Defining Nafion Ionomer Roles for Enhancing Alkaline Oxygen Evolution Electrocatalysis

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.