CA1161792A - Bonding silver particles in silver coating on electrode substrate - Google Patents
Bonding silver particles in silver coating on electrode substrateInfo
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- CA1161792A CA1161792A CA000392257A CA392257A CA1161792A CA 1161792 A CA1161792 A CA 1161792A CA 000392257 A CA000392257 A CA 000392257A CA 392257 A CA392257 A CA 392257A CA 1161792 A CA1161792 A CA 1161792A
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Abstract
ABSTRACT OF THE DISCLOSURE
An electrode is prepared by dispersion-plating a metal layer containing exposed porous particles of Ag on an electrode substrate. If desired, a middle layer is formed between the electrode substrate and the metal layer. The porous particles can be formed by etching an alloy of the particles.
An electrode is prepared by dispersion-plating a metal layer containing exposed porous particles of Ag on an electrode substrate. If desired, a middle layer is formed between the electrode substrate and the metal layer. The porous particles can be formed by etching an alloy of the particles.
Description
7~3;~J
The prese-nt invention relates to an electrode to be used in the electrolysis of an aqueous solution at low overvoltage.
More particularly, it relates to a cathode having a low hydrogen overvoltage.
Various anticorrosive electrodes have been used in the electrolysis of aqueous solution to obtain electrolyzed products, such as the electrolysis of an aqueous solution of an alkali metal chloride to obtain an alkali metal hydroxide and chlorine.
When the overvoltage of the electrode in the electroly-sis of an aqueous solution, such as an aqueous solution ofalkalimetal chloride, is lowered, the electric power consumption can be ;~ reduced and the electrolyzed product can be obtained at low cost.
In order to reduce the chlorine overvoltage of the anode, various studies have been made on the materials of the substrate and the treatments. Some of them have been practically employed.
It has become necessary to use an electrode having a low hydrogen overvoltage and anticorrosive characteristics since the diaphragm method for electrolysis has been developed.
In the conventional electrolysis of an aqueous solution of an alkali metal chloride using an asbestos diaphragm, iron mesh has been used as the cathode.
It has been proposed to treat a surface of an iron sub-strate by a sand blast treatment in order to reduce a hydrogen over-voltage of the iron substrate (for example, Surface Treatment Hand-book, Page 541-542(Sangyotosho) by Saka Tajima). However, the as-bestos diaphragm method has the disadvantages of a low concentra-tion of sodium hydroxide such as about 10 to 13 wt.% and contamina-tion with sodium chloride of the aqueous solution of sodium hydrox-ide. Accordingly, the electrolysis of an aqueous solution of an alkali metal chloride using an ion exchange membrane as a diaph-ragm has been studied, developed and practically used.
z ~ n accordance with the latter method, an aqueous solu-tion of sodium hydroxide having a high concentration of 25 to 40 wt.% may be obtained. When the iron substrate is used as a cathode in the electrolysis, the iron substrate is broken by stress cor-rosion cracking or a part of the iron substrate is dlssolved in the catholyte because of the high concentration of sodium hydrox-ide and the high temperature, such as 80C to 120C, in the elec-trolysis.
The present invention provides an electrode which is non-corrosive to an alkali metal hydroxide and which effectively reduces the hydrogen overvoltage for a long time in the electroly-sis .
According to the present invention there is provided anelectrode comprising a metal layer containing exposed particles of Ag formed on an electrode substrate.
On the surface of the electrode of the present invention, many particles of Ag are bonded to form porous layers.
The electrode of the present invention comprises many exposed particles of Ag having a low hydrogen overvoltage on the surface of the electrode to provide a fine porous condition of the surface so that the activity of the electrode is high and the hy-drogen overvoltage of the electrode can be effectively reduced by a synergistic effect.
The exposed particles of Ag are firmly bonded in the me-tal layer formed on the electrode substrate and thus do not deter-iorate substantially prolonging thereby the maintenance of the low hydrogen overvoltage.
The electrode substrate may be made of suitable electri-cally conductive metal such as Ti, Zr, Fe, Ni, V, Mo, Cu, Ag, Mn, platinum group metals, graphite and Cr and alloys thereof and pre-ferably Fe and Fe-alloys (Fe-Ni alloy, Fe-Cr alloy and Fe-Ni-Cr alloy~, Ni and Ni-alloys ~Ni-Cu alloy and Ni-Cr alloy), Cu and Cu-.,.~6~L~7~
alloys, and especially Fe, Cu, Ni, Fe-Ni alloys and Fe-Ni-Cr-alloys.
The electrode substrate may have dimensions suitable for its use as the electrode.
The electxode may be in the form of a plate, porous and net (expanded metal) or parallel screens which can be flat, curved or cylindrical.
The exposed particles of Ay may be made of the metal it-self or an alloy having silver as main component or a combination of the metal of the alloy.
When the composite or the alloy having silver as the main component is used, a metal which does not substantially adversely affect the reduction of the hydrogen overvoltage, such as Al, Zn, Mg, Si, Sb or Sn is used depending upon the content of the addi-tional metal.
The average particle size of the particles is usually in the range of 0.1 to 100 ~ though it depends upon the dispersibili-ty oftheparticles. From the viewpoint of porosity on the surface of the electrode, the average particle size is preferably in the range of 0.9 to 50 ~, especially 1 to 30~.
