EP1624966A2

EP1624966A2 - Catalyst layer, suitable catalyst paste, and method for the production thereof

Info

Publication number: EP1624966A2
Application number: EP04730194A
Authority: EP
Inventors: Kai Jakoby; Morten Schonert; Andreas GLÜSEN; Carola Schlumbohm; Detlef Stolten
Original assignee: Forschungszentrum Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 2003-05-06
Filing date: 2004-04-29
Publication date: 2006-02-15
Also published as: DE10320320B4; WO2004098773A3; DE10320320A1; WO2004098773A2

Abstract

Known prior art catalyst pastes are, as a rule, to liquid for coating by means of silk-screen printing or colour ductors. The invention relates to a method for the production of a catalyst paste, containing an acidic ion exchanger and specific base polymers, which is especially suitable for coating methods such as silk-screen printing and colour ductors due to the viscosity thereof. A catalyst layer produced with said inventive catalyst paste does not regularly display any loss in performance compared to catalyst layers produced by prior art standard catalyst pastes. Even improved performance properties are partially obtained.

Description

Catalyst layer, suitable catalyst paste, and the production process thereof

The invention relates to a method for producing a catalyst layer, in particular a catalyst layer, which is suitable for use in a fuel cell.

State of the art

Commercially available standard catalyst powders are known from the prior art and are usually applied by spraying or pouring. Furthermore, other application methods for applying a catalyst paste are known which are particularly suitable for large-scale applications. Difficulties regularly arise when a certain minimum viscosity of the paste to be applied is required for a technically relevant application process, such as knife coating or screen printing. The commercially available standard catalyst powders and the conventional methods for producing a corresponding catalyst paste do not regularly meet these requirements for viscosity. The catalyst pastes produced in the conventional way are generally very thin, ie they have a viscosity below 50 mPas and are therefore not suitable for some technically relevant application processes (eg doctor blades, screen printing). So far, suitable catalyst pastes have been produced that lead to an efficient catalyst layer. Only then was a process sought, with which precisely this paste could be processed. This meant that the application process was adapted to the paste properties of the selected paste. It was often disadvantageous to select only those application methods which are not suitable for large-scale implementation.

The attempt to adapt the catalyst pastes to a more advantageous application process using conventional thickeners regularly led to catalyst layers which no longer met the required performance requirements. In order to bring about the required increase in viscosity, the thickeners used up to now are added to the paste in such an amount that the properties of the catalyst layer produced generally deteriorate disadvantageously.

As is known, polymer networks lead to an increase in the viscosity of pastes even with a low polymer content. However, the addition of crosslinked polymers to catalyst pastes is generally not expedient, since crosslinked polymers are poorly soluble and it is very difficult to introduce all components of the paste homogeneously into the network. However, a homogeneous distribution of the catalyst particles is often imperative for the performance of the finished catalyst layer. In addition, the process steps with high shear stress required for the dispersion production regularly lead to the destruction of the polymer network. To transfer the technology to the doctor blades of a coating machine, it is necessary, for example, to set the viscosity of the catalyst pastes in a range of at least 200 mPas. In contrast, the conventional pastes for producing, for example, an anodic PtRu catalyst layer have different paste properties. In the case of carbon-supported PtRu-C catalysts, increasing the solid-solvent ratio from 0.2 to 0.6 can significantly increase the paste viscosity from 15 to 150 mPas. However, such a marked increase in the solid / solvent ratio usually leads to instability of the paste, since the solubility limit is often reached. This procedure is therefore unsuitable for setting the paste viscosity. It has also been found to be disadvantageous that, when knife coating, the high solids content of the paste results in very thick layers with poor mechanical properties and excessive PtRu coatings.

Task and solution

The object of the invention is to provide a catalyst layer which can be applied using a simple application method, such as, for example, knife coating or screen printing, and which, as a finished layer, does not have any significant performance losses compared to layers which are applied using other application methods. The object of the invention is furthermore one for such a catalyst layer and the provide suitable catalyst paste available above.

A first object of the invention is achieved by a viscous catalyst paste according to the main claim. A further object is achieved by a method for producing this catalyst paste according to auxiliary claim 6, and by a method for producing a catalyst layer according to auxiliary claim 14. Advantageous embodiments can be found in the claims which refer back to each of them.

Subject of the invention

In the context of the invention, it was found that the addition of a basic additive to a catalyst paste comprising fully supported and / or unsupported catalyst particles and at least one acidic ion exchange polymer leads to an advantageous increase in the viscosity of the paste. The advantage of the process according to the invention lies in the small amount of additive, which on the one hand leads to a significant increase in viscosity, but on the other hand does not regularly cause any deterioration in the catalyst properties.

The advantageous mode of action of basic additives, in particular of basic polymers and / or copolymers as an additive to a catalyst paste, is based on an acid-base reaction which occurs regularly between an acidic ion exchange polymer with a basic additive as an additive and which is present in the paste as standard , As suitable basic additives in Polymers and copolymers with basic monomer units are to be understood in the sense of the invention. Mixtures of different polymers and / or copolymers with basic monomer units have also proven to be suitable as basic additives.

