recent progress in metal–organic frameworks and their ... · for eventual industrial...

Recent Progress in Metal–Organic Frameworks and TheirDerived Nanostructures for Energy and EnvironmentalApplicationsZhiqiang Xie, Wangwang Xu, Xiaodan Cui, and Ying Wang*[a]

ChemSusChem 2017, 10, 1645 – 1663 T 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim1645

ReviewsDOI: 10.1002/cssc.201601855

1. Introduction

Energy and environmental issues have become the top priorityamong a series of global issues for the next 50 years. Nowa-

days, fossil fuels are the dominant source to satisfy the energy

needs of humanity. However, these resources are being deplet-ed fast and usually lead to environmental problems, such as

air pollution and global warming, owing to emissions of ni-trous oxide, methane, carbon dioxide, and other gases contain-

ing volatile organic compounds.[1] Much progress has beenmade to overcome the abovementioned challenges by devel-

oping new energy technologies, including rechargeable batter-

ies, supercapacitors (SCs), and solar cells. In the past severaldecades, electrochemical energy storage/conversion systems

have been widely used as power supplies for many portableelectronics and electric vehicles (EVs).[2] Nevertheless, driven by

ever-growing demands for green technologies to solve urgentsustainable energy and environmental issues, a vast variety of

nanomaterials have been prepared for applications as high-

performance electrode materials in batteries, SCs, and solarcells ; related research is reported rapidly on a daily basis. It is

believed that the novel design of nanostructured electrodematerials is crucial for gaining maximum energy efficiencies in

electrochemical energy storage/conversion systems, and low-cost synthetic routes of nanomaterials are important for their

large-scale production for eventual commercialization.[3]

In recent years, dwindling freshwater caused by severe pol-lution from industrial wastewater has been considered as oneof the most critical global environmental issues.[4] In particular,heavy-metal pollution in water supply systems has become of

great concern to public health and the environment becauseheavy metals are nonbiodegradable and tend to accumulate in

the environment.[5] In this regard, many nanomaterials, such asporous carbons, transition-metal oxides (TMOs), mixed TMOs,and transition-metal oxide–carbon (TMO@C) composites, have

been explored lately for their applications in environmentalcleaning.

Metal–organic frameworks (MOFs), as a very promising cate-gory of porous nanomaterials, have attracted increasing inter-

est from research communities, since they were first obtained

in the late 1990s by Yaghi and Li.[6] MOFs are generally consid-

ered to be crystalline solids constructed from metal ions/clus-ters and organic linkers. These highly porous materials possess

many remarkable characteristics, including extremely high sur-face areas, as well as tunable pore volumes and pore size dis-

tributions, by properly varying the metal and organic units

during the synthetic process.[7] To date, MOFs have been inten-sively explored for different applications in gas separation,[8, 9]

optoelectronics,[10, 11] proton conduction,[12] catalysis,[13] drug de-livery,[14] and so forth.

In the past five years, MOFs have been utilized to addresschallenges in the fields of energy and environmental cleaning.

In the case of energy applications, MOFs have been applied as

ideal templates to prepare various nanostructured materials,such as heteroatom-doped carbons, TMOs, and TMO@C com-

posites. The MOF-derived nanostructures offer many uniqueadvantages: 1) their chemical compositions can be easily

tuned by designing various MOFs combined with specific ther-mal treatment; 2) MOF-derived nanostructures provide con-

trolled porosity and a huge surface area that facilitates easy

access of electrolyte into the electrode and ensures a largeelectrolyte/electrode contact area; 3) electron and ion diffusion

lengths can be significantly shortened, which is very beneficialfor enhanced rate performance of the MOF-derived nanostruc-

tured electrodes; and 4) low-cost, easy synthesis of MOF-de-rived nanostructures allows for potential large-scale production

for eventual industrial applications. In addition to energy appli-

cations, various functional groups can be incorporated intopristine MOFs for potential applications in environmental

cleaning,[15, 16] especially for the detection and removal ofheavy metals in wastewater.[17, 18] Different detection and ad-sorption mechanisms of heavy-metal ions have been proposed.However, a number of factors (pH, temperature, etc.) have

been observed to significantly affect the structural stability ofMOFs and their performances in terms of detection limit andadsorption capacity of heavy-metal ions in aqueous systems.

Herein, we summarize recent research progress on the appli-cations of pristine MOFs and MOF-derived nanostructures in

emerging energy- and environment-related areas, for example,rechargeable lithium-ion batteries (LIBs), sodium-ion batteries

(SIBs), SCs, dye-sensitized solar cells (DSSCs), and detection/re-moval of heavy-metal ions in aqueous systems. In addition, be-cause there is lack of in-depth understanding of the design

and synthesis of MOF-derived nanostructures and functionali-zation of pristine MOFs in the literature, we collectively sum-

marize the scattered data, highlight the main hurdles, and pro-

Metal–organic frameworks (MOFs), as a very promising catego-ry of porous materials, have attracted increasing interest from

research communities due to their extremely high surfaceareas, diverse nanostructures, and unique properties. In recent

years, there is a growing body of evidence to indicate thatMOFs can function as ideal templates to prepare various nano-structured materials for energy and environmental cleaning ap-

plications. Recent progress in the design and synthesis of

MOFs and MOF-derived nanomaterials for particular applica-tions in lithium-ion batteries, sodium-ion batteries, supercapa-

citors, dye-sensitized solar cells, and heavy-metal-ion detectionand removal is reviewed herein. In addition, the remaining

major challenges in the above fields are discussed and someperspectives for future research efforts in the development of

MOFs are also provided.

[a] Z. Xie, W. Xu, X. Cui, Prof. Y. WangDepartment of Mechanical & Industrial EngineeringLouisiana State University, Baton Rouge, LA 70803 (USA)E-mail : [email protected]

ChemSusChem 2017, 10, 1645 – 1663 www.chemsuschem.org T 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim1646

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http://www.chemsuschem.org

vide new insights into future research trends for promising ap-plications of MOFs in energy and environmental fields.

2. Brief Overview of MOFs

MOFs are a family of intriguing porous solids with well-definedcrystalline structures and extremely large surface areas. They

are generally synthesized through a coordination reaction be-tween metal clusters/ions and proper organic ligands. The

commonly used metal clusters/ions are transition metals andsome lanthanides.[19, 20] The organic ligands function as bridgesto link metal ions within the MOFs. Ligands containing pyridyland cyano groups, crown ethers, polyamines, phosphonates,

or carboxylates are mostly chosen for the preparation of MOFsunder specific synthetic conditions.[19, 20] Thanks to a vast varie-ty of primary building blocks, 20 000 different MOFs with con-

trolled sizes, shapes, and properties, have been reported so far.It is not surprising that primary building blocks (metal clusters/

ions and organic ligands) of MOFs play a very crucial role indetermining their intrinsic physicochemical properties and

structural features. The shape and pore size of various MOFs

are distinctly different from each other, owing to the use of di-verse primary building blocks. However, it should be noted

that the synthetic approaches and processing parameters mustbe taken into consideration as well.

MOFs can be facilely prepared by using various synthesismethods, such as solvothermal, microwave-assisted heating,

electrochemical, and mechanochemical methods.[20] Among

them, conventional solvothermal synthesis of MOFs usuallytakes from a few hours to several days over a large reaction

temperature range (80–250 8C). To produce MOFs more effi-ciently, new synthetic approaches have been developed in the

past decade. For example, microwave-assisted synthesis canachieve the rapid production of MOFs under hydrothermal

conditions.[20] The advantages of this synthetic approach in-

clude decreased crystallization time, a narrow particle size dis-tribution, good morphology control, and high yields of MOFs.

The first environmentally benign electrochemical synthesis ofMOFs was reported for the large-scale production of a copper-

based MOF (HKUST-1) in 2005. Instead of using metal salts, thissynthetic method utilizes metal ions supplied by anodic disso-

lution of a bulk metal plate. Such an approach has lately beenapplied to produce various MOF thin films, such as a zinc-

based MOF (ZIF-8) and aluminum-based MOFs (Al-MIL-100, Al-MIL-53-NH2). Mechanochemical synthesis has also been devel-oped for more efficient production of MOFs, and offers many

advantages. For instance, mechanochemical reactions canoccur without the use of any solvents. In addition, there are no

specific requirements for temperature and pressure. Interest-ingly, in this approach, metal ions are supplied by metal oxides

instead of commonly used metal salts to obtain MOFs. All

aforementioned synthetic techniques have been systematicallystudied over the years and show great potential in large-scale

production of MOFs for industrial applications.Benefiting from the rich variety of MOFs and their well-es-

tablished synthetic methods, MOFs can be designed to possessdesirable structural features and promising properties. The

Zhiqiang Xie received his Master’s

degree in materials engineering from

the University of Dayton, Ohio. He is

currently a Ph.D. student in the Me-

chanical and Industrial Engineering De-

partment at Louisiana State University

under the supervision of Dr. Ying

Wang. His current research focuses on

the synthesis of various nanostruc-

tured electrode materials for recharge-

able lithium-ion batteries, solar cells,

and environmental cleaning.

Wangwang Xu obtained his Bachelor’s

degree in 2012 and Master’s degree in

2014 from Wuhan University of Tech-

nology. He is currently a Ph.D. student

in the Mechanical & Industrial Engi-

neering Department at Louisiana State

University under the supervision of Dr.

Ying Wang. His research interests are

in the development of advanced mate-

rials for lithium-ion batteries and su-

percapacitors.

Xiaodan Cui received her Bachelor

degree in Chemical Engineering from

Tianjin University and Master’s degree

from University of Illinois at Chicago.

