Preparation and characterization of 10 mol% Gd doped CeO2 (GDC) electrolyte for SOFC applications

Ceria-based materials are prospective electrolytes for low and intermediate temperature solid oxide fuel cells. In the present work, fully dense CeO₂ ceramics doped with 10 mol% gadolinium (Gd_0.1Ce_0.9O_1.95, GDC) have been prepared with a Pechini method. Characterization studies were realized with thermo-gravimetric analysis (TGA), differential thermal analysis (DTA), mass spectroscopy (MS), high temperature FT-IR (HT-FTIR) and X-ray diffraction analysis (XRD). A single-phase with a fluorite type structure was found to form at a relatively low calcination temperature of 500 °C. Dense GDC pellets having 98% of the relative density were obtained at sintering temperature of 1400 °C/6 h, which gave significantly higher total ionic conductivity of 3.4×10⁻² S cm⁻¹ at 500 °C in air. The present work showed that the Pechini method is a relatively low-temperature preparation technique to synthesize Gd_0.1Ce_0.9O_1.95 powders that provided high sinterability and good ionic conductivity.     

Re-examination of bulk and grain boundary conductivities of Ce1−xGdxO2−δ ceramics

Powders of gadolinium-doped ceria solid solutions, Ce_1−xGd_xO_2−δ (x = 0.05, 0.1, 0.2, 0.3 and 0.4), were prepared by a freeze-drying precursor route. Dense ceramic pellets with average grain sizes in the range of several microns were obtained after sintering at 1600 °C. Cobalt nitrate was added to the powders to obtain dense ceramic samples with grain sizes in the submicrometer range at 1150 °C. The ionic conduction was analysed by impedance spectroscopy in air, to de-convolute the bulk and grain boundary contributions. The bulk conductivity at low temperature clearly decreases with increasing content of Gd whereas the activation energy increases. An alternative method is proposed to analyse the extent of defect interactions on conduction. For samples without addition of Co, the specific grain boundary conductivity increases with increasing Gd content. Addition of cobalt does not alter the bulk properties but produces an important increase in the specific grain boundary conductivity, mainly in samples with lower Gd-concentration (x = 0.05 and 0.1). Segregation of Gd and its strong interaction with charge carriers may explain the blocking effects of grain boundaries.     

Microstructural and electrical behavior of Bi4V2−xCuxO11−δ (0 ≤ x ≤ 0.4)

Cation substituted bismuth vanadate possesses high oxygen ion conductivity at lower temperatures. The ionic conductivity of this material at 300 °C is 50–100 times more than any other solid electrolyte. Three phases (α, β, γ) are observed in the substituted compound; α and γ are low and high conducting phase, respectively. Samples of Bi₄V_2−xCu_xO_11−δ (x = 0–0.4) were prepared by solid-state reaction technique. Impedance spectroscopy measurements were carried out in the frequency range of 100 Hz to 100 kHz using gold sputtered cylindrical shaped pellets to obtain bulk ionic conductivities as a function of the substitution and temperature. The change of slopes observed in the Arrhenius plots is in agreement with the phase transitions for all the compositions. The highest ionic conductivity of the Cu-substituted compound was observed in Bi₄V_1.8Cu_0.2O_11−δ which is attributed to its lower activation energy. Microstructural studies indicated the stabilization of high temperature γ-phase at low temperature in those samples whose ionic conductivity observed was higher.     

Electrochemical and structural study of Ce0.8Sm0.2−xLaxO1.9 electrolyte materials for SOFC

Ce_0.8Sm_0.2−xLa_xO_1.9 powders, denoted as La_xSDC (for x=0, 0.01, 0.03, 0.05, 0.07 and 0.1), were synthesized via the mechanical milling reaction method. The La³⁺ doping content has a remarkable influence on structural and electrical properties. The phase identification and morphology were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Lattice parameters were calculated by the Rietveld method. It was observed that the lattice parameter values in Ce_0.8Sm_0.2−xLa_xO_1.9 systems obey Vegard's law. The pellets were then sintered at 1500 °C in air for 7 h. The relative densities of these pellets were over 93.7%.The electrical conductivity was studied using two-probe impedance spectroscopy and results showed that the conductivity of Ce_0.8Sm_0.2−xLa_xO_1.9 first increased and then decreased with La dopant content x. Results also showed that Ce_0.8Sm_0.17La_0.03O_1.9 had the highest electrical conductivity, σ₇₀₀ °C equal to 3.8×10⁻² Scm⁻¹ and an activation energy equal to 0.77 eV. It was therefore concluded that co-doping with the appropriate amount of La can further improve the electrical properties of ceria electrolytes.     

