Novel Materials For Electrolytes And Anodes Of Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) is an all-state energy conversion device which can efficiently produce electricity by electrochemically combining fuel and oxidant gases. In Chapter1, a brief introduction is presented on the structure, the working principle, as well as the main component materials of SOFC. During the commercialization process, there are still many issues to overcome, which requires further development on both the key materials and the fabrication technologies. In this dissertation, the direct utilization of hydrocarbon fuels (Chapter2,3and4) and the intermediate temperature operation (Chapter5) are concerned for the further development of SOFC.Conventional Ni-based anodes exhibit excellent electrochemical performance when hydrogen is used as the fuel. When hydrogen carbon, such as methane is directly used as the fuel in an SOFC system, however, the performance decreases obviously with the susceptibility of nickel based anodes to carbon deposition. There are two main approaches for the direct utilization of hydrocarbon fuels, including the optimization of nickel based anodes (Chapter3) and the development of alternative materials(Chapter4). Wet ion-impregnation is one of the most effective and commonly used methods of introducing catalytic active nano-particles for the optimization of nickel based anodes. Therefore, in Chapter2, the latest studies of ion-impregnation techniques are reviewed in the fabrication of nano-structured anodes in SOFC, either from the literatures or from some own research in our group.Although it has been widely proved that nano-structured Sm0.2Ce0.8O1.9(SDC) particles impregnated Ni-based anodes could exhibit both higher performance and good long-term stability when methane, iso-octane and other hydrocarbons are used as the fuel, it is still lack of deep understandings on the mechanism of the coking tolerance of such nano-structured anodes. Therefore, in chapter3, the catalytic cracking of methane over SDC coated and. conventional Ni-based anodes are compared under normal atmospheric pressure, using tunable synchrotron VUV photoionization mass spectrometry (PIMS) combining with molecular-beam mass spectrometry (MBMS) technique. It is shown that SDC nanoparticles coating can significantly decrease the decomposition temperature of methane while increasing its conversion ratio. In addition, ethylene is observed as a decomposition product on the nano-sized SDC modified Ni-SDC catalysts. Considering the fact that ethylene, which is generally recognized as a precursor of carbon deposition, can lead to very quick generation of carbon, our results indicates that the coating of SDC nanoparticles can effectively suppress carbon deposition by blocking the direct contact between hydrocarbon fuels and Ni particles in the anodes.Besides appropriate optimizations of conventional Ni-based anodes, another effective approach to avoid carbon deposition is developing alternative materials, such as perovskites, as a substitution of Ni-based anodes. In chapter4, to search good candidates for SOFC anodes, doped SrSnO3materials are preliminarily investigated. The (Sr0.95La0.05)0.9SnO3-δ ceramic is synthesized using the solid state reaction method, and a pure orthorhombic perovskite phase is observed. Moreover, the perovskite structure is quite stable in high temperature (800℃) reducing condition, and its conductivity is about14Scm-1at800℃in wet H2(3%H2O) atmosphere, which is sufficient to be SOFC anode. In addition, no observable impurity phases are formed when (Sr0.95La0.05)0.9SnO3-δ powders are mixed and co-fired at1000℃with SDC and yttria-stabilized zirconia (YSZ), respectively, and the average thermal-expansion coefficient (TEC) of (Sr0.95La0.05)0.9SnO3-δ is also close to those of the two electrolytes, thus indicating that (Sr0.95La0.05)0.9SnO3-δ is chemically and thermally compatible with traditional SOFC electrolyte materials, further improving its potential as SOFC anode materials. Subsequently, more A site deficient samples are tried, as a series of (Sr0.95La0.05)xSnO3-δ (x=0.9,0.85,0.8,0.75,0.7,0.65) materials are synthesized, all of which show pure perovskite structures. When exposed to reducing atmosphere, however, while the other samples can maintain their perovskite structures, the precipitation of tin can be detected on (Sr0.95La0.05)xSnO3-δ (x=0.7,0.65) ceramics. The conductivity measurement shows that with the increasing of A site deficiency, the conductivity of (Sr0.9sLa0.05)xSnO3-δ (x=0.9,0.85,0.8,0.75) becomes higher, and (Sr0.95La0.05)0.9SnO3-δ exhibits the highest conductivity, which is about24Scm-1at800℃in wet H2atmosphere.Besides the direct utilization of hydrocarbon fuels, lowering down the operation temperature to intermediate temperatures is another main object of SOFC commercialization, which can be achieved by developing novel highly conductive electrolyte materials, as well as by decreasing the electrolyte membrane thickness. As is recently reported that some interstitial oxygen ion conductive materials, such as melilite type La1.54Si0.46Ga3O727could behave very high oxygen ionic conductivity, the La1.54Sr0.46Ga3O7.27materials and its derivatives, are systematically investigated on phase structures and electrical properties in Chapter5for their potential use as SOFC electrolytes. All of the ceramics, including Ln1+xSr1-xGa307-x/2(Ln=La, Pr, Nd, Sm, Eu, Gd, Dy, Yb, Y,-0.1<x<0.54), La1.54Sro.46Ga3+x07.27-δ (x=-0.10,-0.05,0.00,0.05) and La1.54Sr0.46Ga2.95M0.05O7.27-5(M=A1, In, Zn, Ge) series are synthesized using the solid state reaction method. A fairly pure melilite phase is only observed for LaL54Sr0.46Ga3O7.27, Lni.2Sr0.8Ga3O7.1(Ln=La and Pr) and Ln1.1Sr0.9Ga3O7.05(Ln=La, Pr, Nd, and Sm) ceramics. Among these pure melilite materials, La1.54Sr0.46Ga3O7.27affords the highest conductivity due to its higher La/Sr ratio (1.54/0.46), thus could be a candidate of SOFC electrolyte. For La1.54Sr0.46Ga3+xO7.27-δ (x=-0.10,-0.05,0.00,0.05) series, the melilite phase is very sensitive to Ga composition and sintering temperature. Either a slight Ga excess or a low temperature sintering (≤1300℃) is not benefit for obtaining pure melilite structures. And a small amount of Ga deficiency (1.67%) or a high temperature (1450℃) sintering leads to higher conductivities, which may be attributed to the distortion of the melilite structure for the shortage of Ga. Also, when Ga3+is partially (1.67%) replaced by Al3+or Zn2+, fairly pure melilite phases can be formed after sintering at1350℃. In addition, the introduction of Al3+and Zn2+shows excellent effects of inhibiting the grain growth in such materials, thus increasing the ionic conductivities in certain temperature ranges.