Transport Properties of Hydroxide and Proton Conducting Membranes

TLDR

Anion exchange membranes are gaining prominence in alkaline electrochemical devices, yet their transport behavior remains insufficiently understood. The authors synthesized a chloromethylated polysulfone polymer, confirmed its structure by ¹H NMR, cast it into films, and introduced benzyltrimethylammonium groups by aqueous trimethylamine soaking. The resulting membranes swell in water, exhibit ionic conductivities that depend on ion‑exchange capacity, and show higher water mobility than proton‑exchange membranes but lower overall transport performance, informing future design goals.

Abstract

Hydroxide anion conducting solid polymer membranes, also termed anion exchange membranes, are becoming important materials for electrochemical technology, and activity in this field, spurred by renewed interest in alkaline fuel cells, is experiencing a resurgence. Solid polymer anion exchange membranes enable alkaline electrochemistry in devices such as fuel cells and electrolyzers and serve as a counterpoint to proton exchange membranes, of which there is a large body of literature. For their seeming importance, the details of transport in alkaline exchange membranes has not been explored thoroughly. In this work, a chloromethylated polymer with a polysulfone backbone was synthesized. 1H NMR spectroscopy was performed to determine the chloromethyl content and its position on the polymer structure. The chloromethylated polymer was solution cast to form clear, creasable films, and subsequent soaking of these films in aqueous trimethylamine gave benzyltrimethylammonium groups. The resulting anion exchange membranes swell in water and show varying degrees of ionic conductivity depending on their ion exchange capacity. The water mobility in the anion exchange membranes was greater than in previously studied proton exchange membranes; however, the transport properties in these new materials were lower than what might have been expected from the water behavior. This comparison gives some insight as to future anion exchange membrane design objectives.