Samuel Georges

Reducibility of fast oxide-ion conductors La2−xRxMo2−yWyO9(R = Nd, Gd)

The substitutional range and cell parameter evolution of fast oxide-ion conductors La2−xRxMo2−yWyO9 (R = Nd, Gd) are investigated. In the whole series, the cubic β-La2Mo2O9 structural type is stabilized at room temperature. The effects on reducibility of both single and double substitutions are presented. Lanthanum substitution by rare earth appeared to be responsible for an increase in the reducibility and a strong but reversible amorphization under dilute hydrogen. On the contrary, the favourable role of tungsten on the compound stability under reducing conditions is evidenced: it depletes oxygen loss while making the La2Mo2O9 structural type less affected by it.

Modeling of a SOFC Fueled by Methane:Anode Barrier to Allow Gradual Internal Reforming Without Coking

Natural gas appears to be a highly attractive fuel for solid oxide fuel cell systems. To avoid the cooling effect occurring in direct internal reforming, gradual internal reforming (GIR) can be used. GIR is based on local coupling between steam reforming and hydrogen oxidation. The steam required for the reforming reaction is obtained from hydrogen oxidation on the anode side. Previous studies have demonstrated that the cooling effect has disappeared. However, with GIR, the risk of carbon formation is greater. To deal with this issue, a different cell configuration was studied. This configuration combines a catalyst layer with a cermet anode, allowing GIR without coking. The study comprised simulations, using the CFD Research Corporation software package, of the behavior of a tubular solid oxide fuel cell when using GIR. A thermodynamic study based on the partial pressure distributions within the cell was also carried out to investigate the occurrence of carbon formation. A parametric analysis of the reforming rate and the thickness of the layer were then performed. The simulations indicate that the risk of carbon deposition is strongly reduced if the configuration is used for a catalyst layer of 900 μm and at a reforming rate in the catalyst only ten times higher than the reforming rate in nickel.

Comparison of Structural and Electrical Properties of Barium Zirconate Pellets and Thin Films

BaZr 0.84 Y 0.16 O 3-α coatings and pellets were prepared by co-sputtering and glycine-nitrate process, respectively, to compare their structural and electrical properties. Both the films and ceramics present the BaZrO 3 perovskite structure. Structural, morphological, and electrical observations indicate a complex effect of ZnO addition in BZY ceramics. ZnO strongly affects the microstructure and electrical properties of the samples, whereas no trace of ZnO can be detected. All the observations indicate that Zn 2+ is at least partially substituted to Zr 4+ in the perovskite during the sintering process and that the presence of ZnO strongly affects the sintering behavior. The optimal conductivity/density ratio is obtained for 1 wt % ZnO. This sample was taken as a reference to determine the properties of the films. For the coating, the scanning electron microscope observations revealed that the as-deposited coatings are dense. The films are ~100 times more resistive than the pellets. This difference is due to a blocking effect related to the collecting system particularly strong in hydrogen.

Synthesis, crystal structure, sintering and electrical properties of a new alluaudite-like triple molybdate K0.13Na3.87MgMo3O12

A new triple molybdate K0.13Na3.87MgMo3O12 was synthesized by solid state reaction. The crystal structure has been determined by single X-ray diffraction and the electrical conductivity measured by impedance spectroscopy. The title compound crystallizes in the monoclinic space group C2/c with a = 12.9325 (8) A, b = 13.5537 (9) A, c = 7.1627 (6) A, β = 112.212 (9)°, V = 1162.33 (14) A3 and Z = 4. The final agreement factors are R = 0.0241, wR (F2) = 0.0584, S(F2) = 1.22. The magnesium–molybdate 3D-framework belongs to the alluaudite type. The structure is formed by infinite chains composed of edge-sharing (Mg/Na)2O10 dimmers, which are linked together via bridging MoO4 tetrahedra, yielding to a three-dimensional framework enclosing two distinct types of hexagonal tunnels in which Na+ and K+ cations reside. The structural model is validated by bond valence sum (BVS) and charge distribution (CD) methods. Ball milling is used as mechanical means to reduce the particles sizes of the synthesized powder. At the optimal sintering temperature of 650 °C, a relative density of 81% was obtained. The microstructures were characterized by scanning electron microscopy. The compound undergoes a phase transformation at 528 °C accompanied by an abrupt increase of the electrical conductivity. Above this phase transition, the electrical conductivity reaches 10−2 S cm−1. Thus K0.13Na3.87Mg(MoO4)3 may be considered as a promising compound for developing new materials with high ionic conductivity.

Oxide ion diffusion in optimised LAMOX materials

A series of optimised materials in the family with parent compound La(2)Mo(2)O(9) have been studied using the isotope exchange depth profile technique and SIMS analysis. The data collected indicate that the oxide ion diffusion coefficients in both the parent compound and optimised materials are significantly higher than those reported for any of the fluorite structured electrolyte materials, with a peak value for the La(1.7)Gd(0.3)Mo(2)O(9) composition of 1.41 x 10(-6) cm(2) s(-1) at 800 degrees C. Further, it was found that the substitution of isotopic oxygen into the ceramic was limited in dry atmospheres but significantly enhanced in a H(2)(18)O/(16)O atmosphere, in common with recent reports on the diffusion/exchange behaviour of the BIMEVOX class of oxide ion conductors.