WO2011069593A1

WO2011069593A1 - Method for producing a catalytic material for electrodes of a fuel cell

Info

Publication number: WO2011069593A1
Application number: PCT/EP2010/006946
Authority: WO
Inventors: Mirko Herrmann; Rouven Scheffler; Ulrich MÄHR
Original assignee: Volkswagen Aktiengesellschaft
Priority date: 2009-12-10
Filing date: 2010-11-15
Publication date: 2011-06-16
Also published as: EP2510570B1; EP2510570A1; DE102009057797A1

Abstract

The invention relates to a method for producing a catalytic material for electrodes of a fuel cell, wherein a noble metal alloy containing at least one metal of the platinum and/or gold group and at least one further transition metal and adhering to an electrically conductive substrate material containing carbon is brought in contact with a solution of a salt or an acid of a noble metal, which in the present solution has a higher positive electrochemical potential than the at least one further transition metal of the noble metal alloy, so that at least part of the at least one transition metal is electrochemically exchanged for atoms of the noble metal on a surface of the noble metal alloy. The invention further relates to a catalytic material for electrodes of a fuel cell and to a fuel cell that comprises electrodes having such a material.

Description

description

Method for producing a catalytic material for electrodes of a fuel cell

The invention relates to a method for producing a catalytic material for electrodes of a fuel cell, a catalytic material producible by the method and a fuel cell with such a catalytic material.

Fuel cells use the chemical transformation of hydrogen and oxygen into water to generate electrical energy. For this purpose, fuel cells contain as a core component, the so-called membrane electrode assembly (MEA for membrane electrode assembly), which is a composite of a proton-conducting membrane and each on both sides of the membrane arranged gas diffusion electrode (anode and cathode). In general, the

Fuel cell formed by a plurality, arranged in stack (stack) MEA, the electrical power to add. During operation of the fuel cell, a fuel, in particular hydrogen H ₂ or a hydrogen-containing gas mixture, is fed to the anode, where an electrochemical oxidation of H ₂ to H ⁺ takes place with emission of electrons. Via the membrane, which separates the reaction spaces gas-tight from each other and electrically isolated, takes place (water-bound or anhydrous) transport of protons H ⁺ from the anode compartment in the cathode compartment. The electrons provided at the anode are supplied to the cathode via an electrical line. The cathode is supplied with oxygen or an oxygen-containing gas mixture, so that a reduction of 0 ₂ to O ^{2 "} under

Recording of the electrons takes place. At the same time, these are governed in the cathode compartment

Oxygen anions with the protons transported across the membrane to form waters. Achieve through the direct conversion of chemical into electrical energy

Fuel cells compared to other electricity generators due to the circumvention of the Carnot factor improved efficiency.

The most advanced fuel cell technology is based on

Polymer electrolyte membranes (PEM), in which the membrane itself from a

Polymer electrolyte exists. In this case, acid-modified polymers, in particular perfluorinated polymers are often used. The most common representative of this class of Polymer electrolyte is a membrane of a sulfonated polytetrafluoroethylene copolymer (trade name: Nation; copolymer of tetrafluoroethylene and a sulfonyl fluoride derivative of a perfluoroalkyl vinyl ether). The electrolytic conduction takes place via hydrated protons, which is why the presence of liquid water is a prerequisite for the proton conductivity. This results in a number of disadvantages. Thus, during operation of the PEM fuel cell moistening the operating gases is required, which means a high overhead system. If the humidification system fails, power losses and irreversible damage to the membrane-electrode assembly are the result. Furthermore, the maximum operating temperature of these fuel cells - also due to the lack of thermal stability of the membranes - limited at standard pressure below 100 ° C. For mobile as well as stationary use, however, operating temperatures above 100 ° C are desirable for many reasons. Thus, the heat transfer increases with increasing difference to the ambient temperature and allows better cooling of the fuel cell stack. Furthermore, the catalytic activity of the electrodes and the tolerance increase

Contamination of the fuel gases with increasing temperature too. At the same time, the viscosity of the electrolytic substances decreases with increasing temperature and improves the

Mass transport to the reactive centers of the electrodes. Finally, at temperatures above 100 ° C, the resulting product water is gaseous and can be better removed from the reaction zone, so that existing in the gas diffusion layer

Gas transport paths (pores and mesh) are kept free and also a washing out of the electrolytes and electrolyte additives is prevented.

