Synthesis and sintering of Gd-doped CeO2 electrolytes with and without 1 at.% CuO dopping for solid oxide fuel cell applications

Nano-sized Ce_0.8Gd_0.2O_2−δ and Ce_0.79Gd_0.2Cu_0.01O_2−δ electrolyte powders were synthesized by the polyvinyl alcohol assisted combustion method, and then characterized by powder characteristics, sintering behaviors and electrical properties. The results demonstrate that the as-synthesized Ce_0.8Gd_0.2O_2−δ and Ce_0.79Gd_0.2Cu_0.01O_2−δ possessed similar powder characteristics, including cubic fluorite crystalline structure, porous foamy morphology and agglomerated secondary particles composed of gas cavities and primary nano crystals. Nevertheless, after ball-milling these two powders exhibited quite different sintering abilities. A significant reduction of about 400 °C in densification temperature of Ce_0.79Gd_0.2Cu_0.01O_2−δ was obtained when compared with Ce_0.8Gd_0.2O_2−δ. The Ce_0.79Gd_0.2Cu_0.01O_2−δ pellets sintered at 1000 °C and the Ce_0.8Gd_0.2O_2−δ sintered at 1400 °C exhibited relative densities of 96.33% and 95.7%, respectively. The sintering of Ce_0.79Gd_0.2Cu_0.01O_2−δ was dominated by the liquid phase process, followed by the evaporation-condensation process, Moreover, Ce_0.79Gd_0.2Cu_0.01O_2−δ shows much higher conductivity of 0.026 S cm⁻¹ than Ce_0.8Gd_0.2O_2−δ (0.0065 S cm⁻¹) at a testing temperature of 600 °C.     

Preparation and characterization of Ce0.8M0.2O2−δ (M = Y,Gd, Sm,Nd, La) solid electrolyte materials for solid oxide fuel cells

Characteristics, such as lattice parameter, theoretical densities, thermal expansion, mechanical properties, microstructure, and ionic conductivities, of Ce_0.8M_0.2O_2−δ (M = Y, Gd, Sm, Nd, La) ceramics prepared by coprecipitation were systematically investigated in this paper. The results revealed that the lattice parameter and density based on the oxygen vacancy radius generally agreed with experimental results. Ce_0.8Sm_0.2O_2−δ ceramic sintered at 1500 °C for 5 h possessed the maximum ionic conductivity, σ_800 °C = 6.54 × 10⁻² S cm⁻¹, with minimum activation energy, E_a = 0.7443 eV, among Ce_0.8M_0.2O_2−δ (M = Y, Gd, Sm, Nd, La) ceramics. The thermal expansion coefficients of Ce_0.8M_0.2O_2−δ (M = Y, Gd, Sm, Nd, La) were in the range of 15.176–15.571 ppm/°C, which indicates that the rare-earth oxide dopants have insignificant influence on the thermal expansion property. Trivalent, rare-earth oxide doped ceria ceramics revealed high fracture toughness, with the fracture toughness in the range of 6.393–7.003 MPa m^1/2. According to SEM observation, the cracks are limited to one grain diameter; therefore, the high fracture toughness of rare-earth oxide doped ceria may be due to the toughness mechanism of crack deflection at the grain boundary. Based on the results of grain size and mechanical properties, one may conclude that there is no significant dependence of fracture toughness and microhardness for Ce_0.8M_0.2O_2−δ ceramics on grain size. Correlation between the grain size of Ce_0.8M_0.2O_2−δ ceramics and the dopant species can be explained on the basis of the concept of the rate of grain growth being proportional to the boundary mobility M_b. This leads to a conclusion that the diffusion coefficient of La in Ce_0.8La_0.2O_2−δ>Nd in Ce_0.8Nd_0.2O_2−δ>Sm in Ce_0.8Sm_0.2O_2−δ>Gd inCe_0.8Gd_0.2O_2−δ>Y in Ce_0.8Y_0.2O_2−δ.     

