Templated Active Carbons

Templating for pore types. Source Zhang, L. L. and X. S. Zhao. 2009. Chemical Society Reviews, 38,2520-2531. With permission.) 

Zeolite templated carbons exhibit high performance without extensive activation due to the high surface areas and long range orders of their porous structures. Wang et al. [37] used zeolite X (670 m. gr, 1.4 nm pore size) and 8 hr of chemical vapor deposition (CVD) to introduce carbon into the template. The resulting template CNX-2 (2700 m2.g i) had capacitance of 158 Rgy (at 0.25 A.gyi) and energy density of 25 Wh.gyi, respectively, in aqueous electrolyte. Due to the ordered nature of the pore structure, over 97% of the capacitance was retained at rates of 2 A.gyi. The zeolite could produce dense carbon pore structure (1.07 cm. g-i) and improved volumetric capacitance versus most activated carbon materials [37]. 

Retention of capacitive performance with increasing sweep rate and power for various carbon materials. Source Wang, D. W. et al. 2008. Angewandte Chemie, 4 373-376. With permission.) 

g in organic electrolyte at scan rates of 10 mV.sec. Also, Lufrano et al. [42] used CMK-3 carbon generated by SBA-15 (8 nm pore size) to demonstrate 132 Rg i using a flexible Nation electrolyte. 

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Nowadays synthesis of mesoporous materials with zeolite character has been suggested to overcome the problems of week catalytic activity and poor hydrothermal stability of highly silicious materials. So different approaches for the synthesis of this new generation of bimodal porous materials have been described in the literature like dealumination [4] or desilication [5], use of various carbon forms as templates like carbon black, carbon aerosols, mesoporous carbon or carbon replicas [6] have been applied. These mesoporous zeolites potentially improve the efficiency of zeolitic catalysis via increase in external surface area, accessibility of large molecules due to the mesoporosity and hydrothermal stability due to zeolitic crystalline walls. During past few years various research groups emphasized the importance of the synthesis of siliceous materials with micro- and mesoporosity [7-9]. Microwave synthesis had... 

The template carbons demonstrate interesting performance in electric double layer capacitors [109], especially the mesoporous carbons prepared using MgO as the template [105,110], Asymmetric electric double layer capacitors constructed from these mesoporous carbons coupled with amicroporous activated carbon display high capacitance and high rate performance [111]. 

FIGURE 3.25 Diffusion of electrolyte ions into the pores of a conventional activated carbon and the ordered microporous carbon prepared using the zeolite template. 

An exhaustive study has been carried out recently on the synthesis of BN nanotubes and nanowires by various CVD techniques.17 The methods examined include heating boric acid with activated carbon, multi-walled carbon nanotubes, catalytic iron particles or a mixture of activated carbon and iron particles, in the presence of ammonia. With activated carbon, BN nanowires are obtained as the primary product. However, with multi-walled carbon tubes, high yields of pure BN nanotubes are obtained as the major product. BN nanotubes with different structures were obtained on heating boric acid and iron particles in the presence of NH3. Aligned BN nanotubes are obtained when aligned multi-walled nanotubes are used as the templates (Fig. 40). Prior to this report, alignment of BN nanotubes was achieved by the synthesis of the BN nanotubule composites in the pores of the anodic alumina oxide, by the decomposition of 2,4,6-trichloroborazine at 750 °C.116 Attempts had been made earlier to align BN nanotubes by... 

Prior to the 1990s, materials with pore size in the meso range such as silica gels and activated carbons show disordered pore structme with a broad distribution of pore size. In 1992, by using cationic smfactants as template, Mobil scientists... 

Contrarily to Li-ion batteries, the supercapacitor application requires highly developed surface area carbons with micropores adapted to the size of the ions involved in the formation of the electric double layer. In this case, the additional presence of mesopores is crucial to fulfill the demand of fast charge propagation with a minimal time constant. It seems that the most suitable would be to increase the amount of mesopores in KOH activated carbons or to increase the microporosity of the essentially mesoporous template carbons. A further improvement of the materials could be a special carbon doping by the incorporation of heteroatoms able to provide useful pseudocapacitance effects. 

Using the appropriate template cations, macrocycles can be prepared from three different substrates. For example, the complex of copper with multi-dentate amines reacted with formaldehyde and nitromethane or diethyl ma-lonate (not shown) to form a peraza-crown (Comba et al., 1986 Lawrance and O Leary, 1987). The ratio of substrates can vary from 1 2 1 or 1 4 2 (amine formaldehyde activated carbon) depending on reaction conditions... 

The porous skeleton of activated carbon can be used as a template on which to construct other porous materials, for example, Si02, Ti02 and AI2O3. The oxide is first dissolved in supercritical CO2 (see Section 8.13) and then the activated carbon template is coated in the supercritical fluid. The carbon template is removed by treatment with oxygen plasma or by calcination in air at 870 K, leaving a nano-porous ( nano refers to the scale of the pore size) metal oxide with a macroporous structure that mimics that of the activated carbon template. 

High surface areas are normally obtained by using porous materials, and the pore sizes may condition the accessibility of the reactants to the active sites, especially in the case of microporous materials such as activated carbons. Pore diffusion limitations become more important as the pore sizes decrease in addition, the smaller pores may be more easily blocked (e.g., by coke deposition). Therefore, deactivation and diffusion phenomena will in general affect more strongly the performance of microporous carbons. As a result, there has been a drive to develop mesoporous carbon catalysts (such as aerogels, xerogels, and templated carbons) for some applications, especially in the liquid phase. 

Such a well defined nanotexture is interesting for a fundamental study of the double layer capadtance behavior. For a serf of template carbons prepared from different template materials and carbon precursors it has been shown that the cE iacitance, either in aqueous or cnganic medium, increases with the total surface area [66]. As for previous works with activated carbons, the capacitance does not correlate with the BET specific surface area. By contrast, in Fig. 14 a quite linear relationship is observed with the micropore volume determined by CO2 adsorption (micropore size smaller than 0.7 nm [76]). This result confirms that the double layer formed in ultramicropores mainly contributes to tire value of capacitance. Considering the case of flie organic electrolyte, it is obvious that such a pore size is smaller than the diameter of the solvated ions. Hence, it can be concluded that under the application of an electrical polarizaticm, the solvated ions easily diffuse in the mesopores and in the larger micropores for being finally trapped as non-solvated in the small micropores. 

The capacitance performance of a porous carbon prepared by the template method (MC) was compared with a microporous activated carbon presenting a comparable BET specific... 

Triazine-based networks have been produced under ionothermal conditions in molten ZnClj, followed by extraction of residual salt from the materials [15], It is certainly possible that the salt acts here as a template for pore formation indeed, ZnCl has been used previously as a porogen for the formation of activated carbon [27-29],... 

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