US3884792A

US3884792A - Bipolar electrodes

Info

Publication number: US3884792A
Application number: US393945A
Authority: US
Inventors: James D Mcgilvery
Original assignee: Erco Industries Ltd
Current assignee: Tenneco Canada Inc
Priority date: 1972-09-15
Filing date: 1973-09-04
Publication date: 1975-05-20
Anticipated expiration: 1992-05-20
Also published as: CA1094981A

Abstract

A bipolar electrode structure includes an anode face, a layer of an atomic hydrogen permeable base metal providing the cathode and a layer of passivatable metal located between the anode face and the cathode. A layer of electrically conducting metal or alloy resistant to atomic hydrogen flow therethrough is located between the cathode and the layer of passivatable metal to control the flow of atomic hydrogen from the cathode face to the passivatable metal layer.

Description

I Unlted States Patent 191 [111 3,884,792 McGilvery May 20, 1975 BIPOLAR ELECTRODES 3,759,8l3 9/1973 Raetzsch et al 204/256 [75] Inventor: James D. McGilvery, Etobicoke, FOREIGN PATENTS OR APPLICA'HQNS Omam' Canada |.:47,442 4/1969 United Kingdom 204/290 F [73] Assignee: Erco Industries Limited. lslington.

Canada Primary Examiner-F. C. Edmundson [22] Filed: Sept. 4, 1973 Alrorney. Agent, or Firm-Sim & McBurney [21) Appl. No.: 393,945

[ 57] ABSTRACT [30] Foreign A li tion Priority D t A bipolar electrode structure includes an anode face. Sept. 15 [972 Canada [51886 a layer of an atomic hydrogen permeable base metal providing the cathode and a layer of passivatable 52 Us CI H 204 290 F; 204 25 204 2 metal IOCEltfid between the anode face and the cath- [51] Int. Cl B 0lk 3/06 A layer of electrically conducting metal alloy 58 1 Field of Search 204/256 268 290 F resistant ammic hydrogen therehmugh is cated between the cathode and the layer of passivata- [56] References cued ble metal to control the flow of atomic hydrogen from UNITED STATES PATENTS the cathode face to the passlvatable metal layer 3,441,495 4/1969 Colman 204/268 13 Claims, 6 Drawing Figures BIPOLAR ELECTRODES FIELD OF INVENTION This invention relates to bipolar electrodes and more particularly to bipolar electrodes useful in the electrolysis of brine solutions.

BACKGROUND OF THE INVENTION A number of electrolytic chemical manufacturing processes are known in which an important commercial factor is the corrosion of the electrodes by anolyte. Examples of such processes are the electrolysis of brine to form caustic soda and chlorine in a diaphragm or a mercury cathode cell, or and the electrolysis of brine to form sodium chlorate.

In the production of sodium chlorate, brine is electrolysed between closely spaced electrodes so that the anolyte and catholyte, containing chlorine and caustic soda respectively, mingle and react to form sodium hypochlorite. The hypochlorite is passed to a reservoir and allowed to disproportionate relatively slowly into sodium chlorate and sodium chloride. Hydrogen is formed at the cathodes and generally is vented to atmosphere. Commonly the electrolysis is carried out in a tank which is divided into unit cells by thin, closely spaced, parallel, vertical transversely disposed electrode sheets. In this way efficient reaction is achieved in as small a volume as possible with economic use of power. The electrode sheets at either end of the tank are connected to bus bars and function as terminal electrodes. The remaining sheets function as bipolar electrodes each having one face cathodic and one face anodic in successive unit cells.

In the past the most frequently used material for the bipolar electrodes has been graphite. Graphite is considered a satisfactory material for the cathode side of a bipolar electrode, but generally is rather unsatisfactory for the anode side of the bipolar electrode. The anode side tends to be oxidized and to some extent, disintegrates to sludge, and hence wears away. As the electrode wears away and hence the gap between the electrodes increases, there is an increase in the electrical resistance of the cell, with consequent power losses. Further, the rate of wear increases rapidly with temperature, and it is necessary therefore to employ expensive cooling apparatus to maintain the cell temperature at a practical level. Additionally, graphite electrodes are inherently less efficient electrically than metal electrodes.

