Study Of Anode And Electrolyte Materials For Ceria Based Intermediate Temperature Solid Oxide Fuel Cells

Solid Oxide Fuel Cells (SOFCs) are regarded as one of the cleanest and most efficient devices for direct conversion to electricity of a wide variety of fuels, from hydrogen to hydrocarbons, coal gas, and bio-derived renewable fuels. It is believed that broad application of SOFCs for power generation alone can significantly reduce pollutant emission and consumption of fossil fuels.In Chapter1, a brief introduction is presented on the basic concepts and the main component materials of SOFCs. Then it is concluded that for the commercialization of SOFC technologies, it is vital to lower the operation temperature from traditional800-1000℃to700℃or lower, which requires further development of both high performance electrode and electrolyte materials at reduced temperatures. Thus, in the following parts of this thesis, we will mainly focus on the revelation of the reason why nanoparticles impregnation can improve the performance of Ni based anode (Chapter2), the optimization of samaria-doped ceria (SDC) electrolyte synthesis by glycine-nitrate process (GNP)(Chapter3), the fabrication of SDC electrolyte based single cells at reduced co-firing temperature (Chapter4), as well as the design of simplified three-layer stainless steel supported single cells (Chapter5), respectively, with the goal of promoting the development and commercialization of intermediate and low temperature SOFCs.Impregnated nanoparticles are very effective in improving the electrochemical performance of SOFC anodes possibly due to the extension of reaction sites and/or the enhancement of catalytic activity. To date, however, it remains unclear which effect plays a dominant role. Thus, in Chapter2, SDC, pure ceria, samaria, and alumina oxides impregnated Ni-based anodes are fabricated to compare the site extending and the catalytic effects. Except for alumina, the impregnation of the other three nano-sized oxides could substantially enhance the performance of the anodes for the hydrogen oxidation reactions. Moreover, single cells with CeO2and Sm2O3impregnated anodes could exhibit as great performance as those with SDC impregnated anodes. When the impregnation loading reached the optimal value,1.7mmolcm-3, these cells exhibit very high performance, with peak power densities around750mWcm-2. The high performance of CeO2and Sm2O3impregnated anodes demonstrates that the improved performance are mainly attributed to the significantly improved electrochemical activities of the anodes, but not to the extension of triple-phase-boundary, and wet impregnation is indeed an alternative and effective technique to introduce these nano-sized catalytic active oxides into the anode configuration of SOFCs to enhance cell performance, stability and reliability.On the other hand, considerable efforts have been devoted to the development of single cells with thin-film electrolytes of doped ceria, which show much higher ionic conductivities than the state-of-the-art yttria-stabilized zirconia (YSZ) electrolytes at reduced temperatures. A simple and elegant approach to fabrication of dense electrolyte films on porous anode substrates is a co-pressing and co-firing process, significantly reducing the fabrication cost. However, the critical step is the synthesis of electrolyte powders with extremely low apparent density, usually achieved via a GNP. In Chapter3, proper combination of Ce(NO3)3and Ce(NH4)2(NO3)6as mixed cerium source is shown to be more effective in achieving SDC powders with low apparent density for easy fabrication of thin-film electrolyte membrane with very high sintered density and excellent ionic conductivity. In particular, when the molar ratio of the two cerium precursors is around1:1, the derived SDC powders can be readily sintered to high density, exhibiting the highest conductivities (～0.084and～0.020Scm-1at800and600℃, respectively) with the activation energy of～0.70eV. When the molar ratio of Ce(NO3)3to Ce(NH4)2(NO3)6was adjusted to3:1, the derived SDC powders have the lowest apparent density (36.0±0.5mgcm-3), best suited for preparation of dense, thin-film SDC electrolyte membranes on porous anode substrates, a critical step toward low-cost fabrication of high-performance SOFCs.For SDC based electrolyte, one of the main drawbacks which may limit its utilization is its poor sinterability, making it difficult to achieve sufficiently high density below1500℃. Thus, many efforts have been devoted to improving the sinterability of SDC powders in order to effectively reduce the firing temperature, to optimize the electrode microstructure, and to minimize or eliminate unfavorable diffusion and reaction between cell components. In Chapter4, we have successfully fabricated high-performance, single cells by co-pressing followed by co-sintering at a temperature as low as1150℃using highly-active SDC powders derived from a modified GNP which uses75mol%Ce(NO3)3and25mol%Ce(NH4)2(NO3)6as a mixed cerium source as elaborated in Chapter3. In particular, the low firing temperature has resulted in anode microstructures with more appropriate porosity, grain size, and connectivity with the electrolyte, significantly reducing both the ohmic resistance and the electrode polarization resistance and enhancing cell performance. In addition, it is found that the electrode polarization at high current densities is significantly suppressed when operated at650℃.Recently, there have been lots of interests in developing metal-supported SOFCs, driven by their excellent strength, high tolerance to extremely rapid thermal cycling and redox cycling, as well as low material cost. These metal-supported SOFCs are usually four-layer structure consisting of the metal support, the anode, the electrolyte and the cathode. In the last chapter, a simplified three-layer design without the anode interlayer is proposed. The novel design is demonstrated by co-firing YSZ electrolytes and43OL stainless steel substrates, where Ni and doped ceria are impregnated to increase the catalytic activity towards electrochemical oxidation. Peak power density as high as246mWcm-2is obtained at700℃, and good tolerance to five complete redox cycles is also initially demonstrated, suggesting that this design is feasible for high performance metal-supported SOFCs.