Solid Oxide Fuel Cell Components Research Articles

Fabrication of Thin Electrodic Films by Solution Aerosol Thermolysis (SAT)

Thin films of ceramic or ceramic-metal (cermet) electrodes with appropriate structure and morphology significantly affect performance of solid oxide fuel cells (SOFCs. Thus, electrode parameters such as thickness, porosity and tortuosity dictate the values of gaseous effective diffusivities that eventually determine the extent of concentration polarization losses in these high temperature ceramic fuels cells1. In addition, electrode thickness is also a factor dictating concentration polarization losses in working fuel cells which should be as low as possible. Furthermore, it was established that, even total ohmic cell polarization in ceramic fuel cells based on conventional yttria stabilized zirconia (YSZ) electrolyte is significantly affected by electrode microstructure to a greater extent than by electrolyte ohmic resistance. This becomes more evident at low electrolyte thicknesses (i.e. <20 μm) and is related to contiguity factors between metal particles in the fabricated cermets2. On the other hand, nanostructured thin film electrodes have shown great potential for operation in fuel cells due to their enhanced conversion efficiencies compared to their bulk materials counterparts essentially leading to low polarization losses3. In these structures, controlling the electrode film thickness, porosity and tortuosity is of crucial importance as these dictate the magnitude of effective gas diffusivities of reactants and products and the subsequent overall polarization losses4. Therefore, for a given set of chosen materials, the fabrication method adopted for the ceramic fuel cell has to be optimized for the above electrode parameters for optimum performance. In general, innovative fabrication methods with the potential of producing nanostructural components in ceramic fuel cells and possessing inherently the capacity of controlling electrode microstructural characteristics such as porosity, pore size, thickness and uniformity have not given much attention in the literature compared to materials research5. The major advantages of these techniques over the conventional ceramic techniques such as tape casting, are their potential on producing very thin layers of components (i.e. < 1 μm) and the reduction of the subsequent sintering temperatures due to their more active precursor components. Another advantage lies in their capacity of producing nanocomposite electrodes that offer enhanced electrochemical performance6. The spray pyrolysis (SP) technique belongs to this family of deposition methods7 and involves spraying of a solution of metal salts onto a heated substrate. It is robust, easily up scalable and with easily controllable processing parameters. It offers the advantage of accurate control of stoichiometry at droplet level and possess the potential of incorporating the sintering step in the process. It has been used for fabrication of a large number of electroceramic materials and progress in solid oxide fuel cell component fabrication has been reviewed recently5. The technique belongs to the family of molecular precursor deposition methods in which the film is formed on a support via a physical and/or chemical process by appropriately tuning the characteristics of the physicochemical processes involved during deposition in order to achieve the desired functional characteristics. In the present work we focus on the optimization of the technique for fabricating electrodic films. In particular, results are presented of the application of air pressurized spray pyrolysis using aqueous solutions for the production of film cermets of Cu-CeO2, and La0.75Sr0.25MnO3 (LSM) onto dense YSZ substrates. The use of aqueous solutions reduce significantly the cost and the risk factors to personnel health and to the environment. Cu-CeO2 constitutes a potential anode that combines the functionalities of copper ( an electronic conductor) and a good oxidation catalyst (i.e. CeO2) for potential hydrocarbon fuels fed into a SOFC while LSM is the typical state of the art cathode used with a YSZ electrolyte8. The films are characterized by X-ray diffraction and scanning electron microscopy (SEM). References 1W. He et al., Journal of Power Sources, 195, 532 (2010) 2F. Zhao, A. V. Virkar, Journal of Power Sources 141, 79 (2005) 3W. Hea et al., , Nano Energy 1, 828, (2012) 4W. He , J. B. Goodenough, Journal of Power Sources 251, 108 (2014) 5N. E. Kiratzis, Ionics 22,751 (2016) 6S.J. Litzelman et al., Fuel Cells 08, 294 (2008) 7E. Papastergiades et al., Ionics 15, 545 (2009) 8N.E.Kiratzis et al., Journal of Electroceramics 24, 270 (2010) Figure 1

Recent Progress on Advanced Materials for Solid-Oxide Fuel Cells Operating Below 500 °C.

Solid-oxide fuel cells (SOFCs) are electricity generators that can convert the chemical energy in various fuels directly to the electric power with high efficiency. Recent advances in materials and related key components for SOFCs operating at ≈500 °C are summarized here, with a focus on the materials, structures, and techniques development for low-temperature SOFCs, including the analysis of most of the critical parameters affecting the electrochemical performance of the electrolyte, anode, and cathode. New strategies, such as thin-film deposition, exsolution of nanoparticles from perovskites, microwave plasma heating, and finger-like channeled electrodes, are discussed. These recent developments highlight the need for electrodes with higher activity and electrolytes with greater conductivity to generate a high electrochemical performance at lower temperatures.

