Gorer, Alexander (San Jose, CA, US)
Strasser, Peter (Houston, TX, US)
Devenney, Martin (Mountain View, CA, US)
Fan, Qun (Fremont, CA, US)
Giaquinta, Daniel M. (Mountain View, CA, US)
Cendak, Keith James (San Mateo, CA, US)
Oyanagi, Hiroyuki (Utsunomiya, JP)
Urata, Kenta (Utsunomiya, JP)
Plaque It!
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Honda Giken Kogyo Kabushiki Kaisha (Minato-Ku, Tokyo, JP)
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CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Application 60/641,494, filed Jan. 5, 2005, which is incorporated herein by reference.
1. A composition comprising platinum (as platinum metal, platinum oxide, platinum carbide and/or a platinum salt), copper (as copper metal, copper oxide, copper carbide and/or a copper salt) and tungsten (as at least tungsten oxide), wherein the sum of the concentrations of said platinum (as platinum metal, platinum oxide, platinum carbide and/or a platinum salt), said copper (as copper metal, copper oxide, copper carbide and/or a copper salt), and said tungsten (as at least tungsten oxide) is greater than 90 atomic percent.
2. The composition of claim 1 wherein said sum is greater than 92 atomic percent.
3. The composition of claim 1 wherein the concentration of said platinum (as platinum metal, platinum oxide, platinum carbide and/or platinum salt) is at least about 10 atomic percent and less than about 80 atomic percent.
4. The composition of claim 1 wherein the concentration of said tungsten (as at least tungsten oxide) is at least about 10 atomic percent and less than about 80 atomic percent.
5. The composition of claim 1 wherein the concentration of said copper (as copper metal, copper oxide, copper carbide and/or copper salt) is at least about 1 atomic percent and less than about 60 atomic percent.
6. The composition of claim 1 wherein (i) the sum of the concentrations of said platinum (as platinum metal, platinum oxide, platinum carbide and/or platinum salt), said copper (as copper metal, copper oxide, copper carbide and/or copper salt) and said tungsten (as at least tungsten oxide) is greater than 95 atomic percent, and (ii) the concentration of said platinum (as platinum metal, platinum oxide, platinum carbide and/or platinum salt) is greater than about 20 atomic percent and less than about 50 atomic percent.
7. The composition of claim 1 wherein (i) the sum of the concentrations of said platinum (as platinum metal, platinum oxide, platinum carbide and/or platinum salt), said copper (as copper metal, copper oxide, copper carbide and/or copper salt) and said tungsten (as at least tungsten oxide) is greater than 95 atomic percent, and (ii) the concentration of said tungsten (as at least said tungsten oxide) is greater than about 40 atomic percent and less than about 80 atomic percent.
8. The composition of claim 1 wherein (i) the sum of the concentrations of said platinum (as platinum metal, platinum oxide, platinum carbide and/or platinum salt), said copper (as copper metal, copper oxide, copper carbide and/or copper salt) and said tungsten (as at least tungsten oxide) is greater than 95 atomic percent, and (ii) the concentration of copper (as copper metal, copper oxide, copper carbide and/or copper salt) is greater than about 2 atomic percent and less than about 25 atomic percent.
9. The composition of claim 1 wherein the composition consists essentially of said platinum (as platinum metal, platinum oxide, platinum carbide and/or platinum salt), said copper (as copper metal, copper oxide, copper carbide and/or copper salt) and said tungsten (as at least tungsten oxide).
10. The composition of claim 1 wherein the concentration of said copper (as copper metal, copper oxide, copper carbide, and/or copper salt) is greater than about 1 atomic percent.
11. The composition of claim 1 wherein the concentration of said tungsten (as at least tungsten oxide) is greater than about 1 atomic percent.
12. The composition of claim 1 wherein the concentration of said platinum (as platinum metal, platinum oxide, platinum carbide, and/or platinum salt) is greater than about 1 atomic percent.
13. The composition of claim 1, wherein the copper present therein has an average oxidation state of about zero.
14. A supported electrocatalyst powder for use in electrochemical reactor devices, the supported electrocatalyst powder comprising the composition of claim 1 on electrically conductive supports.
15. A composition comprising platinum (as platinum metal, platinum oxide, platinum carbide and/or a platinum salt), copper (as copper metal, copper oxide, copper carbide and/or a copper salt) and tungsten (as at least tungsten oxide), wherein the concentration of said tungsten (as at least tungsten oxide) is greater than 40 atomic percent.
16. The composition of claim 15, wherein the copper present therein has an average oxidation state of about zero.
17. A supported electrocatalyst powder for use in electrochemical reactor devices, the supported electrocatalyst powder comprising the composition of claim 15 on electrically conductive supports.
18. A composition comprising platinum (as platinum metal, platinum oxide, platinum carbide and/or a platinum salt), copper (as copper metal, copper oxide, copper carbide and/or a copper salt) and tungsten (as at least tungsten oxide), wherein the concentration of said platinum (as platinum metal, platinum oxide, platinum carbide and/or platinum salt) is at least 30 atomic percent.
19. The composition of claim 18, wherein the copper present therein has an average oxidation state of about zero.
20. A supported electrocatalyst powder for use in electrochemical reactor devices, the supported electrocatalyst powder comprising the composition of claim 18 on electrically conductive supports.
21. A composition comprising platinum (as platinum metal, platinum oxide, platinum carbide and/or a platinum salt), copper (as copper metal, copper oxide, copper carbide and/or a copper salt) and tungsten (as at least tungsten oxide), wherein the concentration of said copper (as copper metal, copper oxide, copper carbide and/or copper) is less than 20 atomic percent.
22. The composition of claim 21, wherein the copper present therein has an average oxidation state of about zero.
23. A supported electrocatalyst powder for use in electrochemical reactor devices, the supported electrocatalyst powder comprising the composition of claim 21 on electrically conductive supports.
24. A composition comprising platinum (as platinum metal, platinum oxide, platinum carbide and/or a platinum salt), copper (as copper metal, copper oxide, copper carbide and/or a copper salt) and tungsten (as at least tungsten oxide), wherein said composition has a body-centered cubic lattice structure.
25. The composition of claim 24 wherein the concentration of said tungsten (as at least tungsten oxide) exceeds the sum of the concentrations of said platinum (as platinum metal, platinum oxide, platinum carbide, and/or platinum salt) and said copper (as copper metal, copper oxide, copper carbide, and/or copper salt).
26. The composition of claim 24, wherein the copper present therein has an average oxidation state of about zero.
27. A supported electrocatalyst powder for use in electrochemical reactor devices, the supported electrocatalyst powder comprising the composition of claim 24 on electrically conductive supports.
28. A composition comprising platinum (as platinum metal, platinum oxide, platinum carbide and/or a platinum salt), copper (as copper metal, copper oxide, copper carbide and/or a copper salt) and tungsten (as at least tungsten oxide), wherein the composition has a face-centered-cubic lattice structure.
29. The composition of claim 28 wherein the concentration of said tungsten (as at least tungsten oxide) exceeds the sum of the concentrations of said platinum (as platinum metal, platinum oxide, platinum carbide, and/or platinum salt) and said copper (as copper metal, copper oxide, copper carbide, and/or copper salt).
30. The composition of claim 28, wherein the copper present therein has an average oxidation state of about zero.
31. A supported electrocatalyst powder for use in electrochemical reactor devices, the supported electrocatalyst powder comprising the composition of claim 28 on electrically conductive supports.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to compositions which are useful as catalysts in fuel cell electrodes (e.g., electrocatalysts) and other catalytic structures, and which comprise platinum, copper and tungsten.
2. Description of Related Technology
A fuel cell is an electrochemical device for directly converting the chemical energy generated from an oxidation-reduction reaction of a fuel such as hydrogen or hydrocarbon-based fuels and an oxidizer such as oxygen gas (in air) supplied thereto into a low-voltage direct current. Thus, fuel cells chemically combine the molecules of a fuel and an oxidizer without burning, dispensing with the inefficiencies and pollution of traditional combustion.
A fuel cell is generally comprised of a fuel electrode (anode), an oxidizer electrode (cathode), an electrolyte interposed between the electrodes (alkaline or acidic), and means for separately supplying a stream of fuel and a stream of oxidizer to the anode and the cathode, respectively. In operation, fuel supplied to the anode is oxidized, releasing electrons that are conducted via an external circuit to the cathode. At the cathode, the supplied electrons are consumed when the oxidizer is reduced. The current flowing through the external circuit can be made to do useful work.
There are several types of fuel cells, including those having electrolytes of phosphoric acid, molten carbonate, solid oxide, potassium hydroxide, or a proton exchange membrane. A phosphoric acid fuel cell operates at about 160-220° C., and preferably at about 190-200° C. This type of fuel cell is currently being used for multi-megawatt utility power generation and for co-generation systems (i.e., combined heat and power generation) in the 50 to several hundred kilowatts range.
In contrast, proton exchange membrane fuel cells use a solid proton-conducting polymer membrane as the electrolyte. Typically, the polymer membrane is maintained in a hydrated form during operation in order to prevent loss of ionic conduction which limits the operation temperature typically to between about 70 and about 120° C., depending on the operating pressure, and preferably below about 100° C. Proton exchange membrane fuel cells have a much higher power density than liquid electrolyte fuel cells (e.g., phosphoric acid), and can vary output quickly to meet shifts in power demand. Thus, they are suited for applications such as in automobiles and small-scale residential power generation where quick startup is a consideration.
In some applications (e.g., automotive) pure hydrogen gas is the optimum fuel; however, in other applications where a lower operational cost is desirable, a reformed hydrogen-containing gas is an appropriate fuel. A reformed-hydrogen containing gas is produced, for example, by steam-reforming methanol and water at 200-300° C. to a hydrogen-rich fuel gas containing carbon dioxide. Theoretically, the reformate gas consists of 75 vol % hydrogen and 25 vol % carbon dioxide. In practice, however, this gas also contains nitrogen, oxygen and, depending on the degree of purity, varying amounts of carbon monoxide (up to 1 vol %). Although some electronic devices also reform liquid fuel to hydrogen, in some applications the conversion of a liquid fuel directly into electricity is desirable, as then high storage density and system simplicity are combined. In particular, methanol is an especially desirable fuel because it has a high energy density, a low cost, and is produced from renewable resources.
