Electrochemical removal of NOx with porous cell stacks

In this study porous cell stacks were investigated for their ability to remove NO_x electrochemically. The cell stacks were made from laminated tapes of porous electrolyte Ce_0.9Gd_0.1O_1.95 and composite electrodes of La_1−xSr_xMnO₃ (x = 0.15, and 0.5) and ceria doped with Gd or Pr. The cell stacks were infiltrated with nano-particles of pure ceria, Ce_0.9Gd_0.1O_1.95 and Ce_0.8Pr_0.2O_2−δ after sintering. A gas stream containing NO were sent through the cell stack. When the cell stacks were polarised with 0.75 V per cell then it was possible to remove some of the NO_x in the temperature interval of 250-400 °C. The cell stacks infiltrated with ceria showed the highest activity, while the ones infiltrated with Ce_0.9Gd_0.1O_1.95 had the highest selectivity towards NO compared to O₂. When the cell stack was polarised with 1.5 V for each cell it was possible to remove up to 35% of NO present.     

Highly (100)-oriented Ce1 − xFexO2 − δ solid solution films prepared by laser chemical vapor deposition

Ce_1 − xFe_xO_2 − δ solid solution films were prepared on amorphous silica substrates by laser chemical vapor deposition using metal dipivaloylmethanate precursors and a semiconductor InGaAlAs (808 nm in wavelength) laser. X-ray diffraction revealed the formation of single Ce_1 − xFe_xO_2 − δ phase at x ≤ 0.15, while CeO₂ and Fe₂O₃ phases were found for higher Fe content. Highly (100)-oriented Ce_1 − xFe_xO_2 − δ (x = 0.02) films were obtained at laser power, P_L = 50-200 W and deposition temperature, T_dep = 800-1063 K. Lotgering factor (200) was calculated to be above 0.8 for films prepared at P_L = 50-150 W. X-ray photoelectron spectroscopy revealed the presence of Fe³⁺, Ce⁴⁺ and Ce³⁺ on solid solution films. Cross-sectional transmission electron microscope images disclosed a film columnar feather-like structure with a large number of nano-scale interspaces. Deposition rates were 2 or 3 orders of magnitude higher than those reported for conventional metal organic chemical vapor deposition of CeO₂.     

Electrochemical performance of La2NiO4+δ-La0.6Sr0.4Co0.2Fe0.8O3−δ composite cathodes for intermediate temperature solid oxide fuel cells

The composite cathodes La₂NiO_4+δ-La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ were prepared for intermediate temperature solid oxide fuel cells. La₂NiO_4+δ and La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ powders were synthesized successfully by glycine-nitrate process. The effect of composition on the electrochemical performance of the composite electrodes was studied by AC impedance spectroscopy and the optimal calcination temperature was determined when the electrode showed the minimum area specific resistance. The addition of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ to La₂NiO_4+δ electrode decreased the area specific resistance remarkably. The composite electrode with 30 wt% La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ calcined at 1150 °C exhibited the lowest area specific resistance of 0.125 Ω cm², about 60% of the area specific resistance of La₂NiO_4+δ electrode at 700 °C in air. The composite electrode with 30 wt% La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ can be a promising cathode material through the evaluation of electrical conductivity and thermal expansion behavior.     

Novel composite interconnecting ceramics La0.7Ca0.3CrO3−δ/Ce0.8Sm0.2O1.9 for solid oxide fuel cells

