Probing the relationship between bulk and local environments to understand impacts on electrocatalytic oxygen reduction reaction

[Display omitted] •Tuning bulk reaction environment parameters modulates catalyst local environment.•Effect of pH and cation identity on oxygen reduction reaction (ORR) is studied.•Changes in local pH are measured during ORR using an iridium oxide pH sensing probe.•Cation identity provides variable pH-buffering and modulates changes in local pH.•Changes in local pH more directly impact ORR mechanisms and catalytic performance. Local catalyst environments fluctuate during electrochemical reactions which significantly influences reaction efficiency and selectivity. However, elucidating fundamental relationships between local environment and electrocatalytic performance is challenging due to the difficulty in probing this local region. Here, we study the impact of electrolyte composition on the kinetics and mechanism of the oxygen reduction reaction (ORR) for a carbon catalyst. Using a rotating ring-disk electrode equipped with an iridium oxide pH probe, we monitor changes in local pH during ORR. We confirm that local pH changes significantly for near-neutral bulk pH and find that different cations provide variable pH-buffering, thereby modulating overall magnitude and onset of local pH changes. Overall, local pH more strongly dictates changes in mechanisms and performance, while the cation identity modulates local pH to influence these trends. This work highlights the importance relating local pH to electrocatalytic performance to further improve understanding of complex electrochemical processes.