GB2058842A

GB2058842A - Low overvoltage electrode

Info

Publication number: GB2058842A
Application number: GB8018018A
Authority: GB
Original assignee: Olin Corp
Current assignee: Olin Corp
Priority date: 1979-07-02
Filing date: 1980-06-02
Publication date: 1981-04-15
Also published as: NL8003702A; AU5889880A; BR8003885A; IT8048965A0; FR2461023A1; IT1144050B; DE3022751A1

Abstract

A low overvoltage electrode having a metallic film thereon is prepared for use in an electrolytic cell. The low overvoltage electrode is prepared by "sputter depositing" a metallic mixture onto an electrically conductive substrate, to provide a metallic film having a thickness in the range from about 0.01 to about 90 microns.

Description

SPECIFICATION Low overvoltage electrode This invention relates to a method of preparing a low overvoltage electrode for electrolytic cells. In particular, the invention provides such an electrode with a metallic film on the surface thereof which exhibits relatively low overvoltage when employed in electrolytic service.

It is well known that the voltage drop between an anode and a cathode in an electrolytic cell in which gases are generated at the electrodes is made up of a number of components, one of which is the gas overvoltage for the particular electrode employed. In industrial applications of electrolytic cells, it is very important from the viewpoint of operating cost to reduce to a minimum the voltage drop for an electrolytic process. This leads to the use of electrodes having the lowest overvoltage in the system employed. For example, in the electrolysis of an aqueous solution of an alkali metal halide such as an aqueous solution of sodium chloride brine to produce hydrogen, chlorine and sodium hydroxide, the cathode having the lowest hydrogen overvoltage is highly desired.

Overvoltage is defined as H = Ei-Eo, where Ei is the electrode potential under load and Eo is the reversible electrode potential independent of overvoltage including gas overvoltage and general methods of determining the values therefor are found in "Physical Chemistry", Third Edition, Farrington Daniels and Robert A. Alberty, John Wiley s Sons, 1966, in particular pages 265-268, and in "Instrumental Methods of Analysis", Hobart H. Willard, Lynne L. Merritt, Jr. and John A. Dean, D. Van Nostrand Company, Inc., Fourth Edition, 1965, in particular pages 620--621, and "Modern Electrochemistry", Vol. 1 w 2, John O' M. Bockris and A. K. N.Reddy, and "Kinetics of Electrode Processes", Wiley-lnterscience, 1 972, in particular Chapter 2.

Because of the large quantities of chlorine and caustic required by a modern society, millions of tons of these materials are produced annually, principally by electrolysis of aqueous solutions of sodium chloride. A reduction of as little as 0.05 volts in the working voltage of an electrolytic cell translates into a meaningful economic saving, especially in the light of today's every increasing power costs and energy conservation measures.

Several metallic coatings have been proposed for different substrates. U.S. Patent No. 4,080,278, issued March 21, 1978, to Ravier et al (Rhone-Poulenc), discloses nickel and molybdenum among others as metals coatings having 100--500 microns thickness on an iron, steel, or nickel substrate.

U.S. Patent No. 4,116,804, issued September 1978, to C. R. S. Needes (duPont), discloses a Raney nickel coating of at least 75 micron thickness which is preferably coated with a 5-10 micron thick nickel overcoating designed to hold the normally crumbly Raney structure together for better mechanical strength. Only electroplating or electroless plating methods are disclosed. The Needes disclosure indicates that overvoltages on the order of 60 millivolts were achieved, but such results have not been experienced in actual practice. The overvoltages actually obtained are close to 1 50 millivolts or more and the coating experiences spalling, crumbling and generally is rather weak.

Although apparently not applied to the chlor-alkali industry until the present invention, there is a well-known material deposition method known as sputter depositing in which a metallic mixture originally contained on a sacrificial target is bombarded under vacuum by minute particles. The impact of bombarding particles causes transfer of energy from the bombarding particles to surface atoms of a metallic mixture originally contained on the sacrificial target (cathode). The energized surface atoms consequently eject from the metallic mixture of the sacrificial target (cathode) into a surrounding sputtering chamber where a portion of the ejected atoms is intercepted by the electrically conductive substrate to be coated (process target).The intercepted atoms impinge upon the surface of the electrically conductive substrate (process target) and become adhered thereto to form a metallic film adherent to the electrically conductive substrate.

Despite the aforementioned teachings, a need still exists in this particular art for an improved method of depositing a metallic mixture to form a film on an electrically conductive substrate useful as an electrode. In operation of an electrolytic cell, the metallic film should be strongly adherent to the electrically conductive substrate. The electrode should exhibit a high electrochemical activity.

According to one aspect of the invention, there is provided a process for preparing an electrode which comprises applying to the surface of an electrically conductive substrate by a sputter depositing technique, a metallic film of a metallic mixture comprising: a) a first non-noble metal and b) at least one additional metal selected from (i) noble metals, (ii) sacrificial metals (as hereinafter defined) and (iii) a second non-noble metal; until said metallic film has a thickness in the range from about 0.01 to about 90 microns.

