Managing water and product crossover through cation exchange membranes for enhanced zero-gap CO2 electrolyzers

The catholyte-free zero-gap electrolyzer is a promising design for the electrochemical conversion of CO2 to formate at industrial scale, offering low cell voltages and high product concentrations. However, its large-scale deployment is currently hindered by stability issues related to salt crystallization and electrode flooding, as well as product crossover from the cathode to the anode. In this work, we address these challenges by optimizing the membrane transport mechanisms across the (Nafion) cation exchange membrane that separates the two compartments. Building on previous work identifying the cross-membrane water flux from anode to cathode, i.e., membrane permeation (MP), as the primary source of water for cathode salt removal, we further investigate the driving forces governing this transport. Through a comprehensive parametric study, we identified five key parameters that strongly influence MP and thereby cathode salt management: current density, membrane thickness, differential pressure, anolyte concentration, and cation species. Based on these findings, we propose strategies to enhance the cathode water management and mitigate salt crystallization under industrially relevant conditions. Doing so, we found a distinct relationship between MP and formate crossover, with product retention improving considerably at higher MP due to the opposing water flux. Therefore, this work enhances our understanding of water and product crossover in catholyte-free zero-gap electrolyzers and proposes effective measures for stable, reproducible CO2 reduction.