Advances in Low Temperature Coatings for Solid Oxide Fuel Cell Components

In the development of solid-oxide fuel cells, high-performance coatings are essential to successful commercialization. Electrode coating technologies allow the cost-effective deposition of anode and cathodes on cell components at high volumes and high precision. Further, catalytic and corrosion resistant coatings for common steel alloys find application in gas delivery and fuel reforming components of the balance-of-plant. Such corrosion-resistant alloys are also combined with conductive and insulating oxide protective layers to making low-cost steel interconnectors corrosion-resistant at SOFC operating while maintaining low contact resistance and minimizing Cr volatilization. Many of the first-generation coating technologies that have found commercial implementation were designed for cell and stack operating temperatures of 750 °C and higher. In this paper, Nexceris will report on the development of new formulations designed to meet the needs of lower operating temperature SOFCs.

Morphology and Structure of Solid Oxide Fuel Cell (SOFC) Components Fabricated by Spray Pyrolysis

The technique of Solution Aerosol Thermolysis (SAT) was applied for fabrication of thin films of electrodes and electrolytes suitable for operation in Solid Oxide Fuel Cells (SOFCs). Anodic electrodes were chosen for their suitability as suitable electrocatalysts for the direct oxidation of hydrocarbons in these high temperature fuel cells. The technique was optimized in terms of process parameters and their effect on film morphology and crystal structure. Major parameters optimized in aqueous solutions were the substrate temperature, solution flowrate, solution total salts concentration and substrate to nozzle distance given the type of a set of precursor salts.

Oxidation resistance, thermal expansion and area specific resistance of Fe-Cr alloy interconnector for solid oxide fuel cell

It is promising for metal especially ferritic stainless steel (FSS) to be used as interconnector when the solid oxide fuel cell (SOFC) is operated at temperature lower than 800 °C. However, there are many challenges for FSS such as anti-oxidant, poisoning to cathode and high area specific resistance (ASR) when using as SOFC interconnector. The effect of Cr content (12 — 30 mass%) in Fe-Cr alloys on thermal expansion coefficient (TEC), oxidation resistance, microstructure of oxidation scale and ASR was investigated by thermo-gravimetry, scanning electron microscopy, energy dispersive spectroscopy and four-probe DC technique. The TEC of Fe-Cr alloys is (11 — 13) × 10−6 K−1, which excellently matches with other SOFC components. Alloys have excellent oxidation resistance when Cr content exceeds 22 mass% because of the formation of chromium on the surface of alloy. The oxidation rate constants kd and ks decrease rapidly with increasing the Cr content and then increase slowly when the Cr content is higher than 22 mass%. The kinetic results indicate that Cr evaporation must be considered at high temperature for Fe-Cr alloys. After the alloys were oxidized in air at 800 °C for 500 h, log(ASR/T) (T is the absolute temperature) presents linear relationship with 1/T and the conduct activation energy is 0.6 — 0.8 eV (Cr16-30). Optimal Cr content is 22 — 26 mass% considering the oxidation resistance and ASR.

A novel two-layered glass-ceramic sealant design for solid oxide fuel cells

A thermo-mechanical stress develops on the sealant of an solid oxide fuel cell (SOFC) stack because of thermal expansion coefficient (TEC) differences among ceramic electrolyte, metallic interconnector, and glass-ceramic sealant. The stress initiates cracks and usually results in a sealing failure. This study offers a thermo-mechanically stable two-layered sealant to decrease thermo-mechanical stresses among the SOFC components. The novel sealant involves two layers in which TEC of the first layer is close to that of the interconnector, while TEC of the second layer is similar to that of the electrolyte. SiO2, CaO, Al2O3, and Na2O glass-ceramics are selected as a sealant material, and five cases are prepared by changing the amount of Al2O3 to obtain the appropriate properties close to both the interconnector and the electrolyte, separately. Tensile strength, wetting angles, shrinkages, and electrical resistances are measured to determine the sealant properties. To see effect of the reaction on the microstructure, scanning electron microscopy (SEM) tests are conducted for the samples exposed both in air and in crofer 22 APU. Long-term sealing performance of the novel sealant is compared with the traditional designs. While the two-layered sealant keeps 35 kPa pressure for 500 h after 100 thermal cycles, test pressure with traditional design reduces to 31–32 kPa after 50 h. Copyright © 2016 John Wiley & Sons, Ltd.

