GB1565040A - Method for preparing active anodes for electrochemical processes particularly for manufacture of hydrogen - Google Patents

Abstract

Active anodes are produced which are used in electrochemical processes, in particular the electrolysis of water to produce hydrogen. In this method, the anodes are electroplated with nickel after a preceding cleaning and pretreatment before they are anodically activated in an electroplating bath containing a component which releases sulphur. The novel feature is that the anodes are activated in an electroplating bath which contains 10 - 300 g/l of nickel sulphate hydrate and thiosulphate in an amount equivalent to 10 - 200 g/l of sodium thiosulphate and in which the pH value is kept between 4.5 and 6 and the temperature at 30 - 50 DEG C, the activation being carried out for 3 to 5 hours using an anodic current density of 0.2 - 1 A/dm<2>.

Description

(54) METHOD FOR PREPARING ACTIVE ANODES FOR ELECTROCHEMICAL PROCESSES, PARTICULARLY FOR MANUFACTURE OF HYDROGEN (71) We, NORSK HYDRO A.S., a Norwegian Company, of Bygdy alle 2, Oslo 2, Norway, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a method of preparing active anodes for electrochemical processes, particularly for the electrochemical manufacture of hydrogen. First the anode is given a galvanic nickel coating whereupon it is anodically activated in a bath containing a sulphur liberating component. The anode is conventionally cleaned before being coated, and is also given a pre-treatment in order to obtain good adherence for the nickel coating.

Several methods for preparing active electrodes have been suggested. Japanese patent No.

6021 of 1953 describes a method of galvanic activation in a bath containing 40 g/1 of ammonium thiocyanate. The cathodic polarized electrode surface is sulphurized during the activation, and the anodic polarized electrode surface is nitrated and sulphurized. According to the disclosure of this patent, application of thus activated electrodes as anodes for water electrolysis reduces the cell voltage by 0.03 V compared with unactivated nickel anodes.

However, the patent gives no information about the content of nitrogen and sulphur in the electrode surface, nor is treatment of the electrodes before activation mentioned.

Treatment of electrodes before coating can be performed in several ways. German laid open patent application No. 2.620.589 suggests sandblowing and etching for removal of oxide films in order to obtain a rough surface. It is stated that the etching should preferably be performed in a 10% solution of oxalic acid for at least 3 hours, whereupon the electrodes should be dipped in degassed water. The choice of etching agents is not critical and among several possible etching agents mentioned, are hydrofluoric acid/nitric acid solutions. Special conditions during the etching are not specified.

Further, it is suggested in our Norwegian Patent Specification No. 139355 that cathodes be pre-treated by etching in 10 to 25% nitric acid for 5 to 10 minutes at 35 to 45"C.

Energy consumption is an essential cost factor in electrolytic processes. This is proportional to the operating voltage which inter alia comprises overvoltage at the electrodes. In water electrolysis, hydrogen- and oxygen overvoltages by respectively the cathode and the anode, constitute about 35% of the operating voltage when the electrodes are unactivated.

Overvoltage can be reduced by applying activated electrodes. The reduction of the operating volting of such a cell with for example 0.2V, corresponds to energy savings of about 10%.

We have sought to prepare an anode with a low overvoltage and to give the anodes a coating which is active for a longer period than previously used coatings, and which adheres better to the base material and has better mechanical properties than known coatings.

Based on experience with sulphur-containing coatings on cathodes and hints in the literature claiming that anodes with a sulphur-containing coating may have low overvoltage, an attempt was made to activate anodes by galvanic coating in a bath containing a sulphurliberating component.

Among sulphur-liberating components which were applied during the tests with cathodic coating in galvanic baths were thiocyanic acid and its salts, thiosulphate and thiourea. During these tests, a certain lowering of the overvoltage was achieved when the thus prepared electrodes were applied as anodes during water electrolysis. However, the activity decreased after a rather short period.

The reduction of oxygen overvoltage which according to the Japanese patent, was obtained by using anodically activated electrodes was rather modest. Anodic activation in thioureacontaining electrolyte has been tried, but this activation has not given any substantial reduction of the oxygen overvoltage.

However, we found that anodic treatment in a bath containing thiosulphate enabled preparation of active anodes which gave a substantial and lasting reduction of the oxygen overvoltage by water electrolysis. Further tests proved only that not only is the sulphur liberating component important, but that other parameters have to be kept within certain limits during the activation.

Accordingly the present invention provides a method for the preparation of an active anode which comprises galvanically nickel coating an anode and thereafter treating the anode in a galvanic bath containing from 10 to 300 g/1, preferably 10 to 60 g/1, of a hydrate of nickel sulphate and a thiosulphate in an amount corresponding to 10 to 200 g/ 1 preferably 10 to 40 g/1, of sodium thiosulphate at a pH of from 4.5 to 6, preferably 5 to 5.5., and a temperature of from 30 to 50"C, preferably 40 to 45"C, and applying an anodic current density of from 0.2 to 1 A/dm2, preferably 0.3 to 0.5 A/dm2, for 3 to 5 hours, preferably 4 to 5 hours.

