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/SeO 4 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.

Proton dynamics in superprotonic Rb3H(SeO4)2 crystal by broadband dielectric spectroscopy

“…The Arrhenius plot of the bulk conductivity, measured during heating and cooling cycle, is presented in figure 4. Calculated values of the bulk conductivity are in good agreement with the literature [42,43]. Similarly, activation energy of the proton hopping in the superprotonic phase E A = 0.49 eV is in a good agreement with that by Baranov [44].…”

First universality of conductivity spectra in superprotonic (NH4)3H(SO4)2 single crystal

“…Another group of materials with protons as charge carriers, to which the title compound belongs, comprises hydrogen bonded crystalline solids, known as superprotonic conductors. (NH 4 ) 3 H(SO 4 ) 2 is representative of a subgroup M 3 H(XO 4 ) 2 (where M = K, Rb, Cs, NH 4 and X = S, Se) of a group of acidic sulfates and selenates (also known as solid acids), a class of solid electrolytes which have been actively studied over the last three decades, mainly for possible applica tion in fuel cells [10,11]. M 3 H(XO 4 ) 2 acid salts transform into super ionic state through a structural phase transition at T S .…”

Anisotropic Proton Conductivity Arising from Hydrogen-Bond Patterns in Anhydrous Organic Single Crystals, Imidazolium Carboxylates

“…Under this situation, recently, as a new class of proton conductors, molecule-based crystalline materials have attracted attention. − Their crystalline nature would allow a more detailed study on the structure–property relationship and the proton conduction mechanism. In addition, unique assembled structures, dynamic behavior, and proton conducting properties derived from a wide variety of molecules are expected.…”