FR2879626A1 - METHOD FOR ELECTRODEPOSITION OF A METAL FOR OBTAINING SOLID POLYMERIC ELECTRODE ELECTROLYTE CELLS - Google Patents

Abstract

The invention concerns electroplating of a metal on an ion-exchanging polymer to obtain cells with electrodes-solid polymer electrolyte, said method including: a phase of impregnating a polymer with a metal ion solution for a given period, a sequence for obtaining a succession of alternating waves and wherein each of the cycles includes: a) applying, from a time t<SUB>0</SUB>, to the electrodes of the cell a current (and/or a voltage) of predetermined initial amplitude having a first polarity, b) measuring the voltage (and/or the current) at the terminals of the cell, the time measurement starting at t<SUB>0</SUB>, comparing the measured voltage (and/or current) with a first predetermined voltage (and/or current), comparing the time measured with a predetermined inversion period, c) inverting the current (and/or the voltage) from instant t' with application of a current (and/or voltage), when the measured voltage (and/or current) reaches or exceeds the threshold voltage (and/or drops below the threshold current) before said period elapses if the threshold voltage (and/or current) has not yet been reached if not at the end of said period, d) a further step of measuring the voltage (and/or the current). The invention is applicable to the preparation of electrolyzers, to water electrolysis in particular for supplying gas to fuel cells or for regenerating spent cells in situ.

Description

2879626 5

  The present invention relates to a process for electroplating a metal to obtain solid polymer electrolyte-electrolyte cells. It applies more particularly, but not exclusively, to the preparation of electrolysers, the electrolysis of water in particular for the supply of fuel cell gas or the regeneration of used cells in situ.

  In general, the solid polymer electrolyte-electrolyte cells comprise a solid polymer electrolyte or an ion exchange membrane whose thickness is small (approximately 200 m) bearing metal deposits on its two opposite faces which serve as electrodes, one being a cathode and the other anode.

  The membranes sometimes consist of perfluorosulfonic polymer forming a network of fixed cations and further comprising mobile or labile anions within this network, for example FLEMION membranes of the company ASAHI. They are used in chlor-alkali electrolysis.

  Most often, the membranes consist in principle of perfluorosulfonic polymer forming a fixed anion network and further comprising mobile or labile cations within this network.

  An example of the most commonly used membranes are the NAFION membranes developed by the Dupont de Nemours company in which the anions are SO 3 ions and the cations can be of various natures: Na +, K +, Li +. However, it tends to prefer the use of H + protons because the ionic conductivity of the membrane and therefore its performance are better.

  The electrolysis of water can be carried out using such cells by feeding the anode in pure water: an oxygen evolution takes place there while the protons produced by the electrolysis cross the membrane in the direction of the cathode. There is therefore permanent circulation of protons inside the membrane. Hydrogen is released near the cathode.

  In order to increase the energy efficiency of these cells, metal catalysts are used in the form of deposits on the faces of the membrane.

  The choice of the metals deposited on the membrane depends on several factors for a given reaction: the electrocatalytic power which varies according to the metal and the available exchange surface, the chemical stability of the deposit with respect to the reaction, the electrochemical stability of the deposit as a function of the electrode potential, the electronic conductivity which varies according to the porosity of the deposit, the cost.

  However, such cells are very expensive especially because of the use of precious metals, a high cost of manufacturing electrodes and electrochemical performance of the electrodes.

  Indeed, when using cells for the electrolysis of water, the mobile cations being protons, the polymer membranes are then very acidic and require the use of expensive noble metals such as platinum, iridium, ruthenium, gold, rhodium or palladium.

  2879626 3 It is therefore important to try to optimize the ratio of the amount of metal used / energy efficiency, in particular by producing metal deposits on the one hand, of small thickness so as not to hinder the flow of products involved in the process. electrolysis and, on the other hand, porous and rough, that is to say having a maximum exchange surface for a minimum location in order to increase the reactivity of the electrode, a high ratio of active surface area.

