Chemical stability of glass seal interfaces in intermediate temperature solid oxide fuel cells

In intermediate temperature planar solid oxide fuel cell (SOFC) stacks, the interconnect, which is typically made from cost-effective, oxidation-resistant, high-temperature alloys, is typically sealed to the ceramic positive electrode-electrolyte-negative electrode (PEN) by a sealing glass. To maintain the structural stability and minimize the degradation of stack performance, the sealing glass has to be chemically compatible with the PEN and alloy interconnects. In the present study, the chemical compatibility of a barium-calcium-aluminosilicate (BCAS) based glass-ceramic (specifically developed as a sealant in SOFC stacks) with a number of selected oxidation resistant high temperature alloys (and the yttria-stabilized zirconia electrolyte) was evaluated. This paper reports the results of that study, with a particular focus on Crofer22 APU, a new ferritic stainless steel that was developed specifically for SOFC interconnect applications. This paper was presented at the Fuel Cells: Materials, Processing, and Manufacturing Technologies Symposium sponsored by the Energy/Utilities Industrial Sector & Ground Transportation Industrial Sector and the Specialty Materials Critical Technologies Sector at the ASM International Materials Solutions Conference, October 13–15, 2003, in Pittsburgh, PA. The symposium was organized by P. Singh, Pacific Northwest National Laboratory, S.C. Deevi, Philip Morris USA, T. Armstrong, Oak Ridge National Laboratory, and T. Dubois, U.S. Army CECOM.     

Critical issues in catalytic diesel reforming for solid oxide fuel cells

Developing low-cost diesel-reforming catalysts and improving fuel mixing prior to catalytic reforming were addressed as two critical issues under the current study. Ruthenium-doped lanthanum chromite and aluminite were explored as catalysts for the autothermal reforming of diesel fuel. Dodecane was used as a surrogate fuel. Both catalysts yielded nearly 20 moles of hydrogen per mole of dodecane at oxygen-to-carbon ratios of 0.5 and steam-to-carbon ratios of 2 at space velocities near 100,000/h⁻¹. Both catalysts were shown to have good S tolerance when tested with a fuel mixture containing 50 parts per million S in the form of dibenzothiophene. Parallel to catalyst development, the impact of fuel mixing and vaporization through improved liquid injection also is under investigation. This paper was presented at the ASM Materials Solutions Conference & Show held October 18–21, 2004 in Columbus, OH.     

Designing sealing glasses for solid oxide fuel cells

Thermal and chemical properties of “invert” glasses and glass-ceramics developed for hermetic seals for solid oxide fuel cells are described. The glasses crystallize to form thermally stable pyro- and orthosilicate phases with the requisite thermal expansion match to the Y-stabilized ZrO₂ (YSZ) electrolyte. In addition, the glasses bond to Cr-steel substrates at 800–850 °C without forming extensive interfacial reaction products. The thermal expansion characteristics of the glass-ceramics remain essentially unchanged after 28 days at 750 °C. Compositions with lower (≤2 mol%) B₂O₃ contents exhibit the lowest volatilization rates when exposed to wet forming gas at 750 °C. This paper was presented at the ASM Materials Solutions Conference & Show held October 18–21, 2004 in Columbus, OH.     

固体氧化物燃料电池阴极材料第一性原理研究进展

固体氧化物燃料电池(SOFC)具有能量转换效率高和燃料适应性广等突出优势,被认为是未来最有前景的清洁能源技术之一。目前SOFC研究热点是降低工作温度到500～800℃中低温区,以降低运行成本、增加可靠性,进而加速SOFC的商业化进程。阴极作为SOFC的重要组元,合理的设计和优化中低温下对氧还原反应具有较高催化活性的阴极材料至关重要。具有钙钛矿结构或由钙钛矿结构衍生出的层状结构的电子-离子混合导电型（MIECs）氧化物是目前研究最多的SOFC阴极材料。第一性原理可以弥补实验方面信息的缺失,能够提供电子结构、几何参数、吸附能及过渡态等相关信息,可以为合理设计和开发高性能的新型SOFC阴极材料提供科学依据和理论指导。本文通过对钙钛矿阴极氧空位的形成及迁移,氧分子在阴极（包括贵金属引入）表面上的吸附、解离、扩散过程及其规律进行了综述并总结了我们前期的研究成果,最后针对当前研究存在的问题及今后钙钛矿阴极的计算模拟研究方向进行了总结与展望。     

