31 - Conduction mechanisms in materials with volatile molecules

Summary

Compounds containing loosely bonded molecules form by far the majority of all known solid proton conductors. Most of them are hydrates and their conductivity generally strongly depends on their state of hydration (see Chapter 3). Therefore, conduction mechanisms similar to the ones discussed for aqueous solutions have been suggested to hold also for this family of solids. But this is still a matter of ongoing research and in the following, a recent concept will be put forward using, as a background, the previous models. It provides directly a framework for explaining proton conduction in solid hydrates.

The peculiarity of proton transport in aqueous solutions was recognized early on by the exceptionally high limiting equivalent conductivity of the proton (350 cm² Q^–1 at 25 °C¹) compared to that of any other monovalent cation, data for which fall into a very narrow range (39, 50, 74, 78, 77, 74 cm² Ω^–1 for Li ⁺, Na ⁺, K ⁺, Cs ⁺, Rb ⁺, NH₄⁺ at 25 °C¹). The latter similarity is a direct consequence of cation solvation by water molecules, which diffuse at approximately the same rate (2.26 × 10^–5cm²s^–1 at 25 °C).

The earliest ideas about proton conduction in aqueous solutions were stimulated by considerations of the electrolytic decomposition of water in 1806. Grotthuss postulated chains of water dipoles along which electricity may be transported.