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Comparative study of the electrochemical promotion of CO2 hydrogenation on Ru using Na+, K+, H+ and O2 − conducting solid electrolytes

Authors I. Kalaitzidou, M. Makri, D. Theleritis, A. Katsaounis, C.G. Vayenas
Source
Surface Science
Time of Publication: 2015
Abstract The kinetics and the electrochemical promotion of the hydrogenation of CO2 to CH4 and CO are compared for Ru porous catalyst films deposited on Na+, K+, H+ and O2 − conducting solid electrolyte supports. It is found that in all four cases increasing catalyst potential and work function enhances the methanation rate and selectivity. Also in all four cases the rate is positive order in H2 and exhibits a maximum with respect to CO2. At the same time the reverse water gas shift reaction (RWGS) which occurs in parallel exhibits a maximum with increasing pH2pH2 and is positive order in CO2. Also in all cases the selectivity to CH4 increases with increasing pH2pH2 and decreases with increasing pCO2pCO2. These results provide a lucid demonstration of the rules of chemical and electrochemical promotion which imply that (∂r/∂Φ)(∂r/∂pD) > 0 and (∂r/∂Φ)(∂r/∂pA) < 0, where r denotes a catalytic rate, Φ is the catalyst work function and pD and pA denote the electron donor and electron acceptor reactant partial pressures respectively.
Keywords Effect of Ru catalyst support and potential on product selectivity.
Remark In Press, doi:10.1016/j.susc.2015.09.011
Link

Electrochemical promotion of the hydrogenation of CO2 on Ru deposited on a BZY proton conductor

Authors I. Kalaitzidou, A. Katsaounis, T. Norby, C.G. Vayenas
Source
Journal of Catalysis
Volume: 331, Pages: 98–109
Time of Publication: 2015
Abstract he kinetics and the electrochemical promotion of the hydrogenation of CO2 on polycrystalline Ru deposited on BZY (BaZr0.85Y0.15O3−α + 1 wt% NiO), a proton conductor in wet atmospheres, with α ≈ 0.075, was investigated at temperatures 300–450 °C and atmospheric pressure. Methane and CO were the only detectable products and the selectivity to CH4 could be reversibly controlled between 15% and 65% by varying the catalyst potential by less than 1.2 V. The rate and the selectivity to CH4 are very significantly enhanced by proton removal from the catalyst via electrochemically controlled spillover of atomic H from the catalyst surface to the proton-conducting support. The effect is strongly non-Faradaic and the apparent Faradaic efficiency of methanation takes values up to 500 and depends strongly on the porous Ru catalyst film thickness. The observed strong promotional effect, in conjunction with the observed reaction kinetics, is in good agreement with the rules of electrochemical and chemical promotion.
Keywords Hydrogenation of CO2; CO2 methanation; Ru catalyst; RWGS reaction; BZY proton conducting support; Selectivity modification; Electrochemical promotion of catalysis (EPOC); Non-faradaic electrochemical modification of catalytic activity (NEMCA effect)
Remark doi:10.1016/j.jcat.2015.08.023
Link

Electrochemical Promotion of Ir0.5Pt0.5O2/YSZ

Authors S. Balomenou, K. M. Papazisi, D. Tsiplakides
Source
Topics in Catalysis
Volume: 58, Issue: 18, Pages: 1270-1275
Time of Publication: 2015
Abstract A high surface area, nanostructured bimetallic oxide catalyst, Ir0.5Pt0.5O2, deposited on YSZ was studied for the electrochemical promotion of ethylene oxidation. The catalyst was synthesized using the modified Adams fusion method and was characterized regarding its structure, morphology and specific surface area via XPS, XRD, HRTEM, SEM and BET. Regarding the performance for electrochemical promotion, it was found that the rate of ethylene oxidation can be enhanced significantly and in a strongly non-faradaic manner via positive potential application, exhibiting strongly electrophobic behaviour.
Keywords Electrochemical promotion, EPOC, Ir0.5Pt0.5O2, Adams fusion method
Remark Link

Enhanced Carbon Deposition Tolerance of SOFC Anodes Under Triode Operation

Authors Ioanna Petrakopoulou, Dimitrios Tsiplakides, Stella Balomenou
Source
Topics in Catalysis
Volume: 58, Issue: 18, Pages: 1303-1310
Time of Publication: 2015
Abstract The triode fuel cell design and operation concept was applied as an alternative means for controlling and enhancing the carbon tolerance of state-of-the-art solid oxide fuel cell (SOFC) anodes. The triode cell configuration entails the introduction of a third electrode in addition to the anode and cathode, driven by an auxiliary circuit which is run in electrolytic mode. In this way the cell is forced to operate at controlled potential differences that are inaccessible under standard operation, and thus introduces a controllable variable into fuel cell operation. In the present study, the effectiveness of the triode approach was evaluated for the in situ control of the rate of carbon deposition in commercial multilayer NiO–GDC and NiO–YSZ SOFC anodes. The study involved typical and triode operation of SOFC button cells under CH4 steam reforming conditions, and it was found that the application of a small electrolytic current under triode operation resulted in significantly less carbon built-up on the anode compared to the standard SOFC operation.
Keywords SOFC Triode fuel cell operation, Anode degradation, Carbon formation, CH4 steam reforming
Remark Link

