Abstract:
This invention relates to a tubular direct methanol fuel cell. The fuel cell includes: a tubular inner electrode, a tubular outer electrode located concentrically around the inner electrode, and an annular electrolyte conduit located concentric to and in between the inner and outer electrodes. The inside of the inner electrode is a reactant conduit that is fluidly communicable with a first reactant source such that a first reactant is transmittable therethrough. The outside surface of the outer electrode is fluidly communicable with a second reactant source such that a second reactant is transmittable over the outside surface of the outer electrode. The annular electrolyte conduit is fluidly communicable with a fluid electrolyte source such that a fluid electrolyte is transmittable therethrough. One reactant is methanol-containing fuel, and the other reactant is oxidant.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to fuel cells and in particular to direct methanol fuel cells (DMFC).  
         BACKGROUND OF THE INVENTION  
         [0002]    A DMFC uses methanol directly as a reducing agent, similar to how hydrogen acts as the reducing agent for a direct hydrogen fuel cell (DHFC), to produce electrical energy. Methanol is a promising fuel for fuel cell powered vehicles and other devices, as it is stable and liquid at ambient temperatures. For a DHFC system that uses methanol as a fuel source, a reformer must be provided to extract hydrogen from the methanol for use in the DHFC. Such fuel cells systems are known as “indirect” methanol fuel cell systems.  
           [0003]    In contrast, in a direct fuel cell, and in particular a DMFC, liquid or vaporized methanol is supplied directly to the fuel cell without reforming. Therefore, the cost and complexity of providing reformer components in the system is avoided. The following redox reactions occur in a DMFC: 
           CH 3 OH+H 2 O→6H + +CO 2 +6e −   Anode: 
           1½O 2 +6H + +6e − →3H 2 O  Cathode: 
           [0004]    Yielding the net reaction: 
           1½O 2 +CH 3 OH→2H 2 O+CO 2   
           [0005]    The redox reactions result in the production of protons and electrons at the anode. The anode and cathode are electrically connected, through an external load, to provide a path for the electrons to flow from the anode to the cathode. A proton-conducting electrolyte is provided between the electrodes to enable the flow of protons from the anode to the cathode.  
           [0006]    It is known for fuels other than methanol to be used in direct fuel cells. Such fuels may include other simple alcohols, such as ethanol, dimethoxymethane, trimethoxymethane, and formic acid. Also, other oxidants other than air may be used, such as an organic fluid having a high oxygen content or hydrogen peroxide solution.  
           [0007]    The electrolyte used in fuel cells may be either liquid or solid. In the case of a solid electrolyte, a proton exchange membrane (PEM) may be used and may be for example, a hydrated sheet of a perfluorinated ion exchange membrane such as a polyperfluorosulfonic acid membrane, one commercial example being NAFION® sold by E.I. du Pont de Nemours and Co.  
           [0008]    In any of the fuel cells mentioned above, it is important to maintain a separation between the anode and the cathode so as to prevent fuel from directly contacting the cathode and oxidizing thereon. For this reason, PEMs, although functioning well as proton exchangers and/or solid electrolytes, are not as efficient as fuel separators, and a common problem in PEM fuel cells is the incidence of fuel cross over, which occurs when the fuel, prior to oxidation, diffuses through the membrane and contacts the cathode. Apart from the parasitic loss of fuel and oxidant from the system, such 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. Further, other disadvantages of fuel cross over may include structural changes on the cathode surface (e.g. sintering) and poisoning of the reduction catalyst by fuel oxidation products.  
           [0009]    Several approaches to addressing the cross-over and other problems in DMFC have been published. Once such approach is disclosed in PCT application WO 01/39307, which discloses a DMFC with a circulating aqueous electrolyte. The application proposes that controlling the speed of electrolyte circulation controls the build-up of the cross-over gradient in the fuel cell. Also, methanol that has permeated into the electrolyte can be reclaimed in a distillation loop.  
