E.V. Tsipis

Oxygen Ionic and Electronic Transport in Apatite-Type Solid Electrolytes

The oxygen ionic conductivity of apatite-type La 9.83 Si 4.5 Al 1.5-y Fe y O 26±δ (y = 0-1.5), La 10-x Si 6-y Fe y O 26±δ (x = 0-0.77; y = 1-2), and La 7-x Sr 3 Si 6 O 26-δ (x = 0-1) increases with increasing oxygen content. The ion transference numbers, determined by faradaic efficiency measurements at 973-1223 K in air, are close to unity for La 9.83 Si 4.5 Al 1.5-y Fe v O 26+δ and La 10 Si 5 FeO 26.5 , and vary in the range 0.96-0.99 for other compositions. Doping of La 9.83 (Si, Al) 6 O 26 with iron results in an increasing Fe 4+ fraction, which was evaluated by Mossbauer spectroscopy and correlates with partial ionic and p-type electronic conductivities, whereas La-stoichiometric La 10 (Si, Fe)O 26+δ apatites stabilize the Fe 3+ state. Among the studied materials, the highest ionic and electronic transport is observed for La 10 Si 5 FeO 26.5 , where oxygen interstitials are close neighbors of Si-site cations. Data on transference numbers, total conductivity, and Seebeck coefficient as a function of the oxygen partial pressure confirm that the ionic conduction in Fe-substituted apatites remains dominant under solid oxide fuel cell operation conditions. However, reducing p (O 2 ) leads to a drastic decrease in the ionic transport, presumably due to a transition from the prevailing interstitial to a vacancy diffusion mechanism, which is similar to the effect of acceptor doping. Iron additions improve the sinterability of silicate ceramics, increase the n-type electronic conductivity at low p(O 2 ), and probably partly suppress the ionic conductivity drop. The thermal expansion coefficients of apatite solid electrolytes in air are (8.8-9.9) X 10 -6 K -1 at 300-1250 K.

Ionic and p-type electronic transport in zircon-type Ce1 − xAxVO4 ± δ(A = Ca, Sr)

Incorporation of alkaline-earth cations into the zircon-type lattice of Ce1 − xAxVO4 (A = Ca, Sr; x = 0–0.2) leads to higher p-type electronic conductivity, while the tetragonal unit cell volume and Seebeck coefficient decrease due to increasing concentration of electron holes localised on cerium cations. The oxygen ion transference numbers of Ce1 − xCaxVO4 in air, determined by faradaic efficiency measurements, vary in the range from 2 × 10−4 to 6 × 10−3 at 973–1223 K, increasing with temperature. The ionic conductivity is essentially independent of calcium content and decreases with reducing oxygen partial pressure. The activation energy for ionic transport in Ce(Ca)VO4 is 90–125 kJ mol−1. Doping with calcium enhances the stability of cerium orthovanadate at reduced oxygen pressures, shifting the phase decomposition limits down to oxygen activity values of 10−16–10−14 atm at 1023 K. The results on structure, Seebeck coefficient, and the partial p-type electronic and oxygen ionic conductivities suggest the presence of hyperstoichiometric oxygen in the Ce1 − xAxVO4 + δ lattice. The hyperstoichiometry, estimated from Seebeck coefficient data in the p(O2) range from 10−19 to 0.75 atm at 923–1223 K, may achieve 2–3% of the total oxygen content and weakly depends on the temperature and oxygen pressure variations within the zircon phase existence domain. Thermal expansion coefficients of Ce1 − xAxVO4 + δ ceramics in air, calculated from dilatometric data, are in the narrow range (5.6–5.9) × 10−6 K−1 at 400–800 K.

Thermal and mechanical stability of lanthanide-containing glass–ceramic sealants for solid oxide fuel cells