Development of a Coking-Resistant NiSn Anode for the Direct Methane SOFC

Authors N. Bogolowski, B. Iwanschitz and J.-F. Drillet
Source
Fuel Cells
Volume: 15, Issue: 5, Pages: 711–717
Abstract The present work reports on the development of a coking-resistant NiSn-based membrane electrode assembly (MEA) for internal CH4 reforming in solid oxide fuel cells (SOFCs). Catalyst powder was prepared in a centrifugal casting oven by melting stoichiometric amounts of Ni and Sn under vacuum. The formation of Ni3Sn2 intermetallic phase was confirmed by XRD analysis. Catalytic activity for CH4 reforming and stability of the NiSn powder were first evaluated in a quartz glass reactor for 4 h at 600–1,000 °C. The main reaction products H2 and CO were detected by gas chromatography while no carbon formation was detected during the experiments. Then, 3YSZ electrolyte-supported MEAs were fabricated with a Ni3Sn2/YSZ anode and LSM/YSZ cathode and characterized under SOFC conditions. The MEA showed an excellent stability under CH4 atmosphere (3% H2O) at 850 °C over more than 650 h. No substantial decrease in cell potential was observed during this period.
Keywords Anode Material;Intermetallic Phase;Internal Reforming;Methane;Nickel-Tin Alloy;Ni3Sn2;SOFC;Solid Oxide Fuel Cell
Remark DOI: 10.1002/fuce.201400187
Link

Hardening in non-stoichiometric (1−x)Bi0.5Na0.5TiO3–xBaTiO3 lead-free piezoelectric ceramics

Authors Sasiporn Prasertpalichat, David P. Cann
Source
Journal of Materials Science
Volume: 51, Issue: 1, Pages: 476-486
Time of Publication: 2016
Abstract The role of A-site non-stoichiometry was investigated in lead-free piezoelectric ceramics based on compositions in the 1 − x(Bi0.5Na0.5TiO3)–xBaTiO3 system near the morphotropic phase boundary, where x = 0.055, 0.06, and 0.07. Donor doping was introduced through the addition of excess Bi, however, there were no changes in the crystal structure. In contrast, acceptor doping was introduced through the addition of excess Na and was found to promote rhombohedral distortions. A significant improvement of dielectric properties was observed in donor-doped compositions and, in contrast, a degradation in properties was observed in acceptor-doped compositions. Compared to the stoichiometric composition, the acceptor-doped compositions displayed a significant increase in coercive field (E c) which is an indication of domain wall pinning as found in hard Pb(Zr x Ti1−x )O3. This result was further confirmed via remanent polarization hysteresis analyses. Moreover, all A-site acceptor-doped compositions also exhibited an increase in mechanical quality factor (Q m) as well as a decrease in piezoelectric coefficient (d 33), dielectric loss (tan δ), remanent polarization (P r), and dielectric permittivity, which are all the typical characteristics of the effects of “hardening.” The mechanism for the observed hardening in A-site acceptor-doped BNT-based systems is linked to changes in the long-range domain structure and defect chemistry.
Remark Link

Copper Iron Conversion Coating for Solid Oxide Fuel Cell Interconnects

Authors Jan Gustav Grolig, , Patrik Alnegren, Jan Froitzheim, Jan-Erik Svensson
Source
Journal of Power Sources
Volume: 297, Pages: 534–539
Time of Publication: 2015
Abstract A conversion coating of iron and copper was investigated with the purpose of increasing the performance of Sanergy HT as a potential SOFC interconnect material. Samples were exposed to a simulated cathode atmosphere (air, 3 % H2O) for durations of up to 1000 h at 850 °C. Their performance in terms of corrosion, chromium evaporation and electrical resistance (ASR) was monitored and compared to uncoated and cobalt-coated Sanergy HT samples. The copper iron coating had no negative effects on corrosion protection and decreased chromium evaporation by about 80%. An Area Specific Resistance (ASR) of 10 mΩcm2 was reached after 1000 h of exposure. Scanning Electron Microscopy revealed well adherent oxide layers comprised of an inner chromia layer and an outer spinel oxide layer.
Keywords Interconnect; Corrosion; Chromium volatilization; Sanergy HT; SOFC; Area specific resistance
Remark doi:10.1016/j.jpowsour.2015.06.139
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Atomic structure and ionic conductivity of glassy materials based on silver sulfide

