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Effects of Nb5+, Mo6+, and W6+ dopants on the germanate-based apatites as electrolyte for use in solid oxide fuel cells

Authors Sea-Fue Wang, Yung-Fu Hsu, Wan-Ju Lin
Source
International Journal of Hydrogen Energy
Volume: 38, Issue: 27, Pages: 12015–12023
Time of Publication: 2013-09
Abstract Rare information is available in the literature on the cell performance of the solid oxide fuel cells (SOFCs) using apatites known for their good electrical conductivity as electrolyte materials. In this study, La9.5Ge5.5Nb0.5O26.5, La9.5Ge5.5Mo0.5O26.75, and La9.5Ge5.5W0.5O26.75 ceramics were prepared and characterized. The results indicated that the La9.5Ge5.5Nb0.5O26.5 and La9.5Ge5.5W0.5O26.75 ceramics reported hexagonal phase, while the La9.5Ge5.5Mo0.5O26.75 ceramic demonstrated triclinic symmetry. Among the apatities evaluated, La9.5Ge5.5Nb0.5O26.5 sintered at 1450 C showed the best conduction with an electrical conductivity value of 0.045 S/cm at 800 C. Button cells of NiO–SDC/La9.5Ge5.5Nb0.5O26.5/LSCF–SDC were built and revealed good structural integrity. The total ohmic resistance (R0) and interfacial polarization resistance (RP) of the cell read 0.428 and 0.174 Ω cm2 and 0.871 and 1.164 Ω cm2, respectively at 950 and 800 C. The maximum power densities (MPD) of the single cell at 950 and 800 C were respectively 0.363 and 0.095 W cm−2. Without optimizing the anode and cathode as well as hermetic sealing of the cell against the gas, the study found the performance of the single cell with the pure La9.5Ge5.5Nb0.5O26.5 as its electrolyte material superior to those of the SOFC cells with a YSZ electrolyte of comparable thickness shown in the literature.
Keywords Solid oxide fuel cell; Apatite; Impedance; Cell performance
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Transformation from insulating p-type to semiconducting n-type conduction in CaCu3Ti4O12-related Na(Cu5/2Ti1/2)Ti4O12 ceramics

Authors Li, Ming, Sinclair, Derek C.
Source
Journal of Applied Physics
Volume: 114, Issue: 3, Pages: 034106 - 034106-8
Time of Publication: 2013-07
Abstract A double doping mechanism of Na+ + ½ Ti4+ → Ca2+ + ½ Cu2+ on the general formula Ca1-xNax(Cu3-x/2Tix/2)Ti4O12 has been used to prepare a series of isostructural CaCu3Ti4O12 (CCTO)-type perovskites. A complete solid solution exists for 0 ≤ x ≤ 1 and all compositions exhibit incipient ferroelectric behaviour with higher than expected intrinsic relative permittivity. Although CCTO ceramics typically exhibit n-type semiconductivity (room temperature, RT, resistivity of ∼10–100 Ω cm), Na(Cu5/2Ti1/2)Ti4O12 (NCTO) ceramics sintered at 950 C consist of two insulating bulk phases (RT resistivity > 1 GΩ cm), one p-type and the other n-type. With increasing sintering temperature/period, the p-type phase transforms into the n-type phase. During the transformation, the resistivity and activation energy for electrical conduction (Ea ∼ 1.0 eV) of the p-type phase remain unchanged, whereas the n-type phase becomes increasingly conductive with Ea decreasing from ∼ 0.71 to 0.11 eV with increasing sintering temperature. These changes are attributed to small variations in stoichiometry that occur during high temperature ceramic processing with oxygen-loss playing a crucial role.
Remark Link

New double molybdate Na9Fe(MoO4)6: Synthesis, structure, properties

Authors Aleksandra A. Savina, Sergey F. Solodovnikov, Olga M. Basovich, Zoya A. Solodovnikova, Dmitry A. Belov, Konstantin V. Pokholok, Irina A. Gudkova, Sergey Yu. Stefanovich, Bogdan I. Lazoryak, Elena G. Khaikina
Source
Journal of Solid State Chemistry
Volume: 205, Pages: 149–153
Time of Publication: 2013-09
Abstract A new double molybdate Na9Fe(MoO4)6 was synthesized using solid state reactions and studied with X-ray powder diffraction, second harmonic generation (SHG) technique, differential scanning calorimetry, X-ray fluorescence analysis, Mssbauer and dielectric impedance spectroscopy. Single crystals of Na9Fe(MoO4)6 were obtained and its structure was solved (the space group RView the MathML source3, a=14.8264(2), c=19.2402(3) , V=3662.79(9) 3, Z=6, R=0.0132). The structure is related to that of sodium ion conductor II-Na3Fe2(AsO4)3. The basic structure units are polyhedral clusters composed of central FeО6 octahedron sharing edges with three Na(1)О6 octahedra. The clusters share common vertices with bridging МоО4 tetrahedra to form an open 3D framework where the cavities are occupied by Na(2) and Na(3) atoms. The compound melts incongruently at 904.70.2 K. Arrhenius type temperature dependence of electric conductivity σ has been registered in solid state (σ=6.810−2 S сm−1 at 800 K), thus allowing considering Na9Fe(MoO4)6 as a new sodium ion conductor.
Keywords Sodium–iron molybdate; Crystal structure; Solid-state electrolyte
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Study of bulk and grain-boundary conductivity of Ln2+xHf2−xO7−δ (Ln = Sm-Gd; x = 0, 0.096) pyrochlores

