Fuel cell membrane electrode assemblies with improved power outputs

An electrode-membrane combination for use in a fuel cell comprising at least one reactant diffusive, electronically conductive electrode comprising at least one first catalytically active metal and at least one ionically conductive polymer; and at least one ionically conductive membrane contacting the electrode to form an electrode-membrane interfacial region, wherein the interfacial region comprises at least one zone comprising at least one second catalytically active metal and having a zone thickness of about 3 angstroms to about 475 angstroms. Surprisingly improved power output is observed. The zone is preferably deposited by electron beam physical vapor deposition. Substantially spherical nodules are observed for the zone from field-emission SEM analysis.

FIELD OF THE INVENTION
 This invention relates generally to fuel cell membrane electrode assemblies
 with improved power outputs. More particularly, these improved assemblies
 feature a relatively thin zone of catalytically active metal at the
 membrane-electrode interface in addition to catalytically active metal in
 the electrode.
 BACKGROUND OF THE INVENTION
 Fuel cells continue to show great commercial promise throughout the world
 as an alternative to conventional energy sources. This commercial promise
 should continue to grow as energy shortages become more acute,
 environmental regulations become more stringent, and new fuel cell
 applications emerge. See "FUEL CELLS", Encyclopedia of Chemical
 Technology, 4th Ed., Vol. 11, pp. 1098-1121.
 Despite improvements in fuel cell technology, however, long felt needs
 exist to increase power output, reduce initial cost, improve water
 management, and lengthen operational lifetime. Initial cost reduction can
 be most easily achieved by reducing the precious metal content of the fuel
 cell electrode. Such reduction, however, generally results in power output
 loss which blocks commercialization efforts.
 There are different types of fuel cells, but they each produce electrical
 energy by means of chemical reaction. One type of increasing import, the
 "polymer electrolyte membrane fuel cell" (PEMFC), comprises a membrane
 electrode assembly (MEA) typically made of an ionically conducting
 polymeric membrane sandwiched between two electronically conducting
 electrodes. For commercial application, multiple MEAs can be
 electronically connected to form a fuel cell stack (i.e., "stacked").
 Other components associated with typical PEMFCs include gas diffusion
 media and current collectors, the latter of which can also serve as
 bipolar separators and flow field elements. PEMFCs have been reviewed in
 the literature. See S. Srinivasan et al.; J. Power Sources; 29 (1990); pp.
 367-387.
 In a typical PEMFC, a fuel such as hydrogen gas is electrocatalytically
 oxidized at one electrode (anode). At a the other electrode (cathode), an
 oxidizer such as oxygen gas is electrocatalytically reduced. The net
 reaction results in generation of electromotive force. Elevated
 temperature can accelerate this reaction, although one increasingly
 important advantage of the PEMFC is that lower temperatures (e.g.,
 80.degree. C.) can be used. The fuel cell reactions are generally
 catalyzed by precious transition metal, commonly a noble metal such as
 platinum, which is present in both anode and cathode. Because the fuel
 cell is often operated with use of gaseous reactants, typical electrodes
 are porous materials (more generally, reactant diffusive materials) having
 the catalytically active metal at the porous surfaces. The metal can be in
 different morphological forms, but often it is in particulate or dispersed
 form and supported on carbon. Fuel cell performance may depend on the form
 of catalyst. See Poirier et al.; J. Electrochemical Society, vol. 141, no.
 2, February 1994, pp. 425-430.
 Fuel cell systems are complex because the reaction is believed localized at
 a three-phase boundary between ionically conducting membrane, gas, and
 carbon supported catalyst. Because of this localization, addition of
 ionically conductive material to the electrode can result in better
 utilization of catalyst as well as improved interfacial contact with the
 membrane. However, the additional ionic conductor can introduce extra
 cost, especially when perfluorinated conductors are used, and can increase
 the complexity of electrolyte water management, all important to
 commercialization.
 One general approach to minimize loading of expensive catalytic metal has
 been to use smaller catalyst particles. However, long operational
 lifetimes are particularly difficult to achieve with low catalyst
 loadings. Also, catalyst particle size may be unstable and increase by
 agglomeration or sintering.
 Another approach has been to concentrate the metal at the
 membrane-electrode interface. See Ticianelli et al.; Journal of
 Electroanalytical Chemistry and Interfacial Electrochemistry; Vol. 251 No.
 2, Sep. 23, 1988, pp. 275-295. For example, 500 angstrom dense layers of
 metal catalyst reportedly have been sputtered onto certain gas diffusion
 electrodes before sandwiching the ionically conducting membrane between
 the electrodes. Apparently, however, sputtered layers thinner than 500
 angstroms have not been reported, possibly because of the difficulty in
 making uniform thinner layers. Moreover, other types of electrodes and
 deposition techniques may not be suitable, water balance may be upset, and
 testing often is not carried out under commercial conditions. In sum, it
 is recognized that mere depositing a thin layer of catalyst onto the
 electrode does not guarantee a suitable MEA. According to the Srinivasan
 article noted above, sputtering may not be economically feasible compared
 with wet chemical deposition methods. Thus, in general, industry has not
 accepted this approach as realistic.
 Additional technology is described in the patent literature including, for
 example, U.S. Pat. Nos. 3,274,029; 3,492,163; 3,615,948; 3,730,774;
 4,160,856; 4,547,437; 4,686,158; 4,738,904; 4,826,741; 4,876,115;
 4,937,152; 5,151,334; 5,208,112; 5,234,777; 5,338,430; 5,340,665;
 5,500,292; 5,509,189; 5,624,718; 5,686,199; and 5,795,672. In addition,
 deposition technology is described in, for example, U.S. Pat. Nos.
 4,931,152; 5,068,126; 5,192,523; and 5,296,274.
 SUMMARY OF THE INVENTION
 Despite the prejudices existing in the art, the inventors have discovered
 that surprisingly high improvements in power output can be achieved for
 low and ultra-low catalyst MEAs. By introducing a relatively thin zone of
 catalytic metal at the interface between selected electrodes and
 membranes, significantly more power can be produced for the same amount,
 or even less amounts, of catalyst. Moreover, by combining selected
 electrodes and membranes, superior overall fuel cell performance can be
 achieved. The test results, significantly, are promising even under
 commercially realistic conditions.
 In particular, the inventors have discovered an electrode-membrane
 combination comprising at least one reactant diffusive, electronically
 conductive electrode comprising at least one first catalytically active
 metal and at least one ionically conductive polymer; and at least one
 ionically conductive membrane contacting the electrode to form an
 electrode-membrane interfacial region, wherein the interfacial region
 comprises at least one zone comprising at least one second catalytically
 active metal and having a zone thickness of about 3 angstroms to about 475
 angstroms.
 Another aspect of this invention is an electrode-membrane combination
 comprising at least one electronically conducting electrode comprising at
 least one first catalytically active metal and at least one ionically
 conductive polymer; and at least one ionically conducting membrane
 contacting the electrode to form an electrode-membrane interfacial region,
 wherein the interfacial region comprises a vacuum deposited zone
 comprising at least one second catalytically active metal.
