Suppression of Salt Precipitation in Membrane Electrode Assembly Electrolyzers for CO2 reduction reactions by Intermittent Supply of Alkali Metal Cations

The electrolysis of CO2 into value-added products holds great potential for closing the carbon cycle. A membrane electrode assembly (MEA) that uses solid polymer electrolytes is a critical technology for enhancing the energy efficiency for the CO2 reduction reaction (CO2RR) because a thin electrolyte layer can effectively reduce the ohmic resistance. The stable operation of the CO2RR in MEA-based cells with an anion-exchange membrane (AEM), however, is hindered by the accumulation of bicarbonate salts, which are derived from alkali-metal cations in anolytes, on the cathode side. Therefore, for stable operation of CO2 electrolysis using an MEA-based cell, the amount of alkali-metal cations needs to be reduced. However, the minimal amount of alkali-metal cations required to operate the reaction system has not been elucidated. Quantitative analysis of the amount of alkali-metal cations transported from the anode to the cathode is essential to obtain the insight into the necessary amount for the operation. In this study, we conducted a quantitative evaluation of the correlation between the production of high-value C2+ compounds, such as ethylene and ethanol, and the transported alkali-metal cations. 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+. This result suggests a continuous supply of high amount of K+ to the cathode surface is not required for C2+ formation. On the basis of 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) in an MEA-based cell using AEM and Cu nanoparticles as the cathode catalyst. These values of FE and partial current density were almost identical to those when 0.1 M KHCO3 was continuously supplied as the anode solution. Based on this insight, we successfully improved the durability of the system against salt precipitation. The FE for ethylene remained constant for as long as 60 h at 150 mA cm−2 when continuously flowing 0.1 M KHCO3 as anolyte. However, the FE suddenly decreased after 60 h due to the cessation of CO2 gas flow, caused by salt precipitation in the cathode flow channel. On the contrary, by intermittently supplying 0.1 M KHCO3, we confirmed that the amount of visible K+ salt in the cathode flow field remained negligible even after more than 90 hours of stable operation.