The particles are preferably porous on their surfaces so as to give a lower hydrogen overvoltage.
The term "porous on their surfaces" means to be porous on the surface exposed over the metal layer and does not mean to be porous on all of the surfaces of the particles.
Higher porosity is preferable, however excessive porosi-ty causes low mechanical strength and accordingly, the porosity is preferably in the range of 35 to 85%, especially 50 to 80~.
The porosity is measured by the conventional water sub-stituting method.
Various methods have been employed for forming the por-ous surface such as removing metals other than Ag from an alloy having Ag as the main component to form the porous surface; con-~617~
verting Ag into a carbonyl compound thereof and thermally decom-posing the carbonyl compound to form the porous surface; thermally decomposing an organic acid salt of Ag to form the porous surface;
and heating an oxide of Ag in a hydrogen reducing atmosphere to form the porous surface.
From a practical viewpoint, it is preferab]e to employ the method of removing metals other than Ag from an alloy having Ag as the main component. In such a method, the particles are made of an alloy comprising the first type metal component of Ag and the second type metal component selected from the group consist-ing of Al,zn , Mg, Si, Sb and Sn and at least part of the secondtype metal component is removed from the alloy.
Examples of such alloys include Ag-Al alloys, Ag-Zn al-loys, Ag-Mg alloys and Ag-Sn alloys.
From the viewpoint of easy availability, it is prefera-ble to use Ag-Al alloys, such as unleached Raney silver.
The metals of the metal layer for bonding the particles are metals having a high alkali resistance and a capability of bond-ing firmly the particles and are preferably selected from the group ` consisting of Ni, Co and Ag, especially silver which is the main component of the particles.
The thickness of the metal layer ranges from 20 to 200 ~, preferably 25 to 150 ~, especially 30 to 100 ~, since the parti-cles are bonded in the metal layer on the electrode substrate with partial burying in the metal layer.
The present invention will be illustrated by way of the accompanying drawings in which:
Figure l is a sectional view of the surface of the elec-trode of the present invention;
Figure 2 is a sectional view of an elec-trode having the middle layer as the schematic view, 7~Z
Fiyure 3 is a schematic view of a plating bath incorpor-ating the electrode of the present invention; and Figure 4 is a schematic view of another plating bath in-corporating the electrode of the present invention.
As shown in Figure 1, the metal layer 2 is formed on an electrode substrate 1 and particles 3 are Eirmly bonded in the me-tal layer so as to expose parts of the particles above the metal layer.
The content of the particles in the metal layer 2 ranges from 5 to 80 wt.~, preferably 10 to 50 wt.%.
~ It is also preferable to form a middle layer made of me~
- tal selected from the group consisting of Ni, Co, Ag and Cu between the electrode substrate and the metal layer containing the parti-cles whereby the durability-of the electrode is improved.
Such middle layer can be made of the same or different metal as that of the metal layer and is preferably made of the same metal from the viewpoint of the bonding strength to the metal layer.
~ The thickness of the middle layer is ranging 5 to 100 ~, preferably 20 to 80 ~, especially 30 to 50 ~.
In Figure 2, the electrode comprises the electrode sub-strate 1, the middle layer 4, the metal layer 2 containing parti-cles and the particles 3.
Many particles are exposed on the surface of the elec-trode on a macro-scale but the surface of theparicles is micro-porous.
The degree of the porisity de~ermines the reduction of hydrogen overvoltage and is satisfactory at more than 1,000 ~F/cm as a value of a double-layer capacity and preferably more than
The prese-nt invention relates to an electrode to be used in the electrolysis of an aqueous solution at low overvoltage.
More particularly, it relates to a cathode having a low hydrogen overvoltage.
Various anticorrosive electrodes have been used in the electrolysis of aqueous solution to obtain electrolyzed products, such as the electrolysis of an aqueous solution of an alkali metal chloride to obtain an alkali metal hydroxide and chlorine.
When the overvoltage of the electrode in the electroly-sis of an aqueous solution, such as an aqueous solution ofalkalimetal chloride, is lowered, the electric power consumption can be ;~ reduced and the electrolyzed product can be obtained at low cost.
In order to reduce the chlorine overvoltage of the anode, various studies have been made on the materials of the substrate and the treatments. Some of them have been practically employed.
It has become necessary to use an electrode having a low hydrogen overvoltage and anticorrosive characteristics since the diaphragm method for electrolysis has been developed.
In the conventional electrolysis of an aqueous solution of an alkali metal chloride using an asbestos diaphragm, iron mesh has been used as the cathode.