The Numerical Analysis for a Crack in the Cathode Electrode Surface of Planar SOFC

The crack formation and propagation remain as major issues regarding the mechanical crack behavior of solid oxide fuel cell (SOFC), due to the thermomechanical mismatch and the porous nature of the SOFC components. The SOFC consisting of brittle ceramic materials can be damaged by the cracking due to high-temperature during the manufacturing and operation. The crack generated during the manufacturing were can cause the cell performance degradation during operation. For crack evaluation because the SOFC products the required power density through the stack considering the interconnector is required. The purpose of this study is to investigate the effect of the vertical crack at the cathode electrode, and also the effects of the crack on the interconnect channel position in operation for the crack behavior and stress field.

Morphology and Structure of SOFC Components Fabricated By Spray Pyrolysis

Solid oxide fuel cells are expected to play a significant role in the future energy sector mainly due to their inherent high efficiency and low pollutants emissions. These devices typically operate above 800°C by means of a solid oxide electrolyte, typically yttria stabilized zirconia (YSZ) that conducts oxygen ions at sufficiently high conductivities at these temperatures. The cathode is exposed to air or pure oxygen which is reduced to O2 -. State of the art cathodes are based on the perovskite lanthanum strontium manganite La0.9Sr0.1MnO3 (LSM) in a mixture with YSZ which exhibits adequate electronic conductivity at 1000°C, is stable under oxidizing conditions and has a good thermal expansion match with YSZ. The anodic electrode is exposed to the fuel mixture that can even consist of CO which is normally, deleterious for other types of fuel cells. In fact, one major advantage of SOFCs is their suitability to oxidize conventional hydrocarbons and/or biofuels which renders them ideal candidates in connection with renewable sources (see Figure 1). Due to the difference in the oxygen chemical potential between the anode and the cathode, part of the free energy of the oxidation reaction is converted directly to electricity. State of the art anodes, have been based on Ni due to its low cost, again in a mixture with YSZ[i]. Oxidizing directly (i.e. without an intermediate reforming step) conventional hydrocarbons in SOFCs offers a number of advantages that relate to the existing infrastructure for these fuels that in this case, can be used more efficiently and environmentally consciously until their reserves are depleted. Nickel cermets, however, have to be replaced with other types of mixed conducting (i.e. ionic and electronic) electrodes that do not catalyze carbon formation such as composite anodes of the type Cu/CeO2 /YSZ[ii] or materials based on lanthanum chromate such as the perovskite (La0.75Sr0.25)Cr0.5Mn0.5O3 -δ (LSCM)[iii]. High materials and fabrication costs have prevented wide commercialization of these type of fuel cells and, thus, research efforts have been intensified in recent years by many laboratories that address the above issues. Regarding fabrication, there exist two major categories of techniques, namely conventional which are based on a particulate approach and deposition which entail the formation of cell components on a support via a physical and/or chemical process. The latter offer the advantage of in situ and directly controlling final particle size and morphology by appropriately tuning the characteristics of the physicochemical processes involved during deposition. The purpose of this research is to explore the potential of the spray pyrolysis (SP) technique which is a deposition technique for the fabrication of SOFC components of flat plate geometry. It involves spraying of an appropriate solution of ionic salts on a suitable substrate at certain temperature for the production of oxides in film form[iv]. The technique has the potential for the fabrication of electrolytic films with low thickness resulting in lower ohmic losses and for the fabrication of all the cell components in situ. It also offers great flexibility in terms of materials selection and processing parameters to fabricate suitable electrodes and electrolytes for SOFCs. In the present work, we focus on the effect of the substrate temperature and solution concentration on the final morphology of anodic, cathodic and electrolytic films fabricated by SP using environmentally friendly aqueous solutions. Results are presented of the application of this method for the production of anodic film cermets such as Cu-CeO2, Co-CeO2 and Cu-La0.75Sr0.25Cr0.5Mn0.5O3-δ (Cu-LSCM) on dense YSZ substrates. In addition, cathodic electrodes of La0.75Sr0.25MnO3(LSM) on dense YSZ and porous LSM substrates were fabricated. Finally, YSZ films as well as composite structures of the type Cu-LSCM/YSZ were deposited on porous LSM pellets. The effort is concentrated to identify optimized processing conditions that lead to good adhesion. The produced films were characterized by X-ray diffraction and scanning electron microscopy (SEM). [i] Kiratzis N. E., Connor P., Irvine J. T. S.(2010) Preparation and characterization of copper based cermet anodes for use in solid oxide fuel cells at intermediate temperatures. J. Electroceram. 24:270-287. [ii] Park S.D., Vohs J.M., Gorte R.J. (2000) Direct oxidation of hydrocarbons in a solid-oxide fuel cell. Nature 404: 265-267. doi:10.1038/35005040 [iii] Plint S.M., Connor P.A., Tao S., Irvine J.T.S. (2006) Electronic transport in the novel SOFC anode material La1−xSrxCr0.5Mn0.5O3±δ. Solid State Ion. 177: 2005-2008. doi:10.1016/j.ssi.2006.02.025 [iv] Papastergiades E., Argyropoulos S., Rigakis N., Kiratzis N. E. (2009) Fabrication of ceramic electrolytic films by the method of solution aerosol thermolysis (SAT) for solid oxide fuel cells (SOFC). Ionics 15: 545-554. Figure 1