Authors N. V. Melnikova, K. V. Kurochka, O. L. Kheifets, N. I. Kadyrova, Ya. Yu. Volkova
Source
Volume: 79, Issue: 6, Pages: 719-722
Time of Publication: 2015
Abstract The effect of the composition of glassy ionic conductors AgGe1 + x As1–x S3 and the composites based on these materials containing single-walled carbon nanotubes (CNT) AgGe1+x As1–x (S + CNT)3, on the atomic structure and ionic conductivity is analyzed.
Remark Link

Protons in piezoelectric langatate; La3Ga5.5Ta0.5O14

Authors Tor Svendsen Bjřrheim, Vijay Shanmugappirabu, Reidar Haugsrud, Truls E. Norby
Source
Solid State Ionics
Volume: 278, Pages: 275–280
Time of Publication: 2015
Abstract This contribution reports the hydration and electrical transport properties of effectively acceptor doped single crystalline and polycrystalline langatate, La3Ga5.5Ta0.5O14. The electrical properties are investigated over wide ranges of pH2OpH2O, pD2OpD2O and pO2pO2 in the temperature range 400 to 1000 °C. Acceptor doped langatate is dominated by oxygen vacancies in dry atmospheres and at high temperatures, and by protonic defects in wet atmospheres and at lower temperatures. The corresponding standard hydration enthalpy and entropy are − 90 ± 5 kJ/mol and − 130 ± 5 J/mol K, respectively. Further, all compositions display pure proton conductivity in wet atmospheres below 700 °C with a proton mobility enthalpy in the range of 70–75 kJ/mol, depending on doping level and crystallographic direction. Hence, protons are important for the physiochemical properties of langatate even at 1000 °C, and could therefore influence the behavior of langatate-based resonator devices. The proton conductivity is slightly anisotropic, being higher in the X- and Y- than in the Z-direction. At high temperatures and under dry conditions, electron holes and oxide ions dominate the conductivity, and the enthalpy of mobility of vacancies is 140 ± 5 kJ/mol.
Keywords Langatate; Piezoelectric; Defects; Protons; Conductivity
Remark doi:10.1016/j.ssi.2015.06.024
Link

Tetragonal tungsten bronzes Nb8−xW9+xO47−δ: optimization strategies and transport properties of a new n-type thermoelectric oxide

Authors Christophe P. Heinrich, Matthias Schrade, Giacomo Cerretti, Ingo Lieberwirth, Patrick Leidich, Andreas Schmitz, Harald Fjeld, Eckhard Mueller, Terje G. Finstad, Truls Norby and Wolfgang Tremel
Source
Materials Horizons
Issue: 5, Pages: 519-527
Time of Publication: 2015
Abstract Engineering of nanoscaled structures may help controlling the electrical and thermal transport in solids, in particular for thermoelectric applications that require the combination of low thermal conductivity and low electrical resistivity. The tetragonal tungsten bronzes Nb8−xW9+xO47 (TTB) allow a continuous variation of the charge carrier concentration while fulfilling at the same time the concept of a “phonon-glass electron-crystal” through a layered nanostructure defined by intrinsic crystallographic shear planes. The thermoelectric properties of the tetragonal tungsten bronzes Nb8−xW9+xO47−δ (0 < x < 2) were studied in the temperature range from 373 to 973 K. Structural defects and the thermal stability under various oxygen partial pressure pO2 were investigated by means of thermogravimetry, HR-TEM, and XRD. Nb8W9O47−δ was found stable at 973 K and a pO2 of ≈10−15 atm. The oxygen nonstoichiometry δ can reach up to 0.3, depending on the applied atmosphere. By increasing the substitution level x, the electrical resistivity ρ and the Seebeck coefficient S decreased. For x = 2, ρ reached 20 mΩ cm at 973 K, combined with a Seebeck coefficient of approximately −120 μV K−1. The thermal conductivity was low for all samples, ranging from 1.6 to 2.0 W K−1 m−1, attributed to the complex crystal structure. The best thermoelectric figure of merit zT of the investigated samples was 0.043, obtained for x = 2 at 973 K, but it is expected to increase significantly upon a further increase of x. The control of the oxygen non-stoichiometry δ opens a second independent optimization strategy for tetragonal tungsten bronzes.
Remark DOI: 10.1039/C5MH00033E
Link

EuBaCo2O5+δ-Ce0.9Gd0.1O2−δ composite cathodes for intermediate-temperature solid oxide fuel cells: high electrochemical performance and oxygen reduction kinetics

