Development and characterization of thin-film solid oxide fuel cells

Abstract

The emergence of fuel cells (FCs) as important and alternative power generating devices has been accelerated in recent years by the much uplifted environmentalistic concerns and the pessimistic projection of shortage of fossil fuels supply. Among the many types of FC being developed, solid oxide fuel cell (SOFC), which makes use of ionic conducting oxides as electrolytes, has attracted great interest due to its pollutant free operation and high energy conversion efficiency. Currently the size miniaturization and the reduction of operating temperature are the research focuses of SOFC for applications in portable devices. In this connection, the fabrication of ionic conducting oxide thin films and the study of their electrical transport characteristics are necessary. Basically, solid oxide fuel cell composes of two electrodes (the cathode and anode) separated by a solid electrolyte. For the cathode material, La0.85Sr0.15MnO3 (LSMO), La0.7Sr0.3MnO3 and La0.7Sr0.3CoO3 (LSCO) have been chosen to be used in the present studies as the cathode material because of their good electronic conductivity. The bulk target of the LSMO and LSCO are fabricated by the conventional solid state reaction, and then undergoes high temperature sintering process. LSMO and LSCO are all fabricated in thin film form by pulse laser deposition. The structural properties of the bulk targets and the films are confirmed by x-ray diffractometry. For the electrolyte material, thin films of Ce0.8Gd0.2O2-d (CGO), Ce0.8Sm0.2O2-d (CSO) and La1-xSrxGa1-yMgyO3-d (LSGMO) have been fabricated by pulsed laser deposition. We focus our studies on the crystallinity of the as-deposited films and its relationship to the enhanced ionic conductivity. We started off by studying the effect of processing temperature on the conductivity of the ionic conductors. LSGMO perovskite oxide ion conductor thin films of about 300 nm thick were deposited on a LaAlOs (LAO) at various substrate temperatures by pulsed laser deposition. Their structural characteristics were studied by x-ray diffractometry. Polycrystalline and epitaxially grown films were revealed for samples grown at different temperatures. Their ionic conductivities were investigated with a two-probe DC technique as a function of temperature over 400 C - 650 C range in air. The polycrystalline LSGMO films show a conductivity of 0.19 S/cm while the epitaxial LSGMO films exhibit values of about 0.74 S/cm at 600 C. This represents a substantial enhancement from the 0.00028 S/cm of the bulk LSGMO measured at the same temperature. In subsequent studies CSO oxide ion conductor thin films of about 300 nm thick were deposited on a LAO and MgO at the same substrate temperatures by pulsed laser deposition. Their structural characteristics were studied by x-ray diffractometry. Due to structural compatibility, polycrystalline and epitaxially grown films were revealed for samples grown on these two types of substrates. Their ionic conductivities were investigated with a two-probe D.C. and A.C. techniques as a function of temperature over 400 C - 650 C range in air. The polycrystalline CSO films grown on MgO show a conductivity of 0.0132 S/cm while the 45 degree twisted epitaxial CSO films grown on LAO exhibit values of about 0.94 S/cm at 600 C. For the CGO thin films, similar results are also obtained. In order to demonstrate the feasibility of applying thin film technology in SOFCs, in particular FC of a single chamber design, we have fabricated patterned bi-layer and tri-layer structure of the LSCO/LSGMO on LAO substrate and Pt/ LSGMO/ LSCO on LAO. Their good structural characteristics have been revealed by XRD and TEM.