Abstract:
An electrocatalyst based on a highly electroconducting polymer and a transition metal, in which transition metal atoms are covalently bonded to heteroatoms of the backbone monomers of the polymer. The covalently bonded transition metal atoms are nucleation sites for catalytically active transition metal particles. The complex is prepared by complexing a highly electroconducting polymer with transition metal coordination ions and then reducing the transition metal ions to neutral atoms. An electrode for a fuel cell is made by impregnating an electrically conducting sheet with the catalytic complex and drying the impregnated sheet. A fuel cell with a liquid anolyte uses the electrode as its cathode. The anolyte includes an aqueous solution of conjugate polybasic acids buffer, such as H 3 PO 4 —NaH 2 PO 4 —Na 2 HPO 4 , and an alcohol such as methanol as a reductant.

Description:
[0001]    This is a continuation of U.S. patent application Ser. No. 09/503,592, filed Feb. 14, 2000, which is a continuation in part of U.S. patent application Ser. No. 09/377,749 filed Aug. 20, 1999, now U.S. Pat. No. 6,380,126, issued Apr. 30, 2002. 
     
    
     
       FIELD AND BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to electrochemistry and, more particularly, to a new class of electrocatalysts based on highly electroconducting polymers that lave transition metal atoms covalently bonded to backbone heteroatoms, to a gas diffusion electrode including such electrocatalysts, and to a fuel cell based on such an electrode and on an innovative electrolyte.  
           [0003]    Like all electrochemical cells used to produce electricity, a fuel cell consists of an electrolyte sandwiched between two electrodes, a cathode and an anode. The transport of electrical charge from one electrode to another across the electrolyte allows the oxidation of a reductant at the anode and the reduction of an oxidant at the cathode without direct contact of the two reactants. The difference between a fuel cells and other such electrochemical cells is that in a fuel cell, the reactants are continuously replenished. For example, in a fuel cell that combines hydrogen gas with oxygen gas to produce electricity, the hydrogen is oxidized to H +  at the anode, the oxygen is reduced to O −2  at the cathode, the ions diffuse into the electrolyte and combine to form water, and the water diffuses out of the electrolyte.  
           [0004]    One obvious requirement in a hydrogen-oxygen fuel cell is that the gaseous reactants must be able to diffuse into the electrodes. For this reason, these electrodes are called “gas diffusion electrodes”. FIG. 1 is a sketch of a prior art gas diffusion electrode  10 , in cross-section. Bonded to a surface  14  of a porous, electrically conductive support sheet  12  are many small (typically colloidal) catalytically active particles  16 . The function of catalytically active particles  16  is to catalyze the anode and cathode reactions, and to conduct the electrons produced (anode) or consumed (cathode) by the reactions to (anode) or from (cathode) sheet  12 . Common examples of sheet  12  include carbon paper and carbon cloth; but metal (nickel or steel) mesh sheets  12  also are known. The charge carriers of sheet  12  usually are electrons, but sheets  12  in which the charge carriers are protons also are known. Typically, catalytically active particles  16  consist of cores of activated carbon, on the surfaces of which are deposited yet smaller particles of a catalytically active transition metal such as platinum. Surface  14  is the side of electrode  10  that faces the electrolyte in a fuel cell. Particles  16  typically are embedded in a layer  18  of a hydrophobic polymer such as polytetrafluoroethylene (PTFE). The function of hydrophobic layer  18  is to repel water that is formed during the process of electrocatalysts.  
           [0005]    Various methods are known for fabricating electrode  10 . These methods are reviewed by Frost et al. in U.S. Pat. No. 5,702,839, which is incorporated by reference for all purposes as if fully set forth herein. One such method which is reviewed by Frost et al., and which includes screen printing of a co-suspension of carbon particles and particles of a hydrophobic polymer onto sheet  12 , is taught by Goller et al. in U.S. Pat. No. 4,185.131, which also is incorporated by reference for all purposes as if fully set forth herein. Frost et al. then go on to teach their own method for fabricating electrode  10 .  
           [0006]    One field in which fuel cells have yet to realize their potential advantages of low cost and low pollution is that of automotive propulsion.  
