High Efficiency Direct Electrochemical Conversion of CO2 to Liquid Fuel

In this paper, we will discuss a design of an electrochemical cell for CO₂ reduction implementing a proton exchange membrane (PEM) by adding a liquid buffer layer between the membrane and the cathode electrode. This configuration enables production of liquid fuels from CO₂ on Sn gas diffusion electrode at high efficiencies. Specifically, the formate was produced from the electrochemical reduction of CO₂ with an overpotential lower than 0.2 V. At such a low overpotential, the Faradaic efficiency and energetic efficiency of formate production with a buffer layer of 0.1 M KHCO₃ reaches 78.5% and 66.4%, respectively. Moreover, a maximum Faradaic efficiency of ~95% can be achieved. The production rate of formate increases with increasing cell potential to approximately 10 μmol min^-1 cm^-2 at a cell potential of -3.0 V. In contrast, hydrogen evolution reaction dominates at the cathode side in the PEM electrochemical cell without a buffer layer.

The enhancement of CO₂ reduction in the PEM electrochemical cell with a buffer layer is attributed to the optimization of H⁺ and CO₂ concentration at the reactions sites in the cathode. The effects of H⁺ concentration on the Faradaic efficiency were confirmed by the results of CO₂ reduction with a liquid buffer layer with different pH values. As a result, the Faradaic efficiency towards formation of formic acid can be further improved by optimizing the buffer layer thickness and subsequently tuning the H⁺ concentration at the cathode surface. The optimized thickness of the buffer layer was found ~ 3 mm. The buffer layer thickness dependency of H⁺ concentration at the cathode electrode will be reported.