Ordered mesoporous carbide-derived carbons prepared by soft templating

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Ordered mesoporous carbide-derived carbons prepared by soft templating Lars Borchardt a, Martin Oschatz a, Martin Lohe a, Volker Presser b, Yury Gogotsi b, Stefan Kaskel a,* a

Department of Inorganic Chemistry, Dresden University of Technology, Bergstraße 66, D-01062 Dresden, Germany Department of Materials Science & Engineering and A.J. Drexel Nanotechnology Institute (DNI), Drexel University, 3141 Chestnut Street, PA 19104, USA

b

A R T I C L E I N F O

A B S T R A C T

Article history:

Free-standing films of ordered mesoporous silicon and titanium carbide-derived carbons

Received 24 January 2012

have been synthesized using a novel soft templating approach without employing hydro-

Accepted 3 April 2012

fluoric acid. Tetraethyl orthosilicate or titanium citrate, alternatively, and a phenolic resin

Available online 13 April 2012

underwent an evaporation induced self-assembly yielding ordered mesoporous silicon carbide/carbon or titanium carbide/carbon composites. High temperature chlorine treatment transformed these materials conformally into carbide-derived carbons (CDC) while the ordered arrangement of mesopores was maintained. The corresponding hierarchical pore structures consist of narrowly distributed micro- and mesopores (distribution maxima at 1 and 5 nm, respectively) with a high surface area and pore volume of up to 1538 m2/g and 2.53 cm3/g, respectively. Ó 2012 Elsevier Ltd. All rights reserved.

1.

Introduction

Micro- and mesoporous carbons have attracted much interest because they combine beneficial adsorption properties, tunable structure, and high chemical and hydrothermal stability [1]. Accordingly, carbon materials are playing a dominant role in fields such as hydrogen and hydrocarbon storage, gas separation and purification, catalysis, electric and electrochemical energy storage, and capacitive desalination. Most commonly, activated carbons are used in these fields of application. However, studies have shown that the rather broad pore size distribution with irregularly curved pores and surfaces with functional groups limit the performance of activated carbons and an improved performance has been reported for porous carbons with a porosity optimized for a certain application [2]. Motivated by this, various approaches have been developed to synthesize carbons with controllable pore size, such as templat-

ing or selective etching of a carbon-containing precursor [3–4]. A versatile approach to control carbon pore sizes emanates from the selective etching of metal carbides with chlorine gas. These so called carbide-derived carbons (CDCs) were synthesized from numerous binary and ternary carbides (e.g., SiC, TiC, B4C, Ti2AlC) [5]. Even without the application of additional post-synthesis (physical or chemical) activation, this halogen treatment of carbides [6] yields micro- and mesoporous CDCs with a specific surface area (SSA) of up to 2500 m2/g and up to 1.7 cm3/g total pore volume [5,7]. The resulting pore sizes depend largely on the structure of the metal carbide precursor and can be adjusted as a function of the halogen etching temperature with high precision [5,8]. Templating is another technology yielding a high level of control over the pore size and the pore size distribution of carbon materials [9]. As one of the first, Ryoo et al. [10] synthesized mesoporous carbons with an ordered pore arrangement

* Corresponding author. E-mail address: [email protected] (S. Kaskel). 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.04.006

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by a nanocasting process. For this, carbonaceous precursors like sucrose are infiltrated into an ordered mesoporous silica template, followed by pyrolysis and template removal. However, the moderate yield of carbon templating and the use of hydrofluoric acid for the removal of the template are severe complications to scaling the production of templated carbons to obtain large bulk quantities. Resultant replica structures are widely known as CMK carbons which exhibit narrow mesopore size distributions and specific surface areas of up 2000 m2/g [11]. Although templating yields a high level of control over the pore size of ordered mesopores, the diameter of mesopores is often too large to be optimal for many applications. For example, hydrogen storage [12] and CO2 capturing [13] are significantly increased for large pore volumes of sub-nanometer sized micropores and the optimum pore size diameter shifts as a function of the application pressure [14]. Studies on electrochemical double-layer capacitors (EDLCs, also called supercapacitors) document the potential for improvement and increased performance when the pore structure of the electrode material is optimized for a certain electrolyte [15,16]. To optimize the capacitance of supercapacitors it is critical to match the carbon pore size to the ion size for a given electrolyte and sizes of many widely used ions are below 1 nm (usually 0.4–0.8 nm) [17,18]. In addition, micrometer-sized porous carbons particles with a narrow distribution of pores in the sub-nanometer range exhibit significant limitations to ion transport [19] and supercapacitors with ultrafast charge/discharge rates (requiring fast ion redistribution) have only been reported for thin film systems (micro-supercapacitors) [20]. In the last years, we reported a promising new strategy to combine the advantages of both these material classes by synthesizing ordered mesoporous carbide-derived carbons through nanocasting [21]. First, mesoporous silicon carbide with an ordered (hexagonal) pore arrangement is synthesized by infiltrating a polycarbosilane precursor into the pores of ordered mesoporous silica (SBA-15), followed by pyrolysis, and template removal [22]. In the next step, silicon atoms are selectively removed by high temperature chlorine treatment while the ordered structure of the silicon carbide precursor is maintained. The resulting ordered mesoporous carbide-derived carbons (OM–CDC) exhibit both micropores and mesop-