           [0007]    Internal combustion engines, in comparison with other types of engine technology such as electrical engines and engines powered by fuel cells, consume the greatest amount of fuel and also release the greatest amount of pollutants. Moreover, internal combustion engines operating on the Otto cycle have an operating efficiency of at most only 32%, while internal combustion engines operating on the Diesel cycle have an operating efficiency of at most only 40%.  
           [0008]    Considerable effort has been expended by corporations, universities, government institutions and private individuals on finding a realistic commercial alternative to the internal combustion engine. Ideally, automobiles with such an alternative power source must be no more expensive to build and operate than vehicles with internal combustion engines. Moreover, the production of pollutants must be reduced, if not eliminated, relative to the internal combustion engine. Alternatives include electrical engines utilizing battery power, electrical engines tapping solar energy, methane gas engines and fuel cell engines. These alternatives is also have been combined with internal combustion engines in hybrid vehicles. So far, no practical solution has been attained.  
           [0009]    Fuel cells have emerged in the last decade as one of the most promising new technologies for meeting global electric power needs well into the twenty-first century. Fuel cells are inherently clean and remarkably efficient, and have been shown by the U.S. Department of Energy&#39;s Federal energy Technology Center and its industrial partners to supply electricity reliably while reducing emissions of carbon dioxide by 40 to 60 percent. Fuel cells produce negligible harmful emissions and operate so quietly that they can be used in residential neighborhoods.  
           [0010]    Nevertheless, fuel cells have not yet provided a viable solution in the automotive field. Generally, engines using fuel cells have been too expensive to manufacture.  
           [0011]    One important class of fuel cell is the proton exchange membrane (PEM) fuel cell, in which the electrolyte is a proton exchange membrane made of a material such as a PTFE-based ionomer such as Nafion®, available from E. I. DuPont de Namours and Company, Wilmington Del. Fuel cells of this class have much higher output power densities than fuel cells of competing classes, such as phosphoric acid liquid electrolyte fuel cells. Therefore, although PEM fuel cells operate only at relatively low temperatures, up to at most about 120° C. (vs., for example, up to 210° C. in the case of phosphoric acid liquid electrolyte fuel cells), PEM fuel cells show great promise for use in residential and small vehicle settings.  
           [0012]    Other disadvantages of PEM fuel cells include the following:  
           [0013]    1. The most efficient catalytic particles are platinum particles. Platinum is relatively costly.  
           [0014]    2. Hydrogen gas for domestic use typically is produced by the reforming of natural gas. One by product of this reforming is carbon monoxide, which poisons platinum catalysts.  
           [0015]    3. The efficiency of the cell depends on good electrical contact between particles  16  and sheet  12 . This contact tends to be degraded over time, as a consequence of the gradual poisoning of the catalyst, and also as a consequence of environmental insults such as vibration.  
           [0016]    Another important class of fuel cell is the liquid fuel cell, i.e., a fuel cell whose reductant (fuel) is a liquid, particularly an alcohol such as methanol. A methanol-air fuel cell using a neutral or slightly acidic liquid electrolyte has been a leading candidate for the production of electrical energy, because alcohols such as methanol are easy to produce by fermentation or by liquefaction of coal. Typically, the methanol fuel is mixed with the electrolyte to form an “anolyte”. The methanol reacts with water at the anode to produce carbon dioxide and hydrogen ions. The hydrogen ions diffuse to the cathode, where they react with atmospheric oxygen to produce water.  
           [0017]    In all fuel cells that use liquid anolytes, the composition of the anolyte is an important design consideration. The anolyte should have both high electrical conductivity and high ionic mobility at the optimal fuel concentration. Acidic solutions are most commonly used. Unfortunately, these acidic solutions need relatively high reaction temperatures (as in the case of phosphoric acid), at which temperatures these solutions tend to passivate or even destroy the anodes. Anolytes with pHs close to 7 are more anode-friendly but have insufficient electrical conductivity. Consequently, almost all prior art direct methanol fuel cells are based on solid polymer electrolyte (SPE) membranes. Fuel cells based on SPE membranes have other advantages over fuel cells based on liquid anolytes, including higher ultimate power densities and longer operating lifetimes. The disadvantages of SPE membrane fuel cells, relative to liquid electrolyte fuel cells, include necessarily complicated fuel feeding systems and very low methanol concentrations in the membranes. These disadvantages make the miniaturization of SPE membrane fuel cells almost impossible.  