She is currently a Ph.D. candidate in

mechanical engineering at Louisiana

State University under the supervision

of Dr. Ying Wang. Her current research

focuses on novel nanomaterials for

solar cells, photocatalysts, and lithium-

ion batteries.

Dr. Ying Wang is currently an Associate

Professor in the Department of Me-

chanical and Industrial Engineering at

Louisiana State University. She received

her Ph.D. in materials science and en-

gineering from the University of Wash-

ington, her Master’s degree in chemis-

try from Harvard University, and her

Bachelor’s degree in chemical physics

from the University of Science and

Technology of China. Her research fo-

cuses on the synthesis of novel nano-

materials for energy- and environment-related applications. She

has published 65 journal articles that have received over 3800

Google Scholar citations. Her recent awards include an LSU Alumni

Association Rising Faculty Research award, an LSU Rainmaker

award, a Roy Paul Daniels Distinguished Professorship, and

a Ralph E. Powe Junior Faculty Enhancement Award.


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design of MOFs is mainly determined by their applications indifferent fields. In the last five years, significant efforts have

been underway to achieve various novel MOF-derived nano-structures and functionalized MOFs, particularly for some

emerging applications, including LIBs, SIBs, SCs, DSSCs, and de-tection/removal of heavy-metal ions in wastewater. These very

promising novel materials and their excellent performances inthe abovementioned applications are discussed in detail

herein.

3. MOF-Derived Nanostructures for LIBs

LIBs have become popular power supplies, not only for various

portable electronic devices, but also for EVs, due to theirunique advantages, such as high energy and high power den-sity, environmental benignity, long lifetime, and no memory ef-

fects.[21] Metallic lithium was initially used as an anode for LIBs,owing to its very high theoretical capacity of about

3840 mA h g@1 and low reduction potential.[22] However, thegrowth of dendrites during repeated charge/discharge pro-

cesses leads to a short circuit within the LIBs, thereby resulting

in safety issues. As such, graphite has replaced metallic lithiumas an anode for commercial LIBs, thanks to its low cost and

better safety, although it possesses a much lower capacity of372 mA h g@1. The graphite anode can store charges through

an insertion mechanism, in which lithium ions are reversibly in-serted into graphite. Nevertheless, practical capacity and cycle

life of the graphite anode are usually compromised due to anunavoidable blockage of insertion sites in graphite. Driven by

the pressing demands for high-energy, high-power, and high-

stability LIBs, great efforts have been focused on the develop-ment of superior electrode materials with complex nanostruc-

ture designs in recent decades. However, synthetic processesfor these electrode materials usually require multistep, compli-

cated procedures and/or very expensive equipment, which arenot practical for potential commercialization in industry. Fur-

thermore, the limited delivery capacity and rate capability of

commercial LIBs still severely hinder their future application inEVs. To carefully address these challenges, a variety of MOFs

and MOF-derived nanomaterials have been extensively studiedas electrodes for LIBs in recent years.

3.1. Pristine MOFs for LIBs

Most pristine MOFs exhibit poor lithium storage propertiesand are thus generally not considered to be applicable in LIBs

by direct use. For example, in 2006, Li et al. first studied pris-tine MOF-177 (Zn) as an anode for LIBs;[23] this material dis-

plays an initial discharge capacity of 400 mA h g@1 and a chargecapacity of 105 mA h g@1. More recently, Co3[Co(CN)6]2,[24]

Mn3[Co(CN)6]2,[24] and Zn(IM)1.5(abIM)0.5[25] were also evaluated

as anodes for LIBs. Similarly, they demonstrate very low initialcapacities ranging from 100 to 350 mA h g@1 at low current

densities (20 to 50 mA g@1) with fast capacity fading upon sev-eral tens of electrochemical cycles. Such poor lithium storage

properties are mainly ascribed to intrinsic low electric conduc-tivity of MOFs and large irreversible capacity loss.

Apart from the direct use of pristine MOFs as electrode ma-terials for LIBs, a large number of MOF-derived nanostructures

with desirable electrochemical properties have been designedand synthesized by using various MOFs as sacrificial templates.Generally, heat treatments of MOFs at proper temperaturesunder air, inert, or sulfur atmosphere conditions could producea rich variety of nanomaterials, such as porous carbons, metalsulfides, metal oxides, metal carbides, and metal oxide/carbon

composites. It has been widely studied that these MOF-derivednanomaterials display superior performances as electrode ma-terials in LIBs, in comparison with other materials prepared byconventional methods,[26–28] as detailed below.

3.2. MOF-derived carbons for LIBs

Nanoporous carbons (NPCs) with unique structure/morphology

and heteroatom doping (N or S) have attracted extensive inter-est for various energy-related applications, owing to an abun-

dance of resources, as well as good chemical and thermal sta-bility.[29] To date, many carbonaceous materials have been ob-

tained by using complicated processing conditions, including

carbon nanofibers,[30] nitrogen-doped graphene,[31] porouscarbon,[32] hollow carbon spheres,[33] and carbon nanobeads.[34]

Unfortunately, most of them still possess unsatisfactory batteryperformances due to limited available sites for lithium storage,

low electric conductivity, and poor structural stability. Benefit-ing from extremely high surface areas and well-defined nano-

porous structures, MOFs have demonstrated to be ideal start-

ing materials and sacrificial templates for preparing porous car-bons with desirable structural features and properties.

Generally, MOF-derived carbons can be prepared by one-step pyrolysis of MOFs in an inert atmosphere with or without

a post-acid washing treatment. To obtain carbons with largepore volumes and high specific surface areas, it is of great sig-

nificance to completely remove metallic species within the

MOF-derived carbons. For zinc-based MOFs, high temperature(ca. 900 8C) is required to derive highly porous carbons without

post treatment.[35] During carbonization in an inert atmo-sphere, metallic zinc is oxidized to zinc oxide due to disintegra-tion of MOF structures; meanwhile, decomposition of the or-ganic linkers leads to the formation of carbons. The resulting

carbons can further reduce zinc oxide into metallic zinc (boil-ing point&908 8C) and then zinc could evaporate at 900 8C,

thereby leading to highly porous carbons. However, for MOFs

containing metals (Co, Cu, Fe, etc.) with higher boiling pointsthan zinc, the post-acid washing treatment is necessary to

completely remove metallic species remaining within the MOF-derived carbons.[36]

For particular applications as anode materials in LIBs, nitro-gen-doped carbons can be simply obtained through pyrolysis

of specific MOFs due to a rich nitrogen content in the organic

ligands. For example, Zheng et al. reported a ZIF-8-derived ni-trogen-doped graphene analogous product obtained by

a facile carbonization of ZIF-8 particles at 800 8C for 8 h in N2,followed by acid washing treatment.[37] The polyhedron-like

graphene-analogous particles contain as much as 17.72 wt %nitrogen and exhibit a surface area of 634.6 m2 g@1. Such


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unique nitrogen-doped carbon demonstrates an extremelyhigh capacity of 2132 mA h g@1 at 100 mA g@1 after 50 cycles.

More impressively, it delivers a capacity of 785 mA h g@1 at5 Ag@1 over 1000 cycles. To date, these results stand out as one

of the best performances among various carbonaceous anodematerials in LIBs. Such remarkable performance from ZIF-8-de-

rived carbons is largely attributed to their high nitrogen-doping level and large surface area. In addition, this work dem-

onstrates that the nitrogen doping level and porosity of ZIF-8-

derived carbons can be optimized by the selection of an ap-propriate carbonization temperature. Similarly, Zuo et al. pre-

pared 3D porous carbons by annealing treatment of MOF-5 at900 8C in N2, which displayed a capacity of 1015 mA h g@1 over

100 cycles, showing superior cycling performance.[38] More re-cently, Li et al. designed and synthesized porous carbons with

multifractal structures composed of macropores, mesopores,

sub-nanopores, and closed pores, through the vacuum pyroly-sis of a zinc-based MOF at 1000 8C.[39] The obtained porous car-

bons display a capacity of 2458 mA h g@1 at 0.2 C (1 C =

370 mA g@1), which is significantly higher than that of porous

carbons obtained from pyrolysis under N2 (1600 mA h g@1). Thisis because that vacuum pyrolysis at 1000 8C can lead to the

formation of pores with a more complex surface fractal struc-

ture and larger closed pore volume compared with that of thepyrolysis process under N2 at the same temperature. This study

reveals that the fractal structure of porous carbons and closedpores can effectively enhance lithium surface and bulk storage.

However, this high-temperature pyrolysis resulted in lowcarbon yields based on MOF precursors (2.19 wt % under

vacuum; 2.26 wt % under N2) ; this will pose a new challenge

for its large-scale production in industry.