In view of the above-described disadvantages of graphite bipolar electrodes, attention has been directed to finding other materials more Suitable for bipolar electrodes.

The anolyte in a brine electrolysis cell is highly corrosive and apparently rules out the use of base metals, such as iron, for use in the provision of the anodic face. Noble metals of the platinum group are capable of withstanding the anolyte, but their cost prevents their economic use in commercial operations.

It has been proposed to overcome these latter problems by coating titanium with a very thin layer of platinum or other noble metal or alloy thereof, or of a conductive or semiconductive oxide, for example, an oxide of a platinum metal or mixtures thereof. The titanium provides a conductive base, but cannot be utilized as an electrode since the anodic surface is rendered passive by the formation of an inert, nonconducting layer of titanium oxide. The platinum layer on the titanium functions as a conductive surface of the titanium and the layer may be porous since it is unnecessary to protect the passivated surface of the titanium. In this way, the quantity of noble metal used in the electrode may be reduced to an economically viable level. Such platinised titanium electrodes have enjoyed some success as monopolar anodes in caustic/chlorine and chlorate cells. However, attempts to use coated titanium sheets as bipolar electrodes in chlorate cells have failed.

One reason for this failure is that the cathodic face of the electrode can absorb hydrogen atoms generated there and these hydrogen atoms can migrate to all parts of the titanium sheet forming hydrides which cause expansion of the titanium lattice with resultant distortion and disintegration of the sheet, weakening of its structure and loss, or peeling off, of the platinum layer from its anodic surface.

SUMMARY OF INVENTION In accordance with the present invention, there is provided a bipolar structure which utilizes the platinum-coated titanium anodic structure while minimizing the hydriding effects on the titanium. The invention is not limited to titanium, but also is applicable to other passivatable metals capable of detrimental attack by hydrogen, for example, tantalum, niobium, zirconium and hafnium.

The present invention provides a bipolar electrode structure including a layer of a hydrogen degradable passivatable metal, typically titanium or tantalum, a layer of a base metal providing the cathode, and a layer of a metal or alloy thereof resistant to atomic hydrogen flow positioned between the base metal cathode layer and the passivatable metal layer.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic sectional elevation view of bipolar electrode structure in accordance with one embodiment of the present invention;

FIG. 2 is a schematic plan view ofa portion of a cell using a bipolar electrode structure in accordance with a second embodiment of the invention;

FIG. 2A is a detail of the cell structure of FIG. 2;

FIG. 3 is a schematic plan view of a detail of a cell using a bipolar electrode structure in accordance with a third embodiment of the invention;

FIG. 4 is a schematic plan view of a portion of a cell using a bipolar electrode structure in accordance with a fourth embodiment of the invention; and

FIG. 5 is a schematic plan view of a portion of a cell using a bipolar electrode structure in accordance with a fifth embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS As illustrated in FIG. 1, a bipolar electrode may be provided in the form of a flat upright sheet consisting of a plurality of flat metallic layers bonded together. A sheet-like electrode of this type may be used as a substitute for the commonly-used graphite electrodes in cell structures of the type discussed above wherein thin, closely-spaced, parallel, vertical transversely disposed electrode sheets.

An end sectional view of the electrode is shown in FIG. I and includes a very thin anode layer 10 which is a platinum group metal or an interalloy of two or more of such metals. Such a platinum group metal may 3 include platinum, osmium, iridium, ruthenium, palladium, rhodium and indium, and the interalloys thereof may include platinum/iridium.

A conductive metal oxide layer including at least one oxide of a platinum group metal also may be employed as the anode layer 10. Such conductive oxides may be comprised of an oxide of a platinum group metal or a mixture of oxides of platinum group metals. Alternatively, an electrically conductive mixture of oxides may be used including at least one oxide of a platinum group metal and at least one oxide selected from oxides of manganese, lead, chromium, cobalt, iron, titanium, tantalum, zirconium and silicon.

