Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/023—Porous and characterised by the material
  • H01M8/0241—Composites
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/0247—Collectors; Separators, e.g. bipolar separators; Interconnectors characterised by the form
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
  • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
  • H01M8/04089—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants
  • H01M8/04119—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying
  • H01M8/04126—Humidifying
  • H01M8/04149—Humidifying by diffusion, e.g. making use of membranes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
  • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
  • H01M8/04089—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants
  • H01M8/04119—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying
  • H01M8/04156—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying with product water removal
  • H01M8/04171—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying with product water removal using adsorbents, wicks or hydrophilic material
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/10—Fuel cells with solid electrolytes
  • H01M8/1007—Fuel cells with solid electrolytes with both reactants being gaseous or vaporised
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/10—Fuel cells with solid electrolytes
  • H01M2008/1095—Fuel cells with polymeric electrolytes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M2300/00—Electrolytes
  • H01M2300/0017—Non-aqueous electrolytes
  • H01M2300/0065—Solid electrolytes
  • H01M2300/0082—Organic polymers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/0247—Collectors; Separators, e.g. bipolar separators; Interconnectors characterised by the form
  • H01M8/0254—Collectors; Separators, e.g. bipolar separators; Interconnectors characterised by the form corrugated or undulated
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells

Definitions

• This invention relates to fuel cells and, more particularly, to fuel cells having polymer electrolyte membranes.
• This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
• a fuel cell as used herein, provides for the direct production of electrical energy through an electrochemical reaction involving fuel and gas reactants, which may be typically hydrogen and oxygen.
• a single cell comprises an anode structure, cathode structure, and an electrolyte separating the electrodes.
• a particular form of electrolyte is a polymer electrolyte membrane for ionic transport between the electrodes.
• MEA membrane/electrode assembly
• the MEA hydration level will tend to drop even when the reactant gases are at saturated water vapor conditions because the water uptake of perfluorosulfonate membranes is less when a membrane is vapor equilibrated rather than liquid equilibrated.
• the electro-osmotic drag of water with the ionic flux from the anode to the cathode across the membrane tends to dry out the anode.
• An additional problem is that the cathode then tends to flood because of the ORR generated water as well as the water dragged across the membrane. In general, it is difficult to maintain optimum hydration levels under all operating conditions.
• the water produced in the ORR reaction is sufficient to maintain adequate hydration. It is still difficult to maintain adequate performance outside of a particular operating envelope, which typically does not include low current densities, typical operating temperatures (80-90 °C), or near ambient reactant pressures, without humidification of at least the anode region.
• Some approaches to humidification systems or to humidification control entail some means of introducing humidification plates within the individual cells.
• International Fuel Cells IFC, South Windsor, CT
• the water is supplied to the cell by humidifying the reactant gas streams.
• the gas streams are humidified by flowing the reactants through a humidity exchanger.
• the exchanger and its associated control system tend to increase system size and complexity.
• Control of the stack is also complicated if the cells are humidified via the reactant streams because the two systems then become coupled. Often, the optimal hydration level may not match the most efficient reactant flowrate, and it may be difficult, for example, to switch-off the cathode humidification if the cell starts to flood, or to increase the humidification level at low current densities when the cell starts to dry out as less water is produced by the ORR. This lack of water is further exacerbated by the tendency of the membrane to take up less water when only vapor humidified. For such reasons, it is difficult to provide optimal (and not excessive) hydration over all current densities with a particular set-up.
• One approach to alleviate these difficulties is to supply sufficient liquid water directly to the membrane in some manner to de-couple the humidification and reactant supply subsystems and maintain the ionomer at the liquid- equilibrated hydration level at all times.
• the best way of decoupling hydration from the other subsystems and assure liquid-equilibrated hydration levels is to introduce liquid water directly to the MEA, as has been done previously by wicking from the periphery through the ionomeric membrane, or to inject water from the periphery through miniature tubes formed in the membrane.