In this paper, an interconnecting ceramic for solid oxide fuel cells was developed, based on the modification from La_0.7Ca_0.3CrO_3−δ by addition of Ce_0.8Sm_0.2O_1.9. It is found that addition of small amount Ce_0.8Sm_0.2O_1.9 into La_0.7Ca_0.3CrO_3−δ dramatically increased the electrical conductivity. For the best system, La_0.7Ca_0.3CrO_3−δ + 5 wt.% Ce_0.8Sm_0.2O_1.9, the electrical conductivity reached 687.8 S cm⁻¹ at 800 °C in air. In H₂ at 800 °C, the specimen with 3 wt.% Ce_0.8Sm_0.2O_1.9 had the maximal electrical conductivity of 7.1 S cm⁻¹. With the increase of Ce_0.8Sm_0.2O_1.9 content the relative density increased, reaching 98.7% when the Ce_0.8Sm_0.2O_1.9 content was 10 wt.%. The average coefficient of thermal expansion at 30-1000 °C in air increased with Ce_0.8Sm_0.2O_1.9 content, ranging from 11.12 × 10⁻⁶ to 12.46 × 10⁻⁶ K⁻¹. The oxygen permeation measurement illustrated a negligible oxygen ionic conduction, indicating it is still an electronically conducting ceramic. Therefore, this material system will be a very promising interconnect for solid oxide fuel cells.     

Effect of B-site doping on Sm0.5Sr0.5MxCo1−xO3−δ properties for IT-SOFC cathode material (M = Fe, Mn)

The crystal structure, thermal expansion rate, electrical conductivity and electrochemical performance of Sm_0.5Sr_0.5M_xCo_1−xO_3−δ (M = Fe, Mn) have been investigated. Two crystal structures have been observed in the specimens of Sm_0.5Sr_0.5Fe_xCo_1−xO_3−δ (SSFC) at room temperature, the perovskite structure of SSFC has an orthorhombic symmetry for 0 ≤ x ≤ 0.4 and a cubic symmetry for 0.5 ≤ x ≤ 0.9. The specimens of Sm_0.5Sr_0.5Mn_xCo_1−xO_3−δ (SSMC) crystallize in an orthorhombic structure. The adjustment of thermal expansion rate to electrolyte, which is one of the main problems of SSC, can be achieved to lower TEC values with more Fe and Mn substitution. Especially, Sm_0.5Sr_0.5Mn_0.8Co_0.2O_3−δ exhibits good thermal compatibility with La_0.8Sr_0.2Ga_0.8Mg_0.2O₃. High electrical conductivities are obtained for all the specimens and they demonstrate above 100 S/cm at 800 °C in SSFC system. The polarization resistance increases with increasing Mn content, Nevertheless, the polarization resistance of SSFC increases with increasing Fe content, but when the amount of Fe reaches to 0.4, the maximum is obtained while the resistance will decrease when the amount of Fe reaches above 0.4. Sm_0.5Sr_0.5Fe_0.8Co_0.2O_3−δ electrode exhibits high catalytic activity for oxygen reduction operating at temperature from 700 to 800 °C.     

Preparation and performance of intermediate-temperature fuel cells based on Gd-doped ceria electrolytes with different compositions

In this work, the effect of two frequently used Gd_xCe_1−xO_2−x/2 electrolytes (x = 0.1 and x = 0.2) on the performance of fuel cells operated at intermediate temperature was studied. The microstructures of ceria electrolytes responsible for the performance were discussed. Electrochemical measurements of as-prepared cells showed that the cell with Gd_0.2Ce_0.8O_1.9 electrolyte had a better performance than that of Gd_0.1Ce_0.9O_1.95. It can be concluded that the increase of grain boundary conductivity of Gd_0.2Ce_0.8O_1.9 electrolyte contributes to its better cell performance.     

Synthesis and characterizations of A-site deficient perovskite Sr0.9Ti0.8−xGaxNb0.2O3

The effects of reduction and Ga-doping on the physicochemical properties of A-site deficient perovskites Sr_0.9Ti_0.8−xGa_xNb_0.2O₃ (x = 0, 0.05, 0.1, 0.15 and 0.2) are reported. With 10% Ga doping, the sample sintered in air and treated at 1400 °C in H₂ atmosphere exhibits the highest electrical conductivity. It is found that the Ga-doping lowers the sinterability but promotes the reduction of Sr_0.9Ti_0.8−xGa_xNb_0.2O₃. The XRD analysis on the reduced samples suggests that some cations are reduced during the treatment. However, without high temperature pre-reduction, the improvement of Ga-doping is limited and the overall cell performance using Sr_0.9Ti_0.8−xGa_xNb_0.2O₃ as an anode without catalysts is still relatively low.     

