The SOFC

Fuel Cells, Engines and Hydrogen - An Exergy Approach Frederick J. Barclay 2006 John Wiley Sons, Ltd 

October 2005, also fails to mention the problem. The industry still does not seem to realise the vitally important crossroads at which it is located, and the cause of its huge losses, as highlighted in the Price Waterhouse review of the fuel cell industry (Price Waterhouse Coopers, 2005). 

The SOFC system, however, has the potential to use natural gas directly, and thereby the opportunity to bypass the hydrogen source problem for stationary non-vehicle applications. The North Western University US patents on direct hydrocarbon oxidation are 2001/6,214,485 Bl, 2002/6,479,178 B2, 2003/0,118,879 and 2004/0,033,405, which deal with special catalysts and anodes. The parallel work at the University of Pennsylvania, http //www.upenn.edu/, is given in McIntosh S and Gorte (2003), correspondence address gorte seas.upenn.edu/. 

A vehicle must carry stored hydrogen, and for one version of the proposed Renault fuel cell a compact liquid hydrogen tank has been developed by Air Liquide. 

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. 

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The measured pressure drops were slightly greater than literature data would indicate for packed carbon beds. However, they are certainly not prohibitive and a successful outcome of the Westinghouse trial of the SOFC guard bed is anticipated. 

Conceptually elegant, the SOFC nonetheless contains inherently expensive materials, such as an electrolyte made from zirconium dioxide stabilized with yttrium oxide, a strontium-doped lanthanum man-gaiiite cathode, and a nickel-doped stabilized zirconia anode. Moreover, no low-cost fabrication methods have yet been devised. 

One leading prototype of a high-temperature fuel cell is the solid oxide fuel cell, or SOFC. The basic principle of the SOFC, like the PEM, is to use an electrolyte layer with high ionic conductivity but very small electronic conductivity. Figure B shows a schematic illustration of a SOFC fuel cell using carbon monoxide as fuel. 

Over the anode, the hydrogen and CO produced via reactions (1) and (2) are then oxidized at the anode by reacting with the oxygen species transported from the cathode. The catalysis of the fuel such as methane at the anode and oxygen at the cathode becomes increasingly important with demanding catalytic activity as the SOFC operation temperature decreases, which is the aim under intensive research efforts. 

To reduce the formation of carbon deposited on the anode side [2], MgO and Ce02 were selected as a modification agent of Ni-YSZ anodic catalyst for the co-generation of syngas and electricity in the SOFC system. It was considered that Ni provides the catalytic activity for the catalytic reforming and electronic conductivity for electrode, and YSZ provides ionic conductivity and a thermal expansion matched with the YSZ electrolyte. 

While the PEM fuel cells appear to be suitable for mobile applications, SOFC technology appears more applicable for stationary applications. The high operating temperature gives it flexibility towards the type of fuel used, which enables, for example, the use of methane. The heat thus generated can be used to produce additional electricity. Consequently, the efficiency of the SOFC is -60 %, compared with 45 % for PEMFC under optimal conditions. 

Why is the PEM fuel cell so sensitive to CO, while the SOFC cell is not ... 

Figure 6.5 provides a detailed process flow diagram of the system. During normal operation air enters the compressor and is compressed to 3 atmospheres. This compressed air passes through the recuperator, where it is preheated and then enters the SOFC. Pressurized fuel from the fuel pump also enters the SOFC, and the electrochemical reactions take place along the cells. The hot pressurized exhaust leaves the SOFC and goes directly to the expander section of the gas... 

There are a number of informative reviews on anodes for SOFCs [1-5], providing details on processing, fabrication, characterization, and electrochemical behavior of anode materials, especially the nickel-yttria stabilized zirconia (Ni-YSZ) cermet anodes. There are also several reviews dedicated to specific topics such as oxide anode materials [6], carbon-tolerant anode materials [7-9], sulfur-tolerant anode materials [10], and the redox cycling behavior of Ni-YSZ cermet anodes [11], In this chapter, we do not attempt to offer a comprehensive survey of the literature on SOFC anode research instead, we focus primarily on some critical issues in the preparation and testing of SOFC anodes, including the processing-property relationships that are well accepted in the SOFC community as well as some apparently contradictory observations reported in the literature. We will also briefly review some recent advancement in the development of alternative anode materials for improved tolerance to sulfur poisoning and carbon deposition. 

Other categories of chromia forming alloys—including Ni(-Fe)-Cr base and Fe(-Ni)-Cr base alloys (e.g., austenitic stainless steels)—have a face-centered cubic (FCC) substrate structure. In comparison to the FSS, the FCC base alloys, in particular the Ni(-Fe)-Cr base alloys, are generally much stronger and potentially more oxidation resistant in the SOFC interconnect operating environment [6, 123-129], However, the FCC Ni(-Fe)-Cr base alloys with sufficient Cr for an appropriate... 

K. Eichler et al., Degradation Effects at Sealing Glasses for the SOFC, Proc. 4th European SOFC Forum, pp. 899-906 (2000). 

To meet the requirements for electronic conductivity in both the SOFC anode and cathode, a metallic electronic conductor, usually nickel, is typically used in the anode, and a conductive perovskite, such as lanthanum strontium manganite (LSM), is typically used in the cathode. Because the electrochemical reactions in fuel cell electrodes can only occur at surfaces where electronic and ionically conductive phases and the gas phase are in contact with each other (Figure 6.1), it is common... 

The introduction of such a layer can dramatically improve the fuel cell performance. For example, in the SOFC with bilayered anode shown in Figure 6.4, the area-specific polarization resistance for a full cell was reduced to 0.48 Hem2 at 800°C from a value of 1.07 Qcm2 with no anode functional layer [24], Use of an immiscible metal oxide phase (Sn()2) as a sacrificial pore former phase has also been demonstrated as a method to introduce different amounts of porosity in a bilayered anode support, and high electrochemical performance was reported for a cell produced from that anode support (0.54 W/cm2 at 650°C) [25], Use of a separate CFL and current collector layer to improve cathode performance has also been frequently reported (see for example reference [23]). 

Hydrocarbons such as natural gas or methane can be reformed internally in the SOFC, which means that these fuels can be fed to the cells directly. Other types of fuel cells require external reforming. The reforming equipment is size-dependent which reduces the modularity. 

Besides the PEMFC being developed for vehicle propulsion, SOFC are being considered for APU applications in vehicles, since they operate at very high temperatures and therefore require long start-up times (an hour or more). In APU applications, the fuel cell can be left running most of the time, or could be started far in advance of an anticipated stop. The principal attraction of SOFCs is their high tolerance to hydrocarbon fuels. The heat of the SOFC can be used in the air-conditioning unit, either as heat or as cold. 

We discuss both the Proton Exchange Membrane as well as the Solid Oxide Fuel Cells in this chapter (PEMFC and SOFC). Both types are in full development, the PEMFC for mobile and stationary applications, and the SOFC for stationary applications as well as for auxiliary power generation for transport. 

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