Thermal stability of lanthanide (Ln = La, Nd, Gd, Yb) containing glass and glass–ceramics (GCs) was characterized for their application as sealants for solid oxide fuel cells (SOFCs). X-ray diffraction (XRD) in conjunction with the Rietveld-RIR and solid-state NMR techniques was employed to quantify the crystalline and amorphous fractions in the glasses sintered/heat treated at 850 °C in air for 1–1000 h. The structure and crystalline phase evolution of Ln containing aluminosilicate glasses depend markedly on the Ln3+ cation field strength over both short and intermediate length scales. Along with diopside, Ln containing silicate apatites, with general formula Ln9.33+2x(Si1−xAlxO4)6O2 (Ln = La, Nd and Gd; with x varying between 0 and 0.33), were observed in the GCs after the heat treatment periods of 1 to 1000 h at 850 °C, leading to moderately higher electrical conductivity. The substantial amount of the remaining glassy phase in Gd2O3-containing GC after 1000 h at 850 °C is likely to confer self-healing properties to this composition, in accord with the oxygen leakage measurements on thermal cycling. 29Si, 27Al and 11B magic-angle spinning (MAS) NMR spectra confirmed the results of the XRD RIR analysis. The values of Weibull characteristic strength and of average flexural strengths for all the GCs are higher than those reported for G-18 commercial glass (51 MPa), with Weibull modulus varying in the range 11.6–34.4 towards good mechanical reliability. Thermal shock resistance of model electrochemical cells made of yttria-stabilized zirconia (YSZ) was evaluated employing quenching from 800 °C in air and water. All the GC seals bonded well to YSZ and Sanergy HT metallic interconnects without gap formation. Suitable thermal expansion coefficient (9.7–11.1 × 10−6 K−1), mechanical reliability, high electrical resistivity, strong adhesion to Sanergy HT interconnects and YSZ, and sufficient thermal shock resistance indicate good suitability of the lanthanide-containing sealants for SOFC applications.

Lanthanum substituted CeNbO4+δ scheelites: mixed conductivity and structure at elevated temperatures

CeNbO4+δ is a room temperature monoclinic material exhibiting a phase change to a tetragonal scheelite polymorph at 1023 K. The material can accommodate oxygen excess up to a composition of CeNbO4.25 in air which is believed to be incorporated on interstitial sites and hence is of interest for solid oxide fuel cell applications. Recent research however pointed to poor stability at low p(O2) making optimisation a necessity. To this end the effect of substituting lanthanum for cerium in CeNbO4+δ has been investigated in terms of redox, structural and mixed conductive behaviour. On increasing lanthanum content the overall oxygen hyperstoichiometry of the as-prepared substituted phases was found to decrease. A reduction in the temperature of the monoclinic fergusonite to tetragonal scheelite phase transformation with increasing lanthanum content was also observed. However, the extent of oxidation with increasing temperature was found to decrease. Four probe dc conductivity measurements in the temperature range 923 K to 1223 K showed that the total conductivity decreased and that the p(O2) dependence of the total ionic conductivity increased with increasing lanthanum content. The ionic transference number, ti, was found to increase to a maximum of approximately 0.75 with lanthanum substitution but at low p(O2) the stability of the tetragonal phase was found to decrease.

Mixed Conductivity and Stability of CaFe2O4 − δ

The total conductivity of CaFe 2 O 4-δ , 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 × 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 (δ) 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 CaFe 2 O 4-δ 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(O 2 ) results in a modest lattice contraction and in the p-n transition indicated by the conductivity and Seebeck coefficient variations. The phase decomposition of CaFe 2 O 4-δ occurs at oxygen chemical potentials between the low-p(O 2 ) stability limit of Ca 2 Fe 2 O 5-δ brownmillerite and the hematite/magnetite boundary in binary Fe-O system.

Melilite glass–ceramic sealants for solid oxide fuel cells: effects of ZrO2 additions assessed by microscopy, diffraction and solid-state NMR

The influence of adding 0–5 mol% zirconia (ZrO2) to a series of melt-quenched alkaline-earth aluminosilicate glasses designed in the gehlenite (Ca2Al2SiO7)–akermanite (Ca2MgSi2O7) system has been investigated for their potential application as sealants for solid oxide fuel cells (SOFCs). The work was implemented with a dual aim of improving the sintering ability of the glass system under consideration and gaining insight into the structural changes induced by ZrO2 additions in the glasses consequentially leading to their enhanced long-term thermal stability. That the degree of condensation of SiO4 tetrahedra increased with increasing amounts of zirconia was confirmed by 29Si magic-angle (MAS) NMR. 1D 27Al, 11B MAS as well as two-dimensional (2D) 11B MQMAS/STMAS NMR experiments gave structural insight into the number and nature of aluminum and boron sites found in the glass and glass–ceramic (GC) samples. Irrespective of the heat treatment time, increasing the zirconia content in glasses suppressed their tendency towards devitrification, while the glasses exhibited good sintering behavior resulting in mechanically strong GCs with higher amounts of residual glassy phase making them suitable for self-healing during SOFC operation. All the GCs exhibited low total electrical conductivity; appropriate coefficients of thermal expansion (CTE), good joining and minimal reactivity with SOFC metallic components at the fuel cell operating temperature, thus, qualifying them for further appraisal in SOFC stacks.

