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.     

Improved electrical performance and sintering ability of the composite interconnect La0.7Ca0.3CrO3−δ/Ce0.8Nd0.2O1.9 for solid oxide fuel cells

Significant improvements on sintering characteristic and electrical performance of traditional interconnect La_0.7Ca_0.3CrO_3−δ were presented in this paper. For a composite interconnecting ceramic La_0.7Ca_0.3CrO_3−δ/Ce_0.8Nd_0.2O_1.9, it was found that the addition of Ce_0.8Nd_0.2O_1.9 significantly increased the electrical conductivity of La_0.7Ca_0.3CrO_3−δ both in air and in hydrogen. Among all the investigated specimens, La_0.7Ca_0.3CrO_3−δ with 5 wt% Ce_0.8Nd_0.2O_1.9 possessed the maximal electrical conductivity. In air and hydrogen, the maximal electrical conductivity at 800 °C were 55.4 S cm⁻¹ and 5.0 S cm⁻¹, respectively, which increased significantly as compared with La_0.7Ca_0.3CrO_3−δ under the same conditions. With the increase of Ce_0.8Nd_0.2O_1.9 content the relative density increased, reaching 97.1% from 93.9% of La_0.7Ca_0.3CrO_3−δ. This indicated that Ce_0.8Nd_0.2O_1.9 functioned as an effective sintering aid in enhancing the sinterability of the powders. The average coefficient of thermal expansion at 30-1000 °C in air increased with Ce_0.8Nd_0.2O_1.9 content. Most coefficients of thermal expansion of specimens are compatible with other cell components. The oxygen permeation measurement illustrated a negligible oxygen ionic conduction, indicating it is still an electronically conducting ceramic. Results indicate that this composite is suitable to be used as a high-performance interconnect for intermediate temperature solid oxide fuel cells.     

Optimization of the interface polarization of the La2NiO4-based cathode working with the Ce1–xSmxO2–δ electrolyte system

The performance of La₂NiO₄ cathode material and Ce_1–xSm_xO_2–δ (x = 0.1, 0.2, 0.3, 0.4) electrolyte system was analyzed. Ceria-based materials were prepared by the freeze-drying precursor route whereas La₂NiO₄ was prepared by the nitrate–citrate procedure. Electrolyte pellets were obtained after sintering the powders at 1600 °C for 10 h. Also dense ceria-based electrolytes samples were obtained by calcining the powders at 1150 °C after the addition of 2 mol%-Co. Interface polarization measurements were performed by impedance spectroscopy in air at open circuit voltage, using symmetrical cells prepared after the deposition of porous La₂NiO₄-electrodes on the Ce_1–xSm_xO_2–δ system. X-ray diffraction (XRD) of cathode materials after using in symmetrical cells confirmed no significant reaction between La₂NiO₄ and ceria-based electrolytes. The efficiency of the cathode material is highly dependent on the composition of the electrolyte, and low-content Sm-doped ceria samples revealed an important decrease in the performance of the system. Differences in electrochemical behaviour were attributed principally to the oxide ion transference between cathode and electrolyte, and were correlated to the conductivity of the electrolyte. In this way cobalt-doped electrolytes with a Sm-content ≤30% perform better than free-cobalt samples due to the increase in grain boundary conductivity. Finally, composites of the ceria-based materials and La₂NiO₄ to use as cathode were prepared and an important increase of the interface performance was observed compared to La₂NiO₄ pure cathode. Predictions of maximun power density were obtained by the mixed transport properties of the electrolytes and by the interface polarization results. The use of composite materials could allow to increase the performance of the cell from 170 mW cm⁻² for pure La₂NiO₄ cathode, to 370 mW cm⁻² for La₂NiO₄–Ce_0.8Sm_0.2O_2–δ cathode, both working with Ce_0.8Sm_0.2O_2–δ electrolyte 300 μm in thickness and Ni–Ce_0.8Sm_0.2O_2–δ as anode at 800 °C.     

