Mechanism of Tetraalkylammonium Headgroup Degradation in Alkaline Fuel Cell Membranes

Abstract

Cationic headgroups such as tetramethylammonium (TMA) undergo degradation in alkaline conditions through two different mechanisms. In the first mechanism, a hydroxide ion performs an SN2 attack on the methyl groups and directly forms methanol. In the second mechanism, an ylide (trimethylammonium methylide) and a water molecule are formed by the abstraction of a proton from a methyl group. The ylide subsequently reacts with water to form methanol. Both pathways have the same overall barrier as observed in our reaction path calculations with density functional theory. The ylide mechanism is verified by H−D exchange observed between the aqueous phase and the cationic head group. We also discuss the effect of the medium and the water content on the calculated reaction barriers. Good solvation of the head-groups and hydroxide ions is essential for the overall chemical stability of alkaline membranes.