Hydrogen oxidation reaction in alkaline media: Relationship between electrocatalysis and electrochemical double-layer structure

Enhancing the sluggish kinetics of electrochemical hydrogen-oxidation reaction in high pH environments is of crucial importance considering its applications in alkaline-membrane fuel cells (AMFC) and regenerative hydrogen electrodes for energy storage. Alkaline H2-oxidation to form water involves reaction between H-adsorbed intermediates and hydroxide anions wherein the nature/source of the latter plays a crucial role. Here, we take a systematic approach to understand why H2-oxidation kinetics is slower in alkaline media compared to acid. While recently reported models focus on surface-adsorbate bond strength optimization, we herein show that the alkaline H2-oxidation mechanism is fundamentally different due to a complex interplay between electrocatalysis and electrochemical double-layer structure. A heretofore unknown modern rendition of the double-layer structure is proposed wherein specifically adsorbed (M-OHad) and quasi-specifically adsorbed (M-Had/upd…OHq-ad) reactive hydroxide-species localized in the compact part of the electrochemical double-layer is shown to define H2-oxidation kinetics on monometallic and bimetallic catalyst surfaces at high pH.