METHOD FOR PRODUCING CATALYST LAYERS FOR FUEL CELLS

Abstract

The invention relates to a method for producing a catalyst material (47) comprising catalytically active nanoparticles (47), in particular for electrodes (7, 8, 45) with catalyst layers (30) as catalysts for a fuel cell (2), having the steps of: providing (52) a first starting material comprising a first metal, providing (53) a second starting material comprising a second metal, mixing the first starting material and the second starting material in order to form a reactant material, and thermally treating (56) the reactant material so that catalytically active nanoparticles (47) are produced from the first starting material and the second starting material and the first and second metal are connected together in order to at least partly form an alloy of the first and second metal in the catalytically active nanoparticles (47) such that catalytically active nanoparticles (47) are produced as an intermediate material comprising the alloy of the first and second metal. The content of the second metal and/or the second starting material on the surface (48) of the catalytically active nanoparticles (47) is reduced in the intermediate material so that a product material is produced from the intermediate material as the catalyst material (47).

Claims

1. A method for producing a catalyst material (47) comprising catalytically active nanoparticles (47), the method comprising the steps of: providing (52) a first starting material comprising a first metal, providing (53) a second starting material comprising a second metal, mixing the first starting material and the second starting material to form a reactant material, and thermally treating (56) the reactant material so that catalytically active nanoparticles (47) are produced from the first starting material and the second starting material, and the first and second metal are connected together in order to at least partly form an alloy of the first and second metals in the catalytically active nanoparticles (47) such that catalytically active nanoparticles (47) are produced as an intermediate material comprising the alloy of the first and second metals, wherein in the intermediate material, content of the second metal and/or the second starting material on a surface (48) of the catalytically active nanoparticles (47) is reduced so that a product material is produced from the intermediate material as the catalyst material (47).

2. The method according to claim 1, wherein on the surface (48) of the catalytically active nanoparticles (47), a proportion of the second metal and/or of the second starting material is reduced with a fluid by rinsing (60) the intermediate material with the fluid so that the second metal and/or the second starting material is taken up in the fluid and is removed from the intermediate material as a result of a flow of the fluid.

3. The method according to claim 2, wherein a pH level of the fluid is greater than 7, 9, or 11.

4. The method according to claim 2, wherein the fluid comprises sodium hydroxide, and/or potassium hydroxide, and/or ammonia, and/or tetramethyl ammonium hydroxide, and/or alcohol, and/or water.

5. The method according to claim 1, wherein on the surface (48) of the catalytically active nanoparticles (47), a proportion of the second metal and/or the second starting material is reduced by at least 20%, 30%, 50%, 70%, or 90%.

6. The method according to claim 1, wherein after the thermal treatment (56) of the intermediate material, following the production of the catalytically active nanoparticles (47), and while proportions of the second metal and/or of the second starting material on the surface (48) of the catalytically active nanoparticles (47) are reduced, the proportion of the second metal and/or of the second starting material is maintained at substantially constant levels in an interior (50) of the catalytically active nanoparticles (47).

7. The method according to claim 1, wherein the product material is subjected to an additional thermal treatment (61) after proportions of the second metal and/or the second starting material on the surface (48) of the catalytically active nanoparticles (47) have been reduced.

8. The method according to claim 7, wherein during the additional thermal treatment (61), the product material having the catalytically active nanoparticles (47) is at a temperature between 100? C. and 1200? C.

9. The method according to claim 7, wherein during the additional thermal treatment (61), the product material having the catalytically active nanoparticles (47) is exposed (62) to a process gas.

10. The method according to claim 1, wherein the first starting material comprises a chemical compound as a precursor with the first metal and at least one chemical element.

11. The method according to claim 1, wherein the second starting material includes metal oxide nanoparticles from a compound between a second metal and oxygen, as a solution with the metal oxide nanoparticles, and/or as a solution with a salt with the second metal, and/or as a solution configured as a complex with the second metal.

12. The method according to claim 1, wherein the first metal is a noble metal, and/or the second metal is a transition metal.

