CA1051088A

CA1051088A - Production of a metal alloy electrode using chemical reduction

Info

Publication number: CA1051088A
Application number: CA247,305A
Authority: CA
Inventors: Marinus Alfenaar; Cornelis G.M. Van De Moesdijk
Original assignee: Stamicarbon BV
Current assignee: Stamicarbon BV
Priority date: 1975-03-11
Filing date: 1976-03-08
Publication date: 1979-03-20
Also published as: IT1057949B; BE839423A; US4127468A; GB1540888A; DE2610285A1; NL7502841A; FR2304185B1; FR2304185A1; JPS51114699A

Abstract

ABSTRACT OF THE DISCLOSURE

Metal electrodes are prepared by contacting a basis-metal electrode with a solution which contains as alloying element compound, said basis-metal electrode comprising a basis-metal which is present in a finely-divided or porous state and is selected from the group consisting of the noble metals from the Groups IB, IIB or VIII of the Periodic Table of the Elements or an alloy of at least one of the metals, said alloying element being selected from the group consisting of an element from Groups IIIA, IVA, VA, VLA, VIII, IB, IIB, VIIB or combinations thereof of the Periodic Table of the Elements; and reducing in situ said alloying-element compound to form the free-alloying element whereby said alloying element forms an alloy with said basis-metal.

Description

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This invention rela~es to a proce~s of preparing metal electrodes, and particularly relates to a process whereby metal ~ -electrodes~ or the surface thereof~ may be rendered elec~ro~
catalytically active or have their electro~catalytic activity increased. The electrodes according to the inventinn may be used in current generating (fuel cells) or current consuming (electrolytic) processes, particularly in gas diffusion pro-cesses, and in certain instances may be modified in situ of the processes in which they are used.
Cataly~ically active electrodes wherein the catalytic activity is provided by an alloy of particular metals have previously been prepared by mixing a suitable metal alloy powder with a powdered carrier material, and if a porous electrode is required with a pore forming material, and the mixture compressed and sintered into the required electrode shape. When a soluble pore~forming material e.g. sodium sulphate, is incorporated in the electrode shape, this is then leached out with hot water. Such a method however is complicated and~expensive and may result in an inferior product due to the high temperatures used in the compression and sintering step which may adversely affect the performance of the electrode in gas-diffusion processes.
The present invention is directed to means whereby metal electrodes may be converted to a catalytically active form by generating alloying elements in situ, which alloy with metals or alloys already present in the electrodes either in a surface layer of the electrode or throughout the electrode.
The invention provides a process for the production of a metal alloy electrode, which comprises modifying a metal ~ .
elec~rode, which is a nobel metal electrode, by contacting said metal electrode with a solution of a compound of an element which when alloyed with the metal of the electrode

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provides an electro-catalytic eEfect, and reducing the said compound to the element ln contact with the metal electrode whereby the reduced element and electrode metal become alloyed.
The alloying effect may take place throughout the body of the electrode, or only on the surface thereof. The electrode to be treated may comprise the main electrode metal and an electro-catalytic alloy component e.g. a surface layer, or may consis~ of electro-catalytic alloy, the process of the lnvention being directed to enhancing the catalytic activity of ~he said alloy component. Preferably the electrode is a porous elec~rode and/or the electrode metal is in a finely divided state whereby contact of the electrode metal with the treating solution is expedited.
In particular embodiments the metal and/or an electro-catalytic alloy component thereof is associated on a ~inely-divided state on a carrier material, particularly and elec-trically-conducting carrier e.g. carbon. The metal-ladan carrier particles may be bonded together with a bonding agent.
Alternatively the electrode metal is in a finely-divided state ; ;~
dispersed in a porous matrix of carrier material and/or bonding agent. The electrode metal particle size is preferably between about 10 angstroms and about lOJ~m.
A non-powdery, but porous coherent metal electrode may be obtainad by sintering powder o~ the relevant metal.
The poro~ity~ i~e. the ratio between the volume occupied by the pores (or the volume not occupied by the particular ~
material), ~ ;