           [0018]    Highly electroconducting polymers (HECP) are a class of polymers whose electrical resistivities are comparable to the resistivities of metals, in the range 0.1 to 100 siemens/cm. Typical examples of HECPs include polyaniline, polypyrrole, polythiophene and polyfuran. These HECPs include heteroatoms (N, N, S and O respectively) in their backbone monomers. Rajeshwar et al., in U.S. Pat. No. 5,334,292, , which is incorporated by reference for all purposes as if fully set forth herein, teach an improved electrode  10  in which particles  16  and layer  18  are replaced by a layer of a HECP polymer within which catalytically active colloidal particles, for example, platinum particles as small as 10 nanometers across, are dispersed uniformly. This electrode has the following advantages over electrode  10 :  
           [0019]    1. The three-dimensional disposition of the catalytically active particles in the electrode of Rajeshwar et al. gives that electrode higher catalytic activity per unit volume, hence per unit weight, than electrode  10 , in which particles  16  are distributed two-dimensionally along surface  14 .  
           [0020]    2. This increased specific catalytic activity allows the use of a smaller amount of costly catalytic materials such as platinum in the electrode of Raheshwar et al, than in electrode  10 .  
           [0021]    3. That the catalytically active particles of Rajeshwar et al. are embedded in an electrically conductive medium (the HECP), which in turn is in contact with surface  14  all along surface  14 , provides the catalytically active particles of Rajeshwar et al. with a much better electrical contact to sheet  12  than particles  16  have to sheet  12  in electrode  10 .  
           [0022]    4. The embedding of the catalytically active particles of Rajeshwar et al. in a polymer matrix also provides the electrode of Rajeshwar et al. with better mechanical stability, in terms of resistance to disruption by impact and vibration, than electrode  10 .  
           [0023]    5. It is believed that the HECP matrix provides resistance to carbon monoxide poisoning of colloidal platinum particles.  
         SUMMARY OF THE INVENTION  
         [0024]    According to the present invention there is provided a heterogeneous catalyst including: (a) a highly electroconducting polymer having at least one heteroatom per backbone monomer unit thereof and a plurality of transition metal atoms covalently bonded to at least a portion of the heteroatoms.  
           [0025]    According to the present invention there is provided a method of preparing a catalyst, including the steps of: (a) complexing a highly electroconducting polymer with a plurality of transition metal coordination complexes to produce a polymer-transition metal complex; and (b) reducing the polymer-transition metal complex to place the transition metal in a neutral oxidation state.  
           [0026]    According to the present invention there is provided an electrode including: (a) an electrically conducting sheet; and (b) a catalytic polymer film, bonded to one side of the sheet, the catalytic polymer film including a highly electroconducting polymer having at least one heteroatom per backbone monomer unit thereof and a plurality of transition metal atoms covalently bonded to at least a portion of the heteroatoms.  
           [0027]    According to the present invention there is provided a fuel cell including: (a) an anode; (b) a cathode; (c) an electrolyte sandwiched between the anode and the cathode; with at least one electrode, selected from the group consisting of the anode and the cathode, including: (i) an electrically conducting sheet and (ii) a catalytic polymer film, bonded to a side of the sheet that faces the electrolyte, the catalytic polymer film including a highly electroconducting polymer having at least one heteroatom per backbone monomer unit thereof and a plurality of transition metal atoms covalently bonded to at least a portion of the heteroatoms.  
           [0028]    According to the present invention there is provided a method for producing an electrode, including the steps of: (a) complexing a highly electroconducting polymer with a plurality of transition metal coordination complexes to produce a polymer-transition metal complex: (b) reducing the polymer-transition metal complex to place the transition metal in a neutral oxidation state; (c) dispersing the reduced polymer-transition metal complex in a solvent; and (d) impregnating an electrically conducting sheet with the dispersion.  
