Quantifying the Reduction of Kinetic Overpotential on Magnetic Electrocatalysts under Magnetic Fields

The direct enhancement of electrocatalytic oxygen evolution reaction (OER) by a magnetic field is a novel strategy for high-efficiency alkaline water electrolysers. The influence of a magnetic field on either electrochemical equilibria or electron-transfer kinetics remains controversial, partially due to the difficulty of eliminating the mass-transfer related effects. In this work, a magneto-electrochemical system, which contains a thin-layer flow cell allowing a forced convection flow, is designed to decouple the magnetic effects on mass transfer from those on the reaction kinetics. When OER is catalyzed by magnetic materials, the active species on the surface, rather than the composition and magnetic property of the bulk material, determine the magneto-enhancement of the reaction kinetics. The enhancement at monometallic active sites follow the order Ni3+ < Co3+ < Fe3+, corresponding to the change in kinetic overpotential (ΔEk) of 2.7, 18.8, and 42.9 mV at 1 mA·cm-2, respectively. It was assumed that a magnetic field acts on the spin moments of active sites, leading to variations in the d-band center and the electrochemical equilibria. Here we show that the magneto-enhancement follows the same trend of the average spin moments in the order Ni < Co < Fe. Thus, the spin state of active sites determines the ΔEk on magnetic electrocatalysts.