Planar SOFCs

Planar solid oxide fuel cells are built analogously to other kinds of fuel cells, such as polymer electrolyte fuel cells. Usually, one of the electrodes (the fuel anode or the oxygen cathode) serves as support for the membrane-electrode assembly. To this end, it is relatively thick (up to 2 mm), and thin layers of the electrolyte and 

A large problem in high-temperature fuel cells is careful sealing. The gas compartments of the fuel and oxygen (or air) electrodes should be protected against the entry of the wrong gas. The joints and welds of all inner channels and gas manifolds should be free of gas leaks. For planar cells this is a much harder problem than for tubular cells, as their entire perimeter must be sealed. Also, planar cells usually have a larger number of gas feeds. 

Chemically and thermally resistant steels contain considerable quantities of chromium (more than 20%). On the side of the oxygen, cathode metalhc chromium is oxidized to Cr203. Depending on the temperature and oxygen partial pressure, this oxide may further oxidize to volatile compounds CrOg and Cr02(0H)2, which could then settle at the cathode/electrolyte interface and hinder oxygen reduction. Such an evaporation of chromium is the basic difficulty in the use of metallic bipolar plates. 

All the problems and difficulties fisted wifi arise precisely at the high working temperatures of conventional solid oxide fuel cells. For this reason, many research 

Considerable difficulties turned up, however, in the development of flat SOFCs and have so far not been definitely resolved, thus preventing broad commercialization of such cells. The difficulties are related primarily to the fact that the selection of materials having sufficient chemical and mechanical strength for operation at temperatures of 900 to 1000°C in the presence of oxygen and/or hydrogen is rather restricted. This holds true both for the materials of electrodes and electrolyte and for various structural materials. 

The major problems arising in the development and manufacturing of planar-type SOFC are described below. 

A large problem in high-temperature fuel cells is careful sealing. The gas compartments of the fuel and oxygen (or air) electrodes should be protected against 

In recent years, therefore, development of rigid and elastic sealants on the basis of metals and ceramics was initiated. By using multiphase materials, one can adjust the elastic properties and wetting of surfaces in contact. More details concerning the various sealing materials under study today may be found in a review by Fergus (2005). 

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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... 

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]. 

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. 

A. Khandkar, S. Elangovan, "Planar SOFC Development Status," Proceedings of the Second Annual Fuel Cells Contractors Review Meeting, U.S. DOE/METC, p. 152, May, 1990. 

A. Khandkar et al., "Planar SOFC Technology Status and Overview," Ceramatec, Inc., Fuel Cell Seminar Program and Abstracts, 1992 Fuel Cell Seminar, Tucson, AZ,... 

Figure 42. Various planar SOFC configurations (revised from ref 189).

The Australian company Ceramic Fuel Cells Limited (CFCL), a leader in planar SOFCs, is seeking to identify potential partners to create market-ready packages that would incorporate solid oxide fuel cells. 

The CORE-SOFC Project was designed to improve the durability of planar SOFC systems to a level acceptable for commercial operation. The work focuses mainly on materials selection for interconnects, contact layers and protective coatings to minimise corrosion between metallic and ceramic parts to achieve reliable and thermally-cyclable SOFCs. In all work packages, cells and stacks will be analysed by advanced chemical and ceramographic methods. 

Construct and evaluate a novel, planar SOFC stack of - 20 kWe that internally reforms natural gas and delivers an efficiency of > 50% (LHV). The stack should be capable of manufacture using a viable (if not yet proven) process, and with materials costs of less than US 300/kW 2005... 

Atmospheric Plasma Spray (Triplex APS). These deposition techniques will be used within this project to deposit the various layers in planar SOFC. 

Fig. 1.2 Principles, functions and schematic of a flat planar SOFC where the PEN is stacked between two bipolar plates (interconnect with gas flow fields).

Planar SOFC, in particular, monolithic designs (MHI) are capable of high (volumetric) power densities most favoured by direct and short current paths across the stack components. The PEN is principally square, rectangular and circular (Ceramic Fuel Cells Limited (CFCL), Mitsubishi Materials Corp., SulzerHexis) in shape with active surface areas of 100-200 cm2 (15.5-31 in2). A drawback of this design is that it often necessitates the use of high temperature sealants for application at the in-... 

The laminar flow assumption eliminates the non-linear term in the partial differential equations system (3.3), thus significantly reducing the computational cost. In addition, the present formulation often admits an exact solution. For example, in the case of an incompressible 2D laminar flow between two motionless parallel plates (i.e. planar SOFC configuration of Figure 3.1), Equation (3.29) reduces to ... 

Another simplification of the model, widely used when modelling SOFC operation, is to consider the Positive electrode/Electrolyte/Negative electrode (PEN), as a lumped structure. Figure 3. 4 shows a schematic representation of a planar SOFC, when the PEN structure is considered in a ID domain. 

Fig. 3.7 Current collector/gas channel of a flat planar SOFC.

When a planar SOFC is considered, expressions (3.75) and (3.76) are employed for each layer (i.e. anode, cathode and electrolyte), thus obtaining the PEN resistance ... 

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