To overcome these problems, high temperature polymer electrolyte membrane fuel cells (HT-PEM fuel cells) have been developed which operate at operating temperatures of 120 to 180 ° C and which require little or no humidification. The electrolytic conductivity of the membranes used here is based on liquid, bound by electrostatic complex binding to the polymer backbone electrolyte,

in particular acids or bases, which ensure proton conductivity even when the membrane is completely dry above the boiling point of water. The most promising approach involves the use of acid-doped basic N-heterocyclic polymers, in particular polyazoles, where the proton conduction is based on an acid bound as a complex in the polymer. For example

Polybenzimidazole (PBI) high-temperature membranes filled with acids, such as

Phosphoric acid, sulfuric acid or others, are described in US 5,525,436, US 5,716,727, US 5,599,639, WO 01/18894 A, WO 99/04445 A, EP 0 983 134 B and EP 0 954 544 B. The electrodes, in particular of HT-PEM fuel cells, usually each have one, the membrane facing catalyst layer, which is applied to a gas-permeable substrate, the so-called gas diffusion layer (GDL for gas diffusion layer), for the homogeneous supply of the reaction gases. The catalyst layer contains an electrically conductive porous carrier material, for example carbon particles, on which the catalytically active component is present in supported form. Many precious metal catalysts have been investigated for their suitability in PBP / phosphoric acid-based HT-PEM fuel cells. Due to the high stability, virtually exclusively platinum has prevailed commercially. In addition, the catalyst layer may contain further additives, for example polymeric binders and / or

Contain components for hydrophobization.

The slowest step taking place in the fuel cell reaction is the cathodic

Oxygen reduction (ORR for Oxygen Reduction Reaction), and in particular the adsorption of the oxygen anions O ^{2 "} on the surface of the catalytic component in this process A promising approach for increasing the rate of adsorption is the use of platinum alloy catalysts and at least one other

Transition metal. In particular, binary alloys PtM (with M = Co, Cu, Fe, Cr, Ru, Ir or Au) or ternary systems PtXY (eg with X = Co and Y = Ni, X = Cr and Y = Co or X = Ru and Y = Ir) are used. The effects of transition metals on the reaction kinetics have been described, for example, by Zhang et al. ("Mixed-Metal Pt Monolayer Electrocatalysis for

Enhanced Oxygen Reduction Kinetics, JACS 127 (2005), 12480-12481), and the transition metals have an impact on stability, activity, and tolerance to impurities such as those described for gold (Zhang et al .: Stabilization of Platinum Oxygen-Reduction Electrocatalysts Using Gold Clusters ", Science 315 (2007), 220-222).

A disadvantage of this approach is that the catalytic surface of the platinum is partially occupied by atoms of the transition metal and thus is not available for the reduction of oxygen anions, resulting in a certain loss of power. To address this problem, it is known to subject the platinum alloy to acid or alkali washing. In this case, the corresponding transition metal is partially dissolved out of the surface of the metal alloy while remaining inside the alloy particles to accelerate the adsorption of the O ^{2 "} species, but this process also leads to a disadvantageous destruction of the surface structure the platinum particle and thus to a power reduction of the fuel cell and a loss of stability comes. The object of the present invention is therefore to provide a catalyst for fuel cells with an improved power density and a method for its production.

These objects are achieved by a method for producing a catalytic material for electrodes of a fuel cell, a processable by the catalytic material and a fuel cell with the features of the independent claims. Advantageous embodiments of the invention are the subject of the dependent claims.