Assessment of a novel solid oxide fuel cell tri-generation system for building applications

The paper provides a performance analysis assessment of a novel solid oxide fuel cell (SOFC) liquid desiccant tri-generation system for building applications. The work presented serves to build upon the current literature related to experimental evaluations of SOFC tri-generation systems, particularly in domestic built environment applications. The proposed SOFC liquid desiccant tri-generation system will be the first-of-its-kind. No research activity is reported on the integration of SOFC, or any fuel cell, with liquid desiccant air conditioning in a tri-generation system configuration. The novel tri-generation system is suited to applications that require simultaneous electrical power, heating and dehumidification/cooling. There are several specific benefits to the integration of SOFC and liquid desiccant air conditioning technology, including; very high operational electrical efficiencies even at low system capacities and the ability to utilise low-grade thermal energy in a (useful) cooling process. Furthermore, the novel tri-generation system has the potential to increase thermal energy utilisation and thus the access to the benefits achievable from on-site electrical generation, primarily; reduced emissions and operating costs.Using empirical SOFC and liquid desiccant component data, an energetic, economic and environmental performance analysis assessment of the novel system is presented. Significant conclusions from the work include: (1) SOFC and liquid desiccant are a viable technological pairing in the development of an efficient and effective tri-generation system. High tri-generation efficiencies in the range of 68–71% are attainable. (2) The inclusion of liquid desiccant provides an efficiency increase of 9–15% compared to SOFC electrical operation only, demonstrating the potential of the system in building applications that require simultaneous electrical power, heating and/or dehumidification/cooling. (3) Compared to an equivalent base case system, the novel tri-generation system is currently only economically viable with a government’s financial support. SOFC capital cost and stack replacement are the largest inhibitors to economic viability. Environmental performance is closely linked to electrical emission factor, and thus performance is heavily country dependent. (4) The economic and environmental feasibility of the novel tri-generation system will improve with predicted SOFC capital cost reductions and the transition to clean hydrogen production.

Modeling of an anode supported solid oxide fuel cell focusing on thermal stresses

A mechanical failure of a single component is sufficient to cause a solid oxide fuel cell (SOFC) breakdown. As an unfavorable issue for interfering the stable operation of SOFCs, thermal stress stemming from temperature gradient and mechanical mismatch can result in crack damage. Therefore, it is strongly significant to clarify the relationship of mechanical properties of the cell materials with distribution of the stress by taking into account the electrochemical reactions. A complete three-dimensional model for a planar anode-supported SOFC has been proposed and established in this study, which includes governing equations for momentum, gas-phase species, heat, electron and ion transport. The thermal gradients caused by the electrochemical reactions and heat transport processes of the counterflow leading to a maximum thermal stress is slightly larger than that is induced by the coflow. The influence of mechanical mismatch is analyzed and the results indicate that the strength of stress at two sides of a cell tends to be enlarged under fixed constraint conditions. Furthermore, the functional buffer layers can affect the stress between different components and inhibit the extent of degradation. This investigation is expected to offer a path to improve the matches of SOFC components and optimize the stack design.

