Microstructure and ionic conductivity properties of gadolinia doped ceria (GdxCe1−xO2−x/2) electrolytes for intermediate temperature SOFCs prepared by the polyol method

Gd_0.1Ce_0.9O_1.95 and Gd_0.2Ce_0.8O_1.9 powders were prepared through the polyol process without using any protective agent. Microstructural and physical properties of the samples were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry (TG) and impedance analysis methods. The results of the thermogravimetry/differential thermal analysis (TG/DTA) and XRD indicated that a single-phase fluorite structure formed at the relatively low calcination temperature of 500 °C. The XRD patterns of the samples revealed that the crystallite size of the samples increased as calcination temperatures increased. The sintering behavior and ionic conductivity of pellets prepared from gadolinia doped ceria (GDC) powders, which were calcined at 500 °C, were also investigated. The relative densities of the pellets, which were sintered at temperatures above 1300 °C, were higher than 95%. The results of the impedance spectroscopy revealed that the GDC-20 sample that was sintered at 1400 °C exhibited an ionic conductivity of 3.25×10⁻² S cm⁻¹ at 800 °C in air. This result clearly indicates that GDC powder with adequate ionic conductivity can be prepared through the polyol process at low temperatures.     

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.     

Preparation and characterization of reticular SSC cathode films by electrostatic spray deposition

Sm_0.5Sr_0.5CoO_3−δ (SSC) cathode films were deposited on CGO (Gd_0.1Ce_0.9O_1.95) electrolyte substrates by electrostatic spray deposition to prepare SSC/CGO/SSC symmetrical cells. Deposition parameters were changed systematically to examine their effects on film microstructure and electrode performance. A set of deposition parameters including a 0.01 M precursor solution containing metal nitrates in a mixture solvent of de-ionized water (0.6 vol%), ethanol (1.5 vol%) and diethyl butyl carbitol (97.9 vol%), a flow rate of 6 ml/h for precursor solution, a deposition temperature of 350 °C and an imposed electric field of 10 kV/3 cm was identified for preparation of films with a highly porous reticular structure. The superior performance of a reticular SSC electrode was evidenced by its low interfacial resistances of 0.275 and 0.018 Ω cm² measured in 500 and 700 °C, respectively. These values were one-half to one order of magnitude smaller than that of the screen-printed or slurry-painted electrodes.     

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.     

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 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.     

A bi-layer cathode based on lanthanum based cobalt- and iron-containing perovskite and gadolinium doped ceria for thin yttria stabilized zirconia electrolyte solid oxide fuel cells

Composite cathodes based on La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) are investigated for lower operating temperature (<750 °C) applications of a solid oxide fuel cell (SOFC). To enhance a charge transfer, a bi-layer SOFC cathode is proposed, which has a LSCF–Ce_0.9Gd_0.1O_1.95 (GDC) composite layer and a pure LSCF layer. The bi-layer cathode SOFC shows a current density of 0.65 A cm⁻² at 0.8 V and 660 °C, which is higher than a LSCF–GDC composite single-layer cathode SOFC cell of 0.35 A cm⁻². The charge transfer polarizations in the bi-layer cathode SOFC are 0.14 Ω cm² and 0.35 Ω cm² at 760 °C and 660 °C, respectively, which are lower than those in the single-layer cathode cell of 0.23 Ω cm² and 0.66 Ω cm². The impedances characterized with a fitting model show that the lowered charge transfer polarization in the bi-layer cathode is a dominant factor in reducing the total polarization of SOFC.     

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.     

High temperature phase stabilities and electrochemical properties of InBaCo4−xZnxO7 cathodes for intermediate temperature solid oxide fuel cells

InBaCo_4−xZn_xO₇ oxides have been synthesized and characterized as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFC). The effect of Zn substitution for Co on the structure, phase stability, thermal expansion, and electrochemical properties of the InBaCo_4−xZn_xO₇ has been investigated. The increase in the Zn content from x = 1 to 1.5 improves the high temperature phase stability at 600 °C and 700 °C for 100 h, and chemical stability against a Gd_0.2Ce_0.8O_1.9 (GDC) electrolyte. Thermal expansion coefficient (TEC) values of the InBaCo_4−xZn_xO₇ (x = 1, 1.5, 2) specimens were determined to be 8.6 × 10⁻⁶ to 9.6 × 10⁻⁶/°C in the range of 80–900 °C, which provides good thermal expansion compatibility with the standard SOFC electrolyte materials. The InBaCo_4−xZn_xO₇ + GDC (50:50 wt.%) composite cathodes exhibit improved cathode performances compared to those obtained from the simple InBaCo_4−xZn_xO₇ cathodes due to the extended triple-phase boundary (TPB) and enhanced oxide-ion conductivity through the GDC portion in the composites.     

