SE424008B - PROCEDURE FOR PREPARING ACTIVE ANODES FOR USING AVID ELECTROCHEMICAL SPLITING OF WATER - Google Patents

Description

U0 7802991-s 2 search 2 620 589 sandblasting and etching for removing oxide film and for achieving a raw surface. The etching is recommended to be carried out in a 10% oxalic acid solution for at least 3 hours, after which the electrodes are dipped in degassed water. It is stated that the choice of etchant is not critical and among several possible etchants, hydrofluoric acid / nitric acid solutions are mentioned without specifying the conditions during the etching.

Furthermore, it is known from Norwegian patent specification 139355 to pretreat cathodes by etching in 10 to 25% nitric acid for 5-10 minutes at 35-H5 ° C. _ Energy consumption is a significant cost factor in electrolysis processes. It is proportional to the operating voltage, which includes the overvoltages at the electrodes. In water electrolysis, the hydrogen and oxygen overvoltages at the cathode and the ando together together constitute about 35% of the operating voltage, when the electrical overvoltage can be reduced by use. of activated electrodes. operating voltage with, for example, 0.2 V corresponds to an energy saving of approximately 10%. The object of the investigations and experiments which led to the method of the present invention was to obtain an improved anode with low overvoltage. A further object was to give the anode a coating which is activated for a longer time than previously used coatings and which adheres better to the base material and which has better mechanical properties than the known coatings. Based on experience with sulfur-containing coatings on cathodes and indications in the literature that anodes with sulfur-containing coatings may have low overvoltages, attempts were made to activate anodes by galvanic application of coatings in baths containing a sulfur-releasing component.

Examples of sulfur-releasing components used in cathodic coating experiments in galvanic baths are thiocyanic acid and its salts, thiosulfate and thiourea. a certain reduction of the overvoltage was achieved when electrodes thus produced were used as anodes during water electrolysis, but the activity decreased after a relatively short time.

In these experiments, the reduction of the oxygen surge which, according to the Japanese patent specification, was achieved by using anodically activated electrodes is moderate. Anodic activation in thiourea-containing electrolyte has been attempted, but has also not produced a significant reduction in oxygen overvoltage. "In contrast to these experiences, it was surprisingly found that by anodic treatment in baths containing thiosulphate it was possible to produce active anodes which gave a significant and lasting reduction in the oxygen overvoltage in the water electrolysis.

Further experiments showed that it is not only the sulfur-releasing component that is important, but that the other parameters during activation must also be kept within certain limits.

Activation of anodes according to the present invention is performed as stated in the claims.

To investigate the significance of the various parameters for the anode's oxygen overvoltage, some initial experiments were performed. The oxygen overvoltage is measured in a water-splitting cell with 25% potassium hydroxide solution as electrolyte solution, using a temperature of 80 ° C and an anodic current density of 10 A / dmz. Inactivated nickel anodes were used as a reference.

In addition to the = sulfur-releasing component, thiosulphate, the activation bath contains nickel sulphate hydrate (NiSO 4 .7H 2 O), buffering agent and any additional conductivity-promoting salts. Acetate buffer has been used as a buffering agent, but other buffering agents suitable for the current pH range can also be used.

As for the concentration of thiosulfate in the galvanic bath, experiments showed that good coatings were obtained at sodium thiosulfate concentrations of 10-200 g / l. As shown in the following table, anodes produced in these experiments gave oxygen surges of 248-263 mV.

Table 02 overvoltage in mv After 2O0¿g fl g1 Thiosulphate, g / 1 At start-up 210 260, 218 2148 in 2116 258 no 223 255 100 235 255 200 2114 263 In other experiments the concentration of nickel sulphate hydrate was varied between 10 and 300 g / l . this concentration range.

Active anodes were obtained within a total of 55 g. Anodic current density 7802991 5 out In further experiments, the pH, bath temperature and current density were varied under otherwise constant conditions. These experiments showed that anodes with good activity could be obtained, when the pH was kept between U, 5 and 6, the bath temperature between 30 and 50 ° C and the current density between 0.2 and 1 A / dmz during activation.

The following example shows how the anode can be activated according to the invention.

Example 1 The anode plates were treated anodically after a possible degreasing and sandblasting in a 70% HZSOÄ solution and then pickled in H01. The anodes were then given a galvanic nickel coating of 5 g / dm 2 before the activation itself.

The activation of the anodes was performed in a galvanic bath with the following composition: NiSOu.7H2O 20 g / l 'Na2S2O3.5H2O 30 g / l' CH3COOH 'Ä g / l NaOH 2.5 g / l Bath pH' 5.5 Temperature UOOC 0 , 3 A / dmz Use electrolytic time 5 h Stirring was used in the form of air purge in the bath. During the course of the treatment, the anode decreased in weight corresponding to 1.5 g / dmz, by releasing nickel at the same time as sulfur is taken up in the remaining- After activation, the anode coating contained During the nickel coating._ 7ü% nickel and 26% sulfur.

Anodes prepared in the manner set forth in this example are used as anodes in a water-splitting cell with 25% potassium hydroxide solution as electrolyte. The operating temperature was 80 ° C and the current density was 10 A / dmz. During a continuous operating time of 4 months, oxygen overvoltages of ZHO-260 mV were measured.

Example 2 - 7 - The anodes were pretreated as indicated in Example 1, and the activation was carried out in a galvanic bath having the following composition: nisou.7H2O 50 g / l Na¿s2o3.5H2o io g / l cH3cooH 4 g / l NaOH 2 g / 1 7802991-5 Bath pH 5.0 Temperature '45 ° C Anoanic current: anna: 0, 5 A / dm? Use electrolytic time 5 h During the course of the treatment, the anode decreased in weight corresponding to 2 g / dmz. After activation, the anode coating 76 contained 2 nickel and 24% sulfur. When using anodes thus prepared in water-splitting cells, as indicated in Example 1, oxygen overvoltages of 250 mV were measured.

Anodes produced according to the present invention have been used, inter alia, in technical water electrolysis cells for several years. They have been shown to retain activity throughout their life. The coating of the anodes has also been shown to have better mechanical properties than, for example, anodically activated electrodes, which have been treated in galvanic baths with ammonium thiocyanate.

Oxygen voltages to anodes produced according to the invention are 100-150 mV lower than for inactivated nickel anodes.

A further advantage of the present invention is that the costs of activation are relatively low and that it can be carried out under easily controllable and reliable conditions.

Claims (2)

7802991-5 PATENT CLAIMS A process for the preparation of active anodes for use in electrochemical cleavage of water, wherein the anodes after prior purification and pretreatment are given a galvanic nickel coating before being activated anodically in a galvanic had containing a sulfur-releasing component, characterized in that the anodes are activated in a galvanic bid containing 10-300 g / l nickel sulphate hydrate and thiosulphate in an amount corresponding to 10-200 g / l sodium thiosulphate, the pH being kept at 4.5-6, the temperature at 30-SOOC and the activation being carried out using of an anodic current density of 0.2-1 A / dmz for 3-5 hours. Process according to Claim 1, characterized in that the activation of the anode is carried out in a bath of 10 to 60 g / l of nickel sulphate hydrate and thiosulphate in an amount corresponding to 10 to 40 g / l of sodium thiosulphate, the pH being maintained S-5.5, the temperature of 40-45 ° C and the activation are carried out using an anodic current density of 0.3-0.5 A / dmz for 4-5 hours. REQUIRED PUBLICATIONS: