WO1999053557A1

WO1999053557A1 - Improved composition of a selective oxidation catalyst for use in fuel cells

Info

Publication number: WO1999053557A1
Application number: PCT/EP1999/002527
Authority: WO
Inventors: James R. Giallombardo; Emory S. De Castro; Robert J. Allen
Original assignee: De Nora Elettrodi S.P.A.
Priority date: 1998-04-14
Filing date: 1999-04-14
Publication date: 1999-10-21
Also published as: BR9909616A; EP1078406A1; CN1299523A; AU3816899A; CA2327769A1; KR20010071152A; JP2002511639A; US6379834B1; AU745966B2; CN1189966C; US6165636A; KR100431019B1

Abstract

Formulations of platinum - molybdenum alloys for use as anode catalysts. These electrocatalysts find utility as a constituent of gas diffusion electrodes for use in fuel cells that operate at less than 180 °C or in applications whereupon hydrogen is oxidized in the presence of carbon monoxide or other platinum inhibiting substances. The new formulations derive unexpected activity through creating highly dispersed alloy particles of up to approximately 300 Å on carbon supports. The desired activity is achieved by carefully controlling the platinum to molybdenum ratio during preparation and judiciously selecting a proper loading of alloy on the carbon support.

Description

IMPROVED COMPOSITION OF A SELECTIVE OXIDATION CATALYST

FOR USE IN FUEL CELLS

STATE OF THE ART

As mankind expands his presence and activity throughout the world, he is often

limited by the availability of electrical energy to support his endeavors. Fuel

Cells offer one solution to this dilemma by directly deriving electricity from

chemical feedstocks such as oxygen and hydrogen. The Fuel Cell approach also

offers the potential to reduce pollution problems inherent in direct combustion

technology. Applications for Fuel Cells include power for vehicular traction,

stationary power for home and industry, and power supplies for marine use.

However, pure hydrogen fuel is not always available, and the development of

distribution means for hydrogen is uncertain.

In order for the Fuel Cell technology to realize the potential as a generic energy

source, flexibility in the choice of fuel is needed. Large-scale technology such as

Solid Oxide Fuel Cells (SOFC) and Phosphoric Acid Fuel Cells (PAFC) achieve

some feed flexibility by operating at high temperatures, and thus "burn" some of

the anode contaminants mat typically result from deriving hydrogen from carbon-

containing feedstocks such as methane or propane. Both PAFC and SOFC

technology are not amenable to the smaller scales (approximately <200Kwatts)

envisioned for automotive, and other applications cited above.

The Polymer Electrolyte Membrane Fuel Cell (PEMFC) is often cited as the

appropriate energy source for applications requiring less than around 200 kWatts,

and also for devices needing as little as a few hundred watts. This class of fuel

cell operates at less than 180^°C, and more typically around 70^°C due to the 2 limitations in the stability of the polymer electrolyte membrane. There is great

enthusiasm behind the PEMFC approach based on this system's lack of liquid

electrolyte, ease of construction, and high specific power as a function of volume

or mass.

In order to impart some fuel flexibility for the PEMFC, an additional fuel-

reforming component is needed. The "reformer" converts hydrogen-containing

substances such as methane, propane, methanol, ethanol, and gasoline into

hydrogen gas, carbon monoxide, and carbon dioxide through either a steam

reformation reaction, partial oxidation, or a combination of both. Reformer

technology has now advanced to the state whereby commercially units are

available. For example, a newly formed company Epyx (Acorn Park, Cambridge,

MA) offers a fuel processor that converts gasoline into hydrogen. Johnson

Matthey PLC (London, UK) offers a HotSpot™ fuel processor that converts

methanol using a combination of steam reforming and partial oxidation. For both

these technologies, the untreated output is hydrogen and approximately 1-2%

carbon monoxide. Through additional clean-up, the carbon monoxide can be

reduced to around 50 ppm or less.

Platinum has long been acknowledged as the best anode catalyst for hydrogen.

Early fuel cells employed particles of platinum black mixed with a binder as a

component in gas diffusion electrodes. The use of platinum black for hydrogen

has been largely supplemented by the highly disperse and very active catalysts

created by the methods similar to that found in Petrow and Allen, U.S. Patent 3 4,082,699. This patent teaches the use of using finely divided carbon particles

such as carbon black as the substrate for small (tens of angstroms) particles of the

noble metal. Thus called a "supported" catalyst, this methodology has shown

superior performance and utilization of the catalyst in electrochemical

applications. However, while supported platinum catalysts have demonstrated

high activity for hydrogen oxidation, this proclivity for facile kinetics is severely

retarded with carbon monoxide concentrations of only a few ppm.

Thus, with a fuel processor technology producing hydrogen streams containing

around 50 ppm CO and platinum-based gas diffusion anodes being poisoned

slowly with as little as 1 ppm, there is a clear need for a CO tolerant catalyst.