Surface diffusion, migration, and conjugated processes at heterophase interfaces between WO3 and MeWO4 (Me = Ca, Sr, Ba)

With the aid of a complex of methods it is demonstrated that at heterophase interfaces between WO3 and MeWO4 (Me = Ca, Sr, Ba) there occurs penetration of components WO3 and MeWO4 into one another under spontaneous conditions and after the imposition of an electric field. Experimental data concerning the electrosurface migration in potentiostatic and galvanostatic regimes are compared. It is demonstrated that the amount of WO3 transported onto inner surface of MeWO4 is defined by the magnitude of the electric charges passed through the system but does not depend on the I–U parameters of experiment. It is established that the magnitude of the faradaic efficiency of the WO3 transport in an electric field at 900°C is close for all compounds of the type MeWO4 (Me = Ca, Sr, Ba) and amounts to 0.42 ± 0.02 for a galvanostatic regime of the process. Methods of x-ray diffraction analysis, x-ray-fluorescence analysis, XPS, and electron microscopy are employed to explore the properties and compositions of regions adjacent to the WO3|MeWO4 interface after experiments in spontaneous and field-induced regimes. Data are obtained that confirm the reality of formation of nonautonomous phase MeW-s and its crucial role in the origin and mechanism of processes that occur at the heterophase interface WO3|MeWO4. The real architecture of the interface may be portrayed by the scheme WO3⋮MeW-s|MeW-s⋮MeWO4, which reflects penetration of MeW-s into both initial briquettes. The reasons for the loss of weight of briquettes of MeWO4 when annealed in contact with WO3 under spontaneous conditions are analyzed. It is shown that the weight loss may be caused by congruent sublimation of the MeW-s phase, which is directly connected with its low surface energy and relatively low sublimation energy.

Stability and functional properties of Sr0.7Ce0.3MnO3 − δ as cathode material for solid oxide fuel cells

Studies of oxygen diffusion, interphase exchange, specific electric conductivity, and thermal expansion showed that perovskite-like Sr0.7Ce0.3MnO3 − δ (SCMO) as a potential cathode material for solid oxide fuel cells (SOFCs) has considerable advantages over the conventional materials based on lanthanum-strontium manganites. To prevent the interactions of SCMO with solid electrolyte membranes of stabilized zirconia and lanthanum gallate, it is necessary to deposit protective layers of solid solutions based on cerium oxide, which do not form new phases in contact with SCMO and electrolytes. The trials of model SOFCs with porous SCMO-based cathodes demonstrated satisfactory electrochemical and endurance characteristics of these electrodes.

Specific features of bismuth oxide reactivity in spontaneous and electrical field-stimulated interactions

Abstract Solid state reactions of Bi 2 O 3 with Me x O y oxides were studied by means of the Bi 2 O 3 ∣Me x O y coupled anneals in model cells where Me x O y were the oxides of II–VI groups and 3-d metals. The reactions were performed both in spontaneous and external electrical field applied modes. The following main features are revealed and discussed: the dependence of Bi 2 O 3 diffusion ability, mass- and charge-transfer routes upon the oxidation degree of metal in partner oxide, formation of O 2− solid electrolyte phases at the Bi 2 O 3 ∣Me x O y interface, the drastic influence of the reaction rate and product composition upon the external voltage and polarity for reactions with 3-d metals and Cr oxides connected with easy change of Me oxidation degree at the support/product interface.

Effect of size factor on mechanism of interaction between Al2O3 and Bi2O3 and conductivity of composite on their basis

The fundamental differences in the mechanism of interaction between nanosized (Al2O3N) and microsized (Al2O3M) and Bi2O3 and routes of charge transfer in the resulting composite consisting of Bi2O3M, reaction products, and Al2O3N are shown. The size effect is manifested by a strong adhesion of Al2O3N grains to microsized Bi2O3M grains and by the high reactivity of Al2O3N. Both factors result in the encapsulation of Bi2O3M grains in a shell of low-conducting Al2O3N grains and interaction products. The α → δ-Bi2O3 phase transition at 730°C is registered using the HTXRD and DSC techniques, but it does not appear in the temperature dependence of conductivity because it occurs within mutually isolated Bi2O3M grains. Further heating to 780°C results in the solid-phase decomposition of interaction products and the separation of the δ-Bi2O3 phase. Last, a connected charge percolation matrix is formed of highly conducting δ-Bi2O3 grains. Due to this, a 2.5-fold conductivity jump is observed in the range of 770–800°C. The above-described model is confirmed by measurements of σ(T) for four model cells and by the presence of three concentration ranges for the [Al2O3N]/[Bi2O3M] ratio, which have different shapes of the σ(T) dependence and different locations of the σ jump in the range of 730–805°C.