotors.com/powerwall, accessed August 9, 2015.

[11] Advanced Research Projects Agency -Energy, U.S. Department of Energy, ‘GRIDSProgram Overview’, http://arpa-e.energy.gov/sites/default/files/documents/files/GRIDS_ProgramOverview.pdf, 2010.

[12] J. M. Tarascon, M. Armand, Nature 2001, 414,359.

[13] A. Weber, M. Mench, J. Meyers, P. Ross, J.Gostick, Q. Liu, J. Appl. Electrochem. 2011, 41,1137.

[14] N. Trung, R. F. Savinell, Electrochem. Soc.Interface 2010, 19, 54.

[15] M. Skyllas-Kazacos, M. H. Chakrabarti, S.A. Hajimolana, F. S. Mjalli, M. Saleem, J.Electrochem. Soc. 2011, 158, R55.

[16] G. Kear, A. A. Shah, F. C. Walsh, Int. J. Ener.Res. 2012, 36, 1105.

[17] A. Parasuraman, T. M. Lim, C. Menictas, M.Skyllas-Kazacos, Electrochim. Acta 2013, 101,27.

[18] D. Lloyd, E.Magdalena, L. Sanz, L.Murtomäki,K. Kontturi, J. Power Sources 2015, 292, 87.

quirements. Its ability to produce hydrogenon demand provides an additional degreeof freedom compared to classical batter-ies, increasing the versatility of large-scaleelectrochemical energy storage in a renew-able energy grid.


[1] U.S. Energy InformationAdministration OnlineDatabase, http://www.eia.gov, accessed August7, 2015.

[2] Eurostat SHARES Online Database – 2013,http://ec.europa.eu/eurostat/web/energy/data/shares, accessed August 7, 2015.

[3] European Commision, ‘Directive 2009/28/ECof the European Parliament and of the Councilof 23 April 2009’, Official Journal of theEuropean Union, 2009, 140, 16.

[4] Electric Power Research Institute, ‘EPRI-DOEHandbook of Energy Storage for Transmissionand Distribution Applications’, Report No.:1001834, 2003.

[5] Electric Power Research Institute, ‘ElectricityEnergy Storage Technology Options’, ReportNo.: 1020676, 2010.

[6] J. Auer, J. Keil, ‘State-of-the-Art ElectricityStorage Systems’, in Current Issues, DeutscheBank Research, 2012.

[7] S. Savvantidou, T. Sasaki, A. Yuen, ‘StorageBatteries: A Third Growth Market’, in CitiGPS: Global Perspectives & Solutions, CitiGroup, 2015.


*Correspondence: Prof. Dr. C. CopéretDepartment of Chemistry and Applied BiosciencesETH ZürichVladimir-Prelog-Weg 1–5CH-8093 ZürichE-mail: [email protected]

CO2 Hydrogenation: SupportedNanoparticles vs. Immobilized Catalysts

Shohei Tada, Indre Thiel, Hung-Kun Lo, and Christophe Copéret*

Abstract: The conversion of CO2 to more valuable chemicals has been the focus of intense research over thepast decades, and this field has become particularly important in view of the continuous increase of CO2 levelsin our atmosphere and the need to find alternative ways to store excess energy into fuels. In this review we willdiscuss different strategies for CO2 conversion with heterogeneous and homogeneous catalysts. In addition,we will introduce some promising research concerning the immobilization of homogeneous catalysts onheterogeneous supports, as a hybrid of hetero- and homogeneous catalysts.

Keywords: CO2 · Heterogeneous catalysis · Hydrogenation · Immobilization

Introduction

Over the last decades the global CO2

emission has continuously increased withfossil fuel combustion and industrial pro-cesses such as cement and metal produc-tion contributing the largest share. In 2013a new record of 35.3 billion tons of emittedCO

2has been reached.[1] Due to tougher

policies on the emission of green housegases a main focus has been to adapt long-standing processes to reduce CO

2emis-

sion in the first place. However, the simplecapture and storage of CO

2would increase

the energy requirements of an industrialplant by 25–40% presenting challenges inthe form of gas separation and fixation.[2]Consequently the simple usage of CO

2as

a carbon source and C1-building block forthe synthesis of more valuable chemicalsor fuels would not only reduce the overallemission but also present a solution in re-spect to finding alternatives for fossil fuels.Therefore nowadays – at least from the re-search point of view – CO

2can be consid-

ered as an abundant carbon source (Fig. 1)and a part of the methanol economy.[3]

Nevertheless activation of the CO2

molecule is challenging and requires highenergies in form of high temperatures,high pressures and/or the use of active re-actants. Essentially its considerable Gibbsfree energy of formation (CO

2∆G°

298.15K=

–394.4 kJ/mol) has to be overcome. Onesolution to this thermodynamic problemis the usage of co-reactants with higherGibbs free energy such as H

2, methanol, or

even epoxides, the latter being used in thesynthesis of (cyclic) carbonates from CO

2.

The splitting of the C=O double bond andthe formation of a C–H or C–C bond canbe achieved if a reducing agent is used inthe presence of a catalyst. Since renewableenergies have been the focus of currentresearch, the usage of H

2as the reducing

agent – provided it comes from renewable/excess energy – would greatly contributeto a more environmentally friendly conver-sion of CO

2and to incorporate CO

2in the

fuel cycle.The application of various mainly

transition metal catalysts lowers this ac-tivation energy and allows the conversionof CO

2to hydrogenation products such

as CO, methane, methanol and dimethylether (DME), formic acid and dimethyl-formamide (DMF), but also more complexmolecules like (cyclic) carbonates or car-boxylic acids.[4]

This review will focus on selected ex-amples of heterogeneous CO

2hydrogena-

tion catalysts and compare them to im-mobilized homogeneous hydrogenationcatalysts.

Supported Nanoparticles as CO2Hydrogenation Catalysts

As shown in Fig. 1, the hydrogena-tion of CO

2– depending on the catalytic

systems and the reaction conditions – canlead to various products, mainly methane(methanation), hydrocarbons (related toFischer-Tropsch – FT), CO (reverse watergas shift) and methanol. Below we will il-lustrate each reaction and the current stateof the art.

Reverse Water Gas Shift ReactionThe reverse water gas shift (RWGS)

reaction corresponds to the hydrogenationof CO

2into CO and H

2O (Eqn. (1)) and is

considered to be an intermediate step forCH

4and olefin production in FT-related re-

Fig. 1. CO2 hydrogenation with heterogeneous and homogeneous catalysts, a general view.


CO + 2 H2 CH

3OH (3)

CO2+ 3 H

2 CH3OH + H

2O (4)

The interaction of Cu with ZnO greatlyenhances the activity and selectivity ofthe methanol synthesis. Possible expla-nations involve the stabilization of Cu(i)centers[28c] or the better dispersion of Cuspecies by ZnO. It has also been proposedthat the active sites are defective surfac-es of nanoparticulate Cu over Cu/ZnO/Al

2O

3.[28a] ZrO

2is also known as one of

the promising supports and/or promotersfor methanol production from CO

2. For

instance, methanol production turnoverfrequency is 27 times higher (7.3 × 10–2

s–1) with Cu/ZrO2/SiO

2than with Cu/SiO

2(2.7 × 10–3 s–1).[9a] In addition, selectivityof methanol over Cu/ZrO

2/SiO

2was four

times higher (43%) than that over Cu/SiO2

(11%).[9a] This effect of ZrO2is likely due

to a combination of parameters: improve-ment of Cu dispersion,[30] increase in CO

2adsorption[31] and increased number ofedges, corners, defects and oxygen vacan-cies by incorporation of Cu into nanocrys-talline ZrO

2.[32] It has also been suggested

that Cuδ+ sites at the interface between Cu0

and ZrO2play a key role in the methanol

synthesis.[28c]Recently, catalysts based on Cu/CeO

2have been reported as an alternative to tra-ditionalCu/ZnOandCu/ZrO

2systems, cre-

ating a metal-oxide interface that allows abetter adsorption and activation of CO

2.[10]

Worthy of note the ternary catalyst Cu/CeO

x/TiO

2(110) showed even higher ac-

tivity for methanol production with TOFof 8.1 s–1 at 575 K (Table 1) compared toCu/CeO

2(111) and Cu/TiO

2(110). Ni

5Ga

3/

SiO2catalysts are also superior to Cu/

ZnO/Al2O

3owing to low CO production

via RWGS.[33] Very recently, hybrid oxidecatalyst based on MnO

xand mesoporous

found that CeO2increases the amount of

adsorbed CO2[21c] and promotes the conver-

sion of thus-formed surface carbonate intoCO.[22] It has been proposed to increasethe rates of the first steps associated withthe RWGS reaction (Eqn. (1)). In addition,since CeO

2-containing catalysts show high

activity in the methanation of CO, it is nottoo surprising that the overall CO

2metha-

nation is favored on these catalysts as well,leading to high CH

4selectivity (close to

100%).[21c,22] ZrO2-containing materials

are also interesting support candidates forCO

2methanation.[21a] In the related sys-

tem Ni/Ce0.78

Zr0.28

O2the incorporation of a

part of Ni species in the fluorite-structuredCe

0.78Zr

0.28O

2improved the stability of the

catalysts for CO2methanation at 350 °C

(H2/CO

2/N

2= 36/9/10, GHSV = 43,000 h–1

and TOF = 0.4 s–1, Table 1).[16,21b] In addi-tion, Ni sintering can be suppressed by theaddition of noble metals (such as Ru andRh) to the Ni catalyst, leading to longercatalyst lifetime.[26] Adding Pt to Co cata-lysts also led to an increased catalyst life-time.[17] The additions of promoters suchas Na, K and La to Ru/TiO

2catalysts can

also improve the rate in CO2methanation

as well.[27]

Methanol and Related DimethylEther Synthesis

In the 1960s ICI developed an effi-cient low-pressure process (50–100 bar,200–300 ºC) for methanol synthesis fromsyngas (Eqn. (3)) using Cu/ZnO/Al

2O

3catalysts. More recently, this catalyst andrelated systems have also been investigatedfor CO

2hydrogenation to methanol (Eqn.

(4)). For Cu catalysts, the nature of theactive sites and the elementary steps arestill debated, possibly involving Cu0 and/or Cu+. The effect of support and promoteron the Cu catalysts for methanol synthe-sis has been investigated in depth; for ex-ample, reported supports/promoters areZnO,[28] ZrO

2,[9a,28c] MgO,[28b] TiO

2,[10] and

Ga2O

3.[29]

actions. This endothermic RWGS reaction– favored at high temperature – is catalyzedby several metals. Much attention has beengiven to Ni,[5] Pt,[6] Cu,[6] and Au.[7] Threedifferent mechanisms have been proposedfor the RWGS reaction: (i) direct dissocia-tion of CO

2into CO and O*,[8] (ii) forma-

tion of formate (HCOO)[9] or (iii) carboxyl(COOH) intermediates.[5,8b,10] While thesystems are typically very complex, it hasbeen proposed that the active site underreaction conditions (CO/CO

2/H

2/H

2O) in

CeO2supported Pt and Au catalysts cor-

responds to partially oxidized Pt and Auspecies.[11] In addition, based on a kineticapproach of the corresponding reverse re-action (RWGS), corner atoms of Au NPson TiO

2are the most likely active sites.[12]

CO2+ H

2→ CO + H

2O (1)

As discussed above, several reactionpathways are possible. The direct disso-ciation pathway involves splitting CO

2on

the surface of the metal surface into CO*and O*; the surface O* being then reducedby H

2or surface H to H

2O*, leading after

desorption to CO and H2O. In the formate

mechanism, following the initial H trans-fer to CO

2, formate species are formed and

decomposed into CO.[9b] Alternatively,surface H* can react with CO

2to gener-

ate M-COOH intermediates, which thenevolve into CO and surface OH groups.This mechanism has first been proposedby calculations using Cu (111) surfaces.[13]While proposed to be favored on Pt, Ag,and Pd, direct CO

2dissociation would be

favored on Cu, Rh, and Ni.[8b] This processhas also been proposed to be favored at theinterface between CeO

xand Cu in CeO

x/

Cu(111).[10] While under debate, it is clearthat the metal the support and the reac-tion conditions can favor one or the othermechanism, and much work has to be un-dertaken to understand these systems at amolecular level.

MethanationMethane can be obtained via the hydro-

genation of CO2(Eqn. (2)). While mainly

investigated with supported Ni,[21] noblemetals such as Ru,[9b,22] Rh[23] and Pd[8c,24]are also known to participate in this reac-tion. Since CO

2methanation is exother-

mic, lower reaction temperatures favorhigh methane yields.

CO2+ 4 H

2→ CH

4+ 2 H

2O (2)

The performance of CO2methana-

tion catalysts is affected by the nature ofthe support materials. For example in thecase of CeO

2-based catalysts[21c,22,25] it was

Table 1. Selected catalysts for CO, CH4, and MeOH production via CO2 hydrogenation.

Catalysts Temp./ °C Pressure/ bar Product TOF/ s-1 Ref.

Cu(110) 237 5.1 CO 0.01 [14]

Pt/TiO2

300 1 CO 0.10 [15]

Ni/CeO2-ZrO

2350 1 CH

40.429 [16]

Rh/γ-Al2O

3200 1 CH

40.010 [8c]

Pt/MCF-17+ Co/MCF-17 250 1 CH4

0.038 [17]

Ru/Al2O

3350 1 CH

40.03 [18]

Ni-Zr alloy 300 1 CH4

0.054 [19]

Cu-Zn-Al-Zr oxides 270 50 MeOH 0.009 [20]

Cu/ZrO2/SiO

2250 6.5 MeOH 0.073 [9a]

Cu/CeOx/TiO2(110) 303 5 MeOH 8.1 [10]


conversion of CO2to formic acid is highly

reversible in the presence of the catalyst;hence the employment of a base to removethe formic acid from the equilibrium isnecessary.

State of the Art in HomogeneousCatalysis

Pioneering work on the hydrogenationreactions of CO

2started in the 1970s, using

various transition-metal complexes of Ru,Os, Rh, Ir and Pt.[40] Greater performanceswere reached much later using Ru cata-lysts such as 2-(a) and supercritical CO

2(scCO

2) as a solvent, resolving miscibility

issues reported previously.[41]Since 2000 ruthenium[42] and iridi-

um[43] have been used the most frequentlyfor the hydrogenation of CO

2to formic

acid derivatives (Figs. 2 and 3). Worthyof note, [RuCl

2(PTA)

4] (PTA = 1,3,5-tri-

been proposed to play the role of structuralpromoter as well, leading to higher disper-sion of Fe species on support materials.[38]The presence of noble metals such as Ruon Co-K catalysts further enhanced theconversion of CO

2and the selectivity of

C5+ hydrocarbons.[37b]

Immobilized HomogeneousCatalysts for CO2 Hydrogenation

The most prominent reactions in ho-mogeneous CO

2hydrogenation are the

syntheses of formic acid and methanol,however the generation of formic acid andits derivatives yields much higher TONsand TOFs than that of methanol, since theconversion of CO

2to methanol usually re-

quires the combination of several catalystsor concurrent reactions. In general, the

spinel Co3O

4catalyzes the CO

2conversion

to methanol in higher yields than the in-dividual catalysts MnO

x-SiO

2and Co

3O

4respectively,[34] implying that the interfaceof MnO

xand Co

3O

4contains specific ac-

tive sites for the CO2conversion.

Dimethyl ether (DME) is a usefulchemical and an attractive alternative toliquefied petroleum gas (LPG), and canbe synthesized by a multi-step processinvolving methanol production via CO

2hydrogenation to methanol (Eqn. (3)) andsubsequent methanol dehydration to DME(Eqn. (5)).

2 CH3OH CH

3OCH

3+ H

2O (5)

In a one-step DME synthesis approach,methanol consumption viamethanol dehy-dration can abate the catalyst’s surface con-centration of the intermediate methanol,and in turn overcome the equilibrium limi-tation of CO

2conversion at low tempera-

tures. Methanol dehydration takes place onγ-Al

2O

3[35] and acidic zeolites (HZSM-5[35]

and SAPO[36]). Therefore DME can besynthesized directly over physical mix-tures of solid acids and methanol synthesiscatalysts. Note that methanol dehydrationis an exothermic reaction, leading to higherDME selectivity at lower reaction tem-peratures. It is reported that the methanolsynthesis is the rate-determining step whenCu-based methanol synthesis catalystswere physically mixed with high aciditymaterials.[36]

OlefinsThe hydrogenation of CO

2to olefins

derives from the combination of RWGS re-action (Eqn. (1)) converting CO

2to CO and

the Fischer-Tropsch (FT) process (Eqn.(6)), yielding hydrocarbons.

CO + 2 H2→ (CH

2) + H

2O (6)

Accordingly, the reaction has beenstudied mainly on traditional catalystsfor FT synthesis such as iron and cobaltcatalysts with promoters. The olefins arelikely produced on surface carbides, whichare formed in situ via decomposition of COon metal surfaces, following the Boudardreaction (Eqn. (7)).

CO + CO → C + CO2

(7)

The role of promoters, such as K,[37]Na,[37b] Li,[37b] Mn[37a,38] and La[39] species,is considered to enhance the activity of thecatalyst in RWGS (due to improvement ofCO/CO

2adsorption) and the carburization

of Fe or Co species. In addition, Mn has

Ru

Cl

RMe3P PMe3

Me3P PMe3

Ru

PMe3

ClMe3P OAc

Me3P PMe3

Sc(OTf)3

NNRu CO

PtBu2H

++

N

NEt2

Ru CO

PtBu2H

HNTf2

RuPh2P

PPh2

PPh2

+

P

N NN

RuPh2P

CO

PPh2

N

Cl

H

H

3

+Na-O3S

P Rh Cl

SO3-Na+

+Na-O3S

1 Leitner et al., 1993 [40d]

CO2 to formateTON = 3400

CO2 to formate2-(a) Noyori et al., 1994 [41a]

R=Cl, TON = 72002-(b) Jessop et al., 2002 [42b]

R=OAc, TOF = 95000 h-1

Ru

PMe2

PMe2Me2P H

Me2P H

3 Baiker et al., 2007 [42a]

CO2 to formate

5 Sanford et al., 2011[42d]CO2 to methanolTON = 21

6 Sanford et al., 2013 [42c]


PTA =

7 Laurenczy et al., 2014 [42g]

CO2 to formic acidTON = 749

8 Beller et al., 2014 [42k]

CO2 to formateTON > 18000

10 Leitner et al., 2015 [42f]

CO2 to methanolTON = 442

Ru Cl

N N

N

N

+

PF6-

4 Peris et al., 2010 [42i]

CO2 to formateTON = 23000 (with H2)

874 (with iPrOH)

[RuCl2(PTA)4]

N

N

N

N

N

Mes

Mes

Ru COH

Br

11 Pidko et al., 2015 [46]

CO2 to formateTOF = 99100h-1

N(tBu)2P P(tBu)2Ru

COClH

9 Pidko et al., 2014 [44]

CO2 to formateTOF = 1892000 h-1

NN P(tBu)2Ru

CO HCl

12 Milstein et al., 2015 [49]

Captured CO2 (oxazolidinone)to methanolYield: 92%

Fig. 2. Selected ruthenium catalysts used in CO2 hydrogenation reactions.

N(iPr)2P P(iPr)2Ir

HHH

IrIO O

NNN N

SO3-K+SO3-K+

I

N

IrH

HH

P(tBu2)(tBu2)PN N

N N

-O O-

-O O-

Ir Ir*CpH2O

Cp*OH2

14 Peris et al., 2011 [42j]

CO2 to formateTON = 19000 (with H2)

2700 (with iPrOH)

13 Nozaki et al., 2009 [43a]

CO2 to formateTON = 3.5×106

16 Zhou et al., 2015 [43d]


15 Fujita et al., 2012 [43b]


Fig. 3. Iridium catalysts used in CO2 hydrogenation reactions.


of a poly(styrene)-block-poly(ethyleneglycol) polymer (ArgoGel-NH

2®) contain-

ing free amine moieties.[55] Condensationof the free amine with formaldehyde and asecondary phosphine led to supported bi-dentate phosphine ligands that could eas-ily substitute the ligands in [RuCl

2(PPh

3)3]

or cis-[RuH2(PPh

3)4] yielding the bound

compounds 26–27 and 28–29 respectively(Fig. 5). The functionalized beads showeda similar activity compared to the homo-geneous equivalent in the synthesis ofDMF in supercritical CO

2(scCO

2) at 100

°C and could easily be recovered and re-used. 26 in contrast to 27–29 showed nodecrease in activity over four cycles (TON1560–1960). Similar anchoring strategieshave described for the coordination of IrCl

3or RuCl

3to hybrid organosilica materials

containing propylamine or alkylphosphine

24 or 25 (no TEOS added) reach TOFs of3030 h–1 and 2300 h–1 respectively, whiledilution with 20 to 200 equiv. TEOS dra-matically decreased the TOFs to 500–970h–1.[52c]

Alternatively, immobilization of[Ru(TPPTS)

2]4– (TPPTS = tris(3-sulfo-

phenyl) phosphine trisodium salt) waspossible on a Dowex ion exchange resin,phosphine functionalized polymers as wellas zeolites.[54] While solely applied for thedecomposition of formic acid into H

2and

CO2, both the ion exchange resin and the

polymer support led to stable catalysts thatdid not leach and maintained their activityover several cycles. Most of the zeoliteshowever could not withstand the acidic re-action conditions.

Furthermore, immobilization was alsoaccomplishedby thepost-functionalization

aza-7- phosphaadamantane) (7) allows theconversion of CO

2to formic acid without

the addition of base at low temperaturesin DMSO.[42g] Applying other hydrogensources than H

2, such as isopropanol or

methanol, ruthenium and iridium complexwith N-heterocyclic carbenes (4, 14)[42h–j]as well as PNP pincer-type[42k,44] ligands(8, 9) were used to convert CO

2to formate.

Iridium catalysts have also proven tobe very effective to promote CO

2hydro-

genation. IrIII-pincer trihydride complex 13catalyzes the hydrogenation of CO

2into

potassium formate (HCOOK) with a TONof 3.5×106 and TOF of 150 000 h–1 usingan aqueous KOH solution at 120 °C and6 MPa (H

2:CO

2=1:1).[43a,45] The pincer li-

gand, which is believed to be non-innocentin the catalytic cycle,[43a] seems to be supe-rior to other ligand systems such as N^N-bidentate (15),[43b,c] imine-diphosphine(16)[43d,e] or N-heterocyclic carbenes.[46]

Non-precious-metal-based catalystsbased on Fe,[47] Co[48] and Cu[49] have alsobeen discovered (Fig. 4), but suffer fromrelatively low TON/TOF compared to theprecious-metal-based catalysts.

More recently the direct synthesis ofmethanol from CO

2with homogenous

catalysts was reported using (i) a cascadereaction applying two different rutheniumcatalysts together with Sc(OTf)

3as a Bron-

sted/Lewis acid catalyst (5),[42c,d] (ii) a tan-dem capture of CO

2with aminoethanols

combined with the subsequent hydrogena-tion with a PNN Ru catalyst (12)[50] and(iii) a specific ruthenium phosphine com-plex, generated in situ from [Ru(acac)

3]

and the tridentate ligand Triphos (1,1,1-tris(diphenylphosphinomethyl)ethane) (10)in the presence of HNTf

2as an acid co-

catalyst.[42e,f]

Immobilization StrategiesWith the associated difficulty to sepa-

rate reactants/products from the activephase, a large research effort has focusedon immobilizinghighlyactivehomogenouscatalysts on a variety of supports. Differentstrategies have been applied including en-capsulation, intercalation or entrapment ofthe catalyst as well as anchoring the ligandto a support.[51] However, efficient, immo-bilized CO

2hydrogenation catalysts are

still rare. Organo-silica hybrid materialsbased on the co-condensation of silylat-ed derivatives of [Ru(dppp)

2Cl

2] (dppp =

bis-(diphenylphosphino)propane) (Fig. 5)with tetraorthosilicate (TEOS) were thefirst to be reported.[52] While molecular[Ru(dppm)

2Cl

2] and [Ru(dppp)

2Cl

2] dis-

play very good activities for the formationof DMF from CO

2and H

2with TOFs of

190000 h–1 and 2650 h–1 respectively,[53]the corresponding immobilized systemsdisplay much lower TOFs: materials pure-ly consisting of the silylated precursors

N(tBu)2P P(tBu)2Fe

COHH

17 Milstein et al., 2011 [47a]


SiFeCl

PPh2Ph2PPh2P

22 Peters et al., 2015 [47b]


PFeF

PPh2Ph2PPh2P

18 Beller et al., 2012 [47d]


BF4-Co

Ph2PPPh2

PPh2

H2

19 Beller et al., 2012 [48a]


CoP PP P

Me2 Me2

Me2Me2 H

20 Linehan et al., 2013 [48b]


CoH2O

*Cp N

N

OH

OH

21 Fujita et al., 2013 [48c]


CuPh2P

PPh2

Ph2P NCMe

23 Appel et al., 2015 [49]


PF6-

Fig. 4. Non-precious-metal-based catalysts used in CO2 hydrogenation reactions.

RuCl

Cl

PP (CH2)n

Si(OEt)3Ph

PhSi(OEt)3

PP

(CH2)n

Ph

Ph

(EtO)3Si

(EtO)3Si

24 [Ru(bspm)Cl2] n = 125 [Ru(bspp)Cl2] n = 3

RuCl

Cl

P

P

R

R

P

P

R

R

26 R = Ph27 R = Et

NN RuH

PPh3

HPPh3

P

P

R

R

N

= PS-PEG 28 R = Ph29 R = Et

Fig. 5. Immobilization strategies for RuII-hydrogenation catalysts.

SiO

OOSi

Si

Si

OOO

SiOTMSO

O

O

O

OO O

NN MesRuCl

Cl

SiO

OOSi

Si

Si

OOO

SiOTMS

O

O

O

O

OO O

NN MesRu(PMe3)yCl

Cl

PMe3

30 M-RuCym 31 M-RuPMe3Fig. 6. Immobilized Ru-NHC complexes on mesoporous silica.


moities. While reported as active and re-cyclable, the nature of the catalyst is notclear.[56]

More recently, the immobilization of aruthenium N-heterocyclic carbene (NHC)complex for the CO

2hydrogenation to am-

ides was accomplished by a different strat-egy, where the density of the organic func-tionalities (ligands) is controlled througha templating approach and the metal iscovalently anchored to a N-heterocycliccarbene ligand.[57] The immobilized cata-lyst is prepared by reacting imidazolium-functionalized mesoporous silica with[RuCl

2(p-cymene)

2] yielding the sur-

face bound Ru-NHC compound in 60%yield (30).[58] Substitution of the cymeneligand with PMe

3resulted in the respective

surface bound Ru-phosphine-NHC com-pound (31) (Fig. 6).

Hydrogenation reactions of CO2in the

presence of pyrrolidine yielding 1-formyl-pyrrolidine (50 bar CO

2, 30 bar H

2, 100 °C)

showed that 30 had a very low TON while31was almost as active as [RuCl

2(PMe

3)4],

which is one of the best catalysts under thechosen reaction conditions (TON 2900 and3100 respectively). However metal leach-ing proved to be a problem concerning therecyclability of the materials.

Conclusion

Heterogeneous catalysts are alreadyused in industrial applications due to theirhigh thermal stability and recyclabilityleading to low operation costs for chemicalprocesses.[59] Generally in ambient pres-sure reactions CO

2is converted into CO

over transitionmetal catalysts and into CH4

over Ni, Ru and Rh catalysts, whereas CO2

is hydrogenated to methanol (and dimethylether) under high-pressure over Cu-basedcatalysts and to olefins over Fe- and Co-based catalysts. ZnO, ZrO

2and CeO

2are

so far the best support materials for most ofthe CO

2hydrogenation reactions, probably

due to their large CO2adsorption capacity

and high activity towards the conversion ofCO

2into CO. The addition of alkali or lan-

thanide metals has been shown to enhanceCO

2conversion and help to furnish olefins

under high pressure.On the other hand, numerous homo-

geneous catalysts have appeared more re-cently with tailored properties by tuningof the organic ligands. They have becomeconsequently more active and/or selective,but they still suffer from lower thermal sta-bility, difficulty of regeneration as well asseparation from the products, limiting theirindustrial applications. While immobiliza-tion on supports appears to be a promisingstrategy, immobilized CO

2hydrogenation

catalysts always show lower TON/TOFscompared to the homogeneous analogues.

Here deactivation by interaction with thesupport, metal leaching and simple regen-eration protocols still present great chal-lenges, which need to be surmounted. Theright combination and choice of support,linker and homogeneous catalyst will beimportant for the generation of stable andactive immobilized catalysts.

AcknowledgementThe authors acknowledge funding by the

SCCER (REF-1115-40004) as well as the ETH.S.T., I.T. and H.-K. L. were also supported bythe Japan Society for the Promotion of Science(JSPS, No. 15J10157), the German AcademicExchange Service (DAAD), and the SNF (SNFIZK0Z2_160957), respectively.


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*Correspondence: Prof. Dr. P. J. DysonInstitut des Sciences et Ingénierie ChimiquesEcole Polytechnique Fédérale de Lausanne (EPFL)CH-1015 Lausanne, SwitzerlandE-mail: [email protected]

Soft Approaches to CO2 Activation

Shoubhik Das, Felix D. Bobbink, Aswin Gopakumar, and Paul J. Dyson*

Abstract: The utilization of CO2 as a C1 synthon is becoming increasingly important as a feedstock derivedfrom carbon capture and storage technologies. Herein, we describe some of our recent research on carbondioxide valorization, notably, using organocatalysts to convert CO2 into carboxylic acid, ester, formyl andmethyl groups on various organic molecules. We describe these studies within the broader context of CO2

capture and valorization and suggest approaches for future research.

Keywords: Catalysis · CO2 activation · Green chemistry · Sustainable chemistry

Valorization of CO2is currently re-

ceiving increasing attention as it is non-toxic, cheap and is becoming increasinglyabundant in the atmosphere,[1] which has adirect impact on climate change.[2] How-ever, the activation of CO

2is challenging,

as CO2is highly inert due to the carbon

being in the highest oxidized form, whichleads to high thermodynamic and kineticstability.[3] Energy-rich substrates such asepoxides[4] and aziridines[5] can overcomethis high activation barrier under harsh re-action conditions (Scheme 1). In addition,strong nucleophiles such as Grignard re-agents,[6] organolithium,[7] organoboron[8]and organozinc[9] reagents have been alsoused to form new C–C bonds encompass-ing CO

2(Scheme 1). The reduction of CO

2to simple compounds such as formic acidand methanol with homogeneous transi-tion metal catalysts has also been a field ofintensive research.[10]

It should benoted that these procedures,although employing energetic reagents, of-ten require high pressures of CO

2and harsh

reaction conditions, which tends to hinderthe development of these methodologies

on an industrial scale. Despite these dif-ficulties some processes are being com-mercialized, for example, Bayer recentlyannounced that it will produce polyolsincorporating 20% CO

2in an installation

with a 5,000 metric ton capacity (Scheme2).[11] However, to reach more widespreadapplications, on a scale that would havea sizeable impact on carbon capture andsequestration technologies,[12] reactionconditions preferably need to be mild, i.e.at atmospheric pressure, and take place atmoderate temperatures. For this purposefinding new catalysts is highly important.

Organocatalysts such asN-heterocycliccarbenes (NHCs) are able to activate CO

2at atmospheric pressure, potentially allow-ing the harsh reaction conditions usuallyrequired for CO

2activation to be over-

come.[14] In general, organocatalysts areless expensive than metal-based catalysts,are readily accessible and, consequently,represent an interesting avenue for furtherinvestigations.[15] Moreover, these smallorganic molecules are typically non-toxicand environmentally friendly. Based onthese above-mentioned properties, com-

bined with our on-going research activi-ties on imidazolium salts,[16] we decidedto evaluate the ability of carbene catalyststo fix CO

2onto organic molecules such as

amines and alkynes to provide easy accessto a range of valuable molecules, e.g. N-methylated amines, N-formylated amines,alkynyl carboxylic acids or esters, etc.

Amine derivatives are important in-termediates in the chemical and pharma-ceutical industries, the functionality beingfound in agrochemicals, dyes and flavors,fragrances and medicines.[17] We foundthat NHCs can methylate different aro-matic, heteroaromatic and aliphatic aminesusing CO

2as the carbon source with di-

phenylsilane as the reductant under ambi-ent conditions and in high yield.[18] Aro-matic, heteroaromatic and aliphatic aminesreact smoothly leading to excellent yields(Scheme 3). Cantat and coworkers havealso shown that amines can be methylatedusing hydroboranes and a proazaphospha-trane as organocatalyst.[19]

Both electron-donating and -withdraw-ing substituents on the aromatic ring at thepara position react well whereas amines

Scheme 1. Examples of reactions using CO2 as a reagent with highly reactive substrates.

Scheme 2. An example of the synthesis of polycarbonate polyols using CO2 as a C1 source.[13]


uct as a major (only) product is of value asN-formylated compounds are also versatilecompounds employed as intermediates inorganic synthesis and the formyl group isalso present in pharmaceuticals.[21] Formylgroups are often used as protecting groupsin organic synthesis and, in particular, N-formylated amino acid esters and peptideshave been used widely in peptide synthesisas well as for precursors of isocyanides thatfind use in multicomponent reactions.[22]

We discovered a thiazolium carbene,closely related to vitamin B1, which isa potent N-formylation catalyst thatoperates under CO

2at atmospheric pres-

sure.[23] The catalyst is selective for differ-ent primary amines and amino acid esters.Aromatic, heteroaromatic, alicyclic andaliphatic amines afforded yields of up to90% (Scheme 5).

Different amino acids such as me-thionine and tryptophan ethyl ester reactsmoothly under the optimized reactionconditions (7.5 mol% thiazolium carbenecatalyst, 1 atm. CO

2, PMHS, DMA, 50 °C).

Moreover, para-bromo-substituted aminesafford the corresponding products in 80%yield without any signs of reductive deha-logenation taking place. The catalyst is al-so tolerant to heteroaromatic amines suchas furan derivatives. We also performedthe reaction on a multigram scale withouttheN-methylated products being observed.Interestingly, N-methylated products canbe obtained with the thiazolium carbenecatalyst at higher temperatures, i.e. 100 °C(Scheme 6). As mentioned above, the

tial of the reaction it was used to preparenaftifine, an antifungal drug for the topicaltreatment of fungal infections in a two-step catalytic procedure (Scheme 4). TheNHC catalyst was also used to prepare 13C-labelled naftifine using 13CO

2in 78% yield.

During the N-methylation reaction, thecorresponding N-formyl compound wasobserved as an isolatable intermediate.Therefore, tuning the catalyst and catalyticconditions to recover N-formylated prod-

with electron-withdrawing groups are lessreactive and require prolonged reactiontimes. Steric hindrance on both sides of theamine has minimal effect on the yield ofthe reaction. Various symmetric and non-symmetric amines were also reacted underthe optimized reaction conditions (5 mol%IMes, 1 atm. CO

2, 3 equiv. Ph

2SiH

2, DMF,

50 °C) and the procedure worked well withthese substrates. The carbene catalyst IMesis also tolerant to heteroaromatic aminessuch as picoline, indoline and 1,2,3,4-tetra-hydroquinoline. Additionally, primaryamines react in a similar fashion to second-ary amines, selectively forming dimethyl-ated products. We also evaluated the useof IMes in the N-methylation of complexmolecules such as nortriptyline, cinacal-cet, duloxetine and sertraline obtainingpure products in good yields without deg-radation or separation problems (Fig. 1).

The functional group tolerance of areaction potentially enhances the overallsustainability of the route in the synthesisof complex structures, since protecting anddeprotecting steps are not necessarily re-quired, leading to a high atom economy.The selectivity of the carbene catalyst wasevaluated using some particularly chal-lenging substrates with different func-tional groups attached to different parts ofthe amine. Remarkably, it was found thatnitrile, nitro, double and triple carbon–carbon bonds, ether, and ester substitutedamines were well tolerated, providing thecorresponding N-methylated amines ingood to excellent yield. Moreover, meth-ylation took place chemoselectively evenin the presence of a ketone group, whichcould have been reduced by the silane.[20]To demonstrate the chemoselective poten-

Scheme 3. Different methylated-amines prepared using CO2 and the IMes NHC catalyst.

Fig. 1. Drug mol-ecules prepared usingthe IMes catalyst.

Scheme 4. Application of the IMes catalyst for the synthesis of complex structures.


temperatures (50–80 °C). Such conditionsalso enable a sinter-free environment tothe unsupported palladium nanoparticles,which can be detrimental to recyclingexperiments.

The selectivity towards N-formylatedor N-methylated products depends on thenature of the palladium nanoparticles.From the results obtained so far, there isnot an obvious correlation between theparticle size and the selectivity, but cata-lysts such as palladium on activated carbon(commercial grade, Pd content 10 wt%),did not promote either reaction.

In summary, despite the rise in strate-gies for avoiding CO

2production, gigatons

of CO2are produced each year to fulfil the

world’s energy and chemical demands.And although there is a considerable mis-match in the amount of CO

2produced

and the amount that would be consumedfor chemical applications, the long-termgoal to produce fuels such as methane,methanol and hydrocarbons from CO

2us-

ing renewable energy could one day enddependence on fossil fuels. Hence, usingCO

2as a C1 carbon source warrants inten-

hydrogen, but operate under harsh reactionconditions and require prolonged reactiontimes to achieve high yields. In contrast,our hydrosilylation approach functionsunder ambient pressures at relatively low

catalyst is closely related to vitamin B1,which in nature is used to decarboxylatepyruvate, a metabolite obtained fromglycolysis.[24]

We have extended the concept of fixingCO

2via organocatalysts towards terminal

alkynes.[25] In terminal alkynes the protonis suitably acidic so the addition of a baseleads to the formation of the correspond-ing acetylide anion. Acetylide anions arestrong nucleophiles that can spontaneouslyattack a weak electrophile such as CO

2to

generate alkynyl carboxylic acids.Alkynesfunctionalized with carboxylic acids arewidely found in medicinally relevant com-pounds and they also find uses as synthonsin organic synthesis. While several metal-based catalysts have been reported for thisreaction, we found that the thiazoliumcarbene compound catalyzes the carbox-ylation reaction to give the correspondingalkynyl carboxylic acids or esters, depend-ing on the conditions, in excellent yield(Scheme 7). This protocol opens up accessto a pool of highly functionalized propiolicacids from CO

2.

In addition to our studies on homog-enous organocatalysts we have been study-ing dispersed transition metals nanoparti-cles that catalyse selective N-formylationand N-methylation reactions (Fig. 2). Ingeneral, homogeneous catalysts usuallygive better selectivities than their hetero-geneous counterparts as they tend to op-erate under milder conditions.[26] Never-theless, we discovered a viable palladiumnanoparticle catalyst (palladium is knownto efficiently activate/store hydrogen),[27]for these reactions using diphenylsilaneas the hydrogen source. A wide varietyof palladium nanoparticles were preparedand compared to classical heterogeneoussystems such as Pd/Al

2O

3, CuAlO

xand

Pd/CuZrO2and Pt-MoO

x/TiO

2.[28] These

reported heterogeneous catalysts employ

Scheme 6. Temperature controlled reaction of amines with the thiazolium carbene catalyst relatedto vitamin B1.

Scheme 7. Reaction of terminal alkynes to afford esters using the thiazolium carbene catalyst andCO2 as the source of the carboxylate moiety.

Scheme 5. Use of a thiazolium carbene catalyst for the N-formylation of amines using CO2 as acarbon source: PMHS = Polymethylhydrosiloxane.

Fig. 2. TEM images of unsupported palladium nanoparticles prepared using (left) ascorbicacid and (right) formaldehyde as the reducing agent. Both types of nanoparticles catalyse theN-methylation reaction.


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[24] M. Lobell, D. H. G. Crout, J. Am. Chem. Soc.1996, 118, 1867.

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sive study and all types of catalysts, i.e.homogeneous, heterogeneous, organo andenzymatic.

In the examples provided herein, arelatively small fraction of the product isderived fromCO

2and, for a greater impact,

the majority of the carbon atoms in any or-ganic product should be derived from CO

2.

Progress in this direction should be fea-sible by combining the selective catalystsdescribed here with a process that gener-ates base chemicals from CO

2. A CO

2re-

finery powered by renewable energy couldultimately produce not only fuels, but alsosophisticated organic structures that meetthe needs of a modern society.


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*Correspondence: Dr. J. Dursta, Dr. A. Rudnevb

E-mail: [email protected]; [email protected] LaboratoryPaul Scherrer Institut (PSI)CH-5232 Villigen, SwitzerlandbDepartment of Chemistry and BiochemistryUniversity of BernFreiestrasse 3, CH-3012 Bern, SwitzerlandcA.N. Frumkin Institute of Physical Chemistry andElectrochemistryRussian Academy of SciencesLeninskii pr. 31, Moscow 119991, RussiadLaboratory of Physical ChemistryETH Zürich, 8093 Zürich, Switzerland*These authors contributed equally to this work

Electrochemical CO2 Reduction –A Critical View on Fundamentals,Materials and Applications

Julien Durst*a, Alexander Rudnev*bc, Abhijit Duttab, Yongchun Fub, Juan Herranza,

Veerabhadrarao Kaliginedib, Akiyoshi Kuzumeb, Anastasia A. Permyakovaa, Yohan Paratchaa,Peter Broekmannb, and Thomas J. Schmidtad

Abstract: The electrochemical reduction of CO2 has been extensively studied over the past decades. Never-theless, this topic has been tackled so far only by using a very fundamental approach and mostly by trying toimprove kinetics and selectivities toward specific products in half-cell configurations and liquid-based electro-lytes. The main drawback of this approach is that, due to the low solubility of CO2 in water, the maximum CO2

reduction current which could be drawn falls in the range of 0.01–0.02 A cm–2. This is at least an order of mag-nitude lower current density than the requirement to make CO2-electrolysis a technically and economically fea-sible option for transformation of CO2 into chemical feedstock or fuel thereby closing the CO2 cycle. This workattempts to give a short overview on the status of electrochemical CO2 reduction with respect to challenges atthe electrolysis cell as well as at the catalyst level. We will critically discuss possible pathways to increase bothoperating current density and conversion efficiency in order to close the gap with established energy conver-sion technologies.

Keywords: CO2 reduction reaction · Electrolyzer · Energy conversion · Gas diffusion electrode ·Power-to-gas/liquid

1. Introduction

Reducing the emissions of greenhousegases by increasing the fraction of renew-able energies at the expense of fossil fu-els is one of the most important scientific,technological and economic challenges hu-mankind is currently facing.[1] To achievethis aim and to tackle the undesired effectsof climate change, considerable efforts arebeing undertaken worldwide to develop ef-fective CO

2capture and storage technolo-

gies.[2–4]Based on these, one can think howto re-cycle CO

2to more valuable products.

Its electrochemical conversion into car-bon-neutral products might be consideredas one promising approach towards reduc-ing atmospheric CO

2and storing a surplus

of renewable energies at the same time. In

principle, the electrochemical CO2reduc-

tion reaction (CO2RR) could be performed

in an electrolysis type of device, called aCO

2-electrolyzer or co-electrolyzer. This

energy conversion device can be consid-ered as the central part of power-to-gas/power-to-liquid processes that operate us-ing the excess of electricity generated fromrenewable sources.[5,6] In analogy with awater electrolyzer, a CO

2-electrolysis cell

is fed with H2O at the anode, where the

oxygen evolution reaction (OER) occurs,whereas CO

2is supplied to the cathode

where it is electrochemically reduced. Theelectric energy would be chemically storedeither in the form of feedstock chemicals(starting material for further synthesis) oras fuels. Ideally, CO

2RR should yield to a

single energy-rich compound. However,selective CO

2conversion into specific re-

action products remains a challenging taskat present due to the multiple proton-cou-pled electron transfer steps involved in thisreaction.[7]

This work attempts to give a short over-view on the status of electrochemical CO

2reduction with respect to challenges at thecatalysts as well as at the electrolysis celllevel. It critically discusses possible path-ways to increase both operating currentdensity and conversion efficiency in orderto make co-electrolysis a technically andeconomically feasible option for the trans-formation of CO

2into a chemical feed-

stock or fuel thereby closing the CO2cycle.

