Cathode having high durability and low hydrogen overvoltage and process for the production thereof

A cathode having high durability and low hydrogen overvoltage comprising an electrode substrate and an alloy layer formed thereon, characterized in that said alloy layer is made of an alloy comprising Component X selected from the group consisting of nickel, cobalt and a mixture thereof, Component Y selected from the group consisting of aluminum, zinc, magnesium and silicon, and Component Z selected from the group consisting of a noble metal and rhenium, and having a composition of Components X, Y and Z falling within the range defined by the following points A, B, C and D with reference to the diagram of FIG. 1: PA1 A: X=99.6 wt. %, Y=0 wt. %, Z=0.4 wt. % PA1 B: X=79.6 wt. %, Y=20 wt. %, Z=0.4 wt. % PA1 C: X=10 wt. %, Y=20 wt. %, Z=70 wt. % PA1 D: X=12.5 wt. %, Y=0 wt. %, Z=87.5 wt. %.

The present invention relates to a cathode having high durability and low 
hydrogen overvoltage and a process for the production thereof. More 
particularly, the present invention relates to a cathode which hardly 
undergoes degradation of its properties even when subjected to an 
oxidizing environment and which has a low hydrogen overvoltage 
characteristic. 
Various types of cathodes have been proposed as cathodes having low 
hydrogen overvoltage, particularly as cathodes for electrolysis of an 
aqueous solution of an alkali metal halide. Among them, the electrode 
proposed by the present applicant and disclosed in Japanese Unexamined 
Patent Publication No. 112785/1979, has superior effects with respect to 
low hydrogen overvoltage and its durability as compared with the 
electrodes known by that time. However, from a further research, the 
present inventors have found that the electrode disclosed in the above 
publication does not necessarily provide adequate durability in some 
cases. The present inventors have conducted extensive researches to solve 
the problem and finally accomplished the present invention. 
As an industrial process for the production of chlorine and an alkali metal 
hydroxide, it is well known to obtain a halogen gas from an anode 
compartment and an aqueous alkali metal hydroxide solution and hydrogen 
gas from a cathode compartment in the electrolysis of an aqueous solution 
of an alkali metal halide in an electrolytic cell. As a cathode for such 
an electrolytic cell, it is preferred to use a cathode having low hydrogen 
overvoltage as mentioned above. During the operation of the electrolytic 
cell, it sometimes happens for various reasons that the operation must be 
stopped. In such a case, especially when the cell is short-circuited or 
the cathode is kept in a concentrated alkali metal hydroxide solution at 
high temperatures for a long time without supplying electricity, it has 
been found that the hydrogen overvoltage increases when the operation is 
resumed. The present inventors have studied this phenomenon, and finally 
found that nickel or cobalt as an electrochemically active component of 
Raney nickel particles or Raney cobalt particles is oxidized into nickel 
hydroxide or cobalt hydroxide, whereby the electrochemical activity is 
deteriorated, i.e. the hydrogen overvoltage increases. Further, it has 
been found that this deterioration can effectively be prevented by 
incorporating a third component selected from the group consisting of a 
noble metal and rhenium into known metal particles comprising a first 
component such as nickel or cobalt and a second component such as 
aluminum, zinc, magnesium or silicon, and that not only such metal 
particles but also an electrode having a surface layer having the same 
composition is equally effective. The present invention has been 
accomplished based on these discoveries. 
Thus, present invention provides a cathode having high durability and low 
hydrogen overvoltage comprising an electrode substrate and an alloy layer 
formed thereon, characterized in that the alloy layer is made of an alloy 
comprising Component X selected from the group consisting of nickel, 
cobalt and a mixture thereof, Component Y selected from the group 
consisting of aluminum, zinc, magnesium and silicon, and Component Z 
selected from the group consisting of a noble metal and rhenium, and 
having a composition of Components X, Y and Z falling within the range 
defined by points, A, B, C and D of FIG. 1. 
The present invention also provides a process for producing a cathode 
having high durability and low hydrogen overvoltage, which comprises 
applying onto an electrode substrate an electrochemically active alloy 
comprising Component X selected from the group consisting of nickel, 
cobalt and a mixture thereof, Component Y selected from the group 
consisting of aluminum, zinc, magnesium and silicon, and Component Z 
selected from the group consisting of a noble metal and rhenium, and 
having a composition falling within the range defined by point A', B', C' 
and D' of FIG. 4, by depositing particles of said alloy on the electrode 
substrate by a composite coating method, or forming an uniform layer of 
said alloy on the electrode substrate by a coating method, a dipping 
method or a sintering method. 