Authors A. V. Shlyakhtina, S. N. Savvin, A. V. Levchenko, A. V. Knotko, Petra Fedtke, Andreas Busch, Torsten Barfels, Marion Wienecke, L. G. Shcherbakova
Source
Journal of Electroceramics
Volume: 24, Issue: 4, Pages: 300-307
Time of Publication: 2010-06
Abstract The electrical conductivity of new solid electrolytes Eu2.096Hf1.904O6.952 and Gd2Hf2O7 have been compared with those for different pyrochlores including titanates and zirconates Ln2+xМ2−xO7−δ (Ln = Sm-Lu; M = Ti, Zr; x = 0−0.81). Impedance spectroscopy data demonstrate that Eu2.096Hf1.904O6.952 and Gd2Hf2O7 synthesized from mechanically activated oxides have high ionic conductivity, comparable to that of their zirconate analogues. The bulk and grain-boundary components of conductivity in Sm2.096Hf1.904O6.952 (Тsynth = 1600С), Eu2.096Hf1.904O6.952 and Gd2Hf2O7 (Тsynth = 1670С) have been determined. The highest bulk conductivity is offered by the disordered pyrochlores prepared at 1600C and 1670C: ~1.5  10−4 S/cm for Sm2.096Hf1.904O6.952, 5  10−3 S/cm for Eu2.096Hf1.904O6.952 and 3  10−3 S/cm for Gd2Hf2O7 at 780С, respectively. The conductivity of the fluorite-like phases at the phase boundaries of the Ln2+xМ2−xO7−δ (Ln = Eu, Gd; M = Zr, Hf; x ~ 0.286) solid solutions, as well as that of the high-temperature fluorite-like phases Ln2+xМ2−xO7−δ (Ln = Eu, Gd; M = Zr, Hf; x = 0−0.286), is lower than the conductivity of the disordered pyrochlores Ln2+xМ2−xO7−δ (Ln = Eu, Gd; M = Zr, Hf; x = 0−0.096).
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Ionic conduction in glasses in the MnNbOF5-BaF2-BiF3 system

Authors S. A. Polyshchuk, L. N. Ignat’eva, S. L. Sinebryukhov, S. V. Gnedenkov, A. B. Podgorbunsky, N. N. Savchenko, A. B. Slobodyuk, V. M. Bouznik
Source
Russian Journal of Inorganic Chemistry
Volume: 58, Issue: 4, Pages: 387-391
Time of Publication: 2013-04
Abstract The electrical conductivity of oxyfluoride glasses in the MnNbOF5-BaF2-BiF3 system in the temperature range 299–550 K was studied by impedance spectroscopy. It was shown that the conductivity is mainly caused by fluoride ions forming fluorobismuth polyhedra in the glass structure, being as high as 7.46 10−3 S/cm (533 K) in the 20MnNbOF5-30BaF2-50BiF3 system reaches, which is at the level of the best values for fluoride glasses.
Remark Link

Solid-state photoelectrochemical H2 generation with gaseous reactants

Authors Kingsley O. Iwu, Augustinas Galeckas, Andrej Yu. Kuznetsov, Truls Norby
Source
Electrochimica Acta
Volume: 97, Pages: 320–325
Time of Publication: 2013-05
Abstract Photocurrent and H2 production were demonstrated in an all solid-state photoelectrochemical cell employing gaseous methanol and water vapour at the photoanode. Open circuit photovoltage of around -0.4 V and short circuit photocurrent of up to 250 μA/cm2 were obtained. At positive bias, photocurrent generation was limited by the irradiance, i.e., the amount of photogenerated charge carriers at the anode. Time constants and impedance spectra showed an electrochemical capacitance of the cell of about 15 μF/cm2 in the dark, which increased with increasing irradiance. With only water vapour at the anode, the short circuit photocurrent was about 6% of the value with gaseous methanol and water vapour. The photoanode and electrocatalyst on carbon paper support were affixed to the proton conducting membrane using Nafion as adhesive, an approach that yielded photocurrents up to 15 times better than that of a cell assembled by hot-pressing, in spite of the overall cell resistance of the latter being up to 5 times less than that of the former. This is attributed, at least partially, to reactants being more readily available at the photoanode of the better performing cell.
Keywords Photoelectrochemical; hydrogen; TiO2; solid-state; Nafion
Remark Link

