Int. J. Hydrogen Energy, Vol. 14, No. 12, pp. 915-925, 1989.
0360-3199/89 $3.00+ 0.00 Pergamon Press pie. © 1989 InternationalAssociationfor Hydrogen Energy.
Printed in Great Britain.
FUEL
CELL
RESEARCH
AND
DEVELOPMENT
PROJECTS
IN
AUSTRIA* K. KORDESCH, CH. GRUBER, J. GSELLMANN,P. KALAL, J. C. T. OLIVEIRA,K.-H. STEININGER, O. TAGHEZOUTand G. WINKLER Technical University Graz, 8010 Graz, Austria and K. TOMANTSCHGER Battery Technologies Inc., Mississauga, Ontario, Canada, L5L IJ9
(Received for publication 17 May 1989) Abstract--A five year program on energy storage and fuel cell research has been established in Austria, with the goal to find out what types of batteries and fuel cells could possibly be optimized for commercial applications. In the background there are environmental considerations and others, leading to a future hydrogen economy, also possibly to a use in electric vehicles. The search of the European Space Agency for a suitable system for the HERMES manned space vehicle is contributing considerably to the future prospects. The work in Austria is mainly oriented to low temperature alkaline fuel cell systems and the studies are complementary to a similarly oriented program in Canada, originally started by the Institute for Hydrogen Systems in Mississauga, subsequently continued by private companies in cooperation with the University of Toronto.
1. I N T R O D U C T I O N In 1984 the Austrian Fonds for the Support of Science established a five year program on electrochemical energy storage and fuel cell research at the Technical University Graz. The main goal of this program is to find out what types of fuel cells could possibly be optimized for low cost commercial (industrial) applications. In 1986 the European Space Agency (ESA/ESTEC) started the development of a re-usable spacecraft for the Agency's manned missions. This spacecraft, named HERMES, will be equipped with alkaline fuel cells as the main electrical power source and as water supply for the crew. The efforts to establish the most efficient, reliable and adequate technology for this purpose are shared by industrial and academic groups throughout Europe. These two programs, although aimed at different targets with basically different requirements, have common topics and many of the technological developments can be shared between them. As far as the work in Austria is concerned, the research efforts have been directed to the all-carbonelectrode cell construction as the ultimate goal. The reasons are that carbon electrodes can be used universally, either in circulating or immobilized alkaline dec-
trolyte systems or in acidic (matrix- or polymer membrane type) batteries. The requirements of large geometrical electrode areas in industrial applications indicate the need for a bipolar cell stack construction. The use in vehicles requires a hybrid configuration which is, in the opinions of the authors, presently best implemented by a fuel cell/zinc-bromine battery combination.
2. I N D U S T R I A L FUEL CELLS Most of the principles, basic reactions and technical constructions of the different types of fuel cells investigated today, are described in the now classical, but still applicable book of W. Vielstich [1]. However, in the past twenty years it has become more and more evident that the indispensible requirements for fuel cell systems to become economically viable and therefore commercially acceptable, are the following: mass-producibility, low
cost, simplicity, air operation and long life. This is especially true for direct alkaline or indirect acidic systems (with a reformer unit) to be used in electric vehicles [2]. The use of oxygen is a consideration for all space and some specific industrial applications. Larger power plants operate best at high temperatures in order to be able to utilize the off-heat. Molten carbonateand solid oxide fuel cells belong to this category. The room temperature starting capability is a necessity for electric vehicle applications [3]. The solid polymer elec-
*This paper has been dedicated to Prof. Dr Wolf Vielstich, Universit~.t Bonn, at the occasion of his 65th Birthday. 915
