Observing Local Ph Changes Using a Rotating Ring-Disk Electrode Functionalized with a Potentiometric Ph-Sensing Probe

Electrochemical reactions involving protons and hydroxide ions are significantly impacted by changes in the local pH near the catalyst surface. Therefore, it is useful to quantify the catalyst local pH to better understand the impact on overall reaction efficiency and selectivity. While it is difficult to experimentally probe the catalyst/electrolyte interface, this regime can be monitored indirectly using pH-sensitive materials. In this work, we investigate the use of a rotating ring-disk electrode coupled with a pH-sensing probe to track changes in proton concentration near the catalyst surface for the oxygen reduction reaction under well-defined mass transport conditions. We further examine the limitations and describe methods for improving the robustness of this experimental platform. Out of the electrode support and probe materials examined, we find that iridium oxide electrodeposited using cyclic voltammetry onto gold substrates exhibiting high surface area and moderate porosity demonstrates the highest, fastest, and most stable pH-potential response, enabling reliable measurements in under 10 s. Using an analytical convective-diffusion equation, we also estimate the disk local pH under varied operating conditions (e.g., current density and rotation rate) and reaction environments (e.g., bulk pH). This work outlines best practices for applying this technique and provides insights into the impact of relevant reaction environment conditions on the catalytic performance.