2. Identifying Valuable CO2RRProducts

Table 1 provides a simple cost analysisfor all major CO

2reaction products that

can be obtained from a CO2-electrolysis

cell. For designing and establishing aneconomically reasonable CO

2conversion

process one needs first to estimate the totalcosts for the electrochemical production ofspecific CO

2RR products and to compare

these in a second step with data from well-established chemical synthesis routes. Asa benchmark for our approach, we usedata for H

2production from alkaline wa-

ter electrolysis, an already established andcommercially available energy storagetechnology. In the case of alkaline waterelectrolysis, large-scale energy storageplants can daily produce ∼1000 kg

H2with

an electric energy at $0.05 kWh–1. Underthese conditions, the price for H

2produc-

tion reaches ∼ $4 kgH2

–1 (Table 1). Maincontributions to the total production costsoriginate from electricity (58%) and capi-tal expenses (32%).[8,9] The most severedrawback of the alkaline water electroly-sis technology is its low operating currentdensity of ∼0.2 A cm–2. Nevertheless, whencompared to proton exchange membrane(PEM) electrolyzers, which are operated atcurrent densities that are up to one order ofmagnitude higher than those of the alkalineelectrolyzers, there is no major benefit us-ing PEM-based electrolyzers. The reasons


production of 210,000Mt. CO in combina-tion with H

2(syngas) serves as an impor-

tant chemical precursor for a number ofindustrial processes (e.g. Fischer-Tropschsynthesis). Another interesting product ofthe CO

2RR is formate (it should be kept

in mind that only formic acid, obtained bythe protonation of formate, is a valuableproduct). Estimated productions costs arefactors of 2–4 lower than the current mar-ket price for formate/formic acid ($0.34vs. $0.8–1.2 kg–1, Table 1). Formic acid iswidely used as a preservative and an anti-bacterial agent in livestock feed. The mar-ket for formic acid with a yearly produc-tion of 0.8 Mt is, however, much smallercompared to the global demand for CO/syngas. An electrochemical conversion ofCO

2into formate/formic acid has a high

potential to become an economically com-petitive process.

3. Energy Efficiency of CO2Electrolysis

The cost analysis presented above re-lies on the basic assumption that CO

2–elec-

trolysis cells would reach similar currentdensities to those featured in alkaline wa-ter electrolyzers (0.2 A cm

geo–2).[25] How-

ever, such large current densities cannot bereached by CO

2electrolyzers based on liq-

uid aqueous reaction environments. Thisgeneral limitation originates from the lowsolubility of CO

2in aqueous electrolyte

solutions (∼30 mM in H2O at atmospheric

pressure) thus resulting in diffusion-limit-ed current densities which typically do notexceed values of 0.03 A cm

geo–2. These are

one order of magnitude below the currentdensities reported for alkaline water elec-

normalized to its mass. Before comment-ing on the CO

2RR product costs, it has to

be stressed that CO2RR kinetics are signif-

icantly slower compared to the hydrogenevolution reaction (HER) kinetics thus re-sulting in much higher CO

2RR overpoten-

tials that need to be applied to the cathode.This actually leads to a lower energy ef-ficiency of the CO

2R-electrolysis, roughly

half of that of an alkaline water electrolyz-er (for a more detailed discussion we referto the next section). As a consequence, theCO

2RR product costs will rise accordingly

(let us assume by a factor of 2) due to theincreased energy consumption during op-eration. More realistic production costs forCH

4, C

2H

4, CO, HCOO– and CH

3OH are

therefore expected to be $4, $3.2, $0.48,$0.34 and $1.4 kg–1, respectively. Froman analysis of Table 1 it becomes obvi-ous that the CO

2-electrolysis will not be

competitive for all of the possible reac-tion products. In particular the productioncosts for CH

4and C

2H

4($4 and $3.2 kg–1,

respectively) will be far higher comparedto more conventional production routes($0.08 and $1.4 kg–1, respectively). Thisprice difference becomes even larger formethane when considering its extractionfrom natural gas. The conversion of CO

2into CH

4and C

2H

4by electrolysis appears

therefore to be highly counterproductive,at least from an economic point of view.The same is true for methanol, which canbe considered as an energy carrier such asH

2.[12]What seems to be much more prom-

ising is the generation of CO from CO2.

CO production costs ranging from $0.27to $0.54 kg–1 are well below the currentmarket price of $0.65 kg–1 (Table 1). Inaddition, the global market for CO is ex-tremely large as reflected by the annual CO

are related to higher component costs incase of the PEM electrolyzers. In addition,the PEM electrolyzers typically target onlysmall forecourt applications (daily produc-tion of ca. ∼10–100 kg

H2).[10,11]

Based on these numbers, it is possibleto estimate the production costs for specificCO

2RR products under the assumption that

the capital costs are similar for both alka-line water and CO

2electrolyzers. In a first

step, we calculate the production volumesper electrolysis unit by assuming similaroperating current densities (∼0.2 A cm–2)as applied in alkaline water electrolyzers.Eqn. (1) is used to convert the productionvolume of H

2(V

H2in kg

H2h–1) into the re-

spective production volumes of specificCO

2RR products (V

CxHyOzin kg h–1) via the

ratio of the molar masses (M in g mol–1)and the number of electrons exchanged toproduce 1 mol of product (n

e–, 2 for H

2/

CO/HCOO–, 6 for CH3OH, 8 for CH

4, 12

for C2H

4).

[1]𝑉𝑉 = 𝑉𝑉 ∙ ∙ −,−,Production volumes per electroly-

sis unit are listed in Table 1 for variousCO

2RR products. Further assuming that

a CO2electrolysis cell operates at a simi-

lar energy efficiency as an alkaline waterelectrolyzer, the production costs of 1 molCO

2RR product become a fixed num-

ber, namely $8 10–3 molproduct

–1, no matterwhich specific product is considered. Theproduction cost of a specific CO

2RR prod-

uct per unit of mass can then be calculatedby multiplying $8 10–3 mol

product–1 with

the molar mass of the respective CO2RR

product. As can be seen in Table 1, due toits low molecular weight, hydrogen is themost expensive electrolysis product when

Table 1. Current and estimated costs of production by CO2-electrolysis for H2, CH4, C2H4, HCOO– and CH3OH.

Product Produced by Currentmarketprice

Currentproductionvolume

Production priceby electrolysisa

Production volume perelectrolysis unitb

[$ kg–1] [Mt y–1] [$ kg–1] [Mt y–1]

H2

steam reforming, partial oxidationof methane or gasification of coal[13]

2-4[14] 65 4 0.0003

CH4

methanogenesis or hydrogenationof CO

2[15]

<0.08[15] 2400[16] 2-4 0.0007

C2H

4pyrolysis or vapocracking[17] 0.8-1.5[18] 141[17] 1.6–3.2 0.0009

CO Boudouard reaction[19] 0.65 210000 0.27–0.54 0.005

HCOO– /HCOOH hydrolysis from methyl formate andformamide[20] or by-product of ace-tic acid production

0.8-1.2[20] 0.8[21] 0.17–0.34 0.008

CH3OH From natural gas, coal, biomass,

waste[22,23]0.4-0.6[23,24] 100[23] 0.70–1.4 0.002

aThe lowest price is calculated by assuming the cell device efficiency of an alkaline water electrolyzer. The highest price is obtained by consideringthat a co-electrolysis device is operating at half the efficiency of an alkaline water electrolyzer and so that the production price of individual productswill be twice higher.bEstimated on the basis of a daily production from an alkaline water electrolyzer of ca. 1000 kgH2


ideal aqueous environment to a gas diffu-sion configuration.[30–36] Catalyst materialsthat perform best toward specific CO

2RR

product are listed in Table 2. Their kineticperformance can be derived from Fig. 1Awhich relates current density and appliedoverpotential in a semi-logarithmic plot(η

kinvs log(i

kin)). The linear correlations

observed between ηkin

and log(ikin) in Fig.

1A are clear fingerprints of Tafel behav-iors in this specific current density range(with 100–150 mV decade–1 Tafel slopes),which confirm that these curves are domi-nantly charge-transfer controlled and notlimited by mass transport. On the basisof this catalytic performance for modelelectrodes having roughness factors of ca.1 cm

metal2 cm

geo–2, we compute kinetically

controlled curves (not shown) for technicalelectrodes prepared from metal nanopar-ticles with different diameters and corre-sponding surface areas,[37–39] again listed inTable 2. For doing this, we assume that thekinetic performance of the catalysts willnot be affected by the transition betweenaqueous and gas-phase reaction media (asit has been demonstrated for fuel cells[40])nor by particle-size effects.

Based on the CO2RR and OER kinetic

overpotentials (Fig. 1A) and the electrolyteresistances (Table 4), the cell voltage E

cellis calculated from Eqn. (2) as function ofthe applied current and is shown for thevarious electrolysis cells in Fig. 1B. Fromthese theoretical polarization curves, de-vice efficiencies (ξ

Electrolyzer) are estimated

using Eqn. (3):

[3]ξElectrolyzer =∆H0∆G0 .

ErevEcell

∆H0 and ∆G0 in Eqn. (3) relate to en-thalpies and respective Gibbs free energiesof the overall (co-)electrolyzer cell reac-tions as listed in Table 3. The validity ofour approach can be verified on the basisof the calculated efficiency for a PEM elec-trolyzer, in good agreement with perfor-mance data reported elsewhere.[41] It can beseen in Fig. 1A that CO andHCOO– are the

trolyzers. From a technical point of view,there could be in addition a severe contam-ination issue associated with aqueous reac-tion environments. A ppm level of metalliccontaminations, typically present in aque-ous electrolyte solutions, would already besufficient to irreversibly poison the catalystsurface during CO

2electrolysis, e.g. with

Fe trace contaminants that get electro-plated during operation.[26] These metalliccontaminations further shift the selectivityof the electrode reaction towards hydrogenformation thus lowering the faradaic ef-ficiency (FE), ratio of CO

2RR current to

total current, for specific products of theCO

2RR. These circumstances require a

careful and most likely rather costly puri-fication of the electrolyte solutions for theCO

2RR. Contributions to the production

costs originating from these extra electro-lyte purification steps are not consideredin Table 1.

Under the assumption of only kineticand ohmic losses, the cell potential E

cellfor

the CO2electrolysis can be derived on the

basis of Eqn. (2):

Ecell

= Erev+ η

anode+ η

cathode+ i∙R

ohmic[2]

Erev

corresponds to the reversible po-tential (listed in Table 3 further down).The i∙R

ohmicterm in Eqn. (2) accounts for

voltage losses caused by the finite ionicconductivity of the electrolyte solution(see Table 4 for typical R

ohmicvalues). The

ηanode

and ηcathode

terms in Eqn. (2) refer tothe overpotentials of the anodic (OER)and cathodic (CO

2RR) half-cell reactions,

respectively. Reliable information on theCO

2RR and OER reaction kinetics specifi-

cally for gas diffusion configurations are,however, rare in literature.[27–29] For thesake of simplicity we therefore estimateCO

2RR and OER overpotentials from ex-

perimental data available for polycrystal-line catalyst materials in aqueous reactionenvironments. For such considerations weassume that the particular catalyst perfor-mance does not alter when going from an

Table 2. Summary of the half reactions and corresponding catalysts at play in (co-)electrolysis cells, along with the electrode roughness factor valuesprojected on the basis of the catalyst loading and average particle size.

Half Cell Reaction Catalyst Loadinga

[mgcat·cm

geo–2]

dpart

b

[nm]Surface areac

[ mmetal

2·gcat

–1 ]Roughness Factor[cm

metal2·cm

geo–2]

CO2+ 8H+ + 8e– CH

4+ 2H

2O

Cu 5 50 13 6702CO

2+ 12H+ + 12e– C

2H

4+ 4H

2O

CO2+ 2H+ + 2e– CO + H

2O Ag 5 100 5 270

CO2+ H+ + 2e– HCOO- Sn 5 100 7 430

2H+ + 2e– H2

Pt 0.5 3 90 450

H2O ½O

2+ 2H+ + 2e– IrO

21.5 10 50 750

aTypical catalyst loading values in alkaline and PEM-electrolyzers.[43] bAverage particle size diameters based on values reported in the literature for Cublack particles,[39] carbon-supported Ag-nanoparticles,[37] and battery Sn-anodes.[38] The values for Pt and IrO2 are typical of fuel cells and electroly-zers.[43] cAssuming spherical metal particles with all of their area exposed to the reaction medium.

Fig. 1. Tafel plots and computed (co-)electro-lyzer polarization curves and efficiencies. (A)Relation between partial CO2-reduction currentsand overpotentials in 0.1–0.5 M KHCO3 for thereduction of CO2 on Cu, Ag or Sn, to yield C2H4

and CH4,[31] CO[34] or formate,[32] respectively. The

OER curve is estimated on the basis of the datafor the Ir-surface,[30] whereby similar activitiesare observed in acid and alkaline electrolyte.The HER curves are computed using the Butler-Volmer equation and a transfer coefficient of 0.5and considering the exchange current densityvalues at 25 °C reported in refs. [35] and [42](for alkaline and acid media, respectively). (B)Polarization curves computed on the basis of thekinetic data in (A), calculating the geometric cur-rents on the basis of the roughness factors listedin Table 2 and estimating Ecell with Eqn. (2) andthe Rohmic-values in Table 4. (C) Corresponding(co-)electrolyzer efficiencies, computed by sub-stituting the Ecell values in (B) and the thermody-namic data in Table 3 into Eqn. (3).


are reported to be significantly higher atelevated pHs. In this context it is interest-ing to note that the initially high pH of 14in a pristine AEM cannot be maintained inpresence of CO

2, where the following equi-

librium reaction occurs:[50]

(OH–)membrane

+ CO2 (HCO

3– )

membrane)

and the pH is expected to regulate in therange of 7–10.

A CO2electrolysis test based on an

AEM electrolyte has been conducted us-ing a silver-based gas diffusion electrodeas cathode material.[27] Here, the HER wasstill favored over the CO

2RR thus resulting

in low FEs in the range of only 1%.[27] Fur-ther studies utilizing the most recent andstable versions of AEMs[46] would needto be undertaken in order to rationalizethese results and to clarify whether a CO

2-

electrolysis configuration adapted from anAEM electrolyzer can be a suitable solu-tion.

4.3 CO2 Electrolysis at Neutral pHConditions

Studies performed in liquid electrolytesolutions identified an optimum pH rangefor the CO

2RR from 7 to 10. It has further

been reported that not only the pH but alsothe nature of anionic and cationic speciesin the aqueous electrolyte solution has agreat influence on the particular mecha-nism of CO

2reduction and the resulting

FEs (the interested reader is referred to

since PEM electrolyzer configurations al-low 3 orders of magnitude higher currentthan electrochemical measurements inliquid electrolyte to be achieved, and as-suming a Tafel slope for the HER of ca.120 mV decade–1,[36] an electrode interfacein a PEM electrolyzer can be polarized atca. 360 mV lower potential than in a liq-uid-based electrochemical device. None-theless, this extended potential domainprobed in the work of Delacourt et al. didnot allow them to detect any CO

2reduction

product. As a conclusion, it is more thanlikely that a CO

2-electrolysis configuration

adapted from a PEM electrolyzer wouldnot be a suitable solution.

4.2 CO2 Electrolysis in an AEM-typeConfiguration (High / Neutral pHConditions)

The counterpart of the PEM technol-ogy in terms of pH conditions is an anionexchange membrane (AEM). Here, OH–

species are exchanged through quaternaryammonium moieties that are covalentlyattached to the polymer backbone of themembrane. The AEM regulates the pH atthe electrode/membrane interface to a val-ue close to 14.[45,46]Moreover, the use of anAEM, in a so-called AEM electrolyzer de-vice, would allow similar current densitiesto PEM electrolyzers with the advantage ofusing noble-metal free anode electrodes forthe oxygen evolution reaction.[47–49] It is inparticular the alkaline pH which makes theAEM configuration attractive for the CO

2electrolysis since the FEs of the CO

2RR

CO2RR products generated with the lowest

overpotential (–0.6 and –0.45 V at 0.2 Acm

geo–2), as opposed to C

2H

4and CH

4(–0.9

and –1.1 V at 0.2 A cmgeo

–2). This is in linewith our previous conclusions that CO andHCCO– are the most economically inter-esting products to be considered from theCO

2RR (Table 1). Ultimately, when com-

paring all electrolyzers efficiencies (Fig.1C), the CO

2to CO or HCOO– electrolyz-

ers have efficiencies in the range 55–60%at 0.2 A cm

geo–2, close to that of an alkaline

water electrolyzer (80%). Moreover, theprojected efficiency of the CO

2-electrol-

ysis cells would certainly benefit fromimprovements in CO

2-reduction electroca-

talysis, and from the development of mem-branes with better ionic conductivities.This optimization is therefore addressed inthe following sections.

4. CO2 Electrolysis Cell Design

Our cost estimation for the diverseCO

2RR products (Table 1) was based on

the assumption that the electrolysis deviceoperates at the same current density as analkaline water electrolyzer (0.2 A cm

geo–2).

However, as already discussed above,these current densities cannot be achievedby electrolysis cells that use liquid elec-trolytes as source for dissolved CO

2re-

actants. In the following we will reviewseveral electrolysis cell designs that wouldallow achieving high current densities forthe CO

2RR. These different types of elec-

trolyzers can be classified according to thenature of the electrolyte used in these de-vices.

4.1 CO2 Electrolysis at Low pHConditions

PEM electrolysis configurations,based on the use of 50–100 μm thick pro-ton exchange membrane acting as elec-trolyte and separator between the anodeand cathode, allow an order of magnitudelarger current densities than alkaline waterelectrolyzers.[41] The pH in the membraneand at the gas diffusion electrode/electro-lyte interfaces is highly acidic with pH ≈ 0where the HER, considered as a parasiticside reaction for the CO

2RR, proceeds at

the highest rates compared to other pHconditions.[42]As an example, Delacourt etal. used a PEM electrolysis configurationwith a silver-based GDE as cathode thatwas fed with gaseous CO

2as reactant.[27]

In this case no CO2reduction product was

detected (only H2) although silver is con-

sidered as the most active catalyst materialfor the CO

2to CO pathway. Even if most

reports claim that the high HER currentsare the sole reason why cathode electrodescannot be polarized below CO

2reduction

onset potential, it should be noted that

Table 3. Full cell reactions and corresponding enthalpy, free energy and reversible potential values(at standard conditions and 25 °C) for the (co-)electrolysis cells considered in this study.

Overall (co-)electrolyzer reaction–ΔH0

[kJ mol–1]

–ΔG0

[kJ mol–1]

Erev

[V]

H2O (l) H

2+ ½O

2286.0 237.3 1.23

CO2+ H

2O (l) HCOOH + ½O

2270.3 285.5 1.48

CO2 CO + ½O

2283.1 257.2 1.34

CO2+ 2H

2O (l) CH

4+ 2O

2890.8 818.4 1.06

2CO2+ 2H

2O (l) C

2H

4+ 3O

21411.2 1331.2 1.15

Table 4. Ionic conductivity and corresponding resistivity values for the membrane electrolytesimplemented in the (co-)electrolyzers.

Device MembraneIonic

conductivitya

[mS·cm–1]

Rohmic

b

[Ω·cmgeo

2]

PEM-electrolyzer Perfluorosulfonated 100 0.05

Alkaline water electrolyzers 25-35% KOH – 1[9]

Co-electrolysis cell Anion-exchange,carbonated 7 0.700

aConductivity values for OH-exchanged and carbonated alkaline membranes extracted fromref. [44]. bEstimated on the basis of 50 µm thick membranes.


ref. [7]). Several electrolysis designs havealready been reported in literature for suchnear-neutral pH conditions and these cellconfigurations can be grouped into twomain kinds: either the electrolyte remainsstagnant, e.g. immobilized by a matrix, orthe liquid electrolyte is flushed in a flow-cell type of reactor. A prime example ofthe first kind of cell design is proposed inthe work of Delacourt et al. where a Na-fion® membrane in a potassium-form wasutilized.[27] Some features of this approachresemble the design of a PEM electrolyzer.However, in this present case the carriersfor the ion current are the K+ ions that aretransported across the polymer membrane.Moreover, the reactants are dissolved inliquid (aqueous) media and transported byconvection to the anode (e.g. KOH solu-tion for OER) and cathode (CO

2saturated

0.5M KHCO3for CO

2RR), respectively, in

order to balance exchanged charges fromthe cathodic and anodic reactions. From aperformance point of view, the small cur-rents (∼0.02 A cm

geo–2) can be attributed

to limitations caused by the solubility ofCO

2in the KHCO

3solution. Moreover, the

FEs (40% of the total currents) are still farbelow the expected FEs reported for sil-ver catalysts.[27] A similar approach as theone just described would consist of usingAEM electrolysis cell configuration withthe circulation of a carbonate/bicarbonatesolution in the cathode compartment. Thisapproach has been demonstrated to be ef-fective for water electrolysis[51] and couldbe test-proofed for CO

2electrolysis. Both

of these cell configurations are depicted inFig. 2A. Overall, several drawbacks arisefrom the use of a neutral immobilizedelectrolyte configuration. First, the currentcarrier (K+, HCO3–) has to be supplied bythe catholyte. This implies that the cath-ode interface would again consist of a CO

2-

saturated liquid electrolyte, and so that theCO

2-electrolysis cell would be limited to

small current densities (0.01–0.03 A cm–2)even though this limiting current could beincreased by working under higher CO

2pressure conditions.[52] The second draw-back is related to the high level of purityrequired for the catholyte, where traces ofmetal cations at the ppm level could lead tomuch higher rates of hydrogen evolved atthe expense of CO

2reduced.[26] Finally the

durability of this configuration might be anissue since continuous operation will ulti-mately lead to the build up of a pH gradientbetween both electrodes.

To overcome some of these technicallimitations, Delacourt et al. introduced adual solid electrolyte configuration con-sisting of an 800 μm thick glass fiber im-pregnated with 0.5 M KHCO

3and being

in contact with the cathode whereas an ad-ditional PEM is in contact with the anode.The cathode can be fed with a humidified

stream of CO2whereas the anode is ex-

posed to a liquid aqueous solution for theOER.[27] With this dual solid electrolytedesign CO

2RR current densities of up to

∼0.140 A cm–2 were achieved for the re-duction of CO

2to CO on a silver-based

GDE.[53] These current densities are atpresent the highest reported in literaturefor the CO

2RR. However, there are sev-

eral drawbacks associated with the celldesign proposed by Delacourt et al. First,by having the anode and cathode operatingunder different pH conditions, one intro-duces an additional loss to the cell voltage(0.059 V per pH unit difference betweenanode and cathode). The second drawbackis related to the overall thickness of thebuffer layer. Assuming an ionic conductiv-ity of 10 mS cm–1, the ohmic drop acrossthe 800 μm buffer layer would amount to1.6 V for a current density of 0.200 A cm–2.This tremendous IR drop would be highlydisadvantageous in terms of cell perfor-mance. The overall cell efficiency could,however, be significantly improved forinstance by replacing the 800 μm buf-fer layer by a 50 μm thick AEM (such asthose used for alkaline fuel cell applica-tions[54]). Another alternative to the glassfiber used by Delacourt et al. would be aOH– or HCO

3–-doped polybenzimidazole

(PBI) membrane, as depicted in Fig. 2B.This membrane, when doped with H

3PO

4,

is typically used in high temperature fuelcells,[55] and some attempts were alreadymade to incorporate KOH into the mem-brane for alkaline electrolysis of fuel celloperation.[56,57] Such a modified cell de-sign with a dual electrolyte configuration,also enabling straightforward collection ofgaseous and liquid products, is foreseen

as one of the most promising electrolysisconfiguration.

An alternative approach for an im-proved cell design enabling higher CO

2RR

current densities was developed by Kenisand coworkers (for the detailed descrip-tion of this cell design the interested readermight refer to refs [28,58–60]). This de-sign is based on a combined (liquid) flowcell and gas diffusion type of reactor wherea liquid electrolyte is flushed between twofixed GDEs. This concept of a ‘floating’GDE is known to enable very fast diffu-sion rates.[61] Products of the CO

2RR (e.g.

non-volatile formate) are then released intothe liquid electrolyte stream on the innerside of the cathode (Fig. 2A). With regardto the CO

2RR, high current densities of

0.130 A cm–2 for HCOO– production[28]and 0.06 A cm–2 for CO production havebeen reported for this cell design.[62] Theseresults also prove the versatility of this celldesign. Not only (volatile) gaseous reac-tion products (e.g. CO) can be obtained athigh current densities but also liquid (non-volatile) products such as formate.

As electrolyte, a highly concentratedaqueous (bi)carbonate could be used. Thehigh solubility of the Cs and Rb salts (upto 25 mol%) can provide a highly con-centrated electrolyte with conductivitiesup to 100 mS cm–1 and allow operation attemperatures above 100 °C at atmosphericpressure.[63,64] It should be kept in mindthat since liquid electrolytes are involvedin this approach, the same concerns regard-ing the purity of the electrolytes need tobe addressed, as discussed above. Alterna-tively, also non-aqueous electrolyte solu-tions might be used in this configuration.Their CO

2solubility is higher than in aque-

B) Nafion//AEM or Nafion//PBI+KHCO3

GDECO2

Cathode: 3CO2 + H2O + 2e- CO + 2HCO3-

Nafion ®

AEM/ PBI+KHCO3

H+H2O HCO3

-

Anode: 2H2O O2 + 4H+ +4e-

H2O

A) PBI+KHCO3 or AEM

CO2

Cathode: 3CO2 + H2O + 2e- CO + 2HCO3-

AEM or PBI + KHCO3HCO3-

Anode: 4HCO3- O2 + 2H2O + 4CO2 + 4e-

H2O

GDE

GDE

GDE

Current collectorFlowfields

Fig. 2. Graphicalsummary of the CO2-electrolysis configu-rations operating atsimilar current densi-ties as an alkalinewater electrolyzercell. Note that all theelectrochemical reac-tions are written hereto yield CO, but couldhave been writtento yield any kind ofother CO2 reductionproduct.


5.2 Electrodes for CarbonMonoxide Production

The reduction of CO2to CO (or syn-

gas, CO + H2) is very attractive, as it can

be used as a feedstock for synthetic fuelproduction via Fischer-Tropsch processes.Electrochemical CO formation from CO

2is favored by Ag, Au and Zn catalysts.[7]Use of water-free electrolyte solutionssuch as aprotic solvents and ionic liquidsalso increases the FE for CO production.Fig. 3B displays the relationship betweenoperating potentials and partial currentdensities for CO formation on differentelectrodes taken from various full-cellstudies. One can see that GDEs with un-supported Ag NPs display relatively goodperformance reaching partial current den-sities up to 0.115 A cm–2 at E =-0.8 V

RHE.

A considerably lower performance and FEfor COwas found for carbon-supportedAgNPs (40 wt%, dot-centered squares in Fig.3B):[91] in order to obtain partial currentdensities of CO production one neededto apply 0.2–0.3 V more negative poten-tial as compared to unsupported Ag NPs(solid squares). However, the use of TiO

2as a support with 40 wt% loading of Ag(empty squares) allowed reaching similarkinetics of CO

2RR as unsupportedAg NPs

(solid squares) but with much lower silverloading. In general, it is better to avoid acarbon support, as it increases the currentefficiency of HER. Due to the high surfacearea of the carbon support, the contributionof HER in overall cathodic process can bevery significant.[92] The energy efficienciesfor CO production are 40–60% at partialcurrent densities up to 0.115 A cm–2, whichis 50% higher than the efficiencies for for-mate production (Fig. 1B). Normally, theFE of HER increases with increasing thecell voltage. However, since hydrogen gasis a component of syngas, total energy effi-ciencies for CO+H

2production are similar

to those depicted in Fig. 1C. Importantly,the CO/H

2ratio can be readily tuned by

changing cell voltage[27,29] and potentiallyby the right choice of the catalyst system.

are used. Fig. 3 demonstrates the plot ofoperational potentials and correspondingpartial current densities for formate andCO production in CO

2electroreduction

on different GDEs. A few data points aregiven for non-GDEs.

It is rather difficult to compare the per-formances of different catalyst materialsreported by different research groups, asthe CO

2electrolysis was conducted under

different conditions (electrolyte, pH, cellconfiguration). However, we can distin-guish certain trends. The highest currentdensities for formate production, up to~0.13 A cm–2, were obtained on unsup-ported Sn GDEs[28] (solid squares in Fig.3A).We notice that among s- and p-metals,Sn catalysts seem to be the most promis-ing catalytic material, as it is rather inex-pensive, less toxic than e.g. lead, and hasa very good selectivity for formate. A fewrecent studies explored SnO

2as a catalyst

for CO2electroreduction and reported bet-

ter kinetics and selectivity toward formateproduction at such catalysts as compared toSn.[77,80,82] However, the stability of SnO

2under operando conditions (rather negativepotential of CO

2reduction), or any oxide

phase in general, is still an issue to be ad-dressed. Rather low overpotentials werefound for formate production on Pd[79] andRu-Pd[83] catalysts (star and pentagon inFig. 3A) with FEs up to 100% operatingpotentials approximately 0.5 V below thevalues observed for Sn catalysts. However,the high price of Pd needs to be consid-ered before implementing such a catalystin CO

2electrolysis devices.

Review of the literature data showedthat the energy efficiency for formateproduction on different catalysts as ob-tained in full-cell studies typically hasnot exceeded 50% even at current densi-ties <0.02 A cm–2 and dropped with highercurrent densities. Such energy efficiencyis considerably smaller than the expectedmaximal efficiency calculated above (Fig.1C), and is not satisfactory yet.

ous environments (e.g. acetonitrile showsa CO

2solubility which is higher by factor

8 compared to water). The same is validfor room-temperature ionic liquids(RTILs).[65,66] The use of ionic liquidsis also attractive due to their ability tocapture selectively CO

2from a diluted

gas stream.[66] Their use in such a flow-cell design is, however, restricted to thosehaving a low viscosity. What makes ILspromising electrolytes also for such flowcell devices is their ability to even catalyzethe CO

2RR. It was demonstrated that im-

idazolium-based RTILs dramatically de-crease the overpotential for CO

2reduction

particularly in water/RTIL mixtures.[67]Rosen et al. used 18 mol% Emim-BF

4(1-ethyl-3-methylimidazolium tetrafluo-roborate) in water as a catholyte in a flowelectrochemical reactor. A few recentworks have also demonstrated highly se-lective CO

2RR toward CO on nanostruc-

tured metallic catalysts (Ag, Bi) in RTILsand RTIL/acetonitrile.[68–70]

In this part, we have reviewed all thepossible CO

2-electrolysis cell configura-

tions, which would allow closing the gapin terms of operating current density withrespect to an alkaline water electrolyzer.All these arrangements rely on the use ofa GDE for an optimal transport of reactantand reaction products in the gas phase. Inorder to maximize the number of triplephase boundaries (defined as the presenceat the same place of an active catalyst site,CO

2and electrolyte) and to decrease the

kinetic overpotential, the next step shouldconsequently be how to design such GDEwith the highest possible roughness factorsi.e. high cm2

catalystcm–2

geovalues. This part

will therefore focus on understanding howCO

2reduction kinetics could be increased,

and in the meantime HER currents be sup-pressed, when engineering electrocatalystsat the micro- and the nano-scale.

5. Electrode Materials for CO2Electrolysis

5.1 Electrodes for Formate Produc-tion

The products and rates of CO2elec-

troreduction are affected strongly by thenature and structure of catalytic materialsas well as electrolyte solution composi-tion. Numerous half- and full-cell studieswere performed to elucidate the key factorsinfluencing CO

2conversion. As we men-

tioned above, formic acid and CO wouldbe desirable products in CO

2electroreduc-

tion. The catalytic materials selective forformate production are listed in a recentreview article[71] and include: metallic Pb,Hg, In, Sn,[28,72–77] Pd,[78,79] SnO

2,[32,77,80]

andmetallo-organic complexes.[81] In orderto reach sufficient current densities GDEs

Fig. 3. Potential versus partial current density obtained on GDEs of different compositions for for-mation of: (A) formate/formic acid[28,72,76,81,83–87] and (B) CO.[53,59,81,88–91] Solid symbols correspond tounsupported metallic catalysts, dot-centered symbols – to carbon supported, empty symbols – toTiO2-supported. Non-GDEs are indicated in the legend. Details are given in the text.


the CO2electrocatalytic reduction in the

presence of organic ligands or metal com-plexes. Based on these results, we believethat the use of metal nanoparticles modi-fied with nitrogen-rich metal complexesor ligands (Fig. 4D) might be a promisingapproach towards the development of ef-fective catalysts for CO

2RR with a well-

controlled catalyst selectivity and stability.

6. Conclusion

The direct electrochemical conversionof CO

2into more valuable products can be

considered as a highly promising approachfor the concentration reduction of atmo-spheric CO

2and at the same time for the

storage of a surplus of renewable energy(e.g. from solar and wind sources) in formof a reduced carbon compound. A carefulanalysis of the estimated production costsand process efficiencies revealed that CO(or syngas) and formate are the economi-cally most favorable reaction products ofsuch CO

2conversion. Their electrochemi-

cal generation can be considered as poten-tially competitive with their conventionaland well-established routes of production.

In this critical review we, however,identified a number of challenges whichstill need to be addressed before CO

2elec-

trolysis can become economically viable.New designs of CO

2electrolyzers need to

be developed which allow for much highercurrent densities (0.2 A cm

geo–2) than re-

cently reported for state-of-the-art CO2

electrolyzer set-ups. In conventional elec-trolyzers where an aqueous environmentserves as CO

2source it is the CO

2mass

transfer which typically limits the CO2

conversion rate. An improved CO2mass

transfer can be achieved by using non-aqueous electrolytes (e.g. ionic liquids)that reveal a much higher CO

2solubility

(>one order of magnitude as compared toaqueous media). An alternative and mostlikely even more promising approach totackle CO

2mass transfer issues is based on

a gas-diffusion type of cell design wherethe cathode is directly fed with the gas-eous CO

2reactant. Concepts of alkaline

and acidic water electrolyzers with protonexchange membranes (PEMs) and anionexchange membranes (AEMs) as their keyelements can be in part transferred to thedesign of a more efficient CO

2electrolyzer.

These concepts and in particular the mem-brane design still need to be adjusted to thespecific requirements of the CO

2electro-

reduction reaction. The same is valid forthe design and chemical composition ofthe gas diffusion electrodes. In the case ofthe CO

2electroreduction there is no need

to disperse the electrocatalytically activematerial on a carbon support since thecommon catalysts for CO

2electroreduc-

tion are abundant and cheap (e.g. Sn for

obtained in a flow cell on carbon-support-ed, nitrogen-based organometallic Ag cat-alysts.[89]At the same potential, the currentdensities of CO formation form CO

2on

e.g. silver 3,5-diamino-1,2,4-triazole sup-ported on carbon (AgDAT/C) were similarto those achieved onAg-based GDEs (star-symbols in Fig. 3B), but comparatively atmuch lower silver loading.

Alternatively, nitrogen-rich ligands(for example pyridine, bipyridine, benz-imidazole and their polymers) and metalcomplexes with transition metal centerswhich are dissolved in an electrolyte solu-tion are also promising (co-)catalysts forelectrochemical reduction of CO

2to CO

or formate (Fig. 4C).[95,96] By tuning thestructure of the metal complex/ligand, onecan tune the stability of the CO

2-adduct

which dictates the selectivity of the finalproduct formation. In the past four de-cades, numerous metal complexes basedon the transition metals ruthenium, rhe-nium, iridium, cobalt, nickel, palladium,silver, copper, iron and manganese (bothmono- and dinuclear) were proposed forCO

2electroreduction, which are based

on different families of metal complexeswith macrocyclic ligands, with phosphineligands and with polypyridyl ligands in theelectrolyte solution.[95,96] Although someproposed mechanisms suggested such ad-ditives acted as homogeneous catalystsfor CO

2RR, it seems that in the presence

of transition metal complexes or nitrogen-rich ligands the efficiency of CO

2reduction

also depends on the cathode material. Asan example, nickel cyclams chemisorbedon mercury were reported to show an en-hanced catalytic activity compared an in-ert electrode such as glassy carbon.[95,97–99]This fact indicates that the ability of anelectrode material to adsorb the organiccompounds plays an important role in

A recent review postulated the impor-tance of the catalyst morphology for ki-netics and even selectivity of CO

2RR.[70]

Many examples demonstrated that nano-structured and/or nanosized electrodescould significantly decrease the overpoten-tial of CO

2RR as compared to conventional

bulk electrodes. The use of such catalystsalso allows high current densities to bereached (respective to geometric area andthe mass of catalyst material), which isdue to not only a large actual surface area,but also a larger number of active sites forCO

2RR on nanostructured surfaces.[93] The

catalytic effect of metallic nanostructuredelectrodes can be enhanced by introducingadditional foreign metals, forming bime-tallic nanostructures, such as alloys, core-shell and thin-film configurations[94] (Fig.4A). The catalytic properties of bimetalliccatalysts can be tuned by the choice of for-eign metals, chemical composition, mor-phology of the nanostructures (size, shapeand configuration) and capping agents,providing intrinsic functionality differ-ent from that of mono-metallic catalysts.Bimetallic catalysts have been studied formany electrochemical reactions, such asoxygen reduction, hydrogen evolution, COand alcohol oxidation. However, they arescarcely studied on CO

2RR to date.

5.3 Electrodes Modified withLigands and Complexes

Besides metal surfaces and nanopar-ticles, nitrogen-rich ligands or metal com-plexes deposited on the electrode surfacealso showed promising results for elec-troreduction CO

2to formate or CO (Fig.

4B). For example, FEs close to 100% at 70mA cm–2 were achieved on metal-pthtalo-cyanine complexes M-Pc, where M = Co,Ni, Pd, Ag[81,88,89] (dot-centered romb andtriangle in Fig. 3B). Promising results were

Fig. 4. Schematicrepresentation of dif-ferent types of (co-)catalysts for CO2

electroreduction. Redspheres representnitrogen atoms.


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AcknowledgmentThis work was supported by the CTI Swiss

Competence Center for Energy Research(SCCER Heat and Electricity Storage).


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*Correspondence: Prof. Dr. J. WorlitschekGroup Thermal Energy Storage (TES)Competence Centre Thermal Energy Systems &Process EngineeringLucerne University of Applied Sciences and ArtsCH-6048 Horw, SwitzerlandE-mail: [email protected]

Storage of Heat, Cold and Electricity

Anastasia Stamatiou, Andreas Ammann, Andreas Abdon, Ludger J. Fischer, Damian Gwerder,and Jörg Worlitschek*

Abstract: A promising energy storage system is presented based on the combination of a heat pump, a heatengine, a hot and a cold storage. It can be operated as a pure bulk electricity storage (alternative to PumpedHeat Electrical Storage (PHES)/ Compressed Air Energy Storage (CAES)) or as combined storage of heat,cold and electricity. Both variations have been evaluated using a steady state, thermodynamic model and twopromising concepts are proposed: A transcritical CO2 cycle for the pure electricity storage and a subcriticalNH3 cycle for combined storage of electricity, heat and cold. Parametric studies are used to evaluate theinfluence of different parameters on the roundtrip efficiency of the storage system.

Keywords: Dual Energy Storage & Converter · Electricity storage · Power-to-Heat · Reversible heat pump

Introduction

Storage technologies are bound to playan important role in the integration of re-newable energy sources and in increasingthe energy utilization efficiency in theupcoming years. The development of ef-ficient and cost-effective storage technolo-gies in all energy sectors is crucial to thistask.[1] Along with the development of in-novative electricity, thermal and chemicalstorage, the establishment of systems thatcan provide flexibility between energyforms will also gain importance.[2]

A promising storage system based onthe combination of a heat pump, a heat en-gine, a cold and a hot storage is presented(Fig. 1). The system can be designed toserve as pure bulk electricity storage or acombination of heat, cold and electricitystorage. The underlining operating prin-ciple is that during periods of excess elec-tricity generation, a heat pump is operatedto charge a hot and in some cases also acold storage. In periods of excess electric-ity demand, the system can be operated inreverse as a heat engine, using the stored

temperature difference to generate elec-tricity. This system variation is particularlyinteresting for bulk electricity storage in apower output range higher than 10 MW. Itcan be viewed as an alternative to Pumped-storage Hydroelectricity (PSH or PHES)and Compressed Air Electricity Storage(CAES), with the additional advantages ofa higher energy density and the possibilityof site-independent installation.

The first reports of this type of storagesystem can be found as early as 1924[3] andwere continued in the 1970s[4] under theterm ‘reversible heat pumping’. In recentyears, this technology has been the subjectof several independent academic and in-dustrial investigations with varying systemspecifications (working fluid, temperaturelevels, storing material, etc.). Most inves-tigators named the system differently (e.g.ETES,[5] PTES,[6] CHEST,[7] TEES,[8] andothers[9]), making the identification of rel-evant projects challenging.

The authors propose a thermally openvariation of the technology, named DualEnergy Storage & Converter, where thetwo storage units can be used, not only as ameans to store electricity but also to coverheating and cooling demands. This versionoffers additional flexibility between ener-gy forms (power, heat and cold) and canbe implemented in applications which pos-sess a relatively high demand of all threeenergy forms during the whole year (e.g.food & beverages industry). A thermallyopen variation of the reversible heat pumphas been previously considered for build-ing applications using water storage for thehigh temperature side but no storage on thelow temperature side.[10]

This work presents the evaluation pro-cess of different variations of the storagesystem by means of a simple thermody-namic tool. Two promising systems areselected and are shown in detail in termsof temperature–entropy diagrams.

Cold Storage

High TemperatureStorage

Heat Pump Heat Engine

Thermally closed variation

Thermally open variation

Heat

Cold

Fig. 1. Schematic of the DESC process comprising a heat pump, a heat engine, a hot and a coldstorage.


Methods

A simple steady-state thermodynamictool has been developed for the scanningof different variations of the storage sys-tem shown in Fig. 1. The tool is based onMatlab[11] and extracts the thermodynamicproperties from the NIST-REFPROP data-base.[12] It calculates the thermodynamicpoints describing the basic processes tak-ing place during the heat pump and Ran-kine cycle (e.g. evaporation, expansion,condensation, etc.). Both the inefficienciesintroduced by the machines and the exergylosses during heat exchange are consid-ered but pressure drops and heat losses areneglected. The model was verified usingindependently calculated values describ-ing the ABB concept with transcriticalCO

2. It was subsequently used to carry

out parametric studies of the systems con-sidered and assess variations of the tech-nology (working fluids, number of stages,etc.).

Results and Discussion

Themain characteristics of the two sys-tem variations selected to be presented inthis work are shown in Table 1. ETES is abulk electricity storage concept developedbyABB[5]while DESC is a combined heat,cold and electricity storage concept devel-oped by the authors.

