Planar SOFC

There are two types of SOFC planar type and tubular type. An SOFC operates at 800 °C or higher temperatures. Although it can operate with high power density due to its high temperature, the stability of the component materials should be checked for a longtime operation. 

Figure 38.3 (a) CH4 fuel processing for the SOFC and PEMFC. (b) Two types of SOFCs planar SOFC and tubular SOFC and typical three-phase boundary at the anode and the cathode. 

Fiffure 2.14 Different cross-Jlow designs for SOFC planar configuration containing ahernale fuel and air lagers (1 986 and 1989)... 

Figure C shows an electron photomicrograph of a broken planar SOFC. The thick portion on the left is the porous anode structure. This is an anode-supported cell, meaning that in addition to collecting current and supporting the anode reaction, the anode layer stiffens the whole cell. The layer on the right is the cathode, and the interface between the two is the thin electrolyte. One of the challenges of this design is to ensure that the rates of expansion of the cathode and the anode match. If the anode expands faster than the cathode, the planar cell tends to curl like a potato chip when the temperature changes.

The slurry process has been enhanced with vacuum to fabricate planar SOFCs [78], This method is of low cost and thus has been widely used to develop low-cost SOFCs. However, together with other liquid precursor methods such as sol-gel and spray pyrolysis, it is time, labor, and energy intensive because the coating-drying-sintering has to be repeated in order to avoid cracking formation. 

Ivers-Tiffee E, Wersing W, SchieBl M, and Greiner H. Ceramic and Metallic Components for a Planar SOFC. Berichte der Bunsen-Gesellschaftfur Physikalische Chemie 1990 94 978-981. 

Basu RN, Blafi G, Buchkremer HP, Stover D, Tietz F, Wessel E, et al. Fabrication of simplified anode supported planar SOFCs—a recent attempt. In Yokokawa H, Singhal SC, editors. Proceedings of the Seventh International Symposium on Solid Oxide Fuel Cells (SOFC-VII), Pennington, NJ The Electrochemical Society, 2001 2001(16) 995-1001. 

Wen T-L, Wang D, Chen M, Tu H, Lu Z, Zhang Z, Nie H, and Huang W. Material research for planar SOFC stack. Solid State Ionics 2002 148 513-519. 

Historically, one of two techniques has typically been used to seal a planar SOFC stack glass joining or compressive sealing. Glass was originally used because it is simple to make and apply. The first requirement for a rigid seal is that the seal s... 

FIGURE 5.1 Schematics of edge sealing of planar cells (above) and external gas manifold seals (below) used for a simple cross-flow SOFC stack design. 

SOFC electrodes are commonly produced in two layers an anode or cathode functional layer (AFL or CFL), and a current collector layer that can also serve as a mechanical or structural support layer or gas diffusion layer. The support layer is often an anode composite plate for planar SOFCs and a cathode composite tube for tubular SOFCs. Typically the functional layers are produced with a higher surface area and finer microstructure to maximize the electrochemical activity of the layer nearest the electrolyte where the reaction takes place. A coarser structure is generally used near the electrode surface in contact with the current collector or interconnect to allow more rapid diffusion of reactant gases to, and product gases from, the reaction sites. A typical microstructure of an SOFC cross-section showing both an anode support layer and an AFL is shown in Figure 6.4 [24],... 

Both tubular and planar SOFCs are typically fabricated using one of the cell layers as the structural support layer with a fairly large thickness, on the order of millimeters or hundreds of micrometers, with the other components present as thinner layers of 10s of micrometers for the electrodes and 5 to 40 micrometers for the electrolyte. 

Planar SOFCs have received increasing attention recently as an alternative to tubular cells due to their higher power densities, short current paths, and corresponding... 

In planar SOFCs, individual cathode, anode, and electrolyte layers have been deposited by PS [109-111], as well as coatings on interconnect materials and full cells [108, 110, 112]. In addition to the interconnect layers themselves in tubular SOFCs, dense protective layers with good adhesion have also been deposited to protect planar SOFC interconnects from oxidation [110], and diffusion barriers to inhibit inter-diffusion between the interconnects and anodes have been produced by PS [113]. 

Wang Z, Qian J, Cao J, Wang S, and Wen TJ. A study of multilayer tape casting method for anode-supported planar type solid oxide fuel cells (SOFCs). J. Alloys Compounds 2007 437 264—268. 

SOFC are produced with either tubular or planar stack configurations investments for planar design are a rough estimate, as no prototypes exist. Specific investments for PAFC are in the range 4000- 4500/kW (IEA, 2007). For further fuel-cell R D needs see IEA (2005). 

SOFCo, a limited partnership of Babcock and Wilcox (a McDermott International Company) and Ceramatec (an Elkem company), has tested a planar SOFC unit for the MEP program that will operate on logistic fuels. Honeywell tested their MEP unit on logistic fuel. 

Active technology development efforts in both PEFC and planar SOFC technology, driven primarily by the interest in distributed generation and automotive propulsion markets, have achieved significant progress in the development of these technologies. For distributed power applications refined and even early commercial prototypes are being constructed. 

All are focused on small to medium sized distributed generation and on-site generation markets. Only Global Thermoelectric (Calgary, Canada) has been active in the application of its technology to APUs. A recently conducted a detailed conceptual design and cost estimate of a 5-kWnet SOFC-based truck APU conclude that, provided continued improvement in several technology areas, planar SOFCs could ultimately become a realistic option for this mass-market application. 

Arthur D. Little has carried out cost structure studies for a variety of fuel cell technologies for a wide range of applications, including SOFC tubular, planar and PEM technologies. Because phenomena at many levels of abstraction have a significant impact on performance and cost, they have developed a multi-level system performance and cost modeling approach (see Figure 1-15). At the most elementary level, it includes fundamental chemical reachon/reactor models for the fuel processor and fuel cell as one-dimensional systems. 

The main difference in SOFC stack cost structure as compared to PEFC cost relates to the simpler system configuration of the SOFC-based system. This is mainly due to the fact that SOFC stacks do not contain the type of high-cost precious metals that PEFCs contain. This is off-set in part by the relatively complex manufacturing process required for the manufacture of the SOFC electrode electrolyte plates and by the somewhat lower power density in SOFC systems. Low temperature operation (enabled with electrode supported planar configuration) enables the use of low cost metallic interconnects which can be manufactured with conventional metal forming operations. 

Recently, the planar structures using bipolar current collection have received more consideration for SOFCs because of new gas sealing and fabrication techniques. 

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