The current state-of-the-art CO tolerant electrocatalyst is a platinum ruthenium

bimetallic alloy (Pt:Ru) and is available commercially in supported form (E-TEK,

Inc., Natick, MA). The mechanism for CO tolerance is believed to involve the

nucleation of oxygen containing species (OH_aa_s) on the ruthenium site such that

platinum-adsorbed CO can participate in a bimolecular reaction with the

activated oxygen thereby freeing the platinum site for hydrogen oxidation.

However, the ruthenium site is also prone to poisoning by CO at higher

concentrations of CO, and the important nucleation of oxygen containing species

is then inhibited (H.A. Gasteiger, N.M. Markovic, and P.N. Ross; J. Physical

Chemistry, Vol. 99, No. 22, 1995, p 8945). Although Pt:Ru has been optimized

and thoroughly studied to show that an alloy composed of Pt:Ru in the atomic

ratio of 1: 1 yields the best tolerance to CO, this bimetallic catalyst functions only 4 at around 10 ppm CO or less because of the eventual poisoning of the ruthenium

site.

A recent monograph reviewing bimetallic electrocatalysts has summarized

several important facts in the preparation and activity of electrocatalysts (P.N.

Ross: "The Science of Electrocatalysis on Bimetallic Surfaces," in Frontiers in

Electrochemistry. Vol. 4, J. Lipowski and P. N. Ross Jr., Wiley-Interscience,

New York, NY, 1997). The activity of a bimetallic catalyst is dependent on

electronic and structural effects. Electronic properties are determined by the

electron configuration of the alloying elements while structural properties are

determined by both the selection of alloying elements and the method of

preparation of the alloy itself. This last observation is important in the design of

CO tolerant catalysts. For example, a Pt:Ru alloy prepared by sputtering a bulk

alloy, annealing a bulk alloy, or depositing a submonolayer of ruthenium on

platinum all yield fundamentally different catalytic properties (P.N. Ross, p 19).

The precept that alloy formation methodology influences catalyst function

follows from the creation of three zones in every bimetallic catalyst: metal "A",

metal "B", and an intermixed zone "A-B". The distribution of these zones

deteπnines activity.

Another important property noted by Ross in the monograph is that the

phenomenon of surface segregation in bimetallic alloys has often been neglected.

Surface segregation is the enrichment of one element at the surface relative to the

bulk, and in our case would be dominated by platinum in an alloy of 4d elements 5 with the exception of silver and tin (Ross, p. 51).

In summary, there is ample evidence to show that electrocatalysts can differ in

their activity due to preparation methods. Another difference arises from

dissimilarities between the bulk and surface compositions of the alloy. For these

two reasons, we expect even greater contrasts to occur between bimetallic alloys

prepared as bulk metals compared to alloys prepared as very small (10 to 300 A)

supported particles.

Molybdenum has been observed to play a catalytic role in the oxidation of small

organic molecules otherwise known as "Cl" molecules (to designate one carbon

atom). As early as 1965, a molybdenum platinum black complex was implicated

in the catalytic oxidation of formaldehyde and methanol in sulfuric acid (J.A.

Shropshire; Journal of the Electrochemical Society, vol. 112, 1965, p. 465).

Although the molybdenum was added as a soluble salt, it was reduced and

deposited onto the platinum black electrode. Later on, several others took note of

this property of molybdenum and tried to intentionally create platinum alloys.

H. Kita et al. confirm that a platinum molybdenum complex formed through

reduction of the metal salt onto the surface of the platinum foil electrode can

catalyze methanol oxidation (H. Kita et al.; J. Electroanalytical Chemistry, vol.

248, 1988, p.181). H. Kita extended this work to creating a membrane electrode

assembly (MEA) of chemically deposited platinum and molybdenum on Nafion,

to be used in a PEMFC. As before, the fuel here is methanol (H. Kita et al.;

Electrochemistry In Transition. Oliver Murphy et al., Eds., Plenum Press, New 6 York, 1993, p. 619). These are both examples of forming an alloy through

deposition of a submonolayer of molybdenum onto platinum, although no high

surface area support is used.

Masahiro Watanabe discloses the use of vacuum sputtering to form an alloy of

Ni, Co, Mn or Au with Pt, Pd, or Ru. The object of this patent is to provide a CO

tolerant anode catalyst for the PEMFC (Masahiro Watanabe: Japan Patent

Application No. H6-225840, August 27, 1994). Although this patent directs

towards a preferred alloy consisting of Pt with Ni, Co, Mn, or Au, a comparison

example of Pt with Mo is shown whereby sustained currents for hydrogen

oxidation in the presence of CO dissolved in sulfuric acid are recorded. The

example employs a rotating disk electrode coated with an alloy formed by

simultaneous argon sputtering under reduced pressure. While the patent

emphasizes the use of sputter coating, some mention is made to carbon supported

alloys prepared by the usual thermal decomposition methods. However, there is

no description or teaching as to how the properties achieved in a sputter-coated

alloy could be obtained by thermal decomposition onto carbon black.