Now, the present invention will be described in detail with reference to 
the preferred embodiments. 
In the accompanying drawings, FIG. 1 is a diagram of a three-component 
composition comprising X=Ni or Co, Y=Al, Zn, Mg, or Si and Z=a noble metal 
or rhenium, and the composition within the range defined by points A, B, C 
and D represents the electrochemically active alloy composition of the 
alloy layer of the cathode according to the present invention. 
FIG. 2 is a cross sectional view of the surface portion of an electrode of 
the present invention. 
FIG. 3 is a cross sectional view of the surface portion of another 
electrode according to the present invention. 
FIG. 4 is a diagram of a three-component composition comprising X=Ni or Co, 
Y=Al, Zn, Mg or Si and Z=a noble metal or rhenium, and the composition 
within the range of points A', B', C' and D' represents the composition of 
the electrochemically active alloy to be used in the process of the 
present invention. 
In the present invention, the noble metal is meant for gold, silver and a 
platinum group metal such as platinum, rhodium, ruthenium, palladium, 
oxmium or iridium, as is well known. 
FIG. 1 is a diagram of the three-component composition comprising Component 
X selected from the group consisting of nickel, cobalt and a mixture 
thereof, Component Y selected from the group consisting of aluminum, zinc, 
magnesium and silicon, and Component Z selected from the group consisting 
of a noble metal and rhenium. The alloy composition of the alloy layer of 
the cathode according to the present invention is within the range defined 
by points A, B, C and D of FIG. 1. The alloy composition is preferably 
within the range defined by points E, F, G and H, further preferably 
within the range defined by points E, F, I and J. 
The proportions of Components X, Y and Z at points A, B, C and D are as 
follows. 
A: X=99.6 wt. %, Y=0 wt. %, Z=0.4 wt. % 
B: X=79.6 wt. %, Y=20 wt. %, Z=0.4 wt. % 
C: X=10 wt. %, Y=20 wt. %, Z=70 wt. % 
D: X=12.5 wt. %, Y=0 wt. %, Z=87.5 wt. %. 
Likewise, the proportions of Components X, Y and Z at points E, F, G, H, I 
and J are as follows: 
E: X=98.4 wt. %, Y=0 wt. %, Z=1.6 wt. % 
F: X=78.4 wt. %, Y=20 wt. %, Z=1.6 wt. % 
G: X=40 wt. %, Y=20 wt. %, Z=40 wt. % 
H: X=40 wt. %, Y=0 wt. %, Z=60 wt. % 
I: X=60 wt. %, Y=20 wt. %, Z=20 wt. % 
J: X=80 wt. %, Y=0 wt. %, Z=20 wt. %. 
The effect of the present invention is obtained by incorporating a 
component selected from the group consisting of a noble metal and rhenium, 
as one component of the alloy composition. However, the reason why the 
deterioration of the electrochemical activity is prevented by the 
incorporation of this component, has not yet been clearly understood. 
However, it is conceivable that nickel hydroxide or cobalt hydroxide is 
reduced to originally active nickel or cobalt by the incorporation of this 
component. Further, it has been confirmed that among the metals of this 
component, platinum, rhodium and ruthenium are most effective to provide 
the effect of the present invention. Namely, when platinum, rhodium or 
ruthenium is used, it is possible to maintain the hydrogen overvoltage at 
an extreamly low level for a long period of time even under a severe 
environmental condition. 
The alloy for the cathode of the present invention should have a 
composition within the range defined by points A, B, C and D of FIG. 1 
because if the alloy has a composition outside the above range, there will 
be disadvantages such that the hydrogen overvoltage can not be maintained 
at a low level for an extended period of time or the hydrogen overvoltage 
tends to be high from the beginning, or even if a noble metal or rhenium 
is added in a great amount exceeding this range, no further reduction of 
the hydrogen overvoltage or no further improvement in the durability can 
be expected. 
When the above alloy is in particle form, the average particle size may 
usually be in a range of 0.1 to 100 .mu.m although it depends upon the 
porosity of the electrode surface and the dispersibility of the particles 
for the production of an electrode, which will be described hereinafter. 
Within the above range, the particle size is preferably from 0.9 to 50 
.mu.m, more preferably from 1 to 30 .mu.m, from the viewpoint of e.g. the 
porosity of the electrode surface. 
