Breaking the current limitation of electrochemical CO2 reduction via a silica-hydroxide cycle

Abstract

Alkaline local pH during a vigorous electrochemical CO2 reduction reaction (CO2RR) can improve the activity and selectivity of CO2RR. However, it also leads to an alkalinity problem in that hydroxide ions obstruct the mass transfer of CO2 to the active site, thereby limiting the current density. In this study, we introduce a silica-hydroxide cycle, which moderates the local pH by redistributing hydroxide ions, analogous to the carbonate-silicate cycle responsible for the drawdown of atmospheric CO2 on Earth. In the membrane electrode assembly (MEA) of a CO2 electrolyzer, SiO2 undergoes weathering due to the high local pH and consequently consumes OH-, reducing the pH within the MEA. The dissolved silicate ions move to the membrane and are almost regenerated to SiO2 with release of OH-. Geological and spectral observations suggest that the silica-hydroxide cycle reduces the local pH thereby enhancing mass transfer of CO2, breaking the limitation of current density for CO2RR. Our work proposes new chemical approaches to increase current density, mainly improved by physical methods, and contributes valuable insight for improving a variety of electrochemical systems.