The anode layer is provided on a passivatable metal layer 12, commonly titanium, but also including tantalum, niobium, hafnium and zirconium. Alloys predominating in one or more of these metals also may provide the passivatable metal layer [2.

The passivatable metal layer 12 is rendered chemically passive by the corrosive anolyte, by virtue of the formation of an inert oxide layer thereon. Hence, the very thin anode layer 10 may be incoherent, if desired.

The metals which form the passivatable metal layer 12 to some extent are affected by hydrogen, whether by the formation of hydrides or embrittlement caused by hydrogen adsorption, and hence cannot be used to provide the cathode face of the electrode.

A cathode layer 14 is provided, generally constructed of a base metal. The expression base metal is used herein to refer to those inexpensive metals which are commercially available for common constructional purposes. Base metals are characterized by low cost, ready availability and adequate resistances to chemical corrosion when used as a cathode. Base metals include iron, aluminum, nickel, lead, tin and zinc.

The term base metal" also includes alloys such as mild steel, stainless steel, bronze, brass, monel and cast iron. The base metal preferably is chemically resistant to the catholyte and has a high electrical conductivity. Low carbon mild steel is a preferred material, and has a low hydrogen overvoltage.

It is possible to employ base metals which are not fully chemically resistant to the catholyte, in which case a protective layer of a chemically-resistant metal such as copper or iron usually is provided on the cathode surfaces thereof.

The above-mentioned base metals are permeable to a greater or lesser extent to atomic hydrogen and hence atomic hydrogen may pass through the base metal layer 14 towards the titanium metal layer 12. In accordance with the present invention, therefore, there is provided a layer 16 of a metal resistant to atomic hydrogen flow therethrough, herein referred to as a barrier metal".

A veriety of metals may be used to provide the barrier layer 16, and examples of suitable metals include gold, tin, lead, nickel, cobalt, chromium, tungsten, molybdenum, cadmium and copper, and alloys thereof resistant to atomic hydrogen flow therethrough.

The relative thickness of the layers l2, l4 and 16 may vary widely depending on a number of factors. Usually one or the other of the passivatable metal layer 12 and the base metal layer 14 is of such a thickness to provide the supporting structure of the electrode. Usually the supporting structure is provided by the base metal layer 14 in view of the lower cost of base metals as compared with that of the passivatable metals, and, in this structure, the passivatable metal layer 12 is thin in comparison to the base metal layer 14. In this structure the layer 16 is sufficiently thick and rigid to be selfsupporting and not to buckle under the pull of any differential thermal expansion between the base metal, the barrier metal and the passivatable metal. If desired, however, the base metal layer 14 and the passivatable metal layer 12 may be substantially the thickness to combine with the barrier metal layer 16 to provide the support for the electrode.

The thickness of the barrier metal layers 16 may vary widely, and generally depends on the metal chosen. For those metals which are substantially impervious to atomic hydrogen, such as gold, a thin film only is required to prevent the hydriding of the titanium, although thicker layers may be provided however, if desired. For those metals which have a finite atomic hydrogen permeability, although substantially lower than the permeability of the base metal, a thicker layer 16 preferably is used, the thickness being chosen to providea significant degree of protection to the passivatable metal layer 12.

The absolute thickness of the layers depends on the total thickness required for the electrode and the relative thickness, as discussed above. In a typical elec trode structure, a mild steel cathode layer 14 of 0.5 inch, a copper barrier metal layer l6 of 0.1 inch and a titanium metal layer 12 of 0.1 inch thicknesses may be provided.

The anode layer 10 generally is as thin as possible, in view of the cost of the platinum group metals.

Turning now to consideration of FIGS. 2 and 2A, there is a plan view of a cell structure utilizing bipolar electrodes. The electrodes in this embodiment differ in form from that illustrated in FIG. I, but nevertheless incorporate the present invention. A cell 50 includes a plurality of cathode sheet like layers 52 arranged parallel to each other in a plurality of rows 54 with sheet like anode layer 56 being positioned between each adjacent parallel pair of cathode layers 52.

