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.

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Anion exchange membranes (AEMs) are of interest as hydroxide conducting polymer electrolytes in electrochemical devices like fuel cells and electrolyzers. AEMs require hydroxide stable covalently tetherable cations to ensure required conductivity. Benzyltrimethylammonium (BTMA) has been the covalently tetherable cation that has been most often employed in anion exchange membranes because it is reasonably basic, compact (limited number of atoms per charge), and easily/cheaply synthesized. Several reports exist that have investigated hydroxide stability of BTMA under specific conditions, but consistency within these reports and comparisons between them have not yet been made. While the hydroxide stability of BTMA has been believed to be a limitation for AEMs, this stability has not been thoroughly reported. We have found that several methods reported have inherent flaws in their findings due to the difficulty of performing degradation experiments at high temperature and high pH. In order to address these shortcomings, we have developed a reliable, standardized method of determining cation degradation under conditions similar/relevant to those expected in electrochemical devices. The experimental method has been employed to determine BTMA stabilities at varying cation concentrations and elevated temperatures, and has resulted in improved experimental accuracy and reproducibility. Alkaline membrane fuel cells (AMFCs) employing anion exchange membranes (AEMs) are of increasing interest in fuel cell research as they potentially enable the use of non-Pt fuel cell catalysts, a primary cost limitation of proton exchange membrane fuel cells.

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