NiMo-calcium-doped ceria catalysts for inert-substrate-supported tubular solid oxide fuel cells running on isooctane

A calcium-doped ceria (Ce_1-xCa_xO_2−δ, 0 ≤ x ≤ 0.3) has been applied as a ceramic support in NiMo-based catalysts for an internal reforming tubular solid oxide fuel cell running on isooctane. Introducing calcium into the CeO₂-based ceramic was found to improve conductivity of Ce_1-xCa_xO_2−δ. The Ce_0.9Ca_0.1O_2−δ (x = 0.10) sample exhibited an optimum conductivity of 0.045 S cm⁻¹ at 750 °C. The transport of oxygen ions in Ce_1-xCa_xO_2−δ promoted the catalytic partial oxidation of isooctane in the NiMo–Ce_1-xCa_xO_2−δ catalyst, which increased the fuel conversion as well as H₂ and CO yields. As a result, the NiMo–Ce_0.9Ca_0.1O_2−δ (x = 0.10) catalyst exhibited a high isooctane conversion of 98%, and the H₂ and CO yields achieved 74% and 83%, respectively, for reforming of isooctane and air at the O/C ratio of 1.0 at 750 °C. Furthermore, the NiMo–Ce_0.9Ca_0.1O_2−δ catalyst has been applied as an internal reforming layer for an inert-substrate-supported tubular solid oxide fuel cell running on isooctane/air. Due to its high reforming activity, the single cell presented an initial maximum power density of 355 mW cm⁻² in isooctane/air at 750 °C and displayed stable electrochemical performance during ~30 h operation. These results demonstrated the application feasibility of the NiMo–Ce_0.9Ca_0.1O_2−δ catalyst for direct internal reforming solid oxide fuel cells running on isooctane/air.     

The microstructures and property analysis of aliovalent cations (Sm,Mg, Ca,Sr, Ba) co-doped ceria-base electrolytes after an aging treatment

Ce_0.8Sm_0.15R_0.05O_2−δ (R = Sm, Mg, Ca, Sr, and Ba) specimens were successfully prepared using a solid-state reaction, and they were used in an intermediate temperate solid oxide fuel cell (IT-SOFC) electrolyte. This study focused on the effects of co-doping and an aging treatment for the conductivities and microstructures of CeO₂-based ceramics and also analyzed the variation of the conductivity in the reducing atmosphere. The study showed that the conductivities of the CeO₂-based materials have a higher conductivity at 500–800 °C by the co-doping aliovalent cations Sm and R. The conductivity increased with the increasing oxygen vacancies that were induced from charge compensation. The XRD and EDS analyses showed that the MgO and BaCeO₃ phases appeared in the Ce_0.8Sm_0.15Mg_0.05O_2−δ and Ce_0.8Sm_0.15Ba_0.05O_2−δ specimens, respectively. The conductivity of the Ce_0.8Sm_0.15Ca_0.05O_2−δ specimens was higher, approximately 0.0837 S/cm at 800 °C in the air. The thermal expansion coefficient (TEC) in all samples was ca. 11–15 × 10⁻⁶/°C at 200–800 °C. After an aging treatment at 700 °C for a holding time of 1000 h, the conductivities of all samples showed almost no change. However, the conductivity in Ce_0.8Sm_0.15Ca_0.05O_2−δ decreased from 0.0837 to 0.0581 S/cm, and the grain size increased. The conductivities of the CeO₂-based specimens were also measured under a 5%H₂–95%N₂ atmosphere, and the conductivity greatly increased in the reducing atmosphere because the Ce⁴⁺ ions reduced to Ce³⁺ ions.     

Mn-rich SmBaCo0.5Mn1.5O5+δ double perovskite cathode material for SOFCs

SmBaCo_0.5Mn_1.5O_5+δ oxide with Sm-Ba cation-ordered perovskite-type structure is synthesized and examined in relation to whole RBaCo_0.5Mn_1.5O_5+δ series (R: selected rare earth elements). Presence of Sm and 3:1 ratio of Mn to Co allows to balance physicochemical properties of the composition, with moderate thermal expansion coefficient value of 18.70(1)·10⁻⁶ K⁻¹ in 300–900 °C range, high concentration of disordered oxygen vacancies in 600–900 °C range (δ = 0.16 at 900 °C), and good transport properties with electrical conductivity reaching 33 S cm⁻¹ at 900 °C in air. Consequently, the compound enables to manufacture catalytically-active cathode, with good electrochemical performance measured for the electrolyte-supported laboratory-scale solid oxide fuel cell with Ni-Gd_1.9Ce_0.1O_2-δ|La_0.4Ce_0.6O_2-δ|La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ|SmBaCo_0.5Mn_1.5O_5+δ configuration, for which 1060 mW cm⁻² power density is observed at 900 °C. Furthermore, the tested symmetrical SmBaCo_0.5Mn_1.5O_5+δ|La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ|SmBaCo_0.5Mn_1.5O_5+δ cell delivers 377 mW cm⁻² power density at 850 °C, which is a promising result.     