Authors Zhan Shi, Tian Xia, Fuchang Meng, Jingping Wang, Shengming Wu, Jie Lian, Hui Zhao, Chunbo Xu
Source
Electrochimica Acta
Volume: 174, Pages: 608–614
Time of Publication: 2015
Abstract The characteristics and electrochemical performance of double perovskite EuBaCo2O5+δ (EBCO) have been investigated as a composite cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The thermal expansion coefficients can be effectively reduced in the case of EBCO-Ce0.9Gd0.1O2−δ (CGO) composite cathodes. No chemical reactions between EBCO cathode and CGO electrolyte are observed after sintering at 1000 °C for 24 h. The maximum electrical conductivities of EBCO-CGO materials reach 28-77 S cm−1 with the change of CGO weight ratio from 40 wt. % to 5 wt. %. Among all these components, the EBCO-10 wt. % CGO (EBCO-10CGO) composite cathode gives the lowest area-specific resistance of 0.055 and 0.26 Ω cm2 in air at 700 and 600 °C, respectively. The maximum power density of Ni-CGO anode-supported single cell consisted of the EBCO-10CGO composite cathode and CGO electrolyte achieves 0.81 W cm−2 at 700 °C. These results indicate that the EBCO-10CGO composite materials can be used as a promising cathode candidate for IT-SOFCs. Furthermore, the rate-limiting steps for the oxygen reduction reaction at the EBCO-10CGO composite cathode interface are determined to be the charge transfer and dissociation of adsorbed molecule oxygen processes.
Keywords Intermediate-temperature solid oxide fuel cells; cathode materials; electrochemical performance; oxygen reduction kinetics
Remark doi:10.1016/j.electacta.2015.06.059
Link

Multilayer ceramic capacitors based on relaxor BaTiO3-Bi(Zn1/2Ti1/2)O3 for temperature stable and high energy density capacitor applications

Authors Nitish Kumar, Aleksey Ionin, Troy Ansell, Seongtae Kwon, Wesley Hackenberger and David Cann
Source
Applied Physics Letters
Volume: 106, Pages: 252901
Time of Publication: 2015
Abstract The need for miniaturization without compromising cost and performance continues to motivate research in advanced capacitor devices. In this report, multilayerceramiccapacitors based on relaxor BaTiO3-Bi(Zn1/2Ti1/2)O3 (BT-BZT) were fabricated and characterized. In bulk ceramic embodiments, BT-BZT has been shown to exhibit relative permittivities greater than 1000, high resistivities (ρ > 1 GΩ-cm at 300 °C), and negligible saturation up to fields as high as 150 kV/cm. Multilayercapacitor embodiments were fabricated and found to exhibit similar dielectric and resistivity properties. The energy density for the multilayerceramics reached values of ∼2.8 J/cm3 at room temperature at an applied electric field of ∼330 kV/cm. This represents a significant improvement compared to commercially available multilayercapacitors. The dielectric properties were also found to be stable over a wide range of temperatures with a temperature coefficient of approximately −2000 ppm/K measured from 50 to 350 °C, an important criteria for high temperature applications. Finally, the compatibility of inexpensive Ag-Pd electrodes with these ceramics was also demonstrated, which can have implications on minimizing the device cost.
Remark http://dx.doi.org/10.1063/1.4922947
Link

Electrical conductivity of Zn-doped high temperature proton conductor LaNbO4

Authors Yong Cao, Yuan Tan, Dong Yan, , Bo Chi, Jian Pu, Li Jian
Source
Solid State Ionics
Volume: 278, Pages: 152–156
Time of Publication: 2015
Abstract Zn-doped LaNbO4 (La1 − xZnxNbO4 − δ, LZ100x) was prepared by a solid-state reaction method with x = 0, 0.005, 0.01, 0.015, 0.03 and 0.05 and investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and conductivity measurement. There were no XRD and TEM evidences of formed secondary phases in the composition range of x ≤ 0.03 due to the sensitivity. However, the solubility of Zn, less than 1.0 mol.%, was reasonable, according the variety of the grain sizes, conductivity, as well as the activation energy for the conductivity, with the increasing concentration of Zn. The conductivity of LaNbO4 was improved by one to two orders of magnitude with Zn doping in the research range; and the highest conductivity of 9.8 × 10− 4 S cm− 1 was obtained with LZ0.5 at 900 °C in wet air.
Keywords LaNbO4; Conductivity; Zn doping; Grain size
Remark doi:10.1016/j.ssi.2015.06.011
Link

Gd- and Pr-based double perovskite cobaltites as oxygen electrodes for proton ceramic fuel cells and electrolyser cells

