Active sites and factors influencing them for efficient oxygen reduction reaction in metal-N coordinated pyrolyzed and non-pyrolyzed catalysts: a review

With increasing demand for clean energy and approaching commercialization of polymer electrolyte membrane fuel cells (PEMFCs), replacing expensive Pt-based cathode catalysts with much cheaper non-precious metal (NPM) catalysts has become absolutely essential. This review highlights the parameters that have been considered vital to improving the overall performance of the NPM-based catalysts for oxygen reduction reaction (ORR). In the present review, we focus on well-known catalytic systems in three categories of NPM catalysts, i.e. biomimetic heme-copper oxidase enzymes, non-pyrolyzed/polymeric systems, and pyrolyzed NPM-nitrogen-doped carbon (M-N/C) (M = Fe, Ni, Co, etc.) catalysts. The ORR mechanism on the reported active sites and the effect of varying their local environments are considered and discussed in detail. Among all the catalysts, only pyrolyzed M-N/C catalysts have shown activity and stability much closer to that of the state-of-the-art commercial carbon-supported platinum (Pt/C) catalyst. Although great heights have been climbed in pyrolyzed M-N/C-based catalysts, still general consensuses need to be established regarding the active sites in the NMP-based M-N/C catalysts to help enhance the activity and stability of the catalytic system. By comparing the ORR mechanisms of the three studied systems, various similarities between the active sites are identified and reported comprehensively. On the basis of the information amassed, some future directions for improving the activity, selectivity, and durability of the NPM-based catalysts are also discussed.