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 invention relates to a method for the electrolytic production of hydrogen on a cathode, in particular in an alkaline solution.
• 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.
• metal ions with low hydrogen overvoltage are introduced into the catholyte and the plating of these ions, in the metallic state in situ on the cathode, is carried out for 1 'electrolysis.
• the cathode can be made of copper, steel or any other suitable material; copper cathodes are however especially recommended, in association with metal plating ions selected from iron, nickel, chromium, molybdenum and vanadium.
• Copper cathodes used in accordance with the preferred embodiment of this known method however, also have the disadvantage of undergoing progressive corrosion during electrolysis.
• the overvoltage on hydrogen evolution on copper cathodes is generally high and experience has shown that, despite the gain achieved on the overvoltage by the addition of plating ions to the electrolysis bath, the overall voltage remained abnormally high.
• Electrodes intended for electrochemical oxidation processes the active surface of which has a dendritic structure. This is how, in document DE-A-2037968, an electrolysis cell is described, a positive electrode of which comprises nickel dendrites.
• Document FR-A-1427244 describes a porous electrode comprising, on a nickel substrate, a dendritic coating of active metal, this electrode being intended to be the seat of an electrochemical oxidation reaction in a fuel cell .
• the invention aims to provide a method for the electrolytic production of hydrogen, which allows a gain on the electrolysis voltage, significantly greater than the gains which can be obtained with the cathodes and the known methods described above, and which does not have the disadvantages.
• the invention therefore relates to a process for the electrolytic production of hydrogen, on a cathode, according to which an electrode is used for the cathode, the active surface of which comprises a nickel substrate and a coating layer of nickel dendrites. or cobalt.
• the dendrites of the coating layer of the cathode are single crystals of small dimensions, having a branched structure, very aerated, resulting from the interruption of the growth of crystalline germs (A. DE SY and J. VIDTS, "'Treaty of structural metallurgy", 1962, NICl and DUNOD, pages 38 and 39).
• the nickel substrate can have any shape suitable for the destination of the cathode. It can be, for example, a solid or perforated plate, a wire, a trellis or a stack of beads. It can have a smooth surface state; a rough surface finish is preferred, however, since it generally lends itself to better adhesion of the dendrite layer.
• the nickel substrate is preferably constituted by a nickel film applied to a support made of a material which is better conductive of electricity than nickel, for example copper or in aluminium. In this embodiment of the invention, the nickel film must be impermeable to electrolytes, when the material used for the underlying support is likely to be degraded on contact with these electrolytes.
• the nickel film can be either impermeable or permeable, an impermeable film being however preferable in all cases.
• the thickness to be given to the nickel film depends on various parameters, and in particular on the nature and the surface condition of the underlying support, and it must be at least sufficient to resist being torn off under the effect of thermal expansion of the support or by erosion on contact with the electrolyte. In practice, in the case where the support is made of copper, good results have been obtained with nickel films with a thickness of between 5 and 100 microns, more especially between 10 and 75 microns.
• the dendrite coating layer be substantially uniform on the nickel substrate and in an amount at least equal to 0.0005 g per dm 2 of area of the substrate.
• the maximum admissible value for the thickness of the layer of dendrites depends on various factors and it is fixed in particular by the advantage of preserving a homogeneous active surface on the electrode and of avoiding a modification of the geometry of the cathode.
• a layer of dendrites of exaggerated thickness may indeed be torn locally from the substrate under the action of turbulence created by the release of hydrogen; in the case of openwork cathodes, it also risks causing poorly controlled obstruction of the cathode openings.
• the coating layer of dendrites does not exceed 25 g and preferably 15 g per dm 2 of area of the substrate.
• Cathodes which have been found to be particularly advantageous are those in which the dendrite coating layer has a weight of between 0.001 and 10 g per dm 2 of area of the substrate, the values of between 0.002 and 5 g and especially those at least equal. at 1 g per dm 2 of substrate area generally leading to the best results.
• the dendrite coating layer of the cathode can be produced by any suitable means.
• a cathode is used whose coating layer of dendrites is an electrolytic deposit of nickel or cobalt, which has been produced in an electrolyte containing nickel or cobalt ions, where the cathode is the seat of a proton reduction.
