Solid polymer fuel cells: An alternative to batteries in electric vehicles—An overview

Solid polymer fuel cells: An alternative to batteries in electric vehicles—An overview

ht. J. Hydrogen Energy, Vol. 20, No. I, pp. 521-529, 1995 Copyright Pergamon 0 1995 International for Hydrogen Energy Elsevier Science Ltd Printed ...

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ht. J. Hydrogen Energy, Vol. 20, No. I, pp. 521-529, 1995 Copyright

Pergamon

0 1995 International

for Hydrogen Energy Elsevier Science Ltd Printed in Great Britain. All rights reserved 0360-3199195 S9.50 + 0.00

0360-3199(94)E0012-N

SOLID

Association

POLYMER FUEL CELLS: AN ALTERNATIVE TO BATTERIES IN ELECTRIC VEHICLES-AN OVERVIEW R. E. BILLINGS and M. SANCHEZ International Academy of Science,Independence, MO 64057, U.S.A.

(Received for publication

14 February

1994)

the world becomes increasingly concerned about our environment and the effects various pollutants have on our planet, increasing focus is being placed on clean energy alternatives for satisfying growing energy demands. With legislation in California requiring that 2 % of total vehicle sales in 1998 be zero emission vehicles, the automobile industry is in urgent need of a nonpolluting energy solution. Although electric batteries offer a clean source of energy, vehicle range is limited and depreciation costs are high. The hydrogen fuel cell is another zero emission alternative offering greater vehicle range and lower depreciation charges. An overview of several recent solid polymer fuel cell projects is presented. Abstract-As

INTRODUCTION Hydrogen

Although the fuel cell was developed in 1839, a major push toward advancement and commercialization of this important technology has only been underway the past 15 years. Today, more than 200 million U.S. dollars are spent worldwide each year on fuel cell research and development [l], in search of a solution to meet our growing energy demands while protecting the environment. Various fuels can be used in different types of fuel cells but the hydrogen fuel cell is of particular interest from the environmentalist’s point of view. In the case of such a fuel cell, hydrogen and air are continuously provided to the cell, with water and energy being electrochemically produced with zero emissions. Fuel cells have no moving parts, make no noise and utilize hydrogen very efficiently. The basic chemistry of the hydrogen fuel cell, as depicted in Fig. 1, is as follows.

Hydrogen

Exhaust

4

SolId Polymer

Electrolyte

Fig. 1. The solid polymer electrolyte fuel cell.

The overall fuel cell reaction follows. Overall reaction: H, + +O, + H,O

Hydrogen flows into the anode where it dissociates in the presence of a catalyst, forming hydrogen ions (protons) and giving up electrons to the anode. Anode reaction: H, --f 2Hf + 2e-

(1)

Meanwhile, oxygen is being supplied to the cathode. Hydrogen ions are transported through an electrolyte (an ion-conductive substance), as the liberated electrons flow, by way of an electrical cable, through an external load to the cathode to participate in the water-forming reaction. Cathode reaction: 2Hf + 2e- + $0, ---)H,O

(2)

(3) Each individual cell (anode/cathode sandwich) typically produces close to 1 V Cells are assembled in series and constructed into stacks to acquire the desired voltage. Over the years, various types of fuel cells have been developed. Often, these fuel cells are categorized by the electrolyte incorporated into their design. Among the most common configurations are the phosphoric acid, molten carbonate, solid oxide, solid polymer electrolyte and alkaline fuel cells. Of these, only the solid polymer fuel cell offers advantages of low operating temperatures and a safe, nonvolatile electrolyte-two important considerations when choosing an appropriate fuel cell system for transportation requirements.

