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The preceding sections provide parametric performance based on various referenced data at different operating conditions. It is suggested that the following set of equations could be used for performance adjustments unless the reader prefers other data or correlations.

Parameter Equation Comments

Pressure ∆Vp(mV) = 59 log 2 1 P P 19 1 atm < P < 10 atm (5-8) Temperature43 ∆VT(mV) = 0.008(T2 - T1)( °C) * J ∆VT(mV) = 0.04(T2 - T1)( °C) * J 900°C < T < 1050°C (5-11) 800°C < T < 900°C (5-12) Oxidant ∆VCathode(mV) = 92 log

(P ) (P ) 2 2 O 2 O 1 20 0.16 P P 0.20 2 O Total ≤ ≤ 21 (5-14) Fuel ∆VAnode(mV) = 172 log(P / P ) (P / P ) 2 2 2 2 H H O 2 H H O 1 22 0.9 < P / PH2 H O2 < 6.9 T=1000°C, with air23 (5-16) Current Density ∆ VJ(mV) = 0.73∆J 50 < J < 400 mA/cm 2 (5-18) P = 1 atm., T = 1000°C

5.4 Reference

1. N.Q. Minh, "Ceramic Fuel Cells," J. Am. Ceram. Soc., p. 76 [3]563-88, 1993. 2. Courtesy of Siemens Westinghouse.

3. T.H. Etsell, S.N. Flengas, J. Electrochem. Soc., p. 118, 1890 (1971).

4. A.O. Isenberg, in Proceedings of the Symposium on Electrode Materials and Processes for Energy Conversion and Storage, edited by J.D.E. McIntyre, S. Srinivasan and F.G. Will, The Electrochemical Society, Inc., Pennington, NJ, 1977, p. 682.

5. A.O. Isenberg, in Proceedings of the Symposium on Electrode Materials and Processes for Energy Conversion and Storage, edited by J.D.E. McIntyre, S. Srinivasan and F.G. Will, The Electrochemical Society, Inc., Pennington, NJ, 1977, p. 572.

6. D.C. Fee, S.A. Zwick, J.P. Ackerman, in Proceedings of the Conference on High Temperature Solid Oxide Electrolytes, held at Brookhaven National Laboratory, August 16-17, 1983, BNL 51728, compiled by F.J. Salzano, October 1983, p. 29.

7. D.W. Dees, T.D. Claar, T.E. Easler, D.C. Fee, F.C. Mrazek, J. Electrochem. Soc., p. 134, 2141, 1987.

8. E.F. Sverdrup, C.J. Warde, A.D. Glasser, in From Electrocatalysis to Fuel Cells, Edited by G. Sandstede, University of Washington Press, Seattle, WA, 1972, p. 255.

9. D.H. Archer, L. Elikan, R.L. Zahradnik, in Hydrocarbon Fuel Cell Technology, Edited by

B.S. Baker, Academic Press, New York, NY, 1965, p. 51.

10. D.H. Archer, J.J. Alles, W.A. English, L. Elikan, E.F. Sverdrup, R.L. Zahradnik, in Fuel Cell Systems, Advances in Chemistry Series 47, edited by R.F. Gould, American Chemical Society, Washington, DC, 1965, p. 332.

11. M.S.S. Hsu, W.E. Morrow, J.B. Goodenough, in Proceedings of the 10th Intersociety Energy Conversion Engineering Conference, The Institute of Electrical and Electronics Engineering, Inc., New York, NY, 1975, p. 555.

12. M.S.S. Hsu, T.B. Reed, in Proceedings of the 11th Intersociety Energy Conversion Engineering Conference, American Institute of Chemical Engineers, New York, NY, 1976, p. 1.

13. M. Hsu, "Zirconia Fuel Cell Power System," 1985 Fuel Cell Seminar Abstracts, 1985 Fuel Cell Seminar, Tucson, AZ, May 19-22, 1985.