 A further aspect of this invention is an electrode-membrane combination
 comprising at least one reactant diffusive, electronically conducting
 electrode comprising (i) eat least one first catalytically active metal
 dispersed throughout the electrode; (ii) at least one tonically conductive
 polymer, and (iii) a vacuum deposited zone comprising at least one second
 catalytically active metal; an ionically conducting membrane contacting
 the electrode to form an electrode-membrane interface, wherein the zone of
 at least one second catalytically active metal is concentrated in the
 electrode at the electrode-membrane interface.
 Another aspect is an article comprising at least one reactant diffusive,
 electronically conductive electrode comprising at least one first
 catalytically active metal and at least one ionically conductive polymer;
 and at least one ionically conductive membrane contacting the electrode to
 form an electrode-membrane interfacial region, wherein the interfacial
 region comprises at least one zone comprising at least one second
 catalytically active metal and having a zone loading of about 0.0006
 mg/cm.sup.2 to about 0.12 mg/cm.sup.2.
 Moreover, the invention includes an electrode-membrane combination
 comprising at least one reactant diffusive, electronically conductive
 electrode comprising at least one first catalytically active metal and at
 least one ionically conductive polymer; and at least one ionically
 conductive membrane contacting the electrode to form an electrode-membrane
 interfacial region, wherein the interfacial region comprises at least one
 zone comprising at least one second catalytically active metal having a
 form including substantially spherical nodules.
 Another aspect, moreover, is a membrane electrode assembly comprising the
 combination of first and second reactant diffusive, electronically
 conducting electrodes, and at least one ionically conducting membrane
 sandwiched between and contacting the first and second electrodes to form
 first and a second membrane-electrode interfacial regions, respectively,
 wherein the first and second electrodes each comprise at least one
 ionically conductive polymer and at least one catalytically active first
 metal, and wherein at least one of the two interfacial regions comprises a
 zone of at least one catalytically active second metal having a zone
 loading between about 0.0006 mg/cm.sup.2 and about 0.12 mg/cm.sup.2.
 The inventors have also discovered a membrane electrode assembly comprising
 the combination of first and second reactant diffusive, electronically
 conducting electrodes, and at least one ionically conducting membrane
 sandwiched between and contacting the first and second electrodes to form
 first and a second membrane-electrode interfacial regions, respectively,
 wherein the first and second electrodes each comprise ionically conductive
 polymer and at least one catalytically active first metal, and wherein at
 least one of the two interfacial regions comprise a zone of catalytically
 active second metal having a form including substantially spherical
 nodules.
 Other aspects of the invention include fuel cell stacks and transportation
 vehicles comprising the combinations and assemblies according to the
 invention.
 Finally, the invention also includes a method of improving the power output
 of a fuel cell membrane electrode assembly comprising the combination of
 steps of providing assembly elements including (i) at least one reactant
 diffusive, electronically conductive electrode comprising at least one
 ionically conductive polymer and at least one first catalytically active
 metal dispersed throughout the electrode, and (ii) at least one ionically
 conductive membrane; depositing onto at least one of the assembly elements
 a zone of at least one second catalytically active metal having a zone
 thickness between about 3 angstroms and about 475 angstroms, wherein the
 zone deposition is (i) a direct deposition onto the assembly element, or
 (ii) an indirect deposition onto the assembly element wherein the
 deposited zone is first deposited onto a substrate and then transferred
 from the substrate onto the assembly element, and optionally assembling
 the membrane electrode assembly from the assembly elements.
 In addition to improved power output with better catalyst utilization, a
 further important advantage is that multiple methods can be used to
 prepare the structures, and that these multiple methods can be tailored to
 different commercial applications. More precise design and control is now
 possible. Also noteworthy are that the zone of catalyst metal does not
 substantially upset the water balance of the fuel cell system, that the
 invention can be applied to different fuel cell reactants, and that
 process scalability has been demonstrated. In sum, the invention is
 commercially realistic.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 illustrates a cross-section of a planar geometry MEA according to
 this invention. The z-direction is shown coplanar with the page and
 perpendicular to the plane of the MEA. Components 1 and 3 represent
 electronically conductive electrodes (first and second electrodes) which
 each contact and together sandwich an ionically conductive polymeric
 membrane 2. The electrodes includes catalytically active metal. Regions 4
 and 5 represent first and second interfacial regions. The regions separate
 the membrane 2 from the first and second electrodes (1 and 3). The MEA
 comprises two half cells formed by combination of electrode 1 and membrane
 2 (without electrode 3) or by combination of electrode 3 and membrane 2
 (without electrode 1).
 FIG. 2 illustrates a half cell according to this invention comprising the
 first electrode 1 and the ionically conductive membrane 2 which together
 contact and form interfacial region 4. The extent of the interfacial
 region can depend on, for example, (i) the method by which the membrane
 and electrode are brought into contact, and (ii) the surface roughness and
 porosity of the membrane and electrode. Irrespective of how the half cell
 is formed, however, this interfacial region comprises a zone 6 of
 catalytically active metal which optionally is the same catalytically
 active metal present in the electrode (a first metal) . However, the
 catalytically active metal of zone 6 (a second metal) can be deposited in
 a separate step from catalytically active metal in electrode 1. The second
 metal can be a different metal entirely from the first metal, or it can be
 the same metal but have a different structure or morphology. Mixtures of
 metals can be used so that, for example, the zone 6 comprises at least two
 different second catalytically active metals or the electrode comprises at
 least two different first catalytically active metals.
 FIG. 3 illustrates by means of a cross-sectional view of an electrode a
 preferred embodiment of this invention (see Example 2 below). The
 electrode comprises ionically conducting perfluorinated ionomer fused with
 particles of carbon supported platinum catalyst. In addition, the
 electrode comprises a vacuum deposited zone of platinum which helps form a
 z-gradient step function of catalytically active metal.
 FIG. 4 further represents the z-gradient step function concept of this
 invention. In this representation, concentration of catalytically active
 metal in the electrode is shown as a function of distance from the
 membrane. The catalytically active metal can be either metal which is
 present originally in the electrode (i.e., a first metal) or metal which
 is deposited separately (i.e., a second metal). Initially, in region A,
 the catalytically active metal is entirely or substantially the second
 metal, and the electrode is substantially pure metal free from carbon or
 ionically conductive polymer. Then, a region B exists wherein the
 concentration of second metal drops. The slope in region B can vary
 depending on, for example, surface roughness, electrode porosity,
 homogeneity, preparation method, and other experimental factors. The slope
 can include a linear or substantially linear portion. Finally, a region C
 exists wherein the concentration of catalytically active metal is due to
 the first metal originally present in the electrode before deposition of
 the second. If desired, region C can include a gradient in concentration
 of first catalytically active metal with higher concentrations toward the
 membrane.
