carbon aerogels and monoliths: control of porosity and nanoarchitecture via sol–gel routes

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Review

Carbon Aerogels and Monoliths: Control ofPorosity and Nanoarchitecture via Sol-Gel routes

Markus Antonietti, Nina Fechler, and Tim-Patrick FellingerChem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm402239e • Publication Date (Web): 06 Sep 2013

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Carbon Aerogels and Monoliths: Control of Porosity and

Nanoarchitecture via Sol-Gel routes

Journal: Chemistry of Materials

Manuscript ID: cm-2013-02239e.R1

Manuscript Type: Review

Date Submitted by the Author: 30-Jul-2013

Complete List of Authors: Antonietti, Markus; Max Planck Institute of Colloids and Interfaces, Colloid Chemistry

Fechler, Nina; Max-Planck-Institute of Colloids and Interfaces, Colloid

Chemistry Fellinger, Tim-Patrick; Max-Planck Institute of Colloid and Interfaces, Colloids

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Page 1 of 35 Chemistry of Materials

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3 Carbon Aerogels and Monoliths: Control of Porosity and 4

5 Nanoarchitecture via Sol-Gel routes

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10 Markus Antonietti*, Nina Fechler, Tim-Patrick Fellinger, 11

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13 Max-Planck-Institute of Colloids and Interfaces, Department of Colloid Chemistry, Am Mühlenberg 1, 14424

14 Potsdam-Golm, Germany

15

16 Abstract

17

18

19 The synthesis of carbon aerogels by sol-gel like processes, i.e. hard templating, phase

20 demixing, hydrothermal carbonization techniques, as well as by ionothermal syntheses are 21

22 reviewed. In all these techniques, we start with a liquid reaction solution, while – controlled 23

24 by experimental parameters and structure-directing additives – a porous carbon material with

25 high conductivity, high pore volume and high specific surface area is obtained. Many of these 26

27 synthesis approaches give the resulting material in simple, rather sustainable processes, and 28

29 the structures can be employed directly after isolation without further activation processes.

30 The article will discuss also some applications, such as battery and electrode materials as well 31

32 as catalyst supports. 33

34

35 Keywords: carbon aerogels, Sol-Gel chemistry of carbon, hydrothermal carbonization,

37 salt templating 38

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40 TOC 41

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52 Modern Sol-Gel recipes allow for the generation of carbon monoliths and aerogels with 53

54 defined structure. This review summarizes recent advances in this field, generating valuable 55

carbon materials with refined nanostructure and extended porosity by rational synthesis

57 instead of activation.

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Chemistry of Materials Page 2 of 35

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4 Introduction

6 Porous materials have become indispensable in everyday life, however, when compared to 7

8 other classes of porous materials such as zeolites, porous silica, MOFs and many others,

9

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11 carbons are rather competitive. This is because carbon materials are light in weight while at

12

13 the same time they possess an extraordinary chemical, mechanical and thermal stability and 14

15 tunable electrical properties.1

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17 In order to introduce an adequate porosity, especially for systems with ultrahigh specific 18

19 surface areas (apparent surface area beyond 2000 m

2g

-1), activation methods are still the most

20

21

22 widely applied techniques.2, 3

Yet, the additional post-activation step is not only time and 23

24 energy consuming but the pores are etched into the carbon material, and pore formation has to 25

26 be accompanied by significant mass loss, resulting in overall low yield. Especially in the case 27

28 of functionalized carbon materials, i.e. the broad range of heteroatom-containing carbons,

29

30

31 activation techniques changes gravely the chemical composition and counteract the original

32

33 intended functionalization.4

This is why bottom-up, rational chemical design or synthesis 34

35 techniques for such porous carbon which make activation obsolete have been developed. 36

37 Nowadays, a very lively approach to address this problem is the “brick and mortar-approach” 38

39 where carbon aerogels are assembled of preformed carbon nanostructures, such as graphene,

40

41

42 nanotubes or fibers, which are then connected to the final structures.5-7 This, however, is such 43

44 a broad activity that it is simply for volume reasons out of the scope of the present review. 45

46 Instead, we want to focus on sol-gel techniques where we start from a liquid solution (“sol”), 47

48 intermediately form a sol, and end up with nanostructured material (“gel”), all that to finally

49

50

51 generate porous (monolithic) aerogels.

52

53 Both, aerogels and xerogels, are materials of monolithic character, i.e. they are constituted of 54

55 a single piece. In general, monoliths are not necessarily porous, however, in the case of aero- 56

57 and xerogels the materials comprise a continuous phase formed through e.g. a sol-gel process.

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3 Here, depending on the degree of condensation and crystallinity, the network-like structures 4

5 contain free space and thus varying porosities. Whether a material becomes one or the other is 6

7 determined by the drying history. Aerogels are commonly prepared via a colloidal gel and 8

9 subsequent extraction of pore solvent or template, in many cases e.g. by a supercritical fluid

11

12 (e.g. CO2) where structure collapse can be avoided due to low surface tension. In contrast, 13

14 xerogels are obtained after drying under ambient conditions which leads to dramatic structural 15

16 changes. Therefore, low-density aerogels feature porosities up to 90-98 vol% while xerogels 17

18 show only 50 vol%. Aerogels present desirable physical properties such as excellent mass

19

20

21 transfer properties, low density, thermal conductivity, speed of sound and dielectric 22

23 permittivity.8-10

Carbonaceous aerogels, either as powders or monoliths, are lightweight, 24

25 nanostructured materials with wide potential applications in sorption, catalysis, in membranes 26

27 and coatings, as acoustic and thermal insulators, and electrode materials.11-18

Since the first 28

29 reports on silica aerogels in 1931,

2 aerogels have been prepared from many materials, such as

31

32 metal oxides, alumina or metal chalcogenides.3, 9, 17-25 In 1989, the first organic aerogels were 33

34 reported by Pekala et al. which were formed by the condensation of resorcinol-formaldehyde 35

36 (RF) in the presence of acid or base catalysts.12

The resulting aerogels still contain abundant 37

38 oxygen functionalities and are therefore classified as organic aerogels, yet they may be

39

40 converted into carbon aerogels via pyrolysis.

2, 6 Furthermore, the addition of e.g. melamine

42

43 into the RF-system was presented as a suitable method to synthesize nitrogen-doped 44

45 aerogels,26, 27

Due to the similar physical principles between silica and carbon synthesis and 46

47 the equivalence of the resulting structures, we can call such approaches “sol-gel-like” 48

49 syntheses of carbons.

