Cathode for electrolysis of alkali metal chloride

A porous cathode for electrolysis of an alkali metal chloride comprises a substrate and a coating comprising, a catalyst, a water repellent material and a pore forming agent. The water repellent material is a combination of a first fluorinated resin having a particle diameter of 0.1 to 20.mu. and a melting point of 300.degree. to 340.degree. C. and a second fluorinated resin having a particle diameter of 1 to 50.mu. and a melting point of 160.degree. to 320.degree. C. and said first resin has higher melting point than that of said second resin.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a cathode for electrolysis of an alkali 
metal chloride. More particularly, it relates to a cathode which requires 
a lower cell voltage in a process for producing an alkali metal hydroxide 
by an electrolysis of an aqueous solution of an alkali metal chloride by 
using a cation exchange membrane. 
2. Description of the Prior Art 
As a process for producing an alkali metal hydroxide by an electrolysis of 
an aqueous solution of an alkali metal chloride, a diaphragm method has 
been mainly employed instead of a mercury metod in view of a prevention of 
a public pollution. 
It has been proposed to use an ion exchange membrane in place of asbestos 
as a diaphragm to produce an alkali metal hydroxide by electrolyzing an 
aqueous solution of an alkali metal chloride so as to obtain an alkali 
metal hydroxide having high purity and high concentration. 
On the other hand, it has been proposed to save energy in the world. From 
the viewpoint, it has been required to minimize a cell voltage in such 
technology. 
Certain processes have been proposed as processes for lowering a cell 
voltage. Among these processes, as a relatively advantageous process, it 
has been proposed to reduce a cell voltage by using a porous gas permeable 
cathode by dividing a cathode compartment into two parts; filling a 
catholyte in the part partitioned by the cathode and a membrane; feeding 
an oxygen-containing gas such as air in a space between the cathode and a 
wall of the cell so as to diffuse the gas in the gas permeable cathode 
whereby hydroxyl group is rapidly formed by reacting water with oxygen. 
Such cathode is prepared by incorporating silver or a platinum group metal 
as a catalyst so as to accelerate the formation of hydroxyl group and 
incorporating a water repellent material such as polytetrafluoroethylene 
and paraffin so as to prevent a leakage of the resulting alkali metal 
hydroxide or water. 
In the conventional process a desired porous cathode has been prepared by 
mixing the catalytic component with an aqueous dispersion of 
polytetrafluoroethylene, etc. as the resin having such water repellency. 
Polytetrafluoroethylene is used for said purpose imparting water repellency 
as well as maintaining configuration as the cathode by combining with the 
catalytic component. The resulting cathode, however, is gradually broken 
down to reduce the water repellency and the function as the cathode is 
deteriorated. 
The inventors have studied the reason why such phenomenon is caused, and 
have found that it relates to the particle size and the melting point of 
polytetrafluoroethylene. 
The polytetrafluoroethylene used in the conventional process is in a form 
of rough particles having an average diameter of 500.mu. and has a melting 
point of about 327.degree. C. When only such polytetrafluoroethylene resin 
is used, it has been difficult to prevent the deterioration of the 
function of the cathode even though the process for preparing the cathode 
is improved. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a porous gas permeable 
cathode which imparts stable water repellency for a long period. 
The object of the present invention has been attained by using two kinds of 
fluorinated resins having different particle sizes and different melting 
points. 
The object of the present invention has been attained by providing a porous 
cathode for an electrolysis of an alkali metal chloride by using a 
substrate, a catalyst, a water repellent component and a pore-forming 
agent, wherein a fluorinated resin having a particle diameter of 0.1 to 
20.mu. and a melting point of 300.degree. to 340.degree. C. (first resin) 
and a fluorinated resin having a particle diameter of 1 to 50.mu. and a 
melting point of 160.degree. to 320.degree. C. (second resin) which has 
larger particle diameter and lower melting point than those of the first 
resin are used as the water repellent component.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The first resin is used for firmly bonding the catalyst particles and is 
preferably a resin having a smaller diameter and higher melting point. 
When the resin particles are mixed with the catalyst particles, the 
particles having higher melting point tend to oriented in a fibrous state 
thereby firmly bonding the catalyst particles. 
The second resin is used for imparting desired water repellency to the 
cathode and improving fabricability and is preferably a resin having 
relatively low melting point and larger particle diameter. The second 
resin has relatively larger diameter to satisfactorily fill spaces between 
the catalyst particles thereby improving the water repellency. The second 
resin has lower melting point than the first resin whereby it has low 
friction coefficient and superior lubricating property and is easily 
dispersible in a powder or a liquid medium. It remarkably improves 
fabricability in a roll-molding or an extrusion molding of a composition 
of the resins and the catalyst particles. 