For the oxidation and reduction reactions in a fuel cell to proceed at useful rates, especially at operating temperatures below about 300° C., electrocatalyst materials are typically provided at the electrodes. Initially, fuel cells used electrocatalysts made of a single metal, usually platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), osmium (Os), silver (Ag) or gold (Au), because they are able to withstand the corrosive environment. In general, platinum is considered to be the most efficient and stable single-metal electrocatalyst for fuel cells operating below about 300° C.
While the above-noted elements were first used in fuel cells in metallic powder form, later techniques were developed to disperse these metals over the surface of electrically conductive supports (e.g., carbon black) to increase the surface area of the electrocatalyst. An increase in the surface area of the electrocatalyst in turn increases the number of reactive sites, leading to improved efficiency of the cell. Nevertheless, fuel cell performance typically declines over time because the presence of electrolyte, high temperatures and molecular oxygen dissolve the electrocatalyst and/or sinter the dispersed electrocatalyst by surface migration or dissolution/re-precipitation.
Although platinum is considered to be the most efficient and stable single-metal electrocatalyst for fuel cells, it is costly. Additionally, an increase in electrocatalyst activity over platinum is desirable, if not necessary, for wide-scale commercialization of fuel cell technology. However, the development of cathode fuel cell electrocatalyst materials faces longstanding challenges. The greatest challenge is the improvement of the electrode kinetics of the oxygen reduction reaction. In fact, sluggish electrochemical reaction kinetics has prevented electrocatalysts from attaining the thermodynamic reversible electrode potential for oxygen reduction. This is reflected in exchange current densities of around 10 −10 to 10 −12 A/cm 2 for oxygen reduction on, for example, Pt at low and medium temperatures. A factor contributing to this phenomenon includes the fact that the desired reduction of oxygen to water is a four-electron transfer reaction and typically involves breaking a strong O—O bond early in the reaction. In addition, the open circuit voltage is lowered from the thermodynamic potential for oxygen reduction due to the formation of peroxide and possible platinum oxides that inhibit the reaction. A second challenge is the stability of the oxygen electrode (cathode) during long-term operation. Specifically, a fuel cell cathode operates in a regime in which even the most unreactive metals are not completely stable. Thus, alloy compositions that contain non-noble metal elements may have a rate of corrosion that would negatively impact the projected lifetime of a fuel cell. Corrosion may be more severe when the cell is operating near open circuit conditions—the most desirable potential for thermodynamic efficiency.
Electrocatalyst materials at the anode also face challenges during fuel cell operation. Specifically, as the concentration of carbon monoxide (CO) rises above about 10 ppm in the fuel the surface of the electrocatalyst can be rapidly poisoned. As a result, platinum (by itself) is a poor electrocatalyst if the fuel stream contains carbon monoxide (e.g., reformed-hydrogen gas typically exceeds 100 ppm). Liquid hydrocarbon-based fuels (e.g., methanol) present an even greater poisoning problem. Specifically, the surface of the platinum becomes blocked with the adsorbed intermediate, carbon monoxide (CO). It has been reported that H 2 O plays a key role in the removal of such poisoning species in accordance with the following reactions:
Pt+CH 3 OH→Pt—CO+4H + +4e − (1);
Pt+H 2 O→Pt—OH+H + +e − (2); and
Pt—CO+Pt—OH→2Pt+CO 2 +H + +e − (3).
As indicated by the foregoing reactions, the methanol is adsorbed and partially oxidized by platinum on the surface of the electrode (1). Adsorbed OH, from the hydrolysis of water, reacts with the adsorbed CO to produce carbon dioxide and a proton (2,3). However, platinum does not form OH species rapidly at the potentials where fuel cell electrodes operate (e.g., 200 mV-1.5 V). As a result, step (3) is the slowest step in the sequence, limiting the rate of CO removal, thereby allowing poisoning of the electrocatalyst to occur. This applies in particular to a proton exchange membrane fuel cell which is especially sensitive to CO poisoning because of its low operating temperatures.
One approach for improving the cathodic performance of an electrocatalyst during the reduction of oxygen and/or the anodic performance during the oxidation of hydrogen or methanol is to employ an electrocatalyst which is more active, corrosion resistant, and/or more poison tolerant. For example, increased tolerance to CO has been reported by alloying platinum and ruthenium at a 50:50 atomic ratio (see, D. Chu and S. Gillman, J. Electrochem. Soc. 1996, 143, 1685). The electrocatalysts proposed to-date, however, leave room for further improvement.
BRIEF SUMMARY OF THE INVENTION
Briefly, therefore, the present invention is directed to a composition for use as a catalyst in oxidation or reduction reactions, in for example fuel cells, the composition comprising platinum, copper and tungsten, or an oxide, carbide or salt of one or more of said platinum, copper and tungsten, wherein said composition has a body-centered-cubic lattice structure.
The present invention is further directed to a composition for use as a catalyst in oxidation or reduction reactions, in for example fuel cells, the composition comprising platinum, copper and tungsten, or an oxide, carbide or salt of one or more of said platinum, copper and tungsten, wherein said composition has a face-centered-cubic lattice structure.
The present invention is still further directed to a composition for use as a catalyst in oxidation or reduction reactions, in for example fuel cells, the composition comprising platinum, copper and tungsten, or an oxide, carbide or salt of one or more of said platinum, copper and tungsten, wherein the sum of the concentrations of platinum, copper and tungsten, or an oxide, carbide or salt thereof, is greater than 90 atomic percent.
The present invention is still further directed to a composition for use as a catalyst in oxidation or reduction reactions, in for example fuel cells, the composition comprising platinum, copper and tungsten, or an oxide, carbide or salt of one or more of said platinum, copper and tungsten, wherein the concentration of tungsten, or an oxide, carbide or salt thereof, is greater than 40 atomic percent.
The present invention is still further directed to a composition for use as a catalyst in oxidation or reduction reactions, in for example fuel cells, the composition comprising platinum, copper and tungsten, or an oxide, carbide or salt of one or more of said platinum, copper and tungsten, wherein the concentration of platinum, or an oxide, carbide or salt thereof, is at least 30 atomic percent.
The present invention is still further directed to a composition for use as a catalyst in oxidation or reduction reactions, in for example fuel cells, the composition comprising platinum, copper and tungsten, or an oxide, carbide or salt of one or more of said platinum, copper and tungsten, wherein the concentration of copper, or an oxide, carbide or salt thereof, is less than 20 atomic percent.
The present invention is still further directed to a composition for use as a catalyst in oxidation or reduction reactions, in for example fuel cells, the composition consisting essentially of platinum, copper and tungsten, or an oxide, carbide or salt of one or more of said platinum, copper and tungsten.
The present invention is still further directed to one or more of the foregoing catalyst compositions wherein said catalyst composition comprises an alloy of the recited metals, or alternatively wherein said catalyst consists essentially of an alloy of the recited metals.
The present invention is still further directed to a supported electrocatalyst powder for use in electrochemical reactor devices, the supported electrocatalyst powder comprising any of the foregoing catalyst compositions on electrically conductive support particles.
The present invention is still further directed to a fuel cell electrode, the fuel cell electrode comprising electrocatalyst particles and an electrode substrate upon which the electrocatalyst particles are deposited, the electrocatalyst particles comprising any of the foregoing catalyst compositions.
The present invention is still further directed to a fuel cell comprising an anode, a cathode, a proton exchange membrane between the anode and the cathode, and any of the foregoing catalyst compositions, for the catalytic oxidation of a hydrogen-containing fuel or the catalytic reduction of oxygen.
The present invention is still further directed to a method for the electrochemical conversion of a hydrogen-containing fuel and oxygen to reaction products and electricity in a fuel cell comprising an anode, a cathode, a proton exchange membrane therebetween, any of the foregoing catalyst compositions, and an electrically conductive external circuit connecting the anode and cathode. The method comprises contacting the hydrogen-containing fuel or the oxygen and any of the foregoing catalyst compositions to catalytically oxidize the hydrogen-containing fuel or catalytically reduce the oxygen.
The present invention is still further directed to a fuel cell electrolyte membrane, and/or a fuel cell electrode, having deposited on a surface thereof a layer of an unsupported catalyst composition, said unsupported catalyst composition layer comprising any of the foregoing catalyst compositions.
The present invention is still further directed to a method for preparing one of the foregoing catalyst compositions from a catalyst precursor composition, said precursor composition comprising platinum, copper and tungsten, or an oxide, carbide or salt thereof. The process comprises subjecting said precursor composition to conditions sufficient to remove a portion of the tungsten and/or copper present therein, such that the resulting catalyst composition comprises platinum, copper and tungsten, or an oxide, carbide or salt thereof, as set forth above.
In one preferred embodiment of the above-noted method, the catalyst precursor composition is contacted with an acidic solution to solubilize a portion of the tungsten and/or copper present therein. In an alternative embodiment, this method comprises subjecting the catalyst precursor composition to an electrochemical reaction, wherein for example a hydrogen-containing fuel and oxygen are converted to reaction products and electricity in a fuel cell comprising an anode, a cathode, a proton exchange membrane therebetween, the catalyst precursor composition, and an electrically conductive external circuit connecting the anode and cathode. By contacting the hydrogen-containing fuel or the oxygen and the catalyst precursor composition, the hydrogen-containing fuel is oxidized and/or the oxygen is catalytically reduced. As part of this reaction, tungsten and/or copper are dissolved in situ from the catalyst precursor composition.
The foregoing, as well as other features and advantages of the present invention, will become more apparent from the following description and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a photograph of a TEM image of a carbon support with catalyst nanoparticles deposited thereon, in accordance with the present invention.
