Patent Publication Number: US-8114554-B2

Title: Enhanced fuel delivery for direct methanol fuel cells

Description:
BACKGROUND 
     This invention relates to powering of portable electronic devices. 
     Portable electronic devices are normally powered with either a primary or a rechargeable battery. Growth in the portable electronic device market, as well as, changes in usage patterns, has provided opportunities for rechargeable sources of power to power an electronic device. While primary batteries have a greater energy density, their internal resistance is larger, and primary batteries are less suitable in high drain (&gt;0.2 C rate of discharge) electronic devices. Rechargeable batteries can handle large loads but do not have sufficient energy capacity for many applications. 
     Fuel cells incorporated into power sources for portable devices promise longer runtimes than conventional battery systems, due to the ability to use high-energy content fuels. Several fuel cell technologies are currently under development for commercialization in portable power applications, such as direct methanol fuel cells (DMFC) and hydrogen polymer electrolyte membrane (PEM) fuel cells. 
     In a DMFC, the fuel is methanol or mixtures of water and methanol. Methanol or methanol mixtures are delivered as a liquid to an anode chamber in a DMFC, where methanol is oxidized as part of the electrochemical conversion of fuel to electricity. An operational challenge in DMFC systems is “methanol crossover” a phenomenon where at above about 3% methanol concentration in the anode chamber, an unacceptably high amount of methanol migrates across a polymer electrolyte membrane and causes both parasitic losses (reducing runtime) and mixed potentials differences at the cathode causing reduced output power. 
     SUMMARY 
     Described are embodiments to enhance the rate of fuel vaporization to deliver fuel as a vapor to fuel cells. An enhanced membrane is disposed in a fuel cartridge or fuel reservoir to provide fuel as a vapor. The fuel cartridge or reservoir is configured to absorb ambient heat from a device powered by the fuel cell, to increase heat adjacent the membrane and provide a concomitant increase in a rate of vaporization of the fuel. The rate of fuel vaporization is proportional to a surface area of the membrane, and exponentially related to changes in temperature. This permits compact fuel reservoir or fuel cartridge systems. By providing compact fuel reservoir or fuel cartridge systems vapor phase delivery of methanol fuel can be provided at higher rates to enable higher power DMFC systems. 
     According to an aspect of the invention, a fuel cartridge includes a housing, a fuel egress port supported by the housing, and a heat-producing element disposed in thermal communication with an interior portion of the housing. 
     Other embodiments are within the scope of the claims. The fuel cartridge includes a surface area enhanced planar vaporization membrane residing in the fuel cartridge, the surface area enhanced planar vaporization membrane disposed in thermal communication with the heat-producing element. The surface area enhanced planar vaporization membrane is disposed about a substantial portion of an interior perimeter of the housing to provide a high surface area membrane. The surface area enhanced planar vaporization membrane is a composite membrane can be comprised of multiple layers or folds of polymer membrane to increase vapor permeation surface area. The surface area enhanced planar vaporization membrane can be arranged as a series of folds. The surface area enhanced planar vaporization membrane is a polymer membrane provided with macroscopically irregular and/or microscopically roughened membrane surfaces to increase the effective membrane surface area for pre-evaporation. 
     The heating element is disposed within the housing adjacent the surface area enhanced planar vaporization membrane that spaces a liquid source of hydrogen containing compound or carbonaceous fuel from a vapor phase of the source of hydrogen containing compound or carbonaceous fuel. The cartridge supplies a source of fuel to a direct methanol fuel cell, and the fuel cartridge contains a liquid source of hydrogen containing compound or carbonaceous fuel. The heating element is a wire disposed in thermal communication with the interior of the cartridge and spaces a vapor portion of the cartridge from a liquid reservoir of the cartridge. 
     According to an additional aspect of the invention, a fuel cartridge includes a housing, and a fuel egress port supported by the housing. The cartridge also includes a bladder for containing a source of fuel and a piston that is urged against the bladder. 