The melting point of the resin should be considered depending upon the 
desired characteristics of the resin for example, characteristics as a 
binder or characteristics for fabricability or water repellency. 
The first resin should have a melting point of 300.degree. to 340.degree. 
C. preferably 310.degree. to 335.degree. C. in view of the resin for the 
fibrous form. The second resin should have a melting point of 160.degree. 
to 320.degree. C. preferably 170.degree. to 310.degree. C. in view of the 
resin for spreadability and water repellency. The melting point of the 
first resin should be higher than the melting point of the second resin. 
The difference of the melting point is depending upon the level of the 
melting points of the resins and is usually greater than 5.degree. C. 
preferably greater than 10.degree. C. especially greater than 20.degree. 
C. 
A ratio of a particle diameter of said second resin to that of said first 
resin is in a range of 2 to 100 preferably 5 to 30. 
The particle diameter of the second resin is substantially greater than the 
particle diameter of the first resin at a rate of more than two times 
preferably more than 5 times. 
The first resin used in the present invention is preferably 
polytetrafluoroethylene, or tetrafluoroethylene-perfluoroalkyl 
perfluorovinyl ether copolymer. 
The second resin is preferably polytetrafluoroethylene, 
tetrafluoroethylene-hexafluoropropene copolymer, 
polychlorotrifluoroethylene, tetrafluoroethylene-perfluoroalkyl 
perfluorovinyl ether copolymer or ethylene-tetrafluoroethylene copolymer. 
One or more kinds of the resins can be used for the first resin or the 
second resin. 
It is especially preferable to use polytetrafluoroethylene as the first 
resin and polytetrafluoroethylene or 
tetrafluoroethylene-hexafluoropropylene copolymer as the second resin in 
view of excellent water repellency for a long time. It is preferable to 
combine the first resin and the second resin at a ratio of 10 to 98 wt.% 
of the first resin to 2 to 90 wt.% of the second resin preferably at a 
ratio of 20 to 95 wt.% of the first resin to 5 to 80 wt.% of the second 
resin, especially at a ratio of 20 to 70 wt.% of the first resin to 30 to 
80 wt.% of the second resin. 
It is not preferable to be out of said range since a desired result may not 
be obtained. 
The total amount of the first and second resins is depending upon the kind 
and properties of the materials for the substrate and is usually at a 
ratio of about 2 to 80 wt.% based on the total components for the cathode. 
When the ratio of the resins is less than said range, a liquid leakage or a 
peeling of the catalyst from the substrate is caused, whereas when it is 
more than said range, the electric resistance is higher to cause inferior 
characteristics, because of the covering of the surface of the catalyst 
with the water repellent resin. It is the optimum to incorporate the 
resins at a ratio 5 to 60 wt.%, since there is not any trouble of the 
liquid leakage or the peeling of the catalyst and the catalytic activity 
is not substantially lost. 
The substrate used in the present invention is to maintain the form of the 
cathode. The material for the substrate is usually carbon. It is also 
possible to use a porous plate made of a metal such as nickel, iron and 
stainless steel or non-porous metal strands, bundled or plied regularly or 
irregularly. In the case of the carbon substrate, a porous molded 
substrate such as the porous metal substrate can be used. It is optimum to 
prepare a porous cathode comprising the desired components which is 
obtained by heating a molded mixture of carbon powder, a pore forming 
agent and desired additives. 
The catalyst used in the present invention is to improve the reaction 
velocity of the electrode reaction for forming hydroxyl group by reacting 
oxygen with water. The catalyst can be platinum group metals such as Pt 
and Pd; silver; alloys such as Raney silver; spinel compounds such as 
Co.Fe.Al.sub.2 O.sub.3 perovskite ionic crystals such as La.NiO.sub.3 ; 
metal phthalocyanines such as cobalt phthalocyanine and a mixture thereof. 
The amount of the catalyst is depending upon the kind of the catalyst and 
is usually in a range of about 0.01 to 200 mg./cm.sup.2. When the amount 
of the catalyst is less than the range, the reaction velocity for forming 
hydroxyl group is not satisfactorily high enough in an industrial process. 
When the amount of the catalyst is more than the range, any greater effect 
is not expected but a cost of the cathode is disadvantageously higher. It 
is especially preferable to be in a range of 0.1 to 100 mg./cm..sup.2, 
because a electrochemically satisfactory activity is imparted. It is 
optimum to use platinum, palladium or silver as the catalyst because the 
catalytic activity for forming hydroxylation ion is remarkably high. 
