FR2624885A1 - Electrodes-solid polymeric electrolyte system usable, for example, for the electrolysis of water, and process for its manufacture - Google Patents

Abstract

Process for manufacturing a cell consisting of a polymer membrane 1 and at least one surface deposit 2, 3 of metal, and corresponding cell. Cationic salts containing the metal are placed on the surfaces to be coated and allowed to exchange in the membrane, after which a reduction is carried out. The metal precipitates both inside and outside the membrane 1 and therefore adheres better to the latter, while providing superior electrochemical performance. Application to the electrolysis of water.

Description

SOLID POLYMER ELECTRODE-ELECTROLYTE ASSEMBLY AND METHOD THEREOF
MANUFACTURING
DESCRIPTION
The subject of the present invention is a solid electrolyte-electrolyte polymer assembly and its manufacturing method.

A solid polymer electrolyte-electrolyte assembly consists of a solid polymer electrolyte or cation exchange membrane whose thickness is low (about 200 m), carrying metal deposits on its two opposite faces which serve as electrodes, one cathode and the other anode. The membranes consist in principle of perfluorosulfonic polymer forming an array of fixed anions and further comprising mobile or labile cations within this network.
Nafion produces Dupont de Nemours, in which the anions are SOR ions and the cations can be of various natures
+ +, +
Na, K, Li #, but it tends to prefer the use of protons H because the ionic conductivity of the membrane and therefore its performance are better.

The applications of this type of electrolyte solid polymer electrolyte assembly relate to electrolysers, fuel cells and sensors. For example the electrolysis of water can be carried out using such a set by feeding
The anode in pure water: a release of oxygen takes place there while the protons produced by the electrolysis cross the membrane in the direction of the cathode which attracts them.

There is therefore permanent circulation of protons inside the membrane. Hydrogen is released at the cathode.

 This type of cell has several advantages over conventional electrolysis equipment, in particular compactness and safety, since the electrolyzed liquid is pure water while the other processes use potassium hydroxide solutions (KOH), and good energy efficiency. Problems are however posed by metal deposits that must be made on opposite surfaces of the membrane and which also serve to catalyze the electrochemical reaction. They must first be rough and porous to increase the reactivity of the electrode. It is also preferable that these metal deposits are not too thick not to hinder the flow of products involved during electrolysis. sufficient adhesion with the membrane must be achieved to avoid detachments produced by the formation of gas at the junction of the membrane and the deposit. This is particularly important in electrolysis.

 These problems are even more acute with membranes of Nafion when the mobile cations are protons, because these membranes are then very acidic and require the use of very expensive noble metals such as platinum, iridium, ruthenium, L '. gold, rhodium or palladium.

According to a known method described in the patent
No. 4,272,353, these deposits are made by sintering powder (which must be sufficiently fine) at a temperature <1800 ° C.) and at high pressure on the membrane previously subjected to an abrasion treatment. However, it is difficult to obtain deposits that are sufficiently thin and sufficiently porous. Finally, no intimate contact is made between the membrane and the deposit, which can lead to detachments, especially in the case of application to the electrolysis of water where there would be a gas evolution at the level of the solid polymer electrolyte-electrolyte interface.

According to a different process described in European Patent No. 0 120 212, a paste made from a mixture of
Nafion and electrocatalytic metal which is then spread on a membrane of Nafion. A heat treatment makes it possible to obtain a collage. It is then perfectly marten the amount of deposited metal, and the intimate mixture of Nafion and metal made in the dough ensures a good energy efficiency.

However, the adhesion of the paste on the membrane is not ensured and, moreover, the production of the dough requires the use of organic solvents likely to poison the catalyst that is the metal.

