Constrained Sintering in Fabrication of Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are inevitably affected by the tensile stress field imposed by the rigid substrate during constrained sintering, which strongly affects microstructural evolution and flaw generation in the fabrication process and subsequent operation. In the case of sintering a composite cathode, one component acts as a continuous matrix phase while the other acts as a dispersed phase depending upon the initial composition and packing structure. The clustering of dispersed particles in the matrix has significant effects on the final microstructure, and strong rigidity of the clusters covering the entire cathode volume is desirable to obtain stable pore structure. The local constraints developed around the dispersed particles and their clusters effectively suppress generation of major process flaws, and microstructural features such as triple phase boundary and porosity could be readily controlled by adjusting the content and size of the dispersed particles. However, in the fabrication of the dense electrolyte layer via the chemical solution deposition route using slow-sintering nanoparticles dispersed in a sol matrix, the rigidity of the cluster should be minimized for the fine matrix to continuously densify, and special care should be taken in selecting the size of the dispersed particles to optimize the thermodynamic stability criteria of the grain size and film thickness. The principles of constrained sintering presented in this paper could be used as basic guidelines for realizing the ideal microstructure of SOFCs.

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title: Materials and technologies for SOFC-components
abstract: Solid oxide fuel cells are a forward looking technology for a highly efficient, environmental friendly power generation. A SOFC is a multilayer structure consisting of ceramic and metallic materials with different electrical transport properties. All components have to show a well adjusted thermal expansion behavior, chemical compatibility of material interfaces and chemical stability in the prevailing atmospheres. The performance of SOFC single cells is not only determined by intrinsic material properties. There is a significant influence due to the fabrication technology respectively the microstructure at the electrode/electrolyte-interfaces. The performance of cells can only be improved by the application of elevated materials using appropriate technologies.

title: A numerical tool to estimate SOFC mechanical degradation: Case of the planar cell configuration
abstract: The stress state and cell degradation have been modelled considering the classical solid oxide fuel cells (SOFCs) materials (Ni–YSZ//YSZ//LSM). The anode-supported cell is found to be un-cracked after its elaboration. However, the high level of energy stored in the thin electrolyte layer lowers the cell robustness as it constitutes the driving force for interfacial delamination. The elaboration of the electrolyte supported cell leads to the anode layer cracking. The heating up to 800 °C and the cermet reduction allow relaxing the stress level in the cell. For the anode-supported cell, the cermet re-oxidation induces cathode damage as soon as the anodic strain exceeds 0.05–0.09% whereas the electrolyte fracture occurs for strain ranging between 0.12% and 0.15%. For the electrolyte supported cell, the cermet re-oxidation induces a triggered risk of anode/electrolyte delamination when the anodic strain is higher than 0.3–0.35%. The risk of rupture initiation in cell singularities has been also investigated. A pure material electrode/electrolyte singularity is harmless. A perpendicular corner at the electrode/electrolyte free edge constitutes a harmful singularity.

title: Materials and concepts for solid oxide fuel cells (SOFCs) in stationary and mobile applications
abstract: Solid oxide fuel cells (SOFCs) are a forward-looking technology for highly efficient, environmentally–friendly power generation. The electrochemically active single cell is a multilayer structure consisting of ceramic and metallic materials with different electrical transport properties. Materials for electrodes should exhibit electronic and ionic conductivity in combination with porosity and catalytic activity, whereas the electrolyte has to be a purely ionically conducting and gas-tight membrane. All components have to show well-adjusted thermal expansion behavior, chemical compatibility of material interfaces and chemical stability in the prevailing temperature and gas atmosphere. The performance and long-term stability of the single cell is significantly increased by the use of suitable materials, a proper design of the cell and an optimized microstructure at the electrode/electrolyte interfaces. The application of appropriate technologies for the production of single cells with optimized microstructure becomes even more important for highly efficient SOFCs operating in the medium and low temperature range.