Further, the alloy layer of the present invention is preferably porous at 
its surface so as to provide a low hydrogen overvoltage. 
In the case where the alloy is in particle form, the porous surface does 
not necessarily mean that the entire surface of the particles is porous, 
and it is sufficient that only the portions of the surface exposed on the 
above-mentioned alloy layer are porous. In the case where the alloy is 
provided on the electrode substrate in the form of a layered structure 
such as a plated layer, the porosity may be provided by the 
irregularities, i.e. concavities and convexities, of the layer surface. 
In general, the greater the porosity, the better. However, an excessive 
porosity tends to lead to poor mechanical strength. Accordingly, the 
porosity is preferably from 20 to 90%. Within this range, the porosity is 
more preferably from 35 to 85%, particularly from 50 to 80%. 
The porosity is measured by a conventional water substituting method. 
Various methods may be employed to form a porous surface. Whether or not 
the alloy is in particle form, it is preferred to employ a method wherein 
the porosity is provided, for instance, by partially or entirely removing 
Component Y from an alloy comprising Components X, Y and Z. 
In this case, it is particularly preferred to employ a method which 
comprises treating an alloy comprising predetermined proportions of 
uniformly distributed Components X, Y and Z, with an alkali metal 
hydroxide to remove at least partially the metal of Component Y. In the 
case where the cathode of the present invention is used as a cathode for 
producing an alkali metal hydroxide by the hydrolysis of an aqueous 
solution of an alkali metal halide, it is not necessarily required to 
pretreat it with an alkali hydroxide prior to mounting it in the 
electrolytic cell. In such a case, the catholyte is a solution of an 
alkali metal hydroxide, and the metal of Component Y is gradually removed 
during the electrolysis, whereby a desired cathode is obtainable. 
Various combinations of the above-mentioned metal particles may be used as 
the composition of cathode. Typical combinations include Ni-Al-Pt, 
Ni-Al-Rh, Ni-Al-Ru, Ni-Zn-Pt, Ni-Zn-Rh, Ni-Zn-Ru, Ni-Si-Pt, Ni-Si-Rh, 
Ni-Si-Ru, Co-Al-Pt, Co-Al-Rh, Co-Al-Ru, Co-Zn-Pt, Co-Zn-Rh, Co-Zn-Ru, 
Co-Si-Pt, Co-Si-Rh, Co-Si-Ru, Ni-Mg-Pt, Ni-Mg-Rh, Ni-Mg-Ru, Co-Mg-Pt, 
Co-Mg-Rh and Co-Mg-Ru. 
Particularly preferred combinations among them are Ni-Al-Pt, Ni-Al-Rh, 
Ni-Al-Ru, Co-Al-Pt, Co-Al-Rh and Co-Al-Ru. 
The conditions for the alkali metal hydroxide treatment may vary depending 
upon the composition of the particular alloy. However, in the case of the 
alloy having the composition mentioned hereinafter, it is preferred to 
immerse it in an aqueous solution having an alkali metal hydroxide 
concentration (as calculated as NaOH) of from 10 to 35% by weight at a 
temperature of from 10.degree. to 50.degree. C. for from 0.5 to 3 hours. 
These conditions are selected to readily remove Component Y. 
Further, Component Z is the one which is not removed by the above-mentioned 
alkali treatment. 
In the case where the above-mentioned alloy is in particle form, the layer 
for firmly bonding the particles to the metal substrate is preferably made 
of the same metal as Component X of the alloy particles. 
Thus, in the case where the alloy is in particle form, numerous alloy 
particles are bonded on the electrode surface of the cathode of the 
present invention, whereby the surface of the cathode macroscopically 
presents a fine porous structure. 
In the case where the surface of the electrode substrate is uniformly 
coated with an alloy layer, no binder metal layer exists as opposed to the 
case where the alloy particles are used. 
Thus, in the cathode of the present invention, the electrode surface is 
covered with an alloy containing nickel and/or cobalt having by itself a 
low hydrogen overvoltage, and, as mentioned above, the electrode surface 
has a fine porous structure to present a larger electrochemically active 
surface area, whereby the hydrogen overvoltage can be effectively reduced 
by the synergistic effect. 
Further, in the case where the alloy particles are used in the present 
invention, they are firmly bonded to the electrode surface by the layer 
composed of the above-mentioned metal, whereby deterioration due to the 
falling off of the bonded particles is minimized and the superior effect 
for the maintenance of the low hydrogen overvoltage will be ensured. 