 According to another method described in US Pat. No. 4,326,930, one applies on either side of a membrane previously exchanged with metal salts to electrochemically reduce two current leads, the anode and the other cathode, connected to each other. at the terminals of an external generator and a current is passed through this circuit. This results in an electrochemical reduction of the salts of the metal to be reduced firstly on the cathodic feed of current, then towards the inside of the membrane. In this process, the offset is electrochemically formed on the current cathode feed. This requires to take off said defeat of the cathode before using the membrane. Significant risks of deterioration are related to this operation. Moreover, the electrochemical process used involves a loss of noble metal by further oxidation of the salts at the anode and the formation of a metal precipitation in the center of the membrane, adding to the disadvantages already mentioned a significant risk of short circuit.

Finally, the offset obtained is not very rough (the ratio of the active area to the geometrical area is at most 150).

According to another process (Japanese Patent No. 55 39934), anionic salts of the metal are brought into contact with the membrane, which are then reduced. Anions containing the metal can not penetrate the membrane. In addition, the reducer must pass through the membrane to react with the precursor salt. No more than in the other processes, it is possible to guarantee a good adhesion of the deposited metal on the membrane, and moreover
the vexation here is relatively smooth, which requires prior ion bombardment of the membrane to increase its specific surface area.

 The invention overcomes these disadvantages and provides good adhesion between the membrane and the electrodes. The invention proposes a solid polymer electrolyte electrode assembly comprising a mobile cation polymer and a metal element, characterized in that, on at least one of the faces of said membrane, the metal, generally noble and selected from platinum, is iridium. , gold, rhodium, ruthenium, silver or one of their alloys, oxides or mixtures, is in the form of a dispersion in very fine grains in electrical contact with each other, localized under a thin layer in the polymer mobile cation and that the concentration of said metal element increases from the inside to the outside of said mobile cation polymer to form a continuous and porous layer on its surface.

 The invention also proposes a conductive polymer electrolyte electrolyte assembly with mobile cations, characterized in that the faces of the membrane are each covered with different metallic compounds or with several metal compounds, deposited successively or simultaneously.

 The invention also proposes a method for producing this solid electrolyte-electrolyte polymer assembly and which makes it possible in particular to obtain good adhesion between the membrane and the metal element. Indeed, with this method, there is no clear separation between the electrode and the membrane but a junction having imbrications: intimate contact is then made. A specific surface (ratio of the active area over the geometric area) and a large porosity are also obtained.

 The invention also provides the means to adjust the thickness of the obtained despite, and in particular to make it as thin as it is sufficient to maintain a good energy performance. Substantial savings are then made in metals and / or expensive alloys.

More specifically, the subject of the invention is a method for producing a solid polymer electrolyte-electrolyte assembly whose electrolyte contains an array of fixed anions and mobile cations, having at least one surface electrode formed of a metal deposit, characterized in that this deposit is obtained by at least one cycle comprising the following steps - keeping in contact with the face to cover cations
(belonging to one or more precursor salts) containing at least
less a metal to be part of the metal part during
enough time to let at least
partially cations containing the metal in the electrolyte
by exchange with mobile cations originally carried by the
membrane, and - reduce the cations containing the metal to precipitate
this metal in a localized and controlled way.

In this second step, the reducing element begins to penetrate into the electrolyte on one or both sides and chemically reduces on site the state of metal or alloys or mixtures. The salts of the metal cations having previously diffused into the solid polymer electrolyte. A concentration gradient of the precursor salt (s) within
The conductive polymer electrolyte is created which causes reverse diffusion of the still unreduced salts in the solid polymer electrolyte from the inside to the outside. As the reduction is continued, a porous continuous layer is obtained outside and on the faces of the conductive polymer electrolyte, an electrical contact with the metal grains precipitated inside.

 The steps of this process can also be repeated several times.

 In a particular embodiment of the invention, the solid electrolyte-electrolyte polymer assembly prepared according to the above method is modified on all or part of the inner surface of one or both electrodes. For this, the preceding steps are followed by at least one electrochemical deposition cycle of a lethal metal or a mixture or alloy of metals taken in particular from the group consisting of iridium, rhodium, ruthenium, gold, silver or else Their oxides. Thus, the electrochemical performances are improved. In particular the electrode overvoltages are reduced and their active surface is increased.