Thermal Properties of Perovskite-Type Oxides La0.6Sr0.4Co1-XFexO3-δ (0 ≤ x ≤ 1.0)

Introduction Solid oxide fuel cells (SOFCs) are an electrochemical conversion system with high efficiency and low-emission of pollution. In order to develop a commercial SOFC system with high performance and long-term stability, it is important to understand temperature distribution in the cell and stack [1]. For this purpose, thermal properties of SOFC components should be evaluated particularly under SOFC operating conditions, e.g. at elevated temperatures and under various oxygen partial pressures. Although there exist several reports on the thermal properties of the SOFC components, most of the data were measured at room temperature or in some specific atmospheric condition. [2-3] Thus, we aimed to investigate thermal properties of SOFC components as functions of temperature and oxygen partial pressure. Perovskite-type oxides La0.6Sr0.4Co1-x Fe x O3-δ (LSCF) are selected for this work. They are known to exhibit oxygen nonstoichiometry changes depending on temperature and oxygen potential. [4-7] The variation in oxygen vacancy concentration is known to influence electrochemical properties and mechanical properties. However, little is known about the effect on the thermal properties although. There exist several reports on the thermal properties of the LSCF. In this study, thermal diffusivities, heat capacities and thermal conductivities have been investigated by using the laser flash method and TG-DSC at high temperatures up to 1273 K under controlled oxygen atmospheres. Dependencies of the thermal properties of LSCF on temperature, oxygen partial pressure and the material composition will be discussed in terms of oxygen nonsotichiometry and crystal structures. Experimental Powders of La0.6Sr0.4Co1-x Fe x O3-δ (LSCF) were prepared by a conventional Pechini method. Obtained powders were hydrostatically pressed at 150 MPa into compacts, and then sintered in air at 1573K-1473K for 6h. For the thermal diffusivity measurement, the sintered compacts were cut into rectangles (c.a. 10 x 10 x 1 mm). For the TG-DSC measurement, the sintered compacts were cut smaller than Pt crucible. The phase of sintered samples was characterized by XRD to investigate any phase change after sintering. Thermal diffusivities and heat capacities of the LSCF were measured by using the laser flash method (LFA 457 Micro Flash, NETZSCH) and TG-DSC (STA 447, NETZSCH) in the oxygen partial pressure range from 10-4 to 0.2 bar and in the temperature range from R.T. to 1173 K. In order to control the atmospheres, a gas mixing system and an oxygen sensor are additionally attached to the laser flash system. The thermal conductivities (λ) of LSCF were calculated by using Eq(1). α : thermal diffusivity of LSCF with respect to temperature, ρ : the density of sintered LSCF sample and Cp: heat capacity of the LSCF as a function of temperature. λ = α·ρ·Cp (1) Results and Discussion In this study, the thermal properties of perovskite-type oxides La0.6Sr0.4Co1-x Fe x O3-δ (LSCF) were investigated. The thermal diffusivities of LSCF were dependence of temperature as well as oxygen partial pressure and Co contents ratio in intermediate temperatures (873K to 1173K).[Fig.1] The thermal diffusivity of LSCF decreased gradually as p(O2) decreased at all investigated temperatures, and decreased as temperature increased in all investigated p(O2) range. The thermal diffusivity depended on oxygen partial pressure more significant in lower oxygen partial pressure and at higher temperature. The thermal diffusivity increases with Co contents ratio increases and each composition increases with temperature through a maximum, then decreases. Heat capacity and thermal conductivity of LSCF (x=0, 0.8, 1.0) were almost constant from R.T to 873K, however increasing with increasing temperature over 873K. However, measured heat capacity compare with the Neumann-Kopp rule-specific heat of LSCF (x=0, 0.8, 1.0) were not consistent. LSCF is known to show the oxygen nonstoichiometry changes at relatively higher temperatures. Therefore, we suggest these dependenciese could be interpreted by the change in the oxygen nonstoichiometry, meaning that the thermal properties of LSCF were significantly affected by the oxygen nonstoichiometry.

A Brief Description of High Temperature Solid Oxide Fuel Cell’s Operation, Materials, Design, Fabrication Technologies and Performance

Today’s world needs highly efficient systems that can fulfill the growing demand for energy. One of the promising solutions is the fuel cell. Solid oxide fuel cell (SOFC) is considered by many developed countries as an alternative solution of energy in near future. A lot of efforts have been made during last decade to make it commercial by reducing its cost and increasing its durability. Different materials, designs and fabrication technologies have been developed and tested to make it more cost effective and stable. This article is focused on the advancements made in the field of high temperature SOFC. High temperature SOFC does not need any precious catalyst for its operation, unlike in other types of fuel cell. Different conventional and innovative materials have been discussed along with properties and effects on the performance of SOFC’s components (electrolyte anode, cathode, interconnect and sealing materials). Advancements made in the field of cell and stack design are also explored along with hurdles coming in their fabrication and performance. This article also gives an overview of methods required for the fabrication of different components of SOFC. The flexibility of SOFC in terms fuel has also been discussed. Performance of the SOFC with varying combination of electrolyte, anode, cathode and fuel is also described in this article.

