Abstract
The disclosure relates to a supported catalyst material for a fuel cell. This comprises an electrically conductive, carbon-based carrier material and catalytic structures deposited or grown on the carrier material with a multilayer structure. The core layer comprises an electrically conductive bulk material, with the bulk material in direct contact with the carbon-based carrier material. The thin surface layer has a catalytically active noble metal or an alloy thereof. The preparation is carried out directly onto the carrier material with the deposition of the corresponding starting materials from the gas phase.
Claims
1. A supported catalyst material for a fuel cell, comprising: a porous, electrically conductive carbon-based carrier material; catalytic structures on the carrier material, the catalytic structures having a multilayer structure that includes from inside to outside, at least: a core layer of an electrically conductive bulk material, the bulk material in direct contact with the carbon-based carrier material; and a surface layer of a catalytically active noble metal or an alloy.
2. The catalyst material according to claim 1 wherein the bulk material of the core layer is selected from the group comprising nitrides, carbides, borides, oxides of metals and combinations thereof.
3. The catalyst material according to claim 1 wherein a bonding agent layer, including tungsten, is arranged between the core layer and the surface layer.
4. The catalyst material according to claim 1 wherein an intermediate layer including a noble metal alloy is arranged between the core layer and the surface layer.
5. The catalyst material according to claim 1 wherein an intermediate layer including a noble metal alloy is arranged between an adhesion-promoting layer and the surface layer.
6. The catalyst material according to claim 1 wherein one of crystal edges and crystal corners of the surface layer are covered by a local protective layer.
7. The catalyst material according to claim 1 wherein the carbon-based carrier material has a porous particulate structure, having one of spheroids and fibers.
8. The catalyst material according to claim 7 wherein the porous particular structure includes is selected from the group including carbon nanostructures, graphite, volcanic graphite carbon, graphene, ketjen black, acetylene black, furnace black, carbon black, activated carbon and meso phase carbon.
9. The catalyst material according to claim 1 wherein the core layer, is at least selectively covalently or materially bonded to the carbon-based carrier material.
10. 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 converter arranged on at least one side of the carrier; and a coating that includes: a supported catalyst material that includes: a porous, electrically conductive carbon-based carrier material; catalytic structures on the carrier material, the catalytic structures having a multilayer structure that includes a core layer of an electrically conductive bulk material, the bulk material in direct contact with the carbon-based carrier material and a surface layer of a catalytically active noble metal.
11. The electrode structure of claim 10 wherein a bonding agent layer, including tungsten, is arranged between the core layer and the surface layer.
12. A process for preparing a catalyst material, comprising: providing a carbon-based carrier material; generating an anchor on the carrier material, selected from a doping with a foreign material, a functional group, an electric charge, a free electron pair, and a crystal lattice error; forming core layers by depositing an electrically conductive volume material from the gas phase on and around the anchors and directly on the carbon-based carrier material; and depositing a layer comprising a catalytically active noble metal or an alloy onto the core layers.
13. The process of claim 12, further comprising forming a bonding agent layer between the core layer and the surface layer.
Description
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0041] The disclosure is in the following explained in embodiment examples on the basis of the respective drawings. They show:
[0042] FIGS. 1A-1G shows process steps for producing a supported catalyst material according to one embodiment of the present disclosure;
[0043] FIGS. 2A and 2B is a schematic plan view of catalytic structures of a supported catalyst material according to embodiments of the present disclosure with a protective layer (A) on crystal corners and (B) on crystal edges of the surface layer; and
[0044] FIG. 3 is a sectional view of a fuel cell with a catalyst material according to the disclosure.
DETAILED DESCRIPTION
[0045] FIG. 1 shows in a highly schematic manner the method according to the disclosure for producing a catalyst material for a fuel cell by means of various intermediates.
[0046] The method begins with the provision of a carbon-based carrier material 21 according to FIG. 1A. Here, only a surface cut-out of the particulate material 21 is shown. For example, the carrier material can comprise a carbon nanostructures, for example carbon nanotubes, carbon nanorods, carbon nanofibers, carbon nanobands; as well as graphite, volcano, graphitized carbon, graphene, ketjen black, acetylene black, furnace black, carbon black, activated carbon and meso phase carbon.