Hydrogen production by catalytic partial oxidation of coke oven gas in BaCo0.7Fe0.2Nb0.1O3−δ membranes with surface modification

The perovskite-type oxygen separation membranes BaCo_0.7Fe_0.2Nb_0.1O_3−δ (BCFN) combined with Ce_0.8Re_0.2O_2−δ (Re = Sm, Gd) surface modification layers was investigated for hydrogen production by partial oxidation reforming of coke oven gas (COG). The Ce_0.8Re_0.2O_2−δ materials improve the oxygen permeation flux of the BCFN membrane by 8-31% under the COG atmosphere at 875 °C. The high oxygen permeation flux achieved using the Ce_0.8Gd_0.2O_2−δ surface-coating layer in this work is quite encouraging with a maximum value reaching 21.9 ml min⁻¹ cm⁻² at 900 °C. Characterization of the membrane surfaces by SEM and XRD after 100 h long life test show that the Ce_0.8Gd_0.2O_2−δ surface-coating layer on the permeation side can dramatically withstand corrosion of the hash strong reductive working conditions, which will be promising surface modification material in the catalytic partial oxidation reforming of COG using oxygen-permeable ceramics.     

CeO2 based materials doped with lanthanides for applications in intermediate temperature electrochemical devices

The present work aims at the investigation of the influence of different dopants’ ionic radius and concentration on the lattice parameters and the density of Ce_1−xLn_xO_2−δ (x = 0-0.20; Ln = La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb) solid solutions. Moreover, the temperature dependence of the linear expansion of Ce_0.8Ln_0.2O_2−δ ceramics is examined in the range of 300-1173 K and the respective thermal expansion coefficients are calculated. Finally, the total electrical conductivity of Ce_1−xLn_xO_2−δ (x = 0.15-0.20) and multi-component Ce_(1−x)Ln_x/2Ln’_x/2O_2−δ (x = 0.20; Ln = Sm, La, Gd, Dy and Ln’ = Dy, Nd, Er, Y) systems is studied in a wide range of temperatures in air atmosphere, as well as in a wide range of oxygen partial pressures at 1023 K and 1173 K. According to the experimental results of the present work, at the highest examined temperature of 1173 K and in air atmosphere, the maximum values of total electrical conductivity are observed in the cases of Ce_0.8Nd_0.2O_2−δ and Ce_0.8Sm_0.2O_2−δ.     

Investigation of Ni2+-doped ceria nanorods as the anode catalysts for reduced-temperature solid oxide fuel cells

Tailoring the surface chemistry of oxides has been widely used to adjust their catalytic behavior in the energy conversion and storage devices. Herein, nanorods of Ni²⁺-doped ceria (Ce_1-xNi_xO_2-δ, x = 0, 0.05, 0.1, 0.15) are synthesized via a modified hydrothermal method, and evaluated as the anode catalysts for reduced-temperature solid oxide fuel cells (SOFCs). X-Ray diffraction patterns of as-synthesized powders in air imply successful incorporation of Ni²⁺ into the fluorite lattice of ceria for x = 0.05 and 0.1, with a secondary phase of NiO observed for x = 0.15. Transmission electron microscopy (TEM) examination confirms a rod-like morphology with a diameter of 10–13 nm and a length of 55–105 nm. Exposure of these powders in H₂ at 600°C results in exsolution of some spherical Ni particles of 11 nm in diameter. Electrochemical measurements on both symmetrical anode fuel cells and functioning cathode-supported fuel cells show an order of the catalytic activity toward hydrogen oxidation - CeO_2-δ < Ce_0·95Ni_0·05O_2-δ < Ce_0·9Ni_0·1O_2-δ. The anode polarization resistances in 97% H₂ – 3% H₂O are 0.24, 0.31 and 0.37 Ω?cm² for Ce_0·9Ni_0·1O_2-δ, Ce_0·95Ni_0·05O_2-δ and CeO_2-δ at 600°C, respectively. Thin (La_0·9Sr_0.1) (Ga_0.8Mg_0.2)O_3-δ-electrolyte fuel cells with nanostructured SmBa_0.5Sr_0·5Co₂O_5+δ cathodes and Ce_0·9Ni_0·1O_2-δ anodes yield the highest power densities among the investigated series of anodes, e.g., 820 mW?cm^?2 in 97% H₂ – 3% H₂O and 598 mW?cm^?2 in 68% CH₃OH - 32% N₂. XPS analyses of reduced nanorods indicate that the highest catalytic activities of Ce_0·9Ni_0·1O_2-δ toward fuel oxidation reactions should be correlated to the presence of highly active Ni nanoparticles and increased surface active oxygen, as confirmed by substantially facilitated extraction of the lattice oxygen on the surface by H₂ in temperature-programmed reduction (TPR) measurements.     

