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

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 TiB2 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 TiB2 could be achieved with a large electrode-area of 30 cm2 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 cm2, 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.