Each cathode layer 52 is joined to an anode layer 56 ofa laterally adjacent row 54 in any convenient manner to provide a bipolar electrode. Each anode layer 56 may be provided by platinum coated titanium or any other desired combination as discussed above with reference to FIG. 1. The cathode layers 52 may be constructed of any of the base metals discussed above with reference to FIG. I, typically mild steel.

Spacers S8 of any convenient inert non-conductive material, for example, silicon rubber are provided to maintain the anode layers 56 and cathode layers 52 in spaced apart relation in the cell 50, and to separate the rows 54 from fluid flow relationship with each other.

In accordance with the present invention, a barrier metal layer 60 is provided between the anode layer 56 and the cathode layer 52 of each bipolar electrode, as may be seen more clearly in FIG. 2A, in order to minimize the passage of atomic hydrogen through the cathode layer 52 to the anode layer 56, Any of the barrier metals discussed above with reference to FIG 1 may be used, typically copper.

Considering now the embodiment of FIG. 3, there is shown a detail of an alternative cell construction. A bipolar electrode consists of a sheet-like anode 72 and a sheet-like cathode 74 joined to each other through a core 76, to which the anode 72 and cathode 74 are securcd such as by capped tungsten screws 78. The unit cells in which the anode 72 and cathode 74 are located are separated by barriers 80 constructed of any convenient inert, non-conductive material. The barriers 80 are perforated by holes the edges of which fit into peripheral channels 82 provided in the core 76 and are sealed by suitable O-ring seals 84 against liquid and gas flow between the unit cells. The metal cores 76 may be circular or other suitable shape.

The anode 72 may be provided by platinum coated titanium, or other combination of materials is discussed above with reference to FIG. 1. The cathode 74 may be provided by mild steel, or other base metal as discussed above with reference to FIG. 1.

In accordance with this invention a barrier metal layer 86, of any convenient metal, typically copper, is provided in the core 76 to minimize passage of atomic hydrogen through the core 76 to the titanium layer remainder of the core 76 attached to the cathode 74 being constructed of the same material as the cathode 74, and the remainder of the core 76 attached to the anode 72 being constructed of the same material as the anode 72.

In the embodiment of FIG. 4, there is shown a plan view detail of a cell 100 including a cell box wall 102 of suitable inert non-conductive material, typically polypropylene, and a sheet-like unit cell divider 104 constructed of titanium, or other passivatable metal, and dimensioned to support a plurality of individual anodes and cathodes.

Mounted to one side of the cell divider 104 is a plurality of individual sheet-like anodes 106 which extend generally perpendicularly from one face 107 of the cell divider 104. The sheet-like anodes 106 may be formed of titanium of the metal from which the separator 104 is formed, and have platinised outer surfaces at least over their length which interleaves with cathode plates 108 which are attached to the next separator in the cell 100.

A plurality of cathode plates 110 is attached to the face 111 of the separator 104 opposite to that from which the anodes 106 project to provide a bipolar electrode structure. The cathode plates 110 are attached in pairs to titanium lugs 112 which project from the face 111 of the separator 104. Bolts 114, secured by nuts 116 pass through each pair of cathode plates 110 and the appropriate lugs 112. For each pair of cathode plates 110 there are provided at least two such lugs 112, located respectively adjacent the top and adjacent the bottom of the separator 104.

The bolts 114 and nuts 116 usually are constructed of the same material as the cathode plates 110 which themselves may be constructed of any convenient base metal, typically mild steel, as discussed above with reference to FIG. 1.

In accordance with the present invention, two copper washers or spacers 118 are sandwiched between the titanium lugs 112 and the mild steel cathode plates 110 to minimize the migration of hydrogen from the cathode plates 110 to the titanium lugs 112 and separator 104. Any other convenient barrier metal may be used to provide the washers 118, as discussed above with reference to FIG. 1.

The joins of adjacent metals exposed to the cell liquor at the mounting points of the cathode plates 110 usually are coated with an inert non-conductive material to minimize corrosion at these points. In addition, similar calking of the cathode face 1 l 1 of the separator 104 usually is carried out to prevent hydriding of the titanium separator.

