Elucidation of the oxygen evolution reaction mechanism on platinum in an alkaline medium

Abstract

The demand for green hydrogen is at an all-time high, and its zero emissions offer an attractive alternative to fossil fuels as a sustainable source of energy, thereby helping mitigate climate change and conserve fragile ecosystems. The anodic oxygen evolution reaction (OER) concurrent with hydrogen production at the cathode during water electrolysis exhibits sluggish kinetics, increasing the power requirements and hence the production cost. An understanding of the mechanism of the OER is key to engineering highly active and cost-effective electrodes to catalyze the OER. This study identifies the mechanism of the OER on a Pt electrode in an alkaline environment. Mass transfer effects are accounted for by employing an inverted rotating disc electrode to obtain anodic polarization data while eliminating the challenge of bubble-induced surface blockage encountered in traditional rotating disk electrodes. The NaOH concentration was varied from 5 mM to 50 mM, and the electrode rotational speed was also varied. These results confirm that the overall hydroxyl ion oxidation reaction is mass transfer limited at higher overpotentials. The traditional adsorbate evolution mechanism (AEM), which involves four steps with three intermediates, viz. ∗OH, ∗O and ∗OOH could not adequately model the potentiodynamic polarization results. A modified AEM, wherein an additional O₂ evolution pathway involving the recombination of two ∗O intermediates, was found to predict the experimental observations well. The variation in the fractional surface coverage of the intermediates with overpotential was also predicted. The model predicts that the main reaction pathway is the formation of ∗OH and ∗O, followed by the recombination of two ∗O species to release oxygen.