Oxygen reduction on carbon nitride supported mono-/plurimetallic catalysts

This review concerns electrocatalysts for the oxygen reduction reaction (ORR) for application at the cathode of polymer electrolyte membrane fuel cells (PEMFCs) characterized by the presence of both carbon and nitrogen atoms in their chemical composition. Three main families of electrocatalysts are reviewed: platinum- and palladium- based carbon nitride electrocatalysts, M/N-C electrocatalysts and metal-free nitrogen-doped carbons CNx electrocatalysts. The review starts with a brief summary of the proposed mechanisms for the ORR on these materials; then, it describes each family of electrocatalysts in terms of preparation, main structural characteristics and main features controlling the ORR performance. The review is completed by a section where the ORR performance of the various systems is benchmarked vs. the requirements set for practical applications. It is shown that the presence of nitrogen may result in platinum- or palladium based electrocatalysts with a better durability and tolerance to methanol with respect to commercial, carbon-supported systems. Nitrogen is also necessary to prepare platinum-free ORR materials, both M/N-C and metal-free nitrogen-doped carbons CNx. These electrocatalysts, while extremely interesting owing to their low cost and good methanol tolerance, still require a significant research effort to enhance their activity and durability to match the requirements necessary for practical applications.