Intermediate temperature solid oxide fuel cell using (La,Sr)(Co,Fe)O3-based cathodes

Anode-supported SOFCs involving LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ)-based cathodes are fabricated utilizing GDC interlayer on 8YSZ electrolyte for intermediate temperatures. The GDC interlayer between the LSCF cathode and YSZ electrolyte is used to prevent formation of insulating phases such as SrZrO₃ or La₂Zr₂O₇. The cell performance with the GDC interlayer was ten times better than the one without GDC at operating temperature of 750 °C. However, the observed power density (370 mW/cm²) was lower than the value reported in the literature. This can be attributed to an imperfect GDC interlayer in the present study. The GDC interlayer was porous and non-uniform, so that adverse interfacial reactions could not be completely prevented. The chemical incompatibility at the interface was evaluated by SEM and EPMA, which explains the dependence of cell performance on sintering temperatures of the GDC interlayer.     

Gadolinia-doped ceria films deposited by RF reactive magnetron sputtering

Gadolinia-doped ceria (GDC) films were prepared by RF reactive magnetron sputtering from a Gd-10 at.% Ce alloy target in reactive O₂/Ar gas mixtures and annealed at 700 °C for 2 h. Material characteristics and chemical compositions of GDC films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Electrical behaviors were measured by AC impedance in the range of 500–700 °C at OCV for air condition. The microstructure of GDC films was found to be an assembly of columnar crystallites with a cubic fluorite structure. The total conductivity of 700 °C-annealed GDC (GDC-1) with the obtained composition of (Ce_0.911Gd_0.089)O_1.938 was higher than that of bulk yttria-stabilized zirconia (YSZ), but smaller than bulk GDC. The governing mechanism of conduction of sputtered-GDC electrolyte films was mainly governed by a grain boundary process, which resulted in a blocking effect and the lower conductivity of thin films than that of bulk GDC samples. Our results suggested that sputtered-GDC films with a comparable conductivity can be used as solid electrolyte layers for a solid oxide fuel cell (SOFC) system as compared to the well-known YSZ.     

Magnetoresistance and transport properties of different impurity doped La0.67Ca0.33MnO3 composite

An enhanced magnetoresistance and a two-fold effect result from impurity dopant were observed in composites of La_0.67Ca_0.33MnO₃/YSZ and La_0.67Ca_0.33MnO₃/Fe₃O₄. Where YSZ represents yttria-stabilized zirconia and the doping level of both YSZ and Fe₃O₄ is 1 mol%. Different electrical and magnetic transport properties, in particular a lower field magnetization behavior, were observed between pure La_0.67Ca_0.33MnO₃ and the impurity doped La_0.67Ca_0.33MnO₃ composites. Compared with pure La_0.67Ca_0.33MnO₃, a possible interpretation is presented by considering the influences of YSZ and Fe₃O₄ on the structure of grain boundaries and/or surfaces of La_0.67Ca_0.33MnO₃grains.     

Kinetic demixing and grain boundary conductivity of yttria-doped zirconia part I - experimental observations

This work is directed towards a comprehensive study on the role of the microstructure and local chemistry of grain boundaries on the ionic conductivity of yttria (9 mol%)-stabilized zirconia and YSZ-alumina composites. It has been performed on samples prepared from two batches of YSZ powders containing ≈1.0 or 1.6 wt% SiO₂. Electrical conductivity measurements show that the grain boundary conductivity (σ_gb) increases with the sintering temperature and the cooling rate at the end of sintering or when the amount of Si in the ceramic decreases. Alumina additions lead to a decrease in σ_gb of the samples containing 1.0 wt% SiO₂, while σ_gb passes through a maximum in the highly silicon contaminated materials. These results coupled with TEM X-ray microanalysis, which have shown important gradients of the concentration ratio Al/Si in the grains, near the second phase, and in the glassy precipitates, suggest a competitive effect between the insulating alumina particles and the strong interaction of Al₂O₃ for SiO₂, removing it from grain boundary localities. On the other hand, XPS analyses show that Si and Y segregate near the interfaces. Analysis of these results suggests a kinetic demixing process and allow us to explain the beneficial effect of a faster cooling rate at the end of sintering by the lower amount of Si rejected in grain-boundary localities. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Partial electronic conductivity and electrolytic domain of bilayer electrolyte Zr0.84Y0.16O1.92/Ce0.9Gd0.1O1.95