 Both the first catalytically active metal and the second catalytically
 active metal can be present in mixtures of catalytically active metals
 without change in this concept of a z-gradient step function shown in FIG.
 4. If metal mixtures are present, then the concentrations of each metal
 would be added to yield the total concentration.
 Although the theory of the present invention is not fully understood, it is
 believed that an unexpected synergistic interaction can occur between the
 first catalytically active metal and the zone of deposited second
 catalytically active metal. As a result, significant power increases can
 be observed without substantial increase in metal loading, particularly
 when selected deposition methods are used.
 This invention is widely applicable in fuel cell technology, particularly
 PEMFC technology. The fuel is preferably a gas such as hydrogen, but
 liquid fuels such as alcohols, including methanol, can also be used.
 Hydrocarbons including reformed gasoline or diesel fuel can also be used
 to provide fuel.
 When reformate fuel is used, a plurality of catalytically active metals
 (e.g., bimetallic) can be used to improve performance and reduce poisoning
 effects. In particular, carbon monoxide poisoning can be a problem even at
 levels as low as 5-100 ppm of carbon monoxide. For example, in this
 embodiment, the interfacial region can comprise at least two of the second
 catalytically active metals different from each other. Also, the electrode
 can comprise at least two of the first catalytically active metals
 different from each other. In this embodiment, the plurality of
 catalytically active metals is preferably at the anode. The plurality of
 metals can include three, four, and even more different metals if desired.
 Metal alloying preferably occurs. For bimetallic systems, preferred
 combinations include Pt--Ru, Pt--Sn, Pt--Co, and Pt--Cr, and the most
 preferred combination is Pt--Ru. Preferably, substantially equal amounts
 of each metal are present. Hence, a bimetallic combination is preferably a
 50/50 mixture or alloy.
 The reactant diffusive, electronically conductive electrodes, including
 cathode and anode, can be prefabricated before they are contacted with the
 tonically conductive membrane or subjected to the deposition of the second
 catalytically active metal. In general, conventional gas diffusion
 electrodes are commercially available and can be used either directly or
 with modification. For example, low platinum loading electrodes can be
 obtained from E-TEK, Inc. (Natick, Mass.) or from Electrochem, Inc.
 Electrodes should comprise components which provide structural integrity,
 effective water management, diffusivity to reactants including porosity or
 diffusivity to gases, electronic conductivity, catalytic activity,
 processability, and good interfacial contact with the membrane. The
 structure of the electrode is not particularly limited provided that these
 functional attributes are present. At least one ionically conductive
 polymer should be present as part of the electrode to increase catalyst
 utilization.
 The electrodes generally can be of substantially planar geometry. Planar
 means an article or form made so as to have length and width dimensions,
 or radial dimensions, much greater than the thickness dimension. Examples
 of such articles include polymeric films or membranes, paper sheets, and
 textile fabrics. Once formed, such planar articles can be used as an
 essentially flat article, or wound, folded, or twisted into more complex
 configurations.
 The electrodes are at least partially porous, wherein porous means a
 structure of interconnected pores or voids such that continuous passages
 and pathways throughout a material are provided. More generally, the
 electrode should allow reactants to diffuse through the electrode at
 commercially usable rates.
 Electrode preparation and other aspects of fuel cell technology are
 described in, for example, U.S. Pat. Nos. 5,211,984 and 5,234,777 to
 Wilson, which are hereby incorporated by reference. For example, Wilson
 teaches use of catalyst-containing inks and transfer methods to fabricate
 electrodes comprising ionically conductive polymer and metal catalyst. In
 these patents, an uncatalyzed porous electrode is placed against a film of
 catalyst during fuel cell assembly to form a gas diffusion backing for the
 catalyst film. However, the catalyst films in Wilson, unlike those of this
 invention, have little if any porosity.
 Preferred electrodes are formed of electrically conductive particulate
 materials, which may include catalyst materials, held together by a
 polymeric binder. If desired, hydrophobic binders such as
 polytetrafluoroethylene can be used. Ion exchange resin can be used as
 binder. Expanded or porous polytetrafluoroethylene can be used. In
 particular, a preferred electrode can be prepared by the following
 procedure
 ("procedure A"):
 A dispersion of 5 g of carbon black-platinum (50 wt. %) particles (from NE
 Chemcat Co.) in 40 g 2-methyl-1-propyl alcohol is prepared. To the
 dispersion is added a liquid composition of isopropyl alcohol containing 9
 wt. % Nafion.RTM. perfluorosulfonic acid resin (DuPont) and thoroughly
 mixed, with the aid of ultrasonic agitation, to form a liquid mixture,
 having a relative concentration of 50 wt. % ion exchange resin and 50 wt.
 % carbon black supported platinum. The liquid mixture is painted by brush
 to impregnate a porous expanded polytetrafluoroethylene electrode-support
 film (thickness--16 micrometers; pore volume 94%; IBP 0.12 kg/cm.sup.2).
 Solvent is removed by air drying. The composite structure is heat treated
 at 120.degree. C. for 24 hours to complete the procedure.
 This procedure A also can be carried out, for example, with use of at least
 25 wt. % catalyst (carbon black-platinum) with the balance being
 perfluorinated ionomer polymer. Preferably, the electrode in this
 composite structure has some porosity and is reactant diffusive.
 For use as an electrode support, the porous or expanded
 polytetrafluoroethylene film should be thin and can have, for example, a
 thickness of about 3 microns to about 200 microns, and more particularly,
 about 3 microns to about 30 microns, and preferably about five microns to
 about 20 microns. This relatively thin catalyst-containing electrode can
 be contacted with other electrically conducting components which, for
 example, do not contain catalyst and provide passageway for reactants.
 The pore volume of the electrode support can be, for example, about 60% to
 about 95%, and preferably, about 85% to about 95%. The maximum pore size
 defined by an isopropanol bubble point (IBP) can be, for example, about
 0.05 kg/cm.sup.2 to about 0.5 kg/cm.sup.2, and preferably, about 0.05
 kg/cm.sup.2 to about 0.3 kg/cm.sup.2. The Bubble Point was measured
 according to the procedures of ASTM F316-86. Isopropyl alcohol was used as
 the wetting fluid to fill the pores of the test specimen. The Bubble Point
 is the pressure of air required to displace the isopropyl alcohol from the
 largest pores of the test specimen and create the first continuous stream
 of bubbles detectable by their rise through a layer of isopropyl alcohol
 covering the porous media. This measurement provides an estimation of
 maximum pore size.
 Before deposition of the zone of second catalytically active metal, the
 electrode preferably has relatively low level of catalyst loading such as,
 for example, about 0.01 mg/cm.sup.2 to about 1 mg/cm.sup.2, and
 preferably, about 0.02 mg/cm.sup.2 to about 0.5 mg/cm.sup.2, and more
 preferably, about 0.05 mg/cm.sup.2 to about 0.4 mg/cm.sup.2. Preferably,
 it is less than about 0.3 mg/cm.sup.2. Preferably, the total catalyst
 loading for a single MEA is less than about 0.65 mg/cm.sup.2, and more
 preferably, less than about 0.2 mg/cm.sup.2.