According to the method of the invention, a noble metal alloy adhering to an electrically conductive, carbonaceous carrier material comprising at least one metal of the platinum and / or gold group and at least one further transition material is brought into contact with a solution of a salt or an acid of a noble metal. In this case, the dissolved noble metal in the present solution, a higher positive electrochemical

As a result, on a surface of the noble metal alloy, electrochemical exchange of at least a portion of the at least one transition metal with atoms of the noble metal occurs Thus, the electrochemical potential of the noble metal anion of the solution acts as an oxidizing agent for the less noble metal transition metal (oxidation state 0) of the noble metal alloy, wherein the transition metal gives off electrons to the noble metal and goes into solution while the noble metal is reduced and with in metallic form

(Oxidation state 0) deposits on the surface of the noble metal alloy.

The result of the inventive process described above is one

carbon-supported noble metal alloy, wherein the concentration of the at least one metal of the platinum and / or gold group is higher on its surface than in its interior. This catalytic material, which is another aspect of the invention, is characterized by an undestroyed surface of the alloy and, due to the high degree of

Surface concentration of platinum or gold group metal has a high catalytic

Performance. Nevertheless, due mainly to the interior of the

Precious metal alloy particles existing transition metal to observe an accelerated adsorption of oxygen ions O ^{2 "} during the fuel cell process. In a preferred embodiment of the method, the at least one metal of the platinum and / or gold group of the noble metal alloy is an element of the group Ru, Rh, Pd, Os, Ir, Pt, Au and / or Ag. In particular, it is selected from the group Pt, Pd, Au, Ag, Ir and Ru, particular preference being given to using platinum Pt, since this has the highest stability and catalytic activity of the fuel cell reaction.

The at least one transition metal element of the noble metal alloy is in an advantageous embodiment of the invention, an element of the first, fifth, sixth, seventh or eighth subgroup of the Periodic Table of the Elements (PSE). It is preferably selected from Fe, Co, Ni, Cr, Cu, Ir and Ru. These transition metals lead to a particularly rapid adsorption of oxygen anions to the catalyst.

Preferred noble metal alloys are for example binary alloys of

Composition PtM, wherein M can be an element of the group Co, Cu, Fe, Cr, Ir or Ru. Alternatively, the noble metal alloy may be a ternary alloy of the composition PtXY, wherein X and Y are unequally selected and are each an element of the group Co, Ni, Cu, Fe, Cr, Ir and Ru.

The noble metal, which is used in the form of a salt or an acid in the solution, may be composed of the same elements as the metal of the platinum or gold group of the

Precious metal alloy can be selected. It can be identical to this or

be chosen differently to this.

The carbonaceous support material used is preferably graphite or a modification thereof. For example, a product known by the trade name Vulcan XC-72 may be used as the carbonaceous carrier. Structurally, the carbon support is preferably in the form of particles having particle diameters in the range of 20 to 500 nm.

After the washing according to the invention of the catalytic material with the noble metal salt or the noble metal acid, the material thus treated is washed with water or an organic solvent in order to remove the salt or acid residues as well as the dissolved transition metal. Subsequently, the material is dried.

Usually, the catalytic material is applied to a gas diffusion layer, which serves the uniform distribution of the reaction gases and usually from a carbon-based tissue. Such a structure of gas diffusion layer (GDL) and an applied layer of the catalytic material is also referred to as a gas diffusion electrode.

Another aspect of the present invention relates to a fuel cell having a single cell having at least one membrane-electrode assembly and containing a polymer electrolyte membrane sandwiched between two electrodes, the electrodes comprising a catalytic material according to the present invention. The fuel cell is preferably a high-temperature polymer electrolyte membrane fuel cell, the proton conduction of which is realized via an electrolyte complexed to the polymer electrolyte membrane. In particular, one can here with

Phosphoric acid H ₃ P0 _4- treated membrane of polybenzimidazole PBI can be used.

