CA1112607A - Transition metal oxide electrodes - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Disclosed is an electrode for use in electrochemical processes wherein a metal substrate made of a valve metal such as titanium carries a semi-conducting intermediate coating consisting of a combination of tin and antimony oxides laid down upon the valve metal substrate in a series of layers and a top coating consisting of an oxide selected from the group of chromium, manganese, iron, cobalt, nickel, molybdenum or tungsten applied in a series of layers.

Description

TRANSITION METAL OXIDE El ECT~ODES

BACKGl~OUND OF THE INVENTION

This invention generally relates to electrodes Eor use in electrochemical processes, having a valve metal substrate carrying a semi-conducting intermediate coating consisting of tin and antimony oxides with a top coating consisting of oxides selected from the group of chromium, manganese, iron, cobalt, nickel, molybdenum or tungsten to provide an electrode at considerably less cost while obtaining Iow cell ~roltages for given current denslties. More particularly the present disclosure relates to a much improved electrode having a valve metal substrate, such as titanium, carrying a semi-conducting intermediate coating ~ consisting of tin and antimonycompounds applied in a series of layers and baked to their respective oxides; and a top coating consisting of oxides of transition metals selected from ~the group of chromium, manganese, iron, cobalt~ nickel~ molybdenum or tungsten applied in a series of several layers and baked into their respective oxide~forms.
.- , ~ Electrochemical methods~ of manufactur~e are becoming ever mcreasingly important to the chemical industry due to their greater ecological acceptabllity, potential for energy conservation,~and the resultant cost reductlons possible. There-fore, a great deal of ~research and development effort has been applied to electr~
chemical~ processes and the hardware for these processes. One major element of the hardware aspect is the electrode itself. The~ object has been to ;provide: an electrode which ~ill withstand~the corrosive environmen~ within an ele~trolytic cell; an efficient means for electrochemical production; and an electrode cost withln~the range of .
commercial feasibility. Only a iew materials may effectlvely constitute an electrode especially to be used as an anode because of the susceptability of most other substances to the intense corrosive conditions. Among these materials are: graphite, nickel, lead, lead alloy, platinum, or platinized titanium. Electrodes of this type have limited applications because of the various disadvantages such as: a lack of dimensional stability; high cost; chemical activity; contamination of the electrolyte;
contamination of a cathode deposit; sensitivity to impurities; or high oxygen ~ .

overvoltages. Overvoltage xe~ers to the excess eleatrical potential at w~ich t~e des~red element ~s d~scharged at the electrode surface.
The history of electrodes is replete with examples of attempts and proposals to overccme scme of the problems associated with the electrode in an electrolytic cell, none of which seems to have accomplished an optimization of the desirable characteristics for an electrode to be used in an electrolytic cell. Currently, in an electrowinning process for example the cell is operated ;~
at a ralatively low current density o~ less than 1 ampere per square inch (155 milliamperes per square centimeter). The problem in this case is to find an electrode which will haYe many of the desirable characteristics listed above and addition-ally have a low half cell voltage at given current densities so as to conserve a considerable amount of energy in the electro-chemical process. It is known for instance that platinum is an excellent material for use in an anode in an electrode in an electrowinning process and satisfies many of the above-mentioned characteristics. However, platinum is expensive and hence has not been found suitable for industrial use to date. Carbon and lead alloy electrodes have been gènerally used, but the carbon - .
anode has th~ disadvantage that it greatly pollutes the electrolyte due to the fast wearing and has an increasingly higher electrical resistance which results in the increase of the half cell potential. This higher half cell potential causes the electxolytic cell to consume more electrical power than is desirable. The disadvantages of the lead alloy anode are that the lead dissolves in the electrolyte and the resulting solute is deposited on the cathode subsequently resulting in a decrease " ' , ' I

