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 MnCo2O4.5 enhances Co-O covalency and 4f-3d 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-MnCo2O4.5 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.
Deng et al. (Fri,) studied this question.