The pore forming agent used in the process can be nickel salts of 
carboxylic acids such as nickel formate, nickel citrate, nickel stearate 
and nickel oxalate; cobalt salts of carboxylic acids such as cobalt 
formate, cobalt citrate, cobalt stearate, and cobalt oxalate; silver salts 
of carboxylic acids such as silver citrate, silver acetate, silver 
oxalate, silver benzoate and silver lactate and a mixture thereof. 
It is optimum to use nickel formate, cobalt formate or silver citrate as 
the pore forming agent, since it impart the optimum pore forming function. 
The amount of the pore forming agent is depending upon the kind of the 
substance in which pores are formed and the kind of the pore forming agent 
and is usually in a range of about 5 to 60 wt.%. When the amount of the 
pore forming agent is less than the range, a desired pore forming effect 
can not be expected whereas when it is more than the range, many ununiform 
pores are formed to cause remarkably inferior cathode performance. 
It is optimum to use the pore forming agent at the ratio of 10 to 50 wt.% 
because the optimum cathode can be obtained. 
In the porous cathode of the present invention, when the pore diameter is 
too small, oxygen gas can not be satisfactorily diffused to result in 
inferior function, whereas when it is too large, the electrolyte is leaked 
to result in less area of the three phases of the electrolyte, the 
catalyst and oxygen which are contacted with each other. The average pore 
diameter of the cathode is usually preferably to be in a range of about 
0.01 to 30.mu.. 
The porosity highly affects to the characteristics for electrolysis. When 
the porosity is too small, the oxygen-containing gas can not be 
satisfactorily diffused whereas when the porosity is too large, the 
electrolyte is leaked or the strength is too low. The porosity of the 
cathode is preferably in a range of about 20 to 80%. 
The air permeable coefficient of the cathode is also important factor for 
the characteristics. When the air permeable coefficient is too small, the 
oxygen-containing gas can not be satisfactorily diffused whereas when it 
is too large, the oxygen-containing gas is bubbled into the catholyte and 
the desired result can not be obtained. Therefore, the air permeable 
coefficient is usually preferable in a range of 10.sup.-5 to 10.sup.-1 
mol./cm.sup.2.min.cmHg. 
The cathode having the characteristics of an average pore diameter of 0.05 
to 20.mu., a porosity of 30 to 70% and an air permeable coefficient of 
10.sup.-4 to 10.sup.-1 mol./cm.sup.2.cmHg is optimum because any liquid 
leakage is not caused, a satisfactory inner surface area is given and a 
satisfactory gas diffusion effect can be expected. 
In the preparation of the cathode, for example, using carbon as the 
material for the substrate, an aqueous solution of a water soluble salt of 
catalyst such as noble metal chloride, nitrates and sulfates is prepared 
and carbon powder is admixed to support the noble metal salt on the carbon 
powder and then, the noble metal salt is converted into the noble metal by 
reducing it with a reducing agent such as hydrazine and sodium boron 
hydride or by thermal decomposition after the evaporation of water. Then, 
the water repellent materials such as resins, the pore forming agent, and 
a medium for the paste such as water and alcohols are admixed with the 
product and the mixture is kneaded and rolled to mold it in a desired 
thickness. The resulting sheet is bonded on a current collector such as a 
nickel net by means of a pressing process and then, it is baked at 
200.degree. to 380.degree. C. for 10 to 90 minutes in an inert gas 
atmosphere to obtain a porous cathode. It is also possible to obtain the 
porous cathode by mixing the carbon powder, the catalyst source, the pore 
forming agent and the water repellent materials and baking the mixture 
under the reducing condition such as in an inert gas atmosphere or a 
reducing gas atmosphere. 
In the process for producing an alkali metal hydroxide by an electrolysis 
of an aqueous solution of an alkali metal chloride by using the cathode 
obtained by the present invention, as referring to FIG. 1, an electric 
cell (1) is partitioned with a cation exchange membrane (2) by the 
conventional manner into an anode compartment equipped with an anode (3) 
and a cathode compartment (5). An oxygen-containing gas (air) feeding 
compartment (7) is formed by the cathode (6) in the cathode compartment 
(5). It further comprises an inlet (8) for an aqueous solution of an 
alkali metal chloride such as sodium chloride as an electrolyte; an outlet 
(9) for the aqueous solution; an inlet (10) for water into the cathode 
compartment; an outlet (11) for the resulting alkali metal hydroxide; and 
an inlet (12) and an outlet (13) of the oxygen-containing gas. 
