Browsing by Author "Tsipis, EV"
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- ItemBehavior of (La,Sr)CoO3- and La2NiO4-based ceramic anodes in alkaline media: compositional and microstructural factors(Springer, 2008-01) Poznyak, SK; Kharton, VV; Frade, JR; Yaremchenko, AA; Tsipis, EV; Yakovlev, SO; Marozau, IPThe behavior of dense ceramic anodes made of perovskite-type La1-x-ySrxCo1-zAlzO3-δ (x=0.30-0.70; y=0-0.05; z=0-0.20) and K2NiF4- type La2Ni1-xMexO4+δ (Me=Co, Cu; x=0-0.20) indicates significant influence of metal hydroxide formation at the electrode surface on the oxygen evolution reaction (OER) kinetics in alkaline solutions. The overpotential of cobaltite electrodes was found to decrease with time, while cyclic voltammetry shows the appearance of redox peaks characteristic of Co(OH)(2)/CoOOH. This is accompanied with increasing effective capacitance estimated from the impedance spectroscopy data, because of roughening of the ceramic surface. The steady-state polarization curves of (La,Sr)CoO3-δ in the OER range, including the Tafel slope, are very similar to those of model Co(OH)(2)-La(OH)(3) composite films where the introduction of lanthanum hydroxide leads to decreasing electrochemical activity. La2NiO4-based anodes exhibit a low electrochemical performance and poor stability. The effects of oxygen nonstoichiometry of the perovskite-related phases are rather negligible at high overpotentials but become significant when the polarization decreases, a result of increasing role of oxygen intercalation processes. The maximum electrocatalytic activity to OER was observed for A-site-deficient (La0.3Sr0.7)(0.97)CoO3-δ, where the lanthanum content is relatively low and the Co4+ concentration determined by thermogravimetric analysis is highest compared to other cobaltites. Applying microporous layers made of template-synthesized nanocrystalline (La0.3Sr0.7)(0.97)CoO3-δ leads to an improved anode performance, although the effects of microstructure and thickness are modest, suggesting a narrow electrochemical reaction zone. Further enhancement of the OER kinetics can be achieved by electrodeposition of cobalt hydroxide- and nickel hydroxide- based films. © 2008, Springer.
- ItemChemically induced expansion of La2NiO4+ δ-based materials(American Chemical Society, 2007-03-21) Kharton, VV; Kovalevsky, AV; Avdeev, M; Tsipis, EV; Patrakeev, MV; Yaremchenko, AA; Naumovich, EN; Frade, JRThe equilibrium chemical strains induced by the oxygen hyperstoichiometry variations in mixed-conducting La2Ni1-xMxO4+δ (M = Fe, Co, Cu; x = 0−0.2) with K2NiF4-type structure, were studied by controlled-atmosphere dilatometry at 923−1223 K in the oxygen partial pressure range 5 × 10-4 to 0.7 atm. In combination with the oxygen content measured by coulometric titration and thermogravimetry, the results reveal a very low chemical expansivity, favorable for high-temperature electrochemical applications. Under oxidizing conditions, the isothermal expansion relative to atmospheric oxygen pressure (εC) is less than 0.02%. The ratio between these values and the corresponding nonstoichiometry increment varies from −3 × 10-3 to 6 × 10-3, which is much lower compared to most permeable mixed conductors derived from perovskite-like cobaltites and ferrites. Consequently, the chemical contribution to apparent thermal expansion coefficients at a fixed oxygen pressure, (13.7−15.1) × 10-6 K-1, does not exceed 5%. The high-temperature X-ray diffraction studies showed that this behavior results from strongly anisotropic expansion of the K2NiF4-type lattice, namely the opposing variations of the unit-cell parameters on changing oxygen stoichiometry. © 2007, American Chemical Society
- ItemGeometric parameterization of the YBaCo4O7 structure type: implications for stability of the hexagonal form and oxygen uptake(Elsevier, 2010-10) Avdeev, M; Kharton, VV; Tsipis, EVWe explore the stability of the hexagonal form of MBaCo4O7 cobaltites in terms of geometric characteristics of the crystal structure and Global Instability Index (GII) based on the bond-valence considerations. Mismatch between an M3+/2+ and the three-dimensional network of CoO4 tetrahedra, whether expressed using an M ionic radii or GII, is shown to essentially determine both the temperature of structural transition to an orthorhombic modification and oxygen storage properties. A number of M cations not reported in the literature are identified to be suitable for the octahedral sites in an MBaCo4O7 structure. © 2010, Elsevier Ltd.