It has been proposed to treat a surface of an iron sub-strate by a sand blast treatment in order to reduce a hydrogen over-voltage of the iron substrate (for example, Surface Treatment Hand-book, Page 541-542(Sangyotosho) by Saka Tajima). However, the as-bestos diaphragm method has the disadvantages of a low concentra-tion of sodium hydroxide such as about 10 to 13 wt.% and contamina-tion with sodium chloride of the aqueous solution of sodium hydrox-ide. Accordingly, the electrolysis of an aqueous solution of an alkali metal chloride using an ion exchange membrane as a diaph-ragm has been studied, developed and practically used.
z ~ n accordance with the latter method, an aqueous solu-tion of sodium hydroxide having a high concentration of 25 to 40 wt.% may be obtained. When the iron substrate is used as a cathode in the electrolysis, the iron substrate is broken by stress cor-rosion cracking or a part of the iron substrate is dlssolved in the catholyte because of the high concentration of sodium hydrox-ide and the high temperature, such as 80C to 120C, in the elec-trolysis.
The present invention provides an electrode which is non-corrosive to an alkali metal hydroxide and which effectively reduces the hydrogen overvoltage for a long time in the electroly-sis .
According to the present invention there is provided anelectrode comprising a metal layer containing exposed particles of Ag formed on an electrode substrate.
On the surface of the electrode of the present invention, many particles of Ag are bonded to form porous layers.
The electrode of the present invention comprises many exposed particles of Ag having a low hydrogen overvoltage on the surface of the electrode to provide a fine porous condition of the surface so that the activity of the electrode is high and the hy-drogen overvoltage of the electrode can be effectively reduced by a synergistic effect.
The exposed particles of Ag are firmly bonded in the me-tal layer formed on the electrode substrate and thus do not deter-iorate substantially prolonging thereby the maintenance of the low hydrogen overvoltage.
The electrode substrate may be made of suitable electri-cally conductive metal such as Ti, Zr, Fe, Ni, V, Mo, Cu, Ag, Mn, platinum group metals, graphite and Cr and alloys thereof and pre-ferably Fe and Fe-alloys (Fe-Ni alloy, Fe-Cr alloy and Fe-Ni-Cr alloy~, Ni and Ni-alloys ~Ni-Cu alloy and Ni-Cr alloy), Cu and Cu-.,.~6~L~7~
alloys, and especially Fe, Cu, Ni, Fe-Ni alloys and Fe-Ni-Cr-alloys.
The electrode substrate may have dimensions suitable for its use as the electrode.
The electxode may be in the form of a plate, porous and net (expanded metal) or parallel screens which can be flat, curved or cylindrical.
The exposed particles of Ay may be made of the metal it-self or an alloy having silver as main component or a combination of the metal of the alloy.
When the composite or the alloy having silver as the main component is used, a metal which does not substantially adversely affect the reduction of the hydrogen overvoltage, such as Al, Zn, Mg, Si, Sb or Sn is used depending upon the content of the addi-tional metal.
The average particle size of the particles is usually in the range of 0.1 to 100 ~ though it depends upon the dispersibili-ty oftheparticles. From the viewpoint of porosity on the surface of the electrode, the average particle size is preferably in the range of 0.9 to 50 ~, especially 1 to 30~.
The particles are preferably porous on their surfaces so as to give a lower hydrogen overvoltage.
The term "porous on their surfaces" means to be porous on the surface exposed over the metal layer and does not mean to be porous on all of the surfaces of the particles.
Higher porosity is preferable, however excessive porosi-ty causes low mechanical strength and accordingly, the porosity is preferably in the range of 35 to 85%, especially 50 to 80~.
The porosity is measured by the conventional water sub-stituting method.
Various methods have been employed for forming the por-ous surface such as removing metals other than Ag from an alloy having Ag as the main component to form the porous surface; con-~617~
verting Ag into a carbonyl compound thereof and thermally decom-posing the carbonyl compound to form the porous surface; thermally decomposing an organic acid salt of Ag to form the porous surface;
and heating an oxide of Ag in a hydrogen reducing atmosphere to form the porous surface.
From a practical viewpoint, it is preferab]e to employ the method of removing metals other than Ag from an alloy having Ag as the main component. In such a method, the particles are made of an alloy comprising the first type metal component of Ag and the second type metal component selected from the group consist-ing of Al,zn , Mg, Si, Sb and Sn and at least part of the secondtype metal component is removed from the alloy.
Examples of such alloys include Ag-Al alloys, Ag-Zn al-loys, Ag-Mg alloys and Ag-Sn alloys.
From the viewpoint of easy availability, it is prefera-ble to use Ag-Al alloys, such as unleached Raney silver.
The metals of the metal layer for bonding the particles are metals having a high alkali resistance and a capability of bond-ing firmly the particles and are preferably selected from the group ` consisting of Ni, Co and Ag, especially silver which is the main component of the particles.
The thickness of the metal layer ranges from 20 to 200 ~, preferably 25 to 150 ~, especially 30 to 100 ~, since the parti-cles are bonded in the metal layer on the electrode substrate with partial burying in the metal layer.
The present invention will be illustrated by way of the accompanying drawings in which:
Figure l is a sectional view of the surface of the elec-trode of the present invention;
Figure 2 is a sectional view of an elec-trode having the middle layer as the schematic view, 7~Z
Fiyure 3 is a schematic view of a plating bath incorpor-ating the electrode of the present invention; and Figure 4 is a schematic view of another plating bath in-corporating the electrode of the present invention.
As shown in Figure 1, the metal layer 2 is formed on an electrode substrate 1 and particles 3 are Eirmly bonded in the me-tal layer so as to expose parts of the particles above the metal layer.