3.3. MOF-derived TMOs for LIBs

Most TMOs (Co3O4,[40] TiO2,[41] SnO2,[42] Fe2O3,[43] ZnO,[44] CuO,[45]

etc.) have been reported as conversion-type anode materialsfor high-performance LIBs, owing to their high theoretical ca-

pacities, chemical stability, and environmental friendliness. Un-fortunately, the fundamental problems of TMOs, such as drastic

volume changes during cycling, serious particle aggregation,and low intrinsic electric conductivity, lead to severe pulveriza-tion of electrodes, thereby resulting in fast capacity decayupon prolonged cycling and inferior rate performance. Recent-

ly, it has been widely reported that most TMOs could be direct-ly derived from MOFs by a facile calcination in air. The metalions/clusters are homogeneously distributed in MOFs; there-fore, the size or even shape of TMOs can be well controlled byusing proper annealing treatment, which is crucial for enhanc-

ing electrochemical performance of TMOs in LIBs. More impor-tantly, versatile MOFs can serve as templates to prepare vari-

ous nanostructured metal oxides with nanoporous structuresand high surface areas, which will deliver a higher capacityand better cycling stability relative to those metal oxides with

nonporous structures and low surface areas.[46] For example,Xu et al. used MIL-88 as a sacrificial template to synthesize

spindle-like porous a-Fe2O3, through a two-step calcinationprocess.[47] Briefly, as-prepared MIL-88 was first annealed in N2

at 500 8C to form FeOx–C composites. Afterwards, the resultantFeOx–C composites were sintered in air at 380 8C. In this way,

interconnected a-Fe2O3 nanoparticles were obtained with spin-dle-like morphology. The resultant sample consists of Fe2O3

nanoparticles with sizes of less than 20 nm. This spindle-like a-Fe2O3 provides a capacity of 911 mA h g@1 after 50 cycles at

200 mA g@1 and 424 mA h g@1 at an extremely high current den-sity of 10 000 mA g@1 (10 C). Different from the two-step calci-nation procedure described above, Banerjee et al. prepared a-

Fe2O3 by direct calcination of Fe-MOF in air ; this showed inferi-or rate capability compared with that of spindle-like porous a-Fe2O3.[48] These results suggest that heat treatment plays a cru-cial role in optimization of the lithium storage properties.

Among TMOs, Co3O4 stands out as an attractive anode mate-rial for LIBs, owing to its high theoretical capacity of about

980 mA h g@1.[39] To date, many Co3O4 nanostructures with dif-

ferent microstructures and morphologies have been designedand synthesized by using cobalt-based MOFs as precursors

under specific synthetic conditions. For example, Shao et al.synthesized ZIF-67-derived Co3O4 with a ball-in-dodecahedron

nanostructure.[49] Such unique nanostructured Co3O4 deliversa capacity as high as 1550 mA h g@1 at 100 mA g@1 and remains

at 1335 mA h g@1 after 100 cycles. The authors claimed that the

porous ball-in-dodecahedron nanostructure could effectivelyshorten charge transport pathways and facilitate electrolyte

access into the electrodes, thereby resulting in significantly en-hanced lithium storage properties. Liu et al. obtained porous

Co3O4 by one-step calcination of [Co3(NDC)3(DMF)4] (NDC = 2,6-naphthalene dicarboxylate) in air.[50] The resultant Co3O4 prod-

uct provides a high capacity of 965 mA h g@1 at 50 mA g@1 after

50 cycles, showing excellent cycling stability. Wu et al. reportedZIF-67-derived Co3O4 hollow dodecahedra synthesized by two-

step heat treatment of ZIF-67 at 350 8C in N2, followed by calci-nation in air.[51] As-prepared Co3O4 displays a capacity of

780 mA h g@1 after 100 cycles. Similarly, other cobalt-basedMOFs, such as MOF-71 and [Co(HO-BDC)(bbb)] (HO-BDC =

5-hydroxylsophthalic acid, bbb = 1,4-bis(benzimidazol-1-yl)bu-

tane), were also reported as sacrificial templates for the synthe-sis of Co3O4 nanostructures, which exhibited comparable elec-

trochemical performance to that of ZIF-67-derived Co3O4.[52, 53]

In addition to MOF-derived Fe2O3 and Co3O4 nanostructuresoutlined above, many other metal oxides have been preparedand demonstrated distinctly improved electrochemical per-

formances in LIBs. For instance, Hu et al. prepared MOF-199-derived CuO/Cu2O hollow polyhedra with a porous structure,which provided a capacity of 740 mA h g@1 at 100 mA g@1.[54]

Similarly, Banerjee et al. prepared the CuO nanostructure bypyrolysis of MOF-199 at 550 8C for 2 h in air, which provided

a capacity of about 538 mA h g@1 at 50 mA g@1.[55] Althoughthey both used MOF-199 as precursors, different chemical

compositions of final products could be obtained through con-

trol of heat treatment conditions, which significantly affectedtheir electrochemical performances in LIBs. Wang et al. ob-

tained a MIL-125-derived anatase TiO2 with a porous struc-ture.[56] It delivered capacities of 166 and 106 mA h g@1 at 1 and

5 C, respectively, during 500 cycles. Such impressive capacityand rate capability are mainly ascribed to the high surface area


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(ca. 220 m2 g@1) and porous structure, which can effectivelycushion volume changes upon cycling and facilitate access of

the electrolyte.Mixed transition-metal oxides (MTMOs) and their compo-

sites, with precise chemical compositions and different nano-structures, have been synthesized by applying heterometallic

MOFs as sacrificial templates. It should be noted that MTMOsrefer to metal oxides with two different metal cations, which

are different from physical mixtures of two metal oxides.[57] Ac-

cording to previous reports, MTMOs demonstrate significantlyenhanced lithium storage properties compared with those ofsingle metal oxide materials. This phenomenon is probably as-cribed to their improved electrical conductivity and betterelectrochemical reaction activity. For instance, Huang et al. syn-thesized Fe2O3@NiCo2O4 nanocages by one-step heat treat-

ment of core–shell Co3[Fe(CN)6]2@Ni3[Co(CN)6]2 nanocubes in

air.[58] Specifically, Prussian blue analogue Ni3[Co(CN)6]2 nano-cubes were first prepared by a coprecipitation reaction with

NiCl2, K3[Co(CN)6] , and sodium citrate in distilled water at roomtemperature. The obtained Ni3[Co(CN)6]2 nanocubes were then

used as seeds for epitaxial deposition of Co3[Fe(CN)6]2, whichformed core–shell Co3[Fe(CN)6]2@Ni3[Co(CN)6]2 nanocubes due

to their similar crystal structures. During this synthesis, sodium

citrate was used as an additive to reduce the coordination ratebetween the metal ions and organic ligands, thereby resulting

in a uniform coating of Co3[Fe(CN)6]2 on the Ni3[Co(CN)6]2

nanocubes. After heat treatment in air at 450 8C for 6 h, the

porous Fe2O3@NiCo2O4 nanocages were obtained. TheFe2O3@NiCo2O4 nanocages deliver a high capacity of

1079.6 mA h g@1 at 100 mA g@1 after 100 cycles. Such remark-

able performance can be ascribed to the porous hollow struc-ture and synergistic effects of each component in the compo-

site. Similarly, various mixed-metal oxides with porous hollowstructures, such as NiFe2O4/Fe2O3 nanotubes,[59] CoFe2O4 nano-

cubes,[60] and Mn1.8Fe1.2O4 nanocubes,[61] have been constructedby rationally designing heterometallic MOFs as the templates.Among them, CoFe2O4 nanocubes show a high capacity of

815 mA h g@1 at 20 C, exceeding the capacity value of mostpreviously reported anode materials for LIBs to date. In thatwork, Guo et al. studied the formation mechanism of theporous hollow structure by observing structural changes of

CoFe2O4 nanocubes synthesized from the calcination ofCo[Fe(CN)6]0.667 at different temperatures.[60] The results showed

that only solid nanocubes were formed under calcination at200 8C. As the calcination temperature was increased from 200to 350 8C, the solid nanocubes gradually became porous and

finally transformed into the hollow structure when the temper-ature reached 350 8C. The formation of this hollow structure is

mainly due to the fast release of CO2 and NxOy gases duringthe calcination process, resulting in the CoFe2O4 shell and an

interior cavity. However, when the temperature was increased

to 450 8C, the hollow nanocubes were broken into individualnanoparticles. This work reveals that the calcination tempera-

ture plays a crucial role in optimizing the structure of MOF-de-rived metal oxides.

More recently, Xu et al. designed and prepared a Co3O4@TiO2

integrated hollow nanostructure through a cation-exchange re-

action between Ti4 + and Co2+ on the surface of ZIF-67 in DMF,followed by annealing treatment of Ti4+-exchanged ZIF-67 for

2 h in air.[62] The morphology and chemical composition of theCo3O4@TiO2 nanostructure can be clearly observed from the

SEM images and energy-dispersive X-ray spectroscopy (EDS) el-emental mapping results in Figure 1. This composite is de-

signed to combine the merits of high capacity from Co3O4 and

excellent electrochemical stability, as well as good electric con-

ductivity of TiO2. This Co3O4@TiO2 nanostructure delivers a ca-pacity of 642 mA h g@1 at 500 mA g@1 after 200 cycles, which is

much higher than that of bare Co3O4 hollow polyhedra

(350 mA h g@1). The same authors also reported crystallineCo3O4–carbon@amorphous FeOOH interwoven hollow poly-

hedrons synthesized through thermal treatment of ZIF-67 inair, followed by solution-phase growth of amorphousFeOOH.[63] Interestingly, in this synthesis, well-defined Co3O4–carbon hollow polyhedrons serve as a growth platform for the

growth of amorphous FeOOH nanowires. As we can see fromFigure 2, numerous Co3O4 hollow polyhedrons are intercon-nected by FeOOH nanowires. Such an intriguing nanostructure

can effectively buffer against drastic volume changes upon cy-cling in LIBs. As a result, the obtained heterostructured hollow

polyhedrons deliver a capacity of 603 mA h g@1 at 200 mA g@1

after 100 cycles, showing much better performance in compari-

son with that of bare Co3O4 hollow polyhedrons. This work will

shed light on the design of mixed-metal oxides as high-per-formance anode materials for LIBs.

Figure 1. SEM images of a) ZIF-67 hollow polyhedrons, b) Ti4+-exchangedZIF-67 hollow polyhedrons, and c) Co3O4/TiO2 composite hollow poly-hedrons. Insets depict magnifications (scale bar = 300 nm). d) SEM image ofCo3O4/TiO2 composite hollow polyhedrons and corresponding EDS map-pings of Ti, Co, and O elements (scale bars = 1 mm). Reproduced fromref. [62] with permission from Elsevier.