Preparation and microstructural characterization of (Nd1−xGdx)2(Ce1−xZrx)2O7 solid solutions

(Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ (0 ≤ x ≤ 1.0) powders with an average particle size of 100 nm were synthesized with chemical-coprecipitation and calcination method, and were characterized by X-ray diffractometry and scanning electron microscopy. The sintering behaviour of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ powders was studied by pressureless sintering at 1600–1700 °C for 10 h in air. The relative densities of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions increase with increasing the sintering temperature, and gradually decrease with increasing the content of neodymium and cerium at identical temperature levels. (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions have a single phase of defect fluorite-type structure among all the composition combinations studied. The lattice parameters of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions agree well with the Vegard's rule.     

Effect of Sm and Gd dopants on structural characteristics and ionic conductivity of ceria

Sm- and Gd-doped ceria electrolytes Ce_0.9Gd_0.1O_1.95 (GDC) and Ce_0.9Sm_0.1O_1.95 (SDC) were prepared by using the Pechini method. The microstructural and physical properties of the samples were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry/differential thermal analysis (TG/DTA) and Fourier Transform Infrared Spectroscopy (FTIR). The TG/DTA and XRD results indicated that a single-phase fluorite structure formed at a relatively low calcination temperature, 400 °C. The XRD patterns of the samples revealed that the crystallization of the SDC powders was superior than that of the GDC powders at 400 °C. The sintering behavior and ionic conductivity of the GDC and SDC pellets were also investigated. The sintering results showed that the SDC samples were found to have higher sinterability than the GDC samples at a relatively low sintering temperature, 1300 °C, a significantly lower temperature than 1650 °C, which is required for ceria solid electrolytes prepared by solid state techniques. The impedance spectroscopy results revealed that SDC has a higher ionic conductivity compared to GDC.     

Microwave-assisted synthesis of gadolinia-doped ceria powders for solid oxide fuel cells

Gadolinia doped ceria (GDC) is an attractive electrolyte material for intermediate temperature solid oxide fuel cells (IT-SOFCs) for its high ionic conductivity at low temperature (500-700 °C). A number of different methods are currently used to prepare nano-sized doped-ceria powder. Among the others, precipitation in solution remains the best method to obtain well-dispersed particles of controlled properties. In this work, nanocrystalline Ce_1−xGd_xO_2−δ (GDC) particles were produced by polyol microwave assisted method in very mild conditions (170 °C, 2 h, 1 atm). The as-synthesized powder showed good sinterability and ionic conductivity comparable to the ones of the corresponding nanometric commercial GDC.     

Synthesis and NTC properties of YCr1−xMnxO3 ceramics sintered under nitrogen atmosphere

YCr_1−xMn_xO₃ (0 ≤ x ≤ 0.8) negative temperature coefficient (NTC) compositions were synthesized by classical solid state reaction at 1200 °C, and sintered under nitrogen atmosphere at 1500 °C and 1600 °C. XRD patterns analysis has revealed that for x ≤ 0.6, the structure consists of a solid solution of an orthorhombic perovskite YCrO₃ phase with Mn substitute for Cr. For x ≥ 0.8, a second phase with a structure similar to the hexagonal YMnO₃ phase appears. SEM images and calculated open porosity have shown that the substitution of Mn for Cr results in a decrease in porosity. Whatever the sintering temperature, the electrical characterizations (between 25 and 900 °C) have shown that the increase in the manganese content involves the decrease in both resistivity and material constant B (parameter which characterizes the thermal sensitivity of material) when x ≤ 0.6. The magnitude order of the resistivity at 25 °C is of 10⁴-10⁸ Ω cm and activation energies vary from 0.28 to 0.99 eV at low and high temperatures, respectively.     

Synthesis and electrical properties of Ce0.8Sm0.2O1.9 ceramics for IT-SOFC electrolytes by urea-combustion technique

Fine Ce_0.8Sm_0.2O_1.9 (SDC) powders with a fluorite cubic phase were prepared using a urea-combustion technique. The sinterability and microstructural evolution of the resulting ceramics were investigated. The results indicate that the ceramics sintered above 1350 °C display relative densities of about 98.5% along with significant grain growth. With respect to their electrical conduction properties, the specimens sintered above 1350 °C exhibit an excellent total ionic conductivity of 0.082 S cm⁻¹ at 800 °C in air. However, when measured in an N₂ + 33.3%H₂ atmosphere, a pronounced Warburg feature appears in the impedance plots of the SDC ceramics, together with a significant increase of the total conductivity at measuring temperatures above 550 °C, due to the introduction of a mixed Ce⁴⁺/Ce³⁺ valence state and the generation of the electronic conduction in the reducing atmosphere.     

