Introduction
Electrochemistry can transform abundant resources like water and carbon dioxide into useful chemical and sustainable fuels. These reactions often involve protons, which are either consumed or produced at the surface of electrodes. The concentration of protons at the electrode (i.e., local pH) fundamentally governs the efficiency and selectivity of myriad electrochemical transformations (e.g., the oxygen evolution reaction [OER]). As more and more protons are consumed or produced during operation, however, this local pH can change dramatically. Taking bulk pH, buffer composition, and mass-transport into account, we develop an accessible and robust model for describing this local pH. Our model explores how pH gradients form and dissipate during operation, which we correspondingly validate using rotating (ring) disc electrodes. We may employ this model to predict the local pH over a wide range of current densities, including under industrially relevant conditions, and propose that dramatic changes in local pH may be inevitable regardless of bulk conditions. The complicating effects of morphology on local pH are further described to highlight how understanding and controlling this environment is crucial to improving the efficiency of electrochemical transformations.