The content of the particles in the metal layer 2 ranges from 5 to 80 wt.~, preferably 10 to 50 wt.%.
~ It is also preferable to form a middle layer made of me~
- tal selected from the group consisting of Ni, Co, Ag and Cu between the electrode substrate and the metal layer containing the parti-cles whereby the durability-of the electrode is improved.
Such middle layer can be made of the same or different metal as that of the metal layer and is preferably made of the same metal from the viewpoint of the bonding strength to the metal layer.
~ The thickness of the middle layer is ranging 5 to 100 ~, preferably 20 to 80 ~, especially 30 to 50 ~.
In Figure 2, the electrode comprises the electrode sub-strate 1, the middle layer 4, the metal layer 2 containing parti-cles and the particles 3.
Many particles are exposed on the surface of the elec-trode on a macro-scale but the surface of theparicles is micro-porous.
The degree of the porisity de~ermines the reduction of hydrogen overvoltage and is satisfactory at more than 1,000 ~F/cm as a value of a double-layer capacity and preferably more than
2,000 ~F/cm , especially more than 5,000 ~F/cm .
The electrical double layer capacity is electrostatic capacity of the electric double layer formed by distributing re-7~2 latively positive and negative ions at a short distance near the surface of the electrode when dipping the elec-trode in an electro-lyte and it is measured as a differential capacity.
The capacity is increased depending upon irlcreasing the specific surface of the electrode. Thus, the electrical double layer capacity of the surface of the electrode is increased depend-ing upon increasing the porosity of the surface and a surface area of the electrode. The electrochemically effective surface area of the electrode that is the porosity of the surface of the electrode can be considered by the electrical double layer capacity.
The electrical double layer capacity is varied depending upon the temperature at the measurement and the kind and concentra-tion of the electrolyte~ and on the potential and the electrical double layer capacity in the specification means values measured by the following method.
A test piece (electrode) was immersed in an aqueous so-lution of 40 wt.% of NaOH at 25C and a platinum electrode having platinum black coat (platinized platinum plate) having a specific area of about 100 times the area of the test piece is immersed and the cell-impedance under the conditions is measured by Kohlrausch brid~e to obtain an electrical double layer capacity.
Various methods for coating the surface layer on the elec-trode for example, a dispersion coating method, a melt spraying method can be employed.
The dispersion coating method is especially preferable since the particles can be bonded in the metal layer in the pre-sent invention.
In the dispersion coating method, the particles are sus-pended in the plating bath in which electroplating is carried out and they are codeposited on the substrate with the plated metal.
In order to maintain the dispersing conditions, various methods such as a mechanical stirring method, an air mixing method, 7~
a liquid circulating method, all ultrasonic ~ibrating method and a fluidized bed method can be employed.
When the dispersion coating method is employed by using conductive particles, the electrodeposited material is dendritic and has low strength as disclosed in R. Bazzard, Trans, Inst.
Metal Finishing, 1972, 50 63; J. Foster et al, ibid, 1976, 54 178).
It has been found, in accordance with detailed studies on the dispersion coating method, that the electrodeposited ma-terial is dentritic and has relatively low strength when the stir-ring is not vigorous whereas the electrodeposited material is not substantially dendritic and has high strength and hydrogen over-voltage is low enough when the stirring is vigorous. When the stirring is too vigorous, the amount of the codeposition of the particles is decreased to form a smooth electrodeposition and the hydrogen overvoltage becomes high, though the strength of the metal layer and the bonding strength are sufficientl~ high.
It has been found that the hydrogen overvoltage, strength and shape of the electrodeposition in the dispersion coating me-thod are closely related with the conditions of the dispersion.
In a preparation of an industrial size electrode, if non-uniform codeposition is partially formed, hydrogen overvoltage is low increasing the.current in areas of great codeposition whereas hydrogen overvoltage is high decreasing the current at parts of less codeposition. The current line distribution is dlsadvantage-ously greatly disturbed.
It is important to codeposit uniformly, that is to carry out a dispersion coating under uniform stirring conditions.
Various uniform codeposition methods have been studied, As the result, it has been found that a dispersion coating method comprising using a vertically vibrating perforated plate at a lower part of the plating bath vessel is preferable. It has been found that a method of ~Iniformly stirring the plating bath by injecting an inert gas, such as N2 gas, or a reducing gas, such as H2 gas, into a plating bath vessel is also preferable.
As the result of the studies on a method of uniformly stirring a plating bath by recycling it, it has been found that a plating method comprising flowing a plating solution having dis-persed particles from the lower part to the upper part of a coated plate di.;posed between a pair of anodes is also preferable. In such case, it is also preferable to stir the bath while injecting an inert gas or a reducing gas.
When a silver layer is the metal layer, it is possible to use a silver plating bath (Ag CN 36 g/liter; KCN 60 g/liter and K2CO3 15 g/liter)-It is preferable to use the above-mentioned bath, how-ever the bath is not critical and various silver plating baths can be used.
Particles containing Ag are dispersed in said plating bath. The type and size of the particles are described above.
Particles containing a metal selected from Ni, Co or Ag are dispersed in said plating bath. The type and size of the particles are describad above.