FIG. 2 is an exploded, schematic structural view showing members of a fuel cell.
FIG. 3 is cross-sectional view of the assembled fuel cell of FIG. 2.
FIG. 4 is a photograph of an electrode array comprising thin film catalyst compositions deposited on individually addressable electrodes, in accordance with the present invention.
It is to be noted that corresponding reference characters indicate corresponding parts throughout the drawings.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to a composition having catalytic activity for use in, for example, oxidation and/or reduction reactions of interest in a polyelectrolyte membrane fuel cell (e.g., an electrocatalyst), the composition comprising, as further detailed herein, platinum, copper and tungsten, a portion of one or more of which may optionally be present in the form of a metal oxide or carbide or salt. Advantageously and surprisingly, it has been discovered that the catalyst compositions of the present invention may exhibit favorable electrocatalytic activity while having reduced amounts of platinum, as compared to, for example, a platinum standard.
In this regard it is to be noted that, in general, it is desirable, but not essential, to reduce the cost of a catalyst composition to be used in such reactions, particularly when used in fuel cells. One method of reducing the cost of the catalyst composition is to decrease the amount of noble metals (such as platinum) used to produce it. Typically, however, as the concentrations of noble metals are decreased, catalyst compositions tend to become more susceptible to corrosion and/or the absolute activity may be diminished. Thus, it is typically desirable to achieve the most activity per weight percent of noble metals (see, e.g., End Current Density/Weight Fraction of Pt, as set forth in Tables A-E, infra). Preferably, this is accomplished without compromising, for example, the life cycle of the fuel cell in which the catalyst composition is placed. In addition to, or as an alternative to, reducing cost by limiting the noble metal concentration, a catalyst composition of the present invention may be selected because it represents an improvement in corrosion resistance and/or activity compared to platinum (e.g., at least a 3 times increase in electrocatalytic activity compared to platinum).
The present invention is thus directed to a composition that has catalytic activity in oxidation and/or reduction reactions, and that comprises platinum, copper and tungsten, and optionally an oxide, carbide or salt thereof. It is to be noted that the catalyst composition of the present invention may be in the form of an alloy of these metals, the composition for example consisting essentially of an alloy containing these metals. Alternatively, the catalyst composition of the present invention may comprise these metals, a portion of which is in the form of an alloy, the composition for example having alloy particles inter-mixed with oxide, carbide or salt particles as a coating, as a pseudo-support, and/or a simple mixture.
It is to be further noted that the catalyst composition of the present invention comprises an amount of platinum, copper and tungsten, and optionally an oxide, carbide or salt thereof, which is sufficient for each to play a role in the catalytic activity and/or crystallographic structure of the catalyst composition. Stated another way, the concentrations of platinum, copper and tungsten, and optionally an oxide, carbide or salt thereof, in the present catalyst composition are such that the presence of the each of these would not be considered an impurity therein. For example, when present, the concentrations of each of platinum, copper and tungsten, and optionally an oxide, carbide or salt thereof, are at least about 0.1, 0.5, 1 or even 2 atomic percent, wherein the sum of the concentrations thereof is greater than 90 atomic percent, 92 atomic percent, 94 atomic percent, 96 atomic percent, 98 atomic percent, or even 99 atomic percent.
In this regard it is to be still further noted that the catalyst composition of the present invention may optionally consist essentially of platinum, copper and tungsten, including an oxide, carbide or salt thereof (e.g., impurities that play little, if any, role in the catalytic activity and/or crystallographic structure of the catalyst may be present to some degree), the concentrations of the metals, or an oxide, carbide or salt thereof, being within any one or more of the ranges for an individual metal as set forth herein, or for the combination of metals. Stated another way, the concentration of a metallic or non-metallic element other than platinum, copper and tungsten, or an oxide, carbide or salt thereof, may optionally not exceed what would be considered an impurity (e.g., less than 1, 0.5, 0.1, or 0.01 atomic percent).
In view of the foregoing, it is to be understood that the catalyst composition of the present invention may comprise, or consist essentially of, platinum, copper and tungsten metals. Alternatively, the catalyst composition of the present invention may comprise, or consist essentially of, platinum, copper and tungsten, wherein a portion of one or more of these components is in the form of oxides and/or carbides and/or salts.
It is to be further noted that in one or more embodiments of the present invention, platinum, copper and/or tungsten may be substantially in their metallic oxidation states. Stated another way, in one or more embodiments of the present invention, the average oxidation state of platinum, copper and/or tungsten may be at or near zero. In one particular embodiment, the copper present in one or more of the catalyst compositions set forth herein has an average oxidation state that is at or near zero.
In view of the foregoing, it is to be understood that although in such embodiments there may be portions of the catalyst composition wherein the oxidation states of one or more of platinum, copper and tungsten are greater than zero, the average oxidation states of these elements throughout the entire composition will be less than the lowest commonly occurring oxidation state for that particular element (e.g., the lowest commonly occurring oxidation states for platinum, tungsten and copper are 2, 4 and 1, respectively). Therefore, the average oxidation state of platinum may be, for example, less than 2, 1.5, 1, 0.5, 0.1, or 0.01, or even zero, wile the average oxidation state of tungsten may be, for example, less than about 4, 3.5, 3, 2.5, 2, 1.5, 1, 0.5, 0.1, 0.01, or even zero, and the average oxidation state of copper may be less than 1, 0.5, 0.1, or 0.01, or even zero.
It is to be still further noted, however, that in an alternative embodiment of the present invention, the platinum, copper and/or tungsten may not be substantially present in their metallic oxidation states. Stated another way, in one or more embodiments of the present invention, the platinum, copper and/or tungsten in the catalyst composition may have an average oxidation state that is greater than zero (the platinum, copper and/or tungsten being present in the catalyst, for example, as an oxide, carbide or salt, as previously noted). In fact, in one particular embodiment of the present invention, one or more of the catalyst compositions set forth herein comprises a tungsten oxide (e.g., WO 2 or WO 3 ). Without being held to any particular theory, and as further discussed herein below, it is believed that unreacted tungsten may be present in one or more of the compositions of the present invention as tungsten oxide. Tungsten oxide may be present due to, at least in part, the use of certain tungsten precursors or the incomplete reduction of the tungsten precursors during the annealing procedure. Unreacted tungsten may be present in the form of WO 2 after annealing. Unreacted tungsten may also be present in the form of WO 3 after a washing procedure, which is discussed further herein below.
1. Cataltst Compositions
A. Constituent Concentrations
As previously disclosed, the catalyst composition of the present invention comprises platinum, which may be in the form of for example platinum metal, platinum oxide and/or platinum carbide and/or a platinum salt. The concentration of platinum (e.g., platinum metal, platinum oxide, platinum carbide and/or platinum salt) in the present composition is typically greater than about 10 atomic percent, and preferably is greater than about 15 atomic percent, about 20 atomic percent, or even about 25 atomic percent, and is typically less than about 80 atomic percent, about 75 atomic percent, about 70 atomic percent, about 65 atomic percent, about 60 atomic percent, about 55 atomic percent, about 50 atomic percent, about 45 atomic percent, or even about 40 atomic percent. For example, the concentration of platinum metal, platinum oxide, platinum carbide and/or platinum salt may typically be in the range of about 10 to about 80 atomic percent, preferably about 15 to about 70 atomic percent, more preferably about 20 to about 50 atomic percent, or even more preferably from 25 to about 40 atomic percent.
In this regard it is to be noted, however, that the scope of the present invention is intended to encompass all of the various platinum concentration range permutations possible herein, in view of the above-noted maxima and minima.
The catalyst composition of the present invention also comprises tungsten, which may be in the form of for example tungsten metal, tungsten oxide, tungsten carbide and/or tungsten salt. The concentration of tungsten (e.g., tungsten metal, tungsten oxide, tungsten carbide and/or tungsten salt) in the present composition may also vary within a large compositional range. Typically, however, the concentration of tungsten (e.g., tungsten metal, tungsten oxide, tungsten carbide and/or tungsten salt) is greater than about 10 atomic percent, and preferably is greater than about 20 atomic percent, about 30 atomic percent, about 40 atomic percent, or even about 45 atomic percent, and is typically less than about 80 atomic percent, about 75 atomic percent, about 70 atomic percent, about 65 atomic percent, or even about 60 atomic percent. For example, the concentration of tungsten metal, tungsten oxide, tungsten carbide and/or tungsten salt may typically be in the range of about 10 to about 80 atomic percent, preferably about 20 to about 75 atomic percent, more preferably about 30 to about 70 atomic percent, more preferably about 40 to about 65 atomic percent, or more preferably about 45 to about 60 atomic percent.
In this regard it is to be noted, however, that the scope of the present invention is intended to encompass all of the various tungsten concentration range permutations possible herein, in view of the above-noted maxima and minima.
The catalyst composition of the present invention also comprises copper, which may be in the form of for example copper metal, copper oxide, copper carbide and/or copper salt. The concentration of copper (e.g., copper metal, copper oxide, copper carbide and/or copper salt) in the present composition may, like platinum and tungsten, also vary within a large compositional range. Typically, however, the concentration of copper (e.g., copper metal, copper oxide, copper carbide, and/or copper salt) is greater than about 1 atomic percent, and preferably is greater than about 2 atomic percent, about 3 atomic percent, about 4 atomic percent, about 5 atomic percent, or even about 6 atomic percent, and is typically less than about 70 atomic percent, about 60 atomic percent, about 50 atomic percent, about 40 atomic percent, about 30 atomic percent, or even about 20 atomic percent. For example, the concentration of copper metal, copper oxide, copper carbide and/or copper salt may typically be in the range of about 1 to about 60 atomic percent, preferably about 2 to about 50 atomic percent, more preferably about 3 to about 40 atomic percent, more preferably about 4 to about 30 atomic percent, or more preferably about 5 to about 20 atomic percent.
In this regard it is to be noted, however, that the scope of the present invention is intended to encompass all of the various copper concentration range permutations possible herein, in view of the above-noted maxima and minima.
