Macroporous Electrically Conducting Carbon Networks by Pyrolysis of Isocyanate-Cross-Linked Resorcinol-Formaldehyde Aerogels

Carbon (C) aerogels are made by pyrolysis of resorcinol-formaldehyde (RF) aerogels under Ar, and they combine electrical conductivity with a high open mesoporosity. However, because macropores are known to facilitate mass transfer, macroporous C-aerogels could be useful for application in separations or as fuel cell and battery electrodes. Macropores are typically incorporated in C-aerogels during gelation of the RF precursors by using either "hard" templating with silica or polystyrene beads, or "soft" templating with surfactants. Here, we report an alternative method, where open macroporosity is introduced by pyrolyzing RF aerogels whose skeletal nanoparticles have been cross-linked covalently with an isocyanate-derived polymer that coats conformally the entire RF framework. The structural, physical, and chemical evolution of the X-RF network was monitored at various stages during pyrolysis by DSC, TGA, SEM, N-2 adsorption porosimetry, and C-13 NMR. The accumulated evidence shows that the cross-linker first loses its chemical bonding with the skeletal nanoparticles and then melts, exerting surface tension forces on the RF framework, which cause a partial structural collapse that creates macropores. The xerogel-like internal texture of the macroporous walls is responsible for close contact of the carbon skeletal nanoparticles, resulting in an about 7 x lower bulk electrical resistivity of the macroporous material relative to the corresponding mesoporous network, which is obtained by pyrolysis of native (i.e., non-crosslinked) RF aerogels. The new macroporous material was evaluated electrochemically for possible application as an electrode in batteries and fuel cells.