Two-Membrane Dual Non-Aqueous/Aqueous Electrolyte Flow Cell Operation for Electrochemical Conversion of CO2 to Methyl Formate.

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.