Characterization and Optimization of La_0.97Ni_0.5Co_0.5O_3−δ-Based Air-Electrodes for Solid Oxide Cells

On the basis of previous studies of perovskites in the quasi-ternary system LaFeO₃–LaCoO₃–LaNiO₃, LaNi_0.5Co_0.5O₃ (LNC) is chosen as the most promising air-electrode material in the series for solid oxide cells (SOCs). In the present study, A-site deficiency of LNC is discussed and La_0.97Ni_0.5Co_0.5O₃ (LNC97) is selected as the optimal composition. Compatibility of LNC97 with 8 mol % Y₂O₃ stabilized ZrO₂ (8YSZ) is analyzed and compared with that of the state-of-the-art air-electrode La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) and 8YSZ. Targeting to the requirements of high-performance SOC air-electrodes (high electronic and ionic conductivity and high catalytic activity for the oxygen reduction reaction), LNC97-based air-electrodes are tailored, characterized and optimized by symmetric-cell tests. Principles of air-electrode design for SOCs are revealed accordingly. Long-term measurement of the symmetric cells over a period of 1000 h is performed and possible degradation mechanisms are discussed. Full cells based on optimized LNC97 air-electrodes are also tested. Lower reactivity with 8YSZ in comparison to LSCF and a similar performance render LNC97 a very competitive candidate to substitute LSCF as air-electrode material of choice for SOCs

A- and B‑Site Ordering in the A‑Cation-Deficient Perovskite Series La_2–<i>x</i>NiTiO_6−δ (0 ≤ <i>x</i> < 0.20) and Evaluation as Potential Cathodes for Solid Oxide Fuel Cells

The La_2–<i>x</i>NiTiO_6−δ (0 ≤ <i>x</i> < 0.2) series has been investigated in order to assess its possible use as a solid oxide fuel cell (SOFC) cathode material. These perovskite-like oxides exhibit monoclinic symmetry, as determined by a series of high-resolution structural techniques (X-ray diffraction (XRD), neuron powder diffraction (NPD), selected-area electron diffraction (SAED), and transmission electron microscopy (TEM)). Ni and Ti order over the B-site and, unusually, for <i>x</i> > 0, the A-site ions are also ordered along the <i>c</i>-axis in alternate La-rich and □-rich layers (where □ represents a vacancy). Structural determination combined with accurate compositional and magnetic characterization indicates a change in the predominant charge-compensating mechanism of A-site vacancies with composition. For <i>x</i> = 0.1, oxygen-vacancy formation seems to be the main-charge compensating mechanism, whereas, for <i>x</i> = 0.2, partial replacement of Ni by Ti in the B-substructure is dominant. In addition, a small amount of trivalent nickel is present in all samples. The composition dependence of the electrical conductivity of La_2–<i>x</i>NiTiO_6−δ (<i>x</i> = 0, 0.1, 0.2), investigated by impedance spectroscopy, as a function of temperature and oxygen partial pressure, is successfully interpreted on the basis of the relevant charge-compensating mechanisms and associated valence states. Thermal and chemical stability have also been studied in order to perform a preliminary electrochemical characterization as prospective cathode materials for SOFCs. The material La_1.80NiTiO_6‑δ exhibits excellent stability under oxidizing conditions and a polarization resistance of ∼0.5 Ω cm² at 1073 K with a yttria-stabilized zirconia (YSZ) electrolyte, slightly lower than that of the state-of-the-art La_1–<i>x</i>Sr_<i>x</i>MnO₃ (LSM)-based cathodes. A higher thermal stability and a better chemical compatibility of La_1.80NiTiO_6−δ with common electrolytes (e.g., YSZ), in comparison with LSM, suggests that this oxide warrants further study and optimization as a prospective improved cathode material for SOFCs

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

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