The chemical stability and conductivity of BaCe0.9−xYxNb0.1O3−σ proton-conductive electrolyte for SOFC

BaCe_0.8Y_0.2O_3−δ and BaCe_0.9−xY_xNb_0.1O_3−δ (x = 0.1, 0.15, 0.2, 0.25, 0.3, 0.35) were prepared by a solid-state reactions. It was found that the BaCe_0.8Y_0.2O_3−δ samples decomposed into CeO₂ and BaCO₃ after being exposed in the atmosphere (3% CO₂ + 3% H₂O + 94% N₂) at 700 °C for 10 h. However, samples containing Nb remains unchanged in the same conditions, demonstrating a better stability in the presence of CO₂ and H₂O. The conductivity of BaCe_0.9−xY_xNb_0.1O_3−δ increased with the increase of Y content (x ≤ 0.30), and the highest value was observed at x = 0.30 where a significant decrease in conductivity took place at x = 0.35. The conductivity of BaCe_0.6Y_0.3Nb_0.1O_3−δ reaches 0.01 S/cm in humid hydrogen at 700 °C, slight lower than BaCe_0.8Y_0.2O_3−δ, 0.012 S/cm in the same conditions. Fuel cell with BaCe_0.6Y_0.3Nb_0.1O_3−δ as-prepared was successfully prepared and humidified hydrogen was supplied as fuels in evaluating the fuel cell performance. The open circuit voltage, peak power density and interfacial resistance at 700 °C were 1.02 V, 345 mW/cm² and 0.27 Ω cm², respectively.     

Performance of Ni/ScSZ cermet anode modified by coating with Gd0.2Ce0.8O2 for a SOFC

A Ni/scandia-stabilized zirconia (ScSZ) cermet anode was modified by coating with nano-sized gadolinium-doped ceria (GDC, Gd_0.2Ce_0.8O₂) within the pores of the anode for a solid oxide fuel cell (SOFC). X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed in the anode characterizations. Open circuit voltages (OCVs) increased from 1.027 to 1.078 V, and the maximum power densities increased from 238 to 825 mW/cm², as the operating temperature of a SOFC with 2.0 wt.%GDC-coated Ni/ScSZ anode was increased from 700 to 850 °C in humidified hydrogen. The coating of nano-sized Gd_0.2Ce_0.8O₂ particle within the pores of the porous Ni/ScSZ anode significantly improved the performance of anode supported cell. Electrochemical impedance spectra (EIS) illustrated that the cell with Ni/ScSZ anode exhibited far greater impedances than the cell with 2.0 wt.%GDC-coated Ni/ScSZ anode. Consequently, 2.0 wt.%GDC-coated Ni/ScSZ anode could be used as a novel anode material for a SOFC due to better electrochemical performance.     

High performance Gd and Y co-doped ceria-based electrolytes for intermediate temperature solid oxide fuel cells

Co-doped ceria of Ce_1−xGd_x−yY_yO_2−0.5x, wherein x = 0.15 and 0.2, 0 ≤ y ≤ x, were prepared by glycine-nitrate method. Their structures and ionic conductivities were characterized by X-ray diffraction (XRD) and AC impedance spectroscopy (IS). All the electrolytes were found to be ceria-based solid solutions of fluorite type structures. Co-doping was found to effectively enhance the conductivity. In comparison to the singly doped ceria, the co-doped ceria showed much higher ionic conductivities at 673-973 K. At 773 K, the ionic conductivity of Ce_0.8Gd_0.05Y_0.15O_1.9 is 0.013 S cm⁻¹ which is three times as high as that of Ce_0.8Gd_0.2O_1.9. These Gd³⁺and Y³⁺ co-doped ceria are ideal electrolyte materials of intermediate temperature solid oxide fuel cells (SOFCs).     