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3.4. MOF-derived metal oxide/carbon composites for LIBs

To effectively cushion against drastic volume changes of TMOsduring cycling and improve their electrical conductivity, a car-

bonaceous matrix has been incorporated with TMOs to formTMO@C composites. The TMO particle sizes in the TMO@C

composites cannot be well controlled by conventional chemi-

cal synthetic methods, due to the severe agglomeration of par-ticles, which is detrimental to the battery performance. Howev-

er, the TMO@C composites synthesized by using MOFs as start-ing materials and templates would produce well-dispersed

TMO nanoparticles within the nanoporous carbonaceousmatrix.[64, 65] For example, Yang et al. synthesized ZnO quantumdots that were well dispersed within the carbonaceous matrix

by controlled pyrolysis of IRMOF-1 at 900 8C in an inert gas at-mosphere.[64] The obtained ZnO–C composite provided a ca-

pacity of 1200 mA h g@1 at 75 mA g@1. This method offers a gen-eral approach to the preparation of TMO@C nanostructures for

high-performance anode materials. Zou et al. prepared MOF-derived ZnO/ZnFe2O4/C hollow octahedrons, in which ZnO/

ZnFe2O4 ultrafine nanoparticles (ca. 5 nm) were well dispersed

within a highly porous carbonaceous matrix.[65] Such a ZnO/ZnFe2O4/C composite delivers a capacity of 762 mA h g@1 at

10 A g@1, showing an outstanding rate performance.

3.5. MOF–carbonaceous material hybrids for LIBs

To further boost the performances of LIBs, many efforts have

been made to design various MOF–carbonaceous material hy-brids as electrode materials, which can promisingly combine

the merits of MOF-derived active materials and flexible, robust,and conductive carbonaceous materials, such as multiwalled

carbon nanotubes (MWCNTs), graphene, and carbon cloth. Asshown in Figure 3 a, Huang et al. synthesized MWCNTs/Co3O4

nanocomposites by the annealing treatment of as-preparedZIF-67 with MWCNTs inserted (MWCNTs/ZIF-67) at 400 8C in

air.[66] Specifically, the MWCNTs were first functionalized withcarboxylic groups (@COOH) by using a mixture of acids, HNO3

and H2SO4, at 80 8C for 3 h. Afterwards, the pretreated MWCNTswere well dispersed in methanol with ZIF-67 precursors and

PVP as a surfactant. In this step, functional groups on theMWCNTs can serve as the nucleation centers for the growth ofZIF-67. Finally, the MWCNTs were inserted into the ZIF-67 poly-

hedra to form the MWCNTs/ZIF-67 composite. After calcinationat 400 8C in air, MWCNTs/Co3O4 was obtained. Such a hybrid

nanostructure displays a capacity of 813 mA h g@1 at100 mA g@1 after 100 electrochemical cycles, showing signifi-

cantly enhanced cycling stability compared with that of bareCo3O4 polyhedrons. Such improved electrochemical per-

formance is mainly attributed to the porous structure and syn-

ergistic effect between MWCNTs and Co3O4. The same authorsalso synthesized MWCNTs/ZnCo2O4 nanocomposites by using

heterometallic MOFs. However, the MWCNTs/ZnCo2O4 nano-composites exhibit even worse electrochemical performance

than that of MWCNTs/Co3O4 nanocomposites, which suggeststhat ZnCo2O4 does not show superiority in electrochemical

properties over similar nanostructured Co3O4. As we can see

from Figure 3 b, Chen et al. reported the synthesis of tubularstructures consisting of ZIF-67-derived Co3O4 hollow nanoparti-

cles and in situ formed carbon nanotubes (CNTs) througha multistep route.[67] In this work, the authors first prepared

PAN–cobalt acetate (PAN–Co(Ac)2) nanofibers through an elec-trospinning method. Then the ZIF-67 crystals grew on the

PAN–Co(Ac)2 nanofibers in ethanol at room temperature, form-

ing core–shell PAN–Co(Ac)2@ZIF-67 nanofibers, owing to thecoordination reaction of cobalt ions and the organic ligand

2-methylimidazole. Afterwards, the obtained core–shell PAN-Co(Ac)2@ZIF-67 nanofibers were transformed into ZIF-67 micro-

tubes by dissolving PAN and Co(Ac)2 in DMF at 50 8C. The as-prepared ZIF-67 microtubes were further transformed intoCNT/Co–carbon by annealing in an atmosphere of Ar/H2 at

750 8C for 2 h. Finally, CNT/Co3O4 microtubes were obtainedafter calcination at 360 8C for 10 min. Benefiting from the tubu-lar structure and void space within Co3O4 nanoparticles, theproblem of large volume changes of Co3O4 can be effectivelyalleviated. In addition, CNTs within the composite can improveelectrical conductivity of the entire electrode, thereby enhanc-

ing the rate capability. As a result, the CNT/Co3O4 nanocompo-site provides a high capacity of 1281 mA h g@1 at 100 mA g@1

and maintains capacities of 782 and 577 mA h g@1 after

200 cycles at 1 and 4 A g@1, respectively.Graphene, as a two-dimensional carbon material, has been

reported as an ideal platform for the growth of various nano-particles for wide applications, thanks to its high surface area,

outstanding electrical conductivity, and chemical stability. Re-

cently, Xie et al. prepared PNCs@Gr with a sandwich-like mor-phology through the one-step pyrolysis of ZIF-8 nanocrystals

grown on GO sheets (Figure 3 c).[68] During the synthesis, GOsheets with rich functional groups served as the platform for

the growth of ZIF-8 crystals. PVP was used to facilitate uniformgrowth of ZIF-8 on GO sheets. After pyrolysis of ZIF-8@GO at

Figure 2. a) Schematic illustration of the interwoven heterostructure.b, c) SEM images of the Co3O4–carbon@FeOOH interwoven hollow poly-hedron structure. Reproduced from ref. [63] with permission from the RoyalSociety of Chemistry.


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700 8C in N2 for 5 h, sandwich-like PCNs@Gr was obtained. It is

believed that GO can be thermally reduced into conductivegraphene during this pyrolysis process. PNCs@Gr provides a ca-

pacity of about 530 mA h g@1 after 400 cycles at 5 A g@1. Suchelectrochemical performance improvement of PCNs@Gr can be

ascribed to its excellent electric conductivity, highly porous

structure, and high nitrogen-doping level. Similarly, Cao et al.reported the growth of Fe-MOF (MIL-88) particles on 3DGN

(MOF/3DGN).[69] After heat treatment of MOF/3DGN in air,metal oxide/3DGN nanocomposite was prepared (Figure 3 d).

Figure 3. Schematic illustrations showing the synthetic processes for different MOF–carbonaceous materials hybrids: a) MWCNTs/Co3O4 hollow polyhedral ;[66]

b) tubular CNT/Co3O4 ;[67] c) sandwich-like porous carbons (PNCs@Gr);[68] d) flexible Fe2O3/3D graphene networks.[69] PVP = polyvinylpyrrolidone, PAN = polyacry-lonitrile, GO = graphene oxide, PNCs@Gr = nitrogen-doped carbon/graphene composite, 3DGN = 3D graphene network. Reproduced with permissions fromrefs. [66]–[69].


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The Fe2O3/3DGN nanocomposite provides a higher capacity of864 mA h g@1 after 50 cycles than that of bare Fe2O3

(24.3 mA h g@1). Such enhanced battery performance is mainlyascribed to the 3DGN substrate, which can effectively enhance

the electrical conductivity and mechanical integrity of theentire electrode. Zhang et al. prepared ZnO@ZnO quantum

dot/carbon nanorod arrays on a carbon cloth substrate, inwhich the flexible carbon cloth served as the current collector

in the battery cell.[70] The ZnO@ZnO QDs/C composite displays

a high capacity of 1055 mA h g@1 at 100 mA g@1.

4. MOF-Derived Nanostructures for SIBs

Sodium possesses a lower theoretical capacity of 1160 mA h g@1

and a higher reduction potential than that of lithium. However,the price of sodium is $250–300 per ton, which is significantly

cheaper than that of lithium ($5800 per ton). Therefore, in thelast decade, SIBs have been widely considered as a promising

alternative to current LIBs, due to the low cost and abundantsodium resources on earth, which are advantageous, especially

when large amounts of alkali are demanded for future applica-

tions in EVs or stationary storage. Although the operating prin-ciples of SIBs are identical to that of the working mechanism

in LIBs, SIBs face more challenges than LIBs. For example,safety issues are more problematic than those of lithium be-

cause sodium shows much higher reactivity with organic sol-vents in electrolyte solutions, which results in severe growth of

dendrites over electrochemical cycles. In addition, the low

melting point (97.7 8C) of sodium may cause some safetyissues when used as electrodes for SIBs. Therefore, sodium is

not a suitable anode material for SIBs.Similar to LIBs, many anode materials have been developed

for SIBs, such as metal oxides, alloys, metal nitrides, phosphors,and carbonaceous materials. Unfortunately, most of the previ-

ously reported SIBs suffer from much worse electrochemical

performances than those of LIBs, mainly due to sluggishsodium intercalation/deintercalation kinetics caused by the

larger size of Na+ (1.02 a radius) than that of Li+ (0.76 aradius). To date, sodium intercalation chemistry is still not fullyunderstood. Notably, many materials that demonstrated out-standing electrochemical performance for LIBs may show infe-

rior performance for SIBs. Therefore, it is still highly desirableto better understand the sodium intercalation/deintercalation

process and further explore novel electrode materials for SIBs.In recent years, MOF-derived electrode materials have been

widely studied for SIBs. For instance, Fan et al. synthesized an

amorphous carbon nitride (ACN) composite by facile pyrolysisof ZIF-8 at 700 8C.[71] As shown in Figure 4 a, the ACN compo-

site shows a polyhedron-like morphology with uniform ele-mental distributions of C, N, O, and Zn throughout the materi-

al. The composite displays a capacity of 430 mA h g@1 at

83 mA g@1 and 146 mA h g@1 at 8.33 Ag@1. The ACN compositealso maintains a capacity of 146 mA h g@1 at a high specific cur-

rent of 1.67 A g@1 after 2000 cycles. To date, this work showsthe best rate performance and cycling stability among previ-

ously reported anode materials for SIBs. Such impressive per-formances are largely attributed to the high nitrogen content

