Cathodes support

Tubular SOFC cathode supports with diameters or distance between flat faces on the order of 1 to 2 cm are commonly prepared by extrusion. Extrusion is a wet-ceramic process used to prepare tubes, and one which facilitates the formation... 

FIGURE 6.5 Tubular cathode-supported solid oxide fuel cell developed by Siemens Power Generation [48]. Reprinted from [48] with permission from Elsevier. 

Slurry containing the cathode powder is placed into a cylindrically shaped die with an open end, as shown in Figure 6.7 (a), and then a mandril is inserted into the tube that extrudes the slurry into the shape of a hollow tube with a closed end on one side and an open end on the other side, as shown in Figure 6.7 (b). The electrolyte, anode, and interconnect layers are then deposited onto the cathode support tube, using combinations of processes described in Section 6.2.2. 

Although cathode-supported tubular SOFCs in large-scale stacks are the type of SOFC stack most widely commercialized, recent alternative tubular cell designs have been developed with anode-supported designs for smaller-power applications. Cells in these stacks have diameters on the order of several millimeters rather than centimeters,... 

Chen XJ, Liu QL, Chan SH, Brandon NP, and Khor KA. High performance cathode-supported SOFC with perovskite anode operating in weakly humidified hydrogen and methane. Electrochem. Commun. 2007 9 767-772. 

Figure 46. Performance characteristics of a cathode-supported thin film Ni—YSZ/YSZ/LSM fuel cell at 600 °C in humidified H2 and air with and without a dense protective yttria-doped ceria (YDC) protection layer introduced between the porous LSM cathode and the thin-film electrolyte. (Reprinted with permission from ref 296. Copyright 1997 Elsevier.)...

Any one of the three components in SOFC, the cathode, anode, or electrolyte, can provide the structural support for the cells. Traditionally, the electrolyte has been used as the support however, this approach requires the use of thick electrolytes, which in turn requires high operating temperatures. Electrode-supported cells allow the use of thin electrolytes. The Siemens—Westinghouse Corporation has developed a cathode-supported design,although this has required electrochemical vapor deposition of the YSZ electrolyte. Most other groups have focused on anode-supported cells. In all cases, it is important to maintain chemical compatibility of those parts that come in contact and to match the thermal expansion coefficients of the various components. A large amount of research has been devoted to these important issues, and we refer the interested reader to other reviews. 

Planar SOFCs are composed of flat, ultra-thin ceramic plates, which allow them to operate at 800°C or even less, and enable less exotic construction materials. P-SOFCs can be either electrode- or electrolyte- supported. Electrolyte-supported cells use YSZ membranes of about 100 pm thickness, the ohmic contribution of which is still high for operation below 900°C. In electrode-supported cells, the supporting component can either be the anode or the cathode. In these designs, the electrolyte is typically between 5-30 pm, while the electrode thickness can be between 250 pm - 2 mm. In the cathode-supported design, the YSZ electrolyte and the LSM coefficients of thermal expansion are well matched, placing no restrictions on electrolyte thickness. In anode-supported cells, the thermal expansion coefficient of Ni-YSZ cermets is greater than that of the YSZ... 

Figure 9 shows one of the 21/2 ft2 electrolyte and electrode assemblies which, in conjunction with a molded carbon current collector (shown in Figure 7) and suitable anode and cathode supports, form a bipolar cell. The cells are assembled between pneumatically loaded end plates to form an electrolysis module. 

Self-supported SOFC can be classified into anode-supported and cathode-supported fuel cells. The SOFC assembly for laboratory testing has a shape of button with 1 - 2 cm in diameter and less than 500 im in thickness. The majority of these button cells are anode-supported cells due to the easy of their fabrication as compared with that of the cathode-supported cell. These self-supported fuel cell usually possess thin (5-20 p,m) electrolyte and can operate at reduced temperatures (< 800 °C). The low temperature operation is the key to decrease... 

X-ray tube-assisted GD A Grimm-type GD lamp was modifled by incorporating a 15 mm thick PTFE support to insulate the cathode support from the anode body. With this... 

A Jr caMcarx-s. Carbon —/ L y Ribbed Carbon Collector with M and E Supports Cathode Support... 

Flange for compression Insulator plate Cathode side bus bar Cathodic end plate Cathodic support collector... 

In the case of a solid oxide fuel cell, the anode or cathode support can be relatively thin because the component does not need to bear a very high load. However, in the case of oxygen transport membranes, the porous support needs to withstand a differential pressure of 20 atm or greater. Therefore, porous supports which are several millimeters thick are often considered. Alternatively, other concepts for strengthening the support structure are considered. These can include internal structures such as multichannel tubes, distinct solid porous inserts in tubes [21] and support braces in planar geometries. Examples of such structures are shown in Fig. 6.5 and 6.6. 

The rectangular walls were sandwiched between the cathode (supported by a bottom) and a cover. An 0-ring ensured watertightness of the cell (fig. 3.3>i) whereas between cover and walls, a rubber sealing was placed. 

The three components of the fuel cell, anode, cathode, and electrolyte form a membrane-electrolyte assembly, as, by analogy with polymer electrolyte fuel cells, one may regard the thin layer of solid electrolyte as a membrane. Any one of the three membrane-electrode assembly components can be selected as the entire fuel cell s support and made relatively thick (up to 2 mm) in order to provide mechanical stability. The other two components are then applied to this support in a different way as thin layers (tenths of a millimeter). Accordingly, one has anode-supported, electrolyte-supported, and cathode-supported fuel cells. Sometimes though an independent metal or ceramic substrate is used to which, then, the three functional layers are applied. 

Though SOFC can be either of anode, electrolyte or cathode supported, in the case of cells running on hydrocarbon fuels, anode supported cells may be preferable to the others for the reasons of internal reforming. However, the optimal anode thickness required to support the cell mechanically and to achieve the desired level of internal reforming and optimal cell performance is rather a difficult task. 

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