• the electrolyte is preferably an aqueous electrolyte, more particularly water, or an aqueous solution of alkali metal chloride or hydroxide, containing nickel or cobalt ions.
• aqueous solutions of alkali metal hydroxide in particular sodium hydroxide, containing from 20 to 35% by weight of alkali metal hydroxide and, preferably, approximately 30% by weight of alkali metal hydroxide.
• the cathode is brought to an adequate potential to be the seat of a reduction of protons.
• the choice of cathode potential which should be imposed on the cathode depends on various parameters and in particular on the nature of the nickel layer (in particular its surface state, the state of its crystal lattice, the possible presence of impurities and, where appropriate, its porosity), the choice of electrolyte used and its concentration. It can be determined in each particular case by routine laboratory work.
• the cathodic potential must be fixed between -1.30 and -2 V, the most often between -1.55 and -1.65 V compared to a reference calomel electrode, with saturated potassium chloride solution.
• the quantity of nickel or cobalt ions to be used in the electrolyte depends on various parameters, in particular on the geometry of the cathode, the thickness or the weight desired for the coating layer of dendrites, the surface area nickel substrate, the nature of the electrolyte and its volume. As a general rule, it can be easily determined in each particular case by routine laboratory work.
• the nickel or cobalt ions can be introduced into the electrolyte all at once or either continuously or intermittently. They can be introduced into the electrolyte by any suitable means, for example by dissolution of a soluble nickel or cobalt compound, such as nickel or cobalt chloride, or by controlled corrosion of a structure (for example a wire, plate or lattice) made of nickel, cobalt or an alloy or compound of these metals, brought to an anode potential regulated in the electrolyte.
• An interesting means consists in dispersing in the electrolyte a powder of nickel or cobalt, or of a compound or alloy of these metals, the oxides being preferred. In this embodiment of the invention, it is desirable to use a powder as fine as possible.
• powders are used in which the average particle diameter is less than 50 microns and preferably does not exceed 35 microns.
• Powders which are generally well suited are those in which the average particle diameter is between 1 and 32 microns, the best results having been obtained with powders whose average particle diameter is less than 25 microns.
• a cathode is used, the active surface of which comprises, between the nickel substrate and the dendrite coating layer, a porous intermediate layer, intended to reinforce the attachment of the dendrites to the substrate or to improve the electrochemical properties of the cathode.
• the porous intermediate layer is advantageously made of an electrically conductive material, having good electrochemical properties, this material possibly being, for example, a platinum group metal or an oxidized metallic compound of the spinel type, such as those described in Patent EP-A-8476 (SOLVAY & Cie).
• the porous intermediate layer is made of platinum or is obtained by spraying a nickel oxide powder in a plasma jet.
• the cathode used in the method according to the invention can be prefabricated.
• the dendrite coating layer is formed in situ on the cathode mounted in the electrolysis cell for which it is intended.
• the cathode provided with the nickel substrate and possibly with an intermediate layer. It may also be necessary to periodically regenerate the dendrite coating layer to account for a progressive destruction thereof, for example under the effect of erosion caused by the alkaline solution or the hydrogen gas produced.
• the frequency and extent of the regenerations depend on the speed at which the dendrite coating layer is eroded or torn from the cathode; this speed itself depends on a large number of parameters, including in particular the nature of the nickel substrate, the possible presence of a porous intermediate layer between the substrate and the dendrite coating layer, the turbulence and the viscosity of the alkaline solution and the flow of hydrogen produced.
• the frequency and extent of the regenerations must therefore be determined in each particular case, which can be easily done by routine laboratory work.
• the process according to the invention finds a particularly advantageous application for the electrolytic production of hydrogen in an alkaline solution, in particular in cells with a permeable diaphragm or a membrane with selective permeability for the electrolysis of sodium chloride brines, such as those described, by way of example, in patents FR-A-2,164,623, 2,223,083, 2,230,411, 2,248,335 and 2,387,897 (SOLVAY & Cie).
• 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 cathode according to the invention the active surface of which consisted of a nickel substrate and a coating layer of nickel dendrites.
• a temporary cathode formed of a nickel disc we first placed in the cell, a temporary cathode formed of a nickel disc; to form the layer of nickel dendrites on the disk used as substrate, the anode chamber and the cathode chamber were supplied respectively with the aqueous solution of sodium chloride and the dilute solution of sodium hydroxide and the electrolysis was started with the nickel disc serving as cathode, under the nominal current density of 3 kA / m 2.
