CA1072915A - Cathode surfaces having a low hydrogen overvoltage - Google Patents

Abstract

ABSTRACT OF DISCLOSURE

A process for the preparation of a cathode having low hydrogen overvoltage is disclosed which comprises depositing on at least a portion of a core support formed from a material selected from the Periodic Table consisting of elements of groups IB, IVB, VB, VIIB, VIII, graphite and mixtures thereof, an alloy comprised of at least one base metal selected from the group consisting of nickel, cobalt, chromium, manganese and iron and at least one sacrificial metal, which is less noble than the base metal, selected from the group consisting of zinc, aluminum, magnesium and tin, and thereafter removing at least a portion of the sacrificial metal from the de-posited alloy.

Description

~7z~5 BACKGROUND OF THE INV~N~ION
In the operation of electrolytic cells, such as the electroly-sis of brine to produce chlorine and hydrogen, a problem of particular intensity is the loss of power efficiency occasioned by voltage drop (i.e. hydrogen overvoltage) at the cathode of the cell. An overvoltage improvement of as little as 0.1 volt may materially simplify the con-structional design of the cell and substantially improve the economics thereof. It is known that a significant amount of the hydrogen over-voltage is caused by the design of the cathode especially as related to the materials from which it is constructed. Electrocatalytic activity of the cathode core material is important to the reduction of hydrogen overvoltage, however industrial economics play a great part in limiting the plausibility of use of the more highly electrocatalytically active metals, such as platinum and other noble metals and noble metal alloys.
Initial cost of these metals is higher and loss thereof, due to the corrosive medium to which they are subjected, increase their operating cost. In industrial applications then, it becomes very important, from the point of view of operating costs, to economically reduce to a minimum the overvoltage of an electrolytic cell process using inexpensively produced cathodes having the Lowest overvoltage potentials in the systems employed. As a result, experimental approach has centered about the feasibility of modifying relatively inexpensive core materials such as iron, steel, graphite, copper or alloys thereof to produce a red~ced overvoltage.
Methods have been advanced to decrease the hydrogen overvoltage by modification of the cathode, which include clsdding and coating a base metal core with a higher surface active material.

107Z~15 U.S. Patent 3,291,714 discloses that certain alloys may be deposited on suitably pretreated titanium cores to provide cathcdes of a lower hydrogen overvoltage and U.S. Patent 3,291,714 discloses certain alloys can be deposited on metallic, particularly steel, cathodes to reduce overvoltage. It is also known in the art to use finely divided palladium or platinum coating on iron core cathodes. Each of these methods has some effect upon reducing hydrogen overvoltage, however, in most instances the overvoltage reduction is minimal and, where precious metals are deposited, extremely costly.
Accordingly, it i9 an object of the instant invention to provide a cathode having increased resistance to corrosion.
It i9 another object of the invention to provide a cathode having a decreased hydrogen overvoltage. A further object is to provide a simple method for preparing a cathode having increased corrosion resistance and decreased hydrogen overvoltage. A still further object is to provide an inexpensive process for the preparation of a cathode having increased corrosion resistance and decreased hydrogen overvoltage. These and other objects will become apparent with the following explanation.
In accordance with the foregoing, a novel cathode and process for preparation thereof is provided comprising a core support formed from a material selected from the Periodic Table, from the Handbook of Chemistry, by ~. A. Lange, 10th Edition, 1961, consisting of elements of groups IB, IVB, VB, VIIB, graphite and mixtures thereof having a microporous surface formed by a deposit on at least a portion of the core support of an alloy of at least one base metal of the group consisting of chromium, man-; ~anese and at least one sacri~icial metal which is les~ noble than the base metal, selected from the group consi~ting ofaluminum, magnesium, tin, gallium, lead, cadmium, bismith and antimony, at least a portion of the sacrificial metal having been ~J~3 ~.,~