51

52 Carbon aerogel preparation from inexpensive biomass-derived precursors has recently 53

54 complemented the RF-approach due to economic, process and chemistry advantages 28

55

56 Recently our group has found that the hydrothermal carbonization (HTC) of saccharides 57

58 allows carbonaceous material synthesis in a sol–gel type process,

29 which means that methods

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3 for structuring of inorganic solids (e.g. silica) can potentially be transferred to sustainable 4

5 carbon material preparation. For example, early reports have described the preparation of 6

7 functional micrometer-sized carbonaceous spheres from glucose in a Stöber-like process.30

8

9 This HTC is a process which can simplistically be described as the technical acceleration of

11

12 natural biomass coalification down to the timescale of hours and days rather than millions of 13

14 years. Importantly, cheap and readily available precursors, e.g. simple carbohydrates or even 15

16 biowaste, can be turned into valuable carbonaceous materials using HTC.1,31

Using the more 17

18 general principles of sol-gel chemistry, HTC can therefore be also applied to turn

19

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21 carbohydrates into carbon aerogels. 22

23 Another important aspect to be covered in the field of porous materials is the generation of 24

25 hierarchical pore systems.23, 32, 33

Such hierarchical structures are characterized by the 26

27 presence of macropores (>50 nm) along with micro- and/or mesopores. The presence of 28

29 macropores is desirable as these bigger pores can act as a transport system for liquids and

31

32 gases, thus increasing the accessibility of the smaller pores. Hierarchical pores have been 33

34 successfully established in silica gels using spinodal phase separation between poly(ethylene 35

36 glycol) and silica oligomers.21

This process leads to the development of well-defined 37

38 mesoporosity and a bicontinuous macropore network, which has already been exploited for

39

40

41 the fabrication of chromatographic devices exhibiting superior performance, as evidenced by 42

43 simultaneous high plate numbers and short separation times.22

It is clear that carbon aerogels 44

45 should favorably possess exactly such a hierarchical porosity to optimize transport behavior 46

47 and overall specific surface effects at the same time.34

48

49 This review will focus on summarizing the diverse approaches towards making such carbon

51

52 aerogels, however with some focus on the modern sol-gel chemistry-like alternative 53

54 approaches and sustainable routes. We will also discuss some potential applications of the 55

56 resulting systems, as application requirements can be considered already in synthesis design. 57

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3 The early experiments: Carbon monoliths synthesis using silica hard templating 4

5 The most traditional bottom-up synthesis of porous carbons is presumably via the use of

6

7

8 templates as “spaceholders” for pores. In the case of “hard templating”, a porous inorganic 9

10 template (usually porous silica) is soaked with a carbon precursor such as furfuryl alcohol. 11

12 After carbonization, the template is removed by dissolution, and porous carbon with a 13

14 controllable pore size is obtained. This was pioneered by Ryoo et al. who could fill periodic

15

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17 mesoporous silica with carbon precursors and turned them into inverted carbon structures

18

19 with regular mesoporosity.35, 36

This approach is demanding, however, porous carbons with 20

21 very developed surface area being useful for many applications can be achieved. Meanwhile, 22

23 many structurally diverse silicas have been replicated, and we can only point to 24

25 recommendable reviews e.g. by Lu and Schüth.

37

26

27

28 On the high end side of those techniques, especially for the preparation of carbons with super 29

30 high surface areas, one finds templating of zeolites, with apparent Brunauer-Emmett-Teller

32

33 (BET) surface areas of up to2000 - 3800 m2g-1.38

34

35 At present, most of the carbon materials reported made by hard templating were obtained as 36

37 powders with still rather high structural density. The synthesis of very large porous carbon 38

39 monoliths by silica nanocasting is more challenging. Such porous carbon monoliths can be

40

41

42 directly used as catalyst supports and electrodes in electrochemical devices. The synthesis of 43

44 carbon monoliths made of mesophase pitch as a conductive carbon source, using silica 45

46 monoliths as a template for nanocasting, was described in Ref 34 ( Fig. 1).44

The parental 47

48 monoliths are easily accessible by the Nakanishi process 21

and are widely used for high- 49

50 performance liquid chromatography (HPLC). The successfully obtained hierarchically porous

52

53 carbon monoliths have different shapes such as long rods with diameters of ~4 mm and 54

55 lengths of ~70 mm (Fig. 1a) or larger tablets, here with diameters of ~27 mm and thicknesses 56

57 of ~5 mm (Fig. 1b). The shapes and the dimensions of the carbon monolith resemble those of

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3 the silica templates. These micro- and nanostructure of the synthesized carbon monolith is 4

5 depicted in a magnified series of both, scanning electron microscopy (SEM) and transmission 6

7 electron microscopy (TEM) images to illustrate structural control over the micrometer and the 8

9 nanometer range (Figure 1c–f). An SEM image of the silica template is also shown for

11

12 comparison (Figure 1c). One can clearly observe the framework structure of carbon, obtained 13

14 by replication of the corresponding silica system, and the micrometer-sized transport pores 15

16 which later assure optimal accessibility of the functional structure. 17

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3 Figure 1. Photographs representing silica monoliths used as a mold (white) and nanocasting carbon (carbonized at 700 °C) replica (black). c) SEM image of silica template at a lower magnification, d SEM images of nanocasting carbon (carbonized at 700 °C) replica at a lower

4 magnification, e at a higher magnification, and f) TEM image of nanocasting carbon replica. Pictures taken with permission from ref.34

5

6

7 The connecting carbon bridges are, however, nanoporous themselves, as evidenced by TEM 8

imaging (Fig. 1d). Nitrogen-sorption data of the synthesized carbon monolith revealed an 9

10 average mesopore diameter of 7.3 nm (as evaluated by non-local density functional theory 11

12 (NLDFT) model) and a specific BET surface area of about 330 m2 g-1. For the given sample, 13

the volume ratio of meso- to micropores is about 10:1, exhibiting very low micropore content. 14

15 It is rather unusual for thermally treated non-graphitic carbons to be practically free of 16

17 micropores, and this turned out to be very beneficial for the lithium-insertion/intercalation 18

behavior, in lithium ion batteries as will be discussed in the application part of this review. 19

20 Another approach towards such hierarchical, but ordered mesoporous carbons was described 21

22 by Stein and coworkers in their so-called “3dom” (for 3-dim ordered mesoporous)-technology 23 39

. Here, first an opal-type structure of ordered silica spheres (with rather flexible sizes) is 24

25 prepared, which is then filled with the carbon precursor mixture. After carbonization and 26

27 removal of the opal template, the spherical pores constitute the organized transport system 28

while the carbon formed in the interstitial framework can be made micro- and mesoporous as 29

30 such (Figure 2) 31

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46 Figure 2: SEM of 3dom hard carbons; a) low magnification illustrates the long range order of the the opals; b) high magnification the local

47 perfection, including the cubic interlinks between macropores. Taken with permission from ref.39

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50 Despite the beauty of the 3dom-approach, a restriction for the generation of monoliths is that 51 the opal templates are best produced as rather flat specimens only. This of course is a 52

53 demanded morphology in many electrochemical applications. 54

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3 Carbon monoliths using soft templates and spinodal decomposition 4

Soft templates are -by the involved physicochemical principles- more complicated to apply,

6 but extend hard templates in aspects regularity, size and morphology control, and of course by 7

8 the simplicity of the process. 9

10 Recently, Zhao et al. have reported the formation of ordered mesoporous carbon structures by

11 the condensation of phenolic precursors around micelles followed by subsequent 12

13 transformation into the porous material by simple heat treatment.40

These carbons are 14

15 obtained as zeolite-like crystalline powders, with still rather high structural density, and

16 carbon aerogels are difficult to make. Another challenge for such templating approaches is the 17

18 choice of the precursor; the commonly used ones (sucrose, furfuryl alcohol, or phenolic 19

20 resins) have to be sufficiently polar to enable good compatibility with the template, but are

