The high temperature oxidation of ferritic stainless steel interconnects results in chromium poisoning of the solid oxide fuel cell (SOFC) electrodes, which is a limiting factor for their utilisation as SOFC interconnects. Chromium-rich spinel materials were studied as electrode supports that would be situated at the interface between interconnect and electrode, in order to reduce the effect of chromium poisoning of the electrodes. The main goal of this thesis was to find chromium-rich spinel materials with good electrical conductivity (σ ≥ 0.1 S∙cm⁻¹) in air and reducing atmosphere, chemically and mechanically stable in SOFC testing conditions. The structure and properties of newly formulated chromium-rich spinels, such as Mn₁₊ₓCr₂₋ₓO₄ (x = 0, 0.5), MnFeₓCr₂₋ₓO₄ (x = 0.1, 1), MgMnCrO₄, MnLiₓCr₂₋ₓO₄ (x = 0.1) and MgMₓCr₂₋ₓO₄, (M = Li, Mg, Ti, Fe, Cu, Ga) were studied aiming at their application as electrode support material for solid oxide fuel cells. Cation distributions were determined by Rietveld refinement from X-ray diffraction (XRD), within the limits of XRD precision and correlated with electrical properties determined experimentally. The chemical stability in reducing conditions was studied and the reduction effects upon materials were evaluated by XRD phase analysis and microstructure analysis. It was found that MnMₓCr₂₋ₓO₄ materials have a limited stability to reduction, only MnCr₂O₄ proved to have good stability when reduced, with negative influence for its p-type semiconductor conductivity. Even though MnFeCrO₄ had limited stability to reduction, in reducing conditions the conductivity changed from p-type to n-type semiconductor. A similar behaviour to reduction was observed for MgFeCrO₄. Also the mechanical and chemical compatibility of some spinels with YSZ was studied in terms of thermal expansion coefficient (TEC/K⁻¹), sintering step and possible chemical reactions. Lithium titanate spinels, starting with LiCrTiO₄, were investigated in terms of structure, properties and spinel - ramsdellite phase transition temperature also with the purpose of new material development. For these materials positive results were obtained in conductivity and chemical stability in reducing conditions. The performance of MnFeCrO₄ and MgFeCrO₄ as electrode support materials was investigated when used alone or impregnated with (La₀.₇₅Sr₀.₂₅)₀.₉₇Cr₀.₅Mn₀.₅O₃, La₀.₈Sr₀.₂FeO₃, Ce₀.₉Gd₀.₁O₂, CeO₂ or Pd. Composite anodes for SOFC were prepared by aqueous infiltration of nitrate salts into porous MnFeCrO₄ and MgFeCrO₄ scaffolds and studied by electrochemical impedance spectroscopy (EIS) in symmetrical cell configuration. The performance of the composite anodes was evaluated in humidified 5%H₂/Ar in order to understand their stability and performance at 850 °C or lower temperature with respect to the porous substrates. It was found that all the impregnated phases adhere very well to the spinel and considerably enhance performance and stability to a level required for SOFC applications. An interesting next step in this work would be to apply such spinel materials on steel interconnects, integrate them into testing SOFC devices and evaluate their effect upon chromium poisoning of the electrodes.

Solid Oxide Fuel Cells (SOFC) are electrochemical energy conversion devices which allow fuel gases, e.g. hydrogen or natural gas, to be converted to electricity and heat at much high efficiencies than combustion-based energy conversion technologies. SOFC are particularly suited to employment in stationary energy conversion applications, e.g. micro-combined heat and power (μ-CHP) and base load, which are certain to play a large role in worldwide decentralisation of power distribution and supply over the coming decades. Use of high-temperature SOFC technology within these systems is also a vital requirement in order to utilise fuel gases which are readily available in different areas of the world. Unfortunately, the limiting factor to the long-term commercialisation of SOFC systems is the redox instability, coking intolerance and sulphur poisoning of the state-of-the-art Ni-based cermet composite anode material. This research explores the ‘powder to power' development of alternative SOFC anode catalyst systems by impregnation of an A-site deficient La0.20Sr0.25Ca0.45TiO3 (LSCT[sub](A-)) anode ‘backbone' microstructure with coatings of ceria-based oxide ion conductors and metallic electrocatalyst particles, in order to create a SOFC anode which exhibits high redox stability, tolerance to sulphur poisoning and low voltage degradation rates under operating conditions. A 75 weight percent (wt. %) solids loading LSCT[sub](A-) ink, exhibiting ideal properties for screen printing of thick-film SOFC anode layers, was screen printed with 325 and 230 mesh counts (per inch) screens onto electrolyte supports. Sintering of anode layers between 1250 °C and 1350 °C for 1 to 2 hours indicated that microstructures printed with the 230 mesh screen provided a higher porosity and improved grain connectivity than those printed with the 325 mesh screen. Sintering anode layers at 1350 °C for 2 hours provided an anode microstructure with an advantageous combination of lateral grain connectivity and porosity, giving rise to an ‘effective' electrical conductivity of 17.5 S cm−1 at 850 °C. Impregnation of this optimised LSCT[sub](A-) anode scaffold with 13-16 wt. % (of the anode mass) Ce0.80Gd0.20O1.90 (CGO) and either Ni (5 wt. %), Pd, Pt, Rh or Ru (2-3 wt. %) and integration into SOFC resulted in achievement of Area Specific Resistances (ASR) of as low as 0.39 Ω cm−2, using thick (160 μm) 6ScSZ electrolytes. Durability testing of SOFC with Ni/CGO, Ni/CeO2, Pt/CGO and Rh/CGO impregnated LSCT[sub](A-) anodes was subsequently carried out in industrial button cell test rigs at HEXIS AG, Winterthur, Switzerland. Both Ni/CGO and Pt/CGO cells showed unacceptable levels of degradation (14.9% and 13.4%, respectively) during a ~960 hour period of operation, including redox/thermo/thermoredox cycling treatments. Significantly, by exchanging the CGO component for the CeO2 component in the SOFC containing Ni, the degradation over the same time period was almost halved. Most importantly, galvanostatic operation of the SOFC with a Rh/CGO impregnated anode for >3000 hours (without cycling treatments) resulted in an average voltage degradation rate of < 1.9% kh−1 which, to the author's knowledge, has not previously been reported for an alternative, SrTiO3-based anode material. Finally, transfer of the Rh/CGO impregnated LSCT[sub](A-) anode to industrial short stack (5 cells) scale at HEXIS AG revealed that operation in relevant conditions, with low gas flow rates, resulted in accelerated degradation of the Rh/CGO anode. During a 1451 hour period of galvanostatic operation, with redox cycles and overload treatments, a voltage degradation of 19.2% was observed. Redox cycling was noted to briefly recover performance of the stack before rapidly degrading back to the pre-redox cycling performance, though redox cycling does not affect this anode detrimentally. Instead, a more severe, underlying degradation mechanism, most likely caused by instability and agglomeration of Rh nanoparticles under operating conditions, is responsible for this observed degradation. Furthermore, exposure of the SOFC to fuel utilisations of >100% (overloading) had little effect on the Rh/CGO co-impregnated LSCT[sub](A-) anodes, giving a direct advantage over the standard HEXIS SOFC. Finally, elevated ohmic resistances caused by imperfect contacting with the Ni-based current collector materials highlighted that a new method of current collection must be developed for use with these anode materials.

