(706f) Potentiometric Quantification of Interfacial pH Swings during High Current Density Electrocatalysis

The local proton activity at electrode-solution interfaces plays a major role in determining efficiencies of technologically relevant electrocatalytic transformations, including hydrogen evolution/oxidation, oxygen evolution/reduction, and carbon dioxide reduction. Since each of these electrochemical reactions necessarily consumes or produces protons, the interfacial proton activity is commonly profoundly different, compared with the bulk solution during active electrolysis. Understanding the underlying factors that influence polarization-induced interfacial pH swing is therefore essential for designing next-generation electrolyzers. To this end, we develop a technique that measures interfacial pH swing on complex gas diffusion electrodes and is uniquely amenable to high current conditions relevant in functional electrolyzer devices.

Specifically, we quantify polarization-induced interfacial pH swings by measuring the open circuit potential (OCP) set by the H₂/H⁺ equilibrium on Pt immediately after the termination of electrolysis. Since interfacial pH gradients result from the transport characteristic of the interface, not the catalyst itself, we envision that these studies, which require the use of a Pt catalyst, can be used to derive broader insight into how the electrolyte composition, current density, and ionomers influence the interfacial pH. We first validate that the post-electrolysis OCP decay transient method can capture the anticipated effects of current density, buffering capacity, and transport to report directly on the interfacial pH swing experienced by a mesostructured Pt GDE across a range of current densities from −10 to −100 mA cm⁻². We then apply this method to quantify how the incorporation of (1) alkali cations in the bulk electrolyte and (2) ion-exchange membrane overlayers at the interface serves to modulate the interfacial microenvironment and how it affects polarization-induced pH swings. Experimentally measured interfacial pH swing agree quantitatively with a zero-parameter analytical transport model which predicts and quantifies the thickness of the diffusion boundary layer during catalysis with quiescent, stirred, and flowing electrolytes.