It is to be further noted that the catalyst composition of the present invention may encompass any of the various combinations of platinum, copper and tungsten concentrations and/or ranges of concentrations set forth above without departing from its intended scope. For example, for those embodiments wherein the catalyst composition of the present invention comprises platinum, copper and tungsten, each of which may independently be in the form of its metal, oxide, carbide, salt, or a mixture thereof, the sum of the concentrations thereof may be greater than 90, 92 or even 95, atomic percent, and (i) the concentration of platinum (i.e., platinum metal, oxide, carbide and/or salt) may be greater than about 20 atomic percent and less than about 50 atomic percent, or greater than about 25 atomic percent and less than about 40 atomic percent; (ii) the concentration of tungsten (i.e., tungsten metal, oxide, carbide and/or salt) may be greater than about 40 atomic percent and less than about 80 atomic percent, or greater than about 45 atomic percent and less than about 60 atomic percent; and, (iii) the concentration of copper (i.e., copper metal, oxide, carbide and/or salt) may be greater than about 2 atomic percent and less than about 25 atomic percent, or greater than about 5 atomic percent and less than about 20 atomic percent.
The present invention may additionally, or alternatively, encompass catalyst compositions wherein the sum of tungsten and copper (i.e., tungsten or copper metal, oxide, carbide and/or salt) is at least 60 atomic percent, 65 atomic percent, 70 atomic percent, 75 atomic percent, or even 80 atomic percent. For example, the present invention may encompass catalyst compositions wherein: (i) the concentration of platinum (i.e., platinum metal, oxide, carbide and/or salt) is greater than about 30 atomic percent and less than about 40 atomic percent, the concentration of copper (i.e., copper metal, oxide, carbide and/or salt) is greater than about 8 atomic percent and less than about 35 atomic percent, and the concentration of tungsten (i.e., tungsten metal, oxide, carbide and/or salt) is greater than 30 atomic percent and less than about 55 atomic percent.
B. Atomic Ratios of Constituents
The catalyst compositions of the present invention may alternatively or additionally be characterized in terms of atomic ratios between the constituents. More specifically, it has been discovered that when used as a catalyst one or more electrocatalytic properties (e.g. activity) of the compositions of the present invention may be controlled by, for example, controlling the ratio of tungsten to copper. Specifically, it has been discovered that activity may be altered (e.g., increased), in for example a catalyst composition wherein the concentration of platinum is less than about 50 atomic percent, by controlling the atomic ratio of tungsten to copper, in what ever forms each are independently present in (i.e., metal, oxide, carbide and/or salt), such that this ratio is greater than about 1:1 (e.g., about 1.2:1, about 1.4:1, about 1.6:1, about 1.8:1 or more), with tungsten being in excess relative to copper. In such an embodiment, the atomic ratio of tungsten (e.g., metal, oxide, carbide and/or salt) to copper (e.g., metal, oxide, carbide and/or salt) may preferably be greater than about 2:1, more preferably greater than about 3:1, still more preferably greater than about 4:1, still more preferably greater than about 5:1, still more preferably greater than about 6:1, still more preferably greater than about 7:1, still more preferably greater than about 8:1, still more preferably greater than about 9:1, or still more preferably greater than about 10:1.
C. Compositional Drift
As has been reported elsewhere, subjecting a catalyst composition to an electrocatalytic reaction (e.g., the operation of a fuel cell) may change the composition by leaching one or more constituents (e.g., tungsten and/or copper) from the catalyst (see, e.g., Catalysis for Low Temperature Fuel Cells Part 1: The Cathode Challenges, T. R. Ralph and M. P. Hogarth, Platinum Metals Rev., 2002, 46, (1), p. 3-14). Without being held to any particular theory, it is believed that this leaching effect may potentially act to increase the activity of the catalyst by increasing the surface area and/or by changing the surface composition of the catalyst. In fact, the purposeful leaching of catalyst compositions after synthesis to increase the surface area has been disclosed by Itoh et al. (see, e.g., U.S. Pat. No. 5,876,867 which is incorporated herein by reference).
Accordingly, it is to be noted that the concentrations, concentration ranges, and atomic ratios detailed herein for the catalyst compositions of the present invention are intended to include the bulk stoichiometries, any surface stoichiometries resulting therefrom, and modifications of the bulk and/or surface stoichiometries that result by subjecting the catalyst compositions of the present invention to a reaction (e.g., an electrocatalytic reaction) of interest.
D. Structural Characteristics
A catalyst composition of the present invention, such as Pt 35 Cu 10 W 55 , may exhibit a variety of structural characteristics, as measured using techniques known in the art (e.g., XRD). For example, depending on the annealing conditions used to prepare the composition (as further discussed herein below), a face-centered-cubic (fcc) lattice structure may be present that may exhibit a lattice constant smaller than Pt (see, e.g., sample HFC 1840 in Table E, annealed at about 675° C. for about 10 hours), or a lattice constant larger than Pt (see, e.g., sample HFC 1770, annealed at about 925° C. for about 2 hours). Without being held to any particular theory, one possible explanation for this behavior relates to the respective metallic radii of Pt, Cu and W; the metallic radius of Cu is smaller than Pt, while the metallic radius of W is larger than Pt. For a given annealing time (e.g., some time between about 2 and about 10 hours), at low annealing temperatures (e.g., temperatures less than about 850° C., 800° C., 750° C., 700° C., or even 650° C.) it is possible that Cu may react with Pt, resulting in a lattice parameter smaller than Pt, while W remains largely unreacted. Conversely, for the same or similar annealing time, at high annealing temperatures (e.g., temperature greater than about 650° C., 700° C., 750° C., 800° C. or even 850° C.) it is possible that both Cu and W may react with Pt, resulting in a lattice parameter that is larger than Pt.
Additionally, for the same composition, a body-centered-cubic (bcc) lattice structure may be present. It has been observed that the bcc lattice structure may preferentially be formed using annealing conditions between those described above (e.g., between about 650° C. and 800° C., or about 700° C. and 750° C., and annealing time of about 5 to about 10 hours). In other words, for certain catalyst compositions, the choice of annealing conditions may result in 3 different types of XRD profiles.
Furthermore, it has been observed that the relative activity of a catalyst composition appears to show a dependence on the choice of annealing conditions. In a general sense, it appears that a catalyst composition, for example Pt 35 Cu 10 W 55 , exhibiting a lattice constant larger than Pt (that is, a sample that may have completely reacted, see e.g. sample 1770 or 1774), displays a lower relative activity than a catalyst of the same composition that exhibits a lattice constant smaller than Pt (that is, a sample that had been annealed at lower temperatures and has not completely reacted). In part, however, this effect may be related to the fact that the sample annealed at higher temperatures would possess larger particles. In between these two ranges, however, are the conditions in which the catalyst composition exhibits the bcc lattice structure. It has been observed that samples which are annealed in such a way as to exhibit the bcc lattice structure, particularly those which are tungsten-rich, appear to show the highest relative activity.
In view of the foregoing, it is to be noted that the present invention is additionally directed to a composition comprising platinum, copper and tungsten, or an oxide, carbide and/or salt of one or more of said platinum, copper and tungsten, wherein said composition has a body-centered-cubic lattice structure. Alternatively, the present invention is directed to a composition comprising platinum, copper and tungsten, or an oxide, carbide and/or salt of one or more of said platinum, copper and tungsten, wherein said composition has a face-centered-cubic lattice structure. Optionally, one or both of these compositions may be “tungsten-rich”.
In this regard it is to be noted that, as used herein, “tungsten-rich” refers to a composition wherein the concentration of tungsten, including an oxide, carbide and/or salt thereof, is greater than the concentration of either platinum, including an oxide, carbide and/or salt thereof, or copper, including an oxide, carbide and/or salt thereof. Optionally, “tungsten-rich” may refer to a composition wherein the concentration of tungsten, including an oxide, carbide and/or salt thereof, exceeds the sum of the concentrations of platinum and copper, including oxides, carbides and/or salts thereof, and thus is the predominant component thereof. For example, in one embodiment, “tungsten-rich” may refer to a composition having a concentration of tungsten, including an oxide, carbide and/or salt thereof, that is greater than about 50 atomic percent, about 55 atomic percent, about 60 atomic percent, about 65 atomic percent, about 70 atomic percent, about 75 atomic percent, or more (see, e.g., samples 1102 and 1172 to 1856).
2. Catalyst Composition Precursors—Washing/Leaching
With respect to the above-noted compositional drift that has been observed in use, it is to be further noted that, as illustrated by the results presented in, for example, Table E herein (under the heading “Washed Powders”), the catalyst compositions of the present invention, once prepared, may optionally be subjected to a washing procedure, in order to remove, for example tungsten and/or copper therefrom. Such a procedure may be advantageous because it acts to remove at least a portion of the metal or metals that may otherwise leach from the catalyst composition when in use (e.g., when used in a fuel cell), thus acting to extend the useful life of the device in which it is being used (by removing metals that would otherwise leach, and act as contaminants).
The present invention is therefore additionally directed to a method for the preparation of a catalyst composition from a catalyst precursor composition, said precursor composition comprising platinum, copper and tungsten. Generally speaking, the process comprises subjecting said precursor composition to conditions sufficient to remove a portion of the tungsten and/or copper present therein, such that a catalyst composition, as set forth elsewhere herein is obtained (the catalyst composition comprising, for example, platinum, copper and tungsten, a portion of each of which may be in the form of a metal, oxide, carbide and/or salt, wherein the sum of the concentrations thereof is greater than 90 atomic percent).