 At least one first catalytically active metal is distributed throughout the
 porous surface of the electrodes. Catalytically active means that the
 metal is in some way helping to provide catalysis. The first and second
 catalytically active metals can be and preferably are the same metals.
 Both the first and second catalytically active metals can be, for example,
 noble metals or Group VIII metals. Particular examples include Pt, Pd, Ru,
 Rh, Ir, Ag, Au, Os, Re, Cu, Ni, Fe, Cr, Mo, Co, W, Mn, Al, Zn, Sn, with
 preferred metals being Ni, Pd, Pt, and the most preferred being Pt. If
 desired, a plurality of catalytically active metals (e.g., bimetallic) can
 also be selected from this list. Co-catalysts and promoters can also be
 present such as, for example, C, Ni, Al, Na, Cr, and Sn. Any conventional
 agents to enhance fuel cell performance can be used.
 The first catalytically active metal is preferably in the form of metal
 loaded carbon particles. For example, the carbon particles can be loaded
 with metal in amounts of at least 10 wt. % metal, and preferably, at least
 20 wt. % metal. Preferably, the first catalytically active metal is
 relatively uniformly distributed and randomly dispersed throughout the
 electrode. Electrodes can be, for example, formed from particles of high
 surface area carbon, such as Vulcan XC72 (about 200 m.sup.2 /g) or Black
 Pearls 2000 (about 1000 m.sup.2 /g) available from Cabot, Boston, Mass.
 which are loaded with particles of platinum of about 20 angstrom to about
 50 angstrom size to an electrode area loading of about 0.35 mg/cm.sup.2.
 In addition to supported metal catalyst, the electrode should further
 comprise ionically conductive polymer to improve the contact of the
 electrode to the membrane and increase catalyst utilization. The ionically
 conductive polymer of the membrane (a "first ionically conductive
 polymer") can be substantially the same or different than the ionically
 conductive polymer of the electrode (a "second ionically conductive
 polymer"), although they preferably are substantially the same.
 Substantially the same means that, for example, the two ionically
 conductive materials, for example, can be selected to have different
 equivalent weights although having the same general chemical identity.
 The electrode can further comprise at least one hydrophobic component such
 as a fluorinated polymer, preferably a perfluorinated polymer such as
 polytetrafluoroethylene. If desired, this hydrophobic component can be
 concentrated at the electrode-membrane interface. Other examples include
 tetrafluoroethylene/(perfluoroalkyl) vinyl ether copolymer (PFA), or
 tetrafluoroethylene/hexafluoropropylene copolymer (FEP). This fluorinated
 hydrophobic component can help improve water repellency in the electrode
 structure.
 A pore-forming agent or sacrificial filler also can be included in the
 electrode such as, for example, ammonium bicarbonate, sodium chloride, or
 calcium carbonate. This agent can be removed by, for example, heating or
 leaching to create voids and improve gas diffusivity. Gas diffusivity can
 be tailored to the application.
 The electrode can further comprise at least one solvent used during
 electrode preparation. However, solvent may slowly evaporate from the
 electrode. Hence, solvent initially present may not be present at a later
 time. Solvents are known in the art for electrode ink preparations.
 Exemplary solvents include polar solvents and alcohols.
 The ionically conductive membrane should provide, for example, strength,
 high ionic conductance, and good interfacial contact with the electrode.
 The structure of the membrane is not particularly limited provided that
 these functional attributes are present. Reinforced composite membranes
 are preferred.
 The membrane is preferably made largely of one or more fluorinated
 polymers, and preferably, mixtures of perfluorinated polymer and
 fluorinated ion exchange resin. In a preferred embodiment, the membrane is
 prepared from porous or expanded polytetrafluoroethylene which is
 impregnated with ion exchange resin such as a sulfonated perfluorinated
 ionomer including NAFION.RTM. (EW can be, for example, 1100). Similar
 ionomers such as, for example, FLEMION.RTM. (Asahi Glass) can also be
 used. Substantially all (&gt;90%) of the open porous volume can be
 impregnated so that a high Gurley number (&gt;10,000 seconds) is provided.
 Impregnated membranes are described in, for example, U.S. Pat. Nos.
 5,547,551; 5,635,041; and 5,599,614 to Bahar et al., which are hereby
 incorporated by reference. These patents describe test procedures and
 characteristics of the membranes.
 Membranes can be prepared with use of a base material made in accordance
 with the teachings of U.S. Pat. No. 3,593,566 incorporated herein by
 reference. Base materials are available in various forms from W. L. Gore
 and Associates, Inc. (Elkton, Md). Such a base material has a porosity of
 greater than 35. Preferably, the porosity is between about 70% and 95%.
 The porous microstructure can comprise (i) nodes interconnected by
 fibrils, or (ii) fibrils.
 Average pore size for the base material can be, for example, about 0.05
 microns to about 0.4 microns. The pore size distribution value can be, for
 example, about 1.05 to about 1.20. Pore size measurements are made by the
 Coulter Porometer.TM., manufactured by Coulter Electronics, Inc. (Hialeah,
 Fla.). The Coulter Porometer is an instrument that provides automated
 measurement of pore size distributions in porous media using the liquid
 displacement method described in ASTM Standard E1298-89. The Porometer
 determines the pore size distribution of a sample by increasing air
 pressure on the sample and measuring the resulting flow. This distribution
 is a measure of the degree of uniformity of the membrane (i.e., a narrow
 distribution means there is little difference between the smallest and
 largest pore size). The Porometer also calculates the mean flow pore size.
 By definition, half of the fluid flow through the filter occurs through
 pores that are above or below this size.
 High Gurley numbers are preferred for the membrane. The Gurley air flow
 test measures the time in seconds for 100 cc of air to flow through a one
 square inch sample at 4.88 inches of water pressure in a Gurley Densometer
 (ASTM 0726-58). The sample is placed between the clamp plates. The
 cylinder is then dropped gently. The automatic timer (or stopwatch) is
 used to record the time (seconds) required for a specific volume recited
 above to be displaced by the cylinder. This time is the Gurley number. The
 Frazier air flow test is similar but is mostly used for much thinner or
 open membranes. The test reports flow in cubic feet per minute per square
 foot of material at 0.5 inches water pressure.
 The composite membrane is preferably thin having a thickness of, for
 example, more than about 3 microns, but less than about 75 microns, and
 more preferably, less than about 50 microns, and even more preferably,
 less than about 30 microns. About 20 microns and less is most preferred.
 Membrane thickness can be determined with use of a snap gauge such as, for
 example, Johannes Kafer Co. Model No. F1000/302). Measurements are taken
 in at least four areas in each specimen.
