Coated substrate with a thin conformal film made of precious metal

Abstract

Coated substrate comprising: a plurality of substrate particles (30); an adhesive layer (36) disposed over each substrate particle, the adhesive layer (36) comprising a component selected from the group consisting of tungsten metal, a tungsten alloy, tungsten oxide, tungsten nitride, and tungsten carbide; and a precious metal layer (40) disposed over the adhesion layer, wherein the precious metal is selected from the group consisting of platinum, gold, palladium, iridium and alloys thereof, and combinations thereof; characterized in that the substrate further comprises an intermediate layer (34) disposed between each substrate particle (30) and the adhesion layer (36); and in that the adhesion layer (36) comprises a lattice-disrupting layer (36") between two tungsten metal layers (36') different from the lattice-disrupting layer (36").

Description

TECHNICAL FIELD

[0001] The present invention relates to catalyst layers used in fuel cell applications and in particular to a coated substrate with a thin conformal film of noble metal according to the preamble of claim 1, as known for example from US 2009 / 0 054 228 A1 and WO 2013 / 144 631 A1. BACKGROUND

[0002] Fuel cells are used as a source of electrical energy in many applications. In particular, fuel cells are being proposed for use in automotive vehicles as a replacement for internal combustion engines. A commonly used fuel cell design uses a solid polymer electrolyte ("SPE") membrane or a proton exchange membrane ("PEM") to provide ion transport between the anode and cathode.

[0003] In proton exchange membrane fuel cells, hydrogen is supplied to the anode as fuel and oxygen is supplied to the cathode as oxidant. The oxygen can be in either pure form (O2) or as air (a mixture of O2 and N2). PEM fuel cells typically comprise a membrane electrode assembly ("MEA") in which a solid polymer membrane has an anode catalyst on one side and a cathode catalyst on the opposite side. The anode and cathode layers of a typical PEM fuel cell are formed from porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper, to allow the fuel and oxidant to disperse across the surface of the membrane facing the fuel and oxidant supply electrodes, respectively.Each electrode has finely dispersed catalyst particles (e.g., platinum particles) supported on carbon particles to promote the oxidation of hydrogen at the anode and the 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 then removed 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 act as current collectors for the anode and cathode and include channels and openings formed therein suitable for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts.To produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton-permeable, electrically non-conductive, and gas-impermeable. In typical applications, fuel cells are deployed in arrays of many individual fuel cells arranged in stacks to deliver high levels of electrical power.

[0004] Carbon black and carbon nanotubes are preferred supports for fuel cell electrocatalysts due to their excellent electrical conductivity, good mechanical and chemical stability, and low cost. Catalyst layers typically use platinum and / or a platinum alloy, which are fabricated into very fine platinum particles to improve overall activity by increasing surface area. However, the inert surface of the carbon makes it very difficult for the metal to adhere. Nucleation of platinum on carbon surfaces has proven challenging, resulting in large platinum particles as well as particle migration and agglomeration. In addition, due to the high surface energy of platinum, it is very difficult to coat a thin and flat surface.Scanning electron micrographs demonstrate the tendency of these state-of-the-art coatings to agglomerate without forming a smooth, homogeneous layer. The weak interaction between platinum and carbon surfaces hampers nucleation and the formation of a uniform film. In particular, due to poor nucleation and poor layer growth, a relatively large minimum layer thickness is necessary to ensure electrical conductivity.

[0005] A commercially available nanostructured thin film (NSTF) catalyst is produced by sputtering Pt onto a self-assembled perylene red dye support. The support is electrically non-conductive, so electrons must be transported through the coated continuous layer of platinum. On the other hand, these catalyst types are more durable than conventional carbon-supported platinum nanoparticles due to the smooth Pt surface and the superior stability of the support. It has been observed that some of the prior art sputtered films are not uniform due to the line-of-sight nature of the sputtering process.

[0006] Accordingly, there is a need for improved materials for the formation of fuel cell catalyst materials. SUMMARY

[0007] The present invention solves one or more problems of the prior art by providing, according to at least one embodiment, a coated substrate having the features of claim 1 for forming fuel cell catalyst layers. The coated substrates comprise a plurality of substrate particles. An adhesion layer is disposed over each substrate particle, and a noble metal layer is disposed over the adhesion layer. The adhesion layer comprises a tungsten metal layer, a tungsten alloy layer, a WO x -layer, a W x N-layer or a W xC layer. Typically, the coated substrate is used in fuel cell applications, such as providing the catalyst particles used in the cathode and / or anode catalyst layers. Advantageously, the present embodiment provides catalysts with higher activity and durability compared to conventional dispersion-supported catalysts. Furthermore, catalyst manufacturing costs are reduced and chemical / precious metal waste is minimized because the process is a dry process.