13. A method for producing catalyst layers (30), the method comprising the steps of: providing catalyst material (47) comprising catalytically acting nanoparticles (47), providing carrier layers (46) for adhesion of catalyst material (47), applying the catalyst material (47) to the carrier layers (46), such that the catalyst material (47) is adhered to the carrier layers (46) and made from the carrier layers (46) of the catalyst layers (30), wherein the catalyst material (47) is provided by performing a method according to claim 1.

14. A fuel cell unit (1) as a fuel cell stack for electrochemically generating electrical energy, comprising fuel cells (2) arranged as stacks, the fuel cells (2) each comprising a proton exchange membrane (5), an anode (7, 45), a cathode (8, 45), wherein the anode (7, 45) and/or cathode (8, 45) each comprises a catalyst layer (30) with catalytically active nanoparticles (47) with an alloy of a first and second metal, a bipolar plate (10), and gas diffusion layers (9), wherein on a surface (48) of the catalytically active nanoparticles (47), a proportion of the second metal and/or a second starting material is greater than in an interior (50) of the nanoparticles (47).

15. A method for producing a fuel cell unit (1) as a fuel cell stack (1) for electrochemically generating electrical energy, having the steps of: providing components (5, 6, 7, 8, 9, 10) of the fuel cells (2), including anodes (7, 45) and/or cathodes (8, 45), each comprising a catalyst layer (30) with catalytically active nanoparticles (47) with an alloy of a first and second metal, connecting the components (5, 6, 7, 8, 9, 10) of the fuel cells (2) to the fuel cells (2), stacking the fuel cells (2) such that a fuel cell unit (1) is formed, wherein the catalyst layers (30) are provided by performing a method according to claim 13, and on the surface (48) of the catalytically active nanoparticles (47), a proportion of the second metal and/or the second starting material is greater than in an interior (50) of the nanoparticles (47).

16. The method according to claim 1, wherein the catalyst material (47) comprising catalytically active nanoparticles (47) is for electrodes (7, 8, 45) with catalyst layers (30) as catalysts for a fuel cell (2).

17. The method according to claim 2, wherein the fluid is a liquid and the second metal and/or the second starting material is dissolved in the fluid.

18. The method according to claim 8, wherein during the additional thermal treatment (61), the product material having the catalytically active nanoparticles (47) is at a temperature between 200? C. and 800? C.

19. The method according to claim 10, wherein the at least one chemical element includes hydrogen and/or oxygen and/or nitrogen and/or chlorine.

20. The method according to claim 12, wherein the first metal includes palladium (Pd), and/or platinum (Pt) and/or rhodium (Rh) and/or ruthenium (Ru) and/or iridium (Ir) and/or osmium (Os), and/or the second metal includes chromium (Cr), and/or molybdenum (Mo), and/or tungsten (W).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Exemplary embodiments of the invention are explained in greater detail hereinafter with reference to the accompanying drawings. Shown are:

[0066] FIG. 1 a highly simplified exploded view of a fuel cell system with components of a fuel cell,

[0067] FIG. 2 a perspective view of a portion of a fuel cell,

[0068] FIG. 3 a longitudinal section through a fuel cell,

[0069] FIG. 4 a perspective view of a fuel cell unit as a fuel cell stack, i.e. a fuel cell stack,

[0070] FIG. 5 a section through the fuel cell unit according to FIG. 4,

[0071] FIG. 6 a perspective view of a membrane electrode assembly,

[0072] FIG. 7 a perspective view of an anode and cathode as an electrode,

[0073] FIG. 8 a section through a nanoparticle, and

[0074] FIG. 9 a highly schematic representation of method steps for performing the method of producing the catalyst material.

DETAILED DESCRIPTION

[0075] In FIGS. 1 to 3, the basic construction of a fuel cell 2 is shown as a PEM fuel cell 3 (polymer electrolyte fuel cell 3). The principle of fuel cells 2 is that electrical energy or electrical current is generated by means of an electrochemical reaction. Hydrogen H.sub.2 is conducted to an anode 7 as a gaseous fuel, and the anode 7 forms the negative pole. A gaseous oxidant, i.e., air with oxygen, is conducted to a cathode 8, i.e., the oxygen in the air provides the necessary gaseous oxidant. A reduction (electron uptake) takes place on the cathode 8. The oxidation as electron output is performed at the anode 7.