916
K. KORDESCH et al.
trolyte (SPE) or proton exchange membranes (PEM) are in this group. For the Austrian Fuel Cell Program the decision was made towards the development of hydrophobic carbon electrodes as the ones most likely to fulfil the basic requirements for low temperature cells of all types. The fabrication methods of these electrodes, their constituent materials, their electrochemical performance and lifetime are under study. At the Technical University GraZ, the development work includes the preparation of the raw materials (e.g. carbon materials), as well as the study of the welding, pressing, rolling and spraying processes [4]. 3. CARBON MATERIALS [5, 6, 7] 3.1. Industrial carbon Industrial carbon materials are used for molds, structural shapes, electrodes, bipolar plates with grooved manifolds, special tubing for heat exchangers, etc. Petroleum-coke is one of the most frequently used carbon sources, it must be crushed and calcined; binders (pitch) must be pulverized and classified. The "formula" is prepared by weighing the ingredients and hot mixing; the result is the "green mix" which goes into the forming operation: hot or cold molding or extrusion. Baking is the next step. To produce electrographite, the carbon is again packed in granular coke and heated electrically. 3.2. Sources Petroleum coke. Heavy aromatic residue from refining operations. Coking must be done to remove volatiles, in batch-, fluidized-bed and delayed-coking processes. Anthracite coal or natural (active) carbon from organic matter has a high ash content which must be leached. Pitch-coke is a result of coking coal-tar (high ash content). Carbon blacks (lampblack, acetylene black) are usually steam activated to be usable as large-surface materials. Coal-tar pitch. A by-product of the production of metallurgical coke, after distillation of coal-tar. If the carbon to hydrogen ratio is large, it indicates aromatic sources. Upon pyrolysis, in the baking process, 55% of the carbon is contained in a residue of graphitizable nature. Resins (polymeric materials). Like phenolformaldehyde, usually thermo setting, they are frequently used as binders and impregnating materials. Additives. Lubricants or extrusion aids are also used in small quantities. They may be heavy processing oils.
range of 25-500 bar. Large extruders produce up to 60 cm diameter electrodes with 2000 ton hydraulic systems. The extrusion speed may be up to 60 cm min-1. Compressive molding uses dies of the desired shape. Double acting presses go up to 15-20,000 ton pressure. Molded products have less flaws than extruded articles. Thermo-setting binders can be used. Purification. Impurities may be metals, non-metals (sulfur) or gases. Their origin is either the raw material, packing, or the mechanical mixing-, grinding- or machining-operation. Purification by heating removes sulfur, graphitization boils off metals, like iron. Carbides of V, Ti or B enter the lattice as substantial impurities. Chlorine and fluorine (Freon 12) have been used as flushing gases to form volatile metal-halogen compounds during graphitization. 3.4. Products The "green" carbon. A thermoplastic material which loses its room temperature mechanical strength if sufficient binder is converted to coke (500°C). Packing and supporting is therefore important. A 950°C baked product may contain 85% carbonized original material and 15% carbonized binder. Large volumes of gases are released during the initial phases. Heating rates must be slow. Dehydrogenation occurs between 500 and 950°C together with cross-linking and polymerization of the binder. "Manufactured" carbons. The density is less than that of graphite. The porosity of a carbon material with an apparent density of 1.6 g cm 3 is about 30%. For many applications, especially for construction material to be used in bipolar plates for matrix fuel cells, a dense material is needed. Subsequent pyrolysis of impregnated materials (pitch or resins) deposits secondary carbon in the open pores. One additional impregnation may increase the density of 0.1 to 0.25gcm -3. It may be repeated with intervening baking cycles, however, with diminishing returns. The definition of a closed pores material may be the inaccessibility to helium gas (2.8/~ pores). Fine grained graphite from petroleum coke with a porosity of 30% has 3-10% closed pore volume. Impregnation is done in autoclaves under vacuum, at liquid pitch temperatures. "Bonded carbon materials". This group includes the plastic-bonded conductive materials and products used in the structures of fuel cell electrodes and bipolar plates (see the following sections).