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and the total volume of the relevant layer, is preferably at lea~t 50%.
The metal in the initial metal electrode may be any metal, depending on the use to which the electrode is to be put. Particular metals are those of Groups VIII, IB and IIB
of the Periodic System of Elements according to Mendeleef and ~ ;
particularly the Noble metals i.e. ruthenlum, rhodium, palla-dium, osm.ium, iridium, platinum, silver and gold, alloys of two or more of such me~als, or metallic alloys of one or more of such metals with other elements. Particularly preferred electrode metals are palladium, platinum, and palladium/pla-tinum, platinum/rhodium and platinum/iridium alloys.
The said electrode metal is preferably in finely-dlvided formO particularly supported on a porous carrier material e.g. porous carbon, bonded with a bonding material e.g. polyethylene, poly (tetrafluorethylene~ or poly(vinyl-chloride). During manufacture a pore-forming agent e.g. so-dium sulphate, sodium carbonate or ammonium carbonate, may -~
be incorporated in the powder composition and leached out after compressing and possibly sintering, to provide a finely-.
divided porous shaped mass. It may con~titute the electrode in toto, or be present as a finely-divided layer on a metal ~-electrode e.g~ a titanium electrode, or as a component of an electrode comprising a hydrohobic or hydrophobic porous ele-ment e.g. a gas-diffusion electrode comprising a hydrophobic porous element coated on one side with bonded porous carrier material carrylng finely-divided electrode metal.
As alloying element any metal or non-metal may be used which forms a metallic alloy with the electrode metal ' ~ 4.
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under the reaction conditions, particularly one or more elements from ~roups IIIA, IVA, VA, VIA, VIII, IB, IIB and VIIB ~-of the Periodic System of Elements according to Mendelee~.
Examples are technetium, rhenium, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, platinum, copper, silver;
gold, cadmium, mercury, gallium, indium, thallium, germanium, tin, lead, arsenic, antimony, bismuth, sulphur, sel~nium and tellurium. ;
The compound of the said alloying element used as ::
treating material in the process according to the invantion may be a compound of a metal present in thë` cationic state, the metal cation possibly being sequestered with complex~
forming ligands. Examples of suitable compounds of this type.
are copper (II) sulphate, antimony (III) chloride, lead (II) nitrate, cadmium nitrate, gold (III) chloride, bismuth nitrate, ~
potassium antimonyl tartrate, mercury (II) nitrate, gallium -sulphate, indium sulphate, tellurium nitrate, tin (II) chlo~
ride, and silver nitrate. Alternatively the alloying element may be present in the anionic state, for instance as potassium tellurate, hexachloroplatinic acid, tetrachloropalladic acid, ; :, ` ~ perrhenic acid or disodium hydrogen arsenate. In a further ;
embodiment the compound may be of an organic nature, ~or in~
stance germanium methoxide and alkyltin compounds. Oxides, such '~!'~ ~''`'`~
as germanium dioxide, copper ~II) oxide, arsenic trioxide, and `
lead oxide, which are soluble in the reaction mixture at the `~
reduction conditions, may be used. It is not necessary for the compounds to be fully dissolved in the liquid as any non-dissolved part may become dissol~ed during the treatment as the disso1ved portion bAoome~ reduced. If necessary the com- ~ ~

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~5~88 ~ pound used may contain more -than on~ alloying element, for example tinplat.inumchloride. .~.
The solution used may be an aquous solution although an organic solvent in which the compound of the alloying ~ ~
5 element is sufficiently soluble, may be used, for example al- ;
cohols such as methanol, ethanol, the propanolsr or mixtures ~.
thereof with water, especially in combination with compounds of antimony or bismuth.
The pH of the solution during khe subsequent reduc~ ~;
tion step is not without imEortance. At. too low a pM there . .
is a risk of dissolution of the electrode metal, while at too high a pH there is a possibility that alloying of the reduced element with the electrode metal being inhibited for each ~:
reacting system there is an optimum pH. In practice a pH of ~;
between 1 and 5 produces good results, although also pH-va-lues outside this range can be used. The reaction medium may be acidified for instance with sulphuric acid, nitric acid, .';`
hydroch~oric acid, phosphoric acid, formic acid or acetic acid. .-. ~:
. ~ : , It may be advisable sometimes to buffer the solution, especial-O ly if there~is a risk of the electrode metal becoming dissolved as a result of too low a pH.
The required composition of the alloy formed can be `~
: regulated by continuous control of the composition of the so-lution used. In order to avoid too high a concentration of alloy-ing element relative to the electrode metal, khe solution may from time-to-time be replaced by a li~uid which is free from .
: element to be alloyed, for instance a solution of an inert electrolyte. The alloying may be completed by continulng the treatment with the reducing agent.