A recent publication indicates the potential for Pt:Mo as a CO tolerant catalyst

superior to Pt:Ru (B.N. Grgur et al.; Journal of Physical Chemistry (B), vol. 101,

no. 20, 1997, p. 3910). In this paper, a sample of Pt₇₅Mo₂₅ alloy is prepared as a

bulk crystal by arc melting of the pure elements in an argon atmosphere and

homogenizing with a heat treatment. The authors show that the resulting boule

possessed a uniform metal alloy composition from the interior bulk to the 7 surface. This well characterized surface is formed into a rotating electrode disk

and shows oxidation of hydrogen in a mixed gas of H₂/CO. The authors put forth

evidence that the molybdenum may participate in a greater rate of CO oxidation

compared to the ruthenium. Furthermore, the authors point out that ruthenium

and platinum do not differ much in that they both absorb H₂ and CO, possess

quasireversible OH_ads states, and are electrocatalysts for H₂ and CO: the alloying

process does not produce a fundamental change in the properties of either metal.

On the other hand, molybdenum is significantly different than platinum, and

formation of the alloy produces a material with substantial differences in the

intrinsic chemical properties. While the authors relate a surface with unexpected

catalytic properties, there is no mention of how one could translate the properties

discovered in this bulk alloy to the highly disperse carbon supported catalysts

employed in gas diffusion electrodes.

There has been some effort in the patent literature to create the supported Pt:Mo

alloy on carbon blacks. Landsman et al. in U.S. Patent 4,316,944 describe a

method to form noble metal chromium alloys on carbon black for eventual

incorporation into a cathode of a fuel cell. In this case, the inventors were

seeking superior oxygen reduction catalysts for use with PAFC. They make use

of a powder of already-dispersed platinum on metal and a solution of ammonium

chromate. The addition of dilute hydrochloric acid was added to cause the

adsorption of the chromium species on the supported catalyst. Heat treatment in

nitrogen was used to form the platinum chromium alloy. Although Pt:Mo 8 appears in a table of results as a cathode catalyst, no details are given to its

preparation, metal :metal ratio, or metal on carbon weight loading.

Thus, there is a need to show a method of preparation and formulation

requirements that preserve the unexpected CO tolerant properties of Pt:Mo on

carbon black supports that would then allow this alloy to be readily incorporated

into gas diffusion electrodes or membrane electrode assemblies (MEAs).

DESCRIPTION OF THE INVENTION

It is an object of this invention to provide an improved high-surface area

formulation of platinum:molybdenum on a carbon support whereby: the bulk

atomic ratio of Pt:Mo is between 99: 1 and 1: 1, preferably between 3: 1 and 5:1,

and more preferably 4: 1; and the metal loading of alloy on carbon support is

between 1% and 80% total metal on carbon, preferably between 20% and 40%

It is a further object of this invention to provide an anode catalyst for a fuel cell

whereby hydrogen can be oxidized in the presence of carbon monoxide.

It is also an object of this invention to provide a method of manufacturing

supported platinum molybdenum alloy with highly desirable surface activity.

It is a final object of this invention to provide an anode catalyst with high activity

for the direct oxidation of small organic molecules such as methanol.

Amongst the aforementioned methods of forming a bimetallic alloy, we have

found that a combination of deposition and bulk annealing forms the most potent

form of the alloy. As has been previously established, the precipitation of metal

salts onto carbon black supports can yield highly disperse formulations of metal. For example, through the teachings of Petrow and Allen, a complex of platinum

sulfite acid produces extremely small and well-dispersed particles of platinum on

carbon black. The Table below illustrates the relationship between weight

loading on carbon black (here Vulcan XC-72), the resulting average platinum

crystallite size, and the effective platinum surface area.

Table 1 : Weight Loading of Platinum as a function of crystallite size and surface

area.

Catalyst loading on Average Pt Particle Size Pt Surface Area

Vulcan XC-72, in A m²/g

%(wt/wt)

10 20 140

20 25 112

30 32 88

40 39 72

60 88 32

80 250 11

Reproduced from E-TEK, Inc. Gas Diffusion Electrod es and Catalyst Materials,

Catalog, 1998, p 15.

While there are clear trends with regards to particle size and effective surface

area, it is important to note that the specific activity of the catalyst follows a 10 trend as well. As reviewed by Markovic, Gasteiger, and Ross in The Journal of

the Electrochemical Society, Vol. 144, No. 5, May 1997, p 1591, the oxygen

reduction rate and hence activity of platinum can be highly sensitive to the type

and abundance of crystal face (111, 100, and 110). Furthermore, Markovic et al.

point that the platinum crystallite size controls the relative abundance of the

various face geometries. Since the activity of a CO tolerant alloy depends on the

final structure of the alloy crystal, control of metal loading, particle size, and

distribution of particle size all play a vital role as well as the actual method of

alloy formation.