Selection and surface treatment of alloys in solid oxide fuel cell systems

Cost and performance considerations determine the selection of various component materials in a solid oxide fuel cell power system. While the use of commercial alloys provides an opportunity for cost reduction, the requirements of different components vary widely. The interconnect materials must provide low electrical resistance while isolating the oxidizing and reducing gases. The air preheat and heat-exchanger components face high-temperature oxidizing conditions, while the fuel feed and reformer sections encounter highly reducing atmospheres that may contain varying levels of sulfur and CO, and thus are prone to sulfidation and metal dusting. Thus, each of the components requires both a judicious selection of the alloy composition and appropriate surface treatments. Coating processes were developed for two classes of alloy materials for potential use as interconnects and fuel feed hardware or process piping. This paper was presented at the ASM Materials Solutions Conference & Show held October 18–21, 2004 in Columbus, OH.     

New sealing concept for planar solid oxide fuel cells

A key element in developing high-performance planar solid oxide fuel-cell stacks is the hermetic seal between the metal and ceramic components. Two methods of sealing are commonly used: (a) rigid joining and (b) compressive sealing. Each method has its own set of advantages and design constraints. An alternative approach is currently under development that appears to combine some of the advantages of the other two techniques, including hermeticity, mechanical integrity, and minimization of interfacial stresses in the joint substrate materials, particularly the ceramic cell. The new sealing concept relies on a plastically deformable metal seal; one that offers a quasi-dynamic mechanical response in that it is adherent to both sealing surfaces, i.e., non-sliding, but readily yields or deforms under thermally generated stresses. In this way, it mitigates the development of stresses in the adjacent ceramic and metal components even when a significant difference in thermal expansion exists between the two materials. The pre-experimental design of the seal, initial proof-of-principle results on small test specimens, and finite-element analyses aimed at scaling the seal to prototypical sizes and geometries are described herein. This paper was presented at the ASM Materials Solutions Conference & Show held October 18–21, 2004 in Columbus, OH.     

Ceramic materials containing rare earth oxides for solid oxide fuel cell

Materials for a solid oxide fuel cell were investigated aiming especially at low temperature operation of the cell. Although yttria-stabilized zirconia has been most popularly investigated as an electrolyte for the cell, the conductivity reaches the allowable level only around or higher than 1000 °C. The use of a ceria-based electrolyte, especially samaria doped ceria, significantly lowered the operation temperature of the cell due to its high oxide ion conductivity. The reduction of ceria with H₂ and resultant electronic conduction could be avoided by the coating of YSZ on to the anode side of the ceria. The ceria layer facing the air electrode is effective in reducing cathodic polarization. Ni-ceria cermet exhibited higher fuel electrode performance than Ni-YSZ cermet in lowering polarization.     

High-temperature seals for solid oxide fuel cells (SOFC)

A functioning solid oxide fuel-cell (SOFC) may require all types of seals, such as metal-metal, metal-ceramic, and ceramic-ceramic. These seals must function at high temperatures between 600 and 900 °C and in the oxidizing and reducing environments of fuels and air. Among the different types of seals, the metal-metal seals can be readily fabricated using metal joining, soldering, and brazing techniques. However, metal-ceramic and ceramic-ceramic seals require significant research and development because the brittle nature of ceramics/glasses can lead to fracture and loss of seal integrity and functionality. Consequently, any seals involving ceramics/glasses also require significant attention and technology development for reliable SOFC operation. This paper is prepared to primarily address the needs and possible approaches for high-temperature seals for SOFC and seals fabricated using some of these approaches. A new concept of self-healing glass seals is proposed for making seals among material combinations with a significant expansion mismatches. This paper was presented at the ASM Materials Solutions Conference & Show held October 18–21, 2004 in Columbus, OH.     