When an alloy made of the first metal of Ag and the se-cond metal of Al, Nz, Mg, Si, Sb or Sn is used, as the particles it is preferable to treat the particles with an alkali metal hy-droxide as described below. The alloy is preferably the unleach ed Raney silver as described.
The particles may be made of the first metal only or of an alloy of the first metal and second metal from which a part of the second metal is removed. In such case, it is unnecessary to treat the particles with an alkali metal hydroxide. Such alloy can be a leached Raney silver.
In such case, it is preferably to partially form an oxide layer on the surface of the particles to stabilize the surface from the viewpoint of handling. In particular, a co~nercially avail-able stabilized Raney silver can be used.
The oxlde coatiny on the particles may be removed by re-ducing the oxide with the hydrogen generated when the electrode is used as a cathode in the electrolysis of an aqueous solution of an alkali metal chloride. The oxide coating may be removed by reducing it, before using the electrode, (for example, heating the electrode in hydrogen).
The concentration of the particles in the bath is pre-ferably in the range of 1 g/liter to 200 g/liter for an improvement of bonding of the particles on the surface ofkhe electrode. The temperature in the dispersion coating method is preferably in the range of 20C to 80C and the current density is preferably in the rangeof 1 A/dm to 20 A/dm .
It is possible to add a desired additive for reducing strain or a desired additive for improving a codeposition in the plating bath.
It is also possible to heat or to repeat the nickel plating after the dispersion coating in order to improve the bond-ing between the particles and the metal layer.
As described, when the middle layer is formed between the electrode substrate and the metal layer containing the parti-cles, the electrode substrate is firstly coated by nickel plating, cobalt plating, silver plating or copper plating and then, the metal layer containing the particles is formed on the middle la-yer by a dispersion coating method or a melt spraying method.
In the formation of the middle layer, various plating baths can be used and the conventional copper plating bath can be also used.
Thus, the electrode having the particles coated through the metal layer on the electrode substrate can be obtained.
Thus, when desired, the resulting electrode is treated _ g _ with an alkali metal hydroxide (for example, an aqueous solution of an alkali metal hydroxide~ to remove at least part of the me-tal component other than Ag in the alloy of the particles.
In the treatment, the concentration of the aqueous solu-tion of the alkali metal hydroxide such as NaOH is preferably in a range of 5 to 40 wt.~ and the temperature is preferably in the range of 50~C to 150C.
When the particles made of the alloy of the first metal and the second metal are used, it is preferable to carry out the alkali metal hydroxide treatment. However it is possible to carry out the electrolysis of an alkali metal chloride in an electro-lytic cell equipped with the electrode. Thus, the second metal component is dissolved during the electrolysis, to decrease the hydrogen overvoltage of the electrode though the resulting aqueous solution of an alkali metal hydroxide is slightly contaminated with the second metal ions due to dissolution.
The eleGtrode of the present invention can~be used as an electrode especially as a cathode ~or the electrolysis of an aqueous solution of an alkali metal cloride in an ion exchange mem-brane process, and it can be also used as an electrode for the elec-trolysis of an aqueous solution of an alkali metal chloride or the electrolysis of water with a porous diaphragm, such as an asbestos diaphragm.
The present invention will be further illustrated by certain examples and references which are provided for purposes of illustration only.
EXAMPLE 1:
Powdery leached Raney silver was dispersed into a silver bath (AgCN 100 g/liter; KCN 100 g/liter; K2CO3 15 g/liter; KOH
The electrical double layer capacity is electrostatic capacity of the electric double layer formed by distributing re-7~2 latively positive and negative ions at a short distance near the surface of the electrode when dipping the elec-trode in an electro-lyte and it is measured as a differential capacity.
The capacity is increased depending upon irlcreasing the specific surface of the electrode. Thus, the electrical double layer capacity of the surface of the electrode is increased depend-ing upon increasing the porosity of the surface and a surface area of the electrode. The electrochemically effective surface area of the electrode that is the porosity of the surface of the electrode can be considered by the electrical double layer capacity.
The electrical double layer capacity is varied depending upon the temperature at the measurement and the kind and concentra-tion of the electrolyte~ and on the potential and the electrical double layer capacity in the specification means values measured by the following method.
A test piece (electrode) was immersed in an aqueous so-lution of 40 wt.% of NaOH at 25C and a platinum electrode having platinum black coat (platinized platinum plate) having a specific area of about 100 times the area of the test piece is immersed and the cell-impedance under the conditions is measured by Kohlrausch brid~e to obtain an electrical double layer capacity.
Various methods for coating the surface layer on the elec-trode for example, a dispersion coating method, a melt spraying method can be employed.
The dispersion coating method is especially preferable since the particles can be bonded in the metal layer in the pre-sent invention.
In the dispersion coating method, the particles are sus-pended in the plating bath in which electroplating is carried out and they are codeposited on the substrate with the plated metal.
In order to maintain the dispersing conditions, various methods such as a mechanical stirring method, an air mixing method, 7~
a liquid circulating method, all ultrasonic ~ibrating method and a fluidized bed method can be employed.