ores. At the same time, OM–CDCs have, compared to conventional, not-activated CDC, significantly larger specific surface areas (3000 m2/g) and total pore volumes (2 cm3/g) [21]. OM–CDC has been shown to yield a high gas uptake for hydrogen (50.9 mg/g, 40 bar, 196 °C) and methane storage (208 mg/g, 100 bar, 25 °C) [23]. As an electrode material for supercapacitors, it exhibits up to 170 F/g in organic and 200 F/g in aqueous electrolytes, which is one of the highest values so far for pure carbons without the presence of pseudocapacitance [24–26]. However, large scale utilization of OM–CDC has been complicated as the reported synthesis employs harmful chemicals (hydrofluoric acid), is expensive, time-consuming, and the yield is very low (<10%). Herein we report a new pathway for scalable OM–CDC synthesis. A non-ionic triblock copolymer (Pluronic F127) was used as a soft template for the self-assembly of different compositions of resin and metal complexes [27,28]. The aged polymer was subsequently pyrolized into ordered mesoporous metal carbide/carbon composites by carbothermal reduction followed by metal atoms extraction by high temperature chlorine treatment. The resulting carbon materials exhibit a high specific surface area (<1538 m2/g) and a hierarchical pore structure of micro- and mesopores while the hexagonal arrangement of the pores can be preserved.

2.

Experimental section

A scheme showing the steps for the preparation of OM–SiC– CDCs and OM–TiC–CDCs, comprising the preparation of ordered mesoporous silicon [28] and titanium [27] carbide/carbon composite from tetraethyl orthosilicate and titanium citrate (Ti(cit)) [29] precursors as well as the subsequent chlorine treatment is given in Fig. 1. Our experimental setup for the pyrolization limited the final sample size to several cm2 while the film size of the cured polymer was larger than 1 m2. The final product was a free-standing thin film of pure carbon (the chemical composition of the CDC films is provided in Table 1). Detailed experimental procedures as well as information on characterization techniques are provided with the Supporting Information.

Fig. 1 – Scheme for the synthesis of OM–SiC–CDC and OM–TiC–CDC. Even large quantities of the material can be synthesized (in the range of m2 sample size) provided that large furnaces for the pyrolization process are used.

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Table 1 – Elemental composition (in wt.%) derived from EDX for SiC/C and TiC/C composites and CDCs. Sample

C

Si

Ti

O

TiC-1 TiC-1-1300 TiC-2 TiC-3 SiC-1 SiC-1-1300 SiC-2 SiC-3 All CDCsa

88.6 92.6 76.6 60.7 76.8 85.6 70.9 27.8 >99.5

– – – – 11.6 8.0 17.3 32.8 <0.5

6.4 7.4 14.5 28.5 – – – – <0.5

5.0 0.0 8.9 10.8 11.6 6.4 11.6 39.4 <0.5

a All CDCs except SiC–CDC-3 which could not be transferred to the corresponding CDC.

3.

Results and discussion

3.1.

Wide-angle X-ray diffraction and composition

3.1.1.

