Understanding the Effects of Electrolyzer Geometry and Flow Conditions on Methanol Formation via Gas-Fed Electrochemical CO₂ Reduction

Electrochemical reduction of CO2 to methanol is a promising approach for renewable energy storage in chemical bonds. Immobilized cobalt phthalocyanine (CoPc) is capable of converting carbon dioxide to methanol via a cascade catalysis mechanism involving CO as the intermediate. However, weak binding of CO to CoPc leads to a low single-pass efficiency in CO2 flow electrolyzers with gas diffusion electrodes. We show that by controlling the relative concentrations of CO2 and CO near CoPc, we can enhance methanol production in flow electrolyzers. By adjusting the gas flow rate and the CO2 partial pressure, we achieve methanol partial current densities of 20 mA/cm2 at a large Faradaic efficiency of 46%. Our 3D multiphysics model predictions uniquely showcase that the factors that control methanol activity and selectivity change as a function of flow rate. While the ratio of local CO to CO2 concentration shows strong positive correlations with methanol production for gas flow rates larger than 5 mL/min, we hypothesize methanol production is controlled by the local pH for the smaller flow rates (<2 mL/min). Overall, our integrated computational–experimental findings provide valuable insights into the effects of design and operating conditions of CO2 flow electrolyzers with CoPc catalysts for producing drop-in methanol fuel.