Partial electronic conductivity and total conductivity have been determined by Hebb-Wagner polarization method and a.c. impedance spectroscopy, respectively, on bilayer electrolyte Zr_0.84Y_0.16O_1.92(YSZ)/Ce_0.9Gd_0.1O_1.95(GDC) with thickness ratios 10^− 3/1 and 10^− 4/1 at 800°, 900° and 1000 °C, respectively. While their ionic conductivities remain close to that of GDC, the electronic conductivities are suppressed the more from that of GDC towards that of YSZ the higher the thickness ratio, as expected. Even when the GDC-side is exposed to reducing atmosphere, the electronic conductivity is also suppressed, but to a less extent. It is suggested that oxygen activity distribution is discontinuous across the YSZ/GDC interface under ion-blocking condition, refuting the “continuity hypothesis” that has been usually adopted in calculating the oxygen activity distribution across a multilayer of mixed conductor oxides. The electrolytic domain widths of the bilayer electrolyte are reported depending on temperature, thickness ratio and direction of oxygen activity gradient imposed.     

Oxygen permeation characteristics of zirconia with surface modification

Yttria-stabilized zirconia (YSZ) can be used as an oxygen-permeating membrane at elevated temperature (> 1400 °C) due to its chemical and mechanical stability. It was previously shown that the oxygen transport through YSZ membrane in reducing oxygen partial pressure (P_O2) was highly influenced by the surface-exchange kinetics that can be improved by porous surface coating layers such as YSZ, GDC (Gd-doped ceria) or YSZ–GDC mixture [H.J. Park, G.M. Choi, J. Eur. Ceram. Soc. 25 (2005) 2577]. However, the increased oxygen flux was still lower than that estimated assuming bulk-diffusion limit and rapidly decreased with time due to the sintering of coating layers and the reaction between bulk YSZ and coating layers. In this study, the oxygen fluxes through YSZ with LaCrO₃, GDC + LaCrO₃ (bilayer), LaCrO₃ + 5 wt.% GDC (mixture), or LaCr_0.7Co_0.3O₃ coatings were measured under controlled P_O2 gradient (permeate-side P_O2: ∼ 3 × 10^− 12 atm, feed-side P_O2: 2 × 10^− 10–2 × 10^− 8 atm) at 1600 °C. The oxygen flux drastically increased with these coatings. The highest increase in oxygen flux was shown with GDC + LaCrO₃ (bilayer) coating and was maintained for a long time. The presence of highly catalytic Ce ions while maintaining porous structure in the coating layer may explain the observation. The prevention of formation of resistive layer due to ceria coating may also be partly responsible for the observation.     

Different conduction behaviors of grain boundaries in SiO2-containing 8YSZ and CGO20 electrolytes

Both doped zirconia and ceria have been widely recognized as promising electrolytes in solid oxide fuel cells (SOFC). Total conductivity is an important parameter to evaluate solid electrolytes. It is well know that the contribution to the total conductivity by grain boundaries is especially pronounced for SiO₂-contaminated electrolytes. In this study, we report on the different conduction behaviors of grain boundaries (GB) found in SiO₂-containing (impure) 8YSZ (8 mol% Y₂O₃-doped ZrO₂) and CGO20 (10 mol% Gd₂O₃-doped CeO₂) ceramics. In the grain size range (∼ 0.5–10 μm) studied, the GB conductivity of impure CGO20 ceramics constantly decreases with increasing grain size, in contrast to that observed in impure 8YSZ electrolytes whose GB conductivity increases almost linearly with grain size. It is also found that the variation in GB conductivity versus grain size is different from case to case, depending on the sintering/annealing conditions used to fabricate the ceramics. Two mechanisms were proposed to explain the GB behaviors of the impure 8YSZ and CGO20 ceramics. For doped ceria, the GB phases are supposed to be inert, which do not react with or dissolve into the matrix. Increasing sintering temperature leads to not only grain growth but also change in viscosity and wetting nature of the GB phases. These two factors promote further propagation of the GB phases along the grain boundaries, leading to an increased GB coverage fraction. For doped zirconia, however, the major factor dominating the GB conduction is the further dissolution of SiO₂ into zirconia lattice as a result of increase in sintering temperature or/and time. In addition, we will also evaluate and discuss the validities of the three models that are widely used to analyze the GB conduction in solid electrolytes.     