TiC/C and TiC–CDC

The amount of micropores generated by chlorine treatment of carbides corresponds to the amount of metal atoms in the precursor and, thus, the resin/organic metal complex composition was gradually changed in our experiments. TiC-1 was synthesized by using a gravimetric resin/metal complex ratio of 3:6, which yields a titanium content of 6.4 wt.% in the resulting TiC/C composite (Table 1). For sample TiC-2, the titanium content was increased to 14.5 wt.% by using a resin/metal complex ratio of 1:6. TiC-3 represents a resin-free sample with a Ti content of 28.5 wt.%. These materials (TiC-1, 2, 3; all pyrolized at 1000 °C) and a TiC-1-800 reference sample (pyrolized at 800 °C) do not show Bragg reflections corresponding to cubic TiC in the 2h range between 30° and 80° (Fig. 2). The samples with a high Ti content (TiC-2 and TiC3) show pronounced peaks which can be attributed to anatase (TiO2) and this phase is also found for the low Ti content sample TiC-1-800 (6.4 wt.% Ti). As the pyrolysis temperature is increased to 1300 °C, carbothermal reduction is accomplished (TiO2 + 3C ! TiC + 2CO) as indicated by the disappearance of the TiO2 peaks and the appearance of peaks associated with TiC [27,30]. A broad peak at 43° 2h was observed, corresponding to carbon (Fig. 2). The overall reaction process is comprised of several steps at different temperatures. At low temperatures, the titanium citrate is decomposed to titanium dioxide at 200–400 °C (Figure S2). Further heating leads to the reaction of titanium dioxide with carbon according to (TiO2 + 3C ! TiC + 2CO) [27]. Thus, TiC-1-800 represents the status of the reaction where TiO2 is already formed but has not reacted with carbon to form TiC so far. By using the Scherrer equation the size of these TiO2 particles was estimated from the (2 0 0) peak of anatase to be 7 nm. This confirms that by using a titanium citrate complex [31] the aggregation and growth of large TiOx domains can be avoided and this can be explained by complexation and esterification reactions of titanium citrate with the resolic hydroxyl groups [27,32]. Chlorine treatment selectively etches titanium following the reaction TiC + 2 Cl2 ! TiCl4 + C [33]. Initially present

Fig. 2 – X-ray diffraction patterns for TiC/C and SiC/C composite materials and the theoretical peak positions for graphitic carbon, cubic TiC, and anatase including the corresponding PDF (powder diffraction file) numbers.

carbon in the TiC/C composite does not participate in the reaction and remains in the material. If TiO2 is present, a Kroll-like process is responsible for the titanium removal (TiO2 + 2 C + 2 Cl2 ! TiCl4 + 2 CO) [34]. In both cases, chlorine treatment removes titanium from the precursor and the resulting carbons contain less than 0.5 wt.% of titanium and oxygen after CDC synthesis (Table 1).

3.1.2.

SiC/C and SiC–CDC

SiC/C composite materials were synthesized in a similar manner as the TiC/C samples [28]. Different ratios of tetraethyl orthosilicate (TEOS) and resin were used to form ordered structures as a result of co-assembly with the triblock copolymer Pluronic F127. We studied the structural features as a function of silicon content; therefore, the silicon content was varied chosen to be 11.6, 17.3 and 32.8 wt.% (SiC-1, SiC-2, and SiC-3, respectively; Table 1). In contrast to the TiC/C composites, all SiC/C materials were more amorphous as seen from the XRD pattern (Fig. 2); the only peak (located at 43° 2h) is associated with disordered carbon. Neither SiC/C pyrolized at 1300 °C nor SiC/C with a high Si content show peaks corresponding to crystalline SiC or any silica phase. As reported in Ref. [28], this

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synthesis yields material that start forming crystalline silicon carbide at around 1300–1450 °C which has been studied by 29 Si MAS NMR and the onset of crystallization was correlated to the appearance of a new single resonance with a chemical shift of 20 ppm. The chlorine treatment of pure silicon carbide yields, just as for TiC, pure carbon: SiC + 2 Cl2 ! SiCl4 + C [35]. However, we note that silica etching is limited to SiC, SiCN, and Si–O– C [5], but chlorine gas does not react with silica (SiO2); therefore, resin free material (i.e., SiC-3) was not transformed into the SiC–CDC (Table 1).

