The critical role of electrochemically activated adsorbates in neutral OER

Abstract

Developing efficient electrocatalysts for the oxygen evolution reaction (OER) under neutral conditions is important for microbial electrolysis cells (MECs). However, the OER kinetics in neutral electrolytes at present are extremely sluggish, resulting in high overpotentials that greatly limit the energy conversion efficiencies of MECs. Previous studies failed to probe the adsorbates on surface metal sites of catalysts at the atomic scale and elucidate their influence on the catalytic activities, which has impeded the rational design of efficient neutral OER catalysts with optimal surface structures. Here, using in situ transmission electron microscopy (TEM), in situ X-ray photoelectron spectroscopy (XPS) and in situ low-energy ion scattering studies, we have identified, for the first time, that the electrochemically activated adsorbates on surface metal sites play a critical role in boosting the neutral OER activities of Ru-Ir binary oxide (RuxIryO2) catalysts. The adsorbate-activated RuxIryO2 on a glassy carbon electrode achieved a low overpotential of 324 mV at 10 mA cm−2 in neutral electrolyte, with a 36-fold improvement in turnover frequency compared with that of IrO2 benchmark. Upon application in an MEC system, the resulting full cell showed a decreased voltage of 1.8 V, 200 mV lower than the best value reported to date, facilitating efficient synthesis of poly(3-hydroxybutyrate) from bioelectrochemical CO2 reduction. Density functional theory (DFT) studies revealed that the enhanced OER activity of RuxIryO2 catalyst arose from local structural distortion of adjacent adsorbate-covered Ru octahedra at the catalyst surface and the consequently decreased adsorption energies of OER intermediates on Ir active center.