Quantifying the Reduction of OER Overpotential on Magnetic Electrocatalysts Under Magnetic Fields

Magnetic‐field enhancement of the oxygen evolution reaction (OER) represents a promising route toward more efficient alkaline water electrolyzers, yet its origin remains debated due to overlapping effects of mass transport and reaction kinetics. Here, we present a general experimental strategy that employs strong forced convection to suppress uncontrolled transport arising from natural diffusion and magnetohydrodynamic (MHD) flows. Using polycrystalline Au electrodes, we show that this approach resolves subtle OER variations under controlled flow and field conditions. Notably, spontaneous MHD flows near hard‐magnetic electrodes are identified for the first time, highlighting a major complication in interpreting magnetic effects. Forced convection eliminates these artifacts, enabling reliable quantification of intrinsic activity changes. Systematic analysis of 3d transition‐metal catalysts reveals a clear composition dependence: Fe‐based catalysts exhibit the strongest magnetic enhancement, followed by Mn and Co, whereas Ni shows minimal response. Moreover, synergistic interactions between different elements further modulate the effect. By decoupling magnetic influences on mass transport from those on kinetics, this method provides a universal framework to assess how magnetic fields alter electrocatalysis.