According to another aspect of the invention, there is provided a process for preparing the surface of an electrically conductive substrate which comprises: (a) washing said electrically conductive substrate with an organic solvent, thereby forming a washed electrically conductive substrate; (b) contacting said washed electrically conductive substrate with an organic alcohol thereby forming an organic alcohol soaked electrically conductive substrate; (c) contacting said organic alcohol soaked electrically conductive substrate with an inorganic acid thereby forming an acid contacted electrically conductive substrate; (d) contacting said acid contacted electrically conductive substrate with water thereby forming a water-washed electrically conductive substrate;; (e) contacting said water-washed electrically conductive substrate with an organic alcohol thereby forming a cleaned electrically conductive substrate.

According to a further aspect of the invention, there is provided a low overvoltage electrode having a metallic film thereon of less than 90 microns in thickness which is prepared by sputter coating a metallic mixture onto an electrically conductive substrate. The electrode is particularly suited for use as a cathode.

According to a yet further aspect of the invention there is provided a low overvoltage electrode for electrolysis of an alkaline medium which comprises: a conductive metal core; a nickel inner layer surrounding said core; a Raney nickel-containing middle layer surrounding said inner layer and core; a nickel-containing outer layer surrounding said inner layer, middle layer and core. The electrode is particularly suited for use as a cathode.

Preferred embodiments of the invention hereinafter described provide an electrode having very low overvoltage and substantially long service life. The electrode has a long life, low overvoltage metallic film which is substantially thinner than prior art metallic films and it exhibits high electrochemical activity in electrolytic service.

In the process of this invention, a metallic mixture is sputter deposited onto the surface of an electrically conductive substrate to form a metallic film adherent thereon and electrically conductive thereto.

The electrically conductive substrate is any electrically conductive material having the needed mechanical properties and chemical resistance to the electrolyte solution in which it is to be employed.

The electrically conductive substrate can be any electrically conductive substrate which is compatible with the metallic film and which will resist the attack of the contents of the electrochemical cell after being sputter deposited to prepare an electrode exhibiting low overvoltage. The electrically conductive substrate can be prepared from any suitable conducting material, such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, carbon, copper, nickel, aluminum, graphite, electrically conductive polymer such as polyacetylene, polyelectrolytes and semiconductors such as silicon, germanium and the like, mixtures thereof and alloys thereof.Preferred electrically conductive substrates are iron, copper, nickel, titanium, and mixtures or alloys thereof, such as nickel coated copper and nickel coated tin.

Another preferred electrically conductive substrate material is nickel coated copper in the form of an expanded louvered mesh electrode which can be of conventional monopolar design or bipolar design.

The louvers of such an electrode can be oriented either toward or away from a membrane or diaphragm when used in a membrane type chlor-alkali cell. For example, a Raney nickel such as that from a Ni2AI3 precursor disclosed in U.S. Patent No. 4,116,804 or other Raney surface could be advantageously used as an electrically conductive substrate which is sputter deposited with a metallic film by the process of this invention. Also, it is noted that a steel, expanded metal, gas-directing cathode could be employed as an electrically conductive substrate which is sputter deposited with a metallic film in accordance with the process of this invention without any Raney treatment and achieve almost the same lower overvoltages that the Raney nickel coating exhibit.

Typically, electrically conductive substrates include materials employed as electrodes in electrolytic cells, for example, permselective membrane type monopolar or bipolar filter press cells employed in the electrolysis of aqueous solutions of alkali metal halide solutions. Also included are materials employed as electrodes in porous or semi-porous diaphragm cells and in ion-exchange membrane cells such as permselective membrane cells and the like.

The electrically conductive substrate employed, such as an electrode, may have any given shape or size, which is adapted to the cell in which the electrically conductive substrate is employed as an electrode. The electrically conductive substrate may have the shape of a wire, tube, rod, flat or curved plate, perforated plate, expanded metal, wire gauze, gauze, or porous mixture such as fused metal powder. The surface of the electrically conductive substrate may be microporous, smooth, reticulate as well as sintered.

The preferred method of depositing the coating onto an electrically conductive substrate includes first thoroughly cleaning the surface of the electrically conductive substrate.

The surfaces of the electrically conductive substrate are preferably cleaned in any suitable container or cleaning bath to remove any contaminants that could diminish adherence of the coating to the cathode by means such as vapor degreasing, alkaline or acid cleaning, chemical etching, sandblasting and the like. The term "clean" as used herein in reference to surfaces means an electrically conductive substrate surface that is sufficiently free from objectionable organic or inorganic films to allow sputter deposition of metallic mixtures thereupon. All or part of the electrically conductive substrate surface may be cleaned depending on the type of electrolytic cell in which the cathode is to be employed.

The electrically conductive substrate is rinsed and cleaned in any suitable manner common in the electro and sputter deposition art in order to provide a clean surface on the cathodes. A known cleaner may be used for this purpose. An acid pickle, such as with hydrochloric acid, following cleaning is also common in the plating art in order to neutralize any residual alkaline cleaner and also to remove any oxide ions by degassing from the cathodes to form a clean cathode.