(20.4 wt %) in the ACS composite and its high surface area of427 m2 g@1. Qu et al. synthesized ZIF-8-derived microporous

carbon (ZIF-C) at a higher temperature 930 8C.[72] The specific

surface area of ZIF-C is 1251 m2 g@1, which is almost four timesthat of the ACN composite; however, the capacity and rate

performance of ZIF-C in SIBs are much worse than those of theACN composite. ZIF-C provides a low capacity of 136 mA h g@1

at 50 mA g@1. It is worth mentioning that the nitrogen contentof ZIF-C is only 3.1 wt %, which is much lower than that of theACN composite (20.4 wt %). Therefore, the nitrogen-doping

level plays a crucial role in determining the electrochemicalperformances in SIBs. Wang et al. fabricated Co3O4@NC

through a two-step heat treatment of ZIF-67 precursors in dif-ferent gas atmospheres.[73] The formation process is illustratedin Figure 4 b. Briefly, the ZIF-67 precursors were annealed in Arto produce the Co@N-doped carbon intermediate. Afterwards,

the intermediate was calcined in air to obtain Co3O4@NC,whereas the Co3O4 nanoparticles were encapsulated in the N-doped carbon shell. When applied as anode materials for SIBs,

Co3O4@NC provides capacities of 506, 317, and 263 mA h g@1 at100, 400, and 1000 mA g@1, respectively. It also delivers a rever-

sible capacity of 175 mA h g@1 after 1100 cycles at 1 A g@1. Theauthors claimed that such improved capacity and cycling sta-

bility were attributed to the capacitive behavior and full con-

version between Co3O4 and metallic Co during the sodiation/desodiation process. As shown in Figure 4 c, Chen et al. ob-

tained NCO-NBs by an annealing treatment of ZIF-67.[74] NCO-NBs show outstanding performances in LIBs, with a reversible

capacity of 1080 mA h g@1 at 500 mA g@1 after 150 cycles and884 mA h g@1 after 200 cycles at 2000 mA g@1. However, when

Figure 4. a) EDS results for C, N, O, and Zn of the ACN composite. Repro-duced from ref. [71]. b) Illustration of the formation process of Co3O4@N-doped carbon (Co3O4@NC); from left to right: SEM images of ZIF-67 precur-sor; Co@NC intermediate; Co3O4@NC; and schematic illustration of the sub-unit composition of Co3O4@NC, which is composed of a Co3O4 core anda NC shell. Reproduced from ref. [73] with permission from the Royal Societyof Chemistry. c) Schematic illustration of the synthetic process of hollowporous NiCo2O4-nanobox composites (NCO-NBs). Reproduced from ref. [74]with permission from the Royal Society of Chemistry.


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evaluated as anode materials in SIBs, they display much worseelectrochemical performances than those of LIBs. The capacity

of NCO-NBs fade fast from 826 to 328 mA h g@1 after 30 cycles.In addition to the widely reported ZIF-8 and ZIF-67 tem-

plates, copper-based MOFs have also been used to preparenanostructured anode materials in SIBs. For instance, Zhang

et al. synthesized porous CuO/Cu2O hollow octahedrons(CHOs) by direct pyrolysis of Cu-based MOF ([Cu3(btc)2]n ; btc =

benzene-1,3,5-tricarboxylate; Figure 5).[75] After 50 electrochem-

ical cycles, the optimal sample (CHO-1) displays a capacity of

415 mA h g@1 at 50 mA g@1 and about 212 mA h g@1 after400 cycles at 1 A g@1, and remains about 165 mA h g@1 after

1000 cycles at 2 Ag@1. More recently, Nie et al. reported a flexi-ble cathode material composed of Prussian blue analogue

(FeFe(CN)6)/carbon cloth for application in SIBs.[76] The ob-tained FeFe(CN)6/carbon cloth composites demonstrate a ca-

pacity of 82 mA h g@1 at 0.2 C and good capacity retention of

81.2 % over 1000 cycles.

5. MOF-Derived Nanostructures for SCs

SCs with high power densities and remarkable cyclability haveattracted tremendous interest due to their balance of power

density and energy density. In comparison with batteries, al-though SCs display lower energy density than batteries, theycan release the stored electrochemical energy in a short periodof time, supplying very high power density.[77] Moreover, unlike

batteries, SCs are able to operate durably for a long time withexcellent capacity retention. In general, SCs are classified into

two main categories based on the category of active materialsand different charge storage mechanisms. One category in-

volves electrochemical double-layer capacitors (EDLCs), which

can store charge on their internal surface electrostatically.[78]

For EDLCs, commonly used active materials are carbon-basedmaterials, such as carbon nanofiber,[79] single/multiwalledCNT,[80] and porous carbon.[81] The other category of SCs, called

pseudocapacitors, can store charge by reversible and rapid sur-face redox reactions.[82] Most TMOs and certain conducting

polymers are widely used as pseudocapacitive materials. These

two types of SCs are currently available commercially and arepromising for applications as load-leveling applications and

handheld devices. To further enhance the storage capabilityand power density of SCs, large surface area and excellent

electric conductivity are the two main prerequisites for activematerials. Recently, emerging as novel porous materials, MOFs

have been deemed as potential active materials for SCs, bene-

fiting from their adjustable pore sizes, large specific surfaceareas, and incorporated redox metal centers.

Recently, tremendous efforts have been focused on the de-velopment of MOF-based SCs. The application of MOF-based

SCs can be divided into four methods based on electrode ma-terials : 1) some pristine MOFs can be directly used to store

energy through the physisorption of charge on internal surfa-

ces or reversible redox reactions of metal centers in MOFs;2) porous carbon nanomaterials obtained by pyrolyzing MOFs

can be applied as electrodes with large capacitance and highconductivity for SCs; 3) by applying MOFs as templates, porous

metal oxide nano-/heterostructures can be designed and usedas superior electrodes for SCs; and 4) MOF-derived metal oxy-

hydrogen/phosphate nanostructures can be used as novel

electrodes for SCs.

5.1. Pristine MOFs for SCs

The large specific area gives MOFs unique superiority for useas electrodes for SCs. However, the high specific surface area

of MOFs is usually accompanied by very poor electrical con-

ductivity, resulting in the poor electrochemical performance ofactive materials, even at a low specific current. To achieve en-

hanced performance of MOFs for SCs, tremendous efforts havebeen focused on two strategies to improve the conductivity :

1) highly conductive carbonaceous materials (graphene, CNTs,etc.) are used to hybridize with MOFs; and 2) modification ofcrystallinity, such as through doping. Recently, much progress

has been made on the utilization of pristine MOF/graphenecomposites as electrodes for SCs. Choi et al. reported that MOF

nanocrystals could be hybridized with graphene as electrodematerials for SCs for the first time.[83] The films were placed on

both sides of a separator membrane and soaked by a solutionof an electrolyte. Among 23 different MOFs with multiple met-

Figure 5. SEM (a, b) and TEM (c, d) images of CHO-1 at different magnifica-tions; e) the long-term cycling performance of CHO-1 at high specific cur-rents of 1 and 2 A g@1, respectively. Reproduced from ref. [75] with permis-sion from the Royal Society of Chemistry.


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allic ions and organic ligands, zirconium MOFs doped with gra-phene exhibited extraordinarily high capacitance of

5.09 mF cm@2 over at least 10 000 cycles, which was about 6times the capacitance of SCs based on benchmark commercial

activated carbons. Moreover, Srimuk et al. synthesizeda HKUST-1 MOF loaded on reduced graphene oxide (rGO).[84]

This new mesoporous material provides a capacitance of385 F g@1 at 1 A g@1, whereas pure HKUST-1 MOF stores only0.5 F g@1.

Doping with metal species is another strategy to effectivelyenhance the electrical conductivity of MOFs. For example,Yang et al. synthesized Zn-doped Ni-MOF as an active materialfor SCs.[85] The Zn-doped Ni-MOF exhibits high specific capaci-

tance, good cyclability, and good rate capability. At rates of0.25 and 10 A g@1, the Zn-doped Ni-MOF delivers high capaci-

tances of 1620 and 854 F g@1, respectively, for 3000 cycles with

capacitance retention of over 91 %, demonstrating excellentperformance. Another example, Co8-MOF-5 electrode with Zn

cations partially substituted by Co cations, has been reportedfor enhanced performance of SCs.[86] The mixed-metal-based

MOF structure shows much better SC performance than thatof most single metal oxide based electrodes, owing to im-

proved electric conductivity. Recently, Banerjee et al. demon-

strated the enhanced electric conductivity of Ni-doped MOF-5by using electrochemical impedance spectroscopy (EIS).[87] In

comparison with a high charge-transfer resistance of 184 mW

for pure MOF-5, the Ni-doped MOF-5 exhibits much lower re-

sistance of only 74 mW. With reduced charge-transfer resist-ance, the Ni-doped MOF-5 provides a dramatically increased

capacitance of 240 F g@1 at a current density of 50 mA g@1,

whereas the capacitance of pure MOF-5 is only 100 F g@1.