         SUMMARY OF THE INVENTION  
         [0010]    It is an object of the invention to provide a DMFC system that improves upon the state of the art. One specific objective is to provide improvements to a DMFC system that uses a circulating aqueous electrolyte. Another specific objective is to provide an improved geometric configuration for a DMFC that enables the relative areas of cathode and anode to be scaled up or down depending on the operating requirements of the fuel cell.  
           [0011]    According to one aspect of the invention, there is provided a direct methanol fuel cell comprising:  
           [0012]    (a) a tubular inner electrode, the inside of which is a reactant conduit that is fluidly communicable with a first reactant source such that a first reactant stream is transmittable therethrough;  
           [0013]    (b) a tubular outer electrode located concentrically around the inner electrode, the outside surface of which is fluidly communicable with a second reactant source such that a second reactant stream is transmittable over the outside surface of the outer electrode; and  
           [0014]    (c) an annular electrolyte conduit located concentric to and in between the inner and outer electrodes, and being fluidly communicable with a fluid electrolyte source such that a fluid electrolyte stream is transmittable therethrough.  
           [0015]    One reactant is methanol-containing fuel, and the other reactant is oxidant. The tubular design of the anode and cathodes enables their relative areas to be scaled up or down depending on the operational requirements of the fuel cell. In particular, catalyst utilization typically is reduced at higher loadings relative to lower loadings and therefore the cost of the catalyst will increase disproportionately to the increased catalyst loading (ie activity is not a linear function of the loading). Since the cathode may be more active than the anode for a particular catalyst type or catalyst loading, or vice versa, it is advantageous to keep the catalyst loadings constant and as low as possible, and vary the area of the electrodes. This increases utilization of the catalyst and reduces cost. The concentric tubular design provides this design flexibility, which is not available in planar designs.  
           [0016]    The inner electrode may be an anode, and the outer electrode a cathode; in such a case, the first reactant is the fuel and the second reactant is the oxidant. Alternatively, the inner electrode may be a cathode, and the outer electrode an anode; in such a case, the first reactant is the oxidant and the second reactant is the fuel.  
           [0017]    The fuel cell may further comprise a tubular proton-conducting, substantially reactant and electrolyte impermeable membrane separator located concentric to and between the anode and cathode. The membrane separator may be spaced apart from one of the electrodes to form an annular electrolyte conduit therebetween. For example, the electrolyte conduit may be between the separator and the cathode. The electrolyte conduit is fluidly communicable with an electrolyte source such that a fluid electrolyte is transmittable therethrough, and in particular, can be circulated between the electrolyte source and the electrolyte conduit. The membrane may be selected from the group of perfluorinated proton exchange membrane, microporous plastic membranes, carbon tubes, ceramic tubes, asbestos-based membranes, or any other material serving the function of a semi-permeable separator.  
           [0018]    The anode may be substantially fuel permeable. In such a case, the fuel conduit has an inlet for the flow of fuel into the fuel cell and the fuel cell further comprises an annular fuel discharge conduit concentric to and between the anode and the separator with a fuel outlet for the flow of permeated fuel and anode reaction products out of the fuel cell. In particular, the fuel cell may be closed to fuel flow at one end, and the fuel inlet and fuel outlet may be located at the other end.  
           [0019]    Alternatively, the anode may be substantially fuel impermeable. In such a case, the fuel cell further comprises an annular second electrolyte conduit concentric to and between the anode and separator for the flow of a second fluid electrolyte stream therethrough. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0020]    [0020]FIG. 1 is a schematic side view of a tubular direct methanol fuel cell having an inner anode tube with high fuel permeability, and an outer cathode tube.  
         [0021]    [0021]FIG. 2 is a schematic side view of a tubular direct methanol fuel cell having an inner cathode tube, and an outer anode tube with high fuel permeability.  