In one preferred embodiment, manufacture of platinum-molybdenum alloys

begins by first depositing platinum on a carbon black. Colloidal particles of Pt

oxide are deposited on a carbon support from an aqueous solution of a platinum

precursor containing the support material. In order to form a colloid, the platinum

containing species can be subjected to an oxidizing agent or the solution can be

simply evaporated. Although Pt sulfite acid is the preferred choice for the

precursor, chloroplatinic acid could alternatively be used. In a second step,

discrete particles of Mo oxide are deposited on the Pt oxide containing carbon

support by adsorption of colloidal Mo oxide or Mo blue, formed in situ by mild

reduction of a solution containing a Mo precursor, for instance an ammonium

molybdate solution or a solution containing Mo with alkali hydroxide. Several

chemical reducing agents may be employed as well known to one skilled in the

art, for example hydrazine, formic acid, formaldehyde, oxalic acid, or metals 11

having a sufficiently low potential such as molybdenum and zinc: another method

for reducing the Mo containing solution consists in feeding said solution to an

electrochemical cell, applying direct current thereto and reducing the Mo

precursor at the cathode. After drying, the catalyst is first subjected to a reducing

atmosphere between 500 and 900°C, and then alloyed at higher temperature (for

instance at 900 to 1200°C) in the same reducing atmosphere or in an inert one: in

one preferred embodiment, it may be reduced at 500-800°C in H₂ gas, then heat

treated at 800-1200°C in Ar gas to form the alloy phase of Pt and Mo. In another

preferred embodiment, reduction and alloying are both performed in a H₂

environment between 500 and 1200°C, either in a single or in two subsequent

temperature steps. This general method is applicable to preparations of Pt:Mo

alloys supported on amorphous and/or graphitic carbon materials with a ratio of

Mo alloyed with Pt from 1 to 50 atomic % and a total metal loading on the

carbon support from 1-90%. It is however preferred that the total metal loading

be comprised between 10 and 40%. This method produces a carbon supported

Pt:Mo alloy catalyst with a metal particle size of approximately 300 A or less.

Other methods for preparing carbon supported Pt:Mo alloys of the same

characteristics will be given in detail in the following examples.

Catalysts produced in this manner are readily incorporated into gas diffusion

electrodes For example Pt:Mo catalysts thus prepared can be incorporated into

structures similar to the commercially available ELAT^® (E-TEK, Inc., Natick, 12

MA). Here, a carbon cloth serves as the web. A layer of Shawinigan Acetylene

Black (SAB) mixed with polytetrafluoroethylene binder (e.g. Teflon^®

commercialized by DuPont, Wilmington, DE) serves as the wetproofing layer on

each side of the web. Finally, layers of carbon black such as Vulcan XC-72 with

the alloy Pt:Mo are coated onto one side of the assembly: preferably, the specific

loading of metal with respect to the active area is comprised between 0.1 and 5

mg/cm². After the final coat, the assembly may be sintered in air at a temperature

sufficient to cause the binder to flow, typically 300-350^°C. Allen et al. in U.S.

Patent 4,293,396 further describe the construction of this type of gas diffusion

electrode. Such catalysts can also be incorporated in other gas diffusion

electrode structures, for example the electrodes in co-pending patent "Improved

Structures and Methods of Manufacture for Gas Diffusion Electrodes and

Electrode Components" are suitable as well as described in U.S. provisional

application Serial No. 60/070,342 filed January 2, 1998.

These carbon-supported alloys can also be deposited onto the surface of an ion

conducting membrane such as Nafion^® or Gore Select^® commercialized

respectively by DuPont and Gore and Associates, Elkton, MD. Wilson and

references therein have described methods for such operations in U.S. Patent

5,234,777. In general, depositing the catalyst on the membrane through a "decal"

method (see Wilson) can create a membrane electrode assembly, or one can

apply a paint or ink of catalyst to the membrane, or a catalyzed gas diffusion

electrode can be mechanically or heat-pressed against the membrane. 13 For the examples listed here, we have employed a catalyzed gas diffusion

electrode similar to that described in Allen et al. pressed against a Nafion

membrane. However, fuel cell tests can be highly dependent on system

configuration. For example, the mechanical geometry one uses to make contact

between the electrode and the membrane, the flow field geometry employed to

feed gasses to anode and cathode, and the method and manner of providing

hydrated gasses to the cell can all affect the cell performance. In order to evaluate

catalyst performance in the absence of system variables but still as an active

component of a gas diffusion electrode, we also employ a simple three-electrode

test method.

The three-electrode or "half cell" method fits 1 c^m2 sample of gas diffusion

electrode into an inert holder. The gas-feed side of the gas diffusion electrode is

positioned into a plenum whereby an excess of oxygen, air, hydrogen, or

hydrogen containing levels of CO is passed at low pressures (on the order of 10

mm of water or less). The face containing the catalyst (that would normally be

against the membrane of a PEMFC) is held in a 0.5M H₂S0₄ solution at a fixed

temperature. The counter electrode is placed directly across the working

electrode, and a reference electrode is held in-between the two. The fixed

geometry is maintained between the three electrodes through a specially

constructed cap. A potentiostat is employed to control the potential and measure

the current.