The first thermodynamic evaluationwas performed for the ETES system andcan be seen in Fig. 2. The concept is basedon transcritical CO

2as working fluid for

thermally closed cycles (see Table 1). Thered line represents the heat pump cycle,the blue the Rankine cycle. Water is usedfor the sensible high temperature storageat 120 °C whereas ice is used for the lowtemperature storage at 0 °C. As seen inFig. 2, the maximum operating pressure is136.3 bar. The temperature levels and stor-age units are chosen to create an optimummatch between the condensation/evapora-tion processes of the working fluid and thebehavior of the storage media (water/ice)during charging/discharging. The abilityto utilize water as the storage medium onboth sides represents one of the biggest ad-vantages of this variation of the reversibleheat pump concept. The model was veri-fied using independently calculated valuesprovided by ABB. After the verification,the model was used to carry out parametricstudies examining the influence of differ-ent parameters on the roundtrip efficiencyof ETES. The parameters considered were:(i) temperature of hot storage, (ii) tempera-ture of cold storage, (iii) ambient tempera-ture, (iv) ∆T during heat transfer and (v)isentropic efficiencies of the mechanicalcomponents. Fig. 3 presents the results of

Table 1. Characteristics of technology variations presented in Figs 1–4

Name Energy form Workingfluid

Storageunits (hot/cold)

Criteria forchoice of T levels

Application

Input Output

ETES El. El. CO2

Sensible /Latent (ice)

High roundtripefficiency;water as storagematerial

Bulkelectricitystorage

DESC El,heat

El.,heat,cold

NH3

Latent /Latent

Optimized forcustomer-relevantT levels

Heat, cold,electricityand storage,and supply

1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

0

20

40

60

80

100

120

EntropyJ

kg ·K

Tem

perature

[C]

Pres. HP Condensation = 136.3 bar

Pres. HP Evaporation = 33.0 barPres. RC Condensation = 37.7 barPres. RC Evaporation = 125.4 bar

Working Fluid: Carbon dioxide

Fig. 2. Output of thermodynamic modeling for ETES concept with transcritical CO2 as describedin Table 1.

0

5

10

15 100 120 140 160 180 200

0.1

0.2

0.3

0.4

0.5

THST max [°C]Δ THE [°C]

ηE

tE

Fig. 3. Surface plotshowing the roundtripefficiency of the tran-scritical CO2 system,ηEtE, as a functionof the ∆T during theheat exchange andthe maximum tem-perature of the hightemperature storageTHST.


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[7] W. Steinmann, Energy 2014, 69, 543.[8] Y.-M. Kim, D.-G. Shin, S.-Y. Lee, D. Favrat,

Energy 2013, 49, 484.[9] T. Desrues, J. Ruer, P. Marty, J. F. Fourmigué,

Appl. Therm. Eng. 2010, 30, 425.[10] a) O. Dumont, S. Quoilin, V. Lemort, Int. J.

Refrig. 2015, 54, 190; b) S. Schimpf, R. Span,Ener. Conv. Manag. 2015, 94, 430.

[11] Matlab and Statistics Toolbox, The MathWorksInc., 2014, Massachusetts, USA.

[12] E. Lemmon, M. Huber, M. McLinden, ‘NISTReference Fluid Thermodynamic and TransportProperties - REFPROP’, National Instituteof Standards and Technology, 2010, Boulder,USA.

age and a subcritical NH3cycle for com-

bined storage of electricity, heat and cold.The CO

2system has a maximum operating

temperature of 120 °C and can be com-bined with water as a storage medium onboth the hot and cold sides, which offersa valuable simplicity to the ETES system.The NH

3-based DESC system on the other

hand can be integrated in a large numberof industrial applications which requirecold at 0 °C and heat at ~95 °C. The highheat of vaporisation of NH

3and high en-

ergy density of the latent heat storage unitsincrease system compactness. The devel-oped steady-state thermodynamic tool issuitable for a first scanning/evaluation ofdifferent storage systems variations. It canalso be successfully used to perform para-metric studies and find optimal operationpoints. For a more detailed estimation ofthe system potential, the tool has to be ex-tended to capture transient phenomena andto include pressure drops and heat losses.

AcknowledgmentsThe authors thank the CTI (Commission

for Technology and Innovation) for the finan-cial support through SCCER Hae and ABB forproviding data and support for the thermody-namic modelling of the ETES process.


one parametric analysis showing the evo-lution of the ETES roundtrip efficiency asa function of the maximum temperatureof the hot storage and the ∆T during heattransfer. As expected, an increase in theheat exchanger ∆T results in a dramaticdecrease in the ETES roundtrip efficiencyηEtE. In contrast, an increase in the maxi-

mum temperature of the hot storage resultsin an increase in the η

EtE.

The thermodynamic model was modi-fied to describe the behavior of other re-versible heat pump variations. Other work-ing fluids were evaluated such as isobutaneand ammonia. A promising variation forthe combined storage uses NH

3as a work-

ing fluid (Fig. 4). Similarly to Fig. 2, thered line describes the heat pump cycle andthe blue line the Rankine cycle. Both pro-cesses comprise two stages providing theflexibility of charging/discharging the hotand the cold storage separately. Both theproduced heat and cold will be stored inlatent heat storage units to ensure an op-timal match with the temperature profileof the subcritical cycles. The storage tem-peratures have been chosen at 0 °C and95 °C because they represent two key tem-perature levels for industrial process heatand cold. Despite its toxicity, ammoniais a promising natural refrigerant with avery high enthalpy of vaporization. It hasbeen used widely in refrigeration and thereare already commercially available heatpumps that can deliver the temperaturesrequired for this DESC variation. One bigdisadvantage is the high overheating dur-ing the heat pump operation which impos-es challenges for the lubricant and inducesmechanical stress on the components. Theexpansion processes during the heat enginemode take place in the two-phase regionwhich imposes constraints on the choice ofthe expander. At the maximum operatingpressure of 60 bar, even though not as highas the one demanded by the transcriticalCO

2cycle, it could still impose challenges

in the component development and opera-tion.

Conclusions

A promising energy storage systemis presented. It can be operated as a purebulk electricity storage (alternative toPHES/CAES) or as combined storage ofheat, cold and electricity. Both variationshave been evaluated using a steady state,thermodynamic model and two promisingconcepts are being proposed: A transcriti-cal CO

2cycle for the pure electricity stor-

1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500

0

20

40

60

80

100

120

140

160

180

EntropyJ

kg ·K

Tem

perature

[C]

Pres. HP Condensation = 60.1 barPres. HP Interstage = 17.5 barPres. HP Evaporation = 3.3 barPres. RC Condensation = 5.0 barPres. RC Interstage = 17.5 barPres. RC Evaporation = 52.2 bar

Working Fluid: AMMONIA

Fig. 4. Results of thermodynamic evaluation for DESC concept with subcritical NH3 as describedin Table 1.



*Correspondence: Prof. Dr. S. Haussenera

E-mail: [email protected]École Polytechnique Fédérale de Lausanne (EPFL)Institute of Mechanical Engineering, LRESECH-1015 Lausanne, SwitzerlandbScuola universitaria professionale della Svizzeraitaliana (SUPSI)Department for Innovative Technologies, ICIMSICH-6928 Manno, Switzerland$Equally contributing authors

Phase Change Material Systems for HighTemperature Heat Storage

David Y. S. Perraudina$, Selmar R. Bindera$, Ehsan Rezaeiab, Alberto Ortonaab, and SophiaHaussener*a

Abstract: Efficient, cost effective, and stable high-temperature heat storage material systems are important inapplications such as high-temperature industrial processes (metal processing, cement and glass manufactur-ing, etc.), or electricity storage using advanced adiabatic compressed air energy storage. Incorporating phasechange media into heat storage systems provides an advantage of storing and releasing heat at nearly con-stant temperature, allowing steady and optimized operation of the downstream processes. The choice of, andcompatibility of materials and encapsulation for the phase change section is crucial, as these must guaranteegood and stable performance and long lifetime at low cost. Detailed knowledge of the material properties andstability, and the coupled heat transfer, phase change, and fluid flow are required to allow for performanceand lifetime predictions. We present coupled experimental-numerical techniques allowing prediction of thelong-term performance of a phase change material-based high-temperature heat storage system. The experi-mental investigations focus on determination of material properties (melting temperature, heat of fusion, etc.)and phase change material and encapsulation interaction (stability, interface reactions, etc.). The computa-tional investigations focus on an understanding of the multi-mode heat transfer, fluid flow, and phase changeprocesses in order to design the material system for enhanced performance. The importance of both the ex-perimental and numerical approaches is highlighted and we give an example of how both approaches can becomplementarily used for the investigation of long-term performance.

Keywords: High-temperature heat storage · Latent heat of fusion · Multimode heat transfer ·Phase change materials · Thermal energy storage

1. Introduction

The Swiss industrial sector uses about50% of its energy for process heat.[1] Thisprocess heat is provided by fuels (74%)and electricity (26%). Based on the Euro-pean heat market statistics 40%[2] of thatheat is high-temperature heat (>400 °C),thus accounting for an estimated 4.6%of the total end energy consumption inSwitzerland. The metal processing andchemical industries are main consumers.The large exergy content of high-temperature heat, the non-continuous in-dustrial processing routes, and the inter-mittency of some heat sources are driversto develop high-temperature heat storage

systems. Other applications for high-temperature heat storage include concen-trated solar power and electricity storageby advanced adiabatic compressed airenergy storage (AA-CAES).[3] The latterrepresents one of the main complementinglarge-scale electricity storage approachesto pumped hydro electricity storage.

The design of thermal energy storagesystems highly depends on the applica-tion, which defines its temperature level,energy storage capacity, storage duration,and charge and discharge rates. In additionto the technical and performance require-ments, long lifetime, low cost, high volu-metric energy density, and stable operationare key factors for a practical heat storagesystem.

Heat can be stored in the form of sen-sible and latent heat. Latent heat storagesystems often use the solid/liquid phasetransition due to volumetric constraints.If a material with a well-defined melt-ing point is used (such as pure elements,compounds, or eutectics) then storingheat by means of a phase change makesit possible to stabilize the temperature atwhich heat is discharged. The downstreamprocesses can subsequently operate stablyand optimally at a specified temperature.As sensible heat storage materials tend tobe less costly than phase change materi-als (PCMs), combined sensible-latent heat

storage systems are developed to providestabilized output temperatures at reducedcost.[4] An additional challenge of PCMsis the requirements of containment in themolten stage. The stability of the interfacebetween the PCM and the encapsulation iscrucial for high and stable long-term per-formance, especially in high-temperatureenvironments. Complex interlayers can en-sure the mechanical and chemical stabilitywhile maintaining efficient heat transfer.

A generic latent heat storage systemis shown in Fig. 1 utilizing a heat transferfluid (HTF) to provide and evacuate thehigh-temperature heat. Heat is transferredfrom the hot HTF to the solid PCM dur-ing charging. The PCM undergoes meltingand thus absorbs the latent heat of fusionwithin a narrow temperature range. Whendischarging, the reverse process occurs. Apart of the heat can be stored as sensibleheat in the encapsulation and interlayer,and in the PCM if it is completely meltedand further heated above the melting tem-perature. The material combinations usedfor the heat storage system (PCM, inter-layer, encapsulation) must be chosen care-fully. The mechanical stability and goodheat transfer performance must be ensuredwhile guaranteeing chemical stability andinertness of the interfaces and interlay-ers. Detailed analysis of the heat transfer,fluid flow and phase change processes is


3. Numerical Simulation

For the development of an optimized,high-performance material system design,the coupled multi-mode heat transfer, fluidflow, and phase change processes have tobe understood. Generally, convective heattransfer dominates in the heat exchangebetween the HTF and the PCM, and con-duction the heat transfer within the PCM.However, convection plays a role in theliquid phase of the PCM if a flow is in-duced and heat conduction is low, andradiative heat transfer from the surface ofthe opaque encapsulated PCM plays a roleat high temperatures. Two main processeslimit the charging and discharging ratesof encapsulated metallic PCMs; the rateat which heat is transferred between theHTF and the encapsulation (further to thePCM) and the rate of melting. Potentially,evolving or pre-applied interlayers mightprovide additional conductive resistancedue to changes in conductivity or imperfectlayer contact. The performance predictionthus requires simulation of conductive,convective, radiative heat transfer, fluidflow, and phase change.

Back-of-the-envelope calculations pre-dict the convective heat transfer betweenthe HTF and the encapsulated metal PCMto be limiting. Indeed, in the experimentalcampaign (see Fig. 2) it took 20 minutes totransfer 15.12 kJ of heat in order to melt

of fusion), can be of limited applicabilitysince the properties of bulk PCMs can varyfrom that of small samples (in the order ofmg) used in this technique. The T-historymethod can be employed to investigate thethermo-physical properties (melting tem-perature, sub-cooling, specific heat capac-ity and heat of fusion) of different PCMsby melting and solidifying at controlledenvironment temperatures.[12] This typeof measurement provides a fast and non-destructive way to measure bulk propertiesfor encapsulated systems.A set of assump-tions on sample geometry (i.e. cylindricalsymmetry and aspect ratio) and heat trans-fer to the environment have to be verifiedfor the results to be reliable. A typical T-history measurement for Al-12Si is shownin Fig. 2 allowing for the determination ofthe melting temperature and sub-cooling.

To address the stability of PCM and en-capsulation the above-mentioned thermo-physical characterization techniques canbe employed in sequence with typical loadcycles. In addition, post mortem inspectionof encapsulated PCM samples can giveinsight into degradation mechanisms andextent. In the case of steel encapsulatedaluminum alloys an intermetallic reactionproduct forms at the interface and can bequantified visually after cross-sectioning.The formed iron aluminides are brittle andmay deteriorate the mechanical propertiesof the encapsulation.[13,14]

required to predict and enhance the systemperformance while investigation of the in-teractions between the components and atthe interfaces are required to predict andenhance the durability, stability, and sys-tem lifetime.

2. Phase Change Materials (PCM)

Phase change materials are commonlyclassified as organic or inorganic.[5] Or-ganic materials are rarely stable above400 °C making them unworkable for high-temperature heat storage. Among the in-organic materials, salts are often cheaperwhile metals outperform their counterpartsin terms of high thermal conductivity andheat of fusion (see Table 1). Heat transferenhancement techniques, such as the im-mersion of metal meshes, have been in-vestigated for salts[5–8] but result in morecomplex material systems with reducedper volume and mass amount of activePCM material.

The detailed knowledge of materialproperties, such as melting temperature,heat of fusion, heat capacity, density, ther-mal conductivity, sub-cooling and thermalexpansion coefficient, is of paramount im-portance when evaluating the performanceof phase change materials. Data can be ob-tained from literature for elements andwellknowncompounds, but arenot always read-ily available or consistent for new PCMsunder consideration. Differential scanningcalorimetry (DSC), which is typicallyused to determine phase transformationsand associated energies (glass transitiontemperature, melting temperature, heat

Table 1. Thermo-physical properties of selected inorganic PCM. Metal alloy conductivities for thesolid and liquid phases, salt conductivities for the solid phase.

Classification Compound Meltingtemperature[°C]

Heat offusion[Jg–1]

Thermalconductivity[Wm–1K–1]

Al 12Si 575[9] 560[9] 180/70[10]

Metal alloy Al 12Si 5Mg 560[11] 545[11] 200/70[11]

Al 33Cu 548[11] 351[11] 130/80[11]

NaNO3

306[8] 172[8] 0.6[8]

Salts NaCl 800[8] 492[8] 5[8]

Na2CO

3854[8] 275.7[8] 2[8]

HTF

HTF

PCM

PCM

PCM

PCM

PCM

heat transfer

ceramic coating

metallicencapsulation

Fig. 1. Generic latent heat thermal energy stor-age system in its charging state incorporatinghot HTF, solid PCM, encapsulation, and stabi-lizing interlayers.

0 0.5 1 1.5 2 2.5 3 3.5 4650

700

750

800

850

900

849850851852

subcooling

time [h]

temperature

[K]

surrounding TPCM T

Fig. 2. Temperaturemeasurement forAl-12Si encapsu-lated in an AISI 316Lsteel tube (2.13 cmouter diameter, 2.53mm wall thickness).Preset oven tem-perature ramp duringheating (charging) isindicated in green.Oven was turned offduring cooling (dis-charging).


same temperature boundary conditions asin Fig. 2 were used and a heat transfer coef-ficient of 300 W/m2/K between surround-ing air and cylinder was assumed. Theproperties of a Fe

0.6Al

0.4interlayer were

used and it was assumed that the interlayergrew symmetrically with respect to the ini-tial interface.

Fig. 4 illustrates the amount of sensi-ble and latent heat stored as the cylinder isheated (charged) and cooled (discharged).The simulation was repeated for differentinterlayer thicknesses to highlight the de-crease in performance. As the PCM wasconsumed, less heat could be stored in thelatent heat of the PCM. Instead more heatwas stored as sensible heat in the encap-sulation and interlayer. The charge anddischarge rates remained similar, however,the duration of melting and solidificationdecreased. For a heat storage system, thiseffectively means that a full charging cyclestores less heat and the discharge phase atconstant temperature becomes shorter as

fit. The growth kinetics of the intermetalliclayer is generally accepted to be diffusioncontrolled[18] and monotonic growth overtime is expected.[19] Since PCM is con-sumed to form high melting products (T

m≥1160 °C[13]) which act as a sensible heatmaterial, the performance is expected to bealtered. At the same time the mechanicalstability is affected, as the interlayer is hard(up to 1000 HV[14]) and brittle enough tofracture from thermal stresses upon cool-ing the sample to ambient conditions.

4.2 Numerical SimulationThe impact of interlayer growth on

performance was predicted by applyingnumerical simulations and using the mea-sured interlayer growth thickness. A one-dimensional model was applied and the en-thalpy method was used.[15] No convectionwas assumed and changes in density ofthe aluminum alloy were neglected, suchthat no empirical constants were requiredas input to the model. For comparison, the

27g ofAl-12Si. This illustrates the need formaterial system designs that increase thespecific surface per unit mass of PCM suchas provided by porous structures.

For the comparison of different mate-rial system designs, numerical simulationsare the tool of choice. The phase changeis attributed to the general class of freeboundary problem due to the presenceof melting or solidification fronts whichevolve and represent a discontinuity inmaterial properties. Early attempts of nu-merical solution are documented by Vol-ler et al.[15] The work of Voller led to theenthalpy porosity-method,[16] the state-of-the art method for phase change simula-tions implemented in commercial software(ANSYS, COMSOL). While such simula-tions allow to better understand the physicsof multi-mode heat transfer processes, ithas been demonstrated,[17] that the require-ment for semi-empirical constants has asignificant influence on the solution. Dueto their weak relation to first principles,their tuning with reference to experimentsis necessary and the predictive power ofthe method is limited. This underlines thenecessity of a dual approach, includingsimulations and experimental validation.

4. Chemical Stability andPerformance Losses of PCM inAA-CAES Application

The following illustrates current effortsat our laboratory to develop tailored high-performing and stable high-temperatureheat storagematerial systems. Specifically,we illustrate the effect of chemical degra-dation on the performance of a heat stor-age system. For that purpose the chemicalstability of an aluminum alloy Al-12Siencapsulated in an AISI 316L steel tube –relevant in AA-CAES application – wasinvestigated experimentally and the resultswere included in a numerical simulation toassess the impact of chemical degradationon the long-term heat storage characteris-tics.

4.1 Experimental DataFor the investigation of chemical sta-

bility, 20 AISI 316L steel cylinders with21.3 mm outer diameter, 6 cm height, and2.53 mm wall thickness were filled withapprox. 27 g ofAl-12Si and heated in a fur-nace to 700 °C.After different time periodsthe samples were removed from the ovenand cooled to ambient. Cross-sectioningrevealed, in accordance with literature[18,19]that an iron- and aluminum-rich interme-tallic interlayer had formed as the PCMreacted with the encapsulation (confirmedby SEM-EDX). The quantified growth ofthe intermetallic layer thickness is illus-trated in Fig. 3 together with an empirical

0 10 20 30 40 50 60 70 80 900

500

1,000

1,500

2,000

fit quality: R2 = 0.989

time [d]

interlayer

thickness[µm]

measured interlayer thickness

empirical fit 466µm · t1d

0.264

discharge time at constant temperature

0

5

10

15

20

discharge

timeof

latentheat[m

in]

Fig. 3. Measured and fitted intermetallic layer growth for Al-12Si encapsulated in an AISI 316Lsteel cylinder and decrease in discharge time at constant temperature.

Fig. 4. Numerical prediction of the stored heat form (sensible – h0 = h(T=300K) = 0 – and latent) ofthe AISI 316L steel encapsulated Al-12Si during heating (charging) and cooling (discharging) ini-tially (solid lines) and for different thicknesses of the formed Fe-Al intermetallic layer(dotted lines).


[4] G. Zanganeh, R. Khanna, C.Walser,A. Pedretti,A. Haselbacher, A. Steinfeld, Sol. Energy 2015,114, 77.

[5] B. Cárdenas, N. León, Renew. Sustain. EnergyRev. 2013, 27, 724.

[6] L. Fan, J. M. Khodadadi, Renew. Sustain.Energy Rev. 2011, 15, 24.

[7] O. Mesalhy, K. Lafdi, A. Elgafy, K. Bowman,Energy Convers. Manag. 2005, 46, 847.

[8] M. Liu, W: Saman, F. Bruno, Renew. Sustain.Energy Rev. 2012, 16, 2118.

[9] X. Wang, J. Liu, Y. Zhang, H. Di, Y. Jiang,Energy Convers. Manag. 2006, 47, 2211.

[10] D. Farkas, C. E. Birchenall, Metall. Trans. A1985, 16, 323.

[11] A. F. Riechman, C. E. Birchenall,Metall. Trans.A 1980, 11, 1415.

[12] Z. Yinping, J. Yi, J. Yi, Meas. Sci. Technol.1999, 10, 201.

[13] K. Bouché, F. Barbier, A. Coulet, Mater. Sci.Eng. A 1998, 249, 167.

[14] S. Kobayashi, T. Yakou, Mater. Sci. Eng. A2002, 338, 44.

[15] V. Voller, M. Cross, Int. J. Heat Mass Transf.1981, 24, 545.

[16] V. R. Voller, C. Prakash, Int. J. Heat MassTransf. 1987, 30, 1709.

[17] C. A. Kherabidi, D. Groulx, CHT-15 2015, Mayissue.

[18] G. Pasche, M. Scheel, R. Schäublin, C. Hébert,M. Rappaz, A. Hessler-Wyser, Metall. Mater.Trans. A Phys. Metall. Mater. Sci. 2013, 44,4119.

[19] V. N. Yeremenko, Y. V. Natanzon, V. I. Dybkov,J. Mater. Sci. 1981, 16, 1748.

We show how combined experimental andnumerical methods can be used to quan-tify performance and degradation, and toprovide design guidelines for tailored heatstorage solutions guaranteeing high andstable performance over long lifetime atlow costs.

AcknowledgementsThismaterial is based uponwork performed

in cooperation with CTI Swiss CompetenceCenters for Energy Research (SCCER Heatand Electricity Storage), and with the finan-cial support of the National Research Program‘Energy Turnaround’ (NRP 70) of the SwissNational Science Foundation (SNSF) underGrant #153780. Further information on theNational Research Program can be found atwww.nrp70.ch. We thank Dr. Ludger Weberand Prof. Dr. Andreas Mortensen, LMM atEPFL, for fruitful discussions.


[1] Bundesamt fur Energie, (BFE), ‘Analyse desschweizerischen Energieverbrauchs 2000–2013nach Verwendungszwecken’, 2014.

[2] E. Power, ‘The European Heat Market –ECOHEATCOOLWork package 1’, 2006.

[3] L. Geissbuhler, S. Zavattoni, M. Barbato,G. Zanganeh, A. Haselbacher, A. Steinfeld,CHIMIA 2015, 69, 799.

the chemical degradation advances. Basedon our model assumptions, in a mate-rial system with an interlayer of 1.6 mm,19 wt% of the PCM material was con-sumed and the discharging time at constanttemperature was reduced by 10%. The re-sults presented in Figs 3 and 4 can be usedto advance the accuracy of computationalperformance models of combined sensi-ble-latent heat storage systems, such as thetype described by Geissbuhler et al.,[3] andmodels of complete AA-CAES electricitystorage plants.

5. Conclusions

The development of tailored, encapsu-lated phase change material systems is cru-cial for the design of advanced high-tem-perature heat storage technologies, whichare essential for enhancing the efficiencyof applications such as industrial high-temperature processes, electricity stor-age by AA-CAES, and concentrated solarpower. Our current research focuses on thechallenges of finding appropriate materialsystems characterized by enhanced heattransfer and stable long-term performance.



*Correspondence: Dr. X. Daguenet-Fricka

E-mail: [email protected] for Solar Technology SPFUniversity for Applied Sciences-HSROberseestr. 10, CH-8640 Rapperswil, SwitzerlandbEMPAÜberlandstrasse 129, CH-8600 Dübendorf,SwitzerlandcKingspan Environmental Ltd180 Gilford Road, Portadown,Craigavon BT63 5LF, UK

Seasonal Solar Thermal AbsorptionEnergy Storage Development

Xavier Daguenet-Frick*a, Paul Gantenbeina, Mathias Rommela, Benjamin Fumeyb, Robert Weberb,Kanishka Goonesekerac, and Tommy Williamsonc

Abstract: This article describes a thermochemical seasonal storage with emphasis on the development of areaction zone for an absorption/desorption unit. The heat and mass exchanges are modelled and the designof a suitable reaction zone is explained. A tube bundle concept is retained for the heat and mass exchangersand the units are manufactured and commissioned. Furthermore, experimental results of both absorption anddesorption processes are presented and the exchanged power is compared to the results of the simulations.

Keywords: Adsorption · Desorption · Falling film tube bundle · Seasonal solar thermal energy storage ·Thermochemical heat storage

1. Introduction

Solar thermal energy storage for heat-ing and cooling systems is a priority goalin the renewable energy future.

High thermal losses occurring by theuse of sensible thermal energy in materi-als over the long term, i.e. seasonal stor-age and a low volumetric energy density(typically, water storages) are drawbacks.Therefore, numerous research groups areapplying various approaches that aim to re-duce the required tank volume assigned forseasonal thermal storage.[1] Phase-changematerials (PCM) and thermochemical ma-terials (TCM) are promising approacheswith the potential for increased volumetricenergy density. In a TCM-based thermalstorage system, thermal energy is usedto separate the storage medium work-ing pair in its components. Reversing theprocess releases thermal energy throughan exothermal reaction. The storage mate-rial composed of two reactants forming aproduct are usually aqueous salt solutions(NaOH, LiCl, LiBr, etc.). A recent over-view of sensible, latent and thermochemi-cal storage concepts indicated the specificadvantages and disadvantages.[2]

The closed sorption heat storage ap-proach (only heat is exchanged with theenvironment, no exchange of substances)

functions as a continuous but not full cycleliquid state absorption heat pump.[3] Theprocess operates in the planned tempera-ture and pressure range under exclusion ofnon-condensing gases. No thermal insu-lation is required for the storage tanks aslong as they are kept at room temperature.In fact, except during the loading/unload-ing operation, no thermal losses occur overthe time as it is the case for conventionalwater storage.

In a first part of this article, the work-ing principle of the thermochemical stor-age will be described and focus will be puton the heat and mass exchanger, a centralcomponent of the facility. The modellingas well as the manufacture and commis-sioning of the heat and mass exchangerwill be presented in a second part. Experi-mental measurements and results in bothabsorption and desorption modes will thenbe shown in a third part.

2. Thermochemical StorageConcept

2.1 Working PrincipleThe absorption/desorption storage con-

cept works with a sorbent (in the presentstudy aqueous sodium hydroxide) and asorbate (water).[3] In summer, during thecharging process, the solar energy pro-vided by the thermal panels is used in thedesorber to increase the sorbent concentra-tion (partial evaporation of water from thesodium hydroxide solution under reducedpressure). The water vapour is condensedand releases its latent heat to the environ-ment (through a ground heat exchanger)and the water is stored at room temperaturein its liquid phase.

The concentrated sorbent is separatelystored at room temperature until winter,

when the storage is discharged through op-erating in the reverse direction. Then, thesorbate is evaporated under reduced pres-sure in the evaporator using ground heatat a low temperature level. This vapourflows to the absorber and is absorbed bythe concentrated sorbent solution, releas-ing heat at a sufficiently high temperaturelevel to satisfy the building’s heating re-quirements.

Thus, this storage design concept isbased upon the thermally driven heat pumpprinciple.

The concept of this storage allows aseparation of the power unit (heat andmassexchangers) and the energy unit (reactantand product storage tanks) – like in gas oroil burner thermal energy system sources.Furthermore, because of the seasonallyseparated process steps of desorption insummer and absorption in winter one heatand mass exchange component for desorp-tion/absorption and one for condensation/evaporation can be used.

As aqueous sodium hydroxide is avail-able at low cost and as water vapour ab-sorption in aqueous sodium hydroxide hasa considerably higher volumetric energydensity compared to sensible thermal stor-age systems (theoretically up to six timesfor a concentration decrease from 50 to25 %wt), Weber et al.[3] investigated aclosed sorption heat storage based on so-dium hydroxide and water. In the presentwork, the reaction zone of an absorption/desorption concept with sodium hydroxide(NaOH) and water will be highlighted andinvestigated, focussing on the design of thecore components.

2.2 Heat and Mass ExchangerConception

A compact geometric design of the heatand mass exchangers is essential in order


pled heat, mass and momentum equationsare solved under steady state conditions.

The present model considers only onevertical tube column of a falling film heat-exchanger and the scaling of the power isdone linearly (fluid distribution inside andoutside of the tubes in parallel). For eachtube (or horizontal tube row with index nin the scaling, see Fig. 2a), the tempera-ture is calculated inside and outside of thetube, and on the tube surface with thesame index as shown in Fig. 2. The heatexchange coefficient (h

i) inside of each

tube is calculated according the Sieder andTate or Colburn correlation depending onthe flow regime.[7] For the heat exchangecoefficient calculation outside of the tube(h

e), the model convergence was studied

using different kinds of correlations. Thecalculations showed that the prediction ofthe heat transfer coefficients at low massflow rates has limited accuracy. For thedesorber/absorber sizing, it was therefore

creases the heat and mass transfer. Deng[7]describes that increasing the flow rate(volume flow) per area is favourable forhigher heat/mass transfer rates because animproved wetting of the tube surface by ahigher volume flow is achieved. Neverthe-less, recirculation requires additional elec-tric energy for pumping. To reduce this par-asitic energy consumption the challenge isto find a parameter range in which steadystate operation can meet the required ther-mal power without recirculation.

3. Design and Manufacture of theHeat and Mass Exchanger

3.1 Heat and Mass ExchangerModelling

The modelling of the heat and massexchanger tries to be representative for thereality of the system in regard of the opera-tion set points. In the simulation, the cou-

to keep the high volumetric energy densitybenefits of the chosen concept and mate-rial. Both the heat and the mass exchangesare directly correlated to the transfer area.For this transfer zone, the falling filmtechnology with horizontal tubes[4] waschosen because of the large contact areabetween vapour and the liquid fluid as wellas the solution mixing that occurs when thedroplets fall from one tube to another. Thatenables the volume of the heat and massexchanger area to be decreased.[5] In or-der to facilitate the vapour transfers in thefalling film,[6] all non-condensable gasesare removed from the reaction zone (tubebundles seated in evacuated container, seeFig. 1).

As mentioned above, the sequentialrunning of the heat storage allows a combi-nation of desorption/absorption (A/D) andevaporation/condensation (E/C) processeseach confined to one component and anincrease of the storage volumetric energydensity. However, the challenge in the de-sign of the components is to surmount thedifferent heat and mass transfer rates in theprocess steps in one component.

A critical point in the design is the ho-mogeneous distribution of the fluids on theouter tube surface to achieve an efficientheat/mass transfer. Therefore, special at-tention was applied to design the nozzlemanifold (in yellow in Fig. 1) that spraysthe fluid on the top of the tube bundle.Because of corrosion, stainless steel (DIN1.4404) is used.

Previous experimental studies on theLiBr-H

2O absorption system showed that

a certain degree of fluid recirculation in-

Fig. 1. CAD cross-sectional view of theE/C unit (manifoldand tube bundle).

Fig. 2. (a) Tube bundle model schematic with the indicated variables used in the modelling; (b) mass flow rate (right vertical axis), average tempera-tures and solution concentration (right vertical axis) development in the desorber in function of the tube number n.


expected depending on the pressure levelinside of the reaction zone) and on theother hand the feed through should avoidthe transfer of liquid splashes from onecontainer to the other. Additionally, the va-pour feed through should only act as masstransfer unit and therefore form a thermalinfra-red barrier. A nickel plated and bentmetal sheet is implemented for this taskand will predominantly form a radiationshield (radiative disconnection due to thehigh reflectivity of nickel in the infrared).

The manifolds placed at the top of thetube bundle should ensure a homogeneousfluid distribution above the tubes, tak-ing advantage of the experimental resultsobtained with a preliminary test rig. Par-ticularly tricky was the manufacture of thenozzles in DIN 1.4404 stainless steel alloy.In preference to several other possible de-signs, a version with nozzles directly ma-chined in a stainless steel nozzles plate wasretained. With this version a high flexibil-ity of the nozzle geometry is reached, en-abling a good liquid distribution. A draw-back of the chosen design is its high price.

Special attention was paid to the fluiddistribution on the absorber/desorber tubebundle unit as this heat and mass exchang-er is used without fluid recirculation. Likein the preliminary experimentation testrig, the caustic soda mixture enters the

windows allow a view to the top of the tubebundles, which are installed directly underthe feedmanifold. Except for the both fluidoutlets at the bottom of the containers anda temperature sensor feed through in theA/C envelope, all connections and feedthroughs are located on the tube bundlesflanges (in blue in Fig. 3). These connec-tions enable the feed of the tube manifoldswith caustic soda solution on the A/D sideand water on the E/C side. Two other feedthrough connections placed on the flangesare for the evacuation of the containers andthe operation pressure measurement. Themanifolds are integrated into the flangesfor the inlet and outlet of the tube bundlesheating and cooling fluid and further feedthrough are for the temperature sensorsmeasuring the heating and cooling fluidtemperature inside the tube bundle tubesas well as outside these tubes (outside tubewall temperatures).

For process and handling reasons aswell as for fluid separation, both A/D andE/C units are placed in different containers(Fig. 3). The vapour feed through connectsboth units, enabling the required exchang-es of vapour in both directions (evaporatorto absorber or desorber to condenser). Onone hand the vapour pressure loss throughthe feed through should be as low as pos-sible (a value between 50 and 120 Pa is

decided to use a linear regression based onthe experimental data gathered at lowmassflow rates by Lee[9] instead of applying thecorrelations established by Owens[10] andHu and Jacobi.[11,12]

In the design process a multi-parameterproblem has to be solved according to theset boundary conditions. This comprisesthe definition of the tube bundle geometry:number of tube columns and tube rows,tube length (L), outer and inner tube diam-eters (D

e, D

i) as well as tube spacing.

Desorber modelling results are shownin Fig. 2b: the development of tempera-tures (T

e: fluid temperature outside of the

tube; Tw: wall temperature; T

i: fluid tem-

perature inside of the tube), of the sodiumhydroxide concentration (wt) and of themass flow rate outside of the tubes (m

et) are

depicted over a 4×16 tube bundle arrange-ment in function of tube number n (withbottom-up method enumeration, n=1 is thelowermost tube of the tube bundle). Theheat transfer fluid inside the tubes of thiscounter current heat and mass exchanger,cools down while solar thermal panelsheating up this fluid. Outside the tubes,the sorbent is fed by the fluid manifoldnozzles with a temperature lower than itssaturation temperature. Therefore, T

eraises

rapidly until the saturation temperature isreached in uppermost tubes of the tubebundle. Then, partial evaporation occurs,the sorbent concentration (wt) increasesand its temperature follows the mixturesaturation temperature.As the heat transfercoefficient inside of the tube is higher thanoutside (see Table 1), the wall temperatureTwis closer to T

ithan to T

e.

Table 1 shows that for the desorber/ab-sorber combined heat and mass exchangerunit, the required heat flux (Q) of about12 kW for the desorber and 8 kW for theabsorber is reached with a tube bundle ge-ometry of 4 columns and 18 rows (tubelength of L = 300 mm and outside tube di-ameter of D

e= 10 mm). This tube bundle

geometry also minimizes the auxiliary en-ergy consumption of the circulating pumpbecause of moderate pressure losses (cal-culation inside of the tubes).

In the same way, the tube bundle con-figuration of the evaporator/condenserunit was determined: 16 columns of 12rows (same tube diameter D

e, tube length

L = 700 mm). In order to increase the heattransfer coefficient outside the tubes, fluidrecirculation is planned for the operationof this unit.

3.2 Heat and Mass ExchangerManufacture and Commissioning

Amodular concept (each component iseasily dismountable) as well as a limitednumber of vacuum sealing gaskets wereother challenges in the vacuum envelopedesign.As showed in Fig. 3, two inspection

Table 1. Modelling of the desorber/absorber unit under the worst working conditions: principlequantities.

Desorber Absorber

mi

[kg/s] 0.300 0.100

Ti(1) [°C] 95 14

Ti(N+1) [°C] 80.5 33

hi

[W/(m2*K)] 7932 2733

Pe

[Pa] 7050 590

me(N+1) [kg/s] 0.0094 0.0048

me(1) [kg/s] 0.0056 0.0080

Te(N+1) [°C] 18.0 18.0

Te(1) [°C] 79.3 43.0

he

[W/(m2*K)] 984 514

Q [kW] 12.21 7.95

Fig. 3. CAD drawingof the reaction zonewith both A/D (left)and E/C unit (right).


tion) is quite a lot lower than expected;only a small concentration decrease of theinitial 50wt% sodium hydroxide solutionis reached at the outlet of the absorber unit.Therefore, instead of emulating annual op-eration of a building, it was decided to runmeasurements in steady state conditions inorder to characterize the heat and mass ex-changers and to compare the experimentalresults with those obtained from the nu-merical modeling. The aim is to find outthe weak points of the heat and mass ex-changers to further increase the exchangedpower value for the absorption process.The measurement points presented inFig. 5 are averaged values obtained during30 minutes steady state experiments.

An optical inspection shows that dur-ing the discharging process only a fraction(about 50–60%) of the absorber tube bun-dle surface is wetted. Besides the depen-dence of the exchanger power on the tem-perature difference between the evaporatorand the absorber (Fig. 5, left), it was alsonoticed that this power depends on the so-

4. Measurements

4.1 Absorption ProcessThe first non-isothermal experiment

campaign shows that the exchanged powerduring the discharging process (absorp-

top of the manifold from both sides of aperforated feed tube. The expected pres-sure losses through the perforation holesare 3 to 5 time higher than those due tothe fluid flow along inside the tube, ensur-ing an equalised fluid distribution between

both ends of the manifold. Furthermore, afluid film formation is aimed on the bot-tom plate of the manifold (the plate withthe nozzles) ensuring a homogeneous fluiddistribution on the manifold nozzles plateon each side of the feed tube. Fig. 4 (left)shows the proper working of the manifoldwith water as working fluid: the dropletsare correctly falling under the nozzles andhit the uppermost tubes of the tube bundleon their total length. An optical methoddeveloped to quantify the wetting of theheat and mass exchanger tube arrangement(see Fig. 4, right) also enabled us to vali-date that the influence of the temperaturesensors mounted outside of the tubes (filmtemperature measurement Te) on the fluidflow is small.

Fig. 4. Side view picture of the falling film on the A/D tube bundle heat and mass exchanger (left)and of the A/D heat and mass heat exchanger during the fluid distribution tests (right).

Fig. 5. Development of the power (Φ) in function of the temperature difference (∆T) between both absorber and evaporator chamber during discharg-ing process (left) and of the absorption power in function of the linear mass flux (Γ) arriving on the absorber (right).

Fig. 6. Development of the power (Φ) in function of the temperature difference (∆T) between bothdesorber and condenser chamber during charging process (right).


dium hydroxide mass flux flowing over theabsorber (Fig. 5, right). An increase of thesodium lye mass flux leads to a better tubewetting, showing that this parameter is alimiting factor for the exchanged power onthe absorber side.

4.2 Desorption ProcessDuring the charging process (sorbent

desorption with an initial sodium hydrox-ide concentration of 30wt%), it seems thatthe exchanged power only depends on thetemperature difference between the de-sorber and the condenser (Fig. 6), a highertemperature difference leads to a higherpressure difference between both units andtherefore an increased vapour transfer rate.

The wetting of both tube bundle surfac-es as well as the exchanged power on bothdesorber and condenser are appropriate.For a temperature difference of 60 K (simi-lar to the boundary conditions taken for themodeling), a power of 9.5 kW (about 25 %less than predicted) can be reached.

5. Conclusion

According to this first comparison be-tween the experimental and the simulationresults, it looks like that under low sor-bent flow rates the modelling accuracy islimited, especially for the absorption pro-cess. As the discrepancy between the realextracted power and the expected value ismuch higher for the absorption processthan for the desorption process, the com-bination of the absorber and desorber unitcan be questioned. Nevertheless, the evap-orator/condenser unit works well, prob-ably due to the fluid recirculation and indesorption modus, theA/D tube bundle un-der-performs the simulation by only 25 %.

In the near future, more extensive com-parisons between the modelling and exper-iments will be carried out. In parallel, forthe absorber heat and mass exchanger, thewetting should be optimised and/or a newconcept should be found. One step towardsthis target is performing measurements ona down-scaled experimental set-up.

AcknowledgementsFinancial support by the European Union

in the frame of FP 7 under the grant num-ber 295568 and of the University of AppliedSciences Rapperswil as well as of the SwissCTI (SCCER) is gratefully acknowledged.


[1] J.-C. Hadorn, ‘Thermal Energy Storage forSolar and Low Energy Buildings: State of theArt’, International Energy Agency, 2005.

[2] ‘Advances in Thermal Energy Storage Systems:Methods and Applications’, 1st ed., Ed. L. F.Cabeza, Boston, MA: Woodhead Publishing,2014.

[3] R. Weber, V. Dorer, Vacuum 2008, 82, 708.[4] J. R. Thome, ‘Wolverine Engineering Data

Book III’, 2010, Wolverine Tube, inc.[5] J. F. Roques, J. F. Thome, Heat Transf. Eng.

2003, 24, 40.[6] K. J. Kim, N. S. Berman, D. S. C. Chau, B. D.

Wood, Int. J. Refrig. 1995, 18, 486.[7] S. Deng, Int. J. Refrig. 1999, 22, 293.[8] M. Jakob, ‘Heat Transfer’, Wiley, 1949.[9] S. Lee, ‘Development of techniques for in-

situ measurement of heat and mass transfer inammonia–water absorption systems’, GeorgiaInstitute of Technology, 2007.

[10] W. L. Owens, ‘Correlation of thin film evapo-ration heat transfer coefficients for horizontaltubes’, Miami Beach, 1978, pp. 20–22.

[11] X. Hu, A. M. Jacobi, J. Heat Transf. 1996, 118,616.

[12] X. Hu, A. M. Jacobi, J. Heat Transf. 1996, 118,626.