  The properties (thickness, porosity, roughness) of such a metal deposit originate from a phenomenon relating to the growth of metal deposits, for example of noble metal, on a surface of a different nature. Indeed, it turns out that this phenomenon is due to the fact that the energy necessary for the formation of a nucleus is higher than the energy necessary for the growth of an existing nucleus. Thus, rather than forming a regular monolayer on the whole surface and then stacking the monolayers, we first observe a growth in isolated islands called nuclei. Then, a deposition continues to grow from nuclei in dendritic growth.

  However, to have a deposit with properties of porosity and satisfactory roughness, it is necessary to promote a development according to a fractal model that is to say in a direction parallel to the surface and not dendritic that is to say in a normal direction relative to the surface.

  Finally, these deposits must have sufficient adhesion with the membrane to avoid detachments produced by the formation of gas at the junction of the membrane and the deposit.

  Various solutions have already been envisaged: WO 00/28114 (US 6,080,504) discloses a proton exchange membrane fuel cell gas diffusion electrode made by electroplating a nanocrystalline catalytic metal onto a substrate. This result is obtained by contacting an electrically conductive substrate and a counter electrode with an electroplating bath containing metal ions (Pt, Pd, Ru, Rh) to be deposited on the substrate and making passing a pulsed electric current between the substrate and the counter-electrode, the amplitude variation of the current being pre-determined. The pulses of said current are cathodic with respect to the substrate and have a short activity time and / or a short working cycle with a frequency of between about 10 hertz and about 5000 hertz. In a preferred embodiment, the electric current is a modulated inversion electrical current having pulses which are cathodic with respect to the substrate and pulses which are anodic with respect to the substrate, the cathode pulses having a short activity time and / or a short cycle time.

  The method described in this document provides a deposit on one side. However, in the case of electrochemical cells with solid polymer electrolyte electrode, it is necessary to obtain a symmetrical deposit on both sides of the membrane.

  In addition, the electrode on which the deposit is made remains in contact with the bath containing the metal throughout the duration of the deposit. The amount of salts entering the membrane during impregnation is high and the salts are diffused over large distances in the membrane. As a result, the deposit is deep beneath the surface of the membrane. However, during the electrochemical reaction, gas bubbles are formed in the solid electrolyte and then cause high mechanical stresses that can tear the membrane surface and wear prematurely deposit.

  US 5,084,144 discloses a method of manufacturing a high efficiency gas diffusion electrode containing a catalytic metal. This electrode is especially used in electrochemical cells with solid polymer electrode. A gas diffusion electrode is made of a gas-permeable face and, opposite to it, a catalytic face containing an electrically conductive or semiconductor carrier material.

  The method comprises: a step of impregnating the catalytic face of a solution containing a fluorinated cation exchange polymer dissolved in a polar solvent, a so-called plating step during which the electrode thus treated is placed in a solution containing cations of a metal in an oxidized form, application of a direct current and interruption of this current so as to obtain a deposit having a particle size> 10 nm and for which the metal charge obtained is understood between 0.01 and 2 mg per cm 2 of the catalytic surface.

  The process described in this document makes it possible to obtain a deposition of the catalytic metal on a carbon surface and not on a NAFION type polymer membrane, for example.

  Moreover, the roughness of the deposit obtained by this method is too low to obtain a good energy efficiency.

  In addition, for the preparation of electrolysers of water, given the formation of hydrogen and oxygen gas, the electrochemical interface must be rough and almost two-dimensional (<0.01 microns) it is that is to say that the deposit must not be too deep below the surface of the membrane, gas bubbles being able to form in the solid electrolyte thus leading to high mechanical stresses that can tear the membrane on the surface and prematurely wear the deposit .

  None of the solutions encountered takes into account the decrease in the ion concentration in the membrane as the reduction of said ions and consequently, the decrease in the rate of input of material to the interface with time. . However, there is no reason to apply pre-programmed frequencies and even less identical: in fact a not high enough frequency will promote dendritic growth and many cations will diffuse towards the center of the membrane and thus be lost for deposit and difficult to recycle. Too high a frequency will quickly become unnecessary ions that do not have time to reach the electrode-membrane interface.