When the dispersion coating method is employed by using conductive particles, the electrodeposited material is dendritic and has low strength as disclosed in R. Bazzard, Trans, Inst.
Metal Finishing, 1972, 50 63; J. Foster et al, ibid, 1976, 54 178).
It has been found, in accordance with detailed studies on the dispersion coating method, that the electrodeposited ma-terial is dentritic and has relatively low strength when the stir-ring is not vigorous whereas the electrodeposited material is not substantially dendritic and has high strength and hydrogen over-voltage is low enough when the stirring is vigorous. When the stirring is too vigorous, the amount of the codeposition of the particles is decreased to form a smooth electrodeposition and the hydrogen overvoltage becomes high, though the strength of the metal layer and the bonding strength are sufficientl~ high.
It has been found that the hydrogen overvoltage, strength and shape of the electrodeposition in the dispersion coating me-thod are closely related with the conditions of the dispersion.
In a preparation of an industrial size electrode, if non-uniform codeposition is partially formed, hydrogen overvoltage is low increasing the.current in areas of great codeposition whereas hydrogen overvoltage is high decreasing the current at parts of less codeposition. The current line distribution is dlsadvantage-ously greatly disturbed.
It is important to codeposit uniformly, that is to carry out a dispersion coating under uniform stirring conditions.
Various uniform codeposition methods have been studied, As the result, it has been found that a dispersion coating method comprising using a vertically vibrating perforated plate at a lower part of the plating bath vessel is preferable. It has been found that a method of ~Iniformly stirring the plating bath by injecting an inert gas, such as N2 gas, or a reducing gas, such as H2 gas, into a plating bath vessel is also preferable.
As the result of the studies on a method of uniformly stirring a plating bath by recycling it, it has been found that a plating method comprising flowing a plating solution having dis-persed particles from the lower part to the upper part of a coated plate di.;posed between a pair of anodes is also preferable. In such case, it is also preferable to stir the bath while injecting an inert gas or a reducing gas.
When a silver layer is the metal layer, it is possible to use a silver plating bath (Ag CN 36 g/liter; KCN 60 g/liter and K2CO3 15 g/liter)-It is preferable to use the above-mentioned bath, how-ever the bath is not critical and various silver plating baths can be used.
Particles containing Ag are dispersed in said plating bath. The type and size of the particles are described above.
Particles containing a metal selected from Ni, Co or Ag are dispersed in said plating bath. The type and size of the particles are describad above.
When an alloy made of the first metal of Ag and the se-cond metal of Al, Nz, Mg, Si, Sb or Sn is used, as the particles it is preferable to treat the particles with an alkali metal hy-droxide as described below. The alloy is preferably the unleach ed Raney silver as described.
The particles may be made of the first metal only or of an alloy of the first metal and second metal from which a part of the second metal is removed. In such case, it is unnecessary to treat the particles with an alkali metal hydroxide. Such alloy can be a leached Raney silver.
In such case, it is preferably to partially form an oxide layer on the surface of the particles to stabilize the surface from the viewpoint of handling. In particular, a co~nercially avail-able stabilized Raney silver can be used.
The oxlde coatiny on the particles may be removed by re-ducing the oxide with the hydrogen generated when the electrode is used as a cathode in the electrolysis of an aqueous solution of an alkali metal chloride. The oxide coating may be removed by reducing it, before using the electrode, (for example, heating the electrode in hydrogen).
The concentration of the particles in the bath is pre-ferably in the range of 1 g/liter to 200 g/liter for an improvement of bonding of the particles on the surface ofkhe electrode. The temperature in the dispersion coating method is preferably in the range of 20C to 80C and the current density is preferably in the rangeof 1 A/dm to 20 A/dm .
It is possible to add a desired additive for reducing strain or a desired additive for improving a codeposition in the plating bath.
It is also possible to heat or to repeat the nickel plating after the dispersion coating in order to improve the bond-ing between the particles and the metal layer.
As described, when the middle layer is formed between the electrode substrate and the metal layer containing the parti-cles, the electrode substrate is firstly coated by nickel plating, cobalt plating, silver plating or copper plating and then, the metal layer containing the particles is formed on the middle la-yer by a dispersion coating method or a melt spraying method.
In the formation of the middle layer, various plating baths can be used and the conventional copper plating bath can be also used.
Thus, the electrode having the particles coated through the metal layer on the electrode substrate can be obtained.
Thus, when desired, the resulting electrode is treated _ g _ with an alkali metal hydroxide (for example, an aqueous solution of an alkali metal hydroxide~ to remove at least part of the me-tal component other than Ag in the alloy of the particles.
In the treatment, the concentration of the aqueous solu-tion of the alkali metal hydroxide such as NaOH is preferably in a range of 5 to 40 wt.~ and the temperature is preferably in the range of 50~C to 150C.
When the particles made of the alloy of the first metal and the second metal are used, it is preferable to carry out the alkali metal hydroxide treatment. However it is possible to carry out the electrolysis of an alkali metal chloride in an electro-lytic cell equipped with the electrode. Thus, the second metal component is dissolved during the electrolysis, to decrease the hydrogen overvoltage of the electrode though the resulting aqueous solution of an alkali metal hydroxide is slightly contaminated with the second metal ions due to dissolution.
