| In the first part of this thesis, composite cathodes containing (LaSr)MnO 3 (LSM) or (La,Sr)(Co,Fe)O3 (LSCF) mixed with Y2O 3-stabilized ZrO2 (YSZ) or (CeGd)O3 (GDC) were studied for potential applications in low-temperature solid oxide fuel cells (SOFCs). LSM is the standard cathode used in SOFCs, however composite electrodes can provide a greater density of triple-phase-boundaries, allowing electrochemical reactions to proceed more readily. For each of the composite cathodes, electrical measurements were made using impedance spectroscopy to evaluate electrochemical behavior. The cathodes were characterized over a temperature range of 500--850°C at oxygen partial pressures between 10--3 -- 1atm. The results showed that adding 50wt% YSZ to LSM caused the interfacial resistance of the electrode to decrease by a factor of ≈ 3. Further improvement was observed in LSM-GDC composite cathodes. Most likely, the higher ionic conductivity of GDC, in comparison to YSZ, played a part in increasing oxygen reduction rates. The interfacial resistance was extremely small for cathodes containing a 50-50wt% mixture of LSCF and GDC. For a given temperature, the LSCF-GDC cathode had an electrode resistance that was about 2 orders of magnitude lower than that for the standard LSM cathode. Analysis of the reaction mechanisms suggested oxygen adsorption and dissociation were limiting steps in the LSM-YSZ cathodes. Adsorption also seemed to be a limiting mechanism for the LSM-GDC cathodes. Though specific mechanisms were not identified for the LSCF-GDC composite cathodes, it appeared electrochemical reactions could have been rate limiting.;Demonstrated in the second part of this thesis is the feasibility of direct electrochemical oxidation of methane and ethane without carbon deposition at low-temperature SOFCs. Power densities up to 0.37 W/cm2 at 650°C were measured for single cells operating with pure methane as the fuel and atmospheric-pressure air as the oxidant. The measured power densities were competitive with fuel cells operated on hydrogen. Key factors for successful methane operation were the low operating temperatures and the incorporation of (Y2O3)0.15(CeO2)0.85 -- YDC in the Ni-based anodes. In addition, it was found that reducing the anode Ni content enabled carbon-free operation with ethane fuel at 500°C. |
