Axial Phosphate Coordination Driven Spin State Change in FeN ₄ Site for Stable Oxygen Reduction

Abstract Understanding the structure–performance relationships of single‐atom catalysts (SACs) under realistic operating conditions remains a major challenge in electrocatalysis. In this study, axial phosphate groups (PO 4 ) were introduced into the microenvironment of Fe single‐atom sites embedded in cellulose‐derived carbon (P‐FeN 4 /CC), inducing a dynamic transformation of the crystal field from D 4 h to C 4 v and modulating the spin state through charge redistribution to better accommodate the catalytic reaction. The ligand‐induced spin tunability combined with magnetic field‐driven kinetic control enhances the intrinsic activity, selectivity, and stability for the oxygen reduction reaction (ORR). The P‐FeN 4 /CC catalyst demonstrates a half‐wave potential of 0.97 V, an ultra‐high kinetic current density of 179.2 mA cm −2 at 0.85 V, and retains over 90.5% of its current after 136 h, considerably outperforming Pt/C‐20%. In practical applications, liquid zinc‐air batteries (ZABs) achieve a peak power density of 280.1 mW cm −2 and an impressive cycle life of 11,130 cycles (3,710 h), while flexible ZABs deliver 81 mW cm −2 and operate stably for more than 160 h. Theoretical calculations and in situ spectroscopy confirmed the critical role of axial PO 4 ‐induced modulation of Fe centers in enhancing ORR performance, offering new insights into the rational design of high‐performance SACs.