 In addition, the membranes should have high ionic conductance, preferably
 greater than about 8.5 mhos/cm.sup.2, and more particularly, greater than
 about 22 mhos/cm.sup.2. Ionic conductance can be tested using a Palico
 9100-2 type test system. This test system consisted of a bath of 1 molar
 sulfuric acid maintained at a constant temperature of 25.degree. C.
 Submerged in the bath were four probes used for imposing current and
 measuring voltage by a standard "Kelvin" four-terminal measurement
 technique. A device capable of holding a separator, such as the sample
 membrane to be tested, was located between the probes. First, a square
 wave current signal was introduced into the bath, without a separator in
 place, and the resulting square wave voltage was measured. This provided
 an indication of the resistance of the acid bath. The sample membrane was
 then placed in the membrane-holding device, and a second square wave
 current signal was introduced into the bath. The resulting square wave
 voltage was measured between the probes. This was a measurement of the
 resistance due to the membrane and the bath. By subtracting this number
 from the first, the resistance due to the membrane alone was found.
 Impregnated composite membranes can be prepared by repeatedly contacting
 one or both sides of the base porous substrate with a solution of
 ionically conductive polymer. Surfactants can be used to impregnate. In
 each impregnation step, solvent can be removed and heating carried out to
 help bind or lock the jonically conductive polymer in the base substrate.
 Particularly preferred membranes include those known as GORE-SELECT.RTM.
 available from W. L. Gore and Associates, Inc (Elkton, Md.).
 An important advantage of this invention is in avoiding difficulties of
 combining thin membranes with an electrode by traditional methods like
 hot-pressing. Membrane damage can occur. The electrode-membrane
 combination should be mechanically and electrochemically compatible.
 The electrode is brought into contact with the membrane to form an
 interfacial region. At the interfacial region, both membrane and electrode
 can influence activity occurring at the region. This interfacial region,
 like the membrane and the electrode, generally is substantially planar. At
 this interfacial region resides a zone, which preferably is a layer or
 coating, of the second catalytically active metal which unexpectedly and
 substantially improves the power output of the fuel cell. The interfacial
 region may not be perfectly homogeneous because the mating surfaces can
 have, for example, softness, inhomogeneity, and surface roughness.
 However, the zone of second catalytically active metal is associated more
 with the electrode side of the interface than the membrane side because
 the zone, like the electrode, is electronically conductive. Nevertheless,
 it can be possible in some cases for some of the zone to be associated
 with the membrane as well depending on the process used to generate the
 interface and the zone of second catalytically active metal.
 The incorporation of the zone of second catalytically active metal at the
 interfacial region can result in a large percent increase in current
 density (mA/cm.sup.2), and also power output (p=I.times.V), at a given
 voltage on a polarization curve (e.g., 0.6 V) compared to a reference MEA
 without the zone of second catalytically active metal. This percentage
 increase can be as high as 20% or more, and preferably, 30%, and more
 preferably, 40% or more. In some cases, improvements over 90% have been
 observed.
 Surprisingly, greater percent increases can be found for thinner zones.
 Hence, an important advantage of this invention is that high percent power
 increases can be observed with introduction of only a thin catalytic
 layer, and an R ratio can be defined as:
EQU percent increase in current density/zone thickness (.ANG.)
 wherein current densities are measured at 0.6 V in the polarization curve
 under steady-state conditions. Cell temperature should be between about
 60.degree. C. and about 80.degree. C., and preferably, about 65.degree. C.
 For example, this R ratio is about 0.7 when a 33% percent increase is
 found for deposition of a 50 angstrom layer (see working examples).
 Similarly, this R ratio is about 0.9 when a 46% increase is found for
 deposition of a 50 angstrom layer. Surprisingly, R can be greater than 22
 (22.6) when a 113% increase is found for a 5 angstrom coating. Hence, a
 surprising feature of this invention is R values greater than 0.5,
 preferably greater than 1, more preferably greater than 5, more preferably
 greater than 10, and even more preferably greater than 20. If desired, the
 R value can be less than 50, and preferably less than 30, if the system
 needs to be tailored to a particular application. Calculation of this R
 ratio assumes that some fuel cell reaction occurs in the absence of the
 zone of second catalytically active metal.
 The thickness of the zone of second catalytically active metal, which
 represents average thickness, can be determined by methods known in the
 art. These methods include use of, for example, a microbalance together
 with use of deposition rate and deposition time (e.g., 1 .ANG./sec
 deposition rate for 50 seconds of deposition yields approximately 50 .ANG.
 average thickness). Calibration curves can be established to help
 determine thickness. In general, the thickness can be about 3 angstroms to
 about 475 angstroms, and more particularly, about 5 angstroms to about 250
 angstroms, and even more particularly, about 5 angstroms to about 50
 angstroms. Thicknesses much greater than about 475 angstroms, in general,
 can reduce layer uniformity and possibly block diffusion. However, the
 degree to which diffusion is blocked can depend on the structure of the
 zone.
 Examples of loadings of the zone of at least one second catalytically
 active metal include about 0.0006 mg/cm.sup.2 to about 0.12 mg/cm.sup.2,
 and more particularly about 0.0007 mg/cm.sup.2 to about 0.09 mg/cm.sup.2,
 and more particularly, 0.001 mg/cm.sup.2 to about 0.05 mg/cm.sup.2, and
 more particularly, about 0.005 mg/cm.sup.2 to about 0.02 mg/cm.sup.2.
 Typical vacuum deposition methods include chemical vapor deposition,
 physical vapor or thermal deposition, cathodic arc deposition, ion
 sputtering, and ion beam assisted deposition (IBAD). A method which
 requires less vacuum is jet vapor deposition. Because the materials are
 deposited in vacuum (typically less than 13.3 mPa, or 1.times.10.sup.-4
 torr), contamination of the films can be minimized while maintaining good
 control over film thickness and uniformity. Deposition over large areas
 can be achieved via reel-to-reel or web coating processes. The present
 invention makes use of these and other vacuum deposition techniques,
 particularly magnetron sputtering and physical vapor deposition.
 Most preferably, electron beam--physical vapor deposition (EB-PVD) is used.
 Deposition rates can range, for example, from 0.1 .ANG./sec to 10
 .ANG./sec. If necessary, heating of the substrate can be limited.
 In addition, methods such as, for example, combustion chemical vapor
 deposition (CCVD) can be used which do not require a vacuum. Wet chemical
 methods can be used but are not preferred.
 The structure or morphology of the deposited zone of the at least one
 second catalytically active metal can depend on, for example, the
 deposition method and the loading of the second catalytically active
 metal. The structure can be analyzed by, for example, field-emission
 scanning electron microscopy (FE-SEM). This analysis shows that relatively
 uniform zones of the second catalytically active metal are formed. This
 substantial uniformity is present irrespective of the type of film
 morphology present. In general, sputter deposition can provide more dense
 zones than thermal evaporation methods such as EB-PVD. In general, the
 EB-PVD zones can exhibit a greater degree of surface texture. Although the
 theory and detailed structure of the present invention are not fully
 understood, the excellent power improvements found herein may be due to
 the relatively open surface texture. This openness may provide, for
 example, better reactant transport and more surface area for reaction.