- ItemIonic conductivity and thermal expansion of anion-deficient Sr11Mo4O23 perovskite(Springer Nature, 2020-07-07) Kharton, VV; Tsipis, EV; Kolotygin, VA; Avdeev, M; Kennedy, BJTransport properties of perovskite-type Sr11Mo4O23 and composite Sr11Mo4O23 - 1 wt% Al2O3 were studied at 400–1300 K in the oxygen partial pressure range from 0.21 down to 10−19 atm. The electromotive force and faradaic efficiency measurements, in combination with the energy-dispersive spectroscopy of the fractured electrochemical cells, unambiguously showed prevailing role of the oxygen ionic conductivity under oxidizing conditions. At temperatures above 600 K, protonic and cationic transport can be neglected. The oxygen ion transference numbers vary in the range of 0.95–1.00 at 973–1223 K. At temperatures lower than 550 K, the total conductivity of Sr11Mo4O23 - 1 wt% Al2O3 composite measured by impedance spectroscopy tends to increase in wet atmospheres, thus indicating that hydration and protonic transport become significant. Reducing oxygen partial pressure below 10−10–10−9 atm leads to a significant increase in the n-type electronic conduction. The average thermal expansion coefficients in oxidizing atmospheres are (14.3–15.0) × 10−6 K−1 at 340–740 K and (18.3–19.2) × 10−6 K−1 at 870–1370 K. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature
- ItemLifting the geometric frustration through a monoclinic distortion in “114” YBaFe4O7.0: Magnetism and transport(Elsevier, 2013-09-01) Duffort, V; Sarkar, T; Caignaert, V; Pralong, V; Raveau, B; Avdeev, M; Cervellino, A; Waerenborgh, JC; Tsipis, EVThe possibility to lift the geometric frustration in the “114” stoichiomeric tetragonal oxide YBaFe4O7.0 by decreasing the temperature has been investigated using neutron and synchrotron powder diffraction techniques. Besides the structural transition from tetragonal to monoclinic symmetry that appears at TS=180 K, a magnetic transition is observed below TN=95 K. The latter corresponds to a lifting of the 3D geometric frustration toward an antiferromagnetic long range ordering, never observed to date in a cubic based “114’” oxide. The magnetic structure, characterized by the propagation vector k1=(0,0,½), shows that one iron Fe2 exhibits a larger magnetic moment than the three others, suggesting a possible charge ordering according to the formula YBaFe 3 + Fe 3 2 + O 7.0 . The magnetic M(T) and χ′(T) curves, in agreement with neutron data, confirm the structural and magnetic transitions and evidence the coexistence of residual magnetic frustration. Moreover, the transport measurements show a resistive transition from a thermally activated conduction mechanism to a variable range hopping mechanism at TS=180 K, with a significant increase of the dependence of the resistivity vs. temperature. Mössbauer spectroscopy clearly evidences a change in the electronic configuration of the iron framework at the structural transition as well as coexistence of several oxidation states. The role of barium underbonding in these transitions is discussed. © 2013, Elsevier Ltd.
- ItemMagnetic structure of Sr2Fe2O5 brownmillerite by single-crystal Mössbauer spectroscopy(Academic Press Inc Elsevier Science, 2013-09-01) Waerenborgh, JC; Tsipis, EV; Auckett, JE; Ling, CD; Kharton, VVIn order to determine orientation of the Fe3+ magnetic moments and electric field gradient (efg) axes in the brownmillerite-type strontium ferrite structure for both iron sublattices where the efg tensor is not axially symmetric, the Mössbauer spectra of powdered and oriented single-crystal Sr2Fe2O5 were analyzed by solving the complete Hamiltonian for hyperfine interactions in the excited and ground states of the 57Fe nuclei. The magnetic moments of both octahedrally and tetrahedrally coordinated iron cations lie on the ac-plane of the orthorhombic unit cell and are parallel to the shortest c-axis, whilst the main efg axes are parallel to the longest crystallographic axis, b. This orientation is similar to that in Ca2Fe2O5, in spite of the structural differences of strontium and calcium ferrite brownmillerites at low temperatures. © 2013, Elsevier Ltd.
