Oxygen Reduction Reaction Catalysts for PEMFCs: Mechanisms, Classification and Optimization Strategies

Proton exchange membrane fuel cells (PEMFCs) are a central technology for the efficient utilization of hydrogen energy, but their performance is fundamentally limited by the sluggish kinetics and high overpotential of the cathode oxygen reduction reaction (ORR). Moreover, the currently used platinum-based catalysts are expensive, suffer from resource scarcity, and are readily poisoned, all of which impede the commercialization of PEMFCs. Hence, this paper provides a thorough review of the reaction mechanisms of ORR. The text first clearly and logically distinguishes four-electron from two-electron pathways and discusses their relevance to fuel cells, then naturally proceeds to summarize and compare the classification, design strategies, and synthesis methods of precious metal and non-precious metal catalysts. More importantly, it systematically presents the key strategies for improving ORR catalyst performance from three well-defined dimensions: microstructure regulation, development of low-platinum/non-precious metal substitutes, and surface engineering/doping modification. Following this, current challenges are analyzed, and future directions are suggested with clear theoretical and technical implications for designing high-performance, low-cost, and highly stable ORR catalysts.