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Single-step synthesis and deposition of composite solid oxide fuel cell cathodes of composition (Nd,Gd)1/3Sr2/3CoO3−δ : Ce0.9(Gd,Nd)0.1O1.95 (70 : 30 w : w) with and without the addition of Ag nanoparticles (Ag NPs) was achieved employing spray pyrolysis and low-temperature sintering at 900 °C. Spray-pyrolysis-prepared materials offered improved microstructure with smaller particle size and higher porosity (extended triple phase boundary) in comparison to slurry-coated electrodes of analogous composition. Impedance spectroscopy of symmetrical cells of the composite air electrodes indicated a lower resistive response of the spray-pyrolysis-prepared electrodes with lower overall polarisation resistance (Rp), attributable to improved oxygen surface exchange and oxide-ion diffusion processes. Further reduction in Rp was achieved on decoration of the composite air electrodes with Ag NPs; the decorated composites were characterised by high resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Ag K-edge X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) of Ce0.9Gd0.1O2−δ:Ag spray-pyrolysed composites indicated that the Ag nanoparticles exhibit a core–shell structure with a Ag2O outer layer calculated to be two unit cells in thickness. The outer particle shell is suggested to participate in the enhanced electrochemical response, lowering the electrode polarisation response of symmetrical cells below 700 °C. Anode-supported single-cell fuel cells corresponding to the composite air electrode with Ag NPs and a thin YSZ-CGO electrolyte provided a maximum power density of 0.6 W cm−2 at 700 °C. [ABSTRACT FROM AUTHOR]

In recent years, transition metal oxynitrides have gained increasing attention due to offering attractive properties, such as high electronic and thermal conductivity, high melting points and hardness, as well as high catalytic activity. Niobium oxynitrides, especially, have been suggested for a diverse range of applications, including potential electrocatalysts (e.g., water splitting, nitrogen reduction, etc.), antibacterial agents, superconducting materials, and coatings, among others. One of the most promising ways to synthesize these materials is by ammonolysis. However, by this route the final composition can be heavily dependent on the ammonolysis conditions, with this factor currently understudied in the available literature. Thus, in this work, we carefully explore the impact of ammonolysis conditions on the crystalline phase formation of niobium oxynitride compounds. Several techniques were used to fully characterize the materials, including XRD, SEM-EDS, TGA/DSC, XPS, and chemical analysis. Depending on the ammonolysis temperature and time, different composition-dependent changes in crystallographic structure can be induced across δ-NbNxOy (cubic) → Nb4(N,O)5 (tetragonal) → ε-NbN (hexagonal) phases. Potential defect-chemistry models were proposed to support measured compositional variations. A detailed kinetic analysis was then performed on the cubic materials to understand how composition can influence the thermal oxidation behaviour within this structure-type. An Fn-type reaction was used to fit the experimental and calculated data. The selection of the Avrami model, which describes the crystallization kinetics, was supported by the identification of amorphous-crystalline phase transformations during thermal oxidation. This work demonstrates that samples with higher N-content are more resistive to oxidation and shows that the anion composition, cation/anion ratio, and the crystallographic structure can be tailored by careful control of ammonolysis conditions. [ABSTRACT FROM AUTHOR]

LiPON films are one of the most relevant solid electrolytes in Li-ion micro-batteries for low power applications, and the relationships between composition, processing parameters and ionic conductivity have challenged the materials community for the last three decades. Although films have almost exclusively been processed by deposition of Li3PO4 polycrystalline pellets, the amorphous nature of LiPON has permitted the study of compositions with glass-forming ability where the effect of improving the conductivity by nitrogen doping is similar. The aim of this work is to form a general model that allows us to calculate the ionic conductivity of LiPON solid electrolytes for whichever composition. The modelling is based on the application of the fundamental laws of chemical equilibrium to elucidate the different chemical species present for each given system composition. For this purpose, a program in the Matlab software was written, capable of solving simultaneous and competitive chemical equilibria. The method, known as the associated solutions model, was applied to the study of the Li2O–P2O5 system, which is the starting point of the present work. The model assumes that the properties of a given glass or amorphous solid, such as the simulated LiPON films, can be obtained as a function of the weighted contributions from each stable compound in the respective composition diagram. Here, Li2PO2N oxynitride is used as the main phase and, to validate the computed chemical equilibria, nitridation of crystalline Li4P2O7 has been performed by thermal ammonolysis. The combination of the calculations of chemical equilibria together with the structural characterization by XRD and NMR of the products of nitridation from Li4P2O7 allowed us to predict both the formation of Li2PO2N and the existence of a second oxynitride stoichiometric compound with formula LiPO1.5N that would be obtained by nitridation of LiPO3. The solution of the system of equilibria was made possible on development of the Gibbs free energy functions of formation of both oxynitride phases, and the calculation of the activation energy and ionic conductivity in either LiPON thin films or oxynitride glasses, offering comparable values to those found in the literature. [ABSTRACT FROM AUTHOR]