The anode used for the present invention can be a metal electrode having 
dimensional stability such as niobium, titanium or tantalum substrate 
coated with a metal oxide such as oxides of ruthenium and rhodium iridium; 
or graphite. The metallic anode is preferably used because the cell 
voltage can be lower than those of the other anodes. 
The cation exchange membrane on which the electrode layer is formed, can be 
made of a polymer having cation exchange groups such as carboxylic acid 
groups, sulfonic acid groups, phosphoric acid groups and phenolic hydroxy 
groups. Suitable polymers include copolymers of a vinyl monomer such as 
tetrafluoroethylene and chlorotriluoroethylene and a perfluorovinyl 
monomer having an ion-exchange group such as sulfonic acid group, 
carboxylic acid group and phosphoric acid group or a reactive group which 
can be converted into the ion-exchange group. It is also possible to use a 
membrane of a polymer of trifluoroethylene in which ion-exchange groups 
such as sulfonic acid group are introduced or a polymer of styrene-divinyl 
benzene in which sulfonic acid groups are introduced. 
The cation exchange membrane is preferably made of a fluorinated polymer 
having the following units, since an alkali metal hydroxide having high 
purity can be obtained at relatively high current efficiency. 
##STR1## 
wherein X represents fluorine, chlorine or hydrogen atom or --CF.sub.3 ; 
X' represents X or CF.sub.3 (CF.sub.2).sub.m ; m represents an integer of 
1 to 5, and Y represents --P--A or --O--(CF.sub.2).sub.n (P,Q,R)A, and P 
represents --CF.sub.2).sub.a (CXX').sub.b (CF.sub.2).sub.c ; Q represents 
--CF.sub.2 --O--CXX').sub.d ; and R represents --(CXX'--OCF.sub.2)hd e; 
(P,Q,R) means to arrange at least each one of P, Q and R in a desired 
order; X and X' are defined above; n is 0 to 1 and a, b, c, d and e are 
respectively 0 to 6; A represents --COOM or --SO.sub.3 M, or a functional 
group which is convertible into --COOM or --SO.sub.3 M by a hydrolysis or 
a neutralization such as --CN, --COF, --COOR.sub.1, --SO.sub.2 F, 
--CONR.sub.2 R.sub.3 and --SO.sub.2 NR.sub.2 R.sub.3 and M represents 
hydrogen or an alkali metal atom; R.sub.1 represents a C.sub.1 -C.sub.10 
alkyl group; R.sub.2 and R.sub.3 represent H or a C.sub.1 -C.sub.10 alkyl 
group. 
The typical examples of Y have the structures bonding A to a fluorocarbon 
group such as 
##STR2## 
x, y and z respectively represents an integer of 1 to 10; Z and Rf 
represent -F or a C.sub.1 -C.sub.10 perfluoroalkyl group. 
It is preferable to use a fluorinated cation exchange membrane having a ion 
exchange group content of 0.5 to 4.0 meq/g dry resin since the current 
efficiency can be greater than 90% even though a concentration of sodium 
hydroxide is higher than 40%. When the ion exchange group content is in a 
range of 1.0 to 2.0 meq/g. dry resin, sodium hydroxide having such high 
concentration can be produced at high current efficiency for a long time 
in stable condition. 
It is preferable that the units (N) is 1 to 40 mol % especially 3 to 20 mol 
% so as to give the above-mentioned ion exchange capacity of the membrane 
in the case of the copolymer having units (M) and (N). The thickness of 
the membrane is preferably in a range of 20 to 600.mu. especially about 50 
to 400.mu.. 
The alkali metal chloride used for the electrolysis is usually sodium 
chloride and can be other alkali metal chlorides such as potassium 
chloride and lithium chloride. 
The present invention will be further illustrated by certain examples and 
references which are provided for purposes of illustration only and are 
not intended to limit the present invention. 
EXAMPLE 1 
A mixture of 55 wt.% of amicron silver having a particle diameter of about 
700 .ANG., 15 wt.% of powdery activated carbon and 15 wt.% of nickel 
formate powder was thoroughly mixed and an aqueous dispersion containing 
polytetrafluoroethylene (m.p. 327.degree. C.) having a particle diameter 
of 0.2 micron at a ratio of 60 wt.% was added at a ratio of 10 wt.% as a 
solid component, and a powdery polytetrafluoroethylene (m.p. 300.degree. 