Electrochemical evaluation of mixed ionic electronic perovskite cathode LaNi1-xCoxO3-δ for IT-SOFC synthesized by high temperature decomposition

The cobalt doped perovskite cathode material LaNi_1-xCo_xO_3-δ (x = 0.4, 0.6, 0.8) synthesized by cost effective high temperature decomposition is investigated as mixed ionic electronic conductor (MIEC) for intermediate temperature solid oxide fuel cell (IT-SOFC). LaNiO₃ is known for its high electronic conductivity and to introduce more oxygen vacancies for enhancing its ionic conductivity, Ni at B site is substituted by Co. XRD analysis showed perovskite structure for all samples with no additional phases, which was also confirmed by FTIR results. Microstructure analysis revealed well connected and porous structure for LaNi_1-xCo_xO_3-δ (x = 0.6) compared to other compositions. The elemental analysis using EDX confirmed presence of lanthanum, nickel, and cobalt within all samples. No prominent weight loss was observed during TGA analysis. The highest value of conductivity was obtained for LaNi_1-xCo_xO_3-δ (x = 0.6) due to its porous and networked structure of sub micrometric grains. The superior performance is attained for the cell based on LaNi_1-xCo_xO_3-δ (x = 0.6) cathode with maximum power density of 0.45 Wcm^?2 compared to other composition which can be attributed to its well connected and porous structure that caused enhanced electrochemical reaction at triple phase boundary (TPB). It was therefore deduced that LaNi_1-xCo_xO_3-δ (x = 0.6) is promising composition to be used as MIEC cathode for IT-SOFC.     

Dopant-induced surface activation of ceria nanorods for electro-oxidation of hydrogen and propane in solid oxide fuel cells

Ceria is an excellent oxide catalyst to break H₂ in the absence of noble metals and has shown great promise for potential applications in diverse technological fields. The catalytic activity of ceria is critically linked to surface composition and structure. Herein, selective doping with moderate lanthanide ions is reported to regulate surface oxygen vacancies and bonded adsorbates of ceria nanorods so as to finely tune their activities toward electro-oxidation of H₂ and C₃H₈ in reduced-temperature solid oxide fuel cells. Lanthanide doped ceria nanorods are hydrothermally synthesized, and electrochemically evaluated as the anode catalysts for reduced-temperature SOFCs. Measurements of anode polarization resistances and fuel cell power densities show a catalytic activity in the order of Ce_0.8Pr_0.2O_2-δ < Ce_0.8Gd_0.2O_2-δ < Ce_0.8Sm_0.2O_2-δ. Probing the surface structure with hydrogen temperature-programmed reduction, UV-Raman and XPS reveals that such catalytic activities are essentially determined by surface reducibility, availability of surface oxygen vacancies and strongly bonded hydroxyls.     

Stability considerations of semiconductor ionic fuel cells from a LNCA (LiNi0.8Co0.15Al0.05O2-δ) and Sm-doped ceria (SDC; Ce0.8Sm0.2O2-δ) composite electrolyte

