FI60725B - FOERFARANDE FOER FRAMSTAELLNING AV AKTIVA ANODER FOER ELECTROCHEMICAL PROCESSER SPECIFICALLY FORWARDING VENTILATION - Google Patents

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Patent ceddelat (51) Kv.ik? / «Nt.a.3 c 25 B 11/00, C 25 D 5/12 FINLAND - FINLAND (21) - P * Mn * n * eknlng 780789 (22) Hakamlipilvi ~ AmBknlngtdag 13. 0 3 · 7 8 (23) AlkupilvS - Glltlghttsdag 13.03.78 (41) Tulhit JulklMkft - Bllvlt offantllg 01.10.78

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Norway-Norway (NO) 77H21 (71) Norsk Hydro a.s, Bygd ^ y alli 2, Oslo 2, Norway-Norway (NO) (72) Johan B. Holte, Blommenholm, Olav A. Eide, Rjukan,

Norja-Norge (NO) (7M Oy Kolster Ah (5M Method for the manufacture of active anodes for electrochemical processes, in particular for water dispersal) - For the production of active anodes for electrochemical processes

The present invention relates to a process for the preparation of active anodes for electrochemical processes, in particular for water decomposition. The anodes are first provided with a layer of galvanic nickel and then anodically activated in a bath of sulfur donating compound. For coating, the anode is conventionally cleaned and pretreated to achieve good adhesion of the nickel coating.

Several methods for activating electrodes are already known. Japanese Patent No. 6021 of 1953 discloses a method in which activation is performed galvanically in a bath containing 40 g / l of ammonium thiocyanate. Upon activation, the cathodically polarized electrode surface is sulfurized and the anodically polarized electrode surface is nitrided or sulfurized. When the electrodes thus activated are used as anodes in water electrolysis, the cell voltage shows a decrease of 0.03 V compared to non-activated nickel electrodes. The patent does not state the nitrogen or sulfur content of the electrode surface, nor does it mention the pretreatment of the electrodes for activation.

Electrode pretreatment \ arten can be done in several ways. Thus, German Application No. 2,620,589 discloses sandblasting and etching to remove an oxide film and obtain a roughened surface. It is recommended that the etching be performed in a 10% oxalic acid solution for at least 3 hours, after which the electrodes are immersed in deionized water. It is mentioned that the choice of etchant is not critical and of the several possible etchants, hydrofluoric acid / nitric acid solutions are mentioned without defining the conditions during etching.

Furthermore, the pretreatment of the cathodes by etching in 10-25% nitric acid for 5-10 minutes at a temperature of 35-45 ° C is known from the applicant's Finnish patent application No. 780,414.

Energy consumption is an essential cost factor in electrolysis processes. This is proportional to the operating voltage, which includes, but is not limited to, overvoltages at the electrodes. In water electrolysis, hydrogen and oxygen overvoltages with respect to the cathode and anode together account for about 35% of the operating voltage when the electrodes are inactivated. Overvoltages can be reduced by using activated electrodes. Reducing the operating voltage of the water dispensing cell, for example by 0.2 V, corresponds to an energy saving of about 10%.

The purpose of the research and experiments which led to the manufacturing method according to the present invention was to prepare a low overvoltage pretreated anode. Another object was to provide the anode with a coating that is active for a longer period of time than the currently used coating, which adheres better to the substrate material and has better mechanical properties than the known coating.

Experience with sulfur-coated cathodes and references in the literature that low-voltage surges to anodes with sulfur-coated coatings led to an experiment to activate the anodes by coating them in a bath containing a galvanic sulfur donor.

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Examples of sulfur donors that can be used in experiments for coating the cathode in a galvanic bath include thiocyanic acid and its salts, thiosulfate and thiourea. In these experiments, a certain decrease in overvoltage was achieved when the electrodes thus prepared were used as anodes in water electrolysis, but the activity decreased after a relatively short time.

The reduction in oxygen overvoltage obtained by the use of anodically activated electrodes according to the Japanese patent is negligible. Anodic activation has been attempted in a thiourea-containing electrolyte, but a significant reduction in oxygen overvoltage has not been achieved.