The inventive "precious metal wash" are on the surface of the

Noble metal alloy targeted metals, namely those of at least one other

Transition metal, exchanged to influence the reaction kinetics for the ORR. Base metals (the transition metals) on the surface of the alloy particles are replaced by nobler transition metals and unlike the prior art are not only dissolved out. Thus, the sensitive surface structure is retained for the reaction and is not destroyed by defects from the liberated transition metals.

The invention will be described below in embodiments with reference to the associated

Figures explained in more detail. Show it:

FIG. 1A shows a highly schematic fuel cell;

FIG. 1B shows a detail from FIG. 1A with a membrane-electrode unit;

Figure 1 C is a more detailed illustration of a membrane-electrode assembly according to the present invention; and

Figure 2 shows the performance of a membrane-electrode unit with

Gas diffusion electrodes containing a catalytic material according to the invention (PtNiCo-Pt). FIG. 1A shows in a highly schematic representation a fuel cell 10 with a fuel cell stack consisting of a multiplicity of individual cells 12, each of which has a membrane-electrode unit 14 (MEA). Such is shown in Figure 1 B in an enlarged sectional view. A somewhat more detailed representation of a section of the membrane-electrode unit 14 is shown in FIG. 1C also in a sectional view.

As can be seen in FIGS. 1B and 1C, the MEA 14 comprises a proton-conducting (essentially anhydrous) polymer electrolyte membrane 16, which consists of a suitable polymer electrolyte membrane 16

Polymer material 24 is formed and impregnated with at least one electrolyte 26.

For example, the polymer material may be a polymer from the group of polyazoles and polyphosphazohe. In particular, polybenzimidazoles, polypyridines,

Polypyrimidines, polyimidazoles, polybenzothiazoles, polybenzoxazoles, polyoxadiazoles,

Polychinoxalines, polythiadiazoles, poly (tetrazapyrene), polyvinylpyridines, polyvinylimidazoles.

The anhydrous proton conduction of the polymer membrane 16 or the

Polymeric material 24 is based on the electrolyte 26, which is in particular a solution of a high-boiling temperature-resistant electrolyte. Preferably, it is an acid such as phosphoric acid, phosphinic acid, phosphonic acid, nitric acid, hydrochloric acid, formic acid, acetic acid, trifluoroacetic acid, sulfuric acid, sulfonic acid, a particular (per) halogenated alkyl or aryl sulfonic acid or (per) halogenated alkyl or

Arylphosphonic acid, especially methanesulfonic acid or phenylsulfonic acid. Also suitable are phosphoric acid alkyl or aryl esters, heteropolyacids, such as hexafluoroglutaric acid (HFGA) or squarric acid (SA). Alternatively, the electrolyte 26 may be a base, especially an alkali or alkaline earth hydroxide, such as potassium hydroxide, sodium hydroxide or lithium hydroxide. It is also possible to use polysiloxanes or nitrogen-containing heterocycles as electrolyte 26 or addition of electrolyte, for example imides, imidazoles, triazoles and derivatives of these, in particular perfluorosulfonimides. Likewise, ionic liquids, such as 1-butyl-3-methylimidazolium trifluoromethanesulfonite, are suitable as the electrolyte. All of the aforementioned electrolytes can also be used as a derivative or salts. It is also conceivable to use a mixture of various of the abovementioned electrolytes for impregnating the polymer material 24.

Preference is given to using proton exchange membranes which are formed by impregnation of a temperature-resistant basic polymer with an acid. In the present For example, a membrane of polybenzimidazole (PBI) is used as the anhydrous polymer material 24 in which phosphoric acid is bonded as the electrolyte 26.

