WO1999018623A2

WO1999018623A2 - Electrode with a layer of carbon monoxide-sensitive catalyst particles

Info

Publication number: WO1999018623A2
Application number: PCT/DE1998/002953
Authority: WO
Inventors: Frank Henglein; Wolfgang Unkauf; Andreas Friedrich; Dieter Meissner; Ulrich Stimming; Roland Vogel; Andrea Marmann
Original assignee: Forschungszentrum Jülich GmbH
Priority date: 1997-10-06
Filing date: 1998-10-05
Publication date: 1999-04-15
Also published as: EP1029379A2; WO1999018623A3; DE19744028A1; DE19744028C2; AU1223999A

Abstract

The invention relates to an electrode with a layer of precious metal particles. The electrode comprises an electroconductive substrate on which catalytically active precious metal particles have been deposited. The average size of the contiguous surfaces of said catalytically active precious metal particles exceeds a minimum value. The minimum value is selected in such a way that the carbon monoxide compatibility of the catalytically active precious metal particles is constant after the minimum value has been exceeded. It has been established that, when this minimum value is not achieved, the carbon monoxide compatibility of the catalytically active precious metal deteriorates. In order to achieve good carbon monoxide compatibility, the minimum value of the contiguous surfaces is used. Contiguous large surfaces within the meaning of the invention can be obtained by means of sufficiently large particles and/or sufficiently high particle densities. In general, a substrate on which particles have been deposited in accordance with the invention can be used to advantage for the catalytic oxidation of carbon monoxide.

Description

Electrode with applied carbon monoxide sensitive catalyst particles

Technical field:

The invention relates to an electrode which has an electrically conductive substrate. On the substrate are out. Precious metal existing particles brought. The noble metal is catalytically active, e.g. B. to oxidize methanol, hydrogen or carbon monoxide. The invention further relates to the use of a substrate with applied particles consisting of noble metal.

State of the art:

From the document DE 44 47 132 C2 is an electrode of the type mentioned for one

Phosphorus fuel cell is known which comprises a diffusion and a catalyst layer. The catalyst layer has porous bodies on which a catalyst is dispersed. One goal is the economical use of the catalyst material.

Furthermore, EP 0 275 466 A1 describes a membrane-electrode structure in which there are catalyst particles with a diameter of up to approximately 1 cm on an ion exchange membrane. The aim is to create suitable membrane electrode assemblies.

From the publication "P.R. van Rheenen et al., Journal of Solid State Chemistry 67, 151-169 (1987)", the production of platinum particles with diameters from a few 10 nm to a few 100 nm on substrates is known.

An electrode of the type mentioned initially comprises an electrically conductive substrate, on which the particles consisting of noble metal are applied. The electrode is used in fuel cells, for example.

A fuel cell, known for example from the publication DE 42 41 150 Cl, has a layer system consisting of cathode, electrolyte and anode. The cathode becomes an oxidizing agent such as air or oxygen and the anode becomes a fuel, e.g. B.

Hydrogen or methanol supplied. The cathode and anode of a fuel cell generally have a continuous porosity, so that the two operating media, namely the oxidizing agent and the fuel, can be supplied to the active areas of the electrodes and the product water can be removed.

There are so-called PEM fuel cells in which proton-conducting membranes are used as the electrolyte. The operating temperatures are below 130 ° C, so as not to destroy the commonly used Nafion membrane.

At the anode of a PEM fuel cell, the fuel is formed in the presence of the fuel

Precious metal catalyst Pro_onen. The protons pass through the electrolyte and combine on the cathode side with the oxygen from the oxidizing agent to form water. Electrons are released and electrical energy is generated.

The electrically conductive, porous electrode in a fuel cell can consist of highly porous carbon. Platinum metals, in particular z. B. platinum or

Platinum-ruthenium existing particles applied to the electrode.