Conductivity of CGO and CSO ceramics obtained from freeze-dried precursors

Ce_0.8Gd_0.2O_1.9 (CGO) and Ce_0.8Sm_0.2O_1.9 (CSO) have been prepared as polycrystalline materials using a freeze-dried precursor. This method yields amorphous nanometric powders. Crystallization of the fluorite phase occurred on heating at 600 °C or higher temperatures. The grain size of freeze-dried powders increases to about 100 nm after calcination at 800 °C, or about 200 nm after firing at 1000 °C. Freeze-dried powders were used to prepare dense ceramic disks by sintering at 1400 °C. Some disks were sintered at 1000 °C by adding small amounts of cobalt nitrate solution to assist the densification. The electrical conductivity results obtained for these gadolinia-doped ceria and samaria-doped ceria ceramics are similar to those obtained for CGO pellets obtained from commercial nanopowders (Rhodia). Though the bulk conductivity of CSO is probably higher than that of CGO, its grain boundary conductivity is inferior, and tends to control the overall behaviour, at least at relatively low temperatures.     

Characterization of Ce0.9Gd0.1O1.95 powders synthesized by spray drying

Ce_0.9Gd_0.1O_1.95 powders were synthesized by spray drying and successive calcinations. The phase purity, BET surface area, and particle morphology of as-sprayed and calcined powders were characterized. After calcination above 300 °C, the powders were single phase and showed a BET surface area of 68 m²/g when calcined at 300 °C. The conductivity, in air, of sintered pellets was measured by electrochemical impedance spectroscopy (EIS) and it was found to be comparable with literature values. The activation energy for the total conductivity was around 0.83 eV. The powder calcined at lower temperature showed better sinterability and higher total conductivity due to an increased bulk conductivity.     

Ionic transport properties and their variations with grain size in nanostructured double doped cerias such as Ce0.8Gd0.1Pr0.1O2−δ and Ce0.8Gd0.15Pr0.05O2−δ

This work presents the ionic transport properties in some nanocrystalline double doped cerias, i.e., Ce_0.8Gd_0.1Pr_0.1O_2−δ and Ce_0.8Gd_0.15Pr_0.05O_2−δ with various average grain sizes, in the intermediate temperature region. The correlations between electrical and dielectric properties of these materials have been established and variation of conductivity with respect to temperature has been thoroughly discussed. All the materials are found to be ionic in nature and show high value of ionic conductivity at intermediate temperatures. The nanocrystalline Ce_0.8Gd_0.1Pr_0.1O_2−δ material (irrespective of grain size), shows lowest association energy, i.e., 0.03 eV, which is close to the theoretically predicted lowest value (0.02 eV) in double doped ceria. A repulsive force is expected between the free oxygen vacancies at the grain boundary regions at higher temperatures, which restricts the rise in grain boundary conductivity and results in decrease in total conductivity.     

Synthesis of nano-crystalline Ce0.9Gd0.1O1.95 electrolyte by novel sol–gel thermolysis process for IT-SOFCs

In the present work, nano-crystalline Ce_0.9Gd_0.1O_1.95 (GDC) powder has been successfully prepared by a novel sol–gel thermolysis method using a unique combination of urea and PVA. The gel precursor obtained during the process was calcined at 400 and 600 °C for 2 h. A range of analyzing techniques including XRD, TGA, BET, SEM, EDS and TEM were employed to characterize the physical and chemical properties of obtained powders. GDC gel precursors calcined at 400 and 600 °C were found to have an average crystallite size of 10 and 19 nm, respectively. From the result of XRD patterns, we found that well-crystalline cubic fluorite structure GDC was obtained by calcining the precursor gel at 400 and 600 °C. It has been also found that the sintered samples with lower temperature calcined powder showed better sinterability as well as higher ionic conductivity of 2.21 × 10⁻² S cm⁻¹ at 700 °C in air.     