The cathode plates generally are spaced a small distance from the separator 104 at their inboard end to encourage molecular hydrogen production rather than atomic hydrogen transfer to the titanium separator 104.

If desired, copper foil or sheeting may be provided between the cathode plates 110 and the separator 104 and similarly copper foil or a copper sleeve may be provided between the bolts 114 and the lugs 112.

Turning now to FIG. 5, there is shown yet another cell structure 120 wherein the present invention is utilized. A plurality of titanium sheet anodes 122 secured generally perpendicularly to titanium anode sheet 124. The anodes 122 are located in one unit cell and interleave a plurality of cathode sheets in similar manner to the manner in which cathode sheets 125 of the adjacent unit cell interleave with the anode sheets 126 therein. The cathode sheets, typically of mild steel are mounted substantially perpendicularly to a mild steel support sheet 128. Each group of cathode sheets 125 is secured in electrical flow relation with the anodes in an adjacent unit cell, typically by at least two electrically conductive connectors 130, only one of which is shown in FIG. 5, thereby providing a bipolar electrode structure. The unit cells are separated by an inert, nonconducting wall 132, and the connectors 130 pass through the wall 132.

The anode and cathode structures are spaced from the wall 132 by

appropriate spacers

134 and 136 respectively. The spacer 134 is in the form of a threaded titanium boss into which passes the connector 130, in the form of a stud. The spacer 136 is in the form of a mild steel sleeve through which the stud 130 passes.

In accordance with the present invention, the connectors 130 are constructed of a barrier metal, typically copper.

It will be seen therefore that the present invention is applicable to a wide variety of bipolar electrode structures to prevent or at least minimize atomic hydrogen migration to the passivatable metal layer.

By providing a barrier metal layer 16, and hence controlling the atomic hydrogen flow between the cathodic layer 14 and the titanium layer 12, it is possible to realize the beneficial properties of titanium in the anodic portion of the electrode while at the same time reducing the hydriding problems mentioned above.

EXAMPLES The invention is illustrated further by the following Examples.

EXAMPLE I A composite metal electrode consisting of a titanium plate l/16" thick, the anodic face carrying a layer of platinum about 50 micro inches thick and the cathode face carrying a layer of electro-deposited copper about 0.008 inches thick was operated as a bipolar electrode in a chlorate cell. The current density was about l.0 amps per sq. inch, the pH about 6.8 and the operating temperature about 90C. The electrode was operated for 213 hours. Subsequent metallurgical examination showed no evidence for the presence of titanium hydride in the bulk titanium and a relatively low and uniform concentration of hydrogen across the thickness of the electrode corresponding to an absorption of hydrogen of 5 X l gram atoms per cm of electrode face.

An electrode consisting of a 1/16 inch titanium plate coated on the anodic face with about 50 micro inches of platinum having been operated for a comparable length of time (lOOO hours) in the same chlorate cell showed on examination a thickness of about 0.25 mm of Til-l on the cathode surface and compared to the former electrode, a far higher concentration of hydrogen across the thickness of the electrode. The average concentration of hydrogen across the electrode corresponds to an absorption of hydrogen of about 3.5 X gram atoms per cm of electrode face. The presence of the copper film on the cathode surface therefore decreased the amount of absorption of hydrogen by a factor of at least 70.

EXAMPLE 2 An electrode comprising a titanium sheet (0.035 inches thick) coated on the anodic face with a conductive mixture of ruthenium and titanium oxides and on the cathode face with successively electro-deposited layer of copper (0. l4 mils) and iron (l.8 mils) was operated as a bipolar electrode in a chlorate cell of filter press design. The current density, temperature and pH were 2 amps per sq. inch, 60C and 6.8 respectively. The electrode was operated continuously for a period of 10 weeks. Subsequent metallurgical examination of the electrode showed no evidence of titanium hydride formation in the titanium except in a few isolated areas where rupture of the electro deposited copper film had occurred. In these cases small areas of hydride were evident at the surface of the titanium.

Modifications are possible within the scope of the invention.

