Abstract:
The present invention concerns a fuel reservoir for a liquid fuel cell particularly useful for portable electronic devices in which the fuel reservoir can deliver the liquid fuel regardless of the orientation. The fuel reservoir comprises (a) a container defining a cavity for holding the liquid fuel; (b) a wicking structure positioned within the cavity and into which at least a portion of the liquid fuel wicks and from which said liquid fuel subsequently may be discharged or delivered, such as by pumping or wicking. The wicking structure is formed from a wicking material with a free rise wick height greater than at least one half of the longest dimension of the wicking structure. Among materials with such wicking capability are foams, matted, bundled or woven fibers and nonwoven fibers. The container may have a generally flat and thin profile, formed as a pouch or envelope with substantially planar top and bottom faces of flexible film material, such that the container holding the wicking structure and filled with the liquid fuel can be bent or shaped.

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 60/509,035, entitled “Liquid Fuel Delivery System for Fuel Cells”, filed on Jun. 28, 2001 by the instant inventors, the disclosure of which is incorporated by reference. 
    
    
     This invention relates to liquid fuel cells in which the liquid fuel is indirectly or preferably directly oxidized at the anode. In particular, it relates to the reservoir for holding and metering or delivering the liquid fuel to the anode of a liquid fuel cell. This invention also relates to liquid fuel feed systems for micro fuel cell reformers. 
     BACKGROUND OF THE INVENTION 
     Electrochemical fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes (an anode and a cathode). An electrocatalyst is needed to induce the desired electrochemical reactions at the electrodes. Liquid feed solid polymer fuel cells operate in a temperature range of from about 0° C. to the boiling point of the fuel, i.e., for methanol about 65° C., and are particularly preferred for portable applications. Solid polymer fuel cells include a membrane electrode assembly (“MEA”), which comprises a solid polymer electrolyte or proton-exchange membrane, sometimes abbreviated “PEM”, disposed between two electrode layers. Flow field plates for directing the reactants across one surface of each electrode are generally disposed on each side of the membrane electrode assembly. 
     A broad range of reactants have been contemplated for use in solid polymer fuel cells, and such reactants may be delivered in gaseous or liquid streams. The oxidant stream may be substantially pure oxygen gas, but preferably a dilute oxygen stream such as found in air, is used. The fuel stream may be substantially pure hydrogen gas, or a liquid organic fuel mixture. A fuel cell operating with a liquid fuel stream wherein the fuel is reacted electrochemically at the anode (directly oxidized) is known as a direct liquid feed fuel cell. 
     A direct methanol fuel cell (“DMFC”) is one type of direct liquid feed fuel cell in which the fuel (liquid methanol) is directly oxidized at the anode. The following reactions occur:
 
Anode: CH 3 OH+H 2 O→6H + +CO 2 +6e − 
 
Cathode: 1.5O 2 +6H + +6e − →3H 2 O
 
The hydrogen ions (H + ) pass through the membrane and combine with oxygen and electrons on the cathode side producing water. Electrons (e − ) cannot pass through the membrane, and therefore flow from the anode to the cathode through an external circuit driving an electric load that consumes the power generated by the cell. The products of the reactions at the anode and cathode are carbon dioxide (CO 2 ) and water (H 2 O), respectively. The open circuit voltage from a single cell is about 0.7 volts. Several direct methanol fuel cells are stacked in series to obtain greater voltage.
 
     Other liquid fuels may be used in direct liquid fuel cells besides methanol—i.e., other simple alcohols, such as ethanol, or dimethoxymethane, trimethoxymethane and formic acid. Further, the oxidant may be provided in the form of an organic fluid having a high oxygen concentration—ie., a hydrogen peroxide solution. 
     A direct methanol fuel cell may be operated on aqueous methanol vapor, but most commonly a liquid feed of a diluted aqueous methanol fuel solution is used. It is important to maintain separation between the anode and the cathode to prevent fuel from directly contacting the cathode and oxidizing thereon (called “cross-over”). Cross-over results in a short circuit in the cell since the electrons resulting from the oxidation reaction do not follow the current path between the electrodes. To reduce the potential for cross-over of methanol fuel from the anode to the cathode side through the MEA, very dilute solutions of methanol (for example, about 5% methanol in water) are typically used as the fuel streams in liquid feed DMFCs. 
