The Effect of Water Management and Cell Compression in Bipolar Membrane CO ₂ Electrolysis to CO

ABSTRACT In the electrochemical reduction of CO ₂ , it is still unclear which membrane type will prevail in the long run. Major drawbacks of state‐of‐the‐art zero‐gap CO ₂ electrolyzers using anion exchange membranes are carbonate crossover and salt precipitation. Bipolar membranes in forward bias mode promise to solve those challenges but suffer from higher cell potential, as well as lower operational current density and durability. This study therefore employs in‐operando neutron imaging to elucidate the mechanisms of water management and cell compression for bipolar membrane CO ₂ electrolyzers with porous anion exchange layers. The measurements reveal increasing humidification of the cathode with increasing current density, which leads to rapid gas diffusion electrode flooding due to water formation at the membrane interface. Considering these findings, the water management was improved by optimizing the inlet gas humidification (to 0% relative humidity) and the porosity of the anion exchange membrane layer (to 0.8%). Furthermore, the neutron images show that the gas diffusion electrode insufficiently supports the bipolar membrane, leading to membrane delamination. By introducing a rigid cathodic porous transport layer, higher and more homogenous cell compression was achieved, increasing the Faradaic efficiency for CO to 97% and the maximum CO partial current density to 241 mA cm ⁻² .