Recent advances in composite materials, especially semiconductor materials incorporating ionic conductor materials, have led to significant improvements in the performance of low-temperature fuel cells. In this paper, we present a semiconductor LNCA (LiNi_0.8Co_0.15Al_0.05O_2-δ) which is often used as electrode material and ionic Sm-doped ceria (SDC; Ce_0.8Sm_0.2O_2-δ) composite electrolyte, sandwiched between LNCA thin-layer electrodes in a configuration of Ni-LNCA/SDC-LNCA/LNCA-Ni. The incorporation of the semiconductor LNCA into the SDC electrolyte with optimized weight ratios resulted in a significant power improvement, from 345 mW cm^?2 with a pure SDC electrolyte to 995 mW cm^?2 with the ionic-semiconductor SDC-LNCA one where the corresponding ionic conductivity reaches 0.255 S cm^?1 at 550 °C. Interestingly, the coexistence of ionic and electron conduction in the SDC-LNCA membrane displayed not any electronic short-circuiting but enhanced the device power outputs. This study demonstrates a new fuel cell working principle and simplifies technologies of applying functional ionic-semiconductor membranes and symmetrical electrodes to replace conventional electrolyte and electrochemical technologies for a new generation of fuel cells, different from the conventional complex anode, electrolyte, and cathode configuration.     

Preparation and characterization of Ce1−x(Gd0.5Pr0.5)xO2 electrolyte for IT-SOFCs

The Ce_1−x(Gd_0.5Pr_0.5)_xO₂ (x = 0–0.24) compositions were synthesized through the sol–gel process followed by low temperature combustion. X-ray diffraction data analysis showed that all the samples exhibit a cubic structure with single phase. The lattice parameter was calculated by rietveld refinement of XRD patterns. Dense ceramics were prepared by sintering the pellets at 1300 °C. The relative density of the samples was over 98%. The surface morphology was studied by Scanning electron microscopy (SEM). Chemical composition was analyzed by Energy dispersive spectroscopy (EDX). A.C. impedance spectroscopy measurements were carried out to study the grain, grain boundary and total ionic conductivity of co-doped ceria samples in the temperature range 150–700 °C. The Ce_0.84(Gd_0.5Pr_0.5)_0.16O₂ composition showed highest grain ionic conductivity i.e., 1.059 × 10⁻² S/cm at 500 °C which is 11.5% higher than the Ce_0.9Gd_0.1O₂ (with an activation energy 0.62 eV). At intermediate temperatures, the Ce_1−x(Gd_0.5Pr_0.5)_xO₂ materials were found to be ionic in nature.     

Crystal structure and functional properties of Nd1.6Ca0.4Ni1-yCuyO4+δ as prospective cathode materials for intermediate temperature solid oxide fuel cells

Complex oxides Nd_1.6Ca_0.4Ni_1-yCu_yO_4+δ (y = 0.0–0.4) have been prepared by a pyrolysis of glycerol-nitrate compositions. According to the X-ray diffraction analysis, the materials are single-phase up to y = 0.3 and crystallize in an orthorhombic structure (Bmab) at room temperature. High-temperature studies assert that they all undergo a phase transition from orthorhombic to tetragonal (I4/mmm) structure in a range of 300–400 °C. With Cu doping, the over-stoichiometric oxygen content δ decreases from 0.07 (y = 0.0) down to 0.00 (y = 0.3). The studies on the compact samples reveal the maximum value of total conductivity (165 S cm^?1 at 420 °C) and the minimum value of the linear coefficient of thermal expansion (11.9·10^?6 K^?1 in a range of 400–1000 °C in air) at y = 0.2. Chemical compatibility of the Nd_1.6Cа_0.4Ni_1-yCu_yO_4+δ (y = 0.0, 0.2) oxides with oxygen- and proton conducting electrolytes (Ce_0.9Gd_0.1O_1.95, Ce_0.8Sm_0.2O_1.9 and BaCe_0.5Zr_0.3Y_0.1Yb_0.1O_3-δ) up to a temperature of 1100 °C is demonstrated.     

The effect of co-dopant addition on the properties of Ln0.2Ce0.8O2−δ (Ln = Gd,Sm, La) solid-state electrolyte

The present work aims at the investigation of Ln_0.2Ce_0.8O_2−δ (where Ln = Sm, La, Gd) structural and electrical properties when in the Ln sub-lattice, Ba²⁺ and Sr²⁺ with ionic radii 1.42 and 1.26 Å, respectively, are introduced. The conductivity measurements were held both in air and in H₂ + 3%H₂O atmosphere using the 4-probe dc technique at the temperature range of 600–900 °C. Among all the samples, the highest value of electrical conductivity is obtained in the case of (Sm_0.75Sr_0.2Ba_0.05)_0.2Ce_0.8O_2−δ, both in air and in hydrogen atmosphere. In the case of H₂ + 3%H₂O the conductivity of the co-doped compounds increases in comparison with air. Moreover, the dependence of conductivity on the oxygen partial pressure, measured at the PO₂$P_{{\text{O}}_2}$ range of 0.21–10⁻²² atm, showed that the electrolytic area of alkaline-earth metals doped Ln_0.2Ce_0.8O_2−δ is considerably enhanced, as predicted by the theory. Finally, by comparing the thermal expansion coefficients of the different materials (TEC), the thermo-mechanical compatibility between the co-doped and the other cell components was also investigated.     

