Electrochemical CO₂ reduction to methyl formate in methanol was demonstrated in a flow cell electrolyzer. A two-membrane configuration was used to prevent flooding of the non-aqueous solvent at the cathode, which was severely limiting during single-membrane operation. The effect of water content in the methanol was investigated, highlighting a tradeoff between product selectivity and current density.

Abstract

Recently, tandem cathodic reactions have been demonstrated in non-aqueous solvents to couple CO₂ reduction to a secondary reaction to create novel species that are not produced in aqueous CO₂ electrolysis. One reaction that can be performed with high selectivity and durability is the electrochemical conversion of CO₂ to formic acid and in-situ esterification with methanol to produce methyl formate. However, the translation to a high-performance flow electrolyzer is far from trivial, as the non-aqueous catholyte leads to reactor challenges including flooding the gas diffusion electrode. Here, a two-membrane flow electrolyzer with both anion and cation exchange membranes was used with flowing methanol catholyte and aqueous anolyte. This design prevented methanol from flooding the cathode, which was a pervasive limiting issue for electrolyzers with a single membrane. Methyl formate production at 42.9 % faradaic efficiency was achieved with pure methanol in a flow electrolyzer with stable performance beyond 80 min. However, low-water-content catholyte compositions also led to increased cell resistance and lower operating current densities. Thus, with the present ionomer materials there is a tradeoff between methyl formate selectivity and current density depending on water concentration, highlighting a need for new ionomers tailored for desirable non-aqueous solvents such as methanol.