     According to an additional aspect of the invention, a fuel cartridge includes a housing, a vaporization membrane, a fuel egress port supported by the housing, and a piston that is urged against the vaporization membrane, with the vaporization membrane providing a chamber in the fuel cartridge in vapor communication with the fuel cell anode. 
     According to an additional aspect of the invention, a fuel cartridge includes an inner housing having a opening to allow vapor to escape, a vaporization membrane and a piston that is urged against the vaporization membrane, with the vaporization membrane providing a chamber in the inner housing in vapor communication with the opening, and an outer housing disposed around at least a portion of the inner housing, forming an outer chamber about the inner housing, with the outer chamber being in vapor communication with the chamber in the inner housing. 
     Such approaches allow the fuel cell to operate without a need for pumps or other active controls to maintain low methanol activity in the anode. The approach also enables high rates of vapor delivery and thus permits higher power DMFC systems than prior approaches for a specified cell size and geometry. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are block diagrams depicting an electronic device powered by a fuel cell. 
         FIGS. 2A-2E  are diagrams depicting arrangements of polymer membranes in fuel cartridges. 
         FIG. 3  is a diagram depicting a fuel cartridge having a local heating arrangement. 
         FIG. 4  is a diagram depicting a prismatic fuel cartridge having a bladder and local heating arrangement. 
         FIG. 4A  is a diagram depicting aspects of a valve for the fuel cartridge. 
         FIGS. 5-7  are diagrams depicting various arrangements for inducing vapor pressure differentials in fuel cartridges. 
         FIG. 8  is a diagram depicting a powered device and construction details of the fuel cartridge. 
         FIG. 9  is a plot of methanol vapor pressure with temperature changes. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1A , a portable powered, electronic device  10  (hereafter device  10 ) is shown. The device  10  includes a housing (not shown) having a compartment (not shown) to house an energy source, e.g., a fuel cartridge  12 . The device  10  also includes an interconnect  16  to interface a fuel cartridge  12  that supplies a source of fuel (methanol or solutions of methanol or containing and/or carbonaceous compound or mixture of such compounds to deliver a form of hydrogen) to a fuel cell  18  as a vapor rather than a liquid. The fuel cartridge  12  includes a membrane, generally denoted as  44 , which partitions a liquid phase of the fuel to a vapor phase that can be delivered to an egress of the fuel cartridge  12  and into the fuel cell  18 . Embodiments of the membrane  44  are described in  FIGS. 2A-2E  below. Although a fuel cartridge is described, other embodiments of a fuel container are included such as a reservoir  13  as shown in  FIG. 1B . In that instance, the fuel reservoir  13  would include membrane  44  and be arranged to either receive fuel from the fuel cartridge  12  having the membrane  44  or be replenished with liquid fuel directly from a cartridge  12  or via a non-fixed source of the fuel, such as by pouring liquid fuel into the reservoir  13 . 
     In some embodiments the fuel cell  18  is a direct methanol fuel cell (DMFC). Optionally, the interconnect  16  interfaces either a battery source of power, e.g., primary or secondary, e.g., rechargeable batteries (not shown) or the fuel cartridge  12 . Such an interconnect  16  can distinguish between a fuel cartridge and a battery and provides a convenient technique to allow a fuel cell-powered device to operate under battery power in situations where a fuel cartridge is temporarily unavailable. Device  10  can be any type of portable device. Non-limiting examples include a mobile phone, portable computer or audio/video device. 
     Referring to  FIGS. 2A-2C , a fuel cartridge  12  has a fuel delivery interface, that is complementary to the interconnect  16  ( FIG. 1 ), including an egress port  32 , as shown. The fuel cartridge  12  includes a mechanism to enhance the rate of delivery of fuel in a vapor state to fuel cells, via use of a surface area-enhanced planar vaporization membrane  44  residing in the fuel cartridge  12 , which supplies fuel to the direct methanol fuel cell (DMFC). 