 At relatively thin zone thicknesses of, for example, five angstroms, the
 FE-SEM analysis of the electrode can reveal small but measurable increases
 in field brightness compared to the reference electrode without a
 deposited zone. Surprisingly, relatively uniform deposition was observed.
 At thicker thicknesses of, for example, 50 angstroms, the FE-SEM analysis
 can reveal substantially spherical nodules of deposited metal
 approximately 25 nm to about 100 nm, and in particular, about 30 nm to
 about 70 nm, and more particularly, about 50 nm in diameter. At even
 greater thicknesses of, for example, 500 angstroms, the FE-SEM analysis
 can reveal, in addition to the substantially spherical nodules, rod-shaped
 structures in which the rods have diameters of about 20 nm to about 100
 nm, and more particularly, about 20 nm to about 60 nm, and even more
 particularly, about 40 nm. The rod length can vary. Whisker or hair-like
 morphology can be produced.
 Several methods can be used to assemble the half cell or MEA which
 incorporates the zone. In describing these methods, assembly elements
 include the electrode and the membrane. The zone can be deposited on
 assembly elements either directly or indirectly. In direct deposition, the
 zone is deposited directly on the electrode, the membrane, or both as part
 of MEA assembly. In indirect deposition, however, the zone is initially
 deposited onto a substrate, not an assembly element, and then the zone is
 transferred from the substrate to the assembly element, preferably the
 membrane. The substrate can be, for example, low surface energy support
 such as skived polytetrafluoroethylene which allows for ready transfer and
 preservation of the zone.
 Additional components and conventional methods can be used to assemble fuel
 cells and stacks. For example, gas diffusion media include CARBELO.RTM. CL
 available from W. L. Gore and Associates, Inc. MEAs known as PRIMEA.RTM.
 (including 5000 and 5510 series) are also available from W. L. Gore and
 Associates, Inc. Fuel cell gaskets can be made of, for example,
 GORE-TEX.RTM., also available from W. L. Gore and Associates, Inc. The
 present invention is not particularly limited by these additional
 components and methods.
 The invention is versatile and can be used in a variety of applications
 including: (i) transportation vehicles such as cars, trucks, and buses
 which have requirements including high power density and low cost; (ii)
 stationary power applications, wherein high efficiency and long life are
 required; and (iii) portable power applications such as portable
 television, fans, and other consumer products. Methods to use fuel cells
 in these applications are known.
 Surprisingly, MEAs according to this invention can provide catalyst mass
 activities greater than 2,500 mA/mg of catalytically active metal, and
 preferably, greater than 5,000 mA/mg of catalytically active metal. At
 this catalyst mass activity level, commercialization becomes feasible. The
 zone of second catalytically active metal does not modify important
 commercial considerations such as the existing water balance. Hence, MEAs
 according to the present invention can be operated under the same
 temperature and humidification conditions.
 Additional fuel cell technology is described in, for example, the
 references cited in the background as well as the following references,
 which are hereby incorporated by reference: (i) "High performance proton
 exchange membrane fuel cells with sputter-deposited Pt layer electrodes";
 Hirano et al.; Electrochimica Acta, vol. 42, No. 10, pp. 1587-1593 (1997);
 (ii) "Effect of sputtered film of platinum on low platinum loading
 electrodes on electrode kinetics of oxygen reduction in proton exchange
 membrane fuel cells"; Mukerjee et al.; Electrochimica Acta, vol. 38, No.
 12, pp. 1661-1669 (1993); (iii) "Sputtered fuel cell electrodes"; Weber et
 al.; J. Electrochem. Soc., June 1987, pp. 1416-1419; and (iv) "Anodic
 oxidation of methanol at a gold modified platinum electrocatalyst prepared
 by RF sputtering on a glassy carbon support"; Electrochimica Acta, Vol.
 36, No. 5/6, pp. 947-951, 1991.
 The invention is further described by means of the following non-limiting
 examples.
 EXAMPLES
 General Procedures
 In each example, unless otherwise noted, the ionically conductive membrane
 (proton exchange membrane, PEM) was 20 microns thick. The membrane was a
 fully impregnated membrane of high Gurley number (&gt;10,000 seconds) and
 high ionic conductance prepared by impregnating expanded
 polytetrafluoroethylene with a perfluorinated sulfonic acid resin
 (FLEMION.RTM., EW 950) as described in U.S. Pat. Nos. 5,547,551;
 5,635,041; and 5,599,614 to Bahar et al. The membrane is called
 GORE-SELECT.RTM. and is available from W. L. Gore and Associates, Inc.
 The electrode comprising the first catalytically active metal, unless
 otherwise noted, was prepared as described above for Procedure A to
 generate a target metal loading. The membrane comprises Pt supported on
 carbon, ionically conductive polymer, and solvent. The electrodes had
 platinum loadings which ranged from 0.05 mg Pt/cm.sup.2 to 0.4 mg
 Pt/cm.sup.2.
 In Examples 2 and 4 below, a zone of second catalytically active metal was
 coated or deposited onto a substrate, either electrode or membrane, by
 electron beam physical vapor deposition (EB-PVD). In this procedure, a
 substrate, typically 6 in..times.6 in., was mounted onto a 4-point holder
 carrousel in a vacuum chamber, where each holder was mounted on a rotating
 axis, each of which could rotate about the main axis of the carrousel. A
 platinum target was prepared by melting 99.95% purity platinum coins in a
 2 in..times.2 in. crucible in the vacuum chamber (1.5 m diameter, 2 m
 long), followed by recooling. The crucible was also located in the vacuum
 chamber. The chamber was then evacuated to less than 10.sup.-4 torr (e.g.,
 5.times.10.sup.-5 torr) using a diffusion pump. The platinum target was
 then evaporated using an electron beam for heating, and platinum was
 condensed onto the substrate. A real uniformity of the deposited coating
 was ensured by rotating the sample about both rotational axes of the
 holder during deposition. The amount of platinum zone deposited was
 measured using a vibrating crystal microbalance, calibration curves, and
 deposition rates and times. Zone thickness and loading amounts were
 calculated.
 In Examples 1-3 and 5 below, I-V measurements were obtained after the MEA
 had reached steady state.
 In each Example, the area of the cathode and anode contacting the membrane
 were substantially the same. In practicing this invention, however, these
 areas do not need to be the same.
 Unless otherwise noted, MEA testing was carried out with: 25 cm.sup.2
 electrode active area; ELAT.RTM. gas diffusion media (available from
 E-TEK, Inc., Natick, Mass.); clamping at 200 lb in/bolt torque; and GLOBE
 TECH.RTM. computer controlled fuel cell test station. The gas diffusion
 media was believed to comprise approximately 70% graphite cloth and 30%
 polytetrafluoroethylene. Clamping assured compression of the MEA to the
 flow field and diffusers.
