Spray pyrolytic synthesis of samarium doped ceria (Ce0.8Sm0.2O1.9) films for solid oxide fuel cell applications

Uniform, adherent, single phase samarium doped ceria films have been successfully deposited by spray pyrolysis technique for their application in solid oxide fuel cell. These films have been deposited at different substrate temperatures on glass substrate and subsequently heat treated in tube furnace. Effect of substrate temperature and annealing temperature on phase formation was studied with thermo-gravimetric analysis and differential temperature analysis, X-ray diffraction, scanning electron microscope, and energy dispersive X-ray analysis techniques. These studies showed the formation of single phase Ce_0.8Sm_0.2O_1.9 films, at substrate temperature 400 °C and annealing temperature 550 °C. Electrical resistivity of the films, at room temperature was of the order of 10⁷ Ω cm while at 400 °C it is found to be of the order of 10¹ Ω cm. This reveals the use of these films for making low temperature solid oxide fuel cells.     

Effects of atomization conditions on morphology and SOFC anode performance of spray pyrolyzed NiO–Sm0.2Ce0.8O1.9 composite particles

NiO–Sm_0.2Ce_0.8O_1.9 (NiO–SDC) composite particles were synthesized by spray pyrolysis (SP). SP resulted in composite particles of NiO enveloped with SDC and these capsule-type composite particles would reduce aggregation of Ni during the reduction from NiO to Ni metals. SOFC anode microstructures and morphologies of NiO–SDC composite precursor particles much affects on SOFC power densities or anode polarization. Therefore, we focused on atomizing conditions of SP process. Relationship between ultrasonic atomization conditions and morphologies of NiO–SDC composites were investigated by controlling temperatures of atomization vessels. The atomizing temperature changed concentration of mists in the vessel, and mean particle size and particle size distribution were increased with an increase in temperature of the atomization vessels. Some extremely large particles were observed by synthesizing at higher atomization temperatures. Large particles contained voids in the particles. The voids in the composite particles would play a role of pore-formers. SOFC measurement showed the synthesis at the atomizing temperature of 30 °C resulted in the high-performance anode. The atomizing process of SP much affected morphology of anode precursor particles, and the atomizing conditions were important to improve anode performance.     

Chemical and redox stabilities of a solid oxide fuel cell with BaCe0.8Y0.2O3−α functioning as an electrolyte and as an anode

The operation of a solid oxide fuel cell (SOFC) based on BaCe_0.8Y_0.2O_3−α (BCY20) at 800 °C was studied without using an anode material. A porous, Ce-rich phase with a fluorite structure was formed at a depth of approximately 10 μm from the BCY20 surface by heat treatment at 1700 °C. This was due to the vaporization of BaO from the BCY20 surface. This treatment improved the cell performance and chemical stability to CO₂ because the Ce-rich phase functioned as an electrically conducting and protective layer. The heat-treated BCY20 also had better chemical and redox stabilities over a Ni–Ce_0.8Sm_0.2O_1.9 (SDC) cermet anode attached to the SDC electrolyte. The cell with the heat-treated BCY20 operated well on unhumidified methane, ethane, propane, and butane without carbon deposition, while the cell with the Ni–SDC cermet anode degraded within a few hours. Moreover, the BCY20 showed higher tolerance to 10 ppm H₂S and stability over 20 times redox cycling in comparison to the Ni–SDC cermet anode.     

A simple MOD method to grow a single buffer layer of Ce0.8Gd0.2O1.9 (CGO) for coated conductors

A single Ce_0.8Gd_0.2O_1.9 (CGO) buffer layer was successfully grown on the home-made textured Ni–5 at.%W (Ni–5W) substrates for YBCO coated conductors by a simple metal–organic deposition (MOD) technique. The precursor solution was prepared using a newly developed process and only contained common metal–organic salts of both Ce and Gd dissolved into a propionic acid solvent. The precursor solution at 0.4 M concentration was spin coated on short samples of Ni–5W substrates and heat-treated at 1100 °C in a mixture gas of 5% H₂ in Ar for an hour. X-ray studies indicated that the CGO films had good out-of-plane and in-plane textures with full-width-half-maximum values of 4.18° and 6.19°, respectively. Atomic force microscope (AFM) investigations of the CGO films revealed that most of the grain boundary grooves on the Ni–5W surface were found to be well covered by CGO layers, which had a fairly dense and smooth microstructure without cracks and porosity. These results indicate that our MOD technique is very promising for further development of single buffer layer architecture for YBCO coated conductors, due to its low cost and simple process.     