A preferred cleaning procedure comprises (a) washing the electrically conductive substrate in an organic solvent such as trichloroethylene, (b) soaking the washed electrically conductive substrate in an organic alcohol such as isopropyl alcohol, (c) soaking the alcohol soaked electrically conductive substrate in an aqueous solution of an inorganic acid such as an aqueous solution of hydrochloric acid for a time in the range from about 1 to about 30 minutes, (d) washing the acid soaked electrically conductive substrate in water, preferably deionized water, for a time in the range from about 1 to about 30 minutes, (e) soaking the washed electrically conductive substrate in an organic alcohol such as isopropyl alcohol, for a time in the range from about 1 to about 60 minutes to form a cleaned electrically conductive substrate.The cleaned electrically conductive substrate is then (f) dried, preferably with dry nitrogen, until the surface of the cleaned substrate appears devoid of any trace of liquid. The dried cleaned electrically conductive substrate is then visually inspected to ensure that the surface is thoroughly clean and dry.

First and second non-noble metals sputter deposited on the surface of an electrically conductive substrate as part of the metallic film thereon, are generally selected from copper, nickel, molybdenum, cobalt, manganese, chromium, iron, titanium, mixtures and alloys thereof. Alloys of this group include nickel-aluminum, nickel-zinc, and nickel-tin.

Two non-noble metals may be simultaneously sputter deposited on the surface of an electrically conductive substrate for example, nickel and molybdenum may be simultaneously sputter deposited on an electrically conductive substrate such as nickel or steel. A preferred nickel molybdenum film mixture comprises NiaMob where a and b represent only atom numerical percent, a + b total 100 percent and a is in the range from about 5 to about 95 and preferably from about 1 5 to about 85 percent.

Noble metals may also be sputter deposited on the surface of an electrically conductive substrate to become part of a metallic film thereon.

The term "noble metal" is employed throughout the claims and description to include ali those metals which are chemically inert especially with respect to oxygen. Noble metals are generally selected from silver, gold, rhodium, iridium, palladium, platinum, ruthenium, osmium, mixtures and the alloys thereof.

One or more sacrificial metals may be simultaneously sputter deposited on the surface of the electrically conductive substrate along with the noble metal or non-noble metal in a metallic film and selectively removed later from the metallic film preferably without removal of significant amounts of the non-noble metal to form a metallic film having a relatively high microporous surface. The selective removal of the sacrificial metal later can be achieved by differences in the solubility in a solvent and by differences in electrochemical activity.Accordingly, useful sacrificial metals are metals which can be alloyed with the chosen non-noble metals, and which can later be selectively removed from the sputter deposited metallic film, and which do not unfavorably influence the potential drop when a portion of the sacrificial metal remains on the electrically conductive substrate after the selective removal operation.

The term "sacrificial metal" is employed throughout the description and claims to include materials, which are useful with one or more of the non-noble metals and noble metals, such as for example, aluminum, magnesium, gallium, tin, lead, cadmium, bismuth, antimony, zinc, mixtures thereof and alloys thereof, as well as non-metallic materials such as carbon, graphite, and phosphorus. Preferred sacrificial metals are aluminum, zinc, magnesium, tin, mixtures thereof and the like.

The above-mentioned sacrificial metals are selectively adapted to each of the non-noble or noble metals, in connection with the intended removal process of the sacrificial metal and in connection with the intended use of the electrode. One or more of the sacrificial metals may be suitable with one or more of the non-noble or noble metals.

The term "metallic mixture" is employed throughout the description and claims to mean the composition of metals employed simultaneously as sacrificial targets in the sputtering deposition process. The metallic mixture includes targets having only individual metals thereon as well as targets having more than one metal thereon such as alloys which are to be simultaneously sputter deposited on an electrically conductive substrate.

Generally, two or more of the aforementioned metallic mixtures may be sputter deposited simultaneously onto the electrically conductive substrate. However, if desired, an alloy or mixture thereof may be first prepared of two or more metals and the alloy or mixture may then be sputter deposited onto the electrically conductive substrate. Alternatively, two or more alloys, or two or more mixtures, may be simultaneously sputter deposited on the electrically conductive substrate.

In one embodiment of the invention, at least one non-noble metal and at least one sacrificial metal are simultaneously sputter deposited on an electrically conductive substrate. Two non-noble metals and one sacrificial metal may be simultaneously sputter deposited onto an electrically conductive substrate.

in this particular embodiment, preferred non-noble metals are nickel and molybdenum and a preferred sacrificial metal is aluminum. A preferred metallic film prepared by the process of this invention comprises NixMOrAlz where the subscripts x, y, z represent only atom numerical percent of nickel, molybdenum, and aluminum respectively and are exclusive of any carbon or oxygen which may be present in the metallic film. In a preferred metallic film of this embodiment, x is in the range from about 5 to about 50 and preferably from about 10 to about 40 percent, z is in the range from about 5 to about 45 and preferrably from about 10 to about 40 percent, x is independent of z, z is independent of x, and x + y + z total 100 percent.

Metallic film prepared by the process of this invention may additionally comprise oxygen and/or carbon. Without being bound by theory, it is believed that the oxygen and/or carbon is present in the metallic film as a result of the sputter deposition technique or exposure to air or oxygen.

In a second embodiment of the invention, at least one noble metal and at least one non-noble metal are simultaneously sputter deposited onto an electrically conductive substrate.

In a third embodiment of the invention, at least one noble metal, at least one non-noble metal and at least one sacrificial metal are simultaneously sputter deposited onto an electrically conductive substrate.