*Correspondence: Prof. K. SivulaÉcole Polytechnique Fédérale de Lausanne (EPFL)Institute of Chemistry and Chemical EngineeringCH H4 565, Station 6CH-1015 LausanneE-mail: [email protected]

Challenges towards Economic FuelGeneration from Renewable Electricity:The Need for Efficient Electro-Catalysis

Florian Le Formal, Wiktor S. Bourée, Mathieu S. Prévot, and Kevin Sivula*

Abstract: Utilizing renewable sources of energy is very attractive to provide the growing population on earthin the future but demands the development of efficient storage to mitigate their intermittent nature. Chemi-cal storage, with energy stored in the bonds of chemical compounds such as hydrogen or carbon-containingmolecules, is promising as these energy vectors can be reserved and transported easily. In this review, we aimto present the advantages and drawbacks of the main water electrolysis technologies available today: alkalineand PEM electrolysis. The choice of electrode materials for utilization in very basic and very acid conditions isdiscussed, with specific focus on anodes for the oxygen evolution reaction, considered as the most demand-ing and energy consuming reaction in an electrolyzer. State-of-the-art performance of materials academicallydeveloped for two alternative technologies: electrolysis in neutral or seawater, and the direct electrochemicalconversion from solar to hydrogen are also introduced.

Keywords: Electrochemical water splitting · Energy storage · Hydrogen · Oxygen evolution reaction

1. Introduction

The current global energy supplysystem, mainly based on fossil fuels andnuclear power, will change dramatically inthe next decades. The accelerated deple-tion of non-renewable energy resourcesand ecological consequences associatedto their use are a major concern for bothpolicy makers and the general population.In a recent review of different possiblescenarios for the Swiss electricity systemuntil the year 2050, renewable sources ofenergy are forecasted to be deployed on alarge scale after a transition period basedon natural gas-powered generation.[1] Inaddition to reducing the climate impactfrom fossil fuels, enhanced utilization ofrenewable energy is expected to help bringelectricity to the 1.6 billion people in theworld currently without access to energy,meeting the energy demand of a growingpopulation and ensuring stable and secureenergy access for all nations.[2]

Globally, renewable energy can be ex-tracted from a few key available resources:hydroelectric (0.5 terawatts (TW), avail-able at maximum), from all tides & oceancurrents (2 TW), geothermal integratedover all of the land area (12 TW), globallyextractable wind power (2–4 TW), and so-lar energy illuminating the earth (120,000TW).[3] In total, the maximum energy thatcan be converted from these sustainablesources substantially exceeds the energyconsumed by humans on earth today, (ca.600 EJ per annum corresponding to anaverage consumption rate of 17 TW), andalso the predicted usage rate for 2050 and2100 (40.3 and 48.8 TW respectively).[4]Additionally, it is also striking that the con-version of only 0.05% of the solar energy(or 0.5% considering solar cells with 10%conversion efficiency) would be sufficientto secure the energy demand for the nextcentury.

However, themajor limitations towardsexpanded use of renewable energy sourcesin the global energy portfolio are currentlytheir availability and intermittency.[5,6]These drawbacks can be overcome throughthe conversion and storage of renewableenergy into a stable but accessible form al-lowing energy use when needed.[6] Energystorage is especially needed to accommo-date the disaccord between the times ofenergy peak production and of peak con-sumption as well as transporting energyfrom where it is harvested to where it isused.[1] Fig. 1 shows an example of the dis-crepancy between the load on the electricnetwork and the energy generated fromrenewable sources (wind, solar and wave)

in the Pacific Northwest of the U.S.[7] Alldata, originating from both real and simu-lated sources, show the daily average foreach category through the year 2008 (pu:penetration units defined as the ratio of thepeak load to the peak generation within theyear). The load, solar, and wave data setsexhibit both diurnal and seasonal variabil-ity, while the wind generation appears to beless seasonally correlated.

Wind and wave data also show a largedisparity in power generated each day, incontrast to the load. Notably the largestload values are in the end and beginning ofthe year (the winter months), which is thesame time when solar output is negligible.This set of data demonstrates the neces-sity of storing the energy from renewablesources on a short timescale (i.e. daily toaccommodate wind / wave variability andnight / day divergence in generation andusage) and on a longer period—on the timescale of a year for solar or wave energy, forexample.

Chemical storage, i.e. storing energy inthe bonds of molecules such as hydrogenor simple carbon-based compounds (e.g.methane, methanol or formic acid), is par-ticularly attractive as this method does notexhibit limitation to the storage time. In-deed there are potentially very few lossesduring the storing period (depending onthe stability of the compounds) comparedto electrochemical energy storage (e.g. inLi-ion batteries). Moreover, H

2or carbon-

based fuels could be integrated to existingdistribution systems for fossil gas or oil.[8]

One potential way to form these chemi-cal energy storage vectors from electricity


systems have comparable efficiencies andoperate at similar temperature (50–100 °C)and pressure (<30 bars).[9,10] Alkaline wa-ter electrolysis is widely recognized asthe most mature and the most widespreadtechnology. It offers the possibility to uselarge area electrodes but its large-scaledeployment is restricted by the maximumcurrent density and the purity of the hy-drogen generated (99.5–99.9%).[11,12] PEMelectrolyzers have a slight advantage interms of gas purity (>99.99%), efficiencyand produced-hydrogen cost but sufferfrom poor stability.[9,13] The operatingprinciples of an alkaline electrolyzer and aPEM electrolyzer are illustrated in Fig. 2aand 2b respectively.

The main differences between thetwo types of electrolyzer are the electro-lyte and the membrane separating the twoelectrodes. While in an alkaline electroly-sis cell, the two electrodes are separated bya gas-tight diaphragm submerged in a liq-uid electrolyte, a solid proton-conductingpolymer membrane is used to isolate theoppositely charged electrodes in a PEMcell (typically Nafion™). The higher per-meability to gas of the diaphragm as com-pared to a PEM reduces the efficiency ofthe alkaline system due to oxygen diffu-sion on the cathodic side.[12]

The electrolyte used for a alkalineelectrolyzer is highly basic, usually a20–40 wt% aqueous solution of potas-sium hydroxide (KOH), which is preferredover sodium hydroxide (NaOH) due toits higher ionic conductivity. In contrast,in a PEM electrolyzer, high purity water(<1 μS cm–1) is required for the cell to op-erate at high efficiency, increasing the costof this technology.[11] The difference in theemployed pH results in different chemicalprocesses occurring on the electrodes.

In the alkaline system, water is reducedat the cathode according to Eqn. (1), evolv-ing hydrogen and generating hydroxylanions. These hydroxyl groups migratethrough the ion-permeable diaphragm toreach the anode side, where they are oxi-dized (Eqn. (2)) to generate oxygen andextract the four electrons required for thereduction.

Alkaline / Cathode:4H

2O + 4e– → 2H

2(g)+ 4OH– (1)

Alkaline / Anode:4OH– → O

2(g)+ 2H

2O + 4e– (2)

In the PEM system, water is oxidizedat the anode, generating four protons andtransferring four electrons to the externalelectric circuit. Protons are transferred tothe cathodic side through the proton ex-change membrane and react with the four

H2produced from fossil fuels. Thus there

is a strong motivation to optimize watersplitting technology to reduce the price.In this mini-review, we focus on the elec-trochemical production of hydrogen viawater splitting, with particular attentionto the development of the oxidation elec-trode, where the majority of the lossesremain at present. We will present firstthe two main systems already well devel-oped for H

2production: alkaline and PEM

electrolyzers, and discuss their limitationsand challenges toward reducing the price.Next we discuss the prospect to conductwater electrolysis in neutral pH and sea-water, as this approach is industrially veryattractive. In the last section, we will givea brief overview of the direct electrochemi-cal solar-to-hydrogen conversion systems.

2. Overview of CommercialElectrolyzers and ElectrodeCharacterization

Three electrolyzer technologies arecurrently well-developed and availablecommercially: conventional alkaline elec-trolyzers (with liquid electrolyte), ProtonExchange Membrane (PEM) electrolyzersand most recently anion exchange mem-brane (AEMor alkaline Polymer ExchangeMembrane) electrolyzers have emerged.The latter technology will not be discussedin detail as it is still under development andlimited to very specific applications.[9]

Both alkaline and PEM electrolysis

is to use an electrochemical cell, where adifference in electric potential (voltage)drives a non-spontaneous reaction. In anelectrochemical cell the overall reaction iscomposed of two half-reactions that occurat distinct sites in the cell: the oxidation re-action at the anode and the reduction reac-tion at the cathode. In order to produce hy-drogen, water – the most abundant sourceof hydrogen on earth – can be electrochem-ically reduced at the cathode to make H

2.

Carbon-based fuels can be formed eitherby directly reducing CO

2at the cathode or

indirectly: by using the hydrogen producedin a water electrolysis cell to drive the re-verse water gas shift and Fischer-Tropschprocesses separately (to produce CO fromCO

2and subsequently reduce it to form

CnH

(2n+2)compounds with hydrogen). In

both cases, the corresponding oxidationreaction is most conveniently water oxida-tion to produce molecular oxygen at theanode. Indeed, due to the abundance of wa-ter and its relatively low free Gibbs energyof dissociation (237.1 kJ corresponding toan oxidation potential of 1.229 V at stan-dard conditions according to Nernst equa-tion), water is the most obvious candidateto provide electrons (i.e. to be oxidized) inthe electrochemical cell.

While H2and O

2production via water

splitting is an attractive route for storing re-newable energy, a hydrogen-based energyeconomy has been sluggish to be adoptedby society as, in part, the price of the hy-drogen produced via renewable electric-ity electrolysis is 5–10 times greater than

Fig. 1. Plots for load (top left), wind (top right), solar (bottom left) and wave (bottom right) powermeasured or simulated in the North Pacific region (U.S.) through the year 2008. © 2011 IEEE.Reprinted, with permission, from Sustainable Energy, IEEE Trans. 2011, 2, 321–328.


fer between the chemical species and theelectrodes requires overcoming an energybarrier that depends strongly on the cata-lytic properties of the electrode materials.Usually, the anodic half-reaction requires amuch higher activation overpotential thanthe cathodic half-reaction due to the com-plex multi-electron transfer route requiredto form molecular O

2.[11,14] The next sec-

tions will thus accordingly focus on thecatalytic properties of materials developedas water splitting electrodes, with particu-lar emphasis on those used for the most en-ergy demanding reaction, water oxidation.

3. Electrode Materials for AlkalineElectrolysis

The requirements to select electrodematerials for alkaline water electrolysis in-clude good corrosion resistance, high elec-tronic conductivity and high catalytic activ-ity with regard to the two reactions of inter-est (the hydrogen evolution reaction, HER,at the cathode and the oxygen evolution re-action, OER, at the anode). As mentionedbefore, the overall water splitting reactionis mainly limited by the slow kinetics ofthe OER at the anode.[11,14] Therefore tre-mendous research efforts have been dedi-cated to the search of a low-cost and effi-cient electrocatalyst for oxygen evolution.

Stainless steel and lead oxide were firstidentified as inexpensive electrode materi-als, with relatively lowoverpotential for theOER, but their chemical stability at suffi-ciently high voltage in highly concentratedalkaline solutions limit their applicability.Nowadays, IrO

2and RuO

2are considered

the benchmark materials for OER in alka-line conditions, presenting overpotentialsof 0.32 and 0.29 V respectively for a cur-rent density of 10 mA cm–2.[15,16] The ac-tivity of iridium-based electrodes towards

on the cathode, resulting in a negative mea-sured current (i.e. electrons moving fromthe electrode into the electrolyte). Thiscurrent increases in magnitude when shift-ing the potential cathodically as the drivingforce for reduction is enhanced. We definehere the overpotential for reduction, η

red,

as the difference of potential between thatwhich is applied and the thermodynamicreduction potential of water to reach acertain current density (e.g. –10 mA cm–2

in our example in Fig. 2c). Analogously,when probing the anode as the workingelectrode, a positive current can onset oncethe potential exceeds the thermodynamicpotential of water oxidation (1.229 V vsRHE), when water molecules or hydroxylanions are converted to O

2on the anode

material. The overpotential for water oxi-dation, η

ox, is therefore defined as the dif-

ference of between the potential appliedand the potential of water oxidation, toreach a certain current density (+10 mAcm–2 in our example).

The overall voltage required, shown inFig. 2c, corresponds to the voltage neededto operate an electrolyzer with a certaincurrent density (e.g. 10 mA cm–2) ignoringthe ohmic losses in the electrolyte or mem-brane. It equals the sum of the reversiblepotential for water reduction and oxidation(1.229V) and the overpotentials for the re-duction and oxidation reactions. This over-all voltage must be minimized in order toincrease the energy conversion efficiency,consequently decreasing the price of thestorage.While additional overpotential canarise from ohmic losses due to the electro-lyzer geometry, bubble formation, or ionconduction in the membrane and in theelectrolyte, these can be reduced throughcell engineering. Indeed the most signifi-cant losses come from the overpotentialsrequired to kinetically activate the elec-trochemical reactions. The charge trans-

electrons from the electric circuit, resultingin the formation of hydrogen gas. Thesereactions are summarized in Eqn. (3) andEqn. (4):

PEM / Cathode:4H+ + 4e– → 2H

2(g)

(3)

PEM / Anode:2H

2O → O

2(g)+ 4H+ + 4e– (4)

The same overall reaction occurs inboth systems, which corresponds to thewater dissociation reaction:

2H2O → O

2+ 2H

2(g)(5)

Typical electrochemical characteriza-tion (shown in Fig. 2c) of water splittinganodes (red) and cathodes (blue) is usuallyperformed in a three-electrode (potentio-static) setup, to probe specifically one ofthe electrodes for research development.The potential of the working electrode(the electrode under test) is varied againsta reference electrode, whose potential isfixed in the electrolyte. The current gen-erated by the reaction occurring on theworking electrode is transferred to a thirdelectrode (the counter electrode) whichadapts its potential according to the cur-rent and the resistivity of the electrolyte.This method enables isolating the processat the working electrode and disregards thelosses related to the reaction occurring onthe counter electrode.

When the applied potential reaches acertain potential below the reduction po-tential of water, i.e. 0 V vs. the reversiblehydrogen reference electrode (RHE), wa-ter molecules or protons can start to react

Fig. 2. Scheme of conventional electrolyzers: a) Alkaline and b) PEM based, which operate in basic and acidic pH respectively. c) Typical electro-chemical characterization, shown as the current density running through the electrode versus the applied potential against the reversible hydrogenreference electrode (RHE), for an electrode working as an anode (red) and a cathode (blue).


magnetic susceptibility inside the materialinduced by an external magnetizing field(H). Fig. 4 shows the magnetic response ofthree NiFe electrodes, electrodeposited ata constant current density of either 25 mAcm–2 or 250 mA cm–2 in presence of ammo-nium sulfate (sample A and C respective-ly), as well as a reference sample depositedat 105 mA cm–2 without (NH

4)2SO

4. The

good performance of the electrodepositedsamples with the ammonium salt havebeen correlated to the ferro/ferrimagne-tism exhibited by sample A and C at 300K. In contrast, sample B shows paramag-netic behavior, reflecting the chemical andphysical complexity of the catalyst and theslight changes in crystal organization de-pending on the deposition parameters.

Theoretical modeling and experimentshave also evidenced a transition throughhigh spin states in the CaMn

3O

4catalytic

cluster of Photosystem II during water ox-idation.[28,29] Oxygen-activating enzymeshave also been characterized with a high-spin heme iron, non-heme iron and coppercatalytic sites.[30,31] Therefore, the correla-tion between the ferromagnetic characterand suitable catalytic properties in NiFeelectrodes suggest strongly that the highspin states in the catalyst facilitate theelectron spin inversion required for oxygenevolution. Further investigations are nev-ertheless necessary to characterize the de-sired properties for the catalyst and to fullyunderstand the O

2formation mechanism.

The considerable catalytic perfor-mance of nickel iron alloys for the OERreaction renewed the interest in inexpen-sive and large-scale produced materials

worth noting that all the aforementionedmaterials succeed in showing good stabili-ty in performance over long term measure-ments (24 h), except for NiCo and NiFeCo.

From this report, nickel-, iron- and co-balt-based electrodes, especially the onescontaining two or three different metals,seem to have the best catalytic propertiestowards OER in alkaline solution. This wascorroborated by recent studies of iron-nick-el alloys that showed considerable catalyticperformance. Maximum performance wasobtained for films or particles containingbetween 30 and 40% Fe, achieving 10 mAcm–2 at overpotentials below 300 mV.[23,24]The iron incorporation inside the nickeloxide/hydroxide layer has been evidencedto modify the environment of the Ni–Obond, reducing the electrochemical oxida-tion of Ni(OH)

2to NiOOH,[24] and activat-

ing Ni catalytic centers (probably Ni2+)throughout the catalytic film.[25] Theseresults are also consistent with the betterperformance of Ni films after aging, whichindicates that previous reports of highly ac-tive Ni(OH)

2-based OER catalysts include

Fe impurities.[24,25] Using this material, Luet al. have recently obtained the highestcatalytic activity for non-noble metal elec-trodes, when electrodeposited NiFe onto anickel foam mesostructure, with a recordcurrent density of 500 mA cm–2 at an over-potential of only 240 mV in 10 MKOH.[26]

Another study of nickel-iron electrodesfor oxygen evolution evidenced an addi-tional feature of highly active materialsfor this application using SQUID magne-tometer measurements.[27] This techniqueallows the detection of extremely subtle

oxygen evolution has been shown to beindependent of pH, widening the potentialapplication of this material.[17] Neverthe-less, these precious metals do not show un-limited stability in alkaline solutions,[18] arecostly, and their supply is not sustainable.Therefore, they are not suitable for large-scale TW applications. In order to find alow-cost OER electrocatalyst, researchersare now concentrating their efforts on first-row transition metals and their composites,for instance cobalt phosphate composites,nickel borate, cobalt oxide nanoparticlesand manganese oxide thin films.[19–22] Allthese materials offer satisfying OER activ-ity, with overpotentials lower than 0.4 Vfor a current density of 10 mA cm–2, andsignificantly lower fabrication costs whencompared to ruthenium- or iridium-basedmaterials.

Recently, Jaramillo and coworkers test-ed a large range of first row transition met-als for the OER in both alkaline and acidicconditions.[15,16] All electrode materialswere electrodeposited on glassy carbon(GC) substrates using a similar protocolfor each of them, in order to benchmark theassessment and the performance of anodematerials for water oxidation. Due to thehighly oxidizing conditions experiencedduring OER, the electrocatalysts are likelyconverted to oxides or oxyhydroxides butthey are named according to their metalcomposition for simplicity. Comparisonof the electrode activities in 1 M NaOHis shown on Fig. 3. The magnitude of theoverpotential required to achieve a cur-rent density of 10 mA cm–2 after 2 hoursof operation is shown on the y-axis whilethe same overpotential recorded immedi-ately after immersion in the electrode isshown on x-axis. This type of plot givesvaluable information on both electrodeperformance and short-term stability, i.e. ifthere is no change in activity, the materialis represented on the black dotted 45° line.

Except for ruthenium, most catalyststested in this study can achieve the chosencurrent density at an overpotential between0.35 and 0.5 V, which is slightly higherthan the target overpotential selected in thisstudy (0.35 V, represented by the dashedblue lines in Fig. 3). However, one can no-tice the good performance of NiMoFe, theonly non-noble metal catalyst able to func-tion at a potential below the target, duringinitial test and after two hours of operation.This result is even more encouraging tak-ing in account that the active area (real area× roughness factor) was smaller than theone measured for the ruthenium referenceelectrode, which means that the specificcatalytic activity (activity normalized tothe active area) was actually higher. Otherpromising materials that demonstratedoverpotentials lower than 0.4 V includeCo-P, CoFe, NiCo, NiFe and NiFeCo. It is

Fig. 3. OER catalytic activity of electrodeposited materials after 2 h ofoperation is shown against the catalytic activity measured immediatelyafter immersion in alkaline solution. The dotted line represents the idealstability with no change in activity during the 2 first hours of operation andthe color scale is indicative of the thin film roughness (pale green to blackincreasing roughness). Reprinted with permission from J. Am. Chem. Soc.2015, 137, 4347–4357. Copyright 2015 American Chemical Society.


such as nickel containing stainless steel:AISI 304 (8% Ni, 18–20% Cr), AISI 316(10% Ni, 18% Cr) and AISI 316L (sameas 316, low in carbon). Two recent stud-ies[32,33] show that steel samples require apretreatment to form an oxide layer at thesurface, which acts as the catalyst. Electro-oxidation of anAISI 304 metal alloy underparticularly harsh conditions (at a currentdensity of ~1.8 A cm–2 in 7.2 M NaOH for300 min), results in the formation of anultrathin film, depleted of Cr on the sur-face and composed of 67% Ni / 33% Fe.Catalytic performance of such film (namedElox300) is compared to untreated AISI304 alloy in Fig. 5. The surface-modifiedmetal sample exhibits remarkable currentvoltage characteristics, achieving 10 mAcm–2 at an overpotential of 270 mV in 0.1M KOH, and 12 mA cm–2 at only 212 mVin 1 M KOH.[32] These surface-oxidizedsteel samples proved to be inert. More-over, X-ray photoelectron spectroscopy(XPS) performed on this film suggest thatγ-NiOOH constitutes the catalytic activespecies on the surface of the electrode,consistent with the negative current waveobserved at potential 1.2–1.42 V vs. RHE(Fig. 5), which is attributed to the Ni(iii)/Ni(ii) redox couple.

Nickel enrichment of the steel surfacewas also obtained upon aging a film ofAISI 316L in 5 M LiOH electrolyte at 0.8V vs. Hg/HgO reference electrode (ca. 1.7V vs. RHE) for 250 h.[33] The metallic sur-face composition was found to be 83% Ni,10% Fe and 7% Cr and Ni(OH)

2, NiOOH,

FeOOH as well as Cr2O

3were detected in

the oxide layer with XPS. Such film showshigh electrocatalytic activity towards OERafter an induction time of 250 h and stableperformances over 3000 h of operation.

The increase in OER activity for theaged and pretreated steel films have beenrationalized with the increase in rough-ness and a synergetic effect (hypo-hyper

d-orbital interbonding effect) due to thepresence of dispersed Fe and/or Cr in theoxide layer. These recent results, espe-cially those concerning the stability of thesamples, hold significant promise for high-performance OER catalyst in the futureand should be further studied to uncoverthe full potential of low cost steel elec-trodes.

In addition to the development ofconventional alkaline electrolysis cellsoperating at temperatures below 100 °C,researchers are also exploring high-temperature electrolysis (keeping concen-trated KOH as the electrolyte). Increasingthe operating temperature over 100 °Cmodifies the thermodynamics of thesystem (reducing the potential required)and enhances electrode performance byaccelerating kinetic processes. Particularly,ionic conductivities of NaOH and KOHsolutions are increased significantly whenthe temperature reaches 200–250 °C.[34]For the electrolyte to remain in the liquid

phase, it is necessary to apply high pres-sure (30–100 bars)[35] or to immobilizethe electrolyte in porous material such asstrontium titanate (SrTiO

3).[36] Moreover,

the relative volume of the formed hydrogenor oxygen gas bubbles is also lowered withincreasing pressure, which subsequentlydecreases losses in the catalytic area of thecell (ohmic losses).[37] The effect of tem-perature has been shown to be more pro-nounced on the oxygen evolution reactionthan the hydrogen evolution reaction: for acurrent density of 0.25 A cm–2 applied onpolished Ni electrodes, the anodic overpo-tential decreases from 0.53 V to less than0.05 V, whereas the cathodic was reducedfrom –0.43 to –0.19 V by increasing thetemperature from 80 to 264 °C.[38] In termsof cathode materials, nickel alloys (Ni-Ti,Ni-Co and Ni-Mo) have demonstratedslightly better performance than pure nick-el at high temperature.[35,39,40] For anodes,spinel and perovskites structured materi-als, such as Co

3O

4, NiCo

2O

4, LaNiO

3and

La0.5Ni

0.5CoO

3were shown to provide

the lowest overpotential as compared topure nickel electrodes.[35,41,42] However,the increased thermal degradation at hightemperature may be problematic for large-scale application.

4. Electrode Materials for PEMElectrolysis

Historically, alkaline electrolyzers havebeen the first widely developed devices forwater electrolysis and still make up mostof the electrolyzers that can be found onthe market nowadays.As discussed before,the design of such cells, see Fig. 2, offersseveral advantages, including an inexpen-sive microporous ceramic diaphragm andelectrodes that can be made of relativelysimple and cheap materials such as nickel,

Fig. 4. SQUID fieldsweep of NiFe oxideelectrodeposited indifferent conditions:A, at 25 mA cm–2 with25 mM (NH4)2SO4,B at 105 mA cm–2

without any salt andC, 250 mA cm–2 with25 mM (NH4)2SO4.The inset provides abetter perspective ofthe scale of sampleA’s magnetization.Reprinted with per-mission from J. Phys.Chem. C 2008, 112,3655–3666. Copyright2008 AmericanChemical Society.

Fig. 5. Cyclicvoltamograms of AISI304 alloy and pre-treated AISI 304 sam-ple (electroxidized at1.8 A cm–2 in 7.2 MNaOH for 300 min).The recorded cur-rent density is shownagainst the appliedpotential with respectto RHE (bottom) andthe overpotential (topaxis). Reproducedfrom Energy Environ.Sci. 2015, doi:10.1039/C5EE01601Kwith permission ofThe Royal Society ofChemistry.


laboratories have been and are still lookingfor alternative catalytic materials able tocompletely replace the current generationof PGM catalysts. In the next paragraph,we will therefore present the progressmade over the last four decades in cata-lyst optimization for the HER and OER inacidic conditions.

The highly acidic environment createdby the membrane is favorable to HER ca-talysis, since it is typically easier to reduceprotons to hydrogen when they are pres-ent at high concentration. Platinum is wellknown for being an extremely efficientcatalyst for water reduction, with an over-potential (η

red) lower than 0.05 V to pro-

duce a current density of 10 mA cm–2 in1MH

2SO

4. In fact, to date, it is still the best

monoatomic catalyst available for HER.As such, it has been traditionally used atthe cathode of PEM electrolyzers devel-oped at the laboratory scale, but its scar-city prevents a realistic use on a large scale.Still, its performance is usually used as thebenchmark that alternative cheaper materi-als should approach. In addition to its pro-hibitive cost, platinum suffers from beingvery easily poisoned by trace amounts ofmetallic contamination in the feed water.Metallic ions will indeed undergo under-potential deposition (UPD) and complete-ly cover the surface of the platinum, seri-ously decreasing its catalytic performance.For these reasons, a considerable amountof work has been dedicated to designingand optimizing robust earth-abundant elec-trocatalysts able to compete with the effi-ciency of platinum. Typically non-noblemetal catalysts are constructed around Fe,Ni, Cu, Co, Mo and W, which are ordersof magnitude more abundant than Pt inthe Earth’s crust, with Fe and Ni being themost abundant among them. Since none ofthese elements is a good HER catalyst byitself, it is necessary to combine them withothers to produce competitive materials.Recently, molybdenum disulfide (MoS

2)

has received a lot of attention. For a longtime, it was considered to be completelyinactive towards the HER. While this istrue for bulk crystalline MoS

2, it was pre-

dicted in 2005[53] and shown in 2007[54]that MoS

2was actually very active under

its nanocrystalline morphology. More spe-cifically, the edge sites of MoS

2nanoflakes

have been demonstrated to be responsibleto this catalytic behavior. Since then, MoS

2has been extensively studied, and has beenreported with very good performances inacidic electrolyte: a current of 10 mA cm–2

was obtained for ηred

<0.2 V with chemi-cally exfoliated MoS

2.[55,56] Interestingly,

amorphous molybdenum sulfide, MoSx,

also produces very good catalytic wa-ter reduction, with a reported current of15 mA cm–2 at η

red= 0.2 V.[57] In another

work, a nickel-molybdenum nitride cata-

ated with gas mixing. Moreover, it allowsthe electrolyzer to function under a widerange of partial load and even potentiallyunder overload. This makes the systemvery dynamic as it can adapt to large in-put current variations. On the other hand,PEM electrolyzers still present one bigdrawback compared to the alkaline alterna-tive: they are much more expensive due tothe very acidic operation regime imposedby the membrane. Indeed this prevents theuse of most metals – which would quicklycorrode during operation, especially underthe oxidative conditions at the anode – andforces the use of more expensive materi-als for the electrodes (typically titanium-based alloys) and the electrocatalysts, aswill be developed further in this section.

Despite its higher cost, the PEM elec-trolyzer design is very attractive when itcomes to running the device with a renew-able source of electricity. Indeed, the in-termittency of wind and, especially, solarenergy makes them very hard to use effi-ciently with an alkaline electrolyzer thathas to be shut down under 40% partial loadfor safety reasons.[12] On the other hand,PEM electrolysis can be performed underalmost any fraction of the nominal load(although the range does decrease with in-creasing pressure).[47] This ability, coupledwith the very quick response of electro-chemical reactions and proton transportto changes in current density (contrary tothe much more inert ion transport in liq-uid electrolytes), makes the PEM electro-lyzer a very attractive candidate for therenewable production of highly pure andalready-compressed hydrogen gas (e.g. ina PV + electrolyzer configuration).

The first report of a PEM electrolyzergoes back to 1973 and already producedmuch higher current densities than state-of-the-art alkaline electrolyzers (1 A cm–2

for an applied voltage of 1.88 V) that werereported to be stable for over 15000 h.[12]Unfortunately, due to the previously men-tioned tendency of most metals to corrodein acidic conditions, the authors had to useplatinum black as the HER catalyst andiridium as the OER catalyst. Since thisfirst report, the very high cost of catalystsbased on the platinum groupmetals (PGM)has remained one of the biggest obstaclesto the development of cost-efficient PEMelectrolyzers. Indeed, even in more mod-ern commercial electrolyzers, PGM cata-lysts still represent a significant portion ofthe overall material cost,[52] despite theirrelatively low loading: ca. 2 mg cm–2 forthe Pt at the cathode, and ca. 6 mg cm–2

of Ir at the anode. Moreover, and perhapsmore importantly, beyond the cost of thesePGMs, their scarcity (especially for Ir)completely prevents their use on the glob-al scale associated with a hydrogen-basedeconomy. This is the reason why research

stainless steel or other first row transitionmetal oxides.[41,43,44]

However, this technology presents atleast three major drawbacks.[12,43] (1) Theoutput current density is relatively low(typically 0.2–0.4 A.cm–2). This is mostlydue to large ohmic losses across the elec-trolyte and thick diaphragm (typically be-tween 500 nm and 1 μm to efficiently pre-vent gas diffusion). (2) It is impossible tooperate at high pressure. This prevents thecreation of a compact system, and forcesthe post-compression of hydrogen for stor-age. (3) It has a very low partial-load range,because under low load (usually <40% ofthe nominal load), the oxygen produc-tion rate is low enough that the smallamount of hydrogen diffusing throughthe diaphragm (independent of the load)can create an explosive mixture (>4% H

2in O

2)[45] in the system. This makes alka-

line electrolyzers less suitable for the dy-namic requirements of storing the highlyvariable renewable energy.

To address these three issues, a differ-ent design has been proposed as a promis-ing alternative: the proton-exchange mem-brane (PEM) electrolyzer. The configu-ration of a PEM electrolyzer is depictedin Fig. 2. In this design, water circulatesthrough the anode, while the hydrogen isproduced in a dry environment at the cath-ode. Similarly to the alkaline design, theanode and cathode are coated with electro-catalysts for the OER and HER respective-ly. Nevertheless, a major difference withalkaline electrolysis lies in the presenceof a solid electrolyte: a very thin (20–300nm) humidified acidic membrane (typi-cally Nafion™),[46,47] transports protonsfrom the anode to the cathode, where theydirectly recombine with electrons fromthe external circuit to yield gaseous hy-drogen. This proton-exchange membraneis also responsible for the acidic environ-ment in which the electrodes are requiredto function. PEM electrolyzers possessseveral advantages compared to their al-kaline counterparts. First they can operateat much higher current densities (typicallyhigher than 2 A cm–2),[48,49] which allowsoperational cost reduction. This improve-ment comes from themuch lower thicknessand much higher conductivity of the PEMcompared to the alkaline diaphragm,whichin turn reduces ohmic losses between theelectrodes. Second, it can operate undermuch higher pressures (up to 350 bar),[52]and even under differential pressure, whereonly the cathode size is pressurized to ex-tract hydrogen without producing danger-ous pressurized oxygen on the anode side.Finally, and perhaps most importantly, thePEM has an extremely low permeabil-ity to hydrogen.[50,51] This ensures a veryhigh purity of the output hydrogen stream,while removing the safety issues associ-


selective membranes or catalysts may belimiting large-scale application. More-over, impurities in the water feedstockcan dramatically decrease the lifetime ofelectrochemical cells. As such, water split-ting electrocatalysts have been studied inneutral pH conditions. Neutral pH mediaare promising systems since they are lesscorrosive toward active materials (for ex-ample catalysts) and other system com-ponents (for example: piping).[70] How-ever, no efficient and safe electrolysis ina sustained pH-neutral environment hasbeen yet developed. Therefore, taking ad-vantage of the large amount of water onearth, it is crucial to study systems that aresustainable and do not require expensiveequipment for water electrolysis. Most ofthe time, depending on the source, seawa-ter contains many species such as Na+, Cl–,Mg2+, Ca2+, K+, SO

42–, Br+, CO

32–. The latter

chemicals might interfere with the waterreduction (hydrogen evolution) and/or wa-ter oxidation (oxygen evolution).

In seawater, the large numbers of chlo-rine ions can be preferably oxidized (in-stead of H

2O oxidation) because of the

lower overpotential for chlorine evolution.Two main reactions can occur: chlorineevolution reaction (Eqn. (6)) and wateroxidation reaction (Eqn. (7)):

2Cl– → Cl2(g)

+ 2e– (6)

2H2O → 4H+ + O

2(g)+ 4e– (7)

cades, no alternative metal or metal oxidehas been found to come anywhere close toiridium, or ruthenium, in performances, asillustrated in Fig. 6.

Overall, PEM electrolysis seems to bethe most promising short term option forthe large-scale production of high-purityhydrogen, especially in the context of thedevelopment of renewable energy sourc-es. Indeed, the PEM technology is betteradapted to deal with the intermittent cur-rent typically provided by these sources,as it can very quickly adapt to any changein input current, without creating explo-sion hazards in the system. However, thematerial cost of PEM devices remainshigh, and electrocatalyst coatings—al-though they represent a tiny fraction inmass—still represent a significant fractionof the overall cost. More precisely, whilecheap HER catalysts have been developedin recent year, and seem very likely to beintroduced in industrially produced elec-trolyzers, there is still no viable replace-ment for iridium-based electrocatalysts atthe anode. Finding an earth-abundant OEC(Oxygen Evolution Catalyst) able to oper-ate in acidic conditions would acceleratesignificantly the development of the waterelectrolysis technology, and as such, re-mains a critical field of research.

5. Water Splitting in OtherConditions: Sea Water / Neutral pH

As previously discussed, extreme pHand high purity water systems are efficientfor water electrolysis but the cost of suchsystems with, for example expensive ion-

lyst was reported to produce ca. 4.5 mAcm–2 at η

red= 0.2 V.[58] Nickel phosphide

(Ni2P) and cobalt phosphide (CoP) have

also been recently reported with excellentcatalytic abilities. Nanostructured Ni

2P

was measured to produce 20 mA cm–2 atη

red= 0.13 V,[59] and was measured, by a

different group, to be stable for more thantwo days.[60] On the other hand CoP hasbeen reported to produce 10 mA cm–2 atη

red= 85 mV.[61] Other efficient catalysts

reported in the recent literature are listed ina review from 2015 by Zou and Zhang.[62]A more direct and accurate comparison ofthe performances of different HER cata-lysts in acidic conditions has been recentlypublished by Jaramillo and coworkers(Fig. 6).[15] In this study they benchmarkthe currents produced by several materi-als under identical conditions. Unsurpris-ingly, platinum remains the best HER cata-lyst, but interestingly, several alloys, suchas NiMo, CoMo, NiCoMo or NiW comevery close in performance, and also displayvery good stability. Overall, a variety ofvery good alternative materials have beenproposed to replace Pt as HER catalystin acidic conditions. Moreover, given thescalable deposition of most of these ma-terials, it seems like they could be easilyapplied on an industrial scale.

Unfortunately, replacing PGM as cata-lyst for OER is a much tougher challenge.Indeed, as mentioned before, most metalsare easily corroded under oxidative con-ditions in acid. Since the oxidation of themetal is easier than the oxidation of water,the catalyst oxidizes and since most metalcations are soluble in acid the electrode dis-solves into the electrolyte. State-of-the-artOER acid catalysis has been achieved withiridium or iridium oxide since the very ear-ly work on PEM electrolysis. Despite thelarge amount of work produced in the fieldsince these first reports, and contrary to thesuccess encountered with earth-abundantHER catalysts, no viable replacement hasbeen found so far. Iridium being the rar-est element in the Earth’s crust, replacingit with any other element would alreadyrepresent progress. Ruthenium has beenfound to be more active than iridium, but ithas also been found to corrode after sometime.[63,64] On the hand, Ir-Ru binary mix-tures show very good catalytic propertiesand stability,[65,66] for an Ir content as lowas 20 mol%. However, ruthenium is almostas scarce as iridium and does not reallyrepresent a viable replacement for large-scale applications. Other works reportedthat diluting Ir in even more abundant ele-ments, such as Nb,[67] or Sb[68] was also aviable approach to reduce Ir content in thecatalyst. An interesting study described amixture of SnO

2, Ta

2O

5and IrO

2that per-

formed efficiently even at low Ir content(15%).[69] Unfortunately, in the past de-

Fig. 6. OER catalytic activity of electrodeposited materials after 2 h of op-eration is shown against the catalytic activity measured immediately afterimmersion in sulphuric acid solution. The dotted line represents the idealstability with no change in activity during the 2 first hours of operation andthe different colors are indicative of the thin film roughness (pale green toblack increasing roughness). Reprinted with permission from J. Am. Chem.Soc. 2015, 137, 4347–4357. Copyright 2015 American Chemical Society.


onstrated by Fujishima and Honda in 1972using TiO

2in an aqueous electrolyte at pH

4.7.[80] TiO2offers excellent stability under

the harsh conditions of water spitting butsuffers from its semiconductor band gapenergy (3.2 eV) that limits the maximumsolar-to-hydrogen (STH) conversion ef-ficiency to less than 2%. Tremendous ef-forts have been deployed to find a materialwith high light absorption (low band gap),and electronic bands that straddle the waterredox couples (band gap should be about1.7–1.8 eV to account for overpotentials).Unfortunately, this ideal material has notbeen developed yet.

This drawback can be addressed usingtwo semiconductors in tandem to generatesufficient energyandalsooptimize the frac-tion of solar energy collected. A schemerepresenting the operational principle of atandem cell using a n-type semiconductoras a photoanode and a p-type semiconduc-tor as a photocathode is shown in Fig. 7a.The shorter wavelengths photons are ab-sorbed in the first material (photoanode inour example), generating a hole that canoxidize water. Longer wavelengths, whichhave not been absorbed, are transmittedto the second electrode (photocathode),where the photogenerated electrons canreduce water. Both majority carriers (elec-

6. Photoelectrochemical WaterSplitting

A simpler approach to convert renew-able energy into a transportable fuel is todirectly interface a semiconductor able toabsorb light with an electrolyte to performthe water photolysis. This route couldpotentially reduce costs of building twoseparate devices and eliminate the lossesrelated to the electricity transport betweenwhere the renewable energy is harvestedand the electrolyzer, where it is convertedinto a fuel.[79]

Absorption of a photon in a semicon-ductor will promote an electron from thevalence band to the conduction band, gen-erating subsequently an electron-hole (orhole) in the valence band. These chargecarriers can be separated spatially by thespace charge field (an electric field pro-duced by the equilibration of the semicon-ductor Fermi level and the electrolyte uponcontact). With appropriate semiconductorband-edge positions (i.e. with a valenceband lower in energy than the water oxi-dation potential and a conduction higherthan the water reduction potential), the freephotogenerated hole is then able to oxidizewater while the electron can reduce wateron the other side. This has been first dem-

One approach to overcome the chlorineevolution is to insert a selective ion mem-brane in the electrochemical cell. The useof a cation-selective membrane was shownto be very beneficial in enhancing the oxy-gen evolution at the IrO

2/Ti electrode as

an anode.[71] The surface of the latter elec-trode was modified by a perm-selectivepolymer (Nafion), which hinders chlorideion transport to the electrode surface and,as a result, suppresses chlorine evolutionand increases oxygen evolution reaction bya factor of 2.Another study on the effect ofions contained in seawater was reported re-cently.[72] The hydrogen evolution rate wasobserved to vary depending on the natureof ionic species. Especially in the presenceof MgCl

2species, the hydrogen evolution

rate was drastically lowered. For example,to overcome the problem, magnesium ioncan be suppressed from seawater by poly-electrolyte multilayer membrane so thathydrogen evolution rate could be expectedto increase.[73]But the use of such addition-al membrane may increase the overall costof production of water splitting. Anotherstudy focused on manganese-tungsten ox-ide for iridium oxide electrodes in a 0.5 MNaCl at pH 8.0.[74]

Studies of oxygen-evolving catalysts inneutral and natural waters are important tolower the cost of hydrogen-based energystorage.[75] Research of oxygen-evolvingcatalysts has been recently focused on ac-tive catalysts under neutral conditions. Afacile method for deposition of a cobaltoxygen-evolving catalyst (Co-OEC) hasbeen studied under pH 7. The Co-OEC hasshown high current density (100 mA cm–2

at an overpotential of 442 mV) in near neu-tral conditions (pH 9.2)[22] and in naturalwaters and seawater.[75]Modest overpoten-tials at 1 mA cm–2 were observed with theCo-OEC compared to a standard Ni elec-trode in natural water and seawater. Noc-era and co-workers also investigated analternative to the Co-OEC by developinga nickel-borate oxygen-evolving catalystthat is stable under near neutral conditions(pH 9.2). Ni-based oxide films evolves ox-ygen with a current density of 1 mA cm–2

at an overpotential of 425 mV.[76] Anotherstudy showed an alternative with a sil-ver oxygen-evolution catalyst (Ag-OEC)reaching an overpotential of 318 mV at 1mA cm–2.[77] More recently, a Janus cobalt-based catalyst was developed for watersplitting in neutral conditions. It is the firsttime non-noble metal catalysts have beendeveloped for both reduction and oxida-tion of water. The modest overpotentialsachieved for both oxygen and hydrogenevolution are promising for photocatalystssystems and for lowering the cost of elec-trochemical systems.[78]

Fig. 7. a) Electron en-ergy scheme of PECwater splitting usinga dual-absorber tan-dem cell. Two pho-tons, one absorbed inthe photoanode, onein the photocatode,are used to build thenecessary potentialto dissociate water(Reprinted from J.Phys. Chem. C 2013,117, 17879–17893.Copyright 2013 theAmerican ChemicalSociety). b) Typicalcurrent-voltagecharacterization of aphotoanode (red) anda photocathode (blue)under illumination.The current is shownagainst the potentialapplied to the elec-trode versus the RHEreference electrode.The tandem cell us-ing the electrodesdescribed in red andblue will operate atthe current set bythe crossing of theanodic photocurrent(red) and the inverseof the cathodic pho-tocurrent (broken blueline).


can now be achieved at less than 300 mVoverpotential using inexpensive materialslike steel or NiFe. Unfortunately alkalineelectrolyzers are poorly suitable for the dy-namic requirements of storing the highlyvariable renewable energy due to the lowpartial-load range. In contrast, PEM elec-trolyzer do not suffer from dynamic loadlimitations or limited maximum currentdensities as the proton exchange mem-brane is a better gas diffusion blocker. Thehighly acidic environment engendered bythe membrane helps the facile catalysisof HER but limits the choice of materialfor the OER catalysis to expensive andrare metals such as Ir or Ru. This is themain disadvantage for this technology andhindered its expansion on the market. Theidentification of an inexpensive and stablewater oxidation catalyst for use in PEMelectrolyzers would be a major break-through for these devices.

