Structural and electrical properties of γ-alumina - lithium sulphate films

Li₂SO₄ as a model ionic conductor has received very much attention over several decades. Especially, in recent years Li₂SO₄ and Li₂SO₄-Al₂O₃ have been mentioned as promising proton conducting electrolytes for applications such as intermediate temperature fuel cells and novel cogeneration systems regarding H₂S handling devices. This has encouraged us to strive towards further improvement of the properties of the materials to meet the demands of the applications. In order to improve the properties of this system, a new process, a suspension technique, has been recently developed to prepare nanostructured powder and thin film Li₂SO₄-Al₂O₃ membranes. The powders and thin films have a well crystallised structure composed of two phases, Li₂SO₄ and γ-Al₂O₃, and excellent mechanical strength. The thin film thickness is in the scale of a few to several mm with a smooth and shining surface and a homogeneous macroscopic structure. It is a very interesting phenomenon that all samples show no significant conductivity increase at the temperature of the phase transition (∼ 577 °C) from β to α phase of pure Li₂SO₄. This transition has important significance for applications. The conductivity of the two-phase film materials has been greatly enhanced, where the xLi₂SO₄-(1-x)Al₂O₃ (x=58) sample shows the highest conductivity, about 1 S/cm at 600 °C; the activation energy decreases with increasing Li₂SO₄ content. These results agree with the so called composite effect for the conductivity enhancement observed earlier for two-phase bulk materials. Based on the four-step proton conducting mechanism in sulphate-based materials, this work may propose a new mechanism. The protons might jump in a water network associated with the water molecular re-orientation, which is accompanied with the single proton jump of the four-step transportation among SO ₄ ²⁻ groups from one Li₂SO₄ molecule to another. The former mechanism occur in the interfacial region between the Li₂SO₄ and the Al₂O₃ grains, while the latter occur in the bulk of the Li₂SO₄ grains. These thin film materials are intended for use as proton conducting ceramic membranes in applications such as desulphurisation and fuel cell co-generation plants. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.