Low over-voltage electrodes for alkaline electrolytes

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these metals. The compounds used must be capable of

LOW OVER-VOLTAGE ELECTRODES FOR thermal decomposition to their oxides. Subsequently, ALKALINE ELECTROLYTES the oxide coated substrate is cured in a reducing atmo sphere.

(1) Field of the Invention SUMMARY OF THE INVENTION The invention relates to improved electrodes for use An insoluble electrode, particularly an anode, for use in electrolytic cells utilizing alkaline electrolytes. in an electrochemical cell, especially an electrolytic cell (2) Description of the Prior Art where the electrode is an anode at which oxygen is In an electrochemical cell having as basic compo 10 evolved, can be prepared by coating an electrically nents at least one anode and one cathode and an electro conductive substrate with an effective electrocatalytic lyte, a chemical reaction may be achieved such as the amount of cobalt and tungsten compounds, such as the oxidation or reduction of a chemical compound, as in an nitrates and chlorides. The coating can be applied to electrolytic cell or the conversion of chemical energy in said substrate from a homogeneous solution of a mixture a fuel into a low voltage direct current, as in a fuel cell. 15 of compounds of cobalt and tungsten. Said compounds When the electrodes in such a cell are of relatively are converted by thermodecomposition to their oxides inexpensive material such as for instance iron or nickel, subsequent to application of the coating to the electri the electrodes tend to have low activity. The problem is cally conductive substrate. The electrodes are stable to particularly acute in electrochemical cells used, for dissolution in strongly alkaline anolytes and exhibit low example, in the electrolysis of water to produce hydro 20 gen and oxygen utilizing an alkaline electrolyte (for over-voltage initially and after long periods of service. instance a 25% aqueous solution of potassium hydrox DETAILED DESCRIPTION OF THE ide). INVENTION The use of nickel as an anode material for commercial Nickel is well known as the standard anode material water electrolyzers is unsatisfactory because the over 25 for commercial water electrolyzers because of its good voltage for oxygen evolution on nickel is high and in chemical stability in the normally employed 25-30% by creases with length of service. Electrode coatings of weight concentration alkaline electrolyte. However, mixed ruthenium-titanium oxides are useful for the pro over the service life of the nickel electrode, the over duction of oxygen in acidic solutions but the chemical voltage for oxygen evolution increases. Reduced effi stability of such anodes in a strongly alkaline environ 30 ciency results, ment, as used in water electrolyzers, is inadequate. current density.asThis indicated by low levels of operational Graphite which is useful as an anode for chlorine pro operation of the cell.leads to high capital costs for the Low electrolyte concentrations duction is rapidly destroyed by oxygen if used for water electrolysis. such as 3 to 5% by weight alkali as used in the produc U.S. Pat. No. 4,342,792, electrocatalysts are disclosed 35 tion of alkaline hydrogen peroxide are much more cor rosive to a nickel electrode.

which can be coated over a metal electrode substrate to provide an electrode of high activity and stability when The voltage or potential that is required in the opera used as an anode in a strongly alkaline electrolyte. Such tion of an electrochemical cell such as an electrolytic anodes are produced by coating said electrode substrate cell includes the total of (1) the decomposition voltage with a homogeneous solution of a mixture of (1) at least of the compound being electrolyzed, (2) the voltage one compound from the group of compounds consisting required to overcome the resistance of the electrolyte, of iron, cobalt, nickel, and manganese, (2) at least one and (3) the voltage required to overcome the resistance compound from the group of compounds consisting of of the electrical connections within the cell. In addition, molybdenum, tungsten, and vanadium, and (3) at least a potential known as "over-voltage'or "over-potential' one rare earth metal selected from the compounds of 45 is also required in the operation of the cell. The anode lanthanides having an atomic number of 57-71 inclu over-voltage is the difference between the thermody sive. When such compounds are coated on an electrode namic potential of the oxygen evolving anode (for in substrate, if such compounds are not oxides, the com stance, when utilized for water electrolysis of a strongly pound must be capable of thermodecomposition to the alkaline anolyte) when the anode is at equilibrium and corresponding metal oxide. The oxide coated substrate 50 the potential of an anode on which oxygen is evolved is thereafter cured in a reducing atmosphere. due to an impressed electric current. The anode over In U.S. Pat. No. 4,428,805, electrodes for oxygen voltage is related to such factors as the mechanism of manufacture are disclosed. The electrodes are prepared oxygen evolution and desorption, the current density, by coating an electroconductive substrate with a first the temperature and the composition of the electrolyte, coating of one or more oxides of the metals tin, lead, 55 the anode material, and the surface area of the anode. antimony, aluminum, and indium followed by a second In recent years, increasing attention has been directed coating of a monometal or a polymetal oxide having a toward improving the oxygen over-voltage characteris spinel structure. tics of electrolytic cell anodes, particularly those anodes In U.S. Pat. No. 4,464,239, lithiated cobalto-cobaltic utilized in the electrolysis of water as well as in the oxides are used as coatings for electrode substrates as a 60 production of hydrogen peroxide where a strongly means for reducing the electrode over-voltage in a alkaline anolyte is utilized, for instance, a mixture com water electrolysis cell having an alkaline electrolyte. prising an alkali metal halide and 3 to 5% by weight an In European Patent Publication No. 0,009,406, elec alkali metal hydroxide. Electrolytic cells for the pro trodes are disclosed having electrocatalytic coatings of duction of an alkaline hydrogen peroxide preferably the nickel-molybdenum type including mixtures of co- 65 have at least two electrodes, an anode and a cathode, balt and tungsten. Such electrodes are coated on elec separated by a liquid permeable separator. Preferably trode substrates such as nickel, iron, copper, and tita the cathode is in physical contact with the separator and nium and their alloys from a solution of compounds of is porous and self-draining. In addition to having a re