3.2. Low-angle X-ray electron microscopy 3.2.1.

diffraction

and

transmission

TiC/C and TiC–CDC

Low angle X-ray diffraction of TiC-1 showed one sharp (1, 0) peak at 0.95° 2h (Fig. 3A) which is not a TiC peak but a structural peak associated with the ordered mesoporous structure. For a 2-D hexagonal pore arrangement, also seen from transmission electron micrographs, the corresponding lattice constant is calculated to be 10.7 nm. After titanium has been extracted, the ordered hexagonal arrangement of pores is completely preserved for TiC–CDC-1 (Fig. 4) and the 2-D pore arrangement lattice constant of the corresponding carbon is identical with the former carbide. For samples pyrolized at higher temperature (TiC-1-1300 and TiC–CDC-1-1300) the peak intensity decreases, while the peak position remains constant. It is known from literature and theoretical simulation studies of materials with ordered pore structures that a drop of the peak intensity does not directly correlate with a decrease in structural order [36]; however, previous studies on ordered mesoporous materials have shown that synthesis temperatures of about 1300 °C lead to sintering processes and induce the collapse of the ordered structure. For a higher Tiprecursor/resin ratio (TiC-2), the (1, 0) peak shifts to a larger 2h value (1.2°). This corresponds to the shrinking of the 2-D

Fig. 3 – Low angle X-ray diffraction pattern of TiC/C and TiC– CDC (A), and SiC/C and SiC–CDC (B).

pore arrangement lattice constant from 10.7 nm for TiC-1 to 8.5 nm for TiC-2 (21%) which is in agreement with N2 physisorption data where a decrease in the average pore size can be seen as well (Section 3.3). Further decrease in the resin/ Ti-precursor ratio to a level of complete resin absence leads to composite materials (TiC-3) that do not show any ordering in low-angle X-ray diffraction experiments.

3.2.2.

SiC/C and SiC–CDC

All SiC/C composites (Fig. 3B) show the characteristic (1, 0) peak independently of the Si-resol/precursor ratio. The peak positions of SiC-1 and SiC-2 are 2h = 0.92° (a0 = 11.1 nm) and 2h = 0.94° (a0 = 10.9 nm), respectively. The peak of SiC-3 is shifted to lower angles (0.85° 2h), correlating with a larger lattice constant (a0 = 12 nm). Low-angle X-ray diffraction patterns of the corresponding CDC show that only sample SiC– CDC-1 is ordered, whereas SiC–CDC-2 and SiC–CDC-3 are not. The highly ordered pore structure of TiC-1, SiC-1 and the corresponding CDCs is also confirmed by transmission electron microscopy (Fig. 4).

3.3.

Nitrogen physisorption

3.3.1.

TiC/C and TiC–CDC

Fig. 5A depicts the N2-isotherms at 196 °C of TiC/C composites synthesized by the co-assembly of titanium citrate, Pluronic F127, and different amounts of resin. The sample with the highest content of resin (33 wt.%, TiC-1) shows the largest specific surface area (569 m2/g) and the highest pore volume (0.45 cm3/g; Table 2). The shape of the isotherm is type IV, indicative of very narrowly distributed mesopores. Deconvolution with a NLDFT (non-local density functional theory) [37] algorithm and assuming cylindrical pore geometry, the average (volume weighted) pore diameter of the mesopores was calculated to be 5.2 nm TiC-1 (Fig. 5B).

Fig. 4 – Transmission electron micrographs of TiC-1 (A), TiC– CDC-1 (B), SiC-1 (C), and SiC–CDC-1 (D).

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By reducing the resin amount to 14 wt.% (TiC-2) and 0 wt.% (TiC-3), the specific surface area (pore volume) is reduced to 500 m2/g (0.30 cm3/g) and 248 m2/g (0.14 cm3/g), respectively. The corresponding isotherms assume a type-I shape and the hysteresis loop disappears entirely. While NLDFT pore size analysis still shows a broad distribution of mesopores

for TiC-2 around 5.2 nm, TiC-3 no longer shows mesopores; this is in agreement with low-angle X-ray pattern (Section 3.2). Since TiC-1 (highest resin content) exhibits the highest porosity, the influence of the pyrolysis temperature on the adsorption properties was investigated. Both, rising the pyrolysis temperature to 1300 °C as well as decreasing it to 800 °C

Fig. 5 – N2 sorption isotherms at -196 °C and corresponding NLDFT pore size distribution for TiC/C, and TiC–CDC (A, B) and SiC/C and SiC–CDC (C, D).

