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 imidazolebased 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 1 H 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 1 H NMR line width trends observed for the hydrogenbonded protons. 1 H 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.

Probing Proton Mobility in Polyvinazene and its Sulfonated Derivatives Using 1H Solid‐State NMR

“…Good agreement has been established in many cases between activation energies for local proton dynamics, as probe by variable temperature proton NMR, and macroscopic ion transport, as probe through impedance spectroscopy measurements of conductivity. [17,18] Figure 3 shows the Back-to-Back (BaBa) homonuclear double quantum filtering NMR sequence, [19][20][21] utilized here to distinguish mobile from immobile protons. The basis of the sequence is the homonuclear dipolar coupling, which is orientation-dependent, and therefore motionally averaged, if a proton experiences dynamics on a timescale faster than the rotor period.…”

Polymeric organic–inorganic proton-exchange membranes for fuel cells produced by the sol–gel method

“…Apart from fuel cells the PEMs are also promising for use in chemical separation [38,42], sensors [8,35,38,42,45,49,56,67,109], energy storage devices [42], windows with controllable optical transmission ("smart windows") [67], electrochromic displays [35,45], and the electrolysis of water [38].…”

“…Investigations of the temperature dependence of the conductivity showed that the Grotthus mechanism is realized at low and moderate temperatures whereas the contribution of the vehicle mechanism increases at high temperatures [32,40]. The temperature dependence of the conductivities of systems where the proton transfer is due to segmental motion of the polymer chains is described by the Vogel-Tamman-Fulcher equation [32,41,49,[52][53][54]67]. In other cases the temperature dependence of the PEMs obeys the Arrhenius equation [8,36,49,62,[68][69][70].…”

Bio-Inspired Membranes for Advanced Polymer Electrolyte Fuel Cells. Anhydrous Proton-Conducting Membrane via Molecular Self-Assembly

“…The acid-base pairs were shown proved to form proton-conducting pathways by direct proton-transfer reactions between the two moieties, and the membrane exhibited a large conductivity in spite of the absence of mobile liquid molecules, such as water. [77][78][79][80] The temperature dependence of the conductivities and activation energy of conduction suggests a novel proton-transfer reaction among the electrochemical network of the acid-base pairs in the solid polymeric materials, which is much different from those in the hydrated protons in water phase. Table 2 summarizes the results of conducting properties of the composite membrane studied in the present work.…”

“…[77][78][79][80] In these cases, a Grotthuss-type diffusion mechanism has been proposed, in which proton transfer occurs from protonated molecules to neighboring non-protonated molecules. Namely, protonated and non-protonated nitrogen atoms in the heterocycles may act as donors and acceptors in proton-transfer reactions.…”