     As shown in  FIGS. 2A-2C , the cartridge outlet can be an egress port. One benefit to a narrow egress is that the cartridge  12  could be more amenable to additional functionality, such as being easily inserted or removed from a device without significant loss of fuel. Another advantaged of a narrow opening is that optional resistive heating could be more finely controlled with a narrow egress (as discussed below). A pinching mechanism, for example could also be used to further restrict flow if desired. 
     Another approach to the egress port is as an open cavity that separates the cartridge  12  from the fuel cell anode (not shown). An open cavity outlet would not disadvantageously restrict vapor diffusion to the anode, as could happen with a narrow egress. The open cavity outlet could be approximately as wide as the cartridge  12  to allow maximum transport to the anode of the DMFC. Thus, the cartridge  12  could have a temporary cover or the like covering the opening, which is removed during use. In some embodiments, the cartridge  12  could have a portion of the membrane  44  disposed across the opening in the cartridge  12 . In general, a large opening is preferred. 
     The membrane  44  can be fabricated from a variety of polymer materials, including polyurethanes, silicones, poly(trimethylsilylpropyne), and others. Fabrication of the polymer can include introducing microporosity to govern the vaporization process (via a vaporization mechanism) or a dense membrane structure. The membrane can also be fabricated from a sintered metal disc, coated or uncoated with polymer, to achieve a similar vaporization performance. 
     Different surface area enhanced planar vaporization membranes  44  to enhance and stabilizing the rate of fuel delivery are shown in  FIGS. 2A-2E  including a polymer membrane  46  disposed about a substantial portion of an interior perimeter of the fuel cartridge  12  to provide a high surface area membrane.  FIG. 2B  shows a composite membrane  48  comprised of multiple layers or folds of polymer membrane to increase vapor permeation surface area. A membrane  50  can be arranged as a series of folds such as shown in  FIG. 2C .  FIGS. 2D and 2E  show another technique where a polymer membrane  52  is provided with macroscopically irregular (shown) or microscopically (not shown) roughened membrane surfaces to increase the effective membrane surface area for vaporization. 
     Referring to  FIG. 2A , a gas permeable membrane  46  is shown. The gas permeable membrane  46  spaces a liquid source of methanol  62  from a vapor phase  64  of methanol. Vapor occupies the interstitial volume between the membrane  46  and interior walls of the cartridge  12 . Rather than use the membrane in planar geometry at the egress port  42 , the membrane  46  is chosen to surround the fuel volume and is disposed about an interior portion of the wall  65  of the fuel cartridge  12  enabling increased membrane area, and enhanced delivery rate of methanol in a vapor phase to the egress port  42 , for a given cartridge or reservoir size. The rate of fuel delivery is proportional to the surface area of the planar membrane  46 . The membrane  46  augments the rate of fuel delivery in a vapor phase and can be used with regular or compact fuel reservoir or fuel cartridge systems to provide high rates of methanol fuel vapor to high power DMFC powered devices. 
     Referring to  FIG. 2B , a multilayer membrane  48  includes a series of layers  48   a  or folds of polymer membrane disposed about a periphery of the cartridge  12  to increase membrane surface area. An example of the multilayer membrane  48  as wound-cell includes vaporization membrane  48   a  disposed over a first surface of a substrate  48   b  of porous material that holds methanol in a liquid state within pores of the material to enable the liquid methanol to migrate to the membrane  48   a  and convert to a vapor phase. The membrane is fabricated from one of a variety of polymer systems, including polyurethanes, silicones, poly(trimethylsilyl-propyne), and other polymeric compositions, including composites. Fabrication of the polymer can include introducing microporosity to govern the vaporization process (via a vaporization mechanism) or a dense membrane structure. 