The eleGtrode of the present invention can~be used as an electrode especially as a cathode ~or the electrolysis of an aqueous solution of an alkali metal cloride in an ion exchange mem-brane process, and it can be also used as an electrode for the elec-trolysis of an aqueous solution of an alkali metal chloride or the electrolysis of water with a porous diaphragm, such as an asbestos diaphragm.
The present invention will be further illustrated by certain examples and references which are provided for purposes of illustration only.
EXAMPLE 1:
Powdery leached Raney silver was dispersed into a silver bath (AgCN 100 g/liter; KCN 100 g/liter; K2CO3 15 g/liter; KOH
3 g/liter) at the concentration of 100 g/liter and a silver plate was used as an anode and a copper plate was used as a cathode and plating was carried out under the conditions of current density of 6 A/dm at 5QC for 60 minutes.
The silver plated layer had a thickness of about 190 ~
and the content of Raney silvex particles in the silYer plated la-yer was about 35 wt.%. The resulting plated silver layer had an electrical double layer capacity of 4~,000 ~F/cm2. The electrical double layer capacity was measured as follows.
A test piece and a platinum plate coated with platinum black having a specific surface area 100 times the surface area of the piece were immersed in an aqueous solution of NaOH of 40 at 25C, forming a pair of electrodes. The cell-impedance was measured with the Kohlrausch bridge and then the electrical double layer capacity of the test piece was calculated from it.
The electrode potential of the plated silver plate as a cathode versus a saturated calomel electrode as a reference elec-trode was measured in a 40 wt.% aqueous solution of NaOH at 90C
and at 20 A/dm .
As the result, a hydrogen voltage was 130 m V under the conditions described above.
EXAMPLE 2:
Powdery leached ~aney silver was dispersed into a sil-ver bath (AgCN 100 g/liter; KCN 100 g/liter; K~CO3 15 g/liter;
KOH 3 g/liter) at the concentration of 200 g/liter. The resulting dispersion was charged into an electrical plating vessel of Figuxe 3 wherein a perforated plate 5 was moved vertically in the lower part of the vessel and nitrogen gas was injected downwardly through a bubbler 6 and a plate 9 for plating was disposed between a pair of silver electrodes 7, 8 having substantially same area. The perforated plate was moved at a stroke of about 20~ of the height of the bath at 100 Hz/mîn. and the nitrogen gas was injected at a rate of 10 liter/min.dm of the area of the bottom of the vessel.
An electrode substrate 9 immersed in the plating bath as a' cathode was expanded copper metal. The plating was carried out at 50C
.~
~ 2 at a current density of 6 ~/dm for 1 hour to form a yrayish black layer wherein the thickness of the plated nickel layer was about 220 ~ and the content of the leached Raney silver particles in the plated silver layer was about 45 wt.~. The plated silver layer was uniform. The resulting plated silver layer had an electrical double layer capacity of 7,500 ~F/cm2 and a hydrogen overvoltage of 110 mV under the conditions of Example 1.
Example 3:
Powdery leached Raney silver was dipsersed into a sil-ver bath (AgCN 40 g/liter, KCN 60 g/liter; K2CO3 15 g/liter; KOH
2 g/liter) at a concentration of 100 g/liter. The dispersion was fed into a plating vessel 11 shown in Figure 4 wherein a copper plate 12 for plating was disposed between a pair of silver anodes 13, 14 having substantially same area and a plating was carried out with recycling of the dispersion at 50C at a linear flow rate of 70 cm/sec. in the vessel by a pump at a current den-; sity of 6 A/dm for 60 minutes. A grayish black layer was formed and the thickness of the plated silver layer was about 180 ~ and the content of the leached Raney silver in the silver layer was about 30 wt.~. The plated silver layer was uniform. The result-ing plated silver layer had an electrical double layer capacity of 5,000 ~F/cm and a hydrogen overvoltage of 130 m~ under the con-ditions of Example 1.
The silver plated layer had a thickness of about 190 ~
and the content of Raney silvex particles in the silYer plated la-yer was about 35 wt.%. The resulting plated silver layer had an electrical double layer capacity of 4~,000 ~F/cm2. The electrical double layer capacity was measured as follows.
A test piece and a platinum plate coated with platinum black having a specific surface area 100 times the surface area of the piece were immersed in an aqueous solution of NaOH of 40 at 25C, forming a pair of electrodes. The cell-impedance was measured with the Kohlrausch bridge and then the electrical double layer capacity of the test piece was calculated from it.
The electrode potential of the plated silver plate as a cathode versus a saturated calomel electrode as a reference elec-trode was measured in a 40 wt.% aqueous solution of NaOH at 90C
and at 20 A/dm .
As the result, a hydrogen voltage was 130 m V under the conditions described above.