The hydrogen required for a fuel cell can be generated from methanol through an external refor ization reaction. In the so-called direct methanol fuel cell, for. B. known from EP 0 068 508 Bl, methanol is electrochemically oxidized directly at the anode to protons and CO ₂ ; ,

The hydrogen-rich gas obtained through a reforming reaction is contaminated with carbon monoxide, among other things. From the publication "Journal αf Power Sources, Vol. 29, 1990, page 251" it is known that the carbon monoxide poisons the catalyst of the anode, in particular at low temperatures of 80 ° C. The performance of the fuel cell drops accordingly. In general, attempts are made to maximize the surfaces of catalysts in order to achieve good efficiencies. Such as B. from the publication "K. Aika et al., J. Res. Inst. Catalysis Hokkaido Univ., Vol. 24, No. 1, pp. 54-64 (1976)", the catalytically active material is therefore as possible applied fine-grained on a substrate. Particle sizes of 2 - 5 nm are common for electrodes for fuel cells.

In order to save expensive catalyst material, the particle density, that is to say the number of particles per unit area, is also regularly low. The individual particles then do not regularly touch each other.

It is also known to completely manufacture a fuel cell electrode from a noble metal. However, electrodes manufactured in this way are comparatively expensive, since catalytically active noble metals such as platinum or ruthenium are very expensive.

Presentation of the invention:

The object of the invention is to create an inexpensive, catalytically active electrode with a comparatively good carbon monoxide compatibility. The object of the invention is also to provide an advantageous use.

The task is accomplished by a device with the

Features of the first claim solved. Beneficial Refinements result from the subordinate claims. The use can be found in the subsidiary claim.

The electrode with the features of claim 1 comprises an electrically conductive substrate. Catalytically active particles consisting of noble metal, in particular of platinum metal such as Ru or Pt, are applied to the substrate. The particles are chosen so large and / or applied so densely that there are coherent surfaces consisting of the noble metal which (on average) exceed a minimum value. The minimum value is chosen so that when it is exceeded, the oxidation of carbon monoxide or the carbon monoxide compatibility of the catalytically active particles consisting of noble metal is practically (essentially) constant. If the minimum value is undershot, the oxidation of carbon monoxide or the carbon monoxide compatibility of the catalytically active particles consisting of noble metal deteriorate. The catalytic effect is then reduced comparatively strongly by adsorption of carbon monoxide.

In contrast to the prior art, z. B. the particle density chosen to be comparatively large, so that a comparatively large number of catalytically active particles touch each other. From a sufficiently high density, large, coherent surfaces are then formed which have a minimum size within the meaning of claim 1.

The particles can be combined to form agglomerates and thus coherent surfaces with the Form features of the first claim. A high density of the agglomerates is then not necessary.

The particle sizes can therefore be chosen so large that the particle surface already has or exceeds the minimum size as claimed.

In one embodiment of the invention, the minimum diameter D _{ai in} the particles or agglomerates is selected such that from D _m -, the carbon monoxide compatibility or carbon monoxide _{oxidation is} practically no longer improved with increasing diameter of the particles. On the other hand, diameters that are smaller than D _^ - have a significantly poorer carbon monoxide tolerance or

Carbon monoxide oxidation of the catalytically active precious metal.

It has been shown that the carbon monoxide tolerance of z. B. from platinum

Below a diameter D ~ _ _n, ie from Un ^¬ drops below an minimum size of the continuous surface of the catalytically active material deteriorates. For example, the carbon monoxide tolerance was about 10 nm

Platinum particles are already measurably worse than those of approximately 15 nm large platinum particles. However, a particle size above 15 n no longer changes the carbon monoxide tolerance. It was found that the carbon monoxide compatibility of an electrode with 15 nm catalyst particles applied was equal to that Carbon monoxide compatibility of an electrode made entirely of platinum.

The approx. 10 nm or approx. 15 nm large platinum particles can consist of - e.g. B. about 4 nm large - particles to agglomerates.

If platinum is provided as the noble metal catalyst, D-_, _{n is} accordingly 10 nm to 20 nm.

In a further embodiment of the invention, the catalyst particles or agglomerates are applied to the substrate of the electrode with such a high density that there are coherent surfaces with the features as claimed.

It was found that the

Carbon monoxide compatibility is improved with increasing density of the catalyst particles on the substrate. Particle density is the number of particles per unit area. For example, a substrate made of carbon with 10 nm platinum particles applied with a low particle density had the same carbon monoxide compatibility as a carbon substrate with 3 nm platinum particles applied with a significantly higher particle density.

