Efficient Bicarbonate Electrolysis to Formate Enabled via Ionomer Surface Modification in Cation Exchange Membrane Electrolyzers

Electrochemical CO₂ reduction (CO₂RR) is a promising method for converting CO₂ into valuable chemicals, with formate being a particularly viable product. However, current gas-fed CO₂RR systems rely on highly pure CO₂ feed gases and are incompatible with point-source CO₂ emissions without prior capture and concentration. Bicarbonate electrolysis offers a potential solution by bridging the gap between CO₂ emissions and utilization. However, existing electrolyzer configurations, especially those using bipolar membranes (BPM), require high working voltages and suffer from poor energy efficiency. Here, we present a cation exchange membrane (CEM)-based membrane electrode assembly (MEA) incorporating a surface-modified bismuth cathode catalyst. The success of this approach is attributed to two key factors: the use of the positively charged ionomer PiperION for surface modification, which creates a favorable cathode microenvironment; the single CEM that enhances proton flux from the anode to the cathode while reducing ionic impedance. The CEM-based MEA demonstrates a formate faradaic efficiency of up to 80%, with a significant 1.5 V reduction in operating voltage compared to BPM-based MEAs at 300 mA cm^-2. Additionally, the CEM-based MEA exhibits excellent tolerance to O₂ impurities and maintains high performance even with simulated flue gas, making it suitable for direct CO₂ utilization from point sources.