           [0029]    According to the present invention there is provided a fuel cell, including: (a) an anode; (b) a cathode; (c) a liquid electrolyte between the anode and the cathode; herein the cathode includes: (i) an electrically conducting sheet, and (ii) a catalytic polymer film, bonded to a side of the sheet that faces the electrolyte, the catalytic polymer film including a highly electroconducting polymer having at least one heteroatom per backbone monomer unit thereof and a plurality of transition metal atoms covalently bonded to at least a portion of the heteroatoms; and wherein the electrolyte includes an alcohol.  
           [0030]    According to the present invention there is provided a fuel cell, including: (a) an anode; (b) a cathode including a plurality of catalytically active transition metal particles; and (c) a liquid electrolyte, between the anode and the cathode, the electrolyte including: (i) an alcohol, and (ii) an aqueous solution of at least two conjugate polybasic acids.  
           [0031]    According to the present invention there is provided a method of producing electrical current, including the steps of: (a) providing a fuel cell including: (i) an anode, and (ii) a cathode including: (A) an electrically conducting sheet, and (B) a catalytic polymer film, bonded to a side of the sheet that faces the anode, the catalytic polymer film including a highly electroconducting polymer having at least one heteroatom per backbone monomer unit thereof and a plurality of transition metal atoms covalently bonded to at least a portion of the heteroatoms; (b) providing a liquid anolyte including an alcohol fuel; and (c) introducing the liquid anolyte into the fuel cell, between the anode and the cathode.  
           [0032]    According to the present invention there is provided a method of producing electrical current, including the steps of: (a) providing a fuel cell including: (i) an anode, and (ii) a cathode including a plurality of catalytically active transition metal particles; (b) providing a liquid anolyte including: (i) an alcohol fuel, and (ii) an aqueous solution of at least two conjugate polybasic acids; and (iii) introducing the liquid anolyte into the fuel cell, between the anode and the cathode.  
           [0033]    The present invention is based on an innovative electrode that is similar to the electrode of Rajeshwar et al., but has vet smaller catalytic particles, some possibly as small as single atoms of a transition metal, dispersed within a matrix of a HECP having heteroatoms in its backbone monomers. The preferred HECPs include polyaniline, polypyrrole, polythiophene and polyfuran. The preferred transition metals include platinum, iridium, osmium, rhenium, ruthenium, rhodium, palladium, iron, cobalt, nickel, chromium, manganese, copper and vanadium. The most preferred HECPs are polyaniline and polypyrrole. The most preferred transition metals are platinum and iridium. The transition metal catalytic particles are believed to be nucleated around single transition metal atoms that are covalently bonded to some of the backbone heteroatoms of the HECPs.  
           [0034]    The HECP-transition metal complex of the present invention is prepared by complexing the HECP, preferably in a basic form, with coordination complexes of the transition metal, and then reducing the transition metal cations of the resulting HECP-transition metal coordination complex to a neutral oxidation state. Preferably, the reduction is effected by mixing the complex with an aqueous solution of a reductant such as sodium borohydride, formaldehyde and hydrazine.  
           [0035]    The HECP-transition metal complex of the present invention, in and of itself, constitutes a catalyst having applications beyond its primary application as a component of a fuel cell electrode. Therefore, the HECP-transition metal complex of the present invention also is referred to herein as a “catalytic HECP complex”. For use in a PEM fuel cell electrode, the catalytic HECP complex of the present invention preferably includes a polytetrafluoroethylene (PTFE)-based ionomer. Optionally, the catalytically active transition metal particles of the present invention are supplemented by prior art catalytically active particles  16 , deposited on surface  14  of sheet  12  by the methods of the prior art, for example, by screen-printing a paste of particles  16  onto surface  14 . Following this deposition, sheet  12  is impregnated with a dispersion of the catalytic HECP complex of the present invention in a solvent, and is subsequently dried. Preferred solvents include xylene, chloroform, toluene and m-cresol, and the dispersion is stabilized by a stabilizer such as a sulfonic acid.  