In the pastes according to the invention, the basic additives build polymer networks during the production of the pastes by crosslinking the ion-conductive polymer (an acid) present, for example with a basic polymer. The formation of a network of basic and acidic polymers is known in principle, but because of the complexity of the catalyst paste system, it cannot be used here in a simple and obvious way.

The requirements for the method according to the invention for the production of a catalyst paste include

the selection of special basic additives, in particular polymers and / or copolymers, which are suitable for use in a catalyst paste,

the selection of the appropriate amount of basic additives, which at the same time brings about an optimal viscosity of the paste and an optimal performance of the layer,

- The selection or adaptation of a dispersion process for the production of the catalyst layers in order to achieve the formation of a network.

For example, polyethyleneimine is known from the literature as a basic polymer which tends to gel with acidic polymers. For use in a catalyst paste, however, polyethyleneimine has proven to be not shown usable.

In the context of this invention, basic polymers with a ring system comprising a nitrogen atom have been found to be particularly effective for increasing the viscosity in a catalyst paste. These advantageous basic additives include, for example, polymers and / or copolymers with monomer units comprising pyridine rings, such as 4-vinylpyridine or also 2-vinylpyridine. Polymers and / or copolymers with pyrrole-

Monomer units. Also included in the scope of this invention are basic additives which are present as basic polymer mixtures, basic copolymer mixtures or as a polymer-copolymer mixture.

He has surprisingly turned out to be this

Network formation shows the desired effect even with very small additions (> 0.1% by weight) of a suitable basic additive. Additions to a conventional catalyst paste of only 0.1 to 3% by weight, in particular from 0.4 to 2% by weight, and particularly advantageously from 0.6 to 1.5% by weight, have proven to be advantageous. These additions advantageously bring about an increase in the viscosity (> than 100 mPas, in particular> 200 mPas) of the catalyst paste such that it can be used in application processes such as screen printing or knife coating. The percentages by weight relate to the solids content of basic additive based on the mass of dried catalyst layer. The viscosity is measured at a shear rate of 100 per second. The basic additive is advantageously added in the form of a solution. The use of such small amounts of basic additive does not regularly adversely affect the performance of the catalyst layer produced. If necessary, especially with higher additions, a slight impairment of performance can be reversed or even overcompensated for by post-treatment of the finished catalyst layer in a dilute acid. In particular, sulfuric acid has proven to be advantageous as a dilute acid. The aforementioned effect can even be improved by heating up to 90 ° C.

With the aforementioned basic additives, gel formation usually occurs in contact with the acidic ion exchange polymer. The formation of the gel, and thus the desired increase in viscosity, is, however, regularly dependent on the type of addition of the basic additive to the remaining catalyst paste. A person skilled in the art of rheology knows that gel formation can be avoided by metering in the components too slowly or too quickly, or that a gel initially formed can be destroyed again by high shear forces. The mixing process must therefore be based on the components accordingly.

Ideally, for the preparation of the catalyst layer, the dispersion of supported and / or unsupported catalyst particles and acidic ion exchange polymer is first prepared as is customary in the prior art, in order to ensure a uniform distribution of the catalyst particles. Only then is the basic additive, for example in the form of a basic polymer and / or copolymer, advantageously admittedly added as a solution.

A method has proven to be particularly effective in which a basic polymer solution is layered over a dispersion of supported and / or unsupported catalyst particles and acidic ion exchange polymer. The two phases are mixed by shaking and the gel formation occurs regularly. A subsequent ultrasound treatment can advantageously further increase the homogenization.

In the context of this invention, the new, viscous catalyst paste is regarded as an intermediate product for further processing into a catalyst layer as an end product.

Special description part

The subject matter of the invention is explained in more detail below with reference to figures, without the subject matter of the invention being restricted thereby. Show it

Figure 1: Half-cell measurement at 80 ° C, in which the anode comprises a catalyst layer: bold curve: catalyst paste with 0.9 wt .-% PVP, thin curve: catalyst paste without the addition of PVP.

Figure 2: Half-cell measurement at 50 ° C, in which the anode comprises a catalyst layer:

Catalyst paste with 1.5% by weight PVP with aftertreatment (bold curve), catalyst paste with 1.5% by weight PVP without aftertreatment (thin Curve) and catalyst paste without the addition of PVP (dashed curve).

In the context of this invention, a catalyst paste is made available for the production of a catalyst layer, in which a basic paste, in particular a basic polymer, preferably poly (4-vinylpyridine) (PVP), is added to a catalyst paste in such an amount, so that the catalyst paste now present has a viscosity of more than 100 mPas, in particular more than 200 mPas, which is regularly required for application by means of a doctor blade or screen printing.

In the case of the anodic PtRu catalysts, it has been found that the viscosities of the pastes are increased suddenly by a small amount of polymer additive added. At the same time, the electrochemical performance of the layers is impaired as little as possible. In particular, the influence of the additive polyvinylpyridine (PVP) on the viscosity and the electrochemical performance of the pastes was found to be particularly advantageous. The basic functional groups of polyvinylpyridine, through interaction with sulfonic acid groups of Nafions ^{®, lead} to the formation of a polymer gel crosslinked via ion clusters in the catalyst paste.