The invention is now better described by means of the following examples, 14 which are only intended to illustrate but not limit the extent and application of

this invention, and resorting to the figures, wherein ;

Figure 1 shows the potentiostated current - potential curves for samples of

Standard ELAT® with 1 mg Pt/cm², 30 % Pt/C in 0.5M H₂S0₄, at approximately

55 C, with and without 100 ppm CO in hydrogen. Platinum foil 3x2 cm serves as

the counter electrode. A standard calomel electrode serves as the reference.

Reported potentials are corrected for IR using the current interrupt method.

Figure 2 shows potentiostated current - potential curves for samples of Standard

ELAT™ with 1 mg Pt₅₀:Ru₅o/cm², 30%Metal/C in 0.5M H₂S0₄, at approximately

55 C, with and without 100 ppm CO in hydrogen. Platinum foil 3x2 cm serves as

the counter electrode. A standard calomel electrode serves as the reference.

Reported potentials are corrected for IR using the current interrupt method.

Figure 3 shows potentiostated current - potential curves for samples of Standard

ELAT™ with 1 mg Pt₇₅:Mo₂₅/cm², 30% Pt/C in 0.5M H₂S0₄, at approximately

55^°C, with and without 100 ppm CO in hydrogen. Platinum foil 3x2 cm serves as

the counter electrode. A standard calomel electrode serves as the reference.

Reported potentials are corrected for IR using the current interrupt method.

Figure 4 shows a calculation of percent loss of hydrogen current due to 100 ppm

CO vs. applied potential, from the tests of Figures 1,2, and 3. Based on average

of three or more samples. Conditions as in Figure 1.

Figure 5 shows a comparison of anode catalysts (Pt, PtsoRuso, Pt₉₅:Sn5, and

Pt₇5:Mo₂5) in standard ELAT™ Gas Diffusion Electrodes, 1.0 mg/cm² total metal 15 loading using 30% Metal/C, 16 cm² Active Area, Nafion 115, Pressure for

Fuel/Air - 3.5/4.0 BarA , temperature 70^°C, with a hydrogen contamination of 16

ppm CO.

Figure 6 shows a comparison of anode catalysts (Pt, Pt₅₀Ru₅₀, Pt₉₅:Sn₅, and

Pt₇5:Mo₂₅) in standard ELAT™ Gas Diffusion Electrodes, 1.0 mg/cm² total metal

loading using 30% Metal/C, 16 cm² Active Area, Nafion 115, Pressure for

Fuel Air - 3.5/4.0 BarA , temperature 70 C, with a hydrogen contamination of

100 ppm CO.

Figure 7 shows a comparison of anode catalysts (Pt, Pt₅₀Ru₅o, Pt ₅:Sn₅, and

loading using 30% Metal/C, 16 cm² Active Area, Nafion 115, Pressure for

Fuel/ Air - 3.5/4.0 BarA , temperature 70^°C, with a hydrogen contamination of

970 ppm CO.

Figure 8 shows a comparison of Anode Catalysts (Pt, Pt₅₀Ru5₀, Pt₈o:Mo₂o) in

standard ELAT™ Gas Diffusion Electrodes, 1.0 mg/cm² total metal loading using

30% Metal C, 16 cm² Active Area, Nafion 115, Pressure for Fuel/Air - 3.5/4.0

BarA , temperature 70^°C, with a hydrogen contamination of 22 and 103 ppm CO

EXAMPLE 1

A catalyst composed of 30 wt.% alloy on Vulcan XC-72 whereby the alloy is

Pt₇5Mo₂₅ atomic percent begins with the preparation of platinum on carbon

according to the method described by Petrow and Allen (U.S. Patent 4,082,699)

and is briefly summarized below. 16

A solution containing 38.66 ml of a 200 g/1 platinum (II) sulfite acid solution in

1.3 1 of deionized H₂0 is neutralized to pH 4.0 with a dilute (~ 1M) NH₄OH

solution. 21 g of Vulcan XC -72 is slurried with the platinum solution, then

dispersed ultrasonically to achieve a homogenous mixture. Using a magnetic

stirrer to maintain adequate mixing, 125 ml of a 30 wt% H₂0₂ solution is added

over the course of -30 minutes. The slurry is allowed to stir for 1 hour, then the

pH is adjusted to 4.0 with a dilute NH₄OH solution. 75 ml of 30 wt% H₂0₂

solution are added over the course of -20 minutes and the slurry is stirred for 1

hour. The pH of the slurry is again adjusted to 4.0, then the slurry is heated to

70°C . The solids are filtered to remove the supernatant liquid, washed with hot

deionized H₂0 to remove any soluble salts, then dried at 125°C to remove

moisture.

In a second step, the platinum containing carbon catalyst prepared above is

ground to a powder, then dispersed ultrasonically in 500 ml of deionized H₂0.

