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.