SOFC Cell Designs

Fig. 3a. Schematic view of SOFC cell design tubular type.

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. 

FIGURE 6.6 High power density (HPD) SOFC, consisting of a flattened tube with two flat faces. The vertical struts between the two flat faces provide shorter paths for the electronic current collection, eliminating the need for all of the electronic current to travel around the circumference of the cathode, as in the standard tubular cell design shown in Figure 6.5 [48], Reprinted from [48] with permission from Elsevier. 

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,... 

Oxidant Utilization In addition to the obvious trade-ofFbetween cell performance and compressor or blower auxiliary power, oxidant flow and utilization in the cell often are determined by other design objectives. For example, in the MCFC and SOFC cells, the oxidant flow is determined by the required cooling. This tends to yield oxidant utilizations that are fairly low (-25%). In a water-cooled PAFC, the oxidant utilization based on cell performance and a minimized auxiliary load and capital cost is in the range of 50 to 70%. 

Ferguson J.R., 1992, SOFC two dimensional unit cell modeling. SOFC Stack Design Tool, International Energy Agency Final Report. 

In this section we examine the primary transient phenomena that are of interest to SOFC analysis, and provide the fundamental model equations for each one. Examples for the use of these models are given in later sections. While the focus is on reduced-order models (lumped and one-dimensional), depending on the needs of the fuel cell designer, this may, or may not be justifiable. Each fuel cell model developer needs to ensure that the solution approach taken will provide the information needed for the problem at hand. For the goal of calculating overall cell performance, however, it is often that one-dimensional methods such as outlined below will be viable. 

In Part Two, the reader is provided with practical examples of how the general equations defined in part one can be simplified/adapted to specific cases. The examples cover the most widely employed single cell designs, for steady-state and dynamic conditions, and evolve towards stack and system modehng. Finally, Chapter 10 introduces the reader to the problem of mechanical stresses in SOFC, and shows an approach for modeling mechanical stresses induced by the operating conditions. 

In multiple-stack installations, it is important to control the performance of each stack separately to ensure that one stack cannot discharge into another. This is necessary, because the manufacturing of identical stacks is just about impossible with the current means of manufacturing in the industry. This is particularly a problem for active anode SOFCs and molten carbonate cell designs, because the 02 drawn through the cell electrolyte can oxidize and destroy the catalytic ability of the cell. 

Figure 2.56 shows a variety of stacked cell designs employed by SOFCs. Since an individual fuel cell produces a low voltage (typically < 1V), a number of cells are connected in series forming a fuel cell stack. An interconnect comprising a high-density material is used between the repeating anode-electrolyte-cathode units of... 

Only a few materials can sustain the working conditions of SOFC without significant changes after long term operation. Most of the materials used in SOFC are relatively expensive. The expected high efficiency of a SOFC is quite attractive and many cell designs have been proposed. Many attempts have also been made to lower the operating temperature in order to reduce the demands on the materials. 

One remarkable aspect is that, frequently, a variety of products result from electrochemical reactions in the fuel cell, so that selectivity for a given product is another factor to be considered in cell design. For instance, SOFC cells using bismuth... 

Cell design is also an important factor to improve the performance of SOFC stack/module [15-18]. Use of small diameter SOFC may also give opportunity to reduce operating temperature by increasing the volumetric power density [19]. Thus, they are expected to accelerate the progress of SOFC systems which can be applied to portable devices and auxiliary power units for automobile. 

System design, performance and cost of such SOFC depend on the properties of innovative materials and advanced ceramics. The careful design and manufacture of these materials is essential to system integration and performance, and may ultimately determine the success or failure of the technology. Important factors include stability, durability, processibility catalytic, electro-chemical and ionic properties. Figure 9-24 shows SEM image of ceramic layers used in a SOFC cell. 

In contrast to stationary applications, portable applications require frequent start and stop procedures. Therefore for SOFC, a robust cell design and adapted electrode-electrolyte assemblies are an important issue. Frequent thermal cycles between room temperature and an operation temperature of about 600-800 °C pose challenges to the layered system consisting of solid anode, ceranfic electrolyte and solid cathode with respect to thermal and mechanical stability. For several years, different approaches to developing tubular nficro SOFC have been undertaken but did not lead to a commercial product yet. As SOFC can be operated with pure hydrogen, reformate and hydrocarbons as fuel as well - the latter option means direct internal reforming at the anode catalyst — various investigations focused on reduced operation temperature and a parallel conversion of fuels [21]. 

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. 

Since the begiiming of SC-SOFC research in the 1990s, significant progress in this technology has been made in terms of cell design, fabrication, materials, and generated power output. Due to fuel cell operation in a fuel-oxidant gas... 

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