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As reducing agent a reducing gas is preferably employed,' particularly hydrogen gas. rrhis is of particular advantage in `~
the case of gas-diffusion electrodes, the~reducing ga~ being advantageously supplied to the gas side of the electrode9 ~he quantity of gas supplied may be small, for ins~ance about 100 N-ml of 'hydrogen gas an hour per gram of dissolved alloying element, higher or lower quan~ities may be used. Other redu-cing gases for example carbon monoxide, inert-gas-containing hydrogen and sulphur dioxide may be used. If necessary, the ~`~
reducing agent may be generated in situ, for instance, hydro-gen gas being generated hy electrolysis. Instead of reducing gases, non-gaseous reducing agents may be used, for example ' hydrazine.
The period of time needed for alloying of the elec-15 trode metal to take place depends on various factors. Generally ~'-the alloying reaction re~uires a longer time than the reduc-tion reaction. In proportion as the attainable surface area of ~ -the electrode-metal per unit of'weight is larger, i.e. at an increasing porosity or decreasing metal particle size, the ''-`
20 alloying process develops more rapidly. The temperature at -' which the reaction takes place affects the allo~ing reaction, The-alloying pro¢ess-may-pro~eed-at a temperature between 20C
and the boiling point of the solution, preferably at a tempera- ' ture from 60 to 90C. The higher the temperature is, the more rapidly the alloyiny reaction proceeds, but also the greater will be any dissolution of t'he electrode metal and the lesser will be the dissolution of a gaseous reducing agent~ e~g. hy~
drogen, in the reaction medium, which is unfavourable to the ` ' reduction reaction. Also the factors determing the transfer of 7.

e.g. reducing gas to the gas-liquid interface are ~f impo~tance, such as the pressure of the gas over the liquid, the solubility of the gas in the liquid, and the dimensions of the gas bubbles in the liquid. Further the extent to which the transport of ions in the boundary layer bet~een the solid phase and the gas phase is, possibly, accelerated by stirring plays a part. ~-The process according to the invention has particular ;~
advantages in that a series of electrodes differing in alloy, but not in electrode metal, and having standardized properties, can be prepared. The sta~ing electrodes can be made on a large ~ ;
scale~and thereafté~ trbate~ according to the-invention as required. A further advantage is that the process according to the invention makes it possible for metal electrodes to be prepared in which the electrode metal is modified only super-ficially, for instance to a thickness of one or a few atomlayers, so that electrodes having special electrocatalytic properties can be obtained.
By the process according to the invention different types of electrodes can be made. For instance, electrodes can be obtained for both current-fenerating purposes, as those for use in fuel cells, and current-consuming purposes e.g. for use in electrolysis. The porosity of the electrodes may vary from very slight or absent to very large, and the electrode may be of a hydrophobic as well as of a hydrophilic nature. The process accordilng to the invention i8 particularly advantageous for gaseous-diffusion electrodes.
With non-porous or slightly porous electrodes the process according to the invbntion is preferably carried out so that reducing gas is dispersed in fine bubbles in the solu-' 8.