As an alternative to conventional elec-trolyzers, research is also focusing ondeveloping systems able to function innatural water – mostly at neutral pH – andseawater, but demonstrated performancesremain quite low in this area. Another di-rection explored by researchers is the directconversion of solar energy into a transport-able fuel, through photoelectrochemicalwater splitting. Despite recent progress inthe domain, these systems still suffer frompoor efficiency and/or poor stability thathinders a large scale application. Furtherefforts are still needed in order for semi-conductor-based devices to compete withalready commercialized technologies.


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[2] IECWhite Paper, 2015.[3] P. V. Kamat, J. Phys. Chem. C 2007, 111, 2834.[4] N. S. Lewis, D. G. Nocera, Proc. Natl. Acad.

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[7] D. A. Halamay, T. K. A. Brekken, A. Simmons,S. McArthur, IEEE T. Sustain. Energ. 2011, 2,321.

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[9] L. Bertuccioli, A. Chan, D. Hart, F. Lehner, B.Madden, E. Standen, in ‘Development of WaterElectrolysis in the European Union’, reporton behalf of ‘Fuel Cells and Hydrogen JointUndertaking’, www.fch.ju.eu, 2014.

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inum counter electrode but performanceswere limited by the late photocurrent onsetof the photoanode.[88] BiVO

4has also been

used in a tandem cell, in combination witha Cu

2O photocathode[89] or with a Si solar

cell.[90] However, these studied evidencedroutes for improvement as they showedinstability due to detachment of the CoPicatalyst and restricted current originatedfrom bismuth vanadate limited absorptionof the solar spectrum.

In terms of photocathode materials, re-cent studies demonstrated the potential ofusing Cu

2O, which showed relative stabil-

ity and current densities of up to –7.6 mAcm–2 at 0 V vs. RHE.[91] Si, GaP or GaAsare other materials vigorously developedin the field but suffer from poor stability inwater and require careful protection withoverlayers.[92] p-type GaInP

2has also been

investigated for water photoreduction andreached a record STH efficiency of 12.4%in a stand-alone monolithic device, exhib-iting however rapid degradation.[93]

Overall while the development of an in-tegrated PEC water splitting device offersa simple way to directly convert our mostabundant renewable energy source directlyto a chemical storage vector, or solar fuel,considerable efforts are needed to increasethe device stability and decrease the cost inorder to be economically competitive withtraditional solar-to-hydrogen conversionmethods (i.e. PV + electrolysis).

7. Conclusion and Outlook

Efficient energy storage is required toenable a global energy economy basedon renewable energies. For this purpose,chemical storage offers attractive possi-bilities as it can store the electricity pro-duced from any kind of renewable energyinto the bonds of chemical compounds,which themselves can be stored indefi-nitely and/or transported. Hydrogen andsimple carbon-containing compounds(e.g. formic acid, methane, and methanol)are examples of promising energy carriersthat can be formed from electricity throughelectrolysis.

Today, two major technologies lead themarket of electrolyzers: the Alkaline andProton Exchange Membrane types. Al-kaline electrolyzers operate in very basicconditions, which offer several advantag-es, including an inexpensive microporousceramic diaphragm and electrodes that canbe made of relatively simple and cheapmaterials. Its main drawbacks are the lowcurrent density achievable, mostly due tothe oxygen diffusion from the anode tothe cathode, and significant voltage lossesoriginated from poor oxygen evolutioncatalysis on the anode. Recent progresshowever showed that reasonable current

tron in a n-type photoanode and holes in ap-type photocathode) drift into the bulk ofthe semiconductor and recombine in orderto close the electric circuit.

Specific performances of a typical pho-toanode and photocathode are shown inFig. 7b, in red and blue respectively. Theonset of the photocurrent appears belowthe energy of the respective redox couple(at potentials cathodic to +1.229 V vs.RHE for the photoanode and anodic to 0Vvs. RHE for the photocathode), as the reac-tion is performed by the minority chargecarriers (holes in the photoanode and elec-trons in the photocathode). The potentialof an electrode being defined by the bulkFermi level, that corresponds to majoritycarriers. In both cases, the photocurrentplateaus, depending on the conversion ef-ficiencies and the light absorption of eachmaterial, before the onset potential of thedark current (sharp rise in current due toreaction with majority carriers).

Unlike the case for (dark) electrolysiselectrodes, the overpotential is defined fora photoelectrode as the difference betweenthe quasi Fermi level of the minority carri-ers and the redox energy level (see Fig. 7a).The operating potential and operating cur-rent of the cell is defined by the interceptof the photoanode current–voltage char-acterization with the inverse of the photo-cathode one (see Fig. 7b). The inverse ofthe cathodic current is used as the cathodiccurrent is generally considered negative(electrons entering the electrode) whereasthe anodic current is positive (electrons areextracted from the anode. This current willcorrespond to the amount of gas producedaccording to Eqns (1) and (2) in case of al-kaline electrolyte (Eqns (3) and (4) in acid-ic conditions) and described in section 2.

For a tandem cell, as depicted in Fig. 7,calculations have predicted a maximumSTH conversion efficiency of 21.6% as-suming 1.0 eV losses per photons (chargethermalization) using optimum band gapvalues of 1.89 and 1.34 eV for the first andsecond absorber respectively.[79] This cor-responds to a certain improvement as com-pared to the 12.7% resulting from calcula-tions performed for a single semiconductorassuming similar losses.

There is a large number of literaturereviews concerning the development ad-vances of materials that can be used inphotoelectrochemical tandem cells.[81–84]Amongst the most promising photoanodematerials, one can cite the long-studiedFe

2O

3(stable ina long rangeofpH4–14),[85]

WO3that operates in acidic conditions,[86]

and BiVO4that can be used around neutral

pH.[87]Hematite and tungsten trioxide havebeen tested in a tandem cell configuration,using a dye-sensitized solar cell instead ofa photocathode to promote electrons at anenergy sufficient to reduce water on a plat-


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*Correspondence: Dr. A. HaselbacheraaDepartment of Mechanical and Process EngineeringETH ZurichCH-8092 Zurich, SwitzerlandbDepartment of Innovative TechnologiesSUPSI, CH-6928 Manno, SwitzerlandcAirlight Energy Manufacturing SAVia Industria 10, CH-6710 Biasca, SwitzerlandE-mail: [email protected]

Experimental and Numerical Investigationof Combined Sensible/Latent ThermalEnergy Storage for High-TemperatureApplications

Lukas Geissbühlera, Simone Zavattonib, Maurizio Barbatob, Giw Zanganehc, Andreas Haselbacher*a,and Aldo Steinfelda

Abstract: Combined sensible/latent heat storage allows the heat-transfer fluid outflow temperature during dis-charging to be stabilized. A lab-scale combined storage consisting of a packed bed of rocks and steel-encap-sulated AlSi12 was investigated experimentally and numerically. Due to the small tank-to-particle diameter ratioof the lab-scale storage, void-fraction variations were not negligible, leading to channeling effects that cannotbe resolved in 1D heat-transfer models. The void-fraction variations and channeling effects can be resolved in2D models of the flow and heat transfer in the storage. The resulting so-called bypass fraction extracted fromthe 2D model was used in the 1D model and led to good agreement with experimental measurements.

Keywords: Packed bed · Phase change material · Simulation · Thermal energy storage · Thermocline

1. Introduction

The fluctuating nature of intermittentrenewable energy sources such as windand solar requires short- and long-termenergy storage to guarantee the powersupply. At present, pumped hydro storage(PHS) is the main option for short-termlarge-scale storage. Market conditionsthreaten the economic viability of PHS,however. Compressed air energy storage(CAES) is an alternative technology thathas been proven at industrial scale.[1] Be-cause CAES plants waste the heat gener-ated during compression, it must be resup-plied prior to expansion, leading to cycleefficiencies of about 40–55%. In advancedadiabatic compressed air energy storage

(AA-CAES), the heat produced duringcompression is stored in a thermal energystorage (TES), resulting in projected cycleefficiencies of 70–75%, which are compa-rable to PHS’s cycle efficiencies.[2]Advan-tages of AA-CAES compared to PHS arelower estimated capital costs[3] and smallerland requirements. For these reasons, AA-CAES is potentially an attractive alterna-tive to PHS for Switzerland. Because thehigh efficiencies of AA-CAES plants aredue to the integration of a TES for tem-peratures of up to 650 °C, this study is fo-cused on the experimental and numericalinvestigation of TES for high-temperatureapplications. The results presented are rel-evant as well to waste heat recovery in in-dustrial processes and concentrated solarpower plants.

Thermocline TES represents an effi-cient and cost-effectiveway of storing ther-mal energy.[4] In prior work, a packed bedof rocks as sensible heat storage materialand air as heat transfer fluid (HTF) wasexperimentally shown to yield 95% over-all (charging-discharging) efficiency.[5–7]A drawback of thermocline TES is the de-creasing HTF outflow temperature duringdischarging, which can reduce the cycleefficiency of AA-CAES. This drop can bereduced by oversizing the storage or in-creasing its height-to-diameter ratio at theexpense of higher pressure drops[7] and/orincreasing material costs. Another way ofavoiding the temperature drop is throughlatent TES based on phase-change materi-als (PCM). Because the phase change oc-curs at constant temperature, latent TES

can, in principle, stabilize the HTF outflowtemperature during discharging. Among alarge number of potential PCMs,metals areattractive because they offer high heats offusion and high thermal conductivities.[8]However, intermetallic layers can formbetween the encapsulation and the PCM,impacting their performance.[9] Anotherdrawback of PCMs is that they are ex-pensive compared to sensible heat storagematerials. For these reasons, our interesthas centered on combined sensible/latentTESwith the twofold aim of stabilizing theHTF outflow temperature during discharg-ing compared to sensible-only TES and re-ducing costs compared to latent-only TES.The combined storage concept was inves-tigated experimentally and numerically atthe laboratory scale in previous work.[10–14]

For the numerical analysis of TES, 1Dmodels are often used, especially for para-metric studies, because the computationalcost is much lower than for 2D or 3D mod-els. The simplification to 1D can be justi-fied if the tank-to-particle-diameter ratiois large, which means that radial gradientsare small over the entire cross section ex-cept near a small annular region close tothe storage walls. In that region, where thelength scales of the flow and heat transferare comparable to the particle diameter, 1Dmodels are not valid. This means that 1Dmodels are more accurate for large-scalethan laboratory-scale TES. Therefore, theobjectives of this article are: (i) to validatea 1D model of a lab-scale combined sen-sible/latent heat storage and (ii) to compare1D and 2D simulations of that storage.


due to variations of the void fraction. Asstated in the introduction, these variationsare important because the tank-to-particle-diameter ratio for the lab-scale storage issmall. To reduce the computational cost,the 2Dmodel is based on the assumption ofaxisymmetry of the flow and heat transferin the storage. The model solves the mass,momentum, and energy conservationand turbulence transport equations usingANSYS Fluent 15.0. Turbulence effectsare accounted for with the realizable k–εmodel[23] with enhanced wall treatment.[24]Grid-independent results were obtainedwith 360’000 quadrilateral cells. Thepacked bed and the tube rows are modelledusing the porous media approach.[25]As inthe 1Dmodel, the solid and fluid phases areassumed to be in thermal non-equilibrium.The solid-to-fluid heat-transfer coefficientis computed with Eqns. (3) and (6) for thepacked bed and the tube bundle, respec-tively. Air is assumed to be an ideal gas

Further details of themodel of the latentheat section are available in refs [14,15].

2.1.3 Storage Wall and InsulationThe radial and axial temperature distri-

bution in the storagewall and the insulationis determined from Eqn. (8), where q

boundaryrepresents the heat transfer at the boundar-ies of the wall and the insulation. At theinner boundary, convective heat transfer tothe heat transfer fluid and conductive-radi-ative heat transfer to the packed bed are ac-counted for, see the last terms in Eqns. (1),(2), and (5). Free convection heat transferis assumed at the outer boundary.[17]At theheight of the topmost tube row, measuredinsulation temperatures in each materialare imposed as boundary conditions.

2.2 2D ModelThe primary purpose of the 2D model

is to quantify channeling, i.e. the radialvariations in the flow and heat transfer

2. Modelling

2.1 1D ModelThe 1D heat-transfer model[14,15] is

formulated separately for the sensible andlatent heat sections and the storage struc-ture with the insulation. Fluid, solid, andmolten phases are considered and convec-tive, conductive, and radiative heat transfermechanisms are included. The descriptionbelow assumes that the PCM is encapsu-lated in tubes and is restricted for brevity;a more detailed description can be found inrefs [14,15]. Time integration is performedimplicitly and spatial derivatives are ap-proximated with second-order accuracy.The enthalpy method[16] is used to solvethe equations of the PCM.

2.1.1 Sensible Heat SectionThe energy conservation equations for

the fluid and solid phases are given by Eqns(1) and (2), where the symbols are definedin the nomenclature. Temperature-depen-dent rock and air properties are implement-ed and are taken from refs [6,13,17]. Thevolumetric heat-transfer coefficient h

v,rocksis determined from Eqn. (3) for the convec-tive heat-transfer coefficient per unit areahrocks

[18] with hv,rocks

= ashrocks

and as= 6φ

s/d

p.

The lateral wall convective heat-transfercoefficient h

w,convis taken from ref. [19]

and the conductive-radiative wall heat-transfer coefficient h

w,cond-radis taken from

ref. [20]. The effective axial conductivitykeffis calculated according to ref. [21]. The

term qinterface,rad

represents the radiative heattransfer between the last row of tubes andthe top of the packed bed of rocks. Becauseof the small value of the tank-to-particle-diameter ratio, a bypass fraction of 10%was used in the sensible heat section. Thisvalue was obtained from simulations withthe 2D model described below.

2.1.2 Latent Heat SectionThe energy conservation equations for

the fluid and the encapsulation are given inEqns (4) and (5).

A volumetric air-to-tube heat-transfercoefficient h

v,encis used to compute q

enc,gand is calculated from a correlation (Eqn.(6)) for the convective heat-transfer coef-ficient per unit area h

enc,[18] where h

v,enc=

aenchenc

and aenc

= 103.45, Crow

= 0.95, Prg

is the gas Prandtl number evaluated atthe temperature of the gas, Pr

sis the gas

Prandtl number evaluated at the tempera-ture of the solid, and Re

maxis based on the

maximum interstitial velocity umax

= Stu0/

(St–d

tubes). The term q

top,radaccounts for the

radiative exchange between the perforatedplate (Fig. 1) and the topmost row of tubes.An energy conservation equation (Eqn.(7)) is solved for each row of encapsulatedPCM.

(2)𝜙𝜙s s s = 𝑘𝑘eff s + ℎv,rocks 𝑇𝑇g − 𝑇𝑇s − 𝑞𝑞interface,rad + 𝑎𝑎wℎw,cond-‐rad 𝑇𝑇w − 𝑇𝑇s

(5)𝜙𝜙enc enc enc = −𝑞𝑞enc,g + 𝑞𝑞cond,enc + 𝑞𝑞rad,enc + 𝑞𝑞pcm,enc + 𝑞𝑞interface,rad + 𝑞𝑞top,rad +w cont,w enctank− enc tanktank− enc 𝑇𝑇w − 𝑇𝑇enc

(4)1 − 𝜙𝜙epcm g g + 1 − 𝜙𝜙epcm g g = 𝑞𝑞enc,g(3)𝑁𝑁𝑁𝑁rocks = rocks pg = .− s 𝑅𝑅𝑅𝑅 . 𝑃𝑃𝑃𝑃g / 90 ≲ 𝑅𝑅𝑅𝑅 ≲ 4000,𝑃𝑃𝑃𝑃 ≈ 0.7

(1)1 − 𝜙𝜙s g g + 1 − 𝜙𝜙s g g = ℎv 𝑇𝑇s − 𝑇𝑇g + 𝑎𝑎wℎw,conv 𝑇𝑇w − 𝑇𝑇g

(9)𝜙𝜙 𝑟𝑟 = 𝜙𝜙 , 1 + 𝐶𝐶 exp −𝐶𝐶 −(8)str str = 𝑘𝑘str𝑟𝑟 str + 𝑘𝑘str str + 𝑞𝑞boundary(7)pcm pcm = 𝑘𝑘pcm𝑟𝑟 pcm − 𝑞𝑞pcm,enc(6)𝑁𝑁𝑁𝑁enc = enc tubesg = 0.51𝐶𝐶row𝑅𝑅𝑅𝑅max/ 𝑃𝑃𝑃𝑃g . gs / 40 < 𝑅𝑅𝑅𝑅max < 1000


tom. The total capacity of the combinedstorage is 42.4 kWh

th.

4. Results

Two experimental runs under identicalconditions were performed to obtain thetemperature measurements at all desiredpositions, as only 26 thermocouple portscould be used in each run. To check repro-ducibility, 9 of the 26 thermocouples wereconnected in both runs. For these thermo-couples, it was found that themean temper-ature differences between the runs did notexceed 8 K or 1.4%. Figs 2, 3, and 4 showcomparisons of the measured and simu-lated PCM, packed-bed, and tank-wall andinsulation temperatures during one charge-discharge cycle with ∆t

c= 3.25 h, respec-

tively. The symbols and colors correspondto those used in Fig. 1. The predictions bythe 1D model are superior to those by the2D model for the latent section. Duringdischarging, the predictions of the packed-bed temperatures by the 2D model aremore accurate than those of the 1D model,however. Fig. 5 presents the temperaturedistribution at the end of charging. The 2Dmodel results clearly show the substantialradial gradients caused by the small valuesof the tank-to-particle diameter ratio.

5. Conclusions and Outlook

Simulations with 1D and 2D models ofa lab-scale combined sensible/latent heatstorage were validated with experimentalresults. Radial gradients were significantbecause of the small tank-to-particle di-ameter ratios and were represented wellby the 2Dmodel. The 2D simulations wereused to extract the bypass fraction that wasemployed to represent the unresolved ra-dial gradients in the 1D model. For largertank-to-particle diameter ratios, as encoun-tered in industrial-scale storage units, the

3. Experimental Setup

The combined sensible/latent heat lab-scale TES consists of encapsulated AlSi

12on top of a packed bed of rocks in an in-sulated cylindrical tank. A schematic withdimensions and the locations of the ther-mocouples is shown in Fig. 1. The inlettemperature of the air above the topmosttube row was measured by a shieldedthermocouple. A radiation correction wasapplied to the measured temperature, seeref. [15]. The packed bed consisted ofrocks with an average diameter of 32 mmand a total mass of 245 kg. The averagevoid fraction of the bed wasmeasured to be0.4.[13] AlSi

12was chosen as the PCM due

to its melting temperature being suitablefor AA-CAES and CSP applications aswell as due to its high heat of fusion, highthermal conductivity, and comparativelylow cost.[8] The encapsulation was made ofAISI 316 steel tubes with an inner diameterof 16 mm and a wall thickness of 1 mm.Four rows of 17 tubes each were stackedat angles of 45°. The masses of the PCMand encapsulation were 9.27 kg and 13.17kg, respectively. Thermal conductivities,heat capacities, and densities of the PCMand the encapsulation are given in ref. [14].The heat of fusion of the PCM is 466 kJ/kg with a melting range of 4 K.[13] The for-mation of an intermetallic layer betweenencapsulation and PCMwas neglected, butis under investigation in a companion proj-ect.[9] The tank was made of 3 mm thickstainless AISI 304 steel and was insulatedwith Microtherm®, felt, and rockwool. Thethicknesses of the insulation layers as wellas the thermophysical properties of thetank and insulation materials are given inref. [14]. Two perforated steel plates of 20mm thickness were used for flow homog-enization at the top and the bottom of thestorage. The storage was charged with airat up to 595 °C from the top and dischargedwith air at room temperature from the bot-

with temperature-dependent properties.[17]Temperature-dependent properties of sol-id materials are also implemented.[6,13.17]Thermal losses by convection and radia-tion are considered assuming a surround-ings temperature of 20 °C and a convectiveheat-transfer coefficient of 5 W/m2K. ThePISO and PRESTO[26] methods were usedto couple the velocity and pressure fieldsand to solve the pressure-correction equa-tion. Convergence was considered to havebeen achieved when the mass, momentum,and turbulence residuals were below 10–5

and the energy residual was below 10–8.

2.2.1 Sensible Heat SectionFor randomly packed spherical par-

ticles of uniform diameter, the void frac-tion in the bulk region ranges between0.36–0.42.[27] The packing structure is af-fected by the tank wall for a distance of ap-proximately 5d

p. In this near-wall region,

the void fraction distribution follows adamped oscillatory variation, from a valueclose to unity at the wall to a minimum ofapproximately 0.2 at a distance of aboutdp/2 from the wall. For a distance great-

er than about 5dpfrom the wall, the void

fraction approaches the value in the bulkregion. For packed beds of non-sphericaland non-homogeneous particles, the varia-tion of the void fraction in the radial direc-tion is better described by an exponentialdecay affecting the packing structure for adistance of about 2–3d

p.[28] The near-wall

void fraction variation leads to channeling,which is important if the tank-to-particlediameter ratio is lower than 25-30.[29,30]Since the lab-scale storage is character-ized by a diameter ratio of about 12.5, theradial void-fraction distribution is includedin the 2D model,[31] see Eqn. (9), where C

1= 1.2 and C

2= 2.0.[32]An effective thermal

conductivity,[21,33] implemented in Fluentthrough a user-defined function, was usedto account for the conduction- and radia-tion-driven heat transfer in the packed bed.

2.2.2 Latent Heat SectionThe effective heat-capacity method[34]

was used to model the phase transition ofthe PCM. This method allows the phasetransition to be modeled without the needfor explicit tracking of the phase boundaryby combining the latent heat of fusion withthe specific heat. This so-called effectiveheat capacity was defined as a piecewiselinear function of the temperature. ThePCM and encapsulation were modeled asa single material with equivalent thermo-physical properties. A sensitivity analysiswas performed to ensure that the time stepdid not affect the phase transition. Radia-tion from the top plate to the topmost tuberow was accounted for by adding a sourceterm that was extracted from the 1Dmodel.

Rocks

Insulation

PCMPlate

Plate

x

394

1270

90120

1680

200

200

x x

xx

x x

x+x

xx xxxx

xxxx x

x

x

x410

836

1083

1237

1273

1348

x

Fig. 1. Schematic ofcombined sensible/latent heat TES andthermocouple loca-tions (circles andcrosses). All dimen-sions in mm.


effect of radial gradients will decreasesignificantly and the validated 1D modelcan therefore be applied with confidence.Future work includes improved modelingof radiation effects in the 2D model andincorporation of correlations for the evo-lution of intermetallic layers between thePCM and the encapsulation.[9]

NomenclatureLatin charactersa Surface area per unit volume [m2/m3]c Heat capacity [J/kgK]d Diameter [m]e Specific internal energy [J/kg]f Area fraction [-]h Specific enthalpy [J/kg]

Heat-transfer coefficient [W/m2K]hv

Volumetric heat-transfercoefficient [W/m3K]

k Thermal conductivity [W/mK]m Mass [kg]N Number of layers / units [-]q Volumetric heat flux [W/m3]r Radius [m]St

Transverse pitchbetween tubes [m]

T Temperature [K]t Time [s]u Interstitial velocity [m/s]u0

Superficial velocity [m/s]V Volume [m3]x Axial coordinate [m]

Greek charactersε Emissivity [-]ρ Density [kg/m3]μ Dynamic viscosity [kg/ms]σ Stefan-Boltzmann constant,

5.6704·10–8 [W/m2K4]φ Volume fraction [-]

Dimensionless numbersNu Nusselt number, hd/k

g[-]

Pr Prandtl number, μcp/k

g[-]

Re0

Superficial Reynoldsnumber, ρ

gu0dp/μ [-]

Remax

Reynolds number for tubes,rgumaxdtubes

/m [-]

Subscripts0 Undisturbed flow∞ Surroundingsc Chargingcond Conductivecont Contactconv Convectived Dischargingeff Effectiveenc Encapsulationepcm Encapsulation and PCMg Gasmax Maximump Particlerad Radiatives Solidstr Storage wall and insulationv Volumetricw Wall

0.4 0.6 0.8 1 1.2 1.4500

520

540

560

580

600

t/Δtc [−]

T[C]

Fig. 2. Comparison ofPCM temperatures.Circles representexperimental mea-surements and solidand dashed lines rep-resent the simulatedsolid/molten phasetemperatures fromthe 1D and 2D mod-els, respectively.

0 0.5 1 1.5 2

200

400

600

t/Δtc [−]

T[C]

Fig. 3. Comparisonof packed-bed tem-peratures. Bulletsrepresent experimen-tal measurementsand solid and dashedlines represent thesimulated air tem-peratures from the1D and 2D models,respectively.

0 0.5 1 1.5 2 2.5 3

200

400

600

t/Δtc [−]

T[

C]

Fig. 4. Comparison oftank wall and insula-tion temperatures.Markers representexperimental mea-surements and solidand dashed lines rep-resent the simulatedtemperatures fromthe 1D and 2D mod-els, respectively.

Fig. 5. Temperaturedistributions of the1D (left) and 2D (right)models at the endof charging. The airtemperature is shownin the storage itselfwhereas the solidtemperature is shownin the storage wallsand insulation.


AcknowledgmentsFunding by the Swiss Commission for

Technology and Innovation through the SwissCompetence Center for Energy Researchfor Heat and Electricity Storage, and by theEuropean Union under the 7th FrameworkProgram – Grant No. 312643 (SFERA-II) – isgratefully acknowledged.


[1] F. Crotogino, ‘Compressed air energy storagecaverns to integrate fluctuating wind energywithin transmission grids in Germany’, in‘Underground Storage of CO

2and Energy’, Ed.

J. S. Yoon, CRC Press, 2010, pp. 279-284.[2] S. Zunft, C. Jakiel, M. Koller, C. Bullough,

‘Adiabatic compressed air energy stor-age for the grid integration of wind power’,Sixth International Workshop on Large-ScaleIntegration of Wind Power and TransmissionNetworks for Offshore Windfarms, 2006.

[3] G. Locatelli, E. Palerma, M. Mancini, Energy2015, 83, 15.

[4] J. E. Pacheco, S. K. Showalter, W. J. Kolb, J.Sol. Energy Eng. 2002, 124, 153.

[5] M. Hänchen, S. Bruckner, A. Steinfeld, Appl.Therm. Eng. 2011, 31, 1798.

[6] G. Zanganeh, A. Pedretti, S. Zavattoni, M.Barbato, A. Steinfeld, Sol. Energy 2012, 86,3084.

[7] G. Zanganeh, A. Pedretti, A. Haselbacher, A.Steinfeld, Appl. Energy 2015, 137, 812.

[8] M. M. Kenisarin, Renew. Sust. Energy Rev.2010, 14, 955.

[9] D. Y. S. Perraudin, S. R. Binder, E. Rezaei, A.Ortona, S. Haussener, CHIMIA 2015, 69, 780.

[10] R. Ratzesberger, B. Beine, E. Hahne, VDIBerichte 1994, 1168, 467.

[11] E. Hahne, U. Taut, U. Gross, Solar WorldCongress 1991, 2, 1937.

[12] G. Zanganeh, M. Commerford, A. Haselbacher,A. Pedretti, A. Steinfeld, Appl. Therm. Eng.2014, 70, 316.

[13] G. Zanganeh, R. Khanna, C.Walser,A. Pedretti,A. Haselbacher, A. Steinfeld, Sol. Energy 2015,114, 77.

[14] L. Geissbuhler, M. Kolman, G. Zanganeh, A.Haselbacher, A. Steinfeld, ‘Analysis of indus-trial-scale high-temperature combined sensible/latent thermal energy storage’, in ASME-ATI-UIT Conference on Thermal Energy Systems:Production, Storage Utilization and theEnvironment, Napoli, 2015.

[15] L. Geissbuhler, M. Kolman, G. Zanganeh, A.Haselbacher, A. Steinfeld, Appl. Therm. Eng.2015, submitted for publication.

[16] V. R. Voller, Numer. Heat Transfer B 1990, 17,155.

[17] F. P. Incropera, D. P. Dewitt, T. L. Bergman, A.S. Lavince, ‘Fundamentals of Heat and MassTransfer’, John Wiley & Sons, Hoboken, NJ,2007.

[18] A. S. Gupta, G. Thodos, AIChE J. 1963, 9, 751.[19] J. Beek, Adv. Chem. Eng. 1962, 3, 203.[20] K. Ofuchi, D. Kunii, Int. J. Heat Mass Transfer

1965, 8, 749.[21] D. Kunii, J. M. Smith, AIChE J. 1960, 6, 71.[22] A. Zukauskas, ‘Heat transfer from tubes in

crossflow’, in ‘Advances in Heat Transfer’, vol.8, Eds J. P. Hartnett, T. F. Irvine, Elsevier, 1972,pp. 93-160.

[23] T. H. Shih, W. W. Liou, A. Shabbir, Z. Yang, J.Zhu, Comput. Fluids 1995, 24, 227.

[24] J. Tu, G. H.Yeoh, C. Liun, ‘Computational FluidDynamics - A practical approach’, Butterworth-Heinemann, 2008.

[25] D. A. Nield, A. Bejan, ‘Convection in porousmedia’ 3rd ed., USA, Springer, 2006.

[26] ANSYS, FLUENT - Theory guide, 2013.[27] A. E. Scheidegger, ‘The Physics of Fluid Flow

through Porous Media’, 3rd ed., Toronto,University of Toronto Press, 1974.

[28] VDI Heat Atlas, 2nd ed., Germany, Springer,2010.

[29] D. E. Beasley, J. A. Clark, Int. J. Heat MassTransfer 1984, 27, 1659.

[30] A. M. Ribeiro, P. Neto, C. Pinho, Int. Rev.Chem. Engin. 2010, 2, 40.

[31] M. L. Hunt, C. L. Tien, Chem. Eng. Sci. 1990,45, 55.

[32] D. Vortmeyer, J. Schuster, Chem. Eng. Sci.1983, 38, 1691.

[33] S.Yagi, D. Kunii, AIChE J. 1957, 3, 373.[34] D. Poirier, M. Salcudean, J. Heat Transf. 1988,

10, 562.

804 CHIMIA 2015, 69, No. 12 corrigendum

In this article (CHIMIA 2015, 69, 52–56), the name of author Etienne Vermeirssen is corrected.


52 CHIMIA 2015, 69, No. 1-2 note

Scientific Basis for Regulatory Decision-Making of NanomaterialsReport on the Workshop, 20–21 January2014, Center of Applied Ecotoxicology,Dübendorf

Christoph Studera, Lothar Aicherb, Bojan Gasicc, Natalie von Goetzd, Peter Hoete,Jörg Huwylerf, Ralf Kägig, Robert Kaseh, Andrej Kobei, Bernd Nowackj,Barbara Rothen-Rutishauserk, Kristin Schirmerl, Gregor Schneiderm, Etienne Vermeirssenh,Peter Wickj, and Tobias Walser*a

columns CHIMIA 2015, 69, No. 12 805

Swiss Science Concentrates

A CHIMIA ColumnShort Abstracts of Interesting Recent Publications of Swiss Origin

doi:10.2533/chimia.2015.805

Prepared by Caroline D. Bösch, Markus Probst, Yuliia Vyborna, Mykhailo Vybornyi, Simon M. Langenegger and Robert Häner*Do you want your article to appear in the SWISS SCIENCE CONCENTRATES highlight?Please contact [email protected]

Low-Valent Iron: An Fe(i) Ate Compound as a BuildingBlock for a Linear Trinuclear Fe Cluster

C. Lichtenberg*, L. Viciu, M. Vogt, R. E. Rodríguez-Lugo, M.Adelhardt, J. Sutter, M. M. Khusniyarov, K. Meyer, B. de Bruin,E. Bill, and H. Grutzmacher*, Chem. Commun. 2015, 51, 13890.ETH ZurichThe cluster structure of polynuclear iron compounds can deter-mine the characteristics of (multi-)redox processes, enable coop-erative reactivity, and allow the precise adjustability of magneticproperties. Lichtenberg, Grutzmacher and co-workers present alow-valent trinuclear iron complex with an unusual linear Fe(i)–Fe(ii)–Fe(i) unit. The metal complex is prepared by a rationalapproach using a salt metathesis reaction between a new anionic

Fe(i) containing het-erocycle and FeCl

2.

Its electronic struc-ture was studied bysingle crystal XRDanalysis, EPR andMössbauer spec-troscopy, and mag-netic susceptibilitymeasurements.

Combined Operando X-ray Diffraction–Electrochemical Impedance Spectroscopy DetectingSolid Solution Reactions of LiFePO4 in Batteries

M. Hess*, T. Sasaki*, C. Villevieille, and P. Novák, Nat. Com-mun. 2015, 6, 8169. PSI VilligenLithium-ion batteries arewidely used for portable applications to-day. Unfortunately, they often suffer from limited recharge rates.Hess, Sasaki and collaborators report a combination of high-reso-lution operando synchrotron X-ray diffraction and electrochemi-cal impedance spectroscopy to directly track non-equilibriumintermediate phases in lithium-ion battery materials. This tech-nique uses a bulb exposure of X-rays on battery materials similarto the bulb exposure on standard photo cameras. LiFePO

4, for ex-

ample, is known to undergo phase separation when cycled underlow-current-density conditions. Operando X-ray diffraction un-

der ultra-high-ratealternating currentand direct currentexcitation revealsa continuous butcurrent-dependent,solid solution reac-tion between LiFe-PO

4and FePO

4.

This study changesthe understandingof the intercalationdynamics in LiFe-PO

4.

Solid-State Reversible Nucleophilic Addition ina Highly Flexible MOF

A. Lanza*, L. S. Germann, M. Fisch, N. Casati, and P. Macchi*,J. Am. Chem. Soc. 2015, 137, 13072. University of Bern and PSIVilligenIn thisp a p e r ,L a n z a ,M a c c h iand co-wo r ke r sdescr ibea flexible and porous MOF based on CoII connectors and ben-zotriazolide-5-carboxylato linkers. The MOF reacts selectivelywith guest molecules trapped in the channels during the samplepreparation or after an exchange process. Upon mild compres-sion or cooling, the Co atoms are able to extend their coordina-tion, binding the nucleophilic guest molecules. The transforma-tion involves all Co atoms with methanol as guest, whereas onlypart of them with the larger dimethylformamide. The addition isreversible and upon decompression or heating, the initial phaseis reobtained. This peculiar example of chemisorption may haveenormous implications for gas storage, selective sieving andpotentially for catalysis. Further study is ongoing in the sameresearch group.

Enantioselection on Heterogeneous Noble MetalCatalyst: Proline-Induced Asymmetry in theHydrogenation of Isophorone on Pd Catalyst

L. Rodríguez-García, K. Hungerbuhler, A. Baiker, and F. Meem-ken*, J. Am. Chem. Soc. 2015, 137, 12121. ETH ZurichIn the (S)-proline-mediated asymmetric hydrogenation of isopho-rone (IP) on supported Pd catalyst, excellent enantioselectivity isachieved, with an enantiomeric excess of up to 99%. The role ofthe heterogeneous catalyst has been the subject of a controversialdebate. Meemken and collaborators investigated the enantiose-lectivity-controlling steps on the metal surface using attenuatedtotal reflection infrared spectroscopy. The results demonstrate theexistence of two competing enantioselective processes leading toopposing enantioselection. Depending on surface coverage of thePd catalyst, the reaction is controlled either by kinetic resolution((S)-pathway)orbychiral catalysis ((R)-pathway).Theunravelled(R)-reaction pathway emphasizes an intriguing strategy for induc-ing chiral-ity in het-erogeneousasymmetriccatalysis.

806 CHIMIA 2015, 69, No. 12 columns

doi:10.2533/chimia.2015.806

Highlights of Analytical Sciences in Switzerland

Division of Analytical SciencesA Division of the Swiss Chemical Society

Deep UV-LED Based Absorbance Detectors forNarrow-Bore HPLC and Capillary Electrophoresis

Duy Anh Bui and Peter C. Hauser*

*Correspondence: Prof. P.C. Hauser, Department of Chemistry, University ofBasel, Spitalstrasse 51, CH-4051 Basel, E-mail: [email protected]

Keywords: Capillary detection · CE · Deep UV-LED · HPLC ·UV photodiode

The most common detection method for the analytical sepa-ration techniques of HPLC and capillary electrophoresis (CE) isabsorbance measurement in the deep-UV range (below 300 nm)as a large number of organic species absorb in this wavelengthregion. Conventional UV detectors are based on deuterium dis-charge lamps coupled to a monochromator for wavelength se-lection. Light-emitting diodes (LEDs) for this wavelength rangehave been produced in recent years. They have bandwidths oftypically 30 nm, which makes them well suited for direct ab-sorbance measurements of molecules without requiring a mono-chromator. Only UV-photodiodes and a log-ratio amplifier inte-grated circuit for emulating Lambert-Beer’s law are required tocomplete the electronic circuitry.

Narrow-bore HPLC has primarily been developed for usewithmass-spectrometric detection, for which only small amountsof analytes are sufficient. However, the savings in eluent con-sumption makes this approach also attractive for use with opticaldetection when ultimate sensitivity is not required. In CE narrowchannels are essential to limit the Joule heating associated withthe ionic current along the separation path.

The design of LED-based detectors for these narrow gaugemethods is more challenging than for standard HPLC. Due to thesmall available volumes, the construction of dedicated Z-shapedflow cells is not possible and the measurement has to be madetransverse to the flow path. The narrowness of the necessary ap-ertures requires careful attention to efficient light coupling andavoidance of stray light. Highmechanical stability is also requiredin order to minimize noise due to mechanical fluctuations. De-spite these hurdles excellent performance with regard to baselinenoise (low μAU range), reproducibility of peak areas (~1%), andlinearity of calibration curves (correlation coefficients >0.999)could be obtained with LEDs of the commonly used wavelengthsof 255 and 280 nm for both, narrow-bore HPLC (250 mm ID)and CE (50 mm ID).

The inexpensive LED-based devices display a capabilitycomparable to standard commercial detectors. Their com-pact size and low power requirements make them also suit-able for portable battery-powered instruments.

Received: September 30, 2015

ReferencesB. Bomastyk, I. Petrovic, P. C. Hauser, J. Chromatogr. A 2011, 1218, 3750.D. A. Bui, B. Bomastyk, P. C. Hauser, J. Sep. Sci. 2013, 36, 3152.D. A. Bui, P. C. Hauser, J. Chromatogr. A, 2015, 1421, 203..

Can you show us your analytical highlight?Please contact: Dr. Veronika R. Meyer, Unterstrasse 58, CH-9000 St.GallenTel.: +41 71 222 16 81, E-mail: [email protected]

The detector cell for capillary electrophoresis.

7

6

5

4

3

2

1

0A

bsor

banc

e(m

AU

)350300250200150100

Time (s)

Sulfanilic acid

4-Nitrobenzoic

4-Hydroxybenzoic acid

4-Aminobenzoic

acid

acid

Detection of aromatic acids in capillary electrophoresis using a 50 µmID capillary with a 255 nm LED.

columns CHIMIA 2015, 69, No. 12 807doi:10.2533/chimia.2015.807

Universities of Applied Sciences

Fachhochschulen – Hautes Ecoles SpécialiséesFHHES

Effect of Experimental Parameters on Water SplittingUsing a Hematite Photoanode

Olivier Vorlet*a, Francesco Giordanoa, Thierry Chappuisa,and Christoph Ellertb

*Correspondence: O. Vorleta, E-mail: [email protected]; aHES-SO Haute EcoleSpécialisée de Suisse occidentale, Haute école d’ingénierie et d’architecture deFribourg, Institut ChemTech, CH-1705 Fribourg; bHES-SO Haute Ecole Spéciali-sée de Suisse occidentale, HES-SO Valais-Wallis, Institut Systèmes industriels,CH-1950 Sion

Abstract: Many studies designate hematite as a promisingmaterial for direct water splitting into hydrogen and oxygen. Fora real outdoor application, it is important to consider hourly andseasonal conditions like temperature and sunlight intensity. Theperformance of an undoped hematite thin-film photoanode wastested in a photoelectrochemical cell under varying conditionsof temperature and light intensity. Both parameters show apositive effect on performance under outdoor conditions.

Keywords: Hematite · Light intensity · Photoelectrochemicalcell · Temperature · Water splitting

Introduction

The production of hydrogen from renewable intermittent en-ergy sources provides a solution for storage of clean energy. Thisenables the production of liquid fuels such as methanol, whichcan be used in existing infrastructure. Solar-induced water split-ting using a hematite (α-Fe

2O

3) photoanode in a photoelectro-

chemical (PEC) cell is one of the most promising technologies.α-Fe

2O

3is an inexpensive material with high chemical sta-

bility in alkaline conditions. The band gap of about 2.0–2.2 eVprovides good absorption in the visible spectrum up to 590 nmwith a theoretical conversion up to 20% of incident solar energyinto hydrogen. However, several factors limit the practical perfor-mance of hematite for solar water splitting, such as high rate ofcarrier recombination, poor charge transport and short lifetime ofminority carriers. The phenomena that occur in a PEC cell are notyet fully understood, but it is clear that the semiconductor/elec-trolyte interface holds a great importance for the performance.[1]

For commercial application, it is important to consider hourlyand seasonal conditions such as variation of sunlight intensityand temperature in outdoor conditions. Several simple modelshave been proposed to predict the behavior of a PEC cell as afunction of temperature and light intensity. The Gartner modeldescribes the electron/hole formation in the space-charge layerof the semiconductor:[2]

(1)11ph

WeI qL

αφ

α

−⎛ ⎞= −⎜ ⎟

⎜ ⎟+⎝ ⎠

where, Iphis the photocurrent, q is the charge of the electron, φ

is the photon flux, α is the absorption coefficient,W is the widthof the space-charge layer and L is the carrier diffusion length(2–4 nm for α-Fe

2O

3). The Gartner model assumes that every

hole contributes to the photocurrent. This model predicts a linearrelationship between the luminous flux and the photocurrent.

The temperature dependence can be explained in severalways. Carrier diffusion length is defined as:

(2)L Dτ=

where D is the diffusion coefficient and τ is the minority carrierlifetime, which are two temperature-dependent parameters. Thecarrier lifetime (τ) is weakly dependent on temperature.[3]Carrierdiffusion coefficients D

n(for electrons) and D

p(for holes) are

related to minority carrier mobility (μ) and temperature by theEinstein equation:[3,4]

(3)Bi ik TDq

µ=

Finally, the empirical Varshni equation gives the band gapvariation as a function of temperature:

(4)( ) ( )2

0g gATE T ET B

= −+

where A and B are fitting parameters of a given semiconductorand E

g(0) is the band gap at zero kelvin.[3]

Experimental Part

Photocurrent measurements were performed in a ‘cappuc-cino’ photoelectrochemical cell with a standard three-electrodeconfiguration. The hematite photoanode[5] was used as work-ing electrode (WE), a platinum foil electrode (Metrohm ref6.0305.100) as counter-electrode (CE) and anAg/AgCl KCL 3M(Metrohm ref 6.0733.100) as reference electrode. The cell wasfilled with about 20 ml of electrolyte aqueous solution of 1.0 MNaOH (pH = 13.6) prepared with Milli-Q water. Only 0.28 cm2

of the photoanode was in contact with the electrolyte and wasexposed to sunlight. To obtain the photocurrent density-voltage(J-V) curves, an external potential bias is applied between theworking electrode and the counter electrode at a scan rate of10 mV s–1 by a Bio-Logic SP-50 potentiostat. The potential (E

we)

was measured between the working electrode and the referenceelectrode and reported against the reversible hydrogen electrode(RHE) according to the Nernst equation:

(5)= + +0.059oRHE we AgClE E E pH

with E0AgCl

= 0.210 V at 25 °C. The temperature dependence ofabout 1 mV/°C is neglected. The photoanode was illuminatedwith a Xenon short arc lamp (Osram XBO R 180/45C) withintegrated parabolic reflector and IR short pass filter (Thorlabs

Universities of Applied Sciences

Fachhochschulen – Hautes Ecoles SpécialiséesFHHES


FEL0800, cutoff 800 nm). The incident illumination power wasmeasured with a calibrated photodiode (Thorlabs S120VC) andadjusted with the lamp power supply (Lumina Power XLB-300).