The electrochemical deposition comprises the following two steps: - keeping in contact with a solution of a salt or a mixture of
cationic metal precursor salts with the whole
solid polymer electrolyte electrodes previously prepared
for a time sufficient to let at least
partially Cations of said precursor salt (s)
with the labile cations borne initially by the electrolyte, - application of a potential difference between the one or more
electrodes previously deposited on the polymer electrolyte
solid in the previous steps. In the case where only one
electrode is deposited, a metal counter-electrode is
placed near the electrode-free face deposited in a
conductive solution.The applied current is maintained during
sufficient time to reduce or oxidize in the direction of
polarization the precursor salts on the electrode or electrodes to
edit. Reduction or oxidation occurs in contact with
metal deposit already made by chemical means, therefore å
inside the solid polymer electrolyte. We realize
a solid polymer electrolyte-electrolyte assembly where the metal
most expensive noble is deposited in limited quantity, and moreover
protected by the matrix of the solid polymer electrolyte. it
is advantageous when there are gaseous releases at
electrolyte during electrolysis.

The invention will now be described with the help of the following appended figures for illustrative and non-limiting purposes:
FIG. 1 schematically represents an electrochemical cell used in a particular application the electrolysis of water,
FIG. 2 schematically shows the structure of a solid polymer electrolyte-electrolyte assembly of the invention,
FIGS. 3a and 3b illustrate the phenomena involved in producing the solid electrolyte polymer electrolyte assembly.

 In FIG. 1, the membrane is represented by 1.

According to the usual technique, it is assumed in Nafion. Its thickness is about 200 μm and it bears on its opposite sides two metal deposits Z and 3. These deposits 2 and 3 are made of noble metal to prevent the membrane 1 corrodes them; they play a double electrical role of supply of current to the membrane 1 and catalytic by promoting the electrolysis reaction. According to the first role, a first porous and electrically conductive current feed 4 is installed on a deposit 2 and a second porous and electronically conductive current feed 5 is installed on the other 3.

The membrane 1 thus separates the outside into two compartments 6 and 7. To carry out the electrolysis of water, which is a very common application of these membranes, a water supply is installed which reaches the defect 2 along the arrow 8. The water is decomposed at the interface and a release of oxygen is formed and is recovered in the compartment 6. On the other hand, attracted by the cathode, the protons produced by the oxidation of
The water passes through the membrane 1 along the arrow 9 and finally reaches the opposite metal deposit 3. A hydrogen evolution then occurs in the corresponding compartment 7 essentially from the junction of the membrane 1 and this deposit 3.

The chemical stability of the Nafion 1 membrane is guaranteed by the existence of a network of fixed SO anions in
+ 3 this membrane 1. Only the cations, here H, are displaced during the reaction.

 It is immediately understood by means of this figure that the gaseous releases, especially at the junction of the membrane 1 and deposits 2 and 3, may cause the detachment of the latter and that secondly deposits 2 and 3 too thick and not very porous counteract the passage of products of the reaction.

FIG. 2 represents the structure of a metal electrode deposited according to the invention on one of the faces of a solid polymer electrolyte membrane. This is a photograph magnified 6000 times. Three different zones
appear: a, b, c.

Zone a corresponds to the external part of the metallic deposit, outside the surface of the membrane 1. Its
thickness is about 1.5> lm. It is a porous deposit, adhering
solidly to the membrane 1 since it extends inside
this one on an area b. Zone c constitutes the electrolyte
solid polymer without metal.

Zone b has small metal particles
spherical small diameter (from 5 to a few tens of nanometers).

