CA1070290A

CA1070290A - Method for the production of a catalyst suitable for fuel cell electrodes

Info

Publication number: CA1070290A
Application number: CA217,486A
Authority: CA
Inventors: Martinus M.P. Janssen; Jan Moolhuysen
Original assignee: Shell Canada Ltd
Current assignee: Shell Canada Ltd
Priority date: 1974-02-19
Filing date: 1975-01-07
Publication date: 1980-01-22
Also published as: SE7501741L; FR2261059B1; SE408980B; GB1501102A; DE2506568A1; AU499581B2; FR2261059A1; JPS50143040A; NL7501816A

Abstract

A B S T R A C T

Method for the production of a catalyst, suitable for fuel cell electrodes, which catalyst comprises as catalyst metal a transition metal of the eighth group of the periodic table and a co-catalyst, characterized in that the catalyst metal is covered with hydrogen and then immersed in a solution of a compound of the co-catalyst. The catalyst thus obtained has an improved activity.

Description

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This invention relates to a method for the production of a catalyst suitable for fuel cell electrodes, to a catalyst thus prepared, to a fuel cell electrode comprising this catalyst and to a fuel cell comprising this electrode.
The f`uel cell may be based on any liquid electrolyte, but an ac;d or carbonat;c solution in water is preferred.
Catalysts for the electrochemical oxidation of compounds suitable as fuel dissolved in an electrolyte in fuel cells, such as methanol~ formaldehyde and ~ormic acid, in particular methanol, which catalysts comprise a catalyst metal and a co-catalyst are known. As catalyst metal a transition metal of the VIIIth group of the Periodic Table such as Ni, Pt, Pd~ Rh, Ir, 0s and Ru can be used.
In the case of methanol, the following anodic reaction takes place on the catalyst metal surface:
CH30H ~ H20 ~ C02 + 6 H + 6 e This reaction can be accelerated by using a co-catalyst.
Suitable co-catalysts are elements such as Pb, ~1, Sn, As, Sb, Bi and Re.
Such catalysts are e.g. known from British patent ~-specification 1~106g708 and USA patent specification 3,340,097. They can be produced by co-deposition from an aqueous solution containin~ the Pt metal and co-catalyst ~ ~
in dissolved form. This co-deposition can take place by ~ -.. .:
electrochemical reduction o'f a solution of salts of the -metals on a suitable substrate or by impregnation of the substrate with such a solution followed by chemical reductionO

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The co-deposited catalysts thus obtained have a very good electrocatalytic activity. ~lowever, their activity cannot be improved further since their dispersion (specific surface area) cannot be in~
creased easily. On the other hand, the new catalyst preparation pro-cedure described here involves an impregnation with a co-catalyst of a group VIII metal, the dispersion of which can be varied much more easily and chosen to be higher than those of co-deposited catalys~s ~e.g. 100 m2/g against e.g. 30-50 m2/g).
Two other advantages o~ the new preparation method are:
First, it enables easy catalysts preparation as well as regeneration ~the latter e.g. in case of co-catalyst losses during operation).
Secondly, the method can be used successfully in the preparation of virtually any catalyst-cocatalyst system, which does not apply in case of for example electrocodeposition.
Therefore, with the new preparation method catalysts can be obtained with an improved activity with respect to those known thus far.
Accordingly, the present invention provides method for the ~-production of a composite catalyst, suitable for use in forming an anode for a fuel cell, which comprises applying to the surface of a catalyst metal, which is a transition metal of the platinum group of metals, a co-catalyst, characterized in that the catalyst metal is covered with a layer of hydrogen without absorption of hydrogen into the body of the catalyst metal and a co-catalyst is applied to the thus covered catalyst by immersion in a non-etching solution of a ~ -soluble chemical substance comprising the co-catalyst, the applied co-catalyst forming a discontinuous layer on the catalyst metal.

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Co-c~talysts that can be applied to the surface of the catalyst metal by this method include Cu, Ag, Au, Zn, Cd, Hg, B, Al, In, Tl, Si, ~e, Sn, Pb, P, As, Sb, Bi, S, Se, Te, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo~ W, Mn, Re, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, rare earth metals, Th, U and/or their oxides or hydroxides. The catalysts thus obtained can be used for the oxidation of methanol, formaldehyde and formic acid but also of carbon monoxide or hydrogen/
carbon monoxide mixtures.
The preferred metal of the eighth group of the periodic table is Pt and the preferred co-catalysts for methanol oxidation are Sn, Ti, Ta, Re, Ru, Os and/or their oxides or hydroxides. More particularly, optimal results will be obtained with STI, Ta or Re and/or their oxides or hydroxides. Mixtures and/or alloys of group eight transition metals and of co-catalysts can also be used.
The group eight transition metal can be deposited di- `~, rectly onto an electrically conductive substrate by electrodeposition. `~
It can also be applied to the substrate as a finely divided metallic or oxidic powder by using a binder. Furthermore, . . .
it can be deposited OTltO an electrically conductive support, such as carbon, by impregnation followed by chemical or electrochemical reduction. The supported group eight transition metal thus obtained ~ ~
is applied to the substrate by using a binder. `;
The substrate can be non-porous, but is preferably porous, such as a porous carbonaceous material or a ~

