Thesis (Ph.D.) - University of St Andrews, January 2008.

Thesis (Ph.D.) - University of St Andrews, October 2009.

Thesis (MS)--Montana State University--Bozeman, 2009. / Typescript. Chairperson, Graduate Committee: Steven R. Shaw. Includes bibliographical references (leaves 63-65).

Thesis (Ph.D.) -- McMaster University, 2001. / Includes bibliographical references. Also available via World Wide Web.

Thesis (M.S.)--West Virginia University, 2006. / Title from document title page. Document formatted into pages; contains vi, 100 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 98-100).

Thesis (M.S)--Ohio University, March, 2005. / Title from PDF t.p. Includes bibliographical references (p. 132-134)

Thesis (Ph. D.)--Montana State University--Bozeman, 2007. / Typescript. Chairperson, Graduate Committee: Max Deibert. Includes bibliographical references (leaves 74-77).

Nanostructured materials with high specific surface areas and high pore volumes are of interest for applications in solid oxide fuel cells (SOFCs). This study describes the use of novel preparative methods for obtaining nanostructured samarium-doped ceria (SDC) with a view to its application as an anode material in SOFCs. The strategy employed in this work was based on the nanocasting concept. Template materials with a polymer, carbon or silica framework are first obtained using a self-assembly process. These materials have long range networks of ordered mesopore channels and so act as templating moulds. From a three step procedure of precursor impregnation, in-situ formation of SDC by calcination and template removal, SDC with the inverse pore structure of the template is created. Novel methods for producing such SDC materials were applied and the products evaluated. As silica templates have wide ranging applications involving exposure to high temperatures -not least in nanocasting- it was desirable to understand the thermal stability of these materials over a range of temperatures. A systematic study was conducted on three representative silica templates. An inherent problem in nanocasting from silica templates is retention of residual silica after the template removal step. A detailed investigation into these alternative wet chemistry procedures was undertaken. To circumvent the silica problem completely, a number of alternative templates made of mesoporous carbon were considered. A range of ordered mesoporous carbons were prepared and evaluated as templates. To provide a comparator for the ordered SDC materials, a simple combustion method was used to prepare an SDC product without the influence of a structure directing template. The techniques of TEM, SEM-EDX, UV–Vis spectroscopy, MAS-NMR, PXRD and gas physisorption were used to characterise the physical and chemical properties of the products in the bulk and at the nanoscale.

Oxide ion conductors have received much attention in recent years due to their application as solid oxide fuel cells (SOFC) electrolytes. A strong correlation exists between the oxide ion conductivity and the crystal structure of an oxide ion conductor. Consequently, to develop novel electrolytes it is important to discover new structural families of oxide ion conducting materials. In the present study, the electronic properties and crystal structure of the hexagonal perovskite derivative Ba3MoNbO8.5 are reported. Ionic transport number and variable oxygen partial pressure conductivity measurements evidenced that Ba3MoNbO8.5 presents solid oxide ion conduction. A bulk conductivity of 2.2 x 10-3 S cm-1 at 600 C was observed, which is comparable to other leading oxide ion conductors. Ba3MoNbO8.5 is the first hexagonal perovskite derivative to exhibit fast solid oxide ion conductivity. The Ba3MoNbO8.5 structure was described by a hybrid structural model composed by a superimposition of the 9R hexagonal perovskite and palmierite structures. (Mo/Nb)O4 units coexist with (Mo/Nb)O6 units within the structure, forming a disordered arrangement of Mo/Nb tetrahedra and octahedra. Variable temperature neutron diffraction experiments allowed determination of the structural factors at the basis of the oxide ion conduction. In particular, the flexible coordination of the Mo/Nb cations and the distortion of the Mo(1)/Nb(1) polyhedra are thought to enhance the electrical properties so that a conductivity comparable to other leading solid oxide ion conductors is observed. Study of the electrical and structural features of the Ba3Mo1 xNb1+xO8.5-x/2 (x = 0.10, 0.20, 0.30) series also evidenced that the relative ratio of (Mo/Nb)O4 tetrahedra to (Mo/Nb)O6 octahedra and the disorder of the oxygen sub lattice are important for the conduction of the Ba3MoNbO8.5 system. Impedance spectroscopy measurements on the hexagonal perovskite derivatives Ba7MoNb4O20 and Ba3WNbO8.5 showed evidence of ionic contributions in these systems. In addition, neutron diffraction experiments revealed that both Ba7MoNb4O20 and Ba3WNbO8.5 exhibit structural characteristics analogues to Ba3MoNbO8.5. The results of the present study indicate the prospect of designing new oxide ion conductors with mixed tetrahedral and octahedral d-metal units in the hexagonal perovskite family.