14. M. Hsu, "Zirconia Fuel Cell Power System Planar Stack Development," Fuel Cell Abstracts, 1986 Fuel Cell Seminar, Tucson, AZ, October 26-29, 1986.

15. Fuel Cells, DOE/METC-86/0241, Technology Status Report, Morgantown Energy Technology Center, Morgantown, WV, 1986.

16. N.Q. Minh, "High-Temperature Fuel Cells, Part 2: The Solid Oxide Cell," ChemTech, Vol. 21, February, 1991.

17. Y. Jatsuzaki, et al., "High Power Density SOFC Development at Tokyo Gas," Fuel Cell Program and Abstracts, 1992 Fuel Cell Seminar, Tucson, Arizona, November 29 - December 2, 1992.

18. N. Minh, K. Barr, P. Kelly, K. Montgomery, "AlliedSignal Solid Oxide Fuel Cell Technology," Program and Abstracts, 1996 Fuel Cell Seminar, pg. 40-43, 1996.

19. "SOFCo Profile Information for DOE Handbook," Notes from SOFCo, 1998.

20. K. Krist, "Gas Research Institute's Fundamental Research on Intermediate-Temperature Planar Solid Oxide Fuel Cells," in Fuel Cell Program and Abstracts, 1992 Fuel Cell Seminar, Tucson, AZ, November 29 - December 2, 1992.

21. B.L. Halpern, J.W. Golz, Y. Di, "Jet Vapor Deposition of Thin Films for Solid Oxide and Other Fuel Cell Applications," in Proceedings of the Fourth Annual Fuel Cells Contractors Review Meeting, U.S. DOE/METC, July, 1992.

22. H.U. Anderson, M.M. Nasrallah, "Characterization of Oxides for Electrical Delivery Systems," An EPRI/GRI Fuel Cell Workshop on Fuel Cell Technology Research and Development, New Orleans, LA, April 13-14, 1993.

23. O. Yamamoto, et al., "Zirconia Based Solid Ion Conductors," The International Fuel Cell Conference Proceedings, NEDO/MITI, Tokyo, Japan, 1992.

24. S.C. Singhal, "Recent Progress in Tubular Solid Oxide Fuel Cell Technology," Proceedings of the Fifth International Symposium on Solid Oxide Fuel Cells (SOFC-V), The Electrochemical Society, Inc., Pennington, NJ, 1997.

25. Y. Matsuzaki, et al., "High Power Density SOFC Development at Tokyo Gas," Fuel Cell Program and Abstracts, 1992 Fuel Cell Seminar, Tucson, AZ, November 29 - December 2, 1992.

26. C. Bagger, "Improved Production Methods for YSZ Electrolyte and Ni-YSZ Anode for SOFC," Fuel Cell Program and Abstracts, 1992 Fuel Cell Seminar, Tucson, AZ, November 29 - December 2, 1992.

27. J.L. Bates, "Alternative Materials for Solid Oxide Fuel Cells: Factors Affecting Air-Sintering of Chromite Interconnections," Proceedings of the Fourth Annual Fuel Cells Contractors Review Meeting, U.S. DOE/METC, July, 1992.

28. A.R. Nicoll, G. Barbezat, A. Salito, "The Potential of Plasma Spraying for the Deposition of Coatings on SOFC Components," The International Fuel Cell Conference Proceedings, NEDO/MITI, Tokyo, Japan, 1992.

29. F. Uchiyama, et al., "ETL Multi-Layer Spray Coating for SOFC Component," The International Fuel Cell Conference Proceedings, NEDO/MITI, Tokyo, Japan, 1992.

30. C. Tanner, et al., "Fabrication and Characterization of Ceria-Based Electrolytes," An EPRI/GRI Fuel Cell Workshop on Fuel Cell Technology Research and Development, New Orleans, LA, April 13-14, 1993.