ChemInform Abstract: Development of Bilayer Glass‐Ceramic SOFC Sealants via Optimizing the Chemical Composition of Glasses — A Review

AbstractReview: 112 refs.

Selected aspects of the design and diagnostics of solid oxide fuel cells

An increased growth in demand for energy accompanied by efforts to limit its negative impact on the environment is forcing society to seek new, more efficient energy sources. Fuel cells are one of the most promising solutions among the widely developed new generation of electrical generators. Fuel cells directly convert chemical fuel into electricity. Water and waste heat are by-products of fuel cell operation. Solid oxide fuel cells (SOFCs) have proven to be one of the most interesting solutions among the five types of technologically advanced fuel cells, for their ability to operate at temperatures above 800°C. Furthermore, SOFCs are characterized by other advantages in comparison to PEMFCs, including: (1) no need to use expensive catalysts (e.g. platinum, the price of which is high, and its resources limited), (2) the possibility of direct, internal conversion of hydrocarbon fuels, (3) lower sensitivity to contaminants (in particular, hydrogen fuel containing CO, which is useful for SOFCs instead of the platinum catalyst poisoners), and (4) the possibility of using waste heat in a gas turbine, or for heating or other industrial purposes. The paper discusses selected issues regarding the construction and characteristics of planar solid oxide fuel cells. Selected results of the following electrochemical investigations: Ba0.9 Ca0.1 Ce0.9 Y0.1 O3 -based proton electrolyte as possible components of SOFCs operating at intermediate temperature range (500-700°C).

ChemInform Abstract: Progress in Material Selection for Solid Oxide Fuel Cell Technology: A Review

AbstractReview: 1146 refs.

Electrochemical Performance of SOFC Components Fabricated by Spray Pyrolysis Method

The method of Spray Pyrolysis (SP) was applied to prepare thin films of electrodes and electrolytes for Solid Oxide Fuel Cell (SOFC) components. These devices which operate typically above 500°C due to the adequateness of ionic conductance requirement of their ceramic electrolyte, offer a great fuel flexibility and operation at high efficiencies with low environmental burdens. They typically operate with either an Yttria (8% mole) Stabilized Zirconia (YSZ) which is suitable above 800°C or a Gadolinia (10-20% mole) doped Ceria electrolyte which exhibits better conductivities within an intermediate temperature range (i.e. 500-750°C). Various conventional fabrication methods such as tape casting and screen printing involve slurry preparation which includes ball milling, use of organic additives and sintering at high temperatures often above 1000 °C, all of which contribute to increasing the fabrication cost of a SOFC. On the other hand, the spray pyrolysis technique is simple and it basically involves spraying of a solution of appropriate salts onto a heated substrate on which the desired film of either electrolyte or electrode is obtained after thermal decomposition of the precursors. By appropriately tuning process parameters such as substrate temperature, solution flowrates and type and concentration of salts and solvents, thin films of the desired composition can be obtained. In fact, if sintering is also done in situ, the process eliminates intermediate steps and even conduces to a lower sintering temperature due to its reliance on a molecular rather than a conventional particulate approach for thin film production. The process also offers the possibility of producing all cell components in situ and cell operation at lower temperatures due to the lower ohmic resistances exhibited by thin ceramic films. The aim of this work was to operate a SOFC with all its components fabricated in situ by SP and exhibiting an electrochemical performance at least equivalent with that obtained by a conventionally made SOFC. Anodic electrodes were chosen for their performance as suitable electrocatalysts for the direct oxidation of hydrocarbons in these high temperature fuel cells. The strategy, here, is to use a good electronic conductor like Cu for enhanced conductivity but inert to undesired reactions such as coking in conjunction with a good catalyst such as CeO2 for hydrocarbon oxidation. As a cathodic electrode the perovskite (La0.8Sr0.2)0.95MnO3 was used and deposited for all the experiments due to its excellent thermal and chemical compatibility with YSZ electrolyte. Thus, results will be presented of the application of this method for the production of composites consisting of Cu-CeO2/YSZ/LSM and Cu-La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM) /YSZ/LSM deposited by SP on porous support substrates of lanthanum strontium manganite (LSM) of 2 mm thickness. Furthermore, the same type of anodic and cathodic electrodes were deposited on dense YSZ substrates and compared in terms of morphology and electrochemical performance with the previous composite structures. Film microstructure was analyzed by scanning electron microscopy (SEM) and was found that thin defect-free films could be produced on porous and dense substrates. X-ray diffraction (XRD) analysis showed that the films were crystalline after the deposition at relatively low (i.e. 700°C) post-deposition sintering temperatures. Electrochemical tests of composite structures consisting of Cu-CeO2/YSZ/LSM and Cu-LSCM/YSZ/LSM showed that it is possible to obtain a SOFC with sufficient electrochemical performance fabricated by spray pyrolysis.

