Method of making an active cathode

Description

[0001] The present invention relates generally to a low hydrogen overvoltage cathode for the electrolysis of aqueous solutions, and more particularly to a method of making a cathode capable of electrolyzing an aqueous solution of alkali metal halides or hydroxides at low hydrogen over-voltage.

[0002] When an aqueous solution such as those of alkali metal halides or hydroxides is electrolyzed by diaphragm, ion exchange membrane and other processes, hydrogen is evolved at the cathode. So far, cathodes have been formed of a material composed mainly of mild steel. However, a serious problem with mild steelis its high hydrogen overvoltage. For this reason it has been proposed to use various electrodes of low hydrogen overvoltage as an active cathode. The term "active cathode" used herein is understood to refer to a cathode showing hydrogen overvoltage lower than those of conventionally used materials composed mainly of mild steel. For such an active cathode, nickel, cobalt, and elements of the platinum group which may be used alone or in admixture, or oxides thereof have been used as electrode catalyst substances. These electrode catalyst substances have been formed on a cathode substrate by suitable processes such as electroplating, electroless plating, dispersion electroplating, spraying, and dipping.

[0003] An active cathode is now required to have low hydrogen overvoltage and be protected against any degradation when an electrolytic cell is not in operation or it is removed from an electrolytic cell for the purpose of replacing ion exchange membranes with new ones, and for other purposes. It is also required that even when the active cathode is used in an ion exchange membrane electrolytic cell while it comes into close contact with an ion exchange membrane, any contamination of the ion exchange membrane by the electrode catalyst substance be prevented. In addition, the active cathode is required to be low in production cost.

[0004] Known among active cathodes heretofore proposed in the art are an active cathode formed of Raney nickel by the co-plating of nickel and aluminum (see US-A-4170536, JP-B-85015712,JP-B-86036590, and US-A-4536259), and an active cathode co-plated with Raney nickel and a hydrogen occluded alloy (see JP-B-86012032). These electrodes, albeit having an advantage of being low in terms of hydrogen overvoltage, are disadvantageous in that the co-plated cathode substrates cannot firmly be welded to the frame of an electrolytic chamber because aluminum exists in the cathode material. Another disadvantage is that when the cathode substrates are welded to the frame of an electrolytic chamber after rid of aluminum by an alkali treatment, the electrode catalyst degrades because of undergoing oxidation in the air, thus failing to obtain any desired low hydrogen overvoltage.

[0005] An object of the present invention is to provide an active cathode which is substantially free from such problems as arising in the case of conventional co-plated active cathodes containing aluminum, and in which any degradation of the electrode catalyst due to oxidation in the air is substantially avoided.

[0006] This object has been achieved by a method of making an active cathode by forming an nickel aluminum alloy-containing film on an electrically conductive substrate, treating the substrate with alkali to allow aluminum to dissolve therein, and then dipping the substrate in an oxidizing agent solution, so that the active cathode is stabilized.

[0007] Preferably, the oxidizing agent solution used herein is a solution containing at least one member selected from the group consisting of peroxides, percarbonates, and perborates.

[0008] According to the present invention, the nickel aluminum alloy-containing film is formed on the cathode substrate by plating. Then, the cathode substrate is treated with alkali so that aluminum can dissolve therein for its removal. Finally, the cathode substrate is dipped in the oxidizing agent solution to coat the surface of the electrode of high activity with a thin yet dense stabilized film, so that the electrode can remain stabilized during storage or upon attached to an electrode frame.

[0009] A metallic substrate, when plated with a nickel aluminum alloy-containing film and treated with alkali so that aluminum can dissolve therein for its removal, is used as an electrode of high activity, but reacts rapidly with atmospheric oxygen in a dry state, resulting in a drop of its activity. According to the present invention, it has now been found that any degradation of the electrode can be prevented without causing a substantial drop of electrode performance by forming a thin yet dense stabilized film on the surface of the electrode.

[0010] The protective film to be formed on the surface of the electrode so as to stabilize the electrode is preferably a nickel hydroxide film. This may be formed by treating the nickel aluminum composite alloy with alkali to allow aluminum to dissolve therein, and dipping it in the oxidizing agent solution.