Authors Ragnar Strandbakke, Vladimir A. Cherepanov, Andrey Yu. Zuev, Dmitry S. Tsvetkov, Christos Argirusis, Georgia Sourkouni, Stephan Prünte, Truls Norby
Source
Solid State Ionics
Volume: 278, Pages: 120–132
Time of Publication: 2015
Abstract Double perovskite oxides BaGd0.8La0.2Co2O6−δ (BGLC), BaGdCo1.8Fe0.2O6−δ (BGCF), BaPrCo2O6−δ (BPC) and BaPrCo1.4Fe0.6O6−δ (BPCF) were investigated as oxygen electrodes on mixed conducting BaZr0.7Ce0.2Y0.1O3 (BZCY72) electrolyte using impedance spectroscopy vs temperature, pO2, and pH2O. We propose and have applied a novel approach to extract and parameterise the charge transfer and diffusion impedances of the electrode reactions in a system comprising charge transport of protons, oxide ions, and electrons. Given by the properties of the BZCY72, transport of protons dominates at lower temperatures and high pH2O, oxide ions at higher temperatures, and electron holes increasingly at high temperatures and high pO2. The electrodes showed good performance, with the lowest total apparent polarisation resistance for BGLC/BZCY72 being 0.05 and 10 Ωcm2 at 650 and 350 °C, respectively. The low temperature rate limiting reaction step is a surface related process, involving protonic species, with an activation energy of approximately 50 kJ mol−1 for BGLC/BZCY72. The oxide ion transport taking over at higher temperatures exhibits a higher activation energy typical of SOFC cathodes. Thermogravimetric studies revealed that BGLC exhibits considerable protonation at 300–400 °C, which may be interpreted as hydration with an enthalpy of approximately –50 kJ mol−1. The resulting mixed proton electron conduction may explain its good performance as electrode on BZCY72.
Keywords PCFC; PCEC; P-MIEC; Proton conductor; Mixed conductivity; Double perovskite
Remark doi:10.1016/j.ssi.2015.05.014
Link

Praseodymium-deficiency Pr0.94BaCo2O6-δ double perovskite: A promising high performance cathode material for intermediate-temperature solid oxide fuel cells

Authors Fuchang Meng, Tian Xia, Jingping Wang, Zhan Shi, Hui Zhao
Source
Journal of Power Sources
Volume: 239, Pages: 741–750
Time of Publication: 2015
Abstract Praseodymium-deficiency Pr0.94BaCo2O6-δ (P0.94BCO) double perovskite has been evaluated as a cathode material for intermediate-temperature solid oxide fuel cells. X-ray diffraction pattern shows the orthorhombic structure with double lattice parameters from the primitive perovskite cell in Pmmm space group. P0.94BCO has a good chemical compatibility with Ce0.9Gd0.1O1.95 (CGO) electrolyte even at 1000 °C for 24 h. It is observed that the Pr-deficiency can introduce the extra oxygen vacancies in P0.94BCO, further enhancing its electrocatalytic activity for oxygen reduction reaction. P0.94BCO demonstrates the promising cathode performance as evidenced by low polarization are-specific resistance (ASR), e. g. 0.11 Ω cm2 and low cathodic overpotential e. g. −56 mV at a current density of −78 mA cm−2 at 600 °C in air. These features are comparable to those of the benchmark cathode Ba0.5Sr0.5Co0.8Fe0.2O3-δ. The fuel cell CGO-Ni|CGO|P0.94BCO presents the attractive peak power density of 1.05 W cm−2 at 600 °C. Furthermore, the oxygen reduction kinetics of P0.94BCO material is also investigated, and the rate-limiting steps for oxygen reduction reaction are determined.
Keywords Intermediate-temperature solid oxide fuel cell; Cathode material; Double perovskite; Electrochemical performance; Oxygen reduction reaction
Remark doi:10.1016/j.jpowsour.2015.06.007
Link

Reduced long term electrical resistance in Ce/Co-coated ferritic stainless steel for solid oxide fuel cell metallic interconnects

Authors Anna Magrasóa, Hannes Falk-Windisch, Jan Froitzheim, Jan-Erik Svensson, Reidar Haugsrud
Source
International Journal of Hydrogen Energy
Volume: 40, Issue: 27, Pages: 8579–8585
Abstract The present study is focused on the influence of selected coatings on a ferritic stainless steel (Sanergy HT™, Sandvik) on the evolution of the area specific resistance (ASR) as a function of time at high temperature. The samples are exposed in humidified air at 850 °C for up to 4200 h. It combines long-term ASR measurements with the thermogravimetric behavior and microstructural analysis of the cross sections by scanning electron microscopy. The results show that uncoated and Co-coated Sanergy HT™ exhibit similar oxidation kinetics and comparable ASRs, while a combined Ce/Co coating improves oxidation resistance and, consequently, reduces the ASR significantly. Other reports have earlier shown that Co- (and Ce/Co)-coated Sanergy HT™ reduces the evaporation of volatile chromium species. Overall, the study indicates that Ce/Co-coatings will render substantially improved performance for ferritic steel interconnects for solid oxide fuel cells.
Keywords Metallic coating; SOFC; Interconnects; Stainless steel; Conductivity; ASR
Remark doi:10.1016/j.ijhydene.2015.04.147
Link

