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

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 (PO₄) were introduced into the microenvironment of Fe single-atom sites embedded in cellulose-derived carbon (P-FeN₄/CC), inducing a dynamic transformation of the crystal field from D₄h to C₄v 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-FeN₄/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 PO₄-induced modulation of Fe centers in enhancing ORR performance, offering new insights into the rational design of high-performance SACs.