The PEC cell was placed in a thermostatic box. The tempera-ture wasmeasured by a thermocouple immersed in the electrolyteand is controlled with a Peltier element.

Effect of Sunlight Intensity

Power density was varied from 30 to 100 mW cm–2 whichcorresponds to an illumination of 0.3 to 1.0 sun equivalent. Theelectrolyte temperature is kept constant at 26 °C. The J-V curveshows a characteristic curve with a saturation region between1.4 and 1.7V

RHEas a flat area (plateau)[5] defined by an inflection

point Jpl (Fig. 1).

Fig. 1. Photocurrent density-voltage (J-V) diagram of the hematitephotoanode at different light intensities from about 0.3 to 1.0 sunequivalent.

Fig. 2. Linear relationship between light intensity in sun equivalent andthe generated photocurrent density.

The current density observed at 1.23VRHE or at Jpl (pla-teau) follows a nearly linear relationship (Fig. 2) in accordancewith Eqn. (1). No saturation effect is observed with increasingillumination. The non-linearity for higher illuminations can beexplained by the increased hole recombination rate observedwithhigher generation rate.[6]

Effect of Temperature

The temperature varied from 25 °C to maximum 50 °C to pre-vent evaporation of the electrolyte. The electrolyte was replacedat each experiment (Fig. 3).

The temperature has an effect on position of the inflectionpoint defined in the photocurrent saturation region. The photo-voltage show a consistent shift to lower potential. The photo-current increases with temperature but we can see a saturationeffect beyond about 35 °C (Fig. 4). The photocurrent increase ismainly due to the improved minority carrier mobility and there-fore the carrier diffusion length. This fact is weakly countered bythe minority carrier lifetime which typically decreases slightlywith temperature.[7]

Conclusion

This study shows a positive effect of sunlight intensity andtemperature on a photoelectrochemical cell for water splittingwith hematite photoanode. The photocurrent increases linearlywith sunlight without significant saturation effect. The advan-tageous temperature effect avoids the loss of efficiency due toPEC cell overheating. For a commercial application, the pho-toelectrochemical cell does not require a cooling system underconventional operating conditions.

Received: October 30, 2015

[1] I. Cesar, K. Sivula,A. Kay, R. Zboril, M. Grätzel, J. Phys. Chem. 2009, 113,772.

[2] P. Cendula, D. Tilley, S. Gimenez, J. Bisquert, M. Schmid, M. Grätzel, J.Schumacher, J. Phys. Chem. 2014, 118, 29599.

[3] P. Dias, T. Lopes, L.Andrade,A. Mendes, J. Power. Sources 2014, 272, 567.[4] L. Andrade, T. Lopes, H. A. Ribeiro, A. Mendes, Int. J. Hydrogen. Energ.

2011, 36, 175.[5] F. Le Formal, K. Sivula, M. Grätzel, J. Phys. Chem. 2012, 116, 26707.[6] T. Lindgren, H. Wang, N.Beermann, L. Vayssieres, A. Hagfeldt, S-E

Lindquist, Sol. Energ. Mat. Sol. C. 2002, 71, 231.[7] M. Ichimura, H. Tajiri, T. Ito, E.Arai, J. Electrochem. Soc. 1998, 145, 3265.

Fig. 4. Drift of inflection point (Jpl) as function of temperature.

Fig. 3. Photocurrent density-voltage (J-V) diagram of the hematitephotoanode at various temperatures from about 25 °C to 50 °C.

columns CHIMIA 2015, 69, No. 12 809doi:10.2533/chimia.2015.809 Chimia 69 (2015) 809–811 © Schweizerische Chemische Gesellschaft

biotechnet SwitzerlandHot from the press!

Platform for a Technological Leap in Antibiotics

Elsbeth Heinzelmann, science + technology journalist

Abstract: NTN Swiss Biotech™ brings together the SwissBiotech Association SBA, which is involved in regulatory, finan-cial and legal issues, and biotechnet Switzerland, which is ac-tive in translational R&D, to provide a technology base for jointprojects. Biotechnet aims to push promising domains by creat-ing topic-oriented platforms that enable academia and industryto work together to produce R&D results of major importance tosociety and the economy. The first activity initiated by biotech-net is the Antibiotics Platform that has now been launched.

Keywords: Antimicrobial-resistant organisms · Gram-negativebacteria · Klebsiella pneumoniae · Multidrug-resistant strains ·Pathogenesis of tuberculosis

Antimicrobial-resistant organisms are insidious: their rapidspread around the world in recent years represents a great riskfor humanity. It seems that we can keep Gram-positive patho-gens in check for the time being, but the increasing incidence ofantibiotic-resistant Gram-negative infections is forcing us to ac-cord high priority to combating this type of bacterial resistance.

A Clear Objective in Mind

“For multidrug-resistant (MDR) strains, in particular, there isoften a lack of treatment options as they have become resistant tovirtually all available antibiotics”, explains Prof.Markus Seeger,research group leader in the Institute of Medical Microbiologyat the University of Zurich. “Gram-negative bacteria are gener-ally much less permeable to antibiotics and thus generally lesssusceptible to naturally occurring antibiotics. This renders thediscovery of novel antibiotics very challenging but at the sametime rewarding.” He is the head of the Antibiotics Platform cre-ated by the NTN Swiss Biotech™ to combine the expertise ofoutstanding scientists from academia and industry in commonprojects supported by the Commission for Technology and In-novation (CTI). The reason for this step is that – following aperiod of stagnation – research and development in antibiotics aretoday globally in full swing. A key role in this domain is playedby Switzerland with some highly innovative SMEs and start-upsthat were unfortunately not well networked in the past. This hasnow changed with the Antibiotics Platform that involves all spe-cialists within the sector to drive things forward both rapidly andefficiently.

In this close-knit network most industry partners have aclear emphasis on the discovery of antibiotics for the treatmentof Klebsiella pneumoniae, Pseudomonas aeruginosa, Acineto-bacter baumannii and Escherichia coli. These pathogens havebecome a major problem in the context of hospital-acquired(nosocomial) infections. In addition to these Gram-negative bac-teria, multidrug-resistant Mycobacterium tuberculosis (MDR-TB) – the causative agent of most cases of tuberculosis – has be-come a global threat. Members of the Antibiotics Platform fromindustry – among them BioVersys – and from academia – such

as the microbiologist Stewart Thomas Cole, Head of the ColeLaboratory at EPFL – are working on the development of drugsto treat MDR-TB. In a joint start-up meeting in Bern in early au-tumn 2015, partners from academia and industry explained theirviews and objectives in forging plans for a common AntibioticsPlatform.

Everyone Contributes Individual Excellence

TheCenter of Organic andMedicinal Chemistry (ZHWWae-denswil) places particular emphasis on the design and synthesispart of the drug discovery process. In order to improve the po-tency, selectivity and pharmacokinetic profiles of modulators ofvalidated drug targets, the group rationally designs and synthe-sizes novel scaffolds and focused libraries based on X-ray dataof co-crystal structures. “Our iterative process of computationaldesign, organic synthesis and biological testing leads to detailedstructure-activity relationships for therapeutically relevant drugtargets”, explains Prof. Rainer Riedl, head of the Center. “Ourmedical chemistry expertise allows for the development of clini-cal drug candidates for the treatment of bacterial infections.”

Structural understanding of the therapeutic target allows the rationaldesign of optimized drug molecules. J. Lanz, R. Riedl, ChemMedChem2015, 10, 451–454.

Prof. Stewart T. Cole is Director of theGlobal Health Instituteat EPFL. As the Scientific Coordinator of three successful large-scale EC Framework projects he has a track record of bilateralcooperation with academic institutions, SMEs and pharmaceuti-cal companies. His world-class research unit is dedicated to TBdrug discovery, unravelling the pathogenesis of tuberculosis andstudying the phylogeography of leprosy. “I see a decisive trendtowards more microbiome analyses and bioinformatics as wellas sequence-driven research”, says the microbiologist. “There isa major need in the context of rapid diagnostics. We could con-tribute to industrial partners of the platform the use of biosafetylevel III facilities that already exist at EPFL, in Bern and at UZH.”

At the Laboratory of Molecular Evolution, University of Ba-sel, the group working with Dr. Marc Creus is analyzing the


evolution of resistance to antibiotics in Gram-negative bacteriaby means of next-generation sequencing. As an example he citesa detailed clinical case report of a patient infected with a strainof Klebsiella pneumoniae, which developed multidrug antibioticresistance during hospital treatment in Switzerland. In collabora-tion with an industrial partner, the Creus group has also appliedsimilar sequencing technology to explore the potential adaptationof Gram-negative pathogens to a novel siderophore antibiotic thatis currently undergoing clinical trials. “Exploring the evolution ofresistance in the laboratory even before an antibiotic is marketedmay be extremely informative in understanding the mechanismof action of the drug and may also help to minimize the evolutionof resistance later on in the clinical setting.”

The Institute for Chemistry & Bioanalytics at the FHNWMuttenz is particularly active in the research fields biochemis-try, bioanalytics & diagnostics, bio-nanotechnology and organicsynthesis & chemical engineering.

The bioanalytics group contributed to BioVersys’s CTI proj-ects by providing kinetic binding parameters as indicators forlead identification and optimization. The crystallization of drug-target complexes increases understanding of the binding pocketand allows the development of strategies towards the synthesis ofbetter binders. Together with the organic synthesis group at theInstitute, this comprehensive expertise in characterizing highlyspecific drug-target interactions using both kinetic and thermo-dynamic binding parameters with cutting-edge methodologyforms a good basis for running integrated drug discovery proj-ects. All the members of the organic synthesis group worked formany years in the pharmaceutical industry and thus provide verysound experience in medicinal chemistry, which has enabled theInstitute to successfully run various projects in lead discoveryand lead optimization both with small biotech and big pharma-ceutical companies. In addition to this line of investigation, Prof.Eric Kuebler, whose focus is DNA analysis and mutagenesis,recently started a project with BioVersys to investigate potentialresistance mechanisms related to BioVersys’s anti-mycobacterialcompounds.

The biotechnet has for several years had a close relationshipwith the Laboratory of Molecular Microbiology and Biotechnol-ogy run by Prof.AnnaMaria Puglia at theUniversity of Palermo.

Rapid Exchange of Know-how

SNSF Professor Markus Seeger is interested in the structureand function of antibiotic efflux. In particular, tripartite effluxsuch as the well-studied Acr/AcrB/TolC protein complex fromEscherichia coli are major contributors to the drug-resistancein Gram-negative ESKAPE pathogens. But ABC transporters,which pump antibiotics out of the cell at the expense of ATPhydrolysis, are also mediators of intrinsic antibiotic resistance,mainly in Gram-positive cells. “We want to understand the mo-lecular details of the pumping mechanisms that lead to antibioticefflux”, explains Markus Seeger. “For this purpose, we use X-ray crystallography, antibiotic binding and transport studies andmutational analysis of residues lining the antibiotic efflux tun-nel.” Crystal structures determined by him and his group featuretunnels inside the protein and imply a ‘peristaltic mode’ of drugtransport which could account for the wide substrate specificityobserved.

Prof. Vincent Perreten at the Institute of Veterinary Bacte-riology, University of Bern, performs epidemiological studiesof antibiotic resistance using microarray technology and next-generation sequencing. This involves clinical samples from com-panion and food-producing animals, with monitoring of samplesfrom healthy animals at the slaughterhouse and food samples ofanimal origin, i.e. meat.

Dr. Daniel Obrecht, co-founder and CSO of Polyphor Ltd. inAllschwil, is focusing on the discovery and development of mac-rocycle drugs to address areas with a high level of unmet medicalneed. Currently, the company is seeking to develop novel anti-biotics against MDR ESKAPE pathogens such as Klebsiella sppand Acinetobacter spp where multi-drug resistance is becominga real threat. Further areas of interest are proteomic analyses ofouter-membrane proteins in Gram-negative bacteria and photo-affinity labelling of antibiotics for target identification. “I seea major need in rapid diagnostics of Gram-negative bacteria inhospitals, including Pseudomonas infections”, comments DanielObrecht. “A domain of more general interest is the effect of drugson the host defence, in particular in the context of sepsis.”

Electron micrograph of the Gram-negative ESKAPE pathogenEscherichia coli.

A Platform of ‘Give and Take’

For years, Laves Arzneimittel GmbH in Schötz has concen-trated on the exploration and development of preparations basedon theEscherichia coli strain discovered in 1931 byDr.WolfgangLaves. Hans-Dieter Grimmecke, Head of R&D, wants to under-

ABC transporters pump antibiotics out of the cell and thereby confermultidrug resistance. R. J. Dawson, K. P. Locher, Nature 2006, 443,180-185. M. Hohl, C. Briand, M. G. Grütter, M. A. Seeger, Nat. Struct.Mol. Biol. 2012, 19, 395-402.


stand the biofilm of the gut: “Probiotic bacteria of the future willprobably originate from the gut microflora”, he says. “A promis-ing field of application for probiotics is the animal industry, inparticular in the context of reducing or even banning antibioticsin livestock production.”

As long ago as 2004, Actelion Pharmaceuticals Ltd. initiateda research programme in antibiotics, focusing on novel chemi-cal scaffolds with new mechanisms of action against establishedtargets. Goals are the intravenous treatment of severe hospital in-fections and oral antibiotics for community-acquired infections.“Drug transport – influx and efflux – is particularly interestingin the context of Gram-negative bacteria”, explains Daniel Ritz,head ofAnti-Infectives Biology. “They are muchmore difficult totreat than Gram-positive bacteria, mainly because of the presenceof the outer membrane and the action of tripartite efflux pumps.”

“Networking with experts in the field of bacterial infectionsis crucial for the advancement of antibacterial research and thedevelopment of novel antibiotics”, says Dr. Juerg Dreier. He isgroup leader Biochemistry& Screening and project leader of pre-clinical antibacterial projects at Basilea Pharmaceutica Interna-tional Ltd. Basilea is focusing on the development of antibiotics,antifungals and oncology drugs. The company has a portfolio ofcommercial-stage drugs as well as a pipeline of innovative early-stage anti-infective and oncology product candidates. Throughthe integrated research, development and commercial operationsof its Swiss subsidiary Basilea Pharmaceutica International Ltd.,the company focuses on providing innovative pharmaceuticalproducts in the therapeutic areas of bacterial infections, fungalinfections and oncology, targeting the medical challenge of risingresistance and non-response to current treatment options.

BioVersys AG in Basel develops small chemical moleculesthat switch off drug resistance on a gene-regulatory level withinbacteria, known as TRICs (Transcriptional Regulator Inhibitory

Compounds). “We are working on an anti-tuberculosis projectand several approaches to treating Gram-negative infections. Wewant to collaborate in the areas of X-ray crystallography and insilicomodelling and obtain access to strain collections”, says Dr.Michel Pieren, Group Leader Drug Discovery. “We have recent-ly developed a novel technique to create targeted gene deletionsin clinically relevant Gram-negative pathogens without the use ofantibiotic markers. This is very useful when generating knock-outs in multidrug-resistant Gram-negative bacteria.” This methodis of potential interest for other platform members.

Remain Modest, Be Powerful!

The platform members will now work out common areasof interest that are not affected by intellectual property issues.A priority seems to be the establishment of general methods tostudy plasma protein binding. “There is a strong commitmentfrom academia and industry”, Markus Seeger concludes. “Weare not seeking to grow in terms of members, but want to realizea lean, effective organization with clear targets to produce vis-ible results that receive international attention.” For Prof. DanielGygax, President of biotechnet Switzerland, the clear aim is toincrease competitiveness by creating interactive value: “Our fo-cus is on linking the core competencies of companies with thescientific knowledge and experience of academia. We have to‘plug the holes’ in know-how and technology along the entirevalue chain!”

Homepage: www.biotechnet.ch

Received: November 2, 2015


doi:10.2533/chimia.2015.812

Chemical Landmark 2015 – Designation of the FormerInstitute of Chemistry of the University of Fribourg

Leo Merz*

*Correspondence: Dr. L. Merz, Swiss Academy of Sciences (SCNAT), PlatformChemistry, Laupenstr. 7, Postfach, CH-3001 Bern, E-mail: [email protected]

On the 13th of October, the Swiss Academy of SciencesSCNAT designated the former institute of chemistry of the Uni-versity of Fribourg as «Chemical Landmark». The award cere-mony took place in the course of the «Fribourg ChaimWeizmannLecture». The combined festivities attracted an audience of morethan 200 participants.

The chemical institute was established in the former waggon factory(center).[2]

Prof. Dr.Katharina Fromm (SCNAT, Uni FR) moderated theprogram and welcomed the guests. Dr. Jürg Pfister, secretarygeneral of SCNAT, briefly introduced SCNAT and the «Chemi-cal Landmark» program, Prof. Dr. Andreas Zumbühl presentedthe Fribourg Chemical Society, and Beat Vonlanthen (Conseild’État FR) proudly conveyed the congratulations of the cantonalgovernment of Fribourg.

In his laudation, Prof. em. Dr. Alexander von Zelewsky pro-vided an insight into the history of the institutes. The universitywas founded upon the initiative of the conseil d’état GeorgesPython in 1886. Shortly after, in 1896 the faculty of sciencewas opened. Chemistry was installed in a former waggon fac-tory, which had been used as an arsenal for the artillery. Alreadythe first institute of chemistry was from the beginning commit-ted to bilingualism. The first appointed chemistry professor wasA. Bistrzycki from Germany. The first associate professor RenéThomas-Mamert was called from France. The academic careerladder was different at that time: Thomas-Mamert started inFribourg in 1896, finished his doctorate in 1897 and was sub-sequently promoted to full professor. Bistrzycki already had agroup of eleven students whom he brought with him to the newand unknown University of Fribourg. Among them was ChaimWeizmann, eponym of the «Fribourg Chaim Weizmann Lec-

ture», who later became the first president of Israel, co-foundedthe Hebrew University of Jerusalem, and founded the researchinstitute that was later called Weizmann-Institute.

Impression of the audience.

In the early 20th century, a descendant from an old Fribourgfamily, Henri de Diesbach, was appointed professor for chem-istry. Working in his group, Edmond von der Weid was the firstto publish the synthesis of phthalocyanine.[1] Amazed about thestability and colourfulness of the molecule, they offered it to thechemical industry in Basel, who was not interested at that time.The technological importance of phthalocyanine was not yet rec-ognised, and the use and mass production started a couple ofyears later in England. Even today, it plays an important role. In2000, more than 10’000 tons of phthalocyanines were produced,and 25% of all artificial organic pigments are phthalocyanine de-rivatives. In honour of this discovery, the «Chemical Landmark»plaque was anodized turquoise with a phthalocyanine dye.

Old sample of Cu-phthalocyanine from the de Diesbach group.


Von Zelewsky, who himself had worked in the old chemistryinstitute, flavoured his laudation with several anecdotes. He toldstories about chemists using chemical tricks to enter the buildingduring holidays to do their research and of (photo-)reactions thatcould be accomplished in the laboratories in Basel but not in thedark laboratories in Fribourg.

The unveiling of the commemorative plaque concluded the«Chemical Landmark» part of the event. The plaque was attachedat an entrance of the original building and is now visible to thepublic.

The second part of the festivities was the Chaim WeizmannLecture by Nobel laureate Prof. Alan Heeger, from the Univer-sity of California, Santa Barbara.

At the concluding apéro the audience had the opportunity totalk to the Nobel laureate Alan Heeger, the ambassador of IsraelY. Caspi, the conseil d’état B. Vonlanthen and of course the re-searchers.

Additional information on the «Chemical Landmarks» maybe found at chemicallandmarks.ch.

Additional information about the «Platform Chemistry» andits activities may be found at chemistry.scnat.ch.

Received: October 28, 2015

[1] H. de Diesbach, E. von der Weid, Helv. Chim. Acta, 1927, 10, 886;DOI:10.1002/hlca.192701001110

[2] Historical photograph: Service des biens culturels Fribourg, Fonds HéribertReiners. Current photographs: Charles Ellena and Leo Merz.

Katharina Fromm, Alexander von Zelewsky, Beat Vonlanthen, AstridEpiney and Jürg Pfister unveil the «Chemical Landmark» plaque.

Alan Heeger speaking about creativity, discovery and risk in research.

Impressions from the apéro.

A814 CHIMIA 2015, 69, No. 12 scg ssc scs

www.scg.chSociety News and Announcements

SCGSchweizerischeChemischeGesellschaft

SSCSociétéSuissede Chimie

SCSSwissChemicalSociety

news

SCS Spring Meeting 2016 –Registration open!

Topic: «Green Chemistry»

Date: April 22, 2016, 09.30–17.00Location: University of Zurich,

Department of ChemistryIrchel Campus, Winterthurerstrasse 190,8057 Zurich

The SwissChemical Society SpringMeet-ing is a one-day symposium and provides ahigh quality program with national and inter-national speakers. It is also the platform forthe Werner Price Ceremony and the WernerAward Lecture.

As a general rule the Society holds its gen-eral assembly during the lunch break.

RegistrationFree admission. However, a registration is

required via the online form on http://scg.ch/springmeeting/2016/

Program09.30 Welcome coffee, registration10.00 Opening, welcome

Prof. Roger Alberto, Prof. Greta Patzke10.05 Prof.Martyn Poliakoff, University of Nottingham11.00 Prof. Paul T. Anastas, Yale University12.00 Ceremony Werner Prize 2016

Werner Lecture12.50 Lunch / Commercial Exhibition13.30 SCS General Assembly 201614.00 Dr. Thomas Güttinger, Process Development

Specialty Chemicals at BASF15.00 Prof.Walter Leitner, RWTHAachen16.00 Prof. Paul J. Dyson, EPF Lausanne17.00 Closing remarks

Apéro and networking event

Supporters

European Photochemistry Association:Call for Nominations from Members

The Porter Medal 2016Deadline for nominations: 31st January 2016The Porter Medal is awarded every two years to the scientist

who, in the opinion of the European PhotochemistryAssociation,the Inter-American Photochemistry Association and the Asianand Oceania Photochemistry Association has contributed mostto the subject of Photochemistry.

Named for the late George Porter FRS, Nobel Laureate, thePorter Medal is awarded to the scientist who in the opinion ofthe judges, has contributed most the science of Photochemistrywith emphasis on more physical matters, thus reflecting GeorgePorter’s own interests.

More details on the webpage: www.portermedal.com

EPA Prize for Best PhD Thesis in PhotochemistryDeadline for nominations: 31st December 2015The Fifth EPA Prize for the best PhD thesis in Photochem-

istry will be attributed during the CECP 2016 Meeting in BadHofgastein, Austria. (February 13th to 18th). The prize is 1000 €,plus travel costs to Bad Hofgastein (within the limit of 300 €),and ONE FREEYEAR of EPA membership.

More details on the webpage: www.photochemistry.eu/phd_price.php

a warm welcome to our new members!

Period: 24.10.2015 – 13.11.2015

NikosAgorastos, Thalwil – Carlo Perotto,Adliswil – Carl PhilippRosenau, Zurich.

scg ssc scs CHIMIA 2015, 69, No. 12 A815

RegistrationRegistration is now open for the sixth EuCheMS.You can en-

joy early-bird discounts if you register now.You can be connectedto all the news and updates with the Congress newsletter and seeall services included and registration possibilities.

Plenary Speakers ConfirmedOutstanding speakers have already confirmed their atten-

dance. 6 Nobel Laureates will participate in the Congress of-fering plenary lectures. Ada Yonah, Richard Schrock, AaronCiechanover, Harold Kroto and Jean-Marie Lehn will be plenaryspeakers at 6th EuChems in Seville.

Avelino Corma, Spain’s most awarded chemist and 2014Prince of Asturias award for Technical and Scientific Research,the most prestigious in this country, has also confirmed his as-sistance and joins these spectacular group of plenary lecturers.

The European Chemistry Congresses (ECC) are the mostprominent events for the European chemistry community.They constitute a joint endeavour of the national chemicalsociety and the EuCheMS Divisions and Working Parties.Chemists from all parts of Europe come together to presentand discuss the latest achievements in cutting edge chemicalsciences. There is no other occasion where chemists fromdifferent countries, different areas of chemistry and differ-ent professional backgrounds can converge in one place. TheECC are unique forums to foster transnational dialogue andcollaboration, to encourage the dialogue between the dif-ferent branches of chemistry, to bring academia, industryand decision makers together and to emphasize the impactof chemistry and chemical research on our society. Specialattention is given to all activities that help promote the ca-reers of young scientists. A high level Scientific Committeeensures the highest possible quality of the scientific contribu-tions with a regionally and thematically balanced program ofexciting cutting edge chemistry.

6th EuCheMS Congress, Sevilla, 11.–15.09.2016

6th EuCheMS is less than a year awayCountdown has started for 6th EuCheMS Congress, now less

than a year away to take place in Seville, Spain next 11–15 Sep-tember, 2016. Following, you will find some of the major Con-gress milestones.

Call for abstracts OPENThe Call for abstracts process is already available. Guide-

lines, information for authors, submissions and important datesas well as online submission form are available in the congresswebsite

The online abstract submission deadline is April 1st, 2016.

Topic ConvenersTopic Conveners have been recently appointed. Thus, every

topic is already assigned to a European convener specialised inthe issue.You can review their background and experience.

SCSSwiss ChemicalSocietyMerry Christmas

Hoping many happy moments will find their way to you,leaving lovely memories you’ll treasure all the year through.

We wish you a Merry Christmas and a Happy New Year.

Your team from the Head Office

Sarah & David

A816 CHIMIA 2015, 69, No. 12 scg ssc scs

scs prize winners 2016

It’s our pleasure to announce the winners of the 2016 SCSawards.We would like to sincerely congratulate all prize winnersand are looking forward to the ceremonies that will take place atone of our events during the next year.

Paracelsus Prize 2016CHF 20,000 and medal in gold.The Paracelsus Prize is awarded to an internationally

outstanding scientist for his or her lifetime achievements inchemical research. It is awarded every two years;

The Swiss Chemical Society awards theParacelsus Prize 2016 to

Prof. Michael Graetzel,EPF Lausanne,

for his invention and development of thedye-sensitized solar cell.

Professor at the Ecole Polytechnique de Lausanne, MichaelGraetzel directs the Laboratory of Photonics and Interfaces. Hepioneered the use of mesoscopic materials in energy conversionsystems, in particular photovoltaic cells, lithium ion batteriesand photo-electrochemical devices for the splitting of water intohydrogen and oxygen by sunlight. He discovered a new type ofsolar cell based on dye-sensitized nanocrystalline oxide films(Picture and portrait text from http://lpi.epfl.ch/graetzel).

Paracelsus Award winners since 20022014: Prof. Richard R. Schrock, MIT, USA2012: Prof. Bernd Giese, University of Basel and Fribourg2010: Prof. Steven V. Ley, Cambridge, U.K.2008: Prof. Ben L. Feringa, Groningen, NL2006: Prof. Sir Jack E. Baldwin, Oxford, U.K.2004: Prof. George M. Whitesides, Cambridge, USA2002: Prof. Martin Quack, ZurichAll winners on http://scg.ch/awards

Werner Prize 2016CHF 10,000 and medal in bronze.The Werner Prize is awarded to a promising young Swiss

scientist or scientist working in Switzerland for outstandingindependent chemical research. At the time of the award thecandidate may not be a tenured professor or someone in a higherposition in industry, and should be younger than 40. The prize isawarded annually.

The Swiss Chemical Society awards theWerner Prize 2016 to

Prof. Maksym Kavalenko,ETH Zurich,

for his innovative studies in the chemistry,physics and applications of inorganicnanostructures.

Maksym Kovalenko has been an assistant professor (tenure-track) of inorganic functional materials at ETH Zurich sinceJuly 2011. He is also affiliated with Empa - the Swiss FederalLaboratories for Materials Science and Technology. The researchactivities of his group are carried out at both institutions (portraittext from http://www.old-lac.ethz.ch/kovalenkolab.html).

Werner Prize Winners since 20102015: Prof. Gilles Gasser, University of Zurich2014: Prof. Clémence Corminboeuf, EPF Lausanne,

Prof. JérômeWaser, EPF Lausanne2013: Prof. Cristina Nevado, University of Zurich,

Prof. Clément Mazet, University of Geneva2012: Prof. Nicolai Cramer, EPF Lausanne2011: Prof. Dr. Xile Hu, EPF Lausanne,

Prof. Dr. Reto Dorta, University of Zurich2010: Dr. Sandrine Gerber, EPF LausanneAll winners on http://scg.ch/awards

Sandmeyer Award 2016CHF 10,000 for individuals or CHF 20,000 for groupsThe Sandmeyer Prize is awarded to a group for outstanding

work in industrial or applied chemistry. The work must becompleted in Switzerland or with the involvement of a Swissnational. The prize is awarded annually.

The Swiss Chemical Society awardsthe Sandmeyer Prize 2016 to the teamcomprising researchers from SikaTechnology AG, ETH Zurich and theUniversity of Colorado Boulder, namelyDr. MartinWeibel, Sika Technology AGDr. Thomas Müller, Sika DeutschlandGmbHDr. Ratan K. Mishra, ETH ZurichProf. Robert J. Flatt, ETH ZurichProf. Hendrik Heinz, University ofColorado Boulder,

for their experimental and modelling studies of new commercialorganic additives for the grinding of inorganic solids.

Sandmeyer Prize Winners since 20112015: Actelion Pharmaceuticals Ltd, Hochschule fur Technik

und Architektur Fribourg and Swissi Process Safety2014: Syngenta Crop Protection Munchwilen AG2013: Clariant Group R&D and CNRS-Université de

Strasbourg2012: Solvias AG2011: BASF Schweiz AGAll winners on http://scg.ch/awards

Dr. Max Lüthi Award 2016CHF 1’000 and medal in bronzeThe Dr. Max Luthi Award is presented for an outstanding

diploma thesis in Chemistry conducted at a Swiss University ofApplied Sciences. The prize is awarded annually.

The Swiss Chemical Society awards theDr. Max Luthi Prize 2016 to

Mr. Flavio Gall,ZHAWWädenswil,

for his Bachelor Diploma studies onthe design and synthesis of cyclicmetalloprotease inhibitors.

Dr. Max Lüthi Prize Winners since 20102015: Yvan Mongbanziama, HEIA Fribourg2014: Yannick Stöferle, ZHAWWädenswil2013: Peter Elmiger, ZHAWWädenswil

Christophe Laporte, EIA Fribourg2012: Lucie Sägesser, ZHAWWädenswil2011: Michael Brand, Zurich2010: Benjamin Otter, FHNWAll winners on http://scg.ch/awards

scg ssc scs CHIMIA 2015, 69, No. 12 A817

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Im Rahmen des Weiterbildungsprogramms organisieren odererarbeiten wir gemeinsam mit Ihnen InCompany-Schulungen und-Trainings nach IhrenVorstellungen und Bedurfnissen. ProfitierenSie davon, dass wir fur Sie

• Inhalte an firmenspezifischeAnforderungen undWunscheanpassen

• Frage- und Problemstellungen in Ihrem Einsatzgebiet gezieltbehandeln

• praktische Übungen gegebenenfalls an Ihren Geräten durch-fuhren

• Trainings bei Bedarf auch in französischer oder englischerSprache durchfuhren

Ein weiterer Vorteil der InCompany-Trainings:fur Ihre Mitarbeiterinnen und Mitarbeiter fallen keine Reise-

und Übernachtungskosten an!Experten stehen Ihnen fur eine persönliche Bedarfsabklärung

und Beratung gerne zur Verfugung.

Sie erreichen uns uberSekretariat Weiterbildung SCG/DASVerena Schmid/LiloWeishauptc/o EAWAGÜberlandstrasse 133, 8600 DubendorfTelefon 058 765 52 00E-Mail: [email protected]/kurse

A818 CHIMIA 2015, 69, No. 12 information

InformationNews – Honors – Workshops – Conferences – Lectures

conferences in switzerland

01.01. – 30.04.2016

14th Swiss Snow Symposium 2016 by SYCA22.01.2016–24.01.2016Panorama Hotel Alphubel, Saas-FeeScience meets Snow! Symposium for young chemists up to 36years with an interesting combination of science, networking,fun & sports.http://scg.ch/snowsymposium/2016

20th SASP 201607.02.2016–12.02.2016Sunstar Parkhotel Davos, DavosThe Symposium onAtomic, Cluster and Surface Physics (SASP)is one in a continuing biennial series of conferences (the last onewas held in Obergurgl,Austria in 2014) that promotes the growthof scientific knowledge and exchange of information amongscientists in the field of atomic, molecular, and surface physics.http://icbc.zhaw.ch/de/science/institute-zentren/icbc/sasp-2016.html

23rd International Academic Conference on Development inScience and Technology (IACDST)11.02.2016Hotel Allegra, KlotenIACDST aims to bring together leading academic scientists,researchers and research scholars to exchange and sharetheir experiences and research results about all aspects ofDevelopment in Science and Technology. It also provides thepremier interdisciplinary forum for researchers, practitionersand educators to present and discuss the most recent innovations,trends, and concerns, practical challenges encountered and thesolutions adopted in the fields of Development in Science andTechnology.http://academicsworld.org/Conference/Switzerland2016/IACDST/

23rd International Conference on Environmental Scienceand Development (ICESD)12.02.2016Hotel Allegra, KlotenICESD aims to bring together leading academic scientists,researchers and research scholars to exchange and sharetheir experiences and research results about all aspects ofEnvironmental Science and Development. It also provides thepremier interdisciplinary forum for researchers, practitionersand educators to present and discuss the most recent innovations,trends, and concerns, practical challenges encountered and thesolutions adopted in the fields of Environmental Science andDevelopment.http://academicsworld.org/Conference/Switzerland2016/ICESD/

LS2 Annual Meeting 201615.02.2016–16.02.2016Amphipôle, University of Lausanne, LausanneAt this meeting, LS2 will bring together Swiss Life Scientistsfrom academic backgrounds as well as our industry partners withresearchers from all across Europe to explore the large spectrumof ‘Interdisciplinary Sciences’; Listen in on the exciting findingsin the field of Chemical Biology over Synthetic Biology toProteomics and ‘Classic’ Molecular and Cell Biology (andmany more), discuss opportunities and challenges of the SwissResearch funding system and the life beyond academia in non-academic professions, and be a part of the emotional debate oncareers of men and women in the Life Sciences.http://ls2-annual-meeting.ch

Research breakthroughs and social impact: Young scientistsdebate synthetic biology16.02.2016Amphipôle, University of Lausanne, LausanneThis event is part of the LS2 Annual Meeting 2016.Emerging technologies have the potential to offer new solutionsfor society’s challenges but also raise concerns about safety andethical implications. Synthetic biology is a maturing disciplineaiming tomodify, rebuild and design biological systems. Possibleapplications include production of biofuels in microalgae,synthesis of therapeutics in yeast cells or removal of pollutants’from the environment using bacteria.

Swiss Symposium on Lab Automation 201617.03.2016Hochschule fur Technik, Institute for Lab Automation andMechatronics, RapperswilThe Institute for Lab Automation and Mechatronics (ILT)annually invites to the Swiss Symposium on Lab Automation,where presentations are given by and for experts in the fieldof automation and instrumentation in the medical and lifescience sector. In 2016, the Symposium’s focus is on Labor 4.0:modern technologies and the smart lab of the future. Alongsidethe presentation, the exhibition provides an ideal networkingplatform and gives companies and institutions the opportunity topresent their skills and working areas.https://www.ilt.hsr.ch/index.php?id=14274

36th International Conference on Environment and NaturalScience (ICENS)04.04.2016Hotel Allegra, KlotenThe idea of the conference is for the scientists, scholars,engineers and students from universities all around the worldand industry to present ongoing research activities, and henceto foster research relations between academia and industry. Thisconference provides opportunities for delegates to exchangenew ideas and application experiences face to face, to establishbusiness or research relations and to find global partners forfuture collaboration.http://iserd.co/Conference/Zurich-Switzerland/ICENS/

information CHIMIA 2015, 69, No. 12 A819

SCS Spring Meeting 201622.04.2016University of Zurich, Irchel Campus, ZurichSymposium topic: «Green Chemistry»The SCS Spring Meeting is a one-day symposium and providesa high quality program with national and international speakersof a certain topic. It is also the platform for the Werner PrizeCeremony and the Werner Award Lecture.http://scg.ch/springmeeting/2016

lectures

01.01.16 - 28.02.2016

University of Bern, Anorganische, Analytische undPhysikalische ChemieDepartment of Chemistry and Biochemistry, Lecture hall S 379

05.02.2016 Prof. Horst Köppel, Universität Heidelberg,11.00 h Germany

Title to be announced

University of FribourgMain auditorium, Chemistry Department

28.01.201609.30 h Prof. Petra Swiderek, University of Bremen,

GermanyThe intriguing world of electrons: Auroras,radiotherapy, nanofabrication and more

10.15 h Prof. Michael Allan, University of FribourgFree electrons and molecules – a lifelongpassion

11.00 h Prof. Luisa de Cola, University of Strasbourg,FranceTitle to be announced

12.00 h Prof. Peter Belser, University of FribourgEnergy Transfer - Everywhere

14.00 h Prof. Frank Neese, Max Planck Institute forChemical Energy Conversion, Mülheim an derRuhr, GermanyAb initio Ligand Field Theory for d- andf-elements. A powerful link between theory andexperiment

14.45 h Prof. Claude Daul, University of FribourgThe amazing power of simple ideas

15.45 h Prof. Roman Fasel, University of BernCarbon nanomaterials

16.30 h Prof. Titus Andreas Jenny, University ofFribourgCarbon: a life-long love-story

University of Geneva, Chimie OrganiqueSciences II, Auditoire A. Pictet A100

21.01.2016 Prof. Vincent Gandon, Université Paris-Sud,16.30 h France

Title to be announced

18.02.2016 Prof. Konrad Tiefenbacher, Department of16.30 h Chemistry, Technische Universität München,

Munich, GermanyEnzyme-like Catalysis in Self-AssembledAromatic Cavities

University of Geneva, Société Chimique de GenèveSciences II, Auditoire A. Pictet A100

18.01.2016 Prof. Philippe Reymond, Department of Plant17.30 h Molecular Biology, University of Lausanne

Plant defenses against insect attack

ETH Zürich, Laboratory of Inorganic ChemistryHönggerberg HCI, J7

12.01.2016 Dr. Aleix Comas-Vives, ETH Zürich17.15 h The role of interfacial water on water

desalination, hydrates management, and oil &gas production

The complete and updated lecture calendar is available onwww.scg.ch/lectures

A820 CHIMIA 2015, 69, No. 12 impressum

www.chimia.ch

International Journal for Chemistryand

Official Membership Journalof the Swiss Chemical Society (SCS)and its Divisions

DivisionsAnalytical Sciences www.scg.ch/dasFundamental Research www.scg.ch/dfrIndustrial & Applied Chemistry www.scg.ch/diacMedicinal Chemistry & Chemical Biology www.scg.ch/dmccbPolymers, Colloids & Interfaces www.scg.ch/dpci

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Editorial BoardM. P. Brändle, ZürichC. Corminboeuf, LausanneP. J. Dyson, LausanneK.-H. Ernst, DübendorfR. Häner, BernM. Koller, KönizR. W. Kunz, ZürichP. Maienfisch, BaselR. Marti, FribourgM. G. Schlageter, BaselJ. Stohner, ZürichS. Sulzer-Mosse, Stein

Advisory BoardF. Merkt, Zürich (former DFR)K.-H. Altmann, Zürich (DMCCB)W. Jucker, Sisseln (DIAC)G. Hopfgartner, Genève (DAS)A. Baiker, ZürichJ. Bode, ZurichE. Felder, BaselD. Gygax, MuttenzK. Hungerbühler, ZürichH.-A. Klok, GenèveC. Leumann, BernF. Marechal, LausanneV. R. Meyer, St. GallenM. Missbach, BaselC. Nevado, ZurichT. Weller, Allschwil

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EuCheMSEuCheMSN E W S L E T T E R

November 2015

Countdown for Seville startsA lot has happened in the organisation

of the 6th EuCheMS Chemistry Congress

which will take place in Seville (Spain) on

11 to 15 September 2016.

Themes and topics

Organisers and the scientific committee

announced eight congress themes and 29

topics that cover a comprehensive range of

chemistry aspects (see next page).

http://euchems-seville2016.eu/euchems-

themes

Exhibition

The exhibition floor plan is available on

the website. Both sponsors and exhibitors

are quickly reserving spaces so organisers

recommend early booking.

http://euchems-seville2016.eu/exhibition-

booking-form

Plenary speakers

Outstanding speakers have already con-

firmed their attendance. Five Nobel Laur-

eates and Avelino Corma, Spain’s most

awarded chemist, will serve as plenary

speakers.


plenary-speakers

Sponsors support the congress

Some major chemical and engineering

companies as well as one of Europe’s major

chemistry societies have already confirmed

their support of the EuCheMS Chemistry

Congress. The Compañia Española de Petró-

leos (Cepsa), the Royal Society of Chemistry,

Repsol, Dow Chemical and Técnicas Reuni-

das have all declared their sponsorship. All

sponsors are highlighted on the congress

website and in communication materials.

Promotion

The congress organisers and the chair of

the scientific committee have visited some

of the major chemical societies to engage

their commitment and support for the con-

gress. Last June the organisers also at-

tended Achema 2015 in Frankfurt (Ger-

many) to advertise the congress to the

166,000 participants and 3,813 exhibitors

of the chemical, pharmaceutical and food

industry. In July the congress was also fea-

tured at the XXXV Biannual Congress of the

Spanish Royal Society of Chemistry in A Co-

ruña (Spain).

Congress newsletter

Up to now two issues of the EuCheMS

Congress newsletter have been prepared

and distributed among major stakeholders,

preregistered attendees and the chemical

community in general.


latest-newsletters

Press room

Interviews with congress organisers

and major chemical societies’ leaders are

published on an exclusive basis on the

congress website’s press room. Among

others David Cole-Hamilton, EuCheMS

president, and Peter G. Edwards, chair of

the scientific committee, share their vision

about the EuCheMS congresses, chemistry,

education and scientific communication as

well as other issues like the present and fu-

ture of chemistry societies in Europe.

http://euchems-seville2016.eu/portfolio_

category/press-room

Ángela López Berrocal

[email protected]

Young chemists in Boston

Five years ago, the European Young Chemists’

Network (EYCN) and the Younger Chemists

Committee of the American Chemical Society

(YCC-ACS) created theYoung Chemists Crossing

Borders (YCCB) programme. Every second year,

YCC-ACS invites six European students and a

delegation from the Jungchemikerforum (JCF)

of theGesellschaft Deutscher Chemiker (GDCh)

to ACS conferences. In return, EYCN brings six

American students to a EuCheMS event.

Last year, six young Americans visited the

EuCheMS Congress in Istanbul. In August this

year, the YCCB 2015 awardees Sophie Carenco

(France), Grégory Chatel (France), Thomas

McGlone (UK), Madeline Kavanagh (UK), Sebas-

tian Sobottka (Germany) and Bart Verbraeken

(Belgium) enjoyed the 250th ACS Meeting and

Exposition in Boston. YCCB students partici-

pated in symposia and presented their work in

poster sessions and oral communications. In

addition (and thanks to the organisers at YCC-

ACS and the North-Eastern Section of ACS) the

students visited MIT, Harvard, theWoods Hole

Oceanographic Institute and industries such

as Genzyme. Moreover, a variety of social

events were organised. EuCheMS president

David Cole-Hamilton hosted a traditional New

England Clambake, sponsored by EuCheMS,

RSC and GDCh.

