CO residence time modulates multi-carbon formation rates in a zero-gap Cu based CO2 electrolyzer

Abstract Carbon dioxide (CO 2 ) electrolysis on copper (Cu) catalysts has attracted interest for 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 C 2+ selectivity of > 80 % at 200 mA cm ‑2 over broad conversion conditions. The importance of CO pooling is demonstrated through residence time distribution curves, varying flow fields (serpentine/parallel/interdigitated), and flow rate. 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 reach similar selectivity over broad flow rates. Critically, we show that parts of the catalyst area are predominantly reducing 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.