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removed. The coating on the electrode i9 prepared by a process comprising depositing on at least a portion of the core support, an alloy of said at least one base metal with said at least one sacrificial metal and thereafter removing at least a portion of said sacrificial metal from the so deposited alloy. Further, an electrolytic cell is provided having a cathode formed by the above process.
By microporous, it is meant that the surface is sub-stantially porous, about 5~/O of which are of a size less than about 10 microns and preferably about 9~/O. -Cathodes prepared by the aforedescribed method have been shown to have decreased hydrogen overvoltage, electrolytic cells so equipped have increased operating efficiency and cathodes so coated have been found to have increased corrosion resistance.
The core support structure of the cathode may be of any convenient size or shape as is conducive to the particular cell in which it is operated. It may be in the form of a wire, tube, rod, planner or curved sheet, perforated sheet, expanded metal, foraminous metal, gauze, porous composition as fused metal powder, The core support may be prepared from any suit-able conductive material as afore-described such as titanium, æirconium, vanadium, columbium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, carbon and mixtures thereof. The material chosen should be suitable to the construction of the desirable form. Preferred core structure materials are iron, copper, nickel, chromium, graphite and mixtures or alloys thereof. Particular preferred core materials ~7Z9~5 are iron and alloys thereof, especially steel such as carbon steels, iron/nickel alloys and stainless steels such as iron/chromium alloys and iron/nickel/chromium alloys. Other preferred core materials are mixtures of iron and copper and nickel based alloys such as nickel/
copper alloys, nickel/iron alloys, nickel/cobalt alloys and nickel/
chromium alloys.
In accordance with this invention the cathode core is initially prepared by coating at least a portion of the core with an alloy com-prising at least one of the desired base metals selected from the group consisting of copper, nickel, cobalt, manganese, chromium and iron, to-gether with a secondary sacrificial metal. The secondary sacrificial metal must be such that it may be selectively removed from the alloy coating and preferably without removal of significant amounts of the base metal. Selective removal may be achieved through differences in-cluding solvent solubility and electrochemical activity. Accordingly,operable sacrificial metals include these metals which will alloy with the selected base metal, may be selectively removed from the applied coating, and will not adversely increase cathodic potential drop if some metal remains on the cathode after selective removal processing.
Typical sacrificial metals operable with one or more of the base metals include aluminum, zinc, magnesium, gallium, tin, lead, cadmium, bismuth ; and antimony.
It is to be understood however that each of the above sacrificial metals be selectively matched with each of the base metals as is suit-able to the sacrificial metal removal process contemplated and as is - suitable to the cathode use. One or more sacrificial metals may be suitable with one or more of the base metals. Preferred sacrificial metals include those selected from the group consisting of aluminum, zinc, magnesium and tin.

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1C~7Z~5 Preferred alloys include those selected from the group consisting of nickel-aluminum and nickel-zinc.
An especially preferred alloy is a nickel/zinc ~ -alloy particularly the gamma nickel/zinc alloy.
The coating of the instant invention may be applied in various ways to provide the desired mixed coatings. Thus for example, the alloy may be mixed with a commercial resinate and the so prepared rèsinate sprayed or otherwise deposited on the core material. It may ; then go through various heating, baking, etc., steps to assure proper adhesion to the core. U. S. Patent 3,649,485 describes various methods of coatings which are applicable herewith. The metal alloy may be deposited by electrodeposition including electroplating, by sintering a mixture of the powdered alloy metals under the application of heat, with or without pressure, by roll-bonding, vacuum depositing, thermo decomposition of metal organic compounds, metal spraying or rolling the powdered alloy or a mixture of the powdered metals onto the cathode core material, or by painting metallizing solutions of the alloy onto the core material and subsequently firing. These and other methods are deYcribed in U. S. Patent 3,291,714 and are applicable herewith.
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72~5 Still other methods such as chemical vapor deposition, ion plating or sputtering are also operablc herewith. It is preferred however to appLy the alloy by electrodeposition, electroplating, chemical deposition, spray deposition or dipping in a molten metal bath.
The prior art is replete with varying methods of coating cathodes which are applicable herewith. It is desirable to obtaining maximum overvoltage reduction that the coating be continuous and without exposure of the cathode core to the corrosive electrolytic cell medium.
Exposure of the core may produce a mixed potential which may decrease the effect of the coating by initiating a battery effect. It is to be understood however that even partial coatings, i.e. spot coating, on the cathode provide decreased overvoltage.
Upon the attaining of a desirable metal alloy coating on the cathode the microporous surface may be readily prepared by the removal of at least a portion of the alloy material. rhe preferred method is to treat the alloy coated core structure with an alkali solution sufficient to dissolve the sacrificial metal therein without effecting the base metal. A portion of the base metal may also be removed therefrom without significant detriment to the function of the co&ting.
It has been surprisingly found that the microporous surface thereby creatcd maintains a significant resistance to corrosion of thc core material while decreasing the hydrogen overvoltage. The sacrificial metal may be removed by any convenient solvent which will selectively act thereon, however it has been found especially convenient to apply a strong caustic solution for acceptable results. Placement of the untreated alloyed cathode in the electrolytic cell will itself~result in dissolution of the sacrificial métal causing a gradual decrease in hydrogen overvoltage as the microporosity increases. Still further, -removal of the secondary metal may be achieved electrolytically by . .

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107Z9~5 selectively deplating the sacrificial metal. Such methods are usually slow and costly and therefore, in the most part, uneco-nomical. In the case of certain selective sacrificial metals such as magnesium, an acid may be selectively used to create the microporous surface by dissolution.
Selective chemically inert materials may also be added to the coating to increase surface area. Such materials include but are not limited to sulphates, phosphates, silicates, borates, hy-droxides, graphite, carbon, Teflon (registered trademark), inor-ganic oxides, magnetites, etc. Those of skill in the art will not find it difficult to choose a suitable pore-former from the dis-closure of the specification. They may be electrophoretically or otherwise deposited. Care must be taken in the sacrificial metal removal to avoid removal of the inert materials.
The following examples have been provided to further delineate the invent:on and are not meant as a llmitation thereof.