Barium zirconate-based perovskites (BZY) are intensely studied proton conductors for high-temperature electrochemical applications; however, their refractory nature means excessive temperatures are required for sintering (∼1700 °C). Although different strategies have been employed to improve densification, a high-temperature step (1300–1700 °C) is generally required to achieve sufficient grain growth and good electrical properties. Here, sol–gel synthesis using alkoxide precursors is employed to obtain BaZr0.8Y0.2O3−δ sinter-reactive powders with Ba excess at 750 °C. High relative densities (∼98%) of ceramic bodies with an average grain size ∼190 nm were obtained on sintering isostatically-pressed pellets at the uniquely low temperature of 1200 °C for 4 hours on addition of 4% mol ZnO as a sintering agent to the Ba-excess sol–gel powders. Impedance spectroscopy indicates that the resistance of the greater grain-boundary volume arising from the low densification temperature is largely offset by a higher specific grain-boundary conductivity than that of the same composition sintered at 1650 °C with larger grain size. Conductivity measurements for prolonged periods in dry and wet (pH2O ∼ 0.022 atm) 90% N2 : 10% H2 and 90% Ar : 10% CO2 atmospheres at 500 °C verified that Ba excess in the starting composition did not contribute to deteriorating chemical stability. Processing of sol–gel BZY nanopowders with Ba excess combined with ZnO sintering aid is thus demonstrated as a successful low-temperature route for obtaining BZY with high density, stability and sufficient grain growth for electrochemical applications. [ABSTRACT FROM AUTHOR]

Protonic ceramic fuel cells (PCFCs) are promising electrochemical devices for the efficient and clean conversion of hydrogen and low hydrocarbons into electrical energy. Their intermediate operation temperature (500–800 °C) proffers advantages in terms of greater component compatibility, unnecessity of expensive noble metals for the electrocatalyst, and no dilution of the fuel electrode due to water formation. Nevertheless, the lower operating temperature, in comparison to classic solid oxide fuel cells, places significant demands on the cathode as the reaction kinetics are slower than those related to fuel oxidation in the anode or ion migration in the electrolyte. Cathode design and composition are therefore of crucial importance for the cell performance at low temperature. The different approaches that have been adopted for cathode materials research can be broadly classified into the categories of protonic–electronic conductors, oxide-ionic–electronic conductors, triple-conducting oxides, and composite electrodes composed of oxides from two of the other categories. Here, we review the relatively short history of PCFC cathode research, discussing trends, highlights, and recent progress. Current understanding of reaction mechanisms is also discussed. [ABSTRACT FROM AUTHOR]

Synthesis of the proton-conducting electrolyte SrZr0.9Y0.1O3−δ(SZY) was undertaken by the sol–gel method employing an all-alkoxide route from reaction of strontium alkoxide produced in-situ and commercial zirconium and yttrium alkoxides. The solution was homogenized by a previous ligand exchange in 2-methoxyethanol to control the polycondensation rate and achieve SZY at the low final firing temperature of 700–800 °C. SZY thin films (~270 nm) were assisted by dip-coating on different substrates and characterized by scanning and transmission electron microscopy, grazing X-ray diffraction and confocal micro-Raman spectroscopy, revealing well-crystallized SZY phase with orthorhombic symmetry (space group, Pnma). Impedance spectroscopy of a thin film deposited on a quartz substrate revealed that protons contribute to transport in wet conditions as confirmed by a lower conductivity in D2O-humidified air (1.02 eV) compared to H2O-wetted air (0.99 eV), with the difference in activation energy consistent with a conductive isotope effect. Highlights: Proton-conducting electrolytes SrZr0.9Y0.1O3−δ thin-films were successfully obtained. Novel all-alkoxide sol–gel route via in-situ synthesis of triple alkoxide was performed. Ligand exchange in 2-methoxyethanol allows soft thermal treatment SZY thin-film was well crystallized at low temperature and time (700 °C, 1 h). Protonic electrical contribution is present in wet conditions. SZY thin-films synthesized at 700 °C present similar conductivity than a pellet at 1300 °C. [ABSTRACT FROM AUTHOR]

