Method for producing a supported catalyst material for a fuel cell

Abstract

The invention relates to a method for producing a supported catalyst material for a fuel-cell electrode, as well as a catalyst material that can be produced using said method. In the method, first, a carbide-forming substance is deposited from the gas phase onto the carbon-based carrier material to produce a carbide-containing layer and, then, a catalytically-active precious metal or an alloy thereof from the gas phase is deposited to form a catalytic layer. By chemical reaction of the carbide-forming substance with the carbon, very stable carbide bonds are formed at the interface, while an alloy phase of the two forms at the interface between carbide-forming substance and precious metal. Overall, a very stable adhesion of the catalytic precious metal to the substrate results, whereby degradation effects are reduced, and the life of the material is extended.

Claims

1. A method for manufacturing a supported catalyst material for a fuel-cell electrode, comprising: forming a carbide-containing layer by depositing a carbide-forming substance from a gas phase onto an electrically conductive carbon-based carrier material, the carbide-forming substance reacting with carbon of the carrier material; and forming a catalytic layer by depositing a catalytically-active precious metal or an alloy of such a precious metal from the gas phase; wherein the depositing of the carbide-forming substance and the depositing of the catalytically-active precious metal or the alloy of the precious metal is in a time-overlapping manner, producing a gradual enrichment of the catalytically-active precious metal or its alloy.

2. The method according to claim 1, wherein the carbide-forming substance is selected from the group comprising titanium, zirconium, hafnium, tungsten, molybdenum, boron, vanadium, aluminum, scandium, yttrium, silicon, chromium, and nickel, or a mixture of these.

3. The method according to claim 1, wherein the carbide-containing layer has an average thickness in the range of 1 to 50 atomic layers.

4. The method according to claim 1, wherein the catalytically-active precious metal or its alloy comprises platinum, ruthenium, rhodium, palladium, osmium, iridium, or an alloy of these metals.

5. The method according to claim 1, further comprising forming defect sites on a surface of the carbon-based carrier material before depositing the carbide-forming substance.

6. The method according to claim 1, wherein, after depositing the carbide-forming substance and before depositing the catalytically-active precious metal or its alloy, a diffusion barrier layer is deposited, selected from the group comprising gold, palladium, ruthenium, tungsten, osmium, rhodium, and iridium, or a mixture or alloy thereof.

7. A supported catalyst material for a fuel-cell electrode comprising: an electrically-conductive, carbon-based carrier material; a carbide-containing layer on the carrier material; and a catalytic layer of a catalytically-active precious metal or an alloy of such on the surface of the carbide-containing layer, wherein the carbide-containing layer and the catalytic layer are produced by a method comprising depositing a carbide-forming substance and depositing the catalytically-active precious metal or the alloy thereof in a time-overlapping manner, thereby producing a gradual enrichment of the catalytically-active precious metal or its alloy at the surface of the carbide-containing layer.

8. An electrode structure for a fuel-cell, comprising: a flat carrier, selected from a polymer electrolyte membrane and a gas-permeable, electrically-conductive substrate; and a catalytic coating comprising: an electrically-conductive, carbon-based carrier material; a carbide-containing layer on the carrier material; and a catalytic layer of a catalytically-active precious metal or an alloy of such on the surface of the carbide-containing layer, wherein said catalytic coating is arranged on at least one flat side of the carrier, wherein the carbide-containing layer and the catalytic layer are produced by a method comprising depositing a carbide-forming substance and depositing the catalytically-active precious metal or the alloy thereof in a time-overlapping manner, thereby producing a gradual enrichment of the catalytically-active precious metal or its alloy at the surface of the carbide-containing layer.

9. The method according to claim 1, further comprising forming covalent bonds between carbon atoms of a surface of the carbon-based carrier material and chemical groups that promote carbide formation before depositing the carbide-forming sub stance.

10. The supported catalyst material according to claim 7, wherein the carbide-containing layer has an average thickness in the range of 1 to 50 atomic layers.