The oxidation reaction of carbon monoxide depends on the size of the connected surface of the catalyst. If a certain size of the coherent surface is undershot, it deteriorates Oxidation of carbon monoxide-carbon monoxide is then increasingly adsorbed on the catalyst and thus inhibits the catalytic effects. In this sense, the carbon monoxide tolerance decreases.

From a particle size of approx. 15 nm, the carbon monoxide compatibility could not be changed by varying the density.

Increasing densities have the consequence that the individual particles increasingly touch each other. This creates increasingly larger, coherent surfaces. There is a minimum density above which the carbon monoxide tolerance is constant. From this minimum density there are contiguous areas with the features of claim 1.

In order not only to ensure good carbon monoxide compatibility or good carbon monoxide oxidation, but also to maintain a large catalyst surface area, it is advisable to limit the particle size. The particles therefore preferably have a maximum diameter D ~ _ΛX , the z. B. a maximum of five times the minimum diameter. Then D-, _.x <5 • D- _^ . D _M <2 • D _π preferably applies

In an advantageous embodiment, the mean diameters of platinum particles or agglomerates are therefore not larger than approx. 70 to 80 nm, preferably not larger than approx. 40 nm. Otherwise, the provision of large, catalytically active surfaces is comparatively expensive, since a disproportionate amount of precious metal is consumed becomes. For the same reasons, the density of the particles made of noble metal should not exceed a maximum value.

In general, a coherent surface should therefore have a maximum size that does not significantly exceed the minimum value. According to the invention, it is proposed that the coherent surfaces of the noble metal particles be at most one order of magnitude

(Factor 10) should be greater than the minimum value.

The invention is based on the knowledge that catalyst particles consisting of noble metal, the catalytic effects of which can be damaged by carbon monoxide, should in no way be as small as possible and at the same time be applied to a substrate with a low density. If the carbon monoxide compatibility or the oxidation of carbon monoxide is to be optimized, particles are to be applied to a substrate in such a way that there are coherent, in particular closed surfaces of the catalyst or of the catalytically active material which do not fall below a minimum size. Contiguous surfaces that exceed a minimum size within the meaning of the claims can be achieved by means of sufficiently large particles and / or sufficiently large densities of the particles on the substrate.

In an advantageous embodiment of the invention, an electrically conductive substrate with applied, large, contiguous catalyst surfaces as an electrode in a fuel cell and preferably used in a direct methanol fuel cell.

The anode in particular is from

Carbon monoxide poisoning of the catalyst affected. Therefore, the anode preferably has the features as claimed.

Carbon monoxide poisoning and the associated loss in performance of the anode catalyst can thus be reduced with little financial outlay.

In general, a substrate with the applied particles can advantageously be used for the catalytic oxidation of carbon monoxide. Examples of applications are the catalytic oxidation of hydrogen-rich, carbon monoxide-containing synthesis gases or the catalytic oxidation of methanol. In the above two examples, the good oxidation of carbon monoxide is of interest in order to maintain the catalytic effect for further oxidation reactions. The catalytic effect is less affected compared to the prior art by carbon monoxide in the hydrogen-rich gas. The substrate does not have to be electrically conductive for this purpose. For comparable reasons, the use of the substrate for methanol oxidation is advantageous since CO is released during the methanol oxidation. In a direct methanol fuel cell in particular, the above is

Use economically of interest. Of course, the oxidation of carbon monoxide to carbon dioxide can also be the exclusive goal.

Brief description of the drawings and best way to carry out the invention:

Figure 1 shows an image of an electrode with platinum particles. Shown are platinum agglomerates 1 which are applied to a carbon substrate 2 (light areas). The agglomerates 1 are composed of 4 nm particles. The image also shows dark areas 3, which are formed by boron salts. The boron salts are production-related residues from colloid solutions (see publication "P. R-van Rheenen et al., Journal of solid State chemistry 67, 151-169 (1987)").

Figure 2 shows the size distribution of the agglomerates shown in Figure 1. The majority of the agglomerates have a diameter of 15 nm.

