Influence of drying on the morphology of resorcinol-formaldehyde-based carbon gels

The method of drying resorcinol-formaldehyde (RF) gels influences the structure and gas adsorption capacity of the resulting networks as well as of their carbonized derivatives. Small angle X-ray scattering (SAXS) reveals the presence of submicroscopic clusters both in the RF gels and in the carbon gels, whose size, of the order of tens of nanometres, is consistent with SEM observations. Xerogels, obtained by heating the RF hydrogel in an inert atmosphere, have the most compact structure and display the lowest specific surface area (<900 m2/g). The highest surface area is found in the gel lyophilized after freeze-drying in t-butanol (>2500 m2/g), but the resulting carbon cryogel is not structurally stable, the measured surface area decreasing with time. Supercritical extraction with liquid carbon dioxide yields aerogels with an intermediate value for the surface area (ca. 1000 m2/g). Carbonization causes a reduction in the size of the elementary spheres (from 27 to 8 nm) in the case of the aerogels, while an increase (from 10 to 22 nm) is observed for the xerogels. The adsorption sites in the carbon gels are located principally in irregular spaces between monomer units. For the carbonized cryogels the specific surface area derived from SAXS is greater than that found by low temperature gas adsorption. This phenomenon is interpreted in terms of restricted pore size, which limits the access of gas molecules. By contrast, in the RF gels the SAXS-derived surface areas are substantially smaller than those measured by gas adsorption.