11. The supported catalyst material according to claim 7, wherein the carbide-containing layer has an average thickness in the range of 1 to 20 atomic layers.

12. The supported catalyst material according to claim 7, wherein a surface of the carbon-based carrier material comprises defect sites.

13. The supported catalyst material according to claim 7, wherein a surface of the carbon-based carrier material comprises carbon atoms covalently bonded to chemical groups that promote carbide formation.

14. The supported catalyst material according to claim 7, wherein the catalytically-active precious metal or its alloy comprises platinum, ruthenium, rhodium, palladium, osmium, iridium, or an alloy of these metals.

15. The electrode structure according to claim 8, wherein the carbide-containing layer has an average thickness in the range of 1 to 50 atomic layers.

16. The electrode structure according to claim 8, wherein the surface of the carbide-containing layer comprises defect sites.

17. The electrode structure according to claim 8, wherein the surface of the carbon-based carrier material comprises carbon atoms covalently bonded to chemical groups that promote carbide formation.

18. The electrode structure according to claim 8, wherein the catalytically-active precious metal or its alloy comprises platinum, ruthenium, rhodium, palladium, osmium, iridium, or an alloy of these metals.

19. The method according to claim 1, wherein producing a gradual enrichment of the catalytically-active precious metal or its alloy comprises changing the relative proportions of the carbide-forming substance and the precious metal or alloy of such a precious metal in the gas phase continuously during depositing of the carbide-forming substance and the depositing of the catalytically-active precious metal or the alloy of the precious metal.

Description

(1) The invention is explained below in exemplary embodiments, with reference to the respective drawings. The following are shown:

(2) FIG. 1 a schematic representation of a supported catalyst material according to a first embodiment of the invention,

(3) FIG. 2 a schematic representation of a supported catalyst material according to a second embodiment of the invention,

(4) FIG. 3 a schematic representation of a supported catalyst material according to a third embodiment of the invention,

(5) FIG. 4 a schematic representation of a supported catalyst material according to a fourth embodiment of the invention,

(6) FIG. 5 a schematic representation of a supported catalyst material according to a fifth embodiment of the invention,

(7) FIG. 6 a schematic representation of a supported catalyst material at the molecular level according to the fifth embodiment of the invention,

(8) FIG. 7 a schematic representation of a supported catalyst material according to a sixth embodiment of the invention, and

(9) FIG. 8 a sectional view of a fuel-cell with a catalyst material according to the invention.

(10) FIGS. 1 through 7 show highly schematic and idealized supported catalyst materials according to the present invention. In each case, a very greatly enlarged section of the material is depicted as a section through an exemplary structure. The same reference numerals are used here for identical elements and are not explained in detail for each embodiment.

(11) The described supported catalyst material according to FIG. 1, designated overall as 20, has an electrically-conductive, carbon-based carrier material 21 selected from the aforementioned materials—in this case, for example, carbon black. A deposited, carbide-containing layer 22, which is selected from one of the aforementioned materials—in this case, for example, titanium Ti—is arranged on the carrier material 21. At least at the interface between the carrier material 21 and the carbide-containing layer 22, carbides—in this case, titanium carbide TiC—or carbide-containing bonds are formed, i.e., this leads to a chemical—in particular, covalent—bond between these two layers 21, 22. A deposited catalytic layer 23 of a catalytically-active precious metal or an alloy of such that is selected from one of the aforementioned materials—here, for example, platinum Pt—is arranged on the surface of the carbide-containing layer 22. An alloy of the carbide former Ti and the precious metal Pt is formed at the interface between the carbide-containing layer 22 and the catalytic layer 23, so that, here also, a material-to-material bond is present. The structure of carbide-containing layer 22 and catalytic layer 23 is in the form a plurality of discrete catalytic structures or particles, of which only one is shown here.

(12) The catalyst material according to FIG. 2 differs from the one in FIG. 1 in that the catalytic structures are grown on both sides on the carrier material 21. Deviating from the symmetrical arrangement shown, the structures may also be arranged non-symmetrically on the two sides of the carrier material 21.