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THE SOLID

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FUEL CELL

The electrolyte utilized in the solid polymer fuel cell is substantially different than that used in other fuel cell configurations. Under normal conditions, the dry electrolyte looks and feels like a thin sheet of transparent plastic. But, when placed in water, this unusual substance significantly expands and a highly acidic environment, comparable to that of 10 wt% sulfuric acid solution [Z], is produced in the hydrated membrane. The idea of using an organic ion-exchange membrane in solid polymer fuel cells was first introduced by Grubb in the 1950s [3]. These early membranes included hydrocarbon-type polymers which were found to be inadequate for fuel cell applications because of their chemical instability [4]. In the early 1960s shortly after development of the stable and non-reactive substance [email protected], the Du Pont Company’s Plastics Exploratory Research Group, working on new derivatives of this revolutionary product, stumbled upon a new substance that had the interesting characteristic of being able to interact with the environment [S]. These researchers found that if they attached sulfonic acid groups to the ends of the perfluorinated chains, the resultant product was proton selective-in other words, cations (positively-charged ions) were allowed to pass through this substance, while anions (negatively-charged ions) were rejected. Soon researchers at Du Pont discovered the importance of this [email protected] derivative for chloralkali applications-hence, a major effort was put forth optimizing this product, later named [email protected], for this particular industry. Nonetheless, various research efforts have demonstrated the usefulness of [email protected] in other technological fields, including electrochemistry. In 1966, General Electric demonstrated the potential of using solid polymer electrolytes in fuel cells when they constructed a unit for use in the U.S. Gemini Space Program [6]. Even today, [email protected] is still the most widely tested ion-exchange membrane used in solid polymer fuel cell research [7].

and M. SANCHEZ

The basic chemistry for the solid polymer electrolyte when used in a hydrogen/oxygen fuel cell is demonstrated in Fig. 2. The positively-charged hydrogen ions, or protons, generated at the anode are attracted to the negatively-charged sulfonic acid radicals. These sulfonic acid groups provide reactive sites for the hydrogen ions which are then allowed passage through the membrane. Transported hydrogen ions chemically combine with oxygen on the other side of the membrane producing water. Over the years, other manufacturers have begun producing similar solid polymer electrolytes, providing attractive alternatives to [email protected] in solid polymer fuel cells. Japanese companies, including Asahi Chemical and Chlorine Engineers, have developed their own ionexchange membranes. Here in the United States, a new series of perfluorinated membranes has recently become available from the Dow Chemical Company [8,9]. Like Nafiona, the Dow experimental membrane also has a [email protected] backbone; however, the side chain containing the sulfonic acid group is shorter. Like the Du Pont product, the Dow membrane has also produced favorable results in solid polymer fuel cells although this polymer was also developed specifically for the chloralkali industry. One of the major differences in the Dow polymer is that it absorbs approximately half the amount of water absorbed by [email protected] Nonetheless, the experimental membrane still possesses many of the exceptional ionexchange properties of [email protected] and several researchers have achieved significant improvements in fuel cell performance utilizing this product. Traditionally, conventional solid polymer fuel cells have required as much as 4 mg cmm2 of platinum black on both the cathode and the anode [lo], for a total loading of 8 mg Pt per cm’. Typically, this catalyst has been hot-pressed directly to the membrane utilizing a [email protected] binder. These prepared membranes are then, often, bonded to the surface of porous electrodes by using heat and pressure. This process is believed to be necessary to produce the required contact between the electrocatalyst and the membrane surface [ll]. RECENT RESEARCH

SOLID POLYMER FUEL CELL AND DEVELOPMENT PROJECTS

Solid polymer fuel cell technology has progressed considerably in recent times: catalyst loading densities have decreased and fuel cell performance has increased [12-181. An overview of several solid polymer fuel cell research projects conducted between 1990 and the present time are discussed below. International

Solid Polymer

Electrolyte

Fig. 2. Basic chemistry of the solid polymer electrolyte.

