Gas-Induced Structural Damages in Forward-Bias Bipolar Membrane CO₂ Electrolysis Studied by Fast X-ray Tomography

Forward-bias bipolar membrane (BPM) CO₂ coelectrolysis (CO2ELY) aims at overcoming the issues of salt precipitation and CO₂ crossover in anion exchange membrane CO2ELY. Increasing the stability of BPM-CO2ELY is crucial for widespread application of the technique. In this study, we employ time-resolved X-ray tomographic microscopy to elucidate the structural dynamics that occur within the electrochemical cell during operation under various conditions. Using advanced image processing methods, including custom 4D machine learning segmentation, we can visualize and quantify damages in the membrane and anode catalyst layer (CL). We compare our results to a CO₂ transport model and hypothesize gaseous CO₂ as the cause of the observed damages. At any operation condition, CO₂ is formed at the junction in the center of the BPM by recombination of carbonate ions. CO₂ migrates to the anode by diffusion and goes into the gas phase at the interface of the membrane and anode CL. After sufficient CO₂ accumulation and pressure buildup after only tens of minutes, small irreversible holes break into the CL distributed over the entire active area. Additionally, at higher current densities, the CO₂ accumulation leads to membrane delamination at the BPM junction. Despite the clear degradation processes, we do not observe an obvious direct effect on the electrochemical performance. The poor stability of BPM-CO2ELY remains an open question.