NMR As a Tool for the Study and Rational Design of Aems

Abstract

The use of anion exchange membranes (AEMs) as separators in alkaline polymer electrolyte fuel cells (AFCs) has received considerable interest over the past decade. AFCs offer several advantages over more traditional PEM fuel cells, in particular operating at high pH enables the use of non-precious metal catalysts, drastically reducing the cost of the cells. However, widespread adoption of AEMs has been hampered due to poor stability, in part due to base-induced decomposition of the polymer backbone and cation group as well as conversion to carbonate form, all of which contribute to reduction of ion exchange capacity (IEC), conductivity, and mechanical strength. Furthermore, there is an insufficient understanding on how the polymer nanostructure and microstructure affects stability, transport, and cell performance. Nuclear magnetic resonance (NMR) spectroscopy is well suited to investigate these issues. We will demonstrate how 1D and 2D NMR spectroscopy may be utilized to characterize the degradation of AEMs using both model cations and ex situ membranes. Measurements of relaxation, e.g. T1, for a series of AEMs will be correlated with ionic conductivity. We also have used pulsed-field gradient NMR experiments to measure water self-diffusion to assist in deconvoluting how tortuosity, connectivity, and the mobility of water in hydrophobic and hydrophilic domains are governed by the nature of the cation and polymer backbone. The ultimate goal of this work is to develop a structure-activity relationship to assist in the design of novel materials with desirable properties.