Academy of Science

The Billings cell, developed at the International Academy of Science, employs an advanced state-of-the-art solid polymer fuel cell design vastly different from that of traditional fuel cells available today. One major difference in the Billings cell is that it is far more compact than the cells offered by many other fuel cell manufac-

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Ballard Power Systems

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Fig. 3. Effect of platinum loading on fuel cell performance. Cell platinum loading and operating conditions: (a) Air, 0.90 mg Pt per cm2; (A) Air, 20.0 mg Pt per cm’; (0) 0,, 0.90 mg Pt per cm’; (m) 0,, 20.0 mg Pt per cm’. Gas pressure: 1 atm. Cell temperature: 50°C. Source: Texas A&M University, 1991.

turers. It is also one of the few available solid polymer fuel cells specifically designed to be able to function bidirectionally-as a fuel cell, hydrogen and oxygen from the air are chemically combined to generate the required energy; alternatively, as an electrolyzer, water and electricity are provided so that hydrogen and oxygen can be produced. A proprietary method of catalyst application provides satisfactory results with total cell loading densities of less than 1 mg cm-‘. Presently, system life testing is being conducted and additional research is underway to further minimize catalyst loadings. The overall cell design is being carefully evaluated and further experimentation shall be carried out in order to optimize the complete solid polymer fuel cell system in terms of performance, reliability and cost. It is anticipated that the Billings cell will be ready for commercialization in the near future.

1.0

,

/

The anodes and cathodes of the Ballard solid polymer fuel cell [19] are prepared by applying a small amount of platinum black to one surface of a thin sheet of porous carbon paper which has been ‘wet-proofed’ with [email protected] The electrolyte is then hot-pressed between the anode and cathode, producing a membrane/electrode assembly. This assembly, which is the heart of the fuel cell, measures less than 1 mm in thickness. On the backs of the anode and cathode are connected channeled graphite flow field plates. The channels are used to supply reactant gases, while the ridges between the channels make electrical contact with the electrodes and conduct current to the external circuit. Product water is rejected from the back of the cathode into the oxidant gas stream where it is carried out of the fuel cell by the excess oxidant flow. Recent developments by Ballard Power Systems, based on the Dow experimental membrane [ 151, have reduced the size of the fuel cell, making possible the use of their cell in motive power applications. Texas A&M

University

Researchers at Texas A&M University are studying ways to improve platinum loading in solid polymer fuel cells. Utilizing a cell fabrication technique developed by Los Alamos National Laboratory, scientists at Texas A&M have been able to reduce cell platinum loading to 0.90 mg cme2 with minor impact on fuel cell performance

cw. In the reported experiments, a 5 cm2 fuel cell was operated at 50°C with inlet gas pressures of 1 atm Data were collected using both pure oxygen and air as the

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OVERVIEW

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Fig. 4. Effect of gas pressure on fuel cell performance. Oxidant gas pressure: (A) Air at 1 atm; (A) Air at 5 atm; (0) 0, at 1 atm; and (m) 0, at 5 atm. Cell platinum loading: 0.90 mg cm-‘. Cell temperature: 50°C. Source: Texas A&M University, 1991.

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(mAlcm2)

Fig. 5. Fuel cell performance of phase inversion prepared electrodes utilizing cast [email protected] (m) Protech commercially prepared electrode with 0.35 mg Pt per cm*; (0) Fluka 5 % prepared by phase inversion with 0.3 mg Pt per cm’; (A) Fluka 10% prepared by phase inversion with 0.6 mg Pt per cm’. Electrolyte: 3 mil Nafion:’ cast from solution. Cell temperature: 50°C. Source: Weizmann Institute of Materials Research and the Polymer Institute. 1990.

R. E. BILLINGS and M. SANCHEZ

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Fig. 6. Fuel cell performance of phase inversion prepared electrodes utilizing commercially available Nafion”. ( n ) Protech commercially prepared electrode with 0.35 mg Pt per cm’; (0) Fluka 5 % prepared by phase inversion with 0.3 mg Pt per cm’; (A) Fluka 10% prepared by phase inversion with 0.6 mg Pt per cm’. Electrolyte: 7 mil [email protected] 117. Cell temperature: 50°C. Source:

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(mA/cmZ)

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(mA/cms)

Fig. 8. Regenerative electrolyzer performance with NafionE electrolyte and Pt/lr-on-C as oxygen electrode. ( W) Cycle 1; (0) cycle 2; and (A) cycle 3. Cell temperature: 65°C. Source: BCS Technology Inc., 1993.