         [0022]    [0022]FIG. 3 is a schematic side view of a tubular direct methanol fuel cell having an inner anode tube with low fuel-permeability, an outer cathode tube, and a membrane separator in between the anode and cathode tubes.  
         [0023]    [0023]FIG. 4 is a schematic side view of a tubular direct methanol fuel cell similar to the fuel cell shown in FIG. 3 but without a membrane separator in between the anode and cathode tubes.  
         [0024]    [0024]FIG. 5 is a schematic side view of a tubular direct methanol fuel cell having an inner cathode tube, an outer anode tube with low permeability, and a separator membrane between the anode and cathode tubes.  
         [0025]    [0025]FIG. 6 is a schematic side view of a tubular direct methanol fuel cell similar to the fuel cell shown in FIG. 5 but without a membrane separator in between the anode and cathode tubes. 
     
    
     DETAILED DESCRIPTION  
       [0026]    Referring to FIG. 1 and according to a first embodiment of the invention, a tubular DMFC fuel cell  10  comprises a tubular fuel electrode  20  (anode), a tubular separation membrane  22  concentric to and outside of the anode  20 , and a tubular air electrode  21  (cathode) concentric to and spaced outside of the separation membrane  22 . The anode  20 , separation membrane  22 , and cathode  21  are capped at each end by first and second end caps  23 ,  24 . Although not shown in FIG. 1, the electrodes  20 ,  21  are electrically connected to an external circuit as known in the art, to conduct the flow of electrons generated in the fuel cell  10 .  
         [0027]    The diameters of the anode  20 , separation membrane  22 , and cathode  21  are selected to provide annular channels in between the anode and separation membrane (defined hereafter as “fuel discharge channel”  25 ) and in between the separation membrane and cathode (defined hereafter as “electrolyte channel”  26 ). The first end cap  23  has a generally circular fuel inlet  27  that is in fluid flow communication with the inside of the anode  20  (“fuel feed channel”  28 ), an annular fuel outlet  29  in fluid flow communication with the fuel discharge channel  25 , and, an annular electrolyte opening  30  in fluid flow communication with the electrolyte channel  26 .  
         [0028]    The second end cap  24  has an annular electrolyte flow opening  32  in fluid flow communication with the electrolyte channel  26 . The second end cap  24  is closed to the fuel feed channel  28  and the fuel discharge channel  25 .  
         [0029]    The electrodes  20 ,  21  are formed from DMFC electrode materials as known in the art, such as activated carbon tubes or composite carbon tubes formed from activated carbon or carbon-based materials such as carbon cloth, carbon fibers, graphite, and binders. Other suitable materials for the electrodes include ceramics, polymers, metals or other semi-permeable substances or structures. The electrodes could also be made of composite materials such as formed, sprayed or layered PTFE bonded porous tubes on woven carbon (graphite) sheets or fleece, or other composite materials as would be used in planar electrodes. The thickness of the electrodes  20 ,  21  are selected such that the electrodes  20 ,  21  are self-supporting; however, the fuel cell  10  may be optionally provided with a support structure such as a porous ceramic, polymer or stainless steel grid or foam-like matrix or similar supporting structure located within either or both of the two electrode tubes (not shown in the figure). such support structures are particularly desirable when the electrode walls are thin and do not have significant structural integrity. An example of such supported electrode is a stainless steel screen supported plate (foil) structure layered with mixtures of activated carbon and suitable catalyst and pore forming fillers (e.g. bicarbonates) or repellent binders (e.g. PTFE or PE).  