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duced oxygen over-voltage, an anode for such purposes The solvent can be an aqueous solution or a mixture of should also be constructed from materials which are an aqueous and organic solvent or an organic solvent inexpensive, easy to fabricate, mechanically strong, and solution of the compounds. For example, a lower alkyl capable of withstanding the environmental conditions compound such as methanol, ethanol, propanol, isopro of the electrolytic cell, and particularly capable of re 5 panol or formamide or dimethyl formamide. The choice sisting dissolution in the alkaline anolyte. of a particular solvent will depend upon the solubility of The problem of increased over-potential with in the desired compounds of cobalt and tungsten. creasing service of nickel anodes under acidic condi If the homogeneous solution is a liquid, it can be tions has been lessened by the recent adoption of coat applied to the electroconductive substrate to be coated ings on electroconductive substrates of noble metals of 10 by dipping, rolling, spraying, or brushing. The coated Group VIII of the Periodic Table of the Elements. electroconductive substrate is thereafter heated in air at However, use of expensive metal coatings such as ru an elevated temperature to decompose the metal com thenium oxide in the production of anodes for oxygen pounds, if not oxides, to the corresponding oxides. The evolution has met with the problem of dissolution of the decomposition is suitably carried out at a temperature electrode coating in an alkaline electrolyte. Those met 15 between 250' centigrade and 1200' centigrade, prefera als, which, when coated on electroconductive sub bly between 350 centigrade and 800 centigrade, most strates, do not dissolve in strongly alkaline anolytes preferably between about 350 to about 550 centigrade. during oxygen evolution, will generally be covered The operation of applying a coating of the homogene with an oxide film and suffer a loss of activity with ous solution to the electroconductive substrate fol increasing service. The electrodes of European Patent 20 lowed by thermodecomposition to the oxides can be Application No. 0,009,406 having electrode catalyst repeated successively to ensure adequate coverage of coatings such as the mixed nickel-molybdenum type the substrate with the metal oxides so as to provide a which subsequent to deposition are decomposed to their coating thickness of about 2 to about 200 microns. Coat oxides by heating and thereafter exposed to a reducing ing thicknesses of from about 10 to about 50 microns are atmosphere at elevated temperature, show a marked 25 preferred while coatings of less than about 10 microns over-voltage improvement over those disclosed hereto in thickness usually do not have acceptable durability fore. Useful electroconductive substrates for use with and coatings of more than 50 microns usually do not such electrode catalyst coatings have been disclosed in produce any additional operating advantages. the prior art as relatively inexpensive materials such as The concentrations and relative proportions of the nickel, iron, copper, titanium, and alloys thereof or of 30 cobalt and tungsten compounds used in the homogene other metallic substances coated with any of these mate ous solution generally is respectively in the range of rials. about 1:1 to about 5:1, but higher or lower proportions The electrodes of the present invention have been can be used. The concentration of the cobalt and tung found to be more effective when used in water electrol sten compounds in the coating bath is not critical. Par ysis and particularly effective when used in the produc 35 ticularly good coatings are produced when the concen tion of an alkaline hydrogen peroxide using an alkali tration of the cobalt ions in the bath is within the range concentration of about 3 to about 5% by weight. Such of about 0.5% by weight to about 5% by weight and electrodes are prepared utilizing coatings of compounds when the relative proportion of tungsten ions to cobalt of cobalt and tungsten over an electroconductive sub ions in the bath is maintained at about 0.5:1. strate. Preferably, the cobalt and tungsten compounds 40 The deposit of the homogeneous solution of cobalt are deposited as mixtures on an electroconductive sub and tungsten compounds or their oxides may be ob strate consisting of nickel or a nickel coated electrocon tained by use of a sequential application of a mixture, an ductive substrate such as nickel coated steel. The mix alloy, or an intermetallic compound, depending upon tures are deposited from a homogeneous solution of the the particular conditions utilized in depositing the coat cobalt and tungsten compounds which are capable of 45 ing. Since any of these particular combinations of met being thermally decomposed to the oxides. Such com als are within the scope of the present invention, the pounds can be, for instance, the nitrates or chlorides of term "codeposit', or form thereof, as used in the present cobalt and tungsten. Generally the weight ratio of co specification and claims, includes any of the various balt to tungsten utilized in the preparation of the elec alloys, compounds and intermetallic phases of the co trodes of the invention is respectively about 1:1 to about 50 balt and tungsten compounds or oxides of said com 5:1. pounds and does not imply any particular method of The homogeneous solution of the cobalt and metal application or process of formulation with respect to compounds utilized for coating the electroconductive these metal compounds used as electrocatalysts. While substrates in the formation of the anodes of the inven the electroconductive substrates to be coated most pref tion is defined as an intimate mixture of the respective 55 erably are of nickel and nickel coated steel, other elec solid metal compounds in their finely divided state, or a trically conductive metal substrates can be used such as solid solution of the metal compounds, or a solution of stainless steel or titanium or any electrically conductive the compounds in a solvent. An intimate mixture of the metal substrate if coated with nickel. solid metal compounds can be prepared in advance or The cobalt compounds used in making the homoge the compounds can be mixed immediately prior to 60 neous solution with tungsten compounds can be any contact with the electroconductive substrate to be thermally decomposable oxidizable compound which coated. For instance, the compounds of cobalt and when heated in the above prescribed heating range will tungsten can be applied onto the electroconductive form an oxide of cobalt. The compound can be organic substrate either separately or simultaneously. The com such as cobalt octoate (cobalt 2-ethylhexanoate) but is pounds of cobalt and tungsten can be sprayed directly 65 preferably an inorganic compound such as cobalt ni onto the electroconductive substrate. Alternatively the trate, cobalt chloride, cobalt hydroxide, cobalt carbon cobalt and tungsten compounds can be present in a ate, and the like. Cobalt nitrate and cobalt chloride are homogeneous solution in a solvent for the compounds. especially preferred.

