Nigel M. Sammes

Physical, chemical and electrochemical properties of pure and doped ceria

This paper gives an extract of available data on the physical, chemical, electrochemical and mechanical properties of pure and doped ceria, predominantly in the temperature range from 200 to 1000°C. Several areas are pointed out where further research is needed in order to make a better basis for the evaluation of the real potential and limits for the practical application of ceria in solid oxide fuel cells and other solid state electrochemical devices.

Sintering and thermal expansion characterization of Al-doped and Co-doped lanthanum strontium chromites synthesized by the Pechini method

Abstract The air-sintering and thermal expansion characteristics of Al-doped and Co-doped lanthanum strontium chromites synthesized by the Pechini method have been discussed as interconnect materials in high-temperature solid oxide fuel cells (SOFC). Fine perovskite powders with an average particle size of 0.24 μm, and with a surface area of 3.5 m2/g, calcined at 800 °C for 1 h, were obtained. After the Co-doped samples were heated at 1600 °C for 10 h, surface analysis showed the materials to have a densely packed structure with little sign of porosity. It was, however, found that large pores remained inside the samples. By energy dispersive X-ray analysis, it was clarified that the composition on the surface shifted to a B-site lean region by cobalt-vaporization. The La0.9Sr0.1Cr0.95Al0.02Co0.02O3 sample with a slight chromium-deficiency enhanced bulk diffusion sintering, and showed the relative densities to be ≥94% when sintered at 1600 °C for 20 h. The average thermal expansion coefficient (TEC) of this material in the temperature range from 50 to 1000 °C was 10.4×10−6/°C in air and 11.2×10−6/°C in a H2 atmosphere. The La0.9Sr0.1Cr0.95Al0.02Co0.02O3 powder synthesized by the Pechini method is the one that is suitable for use as the interconnect material for solid oxide fuel cells.

Thermal-expansion behaviors and mechanisms for Ca- or Sr-doped lanthanum manganite perovskites under oxidizing atmospheres

The thermal expansion behavior and mechanism of A-site-deficient lanthanum manganite perovskites, La{sub 1{minus}x}MnO{sub 3} (0 < x {le} 0.1), and alkaline earth metal (AE)-doped lanthanum manganite perovskites, La{sub 1{minus}x}AE{sub x}MnO{sub 3} (AE = Ca and Sr, 0 {le} x {le} 0.4), under oxidizing atmospheres have been investigated. The average linear thermal expansion coefficients of the AE-doped lanthanum manganites decreased with increasing Ca content up to 20 mol % and with increasing Sr content up to 10 mol %, and then increased. During the thermal cycle measurement, the La{sub 1{minus}x}MnO{sub 3} (0 {le} x {le} 0.1) perovskite samples and the La{sub 1{minus}x}AE{sub x}MnO{sub 3} perovskites (0 {le} x {le} 0.3) shrank, while no anomalous shrinkage behavior was observed for the La{sub 0.6}AE{sub 0.4}MnO{sub 3} samples. From the results of high-temperature X-ray diffraction analysis, no dependency of cell volume of the perovskites on aging time at selected temperatures and on thermal cycles was observed.

Sintering behavior of Ca- or Sr-doped LaCrO3 perovskites including second phase of AECrO4 (AE=Sr, Ca) in air

Abstract The sintering characteristics and mechanisms of stoichiometric alkaline earth metal (AE)-doped lanthanum chromites, including the second phase of AECrO4 with low-melting point in air, (La1−xAExCrO3, AE=Ca and Sr, 0≤x≤0.3), have been discussed. When x≤0.2, the sintering properties of La1−xCaxCrO3 are lower than those of La1−xSrxCrO3. However, the reverse results were observed at x=0.3. The mixtures of LaCrO3 and AECrO4 showed low sintering properties. It was found that the reaction of the second phases with LaCrO3 started at temperatures lower than the melting point. Thermal expansion coefficients (TECs) in the temperature range from 50 to 1000°C were 8.6×10−6/°C for LaCrO3, 13.1×10−6/°C for CaCrO4, and 17.5×10−6/°C for SrCrO4. The second phases with larger TEC among the LaCrO3 particles disturb the sintering characteristics of the LaCrO3 particles during the high temperature heat treatment.

Small-scale fuel cells for residential applications

Abstract The market and technical requirements for small-scale fuel cells in residential applications are investigated, focusing on the 1 to 10 kW range. In particular, the peculiar features of the New Zealand situation are explored, with its specific energy resources and demands. It is shown that various technologies could be applied, with PEM, SOFC, PAFC and AFC competing on almost equal terms, with cost targets of 500 to 700 EUR/kW. The attributes and disadvantages are discussed, with a number of technology gaps being identified, and some solutions proposed. Two new developments in the PEM and SOFC systems are compared in relation to their use in domestic applications. The obvious premium application of fuel cells in New Zealand exists where grid connection is expensive. Other priority markets are also studied.