3.3. Processes
4. EXPERIMENTAL M A N U F A C T U R I N G A N D E LECTRODE PRODUCTION STUDIES 4. I. Production steps
Calcination. The calcined carbon is ground, pulverized and sized. The particles are referred to as "flour" and the classification is done in accordance with standardized particle size groups (see ASTM sieve sizes). Extrusion. Now more often used than molding. Rods or tubes can also be extruded through special nozzels. A temperature of 100°C is typical, pressures are in the
Figure 1 shows the production steps which are used to manufacture dual-layer carbon electrodes with PTFEpowders. When pressed into a metal screen the electrodes are used in alkaline electrolyte cells. When supported on a carbon paper they are suitable for acidic cells. Noticeable are the different ways to produce the gas diffusion layer (from a mixture of acetylene black,
FUEL CELL R & D IN AUSTRIA
S OC RR CE AERNBON, PRE SS~NG
A BC LA E C TK YLENE I . SUGAR J LIOUID D LA F IYFEU R SO I N IROLLIN6~ ROLLING"--~'~'~'l PTFE POWDER [ELECTR6DE~ ICOM~OSITE PRESSN I6 WASHING
~
'NEING,
i]~ LIQUIO
FINISHED
GRAPHITEl SUGAR I PTFE J I I POWDER
CATALYS'i
ELECTRODE Fig. 1. Production steps for the manufacturing of dual-layer carbon electrodes using PTFE as a powder only (M. Schautz, 1982).
sugar and PTFE-powder) and the catalysed layer (from Vulcan carbon, sugar carbon, Pt-black and PTFEpowder). An alternative way is the catalysation by impregnation of the carbon with Pt-salt after the electrode is finished as a structure. Figure 2 shows the production steps which are used to manufacture the electrode layers from a mixture of PTFE-suspension and carbon powders. A typical carbon-catalyst mixture consists of Vulcan Carbon and 12.5% Pt-black. Such electrodes showed greater resistance to flooding on open circuit than the electrodes described before. Improvements in stability were achieved by heat treating the carbon [8]. 4.2. Carbon collectors and substrates The carbon can be current collector and catalyst carrier at the same time. Production of carbon tubes or plates was done by pressing or extruding a mixture of carbon black or graphite, pitch and fuel oil. The shaped carbon bodies were then baked at e.g., 1000°C for several hours. For catalyst carrier-carbon a surface steam treatment followed. Bipolar constructions became necessary for better current collection: edge-collection is not possible with larger carbon plates. The porous supports on which the active catalysed carbon layers are placed are produced as "carbon paper" or "felt". "Novolack", phenol formaldehyde resin fibres (United Technologies Corp.) or Nylon or Rayon fibres (Union Carbide Corp.) are carbonized at, e.g. 1300°C, cut and made into thin sheets by paper making techniques. This paper is coated by pyrolytic carbon to improve conductivity. For this purpose methane is decomposed at reduced pressures at temperatures up to 2000°C; the deposited carbon has an extremely wellreproducible crystal system. Porous carbon substrates are now available in large sheets from commercial vendors. These products are now taylor-made from carbon product manufacturers. These substrates contain no plastic binder after the carbonization. Different from the carrier materials just discussed are the PTFE-treated substrates, which are later subjected to a sintering heat treatment. The purpose of the PTFE (or
917
PC, PP) is to impart a hydrophobic quality to the substrate and to prepare the material for the additional layering with plastic-bonded, active and catalysed materials. Impermeable carbon sheets are required tbr bipolar electrodes. Several commercially available graphitic foils of good conductivity are available to serve as supports for active layers or as separation walls in fuel cell gas manifolds. They also find use in primary cell stacks (6 V Polaroid batteries) to connect the different polarity electrodes (e.g. C O N D U L O N foil). After fabrication, the electrodes are submitted to optical checks of the porosity distribution in a "gas penetration test" device using alcohols, water and electrolyte as interface test liquids (of different surface tension). The electrode materials, namely carbon blacks, polytetrafluoroethylene (PTFE) and catalysts have been used in various proportions and the effect of variations are analysed through the subsequent performance tests [9, 10]. The tests include polarization curves for performance analysis and endurance under continuous load. A typical test bench holds 30 single experimental cells with 3 cm: and also 10 cm 2 (area) electrodes and a microcomputer based data acquisition system [11]. A particular study on carbon activation is being done at the Technical University Graz, to obtain better chemical and physical characteristics of such materials. During the activation procedure surface groups are eliminated from the carbon black particles, thus increasing their surface area and improving the resistance to chemical attack. The materials obtained under different activation conditions are characterized by porosimetry and B.E.T. techniques [12, 13]. PTFE is used as binding and hydrophobic agent. It has been used in two forms, as suspension or granulated. By the use of granulated PTFE the effect of different particle sizes has also been investigated. 4.3. The catalyst As early as 1976, the Pt-content was lowered to 0.5 mg cm -2. The best performances were obtained with
SCREEN ORCARBON RRESSN IO J I ~
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Fig. 2. Production steps for the manufacturing of dual-layer carbon electrodes using PTFE as powder and in aqueous suspension (1984).
