Anhydrous proton conductors for use as solid electrolytes

Abstract

Anhydrous proton conductors in which proton transport proceeds via structure diffusion are interesting alternatives for membrane materials in hydrogen/air fuel cells. Understanding the mechanism of proton diffusion in candidate materials is necessary to improve their performance. We present the results of our studies of two classes of proton conductors in which conductivity does not rely on the presence of water molecules in their structure: crystalline hydrogen sulphates and selenates and new salts of nitrogen-containing heterocyclic molecules combined with a series of dicarboxylic acids. In the former group, superprotonic conductivity results from a high molecular dynamics of S/SeO4 groups, which creates an excess of structurally equivalent positions for protons. Ferroelastic properties of the crystals determine their physical behaviour at high temperatures. In particular, ferroelasticity makes a smooth and reversible character of superionic phase transition possible. In the second group of materials, the heterocycles exhibit extensive specific interactions that result in a fluctuating network of H-bonds. Thus, the proton conduction in these materials depends on the number of protons, which can be involved in the diffusion and on the fluctuation rate of the hydrogen bond network.