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~ s~tion of the compound of the alloying element and that this solution is led along the electrode.
prepared Porous, liquid throughflow electrodes are preferably by passing a ~olution of the compound of the alloying element in which the reduction agent is dissolved through the liquid throughflow starting electrode. In this case it may be of particular advantage to have hydrogen ormed in the ~qscent state at the surface of th electrode by means of an electric current. The liquid containing the hydrogen moves into the ;~
pores of the electrode, the alloying element there being libe~
rated by reduction and alloying with the pore-bounding elec-trode-metal.
Gaseous diffusion electrodes preferably are contacted with the reducing gas on the gas sidè, whilst the solution of ~-~
the compound of the element to be alloyed is circulating on the liquid side. The gas enters the pores on the gas side, the liquid on the liquid side. Reduction takes place in the pores near the gas-liquid contact surface area. Since with this type of electrode the supply of the reducing gas, and also the ~ ;
20 supply and discharge of the compound to be alloyed orthe `
finished~product, are optimal, the alloying process generally ', ~ proceeds 3~apidly~
If the electrode is applied as anode in a fuel cell ~ with hydrogen for fual, a modification of the electrocatalytic j 25 properties may be realized during the operation of the fuel cell,~
The same applies if the electrode is applied as cathode in water ~i electxolysis. In these instances the operation need not be in-i terrupted. It will suffice to add a compound of the element to , be alloyed in the requixed quantity to the electrolyte. ~he ? 9 .
.1 ' ' ' ~ , ' ' ' ' ,, ' ' applicability of this en~odiment of the process according to the invention depends on the nature o~ the electrode metal, -the nature o~ the element to be alloyed and-its-aompound,-the nature of the electrolyte and the operating conditions.
S Although in the process according to the invention, it is possible to generate the element to be alloyed electroly~
tically in situ and the element then becomes alloye~ with the finely divided or highly porous electrode metal, a chemical reducing agent may be employed, which enables a homoyeneous distribution of the element to be alloyed over the electrode ;~
metal surface area.
Generally it is advantageous first to incorporate the electrode to be treated in an electro-chernical cel~ and to ef-fect the invention in the cell.
The following Examples of the invention are provided. ~-~''' ` .

$xample I
An electrode consisting of titanium metal to which a S0-~m thick coating layer of very finely divided platinum-iridium with 30 wt.-% Ir is applied by vaporization, was con-20 tacted with an aqueous solution of perrhenic acid having a ~ `
, ~ concentration of l.O g/l, which had been acidified to a p~ of ~ ~
~ . , .2,5 with glacial acetic acid.
The solution was maintained at 80C. Hydrogen gas at 1 atm. was introduced and dispersed in fine bubblesby intensive stirring. The introduction rate was such that invariably more hydrogen was supplied than consumed per unit time.
Af~er 72 hours all rhenium had disappeared from the solution, and it was established that the rhenium was not pre-sent as coating layer but had been completely incorporated by 10.

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the platinum- ididium crystal lattice.

Exam~le II
A gas-diffusion electrode having a thickness of 300~m was constructed of a hydrophobic porous layer oE poly (tetra- -fluorethylene) with a porosity of 50-70% and a thickness of about 180 ~m, and a second porous layer with a porosity of about 50% which consisted oE active carbon with 10 wt.-%
Pd/Pt (9:1), to which approximately 15% of poly(tetrafluor-ethylene) had been added. The electrode was incorporated in a fuel cell. On the hydrophobic side a yas and on the carbon ;~
side an electrolyte, may be circulated.
For electrolyte a freshly prepared sodium-antimonylni~
trate solution was used containing about 100 mg of sodium-anti- -monylnitrate per litre. The pH value was maintained at abouk 1 and hydrogen gas circulated o~ the gas side. After 24 hours, the antimony had disappeared fxom the solution, and it was es-tablished that the antimony had been incorporated in the crys-tal lattice of the Pd/Pt alloy.
,~, xample III ~
' ~ ' An operating hydrogen-air fuel cell equipped with ?
20 electrodes of the type described in Example II contained as ~ `
circulating electrolyte à 30 wt.-% solution of sodium hydroxide in water. The anode, which was fed by hydrogen, contained as catalyst material Pd/Pt (9:1) in a quantity of 10 wt.-% on car- -bon as carrier material.
50 mg/litre of germanium dioxide were dissolved in the electrolyte solution. The fuel cell in operation yielded a ~`

current density of 10 100 mA/cm at a temperature of 65 C. Af-ter 4 hours the germanium had disappeàred from the solution and a Ge/Pd/Pt alloy has formed.