In one preferred embodiment of the above-noted method, the catalyst precursor composition is contacted with an acidic solution to wash or remove a portion of the tungsten and/or copper present therein out of the precursor. For example, a given weight of the catalyst precursor composition may be contacted with a quantity of a perchloric acid (HClO 4 ) solution (e.g., 1 M), heated (e.g., about 90 to about 95° C.) for a period of time (e.g., about 60 minutes), filtered, and then repeatedly washed with water. The precursor composition is preferably washed a second time, the solid cake isolated from the first filtration step being collected and then subjected to substantially the same sequence of steps as previously performed, the cake being agitated sufficiently to break it apart prior to and/or during the time spent heating the cake/acid solution mixture back up to the desired temperature. After the final filtration has been performed, the isolated cake is dried (e.g., heated at about 90° C. for about 48 hours).
It is to be noted, however, that in an alternative embodiment the catalyst precursor composition may be exposed to conditions common within a fuel cell (e.g., immersion in an electrochemical cell containing an aqueous 0.5 M H 2 SO 4 electrolyte solution maintained at room temperature, such as described in Example 4, herein below), in order to leach tungsten and/or copper from the precursor. Alternatively, the precursor may be directly subjected to an electrochemical reaction wherein, for example, a hydrogen-containing fuel and oxygen are converted to reaction products and electricity in a fuel cell comprising an anode, a cathode, a proton exchange membrane therebetween, the catalyst precursor composition, and an electrically conductive external circuit connecting the anode and cathode. By contacting the hydrogen-containing fuel or the oxygen and the catalyst precursor composition, the hydrogen-containing fuel is oxidized and/or the oxygen is catalytically reduced. As part of this reaction, tungsten and/or copper are thus dissolved in situ from the catalyst precursor composition. After this reaction has been allowed to continue for a length of time sufficient to obtain a substantially stable composition (i.e., a composition wherein the concentration of platinum, copper and/or tungsten remain substantially constant), the composition may be removed from the cell and used as a catalyst composition in a future fuel cell reaction of interest.
It is to be still further noted that the process for removing a portion of, for example, the tungsten and/or copper from the catalyst composition precursor may be other than herein described without departing from the scope of the present invention. For example, alternative solutions may be used (e.g., HCF 3 SO 3 H, NAFION™, HNO 3 , HCl, H 2 SO 4 , CH 3 CO 2 H), and/or alternative concentrations (e.g., about 0.05 M, 0.1 M, 0.5 M, 1 M, 2 M, 3 M, 4 M, 5 M, etc.), and/or alternative temperatures (e.g., about 25° C., 35° C., 45° C., 55° C., 65° C., 75° C., 85° C., etc.) and/or alternative washing times or durations (e.g., about 5 minutes, 10 minutes, 20 minutes, 30 minutes, 40 minutes, 50 minutes, 60 minutes or more), and/or alternative numbers of washing cycles (e.g., 1, 2, 3, 4, 5 or more), and/or alternative washing techniques (e.g., those involving centrifugation, sonication, soaking, electrochemical techniques, or a combination thereof), and/or alternative washing atmospheres (e.g., ambient, oxygen-enriched, argon), as well as various combinations thereof (selected using means common in the art).
3. Formation of Catalyst Composition Precursors Comprising/Consisting Essentially of an Alloy
As previously noted, the catalyst composition, and/or the catalyst composition precursor, of the present invention may consist essentially of an alloy of platinum, copper and tungsten. Alternatively, the catalyst composition, and/or the catalyst composition precursor, of the present invention may comprise an alloy of platinum, copper and tungsten; that is, one or both of these may alternatively comprise an alloy of these metals, and optionally one or more of these metals in a non-alloy form (e.g., a platinum, copper and/or tungsten salt and/or oxide and/or carbide).
Such alloys may be formed by a variety of methods. For example, the appropriate amounts of the constituents (e.g., metals) may be mixed together and heated to a temperature above the respective melting points to form a molten solution of the metals that is cooled and allowed to solidify.
Typically, the catalyst compositions of the present invention, and/or the precursors thereto, are used in a powder form to increase the surface area, which in turn increases the number of reactive sites, and thus leads to improved efficiency of the cell in which the catalyst compositions are being used. Thus, a formed catalyst composition alloy, and/or the precursor thereto, may be transformed into a powder after being solidified (e.g., by grinding), or during solidification (e.g., spraying molten alloy and allowing the droplets to solidify). In this regard it is to be noted, however, that in some instances it may be advantageous to evaluate alloys for electrocatalytic activity in a non-powder form, such as a film, as further described and illustrated elsewhere herein (see, e.g., Examples 1 and 2, infra).
To further increase surface area and efficiency, a catalyst composition alloy (i.e., a catalyst composition comprising or consisting essentially of an alloy), and/or the precursor thereto, may be deposited over the surface of electrically conductive supports (e.g., carbon black) for use in a fuel cell. One method for loading a catalyst composition or precursor alloy onto supports typically comprises depositing metal-containing (e.g., platinum, copper and/or tungsten) compounds onto the supports, converting these compounds to metallic form, and then alloying the metals using a heat-treatment in a reducing atmosphere (e.g., an atmosphere comprising an inert gas such as argon and/or a reducing gas such as hydrogen). One method for depositing these compounds involves the chemical precipitation thereof onto the supports. The chemical precipitation method is typically accomplished by mixing supports and sources of the metal compounds (e.g., an aqueous solution comprising one or more inorganic metal salts) at a concentration sufficient to obtain the desired loading of the catalyst composition, or precursor thereto, on the supports, after which precipitation of the compounds is initiated (e.g., by adding an ammonium hydroxide solution). The slurry is then typically filtered from the liquid under vacuum, washed with deionized water, and dried to yield a powder that comprises the metal compounds on the supports.
Another method for depositing the metal compounds comprises forming a suspension comprising a solution and supports suspended therein, wherein the solution comprises a solvent portion and a solute portion that comprises the metal compound(s) being deposited. The suspension is frozen to deposit (e.g., precipitate) the compound(s) on the support particles. The frozen suspension is then freeze-dried to remove the solvent portion, leaving a freeze-dried powder comprising the supports and the deposits of the metal compound(s) on the supports.
Since the process may involve sublimation of the solvent portion from the frozen suspension, the solvent portion of the solution in which the supports are suspended preferably has an appreciable vapor pressure below its freezing point. Examples of such sublimable solvents that also dissolve many metal-containing compounds and metals include water, alcohols (e.g., methanol, ethanol, etc.), acetic acid, carbon tetrachloride, ammonia, 1,2-dichloroethane, N,N-dimethylformamide, formamide, etc.
The solution in which the supports are dispersed/suspended provides the means for delivering the metal species which is to be deposited onto the surfaces of the supports. The metal species may be the final desired form, but in many instances it is not. If the metal species is not a final desired form, the deposited metal species may be subsequently converted to the final desired form. Examples of such metal species that may be subsequently converted include inorganic and organic metal compounds such as metal halides, sulfates, carbonates, nitrates, nitrites, oxalates, acetates, formates, etc. Conversion to the final desired form may be made by thermal decomposition, chemical reduction, or other reaction. Thermal decomposition, for example, is brought about by heating the deposited metal species to obtain a different solid material and a gaseous material. In general, as is known, thermal decomposition of halides, sulfates, carbonates, nitrates, nitrites, oxalates, acetates, and formates may be carried out at temperatures between about 200 and about 1,200° C.
If conversion of the deposited metal species to the final desired form is to occur, the deposited metal species is usually selected such that any unwanted by-products from the conversion can be removed from the final product. For example, during thermal decomposition the unwanted decomposition products are typically volatilized. To yield a final product that is a metal alloy, the deposited metal species are typically selected so that the powder comprising the deposited metal species may be reduced without significantly altering the uniformity of the metal deposits on the surface of the supports and/or without significantly altering the particle size of the final powder (e.g., through agglomeration).
Nearly any metal may be deposited onto supports by one or more of the processes noted herein, provided that the metal or compound containing the metal is capable of being dissolved in a suitable medium (i.e., a solvent). Likewise, nearly any metal may be combined with, or alloyed with, any other metal provided the metals or metal-containing compounds are soluble in a suitable medium.
The solute portion may comprise an organometallic compound and/or an inorganic metal-containing compound as a source of the metal species being deposited. In general, organometallic compounds are more costly, may contain more impurities than inorganic metal-containing compounds, and may require organic solvents. Organic solvents are more costly than water and typically require procedures and/or treatments to control purity or negate toxicity. As such, organometallic compounds and organic solvents are generally not preferred. Examples of appropriate inorganic salts include Cu(NO 3 ) 2 .2.5H 2 O, and (NH 4 ) 10 W 12 O 41 .5H 2 O or (NH 4 ) 6 W 12 O 39 .5H 2 O. Such salts are highly soluble in water and, as a result, water is often considered to be a preferred solvent. In some instances, it is desirable for an inorganic metal-containing compound to be dissolved in an acidic solution prior to being mixed with other inorganic metal-containing compounds.
To form a catalyst alloy, or catalyst precursor alloy, having a particular composition or stoichiometry, the amounts of the various metal-containing source compounds necessary to achieve that composition are determined in view thereof. If the supports have a pre-deposited metal, the loading of the pre-deposited metal on the supports is typically taken into account when calculating the necessary amounts of metal-containing source compounds. After the appropriate amounts of the metal-containing compounds are determined, the solution may be prepared by any appropriate method. For example, if all the selected metal-containing source compounds are soluble at the desired concentration in the same solvent at room temperature, they may merely be mixed with the solvent. Alternatively, the suspending solution may be formed by mixing source solutions, wherein a source solution comprises a particular metal-containing source compound at a particular concentration. If, however, all of the selected compounds are not soluble at the same temperature when mixed together (either as powders in a solvent or as source solutions), the temperature of the mixture may be increased to increase the solubility limit of one or more of the source compounds so that the suspending solution may be formed. In addition to adjusting solubility with temperature, the stability of the suspending solution may be adjusted, for example, by the addition of a buffer, by the addition of a complexing agent, and/or by adjusting the pH.