An ammonium molybdate solution is prepared by dissolving 1.902 g of M0O3 in

-25 ml of concentrated NH₄OH solution and removing the excess ammonia by

heating and stirring. This clear solution is added to the platinum catalyst slurry

under stirring and the pH is adjusted to -1.8 with dilute H₂S0₄. One ml of a 16

wt% N₂H₄ solution is added to form colloidal Mo0₃._x (molybdenum blue) in-situ

and the slurry allowed to stir - 8 hours. The addition of the reducing agent is

repeated twice more over 24 hours to ensure a complete reaction, then the slurry

is heated to 70°C. The solids are filtered to remove the supernatant liquid, 17 washed with hot deionized H₂0 to remove any soluble salts, then dried at 125°C

to remove moisture. After grinding to a powder, the catalyst is hydrogen reduced

at 800°C for 1 hour, then heat treated at 1000°C for 1 hour in flowing argon gas

to form the alloy phase.

EXAMPLE 2

A catalyst composed of 30 wt.% alloy on Vulcan XC-72 whereby the alloy is

Pt8oMo₂o atomic percent follows that of Example 1 except 40.07 ml of a 200 g/1

platinum (II) sulfite acid solution is substituted in the first step and 1.478 g of

Mo0₃ is substituted in the second step.

EXAMPLE 3

A catalyst composed of 30 wt.% alloy on Vulcan XC-72 whereby the alloy is

PtgsMois atomic percent follows that of Example 1 except 40.97 ml of a 200 g/1

platmum (II) sulfite acid solution is substituted in the first step and 1.209 g of

M0O3 is substituted in the second step.

EXAMPLE 4

A catalyst composed of 30 wt.% alloy on Vulcan XC-72 whereby the alloy is

Pt₇5Mo₂₅ atomic percent follows that of Example 1 except that the colloidal

solution of Mo0_3-x (Molybdenum Blue) is prepared separately, following the

same general method described to form this species in situ., then added to the

platinum on carbon slurry. The colloidal Mo0₃._x particles are readily adsorbed

on the carbon surface adjacent to the deposited platinum. After filtration and

drying, the alloy phase is formed as previously described. 18

EXAMPLE 5

A catalyst composed of 30 wt.% alloy on Vulcan XC-72 whereby the alloy is

Pt₇₅Mo₂5 atomic percent follows that of Example 1 except that a colloidal

solution of PtO_x is prepared by evaporation of the platinum (II) sulfite acid

solution to dryness, then dissolving the solids in H 0 to form a stable colloidal

dispersion. A colloidal solution of Mo0_3-x (Molybdenum Blue) is also prepared

separately following the same general method used to form this species in situ.

The two colloidal dispersions are then added concurrently to a slurry of Vulcan

XC-72 in H₂0 allowing the PtO_x and Mo0₃._x species to adsorb on the carbon

surface. After filtration and drying, the alloy phase is formed as previously

described.

COMPARATIVE EXAMPLE 6

A catalyst composed of 30 wt.% platinum on Vulcan XC-72 is prepared as

follows. The platinum addition method as described in Example 1 is followed

except now the amount of platinum (II) sulfite acid solution added is 45.00 ml,

and after drying, the 30 wt.% platinum on Vulcan catalyst powder is H₂ reduced

at 500°C for V₂ hour.

COMPARATIVE EXAMPLE 7

A catalyst composed of 30 wt.% alloy on Vulcan XC-72 whereby the alloy is

Pt5oRu₅o atomic percent is prepared as follows. The platinum addition method as

described in Example 1 is followed except now a combination of 29.64 ml of

platinum (II) sulfite acid solution and 76.80 ml of ruthenium (II) sulfite acid 19 solution is added to 1.3 1 of deionized H₂0. Oxidation of the mixed sulfite acid

solution with 30 wt.% H₂0₂ results in a mixed transient colloidal solution

containing discrete particles of PtO_x and RuO_x that adsorb simultaneously on the

carbon surface. After drying, the 30 wt.% Pt₅oRu₅o on Vulcan catalyst powder is

H₂ reduced at 230-250°C for 1 hour to form the alloy phase.

COMPARATIVE EXAMPLE 8

A catalyst composed of 30 wt.% alloy on Vulcan XC-72 whereby the alloy is

Pt95Sn₅ atomic percent follows the method described in Example 1 except that the

amount of platinum (II) sulfite acid solution added is 43.60 ml in the initial step.

In the second step, 2.364 g of a stable Sn0₂ colloid, commercially available from

Nyacol Products Inc., Ashland, MA, (15 wt.% Sn0₂) is added to the Pt on

Vulcan XC-72 catalyst powder slurry and the discrete Sn0₂ particles are readily

adsorbed on the platinized carbon surface. After filtration and drying, the

catalyst powder is H₂ reduced at 500°C for Vz hour then heat treated at 900°C for

1 hour under flowing argon to form the alloy phase.

