Abstract:
A method for making platinum group metal (PGM) alloys for fuel cell applications includes a step of heating a substrate to a predetermined temperature. The substrate is contacted with a vapor of a PGM-containing compound and then with a vapor of an early transition metal-containing compound. These contacting steps are repeated a plurality of times to form a PGM alloy layer on the carbon particles. The present method allows the PGM alloy layer to be built up monolayer-by-monolayer thereby providing for uniform coating on a support with high porosity or complex morphology. Advantageously, the present embodiment provides a method for preparing a catalyst with higher activity and durability than current alloy catalysts.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application No. 62/005,410 filed May 30, 2014, the disclosure of which is incorporated herewith in its entirety by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    In at least one aspect, the present rejection is related to corrosion resistant carbon supports for fuel cell and battery applications. 
       BACKGROUND 
       [0003]    Fuel cells are used as an electrical power source in many applications. In particular, fuel cells are proposed for use in automobiles to replace internal combustion engines. A commonly used fuel cell design uses a solid polymer electrolyte (“SPE”) membrane or proton exchange membrane (“PEM”) to provide ion transport between the anode and cathode. 
         [0004]    In proton exchange membrane type fuel cells, hydrogen is supplied to the anode as fuel and oxygen is supplied to the cathode as the oxidant. The oxygen can either be in pure form (O 2 ) or air (a mixture of O 2  and N 2 ). PEM fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face, and a cathode catalyst on the opposite face. The anode and cathode layers of a typical PEM fuel cell are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel and oxidant to disperse over the surface of the membrane facing the fuel- and oxidant-supply electrodes, respectively. Each electrode has finely divided catalyst particles (for example, platinum particles) supported on carbon particles to promote oxidation of hydrogen at the anode and reduction of oxygen at the cathode. Protons flow from the anode through the ionically conductive polymer membrane to the cathode where they combine with oxygen to form water which is discharged from the cell. The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”) which, in turn, are sandwiched between a pair of non-porous, electrically conductive elements or plates. The plates function as current collectors for the anode and the cathode, and contain appropriate channels and openings formed therein for distributing the fuel cell&#39;s gaseous reactants over the surface of respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cell stacks in order to provide high levels of electrical power. 
         [0005]    Alloying of Pt with transition metals (Co, Ni, etc.) is commonly attempted to increase the activity of the catalyst but the stability of these metals leads to losses of activity and performance. Accordingly, there is a need for improved methodology for making carbon supported electrocatalysts for fuel cell applications. Theoretical and experimental studies suggest both high activity and stability in alloys of Pt and early transition metals, more specifically yttrium and scandium. However, the high affinity of oxygen of these elements makes it very difficult to form small dispersed particles or sufficiently thin film to make the catalyst economically viable. Very high temperature which causes particle growth also is required to prepare these alloys. To our knowledge, successful preparation has only been achieved with a sputtering method which is not a controllable means to form nanoparticles. 
         [0006]    Accordingly, there is a need for improved methods of forming catalysts for fuel cells with higher catalytic activity than currently available. 
       SUMMARY 
       [0007]    The present invention solves one or more problems of the prior art by providing in at least one embodiment, a method for making platinum alloys for fuel cell applications. The method includes a step of heating a substrate to a predetermined temperature. The substrate is contacted with a vapor of a platinum group metal (PGM) containing compound to form a layer of PGM-containing compound precursors disposed over the substrate. The substrate is also contacted with a vapor of an early transition metal-containing compound to form a layer of early transition metal-containing precursors disposed over the substrate. The layer of PGM-containing compound residues and the layer of early transition metal-containing precursors are contacted with a hydrogen plasma to form a monolayer of PGM alloy. The steps of contacting the substrate with the PGM-containing compound and the early transition metal compound, and the hydrogen plasma are repeated a plurality of times to form a platinum alloy layer of predetermined thickness on the substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a cross sectional view of a fuel cell incorporating a substrate coated with a platinum alloy layer; 
           [0009]      FIG. 2  provides a schematic flowchart illustrating a method for making a substrate coated with a platinum alloy layer; 
           [0010]      FIG. 3  provides a schematic flowchart illustrating a method for making a substrate coated with a platinum alloy layer; 
           [0011]      FIG. 4  provides a schematic flowchart illustrating a method for making a substrate coated with a platinum alloy layer; and 
           [0012]      FIG. 5  is a schematic diagram of an atomic layer deposition system. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present invention which constitute the best modes of practicing the invention presently known to the inventors. The Figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for any aspect of the invention and/or as a representative basis for teaching one skilled in the art to variously employ the present invention. 
