Impact of Cathode Components’ Configuration on the Performance of Forward-Bias Bipolar Membrane CO₂-Electrolyzers

Zero-gap CO₂-electrolyzers using a forward bias bipolar membrane (BPM) are becoming increasingly appealing, since this configuration addresses the issues of CO₂ pumping and salt precipitation observed with other approaches. However, such CO₂-electrolyzers often suffer from BPM-delamination caused by the generation of water and gaseous CO₂ at the junction between cation- and anion-exchange membranes. To circumvent this, in this study we used a rigid titanium porous transport layer (PTL) at the cathode to mechanically suppress the membrane delamination and managed to operate such cells at current densities >100 mA·cm^-2. In doing so, we compared the performance differences caused by the implementation of a catalyst-coated membrane (CCM) or a gas diffusion electrode (GDE) at the cell's cathode. These combinations of diffusion media and catalyst layer (CL) deposition approaches result in five different configurations that systematically featured a current-driven rise in high-frequency resistance (HFR) and CO selectivity when operated at current densities <100 mA·cm^-2, whereas at current densities >100 mA·cm^-2, both HFR and CO selectivity decreased. By determining the water balance at the cathode compartment and BPM-junction, we propose that variations in membrane-CL humidification are tied to this unambiguous correlation between HFR and selectivity across all tested configurations, which we attribute to the concomitant changes in water and ion distribution (and thus pH) along this key operational interface.