[0047] The carbon-based carrier material 21 is subjected to a surface treatment according to FIG. 1B in order to produce local anchors 22 on the carbon surface, which in the subsequent step serve as a docking point for the volume material. The anchor 22 is preferably a doping of the carbon, with a foreign material, which is introduced as an extraneous atom into the crystal lattice of the carbon material instead of a carbon atom or as intercalation in intermediate lattice positions. Preference is given to doping with hydrogen, oxygen, nitrogen and/or boron. In order to introduce these foreign atoms, treatment of the carbon carrier material 21 is carried out by means of a suitable reactant or a suitable process. In the case of oxygen functionality, treatment with an oxidizing acid, such as sulfuric acid and/or nitric acid of high concentration, can be carried out, as well as supplementary or alternatively a treatment by means of oxygen plasma. The functionalization with nitrogen can be effected by means of a precursor compound such as aniline or with reactive gases, such as ammonia, hydrazine or the like. A functionalization with boron is possible with the aid of inorganic or organic boron compounds. Subsequent to this pretreatment, tempering can be carried out in order to assist the diffusion of the doping element into the graphite structure or into the carbon lattice or the formation of functional groups such as carboxyl, carbonyl, amine, imine or amide groups.
[0048] In the subsequent step, according to FIG. 1C a deposition of an electrically conductive volume material from the gas phase takes place directly on the carbon-based carrier material, forming discrete core layers 23, for example in the form of spherical halves. The anchors 22 serve, on the one hand, as the nucleus or starting point for the deposition and layer-building process, and on the other hand ensure the formation of a stable, in particular, chemical, bonding of the volume material to the carrier material, that is to say the fixing and immobilization on the latter. In this way, it can also be achieved, in particular, that the resulting structures are spatially separated from one another on the carbon carrier 21 and do not grow together by sintering processes. In particular, metal nitrides, metal carbides, metal borides, metal oxides and combinations thereof and mixtures thereof are used as the volume material for this core layer 23. Preferably, a metal nitride, for example titanium nitride, or a metal carbide as a volume material for the core layer 23 is produced. The deposition of these “anchoring materials” on the preconditioned carbon surface takes place by means of a chemical or physical gas deposition process, such as CVD, ALD, PVD, PLD, etc. In the case of a chemical gas deposition process, a suitable reactive precursor of the volume material is used or in the case of a chemical vapor deposition process physical gas deposition method is a target which consists directly of the volume material to be deposited. These starting materials are transferred into the gas phase by a suitable technique and transported into a vacuum chamber in which the carrier material 21 is arranged as a substrate. The deposition of the material on the carrier material 21 then begins on the anchors 22. The corresponding metal of the volume material, for example titanium, is, for example, vaporized in the form of a reactive precursor in the form of a metal-organic compound and transported into the vacuum chamber. At the same time, either a source of nitrogen, a carbon source, a source of boron, or an oxygen source for producing the corresponding nitride, carbide, boride or oxide or a combination of these mixed compound sources is transferred into the gas phase and also transported into the vacuum chamber. This is followed by deposition on the mostly heated carrier material and reaction of this precursor compound, with the corresponding volume material. The layer thickness is controlled by the concentration of the reactants or the gas volume flow, as well as the process duration and is preferably 5 to 20 nm. Optionally the crystallization of the volume material can be supported in a subsequent annealing step.
[0049] In an optional subsequent process step according to FIG. 1D a bonding agent layer 24, for example tungsten W, is applied, for which a chemical or physical gas separation process is also used. The adhesion-imparting layer 24 preferably has an average layer thickness of 1 to 2 atomic layers.
[0050] Subsequently, according FIG. 1E the application of an intermediate layer 25, takes place, for which a chemical or physical gas deposition process is also applied. The intermediate layer 25 is an alloy of a catalytically active noble metal, for example, an alloy of platinum and/or palladium, with at least one alloying element which is selected in such a way that the alloy formed has the highest possible thermodynamic stability. Examples of suitable alloying elements are nickel and/or cobalt. A chemical gas deposition process, such as CVD or ALD, is preferably used in which suitable reactive precursor compounds of the corresponding metals are used. These reactive precursors are, in particular, organometallic compounds of the respective elements. Suitable organometallic compounds are, for example, alkyls, alkenyls or alkoxides. In the case of platinum, for example, trimethyl (methylcyclopentadienyl) platinum is used. In order to deposit the metal or the metals metallically, that is to say with the oxidation stage zero, a suitable reducing agent with a sufficient reduction potential, for example hydrogen or a hydrogen-nitrogen mixture, is added to the gas atmosphere in the vacuum chamber or the carrier gas. As a result, the intermediate layer 25 is produced from the corresponding noble metal alloy with a preferred average layer thickness of 4 to 6 atomic layers.
