Monolithic SOFC

Because this design has relatively low power density, recent work has focused on a monolithic SOFC, since this could have faster cell chemistry kinetics. The very high temperatures do, however, present sealing and cracking problems between the electrochemically active area and the gas manifolds. 

The concept of monolithic module design is associated with Argonne National Laboratories [111, 112]. Power density of about 8 kW/kg or 4kW/1 and fuel efficiency over 50% are expected to be achieved with monolithic SOFCs. The monolithic structure started with a co-flow version where the cell consists of a honeycomb-like array of adjacent fuel and oxidant channels that look like corrugated paperboard, as shown in Fig. 42. Multilayer laminates of the active cell components (anode-elec-ttolyte-cathode) are appropriately corrugated and stacked alternatively between flat multilayer laminates in the following sequence anode- interconnection material-cathode. Tape casting [111] and hot-roll calendering [113, 114] are used to fabricate the monolithic structure. A cross-flow version where oxidant and fuel channels are... 

Fig. 43. Crossflow monolithic SOFC. From ref. [114]. This paper was originally presented at the Fall 1989 Meeting of The Electrochemical Society held in Hollywood, Florida.

Fig. 44. Current-voltage characteristics of monolithic SOFC at 1000 °C using pure hydrogen. From ref. [100],...

Fig. 13.22. The monolithic SOFC concept of Argonne National Laboratory. Anode nickel-yttria-stabilized zirconia. Cathode strontium-doped lanthanum manganite. Interconnect doped lanthanum chromite, a, Interconnection b, electron-ion path c, anode d, electrolyte e, cathode. (Reprinted from K. Kordesch,...

Figure 7.10 Schematic of monolithic SOFC design. (Image courtesy of Siemens Power Generation.)...

Fig. 4.10 Demonstration of eurrent path in monolithic SOFC design (Minh 2004)...

An evolution of the flat planar design is the monolithic SOFC (Fig. 10). This consists of corrugated thin cell components. The result is that the volumetric power density is very high. On the other hand, the structure is more complex to realize, as well as the manifold system compared to the flat planar one. Based on this configuration, the Japanese Chubu Electric Power Company, INC. (CEPCO) and Mitsubishi Heavy Industries developed the so-called MOLB (Mono-block Layer Built) cells (Nakanishi et al. 2004). 

Fig. 9 Schematic cross-section of a flat planar SOFC. Fig. 10 Schematic cross section of a monolithic SOFC...

Fig. 11.26 SOFC configuration (a) Westinghouse single tube configuration (b) Argonne multichannel monolithic configuration (c) planar configuration.

An alternative SOFC configuration, based on the corrugated monolithic concept, was first proposed by the Argonne National Laboratories (Fee et al, 1986). The design is shown schematically in Fig. 11.26(h). The cells incorporate two triple layer subassemblies each prepared by... 

Planar SOFC, in particular, monolithic designs (MHI) are capable of high (volumetric) power densities most favoured by direct and short current paths across the stack components. The PEN is principally square, rectangular and circular (Ceramic Fuel Cells Limited (CFCL), Mitsubishi Materials Corp., SulzerHexis) in shape with active surface areas of 100-200 cm2 (15.5-31 in2). A drawback of this design is that it often necessitates the use of high temperature sealants for application at the in-... 

SOFC can be manufactured in different geometrical configurations, i.e. planar, tubular or monolithic. Regardless of the geometrical configuration, a solid oxide fuel cell is always composed of two porous electrodes (anode and cathode), a dense electrolyte, an anodic and a cathodic gas channel and two current collectors. For the sake of simplicity the planar configuration is taken as reference, as shown in Figure 3.1. 

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. 

Different concepts for SOFCs have been developed Flat plates offer the possibility of easier stacking while tubular designs have smaller sealing problems. Monolithic plate and even single chamber designs have been considered and investigated. 

Lenz et al. [73] described the development of a 3 kW monolithic steam-supported partial oxidation reactor for jet fuel, which was developed to supply a solid oxide fuel cell (SOFC). The prototype reactor was composed of a ceramic honeycomb monolith (400 cpsi) operated between 950 C at the reactor inlet and 700°C at the reactor outlet [74]. The radial temperature gradient amoimted to 50 K which was attributed to inhomogeneous mixing at the reactor inlet. The feed composition corresponded to S/C ratio of 1.75 and O/C ratio of 1.0 at 50 000 h GHSV. Under these conditions, about 12 vol.% of each carbon monoxide and carbon dioxide were detected in the reformate, while methane was below the detection limit. Later, Lenz et al. [74] described a combination of three monolithic reactors coated with platinum/rhodium catalyst switched in series for jet fuel autothermal reforming. An optimum S/C ratio of 1.5 and an optimum O/C ratio of 0.83 were determined. Under these conditions 78.5% efficiency at 50 000 h GHSV was achieved. The conversion did not exceed 92.5%. In the product of these... 

Conventional SOFCs exist in several design variants. The basic variants are tubular and planar cells. Monolithic cells joined the first two variants around 1990. The specific design and operating features of these and other variants are described in subsequent sections of this chapter. Those factors that are common for all variants of conventional SOFCs are described in the present section. 

Figure 8.8 Monolithic design of SOFCs. (From Yamamoto, 2000, with permission from Elsevier.)...

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