     The membrane  48  can also be fabricated from a sintered metal disc, coated or uncoated with polymer, to achieve a similar vaporization performance. The substrate  48   a  is comprised of one of a variety of polymer systems, including polyethylene, polypropylene, nylon, polyurethane, or other analogous polymers or composites of one or more of these polymers. The substrate  48   a  can also be fabricated from a sintered metal form, coated or uncoated with polymer, to achieve a similar performance. 
     In some embodiments the material of substrate  48   a  can have further qualities of a “sponge-like” material. An opposite surface of the sponge material  46   b  is coated with a methanol-impermeable layer  48   c , which can be fabricated from materials such as a cross-linked rubber, a polymer/inorganic composite, a surface treated material such as surface fluorinated high density polyethylene, or other methanol-impermeable material. 
     This three-layer arrangement  48   a - 48   c  can be wound and placed into a cylindrical container that comprises the cartridge  12 , with an array of gaps between the vaporization membrane  48   a  and the methanol-impermeable layer  48   c  providing a path for transporting a high flux of methanol vapor to an anode chamber in the fuel cell. This multilayer membrane  48  can provide a very high flux of methanol vapor from a relatively compact fuel reservoir or fuel cartridge  12 . The three-layer arrangement  48   a - 48   c  can also be arranged as a series of planar layers and disposed in housings of various shapes and in various configurations, such as disposed about a periphery of the housing, at the egress port of the housing in prismatic shaped cells as in  FIG. 4  and so forth. 
     Various intermediate arrangements between the high surface area of a wound-cell arrangement ( FIG. 2B ) and the rectangular, liquid-fuel-surrounding membrane ( FIG. 2A ) are possible. For instance, intermediately dense folded membrane  50  such as shown in  FIG. 2C  can balance high fluxes obtained in the multilayer configuration and the low membrane volume (i.e., high fuel energy density) of option ( FIG. 2A ). The gas permeable membrane  50  would extend between interior walls of the fuel cartridge  12  providing a vapor chamber  51  adjacent the egress port  32  of the fuel cartridge  12 . 
     Referring to  FIGS. 2D ,  2 E, another approach to provide a rate enhancement polymer membrane  52  is by providing a random or patterned roughening of the membrane surface (FIG.  2 E). The gas permeable membrane  52  is disposed between interior walls of the fuel cartridge  12  and provides a vapor chamber  51  adjacent the egress port  32  of the fuel cartridge  12 . The roughening can be on one or both sides of the membrane. One side of the membrane (commonly the vapor side) may limit the permeation rate. It is preferable to enhance the permeation-rate-limiting side of the membrane. 
     While room temperature vapor phase delivery of methanol to the anode of a fuel cell using a passive, a gas permeable membrane placed parallel to and overlapping the anode layer in the fuel cell can work well for low power (&lt;3 W) DMFC systems) such an approach may not provide sufficient methanol vapor flux to sustain higher power operation. This is due to fundamental limitations in the membrane-enabled vaporization process. The flux of methanol per unit area of membrane is sufficient to maintain oxidation of methanol at reasonable rates for a similar area of the anode. However, above a power range of several Watts, the area of the membrane needs to grow unreasonably large to maintain the methanol flux needed to sustain fuel cell operation at higher power. A fuel cartridge with the geometric dimensions needed to provide the flat membrane area for higher power operation is not convenient for consumer use. In addition, large membranes can be mechanically unstable and have a higher likelihood of mechanical failure over time. Dependent upon operating point and choice of membrane material, an example power range of, e.g., 1 W could require a membrane area of 0.7 cm 2 , whereas a 5 watt application could require a membrane area of 3.3 cm 2 . At 3.3 cm 2  and higher this becomes impractical for many consumer applications because it requires a very large membrane surface area. 
     Localized heating can be used in conjunction with the above approaches, either via a resistive element that is disposed in the cartridge or by use of heat generated from the electronic device. 
