Nanoscale clusters in the high performance thermoelectric AgPbmSbTem+2

The local structure of the AgPbmSbTem+2 series of thermoelectric materials has been studied using the atomic pair distribution function (PDF) method. Three candidate-models were attempted for the structure of this class of materials using either a one-phase or a two-phase modeling procedure. Combining modeling the PDF with HRTEM data we show that AgPbmSbTem+2 contains nanoscale inclusions with composition close to AgPb3SbTe5 randomly embedded in a PbTe matrix.Comment: 7 pages, 5 figures, 2 tables, submitted to PR

Crystal structure and electrochemical properties vs. Na+ of the sodium fluorophosphate Na1.5VOPO4F0.5

Pure Na1.5VOPO4F0.5 was prepared by tuning the synthesis conditions previously reported by Barker et al. Using FTIR, Rietveld analysis and atomic absorption measurements, the stoichiometry and structure were unambiguously determined. The refined structure shows the same framework as the one ascribed to "NaVPO4F" but clearly underline the presence of two different sodium sites (8h and 8j), one fluorine site (2a) and one octahedral V4+ site [VO5F]. We further examined the Na+ insertion mechanism of this phase whose signature was similar to the one of "NaVPO4F" and "Na3V2(PO4)2F3". Namely two voltage plateaux are found at 3.6 and 4.0 V vs. Na+/Na and are characteristics of two bi-phasic transitions. However the overall reversible capacity does not exceed 0.56 Na ion per formula unit, and is furthermore hindered by a high voltage phenomenon, most likely linked to electrolyte degradation. Finally AC impedance measurements carried out on a dense pellet showed a RT ionic conductivity of 1.8 × 10-7   S / cm with an activation energy of 0.43 eV. © 2006 Elsevier SAS. All rights reserved

Crystal structures of new silver ion conductors Ag7Fe3(X2O7)(4) (X = P, As)

International audienceThe crystal structures of new Ag7Fe3(X2O7)(4) (X = P, As) compounds, prepared through ion exchange from their sodium analogs, are reported. They adopt the monoclinic crystal system and exhibit an Ag ordering on cooling evidenced by a lowering of the symmetry from C-centered to primitive Bravais lattices. Crystal structures were determined from single-crystal X-ray diffraction at 100 K and 298 K for each composition. The structure consists of FeO6 octahedra sharing their corners with P2O7 dimers to form a three-dimensional framework [Fe-3(P2O7)(4)](7-) into which the silver ions are located. The differences between the four structures lie on the distribution of the silver ions within this framework, at the origin of a strong anisotropy in conductivity. Temperature displacement factors on Ag sites are generally higher in the arsenate than in the phosphate, in good correlation with conductivity data

Crystal structures and sodium/silver distributions within the ionic conductors Na5Ag2Fe3(As2O7)(4) and Na2Ag5Fe3(P2O7)(4)

International audienceThe crystal structures of new Na5Ag2Fe3(As2O7)(4) and Na2Ag5Fe3(P2O7)(4) compounds, prepared through ion exchange from Na7Fe3(X2O7)(4) (X = P, As) are reported. They crystallize in the monoclinic C2/c space group and exhibit a Na/Ag ordering on cooling. Crystal structures were determined from single crystal X-ray diffraction at 100 and 298 K for each composition. The structure consists of FeO6 octahedra sharing their corners with X2O7 dimers (X = P, As) to form a three-dimensional framework [Fe-3(X2O7)(4)](7-) into which the sodium/silver ions are located. The differences between the four structures lie on the distribution of the sodium/silver ions within this framework giving a developed insight of the ionic diffusion paths

Structural investigation of composite phases Ba1+x[(NaxMn1-x)O-3] with x similar or equal to 2/7, 5/17 and 1/3; exotic Mn4.5+ valence