Preparation, characterization and electrical conductivity studies of NdYb1−xGdxZr2O7 (0 ≤ x ≤ 1.0) ceramics

Several compositions of NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 1.0) ceramics were prepared by pressureless-sintering method at 1973 K for 10 h in air. The relative density, microstructure and electrical conductivity of NdYb_1−xGd_xZr₂O₇ ceramics were analyzed by the Archimedes method, X-ray diffraction, scanning electron microscopy and impedance plots measurements. NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 0.3) ceramics have a single phase of defect fluorite-type structure, and NdYb_1−xGd_xZr₂O₇ (0.7 ≤ x ≤ 1.0) ceramics exhibit a single phase of pyrochlore-type structure; however, the NdYb_0.5Gd_0.5Zr₂O₇ composition shows mixed phases of both defect fluorite-type and pyrochlore-type structures. The measured values of the grain conductivity obey the Arrhenius relation. The grain conductivity of each composition in NdYb_1−xGd_xZr₂O₇ ceramics gradually increases with increasing temperature from 673 to 1173 K. NdYb_1−xGd_xZr₂O₇ ceramics are oxide-ion conductor in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The highest grain conductivity value obtained in this work is 1.79 × 10⁻² S cm⁻¹ at 1173 K for NdYb_0.3Gd_0.7Zr₂O₇ composition.     

A simple and effective process for fabrication of Gd0.1Ce0.9O1.95 solid electrolyte ceramics

Ce_0.9Gd_0.1O_1.95 ceramics were prepared using a simple and effective process in this study. Without any prior calcination, the mixture of raw materials was pressed and sintered directly. The reaction of the raw materials occurred during the heating up period by passing the calcination stage in the conventional solid-state reaction method. More than 99.5% of theoretical density was obtained for Ce_0.9Gd_0.1O_1.95 sintering at 1500–1600 °C. Fine grains (<1 μm) formed in pellets sintered at 1450 °C. The homogeneity of grains increased with the sintering temperature. The grains grew to >4.5 μm in pellets sintered at 1600 °C. The reactive-sintering process is proved to be a simple and effective method in preparing Ce_0.9Gd_0.1O_1.95 ceramics for solid electrolyte application.     

Synthesis and transport properties in La2−xAxMo2O9−δ (A = Ca, Sr, Ba, K) series

The La_2−xA_xMo₂O_9−δ (A = Ca²⁺, Sr²⁺, Ba²⁺ and K⁺) series has been synthesised as nanocrystalline materials via a modification of the freeze-drying method. The resulting materials have been characterised by X-ray diffraction (XRD), thermal analysis (TG/DTA, DSC), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The high-temperature β-polymorph is stabilised for dopant content x > 0.01. The nanocrystalline powders were used to obtain dense ceramic materials with optimised microstructure and relative density >95%. The overall conductivity determined by impedance spectroscopy depends on both the ionic radius and dopant content. The conductivity decreases slightly as the dopant content increases in addition a maximum conductivity value was found for Sr²⁺ substitution, which show an ionic radii slightly higher than La³⁺ (e.g. 0.08 S cm⁻¹ for La₂Mo₂O₉ and 0.06 S cm⁻¹ for La_1.9Sr_0.1Mo₂O_9−δ at 973 K). The creation of extrinsic vacancies upon substitution results in a wider stability range under reducing conditions and prevents amorphisation, although the stability is not enhanced significantly when compared to samples with higher tungsten content. These materials present high thermal expansion coefficients in the range of (13-16) × 10⁻⁶ K⁻¹ between room temperature and 753 K and (18-20) × 10⁻⁶ K⁻¹ above 823 K. The ionic transport numbers determined by a modified emf method remain above 0.98 under an oxygen partial pressure gradient of O₂/air and decreases substantially under wet 5% H₂-Ar/air when approaching to the degradation temperature above 973 K due to an increase of the electronic contribution to the overall conductivity.     