21 not optimal with regards to transformation into highly conductive carbon because of 22

23 insufficiently extended aromatic rings. However, a major challenge in the fabrication of 24

25 tailored mesoporous carbon is to achieve good conductivity (i.e., extended graphene units)

26 and mesoporosity at the same time. In general, high conductivity in carbon is obtained by 27

28 high-temperature heat treatment; however, such treatment destroys the mesoporous structure 29

due to pore collapse arising from changes in the structure towards graphene units.41

31 Adelhelm et al. presented a soft-templating-based methodology to synthesize carbons with 32

33 meso- and macroporosity in a one-step process, taking advantage of the phase separation 34

35 (spinodal decomposition) of mesophase pitch (MP) as the carbon precursor and an organic

36 polymer as a template.42

In general, MP is a known, highly suitable carbon precursor for 37

38 electrochemical applications because it consists of extended, condensed polyaromatic 39

40 moieties, thus exhibiting significantly improved carbonization behavior as compared to sugar-

41 based precursors or phenolic resins. The approach starts from generating a homogeneous 42

43 solution of MP and an appropriate non-carbonisable polymer (e.g. polystyrene (PS) or 44

45 poly(methyl methacrylate (PMMA) The polymers have been selected for compatibility with

46 MP in a volatile solvent, such as chloroform. After the formation of a homogeneous solution, 47

48 a catalyst like FeCl3 is added to spur the carbonization process. The main idea behind this 49

50 approach is to induce continuously increasing incompatibility between MP and the polymer

51 during the evaporation of the solvent and the subsequent carbonization step. Here, the induced 52

53 incompatibility has to be moderated to allow for controlled spinodal phase separation of the 54

55 polymer-rich and MP-rich domains on both the micrometer and nanometer scale. In the

56 spinodal case, the microphase separation of MP and the polymer results in the formation of a 57

58 bicontinuous, sponge-like structure, as pioneered by Nakanishi et al. for silica.21

Also, similar

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3 to the Nakanishi process, meso- and macroscale phase separations have been further 4

controlled by carefully chosen heat-treatment protocols at about 250–300 °C, i.e., below the

6 decomposition temperature of the polymer. Annealing stabilizes the mesostructure by 7

8 establishing a network of connected MP species, thus improving the mechanical stability of 9

the material and enabling the structures to avoid pore collapse after removal of the polymer

11 template during carbonization at 600 °C under a nitrogen atmosphere. In this respect, 12

13 polymers such as PMMA and PS are particularly beneficial, as they predominantly 14

decompose/depolymerize into volatile fragments under the applied conditions, thereby

16 generating the pore structure. The polymer-based process also allows for the generation of 17

18 films and monolithic carbon structures with dimensions of up to several centimeters using 19

appropriate scaffolds (Fig. 3a).

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51 Figure 3. a) Photograph of a polystyrene-templated monolith. Scanning electron microscopy images

52 showing the macroporous structure of a sample carbonized at b) 340 °C (33 wt% PS) and c) 600 °C

53 (66 wt% PS). d) Hg porosimetry measurements of a sample carbonized at 340 °C (33 wt% PS).

54 Pictures taken with permission from ref.42. 55

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3 Carbon synthesis by HTC using Albumin as a moderator 4

It was already stated above that HTC can be understood as a sol-gel chemistry of

6 carbon nanostructures, with carbohydrates acting as the soluble starting monomer in a water 7

8 based synthesis. In fact, hydrothermal carbonization is reminiscent of the famous bottom - up 9

approach to synthesize micron - sized carbonaceous materials: the polycondensation of RF

11 mixtures towards the respective resins with subsequent carbonization.[43]

The two main 12

13 reaction steps, namely (1) building a reactive precursor followed by (2) polymerisation, and 14

the comparable particle morphology generated in water as solvent, indicate mechanistic

16 similarities for the respective material formation (Figure 4). Importantly, as already indicated 17

18 by the dark brown color, the materials after hydrothermal carbonization already contain 19

conjugated aromatic moieties, increasing the graphitizability of HTC as compared to

21 carbohydrates. 22

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45 Figure 4 : Two - step reaction as precondition for resinification chemistry in “classical” RF system (A) and

46 schematic HTC system (B). 47

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49 Titirici et al developed a reaction system on the basis of water, glucose, and albumin mixed in

50

51 a ratio of 9 : 1 : 0.15 (w/w/w).44, 45

Albumin herein acts as both the nitrogen source and the 52

53 phase demixing stabilizer, essentially defining the elaborated nanostructure in a dynamically 54

controlled fashion. After supercritical (sc) CO drying low density, hierarchically structured 55

56 N-doped monolithic carbon aerogels could be obtained. It is to be emphasized that in the 57

58 closed system the absolute material porosity is predetermined by the H2O to biomolecules

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3 ratio, where about 50% of the added biomass were recovered as hydrothermal carbon. The 4

given recipe therefore generates carbon aerogels with 95 vol% porosity, but in general leads

6 to stable gels with up to 97 vol% porosity can be obtained. Structural details, including X-ray 7

8 photoelectron spectroscopy (XPS), solid state nuclear magnetic resonance (ssNMR) and 9

Fourier transformation infrared (FTIR) of the as made carbons are analyzed in detail in the

11 original paper.44

12

13 Thermally treated Carbogels (900 °C) maintain the parental morphology of a continuous 14

hierarchical nano-network (Fig. 5(C)). A combination of SEM and high resolution (HR)TEM

16 images of heat treated Carbogels nicely demonstrates the unusual coral-like continuous 17

18 carbonaceous nano-architecture (Fig. 5). The hyper-branched network has walls of disordered 19

graphitic-like sheets of ca. 10–15 nm thickness composed of ~2–3 short carbon layers with

21 only local stacking order. It must be emphasized that the resulting structures are in the size 22

23 range of disordered multi-walled carbon nanotubes, but are derived from biomolecules in a 24

25 simple process, only.

26 Given the structural similarities to corresponding monolithic silica (2) a similar underlying 27

28 formation mechanism was assumed: After sugar dehydration, carbon precursors demix from 29

30 the aqueous phase in a spinodal fashion, which in this case is stopped from further ripening

31 towards larger droplets by an early and efficient reaction with the water-based proteins and 32

33 coupled structural cross-linking. 34

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48 Fig.5 a) Low magnification SEM image of monolithic “Carbogel” materials after ScCO2 drying; after carbonization at T =

49 550 ◦C; b & c) HR-TEM images of Carbogels prepared at 900 °C in differet magnifications. Pictures taken with permission

50 from ref.44.

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52

53 The homogeneous solution character at the beginning of the HTC sol-gel synthesis allows for

54 chemical manipulation on a molecular scale. This option is attractive for the achievement of 55

56 homogeneously doped carbons in contrast to “only” surface-doped carbons, which can be 57

58 obtained by post-modification. In a successful approach to obtain nitrogen and sulphur co-

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3 doped carbons S-(2-thienyl)-L-cysteine (TC) or 2-thiophene carboxaldehyde (TCA) where 4

added to the original hydrothermal carbonization recipe. Interestingly, the use of TCA

6 additives did not alter the reactivity of the rather sensitive system in the sense that aerogel 7

8 formation could be achieved in exactly the same manner.46

After carbonization at 900 °C, the 9

obtained materials showed reduced pore volumes, but similar specific surface areas and

11 importantly additional abundance of structurally integrated sulfur. This example is illustrating 12

13 the general advantage of a bottom-up approach to generate porous carbons with distinctly 14

altered physical as well as chemical properties.

