Water dissociation efficiencies control the viability of reverse-bias bipolar membranes for CO₂ electrolysis

Bipolar membranes operated under reverse-bias (r-BPM) provide the only potential route to use anodes free of platinum group metals in CO₂ electrolyzers when paired with the oxygen evolution reaction. Under 100% water dissociation efficiency (WDE) conditions, the OH^- generated by a r-BPM fully replenishes the OH^- consumed by the oxygen evolution reaction, 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 measured the WDE of r-BPMs in a membrane-electrode assembly configuration as a function of the 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 modeling that WDEs >99.8% are required to operate for >10,000 h with reasonable anolyte volumes. Our results show that r-BPMs CO₂ electrolyzers require additional strategies, such as reverting to platinum group metal anodes or regenerating the anolyte, to operate stably at an industrial scale.