           [0036]    The electrode of the present invention also is an ideal cathode for a liquid anolyte fuel cell that uses an alcohol such as methanol as the reductant. Preferably, the alcohol constitutes between 10% and 45% of the anolyte by weight. The preferred electrolyte is an aqueous solution of at least two conjugate polybasic acids. Conjugate polybasic acids are acids that share a common anion. For example, the acids H 3 PO 4 , NaH 2 PO 4  and Na 2 HPO 4 , share PO 4   −3  as a common anion. Preferably, the more acidic acid or acids are present in stoichiometric excess over the least acidic acid, and the solution has a pH of at most 3.5. Such an electrolyte has relatively high electrical conductivity and relatively high ionic mobility, and supports a relatively high concentration of the alcohol reductant. The scope of the present invention includes fuel cells based on such anolytes and using, as their cathodes, not only the electrode of the present invention, but also any electrode based on catalytically active transition metal particles, for example, the electrode of Rajeshwar et al. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]    The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:  
         [0038]    [0038]FIG. 1 shows a prior art gas diffusion electrode;  
         [0039]    [0039]FIG. 2 shows three of the steps in the preparation of a polyaniline-platinum complex;  
         [0040]    [0040]FIG. 3 is a schematic illustration of a PEM fuel cell of the present invention;  
         [0041]    [0041]FIG. 4A is an x-ray photoelectron spectrum of polyaniline;  
         [0042]    [0042]FIG. 4B is an x-ray photoelectron spectrum of a polyaniline-platinum complex;  
         [0043]    [0043]FIG. 5 is an enlarged superposition of the nitrogen 1s lines of the spectra of FIGS. 4A and 4B;  
         [0044]    [0044]FIG. 6A is an infrared spectrum of polyaniline;  
         [0045]    [0045]FIG. 6B is an infrared spectrum of a polyaniline-platinum complex. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0046]    The present invention is of a catalytic HECP complex that includes catalytically active transition metal particles nucleated around transition metal atoms covalently bound to backbone heteroatoms thereof. The present invention can be used as a component of an electrode in a PEM fuel cell and in a fuel cell that has a liquid anolyte. The scope of the present invention also includes an electrode that incorporates the catalytic HECP complex, a fuel cell that includes the electrode, and method of preparing the catalytic HECP complex and the electrode.  
         [0047]    The principles and operation of a catalytic HECP complex and of a fuel cell based thereon according to the present invention may be better understood with reference to the drawings and the accompanying, description.  
         [0048]    Referring again to the drawings, FIG. 2 shows three of the steps in the preparation of one of the most preferred embodiments of the catalytic HECP complex of the present invention, in which the HECP is polyaniline and the transition metal is platinum. FIG. 2A shows a polyaniline chain. FIG. 2B shows a complex of polyaniline with PtCl 4  coordination complexes. FIG. 2C shows the complex of FIG. 2B after reduction of the Pt +1  of FIG. 2B to Pt 0 .  
         [0049]    [0049]FIG. 1B of Rajeshwar et al. serves to illustrate, schematically, an electrode of the present invention. What Rajeshwar et al. label as a “microporous film” should be understood as the catalytic HECP complex of the present invention. The particles labeled “Catalyst” by Rajeshwar et al. are to be understood as the catalytically active transition metal particles of the present invention, not drawn to scale, as many the catalytically active particles of the present invention are believed to be only several atoms in size, or even as small as a single atom.  
         [0050]    [0050]FIG. 3 shows, schematically, a PEM fuel cell  20  of the present invention. Fuel cell  20  consists of a PEM electrolyte  50  sandwiched between an anode  30  and a cathode  40 . Anode  30  consists of a graphite plate  22  bearing a carbon sheet  24 . The side of carbon sheet  24  that faces electrolyte  50  is covered by a layer of catalytic HECP complex  26 . Similarly, cathode  40  consists of a graphite plate  32  bearing a carbon sheet  34 . The side of carbon sheet  34  that faces electrolyte  50  is covered by a layer of catalytic HECP complex  36 . The side of graphite plate  22  that bears carbon sheet  24  includes grooves  23  for admitting hydrogen gas. Similarly, the side of graphite plate  32  that bears carbon sheet  34  includes grooves  33  for admitting oxygen gas.  