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19 Carbon synthesis by HTC using Borax mediated process

21 In another set of experiments following the teaching of RF systems, albumine could be 22

23 replaced by simple, inorganic salts which interact with the carbohydrate and the carbon 24

surface in an appropriate fashion, namely borax or borate salts.47, 48

In the RF system reactions

26 free of catalyst lead to precipitation of micrometer sized particles,49

while aerogels composed 27

28 of interconnected nanoparticles can be achieved either by acid or base catalysis. The molar 29

ratio of resorcinol to catalyst allows the control of the final particle size, basically by

31 adjusting the number of “seeds”.50 In the borax-mediated aerogel formation from glucose, the 32

33 particle size and hence surface area are controlled by the amount of borax added to the initial 34

reaction mixture. The more borax is added, the smaller the seed particles and hence the higher

36 the surface area. A proposed mechanistic explanation was given elsewhere.47 Briefly, borax 37

38 was assumed to increase the overall reactivity between HTC intermediates due to a secondary 39

catalytic effect. According to the LaMer model, this accelerated reaction rate rapidly results in

41 a critical supersaturation of small hydrothermal carbon and hence a nucleation burst.51

The 42

43 large number of seeds results in smaller particles in the growth phase. The small particles 44

together with the additional gelating effect of borax give rise to the aerogel morphology by

46 aggregation and covalent crosslinking amongst each other, finally giving mechanically very 47

48 robust monoliths. (Fig. 6.) 49

Again taking advantage of the homogeneous bottom-up process, co-addition of 2-pyrrol-

51 carboxaldehyde (PCA) was used to enable simultaneous nitrogen-doping of the carbon 52

53 aerogels. It could be shown that both the particle size (and hence surface area) and the 54

nitrogen content could be tuned independently by varying the amounts of borax or PCA used.

56 Pyrolysis at 900 °C of the organic aerogels rendered the resulting carbon aerogels electrically 57

58 highly conducting. In a typical experiment, 0.8 g PCA, 6.0 g glucose and 14.0 g water were

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3 used, i.e. the whole recipe was made to address an overall porosity of the carbon aerogels of 4

about 90 vol%. This indeed could be confirmed by the analytical data.

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21 Fig.6: Borax mediated HTC aerogel monoliths and TEM images of monoliths with a) 150mg, b) 300mg, c)

22 600mg borax in the recipe.

23

24

25

26 The TEM images (Figure 6, right ) show the typical borax-mediated aerogel morphology i.e. a 27 matrix comprising interconnected particles. The comparison of Figure 7 a to c nicely 28

29 demonstrates the decrease in particle size with increasing borax amount, going from an 30

31 average diameter of 75 nm, to 23 nm and finally to 15.7 nm. This nicely corresponds to the 32 typical size range of silica aerogels. Addition of PCA as a nitrogen donor in a wider range 33

34 does not change this architecture and particle size. These results indicated a similar reactivity 35

36 of PCA as compared to HMF and a low sensitivity of the borax system towards 37 functionalization. 38

39 The primary aerogels obtained after HTC at 180 °C contained around 65 wt % carbon and can 40

41 therefore be classified as “organic” aerogels. In order to obtain carbon aerogels with an 42

increased conductivity and material stability, post-carbonization at 900 °C was carried out. 43

44 These carbon aerogels contained around 90 wt % carbon and have retained their relative 45

46 heteroatom content after pyrolysis. The electronic conductivity of all those samples was rather 47

high, while the conductivity increases with increasing particle size. Their use in 48

49 electrocatalytic applications is discussed below in the application paragraph. 50

51

52 Carbon monolith synthesis with HTC under hypersaline conditions using phase

53

54 separation. 55

56 The up to now final simplification of carbon aerogel synthesis was accomplished by the 57 application of hypersaline conditions, i.e. aqueous salt solutions close to the saturation limit.

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1

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3 Using very hydrophilic ions at the same time lowers the partial pressure of water and changes 4

its structure so that gel formation can be performed under less extreme conditions, gaining salt

6 concentration and salt type as additional parameters of structure control. Recycling of the 7

8 reaction medium in all these cases is very simple: the salt is washed away with water, filtered, 9

and can be reused after evaporation of the water.

11 In ref. 52, water containing salt melts (i.e. ZnCl2 or eutectic mixtures of LiCl/ZnCl2

12

13 (Li-Zn-x/y), NaCl/ZnCl2 (Na-Zn-x/y) and KCl/ZnCl2 (K-Zn-x/y), where x/y denotes the mass 14

ratio of the salts) were used as a reaction medium for the hydrothermal carbonization of

16 glucose.52 In presence of ZnCl2, and only little amount of, water, porous carbonaceous 17

18 structures with a typical “aerogel” structure and surface areas of 400 - 650 m2/g were

19 obtained. It is to be underlined that the samples could be purified by simple washing with

21 water and ordinary drying, i.e. no supercritical CO2 drying was necessary to prevent collapse 22

23 of the nanostructured carbonaceous aerogels due to capillary forces. This means that the as 24

25 formed still organic structures were unusually robust as well as highly cross-linked. It was

26 shown that both the particle size and hence the specific surface area as well as the nitrogen 27

28 content could be varied by the starting products and the salt mixtures used. 29

30 In a mechanistic discussion, obviously also hypersaline conditions offer the possibility to

31 stabilize the surface of as formed primary nanoparticles to avoid excessive Ostwald ripening 32

33 or particle growth. These primary particles at sufficiently high concentration then turn 34

35 collectively unstable, undergo spinodal phase separation and cross-linking towards the final

36 porous carbon gels. At intermediate concentrations, porous “soots” are obtained which are 37

38 composed of interconnected particles, but are still dispersible as such. The more salt is added, 39

40 the smaller the primary particles are and hence the higher is the surface area. Again, the

41 absolute porosity is controlled- by the relative amount of glucose and salt that it is directly in 42

43 the hand of the scientist. 44

45 On the one hand, it was also found that neither water-free salts nor “hard salts”, e.g. NaCl,

46 show a beneficial influence on the sample morphology. On the other hand, surface area could 47

48 be successfully introduced into the carbon materials when the used salts were rather 49

50 hygroscopic. At the same time, the salt mixtures had to contain at least some water, both to

51 ensure a liquid reaction medium, but also to enhance surface stabilization which is most 52

53 probably indirectly provided via the hydration water. In contrast, highly diluted media do not 54

55 result in any porous materials, thus confirming the prerequisite of hypersaline environment as

56 advantageous reaction medium to form monolithic, aerogel-like carbons (Figure 7).

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15 Figure 7: SEM micrographs of carbonaceous materials obtained from reaction mixtures of Glucose,

16

17 water and zinc chloride using highly diluted (left) and hypersaline (right) conditions. Fig 7c: TEM 18

picture of a hypersaline sample, illustrating the primary particle structure.