EXAMPLE 2:
Powdery leached ~aney silver was dispersed into a sil-ver bath (AgCN 100 g/liter; KCN 100 g/liter; K~CO3 15 g/liter;
KOH 3 g/liter) at the concentration of 200 g/liter. The resulting dispersion was charged into an electrical plating vessel of Figuxe 3 wherein a perforated plate 5 was moved vertically in the lower part of the vessel and nitrogen gas was injected downwardly through a bubbler 6 and a plate 9 for plating was disposed between a pair of silver electrodes 7, 8 having substantially same area. The perforated plate was moved at a stroke of about 20~ of the height of the bath at 100 Hz/mîn. and the nitrogen gas was injected at a rate of 10 liter/min.dm of the area of the bottom of the vessel.
An electrode substrate 9 immersed in the plating bath as a' cathode was expanded copper metal. The plating was carried out at 50C
.~
~ 2 at a current density of 6 ~/dm for 1 hour to form a yrayish black layer wherein the thickness of the plated nickel layer was about 220 ~ and the content of the leached Raney silver particles in the plated silver layer was about 45 wt.~. The plated silver layer was uniform. The resulting plated silver layer had an electrical double layer capacity of 7,500 ~F/cm2 and a hydrogen overvoltage of 110 mV under the conditions of Example 1.
Example 3:
Powdery leached Raney silver was dipsersed into a sil-ver bath (AgCN 40 g/liter, KCN 60 g/liter; K2CO3 15 g/liter; KOH
2 g/liter) at a concentration of 100 g/liter. The dispersion was fed into a plating vessel 11 shown in Figure 4 wherein a copper plate 12 for plating was disposed between a pair of silver anodes 13, 14 having substantially same area and a plating was carried out with recycling of the dispersion at 50C at a linear flow rate of 70 cm/sec. in the vessel by a pump at a current den-; sity of 6 A/dm for 60 minutes. A grayish black layer was formed and the thickness of the plated silver layer was about 180 ~ and the content of the leached Raney silver in the silver layer was about 30 wt.~. The plated silver layer was uniform. The result-ing plated silver layer had an electrical double layer capacity of 5,000 ~F/cm and a hydrogen overvoltage of 130 m~ under the con-ditions of Example 1.
Claims (14)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An electrode which comprises a plated metal layer made of silver which has firmly bonded thereto 5 to 80% by weight of partially exposed particles made of silver on an electrode substrate.
2. An electrode according to claim 1, having an elec-trical double layer capacity of the surface greater than 1,000 µF/cm2.
3. An electrode according to claim 1, wherein the con-tent of the particles in the plated metal layer is in a range of 10 to 50 wt.%.
4. An electrode according to claim 1, wherein a middle layer made of at least one of nickel, cobalt, silver and copper is formed between the electrode substrate and the plated metal layer containing the particles.
5. An electrode according to claim 1, wherein the parti-cles are formed by removing at least part of a second metal compo-nent from particles made of an alloy of a first metal component of Ag and the second metal component selected from the group consist-ing of Al, Zn, Mg, Si, Sb and Sn.
6. An electrode according to claim 1 or 5, wherein the particles are made of leached Raney silver.
7. A process for preparing an electrode which comprises bonding particles made of Ag in a metal layer made of Ag on an electrode substrate to provide 5 to 80% by weight of said particles.
8. A process according to claim 7, wherein the parti-cles and the metal layer are bonded on the electrode substrate by a dispersion coating method.
9. A process according to claim 7 or 8, wherein the par-ticles are formed by removing at least part of a second metal com-ponent from particles made of an alloy of a first metal component of Ag and the second metal component selected from the group con-sisting of Al, Zn , Mg, Si, Sb and Sn.
10. A process according to claim 7, wherein particles made of an alloy of a first metal component of Ag and a second metal component selected from the group consisting of Al, Zn, Mg, Si, Sb and Sn are bonded in the metal layer made of Ag on the elec-trode substrate and treated with an alkaline solution to dissolve at least part of the second metal component from the particles.
11. A process according to claim 8, wherein the disper-sion is uniformly dispersed by moving a perforated plate vertical-ly in an up and down direction in a lower part of a plating ves-sel in the dispersion coating method.
12. A process according to claim 8, wherein the disper-sion is uniformly dispersed by feeding the dispersion containing the particles into a plating vessel.
13. A process according to claim 11, wherein a gas is bubbled from the bottom of the plating vessel in the dispersion coating method.
14. A process according to claim 13, wherein the gas is an inert gas or a reducing gas.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000392257A CA1161792A (en) | 1978-02-24 | 1981-12-14 | Bonding silver particles in silver coating on electrode substrate |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP1992578A JPS54112785A (en) | 1978-02-24 | 1978-02-24 | Electrode and manufacture thereof |
| JP19925/1978 | 1978-02-24 | ||
| CA000321783A CA1143325A (en) | 1978-02-24 | 1979-02-19 | Process for preparing a low hydrogen overvoltage electrode by decomposition method |
| CA000392257A CA1161792A (en) | 1978-02-24 | 1981-12-14 | Bonding silver particles in silver coating on electrode substrate |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1161792A true CA1161792A (en) | 1984-02-07 |
Family
ID=27166083
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000392257A Expired CA1161792A (en) | 1978-02-24 | 1981-12-14 | Bonding silver particles in silver coating on electrode substrate |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1161792A (en) |
-
1981
- 1981-12-14 CA CA000392257A patent/CA1161792A/en not_active Expired
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