Direct observation of the local microenvironment in inhomogeneous CO 2 reduction gas diffusion electrodes via versatile pOH imaging

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Journal Article

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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.


Energy & Environmental Science

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