Interfacial electric properties of beta″-alumina and electrode by AC impedance

Abstract

Abstract In order to enhance cell power density and to study the interfacial electric property between beta″-alumina and an electrode, test cells of Na(l)/beta″-alumina/M, where M=TiN or TiB 2 or Na–Sn or Na–Pb molten alloys as electrode materials, were set up and run within the temperature range of 400°–800°C. The performance of the test cells and the interfacial electric properties were investigated by measuring current–voltage characteristics and AC impedance. The maximum power density of 0.18 W cm −2 for TiN and 0.24 W cm −2 for TiB 2 could be achieved with a large electrode-area of 30 cm 2 at 800°C. A simplified model and equivalent circuit were given, based on the impedance data. The effect of microstructure of the porous electrode and roughness of the beta″-tube on the cell electric performance and impedance has been studied and discussed. The electron-transport through the porous electrode to the interface of the electrode and the beta″-tube surface is the control step for the electrode reaction, Na + +e→Na, rather than the mass-transport step, for a cell of Na(l)/beta″-alumina/porous thin film electrode. The AC impedance data demonstrated that wetting of the beta″-alumina electrolyte plays an important roll in reducing the cell resistance for the molten Na–Sn or Na–Pb electrode, and the molten alloy electrodes have a smaller cell-resistance, 0.3–0.35 Ω cm 2 , at 700°C after 10–20 h. The comparison with sputtered thin, porous film electrodes, showed that the microstructure and thickness of electrode, and the interfacial resistance between electrode and the surface of the beta″-alumina is crucial to enhance cell power density.