Intrinsically proton conducting polymers with imidazole as proton solvent tethered to a polysiloxane backbone via a flexible spacer have been synthesized. Apart from the standard characterization also their thermal properties and transport behavior have been investigated. The materials exhibit proton conductivity as a consequence of self‐dissociation of the imidazole moieties and “structure diffusion” of the resulting defects. In particular, no liquid phase such as water or monomeric imidazole is needed for the observed proton conductivities.To study the influence of the tether structure on the transport properties, cyclic oligomers and open chain polymers with different spacer lengths have been synthesized.The materials are thermally stable up to 200 °C and become soft around room temperature. The conductivity exhibits VTF and WLF behavior with maximum conductivities around σ = 1.5.10–3 S cm–1 at T = 160 °C. The activation volume of the conductivity as derived from pressure dependent measurements is found to be unusually high. The lowest activation volumes and the highest conductivities are observed for the materials with the highest segmental mobilities, i.e. the longest spacers.Proton self‐diffusion coefficients as obtained from PFG NMR diffusion measurements are significantly higher than expected from the proton conductivities obtained by dielectric spectroscopy. This corresponds to unusually high Haven ratios which have been interpreted by correlated proton transfers allowing for fast proton diffusion while minimizing the separation of ionic charge carriers.
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.
Proton-conducting polymers have attracted much attention due to their promising application as membranes in fuel cells. The most commonly used polymer has been a perfluorinated ionomer known by its trade name, Nafion. It is well known that, including Nafion, the proton-transport properties of water-swollen perfluorinated ionomers largely depend on their water content, which limits their application to temperatures and pressures below the dew point of water. [1,2] This puts several constraints on the use, as fuels, of hydrogen-rich reformats, which usually contain traces of carbon monoxide or methanol, as they require somewhat higher temperatures. Therefore, it is worthwhile to seek distinctly different proton-conducting materials. One approach is based on the complexation of basic polymers (e.g., polybenzimidazole) with oxo-acids (e.g., phosphoric acid).[3±5] Another approach is the substitution of water by heterocycles, such as imidazole, pyrazole, and benzimidazole.[6±10] They are known to present high-proton conductivity that is comparable to the value of hydrated membranes, especially at high temperatures ranging between 150 and 250 C. The approach of employing these heterocycles as proton solvents in fuel-cell membranes may require their immobilization to prevent the leakage of heterocycles during fuel-cell operation. The first attempt towards full immobilization of a heterocycle as a proton solvent in completely water-free electrolytes has been recently developed by Schuster et al. [9] They reported that structure diffusion, which consists of intermolecular proton transfer and structural reorganization by a hydrogen-bond breaking and forming process, is the dominant proton-conducting mechanism in the imidazole-terminated ethylene oxide oligomers. However, this material is not yet available for practical applications, mainly because of its liquid-like physical state. Following this model system, in this study, a new waterfree proton-conducting solid electrolyte based on an organic/ inorganic hybrid is presented. Since a siloxane oligomer functionalized with imidazole-terminated side chains (CImSS) [10] has been reported as a material with a good proton-transport ability as well as a good self-dissociation constant, this moiety is employed as a candidate for a proton transporter, although it shows a viscous liquid characteristic. The CImSS has been synthesized according to a literature procedure [10] (Fig. 1). As a mechanical framework, silicates prepared by a sol±gel synthesis of silane monomers have been chosen. Two silane monomers, such as tetraethoxysilane (TEOS) and methyltriethoxysilane (MTEOS), were employed after consideration of their different polarities. The polarity difference between the CImSS and the silane monomers could affect the phase separation during the blending accompanied by the sol±gel synthesis. The morphologies of the CImSS/silicate hybrids were investigated by field-emission scanning electron microscopy (FE-SEM) measurements (Fig. 2). The physical appearance of the CImSS/MTEOS-based...
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