[0008] In another refinement, a coated substrate is provided for forming fuel cell catalyst layers. The coated substrate comprises a plurality of substrate particles. The substrate particles have an average spatial dimension of about 20 nm to 1 micrometer and a length-to-width aspect ratio of greater than 10. The coated substrate also comprises an adhesion layer disposed over each substrate particle and a noble metal layer disposed over the adhesion layer. The adhesion layer comprises a component selected from the group consisting of tungsten metal, a tungsten alloy, tungsten oxide, tungsten nitride, and tungsten carbide. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Exemplary embodiments of the present invention will be better understood from the detailed description and the accompanying drawings, in which Fig. 1 shows a cross-section of a fuel cell comprising a plurality of noble metal-coated substrate particles, Fig. Figure 2A shows a schematic cross-section of a portion of an unstressed coated substrate particle and Fig. Figure 2B shows a schematic cross-section of a portion of a coated substrate particle according to the invention. DETAILED DESCRIPTION

[0010] Reference will now be made in detail to presently preferred compositions, embodiments, and methods of the present invention which are presently known to the inventors as the best modes for carrying out the invention. The figures are not necessarily to scale. It should be understood, however, 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 merely intended to provide a representative basis for each aspect of the invention and / or as a representative basis for teaching one skilled in the art to variously employ the present invention.

[0011] Except in the examples or where expressly stated otherwise, all numerical references in this specification describing amounts of material or reaction and / or usage conditions are to be understood as if preceded by the word "about." Practices within the stated numerical limits are generally preferred. Also, unless expressly stated to the contrary, percent, parts of, and ratios are by weight. The term "polymer" includes "oligomer," "copolymer," "terpolymer," and the like. 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 members of the group or class are equally suitable or preferred. All molecular weights given for polymers refer to number average molecular weights.A description of components in chemical terms refers to the components at the time of addition to each combination specified in the description and does not necessarily preclude chemical interactions among the components of a mixture after mixing. The first definition of an acronym or other abbreviation applies to all subsequent uses of the same abbreviation and, accordingly, to the normal grammatical variations of the original abbreviation. And, unless expressly stated to the contrary, the measurement of a property is made by the same technique referred to previously or subsequently for the same property.

[0012] It should also be noted that, as used in the specification and the appended claims, the singular form "a" and "the" also includes plural references unless the context clearly indicates otherwise. For example, a reference to a component in the singular form is intended to encompass a plurality of components.

[0013] In one embodiment, the prefix "nano" means that the described particles have at least one spatial dimension of between 1 nanometer and about 100 nanometers. In a variation, the prefix "nano" means that the described particles have at least one spatial dimension of between about 10 nanometers and about 80 nanometers. In another variation, the prefix "nano" means that the described particles have at least one spatial dimension of between about 20 nanometers and about 50 nanometers.

[0014] With reference to Fig. 1, a fuel cell comprising a plurality of noble metal-coated substrate particles is provided. The fuel cell 10 includes the membrane electrode assembly 12, which includes an anode catalyst layer 14, a cathode catalyst layer 16, and an ion-conductive membrane (i.e., proton exchange membrane) 20. One or both of the anode catalyst layer 14 and the cathode catalyst layer 16 comprise a plurality of noble metal-coated substrate particles, which are prepared as explained below. The proton-conductive (i.e., ion-conductive) membrane 20 is sandwiched between the anode catalyst layer 14 and the cathode catalyst layer 16, with the anode catalyst layer 14 disposed over the first side of the proton-conductive membrane 20 and the catalyst layer 16 disposed over the first side of the proton-conductive membrane 20. The fuel cell 10 also includes porous gas diffusion layers 22 and 24.The gas diffusion layer 22 is disposed over the anode catalyst layer 14, whereas the gas diffusion layer 24 is disposed over the cathode catalyst layer 16. In yet another variation, the fuel cell 10 includes an anode flow field plate 26 disposed over the gas diffusion layer 22 and a cathode flow field plate 28 disposed over the gas diffusion layer 24. During operation, hydrogen is supplied as fuel to the anode catalyst layer 14 and oxygen is supplied as an oxidant to the cathode catalyst layer 16, resulting in electricity as a result of the electrochemical processes taking place therein.