Each of the two outer membrane surfaces is followed by a gas diffusion electrode 18a and 18b, namely a cathode-connected electrode 18a on the cathode side of the membrane 16 and an anode-connected electrode 18b on the anode side. The

Gas diffusion electrodes 18a and 18b each include a microporous catalyst layer 20a and 20b which contact the polymer electrolyte membrane 16 on both sides. The

Catalyst layers 20a, 20b contain as actual reactive centers of the electrodes, a catalytic material according to the present invention, which is a binary, ternary or higher noble metal alloy as a catalytically active substance. This comprises at least one element of the platinum and / or gold group, in particular, such as platinum, iridium or ruthenium, palladium, gold and / or silver, and at least one further transition metal, such as chromium, cobalt, nickel, iron, copper, iridium and / or ruthenium. The catalytic substance is preferably fixed on a porous, electrically conductive carrier material. For the carrier material gas-permeable electrically conductive carbon materials, such as gas-permeable particles, fabrics and felts carbon-based in question. An electrically conductive connection of the reaction centers of the electrodes to an external circuit (not shown) is realized via the carrier material of the catalyst layers 20a and 20b.

The gas diffusion electrodes 18a and 18b also each comprise a gas diffusion layer (GDL) 22a and 22b, which are connected to the outer, from the

Polymer electrolyte membrane 16 facing away from surfaces of the catalyst layer 20a

or 20b. Function of the GDL 22a, 22b is a uniform flow of the catalyst layers 20a, 20b with the reaction gases oxygen

or to ensure air on the cathode side and hydrogen on the anode side. Furthermore, the GDL 18a, 18b adjacent to the catalyst layer 20a, 20b may still have a thin microporous layer, for example carbon based (not shown). Not shown in Figures 1 B and 1C are also so-called bipolar plates (BP), which connect to both sides of the MEA composite and provide for the supply of process gases and the discharge of product water and also the individual MEA 14 im

Fuel cell stack 12 gas-tight separate from each other.

According to the invention, the preparation of the catalytic material for the catalyst layers 20A and 20B is carried out by subjecting the carbon-supported noble metal alloy to a wash with a solution of a noble metal salt or noble metal acid. Important in the Selection of the noble metal of the washing solution is that this has a higher positive electrochemical potential than the transition metal to be removed from the surface of the noble metal alloy. In this case, the noble metal of the washing solution can basically be selected from the same metal of the platinum or gold group of the catalytic noble metal alloy, for example platinum. Exemplary embodiments for the preparation of the catalytic material according to the invention are described below.

In principle, the corresponding noble metal salt or the noble metal acid is dissolved in distilled water in a suitable vessel. For example, this is a

Precious metal content of 1 to 10 mmol, in particular from 2 to 3 mmol in 100 ml H ₂ 0 set. For example, 4 g of the carbon-supported noble metal alloy are added to this solution and stirred for 90 minutes. The solution is then diluted with a further 100 ml of H ₂ O and stirred for a further 12 to 20 hours. The product thus treated is washed several times with distilled water, filtered and dried, for example, over silica rock beads.

The finished catalyst can then be processed with a pore-forming agent in an organic liquid, for example Ν, Ν-dimethylacetamide to a paste and with a

Drawing knife are applied to a gas diffusion layer. The coated GDL is dried in a vacuum oven at 150 ° C and a pressure of 300 mbar for 16 hours. After drying, the coated GDL is a ready-to-use electrode for the HT fuel cell.

Particularly preferred noble metal alloys and their molar or atomic compositions are listed in Table 1.

Table 1 :

alloy catalysts

Pt-Ni-Co 3: 0.5: 0.5

3: 3: 1

3: 6: 3

2: 1: 1

Pt-Ir 3: 1

2: 1

Pt-Ru 3: 1 2: 1

Pt-Ru-Ir 3: 0.5: 0.5

Pt-Co 3: 1

1: 1

Preferred salt or acid compounds of noble metals suitable for the solution of

Precious metal washing can be used, are listed in Table 2.

