Molecular catalysts for the oxygen reduction reaction based on earth abundant transition metals: progress, limitations, and future opportunities

The oxygen reduction reaction (ORR) remains a central kinetic bottleneck in electrochemical energy conversion technologies, including polymer electrolyte membrane fuel cells and metal-air batteries. Although the platinum group metals are now predominantly used in ORR catalysis, their high cost and limited availability along with durability issues have motivated significant efforts to use earth abundant replacements. In such a scenario, molecular catalysts that are built around the first-row transition metals have been considered as an attractive platform because of a high degree of control of overactive-site structure, tailored electronic properties and opportunities to get mechanistic insights at a molecular level. In this review, we will critically analyse latest advances in earth-abundant transition-metal molecular catalysts for the ORR, focusing specifically on progress published in or after 2020. We describe basic mechanistic principles that drive ORR activity and selectivity, such as proton-coupled electron transfer, O-O bond activation, and the balance between two-electron and four-electron pathways. Representative catalyst families based on Fe, Co, Ni, Cu, and Mn molecular systems are highlighted, with attention to structure-activity relationships, secondary coordination sphere effects, and strategies to enhance stability under acidic and fuel cell-relevant conditions. Advances in heterogenization, catalyst ionomer interactions, and integration into practical electrode architectures are also discussed. Finally, we identify key challenges and emerging design strategies that will be critical for translating molecular precision into durable, platinum-free ORR catalysts for next-generation energy technologies.