In the present invention, the electrode substrate can be made of a suitable 
electroconductive metal such as Ti, Zr, Fe, Ni, V, Mo, Cu, Ag, Mn, 
platinum group metals, graphite and Cr, and alloys thereof. Among them, it 
is preferred to use Fe, a Fe-alloy (a Fe-Ni alloy, a Fe-Cr alloy or a 
Fe-Ni-Cr alloy), Ni, a Ni-alloy (a Ni-Cu alloy or a Ni-Cr alloy), Cu or a 
Cu-alloy. Particularly preferred materials for the electrode substrate are 
Fe, Cu, Ni, a Fe-Ni alloy and a Fe-Ni-Cr alloy. 
The size and configuration of the electrode substrate may be optionally 
adjusted to conform with the structure of the electrode to be used. For 
instance, the substrate may be in the form of a plate, a foraminous sheet, 
a net (such as an expanded metal) or a parallel screen type, which may be 
flat, curved or cylindrical. 
The thickness of the alloy layer of the present invention is preferably 
from 20 to 200 .mu.m, more preferably from 25 to 150 .mu.m, particularly 
from 30 to 100 .mu.m. 
FIGS. 2 and 3 illustrate cross sections of the electrode surfaces according 
to the present invention. As shown in FIG. 2, a metal layer 2 is formed on 
an electrode substrate 1 with a middle layer 4 interposed between them. 
The metal layer contains electrochemically active metal particles 3, and 
the metal particles are partially exposed on the surface of the layer. The 
proportion of the particles in the layer 2 is preferably from 5 to 80% by 
weight, more preferably from 10 to 50% by weight. The durability of the 
electrode of the present invention can be further improved by providing a 
middle layer composed of a metal selected from the group consisting of Ni, 
Co, Ag and Cu, between the electrode substrate and the metal layer 
containing alloy particles. Such a middle layer may be made of the same or 
different metal as the metal in the above-mentioned metal layer. However, 
in view of the bonding property of the middle layer with the 
above-mentioned metal layer, it is preferred that the middle layer is made 
of the same metal as the above-mentioned metal layer. From the viewpoint 
of e.g. the mechanical strength, the thickness of the middle layer is 
preferably from 5 to 100 .mu.m, more preferably from 20 to 80 .mu.m, 
particularly from 30 to 50 .mu.m. 
However, it is not essential to provide such a middle layer. 
FIG. 3 is a cross sectional view of the cathode of the present invention 
wherein the surface of the electrode substrate is uniformly coated with an 
alloy layer. Reference numeral 1 designates an electrode substrate, 
numeral 5 designates a uniform surface layer made of an electrochemically 
active alloy, and numeral 6 designates a middle layer. 
In the electrode of the present invention as illustrated in FIG. 2, 
numerous particles are exposed on the electrode surface, whereby the 
porosity of the surface layer is mainly provided by the spaces between the 
particles, and the voids formed by the removal of Component Y of the alloy 
also contribute to the porosity. 
As mentioned above, the degree of the porosity relates to the reduction of 
hydrogen overvoltage, and it is sufficient for the purpose of the present 
invention if it provides an electrical double layer capacity of at least 
1000 .mu.F/cm.sup.2. Within this range, the electrical double layer 
capacity is preferably at least 2000 .mu.F/cm.sup.2, more preferably at 
least 5000 .mu.F/cm.sup.2. 
The electrical double layer capacity is an electrostatic capacity of the 
electrical double layers formed by the positive and negative ions 
distributed in a face-to-face relationship with a short distance from each 
other near the surface of the electrode when the electrode is immersed in 
an electrolyte, and it is measured as a differential capacity. 
The capacity increases with an increase of the surface area of the 
electrode. Accordingly, with an increase of the porosity of the electrode 
surface and the consequential increase of the surface area of the 
electrode, the electrical double layer capacity of the electrode surface 
increases. Thus, the electrochemically effective surface area of the 
electrode i.e. the degree of the porosity of the electrode surface can be 
determined by the electrical double layer capacity. 
The electrical double layer capacity varies depending upon the temperature 
at the time of the measurement, the kind and concentration of the 
electrolyte and the electrode potential, and for the purpose of the 
present invention, the electrical double layer capacity is meant for the 
values measured by the following method. 
