Rare-Earth-Induced Intermediate-Spin Co Centers in MnCo ₂ O _4.5 for Sustainable Acidic Water Oxidation

Developing robust oxygen evolution reaction (OER) catalysts for proton exchange membrane water electrolysis (PEMWE) demands concurrent mitigation of insufficient activity and structural instability in acidic media. Herein, we propose a spin-state engineering strategy enabled by rare-earth doping to resolve the intrinsic activity-stability trade-off dilemma. Incorporation of rare-earth cations (Sm³⁺, Nd³⁺, Ho³⁺) into MnCo₂O_4.5 enhances Co-O covalency and 4<i>f</i>-3<i>d</i> coupling, increasing the crystal-field splitting and driving the Co sublattice from a purely high-spin Co²⁺/Mn⁴⁺ toward a mixed-spin Co³⁺/Mn⁴⁺configuration, within which the intermediate-spin Co³⁺ state can stably exist. This spin-state modulation occurs alongside lattice distortion and oxygen-vacancy formation, which together reinforce the spinel framework and mitigate excessive Co overoxidation. The coupled electronic-structural effects lower the adsorption energy barrier, thereby alleviating structural reconstruction. The optimized Sm-MnCo₂O_4.5 catalyst exhibits a low overpotential of 212 mV at 10 mA cm^-2 and sustains operation for 1200 h. When integrated into a PEM electrolyzer, it delivers 0.5 A cm^-2 at 1.73 V for over 300 h. This work establishes rare-earth-mediated spin-state modulation as a fundamental design principle for sustainable non-noble-metal acidic OER catalysts.