PHOSPHORIC ACID FUEL CELLS
5.5 DEVELOPMENT OF LARGE STATIONARY POWER PLANTS
More complete data for the large-scale PAFC-based power plants built or tested in Japan can be found in papers by Hojo et al. (1996) and Kasahara et al.
(2000).
5.6 THE FUTURE OF PAFCs
Toward the end of the 1990s, interest in PAFCs and PAFC-based power plants gradually waned, despite the success that had been achieved, the relatively large number of intermediate-power PC-25 plants built, and the installation of several megawatt-sized power plants in a number of places. On the one hand, this had a strictly economic basis: the high cost of such plants. On the other hand, there were strictly technical problems, that is, insufficient operating reliability in the long term.
Cost of PAFC-Based Power Plants
It was reported by an official of UTC and Toshiba, R. Whitaker (1998), that the joint company, together with several other companies and a number of government agencies, had spent about $200 million on research and technology to develop and manufacture the PC-25 plants. Not surprisingly, part of this
5.6 THE FUTURE OF PAFCs 105
expenditure had to be recovered in the sales price. The production cost of each of the early units was slightly over $1 million, more than $5000/kW. All 144 units delivered up to the time of Whitaker’s article were given away at half of this figure, that is, with a considerable subsidy provided by the company. This subsidy should have served to enlarge the future market. It should be pointed out for comparison that the cost of an alternative power plant in the same class (e.g., wind turbines) is about $500 to $700/kW. A further decrease in the production cost of fuel cell power plants would be possible by lowering the labor cost in higher production volumes. One had hoped for a ‘‘virtuous cycle’’
but experienced a ‘‘vicious cycle,’’ seeing that increases in production and sales volume were not possible without lower prices, but lowering the prices without raising the production volume was equally impossible. Measures have been taken, of course, to get through this impasse, but substantial results have not yet been achieved.
Reliability and Long-Term Operation of PAFC-Based Power Plants PAFC-based power plants of intermediate and large power output are designed to work for 50,000 hours, which is approximately five years. During this period, mandatory checking and tune-ups are performed. Many of the large number of PC-25-type plants installed have worked for the designated period and continue to work. Yet in individual units, malfunctions and a gradual performance decline were seen. According to data reported by Blomen and Mugerwa (1990), almost 95% of the sudden interruptions, particularly in large plants, were caused by a mismatch in the work and the effect of individual components, such as electronic monitoring and controlling equipment, mechanical gear, and sensors: that is, by events not related directly to the fuel cell battery. However, during prolonged operation, processes causing a gradual decline in perfor- mance also take place in the fuel cells.
In 1991, Paffett et al. reported on an autopsy and detailed examination of individual PAFCs that had worked 5000 or 16,000 hours. By various methods, such as electron microscopy and x-ray photoelectron spectroscopy, they were able to show that during 5000 hours of operation, strong corrosion (electro- chemical oxidation) occurred at the carbon support of the platinum catalyst in the catalytic layer of the cathode; this corrosion led to a loss of contact between the support and the catalyst particles. In addition, the highly disperse platinum particles recrystallized and the catalyst’s working surface area decreased. No such effects were seen at the anode (the hydrogen electrode), even after 16,000 hours. The mechanical integrity of the electrodes was quite satisfactory after 5000 hours, but much less so after 16,000 hours. The authors unambiguously attributed the performance loss of the cells to this carbon and platinum corrosion. These corrosion processes have to do with the rather highly positive potential of the oxygen electrode. At lower current densities (more so at open circuit and zero current), the potential shifts even further to the positive side, and the corrosion rate increases further.
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Song et al. (2000) found that a reduction in the rate of gas supply to the electrodes or, worse, the complete cessation of gas supply may in individual cases lead to irreversible performance loss.
5.7 IMPORTANCE OF PAFCs FOR FUEL CELL DEVELOPMENT First data on fuel cells based on concentrated phosphoric acid solutions date from the mid-1960s. After three decades, beginning about 1995, very few papers on phosphoric acid fuel cells have appeared in the scientific literature. Of course, many of the numerous intermediate-size and large power plants that had been built in prior years still function, but work toward their further development and improvement has practically ceased.
The period during which these fuel cells were receiving prime interest lasted only about three decades, but they played an important role in the development of fuel cells as such. For the first time a relatively large-scale industrial production of fuel cell–based power plants was initiated, and such plants were spread widely among many users. As a result, many scientific and business circles worldwide have recognized that fuel cells are an entirely real possibility for applications useful to humankind. During this period of PAFC develop- ment, the first large government schemes and projects for fuel cell research and engineering were implemented in many countries of the world (United States, Japan, Russia, and others).
During PAFC research and development, technical solutions were found which were then adopted successfully in the development of other fuel cell types. This is true in particular for the use of platinum catalysts, not in a pure form but as deposits on a carbon support (e.g., carbon black), leading to a considerable drop in the amount of platinum needed to manufacture fuel cells.
During this period, the influence of traces of carbon monoxide in hydrogen on the performance of platinum catalysts was first investigated. It was shown that Pt–Ru catalysts could be used to reduce the influence of carbon monoxide. It was also shown for the first time that the performance of an oxygen electrode could be improved by using catalysts of platinum alloyed with iron-group metals (Kim et al., 1993; Watanabe et al., 1994).
REFERENCES
Blomen L. J. M. J., M. N. Mugerwa,J. Power Sources,29, 71 (1990).
Grothuss T.,Ann. Chim.,58, 54 (1806).
Grubb W. T., L. W. Niedrach,J. Electrochem. Soc.,110, 1086 (1963).
Hojo N., M. Okuda, M. Nakamura,J. Power Sources,61, 73 (1996).
Kasahara K., M. Morioka, H. Yoshida, H. Shingai,J. Power Sources,86, 298 (2000).
Kim K. T., J. T. Hwang, Y. G. Kim, J. S. Chung,J. Electrochem. Soc.,140, 31 (1993).
Mori T., A. Honji, T. Kahara, Y. Hishinuma,J. Electrochem. Soc.,135, 1104 (1998).
REFERENCES 107
Neergat M., A. K. Shukla,J. Power Sources,102, 317 (2001).
Paffett M. T., W. Hutchinson, J. D. Farr, et al.,J. Power Sources,36, 137 (1991).
Song R.-H., C.-S. Kim, D. R. Shin,J. Power Sources,86, 289 (2000).
Song R.-H., S. Dheenadayalan, D.-R. Shin,J. Power Sources,106, 167 (2002).
Watanabe M., K. Tsurumi, N. Mizukami, et al.,J. Electrochem. Soc.,141, 2659 (1994).
Whitaker R.,J. Power Sources,71, 71 (1998).
Monographs
Kinoshita K.,Electrochemical Oxygen Technology, Wiley, New York, 1992.
Srinivasan S.,Fuel Cells: From Fundamentals to Applications, Springer Science+ Busi- ness Media, New York, 2006.
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CHAPTER 6