Metallic Interconnects for Proton Ceramic Fuel Cells. Oxidation behavior and transport properties under simulated fuel cell conditions

Author Anders Werner Bredvei Skilbred
Source
Time of Publication: 2013-03
Remark Dissertation for the degree of Philosophiae Doctor
Link

Synthesis and characterization of perovskite-type SrxY1−xFeO3−δ (0.63≤x<1.0) and Sr0.75Y0.25Fe1−yMyO3−δ (M= Cr, Mn, Ni), (y=0.2, 0.33, 0.5)

Authors J.J. Biendicho, S. Shafeie, L. Frenck, D. Gavrilova, S. Bhme, A.M. Bettanini, P. Svedlindh, S. Hull, Z. Zhao, S.Ya. Istomin, J. Grins, G. Svensson
Source
Journal of Solid State Chemistry
Volume: 200, Pages: 30-38
Time of Publication: 2013-04
Abstract Abstract Oxygen-deficient ferrates with the cubic perovskite structure SrxY1−xFeO3−δ were prepared in air (0.71≤x≤0.91) as well as in N2 (x=0.75 and 0.79) at 1573 K. The oxygen content of the compounds prepared in air increases with increasing strontium content from 3-δ=2.79(2) for x=0.75 to 3-δ=2.83(2) for x=0.91. Refinement of the crystal structure of Sr0.75Y0.25FeO2.79 using TOF neutron powder diffraction (NPD) data shows high anisotropic atomic displacement parameter (ADP) for the oxygen atom resulting from a substantial cation and anion disorder. Electron diffraction (ED) and high-resolution electron microscopy (HREM) studies of Sr0.75Y0.25FeO2.79 reveal a modulation along <1 0 0>p with G ~0.4<1 0 0>p indicating a local ordering of oxygen vacancies. Magnetic susceptibility measurements at 5–390 K show spin-glass behaviour with dominating antiferromagnetic coupling between the magnetic moments of Fe cations. Among the studied compositions, Sr0.75Y0.25FeO2.79 shows the lowest thermal expansion coefficient (TEC) of 10.5 ppm K−1 in air at 298–673 K. At 773–1173 K TEC increases up to 17.2 ppm K−1 due to substantial reduction of oxygen content. The latter also results in a dramatic decrease of the electrical conductivity in air above 673 K. Partial substitution of Fe by Cr, Mn and Ni according to the formula Sr0.75Y0.25Fe1−yMyO3−δ (y=0.2, 0.33, 0.5) leads to cubic perovskites for all substituents with y=0.2. Their TECs are higher in comparison with un-doped Sr0.75Y0.25FeO2.79. Only M=Ni has increased electrical conductivity compared to un-doped Sr0.75Y0.25FeO2.79.
Keywords Perovskites; Neutron diffraction; Electron diffraction; High-temperature conductivity; Thermal expansion; Magnetic susceptibility
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Optimization of synthesis conditions for rare-earth titanate based oxygen ion conductors

Authors A.V. Shlyakhtina, D.A. Belov, S.Yu. Steafanovich, E.A. Nesterova, O.K. Karyagina, L.G. Shcherbakova
Source
Solid State Ionics
Volume: 230, Pages: 52-58
Time of Publication: 2013-01
Abstract High-density (Yb0.9Ca0.1)2Ti2O6.9, (Yb0.8Ca0.1Tb0.1)2Ti2O7 − δ, and (Dy0.8Ca0.1Tb0.1)2Ti2O7 − δ solid solutions have been prepared through co-precipitation followed by firing for 4 h at 1500 and 1550 C, and their crystal structure (XRD), microstructure (SEM), and oxygen ion conductivity (impedance spectroscopy) have been studied in relation to the firing temperature and precipitant used. As in the case of (Yb0.9Ca0.1)2Ti2O6.9 and (Yb0.8Ca0.1Tb0.1)2Ti2O7 − δ, the optimal synthesis temperature for (Dy0.8Ca0.1Tb0.1)2Ti2O7 − δ is 1500 C. The bulk oxygen ion conductivity of the pyrochlore-like solid solutions (Yb0.9Ca0.1)2Ti2O6.9 is a stronger function of synthesis temperature than that of the (Dy0.8Ca0.1Tb0.1)2Ti2O7 − δ and (Yb0.8Ca0.1Tb0.1)2Ti2O7 − δ solid solutions with more complex A sublattice. The rise of the synthesis temperature from 1500 to 1550 C has detrimental effect on the grain boundary conductivity of the (Yb0.9Ca0.1)2Ti2O6.9 and (Dy0.8Ca0.1Tb0.1)2Ti2O7 − δ ceramics. That effect is connected with a considerable grain-boundary segregation of a calcium-containing phase in the (Yb0.9Ca0.1)2Ti2O6.9 and (Dy0.8Ca0.1Tb0.1)2Ti2O7 − δ. The bulk and grain boundary conductivity of (Dy0.8Ca0.1Tb0.1)2Ti2O7 − δ are independent of the precursor synthesis conditions (homogeneous and non-homogeneous co-precipitation).
Keywords Synthesis; Co-precipitation; Pyrochlore; Doping; Oxide ion conductivity; Impedance spectroscopy
Remark Link