 Catalyst and electrode layers were supported on polytetrafluoroethylene
 backings and were transferred from the backing to the membrane by decal
 methods with hot pressing. Unless otherwise noted, hot pressing was
 carried out for 3 minutes at 160.degree. C. with a 15 ton load. The
 backing was subsequently peeled off, leaving the coated layer(s) bonded to
 one side of the membrane and positioned centrally.
 Reference MEAs, unless otherwise noted, were substantially the same as the
 MEA according to the invention except that no z-gradient zone was present
 in the reference MEA.
 Example 1
 Example 1 illustrates the indirect method wherein the zone of second
 catalytically active metal is first deposited onto a substrate before
 transfer from the substrate to the membrane or electrode.
 A 50 .ANG. platinum coating zone (0.01 mg/cm.sup.2) was deposited at 1
 .ANG./sec onto a skived PTFE substrate backing by EB-PVD. The catalyst
 zone was then transferred onto the membrane by the decal method leaving
 the 50 .ANG. catalyst zone bonded to one side of the membrane and
 positioned centrally. The area of the membrane demarcated by the
 transferred catalyst is the active area. A catalyzed electrode (0.3 mg
 Pt/cm.sup.2) was attached to each side of the catalyzed membrane also
 using the decal method, so as to overlay the active area. Therefore, one
 side of the MEA had a z-gradient zone of platinum at the
 membrane/electrode interface.
 The prepared MEAs with 25 cm.sup.2 active areas were each loaded between
 gaskets in a 25 cm.sup.2 active area fuel cell test fixture or cell. The
 electrode containing the z-gradient zone was placed towards the cathode
 where it would be in contact with the oxidant (air). The test fixture was
 then attached to the fuel cell test station for acquisition of data.
 MEA performance was evaluated with the cell pressure at 0 psig and at 15
 psig. For the 0 psig cell pressure runs, the cell was operated at
 60.degree. C., with hydrogen and air humidified to dew points of
 20.degree. C. and 55.degree. C. respectively. For the 15 psig cell
 pressure runs, the cell was operated at 75.degree. C., with hydrogen and
 air both supplied at 15 psig and humidified to dew points of 30.degree. C.
 and 70.degree. C. Hydrogen and air flow rates were set to 2 and 3.5 times
 the stoichiometric value theoretically needed to produce a given cell
 current output.
 FIG. 5 shows the fuel cell output voltage at various current outputs for
 the MEA at 0 psig. Superior performance was observed in the MEA according
 to the invention compared to a reference MEA which was substantially the
 same except it did not contain the z-gradient catalyst layer. For example,
 at 0.6 V, the MEA according to the invention produced almost 1200
 MA/cm.sup.2 versus only about 820 mA/cm.sup.2 for the reference (a 46%
 increase).
 Similarly, FIG. 6 shows data for the 15 psig cell. Again, the polarization
 analysis showed improved performance over the entire range of current
 densities. At 0.6 V, for example, the MEA containing z-gradient cathode
 produced almost 1600 mA/cm.sup.2 versus only 1200 mA/cm.sup.2 for the
 reference MEA (33% increase) which was substantially the same but did not
 contain the z-gradient cathode. Power density is also plotted in FIG. 6
 (p-I.times.V), and improved power density was also evident.
 FIG. 7 shows an electrocatalyst mass activity analysis for the 15 psig
 cell. The mass activity is the amount of current generated (or
 alternatively power generated) per unit mass of catalyst metal in the
 active area. Hence, mass activity units are mA/mg Pt for current
 generation (and mW/mg Pt for power generation). At 0.6 V, the MEA with
 z-gradient cathode surprisingly produced over 2,500 mA/mg Pt compared to
 only 2,000 mA/mg Pt (i.e., a 25% increase) for the reference MEA which was
 substantially similar but did not contain the z-gradient cathode.
 FIG. 8 shows data for an MEA at 0 psig where the z-gradient catalyst zone
 was part of the anode rather than cathode. Surprisingly, the polarization
 analysis revealed an improvement in performance with a z-gradient anode
 (12% increase at 0.6 V), although the improvement was not as large as for
 the MEA with a z-gradient cathode.
 Example 2
 In this Example, direct deposition of the zone on the electrode was carried
 out at two zone thicknesses. Deposition was carried out by EB-PVD. The
 catalyzed electrodes having the z-gradient deposited thereon had a loading
 of 0.1 mg Pt/cm.sup.2 before deposition. For one sample, the deposition
 rate was 0.2-0.3 .ANG./sec to achieve a 50 .ANG. zone (0.01 mg
 Pt/cm.sup.2). A second electrode was coated at a rate of o.1 .ANG./sec to
 achieve a 5 .ANG. zone (0.001 mg Pt/cm.sup.2). An electrode (anode)
 containing 0.05 mg/cm.sup.2 of platinum was used for both samples.
 MEA performance was again evaluated with the cell pressure at 0 psig and at
 15 psig. For all runs, the cell was operated at 65.degree. C., with
 hydrogen and air both supplied at 0 psig, and humidified to dew points of
 60.degree. C. Hydrogen and air flow rates were set to 1.2 and 3.5 times
 the stoichiometric value theoretically needed to produce a given cell
 current output respectively.
 FIG. 9 shows the improved power output at 0 psig. Improvements in current
 density were observed at 0.6 V from 240 mA/cm.sup.2 for the reference MEA:
 (i) to 460 mA/cm.sup.2 for the 50 .ANG. deposition (92w increase), and
 (ii) to 510 mA/cm.sup.2 for a 5 .ANG. deposition (113% increase).
 Surprisingly, the lower loading (thinner deposition) provided a greater
 percentage increase at this voltage.
 FIG. 10 shows fuel cell performance at 15 psig cell pressure, in terms of
 both current density and power density, for the 5 .ANG. sample. The data
 indicated an increase in current density at 0.6 V from 440 to 860
 mA/cm.sup.2 (95% increase), with a substantial increase in peak power
 density.
 FIG. 10 also shows polarization performance as compensated cell potential
 versus current density at 15 psig. When the polarization curve is
 expressed in terms of compensated potential, the electrocatalytic
 performance of the z-gradient cathode is shown independent of the effects
 of other MEA components. By comparison of compensated potentials, FIG. 10
 showed that improved MEA performance was due to improved cathode
 performance (resulting from z-gradient layer), and not from some other
 spurious secondary effects.
 FIG. 11 shows the corresponding improvement in electrocatalyst mass
 activity and specific power at 15 psig. The observed enhancement in
 electrocatalyst utilization was proportional to the enhancement in
 current/power density.
 Surprisingly, the percent increases in current found at 0.6 V were
 significantly higher in Example 2 compared to Example 1. In addition, the
 MEAs of Example 2 had less precious metal than the MEAs of Example 1.