     The polymer electrolyte membrane (PEM) is a solid, organic polymer, usually polyperfluorosulfonic acid, that comprises the inner core of the membrane electrode assembly (MEA). Commercially available polyperfluorosulfonic acids for use as PEM are sold by E.I. DuPont de Nemours &amp; Company under the trademark NAFION®. The PEM must be hydrated to function properly as a proton (hydrogen ion) exchange membrane and as an electrolyte. 
     For efficient function of the fuel cell, the liquid fuel should be controllably metered or delivered to the anode side. The problem is particularly acute for fuel cells intended to be used in portable applications, such as in consumer electronics and cell phones, where the fuel cell orientation with respect to gravitational forces will vary. Traditional fuel tanks with an outlet at the bottom of a reservoir, and which rely on gravity feed, will cease to deliver fuel when the tank orientation changes. 
     In addition, dipping tube delivery of a liquid fuel within a reservoir varies depending upon the orientation of the tube within the reservoir and the amount of fuel remaining in the reservoir. Referring to  FIG. 1 , a cartridge  10  holds a liquid fuel mixture  12  therein. An outlet tube  14  and an air inlet tube  16  protrude from the cartridge cover  18 . If the cartridge  10  stably remained at this orientation, the fuel mixture could be drawn out from the outlet tube  14  by pumping action, and the volume space taken by the fuel exiting the cartridge  10  filled by air entering through the air inlet tube  16 . However, if the cartridge  10  were tipped on its side, the fuel mixture could be drawn out only so long as the fuel level is above the fuel removal point of the outlet tube. 
     Accordingly, to facilitate use of liquid fuel cells in portable electronic devices, a liquid fuel reservoir that controllable holds and delivers fuel to a liquid fuel cell, regardless of orientation, is desired. A swappable, disposable, replaceable or recyclable liquid fuel reservoir is further desired. It is also desirable to maximize the amount of liquid fuel that the liquid fuel reservoir can hold. 
     SUMMARY OF THE INVENTION 
     According to one embodiment of the invention, a fuel reservoir for a liquid fuel cell comprises 
     (a) a container having a wall and an interior defining a cavity for holding a liquid fuel for a liquid fuel cell; 
     (b) a wicking structure positioned within the cavity and into which at least a portion of the liquid fuel wicks and from which the liquid fuel may be metered, discharged or delivered; and 
     (c) an outlet passageway through the container that communicates with the wicking structure in the cavity. 
     The fuel reservoir of the present invention controllably holds a liquid fuel for the liquid fuel cell. The fuel reservoir can deliver fuel to a liquid fuel cell regardless of orientation because the liquid fuel inside the container is in fluid communication with the outlet passageway without regard to the orientation of the fuel reservoir. The liquid fuel stored in the fuel reservoir can exit the container without being dependent on gravity. 
     Furthermore, the fuel reservoir of the present invention can be selectively attachable to or detachable from a fuel cell. The fuel reservoir can be swappable, disposable or replaceable. The fuel reservoir can also be recyclable or replenishable in that a spent fuel reservoir can be replenished with the liquid fuel via the outlet passageway or an optional liquid fuel inlet having a valve or a membrane, preferably made of fuel resistant rubber, through which the liquid fuel can be introduced into the spent fuel reservoir through a needle or the like to obtain a replenished fuel reservoir, wherein the membrane reseals the cavity after fuel introduction. In one of the embodiments of the recyclable or replenishable fuel reservoir of the present invention, the outlet passageway is fitted with a valve or sealable cap that allows the introduction of the liquid fuel into the spent fuel reservoir and prevents the liquid fuel from leaking out of the replenished fuel reservoir during storage or shipment before the next use. In another embodiment of the recyclable or replenishable fuel reservoir, the fuel reservoir further comprises a liquid fuel inlet fitted with a valve or sealable cap that allows the introduction of the liquid fuel into the spent fuel reservoir and prevents the liquid fuel from leaking out of the replenished fuel reservoir. 
     The wicking structure not only wicks and retains liquids, but permits liquids to be controllably metered or delivered out from such structure. The wicking structure has a geometry having a longest dimension. For a cylindrically shaped wicking structure, the longest dimension may be either its height or its diameter, depending upon the relative dimensions of the cylinder. For a rectangular box-shaped wicking structure, the longest dimension may be either its height or its length or its thickness, depending upon the relative dimensions of the box. For other shapes, such as a square box-shaped reservoir, the longest dimension may be the same in multiple directions. The free rise wick height (a measure of capillarity) of the wicking structure preferably is greater than at least one half of the longest dimension. Most preferably, the free rise wick height is greater than the longest dimension. 