FexCo0.5−xNi0.5-SDC anodes for low-temperature solid oxide fuel cells

Trimetal alloys, Fe_xCo_0.5−xNi_0.5 (x = 0.1, 0.2, 0.25, 0.3, 0.4), were studied as anodes for low-temperature solid oxide fuel cells (LT-SOFCs) based on GDC (Ce_0.9Gd_0.1O_1.95) electrolytes. The alloys were formed by in situ reduction of Fe_xCo_0.5−xNi_0.5O_y composites, which were synthesized using a glycine-nitrate technique. Symmetrical cells consisted of Fe_xCo_0.5−xNi_0.5-SDC electrodes and GDC electrolytes, and single cells consisted of Fe_xCo_0.5−xNi_0.5-SDC (Ce_0.8Sm_0.2O_1.9) anodes, GDC electrolytes, and SSC (Sm_0.5Sr_0.5CoO₃)-SDC cathodes were prepared using a co-pressing and co-firing process. Interfacial polarization resistances and I-V curves of these cells were measured at temperature from 450 to 600 °C. With Fe_0.25Co_0.25Ni_0.5-SDC as anodes, the cells showed the lowest interfacial resistance and highest power density. For example, at 600 °C, the resistance was about 0.11 Ω cm² and power density was about 750 mW cm⁻² when humidified (3% H₂O) hydrogen was used as fuel and stationary air as oxidant. Further, the cell performance was improved when the molar ratio of Fe:Co:Ni approached 1:1:2, i.e. Fe_0.25Co_0.25Ni_0.5. In addition, higher power density and lower interfacial resistance were obtained for cells with the Fe_0.25Co_0.25Ni_0.5-SDC anodes comparing to that with Ni-SDC anodes, which have been usually used for LT-SOFCs. The promising performance of Fe_xCo_0.5−xNi_0.5 as anodes suggests that trimetallic anodes are worth considering for SOFCs that operate at low-temperature.     

Synthesis and characterization of Gd0.1Ce0.9O1.95 nanopowder via an acetic–acrylicmethod

Nanocrystalline Ce_0.9Gd_0.1O_1.95 (GDC) powders are successfully prepared by an acetic–acrylic method using acrylic acid, cerium acetate and gadolinium acetate as the starting materials. The polymeric precursors are characterized by means of TG/DTA, XRD and FT-IR, and the resultant powders are characterized by XRF, BET, SEM and particle size distribution (PSD) analysis. It is shown that the morphology of the oxide particles is dependent on the preparation conditions such as molar ratio of acrylic acid to metallic ions (L/M) and the sort of surfactant. High purity, single phase, homogeneous, nanocrystalline GDC powders with slight aggregation are obtained using ethylene glycol as surfactant, L/M=0.5 and heat treatment above 600 °C. The low application amount and high effect of acrylic acid is attributed to the co-operation of carboxyl group and ethylenic bond. The electrical conductivity of the sintered GDC pellet is 0.053 Scm⁻¹ in air at 750 °C. The present work indicates that the acetic–acrylic method is a relatively green method without any NO_x to synthesize high performance GDC powders.     

Electrical conductivity and redox stability of La2Mo2−xWxO9 materials

A series of compounds La₂Mo_2−xW_xO₉ (x = 0-2) were synthesized using a freeze-dried precursor method at relatively low temperatures (673-823 K). These materials were characterised by thermogravimetric and differential thermal analysis (TG/DTA), differential scanning calorimetric (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM) and dilatometric measurements. Oxygen stoichiometry was evaluated by coulometric titration and thermogravimetric analysis at 873-1273 K. The ionic and electronic conductivities of these materials were analysed by impedance spectroscopy and a Hebb-Wagner ion-blocking method under moderately reducing conditions. The presence of W⁶⁺ leads to an increase of the stability range (about 10⁻¹⁶ Pa for La₂Mo_0.5W_1.5O₉ at 1073 K) and prevents oxygen loss and amorphisation. Within the stability range, the electronic conductivity increases gradually as the temperature increases and as the oxygen partial pressure reduces. This indicates that the electronic transport is mainly n-type as a result of the oxygen-content decreasing in the molybdate lattice. Further reduction of the oxygen partial pressure gave rise to the decomposition of La₂Mo_2−xW_xO₉, leading to the formation of new phases with molybdenum in lower oxidation states, which further enhances the electronic conductivity. The results of the coulometric titration and the thermogravimetric studies under a dry 5% H₂/Ar flow suggest that tungsten doped lanthanum molybdate materials can be used as electrolyte only at low temperature and under moderate reducing conditions.     

Synthesis and properties of (BaxPb1−x)(Zn1/3Nb2/3)O3 relaxor ferroelectric ceramics using a reaction-sintering process

(Ba_xPb_1−x)(Zn_1/3Nb_2/3)O₃ (BPZN; x = 0.06–0.1) relaxor ferroelectric ceramics produced using a reaction-sintering process were investigated. Without any calcination involved, the mixture of raw materials was pressed and sintered directly. BPZN ceramics of 100% perovskite phase were obtained. Highly dense BPZN ceramics with a density higher than 98.5% of theoretical density could be obtained. Maximum dielectric constant K_max 13,500 (at 75 °C), 19,600 (at 50 °C) and 14,800 (at 28 °C) at 1 kHz could be obtained in 6BPZN, 8BPZN and 10BPZN, respectively. Dielectric maximum temperature (T_max) in BPZN ceramics via reaction-sintering process is lower than BPZN ceramics prepared via B-site precursor route.