• Watanabe et al. J. Electrochem. So ⁇ , 140, 3190 (1993) introduces liquid water to the membrane directly through a supply reservoir at the periphery of the electrode. Since the standard perfiuorosulfonate membranes do not wick water particularly well, Watanabe et al. teach a composite layer within the membrane that consists of the ionomer for ionic conductivity and a wicking material to facilitate the transport of the water. Performance improvements over reactant humidification are demonstrated in small cells. In the other liquid hydration approach, Lynntech, Inc. (College Station, TX) impresses miniature channels into conventional perfluoroionomer membranes and injects water from the edge through the tubes thus formed. In these configurations, it may be difficult to wick or pump the water a substantial distance. The membranes need to be relatively thick, and currently available membranes and MEAs can not be directly utilized.
• Another object of the present invention is to provide a relatively simple membrane humidification system for use with a fuel cell.
• One other object of the present invention is to uniformly distribute liquid water to a surface of a membrane. Yet another object of the present invention is to decouple control of membrane humidification from control of reactant flow rate.
• the apparatus of this invention may comprise a polymer electrolyte membrane fuel cell assembly having an anode side and a cathode side separated by the membrane and generating electrical current by electrochemical reactions between a fuel gas and an oxidant.
• the anode side comprises a hydrophobic gas diffusion backing contacting one side of the membrane and having hydrophilic areas therein for providing liquid water directly to the one side of the membrane through the hydrophilic areas of the gas diffusion backing.
• the hydrophilic areas of the gas diffusion backing are formed by sewing a hydrophilic thread through the backing. Liquid water is distributed over the gas diffusion backing in distribution channels that are separate from the fuel distribution channels.
• FIGURE 1 is an exploded cross-sectional view of a unit fuel cell according to one embodiment of the present system.
• FIGURE 2 is a schematic of one water injection system for introducing liquid water into the anode side of a unit fuel cell.
• FIGURE 3 is a performance comparison of conventional fuel gas humidification with liquid water injection into a fuel gas stream according to the present invention.
• FIGURE 4 illustrates high frequency cell resistance comparisons of the fuel cells used to obtain FIGURE 3.
• FIGURES 5A and 5B graphically depict performance parameters under a life test of a fuel cell with water injection into the fuel gas stream according to the present invention.
• FIGURE 6 illustrates performance characteristics of a fuel cell with direct water injection but having a different polymer electrolyte membrane than the membrane used to generate performance curves shown in FIGURES 1 and 2.
• FIGURES 7A and 7B are exploded cross-sections of flow fields for use in the anode plenum of fuel cells with liquid water injection according to the present invention.
• FIGURE 8 is a planar view of a distribution plate having serpentine distribution channels for liquid water and fuel gas on one side of the plate.
• FIGURE 9 is a schematic for a recirculating system for liquid water injection into the anode side of a fuel cell.
• FIGURE 10 is a performance comparison of relatively large area cells for conventional humidification of the reactant gases and liquid water injection water into the anode side of a fuel cell.
• a wicking configuration delivers liquid water to the MEA through the otherwise hydrophobic backing from a liquid water source in the anode plenum.
• a continuous supply of water can be provided over the entire active area of the MEA because there are no space constraints as when the membrane serves as the water conduit.
• a two-part hydrophilic/hydrophobic backing structure is used in order to convey the water from the anode plenum to the membrane.
• a preferred embodiment for the two-part structure is shown in Figure 1 in exploded cross-section (not to scale).
• a unit fuel cell 10 is formed with anode side 12 and cathode side 14 separated by catalyzed membrane 16.
• Cathode side 14 comprises flow field plate 18 with oxidant feed channels 22 and hydrophobic gas diffusion backing 24 adjacent one surface of catalyzed membrane 16.
• Cathode side 12 comprises flow field plate 26 with fuel distribution channels 28 and hydrophobic gas diffusion backing 32 adjacent a second side of catalyzed membrane 16.
• a two-part hydrophobic/hydrophilic structure is formed by wicking thread 34 that has been sewn through conventional hydrophobic gas diffusion backing 32 to supply the hydrophilic component for anode side 12.
• a number of materials may be used for the backings 24, 32 such as PTFE treated carbon paper, as is frequently used, or preferably, a carbon black/PTFE filled carbon cloth gas-diffusion electrode such as non-catalyzed ELAT, from E- TEK, Inc. (Natick, MA).
• a serpentine thread pattern for wicking thread 34 with each stitch about 2 mm long and the rows separated by about 2 mm, two threads convey water toward the membrane about every 2 mm.
