Abstract:
A direct methanol fuel cell (DMFC) having a methanol fuel supply, oxidant supply, and its membrane electrode assembly (MEA) formed of an anode electrode and a cathode electrode with a membrane therebetween, a methanol oxidation catalyst adjacent the anode electrode and the membrane, an oxidant reduction catalyst adjacent the cathode electrode and the membrane, comprises an oxidant reduction catalyst layer of a platinum-chromium alloy so that oxidation at the cathode of methanol that crosses from the anode through the membrane to the cathode is reduced with a concomitant increase of net electrical potential at the cathode electrode.

Description:
RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. patent application Ser. No. 10/260,780, filed Sep. 27, 2002. 
     
    
     STATEMENT REGARDING FEDERAL RIGHTS  
       [0002]     This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION  
       [0003]     The present invention relates generally to direct methanol fuel cells, and, more particularly, to cathode catalyst compositions for direct methanol fuel cells.  
       BACKGROUND OF THE INVENTION  
       [0004]     Fuel cells are considered a possible alternative to direct combustion engines to power transportation vehicles and to possibly furnish electrical energy to a power distribution grid for home and business use. In a fuel cell, fuels are chemically reacted with an oxidant whereby a direct current is produced at a low voltage across individual cells and stacks of cells produce useful power. Catalyst materials promote the chemical reactions of the fuels (typically hydrogen or methanol) and oxidant (typically pure oxygen or air).  
         [0005]     In a generic embodiment shown in  FIG. 1 , a fuel cell  10  includes an anode electrode  14  for the fuel oxidation, a cathode electrode  16  for the oxidant reduction, and a solid-state polymer electrolyte membrane  18  therebetween to provide an ionic conduction path. The combination of anode electrode  14 , cathode electrode  16 , and membrane  18  is conventionally called a membrane electrode assembly (MEA)  12 . A suitable catalyst is disposed adjacent the interfaces of electrode/membrane surfaces  14 / 18  and  16 / 18  so that the fuel is oxidized at the anode/membrane interface  14 / 18  to produce ions that traverse the membrane to complete the oxidant reduction at the cathode/membrane interface  16 / 18 . Fuel  24  is distributed over anode  14  of MEA  12  by fuel distribution plate  22  and unreacted fuel and reaction products  26  are exhausted. Oxidant  32  is distributed over cathode  16  of MEA  12  by oxidant distribution plate  28  and excess oxidant and reaction products  34  are exhausted. As a result, electrons generated at anode  14  travel through an external circuit (not shown) back to cathode  16 . The electrons constitute the flow of electrical current that provides energy to components connected to the external circuit.  
         [0006]     The most common fuel used in the development of polymer electrolyte membrane fuel cells has been hydrogen, either in a pure form or furnished as a reformate from hydrocarbon products. Yet another approach is to directly use a liquid methanol solution in direct methanol fuel cells (DMFCs) to avoid the complications associated with supplying pure hydrogen or providing a separate system for reforming hydrocarbons to provide reformed hydrogen. DMFCs with a solid polymer electrolyte can provide high current density at low temperature and have a relatively simple fuel cell construction. Methanol is a renewable fuel material and can be readily transported and supplied with existing transportation and distribution infrastructure for liquid fuels. Both hydrogen fuel cells and DMFCs have the generic structure shown in  FIG. 1 .  
         [0007]     In a DMFC, catalysts promote electrode reactions at the cathode and the anode, where a metric of performance is the catalytic activity per unit mass of catalytic metal. This metric is directly related to the efficiency and output power of the cells and to the manufactured cost of the cells. Platinum black was an early cathode catalyst in an ion-exchange MEA for hydrogen fuel cells, typically a gas diffusion electrode substrate with one surface coated with platinum black in an amount of 4 to 10 mg cm −2  of the MEA.  
         [0008]     To improve the utilization efficiency of platinum, a catalyst was developed with a platinum alloy supported on conductive carbon or an unsupported platinum alloy, which was mixed with an ion-exchange polymer, coated on an electrode substrate, and joined to an ion-exchange membrane by painting, hot pressing, or the like, to form the MEA. This process permitted the thickness and composition of the catalyst layer to be controlled so that the catalyst was more effectively utilized in the electrode reaction. A loading of platinum or platinum alloy of only 0.1 to 1.0 mg cm −2  was needed to produce a performance equivalent to prior art hydrogen fuel cells.  
