Yan Yan
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 known to be electrically conductive at the reducing fuel gas atmosphere (one atmosphere of 25 % H2/ 75 % Ar). To our knowledge, we were the first to measure directly the current and power density per TPB length. An open circuit voltage of 0.93 V and an ionic current density per length of TPB of 1.24 mAm-1 were measured at 450°C at maximal power. Considering literature values for oxygen diffusion activation on Pt, the active zone supplying atomic oxygen to the triple phase boundary was calculated to be 22 nm wide at most. The active zone of metal is critical for designing and optimizing the microstructure of the composite electrodes with electronic and ionic conducting phases. Nano-porous Pt films thus look as ideal candidates for cathode layers. However, Pt films showed a strong dewetting on YSZ surfaces. We studied the possibility to stabilize a porous Pt structure within a composite film in order to obtain a temperature stable and high catalysis metal/ ionic conductor composite. The investigated materials cover combinations of (Pt, IrO2, Ni)-(YSZ, CGO). Nano-structured electrode films with metal nano-phases and amorphous/nano-crystalline oxide phases were designed and fabricated using a multi-source sputtering system. Metal formation and the presence of nano-phases in an amorphous oxide phase were confirmed by x-ray diffraction and transmission electron microscopy, respectively. The nano-structured composite electrodes showed improved electrochemical performance in oxygen reduction because of the presence of the metal nano-phases leading to longer TPB lines as compared to pure Pt electrodes. However, upon heating, segregation of metal phase in the composite was observed. Pt based composites in addition have shown strong compressive stresses, which promote recrystallization for stress release even more. It has been found that the microstructure and the amount of the metal phase are critical to conductivity. The segregation of IrO2 in the composite was also observed in the IrO2-YSZ and IrO2-CGO composite thin films. IrO2 based composites showed clearly less dewetting phenomena than Pt based composites. The Pt versions integrated onto electrolyte membranes showed all a too small in-plane conductivity to be efficient as PEN cathodes. The cells with a Pt-YSZ composite cathode layer covering the complete YSZ/CGO membrane showed that the cell peak power density was increased by a factor of 5 to 5 mWcm-2 as compared to the ring shaped Pt electrode at the border of the membrane. The open circuit voltage dropped to 0.68 V at 450°C, most likely by increased leakage either through the PEN structure, or through a path outside of the cell. Even though the Pt-YSZ composite film reveals improvements, it cannot be exploited as a good cathode to achieve a sufficient TPB lines due to Pt grains re-crystallization leading to a loss of electrical connectivity. The tested composite cathode layers were thus inadequate to contact the complete membrane area, leading to a too large area specific resistance (ASR) in the interior of the cell.
EPFL2012