31. N.Q. Minh, C.R. Horne, R.A. Gibson, "A Novel and Cost-Effective Fabrication Method for Reduced-Temperature Solid Oxide Fuel Cell Applications," An EPRI/GRI Fuel Cell Workshop on Fuel Cell Technology Research and Development, New Orleans, LA, April 13-14, 1993.

32. W. Bakker, R. Goldstein, "Development of Low Temperature Solid Oxide Fuel Cells," Program and Abstracts, 1996 Fuel Cell Seminar, pp 48-50, 1996.

33. A.F. Sammells, "Perovskite Solid Electrolytes for SOFC," Proceedings of the Fourth Annual Fuel Cells Contractors Review Meeting, U.S. DOE/METC, July, 1992.

34. J. Goodenough, "Solid Oxide Fuel Cells with Gallate Electrolytes," summary data on research supported by EPRI, 1998.

35. I. Bloom, et al., "Electrolyte Development for Intermediate Temperature, Solid Oxide Fuel Cells," Fuel Cell Program and Abstracts, 1992 Fuel Cell Seminar, Tucson, AZ, November 29 - December 2, 1992.

36. I. Bloom, M. Krumpelt, "Intermediate Temperature Electrolytes for SOFC," Proceedings of the Fourth Annual Fuel Cells Contractors Review Meeting, U.S. DOE/METC, July, 1992. 37. H. Tsuneizumi, et al., "Development of Solid Oxide Fuel Cell with Metallic Separator," The

International Fuel Cell Conference Proceedings, NEDO/MITI, Tokyo, Japan, 1992.

38. P. Han, et al., "Novel Oxide Fuel Cells Operating at 600 - 800°C," An EPRI/GRI Fuel Cell Workshop on Fuel Cell Technology Research and Development, New Orleans, LA, April 13-14, 1993.

39. J.T. Brown, Energy, p. 11, 209, 1986.

40. H. Ide et al., "Natural Gas Reformed Fuel Cell Power Generation Systems - A Comparison of Three System Efficiencies," Proceedings of the 24th Intersociety Energy Conversion Engineering Conference, The Institute of Electrical and Electronics Engineers, Washington, D.C., 1989.

41. Data from Allied-Signal Aerospace Company, 1992.

42. C. Zeh, private communication, 2nd edition of Handbook, April 29, 1987.

43. A. Sammells, "Perovskite Electrolytes for SOFC," Proceedings of the Third Annual Fuel Cells Contractors Review Meeting, U.S. DOE/METC, p. 152, June, 1991.

44. A. Khandkar, S. Elangovan, "Planar SOFC Development Status," Proceedings of the Second Annual Fuel Cells Contractors Review Meeting, U.S. DOE/METC, p. 152, May, 1990. 45. C. J. Warde, A. O. Isenberg, J. T. Brown "High-Temperature Solid-Electrolyte Fuel-Cells

Status and Programs at Westinghouse," in Program and Abstracts, ERDA/EPRI Fuel Cell Seminar, Palo Alto, CA, June 29-30 to July 1, 1976.

46. W.J. Dollard, J.T. Brown, "Overview of the Westinghouse Solid Oxide Fuel Cell Program," Fuel Cell Abstracts, 1986 Fuel Cell Seminar, Tucson, AZ, Oct. 26-29, 1986.

47. N. Maskalick, "Contaminant Effects in Solid Oxide Fuel Cells," Proceedings of the Fourth Annual Fuel Cells Contractors Review Meeting, U.S. DOE/METC, July, 1992.

Progress," AlliedSignal, Fuel Cell Seminar Program and Abstracts, 1992 Fuel Cell seminar, 1992.

49. Y. Yoshida et al., "Development of Solid Oxide Fuel Cell," paper provided by Mitsubishi Heavy Industries Ltd.

50. A. Khandkar et al., "Planar SOFC Technology Status and Overview," Ceramatec, Inc., Fuel Cell Seminar Program and Abstracts, 1992 Fuel Cell Seminar, Tucson, AZ, November 29-December 2, 1992.