         [0009]     This reduced loading that has been demonstrated for hydrogen fuel cells has not been achieved for DMFCs. In a DMFC, some methanol crosses through the membrane from the anode and reacts at the cathode, competing with the oxygen reduction reaction for active catalyst surface sites. Reducing the catalyst loading results in fewer active sites available for the oxygen reduction reaction, as well as limiting the ability of the catalyst to handle methanol crossover, with a concomitant reduction in the potential of the DMFC cathode. Thus, methanol crossover to the cathode not only lowers fuel utilization, but also adversely affects the oxygen cathode with overall lower cell performance.  
         [0010]     One way to reduce the effect of methanol crossover on DMFC performance is to simply reduce methanol crossover by developing a membrane that is less permeable to methanol. However, this has not yet been fully realized in the art. Other ways to reduce methanol crossover include lower methanol feed concentration and optimized cell design.  
         [0011]     The present invention recognizes that performance losses associated with methanol crossover arise from the fact that most Pt-based cathode systems are catalytically active to methanol oxidation under normal cell operating conditions with a resulting net cathode potential from oxygen reduction reaction potential reduced by the methanol oxidation reaction. In accordance with the present invention, a Pt-alloy catalyst has been identified that is less catalytically active for methanol oxidation, while having equal or increased catalytic activity for oxygen reduction.  
         [0012]     Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention is a direct methanol fuel cell (DMFC) having a methanol fuel supply, oxidant supply, and a membrane electrode assembly (MEA) formed of an anode electrode and a cathode electrode with a solid polymer electrolyte membrane therebetween, a methanol oxidation catalyst adjacent the anode electrode and the membrane, an oxidant reduction catalyst adjacent the cathode electrode and the membrane, wherein the improvement comprises an oxidant reduction catalyst layer containing platinum-chromium so that oxidation at the cathode of methanol that crosses from the anode through the membrane to the cathode is reduced with a concomitant increase of net electrical potential at the cathode electrode.  
         [0014]     In one embodiment, the platinum-chromium catalyst is supplied at a loading less than 1 mg cm −2  and the cathode catalyst layer includes a perfluorinated ion exchange polymer in a volume percent between 35% and 55%. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:  
         [0016]      FIG. 1  pictorially depicts the component parts of a fuel cell having functions described herein.  
         [0017]      FIG. 2  graphically depicts CO stripping voltammograms for platinum-chromium/C and Pt/C catalysts to illustrate the low catalytic activity of platinum-chromium/C for methanol oxidation relative to Pt/C.  
         [0018]      FIG. 3  graphically depicts polarization plots representing electro-oxidation of crossover methanol at platinum-chromium/C and Pt/C electrodes to further demonstrate the relatively low catalytic activity of platinum-chromium/C for methanol oxidation.  
         [0019]      FIG. 4  graphically compares the performance of two DMFCs, one with a platinum-chromium/C cathode and the other with a Pt/C cathode.  
         [0020]      FIG. 5  graphically compares the performance of three (3) DMFCs with MEAs using platinum-chromium/C with one MEA using Pt/C to illustrate performance improvement consistency.  
         [0021]      FIG. 6  graphically compares the performance effect of Nafion® content in the catalyst layer for platinum-chromium/C and for Pt/C. 
     
    
     DETAILED DESCRIPTION  
       [0022]     In accordance with the present invention, a cathode catalyst of a platinum-chromium alloy provides reduced catalytic activity for methanol reaction at a DMFC cathode while maintaining or increasing catalytic activity for the oxygen reduction reaction at the cathode. The net potential at the cathode from the methanol oxidation and oxygen reduction is increased over the net potential obtained from a conventional carbon supported Pt (Pt/C) cathode catalyst. The platinum-chromium alloy may be unsupported or carbon supported.  