[0076] The redox equations of the electrochemical processes are as follows:

Cathode:

[0077]
O.sub.2+4H.sup.++4e.sup.?-?2H.sub.2O

Anode:

[0078]
2H.sub.2-?4H.sup.++4e.sup.?

Summed Reaction Equation of Cathode and Anode:

[0079]
2H.sub.2+O.sub.2-?2H.sub.2O

[0080] The difference in the normal potentials of the electrode pairs under standard conditions as reversible fuel cell voltage or neutral voltage of the unloaded fuel cell 2 is 1.23 V. This theoretical voltage of 1.23 V is not achieved in practice. At rest and at small currents, voltages above 1.0 V can be achieved and, in operation with larger currents, voltages between 0.5 V and 1.0 V are achieved. The series circuit of multiple fuel cells 2, in particular a fuel cell unit 1 as a fuel cell stack 1 of multiple fuel cells 2 arranged one above the other, has a higher voltage, which corresponds to the number of fuel cells 2 multiplied by the single voltage of a respective fuel cell 2.

[0081] The fuel cell 2 also comprises a proton exchange membrane 5 (PEM), which is arranged between the anode 7 and the cathode 8. The anode 7 and cathode 8 are designed in a layer or disc shape. The PEM 5 functions as an electrolyte, catalyst carrier, and separating device for the reaction gases. The PEM 5 also functions as an electrical insulator and prevents an electrical short circuit between the anode 7 and cathode 8. In general, 12 ?m to 150 ?m thick, proton-conductive films made of perfluorinated and sulfonated polymers are used. The PEM 5 conducts the protons H.sup.+ and substantially blocks ions other than protons H.sup.+ so that charge transport can occur due to the permeability of PEM 5 for the protons H.sup.+. The PEM 5 is substantially impermeable to the reaction gases oxygen O.sub.2 and hydrogen Hz, i.e., it blocks the flow of oxygen O.sub.2 and hydrogen Hz between a gas space 31 at the anode 7 with fuel hydrogen Hz and the gas space 32 at the cathode 8 with air and Oxygen O.sub.2 as oxidizers. The proton conductivity of the PEM 5 increases with increasing temperature and increasing water content.

[0082] On the two sides of the PEM 5, each facing the gas spaces 31, 32, the electrodes 7, 8 are located as the anode 7 and cathode 8. A unit consisting of the PEM 5 and anode 7 as well as cathode 8 is referred to as a membrane electrode assembly 6 (MEA). The electrodes 7, 8 are pressed together with the PEM 5. The electrodes 6, 7 are platinum-containing carbon particles bonded to PTFE (polytetrafluorethylene), FEP (fluorinated ethylene-propylene copolymer), PFA (perfluoroalkoxy), PVDF (polyvinylidene fluoride), and/or PVA (polyvinyl alcohol) and hot-pressed in microporous carbon fiber, glass fiber, or plastic mats. A catalyst layer 30 is typically applied to the electrodes 7, 8 on the side towards the gas spaces 31, 32. The catalyst layer 30 at the gas space 31 with fuel at the anode 7 comprises nanoparticles 47 with a first and second metal alloy that are, respectively, bonded and adhered to a carrier layer 46. For example, the carrier layer 46 comprises graphitized soot particles bonded to a binder. The catalyst layer 30 on the gas space 32 with the oxidizer on the cathode 8 is structured analogously. For example, binders may consist of Nafion? as an ionomer, a PTFE emulsion, or polyvinyl alcohol. Preferably, the electrodes 7, 8, 45 and the catalyst layers 30 are formed from an identical carrier layer 46.

[0083] A gas diffusion layer 9 (GDL) is located on the anode 7 and cathode 8. The gas diffusion layer 9 at the anode 7 evenly distributes the fuel from channels 12 for fuel to the catalyst layer 30 at the anode 7. The gas diffusion layer 9 on the cathode 8 evenly distributes the oxidizer from channels 13 for oxidizer onto the catalyst layer 30 at the cathode 8. The GDL 9 also withdraws reaction water counter to the direction of flow of the reaction gases. Furthermore, the GDL 9 keeps the PEM 5 moist and conducts the power. For example, the GDL 9 is constructed from a hydrophobized carbon paper and a bonded layer of carbon powder.

