Electrode-supported cell designs

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... 

The electrolyte membrane is an oxide ion conductive ceramic, whose thickness depends on the cell design. One may distinguish electrolyte-supported cell from electrode-supported cell (Fig. 15.6). In the first case, anode and cathode are deposited onto both faces of the electrolyte membrane. As a direct consequence, the membrane must be mechanically strong, and a minimal thickness of 100 pm is required. In the case of the electrode-supported cell, the anode is actually the mechanical support of the electrolyte first, and next the cathode on the top. Thus, the electrolyte thickness can be greatly reduced, down to 8 pm for classical SOFC devices. More recently, with the development of micro-SOFC, it can reach 100 nm to 1 pm. 

Most of the discussion in this chapter is centered on cells made with traditional materials such as YSZ electrolyte, Ni + YSZ anode, and LSM + YSZ cathode although its extension to other materials is essentially straightforward. The relative contributions of various polarisations vary widely among the different cell designs anode-supported, cathode-supported, and electrolyte-supported. Ohmic contribution is the smallest in electrode-supported cells due to the thin... 

If the reaction kinetics of the electrode is assumed to be very rapid, mass transfer and ohmic resistance are the dominant resistances. Assuming a reaction zone that coincides with the electrode-electrolyte interface, the diffusion fluxes in stationary operation can be expressed simply in terms of bulk gas partial pressures and gas-phase diffusivities. This is illustrated schematically in Figure 11.8, which compares anode- and cathode-supported cell designs for the simple case of a H2/O2 fuel cell. The decrease in concentration polarisation at the cathode, rjcc- is obvious in the case of an anode-supported cell, while the model shows that concentration polarisation at the anode, tiac is relatively insensitive to anode thickness. The advantage of the mass transfer-based approach is that analytical expressions are obtained for the polarisation behaviour. These are rather simple if activation overpotential is excluded but may still become elaborate in the case of an internally reforming anode where a number of reactions (discussed in Section 11.3) may occur simultaneously within the pores of the anode. 

The conclusion of Huang et al. was supported by Lin et al., who used a three-electrode cell design to monitor the voltage profile of both the anode and cathode in a full lithium ion cell during cycling at low temperatures and found that these cyclings resulted in the deposition of metallic lithium on the... 

Commence design work on heavy-duty tmck cab application using planar electrode supported solid oxide fuel cell and diesel fuel. Finalize specifications (e.g. capacity, duty cycle, volume and weight, vehicle integration issues). 

The rolled cell design has also been studied for the application in Mn02-H2 cells [49]. In that system the influence of zinc on the cycling behaviour is not present and the cycle life is improved. However, it should be noted that rolled Mn02 H2 cells are capacity-wise far below the 4 AA or 7 AAA RAM cell arrangement unless electrodes without supporting metal screen are developed. 

The tubular design is probably the best-known design. It has been developed by Westinghouse (now Siemens Power generation) [8]. The first concept that was pursued by Westinghouse consisted of an air electrode supported fuel cell tube. In earlier days the tubes were made from calcium-stabilized zirconia on which the active cell components were sprayed. Nowadays this porous supported tube (PST) is replaced by a doped lanthanum manganite (LaMn) air electrode tube (AES) that increases the power density by about 35 %. The LaMn tubes are extruded and sintered and serve as the air electrode. The other cell components are deposited on this construction by plasma spraying. 

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