A test piece (i.e. an electrode) is immersed in an aqueous solution 
(25.degree. C.) containing 40% by weight of NaOH and a platinum black 
coated platinum plate having an apparent surface area of about 100 times 
the surface area of the test piece is immersed as a counter electrode, 
whereby a cell-impedance is measured by a vector-impedance meter to obtain 
the electrical double layer capacity of the test piece. 
Various methods may be used for practically forming the surface layer on 
the electrode. For instance, a composite coating method, a melt-coating 
method, a sintering method, an alloy plating method or a melt-dipping 
method may be employed. 
When alloy particles are used, it is particularly preferred to employ a 
composite coating method, since the particles of the present invention can 
thereby effectively be coated on the electrode surface. 
The composite coating method is conducted in such a manner that into an 
aqueous solution containing a kind of metalions to form the metal layer, 
alloy particles mainly composed of e.g. nickel are dispersed to obtain a 
plating bath, and electroplating is carried out in the plating bath by 
using an electrode substrate as a cathode so that the above metal and the 
alloy particles are co-electrodeposited on the electrode substrate. More 
specifically, the particles in the bath are considered to become bipolar 
by the influence of the electric field, and when they approach close to 
the surface of the cathode, the local cathode current density increases 
and when they get in contact with the cathode, they are 
co-electrodeposited with the metal on the substrate by the reduction of 
the metal ions. For instance, when a nickel layer is used as the metal 
layer, a nickel chloride bath, a high nickel chloride bath or a nickel 
chloride-nickel acetate bath may be employed. When a cobalt layer is used 
as a metal layer, a cobalt chloride bath, a high cobalt chloride bath or a 
cobalt chloride-cobalt acetate bath may be employed. 
In this case, the pH of the bath is important. Namely, in many cases, it is 
usual that oxygen is deposited or certain oxide films are formed on the 
surface of electrochemically active metal particles to be dispersed in the 
plating bath. In such a state, the bonding strength of the particles with 
the metal layer will be inadequate, and consequently, it will be likely 
that the particles will fall off during the use as an electrode. In order 
to prevent this from happening, it is necessary to minimize the oxygen 
deposition or the formation of oxide films on the surface of the 
particles. For this purpose, it is preferred to adjust the pH of the 
plating bath to be from 1.5 to 3.0. 
In the process of the present invention, the metal particles or the alloy 
layer is made of an alloy comprising Component X selected from the group 
consisting of nickel, cobalt and a mixture thereof, Component Y selected 
from the group consisting of aluminum, zinc, magnesium and silicon, and 
Component Z selected from the group consisting of a noble metal and 
rhenium, and having a composition falling within the range defined by 
points A', B', C' and D' of FIG. 4. 
The proportions of the alloy Components (X, Y and Z) at point A', B', C' 
and D' in FIG. 4 are as follows: 
A': X=59.8 wt. %, Y=40 wt. %, Z=0.2 wt. % 
B': X=39.8 wt. %, Y=60 wt. %, Z=0.2 wt. % 
C': X=5 wt. %, Y=60 wt. %, Z=35 wt. % 
D': X=12 wt. %, Y=40 wt. %, Z=48 wt. %, 
The composition is preferably within the range defined by points E', F', G' 
and H', more preferably within the range defined by points E', F', I' and 
J'. The proportions of Components X, Y and Z at points E', F', G', H', I' 
and J' are as follows: 
E': X=59.2 wt. %, Y=40 wt. %, Z=0.8 wt. % 
F': X=39.2 wt. %, Y=60 wt. %, Z=0.6 wt. % 
G': X=16 wt. %, Y=60 wt. %, Z=24 wt. % 
H': X=24 wt. %, Y=40 wt. %, Z=36 wt. % 
I': X=30 wt. %, Y=60 wt. %, Z=10 wt. % 
J': X=50 wt. %, Y=40 wt. %, Z=10 wt. %. 
If the composition is outside the above range, there will be disadvantages 
such that no adequate deposition tends to be secured by the composite 
coating process, no adequate bonding strength will be obtained even when 
an adequate amount has been co-deposited, or the electrochemical catalytic 
activity of the electrode after the extraction of Component Y will be 
inadequate. Further, even when the amount of the noble metal exceeds the 
range of the present invention, no additional effectiveness for the 
reduction of the hydrogen overvoltage or no further improvement of the 
durability will be thereby obtained. 