The organisers also invited Fernando Gomollón

Bel (EYCN chair) and Fréderique Backaert (EYCN

current past chair) to participate in the ex-

change programme. They presented their most

recent activities to their American colleagues,

establishing the foundations for YCCB 2016,

which will bring six YCC-ACS students to the

EuCheMS Chemistry Congress 2016 in Spain.

Fernando Gomollón Bel, [email protected]

Participants of the YCCB programme in Boston.

CHIMIA 2015, 69, No. 12 A821

The European Associationfor Chemical and MolecularSciences 2

Organometallic chemistry:21st EuCOMC in Bratislava

A series of successful European Conferences

on Organometallic Chemistry (EuCOMC) con-

tinued with the 21st event in Bratislava, 5 to 9

July. The conference was organised by Slovak

and Czech Chemical Societies under the um-

brella of EuCheMS. More than 220 partici-

pants from 38 countries of five continents ga-

thered in the capital of Slovakia to listen to 56

speakers and to discuss 134 posters in topics

ranging from fundamental organometallic

chemistry via catalysis and materials to envi-

ronmental and bio-aspects of organometallic

compounds.

Distinguished Plenary Lectures were pres-

ented by Alois Fürstner (Germany), Ian

Manners (UK), Masakatsu Shibasaki (Japan),

Georg Süss-Fink (Switzerland) and Maria

Christina White (USA) whereas Young Plenary

Lectures were given by Thibault Cantat

(France), Oleg Filippov (Russia), Peter Fristrup

(Denmark), Giuliano Giambastiani (Italy) and

Roman Šebesta (Slovakia) who substituted for

Franziska Schoenebeck (Germany) being un-

able to come.

For the first time in this series the participants

witnessed the ceremony of the European Prize

for Organometallic Chemistry 2015. The Award

Lecture was delivered by Malcolm L. H. Green

(UK, see the details in the EuCheMSNewsletter

of September 2015).

Two poster prizes sponsored by the RSC jour-

nal Inorganic Chemistry Frontiers documented

the effort of the organisers to encourage and

support the youngest generation of European

organometallic chemists.

Jan Cermák, [email protected]

At the European Conference on Organometallic

Chemistry (EuCOMC) in Bratislava.

IUPAC General Assembly in KoreaAt the 48th General Assembly of IUPAC

in Busan (Korea) from 8 to 14 August the

council elected the officers for the next

biennium and voted for a new intermediate

subscription scheme to overcome the ex-

change rate variations. In his report, Mark

Cesa, IUPAC president, explained the

changes in the management and the secre-

tariat during the last 18months. The execu-

tive director had left and Lynn Soby was

hired as new ED. Also the secretary general

René Deplanque had resigned shortly after

the last General Assembly and was fol-

lowed by Colin Humphris in April 2014.

As new vice president and president

elect Qi-Feng Zhou (China) was elected by

the council. Mark Cesa (USA) will be past

president when Natalia Tarasova (Russia)

becomes president for 2016 and 2017.

Colin Humphris (UK) was elected as treas-

urer and Richard Hartshorn (New Zealand)

as secretary general. The six newly elected

members of the bureau are Mei Hung Chui

(China Taipei), Christopher M. A. Brett (Por-

tugal), Hemda Garelick (UK), Ehud Keinan

(Israel), Kew-Ho Lee (Korea) and Pietro

Tundo (Italy). The re-elected members are

Russell Boyd (Canada), Tavarekere Chan-

drashekar (India), Christopher Ober (USA)

and Kaoru Yamanouchi (Japan).

Michael Dröscher

[email protected]

EuCheMS Chemistry Congress 2016: Themes and topicsThe EuCheMS Chemistry Congress 2016 will

take place in Seville (Spain) on 11 to 15 Sep-

tember 2016. The congress themes and topics

are as follows:

Theme A: Education and society

Chair: Evangelia Varella, Greece

Topic A1: Chemistry education

Topic A2: Chemistry, society and

public engagement

Topic A3: Benefits:Wealth creation and society

Theme B: The environment, energy and

sustainability

Chair: Inmaculada Ortiz, Spain

Topic B1: Sustainable energy and air quality

Topic B2: Environment and natural resources

management

Topic B3: Sustainable chemistry

Topic B4: Food chemistry

Theme C: New chemical compounds:

Synthesis, methods and industrial processes

Chair: Sylviane Sabo-Etienne, France

Topic C1: Synthesis and reactivity in metal

based compounds

Topic C2: Synthesis and reactivity in carbon

based compounds

Topic C3: Methods and mechanisms

Topic C4: Catalysis in solution

Theme D: Catalysis, industry and applications

Chair: Bob Tooze, UK

Topic D1: Chemistry in industry

TopicD2: Industrial processes for the21st century

Topic D3: Catalysis at interfaces

Theme E:Materials, devices and nanochemistry

Chair: Luisa De Cola, France

Topic E1: Materials chemistry

Topic E2: Nanomaterials, devices, technology

and applications

Topic E3: Analytical techniques,

characterisation and properties

Topic E4: Carbon based nanochemistry

Theme F: Properties of matter

Chair: Angela Agostiano, Italy

Topic F1: States of matter

Topic F2: Properties of materials

Topic F3: Polymers

Topic F4: Innovative computational

environments for molecular science

Theme G: Physical, analytical and

experimental methods in chemistry

Chair: Günther Gauglitz, Germany

Topic G1: Analytical and physical methods

Topic G2: Determination of structure and

physical properties

Topic G3: Chemical dynamics

Theme H: Chemistry in the life sciences

Chair: Péter Mátyus, Hungary

Topic H1: Drug discovery and chemical biology

Topic H2: Bio-macrocolecules

Topic H3: Methods and applications

Topic H4: In-silico methods in life sciences

A822 CHIMIA 2015, 69, No. 12

3 www.euchems.org

The EuCheMS Executive BoardThe EuCheMS Newsletter introduces the EuCheMS Executive Board members. This issue

goes on with Wolfram Koch (Germany), Robert Parker (UK) and Igor Tkatchenko (France).

Wolfram Koch has been the executive di-

rector of the Gesellschaft Deutscher Che-

miker since 2002. Before that he was for

seven years professor of theoretical organic

chemistry at TU Berlin, prior to which he

was a senior scientist at IBM research facil-

ities in Heidelberg (Germany) and San Jose

(California).

During his active time in research Wolf-

ram Koch worked on quantum chemical in-

vestigations on properties and reactivity of

open shell transition metal compounds as

well as on spectroscopic properties of small

molecules. Hehas authored and co-authored

ca. 190 papers in peer-reviewed journals

and is the senior author of a textbook on

density functional theory.

Besides being amember of the EuCheMS

Executive Board for more than 15 years,

Wolfram Koch is also a member of the

IUPAC Finance Committee (until the end of

this year), the Administrative Council of the

German Copyright Collective (VG WORT)

and Chairman of the Advisory Board of the

German National Library of Science and

Technology (TIB Hannover). He is a Fellow

of the Royal Society of Chemistry and an

IUPAC Fellow and holds an honorary mem-

bership of the Czech Chemical Society. Very

recently he was appointed an honorary fel-

low of the Chemical Publishing Society Eu-

rope (ChemPubSoc Europe).

Robert Parker is chief executive of the

Royal Society of Chemistry, a learned and

professional organisation with 51,000

members. It has major outputs in edu-

cation, events, policy and publishing. Be-

fore taking up the position as CEO he ran

the very successful publishing wing of the

RSC. Robert Parker has a PhD in chemistry

and is a Chartered Scientist, Chartered

Chemist and Fellow of the Royal Society of

Chemistry as well as being a husband,

father of three adult children and keen gar-

dener and motorcyclist.

Igor Tkatchenko is a professor of organo-

metallic chemistry and homogeneous

catalysis. After postdoctoral research with

Günther Wilke at the Max Planck Institute

for Coal Research in Mülheim/Ruhr (Ger-

many) and five years research in industry,

he worked in France at the Laboratoire de

Chimie de Coordination, CNRS, in Toulouse

and at the Institut de Recherches sur la

Catalyse, CNRS, in Villeurbanne. He then

joined the Institute for Molecular Chemis-

try at the University of Bourgogne, where

he served as research director until his re-

tirement. Igor Tkatchenko serves as the

general secretary of the Société Chimique

de France. He has been appointed to the

EuCheMS Executive Board and has repre-

sented ChemPubSoc Europe for many years

and was a board member of ChemViews

magazine. kjs

EuCheMS at theACS Fall Meeting in BostonNineta Majcen, EuCheMS general secretary,

and David Cole-Hamilton, EuCheMS presi-

dent, attended the ACS Fall Meeting in Bos-

ton in August at the invitation of Diane Grob

Schmidt, ACS president.Wolfram Koch (GDCh)

and Robert Parker (RSC), both EuCheMS

Executive Board members, were also present,

which allowed various meetings to take place

involving EuCheMS and the ACS. One of them

involved Nineta Majcen, Robert Parker and

David Cole-Hamilton from EuCheMS and

Diane Grob Schmidt, Tom Connelly (CEO), Pat

Confalone (chair of the Board) and Denise

Creech (director, Members & Scientific Ad-

vancement) from ACS.

The discussion focussed on how the two or-

ganisations might work more closely to-

gether. It was agreed that we should investi-

gate possible joint symposia, and that the two

organisations would support one another’s

main meetings (ACS Fall and Spring Meetings,

EuCheMS Chemistry Congresses). There was

strong support for the Young Chemists Cross-

ing Borders programme and it was agreed

that the two societies should collaborate in

the next iteration of an employment survey.

ACS have been surveying the employment of

chemists in the US for many years. The publi-

cation of the recent report on chemist em-

ployment in Europe (see R. Salzer, P. Taylor, et

al., Chem. Eur. J., 2015, 21, 9921), carried out

with the support of the European Chemistry

Thematic Network (ECTN) and EuCheMS, indi-

cates that there could be important com-

parative information to obtain through col-

laboration.

As part of the Young Chemists Crossing

Borders programme, six European students

had been selected by the European Young

Chemists’Network to attend the ACS meeting

(see article on page 1). An ACS-GDCh student

exchange was also taking place, so a day trip

involving a visit toWoods Hole Oceanographic

Institution and a clam bake on Cape Cod, sup-

ported by EuCheMS, RSC, GDCh and ACS, was

arranged. This was an excellent opportunity

for further networking.

Nineta Majcen

David Cole-Hamilton

[email protected]

Wolfram Koch Robert Parker Igor Tkatchenko

CHIMIA 2015, 69, No. 12 A823

4

EuCheMS Newsletter

Newsletter coordinator: Karin J. Schmitz

Please send all correspondence andmanuscripts

to [email protected]

Editors:WolframKoch (responsible),

Karin J. Schmitz, UtaNeubauer, Frankfurt amMain

Advisory board:David Cole-Hamilton (Presi-

dent), Ulrich Schubert (Vice-President), Franco

De Angelis (Treasurer), Eckart Ruehl (Member of

Executive Board), Nineta Majcen (Secretary

General).

Layout: Jürgen Bugler, Frankfurt amMain

Production:Nachrichten aus der Chemie

Publisher:Gesellschaft Deutscher Chemiker on

behalf of EuCheMS

Postfach 900440

D-60444 Frankfurt amMain

EuCheMS General Secretary:

NinetaMajcen, Rue duTrône, 62

1050 Brussels, Belgium

[email protected]

www.euchems.eu

EuCheMS is registered as “Association inter-

nationale sans but lucratif” (AISBL, international

non-profit association), AISBL-Registered office:

Rue duTrône, 62, 1050 Brussels, Belgium

The GDCh Science ForumChemistry in Dresden

On 30 August EuCheMS president David Cole-

Hamilton addressed the audience of the

opening ceremony of the GDCh Science Forum

Chemistry in Dresden, one of the largest

chemistry conferences in Europe this year. He

introduced EuCheMS as the single voice of

chemistry in Europe and mentioned a number

of recent EuCheMS activities, such as the

Younger Chemists Crossing Border exchange

programme with the American Chemical So-

ciety and the first European Employment Sur-

vey, coordinated by Reiner Salzer from the

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The GDCh Science Forum Chemistry with the

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gathered more than 1500 chemists. In the

UNESCO International Year of Light, GDCh had

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tute for Biophysical Chemistry in Göttingen,

Germany) as one of the plenary speakers

talked about „Far-field optical nanoscopy:

principles and recent advancements“. K. Barry

Sharpless (The Scripps Research Institute in La

Jolla, USA), Nobel prize laureate 2001, was

awarded the August-Wilhelm-von-Hofmann

lecture. He talked about „Click chemistry –

new directions“. wk/kjs

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Chemistry, www.chimica.unipd.it/wispoc/pubblica

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(ACS),Wolfram Koch and David Cole-Hamilton (both EuCheMS) with his daugther Rosie Cole-Hamilton,

Neil Burford and Youla Tsantrizos (both Chemical Society of Canada) and Tom Connelly (ACS).

tend the Heroes of Chemistry dinner where

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A824 CHIMIA 2015, 69, No. 12

SCHWEIZ. CHEMISCHE GESELLSCHAFT SCGSOCIETE SUISSE DE CHIMIE SSCSWISS CHEMICAL SOCIETY SCS

VOL. 69 (2015)

www.chimia.ch

Editorial BoardM. P. Brändle, ZürichC. Corminboeuf, LausanneP. J. Dyson, LausanneK.-H. Ernst, DübendorfR. Häner, BernM. Koller, KönizR. W. Kunz, ZürichP. Maienfisch, BaselR. Marti, FribourgM. G. Schlageter, BaselJ. Stohner, ZürichS. Sulzer-Mosse, Stein

Advisory BoardF. Merkt, Zürich (former DFR)K.-H. Altmann, Zürich (DMCCB)W. Jucker, Sisseln (DIAC)G. Hopfgartner, Genève (DAS)A. Baiker, ZürichJ. Bode, ZurichE. Felder, BaselD. Gygax, MuttenzK. Hungerbühler, ZürichH.-A. Klok, GenèveC. Leumann, BernF. Marechal, LausanneV. R. Meyer, St. GallenM. Missbach, BaselC. Nevado, ZurichT. Weller, Allschwil

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Register i CHIMIA 2015, 69, No. 12

Author IndexCHIMIA 69 (2015)

Abdon, A., see Worlitschek, J., 777Ackermann, J., see Neidhart, W., 407Adibekian, A., Magauer, T., Conference

Report: The 50thEUCHEMConferenceon Stereochemistry (Burgenstock Con-ference 2015), Brunnen, April 26th–Mai 1st, 2015, 485

Adlhart, C., Fleischli, F. D., Morf, F., Uni-versities of Applied Sciences: SkinConcentrations of Topically AppliedSubstances in Reconstructed HumanEpidermis (RHE) Compared with Hu-man Skin Using in vivo Confocal Ra-man Microscopy, 147

Aghazada, S., see Nazeeruddin, M. K., 253Aghazada, S., see Nazeeruddin, M. K., 274Aicher, L., see Walser, T., 52Aicher, L., see Walser, T., 804Al-Oweini, R., see Kortz, U., 537Albrecht, M., Licini, G., Iridium-mediated

Bond Activation and Water Oxidationas an Exemplary Case of CARISMA,A European Network for the Develop-ment of Catalytic Routines for SmallMolecule Activation, 316

Allardyce, C. S., Bays, R., Thévenaz, N.,Light in Medicine: The Interplay ofChemistry and Light, 10

Ammann, A., see Worlitschek, J., 777Amrein, K., see Neidhart, W., 407Amstutz, V., see Girault, H. H., 284Amstutz, V., see Girault, H. H., 753Ang,W. H., Ong, J. X., Yap, S. Q.,Wong, D.

Y. Q., Chin, C. F., Platinum(iv) Carbo-xylate Prodrug Complexes as VersatilePlatforms for Targeted Chemotherapy,100

Aragay, G., see Ballester P., 652

Bachmann, A. L., see Fierz, B., 624Bachmann, D., see Daeppen, C., 63Bag, N., see Wohland, T., 112Bagnoud-Velásquez, M., Refardt, D., Vuil-

le, F., Ludwig, C., Opportunities forSwitzerland to Contribute to the Pro-duction of Algal Biofuels: the Hydro-thermal Pathway to Bio-Methane, 614

Bährle, C., see van Bokhoven, J. A., 597Bakker, E., Xie, X., Gutiérrez, A., Trofimov,

V., Szilagyia, I., Soldati, T., PotassiumSensitive Optical Nanosensors Contai-ning Voltage Sensitive Dyes, 196

Bakker, E., Cuartero, M., Crespo, G. A.,Thin Layer Samples Controlled by Dy-namic Electrochemistry, 203

Brethauer, S., see Studer, M. H., 572Broekmann, P., see Durst, J., Rudnev, A.,

769Brog, J.-P., see Kwon, N. H., Fromm, K.

M., 734Brönnimann, R., see Emmenegger, L., 708Bryan, L. C., see Fierz, B., 624Bui, D. A., see Hauser, P. C., 806Buyck, T., see Zhu, J., 199

Callini, E., see Züttel, A., 264Callini, E., Kato, S., Mauron, P., Züttel, A.,

Surface Reactions are Crucial for Ener-gy Storage, 269

Callini, E., see Züttel, A., 741Chappuis, T., see Vorlet, O., 807Chen, X., see Yan, N., 120Chevreux, S., see Lemercier, G., 666Chiaia-Hernandez, A. C., see Giger, W.,

488CHIMIA Editorial Board, CHIMIA News,

1CHIMIA Editorial Board, Instructions to

Authors, 2015, 2Chin, C. F., see Ang, W. H., 100Christen, P., see Bieri, S., 231Comas-Vives, A., see Núñez-Zarur, F., 225Comte, P., see Rothen-Rutishauser, B., 68Conte, A., see Neidhart, W., 407Copéret, C., Estes, D. P., The Role of Pro-

ton Transfer in Heterogeneous Trans-formations of Hydrocarbons, 321

Copéret, C., Tada, S., Thiel, I., Lo, H.-K.,CO2

Hydrogenation: Supported Nano-particles vs. Immobilized Catalysts,759

Corsi, C., see Walter, H., 425Cortés-Salazar, F., see Girault, H. H., 284Cortés-Salazar, F., see Girault, H. H., 290Cramer, N., Souillart, L., Enantioselective

Rhodium-catalyzed C–C Bond Activa-tion of Cyclobutanones, 187

Crespo, G. A., see Bakker, E., 203Crochet, A., see Kwon, N. H., Fromm, K.

M., 734Cuartero, M., see Bakker, E., 203Custodis, V., see van Bokhoven, J. A., 597Czerwinski, J., see Rothen-Rutishauser, B.,

68Czerwinski, J., see Heeb, N., 152

Daali, Y., see Daouk, S., 684Dabros, M., Vorlet, O., Marti, R., Riedl,

W., Grundler, G., Vaccari, A., Zinn, M.,Ecker, A., Hinderling, C., Universitiesof Applied Sciences: PAT at the Uni-versities of Applied Sciences, 482

Daeppen, C., Bachmann, D., Pannwitz, A.,Kerschgens, I., Laupheimer, C., Ro-ther, M., Jundt, L., Rickhaus, M., Gan-tenbein, M., Bodoky, I., ConferenceReport: Basel Chemistry Symposium2014: In Memory of Prof. T. Reichs-tein, 63

Ballester P., Espelt, M., Aragay, G., Resol-ving the Magnetic Asymmetry of theInner Space in Self-assembled DimericCapsules Based on Tetraurea-calix[4]pyrrole Components, 652

Balog, S., Polymer and Colloid Highlights:Structural Studies of Radiation-graftedCopolymer Proton Exchange Membra-nes, 69

Baranoff, E., Herbaut, A., UV-visible Ab-sorption Study of the Self-associationof Non-ionic Chromonic Triphenyl-enes TP6EOnM (n = 2, 3, 4) in DiluteAqueous Solutions: Impact of ChainLength on Aggregation, 520

Barbato, M., see Haselbacher, A., 799Bassas-Galià, M., see Mathieu, M., 627Bassetto, V. C., see Girault, H. H., 284Bassil, B. S., see Kortz, U., 537Bays, R., see Allardyce, C. S., 10Beller, M., Laurenczy, G., Editorial: Ca-

talytic Activation of Small Molecules,313

Benetti, E. M., Ramakrishna, S. N., Po-lymer and Colloid Highlights: LateralDeformability of Polymer Brushes byAFM-Based Method, 709

Bertini, F., see Peruzzini, M., Gonsalvi, L.,331

Bettens, R. P. A., Ouyang, J. F.,ModellingWater: A Lifetime Enigma, 104

Bieri, S., Mathon, C., Edder, P., Christen,P., Highlights of Analytical Sciencesin Switzerland: Occurrence of NaturalHepatotoxines in Herbal Teas, 231

Binder, S. R., see Haussener, S., 780Biollaz, S. M. A., see Schildhauer, T. J., 603Biotechnet Switzerland, Biotechnet Swit-

zerland: A Network Sets Things inMotion: TEDD Celebrates its 5th An-niversary, 690

Bisig, C., see Rothen-Rutishauser, B., 68Blanchard-Desce, M., see Lemercier, G.,

666Blaser, H.-U., Looking Back on 35 Years

of Industrial Catalysis, 393Blazina, T., see Winkel, L., 547Bobbink, F. D., see Dyson, P. J., 765Bodenmiller, B., Günther, D., Schwarz, G.,

Wang, H. A. O., Giessen, C., Schapiro,D., Hattendorf, B., Highlights of Ana-lytical Sciences in Switzerland: LaserAblation ICP-MS for Single-Cell-ba-sed Tissue Imaging, 637

Bodmann, K., Marti, R., Conference Re-port: 12. Freiburger Symposium 2015:Smart Solutions in the Chemical Pro-cess & Product Development – CaseStudies from the Chemical Industry,698

Bodnarchuk, M. I., see Kovalenko, M. V.,724

Bodoky, I., see Daeppen, C., 63Bondarenko, A., see Girault, H. H., 290Bonn, A. G., see Wenger, O. S., 10Börner, R., see Sigel, R. K. O., 207Bourée, W. S., see Sivula, K., 789

CHIMIA 2015, 69, No. 12 register ii

Gonsalvi, L., Peruzzini, M., Mellone, I.,Bertini, F., Guerriero, A., New Windin Old Sails: Novel Applications ofTriphos-based Transition Metal Com-plexes as Homogeneous Catalysts forSmall Molecules and Renewables Ac-tivation, 331

Gonzalez, A., see Mathieu, M., 627Goonesekera, K., see Daguenet-Frick, X.,

784Gopakumar, A., see Dyson, P. J., 765Graf-Hausner, U., Rimann, M., Laternser,

S., Keller, H., Leupin, O., BiotechnetSwitzerland: 3D Bioprinted Muscleand Tendon Tissues for Drug Develop-ment, 65

Grass, R. N., see Stark, W. J., 369Graule, T., see Michalow-Mauke, K. A.,

Kröcher, O., 220Gribkov, D., see Walter, H., 425Grundler, G., see Dabros, M., 482Guerriero, A., see Peruzzini, M., Gonsalvi,

L., 331Guijarro, N., see Sivula, K., 30Günther, D., Bodenmiller, B., Schwarz, G.,

Wang, H. A. O., Giessen, C., Schapiro,D., Hattendorf, B., Highlights of Ana-lytical Sciences in Switzerland: LaserAblation ICP-MS for Single-Cell-based Tissue Imaging, 637

Guo, S., see Zhao, J., 524Gutiérrez, A., see Bakker, E., 196Gwerder, D., see Worlitschek, J., 777

Haag, R., see Heeb, N., 152Hagfeldt, A., Editorial: Chemistry and

Light: The International Year of Light,6

Hagfeldt, A., Vlachopoulos, N., Zhang, J.,Dye-sensitized Solar Cells: New Ap-proaches with Organic Solid-state HoleConductors, 41

Häner, R., Bösch, C. D., Probst, M., Vy-borna, Y., Vybornyi, M., Langenegger,S. M., Swiss Science Concentrates 70,153, 233, 302, 370, 495, 549, 638, 710,805

Haselbacher, A., Geissbühler, L., Zavat-toni, S., Barbato, M., Zanganeh, G.,Steinfeld, A., Experimental and Nume-rical Investigation of Combined Sensi-ble/Latent Thermal Energy Storage forHigh-Temperature Applications, 799

Hassan, H. M. A., A Short History of theUse of Plants as Medicines fromAnci-ent Times, 622

Hattendorf, B., see Bodenmiller, B., Gün-ther, D., 637

Haudecoeur, R., see Monchaud, D., 530Hauser, P. C., see Koenka, I. J., 172Hauser, P. C., Bui, D. A., Highlights of

Analytical Sciences in Switzerland:Deep UV-LED Based Absorbance De-tectors for Narrow-Bore HPLC and Ca-pillary Electrophoresis, 806

Fei, Z., see Yan. N., Dyson, P. J., 592Ferri, D., see Michalow-Mauke, K. A.,

Kröcher, O., 220Fierz, B., Bryan, L. C., Bachmann, A. L.,

Conference Report: Molecular andChemical Mechanism in Epigenetics –Swiss Summer School 2015, July12–17, 2015, Hotel Kurhaus, Arolla,Switzerland, 624

Fink, C., see Laurenczy, G., 746Fiorini, E., see Sigel, R. K. O., 207Fischer, L. J., see Worlitschek, J., 777Fleischli, F. D., see Adlhart, C., 147Fleury-Souverain, S., see Daouk, S., 684Four, M., see Lemercier, G., 666Fromm, K. M., Kwon, N. H., Brog, J.-P.,

Maharajan, S., Crochet, A.,Nanomate-rials Meet Li-ion Batteries, 734

Fu, Y., see Durst, J., Rudnev, A., 769Fumey, B., see Daguenet-Frick, X., 784

Gagnon, K. J., see Dalgarno, S. J., 516Gaillard, V., see Mathieu, M., 627Gantenbein, M., see Daeppen, C., 63Gantenbein, P., see Daguenet-Frick, X.,

784Gao, P., see Nazeeruddin, M. K., 253Gao, P., see Nazeeruddin, M. K., 474Gasic, B., see Walser, T., 52Gasic, B., see Walser, T., 804Gasilova, N., see Girault, H. H., 290Gasser, G., Mari, C., Lightening up Ruthe-

nium Complexes to Fight Cancer? 176Gasser, G.,Metal Complexes and Medici-

ne: A Successful Combination, 442Geissbühler, L., see Haselbacher, A., 799Germann, M., Willitsch, S., Tong, X., For-

bidden Vibrational Transitions in ColdMolecular Ions: Experimental Obser-vation and Potential Applications, 213

Giessen, C., see Bodenmiller, B., Günther,D., 637

Giger,W., Chiaia-Hernandez,A.C.,Confe-rence Report: International Conferenceon Contaminated Sediments – Conta-Sed 2015, 8–13 March 2015, MonteVerità, Ascona, Switzerland, 488

Giordano, F., see Vorlet, O., 807Girault, H. H., Lesch, A., Cortés-Salazar,

F., Bassetto, V. C., Amstutz, V., InkjetPrinting Meets Electrochemical Ener-gy Conversion, 284

Girault, H. H., Bondarenko, A., Cortés-Salazar, F., Gasilova, N., Lesch, A.,Qiao, L., Analytical Chemistry at theLaboratoire d’Electrochimie Physiqueet Analytique, 290

Girault, H. H., Dennison, C. R., Vrubel,H., Amstutz, V., Peljo, P., Toghill, K. E.,Redox Flow Batteries, Hydrogen andDistributed Storage, 753

Daguenet-Frick, X., Gantenbein, P., Rom-mel, M., Fumey, B., Weber, R., Goo-nesekera, K., Williamson, T., SeasonalSolar Thermal Absorption Energy Sto-rage Development, 784

Dalgarno, S. J., Fairbairn, R. E., Teat, S. J.,Gagnon, K. J., A Convenient SyntheticRoute to Partial-Cone p-Carboxylato-calix[4]arenes, 516

Daouk, S., Fleury-Souverain, S., Daali, Y.,Development of an LC-MS/MS Me-thod for the Assessment of SelectedActive Pharmaceuticals and Metabo-lites in Wastewaters of a Swiss Univer-sity Hospital, 684

Das, S., see Dyson, P. J., 765Davey, C. A., Exposure to Metals Can Be

Therapeutic, 125Delley, M. F., A Molecular Approach to

Well-defined Metal Sites Supported onOxides with Oxidation State and Nu-clearity Control, 168

Dennison, C. R., see Girault, H. H., 753Dibenedetto, A., Conditions for the Use of

CO2, 353

Döhlert, P., see Enthaler, S., 327Domanski, K., seeNazeeruddin,M. K., 253Domanski, K., seeNazeeruddin,M. K., 253Dünki, S. J., see Opris, D. M., 548Durst, J., Rudnev, A., Dutta, A., Fu, Y.,

Herranz, J., Kaliginedi, V., Kuzume,A., Permyakova, A. A., Paratcha, Y.,Broekmann, P., Schmidt, T. J., Electro-chemical CO

2Reduction – A Critical

View on Fundamentals, Materials andApplications, 769

Dutta, A., see Durst, J., Rudnev, A., 769Dyson, P. J., Yan. N., Siankevich, S., Fei,

Z., Application of Ionic Liquids in theDownstream Processing of Lignocellu-losic Biomass, 592

Dyson, P. J., Das, S., Bobbink, F. D., Go-pakumar, A., Soft Approaches to CO

2Activation, 765

Ecker, A., see Dabros, M., 482Edder, P., see Bieri, S., 231Ellert, C., see Vorlet, O., 807Elsener, M., see Michalow-Mauke, K. A.,

Kröcher, O., 220Emmenegger, L., see Heeb, N., 152Emmenegger, L., Jágerská, J., Brönnimann,

R., Faist, J., Jouy, P., Looser, H., Soltic,P., Tuzson, B., Highlights of AnalyticalSciences in Switzerland: Multi-Com-ponent Trace Gas Spectroscopy UsingDual-Wavelength Quantum CascadeLasers, 708

Enthaler, S., Döhlert, P., Weidauer, M., Ni-trous Oxide-dependent Iron-catalyzedCoupling Reactions of Grignard Rea-gents, 327

Espelt, M., see Ballester P., 652Estes, D. P., see Copéret, C., 321

Fairbairn, R. E., see Dalgarno, S. J., 516Faist, J., see Emmenegger, L., 708

Register iii CHIMIA 2015, 69, No. 12

Kröcher, O., Kambolis, A., Schildhauer, T.J.,COMethanation for Synthetic Natu-ral Gas Production, 608

Kuhn, B., see Neidhart, W., 407Kündig, E. P., Spichiger, D., Editorial: SCS

Laureates and Awards & Fall Meeting2015, 385

Kuzume, A., see Durst, J., Rudnev, A., 769Kwon, N. H., Fromm, K. M., Brog, J.-P.,

Maharajan, S., Crochet, A.,Nanomate-rials Meet Li-ion Batteries, 734

Laguerre, A., see Monchaud, D., 530Laternser, S., see Graf-Hausner, U., 65Laupheimer, C., see Daeppen, C., 63Laurenczy, G., Beller, M., Editorial: Ca-

talytic Activation of Small Molecules,313

Laurenczy, G., Fink, C., Montandon-Clerc,M., Hydrogen Storage in the CarbonDioxide – Formic Acid Cycle, 746

Le Formal, F., see Sivula, K., 30Le Formal, F., see Sivula, K., 789Lee, A. C.-H., see William, A. D., 142Lemercier, G., Mongin, O., Four, M.,

Chevreux, S., Blanchard-Desce, M.,Two-photon Absorption Engineeringof 5-(Fluorenyl)-1,10-phenanthroline-based Ru(ii) Complexes, 666

Lesch, A., see Girault, H. H., 284Lesch, A., see Girault, H. H., 290Leupin, O., see Graf-Hausner, U., 65Levillain, M., see Monchaud, D., 530Li, J., see Xu, Q., 348Li, W., see Wu, X.-F., 345Li, Z., Yang, Y., Evolving P450pyr Mono-

oxygenase for Regio- and Stereoselec-tive Hydroxylations, 136

Licini, G., see Albrecht, A., 316Lin, Z., see Kortz, U., 537Ling-Chen, see Pan, M., 670Liu, W., see Kortz, U., 537Lo, H.-K., see Copéret, C., 759Looser, H., see Emmenegger, L., 708Lu, Y., see Michalow-Mauke, K. A., Krö-

cher, O., 220Ludwig, C., see Bagnoud-Velásquez, M.,

614Ludwig, P. E., Vaucher, A. C., Phong Lê,

T., Suter, Y., Two Bronze Medals forSwitzerland at the 46th InternationalChemistry Olympiad in Hanoi, Viet-nam, 71

Luterbacher, J. S., Héroguel, F. E.,Rozmysłowicz, B., Improving Hetero-geneous Catalyst Stability for Liquid-phase Biomass Conversion and Refor-ming, 582

Lüthi, H. P., Editorial: Laureates: JuniorPrizes, SCS Fall Meeting 2014, 165

Ma, Z., see van Bokhoven, J. A., 597Magauer, T., Adibekian, A., Conference

Report: The 50thEUCHEMConferenceon Stereochemistry (Burgenstock Con-ference 2015), Brunnen, April 26th–Mai 1st, 2015, 485

Hunziker, D., see Neidhart, W., 407Huwyler, J., see Walser, T., 52Huwyler, J., see Walser, T., 804

Jágerská, J., see Emmenegger, L., 708Jahnke, W., Skora, L., Contributions of Bi-

omolecular NMR to Allosteric DrugDiscovery, 421

Jeschke, G., see van Bokhoven, J. A., 597Jessen, H. J., Simon, Y. C., Merz, L.,

SCNAT: The 8th Young Faculty Mee-ting – An Active Crowd Attuned toModern Challenges, 475

Joó, F., see Kathó, A., 339Jouy, P., see Emmenegger, L., 708Jundt, L., see Daeppen, C., 63

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225SCS Foundation Alfred Werner Fund,

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CHIMIA 2015, 69, No. 12 register vi

Bacterial synthesis of metal chelatorsBiotechnet Switzerland: Where Experts

Meet to Exchange Knowledge: Bio-technet’s Summer School onAdvancedBiotechnology, 634

Benzimidazolyl ringDimension Increase via Hierarchical Hy-

drogen Bonding from Simple Pincer-like Mononuclear Complexes, 670

BiocatalysisEvolving P450pyr Monooxygenase for

Regio- and Stereoselective Hydroxyl-ations, 136

BioenergyEditorial: SCCER BIOSWEET – The

Swiss Competence Center for EnergyResearch on Bioenergy, 569

BiofuelOpportunities for Switzerland to Contrib-

ute to the Production of Algal Biofu-els: the Hydrothermal Pathway to Bio-Methane, 614

BiomassValorization of Renewable Carbon Re-

sources for Chemicals, 120New Wind in Old Sails: Novel Applica-

tions ofTriphos-basedTransitionMetalComplexes as Homogeneous Catalystsfor Small Molecules and RenewablesActivation, 331

Application of Ionic Liquids in the Down-stream Processing of LignocellulosicBiomass, 592

Biomass conversionImproving Heterogeneous Catalyst Stabil-

ity for Liquid-phase Biomass Conver-sion and Reforming, 582

Biomethanation microbial processBiotechnet Switzerland: ORION – A

Global Approach to Waste Manage-ment, 365

Biomolecular NMRContributions of Biomolecular NMR to

Allosteric Drug Discovery, 421

Bioorganometallic chemistryMetal Complexes and Medicine: A Suc-

cessful Combination, 442

Bioprinting standardizationBiotechnet Switzerland: 3D Bioprinted

Muscle and Tendon Tissues for DrugDevelopment, 65

BiorefineryBiochemical Conversion Processes of

Lignocellulosic Biomass to Fuels andChemicals – A Review, 572

BiosynthesisUniversities of Applied Sciences: Chemi-

cal Modification of Polyhydroxyal-kanoates (PHAs) for the Preparation ofHybrid Biomaterials, 627

Anaerobic digestion machineBiotechnet Switzerland: ORION – A


Analytical chemistryAnalytical Chemistry at the Laboratoire

d’Electrochimie Physique et Ana-lytique, 290

Anderson-Evans structureSynthesis and Structure of Hexatungs-

tochromate(iii), [H3CrIIIW

6O

24]6–, 537

AnilinesTransition Metal-free Methylation of

Amines with Formaldehyde as the Re-ductant and Methyl Source, 345

Anode materialsEvaluation of Metal Phosphide Nanocrys-

tals asAnodeMaterials for Na-ion Bat-teries, 724

Antibiotic resistanceBiotechnet Switzerland: Where Experts


Anticancer agentPlatinum(iv) Carboxylate Prodrug Com-

plexes as Versatile Platforms for Tar-geted Chemotherapy, 100

Exposure to Metals Can Be Therapeutic,125

Antimicrobial-resistant organismsBiotechnet Switzerland: Platform for a

Technological Leap in Antibiotics, 809

ApoptosisLight in Medicine: The Interplay of Chem-

istry and Light, 10

Applied Research and DevelopmentUniversities of Applied Sciences: PAT at

the Universities of Applied Sciences,482

Aqueous solutionHydrogen Storage in the Carbon Dioxide –

Formic Acid Cycle, 746

ArduinoInstrumentino: An Open-Source Software

for Scientific Instruments, 172

Asymmetric catalysisEnantioselective Rhodium-catalyzed C–C

Bond Activation of Cyclobutanones,187

Atomic force microscopyPolymer and Colloid Highlights: Lateral

Deformability of Polymer Brushes byAFM-Based Method, 709

AutophagyLight in Medicine: The Interplay of Chem-

istry and Light, 10

Bacteria inactivationFe vs. TiO

2Photo-assisted Processes for

Enhancing the Solar Inactivation ofBacteria in Water, 7

Subject IndexCHIMIA 69 (2015)

Adrenal cortexConference Report: Basel Chemistry Sym-

posium 2014: In Memory of Prof. T.Reichstein, 63

AdsorptionSeasonal Solar Thermal Absorption En-

ergy Storage Development, 784Adverse effectsHighlights of Analytical Sciences in Swit-

zerland: A Fast and Reliable in vi-tro Method for Screening of ExhaustEmission Toxicity in Lung Cells, 68

Aerosol mass spectroscopyHighlights of Analytical Sciences in Swit-

zerland: What are the Sources of Aero-sols during Haze Events in China? 368

AggregationUV-visible Absorption Study of the Self-

association of Non-ionic ChromonicTriphenylenes TP6EOnM (n = 2, 3, 4)in DiluteAqueous Solutions: Impact ofChain Length on Aggregation, 520

Air pollutionHighlights of Analytical Sciences in Swit-

zerland: What are the Sources of Aero-sols during Haze Events in China?, 368

AldehydesEffect of 2-Propanol on the Transfer Hy-

drogenation of Aldehydes by AqueousSodium Formate using a Rhodium(i)-sulfonated Triphenylphosphine Cata-lyst, 339

Algae culturesBiotechnet Switzerland: The Management

Centre Innsbruck – Keeping one stepahead with algae innovation, 362

AlkenesMetathesis by Molybdenum and Tungsten

Catalysts, 388AlkynesMetathesis by Molybdenum and Tungsten

Catalysts, 388Allosteric drug discoveryContributions of Biomolecular NMR to

Allosteric Drug Discovery, 421Alternative test methodBiotechnet Switzerland: 3D Bioprinted


AminoacylaseWhat Makes Us Smell: The Biochemistry

of Body Odour and the Design of NewDeodorant Ingredients, 414

Register vii CHIMIA 2015, 69, No. 12

Cellulose nanofibersPolymer and Colloid Highlights: Func-

tional Materials from Cellulose Nano-fibers, 232

Charge decomposition analysisThe Effect of the Electronic Nature of

Spectator Ligands in the C–H BondActivation of Ethylene by Cr(iii) Sili-cates: An ab initio Study, 225

Chemical analysisPhotoelectron Spectroscopy for Chemical

Analysis, 22

Chemical biologyConference Report: Molecular and Chemi-

cal Mechanism in Epigenetics – SwissSummer School 2015, July 12–17,2015, Hotel Kurhaus, Arolla, Switzer-land, 624

Chemical Landmark 2015SCNAT: Chemical Landmark 2015 – Des-

ignation of the Former Institute ofChemistry of the University of Fri-bourg, 812

Chemical productionConference Report: 12. Freiburger Sym-

posium 2015: Smart Solutions in theChemical Process & Product Develop-ment – Case Studies from the ChemicalIndustry, 698

Chemical research in SingaporeEditorial: Singapore – Swiss Connections,

97

Chemical safetyHighlights of Analytical Sciences in Swit-

zerland: Mass Spectrometric ProteomeAnalysis of Small Three-DimensionalMicrotissues Allows for the Quantita-tive Description of Toxic Effects ofDrugs, 494

Chemical spaceManyMolecular Properties from One Ker-

nel in Chemical Space, 182

Chemicals and fuelsConditions for the Use of CO

2, 353

CHIMIACHIMIA News, 1Instructions to Authors 2015, 2

ChitinValorization of Renewable Carbon Re-

sources for Chemicals, 120

Chlamydomonas reinhardtiiBiotechnet Switzerland: The Management


ChromatinExposure to Metals Can Be Therapeutic,

125Conference Report: Molecular and Chemi-


CARISMAIridium-mediatedBondActivation andWa-

ter Oxidation as an Exemplary Case ofCARISMA, A European Network forthe Development of Catalytic Routinesfor Small Molecule Activation, 316

CatalysisValorization of Renewable Carbon Re-

sources for Chemicals, 120Conditions for the Use of CO

2, 353

Heterogenised Molecular Catalysts for theReduction of CO

2to Fuels, 435

Chemicals from Lignin by Catalytic FastPyrolysis, from Product Control to Re-action Mechanism, 597

Soft Approaches to CO2Activation, 765

Catalyst deactivationCOMethanation for Synthetic Natural Gas

Production, 608

Catalyst layersInkjet Printing Meets Electrochemical En-

ergy Conversion, 284

Catalyst stabilizationImproving Heterogeneous Catalyst Stabil-


Catalytic activation of small moleculesEditorial: Catalytic Activation of Small

Molecules, 313

Catalytic fixed bed reactorReactors for Catalytic Methanation in the

Conversion of Biomass to SyntheticNatural Gas (SNG), 603

CathodeNanomaterials Meet Li-ion Batteries, 734

CEHighlights of Analytical Sciences in Swit-

zerland: Deep UV-LED Based Ab-sorbance Detectors for Narrow-BoreHPLC and Capillary Electrophoresis,806

Cell biologyConference Report: Molecular and Chemi-


3D Cell cultureBiotechnet Switzerland: 3D Bioprinted


Moving 3D Cell Cultures from Bench toPractice – TEDD Annual Meeting atZHAW Waedenswil 22 October 2015,694

CellulaseBiochemical Conversion Processes of


CelluloseValorization of Renewable Carbon Re-


Biotechnet SwitzerlandBiotechnet Switzerland: Biotech Meets

Chemistry: Roche Invests in Custom-ized Training, 298

BodipyTriplet–Triplet Energy Transfer Study in

Hydrogen Bonding Systems, 524

Bürgenstock conference 2015Conference Report: The 50th EUCHEM

Conference on Stereochemistry(Burgenstock Conference 2015), Brun-nen, April 26th–May 1st, 2015, 485

