Hydroxide Degradation Pathways for Imidazolium Cations: A DFT Study

Imidazolium cations are promising candidates as covalently tetherable cations for application in anion exchange membranes. They have generated specific interest in alkaline membrane fuel cell applications where ammonium-based cations have been the most commonly applied but have been found to be susceptible to hydroxide attack. In the search for high stability cations, a detailed understanding of the degradation pathways and reaction barriers is required. In this work, we investigate imidazolium and benzimidazolium cations in the presence of hydroxide using density functional theory calculations for their potential in alkaline membrane fuel cells. The dominant degradation pathway for these cations is predicted to be the nucleophilic addition-elimination pathway at the C-2 atom position on the imidazolium ring. Steric interferences, introduced by substitutions at the C-2, C-4, and C-5 atom positions, were investigated and found to have a significant, positive impact on calculated degradation energy barriers. Benzimidazolium cations, with their larger conjugated systems, are predicted to degrade much faster than their imidazolium counterparts. The reported results provide important insight into designing stable cations for anion exchange membranes. Some of the molecules studied have significantly increased degradation energy barriers suggesting that they could possess significantly improved (several orders of magnitude) durability, compared to traditional cations and potentially enable new applications.