Open Circuit Potential Decay Transients Quantify Non-Equilibrium Local pH During Electrocatalysis

Many key energy conversion reactions are proton-coupled electron transfer (PCET) reactions that consume or generate protons at electrode surfaces. Thus, catalytic turnover can generate non-equilibrium local pH environments at the surface that differ substantially from that of the bulk. Quantitative insight into the magnitude of this interfacial pH swing is a prerequisite for understanding and designing efficient systems for energy conversion, but is difficult to measure, particularly under high current density operation; with complex gas diffusion electrodes (GDEs); and with membrane-decorated surfaces employed in functional devices. Herein, we develop and validate a general methodology for experimentally quantifying interfacial pH swings using open circuit potential (OCP) decay transients. Using this method, we quantify the impact of buffer strength, supporting electrolyte composition, and the presence of cation exchange polymer overlayers on the polarization-induced pH swing on Pt GDEs. We find that modest current densities of −30 mA cm−2 are sufficient to sustain pH swings of > 2 pH units, even for strongly buffered solutions. Meanwhile, the addition of alkali supporting electrolyte to unbuffered, acidic electrolyte can induce pH swings so large that the polarized electrode environment becomes strongly alkaline. The presence of a Nafion polymer overlayer containing fixed anionic charges serves to further augment the interfacial pH swing, resulting in a similar pH swing at half the applied current density. The transport characteristics of these systems were analytically modelled, enabling direct calculation of boundary layer thickness and quantitative prediction of the OCP decay transient. These studies establish methods for quantifying local pH swings and highlight the dramatic variation in local pH relative to the bulk under many electrolyte conditions.