Regulating d-orbital spin state of Fe in single-atom electrocatalyst for boosting oxygen reduction activity in neutral electrolyte

Abstract

Oxygen reduction reaction (ORR) in neutral electrolyte is urgently needed in various areas, such as metal-air batteries. However, the N-coordinated transition-metal single-atom electrocatalysts confront sluggish catalytic kinetics due to the inappropriate electronic structure and the as-resulted unreasonable adsorption strength towards oxygen-containing intermediates. In this work, we develop a strategy to tune the Fe d-orbital spin state by introducing inert Si atom into the first coordination sphere of Fe-N₄ moieties. The experimental and theoretical results suggest that Si atom generates the coordination field distortion of Fe and induces the Fe d-orbital spin state transforming from low to medium spin state. The optimized spin-electron filled state (t_2g⁴e_g¹) of Fe sites weakens the adsorption strength to intermediates and reduces the energy barrier of *OH desorption. Consequently, Fe-Si/NC catalyst exhibits superior ORR performance compared with that of Fe-NC and commercial Pt/C, showing a more positive half-wave potential of 0.751 V (vs. RHE) in 0.1 mol/L phosphate buffered saline. In addition, Fe-Si/NC-based neutral zinc-air batteries show a maximum power density of 108.9 mW cm⁻² and long-term stability for more than 200 h. This work represents the possibility of constructing distorted coordination configurations of single-atom catalysts to modulate electronic structure and enhance ORR activity in neutral electrolyte.