Evaluation of ferritic stainless steel for use as metal interconnects for solid oxide fuel cells

Interconnect development for planar solid oxide fuel cells is considered a vital technical area requiring focused research to meet the performance and cost goals. A commercial ferritic stainless steel composition for oxidation resistance properties was investigated by measuring the weight gain due to air exposure at fuel cell operating temperature. A surface treatment process was found to produce a dense, adherent scale and to reduce the oxide scale growth rate significantly. A process was also identified for coating the surface of the alloy to reduce the in-plane resistance and potentially to inhibit chromium oxide evaporation. The combination of treatments provided a very low resistance through the scale. The resistance measured was as low as 10 mΩ-cm² air. The resistance value was stable over several thermal cycles. The treated samples were exposed to a variety of atmospheres that were relevant in fuel cell operation to evaluate changes in scale morphology. Analysis of the scale after such exposure showed the presence of a stable composition. When exposed to a dual atmosphere (air and hydrogen on opposite sides of the metal sheet), however, the scale composition contained a mixture of phases. Additional process modifications are planned to reduce the effect of dual-atmosphere exposure. This paper was presented at the Fuel Cells: Materials, Processing, and Manufacturing Technologies Symposium sponsored by the Energy/Utilities Industrial Sector & Ground Transportation Industrial Sector and the Specialty Materials Critical Technologies Sector at the ASM International Materials Solutions Conference, October 13–15, 2003, in Pittsburgh, PA. The symposium was organized by P. Singh, Pacific Northwest National Laboratory. S.C. Deevi, Philip Morris USA, T. Armstrong, Oak Ridge National Laboratory, and T. Dubois, U.S. Army CECOM.     

Development of metal supported solid oxide fuel cells for operation at 500–600 °C

A novel metal-supported solid oxide fuel cell has been developed that is capable of operating at temperatures of 500–600 °C. The rationale behind the materials used to construct this fuel cell type is given, and results are presented from cell testing on hydrogen and reformed natural gas, including durability trials of some 2500 h duration. This new fuel cell variant is shown to be tolerant of carbon monoxide, durable, robust to thermal and redox cycling, and capable of delivering technologically relevant power densities. This paper was presented at the Fuel Cells: Materials, Processing, and Manufacturing Technologies Symposium sponsored by the Energy/Utilities Industrial Sector & Ground Transportation Industrial Sector and the Specialty Materials Critical Technologies Sector at the ASM International Materials Solutions Conference, October 13–15, 2003, in Pittsburgh, PA. The symposium was organized by P. Singh, Pacific Northwest National Laboratory, S.C. Deevi, Philip Morris USA, T. Armstrong, Oak Ridge National Laboratory, and T. Dubois, U.S. Army CECOM.     

Gas phase deposition of diffusion barriers for metal substrates in solid oxide fuel cells

One way to improve the mechanical properties of solid oxide fuel cells is the development of metal supported designs. This type of SOFC offers improved thermal shock resistance, reduced temperature gradients due to the greater thermal conductivity of the metal, and lower operating temperatures. Switching from ceramic supports to metal supports also allows the uses of conventional metal joining and forming techniques and could significantly reduce the material and manufacture costs. However, one persistent problem needs to be solved: inter-diffusion of chemical elements contained in the metal substrates and in the anodes of SOFC leads to degradation, which is to be prevented by protective coatings. In order to address the issues of sintering and delamination for metal supported SOFC, the deposition of gadolinia doped ceria on metal substrates made of Crofer 22 APU has been done by electron beam evaporation and reactive spray deposition technique, as two direct deposition techniques that will not require a sintering step, respectively. Additionally, the effect of ion-assistance on layers made by electron beam evaporation was studied. Because metal supported fuel cells aim at low/intermediate operating temperatures, reducing the thickness of these protective coatings is crucial, since layer thickness is directly correlated to its ohmic resistance. A layer of nickel was applied by magnetron sputtering to prove the effectiveness of the deposited diffusion barrier layers.     

Dip coating technique in fabrication of cone-shaped anode-supported solid oxide fuel cells

A simple and cost-effective dip coating technique was successfully developed to fabricate NiO-YSZ anode substrates for cone-shaped anode-supported solid oxide fuel cells. A single cell, NiO-YSZ/YSZ/LSM-YSZ, was assembled and tested to demonstrate the feasibility of the technique applied. Using humidified hydrogen (75 ml/min) as fuel and ambient air as oxidant, the maximum power density of the cell was 0.78 and 1.0 W/cm² at 800 and 850 °C, respectively. The observed open-circuit voltages (OCV) was closed to the theoretical value and the scanning electron microscope (SEM) results revealed that the microstructures of the anode substrate and the cathode layer are porous and the electrolyte film is dense.     

