There are five main types of fuel cells currently undergoing research. They are all at differing levels of commercialisation and are being considered for different applications. Each of the fuel cells operates at different temperatures and utilises different electrolytes. All of the fuel cells considered operate essentially on hydrogen though those operating at higher temperatures offer the possibility of internal conversion (reforming) of hydrocarbon fuels to yield hydrogen. The higher temperature fuel cells also offer advantages in terms of efficiency. It is these fuel cells which rely on ceramic electrolytes for their operation.
The five main types of fuel cell are:
• Phosphoric Acid Fuel Cell (PAFC)
• Alkaline Fuel Cells (AFC)
• Molten Carbonate Fuel Cells (MCFC)
• Solid Polymer Fuel Cells (SPFC)
• Solid Oxide Fuel Cells (SOFC)
Phosphoric Acid Fuel Cell (PAFC)
In this design the phosphoric acid electrolyte is contained between two porous graphite electrodes coated with a platinum catalyst. Hydrogen is used as the fuel and oxygen as the oxidant. The operating temperature is between 180°C and 210°C. This fuel cell is the closest to commercialisation for large-scale power generation. This is because its development started earlier and a great deal of effort and finance was put into the PAFC during the initial stages [. The electrical efficiency of the PAFC units are ~40% with combined heat and power units achieving ~70% [.
As of August 1999, a total of 180 plants with a total rated power of 46.26MW had been installed in Japan. Among these plants, 106 were installed by gas utilities with the other 74 being sited in breweries, water and sewage works, electrolyser plant and at Epson’s Nagano Works ].
Disadvantages with PAFC’s
There are several disadvantages associated with the PAFC design. These include the need to use the expensive noble metal, platinum, as electrodes. Furthermore the electrodes are susceptible to CO poisoning and the electrolyte in the fuel cells is a corrosive liquid which, is consumed during operation .
Alkaline Fuel Cells (AFC)
In the AFC the aqueous potassium hydroxide acts not only as an electrolyte but also as a coolant. It conducts hydroxyl ions from cathode to anode and depending upon the weight percentage of the hydroxide in the electrolyte can operate at temperatures between 60 and 120°C or as high as 250°C . The cells require very pure hydrogen and oxygen as the fuel and oxidant as they cannot tolerate even the atmospheric levels of carbon dioxide. Due to the requirement for pure gases, the AFC would not be economically viable for terrestrial power applications. However, they do demonstrate high power densities and have an established space application. Today there are three AFC power plants used in the space shuttle Orbiter, supplying 12kW maximum power .
Molten Carbonate Fuel Cells (MCFC)
The MCFC has a molten mixture of alkali carbonates, Li2CO3 and K2CO3, as the electrolyte. Both the electrodes are nickel based, the anode consisting of a nickel chromium mix. The 10% chromium is required to maintain the porosity of the anode structure. These cells can be run on a variety of fuels and one of their interesting features is their operating temperature (~ 650°C), which is high enough for the direct conversion of natural gas to be performed at the anode .
The potential market for MCFC systems is thought to be in co-generation applications. The electrical efficiency of the MCFC unit is ~ 50% and in combined heat and power applications efficiencies could reach up to 90% . Currently, the MCFC technology is entering the 0.1-2MW demonstration phase in order to confirm the initial indications of performance and efficiency .
Disadvantages with MCFC’s
Unfortunately there are problems associated with this technology, due to the cell reactions, and the molten nature of the electrolyte. The main product at the anode is carbon dioxide, as nearly all the CO from the conversion of methane is converted to CO2 by the water gas shift reaction. This needs to be separated and recycled to the cathode where the addition of CO2 to the feed gas is required to provide the CO32- ions for transport through the electrolyte . Systems to achieve this recycling process are more expensive than the fuel stack itself. In addition, corrosion problems have been associated with the molten electrolyte .
Solid Polymer Fuel Cells (SPFC)
The SPFC consists of a proton-conducting polymer electrolyte, usually based on perfluorinated sulphonic acid polymers, with a thin layer of platinum-based catalyst and gas porous electrode support material on each side. It is referred to as a membrane electrode assembly (MEA). The MEA is less than a millimetre thick. Due to its low operating temperature (60-90°C), platinum based materials are the only practical catalysts for the SPFC . Hydrogen is used as the fuel at the anode (or in some cases methanol ) and air as the oxidant, with water and heat as the only by-products .
The SPFC has real market potential for mobile applications such as portable power and electric transport which is due to its relatively short warm up time and high power density. In 1995 and 1996 several field tests of prototype vehicles equipped with SPFC stacks (up to 260kW) were started in Canada, Europe and Japan . Several companies including Daimler-Benz and Toyota Motor Corporation have demonstrated prototype fuel cell powered vehicles at motor shows around the world. The United States Council for Automotive Research (USCAR), which represents Daimler Chrysler, Ford and General Motors hopes to manufacture production prototype cars by 2004, but realises that the SPFC technology faces significant technical challenges before the production of prototype cars can be realised .
Disadvantages with SPFC’s
The problems that have been identified with SPFC are those of cost and the type of fuel that can be utilised in the vehicles. It is the cost of the materials used in the manufacture of the stack components coupled with the need for hydrogen that make the SPFC expensive. Research has been carried out into lowering the cost of the stack. One of proposed ideas is to maximise the effective MEA area, which can be achieved by reducing the sealing requirements. On-board methanol reformers are also being developed which will allow the liquid methanol to be reformed into hydrogen directly in the stack .
Solid Oxide Fuel Cells (SOFC)
Solid Oxide Fuel Cells (SOFCs) offer several potential advantages over other fuel cell systems because the high operating temperature gives flexibility in the choice of fuel, and in particular allows the possibility of running the cell directly on natural gas or other hydrocarbon fuels, internally reforming the fuel within the fuel cell. The SOFC is the ideal device for small-scale application offering tremendous potential for clean and economic production of electrical power as well as higher efficiencies through improved heat utilisation.
