Quantitative Analysis and Manipulation of Alkali Metal Cations at the Cathode Surface in Membrane Electrode Assembly Electrolyzers for CO 2 Reduction Reactions.

The stable operation of the CO 2 reduction reaction (CO 2 RR) in membrane electrode assembly (MEA) electrolyzers is known to be hindered by the accumulation of bicarbonate salt, which are derived from alkali metal cations in anolytes, on the cathode side. In this study, we conducted a quantitative evaluation of the correlation between the CO 2 RR activity and the transported alkali metal cations in MEA electrolyzers. As a result, although the presence of transported alkali metal cations on the cathode surface significantly contributes to the generation of C 2+ compounds, the rate of K + ion transport did not match the selectivity of C 2+ , suggesting that a continuous supply of high amount of K + to the cathode surface is not required for C 2+ formation. Based on these findings, we achieved a faradaic efficiency (FE) and a partial current density for C 2+ of 77 % and 230 mA cm -2 , respectively, even after switching the anode solution from 0.1 M KHCO 3 to a dilute K + solution (<7 mM). These values were almost identical to those when 0.1 M KHCO 3 was continuously supplied. Based on this insight, we successfully improved the durability of the system against salt precipitation by intermittently supplying concentrated KHCO 3 , compared with the continuous supply.