918
K. KORDESCH et al.
steam activated Vulcan X-72 (Cabot Corp.) and steam activated Shawinigan black (Gulf Oil Corp.). Most remarkable was the low CO sensitivity of the Pt-catalyst on spinel-containing carbon [14]. It should be noted that active carbon materials of natural origin contain ashresidues which are deterimental to the long-time hydrophobicity of the electrodes. The catalysts add just a small portion to the cost of a commercial fuel cell system. It seems unlikely that any other than noble metals, evenly dispersed on large surface supports (e.g. various activated carbon blacks) could fulfil the simultaneous needs for high current density and long life. Platinum and palladium have been tested as catalysts and the method of their application has also been studied.
KOH (]AS B
5. I. A reversible storage system
The original idea of F. Bacon in 1946 [15] was the storage of energy by means of electrolysis of water and storing the gases, later recombining them in the same unit to produce electricity. E. W. Justi and A. Winsel [16] used double porous Raney electrodes ("valve electrodes") in their reversible fuel cells. (See also W. Vielstich [1], electrolysis and storage: pp. 330-337, Electrochemical enrichment of deuterium and tritium: pp. 343-357.) A combined energy storage device is especially suited for photovoltaic space power applications [17]. The efficiency of the electrolysis unit may be 90% and that of the fuel cell near 60%, a total of 54%. This value is actually lower than values reached with regular rechargeable batteries, but the energy storage density by weight (Wh kg -l) is larger, a figure which is important for space applications, but not so for stationary applications on earth, which may use underground gas storage. It has been suggested to store the gases in space by means of balloons, thus avoiding the power for gas compression. Unfortunately, the performance of porous metal electrodes requires pressure or special catalysts, they usually do not perform well on air because the 80% nitrogen present reduces the partial pressure of the oxygen gas and the solubility of the oxygen in the electrolyte layers covering "wetted" (hydrophilic) electrodes. 5.2. Reversible (bifunetional) electrodes
Special electrodes feature a nickel layer of dual poresstructure on the electrolyte side and a PTFE-bonded (no-noble metal catalysed) carbon layer on the oxygen (air) side. During the electrolysis or charging period, the oxygen evolves from the nickel surface and fills the large pores of the porous Ni-structure with gas, blocking to a large extent the ionic pathways to the carbon. During the discharge cycle of the fuel cell, iron/air or zinc/air cell the carbon layer is operating as a regular oxygen electrode because the pores are refilled. This improved electrode avoids the necessity to use three-electrode-systems with the known mechanical-
OXYSEN
ELECTROLYTE
consumption
02- evotution
POROUS
POROUS
NICKEL (A)
CARBON {C)
hydrophitic
hydrophobic A B (B):
5. THE ELECTROLYSIS/FUEL CELL SYSTEM
C
IC
course porous Ni
B/Ct: interfclca
KOH/GAS
Fig. 3. The reversible oxygen (air) electrode consists of a dual porosity nickel sheet and a PTFE-bonded carbon layer. The fine nickel layer faces the electrolyte and the carbon PTFE-layer is supplied with oxygen (air) when the electrode operates as a fuel cell cathode.