         [0030]    The electrodes  20 ,  21  are porous. The porosity of the electrodes  20 ,  21  can be controlled through the admixture of or coating with polytetrafluoroethylene (PTFE), Nafion® or other materials known to persons skilled in the art. The porosity of the anode  20  is in the order of 1% to 10% and thus is permeable to methanol fuel. The porosity of the cathode  21  is in the range of a normal 3-phase electrode and thus is permeable to air/oxidant but not to the electrolyte. The opposing surfaces of the electrodes  20  and  21  are coated with a thin catalyst layer for the purposes of catalyzing the oxidation and reduction reactions of the fuel cell  10 . The catalyst material is preferably platinum (Pt), or a Pt and Ruthenium (Ru) combination. However, other catalysts may be used for the fuel cell  10 , such as, other noble metals, molybdenum, Pt alloys, etc. and will be apparent to persons skilled in the art. The catalyst material is dispersed throughout the body of the electrodes  20  and  21 ; alternatively, the catalyst may be applied onto the surface of the electrodes  20 ,  21 . In particular, the anode  20  may be coated with a 50/50 Pt/Ru mixture at a density of 1 mg/.sq cm 2  or less, while the cathode is coated with Pt at a density of 4 mg/sq cm 2  or less, or at the discretion of the person skilled in the art.  
         [0031]    The walls of the electrodes  20 ,  21  may be coated with or annealed to a conductive material such as gold (not shown) to serve as a conductor for edge collection for those materials that are not sufficiently conductive to act as low resistance conductors in their own right. The amount of coating can be either the entire surface for low conductivity materials, or only selected regions, or with thin traces forming a grid or network to provide sufficient conductivity for moderately conductive materials.  
         [0032]    The membrane separator  22  material is selectable from semi-permeable materials that perform the function of a mechanical separator that is substantially impermeable to liquid fuel and electrolyte and provide efficient proton conduction, as is known to practitioners of the art. Examples of such materials include Nafion®, Gore-Tex®, microporous plastic membranes, ceramic tubes, carbon tubes, etc.  
         [0033]    The ratio of the anode and cathode diameters may be varied depending on the desired active area of the anode  20  relative to the active area of the cathode  21  to accommodate the different activities of the catalysts utilized. In particular, catalyst utilization typically is reduced at higher loadings relative to lower loadings and therefore the cost of the catalyst will increase disproportionately to the increased catalyst loading (i.e. activity is not a linear function of the loading). Since the cathode may be more active than the anode for a particular catalyst type or catalyst loading, or vice versa, it is advantageous to keep the catalyst loadings constant and as low as possible, and vary the area of the electrodes. This increases utilization of the catalyst and reduces cost. The concentric tubular design provides this design flexibility, which is not available in planar designs.  
         [0034]    A suitable fuel used in the fuel cell  10  is a mixture of methanol in water with sulfuric acid. Alternatively, the fuel can be any of or combination of methanol, ethanol or other alcohols, glycols or similar compounds. Preferably, the methanol or other suitable fuel is in liquid form. A direct methanol liquid feed fuel cell has certain advantages over gas-feed fuel cells, including elimination of a fuel vaporizer and its associated heat source and controls, elimination of complex humidification and thermal management systems, and the use of a liquid methanol/water mixture as a fuel and as a stack coolant.  
         [0035]    In operation, methanol fuel is flowed into the fuel cell  10  via the fuel inlet  27 , and into the fuel feed channel  28 . The fuel permeates through the porous anode  20  and is oxidized at the catalyst layer of the anode  20  to produce protons and other reaction products. Protons conduct from the anode through the fuel mixture in the fuel discharge channel  25 , through the membrane separator  22 , through the electrolyte solution in the electrolyte channel  26  and to the cathode  21  where they electrochemically react with oxidant. Electrons liberated in the electrochemical reaction at the anode  20  are conducted via an electrical connection (not shown) from the anode  20  through an external load (to do work), and return to the cathode  21 . The flow rate of fuel into and out of the fuel cell  10  can be varied depending on the operating conditions; for example, for a fuel cell  10  in the order of about 10 ml internal volume, the fuel flow rate is about 2 ml per minute.  
