FR2536091A1 - PROCESS FOR THE CATALYTIC ACTIVATION OF ANODES AND CATHODES BY IN SITU FORMATION OF ELECTROCATALYSTS UNDER IDENTICAL CONDITIONS OR ALMOST IDENTICAL TO OPERATIONAL CONDITIONS - Google Patents

Abstract

THE PROCESS FOR THE CATALYTIC ACTIVATION OF ELECTRODES IS CHARACTERIZED BY APPLYING A LAYER OF THE ELECTROCATALYZER ADHESIVE TO THE SURFACE OF THE ELECTRODE, DIRECTLY IN THE ELECTROLYZER AND IN OPERATING CONDITIONS (PRESSURE, TEMPERATURE, COMPOSITION OF ELECTROLYTE, CURRENT DENSITY, ELECTRODE POTENTIAL) WHICH ARE PRACTICALLY IDENTICAL TO THE NORMAL OPERATING CONDITIONS OF THE DESIRED ELECTROLYSIS.

Description

1 If 3609 l-

  The invention relates to a method for activating

catalytic of electrodes.

  Electrode activations are implemented

  in various processes, We will cite as examples of activa-

  cathode coating of electrodes by cathodic deposition

  of a metal with electrocatalytic activity or of a metal-

  lique or an alloy of which a component can also then be dissolved chemically or electrochemically for the purpose of elevation

  of the specific surface, as well as the spray coating

  tion of plasma, by vacuum vaporization or by implantation of ions. Examples of activating anodes include the so-called spray-sinter process. In this case, solutions of the metal salts, the ions of which are necessary for the formation

  catalyst often based on oxides, are applied by spraying

  sation, brush application or soaking on the electrical surface

  trode, then dried and transformed by heating or sintering at controlled temperature and atmosphere, for example in layers of oxides which constitute the catalyst and have a composition

  more or less precisely defined.

  All these methods are characterized in that the implementation of the activation of the electrodes is separated in

  time and space of the electrolysis process.

  Only in exceptional cases is it possible to manufacture, in the formation in particular of oxide catalysts, a catalyst which is thermodynamically and mechanically

  stable in subsequent use in the electro-

  lysis under the conditions previously fixed (potential of electro

  trode, electrolyte composition, etc.).

  The result is often insufficient or not fully sufficient long-term stability of the electrocatalyst,

  with the need for frequent reactivation.

  The method according to the invention is characterized in that a layer of the electrocatalyst is adhered to the surface of the electrode directly in the electrolyser and under operating conditions (pressure, temperature, composition of the electrolyte,

253609 1 '

  current density and / or electrode potential) which are practically identical to the normal operating conditions for the desired electrolysis. The electrolysis is carried out or continued in the process according to the invention under only slightly modified conditions, with addition to the electrolyte of metal ions, for example of transition metal ions or other cations , or

  of anions or molecules which are necessary for the formatio chi-

  mique or electrochemical of the desired electrocatalyst, in the form of salts or complexes or in the form of more or less soluble compounds, at the lowest possible concentration and in the minimum amount which is just necessary for the formation of an active catalyst layer In For this purpose, it is often possible to carry out the activation in relatively short durations, without appreciably modifying the composition of the electrolyte By this anodic in situ formation (for example, of catalysts with oxides)

  or cathodic (for example, metallic catalysts) of electro-

  catalysts, the components of the catalyst reach the

  electrodes by means of electrolytes (depending on the purchase: disso-

  homogeneous lution, transport of substance on the electrode, formation by cathodic or anodic formation and deposition of the electrocatalyst), which makes an additional mechanical operating stage superfluous,

  such as spraying.

  The in situ application according to the invention makes it possible to obtain that the electrocatalyst is precipitated on the electrode according to the training conditions which practically correspond to the operating conditions of the electrolysis, in a form (composition

  chemical, modification, atomic, microscopic and macro structure

  scopic) in which it has the greatest possible chemical and mechanical stability under the desired operating conditions. In addition to the formation of the most thermodynamically stable phase, there is indeed the deposition of a structure.

  mechanically very stable, because the precipitation of electrocataly-

  occurs under mechanical conditions (for example,

  tion of bubbles on the electrodes producing a release of gas or high relative movement of the electrolyte against the electrode by pumping or siphon effect of the released gases), which correspond

  also practically under the practical conditions of electrolysis.

