Inverse magneto-ionics enhances oxygen evolution reaction

Applying magnetic fields to electrodes has been reported to enhance electrochemical reactions, leading to numerous studies and the hypothesis that externally applied magnetic fields manipulate direct spin exchange at the electrode–electrolyte interface to control reaction kinetics. However, a closer analysis suggests that this mechanism is unlikely to play a significant role, whereas the influence of magnetic gradient forces on ion migration is well established. Here, we provide experimental evidence supporting the latter mechanism for the oxygen evolution reaction (OER) on nickel. Moderate magnetic fields increase the electrochemical current density by up to 20%. For pristine samples, the enhancement is both field-and time-dependent. We demonstrate that the magnetic field effect does not arise from direct spin alignment at the electrode surface or from the magnetic domain structure of the underlying ferromagnetic film. Instead, the external magnetic field indirectly enhances the electrochemical performance through magnetic gradient forces that promote the growth of the electrocatalytically active nickel oxyhydroxide surface layer by driving the exsolution of paramagnetic Ni species during the OER. With the structure of the passivation layer of Ni and its evolution being common in similar d-metal (alloy) electrode materials, the magnetic field effect by ion migration is likely to be generally valid.