Understanding the Rheology of Screen-Printing Inks for the Fabrication of SOFC Thick Films

Fabrication of solid oxide fuel cell (SOFC) components via a simple and economical technique is critical to lower the overall manufacturing cost of SOFCs. In this regard, screen-printing is widely used to fabricate SOFC components having thickness in the range of 10 to 100 μm. Fabrication of SOFC components by screen-printing using an optimized ink is very significant for the production of high quality films with improved performance. To realize this, the rheological properties of the ink, including static and dynamic properties, need to be understood deeply for the application of the ink at a specific printer setting. Understanding the relationship between the composition and rheology of the inks may enhance the properties and performance of the resultant screen-printed films. The effect of ink composition consisting of a solid, a binder, a solvent and a dispersant on the rheological properties and performance of resultant films must be understood for the fabrication of an optimized ink. These effects can be optimized by measuring the ink rheological parameters such as viscosity, yield stress, thixotropy and viscoelasticity. Furthermore, these parameters must be related to the particle network strength in the fabricated inks and mechanical, electrical and electrochemical performance of the resultant films. In this study, the rheological properties of NiO/ScSZ anode inks are studied and correlated to the particle network strength, screen-printability, microstructure, mechanical hardness, electrical performance and electrochemical performance. The initial study showed that inks having particle network strength and complex modulus in the range of 100-200 Pa and 500-4000 Pa, respectively, have been suggested suitable for screen-printing application. In addition, these inks showed suitable viscosity and thixotropy for screen-printing application. These range was exhibited by inks having binder and solids loading in the range of 1-3 wt% and 25-28 vol%, respectively (wt% refer to wt% of solids loading and vol% refers to vol% of final ink composition). The mechanical hardness, conductivity and electrochemical performance of screen-printed films produced using these inks also showed improved properties with increasing particle network strength in the fabricated inks. These properties were further confirmed by investigating the SEM images of sintered films. Figure 1

High Temperature Strength and Elastic Properties of Doped Ceria under Various Oxygen Partial Pressures

The mechanical endurance of brittle components under operating conditions is extremely important to ensure the reliability of Solid Oxide Fuel Cell (SOFC). To prevent a decline in SOFC performance due to mechanical damages, it is essential to deal with the mechanical design which was considered the mechanical properties changing under the operating conditions. Electrochemical properties of the oxygen-ion conducting ceramics were revealed to be affected by not only the temperature but also the oxygen partial pressure. On the other hand, because of the limitation of technique, the mechanical properties were still unclear, have been revealed only under the air. Our group therefore developed the mechanical testing machine under the temperature and gas controlled conditions [1]. In this study, oxygen partial pressure dependence of fracture strength and the Young’s modulus of ceria doped with 10 mol.% Gadolinia and 10, 20 and 30 mol.% Yttria were investigated by means of small punch testing method under controlled the temperature (1073 K) and the oxygen partial pressure (log(p(O2)/atm) = -1.1, 17.4, -21.4, -22.0). Figure 1 shows the oxygen partial pressure dependence of (a) Young’s modulus and (b) fracture strength for 10GDC at 1073 K. For purpose of comparison, stoichiometric oxide for usual SOFC components, 8YSZ was also shown in Fig. 1. Regardless of p(O2), the Young’s modulus and fracture strength of 8YSZ at 1073 K remained almost constant, and hence there is no dependence of mechanical properties on the oxygen partial pressure. On the other hand, for 10GDC, high temperature mechanical properties were affected by the oxygen partial pressure, namely the oxygen nonstoichiometry. At log(p(O2)/atm) = -21.4, the young’s modulus decreased E SP =162.0 ± 4.1 to 127.7 ± 4.0 GPa, approximately 22 % degradation. This behavior is in good agreement with the by Amezawa et al[2].Furthermore, the fracture strength were increased monotonically with the decreasing the oxygen partial pressure. Under the low oxygen partial pressure, oxygen vacancies which were induced by the oxygen nonstoichiometry and the weakening bonding strength resulting from the chemical expansion can have a significant effect on the mechanical properties, however, this results suggested the presence of improving mechanisms and it exceeds the factors listed above. The oxygen partial pressure dependence of (a) the Young’s modulus of 10, 20 and 30YDC at 1073 K is shown in figure 2. The Young’s modulus under the low oxygen partial pressure decreased monotonically. Regardless of the amount of Y dopant, there is a similar decreasing tendency, and therefore changes in the Young’s modulus of doped ceria seems to depends on the generated oxygen vacancies, not initial ones introduced by doping the rare-earth oxides. Figure 3 shows normalized fracture strength of 10GDC, 10 and 20YDC vs relative reduction expansion of the lattice constant. The relative expansion values of the lattice constant for the rare-earth doped ceria were taken from S. Wang et al [3,4]. Around a/a 0 = 0.30 %, the fracture strength reached a maximum value and then somewhat decrease. Strength decreasing region could be attributed to the too much reduction expansion, and therefore it is suggested that the mechanical damage by the chemical expansion affected the decreasing the fracture strength. In relatively small reduction expansion region, it is expected that the homogenous structure by partial cation reduction of Ce4+ to Ce3+ plays a dominant role in improving a resistance for crack extension.

