Catalyst layer for a fuel cell and method for the production thereof

Abstract

A catalyst layer (20) for a fuel cell and to a method suitable for producing the catalyst layer (20). The catalyst layer (20) includes a catalyst material (22) containing a catalytic material (24) and optionally porous carrier material (23) on which the catalytic material (24) is supported. The catalyst layer also includes mesoporous particles (21) made from hydrophobic material.

Claims

1. A catalyst layer for a cathode of a fuel cell, the catalyst layer comprising: a catalyst material containing a catalytic material; and hydrophobic carbon-based mesoporous particles, the catalyst material and the hydrophobic carbon-based mesoporous particles being on a cathode side of a polymer electrolyte membrane, the hydrophobic carbon-based mesoporous particles being in the form of a hydrophobic material.

2. The catalyst layer as recited in claim 1, wherein the catalytic material is not supported on the hydrophobic carbon-based mesoporous particles.

3. The catalyst layer as recited in claim 1, wherein the hydrophobic carbon-based mesoporous particles have an average pore diameter in the range of 2 nm to 20 nm.

4. The catalyst layer as recited in claim 1, wherein the hydrophobic carbon-based mesoporous particles have a pore volume of at least 2 ml/g.

5. The catalyst layer as recited in claim 1, wherein the hydrophobic carbon-based mesoporous particles have an average size of no more than 1 μm.

6. The catalyst layer as recited in claim 1, further comprising water-retaining particles made of silicon dioxide or carbon.

7. The catalyst layer as recited in claim 1, wherein the catalyst layer has an average content of the hydrophobic carbon-based mesoporous particles of 0.5 g to 5 g per square meter (g/m.sup.2).

8. The catalyst layer as recited in claim 1, further comprising a porous carrier material, the catalytic material being supported on the porous carrier material.

9. The catalyst layer as recited in claim 1 wherein the hydrophobic material is graphite.

10. The catalyst layer as recited in claim 1 wherein the hydrophobic material is carbon black.

11. The catalyst layer as recited in claim 1 further comprising water retaining mesoporous particles.

12. The catalyst layer as recited in claim 11 wherein the water retaining mesoporous particles are made of silicon dioxide or carbon.

13. The catalyst layer as recited in claim 1 wherein the hydrophobic carbon-based mesoporous particles have a hydrophobicity showing a static contact angle of greater than 80 degrees.

14. The catalyst layer as recited in claim 13 wherein the hydrophobic carbon-based mesoporous particles have a hydrophobicity showing a static contact angle of greater than 90 degrees.

15. The catalyst layer as recited in claim 14 wherein the hydrophobic carbon-based mesoporous particles have a hydrophobicity showing a static contact angle of greater than 100 degrees.

16. The catalyst layer as recited in claim 1 wherein the catalyst layer is directly coated on the cathode side of the polymer electrolyte membrane.

17. A cathode of a fuel cell comprising the catalyst layer as recited in claim 1.

18. A fuel cell comprising a polymer electrode membrane having a catalyst layer, the catalyst layer comprising: a polymer electrode membrane; a catalyst material containing a catalytic material; and hydrophobic carbon-based mesoporous particles, the catalyst material and the hydrophobic carbon-based mesoporous particles being on a cathode side of the polymer electrolyte membrane, the hydrophobic carbon-based mesoporous particles being in the form of a hydrophobic material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically shows a sectional view of a PEM fuel cell (individual cell),

(2) FIG. 2 schematically shows a sectional representation of a catalytically coated substrate having a catalyst layer according to the present invention.

DETAILED DESCRIPTION

(3) A schematic sectional view of an individual cell of a PEM fuel cell, identified in its entirety with reference numeral 100, is depicted in FIG. 1 for explaining the structure and its mode of operation.

(4) Fuel cell 100 includes as its core component a membrane electrode assembly 6, which has a polymer electrolyte membrane 1 as well as one electrode 2 each, namely, an anode and a cathode, attached on one or both flat sides of membrane 1. Polymer electrolyte membrane 1 is a cation-, in particular, proton (H.sup.+)-conducting membrane. Electrodes 2 include a catalytic material, which may be supported on an electrically conductive material, for example, a carbon-based material.

(5) A gas diffusion layer 3 is attached to each electrode 2, which is essentially assigned the task of uniformly distributing the supplied process gases over the main surfaces of electrodes 2, and membrane 1.

(6) A bipolar plate 5 is situated on the outside of each gas diffusion layer 3. Bipolar plates have the task of interconnecting individual membrane electrode assemblies 6 of the individual cells in the stack electrically to one another, to cool the fuel cell stack and to supply the process gases to electrodes 2. For this latter purpose, bipolar plates 5 (also called flow field plates) include flow fields 4. Flow fields 4 include, for example, a plurality of flow channels situated in parallel to one another, which are incorporated in the form of furrows or grooves in plates 5. Each bipolar plate normally includes an anode flow field on its one side, which faces anode 2, and a cathode flow field on its other side facing the cathode. In the present case, only one flow field 4, respectively, is depicted for each of the two depicted bipolar plates 5. A fuel, in particular, hydrogen (H.sub.2) is supplied to the anode flow field, whereas an oxygen (O.sub.2)-containing operating medium, in particular, air, is supplied to the cathode flow field.