Multilayered thin films for oxidation protection of Mg2Si thermoelectric material at middle–high temperatures

Authors S. Battiston, S. Boldrini, S. Fiameni, A. Famengo, M. Fabrizio, S. Barison
Source
Thin Solid Films
Volume: 526, Pages: 150–154
Time of Publication: 2012-12
Abstract Multilayered molybdenum silicide-based thin films were deposited via radio frequency magnetron sputtering in order to obtain efficient barrier against oxidation process which affected Mg2Si thermoelectric materials at middle–high temperatures. X ray diffraction, energy dispersive spectroscopy, secondary ion mass spectroscopy, field emission scanning electron microscopy (FE-SEM) and electrical measurements at high temperature were carried out in order to obtain, respectively, the structural, compositional, morphological and electrical characterization of coatings. Furthermore, the mechanical behavior of the thin film/Mg2Si-pellet system was observed in situ as a function of temperature by FE-SEM employing a heating module. Moreover, the barrier properties for oxygen protection after thermal treatment in air at high temperature were qualitatively evaluated.
Keywords Thin film; Thermoelectric material; Magnesium silicide; Molybdenum silicide; Middle–high temperature
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Thin films of SnO2-CeO2 binary oxides obtained by pulsed laser deposition for sensing application

Authors Simona Somacescua, Rares Scurtu, George Epurescu, Rovena Pascu, Bogdana Mitu, Petre Osiceanu, Maria Dinescu
Source
Applied Surface Science
Time of Publication: 2012-11
Abstract Binary tin oxide – cerium oxide thin films with ceria concentrations in the range 10- 30% have been obtained by pulsed laser deposition technique, with or without additional oxygen RF plasma beam assistance. A good preservation of the Ce/Sn atomic concentration and Ce3+ content on the film surface of about 30% was obtained for almost all the investigated conditions of substrate temperature and RF powers. The sharp decrease of the electrical resistance in hydrogen environment at temperatures above 300 C indicates a direct interaction between hydrogen and metal oxides surfaces leading to OH groups formation, as evidenced by XPS measurements. The highest sensitivity (∼40) was attained for the sample with 10% ceria and RF assistance, while the lowest operating temperature (∼250-320 C) was encountered for that with 30% ceria deposited in the presence of RF discharge.
Keywords RF assisted pulsed laser deposition; SnO2-CeO2 binary oxides; Ce3+ and Ce4+ concentration; sensitivity in H2 atmosphere
Remark Available online
Link

Phase formation, electrical properties and morphotropic phase boundary of 0.95Pb(ZrxTi1−x)O3–0.05Pb(Mn1/3Nb2/3)O3 ceramics

Authors Anurak Prasatkhetragarna, Rattikorn Yimnirun
Source
Ceramics International
Volume: 39, Pages: S91–S95
Time of Publication: 2013-05
Abstract Ferroelectric ceramics in specific composition of 0.95Pb(ZrxTi1−x)O3–0.05Pb(Mn1/3Nb2/3)O3 or PZT–PMnN (with x=0.46, 0.48, 0.50, 0.52, and 0.54) have been investigated in order to identify the morphotropic phase boundary (MPB) composition. The effects of Zr/Ti ratio on phase formation, dielectric and ferroelectric properties of the specimens have also been investigated and discussed. X-ray diffraction patterns indicate that the MPB of the tetragonal and rhombohedral phase lies in x=0.52. The crystal structure of PZT–PMnN appeared to change gradually from tetragonal to rhombohedral phase with increasing Zr content. The dielectric and ferroelectric properties measurements also show a maximum value (εr, tan δ and Pr) at Zr/Ti=52/48, while the transition temperature decreases with increasing Zr content.
Keywords Dielectric properties; Ferroelectric properties; MPB
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Oxygen ion conductivity of (Yb0.9 − xTbxCa0.1)2Ti2O7 − δ solid solutions

