Olga A. Marina

Thermal, electrical, and electrocatalytical properties of lanthanum-doped strontium titanate

Abstract Thermal, electrical and electrocatalytical properties of La x Sr 1− x TiO 3 , where x =0.1, 0.2, 0.3, 0.35, and 0.4, perovskite compositions are studied in relation to their potential use as solid oxide fuel cell (SOFC) anode materials. An emphasis is made on the effect of oxidation–reduction cycling on these properties. Depending on the dopant amount, x , and the oxygen partial pressure, La x Sr 1− x TiO 3 possesses an electrical conductivity on the order of 0.01–500 S/cm at 800–1000 °C. The thermal expansion of La x Sr 1− x TiO 3 is close to that of yttria-stabilized zirconia (YSZ). No significant chemical expansion or contraction of La x Sr 1− x TiO 3 with x p O 2 . La x Sr 1− x TiO 3 is found to be dimensionally and chemically stable when subjected to oxidation–reduction cycling. Cell tests demonstrated the potential ability of the doped titanates to be used as SOFC anodes.

A solid oxide fuel cell with a gadolinia-doped ceria anode: preparation and performance

Abstract The application of doped ceria as an anode material in high-temperature solid oxide fuel cells (SOFC) is described. Deposition of an anchoring layer of YSZ particles was used to obtain sufficient adhesion between a porous Ce0.6Gd0.4O1.8 (CG4) anode and an yttria-stabilised zirconia (YSZ) electrolyte without detrimental reaction. Single SOFCs comprising the CG4 anode, a composite strontium-doped lanthanum manganite-based cathode and the YSZ electrolyte were manufactured and tested in H2/H2O and CH4/H2O atmospheres vs. air in the temperature range of 800–1015°C. An area specific internal resistance of 0.39 Ωcm2 at 0.71 V cell voltage and a power density of 470 mW/cm2 was obtained at 1000°C using H2/H2O/N2=9/1.2/89.8 as the fuel and air as oxidant. A current density of 0.25 A/cm2 at an area specific internal resistance of 2 Ωcm2 was obtained with CH4/H2O/N2=33/3/64. No carbon deposition was found on CG4 after cell operation at a steam-to-carbon ratio of 0.3 for 1000 h. Cells sustained several rapid thermal cycles in the temperature interval 200–1000°C and a full redox cycle without degradation.

Nanoscale Effects on Ion Conductance of Layer-by-Layer Structures of Gadolinia-doped Ceria and Zirconia

Layer-by-layer structures of gadolinia-doped ceria and zirconia have been synthesized on Al2O3(0001) using oxygen plasma-assisted molecular beam epitaxy. Oxygen ion conductivity greatly increased with an increasing number of layers compared to bulk polycrystalline yttria-stabilized zirconia and gadolinia-doped ceria electrolytes. The conductivity enhancement in this layered electrolyte is interesting, yet the exact cause for the enhancement remains unknown. For example, the space charge effects that are responsible for analogous conductivity increases in undoped layered halides are suppressed by the much shorter Debye screening length in layered oxides. Therefore, it appears that a combination of lattice strain and extended defects due to lattice mismatch between the heterogeneous structures may contribute to the enhancement of oxygen ionic conductivity in this layered oxide system.

Electrode Performance in Reversible Solid Oxide Fuel Cells

Electrolysis has long been used to dissociate water into its constituents of oxygen and hydrogen. Various electrolyzers have been developed and are commercially available today, including those based on proton exchange membranes, molten carbonate, phosphoric acid, alkaline, and solid oxide technology. 1-5 Some of these are reversible systems capable of operating both as a fuel cell and as an electrolyzer, although fuel cell and electrolyzer functions are carried out in separate subsystems. A reversible fuel cell can take advantage of excess electrical grid capacity during off-peak hours to produce hydrogen fuel, to be utilized later during periods of high electrical demand. The power unit fuel cell is sized for the peaking load in a practical reversible fuel cell, whereas the electrolyzer is rated at a power that can produce sufficient hydrogen to recharge the hydrogen storage capacity over the remaining hours of the day. If energy conversion, electrical to chemical and chemical to electrical, can occur in the same device with reasonable efficiencies, there could be significant overall cost benefits. For solid oxide electrolysis cells SOEC to be of commercial interest, the cost of the hydrogen produced must be competitive with that of other means of production. The cost of electricity is a significant factor in steam electrolysis, comprising 75% to 95% of that of electrolysis-derived hydrogen according to performance and cost

Oxygen transport studies in nanocrystalline ceria films

Oxygen uptake and conductivity were measured by nuclear reaction analysis and ac impedance technique at the intermediate temperature range on sol-gel grown nanocrystalline ceria films with average grain-sizes 7 nm and 38 nm synthesized at 723 K and 1173 K, respectively. Higher oxygen uptake and lower ionic conductivity are observed in ceria films with {approx}7 nm grain-size. High permeation-assisted oxygen diffusion in nanocrystallites combined with oxygen trapping in the disordered region contributes in higher oxygen uptake. However the lower ionic conductivity in the film results from absence of long-range lattice ordering. Relationship between oxygen uptake and conductivity in ceria is discussed in details by considering grain-size dependent defect density, related surface area and enhanced oxygen mobility.

