Asymmetrically varying BPM layer thickness for use in CO2 – to – CO electrolyzers

Abstract

Electrochemical reduction of carbon dioxide (CO2) to carbon monoxide (CO) offers a promising pathway for sustainable chemical production, but its large-scale implementation is limited by challenges related to energy efficiency, selectivity, and membrane-mediated ion transport. Bipolar membrane (BPM) CO2 electrolyzers have emerged as an attractive architecture, as they enable independent control of cathodic and anodic pH environments, therefore allowing the use of earth-abundant anode materials. However, BPM operation in CO2 electrolysis is governed by complex transport phenomena and inherent trade-offs between performance metrics like cell voltage, faradaic efficiency (FE), water dissociation efficiency, and co-ion crossover. In this thesis, the effect of asymmetric bipolar membrane design on CO2-to-CO electrolysis performance is investigated. A series of Nafion/PiperION-based BPMs with systematically varied AEL and CEL thicknesses were fabricated and evaluated in a zero-gap membrane–electrode assembly CO2 electrolyzer. Key performance metrics including cell voltage, faradaic efficiency toward CO, crossover of species through the membrane and water dissociation efficiency (WDE), were quantified under relevant operating conditions. Additionally, results were compared to the commercially available state-of-the-art Fumasep bipolar membranes. The results demonstrate that membrane layer thickness strongly influences ion transport and charge-carrier distribution across the BPM. Increasing AEL thickness effectively suppresses co-ion crossover and improves WDE, but can limit cation transport to the cathode and thereby reduce FE toward CO. In contrast, variations in CEL thickness have a smaller impact on crossover and selectivity for the membrane chemistries studied. Co-ion fluxes across the membrane were shown to be dominated by K+ transport for the Nafion/PiperION membranes, which contrasted with the HCO3 – dominant behaviour found for the Fumasep membranes. This indicates that co-ion transport is also largely dependent on layer chemistry, rather than thickness alone, and the optimization strategy will differ with layer identity. Additionally, the first limiting current density was shown to correlate with co-ion transport behavior, specicically K+ crossover, indicating its potential as a diagnostic tool for evaluating asymmetric BPM performance. Along with these findings, important tradeoffs in BPM CO2 electrolysis were evaluated. Although layer thicknesses were found to enable decoupled control over ion fluxes, important tradeoffs between conductivity and FE to CO remain for Ag based configurations, regardless of layer asymmetry. Overall, this work elucidates how asymmetric BPM layer thicknesses shape transport phenomena and performance trade-offs in CO2 electrolysis. The insights obtained provide guidance for the rational design of bipolar membranes optimized for selective, energy-efficient CO2 conversion and highlight BPM layer thickness as a practical design lever for mitigating key challenges in BPM CO2 electrolyzers.