Performance of fuel cells with proton-conducting ceria-based composite electrolyte and nickel-based electrodes

A ceria-based composite electrolyte with the composition of Ce_0.8Sm_0.2O_1.9 (SDC)–30 wt.% (2Li₂CO₃:1Na₂CO₃) is developed for intermediate temperature fuel cells (ITFCs). Two kinds of SDC powders are used to prepare the composite electrolytes, which are synthesized by oxalate coprecipitation process and glycine–nitrate process, respectively, and denoted as SDC(OCP) and SDC(GNP). Based on each composite electrolyte, two single cells with the electrolyte thickness of 0.3 and 0.5 mm are fabricated by dry-pressing technique, using nickel oxide as anode and lithiated nickel oxide as cathode, respectively. With H₂ as fuel and air as oxidant, all the four cells exhibit excellent performances at 400–600 °C, which can be attributed to the highly ionic conducting electrolyte and the compatible electrodes. The cell performance is influenced by the SDC morphology and the electrolyte thickness. More interestingly, such composite electrolytes are found to be proton conductors at intermediate temperature range for the first time since almost all water is observed at the cathode side during fuel cell operation for all cases. The unusual transport property, excellent cell performance and potential low cost make this kind of composite material a good candidate electrolyte for future cost-effective ITFCs.     

Pr-doped ceria nanoparticles as intermediate temperature ionic conductors

One of the most important challenges for further development of solid oxide fuel cells is to develop new electrolytes which operate at intermediate temperatures. In this sense, the Ce_0.8Pr_0.2O₂ material has been synthesized for the first time using polyol-mediated synthesis route, obtaining well crystallized nanoparticles with a particle size of 2-3 nm. In order to study the thermomechanical properties of the sample, dilatometric measurements have been carried out. The Ce_0.8Pr_0.2O₂ nanostructured sample exhibits a thermal expansion coefficient of 22.1 × 10⁻⁶ K⁻¹, making this material one of the first electrolytes compatible with cobaltites and double perovskites, the most promising cathodes for these devices. The electrochemical behavior of these nanoparticles has been studied and the influence of the particle size on the material properties has been also analyzed. The nanostructured electrolyte presents bulk conductivity much higher than microstructured samples, giving rise to an improvement of the electrolyte performance. Experimental results indicate that Ce_0.8Pr_0.2O₂ nanoparticles exhibit a lot of advantages for their employment as electrolyte material in terms of smaller particle size and better electrochemical performance.     

Addition effects of erbia-stabilized bismuth oxide on ceria-based carbonate composite electrolytes for intermediate temperature-−solid oxide fuel cells

Highly conductive Er_0.2Bi_0.8O_1.5 (ESB) and rare-earth doped ceria solid oxide electrolytes (SOEs) at intermediate temperature (IT) continue to suffer disadvantages in terms of thermodynamic instability and significant electronic conduction, respectively, at low oxygen partial pressure for solid oxide fuel cell (SOFC) operations. It is therefore necessary to improve the low-temperature ionic conductivity in order to enhance the electrolytic domain of these materials and thereby mitigate cell efficiency dissipation by electronic conduction. In this work, an advanced multiphase carbonate composite material based on ceria has been developed to overcome this IT-SOE challenge. This advanced electrolyte is comprise of nanostructured neodymium-doped ceria (NDC) and 38 wt% (Li–0.5Na)₂CO₃ carbonate with a small amount of ESB phase. The addition of 2 wt% ESB in ceria-based materials decreases the grain boundary resistance of the SOEs in the IT range. Further, a small amount of highly conducting ESB phase in the NDC/[(Li–0.5Na)₂CO₃] composite electrolyte increases the overall conductivity of the composite SOEs. The NDC electrolyte containing 38 wt% carbonate shows the highest conductivity of 0.104 Scm⁻¹ at 600 °C, while the conductivity is increased to 0.165 Scm⁻¹ by the addition of 2 wt% ESB. In addition, the activation energy of the multiphase composite electrolytes (0.52 eV) is lower than that of the NDC/carbonates (0.65 eV) in the IT range. This is attributed to the effect of the physical properties of the NDC sample, induced by the light ESB doping, on the ionic conductivity, and this effect is closely associated with the grain boundary property. Furthermore, the interfacial effects of the multiphase materials also contribute to the improved conductivity of this advanced composite electrolyte.     

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.     