  The object of the invention is to eliminate these disadvantages by making it possible to obtain good adhesion of the deposit with the membrane, a low depth of deposition in the membrane and an increased roughness of said deposit, this deposition being carried out on both sides of the membrane. the membrane which implies reasoning on transient concentration profiles.

  For this purpose, it proposes a process for electroplating a metal on an ion exchange polymer to obtain solid polymer electrolyte-electrolyte cells, this process comprising: a phase of impregnation of the polymer with a solution of metal ions during a given period so as to enter only a number of ions necessary and sufficient to perform the deposition, said period being determined as a function of the ion diffusion depth tolerated being understood that, the longer the duration the longer the ions have time to diffuse into the interior of the membrane, a sequence making it possible to obtain a succession of alternating waves and in which each of the cycles of the sequence comprises the following operations: applying, from a time to, to the electrodes of the cell a current (and / or a voltage) of predetermined initial amplitude having a first polarity of in order to achieve an electrochemical reduction of the ions located at the interface of the polymer and one of said electrodes, this reduction generating an increase in the voltage (and / or a decrease in the current) at the terminals of the cell, where the measurement of the voltage (and / or current) at the terminals of the cell, o the measurement of the time flowing from the instant to, o the comparison of the voltage (and / or the current) measured with a first voltage ( and / or a first predetermined current, where the comparison of the measured time with a predetermined inversion period, the inversion of the current (and / or the voltage) from a time t 'with the application of a current (and / or voltage) of the same magnitude as the initial amplitude but of inverted polarity, when the measured voltage (and / or current) reaches or exceeds the voltage (and / or falls below below the threshold current) before the expiry of said period if the voltage (and / or current) of threshold has not yet been reached, except at the expiry of the said period, o a new step of measuring the voltage (or current) at the terminals of the cell, o a new measurement of the time flowing from of the inversion moment t ', where the comparison of the newly measured voltage (and / or current) with a second threshold voltage (and / or a second current) may have the same amplitude as the first voltage (and / or the first current) threshold but inverted polarity, o the inversion of the current (and (or voltage) with the application of a current (and / or voltage) of the same amplitude as the initial amplitude but of reversed polarity with respect to the current of the preceding cycle, when the measured voltage (and / or current) reaches or exceeds the voltage (and / or lowers below the current) threshold before the expiration of said period or if the threshold voltage (and / or current) has not yet been reached if not at the time of the expiration of said period, o then again the measurements of voltage (and / or current) and time, comparisons and reversals of current (and / or voltage).

  In addition, after obtaining a deposit having the desired characteristics, a direct current is applied to allow the remaining ions, if any, to diffuse to a polymer-electrode interface to be reduced.

  Said ion exchange polymer may be an ion exchange membrane.

  Said ion exchange membrane may be a NAFION membrane O 5 developed by the company Dupont de Nemours.

  Said ion exchange membrane may be a FLEMIONO membrane developed by ASAHI.

  Said metal may be one of the following metals or any combination of these metals: platinum, iridium, gold, rhodium, ruthenium, palladium, silver, vanadium, chromium, iron, nickel, cobalt, copper, zinc, tin, antimony, lead , bismuth.

  The duration of impregnation of the polymer by a solution of metal ions being dependent on the nature of the ion, it can be optimized by measuring its diffusion coefficient in the membrane and by modeling the exchange between the labile ions of the polymer and the metal ions.

  The determination of the inversion period of the wave is based on various criteria such as the thickness of the membrane - in the case of a membrane Nafion the number of moles of sulforiates per gram of membrane -, the nature of the salt metal, the diffusion time of the metal salt in particular depending on the nature of the salt, temperature and pressure.

