Optimization of spray pyrolysis for cathode-supported solid oxide fuel cells

Abstract

Cathode-supported solid oxide fuel cells (SOFCs) have been the most reliable devices for direct conversion of fuels in electrical power. However, processing at high temperatures to obtain a gas-tight electrolyte is prohibited due to formation of interfacial secondary phases between the electrolyte and the cathode support. Various deposition techniques such as electrochemical vapor deposition have been successfully employed to deposit thin electrolytes, but with a high cost. Therefore, a fabrication method that can meet the requirements of an industrial application at lower cost is the key for commercialization of this type of SOFCs. The aim of this study was to optimize air-pressurized spray pyrolysis technique for preparation of ultra-thin and dense electrolytes at low temperatures, for cathode-supported SOFCs. This process is cost-effective and easy scalable, suitable for deposition of thin films over large areas. Cathode-supported SOFCs were developed with thin 3.5-5.5μm yttria-stabilized zirconia (YSZ) electrolytes of columnar structure, at deposition temperatures as low as 170°C and deposition rates ≥10 μm h⁻¹. The surface of the composite LSM-YSZ cathode support was modified by spraying a LSM interlayer to reduce the roughness of the substrate and thus, secure a uniform thickness of the post-deposited electrolyte layer. To complete the cell with an anode electrode, cobalt ceria Co-CeO₂ with mixed ionic-electronic conductivity (MIEC) was also deposited by spray pyrolysis. Optimization of spray pyrolysis process parameters revealed the precursor concentration in conjunction with deposition time as the most important parameters to shift the morphology of the film from dense to porous depended on the target film structure. Sintering, from 750°C up to 950°C, proved to suppress the formation of zirconate phases at the interface of the YSZ/LSM that would severely degrade the performance of the cell. The cathode-supported SOFCs were electrochemically tested using 5%H₂/Ar as fuel and air as oxidant within a temperature range of 700-850°C. The measured open circuit voltage values were close to the theoretical ones with a maximum of 1.002 V at 850°C, indicating a gas-tight electrolyte. A power density of 127 mW/cm² at 850°C for a cathode-supported SOFC with a 3.5 μm thick YSZ electrolyte, was achieved. The activation energy of the whole cell was 0.15 eV corroborating the actual ohmic resistance values correspond to the cathode support which is the limiting factor of the cell's performance. Long-term stability test of five days showed a performance degradation to 83 mW/cm² at 850°C due to particle agglomeration of the cobalt metal in the anode electrode and reduction of the catalytic active area. The above indicate spray pyrolysis is an established technique for preparation of thin films for use in cathode-supported SOFCs.