Design of anion exchange membranes for electrochemical carbon dioxide reduction to carbon monoxide

Electrochemical CO₂ reduction technologies provide a platform for transforming renewable electricity, water, and waste CO₂ into synthetic building blocks (e.g., CO) and chemicals. However, CO₂ electrolyzers are not yet available in the market due to poor efficiencies at high reaction rates. Emerging CO₂ electrolyzers containing an anion exchange membrane (AEM) offer promise in overcoming this challenge, but limited design principles exist for AEMs in CO₂ electrolysis application. This thesis reports on my investigation of the designs of AEMs that improve CO₂ electrolyzer performance for CO production. I first interrogate two analogous AEM designs to determine the effect of AEM functional group on product selectivity, cell voltage, and stability. This relationship between the membrane and CO₂ electrolyzer performance has not yet been established at reaction rates greater than 10 mA/cm². I demonstrate that an imidazolium-based AEM achieves a higher cell performance than a trimethylamine analog at reaction rates up to 100 mA/cm². I find that the low water uptake (i.e., water content) and improved chemical stability of an imidazolium group are contributing factors that lead to increased performance for CO production. I explore further how AEM water uptake and thickness, and cathode hydrophobicity impact the amount of water transported to the cathode. Water serves as a proton source for the CO₂ reduction reaction to CO, but excess water at the cathode (i.e., flooding) can block the pores of catalytic sites and impede mass transport of reactants and products. Theoretical simulations predict that flooding will occur at reaction rates greater than 750 mA/cm², but this thesis demonstrates that cathode flooding is an issue at merely 200 mA/cm². I find that thin, low water uptake AEMs paired with hydrophobic cathodes mitigate cathode flooding and improve product selectivity and cell voltage for CO production.