High-temperature electrical testing of a solid oxide fuel cell cathode contact material

The development of high-temperature solid-state devices for energy generation and environmental control applications has advanced remarkably over the past decade. However, there remain a number of technical barriers that still impede widespread commercial application. One of these, for example, is the development of a robust method of conductively joining the mixed-conducting oxide electrodes that lie at the heart of the device to the heat resistant metal interconnect used to transmit power to or from the electrodes and electrochemically active membrane. This study investigated the high-temperature electrical and microstructural characteristics of a series of conductive glass composite paste junctions between two contact materials representative of those used in solid-state electrochemical devices, lanthanum calcium manganate, and 430 stainless steel. This paper was presented at the Fuel Cells: Materials, Processing, and Manufacturing Technologies Symposium sponsored by the Energy/Utilities Industrial Sector & Ground Transportation Industrial Sector and the Specialty Materials Critical Technologies Sector at the ASM International Materials Solutions Conference, October 13–15, 2003, in Pittsburgh, PA. The symposium was organized by P. Singh, Pacific Northwest National Laboratory, S.C. Deevi, Philip Morris USA, T. Armstrong, Oak Ridge National Laboratory, and T. Dubois, U.S. Army CECOM.     

A radiation-based approach to planar solid oxide fuel cell modules

This article describes the initial analysis underlying the design of a core module consisting of a 1 to 3 kW solid oxide fuel cell (SOFC) stack and a radiant air preheater (RAP) module. The design and testing of three SOFC stack/RAP modules was part of a California Energy Commission-sponsored project with the Gas Technology Institute. The objective of the design was to improve the thermal management of an SOFC system through radiant heat transfer from the stack walls to adjacent air preheater panels. The testing of this and subsequent modules has suggested that use of the radiation-based approach significantly improved the management of stack-generated heat. This paper was presented at the ASM Materials Solutions Conference & Show held October 18–21, 2004 in Columbus, OH.     

Integrated fabrication process for solid oxide fuel cells in a triple torch plasma reactor

This paper describes an approach for an integrated manufacturing process for solid oxide fuel cells. The approach is based on successively depositing the different layers of the cell using plasma deposition processes in a controlled-atmosphere chamber. Cells have been manufactured following this approach with minimal changes in process conditions for the different processes. The cells have been evaluated with regard to their materials characteristics and with regard to their electrical performance. The cell performance has been acceptable, with open circuit voltages of about 1 V and power densities between 325 and 460 mW/cm². Process modifications to improve the performance further are possible. The described process has the potential for being easily automated.     

Assembling single cells to create a stack: The case of a 100 W microtubular anode-supported solid oxide fuel cell stack

Microtubular solid oxide fuel cell systems have many desirable characteristics compared with their planar counterparts; however, there are many obstacles and difficulties that must be met to achieve a successful and economically viable manufacturing process and stack design. Anode-supported tubes provide an excellent platform for individual cells. They allow for a thin electrolyte layer, which helps to minimize polarization losses, to be applied to the outside of the tube, thus avoiding the difficulty of coating the inside of an electrolyte or cathode-supported tubes, or the stack design problem of having a fuel chamber if the anode is on the outside of the tube. This article describes the fabrication of a traditional (Ni-YSZ) anode tube via extrusion of a plastic mass through a die of the required dimensions. The anode tubes were dried before firing, and tests were performed on the tubes to determine the effects of prefiring temperature on porosity. The porous tubes had a vacuum applied to the inside while being submerged in aqueous electrolyte slurry. Various parameters were examined, including vacuum pressure, submergence time, and drying conditions, and were studied using microscopy. Cathode coatings (based on both doped lanthanum manganite and doped lanthanum cobaltite) were applied using a brush-painting technique, and were optimized as a function of paint consistency, drying conditions, and firing temperatures. The finished tubes were then stacked in an array to provide the specific current/voltage requirements, using a brazing technique. This article will describe the output characteristics of a single cell and a small stack (of 100 W designed power output). This paper was presented at the ASM Materials Solutions Conference & Show held October 18–21, 2004 in Columbus, OH.     