Authors A. V. Shlyakhtina, D. A. Belov, S. Yu. Stefanovich, O. K. Karyagina and L. G. Shcherbakova
Source
Inorganic Materials
Volume: 48, Issue: 11, Pages: 1126-1130
Time of Publication: 2012-11
Abstract We have studied terbium substitution for ytterbium in (Yb0.9 − x Tb x Ca0.1)2Ti2O7 − δ (x = 0.1, 0.2, 0.3, 0.4) pyrochlore solid solutions synthesized through coprecipitation followed by firing at 1550C. The results indicate that only a small amount of terbium (less than 10%) can be incorporated into the pyrochlore structure of (Yb0.9Ca0.1)2Ti2O6.9 because of the large difference in ionic radius between the terbium and ytterbium cations: Δr = r(TbCN 83+) − r(YbCN 83+) = 0.055 . The oxygen ion conductivity of the (Yb0.9 − x Tb x Ca0.1)2Ti2O7 − δ solid solutions has been determined by impedance spectroscopy in air in the temperature range 300 to 900C. At high temperatures (t > 640C), their bulk conductivity was essentially independent of the Yb/Tb ratio. The observed decrease in density and microstructural changes were insignificant. At relatively low temperatures (t < 640C), the bulk conductivity decreased slightly, and the decrease depended little on terbium concentration.
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H and Li Related Defects in ZnO and their Effect on Electrical Properties

Authors Tor Svendsen Bjrheim , Skjalg Erdal , Klaus Magnus Johansen , Knut Erik Knutsen , and Truls Norby
Source
J. Phys. Chem. C
Volume: 166, Issue: 44, Pages: 23764–23772
Time of Publication: 2012-10
Abstract Li and H are important electrically active impurities in ZnO and this work presents a detailed experimental and computational study of the behavior of H and Li in ZnO, and their effect on its defect structure. We employ AC conductivity measurements as a function of temperature and partial pressure of O2, H2O and D2O, which is combined with first principles density functional theory (DFT) calculations and thermodynamic modeling (TDM) of finite temperature defect structures in undoped and Li doped ZnO. Undoped ZnO is dominated by protons as hydroxide defects (OH_O^•), oxygen vacancies (v_O^(••)) and electrons under a large variety of atmospheric conditions, and we also predict from DFT and TDM the substitutional hydride ion (H_O^•) to dominate concentration-wise under the most reducing conditions at temperatures above 500 C. The equilibrium concentrations of defects in ZnO are small, and dopants such as Li strongly affect the electrical properties. Experimentally, Li doped ZnO is found to be n-type under all available atmospheric conditions and temperatures, with a n-type conductivity significantly lower than that of as-grown ZnO. The n-type conductivity also increases with decreasing p_(O_2 ) and with increasing p_(H_2 O). The observed electrical properties of Li doped ZnO are attributed to dominance of the ionic defects Li_Zn^/, OH_O^•, Li_i^•, v_O^(••), and the neutral complexes (Li_Zn OH_O)^ and (Li_Zn Li_i)^. Although Li doping lowers the Fermi level of as-grown ZnO significantly, low formation energy of the ionic donors, and passivation of Li_Zn^/ in the form of (Li_Zn OH_O)^ and (Li_Zn Li_i)^, prevents realization of significant/stable p-type activity in Li doped ZnO under equilibrium conditions.
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Crystal structure, hydration and ionic conductivity of the inherently oxygen-deficient La2Ce2O7

Authors Vasileios Besikiotis, Christopher S. Knee, Istaq Ahmed, Reidar Haugsrud, Truls Norby
Source
Solid State Ionics
Volume: 228, Pages: 1–7
Time of Publication: 2012-11
Abstract The crystal structure, hydration and ionic conductivity of the inherently oxygen deficient La2Ce2O7 system have been investigated. On the basis of Rietveld analysis of neutron diffraction data, the material is found to adopt a cation disordered oxygen-deficient fluorite structure. Impedance spectroscopy, performed in the temperature range 1000–200 C and as a function of water vapor and oxygen partial pressure, suggests that oxide ion conductivity dominates at high temperatures, while protons are the main charge carrier at temperatures below approximately 450 C. Proton conductivity was confirmed by isotope shifts under H2O and D2O. The dissolution of water was measured by means of thermogravimetry (TG). A defect chemical model is developed to derive hydration thermodynamic parameters based on TG and conductivity data. The hydration enthalpy was, moreover, determined directly by simultaneous TG and differential scanning calorimetry (TG–DSC). The TG–DSC values were in good agreement with those modeled from conductivity and TG data.
Keywords La2Ce2O7; Proton conductivity; Pyrochlore structure; Fluorite structure; Nonstoichiometric oxides
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Microstructure and electrical properties of zirconia and composite nanostructured ceramics sintered by different methods