51. "Research and Development on Fuel Cell Power Generation Technology," FY 1990 Annual Report, NEDO, April, 1991.

52. T. Nakanishi, "Substrate Type, Planar Solid Oxide Fuel Cell," Fuji Electric, Fuel Cell Seminar Program and Abstracts, 1992 Fuel Cell Seminar, Tucson, AZ, November 29 - December 2, 1992.

Polymer electrolyte fuel cells (PEFC) deliver high power density, which offers low weight, cost, and volume. The immobilized electrolyte membrane simplifies sealing in the production process, reduces corrosion, and provides for longer cell and stack life. PEFCs operate at low temperature, allowing for faster startups and immediate response to changes in the demand for power. The PEFC system is seen as the system of choice for vehicular power applications, but is also being developed for smaller scale stationary power. For more detailed technical information, there are excellent overviews of the PEFC (1, 2).

6.1 Cell Components

The use of organic cation exchange membrane polymers in fuel cells was originally conceived by William T. Grubbs (3) in 1959. The desired function of the ion membrane was to provide an ion conductive gas barrier. Strong acids were used to provide a contact between the adjacent membrane and catalytic surfaces. During further development, it was recognized that the cell functioned well without adding acid. As a result, present PEFCs do not use any electrolyte other than the hydrated membrane itself (4). The basic cell consists of a proton conducting membrane, such as a perfluorinated sulfonic acid polymer, sandwiched between two platinum impregnated porous electrodes. The back of the electrodes is made hydrophobic by coating with an appropriate compound, such as Teflon. This wet proof coating provides a path for gas diffusion to the catalyst layer.

The electrochemical reactions of the PEFC are similar to those of the PAFC: hydrogen at the anode provides a proton, freeing an electron in the process that must pass through an external circuit to reach the cathode. The proton, which remains solvated with a certain number of water molecules, diffuses through the membrane to the cathode to react with oxygen and the returning electron (5). Water is subsequently produced at the cathode.

Because of the intrinsic nature of the materials used, a low temperature operation of approximately 80oC is possible. The cell also is able to sustain operation at very high current densities. These attributes lead to a fast start capability and the ability to make a compact and lightweight cell (5). Other beneficial attributes of the cell include no corrosive fluid hazard and lower sensitivity to orientation. As a result, the PEFC is particularly suited for vehicular power application. Transportation applications mean that the fuel of choice will probably be methanol (6), although hydrogen storage on-board in the form of pressurized gas and the partial oxidation of gasoline (7) is being considered. The cell also is being considered for stationary power application, which will use natural gas or other hydrogen-rich gases.

The lower operating temperature of a PEFC results in both advantages and disadvantages. Low temperature operation is advantageous because the cell can start from ambient conditions quickly, especially when pure hydrogen fuel is available. It is a disadvantage in that platinum catalysts are required to promote the electrochemical reaction. Carbon monoxide (CO) binds strongly to platinum sites at temperatures below 150oC, which reduces the sites available for hydrogen chemisorption and electro-oxidation. Because of CO poisoning of the anode, only a few ppm of CO can be tolerated with the platinum catalysis at 80oC. Because reformed hydrocarbons contain about one percent of CO, a mechanism to reduce the level of CO in the fuel gas is needed. The low temperature of operation also means that little if any heat is available from the fuel cell for any endothermic reforming process (8, 9).

Both temperature and pressure have a significant influence on cell performance; the impact of these parameters will be described later. Present cells operate at 80oC, nominally, 0.285 MPa (30 psig) (5), and a range of 0.10 to 1.0 MPa (10 to 100 psig). Using appropriate current collectors and supporting structure, polymer electrolyte fuel cells and electrolysis cells should be capable of operating at pressures up to 3000 psi and differential pressures up to 500 psi (10).