[0011] The active cathode according to the present invention is made by forming a film comprising nickel and aluminum on the substrate, and then coating the film with an electrode catalyst called Raney nickel from which aluminum has been removed by dissolution in alkali and which has a large area and is composed mainly of nickel. As known so far in the art, the stabilization of Raney nickel used for catalysts, etc., is carried out by a dry stabilization process wherein Raney nickel is allowed to stand alone in nitrogen or carbon dioxide gas containing a few % of oxygen for about seven days so that nickel oxide is gradually formed on the surface of Raney nickel. However, this process, because of taking much time for stabilization, is not preferable in view of productivity and so is unsuitable for industrial production.

[0012] According to the present invention, in contrast, Raney nickel is stabilized by covering its surface with nickel hydroxide. It has heretofore been known that when a Raney nickel-containing active cathode oxidizes and degrades due to inverse currents, etc., produced during shut down, nickel hydroxide is formed on the surface of the active cathode. Never until now, however, is it known to stabilize Raney nickel by covering its surface with nickel hydroxide.

[0013] For the substrate of the active cathode according to the present invention, stainless steel, nickel, and steel plated with nickel or cobalt may be used. The substrate used herein may be in suitable forms, for instance, in flat sheet, expanded metal, perforated sheet, network, and rod forms.

[0014] According to the present invention, the electrode substrate is plated with a nickel aluminum alloy-containing film, and is treated with alkali to allow aluminum to dissolve therein for its removal. Then, the thus treated electrode substrate is dipped in the oxidizing agent solution for stabilization, thereby obtaining a stabilized active cathode. For the oxidizing agent solution it is preferable to use hydrogen peroxide or a solution of peroxides,percarbonates and perborates of preferably alkali metals dissolved in alkaline water. Salts, which show alkalinity upon dissolved in water, may be used in combination with water; in other words, it is not necessary to use alkaline water for those salts. For the alkaline water use may be made of aqueous solutions of hydroxides, carbonates and borates of alkali metals. The pH of the aqueous solution used is preferably at least 8, more preferably 9 to 13 inclusive.

[0015] The concentration of the oxidizing agent in the oxidizing agent solution used herein is 0.5 to 10 g/l, preferably 1 to 5 g/l when calculated as effective oxygen concentration. The term "effective oxygen concentration" is here understood to refer to the amount of oxygen determined by iodometry, which is expressed in g/l unit. An effective oxygen concentration less than 0.5 g/l is not preferable because not only is much time needed for stabilization but also the effect on stabilization becomes insufficient.

[0016] The pH of the oxidizing agent solution used herein ranges from 7 to less than 14, preferably from 10 to 13 inclusive. The use of an oxidizing agent solution whose pH is lower than 7 is not preferable because a portion of nickel hydroxide formed on the surface of Raney nickel dissolves, thus making the effect on stabilization insufficient.

[0017] The amount of the oxidizing agent solution used herein is preferably 10 to 300 liters per m² of projected area of the cathode material to be treated. When the amount of the oxidizing agent solution used is less than 10 liters, difficulty is involved in treating a number of cathode substrates at the same time because they come in contact with each other in treating equipment and so may not successfully be positioned therein.

[0018] In the present invention, it is preferable that the cathode is treated with the oxidizing agent solution at a temperature of 10 to 30°C for 5 to 30 minutes. A treating temperature lower than 10°C or a treating time shorter than 5 minutes is not preferable because the stabilizing treatment becomes incomplete. A treating temperature higher than 30°C or a treating time longer than 30 minutes, too, is not preferable because the formation of the film proceeds excessively, resulting in a drop of cathodic activity.

[0019] The present invention will now be explained more specifically with reference to illustrative and comparative examples.

Illustrative Example 1 & Comparative Example 1

[0020] Nickel expanded metal (20 mm x 20 mm) was degreased with an aqueous solution of sodium hydroxide, washed with water, and etched with a solution obtained by diluting concentrated hydrochloric acid with the same amount of water. After water washing, a nickel-containing electrode catalyst layer containing nickel-aluminum alloy particles having a particle diameter of 25 µm and having the following composition was prepared.

Plating Bath Composition:
Nickel chloride	300 g/l
Aluminum chloride	50 g/l
Boric acid	38 g/l
Ni-Al alloy (50:50)	0.9 g/l
pH of Plating Bath: held at 2.25 to 2.30. Plating Bath Temperature: held at 45 to 50°C. Plating Current Density: 3 A/dm2 Plating Time: 90 minutes

[0021] The electrode substrate coated with a film by nickel plating was washed with water, and then dipped in a 20% by weight aqueous solution of sodium hydroxide held at a temperature of 70 to 80°C for 2 hours for aluminum removal. Of 20 samples obtained in this way, 12 was treated for stabilization with hydrogen peroxide under the conditions shown in Table 1.

