Does the Presence of CO2 Affect the Alkaline Stability of Anion-Exchange Membranes?

Abstract

Anion-exchange membranes (AEMs) enable electrochemical energy devices to operate in alkaline environments, allowing the use of earth-abundant, platinum group metal-free catalysts. This makes them highly attractive for applications such as AEM fuel cells (AEMFCs), water electrolyzers (AEMWEs), and oxygen separators (AEMOSs). However, two key challenges still hinder their commercialization: chemical degradation of the membranes and carbonation from atmospheric CO₂, both of which impair conductivity and reduce long-term performance. Building upon our ex-situ novel technique to measure the alkaline stability of quaternary ammonium salts (QAs) and AEMs at different hydroxide hydration levels, in this study we investigate the effect of CO2 on the stability of the AEMs. Specifically, we evaluate the stability of AEM standard benzyltrimethyl ammonium cations (BTMA+) in the presence of mixtures of hydroxide and (bi)carbonate anions at different ratios and hydration levels. Additionally, we introduce a novel technique to simultaneously evaluate (bi)carbonate impact and hydroxide conductivity in AEMs. Our results reveal that the presence of (bi)carbonate anions surrounding BTMA⁺ significantly reduces QA degradation. This phenomenon is further supported by molecular dynamics simulations, which suggest a “salting-out” effect, where water more strongly hydrates hydroxide ions, isolating them from the organic QA sites and reducing degradation probability. These findings provide new insight into AEM behavior in CO₂-containing environments such as ambient air. This study provides new, valuable data to overcome the critical challenge of AEM alkaline stability and opens new avenues for the design of more durable AEM materials, advancing the development of AEMFCs, AEMWEs, and AEMOSs for real-world applications.