     The wicking structure may be made from foams, bundled fibers, matted fibers, woven or nonwoven fibers, or inorganic porous materials. The wicking structure can in general be a porous member made of one or more polymers resistant to the liquid fuel. Preferably, the wicking structure is constructed from a wicking material selected from polyurethane foams (preferably felted polyurethane foams, reticulated polyurethane foams or felted reticulated polyurethane foams), melamine foams, and nonwoven felts or bundles of a polyamide such as nylon, polypropylene, polyester such as polyethylene terephthalate, cellulose, polyethylene, polyacrylonitrile, and mixtures thereof. Alternatively, the wicking structure is preferably constructed from a wicking material selected from polyurethane foams (preferably felted polyurethane foams, reticulated polyurethane foams or felted reticulated polyurethane foams), melamine foams, cellulose foams, nonwoven felts of a polyamide such as nylon, polyethylene, polypropylene, polyester, polyacrylonitrile, or mixtures thereof, bundled, matted or woven fibers of cellulose, polyester such as polyethylene terephthalate, polyethylene, polypropylene, polyacrylonitrile, and mixtures thereof. Certain inorganic porous materials, such as sintered inorganic powders of silica or alumina, can also be used as the wicking material for the wicking structure. 
     If a polyurethane foam is selected for the wicking structure, such foam should have a density in the range of about 0.5 to about 45, preferably about 0.5 to about 25, pounds per cubic foot, and pore sizes in the range of about 10 to about 200 pores per linear inch, more preferably a density in the range of about 0.5 to about 15 pounds per cubic foot and pore sizes in the range of about 40 to about 200 pores per linear inch, most preferably a density in the range of 0.5 to 10 pounds per cubic foot and pore sizes in the range of 75 to 200 pores per linear inch. 
     If a felted polyurethane foam is selected for the wicking structure, such as a felted reticulated polyurethane foam, such felted foam should have a density in the range of about 2 to about 45 pounds per cubic foot and a compression ratio in the range of about 1.1 to about 30, preferably a density in the range of about 3 to about 15 pounds per cubic foot and compression ratio in the range of about 1.1 to about 20, most preferably a density in the range of 3 to 10 pounds per cubic foot and compression ratio in the range of 2.0 to 15. 
     A felted foam is produced by applying heat and pressure sufficient to compress the foam to a fraction of its original thickness. For a compression ratio of 30, the foam is compressed to 1/30 of its original thickness. For a compression ratio of 2, the foam is compressed to ½ of its original thickness. 
     A reticulated foam is produced by removing the cell windows from the cellular polymer structure, leaving a network of strands and thereby increasing the fluid permeability of the resulting reticulated foam. Foams may be reticulated by in situ, chemical or thermal methods, all as known to those of skill in foam production. 
     The wicking material can be permanently or reversibly compressed to from the wicking structure. An example of a permanently compressed wicking material is a felted wicking material. An example of a reversibly compressed wicking structure is a wicking structure formed by compressing a wicking material while the wicking material is being put into the cavity of a container, so that structures such as the walls of the container help keep the wicking material in a compressed state while the wicking material is inside the container. 
     In a particularly preferred embodiment, the wicking structure is made with a foam with a capillarity gradient, such that the flow of the liquid fuel is directed from one region of the structure to another region of the structure as a result of the differential in capillarity between the two regions. One method for producing a foam with a capillarity gradient is to felt the foam to varying degrees of compression along its length. The direction of capillarity flow of liquid is from a lesser compressed region to a greater compressed region. Alternatively, the wicking structure may be made of a composite of individual components of foams or other materials with distinctly different capillarities. The capillarity gradient is such that the capillarity is greatest at the portion of the wicking structure proximate the outlet passageway of the fuel reservoir, and the further distal a portion of the wicking structure is from the outlet passageway the lesser will be the capillarity. With such a capillarity gradient, the liquid fuel in the wicking structure is directed to flow from a point furthest away from the outlet passageway toward the outlet passageway, aiding in the delivery of the liquid fuel by the fuel reservoir. 
     In one of the embodiments, the wicking structure held within the container conforms in shape substantially to the cavity of the container. 
     It is desirable to minimize the volume effectively occupied by the wicking structure inside the container by minimizing the solid volume of the wicking structure in order to maximize the amount of liquid fuel held in the container. Alternatively, to maximize the amount of liquid fuel held in the container, it is desirable to minimize the wicking material volume. The “solid volume” of the wicking structure is the volume occupied by the solid material of the wicking structure. In other words, the “solid volume” is the external volume of the wicking structure minus its void volume. The “wicking material volume” or the “volume of the wicking structure” is the sum of the solid volume and the volume of wicking pores in the wicking material. The wicking material volume is preferably no more than about 50%, more preferably no more than about 25%, and most preferably no more than about 10%, of the volume of the cavity within the container. The void volume of the wicking material is preferably at least about 50%, more preferably about 65% to 98%, and most preferably about 70% to 85%, of the external volume of the wicking material. 