Exploring the reasons for the initiation of Al–O–Al bond formation in alkali-earth alumino silicate glasses is a key topic in the glass-science community. Evidence for the formation of Al–O–Al and Al–NBO bonds in the glass composition 38.7CaO–9.7MgO–12.9Al2O3–38.7SiO2 (CMAS, mol%) has been provided based on Molecular Dynamics (MD) simulations. Analyses in the short-range order confirm that silicon and the majority of aluminium cations form regular tetrahedra. Well-separated homonuclear (Si–O–Si) and heteronuclear (Si–O–Al) cluster regions have been identified. In addition, a channel region (C-Region), separated from the network region, enriched with both NBO and non-framework modifier cations, has also been identified. These findings are in support of the previously proposed extended modified random network (EMRN) model for aluminosilicate glasses. A detailed analysis of the structural distributions revealed that a majority of Al, 51.6%, is found in Si–O–Al links. Although the formation of Al–O–Al and Al–NBO bonds is energetically less favourable, a significant amount of Al is found in Al–O–Al links (33.5%), violating Lowenstein's rule, and the remainder is bonded with non-bridging oxygen (NBO) in the form of Al–NBO (Al–O–(Ca, Mg)). The conditions necessary for the formation of less favourable bonds are attributed to the presence of a high amount of modifier cations in current CMAS glass and their preferable coordination. [ABSTRACT FROM AUTHOR]

A complete solid solution forms between the perovskite proton conductor BaZr0.7Ce0.2Y0.1O3−δ (BZCY72) and BaPr0.9Y0.1O3−δ (BPY) on synthesis by the Pechini method and high-temperature annealing. Phase fields of selected members of the Ba(Zr0.7Ce0.2)1−(x/0.9)PrxY0.1O3−δ series were studied as a function of composition and temperature by high-resolution neutron powder diffraction revealing symmetry changes in the sequence Pnma→Imma→UGRAPHIC DISPLAY="INLINE" ID="UGT1" SRC="UGT1"/→Pm3̅m. Higher symmetry is favoured for low Pr contents and high temperatures, as consideration of tolerance factor suggests. A volume contraction, ascribed to dehydration, is observed by synchrotron X-ray diffraction on heating in air for lower x. Magnetic measurements and structural data support the presence of Pr in the IV valence state on the perovskite B site. Thermogravimetric analysis in CO2 to ∼1253 K indicates better chemical stability for x≤ 0.445, whereas decomposition occurred for higher x. Electrical conductivity increases by over two orders of magnitude in dry air at lower temperature from x = 0.225 to 0.675; total conductivity reaches a value of 0.4 S cm−1 at 1173 K for x = 0.675. The series exhibits electron–hole transport with a positive pO2 dependence which increases with temperature, consistent with participation of oxygen vacancies in charge compensation of the Y3+ acceptor dopant. The activation energy for thermally activated hole hopping in air in the range 523–773 K decreases from ∼1 eV for BZCY72 to ∼0.4 eV for x = 0.675. Conductivity is generally lower in humidified N2 and air (pH2O ≈ 0.023 atm) than the corresponding dry atmospheres, consistent with consumption of holes by less mobile protonic species; however for x≤ 0.225 the lower concentration of electron holes concomitant with higher oxygen-vacancy content in N2 results in slightly higher conductivity in wet conditions due to hydration of vacancies. [ABSTRACT FROM AUTHOR]

