Magnetic Field Sensitivity of Active Sites: A Universal Metric for Distinguishing Spin-Dependent Effects in OER

Mechanistic understanding of OER enhancement under magnetic fields remains challenging across electrocatalysts with different magnetic properties due to the coexistence of multiple spin and non-spin effects. Here, we conduct OER experiments on catalyst layers exhibiting in-plane magnetic anisotropy from collectively oriented ferromagnetic microchains, revealing that traditional electrochemical parameters are inadequate for reliable quantitative comparison of magnetic field enhancement across different experimental systems. We introduce a universal metric, κ, defined as the overpotential reduction per unit magnetic field at high field strengths, which yields nearly identical values for the same catalyst regardless of its magnetic anisotropy. Moreover, κ values establish the magnetic response hierarchy of elements Fe > Co > Ni, aligning with the descending spin states from Fe3+ to Ni3+. Thus, the magnetic field sensitivity of active sites‒and consequently the OER enhancement‒is determined primarily by the intrinsic spin configuration of metal cations at the active sites rather than by the bulk magnetic properties of the oxide matrices. This research demonstrates that the site-specific metric, κ, overcomes the limitations of traditional electrochemical parameters in quantifying magnetic field effects on OER and should be adopted in future research on spin-dependent electrocatalysis.