IT/SOFC

Zhou X, Ma J, Deng F, Meng G, and Liu X. Preparation and properties of ceramic interconnecting materials, I.a07Ca0 Cr() n doped with GDC for IT-SOFCs. J. Power Sources 2006 162 279-285. 

Steele BCH. Appraisal of Ce, yGdy02 y/2 electrolytes for IT-SOFC operation at 500°C. Solid State Ionics 2000 129 95-110. 

Xu X, Xia C, Xiao G, and Peng D. Fabrication and performance of functionally graded cathodes for IT-SOFCs based on doped ceria electrolytes. Solid State Ionics 2005 176 1513-1520. 

Yin Y, Li S, Xia C, and Meng GJ. Electrochemical performance of IT-SOFCs with a double-layer anode. J. Power Sources 2007 167 90-93. 

Lee S, Lim Y, Lee EA, Hwang HJ, and Moon JW. Ba0 5Sr05Co0 8Fe02O3 5 (BSCF) and Lao6Ba04Co02Fe0803 5 (LBCF) cathodes prepared by combined citrate-EDTA method for IT-SOFCs. J. Power Sources 2006 157 848-854. 

SubramaniaA, SaradhaT, and Muzhumathi S. Synthesis of nano-crystalline (Ba05Sr05) Co08Fe0 203 g cathode material by a novel sol-gel thermolysis process for IT-SOFCs. J. Power Sources 2007 165 728-732. 

Meng G, Song H, Xia C, Liu X, and Peng D. Novel CVD techniques for micro- and IT-SOFC fabrication. Fuel Cells 2004 4 48-55. 

An attempt by the author to forecast the design of a plant based on complete IT/SOFCs with concentration cell circulators, integrated with a gas turbine and consuming natural gas, is shown as Figure A.7. 

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. 

For all-ceramic HT/SOFCs the rate of preheating will be slow compared with IT/SOFCs with flexible metallic structural parts between the MEAs. In future SOFCs capable of direct oxidation of hydrocarbons (Section 4.1.11) and without a reformer, the preheating manoeuvres will be simplified by the absence of the reformer. An exception is the small-tube SOFC (Sections 4.11 and 4.12), which can be preheated rapidly. 

In a 1999 letter to Nature (Perry Murray etal., 1999), from North Western University, Illinois, the authors record the first laboratory achievement of useful oxidation rates for direct methane electrochemical oxidation, using an IT/SOFC. The cathode structures were porous lanthanum strontium manganite (LSM) on porous ( 203)0.15 (Ce02)o,85 or YDC. The anodes were cermets, porous YSZ with nickel in the pores. The laboratory operating temperatures were in the range 500-700 °C. The account of the North Western work, reporting on new anode types, continues on pp. 921-924 of Williams (2002). 

A fusion between these direct hydrocarbon proposals and IT/SOFCs such as that of the Mitsubishi Materials Corp. (Section 4.5), or that of Ceres Power Ltd (Section 4.6), could make a hugely competitive, simplified and cheaper future system, with further major development potential to take account of the points in Appendix A of this book, that is fuel cells allied to concentration cell circulators, namely complete fuel cells. Such a development will await judgement of this book, and departure into the history of the application of combustion theory to fuel cells using isothermal oxidation. 

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. 

In a press release dated 17 December 2001, see listed web site, MMTL claimed a new design of IT/SOFC having 1.8 Wcm at 800 °C, a world-record high at the time. Partners are Kansai Electric and Japan Fine Ceramics Centre. The Japanese cells are to be compared with the German FZJ cells described in the press release of Section 4.8, below. A major increase in power density comes from a new electrolyte, namely Lao StQjGao Mgo 1O2 9. 

A modern, non-stoichiometric, fuel-cell-oriented, perovskite material, Cco Gdo iOj 95 (COG), has been developed by Professor Steele at Imperial College London, as recorded in Steele etal. (2000a, b). Professor Steele has found the way, via COG, to a porous ferritic stainless steel, tubular or flat square IT/SOFC operating at 500 °C, superb achievement. 

In Figure 4.7, 800 °C is the operating temperature of the planar IT/SOFC, patented by Ceramic Fuel Cells Ltd of Australia (Gibson etal, 1995 Wolfe etal, 1999 Badwal etal, 1996 1997 Donelson etal, 1998). 

The MCFC and, as in Chapter 4, the IT/SOFC are direct competitors for the stationary power role, with convergent operating temperatures. The MCFC is fixed at its electrolyte melting point, but see Ceres Power in Section 4.6. The modus operandi and the general arrangement of an MCFC are shown in Figures 5.1 and 5.2. 

A so far exclusive feature of the competing IT/SOFC is, however, the nascent ability (Chapter 4) to oxidise methane and other hydrocarbon gases electrochemically, without reform. The future MCFC is likely to have to meet this potential leapfrog of its simplicity and fuel versatility. 

Like the competing IT/SOFC, the MCFC is an incomplete system which has not considered isothermal oxidation with its circulator problem, as defined in this book. The system is also immature as it stands, since the mobility of its electrolyte is a newly recognised fact, for which the author has suggested surface tension gradient as the mechanism. Moreover, a major increase in power density is under consideration, which would interact with electrolyte mobility via higher temperature gradient. 

Because the IT/SOFC integrates well with the gas turbine so as to burn fuel not consumed by isothermal oxidation, and because the gas turbine compressor provides the means to elevate the system temperature to ion conduction level for start-up purposes, the IT/SOFC is a promising choice, with a wide selection of competing types. 

For Figure A.5, which is in no sense a hydrogen mine but a pure power plant of very high productivity, the gas-turbine-integrated IT/SOFC is additionally attractive because it accommodates any of the special techniques for methane oxidation (Perry Murray etal., 1999 Park etal., 1999 Gorte etal., 2000). 

The compressor outlet air temperature is matched to the IT/SOFC. Since the plant would be a large base load system it would have a multipressure steam plant to use its exhaust heat. 

The plant in Figure A.4 can be dealt with in exactly the same way. The reformer and the two fuel cells would be elevated to IT/SOFC conditions, as in Figure A.6. All surplus fuel, heavy hydrocarbons and unoxidised fuel from the three plant sections, together with three hot exhausts, would be swallowed by a gas turbine combustion chamber as above. That would yield a controllable plant, subject to availability of semi-permeable membranes and of isothermal concentration cells, appropriate to IT/SOFC temperatures and gas turbine pressure. 

Figure A.6 Practicality in direct oxidation via IT/SOFC technology...

A.5.1 Concentration Cells for Future IT/SOFC/Gas Turbine Power... 

Figure A.7 IT/SOFC complete with concentration cell circulators and integrated with gas turbine...

Long-term successful operation of the SOFCs requires that the electrolyte possess adequate chemical and structural stability over a wide range of oxygen partial pressures, from air or oxygen to humidified hydrogen or hydrocarbons. The requirements for the electrolyte used in the intermediate-temperature SOFCs (IT SOFCs) include ... 

Steele, B.C.H., Materials for IT-SOFC stacks 35 years R D the inevitability of gradualness Solid State Ionics, 2000, 134, 3-20. 

In all ceramic high-temperature (HT) SOFC systems, LaCr(Mg)O3 and La(Ca)CrO3 are the interconnect materials used. The disadvantages of this include an expensive manufacturing route and a poor ability to withstand rapid temperature changes. Ferritic stainless steel can be used in IT SOFCs (see Figure 12.9). 

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