[0022] After treated for stabilization, the cathodes were estimated for hydrogen overvoltage upon attached to an ion exchange membrane electrolytic cell and currents passed through it. In a hydrogen overvoltage-measuring electrolytic cell, an insoluble electrode for brine electrolysis (made by Permelec Electrode) was used as an anode and Nafion 954 (made by Du Pont) as an ion exchange membrane. Hydrogen overvoltage was determined by the current interrupter method using a mercury-mercury oxide electrode as a reference electrode at a temperature of 80°C and a current density of 30 A/dm² while brine having a concentration of 200 g/l was used as an anodic solution and a 32% by weight aqueous solution of sodium hydroxide as a cathodic solution. The samples upon measured for hydrogen overvoltage were washed with water for sodium hydrogen removal, permitted to stand alone at room temperature in the air for 12 hours, and then again measured for hydrogen overvoltage. The values of the samples for hydrogen overvoltage before and after they were permitted to stand alone are shown in Table 1. For the purpose of comparison, a sample was prepared, which was not treated for stabilization with the oxidizing agent. This sample is designated Sample No. 13 in Table 1.

Table 1

Sample No.	Conc. of hydrogen peroxide (g/l)	pH of dipping solution	Temp. of dipping solution (°C)	Dipping time (min.)
1	3.0	12	20	5
2	3.0	12	20	10
3	3.0	12	20	20
4	3.0	12	20	30
5	1.0	12	20	10
6	2.0	12	20	10
7	5.0	12	20	10
8	3.0	9	20	10
9	3.0	10	20	10
10	3.0	11	20	10
11	3.0	12	10	10
12	3.0	12	30	10
13	No stabilization treatment

Sample No.	Hydrogen Overvoltage (mV)
	Before permitted to stand alone		After permitted to stand alone
1	114		120
2	115		115
3	118		117
4	120		120
5	108		116
6	110		113
7	120		119
8	120		120
9	117		116
10	115		115
11	109		120
12	122		123
13	102		185

[0023] The hydrogen peroxide concentrations are all expressed in terms of available oxygen concentration. The amounts of the dipping solutions are all 50 liters/m² (projected area).

Illustrative Example 2

[0024] The rest of the samples prepared in Example 1 were dipped in aqueous solutions of oxidizing agents other than hydrogen peroxide. The available oxygen concentration of the oxidizing agents was kept constant at 3 g/l, while the amounts of the dipping solutions were all 50 liters/m² (projected area). The thus dipped samples were washed with water for oxidizing agent removal, and then determined for hydrogen overvoltage as substantially described in Example 1. Then, the active cathode samples were removed from the electrolytic cell, permitted to stand alone at room temperature in the air for 12 hours, and again measured for hydrogen overvoltage. The values of the samples for hydrogen overvoltage before and after they were allowed to stand alone are shown in Table 2.

Table 2

Sample No.	Type of oxidizing agent	pH of dipping solution	Temp. of dipping solution (°C)	Dipping time (min.)
14	Sodium Peroxide	13	20	5
15	(ditto)	13	20	10
16	Sodium Percarbonate	12	10	10
17	(ditto)	12	20	10
18	Sodium Perborate	11	20	10
19	(ditto)	11	30	10

Sample No.	Hydrogen Overvoltage (mV)
	Before permitted to stand alone		After permitted to stand alone
14	110		114
15	112		115
16	117		118
17	116		120
18	115		116
19	117		120

[0025] As can be understood from the foregoing, the present invention makes it possible to handle Raney nickel base cathodes in the air with ease, which have heretofore been difficult to handle. The wet treating process using an oxidizing agent solution enables an active cathode to be stabilized by an oxidization treatment within a short period of time. It is also possible to attach an active cathode to an electrode frame because the active cathode has previously been treated with alkali to allow aluminum to dissolve therein for its removal. Furthermore, it is possible to provide a solution to the problem that aluminum dissolves from an active cathode upon the initiation of electrolysis and contaminates sodium hydroxide.

Claims

1. A method of making an active cathode which comprises the steps of forming an aluminum nickel alloy-containing film on an electrically conductive substrate, treating the substrate with alkali to allow aluminum to dissolve therein, and dipping the substrate in an oxidizing agent solution to stabilize said active cathode.

2. The method according to claim 1, wherein the oxidizing agent solution is a solution containing at least one member selected from peroxides, percarbonates, and perborates.