Fabrication and performance of functionally graded cathodes for IT-SOFCs based on doped ceria electrolytes

Functionally graded cathodes were fabricated using ion impregnation and screen-printing techniques for intermediate temperature solid oxide fuel cells (IT-SOFCs) based on Samaria-doped ceria (SDC) electrolytes. The cathodes were composed of La1-xSrxMnO3 (LSM), SDC, and La0.6Sr0.4CoO3-δ (LSC) with electronic conducting components gradually changed from LSM at the electrode/electrolyte interface to LSC at the electrode surface. The compositional graded cathodes fabricated using the ion impregnation method showed much higher electrochemical performance than that using the conventional screen-printing technique. Scanning electron microscopy and electrochemical impedance spectroscopy investigations revealed that the electrochemical performance of graded cathodes depended sensitively on the microstructures that were primarily determined by firing temperatures and fabrication techniques. The cathode graded in composition showed interfacial polarization resistances much lower than that of conventional LSM-SDC electrodes that had similar microstructures and thickness. Impedance spectroscopy was used to study the oxygen reduction kinetics within the graded cathodes. Multi-arcs presented in the impedance spectra measured at low temperature, whereas two depressed arcs at high temperature or low oxygen partial pressure. At high temperature, the rate-limiting steps for the high-frequency arc are possibly attributed to oxygen adsorption and disassociations, while the low-frequency arc to gas-phase diffusion.