Carbon aerogels surface area

It is less well known, but certainly no less important, that even with carbon dioxide as a drying agent, the supercritical drying conditions can also affect the properties of a product. Eor example, in the preparation of titania aerogels, temperature, pressure, the use of either Hquid or supercritical CO2, and the drying duration have all been shown to affect the surface area, pore volume, and pore size distributions of both the as-dried and calcined materials (34,35). The specific effect of using either Hquid or supercritical CO2 is shown in Eigure 3 as an iHustration (36). 

Saliger, R., Fischer, U., Herta, C., and Fricke, J. High surface area carbon aerogels for supercapacitors. J. Non-Cryst. Sol. 225, 1998 81-85. 

First results indicate a dependence of the surface capacitance of untreated carbon aerogels on their microstructure. Micro- and mesopores exhibit different storage capacitances (6.6 and 19.4 pF/cm in 1 M sulfuric acid, respectively).. An optimized thermal activation procedure of low density aerogels at 950°C in controlled CO2- atmosphere leads to an increase of the specific surface area and capacitance. On the other hand, the increase of the capacitive current after anodic oxidation in sulfuric acid is caused by electroactive surface groups, while the BET-surface area remains almost constant. 

In table 1 the densities and surface capacitances of various carbon aerogels are shown. The data reveal that the surface capacitance varies from one species to another.. According to Shi et al. the micro- and mesopore surface areas were separated and the total capacitance of the samples was split up in contibutions from both pore species [3]. The correlation can be expressed as... 

Density and surface capacitance of different carbon aerogels pyrolized at 1050°C. The surface capacitance was derived dividing the capacitance calcuted from the impedance data at 8.25 mHz by the BET surface area. 5-10% error must be assumed for the evaluated data. 

Carbon (C)-aerogels have been investigated for one decade as a promising material for electrochemical applications in supercapacitors, fuel cells and waste water treatment [1,2], C-aerogels are nanoporous, electrically conducting and monolithic materials that provide the unique possibility to tailor the carbon properties on a molecular scale. The surface area and the degree of microporosity can be adjusted almost independently of the overall porosity for which mainly meso- and macropores are responsible. Whereas the mesostructure is determined by the stoichiometry of the reactants in the precursor solution, the pyrolysis conditions control the micropore structure of the material [3,4]. High pyrolysis temperatures will increase the electrical conduchvity [5], an important property for many electrochemical applications. 

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