D. A. Agarkov

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.

Synthesis and properties of fuel cell anodes based on (La0.5 + xSr0.5 − x)1 − yMn0.5Ti0.5O3 − δ (x = 0–0.25, y = 0–0.03)

Results are presented of studying electrochemical properties of perovskite-like solid solutions (La0.5 + xSr0.5 − x)1 − yMn0.5Ti0.5O3 − δ (x = 0–0.25, y = 0–0.03) synthesized using the citrate technique and studied as oxide anodic materials for solid oxide fuel cells (SOFC). X-ray diffraction (XRD) analysis is used to establish that the materials are stable in a wide range of oxygen chemical potential, stable in the presence of 5 ppm H2S in the range of intermediate temperatures, and also chemically compatible with the solid electrolyte of La0.8Sr0.2Ga0.8Mg0.15Co0.05O3 − δ (LSGMC). It is shown that transition to a reducing atmosphere results in a decrease in electron conductivity that produced a significant effect on the electrochemical activity of porous electrodes. Model cells of planar SOFC on a supporting solid-electrolyte membrane (LSGMC) with anodes based on (La0.6Sr0.4)0.97Mn0.5Ti0.5O3 − δ and (La0.75Sr0.25)0.97Mn0.5Ti0.5O3 − δ and a cathode of Sm0.5Sr0.5CoO3 − δ are manufactured and tested using the voltammetry technique.

Preparation of membrane-electrode assemblies of solid oxide fuel cells by co-sintering of electrodes

The results of the development of procedures for forming membrane-electrode assemblies (MEAs) of solid oxide fuel cells (SOFCs) by co-sintering of electrodes and electrochemical tests of MEAs were described. Plates of Hionic™ material (Fuel Cell Materials, United States) with an area of 50 × 50 mm2 were used as a solid electrolyte membrane. The cathode layers were prepared from cation-deficient lanthanum-strontium manganite and the anion conductor 89 mol % ZrO2–10 mol % Sc2O3–1 mol % CeO2 (10Sc1CeSZ) with a carbon black addition for control over the microstructure. The anode layers were formed from the composite NiO/10Sc1CeSZ and by introducing rice starch as a pore-forming agent in the anode current-collecting layer. The thermal treatment mode was optimized based on thermogravimetry and scanning electron microscopy data and the results of testing the electrochemical characteristics of SOFCs to provide the formation of electrochemically active electrodes using one thermal cycle.

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.

Fabrication of membrane–electrode assemblies for solid-oxide fuel cells by joint sintering of electrodes at high temperature

The results on optimizing the procedure of preparation of the electrode system within membrane–electrode assemblies (MEA) of solid-oxide fuel cells (SOFC) by joint sintering of electrodes at the enhanced temperature close to that of anode sintering are presented. The MEA are prepared based on membranes of the anionic conductor HionicTM (Fuel Cell Materials, USA); the cathode is formed based on cation–deficient lanthanum-strontium manganite (La0.8Sr0.2)0.95MnO3 with addition of activated carbon for optimizing its microstructure; the anode is formed on the basis of cermet NiO/10Sc1CeSZ (89 mol % ZrO2, 10 mol % Sc2O3, 1 mol % CeO2). The results of electrochemical testing of model MEA are also shown.

Structure and Transport Properties of Zirconia-Based Solid Solution Crystals Co-Doped with Scandium and Cerium Oxides

The crystals of (ZrO2)1–x(Sc2O3)x(СeO2)0.01 solid solutions (x = 0.08–0.10) were obtained by directional crystallization. The crystals of the grown composites were semitransparent, opalescent, and without cracks and had varying microstructure in the bulk. In the range of compositions under study, it was impossible to obtain optically homogeneous, fully transparent crystals. The crystals grown at a growth rate of 10 mm/h had a nonuniform distribution of ceria along the length of the ingot. The introduction of ceria in an amount of 1 mol % increased the conductivity of the crystals, but the increase in the specific electric conductivity depended on the Sc2O3 content and the phase composition of the crystals. The highest conductivity was inherent in the (ZrO2)0.89(Sc2O3)0.10(CeO2)0.01 crystals.

Electrotransport Characteristics of Ceramic and Single Crystal Materials with the (ZrO2)0.89(Sc2O3)0.10(Y2O3)0.01 Composition

The comparative analysis of electrotransport characteristics and structure of ceramic and single crystal solid electrolytes with the (ZrO2)0.89(Sc2O3)0.10(Y2O3)0.01 composition is carried out before and after their life tests. It is shown that before the life tests, the specific conductivities of single-crystal and ceramic materials virtually coincide. During the 3000 h life tests, the specific ionic conductivity decreases for both single crystal and ceramic samples down to about 0.1 S cm–1 but the degradation of conductivity in single crystal proceeds more slowly as compared with the ceramic material. The reason for degradation of electrotransport characteristics in the single crystal is associated with the transition of its bulk structure from the t′′ phase to a phase with the higher degree of tetragonality, whereas in the ceramic material, in addition to the latter process, a rhombohedral phase appears presumably along grain boundaries.