- ItemMagnetization, Mössbauer and isothermal dilatometric behavior of oxidized YBa(Co,Fe)4O7+δ(Royal Society of Chemistry, 2011-11-08) Waerenborgh, JC; Tsipis, EV; Pereira, LCJ; Avdeev, M; Naumovich, EN; Kharton, VVMössbauer spectroscopy and magnetization studies of YBaCo4-xFexO7+δ (x = 0–0.8) oxidized at 0.21 and 100 atm O2, indicate an increasing role of penta-coordinated Co3+ states when the oxygen content approaches 8–8.5 atoms per formula unit. Strong magnetic correlations are observed in YBaCo4-xFexO8.5 from 2 K up to 55–70 K, whilst the average magnetic moment of Co3+ is lower than that for δ ≤ 0.2, in correlation with the lower 57Fe3+ isomer shifts determined from Mössbauer spectra. The hypothesis on dominant five-fold coordination of cobalt cations was validated by molecular dynamics modeling of YBaCo4O8.5. The iron solubility limit in YBaCo4-xFexO7+δ corresponds to approximately x ≈ 0.7. The oxygen intercalation processes in YBaCo4O7+δ at 470–700 K, analyzed by X-ray diffraction, thermogravimetry and controlled-atmosphere dilatometry, lead to unusual volume expansion opposing to the cobalt cation radius variations. This behavior is associated with increasing cobalt coordination numbers and with rising local distortions and disorder in the crystal lattice on oxidation, predicted by the computer simulations. When the oxygen partial pressure increases from 4 × 10−5 to 1 atm, the linear strain in YBaCo4O7+δ ceramics at 598 K is as high as 0.33%. © Royal Society of Chemistry 2021
- ItemMixed conductivity and stability of CaFe2O4−δ(Electrochemical Society, 2008-03) Kharton, VV; Tsipis, EV; Kolotygin, VA; Avdeev, M; Viskup, AP; Waerenborgh, JC; Frade, JRThe total conductivity of CaFe2O4-delta, studied in the oxygen partial pressure range from 10(-17) to 0.5 atm at 1023-1223 K, is predominantly p-type electronic under oxidizing conditions. The oxygen ion transference numbers determined by the steady-state oxygen permeation and faradaic efficiency measurements vary in the range of 0.2 to 7.2 x 10(-4) at 1123-1273 K, increasing with temperature. No evidence of any significant cationic contribution to the conductivity was found. The Mossbauer spectroscopy, thermogravimetry, and X-ray diffraction (XRD) showed that the orthorhombic lattice of calcium ferrite is essentially intolerant to the oxygen vacancy formation and to doping with lower-valence cations, such as Co and Ni. The oxygen nonstoichiometry (delta) is almost negligible, 0.0046-0.0059 at 973-1223 K and p(O-2) = 10(-5)-0.21 atm, providing a substantial dimensional stability of CaFe2O4-delta ceramics. The average linear thermal expansion coefficients, calculated from the controlled-atmosphere dilatometry and high-temperature XRD data, are (9.6-13.9) x 10(-6) K-1 in the oxygen pressure range from 10(-8) to 0.21 atm at 873-1373 K. Decreasing P(02) results in a modest lattice contraction and in the p-n transition indicated by the conductivity and Seebeck coefficient variations. The phase decomposition of CaFe2O4-delta occurs at oxygen chemical potentials between the low-p(O-2) stability limit of Ca2Fe2O5-delta brownmillerite and the hematite/magnetite boundary in binary Fe-O system. © 2008, Electrochemical Society Inc.
- ItemMossbauer spectroscopy analysis of Fe-57-doped YBaCo4O7+delta: effects of oxygen intercalation(Elsevier, 2009-03) Tsipis, EV; Waerenborgh, JC; Avdeev, M; Kharton, VVMossbauer spectroscopy of layered YBaCo3.96Fe0.04O7+delta (delta=0.02 and 0.80), where 1% cobalt is substituted With 57 Fe isotope, revealed no evidence of charge ordering at 4-293 K. The predominant state of iron cations was found trivalent, irrespective of their coordination and oxygen stoichiometry variations determined by thermogravimetric analysis. The extremely slow kinetics of isothermal oxidation at 598 K in air, and the changes of Fe3+ fractions in the alternating triangular and Kagome layers in oxidized YBaCo3.96Fe0.04O7.80, may suggest that oxygen intercalation is accompanied with a substantial structural reconstruction stagnated due to sluggish cation diffusion. Decreasing temperature below 75-80 K leads to gradual freezing of the iron magnetic moments in inverse correlation with the content of extra oxygen. The formation of metal-oxygen octahedra and resultant structural distortions extend the temperature range where the paramagnetic and frozen states co-exist, down to 45-50 K. © 2008, Elsevier Ltd.