Table 2 – Specific surface area (SSA; derived from the BET equation for 0.05 < p/p0 < 0.2), micro- and mesopore volume (determined from NLDFT) and specific total pore volume (at p/p0 = 0.95) for SiC, TiC, SiC–CDC, and TiC–CDC. Composite precursor

SSA (m2/g)

Vmicropore (cm3/g)

Vmesopore (cm3/g)

Vpore (cm3/g)

Corresponding CDC

SSA (m2/g)

Vmicropore (cm3/g)

Vmesopore (cm3/g)

Vpore (cm3/g)

TiC-1 TiC-1-800 TiC-1-1300 TiC-2 TiC-3 SiC-1 SiC-1-800 SiC-1-1300 SiC-2 SiC-3

569 480 537 500 248 387 505 408 323 14

0.14 0.10 0.12 0.10 0.02 0.06 0.11 0.06 0.23 –

0.31 0.29 0.31 0.20 0.12 0.35 0.30 0.31 0.13 –

0.45 0.39 0.43 0.30 0.14 0.41 0.41 0.37 0.36 0.01

TiC-CDC-1 TiC-CDC-1-800 TiC-CDC-1-1300 TiC-CDC-2 TiC-CDC-3 SiC-CDC-1 SiC-CDC-1-800 SiC-CDC-1-1300 SiC-CDC-2 SiC-CDC-3

952 1021 682 1344 1403 1394 1420 836 1538 13

0.26 0.28 0.16 0.47 0.29 0.20 0.27 0.13 0.63 –

0.46 0.42 0.39 0.41 0.89 1.03 1.15 0.77 1.90 –

0.72 0.70 0.55 0.88 1.18 1.23 1.42 0.90 2.53 0.02

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reduces the specific surface area and the specific pore volume of the resulting composite materials. Hence, an optimum for the pyrolysis of TiC/C composites of 1000 °C was identified. The porosity of the composite materials decreases with decreasing resin amount but the titanium content increases at the same time, leading to a larger porosity after chlorine treatment for samples with high metal and low resin content. Chlorine gas treatment increases the SSA of TiC-1 by 66% to 952 m2/g, 268% for TiC-2 (1344 m2/g), and 564% for TiC-3 (1403 m2/g). The resulting TiC–CDC-3 contains a large amount of mesopores even though none were present in the starting TiC/C composite. This is attributed to the etching of anatase with nanometer-sized crystallite size leaving behind these mesopores [5,34]. The crystallite size calculated from Scherrer equation (7 nm) is in agreement with the resulting pore size in the TiC-CDC-3 material (approx. 5–6 nm). Fig. 5B shows the pore size distributions for TiC–CDC materials.

3.3.2.

SiC/C and SiC–CDC

The samples SiC-1 and SiC-2 (11.6 and 17.3 wt.% Si, respectively) exhibit comparable SSA of 387 and 323 m2/g as well as comparable pore volumes of 0.41 and 0.36 cm3/g (Table 2). The mesopores are distributed between 3 and 8 nm with a maximum at approximately 5 nm. Thus, decreasing the

Fig. 6 – Raman spectra of TiC–CDC, SiC–CDC, and an OM– CDCref reference sample produced by nanocasting (cf. Ref. [23]).

resin-to-silicon-precursor ratio from 6:10 to 3:10 does not have a strong influence on the porosity of the composite. After silicon extraction the SSA of SiC–CDC-1 is increased by a factor of 3.6 to a value of 1394 m2/g and the pore volume by a factor of 3 to 1.23 cm3/g. Due to a higher amount of silicon in SiC–CDC-2, a larger pore volume is generated during the chlorination process, resulting in an increased SSA and pore volume of 1538 m2/g and 2.36 cm3/g, respectively. We note that the pore volume of this material is significantly larger than of conventional CDC and ordered mesoporous CDCs synthesized by nanocasting [21]. Further increasing the silicon content by preparing resinfree samples decreases the pore volume and SSA significantly. Such materials, SiC-3 and SiC–CDC-3, have a lower SSA (13 m2/g) and elemental analysis documents the presence of amorphous silica which cannot be removed by chlorine treatment. Resin appears to be an indispensable additive for silicon carbide formation, but for titanium systems, its content can be decreased or even completely omitted as titania can be selectively extracted by thermal chlorine treatment (Section 3.1) [34].