Nanosized ceria-based ceramics: a comparative study

Ce_(1−x)Gd/Sm _x O_2−δ (x = 0.05–0.2, GDC/SDC) nanometric powder was prepared by glycine-nitrates combustion synthesis, by strictly following uniformity in the preparation route. The thermochemical properties of the obtained precursor were studied by TGA/DTA. Crystallization of the fluorite phase occurred on heating at 800 °C or higher temperature. The grain size of calcined powder was found to be about 15 nm. Densification was studied as a function of dopant content. SDC was found more sinterable than GDC. Crystal structure and microstructure were characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Electrical characterization was carried out using the impedance spectroscopy method in the frequency range of 50 Hz–13 MHz. The bulk conductivity of SDC is higher than GDC pellet for all concentration ranges. The results were analyzed by using the concept of change of the chemical bond ionicity due to the replacement of the host by dopant. Guest/host ionic size, valence mismatch ratio and their consequences are counted semiquantitatively into the configurational and thermal entropy.     

Large room temperature magnetoresistance in YSZ doped La0.67Ba0.33MnO3 composite

The temperature dependence of the resistivity for composite samples of (1−x)La_0.67Ba_0.33MnO₃+xYSZ(LBMO/YSZ) with different YSZ doping level of x has been investigated in a magnetic field range of 0-7000 Oe, where the YSZ represents yttria-stabilized zirconia (8 mol% Y₂O₃+92 mol% ZrO₂). With increasing YSZ doping level, the range of 0-10%, the metal-insulator transition temperature (T_P) decreases. However, the resistivity, specially the low temperature resistivity, increases. Results also show that the YSZ doping level has an important effect on a low field magnetoresistance (LFMR). In the magnetic field of 7000 Oe, a room temperature magnetoresistance value of 20% was observed for the composite with a YSZ doping level of 2%, which is encouraging for potential application of CMR materials at room temperature and low field.     

Zirconia stabilized by Y and Mn: A microstructural characterization

Cubic stabilized ZrO₂ with 8 mol% Y₂O₃ (YSZ) is commonly used as an electrolyte in solid oxide fuel cells (SOFC). One of the most promising cathode materials is La,Sr-manganite (LSM). During manufacture and operation of the SOFC, Mn diffuses from the LSM into YSZ. The structural changes caused by the presence of Mn in the electrolyte under various oxygen partial pressures have been examined by X-ray diffraction. Microstructures of YSZ containing Mn have been examined by transmission electron microscopy and surface changes of the electrolyte due to Mn diffusion were studied by scanning electron microscopy. The results are discussed with respect to properties and stability of the electrolyte and the SOFC. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Defect thermodynamic and transport properties of nanocrystalline Gd-doped ceria

Nanocrystalline CeO₂-doped (5, 7.5, 10, and 15 mol%) Gd₂O₃ powders, with a particle size of about 17 nm, were synthesized through the combustion of glycine/nitrate gels. Dense nanocrystalline materials were obtained by hot uniaxial sintering. Optical microscopy, scanning electron microscopy and transmission electron microscopy examinations, as well as X-ray diffraction analyses, have allowed us to characterize these polycrystals. The grain sizes, included between ∼10 and 80 nm, depend on both the sintering temperature and the amount of dopant. A comparison of the transport properties of these nanocrystalline samples to the values obtained with coarsened grained materials of same composition shows that the ionic conductivity passes through a maximum for mean grain sizes included between 300 and 500 nm. Furthermore, an enhancement of the ionic conductivity is observed when the amount of dopant increases. This was attributed to a grain-size-dependent gadolinium segregation at the periphery of the grains confirmed by X-ray photoelectron spectroscopy characterizations. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, France, 9–15 Sept. 2007.     

Correlation between microstructure and degradation in conductivity for cubic Y2O3-doped ZrO2

The application of Yttria-stabilized Zirconia (YSZ) as solid electrolyte in high-temperature solid oxide fuel cells (SOFC) is well established. However, the strong decrease of the ionic conductivity in 8.5 mol% Y₂O₃-doped ZrO₂ at high temperature has not yet been clarified completely. To contribute to the understanding of the degradation process, transmission electron microscopy (TEM) was applied to analyze the microstructure in YSZ electrolyte substrates in as-sintered and aged material. Selected area electron diffraction and conventional TEM imaging were performed to investigate the evolution of different phases and phase transitions in YSZ. Grain boundary charging and the possible formation of a glassy phase at grain boundaries after aging were investigated using transmission electron holography and high-resolution TEM. The ionic conductivity was characterized by dc-conductivity measurements and impedance spectroscopy.     

