US3824175A - Bipolar electrode - Google Patents

Abstract

AN IMPROVED DIMENSIONALLY-STABLE BIPOLAR ELECTRODE FOR USE IN ELECTROCHEMICAL PROCESSES COMPRISING A VALVE METAL LAYERS, SUITABLE ANODIC MATERIAL ON THE ANODE SIDE OF THE VALVE METAL, A BARRIER LAYER OF CARBON ON THE CATHODE SIDE OF THE VALVE METAL PROTECTED BY A LAYER OF COMMON METAL CATHODIC MATERIAL. SUCH ELECTRODES FUNCTION AT LOW HYDROGEN PERMEABILITY RATES DURING USE IN ELECTROLYTIC PROCESSES.

Description

United States Patent 3,824,175 BIPOLAR ELECTRODE Robert F. Schultz, Niagara Falls, and Edward H. Cook, Jr., Lewiston, N.Y., assignors to Hooker Chemical Corporation, Niagara Falls, N.Y. No Drawing. Filed May 25, 1973, Ser. No. 364,199 Int. Cl. Btllk 1/00, 3/06; C01b 11/26 US. Cl. 204-290 F Claims ABSTRACT OF THE DISCLOSURE This invention relates to electrodes for use in electrolytic cells. More particularly, this invention relates to improved, corrosion-resistant, dimensionally-stable bipolar electrodes particularly useful in the electrolysis of alkali metal chlorides in the production of alkali metal chlorates.

The electrolysis of aqueous solutions of alkali metal chlorides such as sodium chloride and potassium chloride has been conducted commercially for years on a wide scale.

Graphite electrodes have been employed in the past in various alkaline chloride electrolysis operations; however, there have been certain disadvantages which have arisen as a result of the use of graphite. One of the most serious of the disadvantages is the constant attrition of the graphite during the electrolysis operation. Attrition results in the increase of the clearance or spacing between the anode and the cathode which in turn causes an increase in the cell voltage drop and a resulting decreasing efliciency in cell operation.

Graphite anodes have a limited life, generally being on the order of about 1.0 inches in thickness on installation but at the end of 10-12 months of continuous use may be reduced to about 0.25 inches, with the attendant loss in power and efliciency. Such losses, including economic losses, have resulted in the proposed use of metallic electrodes and the use of bipolar cells.

Generally in the production of alkali metal chlorate, bipolar electrodes are now preferentially used which, when arranged suitably in an electrolytic cell, in a spaced electrical series, serve to function both as the anode and cathode in the cell. The electrodes are subjected to an electrical potential while immersed in the alkali metal chloride solution and, electrochemically, alkali metal chlorate is produced, either in the cell itself, or outside the cell upon the standing of the solution.

The advantages of the use of the bipolar cells and bipolar electrodes include:

(a) bipolar cells are relatively simpler and more economical to produce than are monopolar cells;

(b) the electrical contact for supplying current to the electrodes in bipolar cells is applied only through the first and last plates while the current supply to the anodes of monopolar cells must be supplied by electrical contact established with each individual anode;

(c) bipolar cells allow for the use of minimal distances between the electrodes which reduces voltage and allows for a reduction of the volume of electrolyte used.

Platinum group metal-coated electrolytic valve metals such as titanium have been proposed as substitutes for graphite anodes. The metallic electrodes have offered ice several potential advantages over conventional graphite electrodes, as for example, lower overvoltage, lower erosion rates and the resulting electrolytic production of higher purity products. The economic advantages gained by the use of such electrodes, however, must be sufficiently high to overcome the high cost of these metallic electrodes.

A problem existant with bipolar electrodes based on anodic precious metals is that the titanium or valve metal support is attacked by hydrogen during the electrolysis on the cathode side, forming hydrides and causing disintegration of the electrode.

The present invention provides a bipolar electrode having excellent electrolytic use characteristics and excellent durability. The electrodes of the present invention are composite bipolar electrodes having a base layer of valve metal, preferably titanium, an anodic material, preferably a platinum group metal or metal oxide, deposited on one side of the valve metal, and a barrier layer of carbon on the opposing side of the valve metal, overlaid with a layer of suitable cathodic metal.

The electrode comprises a central or inner layer of a valve metal of which the oxide is chemically resistant under anodic conditions to the electrolyte employed. The expression valve metal as employed herein is definitive of a metal which can function generally as a cathode in an electrolytic cell, but not generally as an anode, due to the formation, under anodic conditions, of the oxide of the metal, which oxide once developed is highly resistant to the passage therethrough of electrons.

The preferred valve metal is titanium, although tantalum or colombium may also be advantageously employed.