International audienceStructural models are proposed for three composite compounds of chemical formula Ba1+x[(NaxMn1-x)O-3] (x similar or equal to 2/7, 5/17 and 1/3) by single crystal X-ray diffraction; superspace formalism is used to obtain an unified description of the three phases. The modulation affecting Ba atoms can be easily designed but the competition "occupational/displacive" modulations relating to the Mn/Na metallic columns were particularly difficult to modelize. However, the large amplitude of the displacive modulation affecting the oxygen atoms (similar or equal to +/- 0.7 angstrom) in comparison with that observed for related compounds (similar or equal to +/- 0.3 angstrom) makes it a direct consequence of the Na+ alkali insertion inside the trigonal prisms. Owing to this insertion, the Mn atoms exhibit, in the three phases, an "exotic" oxidation state of about +4.5. In addition, even if the sequence between face sharing MnO6 octahedra and NaO6 trigonal prisms can be analytically deduced from the x value, it is clear that the Na/Mn contrasts play in favour of its accurate determination through the XRD single crystal refinement

BITX: new electrolyte for oxide ion and proton SOFC

International audienceBa2In2(1-x)Ti2xO5+x•1-x compounds (BITx) have been prepared by solid state reaction. Whereas Ba2In2O5 corresponds to an ordered Brownmillerite structure, for 0.15 < x < 1 all members adopt a disordered cubic perovskite structure at RT. This phase transformation induces a drastic increase of the anionic conductivity level. At 700{degree sign}C, 10-2 S.cm-1 is obtained for BIT07. Stable under hydrogen, the x = 0.7 was selected and a SOFC built on BIT07 and Ni/BIT07 cermet was tested. The peak power is about 500mW at 800{degree sign}C. Due to the basicity of these compounds, water uptake of BITx was investigated. The change of the lattice volume upon hydration, inferred from X-ray diffraction data, appears as an important parameter for the characterization of perovskite-type proton conductors. Proton conductivity higher than 1 mS.cm-1 was measured at 400{degree sign}C for BIT02. A ceramic fuel cell based on this material as electrolyte was evaluated

Evaluation of Ba-2(In0.8Ti0.2)(2)O5.2-n(OH)(2n) as a potential electrolyte material for proton-conducting solid oxide fuel cell

International audienceElectrochemical measurements of fuel cells based on proton conductor electrolyte Ba-2(In0.8Ti0.2)(2) O5.2-n(OH)(2n) and prepared through a tape casting process and a co-pressing of anode-composite powder and electrolyte tape were performed at 500 degree

Influence of the autocombustion synthesis conditions and the calcination temperature on the microstructure and electrochemical properties of BaCe0.8Zr0.1Y0.1O3−δ electrolyte material

International audienc

Cathode materials for La0.995Ca0.005NbO4 proton ceramic electrolyte

[EN] The study presents the chemical and mechanical compatibility of the proton conducting electrolyte La0.995Ca0.005NbO4 (LCNO) with the LSM, LSCM and BSCF cathodes and the electrochemical performance of symmetrical cells based on LCNO. After annealing at high temperature the electrolyte-cathode mixtures in air and wet air, the obtained products were analyzed by X-ray powder diffraction (XRPD). The microstructure of the cathode and electrolyte materials and the interfaces were observed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX). The results show that LSCM cathode is chemically and mechanically stable with the LCNO electrolyte although the BSCF cathode reacts with it. Cation diffusion was observed between LSM cathode and LCNO electrolyte after the heat treatment of their mixture at T = 1150 degrees C. The electrochemical study performed on symmetrical cells revealed that the LSCM cathode presents the lowest value of area specific resistance (ASR) compared to the ones of the LSM and BSCF cathodes: ASR(LSCM) = 35 Omega cm(2); ASR(LSM) = 57 Omega cm(2); ASR(BSCF) = 416 Omega cm(2) (in humidified air at 750 degrees C). Finally, a CER-CER approach was used in order to minimize the polarisation resistance of the LSM cathode by mixing LSM and LCNO in different volumetric ratios. The lowest value of ASR for LSM-based composite cathode was obtained by adding 50 vol.% of LCNO to LSM cathode (ASR(LSM/LCNO) = 22 Omega cm(2) in humidified air at 750 degrees C).This work has been performed in the frame of the FP7 Project EFFIPRO "Efficient and robust fuel cell with novel ceramic proton conducting electrolyte" (Grant Agreement 227560).Kravchyk, K.; Quarez, E.; Solis Díaz, C.; Serra Alfaro, JM.; Joubert, O. (2011). Cathode materials for La0.995Ca0.005NbO4 proton ceramic electrolyte. International Journal of Hydrogen Energy. 36(20):13059-13066. https://doi.org/10.1016/j.ijhydene.2011.07.0691305913066362