  The value of the threshold voltage (or current) may depend on the amplitude value in current (or voltage) chosen for the succession of waves. The relationship between this threshold voltage (or current) and the current (or voltage) amplitude comes from the reference current-voltage characteristic curve of the metal used: at a current intensity I corresponds a voltage E, so if one imposes I and I (or E and E), one poses as maximum voltage or threshold voltage E and --E (or as minimum current or threshold current I and I).

  The current amplitude (or voltage) may be constant or variable.

  The shape of the current signal (or voltage) may also be square, Gaussian for example ...

  The application of the current (or voltage) to the terminals of the cell and the reversals of current (or voltage) can be achieved by a current generator, constant or not, alternating (or a voltage generator, constant or not alternative).

  The deposition proceeds as follows: At the beginning, many ions are present at the electrode-membrane interface, the measured current is below the fixed value, the voltage at the terminals of the cell increases rapidly and the voltage of threshold is then quickly reached. Thus, the current inversions are fast, which makes it possible to reduce the ions in the metallic phase at the moment when they are located at the polymer-electrode interface.

  As the deposit is formed, the voltage necessary to respect the fixed current value decreases, the inversion times increase to allow the ions to be renewed at the electrode-polymer interface, but they must be at least equal to the period reversal determined to prevent too deep diffusion of the ions towards the inside of the membrane, this diffusion being accelerated by the electric field. As the ion concentration decreases, the diffusion of the ions inside the polymer remains substantially constant (at a low rate).

  Once the deposition time is reached, the inversions are stopped in favor of a continuous wave to allow the remaining ions, if any, to diffuse to a polymer-electrode interface to be reduced.

  According to a first advantage, the process according to the invention makes it possible to increase the number of nuclei before starting their growth. The resulting deposit 2879626 - 10- is very localized, very rough. It comprises cauliflower-shaped percolating metal nano-grains characteristic of a growth according to a fractal model and is almost two-dimensional with a thickness of between 0.01 and 10 microns and preferably between 0.01 and 0, 1 micron or less than 0.01 microns.

  For an electrode considered, these wave inversions correspond to a succession of negative alternations or cathodic current during which the metal ions and positive alternations or anodic current during which the oxidation of the surface of the metal deposit formed at the previous negative half cycle.

  Consequently, at the beginning of negative alternation, the reduction of the metal cations is done on nuclei oxidized at the surface at the previous positive half cycle; it is found that this favors the formation of new nuclei rather than a growth of the support nuclei, hence the formation of a fractal structure.

  Moreover, for thermodynamic and kinetic reasons, the thickness of the oxide layer is lower at the base of the nuclei. Thus, the nucleation energy of new nuclei must be lower at the base rather than at the top which favors lateral rather than axial growth and, consequently, a thin and very rough deposit.

  According to a third advantage, the metal deposition can be carried out symmetrically on the different faces of a polymer so the cells can operate in both directions by polarity inversion which is an important asset in case of poisoning and loss of performance at course of time.

  According to a fourth advantage, the deposit being electrochemical, its location is identical to that in which the electrolysis of the water takes place, which makes it possible to minimize the quantity of metal used.

  According to a fifth advantage, the polymer may be re-impregnated in a solution of the same or different metal ions.

  According to a sixth advantage, the solution may contain different metal ions in order to perform a simultaneous codéposition of several metals. This codeposition may for example be a platinum and ruthenium codeposition, the complex thus formed being more stable whereas a ruthenium deposit itself followed or preceded by a platinum deposition is degraded very rapidly.

  In this case, the voltage (and / or the threshold current) will be determined according to the current-voltage curve of the metal alloy.

  The impregnation stage of the polymer is very important because it must be fast and very quickly followed by the rinsing of the polymer and the electrolysis deposit, otherwise the ions have time to start diffusing towards the inside of the membrane.

  For this purpose, the method according to the invention can be implemented in its entirety in a device comprising a cell separated into two compartments by a polymer membrane, these two compartments each comprising an inlet and a liquid outlet so as to impregnate the membrane, rinsing it and directly starting the electrodeposition. This avoids any expensive handling time and therefore detrimental to optimal deposition.