Preliminary tests were carried out in order to assay the importance of the different parameters to the oxygen overvoltage of the anode. The oxygen overvoltage was measured in a water decomposing cell with 25% potassium hydroxide solution as the electrolyte, the temperature was 80"C and the anodic current density was 10A/dm2. Unactivated nickel anodes were used as reference.

Beside the sulphur liberating component, thiosulphate, the activation bath contained a hydrate of nickel sulphate (NiSO4.7H20), an acetate solution was used as a buffer, but other buffers suitable for the actual pH range can be used.

The best coating was obtained with a sodium thiosulphate concentration in the galvanic bath of from 10 to 200 girl. Anodes prepared by these tests had, as shown in the following table, oxygen overvoltages of from 248 to 263 mV.

Table 02-overvoltage in mV Thiosulphate g/l At start-up After 200 days 10 210 260 20 218 248 30 246 258 40 223 255 100 235 255 200 244 263 During other tests the concentration of the hydrate of nickel sulphate was varied between 10 and 300 girl. Active anodes were obtained within the whole of this concentration range.

In further tests the pH, bath temperature and current density were varied while the other conditions were kept constant. These tests show that anodes with good activity could be obtained when the pH was from 4.5 to 6 the bath temperature from 30 to 500C, and the current density between from 0.2 to 1 A/dm2 during the activation.

The invention is further illustrated in the following examples.

Example 1 After degreasing the sandblowing. the anode plates were treated anodically in a 70% H2SO4, and thereupon etched in HCL. Then the anodes were given a coating of galvanic nickel of 5 g/dm2 before activation.

Activation of the anodes was carried out in a galvanic bath with the following composition: NiSO4.7H2O 20 gull Na2S203. 5H20 30 gll CH3COOH 4 g/l NaOH 2,5 g/l pH of the bath 5,5 Temperature 400C Anodic current density 0,3 A/dm2 Duration of electrolysis 5 hours The bath was agitated by blowing air through it. During the treatment the weight of the anode decreased corresponding to 1.5 g/dm2, as the nickel dissolved and the sulphur was taken up in the remaining nickel coating. After activation the anode coating contained 74% nickel and 26% sulphur.

Anodes prepared as stated in this example, were used as anodes in a water decomposing cell with 25% potassium hydroxide solution as the electrolyte. The operating temperature was 80"C and current density was 10 A/dm2. During continuous operation for 4 months, oxygen overvoltage was measured at 240 to 260 mV.

Example 2 The anodes were given a pre-treatment as stated in Example 1 and the activation was carried out in the galvanic bath with the following composition: NiSO4. 7H20 50 gull Na2S203 . 5H20 10 g/l CH3COOH 4 g/l NaOH 2 g/l pH of the bath 5,0 Temperature 450C Anodic current density 0,5 A/dm2 Duration of electrolysis 5 hours During the treatment, the weight of the anodes decreased corresponding to 2 g/dm2. After activation, the anode coating contained 76% nickel and 24% sulphur. By use of the thus prepared anodes in a water decomposing cell as stated in Example 1, oxygen overvoltage was measured to 250 mV.

Anodes prepared according to the method of the present invention have been applied inter alia in technical water electrolysis cells for several years. They have proved that they can retain their activity during this time. The coatings of the anodes have also proved to have better mechanical properties than, for example, anodically activated electrodes treated in a galvanic bath with ammonium thiocyanate.

The oxygen overvoltage of anodes prepared according to the invention are 100 to 150 mV lower than that achieved with unactivated nickel anodes.

Another advantage derived from the present invention is that the cost of activation is relatively low, and the activation can be carried out under easily adjustable and reliable conditions.

Claims (9)

WHAT WE CLAIM IS:

1. A method for the preparation of an active anode which comprises galvanically nickel coating an anode and thereafter treating the anode in a galvanic bath containing from 10 to 300 g/l of a hydrate of nickel sulphate and a thiosulphate in an amount corresponding to 10 to 200 g/l of sodium thiosulphate at a pH of from 4.5 to 6 and a temperature of from 30 to 500C and applying an anodic current density of from 0.2 to 1 A/dmw for 3 to 5 hours.

2. A method as claimed in claim 1, wherein the galvanic bath contains from 10 to 60 g/l of a hydrate of nickel sulphate.

3. A method as claimed in claim 1, wherein the galvanic bath contains a thiosulphate in an amount corresponding to 10 to 40 g/l of sodium thiosulphate.

4. A method as claimed in any one of the preceding claims, wherein the pH of the galvanic bath is from 5 to 5.5.

5. A method as claimed in any one of the preceding claims, wherein the temperature of the galvanic bath is from 40 to 450C.

6. A method as claimed in any one of the preceding claims, wherein the anodic current density is from 0.3 to 0.5 A/dm

7. A method as claimed in any one of the preceding claims wherein the anodic current density is applied for 4 to 5 hours.

8. A method substantially as herein described and with reference to either of the Examples.

9. An active anode whenever prepared by a method as claimed in any one of the preceding claims.