In this work, a broad range of analytical methods was applied to the study of the following three materials systems: yttria-stabilised zirconia (YSZ), samarium-doped ceria (SDC) and SDC-supported metal catalysts. YSZ and SDC were studied in the light of their application as solid electrolytes in Solid Oxide Fuel Cells. The SDC-supported metal catalysts were evaluated for application in the reforming of methanol. The conductive properties of YSZ pellets derived from powders of different Y contents and particle size ranges were investigated using Impedance Spectroscopy (IS). Comparative studies of the crystallography (by X-ray Powder Diffraction (XRD)), morphology (by Scanning and Transmission Electron Microscopy (SEM, TEM)), chemical composition (by Energy Dispersive X-ray Spectroscopy (EDX) and Inductively Coupled Plasma Mass Spectroscopy (ICP-MS)) and sintering behaviour (dilatometry) were employed in the overall assessment of the conductivity results collected. Detailed studies of three SDC compositions were performed on nanopowders prepared by a low temperature method developed in the Baker group. Modifications led to a simple and reliable method for producing high quality materials with crystallites of ~10 nm diameter. The products were confirmed by XRD and TEM to be single-phase materials. Thermogravimetric analysis, dilatometry, specific surface area determination, elemental analysis and IS were carried out on these SDC powders. The relationships between particle size, chemical composition, sintering conditions and conductivity were studied in detail allowing optimum sintering conditions to be identified and ionic migration and defect association enthalpies to be calculated. Finally, the interesting results obtained for the SDC nanopowders were a driving force for the preparation of SDC-supported metal catalysts. These were prepared by three different methods and characterised in terms of crystallographic phase, specific surface area and bulk and surface chemical composition. Isothermal catalytic tests showed that all catalysts had some activity for the reforming of methanol and that some compositions showed both very high conversions and high selectivities to hydrogen. These catalysts are of interest for further study and possibly for commercial application.

Cerias, appropriately doped with trivalent rare earth ions in particular, can have high oxide ion conductivity and are attractive as both SOFC (solid oxide fuel cell) electrolytes and anodes. Here, four groups of candidate electrolyte materials were synthesised using a low temperature method in order to determine the effect of multiple doping on their microstructure and ionic conductivity. In an initial study, seven compositions of Ce0.8SmxGd[sub]yNd[sub]zO1.9 (where x, y and z = 0.2, 0.1, 0.0667 or 0 and x + y + z = 0.2) were synthesised and the properties of multiply-doped materials were compared with the corresponding singly-doped parent materials. The effect of co-doping with Gd and Sm was investigated in more detail by preparing and studying five compositions of Ce1−2xSmxGdxO2−x (where x = 0.125, 0.1, 0.0875, 0.075 or 0.05) and seven compositions of Ce0.825SmxGd0.175−xO1.9125 (where x = 0.175, 0.14, 0.105, 0.0875, 0.07, 0.035 or 0). The effect of additional doping with a divalent ion- Ca2+- was studied in six compositions of Ce[sub](0.825+y)Sm[sub](0.0875-y)Gd[sub](0.0875-y)Ca[sub]yO1.9125 (where y = 0, 0.00875, 0.0175, 0.02625, 0.035 or 0.04375). The materials were characterised using scanning and transmission electron microscopy, inductively coupled plasma mass spectrometry and X-ray diffraction. Crystallite sizes were determined in the powders and relative densities and grain size distributions were obtained in sintered pellets. Total, bulk and grain boundary conductivities were obtained using impedance spectroscopy and corresponding activation energies and enthalpies of ion migration and defect association were calculated. The most promising material for SOFCs operating at intermediate temperatures was found to be Ce0.825Sm0.0875Gd0.0875O1.9125 which had a total conductivity at 600 °C of 2.23 S m−1. Lastly, doped ceria materials, primarily Ce0.8Sm0.2O1.9, were employed as catalytic supports for Pd and PdO nanoparticles and these were investigated as SOFC anode materials.