         [0036]    Any gases produced by the reaction at the anode  20  are removed from the fuel cell  10  via fuel discharge outlet  29  along with most of the unconsumed flowing fuel mixture. Since the second cap  24  is closed to the fuel feed channel  28  and the fuel discharge channel  25 , the fuel is fed and discharged at the same end. The amount of unreacted fuel is typically about 90% of the fuel fed into the fuel cell  10 . The separator membrane  22  provides a barrier and stops most of the unreacted fuel in the fuel discharge channel  25  from permeating into the electrolyte channel  26 . Any fuel that may have crossed over into the electrolyte channel  26  is swept out of the fuel cell  10  by the flowing liquid electrolyte stream.  
         [0037]    The liquid electrolyte is suitably a sulfuric acid mixture, but may also be any good conductive salt solution selected from the group of battery electrolytes with a pH of neutral to low acidic values, such as, KSCN, NH 4 SCN, acidified K 2 SO 4 , or selected strong organic acids (Superacids). It is also possible to operate the fuel cell in a basic rather than acidic mode of operation, in which case the choices of electrolyte include potassium or sodium hydroxide, or other strongly dissociating bases. The electrolyte is flowed into the fuel cell  10  through the electrolyte opening  32  of the second end cap  24  and the electrolyte plus any methanol and other potentially damaging products leaked through the membrane separator  22  are flowed out through the electrolyte opening  30  of the first end cap  23 . Because the liquid electrolyte is acidic, the fluid electrolyte does not impede the flow of protons from the anode to the cathode.  
         [0038]    Alternatively, the electrolyte flow may be reversed, entering through the first cap electrolyte opening  30  and exiting through the second cap electrolyte opening  32 . (Note: the arrows in FIG. 1 indicate fluid flows; unidirectional arrows indicate that the fluid flows in the direction indicated and bi-directional arrows indicate that the direction of fluid flow is discretional.)  
         [0039]    The rate of flow of electrolyte is selected to carry out leaked (cross-over) methanol before it can reach the cathode  21 . Factors that affect the rate of methanol cross-over include the electrolyte flow rate through electrolyte channel  26 , the dimensions of the fluid channels inside the fuel cell  10 , and the physical characteristics of the membrane separator  22  (e.g. porosity or permeability). The electrolyte flow rate can be varied depending on the operating conditions desired; typically, the electrolyte flow rate is set to exchange 2× to 5× the volume of electrolyte in the fuel cell  10  per minute.  
         [0040]    The circulating electrolyte also serves as a heat management system as well as a water management system via circulating the electrolyte through exterior heat exchangers.  
         [0041]    The fuel cell  10  may be located in a chamber that is supplied air for use as oxidant; in such case, the outside surface of the cathode is immersed in oxidant. Alternatively, the fuel cell  10  may be contained inside a cylindrical or other suitably shaped container (not shown) and oxidant is delivered to and discharged from the annular channel formed inside and between the container and the cathode.  
         [0042]    Although not shown in FIG. 1, the first and second end caps  23 ,  24  are fluidly coupled to reservoirs, pumps, controls and other ancillary components of a fuel cell system as is known in the art. These components facilitate the delivery and removal of fuel, oxidant and electrolyte to and from the fuel cell  10 . For example, the electrolyte inlet and outlet  30 ,  32  may be fluidly coupled to a methanol distillation recovery loop comprising an electrolyte storage tank, electrolyte pump and a separator (all not shown). Electrolyte is pumped from the storage tank and into the electrolyte conduit, wherein any leaked methanol is collected. The methanol and leaked methanol are then discharged from the electrolyte conduit and transmitted to the separator, wherein the electrolyte and methanol are separated as known in the art; the purified methanol is returned back the storage tank, and the recovered methanol is returned back the methanol supply source (e.g. a supply tank, not shown).  