[0015] With reference to Fig. 2A and Fig. 2B, schematics of a portion of a coated substrate particle are provided, which as in Fig. 1 is used. Fig. 2A illustrates a schematic section of an unclaimed coated substrate particle utilizing a single adhesion layer. The coated substrate particle 30 comprises a substrate particle 32 and is coated with an intermediate layer 34. The intermediate layer 34 is disposed over the substrate particle 32 to protect the substrate during coating. Alternatively, the intermediate layer 34 is an oxide layer or other inexpensive stable layer. Examples of useful oxide layers include aluminum oxides (e.g., Al2O3), silicon oxides (e.g., SiO2), titanium oxides (e.g., TiO2), zirconium oxides (e.g., ZrO2), WO x , TiC, Ti y N, TiO 2-z N z and the like, wherein x is 2.5 to 3, y is 0.7 to 1.1, and z is 0 to 0.5.

[0016] The adhesion layer 36 is arranged over the substrate particle 32 or, if the intermediate layer 34 is present, over the intermediate layer 34. The adhesion layer 36 comprises a component selected from the group consisting of a tungsten alloy layer, tungsten oxide layers (e.g. WO x , where x is 1.5 to 3.0), tungsten nitride layers (e.g. W o N, where o is 0.5 to 2) and / or tungsten carbide layers (e.g. W p C, where p is 1 to 2). According to the invention, the adhesive layer 36 is Fig.2B is formed from a heterogeneous layer, for example, 36' / 36" / 36', where 36' is a W metal layer and 36" is a "lattice-disrupting layer." The lattice-disrupting layer 36" reduces mechanical stresses of the adhesion layer 36 with increasing thickness, which can develop when it experiences changing environmental conditions. The lattice-disrupting layer 36" can be any layer other than the metal layer 36', for example, Al2O3, Al, WO xetc. In one refinement, the thickness of the adhesion layer 36 is between about 0.5 to about 10 nanometers. In another refinement, the thickness of the adhesion layer 36 is between about 0.5 to about 5 nanometers. In yet another refinement, the thickness of the adhesion layer 36 is between about 1 to about 4 nanometers. In one refinement, the intermediate layer 34 contacts the adhesion layer 36. In a variation, the intermediate layer 34 is a metal oxide layer. For example, the intermediate layer 34 comprises a member selected from the group consisting of aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, tungsten oxide, titanium carbide, titanium nitride, titanium oxynitride, and combinations thereof.

[0017] Typically, the thickness of the intermediate layer 34 is about 0.5 to 10 nanometers. A noble metal layer 40 is disposed over the adhesion layer 36. The noble metal layer 40 comprises a noble metal selected from the group consisting of platinum, gold, palladium, iridium, and alloys thereof, as well as combinations thereof. In one refinement, the thickness of the noble metal layer 40 is about 0.5 to 10 nanometers. In another refinement, the thickness of the noble metal layer 40 is about 0.5 to about 5 nanometers. In yet another refinement, the thickness of the noble metal layer 40 is about 0.5 to about 3 nanometers. It should be understood that any of the layers described above may be deposited by a number of techniques known to those skilled in the art. Such deposition techniques include sputtering, chemical vapor deposition, atomic layer deposition, evaporation, chemical deposition, and the like.

[0018] Typically, the coated substrates comprise a plurality of substrate particles. The substrate particles 30 can have any number of shapes. Examples of such shapes include nanorods, nanotubes, nanorafts, electrically non-conductive particles, spherical particles, and the like. Nanorods, nanorafts, and nanotubes are each characterized by having at least one spatial dimension of about 10 nanometers to about 100 nanometers. In one refinement, the substrate particles 30 comprise particles with a length-to-width aspect ratio of 10:1 to 25:1. In another refinement, the substrate particles 30 comprise particles with a length-to-width aspect ratio of 13:1 to 20:1. In yet another refinement, the substrate particles 30 comprise particles with a length-to-width aspect ratio of about 15:1.

[0019] According to another embodiment, an ink composition for forming catalyst layers is provided. Typically, the ink composition is applied to the ionically conductive membrane or to the gas diffusion layers and allowed to dry, thereby forming the anode and / or cathode catalyst layers described above. The ink compositions comprise a solvent system, an ionically conductive polymer dispersed in the solvent system, and a supported catalyst dispersed in the solvent system. The supported catalysts comprise a plurality of the coated substrate particles as described above. The amount of coated substrate particles is typically between 0.1 and 20 weight percent of the total weight of the ink composition.In another refinement, the amount of coated substrate particles is typically between 0.1 and 10 weight percent of the total weight of the ink composition. In yet another refinement, the amount of coated substrate particles is typically between 0.5 and 5 weight percent of the total weight of the ink composition. Typical solvent systems include water, C. 1-4-Alcohols (methanol, ethanol, n-propanol, isopropanol, etc.). Typically, the solvent is present in an amount of about 30 to 99 weight percent of the total weight of the ink composition. In a further refinement, the solvent system is present in an amount of about 40 to 98.9 weight percent of the total weight of the ink composition. In one refinement, the ionically conductive polymers typically comprise protogenic groups such as -SO2X, -PO3H2, -COX, and combinations thereof, where X is -OH, a halogen, or an ester. Examples of suitable ionically conductive polymers include perfluorosulfonic acid (PFSA) polymers, hydrocarbon-based ionomers, sulfonated polyetheretherketone polymers, perfluorocyclobutane polymers, and combinations thereof. In a variation, the ionically conductive polymer is present in an amount of 1 to 20 weight percent of the total weight of the ink composition.It is understood that at least a portion of the protogenic groups is neutralized by the formation of the onium compound. Commercial NAFION. ® Polymer is a particularly useful example of an ion-conductive polymer.