electric switching problems when used in multi-cell batteries. The combination of water electrolysis and hydrogen-oxygen fuel cells still looks very attractive for space and energy storage applications. Variations for low-cost electrodes, e.g. for metal-air cells, are possible. Quite considerable efforts were made to produce reversible oxygen (air) electrodes on the basis of PTFEbonded carbon materials. However, it turned out that the oxidation of the carbon during the 1.8-1.9 V electrolysis operation usually attacked the carbon in a short time. Researchers at Westinghouse R&D Center have obtained up to 250 short cycles with their rechargeable iron/air batteries, confirming these findings [18]. The total lifetime however was not satisfactory. One of the difficulties was the seepage of the KOH electrolyte through the PTFE-bonded structure at advanced cycles. Other workers in the field found the same. Additives improved the life time performance somewhat. Several patents, among them a very early patent of Kordesch [19] tried to circumvent the difficulties of the oxidation of carbon (which was obviously wetted) by letting the oxygen evolve from a third electrode (anode) or from an adjacent porous metal or screen in the electrolysis mode and use the carbon electrode only during the (cathodic) fuel cell operation. This "trick" is only needed on the reversible oxygen electrode because a porous Raney-nickel metal electrode or a noble-metal catalysed carbon electrode as hydrogen electrode can be used reversibly, without any damage from the hydrogen evolution. With the so-called "reversed electrode design" (see Fig. 3) developed by Union Carbide already in the seventies for regular hydrogen/air fuel cells [20], it is possible to minimize the described problems.
FUEL CELL R & D IN AUSTRIA
919
OXYGEIN N O HYDROGENG INASOVOLUHEI COUNTERS
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GAS-CONDITIONERS-| ! E-i HUMIDIFIER DEVICES T~ [
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Fig. 4. Experimental set-up to determine operating parameters of fuel cell electrodes, especially suited for the study of water transpiration data into circulating gas streams.
As in other electrolysis cells, mixing of the gases during the electrolysis should be avoided and a gasseparator must be built into the cell.
pies of 10cm 2 area, taken from the center and edges of large (200cm') experimental electrodes. A detailed electrode mounting apparatus for test cells is shown in Fig. 5.
6. NEW EXPERIMENTAL TEST DATA The experimental set-up for testing various types of fuel cell anodes and cathodes is shown in Fig. 4. The electrodes were mounted in polysulfone frames and sealed with epoxy resin cured at high temperature. A matrix of operating parameter was established in order to find the optimal operating conditions. It turned out that 80-90°C and 10-12 molar KOH and partially water-vapor saturated oxygen or air was beneficial to the life time of the cathodes. Operation at 60-65°C at 6 molar KOH with completely dry oxygen or air was detrimental. The beneficial effect of the relative humidity of the oxygen is very predictable: dry gas enhances the KOH gradient in the cathode by water vaporization, humid gas reduces the gradient. The higher operating temperature helped, because the electrolyte equilibrium concentration automatically increased to larger values as a result of the rapidly increasing water removal through the anode structure. At the hydrogen electrode, water is produced, diluting the electrolyte in the pores, thereby establishing the opposite gradient in concentration as was estimated to exist inside of the oxygen electrode structure. For testing uniformity and longevity a test set-up was used, which tested simultaneously three electrode sam-
10cm2E L E C T ~
Fig. 5. Fuel cell electrode testing: establishing uniformity and longevity.
920
K. KORDESCH et al.
Fig. 6. A sketch of "HERMES", the proposed European Space Vehicle.
