SOFC stack

FIGURE 2.23 Stability of 7-cell SOFC stack tested at 1000°C in pure hydrogen and spent fuel (10 vol% CO + 5 vol% H2 + 85 vol% C02) without and with 50 ppm H2S. (From Feduska, W. and Isenberg, A.O., J. Power Sources, 10 89-102, 1983. Copyright by Elsevier, reproduced with permission.)... 

In addition to the YSZ electrolyte and metallic interconnect, SOFC stack also includes component materials such as seals and manifolds. Sealant materials based on glass or... 

Nevertheless, Ni(-Fe)-Cr base alloys may find application as interconnect materials through the use of innovative SOFC stack and seal designs and novel interconnect structures. For example, a cladding approach has been applied to fabricate a stable composite interconnect structure consisting of FCC Ni-Cr base alloy claddings on a BCC FSS substrate [134,135], The clad structure appeared to be stable over 1000 hours at 800°C in air and exhibited a linear CTE close to that of the FSS, but needs further long-term stability evaluation before its commercial use. 

The aforementioned requirements on surface stability are typical for all exposed areas of the metallic interconnect, as well as other metallic components in an SOFC stack e.g., some designs use metallic frames to support the ceramic cell. In addition, the protection layer for the interconnect or in particular the active areas that... 

Sakai N, Yokokawa H, Horita T, and Yamaji K. Lanthanum chromite-based interconnects as key materials for SOFC stack development. Int. J. Appl. Ceram. Technol. 2004 l 23-30. 

Wen T-L, Wang D, Chen M, Tu H, Lu Z, Zhang Z, Nie H, and Huang W. Material research for planar SOFC stack. Solid State Ionics 2002 148 513-519. 

Historically, one of two techniques has typically been used to seal a planar SOFC stack glass joining or compressive sealing. Glass was originally used because it is simple to make and apply. The first requirement for a rigid seal is that the seal s... 

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 5.2 Schematic of seals typically found in aplanar design SOFC stack with metallic interconnect and metallic internal gas manifold channels (possibly for counter flow pattern of fuel and air gases). 

Over the years, a large number of materials and approaches have been investigated for sealing of SOFC stacks. The best of these approaches have been successful in sealing the cells to acceptable leakage rates. Glass-ceramic and composite seals... 

N. Lahl et al. Aluminosilicate Glass Ceramics as Sealant in SOFC Stacks, Solid Oxide Fuel Cells Vol. VI, Electrochemical Society Proceedings Vol. 99-19, pp. 1057-1066. 

M. Bram et al., Basic investigations on metallic and composite gaskets for an application in SOFC stacks, in Proceedings of the Fifth European Solid Oxide Fuel Cell Forum, J. Huijsmans (ed.), 1-5 July 2002, Lucerne, Switzerland, 2202, pp. 847-854. 

M. Bram et al., Deformation Behavior and Leakage Tests of Alternate Sealing Materials for SOFC Stacks, Journal of Power Sources, 138, pp. 111-119 (2004). 

Each of the components of an SOFC stack anodes, cathodes, electrolytes, and interconnects must be thermally, chemically, mechanically, and dimensionally stable at the operating conditions and compatible with the other layers with which they come into contact in terms of thermal expansion and chemical inter-reaction. They must also have compatible processing characteristics. In addition to those requirements, the individual layers have additional microstructural, property, and processing target requirements, as summarized in Table 6.1. 

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,... 

The main difference in SOFC stack cost structure as compared to PEFC cost relates to the simpler system configuration of the SOFC-based system. This is mainly due to the fact that SOFC stacks do not contain the type of high-cost precious metals that PEFCs contain. This is off-set in part by the relatively complex manufacturing process required for the manufacture of the SOFC electrode electrolyte plates and by the somewhat lower power density in SOFC systems. Low temperature operation (enabled with electrode supported planar configuration) enables the use of low cost metallic interconnects which can be manufactured with conventional metal forming operations. 

Material Flexibility - The type and range of materials is greater at lower temperatures. In particular metallics may be incorporated into SOFC stack designs. 

Given the large number of potential beneficial effects of lowering the nominal operating temperature of the SOFC stack and their corollary affect on system cost, intermediate temperature SOFC concepts are being pursued by many organizations throughout the U.S. and the World. 

The European Union sponsors the Real SOFC program, which aims to raise the durability of p lanarSOFCstackstoa level acceptable for stationary applications to find materials, manufacturing routes and standards suitable for low cost production and, to reduce specific weight and volume of SOFC stacks. The consortium includes 25 of Europe s leading research and industrial organisations. 

Construct and evaluate a novel SOFC stack with the following characteristics power density > 1 kW/litre, capacity > 50 kWe, CO-tolerant to at least 100 ppm, pressure < 2 barg, performance degradation < 1% over 1,000 hours 2003... 

Construct and evaluate a novel, planar SOFC stack of - 20 kWe that internally reforms natural gas and delivers an efficiency of > 50% (LHV). The stack should be capable of manufacture using a viable (if not yet proven) process, and with materials costs of less than US 300/kW 2005... 

Build and evaluate a compact, integrated, self-sustaining system around a - 50 kWe SOFC stack, achieving an electrical efficiency of > 40% (LHV) 2005... 

Build and evaluate a pressurized SOFC stack of - 50 kWe, complete with immediate BoP and control system 2006... 

Interestingly, research has started on single chamber SOFC (SC-SOFC) concepts. However, the SC-SOFC exhibits inherently low power density and is therefore primarily of academic interest. It has the potential to relax cell component requirements and probably to ease manufacture. The principle of SC-SOFC is that it is fed by an air fuel mixture which flows onto the PEN contained in a single compartment, avoiding the use of gas separator plates and high temperature sealants. The fluid may flow simultaneously or sequentially along the electrodes. Both electrodes are either built onto the same side of the electrolyte some distance apart or on opposite sides. Low temperature operation would apparently suppress direct combustion of the air fuel mixture provided the electrode materials chosen are highly selective towards their respective catalytic reactions. SC-SOFC stacks may hold prospects in specific applications where the reaction products are the prime focus. 

Because a complete conversion of the fuel cannot be achieved in practice within the fuel cell, the SOFC stack can be treated like a power generating burner so as to integrate it easily into a system model. The cooling of the stack depends on the excess air ratio. 

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

Fig. 5.1 Commonly used SOFC designs (Celik, 2006). (a) Tubular SOFC, (b) 24 cell tubular SOFC stack, (c) a tubular SOFC module with 48 stacks, (d) 28 cell internally manifolded stack design by Versa Power Systems.

---