Preparation of Cathodes for Thin Film SOFCs

SOFCs are expected to become competitive devices for electrical power generation, but successful application is dependent on decreasing working temperature from 1000 to 800 degrees C, without detrimental effects on resistance and on electrode processes. This requires a reduction of the stabilized zirconia electrolyte thickness and an optimization of the electrodes, especially the cathode, where losses are higher. Strontium doped lanthanum manganites are the most common materials tested as cathodes for SOFCs working at high temperature (1000 degrees C). This cathode material presents high electronic and oxygen-ion conductivities, a thermal expansion coefficient compatible with stabilized zirconia and good catalytic activity. For thin film SOFC devices working at intermediate temperatures (less than 800 degrees C), we have studied the optimization of this type of cathode. Strontium doped lanthanum manganite has been deposited on yttria stabilized zirconia electrolyte substrates by spray-pyrolysis and by RF sputtering. The electrode performances depend strongly on cathode microstructure, influenced by processing conditions. With spray-pyrolysis processes, large porosity is expected. This is important for the supply of oxygen, via O(2) molecules through the pores to the triple phase boundaries, where the gas, the cathode and the electrolyte are in contact and where oxygen reduction may occur. However, large porosity can have a nefaste effect on electronic conductivity. With RF sputtering, denser films with higher electronic conductivity are obtained. But, in that case, the supply of oxygen occurs via adsorbed O-atoms in a diffusion process through the cathode to the electrolyte. Spray-pyrolysis and RF sputtering have been compared relative to electrode properties.