         [0051]    In the examples presented below the HECPs are polyaniline and polypyrrole. Polyaniline was prepared as described by X. Wei and A. Epstein, “Synthesis of highly sulfonated polyaniline”.  Synthetic Metals , vol. 74 pp. 123-125 (1995). (NH 4 ) 2 S 2 O 8  was used as an oxidizer. Polypyrrole was prepared as described in T. H. Chao and J. March. “A study of polypyrrole synthesized with oxidative transition metal ions”.  Journal of Polymer Science, Part A: Polymer Chemistry , vol. 26 pp. 743-753 (1988). FeCl 3  was used as an oxidizer.  
         [0052]    The first four examples are of catalytic HECP complexes of the present invention. The last nine examples are of electrodes that incorporate the catalytic HECP complex of the present invention.  
       EXAMPLE 1  
       [0053]    5.12 g of polyaniline powder (in basic form) and 100 ml of distilled water were mixed if an Erlenmeyer flask for 30 minutes to prepare an aqueous suspension of polyaniline. 110 ml of 0.98% aqueous H 2 PtCl 6  were slowly added to this suspension over the course of 60 minutes while stirring constantly. The resulting polyaniline-PtCl 4  complex was separated by centrifuging and decanting. The resulting solid filtrate was colorless, unlike the initial polyaniline suspension, which was yellow, showing that the H 2 PtCl 6  had reacted with the polyaniline. The filtrate was rinsed with distilled water until the rinse water had a pH of 7.  
         [0054]    A reductant solution was prepared by dissolving 1.5 g NaBH 4  and 0.7 g NaOH in 100 ml of distilled water at room temperature with stirring for 30 minutes. The polyaniline-PtCl 4  complex filtrate was placed in an Erlenmeyer flask, and the reductant solution was added dropwise while stirring over the course of 2 hours and heating gently, to no more than 60° C. The product of this reaction was rinsed with distilled water until the rinse water had a pH of 7, and then was dried at 110° C. for 24 hours.  
       EXAMPLE 2  
       [0055]    As in example 1, 4.73 g of polyaniline were suspended in 100 ml of distilled water. 80 ml of 1.15% aqueous H 2 IrCl 6  were added slowly to the polyaniline suspension over the course of 60 minutes while stirring constantly. The resulting polyaniline-IrCl 4  complex was separated by centrifuging and decanting. The resulting solid seas rinsed with distilled water until the rinse water had a pH of 7.  
         [0056]    The reductant solution of Example 1 was added to the polyaniline-IrC 4  complex over the course of 2 hours at a temperature of 60° C. The product of this reaction was rinsed Faith distilled water until the rinse water had a pH of 7, and then was dried at 110° C. for 24 hours.  
       EXAMPLE 3  
       [0057]    4.90 g of polypyrrole powder were suspended in 100 ml of distilled water. 105 ml of 0.98% aqueous H 2 PtCl 6  were added to the polypyrrole suspension over the course of 60 minutes while stirring constantly. The resulting polypyrrole-PtCl 4  complex was separated by centrifuging and decanting. The resulting solid was rinsed with distilled water until the rinse water had a pH of 7.  
         [0058]    The reductant solution of Example 1 was added to the polypyrrole-PtCl 4  complex at a temperature of 60° C. over the course of 2 hours. The product of this reaction was rinsed with distilled water until the rinse water had a pH of 7, and then was dried at 110° C. for 24 hours.  
       EXAMPLE 4  
       [0059]    As in example 3, 3.92 g of polypyrrole powder were suspended in 100 ml of distilled water. 70 ml of 1.15% aqueous H 2 IrCl 6  were added to the polypyrrole suspension over the course of 60 minutes while stirring constantly. The resulting polypyrrole-IrCl 4 , complex was separated by centrifuging and decanting The resulting solid was rinsed with distilled water until the rinse water had a pH of 7.  
