Oxygen Evolution Reaction in Alkaline Media: Electrical Double Layer and Interfacial Interactions

Abstract

The industrial water splitting process typically occurs in aqueous alkaline electrolyte because of the corrosion issue when conducted in acidic conditions. This process, which involves applying electricity to decompose water into hydrogen and oxygen, is limited primarily by the oxygen evolution reaction (OER). Many open questions and great challenges still orbit around the OER. Neither the OER mechanism nor the ideal catalyst properties in terms of cost, stability and activity have been revealed so far. Therefore, there is a critical need for large-scale applications to enhance the understanding of OER at a fundamental level. Our recent observations mainly focus on the reaction interface (i.e. electrical double layer, EDL) between the catalyst layer surface and the adjacent aqueous electrolyte and the interfacial interactions. We newly-develop the benchmarking methods via the thin film rotating disk and rotating disk ring electrode measurements.1 Key parameters, such as electrochemical active surface area, interfacial oxygen transport, solid-state pseudocapacitance, EDL charging capacitance and reconstruction, can thus be evaluated accurately. Based on the traditional linear sweep voltammetry method, it is found that the OER performance depends strongly on the applied potential scan rate, because of the material-dependent mass transport resistance and continuous reconstruction of various chemical phases.2, 3 Further study confirms that the ionomer blocking effect is a common phenomenon, which is independence of catalyst loadings and types. Nevertheless, as demonstrated by Nafion-free samples, the addition of ionomer is indispensable for efficient catalyst utilization during OER. The optimized ionomer to catalyst loading ratio ranges from 10-30 wt. %. In this range, well-organized catalyst-ionomer networks in EDL can be formed to achieve superior OER activity and electrochemical stability.4 New insights into interfacial structure and interactions are achieved by investigating the roles of electrolyte-containing cations and anions. The necessity to promote inner-sphere OH- adsorption in EDL is emphasized for high-efficiency alkaline OER. Adding an inert supporting electrolyte, NaNO3, into the aqueous KOH complicates the electrocatalytic OER. The non-specially adsorbed Na+ in EDL has a much stronger ionic interaction with OH- than K+, and thus decreases OH- mobility. Finally, a non-covalent model of the gas-liquid interface is proposed to elucidate the improved interfacial oxygen transport when increasing ionic conductivity of bulk electrolyte solution. These findings provide valuable guidance for a molecular picture of EDL, and highlight the prospective approaches to maximize the efficient utilization of OER catalysts.