Classifications

• • 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
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
  • C25B11/091—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds

Definitions

• the present invention relates to a method of manufacturing an electrode for electrochemical processes.
• electrolysis processes it is generally sought to reduce the potentials of electrochemical reactions at the electrodes to as low a value as possible. This is particularly the case in the electrolysis processes in which hydrogen gas is produced on the active surface of a cathode, such as the processes for the electrolysis of water, aqueous solutions of hydrochloric acid and aqueous solutions of sodium chloride.
• the cathodes most commonly used hitherto for the electrolysis of water or aqueous solutions of sodium or potassium chloride have generally consisted of plates or lattices of mild steel. These known cathodes have the advantage of easy implementation and low cost. The overvoltage on the evolution of hydrogen on these known steel cathodes is however relatively high, which increases the cost of the electrolysis processes. Steel cathodes have the additional disadvantage of being the site of progressive corrosion on contact with concentrated aqueous solutions of sodium hydroxide, as they are generally obtained in electrolysis cells with selective permeability membranes. ''
• cathodes have been proposed obtained by applying, on a steel or nickel support, a coating formed of a nickel powder mixed with a polysilicate and then subjecting the nickel powder sintering in a reducing atmosphere, at a temperature above 760 ° C (Journal of the Electrochemical Society, vol.128, N ° 4, April 1981 - DEHALL: "Electrodes for alkaline water electrolysis", pages 740 to 746).
• these known cathodes Compared to the electrodes formed from steel or nickel plates as such, these known cathodes generally allow an improvement in the energy efficiency of the processes for the electrolysis of water or aqueous solutions of sodium chloride.
• the invention aims to provide a method of manufacturing electrodes which, when used as cathodes in electrolysis processes where hydrogen is generated, make it possible to further improve the energy efficiency of the electrolysis process .
• the invention relates to a method of manufacturing an electrode for electrochemical processes, according to which an electrically conductive substrate is coated with a material containing a powder of at least one active metal for the electrochemical reduction of protons and said material is heated on the substrate, successively in an oxidizing atmosphere then in a reducing atmosphere; according to the invention, a material is used in which the aforementioned active metal is in the form of an unsintered powder, associated with colloidal silica.
• the active metal selected must be a metal which can be oxidized by heating in an oxidizing atmosphere, and whose oxide can be reduced to the solid metallic state by heating in a reducing atmosphere.
• the selection of the active metal also depends on the destination of the electrode. In the case where it is intended to serve as a cathode for the electrolytic production of hydrogen in an electrolysis process, it is advantageously selected from cobalt, iron, manganese and nickel.
• the substrate can be made of any electrically conductive material compatible with the active metal and the oxidation and reduction treatments used.
• the active metal is selected from cobalt, iron, manganese and nickel, it is advantageous to choose the material of the substrate from these metals and their alloys.
• the substrate can have any suitable shape with the destination of the electrode. It can be, for example, a solid or perforated plate, a wire, a trellis or a stack of beads. It may have a smooth surface state, a rough surface state being however preferred. It may possibly be linked to an underlying support made of a different material, for example a material that is better conductive of electricity such as copper or aluminum.
• Heating in an oxidizing atmosphere has the function of oxidizing the active metal.
• the choice of temperature, atmosphere and heating time depends on the active metal selected and must therefore be determined in each particular case by routine laboratory work. After heating in an oxidizing atmosphere, the active metal is in the state of metal oxide.
• Heating in a reducing atmosphere has the function of reducing this metal oxide to the state of metal.
• the choice of heating conditions also depends on the active metal selected.
• the material of the electrode which is subjected to heating in an oxidizing atmosphere contains the active metal powder in the unsintered state, associated with colloidal silica.
• an active metal powder is desirable to use as fine as possible.
• a powder is used in which the average particle diameter does not exceed 50 microns and preferably 30 microns. Powders which are generally well suited are those in which the average particle diameter is between 1 and 25 microns, more especially those with an average diameter of less than 20 microns.
• the active metal powder is associated with colloidal silica.
• the optimum amount of colloidal silica to be used depends on various factors, in particular the nature of the active metal and its particle size. In general, a relative amount by weight of colloidal silica in the material is used, comprised between 0.5 and 10% of the weight of active metal, the amounts comprised between 0.8 and 4% of this weight being preferred.
• the colloidal silica can be used in the form of a gel which is mixed as it is with the active metal powder to form the aforementioned material.
