Protonic solid electrolytes: Thermal stability and conductivity of ammonium/hydronium β″-alumina

Abstract

Ammonium/hydronium β-alumina, [NH4H3O]1.67Mg0.67Al10.33O17, is an unusually stable and conductive solid electrolyte for protonic ions. Its conductivity at 200°C, for example, reaches about 10−2 (Ω cm)−1. This paper describes an investigation of the composition, stability, and ionic conductivity of ammonium/hydronium β″-alumina by means of chemical analysis, TGA, DSC, and complex impedance analysis. The results demonstrate that ammonium/hydronium β″-alumina has a preferential composition of (NH4)1.0(H3O)0.67Mg0.67Al10.33O17, which corresponds to a rather elegant, albeit only hypothetical at this time, ordered arrangement of ammonium ions, hydronium ions, and vacancies in the structure. Ammonium/hydronium β″-alumina decomposes below 350°C in discrete and reversible reactions that yield water and ammonia. The conductivity of single crystals of (NH4)1.0(H3O)0.67Mg0.67Al10.33O17 obeys an Arrhenius relationship below 200|MoC with an activation energy of 0.26 eV (σ = 10−3 (Ω cm−1 at 25°C).