         [0014]    Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of,” and ratio values are by weight; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property. 
         [0015]    It is also to be understood that this invention is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way. 
         [0016]    It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components. 
         [0017]    Throughout this application, where publications are referenced, the disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains. 
         [0018]    The term “residue” as used in at least one embodiment refers to that portion of a chemical compound that remains on a substrate after the substrate is contacted with the chemical compound. 
       Abbreviations: 
       [0019]    “ALD” refers to atomic layer deposition. 
         [0020]    “NSTF” refers to nanostructured thin film. 
         [0021]    With reference to  FIG. 1 , a cross sectional view of a fuel cell incorporating the corrosion resistant substrate set forth above is provided. PEM fuel cell  10  includes polymeric ion conducting membrane  12  disposed between cathode electro-catalyst layer  14  and anode electro-catalyst layer  16 . Fuel cell  10  also includes electrically conductive flow field plates  20 ,  22  which include gas channels  24  and  26 . Flow field plates  20 ,  22  are either bipolar plates (illustrated) or unipolar plates (i.e., end plates). In a refinement, flow field plates  20 ,  22  are formed from a metal plate (e.g., stainless steel) optionally coated with a precious metal such as gold or platinum. In another refinement, flow field plates  20 ,  22  are formed from conducting polymers which also are optionally coated with a precious metal. Gas diffusion layers  32  and  34  are also interposed between flow field plates and a catalyst layer. Cathode electro-catalyst layer  14  and anode electro-catalyst layer  16  include catalytic platinum alloys made by the processes set forth below. Advantageously, the platinum alloys enhance the activity of the oxygen reduction reaction when incorporated into cathode electro-catalyst layers. 
         [0022]    With reference to  FIG. 2 , a method for making platinum alloy catalysts for fuel cells is provided. The method includes step a) in which substrate  36  is contacted with a vapor of a platinum group metal-containing compound  38  and an early transition metal-containing compound  40  to form precursor layer  42 . Typically, PGM containing compounds include Pt, Pd, Au, Ru, Ir, Rh, or Os. Typically, early transition metal-containing compounds include Sc, Ti, V, Cr, Mn, Y, Zr, Nb, Mo, Tc, La, Hf, Ta, W, or Re. In a refinement, early transition metal-containing compounds include Sc, Ti, Y, or Zr. Yttrium-containing and scandium-containing compounds are particularly useful. An example of a platinum-containing compound includes, but is not limited to, trimethyl(methylcyclopentadienyl)platinum. Examples of yttrium-containing compounds include, but are not limited to, tris(methylcyclopentadienyl)yttrium and tris[N,N-bis(trimethylsilyl)amide]yttrium. Examples of scandium-containing compounds include, but are not limited to, tris(methylcyclopentadienyl)scandium, tris(cyclopentadienyl)scandium, tris[N,N-(diisopropyl)acetamidinate)scandium, and Sc(2,2,6,6-tetramethyl-3,5-heptanedione). Although virtually any substrate can be used in practicing the method of the present embodiment, suitable examples include TiO 2  supports, and NSTF supports, other metal oxide supports, metal carbide supports, carbon black supports, carbon nanotube supports. In a refinement, the support includes carbon particles having an average spatial dimension from about 10 to 100 nanometers. In step b), precursor layer  42  is contacted with a hydrogen plasma to form a platinum group metal alloy layer  44 . These contacting steps are repeated a plurality of times to form a platinum group metal alloy layer  46  of predetermined thickness on (e.g., contacting the surface of) the substrate  36  as indicated by loop c). In a variation, the contacting steps are repeated from 1 to several thousand deposition cycles depending on the desired thickness of a platinum group metal alloy layer  44 . In a refinement, the contacting steps are repeated for 1 to 5000 deposition cycles. In another refinement, the contacting steps are repeated for 10 to 2000 deposition cycles. In still another refinement, the contacting steps are repeated for 20 to 1000 deposition cycles 