In a fourth embodiment of this invention, a metallic film of a metallic mixture comprises a) a first non-noble metal or a noble metal, and b) at least one additional metal selected from (i) noble metals (ii) sacrificial metal and (iii) non-noble metals until the metallic film has a thickness in the range from about 0.01 to about 90 microns.

In practicing the process of this invention, the dried cleaned electrically conductive substrate may be sputter deposited with a high capacity sputtering device such as, for example, an inverted magnetron, hollow-cathode sputtering system. One such system especially useful as a sputter coating source is the SGuntm Source made by Varian Associates, Vacuum Division, Palo Alto, California. Such a sputter coating source and a preferred cathode target structure are described in U.S. Patent No. 4,100,055, issued July 11, 1978, to R. M. Rainey.This particular sputter coating source features a specially profiled cathode (metallic coating material); a grouped shield and centrally located anode that confine the electron discharge to the region adjacent the cathode surface and provide the fields that initiate the discharge; a magnet assembly that provides a magnetic field in the discharge region, and a cooling jacket that surrounds the cathode assembly In a fifth embodiment, a low overvoltage electrode is prepared by the process of this invention, for the electrolysis of an alkaline medium. The electrode comprises: a conductive metal core, a nickel inner layer surrounding said core; a Raney nickel containing middle layer surrounding said inner layer and conductive metal core; and a nickel containing outer layer surrounding the nickel inner layer, the Raney nickel containing middle layer, and the conductive core.

The number of metals which may be employed as the metallic mixture of the process of this invention is in the range from 2 to about 50 and is preferably from 2 to about 40.

"Sputter depositing" and "sputter deposition" as used herein mean a sputter deposition technique for simutaneously depositing metals of a metallic mixture onto an-electrically conductive substrate in the form of a film, the method being one in which a high voltage is applied between an anode and a cathode (sacrificial target). The metallic mixture contained on the cathode (sacrificial target) is vaporized by positive-ion bombardment, some of this vapor diffusing away from the cathode and depositing as a metallic film on the object to be treated (the electrically conductive substrate).Both RF Sputtering and DC Sputtering are included in the term "sputter coating" as used herein. "Chemical sputtering" where the vapor is formed by a chemical reaction at the cathode, "Reactive Sputtering" where the generated vapor or deposited film reacts with the atmosphere of the apparatus and "ion beam sputtering" or "ion plating" where a focused beam of high energy inert gas ions bombard a target to produce the vapor are also included within the general term "sputter coating" as used herein. In fact, other techniques known as bias sputtering and getter sputtering are included. For a detailed description of the principles of sputter coating, reference is made to Sputter Coating - Its Principles and Potential by J. A.Thornton, S.A.E. publication 730544 (May, 1973). The term "Sputter deposition" also includes D.C. diode Sputtering, or H.F. diode Sputtering employing D.C. H.F., or lonic Magnetron guns, or Triode Sputtering or Reactive Sputtering (C.V.D.) with lonic Activation, as discussed in Metal Finishing, J. J. Bessot, March 1980 in particular p. 21.

During the sputter deposition, metals of a metallic mixture are simultaneously and conductively applied to the cleaned cathode surface to form a film sputter deposited electrode.

In the process of preparing metallic film comprising two non-noble metals and one sacrificial metal by sputter deposition, individual targets comprising each of the non-noble metals and the sacrificial metal may be employed. The electrically conductive substrate is placed in a rotating holder and is sputter deposited from each of the individual targets simultaneously to prepare a metallic film on the electrically conductive substrate. The amount of each target deposited on the electrically conductive substrate to form the metallic film is generally directly proportional to the electric power supplied to the sputtering system of that target. Thus, to obtain a selected nominal composition of a metallic coating, the power is appropriately selected for each target component of that nominal composition.

The sputter deposition is continued until the desired thickness of metallic film is achieved.

Typically the thickness is in the range from about 0.01 to about 90 and preferably from about 0.05 to about 20 and most preferably from about 0.10 to about 10 microns. After sputter deposition is complete, the power is turned off and the sputter deposited electrically conductive substrate is removed from the sputter depositing apparatus.

If desired, those electrically conductive substrates having a metallic film containing one or more sacrificial metals can easily be further processed by selectively removing at least a portion of the sacrificial metal portion to form a leached metallic film adherent to the electrically conductive substrate.

A preferred method of removal is contacting the sputtered electrically conductive substrate which is coated with the metallic film containing one or more sacrificial metals, with an alkali metal hydroxide solution, such as an aqueous solution of sodium hydroxide, potassium hydroxide, lithium hydroxide and mixtures thereof, which is sufficient to selectively dissolve the sacrificial metal without attacking or removing any of other non-noble or metal portions. A leached thin metallic coated electrically conductive substrate is thereby formed. However, a small portion of the other non-noble metals can also be removed without significant damage to the coated substrate.

