Quantifying Spin‐Dependent OER Enhancement Through Butterfly‐Shaped Hysteresis

Understanding the mechanism of magnetic field-enhanced oxygen evolution reaction (OER) remains challenging due to coexisting spin-dependent and spin-independent effects. Here, we conduct OER experiments on catalyst layers exhibiting in-plane magnetic anisotropy from collectively oriented ferromagnetic microchains, reporting the first observation of butterfly-shaped hysteresis in overpotential shifts (ΔE) during complete magnetization cycles. Spin-dependent magnetoresistance is identified as the mechanism determining ΔE responses. We introduce a composition-specific metric κ (overpotential reduction per unit field) that serves as an intrinsic descriptor of surface magnetic sensitivity, independent of bulk magnetic properties and anisotropy, enabling reliable quantitative comparison across diverse catalysts where conventional electrochemical parameters fail. These findings provide both a mechanistic foundation and quantitative methodology for rationally designing magnetic field-enhanced electrocatalysts based on interfacial spin engineering.