Chemical and thermomechanical compatibility between Ni–GDC anode and electrolytes based on ceria

The scope of the present work is to study the thermal and chemical compatibility between a Ni–Ce_0.9Gd_0.1O_1.95 cermet, with 39 vol.% Ni, and two electrolytes based on Ce_0.9Gd_0.1O_1.95 (GDC). The cermet was synthesized as a composite NiO–Ce_0.9Gd_0.1O_1.95 by a polymeric organic complex solution method and subsequently reduced to Ni–Ce_0.9Gd_0.1O_1.95 cermet. The GDC electrolytes were prepared by: (a) chemical precipitation process with nitrates as precursors and (NH₄)OH as precipitant agent and (b) from a commercial submicronic powder modified with 1.0 wt.% Bi₂O₃ for improving the sintering mechanism.The anode was fixed on the electrolyte by isostatic pressing of powders and the obtained sandwich was cosintered between 1350 and 1400 °C for 2 h to obtain dense electrolytes with high ionic conductivity along with well-developed anode/electrolyte interfaces of solid oxide fuel cells. The cosintered anode/electrolyte interfaces were characterized by using scanning electron microscopy. The study of the possible diffusion of nickel from the anode into the electrolyte was performed by EDAX analysis. The reaction products formed into cosintered materials were determined by X-ray diffraction (XRD). It is found that the anode is compatible with both electrolytes up to 1400 °C without formation of new phases at these temperatures even during prolonged treatments.     

TPR,XRD and XPS characterisation of ceria-based materials synthesized by freeze-drying precursor method

A freeze-drying precursor method was used to obtain submicrometric powders of ceria-based materials such as Ce_1−xGd_xO_2−δ (x=0, 0.01, 0.05, 0.10 and 0.20), 80%CeO₂–20%ZrO₂, 80%CeO₂–20%Al₂O₃ and (1−y)Ce_0.99Gd_0.01O_2−δ– (y)Al₂O₃ (y=0.01, 0.02, 0.05, 0.10 and 0.30) at temperatures as low as 400 °C. The phase formation and evolution with the temperature was studied by X-ray diffraction (XRD). Also, temperature programmed reduction (TPR) was performed to investigate the reducibility of the ceramic powders. It was observed that after reduction of the ceria-based materials the fluorite structure of the samples was retained. The TPR profiles showed two peaks which are associated to the surface and bulk ceria reduction processes. Likewise, after the TPR measurements the resulting powders have showed high phase stability and reproducibility. XPS results confirmed the reduction of Ce⁴⁺ to (Ce³⁺+Ce⁴⁺) ratio with alumina doping.     

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.     

Structures and electrical conductivities of Gd3+ and Fe3+ co-doped cerium oxide electrolytes sintered at low temperature for ILT-SOFCs

Gd³⁺ and Fe³⁺ co-doped cerium oxide electrolytes, Ce_0.9Gd_0.1‐xFe_xO_2-δ (x?=?0.00, 0.01, 0.03, 0.05, 0.07, 0.10), were prepared by co-precipitation for ultrafine precursor powders and sintering for densified ceramic pellets. The crystal and microscopic structures were characterized by XRD, FESEM and Raman spectroscopy and their electrical properties were studied by AC impedance spectroscopy and the measurement of single cell's outputs. In comparison with Ce_0.9Gd_0.1O_1.95, the ceramic pellets of Ce_0.9Gd_0.1‐xFe_xO_2-δ with a relative density of 95% can be obtained after sintered at 1000?°C for 5?h, showing a remarkably enhanced sintering performance with a sintering temperature reduction of 500?°C, which might be ascribed to the highly activated migration of constituent species in the cerium oxide lattice doped with Gd³⁺ and Fe³⁺ions. Moreover, the electrical conductivity of Ce_0.9Gd_0.1‐xFe_xO_2-δ can be significantly enhanced depending on the mole fraction x, with Ce_0.9Gd_0.07Fe_0.03O_1.95 exhibiting the highest electrical conductivity of 38 mS/cm at 800?°C, about 36% higher than that of Ce_0.9Gd_0.1O_1.95 electrolyte sintered at 1500?°C for 5?h. So, The Gd³⁺ and Fe³⁺ co-doped cerium oxide would be an excellent candidate electrolyte for ILT SOFCs due to its prominent sintering performance and enhanced electrical conductivity.     

Electrochemical properties of Sr1−xCexMnO3 (0.1≤x≤0.4) – GDC composite cathodes for IT-SOFCs

The electrochemical properties of Sr_1−xCe_xMnO₃ (SCM, 0.1≤x≤0.4)–Gd_0.2Ce_0.8O_2−x (GDC) composite cathodes were determined by impedance spectroscopy. The study focused on the doping effect of Ce in the composite cathodes. Single-phase perovskite was obtained for 0.1≤x≤0.3 in SCM. No reaction occurred between the Sr_0.7Ce_0.3MnO₃ electrode and the GDC electrolyte at an operating temperature of 800 °C for 100 h. In the single phase perovskite region, lattice expansion occurred due to the reduction of Mn⁴⁺ to Mn³⁺ at B-sites, and this was attributed to an increase in Ce content. Ce doping enhanced the electrode performance of SCM–GDC composite cathodes, and best electrode performance was achieved for the Sr_0.7Ce_0.3MnO₃–GDC composite cathode (0.93 Ω cm² and 0.47 Ω cm² at 750 °C and 800 °C, respectively). The improvement in electrode performance was attributed to increases in charge carriers induced by a shift of some Mn from +4 to +3 and to the formation of surface oxygen vacancies caused by Mn⁴⁺ to Mn³⁺ conversion at high temperatures.     