The catalysts as described above are incorporated into a standard gas diffusion

electrode and subjected to small-scale testing free of system variables. Figures 1,

2, and 3 show the results of several samples of each (platinum, PtsoiRuso, and

Pt75:Mθ₂₅) being subjected to either hydrogen or hydrogen contaminated with

lOOppm CO. These are considered "driven" cells in as much as the potentiostat

applies a potential, the feedgas is consumed, and current is developed. In Figure

1 one readily notes the devastating effects of CO on pure supported platinum: 20 current is reduced dramatically. Figure 2 employs the comparative example

Pt₅₀:Ru5o subjected to the same conditions. Here some resistance to poisoning is

noted. Figure 3 is Pt₇₅:Mo ₅ subjected to pure H₂ and H₂ with lOOppm CO. It is

significant to note that at the higher applied potentials (100-200 mV vs. SCE), the

current for the new alloy does not appear to plateau as in the Pt₅₀:Ru₅o. Figure 4

illustrates the resilience of Pt₇₅:Mo₂₅ more clearly. In this Figure, instead of

plotting current on the ordinate axis, the loss of current due to CO poisoning is

plotted as a function of percent. Thus, the current obtained at the electrodes in

hydrogen is compared to the current obtained at lOOppm CO. Thus, pure

platinum results in an approximately 75% loss of current, while Pt₅₀:Ru5₀ is 50%,

and Pt₇₅:Mo₂₅ is around 25%. These results illustrate an improvement over the

current state of the art and verify that forming the platinum molybdenum alloy on

a carbon black support is viable method for preparing a catalyst for high

hydrogen oxidation activity in the presence of moderate levels of CO.

The next set of Figures affirms that the advances observed on the small scale are

operative within a fuel cell system. Figure 5 shows a family of curves generated

on a single 16 c"¹² cell operating as an air/hydrogen fuel cell. The electrodes and

catalysts represented here are prepared as described above. Unlike the previous

experiments, the fuel cell generates current and voltage proportional to the power

available from the system and the load placed on this system. Within this family

of current ~ potential curves two reference examples are displayed. The top

curve labeled "average Pt ELAT - H₂ data" is the case of pure hydrogen over a 21 supported platinum catalyst, i.e., the best case. The bottom curve of the family,

labeled "Standard Pt ELAT" is the example of a supported platinum catalyst

being subjected to the CO contaminated hydrogen feed, i.e., the worst case.

Thus, Figure 5 shows the effects of three different alloy combinations being

subjected to 16 ppm CO in the hydrogen. At this low level of CO, only small

differences arise between the three alloys, although the Pt₇₅:Mo₂₅ appears slightly

better-performing at the higher current densities. Figure 6 is a plot of a similar

family of curves except now there is lOOppm CO contamination. At this level of

CO, one notes that higher currents and voltages are obtained from the Pt₇₅:Mo₂₅

alloy compared to either Pt₅₀:Ru₅₀ or Pt₉₅:Sn₅. Similarly, the plot of Figure 7

shows the same electrodes subjected to 970ppm CO in hydrogen with the same

result: the Pt₇₅:Mo₂₅ alloy provides the greatest resistance to CO poisoning.

A similar alloy is prepared except now the amount of Mo is decreased to form a

Pt₈₀:Mo₂o alloy. Figure 8 compares ELAT electrodes assembled with this

catalyst compared to the standard Pt₅o:Ru₅o catalyst under 22 and 103 ppm CO in

hydrogen. This Figure more clearly shows higher currents being obtained for a

fixed voltage with the Pt:Mo over Pt:Ru, especially over the voltage region of 0.6

to 0.7V, which is considered a more efficient operating voltage for the fuel cell

stack.

Similar experiments were performed over a range of temperatures, from 60 C to

90 C, and currents obtained at 0.7 and 0.6 V are tabulated for comparison. Refer

to Tables 1-4 below. A column within the Tables is the calculation of the percent 22 decrease from pure hydrogen when the alloys are subjected to each level of

carbon monoxide. In all cased, through all temperatures, the Pt₈o:Mo₂₀ shows a

smaller percent decrease than Pt:Ru. In all cased, the Pt₈o:Mo₂₀ catalyst yielded

greater current than the commercially employed Pt:Ru. These results confirm that

the Pt:Mo alloy is an improved anode catalyst for a fuel cell whereby hydrogen

can be oxidized in the presence of carbon monoxide.

