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

Alkaline local pH during vigorous electrochemical CO₂ reduction reaction (CO₂RR) can improve the activity and selectivity of CO₂RR. However, it also leads an alkalinity problem that hydroxide ion obstructs the mass transfer of CO₂ to the active site, thereby limiting the current density. In this study, we introduce the silica-hydroxide cycle, which moderates the local pH by redistributing hydroxide ion, analogous to carbonate-silicate cycle responsible for the drawdown of atmospheric CO₂ on Earth. In the membrane electrode assembly (MEA) of CO₂ electrolyzer, SiO₂ undergoes weathering due to the high local pH and consequently consumes OH^–, reducing the pH within the MEA. The dissolved silicate ion moves to membrane and are almost regenerated to SiO₂ with releasing OH^–. Geological and spectral observations suggest that the silica-hydroxide cycle draw down the local pH thereby enhancing mass transfer of CO₂, breaking 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 variety of electrochemical systems.