A Phosphorus-Bridged Spin Trigger for Oxygen Reduction

Precise control over the spin degree of freedom of catalytic metal centers represents a significant challenge in enhancing the oxygen reduction reaction (ORR). In this work, we report a phosphorus(P)-bridged composite comprising Fe single atoms (SAs) and atomic clusters (ACs), Fe_SA/AC/PNC, wherein the Fe_SA-P-Fe_AC structure functions as an efficient electron channel and, more importantly, a spin trigger. This trigger induces a spin-state transition of Fe^II from low-spin (S = 0) to medium-spin (S = 1), as unequivocally deciphered by advanced spectroscopic and magnetic analyses. This spin-state reconstruction directly optimizes the reaction pathway by enhancing O₂ adsorption and facilitating *OH desorption. The resulting catalyst exhibits exceptional oxygen reduction activity in both acidic and neutral media, with half-wave potentials of 0.852 and 0.831 V, respectively, and achieves a peak power density of 1.35 W cm^-2 in proton exchange membrane fuel cells (PEMFCs). This strategy is universally effective for Co and Ni systems, establishing spin-state engineering as a general principle for designing high-performance non-precious metal catalysts.