Novel Cathodes Materials For Solid Oxide Fuel Cells And Their Appication In Single-Chamber Confirgartions

Solid oxide fuel cell (SOFC) is a device that can directly convert chemical energy to electric energy with very high efficiency and low emissions. With the advantages of fuel flexible, all-solid-state and long-lifetime, much attention around the world has been paid on the research of SOFCs. Current research of SOFCs focuses on lowering the operation temperature range of 500-800°C to promote the commercialization process. However, cathodic overpotential increases considerably with temperature decreasing, and limits the cell performance critically, making the search of high performance cathode urgent. As the hydrocarbon fuels are more economical than hydrogen and are easily obtained, the related research increases in recent years. Furthermore, sealant-free single-chamber SOFC (SC-SOFC) has potential application in portable area, and thus becomes another hot research topic. Based on the research trends, the objective of this work is to investigate the high performance cathode materials and their applications in methane-fueled SC-SOFCs. The cathodes studied were nanosized Sm0.2Ce0.8O1.9 (SDC) modified La1-xSrxMnO3 (LSM), the newly-developed Ba1-xSrxCo0.8Fe0.2O3-δ(BSCF) perovskites and the novel Ba0.5Sr0.5Zn0.2Fe0.8O3–δ(BSZF) cathode that proposed by us firstly.The application of modified LSM cathode in SC-SOFC was firstly studied. With the SDC impregnated LSM cathode, the cell performance was improved significantly. At the furnace temperature of 750oC, the max power density and total cell resistance were 404mW?cm-2 and 1.6??cm-2, which respectively, were 4 times higher and 38.5% of the non-impregnated cell. The enhancement was attributed to the fact that the LSM grains were covered by many ionic conducting SDC nanophase, which generated amount of electrochemically active area and improved the electrochemical properies. The anode-facing-cathode micro-stack using anode-supported cells was proposed. A stack with two single cells generated 371mW at 700oC. Impedance spectra data confirmed that electrode polarization resistance was the primary reason limiting the output of both single cell and stacks. The performance of stacks with 3 or more cells was also limited by the flow field and the size of quartz tube. To solve the problem, another novel design with star-shape was developed, which enables the advantages of uniform flow field, highly shock-resistance and easily obtained higher output. The stack with 4-cells contacted in series generated higher output of 421mW at 750°C, which successfully powered a USB fan. Enlarged stacks are attractive alternatives to batteries in the micropower arena.Ba0.5Sr0.5Co0.8Fe0.2O3–δcathode has demonstrated high performance in both dual- and single-chamber cells. Our research focused on the properties of BaxSr1-xCo0.8Fe0.2O3–δperovskites that were synthesized by a combined EDTA-citric acid complexing method. The phase development of BSCF experienced complexing intermediate process and the baking temperature of≥900°C was need for the formation of pure-phase BSCF. The effect of Ba content on crystal structure, TG, thermal expansion and conductivity were measured. The results showed that the oxygen loss, TEC and conductivity decreased with the increasing of Ba content, indicating the important role of oxygen deficiency. Lattice oxygen began loss at about 400oC, resulting in amount of oxygen vacancy and the thermal reduction of high valance of Co and Fe ions. Abnormal expansion behavior was found for the first time and its mechanism was explained. The conductivity relaxation and size relaxation of BSCF were parimarily studied. The oxygen diffusion coefficients were 5.0×10-9-5.9×10-8 cm2?s-1 between 400°C and 500°C. Moreover, the BSCF55-SDC composite cathodes were studied. Our results showed that the electrochemical performance of BSCF was improved obviously with the addition of SDC electrolyte, and the optimal composition was found to be BSCF-30wt%SDC. Using this composite cathode, the SDC interlayered YSZ film cell generated max power densities of 1090mW?cm-2 and 1200W?cm-2, respectively, when the cathode was fed by air and oxygen flow. Electrode polarization resistance mainly limited the performances. In single-chamber mode, the OCV of the cell reached high value of about 1V at 550°C. At the furnace temperatures of 600°C, the cell output achieved 220mW?cm-2. But the output decreased to 224mW?cm-2 when temperature increased to 700°C, due to the imperfect selective of electrodes at higher temperature.Finally, the cobalt-free Ba0.5Sr0.5Zn0.2Fe0.8O3–δ(BSZF) perovskite, formerly as oxygen permeation membrane, was investigated as a novel cathode for SOFCs. After baking at 950°C for 5h, precursor turned to cubic perovskite powder completely. The room-temperature oxygen nonstoichiometry in BSZF, as determined by iodometric titration experiment, was as high as 0.412, indicating high concentration of vacancy existed which was preferred for oxygen reduction. The electrical conductivity was relatively low with a peak value of 9.4 S?cm-1 at about 590°C, which was mainly caused by the high concentration of oxygen vacancy and the doping of bivalent zinc that doesn't contribute to small polaron conduction. The optimal firing condition of BSZF cathode on SDC was 950°C for 4h. The dependence of Rp with oxygen partial pressure indicated that the rate-limiting step for oxygen reduction was oxygen adsorption/desorption kinetics. The polarization resistance of pure BSZF at 650°C was 0.48??cm2, which is comparable to LSCF cathode. After SDC impregnation, the resistance decreased to 0.28??cm2, which was further activated to 0.21??cm2 after 24h operation. Using BSZF as the cathode, the wet hydrogen fueled Ni+SDC?SDC?BSZF cell exhibited peak power densities of 392mW?cm-2, 208mW?cm-2 and 626mW?cm?2 and 107mW?cm-2 at 650°C,600°C and 550°C respectively, using stationary air as oxidant. When using oxygen as oxidant, improved outputs of 626mW?cm-2 , 353mW?cm-2 and 173mW.cm-2 were obtained. In single-chamber condition, the cell exhibited peak power densities of 202mW?cm-2 and 173mW?cm-2, at the furnace temperatures of 600°C and 550°C respectively. Owning to the self-heating effect, the performances in single chamber mode were comparable to that in dual-chamber cell powered by hydrogen.In conclusion, three kinds of new cathodes were investigated. Impregnated LSM cathodes can enhance the cell performance obviously. A novel star-shaped micro-stack was proposed which can be applied in portable arena. The properties of mixed conducting BSCF and cobalt-free BSZF cathodes were systemically studied, which exhibited attractive performances for intermediate-to-low temperature SOFC and single-chamber SOFC.