Weizmann Institute of Materials Research and the Polymer Institute, 1990.

cell performance, particularly air solid polymer fuel cell. oxidant in the hydrogen polymer fuel cell. A comparison of fuel cell performance for very high and low platinum loadings is presented in Fig. 3. Although a substantial change in cell platinum loadings existed between the two samples, decreasing the catalyst loading only moderately diminished fuel cell performance when oxygen was used. In the case of air, however, a more notable decrease in cell performance was realized due to mass transfer limitations; or, in other words, fewer reactive sites were available for reaction in the low platinum-loaded sample. The effect of increasing reactant gas pressures on fuel cell performance for the low platinum-loaded sample is shown in Fig. 4. Again, data are reported using pure oxygen and air as oxidants. As is clearly apparent, increasing reactant gas pressures notably improves fuel

Weizmann Institute Polymer Institute

in the case of the hydrogen-

of Materials

Research

and the

The Weizmann Institute of Materials Research in Israel, in conjunction with the U.S.-based Polymer Institute, have been investigating alternatives to frequently used techniques for preparing solid polymer fuel cell electrodes. Their procedure gives results equal if not slightly superior to the conventional method. Instead of hot-pressing and sintering mixtures of the catalyst containing carbon black with Teflon8 as is commonly done in traditional fuel cell electrode preparation, these researchers use a process known as phase inversion. In the described method, a polymer is dissolved in a solvent, carbon black containing the catalyst is mixed with the polymer solution, a film is cast and immersed

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Fig. 7. Regenerative fuel cell performance with Nafio# electrolyte and Pt/Ir-on-C as oxygen electrode. ( n ) Cycle 1; (0) cycle 2; and (A)

cycle 3. Cell temperature: 50°C. Source: BCS Technology Inc., 1993.

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Fig. 9. Regenerative fuel cell performance with Dow electrolyte and Pt/Ir-on-C as oxygen electrode. (W) Cycle 1; (0) cycle 2; (A) cycle 4; and (0) cycle 5. Cell temperature: 50°C. Source. BCS Technology Inc., 1993.

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Fig. 10. Regenerative electrolyzer performance with Dow electrolyte and Pt/Ir-on-C as oxygen electrode. (W) Cycle 1; (p) cycle 2; (A) cycle 4; and (0) cycle 5. Cell temperature: 65°C. Source: BCS Technology Inc., 1993.

and the dissolved polymer is then precipitated out of its dissolved form into a highly porous state [21]. Electrodes, containing 0.3 and 0.6 mg Pt per cm2, were prepared using the phase inversion process, and then bonded to the perfluorinated membrane using a procedure similar to that previously used by the Los Alamos group [12]. The resulting polarization curves, comparing the phase inversion electrodes to 0.35 mg Pt per cm2 commercially available electrodes, are presented in Fig. 5. The electrolyte utilized in this experiment was a 3-mil cast Nafion’& membrane. Figure 6 shows similar data collected using commercially available, 7-mil, Nafion’b 117.

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Fig. 12. Effect of ultra low platinum loading on fuel cell performance. Electrode platinum loading and operating conditions: (a) Air, 0.07 mg Pt per cm’; (A) 0,; 0.07 mg Pt per cm’; (a) air, 0.12 mg Pt per cm’; (U) 0,, 0.12 mg Pt per cm’; (0) air, 0.17 mg Pt per cm2; and (+) O,, 0.17 mg Pt per cm2. Ratio of hydrogen to oxidant gas pressure: 3 : 5. Cell temperature: 80°C. Source: Los Alamos National Laboratory, 1992.

in a nonsolvent,

BCS Technology Inc. Based on the cell design developed by Ticianelli, Srinivasan and their coworkers at Texas A&M University

[12, 131, researchers at BCS Technology have designed what they refer to as a “unitized regenerative fuel cell” [221-a solid polymer fuel cell capable of operating in reverse as an electrolyzer. Utilizing a total cell platinum loading of 2 mg cmm2, Pt/Ir-on-C and Pt-on-C were evaluated as catalysts for the oxygen electrode in this fuel cell. Pt/Ir-on-C demonstrated a better electrolyzer performance with slightly lower fuel cell performance. This cell was operated alternately in the fuel cell and electrolyzer modes and data were collected, as shown in Fig 7-10, using both Nafions 117 and the Dow experimental membrane. A slight declination in cell performance resulted from cycle to cycle. This deterioration in performance was believed to be caused by electrode retention of reactant gas humidification water.