• the active metal powder is dispersed in a colloidal silica solution, preferably aqueous, to form the aforementioned material which is then applied as it is, to the state of a liquid suspension, on the substrate by any suitable means, for example by soaking the substrate in the suspension, by coating with a brush or roller or by spraying.
• a colloidal silica solution preferably aqueous
• the maximum admissible concentration of silica in the suspension is imposed by the need to produce a stable colloidal silica solution. It depends on various factors, in particular on the concentration of the active metal suspension and on the possible presence of additives such as stabilizers of the colloidal solution or thickeners.
• the silica content of the colloidal solution should not exceed 30% by weight, values between 3 and 28% and more especially between 10 and 25% being desired.
• the active metal powder can be dispersed in the colloidal silica solution as it is and the resulting suspension applied to the substrate.
• the optimum amount of dilution water is variable depending on the particle size of the active metal powder, the relative amount of active metal added to the colloidal silica solution and the desired viscosity. In practice, good results are obtained by using an amount of dilution water such that the weight content of active metal in the resulting aqueous suspension is between 10 and 80 X, preferably 15 and 60 X, the contents included. between 20 and 50% being especially advantageous.
• the drying is advantageous to subject the aforementioned material to drying on the substrate, before heating it in an oxidizing atmosphere, the drying having the function of removing at least most of the water from the solution colloidal.
• the drying is advantageously regulated so that, at the end of the latter, the water content of the material does not exceed 10%, preferably 5% of the weight of the material. During drying, sintering of the active metal powder should be avoided.
• a colloidal silica solution which additionally contains lithium ions as stabilizing agent.
• the lithium ions can be introduced by any suitable means into the colloidal silica solution, preferably in the form of lithium hydroxide.
• the content of lithium ions in the colloidal solution is preferably adjusted so as to achieve therein a molar ratio Si0 2 : Li0 2 of between 3 and 25, preferably 4 and 10.
• Solutions of colloidal silica especially suitable in the context of the invention are those described in patent US-A-2,668,149 (DU PONT).
• the heating in an oxidizing atmosphere and the heating in a reducing atmosphere are preferably carried out at temperatures for which no melting or sintering of the metal powder is caused.
• heating in an oxidizing atmosphere can be carried out in air, preferably at a temperature not exceeding not 850 ° C and heating in a reducing atmosphere may be carried out in hydrogen at a temperature not exceeding 600 ° C.
• Suitable working temperatures are those between 600 and 800 ° C, and more particularly between 700 and 760 ° C, for heating in an oxidizing atmosphere and those between 300 and 500 ° C, and more particularly between 350 and 450 ° C, for heating in a reducing atmosphere.
• the electrode obtained after heating in a reducing atmosphere can generally, after cooling, be used as it is, in the electrochemical process for which it is intended.
• oxidation treatment after heating in a reducing atmosphere.
• This oxidation treatment can be carried out in ambient air and it is preferably carried out at a temperature above ambient temperature but not exceeding the maximum temperature of heating in a reducing atmosphere.
• a practical way to achieve this is to cool the electrode in the presence of air after heating in a reducing atmosphere.
• a coating containing a metal selected from chromium, molybdenum, cobalt, nickel is applied to the electrode, after heating in a reducing atmosphere, ruthenium, rhodium, palladium, osmium, iridium, platinum, lanthanum and rare earth elements.
• this embodiment of the invention allows an additional gain in electrical voltage in electrochemical processes, and especially in electrolysis processes.
• the metal of the coating can be applied to the electrode by any suitable means, for example by a technique of projection in a plasma jet.
• the coating metal is selected from chromium, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, it is useful to carry out a deposition by process electrolytic. To this end, it has been found to be especially advantageous to carry out the electrolytic deposition of the selected metal, in an electrolyte containing ions of said metal, where the electrode is the site of an electrolytic reduction of protons.
• a layer of a heat-decomposable compound of said metal is first deposited there, then said compound is subjected to a treatment thermal decomposition to release an oxide from said metal, and the oxide is then heated in a reducing atmosphere.
• the thermally decomposable compound can be any compound which, by heating in a controlled atmosphere, releases an oxide of the metal selected for the coating.
• It can be, for example, a nitrate, a sulfate, a phosphate, a chloride, a salt of carboxylic acid such as a formate, an acetate, a propionate or an oxalate. It can be used in the solid state, for example in the form of a powder, or in the liquid state, for example in the form of a molten salt, of a suspension or of a solution.
• the heating temperature and the controlled atmosphere must be chosen according to the metal selected and the thermally decomposable compound used.
• the heating can be carried out in an inert atmosphere (for example in a nitrogen or argon atmosphere).
• an oxidizing atmosphere generally in air, at a temperature below 1000 ° C, preferably not exceeding 850 ° C; temperatures between 100 and 800 ° C and more especially those not exceeding 750 ° C are preferred.