         [0023]     A suitable DMFC has the functional fuel cell structure shown in  FIG. 1 . It will be appreciated that each structural element shown in  FIG. 1  can be implemented in a variety of structures known to those skilled in the art, except as specifically described herein to incorporate the platinum-chromium cathode catalyst.  
         [0024]     As shown below, we have demonstrated that improved DMFC performance is obtained with a platinum-chromium cathode catalyst compared to a Pt/C cathode catalyst. The exemplary platinum-chromium catalyst was a carbon-supported Pt 3 C alloy. In these comparative experiments, the platinum-chromium/C catalyst was about 44 wt % platinum-chromium and the Pt/C catalyst was about 40 wt % Pt. The anode catalyst was a PtRu black or PtRu/C (45 wt %). The anode and cathode catalysts were dispersed in appropriate amounts in water, with an added perfluorinated ion exchange polymer for ionic conduction adjacent the catalysts (e.g., 5% Nafion® solution (1100 EW, Solution Technology, Inc., USA). Exemplary cathode ink compositions were 65 wt % Pt/C and 35 wt % Nafion® and 66 wt % Pt 3 Cr/C and 34 wt % Nafion®; anode ink compositions were 85 wt % PtRu and 15 wt % Nation® or 70 wt % PtRu/C and 30 wt % Nafion®. MEAs were prepared by painting the catalytic inks on membranes of Nation 117®. The desired catalyst loadings are less than about 1.0 mg cm −2  so the cathode catalyst inks were applied to obtain an experimental loading of about 0.6 mg cm −1 . In all cases the geometric active area of the MEA was 5 cm 2 .  
                                           TABLE 1                           Preferred composition of catalyst layers            Catalyst   Catalyst (wt %)   Nafion ® (wt %)                    PtRu black   85   15       PtRu/C   70   30       Pt/C   65   35       Pt 3 Cr/C   66   34                  
 
         [0025]     The reduced catalytic activity of platinum-chromium/C for methanol oxidation compared to Pt/C is demonstrated by the CO stripping voltammograms shown in  FIG. 2  for a Pt loading of 0.6 mg cm −2  in both cases. To obtain these results, DMFCs with platinum-chromium/C cathode catalyst and with Pt/C cathode catalyst were operated in a “driven mode”, with methanol being oxidized at the fuel cell cathode and hydrogen evolving at the fuel cell anode, which acted as a counter/quasi-reference electrode (a dynamic hydrogen electrode, DHE). CO produced during the methanol oxidation was adsorbed onto the electrode surface and then stripped to determine the surface charge density as a measure of catalytic activity. The charge density for surface CO stripping on the platinum-chromium/C cathode was 33 mC cm −2  compared with ˜67 mC cm −2  for the Pt/C cathode. These results clearly indicate a reduced catalytic activity of platinum-chromium/C for methanol oxidation compared with Pt/C.  
         [0026]     Another indication of different activity of methanol towards platinum-chromium/C and Pt/C was obtained in direct measurements of methanol crossover. In these experiments, the cells were again operated in a driven mode, with methanol oxidized at the fuel cell cathode and hydrogen evolved at the fuel cell anode, thereby serving as a hydrogen counter/quasi-reference electrode. After stabilizing the cell at open voltage, a single voltammetric scan was applied to the fuel cell cathode and current response recorded, typically in the range of 0.1-0.6 V. In addition to allowing the magnitude of crossover to be directly examined, the activity of the catalyst towards methanol could also be determined in such an experiment from the kinetic part of the current-potential plots.  
         [0027]     As shown by the plots in  FIG. 3 , the regular Pt/C cathode catalyst is significantly more active towards methanol crossing through the Nafion 117® membrane than the platinum-chromium/C catalyst. For example, at an anode potential of 0.35 V, a typical DMFC operating potential, the current density of methanol oxidation with a platiunum-chromium/C catalyst is about 20 mA cm −2 , much lower than that of 81 mA cm −2  obtained with the Pt/C catalyst. Not surprisingly, the differences in the rate of methanol oxidation disappear once limiting-current conditions are reached on both electrodes, i.e. at a potential higher than 0.5 V. The same current density of ˜130 mA cm −2  is measured in either case, thus attesting to the expected similar permeation rates of methanol through the Nafion 117® membrane used with both platinum-chromium/C and Pt/C cathode catalysts. However, at lower potentials, the cathode using the platinum-chromium/C catalyst that is less sensitive to methanol is expected to remain at a higher net potential than the Pt/C cathode, which is more susceptible to becoming depolarized by methanol.  