Authors Bogdan Stefan Vasile, Ecaterina Andronescu, Cristina Ghitulica, Otilia Ruxandra Vasile, Lavinia Curechiu, Rares Scurtu, Eugeniu Vasile, Roxana Trusca, Livia Pall, Virgil Aldica
Source
Ceramics International
Volume: 39, Issue: 3, Pages: 2535–2543
Time of Publication: 2013-04
Abstract The aim of this study is the preparation and characterization of dense cubic zirconia ceramics and zirconia nanocomposites (reinforced with 5 wt% alumina). The powders were obtained through sol–gel methods and densified using classical sintering and spark plasma sintering (SPS) methods. The obtained ceramics were characterized through X-ray diffraction, scanning electron microscopy and impedance spectroscopy at room and high temperature. The average grain size of cubic zirconia particles was found to be approximately 8 and 2.5 μm for the classical sintering and 99 nm for SPS. The alumina particles in composites have an average grain size of 0.7 μm for classical sintering and 53 nm for SPS ones. The total conductivity for nanocomposites sintered through both methods was also determined.
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Characteristics of SrCo1 − xSnxO3 − δ cathode materials for use in solid oxide fuel cells

Authors Sea-Fue Wang, Yung-Fu Hsu, Chun-Ting Yeh, Chien-Chung Huang, Hsi-Chuan Lu
Source
Solid State Ionics
Volume: 227, Pages: 10–16
Time of Publication: 2012-10
Abstract In this study, introduction of tin ions in the SrCoO3 − δ oxide is attempted to modify its electrochemical behavior for serving as a cathode of intermediate-temperature solid oxide fuel cells (IT-SOFCs). Doping of tin ions appears to stabilize the cubic Pm-3m phase of the SrCo1 − ySnyO3 − δ ceramics but generates SrSnO3 precipitates and inhibits the grain growth as y value rises to a level greater than 10%. Obtained at 550 C, the maximum electrical conductivity of SrCo0.95Sn0.05O3 − δ reads 545 S cm− 1. Single cells with a structure of NiO–Sm0.2Ce0.8O2 − δ (SDC)/SDC/SrCo0.95Sn0.05O3 − δ–SDC are built and characterized. Though SrCo0.95Sn0.05O3 − δ is regarded as an MIEC (mixed ionic/electronic conductivity material), adding SDC to SrCo0.95Sn0.05O3 − δ guarantees good adhesion to and fine electrical contact with the electrolyte layer, thereby contributing to the reduction in R0 and RP values. The single cell with the SrCo0.95Sn0.05O3 − δ–SDC composite cathode at 700 C registers respectively an R0 value of 0.044 Ω cm2 and an RP value of 0.109 Ω cm2. In the absence of microstructure optimization and hermetic sealing of cells, a high power density of 0.847 W cm− 2 is reached. SrCo1 − ySnyO3 − δ thus emerges to be a promising cathode material for IT-SOFCs applications.
Keywords Solid oxide fuel cell; Cathode; Impedance; Cell performance
Remark Link

Investigation of La1−xSrxCrO3−∂ (x ~ 0.1) as Membrane for Hydrogen Production

Authors Yngve Larring, Camilla Vigen, Florian Ahouanto, Marie-Laure Fontaine, Thijs Peters, Jens B. Smith, Truls Norby and Rune Bredesen
Source
Membranes
Volume: 2, Issue: 3, Pages: 665-686
Time of Publication: 2012-09
Abstract Various inorganic membranes have demonstrated good capability to separate hydrogen from other gases at elevated temperatures. Hydrogen-permeable, dense, mixed proton-electron conducting ceramic oxides offer superior selectivity and thermal stability, but chemically robust candidates with higher ambipolar protonic and electronic conductivity are needed. In this work, we present for the first time the results of various investigations of La1−xSrxCrO3−∂ membranes for hydrogen production. We aim in particular to elucidate the material’s complex transport properties, involving co-ionic transport of oxide ions and protons, in addition to electron holes. This opens some new possibilities for efficient heat and mass transfer management in the production of hydrogen. Conductivity measurements as a function of pH2 at constant pO2 exhibit changes that reveal a significant hydration and presence of protons. The flux and production of hydrogen have been measured under different chemical gradients. In particular, the effect of water vapor in the feed and permeate gas stream sides was investigated with the aim of quantifying the ratio of hydrogen production by hydrogen flux from feed to permeate and oxygen flux the opposite way (“water splitting”). Deuterium labeling was used to unambiguously prove flux of hydrogen species.
Keywords hydrogen transport membrane; proton permeation; oxygen permeation; water splitting
Remark Link

CO2 decomposition via oxygen deficient ferrite electrodes using solid oxide electrolyser cell