• the electrolysis voltage measured between the anode and the cathode, stabilized at 3.65 V.
• Example 2 The procedure was as in Example 1, using an aqueous solution of nickel sulfocyanide in place of the nickel chloride solution.
• the electrolysis voltage stabilized at 3.63 V.
• the electrolysis voltage fell to 3.38 V, which corresponds to a gain of 250 mV compared to the initial voltage.
• a cathode according to the invention was used, the active surface of which consisted of a nickel substrate and a coating layer of cobalt dendrites.
• the procedure was as in Example 1, except that the aqueous solution of nickel chloride was replaced by an aqueous solution of cobalt acetate, in a quantity adjusted so that it corresponded to a addition of 1 g of cobalt.
• the electrolysis voltage was fixed at 3.70 V. After the formation of a coating layer of cobalt dendrites on the nickel disc, consecutively on addition of the cobalt acetate solution to the catholyte, the electrolysis voltage fell to 3.46 V, which corresponds to a voltage gain of 240 mV.
• Example 3 The procedure was as in Example 3, the only differences being that the cobalt acetate solution was replaced by an aqueous cobalt chloride solution and that the latter was added to the catholyte in a controlled amount. so that it corresponds to an addition of 2 mg of cobalt.
• the electrolysis voltage was established at 3.67 V.
• the electrolysis voltage dropped to 3.58 V, which corresponds to a gain of 90 mV on the original voltage.
• Example 4 The test of Example 4 was continued, with an additional addition of cobalt chloride solution in a quantity adjusted so that it corresponds to an additional addition of 2 mg of cobalt.
• the electrolysis voltage fell to 3.46 V, resulting in a total gain of 210 mV compared to the original voltage.
• Cobalt oxide powder had an average particle diameter of less than 20 microns.
• the electrolysis voltage was established at 3.68 V.
• the cobalt oxide powder was then dispersed in the catholyte, in two fractions of equal weight each corresponding to 1 g of cobalt.
• the electrolysis voltage went successively to 3.44 V and then to 3.36 V, thus resulting in a gain of 320 mV compared to the original voltage.
• a cathode according to the invention was used, the active surface of which consisted of a nickel substrate and a coating layer of nickel dendrites.
• a provisional cathode consisting of a mild steel disc carrying an impermeable coating of 30 microns of nickel, obtained by electrolytic deposition, was first placed in the cell. This coating is intended to constitute the above-mentioned substrate. .
• a layer of nickel dendrites was then deposited on the substrate and, for this purpose, a nickel oxide powder was dispersed in the catholyte, in a quantity adjusted so that it corresponds to 4 g of nickel.
• the particle size of the nickel oxide powder was characterized by an average particle diameter of less than 20 microns; it was added to the catholyte in four successive fractions of equal weight.
• the electrolysis conditions are given in table 1.
• the total gain on the electrolysis voltage is approximately 300 mV.

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• 1982
  • 1982-12-17 FR FR8221390A patent/FR2538005B1/fr not_active Expired
• 1983
  • 1983-12-13 DE DE8383201758T patent/DE3374950D1/de not_active Expired
  • 1983-12-13 AT AT83201758T patent/ATE31431T1/de not_active IP Right Cessation
  • 1983-12-13 EP EP83201758A patent/EP0113931B1/fr not_active Expired
  • 1983-12-15 CA CA000443351A patent/CA1247047A/fr not_active Expired
  • 1983-12-15 US US06/561,726 patent/US4555317A/en not_active Expired - Fee Related
  • 1983-12-16 NO NO834654A patent/NO159295C/no unknown
  • 1983-12-16 FI FI834649A patent/FI73247C/fi not_active IP Right Cessation
  • 1983-12-16 ES ES528101A patent/ES8406570A1/es not_active Expired
  • 1983-12-16 JP JP58237744A patent/JPS59166689A/ja active Pending
  • 1983-12-16 PT PT77833A patent/PT77833B/pt not_active IP Right Cessation
  • 1983-12-16 BR BR8306939A patent/BR8306939A/pt not_active IP Right Cessation

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