I claim:

1. A bipolar electrode structure comprising an anode surface, a layer of iron or an alloy thereof providing the cathode, a layer of passivatable metal located between the anode surface and the cathode and a layer of an electrically conducting metal or alloy thereof resistant to atomic hydrogen flow therethrough selected from the group consisting of gold, tin, lead, nickel, cobalt, chromium, tungsten, molybdenum, cadmium and copper, and alloys thereof and located between and in electrical contact with the iron or alloy thereof layer and the layer of passivatable metal.

2. The electrode of claim 1 wherein said passivatable metal is titanium or an alloy consisting predominantly of titanium.

3. The electrode of claim I wherein said passivatable metal is tantalum or an alloy consisting predominantly of tantalum.

4. The electrode of claim 1 wherein said anode surface is constituted by an electrically conducting layer of a platinum group metal or an interalloy of two or more of such metals.

5. The electrode of claim 1 wherein said anode surface is constituted by an electrically conducting layer of an oxide of at least one platinum group metal alone or an admixture thereof with at least one oxide of lead. manganese, cobalt, titanium, tantalum, zirconium, silicon and iron.

6. The electrode of claim 1 wherein said layer of metal resistant to atomic hydrogen flow is copper.

7. The electrode of claim I wherein said layer of metal resistant to atomic hydrogen flow is gold.

8. The electrode of claim 1 wherein said layer of metal resistant to atomic hydrogen flow is selected from tin, lead, nickel, cobalt, cadmium and chromium.

9. The electrode of claim 1 wherein said layer of metal resistant to atomic hydrogen flow is molybdenum.

10. The electrode of claim 1 wherein said layer of metal resistant to atomic hydrogen flow is tungsten.

11. The electrode of claim 1 in sheet-like form and consisting of a cathode sheet-like layer of iron or alloy thereof, a sheet-like layer of a passivatable metal, a sheet-like layer of a metal or alloy thereof resistant to atomic hydrogen flow positioned between and bonded to both the layer of passivatable metal and the layer of iron or alloy thereof and an anode surface layer of conducting material.

12. The electrode of claim 11 wherein said layer of metal resistant to atomic hydrogen flow is gold.

13. The electrode of claim 11 wherein said layer of metal resistant to atomic hydrogen flow is copper.

Claims

2. The electrode of claim 1 wherein said passivatable metal is titanium or an alloy consisting predominantly of titanium.
3. The electrode of claim 1 wherein said passivatable metal is tantalum or an alloy consisting predominantly of tantalum.
4. The electrode of claim 1 wherein said anode surface is constituted by an electrically conducting layer of a platinum group metal or an interalloy of two or more of such metals.
5. The electrode of claim 1 wherein said anode surface is constituted by an electrically conducting layer of an oxide Of at least one platinum group metal alone or an admixture thereof with at least one oxide of lead, manganese, cobalt, titanium, tantalum, zirconium, silicon and iron.
6. The electrode of claim 1 wherein said layer of metal resistant to atomic hydrogen flow is copper.
7. The electrode of claim 1 wherein said layer of metal resistant to atomic hydrogen flow is gold.
8. The electrode of claim 1 wherein said layer of metal resistant to atomic hydrogen flow is selected from tin, lead, nickel, cobalt, cadmium and chromium.
9. The electrode of claim 1 wherein said layer of metal resistant to atomic hydrogen flow is molybdenum.
10. The electrode of claim 1 wherein said layer of metal resistant to atomic hydrogen flow is tungsten.
11. The electrode of claim 1 in sheet-like form and consisting of a cathode sheet-like layer of iron or alloy thereof, a sheet-like layer of a passivatable metal, a sheet-like layer of a metal or alloy thereof resistant to atomic hydrogen flow positioned between and bonded to both the layer of passivatable metal and the layer of iron or alloy thereof and an anode surface layer of conducting material.
12. The electrode of claim 11 wherein said layer of metal resistant to atomic hydrogen flow is gold.
13. The electrode of claim 11 wherein said layer of metal resistant to atomic hydrogen flow is copper.