     In an embodiment which minimizes the solid volume occupied by the wicking structure, the wicking structure volume is minimized by providing a wicking structure that extends to the extreme parts of the cavity within the container with the central portion of the cavity substantially devoid of the wicking structure either by making the wicking structure with no or only a minimal amount of wicking material in the central portion of the wicking structure or by substantially perforating the central portion of the wicking structure. With the wicking structure occupying at least the extreme parts of the cavity, all the liquid fuel in the cavity maintains, regardless of orientation, fluid communication with the outlet passageway of the container at least via capillarity. By reducing the amount of the wicking material in the central portion of the wicking structure to a minimum, the wicking structure volume is minimized and, consequently, the amount of the liquid fuel that the fuel reservoir can hold can be maximized. For instance, if the cavity within the container is planar having a square or rectangular shape and eight corners, the wicking structure is disposed at least at or proximate the eight extreme corners of the cavity. If the cavity is planar with the square or rectangular shape, the wicking structure can have a configuration of a square or rectangular sheet with a plurality of perforations, a square or rectangular rim, or a configuration shaped like the alphabet letters “E”, “H”, “K”, “M”, “N”, “X” or “Z”. On the other hand, if the cavity within the container is planar having a round or oval shape, the wicking structure is disposed at least as a circular or oval ring along the curved edge of the cavity. 
     The container of the fuel reservoir may take various shapes, such as a generally cylindrical cartridge comparable in size and shape to disposable dry cell batteries, or other known battery cartridge shapes. Alternatively, and particularly preferred, the container may form a generally planar thin pouch, packet or envelope having flexible top and bottom faces. The envelope may be formed from one or more sheets of a flexible plastic film or a plastic-coated film that are heat-sealed or ultra-sonic welded together at the side edges of the sheets. Such an envelope container is flexibly bendable when filled with liquid fuel, and the wicking structure into which at least a portion of the liquid fuel has wicked retains such liquid and permits metering or delivering of such liquid when the container is so bent. A removable tape may be supplied to cover the outlet passageway when the envelope container is shipped or stored prior to use. 
     A liquid delivery means, such as a pump or a wick, can communicate with the outlet passageway of the fuel reservoir to deliver the liquid fuel out of the container through the outlet passageway. Alternatively, the liquid fuel can flow out of the container via the outlet passageway under the force of gravity. The liquid fuel leaving the container can be delivered to an anode of a liquid fuel cell by gravity or preferably by the action of the liquid delivery means. In one of the embodiments, for instance, the liquid fuel can be delivered to the anode using a wick having differential capillarity with the capillarity in the wick greater in the part proximate the anode than the part proximate the outlet passageway. The liquid fuel can optionally be delivered to the anode by a series of wicks connected together having different capillarities to generate a capillarity gradient in order to direct the flow of the liquid fuel from the outlet passageway to the anode. If the container is made of a rigid material, an air inlet having a one-way valve is provided to the container to permit gas flow into the volume of the container as the liquid fuel exits the container through the outlet passageway. If the container is made of a flexible material, e.g. if the container is a flexible pouch, an air inlet is optional. 
     A further embodiment of the invention is a wicking material for a fuel reservoir for a liquid fuel cell formed from a wicking structure of foam, bundled fibers or nonwoven fibers. Preferably, the wicking structure is constructed from a wicking material selected from polyurethane foams (preferably felted polyurethane foams, reticulated polyurethane foams or felted reticulated polyurethane foams), melamine foam, and nonwoven felts or bundles of polyamide such as nylon, polypropylene, polyester such as polyethylene terephthalate, cellulose, polyethylene, polyacrylonitrile, and mixtures thereof. The wicking structure made from such wicking material not only wicks and retains liquids, but permits liquids to be controllably metered or delivered out from such structure. The free rise wick height (a measure of capillarity) of the wicking structure preferably is greater than at least one half of the longest dimension. Most preferably, the free rise wick height is greater than the longest dimension. 
     In a particularly preferred embodiment, the wicking material has a capillarity gradient, such that the flow of the liquid fuel is directed from one region of the material to another region of the material as a result of the differential in capillarity between the two regions. Alternatively, the wicking material may be formed as a composite of individual structures of the same or different materials with distinctly different capillarities. 
