Noble metal nanoparticles, a process for preparing these and their use

Abstract

Nanoparticles which contain noble metals alone or noble metals in combination with base metals. The nanoparticles are embedded in an aqueous solution of a temporary stabilizer based on a polysaccharide.

Description

INTRODUCTION AND BACKGROUND[0001] The present invention provides noble metal-containing nanoparticles for producing membrane electrode assemblies (MEAS) for fuel cells, in particular for low temperature fuel cells, for example polymer electrolyte membrane fuel cells (PEM fuel cells) and direct methanol fuel cells (DMFC). New types of colloidal solutions which contain the noble metal alone or in association with other metals are described, wherein the metals are in the form of nanoparticles embedded in a temporary stabilizer. The nanoparticles are used to produce electrocatalysts and catalysed components for fuel cells. Using these nanoparticles, catalyzed ionomer membranes, catalyzed gas diffusion electrodes (so-called "backings") and membrane electrode assemblies can be produced.[0002] Fuel cells convert a fuel and an oxidizing agent which are spatially separated from each other at two electrodes into electricity, heat and water. Hydrogen or a hydrogen-rich gas may be used as the f...

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Benefits of technology

[0017] When properly selected, the stabilizers mentioned are able to keep the colloidal preparation of nanoparticles, even in high concentrations, stable for a long time. For this purpose, it has proven advantageous to adjust the ratio by weight of nanoparticles to stabilisers to a value between 10:1 and 1:10, preferably between 5:1 and 1:5.

[0018] The temporary stabilizers used have to be capable of being removed effectively. Particularly important here is an easy decomposition (i.e. breaking down of the main chain in the polymer into low molecular weight fragments). During the course of trials, it has been shown that the polysaccharides are extremely suitable as temporary stabilisers. As described previously, in these compounds, the glycosidic bonds between the individual monosaccharides or sugar monomers break readily when treated with acids or alkalis. They depolymerize and break down into low molecular weight constituents. This decomposition process also takes place during pyrolysis at temperatures up to 250.degree. C. The low molecular weight fragments can be readily removed, for example by washing out.

[0030] Direct use of the nanoparticles for catalysing the various components in a fuel cell is enabled in that the protective colloid, or temporary stabilizer, decomposes under relatively mild conditions and can be washed out so that damage to the components in the fuel cell does not occur. This produces a considerable simplification in and reduction in costs of the production process for membrane electrode assemblies. In addition, the process has the advantage that the high surface area and dispersion of the nanoparticles is retained and is not distorted by high temperature tempering processes. This leads to very good performance by the membrane electrode assemblies prepared in this way so that the platinum loading can be kept low.

[0033] Reprotonation with sulfuric acid can therefore be combined in a simple manner with decomposition of the temporary stabilizer. This simplifies the production process for membrane electrode assemblies. The ionomer membrane catalyzed in this way is then completed with 2 gas diffusion electrodes to give a 5-layered membrane electrode assembly.

[0038] In addition, combinations of the types of use described above are possible. All these methods for catalysing, due to the use of the colloidal noble metal particles according to the invention, lead to high catalytic activity and electrical performance in the membrane electrode assembly, and in the PEM fuel cell.

Problems solved by technology

The process is complicated and expensive and provides only electrocatalysts which are contaminated with sulfur due to using sulfur-containing precursor compounds for the platinum.

It has been shown that these stabilizers adhere firmly to the noble metal surface, due to their long polymethylene main chains, and thus contaminate the catalytically active catalyst surfaces.

For this reason, these nanoparticles are not very suitable as catalytically active species for membrane electrode assemblies in fuel cells.

In addition, our work has shown that this process has considerable disadvantages because it provides nanoparticles which are heavily contaminated with foreign ions such as, for example, chloride or sodium.

The presence of chloride in particular leads to corrosion and reduced resistance to ageing of the catalyst components prepared using this metal colloid preparation.

Surplus reducing agents are destroyed due to treatment at elevated temperatures of up to 95.degree. C.

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