Source
Time of Publication: 2012-09
Abstract Oxygen Deficient Ferrites (ODF) electrodes integrated with Yttria Stabilized Zirconia (YSZ) electrolyte, electrochemically decompose carbon dioxide (CO2) into carbon (C)/carbon monoxide (CO) and oxygen (O2) in a continuous process. The ODF electrodes can be kept active by applying a small potential bias across the electrodes. CO2 and water (H2O) can also be electrolyzed simultaneously to produce syngas (H2+CO) and O2 continuously that can be fed back to the oxy-fuel combustion. With this approach, CO2 can be transformed into a valuable fuel source allowing CO2 neutral use of the hydrocarbon fuels.
Remark United States Patent Application 20120228150
Link

Transient Oxygen Permeation and Surface Catalytic Properties of Lanthanum Cobaltite Membrane under Oxygen–Methane Gradient

Author Tyler T. Norton and Y. S. Lin
Source
Ind. Eng. Chem. Res.
Volume: 51, Issue: 39, Pages: 12917–12925
Time of Publication: 2012-09
Abstract Oxygen permeation through mixed-conducting ceramic membranes in an air/methane gradient is important for their applications in membrane reactors for air separation and partial oxidation of hydrocarbons. This study examines transient characteristics of oxygen permeation and surface catalytic properties of La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) membranes in an oxygen/methane gradient for an extended period of time. Upon exposure to an oxygen/methane gradient, the oxygen permeation flux of the membrane increases to a maximum at around 55 h, then decreases and reaches a steady-state value at around 200 h. The maximum and steady-state flux is approximately 60% and 30% higher than the initial flux of the fresh membrane, respectively. The surface catalytic properties of the membrane exposed to methane also change with the exposure time in a similar fashion. However, the apparent activation energy for oxygen permeation for the membranes at various stages of the transient study is nearly constant while the effects of temperature, feed pressure, and sweep flow rate on catalytic properties are also similar for the fresh and aged membranes. The surface of a LSCF membrane reacts with methane resulting in a formation of a thin porous layer which changes the surface catalytic properties. The membrane surface becomes more active for reaction with increased selectivity for carbon monoxide formation upon exposure to methane. This lowers oxygen partial pressure in the permeate side and increases the driving force for oxygen permeation and, therefore, increases oxygen permeation flux. Under the studied experimental conditions the membrane can reach steady-state for continuous operation.
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Fabrication, sealing and high pressure testing of tubular La2NiO4+δ membranes for air separation

Authors Paul Inge Dahl, Marie-Laure Fontaine, Florian Ahouanto, Christelle Denonville, Ove Paulsen, Yngve Larring, Thijs Peters, Partow Pakdel Henriksen, Rune Bredesen
Source
Energy Procedia
Volume: 23, Pages: 187–196
Time of Publication: 2012-09
Abstract Recent results achieved on fabrication of La2NiO4+δ membranes, sealing technology and performance in pressurized conditions are presented. Porous tubular membrane supports of up to 1 m length are prepared by ceramic extrusion. Asymmetric La2NiO4+δ membranes are prepared by coating dense selective layers of 10-15 μm thickness onto the porous supports. Glass ceramic seals in the system Na2O-CaO-Al2O3-SiO2 are currently being evaluated for joining the membranes with high temperature steel alloys coated with a corrosion resistance protective layer. By adjusting the composition of the glass system the thermal expansion coefficient is tailored to match that of the membrane and steel material. Good seal adherences towards these materials are obtained. Long term oxygen flux measurements (>4000 hours continuous operation) performed on symmetric (dense) La2NiO4+δ membranes are conducted under various conditions (atmosphere, temperature, pressure). The oxygen flux dependency on the oxygen partial pressure (pO2) is investigated by increasing the feed pressure and oxygen content. An asymptotic flux behavior is observed with increasing pO2. It is seen a significant increase in the flux (by a factor of 6) when increasing the feed pO2 from 0.8 to 2.4 bars while a less significant pO2 dependency is observed with further increase.
Keywords Oxygen transport membranes; fabrication; sealing; pressurized testing
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Effects of the microwave heating on the properties of gadolinium-doped cerium oxide prepared by polyol method

Authors A. Gondolini, E. Mercadelli, A. Sanson, S. Albonetti, L. Doubova, S. Boldrini
Source
Journal of the European Ceramic Society
Volume: 33, Issue: 1, Pages: 67–77
Time of Publication: 2013
Abstract Gadolinium doped ceria (GDC) has received a lot of attention as possible electrolyte material for Intermediate-Temperature (500–800 C) Solid Oxide Fuel Cells (IT-SOFC). Microwave heating has been recently considered in combination with precipitation for the production of oxide or non-oxide nano-powders. In this study, crystalline CeO2 powders doped with different amount of gadolinium were successfully prepared by microwave-assisted polyol method under mild conditions and in one single step. The microwave heating was found to strongly influence the morphological properties of the powder especially for low gadolinium content. IR and thermal analyses helped to identify the major reaction path for the formation of the as-observed complex morphologies. Regardless to the morphology, the powders showed good densification behavior and expected electrochemical properties; Ce0.9Gd0.1O1.95 exhibited the highest conductivity.
Keywords Doped ceria; Microwave processing; Ionic conductivity; Fuel cells; Polyol method
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Investigating Reliability on Fuel Cell Model Identification. Part II: An Estimation Method for Stochastic Parameters

