Structural and electrical properties of Ni-YSZ cermet materials

Ceramic-metal composites (cermets) containing yttria-stabilized zirconia, YSZ, and Ni particles are commonly used as anode materials in solid oxide fuel cells. The long-term performance of fuel cells is strictly related to both the structural and electrical properties of anode materials. In order to achieve high mixed electrical conductivity and high activity of electrochemical reactions and hydrocarbon fuel reforming, it is necessary to select an appropriate chemical composition and a suitable method of preparation. Materials containing 8 mol% yttria-stabilized zirconia and Ni were prepared by means of two methods: co-precipitation and impregnation. The structure of the materials was characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and porosity studies. The thermal expansion coefficient (TEC) was determined using the dilathometric method. Electrochemical impedance spectroscopy (EIS) and the Wagner polarization method were used to determine electrical conductivity and the electron transference numbers, respectively.     

Development of anode material based on La-substituted SrTiO3 perovskites doped with manganese and/or gallium for SOFC

Materials based on La-substituted SrTiO₃ perovskites doped with manganese and/or gallium for SOFC have been studied as novel anodes for solid oxide fuel cell. La₄Sr₈Ti₁₁Mn_1−xGa_xO_38−δ (0 ≤ x ≤ 1) oxides were synthesized by solid state reaction and the influences of the manganese and/or gallium content on the structure, morphology, thermal properties and electrical conductivity of these materials has been investigated. All compounds show cubic structure with a space group Pm-3m. These compounds presented high electrical conductivity values under reducing atmosphere between 7.9 and 6.8 S cm⁻¹ at 900 °C. For the composition x ≥ 0.5, the thermal expansion coefficient in both reducing and oxidizing atmosphere are close to that of SOFC electrolytes (8YSZ, CGD). In general, the substitution of Ga by Mn causes a slight reduction in each of the following, lattice parameter, degree of oxygen loss on reduction, thermal expansion coefficient, and electrical conductivity.     

Structural and electrical properties of thin films of Pr0.8Sr0.2Fe0.8Ni0.2O3−δ

Pr_0.8Sr_0.2Fe_0.8Ni_0.2O_3−δ (PN22) films have been deposited at different temperatures on yttria-stabilized zirconia (YSZ) substrates by pulsed laser deposition (PLD) for application to thin film solid oxide fuel cell cathodes. The structure of the films was analysed by X-ray diffraction (XRD) and atomic force microscopy (AFM). A marked influence in the structural properties of the substrate temperature has been found but not of the composition. Samples deposited at temperatures below 700 °C are amorphous, with granular aspect, and with decreasing roughness with the temperature. Meanwhile, the films at 700 °C are polycrystalline and exhibit a needle-shaped surface, with the highest roughness observed. Additionally, the conducting behaviour of the films has been studied by electrochemical impedance spectroscopy (EIS) and their cathodic area specific resistance (ASR) was determined. The main part of the impedance of the testing cells is due to the electrode. The ASR values of the films of PN22 are lower than those of Pr_0.9Sr_0.1Fe_0.8Ni_0.2O_3−δ (PN12), being the lowest 0.5 Ω cm² at 850 °C for the sample PN22 deposited at room temperature.     

Synthesis and properties of Y-doped SrTiO3 as an anode material for SOFCs

Y-doped SrTiO₃ was synthesized via solid-state reaction. The effects of Y-doping on the sinterability and the electrical conductivity of Y_xSr_1−xTiO₃ were investigated. Y-doping can increase the sintering activity and the electrical conductivity of SrTiO₃ when yttrium amount is less than 0.09 in Y_xSr_1−xTiO₃. Excessive yttrium will cause the generation of an insulating phase Y₂Ti₂O₇, which impedes the densification process and decreases the electrical conductivity of Y_xSr_1−xTiO₃ material. With the increased temperature, the electrical conductivity of Y-doped SrTiO₃ increases first and then decreases gradually, showing a mixed conduction behavior of semi-conductors and metals. The optimized Y_0.09Sr_0.91TiO₃ possesses an electrical conductivity on the order of 32.5–195.8 S cm⁻¹ in the temperature range of 25–1000 °C and being 73.7 S cm⁻¹ at 800 °C in forming gas. The thermal cycling in air does not remarkably affect the electrical conductivity and the conduction behavior of Y_0.09Sr_0.91TiO₃ at high temperatures. Y_0.09Sr_0.91TiO₃ displays a relatively stable electrical conductivity at different oxygen partial pressures and excellent chemical compatibility with YSZ at temperatures lower than 1300 °C.     