When the composite coating is conducted in a plating bath in which alloy 
particles are dispersed, the amount of the particles in the bath is 
preferably from 1 to 200 g/l, more preferably from 1 to 50 g/l, 
particularly from 1 to 10 g/l in order to ensure good bonding of the 
particles to the electrode surface. Further, the composite coating 
operation is preferably conducted at a temperature of from 20.degree. to 
80.degree. C., particularly from 30.degree. to 60.degree. C. at a current 
density of 1 to 20 A/dm.sup.2, particularly from 1 to 10 A/dm.sup.2. 
Further, additives such as an additive to reduce the strain of the coating 
or an additive to facilitate co-electrodeposition may optionally be added 
to the plating bath. 
When a middle layer is to be formed between the electrode substrate and the 
particle-containing metal layer as mentioned above, the electrode 
substrate is firstly plated with Ni, Co or Cu, and then the 
particle-containing metal layer is formed thereon by the above-mentioned 
composite coating method or melt-spraying method. 
In such a case, the above-mentioned various plating baths may be employed 
as the plating bath. In the case of the Cu plating, conventional plating 
baths may be employed. 
Thus, it is possible to obtain an electrode wherein the particles of the 
present invention are co-deposited on the electrode substrate with the 
metal layer interposed between them. 
Now, specific methods for uniformly forming an electrochemically active 
alloy layer on the electrode substrate will be described. 
The specific methods include a coating method, a dipping method, a 
sintering method and an electroplating method, as mentioned above. 
As the coating method, it is preferred to employ a method wherein a slender 
rod or powder of the alloy as shown in FIG. 4 is melted and sprayed. For 
this melt spraying, there may be employed a plasma spray apparatus or an 
oxygen-hydrogen flame or oxygen-acetylene flame spray apparatus which is 
commonly used in a melt-coating method. 
The dipping method is a method wherein an electrode substrate is dipped in 
a molten liquid of the above-mentioned alloy to form a coating layer of 
the alloy on the substrate, whereby the temperature of the molten alloy 
liquid is preferably higher by from 50.degree. to 200.degree. C. than the 
melting point of said alloy. For instance, in the case of Ni-Al-Ru, the 
melting point is about 1500.degree. C., and accordingly the dipping is 
conducted at a temperature of about 1600.degree. C. and a coating layer of 
the alloy is formed on the electrode substrate when the dipped substrate 
is taken out. 
The sintering method is a method wherein preliminarily prepared fine 
particles having a particle size of not greater than 100 .mu.m are coated 
on the electrode substrate by using a suitable polymer, particularly an 
aqueous solution of a water-soluble polymer, and then heated to burn off 
the binder and to sinter the particles and bond them to the substrate. 
Usually, the operation is conducted at a temperature lower by from 
100.degree. to 300.degree. C. than the melting point, and the sintering is 
preferably conducted under elevated pressure. 
The electroplating method is a so-called alloy plating method wherein a 
solution (preferably an aqueous solution) of metal salt, of which 
Components X, Y and Z fall within the range shown in FIG. 4, is prepared, 
and an electrode substrate is immersed as a cathode in the solution, 
thereby to conduct electroplating. However, when Component Y is Al or Mg, 
this method can not be employed. This method can be employed when 
Component Y is Zn. Commonly employed conditions may be used as the plating 
conditions. For instance, the electroplating may be conducted at a 
temperature of about 60.degree. C. at a current density of about 1 
A/dm.sup.2 in a solution of the mixture of NiSO.sub.4 .multidot.7H.sub.2 
O, ZnSO.sub.4, KReO.sub.4 and (NH.sub.4).sub.2 SO.sub.4 with its pH 
adjusted at 4.0, whereby an alloy layer of Ni-Zn-Re can be formed. 
It is also effective to deposit a non-electronic conduction substance on 
the surface of the low hydrogen voltage cathode thus obtained. 
When the cathode of the present invention is used as a cathode for 
electrolysis of e.g. an aqueous solution of an alkali metal halide, it 
sometimes happens that the catholyte contains dihypoferrite ion 
(HFeO.sub.2.sup.-) or other ion-containing ions dissolved from the 
material of the surrounding electrolytic cell and these ions discharge on 
the cathode to precipitate an iron compound (for instance iron metal) on 
the cathode. In such a case, the electrochemical activity of the cathode 
surface will be lost and consequently the cathode overvoltage will 
increase. 