Resistivity Enhancement and Transport Mechanisms in (1 − x)BaTiO3–xBi(Zn1/2Ti1/2)O3 and (1 − x)SrTiO3–xBi(Zn1/2Ti1/2)O3

Author Nitish Kumar* andDavid P. Cann
Source
Journal of the American Ceramic Society
Time of Publication: 2015
Abstract Ceramics of composition (1−x)BaTiO3–xBi(Zn1/2Ti1/2)O3 (BT-BZT) were prepared by solid-state synthesis; they have been shown to exhibit excellent properties suited for high-temperature dielectric applications. The X-ray diffraction data showed a single-phase perovskite structure for all the compositions prepared (x ≤ 0.1 BZT). The compositions with less than 0.075 BZT exhibited tetragonal symmetry at room temperature and pseudo-cubic symmetry above it. Most notably, a significant improvement in insulation properties was measured with the addition of BZT. Both low-field AC impedance and high-field direct DC measurements indicated an increase in resistivity of at least two orders of magnitude at 400°C with the addition of just 0.03 BZT (~107 Ω-cm) into the solid solution as compared to pure BT (~105 Ω-cm). This effect was also evident in dielectric loss data, which remained low at higher temperatures as the BZT content increased. In conjunction with band gap measurements, it was also concluded that the conduction mechanism transitioned from extrinsic for pure BT to intrinsic for 0.075 BZT suggesting a change in the fundamental defect equilibrium conditions. It was also shown that this improvement in insulation properties was not limited to BT-BZT, but could also be observed in the paraelectric SrTiO3–BZT system.
Remark DOI: 10.1111/jace.13666, Article first published online
Link

Triode operation for enhancing the performance of H2S-poisoned SOFCs operated under CH4–H2O mixtures

Authors Foteini M. Sapountzi, Michail N. Tsampas, Chunhua Zhao, Antoinette Boreave, Laurence Retailleau, Dario Montinaro, Philippe Vernoux
Source
Solid State Ionics
Volume: 277, Pages: 65–71
Time of Publication: 2015
Abstract Performances of Solid Oxide Fuel Cells (SOFCs) were investigated in triode operation mode under methane steam reforming in the presence of H2S. Both the catalytic performances for methane steam reforming and the electrochemical properties for the electrochemical oxidation of hydrogen of a Ni/GDC anode drastically dropped in the presence of 1 ppm H2S. Poisoned catalytic sites are different from those for the hydrogen electrochemical oxidation. Triode operation, i.e. application of moderate negative currents between the anode and an auxiliary electrode, can improve electrochemical properties, as a result of a local production of H2 coming from H2O electrolysis. Some specific triode operations were found to achieve a thermodynamic efficiency close to the unity to avoid any energy overconsumption.
Keywords SOFC; Triode operation; H2S poisoning; Ni/GDC anode
Remark doi:10.1016/j.ssi.2015.05.003
Link

High-temperature anion and proton conduction in RE3NbO7 (RE = La, Gd, Y, Yb, Lu) compounds

Authors A. Chesnauda, M.-D. Braidab, S. Estradéd, F. Peiród, A. Tarancónf, A. Morataf, G. Dezanneau
Source
Journal of the European Ceramic Society
Volume: 35, Issue: 11, Pages: 3051–3061
Time of Publication: 2015
Abstract The oxide-ion and proton conduction properties of RE3NbO7 (RE = La, Gd, Y, Yb, Lu) compounds were investigated. For the bigger rare-earth cation, i.e. La3+, the compound crystallises in a weberite-type structure and the oxide-ion conductivity is low owing to the lack of intrinsic oxygen vacancies. Consequently, the resultant proton incorporation and conductivity in La3NbO7 are also low. For small rare-earth cations, i.e. from Gd3+ to Lu3+ and for RE = Y, materials adopt a fluorite-like structure confirmed from X-ray powder diffraction. In this latter case, materials include intrinsic oxygen vacancies leading to a higher oxygen conductivity. For these compounds, a proton incorporation takes place at low temperature under wet conditions giving rise to proton conductivity. Nevertheless, both oxygen and proton conductivities are low in these materials, which can be explained by the ordering of oxygen vacancies observed by Transmission Electron Microscopy.
Keywords Protonic ceramic fuel cell; Rare-earth niobate; Combustion synthesis; Fluorite-type structure; Electrical properties
Remark doi:10.1016/j.jeurceramsoc.2015.04.014
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Proton transport properties of the RE3Ga5MO14 (RE = La, Nd and M = Si, Ti, Sn) langasite family of oxides

