Hydroxide Degradation Pathways for Substituted Trimethylammonium Cations: A DFT Study

Substituted trimethylammonium cations serve as small molecule analogues for tetherable cations in anion exchange membranes. In turn, these membranes serve as the basis for alkaline membrane fuel cells by allowing facile conduction of hydroxide. As these cations are susceptible to hydroxide attack, they degrade over time and greatly limit the lifetime of the fuel cell. In this research, we performed density functional theory calculations to investigate the degradation pathways of substituted trimethylammonium cations to probe the relative durability of cation tethering strategies in alkyl and aromatic tethers. Our results show that significant changes in calculated energy barriers occur when substitution groups change. Specifically, we have found that, when available, the Hofmann elimination pathway is the most vulnerable pathway for degradation; however, this barrier is also found to depend on the carbon chain length and number of hydrogens susceptible to Hofmann elimination. S _N2 barriers were also investigated for both methyl groups and substitution groups. The reported findings give important insight into potential tethering strategies for trimethylammonium cations in anion exchange membranes.