CO residence time modulates multi-carbon formation rates in a zero-gap Cu based CO electrolyzer

Carbon dioxide (CO 2 ) electrolysis on copper (Cu) catalysts has attracted interest due to its direct production of C 2+ feedstocks. Using the knowledge that CO 2 reduction on copper is primarily a tandem reaction of CO 2 to CO and CO to C 2+ products, we show that modulating CO concentrations within the liquid catalyst layer allows for a C 2+ selectivity of >80% at 200 mA cm −2 under broad conversion conditions. The importance of CO pooling is demonstrated through residence time distribution curves, varying flow fields (serpentine/parallel/interdigitated), and flow rates. While serpentine flow fields require high conversions to limit CO selectivity and maximize C 2+ selectivity, the longer CO residence times of parallel flow fields achieve similar selectivity over broad flow rates. Critically, we show that parts of the catalyst area predominantly reduce CO instead of CO 2 as supported by CO reduction experiments, transport modelling, and achieving a CO 2 utilization efficiency greater than the theoretical limit of 25% for C 2+ products. This study shows how flow field designs can be used to modulate CO concentrations and hydrocarbon production rates in a zero gap copper based carbon dioxide electrolyzer.