In order to prevent such precipitation, it is effective to partially 
deposit an electrically nonconductive substance such as a 
fluorine-containing resin (for example, PTFE) on the cathode of the 
present invention or on the metal particles exposed on the cathode 
surface. As a specific method for this purpose, it is preferred to employ 
a method as disclosed in Japanese Patent Application No. 126921/1981. 
If necessary, the cathode thus obtained may be subjected to treatment with 
an alkali metal hydroxide (for instance, by immersing it in an aqueous 
alkali metal hydroxide solution) to remove at least partially the metal of 
Component Y in the alloy particles and to form a porous structure on the 
particles or on the surface layer of the electrode. 
The conditions for such treatment are as described above. 
When an alloy comprising the above-mentioned Components X, Y and Z is used, 
it is preferred to conduct the above-mentioned alkali metal hydroxide 
treatment. However, the electrode coated with such an alloy may be mounted 
on an alkali metal chloride electrolytic cell as it is, i.e. without 
subjecting it to the alkali metal hydroxide treatment and the electrolysis 
may be thereby conducted. 
In such a case, the metal of Component Y dissolves during the process of 
the electrolysis, whereby the electrode overvoltage will be reduced. 
Although the resulting aqueous alkali metal hydroxide solution may be 
slightly contaminated with the dissolved metal ions of Component Y, such 
contamination is usually negligible and does not create a problem. 
The electrode of the present invention can be used as an electrode, 
especially as a cathode, for electrolysis of an aqueous solution of an 
alkali metal chloride in an ion exchange membrane process. It may be used 
also as an electrode for electrolysis of an aqueous alkali metal chloride 
solution by means of a porous diaphragm such as an asbestos diaphragm.

Now, the present invention will be described in further detail with 
reference to Examples. 
EXAMPLES 1 to 16 
Alloy powders (200 mesh pass) having the compositions as identified in 
Table 1 were prepared. With respect to Examples 1 to 10 and 14 to 16, low 
hydrogen overvoltage electrodes were prepared by a composite coating 
method in accordance with Example 12 of Japanese Unexamined Patent 
Publication No. 112785/1979. With respect to Examples 11 to 13, low 
hydrogen overvoltage electrodes were prepared by a composite coating 
method in accordance with Example 12 of the same publication except that 
the coating method was modified by replacing NiCl.sub.2 .multidot.6H.sub.2 
O and the Ni plate anode by CoCl.sub.2 .multidot.6H.sub.2 O 
(concentration: 300 g/l) and a Co plate anode, respectively. (However, the 
leaching treatment after the plating was conducted at a temperature of 
50.degree. C.) 
With respect to each electrode thus obtained, the metal particles on the 
electrode were partially sampled and their composition was examined. The 
results are shown in Table 1. Further, the electrical double layer 
capacity was measured by the above-mentioned method, and the results are 
also shown in Table 1. 
Then, tests for resistance against short-circuiting of the cee were 
conducted by using these electrodes as cathodes for a sodium chloride 
electrolytic cell wherein RuO.sub.2 --TiO.sub.2 was used as anodes and a 
fluorine-containing cation exchange membrane (a copolymer of CF=CF.sub.2 
with CF.sub.2 .dbd.CFO(CF.sub.2).sub.3 COOCH.sub.3 manufactured by Asahi 
Glass Company Ltd., an ion-exchange capacity of 1.45 meq/g resin) was used 
as an ion-exchange membrane. Electrolysis was conducted at 90.degree. C. 
at a current density of 20 A/dm.sup.2 using a 3N NaCl solution as the 
anolyte and a 35% NaOH solution as the catholyte. On the third day from 
the initiation of the electrolysis, the following short-circuiting test 
was carried out. 
Firstly, the supply of an electric current from the direct current power 
source was stopped, and the anode and cathode were connected outside the 
electrolytic cell by a copper wire and left to stand for about 15 hours in 
that state. During this period, the electric current from the cathode to 
the anode was measured. The catholyte temperature was maintained at 
90.degree. C. for about 3 hours after the termination of the electrolysis, 
and then the cell was allowed to cool naturally. This operation was 
repeated 5 times and then the cell was left to cool for 15 hours. 
Thereafter, the cathode was taken out and the hydrogen overvoltage was 
measured. The results are shown in Table 1. The hydrogen overvoltage was 
almost the same as the one measured prior to the tests. 
