Fabrication of Thin Film Electrolyte and Nanoporous Electrode Films for IT-SOFCs by Novel Liquid Aerosol Depositions

Abstract

Recently, reducing operation temperature while maintaining high performance is a major consideration for solid oxide fuel cells in order to lower manufacturing costs and secure mechanical durability. In anode-supported SOFCs, the electrolyte ohmic resistance and the cathode polarization resistance are the major contribution to the total loss in a cell. Therefore, one of the key issues is to enhance mass and charge transport through porous cathode, enlarge electrode/electrolyte interfacial area, improve electrochemical properties of cathode and enhance ionic conduction of electrolyte. In addition to searching for new materials, developing new fabrication approaches are effective methodologies to achieve this goal. The objective of this research is to synthesize nanoparticles with high specific surface area and prepare porous cathode films with nanostructured features and dense electrolyte films with thickness of 1~5 µ m by using liquid aerosol depositions such as aerosol flame deposition and electrostatic spray deposition. The final goal is to fabricate an anode-supported single cell with high performance. The aerosol flame deposition was applied to synthesize spherical Gd0.1Ce0.9O2-x and La0.2Sr0.8MnO3-x nanoparticles with high crystallinity. The synthesized powder was composed of particles with two different size distributions. Smaller particles were a few tens of nanometers in diameter and larger particles were approximately a few hundreds of nanometer in diameter. The origin of this bimodal distribution was thought to be two different mechanisms of particle formation. Large particle is formed by the rapid evaporation of liquid solvent in high temperature flame and pyrolysis mechanism of dissolved precursors. Small particle might be formed by the plasma reaction. In an oxy-hydrogen flame, some of ions in the precursor solution don't have enough time to go through the pyrolysis process. Instead, they could remain as ions after the complete evaporation of solvent, and consequently form gaseous thermal plasma. The plasma gases will be supersaturated at the region of lower temperature, and experience the gaseous nuclear and growth process. The correlation between processing conditions and microstructure and properties of nanoparticles prepared using the aerosol flame deposition was investigated by systematically changing deposition parameters, including the concentration of the precursor solution, the hydrogen gas flow rate, the oxygen gas flow rate, solvents, and flame conditions. In particular, the effects of two different atomizers such as ultrasonic and electrostatic atomizers on the size and electrical properties of synthesized particles were investigated. Electrostatic atomization was shown to be the more effective atomization method for decreasing the size of droplets leading to smaller particle with more narrow size distribution. The GDC particles synthesized with electrostatic atomizer exhibited similar ionic conductivity(0.01 S/cm at 700 oC) compared to the reported results. Finally, These results suggest that aerosol flame deposition offers avaliable route for the synthesis of GDC particles for intermediate temperature solid oxide fuel cells. The aerosol flame deposition technique was applied to synthesize the spherical and dense LSM particles with high specific surface area (77 m2g-1) on YSZ substrate and the simultaneous deposition of porous film with porosity of ~30%. The activation energy of AFD-prepared LSM electrode exhibited relatively low value(1.48 eV) compared to those reported by others. The result is related to the good porous microstructure, which enlarges the reduction sites, triple phase boundary, and favors the gas (O2) transportation. However, it is thought that the AFD process is not appropriate method to fabricate dense electrolyte film and porous cathode film with precisely controlled porosity. The electrostatic spray deposition technique was applied to prepare lanthanum strontium manganite films with high specific area (37.34 m2/g) and it was demonstrated that this technique is extremely useful for producing homogeneous and nanoporous metal oxide film with about one micron in thickness. The optimally LSM films were obtained at an electric field of 18 kV, substrate temperature of 219 °C, deposition time of 10 min, nozzle-substrate distance of 4 cm, precursor solution concentration of 0.1M, and solution flow rate of 4.5 ml/h. A fully crystallized perovskite phase LSM layer was obtained by sintering ESD-prepared LSM cathodes at 900 oC and the secondary phases such as La2Zr2O7 and SrZrO3 were not formed. In addition, this sintering temperature is lower than those of conventional fabrication methods of LSM electrodes. The electrochemical resistance of the ESD-prepared LSM cathode decreased from 15 Ωcm2 to 1.2 Ωcm2 with increasing temperature from 546 to 777 oC and the activation energy of the ESD-prepared LSM is 0.81 eV. Considering its capability such as the relatively high specific area, low sintering temperature and low activation energy, this technique is believed to offer a potential route for the production of uniform and nanoporous LSM electrode in a few micrometers thickness which will be applied for the thin film type solid oxide fuel cell. However, it was observed that the ESD-prepared films were not free of pinhole and cracks in especially preparing dense films. This might be attributed to the nonuniform evaporation rate of solvent in droplets. In this study, the electrostatic slurry spray deposition was first demonstrated to fabricate an anode-supported electrolyte for a SOFC unit cell. Characterization of the unit cell showed that the YSZ film was crack-free and dense with good adhesion between the anode and the cathode. An SOFC based on a YSZ electrolyte film of about 2 µ m in thickness was tested at 800 oC with humidified hydrogen as fuel. An open circuit voltage of about 1.1 V was observed at the tested temperature, which was close to the theoretical value, implying that the membrane was dense and the gas permeability of the membrane was insignificant. The polarization resistance of the electrode-electrolyte interfaces and ohmic resistance are 1.2 Ωcm2 and 0.24 Ωcm2 at 800 oC, respectively. Maximum power density of about 490 mW‧ cm-2 was achieved at 800 oC. The results demonstrate that the ESSD technique is a suitable process to fabricate dense and high quality membranes with thickness of 1~5 µ m for application to thin film type IT-SOFCs.