Role of sintering temperature on thermal, electrical and structural properties of Y2Ti2O7 pyrochlores

Pyrochlores exhibit variety of properties which can be tailored by changing the processing conditions. In the present study, the sintering characteristics, thermal expansion coefficient, crystal structure and conductivity behavior of pyrochlores have been studied for different applications. It was observed that sintering at 1550 °C for 12 h exhibits more oxygen deficient YTiO_2.085 phase which shows two order of magnitudes higher conductivity than Y₂Ti₂O₇ phase. The conductivity enhancement in YTiO_2.085 sample is attributed to higher oxygen deficiency which may be created due to transformation of Ti⁴⁺ to Ti³⁺ at low oxygen pressure.     

Structure, thermal stability and electrical properties of Ca(V0.5Mo0.5)O3 as solid oxide fuel cell anode

The Ca(V_0.5Mo_0.5)O₃ perovskite has been prepared in order to study its potential use as anode in SOFC. The crystal structure has been refined, by neutron powder diffraction, in the orthorhombic Pbnm space group (no. 62). The electrical conductivity values were over 525 S cm⁻¹ in the studied temperature range (25-800 °C). The sample is stable under reducing working conditions (H₂/N₂ 10:90, 25-900 °C). This orthorhombic phase transforms at 500 °C in air to the tetragonal I4₁/a scheelite phase. This transition is reversible and, due to the fact that the thermal expansion coefficients of both, the reduced and oxidized phases, are very similar and match well with those of the other cell components ((10-13) × 10⁻⁶ K⁻¹) this materials are presented as excellent candidates as anodes in SOFCs.     

Structural and electrical properties of (Ba1−xSrx)(Zr0.9M0.1)O3, M = Y, La, solid solutions

The aim of the work was to study the structural and electrical properties of the (Ba_1−xSr_x)(Zr_0.9Y_0.1)O₃ and (Ba_1−xSr_x)(Zr_0.9Y_0.1)O₃ solid solutions. The powders of different strontium content (x = 0, 0.03, 0.05 and 0.1) were prepared by a thermal decomposition of organo-metallic precursors containing ethylenediaminetetraacetate acid. Some parameters describing stability and transport properties of the perovskite structure, such as tolerance factor, specific free volume and global instability index, were calculated. It was found that the introduction of strontium into both solid solutions caused the increase of specific free volume and global instability index—these structures became a little less stable but, on the other hand, better ionic conductor. All samples were cubic perovskite and the substitution of strontium for barium caused the decrease of respective lattice parameters. Electrical conductivity measurements were performed by the d.c. four-probe method in controlled gas atmospheres containing Ar, air, H₂ and/or H₂O at the temperature from 300 to 800 °C. It was found that the conductivity depended on a chemical composition of the samples and the atmosphere. In general, the electrical conductivity was higher in wet atmospheres which contained oxygen, being in accordance with the model of a proton transport in the perovskite structure which assumed the presence of the oxygen vacancy. The solid solution containing 5 mol.% of strontium showed the highest conductivity and the lowest activation energy of conductivity regardless of the atmospheres.     