         [0023]    With reference to  FIG. 3 , a method for making a platinum group metal alloy catalyst by atomic layer deposition (ALD) is provided. In step a), substrates  50  are heated to a predetermined temperature in an ALD reactor. In a refinement, substrates  50  are heated to a temperature from about 80° C. to 150° C. Examples of suitable substrates are set forth above. In step b), the substrates  50  are contacted with a vapor of a platinum group metal-containing compound and an early transition metal-containing compound in the reactor to form substrates  52  in which a layer including residues of the platinum group metal-containing compound and early transition metal-containing compound is individually disposed over substrates  50 . Suitable platinum group metal-containing compounds and early transition metal-containing compounds are set forth above. In step p 1 ), the reactor is evacuated and/or substrates  52  are purged with an inert gas (e.g., nitrogen, helium, argon, and the like) by purging the ALD reactor. In step c), substrates  52  are contacted with a H 2  plasma to form a monolayer of a platinum group metal alloy disposed over substrates  50  (item number  54  refers to the coated substrates formed in this step). In step p 2 ), the reactor is evacuated and/or substrates  54  are purged with an inert gas (e.g., nitrogen, helium, argon, and the like). 
         [0024]    Still referring to  FIG. 3 , as illustrated by loop d, steps b, p 1 , c, p 2  are repeated a plurality of times to form a platinum group metal alloy layer  56  of predetermined thickness on substrate  50 . In a refinement, these steps are repeated 1 to 1000 times to build up the thickness of the platinum group metal alloy layer  56  monolayer by monolayer until a desired thickness is achieved. In a refinement, the thickness of the platinum group metal alloy layer  56  is from 0.2 nanometer to 30 nanometers. In another refinement, the thickness of the platinum group metal alloy layer  56  is from 0.2 nanometers to 4 nanometers. In step e), platinum group metal alloy coated substrates  50  are incorporated into cathode catalyst layer  14  and/or anode catalyst layer  16  of fuel cell  10 . 
         [0025]    With reference to  FIG. 4 , a method for making a platinum group metal alloy catalyst by atomic layer deposition (ALD) is provided. In step a), substrates  50  are heated to a predetermined temperature in an atomic layer deposition reactor. In a refinement, substrates  50  are heated to a temperature from about 80° C. to 150° C. Examples for substrates  50  are set forth above. In step b), substrates  50  are contacted with a vapor of a platinum group metal-containing compound to form substrates  62  in which a layer of platinum group metal-containing compound residues are disposed over substrates  50 . Suitable platinum group metal-containing compounds are set forth above. In step p 1 ), the reactor is evacuated and/or substrates  62  are purged with an inert gas (e.g., nitrogen, helium, argon, and the like). In step c), substrates  62  are contacted with a hydrogen plasma to form substrates  64  in which the residues of the platinum group metal-containing compound have been converted to platinum group metal. In step p 2 ), the reactor is evacuated and/or substrates  64  are purged with an inert gas (e.g., nitrogen, helium, argon, and the like). In step d), substrates  64  are contacted with a vapor of an early transition metal-containing compound to form substrates  66  in which a layer of residues of the early transition metal-containing compound are disposed over substrates  50 . In step p 3 ), the reactor is evacuated and/or substrates  66  are purged with an inert gas (e.g., nitrogen, helium, argon, and the like). In step e), substrates  66  are contacted with a hydrogen plasma to form substrates  68  in which the residues of the early transition metal-containing compound have been converted to a monolayer of early transition metal. In step p 4 ), the reactor is evacuated and/or substrates  68  are purged with an inert gas (e.g., nitrogen, helium, argon, and the like). 
