Magnetohydrodynamic Redeposition of Cations Onto the Anode

Abstract

Dissolution of metallic and metal-based catalysts occurs at the anode surfaces. The oxygen evolution reaction (OER) has been reported to cause the catalyst to be dissolved into cationic species during the electrolysis.[1–5] This phenomenon is not limited to non-noble metals (e.g. Fe, Co, Cu), but also occurs with noble metals (e.g. Ru, Rh, Pd, Os, Ir, Pt, Au).[1–4] Even with thermodynamically stable stoichiometric oxide catalysts, the dissolution cannot be completely avoided during the OER.[5] Upon dissolution, the as-formed cationic species will be attracted to the negatively charged cathode surface, resulting in catalyst loss from the anode over time.Mass transport in diffusion layers during electrochemical reactions can be tuned by means of magnetohydrodynamics (MHD). This magnetic convection is a result of the Lorentz force that arises when there is an angular mismatch between the magnetic field and the local current density. Therefore, in the presence of a magnetic field, vortices can be generated around microscale protuberances on the electrode surface, and this phenomenon is often referred to as micro-MHD.[6] Conventional applications of MHD include cathodic electrodeposition and anodic corrosion or electropolishing studies. The difference in application according to the redox tendency of the target reaction is electrochemically rational on the basis of E H–pH diagrams.Despite a rather elusive combination, we presume that the magnetically-induced vortex can contribute to the electrochemical redeposition of as-dissolved cationic species onto the anode surface. Provided that a vortex can enhance the retention of the cations remaining in the vicinity of the surface, the chance of oxidative electrodeposition will increase. If this is the case, the protuberances on the anode are expected to grow under magnetic fields over the course of the OER. This will be realized as a result of facilitated O2 bubble detachment or increased local ionic concentration, or both.In this work, we present that the morphology of the anode surface can change in the presence of a magnetic field. For the demonstration, we prepare a nanometrically-flat electrode consisting of a magnetic superlattice and an electrocatalyst layer. We perform the OER on the flat catalyst surface using magnetic and nonmagnetic samples and compare micrographs of pre-experimental and post-experimental sample surfaces. The findings show that the protuberances build up along their axes on the magnetic sample surface whereas the nonmagnetic sample involves the flattening of the surface.