         [0028]      FIG. 4  graphically compares the performance of a DMFC with an exemplary carbon-supported Pt 3 Cr alloy (Pt 3 Cr/C) cathode catalyst and a DMFC with Pt/C cathode catalyst. The cathode catalyst loadings were 0.6 mg-cm −2 , the anode catalyst loadings were 9.6 mg cm −2  of PtRu black. The cells were operated at 80° C., cathode pressure of 2.7 atm, with an anode feed of 0.5 M methanol. Except for the lowest current density range, below about 20 mA cm −2 , where performance of both catalyst composites is dominated by the high flux of methanol through the Nafion membrane, the Pt 3 Cr/C catalyst consistently showed a 70-80 mV voltage advantage over the reference Pt/C catalyst. This suprisingly large voltage advantage of the Pt 3 Cr/C catalyst has not been observed in hydrogen fuel cells, although a small voltage advantage is realized by Pt 3 Cr/C because of a higher activity of the catalyst in oxygen reduction at a cathode.  
         [0029]     Relative performance of the two cathode catalysts was also tested versus anodes prepared by using carbon-supported PtRu catalyst at a low loading of ˜1.0 mg cm −2 . Three different MEAs using Pt 3 Cr/C cathode and PtRu/C anode were made to test reproducibility of the results obtained with the novel cathode catalyst formulation. Hydrogen-air cell polarization plots were then recorded as initial tests of cathode activity at a cell temperature of 80° C., showing very good and reproducible cathode performance. In particular, a cell current density of 0.2 A cm −2  was reached at 0.83-0.84 V, i.e., at a cell voltage similar to that measured with highly loaded (9.6 mg cm −2 ) unsupported PtRu anode.  
         [0030]     Following operation in hydrogen-air mode, the above cells underwent regular DMFC testing. Polarization plots for the cells with Pt 3 Cr/C and Pt/C cathodes and PtRu/C anodes at 80° C. are shown in  FIG. 5 . All plots represent MEAs having the same catalyst loading and operated under the same fuel cell operating conditions.  FIG. 5  shows again the very significant performance advantage of the platinum-chromium/C catalyst over the reference Pt/C catalyst. As in the testing performed with highly loaded PtRu anodes, the voltage advantage offered by Pt 3 Cr/C catalyst over regular Pt/C catalyst with low PtRu/C anode loadings (˜1.0 mg cm −2 ) was 60-80 mV in the entire range of investigated cell current densities. The results obtained with three different platinum-chromium/C cells were highly reproducible, with current densities remaining within just a few percent from one another, as further shown in  FIG. 5 .  
         [0031]     To further verify the repeatable nature of the results, different preparation batches (OMG) of the cathode catalyst and a catalyst from another manufacturer (E-TEK) were prepared and tested. Methanol-air polarization plots indicate that there is no significant difference between these catalysts over the total current density region.  
         [0032]     In the cathode catalyst layer, an optimal ion-exchange polymer content should minimize both ohmic and mass transport limitations, maximize electrochemical activity and Pt utilization. The influence of polymer content in the catalyst layer with Pt/C or platinum-chromium/C on performance of methanol/air fuel cell is shown in  FIG. 6 . An increase in Nafion® content improves the performance up to 35% or 40% for Pt/C or platinum-chromium/C, respectively, but platinum-chromium/C catalyst shows a significantly greater performance improvement than does Pt/C. However, when too much polymer (beyond ca. 52%) is introduced the current density at 0.45 V decreases because the ohmic and mass transport limitations appear. This is a reasonable result because the ca. 52% Nafion® content means that the volume of Nafion® in the catalyst layer is almost the same as that of the catalyst. It is obvious that platinum-chromium/C is sensitive to the polymer content.  
         [0033]     The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching.  
         [0034]     The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.