Investigation of praseodymium and samarium co-doped ceria as an anode catalyst for DIR-SOFC fueled by biogas

The Pr and Sm co-doped ceria (with up to 20 mol.% of dopants) compounds were examined as catalytic layers on the surface of SOFC anode directly fed by biogas to increase a lifetime and the efficiency of commercially available DIR-SOFC without the usage of an external reformer.The XRD, SEM and EDX methods were used to investigate the structural properties and the composition of fabricated materials. Furthermore, the electrical properties of SOFCs with catalytic layers deposited on the Ni-YSZ anode were examined by a current density-time and current density-voltage dependence measurements in hydrogen (24 h) and biogas (90 h). Composition of the outlet gasses was in situ analysed by the FTIR-based unit.It has been found out that Ce_0.9Sm_0.1O_2-δ and Ce_0.8Pr_0.05Sm_0.15O_2-δ catalytic layers show the highest stability over time and thus are the most attractive candidates as catalytic materials, in comparison with other investigated lanthanide-doped ceria, enhancing direct internal reforming of biogas in SOFCs.     

CuxCo3-xO4-δ (0≤x≤1) coatings for solid oxide fuel cell interconnect applications

In order to obtain the solid oxide fuel cell (SOFC) interconnect coatings with high electrical conductivity, satisfactory protectiveness, and well-fitting thermal expansion, a series of Cu_xCo_3-xO_4-δ (x = 0, 0.5, 0.8, and 1.0) coatings are prepared by supersonic spraying via subsequent sintering. The chemical composition, lattice and morphological structures, electrical properties, and thermal expansion are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), area-specific resistance (ASR), and coefficient of thermal expansion (CTE) measurements. The experimental results show that the formation of CuCo₂O₄ is a reversible and incomplete reaction at the elevated temperature, and the coexistence of CuO, Co₃O₄, and CuCo₂O₄ is inevitable in the coatings. The concentration of the chemicals mentioned above is highly related to the coatings’ Cu:Co molar ratio. The correlation between the chemical composition and the properties is comprehensively studied in this research. The Cu_xCo_3-xO_4-δ coatings exhibit good electrical conductivity when 0 ≤ x ≤ 0.8, satisfactory protectiveness when 0.5 ≤ x ≤ 1.0, and fitting CTE with remarkable robustness through the quick heating-cooling cycles when 0.8 ≤ x ≤ 1.0. In general, Cu_0.8Co_2.2O_4-δ can be an appropriate candidate to meet the advancing interconnect coating demands with high electrical conductivity, satisfactory protectiveness, and well-fitting thermal expansion properties.     

Various synthesis methods of aliovalent-doped ceria and their electrical properties for intermediate temperature solid oxide electrolytes

This article investigates the relationship between ionic conductivity and various processing methods for aliovalent-doped, ceria solid solution particles, as an intermediate temperature-solid oxide electrolyte to explain the wide range of conductivity values that have been reported. The effects of doping material and content on the ionic conductivity are investigated comprehensively in the intermediate temperature range. The chemical routes such as coprecipitation, combustion, and hydrothermal methods are chosen for the synthesis of ceria-based nanopowders, including the conventional solid-state method. The ionic conductivity for the ceria-based electrolytes depends strongly on the lattice parameter (by dopant type and content), processing parameters (particle size, sintering temperature and microstructure), and operating temperature (defect formation and transport). Among other doped-ceria systems, the Nd_0.2Ce_0.8O_2−d electrolyte synthesized by the combustion method exhibits the highest ionic conductivity at 600 °C. Further, a novel composite Nd_0.2Ce_0.8O_2−d electrolyte consisting of a combination of powders (50:50) synthesized by coprecipitation and combustion is designed. This electrolyte demonstrates an ionic conductivity two to four times higher than that of any singly processed electrolytes.     

Electrical properties of ceria Co-doped with Sm and Nd

Ceria co-doped with Sm³⁺ and Nd³⁺ powders are successfully synthesized by citric acid–nitrate low-temperature combustion process. In order to optimize the electrical properties of the series of ceria co-doped with Sm³⁺ and Nd³⁺, the effects of co-doping, doping content and sintering conditions on grain and grain boundary conductivity are investigated in detail. For the series of Ce_0.9(Sm_xNd_1−x)_0.1O_1.95 (x = 0, 0.5, 1) and Ce_1−x(Sm_0.5Nd_0.5)_xO_δ (x = 0.05, 0.10, 0.15, 0.20) sintered under the same condition, Ce_0.9(Sm_0.5Nd_0.5)_0.1O_1.95 exhibits both higher grain and grain boundary conductivity. Compared with Ce_0.9Gd_0.1O_1.95 and Ce_0.8Sm_0.2O_1.9, Ce_0.9(Sm_0.5Nd_0.5)_0.1O_1.95 sintered at 1350–1400 °C shows higher total conductivity with the value of 1.0 × 10⁻² S cm⁻¹ at 550 °C. In addition, it can be found the trends of grain and grain boundary activation energies of Ce_1−x(Sm_0.5Nd_0.5)_xO_δ are both consistent with those of Ce_1−xNd_xO_δ, but different from those of Ce_1−xSm_xO_δ, which can be explained as: the local ordering of oxygen vacancies maybe occurs more easily in Nd-doped ceria than in Sm-doped ceria; the segregation amount of Sm³⁺ is more than that of Nd³⁺ to the grain boundaries in ceria co-doped with Sm³⁺ and Nd³⁺, which is confirmed by X-ray photoelectron spectroscopy (XPS).     

Effect of scandium on the phase composition,microstructure and electrical conductivity of strontium hafnate

Effect of Sc-doping on crystal structure, morphology and conductivity of SrHfO₃ was studied for the first time. SrHf_1-xSc_xO_3-δ (x = 0–0.15) was synthesized by solid state reaction and examined using Rietveld refinement of X-ray diffraction patterns, scanning electron microscopy and four-probe DC technique. It was shown that scandium incorporates into the crystal lattice of SrHfO₃ and SrHf_1-xSc_xO_3-δ crystallizes in the orthorhombic Pbnm space group. Sc-doping enhances the grain growth and sinterability, which results in increased density of ceramics. The conductivity of SrHf_1-xSc_xO_3-δ increases with increasing Sc-content, which is consistent with the common model of oxygen vacancies formation. Increase in conductivity with increasing water vapor partial pressure indicates a significant contribution of protons to charge transport. High ionic conductivity is combined with high ion transference numbers, which makes SrHf_1-xSc_xO_3-δ promising electrolytes for use in electrolysis cells producing clean hydrogen from steam and fuel cells converting chemical energy into electricity.     

Effect of oxygen vacancies on electrical properties of Ce0.8Sm0.1Nd0.1O2−δ electrolyte: An in situ Raman spectroscopic study

Ce_0.8Sm_0.2O_2−δ, Ce_0.8Nd_0.2O_2−δ and Ce_0.8Sm_0.1Nd_0.1O_2−δ samples were prepared by a citrate sol-gel method. Effects of microstructures and oxygen vacancies of the samples on their electrical properties were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), in situ Raman spectroscopy and AC impedance spectroscopy. SEM results indicated that larger grains were formed on the Ce_0.8Nd_0.2O_2−δ and Ce_0.8Sm_0.1Nd_0.1O_2−δ electrolytes compared to that on the Ce_0.8Sm_0.2O_2−δ. In situ Raman spectra suggested that the concentration of oxygen vacancies of the Ce_0.8Sm_0.1Nd_0.1O_2−δ sample was the highest while that of Ce_0.8Sm_0.2O_2−δ was the lowest. It was found that the difference in the electrical conductivity for these electrolytes was closely related to the microstructure and oxygen vacancies of the samples. The highest electrical conductivity obtained on the Ce_0.8Sm_0.1Nd_0.1O_2−δ sample was ascribed to its larger grain size and higher concentration of oxygen vacancies.