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1(~7Z915 EXAMPLE I
Two substantially identical 1~" x l~" wire mesh, mild steel cathodes were degreased, cleaned, rinsed and dried. One of the cathodes was thereafter subjected to electrolytic plating by submerging it in a solution containing 1 mole/liter of NiCl3.6H20, 1 mole/liter of ZnC12 and 30 grams/liter of H3B03, at a pH of 4.0, under a current density of 0.775 A/100 cm2 and a temperature oE about 40C for about 60 minutes.
The plated cathode was then immersed in a 0.5 M aqueous NaOH solution for about 24 hours, two hours thereof at about 90C, the remainLng time at about room temperature for the removal of sacrificial Zn therefrom.
The half cell voltage of both cathodes were then compared by sub~ecting each to testing in a small glass cell under the same conditions.
The glass cell was of two compartments, separated by a perfluoro sulfonic acid membrane diaphragm. The anode was ruthenium oxide coated titanium, the anolyte, saturated brine at a pH of 3.2. The catholyte was 150 g/l aqueous NaOH and the cell temperature was maintalned at 84 centigrade.
Half cell voltages were measured by a Luggin capillary attached to a saturated calomel reference electrode in a separate reservoir. Half cell voltages at varying cell amperages are tabulated in Table I.
TABLE I
Mild Steel Microporous Nickel Plated AmpsCathode (Volts) Steel Cathode (Volts) 1.0 2~85 2.80

2.0 3.40 ` 3.23

3.0 3.85 3.65

4.0 4.30 4.05

5.0 4.70 4 40

6.0 5.07 4.72

7.Q 5.45 5.07

8.0 5.80 , 5.38 .
' EXAMPLE II
Two substantially identical 6" x 5" wire mesh, mild steel cathodes were degreased, cleaned, rinsed and dried. One of the cathodes was thereafter sub~ected to electrolytic plating and causticJsacrificial metal removal by the method of Example I.
Two substantially identical glass 6" x 5" chlorine cells were constructed differing only in that one contained the untreated steel cathode, the other the microporous nickel plated cathode. A circulating catholyte system was employed, whereby each cell shared the same catholyte.
The catholyte was 150 g/l aqueous NaOH and the cell temperature was maintained at 84 centigrade. Each anolyte consisted of saturated b~ine at a pH of 3.2.
Each anode being constructed from ruthenium oxide coated titanium. Luggin capillaries were inserted into each cell and the potential of the cathode surface was measured as in Example I. Table I~ is a tabulation of the half cell voltage obtained at each cathode surface with varying amperage per square inch (ASI) current density.
TABLE II

Mild Steel Microporous Nickel Plated Current Density (ASI) Cathode Voltage Cathode Voltage 0.5 1.29 1.22 1.0 1.38 1.28 2.0 1.49 1.35 3.0 1.54 1.39 EXAMPLE III
Two substantially identical, 1 cm x 5 cm, mild steel, wire mesh cathodes and a third cathode substantially identical to the other cathodes with the exception that it was prepared from nickel were degreased, cleaned rlnsed and dried. One of the mild steel cathodes was electroplated and sacrificial metal was removed by the process of Example I.

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Three substantially identical glass chlorine cells were cons-tructed differLng only in that each encompassed one of the above prepared cathodes. The catholyte solutions was 2.5 M aqueous NaOH; the anolyte a saturated brine maintained at a pH of 4Ø The anode of each cell was a platinum metal anode. Luggin probes were inserted into each cell and - the potential of the cathode surface were measured as in Example I.
Table III is a tabulation of the half cell voltages obtained at each cathode surface with varying current density and varying cell temperature.
TABLE III
Cell Curren2 Density Iron Nickel Microporous Temp. A/dm Cathode MV Cathode Nickel Cathode - - -- --' , ' :'', . I ' ' .

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

A process for the preparation of an electrolytic cathode having a microporous surface comprising, depositing on at least a portion of a core support formed from a material selected from the Periodic Table consisting of elements of Groups IB, IVB, VB, VIIB, graphite and mixtures thereof, an alloy comprised of at least one base metal selected from the group consisting of chromium, manganese, and at least one sacrificial metal, which is less noble than the base metal, selected from the group consistin? of aluminum, magnesium.
tin, gallium, lead, cadmium, bismuth, and antimony, and thereafter removing at least a portion of said sacrificial metal from the deposited alloy.

The process of Claim 1 wherein said core is a material selected from the group consisting of chromium, graphite and mixtures and alloys thereof, and said sacrificial metal is selected from the group consisting of aluminum, magnesium and tin.

The process of Claim 1 wherein said alloy is electroplated on said core support and said sacrificial metal is removed by treating with a caustic solution.

The process of Claim 1 wherein said sacrificial metal is removed by treating with an acid solution.

The process of Claim 1 wherein about 50% of the Pores of said microporous surface are of a size less than about 10 microns.

The process of Claim 1 wherein about 90% of the pores of said microporous surface are of a size less than about 10 microns.