Low-cost transition metal–nitrogen–carbon electrocatalysts for the oxygen reduction reaction: operating conditions from aqueous electrolytes to fuel cells

After decades of effort, the performance of low-cost transition metal–nitrogen–carbon (M–N–C) catalysts has been significantly improved, positioning them as promising catalysts for the oxygen reduction reaction in proton-exchange-membrane fuel cells (PEMFCs). Despite this progress, compared to traditional commercial Pt/C catalysts, the practical application of M–N–C catalysts in PEMFCs is hindered by their inferior performance in acidic environments. In this perspective, we first summarize the current status of M–N–C catalysts in terms of activity and stability, and compare their performance with that of Pt/C catalysts. Then we discuss the fundamental research challenges associated with M–N–C catalysts, which are primarily related to (i) conducting basic research with tests exclusively using oversimplified aqueous electrolytes that limits exploration in practical fuel cell environments; (ii) lacking operando characterization methods under fuel cell working conditions; and (iii) the complexity of catalyst structures and fuel cell operating environments causing difficulty in M–N–C catalyst research. Lastly, we propose key advances that need to be made in the future to address these fundamental challenges, including the rational design of fit-for-purpose catalysts based on more cost-effective and efficient modelling, preparing model/quasi-model catalysts with defined and controllable structures, and developing operando characterization techniques for PEMFCs. By combined study using model/quasi-model catalysts, operando characterization methods and atomistic modeling, we can deeply understand the “structure-performance” relationship of the catalysts at various scales and develop next generation M–N–C catalysts that can meet the increased demand for PEMFCs.