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The tungsten compounds used in making the elec as the alkaline electrolyte an aqueous solution consisting trodes of the present invention can be any thermally of 4% by weight sodium hydroxide and 0.6% by weight decomposable oxidizable compound which when sodium chloride. The initial start up cell voltage was heated in the above prescribed heating range will form 1.68 volts for the anode coated in accordance with the an oxide of tungsten. The compound can be organic teaching of Example 1. (This compares with the initial such as tungsten octoate and the like but is preferably an start up cell voltage for an anode of nickel plated steel inorganic compound such as tungsten nitrate, tungsten of 2.21 volts.) The hydrogen peroxide efficiency of the chloride, tungsten hydroxide, tungsten carbonate, so anode having a cocatalytic coating prepared in accor dium tungstate, and the like. Tungsten nitrate or tung dance with the process of Example 1 was 95% after 100 sten chloride are especially preferred. 10 days of operation of the cell. (This compares with the The following examples illustrate the various aspects hydrogen peroxide efficiency of the nickel plated steel of the invention but are not intended to limit its scope.

Where not otherwise specified throughout this specifi anode which was only 77% after 82 days of operation of the electrolytic cell.)

cation and claims, temperatures are given in degrees The hydrogen peroxide efficiency is the actual centigrade, and parts, percentages, and proportions are 15 amount of hydrogen peroxide produced by the passage by weight. of current divided by the theoretical amount of hydro EXAMPLE 1. gen peroxide expected to be produced as calculated by Coulombs law. For example, if 1.21 grams of hydrogen

Electrodes were prepared in accordance with the peroxide is produced in 40 minutes using a current of 3 invention by preparing a homogeneous solution of 5% 20 by weight cobalt chloride and 1% by weight tungsten amps, then the weight of hydrogen peroxide expected to be produced would be, by Coulombs law:

chloride in isopropanol. The measured weight of cobalt chloride was 1%, the measured weight of tungsten 17.01 x 3.0 x 40 x 60 chloride was 0.5%. Both components were prepared in 96500 = 1.2706 grams a single homogeneous solution but individual solutions 25 could be prepared separately and thereafter mixed to The hydrogen peroxide efficiency would be: form the final solution. The compounds provide a solu tion which is clear and homogeneous. 1.21

A nickel plated steel expanded metal sample was used 2706 X 100 = 95.2% which was degreased in trichloroethane, etched by 30 dipping in hydrochloric acid (about 20% by weight concentration) for a few seconds, and rinsed thoroughly EXAMPLE 4 in distilled water. Before coating, the water as removed Example 1 was repeated using a nickel expanded from the sample by air drying and the sample was dried metal to prepare a coated anode. The anode was utilized in an oven at a temperature of 60 to 90° C. A cocata 35 in an electrolytic cell for the production of an alkaline lytic coating of the above mixture of cobalt and tung hydrogen peroxide. The electrolyte fed to the cell was sten compounds was applied by dipping the nickel a 4% by weight coated steel expanded metal into the homogeneous solu containing 0.5%aqueous by solution of sodium hydroxide weight of sodium chloride. The tion and subsequently drying the coated metal in heated current density was 0.5 amperes per square inch. The air in a furnace at a temperature of 480 centigrade for 40 anode did not show any sign of corrosion up to 60 days a period of 10 to 12 minutes. The operation was re of cell operation.

