Ab initio molecular dynamics reveal multiple hydroxyl-induced degradation pathways of imidazolium-based anion exchange membrane

Abstract

Anion exchange membranes (AEMs) are promising for hydrogen production via water electrolysis. Imidazolium cations provide high thermal stability and performance, but their alkaline degradation mechanisms remain difficult to resolve due to the complexity of the pathways involved. We performed comprehensive ab initio molecular dynamics (AIMD) simulations using density functional theory (DFT) to elucidate the competing degradation pathways of imidazolium-based AEMs. The AIMD simulations identified five unique degradation pathways of the AEMs, including ring-opening, demethylation, disscociation and other side reactions. The transition state (TS) calculations found that all degradation pathways start with the OH⁻ attack on the imidazolium cations, and the subsequent C-N and C-C bond cleavage are the rate-determining steps. The noncovalent interaction analysis shows that the alkyl protection of imidazolium introduces steric hindrance to elevate the energy barriers of OH⁻ attack, which suppresses the formation of the enol-like intermediates for further degradations. The electrostatic potential (ESP) analysis reveals a direct correlation between the charge distributions of imidazolium cations and the nearby motions of OH⁻ anions, where a symmetric backbone framework of imidazolium cations introduces accumulated positive charges, facilitating the OH– attack for degradation. Our findings suggest that controlling OH– attack on imidazolium cations is key to ensuring high alkaline stability, and that using a branched imidazolium cation with an alkyl protecting group is an effective strategy to prevent OH– attack-induced degradation.