Origin of Spin-State Precise Modulation for Enhanced Oxygen Evolution Activity: Effect of Secondary Coordination Sphere

The process of oxygen evolution reaction (OER) is crucial for energy storage and conversion, and the spin electronic structure of catalyst significantly influences its catalytic activity. Precisely regulating the spin electronic structures of metal active centers with intermediate spin (IS) states is challenging but important. This study presents a general method for achieving spin-state precise modulation by altering the secondary coordination sphere (SCS) in Fe-substituted LaCo1-xFexO3 perovskites, denoted as Co6-y-[Co]-Fey (y = 0-6). The concentration-dependent SCSs can precisely regulate the spin state of Co3+ from high-spin (HS) to IS and low-spin (LS) state by tuning the CoO binding energy of primary coordination sphere (PCS) to approximate to 567 KJ mol-1. The binding energy demonstrates a strong negative correlation with the spin state of Co3+, serving as a quantitative descriptor for precise spin-state modulation. Furthermore, a universal optimal doping concentration is proposed for generating IS-state Co3+ with the best OER activity, ranging from 1/(m+1) to 2/(m+1) in M-doped ACo1-xMxOy system with the coordination number of m. As a proof-of-concept, the LaCo7/9Fe2/9O3 with IS Co3+ exhibits significantly enhanced OER activity, almost six times higher than the control samples (without IS Co3+). These findings provide new insights into spin-state modulation for effective OER catalysts. Combining theoretical and experimental studies, the origin of spin-state precise modulation of Co3+ is systematically revealed for enhanced OER: effect of secondary coordination sphere (SCS). Concentration-dependent SCSs (Co6-y-[Co]-Fey) can precisely regulate the spin state of Co3+ by tuning the coordination number (y) of dopant.image