In addition to varying the amounts of the various metals to form alloys having different compositions, this method allows for a wide variation in the loading of the metal onto the supports. This is beneficial because it allows for the activity of a supported catalyst composition (e.g., an electrocatalyst powder) to be maximized. The loading may be controlled in part by adjusting the total concentration of the various metals in the solution while maintaining the relative amounts of the various metals. In fact, the concentrations of the inorganic metal-containing compounds may approach the solubility limit for the solution. Typically, however, the total concentration of inorganic metal-containing compounds in the solution is between about 0.01 M and about 5 M, which is well below the solubility limit. In one embodiment, the total concentration of inorganic metal-containing compounds in the solution is between about 0.1 M and about 1 M. Concentrations below the solubility limit are used because it is desirable to maximize the loading of the supported catalysts without decreasing the surface area of the metal deposits. Depending, for example, on the particular composition, the size of the deposits, and the uniformity of the distribution of deposits on the supports, the loading may typically be between about 5 and about 60 weight percent. In one embodiment, the loading is between about 15 and about 45 or about 55 weight percent, or between about 20 and about 40 or about 50 weight percent. In another embodiment, the loading is about 20 weight percent, about 40 weight percent, or about 50 weight percent.
The supports upon which the metal species (e.g., metal-containing compound) is to be deposited may be of any size and composition that is capable of being dispersed/suspended in the solution during the removal of heat to precipitate the metal species thereon. The maximum size depends on several parameters including agitation of the suspension, density of the supports, specific gravity of the solution, and the rate at which heat is removed from the system. In general, the supports are electrically conductive and are useful for supporting catalytic compounds in fuel cells. Such electrically conductive supports are typically inorganic, for example, carbon supports. However, the electrically conductive supports may comprise an organic material such as an electrically conductive polymer (see, e.g., in U.S. Pat. No. 6,730,350). Carbon supports may be predominantly amorphous or graphitic and they may be prepared commercially, or specifically treated to increase their graphitic nature (e.g., heat treated at a high temperature in vacuum or in an inert gas atmosphere) thereby increasing corrosion resistance. Carbon black support particles may have a Brunauer, Emmett and Teller (BET) surface area up to about 2000 m 2 /g. It has been reported that satisfactory results are achieved using carbon black support particles having a high mesoporous area, e.g., greater than about 75 m 2 /g (see, e.g., Catalysis for Low Temperature Fuel Cells Part 1: The Cathode Challenges, T. R. Ralph and M. P. Hogarth, Platinum Metals Rev., 2002, 46, (1), p. 3-14). Experimental results to-date indicate that a surface area of about 500 m 2 /g is preferred.
In another embodiment, the supports may have a pre-deposited material thereon. For example, when the final composition of the deposits on the carbon supports is a platinum alloy, it may be advantageous to use a carbon supported platinum powder. Such powders are commercially available from companies such as Johnson Matthey, Inc., of New Jersey and E-Tek Div. of De-Nora, N.A., Inc., of Somerset, N.J. and may be selected to have a particular loading of platinum. The amount of platinum loading is selected in order to achieve the desired stoichiometry of the supported metal alloy. Typically, the loading of platinum is between about 5 and about 60 weight percent. Preferably, the loading of platinum is between about 15 and 45 weight percent. The size (i.e., the maximum cross-sectional length) of the platinum deposits is typically less than about 20 nm. For example, the size of the platinum deposits may be less than about 10 nm, 5 nm, 2 nm, or smaller; alternatively, the size of the platinum deposits may be between about 2 and about 3 nm. Experimental results to-date indicate that a desirable supported platinum powder may be further characterized by having a platinum surface area of between about 150 and about 170 m 2 /g (determined by CO adsorption), a combined carbon and platinum surface area of between about 350 and about 400 m 2 /g (determined by N 2 adsorption), and an average support size that is between about 100 and about 300 nm.
The solution and supports are mixed according to any appropriate method to form the dispersion/suspension, using means known in the art. Exemplary methods of mixing include magnetic stirring, insertion of a stirring structure or apparatus (e.g., a rotor), shaking, sonication, or a combination of the foregoing methods. Provided that the supports can be adequately mixed with the solution, the relative amounts of supports and solution may vary over a wide range. For example, when preparing carbon supported catalysts using an aqueous suspension comprising dissolved inorganic metal-containing compounds, the carbon supports typically comprise between about 1 and about 30 weight percent of the suspension. Preferably, however, the carbon supports comprise between about 1 and about 15 weight percent of the suspension, between about 1 and about 10 weight percent of the suspension, between about 3 and about 8 weight percent of the suspension, between about 5 and about 7 weight percent of the suspension, or about 6 weight percent of the suspension.
In this regard it is to be noted that the above-referenced amounts of carbon supports in suspension may apply equally to other, non-carbon supports noted herein, or which are known in the art.
The relative amounts of supports and solution may also be described in terms of volumetric ratios. For example, the dispersion/suspension may have a volumetric ratio of support particles to solution or solvent that is at least about 1:10; that is, the dispersion/suspension may have a volume of solution or solvent that is no more than about 10 times the volume of support particles therein.
In this regard it is to be noted that specifying a minimum volumetric ratio indicates that the volume of support particles may be increased relative to the volume of solution or solvent. As such, the volumetric ratio of support particles to solution or solvent may more preferably be at least about 1:8, about 1:5, or even about 1:2. The volumetric ratio may therefore range, for example, from about 1:2 to about 1:10, or from about 1:5 to about 1:8.
In one method of preparation, the solution and supports described or illustrated herein are mixed using sonication at a power and for a duration sufficient to form a dispersion/suspension in which the pores of the supports are impregnated with the solution and/or the supports are uniformly distributed throughout the solution. If the dispersion/suspension is not uniformly mixed (i.e., the supports are not uniformly impregnated with the solution and/or the supports are not uniformly distributed throughout the solution), the deposits formed on the supports will typically be non-uniform (e.g., the loading of the metal species may vary among the supports, the size of the deposits may vary significantly on a support and/or among the supports, and/or the composition of the deposits may vary among the supports). Although a uniform mixture, or distribution of supports in the solution, is generally preferred, there may be circumstances in which a non-uniform mixture, or distribution of supports in the solution, is desirable.
When a freeze-drying method of preparation is employed, typically the uniformity of the distribution of particles in the dispersion/suspension is maintained throughout the removal of heat therefrom. This uniformity may be maintained by continuing the mixing of the dispersion/suspension as it is being cooled. The uniformity may, however, be maintained without mixing by the viscosity of the dispersion/suspension. The actual viscosity needed to uniformly suspend the support particles depends in large part on the amount of support particle in the dispersion/suspension and the size of the support particles. To a lesser degree, the necessary viscosity depends on the density of the support particles and the specific gravity of the solution. In general, the viscosity is typically sufficient to prevent substantial settling of the support particles as the heat is being removed from the suspension to precipitate the deposits, and/or, if desired, until the dispersion/suspension is solidified by the freezing of the solution or solvent. The degree of settling, if any, may be determined, for example, by examining portions of the solidified or frozen suspension. Typically, substantial settling would be considered to have occurred if the concentration of supports in any two portions varies by more than about ±10%. When preparing a carbon supported catalyst powder, or precursors thereto, in accordance with the freeze-drying method, the viscosity of the suspension/dispersion is typically sufficient to prevent substantial settling for at least about 4 minutes. In fact, the viscosity of the suspension/dispersion may be sufficient to prevent substantial settling for at least about 10 minutes, at least about 30 minutes, at least about 1 hour, at least about 6 hours, at least about 12 hours, at least about 18 hours, or even up to about 2 days. Typically, the viscosity of the dispersion/suspension is at least about 5,000 mPa·s.
Heat is removed from the dispersion/suspension so that at least a part of the solute portion separates from the solvent portion and deposits (e.g., precipitates) a metal species/precipitated metal onto the supports and/or onto any pre-existing deposits (e.g., a pre-deposited metal and/or pre-deposited metal species formed, for example, by precipitation of incompatible solutes). If the concentration of supports in the suspension is sufficient (e.g., within the ranges set forth above) and enough heat is removed, nearly all of the metal species to be deposited is separated from the solvent portion to form deposits (e.g., precipitates) comprising the metal species on the supports. In one embodiment, the heat is removed to solidify or freeze the dispersion/suspension and form a composite comprising the supports/particulate support with deposits comprising the metal species or a precipitated metal on the supports/particulate support, within a matrix of the solvent portion in a solid state. If the concentration of the solute portion in the solution exceeds the ability of the supports to accommodate deposits of the metal species, some of the solute portion may crystallize within the matrix. If this occurs, such crystals are not considered to be a supported powder.
In one embodiment of the present invention, the size of the deposits of the metal species is controlled such that the eventually formed deposits of the catalyst composition alloy, or precursor thereto, are of a size suitable for use as a fuel cell catalyst (e.g., no greater than about 20 nm, about 10 nm, about 5 nm (50 Å), about 3 nm (30 Å), about 2 (20 Å) nm, in size or smaller). As set forth above, control of the alloy deposit size may be accomplished, at least in part, by maintaining a well-impregnated and uniformly distributed suspension throughout the removal of heat from the system. Additionally, the control of the deposit size may be accomplished by rapidly removing heat from the dispersion/suspension as the compound or compounds are depositing on supports.
The rapid heat removal may be accomplished by cooling the dispersion/suspension from a temperature of at least about 20° C. to a temperature below the freezing point of the solvent at a rate of, for example, at least about 20 ° C./minute. In order of increasing preference, heat removal may comprise cooling the dispersion/suspension at a rate of at least about 50, 60, 70, 80, 90, or 100 ° C./minute. As such, the dispersion/suspension may be cooled at a rate that is between about 50 and about 100° C./minute, or at a rate that is between about 60 and about 80° C./minute. Typically, removal of heat is at a rate that allows for the temperature of the suspension to be reduced from a temperature such as room temperature (about 20° C.) or higher (e.g., about 100° C.) to the freezing point of the solution or solvent within a relatively short period of time (e.g., not more than about 10, 5, or 3 minutes).
