In situ local pH measurement of oxygen evolution reaction catalysts

The oxygen evolution reaction (OER) plays a central role in water electrolysis, yet its kinetics, catalyst stability, and degradation pathways are strongly influenced by the local pH at the electrode electrolyte interface rather than the bulk electrolyte pH. This is particularly important for anion exchange membrane (AEM) water electrolyzers, where operation under reduced alkalinity or pure water feed can result in substantial divergence between bulk and local pH. Despite growing interest in catalyst strategies that modulate the local reaction environment, progress has been hindered by the lack of a standardized and accessible method for quantitative local pH measurement. In this work, the rotating ring–disk electrode technique (RRDE) equipped with an iridium oxide pH-sensing ring electrode was developed and systematically evaluated as an in situ method for measuring local pH during the OER. IrO₂ was selected as a benchmark OER catalyst due to its well-established activity and minimal intrinsic pH modulation. Local pH measurements were conducted in unbuffered, phosphate-buffered, and bicarbonate-buffered electrolytes and validated against theoretical mass transport models. The results demonstrate that, when properly implemented, the RRDE technique can provide reliable quantitative measurements of local pH. Accurate interpretation requires careful calibration of the pH-sensing ring, correction for ring electrode ohmic potential shifts, and consideration of electrolyte buffering effects. Significant local acidification was observed under conditions of limited proton or hydroxide transport, low buffering capacity, and high current density, consistent with both modeling predictions and prior literature. Several sources of error inherent to RRDE-based pH measurements were identified and quantified, including nonlinear disk pH versus ring pH relationships, sensor response time and drift, gas bubble interference, and susceptibility of the IrOx sensor to redox active chlorine species in chloride containing electrolytes. Overall, this thesis establishes the RRDE method as a practical and informative tool for probing local pH during OER and provides a foundation for the discovery of catalysts aimed at enabling low-alkalinity and pure-water AEM water electrolysis.