(13) The catalyst materials 20 according to FIGS. 1 and 2 may be formed by first depositing the carbide former Ti via a gas phase deposition process on the carbon-based carrier material 21 to form the carbide-containing layer 22 having the desired layer thickness. This results in an initial local deposition and carbide formation of individual titanium atoms and an increase in the layer 22 caused by accumulation of further titanium atoms on these “cores.” After reaching the desired layer thickness of, for example, 1 to 10 atomic layers of Ti, the deposition of the precious metal Pt on the carbide-containing layer 22 is then accomplished, forming the catalytic layer 23. Once the desired layer thickness of, for example, 1 to 5 atomic layers Pt has been reached, the catalyst material 20 is obtained. Whether the catalytic structures grow on the carrier material 21 on one or both sides depends primarily on the accessibility of the various sides of the carrier material.

(14) The catalyst material 20 shown in FIG. 3 differs from that in FIG. 1 in that the carbon-based carrier material 21 has defect sites 24. In the present example, this is a lattice defect in the crystal lattice of the carbon.

(15) The catalyst material 20 shown in FIG. 4 essentially corresponds to that in FIG. 3, except that, similarly to FIG. 2, the catalytic structures are grown on both sides of the carbon carrier 21.

(16) The catalyst materials 20 according to FIGS. 3 and 4 may be produced with a method analogous to the one for the exemplary embodiments according to FIGS. 1 and 2, except that, prior to the deposition process of the carbide former, the carbon carrier material 21 is subjected to a treatment process—for example, a plasma treatment for generating the defect sites 24. The defect sites 24 produced in this way promote the initial deposition and nucleation of the carbide former.

(17) FIGS. 5 and 6 show another embodiment of a catalyst material 20 according to the invention. In a modification to the materials according to FIGS. 1 through 4, a mixed layer 25 is present here between the carbide-containing layer 22 and the catalytic layer 23, which mixed layer is made up of a mixture or alloy of the carbide former and of the catalytic precious metal—in this case, a Pt—Ti alloy. In particular, the mixed layer 25 is designed in such a way that the proportion of Ti decreases from inside to outside, and the proportion of Pt increases in the opposite direction. Thus, the entire layer structure 22, 25, 23 may also be described as a single, continuous layer, within which the titanium content falls from 100% (inside) to 0% (outer surface), and the platinum content increases from 0% (inside) to 100% on the surface. According to a further embodiment, the mixed layer 25 has a homogeneous Pt—Ti alloy, i.e., without a concentration gradient. Moreover, the layer structure according to FIGS. 5 and 6 can be grown on both sides of the carbon carrier 21 (analogously to FIGS. 2 and 3) and/or be combined with the generation of a defect site 24 in the carbon carrier 21 (analogously to FIGS. 3 and 4).

(18) The material 20 according to FIGS. 5 and 6 can be produced by continuously varying the composition of the gas phase during a continuous vapor deposition process.

(19) Another version of the catalyst material 20 according to the invention is shown in FIG. 7. In a modification to the materials according to FIGS. 1 through 4, a diffusion barrier layer 26 made of one of the aforementioned materials—in this case, for example, gold Au—is present here between the carbide-containing layer 22 and the catalytic layer 23. The diffusion barrier layer 26 prevents diffusion of the non-precious carbide former onto the surface of the catalytic structure and, thus, its dissolution. The material 20 according to FIG. 7 can be produced in that, following the deposition of the carbide former in an additional vapor deposition step, the material of the diffusion barrier layer 26 is deposited onto the layer 22, and, then, the catalytic precious metal is deposited onto the diffusion barrier layer 26.

(20) In order to manufacture an electrode for a fuel-cell, first, a composition (slurry, paste, or the like) is produced from the catalytic material 20 according to the invention and contains a solvent in addition to the catalytic material 20, and may contain further additives—in particular, a polymeric binder. This composition is then applied to a flat support as a coating, for which any coating process, e.g., coating, spraying, scraping, printing, or the like, can be used. The flat carrier is, in particular, a polymer electrolyte membrane of the fuel-cell, which is preferably coated on both sides with the catalytic material. Alternatively, the catalytic coating can also be applied to a gas diffusion layer or to another gas-permeable, electrically-conductive substrate, such as carbon paper.