5.2. MOF-derived carbons for SCs

Various carbon materials, including porous carbons, carbon

nanowires/nanotubes, and graphene, have attracted intenseinterest as very promising electrode materials for SCs due to

their low cost and excellent chemical stability. For EDLCs,energy is stored at the electrode/electrolyte interface based on

the electrochemical double-layer mechanism; therefore, porosi-ty and specific surface area of the electrodes are critical factorsthat affect their electrochemical performance. Introduced as anintriguing family of porous materials, MOFs have gained partic-ular attention and been demonstrated as versatile templates

to obtain micro-/mesoporous carbon materials for high-per-formance EDLC electrodes, owing to their well-designed nano-structures with very high specific surface area and controllablepore size.[88–91]

Carbon materials with exceptional surface areas can beeasily derived from MOFs.[92] In this process, furfuryl alcohol

(FA) was first polymerized in the pores of MOF-5. During the

subsequent carbonization process, highly porous carbon back-bones were produced through the pyrolysis of organic linkers

and vaporization of zinc at a high temperature of 1000 8C. Thespecific surface area of the as-synthesized NPC is 2872 m2 g@1

with a pore size distribution centered at 3.9 nm. The specificcapacitance of the optimal NPC is 222 F g@1 at 50 mA g@1. In

comparison with carbon-based electrodes prepared by otherstrategies, MOF-derived carbon-based nanomaterials exhibitsuperior performance for SCs in terms of specific capacitanceand rate capability. For instance, carbon nanofibers fabricated

by electrospinning can deliver a low capacitance of 64 F g@1 at400 mA g@1.[93] CNT/carbon composites synthesized by a hydro-thermal method can supply 125 F g@1 at 2 A g@1.[94] Such en-hanced electrochemical performances of MOF-derived carbonsare attributed to their high surface area and high porosity.

To further enhance the rate performance, most research ef-forts have been concentrated on enhancing the relatively low

electronic conductivity of MOF-derived carbon materials in twoways: 1) derive porous carbon with a high degree of carboni-zation and nitrogen content; and 2) improve the degree ofgraphitization of NPC. For example, Jeon et al. introduced

a self-sacrificial templating method to synthesize a nitrogen-doped porous carbon.[95] This carbon material shows a large ca-pacitance of 239 F g@1 at 50 mA g@1, which is much higher than

that of nitrogen-free carbon (24 F g@1). Moreover, Liu et al. ob-tained spherical nitrogen-rich porous carbon shells through

the pyrolysis of porous organic frameworks (Figure 6).[96] The

as-prepared nitrogen-rich porous carbon delivers a high capac-itance of 230 F g@1 at 500 mA g@1 over 1500 cycles with about

98 % retention of the initial capacitance; this suggests the sig-nificance of nitrogen dopants on the electrochemical per-

formance. Enhancing the degree of graphitization of carbon isalso very effective to enhance the electrochemical performance

of MOF-derived carbons. For example, Torad et al. synthesized

highly graphitized carbon from ZIF-67 for application inEDLCs.[97] NPCs with a high degree of graphitization are directly

obtained through the direct pyrolysis of ZIF-67. After chemicaletching to remove the cobalt content, NPCs exhibit a high ca-

pacitance of 238 F g@1 at a scan rate of 20 mV s@1, which ismuch larger than that of the activated carbon (112 F g@1) ; this

Figure 6. SEM (a, b) and TEM (c, d) images of porous organic frameworks(a, c) and MOF-derived nitrogen-doped porous carbons (b, d). Reproducedfrom ref. [96] with permission from the American Chemical Society.


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demonstrates the importance of the degree of graphitizationof carbon for application in high-performance SCs.

5.3. MOF-derived TMOs for SCs

TMOs, such as MnO2, Co3O4, V2O5, NiO, and Fe2O3, are com-monly used active materials in SCs worldwide owing to their

remarkable capacities as redox SCs. In most MOFs, the metal isintrinsically monodispersed. This unique feature provides a spe-

cial opportunity for designing and synthesizing various metal

oxides or structures with MOFs as precursors. In this approach,the size and shape of the obtained metal oxide nanomaterials

can be well controlled, which is beneficial for achieving re-markable electrochemical performance from the elec-

trodes.[98–100] Meng et al. reported the synthesis of porousCo3O4 materials through a solid-state conversion process of

cobalt-based MOF materials.[101] As-made Co3O4 exhibits a well-

defined porous structure. At 1 A g@1, the porous Co3O4 materi-

als maintain a capacitance of 150 F g@1 for 3400 cycles. The su-perior performance is attributed to its porous structure and

high specific surface area. Moreover, by applying MOFs as pre-cursors, unique hierarchical metal oxides can also be designed

and derived. For example, Li et al. reported double-shelledNiO/ZnO hollow spheres, which was synthesized by annealing

treatment of bimetallic organic frameworks in air, as novelelectrodes for SCs.[102] As illustrated schematically in Figure 7,the synthetic process can be divided into three steps. In step I,

H2BDC reacts with Ni2 + and Zn2+ ions at high temperature,forming solid spheres. In step II, yolk–shell microspheres areobtained during the hydrothermal process. In step III, the NiO/ZnO double-shelled spheres are synthesized after the calcina-tion of Ni/Zn bimetallic MOFs in air. The NiO/ZnO double shellsexhibit a capacitance as high as 497 F g@1 at 1.3 A g@1, which is

much larger than the capacitance (48 F g@1) of ZnO/CNT nano-

composite synthesized by reactive direct current (DC) magne-tron sputtering.[103] The remarkable electrochemical performan-

Figure 7. Schematic illustration of the formation process of double-shelled NiO/ZnO hollow spheres. SEM images (a, b), TEM images (c, d), and EDS measure-ments (e, f) of the double-shelled NiO/ZnO hollow spheres. Reproduced from ref. [102] with permission from the Royal Society of Chemistry.


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ces are mainly ascribed to the double-shelled nanostructures,which can offer numerous active sites for electrochemical reac-

tions.

5.4. MOF-derived metal hydroxides for SCs

Generally, metal hydroxides can supply higher specific capaci-

tance than that of carbonaceous materials and provide betterelectrochemical stability than that of conducting polymers as

electrodes in SCs. Moreover, within appropriate potential win-dows, they can exhibit electrochemical faradaic reactions. No-

tably, micro-/mesoporous metal hydroxide materials can beprepared by using MOFs as templates for enhanced per-

formance for SCs.[104–107]

As reported by Zhang et al. , Co(OH)2 nanostructures can befacilely synthesized by using ZIF-67 as a template.[108] The de-

rived Co(OH)2 nanostructures exhibit a mesoporous structureand large surface area, which can shorten the charge-transfer

distance, provide good electrolyte access, and enlarge the con-tact area between the active material and electrolyte. There-

fore, such Co(OH)2 nanomaterials display a large capacitance

(604.5 F g@1), long cycling life, and good rate capability. Thisfacile strategy provides unlimited possibility to design high-

performance metal hydroxide based electrodes for SCs.[108, 109]

Moreover, in comparison with Co(OH)2 nanomaterials obtained

by other strategies, MOF-derived Co(OH)2 nanostructures ex-hibit better electrochemical performance. For example,

Co(OH)2 synthesized by solution-phase growth can supply only416 F g@1 at 1 A g@1.[110] The superior performance of MOF-de-

rived metal hydroxides is attributed to their extremely high

specific surface area.

6. MOF-Derived Nanostructures for DSSCs

To fulfill increasing energy demands, the exploration of renew-

able and sustainable energy sources is becoming more andmore urgent. Among various forms of energy, solar energy

stands out as an ideal candidate owing to its unlimited supplyand clean nature. As a leading technology in third-generation

solar cells, DSSCs have received tremendous interest owing totheir easy fabrication, as well as ecological, green, and low-cost

nature. A typical DSSC is composed of three main parts : a dye-sensitized photoanode, a liquid electrolyte, and a counter elec-

trode.[111] When sunlight passes through the transparent elec-trode, the photosensitizers accept incident photons and elec-trons are excited to flow into the photoanode. Then electrons

are collected and transferred to the counter electrode throughan external circuit. Meanwhile, photosensitizer is regenerated

by oxidizing the electrolyte ions, which are reduced back byaccepting electrons from the counter electrode.

To improve the power conversion efficiency (PCE) and stabil-

ity of DSSC, the development of nanostructured materials is ofgreat potential.[112, 113] MOFs, as a novel class of porous materi-

als, appear very appealing for this application. The composi-tion and structure of MOFs can be easily tuned by combining

multiple organic linkers and metal ions, especially earth-abun-dant metals, which can effectively reduce the cost of DSSCs.

Meanwhile, MOFs are also ideal templates for nanostructuredmaterials with excellent electrochemical performance. In the

field of DSSCs, as-prepared MOFs and MOF-derived materialshave been reported to work efficiently as photoanodes andcounter electrodes. In this section, the design and applicationof MOF-based materials in DSSCs is discussed.

6.1. MOF-derived nanostructures as photoanodes

In DSSCs, the photoanode is both a skeleton matrix for dye ad-

sorption and a photocatalyst for charge generation and trans-portation; this makes it the most critical component of DSSCs.