Synthesis, structure and oxygen-storage capacity of Pr1−xZrxO2−δ and Pr1−x−yPdyZrxO2−δ

Nanocrystalline Pr_1−xZr_xO_2−δ (0 ≤ x ≤ 1) and Pr_1−x−yPd_yZr_xO_2−δ (x = 0.50, y = 0.02) solid solutions have been synthesized by a single step solution combustion method. The whole range of solid solution compositions crystallize in cubic fluorite structure. The lattice parameter ‘a’ linearly varied up to x = 1.0. Oxygen-storage capacity (OSC) and redox properties of Pr_1−xZr_xO_2−δ (0.0 ≤ x ≤ 0.8) solid solutions have been investigated by temperature-programmed reduction (TPR) and are compared with those of Ce_1−xZr_xO₂. Pr_1−xZr_xO_2−δ exhibited H₂ uptake and CO oxidation at a lower temperature than Ce_1−xZr_xO₂. Small amount of Pd ion (y = 0.02) substitution was found to bring down the temperature of oxygen release-storage significantly.     

Electrical conductivity and oxygen sensing behavior of SrFe0.2−xTi0.8CoxO3−δ (x = 0.05-0.2)

SrFe_0.2−xTi_0.8Co_xO_3−δ (x = 0.05-0.2) were prepared by solid-state reaction method. Phase characterization and lattice parameter evaluation were done by X-ray diffraction studies. Relative concentrations of iron in various oxidation states in these compositions were estimated using Mossbauer spectroscopy. Electrical conductivities of these bulk samples were measured in various ambient and temperatures using AC impedance spectroscopy. SrFe_0.15Ti_0.8Co_0.05O_3−δ and SrTi_0.8Co_0.2O_3−δ have been found to exhibit good change in electrical conductivity between 21% O₂ and 10 ppm O₂ in argon.     

Phase transitions in Sr1 + xCo0.8Fe0.2O3 − δ oxides

The phase transitions that take place in Sr_1 + xCo_0.8Fe_0.2O_3 − δ (− 0.2 ≤ x ≤ 0.1) oxides are reported here. Thermogravimetric analysis (TGA) showed that the oxides with − 0.2 ≤ x ≤ 0 were prone to undergo oxygen-vacancy disorder-order phase transitions, while others with x = 0.05, 0.1 had more stable crystal structures during oxygen-desorption processes in nitrogen. These results were further confirmed by high-temperature in-situ X-ray techniques. The changes in activation energies of three typical oxides, Sr_1 + xCo_0.8Fe_0.2O_3 − δ (x = − 0.2, 0, 0.1), used as oxygen-permeable membranes were investigated. The phase transitions in Sr_1 + xCo_0.8Fe_0.2O_3 − δ (x = − 0.2, 0) have also been detected in differential scanning calorimetry (DSC) profiles.     

Study of Ln0.6Sr0.4Co0.8Mn0.2O3-δ (Ln=La, Gd, Sm or Nd) as the cathode materials for intermediate temperature SOFC

Perovskite type oxides Ln_0.6Sr_0.4Co_0.8Mn_0.2O_3−δ (Ln=La, Gd, Sm, or Nd) have been prepared by the solid state reaction of corresponding oxides. The crystal parameters of the compositions were determined by XRD powder diffraction, which revealed that all the compositions have orthorhombic structure. The reaction test of all samples with Ce_0.8Gd_0.2O_1.9 was carried out at 1200 °C for 48 h, and no reaction product was detected by XRD. The change in mass of La_0.6Sr_0.4Co_0.8Mn_0.2O_3−δ as a function of temperature was determined by thermogravimetric analysis (TGA). The electrical conductivity of the sintered samples were measured as a function of temperature from 200 to 1000 °C. The highest conductivity of about 1400 S cm⁻¹ was found in La_0.6Sr_0.4Co_0.8Mn_0.2O_3−δ. The cathodic polarization of these oxides electrodes deposited on Ce_0.8Gd_0.2O_1.9 tablet was studied at 500-800 °C in air.     