The heat may be removed from the dispersion/suspension by any appropriate method. For example, a container containing a volume of the dispersion/suspension may be placed within a refrigeration unit such as freeze-dryer, a volume of dispersion/suspension may be contacted with a cooled surface (e.g., a plate or container), a volume of dispersion/suspension in a container may be immersed in, or otherwise contacted with, a cryogenic liquid. Advantageously, the same container may also be used during the formation of the dispersion and/or during the separation of solvent from deposited supports. In one embodiment a cover is placed over an opening of the container. Although the cover may completely prevent the escape of any solid matter from the container, the cover preferably allows for a gas to exit the container while substantially preventing the supports from exiting the container. An example of such a cover includes a stretchable film (e.g., PARAFILM) having holes that are, for example, less than about 500, 400, or 300 μm in size (maximum length across the hole).
In one embodiment the dispersion/suspension is cooled at a rate of at least about 20° C./minute by immersing or contacting a container containing the dispersion/suspension in or with a volume of cryogenic liquid within a cryogenic container sized and shaped so that at least a substantial portion of its surface is contacted with the cryogenic liquid (e.g., at least about 50, 60, 70, 80, or 90 percent of the surface of the dispersion/suspension container). The cryogenic liquid is typically at a temperature that is at least about 20° C. below the freezing point of the solvent. Examples of suitable cryogenic liquids typically include liquid nitrogen, liquid helium, liquid argon, but even less costly media may be utilized (for example, an ice water/hydrous calcium chloride mixture can reach temperatures down to about −55° C., an acetone/dry ice mixture can reach temperatures down to about −78° C., and a diethyl ether/dry ice mixture can reach temperatures down to about −100° C.).
The container may be made of nearly any type of material. Generally, the selected material does not require special handling procedures, can withstand repeated uses without structural failure (e.g., resistant to thermal shock), does not contribute impurities to the suspension (e.g., resistant to chemical attack), and is thermally conductive. For example, plastic vials made from high density polyethylene may be used.
The supports having the deposits thereon may be separated from the solvent portion by any appropriate method such as filtration, evaporation (e.g., by spray-drying), sublimation (e.g., freeze-drying), or a combination thereof. The evaporation or sublimation rate may be enhanced by adding heat (e.g., raising the temperature of the solvent) and/or decreasing the atmospheric pressure to which the solvent is exposed.
In one embodiment a frozen or solidified suspension is freeze-dried to remove the solvent portion therefrom. The freeze-drying may be carried out in any appropriate apparatus, such as a LABCONCO FREEZE DRY SYSTEM (Model 79480). Intuitively, one of skill in the art would typically maintain the temperature of the frozen suspension below the melting point of the solvent (i.e., the solvent is removed by sublimation), in order to prevent agglomeration of the supports. The freeze-drying process described or illustrated herein may be carried out under such conditions. Surprisingly, however, it is not critical that the solvent portion remain fully frozen. Specifically, it has been discovered that a free-flowing, non-agglomerated powder may be prepared even if the solvent is allowed to melt, provided that the pressure within the freeze-dryer is maintained at a level that the evaporation rate of the liquid solvent is faster than the melting rate (e.g., below about 0.2 millibar, 0.000197 atm, or 20 Pa). Thus, there is typically not enough solvent in the liquid state to result in agglomeration of the supports. Advantageously, this can be used to decrease the time needed to remove the solvent portion. Removing the solvent portion results in a free-flowing, non-agglomerated supported powder that comprises the supports/particulate support and deposits comprising one or more metal species or precipitated metals on the supports/particulate support.
To accomplish the conversion of the deposited compound to the desired form of the metal therein, the powder is typically heated in a reducing atmosphere (e.g., an atmosphere containing hydrogen and/or an inert gas such as argon) at a temperature sufficient to decompose the deposited compound. The temperature reached during the thermal treatment is typically at least as high as the decomposition temperature(s) for the deposited compound(s) and not so high as to result in degradation of the supports and agglomeration of the supports and/or the catalyst deposits. Typically, the temperature is between about 60° C. and about 1100° C., between about 100 and about 1000° C., between about 200 and about 800 ° C., or between about 400 and about 600° C. Inorganic metal-containing compounds typically decompose at temperatures between about 600 and 1000° C.
The duration of the heat treatment is typically at least sufficient to substantially convert the deposited compounds to the desired state. In general, the temperature and time are inversely related (i.e., conversion is accomplished in a shorter period of time at higher temperatures and vice versa). At the temperatures typical for converting the inorganic metal-containing compounds to an alloy set forth above, the duration of the heat treatment is typically at least about 30 minutes (e.g., about 1, 2, 4, 6, 8, 10, 12 hours, or longer). For example, the duration may be between about 1 and about 14 hours, about 2 and about 12 hours, or between about 4 and about 6 hours.
Referring to FIG. 1, a carbon supported catalyst alloy powder particle 1 of the present invention, produced in accordance with the freeze-drying method described or illustrated herein, comprises a carbon support 2 and deposits 3 of the catalyst alloy on the support. A particle and a powder comprising said particles may have a loading that is up to about 90 weight percent. However, when a supported catalyst powder is used as a fuel cell catalyst, the loading is typically between about 5 and about 60 weight percent, and is preferably between about 15 and about 45 or about 55 weight percent, or more preferably between about 20 and about 40 or 50 weight percent (e.g., about 20 weight percent, 45 weight percent, or about 50 weight percent). Increasing the loading to greater than about 60 weight percent does not typically result in an increase in the activity. Without being held to a particular theory, it is believed that excess loading covers a portion of the deposited metal and the covered portion cannot catalyze the desired electrochemical reaction. On the other hand, the activity of the supported catalyst typically decreases significantly if the loading is below about 5 weight percent.
The freeze-dry method may be used to produce supported catalyst alloy powders that are heavily loaded with nanoparticle deposits of a catalyst alloy that comprises one or more non-noble metals, wherein the deposits have a relatively narrow size distribution. For example, in one embodiment the supported non-noble metal-containing catalyst alloy powder may have a metal loading of at least about 20 weight percent of the powder, an average deposit size that is no greater than about 10 nm, and a deposit size distribution in which at least about 70 percent of the deposits are within about 50 and 150 percent of the average deposit size. In another embodiment, the metal loading may preferably be between about 20 and about 60 weight percent, and more preferably between about 20 and about 40 weight percent.
The average size of the catalyst alloy deposits is typically no greater than about 5 nm (50 Å). Preferably, however, the average size of the catalyst alloy deposits is no greater than about 3 nm (30 Å), 2 nm (20 Å), or even 1 nm (10 Å). Alternatively, however, the average size of the metal alloy deposits may preferably be between about 3 nm and about 10 nm, or between about 5 nm and about 10 nm. Additionally, the size distribution of the deposits is preferably such that at least about 80 percent of the deposits are within about 75 and 125 percent of the average deposit size.
The freeze-dry method of preparing supported catalyst powders allows for improved control of the stoichiometry of the deposits because the suspension is preferably kept within a single container, the solution is not physically separated from the supports (e.g., by filtration), and freezing results in substantially all of the solute precipitating on the supports. Additionally, the deposits tend to be isolated, small, and uniformly dispersed over the surface of the supports, thereby increasing the overall catalytic activity. Still further, because filtering is not necessary, extremely fine particles are not lost and the supported catalyst powders produced by this method tend to have a greater surface area and activity. Also, the act of depositing the metal species on the supports is fast. For example, immersing a container of the dispersion/suspension in a cryogenic liquid may solidify the dispersion/suspension in about three to four minutes.
4. Unsupported Catalyst Compositions in Electrode/Fuel Cell Applications
It is to be noted that, in another embodiment of the present invention, a catalyst composition (e.g., the catalyst composition comprising or consisting essentially of an alloy of the metal components), and/or the precursor thereto, may be unsupported; that is, a catalyst composition as set forth herein may be employed in the absence of support particles. More specifically, it is to be noted that in another embodiment of the present invention a catalyst composition comprising platinum, copper and tungsten, as defined herein, may be directly deposited (e.g., sputtered) onto, for example: (i) a surface of one or both of the electrodes (e.g., the anode, the cathode or both), and/or (ii) one or both surfaces of a polyelectrolyte membrane, and/or (iii) some other surface, such as a backing for the membrane (e.g., carbon paper).
In this regard it is to be further noted that each constituent (e.g., metal-containing compound) of the composition may be deposited separately, each for example as a separate layer on the surface of the electrode, membrane, etc. Alternatively, two or more constituents may be deposited at the same time. Additionally, when the composition comprises or consists essentially of an alloy of these metals, the alloy may be formed and then deposited, or the constituents thereof may be deposited and then the alloy subsequently formed thereon.
Deposition of the constituent(s) may be achieved using means known in the art, including for example known sputtering techniques (see, e.g., PCT Application No. WO 99/16137, or U.S. Pat. No. 6,171,721 which is incorporated herein by reference). Generally speaking, however, in one approach sputter-deposition is achieved by creating, within a vacuum chamber in an inert atmosphere, a voltage differential between a target component material and the surface onto which the target constituent is to be deposited, in order to dislodge particles from the target constituent material which are then attached to the surface of, for example, an electrode or electrolyte membrane, thus forming a coating of the target constituent thereon. In one embodiment, the constituents are deposited on a polymeric electrolyte membrane, including for example (i) a copolymer membrane of tetrafluoroethylene and perfluoropolyether sulfonic acid (such as the membrane material sold under the trademark NAFION™), (ii) a perfluorinated sulfonic acid polymer (such as the membrane material sold under the trademark ACIPLEX), (iii) polyethylene sulfonic acid polymers, (iv) polyketone sulfonic acids, (v) polybenzimidazole doped with phosphoric acid, (vi) sulfonated polyether sulfones, and (vii) other polyhydrocarbon-based sulfonic acid polymers.