         [0043]    [0043]FIG. 2 illustrates another embodiment of the invention in which the anode  20  and cathode  21  are reversed in relative position, with the anode  20  becoming the outer tubular electrode, and the cathode  21  being the inner tubular electrode. In this embodiment, the assumption is made that the cathode catalyst is more efficient than the anode catalyst, so that a cathode with less surface area is required. As in the first embodiment, the membrane separator  22  is positioned between the two electrodes  20 ,  21 . In this second embodiment, the fuel mixture is directed to the outside surface of the fuel cell  10 , wherein fuel reacts at the anode  20 ; reaction products and unreacted fuel diffuse through the porous anode  20  into the fuel discharge channel  25  (between the membrane separator  22  and the anode  20 ) and is discharged from the fuel cell via fuel outlet  29 . There may be fuel outlets in one or both end caps (a two ended outlet design is shown in FIG. 2). Air or oxidant is transmitted into the fuel cell  10  via an oxidant inlet  38  and into an oxidant flow channel  40  inside of the tubular cathode  21 ; reaction products and air are discharged from the fuel cell  10  through an oxidant outlet  42 . The electrolyte is flowed into the fuel cell  10  through electrolyte opening  30  in the first cap  23  and out through electrolyte opening  32  in the second cap  24 . Operation of the fuel cell  10  is otherwise the same as described in the first embodiment.  
         [0044]    [0044]FIG. 3 illustrates a third embodiment of invention. This embodiment is a variation of the fuel cell  10  of the first embodiment and differs in that the anode  20  is substantially impermeable to the fuel, but is proton conductive. The porosity of the anode  20  is about 10× less than that of the anode  20  of the first embodiment. Therefore, the flow of the fuel mixture passes by but not through the anode  20 ; a fuel opening  44  is provided in the second end cap  24  that is in fluid flow communication with the fuel feed channel  28  such that fuel can be fed into the fuel feed channel  28  via fuel flow opening  27  in the first end cap  23  and discharged via the fuel opening  44  (or vice versa). Catalyst material coats the inner surface of the anode  20  to promote the electrochemical reaction at the fuel feed channel side of the anode  20 . This embodiment of the fuel cell  10  also differs from the first embodiment in that a second electrolyte channel  46  replaces the fuel discharge channel, and has electrolyte inlet  48  and outlet  50  for the feed and discharge of electrolyte fluid. Membrane separator  22  separates the two electrolyte channels  26  and  46 .  
         [0045]    According to fourth embodiment of the invention and as illustrated in FIG. 4, the fuel cell  10  resembles the fuel cell  10  of the third embodiment (shown in FIG. 3) except that no membrane separator exists, thereby combining the two electrolyte channels in the third embodiment into one electrolyte channel  50 . In this case, any leakage of methanol and gases through the anode  20  is removed by electrolyte flow. Alternatively, a separator membrane (not shown) may be located closely beside, in intimate contact with, attached to, or deposited on one or both of electrolyte-facing surfaces of the anode  20  and the cathode  21 , to form separator-anode and separator-cathode assemblies, respectively. Since the membrane separators is semi-permeable (impermeable to liquid fuel and electrolyte but conductive to protons), the separator-anode assembly and/or separator-cathode assembly are also rendered semi-permeable. Another variation of this embodiment is to include a static non-circulating electrolyte between the anode  20  and cathode  21 .  
         [0046]    [0046]FIGS. 5 and 6 are the logical reversal of anode and cathode in the embodiments illustrated in FIGS. 3 and 4 respectively.  
         [0047]    While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the scope and spirit of the invention. For example, a direct methanol fuel cell system as described above may have multiple tubular and concentric anodes inside the outer cathode tube, or vice versa. Another example would be to construct a fuel cell from an array of tubes in a common manifold. In addition, the tubes need not be cylindrical in cross-section; any closed tubular configuration is possible including square and hexagonal cross-sections. Also, the position of the tubes is not restricted to be concentric, but may be moved one relative to another.  
         [0048]    A variant of the device can be used for the purpose of methanol or other fuel destruction. This is accomplished by short circuiting the anode to the cathode and flowing the methanol to be destroyed through the device. The methanol will be oxidized just as in the usual mode of operation.