C–C bond activationEnantioselective Rhodium-catalyzed C–C


C–H activationEvolving P450pyr Monooxygenase for


The Effect of the Electronic Nature ofSpectator Ligands in the C–H BondActivation of Ethylene by Cr(iii) Sili-cates: An ab initio Study, 225

Calix[4]pyrroleResolving the Magnetic Asymmetry of the

Inner Space in Self-assembled DimericCapsules Based on Tetraurea-calix[4]pyrrole Components, 652

Capillary detectionHighlights of Analytical Sciences in Swit-


Carbon captureScreening Materials Relevant for Energy

Technologies, 248

Carbon dioxideElectrocatalysts for the Selective Reduc-

tion of Carbon Dioxide to Useful Prod-ucts, 131

Hydrogen Storage in the Carbon Dioxide –Formic Acid Cycle, 746

CO2Hydrogenation: Supported Nanopar-

ticles vs. Immobilized Catalysts, 759

Carbon dioxide utilization (CDU)New Wind in Old Sails: Novel Applica-


Conditions for the Use of CO2, 353

p-Carboxylatocalix[4]areneA Convenient Synthetic Route to Partial-

Cone p-Carboxylatocalix[4]arenes,516

CardiovascularChallenges and Rewards in Medicinal

Chemistry Targeting Cardiovascularand Metabolic Diseases, 407

CHIMIA 2015, 69, No. 12 register viii

Density functional theory (DFT)The Effect of the Electronic Nature of


Transition Metal Complexes of Bidentateand Tridentate Ligands: From Opto-electronic Studies to SupramolecularAssemblies, 659

DeodorantsWhat Makes Us Smell: The Biochemistry


Desalinator unitThin Layer Samples Controlled by Dy-

namic Electrochemistry, 203

Desensitization to allergiesBiotechnet Switzerland: The Management


DesorptionSeasonal Solar Thermal Absorption En-

ergy Storage Development, 784

Diagnostic biomarkersConference Report: Early Diagnosis – The

Value of Knowledge: The 2014 OltenMeeting, 59

Dielectric elastomer actuatorsPolymer and Colloid Highlights: Polysi-

loxanes with Increased Permittivity asArtificial Muscles, 548

Diesel particle filtersHighlights of Analytical Sciences in Swit-

zerland: Benefit-Risk Assessment ofDiesel Particle Filters (DPFs):AnAna-lytical and a Toxicological Challenge,152

Dimension increaseDimension Increase via Hierarchical Hy-


Directed evolutionEvolving P450pyr Monooxygenase for


DispersionsPolymer and Colloid Highlights: Stable

Ferromagnetic Nanoparticle Disper-sions in Aqueous Solutions, 369

DNAPorphyrin-modified DNA as Construction

Material in Supramolecular Chemistryand Nano-architectonics, 678

DNA structureExposure to Metals Can Be Therapeutic,

125

Domino reactionIntegrated One-Pot Synthesis of 1,3-Ox-

azinan-2-ones from Isocyanoacetatesand Phenyl Vinyl Selenones, 199

Conducting polymerDye-sensitized Solar Cells: New Ap-

proaches with Organic Solid-state HoleConductors, 41

Consolidated bioprocessing (CBP)Biochemical Conversion Processes of


ContaminantsConference Report: International Confer-

ence on Contaminated Sediments –ContaSed 2015, 8–13 March 2015,Monte Verità, Ascona, Switzerland,488

Controlled releaseWhat Makes Us Smell: The Biochemistry


CortisoneConference Report: Basel Chemistry Sym-


COST actionIridium-mediatedBondActivation andWa-


CoulometryThin Layer Samples Controlled by Dy-


Customized trainingBiotechnet Switzerland: Biotech Meets

Chemistry: Roche Invests in Custom-ized Training, 298

Cyclin-dependent kinases (CDKs)Acid Mediated Ring Closing Metathesis:

A Powerful Synthetic Tool Enablingthe Synthesis of Clinical Stage KinaseInhibitors, 142

CyclobutanoneEnantioselective Rhodium-catalyzed C–C


Data acquisitionInstrumentino: An Open-Source Software


DeactivationImproving Heterogeneous Catalyst Stabil-


Deep UV-LEDHighlights of Analytical Sciences in Swit-


DehydrogenationDehydrogenation of Formic Acid by Het-

erogeneous Catalysts, 348

ChromiumA Molecular Approach to Well-defined

Metal Sites Supported on Oxides withOxidation State and Nuclearity Con-trol, 168

Synthesis and Structure of Hexatungs-tochromate(iii), [H

3CrIIIW

6O

24]6–, 537

CinchonaIntegrated One-Pot Synthesis of 1,3-Ox-


CisplatinPlatinum(iv) Carboxylate Prodrug Com-


Clathrochelate complexesFunctionalised Clathrochelate Complexes

– New Building Blocks for Supramo-lecular Structures, 191

ClimateHighlights of Anaytical Sciences in Swit-

zerland: Using Multiple GeochemicalTechniques to Investigate Rainfall as aPotential Source of Selenium to Soils,547

CO methanationCOMethanation for Synthetic Natural Gas

Production, 608

CO2 activationSoft Approaches to CO

2Activation, 765

CO2 captureRecent Advances in Carbon Capture with

Metal–Organic Frameworks, 274

CO2 reductionStorage of Renewable Energy by Reduc-

tion of CO2with Hydrogen, 264

Surface Reactions are Crucial for EnergyStorage, 269


2to Fuels, 435

Electrochemical CO2Reduction – A Criti-

cal View on Fundamentals, Materialsand Applications, 769

Cold molecular ionsForbidden Vibrational Transitions in Cold

Molecular Ions: Experimental Obser-vation and Potential Applications, 213

ComplexRhenium(i)-based Double-heterostranded

Helicates, 675

Composite structureNanomaterials Meet Li-ion Batteries, 734

Computer-control of experimentsInstrumentino: An Open-Source Software


Concentration profilesUniversities of Applied Sciences: Skin

Concentrations of Topically AppliedSubstances in Reconstructed HumanEpidermis (RHE) Compared with Hu-man Skin Using in vivo Confocal Ra-man Microscopy, 147

Register iX CHIMIA 2015, 69, No. 12

Potential Source of Selenium to Soils,547

EpigeneticsConference Report: Molecular and Chemi-


EpoxidationUniversities of Applied Sciences: Chemi-


Eschweiler-Clarke reactionTransition Metal-free Methylation of


Ethylene polymerizationA Molecular Approach to Well-defined


The Effect of the Electronic Nature ofSpectator Ligands in the C–H BondActivation of Ethylene by Cr(iii) Sili-cates: An ab initio Study, 225

EUCHEMS StereochemistryConference Report: The 50th EUCHEM

Conference on Stereochemistry(Burgenstock Conference 2015), Brun-nen, April 26th–May 1st, 2015, 485

Excluded volume effectMimicking the in vivo Environment – The

Effect of Crowding on RNA and Bio-macromolecular Folding and Activity,207

Exhaust emissionHighlights of Analytical Sciences in Swit-


Exposure systemHighlights of Analytical Sciences in Swit-


Falling film tube bundleSeasonal Solar Thermal Absorption En-

ergy Storage Development, 784Fast pyrolysisChemicals from Lignin by Catalytic Fast

Pyrolysis, from Product Control to Re-action Mechanism, 597

FCS diffusion lawCharacterization of Lipid and Cell Mem-

brane Organization by the Fluores-cence Correlation Spectroscopy Diffu-sion Law, 112

FeHydrogen Storage in the Carbon Dioxide –


Electrode fabricationInkjet Printing Meets Electrochemical En-

ergy Conversion, 284

ElectrolysisStoring Renewable Energy in the Hydro-

gen Cycle, 741ElectrolyzerElectrochemical CO

2Reduction – A Criti-


Electron transferPhotoinduced Charge Accumulation in

Molecular Systems, 17

Electrostatic spray ionization massspectrometryAnalytical Chemistry at the Laboratoire

d’Electrochimie Physique et Analyt-ique, 290

Energy conversionInkjet Printing Meets Electrochemical En-

ergy Conversion, 284Electrochemical CO



Energy from biomassBiotechnet Switzerland: ORION – A


Energy researchEditorial: Energypolis: Chemistry for En-

ergy, 245

Energy storageArtificial Photosynthesis with Semicon-

ductor–Liquid Junctions, 30Storage of Renewable Energy by Reduc-



Editorial: SCCER – The Swiss Compe-tence Center for Energy – Energy Stor-age Research in Switzerland, 721

Challenges towards Economic Fuel Gener-ation from Renewable Electricity: TheNeed for Efficient Electro-Catalysis,789

Storing Renewable Energy in the Hydro-gen Cycle, 741

Energy Strategy 2050Opportunities for Switzerland to Contrib-


EnvironmentConference Report: International Confer-


Environmental archivesHighlights of Analytical Sciences in Swit-

zerland: Using Multiple GeochemicalTechniques to Investigate Rainfall as a

DOTASynthetic G-Quartets as Versatile Nanoto-

ols for the Luminescent Detection ofG-Quadruplexes, 530

Drug developmentBiotechnet Switzerland: 3D Bioprinted


Drug quantification in human bloodBiotechnet Switzerland: Where Experts


Drug toxicityHighlights of Analytical Sciences in Swit-


Duel Energy Storage & ConverterStorage of Heat, Cold and Electricity, 777DyeDye-sensitized Solar Cells: New Ap-


Molecular Engineering of Functional Ma-terials for Energy and Opto-ElectronicApplications, 253

Edible vaccinesBiotechnet Switzerland: The Management


Electrical energy storageRedox Flow Batteries, Hydrogen and Dis-

tributed Storage, 753Storage of Heat, Cold and Electricity, 777ElectrocatalysisElectrocatalysts for the Selective Reduc-


Inkjet Printing Meets Electrochemical En-ergy Conversion, 284


2to Fuels, 435

Electrochemical impedancespectroscopyStress-induced Ageing of Lithium-Ion

Batteries, 737Electrochemical water splittingChallenges towards Economic Fuel Gener-

ation from Renewable Electricity: TheNeed for Efficient Electro-Catalysis,789

ElectrochemistryTransition Metal Complexes of Bidentate

and Tridentate Ligands: From Opto-electronic Studies to SupramolecularAssemblies, 659

Electrode engineeringElectrode Engineering of Conversion-

based Negative Electrodes for Na-ionBatteries, 729

CHIMIA 2015, 69, No. 12 register X

lytical and a Toxicological Challenge,152

HelicateRhenium(i)-based Double-heterostranded

Helicates, 675HematiteUniversities of Applied Sciences: Effect

of Experimental Parameters on Wa-ter Splitting Using a Hematite Photo-anode, 807

Herbal teasHighlights of Analytical Sciences in Swit-

zerland: Occurrence of Natural Hepa-totoxines in Herbal Teas, 231

Heterogeneous catalysisThe Role of Proton Transfer in Heteroge-

neous Transformations of Hydrocar-bons, 321

Dehydrogenation of Formic Acid by Het-erogeneous Catalysts, 348

Looking Back on 35 Years of IndustrialCatalysis, 393

Heterogenised Molecular Catalysts for theReduction of CO2

to Fuels, 435Improving Heterogeneous Catalyst Stabil-


CO2Hydrogenation: Supported Nanopar-

ticles vs. Immobilized Catalysts, 759Hierarchical hydrogen bondingDimension Increase via Hierarchical Hy-


High permittivityPolymer and Colloid Highlights: Polysi-

loxanes with Increased Permittivity asArtificial Muscles, 548

High-temperature heat storagePhase Change Material Systems for High

Temperature Heat Storage, 780High-throughput screeningChallenges and Rewards in Medicinal


History of scienceA Short History of the Use of Plants as

Medicines fromAncient Times, 622Hole conductorDye-sensitized Solar Cells: New Ap-


Homogeneous catalysisIridium-mediatedBondActivation andWa-


New Wind in Old Sails: Novel Applica-tions ofTriphos-basedTransitionMetalComplexes as Homogeneous Catalystsfor Small Molecules and RenewablesActivation, 331

Looking Back on 35 Years of IndustrialCatalysis, 393

Functional foams and filmsPolymer and Colloid Highlights: Func-


Functional materialsMolecular Engineering of Functional Ma-

terials for Energy and Opto-ElectronicApplications, 253

Functionalized nanoparticles in releasemechanismBiotechnet Switzerland: Where Experts


G-quadruplexesSynthetic G-Quartets as Versatile Nanoto-


Gamma ray spectrometryHighlights of Analytical Sciences in Swit-

zerland: Gamma Ray Spectrometry ofSewer Sludge – A Useful Tool for theIdentification of Emission Sources in aCity, 301

Gas diffusion electrodeElectrochemical CO2

Reduction – A Criti-cal View on Fundamentals, Materialsand Applications, 769

GenotoxicityHighlights of Analytical Sciences in Swit-


GeochemistryHighlights of Analytical Sciences in Swit-


Gram-negative bacteriaBiotechnet Switzerland: Platform for a

Technological Leap in Antibiotics, 809Graphical user interfaceInstrumentino: An Open-Source Software

for Scientific Instruments, 172Green chemistrySoft Approaches to CO

2Activation, 765

Grignard reagentsNitrous Oxide-dependent Iron-catalyzed

Coupling Reactions of Grignard Re-agents, 327

Grubbs catalystAcid Mediated Ring Closing Metathesis:


Heavy duty vehiclesHighlights of Analytical Sciences in Swit-

zerland: Benefit-Risk Assessment ofDiesel Particle Filters (DPFs):AnAna-

FerromagneticPolymer and Colloid Highlights: Stable


Fixation into biomassConditions for the Use of CO

2, 353

Flame spray synthesisWO

3/CeO

2/TiO

2Catalysts for Selective

Catalytic Reduction of NOxby NH

3:

Effect of the Synthesis Method, 220

Flexible bisplatinum acceptorSelf-assembly of Metallamacrocycles Em-

ploying a New Benzil-based Organo-metallic Bisplatinum(ii) Acceptor, 541

Flue gasRecent Advances in Carbon Capture with


Fluidized bedReactors for Catalytic Methanation in the


5-(Fluorenyl)-1,10-phenanthrolineligandsTwo-photon Absorption Engineering of

5-(Fluorenyl)-1,10-phenanthroline-based Ru(ii) Complexes, 666

Fluorescence correlation spectroscopy(FCS)Characterization of Lipid and Cell Mem-


Food safetyHighlights of Analytical Sciences in Swit-


Forbidden transitionsForbidden Vibrational Transitions in Cold


Force fieldModellingWater: A Lifetime Enigma, 104

FormaldehydeTransition Metal-free Methylation of


Formic acidDehydrogenation of Formic Acid by Het-

erogeneous Catalysts, 348Hydrogen Storage in the Carbon Dioxide –


Freiburger SymposiumConference Report: 12. Freiburger Sym-


FrictionPolymer and Colloid Highlights: Lateral


Register Xi CHIMIA 2015, 69, No. 12

Janus kinase 2 (JAK2)Acid Mediated Ring Closing Metathesis:


Kernel Ridge RegressionManyMolecular Properties from One Ker-

nel in Chemical Space, 182Kinetic monitoringUniversities of Applied Sciences: Chemi-


Klebsiella pneumoniaeBiotechnet Switzerland: Platform for a


Laboratory medicineConference Report: Early Diagnosis – The


LakesConference Report: International Confer-


Laser ablation ICPMSHighlights of Analytical Sciences in Swit-

zerland: Laser Ablation ICP-MS forSingle-Cell-based Tissue Imaging, 637

Latent heat of fusionPhase Change Material Systems for High

Temperature Heat Storage, 780Li-ion batteriesEvaluation of Metal Phosphide Nanocrys-


Nanomaterials Meet Li-ion Batteries, 734Stress-induced Ageing of Lithium-Ion

Batteries, 737Life SciencesUniversities of Applied Sciences: PAT at


Light intensityUniversities of Applied Sciences: Effect


Light-activated prodrugsLightening up Ruthenium Complexes to

Fight Cancer? 176LigninValorization of Renewable Carbon Re-

sources for Chemicals, 120Chemicals from Lignin by Catalytic Fast


Lipid raft modelCharacterization of Lipid and Cell Mem-

brane Organization by the Fluores-

HydroxylationEvolving P450pyr Monooxygenase for


Imaging mass cytometryHighlights of Analytical Sciences in Swit-


ImmobilizationCO

2Hydrogenation: Supported Nanopar-

ticles vs. Immobilized Catalysts, 759Immunoaffinity electrophoresisAnalytical Chemistry at the Laboratoire

d’Electrochimie Physique et Analyt-ique, 290

in vivo applicationUniversities of Applied Sciences: Skin


Industrial catalysisLooking Back on 35 Years of Industrial

Catalysis, 393Inkjet printingInkjet Printing Meets Electrochemical En-

ergy Conversion, 284Polymer and Colloid Highlights: Stable


Intermolecular/intramolecular TTETTriplet–Triplet Energy Transfer Study in

Hydrogen Bonding Systems, 524International Chemistry OlympiadTwo Bronze Medals for Switzerland at the

46th International Chemistry Olympiadin Hanoi, Vietnam, 71

International Year of Light 2015Editorial: Chemistry and Light: The Inter-

nationalYear of Light, 6Ionic liquidsApplication of Ionic Liquids in the Down-

stream Processing of LignocellulosicBiomass, 592

Ionophore-based membranesThin Layer Samples Controlled by Dy-

namic Electrochemistry, 203Iron catalystsNitrous Oxide-dependent Iron-catalyzed


IsocyanoacetatesIntegrated One-Pot Synthesis of 1,3-Ox-


IsopyrazamSedaxane, Isopyrazam and Solatenol™:

Novel Broad-spectrum FungicidesInhibiting Succinate Dehydrogenase(SDH) – Synthesis Challenges andBiological Aspects, 425


Hospital wastewatersDevelopment of an LC-MS/MS Method

for the Assessment of Selected Ac-tive Pharmaceuticals and Metabolitesin Wastewaters of a Swiss UniversityHospital, 684

HPLCHighlights of Analytical Sciences in Swit-


HPLC-MS/MSHighlights of Analytical Sciences in Swit-


Human 3D cellsHighlights of Analytical Sciences in Swit-


Human body odoursWhat Makes Us Smell: The Biochemistry


HydridesStorage of Renewable Energy by Reduc-



HydrogenArtificial Photosynthesis with Semicon-

ductor–Liquid Junctions, 30Storage of Renewable Energy by Reduc-



Dehydrogenation of Formic Acid by Het-erogeneous Catalysts, 348


Redox Flow Batteries, Hydrogen and Dis-tributed Storage, 753

Challenges towards Economic Fuel Gener-ation from Renewable Electricity: TheNeed for Efficient Electro-Catalysis,789

Hydrogen bondTriplet–Triplet Energy Transfer Study in

Hydrogen Bonding Systems, 524

Hydrogen storageHydrogen Storage in the Carbon Dioxide –


HydrogenationCO

2Hydrogenation: Supported Nanopar-

ticles vs. Immobilized Catalysts, 759

Hydrolytic stability and regenerationRecent Advances in Carbon Capture with


CHIMIA 2015, 69, No. 12 register Xii

els: the Hydrothermal Pathway to Bio-Methane, 614

Microalgae-based food and feedproductsBiotechnet Switzerland: The Management


Microbial fuel cellBiotechnet Switzerland: Recovering Valu-

able Phosphates: Chemical Biotech-nology as a Problem Solver for theEnvironment, 296

MicroscopyStress-induced Ageing of Lithium-Ion

Batteries, 737

Mid-IR spectroscopyHighlights of Analytical Sciences in Swit-

zerland: Multi-Component Trace GasSpectroscopy Using Dual-WavelengthQuantum Cascade Lasers, 708

MOF synthesisRecent Advances in Carbon Capture with


Molecular algae biotechnologyBiotechnet Switzerland: The Management


Molecular approachA Molecular Approach to Well-defined


Molecular crowdingMimicking the in vivo Environment – The


Molecular designChallenges and Rewards in Medicinal


Molecular dynamicsModellingWater: A Lifetime Enigma, 104

Molecular encapsulationResolving the Magnetic Asymmetry of the


Molecular engineeringMolecular Engineering of Functional Ma-


Molecular gluePorphyrin-modified DNA as Construction


Molecular propertiesManyMolecular Properties from One Ker-


MolybdenumMetathesis by Molybdenum and Tungsten

Catalysts, 388

Membrane dynamicsCharacterization of Lipid and Cell Mem-


Membrane organizationCharacterization of Lipid and Cell Mem-


Mesoporous oxideDye-sensitized Solar Cells: New Ap-


MetabolicChallenges and Rewards in Medicinal


Metabolic profilingUniversities of Applied Sciences: Novel

Analytical Workflow for Comprehen-sive Non-targeted Phytochemical Met-abolic Profiling, 294

Metal-based drugExposure to Metals Can Be Therapeutic,

125Metal-freeTransition Metal-free Methylation of


Metal–organic frameworks (MOF)Screening Materials Relevant for Energy

Technologies, 248Recent Advances in Carbon Capture with

Metal–Organic Frameworks, 274MetallamacrocycleSelf-assembly of Metallamacrocycles Em-

ploying a New Benzil-based Organo-metallic Bisplatinum(ii) Acceptor, 541

MetalloligandsFunctionalised Clathrochelate Complex-

es – New Building Blocks for Supra-molecular Structures, 191

MetathesisMetathesis by Molybdenum and Tungsten

Catalysts, 388MethanationReactors for Catalytic Methanation in the


Methane storageScreening Materials Relevant for Energy

Technologies, 248N-methylationTransition Metal-free Methylation of


(S)-MetolachlorLooking Back on 35 Years of Industrial

Catalysis, 393MicroalgaeOpportunities for Switzerland to Contrib-

ute to the Production of Algal Biofu-

cence Correlation Spectroscopy Diffu-sion Law, 112

Liquid chromatographyDevelopment of an LC-MS/MS Method


LuminescenceSynthetic G-Quartets as Versatile Nanoto-


Two-photon Absorption Engineering of5-(Fluorenyl)-1,10-phenanthroline-based Ru(ii) Complexes, 666

3D Lung cell modelHighlights of Analytical Sciences in Swit-


Machine learningManyMolecular Properties from One Ker-


Magnetic asymmetryResolving the Magnetic Asymmetry of the


Mass spectrometryUniversities of Applied Sciences: Novel


Highlights of Analytical Sciences in Swit-zerland: Mass Spectrometric ProteomeAnalysis of Small Three-DimensionalMicrotissues Allows for the Quantita-tive Description of Toxic Effects ofDrugs, 494

Development of an LC-MS/MS Methodfor the Assessment of Selected Ac-tive Pharmaceuticals and Metabolitesin Wastewaters of a Swiss UniversityHospital, 684

Materials genomeScreening Materials Relevant for Energy

Technologies, 248

MechanismElectrocatalysts for the Selective Reduc-


Medicinal chemistryChallenges and Rewards in Medicinal


Medicinal inorganic chemistryMetal Complexes and Medicine: A Suc-


Medicinal organometallic chemistryMetal Complexes and Medicine: A Suc-


Register Xiii CHIMIA 2015, 69, No. 12

Optical sensorPotassium Sensitive Optical Nanosensors

Containing Voltage Sensitive Dyes,196

Organic synthesisMetathesis by Molybdenum and Tungsten

Catalysts, 388

OxaliplatinPlatinum(iv) Carboxylate Prodrug Com-


Oxidative MCRIntegrated One-Pot Synthesis of 1,3-Ox-


Oxide supportA Molecular Approach to Well-defined


Oxygen evolution reactionChallenges towards Economic Fuel Gener-

ation from Renewable Electricity: TheNeed for Efficient Electro-Catalysis,789

P450 monooxygenaseEvolving P450pyr Monooxygenase for


Packed bedExperimental and Numerical Investigation

of Combined Sensible/Latent ThermalEnergy Storage for High-TemperatureApplications, 799

PAHsHighlights of Analytical Sciences in Swit-


PalladiumFunctionalised Clathrochelate Complex-


Paper-based microfluidic devicesThin Layer Samples Controlled by Dy-


PATUniversities of Applied Sciences: PAT at


Pathogenesis of tuberculosisBiotechnet Switzerland: Platform for a


PCC BaselConference Report: Basel Chemistry Sym-


PCDD/FsHighlights of Analytical Sciences in Swit-

zerland: Benefit-Risk Assessment ofDiesel Particle Filters (DPFs):AnAna-

NH3

WO3/CeO

2/TiO



3:


Ni/Al2O3

COMethanation for Synthetic Natural GasProduction, 608

NIR emitterTransition Metal Complexes of Bidentate


Nitro-PAHsHighlights of Analytical Sciences in Swit-


Nitrogen oxidesHighlights of Analytical Sciences in Swit-


Nitrous oxideNitrous Oxide-dependent Iron-catalyzed


Nobel PrizeConference Report: Basel Chemistry Sym-


Nox reductionWO

3/CeO

2/TiO



3:


Nuclear magnetic resonanceUniversities of Applied Sciences: Chemi-


NucleosomeExposure to Metals Can Be Therapeutic,

125

o-BiscyclopropylanilineSedaxane, Isopyrazam and Solatenol™:


OceansConference Report: International Confer-


Omics technologiesConference Report: Early Diagnosis – The


Multidrug-resistant strainsBiotechnet Switzerland: Platform for a

Technological Leap in Antibiotics, 809Multimode heat transferPhase Change Material Systems for High

Temperature Heat Storage, 780Multiplexed tissue analysisHighlights of Analytical Sciences in Swit-


Muscle-tendon diseaseBiotechnet Switzerland: 3D Bioprinted


Na-ion batteriesElectrode Engineering of Conversion-


Evaluation of Metal Phosphide Nanocrys-tals asAnodeMaterials for Na-ion Bat-teries, 724

Nano-architechonicsPorphyrin-modified DNA as Construction


Nano-LiCoO2Nanomaterials Meet Li-ion Batteries, 734Nano-LiMnO2Nanomaterials Meet Li-ion Batteries, 734NanocrystalsEvaluation of Metal Phosphide Nanocrys-


NanomaterialsScientific Basis for Regulatory Decision-

Making of Nanomaterials: Report onthe Workshop, 20–21 January 2014,Center of Applied Ecotoxicology,Dubendorf, 52

NanoparticlesArtificial Photosynthesis with Semicon-

ductor–Liquid Junctions, 30Nanoporous materialsScreening Materials Relevant for Energy

Technologies, 248NanospherePotassium Sensitive Optical Nanosensors


Natural productsUniversities of Applied Sciences: Novel


Negative electrodeElectrode Engineering of Conversion-


Neutral photo-FentonFe vs. TiO



CHIMIA 2015, 69, No. 12 register Xiv

PotentialPotassium Sensitive Optical Nanosensors


Potential energy surfaceModellingWater: A Lifetime Enigma, 104

Power-to-GasReactors for Catalytic Methanation in the


Power-to-gas/liquidElectrochemical CO



Power-to-HeatStorage of Heat, Cold and Electricity, 777

Pre-irradiation grafted copolymerelectrolytePolymer and Colloid Highlights: Struc-

tural Studies of Radiation-grafted Co-polymer Proton ExchangeMembranes,69

Precision spectroscopyForbidden Vibrational Transitions in Cold


Process Analytical TechnologiesUniversities of Applied Sciences: PAT at


Process engineeringImproving Heterogeneous Catalyst Stabil-


ProphyrinPorphyrin-modified DNA as Construction


Protein kinases in cancer therapyBiotechnet Switzerland: Where Experts


Protein networksHighlights of Analytical Sciences in Swit-


ProteomicsHighlights of Analytical Sciences in Swit-


Proton exchange membranePolymer and Colloid Highlights: Struc-


Photoelectrochemical cellUniversities of Applied Sciences: Effect


Photoelectrochemical water splittingArtificial Photosynthesis with Semicon-

ductor–Liquid Junctions, 30

Photoelectron spectroscopyPhotoelectron Spectroscopy for Chemical

Analysis, 22

Physicochemical properties of exhaustHighlights of Analytical Sciences in Swit-


Picket-fence modelCharacterization of Lipid and Cell Mem-


PlantsA Short History of the Use of Plants as

Medicines fromAncient Times, 622

Platinum(iv) prodrugsPlatinum(iv) Carboxylate Prodrug Com-


Point-of-care devicesConference Report: Early Diagnosis – The


Point-of-Care Therapeutic Drug Moni-toring devicesBiotechnet Switzerland: Where Experts


PolarizableModellingWater: A Lifetime Enigma, 104

PolyhydroxyalkanoatesUniversities of Applied Sciences: Chemi-


Polymer brushesPolymer and Colloid Highlights: Lateral


Polymer chemistryMetathesis by Molybdenum and Tungsten

Catalysts, 388

Polymers

Scalable Synthesis of Two-dimensionalPolymer Crystals and Exfoliation intoNanometer-thin Sheets, 217

PolyoxometalatesSynthesis and Structure of Hexatungs-

tochromate(III), [H3CrIIIW

6O

24]6–, 537

lytical and a Toxicological Challenge,152

PerovskiteMolecular Engineering of Functional Ma-


Personalized medicineBiotechnet Switzerland: 7th Waedenswil

Day of Chemistry – Personalized Med-icine: Full Speed Ahead! 491

PharmaceuticalsDevelopment of an LC-MS/MS Method


Phase change materialExperimental and Numerical Investigation


Phase Change Material Systems for HighTemperature Heat Storage, 780

Phillips catalystThe Effect of the Electronic Nature of


Phosphate recoveryBiotechnet Switzerland: Recovering Valu-


Phosphate remobilizationBiotechnet Switzerland: Recovering Valu-


Phosphine ligandsHydrogen Storage in the Carbon Dioxide –


Photoactivated chemotherapyLightening up Ruthenium Complexes to

Fight Cancer? 176

PhotocatalysisHeterogenised Molecular Catalysts for the

Reduction of CO2to Fuels, 435

PhotochemistryPhotoinduced Charge Accumulation in

Molecular Systems, 17Scalable Synthesis of Two-dimensional

Polymer Crystals and Exfoliation intoNanometer-thin Sheets, 217

Transition Metal Complexes of Bidentateand Tridentate Ligands: From Opto-electronic Studies to SupramolecularAssemblies, 659

Photodynamic therapyLight in Medicine: The Interplay of Chem-

istry and Light, 10Lightening up Ruthenium Complexes to

Fight Cancer? 176

Register Xv CHIMIA 2015, 69, No. 12

Risk assessmentScientific Basis for Regulatory Decision-


Conference Report: International Confer-ence on Contaminated Sediments–ContaSed 2015, 8–13 March 2015,Monte Verità, Ascona, Switzerland,488

RNAMimicking the in vivo Environment – The


RutheniumLightening up Ruthenium Complexes to

Fight Cancer? 176New Wind in Old Sails: Novel Applica-



Ruthenium(ii) complexesTwo-photon Absorption Engineering of

5-(Fluorenyl)-1,10-phenanthroline-based Ru(ii) Complexes, 666

Scanning electrochemical microscopyAnalytical Chemistry at the Laboratoire

d’Electrochimie Physique et Ana-lytique, 290

SCCER BIOSWEETEditorial: SCCER BIOSWEET – The

Swiss Competence Center for EnergyResearch on Bioenergy, 569

SCCER Heat & Electricity StorageEditorial: SCCER – The Swiss Compe-

tence Center for Energy – Energy Stor-age Research in Switzerland, 721

SchistosomiasisMetal Complexes and Medicine: A Suc-


SCNAT «Platform Chemistry»SCNAT: New President, New BoardMem-

ber and New Chief Science Officer ofthe «Platform Chemistry», 57

SCNAT: 2015 SCNAT/SCS ChemistryTravel Award, 58

SCNAT: The 8thYoung Faculty Meeting –An Active Crowd Attuned to ModernChallenges, 475

SCNAT: 2015 Chemistry Travel Award bySCNAT, SCS and SSFEC, 478

SCNAT: Chemical Landmark 2015 – Des-ignation of the Former Institute ofChemistry of the University of Fri-bourg, 812

Recombinant protein vaccinesBiotechnet Switzerland: One Step ahead

in Cell Cultivation – InternationalAdvanced Training Course at ZHAWWaedenswil, 631

Reconstructed human epidermisUniversities of Applied Sciences: Skin


Redox flow batteriesRedox Flow Batteries, Hydrogen and Dis-

tributed Storage, 753

Regulation of nanomaterialsScientific Basis for Regulatory Decision-


Reichstein, TaddeusConference Report: Basel Chemistry Sym-


RemediationConference Report: International Confer-


Renewable chemicalsValorization of Renewable Carbon Re-


Renewable energyBiotechnet Switzerland: ORION – A


Renewable feedstocksApplication of Ionic Liquids in the Down-


Reversible heat pumpStorage of Heat, Cold and Electricity, 777

RheniumRhenium(i)-based Double-heterostranded

Helicates, 675

RhodiumEnantioselective Rhodium-catalyzed C–C


Rhodium-phosphine catalystEffect of 2-Propanol on the Transfer Hy-


Ring-closing metathesis (RCM)Acid Mediated Ring Closing Metathesis:


Proton transferPhotoinduced Charge Accumulation in

Molecular Systems, 17The Role of Proton Transfer in Heteroge-

neous Transformations of Hydrocar-bons, 321

Purification technologiesPolymer and Colloid Highlights: Func-


Purpose-made instrumentsInstrumentino: An Open-Source Software


PyreneSynthetic G-Quartets as Versatile Nano-

tools for the Luminescent Detection ofG-Quadruplexes, 530

Pyrrolizidine alkaloidsHighlights of Analytical Sciences in Swit-


PythonInstrumentino: An Open-Source Software


Quantum cascade lasersHighlights of Analytical Sciences in Swit-


Quantum chemistryManyMolecular Properties from One Ker-


RadicalsChemicals from Lignin by Catalytic Fast


RadiocarbonHighlights of Analytical Sciences in Swit-


RadiopharmaceuticalsHighlights of Analytical Sciences in Swit-

zerland: Gamma Ray Spectrometry ofSewer Sludge – A Useful Tool for theIdentification of Emission Sources in aCity, 301

Raman spectroscopyUniversities of Applied Sciences: Skin


Reaction mechanismCOMethanation for Synthetic Natural Gas

Production, 608

Reactive oxygen speciesLight in Medicine: The Interplay of Chem-

istry and Light, 10

CHIMIA 2015, 69, No. 12 register Xvi

Solar fuelPhotoinduced Charge Accumulation in

Molecular Systems, 17Artificial Photosynthesis with Semicon-


Solatenol™Sedaxane, Isopyrazam and Solatenol™:


Solid-state chemistryScalable Synthesis of Two-dimensional


Source apportionmentHighlights of Analytical Sciences in Swit-


Specialized well plateBiotechnet Switzerland: 3D Bioprinted


Specific microarray detectionConference Report: Early Diagnosis – The


SpectroscopySurface Reactions are Crucial for Energy

Storage, 269

StabilityPolymer and Colloid Highlights: Stable


Statistical data analysisUniversities of Applied Sciences: Novel


Stress cyclesStress-induced Ageing of Lithium-Ion

Batteries, 737

Structural characterizationUniversities of Applied Sciences: Novel


StructureConference Report: Molecular and Chemi-


Struvite fertilizerBiotechnet Switzerland: Recovering Valu-


Subcellular resolutionHighlights of Analytical Sciences in Swit-


Sewage sludgeBiotechnet Switzerland: Recovering Valu-


Highlights of Analytical Sciences in Swit-zerland: Gamma Ray Spectrometry ofSewer Sludge – A Useful Tool for theIdentification of Emission Sources in aCity, 301

SI-ATRPPolymer and Colloid Highlights: Stable


SiliconesPolymer and Colloid Highlights: Poly-

siloxanes with Increased Permittivityas Artificial Muscles, 548

SimulationExperimental and Numerical Investigation


Single-molecule Förster resonanceenergy transfer (smFRET)Mimicking the in vivo Environment – The


Skin penetrationUniversities of Applied Sciences: Skin


Small-angle neutron scatteringPolymer and Colloid Highlights: Struc-


Small-angle X-ray scatteringPolymer and Colloid Highlights: Struc-


Sn particleElectrode Engineering of Conversion-


Sodium formateEffect of 2-Propanol on the Transfer Hy-


Solar cellDye-sensitized Solar Cells: New Ap-


Solar disinfectionFe vs. TiO



SCRWO

3/CeO

2/TiO



3:


SCS FoundationAlfred Werner Fund, Master’s Student

Scholarships, 496

SDHISedaxane, Isopyrazam and Solatenol™:


Seasonal solar thermal energy storageSeasonal Solar Thermal Absorption En-


SedaxaneSedaxane, Isopyrazam and Solatenol™:


SedimentsConference Report: International Confer-


SelectivityElectrocatalysts for the Selective Reduc-


Recent Advances in Carbon Capture withMetal–Organic Frameworks, 274


SeleniumHighlights of Analytical Sciences in Swit-


SelenonesIntegrated One-Pot Synthesis of 1,3-Ox-


Self-assemblyFunctionalised Clathrochelate Complex-


Self-assembly of Metallamacrocycles Em-ploying a New Benzil-based Organo-metallic Bisplatinum(ii) Acceptor, 541

Self-associationUV-visible Absorption Study of the Self-


SemiconductorArtificial Photosynthesis with Semicon-


Register Xvii CHIMIA 2015, 69, No. 12

Sugar platformBiochemical Conversion Processes of


Supercritical water gasificationOpportunities for Switzerland to Contrib-


Supramolecular assembliesA Convenient Synthetic Route to Partial-

Cone p-Carboxylatocalix[4]arenes,516

Triplet–Triplet Energy Transfer Study inHydrogen Bonding Systems, 524

Porphyrin-modified DNA as ConstructionMaterial in Supramolecular Chemistryand Nano-architectonics, 678

Supramolecular chemistryEditorial: Supramolecular Chemistry, 513Editorial: Supramolecular Chemistry Part

2, 649Transition Metal Complexes of Bidentate


Supramolecular chiralityResolving the Magnetic Asymmetry of the


SupramoleculeRhenium(i)-based Double-heterostranded

Helicates, 675

Surface organometallic chemistryA Molecular Approach to Well-defined


Surface reactionsSurface Reactions are Crucial for Energy

Storage, 269

Surface-initiated polymerizationPolymer and Colloid Highlights: Lateral


Sustainable chemistryApplication of Ionic Liquids in the Down-


Soft Approaches to CO2Activation, 765

Swiss Chemical SocietyAnnual Report 2014, 73KGF-SCS Industrial Science Awards, 234Editorial: SCS Laureates and Awards &

Fall Meeting 2015, 385Review SCS Fall Meeting 2015, 639SCS Prize Winners 2016, 816

SynthesisEvaluation of Metal Phosphide Nanocrys-


Synthetic fuelsStorage of Renewable Energy by Reduc-


Synthetic hydrocarbonsStoring Renewable Energy in the Hydro-

gen Cycle, 741

Synthetic microbial consortiumBiochemical Conversion Processes of


Synthetic natural gas (SNG)Reactors for Catalytic Methanation in the


COMethanation for Synthetic Natural GasProduction, 608

Opportunities for Switzerland to Contrib-ute to the Production of Algal Biofu-els: the Hydrothermal Pathway to Bio-Methane, 614

Targeted chemotherapyPlatinum(iv) Carboxylate Prodrug Com-


TASQSynthetic G-Quartets as Versatile Nanoto-


Technology trendsConference Report: 12. Freiburger Sym-


TemperatureUniversities of Applied Sciences: Effect


TerbiumSynthetic G-Quartets as Versatile Nanoto-


Terrestrial, airborne and lichen algaeBiotechnet Switzerland: The Management


Therapeutic drug monitoringConference Report: Early Diagnosis – The


Thermal energy storageExperimental and Numerical Investigation


Phase Change Material Systems for HighTemperature Heat Storage, 780

Thermochemical heat storageSeasonal Solar Thermal Absorption En-


ThermoclineExperimental and Numerical Investigation


Thin filmsArtificial Photosynthesis with Semicon-


Thin layer sampleThin Layer Samples Controlled by Dy-


TiO2 photo-catalysis

Fe vs. TiO2Photo-assisted Processes for


Tissue Engineering for Drug Develop-ment and Substance Testing (TEDD)

Biotechnet Switzerland: 7th WaedenswilDay of Chemistry – Personalized Med-icine: Full Speed Ahead!, 491

Biotechnet Switzerland: A Network SetsThings in Motion: TEDD Celebratesits 5th Anniversary, 690

Biotechnet Switzerland: Moving 3D CellCultures from Bench to Practice -TEDDAnnualMeeting at ZHAWWae-denswil 22 October 2015, 694

TP6EO2MUV-visible Absorption Study of the Self-


Trace element distributionHighlights of Anaytical Sciences in Swit-


Trace gas analysisHighlights of Analytical Sciences in Swit-


Traditional medicineA Short History of the Use of Plants as

Medicines fromAncient Times, 622

Transfer hydrogenationEffect of 2-Propanol on the Transfer Hy-


Transient absorptionPhotoinduced Charge Accumulation in

Molecular Systems, 17

TriphenyleneUV-visible Absorption Study of the Self-


CHIMIA 2015, 69, No. 12 register Xviii

TriphosNew Wind in Old Sails: Novel Applica-


Troger’s baseRhenium(i)-based Double-heterostranded

Helicates, 675

Tumor cell heterogeneityHighlights of Analytical Sciences in Swit-


Tungsten catalysisMetathesis by Molybdenum and Tungsten

Catalysts, 388

Two-dimensional materialsScalable Synthesis of Two-dimensional


Two-photon absorption

Two-photon Absorption Engineering of5-(Fluorenyl)-1,10-phenanthroline-based Ru(ii) Complexes, 666

UV photodiodeHighlights of Analytical Sciences in Swit-


UV-visible absorptionUV-visible Absorption Study of the Self-


ValinomycinPotassium Sensitive Optical Nanosensors


Valorization of wasteBiotechnet Switzerland: ORION – A


VGCFElectrode Engineering of Conversion-


Vibrational spectroscopyForbidden Vibrational Transitions in Cold


Vitamin CConference Report: Basel Chemistry Sym-


Voltage sensitive dyePotassium Sensitive Optical Nanosensors


WaterEffect of 2-Propanol on the Transfer Hy-


Water modelsModellingWater: A Lifetime Enigma, 104

Water splittingUniversities of Applied Sciences: Effect


Wet impregnation

WO3/CeO

2/TiO



3:


Wound healing

Light in Medicine: The Interplay of Chem-istry and Light, 10

X-ray crystallography

Scalable Synthesis of Two-dimensionalPolymer Crystals and Exfoliation intoNanometer-thin Sheets, 217

XRD

Synthesis and Structure of Hexatungs-tochromate(iii), [H

3CrIIIW

6O

24]6–, 537

Young Faculty Meeting

SCNAT: The 8th Young Faculty Meeting– AnActive CrowdAttuned to ModernChallenges, 475

Zeolite catalysis


Zhan-1B catalyst

Acid Mediated Ring Closing Metathesis:A Powerful Synthetic Tool Enablingthe Synthesis of Clinical Stage KinaseInhibitors, 142

Register XiX CHIMIA 2015, 69, No. 12

CHIMIA REPORT/COMPANY NEWS

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CHIMIA2015,VOLUM

E69,NUM

BER12/15,PAGES

721–824/CHIMIAD

69(12)721–824(2015)

© 2010 Syngenta International AG, Basel, Switzerland. All rights reserved.The SYNGENTA Wordmark and BRINGING PLANT POTENTIAL TO LIFEare registered trademarks of a Syngenta Group Company. www.syngenta.com

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