It is to be noted that the specific amount of each metal or constituent of the composition may be controlled independently, in order to tailor the composition to a given application. In some embodiments, however, the amount of each deposited constituent, or alternatively the amount of the deposited catalyst (e.g., catalyst alloy), may be less than about 5 mg/cm 2 of surface area (e.g., electrode surface area, membrane surface area, etc.), less than about 1 mg/cm 2 , less than about 0.5 mg/cm 2 , less than about 0.1 mg/cm 2 , or even less than about 0.05 mg/cm 2 . In other embodiments, the amount of the deposited constituent, or alternatively the amount of the deposited catalyst (e.g., catalyst alloy), may range from about 0.5 mg/cm 2 to less than about 5 mg/cm 2 , or from about 0.1 mg/cm 2 to less than about 1 mg/cm 2 .
It is to be further noted that the specific amount of each constituent, or the composition, and/or the conditions under which the constituent, or composition, are deposited, may be controlled in order to control the resulting thickness of the constituent, or composition, layer on the surface of the electrode, electrolyte membrane, etc. For example, as determined by means known in the art (e.g., scanning electron microscopy or Rutherford back scattering spectrophotometric method), the deposited layer of the constituent or composition may have a thickness ranging from several angstroms (e.g., about 2, 4, 6, 8, 10 Å or more) to several tens of angstroms (e.g., about 20, 40, 60, 80, 100 Å or more), up to several hundred angstroms (e.g., about 200, 300, 400, 500 Å or more). Additionally, after all of the constituents have been deposited, and optionally alloyed (or, alternatively, after the composition has been deposited, and optionally alloyed), the layer of the composition of the present invention may have a thickness ranging from several tens of angstroms (e.g., about 20, 40, 60, 80, 100 Å or more), up to several hundred angstroms (e.g., about 200, 400, 600, 800, 1000, 1500 Å or more). Thus, in different embodiments the thickness may be, for example, between about 10 and about 500 angstroms (Å), between about 20 and about 200 angstroms (Å), and between about 40 and about 100 angstroms (Å).
It is to be still further noted that in embodiments wherein a composition (or the constituents thereof) is deposited as a thin film on the surface of, for example, an electrode or electrolyte membrane, the various concentrations of platinum, copper and tungsten therein may be as previously described herein. Additionally, in other embodiments, the concentration of platinum, copper and tungsten in the composition may be other than as previously described.
5. Incorporation of the Composition in a Fuel Cell
The compositions of the present invention are particularly suited for use as catalysts in proton exchange membrane fuel cells. As shown in FIGS. 2 and 3, a fuel cell, generally indicated at 20 , comprises a fuel electrode (anode) 22 and an air electrode/oxidizer electrode (cathode) 23 . In between the electrodes 22 and 23 , a proton exchange membrane 21 serves as an electrolyte and is usually a strongly acidic ion exchange membrane, such as a perfluorosulphonic acid-based membrane. Preferably, the proton exchange membrane 21 , the anode 22 , and the cathode 23 are integrated into one body to minimize contact resistance between the electrodes and the proton exchange membrane. Current collectors 24 and 25 engage the anode and the cathode, respectively. A fuel chamber 28 and an air chamber 29 contain the respective reactants and are sealed by sealants 26 and 27 , respectively.
In general, electricity is generated by hydrogen-containing fuel combustion (i.e., the hydrogen-containing fuel and oxygen react to form water, carbon dioxide and electricity). This is accomplished in the above-described fuel cell by introducing the hydrogen-containing fuel F into the fuel chamber 28 , while oxygen O (preferably air) is introduced into the air chamber 29 , whereby an electric current can be immediately transferred between the current collectors 24 and 25 through an outer circuit (not shown). Ideally, the hydrogen-containing fuel is oxidized at the anode 22 to produce hydrogen ions, electrons, and possibly carbon dioxide gas. The hydrogen ions migrate through the strongly acidic proton exchange membrane 21 and react with oxygen and electrons transferred through the outer circuit to the cathode 23 to form water. If the hydrogen-containing fuel F is methanol, it is preferably introduced as a dilute acidic solution to enhance the chemical reaction, thereby increasing power output (e.g., a 0.5 M methanol/0.5 M sulfuric acid solution).
To prevent the loss of ionic conduction in the proton exchange membranes, these typically remain hydrated during operation of the fuel cell. As a result, the material of the proton exchange membrane is typically selected to be resistant to dehydration at temperatures up to between about 100 and about 120° C. Proton exchange membranes usually have reduction and oxidation stability, resistance to acid and hydrolysis, sufficiently low electrical resistivity (e.g., <10 Ω·cm), and low hydrogen or oxygen permeation. Additionally, proton exchange membranes are usually hydrophilic. This ensures proton conduction (by reversed diffusion of water to the anode), and prevents the membrane from drying out thereby reducing the electrical conductivity. For the sake of convenience, the layer thickness of the membranes is typically between 50 and 200 μm. In general, the foregoing properties are achieved with materials that have no aliphatic hydrogen-carbon bonds, which, for example, are achieved by replacing hydrogen with fluorine or by the presence of aromatic structures; the proton conduction results from the incorporation of sulfonic acid groups (high acid strength). Suitable proton-conducting membranes also include perfluorinated sulfonated polymers such as NAFION™ and its derivatives produced by E. I. du Pont de Nemours & Co., Wilmington, Del. NAFION™ is based on a copolymer made from tetrafluoroethylene and perfluorovinylether, and is provided with sulfonic groups working as ion-exchanging groups. Other suitable proton exchange membranes are produced with monomers such as perfluorinated compounds (e.g., octafluorocyclobutane and perfluorobenzene), or even monomers with C—H bonds that do not form any aliphatic H atoms in a plasma polymer, which could constitute attack sites for oxidative breakdown.
The electrodes of the present invention comprise the catalyst compositions of the present invention and an electrode substrate upon which the catalyst is deposited. In one embodiment, the composition is directly deposited on the electrode substrate. In another embodiment, the composition is supported on electrically conductive supports and the supported composition is deposited on the electrode substrate. The electrode may also comprise a proton conductive material that is in contact with the composition. The proton conductive material may facilitate contact between the electrolyte and the composition, and may thus enhance fuel cell performance. Preferably, the electrode is designed to increase cell efficiency by enhancing contact between the reactant (i.e., fuel or oxygen), the electrolyte and the composition. In particular, porous or gas diffusion electrodes are typically used since they allow the fuel/oxidizer to enter the electrode from the face of the electrode exposed to the reactant gas stream (back face), and the electrolyte to penetrate through the face of the electrode exposed to the electrolyte (front face), and reaction products, particularly water, to diffuse out of the electrode.
Preferably, the proton exchange membrane, electrodes, and catalyst composition are in contact with each other. This is typically accomplished by depositing the composition either on the electrode, or on the proton exchange membrane, and then placing the electrode and membrane in contact. The composition of this invention can be deposited on either the electrode or the membrane by a variety of methods, including plasma deposition, powder application (the powder may also be in the form of a slurry, a paste, or an ink), chemical plating, and sputtering. Plasma deposition generally entails depositing a thin layer (e.g., between 3 and 50 μm, preferably between 5 and 20 μm) of a catalyst composition on the membrane using low-pressure plasma. By way of example, an organic platinum compound such as trimethylcyclopentadienyl-platinum is gaseous between 10 −4 and 10 mbar and can be excited using radio-frequency, microwaves, or an electron cyclotron resonance transmitter to deposit platinum on the membrane. According to another procedure, a catalyst powder, for example, is distributed onto the proton exchange membrane surface and integrated at an elevated temperature under pressure. If, however, the amount of catalyst powder exceeds about 2 mg/cm 2 , the inclusion of a binder such as polytetrafluoroethylene is common. Further, the catalyst may be plated onto dispersed small support particles (e.g., the size is typically between 20 and 200 Å, and more preferably between about 20 and 100 Å). This increases the catalyst surface area, which in turn increases the number of reaction sites leading to improved cell efficiency. In one such chemical plating process, for example, a powdery carrier material such as conductive carbon black is contacted with an aqueous solution or aqueous suspension (slurry) of compounds of metallic components constituting the alloy to permit adsorption or impregnation of the metallic compounds or their ions on or in the carrier. Then, while the slurry is stirred at high speed, a dilute solution of suitable fixing agent such as ammonia, hydrazine, formic acid, or formalin is slowly added drop-wise to disperse and deposit the metallic components on the carrier as insoluble compounds or partly reduced fine metal particles.
The loading, or surface concentration, of a composition on the membrane or electrode is based in part on the desired power output and cost for a particular fuel cell. In general, power output increases with increasing concentration; however, there is a level beyond which performance is not improved. Likewise, the cost of a fuel cell increases with increasing concentration. Thus, the surface concentration of composition is selected to meet the application requirements. For example, a fuel cell designed to meet the requirements of a demanding application such as an extraterrestrial vehicle will usually have a surface concentration of the composition sufficient to maximize the fuel cell power output. For less demanding applications, economic considerations dictate that the desired power output be attained with as little of the composition as possible. Typically, the loading of composition is between about 0.01 and about 6 mg/cm 2 . Experimental results to-date indicate that in some embodiments the composition loading is preferably less than about 1 mg/cm 2 , and more preferably between about 0.1 and 1 mg/cm 2 .
To promote contact between the collector, electrode, composition and membrane, the layers are usually compressed at high temperature. The housings of the individual fuel cells are configured in such a way that a good gas supply is ensured, and at the same time the product water can be discharged properly. Typically, several fuel cells are joined to form stacks, so that the total power output is increased to economically feasible levels.
In general, the catalyst compositions and fuel cell electrodes of the present invention may be used to electrocatalyze any fuel containing hydrogen (e.g., hydrogen and reformed-hydrogen fuels). Also, hydrocarbon-based fuels may be used including: saturated hydrocarbons, such as methane (natural gas), ethane, propane and butane; garbage off-gas; oxygenated hydrocarbons, such as methanol and ethanol; fossil fuels, such as gasoline and kerosene; and, mixtures thereof.
To achieve the full ion-conducting property of proton exchange membranes, in some embodiments suitable acids (gases or liquids) are typically added to the fuel. For example, SO 2 , SO