It will be understood that leaching is merely one method of treating the sputter deposited electrically conductive substrate containing sacrificial metal since it is possible to put the sputtered electrically conductive substrate to direct use in an electrolytic cell for the electrolysis of alkali metal halide brines, the alkali metal hydroxide produced during electrolysis effecting the leaching of the sacrificial metal from the sputtered electrically conductive substrate. If, however, contamination by sacrificial metal ion is a problem with respect to the alkali metal hydroxide, then it will be necessary to leach the sputtered electrically conductive substrate prior to placing the same in use.The concentration of aqueous alkali metal hydroxide used as the metal dissolving solution is generally in the range from about 5 to about 50 and preferably from about 10 to about 40 percent alkali metal hydroxide by weight.

The temperature of the alkali metal hydroxide solution is generally in the range from about 200 to about 600, and preferably from about 40 to about 500 C.

After contact with the alkali metal hydroxide solution, the leached metallic film electrically conductive substrate may be employed as an electrode in an electrolytic cell, preferably as a cathode.

Employing the process of this invention, it is now possible to sputter deposit a metallic film of extremely uniform thickness on the surface of an electrically conductive substrate.

It is believed that with the improved uniformity of the sputter deposited metallic film, that metallic film thickness greater than 100 microns would be undesirable, since two harmful effects are present.

First, the thicker films may tend to develop fissures, as in the thicker electrocoatings, and thus corrosion resistance might actually decrease. Secondly, the electrically conductive substrate, which is normally somewhat rough on a microscopic level following cleaning by etching, acid rinsing or other similar cieansing methods, might be "smoothed over" by the thicker coatings which tend to have their own surface characteristics. This "smoothing" effect is undesirable from an overvoltage standpoint as it tends to decrease the effective surface area, which results in higher current density on the electrode surface. "Spalling" or "delamination" are signs that the prior art coatings, do not adhere well over long periods of use. The metallic films of the present invention, however, adhere well even though they are very thin.

The thickness of the metallic film of this invention gives yet another unexpected advantage to the resulting electrode. The electrical conductivity of the metallic film is generally lower than the electrical conductivity of the electrically conductive substrate on which the metallic film is deposited. Thus, use of a relatively thin metallic film rather than a relatively thick metallic film increases overall electrical conductivity, and this is especially true when the thickness of the metallic film is decreased to a few microns or less.

Metallic film prepared by the process of this invention may additionally comprise oxygen and/or carbon. Without being bound by theory, it is believed that the oxygen and/or carbon is present in the metallic film as a result of the sputter deposition technique.

Sputter deposited electrodes, prepared by the process of this invention, unlike prior art Raney nickel electrodes, do not have a crumbly crystalline coating but rather are believed to be amorphous or non-crystalline in nature so that no grain boundaries exist. Hence, it is believed that the possibility of chemical attack along grain boundaries is eliminated as a source of coating spalling.

The nature of the metallic film deposited by sputter deposition thus reduces the number of "fissures" which, it is believed, lead to many of the spalling problems with the very alloys which produce the lower overvoltages. Since sputter deposition produces a non-fissured layer, both the problem of chemical attack at cracks (fissures) and grain boundary attack are greater reduced. Also, the metallic film exhibits a surface uniformity that far surpasses Raney nickel coatings, electrocoatings, and electroless coatings. This surface uniformity is believed to result in a change in the nature of the chemical attack to one of straight overall uniform surface attack at an almost negligibly low rate.

Without being bound by theory, it is believed that by its nature, sputter deposition involves bombarding and imbedding an electrically conductive substrate with atoms or ions from the sputtering source (sacrificial target). This bombardment results in three additional major advantages of sputter coatings. First, the bombardment step cleans the electrically conductive substrate by blasting off impurities or driving the impurities deeper into the electrically conductive substrate and then covering up the impurity with a metallic film. Secondly, the bombardment step causes deposited atoms to penetrate any existing very thin surface films or contaminants and thus adhere better.Thirdly, the imbedding of deposited atoms "nails" the metallic film to the electrically conductive substrate so that it is much more resistant to any "spalling" or "delamination" which has been observed with almost every other low overvoltage coating known when employed in electrolytic service. It is known that that overvoltages are greater for any given metal at increased current density. Thus, the sputter depositing method is surprisingly unexpectedly superior to other coating methods, as it produces a metallic film coating which is relatively unfissured and which is thin enough to retain or "replicate" the surface irregularities of the electrically conductive substrate which lower gas overvoltage.

Electrodes prepared by the process of this invention are preferably employed as cathodes in electrolyzing alkali metal solutions. Typical alkali metal solutions include solution of alkali metal halides for example solutions of alkali metal chlorides such as sodium chloride, potassium chloride and mixtures thereof, alkali metal hydroxide solutions such as solutions of sodium hydroxide, potassium hydroxide.

lithum hydroxide and mixtures thereof. Typical electrolytic cells include diaphragm cells and membrane cells preferably of a filter press configuration wherein the filter press cell is of monopolar or bipolar electrical operation.

Electrodes prepared by the process of this invention may be employed as electrodes in other electrolytic cells such as, for example, anodes or cathodes in electrolyzing water.

Materials suitable for use as membranes with the electrodes prepared by the process of this invention include sulfonic acid substituted perfluorocarbon polymers of the type described in U.S.

