Sulfonated Silica Particles as Proton-Conductive Porous Solid Electrolytes for CO₂ Electrolysis

The carbon dioxide reduction reaction (CO₂RR) offers a promising route for converting CO₂ to valuable chemical feedstocks, addressing both environmental and industrial needs. However, liquid fuels and chemicals produced through the CO₂RR typically contain dissolved aqueous electrolytes, necessitating additional separation processes. Electrochemical CO₂RR reactors that utilize porous solid electrolytes (PSEs) can produce liquid fuels and chemicals free of aqueous electrolytes, but new materials capable of ionic conduction and with sufficient porosity for water flow are needed. In this work, we report silica-based particles that can be used as a PSE for CO₂ electrolysis. These particles were produced by grafting sulfonated silane ligands onto mesoporous silica particles. We investigated two different sulfonated ligands, 2-(4-chlorosulfonylphenyl)-ethyltrimethoxysilane and 3-(trimethoxysilyl)propane-1-sulfonic acid, and varied the particle size from 20 nm to 40 μm to elucidate their impact on ionic conductivity and performance in a continuous-flow CO₂ electrolyzer. The ionic conductivity of the particles improved with decreasing particle size, and the most conductive particles achieved ionic conductivities as high as 5.31 × 10^-2 S cm^-1. We also found a trade-off between flow stability and particle size, with smaller particles being more susceptible to dissolution and dispersion in water, resulting in clogging of the flow reactor. Incorporation of silica PSEs in a CO₂ electrolyzer produced a Faradaic efficiency greater than 90% toward formic acid. This work demonstrates a novel approach to porous solid-state electrolytes with a wide array of applications, including, but not limited to, CO₂ electrolysis.