The effect of sintering on the mechanical properties of SOFC ceramic interconnect materials

Solid oxide fuel cell (SOFC) interconnect materials, strontium and calcium doped lanthanum chromite, were synthesized to investigate the effect of dopant content and sintering temperature on their sinterability. The results show that approximately 96% of the theoretical density could be achieved when LaCrO3, doped with 30 mol% Ca, was sintered in air at 1400°C. However, to get the same sintered density for the strontia doped material, a 1700°C sintering temperature had to be used. The effect of sintering temperature on the fracture strength was also investigated. A maximum fracture strength of 234 MPa for La0.7Sr0.3CrO3−δ and 256 MPa for La0.7Ca0.3CrO3−δwere obtained for both samples sintered at 1700°C.

Ionic conductivity of ceria/yttria stabilized zirconia electrolyte materials

Abstract CeO 2 -based electrolytes are promising candidates for intermediate temperature SOFC applications. However, they are prone to electronic conduction at low partial pressures of oxygen. This paper describes the effect of yttria stabilized zirconia (YSZ) additions on (CeO 2 ) 0.8 (GdO 1.5 ) 0.2 materials between 0 and 20 mol% YSZ [8 mol% YSZ (8YSZ) and 3 mol% YSZ (3YSZ)], on the ionic conductivity. For all samples examined, a single cubic-phase was observed, stable between room temperature and 800 °C. Between 1 and 5 mol% 8YSZ and 3YSZ, the ionic conductivity was very similar to (CeO 2 ) 0.8 (GdO 1.5 ) 0.2 with no YSZ additions. However, the ionic domain increased from P o 2 = 10 −15 to 10 −17 atm, showing greater stability of the system containing YSZ.

Sintering behavior and mechanism of Sr-doped lanthanum chromites with A site excess composition in air

Abstract Sintering characteristics and mechanisms of lanthanum strontium chromites, (La 1− x Sr x ) 1+ y CrO 3 (0≤ x ≤0.3, 0≤ y ≤0.02), in air are discussed in comparison with the results for Ca-doped lanthanum chromites. The sinterability of La 0.9 Sr 0.1+ y CrO 3 increased with increasing alkaline earth (AE) content, but was much lower than that of La 0.9 Ca 0.1+ y CrO 3 . It was found that the ratios of AE to Cr formed a second phase in the sample in the region 0.5 x x 0.9 Sr 0.1 CrO 3 with 2 mol% sintering aid was clarified. With increasing ionic field strength, that is with decreasing basicity, the relative density of the doped La 0.9 Sr 0.1 CrO 3 perovskites decreased.

Extruded Tubular Strontium- and Magnesium-Doped Lanthanum Gallate, Gadolinium-Doped Ceria, and Yttria-Stabilized Zirconia Electrolytes Mechanical and Thermal Properties

Thermal properties and mechanical strength of ceramic fuel-cell components are critical in allowing for rapid start-up and operational stability at high temperatures of solid oxide fuel cell (SOFC) systems. Tubular SOFC electrolytes were prepared using a plastic extrusion technique from a dough containing the electrolyte powders and additives. Strontium- and magnesium-doped lanthanum gallate (LSGM), gadolinium-doped ceria (CGO), and 8 mol % yttria-stabilized zirconia (YSZ) were studied. Burst failure strength, thermal expansion coefficient, and thermal shock resistance of the extruded tubular LSGM, CGO, and YSZ electrolytes were investigated and compared. Three-point bending strength of these three extruded electrolyte materials were tested at room temperature, 600, 800, and 1000°C in air, and the results are discussed.

Novel applications for micro-SOFCs

Abstract The application of micro-solid oxide fuels cells in small systems is discussed. Two types of application are examined, namely, leisure CHP systems and micro-hybrid vehicles. A unique triple layer catalyst–SOFC–catalyst system has been designed utilising propane/butane fuel. The system consists of a co-generating gas burner with a pre-reforming catalyst, a micro-SOFC stack and an oxidation catalyst. The pre-reforming catalyst comprising of Ru metal on Saffil® ceramic wool, was used to partially reform the propane/butane gas prior to entering the fuel cell, preventing carbon formation. The micro-SOFCs were YSZ tubes (Adelan, UK) with nickel/YSZ cermet anodes on the outside and strontium-doped lanthanum manganite cathodes on the inside. Final oxidation was provided by a cordierite honeycomb coated with platinum combustion catalyst producing most of the heat for the fuel cell operation. Initial performance results were obtained and it was shown that a co-generating system could be achieved using a propane/butane fuel supply, piezoelectric ignition system and air supply for the triple catalyst system. The application of this micro-SOFC system for leisure and micro-hybrid vehicles, such as golf trolleys and power-assisted bicycles, is described.