| In this dissertation, a novel method was developed for fabricating SOFC (solid oxide fuel cell) cathode composites by impregnating a porous YSZ (yttria-stabilized zirconia) with perovksites.; First, the impregnation procedure was used to fabricate composites of YSZ with Sr-doped LaFeO3 (LSF) and Sr-doped LaCoO3 (LSCo) at low temperatures. Aqueous solutions of La, Sr, and Fe (or Co) salts were impregnated into porous YSZ and calcined to form perovskite phases, using relatively low calcination temperatures to avoid solid-state reactions. Composites made by this method exhibited a non-random structure, providing reasonable electrical conductivities at loadings below the percolation threshold. The coefficients of thermal expansion achieved a close match to that of YSZ. Electrochemical measurements using symmetric cells indicated that the ASR (area-specific resistances) of these composites could be as low as 0.08 O·cm2 and 0.03 O·cm2 for LSF-YSZ and LSCo-YSZ, respectively, at 973 K. These low ASRs were verified in SOFCs. The electrodes exhibited sufficient strength to allow fabrication of cathode-supported cells.; Composites of La0.8Sr0.2MnO3 and YSZ were also prepared by impregnation method. The effect of calcination temperature on cathode performance was studied using both symmetric cells and SOFCs. Calcination at lower temperatures produces highly porous LSM while higher temperatures, 1323 K or above, LSM tends to form a dense layer over the YSZ. The hysteresis observed upon polarization of these cathodes is associated with the production of microporosity within the dense LSM.; Mixed perovskite La0.8Sr0.2Mn(1-x)CO xO3 can be easily formed by a wet co-impregnation method. CoOx or LSCo can be easily introduced into LSM-YSZ composites without formation of insulating phases. The addition of 10-wt% LSCo to an LSM-YSZ composite dramatically decreased the cathode ASR from 0.45 O·cm 2 to 0.1 O·cm2.; LSM-YSZ composites were able to be prepared by infiltration of LSM nano-particles, of aqueous salt solutions, and of molten salts. All of the composites showed essentially identical performance as SOFC cathodes. AFM images of LSM particles on YSZ(100) suggest that the structures of the LSM-YSZ composites are similar because of LSM mobility. |