Synthesis and characterization of La0.9Sr0.1Ga0.8Mg0.2O3-δ electrolyte for intermediate temperature solid oxide fuel cells (ITSOFC)

Experimental investigations on new materials for application as electrolyte in electrolyte supported planar Intermediate Temperature Solid Oxide Fuel Cells (ITSOFC) operating below 800 °C is in progress at our laboratory. Sr and Mg doped Lanthanum gallate (LSGM) powder was prepared by glycine — nitrate combustion method. The prepared LSGM powder is relatively finer than that prepared through other techniques such as solid-state reaction. The measurements comprising XRD, particle size, density, TGA/DTA were made. Thin sections of circular pellets were fabricated and annealed at different temperatures ranging between 1000 and 1300 °C. The sintering behaviour of LSGM was investigated to obtain information on the densification factor, relative percentage shrinkage/expansion in volume, while annealing and the resulting apparent porosity values. Bismuth oxide is found to be an effective sintering aid in general. So the effect of bismuth oxide addition on LSGM was investigated through sintering studies, XRD, TGA/DTA, SEM and conductivity measurements. The results obtained on LSGM with and without bismuth oxide addition are discussed with respect to the requirement of an electrolyte for ITSOFC applications. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Structural and electrochemical characterization of Ce0.85Ca0.05Sm0.1O1.9 oxide ion electrolyte with Sr-doped LaMnO3 and SmCoO3 cathodes

This paper reports on the electrochemical properties and chemical stability of a recently developed Ca²⁺ and Sm³⁺-doped oxide ion conducting electrolyte, Ce_0.85Ca_0.05Sm_0.1O_1.9 (CCS), employed in an intermediate temperature solid oxide fuel cell (IT-SOFC) using conventional Sm_0.5Sr_0.5CoO₃ (SSC) and La_0.8Sr_0.2MnO₃ (LSM) cathodes in air at elevated temperatures. The materials were prepared by conventional solid-state reactions using their corresponding metal oxides and salts in the temperature range of 1,200–1,450 °C in air. Powder X-ray diffraction (PXRD) and impedance spectroscopy were employed for phase formation, chemical compatibility, and electrochemical characterization. PXRD studies on 1:1 weight ratio of heat-treated (1,000 °C for 3 days) mixtures of SSC or LSM and CCS revealed the presence of fluorite-type and perovskite-like phases. The area-specific resistance (ASR) value in air was lower for SSC cathodes (4.3–0.15 Ω cm²) compared to those of LSM (407–11 Ω cm²) over the investigated temperature range of 600–800 °C. As expected, a significant increase in ASR was observed in Ar as compared to air.     

Helium defectoscopy of cerium gadolinium ceramics Ce0.8Gd0.2O1.9 with a submicrocrystalline structure in the impurity disorder region

The concentration of impurity anion vacancies formed upon the dissociation of gadolinium-vacancy complexes has been determined using helium defectoscopy of the cerium gadolinium ceramics Ce_0.8Gd_0.2O_1.9 with a submicrocrystalline structure in the temperature range T = 740–1123 K and at saturation pressures ranging from 0.05 to 15 MPa. It has been found that the energy of dissociation of gadoliniumvacancy complexes is E _eff ^D = 0.26 ± 0.06 eV, and the energy of dissolution of helium in anion vacancies in the impurity disorder region is E ^P = ?0.31 ± 0.09 eV. The proposed mechanism of dissolution has been confirmed by the investigation of the electrical conductivity of the cerium gadolinium ceramics, as well as by the high-speed molecular dynamics simulation of the dissociation of gadolinium-vacancy complexes. It has been assumed that a decrease in the effective dissolution energy in comparison with the results of the previously performed low-temperature investigations is caused by the mutual repulsion of vacancies formed upon the dissociation of gadolinium-vacancy complexes in highly concentrated solutions of gadolinium in CeO₂ with increasing temperature.     

