Grain Size Effect on Conductivity of LSGM Thin Film Electrolyte for Solid Oxide Fuel Cell

Solid oxide fuel cell (SOFC) consisting of ion conductive ceramics has various advantages such as the high generating efficiency, the utilization of hydrocarbon-containing fuel and the non-necessity of expensive rare metal catalyst. Sr and Mg dopoed Lanthunum Gallate (La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ, LSGM) is a well-known electrolyte for solid oxide fuel cell because of the high conductivity.

It is said that thin film material can exhibit different characterization from bulk material. In our former study, we tried to reveal the mechanism of the conductivity change of LSGM thin film from the bulk. The LSGM thin films with different thickness deposited on sapphire substrate by radio frequency magnetron sputtering was studied. The grains in the thicker film were larger than those in the thinner film. The thicker the film was, the conductivity increased and the activation energy decreased. We suggested this was caused by the smaller bulk resistivity than the grain boundary resistivity. It is necessary to develop the method to separate the resistivity into the bulk resistivity and the grain boundary resistivity to confirm this hypothesis.

In this study, the radio frequency magnetron sputtering was used for obtaining LSGM films on LaSrGaO₄ single crystal substrate. To crystallize and control the grain size in the LSGM film, we conducted post-annealing under various temperature. We evaluated the grain size in the LSGM film with a scanning electron microscope and measured the conductivity by electrochemical impedance spectroscopy. We try to separate the resistivity into the bulk resistivity and the grain boundary resistivity by using an equivalent circuit based on Brick-Layer model. In this paper, we report the grain size effect on the electrical conductivity in LSGM thin film.