Authors Tor S. Bjřrheim, Reidar Haugsrud
Source
Solid State Ionics
Volume: 275, Pages: 29–34
Time of Publication: 2015
Abstract Hydration and proton transport properties of novel, intrinsically acceptor doped compositions within the RE3Ga5MO14 family of oxides have been addressed by means of measurements of the electrical conductivity. Oxygen vacancies and protons charge compensate the acceptor in dry and wet atmospheres, respectively, and all compositions display significant proton conductivity below 1000 °C. The hydration thermodynamics is affected by M-ion substitution, and becomes more favorable in the order Si < Ti < Sn. The enthalpy of proton mobility is also strongly dependent on the M-ion; M = Si, Ti and Sn exhibit enthalpies of proton mobility of 76 ± 3, 61 ± 1 and 80 ± 2 kJ mol− 1, respectively.
Keywords Langasites; Conductivity; Defects; Acceptor; Hydration; Protons
Remark doi:10.1016/j.ssi.2015.03.014
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Diffusion of Nd and Mo in lanthanum tungsten oxide

Authors Einar Vřllestad, Markus Teusner, Roger A. De Souza, Reidar Haugsrud
Source
Solid State Ionics
Volume: 274, Pages: 128–133
Time of Publication: 2015
Abstract Cation diffusion in functional oxides exposed to electrochemical gradients may lead to kinetic demixing or decomposition and, consequently, determine the life-time of the functional component. Here we present chemical diffusion coefficients of Nd and Mo in the mixed proton–electron conductor lanthanum tungsten oxide, La28 − xW4 + xO54 + 3x/2 (LWO), measured at 1000 to 1200 °C in both oxidizing and reducing atmospheres. The bulk diffusivities of Nd and Mo were similar at all temperatures investigated and did not change significantly from oxidizing to reducing conditions. On these bases it is suggested that bulk diffusion of both Nd and Mo occurs via the La2 site on which both cations reside. Based on the low activation energy for bulk transport (~ 200 kJ∙mol− 1) at temperatures below 1200 °C it is proposed that the cation defect concentrations are, in effect, frozen in. Preferential diffusion of Nd along the grain boundaries was rationalized based on space charge effects and depletion of W6 + and Mo6 + near the positively charged grain boundary core. Potential implications of kinetic demixing or decomposition of LWO membranes are also evaluated based on the present results.
Keywords Lanthanum tungstate; Cation diffusion; SIMS; Degradation; Tracer diffusion
Remark doi:10.1016/j.ssi.2015.03.011
Link

Crystal structure and high-temperature properties of the Ruddlesden–Popper phases Sr3−xYx(Fe1.25Ni0.75)O7−δ (0≤x≤0.75)

Authors Louise Samain, Philipp Amshoff, Jordi J. Biendicho, Frank Tietz, Abdelfattah Mahmoud, Raphaël P. Hermann, Sergey Ya. Istomin, Jekabs Grins, Gunnar Svensson
Source
Journal of Solid State Chemistry
Volume: 227, Pages: 45–54
Time of Publication: 2015
Abstract Ruddlesden–Popper n=2 member phases Sr3−xYxFe1.25Ni0.75O7−δ, 0≤x≤0.75, have been investigated by X-ray and neutron powder diffraction, thermogravimetry and Mössbauer spectroscopy. Both samples as-prepared at 1300 °C under N2(g) flow and samples subsequently air-annealed at 900 °C were studied. The as-prepared x=0.75 phase is highly oxygen deficient with δ=1, the O1 atom site being vacant, and the Fe3+/Ni2+ ions having a square pyramidal coordination. For as-prepared phases with lower x values, the Mössbauer spectral data are in good agreement with the presence of both 5- and 4-coordinated Fe3+ ions, implying in addition a partial occupancy of the O3 atom sites that form the basal plane of the square pyramid. The air-annealed x=0.75 sample has a δ value of 0.61(1) and the structure has Fe/Ni ions in both square pyramids and octahedra. Mössbauer spectroscopy shows the phase to contain only Fe3+, implying that all Ni is present as Ni3+. Air-annealed phases with lower x values are found to contain both Fe3+ and Fe4+. For both the as-prepared and the air-annealed samples, the Y3+ cations are found to be mainly located in the perovskite block. The high-temperature thermal expansion of as-prepared and air-annealed x=0.75 phases were investigated by high-temperature X-ray diffraction and dilatometry and the linear thermal expansion coefficient determined to be 14.4 ppm K−1. Electrical conductivity measurements showed that the air-annealed samples have higher conductivity than the as-prepared ones.
Keywords Ruddlesden–Popper structure; Oxygen non-stoichiometry; Crystal structure; Mössbauer spectroscopy; Electrical conductivity; Thermal expansion
Remark doi:10.1016/j.jssc.2015.03.018
Link

Atmosphere dependence of anode reaction of intermediate temperature steam electrolysis using perovskite type proton conductor