6.1.1 Water Management

Water is produced not as steam, but as liquid in a PEFC. A critical requirement of these cells is maintaining a high water content in the electrolyte to ensure high ionic conductivity. The ionic conductivity of the electrolyte is higher when the membrane is fully saturated, and this offers a low resistance to current flow and increases overall efficiency. The water content in the cell is determined by the balance of water or its transport during the reactive mode of operation. Contributing factors to the water transport are the water drag through the cell, back diffusion from the cathode, and the diffusion of any water in the fuel stream through the anode. The water transport is a function of the cell current and the characteristics of the membrane and the electrodes. Water drag refers to the amount of water that is pulled by osmotic action along with the proton (11). Between 1 and 2.5 molecules are dragged with each proton (12). As a result, the ion exchanged can be envisioned as a hydrated proton, H(H2O)n+. The water drag increases at

high current density, and this makes the water balance a potential concern. During actual operation, however, back diffusion of water from the cathode to the anode through the thin membrane results in a net water transport of nearly zero (12, 13). A detailed modeling of the reactions and water balance is beyond the scope of this handbook; References (14) and (15) should be reviewed for specific modeling information.

Water management has a significant impact on cell performance, because at high current densities mass transport issues associated with water formation and distribution limit cell output. Without adequate water management, an imbalance will occur between water production and evaporation within the cell. Adverse effects include dilution of reactant gases by water vapor, flooding of the electrodes, and dehydration of the solid polymer membrane. The adherence of the membrane to the electrode also will be adversely affected if dehydration occurs. Intimate contact between the electrodes and the electrolyte membrane is important because there is no free liquid electrolyte to form a conducting bridge. If more water is exhausted than produced, then it is important to humidify the incoming anode gas. If there is too much humidification, however, the electrode floods, which causes problems with diffusing the gas to the electrode. A smaller current, larger

reactant flow, lower humidity, higher temperature, or lower pressure will result in a water deficit. A higher current, smaller reactant flow, higher humidity, lower temperature, or higher pressure will lead to a water surplus. There have been attempts to control the water in the cell by using external wicking connected to the membrane to either drain or supply water by capillary action. Another alternative is to control the cell water content by humidifying the incoming reactant gases (14). More reliable forms of water management also are being developed based on continuous flow field design and appropriate operating adjustments. A temperature rise can be used between the inlet and outlet of the flow field to increase the water vapor carrying capacity of the gas streams. At least one manufacturer, Ballard Power Systems of Canada, has demonstrated stack designs and automated systems that manage water balances successfully.

6.1.2 State-of-the-Art Components

There has been an accelerated interest in polymer electrolyte fuel cells within the last few years, which has led to improvements in both cost and performance. Development has reached the point where motive power applications appear achievable at an acceptable cost for commercial markets. Noticeable accomplishments in the technology, which have been published, have been made at Ballard Power Systems. PEFC operation at ambient pressure has been validated for over 25,000 hours with a six cell stack without forced air flow, without humidification, and without active cooling (17). Complete fuel cell systems have been demonstrated for a number of transportation applications including public transit buses and passenger automobiles. Recent development has focused on cost reduction and high volume manufacture for the catalyst, membranes, and bipolar plates. This coincides with ongoing research to increase power density, improve water management, operate at ambient conditions, tolerate reformed fuel, and extend stack life. In the descriptions that follow, Ballard Power Systems fuel cells are considered representative of the state-of-the-art because of the company's discernible position in the transportation and stationary fuel cell application fields.

Manufacturing details of the Ballard Power Systems cell and stack design are proprietary (18), but the literature provides some information on the cell and stack design. An example schematic of a manufacturer's cell is shown in Figure 6-1.

