Skin formation on RF aerogel sheets

Aerogel sheets a few hundred μm thick were made from fiber-reinforced resorcinol–formaldehyde (RF) gels with a high molar resorcinol/catalyst (R/C) ratio and a target density of 300 kg/m3. The obtained gels were ambient pressure dried without noticeable shrinkage. The bulk of these samples show a coarse structure with micron-sized particles and pores while on the sheetsʹ surface a nanostructured skin with a thickness of a few microns formed. The surface skin was peeled off the aerogel sheet. In addition, a non-reinforced aerogel film with the same thickness as the peeled-off skin was made. Both were investigated via small angle X-ray scattering (SAXS) and atomic force microscopy (AFM). It was found, that the surface skin on a Novoloid-fiber-reinforced RF aerogel possesses a higher density and smaller entities than the non-reinforced RF aerogel film. This is explained by a local superposition of the catalytic influence of the fibers and the sol–mold interface. If pyrolyzed, the skin covered aerogel sheets are most suitable e.g., as gas diffusion electrodes in PEM fuel cells.