A solid solution forms for Sr3NdNb3−xTixO12−δ with approximate limits 0 ≤x≤ 0.06. The system crystallizes with a 12R-type hexagonal perovskite structure in the space group R3̅, as determined by neutron diffraction and selected area electron diffraction. The electrical properties of the end members have been investigated by impedance spectroscopy in the temperature range 550–800 °C under various gas atmospheres and as a function of oxygen and water-vapour partial pressure. Proton transport dominates under wet oxidising conditions in the temperature range 550–700 °C, as confirmed by the H+/D+ isotope effect. Acceptor doping considerably enhances proton conductivity with a value of 3.3 × 10−6 S cm−1 for the bulk response of x = 0.06 at 700 °C in moistened air. The presence of a −¼ slope for both doped and undoped samples in the range 10−19≤pO2≤ 10−8 atm at 900 °C indicates n-type transport under reducing conditions following the extrinsic model attributable to acceptor centres. The conductivity is essentially independent of pO2 at 600 °C under dry oxidising conditions, consistent with oxide-ion transport; a positive power-law dependence at higher temperature indicates extrinsic behaviour and a significant electron–hole contribution. The dielectric constant at RT of nominally stoichiometric Sr3NdNb3O12 is εr∼ 37, with a moderately high quality factor of Q×f∼ 16 400 GHz at fr∼ 6.4 GHz. The temperature coefficient of resonant frequency of x = 0 is τf∼ 12 ppm °C−1, which lowers to −3 ppm °C−1 for the Ti-doped phase x = 0.06. [ABSTRACT FROM AUTHOR]

Nanocrystallization of LaF3 in a glass of composition 55SiO2-20Al2O3-15Na2O-10LaF3 (mol%) has been achieved by heat treatment above the glass transition temperature. A maximum crystal size of 14 nm has been attained, with the crystalline fraction and crystal size dependent on the time and temperature of thermal treatment. The effect of lanthanum fluoride crystallization is noticeable from the microstructural and compositional changes in the glass matrix, which have been studied using several techniques, including viscosity, dilatometry, X-ray diffraction, and quantitative Rietveld refinement, transmission electron microscopy, and differential scanning calorimetry. The crystallization mechanism is shown to occur via regions of La- and Si-phase separation in the glass, from which the fluoride crystals develop during heat treatment. The interface between the glass matrix and the crystals in the demixed ranges is enriched in network formers, mainly SiO2, creating a viscous barrier, which inhibits further crystal growth and limits the crystal size to the nanometric range. [ABSTRACT FROM AUTHOR]

In this study, transparent oxyfluoride glass-ceramics (GCs) with NaLaF4 nanocrystals (NCs) were prepared by the sol–gel method for the first time. Three different molar ratios of La(CH3COO)3/Na(CH3COO) were used to obtain the GCs, which were sintered at 450, 550 and 650 °C for 1 min. X-ray diffraction (XRD) was employed to follow the evolution of the xerogel during the heat treatments and to study crystal growth for the three temperatures. In all cases, the LaF3 crystalline phase was present, but crystallization of NaLaF4 was only promoted at 650 °C. Thermogravimetric and thermodifferential analysis (TGA-DTA) and Fourier transform infrared spectroscopy (FTIR) were used to analyze the crystallization process. High-resolution transmission electron microscopy (HRTEM) was employed to confirm NaLaF4 crystallization and determine the size distribution. The incorporation of Nd3+ ion into NaLaF4 and LaF3 nanocrystals was confirmed by site-selective emission and excitation spectra. The Nd3+ emission intensities in both phases depend not only on the NaLaF4/LaF3 ratio but also on their emission efficiencies. [ABSTRACT FROM AUTHOR]

Transparent oxyfluoride glass-ceramic films and self-supported layers with composition 80SiO2-20LaF3 doped with Er3+ have been successfully synthesized by sol-gel process for the first time. Crack-free films and self-supported layer with a maximum thickness up to 1.4 µm were obtained after heat treatment at the low temperature of 550 °C for 1 min, resulting in a LaF3 crystal fraction of 18 wt%, as confirmed by quantitative Rietveld refinement. This is the highest value reported up to now for transparent oxyfluoride glass-ceramics prepared by sol-gel. This work provides a new synthesis strategy and opens the way to a wide range of potential applications of oxyfluoride glass-ceramics. The characterization by a wide range of techniques revealed the homogeneous precipitation of LaF3 nanocrystals into the glass matrix. X-ray absorption spectroscopy and electron paramagnetic resonance confirmed that the Er3+ ions are preferentially embedded in the low phonon-energy LaF3 nanocrystals. Moreover, photoluminescence (PL) measurements confirmed the incorporation of dopants in the LaF3 nanocrystals. The effective concentration of rare-earth ions in the LaF3 nanocrystals is also estimated by X-ray absorption spectroscopy. [ABSTRACT FROM AUTHOR]