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The group eight transition metal can be covered with hydrogen by any suitable means, such as by electrochemical adsorption in the hydrogen evolution region, i.e. for pure platinum at a potential below 0.0 V vs. that of the reversible hydrogen electrode (RHE). The latter method of ~
hydrogen covering is preceded by a cleaning treatment o~ `
the normally finely divided metal by alternative oxidation and reduction of the surface which is in the case of pure Pt effected by potential cycling between + 1.6 V and 0~0 V vs. RHE in e.g. 8N H2S04.
The group eight transition metal thus covered with hydrogen is rinsed and then provided with the co-catalyst ~ ;
by immersion lnto a solution of a compound of the ~co catalyst, preferably an aqueous solution of a salt~. an oxide or a hydroxide of the co-catalyst element.
In the table results are given ~or catalysts obtained by the present method;; their loadings vary between 0.1 and 10 mg/cm of platinum. The immersion treatments given are only examples, the conditions can be varied wide~ly.
For example~ instead of SnClLI solution, Na2Sn(OH)6 or SnS04 solutions can also be used successfully. It i5 seen that up to 100-fold increases in specific activity , ~ (current density divided by loading) can be obtained.

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In case the Pt was precovered with oxygen at about 1.0 V vs. RME cat;alysts havin~ an unchanged activity compared with pure Pt were obtained. In case uncovered Pt was used (i.e. pretreated at about ~0.5 V vs. RHE) a catalyst of intermediate activity was obtained.
It is observed that the cocatalyst is in an oxidized ~orm before it is deposited on the metal surface during the immersion. Partial reduction of the cocatalyst compound may take place during the immersion proce`ss by reaction with the hydrogen adsorbed on the Pt.
The two-step preparation process (i.e. hydrogen ~ ~, covering and immersion in separate solutions) described ;
he~re can be replaced by a one-step process when the co-catalyst compound is well~soluble in diluted acid. In : . .
such a case the group eight transition metal is immersed in a solution o~ the cocatalyst in for example 8N H2SO4 by setting the potential near 0.0 V hydrogen covering : , .: .
takes place, immediately followed by adsorption o~ the cocatalyat.
It is also observed, especially in the case of Pt/Sn oatalysts by using the radioactive tracer Sn113, that too large quantities of cocatalyst for optimal ~
catalytic per~ormance can be removed~from the Pt surface ~ -by potential cycling as described before in e.g. 8N H2SO4.
In the case o~ Pt/Sn the rc~dioactive tracer studies show also that the optimal Sn covering remains constant on further cycl1ng and on methanol oxidatiol.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Method for the production of a composite catalyst, suitable for use in forming an anode for a fuel cell, which comprises applying to the surface of a catalyst metal, which is a transition metal of the platinum group of metals, a co-catalyst, characterized in that the catalyst metal is covered with a layer of hydrogen without absorption of hydrogen into the body of the catalyst metal and a co-catalyst is applied to the thus covered catalyst by immersion in a non-etching solution of a soluble chem-ical substance comprising the co-catalyst, the applied co-catalyst forming a discontinuous layer on the catalyst metal.

2. Method according to claim 1, wherein the catalyst metal is Pt.

3. Method according to claim 2, wherein the co-catalyst is Sn, Ta or Re and/or an oxide or hydroxide thereof.

4. Method according to claim 1, 2 or 3, wherein the catalyst metal is deposited directly onto an electrically conductive substrate by electro-deposition.

5. Method according to claim 1, 2 or 3, wherein the catalyst metal is deposited onto an electrically conductive support by impregnation followed by chemical or electrochemical reduction.

6. Method according to claim 1, 2 or 3, wherein the catalyst metal is covered with hydrogen by electrochemical adsorption in the hydrogen evolution region.

7. Method according to claim 1, wherein the covering with hydrogen is preceded by a cleaning treatment of the catalyst metal.

8. Method according to claim 7, wherein said cleaning treatment is effected by potential cycling.

9. Method according to claim 1, 2 or 3, wherein the catalyst metal after covering with hydrogen is rinsed.

10. Method according to claim 1, 2 or 3, where the co-catalyst is provided by immersion of the hydrogen covered catalyst metal into a non-etching aqueous solution of a salt, oxide or hydroxide of the co-catalyst element.

11. Method according to claim 1, 2 or 3, wherein the catalyst metal is immersed in a non-etching acid solution of the co-catalyst, the poten-tial is set near 0.0 V so that hydrogen covering takes place immediately followed by adsorption of the co-catalyst.

12. Method according to claim 1, 2 or 3, wherein the loading of the catalyst metal is between 0.1 and 10 mg/cm2.