Sol Gel Method

Preparation of mesoporous carbons by carbonizing organic gels was initiated by Pekala [101]. In his study, resorci-nol was condensed with formaldehyde in alkaline solution. A schematic diagram of the resorcinol-formaldehyde (RF) gelation mechanism is presented in Figure 30. Resorcinol was first substituted with hydroxymethyl group by a catalytic action of alkaline reagent. The products condensed into surface functionalized polymer "clusters" that cross-link to form a gel. Pekala and co-workers carbonized the organic gel and found that the resulting carbon aerogel has high porosity (>80%) and a high surface area (400-900 m2/g) [102]. The mesoporosity of the carbon aerogels can be ascribed to a network structure that was inherited from the original organic aerogel.

Tamon et al. conducted exhaustive studies on the relation between the porous nature of the carbon aerogels and their sol-gel conditions and showed that the pore size and the pore volume can be controlled by changing the mixing ratios of resorcinol, formaldehyde, water, and catalyst [103]. They tried to use freeze drying instead of supercritical drying for

Figure 30. Formation of a silicalike organic gels from resorcinol. Reprinted with permission from [101], R. W. Pekala, J. Mater. Sci. 24, 3221 (1984). © 1984, Kluwer Academic Publishers.

drying wet gels, because the former is more economical than the latter [104]. Hanzawa et al. conducted high temperature treatment of the carbon aerogel [105]. They found that micropores disappeared by 2273 K, but half of the meso-pores could survive even after a heat treatment at 3073 K.

Resorcinol is not the only reagent for the preparation of organic gel. Gel formation was also observed for phenolic-furfural [106], melamine-formaldehyde [107], polyurethane [108], and mixed cresol-formaldehyde [109] systems. Finding cheaper raw materials and developing a cost-effective and time-saving drying method should be required for the practical uses of sol-gel carbons.

The application fields of carbon gels will be expanded, because the gels can be formed into desirable forms such as monoliths, wafers, films, spheres, and irregular powders [110]. In particular, monolith or film type carbon gels are suitable for electrochemical usage such as electrodes for EDLC [111, 112], for capacitive deionization [113], porous electrodes for fuel cells [114], and analytical electrodes [115].

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