Cation Doping in Manganese Oxide Nanorods via Ionic Radius Modulation for Enhanced Oxygen Reduction Reaction Activity

Unlike previous OMS-2 doping studies focused on single-ion modification, nanostructured OMS-2 were engineered via systematic cationic substitution of Group IA alkali metals (Li+, Na+, Rb+, and Cs+) with well-defined ionic radius differences into tunnel vacancies. Structural characterization confirms the predominant occupancy of the introduced cations at tunnel vacancy sites without inducing secondary phases or morphological alterations. Notably, our systematic approach reveals that the ionic radius synergistically modulates vacancy occupancy, lattice distortion, and electronic properties, establishing a clear radius-stress-electronic structure–performance correlation. This nanoscale modulation synergistically enhances oxygen adsorption energetics and electronic conductivity to modulate its oxygen reduction reaction (ORR) performance. These structural-electronic synergies correlate directly with ORR activity, as evidenced by Rb+-doped nanorods achieving a half-wave potential (E1/2) of 0.815 V vs RHE, significantly outperforming pristine OMS-2 (0.752 V). Density functional theory calculations reveal that cation size governs the d-band center position and bandgap narrowing, establishing a mechanistic link between ionic radius and catalytic enhancement. This work advances beyond empirical screening by providing a generalizable design strategy for tunnel-structured catalysts via systematic ionic radius engineering, resolving intrinsic activity limitations of transition metal oxide catalysts, and offering a rational pathway to high-performance doped electrocatalysts.