20

21

22 Here, it is also to mention that the carbonaceous materials obtained from hypersaline 23

conditions reveal a darker color whereas the materials from highly diluted reaction mixtures 24

25 are light brown. This, together with an increased carbon content of around 70 wt% for 26

27 hypersaline conditions, indicates an increased degree of aromatization in the presence of salt. 28

Representative SEM and TEM micrographs of the resulting samples prepared from the salt 29

30 mixtures under hypersaline conditions are shown in Figure 7 b and c. The material is 31

32 composed of primary rough carbon frazzles in the 10 nm range, which are interconnected to 33

give the porous aerogel structure with extended pore transport systems. This fine particle 34

35 morphology already indicates a very effective surface stabilization of the material throughout 36

37 the synthesis in hypersaline conditions. Note that the hydrothermal carbonization of glucose 38

without the presence of a salt agent generally results in carbonaceous particles that are several 39

40 orders of magnitude larger than observed here (approximately 200 nm and larger, comparable 41

42 to the default sample in Figure 7 a.1

43 Compared to the work discussed above in the previous paragraphs, these samples are

44

45 constituted of smaller particles than the albumin-derived aerogels and are comparable to the 46

47 very best borax-mediated aerogel samples. Again, it is to emphasize that the carbons could be 48

recovered from simple washing with water instead of elaborated supercritical CO drying, a 49

50 behavior, even hardly found for silica aerogels which are also preferentially dried in a 51

52 supercritical fashion or after solvent exchange. Nitrogen sorption measurements were carried 53

out for detailed porosity analysis (Figure 8). 54

55

56

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range (salt melts are operative between 300 °C – 1000 °C). Meanwhile, it is safe to state that

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21 Figure 8: Nitrogen sorption isotherms of three carbon aerogels synthesized with Li-Zn 15/3 (black),

22 Na-Zn13/3 (red) and K-Zn14/3 (green) eutectic salt mixtures. For details, see ref 52 23

24

25 All three isotherms are characterized by the typical surface nitrogen sorption, with the 26

interstitial pores between the particles just starting to be visible in the higher pressure range. 27

28 The medium pressure range is similar for all three samples and due to surface adsorption onto 29

30 very small nanoparticles, supporting the observations by electron microscopy. Interestingly 31

and very unusual for low temperature hydrothermal carbons, we also find a distinct 32

33 microporosity which is most pronounced for the ZnCl2/LiCl salt. This observation points 34

35 towards an additional imprinting of simple salts in the particles, presumably zinc and lithium 36

entities. It is an exciting question if this imprinted salt would also be recognized in a later 37

38 rebinding event, but this was not analyzed. The as obtained crude hydrophilic carbonaceous 39

40 material could further be transferred into the corresponding rather hydrophobic carbons by 41

post-calcination at elevated temperatures, also leading to a significantly enhanced surface area 42

43 as a result of further water elimination.53,54

44

45

46

47

48 Carbon aerogel synthesis using salt melts: salt as a solvent and “molecular” template 49

50

51 After exploration of hypersaline conditions, it is a natural next step to explore the same

52

53

54 sol-gel reactivity schemes in molten salt systems, now without water. It is an exciting

55

56 question if sol-gel chemistry also exists in those solvents and in that intermediary temperature

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contain nitrogen (N) and boron (B) since these atoms are capable of adding favorable

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1

2

3 the same operative physico-chemical principles can be found and applied under ionothermal 4

5 conditions, too. This technique is now called “salt templating”.55

Here, a non-carbonizable 6

7 inorganic salt is mixed with a carbon precursor which is condensed and scaffolded in the 8

9 presence of the molten salt at elevated temperatures. If –by appropriate choice of the reagent-

11

12 miscibility between the salt melt and the carbonizing material is kept over a main part of the 13

14 reaction pathway, the resulting carbon shows a high specific surface area with the pore size 15

16 corresponding to ion pairs, salt clusters and their percolation structures. As for the 17

18 carbonaceous materials obtained from hypersaline conditions described in the previous

19

20

21 section, the salt phase is easily removed by simple washing with water, while the carbon is 22

23 not etched as such. Simple closed-loop processes including salt recycling are conceivable 24

25 which result in the demanded high surface area carbons in high yields with structural and 26

27 chemical functionality. 28

29 It is important to note that – in spite of similar notations- this “salt templating” is different

31

32 from the synthesis of macroporous polymers, where freshly ground salt crystals (crystallite 33

34 size 0.2 to 500 µm) in a non-solvent are used as template.56, 57

This solvent casting/ particle 35

36 leaching method is rather based on the formation of the respective material around 37

38 macroscopic irregular crystals than porosity generation due to “molecularly” dissolved ions

39

40

41 and phase separation. The synthesis of low surface area inorganic scaffolds via salt-inclusion 42

43 was also reported.58

However, this is not to be mixed up with the approach discussed here, 44

45 where a salt melt enables the generation of a factor of 1000 smaller pores and correspondingly 46

47 higher surface areas. In “molecular” sol-gel salt-templating, the appropriate choice of cation 48

49 size and counterion controls the minimal pore size as well as miscibility with the reaction

51

52 medium via adjustment of the polarizability. 53

54 A convenient choice for carbon precursors are ionic liquids (IL), being miscible with the salt 55

56 template and unreactive until elevated temperatures.59-62

In ref. 55, the ILs were chosen to

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3 properties to carbon networks when structurally incorporated.55, 63-66

By employing different 4

5 eutectic salt mixtures which possess low melting points and are homogeneously miscible with 6

7 diverse ILs, carbon aerogels with unusually high apparent surface areas could be synthesized. 8

9 Depending on the salt nature and amount, micro- to mesoporous materials with apparent

11

12 surface areas up to 2000 m2g-1 are obtained in one step, while preserving the targeted 13

14 chemical functionality brought in by the monomer composition. At the same time, the product 15

16 yield increased and did not depend on the amount of salt. This is believed to result from 17

18 increased electrostatic interactions of the precursor molecules and/or further intermediates and

19

20

21 the template salt. With regard to literature, the increased carbon yield obtained with ZnCl2

22

23 containing eutectic salt mixtures is a rather surprising effect, since ZnCl2 was mostly 24

25 described as an activation agent.67, 68

Previously observed high carbon yields in this case 26

27 could be explained by the dehydrating effect of ZnCl2 69

. 28

29 In Ref.55, it was shown that the salt templating approach is a combination of templating (of

31

32 ion pairs and small salt clusters) and phase separation, but shows no indications for an 33

34 activation process. This was also supported by the fact that a high heteroatom-doping level 35

36 could be maintained even in the high surface area materials using a high amount of salt. Bulk 37

38 elemental analysis, XPS and some other structural analytical data have been reported in the

39

40

41 original paper. 42

43 TEM and SEM pictures of the washed products reveal the morphology of the materials to be 44

45 dependent on the nature of the eutectic porogen which is exemplarily shown for carbon 46

47 aerogels based on N-butyl-3-methylimidazolium-dicyandiamide (Bmp-dca, Fig. 9). 48

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model method for slit pores, respectively.

