Pure-Water-Fed Forward-Bias Bipolar Membrane CO₂ Electrolyzer

Coupling renewable electricity to reduce carbon dioxide (CO₂) electrochemically into carbon feedstocks offers a promising pathway to produce chemical fuels sustainably. While there has been success in developing materials and theory for CO₂ reduction, the widespread deployment of CO₂ electrolyzers has been hindered by challenges in the reactor design and operational stability due to CO₂ crossover and (bi)carbonate salt precipitation. Herein, we design asymmetrical bipolar membranes assembled into a zero-gap CO₂ electrolyzer fed with pure water, solving both challenges. By investigating and optimizing the anion-exchange-layer thickness, cathode differential pressure, and cell temperature, the forward-bias bipolar membrane CO₂ electrolyzer achieves a CO faradic efficiency over 80% with a partial current density over 200 mA cm^-2 at less than 3.0 V with negligible CO₂ crossover. In addition, this electrolyzer achieves 0.61 and 2.1 mV h^-1 decay rates at 150 and 300 mA cm^-2 for 200 and 100 h, respectively. Postmortem analysis indicates that the deterioration of catalyst/polymer-electrolyte interfaces resulted from catalyst structural change, and ionomer degradation at reductive potential shows the decay mechanism. All these results point to the future research direction and show a promising pathway to deploy CO₂ electrolyzers at scale for industrial applications.