Trilayer Polymer Electrolytes Enable Carbon-Efficient CO₂ to Multicarbon Product Conversion in Alkaline Electrolyzers

The electrochemical CO₂ reduction reaction (CO₂RR) is an appealing method for carbon utilization. Alkaline CO₂ electrolyzers exhibit high CO₂RR activity, low full-cell voltages, and cost-effectiveness. However, the issue of CO₂ loss caused by (bi)carbonate formation leads to excessive energy consumption, rendering the process economically impractical. In this study, we propose a trilayer polymer electrolyte (TPE) comprising a perforated anion exchange membrane (PAEM) and a bipolar membrane (BPM) to facilitate alkaline CO₂RR. This TPE enables the coexistence of high alkalinity near the catalyst surface and the H⁺ flux at the interface between the PAEM and the cation exchange layer (CEL) of the BPM, conditions favoring both CO₂ reduction to multicarbon products and (bi)carbonate removal in KOH-fed membrane electrode assembly (MEA) reactors. As a result, we achieve a Faradaic efficiency (FE) of approximately 46 % for C₂H₄, corresponding to a C₂₊ FE of 64 % at 260 mA cm^-2, with a CO₂-to-C₂H₄ single-pass conversion (SPC) of approximately 32 % at 140 mA cm^-2-nearly 1.3 times the limiting SPC in conventional AEM-MEA electrolyzers. Furthermore, coupling CO₂ reduction with formaldehyde oxidation reaction (FOR) in the TPE-MEA electrolyzer reduces the full-cell voltage to 2.3 V at 100 mA cm^-2 without compromising the C₂H₄ FE.