V. V. Kharton

A (3 + 3)-Dimensional “Hypercubic” Oxide-Ionic Conductor: Type II Bi2O3–Nb2O5

The high-temperature cubic form of bismuth oxide, δ-Bi2O3, is the best intermediate-temperature oxide-ionic conductor known. The most elegant way of stabilizing δ-Bi2O3 to room temperature, while preserving a large part of its conductivity, is by doping with higher valent transition metals to create wide solid-solutions fields with exceedingly rare and complex (3 + 3)-dimensional incommensurately modulated hypercubic structures. These materials remain poorly understood because no such structure has ever been quantitatively solved and refined, due to both the complexity of the problem and a lack of adequate experimental data. We have addressed this by growing a large (centimeter scale) crystal using a novel refluxing floating-zone method, collecting high-quality single-crystal neutron diffraction data, and treating its structure together with X-ray diffraction data within the superspace symmetry formalism. The structure can be understood as an inflated pyrochlore, in which corner-connected NbO6 octahedral chains move smoothly apart to accommodate the solid solution. While some oxide vacancies are ordered into these chains, the rest are distributed throughout a continuous three-dimensional network of wide δ-Bi2O3-like channels, explaining the high oxide-ionic conductivity compared to commensurately modulated phases in the same pseudobinary system.

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.

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.

Development of bilayer glass-ceramic SOFC sealants via optimizing the chemical composition of glasses—a review

Glasses and glass-ceramics (GCs), in particular alkaline-earth alumino silicate-based compositions, are becoming the most common sealing materials for gas-tight sealing applications in solid oxide fuel cells (SOFCs). The present review aims at reporting the systematic procedures put forward developing a novel concept of diopside-based bilayer GC seal, which contains a rigid GC and a self-healing (SH) GC. The concept behind the bilayer GCs is (i) a small gradient in the coefficient of thermal expansion (CTE) will lead to lower thermal expansion mismatch between the sealing layers and other SOFC components, thus providing enhanced mechanical reliability for the stack; and (ii) cracks produced due to minor thermal stresses in the rigid GC layer can be healed by the SH GC layer due to sufficient amorphous content. In general, at high temperature, highly crystallized glass behaves as a rigid glass. On the other hand, due to low viscosity behavior, partially crystallized glass provides a SH behavior. Various glasses in the field of diopside crystalline materials have been systematically designed by varying the chemical composition of glass to achieve desired combination of functional properties for the rigid and SH GC layers. The glass Sr-0.3 where Sr replaced 30xa0% of Ca was revealed as a highly reliable rigid GC seal for high-temperature electrochemical applications. On the other hand, SH features have been achieved in 30xa0% Sr-containing diopside-based glass with Gd2O3 for MgOu2009+u2009SiO2 substitution (denoted as Gd-0.3). These GCs exhibit similar thermal properties and excellent thermal stability along a period of 1000xa0h, while differing in their amorphous fractions, and revealed excellent thermal stability along a period of 1000xa0h. The bilayered GC synthesized from Sr-0.3 and Gd-0.3 showed good wetting and bonding ability to the SOFC metallic Crofer22APU components. The results revealed superior performance for the newly proposed bilayer GCs in comparison to single-layer sealants.

Analysis of electric properties of ZrO2-Y2O3 single crystals using teraherz IR and impedance spectroscopy techniques

The data obtained by impedance spectroscopy (1 Hz to 32 MHz) and broad-band dielectric spectroscopy (30 GHz-150 THz) are presented for crystals based on zirconia doped by 1.5–30 mol % Y2O3 or 10 mol % Sc2O3 and 1 mol % Y2O3. The maximum of ionic conductivity is confirmed for the latter composition in the working temperature range of solid oxide fuel cells where the doping by scandium and yttrium oxides makes it possible to obtain isotropic single crystals. Dependences of dielectric permeability and high-frequency conductivity of materials on the composition of crystals and temperature are presented.

Kinetics of NiO reduction and morphological changes in composite anodes of solid oxide fuel cells: Estimate using Raman scattering technique

The kinetics of nickel reduction and morphological changes in Ni–10Sc1CeSZ composite anodes in intermediate-temperature solid oxide fuel cells (SOFC) are studied using the Raman spectroscopy technique with the help of application of optically transparent single crystal solid electrolyte membranes and also the thermogravimetric analysis technique. It is shown that the first reduction cycle differs considerably from all the further ones, which is related to morphological changes of nickel grains occurring during the first reduction cycle. A general scheme of occurrence of the process is suggested in studies of model cells using the Raman spectroscopy technique and also in the case of thermogravimetric analysis of powders; it explains the causes for significant differences between the total duration of the process as measured using different techniques. The results of the work can be used for optimization of the mode of initial reduction of the anodic SOFC electrode.

