Limitations of Liquid Electrolyte RDE Experiments for the Determination of OER Catalyst Activity: The Effect of the Catalyst Layer Thickness

Developing novel oxygen evolution reaction (OER) noble metal catalysts with a low metal packing density is key for the wider large-scale implementation of polymer electrolyte membrane water electrolysis (PEMWE). OER activities of these new catalysts are commonly measured in liquid electrolytes using half-cell configurations, such as rotating-ring-disk electrodes (RDE). Recent studies in liquid electrolyte cells showed that the accumulation of microscopic oxygen bubbles within the OER catalyst layer causes shielding of active catalyst sites. In this study, three different OER catalysts were screened for their activity at different loadings in a liquid electrolyte RDE setup. Potential sweeps using bare Ir black, a commercially available IrO 2 /TiO 2 and a homemade Ir/ATO (antimony doped tin oxide) catalyst with different loadings were performed. It was demonstrated that the mass activity of the Ir/ATO catalyst decreases by more than 50% with a catalyst loading increase, which is attributed to the accumulation of microscopic oxygen bubbles within the catalyst layer and was correlated to the coating thickness of the catalyst layer. We suggest screening the OER catalyst activity of low packing catalyst materials in a loading analysis by testing minimum three different loadings within the kinetic Tafel slope region to avoid underestimation of the catalyst activity.