• the actual region of membrane 6 that is in contact with wick 34 is even greater because of the portion of wick 34 stitches that overlie backing 26 on both sides. This allows take-up and delivery of the liquid water over a greater area.
• liquid water is supplied from direct injection of water droplets in fuel gas channels 28 (see Figure 2)or from separate water-filled channels in the anode plate (see Figure 7A, 7B, and 8) that may be arrayed in a number of configurations.
• wicking thread 34 is readily impressed into the originally 350 ⁇ m thick gas diffusion backing 32.
• backing 32 deforms sufficiently to close the needle holes and accommodate wicks 34 to provide good interfacial contacts of the various materials.
• Wicking 34 can be sewn into conventional, catalyzed gas diffusion backings 32 that are then impregnated with ionomer and hot-pressed to membrane 16 (e.g., I. D. Raistrick, US Patent No. 4,876,115, incorporated herein by reference). Because of the higher catalyst utilization efficiencies and robust performance, catalyzed membranes 16 (or MEAs) are used and prepared in accordance with US Patents 5,211 ,984 and 5,234,777, both incorporated herein by reference. As such, the platinum catalyst loading of membrane 16 is in the neighborhood of only 0.12 mg Pt/cm 2 /electrode. A loading as low as 0.03 mg Pt/cm 2 has been shown to be effective on anode side 12.
• a first exemplary embodiment of a unit fuel cell according to the present invention comprised a standard 5 cm 2 catalyzed National 115 membrane (DuPont) using standard E-TEK gas-diffusion backings in conventional hardware.
• the only difference between this cell and a conventional cell is that wicking material 34 was sewn into anode backing 32 in a top-stitch using a conventional sewing machine. The thread was sewn in a serpentine pattern with roughly 2 mm between stitches and between rows.
• the wick material was a continuous multi- filament polyester thread that is about 80 ⁇ m in diameter available as U151 from G ⁇ ntermann of America.
• FIG. 2 schematically depicts experimental apparatus for injecting liquid water directly into a unit fuel cell 40.
• Fuel gas 44, H 2 is input to fuel cell 40 through meter 46. Any unreacted gas and accumulated water forms output 52, which is output from fuel cell 40 through back-pressure regulator 48 to control flow through anode side 42 of cell 40.
• Liquid water is contained in reservoir 54 and injected into fuel gas 44 by a controllable pump 56, which may be any convenient injection device, such as a piston pump, positive displacement pump, syringe pump, or the like.
• a droplet of water was introduced each second into the anode 42 reactant inlet.
• Total water flow rate was about 1.5 ml/min with no reactant humidification.
• the wicking backing was used only on anode side 42 because of the tendency of the anode side 42 to dry out and the preponderance of water at the cathode side 43 (due to the cathode reaction) at higher current densities.
• Figure 3 depicts polarization curves for two basic fuel cells with membranes of National 115; one configured with an anode wicking backing (closed circles) as shown in Figure 1 and the other a conventional cell (open circles) with conventional reactant humidification.
• the cell with the wicking backing outperformed the conventional cell over the entire current density range even though the latter had relatively aggressive humidification conditions (anode and cathode humidifiers at 110 and 90 °C, respectively).
• anode and cathode humidifiers at 110 and 90 °C, respectively.
• the performance higher at the low current densities where the higher hydration conditions improves the ORR but it was also greater at the higher ranges where the non-humidified cathode gases used in the wicking scheme probably improved the mass transport of the oxygen.
• Figure 4 depicts the high frequency (8 kHz) cell resistance for the two cells having the characteristics shown in Figure 3.
• the resistance of the conventionally humidified cell (open circles) is much greater at the higher current densities where the anode side of the membrane tends to dry out due to the electro-osmotic drag of water away from this side.
• the run with water injection according to the present invention shows relatively constant, and ultimately much lower, cell resistance. It is evident that the droplets of injected water are contacting the plenum side of the wicking thread, where the water is carried across the backing to supply the membrane directly with liquid water. While improving the cell performance is obviously desirable, it is important to note that a much simpler and energy effective hydration scheme is realized than with reactant humidification.
• threads e.g., polymer blends or special polymer fibers
• other coatings e.g., polyaniline or poiypyrrole.
