The electrocatalytic oxygen evolution reaction (OER) is not strictly a surface reaction, because it takes place in a thin hydrous electrical double layer (EDL). In this work, we perform an in-depth study of Nafion ionomer functions in both catalyst inks and catalyst layers (CL) for high-efficiency OER in alkaline media. Based on cyclic voltammetry results, an analysis method is developed to characterize pseudocapacitance and EDL charging capacitance independently. This enables accurate quantification of interfacial charge transport behavior and active sites for OER. Zeta potential measurements confirm that Nafion ionomer serves as a stabilizing and binding agent in catalyst-solvent inks. The data obtained from the half-cell tests in 1 M KOH reveal that increasing ionomer content in CL reduces OER performance due to higher mass transport resistance and less active sites. The sample with I/C = 1/24 (weight ratio of Nafion ionomer to IrOx catalyst) exhibits an approximately 1.7-times higher OER activity than that of I/C = 2/1. Furthermore, the ionomer blocking effect is found to be a common phenomenon, which was observed in a wide range of catalyst loadings and three different catalyst materials. Nevertheless, as demonstrated by Nafion-free samples, the addition of Nafion is indispensable for efficient catalyst utilization. Our study shows that the optimized ionomer content in the CL is 10-30 wt % of catalyst loading. Within this range, Nafion, catalyst particles, and electrolyte solution form efficient interaction, resulting in good connectivity of the charge conduction paths without inhibiting the gas diffusion.

Traditional understanding of electrocatalytic reactions generally focuses on either covalent interactions between adsorbates and the reaction interface (i.e., electrical double layer, EDL) or electrostatic interactions between electrolyte ions. Here, our work provides valuable insights into interfacial structure and ionic interactions during alkaline oxygen evolution reaction (OER). The importance of inner-sphere OH^- adsorption is demonstrated as the IrO_x activity in 4.0 M KOH is 6.5 times higher than that in 0.1 M KOH. Adding NaNO₃ as a supporting electrolyte, which is found to be inert for long-term stability, complicates the electrocatalytic reaction in a half cell. The nonspecially adsorbed Na⁺ in the outer compact interfacial layer is suggested to form a stronger noncovalent interaction with OH^- through hydrogen bond than adsorbed K⁺, leading to the decrease of interfacial OH^- mobility. This hypothesis highlights the importance of outer-sphere adsorption for the OER, which is generally recognized as a pure inner-sphere process. Meanwhile, based on our experimental observations, the pseudocapacitive behavior of solid-state redox might be more reliable in quantifying active sites for OER than that measured from the conventional EDL charging capacitive process. The interfacial oxygen transport is observed to improve with increasing electrolyte conductivity, ascribing to the increased accessible active sites. The durability results in a liquid alkaline electrolyzer which shows that adding NaNO₃ into KOH solution leads to additional degradation of OER activity and long-term stability. These findings provide an improved understanding of the mechanistic details and structural motifs required for efficient and robust electrocatalysis.