33 - Synthesis of polycrystalline H₃O⁺ and NH₄⁺ -β″/β-Al₂O₃ and potential applications in steam-electrolysis/fuel-cells

Summary

Introduction

Proton conduction in β- and (β″-Al₂O₃ is of interest because of potential use in steam-electrolysis/H₂/O₂ fuel-cells. Single crystal H₃0⁺/-β″-Al₂O₃ was reported by Farrington & Briant to be a fast ion conductor with a proton conductivity of 10^–4–10^–5 (Ω cm)^–1 at 25 °C. These encouraging results led to the first fabrication, characterization and use of polycrystalline H₃O ⁺ -β″ /β-A1₂O₃².

A second electrolyte with a higher conductivity but a lower thermal stability is NH₄⁺ -β″-Al₂0₃². This system, NH₄⁺/H₃0⁺ -β″-Al₂0₃, is the best conductor of protons (σ = 7 × 10^–4 (Ω cm)^–1 rising to 2 × 10^–2 (Ω cm)^–1 at 200 °C). All previous work has been on single crystals (see Chapters 13 & 23) or powders. This chapter describes the synthesis of NH₄⁺O ⁺ -β″-Al₂O₃ polycrystals.

Ion conducting structure of β″- and β-aluminas

The β-A1₂O₃ family of compounds is constructed from oxygen and aluminium ‘spinel blocks’ with intervening ‘conduction planes’. In βA1₂O₃ there are two spinel blocks per unit cell and in β″-A1₂O₃ there are three. The upper oxygen layer of the spinel block is mirrored across the conduction plane in β-Al₂O₃ but this symmetry is lost in β″-A1₂O₃. The alkali ion ‘Beevers-Ross’ site in the β-Al₂O₃ conduction planes is octahedral. The equivalent site in β″-A1₂O₃ is tetrahedral and smaller. Thus K⁺ (r = 0.28 nm) promotes formation of β-A1₂O₃ and Na⁺ (r = 0.196 nm), β″- A1₂O₃.