Near the surface of the membrane, in part b1 of
zone b, the particles are in electrical contact with the external deposit and in electrical contact with each other. This gives the electrode excellent adhesion to the membrane and a large specific surface area greater than 400. It is in fact the part bl which is the most important of these two points of view of mechanical strength and electrochemical efficiency. As one sinks into the solid electrolyte, the particle density decreases. They cease to be in electrical contact with each other in part b2 of zone b.

 The particles present in part b2 of zone b correspond to a small loss of catalyst because of their small size and low density. The joint thickness of parts b1 and b2 is June 6th. As will be seen below, however, the thicknesses given here are not the only ones that can be obtained with the invention but depend on them. contrary to manufacturing factors. They only correspond to an example.

Referring now to FIG. 3a, which shows the evolution over time of the concentration profiles of a precursor cationic salt inside the solid polymer electrolyte, when a sample is immersed in a solution of one of these salts. The same exchange phenomena occur when
The solid polymer electrolyte is immersed in a solution of a mixture of these salts: there is then simultaneous incorporation of several precursor salts. The abscissas are taken according to the thickness of the membrane 1, and the ends of the diagram correspond to the opposite surfaces of this membrane 1. The ordinates of FIG. 3a correspond to concentration graduations. The profiles 11 to 15 represent what occurs. if the surfaces of the membrane 1 are brought into contact with cations containing the metal or metals to be deposited.

These cations can be for example salts
Cationic 2+ 3+ such as Pt (NH) or Ir (HO) t Ru3 +, Ru (OH)
34 2 6 or any other cation in solution containing the metals of the group already mentioned. They tend to replace by exchange and diffusion the cations originally present in membrane 1 and mobile as we have already seen. The replacement is immediate to the surfaces and slower in depth. The profile represents the evolution of the penetration according to the depth: the concentration of cations containing the metal to be deposited describes very quickly according to the depth and ends up becoming null, and thus leaves a relatively important thickness with the heart of the membrane 1 still containing only primitive cations.A more advanced stage represented by the profile 12, this untouched area has disappeared and so we find cations containing the metal throughout the membrane 1. Their concentration in the heart remains low. At even more advanced stages, represented by the profiles 13 and 14, the differences in concentrations between the surfaces and the heart gradually decrease as the primitive cations are evacuated. Finally, according to profile 15, all the primitive cations have disappeared and have been replaced by cations comprising the metal to be deposited and the concentration thereof is uniform irrespective of the location of the membrane 1.

 The phenomenon of progressive exchange of the cations initially present in the solid polymer electrolyte by the metal cations of the precursor salt or salts shown in FIG. 3a is used in the invention in order to locally precipitate the metal (s) (if mixtures of precursor salts are used).

 For optimum performance, the solid polymer electrolyte is immersed in a solution of a precursor cationic salt for a long time (15 minutes at a salt concentration of 0.01 mol / l) in order to reach the profile 11 (FIGS. and 3b) which corresponds to a partial exchange.

Then, the solid polymer electrolyte is removed from the solution, rinsed with pure water and brought into contact with a reducing agent, for example sodium borohydride NaBH, diluted in
4 pure water at 3 g / l, but any other reducer may be suitable.

The reducer enters the solid polymer electrolyte and reduces the cationic salts it encounters from the surface to the metal state. Because of this chemical phenomenon of reduction and therefore of reduction of the concentration of salts, a physical phenomenon of diffusion of the salts has not occurred which has not precipitated from the inside towards the outside and which is therefore inverse of the previous diffusion. This phenomenon has the effect of creating a metal precipitation profile 21 closer to the surface of
The solid polymer electrolyte as the salt concentration profile 11 at the beginning of the reduction. At the same time, there is the appearance of a metal deposit delimited by the profile 21 'to
The outside of the solid polymer electrolyte which corresponds to the zone a of FIG.

