Quantitative Analysis and Manipulation of Alkali Metal Cations at the Cathode Surface in Membrane Electrode Assembly Electrolyzers for CO2 Reduction Reactions
Authors: Kato Shintaro , Ito Shotaro , Nakahata Shoko , Kurihara Ryo , Harada Takashi , Nakanishi Shuji , Kamiya Kazuhide 
Published: December 2024

The electrochemical CO2 reduction reaction (CO2RR) has garnered significant attention due to its potential for achieving high CO2 conversion rates under ambient temperature and pressure conditions.[1-3] For practical applications of CO2RR, improving energy efficiency, enhancing selectivity toward high-value products, and increasing production rates are critical. In particular, the production of multi-carbon compounds (C2+, such as ethylene, ethanol, acetic acid, and n-propanol), with high current density and low energy consumption, is of paramount importance.
Recently, we successfully increased the partial current density for CO2 electroreduction to C2+ products to a record value of 1.7 A cm−2 using gas diffusion electrodes (GDEs) carrying Cu nanoparticles.[4,5]. However, this study employed liquid electrolytes as the catholyte, which led to an ohmic drop in the electrolyte, resulting in decreased energy efficiency, particularly under high-current electrolysis conditions. To address this issue, researchers have begun utilizing membrane electrode assembly (MEA) cells with solid polymer electrolytes for CO2RR.[6] In previous studies, anion-exchange membranes (AEMs), Cu-based catalysts loaded on GDEs, and concentrated alkaline solutions, such as 1 M KOH or 0.1 M KHCO3, were used as electrolytes, cathodes, and anolytes, respectively; these configurations have become standard for MEA-based CO2RR systems. However, alkali metal cations are transported from the anode to the cathode through the AEM, leading to the formation of bicarbonate salts at the cathode, which disrupts stable CO2RR operation. Therefore, to achieve stable CO2 electrolysis in an MEA-based cell, the amount of alkali metal cations must be reduced or eliminated. However, the quantity of alkali metal cations transported from the anode to the cathode has yet to be quantitatively analyzed.
In this study, we quantify the transfer of alkali metal cations from the anode to the cathode and investigate their role in C2+ compound formation, determining the optimal concentration of alkali metal cations required for stable operation.[7] While the presence of alkali metal cations at the cathode surface significantly contributes to C2+ production, the transport rate of K+ ions did not correlate with C2+ selectivity, suggesting that a continuous, high supply of K+ to the cathode surface is not essential for C2+ formation (Figure). Based on these findings, we achieved a faradaic efficiency (FE) and partial current density for C2+ products of 77% and 230 mA cm−2, respectively, even after switching the anode solution from 0.1 M KHCO3 to a diluted K+ solution (<7 mM). These results were nearly identical to those obtained with continuous 0.1 M KHCO3 supply. Furthermore, by intermittently supplying concentrated KHCO3, we significantly improved system durability against salt precipitation compared to continuous supply.
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Figure 1