(21) FIG. 8 shows the structure of such a fuel-cell 10 in a schematic sectional view. The core piece of the fuel-cell 10 is a membrane electrode arrangement (MEA), designated overall by reference numeral 14. The MEA 14 comprises a polymer electrolyte membrane 11, two catalytic electrodes or catalytic coatings arranged on its flat sides, viz., an anode 12a and a cathode 12k, as well as two gas diffusion layers 13 arranged on both of its sides. The polymer electrolyte membrane 11 is an ion-conducting—in particular, proton-conducting—polymer, e.g., a product marketed under the trade name, Nafion®. The catalytic electrodes 12a, 12k comprise the catalytic material according to the invention and, in the illustrated example, are designed as a double-sided coating of the membrane 11. The gas diffusion layers 13 consist of a gas-permeable, electrically-conductive material which, for example, has the structure of a foam or a fiber structure or the like, and serves to distribute the reaction gases to the electrodes 12a and 12k. Bipolar plates 15, viz., an anode plate 15a and a cathode plate 15k, are connected to the membrane electrode arrangement 14 on both sides. Usually, a plurality of such individual cells 10 are stacked into a fuel-cell stack so that each bipolar plate is composed of an anode plate 15a and a cathode plate 15k. The bipolar plates 15a, 15k each comprise a structure of reactant channels 16, which are open in the direction of the gas diffusion layers 13 and serve to supply and distribute the reactants of the fuel-cell. Thus, via the reactant channels 16 of the anode plate 15a, the fuel—here, hydrogen H.sub.2—is supplied, and, via the corresponding channels 16 of the cathode plate 15k, oxygen O.sub.2 or an oxygen-containing gas mixture—in particular, air. The bipolar plates 15a, 15k are, via an external circuit 18, connected to one another and to an electrical load 19—for example, a traction motor for an electric vehicle or a battery.

(22) During operation of the fuel-cell 10, the hydrogen is supplied via the reactant channels 16 of the anode plate 15a, distributed via the gas diffusion layer 13 on the anode side, and fed to the catalytic anode 12a. Here, a catalytic dissociation and oxidation of hydrogen H.sub.2 to protons H.sup.+ takes place, with release of electrons, which are removed via the circuit 18. On the other hand, via the cathode plate 15k, the oxygen is conducted to the catalytic cathode 12k via the cathode-side gas diffusion layer 13. At the same time, the proteins H.sup.+ formed on the anode side diffuse across the polymer electrolyte membrane 11 in the direction of the cathode 12k. In this case, the supplied atmospheric oxygen reacts to the catalytic precious metal, taking up the electrons supplied via the external circuit 18 with the protons to form water, which is discharged from the fuel-cell 10 with the reaction gas. The electrical load 19 can be supplied by the electrical current flow thus generated.

(23) The catalyst material 20 according to the present invention may be used for the anode 12a and/or the cathode 12k of fuel-cells. The fuel-cell 10 equipped with the catalytic material 20 according to the invention is characterized in that the catalytic electrodes 12a, 12k have a low corrosion tendency, and thus high long-term stability. At the same time, comparatively little catalytic precious metal is required, since the main volume of the catalytic material of the electrodes is formed by a comparatively inexpensive material.

LIST OF REFERENCE SYMBOLS

(24) 10 Fuel-cell 11 Polymer electrolyte membrane 12 Catalytic electrode 12a Anode 12k Cathode 13 Gas diffusion layer 14 Membrane electrode arrangement 15 Bipolar plate 15a Anode plate 15k Cathode plate 16 Reactant channel 17 Coolant channel 18 Circuit 19 Electrical user/electrical load 20 Supported catalyst material 21 Carbon-based carrier material 22 Carbide-containing layer 23 Catalytic layer 24 Defect site 25 Mix layer 26 Diffusion barrier layer