Oxygen-ion conducting electrolyte materials for solid oxide fuel cells

Oxygen-ion conducting solid electrolyte systems have been reviewed with specific emphasis on their use in solid oxide fuel cells. The relationships between phase assemblage, electrolyte stability and ionic conductivity have been discussed. The role of parameters such as sintering temperature and atmosphere which influence the segregation of impurities, present in the starting ceramic powders, at grain boundaries and at the external surface of the electrolyte compacts has been emphasised. The stability of various electrolyte materials in contact with other fuel cell components and in fuel environments has been discussed in detail. The ageing behaviour at fuel cell operating temperatures has been described. Data on ionic conductivity, mechanical and thermal properties have been presented for a number of electrolyte materials.     

Electrical properties of triple-doped bismuth oxide electrolyte for solid oxide fuel cells

In this study, the quaternary solid solutions of (Bi₂O₃)_(0.8?x)(Tb₄O₇)_0.1(Ho₂O₃)_0.1(Dy₂O₃)_x (x = 0.05, 0.10, 0.15, 0.20) as an electrolyte were synthesized for solid oxide fuel cells by the technique of solid-state synthesis.

The products were characterized by X-ray powder diffraction, differential thermal analysis/thermal gravimetry and the four-point probe technique (4PPT). The total electrical conductivity is measured on the temperature and the doped concentration by 4PPT.

All samples have been obtained as the δ-phase. According to the measurements of the 4PPT, the electrical conductivities of the samples increase with the temperature but decrease with the amount of doping rate. The value of the highest conductivity (σ) is found as 1.02?×?10^?1 S cm^?1 for the system of (Bi₂O₃)_0.75(Tb₄O₇)_0.1(Ho₂O₃)_0.1(Dy₂O₃)_0.05 at 850 °C. The thermal gravimetry (TG) curve shows that there is no mass loss of sample during the measurement. The analyses of differential thermal reveal that there are neither endothermic peaks nor exothermic peaks during the heating and cooling cycles (ranging from 30 to 1000 °C).     

A one-chamber solid electrolyte fuel cell for chemical cogeneration

A one chamber solid oxide fuel cell (SOFC) which works in a flow of a mixture of CH₄ and air has been studied in the present author's laboratory. This new type cell consists of Pt | Solid Electrolyte | Au, in which both electrodes are exposed to the CH₄₊₂ai r mixture with a CH₄/O₂ ratio of 2 at elevated temperatures, and it generates an electric power producing hydrogen and carbon monoxide (synthetic gas) with a H₂/CO ratio of about 2. A large difference in catalytic activity for the partial oxidation of CH₄ between the Pt and Au electrode materials leads to an oxygen concentration cell which can generate an electric power between them. In this paper the author describes such a one-chamber fuel cell with respect to their structural feature, performance as a fuel cell and their component materials citing the experimental results made by his laboratory's group. Paper presented at the 97th Xiangshan Science Conference on New Solid State Fuel Cells, Xiangshan, Beijing, China, June 14–17, 1998.     

高温固体氧化物燃料电池实验演示

介绍一种高效低污染的新型能源－高温固体氧化物燃料电池的工作原理及其演示实验装置。     

NiO+YSZ anode substrate for screen-printing fabrication of YSZ electrolyte film in solid oxide fuel cell

Anode substrate has a great effect on screen-printing fabrication of yttria-stabilized zirconia (YSZ) electrolyte film and cell performance. In this work, NiO+YSZ anode substrate was prepared by a conventional ceramic sintering method, on which dense YSZ electrolyte film was successfully fabricated by screen-printing method. Microstructure of the anode substrate and cell performance were investigated. The optimal amount of addition of starch to the anode substrate was 20 wt%. The optimal temperature for pre-sintering of NiO powder was 800 °C. A single cell with the NiO powder pre-sintered at 800 °C exhibited the highest power density of 0.95 W cm⁻² at 700 °C.     