- ItemOxygen nonstoichiometry, chemical expansion, mixed conductivity, and anodic behavior of Mo-substituted Sr3Fe2O7-δ(Elsevier, 2010-07-26) Kharton, VV; Patrakeev, MV; Tsipis, EV; Avdeev, M; Naumovich, EN; Anikina, PV; Waerenborgh, JCThe incorporation of molybdenum in the Ruddlesden-Popper type Sr3Fe2-xMoxO7-δ (x = 0–0.1) decreases oxygen deficiency, thermal expansion and electron-hole transport, and increases n-type electronic conductivity in reducing atmospheres. The oxygen ionic conduction remains essentially unaffected by doping. The equilibrium p(O2)–T–δ diagram of Sr3Fe1.9Mo0.1O7-δ, collected in oxygen partial pressure ranges from 10− 20 to 0.7 atm at 973–1223 K, can be adequately described by a defect model accounting for the energetic nonequivalence of apical O1 and equatorial O3 sites in the layered structure, in combination with iron disproportionation and stable octahedral coordination of Mo6+ and Mo5+ cations. The calculated enthalpy of anion exchange between the O1 and O3 positions, 0.49–0.51 eV, is in agreement with the values predicted by the atomistic computer simulation technique. The high-temperature X-ray diffraction studies showed a strongly anisotropic expansion of the Ruddlesden-Popper lattice on reduction, leading to very low chemical strains favorable for electrochemical applications. At 298–1223 K and oxygen pressures from 10− 8 to 0.21 atm, the linear thermal expansion coefficient of Sr3Fe1.9Mo0.1O7-δ varies in the narrow range (12.9–14.2) × 10− 6 K− 1. The relatively low level of n-type electronic conductivity leads, however, to a poor performance of porous Sr3Fe1.9Mo0.1O7-δ anodes in contact with lanthanum gallate-based solid electrolyte under reducing conditions. © 2010, Elsevier Ltd.
- ItemOxygen permeability, stability and electrochemical behavior of Pr2NiO4+δ-based materials(Springer, 2007-08) Kovalevsky, AV; Kharton, VV; Yaremchenko, AA; Pivak, YV; Tsipis, EV; Yakovlev, SO; Markov, AA; Naumovich, EN; Frade, JRThe high-temperature electronic and ionic transport properties, thermal expansion and stability of dense Pr2NiO4+δ Pr2Ni0.9Fe0.1O4+δ ceramics have been appraised in comparison with K2NiF4-type lanthanum nickelate. Under oxidizing conditions, the extensive oxygen uptake at temperatures below 1073-1223 K leads to reversible decomposition of Pr2NiO4-based solid solutions into Ruddlesden-Popper type Pr4Ni3O10 and praseodymium oxide phases. The substitution of nickel with copper decreases the oxygen content and phase transition temperature, whilst the incorporation of iron cations has opposite effects. Both types of doping tend to decrease stability in reducing atmospheres as estimated from the oxygen partial pressure dependencies of total conductivity and Seebeck coefficient. The steady-state oxygen permeability of Pr2NiO4+δ ceramics at 1173-1223 K, limited by both surface-exchange kinetics and bulk ionic conduction, is similar to that of La2NiO4+δ. The phase transformation on cooling results in considerably higher electronic conductivity and oxygen permeation, but is associated also with significant volume changes revealed by dilatometry. At 973-1073 K, porous Pr2Ni0.8Cu0.2O4+δ electrodes deposited onto lanthanum gallate-based solid electrolyte exhibit lower anodic overpotentials compared to Pr2Ni0.8Cu0.2O4+δ, whilst cathodic reduction decreases their performance. © 2007, Springer.
- ItemPhase behavior and mixed ionic-electronic conductivity of Ba4Sb2O9(Elsevier Science BV., 2013-03-21) Dunstan, MT; Pavan, AF; Kharton, VV; Avdeev, M; Kimpton, JA; Kolotygin, VA; Tsipis, EV; Ling, CDThe 6H-type perovskite phase Ba4Sb2O9, which decomposes in air below 600 K, is found to survive to room temperature in a CO2-free atmosphere. It shows substantial mixed protonic, oxide ionic and electronic conductivity. Compared to Ba4Nb2O9 and Ba4Ta2O9, Ba4Sb2O9 shows higher ionic conductivity due to the relatively easy reducibility of Sb5 +, but lower electronic conductivity due to the predominantly n-type conductivity provided by the Sb5 +/Sb3 + redox couple which leads to reduced hole concentration under oxidizing conditions. Variable temperature synchrotron X-ray diffraction studies carried out in situ under controlled atmospheres reveal a strong monoclinic distortion below 1150 K. The hexagonal to monoclinic transition is slow, does not show second-order behavior, is strongly dependent on atmosphere, and coincides with the loss of ~ 0.4 molecules of H2O per formula unit of Ba4Sb2O9. All of this suggests an important structural role for protons or hydroxide ions in the monoclinic phase. © 2013, Elsevier Ltd.