Electrolysis of CO₂ to Syngas in Bipolar Membrane-Based Electrochemical Cells

The electrolysis of CO₂ to syngas (CO + H₂) using nonprecious metal electrocatalysts was studied 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 or Bi/ionic liquid cathode electrocatalysts. In both cases, stable currents were observed over a period of hours with an aqueous alkaline electrolyte and NiFeO_x electrocatalyst on the anode side of the cell. 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 agreement with earlier reports, the Faradaic efficiency for CO₂ reduction to CO was high at low overpotential. In the liquid-phase bipolar membrane cell, the Faradaic efficiency was stable at about 50% at 30 mA/cm² current density. In the gas-phase cell, current densities up to 200 mA/cm² could be obtained, albeit at lower 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 1 h, most likely because of dewetting of the ionic liquid from the Bi catalyst surface. The effective management of protons in bipolar membrane cells enables stable operation and the possibility of practical CO₂ electrolysis at high current densities.