The electrochemical performances of the catalyst layers with the addition of PVP on the anode diffusion layer produced by the method according to the invention were evaluated. For this purpose, the pastes were applied to diffusion layers with a table doctor device. gene and then processed to membrane electrode assemblies (MEAs) with an area of 2 cm ² . Subsequently, half-cell measurements were carried out in which the measurement was carried out against a hydrogen-developing cathode as a reference electrode in order to be able to exclude influences on the performance curves on the cathode side. A voltage was applied between the anode and cathode (anode positive) and the current was measured. 1 molar methanol solution is oxidized at the anode, hydrogen is developed at the cathode. The anode potential is related to an electroless hydrogen development electrode in the cathode compartment. The higher the current densities that are achieved with the lowest possible anode potentials, the better the performance of the tested anode for methanol oxidation. A correction for Ohm's losses is made. Since different samples usually have slightly different precious metal assignments, the precious metal assignment is standardized.

The results, which can be seen in FIG. 1, show that the power curves of the PVP layers with a proportion of 0.9% by weight are comparable to those of an analog layer without PVP.

At higher PVP contents (> 1.5% by weight), the doctored gas diffusion electrodes were treated for one hour at 80 ° C. in dilute sulfuric acid in order to achieve comparable performance curves (see FIG. 2). With PVP contents of 0.9% by weight, this aftertreatment led to only minor improvements. The results show that the catalyst paste according to the invention mixed with basic additives as a finished catalyst layer performs just as well as conventional, low-viscosity catalyst pastes, but has the great advantage that it can be applied using technical application methods.

In one embodiment, the parameters for increasing the viscosity of a PtRu catalyst paste with polyvinylpyridine as a basic additive are listed below.

Approach for the catalyst paste:

3.0 g Pt / Ru-C (40% precious metal) 6.0 ml water 6.0 ml 2-propanol 8.55 g Nation ^® solution (15% by weight)

Treatment of the catalyst paste: 10 min ultrasonic bath 3 min Ultraturrax

Approach for the basic additive: 342 mg PVP solution (210 mg PVP in 2 ml 2-propanol

11.6% by weight) corresponds to 0.92% by weight of PVP in the solid

Addition of the additive:

The PVP solution is carefully run down the wall of the vessel containing the catalyst paste. The catalyst paste is overlaid by the solution containing the additive. Shake for 1 min

10 min ultrasonic bath

Due to the above-mentioned type of mixing, a polymer network is formed in a particularly simple manner, which leads to a significant increase in viscosity.

Claims

claims

1. Catalyst layer comprising supported or unsupported catalyst particles and at least one acidic ion exchange polymer, characterized in that the catalyst paste comprises at least one polymer or copolymer comprising basic monomer units with a nitrogen-containing ring system as a basic additive in a proportion of 0.1 to 3% by weight .-%, based on a dried catalyst layer produced therefrom.

2. Catalyst layer according to claim 1, which the basic additive in a proportion of 0.4 to 2 wt .-%, in particular with a proportion of 0.6 to

1.5 wt .-% comprises.

3. Catalyst layer according to one of the preceding

Claims 1 to 2, which comprises at least one polymer or copolymer comprising 4-vinylpyridine and / or 2-vinylpyridine and / or pyrrole as a basic additive.

4. A method for producing a catalyst layer according to one of claims 1 to 3 on a substrate, with the steps

- For the production of a paste, at least one polymer or a copolymer comprising basic monomer units with a nitrogen-containing ring system as a mixture of supported or unsupported catalyst particles and at least one acidic ion exchange polymer basic additive with a proportion between 0.1 and 3% by weight, based on a dried catalyst layer produced therefrom, added and homogenized in such a way that the viscosity of the entire paste increases to more than 100 mPas, in particular to more than 200 mPas , - The paste thus produced is applied to the substrate.

5. The method according to claim 4, in which the base additive is added in a proportion of between 0.4 and 2% by weight, in particular between 0.6 and 1.5% by weight, to produce the paste.

6. The method according to any one of claims 4 to 5, in which for the preparation of the paste at least one polymer or copolymer comprising 4-vinylpyridine or 2-

Vinyl pyridine or pyrrole is added as monomer units.

7. The method according to any one of claims 4 to 6, in which the basic additive is added in the form of a solution for the preparation of the paste.

8. The method according to claim 4 to 7, in which for the preparation of the paste the basic additive is layered as a solution over the mixture of supported or unsupported catalyst particles and at least one acidic ion exchange polymer and is then homogenized by shaking.

9. The method according to claim 4 to 8, wherein the catalyst paste is applied by doctor blade or screen printing on the substrate.

10. The method according to any one of claims 4 to 9, wherein the applied catalyst layer is subjected to an aftertreatment.

11. The method according to any one of claims 4 to 10, wherein the applied catalyst layer is treated at temperatures up to 90 ° C in a dilute acid.

12. The method according to any one of claims 4 to 11, wherein the anode diffusion layer of a fuel cell is selected as the substrate.