The SOFC has a solid ceramic electrolyte, which must conduct either oxygen ions or hydrogen ions. Most of the research to date has concentrated on oxygen ion conducting materials. The most common electrolyte material is yttria stabilised zirconia, (YSZ), which has been used as a solid electrolyte since 1937, although more recent research has focused on alternative electrolyte materials, which can demonstrate high ionic conductivity at lower operating temperatures. The operating temperature of the SOFC is currently between 700-1000°C; the high temperature can support direct methane conversion at the anode although this can lead to problems of carbon deposition. Air is used as the oxidant at the cathode. The most common materials for the two electrodes are a nickel-YSZ cermet for the anode and a LSM perovskite cathode.
During the 1980s the Westinghouse Electric program dominated development of the SOFC. Osaka Gas and Tokyo Gas successfully obtained operating performance of 3kW modules and collaborated with Westinghouse in the construction of 25kW units which were designed to operate on natural gas with internal reforming of the gas occurring at the anode . The next stage of development was the construction of a 100kW prototype stack, which was operated at 1000°C and demonstrated 80% fuel utilisation. The system was run for over 4000 hours before it was shut down due to excessive stack resistance . Their next project, which the new company Siemens-Westinghouse is overseeing, commenced at the end of 1999 was the construction of a 250kW pressurised SOFC-Gas Turbine hybrid System. It is estimated that the hybrid system can achieve total efficiencies of over 90%.
Since the 1990s there has been a huge amount of interest into SOFCs with many companies, such as Rolls Royce, UK , Sulzer Hexis, Switzerland , CFCL, Australia  and utility services around the world, investing time and effort into the development of SOFCs. One of the main emphasis of current work is in low cost manufacture and improvements in the stability of the cells. One way for the SOFCs to be more cost effective is to operate at a lower temperature as this will reduce the demands on the stack components of the SOFC. However, there are still some fundamental problems that need to be solved before the routine operation of lower temperature SOFCs can be achieved.
Advantages of SOFC’s
There are several advantages to using SOFC systems for practical power generation as compared with the other types of fuel cell. SOFC’s have a solid electrolyte, which eliminates the corrosion and liquid management problems of the PAFC and MCFCs. Generally non-precious materials are used in the SOFC components unlike the platinum electrodes needed in the PAFC and SPFC and the ability of SOFC’s to directly reform commercially available fuel at the anode.
1. C. O’Driscoll, Chemistry in Britain, (1995), 656-658.
2. T. Wananbe; Proceedings of the Institute of Mechanical Engineers, Vol 211 Part A, (1997), 113-119.
3. K. Kasahara, M. Morioka, H. Yoshida, H. Shingai, J. Power Sources, 86 (2000) 298-301.
4. L.J.M.J. Blomen and M.N. Mugerwa, Fuel Cell Systems, Plenum Press, (1993) 19-485.
5. R.J. Berger, Applied Catalysis, A-General, 143, (1996), 343-365.
6 J.P.P. Hujismans, Molten Carbonate Fuel Cell (MCFC), ECN Document, ECN-B—96-035, (1996) p 1.
7 J.P.P. Huijmans, G.J. Kraaij, R.C. Makkus, G. Rietveld, E.F. Sitters, H.Th.J.Reijers, J. Power Sources, 86, (2000) 117-121.
8 V. Plzak, B. Rohland and H. Wendt, Modern Aspects of Electrochemistry, Plenum Press (1994).
9 J.H Hirschenhofer, D.B. Stauffer, R.R. Engleman, Fuel Cells A Handbook (Revision 3), US Department of Energy, (1994).
10. S.B. van der Molen, Fuel Cells, ECN Document, ECN-B—96-034, (1996) p 3.
11. X. Ren, S.M. Wilson and S. Gottesfeld Proceedings of the First International Symposium on Proton Conducting Membrane Fuel Cells, eds S. Gottesfeld, G. Halpert and A. Landgrebe, Electrochem. Soc., PV95-23 , (1995), p 199.
12. T.R. Ralph and G.A. Hards, Chemistry and Industry, (May 1998), 337-342.
13. R.K.A.M. Mallant, Solid Polymer Fuel Cells, ECN Document, ECN-B—96-037, (1996) p 1.
14. S.G. Chalk, J.F. Miller and F.W. Wagner, J. of Power Sources, 86, (2000) 40-51.
15. M.P. Hogarth and G.A. Hards, Platinum Metals Review, 40, (1996) 150.
16. S.C. Singhal, Proceedings of the Fifth International Symposium on Solid Oxide Fuel Cells, eds U. Stimming, S.C. Singhal, H. Tagawa, W. Lehnert, Electrochem. Soc. Vol 97-40, (1997) p 37-50.
17. S.C. Singhal, , Proceedings of the Sixth International Symposium on Solid Oxide Fuel Cells, eds S.C. Singhal, and M. Dokiya, Electrochem. Soc. Vol 99-19, (1999) p 39-50.
18. F.J. Gardner, M.J. Day, N.P. Brandon, M.N. Pahley and M. Cassidy, J. Power Sources, 86, (2000), 122-129.
19. R. Diethelm , M. Schmidt, K. Honegger and E. Batawi, Proceedings of the Sixth International Symposium on Solid Oxide Fuel Cells, eds S.C. Singhal, and M. Dokiya, Electrochem. Soc. Vol 99-19, (1999) p 60-67.
20. B. Godfrey, K. Foger, R. Gillespie, R. Bolden and S.P.S. Badwal, J. Power Sources, 86, (2000), 68-73.