Direct observation of the local microenvironment in inhomogeneous CO₂ reduction gas diffusion electrodes via versatile pOH imaging

We report how the micrometer-scale morphology of a carbon dioxide reduction (CO₂R) gas diffusion electrode (GDE) affects the mass transport properties and with it, the local CO₂R performance. We developed a technique to probe the microenvironment in a CO₂R GDE via local pOH imaging with time- and three-dimensional spatial, micrometer-scale resolution. The local activity of hydroxide anions (OH⁻), represented by the pOH value, around a GDE in contact with an aqueous electrolyte is a crucial parameter that governs the catalytic activity and CO₂R selectivity. Here, we use fluorescence confocal laser scanning microscopy (CLSM) to create maps of the local pOH around a copper GDE by combining two ratiometric fluorescent dyes, one of which is demonstrated as a pOH sensor for the first time in this work. We observe that the local pOH decreases when current is applied due to the creation of OH⁻ as a byproduct of CO₂R. Interestingly, the pOH is lower inside microtrenches compared to the electrode surface and decreases further as trenches become more narrow due to enhanced trapping of OH⁻. We support our experimental results with multiphysics simulations that correlate exceptionally well with measurements. These simulations additionally suggest that the decreased pOH inside microcavities in the surface of a CO₂R GDE leads to locally enhanced selectivity towards multicarbon (C₂₊) products. This study suggests that narrow microstructures on the length scale of 5 μm in a GDE surface serve as local CO₂R hotspots, and thus highlights the importance of a GDE's micromorphology on the CO₂R performance.