         [0060]    A reductant solution was prepared by dissolving 1.3 g NaBH 4  and 0.6 g NaOH in 100 ml of distilled water at room temperature with stirring for 30 minutes. This reductant solution was added to the polypyrrole-IrCl 4  complex over the course of 2 hours at a temperature of 60° C. The product of this reaction was rinsed with distilled water until the rinse water had a pH of 7, and then was dried at 110° C. for 24 hours  
         [0061]    Table 1 shows the rate of oxygen gas liberation (cm 3  per mg metal (Pt or Ir) per hour at room temperature and atmospheric pressure) from 20 ml of 10% H 2 O 2  per gram of several catalysts: three prior art catalysts and the four catalysts of Examples 1-4.  
                                     TABLE 1                       catalyst   mass (g)   rate of O 2  release                                10% Pt/polypyrrole of Rajeshwar et al.   0.16   0.7       Polyaniline-Pt of Example 1   0.12   6.7       Polyaniline-Ir of Example 2   0.15   2.2       Polypyrrole-Pt of Example 3   0.15   17.3       Polypyrrole-Ir of Example 4   0.16   3.1                  
 
         [0062]    In examples 5-11, suspensions of HECP-transition metal complexes were prepared by adding a stabilizer to the HECP-transition metal complex, along with sufficient organic solvent to produce a paste-like mixture. Electrodes  24  and  34  were fabricated on Toray® Teflon®-treated carbon sheets, available from Electrochem. Inc. of Woburn Mass. The paste-like mixture was applied to the sheets by screen printing, and the sheets were dried at 120° C. for 5 hours. The platinum content of the resulting impregnated sheets was 0.25 mg/cm 2 . Circular discs 2.5 cm in diameter (4.9 cm 2  in area) were cut from the impregnated carbon sheets PEMs 3.5 cm in diameter, to serve as electrolyte  50 , were cut from a Nafion®-117 sheet.  
         [0063]    Membrane-electrode assemblies (MEAs) that emulate fuel cell  20  were fabricated as sandwich structures of PEMs between impregnated carbon sheets, sintered in a heated press tool by hot pressing. The MEAs were tested in a standard hydrogen—oxygen fuel cell. All tests were conducted at a constant gas pressure P H     2    =P O     2   =40 psig. Voltage-current curves of the MEAs were measured in the range 0.3 V to 0.8V.  
         [0064]    As a control, example 12 used electrodes  24  and  34  prepared as taught by Rajeshwar et al., using colloidal platinum in polypyrrole.  
         [0065]    Table 2 presents the results of examples 5-12. The current densities I are given for a voltage of 0.5 V.  
                                                     TABLE 2                                       I (amp/       example   catalyst   solvent   stabilizer   cm 2 )                                5   2.94 g PPy-Pt complex   xylene   0.29 g DBSA   0.65       6   4.31 g PAni-Pt complex   xylene   0.43 g DBSA   0.98       7   2.40 g PAni-Pt complex   m-cresol   0.24 g CSA   0.84       8   4.37 g PAni-Pt complex   ethanol   0.44 g Nafion   1.22       9   5.05 g PPy-Pt complex   chloro-   0.51 g DBSA   0.60               form       10   4.52 g PPy-Pt complex   m-cresol   0.45 g CSA   0.56       11   3.94 g PAni-Pt complex   toluene   0.40 g DBSA   0.93       12   3.18 g PPy-Pt of   xylene   0.32 g DBSA   0.39           Rajeshwar et al.                                                                          
 
         [0066]    FIGS.  4 - 6  show measurements conducted on the polyaniline-platinum complex of example 1, and on a control sample of polyaniline, that demonstrate that that platinum is indeed covalently bonded to the polyaniline.  
         [0067]    [0067]FIGS. 4A and 4B are x-ray photoelectron spectra of polyaniline and of the polyaniline-platinum complex of example 1, respectively. The spectra were recorded at the Wolfson Applied Materials Research Center of Tel Aviv. Israel, using a Scanning AES/XPS Multitechnique System of PHI, and interpreted according to J. F. Moulder.  Handbook of X - ray Photoelectron Spectroscopy . Perkin-Elmer, 1992. The spectral lines are labeled by the atomic orbitals that are the sources of the photoelectrons. The ordinates are electron flux, in arbitrary units.  
         [0068]    [0068]FIG. 5 is a horizontally expanded superposition of the nitrogen 1s lines of FIGS. 4A and 4B. The nitrogen 1s line of the polyaniline-platinum complex is shifted upward in binding energy relative to the nitrogen 1s line of polyaniline.  