title: Effect of internal and external constraints on sintering behavior of thin film electrolytes for solid oxide fuel cells (SOFCs)
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title: Processing‐Related Fracture Origins: II, Agglomerate Motion and Cracklike Internal Surfaces Caused by Differential Sintering
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title: Thin film yttria-stabilized zirconia electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs) by chemical solution deposition
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title: A high performance ceria based interdiffusion barrier layer prepared by spin-coating
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title: Simulation of a composite cathode in solid oxide fuel cells
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title: Development of LSCF–GDC composite cathodes for low-temperature solid oxide fuel cells with thin film GDC electrolyte
abstract: La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) powder was prepared by glycine–nitrate combustion method. The electrochemical properties of porous LSCF cathodes and LSCF–Gd 0.1 Ce 0.9 O 1.95 (GDC) composite cathodes were evaluated at intermediate/low temperatures of 500–700 °C. The polarization resistance of pure LSCF cathode sintered at 975 °C for 2 h was 1.20 Ω cm 2 at 600 °C. The good performance of pure LSCF cathode is attributed to its unique microstructure—small grain size, high porosity and large surface area. The addition of GDC to LSCF cathode further reduced the polarization resistance. The lowest polarization resistance of 0.17 Ω cm 2 was achieved at 600 °C for LSCF–GDC (40:60 wt%) composite cathode. An anode-supported solid oxide fuel cell (SOFC) was prepared using LSCF–GDC (40:60 wt%) composite as cathode, GDC film (49-μm-thick) as electrolyte, and Ni–GDC (65:35 wt%) as anode. The total electrode polarization resistance was 0.27 Ω cm 2 at 600 °C, which implies that LSCF–GDC (40:60 wt%) composite cathode used in the anode-supported SOFC had a polarization resistance lower than 0.27 Ω cm 2 at 600 °C. The cell generated good performance with the maximum power density of 562, 422, 257 and 139 mW/cm 2 at 650, 600, 550 and 500 °C, respectively.

title: Fabrication of thin-film gadolinia-doped ceria (GDC) interdiffusion barrier layers for intermediate-temperature solid oxide fuel cells (IT-SOFCs) by chemical solution deposition (CSD)
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title: Extremely thin bilayer electrolyte for solid oxide fuel cells (SOFCs) fabricated by chemical solution deposition (CSD).
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title: Characterization of electrical properties of GDC doped A-site deficient LSCF based composite cathode using impedance spectroscopy
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title: Quantitative three-dimensional microstructure of a solid oxide fuel cell cathode
abstract: Solid oxide fuel cells (SOFCs) are being actively developed world wide for clean and efficient electrical generation from fuels such as natural gas, hydrogen, coal, and gasoline. The cathode in state of the art SOFCs is typically a porous composite of electronically-conducting La 1− x Sr x MnO 3 (LSM) and ionically-conducting Y 2 O 3 -stabilized ZrO 2 (YSZ) that facilitates the critical oxygen reduction reaction. Here we describe the three-dimensional characterization and quantification of key structural parameters from an LSM-YSZ cathode, using imaging and volume reconstruction based on focused ion beam – scanning electron microscopy. LSM-YSZ-pore three-phase boundaries (TPBs) were identified. Approximately 1/3 of the TPBs were found to be electrochemically inactive, as they were on isolated LSM particles, yielding an active TPB density of 4.9 μm −2 . Cathode electrochemical modeling, which included a measured YSZ tortuosity of 3.4, yielded an effective TPB resistance of ≈2.5 × 10 5  Ω cm at 800 °C.

title: Inclusion size and sintering of composite powders
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title: Analysis of Sintering of a Composite with a Glass or Ceramic Matrix
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title: Viscoelastic stresses and sintering damage in heterogeneous powder compacts
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title: Effect of Inclusions on Densification: II, Numerical Model
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title: Influence of thickness on the constrained sintering of alumina films
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title: Anisotropic Microstructural Development During the Constrained Sintering of Dip‐Coated Alumina Thin Films
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title: Constrained sintering: A delicate balance of scales
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