(7) The reactions taking place at the anode and the cathode are also depicted in FIG. 1. Accordingly, a catalytic oxidation of hydrogen H.sub.2 to protons H.sup.+ takes place at the anode giving off electrons. The protons migrate through proton-conducting membrane 1 to reach the cathode. There, the supplied oxygen O.sub.2 reacts with the protons yielding water H.sub.2O, the oxygen being reduced and therefore taking up electrons. The electrons of the anode reaction are supplied to the cathode via an outer electrical circuit not depicted. It is apparent from FIG. 1 that the product water of fuel cell 100 accrues on the cathode side of membrane 1.

(8) FIG. 2 shows a detail of a catalytically coated substrate 10 according to the present invention in a schematic sectional representation. Catalytically coated substrate 10 includes a substrate 15 to which a catalyst layer 20 according to the present invention is applied. Catalyst layer 20 may function as a cathode or an anode in a fuel cell. Substrate 15 may be a membrane or a gas diffusion layer. In the specific embodiment depicted herein, substrate 15 is a membrane. Membrane 15 may, in principle, be an arbitrary membrane used in fuel cell technology. These include, for example, polymer electrolyte membranes, the electrolytic conductivity of which is based on its moistening with water (for example, Nafion®), or those which owe their conductivity to an acid bonded to the polymer material, for example, polybenzimidazole doped with phosphoric acid.

(9) Catalyst layer 20 is composed of at least one catalyst material 22 and particles 21. It may also include a solvent, an electrolyte and/or an electron conductor. Catalyst material 22 in the specific embodiment depicted, in turn, includes a catalytic material 24, which is adsorbed on a carrier material 23.

(10) Carrier material 23 functions as an electron conductor. The electron conductor is generally an electrically conductive carbon particle. All carbon materials known in the field of fuel cells or electrolyte cells having high electrical conductivity and a large surface may be used as electrically conductive carbon particles. The surface is, for example, 50 m.sup.2/g to 200 m.sup.2/g. Carbon black, graphite or active carbons are preferably used. Carbon blacks having high conductivity, so-called conductive carbon blacks are particularly preferably used. In addition, carbon may also be used in other modifications, for example, in granular form or as so-called nanotubes.

(11) All materials which promote the chemical processes at the location of catalytically coated substrate 10 may be used as catalytically active material 24. If catalytically coated substrate 10 is used in a fuel cell, the elements, in particular, of the group IIIb, IVb, Vb, VIb, VIIb, VIIIb, Ib, IIb of the Periodic Table of the Elements as well as tin are preferred. The metals of the platinum group, preferably platinum, palladium, iridium, rhodium, ruthenium and/or mixtures thereof are particularly preferred. In addition, alloys of, for example, platinum, cobalt, nickel, iron and/or iridium may also be used as a catalyst. Generally, catalytically active material 24 is present in the oxidation stage ±0. Oxides of catalytic material 24 are also possible, however.

(12) If catalytic material 24 is present on a carrier 23, it may be a chemical bond, preferably however, a physical adsorption of catalytic material 24 on the surface of carrier 23. For this purpose, carrier 23 may be impregnated with catalytic material 24, carrier material 23 being introduced into a salt solution of catalytic material 24 such as, for example, platinum chloride or platinum nitrate, the catalytic metal cation being sorbed on the surface of material 24 and subsequently reduced to metal. As a result of the impregnation, catalytic material 24 is adsorbed on the surface of carrier material 23.

(13) According to the present invention, particles 21 are mesoporous particles 21 made of a hydrophobic material, which are not related to catalytic material 24 in the embodiment depicted. Hydrophobic materials in this case are, in particular, hydrophobic carbons and compounds thereof. The particles are present in catalyst layer 20 in a concentration in the range of 0.5 g to 5 g particles per m.sup.2 of catalyst layer. The particles have on average a size of 1 μm maximum. The highly porous particles 21 show a pore diameter in the range of 2 nm to 50 nm. They have, on average, a pore diameter in the range of 2 nm to 50 nm. A pore diameter of, on average, 20 nm maximum is particularly preferred. A pore volume of at least 2 ml/g is formed depending on the number of pores.

(14) The pores of particles 21 in this case show, in particular, the function of the media transport. Water, in particular, is intended to be removed from catalyst layer 20 in order to prevent a decrease in the performance output or in the service life of a catalytically coated substrate 10. The hydrophobic character of particles 21 supports this function, since it prevents water from accumulating in the interior of the pores.

LIST OF REFERENCE NUMERALS

(15) 100 fuel cell 1 membrane 2 electrode 3 gas diffusion layer—GDL 4 flow field 5 bipolar plate—BPP 6 membrane electrode assembly—MEA 10 catalytically coated substrate 15 substrate/polymer electrolyte membrane 20 catalyst layer 21 particles 22 catalyst material 23 carrier material 24 catalytically active material