Thermal and mechanical properties of LaCoO3 and La0.8Ca0.2CoO3 perovskites

In this work, the thermal and mechanical properties such as coefficient of thermal expansion, strength, Young's modulus and fracture toughness of LaCoO₃ and La_0.8Ca_0.2CoO₃ perovskites have been studied, as well as slow crack growth of La_0.8Ca_0.2CoO₃. The mechanical performance of the two cobaltites have been evaluated in terms of their ferroelastic hysteresis properties such as non-symmetry in bending of both stress and strain distributions, non-linear deformation upon applied load from the arbitrary low stresses, and ferroelastic toughening.     

Investigation of the protonic conduction in Sm doped BaCeO3

In the present work the structural and electrical properties of samarium-doped barium cerate perovskites of BaCe_1−xSm_xO_3−δ formula (with x = 0–0.2), prepared by following the solid state reaction method, are investigated. The crystal structure and microstructure of the samples is determined by employing the techniques of X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). According to the XRD analysis at 0 ≤ x ≤ 0.2 the formed continuous series of BaCe_1−xSm_xO_3−δ solid solutions have the structure of cubic perovskite with orthorhombic distortions. It was found that the relative density of the samples is ∼87% for 0.02 < x < 0.05 and ∼94% for 0.05 < x < 0.25. It was also found that the highest conductivity is observed for x = 0.15. Finally, the thermal expansion of BaCe_1−xSm_xO_3−δ (x = 0–0.2) is studied and the thermal expansion coefficients for the high temperature region are calculated.     

The structure and magnetic properties of magnesium-substituted LaFeO3 perovskite negative electrode material by citrate sol-gel

Perovskite-type composite oxide is a material as high-energy-density Ni-MH batteries. Due to the perovskite type oxide has a special crystal structure, and exhibits a rich variety of physicochemical properties, we study the affect of the Mg²⁺ doping amount, calcination temperature and calcination time for the microstructure, sample composition and magnetic properties of LaFeO₃. XRD patterns showed that all the samples are perovskite orthogonal structure and the space group is Pnma (No. 62). When the calcination temperature is in the range of 400 °C–600 °C, the samples are a single phase, no other impurities generated. When the temperature of 800 °C and 1000 °C, 2θ between 30° and 40° detected the second phase MgFe₂O₄ and miscellaneous phase peak intensity increases with the increase of calcination temperature and strengthen, and the calcination temperature had a direct effect on the grain size of the powder. When the calcination temperature is higher, the grain shape is better and the grain size is larger. The saturation magnetization of samples increases with the increase of Mg²⁺ concentration. The coercive force is decreased with the increase in Mg²⁺ concentration. The saturation magnetization and the residual magnetization of the sample are reduced when the sintering temperature from 400 °C rises to 600 °C. When the sintering temperature is 800 °C, the MgFe₂O₄ impurity phase appear. The iron oxide composition is increased, and the magnetic enhancement of the sample is enhanced. Magnetic studies show that when the calcination temperature is 600 °C, the magnetic parameters of the sample are the best.     

Dy doped SrTiO3: A promising anodic material in solid oxide fuel cells

The perovskite-type oxides, having a general formula ABO₃, are promising candidates for anode materials in solid oxide fuel cells. In particular, doped SrTiO₃ based perovskites are potential mixed ionic-electronic conductors and they are known to have excellent thermal and chemical stability along with carbon and sulfur tolerance. In this work, Dy_xSr_1-xTiO_3-δ system with x = 0.03, 0.05, 0.08 and 0.10 is studied to understand the influence of Dy content on its structural and electrical behavior. Electrochemical properties are measured, both in air and hydrogen atmosphere, and structural characterizations are performed before and after electrochemical tests and compared each other to study the stability. Results show that Dy_xSr_1-xTiO_3-δ powders with x ≤ 0.05, are single phase, while for x ≥ 0.08 a small amount of secondary phases is formed. In air, the conductivity is predominantly mixed ionic-electronic type for x ≤ 0.05, becoming ionic for x ≥ 0.08. It is observed that conductivity, for each composition, increases passing from air to hydrogen and activation energy decreases. Dy_0.05Sr_0.95TiO_3-δ shows the highest conductivity in air whereas Dy_0.08Sr_0.92TiO_3-δ in H₂ atmosphere. Degradation observed by XRD is negligible for x ≤ 0.05 but increases with higher Dy content.     