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Example IV
A porous, gaseous-dif~usion electrode as described in Example II, contained as catalyst material an Ag/Pd alloy (3:97) in a quantity of 10% by weight on carbon. The gas side of the electrode was contacted with hydrogen gas, whilst a solution of 50 mg of coppper (II) sulphate in a 6 wt.-% sul-phuric acid solution in water was circulated on the electro-lyte side. The temperature was maintained at 65C. After 1 hour the copper had disappeared from the solutionO After the hydrogen gas has been passed for a further 48 hours, the pre-cipitated copper became substantially alloyed with the Ag/Pd.
. . .

Example V
A freshly prepared solution of tin (II) sulphate in sulphuric acid, made according to the method of J.D. Donald-son and W. Moser, J. Chem. Soc. 1960, 4000 was diluted to a concentration of approximately 120 mg of tin (II) sulphate/
litre and the pH~adjusted to a value of 1 with sulphuric acid, and subsequently circula~ed for bne weeX at 7SC through a trhough-flow electrode. Electrolyte was circulated through this eLectrode by means of a pressure drop.
The electrode was made up of porous carbon spherules which are bonded together and to which 10% by weight of a fi-nely divided Pd/Rh alloy (1:1) had been applied.
The solution was saturated with hydrogen gas at 5 at- -25 mospheres. After 1 week the tin content of the solution had ~ ~-decreased to 21 mg per litre, and a Sn/Pd/Rh alloy formed on . theelectrode. - -~

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Claims

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

1. A process for the production of a metal alloy elec-trode, which comprises modifying a metal electrode, which is a noble metal electrode, by contacting said metal electrode with a solution of a compound of an element which when alloyed with the metal of the electrode provides an electro-catalytic effect, and reducing the said compound to the element in contact with the metal electrode whereby the reduced element and electrode metal become alloyed.

2. A process according to Claim 1, in which the metal electrode is a platinum, palladium, palladium/platinum, platinum/rhodium or platinum/iridium alloy electrode.

3. A process according to Claim 1, in which the electrode is a porous electrode formed at least in part from finely-divided electrode metal.

4. A process according to Claim 3, in which the finely-divided electrode metal is supported by a porous carrier.

5. A process according to Claim 4, in which the said carrier is porous carbon.

6. A process according to Claim 1, in which the elec-trode metal is in the form of a layer on a substrate.

7. A process according to Claim 6, wherein the substrate is a metal.

8. A process according to Claim 6, wherein the said substrate is a hydrophilic element.

9. A process according to Claim 6, wherein the said substrate is a hydrophobic element.

10. A process according to Claim 1, wherein the said solution is a solution of a compound of an element of one or more of Group IIIA, IVA, VA, VIA, VIII, IB, IIB or VIIB of the Periodic System of Elements of Mendeleef.

11. A process according to Claim 10, wherein the said element is arsenic, antimony, sulphur, mercury, lead, selenium, tellurium, copper, rhenium, bismuth, germanium, indium, tin, cadmium or silver.

12. A process according to Claim 1, wherein the pH of the said solution is between 1 and 5.

13. A process according to Claim 1, wherein the concen-tration of reduced element in the alloy produced is controlled by replacing the said solution with a solution not containing an alloying element.

14. A process according to Claim 1, wherein the reduction is effected by a reducing gas.

15. A process according to Claim 14, wherein the said reducing gas is hydrogen.

16. A process according to Claim 15, wherein the said electrode is a gas-diffusion electrode and the said hydrogen gas is introduced to the said solution in the electrode from the gas side of the electrode.

17. A process according to Claim 15, wherein the electrode is constructed so as to allow the said solution to flow through the electrode, and as reducing agent nascent hydrogen is formed at the electrode surface by electrolysis.

18. A process according to Claim 1, wherein the electrode forms part of an electrochemical cell.