[0084] A bipolar plate 10 lies atop the GDL 9. The electrically conductive bipolar plate 10 serves as a current collector, for diverting water, for conducting the reaction gases through a channel structure 29 and/or a flow field 29, and for dissipating the waste heat, which occurs in particular in the exothermic electrochemical reaction on the cathode 8. To dissipate the waste heat, channels 14 for passing a liquid or gaseous coolant are worked into the bipolar plate 10. The channel structure 29 on the gas space 31 for fuel is formed by channels 12. The channel structure 29 on the gas space 32 for oxidizers is formed by channels 13. For example, metal, conductive plastics, and composites or graphite are used as the material for the bipolar plates 10. The bipolar plate 10 thus comprises the three channel structures 29 formed by the channels 12, 13, and 14 for separately passing fuel, oxidizer, and coolant.

[0085] In a fuel cell unit 1 and/or a fuel cell stack 1 and/or a fuel cell stack 1, multiple fuel cells 2 are arranged so to as to be stacked in alignment (FIGS. 4 and 5). An exploded view of two stacked fuel cells 2 is depicted in FIG. 1. A seal 11 seals the gas spaces 31, 32 in a fluidically sealed manner. In a compressed gas reservoir 21 (FIG. 1), hydrogen H.sub.2 is stored as a fuel at a pressure of, e.g., 350 bar to 700 bar. From the compressed gas reservoir 21, the fuel is conducted through a high pressure conduit 18 to a pressure reducer 20 in order to reduce the pressure of the fuel in a medium pressure conduit 17 of about 10 bar to 20 bar. From the medium pressure conduit 17, the fuel is conducted towards an injector 19. At the injector 19, the pressure of the fuel is reduced to an injection pressure of between 1 bar and 3 bar. From the injector 19, the fuel is supplied to a fuel supply line 16 (FIG. 1) and from the supply line 16 to the fuel channels 12 forming the channel structure 29 for fuel. As a result, the fuel passes through the gas space 31 for the fuel. The gas space 31 for the fuel is formed by the channels 12 and the GDL 9 at the anode 7. After passing through the channels 12, the fuel not consumed in the redox reaction at the anode 7 (and optionally water) are discharged from a controlled humidification means of the anode 7 via a discharge line 15 from the fuel cells 2.

[0086] A gas conveying device 22, designed as, e.g., a blower 23 or a compressor 24, conveys air from the environment as an oxidizer into an oxidizer supply line 25. From the supply line 25, the air is supplied to the oxidizer channels 13, which form a channel structure 29 on the bipolar plates 10 for oxidizers such that the oxidizer passes through the gas space 32 for the oxidizer. The gas space 32 for the oxidizer is formed by the channels 13 and the GDL 9 on the cathode 8. After passing through the channels 13 or the gas space 32 for the oxidizer 32, the oxidizer not consumed on the cathode 8 and the reaction water resulting on the cathode 8 due to the electrochemical redox reaction are discharged from the fuel cells 2 through a discharge line 26. A supply line 27 is used to supply coolant into the channels 14 for coolant, and a discharge line 28 is used to discharge coolant conducted through the channels 14. The supply and discharge lines 15, 16, 25, 26, 27, 28 are shown as separate lines in FIG. 1 for the sake of simplicity. They are in fact constructively designed at the end region near the channels 12, 13, 14 as aligned fluid openings 42 on sealing layers 41 at the end region of the membrane electrode assemblies 6 that are arranged on top of each other (FIG. 6). Fluid openings (not shown) are also designed in a similar manner on plate-shaped extensions (not shown) of the bipolar plates 10, and the fluid openings in the plate-shaped extensions of the bipolar plates 10 align with the fluid openings 42 and the sealing layers 41 of the membrane electrode assemblies 6 for partially forming the supply and discharge lines 15, 16, 25, 26, 27, 28. The fuel cell stack 1, together with the compressed gas reservoir 21 and the gas conveying device 22, form a fuel cell system 4.