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.     

Chemical stability and electrochemical properties of CaMoO3−δ for SOFC anode

In an effort to develop alternative anode materials based on mixed conducting ceramics capable of offering high mixed ionic-electronic conductivity, stability to redox cycles, and limited activity for carbon formation to Ni/YSZ cermets, CaMoO₃ ceramics for application as a solid oxide fuel cell (SOFC) anode material were synthesized as a function of temperature and oxygen partial pressure (pO₂). CaMoO₃ perovskite-dominant powders were obtained by reducing the CaMoO₄ showing a structure of orthorhombic unit cells with the following lattice parameters: a = 5.45 Å, b = 5.58 Å, and c = 7.78 Å. The equilibrium total conductivity of CaMoO₃, measured by DC 4-probe method in 5% H₂/balance N₂ condition (pO₂ ≈ 10⁻²² atm) at various temperatures, decreased with increasing temperature below 400 °C, indicating metallic properties with an activation energy of 0.028 eV. Between 400 °C and 600 °C, the equilibrium total conductivity slightly increased, and finally sharply decreased at 800 °C. The Mo metal precipitation during measurement was thermodynamically proved by the predominance diagram for CaMoO₃. Finally, a fuel cell with CaMoO₃ anode exhibited poor performance with a maximum power density of only 14 mW/cm² at 900 °C, suggesting that further research is needed to enhance the ionic conductivity and thus improve the catalytic properties.     

Effect of Fe2O3 on properties and densification of Ce0.8Gd0.2O2−δ by PCAS

Dense Ce_0.8Gd_0.2O_2−δ was sintered by pulsed current activated sintering (PCAS) within 6 min from Ce_0.8Gd_0.2O_2−δ nanopowder prepared by co-precipitation method. Sintering was accomplished under the combined effects of a pulsed current and mechanical pressure. Highly dense Ce_0.8Gd_0.2O_2−δ with relative density of up to 96.3% was produced under simultaneous application of an 80-MPa pressure and the pulsed current. The effects of Fe₂O₃ additions on the sintering behavior, ionic conductivities, and mechanical properties of the Ce_0.8Gd_0.2O_2−δ were investigated.     

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.     

Inorganic-organic hybrid membranes with anhydrous proton conduction prepared from tetramethoxysilane/methyl-trimethoxysilane/trimethylphosphate and 1-ethyl-3-methylimidazolium-bis (trifluoromethanesulfonyl) imide for H2/O2 fuel cells

Anhydrous proton-conducting inorganic-organic hybrid membranes were prepared by sol-gel process with tetramethoxysilane/methyl-trimethoxysilane/trimethylphosphate and 1-ethyl-3-methylimidazolium-bis (trifluoromethanesulfonyl) imide [EMI][TFSI] ionic liquid as precursors. These hybrid membranes were studied with respect to their structural, thermal, proton conductivity, and hydrogen permeability properties. The Fourier transform infrared spectroscopy (FT-IR) and ³¹P, ¹H, and ¹³C nuclear magnetic resonance (NMR) measurements have shown good chemical stability, and complexation of PO(OCH₃)₃ with [EMI][TFSI] ionic liquid in the studied hybrid membranes. Thermal analysis including TG and DTA confirmed that the membranes were thermally stable up to 330 °C. Thermal stability of the hybrid membranes was significantly enhanced by the presence of inorganic SiO₂ framework and high stability of [TFSI] anion. The effect of [EMI][TFSI] ionic liquid addition on the microstructure of the membranes was studied by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) micrographs and no phase separation at the surfaces of the prepared membranes was observed and also homogeneous distribution of all elements was confirmed. Proton conductivity of all the prepared membranes was measured from −20 °C to 150 °C, and high conductivity of 5.4 × 10⁻³ S/cm was obtained for 40 wt% [EMI][TFSI] doped 40TMOS-50MTMOS-10PO(OCH₃)₃ (mol%) hybrid membrane, at 150 °C under anhydrous conditions. The hydrogen permeability was found to decrease from 1.61 × 10⁻¹¹ to 1.39 × 10⁻¹² mol/cm s Pa for 40 wt% [EMI][TFSI] doped hybrid membrane as the temperature increases from 20 °C to 150 °C. For 40 wt% [EMI][TFSI] doped hybrid membrane, membrane electrode assemblies were prepared and a maximum power density value of 0.22 mW/cm² at 0.47 mA/cm² as well as a current density of 0.76 mA/cm² were obtained at 150 °C under non-humidified conditions when utilized in a H₂/O₂ fuel cell.