FIG. 3 shows a photograph of a further substrate consisting of carbon with applied agglomerates consisting of platinum 1. FIG. 4 illustrates the size distribution of the agglomerates 1 shown in FIG. 3. The majority of the agglomerates 1 here have a diameter of 10 nm.

The curves shown in FIGS. 5 to 7 were determined as follows. It was in a liquid Electrolytes generates a voltage between a (catalytically active) electrode with the platinum particles 1 and a reversible hydrogen electrode. Depending on the set voltage, the current flow of the electrode was measured with the platinum particles and a third electrode. The third electrode was also in the liquid electrolyte.

The current flow I was plotted against the voltage U according to FIGS. 5 to 7. FIG. 5 shows the course for the catalytically active electrode with the 15 nm agglomerates 1 according to FIG. 1. FIG. 6 shows the course for the catalytically active electrode with the 10 nm large agglomerates 1 according to FIG. 3. FIG. 7 shows the course for a catalytic active electrode with 3 nm agglomerates consisting of platinum, which have been applied with a density corresponding to FIGS. 1 and 3 onto a substrate made of carbon.

Figure 5 shows a peak at a voltage of 740 mV. Figure 6 shows a peak at a voltage of 920 mV. Figure 7 shows a peak at a voltage of 1038 mV. If a catalytically active electrode consisting of solid platinum was used, the corresponding peak occurred at a voltage of approximately 740 mV.

The peaks mentioned indicated the voltages required to oxidize carbon monoxide adsorbed on the catalytically active electrodes. These required tensions are a measure of that Carbon monoxide compatibility of the associated catalytically active electrodes. The lower the voltage required, the less the catalytic effect of the platinum is adversely affected by carbon monoxide.

The test results show that electrodes with 15 nm platinum agglomerates have the same carbon monoxide compatibility as solid platinum electrodes. The catalytic effects of platinum agglomerates with smaller diameters of 10 nm and 3 nm, on the other hand, decrease regularly with adsorbed carbon monoxide. If the (initially low) density of the 3 nm agglomerates was increased, the same eventually became the same

Carbon monoxide compatibility as achieved with an electrode made of solid platinum.

Claims

P a t e n t a n s r u c h e

1_ electrode with the characteristics:

- The electrode comprises an electrically conductive substrate (2),

- Catalytically active particles (1) consisting of noble metal are applied to the substrate (2),

the average size of the coherent surfaces of the catalytically active particles (1) consisting of noble metal exceeds a minimum value,

- The minimum value is chosen so that the .Oxidation of carbon monoxide is essentially constant from above the minimum value.

2. Electrode with the features of claim 1 with catalytically active particles consisting of noble metal or agglomerates consisting of the particles (1), which have a minimum diameter D ^ _n and a maximum diameter O____ _x , characterized in that O____ _n the diameter from which the oxidation of carbon monoxide is constant and D "is at most five times greater than D, ^ _" .

3. Electrode with the characteristics according to one of the

Claims 1 or 2, characterized in that the size of the coherent surfaces exceed the minimum value by a maximum of an order of magnitude.

4. Electrode with the features according to one of the preceding claims, characterized in that the catalytically active, consisting of precious metal Particles form agglomerates of 10 nm to 80 nm.

5. Electrode with the features according to one of the preceding claims, characterized in that the substrate has an open porosity and pores are coated with the catalytically active particles (1) consisting of noble metal.

6. Electrode with the features according to one of the preceding claims, characterized in that the substrate (2) consists of carbon.

7. Electrode with the features according to one of the preceding claims, characterized by an electrolyte layer adjacent to the electrode (1, 2).

8. use of a substrate,

- On the catalytically active, consisting of precious metal particles are applied, - the mean size of the connected

Surfaces of the catalytically active particles consisting of precious metal exceed a minimum value,

- And the minimum value is chosen so that the oxidation of carbon monoxide is essentially constant when the minimum value is exceeded, for the catalytic oxidation of carbon monoxide.

9. Use of a substrate according to claim 8 for the oxidation of hydrogen-rich, carbon monoxide-containing gases or for the oxidation of methanol.