Further, the electrode of Example 3 was immersed in a 50% NaOH aqueous 
solution at 140.degree. C. for 3 weeks. To permit adequate contact with 
air, the depth of the container was set as shallow as 7 cm and the top of 
the container was open. The hydrogen overvoltage of this electrode was 
measured before and after the immersing test. The hydrogen overvoltage was 
0.09 V, and no substantial change was observed between the values measured 
before and after the test. 
COMATIVE EXAMPLES 1 to 2 
With respect to Comparative Example 1, a Ni-Al alloy powder composite 
coated electrode was prepared by the coating method of Example 12 in 
Japanese Patent Publication No. 112785/1979. With respect to Comparative 
Example 2, a Co-Al alloy powder composite coated electrode was prepared by 
the coating method of Example 12 of the same publication except that the 
coating method was modified by replacing NiCl.sub.2 .multidot.6H.sub.2 O 
and the Ni plate anode by CoCl.sub.2 .multidot.6H.sub.2 O (concentration: 
300 g/l) and a Co plate anode, respectively. With respect to each 
electrode, the metal particles on the electrode were partially sampled and 
their composition was examined. The results are shown in Table 2. Further, 
the electrical double layer capacity of each electrode is also presented 
in Table 2. 
The short-circuiting test was conducted in the same manner as in Examples 1 
to 16, and the change in the hydrogen overvoltage before and after the 
test was measured. The results are shown in Table 2 together with the 
values of the hydrogen overvoltage measured before the tests. 
COMATIVE EXAMPLES 3 to 6 
Cathodes were prepared in the same manner as the Examples except that the 
alloy powder compositions were changed to those of Comparative Examples 3 
to 6 as identified in Table 2. The electrical double layer capacity of 
each electrode thereby obtained is also presented in Table 2. Further, the 
results of short-circuiting tests conducted in the same manner as the 
Examples are also shown in Table 2. 
Comparative Examples 3 and 4 show that even if the third component is 
incorporated in a great amount, no further improvement of the properties 
is obtained. Comparative Examples 5 and 6 show that if the metal 
compositions of the starting material powders are outside the range 
specified by the present invention, the overvoltage is originally greater 
than the values obtained by using the metal powders with the composition 
of this invention. 
TABLE 1 
__________________________________________________________________________ 
Hydro- Electrical 
gen Composition after 
double 
over- 
the NaOH treat- 
layer 
X (%) Y (%) 
Z (%) voltage 
ment (%) capacity 
Exp. 
Ni Co 
Al 
Zn 
Ru 
Rh 
Pt 
Re 
(V) X Y Z (.mu.F/cm.sup.2) 
__________________________________________________________________________ 
1 49.7 50 0.3 0.11 
93.5 
6 0.5 
21,000 
2 49.5 50 0.5 0.10 
91.2 
8 0.8 
22,000 
3 45 50 5 0.07 
84 8 8 23,000 
4 40 50 10 0.05 
88 5 17 20,000 
5 35 55 10 0.05 
70 10 20 21,000 
6 45 45 10 0.05 
74 10 16 20,000 
7 45 50 5 0.06 
84 7 9 19,000 
8 45 50 5 0.06 
85 8 7 20,000 
9 45 50 5 0.07 
84 8 8 23,000 
10 40 50 
10 0.05 
88 4 18 20,000 
11 45 
50 5 0.07 
85 9 7 25,000 
12 45 50 
5 0.06 
83 8 9 27,000 
13 45 50 5 0.07 
85 9 7 24,000 
14 20 25 
50 5 0.07 
84 9 7 22,000 
15 20 55 25 0.04 
45 5 50 18,000 
16 20 45 35 0.04 
27 6 67 17,000 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
Hydrogen Electrical 
Com- overvoltage 
Composition after 
double 
para- (V) the NaOH treat- 
layer 
tive 
X (%) 
Y (%) 
Z (%) 
Short-circuit 
ment (%) capacity 
Exp. 
Ni 
Co 
Al 
Zn 
Ru 
Pt 
test X Y Z (.mu.F/cm.sup.2) 
__________________________________________________________________________ 
1 50 50 0.08 
0.14 
93 7 0 15,000 
2 50 
50 0.09 
0.15 
94 6 0 16,000 
3 5 55 40 0.05 
0.05 
11 8 81 16,000 
4 5 45 50 
0.05 
0.05 
8 7 85 18,000 
5 80 10 10 0.20 
0.20 
83 5 12 850 
6 80 10 10 
0.21 
0.21 
84 5 11 800 
__________________________________________________________________________