     The approaches described above result in an augmentation of the effective surface area of the membrane arrangement generally  44  (and thus an overall rate of vapor permeation) over a fixed geometric area. An enhanced membrane  44  disposed in a fuel cartridge or fuel reservoir provides fuel delivery as a vapor to fuel cells at a rate proportional to the enhanced surface area of the membrane. The enhanced surface area membrane permits compact fuel reservoir or fuel cartridge systems that can deliver a vapor phase of methanol fuel at higher rates to enable higher power DMFC systems. Such an approach also allows the fuel cell to operate without a need for pumps or other active controls to maintain low methanol activity in the anode. 
     Referring to  FIG. 3 , a resistive heating element  72  is disposed at a vaporization membrane interface  44  to enhance vapor fuel delivery, as shown. The rate of vaporization increases significantly with increases in temperature. The vaporization membrane arrangements described in  FIGS. 2A-2D  can use the heating element  72  as a localized heat source to increase temperature and hence rate of vaporization. The heating element  72 , illustrated in  FIG. 3  is disposed electrically in parallel with the primary load (device  10 ) and is powered by a small fraction of the fuel cell electrical output to provide a net boost in output power. 
     One example of the heating element  72  is a wire, e.g., a coiled wire having a relatively high resistivity characteristic. A typical resistivity characteristic for the heating element  72  as a wire is in a range of 10 to 1 M ohms/cm. The heating element  72  can be comprised of a relatively high resistivity material such as Tungsten. Other materials that can be used include nickel/chrome alloys and others. The high resistivity materials can be coated with a polymer or a precious metal to provide protection against erosion and contamination of the fuel cell. The resistive element  72  is disposed in thermal communication with one of the vaporization membrane  44  arrangements (e.g., any of the embodiments in  FIGS. 2A-2E , or other configurations). 
     The membrane  44  and resistive element  72  provide a vapor chamber  74 , e.g., a space between the liquid fuel  76  with or without the egress port  32  of the cartridge  12  principally occupied by a vapor phase of the fuel. Preferably, the resistive heating element  72  directly contacts the membrane  44 , since as the membrane temperature increase that augments the vaporization rate. The heating element  72  could be on the liquid side or on the vapor side of the membrane  44 , or embedded within the membrane  44 . The latter two options (vapor side and embedded) provide the advantage of minimizing unnecessary heating of the liquid in the cartridge. Additionally, a sintered metal, for example, could serve as both the membrane material and resistive heater. Heat provided by the resistive element  72  enhances the rate of vaporization across the membrane  44  and can improved overall performance when the device  10  powered by the fuel cell is used in relatively cold ambient temperature environments. 
     Referring to  FIG. 4 , another approach  80  can vaporize the liquid fuel, e.g., methanol in a fuel cartridge  12  entirely through a thermal process without the need for a membrane. In this arrangement, power is drawn from the fuel cell (not shown), or supplied through a small battery  82  (button cell, for example) located within or on the fuel cartridge  12  to power a heating mechanism  84 . Here, the heating mechanism  84  is schematically shown without connections to the battery, as a wire disposed at the egress port  32  of the fuel cartridge  12 . 
     The fuel cartridge  12  includes a wall or body, here illustrated as a prismatic battery case  86  including the heating element  84 , and an internal fuel bladder  90  of a fuel impermeable material, e.g., a rubber and the like that is in contact with a movable wall or piston  88  in the interior of the fuel cartridge  12 . A spring  89  applies force to the wall. Guides (not shown) can be used to guide the wall or piston  88  as it moves along the length of the prismatic case. Liquid fuel, e.g., methanol is disposed in the bladder  90 . As liquid is consumed from the fuel cartridge  12  the pressure in the bladder  90  subsides, allowing the force produced by the spring  89  to urge the wall or piston  88  against the bladder  90  to insure that methanol in the bladder  90  is delivered to the egress port  32  of the fuel cartridge  12 . The wall/piston  88  and spring  89  insure uniform delivery of liquid from the bladder  90  independent of case orientation. 
