Quantitative Analysis and Manipulation of Alkali Metal Cations at the Cathode Surface in Membrane Electrode Assembly...

The stable operation of the CO₂ reduction reaction (CO₂RR) 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 CO₂RR 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 C₂₊ compounds, the rate of K⁺ ion transport did not match the selectivity of C₂₊, suggesting that a continuous supply of high amount of K⁺ to the cathode surface is not required for C₂₊ formation. Based on these findings, we achieved a faradaic efficiency (FE) and a partial current density for C₂₊ of 77 % and 230 mA cm^-2, respectively, even after switching the anode solution from 0.1 M KHCO₃ to a dilute K⁺ solution (<7 mM). These values were almost identical to those when 0.1 M KHCO₃ was continuously supplied. Based on this insight, we successfully improved the durability of the system against salt precipitation by intermittently supplying concentrated KHCO₃, compared with the continuous supply.