Table 2:

Examples of the preparation of the catalytic material according to the invention for the

Gas diffusion electrodes 18a and 18b will be described below.

example 1

In a beaker, 158 g of H ₂ PtCl ₆ (H ₂ 0) _x dissolved in 100 ml H ₂ 0. 1 To the solution is added 4 g of alloy catalyst and stirred. The solution is then diluted with an additional 100 ml and stirred for 12 to 20 hours. The product is washed several times with distilled water, filtered, dried and then further processed into electrodes.

Example 2

In a beaker, 1.03 g of HAuCl ₄ (H ₂ O) _{x is dissolved} in 100 ml of H ₂ O. To the solution is added 4 g of alloy catalyst and stirred for 90 minutes. The solution is then diluted with an additional 100 ml and stirred for 12 to 20 hours. The product comes with Washed several times with distilled water, filtered, dried and then processed into electrodes.

Example 3

In a beaker, 0.58 g of RuCl _{3 is dissolved} in 100 ml of H ₂ O. To the solution is added 4 g of alloy catalyst and stirred for 90 minutes. The solution is then diluted with an additional 100 ml and stirred for 12 to 20 hours. The product is washed several times with distilled water, filtered, dried and then further processed into electrodes.

FIG. 2 shows the performance characteristics of the catalyst PtNiCo (3: 0.5: 0.5) prepared according to Example 1, which was treated with hexachloroplatinic acid H ₂ PtCl ₆ . A membrane-electrode assembly according to FIG. 1 containing this catalyst exhibited a power density of up to 0.87 W / cm ² at 0.6 V.

LIST OF REFERENCE NUMBERS

fuel cell

single cell

Membrane Electrode Unit (MEA)

Polymer electrolyte membrane

a first gas diffusion electrode (cathode)

b second gas diffusion electrode (anode)

a cathode-side catalyst layer

b anode-side catalyst layer

a cathode-side gas diffusion layer (GDL) b anode-side gas diffusion layer (GDL)

polymer material

electrolyte

Claims

claims

1. A process for producing a catalytic material for electrodes of a

Fuel cell, one on an electrically conductive, carbon-containing

Supporting noble metal alloy comprising at least one metal of the platinum and / or gold group and at least one further transition metal, with a solution of a salt or an acid of a noble metal, which has a higher positive electrochemical potential in the present solution than the at least one further transition metal the noble metal alloy is brought into contact so that on a surface of the noble metal alloy at least a part of the at least one

Transition metal is electrochemically exchanged for atoms of the noble metal.

2. The method of claim 1, wherein the at least one metal of the platinum or gold group and / or the noble metal independently of one another is an element of the group Ru, Rh, Pd, Os, Ir, Pt, Au, Ag, in particular the group Pt, Pd, Au, Ag, Ir, Ru, preferably Pt.

3. The method according to any one of claims 1 or 2, wherein the at least one further

Transition metal is an element of I., V., VI. VII. And / or VIII. Subgroup of the PSE, in particular an element, the group Fe, Co ,, Ni, Cr, Cu, Ir, Ru.

Method according to one of claims 1 to 3, wherein the noble metal alloy is a binary alloy of composition PtM, with M = Co, Cu, Fe, Cr, Ir or Ru.

A method according to any one of claims 1 to 3, wherein the noble metal alloy is a ternary alloy of the composition PtXY with XY = Co, Ni, Cu, Fe, Cr, Ir or Ru.

6. The method according to any one of the preceding claims, wherein the carbonaceous

Support material is graphite or a modification of graphite.

7. A catalytic material for electrodes of a fuel cell, producible according to one of claims 1 to 6, comprising an electrically conductive, carbonaceous carrier material and adhering to the support material noble metal alloy, the at least one metal the platinum and / or gold group and at least one further transition metal, wherein a concentration of the at least one metal of the platinum and / or gold group on a surface of the noble metal alloy is higher than in an interior thereof.

8. Fuel cell (10). with at least one, a membrane-electrode unit (14)

single cell (12) comprising a polymer electrolyte membrane (16) sandwiched between two electrodes (18a, 18b), said electrodes (18a, 18b) comprising a catalytic material according to claim 7.