| Fuel cell technology holds the promise to change the way power is generated, transmitted, and utilized in our increasing demanding lifestyles. State of the art solid oxide fuel cells (SOFCs) utilize an all ceramic design and operate at 750--1000°C. Lower operating temperatures will significantly improve the economics of power generation using SOFCs. The aim of this dissertation was to evaluate and develop component materials for SOFCs, which could work efficiently at temperatures between 500--750°C.; Erbia stabilized bismuth oxide (ESB) shows one of the highest oxygen ion conductivity among all solid electrolytes. However, due to positional and occupational ordering the conductivity decays below the transition temperature (∼600°C). The effect of direct current bias on the ordering phenomenon in ESB was studied using symmetrical cells with Ag-ESB electrodes. At 500°C, the endotherm, related to reverse transition, is enhanced by the applied bias at short time but with negligible change in conductivity decay. It is proposed that the conductivity decay with anneal time is related more to the positional ordering than occupational ordering. Ag-ESB electrodes showed good performance, though were unstable under high currents at 625°C due to Ag migration with oxygen flux.; Novel bismuth ruthenate based cathodes were evaluated using impedance spectroscopy with symmetric cells on gadolinium doped ceria (GDC) electrolytes. Undoped bismuth ruthenate electrode showed area specific resistance (ASR) of 55.64 Ocm2 at 500°C and 1.45 Ocm 2 at 700°C in air. Doping with similar size Ca2+, Ag+, or Sr2+ on Bi3+ site did not improve the electrode performance significantly, while bismuth ruthenate-ESB composites showed 3--4 times lower electrode ASR. Bismuth ruthenate-ESB (62.5:37.5 wt%) composite showed the best performance of 18.4 Ocm 2 at 500°C and 0.32 Ocm2 at 700°C in air. Addition of the ESB phase is believed to reduce the rate limiting surface diffusion in oxygen reduction reaction.; Anode supported thick film GDC electrolyte unit cells were developed for IT-SOFCs. A colloidal deposition technique was used to fabricate dense, thick GDC electrolyte films on porous Ni-GDC anode supports. Pre-sintering temperature of the anode and final sintering temperature of the anode/electrolyte bilayer were found to be the primary parameters determining the density of the film. The sintering temperature of LSCF-GDC (70:30 wt%) composite cathode was optimized to 1250--1350°C, which resulted in a maximum power density of 0.338 W/cm2 at 0.771 A/cm2, 700°C. Current interrupt showed that apart from the electrolyte layer, the ohmic polarization across the cell has significant contributions from the electrodes. |