         [0069]    [0069]FIGS. 6A and 6B are infrared spectra of polyaniline and of the polyaniline-platinum complex of example 1, respectively. The spectra were recorded at the Chemistry Faculty of Tel Aviv University, Israel, using a Vector-22 IR spectrometer of Bruker Instruments, Inc., and interpreted according to Silverstein, Bassler and Morill, Spectrometric Identification of Organic Compounds. John Wiley and Sons. 1981. The spectral lines in FIGS. 6A and 6B belong to aromatic amines. The marked change in character between the two infrared spectra shows that the platinum of the polyaniline-platinum complex is indeed covalently bonded to the polyaniline.  
         [0070]    [0070]FIG. 3 also serves to illustrate a liquid fuel cell  20  of the present invention. Reference numeral  50  now refers to a liquid, water-based anolyte including an alcohol such as methanol as the reductant. As before, anode  30  is illustrated as an electrically conducting (graphite or aluminum) plate  22  bearing a carbon sheet  24 , with the side of carbon sheet  24  that faces anolyte  50  being covered by a layer of catalytic HECP complex  26 . Similarly, cathode  40  is illustrated as a graphite plate  32  bearing a carbon sheet  34  whose side that faces anolyte  50  is covered by a layer of catalytic HECP complex  36 . Alternatively, anode  30  and/or cathode  40  are any electrodes based on catalytically active transition metal particles, preferably also including carbon-supported catalytically active noble metal (Pt—Ru, Pt—Os, Pt—Pd, Pt—Rh or Pt—Re) particles. The oxidant is either pure oxygen or air, and the side of graphite plate  32  that bears carbon sheet  34  includes grooves  33  for admitting the gaseous oxidant. The alcohol reductant reacts with the water of the anolyte at anode  40  to produce CO 2  and H + . The CO 2  is vented via grooves  23  on the side of graphite plate  22  that bears carbon sheet  24 . The H +  diffuses to cathode  40 , where the H −  reacts with the oxidant to form water. A mechanism (not shown) is provided to exclude, from grooves  23 , air that otherwise would oxidize the alcohol at anode  30 . Most preferably, alcohol reductant is replenished by flossing anolyte  50  through fuel cell  20 .  
         [0071]    Preferably, anolyte  50  is an aqueous solution of at least two conjugate polybasic acids, having a pH of at most 3.5 and having a stoichiometric excess of the more acidic acids over the least acidic acid. Most preferably, anolyte  50  is an aqueous solution of NaH 2 PO 4  and Na 2 HPO 4 , with a stoichiometric excess of NaH 2 HPO 4  over Na 2 HPO 4  and with sufficient H 3 PO 4  added to bring the pH to the desired value below 3.5. Preferably, the reductant is methanol, constituting between 10% and 45% of anolyte  50  by weight.  
       EXAMPLE 13  
       [0072]    Recrystallized NaH 2 PO 4  and Na 2 HPO 4  and twice-distilled deionized water were used to prepare 0.2M solutions that were mixed in the proportion 7.5 parts 0.2M NaH 2 PO 4  to one part 0.2M Na 2 HPO 4 . Sufficient H 3 PO 4  was added to bring the pH to 3.1. To 500 ml of this solution was added enough methanol to produce an anolyte containing 30% methanol by weight.  
         [0073]    The cathode and the anode both were HECP-transition-metal-complex electrodes of the present invention, the cathode was prepared as in Example 8 above. This electrode included 20% by weight of Pt on a carbon sheet. The anode was prepared as in example 6 above, except that a 2:1 mixture of Pt to Ru was substituted for the Pt. This electrode included 20% by weight of Pt and 10% by weight of Ru on a carbon sheet. The carbon sheet of the anode was mounted on a 1 mm thick aluminum plate. Both electrodes were 0.96 cm 2  disks, separated by a gap of 6.5 mm.  
         [0074]    The anolyte was introduced to the gap between the electrodes. The fuel cell produced electrical current at a power density of between 25 and 30 mW/cm 2  for several hours.  
         [0075]    While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.