Recent Advances in MOF-Derived PGM-Free ORR Catalysts: From Active-Site Engineering to Working Cathodes

The oxygen reduction reaction (ORR) remains a major bottleneck in terms of kinetics and durability in fuel cells and zinc–air batteries (ZABs). Metal–organic frameworks (MOFs) are versatile precursors for platinum-group metal (PGM)-free ORR electrocatalysts because their metal distribution, ligand chemistry, guest confinement, morphology, and porosity can be controlled before pyrolysis. This review examines how these precursor characteristics and subsequent thermal conversion govern metal migration; heteroatom retention; carbon ordering; pore evolution; and, ultimately, the nuclearity, coordination environment, and accessibility of the resulting active sites. Recent advances in conventional and asymmetric M–Nx single-atom sites, dual- and multi-atom sites, and single-atom–cluster or nanophase interfaces are critically evaluated, with particular attention to the evidence supporting structural assignments, activity, selectivity, and durability. Half-cell performance is further related to practical fuel-cell and ZAB operation by considering catalyst loading, ionomer or electrolyte contact, gas and water transport, and catalyst-layer degradation. Further progress will require simultaneous optimization of active-site structure, accessible-site density, hierarchical porosity, carbon stability, and electrode architecture, together with standardized testing protocols for reliable translation from rotating disk electrode measurements to working cathodes.