ABSTRACT
Oxide diffusion was studied in two innovative SOFC cathode materials, Ba(2)Co(9)O(14) and Ca(3)Co(4)O(9)+δ derivatives. Although oxygen diffusion was confirmed in the promising material Ba(2)Co(9)O(14), it was not possible to derive accurate transport parameters because of an oxidation process at the sample surface which has still to be clarified. In contrast, oxygen diffusion in the well-known Ca(3)Co(4)O(9)+δ thermoelectric material was improved when calcium was partly substituted with strontium, likely due to an increase of the volume of the rock salt layers in which the conduction process takes place. Although the diffusion coefficient remains low, interestingly, fast kinetics towards the oxygen molecule dissociation reaction were shown with surface exchange coefficients higher than those reported for the best cathode materials in the field. They increased with the strontium content; the Sr atoms potentially play a key role in the mechanism of oxygen molecule dissociation at the solid surface.
ABSTRACT
Four new mixed lead-bismuth oxychloride compounds have been prepared and characterized by single crystal X-ray diffraction. Their crystal structures are described on the basis of the association of distinct building units found in parent Pb or Bi oxychlorides. The new compound PbBi4O6Cl2 is formed of the stacking of 2D positive [Bi2O2](2+) layers and neutral [PbBi2O4](0) double layers separated by Cl(-) anions. Similar motifs with finite lengths are combined together in the new series [Pb(n)Bi(10-n)O13][Bi2O2](n)Cl(4+n). From the structural viewpoint, it is striking that this family of homologous phases is strongly related to Bi24O31Cl10 well-known as Arppe's compound in which the fluorite-like [Bi2O2]n subunit was increased from n = 1 (mixed Bi/Pb Arppe's compound) to n = 2, 3, and 4 new members. The preparation of the respective powders shows the predominant stability of the n = 2 term which was prepared as a single-phase, while other terms have not been obtained in absence of secondary phases. For n = 2, the impedance spectroscopy shows a conductivity value σ â¼ 10(-3) S cm(-1) at 650 °C and suggests a contribution of Cl(-) in the diffusion process. Most remarkable, PbBi4O6Cl2 as well as [Pb2Bi8O13][Bi2O2]2Cl6 show very bright red emission at low temperature, which could be assigned to Bi(3+) transitions by comparison to BaBi4O6Cl2. The different shapes of the excitation spectra lead to the assumption of a complete Pb-Bi energy transfer.
