Investigation of Cation Degradation in Anion Exchange Membranes Using Multi-Dimensional NMR Spectroscopy

Abstract

Low temperature anion exchange polymer electrolyte membrane (AEM) based fuel cells and electrolyzers have been successfully shown to convert hydrogen and oxygen to electricity (and vice-versa) with high efficiencies and energy densities with non-precious group metals. However, a significant challenge to realizing these technologies is the poor alkaline stability of the AEMs. In this work, we present the results of our multi-faceted approach to better understand AEM degradation in alkaline media. Several two-dimensional (2D) NMR techniques were applied to comprehensively elucidate cation degradation in polysulfone-based AEMs exposed to alkaline solutions. The 2D NMR tools utilized were: a) homonuclear correlation spectroscopy (COSY), b) 1H-X (X = 13C or 31P) heteronuclear multiple quantum correlation spectroscopy (HMQC), and c) 1H-13C heteronuclear multiple bond correlation spectroscopy (HMBC). Analysis of the 2D NMR spectra enabled precise identification of the degradation products for several different cation chemistries (quaternary ammonium, imidazolium, and phosphonium types). The observation of multiple degradation products highlighted the fact that the cation groups degraded through fundamentally different mechanisms. A clear understanding of the degradation mechanism is a critical requirement in the process of selecting or designing alternate cation chemistries for alkaline resilient AEMs.