Tuning Spin State of Rock-Salt-Based Oxides by Manipulation of Crystallinity for Efficient Oxygen Electrocatalysis

The spin state, specifically antibonding orbital (e(g)) occupancy, of transition-metal ions is recognized as a descriptor for oxygen electrocatalysts with perovskite or spinel structures, and can be facilely adjusted by varying the valence states of transition metals. However, both perovskites and spinels show unsatisfactory performance even at the optimal spin states. In comparison, some oxides with a rock salt structure (e.g., (NiCoO2)-Co-II-O-II) exhibit higher activity than perovskites and spinels, nevertheless, the rock salt structure excludes valence changes of the transition metals, obstructing further e(g) optimization and performance enhancement. Herein, an innovative strategy is demonstrated to regulate the spin states of Co-II in NiCoO2 via crystallinity manipulation, thus providing a new strategy for e(g) optimization. Remarkably, the catalyst (CNO-8) with a moderate e(g) occupancy (approximate to 1.2) achieves the best electrocatalytic activity, which is among the highest achieved by the state-of-the-art electrocatalysts, namely, an overpotential of approximate to 269 mV at 10 mA cm(-2) for oxygen evolution reaction and an onset potential of 935 mV for oxygen reduction reaction. As an efficient bifunctional catalyst for rechargeable Zn-air batteries, CNO-8 even outperforms the noble metal catalyst (Pt/C + RuO2), demonstrating high potential for practical applications in electrochemical energy conversion.