3.4.

Raman spectroscopy

Raman spectra of CDCs synthesized at 1000 °C, as well as a reference sample (OM–CDCref) [23] derived by a nanocasting approach are presented in Fig. 6. Two broad bands are observed at 1345 and 1600 cm1, which are related to the D-mode (characteristic for disordered carbon with finite sizes of crystallites correlating with an A1g breathing mode) and the graphite G-mode, respectively. A 4-band fitting of symmetrical peaks was required for full deconvolution of the Raman signal in the range between 1000 and 1800 cm1; that is, besides the D- and G-mode, peaks at 1160 cm1 (t1) and 1540 cm1 (t2) were considered, too (Table 3). It can be seen that for TiC–CDCs (i.e., TiC–CDC-1, TiC–CDC2, and TiC–CDC-3) the used amount of resol during the synthesis has an influence on the degree of ordering in the resulting CDC. The ID/IG increases significant when the resol content decreases (Table 3) and the FWHM (full-width at half-maximum) decreases simultaneously. From this, we conclude that carbons synthesized from precursors containing low or no resol exhibit a higher degree of structural order (i.e., are more graphitic). Comparing SiC–CDCs to TiC–CDCs synthesized at the same temperature and with similar molar metal precursor/resol ratio, then we see that SiC–CDC-1 shows a narrower

Table 3 – CDC peak positions, full-width at half-maximum (FWHM), and ID/IG intensity ratios following a four symmetrical line fitting appraoch.[38] Data for a conventional OM-CDC is included. Sample

OM-CDCref TiC-CDC-1 TiC-CDC-2 TiC-CDC-3 SiC-CDC-1

D-mode

t1

G-mode

t2

ID/IG

Position (cm1)

FWHM (cm1)

Position (cm1)

FWHM (cm1)

Position (cm1)

FWHM (cm1)

Position (cm1)

FWHM (cm1)

1160 1160 1160 1160 1160

81 100 125 129 112

1339 1347 1346 1345 1347

154 129 124 109 105

1537 1541 1540 1542 1539

108 117 117 121 122

1598 1603 1603 1603 1604

54 55 56 54 56

1.32 1.44 1.46 1.59 1.58

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FWHM (105 cm1) compared to TiC–CDC-1 (129 cm1). Thus, SiC–CDC exhibits a higher degree of structural ordering of carbon than the analog TiC–CDC. In addition, a reference sample (OM–CDCref) [21,23] was investigated. In Fig. 6 it can be seen that the carbon disorder in this material (FWHMOM-CDC1 ref = 154 cm ) is significantly larger than that of the new mesoporous CDCs discussed in this paper. Peak positions for the SiC/C and TiC/C composites are provided within the Supporting information (Table S2).

4.

Conclusion

Ordered mesoporous silicon and titanium carbide/carbons composites were synthesized by a soft templating approach employing different ratios of tetraethyl orthosilicate or titanium citrate, and phenolic resin. Unlike conventional nanocasting, this approach did not involve the use of any hydrofluoric acid. After metal atom extraction by hightemperature chlorine treatment highly porous carbon materials were obtained with a hierarchical and ordered arrangement of pores. The specific surface area as well as the total pore volume depend on the metal precursor/resin ratio. The higher this ratio, the higher are the surface area and the pore volume, reading 1538 m2/g and 2.53 cm3/g, respectively, for CDC obtained from a resin free precursor. The highest degree of ordering was observed for materials with a gravimetric metal precursor/resin ratio of 2:1. Compared to ordered mesoporous carbide-derived carbons synthesized by nanocasting, this new synthesis method is beneficial with reference to process cost and overall yield, saving time and minimizing the use of toxic chemicals. Also, with this approach, pure and freestanding thin films of OM–CDC can be synthesized. The latter are important for applications such as filtration, blood cleaning, or capacitive desalination.

Acknowledgements LB acknowledges financial support by the Deutscher Akademischer Austauschdienst (DAAD) which provided funding for an extended research stay at Drexel University. VP acknowledges financial support by the Alexander von Humboldt Foundation. YG was funded through the National Science Foundation (NSF) under the International Collaboration in Chemistry Grant No. 0924570.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2012.04.006.

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5 0 ( 2 0 1 2 ) 3 9 8 7 –3 9 9 4

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