DFT insight of hydroxide degradation pathways for heterocyclic quaternary ammonium cations in anion exchange membranes

Abstract

The alkaline stability of the heterocyclic quaternary ammoniums (QAs) is crucial for the long-term operation of anion exchange membrane fuel cells (AEMFCs). Here, we calculate the degradation barriers of 21 heterocyclic QAs and analyze the degradation mechanism systematically by density functional theory (DFT). We have found that the chair conformation and ring structure of heterocyclic QAs can inhibit Hoffman degradation of β-hydrogens and that methyl SN2 is the primary degradation pathway for most heterocyclic QAs. Higher ring strain and the introduction of electron-withdrawing groups are disadvantageous to the stability. The positions of substituted methyl affect the stability of the molecules significantly. Forming bicyclic structures is also an effective strategy to improve stability. Among tethering strategies of anion exchange membranes (AEMs), tethering at the 4-position is more stable than at the 1-position. The superacid-catalyzed condensation reaction is still the best AEM preparation strategy which balances the stability and synthetic difficulty. Compared with the experimental value, the average error of simulation is only 0.56 kcal mol−1. Our prediction method can also be extended to various alkali concentrations, and the results are consistent with the experiments. At low alkali concentrations, the heterocyclic QAs mainly degrade by SN2 reactions. At high alkali concentrations, the groups mainly degrade by Hofmann elimination reactions.