  By the two conditions a) thermodynamic stability of the electrocatalyst, b) mechanical stability of the electrocatalyst, under the desired electrolysis conditions, which are possible by the in situ formation of electrocatalysts, there is the greatest

  possible guarantee of long-term electrocataly-

  against modification and deactivation of chemicals and electro-

chemical and mechanical erosion.

  In addition, the method according to the invention allows renewal of the electrode activation at any time after

  possible deactivation without interruption of the electro-

lysis in progress.

  The following examples illustrate the invention without

however limit its scope.

EXAMPLE 1

  Simultaneous in situ activation of anodes and cathodes for electricity

alkaline water trolysis.

  In a 4 cm microcell with arrangement

  "sandwich" of the anode, the diaphragm and the cathode, we electro-

  lyses water at 900 C in KOH at 50% by weight Current density

  nominal is 1 A / cm 2 (104 A / M 2).

  After 200 h of operation, the cathode has an overvoltage of -350 m V, the anode has an overvoltage of + 350 m V.

  To the electrolyte (1 liter, KOH at 50% by weight), a solution is added.

  tik of cobalt nitrate in an amount which corresponds to 65 mg of cobalt Cobalt dissolves immediately in the electrolyte in the form of a cobalt hydroxy-complex It colored in deep blue, the color of which disappears again in a few minutes After

  min, the overvoltage has improved by 20 m V, while the am-

  improvement of the anode potential is 50 m V During the 300 h

  following operating conditions, no new increase is observed.

  anodic and cathodic overvoltage.

EXAMPLE 2

  Activation of the anode for the alkaline electrolysis of water by

  in situ deposition of a mixed cobalt-manganese oxide on the anode.

  In an experiment, as described in Example 1, after 350 hours of operation, is added to 1 liter of alkaline electrolyte cobalt nitrate and manganese sulfate in an amount which corresponds to 20 mg of cobalt and 20 mg of manganese The salts are dissolved in 2 l of water and added to the electrolyte in 30 h to measure the water loss, the concentration of cobalt or manganese in the alkaline electrolyte never exceeding mol / l In 40 h, the anode overvoltage decreases by 90 m V and

  remains unchanged for the next 200 h.

EXAMPLE 3

  Activation of the cathode for the alkaline electrolysis of water by

  in situ deposition of finely divided iron whiskers on the cathode.

  In an experiment as described in Example 1, we

  add 1 g of Fe 203 to the liter of electrolyte, which has a solution

  bility of about 10-3 mol / l at 90 C in KOH at 50% by weight After h, it deposited on the cathode of fine needles of iron and

  the cathode overvoltage dropped by 100 m V (to -250 m V).

EXAMPLE 4

  Activation of the cathode for the alkaline electrolysis of water by in situ deposition of zinc on a cathode already covered with nickel

of finely divided Raney.

  In an experiment as described in Example 1, a perforated nickel sheet is used as the cathode on which 115 mg per cm 2 of finely divided Raney nickel has been deposited. After h operation, the cathode overvoltage (900 C, KOH at 50% by weight, 1 A / cm 2) is -225 m V. By addition of Zn O (which dissolves immediately in the form of zinc hydroxocomplex) in an amount which corresponds to 0.01 mol / l of zinc in the alkaline electrolyte, the cathodic overvoltage, after temporary increase, falls again by 60 m V

  in 20 h and remains unchanged for the following 50 h.

  253609 t 'It is understood that the invention is not limited

  to the preferred embodiments described above by way of illustration.

  that those skilled in the art can make various modifications thereto.

  tions and various changes without however departing from the scope and spirit of the invention.

Claims (1)

R E V E N D I C A T I O N   Process for the catalytic activation of electrodes,   characterized in that a layer of electrocataly- is applied   adhered to the surface of the electrode, directly in the electro-   lyser and under operating conditions (pressure, temperature,   electrolyte composition, current density, electric potential   trode) which are practically identical to the operating conditions   normal electrolysis sought.