Gas-fed liquid-covered electrodes used for electrochemical reduction of dilute CO2 in a flue gas

Direct electrochemical reduction of dilute CO2 in a flue gas eliminates the energy-consuming preprocesses of capture and condensation for preparing 100% CO2 gas, and hence could decrease the total energy consumption for utilization of emitted CO2. However, it raises two major issues: (i) insufficient supply rate of CO2 to the cathode electrode and (ii) preferential reduction of O2; both lower the Faradaic efficiency of the target reaction of CO2 reduction. To solve these issues, we propose a new concept of gas-fed liquid-covered electrodes (GFLCEs). The reaction gas is fed to the catalyst layer across a thin liquid cover layer formed by a combination of hydrophilic and hydrophobic porous films. Owing to an extremely lower solubility of O2 in an aqueous electrolyte than that of CO2, the O2 concentration in the catalyst layer is sufficiently decreased while securing high CO2 concentration. In the GFLCE, the catalyst is entirely immersed in the electrolyte contrasting to those of conventional gas-diffusion electrodes. Therefore, a CO2 capture medium including monoethanolamine (MEA) added in the electrolyte increases the effective CO2 concentration. We applied the GFLCE concept with MEA to a Ru-complex-based electrocatalyst for formate production, and achieved high Faradaic efficiencies of around 70% under direct feeding of a simulated flue gas of 15% CO2 + 4% O2 + 81% N2 (v/v). The present concept proved here with the MEA additive is a versatile means applicable to existing electrocatalysts. [Display omitted] •A Gas-fed liquid-covered electrode (GFLCE) reduces dilute CO2 in a directly fed O2-containing flue gas.•The GFLCE consists of a catalyst layer and hydrophilic/hydrophobic porous films.•These films block O2 transfer to the catalyst layer while securing high CO2 concentration.•The GFLCE is compatible with CO2-capturing electrolytes and applicable to existing electrocatalysts.•The GFLCE can eliminate energy-consuming preprocesses of capture and condensation for preparing 100% CO2.