     If a polyurethane foam is selected for the wicking material, such foam should have a density in the range of 0.5 to 25 pounds per cubic foot, and pore sizes in the range of 10 to 200 pores per linear inch, preferably a density in the range of 0.5 to 15 pounds per cubic foot and pore sizes in the range of 40 to 200 pores per linear inch, most preferably a density in the range of 0.5 to 10 pounds per cubic foot and pore sizes in the range of 75 to 200 pores per linear inch. 
     If a felted polyurethane foam is selected for the wicking material, such as a felted reticulated polyurethane foam, such foam should have a density in the range of 2 to 45 pounds per cubic foot and a compression ratio in the range of 1.1 to 30, preferably a density in the range of 3 to 15 pounds per cubic foot and compression ratio in the range of 1.1 to 20, most preferably a density in the range of 3 to 10 pounds per cubic foot and compression ratio in the range of 2.0 to 15. 
     The fuel reservoir of the present invention can hold a liquid fuel for an indirect or direct fuel cell. Examples of the liquid fuel that the fuel reservoir can hold for a direct fuel cell are methanol, ethanol, ethylene glycol, dimethoxymethane, trimethoxymethane, formic acid or hydrazine. The liquid fuel that the fuel reservoir can hold for indirect fuel cells or reformers includes liquid hydrocarbons, such as methanol, petroleum and diesel fuel. The fuel reservoir of the present invention preferably contains methanol as the liquid fuel. The methanol in the fuel reservoir is an aqueous mixture of methanol or, preferably, pure methanol. The methanol concentration of the aqueous mixture is preferably at least about 3%, preferably at least about 5%, more preferably at least about 25%, even more preferably at least about 50%, further even more preferably at least about 60%, and most preferably about 70% to about 99%, e.g. about 85%, 90%, 95% or 99%, with the methanol concentration percentage expressed on a weight-to-weight basis. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  is a front elevational view partially broken away of a prior art fuel cartridge for a liquid fuel cell; 
         FIG. 2  is a front elevational view of a liquid fuel reservoir for a fuel cell according to the invention; 
         FIG. 3  is a right side elevational view partially broken away of the liquid fuel reservoir of  FIG. 2 ; 
         FIG. 4  is a top plan view of the liquid fuel reservoir of  FIGS. 2 and 3 ; 
         FIG. 5  is a front elevational view of an alternative liquid fuel reservoir for a fuel cell according to the invention; 
         FIG. 6  is a right side elevational view partially broken away of the alternative liquid fuel reservoir of  FIG. 5 ; 
         FIG. 7  is a front elevational view of an alternative liquid fuel reservoir having no air inlet for a fuel cell according to the invention; 
         FIG. 8  is a right side elevational view partially broken away of the alternative liquid fuel reservoir of  FIG. 7 ; 
         FIG. 9  is a front elevational view of an alternative liquid fuel reservoir having a liquid fuel introduction inlet  56  containing a valve  58  for a fuel cell according to the invention; 
         FIG. 10  is a right side elevational view partially broken away of the alternative liquid fuel reservoir of  FIG. 9 ; 
         FIG. 11  is a front elevational view of an alternative liquid fuel reservoir having a liquid fuel introduction inlet  57  sealed by a membrane  59  made preferably of rubber for a fuel cell according to the invention; 
         FIG. 12  is a right side elevational view partially broken away of the alternative liquid fuel reservoir of  FIG. 11 ; 
         FIG. 13  is a schematic diagram of a liquid fuel reservoir viewed from the front with the volume of the wicking structure minimized according to the invention; 
         FIG. 14  is a schematic diagram of a liquid fuel reservoir viewed from the front with the volume of the wicking structure minimized according to the invention; 
         FIG. 15  is a schematic diagram of a liquid fuel reservoir viewed from the front with the volume of the wicking structure minimized according to the invention; 
         FIG. 16  is a schematic diagram of a liquid fuel reservoir viewed from the front with the volume of the wicking structure minimized according to the invention; 
         FIG. 17  is a schematic diagram of a liquid fuel reservoir viewed from the front with the volume of the wicking structure minimized according to the invention; 
         FIG. 18  is a schematic diagram of a liquid fuel reservoir viewed from the front with the volume of the wicking structure minimized according to the invention; 
         FIG. 19  is a schematic diagram of a liquid fuel reservoir viewed from the front with the volume of the wicking structure minimized according to the invention; 
         FIG. 20  is a schematic diagram of a liquid fuel reservoir viewed from the front with the volume of the wicking structure minimized according to the invention; 
         FIG. 21  is a schematic diagram of a recyclable or rechargeable liquid fuel reservoir with a liquid fuel outlet  78  having a sealable cap  82  containing a membrane  84  preferably made of rubber according to the invention; 
         FIG. 22  is a schematic diagram of a recyclable or rechargeable liquid fuel reservoir having a valve  86  in a liquid fuel outlet  88  according to the invention; 
         FIG. 23  is a schematic diagram of an arrangement of delivering liquid fuel from a fuel reservoir of the invention to the anode of a fuel cell by capillarity. 