Influence of Ag on Chemical and Thermal Compatibility of LSCF-SDCC for LT-SOFC

In addition to the good electrochemical performance criteria in solid oxide fuel cell (SOFC) applications, cathode material must match thermal expansion with other SOFC components. Thus, effects of Ag on thermal mismatch, chemical reactions, and microstructure are investigated. Ag (1 wt% to 5 wt. %) was mixed with La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF6428) and Sm-doped ceria carbonate (SDCC) composite cathode powder. LSCF6428-SDCC-Ag samples were sintered at 600 °C for 2 h. The thermal expansion coefficients (TECs), which were determined using a dilatometer, indicated relatively less TEC mismatch between LSCF-SDCC-Ag cathodes composite and SDCC electrolyte. The average TEC value obtained from 20 °C to 600 °C implied that LSCF-SDCC-A5 (5 wt. % Ag) showed better thermal matching (13.18×10−6 K−1) with SDCC electrolyte (12.84×10−6 K−1) and achieved better compatibility. The X-ray diffraction patterns indicated that the LSCF6428-SDCC-Ag peak increased with the increase in the amount of Ag. Scanning electron microscopy analysis showed that Ag was capable of maintaining the porosity that is required for cathodes (20%–40%). Results showed that Ag exhibited desirable thermal and chemical compatibility with LSCF-SDCC. Thus, LSCF6428-SDCC-Ag can be used as a composite cathode for low-temperature SOFCs.

Development of bilayer glass-ceramic SOFC sealants via optimizing the chemical composition of glasses—a review

Glasses and glass-ceramics (GCs), in particular alkaline-earth alumino silicate-based compositions, are becoming the most common sealing materials for gas-tight sealing applications in solid oxide fuel cells (SOFCs). The present review aims at reporting the systematic procedures put forward developing a novel concept of diopside-based bilayer GC seal, which contains a rigid GC and a self-healing (SH) GC. The concept behind the bilayer GCs is (i) a small gradient in the coefficient of thermal expansion (CTE) will lead to lower thermal expansion mismatch between the sealing layers and other SOFC components, thus providing enhanced mechanical reliability for the stack; and (ii) cracks produced due to minor thermal stresses in the rigid GC layer can be healed by the SH GC layer due to sufficient amorphous content. In general, at high temperature, highly crystallized glass behaves as a rigid glass. On the other hand, due to low viscosity behavior, partially crystallized glass provides a SH behavior. Various glasses in the field of diopside crystalline materials have been systematically designed by varying the chemical composition of glass to achieve desired combination of functional properties for the rigid and SH GC layers. The glass Sr-0.3 where Sr replaced 30 % of Ca was revealed as a highly reliable rigid GC seal for high-temperature electrochemical applications. On the other hand, SH features have been achieved in 30 % Sr-containing diopside-based glass with Gd2O3 for MgO + SiO2 substitution (denoted as Gd-0.3). These GCs exhibit similar thermal properties and excellent thermal stability along a period of 1000 h, while differing in their amorphous fractions, and revealed excellent thermal stability along a period of 1000 h. The bilayered GC synthesized from Sr-0.3 and Gd-0.3 showed good wetting and bonding ability to the SOFC metallic Crofer22APU components. The results revealed superior performance for the newly proposed bilayer GCs in comparison to single-layer sealants.