Synthesis and electrochemical properties of Na-doped Li3V2(PO4)3 cathode materials for Li-ion batteries

Na-doped Li_3−xNa_xV₂(PO₄)₃/C (x = 0.00, 0.01, 0.03, and 0.05) compounds have been prepared by using sol-gel method. The Rietveld refinement results indicate that single-phase Li_3−xNa_xV₂(PO₄)₃/C with monoclinic structure can be obtained. Among three Na-doped samples and the undoped one, Li_2.97Na_0.03V₂(PO₄)₃/C sample has the highest electronic conductivity of 6.74 × 10⁻³ S cm⁻¹. Although the initial specific capacities for all Na-doped samples have no much enhancement at the current rate of 0.2 C, both cycle performance and rate capability have been improved. At the 2.0 C rate, Li_2.97Na_0.03V₂(PO₄)₃/C presents the highest initial capacity of 118.9 mAh g⁻¹ and 12% capacity loss after 80 cycles. The partial substitution of Li with Na (x = 0.03) is favorable for electrochemical rate and cyclic ability due to the enlargement of Li₃V₂(PO₄)₃ unit cells, optimizing the particle size and morphology, as well as resulting in a higher electronic conductivity.     

Structural and electrochemical properties of LiNi1/3Co1/3Mn1/3O2: Calcination temperature dependence

The structure of the layered LiNi_1/3Co_1/3Mn_1/3O₂ has been investigated by powder X-ray diffraction and electron diffraction, and the relationship of the calcination temperature with the crystal structure, morphology and electrochemical properties has been studied. All the unit cell parameters increase monotonically with increasing the calcination temperature. Some of the [00.1] zone electron diffraction patterns for the sample calcined at higher temperature than 1000 °C show extra spots indicating the 2 × 2 ordering in the basal triangular lattice. These results indicate that the high temperature calcination leads to the formation of vacancies in the transition metal layers with the spinel-like ordering. The calcination at higher temperature lowers the specific capacities and degrades the cycle performances, while the packing density of the powder is increased by the sintering. The optimum calcination temperature is 900 °C in order to obtain the electrochemically active and dense packed oxide particles. The decrease of Li composition leads to coprecipitation of the spinel-like second phase in the range of 0.742 ≤ x ≤ 0.884 for Li_xNi_1/3Co_1/3Mn_1/3O₂, when calcined at 900 °C. The Li-deficient samples show the worse electrochemical properties similarly to the stoichiometric samples calcined at high temperature. For the Li-excess samples, no impurity phase has been detected and their cycle performances are improved.     

Synthesis and improved electrochemical performances of porous Li3V2(PO4)3/C spheres as cathode material for lithium-ion batteries

Spherical Li₃V₂(PO₄)₃/C composites are synthesized by a soft chemistry route using hydrazine hydrate as the spheroidizing medium. The electrochemical properties of the materials are investigated by galvanostatic charge-discharge tests, cyclic voltammograms and electrochemical impedance spectrum. The porous Li₃V₂(PO₄)₃/C spheres exhibit better electrochemical performances than the solid ones. The spherical porous Li₃V₂(PO₄)₃/C electrode shows a high discharge capacity of 129.1 and 125.6 mAh g⁻¹ between 3.0 and 4.3 V, and 183.8 and 160.9 mAh g⁻¹ between 3.0 and 4.8 V at 0.2 and 1 C, respectively. Even at a charge-discharge rate of 15 C, this material can still deliver a discharge capacity of 100.5 and 121.5 mAh g⁻¹ in the potential regions of 3.0-4.3 V and 3.0-4.8 V, respectively. The excellent electrochemical performance can be attributed to the porous structure, which can make the lithium ion diffusion and electron transfer more easily across the Li₃V₂(PO₄)₃/electrolyte interfaces, thus resulting in enhanced electrode reaction kinetics and improved electrochemical performance.     