Y0.08Sr0.92FexTi1−xO3−δ perovskite for solid oxide fuel cell anodes

A single phase perovskite Y_0.08Sr_0.92Fe_xTi_1−xO_3−δ (x = 0.05, 0.1,0 0.20, 0.25, 0.40, and 0.50) was fabricated at 1400 °C in air by a solid state reaction method and its electrical conductivity and electrochemical properties as an anode were investigated as a function of the Fe content. Doping with Y for Sr allowed the SrFe_xTi_1−xO_3−δ perovskite to be stable at 800 °C in a reducing atmosphere. At 900 °C, metallic Fe precipitated and the stability of the perovskite phase under a reducing atmosphere decreased as the Fe content increased. The conductivity of Y_0.08Sr_0.92Fe_xTi_1−xO_3−δ (x = 0.40) was greater than that of the x = 0.20 sample. The conductivity of Y_0.08Sr_0.92Fe_xTi_1−xO_3−δ was found to be 2 × 10⁻¹ Scm⁻¹ at 800 °C in H₂. Sintering the Y_0.08Sr_0.92Fe_xTi_1−xO_3−δ anode at 1200 °C was found to be optimum to obtain not only good interfacial adhesion, but also a fine grain structure. The Y_0.08Sr_0.92Fe_0.25Ti_0.75O_3−δ anode exhibited the lowest polarization resistance (0.7 and 1.8 Ωcm² at 800 and 700 °C).     

Properties and performance of La1.6Sr0.4NiO4+δ-Ce0.8Sm0.2O1.9 composite cathodes for intermediate temperature solid oxide fuel cells

La_1.6Sr_0.4NiO_4+δ-Ce_0.8Sm_0.2O_1.9 composite cathodes were prepared successfully using combustion synthesis method for intermediate temperature solid oxide fuel cells. The chemical compatibility, thermal expansion behavior, electrical conductivity and electrode performance were studied. The X-ray diffraction of La_1.6Sr_0.4NiO_4+δ-Ce_0.8Sm_0.2O_1.9 composite result proved a slight reaction between La_1.6Sr_0.4NiO_4+δ and Ce_0.8Sm_0.2O_1.9. Both the thermal expansion coefficient and the electrical conductivity of La_1.6Sr_0.4NiO_4+δ-Ce_0.8Sm_0.2O_1.9 decreased with increasing Ce_0.8Sm_0.2O_1.9 content. AC impedance spectroscopy measurements indicated that the addition of 30 wt% Ce_0.8Sm_0.2O_1.9 to La_1.6Sr_0.4NiO_4+δ exhibited the lowest polarization resistance (0.238 Ωcm²) at 800 °C in air, which was only one fourth of the La_1.6Sr_0.4NiO_4+δ electrode measured at the same temperature.     

Preparation and electrical behavior study of the ceramic interconnect La0.7Ca0.3CrO3−δ with CeO2-based electrolyte Ce0.8Sm0.2O1.9

Based on the conventional interconnect La_0.7Ca_0.3CrO_3−δ, a novel ceramic interconnect for intermediate temperature solid oxide fuel cells was developed. In the air, the electrical conductivities of La_0.7Ca_0.3CrO_3−δ + 5%Ce_0.8Sm_0.2O_1.9 at 600, 700 and 800 °C were 96.7, 146.3 and 687.8 S cm⁻¹, respectively, which increased significantly as compared with La_0.7Ca_0.3CrO_3−δ under the same conditions. Similarly, in pure hydrogen, La_0.7Ca_0.3CrO_3−δ + 3%Ce_0.8Sm_0.2O_1.9 possessed the maximal electrical conductivities which were 4.2, 5.3 and 7.1 S cm⁻¹, respectively at 600, 700 and 800 °C. The crystal structures of La_0.7Ca_0.3CrO_3−δ, La_0.7Ca_0.3CrO_3−δ + 5%Ce_0.8Sm_0.2O_1.9 and La_0.7Ca_0.3CrO_3−δ + 10%Ce_0.8Sm_0.2O_1.9 were single phase with hexagonal symmetry, cubic phase plus some doped ceria impurity and orthorhombic phase plus some doped ceria impurity, respectively. The difference between the crystal structures may account for the difference between the electrical conductivities. The electrical conductivities and sinterability of La_0.7Ca_0.3CrO_3−δ were increased by introducing Ce_0.8Sm_0.2O_1.9, whereas the other properties were not influenced.     

