The Olympic challenge of reverse-bias bipolar membranes for CO2 electrolysis

Bipolar membranes operated under reverse-bias (r-BPM) provide the only potential route to use PGM-free anodes in CO ₂ electrolyzers when paired with the oxygen evolution reaction (OER). Under 100% water dissociation efficiency (WDE) conditions, the OH ^- generated by a r-BPM fully replenishes the OH ^- consumed by the OER, maintaining an alkaline anolyte. However, unwanted co-ion crossover leads to <100% WDEs, gradually causing anolyte acidification and nickel-based anodes to corrode over time. Here we experimentally measure the WDE of r-BPMs in a membrane-electrode assembly configuration as a function of current density, anolyte concentration and cation identity, finding that the highest measured WDE of 98% is insufficient to maintain an alkaline environment over extended operation. We further highlight through modelling that WDEs >99.8% are required to operate for >10,000 hours with reasonable anolyte volumes. Our results show that r-BPMs CO ₂ electrolyzers require additional strategies, such as reverting to PGM anodes or regenerating anolyte, to operate stably at an industrial scale.