• Heating in a reducing atmosphere can generally be carried out in a hydrogen atmosphere, at a temperature not exceeding 600 ° C, usually between 200 and 500 ° C, depending on the metal selected for coating the electrode.
• the electrodes obtained by the process according to the invention find applications in various electrochemical processes, such as, for example, cathodic protection, electrolysis and fuel cells.
• the invention therefore also relates to the use of an electrode obtained by the process according to the invention, as a cathode for the electrolytic production of hydrogen by electrochemical reduction of protons in an aqueous alkaline medium.
• Such use is especially advantageous in electrolysis cells for the production of aqueous solutions of alkali metal hypochlorite, as well as in cells with a permeable diaphragm and a membrane with selective permeability for the electrolysis of aqueous solutions of chloride.
• sodium such as those described in patents FR-A-2 164 623, 2 223 083, 2 230 411, 2 248 335 and 2 387 897 (SOLVAY & Cie).
• an aqueous brine containing 255 g of sodium chloride per kg was electrolysed in a laboratory cell with vertical electrodes, separated by a membrane with selective cation permeability NAFION (DU BRIDGE OF NEMOURS).
• the cylindrical cell included an anode formed of a circular titanium plate, pierced with vertical slits and coated with an active material of mixed crystals, consisting of 50% by weight of ruthenium dioxide and 50% by weight of titanium dioxide.
• the cathode consisted of a non-perforated disc whose constitution is defined in each example.
• each electrode of the cell was 102 cm 2 , and the distance between the anode and the cathode was fixed at 6 mm, the membrane being placed at equal distance from the anode and the cathode.
• the anode chamber was continuously supplied with the above-mentioned aqueous brine and the cathode chamber with a dilute aqueous solution of sodium hydroxide, the concentration of which was adjusted to maintain, in the catholyte, a concentration of about 32% by weight of sodium hydroxide.
• the temperature was continuously maintained at 90 ° C in the cell.
• the density of the electrolysis current was maintained at the fixed value of 3 kA per m 2 of area of the cathode. This produced chlorine at the anode and hydrogen at the cathode.
• a coating composition was prepared, by mixing the following constituents:
• the nickel powder used in this coating composition had a particle size such that its specific surface was approximately equal to 0.6 m 2 / g.
• colloidal silica solution a colloidal solution containing approximately 20% by weight of silica and 2.1 x by weight of lithium oxide was used.
• a polysaccharide As a thickener, a polysaccharide was used.
• a nickel plate serving as a substrate On a nickel plate serving as a substrate, six consecutive layers of this coating composition were applied, the plate being subjected to drying for half an hour in an oven at 70 ° C. after the application of each layer.
• the substrate and its coating were then heated in an oven at 750 ° C for 5 hours, in the presence of air, so as to oxidize practically all of the nickel in the coating. After being cooled, they were treated at 450 ° C for one hour in an oven swept by a stream of hydrogen, then cooled to room temperature, while maintaining the atmosphere of hydrogen in the oven.
• Electrolysis results The electrode obtained at the end of the process which has just been described has been mounted as such as a cathode in the electrolysis cell. During electrolysis, the voltage across the cell was established at 3.29 V.
• Electrolysis results The voltage at the terminals of the electrolysis cell using the electrode thus obtained as a cathode has stabilized at 3.16 V.
• the potential of the cathode has also been measured by means of the Luggin capillary measurement method, connected to a KCl saturated calomel reference electrode (ECS) (Modern Electrochemistry, Bockris and Reddy, Plenum Press, 1970, vol.2, pages 890 and 891).
• ECS KCl saturated calomel reference electrode
• the cell cathode consisted of a plate of sandblasted nickel as it was. During electrolysis, the voltage across the cell stabilized at 3.36 V.
• composition of the cathode The coating composition described in Example 1 was used, which was applied in five successive layers on a nickel plate, the plate being subjected to drying by half - hour in an oven at 70 ° C after the application of each layer.
• the thickness of the coating material thus formed on the nickel plate was about 100 microns and it weighed about 400 g per m 2 of area.
• the plate and its coating were then heated for 30 minutes in an oven at 750 ° C swept by a stream of hydrogen, so as to cause sintering of the nickel powder.
• a comparison of Examples 2 and 8 further shows that the absence of sintering before heating in an oxidizing atmosphere does not harm the cathodic potential.

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• 1983
  • 1983-06-20 FR FR8310285A patent/FR2547598A1/fr active Pending
• 1984
  • 1984-06-04 CA CA000455759A patent/CA1229573A/fr not_active Expired
  • 1984-06-12 DE DE8484200825T patent/DE3469374D1/de not_active Expired
  • 1984-06-12 AT AT84200825T patent/ATE32530T1/de not_active IP Right Cessation
  • 1984-06-12 EP EP84200825A patent/EP0131978B1/de not_active Expired
  • 1984-06-18 PT PT78754A patent/PT78754B/pt not_active IP Right Cessation
  • 1984-06-19 ES ES533526A patent/ES8504968A1/es not_active Expired
  • 1984-06-19 BR BR8403008A patent/BR8403008A/pt not_active IP Right Cessation
  • 1984-06-19 NO NO842456A patent/NO162524C/no unknown
  • 1984-06-19 US US06/622,222 patent/US4534837A/en not_active Expired - Lifetime
  • 1984-06-20 JP JP59127264A patent/JPH0676670B2/ja not_active Expired - Lifetime
  • 1984-06-20 FI FI842495A patent/FI74742C/fi not_active IP Right Cessation

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