[0051] In the subsequent step according to FIG. 1F the surface layer 26 is deposited from the catalytically active noble metal, in particular platinum, palladium or an alloy of both. This deposition also takes place from the gas phase by means of a physical, but preferably a chemical, gas phase deposition process. The procedure corresponds to the deposition of the intermediate layer 25 as described above. The layer thickness of the surface layer 26 is preferably only 1 to 2 atomic layers.
[0052] Instead of a separate deposition of the surface layer 26 by means of gas-phase deposition, the surface layer 26 can be effected by chemical or (electro) chemical de-alloying of the intermediate layer 25. In this case, the less noble alloying constituents are chemically or electrochemically removed from the intermediate layer 25 so that only a shell of the noble metal (platinum and/or palladium) remains on the surface. However, since the result of such a segregation process frequently does not provide a continuous but a discontinuous noble metal layer, a separate gas deposition process for producing the catalytic noble metal surface layer 26 is preferred.
[0053] After each individual deposition step, an annealing step can optionally be carried out in order to promote the crystallization of the deposited layer.
[0054] In another optional process step, the production of local protective layers 27 on corners and/or edges of the crystalline or semi-crystalline surface layer 26 of the catalytic noble metal takes place according to FIG. 1G. Here too, gas separation processes, in particular chemical gas deposition methods, such as CVD or ALD, are used. As a material for the protective layer 27 a material with high corrosion stability is used comprising oxides (for example Al.sub.2O.sub.3, B.sub.2O.sub.3, SiO.sub.2), carbides (for example B.sub.4C, SiC, WC), nitrides (for example BN, AlN, Si.sub.3N.sub.4, TiN) or gold (Au). Since the crystal corners and edges act as crystallization nuclei, the targeted deposition of these protective materials at these sites is readily possible.
[0055] FIG. 1G shows the finished catalyst material 20 according to the disclosure, the detail on the right showing the structure in detail. All deposited layers 23, 24, 25, 26 and 27 are thus produced by means of a gas deposition process. Supported by the anchor point 22 the catalytic structures 28 are constructed such that these are produced separately from one another on the carbon carrier material 21. Depending on the nature of the anchor point 22 a covalent bonding of the volume material of the core layer 23 to the carbon material of the carrier 21, occurs, but at least to a materially bonded connection. The catalytic structures 28 produced in this way have the form of semi-particles with a core-shell structure, whereby the material of the core layer 23 has a direct, material-locking contact with the carbon material of the carrier 21. The catalyst material 20 thus obtained shows very high chemical and mechanical stability with a comparatively low requirement for the catalytic noble metal of the surface layer 26.
[0056] FIG. 2 shows a schematic plan view of catalytic structural elements 28 according to the disclosure, which are provided with local protective layers 27. According to FIG. 2A only the crystal corners are provided with such “edge protection”, whereas according to FIG. 2B the crystal edges have the protective layer 27.
[0057] In order to manufacture an electrode for a fuel cell, a composition (slurry, paste or the like) is first produced from the catalytic material 20 according to the disclosure, which 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, for example 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.
[0058] FIG. 3 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 the flat sides thereof, namely an anode 12a and a cathode 12k, as well as two gas diffusion layers 13 arranged on both sides thereof. The polymer electrolyte membrane 11 is an ion-conducting, in particular proton-conducting polymer, for example a product marketed under the trade name Nafion®. The catalytic electrodes 12a, 12k comprise the catalytic material according to the disclosure and are designed as a double-sided coating of the membrane 11 in the illustrated example. 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 namely 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 15, the fuel, here hydrogen H.sub.2, is supplied 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 connected to one another via an external circuit 18 and to an electrical load 19, for example a traction motor for an electric vehicle or a battery.
[0059] During operation of the fuel cell 10 the hydrogen is supplied via the reactant channels 16 of the anode plate 15a distributed over 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 oxygen supplied to the catalytic noble metal reacts with the protons to the water, which is discharged from the fuel cell 10 with the reaction gas, via the electrons supplied via the external circuit 18. The electrical load 19 can be supplied by the electrical current flow thus generated.
[0060] The catalyst material 20 according to the present disclosure 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 disclosure, is characterized as the catalytic electrodes 12a, 12k have a low corrosion tendency and thus high long-term stability. At the same time, comparatively little catalytic noble metal is required since the main volume of the catalytic material of the electrodes is formed by a comparatively inexpensive material.
[0061] The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments.
[0062] These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.