 A dense metal deposit is thus obtained on the surface, although having a good exchange surface, and formed of small grains whose concentration decreases gradually but rapidly towards the inside of the membrane. These general characteristics are found after reduction of cationic salts having both the profile 11 at the time of reduction that one of the other profiles 12, 13, 14 or 15. However, it is found that the precipitate is more localized at the surface, and therefore more effective, if we start the reduction starting from a profile Il corresponding to a relatively weak diffusion of cationic salts: the precipitates obtained from the other profiles have, all things being equal, zones b2, where the grains are ineffective, more important.

To obtain an external deposit, corresponding to the zone a, more importantly, one operates with a strongly diluted reducing solution. The penetration of the reducer into the membrane is slowed down, so that the precipitation is here even more localized on the surface, especially since it is more
Slow and the diffusion of cations outward is more complete. Knowledge of the profiles 11 to 15 thus makes it possible to precipitate the metal in a localized and controlled manner.

 The process is complete when the cations have been reduced. The membrane and metal deposits are dipped, before being used, in a solution which makes it possible to reintroduce the desired mobile cations into the membrane.

 The inventors have also found that it can be very advantageous to pre-dive the Nafion membrane i in pressurized water at a few bars at 160-1800C (for one or a few quarters of an hour). The modifications that this operation brings to the structure of the membrane 1 can then obtain the same electrochemical performance with metal deposits of content up to ten times lower. The rest of the process is essentially unaffected.

 Given these indications, five examples will now be described in detail, which correspond to conditions deemed favorable, that is to say which make it possible to obtain electrodes which are porous and which have a high active surface area ratio ( greater than 400), which are very localized (a few microns deep inside the solid polymer electrolyte) and which are very adherent (the adhesion is estimated during an operation in electrolysis of water under heavy clearing gaseous, that is to say at current densities of 1 A / cm, and for periods of a few hundred hours.

These very demanding conditions for the systems constitute an excellent criterion of evaluation of the adhesion of the deposits).

 The deposits can be made on only one of the two faces of the solid polymer electrolyte by mounting it in a cell with two compartments; a solution of precursor metal salts and then a reducing agent are then successively circulated in one of these two compartments. The latter may be a gas such as hydrogen but the best results have been obtained with aqueous solutions of borohydrides (for example sodium borohydrides flaSH) or hydrazine (N H).

4 24
The tests carried out were made on samples whose surface varies from a few square centimeters to a few hundred square centimeters, and the results obtained are reproducible. The choice of the solid polymer electrolyte is a function of the characteristics of the products on the market.

In a particular embodiment of the invention, the solid electrolyte-electrolyte electrode assembly previously prepared receives an electrochemical deposit located on the surface of the fine grains of the metal element already precipitated in the internal part of the solid polymer electrolyte. It is thus possible to improve the electrochemical performances of the solid polymer electrolyte-electrolyte assembly and, on the other hand, to save an expensive noble metal by chemically precipitating in the first phase a cheaper noble metal and covering it electrochemically with another more expensive noble metal. and more efficient (for example iridium vis-à-vis platinum) constituting a very thin film. This second noble metal can only be deposited where the electrochemical reaction takes place, therefore inside the solid polymer and obligatorily on the first noble metal, essentially between the grains of the first metal in zone b. Thus, the totality of the second precursor salt is reduced and this last step leads to an increase in the efficiency of the solid polymer electrolyte-electrolyte assembly. The electrochemical reduction further creates metal parts connecting the small grains of the first precipitated metal element which may not be in contact with each other.
electric power, thus further improving the efficiency of the
system.

EXEPLE 1
LXEMPLE # i
A membrane of Nafion (weight equivalent 1100) under
+ 2
H form of 5 cm previously cleaned (for example by
introduction for 30 minutes in a boiling solution
molar nitric acid, then by introduction for one hour
in boiling water at 1000C under atmospheric pressure),
3 is immersed in 10 cm of a solution of Pt (NH) Cl at 0.01
34 mole / liter for 15 minutes.