Patent No. 4,036,714, issued July 19, 1977 to Robert Spitzer, primary amine substituted polymers described in U.S. Patent No. 4,085,071, issued April 18, 1978 to Paul Raphael Resnick et al, polyamine substituted polymers of the type described in U.S. Patent No.4,030,988, issued June 21, 1977 to Walther Gustav Grot and carboxylic acid substituted polymers including those described in U.S. Patent No. 4,065,366, which issued December 27, 1977 to Yoshio Oda et al.

The Examples below demonstrates the invention and the operability of the sputter deposition method in the preparation of low overvoltage cathodes. All parts and percentages are by weight unless otherwise specified.

EXAMPLE 1 The process of this invention was employed to prepare a metallic film on an electrically conductive substrate to give an electrode having a high electrochemical activity and a low hydrogen overvoltage when subsequently employed as a cathode in an electrochemical cell.

TEST A Expanded nickel mesh specimens of electrically conductive substrate having dimensions of 2.5 centimeters x 7.5 centimeters were sputter deposited by the process of this invention. The specimens of expanded nickel mesh were cleaned by: (a) washing in trichloroethylene for five minutes, (b) soaking in 10 percent hydrochloric acid for 10 minutes, (c) washing in deionized water for 1 5 minutes, (d) soaking in isopropyl alcohol for 30 minutes to produce a clean nickel mesh which was then (e) dried with dry hydrogen. The specimens of cleaned expanded nickel mesh were sputter deposited by employing a metallic mixture comprised of three separate targets: (1) a non-noble metal-(nickel), (2) a non-noble metal-(molybdenum), and (3) a sacrificial metal-(aluminum).Three sputter deposition guns were simultaneously employed to sputter deposit proportional amounts of nickel, molybdenum, and aluminum on a first specimen of electrically conductive substrate to produce a thin metallic film having a composition A targeted to be about 45 percent by weight nickel, about 45 percent by weight molybdenum and about 10 percent by weight aluminum. A similar metallic film having a composition B targeted to be about 40 percent by weight nickel, about 40 percent by weight molybdenum and about 20 percent by weight aluminum was sputter deposited on a second electrically conductive substrate. In both preparations, the electrically conductive substrate was placed in a rotating planetary holder.The electric power to the individual three guns (for the individual targets of nickel, molybdenum and aluminum) were adjusted to obtain the compositions A and B respectively as indicated in Table I at the thickness indicated for each of the specimens. The sputter deposition was then terminated upon attaining a desired thickness.

An Auger electron spectroscopy analysis was employed in combination with etching to determine the composition of the metallic film as a function of thickness of the metallic film as measured from the outer surface of the metallic film toward the electrically conductive substrate.

An Auger electron spectroscopy analysis comprises bombarding the metallic film with electrons, and thereafter counting returning electrons which are distinctive of metal atoms in the metallic film.

General methods of employing Auger electron spectroscopy analysis are discussed in detail in Electronic Structure and Reactivity of Metal Surfaces, E. G. Derouane and A. A. Lucas, Plenum Press, NY, 1976 in particular p. 3-6.

In employing the Auger electron spectroscopy analysis, the surface of the metallic film is analyzed.

Thereafter the metallic film is sputter etched to selected depth where the Auger electron spectroscopy analysis is repeated. Thereafter the metallic film is sputter etched to a greater depth and the Auger electron spectroscopy analysis is repeated. The term "sputter etching" as employed in combination with Auger electron spectroscopy refers to removal of a select portion of the metallic film to a selected depth so as to enable an Auger electron spectroscopy analysis to be performed at selected depths.

The results of an Auger electron spectroscopy analysis performed on the first unleached electrically conductive substrate having a metallic film and thickness of about 0.8 microns are summarized in Table I below.

TABLE I Thickness Metal Species Present (As Measured From The Outer Metallic Film Surface Towards The Electrically Conductive Substrate) (Atom Numerical %) Microns Mo Ni Al O C 0 15 15 42 20 8 0.008 48 17 18 10 7 0.021 50 20 16 8 6 0.032 59 25 16 < 1 < 0.058 59 25 16 < 1 < 0.111 59 25 16 < 1 < 1 0.800 59 25 16 < 1 < (The symbol < means less than).

The results of an Auger electron spectroscopy analysis performed on the second unleached electrically conductive substrate having a metallic film thickness of about 0.91 microns are summarized in Table II below.

TABLE II Thickness (Microns) Metal Species Present (As Measured From The Outer Metallic Film Surface Towards The Electrically Conductive Substrate) (Atom Numerical %) Mo Ni Al O C 0 4 17 37 24 18 0.008 34 26 34 4 2 0.016 50 26 24 1 1 0.032 51 25 24 < 1 < 1 0.058 52 25 24 < 1 < 1 0.111 52 25 24 < 1 < 1 0.800 52 25 24 < 1 < 1 (The symbol means less than).

TEST B Following the procedure of Test A of this example, eight electrically conductive substrates were sputter deposited in accordance with the process of this invention. The eight electrically conductive substrates were first separated into two groups of four electrically conductive substrates each. A first group was sputter deposited in accordance with the process of this invention with composition A.

Composition A was targeted to be about 45 percent by weight molybdenum, about 45 percent by weight nickel and about 10 percent aluminum. The second group was sputter deposited in accordance with the process of this invention with composition B. Composition B was targeted to be about 40 percent molybdenum, about 40 percent nickel and about 20 percent aluminum.

