Planar cells and stacks

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. 

Nd2Ni04+5 oxygen electrode material was tested on small size single cells before any further integration into large cells and stacks. Such single cells are planar circular cells with an active area of 3.14 cm2 (diameter 20 mm). 

The beginnings of the SOFC are recorded in an early East German University patent (Mobius and Roland, 1968) which shows awareness of many of the variables still being worked upon today. The oxides of lanthanum, zirconium, yttrium, samarium, europium, terbium, ytterbium, cerium and calcium are mentioned as candidate electrolyte materials. The proposed monolithic planar arrangement has, however, been abandoned by many companies, on the example of Allied Signal. One notable exception is a reversion to a circular planar concept by Ceramic Fuel Cells of Australia, UK (Section 4.7). The Rolls-Royce all-ceramic fuel cell (Section 4.3), which is monolithic and has one compliant feature, namely a gap, is a major exception. One modern trend is towards lower SOFC temperatures, with the intermediate-temperature IT/SOFC allowing the use of cell and stack arrangements with some flexibility and manoeuvrability based on new electrolytes, metallic flow plates, electrodes and interconnects. 

The so-called E- and F-designs for stacks with planar anode substrate type cells and metallic interconnects constitute the work horses at Forschungszentrum Jiilich used for testing SOFC materials, cells and manufacturing processes in cell and stack development since its introduction in the year 2000 [1]. Ferritic interconnects were chosen since they offer a high electric conductivity and thus the potential for high power density in the stacks. On the other hand the ferritic material gives rise to chromium evaporation (which can poison the cathodes) and is prone to massive corrosion at temperatures above approximately 900°C [2],... 

The E-design stack, shown in Fig. lA, consists of a relatively thick metallic interconnect with machined gas channels on both sides and a metallic picture frame for the cell, brazed to the interconnect by a metal solder. Sealing is obtained by glass ceramic sealants applied to the planar cell and frame surfaces using an automated dispenser. A fine Ni-mesh is spot-welded to the fuel side of the interconnect in order to improve the electrical contact between interconnect and anode substrate. On the air side usually a lanthanum cobaltite (LC) contact layer is sprayed onto the ribs of the interconnect. 

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. 

Computer simulations of bulk liquids are usually performed by employing periodic boundary conditions in all three directions of space, in order to eliminate artificial surface effects due to the small number of molecules. Most simulations of interfaces employ parallel planar interfaces. In such simulations, periodic boundary conditions in three dimensions can still be used. The two phases of interest occupy different parts of the simulation cell and two equivalent interfaces are formed. The simulation cell consists of an infinite stack of alternating phases. Care needs to be taken that the two phases are thick enough to allow the neglect of interaction between an interface and its images. An alternative is to use periodic boundary conditions in two dimensions only. The first approach allows the use of readily available programs for three-dimensional lattice sums if, for typical systems, the distance between equivalent interfaces is at least equal to three to five times the width of the cell parallel to the interfaces. The second approach prevents possible interactions between interfaces and their periodic images. 

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. 

The double helix structure of DNA has the hydrophobic bases pointing to the centre of the helix in an almost planar arrangement. These base-pairs are closely stacked perpendicular to the long axis of the chain, and are attracted to each other by Van der Waals forces. The hydrophilic phosphates are negatively charged at the pH of the cell and point to the outside. 

In the case of cell stacks of planar cells in particular, many cells are piled vertically to obtain a high voltage and a mechanical load is sometimes placed on the cells on top of the stack. This load induces a stress inside the cell stacks. 

The Rolls-Royce fuel cell modules and stacks are devoid of compliant features, as the cross-section in Figure 4.3 shows, unless one counts the gap as compliant. Cell improvement is clearly possible via thin (say 6 p.m) electrolyte layers, as employed in a planar IT/SOFC by Global Thermoelectric (Section 4.9) and Steele etal. (2000a, b). Alternatively, modern reduced temperature electrolytes could be used. 

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