The expression chemically resistant under anodic conditions hereinbefore employed, as applied to the valve metal, indicates that the oxide is resistant to the corrosive surrounding electrolyte and is not, to an appreciable extent, subject to erosion, deterioration or to electrolyte attack.

The carbon has a low hydrogen diffusion rate and effectively prevents the migration of cathodically produced hydrogen from reaching the surface of the valve metal. Either carbon as such as graphite may be used as the carbon layer.

One face of the central or inner valve metal layer is adhered to the carbon layer by any means readily available in the art, particularly by sputtering the valve metal onto the carbon layer. The thickness of the layers is not critical, it only being necessary that the thickness of the titanium or other valve metal central or inner layer furnish a self-supporting layer, and that the layer of carbon be of such thickness and characteristics as to provide an essentially hydrogen impermeable barrier layer. Generally, the valve metal layer is on the order of from about 0.10 inch to about 0.70 or 0.80 inches in thickness, with the layer of the carbon being on the order of from about 0.01 inch in thickness to up to about 0.1 inch in thickness, if desired.

At least an operable portion of the opposing face of the central or inner layer has bonded thereto a layer of suitable anodic material chemically resistant under anodic conditions to the electrolyte employed. The term suitable anodic materia as employed herein refers to a material which is electrically conductive, resistant to oxidation and substantially insoluble in the electrolyte. Platinum is the preferred anodic material; however, it is also possible to utilize ruthenium, palladium, osmium, iridium, oxides of these materials, alloys of two or more of the metals, or suitable mixtures thereof.

The cathodic material, or outer cathode side layer may be of a suitable, common cathodic material chemically resistant or insoluble in the electrolyte under cathodic conditions. Such materials as steel, copper, chromium,

cobalt, nickel, iron or alloys of these metals may be used. The cathodic material may be applied to the carbon layer by any suitable means known to the art which is nondestructive to the carbon layer, e.g., electroplating, vacuum deposition, metal spraying or the like. The thickness of the cathodic material layer may vary, but the material is generally utilized as a thin film, about 0.01 inch in thickness or greater.

The anodic material, preferably platinum, can be applied to the anodic side of the valve metal by using such sources of platinum as chloroplatinic acid or a thermallydecomposable metallo-organic compound such as a platinum resinate.

For example, the metallic resinate may be mixed with an organic solvent or diluent, such as terpenes or aromatics, typically oil of turpentine, xylene or toluene, before being applied to the base member. The electrode is heated to decompose and/ or to volatilize the organic matter and other non-metallic components, leaving on the base member a layer of adherent electroconductive platinum.

In producing a metallic anodic coating by such method, care should be taken to avoid oxide formation, for example, by limiting the temperatures of heating or by effecting heating in an oxygen-free atmosphere such as in a vacuum or under a nitrogen or argon blanket.

Heating may be elfected in an air atmosphere; however, temperatures above about 600650 C. are not recommended due to the possibility of oxidizing the valve metal.

In the production of an anodic oxide coating, the temperatures and times of heating are selected that will result in the formation of an oxide, preferably an oxide of a metal of the platinum series of metal, such as ruthenium. The temperature applied may vary depending upon the particular platinum metal used. Typically, the temperature may be in the range of from about 300 to about 600 C., preferably from about 350 C. to about 550 C., with such temperatures applied for periods on the order of from about minutes to about 2 hours. The heating of the metal is most advantageously conducted in an atmosphere containing elemental oxygen such as air or other oxygen-inert gas mixtures although an atmos phere of pure oxygen could be used. The platinum group metal oxide formed is either crystalline of amorphous depending upon the temperature of heating, with the degree of crystallinity increasing as temperatures and duration of heating are increased. Both crystalline, particularly if the crystals are small in size, and non-crystalline coatings have good electroconductivity. Where the coatings have a low degree of crystallinity, improved adhesion and conductivity are noted.

It is not necessary that the anodic material be applied in such a manner as to completely cover the entire surface of the valve metal central or inner layer. However, the total anodic side of the central or inner layer should be coated with the anodic material to the extent that the massed portion of the anodic material essentially cover the anodic side of the valve metal layer.

The anodic layer, preferably a platinum group metal or metal oxide, can be deposited to the extent of 0.0001 inch, although the use of lesser or greater thicknesses may be achieved, depending on the methods of deposition, it only being necessary that the anodic material be present on the anodic side of the central or inner layer in an amount suflicient to function effectively as the anode.