peated several times until a visibly satisfactory film of the metal oxides was formed on the nickel coated steel EXAMPLE 5 expanded metal. After the final dipping operation, the (Control-forming no part of this invention) coated expanded metal was heated for one hour at 480 45 centigrade to convert the coated metal compounds to An uncoated nickel anode was used in an electrolytic their oxides. cell under the condition described in Example 4. Within

EXAMPLE 2

2 days of cell operation, the uncoated anode showed signs of corrosion.

The electrode prepared by the process of Example 1 50 EXAMPLE 6 was tested as an anode in a water electrolysis cell using as an anolyte a 4% by weight aqueous solution of so Example 1 was repeated using a nickel plated copper dium hydroxide. The anode showed a start up potential expanded metal to prepare a coated anode. The anode at 0.45 amps per square inch of 0.56 volts (versus a was tested in a water electrolysis cell using a 4% by saturated calomel electrode). After 104 days of opera 55 weight aqueous sodium hydroxide solution. The initial tion the anode potential was 0.645 volts. The anode anode potential was 0.745 volts (versus saturated calo potential compares favorably with a nickel plated steel mel electrode).

electroconductive substrate used as an anode without EXAMPLE 7 any cocatalytic coating. A nickel plated steel anode showed a start up potential when used in a similar elec 60 (Comparative Example-forming no part of this trolytic cell at 0.45 amps per square inch of 0.661 volts invention) and after 86 days of operation an anode potential of An anode was prepared by applying a cobalt-molyb 0.730 volts. denum coating to a nickel substrated in accordance with EXAMPLE 3 the procedure described in European Patent Applica 65 tion No. 0,009,406 except the oxide-coated substrate

The electrode prepared by the process of Example 1 was not cured in a reducing atmosphere at elevated was also tested in an electrolytic cell utilized for the temperature. The coated anode was tested in a water preparation of an alkaline hydrogen peroxide utilizing electrolysis cell under the conditions described in Ex

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ample 2. The initial anode potential (versus a saturated geneous solution wherein said solvent is selected from calomel electrode) was 0.65 volts at 0.45 amperes per the group consisting of at least one of an aqueous sol square inch. This compares to the initial anode (start up) vent, a fixed aqueous and organic solvent, and an or potential of a nickel plated steel anode coated with ganic solvent.

cobalt and tungsten of 0.56 volts, as described in Exam 5. The method according to claim 4 wherein said ple 2. solvent is a lower alkyl alcohol and said substrate is

EXAMPLE 8

coated with said metal compounds, other than the ox ides, and is thereafter heated at an elevated temperature

(Comparative Example-forming no part of this to convert said compounds to the corresponding oxides. invention) O 6. The method of claim 5 wherein successive applica An anode was prepared by applying a nickel-molyb tions of said homogeneous solution are applied to said substrate followed by successive heating at said ele denum-cerium coating to a nickel substrate in accor vated temperature to convert said metal compounds to dance with the procedure described in U.S. Pat. No. the corresponding oxides.