Porous electrolyte-supported tubular micro-SOFC design

Due to the poor redox cycling resistance of the second generation of μ-SOFCs, a new generation of SOFC has been recently developed using a porous electrolyte-supported structure to overcome this problem. In this research, the porous structure was successfully fabricated with slip casting using calcined YSZ (ZrO₂ + 8 mol% Y₂O₃) with or without graphite as a pore former. Calcination of YSZ powder at 1300-1500 °C prior to making the slip leads to growth of YSZ crystals and particle size which results in a decrease in surface area and powder sinterability. This was found to be an important criterion in developing the porous structure as, due to the high sinterability of non-calcined YSZ, even the addition of graphite is inadequate to generate sufficient open porosity. A dense YSZ electrolyte layer was immediately coated on the porous structure using YSZ calcined at 1300 °C with a sequential slip casting method. Sample thickness was found to be a function of both graphite content as well as YSZ calcination temperature. Physical properties of the porous YSZ supports and SEM analysis of the support and coated electrolyte are presented.     

Characterization of YSr2Fe3O8 − δ as electrode materials for SOFC

YSr₂Fe₃O_8 − δ was prepared by traditional solid state reaction method and characterized by X-ray diffraction, ac impedance, dc conductivity, dilatometry and thermogravimetric analysis for possible use in solid oxide fuel cells (SOFCs). YSr₂Fe₃O_8 − δ crystallizes with tetragonal symmetry in the space group P4/mmm and found to be stable at high temperatures under H₂ and air. Four probe dc electrical conductivity measurements show that the conductivity increases up to 745 K and then decreases with temperature; the highest conductivity σ_745K = 43.5 S cm^− 1. The n-type conductivity at low oxygen partial pressure (pO₂) changes to p-type at high pO₂. Polarization behavior was investigated measuring the ac impedance response in symmetrical cell arrangements in air with YSZ and GDC electrolytes. Cathodic area specific resistance (ASR) varies with firing temperature. The lowest area specific resistance was observed with a GDC electrolyte fired at 1000 °C. In case of YSZ, ASR increases and in case of GDC, ASR decreases in air when electrode firing temperature decreases. At 800 °C ASRs are 0.20 Ω cm² and 0.65 Ω cm² with GDC and YSZ electrolytes, respectively, in air. Fuel cell measurements with symmetrical electrodes were performed using a thin YSZ electrolyte under H₂ at anode and air at cathode, show that the power density is about 0.035 W/cm² at 900 °C.     

La0.8Sr0.2Ni0.4Fe0.6O3–Ce0.8Gd0.2O2–δ Nanocomposite as Mixed Ionic–Electronic Conducting Material for SOFC Cathode and Oxygen Permeable Membranes: Synthesis and Properties

Mixed ionic–electronic conducting nanocomposite La_0.8Sr_0.2Ni_0.4Fe_0.6O₃ (LSNF)–Ce_0.8Gd_0.2O_{2– δ} (GDC) was prepared via ultrasonic dispersion of nanocrystalline powders of perovskite and fluorite oxides in water with addition of surfactant, followed by drying and sintering up to 1300°C. Analysis of the real structure of nanocomposite (studied by XRD and TEM with EDX) and its surface composition (studied by XPS) revealed moderate redistribution of elements between phases favoring their epitaxy. Results of impedance spectroscopy, oxygen isotope exchange, O₂ TPD and H₂ TPR experiments revealed a positive effect of composite interfaces on the oxygen mobility and reactivity agreeing with the ambipolar transport behavior of MIEC composite. Preliminary testing of button-size cell with functionally graded LSNF–GDC cathode layer supported on thin YSZ layer covering Ni/YSZ cermet demonstrated high and stable performance, which is promising for its practical application.     

Effect of ion nitriding on the crystal structure of 3 mol% Y2O3-doped ZrO2 thin-films prepared by the sol-gel method

We investigated the effect of ion nitriding on the crystal structure of 3 mol% Y₂O₃-doped ZrO₂ (3YSZ) thin-films prepared by the sol-gel method. For this purpose, we used X-ray diffractometry to determine the crystalline phases, the lattice parameters, the crystal sizes, and the lattice microstrains, and glow discharge-optical emission spectroscopy to obtain the depth profiles of the elemental chemical composition. We found that nitrogen atoms substitute oxygen atoms in the 3YSZ crystal, thus leading to the formation of unsaturated-substitutional solid solutions with reduced lattice parameters and Zr_0.94Y_0.06O_1.72N_0.17 stoichiometric formula. We also found that ion nitriding does not affect the grain size, but does generate lattice microstrains due to the increase in point defects in the crystalline lattice.     

