Mass Transport-Dependent C–C Bond Formation for CO Electroreduction with Alkali Cations

Electrolyte cation identity has been reported to influence the multicarbon (C 2+ ) selectivity in CO 2 /CO electroreduction. However, most of the previous work for cation size effect is based on H-cell configurations, which may inadvertently distort the underlying mechanism of cation effect due to mass transport limitations, particularly for CO reduction. Here, using GDE-type flow electrolyzers, we report that the selectivity of total C 2+ products on Cu is independent of alkali cation identity (Li +, Na +, K +, and Cs + ) in the absence of the CO transport limitation. Notably, a high concentration of strongly hydrated cation (such as Li + ) inhibits the total C 2+ formation in CO reduction, whereas total C 2+ selectivity is retained upon increasing concentrations of weakly hydrated cation (such as K + ). Further investigations reveal that the CO coverage at a low cation concentration is almost independent of the cation identity, but the CO coverage at highly concentrated cations strongly relies on the alkali cation identity.