High Efficiency Direct Electrochemical Conversion of CO2 to Liquid Fuel

Innovations of Green Process Engineering for Sustainable Energy and Environment
AIChE Annual Meeting
October 30, 2012 - 5:05pm-5:25pm
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+ concentrationon 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.

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