Study of Imidazole-Based Proton-Conducting Composite Materials Using Solid-State NMR

Abstract

The application of solid-state NMR methods to characterize the structure and dynamics of imidazole-based proton-conducting polymeric materials provides insight into the mechanism (Grotthus vs vehicle) of proton-mobility. The presented materials are built on a siloxane backbone, and are of interest as potential new proton-conducting membranes for fuel cells able to function at temperatures above 130 °C. This is expected to improve the CO tolerance of the catalyst in the fuel cell, as compared to water-based systems. High-resolution solid-state 1H NMR is achieved under fast magic-angle spinning (MAS) conditions (30 kHz), and provides resolution of resonances in the hydrogen-bonding region. Homonuclear double quantum filtered (DQF) NMR spectra, acquired using the back-to-back sequence, provided identification of mobile protons. It was found that proton conductivity, observed macroscopically using impedance spectroscopy, is correlated with local proton mobility, observed via 1H NMR line width trends observed for the hydrogen-bonded protons. 1H MAS and DQF NMR experiments show no crystal packing of these materials in contrast to model oligo-ethyleneoxide-tethered imidazole materials (Imi-nEO) studied previously. Comparisons of macroscopic and microscopic measures of proton mobility are also presented in the activation energies of pure and acid-doped siloxane oligomers and polymers functionalized with imidazole. The acid-doped materials show enhanced proton mobility, and hence higher conductivity, relative to the pure material.