Metal-supported cells

This entry is organized as follows In section Historical Aspects of Reliability, Durability and Cost Issues, historical aspects are first described to provide essential points of SOFC stack/system development. The technological features of the first-generation cells, namely, sealless tubular cells, will be described in comparison with the second- and the third-generation cells in critical technological issues these are materials selection of interconnect (oxide or metal), sealing scheme, redox issues of nickel cermets, metal support cells, trade-off relation between reliability and performance, and materials chemistry associated with lowering operation temperature. 

Metal support cells are proposed to realize cost reduction from the materials cost and fabrication cost simultaneously. Compared with nickel which is the major component in the anode support cells, the metal support cells aim at using cheaper Fe-Cr alloy as supporting materials [19]. Instead, the fabrication process becomes rather difficult compared with other cell design. 

The metal support cells can be regarded as the third generation. The major driving... 

Another major driving force for metal support cells is cost reduction [19]. For the anode-supported cells, a relatively large amount of nickel will be used because the nickel cermet becomes the major stmctural supporting component. To achieve a drastic change in row materials cost, therefore, cheaper alloys should be used as the major component. Note also that from the raw materials point of view, ceramics are more expensive than metals in general. 

Despite expected low cost, it is expected that the fabrication technology is much more difficult for metal support cells. This is illustrated in Fig. 18.3c. 

A planar, lightweight stack with metal-supported cells made by plasma spraying, as, for instance, developed by DLR [31], referred to in the following as lightweight plasma-sprayed design ... 

Each of the three functional layers of the cell, anode, electrolyte, and cathode, are deposited onto the substrate using screen printing. In practice, each of the functional components requires multiple layers to provide the necessary functionality and manufacturability. Unlike other SOFC designs and similar to the metal-supported cell, the IP-SOFC uses neither the electrolyte nor electrodes to provide stmctural support. This allows very thin cells with consequently very small material quantities (cf. previous section). The cell functional layers again are made of conventional SOFC materials yttria-stabilized zirconia electrolyte, nickel cermet anode, and rare earth... 

Metal interconnect-supported. Lawrence Berkeley National Laboratory (66), Argonne National Laboratory, and Ceres (67) have pioneered metal-supported cells to minimize mass transfer resistance and the use of (expensive) ceramic materials. In such cells, the electrodes are typically 50 im thick and the electrolyte around 5 tol5 im. While the benefits are obvious, the challenges are to find a materials combination and manufacturing process that avoids corrosion and deformation of the metal and interfacial reactions during manufacturing as well as operation. 

Metal-supported cell Since metal-supported cells are still in the early stages of development, there is no defined stack design associated with metal-supported cells. There is a possibility of achieving gas tightness without using sealing materials. 

Metal-Supported Cells Next Generation SOFC Technology... 

A metal-supported cell and stack concept is ideally based upon a low-cost stainless steel cell support and low-cost stainless steel thin metallic sheet interconnect bonded together with a cost-effective metalfic brazing or welding technique. The concept substimtes brittle ceramics with ductile metallic components and thereby to achieve a desirable, graceful non-catastrophic failure mode of the cells and stacks. The main issue addressed in the development of this concept is the simultaneous achievement of robustness, rehabUity and cost-effectiveness offered by the metalfic materials as well as the high electrochemical performance offered by next generation electrode development based on nano-structured materials. 

TOFC has in collaboration with Ris0/DTU a focussed commitment to develop next generation stack technology based on metal-supported cells to improve reliability, cost-effectiveness and functionality including the objective to reach a robusmess of SOFC stacks which fulfils the requirements under real system load. 

The cell design includes an anode structure, in which nano-sized ceria and nickel particles are formed by solution infiltration in the porous microstructure after fabrication of the half cells. The multilayered structure, comprising the metal supported cell, can be obtained by conventional processing techniques such as tape casting, co-sintering and screen printing. 

The observed performance and the measured degradation rates over 1,000 h for the button cell test and the 5 x 5 cm test are reasonably close and indicate good correlation between the two different test set-up and contacting protocols. The cell performance results show generally lower or at least comparable ASR values than that of many other metal supported SOFCs presented so far in the literature (see e.g. Ref. [31] and references therein). Recently, the TOFC stack with 12 x 12 cm cell footprint has been modified with metal-supported cells and showed performance results close to those for the single cells. 

In case of a metal-supported cell concept, some new important challenges appear that must be overcome primarily concerning materials compatibility, materials processing and interaction, inter-diffusion and vaporisation of metallic alloy elements and finally hot corrosion/oxidation and high temperature creep of the metallic parts. Due to hot corrosion of the metallic layers in the cell the operation temperature has to be lowered to around 600-750 °C, which, on the other hand, leads to new challenges in the development of more active electrode materials. Recently, the METSOFC project has led to a significant progress in performance and durability of the metal-supported cells. 

Metal supported cells, 218 Mixed ionic-electronic, 48, 51, 52... 

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