Authors L. Tsikonis, S. Diethelm, H. Seiler, A. Nakajo, J. Van herle, D. Favrat
Source
Fuel Cells
Time of Publication: 2012-08
Abstract An alternative way to process data from polarization measurements for fuel cell model validation is proposed. The method is based on re- and subsampling of I–V data, with which repetitive estimations are obtained for the model parameters. This way statistics such as standard deviations and correlations between the parameters may be experimentally derived. Histograms may also be produced, approximating the probability distributions that they follow. Two experimental case studies are discussed. In the first case, observations are made on the behavior of the parameter values for two mathematical models. As the number of data points (measurement points) employed in the estimation of the parameters increases, parameters with high variances converge to specific values. On the contrary, parameters with small variances diverge linearly. The parameters' histograms do not usually follow normal distributions rather they show a connection between the number of peaks in the graphs and correlations of the parameters. The second case study is an application on a fast degraded SOFC button cell, where the values and the histograms of the parameters are compared before and after degradation.
Keywords Data Fitting; Design of Experiments; Diagnostics; Fast Degradation; Identification; Parameter Estimation; Polarization Curves; Robust Regression; Solid Oxide Fuel cells; Stein's Paradox
Remark DOI: 10.1002/fuce.201200031
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Extending the family of oxygen ion conductors isostructural with La2Mo2O9

Authors V.I. Voronkova, E.P. Kharitonova, E.I. Orlova, D.A. Belov
Source
Journal of Solid State Chemistry
Volume: 196, Pages: 45–51
Time of Publication: 2012-12
Abstract X-ray diffraction characterization of materials prepared by solid-state reactions in the ternary systems La2Mo2O9–Nd2W2O9–“Nd2Mo2O9” and La2Mo2O9–Pr2W2O9–Pr2Mo2O9 has shown that, in these systems, compounds isostructural with the oxygen ion conductor La2Mo2O9 exist in wide single-phase regions. Partial tungsten substitution for molybdenum may yield stable Ln2Mo2−2xW2xO9 compounds with the La2Mo2O9 structure, where Ln is a rare-earth element different from lanthanum and praseodymium, e.g., neodymium. Tungsten also stabilizes Pr2Mo2O9, which otherwise decomposes above 700 C. A series of continuous solid solution was found in the La2Mo2O9–Pr2Mo2O9 system. Polymorphism of compounds existing in the above ternary systems was studied by differential scanning calorimetry. The conductivity of most of the compounds studied approaches that of lanthanum molybdate.
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BaTiO3–Bi(Zn1/2Ti1/2)O3–BiScO3 Ceramics for High-Temperature Capacitor Applications

Authors Natthaphon Raengthon, Tutu Sebastian, Denis Cumming, Ian M. Reaney, David P. Cann
Source
Journal of the American Ceramic Society
Volume: 95, Issue: 11, Pages: 3554–3561
Time of Publication: 2012-09
Abstract Ceramics based on solid solutions of xBaTiO3–(100−x)(0.5Bi(Zn1/2Ti1/2)O3–0.5BiScO3), where x = 50, 55, and 60 were prepared by solid-state reaction which resulted in a single perovskite phase with pseudocubic symmetry. Dielectric property measurements revealed a high relative permittivity (>1000), which could be modified with the addition of Bi(Zn1/2Ti1/2)O3 (BZT) and BiScO3 (BS) to engineer a temperature-stable dielectric response with a temperature coefficient of permittivity (TCε) as low as −182 ppm/C. By incorporating 2 mol% Ba vacancies into the stoichiometry, the resistivity increased significantly, especially at high temperatures (>200C). Vogel–Fulcher analysis of the permittivity data showed that the materials exhibited freezing of polar nanoregions over the range of 100–150 K. An analysis of optical absorption near the band edge for the Ba-deficient compositions suggested that the enhanced resistivity values were linked to a decrease in the concentration of defect states. An activation energy of ~1.4 eV was obtained from DC resistivity measurements suggesting that an intrinsic conduction mechanism played a major role in the high temperature conductivity. Finally, multilayer capacitors based on these compositions were fabricated, which exhibited dielectric properties comparable to the bulk material. Based on these results, this family of materials has great promise for high-temperature capacitor applications.
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