     The egress port  32  can have a fuel valve integrated with a vaporization heating unit. One embodiment as shown in  FIG. 4A , includes a resistive heating element(s) that is disposed in a constricted area within the valve assembly (not shown). In some embodiments the heat element  84  could be dispensed with. Power for the resistive heaters can be obtained by the button cell battery within or supported on the fuel cartridge, or from the fuel cell power source via external leads (not shown). Other embodiments are possible. 
     Referring to  FIG. 4A , an example of a fuel valve  70  having an integrated vaporization-heating unit is shown. The fuel valve  70  is illustrated as the egress  32  for the embodiment of the cartridge  12  shown in  FIG. 4  including membrane arrangement  46 . The egress  32  is depicted as a valve  33  having an integrated heating element  73 . The valve  33  is supported on the cartridge wall  65  and includes the heating element  73  arranged in any one of a variety of configurations such as disposed in the center of the valve as shown, or disposed about the sidewalls of the valve (not shown) or integrated into the sidewalls (not shown). The heating element is disposed to increase the rate of vaporization across the membrane  46 . The valve can have various mechanisms to secure it to a device during use, such as a bayonet connection, threaded connection and so forth. 
     Referring to  FIG. 5 , an alternative arrangement to enhance vapor delivery is to provide a reduced pressure on the permeate (vapor) side of a vaporization membrane  44  and take advantage of the principle that a pressure decrease (similar to a temperature increase) can boil or evaporate a liquid. Stated differently, a reduced pressure downstream incrementally decreases a vapor concentration of fuel, thus increasing a driving force for permeation of the fuel from the liquid phase to the vapor phase. 
     One mechanism to induce a reduced pressure is to increase volume on a vapor side  90  of the cartridge  12 . The vapor side of the cartridge  12  includes a vapor permeable piston  92  that is urged against liquid  96  in the cartridge  12  by one or more spring mechanisms  94  disposed between the piston  92  and interior regions of the cartridge  12  adjacent the egress port  32  of the cartridge  12 . One embodiment of the piston  92  is as a vaporization membrane  44 . A wire mesh or rigid micro- or macro-porous layer can mechanically support a flexible vaporization layer, (e.g., a fluorocarbon polymer, polyethylene, polypropylene, polycarbonate, polyimide, polysulfone, polysulfide, polyurethane, polyester, cellulose, or paper). The ring piston  92  provides a leak-proof seal while sliding along the cartridge wall. The ring outer diameter nests barely within the cartridge diameter. Also, the ring and adjacent cartridge wall are preferably made of or coated by a fuel repellent and fuel impermeable material to minimize liquid flow leakage into the vapor side. Such a materials or coatings are fluoropolymers, e.g., polytetrafluoroethylene and so forth. In addition for the ring in particular, a sufficiently rigid material is preferred to minimize the ring radial thickness while still providing mechanical stability, allowing for maximum uncovered membrane area. 
     As the liquid volume is depleted, the vapor side increases in volume since the piston  92  travels further away from the egress port  32  expanding the volume on the vapor side of the cartridge  12 . Again, the vaporization membrane  44  contains the fuel in its liquid phase and principally allows only vapor to permeate into the vapor side  90 . The mechanical action can be active (e.g. with the force of springs) or passive (e.g., with liquid displacement alone). Passive actuation relies on low friction of the ring piston. 
     Referring to  FIG. 6 , fuel cell  18  is shown as a fuel cell stack  100  (a single membrane electrode assembly) having an anode  102  and a cathode  103  spaced by a separator  105 . The fuel stack  100  is disposed adjacent the vapor side  90  of the fuel cartridge  12 . Vapor from the fuel cartridge  12  directly flows to an anode electrode  102  of the fuel cell  18 . 
     The volume of expansion induced in the vapor side  90  of the cartridge  18  can be made greater than the contraction volume of the liquid fuel phase by permitting additional expansion of the volume of the vapor chamber  74 . 