4,342,792 except that the oxide coated substrate was not 7. An electrode for use in an electrolytic cell, said cured in a reducing atmosphere at elevated tempera 15 electrode comprising an electrically conductive sub ture. The initial anode potential when tested in a water strate selected from the group consisting of nickel, electrolysis cell was 0.88 volts (versus a saturated calo stainless steel, titanium, and a nickel coated substrate, mel electrode). This compares with an initial anode said electrically conductive substrate having a coating potential of 0.56 volts, as described in Example 2 for an anode having a cobalt-tungsten coating on a nickel 20 thereon of an effective, electrocatalytic amount of the plated steel substrate. oxides of cobalt and tungsten, and said electrode being While this invention has been described with refer produced by the process of claim 1. ence to certain specific embodiments, it will be recog conductive 8. The electrode of claim 7 wherein the electrically nized by those skilled in the art that many variations are 25 steel and said substrate consists of nickel or nickel coated possible without departing from the scope and spirit of electrolytic cell is suitable for reacting a the invention, and it will be understood that it is in liquid with a gas.

9. The electrode of claim 8 wherein the thickness of tended to cover all changes and modifications of the said invention disclosed herein for the purposes of illustra an oxygen coating is about 2 to about 200 microns, said gas is tion which do not constitute departures from the spirit 30 containing gas, and the weight ratio of cobalt and scope of the invention. to tungsten is, respectively, about 1:1 to about 5:1. The embodiments of the invention in which an exclu 10. The electrode of claim 9 wherein said electrode is sive property or priviledge is claimed are defined as an anode and said electrolytic cell is suitable for the follows: electrolysis of a mixture comprising an aqueous solution 1. A method of producing an electrode for use in an 35 of an alkali metal hydroxide to produce an alkaline electrolytic process, said electrode having electrocata hydrogen peroxide aqueous solution.

lysts deposited on an electrically conductive substrate 11. In an electrolytic cell comprising at least two wherein said substrate is selected from the group con electrodes, an anode and a cathode, separated by a liq sisting of nickel, stainless steel, titanium, and a nickel uid permeable separator, wherein said cathode is in coated substrate and wherein said electrocatalysts con physical contact with said separator and is porous and sist of the oxides -of cobalt and tungsten; said method self-draining; the improvement comprising using as said consisting of: -- anode an electrically conductive substrate coated with (A) codepositing on said substrate a homogeneous an effective, electrocatalytic amount of the oxides of solution of compounds of cobalt and tungsten, each cobalt and tungsten, and said anode being produced by of which compound, when not an oxide, being 45 the process of claim 1.

capable of thermodecomposition to the corre 12. The electrolytic cell of claim 11 wherein said sponding oxide and substrate is selected from the group consisting of nickel, (B) thermally decomposing said compounds of cobalt stainless steel, titanium, and a nickel coated substrate and tungsten, which are present other than in the and said cell is suitable for reacting a liquid with a gas. oxide form, to the corresponding oxide. 13. The electrolytic cell of claim 12 wherein said 2. The method of claim 1 wherein said homogeneous 50 substrate consists of nickel or nickel coated steel, the solution consists of a solvent and metal compounds of thickness of said coating is about 2 to about 200 mi cobalt and tungsten in a weight ratio, respectively, of crons, and the weight ratio of cobalt to tungsten is, about 1:1 to about 5:1 and said electrode is an anode. respectively, about 1:1 to about 5:1. 3. The method of claim 2 wherein said homogeneous 55 14. The electrolytic cell of claim 13 wherein said gas solution consists of the nitrates or chlorides of cobalt is an oxygen containing gas and said cell is utilized for and tungsten. the electrolysis of an aqueous mixture comprising an 4. The method of claim 3 wherein said homogeneous alkali metal hydroxide and an alkali metal halide to solution is codeposited on said substrate by brushing, produce an alkaline solution k k k ofsk hydrogen

peroxide.

roll coating, or by dipping said substrate into said homo