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24 Fig. 9 TEM (upper row) and SEM pictures (lower row) of N-dCs using Bmp-dca as precursor templated with LiCl/ZnCl2

25

26 (LiZ, left), NaCl/ZnCl2 (SZ, middle) and KCl/ZnCl2 (PZ, right) at equal mass ratios synthesized at 1000 °C. In SEM, The

27

28 monolithic aerogel character is clearly revealed. Taken with permission from ref.55.

29

30

31

32 The TEM and SEM pictures already show that the surface roughness of the carbons increases 33

34 from the lithium over the sodium to the potassium eutectic, indicating the presence of large 35

36 mesopores or small macropores. Furthermore, in the potassium chloride case, fluffy spherical 37

38 carbon globular aggregates are observed to constitute the materials, where homogeneous 39

40 carbon aerogels are found to constitute all other samples. These aerogels are cohesive and

42

43 withstand even the removal of salt and the washing with water. Also the utilization of 44

45 alternative ionic liquids, revealed the same trend of morphology of the carbon materials. This 46

47 supports the strong dependence of the morphology only on the nature of the porogen salt, 48

49 whereas the nature of the IL plays a minor role, thus a main part of the pore formation indeed

50

51

52 results from a templating mechanism by the salt. 53

54 The actual apparent surface areas and pore size distributions were determined by nitrogen 55

56 sorption measurements (Fig. 10) by applying the BET model and the NLDFT equilibrium

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formation of spherical particles of the carbon phase and their interstitial pores, as observed in

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21 Fig. 10 Upper row: Nitrogen sorption isotherms of Bmp-dca (left), Emim-dca (middle) and Emim-tcb (right) derived carbons templated

22

23 with Li-Zn, Na-Zn and K-Zn at equal mass ratios. Lower row: Schematic representation of pore formation for carbons templated with Li-Zn

24 (left), Na-Zn (middle) and K-Zn (right) at similar mass ratios. Each left image depicts the carbon (grey)/salt (red) composite and each right

25

26 side the carbon aerogel structure after washing. Taken with permission from ref.55.

27

28

29 In all cases a high nitrogen uptake is observed and, compared to the carbons derived from the

30

31

32 pure ILs, the apparent surface areas significantly increase due to salt templating, ranging from 33

34 1100 m2g

-1 up to 2000 m

2g

-1. In accordance to the TEM and SEM images the materials

35

36 templated with the same salt mixture also show a comparable shape of the isotherms 37

38 independent of the nature of the IL used as precursor. In more detail, by the addition of the Li- 39

40 Zn salt melt, the isotherms are of type I which implies a solely microporous structure of the

42

43 carbons, i.e. the salt acts as a ”molecular template”, either forming ion pairs or little salt 44

45 clusters of minimal free energy (Fig. 10 lower left). For carbons templated with Na-Zn the 46

47 sorption isotherms of the materials show a further uptake of N2 in the medium relative 48

49 pressure region as well as a small hysteresis, reflecting a substantial contribution of additional

50

51

52 supermicropores and small mesopores. Finally, for materials templated with K-Zn which has 53

54 the lowest melting point of the used eutectics, an additional uptake in the high relative 55

56 pressure region is observed. This is typical for macropores and is here in accordance to the

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3 SEM. As in classical sol-gel chemistry, this is obviously due to the onset of demixing in even 4

5 earlier phases of the structure formation which results in a continuous, demixed salt phase 6

7 (Fig. 10 lower right). It must be emphasized that an apparent specific surface area of 8

9 2000 m

2g

-1 is much higher than of any zeolite, in the range of activated carbons and even

11

12 approaching the theoretical value of single-layer graphene.[70] In spite of the simplicity of the 13

14 applied one step salt sol-gel synthesis and the aerogel character, this is a notable result. 15

16 Besides the salt nature, also the salt amount has a significant influence on the carbon 17

18 properties. With increasing salt amounts, for Li-Zn and Na-Zn the pore size and volume of the

19

20 final products increase up to 12 nm in diameter and 2.75 ml g

-1, respectively. In case of K-Zn,

22

23 the porosity parameters are relatively constant while the secondary particle size and 24

25 connectivity of the aerogel can be influenced through the amount of salt. Also this behavior is 26

27 comparable to classical sol-gel chemistry of silica or RF-resins and supports the fact that sol- 28

29 gel chemistry can also be performed under ionothermal conditions above 300 °C

31

32

33 Applications

34

35 Battery electrodes 36

37 Interestingly, carbon aerogels were discussed very favorably for a number of applications, e.g. 38

39 for the electrochemical storage of energy in a supercapacitor12, 14, 71

, as a membrane and 40

41 electrode in fuel cells72

, as catalysts supports in both catalysis and electrocatalysis73, 74

, as 42

43 electrodes for Li-batteries

39, as giant stroke artificial muscles

75, as fast sorption materials

76 for

44

45 oil spills, or for capacitive deionization.

77 Already the sheer listing of those properties of

47

48 course explains the excitement of material chemist for the generation and compositional and 49

50 structural control of those carbon aerogels. In the following, we will go through some 51

52 illustrative examples of those applications where optimized transport properties through the 53

54 macropore system in combination with a high specific surface area nanostructure play a

56

57 crucial role.

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3 The use of nanostructured carbon as anodes in the lithium ion battery is presumably one of the 4

5 catchiest applications. Indeed, already in 2007 J. Maier et al. were able to show that carbon 6

7 aerogels made from silica replication can store extraordinary amounts of metallic lithium in 8

9 their structure.

34 In Figure 11, it is seen that the material can store up to 800 mAh/g of energy,

11

12 while the theoretical limit for graphene is 372 mAh/g, only. This clearly points to secondary 13

14 storage mechanisms for metallic lithium besides the traditional intercalation between 15

16 graphene layers. This secondary mechanism is however still sensitive towards fading. 17

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34 Fig.11: Galvanostatic discharge (Li insertion, voltage decreases)/charge (Li extraction, voltage increases) curves of carbon aerogels

carbonized at 700 °C, cycled at a rate of C/5, and b) cycling performance of carbon sample carbonized at 700 °C cycled at a rate of C/5.Taken with permission from ref 37).

36

37

38

39

40 Supercapacitors 41 A similar carbon aerogel was later explored as an electrode in a supercapacitor

78. In those

42

43 experiments, part of the surface and the mesopore system were employed to deposit a second

45

46 phase of a redox active polymer, i.e. polyaniline (PANI). The carbon aerogel with its high 47

48 conductivity is herein used as a current collector, while the remaining hierarchical 49

50 meso/macropore system is used for the liquid contacting of the overall system. Unexpectedly, 51

52 very high and practically rate-independent capacities of 1200 – 1400 F/g were found

53

54

55 (Fig.12a), which in addition turned out to be rather stable in multiple cycling. A 56

57 corresponding PANI/nanotube sample prepared in an otherwise similar fashion gave a by a

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3 factor 5 lower performance, with much lower cycling performance. The Ragone-plot of the 4

5 same data (Fig.12b) revealed a specific energy of about 200 Wh /kg at a specific power of 6

7 10 kW/kg, i.e. a performance which is already in the highly useful region. Also those 8

9 capacities are even slightly above the theoretical value for PANI, i.e. it is obvious that

11

12 additional charges can be stored in the carbon/PANI heterojunction. 13

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27 Fig.12a) as determined supercapacitance of a PANI/carbon aerogel hybrid in direct comparison with a

28

29 corresponding carbon nanotube reinforced sample, b) Ragone plot of the specific power/specific energy

30 performance of the same samples. Taken with permission from ref.78.

