Modeling the Copper Form Cathode of a Bicarbonate CO2 Electrolyzer for Methane Conversion

Abstract Bicarbonate electrolyzers are devices that convert CO2 released in situ from bicarbonate ions into chemicals and fuels without requiring an external source of CO2 gas. Among the CO2-derived chemicals and fuels, methane is an appealing target due to its high heating value (802 kJ/mol CO2). A one-dimensional, steady-state, isothermal multiphysics model has been developed for a copper foam-based cathode electrode of a bicarbonate CO2 electrolyzer aimed at methane production. This model considers species transport due to convection, diffusion, and migration and integrates the catalyzed water-splitting reaction at the interface between the anion exchange layer and the cation exchange layer of the bipolar membrane used in the electrolyzer. The simulated polarization curve and Faradaic efficiencies of methane, hydrogen, and formate are compared with published testing data. The effects of cathode design parameters on the Faradaic efficiencies of hydrogen, methane, and formate production are examined. Simulation results reveal that the Faradaic efficiency for hydrogen production improves with an increase in pore radius, interfacial surface area, and the thickness of the copper foam cathode catalyst layer. Conversely, the Faradaic efficiency for methane production benefits from a smaller pore radius, reduced interfacial area, a thinner cathode catalyst layer or cation exchange membrane layer, and a larger cathode flowrate. For instance, at a current density of 200 mA/cm2, the Faradaic efficiency of methane increases from 16.4% to 18.5% as the pore radius in the cathode catalyst layer decreases from 5 µm to 1 µm. Similar improvements are observed when the interfacial surface area drops from 12 × 104 m−1 to 4 × 104 m−1, the thickness of the cathode catalyst layer decreases from 300 µm to 200 µm, and the thickness of the cation exchange layer reduces from 100 µm to 50 µm. In these cases, the Faradaic efficiencies for methane increase from 11.1% to 16.4%, from 15.7% to 17.6%, and from 16.4% to 17.5%, respectively. Increasing the cathode flowrate from 50 ml/min to 110 ml/min slightly increases methane Faradaic efficiency from 16.39% to 16.52%. The simulation further indicates that contact resistance in the cathode does not impact Faradaic efficiencies; instead, it affects the polarization curve.