Understanding the Formation of CaAl2Si2O8 in Melilite-Based Glass-Ceramics: Combined Diffraction and Spectroscopic Studies

An assessment is undertaken for the formation of anorthite crystalline phase in a melilite-based glass composition (CMAS: 38.7CaO–9.7MgO–12.9Al2O3–38.7SiO2 mol %), used as a sealing material in solid oxide fuel cells, in view of the detrimental effect of anorthite on the sealing properties. Several advanced characterization techniques are employed to assess the material after prolonged heat treatment, including neutron powder diffraction (ND), X-ray powder diffraction (XRD), 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS-NMR), and in situ Raman spectroscopy. ND, 29Si MAS-NMR, and 27Al MAS-NMR results revealed that both Si and Al adopt tetrahedral coordination and participate in the formation of the network structure. In situ XRD measurements for the CMAS glass demonstrate the thermal stability of the glass structure up to 850 °C. Further heat treatment up to 900 °C initiates the precipitation of melilite, a solid solution of akermanite/gehlenite crystalline phase. Qualitative XRD data for glass-ceramics (GCs) produced after heat treatment at 850 °C for 500 h revealed the presence of anorthite along with the melilite crystalline phase. Rietveld refinement of XRD data indicated a high fraction of glassy phase (∼67%) after the formation of crystalline phases. The 29Si MAS-NMR spectra for the CMAS-GC suggest the presence of structural units in the remaining glassy phase with a polymerization degree higher than dimer units, whereas the 27Al MAS-NMR spectra revealed that most Al3+ cations exhibit a 4-fold coordination. In situ Raman spectroscopy data indicate that the formation of anorthite crystalline phase initiated after 240 h of heat treatment at 850 °C owing to the interaction between the gehlenite crystals and the remaining glassy phase.

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.

Stability, mixed conductivity, and thermomechanical properties of perovskite materials for fuel cell electrodes based on La0.5A0.5Mn0.5Ti0.5O3–δ, La0.5Ba0.5Ti0.5Fe0.5O3–δ, and (La0.5А0.5)0.95Cr0.5Fe0.5O3–δ (A = Ca, Ba)

For materials based on ferrites and manganites with Са2+ and Ва2+ cations substituted into А sublattice, the functional properties are studied and the prospects as electrode materials for solid-oxide fuel cells are assessed. The electronic conductivity of materials based on La0.5A0.5Mn0.5Ti0.5O3–δ is shown to decrease with the increase in the ionic radius of alkali-earth substituent; however, for La0.5Ва0.5Mn0.5Ti0.5O3–δ and La0.5Ва0.5Fe0.5Ti0.5O3–δ, the appearance of n-conduction is observed during reduction, which may provide adequate conductivity under anodic conditions. Under the conditions of fuel cell operation, the thermal expansion coefficients of these materials are (13.0–13.5) × 10–6 K–1. The thermal and chemical expansion increases with the increase in the radius of alkali-earth cation; the latter value does not exceed 0.2%, which is acceptable for preparation of electronic layers. The transition of oxygen through membranes based on materials studied is determined to the large extent by the kinetics of surface exchange which depends on the rate of delivery of oxygen vacancies to the surface. Doping of ferrites with chromium or titanium decreases the electronic and ionic conductivity; however, the presence of substituents in В sublattice makes it possible to stabilize the perovskite phase in a wide range of р(О2), decrease the thermal and chemical expansion, and prevent to the large extent the ordering of oxygen vacancies, which allows one to consider these materials as the candidates for electrodes in symmetrical solid-oxide fuel cells.

Oxygen Nonstoichiometry and Transport Properties of Mixed-Conducting Ce 0.6– x La 0.4 Pr x O 2–δ

The oxygen nonstoichiometry and electrical conductivity of fluorite-type solid solutions Ce0.6‒xLa0.4PrxO2–δ (x = 0.1–0.2) were studied in the oxygen partial pressure range 10–19–0.35 atm at 1023–1223 K. It was confirmed that the Pr4+/3+ and Ce4+/3+ redox pairs, which determine the concentration of p- and n-type electron charge carriers, play the dominant roles under oxidizing and reducing conditions, respectively. The conductivity vs. charge carrier concentration dependencies in these conditions are almost linear. Increasing praseodymium content leads to a substantially higher hole conductivity and an expanded range of the oxygen nonstoichiometry variations at high oxygen partial pressures. Under reducing conditions when praseodymium cations become trivalent opposite trends are observed on doping.