Characterizing Cation Chemistry for Anion Exchange Membranes—para-Alkyl-Substituted Benzyltetramethylimidazolium and Benzyltrimethylammonium Salts in Base

Abstract

Base stability and degradation of imidazolium-functional materials have been a focus of research on anion exchange membranes for electrochemical devices. Despite extensive work, mechanistic questions can cloud efforts to correlate results from model compounds and polymeric membranes. Here, we report an NMR study of behavior in aqueous KOH of benzyltetramethylimidazolium and benzyltrimethylammonium salts with and without para-alkyl-substitution to probe whether base attack at the alkyl substituent plays a role in the reaction pathways. In all cases, the major products from the imidazolium salts are consistent with hydrolytic ring fragmentation as the principal mode of decomposition. Also, relative decomposition kinetics for both types of salts provided no conclusive evidence for a change in reaction mechanism upon introduction of the alkyl substituent. While our data do not rule out base attack at the alkyl group in the cases where rate differences were noted, these differences appear to be better attributed to reaction medium effects of submicrometer oil phase droplets arising from salt aggregation.

Examining the Alkaline Stability of Tris(dialkylamino)sulfoniums and Sulfoxoniums

“…Polymer electrolytes with covalently attached cationic groups have attracted interest for transport of hydroxide ions in alkaline fuel cells and electrolyzers. − These polymer electrolytes (known as anion-exchange membranes or AEMs) commonly have pendant ammonium cations to promote hydroxide transport, but concerns about the alkaline stability of ammoniums has led to exploration of alternatives. − The stability of any cation to hydroxide must be considered prior to use in AEMs, and a number of studies have offered insight into the alkaline stability of imidazoliums, − guanidiniums, − alkyl and aryl phosphoniums, − tetraaminophosphoniums, − cobaltoceniums, − and cationic ruthenium complexes . To our knowledge, no extensive alkaline stability studies on sulfur-based cations have been conducted, and only two examples of sulfur-based cations in AEMs have appeared in the literature. , …”