Electrical conductivity of Ce0.8Gd0.2−xDyxO2−δ (0 ≤ x ≤ 0.2) co-doped with Gd and Dy for intermediate-temperature solid oxide fuel cells

High-quality nano-sized Ce_0.8Gd_0.2−xDy_xO_2−δ (0 ≤ x ≤ 0.2) powders are synthesized by a solution combustion process. The calcined powders are composed of a ceria-based single phase with a cubic fluorite structure and are nanocrystalline nature, i.e., 15-24 nm in crystallite size. The addition of an intermediate amount of Dy³⁺ (0.03 ≤ x ≤ 0.16) for Gd³⁺ in Ce_0.8Gd_0.2O_2−δ decreases the electrical conductivity. On the other hand, the doping of a small amount of Dy³⁺ (0.01 ≤ x ≤ 0.02) and of a large amount of Dy³⁺ (0.17 ≤ x ≤ 0.19) leads to an increase in conductivity. The Ce_0.8Gd_0.03Dy_0.17O_2−δ shows the highest electrical conductivity (0.215 S cm⁻¹) at 800 °C.     

Studies on structural,morphological, and electrical properties of Ga3+ and Cu2+ co-doped ceria ceramics as solid electrolyte for IT-SOFCs

The present study highlights the effect of Ga³⁺ and Cu²⁺ co-doping on the crystal structure, surface morphology and ionic conductivity of ceria ceramics in the system Ce_0.8Ga_0.2-xCu_xO_2-δ for potential applications as the solid electrolyte material in the intermediate temperature solid oxide fuel cells (IT-SOFCs). Ultrafine Ce_0.8Ga_0.2-xCu_xO_2-δ (for x = 0, 0.05, 0.1, 0.15, and 0.2) nanopowders were prepared via glycine nitrate auto-combustion method. Phase identification, microstructural, and ionic conductivity of all the ceria ceramics were observed by powder XRD, SEM, TEM, and impedance analyses, respectively. Rietveld structural analysis using powder XRD pattern for all the co-doped systems confirms cubic fluorite type structure having Fm-3m space group, similar to cerium oxide. All these samples were found to have density above 85% after sintering at 1300 °C for 4 h. Raman spectra revealed the oxygen vacancies in all the compositions. Thermal analysis for change in weight and thermal expansion coefficient with temperature were performed by TGA and high temperature XRD measurements, respectively. Thermal expansion coefficient of the developed electrolytes matches with the commonly used electrode materials. The composition Ce_0.8Ga_0.05Cu_0·15O_1.825 was found to demonstrate the maximum ionic conductivity with the least activation energy among all the existing co-doped ceria ceramics. These features make it a promising candidate in the IT-SOFC as the electrolyte material.     

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.     

Effect of Co doping on the properties of Sr0.8Ce0.2MnO3−δ cathode for intermediate-temperature solid-oxide fuel cells

The effect of the Co doping on the structure, electrical conductivity and electrochemical properties of Sr_0.8Ce_0.2MnO_3−δ was investigated. The Co doping decreased the sintering temperature by about 100 °C and cubic structure was synthesized for Sr_0.8Ce_0.2Mn_0.8Co_0.2O_3−δ. The electrical conductivity of Sr_0.8Ce_0.2Mn_0.8Co_0.2O_3−δ reached 102 S cm⁻¹ at 700 °C, which was sufficient to provide low ohmic losses at the cathode. In comparison with Sr_0.8Ce_0.2MnO_3−δ, the area-specific resistance of Sr_0.8Ce_0.2Mn_0.8Co_0.2O_3−δ was 0.10 Ω cm² at 750 °C, which was about 20 times lower than that of Sr_0.8Ce_0.2MnO_3−δ. While the exchange current density i₀ of Sr_0.8Ce_0.2Mn_0.8Co_0.2O_3−δ was 0.49 A cm⁻² at 800 °C, that for Sr_0.8Ce_0.2MnO_3−δ was 0.11 A cm⁻². The results show that the Sr_0.8Ce_0.2Mn_0.8Co_0.2O_3−δ cathode had high catalytic activity for oxygen reduction reaction in the temperature range of 700–800 °C.     

Thermochemical compatibility and polarization behaviors of La0.8Sr0.2Co0.8Ni0.2O3−δ as a cathode material for solid oxide fuel cell

Thermochemical compatibilities with Ce_0.8Gd_0.2O_2−δ (GDC) electrolyte and electrochemical behaviors under the condition of anodic or cathodic current treatment are investigated for La_0.8Sr_0.2Co_0.8Ni_0.2O_3−δ (LSCN) cathode of solid oxide fuel cell (SOFC). X-ray diffractometer (XRD) shows that cation exchange at 1150 °C leads to the formation of solid state solution between the cathode and electrolyte. Considering thermal expansion coefficient (TEC) and conductivity, La_1−xSr_xCo_1−yNi_yO_3−δ with the composition of La_0.8Sr_0.2Co_0.8Ni_0.2O_3−δ is indicated as a promising cathode for intermediate temperature SOFC. Electrochemical measurement reveals that the performance of LSCN cathode shows reversibility under anodic with subsequent cathodic current treatment. Further, the low frequency electrode process is strongly affected by anodic current. While the high frequency arc shows independent relationship with current polarization.     