Therefore, the surface area to volume ratio, optical properties,and electronic properties determine the ultimate performance

of a photoanode. Among various semiconductors, TiO2 receives

most attention due to its high conduction band edge andgood dye-loading affinity. Nowadays, other metal oxides have

started to attract attention from researchers, especially ZnO,which is also a wide band gap semiconductor, but with

a much higher electron mobility. Recently, more and more ef-forts are being devoted to developing nanostructured TiO2

and ZnO with different morphologies, such as nanoparticles,

nanosheets, nanotubes, and hybrid nanostructures.[114, 115]

A ZnO-based photoanode derived from homochiral MOFs

was first reported by Kundu et al. in 2012.[116] Through varyingthe anions (Cl@ and Br@) and annealing atmospheres (N2 and

air), different morphologies resulted, including hexagonalcolumn shaped, rod-shaped, and elliptical aggregation of ZnO

(Figure 8). The as-synthesized ZnO possesses a visible-light

emission centered at l= 510 or 605 nm and delivers DSSCPCEs of 0.15 and 0.14 %. In 2014, Li et al. screen-printed a layer

of MOF-5 on a ZnO film and then hierarchical ZnO parallelepi-peds were obtained after calcination.[117] The parallelepipedlayer serves as a light-scattering layer, which leads to a signifi-

cant improvement of the ZnO-based DSSC; the PCE of the cellincreased by 16.5 % from 3.15 to 3.67 %. Dou et al. first report-

ed MOF-derived TiO2 as a photoanode for DSSCs.[118] MIL-125(Ti) was used as a template for the preparation of hierarchi-

Figure 8. Synthetic routes to different ZnO nanostructures through one-stepthermolysis of porous homochiral MOFs under N2 or air. Reproduced fromref. [116] with permission from the American Chemical Society.


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cal porous anatase TiO2 with a high surface area of 147 m2 [email protected] resulting TiO2 photoanode delivers an overall PCE of

7.20 %, which is even better than that of conventional P25nanoparticle-based DSSCs (6.37 %).

6.2. MOF-derived nanostructures as counter electrodes

The functions of a counter electrode in a DSSC include catalyz-ing redox couples in the electrolyte at the electrode/electrolyte

interface, accepting electrons from the external circuit, andconducting electrons to the electrolyte. Therefore, a functional

counter electrode must possess excellent electrocatalyst activi-ty toward electrolyte redox couples (typically, I3

@/I@) and high

electrical conductivity. In addition, good chemical stability, eco-friendly nature, and low cost are also crucial for an ideal coun-ter electrode. Platinum coated on transparent conductive sub-

strate is the traditional counter electrode for DSSCs.[119] Cur-rently, the exploration of affordable, durable, and efficient

counter electrode material is the main task for DSSC develop-ment.

Due to the high content of carbon from organic linkers, the

design of carbon-based nanostructured counter electrodesfrom MOF templates is of great potential. Through removing

metallic ions in MOFs by various methods, a robust and highlyporous carbon backbone can be achieved. In 2016, Sun et al.

first reported preparing porous carbon spheres as DSSC coun-ter electrodes by thermal carbonization of ZIF-8 in argon at

elevated temperatures.[120] This robust carbon material displays

a surface area of 982.1 m2 g@1. The annealing temperature isvery important for controlling the morphology and porosity.

The optimal sample gave a good electrochemical performancewith a PCE of 7.32 %, which was comparable to the per-

formance of the platinum electrode. In comparison, nitrogen-doped carbon spheres synthesized through a microwave-as-

sisted approach only deliver a PCE of 6.28 %.[121]

Owing to inherent unique nanostructure of MOFs, the prep-aration of an abundant-metal sulfide counter electrode has

also become a hot topic in DSSC research. For example, Hsuet al. reported that a DSSC based on the CoS counter electrode

delivered a PCE of 8.1 % by sulfurizing ZIF-67-derived Co3O4.[122]

More recently, Cui et al. reported the synthesis of CoS2-embed-ded carbon nanocages through directly sulfurizing ZIF-67(Figure 9).[123] To synthesize CoS2-embedded carbon nanocages,

the as-prepared ZIF-67 crystals were transferred into a beakercontaining 0.1 m thioacetamide (TAA), anhydrous ethanol(150 mL), and distilled water (150 mL). Then the mixture was

heated at 90 8C for different times. Afterwards, the sampleswere annealed in N2 at 450 8C for 1 h. The sulfurizing time sig-

nificantly affects the structure and composition of CoS2. Aftersulfurization, uniform hollow spheres with ultrathin walls were

formed. By subsequent thermal annealing in N2, these hollow

spheres are transformed into porous nanocages. As shown inFigure 9 a, it is noticed that the wall thickness of nanocages is

very thin; however, they still maintain great thermal stabilitywithout showing apparent morphology changes during an-

nealing at high temperature. The DSSC PCE is maximized to8.20 %, which is even higher than that of platinum-based

DSSCs (7.88 %). The enhanced DSSC PCE is mainly attributed tothe combined merits of embedded CoS2 particles and conduc-tive carbon matrix, resulting in high electrocatalytic activity,electrical conductivity, and electrochemical stability.

In addition to MOF-derived porous carbons and sulfides,MOF is also an excellent template in the preparation of metalor alloy counter electrodes. As shown in Figure 10, Xie et al. re-ported CoNi alloy@CNT-embedded carbon nanocages (CoN-i@CNT-NCs) by using Co/Ni-based MOFs as precursors.[124] To

prepare CoNi@CNT-NCs, cobalt nitrate hexahydrate (2 g),2-methylimidazole (3.06 g), and different amounts of nickel ni-

trate hexahydrate (100, 200, 400 mg, respectively) were well

mixed in methanol at room temperature and aged for 24 h. Af-terwards, Co/Ni-based MOFs were transformed into CoNi@CNT-

NC by direct pyrolysis at 900 8C for 2 h in an argon atmo-sphere. The optimized sample delivers a high PCE of 9.04 %

and an excellent electrochemical stability in up to 300 cyclicvoltammetry (CV) cycles, whereas a CoNi0.25 alloy counter elec-

Figure 9. a) SEM image of CoS2-embedded nanocages. b) Photocurrent den-sity–voltage (I–V) characteristics of DSSCs with various CoS2-based counterelectrodes synthesized under different sulfurization times [& ZIF-67; * CoS2

(1 h); ~ CoS2 (2 h); ! CoS2 (4 h); ^ CoS2 (8 h); 3 Pt] , measured at100 mW cm@2. Reproduced from ref. [123] with permission from the RoyalSociety of Chemistry.


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trode prepared through a hydrothermal method only displaysa PCE of 8.39 %.[125] All of these MOF-derived materials demon-

strate great potential in preparing low-cost, environmentallyfriendly, stable, and high-performance counter electrode mate-

rials for next-generation DSSCs.

7. MOFs for Environmental Cleaning Applica-tions

Heavy metals are metallic chemical elements that have a densi-

ty greater than 5 g cm@3.[126] Typically, heavy metals include ar-senic, lead, mercury, aluminum, iron, cadmium, cobalt, copper,

manganese, chromium, nickel, and zinc. Some heavy-metalions, such as iron and zinc, are essential nutrients for human

health at very low concentrations. They function either as cata-lysts in metalloproteins or as cofactors/activators of reactions

involving enzymes.[127] These metal nutrients play pivotal rolesin human physiology, such as redox reactions, electron transfer,

and nucleic acid metabolisms. However, even at relatively lowconcentrations, heavy-metal ions tend to form complexes withbiological molecules containing nitrogen, sulfur, and oxygen,which can induce structural changes and bond breakage ofbiological molecules, and therefore, may affect the function of

human tissues and organs, such as central nervous systems,kidneys, livers, skin, bones, or teeth.[128] In addition, the forma-

tion of these complexes can cause various diseases and disor-

ders, such as stomach cramps, skin irritations, vomiting, oreven death.[122, 124]

Heavy-metal-ion pollution has become a serious environ-mental issue owing to the rapid development of modern in-

dustry. There are many types of heavy-metal-ion pollution inindustrial wastewaters from battery manufacturing, smelting,

tanneries, metal plating, and so forth.[129] Heavy metals are notbiodegradable and tend to accumulate in foods, such as

aquatic products, vegetables, and land animals. More impor-tantly, heavy metals can be ingested into human body through

food intake, and thus, cause various serious diseases. For ex-

ample, excessive intake of copper can cause vomiting, convul-sions, and even death. Thus, it is extremely important to devel-

op efficient methods for the detection of heavy-metal pollu-tion in environments such as wastewater. In the past five

years, more and more researchers have been studying the po-tential applications of MOFs as chemical sensors and adsorb-

ents for the detection and removal of heavy-metal ions in

wastewater. Furthermore, the growing body of evidence sug-gests that modification of MOFs with specific functional

groups can effectively enhance the performance of heavy-metal-ion detection and removal in aqueous systems.

7.1. MOFs for heavy-metal-ion detection applications

Recently, luminescent MOFs have been explored to detectheavy-metal ions in aqueous systems.[130] For instance, He

et al.[131] developed a colorimetric detection of palladium(II)ions at various concentrations by utilizing a distinct color

change property of a MOF with pendant allyl thioether units(Figure 11 a and b). The fast color change is probably because

of the synergic effects of multiple functions incorporated into

the porous framework, in which the thioether and alkene func-tional groups lead to efficient adsorption of palladium(II) ions.

Yang et al. designed a fluorescent MIL-53 (Al) for the detectionof Fe3+ with high selectivity and sensitivity in aqueous sys-

tems.[132] As displayed in Figure 11 c, the cation-exchange reac-tion between Al3 + in MIL-53(Al) and Fe3 + leads to quenching

Figure 10. TEM images (a, b) and high-resolution (HR) TEM images (c, d) of CoNi@CNTs-NC. Reproduced from ref. [124] with permission from Elsevier.


