A Straightforward Model\nfor Quantifying Local pH Gradients\nGoverning the Oxygen Evolution Reaction

The production and consumption of protons by an electrocatalyst\nwill, under certain conditions, generate localized microenvironments\nwith properties distinct from those of the bulk solution. These local\nproperties are particularly impactful for reactions involving proton-coupled\nelectron transfer, where the generation of locally basic or acidic\nenvironments may significantly influence the energy efficiency and\nreaction selectivity of the electrocatalyst. Whereas local pH environments\nhave been observed and characterized in reductive half-reactions,\nincluding the CO<sub>2</sub> reduction and hydrogen evolution reactions,\nthe incompatibility of conventional techniques and materials has limited\nstudies in oxidative half-reactions, including the oxygen evolution\nreaction (OER), which provides the reducing equivalents for solar-to-fuels\nelectrolysis. With the straightforward parameters bulk pH, buffer\ncomposition and p<i>K</i><sub>a</sub>, and mass transport,\nwe develop a model for describing local pH as a function of current\ndensity regardless of the microscopic details of the mechanism. Using\nan acid-stable PbO<sub><i>x</i></sub> OER catalyst, we observe\nthe formation and dissipation of pH gradients during the OER and validate\nthe model with voltammetric and potentiometric studies. The model\npredicts how local acidic environments can develop over a narrow OER\ncurrent density window, thus providing further motivation for the\ndevelopment of OER catalysts that are stable to acid, even when operating\nin basic aqueous conditions. More generally, the model is not restricted\nto the OER and is useful for determining the onset of local pH gradients\nfor other electrocatalytic reactions that involve the consumption\nor generation of protons in energy conversion reactions.