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

Bipolar membranes in electrochemical CO conversion cells enable different reaction environments in the CO-reduction and O-evolution compartments. Under ideal conditions, water-splitting in the bipolar membrane allows for platinum-group-metal-free anode materials and high CO 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 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 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 to the catalyst surface, improving Faradaic efficiency and limiting current densities by twofold. Using these insights, we demonstrate a fully platinum-group-metal-free bipolar membrane electrode assembly CO conversion system exhibiting <1% CO/cation crossover rates and 80-90% CO-to-CO utilization efficiency over 150 h operation at 100 mA cm without anolyte replenishment.