The conversion of CO2 to chemical fuels provides an attractive option to reduce greenhouse gases and store chemical energy for future uses. Intense efforts have been focused on the development of heterogeneous and homogeneous catalysts for the CO2 reduction reaction. However, relatively little research has been devoted to the system-level design of CO2 electrolyzers, which should be the next step in developing CO2 conversion technologies. We present here the electrolysis of CO2 to syngas (CO + H2) using non-precious metal electrocatalysts in bipolar membrane-based electrochemical cells. Electrolysis was carried out using aqueous bicarbonate and humidified gaseous CO2 on the cathode side of the cell, with Ag and Bi/ionic liquid cathode electrocatalysts, respectively. In both cases, stable currents were observed over a period of hours with an aqueous alkaline electrolyte and NiFeOx electrocatalyst on the anode side of the cell. In agreement with earlier reports, the Faradaic efficiency for CO2 reduction to CO was high at low overpotential, and in the liquid phase bipolar membrane cell remained stable at 50% at 30 mA/cm2 current density. In contrast, the performance of the cells degraded rapidly when conventional anion- and cation-exchange membranes were used in place of the bipolar membrane. In the gas-phase cell, current densities up to 200 mA/cm2 could be obtained, albeit at low Faradaic efficiency for CO production. At low overpotentials in the gas-phase cathode cell, the Faradaic efficiency for CO production was initially high but dropped within one hour, most likely due to dewetting of the ionic liquid from the Bi catalyst surface. The gas-fed CO2 electrolyzer design bypasses the low CO2 solubility problem in liquid electrolyzers and provides easier extraction of reaction products downstream. The effective management of protons in the bipolar membrane cells enables stable operation and the possibility of practical CO2 electrolysis at high current densities.

Electrolysis of CO2 to syngas in bipolar membrane-based electrochemical cells

253rd National Meeting (2017)

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