Local ionic transport enables selective PGM-free bipolar membrane electrode assembly

Abstract Bipolar membranes in electrochemical CO 2 conversion cells enable different reaction environments in the CO 2 -reduction and oxygen-evolution compartments. Under ideal conditions, water-splitting in the bipolar membrane allows for platinum-group-metal-free anode materials and high CO 2 utilizations. In practice, however, even minor unwanted ion crossover limits stability to short time periods. Here we report the vital role of managing ionic species to improve CO 2 conversion efficiency while preventing acidification of the anodic compartment. Through transport modelling, we identify that an anion-exchange ionomer in the catalyst layer improves local bicarbonate availability and increasing the proton transference number in the bipolar membranes increases CO 2 regeneration and limits K + concentration in the cathode region. Through experiments, we show that a uniform local distribution of bicarbonate ions increases the accessibility of reverted CO 2 to the catalyst surface, improving Faradaic efficiency and limiting current densities by twofold. Using these insights, we demonstrate a fully PGM-free bipolar membrane electrode assembly CO 2 conversion system exhibiting < 1% CO 2 /cation crossover rates and 80–90% CO 2 -to-CO utilization efficiency over 150 h operation at 100 mA cm − 2 without anolyte replenishment.