Fabrication And Characterization Of Novle Anode Materials For Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFC) are electrochemical energy conversion devices with high efficiency. With the development of its commercialization implementation, there exist many issues for SOFCs so far that are material and technique problems, two of them are the choose of anode materials with high catalytic property as well as the stability of materials in use. The research of this thesis aims to improve the anode performance, mainly through:(1) developing a novel Ni based anode with negligible ionic conductivity,(2) improving the perovskite anode material by introducting an oxygen ion-conducting oxide,(3) developing new alternative double perovskite anode material.At first, the SOFC principle and the key component materials were briefly introduced at first. Based on highlighting the present development status and direction for SOFCs, proposal on the thesis work was thus presented in this chapter.In chapter2, Ni-LnOx (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb, Tb) composite anodes were developed for intermediate temperature SOFCs. Unlike the conventionalNi-YSZ and Ni-SDC anodes, this anode was considered to be non-ionic conductive due to the negligible ionic conductivity of LnOx. This meant TPB in the anode should be constricted to the physical interface between the electrolyte and anode so that the anode bulk possessed far small TPB. Even so, when the porosity was high enough to eliminate concentration polarization, the Ni-LnOx (Ln=Sm, Eu, Ce, Gd, Dy, Ho, Er and Yb) anodes exhibit very high performance; power density over600mW cm-2is achieved at600℃, comparable to, if not higher than those with the Ni-SDC anodes when the same cathodes and electrolytes were applied. In addition, Ni-Siri2O3cermets were investigated as the anodes with scandia-stabilized zirconia (ScSZ) electrolytes. Single cells with a Ni-SDC interlayer generated peak power density of410mW cm-2at700℃, and the interfacial polarization is about0.7Ω cm2. The high anode performance with both doped ceria electrolytes and stabilized zirconia electrolytes suggested that the high performance in Ni-LnOx (Ln=Sm, Eu, Ce, Gd, Dy, Ho, Er and Yb) anode was possibly due to the catalytic property of LnOxIn chapter3, taken Ni-LnOx (Ln=Dy, Ho, Er and Yb) cermet anodes as example, more systematic study had been carried out for this anode with negligible ionic conductivity. Temperature programmed reduction of NiO mixed with LnOx has shown that the surface properties of Ni in cermets are affected by the neighboring oxide panicles and that the interaction of NiO with LnOx is strong. High H2consumption implied that LnOx possessed high hydrogen adsoiption capability which might enhance spill-over process, thus promoting the surface diffusion with charge transfer through spillover reactions. The hydrogen spillover effect is further shown with impedance spectroscopy. It is reasonable to conclude that the catalytic behavior of LnOx with the promoting effect through nickel and LnOx coupling via the spillover reactions at the TPB is comparable to that of SDC by the extension of the three-phase boundary regions through oxide ion conduction. A continued concerted effort of theory and experiments is proposed in order to unambiguously elucidate the mechanism of the hydrogen oxidation reaction at these novel anodes.In chapter4, Sr2Fe1.5Mo0.5O6-δ (SFM) perovskite was carefully investigated by modifying and optimizing the electrode microstructure. Its ionic conductivity under cathodic and anodic atmosphere was determined with oxygen permeation measurement. Sm0.8Ce0.2O1.9(SDC) was incorporated into SFM electrode to improve the anodic performance. A strong relation was observed between SDC addition and polarization losses, suggesting that the internal SFM-SDC contacts are active for H2oxidation. The best electrode performance was achieved for the composite with30wt.%SDC addition, resulting in an interfacial polarization resistance of0.258D cm2at700℃for La0.8Sr0.2Ga0.8Mg0.2O3-δ supported single cells. Electrochemical impedance spectroscopy analysis indicated that the high performance of SFM-SDC composite anodes was likely due to the high ionic conductivity and electro-catalytic activity of SDC by promoting the ionic exchange processes. Furthermore, this study had demonstrated that the SFM-SDC composite anodes were highly tolerant against redox cycling and carbon deposition.In chapter5, SrTi0.5Ni0.25Mo0.25O3(STNM) was verified as a potential anode materials. STNM was comprehensively studied in terms of its chemical stability and thermal compatibility with La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) electrolytes, the electronic conductivity and electrochemical performance. The XRD results indicated that SrNi0.5Mo0.5O3was chemically stability with50mol%Ti doped. At600-S00℃the total conductivity of SrTi0.5Ni0.25Mo0.25O3(STNM) was14-20S cm-1, fulfilling the conductivity requirement of anode materials. The thermal expansion coefficient of STNM was closed to that of LSGM. The single cell with STNM anode, SLGM electrolyte and SSC cathode at800℃generated peak power density of335mW cm-2and the interfacial polarization resistance was0.305Ω cm2. It showed the performance not lower than that of the (La0.75Sr0.25)0.9Cr0.5Mn0.5O3-δ anode. Besides, STNM anode exhibited stability for directly use of hydrocarbons as the fuel to some extent. In addition, the bulk oxygen p-band center was calculated with first-principles-based in order to indirectly demonstrate the catalytic activity of oxygen reduced. The performance of STNM cathode is expected to be improved by optimizing the electrode microstructure. The electrode reaction mechanism of STNM needs be further investigated.