Each of the resulting electrodes having a metallic film thereon was leached in 20% sodium hydroxide at 500C for about 60 minutes and then was electrically connected to a reference electrode in an aqueous solution of about 35 percent sodium hydroxide at 900 C. A potentiostat was connected to the electrodes to determine the hydrogen overvoltage, as shown in Table Ill below. The current density was 2 KAlm2.

The use of a potentiostat to determine electrode overvoltage is described with particularity in Interfacial Electrochemistry, An Experimental Approach, by E. Gileadi, et. al., Addison-Wesley Publishing Co. Inc., 1975, in particular pp. 181-195.

TABLE III Composition A Composition B First Group Second Group Nominal Calculated Nominal Calculated Film Hydrogen Film Hydrogen Thickness Overvoltage Thickness Overvoltage Microns (mv) Microns (mv) 0.22 290 0.2 170 0.52 275 0.5 140 0.93 200 0.98 65 1.93 150 1.77 60 TEST C Following the procedure outlined in Test B of this Example, another electrode was prepared by sputter depositing a metallic film on an electrically conductive substrate to a thickness of about 1.8 microns. The composition employed was targeted to be about 45 percent nickel, about 40 percent molybdenum, and about 20 percent aluminum. The unleached sputter deposited metallic film electrically conductive substrate was employed in a divided chlor-alkali electrolytic cell as a cathode.

The electrolytic cell was employed to prepare 30 percent caustic soda, hydrogen and chlorine from an aqueous sodium chloride brine solution. The temperature of the solution was about 850C, a current density of 2KA/cm2 was applied and 30 percent NaOH was produced. A sulfonic acid substituted perfluorocarbon polymer, a Nafion 295, ion-exchange membrane was employed as a separator in the cell. The following calculated hydrogen overpotentials were noted with elapsed time since startup of the electrolytic cell.

Calculated Hydrogen Overpotential Time Since Startup on the Cathode (days) (mv) 1 189.5 3 155.4 19 128.0 24 101.5 26 79.3 33 71.6 42 57.5 45 73.3 49 71.9 56 80.0 60 74.0 75 87.0 81 92.3 87 91.7 90 80.1 No metal contamination or spalling problems were noted. The decrease in calculated hydrogen overpotential on the cathode from day 1 to about day 42 is believed to be due to cathode activation as the sacrificial metal aluminum is leached out of the metallic film by caustic soda produced in the cell.

EXAMPLE 2 A nickel-molybdenum metallic coating of about 0.3 thickness was sputter deposited on a nickel substrate to form a metallic film thereon. The nickel substrate was at least 99 atom numerical percent nickel in plate form and the metallic film deposited thereon has an overall composition of about 60 atom numerical percent molybdenum, about 28 atom numerical percent nickel, about 6 atom numerical percent oxygen, and about 6 atom numerical percent carbon.

A small sample of the sputter deposited coating thus described is masked with epoxy so that only 1 cm2 surface was exposed when the sample was placed in electrolyte solution. A similar coupon of uncoated nickel was similarly masked to expose 1 cm2 for use as a control electrode. Each sample was cathodically polarized in a solution comprising 17% by weight sodium hydroxide at 800 C. The electrode voltage was measured by comparison of its potential with that of a saturated calomel electrode which is in contact with the same electrolyte via a salt bridge. The measurement was carried out with a potentiostat which automatically compensates for ohmic resistance in the electrolyte solution. The electrode voltages are shown in the drawing.In the drawing, the decreased overvoltage for the sputter deposited electrode prepared in this Example is determined at a selected current density by obtaining the difference in the electrode voltage of the sputter deposited electrode and the electrode voltage of the uncoated nickel electrode at that selected current density. For example, at a current density of 2 KA/m2, the overvoltage reduction is -1.46 volts -(1.22 volts) = 0.24 volts or -240mV. As can be seen from the drawing, the metallic coating has decreased the cathode potential by about 240 mV at 2.0 KA/m2 current density.

EXAMPLE 3 Following the procedure employed in Test A of Example 1, three electrically conductive substrates (nickel) having cleaned surfaces were sputter deposited in accordance with the process of this invention.

A first specimen was sputter deposited with a Composition C targeted to be about 66 2/3% molybdenum and about 33 1/3% nickel.

A second specimen was sputter deposited with a Composition D targeted to be about 50% molybdenum and about 50% nickel.

A third specimen was sputter deposited with a composition E targeted to be about 33 1/3% molybdenum and about 66 2/3% nickel.

Following the procedure of Example 1, Test B, the hydrogen overvoltage was measured by using a potentiostat at selected depths for each of the sputter deposited electrodes and is shown in Table IV below. Calculated hydrogen overvoltage is the measured electrode potential -- the reversible hydrogen potential at the corresponding temperature and concentration. The particular potentiostat employed automatically compensated for IR drop.

