Unravelling the key role of ion-exchange membranes in water management and ion crossover for zero-gap CO2 electrolyzers

Abstract

Catholyte-free zero-gap reactor designs for CO₂ reduction show promise for industrial application, yet frequently suffer from stability issues due to salt crystallization. To address this challenge, efforts were made to improve the water management inside the electrolyzer, through which stable operation (>6 h) with negligible salt precipitation (<1 mg cm⁻²), high faradaic efficiencies (FE) to formate (96.7 ± 0.8%) and low cell voltages of 2.45 ± 0.11 V were achieved in a 49 cm² catholyte-free electrolyzer at 100 mA cm⁻². We were able to do so by identifying a currently underestimated role of the membrane concerning the electrolyzer water management. Our findings indicate that, besides its primary function as a selective ion transport medium, the membrane additionally serves as the main pathway through which water is supplied to the catalyst. To emphasize the importance of this critical role, we determined the minimum required water permeation rate by studying the complete carbon balance in the electrolyzer and compared it to the experimentally measured rates. Furthermore, we report that pristine Nafion 117 membranes exhibit intrinsic differences and extents of degradation upon reuse, which in turn affect the membrane permeation and aggravate the performance in subsequent runs. By using thinner Nafion 211 membranes, an increase in the intrinsic membrane permeation and a lower extent of membrane degradation were achieved, while also exacerbating formate crossover. Consequently, this work reinforces our understanding of water and salt management in catholyte-free electrolyzers and establishes an experimentally verifiable quantitative measure to ensure stable and reproducible formate production.