[0087] In the fuel cell unit 1, the fuel cells 2 are arranged between two clamping elements 33 as clamping plates 34. An upper clamping plate 35 lies atop the uppermost fuel cell 2, and a lower clamping plate 36 lies atop the lowermost fuel cell 2. The fuel cell unit 1 comprises approximately 200 to 400 fuel cells 2, not all of which are shown in FIG. 4 for illustrative reasons. The clamping elements 33 impart a compression force on the fuel cells 2. In other words, the upper clamping plate 35 imparts a compression force on the uppermost fuel cell 2, and the lower clamping plate 36 imparts a compression force on the lowermost fuel cell 2. The fuel cell stack 2 is thus tensioned in order to ensure the sealing for the fuel, the oxidizer, and the coolant, in particular due to the elastic seal 11, and also to keep the electrical contact resistance within the fuel cell stack 1 as low as possible. To clamp the fuel cells 2 with the clamping elements 33, four connection devices 39 are designed on the fuel cell unit 1 as bolts 40, which are tensioned. The four bolts 40 are fixedly connected to the clamping plates 34.

[0088] FIG. 6 shows a perspective view of the membrane electrode assembly 6 of the fuel cell unit 1. The layered membrane electrode assembly 6 comprises a layered inner region 38 from the proton exchange membrane 5. The substantially rectangular proton exchange membrane 5 is completely enclosed and framed by two layered sealing layers 41 as a first subgasket 43 and a second subgasket 44. In the inner region 38, the layered proton exchange membrane 5 is arranged between the layered anode 7 and the layered cathode 8 (not visible due to the perspective view in FIG. 6). The sealing layers 41 and thereby the first and second subgaskets comprise the materials of polyethylene naphthalate (PEN) as a thermoplastic. The layered membrane electrode assembly 6 spans an imaginary plane 37 (FIG. 3). Moreover, the bipolar plates 10 and the anodes 7 and cathodes 8 with the catalyst layers 30 and the gas diffusion layers 9 also span fictitious planes 37 aligned parallel to each other.

[0089] For producing the catalyst material 47 comprising catalytically active nanoparticles 47, a provision 51 of a carrier material is performed. The carrier material consists of carbon powder, with a carbon particle size ranging between 20 nm and 40 nm. The carrier material features a high degree of porosity for the attachment or adhesion of a first and a second starting material. The first starting material is also provided 52. The first starting material is a chemical compound as a precursor of a first metal as platinum (Pt) and other chemical elements, e.g., H.sub.2PtCl.sub.6 or Pt(NO.sub.3).sub.2) in a solution, in particular with alcohol, preferably ethylene glycol. Furthermore, the second starting material is provided 53. The second starting material comprises a second metal, i.e., tungsten (W) or molybdenum (Mo) as an oxide of the second metal as oxide nanoparticles, e.g., WO.sub.3, or a solution of a salt of the second metal or a complex with the second metal, in particular as a solution in alcohol, in particular ethylene glycol, or with alcohol and water. Subsequently, a mixing 54 of the first starting material, the second starting material and the carrier material is performed to form a reactant material such that the first and second starting materials at least partially attach 55 and adhere 55 to the carrier material. A material quantity ratio between the first metal and the second metal from 1:3 to 2:1 is advantageous. After mixing 54 the first and second starting materials and the carrier material with the reactant material, the reactant material is dried, so the reactant material is then present as a powder or particulate reactant material.

