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{\textquoteright}s micromorphology on the CO2R performance.
}, year = {2023}, booktitle = {Energy \& Environmental Science}, journal = {Energy \& Environmental Science}, series = {Energy \& Environmental Science}, month = {01/2023}, issn = {1754-5692}, url = {http://xlink.rsc.org/?DOI=D2EE02607D}, doi = {10.1039/D2EE02607D}, language = {eng}, }