United Technologies’ Hamilton Standard Division 1.0 0.9 0.8 0.7 E

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Fig. 11. Performance curve for United Technology fuel cell. Oxygen pressure: 6.8 atm. Cell temperature: 82°C. Source. United Technologies’ Hamilton Standard Division, 1990.

United Technologies has been investigating the use of solid polymer fuel cells for naval applications [23]. These fuel cells need to be able to handle high shock and vibration and have “perfect” sealing of reactant gases. To satisfy these stringent requirements, researchers at the Hamilton Standard Division have developed a “sheet metal” design which they feel offers many advantages over traditional carbon/graphite electrodes. The United Technologies’ design consists of two assemblies: a membrane and electrode assembly and a metal/compression pad assembly, both of which are very flexible. The membrane and electrode assembly utilized in their fuel cell is basically the same as that used in their electrolyzer except that a different catalyst is employed on the oxygen electrode and a special wet-proofing film is included on this side of the cell. This assembly has no rigid structural properties. The metal/compression pad assembly consists of expanded sheet metal screen fluid

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and M. SANCHEZ 1.0 0.9 0.8 0.7 E 0.6 g 0.5 B p 0.4 0.3 0.2 0.1

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Fig. 13. Effect of gas pressure on ultra-low platinum loaded fuel cell performance. Ratio of reactant gas pressures: (a) 1: 1 H,: air; (0) 1:2 H,:air; (0) 3:5 H,:air; (A) 1:l H,:O,; (+) I:2 H,:O,; and (W) 3:5 H,:O,. Electrode platinum loading: 0.17 mg Pt per cm’. Cell temperature: 80°C. Source: Los Alamos National

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Fig. 14. Polarization curves for ultra-low platinum fuel cells utilizing the Dow experimental membrane. Cell oxidant: ( n ) air and (0) 0,. Electrode platinum loading: 0.13 mg Pt per cm’. Ratio ofhydrogen to oxidant gas pressure: 3:5. Cell temperature:

80°C. Source: Los Alamos National Laboratory, 1992.

Laboratory, 1992. Los Alamos National Laboratory

fields, sheet metal separators,individual cell compression pads and nonmetal frames and gaskets. A separate chamber is incorporated for cell coolant. Like the membrane and electrode assembly, the sheet metal/compression pad assembly is very flexible and produces an exceptionally positive seal for separating the various fluids and maintaining a “perfect” overboard seal. A polarization curve for the “sheet metal” fuel cell system is given as Fig. 11.

Fig. 15. The LaserCel lTM fuel cell prototype.

Researchersat Los Alamos National Laboratory, concerned with the high cost of platinum catalysts, have been working on lowering platinum loadings in the solid polymer fuel cell. They have developed a new process whereby a thin film catalyst layer is cast from solutions of suspended Pt/C catalyst and solubilized [email protected] ionomer [24]. The direct application apparently provides enhanced bonding at the interface between the membrane and the catalyst layer. Consequently, fuel cells utilizing

Source: International Academy of Science, 1991.

SOLID POLYMER FUEL CELLS-AN

Load-Following Gas Compressor

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OVERVIEW

LaserCelTM Fuel Cell

Metal Hydride Storage Vessel Fig. 16. LaserCel 1TMhydrogen system. Source: International

these ultra-low platinum-loaded membranes demonstrate superior performance when compared to other lowcatalyst loaded cells. The latest process developed at Los Alamos for casting ultra-low platinum, thin film catalyst layers, involves the direct application of a prepared Pt/C/solubilized [email protected] ink onto the surface of a dry solid polymer electrolyte and then baking the electrode/membrane assembly at 150-190°C. The other side is then prepared in a similar

Academy of Science,

1991.