C.) having a particle diameter of 8 microns was added at a ratio of 5 wt.% 
and the mixture was kneaded. The paste was rolled to obtain a sheet having 
a desired thickness. The sheet was bonded on a nickel net (40 mesh) by a 
press-molding machine under a molding pressure of 1000 kg./cm.sup.2. The 
molded product was baked at 350.degree. C. for 60 minutes in nitrogen gas 
atmosphere to melt-bond polytetrafluoroethylene whereby the water 
repellency and bonding strength were improved and nickel formate was 
thermally decomposed to obtain a porous electrode having an average pore 
diameter of 0.6.mu., a porosity of 56%, and an air permeable coefficient 
of 1.2.times.10.sup.-3 mol./cm.sup.2.min.cmHg. The electrode contained 
silver at a ratio of 50 mg./cm.sup.2. 
The resulting electrode was equipped in the electrolytic cell shown in FIG. 
1 wherein a metal anode made of a titanium substrate coated with ruthenium 
oxide was used and a cation exchange membrane of a fluorinated polymer 
obtained by hydrolyzing a membrane of a copolymer of C.sub.2 F.sub.4 and 
CF.sub.2 .dbd.CFO(CF.sub.2).sub.3 COOCH.sub.3 (thickness=300.mu.; A.sub.R 
=1.48 meq./g.) was used as the membrane. Air (carbon dioxide gas is 
removed) was fed into the gas feeding compartment at a rate of 1 
liter/min. and an electrolysis of 25% aqueous solution of sodium chloride 
was carried out under controlling the feed rates of the aqueous solution 
of sodium chloride and water so as to maintain 30 wt.% of a concentration 
of sodium hydroxide in the cathode compartment. 
In the electrolysis using the cathode of the present invention at a current 
density of 20 A/dm.sup.2, a cell voltage was 2.21 V and a cell voltage was 
increased only for 0.08 V after the operation for 1000 hours. 
EXAMPLE 2 
In accordance with the process of Example 1 except mixing and kneading a 
mixture of 70 wt.% of silver carbonate for a catalyst silver, 10 wt.% of 
powdery activated carbon 15 wt.% of polytetrafluoroethylene (m.p. 
327.degree. C.) having a particle diameter of 0.2 micron and 10 wt.% of 
polytetrafluoroethylene (m.p. 300.degree. C.) having a particle diameter 
of 8 microns a cathode was prepared. The cathode contained silver at a 
ratio of 50 mg./cm.sup.2. 
In accordance with the process of Example 1, except using the cathode, an 
electrolysis was carried out. As a result, a cell voltage at the beginning 
was 2.23 V and the cell voltage was increased only for 0.09 V after the 
operation for 1000 hours. 
EXAMPLE 3 
In accordance with the process of Example 1 except producing powdery 
activated carbon supporting 10 wt.% of palladium obtained by reducing 
palladium chloride on powdery activated carbon with formaline and mixing 
and kneading 85 wt.% of the powdery activated carbon supporting palladium; 
10 wt.% of polytetrafluoroethylene (m.p. 327.degree. C.) having a particle 
diameter of 0.2 micron and 10 wt.% of polytetrafluoroethylene (m.p. 
300.degree. C.) having a particle diameter of 8 microns, a cathode was 
prepared. The cathode contained palladium at a ratio of 2 mg./cm.sup.2. 
In accordance with the process of Example 1 except using the cathode, an 
electrolysis was carried out. As a result, a cell voltage at the beginning 
was 2.21 V and the cell voltage was increased only for 0.08 V after the 
operation for 1000 hours. 
EXAMPLE 4 
In accordance with the process of Example 1 except using 
polytetrafluoroethylene-hexafluoropropene copolymer (m.p. 320.degree. C.) 
having a particle diameter of 17.mu. was used instead of 
polytetrafluoroethylene powder (m.p. 300.degree. C.), a cathode was 
prepared and an electrolysis was carried out. As a result, a cell voltage 
at the beginning was 2.22 V. 
REFERENCE 
In accordance with the process of Example 1 except that the 
polytetrafluoroethylene powder (m.p. 300.degree. C.) was not incorporated, 
a cathode was prepared. The cathode had an average pore diameter of 3.mu., 
a porosity of 72%, and an air permeable coefficient of 1.times.10.sup.3 
mol./cm.sup.2.min.cmHg. 
In accordance with the process of Example 1 except using the cathode, an 
electrolysis was carried out. As a result, a cell voltage at the beginning 
was 2.20 V and the cell voltage was increased for 0.21 V after the 
operation for 1000 hours.