High-velocity DC-VPS for diffusion and protecting barrier layers in solid oxide fuel cells (SOFCs)

High-temperature fuel cells of the solid oxide fuel cell (SOFC) type as direct converter of chemical into electrical energy show a high potential for reducing considerably the specific energy consumption in different application fields. Of particular interest are advanced lightweight planar cells for electricity supply units in cars and other mobile systems. Such cells, in one new design, consist mainly of metallic parts, for example, of ferrite steels. These cells shall operate in the temperature range of 700 to 800 °C where oxidation and diffusion processes can be of detrimental effect on cell performance for long-term operation. Problems arise in particular by diffusion of chromium species from the interconnect or the cell containment into the electrolyte/cathode interface forming insulating phases and by the mutual diffusion of substrate and anode material, for example, iron and chromium from the ferrite into the anode and nickel from the anode into the ferrite, which in both cases reduces performance and system lifetime. Additional intermediate layers of perovskite-type material, (e.g., doped LaCrO₃) applied with high-velocity direct-current vacuum plasma spraying (DC-VPS) can reduce such effects considerably if they are stable and of high electronic conductivity. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Rupture testing as a tool for developing planar solid oxide fuel cell seals

One of the critical issues in designing and fabricating high-performance planar solid oxide fuel cell (pSOFC) stacks is the ability to hermetically seal adjacent metal and ceramic components. In our pSOFC development program, we have designed a testing technique that allows us to screen through the numerous variables involved in developing glass seals. Using this test for example, we have found that the composition of the metal component plays an important role in the strength of the seal. Microstructural analysis of as-sealed specimens revealed that an interfacial reaction zone forms during joining, and it appears that the thickness and composition of this layer are the dominant parameters that control joint strength. In this paper the details of the seal test are reported. The results have proven particularly significant in the development of the next-generation stack design. Supporting microstructural and chemical analyses collected on the test specimens are also presented and used to interpret the seal test results in an effort to identify the necessary steps toward improving glass pSOFC seals. This paper was presented at the Fuel Cells: Materials, Processing, and Manufacturing Technologies Symposium sponsored by the Energy/Utilities Industrial Sector & Ground Transportation Industrial Sector and the Specialty Materials Critical Technologies Sector at the ASM International Materials Solutions Conference, October 13–15, 2003, in Pittsburgh, PA. The symposium was organized by P. Singh, Pacific Northwest National Laboratory, S.C. Deevi, Philip Morris USA, T. Armstrong, Oak Ridge National Laboratory, and T. Dubois, U.S. Army CECOM.     

Near-net-shape forming of metallic bipolar plates for planar solid oxide fuel cells by induction plasma spraying

In one of the present designs of solid oxide fuel cells (SOFC), metallic bipolar plates with gas channels on the surface are used, which consist of a chromium alloy and are manufactured by a time consuming and costly multistep process. To reduce the production time and costs, attempts were made to develop an alternative near-net-shape production method based on RF-induction plasma spray technology. With this process raw powders, as applied for the “conventional” sintering route as well as recycled powders from used bipolar plates, have been applied. The process parameters were adapted to both powders, and the obtained products were qualified. The near-net-shape production requires the formation of a gas channel structure already with the spray process using structured substrates. Therefore, different spray angles occur during the deposition process. The influence of the spray angle on the microstructure of the free-standing parts was investigated. The required gas tightness for grooved profiles with relatively large channel depths and widths can only be achieved using spray angles between 90° and approximately 60°. Then a tilting of the substrate and an adapted design of the gas channel profiles are needed to fulfill the structural requirements for the bipolar plates.     

Evaluation of commercial alloys as cathode current collector for metal-supported tubular solid oxide fuel cells

Ferritic stainless steels have become promising candidate materials for interconnects in tubular metal-supported solid oxide fuel cell stacks. A number of ferritic alloys containing between 18 and 26 mass% Cr and discrete changes in minor alloying elements and reactive elements were isothermally oxidized at 800 °C in air and their electrical resistance was measured with the objective of obtaining an overview of the properties relevant for applications for cathode side interconnect. The alloys containing Mn showed a (Mn,Cr)₃O₄ spinel layer on top of a Cr₂O₃ oxide. The electrical conductivity of the steels forming this kind of oxide layer was higher than the measured for only Cr₂O₃ former or oxide dispersion strengthened alloys and increased when the alloy contained Ti or Nb. Oxide scale spallation was observed for F18TNb and E-Brite, both containing Si. The influence of different cyclic oxidations was studied for the Crofer22APU steel, showing an irregular oxide growth as well as an increase in conductivity of the oxide scale formed when 12-h cycles were applied.