         [0026]    Still referring to  FIG. 4 , as illustrated by loop f, steps b, p 1 , c, p 2 , d, p 3 , e, and p 4  are repeated a plurality of times to form a platinum group metal alloy layer  70  of predetermined thickness disposed over substrates  50 . In a refinement, these steps are repeated 1 to 1000 times to build up the thickness of the platinum group metal alloy layer  56  monolayer by monolayer until a desired thickness is achieved. In a refinement, the thickness of the platinum group metal alloy layer  56  is from 0.2 nanometers to 30 nanometers. In another refinement, the thickness of the platinum group metal alloy layer  56  is from 0.2 nanometers to 4 nanometers. In step g), platinum group metal alloy coated substrates  50  are incorporated into cathode catalyst layer  14  and/or anode catalyst layer  16  of fuel cell  10 . 
         [0027]    With reference to  FIG. 5 , a schematic illustration of an atom layer deposition apparatus for implementing the methods set forth above is provided. Reactor  78  includes vacuum chamber  80  which has platinum group metal-containing compound source  82  with associated pulse valve  84 , early transition metal-containing compound source  86  with associated pulse valve  88 , and purge gas source  90  with associated pulse valve  92 . Reactor  80  also includes hydrogen plasma source  94  which has hydrogen gas source  96 , associated pulse valve  98 , and RF coils  100  with associated power supply  102 . The RF coils induce the H 2  plasma formation as hydrogen-containing gas flows through conduit  102 . In each case, the respective gaseous reactant is introduced by opening of the pulse valve for a predetermined pulse time. Similarly, for purging steps the inert gas is introduced by opening of pulse valve  92  for a predetermined purge time. In one refinement, pulse times and purge times are each independently from about 0.0001 to 200 seconds. In another refinement, pulse and purge times are each independently from about 0.1 to about 10 seconds. 
         [0028]    During coating, the substrates  50  are heated via heater  104  to a temperature suitable to the properties of the chemical precursor(s) and coatings to be formed. In another refinement of the method, the substrate has a temperature from about 80 to 150° C. Similarly, the pressure during film formation is set at a value suitable to the properties of the chemical precursors and coatings to be formed. Vacuum system  106  is used to establish the reactor pressure and remove the reagents and purge gas. In one refinement, the pressure is from about 10 −6  Torr to about 760 Torr. In another refinement, the pressure is from about 0.1 millitorr to about 10 Torr. In still another refinement, the pressure is from about 1 to about 5 Torr. 
         [0029]    The following examples illustrate the various embodiments of the present invention. Those skilled in the art will recognize many variations that are within the spirit of the present invention and scope of the claims. 
         [0030]    A 3M NSTF support is used as a catalyst as a substrate. The NSTF support is a highly-oriented laft-shape substrate made from a self-assembly of an organic compound, perylene red dye. Its high length-to-width aspect ratio of about 15 makes it very difficult to be coated with any metal and especially a high-surface-energy metal such as platinum. It is noted that large amounts of platinum are wasted at the substrate tip when the deposition of platinum alloy is performed with conventional sputtering methods. A 2 nm thick tungsten layer is first deposited by alternating 14 cycles of WF 6  (as W precursor) and Si 2 H 6  (as a reactant). Pt and early transition metals are co-deposited or alternatively-deposited onto the adhesive layer using H 2  plasma. Therefore, 150 cycles of Pt and Y ALD at 120° C. and 100 watts H 2  plasma yield about 3 nm thick film. Examples of the metal precursors include trimethyl(methylcyclopentadienyl)platinum, tris(methylcyclopentadienyl)yttrium, tris [N,N-bis(trimethylsilyl)amide]yttrium, tris(methylcyclopentadienyl)scandium, tris(cyclopentadienyl)scandium, tris[N,N-(diisopropyl)acetamidinate)scandium, and Sc(2,2,6,6-tetramethyl-3,5-heptanedione). 
         [0031]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.