[0090] Subsequently, a thermal treatment 56 of the reactant material is performed with the first and second starting materials and the carrier material, i.e., they are heated to about 500? C. to 1500? C. for a few hours while being arranged 58 in a hydrogen atmosphere. The temperature level affects the size of the nanoparticles 47. The thermal treatment 56 and the arranging 58 in the hydrogen atmosphere are, e.g., performed in a closed furnace (not shown). The thermal treatment 56 causes the first metal as platinum and the second metal as tungsten or molybdenum to combine into nanoparticles 47 with an alloy of the first and second metal 57 as an intermediate material, i.e., the production 57 of catalytically active nanoparticles 47 is performed with an alloy of the first and second metals. During the thermal treatment, the entire second metal is in this case not typically transferred to the alloy with the first metal, such that the first and/or second starting materials, e.g., as tungsten oxide, WO.sub.3, remain in the catalytically active nanoparticles 47, in particular at the surface 48 and/or surface layer 49. The catalytically active nanoparticles 47 from the alloy attach to the carbon particles of the carrier material as an adhesion. The thermal treatment causes the formation of the nanoparticles 47 as an intermediate material in a solution. The nanoparticles 47 with a diameter of between 1.5 and 4.0 nm comprise a surface 48. In the outer marginal regions of the nanoparticles 47, a surface layer 49 with a layer thickness between 3% and 20% of the diameter of the nanoparticles 47 is formed. Within the surface layer 49, the interior 50 of the nanoparticles 47 exists.

[0091] After producing the catalytically active nanoparticles 47 as an intermediate material in the form of a powder, the proportion of the second metal, tungsten, or molybdenum, and/or a tungsten oxide or molybdenum oxide, is reduced 59 in the surface layer 49 by performing a rinse 60 of the intermediate material with sodium hydroxide and alcohol such that a solution is provided during the rinse 60. Sodium hydroxide partially dissolves out tungsten oxide or molybdenum oxide in the surface layer 49, so the remaining proportion of the catalytically active alloy of the first and second metals and the proportion of the first metal as platinum is increased at the surface 48 of the nanoparticles 47 as a solution, and the catalytic effect of the catalyst material 47 as the nanoparticles 47 is also greatly increased as a result, thereby producing a product material. Alcohol in the solution improves dispersion during the rinse 60.

[0092] Subsequently, an additional thermal treatment 61 of the product material with the catalytically active nanoparticles 47 as the catalyst layer 30 is performed, i.e., it is heated to about 200? C. to 700? C. for a few hours while being arranged 62 in a hydrogen atmosphere. During or prior to the additional thermal treatment 61, the product material as a solution is dried into a powder or particulate product material without a liquid. In the surface layer 49 of the nanoparticles 47, the composition between platinum and tungsten is between 95%:5% and 60%:40%.

[0093] To produce catalyst layers 30, in particular, electrodes 7, 8, 45 with catalyst layers 30 for fuel cells 2, the following steps are performed: providing catalyst material 47 according to the description hereinabove comprising catalytically active nanoparticles 47, providing carrier layers 46 for adhesion of catalyst material 47, applying the catalyst material 47 to the carrier layers 46, such that the catalyst material 47 is adhesively adhered to the carrier layers 46 and made from the carrier layers 46 of the catalyst layers 30. The catalyst material 47 acting as the product material is present as a dry powder or dry particles, such that the dry, powdered or particulate product material is mixed into a slurry with water, alcohol and at least one ionomer, e.g., Nafion?, before it is applied to the carrier layer. This slurry is subsequently wet-chemically applied to the carrier layer 46e.g., an electrode 7, 8, 45 or a gas diffusion layer 9in a thin layer, e.g., with a layer thickness of 10 ?m. The slurry as a product material attaches or adheres wet-chemically to the carrier layer 46, such that a catalyst layer 30 is formed on the adjacent area of the carrier layer 46. Drying of the catalyst layer 30 is preferably then performed.

[0094] Overall, significant advantages are associated with the method of producing catalyst material 47 according to the invention, the method of producing catalyst layers 30 according to the invention, the fuel cell unit 1 according to the invention, and the method of producing the fuel cell unit 1 according to the invention. The catalytically active nanoparticles 47 in the catalyst layers 30 of the carrier layers 46 contain a high proportion of platinum and/or the alloy of platinum and tungsten or molybdenum and a low proportion of tungsten oxide or molybdenum oxide in the surface layers 49, such that the fuel cells 2 achieve a high electrical performance per unit area, e.g., cm.sup.2. This represents a particularly important advantage in the application of the fuel cell unit 1 in motor vehicles.