Material Study of Micromachined Thin Film Solid Oxide Fuel Cells

Miniaturized, low temperature Solid Oxide Fuel Cells (SOFCs) that operate below 600 °C are promising for supplying electrical power to portable devices. Thin electrolyte membranes of less than one micrometer thickness have been shown to yield reasonable power densities. It remains, however, challenging to produce membranes with a high performance over a long time. While ohmic losses in the electrolyte can be reduced by reducing its thickness, polarization losses in cathode and anode layers are more difficult to get under control. It is generally found the oxygen reduction reactions (ORR) on the cathode are limiting the performance. The goal of this thesis is to realize the membrane fabrication based on silicon micromachining technology and improve the performance of micro-SOFC devices by studying the relevant properties of the materials in positive electrode-electrolyte-negative electrode (PEN) elements of generic cell configuration. Yttria-stabilized zirconia (YSZ) is the most favored material for the electrolyte in devices of all scales. It potentially meets the requirements for micro-SOFC's as the YSZ film thickness can be scaled down to optimal thickness, even though the membrane becomes very fragile. In this work, sputter deposition was used for YSZ thin films. This process leads to a considerable residual stress that is not compatible with thermal misfit strain, but must be assigned to defects created during sputter deposition or cool down in the sputter chamber. This stress can be a major factor to destabilize the membrane during operation. Indeed, a thermal post processing treatment was necessary for achieving YSZ films with thermal mismatch stress only. We thus studied the impact of deposition conditions and evaluated the impact on film microstructure and morphology, electronic conductivity and mechanical stress. The stress evolution with temperature demonstrates that the residual stress strongly correlated with oxygen gas pressure during deposition and cool down. The compressive stress of as-deposited YSZ films was relaxed by cooling in oxygen atmosphere. The excessive compressive stress is irreversibly removed by annealing at 600°C in air. The relaxation sets in at about 400°C. This phenomenon could be explained by the mobility of additional oxygen vacancies in the edge dislocation core. Therefore, excessive stress could be avoided by tuning deposition conditions and post-treatment. Durable electrolyte membranes were obtained by such processes. Moreover, PEN operation revealed a high concentration of electrochemically active sites on their surface. Since the polarization loss on the cathode is a key issue in cell performance limitation, we studied the simplest geometry with a defined, circular TPB line and a strong catalytic material like platinum as cathode metal in combination with 200 µm diameter electrolyte membranes obtained by silicon micromachining. The anode function was provided by a ceria doped gadolinia (CGO) layer, which is