Operando pH Measurements of OER within a Porous Oxygen-Evolving Electrode

Three-dimensional porous electrodes, such as those in membrane electrode assemblies, are widely employed in electrochemical devices due to their high surface area and achievable current density. The heterogeneous internal structure of the porous electrode microenvironment engenders local concentration gradients owing to mass transport constraints. Localized microenvironments with properties distinct from those of the bulk solution are especially prevalent for electrocatalytic reactions that produce or consume protons, as local basic or acidic environments may significantly influence the energy efficiency and selectivity of proton-coupled electron transfer reactions that are necessary for small-molecule activation. Here, we develop an operando method employing polymer-bound fluorescent sensors (PFS) to directly quantify local pH and the internal proton gradient in a porous electrode for the oxygen evolution reaction catalyzed by IrOx supported on indium-tin oxide (ITO). The PFS uncovers notable pH gradients within a 3-μm-thick layer of the OER electrode in unbuffered electrolyte, even at low current density. Proton transport dynamics were found to be influenced significantly by the proton adsorption properties of the ITO support, revealing a previously unrecognized role of the oxide support in modifying the local pH gradient. The PFS method should prove useful in the study and evaluation of electrochemical devices, including fuel cells, bipolar and other ion-conducting membranes, and electrolyzers.