         FIG. 24  is a schematic diagram of a wedge of wicking material prior to felting; and 
         FIG. 25  is a schematic diagram of the wicking material of  FIG. 24  after felting. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to  FIGS. 2  to  4 , a cartridge container  20  defines a cavity holding a liquid fuel mixture  22 . An outlet tube  24  extends into the container  20  through a cover  28  and the outlet tube  24  communicates between the cavity of the container  20  and outside of the container. An air inlet tube  26  also extends into the container  20  through cover  28 . The air inlet tube  26  may include a one way valve (not shown) so as to prevent liquid from flowing from the container  20 . 
     A wicking structure  32  is provided within the cavity of the container  20 . The wicking structure  32  surrounds the open end of the outlet tube  24  within the cavity of the container  20 . Liquid fuel wicks into the wicking structure  32 . 
     In the embodiment shown in  FIGS. 2  to  4 , the wicking structure is a felted polyurethane foam shaped as a rectangular cube or box. For example, the structure is approximately 10 mm (width)×5 mm (thickness)×90 (height) mm, with the 90 mm height as the longest dimension of the structure. 
                                               The foam was produced with the following mix:                                    Arcol 3020 polyol (from Bayer Corp.)   100   parts           Water   4.7           Dabco NEM (available from Air Products)   1.0           A-1 (available for OSi Specialties/Crompton)   0.1           Dabco T-9 (available from Air Products)   0.17           L-620 (available from OSi Specialties/Crompton)   1.3                        
After mixing for 60 seconds and allowed to degas for 30 seconds, 60 parts of toluene diisocyanate were added. This mixture was mixed for 10 seconds and then placed in a 15″×15″×5″ box to rise and cure for 24 hours. The resulting foam had a density of 1.4 pounds per cubic foot and a pore size of 85 pores per linear inch. The foam was felted by applying heat (360° F.) and pressure sufficient to compress the foam to ⅕ of its original thickness (i.e., compression ratio=5). The heat and compressive pressure were applied for about 30 minutes. The felted foam had a density of 7.0 pounds per cubic foot.
 
     The container  20  is filled with 6 ml. of an aqueous fuel solution containing 95% methanol. The cover  18  to the container comprises a cap with a rubber serum stopper  34 . 
     A pump  30  acts on the outlet tube  24  and draws liquid fuel  22  from the wicking structure  32  through the outlet tube  24 . Only a slight vacuum needs to be placed on the outlet tube  24  to draw the fuel mixture out of the container. Fuel may be drawn out regardless of the orientation of the container. In one test, with the container in its “vertical” orientation as shown in  FIGS. 2  to  4 , 5.0 ml of liquid fuel were drawn out of the fuel reservoir for a fixed pump setting. In a second test, with the container in an “upside-down” orientation (not shown), more than 2.0 ml of liquid fuel were drawn from the fuel reservoir at the same pump setting. While the “upside-down” orientation causes less efficient fuel delivery, fuel delivery was not interrupted, as would be the case for other fuel reservoirs. 
     In an alternate embodiment (not shown), the wicking structure was selected as a non-woven polyester fiber pad shaped into a rectangular cube or box of approximately 10 mm×5 mm×90 mm. The non-woven pad was formed by mixing together bulk fiber (polyester and melt-binder coated sheathed polyester) and forming the mixture with a combed roller into a layer. The layer was removed from the roller with a moving comb and transferred to a conveyor belt. The conveyor belt fed the material to an articulated arm that stacked multiple layers onto a separate conveyor belt. The multiple layers were heated and compressed to the desired final thickness. Similar fuel delivery was achieved with this non-woven polyester fiber wicking structure. 