TABLE IV First Specimen ~ Composition C Measured Film Calculated Thickness Hydogen (Microns) Overvoltage (mV) 0.03 337 0.33 187 0.65* 137 Second Specimen - Composition D Measured Film Calculated Thickness Hydrogen (Microns) Overvoltage (mV) 0.03 307 0.19 247 0.65* 167 Third Specimen - Composition E Measured Film Calculated Thickness Hydrogen (Microns) Overvoltage (mV) 0.03 347 0.23 297 0.65* 237 *Estimated Film Thickness

Claims

1. A process for preparing an electrode which comprises applying to the surface of an electrically conductive substrate by a sputter depositing technique, a metallic film of a metallic mixture comprising: a) a first non-noble metal and b) at least one additional metal selected from (i) noble metals, (ii) sacrificial metals (as hereinbefore defined) and (iii) a second non-noble metal; until said metallic film has a thickness in the range from about 0.01 to about 90 microns.

2. The process of claim 1, wherein said thickness is in the range from about 0.05 to about 20 microns.

3. The process of claim 1, wherein said thickness is in the range from about 0.10 to about 10 microns.

4. The process of claim 1,2 or 3 wherein said additional metal is a sacrificial metal.

5. The process of claim 1,2 or 3, wherein said metallic mixture comprises at least two non-noble metals and at least one sacrificial metal.

6. The process of claim 1, 2 or 3 wherein said metallic mixture comprises at least one noble metal, at least one non-noble metal, and at least one sacrificial metal.

7. The process of any preceding claim, wherein said non-noble metals are selected from copper, nickel, molybdenum, cobalt, manganese, chromium, iron, mixtures thereof and alloys thereof.

8. The process of any preceding claim, wherein said sacrificial metals are selected from aluminum, magnesium, gallium, tin, lead, cadmium, bismuth, antimony, zinc, carbon, steel, phosphorus, mixtures thereof and alloys thereof.

9. The process of any preceding claim, wherein said noble metals are selected from iridium, palladium, platinum, rhodium, silver, gold, mixtures thereof and alloys thereof.

10. The process of any preceding claim, wherein said electrically conductive substrate is selected from iron, copper, nickel, titanium, nickel coated copper, nickel coated tin, alloys thereof all mixtures thereof.

11. The process of any preceding claim, wherein said metallic film also contains oxygen.

12. The process of any preceding claim, wherein said metallic film also contains carbon.

13. The process of any preceding claim, wherein said metallic film also contains oxygen and carbon.

14. The process of any preceding claim, wherein said metallic film is leached with alkali metal hydroxide after the sputter deposition technique is completed.

1 5. The process of any preceding claim, wherein said non-noble metals comprise nickel and molybdenum and alloys thereof.

1 6. The process of any preceding claim, wherein said noble metals comprise palladium and platinum and alloys thereof.

1 7. The process of any preceding claim, wherein said sacrificial metal comprises aluminum.

18. The process of any preceding claim, wherein the composition of said metallic film is Nix Mo, Alz where the subscripts x, y, z represent only the atomic numerical percent of nickel, molybdenum and aluminum respectively, x is in the range from about 5 to about 50 percent, z is in the range from about 5 to about 45 percent, x is independent of z, z is independent of x and x + y + z total 100 percent.

1 9. The process of claim 18, wherein x is in the range from about 10 to about 45 percent and z is in the range from about 10 to about 40 percent.

20. A process for preparing the surface of an electrically conductive substrate which comprises: a) washing said electrically conductive substrate with an organic solvent, thereby forming a washed electrically conductive substrate; (b) contacting said washed electrically conductive substrate with an organic alcohol thereby forming an organic alcohol soaked electrically conductive substrate; (c) contacting said organic alcohol soaked electrically conductive substrate with an inorganic acid thereby forming an acid contacted electrically conductive substrate; (d) contacting said acid contacted electrically conductive substrate with water thereby forming a water washed electrically conductive substrate.

(e) contacting said water washed electrically conductive substrate with an organic alcohol thereby forming a cleaned electrically conductive substrate.

21. The process of claim 20, wherein said cleaned electrically conductive substrate is contacted with dry nitrogen thereby forming a dried, cleaned electrically conductive substrate.

22. The process of any one of claims 1 to 1 9 wherein the surface of said electrically conductive substrate is first prepared by the surface cleaning process of claim 20 or 21.

23. An electrode prepared by the process of any one of claims 1 to 19 and 22.

24. An electrode prepared by the process of claim 6.

25. A low overvoltage electrode having a metallic film thereon of less than 90 microns in thickness which is prepared by sputter coating a metallic mixture onto an electrically conductive substrate.

26. The electrode of claim 25, wherein said metallic film comprises molybdenum and nickel.

27. The electrode of claim 25 or 26 wherein the thickness of said metallic film is in the range from about 0.05 to about 20 microns.

28. The electrode of claim 27, wherein thickness of said metallic film is in the range from about 0.10 to about 10 microns.

29. A low overvoltage electrode for electrolysis of an alkaline medium which comprises: a conductive metal core; a nickel inner layer surrounding said core; a Raney nickel-containing middle layer surrounding said inner layer and core; a nickel-containing outer layer surrounding said inner layer, middle layer and core.

30. The process of claim 15, wherein the composition of said metallic film is NiaMob, where the subscripts a and b represent only the atom numerical percent of nickel and molybdenum respectively, a is in the range from about 5 to about 95 percent and a + b totals 100 percent.

31. The process of claim 30 wherein a is in the range from about 1 5 to about 85 percent.

32. An electrode substantially as hereinbefore described in any of the Examples.