A graphene-directed assembly route to hierarchically porous Co-N_x/C catalysts for high-performance oxygen reduction

The development of non-precious metal catalysts for efficient oxygen reduction is of significance for many advanced electrochemical devices such as fuel cells and metal-air batteries. Herein, we develop a graphene-directed assembly route to synthesize hierarchically nanoporous Co-N_x/C materials with a macro/meso/microporous structure, high specific surface area (i.e. 512 m² g^-1) and excellent conductivity using graphene oxide (GO) supported zeolitic imidazolate framework nanocrystal arrays as a catalyst precursor, followed by the carbonization and acid leaching process. In this route, GO acts as a structure-directing agent to construct ZIF nanocrystal arrays supported on GO nanosheets. During the carbonization process, the resulting reduced graphene oxide functions as a binder and electrical conductor to connect individual ZIF-derived carbon nanoparticles into the macroporous structure and increase the overall conductivity. ZIF nanocrystals themselves are also converted into meso/microporous carbon nanoparticles without using any other template. The hierarchically porous Co-N_x/C materials exhibit high ORR catalytic activity, superior stability and good methanol tolerance under both alkaline and acidic conditions.