     In a further alternate embodiment (not shown), the wicking structure comprised a needled felt. A blend of recycled polyester, polypropylene and nylon fibers were fiber-separated and a comb roller pulled a layer of fiber. The layer was removed from the roller with a moving comb and transferred to a conveyor belt. The conveyor belt fed the material to an articulated arm that stacked multiple layers onto a separate conveyor belt. The multiple layers (with a combined thickness of about 10 inches) were fed through two needling operations in which a bank of barbed needles compact the multiple layers together. Needling also forced some fibers to be pulled through the sample to entangle and hold the final shape of the needled felt together. Similar fuel delivery was achieved with a wicking structure formed as a rectangular cube of the needled felt. 
     Referring next to  FIGS. 5 and 6 , an alternate container of flexible packaging for a fuel reservoir is shown. The flexible fuel delivery pouch, packet or envelope  40  comprises one or more sheets connected together to form the pouch, packet or envelope with sealed edges  42 . Preferably, the sheets are connected by heat-sealing or ultra-sonic welding. The envelope  40  defines a central volume forming a reservoir for a liquid fuel  52  for a fuel cell. An air inlet  44  is provided with a one way valve  46  to prevent liquid fuel from draining from the envelope  40 . The air inlet  44  provides a passageway for air to enter the volume of the envelope as liquid fuel is drawn therefrom. 
     An outlet tube  48  is provided through the envelope  40 . The outlet tube is in fluid communication between the interior volume of the envelope and the fuel cell. Prior to use, the outlet tube  48  may be covered with a covering tape  50 , which is shown in phantom outline in FIG.  5 . The tape covers the opening of the outlet tube  48 . In this way, a pre-filled fuel reservoir may be shipped and stored without leakage of liquid fuel therefrom. The tape  50  is removed when the envelope is installed for use to fuel a fuel cell. 
     A wicking structure  54 , formed from materials noted above with respect to the embodiment in  FIGS. 2  to  4 , is held within the volume of the envelope  40 . Just as with the first embodiment, a pump (not shown in  FIGS. 5 and 6 ) is used to draw liquid fuel from the interior volume of the container through the outlet tube  48 . And like the first embodiment, efficient fuel delivery is independent of the orientation of the envelope and the wicking structure. 
     Preferably, the wicking structure  54  conforms in dimension to the interior volume of the envelope  40 . Because the wicking structure  54  preferably is flexible, and the envelope  40  preferably is formed from flexible film materials, the entire fuel cell delivery system may be bent or flexed for various positions and configurations when in use. Moreover, the envelope  40  in this preferred embodiment is lightweight and formed with substantially planar top and bottom surfaces. 
     Referring to  FIGS. 7 and 8 , another flexible fuel reservoir is shown. The fuel reservoir according to  FIGS. 7 and 8  is similar to the flexible fuel reservoir of  FIGS. 5 and 6  except for the absence of the air inlet  44  and the one way valve  46  as the flexible pouch can collapse as fuel is withdrawn. 
       FIGS. 9 and 10  illustrate another flexible fuel reservoir of the present invention. The flexible fuel reservoir of  FIGS. 9 and 10  is similar to the flexible fuel reservoir according to  FIGS. 7 and 8  except for the presence of a liquid fuel inlet  56  having a valve  58  for the introduction of liquid fuel into the flexible pouch in order to replenish the flexible fuel reservoir with liquid fuel making the fuel reservoir recyclable. 
       FIGS. 11 and 12  illustrate another recyclable, flexible fuel reservoir of the present invention. The recyclable, flexible fuel reservoir of  FIGS. 11 and 12  is similar to the flexible fuel reservoir according to  FIGS. 9 and 10  except for the presence of a liquid fuel inlet  57  sealed with a membrane  59  preferably made of rubber for the introduction of fresh liquid fuel by a syringe or the like into the flexible pouch after some or all of the original liquid fuel has been discharged from the reservoir in order to replenish the spent fuel reservoir with liquid fuel making the fuel reservoir recyclable. Upon puncture, the membrane allows the introduction of the liquid fuel into the cavity, and after liquid fuel introduction the membrane reseals the cavity. 
     Referring to  FIGS. 13 through 20 , several embodiments of fuel reservoirs  100 ,  102 ,  104 ,  106 , 108 ,  110 ,  112  and  114  with the volume of the wicking structures  73 ,  74 ,  75 ,  77 ,  79 ,  81 ,  83  and  85  minimized are shown. Each of the fuel reservoirs comprises a container  72  defining a cavity  76  having a wicking structure  73 ,  74 ,  75 ,  77 ,  79 ,  81 ,  83  or  85 , a liquid fuel outlet passageway  78  and an optional air inlet  80  (depending on whether the container  72  is made of a rigid material). The wicking structures  73 ,  74 ,  75 ,  77 ,  79 ,  81 ,  83  and  85  of these fuel reservoirs occupy at least the extreme parts of the cavity  76 . The wicking structure can have a 3-sided configuration (see FIG.  13 ), square or rectangular configuration (see  FIG. 14 ) or a configuration in the shape of an alphabet letter “H”, “X”, “N”, “M”, “K” or “E” (see  FIGS. 15-20 , respectively). 