Then the membrane is out of this solution, rinsed
superficially to pure water and introduced immediately into
50 cm 3 of a solution of sodium borohydride titrated 3 g / liter for one hour. This reducing solution is renewed after 5, 30 and 45 minutes. All these operations (except the preliminary cleaning) is then repeated a second time, and between these two cycles of operation, the membrane is introduced into an aqueous solution of 0.5 mol / l hydrochloric acid to replace the ions by protons, then rinsed with pure water.

The platinum content on each of the two
2 faces is between 0.8 and 1.5 mg / cm. Cell voltage
2 in electrolysis of water is 2.10 V at 1 A / cm and 800C.

~~ EXAMPLE
A procedure analogous to that described in Example 1 was followed except that the membrane was, after
The preliminary cleaning step, introduced for one hour in pressurized water at 1600C, and only one loading cycleprecipitation was carried out.

In this case, the same electrochemical performances as those of Example 1 were obtained, but the platinum surface content on each of the two faces is included
2 between 0.1 and 0.2 mg / cm
EXAMPLE 3
A procedure similar to that described in Example 1 was followed, but this time the membrane was immersed in a solution containing a mixture of a platinum salt Pt (NH) Cl
34 2 and several cationic salts of iridium.The iridium solution was obtained from K IrCt by dissolution in a
2 6
sodium hydroxide solution at 0.1 mole / liter, at a pH of about 8, adjusted with a perchloric acid solution at 0.1 mole / liter, then by decantation and recovery of the precipitate, form and dissolution of this same precipitate in a solution perchloric acid at 0.1 mole / liter. The platinum salt is then dissolved in this solution. The total iridium content is about 0.01 mole / liter and the platinum salt concentration is 0.01 mole / liter. After two successive cycles of pre-precipitation, the noble metal content on each side is
2 between 0.8 and 1.5 mg / cm (approximately 70X platinum), and in
2 electrolysis of the water is obtained 2.00V at 1 A / cm at 800C.

EXAMPLE 4
A solid polymer electrolyte-electrolyte assembly prepared according to the method described in Example 1 is mounted in a cell with two compartments. Two platinum current feed grids are plated on each of the deposits.

 In one of the two compartments, a solution of cationic salts of iridium at 0.01 mol / liter, as described in Example 3, is introduced for a few minutes (1 to 5).

 In the other compartment, we put pure water.

Then the solution of iridium salts is removed, replaced by pure water and the current is passed through.
2 cell under high current density (between 0.5 and 1 A / cm) for 15 minutes to one hour, the cathode being the electrode where the iridium was introduced.

After that, an iridium content is obtained on a
2 of the two electrodes from 0.1 to 0.3 mg / cm. In electrolysis of water,
Since the electrode containing iridium is used as anode,
2 obtains a cell voltage of 1.80 V at 1 A / cm and 800C.

EXAMPLE 5
A procedure analogous to that described in Example 4 was followed, except that a solution containing a mixture of iridium salts (prepared as indicated in Example 3) was introduced into one of the two compartments. ruthenium (RuCl) at 0.01 mol / liter.

3 - 2
An iridium content of 0.05 to 0.1 ng / cm is obtained
2 and a ruthenium content of 0.08 to 0.15 mg / cm. The
2 performances in electrolysis of water are 1.75 V at 1 A / cm and 800C.

 The method can be generalized to any element that can be chemically or electrochemically reduced. The list of usable noble metals: platinum, iridium, gold, rhodium, ruthenium, palladium, silver can then be extended to the following elements: vanadium, chromium, iron, nickel, cobalt, copper, zinc, tin, antimony, lead, bismuth to obtain pure metal electrodes or any combination of metal or alloy mixtures. For easily oxidizable metals, it should not be considered for use in acidic medium. The fields of application covered by the invention are those of mineral electrosyntheses (for example the electrolysis of water in an acid or alkaline medium, chlor-alkali electrolysis) and organic electrolysis (for example the reduction of oxalic acid), gas sensors (for example those with alcohol) and fuel cells (for example H / O).

