26 - Measuring the true proton conductivity

Summary

A knowledge of the true, bulk proton conductivity has been both desirable and difficult to achieve in the characterization of most known solid proton conductors. It was almost 100 years after the first electrical measurements on ice that von Hippel demonstrated that single crystals of pure ice must have a proton conductivity far below the values reported previously.

Quantitative data on bulk proton transport are required especially for the understanding of proton transport mechanisms (see Chapters 29 & 31) including the implications for the use of solid proton conductors in operational electrochemical cells (see Chapters 32 & 39).

In this Chapter, proton conductivity refers to the displacement of protonic species (e.g. H ⁺, H₃O⁺, O H^–) in small electric fields across a sample close to thermodynamic equilibrium. This conductance relates directly to the self diffusion coefficient of the corresponding species, which may be significantly smaller than the chemical diffusion coefficient, in particular in the presence of another highly mobile species (e.g. conduction electrons in hydrogen bronzes; see Chapter 7).

There is no standard procedure for the measurement of proton conductivity and experimental techniques as well as structural and chemical considerations have to be adapted to the material under investigation.