Table 1 Comparison of Pt4Mo to Pt:Ru at 60°C

Current at 0.7V, 60°C

H2 22 ppm CO %decrease 103 ppm CO %decrease

Pt4:Mo 471 243 -48% 162 -66%

Pt:Ru 459 219 -52% 149 -68%

Current at 0.6V, 60°C

Pt4:Mo 711 356 -50% 272 -62%

Pt:Ru 728 317 -56% 208 -71%

Table 2 Comparison of Pt4:Mo to Pt:Ru at 70°C

Current at 0.7V, 70°C

H2 22 ppm CO %decrease 103 ppm CO %decrease

Pt4:Mo 521 330 -37% 231 -56%

Pt:Ru 530 304 -43% 211 -60%

Current at 0.6V, 70°C

Pt4:Mo 790 492 -38% 365 -54%

Pt:Ru 831 455 -45% 304 -63%

Table 3 Comparisori ι ofPt4:Mo to Pt:Ru at 80°C

Current al : 0.7V, 80°C

H2 22 ppm CO %decrease 103 ppm CO %decrease

Pt4:Mo 541 404 -25% 300 -45%

Pt:Ru 570 371 -35% 273 -52%

Current at 0.6V, 80°C

Pt4:Mo 825 599 -27% 453 -45%

PtrRu 877 555 -37% 398 -55% 23

Table 4 Comparison of Pt4:Mo to Pt:Ru at 90°C

Current at 0.7V, 90°C

H2 22 ppm CO %decrease 103 ppm CO % decrease

Pt4:Mo 578 475 -18% 386 -33%

Pt:Ru 573 461 -20% 343 -40%

Current at 0.6V, 90°C

Pt4:Mo 858 689 -20% 564 -34%

Pt:Ru 891 694 -22% 508 -43%

Even if the invention has been described making reference to specific embodiments, it must be understood that modifications, substitutions, omissions and changes of the same are possible without departing from the spirit thereof and are intended to be encompassed in the appended claims.

Claims

24 We claim:

1. A carbon black supported catalyst for use in gas diffusion electrodes

having an atomic composition of Pt_x:Mo_y whereby X is from 0.5 to 0.9 and Y is

from 0.5 to 0.1 respectively.

2. The carbon black supported catalyst of claim 1 whereby the total metal

loading on carbon black is between 10 and 90% by weight, and preferably

between 10 and 40%.

3. A gas diffusion electrode having a web, a catalyst layer, and optionally a

wet-proofing coating, the catalyst layer comprising 0.1 and 5 mg/cm2 metal of

the catalyst of claim 1.

4. An ion exchange membrane coated on at least one side with the catalyst of

claim 1.

5. The ion exchange membrane of claim 4 wherein the total metal loading is

comprised between 0.1 and 5 mg/cm2.

6. A method of preparing a carbon black supported catalyst for use in gas

diffusion electrodes having an atomic composition of Pt_x:Mo_y wherein X is from

0.5 to 0.9 and Y is from 0.5 to 0.1 respectively, comprising:

• preparing a slurry of carbon black in a platinum containing solution, said

solution further comprising molybdenum;

• adding a reducing agent;

• alloying the two elements in a reducing and/or inert atmosphere at a 25 temperature above 300°C.

7. The method of claim 6 wherein said solutions containing platinum and

molybdenum are a colloidal dispersion of platinum and a colloidal dispersion of

molybdenum.

8. A method of preparing a carbon black supported catalyst for use in gas

0.5 to 0.9 and Y is from 0.5 to 0.1 respectively, comprising first absorbing a

platinum colloidal dispersion onto carbon black followed by absorbing a

colloidal dispersion of molybdenum onto carbon black, and finally alloying the

two elements in a reducing and/or inert atmosphere at a temperature above

300°C.

9. The method of claim 8 wherein said colloidal dispersion of platinum is

obtained upon evaporating a solution of platinum sulfite acid to dryness.

10. The method of claim 8 wherein said colloidal dispersion of platinum is

obtained upon addition of an oxidizing agent to a solution containing a platinum

precursor.

11. The method of claim 8 wherein said colloidal dispersion of molybdenum

is obtained from a solution containing molybdenum species in alkali hydroxides

or ammonia by adding a reducing agent.

12. The method of claim 11 wherein said molybdenum species are selected

from the group containing Mo03, molybdic acid, molybdenum blue or

molybdates. 26

13. The method of claim 6 wherein said alloying of the two elements is

realized by subjecting the catalyst to an inert atmosphere of argon between 500

and 1200°C.

14. The method of claim 6 wherein said alloying of the two elements is

realized by subjecting the catalyst to an atmosphere of hydrogen between 500

and 1200°C.

15. The method of claim 6 wherein said alloying of the two elements is

realized by subjecting the catalyst to a reducing atmosphere of hydrogen between

500 and 900°C, and an inert atmosphere of argon between 900 and 1200°C

16. The method of claim 11 wherein the reducing agent is selected from the

group comprising: hydrazine, molybdenum metal, zinc, formic acid,

formaldehyde, and oxalic acid.

17. A polymer electrolyte membrane fuel cell stack fed on the anode side with

a hydrogen-rich gas mixture containing at least 10 ppm CO, comprising at least

one electrode of claim 3.

18. A polymer electrolyte membrane fuel cell stack fed on the anode side with

a hydrogen-rich gas mixture containing at least 10 ppm CO, comprising at least

one ion exchange membrane of claim 4.