     Referring to  FIG. 7 , an arrangement  110  to enhance vapor delivery by providing additional volume to the vapor phase chamber  74  is shown. The vapor side  90  of the fuel cartridge  12  including piston  92  and internal spring  94 , as in  FIG. 5 , is augmented with an arrangement  110  to increase the effective volume of the vapor chamber  74  of the cartridge  12 . Additional volume is provided to the vapor phase chamber  74  by an external chamber  112  that is disposed around the outer surface of the cartridge  12  and which is in vapor communication with the internal vapor chamber  74 . The external chamber  112  has a vapor impermeable piston  114  that is urged against vapor in the outer chamber  112  in the cartridge  12  by one or more outer spring mechanisms  116  disposed between the vapor impermeable piston  114  and the fuel cell  18 , adjacent the egress port  32  of the cartridge  12 . As the vapor pressure increases, the increase in vapor pressure causes the piston  114  to move in a manner that increases the volume of the external chamber  114 . 
     One embodiment of the vapor impermeable piston  114  is a solid sealing material or metal coated with sealing material such as polyfluoroalkenes, fluoroelastomers, and rubbers, e.g., silicone, fluorosilicone, nitrile neoprene, natural, or polyurethane. A metal core can be included in the ring piston to provide mechanical rigidity. The external chamber  114  may be an expandable gas volume of fuel vapor, anode reaction product, and possibly inert gas (such as nitrogen). The contracting volume opposing the external chamber  114  (i.e., on the opposite side of the ring piston) is preferably vented to an external ambient to avoid pressure buildup inside the external chamber  114 . 
     The expansion may be independent of liquid depletion as shown here with independent springs. Alternatively, the outer ring piston may be connected mechanically (or magnetically if desired) to slide in parallel with the inner piston movement with liquid depletion. Furthermore, the vapor side cavity may be shaped (e.g., cone-like) to allow for an increasing volume expansion as the fuel depletes. Vapor-side expansions greater than the liquid contraction do have the disadvantage of requiring additional overall volume. 
     For control of fuel delivery, the membrane may be synthesized or processed (by localized compression or elongation, for example) to have variable permeability with surface position. For instance, if a non-uniform distribution of fuel to the anode is provided, a position-variable permeability (and thus variable fuel flux) can be provided to even fuel distribution. 
     Referring to  FIG. 8 , the portable powered, electronic device  10  depicted in  FIGS. 1A and 1B , is shown with a housing  11 , having a compartment  14  that houses an energy source, e.g., one of the fuel cartridges  12  as described above. The interconnect  16  interfaces the fuel cartridge  12  that supplies a source of fuel (a form of hydrogen) to the fuel cell (not shown) as a vapor rather than a liquid. The fuel cartridge  12  includes vaporization membrane  44  that partitions a liquid phase of the fuel to a vapor phase that can be delivered to an egress  32  of the fuel cartridge  12 . In some embodiments of the fuel cartridge  12  the walls or at least portions of a wall, e.g.,  12   a  of the fuel cartridge  12  are fabricated from a thermally conductive material, typically a metal. Such an embodiment of a fuel cartridge  12  uses the walls of the fuel cartridge as a heat sink for heat generated by small portable devices like a lap top computers. The metal or conductive material or at least those portions of the cartridge comprised of the conductive material are disposed in thermal communication with a heat-dissipating component  19  within the device  10 . The fuel cartridge is disposed in close proximity to heat dissipating component  19 , e.g., a CPU in a laptop, or within an airflow pattern associated with micro fans (not shown) used in some portable power devices. 
     The fuel cartridge  12  draws heat away from heat dissipating component  19  in the electric device  10 . Heat will be transferred across the thermally conductive wall of the fuel cartridge  12  and will provide a concomitant increase in the pressure of methanol vapor within the cartridge  12 . The increase in vapor pressure enables faster vapor flow through the separator membrane  44 . This technique provides a fuel cartridge  12  with a passive system that provides enhanced methanol vapor pressure and hence greater energy delivery to the fuel cell. In addition, the use of the fuel cartridge  12  as a heat sink may significantly reduce the need for a cooling fan (also an energy drain on the device) to enhance device efficiency and increase run time of the device. The exact configuration of the fuel cartridge  12  could be dependent on the configuration of the device  10 , the amount of heat generated by the device and the presence or absence of a fan. 
