Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/54—Electroplating of non-metallic surfaces
  • C25D5/56—Electroplating of non-metallic surfaces of plastics
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/60—Electroplating characterised by the structure or texture of the layers
  • C25D5/615—Microstructure of the layers, e.g. mixed structure
  • C25D5/617—Crystalline layers
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/17—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof
  • C25B9/19—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms
  • C25B9/23—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms comprising ion-exchange membranes in or on which electrode material is embedded
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/18—Electroplating using modulated, pulsed or reversing current
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/60—Electroplating characterised by the structure or texture of the layers
  • C25D5/605—Surface topography of the layers, e.g. rough, dendritic or nodular layers

Definitions

• 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.
• 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.
• the membranes consist in principle of perfluorosulfonic polymer forming a fixed anion network and further comprising mobile or labile cations within this network.
• NAFION ® membranes developed by the company Dupont de Nemours in wherein the anions are SO 3 "ions and the cations can be of various natures. Na +, K +, Li +, however, tends to favor the use of protons H + for the ionic conductivity of the membrane and thus its yield 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.
• metal catalysts are used in the form of deposits on the faces of the membrane.
• the polymer membranes are then very acidic and require the use of expensive noble metals such as platinum, iridium, ruthenium, gold, rhodium or palladium. It is therefore important to try to optimize the ratio of the amount of metal used / energy efficiency, in particular by making metal deposits on the one hand, of small thickness so as not to hinder the path of the products involved in 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, ie a high active surface area ratio.
• 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.
• WO 00/28114 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 putting in contact an electrically conductive substrate and a counter electrode with an electroplating bath containing ions of a metal (Pt 1 Pd, Ru, Rh) to be deposited on the substrate and 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.
• 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 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.
• the deposit is deep beneath the surface of the membrane.
• 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 semiconductive support 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,
• the method described in this document makes it possible to obtain a deposition of the catalytic metal on a carbon surface and not on a polymeric membrane of the NAFION® type, for example. Moreover, the roughness of the deposit obtained by this method is too low to obtain a good energy efficiency.
• 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 .
• the object of the invention is to eliminate these disadvantages by making it possible to obtain a good adhesion of the deposit with the membrane, a deposition depth in the weak membrane and an increased roughness of said deposit, this deposition being carried out on both sides of the membrane. which implies reasoning on transient concentration profiles.
• each of the cycles of the sequence comprises the following operations: the application, from a time to, to the electrodes of the cell 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 and one of said electrodes, this reduction generating an increase in the voltage ( and / or a decrease of the current) at the terminals of the cell, where the measurement of the voltage (and / or the current) at the terminals of the cell, o the measurement of the time flowing from the moment to, o comparing the voltage (and / or the current) measured with a predetermined first 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
• a direct current is applied to allow the remaining ions, if any, to diffuse to a polymer-electrode interface to be reduced.
• the ion exchange polymer may be an ion exchange membrane.
• Said ion exchange membrane may be a NAFION ® membrane developed by the company Dupont de Nemours.
• Said ion exchange membrane may be a FLEMION® 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 sulfonates per gram of membrane - the nature of the membrane. metal salt, the diffusion time of the metal salt in particular depending on the nature of the salt, the temperature and the 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 current of threshold I and - I).
• the current amplitude (or voltage) may be constant or variable.
• the current amplitude may depend on several such as the nature of the metal, temperature, pressure.
• the shape of the current signal 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 deposit takes place as follows:
• the measured current is below the fixed value, the voltage across the cell increases rapidly and the threshold voltage is quickly reached.
• 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.
• the voltage required 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 inversion period determined to prevent too deep diffusion of the ions towards the inside of the membrane, this diffusion being accelerated by the electric field.
• the concentration of ions decreases, the diffusion of the ions inside the polymer remains substantially constant (at a low rate).
• the process according to the invention makes it possible to increase the number of nuclei before starting their growth.
• the deposit thus obtained 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.
• 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.
• the thickness of the oxide layer is lower at the base of the nuclei.
• 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.
• 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.
• the deposit since the deposit is electrochemical, its location is identical to that in which the electrolysis of the water is carried out, which makes it possible to minimize the quantity of metal used.
• the polymer may be re-impregnated in a solution of the same or different metal ions.
• 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.
• 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.
• 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 .
• 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 face in the case of two different ion solutions, the two times ending at the same time. time.
• this device allows an in situ regeneration of the cells.
• FIG. 1 is a representation of a device implementing the method according to the invention
• FIG. 2 is a representation of a concentration profile inside an ion exchange membrane after impregnation
• Figure 3 is a representation of a succession of current waves applied across the cell as a function of time
• Fig. 4 is a representation of the voltage response of the cell as a function of time
• FIG. 5 is a representation of the voltage as a function of the current density according to the type of deposit produced.
• 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 an equivalent weight of 1100 corresponding to 1100 moles of sulphonates per gram of membrane, is first introduced for half an hour in a boiling mixture of water and sulfuric acid H 2 SO 4 suprapur 1M then one hour in boiling water with resistivity 18 M ⁇ .cm in order to clean it on the surface and to saturate it with water.
• the membrane is then positioned in a cell CU that separates it into two compartments C1, C2, these compartments each having an inlet and a liquid outlet.
• a solution of concentration of 10 ⁇ 2 M platinum tetramine salts [Pt (NH 3 ) ⁇ CI 2 is then circulated in each compartment for fifteen minutes so as to obtain a concentration profile P1 (FIG. 2) and then this solution. is replaced by pure water.
• the membrane is located between two porous titanium plates A1, 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.
• 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 direction of flow and in the form 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,
• the selected current value I +/- 30 mA corresponds on the current-voltage curve of the platinum to a value of +/- 2.5 volt which is fixed as the threshold voltage, which must correspond to a deposit with a density current of 20 mA / cm 2 .
• the maximum duration between two polarity inversions is set to 1 minute.
• 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 CtrIV 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.
• 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 towards zero, the computer CA sends to the generator G a command to impose a direct current to allow the salts, if any, to migrate to one of the membrane-electrode interfaces in order to be reduced. Finally, the computer CA causes the shutdown of the current generator G.
• the computer In the event of a malfunction detected from the information CtrIV, Ctrll, the computer also causes the shutdown of the current generator G.
• the succession of alternating waves applied during the deposition is represented in FIG. 3 and the response of the system in voltage across the current leads in FIG. 4, each point corresponding to a measurement of the voltage.
• the current generator G 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 in the direction of alternating current, which corresponds to the voltage minimum of water decomposition 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. But during the duration of a given alternation, a small amount of hydrogen H 2 is formed on one side and a little oxygen O 2 on the other which remain trapped in the vicinity of the current leads.
• the initial current comes from the inverse recombination of electrolysis: the hydrogen H 2 gives protons H + and the oxygen O 2 is reduced in 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 applied current or voltage may have a variable amplitude.
• the signal form can also be square, Gaussian for example ...

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
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• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Engineering & Computer Science (AREA)
• Crystallography & Structural Chemistry (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Fuel Cell (AREA)
• Battery Electrode And Active Subsutance (AREA)
• Electroplating Methods And Accessories (AREA)
• Electrolytic Production Of Metals (AREA)

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• 2004
  • 2004-12-20 FR FR0413586A patent/FR2879626B1/fr not_active Expired - Fee Related
• 2005
  • 2005-12-19 DE DE602005023214T patent/DE602005023214D1/de active Active
  • 2005-12-19 WO PCT/FR2005/003239 patent/WO2006067337A2/fr active Application Filing
  • 2005-12-19 EP EP05850580A patent/EP1838903B1/de not_active Not-in-force
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