31

32

33 Capacitive Desalination 34

35 Supercapacitive activity is closely related to capacitive desalination, in the latter case the cell 36

is just a flow-through device operative with slightly saline aqueous solutions. Here, as an 37

38 example, we refer to one of the very classic papers 77

as follow-up work describing technical 39

40 improvements rather than novel, better performing carbons is indeed very broad. 41

Fig.13a shows the desalination kinetics of a model NaCl solution at such a capacitive 42

43 desalination stack at a flow rate of 1 l/min and a “binding voltage” of 1.2 Volt, i.e. slightly 44

45 below water electrolysis. Water is desalinated in about 10 min, with more than 90% of the 46

ions bound to the electrodes (Fig.13b). Back-flushing the device without applied voltage 47

48 allows reconstituting for the ion of the original material. 49

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15 Figure 13 Left: Deionization of a fixed volume of 100 µS/cm NaCl solution. Complete recycle of 4 l at a rate of

16

17 1 l/min. The apparatus included 192 aged electrode pairs operated at a cell voltage of 1.2 V. Right: Data for both

18 NaCl and NaNO3 solutions showing the effect of aging on the electrosorption capacity of carbon aerogel 19

20 electrodes. Salt removal a cell voltages ranging from 0.6 to 1.2 V. Complete recycle of a 4 l volume of a solution

21 at a rate of 1 l/min. Data for new electrodes are represented by (1); data for electrodes cycled for several weeks

22 are represented by (2); and data for aged electrodes that have been cycled for several month are represented by

23

24 (3). Note that ® represents aged electrodes that have been regenerated by potential reversal. Taken with

25 permission from ref. 77.

26

27

28 Besides treatment of slightly brackish waters and industrial waste, this of course also allows 29

30 for the enrichment of charged organic products from the water stream, as it occurs in many 31

32 fermentation operations.

34

35 Current systems shows overall capacities of around 15 mg salt/g carbon, but we are allowed 36

37 to expect a multiplication of capacities with the introduction of redox active binding layers.110

38

39

40

41 Chemical catalysis and supports

42

43

44 Also as a support for chemical catalysts, such carbon aerogels can show extraordinary 45

46 performance. To our opinion, the presumably most spectacular experiments were presented by 47

48 Palkovits and Schüth.79

In an attempt to heterogenize the Periana reaction (the monooxidation 49

50 of methane with oxygen), they used an N-doped carbon aerogel made by hydrothermal 51

52 carbonization from a lobster shell as a support for ordinary platinum atoms.

54

55 The support not only survived the extreme reaction conditions (boiling sulfuric acid at a high 56

57 oxygen pressure), but the resulting catalytic system also outperformed the classical

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53


1

2

3 homogeneous system as well as the previous triazine-framework immobilized catalyst (Table 4

5 1).80

This is attributed to the better accessibility of the catalytic sites and the concurrently high 6

7 site density in the carbon aerogel sample. 8

9

10

11 Table 1: Conversion, yields and selectivities to methanol and catalytic activities (determined in 2 different

12 fashions) in a Periana reaction. Entry 1: previous heterogenized catalyst. Entry 2: homogeneous catalyst. Entry 3

13

14 – 5: three repeats with an aerogel supported sample. For details, see original publication 80

. Table and picture

15 copied with permission from ref. 80.

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46 Following a similar strategy, the team of Wang and Li et al. immobilized nanoscopic CuO 47

layers onto the surface of an N-doped carbon aerogel.81

49 Also in this system, a very good reactivity and catalyst stability was found, in this case for the 50

51 heterogeneous C-C, C-N, C-O, and C-S Ullmann coupling. Table 2 summarizes some of the 52

corresponding catalytic data.

54

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1

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3 Table 2: Cu-catalyzed Ullmann type O-, N- and S-arylatition with Arylhalides. Table taken with permission from

4 ref. 87.

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46 Electrocatalysis 47

48 A highly attractive application of carbon aerogels is in electrocatalysis, either in fuel cells, 49

50 electrolyzers, or “just” for the controlled conversion of chemicals. Conductive carbon

51

52

53 aerogels have for instance been used as supports for noble83-85

as well as non-noble 54

55 metal86,87

catalysts in the oxygen reduction reaction (ORR). In 2009 Gong et al. reported 56

57 the first efficient metal-free catalyst for the oxygen reduction reaction (ORR), based on

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1

2

3 nitrogen-doped carbon nanotube arrays.88

Since then, efforts have been increased to find 4

5 metal-free alternatives to conventional platinum catalysts, for which heteroatom doped 6

7 carbon materials have proven to be high potential candidates. This is due to their high 8

9 conductivity, their chemical stability, and their functional patterns.

89-97 Most examples

11

12 involve nitrogen doped carbon materials, but boron94 as well as sulfur 93, 95, 98 have also 13

14 been reported to enhance electrocatalytic activity of the material. Jin et al. reported on RF- 15

16 based nitrogen-doped xerogels synthesized using ammonia as nitrogen source and cobalt 17

18 nitrate as catalyst. The xerogels exhibited high ORR electrocatalytic activity and good

19

20 stability in acidic media.

99 The sol-gel routes presented herein allow for gradual change of

22

23 the chemical composition and/or morphology. This opens the door for structure-property 24

25 relation studies, which are highly interesting in the field of electrocatalysis to get deeper 26

27 understanding of how to potentially substitute noble metal catalysts by carbon. 28

29 The hierarchical pore structure of sol-gel synthesized carbon aerogels is expected to

31

32 optimize mass transport, which is often a limiting factor for electrocatalysis. Conventional 33

34 catalysts are particles dispersed on catalytically inactive supports; therefore the surface 35

36 concentration of active sites is rather low. The heteroatom containing carbon aerogels are 37

38 support and catalyst at once causing high concentration of catalytically active sites, which

39

40

41 could reduce kinetic limitations. 42

43 Exemplarily, we present linear sweep voltammetry (LSV, Fig. 1 left) using a rotating 44

45 disk electrode (RDE) in 0.1 M KOH on albumine derived carbogels co-doped with 46

47 nitrogen and sulfur.46

Featureless voltammetric curves were observed for all samples in 48

49

50 N2-saturated solution. A strong cathodic peak is seen upon saturating the solution with O2,

51

52 showing the catalytic effect of the aerogels towards oxygen reduction. The linear sweep 53

54 voltammograms corresponding to the cathodic currents due to oxygen reduction show an 55

56 interesting co-operative effect of the nitrogen and sulfur doping. Both aerogels are more 57

58 active catalysts as compared to the reference Vulcan carbon, but still not competitive with

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1

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3 commercial platinum catalyst. However, a clear positive shift in the onset potential can be 4

5 stated for the optimized samples. At higher overpotentials, the best samples also give 6

7 higher current densities as compared to platinum, supporting our view of an improved 8

9 catalytic site density within the carbon aerogels. The improved electrocatalytic activity of

11

12 nitrogen and sulfur-co-doped carbon is also reflected in the reaction mechanism. 13

