Synthesis And Properties Of Layered-perovskite Oxide GdBaCo₂O_5+δ As Cathode Material For Solid Oxide Fuel Cell

Solid Oxide Fuel Cells (SOFCs) are considered promising next-generation electric power sources because of their high energy conversion efficiency, low pollution and extensive fuel flexibility. Lowering operating temperature of SOFCs from high temperature of-1000℃to intermediate temperature (IT) range of600-800℃has become one of the main research goals. In this range of temperature, contribution of cathode to the total resistance of the cell becomes dominant due to high activation energy of oxygen reduction reaction taking place in the cathode. Thus, developing of new cathode materials with good electrical conductivity, high-electrocatalytic activity for oxygen reduction and fast oxygen transport kinetics at600-800℃is of great importance for IT-SOFCs.Micro structure influences the cathode electrochemical activity significantly; large specific surface area and enough porosity is required for good electrochemical performance of the cathode. In this work, a new cathode material, GdBaCo2O5+δ (GBCO), with double-layered perovskite structure was synthesized by electrospinning combined with sol-gel method. Crystal structure, thermal behavior and microstructure of the samples were characterized by X-ray diffraction (XRD), thermal gravimetric analysis (TG) and scanning electron microscopy (SEM). Structural and morphological evolution of the GBCO/PVP (Polyvinylpyrrolidone, PVP) spun-fibers with calcination temperatures was studied. Electrochemical performance of GBCO on the Ce0.9Gd0.1O1.95(GDC) electrolyte interface was characterized by Electrochemical Impedance Spectroscopy (ElS) technique. The results showed that a pure orthorhombic GBCO phase was formed after calcination at1000℃, which transformed into a tetragonal structure at1150℃, accompanied by decrease of cell volume. The as-spun GBCO/PVP composite fibers shrank after the calcination with a typical diameter of-50nm at800℃; when calcined at1000℃, the nanofibers were broken and developed as nanorods-like aggregates, which grew up into well-defined microcrystals at a higher calcination temperature of1150℃. GBCO was chemically stable with GDC electrolyte at temperatures up to1025℃.Influence of cathode preparation methods and calcination temperature on the electrochemical performance of GBCO was also studied. The EIS results showed that the GBCO cathode prepared with the low-temperature pre-fired GBCO/PVP nanofibers exhibited much better electrochemical performance than the cathode prepared with GBCO powders. Electrochemical performance of the nanofiber-based GBCO cathode was found to change with the calcination temperature. The GBCO cathode calcined at1000℃exhibited the best electrochemical performance than the cathode prepared at other temperatures. Low area-specific resistance (ASR) with values of0.024Ω·cm2，0.043Ω·cm2,0.099Ω·cm2,0.222Ω·cm2and0.533Ω·cm2at800℃,750℃,700℃,650℃and600℃respectively were obtained for the GBCO cathode. These experimental results have demonstrated that electrospinnin technique provides an effective way of adjusting microstructure of the cathode layer; the GBCO prepared with electrospun nanofibers has high electrocatalytic activity for oxygen reduction reaction at the temperatures of600-800℃, and thus is a promising cathode material for IT-SOFCs.