       FIG. 21  schematically shows an embodiment of a recyclable fuel reservoir according to the present invention. The recyclable fuel reservoir  116  comprises a container  72 , wicking structure  73 , cavity  76 , an optional air inlet  80  and a liquid fuel outlet  78  having a sealable cap  82  and a membrane  84  preferably made of rubber on the sealable cap. After some or all of the original liquid fuel has been discharged from the fuel reservoir, the fuel reservoir can be disconnected from the fuel cell, the opening of the liquid fuel outlet  78  can then be sealed with the sealable cap  82  and fresh liquid fuel can be injected through the membrane  84  to replenish the spent fuel reservoir with liquid fuel. 
       FIG. 22  is a schematic view of another embodiment of a recyclable fuel reservoir according to the present invention. The recyclable fuel reservoir  118  comprises a container  72 , wicking structure  73 , cavity  76 , an optional air inlet  80  and a liquid fuel outlet  88  having a valve  86 . After some or all of the original liquid fuel has been discharged from the fuel reservoir, the valve  86  can be closed and the fuel reservoir is disconnected from the fuel cell. Fresh liquid fuel can be introduced into the spent fuel reservoir through the valve  86  to replenish the spent fuel reservoir with liquid fuel to make the fuel reservoir recyclable or rechargeable. 
       FIG. 23  schematically shows an embodiment in which a swappable fuel reservoir  200  of the present invention is connected to the anode  212  of a fuel cell  210  via a fuel delivery wick  208 . The swappable fuel reservoir  200  comprises a container  204  defining a cavity  206 , which contains a wicking structure  202 , The wicking structure  202  of the fuel reservoir  200  is in contact with the fuel delivery wick  208 . The capillary of the fuel delivery wick  208  is greater than the capillary of the wicking structure  202  so that a capillary gradient is created to deliver liquid fuel from the fuel reservoir  200  to the anode  212  of the fuel cell  210 . 
     In a particularly preferred embodiment, the wicking structure is made with a foam with a capillarity gradient, such that the flow of the liquid fuel is directed from one region of the structure to another region of the structure as a result of the differential in capillarity between the two regions. One method for producing a material with a capillarity gradient is to felt a foam to varying degrees of compression along its length. Another method for producing a material with a capillarity gradient is to assemble a composite of individual components with distinctly different capillarities. The direction of capillarity flow of liquid is from a lower capillarity region to a higher capillarity region. 
       FIGS. 24 and 25  illustrate schematically a method for making a wicking material, such as foam, with a capillarity gradient. As shown in  FIG. 24 , a wedge-shaped slab  60  of foam of consistent density and pore size has a first thickness T 1  at a first end  61  and a second thickness T 2  at a second end  65 . The slab  60  is subjected to a felting step—high temperature compression for a desired time to compress the slab  60  to a consistent thickness T 3 , which is less than the thicknesses T 1  and T 2 . A greater compressive force, represented by arrows  62 , is required to compress the material from T 1  to T 3  at the first end  61  than is the compressive force, represented by arrows  64  required to compress the material from T 2  to T 3  at the second end  65 . 
     The compression ratio of the foam material varies along the length of the felted foam shown in  FIG. 25 , with the greatest compression at the first end  61 A (T 1  to T 3 ) as compared with the second end  65 A (T 2  to T 3 ). The capillary pressure is inversely proportional to the effective capillary radius, and the effective capillary radius decreases with increasing firmness or compression. Arrow  66  in  FIG. 25  represents the direction of capillary flow from the region of lower felt firmness or capillarity to higher felt firmness or capillarity. Thus, if a wicking material or wicking structure is formed with a material or composite material having a capillarity gradient, the liquid fuel wicked into the material may be directed to flow from one region of the material with lower compression ratio to another region with higher compression ratio. 
     The invention has been illustrated by detailed description and examples of the preferred embodiments. Various changes in form and detail will be within the skill of persons skilled in the art. Therefore, the invention must be measured by the claims and not by the description of the examples or the preferred embodiments.