14 Koutecky-Levich analysis unfolds higher electron transfer numbers at different potentials 15

16 (Fig. 14 right) as compared to the purely nitrogen doped carbon aerogel. 17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37 Figure 14 Left: RDE polarization curves at 1600 rpm of doped carbon aerogels compared to 20 wt% Pt@C and

38

39 Vulcan in 0.1 M KOH. Right: Electron transfer numbers at various voltages.

40

41

42

43 At borax mediated aerogels described above, a more detailed analysis of different factors such

44

45 as chemical structure and morphology on the electrocatalytic performance has been 46

47 performed.100 Here, the effect of surface area and of nitrogen content was analyzed 48

independently at samples pyrolized at 900 °C. Analysis of the role of the specific surface 49

50 area/particle size reveals a direct correlation of surface area, onset potential and maximum 51

52 current density. This clearly indicated the homogeneous distribution of catalytic sites on the 53

material surface. Analysis of impact of the dopant concentration showed that the overall 54

55 catalytic performance is also increasing with the dopant concentration, however only up to a 56

57 certain limit as very high loads of nitrification agents negatively influenced the aerogel 58

formation.

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60 29


5

10

15

25


1

2

3 The aerogel which was engineered to maximize catalytic performance showed drastically 4

improved plateau current densities compared to a platinum catalyst (but lower onset

6 potential), excellent long term stability, as well as an ideal 4-electron process. As all carbon 7

8 catalysts, the system turned out completely insensitive against fuel spillover, a known 9

weakness of the traditional Pt-catalysts. Importantly, this approach offered the first

11 carbohydrate-derived, non-metal electrocatalyst, synthesized via a fully sustainable 12

13 hydrothermal carbonization (HTC) technique, which partially even outperformed noble metal 14

platinum catalysts.

16

17

18 Conclusions 19

20 This review described some recent developments in materials chemistry where carbon 21

22 aerogels, including monolithic species, are synthesized via typical processes well-known from 23

24 the sol-gel synthesis of silica. All these processes start with soluble, molecular precursors (in

26

27 many cases sustainable carbohydrates), which throughout a condensation process turn into a 28

29 final, nanostructured gel. Nanoscale architecture, the coupled high specific surface area, as 30

31 well as the secondary transport structure within those gels are controlled by surface 32

33 thermodynamics, additives, and demixing and cross-linking kinetics, i.e. the processes can be

34

35

36 seriously understood as a sol-gel chemistry of carbon. 37

38 As the enormous importance of porous materials has led to the development of several 39

40 synthetic approaches which cannot be described simply by classical terms anymore, it 41

42 becomes meaningful to reconsider the classical distinction between methods for the 43

44

45 introduction of porosity. These are:

46

47 1) Templating 48

49 a. Hard templating: utilization of hard and thermally stable materials such as 50

51 silica or zeolites which need to be dissolved after the synthesis38, 101, 102

52

53 b. Soft templating; utilization of soft and thermally fragile molecules and their

54

55

56 aggregates, such as polymers or surfactants which can be removed by thermal 57

58 decomposition or extraction.103-105

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2 2

25


1

2

3 c. Molecular imprinting: utilization of molecules as template 4

5 d. Phase separation towards bicontinuous structures: utilization of additives such 6

7 as polymers or salts which cause spinodal decomposition throughout the 8

9 material condensation step.

106

11

12 2) Activation 13

14 a. utilization of etching and/or oxidizing agents (powders, solutions) such as 15

16 alkali hydroxides.75, 107, 108

17

18 b. utilization of oxidizing gases such as CO or H O.

109

19

20

21

22 In this article, we presented cases for all categories of templating of carbon structures, i.e. we 23

24 reported on methods, which direct the pore formation in a constructive fashion. We

26

27 differentiated that from methods, which are used to introduce pores by degradation after the 28

29 material synthesis, which simply have the disadvantage of counteracting some of the rational 30

31 synthetic efforts for functionalization and the establishment of electric conductivity pathways 32

33 by materials erosion. In addition, any etching process reduced the mass yield, i.e. is less

34

35

36 efficient and sustainable. 37

38 Throughout our work, we however found out that some of the agents classically considered as 39

40 “activation agents” are rather templating agents or moderators in templating, for instance 41

42 ZnCl2, the use of which was not found to be accompanied by mass loss. This observation 43

44

45 allows – beside rather traditional soft and hard templating- the novel techniques of sol-gel

46

47 chemistry under hypersaline conditions and finally sol-gel chemistry in salt melts. The final 48

49 “salt templating” represents a new technique for the preparation of functional carbons with 50

51 very high apparent surface areas of up to 2000 m2g

-1, higher than for any zeolite and

52

53 approaching the value of single-layer graphene. In salt templating, the pore morphology was

54

55

56 shown to solely depend on the nature of the templating salt. This offers the opportunity of 57

58

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55


1

2

3 tuning the morphologies of the materials from micro- to mesoporous with very high apparent 4

5 specific surface areas. 6

7 A big pull towards such conductive, oxidation stable carbon aerogels comes from the 8

9 application side. We presented in an illustrative fashion some selected cases as electrodes in

11

12 lithium batteries, in supercapacitors, in capacitive desalination, as active catalytic supports, 13

14 and in electrocatalysis. In all those relevant applications, carbon aerogels have the chance to, 15

16 or already do, outperform classical material systems, which makes this area a rather hot actual 17

18 focal point of research.

19

20

21 Thus, the “sol-gel approach” towards carbon aerogels opens the door to sustainable future 22

23 chemistry and materials design which enables for custom-made synthesis of materials, 24

25 optimizing their performance by rational bottom-up design. 26

27

28

29

30 References 31

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18

19

20

21 Curricula vitae of the authors: 22

23 Markus Antonietti has studied Chemistry in Mainz and did his doctorate with Hans Sillescu. His

24 habilitation on nanogels in 1990 fuelled his enthusiasm for complex materials. After a professorship at

25 the University of Marburg he was appointed director for the department of Colloid Chemistry at the 26

MPI of Colloids and Interfaces in 1993. Antonietti’s contributions to the chemical community 27

comprise many aspects, but first of all he is devoted to creativity in research. He is fond of cooking

29 and loud music. Nina Fechler studied Nanostructure and Molecular Sciences at the University of

30 Kassel and the Fraunhofer Institute for Applied Polymer Research, where she was working on

31 thermoresponsive polymers with J. F. Lutz. She obtained her PhD degree from the MPI of Colloids 32 and Interfaces with M. Antonietti in Potsdam/Golm, focussing on porous carbon-based materials from 33

ionic liquids. Currently, she is research coordinator of Colloid Chemistry at the same institute where 34

35 she works on supercapacitors and the extension of the salt approach also to other material classes.

36 Moreover, she enjoys sports. Tim Fellinger is also a master of Nanostructure and Molecular Sciences

37 of the University of Kassel. He completed his PhD at the MPI of Colloids and Interfaces in 2011 with

38 M. Antonietti where he was working on iono- and hydrothermal carbon synthesis and electrochemical 39 applications. Since 2013, he is group leader for “Energy Carbons”. Beside science, he enjoys all types 40

of sports. 41

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carbon aerogels and monoliths: control of porosity and nanoarchitecture via sol–gel routes

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