Energy integration strategies for solid oxide fuel cell systems

Solid oxide fuel cells (SOFCs) have operating temperatures ranging from as low as 600 °C for intermediate temperature operation to above 900 °C for higher temperature operation. These high temperatures are often viewed as a considerable disadvantage from a materials point of view because of the occurrence of unwanted interfacial reactions, stresses as a result of thermal expansivity mismatches, etc. However, higher temperatures are also an advantage of SOFC systems. Fuel pretreatment that may involve such processes as reforming is very often highly endothermic in nature. The high operating temperature of an SOFC allows for efficient system energy integration with the waste heat from the fuel cell being used to drive fuel pretreatment processes. Here, we demonstrate this propensity for energy integration by looking at the use of a novel hydrogen-carrier system working with an SOFC.     

Performance of a double-layer CGO/YSZ electrolyte for solid oxide fuel cells

The use of a double-layer ceria-gadolinia (CGO) - yttria-stabilized zirconia (YSZ) electrolyte has been suggested as an alternative for efficient intermediate temperature operation of Solid Oxide Fuel Cells (SOFC). CGO offers the advantage of high ionic conductivity and good chemical compatibility with Co-containing cathode perovskite materials, while YSZ serves as an electron blocking layer. The main problem for the applicability of such a composite film still remains the formation of a poorly conductive solid solution phase at the CGO/YSZ interface. The microstructure and the elemental distribution of this solid solution phase were examined with the aid of electronic probe microanalysis. Powders with the same composition were synthesized in order to examine their crystal structure and electrical properties, with the objective to propose a suitable gradation at the interface in order to improve the feasibility of CGO/YSZ two- layer composite electrolyte films. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998     

Reactive sputtering deposition and electrochemical characterisation of thin solid electrolyte membranes for solid oxide fuel cells

Solid oxide fuel cells directly convert the chemical energy of a fuel into electricity. To enhance the efficiency of the fuel cells, the thickness of the gastight solid electrolyte membranes should be as thin as possible. Y₂O₃-stabilised ZrO₂ (YSZ) electrolyte films were prepared by reactive sputtering deposition using Zr/Y targets in Ar/O₂ atmospheres. The films were 5 – 8 μm thin and were deposited onto anode substrates made of a NiO/YSZ composite. After deposition of a cathode with the composition La_0.65Sr_0.35MnO₃ the electrochemical properties of such a fuel cell were tested under operating conditions at temperatures between 600 °C and 850 °C. Current-voltage curves were recorded and impedance measurements were performed to calculate apparent activation energies from the fitted resistance data. The conductivity of the YSZ films varied between 4.6·10⁻⁶ S/cm and 2.2·10⁻⁵ S/cm at 400 °C and the fuel cell gave a reasonable power density of 0.4 W/cm² at 0.7 V and 790 °C using H₂ with 3 % H₂O as fuel gas. The gas compositions were varied to distinguish the electrochemical processes of the anode and cathode in the impedance spectra. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Synthesis of LiNi0.6Co0.2Mn0.2O2 cathode material by a carbonate co-precipitation method and its electrochemical characterization

Using Na₂CO₃ and Me(NO₃)₂ (Me = Ni, Co and Mn) as starting materials, the precursor of LiNi_0.6Co_0.2Mn_0.2O₂ cathode material for lithium rechargeable batteries has been synthesized by carbonate co-precipitation. The precursor was mixed with Li₂CO₃ and heated in air. Thermogravimetric analysis (TG–DTA), laser particle size analysis, X-ray diffraction (XRD) and electron scanning microscopy (SEM) were employed to study the reaction process and the structures of the powders. The D₅₀ of precursor was 2.509 μm and the distribution was relatively narrow. The optimum calcination temperature was 850–900 °C. Galvanostatic cell cycling and cyclic voltammetry were also used to evaluate the electrochemical properties. The initial discharge capacity for the powders calcined at 900 °C was about 180 mA h/g at room temperature when cycled between 2.8 and 4.3 V at 0.2 C rate.     