The oxidation of carbon materials and destruction of the hydrophobicity during the electrolysis mode is not the only reason for the short life of reversible carbonPTFE electrodes. Concentration gradients in the pores of the cathodes cause a volume increase of liquid in the structure and the resulting pressure causes electrolyte to exude on the gas side (electrode "weeping"). This phenomenon seems to be controllable now It was found that various types of electrodes obtained from different manufacturers and from our in-house electrode production behaved unpredictably under electrolysis and fuel cells operating conditions and had to be retested. In parallel studies, K. Tomantschger [21] found also, that unprotected "regular" diffusion type oxygen electrodes made from carbon materials can not be used as reversible electrodes. He found that a simple nickelplating of the carbon, improves the life expectancy of electrolysis cathodes considerably. 7. SYSTEM CONSIDERATIONS The bipolar configuration is mandatory for large geometrical electrode areas required for some industrial applications. Further: the system must be able to start at temperatures as close as possible to the ambient temperature. However, the operating temperature must be chosen within a range where the water removal can be accomplished by transpiration through the electrodes into a circulating gas stream, thus avoiding the needs for additional water removal or heat management systems [22]. For all-carbon (bipolar) fuel cell systems, the electrolyte could be either alkaline or acidic and the immobilized electrolyte (matrix), semi-mobile electrolyte (matrix with very low electrolyte flow) and mobile electrolyte concepts are investigated. This study takes into consideration the best choice to fulfil the stringent requirements for compactness, low power losses, optimal distribution of fluids and thermal management [23]. 8. SPACE APPLICATIONS 8. I. Background Fuel cells have already proven to be excellent electric energy sources for space crafts, especially in manned missions where, in addition to their inherent qualities
(e.g. high efficiency and high energy density) they provide drinking water for the crew. Therefore, the ESA re-useable spacecraft HERMES (Fig. 6) will be equipped with an alkaline fuel cell system (Fig. 7) suitable for that purpose [24]. In contrast to the efforts in the U.S., where the fuel cell development had a great impulse on Aerospace work during the NASA space programs [25], fuel cell developments in Europe have always been aimed at terrestrial or undersea applications. 8.2. Circulating electrolyte systems They provide efficient heat removal and fast temperature equilization throughout the whole fuel cell stack. The electrolyte is used as the system's cooling medium. Electrolyte filtration is easily attainable, making the system insensitive to impurities that can dissolve from the electrodes or from the manifolding or framing system. Carbonate accumulation is no problem. The electrolyte level indicates the water balance, which is self-adjusting. Liquid electrolytes are an efficient barrier against cross leaks and provide for the release of gas bubbles resulting from (dissolved) gas into the electrolyte chamber. As disadvantages can be cited the need for a circulating pump, for a gas removing separation device and the occurrence of parasitic currents between the cells of the stack (ca 3% losses). It may be interesting to recall, that such a system (MOLAB) was proposed already in 1964 [26, 27]. Figure 8 shows the diagram of the 5 kW Union Carbide Corp. (UCC) System as it was first presented in 1967 together with Bendix Corp. The circulation of the gases for water removal was done by gas-jets [28]. This principle was later used successfully in a 4 kW Navy Fuel Cell, in a 75 kW UCC/Ford Demonstration Battery and in the 50-150kW UCC/General Motors "Electrovan" Fuel Cell Battery. In 1967 only few persons believed in this possibility and the space engineers also questioned the separation of gases and liquids by centrifugal devices (which was proposed by Air Research Corp,). 9. BATTERY CONSTRUCTION AND OPERATION 9.1. Stack configuration The series connection of cells into higher voltage
FUEL CELL R & D IN AUSTRIA
921