Reviews


of the fluorescence of MIL-53 (Al) owing to the conversion of

MIL-53 (Al) with strong fluorescence to MIL-53 (Fe) with weak

fluorescence. The detection limit is 0.9 mm. Shahat et al. fabri-cated a UiO-66 sensor for colorimetric detection of heavy-

metal ions, including Bi3 + , Pb2+ , Zn2 + , Hg2 + , and Cd2+ , inaqueous systems with the naked eye.[133] This sensor demon-

strates a large range of detectable metal concentrations up to10@10 m.

Different from the abovementioned colorimetric detection

mechanism of heavy-metal ions, Wang et al. developed anelectrochemical sensor in a dynamic flow system to detect

Pd2 + by using a nanocomposite of MWCNTs and Cu-basedMOF (Cu3(BTC)2) nanocrystals.[134] This electrochemical sensor is

capable of detecting nanomolar levels of Pd2 + in aqueous sys-tems. The same authors reported an electrochemical sensorfabricated based on a MOF-5-modified carbon paste electrode

for the sensitive detection of Pd2 + . The detection limit of thiselectrochemical sensor was as low as 4.9 V 10@9 mol L@1.[135]

7.2. MOFs for heavy-metal-ion removal applications

Among various technologies developed for heavy-metal-ion re-

moval from wastewater, the adsorption technique holds greatpromise because of its low cost and simplicity. Nevertheless,currently used adsorbents, such as activated carbon and CNTs,still face many challenges, such as inferior adsorption capacityand low removal efficiency.[136] Therefore, it is urgent to devel-

op new types of adsorbents for high-efficiency removal ofheavy metals from industrial wastewater. In this regard, MOFs

have emerged as very promising adsorbents for water-cleaning

applications, mainly due to their high surface areas, tunablephysiochemical properties, and versatile modifiability. To date,

several pristine MOFs without any surface functionalizationhave been discovered for high efficiency for the selective re-

moval of certain heavy metals from aqueous solutions. For ex-ample, pristine MOF-5, a member of the zinc-based MOF

family, was reported as efficient absorbents for theremoval of copper ions from aqueous systems. A

high adsorption capacity of MOF-5 for Cu2 +

(290 mg g@1) was achieved, and the percentage re-

moval of Cu2 + increases as the temperature of thesolution increases from room temperature to 55 8C.

More recently, Zhang et al. reported that pristinezeolitic imidazolate framework (ZIF-8) could be di-rectly used as a promising absorbent for the removal

of Cu2 + with high adsorption capacity and removalefficiency for Cu2 + in aqueous systems.[137] The ad-

sorption capacity value reaches as high as800 mg g@1, which is much higher than that of most

adsorbents for the removal of Cu2+ in aqueous solu-tion.[138, 139] In addition, ZIF-8 shows highly efficient

removal of Cu2+ in 30 min. The authors proposed

that the adsorption mechanism of Cu2 + by ZIF-8mainly involved a cation-exchange reaction when

the concentration of Cu2+ was less than 200 mg L@1,and the coordination reaction between Cu2 + and

the nitrogen atom on the organic ligand (2-methyl-imidazole), when the concentration of Cu2 + was higher than

200 mg L@1. Zou et al. reported that MOF HKUST-1 showed no

adsorption of Hg2 + , but highly selective adsorption of Pb2 +

and Cd2 + with equilibrium adsorption capacities of 98.18 and

32.45 mg g@1, respectively.[140]

To further improve the adsorption capacity and selectivity of

heavy-metal ions, various functionalization methods have beendeveloped to modify pristine MOFs with desirable functional

groups for the selective removal of heavy-metal ions from

aqueous systems. For example, Yee et al. reported that thiol-laced Zr-MOF demonstrated effective mercury removal, both

from aqueous systems and from the gaseous phase.[141] Thethiol-laced Zr-MOF lowered the Hg2 + content to 84 ppm in

a solution in ethanol. As shown in Figure 12 a, the thioether

Figure 11. a) Schematic illustration of ASMOF-5 crystal structure. b) Photographs ofASMOF-5 crystals under natural light before and after immersion in specific Pd-contain-ing solutions with various concentrations.[131] c) Detection mechanism of Fe3 + by MIL-53in aqueous systems.[132] Reproduced with permission from the American Chemical Soci-ety.

Figure 12. a) Mercury sorption mechanism of thiol-laced Zr-MOFs.[141] b) Ad-sorption mechanism of heavy-metal ions (Cd2 + , Co2 + , Cr2+ , Cu2 + and Pb2+)by TMU-5 (Zn-based MOF).[17] Reproduced with permission from the Ameri-can Chemical Society.


Reviews


groups play a crucial role in Hg2 + uptake from aqueous solu-tions. The incorporation of the very reactive thiol group can

enhance the uptake of various metal ions into MOF pores.However, it is difficult to modify Zn-MOF and Cu-MOF with

thiol groups, mainly due to thiol–metal interactions. As shownin Figure 12 b, Tahmasebi et al. explored TMU-5 (Zn-based

MOF) decorated with azine and imine groups with accessibleLewis basic sites for the adsorption and extraction of certain

heavy-metal ions (Cd2 + , Co2 + , Cr2 + , Cu2 + , and Pb2 +)[17] The de-

tection limits are in the range of 0.01–1.0 mg L@1.

8. Summary and Outlook

Nowadays, due to ever-increasing concerns related to nonre-newable energy depletion and environmental pollution, thereis a pressing need for the development of high-performance

energy storage/conversion technologies and efficient wastewa-ter treatment methods. The search for novel materials with de-

sirable properties has become the focus of recent research ac-tivities. Extensive research has demonstrated that MOFs can

function as ideal templates for the facile preparation of various

nanostructures, such as nitrogen-doped porous carbons, TMOs,TMO@carbon composites, and some other MOF–carbonaceous

material hybrids. These novel MOF-derived nanostructured ma-terials exhibit very impressive electrochemical performances in

LIBs, SIBs, SCs, and DSSCs. Different from the above applica-tions, some MOFs modified by various functional groups have

been explored for environmental cleaning application, espe-

cially for heavy-metal-ion detection and removal in aqueoussystems.

As discussed herein, four main MOF-derived nanostructureswith intriguing properties can be simply synthesized under

specific gas atmospheres and temperatures. First, for MOF-de-rived carbons, nitrogen can be easily doped into the carbons

through the one-step pyrolysis of MOFs at high temperature,

which would be beneficial for improving electrical conductivityand electrochemical activity of MOF-derived carbons. However,

the temperature and organic linkers significantly affect theamount of doped nitrogen in the final carbon products. Gener-

ally, with increasing temperatures, the amount of doped nitro-gen will decrease, due to the instability of nitrogen at high

temperature. In addition, to achieve a high nitrogen dopinglevel, organic linkers that are rich in nitrogen are preferred.

Second, MOF-derived metal oxides can be synthesized through

one- or two-step calcination in air. The obtained metal oxidescan retain similar morphologies to those of the MOFs through

proper control of the calcination temperature and heating/cooling rate. Hollow structures of MOF-derived metal oxides as

electrode materials offer many benefits, such as freedom ofvolume changes, large surface areas, and fast charge and mass

transport. To improve their electrical conductivity and structur-

al stability, a carbon matrix can be incorporated with metaloxides to form metal oxide/carbon composites through car-

bonization of MOFs in an inert gas, followed by calcination inair. Ultrafine metal oxide nanoparticles are well dispersed

within the highly porous carbon matrix. More recently, CNTs,graphene, and carbon cloth are used as flexible, conductive,

and robust substrates for the growth of various MOFs (ZIF-8,ZIF-67, MOF-5, etc.)

For a particular application in environmental cleaning, someluminescent MOFs have been used to detect heavy-metal ions

in aqueous systems based on metal–ligand coordination inter-actions. Fluorescent MIL-53 (Al) and MOFs with thioether and

alkene functional groups have been used to detect Fe3 + andPd2 + , respectively. Interestingly, pristine MOFs can be directlyused as high-performance absorbents for the efficient adsorp-

tion of heavy-metal ions. Among them, pristine MOF-5 andZIF-8, as two emerging adsorbents, have demonstrated veryhigh adsorption capacities of Cu2 + in aqueous medium; theseare superior to other traditional adsorbents. In addition, vari-

ous functional groups, such as thiol, azine, and imine groupshave been incorporated into pristine MOFs for improved ad-

sorption capacity and better selectivity of heavy-metal ions

from wastewater.Despite much progress on the development of various MOF-

derived nanostructures, more precise control of the chemicalcompositions and fine nanostructures still remains challenging

due to a limited understanding of pyrolysis or carbonizationprocesses. Systematic experimental studies and predicative

theoretical modeling are essential for better elucidation and

understanding of the relationship between MOF-derived nano-structures and their performance in various energy applica-

tions. Instead of using singular metallic and organic linkers forthe growth of MOFs, a multimetallic source could be used to

produce heterometallic MOFs, which would be very promisingtemplates for the synthesis of a variety of mixed-metal oxides.

In addition, close collaborations between scientists and indus-

trial partners are vital for accelerating the commercializationand industrialization of MOF-derived materials in the future.

Acknowledgements

We thank the Research Awards Program (RAP) (grant No. :NASA(2014)-RAP-10) and Research Enhancement Award (REA)

sponsored by LaSPACE, and the Chevron Innovative Research

Fund (CIRS) sponsored by Chevron Incorporation.

Keywords: dye-sensitized solar cells · electrochemistry ·heavy-metal ions · metal–organic frameworks · supercapacitors

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Manuscript received: December 16, 2016Revised: January 31, 2017

Accepted Article published: February 2, 2017Final Article published: March 9, 2017


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