An intermediate temperature solid oxide fuel cell utilizing superior oxide ion conducting electrolyte, doubly doped LaGaO3 perovskite

LaGaO₃-based perovskite oxide doped with Sr and Mg exhibits high ionic conduction over a wide oxygen partial pressure. In this study, the stability of the LaGaO₃ based oxide was investigated. It became clear that LaGaO₃ based oxide is very stable for reduction and oxidation. SOFCs utilizing LaGaO₃-based perovskite type oxide for electrolyte were further studied for the decreased temperature solid oxide fuel cells. The power generation characteristics of cells were strongly affected by the electrode, both anode and cathode. It became clear that Ni and LnCoO₃ (Ln: rare earth) are suitable for anode and cathode, respectively. Rare earth cations in the Ln-site of Co-based perovskite cathode also have a great effect on the power generation characteristics. In particular, high power density could be attained in the temperature range from 973 to 1273 K by using doped SmCoO₃ for the cathode. The electrical conductivity of SmCoO₃ increases with increasing Sr amount doped for the Sm site and attained the maximum at Sm_0.5Sr_0.5CoO₃. The cathodic overpotential and the internal cell resistance exhibit almost opposite dependence on the amount of doped Sr. Consequently, the power density of the cell reaches a maximum when Sm_0.5Sr_0.5CoO₃ is used for cathode. On this cell, the maximum power density is as high as 0.58 W/cm² at 1073 K, although a 0.5 mm thick electrolyte is used. Therefore, this study reveals that the LaGaO₃ based oxide for electrolyte and the SmCoO₃ based oxide for cathode are promising for solid oxide fuel cells at intermediate temperature. Paper presented at the 97th Xiangshan Science Conference on New Solid State Fuel Cells, Xiangshan, Beijing, China, June, 14–17, 1998.     

Malonic acid-assisted synthesis of LiNi0.8Co0.2O2 cathode active material for lithium-ion batteries

Polycrystalline LiNi_0.8Co_0.2O₂ was synthesized by a solution route with malonic acid as the complexing agent. The effects of temperature, duration of heat treatment, pH of the precursor solution, and the nature of the solvent employed on the performance characteristics of the product were studied. It was observed that a 12-hour 800 °C heat treatment protocol was necessary to obtain products with optimal electrochemical characteristics. Furthermore, an excess lithium stoichiometry of 1.05 was found to be detrimental to the performance of the cathode material. The beneficial effect of ethanol as a solvent over water on the product characteristics is explained by the presence of solvent molecules in the coordination sphere of the cations. A pH of 7, at which malonic acid is complexed completely with the cations without interference from other nucleophiles, was found to be ideal for the synthesis of the cathode active material from aqueous solutions. With ethanol as the medium, the product formed by a 12-h calcination at 800 °C yielded a first-cycle capacity of 173 mAh/g and a tenth-cycle capacity of 169 mAh/g.     

Spectroelectrochemical cell for in situ studies of solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are able to produce electricity and heat from hydrogen‐ or carbon‐containing fuels with high efficiencies and are considered important cornerstones for future sustainable energy systems. Performance, activation and degradation processes are crucial parameters to control before the technology can achieve breakthrough. They have been widely studied, predominately by electrochemical testing with subsequent micro‐structural analysis. In order to be able to develop better SOFCs, it is important to understand how the measured electrochemical performance depends on materials and structural properties, preferably at the atomic level. A characterization of these properties under operation is desired. As SOFCs operate at temperatures around 1073 K, this is a challenge. A spectroelectrochemical cell was designed that is able to study SOFCs at operating temperatures and in the presence of relevant gases. Simultaneous spectroscopic and electrochemical evaluation by using X‐ray absorption spectroscopy and electrochemical impedance spectroscopy is possible.