Synthesis and properties of Ba0.5Sr0.5(Co0.6Zr0.2)Fe0.2O3−δ perovskite cathode material for intermediate temperature solid-oxide fuel cells

A highly stable perovskite cathode material, Ba_0.5Sr_0.5(Co_0.6Zr_0.2)Fe_0.2O_3−δ (BSCZF) for intermediate temperature solid-oxide fuel cells (IT-SOFCs) was synthesized via the improved EDTA-citric acid complexing technique combined with high-temperature sintering. The product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical impedance spectra (EIS) measurements. An electrolyte-supported BSCZF/SDC/Ni-SDC fuel cell was fabricated to evaluate the performance of the material. The XRD study indicates that the sintering temperature higher than 950 °C is sufficient to the formation of clean single BSCZF perovskite phase. Due to the incorporation of Zr ions, BSCZF perovskite exhibit lower electrical conductivity with higher activation energy but higher structural stability than the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) parent oxide. The maximum electrical conductivity of BSCZF attains 16.9 S cm⁻¹ at around 540 °C. Impedance spectra showed that the ASRs of BSCZF cathode on samaria doped ceria (Ce_0.8Sm_0.2O_1.9, SDC) electrolyte are low but are still slightly larger than those of BSCF at similar conditions. The BSCZF/SDC/Ni-SDC cell exhibited a stable output with the maximum power densities of 30, 75, 139 and 241 mW cm⁻² at 550, 600, 650 and 700 °C, respectively. Due to the high electrochemical performances as well as the excellent stability, BSCZF perovskite may be an attractive cathode material for IT-SOFCs.     

Effect of Sm and Mg co-doping on the properties of ceria-based electrolyte materials for IT-SOFCs

Samples of Sm and Mg co-doped ceria electrolytes of Ce_0.8Sm_0.2−xMg_xO_2−δ (x = 0, 0.05, 0.10, 0.15 and 0.20) were sintered from powders obtained by solid-state reaction method. The structures and electrical properties were characterized by X-ray diffraction (XRD) and AC impedance spectroscopy (IS). The thermal expansion curves of samples were measured and the thermal expansion coefficients between 100 and 800 °C were calculated. Results showed that co-doping with appropriate ratio of Sm and Mg can improve the electrical performance of ceria-based electrolytes. As the substitution amount of Mg for Sm increased up to 50 mol%, the conductivity of the samples maintained almost unchanged or even slightly higher than that doped only with Sm in intermediate temperature. The results suggest that the materials cost for producing the ceria-based intermediate temperature solid oxide fuel cells (IT-SOFCs) may be significantly reduced owing to the cost difference between Mg and Sm.     

Investigation of SmBaCuCoO5+δ double-perovskite as cathode for proton-conducting solid oxide fuel cells

SmBaCuCoO_5+δ, a double-perovskite oxide, was synthesized by the modified Pechini method and developed as cathode material for proton-conducting solid oxide fuel cells. The SmBaCuCoO_5+δ powders calcined at 800 °C, show the double-perovskite structure in powder XRD pattern. SmBaCuCoO_5+δ has a more suitable thermal expansion coefficient than SmBaCo₂O_5+δ for BaCe_0.7Zr_0.1Y_0.2O_3−δ electrolyte-based solid oxide fuel cells. The single cell was tested with humidified hydrogen (∼3% H₂O) as the fuel and static air as the oxidant. The performance of the cell was characterized by DC Electronic Load and AC impedance spectroscopy. The peak power densities reached 355-86 mW cm⁻² in the range of 700-550 °C and the interfacial polarization resistance decreased with increasing operation temperature, from 3.1 Ω cm² at 550 °C to 0.22 Ω cm² at 700 °C. The high power density and low polarization demonstrate that SmBaCuCoO_5+δ is a potential candidate for proton-conducting solid oxide fuel cells.