Improvement of Sintering, Thermal Behavior, and Electrical Properties of Calcium- and Transition Metal-Doped Yttrium Chromite

The A-site calcium doped yttrium chromite was additionally doped with various transition metals on the B-site to improve the sintering, thermal behavior and electrical properties of these ceramics for future use as an interconnect material in high temperature solid oxide fuel cells (SOFC). With 10 % addition of Co, Cu, Ni, Fe, and Mn, the single phase orthorhombic perovskite structure remained stable over a wide range of oxygen partial pressures, as confirmed by X-ray diffraction. The substitution of Cu for chromium remarkably improved the sinterability and allowed full densification in air by sintering at 1400 degrees C. The substitution of Co and Ni significantly improved the electrical conductivity of yttrium chromites in both oxidizing and reducing environments. This was explained by the increase of charge carrier density with nickel and cobalt doping, as confirmed by Seebeck measurements. With 10% of nickel dopant, the electrical conductivity of Y0.8Ca0.2CrO3±δ increased from 12 to 38 S/cm in air and from 2 to 15 S/cm in reducing atmosphere at 950 degrees C. Mn doping had a negative effect on the sintering and electrical conductivity.

Conductivity of Oriented Samaria-Doped Ceria Thin Films Grown by Oxygen-Plasma-Assisted Molecular Beam Epitaxy

We have used oxygen-plasma-assisted molecular beam epitaxy to grow highly oriented Ce 1-x Sm x O 2-δ films on single-crystal c-Al 2 O 3 . The samarium concentration x was varied in the range of 1-33 atom %. It was observed that dominant (111) orientation in Ce 1-x Sm x O 2-δ films can be maintained up to about 10 atom % samarium concentration. Films higher than 10 atom % Sm concentration started to show polycrystalline features. The highest conductivity of 0.04 S cm -1 at 600°C was observed for films with ∼5 atom % Sm concentration. A loss of orientation, triggering an enhanced grain-boundary scattering, appears to be responsible for the decrease in conductivity at higher dopant concentrations.

Ceria Incorporation into YSZ Columnar Nanostructures

We report the growth of porous yttria-stabilized zirconia (YSZ) columnar nanostructures by glancing angle deposition (GLAD) technique. CeO₂ sol solution was incorporated into YSZ creating high interface density columnar nanostructures. Initial experiments suggest higher conductivity in CeO₂/YSZ columnar nanostructures than polycrystalline CeO₂ and lower conductivity than single crystal YSZ in the intermediate temperature range of 600-825 K. GLAD/sol-gel process combination to create high density columnar nanostructures is discussed in the context of solid oxide fuel cells operating at intermediate temperatures.

Ni/YSZ Anode Interactions with Impurities in Coal Gas

Performance of solid oxide fuel cell (SOFC) with nickel/zirconia anodes on synthetic coal gas in the presence of low levels of phosphorus, arsenic, selenium, sulfur, hydrogen chloride, and antimony impurities were evaluated. The presence of phosphorus and arsenic led to the slow and irreversible SOFC degradation due to the formation of secondary phases with nickel, particularly close to the gas inlet. Phosphorus and antimony surface adsorption layers were identified as well. Hydrogen chloride and sulfur interactions with the nickel were limited to the surface adsorption only, whereas selenium exposure also led to the formation of nickel selenide for highly polarized cells.

Effect of coal gas contaminants on solid oxide fuel cell operation

The operation of solid oxide fuel cells (SOFC) was evaluated on simulated coal gas in the presence of several coal gas impurities that are expected to remain in low concentration after warm gas cleanup. Phosphorus, arsenic and sulfur were considered in this study. The presence of phosphorus and arsenic in low, 1-2 ppm, concentrations led to the slow and irreversible SOFC degradation due to the formation of the secondary phases with nickel in the upper part of the nickel-based anode close to the gas inlet. Sulfur interactions with the nickel were limited to the surface only. Cell performance losses due to sulfur exposure were reversible and independent of the presence of other impurities.