Characterization of doped La0.7A0.3Cr0.5Mn0.5O3 − δ (A = Ca,Sr, Ba) electrodes for solid oxide fuel cells

Doped lanthanum manganese chromite based perovskite, La_0.7A_0.3Cr_0.5Mn_0.5O_3 ? δ (LACM, A = Ca, Sr, Ba), on yttria-stabilized zirconia (YSZ) electrolyte is investigated as potential electrode materials for solid oxide fuel cells (SOFCs). The electrical conductivity and electrochemical activity of LACM depend on the A-site dopant. The best electrochemical activity is obtained on the La_0.7Ca_0.3Cr_0.5Mn_0.5O_3 ? δ/YSZ (LCCM/YSZ) composite electrodes. The conductivity of LCCM is 29.9 S cm^? 1 at 800 °C in air, and the electrode polarization resistance (R_E) of the LCCM/YSZ composite cathode for the O₂ reduction reaction is 0.5 Ω cm² at 900 °C. The effect of Gd-doped ceria (GDC) impregnation on the LCCM cathode polarization resistances is also studied. GDC impregnation significantly enhances the electrochemical activity of the LCCM cathode. In the case of the 6.02 mg cm^? 2 GDC-impregnated LCCM cathode, R_E is 0.4 Ω cm² at 800 °C, ~ 60 times smaller than 24.4 Ω cm² measured on a LCCM cathode without the GDC impregnation. Finally the electrochemical activities of the doped lanthanum manganese chromites for the H₂ oxidation reaction are also investigated.     

Impedance spectroscopy study of the sintering of yttria-stabilized zirconia/magnesia composites

In the present study, the sintering of (ZrO₂:8 mol%Y₂O₃)_1 ? m–(MgO)_m, YSZ–mMg composites, with m in the 0–30 mol% range, has been investigated by impedance spectroscopy (IS), dilatometry, and X-ray diffraction. Impedance diagrams were collected at 400 °C after heating the green compacts up to a selected sintering temperature, which was increased stepwise from 800 to 1400 °C. The combined experimental results revealed that the samples can be separated in two categories: below and above the solubility limit of MgO in the YSZ (m ~ 10). Moreover, important microstructural features associated with both the sintering process and solid solution formation of YSZ–mMgO samples were correlated to the electrical properties inferred by IS.     

Microstructure and electrical properties of vanadium-doped zinc oxide-based non-ohmic resistors

The microstructure and non-ohmic properties of the ternary system ZVM were investigated in accordance with Mn₃O₄ content. For all samples, the microstructure of the ternary system ZnO-V ₂O₅-Mn₃O₄ consisted of mainly ZnO grain and secondary phase Zn₃(V O₄)₂. The incorporation of Mn₃O₄ to the binary system ZnO-V ₂O₅ was found to restrict the abnormal grain growth of ZnO. The breakdown voltage in the V-I characteristics increased from 17.5 to 463.5 V/mm with the increase in Mn₃O₄ content. The incorporation of Mn₃O₄ up to 0.5 mol% improved non-ohmic properties by increasing non-ohmic coefficient, whereas the further additions decreased it. The highest non-ohmic coefficient (22.2) was obtained from Mn₃O₄ content of 0.5 mol%. It was found that the highest barrier height at grain boundary was 2.66 eV for Mn₃O₄ content of 0.5 mol%.     

Electrochemical characterization of nanostructured zirconias

Redox stability of cubic nanostructured zirconia ceramics, free of any secondary phases, has been investigated experimentally as a function of grain size. Pure 8 mol% Y₂O₃-doped ZrO₂ powders were synthesized by a spray pyrolysis process and then compacted by uniaxial pressing, followed by cold isostatic pressing. Using appropriate thermal treatments, average grain sizes ranging from 25 to 242 nm and relative densities from 71% up to 98% were obtained. An electrochemical characterization was performed with comparison on ceramics of 3.2 and 7.6 μm and 98% of theoretical density starting from commercial YSZ powder.