The anodic material, as hereinbefore stated, can be deposited on the central or inner layer of the valve metal by any suitable method known to the art. The deposition can be eifected, for example, by using a bath consisting of 4.5 grams platinic chloride and 22 ml. 37 percent hydrochloric acid dissolved in 2800 ml. water. The tem perature is generally maintained between 70 and 85 C. and the current intensity is such that essentially no hydrogen is evolved at the valve metal panel. A graphite anode is used in the bath and the valve metal is made cathodic. The panel is agitated or moved during the plating opera tion, and the current is regulated as to preclude hydrogen involvement at the valve metal, with the anodic platinum metal being deposited at a thickness less than about 0.0001 inch. Minor variations may be effected in the deposition of the precious metal and varying thicknesses may be obtained by suitable modifications in the time consumed in the electroplating operation. Also, simultaneous deposition may be made of more than one component, as for example, by effecting a coating from a solution containing, in addition to platinum, another platinum group metal compound such as ruthenium, such other component being added to the electroplating bath whereby a desired resulting composite coating is obtained by the electrodeposition.

The electrodes of the present invention, as stated, find particular application in the electrolytic production of alkali metal chlorates. In producing chlorates using the electrodes of the present invention, the process may be carried out continuously by passing a solution containing alkali metal chloride through the cells at temperatures generally on the order of up to the boiling point of the electrolyte with the efiluent liquor cooled or concentrated to promote crystallization of the chlorate produced in the cell. Advantageously, a small amount of chromate may be added to the liquor fed to the cell in order to promote chlorate formation, in accordance with methods known in the art.

A typical bipolar electrolytic unit which can be used with the novel electrodes of the present invention consists of a housing having spaced-apart end electrodes with the enclosed space defined by the walls and end electrodes divided intermediate at intervals by the bipolar electrodes into substantially isolated unit cells. For each individual electrolysis cell, there is an individual electrolysis cell, there is an individual reaction zone and an individual electrolysis zone substantially isolated from the reaction and electrolysis zones of the adjoining cells, the term unit cell referring to one of the chambers or sections into which the apparatus is divided by the bipolar electrodes. Such cells makeup permits a good circulation of the electrolyte between the zones.

A bipolar electrolytic cell utilizing the bipolar electrodes described has essentially minimal or no current leakage and voltages on the order of 3.8 to 4.0 volts can be employed, with a current density of about 4 amp/m What is claimed is:

1. A bipolar electrode consisting of a layer of valve metal, at least a portion of the anodic surface of which is conductively covered by a material selected from the group consisting of platinum, palladium, ruthenium, osmium, iridium, mixtures thereof and oxides thereof, a barrier layer of carbon on the cathodic side of the valve metal layer and at least a portion of the exterior surface of the layer of carbon covered by a material selected from the group consisting of steel, copper, chromium, cobalt, nickel iron and alloys thereof.

2. A bipolar electrode as defined by Claim 1 wherein the valve metal is titanium.

3. A bipolar electrode as defined by Claim 1 wherein the anode side covering material is platinum metal.

4. A bipolar electrode as defined by Claim 1 wherein the cathode side covering material is nickel.

5. A bipolar electrode as defined by Claim 1 wherein the cathode side covering material is iron.

6. A bipolar electrode as defined by Claim 1 wherein the anode side covering material is ruthenium oxide.

7. A bipolar electrode as defined by Claim 2 wherein the anode side covering material is platinum metal.

8. A bipolar electrode as defined by Claim 2 wherein the anode said covering material is ruthenium oxide.

9. A bipolar electrode as defined by Claim 7 wherein the cathode side covering material is iron.

References Cited UNITED STATES PATENTS 5 Tirrell 204-180 Hall et a1. 204-256 Jacobson 29-195 A Ono et al 204-290 F Bianchi er a1 204-242 Martinsons 117-221 6 3/1972 Chisholm 204-95 6/1972 King et al 204-284 1/1973 Anthony 204-1 R 4/1963 Stackhouse 29-198 X FOREIGN PATENTS 5/1967 Great Britain 204-290 F US. Cl. X.R..

2%? UNITED STATES PATENT OFFICE GER'llFICATE OF 0RRECTION Patent No 3 g24 175 Dated J l 15 1971.,

1nv nt g7f(@: Robert F. Schultz et a1 It is certified that error appears in the above-identified patent and that eaid Letters Patent are hereby corrected as shown below:

. '1 Column 2, line 42, "such as" should read such or Column 4,

line 59, "nickel iron" line 43,, Such cell-s" should read Such cell line 73, "said" should read side should read nickel, iron Signed and sealed this 5th day of November 1974.

(SEAL) Attest;

MCCOY 1Y1. GIBSON JR. C, MARSHALL DANN Attestmg Officer Commissioner of Patents