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Fig. 7. One of the three fuel cell systems proposed for the HERMES Space Vehicle. It consists of 3 modules, each capable of producing 2 to 4 (max. 6) kW. The voltage range is 85 to 130 V, operating temperature: 80-90°C. The life expectancy is a minimum of 2000 h. stacks, the necessity of circulation of gases and coolants in matrix systems and the existence of electrolyte loops in liquid K O H systems makes a sequential stack construction (out of prefabricated units) necessary. A bipolar current collection is therefore a logical consequence. Unfortunately, this method of current flow incorporates often serious interfacial contact problems which must be solved by pressure devices working pneumatically, with spring tension or with changing cooling loop pressure (e.g. having a thin flexible foil as separation wall). The edge collection of the current requires a good conductivity of the electrode. This is no problem with porous metal electrodes, but requires metal screens, expanded metal or metal felt structures in carbon electrodes. In all cases the current distribution at high current densities is a point in question. Non-uniformities are transferred throughout the stack, equalized by the resistance of the electrolyte layers. The bipolar current collection assures uniformity, but only if there are no "poor contact areas" in or on the electrode structures. In general, an electrode size up to 400 cm: can be handled with edge collection. Beyond that size, the bipolar design has undisputed advantages. Space systems requires a high redundancy and therefore a parallel connection of electrodes, cells and stacks. For systems in the order o f a few kW, which is the case with spacecrafts such as the one planned by ESA, the electrode sizes will be small, therefore allowing the use of the monopolar configuration. .9.2. Product water r e m o v a l [29]
Water removal can be effected by: (a) the gas circulation through an external condenser
and trapping system on one or both electrodes can remove all the product water. In this case the heat of vaporization also contributes to the cooling of the battery. (b) the "static water removal method" into an electrolyte stream kept at a lower temperature. A permeable membrane or porous diffuser plate is needed: ,. (c) the distillation method, by means of a water vapor condensation plate (non-porous), cooled by an adjacent stream of coolant (water, electrolyte or refrigerant) pumped around. In such cases the gas feed is dead-ended (except occasional purging). (d) the water removal from the electrolyte which is pumped into an external water removal unit. The heat of vaporization must be supplied by the waste heat from the stack. In this case there are transfer-losses and a small battery may not produce enough waste heat. Such a unit employs diffusion plates which are similar to porous electrodes, essentially performing the method described under (a), but outside of the battery. Usually hydrogen is used as the dry gas. The external water removal system can employ the static water removal method like described under (b) with no gas circulation needed. Figure 9 shows the design of a bipolar, all-carbon fuel cell battery with a circulating alkaline electrolyte. The fuel is hydrogen, the oxidant air. 10. POROUS CARBON TECHNOLOGY Carbons are lightweight, have a large surface area and are suitable for the deposition of very active catalysts, PTFE-bonded carbon electrodes are easy to manufac-
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ture, even on a large scale. Carbon has the disadvantage of poor conductivity if compared with metals. Therefore, a wire screen as current collector is required when edge current collection is used. The carbon/metal cx)mbination can lead to leakage; the reason are thermal expansion differences due to the poor matching of expansion coefficient. The life expectancy of screen-containing electrodes is considerably lower than that of single-structure electrodes. Without metal collector the conductivity is poor and only a bipolar cell configuration is acceptable. In the U.S., the alkaline electrolyte/carbon system has been investigated and developed by Union Carbide [30]. The goal was set at space and commercial applications and because none developed at that time, further work was stopped in 1978. In France, composite carbon electrodes were used by the Institut Fran~;ais du Petrol (until 1971) and by the Compagnie Generale d'Electricit6 (until 1974). ALSTHOM developed bipolar, all carbon systems for many years, the intentions were to produce liquid fuel-air systems [31]. EXXON Research & Engineering Corp. cooperated with Alsthom in several projects and one of the results was the zinc-bromine rechargeable battery with bipolar conductive plastic electrodes. ALSTHOM-OXY existed in the U.S.A. until 1986, In Belgium, Electrochemische Energieconversie n.v. (ELENCO) develops PTFE-bonded carbon electrodes since the early seventies [32]. The company was founded by a consortium of the Belgian Atomic Energy Co. (AEC), the Dutch State Mines (DSM) and the Bekaert Co. Today the company has a projected production capability of 500,000m 2 of electrodes per year. The system uses circulating KOH. The goal of the firm is the commercial use of fuel cell systems. A city bus with a 10kW H2-air battery was built in 1986. Forty kW systems are developed.