Carbon-doped Sb2S3 thin films: Structural, optical and electrical properties

We report the modification of electrical properties of chemical-bath-deposited antimony sulphide (Sb₂S₃) thin films by thermal diffusion of carbon. Sb₂S₃ thin films were obtained from a chemical bath containing SbCl₃ and Na₂S₂O₃ salts at room temperature (27 °C) on glass substrates. A carbon thin film was deposited on Sb₂S₃ film by arc vacuum evaporation and the Sb₂S₃-C layer was subjected to heating at 300 °C in nitrogen atmosphere or in low vacuum for 30 min. The value of resistivity of Sb₂S₃ thin films was substantially reduced from 10⁸ Ω cm for undoped condition to 10² Ω cm for doped thin films. The doped films, Sb₂S₃:C, retained the orthogonal stibnite structure and the optical band gap energy in comparison with that of undoped Sb₂S₃ thin films. By varying the carbon content (wt%) the electrical resistivity of Sb₂S₃ can be controlled in order to make it suitable for various opto-electronic applications.     

Development of Ag-(BaO)0.11(Bi2O3)0.89 composite cathodes for intermediate temperature solid oxide fuel cells

The electrochemical performances of Ag-(BaO)_0.11(Bi₂O₃)_0.89 (BSB) composite cathodes on Ce_0.8Sm_0.2O_1.9 electrolytes have been investigated for intermediate temperature solid oxide fuel cells (ITSOFCs) using ac impedance spectroscopy from 500 to 700 °C. Results indicate that the electrochemical properties of these composites are quite sensitive to the composition and the microstructure of the cathode. The optimum BSB addition (50% by volume) to Ag resulted in about 20 times lower area specific resistance (ASR) at 650 °C. The ASR values for the Ag50-BSB and Ag cathodes were 0.32 and 6.5 Ω cm² at 650 °C, respectively. The high performances of Ag-BSB cathodes are determined by the high catalytic activity for oxygen dissociation and ionic conductivity of BSB, and by the excellent catalytic activity for oxygen reduction of silver. The maximum power density of the Ag50-BSB cathode was 224 mWcm⁻² at 650 °C, which classify this composite as a promising material for ITSOFC.     

Adjustment of the decomposition path for Na2LiAlH6 by TiF3 addition

The decomposition of Na₂LiAlH₆ is studied by in-situ synchrotron diffraction. By addition of TiF₃ and dehydrogenation-rehydrogenation cycling of the samples new decomposition paths are found. Na₃AlH₆ is formed on decomposition in the presence of TiF₃. The additive brings the system closer to equilibrium, and decomposition through Na₃AlH₆ is demonstrated for the first time. The results are in agreement with previously published computational data. For a cycled sample with 10 mol% TiF₃ Na₂LiAlH₆ decomposes fully into Na₃AlH₆ before further decomposition to NaH and Al. This shows clear changes in the kinetics of the system, and may open possibilities of tailoring the decomposition path by the use of additives.     

La-doped SrTiO3 interconnect materials for anode-supported flat-tubular solid oxide fuel cells

An interconnect layer in an anode-supported flat-tubular solid oxide fuel cell connects electrically unit cells and separates fuel from oxidant in the adjoining cells. Nano-sized La-doped SrTiO₃ for the interconnect is synthesized in this study by the Pechini method using citric acid. The materials with stoichiometric and Sr-deficient compositions are prepared and sintered in an oxidizing atmosphere. The synthesized fine powders exhibit high sinterability, leading to near-full densification. The Sr deficiency plays a crucial role in mechanical, electrical and thermal expansion properties. The interconnect is coated using the synthesized powder on a porous flat-tubular anode support by a screen printing process. The thin and dense layer is obtained after co-sintering in air, and the interconnect/anode interface remains intact upon reduction.