Contrary to the above experiences, it surprisingly proved that the anodic treatment in a thiosulfate bath made it possible to produce active anodes with a considerable and durable reduction in the voltage across the oxygen in water electrolysis. Additional experiments showed that the sulfur-donating substance is not only important, but other parameters in the activation must also be kept within certain limits.

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

To investigate the effect of different parameters on oxygen overvoltage, some preliminary experiments were performed. Oxygen overvoltage was measured in a water dispersion cell using 25% potassium liquor as the electrolyte solution at a temperature of 80 ° C and an anodic current density of 2 10 A / dm. Inactivated nickel anodes were used for comparison.

In addition to the sulfur donating compound, the activation bath contains nickel sulfate hydrate (NiSO 2 .7H 2 O), a buffer, and possibly other conductivity-enhancing salts. Acetate buffer was used as the buffer, but other buffers are also suitable if they can be used in the pH range in question.

With respect to the thiosulfate content in the galvanic bath, experiments showed that good coatings were obtained with a thiosulfate ion content of 10-200 g / l. KUten, the following table shows that the oxygen overvoltage at the anodes prepared in these experiments was 248-263 mV.

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Table

02 ~ overvoltage mV

Thiosulphate g / l Initially after 200 days 10 210 260 20 218 248 30 246 258 40 223 255 100 235 255 200 244 263

In other experiments, nickel sulfate hydrate concentrations ranged from 10 to 300 g / L. Using this entire concentration range, active anodes were obtained.

In the following experiments, the pB value, bath temperature, and current density were varied under otherwise constant conditions. These experiments showed that anodes with good activity can be achieved if the pH is maintained between 4.5-6, the bath temperature between 30-50 ° C and the current density between 0.2-1 A / dm 2 during activation.

The following examples show how the anodes can be activated according to the invention.

Example 1

After possible degreasing and sandblasting, the anode plates were anodized in a 70% I 2 SO 4 bath and then pickled in hydrochloric acid. For activation itself, the anodes were galvanically coated with nickel to a corresponding thickness of 5 g / dm.

Activation of the anodes was performed galvanically in a bath having the following composition:

NiSO4.7H2O 20 g / l

Na 2 S 2 O 3 .5H 2 O 30 g / l CH 3 OH 4 g / l

NaOH 2.5 g / l

Bath pH 5.5

Temperature 40 ° C

Anodic current density 0.3 A / dm Electrolysis time used 5 hours 2 5 60725

Aeration was used as a movement in the bath. During the treatment, the weight of the anodes decreased by 1.5 g / dm because nickel dissolved at the same time as the sulfur was transferred to the residual nickel layer. After activation, the anode coating contained 74% nickel and 26% sulfur.

The anodes prepared according to this example were used as anodes in a water dispersing cell containing 25% potassium liquor as an electrolyte. The operating temperature was 80 C and the current density was 10 A / dm. The anodes were used continuously for 4 months, after which the oxygen overvoltage was measured to be 240-260 mV.

Example 2

The anodes were pretreated as in Example 1 and the activation was performed in a galvanic bath having the following composition:

NiSO4.7H2O 50 g / l

Na 2 S 2 O 3 * 5H 2 O 10 g // 1 CH 3 COOH 4 g / l

Bath pH 5.0

Temperature 45 ° C

2

Anodic current density 0.5 A / dm Electrolysis time used 5 hours At the end of the treatment, the weight of the anodes had decreased to an amount corresponding to 2 g / dm. After activation, the anode coating contained 76% nickel and 24% sulfur. Using the anodes thus prepared as described in Example 1, the oxygen scattering cells were charged with an oxygen overvoltage of 250 mV.

The anodes made according to the present invention have been used in technical water electrolysis cells, among others, for several years.

They have been shown to remain active throughout use. The mechanical properties of the anode coatings have also proven to be better than, for example, anodically activated electrodes treated in a galvanic bath with ammonium thiocyanate. The oxygen overvoltages of the anodes made according to the invention are 100-150 mV lower than those of the non-activated nickel anodes.

A further advantage of the present invention is that the activation costs are relatively low and the activation can be performed under easily controlled and reliable conditions.