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.     

Performance improvement of ceria-based solid oxide fuel cells with yttria-stabilized zirconia as an electronic blocking layer by pulsed laser deposition

A yttria stabilized zirconia (YSZ) layer (∼2 μm) is fabricated by pulsed laser deposition (PLD) technique on an Ce_0.8Sm_0.2O_2−δ (SDC) electrolyte film which is prepared by a co-pressing process on a NiO-SDC anode substrate. La_0.8Sr₀₂MnO_3−δ-YSZ (LSM-YSZ, 70:30 wt.%) cathode is applied onto the SDC/YSZ bilayer electrolytes to form a single cell. The open circuit voltages of the cell increase significantly compared with that of the SDC single electrolyte cell. Preparation of an SDC buffer layer (∼500 nm) on the bilayer electrolytes by PLD method is also studied for reducing the cathode polarization losses with a Sm_0.5Sr_0.5CoO_3−δ-SDC (SSC-SDC, 70:30 wt.%) cathode and preventing the interfacial chemical reaction between YSZ and SSC. The YSZ thin film blocks electrical current leakage in the SDC layer, whereas the SDC buffer layer with the SSC-SDC cathode decreases the cathode polarization losses, which results in the overall enhanced performance.     

Performance of anode-supported solid oxide fuel cell using novel ceria electrolyte

The potential of a novel co-doped ceria material Sm_0.075Nd_0.075Ce_0.85O_2−δ as an electrolyte was investigated under fuel cell operating conditions. Conventional colloidal processing was used to deposit a dense layer of Sm_0.075Nd_0.075Ce_0.85O_2−δ (thickness 10 μm) over a porous Ni-gadolinia doped ceria anode. The current-voltage performance of the cell was measured at intermediate temperatures with 90 cm³ min⁻¹ of air and wet hydrogen flowing on cathode and anode sides, respectively. At 650 °C, the maximum power density of the cell reached an exceptionally high value of 1.43 W cm⁻², with an area specific resistance of 0.105 Ω cm². Impedance measurements show that the power density decrease with decrease in temperature is mainly due to the increase in electrode resistance. The results confirm that Sm_0.075Nd_0.075Ce_0.85O_2−δ is a promising alternative electrolyte for intermediate temperature solid oxide fuel cells.     

Development of solid oxide fuel cell materials for intermediate-to-low temperature operation

The commercialization of solid oxide fuel cell (SOFC) needs the development of functional materials for intermediate-to-low temperature (400-700 °C, ILT) operation. Recently, we have successfully developed new electrolyte materials for ILT-SOFCs, including Ce_0.8Sm_0.2O_1.9 (SDC), BaCe_0.8Sm_0.2O_2.9 (BCSO) and SDC-carbonate composites. Compared with the state-of-the-art yttria-stabilized zirconia (YSZ), these materials exhibit much higher ionic conductivity at ILT range. Especially, SDC-carbonate composites show an ionic conductivity of 10⁻² to 1 Scm⁻¹ between 400 and 600 °C in fuel cell environment. Some new cathode materials were investigated for above electrolyte materials and showed promising performance. Alternative anode materials were developed to directly utilize alcohol fuels. A dry-pressing and co-firing process was employed to fabricate thin SDC and BCSO electrolyte membranes as well as thick SDC-carbonate composite electrolyte with acceptable density on anode substrate. Many efforts have also been made on fabrication of larger-size planar cells and exploitation of reliable sealing materials.     