fashion. Data collected at various platinum loading densities for hydrogen and oxygen/air, while utilizing the Chlorine Engineers’ perfluorinated membrane product, is depicted as Fig. 12. Of particular interest are the curves for 0.12 and 0.17 mg cm-‘-although the electrode platinum loading was increased by 0.05 mg cm-‘, little improvement in overall performance was realized. Fuel cell performance, as affected by reactant gas pressure, is presented in Fig. 13. Because of the detrimen-

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tal effect of an inert nitrogen blanket in the gas diffusion backing of the oxygen electrode, 1 atm of oxygen performed superiorly, in the solid polymer fuel cell, to 5 atm of air at high current densities. In Fig. 14 are presented oxygen and air fuel cell polarization curves for a Dow experimental membrane with electrode loading of 0.13 mg Pt per cm2 applied in a similar manner. This cell is capable of delivering 1.3 W cm-‘. SOLID POLYMER FUEL CELL VEHICLE DEMONSTRATION PROJECTS Several projects aimed at demonstrating the potential of the solid polymer fuel cell in vehicular applications have been and are still being conducted. Below, several of these projects are discussed. International Academy of Science-LaserCel Prototype Vehicle

lTM

Gaseous hydrogen, stored on-board the bus in DOT-

approved glass-wound composite cylinders operating at 200 atm, provides the necessary fuel required for the 150 km projected vehicle range [19].

Phase II will involve conversion of a 12 m transit vehicle using improved hydrogen storage and more compact solid polymer fuel cells [19]. General MotorsILos Alamos National Laboratory

Los Alamos National Laboratory and General Motors have been working on developing a solid polymer fuel cell for vans and passengercars [28]. They plan to carry on-board reformers on these vehicles to provide their hydrogen needs. CONCLUSIONS Recent solid polymer fuel cell research and development is demonstrating the potential of this technology in vehicular applications. Platinum loading densities are being minimized to acceptable levels and fuel cell systems

In 1991,the International Academy of Scienceengaged in a demonstration project with the Pennsylvania Energy are being optimized to achieve ideal vehicle range and Office to build the world’s first fuel cell automobile [25]. performance. Several demonstration projects prove the This prototype, a converted electric Ford Fiesta postal feasibility of this important new technology for meeting vehicle shown in Fig. 15, derives its cruise power from the growing world energy demands without compromisthe LaserCelTMsolid polymer fuel cell installed beneath ing the environment. the hood. In the LaserCel lTM prototype, the fuel cell consists of two stacks of cells inside a single enclosure. REFERENCES The small stack consists of 16 cells, and produces 12 V to power the vehicle’s accessories. The large stack, 1. J. H. Hirschenhofer, IEEEAESSyst. Mug. (November 1992). 2. K. Kinoshita, F. R. McLarnon and E. J. Cairns, Fuel consisting of 135 cells, outputs 100 V, and is used to Cells-A Handbook. U. S. Department of Energy, Morganpower the drive motor. Both hydrogen and air are town, West Virginia (1988). pumped to the cell using a specially designed, load3. W. T. Grubb, Proc. 11th Annual Battery Research and following air compressor designed specifically for this fuel Development Conference.PSC Publications Committee, Red cell application [26]. The cell utilizes hydrogen stored Bank, NJ (1957). on-board in a metal-hydride form-the safestmethod of 4. K. Kinoshita, F. R. McLarnon, and E. J. Cairns, Fuel hydrogen storage presently available [27]. With a 136 Cells-A Handbook. U.S. Department of Energy, Morgankg hydride tank, the vehicle has a 300 km range. Specific town, West Virginia (1988). components of this system can be seen in Fig. 16. 5. W. G. Grot, The Castner Metal Lecture, paper presented at the Society for the Chemical Industry Third London Currently, researchers at the Academy are focused on International-Chlorine Symposium, (June-5-7 1985). optimizing the various components of the fuel cell system, 6. H. A. Liebhafskv and E. J. Cairns. Fuel Cells and Fuel getting all the pieces ready for commercialization. In the Batteries. Wiley,New York (1968). meantime, plans for a dual fuel cell powered Eagle are 7. K. Kinoshita, F. R. McLarnon and E. J. Cairns, Fuel on the slate. This prototype will demonstrate the ability Cells--A Handbook. U.S. Department of Energy, Morganof the fuel cell to provide all the necessary power for town, West Virginia (1988). vehicle operation. 8. G. A. Eisman, In J. W. Van Zee, R. E. White, K. Kinoshita, Ballard Power Systems-The tion Project