• FIG. 6 depicts the polarization and high frequency resistance curves for one such cell operating at 115 °C without any difficulty. The cell resistance is very close to that observed at normal operating temperatures (i.e., roughly at 0.08 ⁇ cm 2 as depicted in Figure 6), which is not normally attained with vapor humidified cells at these higher temperatures.
• sets of channels (water channels 64 and fuel gas channels 66 for fuel cell anode side 60; water channels 74 and fuel gas channels 76 for fuel cell anode side 70) can be formed into either side of a porous, electronically conductive carbon paper 62,
• the anode gas i.e., hydrogen
• liquid water are manifolded to the opposing faces of the paper, and the porous material is rendered either hydrophilic (porous material 62) if the water channels were on the far side or hydrophobic (porous material 72) if the water channels are against the wicking backing 68 and 78, respectively.
• the former configuration tends to collect water in the hydrogen channels which leads to erratic performance and the latter provides results that are similar to those shown in Figures 3 and 4 on the 5 cm 2 scale.
• the system for supplying the water to the wicking backing can be further simplified by placing the hydrogen and liquid water channels in the same plane. This also decreases the amount of space required for the anode flow structure, which decreases the pitch of the unit cell and increases the power density of the stack.
• a 50 cm 2 active area H2/H2O anode flow-field plate 80 that accommodates both reactant gas and water flow-channels in this manner is shown in Figure 8.
• Three channels 82, 84, 86 are formed in a conventional serpentine arrangement.
• the middle channel 86 is manifolded to carry water and the other two channels 82, 84 to carry reactant gas.
• the three channels are preferable to two because this arrangement prevents water channel 86 from doubling back on itself.
• the 50 cm 2 experimental cell 90 has a channel on anode side 92 that is dedicated to the liquid water which thus separates the two flow-streams, as shown in Figure 9.
• a separate recycleable flow loop is thus provided for the water.
• Pump 112 is used as before to inject water 108 into anode side 92 of cell 90, albeit through the separate H2O channel.
• Fuel gas 94 is provided through meter 96 to the separate gas channels for anode side 92.
• Needle valve 98 (or a second back pressure regulator) is used on the stack effluent side 102 of the water loop in order to pressurize the water stream slightly compared to the reactant gasses.
• Effluent 102 is provided through back-pressure regulator 104 to reservoir 106. where fuel gas 108 is separated from effluent water that is returned to reservoir 106.
• the polarization curves for two Nation 115-based 50 cm 2 active area cells are compared in Figure 10.
• One is a standard cell with conventional reactant humidification (open circles), the other utilizes a separate water channel in the anode flow-field as described above as well as a wicking anode backing (closed circles).
• Figure 3 the system shown in the case of the smaller cells
• Figure 9 that delivers liquid water directly to the membrane outperforms the otherwise similar conventional cell over the entire current density range.
• the continuously filled water channel it was necessary to pump only about 3 ml/min into the special plate, and even less would have sufficed as a fair amount of the water was wicked over the ribs separating the water from the hydrogen channels. This can be prevented by selectively applying the plenum side of the wicking over only the water channel.
• the wicking on the membrane side of the backing ideally still will span the hydrogen channels to supply the liquid water in these areas. More important than the increase in performance demonstrated by the wicking backing/H2-H2 ⁇ anode flow field in the comparison of the two non- optimized systems, is the demonstration of a relatively simple hydration approach from a systems perspective.
• De-coupling the water and gas supply subsystems simplifies the optimization of stack performance. Eliminating humidifier modules decreases system volume and improves response.
• the system described herein essentially requires only a pump and a water reservoir, which are required for most hydration schemes anyway.
• a stack system could be yet further simplified by combining the stack cooling with the hydration system by pumping a sufficient amount of cooled water through the hydration channels in the anode plate to remove the excess heat generated by the fuel cell stack.

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• 1998
  • 1998-10-28 EP EP98944427A patent/EP1129500A4/de not_active Withdrawn
  • 1998-10-28 WO PCT/US1998/002895 patent/WO2000025377A1/en active Application Filing
  • 1998-10-28 CA CA002348789A patent/CA2348789A1/en not_active Abandoned
  • 1998-10-28 AU AU91966/98A patent/AU9196698A/en not_active Abandoned

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