Planar SOFC, structure

Anodes for SOFC are again based on Ni, usually Ni cermet materials are used that are more stable than plain Ni metal. NiO anodes are slightly soluble in YSZ electrolyte, but this stabilizes the cubic phase of the electrolyte. A NiO powder mixed with a YSZ powder together with a resin binder produces an anode functional layer onto which YSZ electrolyte can be deposited and sintered. The cathode can then be sprayed onto this layer and form an anode-supported planar SOFC structure [33]. YSZNi anodes can also be produced by vacuum plasma spraying. To fit the thermal expansion mismatch that can occur between the anode and the electrolyte, a zirconia-stabilized anode is preferable. The performance of plasma-sprayed electrodes is similar to that of the more common screen-printed anodes [37]. 

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.

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. 

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. 

From the structure point of view, there are two types of structures of SOFC tubular and planar. Tubular SOFCs have shown some desirable characteristics over systems with planar SOFCs [4]. Tubular SOFCs can alleviate the sealing problem arose by CTE mismatch of planar SOFC therefore, they are robust for repeated cycling under rapid changes in electrical load and in cell operating temperatures. The large form factor tubular SOFC built by Siemens Westinghouse has successfully conducted long-term operation over 70,000 h. Small-scale tubular SOFCs could... 

In planar SOFC, the anode, electrolyte, and the cathode are sandwiched together their geometry is planar. The cells were assembled by stacking planar cells and rigid planar interconnectors alternately. The typical structure of a tabular SOFC is shown in Figure 5-2. 

In the same year, Weil et al. [13] introduced a sealing concept for planar SOFCs. The finite element method (FEM) was used to aid in scaling up a bonded compliant sealant design to a 120 x 120 mm component. The stresses of the cell, foil, brazes, and frame were calculated and compared with experimental fracture and yield stress results. A quarter symmetrical model was used. The commercial software AN SYS was utilized. The tensile stress of the component was predicted, considering thermal cycling from elevated temperature to room temperature. The materials used were mentioned, but no properties were given. Regarding the structural analysis boundary conditions and the failure criteria employed, material models were not depicted. 

These devices feature the use of micro-fabrication techniques adapted from the micro-electronics industry. These encompass substrate etching, thin-fihn deposition, lithography, and film-etching steps. This field has recently been reviewed by Evans et al. [11], and all devices exhibit the beautiful structural quality resulting from the micro-fabrication techniques. An example of a micro-planar SOFC fabricated on a silicon substrate is illustrated in Fig. 19.4 [12]. Figure 19.4a shows the sequence of fabrication steps used to make the edge-supported SOFC membrane which spans an aperture with dimensions 600 x 600 pm. The yttria stabilized zirconia (YSZ) electrolyte, which is only 70-nm thick, was deposited by... 

Planar SOFCs are generally manufactured in three different configurations depending on the structure-supported cell element and operating temperature range as shown in Figure 9.19. These configurations are referred to as (i) electrolyte-supported cell with thick electrolyte layer, (ii) anode-supported cell with thick anode layer, and (iii) cathode-supported cell with thick cathode layer. 

The stainless steel is composed of chromium and Fe-C base alloys which are used due to their pitting resistance. To maintain a body centred cubic ferritic structure, a minimum of 13% chromium is required. Stainless steels can be categorised into duplex, austenitic, ferritic, and martensitic. Carbon and nitrogen are austenitic formers and characterised by a y-austenitic face-centred cubic. For planar SOFC, the stainless steels should contain Cr content 17% considering the fact that chromium may be depleted by interaction with adjoining components. 

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. 

Fig. 1.6 Illustration of a planar-stack, solid-oxide fuel cell (SOFC), where an membrane-electrode assembly (MEA) is sandwiched between an interconnect structure that forms fuel and air channels. There is homogeneous chemical reaction within the flow channels, as well as heterogeneous cehmistry at the channel walls. There are also electrochemical reactions at the electrode interfaces of the channels. A counter-flow situation is illustrated here, but co-flow and cross-flow configurations are also common. Channel cross section dimensions are typically on the order of a millimeter.

Siemens Concept (Fig. 16). The planar concept of SOFC developed by Siemens is similar to the sandwich structure of the PEM single cell (see Fig. 6). A foil of YSZ with a thickness of 150-200 pm acts as the supporting component on which the electrode materials are deposited the single cells are then stacked with interconnectors with the gas channels and sealing elements in between. The gases can be supplied in co-, cross-, or counterflow and can be let to and out of the stack in internal or external... 

Figure 12.18 Example of SOFC geometries, (a) Tubular geometry ofthe Siemens-Westinghouse system (b) Planar structure ofthe Sulzer-Hexis SOFC (for details, see the text).

SOFCs have largely converged on standard configurations, such as tubular or planar, with the structural support provided by the electrolyte, the anode, the metallic intercormector, or an inert porous support material. Each of these concepts has its own combination of advantages and disadvantages. In this section, some unconventional SOFC configurations and devices are discussed, and their performance and potential applications are considered in comparison with the more conventional approaches. This will include microtubular fuel cells, mixed reactant fuel cells, micro-planar fuel cells, and dual proton-oxygen ion fuel cells. 

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