     Configurations of the fuel cartridge  12  can include, a metal or other thermally conductive material wall  12   a  that is combined with remaining, thermally insulating walls  12   b  of the fuel cartridge  12   b . The thermally conductive walls  12   a  would be disposed in direct contact with the heat source  19  in the device or at least in close proximity to the heat source  19 , or in an air flow path (not shown) that is used to remove heat from the heat source  19 . Alternatively, the thermally conductive can be an upper portion of the fuel cartridge  12  adjacent the fuel egress port  32  and in general alignment with the vapor chamber provided in the cartridge. In some embodiments, the housing of the fuel cartridge  12  can be completely comprised of metal or other thermally conductive material. The fuel cartridge can take various shapes including the prismatic type depicted, cylindrical types depicted in  FIGS. 1 ,  2 A- 2 D and so forth. 
     Referring to  FIG. 9 , a plot that depicts changes in methanol vapor pressure with temperature changes is shown. 
     The cartridge  12  is particularly useful with electrical components that generate a large amount of heat during operation. The cartridge  12  would have features that take advantage of heat generating surfaces in the device ideally being placed in direct contact with the fuel cartridge. In some embodiments, the cartridge can be configured as a fuel reservoir and supplement or replace heat sink elements on heat dissipating devices. The cartridge containing the methanol liquid serves as a vapor phase fuel delivery system and a heat sink for the device  10 . Thus, the fuel cartridge acting as a heat sink helps to remove heat from the device  10 , while the heat generated increases the vapor pressure of the methanol vapor and therefore increases the amount of vaporized fuel that can be delivered by the membrane surface to the fuel cell. The fuel cartridge can include external and/or internal fins to increase heat transfer to the methanol fuel. 
     In pervaporation, the fuel is vaporized as it moves through the membrane, rather than being vaporized in advance of the membrane. Some embodiments of the membrane can be considered pervaporation membranes whereas; others can be considered vaporization membranes. For instance, direct heating without a membrane or in advance of the membrane (vapor-vapor permeation) is a direct vaporization process. 
     The approaches described above in  FIGS. 2A-2E  result in an augmentation of the effective surface area of the membrane arrangement generally  44  (and thus an overall rate of vapor permeation) over a fixed geometric area. An enhanced membrane  44  disposed in a fuel cartridge or fuel reservoir provides fuel delivery as a vapor to fuel cells at a rate proportional to the enhanced surface area of the membrane. The arrangements depicted in  FIGS. 3-9  increase vaporization rate exponentially with increases in the temperature of the liquid fuel source. The enhanced surface area membrane and/or heating or pressure reducing mechanisms permit compact fuel reservoir or fuel cartridge systems that can deliver vapor phase of methanol fuel at higher rates to enable higher power DMFC systems. Such an approach also allows the fuel cell to operate without a need for pumps or other active controls to maintain low methanol activity in the anode. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, rather than being a replaceable fuel cartridge, the arrangement can be a permanently attached fuel reservoir that can be replenished periodically through a refilling mechanism. In addition, a fuel cartridge could be used to provide vapor phase methanol fuel to a fuel cell assembly that has a permanently attached fuel reservoir containing a second membrane system. In such a system, the second membrane regulates the flux of vapor phase methanol to the fuel cell in two-stage a manner that may provide more control of vapor delivery than that of a single-stage vaporization approach. The techniques thus apply to a fuel cell assembly with a permanently attached fuel reservoir, or replaceable fuel cell cartridge, or both. Accordingly, other embodiments are within the scope of the following claims.