BallardlBC Bus Demonstra-

Ballard Power Systems,in conjunction with the Province of British Columbia and the government of Canada, has recently converted a diesel city bus for the Vancouver BC Transit [28]. This 9 m vehicle is powered by a 105 kW fuel cell based on Ballard technology. The objective of Phase I of this project was to provide a commercial transit bus, fully-powered by a solid polymer fuel cell, providing the same driver acceptanceas the corresponding diesel counterpart

[19].

and H. S. Burney (eds), Diaphragms, Separators and IonExchange Membranes, p. 156.The Electrochemical Society, Pennington, NJ (1986). 9. B. R. Ezzell, W. P. Carl and W. A. Mod, IndustrialMembrane Processes,AIChE Symposium SeriesVol. 82, No, 248, p. 45. American Institute of Chemical Engineers,New York (1986). 10. A. J. Appleby and E. B. Yeager, Energy 11, 137 (1986). 11. K. Kinoshita, F. R. McLarnon and E. J. Cairns, Fuel Cells-A Handbook. U. S. Department of Energy, Morgantown, West Virginia (1988). 12. E. A. Ticianelli, C. R. Derouin, A. Redondo and S. Srinivasan, J. Electrochem. Sot. 13.5,2209 (1988). 13. E. A. Ticianelli, C. R. Derouin and S. Srinivasan, J. electroanal. Chem. 251, 275 (1988).

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14. S. Srinivasan, E. A. Ticianelli, C. R. Derouin and A. Redondo, J. Power Sources 22, 359 (1988). 15. K. Prater, J. Power Sources 29, 239 (1990). 16. M. Bernardi, In R. E. White and A. J. Appleby (eds), Proc Svmp on Fuel Cells, p. 51. San Francisco (1989). 17. PI J.* Shultz, In R. E.. White and A. J. Appleby (eds), Symp on Fuel Cells. D. 87. San Francisco (1989). 18. S. Srinivasan; 8. Somasundaram, M: Enayetullah, D. Swan and A. J. Appleby, In R. E. White and A. J. Appleby (eds), Proc Symp on Fuel Cells, p. 71. San Francisco (1989). 19. B. N. Corbel1 and K. B. Prater, In T. N. Veziroglu and R. E. Billings (eds), Project Hydrogen ‘91 Conference Proceedings, p. 247. Kansas City (1991). 20. D. H. Swan, 0. A. Velev, I. J. Kakwan, A. C. Ferreira, S. Srinivasan and A. J. Appleby, In T. N. Veziroglu and R. E. Billings (eds), Project Hydrogen ‘91 Conference Proceedings,

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p. 185. Kansas City (1991). 21. J. Manassen and I. Cabasso, Power Sources Symp. p. 408. Cherry Hill, NJ. 22. H. P. Dhar, .I. Appl. Electrochem. 23, 32 (1993). 23. J. F. McElroy, T: M. Molter and R. N. Sexauer; Proc. 34th Int. Power Sources Svmu. v. 403. Cherrv Hill, NJ (1990). 24. M. S. Wilson and S.‘Gbt&feld, X Ele&ochem. Sot. 139, 28 (1992). 25. R. E. Billings, The Hydrogen World View, American Academy of Science, Independence, MO (1991). 26. R. E. Billings, M. Sanchez, P. Cherry and D. B. Eyre, Project Hydrogen ‘91 Conj Proc. p. 257. In T. N Veziroglu and R. E. Billings (eds). Kansas Citv (1991). 27. R. E. B&gs, The Hydrogen korla View, American Academy of Science, Independence, MO (1991). 28. J. Glanz, R & D Msg. p. 36 (1993).