  This device can very easily make it possible to implement an asymmetrical deposit by introducing into each compartment solutions of different metal salts, for example platinum and iridium, for the electrolysis of water to supply hydrogen fuel cells. .

  In the case of an asymmetrical deposit, the method according to the invention operates by taking into account the properties of the two ions by imposing two threshold voltages (and / or two currents) as a function of the alternation and the metal deposit concerned.

  This device also allows a different impregnation time for each side of the membrane for example, to obtain a similar deposit (thickness ..) on each side in the case of two different ion solutions, the two times. ending at the same time.

  Finally, this device allows an in situ regeneration of the cells.

  Embodiments of the invention will be described hereinafter, by way of non-limiting examples, with reference to the accompanying drawings in which: FIG. 1 is a representation of a device implementing the method according to invention; FIG. 2 is a representation of a concentration profile inside an ion exchange membrane after impregnation, FIG. 3 is a representation of a succession of current waves applied to the terminals of the cell according to FIG. time; Fig. 4 is a representation of the voltage response of the cell as a function of time; Figure 5 is a representation of the voltage vs. current density according to the type of deposition performed.

  The example of FIG. 1 illustrates a device making it possible to implement the process for electroplating a metal on an ion exchange polymer according to the invention: A NAFION 1100 membrane, that is to say with a weight equivalent of 1100 corresponding to 1100 moles of sulfonates per gram of membrane is first introduced half an hour in a boiling mixture of water and sulfuric acid H2SO4 suprapur 1M and then one hour in boiling water of resistivity 2879626 -13- 18 MS2.cm to clean it on the surface and saturate it with water.

  The membrane is then positioned in a cell CU which it separates into two compartments C1, C2, these compartments each comprise an inlet and a liquid outlet.

  A solution with a concentration of 10 -2 M in platinum tetramine salts [Pt (NH 3) 4] Cl 2 is then circulated in each compartment for fifteen minutes so as to obtain a concentration profile P1 (FIG. 2), then this solution is replaced by pure water.

  The membrane is located between two porous titanium plates Al, A2 (or spiral platinum wires) fed in series and located on both sides of the membrane with respect to a reference potential (mass) corresponding to the potential of reference of a current generator G. Therefore, the current generator G, a voltage meter MV located between the generator G and the cell CU and the plate A2 are referenced with respect to the reference potential (mass).

  The current generator G is programmable in intensity, in the flow direction and in the shape of the envelope of the generated signal. Its commands are respectively Cdlnt, Cdlnv and CdEnv.

  The computer CA receives the following information: a voltage measured with respect to the mass CtrIV, a current through Ctrll, an envelope of the determined voltage signal CtrlE (square, Gaussian ...).

  The selected current value I = + 1- 30 imA corresponds on the current-voltage curve of the platinum to a value of +/- 2.5 volts which is fixed as the threshold voltage, which must correspond to a deposit with a density current of 20 mA / cm2.

  2879626 - 14 - Voltage measurements are performed every two seconds.

  The maximum duration between two polarity inversions is set to 1 minute.

  The computer CA commands the start of the current generator G and the application, from a time to, to the current leads A1, A2, of a predetermined initial amplitude current I = + 30 mA. to perform an electrochemical reduction of the ions located at the interface of the membrane and of the current supply A1, this reduction generating an increase in the voltage across the cell CU.

  The current value I is kept constant. The voltage increases simultaneously. The measurement of the voltage is taken by the voltage meter MV and the corresponding information CtrlV is taken into account by the computer CA which compares the measured voltage with the first predetermined voltage + 2.5 V and the measured time with the period d predetermined inversion.

  Then, the computer CA controls the inversion of the current with the application of a current of the same amplitude as the initial amplitude but of reversed polarity, ie I = -30 mA, so as to achieve an electrochemical reduction of the ions located at the interface of the membrane and of the current supply A2, when the measured voltage reaches or exceeds the threshold voltage before the expiry of said period or if the threshold voltage has not yet been reached, except at the moment of the expiry of that period.