Authors Takaaki Sakai, Keita Arakawa, Masako Ogushi, Tatsumi Ishihara, Hiroshige Matsumoto, Yuji Okuyama
Source
Journal of Solid State Electrochemistry
Volume: 19, Issue: 6, Pages: 1793-1798
Time of Publication: 2015
Abstract The effect of oxygen partial pressure on anode reaction of steam electrolysis using SrZr0.5Ce0.4Y0.1O3-α (SZCY-541) proton conducting electrolyte was investigated by AC impedance measurement in this work. The anode reaction was enhanced by increasing oxygen partial pressure, and this result was opposite to the expectation from the conventional anode reaction (water splitting reaction). The increase in the electrode reaction conductivity with oxygen chemical potential was proportional to PO21/4 at 600 °C and at higher oxygen partial pressure region of 700 and 800 °C, suggesting the possibility that the enhancement is caused by the increase in hole concentration on the electrolyte surface near the anode.
Remark Link

FD Electrolysis: Co-electrolysis of steam and CO2 in full-ceramic symmetrical SOECs: A strategy for avoiding the use of Hydrogen as a safe gas

Authors Marc Torrell, Sergio García-Rodríguez, Alex Morata, Germán Penelas and Alberto Tarancon
Source
Faraday Discussions
Time of Publication: 2015
Abstract The use of cermets as fuel electrodes for solid oxide electrolysis cells requires permanent circulation of reducing gas, e.g. H2 or CO, so called safe gas, in order to avoid oxidation of the metallic phase. Replacing metallic based electrodes by pure oxides is therefore proposed as an advantage for the industrial application of solid oxide electrolyzers. In this work, full-ceramic symmetrical solid oxide electrolysis cells have been investigated for steam/CO2 co-electrolysis. Electrolyte supported cells with La0.75Sr0.25Cr0.5Mn0.5O3-δ reversible electrodes have been fabricated and tested in co-electrolysis mode using different fuel compositions, from pure H2O to pure CO2, at temperatures of 850şC – 900şC. Electrochemical impedance spectroscopy and galvanostatic measurements have been carried out for the mechanistic understanding of the symmetrical cells performance. The content of H2 and CO in the product gas has been measured by in-line gas micro-chromatography. The effect of employing H2 as a safe gas has been also investigated. Maximum density currents of 750 mA/cm2 and 620 mA/cm2 have been applied at 1.7 V for pure H2O and for H2O:CO2 ratios of 1:1, respectively. Remarkable results were obtained for hydrogen-free fuel compositions, which confirmed the interest of using ceramic oxides as a fuel electrode candidate to reduce or completely avoid the use of safe gas in operation minimizing the contribution of the reverse water shift reaction (RWSR) in the process. H2:CO ratios close to two were obtained for hydrogen-free tests fulfilling the basic requirements for synthetic fuel production. An important increase of the operation voltage was detected under continuous operation leading to a dramatic failure by delamination of the oxygen electrode.
Remark Accepted Manuscript, DOI: 10.1039/C5FD00018A
Link

Doping strategies for increased oxygen permeability of CaTiO3 based membranes

Authors Jonathan M. Polfus, Wen Xing, Martin F. Sunding, Sidsel M. Hanetho, Paul Inge Dahl, Yngve Larring, Marie-Laure Fontaine, Rune Bredesen
Source
Journal of Membrane Science
Volume: 482, Pages: 137–143
Time of Publication: 2015
Abstract Oxygen permeation measurements are performed on dense samples of CaTi0.85Fe0.15O3−δ, CaTi0.75Fe0.15Mg0.05O3−δ and CaTi0.75Fe0.15Mn0.10O3−δ in combination with density functional theory (DFT) calculations and X-ray photoelectron spectroscopy (XPS) in order to assess Mg and Mn as dopants for improving the O2 permeability of CaTi1−xFexO3−δ based oxygen separation membranes. The oxygen permeation measurements were carried out at temperatures ranging between 700 and 1000 °C with feed side oxygen partial pressures between 0.01 and 1 bar. The O2 permeability was experimentally found to be highest for the Mn doped sample over the whole temperature range, reaching 4.2×10−3 ml min−1 cm−1 at 900 °C and 0.21 bar O2 in the feed which corresponds to a 40% increase over the Fe-doped sample and similar to reported values for x=0.2. While the O2 permeability of the Mg doped sample was also higher than the Fe-doped sample, it approached that of the Fe-doped sample above 900 °C. According to the DFT calculations, Mn introduces electronic states within the band gap and will predominately exist in the effectively negative charge state, as indicated by XPS measurements. Mn may therefore improve the ionic and electronic conductivity of CTF based membranes. The results are discussed in terms of the limiting species for ambipolar transport and O2 permeability, i.e., oxygen vacancies and electronic charge carriers.
Keywords Dense ceramic oxygen membrane; Ambipolar transport; Mixed ionic-electronic conduction; CaTiO3; Calcium titanate
Remark doi:10.1016/j.memsci.2015.02.036
Link
norecs.com

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