Effect of Interstitial Alkyl Chain Engineering of Anion Exchange Membrane for Improved CO₂ Electrolyzer

Anion exchange membranes (AEMs) are emerging as crucial components in the electrochemical CO₂ reduction reaction. However, there is a significant gap in the literature regarding structural design strategies for AEMs that achieve balanced water management while suppressing cation crossover—issues that are responsible for cathode flooding and salt precipitation.
Unlike AEMs designed for analogous fuel cells and water electrolysis cells, there has been limited research on the structural optimization of AEMs specifically for zero-gap CO₂RR. To achieve high performance and durability in analogous cells, several recent investigations have focused on various types of poly(arylene piperidinium) (PAP) AEMs, driven by a mature understanding of AEM structure. However, zero-gap CO₂RR systems, which aim for superior electrolyzer performance and durability, often encounter challenges related to imbalanced water management and excessive cation crossover from the anolyte, primarily due to the AEM's imperfect cation exclusion. Despite these challenges, much remains to be understood about the design strategies required to balance water management and suppress cation crossover in AEMs, given the complex water and ion transport mechanisms involved. In the context of CO₂RR, most research has predominantly utilized the commercial Sustainion® X37-50 RT AEM, in contrast to PAP AEMs. Notably, the X37-50 RT exhibits relatively lower ion conductivity compared to PAP AEMs, suggesting that additional properties beyond ion conductivity are necessary to fully meet the requirements of an ideal AEM for CO₂RR cells. However, clear structural guidelines on how to meet these requirements in designing ideal AEMs for CO₂RR are lacking.
Our previous research has demonstrated that AEMs with alkyl chains in the conducting groups and polymer backbone exhibit improved contact properties with the catalyst layer (CL) and possess well-developed ion transport channels in water electrolysis cells. Based on these findings, we hypothesize that AEMs with alkyl chains could provide sufficient water to the cathode CL to facilitate CO₂ conversion and suppress cation crossover, likely due to the densely packed conducting groups, which are related to Donnan exclusion. In this study, we have developed a novel PBFA-QA AEM with a concentrated conducting group in the bisfluorene moiety and an interstitial alkyl chain in the conducting groups and the polymer backbone. The first objective is to facilitate water and ion transport and suppress the cation crossover by creating a well-developed ion transport channel, and the second objective is to guarantee a sufficient water supply by enhancing the contact properties with the CL. Additionally, we also developed PBF-QA without an interstitial alkyl chain in the bifluorene moiety to assess the impact of the alkyl chain in the backbone on the performance of the zero-gap CO₂RR. Through a comparison of PBFA-QA, PBF-QA, and other commercial AEMs (X37-50 RT and PiperION), we report the observation of a pronounced alkyl chain effect of AEMs for the conversion of CO products in CO₂RR cells.
Therefore, our PBFA-QA demonstrated superior performance in CO₂RR with a CO partial current density (j^_CO) of 328.0 mA cm⁻² at 3.2 V, compared to that of PBF-QA (267.3 mA cm⁻²), PiperION (173.6 mA cm⁻²), and X37-50 RT (161.8 mA cm⁻²) at the same operating conditions due to the alkyl chain effect in the conducting groups and the polymer backbone. Furthermore, due to its balanced water management and effective suppression of cation crossover, PBFA-QA demonstrated outstanding in situ durability, with over 93% of Faradaic Efficiency of CO (FE_CO) observed at a high current density of 300 mA cm⁻², which cannot be achieved in the X37-50 RT and PiperION AEMs.