Claims (18)

 A method for manufacturing an assembly consisting of an electrolyte (1) in solid polymer form containing an array of fixed anions and mobile cations, having at least one surface corresponding to an electrode, the electrode being composed of a metal deposit (2, 3), characterized in that this deposit is obtained by at least one cycle comprising the following steps - keeping in contact with the surface to be coated with cations  containing at least one metal to be part of the deposit  metal for a period of time sufficient to allow  at least partially the cations containing the metal in  Electrolyte with mobile cations, and - reduce the cations containing the metal to precipitate the  Localized and controlled way metal.  2. Process for producing a solid polymer electrolyte electrode assembly according to claim 1, characterized in that the cations left in contact with the surface to be coated comprise at least two different metals.  3. A method for manufacturing a solid polymer electrolyte electrode assembly according to any one of claims 1 or 2, characterized in that the electrolyte is perfluorosulfonic polymer.  4. Process for producing a solid polymer electrolyte electrode assembly according to claim 3, characterized in that the metals are selected from platinum, iridium, ruthenium, gold, rhodium, palladium and silver.  5. Process for producing a solid polymer electrolyte electrode assembly according to claim 3, characterized in that the metals are selected from iron and nickel.  A process for producing a solid polymer electrolyte electrode assembly according to any one of claims 1 to 5, characterized in that the cations are reduced by a dilute reducing solution.  7. A method for producing a solid polymer electrolytic electrode assembly according to claims 2 and 4, characterized in that the deposited metals are successively platinum and iridium.  8. A method for manufacturing a solid polymer electrolyte electrode assembly, characterized in that it comprises at least one additional deposit of metal on a prior deposit (2, 3) obtained according to any one of claims 1 to 7, the additional deposit being obtained by the following steps: depositing cations containing the metal on the previous deposit, cathodically polarizing the previous deposit to operate a  reduction of the cations containing the metal.  9. A method for manufacturing a solid electrolyte electrolyte electrode assembly according to any one of claims 1 to 8, characterized in that the electrolyte (1) is previously immersed in pressurized water to 1700C.  10. Electrode (2,3) -electrolyte assembly (1), The electrolyte being a solid polymer containing a fixed network of anions and mobile cations, characterized in that it comprises, on at least one surface of the electrolyte, an electrode consisting of a metal deposit which consists of a first portion (b) in the electrolyte, mixed with the solid polymer and formed of grains of a few nanometers in diameter having an increasing concentration towards the surface, the grains being in electrical contact near the surface; as well as a second part (a) out of the electrolyte, adhering to the electrolyte and the first part.  11. Electrode-electrolyte assembly according to claim 10, characterized in that the metal deposit consists of a pure metal.  12. electrolyte-electrolyte assembly according to claim 10, characterized in that the metal deposit consists of a mixture of metals.  Electrode-electrolyte assembly according to claim 10, characterized in that the metallic deposit consists of at least one metal present in a compound.  14. electrolyte-electrolyte assembly according to claims 10 and 12, characterized in that the second portion (a) of the deposit consists of a first metal and in that the first part (b) of the deposit consists of a mixture grains of the first metal and at least one second metal.  15. Electrode-electrolyte assembly according to claim 14, characterized in that the first metal is platinum and the second metal is iridium.  16. An electrolyte-electrolyte assembly according to any one of claims 11 to 14, characterized in that the metals are selected from platinum, iridium, ruthenium, rhodium, palladium, gold and silver.  17. An electrolyte-electrolyte assembly according to any one of claims 11 to 14, characterized in that the metals are selected from iron and nickel.  Electrode-electrolyte assembly according to one of Claims 10 to 17, characterized in that the solid polymer is perfluorosulfonic.