Structure and Crystallization of Alkaline-Earth Aluminosilicate Glasses: Prevention of the Alumina-Avoidance Principle

Aluminosilicate glasses are considered to follow the Al-avoidance principle, which states that Al-O-Al linkages are energetically less favorable, such that, if there is a possibility for Si-O-Al linkages to occur in a glass composition, Al-O-Al linkages are not formed. The current paper shows that breaching of the Al-avoidance principle is essential for understanding the distribution of network-forming AlO4 and SiO4 structural units in alkaline-earth aluminosilicate glasses. The present study proposes a new modified random network (NMRN) model, which accepts Al-O-Al linkages for aluminosilicate glasses. The NMRN model consists of two regions, a network structure region (NS-Region) composed of well-separated homonuclear and heteronuclear framework species and a channel region (C-Region) of nonbridging oxygens (NBOs) and nonframework cations. The NMRN model accounts for the structural changes and devitrification behavior of aluminosilicate glasses. A parent Ca- and Al-rich melilite-based CaO-MgO-Al2O3-SiO2 (CMAS) glass composition was modified by substituting MgO for CaO and SiO2 for Al2O3 to understand variations in the distribution of network-forming structural units in the NS-region and devitrification behavior upon heat treating. The structural features of the glass and glass-ceramics (GCs) were meticulously assessed by advanced characterization techniques including neutron diffraction (ND), powder X-ray diffraction (XRD), 29Si and 27Al magic angle spinning (MAS)-nuclear magnetic resonance (NMR), and in situ Raman spectroscopy. ND revealed the formation of SiO4 and AlO4 tetrahedral units in all the glass compositions. Simulations of chemical glass compositions based on deconvolution of 29Si MAS NMR spectral analysis indicate the preferred formation of Si-O-Al over Si-O-Si and Al-O-Al linkages and the presence of a high concentration of nonbridging oxygens leading to the formation of a separate NS-region containing both SiO4 and AlO4 tetrahedra (Si/Al) (heteronuclear) in addition to the presence of Al[4]-O-Al[4] bonds; this region coexists with a predominantly SiO4-containing (homonuclear) NS-region. In GCs, obtained after heat treatment at 850 °C for 250 h, the formation of crystalline phases, as revealed from Rietveld refinement of XRD data, may be understood on the basis of the distribution of SiO4 and AlO4 structural units in the NS-region. The in situ Raman spectra of the GCs confirmed the formation of a Si/Al structural region, as well as indicating interaction between the Al/Si region and SiO4-rich region at higher temperatures, leading to the formation of additional crystalline phases.

Influence of the yttria dopant on the structure and properties of (ZrO2)0.91–x (Sc2O3)0.09(Y2O3) х (x = 0–0.02) crystals

We have studied the influence of the yttrium oxide (Y2O3) dopant (1 and 2 mol %) on the phase composition, structure, and electrical properties of ZrO2–9 mol % Sc2O3 solid solution. Stabilization of ZrO2 jointly with 9 mol % Sc2O3 and 2 mol % Y2O3 is shown to allow the acquisition of high phase stability transparent homogeneous crystals with a cubic structure. Their mechanical grinding is established to cause no change in the phase composition of these crystals, whereas the powders retain the initial fluorine structure. The powders preserved the original structure of the fluorite crystals. All the probed crystals reveal high microhardness and low fracture toughness. Increasing the Y2O3 concentration in the crystals led to a reduction of the maximum loads on the indenter, which the sample withstood without cracking. As is shown, the specific conductivity exhibits nonmonotonic behavior depending on the Y2O3 concentration in the crystals. Increasing the Y2O3 content to 2 mol % in the solid electrolyte reduces the conductivity of the crystals in the entire temperature range that is attributed to a decrease in the carrier mobility due to the increasing ion radius of the stabilizing ion.

ВЛИЯНИЕ ЛЕГИРУЮЩЕЙ ПРИМЕСИ ОКСИДА ИТТРИЯ НА СТРУКТУРУ И СВОЙСТВА КРИСТАЛЛОВ (ZrO₂)0,91−x(Sc₂O₃)0,09(Y₂O₃)х (x = 0÷0,02)

We have studied the influence of dopant Y 2 O 3 oxide (1 and 2 mol.%) on the phase composition, structure and electrical properties of the ZrO 2 — 9 mol.% Sc 2 O 3 solid solution. We have shown that stabilization of ZrO 2 jointly with 9 mol.% Sc 2 O 3 and 2 mol.% Y 2 O 3 allows one to obtain transparent homogeneous crystals with a cubic structure which have a high phase stability. Mechanical grinding of these crystals did not lead to a change in the phase composition of the powders. The powders inherited the original structure of the fluorite crystals. All the test crystals had high microhardness and low fracture toughness. Increasing the concentration of Y 2 O 3 in crystals led to the need to reduce maximum loads on the indenter that the sample could withstand without cracking. We have shown that the conductivity varies nonmonotonically with increasing Y 2 O 3 concentration in the crystals. An increase in the Y 2 O 3 content to 2 mol. % in the composition of the solid electrolyte reduces the conductivity of the crystals in entire temperature range which is caused with a decrease in carrier mobility due to increasing ion radius of the stabilizing ion.