T* based La₀.₉Ln₀.₉Sr₀.₂CuO₄ (Ln = Sm & Gd) has been investigated as cathode material for intermediate temperature solid oxide fuel cell using Ce₀.₉Gd₀.₁O₁.₉₅ (GDC) and La₀.₉Sr₀.₁Ga₀.₈Mg₀.₂O₃-δ (LSGM-9182) as the electrolyte material. Both oxides crystallize in tetragonal P4/nmm symmetry. The structural and phase stability has been confirmed up to 800 °C by High temperature XRD studies. The coefficient of thermal expansion (CTE) and oxygen content decrease with decreasing size of the Ln³+ ions from Ln = Sm to Gd. While the decrease in CTE is due to the increasing co-valence of the Ln–O bond, the decrease in electrical conductivity at high temperature is due to the increasing oxide ion vacancies and a bending of the O–Cu–O bonds. The highest value of DC conductivity has been observed for the LSSCu, which showed a metal like temperature dependence. LGSCu showed a semiconductor to metallic temperature dependence of conductivity with a maximum of 25 Scm-¹. From the microstructural characterization and the polarisation resistance measurement of the symmetric cells at temperature ranges from 700 - 800 °C, 900 °C has been chosen as the most suitable sintering temperature and LGSCu has shown the minimum polarization resistance of 0.35 Ωcm² and 0.09 Ωcm² at 800 °C using GDC and LSGM-9182 electrolytes respectively under OCV condition. To improve the ASR of LGSCu, the composite of LGSCu and GDC with varying wt. % of GDC has been optimised and it shows the ASR of 0.12 Ωcm² using GDC as the electrolyte because it enhance the triple phase boundary region. The maximum power density of single-cell SOFCs fabricated with the La₀.₉Ln₀.₉Sr₀.₂CuO₄ (Ln= Sm & Gd) cathodes, La₀.₉Sr₀.₁Ga₀.₈Mg₀.₂O₃-δ (LSGM-9182) electrolyte, and Ni–Ce₀.₉Gd₀.₁O₁.₉₅ cermet anode exhibit 720 and 824 mWcm-² at 800 °C respectively. The phase pure T* Nd₁.₃₂Ce₀.27Sr₀.₄₁CuO₄-δ (NCSCu) has been synthesized by combustion method and its crystal chemistry, thermal and electrochemical properties, and catalytic activity in SOFC were evaluated using LSGM-9182 as the electrolyte. It shows promising performance and can be used as potential cathode materials for IT-SOFC. The effect of B-site Ni and Co substitution for Cu on the structural and electrochemical properties of the T* La₀.₉Gd₀.₉Sr₀.₂CuO₄ has been investigated as cathode materials for intermediate temperature solid oxide fuel cells using LSGM-9182 as the electrolyte. At a given temperature, the electrical conductivity gradually increases with increasing Ni content and the CTE gradually decreases. Ni doping has also improved the electrochemical performance. Sr doped A /A //B₂O₅+δ (A / = Rare Earth, A // = Ba or Sr and B = Transition Metals) layered perovskites improves the electrochemical performance due to the increase in electrical conductivity and smaller size difference between Ln+³ and Sr+². However these layered perovskites suffer from high thermal expansion coefficient (20-23 x 10-6 K-1) which does not match with the state of the art electrolyte materials. B-site transition metal doped layered perovskites of compositions SmBa₀.₅Sr₀.₅Co₂-ₓO₅+δ (M = Cu, Ni, Fe) have been investigated as cathode material for intermediate temperature solid oxide fuel cell using LSGM-9182 as the electrolyte material. Phase purity has been confirmed by XRD technique. The crystal cell parameters have been found out using Rietveld refinement by FULLPROF software. The substitution of Cu, Ni and Fe for Co lowers the CTE of Co-based materials by suppression of the spin state transition of Co³+ which will be highly advantageous for long term SOFC application. The introduction of transition metals exhibit inferior electrochemical performance to pristine cathode using LSGM-9182 as the electrolyte but still shows reasonable power density with advantage of lower CTE value thereby can be explored as promising cathode material for IT-SOFCs.