Grain boundary conductivity of high purity neodymium-doped ceria nanosystem with and without the doping of molybdenum oxide

Solid solutions of Ce_1−xNd_xO_2−x/2 (0.05 ≤ x ≤ 0.2) and (Ce_1−xNd_x)_0.95Mo_0.05O_2−δ (0.05 ≤ x ≤ 0.2) have been synthesized by a modified sol–gel method. Both materials have very low content of SiO₂ (∼27 ppm). Their structures and ionic conductivities were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and electrochemical impedance spectroscopy (EIS). The XRD patterns indicate that these materials are single phases with a cubic fluorite structure. The powders calcined at 300 °C with a crystal size of 5.7 nm have good sinterability, and the relative density could reach above 96% after being sintered at 1450 °C. With the addition of MoO₃, the sintering temperature could be decreased to 1250 °C. Impedance spectroscopy measurement in the temperature range of 250–800 °C indicates that a sharp increase of conductivity is observed when a small amount of Nd₂O₃ is added into ceria, of which Ce_0.85Nd_0.15O_1.925 (15NDC) shows the highest conductivity. With the addition of a small amount of MoO₃, the grain boundary conductivity of 15NDC at 600 °C increases from 2.56 S m⁻¹ to 5.62 S m⁻¹.     

Preparation and evaluation of Ca3−xBixCo4O9−δ (0< x ≤ 0.5) as novel cathodes for intermediate temperature-solid oxide fuel cells

The misfit compounds Ca_3−xBi_xCo₄O_9−δ (x = 0.1–0.5) were successfully synthesized via conventional solid state reaction and evaluated as cathode materials for intermediate temperature-solid oxide fuel cells. The powders were characterized by X-ray diffraction, scanning emission microscopy, X-ray photoelectron spectroscopy, thermogravimetry analysis and oxygen-temperature programmed desorption. The monoclinic Ca_3−xBi_xCo₄O_9−δ powders exhibit good thermal stability and chemical compatibility with Ce_0.8Sm_0.2O_2−γ electrolyte. Among the investigated single-phase samples, Ca_2.9Bi_0.1Co₄O_9−δ shows the maximal conductivity of 655.9 S cm⁻¹ and higher catalytic activity compared with other Ca_3−xBi_xCo₄O_9−δ compositions. Ca_2.9Bi_0.1Co₄O_9−δ also shows the best cathodic performance and its cathode polarization resistance can be further decreased by incorporating 30 wt.% Ce_0.8Sm_0.2O_2−γ. The maximal power densities of the NiO/Ce_0.8Sm_0.2O_2−γ anode-supported button cells fabricated with the Ce_0.8Sm_0.2O_2−γ electrolyte and Ca_2.9Bi_0.1Co₄O_9−δ + 30 wt.% Ce_0.8Sm_0.2O_2−γ cathode reach 430 and 320 mW cm⁻² at 700 and 650 °C respectively.     

Chemically-induced mechanical unstability of samaria-doped ceria electrolyte for solid oxide electrolysis cells

Doped-ceria is an attractive electrolyte material for solid oxide electrolysis cells (SOECs) operated at intermediate temperatures. However, ceria is highly prone to break down under high applied voltages and low oxygen partial pressures at the fuel side. This phenomenon is analyzed for the typical Sm_0.2Ce_0.8O_1.9−δ electrolyte based on the chemically-induced stress, which is caused by the inhomogeneous distribution of oxygen non-stoichiometry throughout the thickness of electrolyte plate. The sensitivities of the maximum tensile stresses are explored under typical SOEC operating parameters such as temperature, applied voltage and oxygen partial pressure. Varying from short-circuit of solid oxide fuel cell (SOFC) mode to high voltage of SOEC conditions, the applied voltage sharpens the maximum tensile stress by seven times and raises the minimum permitted oxygen partial pressure at the cathode-electrolyte interface by a factor of 10^4.5 at most. The analysis results indicate that a ceria-based electrolyte under SOEC conditions denotes a definite trend of collapse at 700 °C even 600 °C, suggesting the inapplicability of doped-ceria electrolyte in SOEC mode.     

Electrochemical effects of cobalt doping on (La,Sr)(Ga,Mg)O3−δ electrolyte prepared by aerosol deposition

(La,Sr)(Ga,Mg)O_3−δ and (La,Sr)(Ga,Mg,Co)O_3−δ electrolytes were aerosol deposited on conventionally sintered NiO-GDC anode substrates at room temperature to minimize reactions between them. Composite cathodes comprising (La,Sr)(Co,Fe)O_3−δ and polyvinylidene fluoride were similarly deposited at room temperature. Both electrolytes and cathode maintained good adhesion. Cobalt in the electrolyte reduced open cell voltage (∼0.8 V vs. ∼1.1 V) probably due to the decrease of ionic transfer number, and increased maximum power density (∼0.8 W/cm² vs. ∼0.5 W/cm² at 650 °C) by increasing ionic conductivity.