[0020] The following examples illustrate various embodiments of the present invention. Those skilled in the art will recognize numerous variations.

[0021] A 3M NSTF™ (nanostructured thin film) support is used as the substrate. The NSTF support is a highly oriented, ledge-shaped substrate made from a self-assembled organic compound (e.g., perylene red dye). Its high aspect ratio of approximately 15 makes it very difficult to coat with any metal. It is even more difficult with a high surface energy metal such as platinum. Scanning electron micrographs indicate the problems associated with coating NSTF supports with platinum using a conventional sputtering process. Due to the line-of-sight nature of sputtering, most of the platinum is deposited on the top surface, but little on the bottom. This is a wasteful use of platinum for a heterogeneous catalyst application.

[0022] To demonstrate the ability to deposit a thin, uniform layer of platinum on a wide variety of substrates using tungsten (W) as an adhesion layer, an NSTF sample is deposited with 14 cycles of tungsten, 1 cycle of Al2O3, and then 14 cycles of W using atomic layer deposition (ALD). The Al2O3 layer is deposited to break the continuity of the W lattice, thereby improving the chemical and mechanical properties of the W layer. In this example, water and Si2H6 are used as reactants to deposit Al2O3 and W, respectively. This results in a W / Al2O3 / W adhesion layer approximately 3 nm thick. Platinum is then deposited onto the resulting layer using a hydrogen plasma (150 cycles of platinum ALD at 120°C, 100 watts from a MeCpPtMe3 precursor). This results in a uniform platinum film approximately 3 nm thick.In our previous work, continuously uniform 1.5 nm films were demonstrated on a W adhesion layer [Appl. Phys. Lett. 101, 111601 (2012)]. Scanning electron micrographs show that the coating of the platinum and tungsten film is very uniform across the length of the NSTF, in contrast to the conventional method. Similarly, elemental contrast electron energy loss spectroscopy mapping of the fabricated platinum / W-ALD / NSTF sample confirms good uniformity.

[0023] A uniform film of platinum can be coated onto a carbon support by first allowing NO2 to form on a sp 2-carbon surface, then the Al2O3 could be evenly coated. Transmission electron microscopy images confirm that Al2O3 is coated on carbon nanotubes. The adhesion layer and the platinum layer can then be evenly deposited on this layer in a similar manner as described above. Although carbon nanotubes were demonstrated here, any carbon substrate can be used.

Claims

[1] Coated substrate comprising: a plurality of substrate particles (30); an adhesive layer (36) disposed over each substrate particle, the adhesive layer (36) comprising a component selected from the group consisting of tungsten metal, a tungsten alloy, tungsten oxide, tungsten nitride, and tungsten carbide; and a precious metal layer (40) disposed over the adhesion layer, wherein the precious metal is selected from the group consisting of platinum, gold, palladium, iridium and alloys thereof, and combinations thereof; characterized by , that the substrate further comprises an intermediate layer (34) disposed between each substrate particle (30) and the adhesion layer (36); and in that the adhesion layer (36) comprises a lattice-disrupting layer (36") between two tungsten metal layers (36') different from the lattice-disrupting layer (36"). [2] The coated substrate of claim 1, wherein the plurality of substrate particles (30) comprises carbon powder, carbon nanorods, electrically non-conductive particles, carbon nanotubes, and combinations thereof. [3] The coated substrate of claim 1, wherein the substrate particles (30) have a length-to-width aspect ratio of 10:1 to 25:

1. [4] The coated substrate of claim 1, wherein the substrate particles (30) have a length-to-width aspect ratio of 13:1 to 20:

1. [5] The coated substrate of claim 1, wherein the substrate particles (30) have a length-to-width aspect ratio of 15:

1. [6] The coated substrate of claim 1, wherein the adhesive layer (36) has a thickness of 0.5 to 5 nanometers. [7] The coated substrate of claim 1, wherein the noble metal layer (40) has a thickness of 0.5 to 5 nanometers.

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