 Example 3
 This example illustrates DC magnetron sputtering compared to EB-PVD. An
 electrode (0.4 mg Pt/cm.sup.2) on a skived PTFE backing was coated by D.C.
 magnetron sputtering. A 0.127 mm thickness, 99.9% purity platinum foil
 served as target, and the vacuum chamber base pressure was maintained at
 8.times.10.sup.-4 torr. More specifically, a vacuum less than 10.sup.-4
 torr was established, and then high purity argon was bled in so that the
 pressure rose to 8.times.10.sup.-4 torr. Platinum deposition rate was
 about 1 .ANG./sec continuous to achieve a platinum loading of 0.01
 mg/cm.sup.2 (50 .ANG.). This sputtered electrode was used as cathode. An
 unsputtered electrode (0.4 mg Pt/cm.sup.2) served as anode.
 MEA performance was evaluated with the cell pressure at 0 psig and at 15
 psig. For the 0 psig cell pressure runs, the cell was operated at
 70.degree. C., with hydrogen and air both supplied at 0 psig, and
 humidified to dew points of 55.degree. C. and 70.degree. C. respectively.
 The 15 psig runs were performed at a cell temperature of 80.degree. C.,
 with hydrogen and air both supplied at 15 psig, and humidified to dew
 points of 600C. and 75.degree. C. respectively. For all runs, hydrogen and
 air flow rates were set to 2 and 3.5 times the stoichiometric values
 respectively.
 FIG. 12 shows that for 0 psig at 0.6 V there is an improvement in current
 density from 820 mA/cm.sup.2 for the reference MEA to 1050 mA/cm.sup.2
 (28% increase) for the sputtered z-gradient MEA. FIG. 13 shows fuel cell
 performance at 15 psig cell pressure. There is an improvement in current
 density from 1200 mA/cm.sup.2 (reference MEA) to 1360 mA/cm.sup.2 for the
 sputtered cathode (13% increase). Hence, the percent increases in Example
 3 were not as great as observed in Example 2.
 Example 4
 Membranes were coated with platinum using EB-PVD and DC magnetron
 sputtering. Loadings for different samples were 0.001, 0.01, 0.05, and 0.1
 mg Pt/cm.sup.2. One side of the membrane was coated. MEAs were prepared
 from the coated membranes.
 Example 5
 A zone of second catalytically active metal (50 .ANG.) was deposited onto
 the membrane by the indirect transfer method. The Pt/skived PTFE was hot
 pressed against the membrane to bond the Pt evaporated layer to the
 membrane by the decal method. The skived PTFE layer was peeled off, thus
 leaving a zone of 50 .ANG. Pt layer bonded to the membrane. Catalyzed
 electrodes (0.3 mg Pt/cm.sup.2) were then attached by hot pressing to form
 a first MEA.
 A second MEA was prepared in which the cathodic active phase was just the
 electrode structure formed by a thin 50 .ANG. Pt layer bonded to the
 membrane. The anode had a loading of 0.2 mg Pt/cm.sup.2.
 Polarization performance was evaluated at 0 psig cell pressure. The
 atmospheric pressure run, having both anode and cathode at 0/0 psig
 respectively, was performed at 60.degree. C. cell temperature with
 hydrogen and air reactants saturated in humidification bottles to ca. 100%
 relative humidity. The anode, hydrogen, and cathode, air, reactants were
 then saturated at 20/60.degree. C., respectively. The reactant flow was
 set to 2/3.5 times the stoichiometric value, for hydrogen and air
 respectively, and the stoichiometric flow was maintained throughout the
 polarization curve.
 FIG. 14 shows the performance of the first and second MEAs. The difference
 in performance observed between the two MEAs indicates that the 50 .ANG.
 layer presents low activity in itself at this low loading, but its
 presence at the interface between the electrocatalyst layer and membrane
 produces a power improvement and improves the electrode current density
 profile.
 FE-SEM Analysis
 FE-SEM analyses were carried out for one comparative sample of an electrode
 with no zone present (FIG. 15) and for 3 samples with different zone
 thicknesses deposited onto the electrode (FIGS. 16-18). For FIGS. 15-18,
 the magnification was 20kX and the electron beam energy was 2 keV. The
 analyses showed relatively uniform zone deposition with FIGS. 15-18 being
 representative. In general, the microstructure was represented by a
 combined spherical nodular and whisker morphology, with the latter
 evidenced at loadings of about 0.1 mg/cm.sup.2 (500 .ANG.) (FIG. 18).
 FIG. 15 was taken from a sample of the cathode used in Example 2 with 0.1
 mg/cm.sup.2 Pt loading but without deposition of the second catalytically
 active metal. FIG. 15 demonstrates the electrode porosity, which allows
 for reactant diffusion, before deposition of the second catalytically
 active metal.
 FIG. 16 was taken from a sample of the Example 2 cathode with a 0.1
 mg/cm.sup.2 Pt loading but with a 5 .ANG. zone deposition (0.001
 mg/cm.sup.2) by EB-PVD. A small but measurable increase in field
 brightness was evident in FIG. 16 compared with the FIG. 15 control. The
 increased brightness was uniform across the Figure which suggested an
 evenly deposited platinum zone. The electrode remained porous and open to
 reactant diffusion despite the deposition.
 FIG. 17 was taken from a sample of the Example 2 cathode with a 0.1
 mg/cm.sup.2 Pt loading but with a 50 .ANG. zone deposition (0.01
 mg/cm.sup.2) by EB-PVD. A further increase in field brightness was
 observed compared with FIG. 16. Spherical platinum nodules were present
 with diameter widths between about 30 and about 70 nm, and generally about
 50 nm. The electrode remained porous and open to reactant diffusion
 despite the deposition.
 FIG. 18 was taken from a sample of the electrode similar to that of Example
 2 but with no Pt loading before the deposition. The electrode was then
 provided with a 500 .ANG. Pt zone by EB-PVD. Again, spherical platinum
 nodules were present with diameter widths between about 25 nm and about
 100 nm, and more particularly, about 30 nm and about 70 nm, and generally
 about 50 nm. In addition, however, rod-shaped structures were also
 present. The width diameter of these rods was about 20 nm to about 60 nm,
 and generally, about 40 nm. The electrode remained porous and open to
 reactant diffusion despite the deposition.
 Data Summary
 Data from these examples are summarized below:

Percent
 increase in
 current at
 0.6 V
 Zone compared to
 Example thickness Pressure reference
 number (.ANG.) (psig) MEA
 1 50 0 46
 1 50 15 33
 2 50 0 92
 2 5 0 113
 2 5 15 95
 3 50 0 28
 3 50 15 13
 The foregoing description of preferred embodiments of the invention have
 been presented for purposes of illustration and description. It is not
 intended to be exhaustive or to limit the invention to the precise form
 disclosed. Hence, many modifications and variations are possible in light
 of the above teaching.