11. THE COMBINATION OF A FUEL CELL AND A RECHARGEABLE BATTERY The secondary battery should have a flat performance curve, the fuel cell (especially a hydogen-air battery) has a sloping curve. This behaviour can be simulated easily by assigning certain resistance values to the batteries. With a suitable selection of the number of cells, there is no need for an intermediate power converter (which is claimed to be needed by some designers), especially if a diode is provided, which prevents charging back into the fuel cell system (see Kordesch Hybrid). Table 1 shows data of a hybrid vehicle using a hydrogen-air fuel cell system and a zinc-bromine rechargeable battery [33]. The calculations have been made with the help of the "MARVEL" program [34]. The cost data are very realistic for a fuel cell hybrid system with a Zn-Br Battery. Presently a high pressureor cryogenic system is considered for the storage of the hydrogen. Figure 10 shows the zinc-bromine battery system developed in Austria. From the data presented it can be concluded that a fuel cell system in a hybrid system with a rechargeable battery is the only conceivable next step in vehicle design, assuming that fossil fuels will get rare and more expensive in the future. Hydrogen can be stored in many ways still to be studied. Air is the logical oxidant. The zinc-bromine battery is presently a good example of a powerful rechargeable companion battery [35]. 12. SUMMARY It was intended to show that alkaline fuel cells are presently developed in Austria within the framework of European Fuel Cell Efforts. The goals are still commer-
K. KORDESCH et al.
924
Table I. Hybrid vehicle, hydrogen-air fuel cell/zinc-bromine battery
Example 1.
The car is a commercially available small car, a converted vehicle with an energy coefficient of
12-15 kW H2-air battery (ca 15kg kW -I, ca 70Wkg -t) (based on 150mA cm -2, 0.75V cell-l, 70°C, 9M KOH) H2-supply: 4.5-5 kg = 75 kWh (55% eft. 500 Wh kg -I) 7 kWh Zn-Br accumulator (ca 15 kg kWh -t, 70 Wh kg +t) Vehicle weight, gross, with 2 persons Total battery weight fracation: with a 34% H2-usage per 100km: range at 75km h-t: (based on about 55% efficiency, 1.5 kg H2 = 25 kWh) Acceeleration: 0-70 km h -t Maximum speed: Peak power (1 rain) corresponding to 150 mA cm -2 (Zn-Br2) High performance (20 min): Fuel cell system plus Zn-Br2system Average performance (continuously, fuel cells at 70°C) Readiness of the fuel cell system (empty on stand-by) Zinc-bromine battery full power (empty on stand-by) Life of fuel cells: 5000 operating h at 50 km h - l = Cycle life of Zn-Br2 system: I000 cycles, each 250 km= Cost of fuel cells: Pt-catalyst only: 3 g (m2 k W - ' ) = The remaining stack components: Cost of Zn-Br system, complete (estimate of 1987) Cost of a 10-15 kW hybrid power plant (mass-produced)
Example 2.
Example 3.
Improvements to the data above (values of 1987): The improved vehicle energy coefficient is: If the hydrogen consumption with the improved vehicle is now only I kg per 100km, the range at 60-80 km h -t is: A recalculation of the values in this Table with the vehicle data from the MARVEL program results in a far lower energy consumption. The program uses a value of: The driving range would then be:
0.18 Wh (km kg)-t 220kg 150 kg 100 kg 1350 kg 470/1350 300km 15 s 90 km h -t 40 kW 30 kW 15kW 30 s 10 s 250,000 km 250,000 km $50 kW -t $200 kW -I $150 kW -I $500 kW -l 0.133 Wh (km kg) -t 450 km
0.080 Wh (km kg) -t 600 km
c i a l batteries, air operated, if possible. The request to build a fuel cell system for the European Space Agency ( E S A / E S T E C ) to operate the manned space vehicle H E R M E S has raised future prospects and a certain spin-off technology is expected to emerge. Research and development efforts at Universities and G o v e r n m e n t Institutions have increased substantially in numbers and funding. The purpose is to select the best possible system. The Technical University Graz is participating in several projects combining space and industrial goals.
Acknowledgements--The authors thank the Austrian Ministry for Science and Research and the Funds for the Support of Scientific Research for the establishment of the fuel cell R&D Program. One of the authors, J. C. T. Oliveira thanks CNPQBrazil for personal financial support. REFERENCES
Fig. 10. This 100 V, 20 kWh zinc-bromine battery system was developed by the Studiengesellschaft fiir Energiespeicher und Antriebssysteme GmbH (S.E.A.) in Miirzzuschlag, A-8680, Austria (1988).
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