Bipolar Membranes Inhibit Product Crossover in CO2 Electrolysis Cells

As electrocatalysts and electrolyzer designs for CO2 reduction continue to improve in terms of current density and product selectivity, product crossover from the cathode to the anode is a loss mechanism that is relatively unexplored. The crossover rates of formate, methanol, and ethanol, which are desirable CO2 reduction products, are compared in electrolyzers containing anion-exchange membranes and bipolar membranes. The crossover of formate, an anionic CO2 reduction product, occurs by electromigration through anion-exchange membranes, and its rate increases linearly with current density. Crossover of electroneutral methanol or ethanol through anion-exchange membranes occurs to a lesser extent through both diffusion and electroosmotic drag, the latter increasing with current density in anion-exchange membranes. In contrast, the outward fluxes of protons and hydroxide ions generated in bipolar membranes inhibit the crossover of both anionic and neutral products, even with membranes that contain high surface area junctions. Calculated electroosmotic drag coefficients for each of the neutral products confirm the better performance of bipolar membranes in terms of product losses.