  The current inversions follow the same process as long as there is a decrease in the amplitude of the envelope of the voltage signal, this decrease being correlated with the increase in the deposition. The calculator CA calculates the variation of the amplitude and when it tends to zero, the computer CA sends the generator G a command to impose a direct current to allow the salts, if any, to migrate to one of the interfaces membrane-electrode in order to be reduced. Finally, the computer CA causes the current generator G to stop. 2879626 -15- It should be noted that the information corresponding to the current applied by the generator G to the terminals of the cell (Ctrll) is taken into account by the calculator AC (check of correct operation of the electrolysis).

  In the event of a malfunction detected from the information CtrlV, Ctrll, the computer also causes the current generator G to stop. The succession of alternating waves applied during the deposition is represented in FIG. 3 and the response of the system in tension to terminals of the current leads in FIG. 4, each point corresponding to a measurement of the voltage.

  The metallic deposition changes according to the inversions and the electrochemical reactions at the membrane / current supply interfaces: When a current supply is subjected to a positive alternation (anodic current), the decomposition of the water takes place according to: H2O1 / 202 + 2H ++ 2e Meanwhile, on the other current supply, two reductions occur in parallel: É the reduction of the protons according to: H + + 1 e *% 2 H2 É the reduction of the salt according to: Pte + + 2 e * Pt In terms of current, the second reduction is negligible in front of the first.

  To impose the predetermined current at a temperature of 25 C, the current generator G must at least impose + 1.23 volts or 1.23 volts according to the direction of the current alternation, which corresponds to the minimum voltage of decomposition water and about 0.2 volts per overvoltage electrode plus an ohmic drop in the membrane. Therefore, during the inversion, the voltage increases very quickly before reaching a landing.

  However, during the duration of a given alternation, a small amount of hydrogen H2 is formed on one side and a little oxygen 02 on the other, which remains in the vicinity of the current leads.

  At the next inversion, the initial current comes from the reverse recombination of electrolysis: hydrogen H2 gives protons H + and oxygen O2 is reduced to water: the tension finally slowly rises again.

  Then, as and when alternating, the platinum surface of each side increases, overvoltages decrease and the voltage increases less and less quickly to a lower and lower equilibrium value.

  Gradually, the threshold voltage is no longer reached before the end of the inversion period.

  The comparison between deposits made according to a conventional method without modulation of the inversion times and deposits made according to the method of the invention is performed by means of voltage curves (V) as a function of the current density (mA. cm-2) of Figure 5. This comparison shows that the method allows to lower the voltage required to obtain a higher current density.

  The invention is not limited to the examples described above.

  Indeed, it will also be possible to consider imposing voltage inversions according to a threshold current.

  The applied current or voltage may have a variable amplitude. The signal form can also be square, Gaussian for example ...

2879626 -17-

Claims (19)