Figure 6-1 PEFC Schematic (19)

The standard electrolyte material presently used in PEFCs is a fully fluorinated Teflon-based material produced by E.I. DuPont de Nemours for space application in the mid-1960s. The DuPont electrolytes have the generic brand name Nafion, and the specific type used most often in present PEFCs is membrane No. 117 (20). The Nafion membranes, which are fully fluorinated polymers, exhibit exceptionally high chemical and thermal stability; and are stable against chemical attack in strong bases, strong oxidizing and reducing acids, H2O2, CI2, H2, and O2 at temperatures

up to 125oC (21). Nafion consists of a fluoropolymer backbone, similar to Teflon, upon which sulfonic acid groups are chemically bonded (22). DuPont fluorinated electrolytes exhibited a substantial improvement in life over previous electrolytes and have achieved over 50,000 hours of operation. The Dow Chemical Company has produced an electrolyte membrane, the XUS 13204.10, which exhibits lower electrical resistance and permits increased current densities than the Nafion membrane, particularly when used in thinner form (18). These membranes exhibit good performance and stability, but their current price is deemed too high for transportation markets. This has led to ongoing research into alternative materials.

The present electrodes are cast as thin films and bonded to the membrane. Low platinum loading electrodes ( _≤0.60 mg Pt/cm2 cathode and _≤0.25 mg Pt/cm2, 0.12 mg Ru/cm2 anode) tested in the Ballard Mark V stack have performed as well as current high platinum loading electrodes (4.0 to 8.0 mg Pt/cm2). These electrodes, which have been produced using a high-volume manufacturing process, have achieved 600 mA/cm2 at 0.7 V. The equivalent platinum loading of these electrodes is 1.5 g Pt/kW (23). To improve utilization of the platinum, a soluble form of the polymer is incorporated into the porosity of the carbon support structure. This increases the interface between the electrocatalyst and the solid polymer electrolyte. Two methods are used to incorporate the polymer solution within the catalyst. In Type A, the polymer is introduced after fabrication of the electrode; in Type B, it is introduced before fabrication. Performance of low platinum loading electrodes (Type B) is shown in Figure 6-2.

0.00 0.20 0.40 0.60 0.80 1.00 0 500 1000 1500 2000

Current Density (mA/cm2)

Cell Potential (V) 0 0.2 0.4 0.6 0.8 1 0 500 1000 1500 2000 Type B cathode, 0.11 mgPt/cm^2 Type B cathode, 0.20 mgPt/cm^2

Figure 6-2 Performance of Low Platinum Loading Electrodes (23)

Most PEFCs currently use machined graphite plates for current collection and distribution, gas distribution, and thermal management. Cooling is accomplished by using a heat transfer fluid, usually water, which is pumped through integrated coolers within the stack. The temperature rise across the cell is kept to less than 10oC. Water cooling and humidification are in series, which results in a need for high quality water. The cooling unit of a cell can be integrated to supply reactants to the membrane electrode assembly (MEA), remove reaction products from the cell, and seal off the various media against each other and the outside (Figure 6-1). The conducting parts of the frames are titanium; non-conducting parts are polysulfone (24).

The primary contaminants of a PEFC are carbon monoxide (CO), carbon dioxide (CO2), and the

hydrocarbon fuel. Reformed hydrocarbon fuels typically contain at least 1% CO. Even small amounts of CO in the gas stream, however, will preferentially adsorb on the platinum catalysts surface and block access of the hydrogen to the catalyst sites. Tests indicate that approximately 10 ppm of CO in the gas stream begins to impact cell performance (6, 25). Fuel processing can reduce CO content to several ppm, but there are system costs associated with increased fuel purification. Platinum/ruthenium catalysts that have intrinsic tolerance to CO are being developed. These electrodes have been shown in controlled laboratory experiments to be CO tolerant up to 200 ppm (26). Although much less significant than CO poisoning, CO2 affects

anode performance through the reaction of CO2 with adsorbed hydrides on platinum. This

reaction is the electrochemical equivalent of the reverse of the water gas shift reaction.