claims   1. A process for electroplating a metal on an ion-exchange polymer for obtaining cells with solid polymer electrolyte-electrolyte, characterized in that it comprises: a phase of impregnation of the polymer (M) with a solution of metal ions during a given period so as to enter only a number of ions necessary and sufficient to effect the deposition, said period being determined as a function of the permissible ion diffusion depth, a sequence permitting to obtain a succession of alternative waves and in which each of the cycles comprises the following operations: the application, from a time to, to the electrodes (Al, A2) of the cell (CU) of a current (and or a voltage) of predetermined initial amplitude having a first polarity so as to achieve an electrochemical reduction of the ions located at the interface of the polymer (M) and of one of said electrodes (Al), this reduction generating an increase in the voltage (and / or a decrease in current) across the cell CU, where the measurement of the voltage (and / or current) across the cell, where the measurement of the time flowing from instant to, where the comparison of the measured voltage (and / or current) with a first predetermined voltage (and / or a first current), where the comparison of the measured time with a predetermined inversion period, o the inversion of the current (and / or the voltage) from a moment t 'with the application of a current (and / or a voltage) of the same amplitude as the initial amplitude but of polarity inverted, when the measured voltage (and / or current) reaches or exceeds the threshold voltage (and / or drops below the current) before the expiry of said period if the voltage (and / or the 18current) threshold has not yet been reached, except at the expiry of the said period, o a new step of measuring the voltage (or current) at the terminals of the cell, o a new measurement of the time flowing from the instant t 'of inversion, o the comparison of the voltage (and / or the current) newly measured with a second threshold voltage (and / or a second current) that may have the same amplitude as the first voltage (and / or the first current) of threshold but of inverted polarity, where the inversion of the current (and / or the voltage) with the application of a current (and / or a voltage) of the same amplitude as the initial amplitude but of inverted polarity with respect to the current of the preceding cycle, when the measured voltage (and / or current) reaches or exceeds the threshold voltage (and / or lowering below the current) before the expiration of said period or if the threshold voltage (and / or current) has not yet been reached, except at the time of the expiration of said period, o then again the measurements of voltage (and / or current) and time, comparisons and invitations ersions of current (and / or voltage).   2. electroplating process according to claim 1, characterized in that it comprises, in addition, after the deposit having the desired characteristics, the application of a direct current to allow the remaining ions if there are any to diffuse to a polymer-electrode interface to be reduced.   3. electrodeposition process according to claim 1, characterized in that said ion exchange polymer is an ion exchange membrane (M).   2879626 - 19- 4. electroplating process according to claim 3, characterized in that said ion exchange membrane is a NAFION membrane (registered trademark).   5. electroplating process according to claim 3, characterized in that said ion exchange membrane is a FLEMION membrane (registered trademark).   6. electroplating process according to claim 1, characterized in that said metal is one of the following metals or any combination of these metals: platinum, iridium, gold, rhodium, ruthenium, palladium, silver, vanadium, chromium, iron , nickel, cobalt, copper, zinc, tin, antimony, lead, bismuth.   7. electroplating process according to claim 1, characterized in that the determination of the inversion period of the wave is based on various criteria such as the thickness of the membrane, the nature of the metal salt, the time of diffusion of the metal salt in particular according to the nature of the salt, the temperature and the pressure.   8. electroplating process according to claim 1, characterized in that the value of the threshold voltage (or current) depends on the current amplitude value (or voltage) chosen for the succession of alternative waves.   9. Electroplating process according to claim 1, characterized in that the current amplitude (or voltage) is constant.   10. Electroplating process according to claim 1, characterized in that the amplitude in current (or in voltage) is variable.   11. electroplating process according to claim 1, characterized in that the application of the current (or voltage) to the terminals of the cell and the reversals of current (or voltage) are performed by a generator 2879626 - 20 - alternating current (G) (or an alternating voltage generator).   12. electroplating process according to claim 1, characterized in that the polymer is re-impregnated in a solution of identical metal ions.   13. Electroplating process according to claim 1, characterized in that the polymer is re-impregnated in a solution of different metal ions.   14. electroplating process according to claim 1, characterized in that the solution contains different metal ions in order to perform a simultaneous codéposition of several metals.   15. electroplating process according to claim 1, characterized in that the metal deposition is carried out symmetrically on the different faces of a polymer.   16. Electroplating process according to claim 1, characterized in that the metal deposition is carried out asymmetrically on the different faces of a polymer.   17. A solid polymer electrolyte-electrolyte cell obtained according to the method according to one of the preceding claims, this cell comprising a polymer comprising a highly localized, very rough deposit formed of percolating metal nano-grains whose structure responds to a fractal model. .   18. Cell according to claim 17, characterized in that the deposit has a thickness of 0.01 and 10 microns and preferably between 0.01 and 0.1 micron.   2879626 - 21 - 19. Cell according to claim 17, characterized in that the deposit has a thickness of less than 0.01 micron.