in the purity of the deposit obtained, and that the oxygen overvoltage becomes too high. Another disadvantage of the lead alloy anode in the instance of copper electrowinning is that the PbO2 changes to a Pb304 which is a poor conductor. Oxygen may penetrate below this layer and flake off the film resulting ~-~
in particles becoming trapped in the deposited copper on a cathode. This causes a degrading of the copper plating which -is very undesirable.
It has been proposed that platinum or other precious metals be applied to a titanium substrate to retain their ,~
attractive electrical characteristics and further red'uce the ';~-;' manufacturing costs. However, even this limited use of precious ',~;'' metals such as platinum which involves a cost in the range of about $30.00 per square foot ($323.00 per square meter) of '~
electrode surface areas is expensive and therefore not desirable for industrial uses. It has also been proposed that the ,~
surfaces of titanium be plated with pl~tinu~
to which another electrolytic deposit either o~ lead dioxide '' or manganese dioxide is to be applied. The electrodes with the lead dioxide coating have the disadvantage of comparitively high "' oxygen overvoltages and both types of coatings have high internal stresses when electrolytically deposited so that-they are -~
liable to be detached from the surface during commercial usage thereby contaminating the electrolyte and the product being deposited on the cathode surface. Thus, the current density of such anodes must be'limited and handling of ~uch anode~s mlst be done with extreme care. Another attempted improvement has been to put ; ~;
a layer of manganese dioxide on thé surface of a titanium substrate w~ic~
is relati~ely porous in nature and b~ld up a nu~ber of layers of the manganese di ~ de soias to present an inte~al coa,ting. Th~s yields ~elatively - 4 - ~ -'~

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low half cell potentials as long as the cuxrent density remains below 0.5 ampere per square inch ~77.5 milliamperes per square centimeter) but as the current density is increased to near 1 ampere per square inch ~155 milliamperes per square centimeter) the half cell potential required rises rather rapidly on this type o~ electrode, resulting in a considerable disadvantage at higher current densities. Therefore, to date, none o$ these proposals have met with much commercial success basically because efficiencies and cost reductions desired have not been achieved to this point.

SUMMARY OF THE I~IVENTI ON
It is therefore an object of the present invention to provide an electrode having the desired operational characteristics which can be manufactured at a cost within the range of commercial feasibility. -;
. , Another object of the present invention is to provide an improvedelectrode for use in an electrolytic cell which will have longer wear characteristics within the given cell `~
environment. -These and other objects of the present invention, together with the advantages thereof over existing and prior art forms which will become apparent to those skilled in the art ;~`
from the detailed disclosure of the present invention as set forth hereinbelow, are accomplished by the improvements herein described and claimed.
It has been found that an improved electrode for use in an electrolytic cell can be made of a valve metal substrate selected from the group of aluminum, molybdenum,niobium, tantalum, titanium, tungsten, zirconium, and alloys thereof; on the surface of the base substrate a semi~conductive intermediate ;", ~2~
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coating of tin and antimony compounds applied and converted to the respective oxides, the intermediate coating of tin and ;
antimony compounds having betweenQ.l t~ 3Q we~ht percent antimony compounds; and on the surface of the semi-conductive intermediate coating a top coating of oxides of transition metals selected from the group of chromium, manganese, iron, cobalt, nickle, ~;
molybdenum or tungsten.
DESCRIPTION OF THE PERFERRED EMBODIMENTS `~
The improved electrode which will overcome many of these disadvantages of the pxior art consists of a valve metal substrate which carrles a semi-conductive intermediate coating of tin and antimony oxides and a top coating of an oxide of a ~
transition metal. The valve metal substrate which forms the base ~ ~-component of the electrode is an electro-conductive metal having sufficient mechanical strength to serve as a support for the coatings and should have hlgh resistance to corrosion when exposed to the interior environment of an electrolytic cell. ~` ~
Typical valve metals include: aluminum, m~ybdenum, niobium, - ~-tantalum, titanium, tungsten, zirconium and alloys thereof. A
perferred valve metal based on cost, availability and electrical and chemical properties is titanium. There are a number of forms that titanium substrate may takè in the manufacture of an electrode, including for example: solid sheet material, expanded metal mesh material with a large percentage l~-open area~ and a porous titanium with a density of 30 to 70 per-cent pure titanilDm which can be produced by cold compacting titanium powder or by a sintering process. Porous titanium ;~
is perferred in the present invention for its long life character-istics along with its relative structural integrity. If desired the porous-titanium can be reinforced with titanium mesh in the - 5a - ~

case of a large electrode.
The semi-conductive intermediate coating of tin and antimony oxides is a tin dio~ide coating that has been modified by adding portions of a suitable organic ~-s~

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material, commonly referred to as a "dopent." The dopent of the present invention is an antimony compound such as SbC13 which forms an oxide when baked in an oxidizing atmosphere. Al~hough the exact form of the antimony in the coating is not certain, it ;s assumed to be present as a Sb2O3 for purposes of weight calculations. The com-5 positions are mixtures of tin dioxide and a minor amount of antimony trioxicle, thelatter being present in an amount of between 0.1 and 30 weight percent, calculated on the basis of total weight percent of SnO2 and Sb2O3. The preferred amount of the antimony trioxide in the present invention is between 3 and 15 weight percent.
There are a number of methods for applymg the semi-conductive 10 intermediate coating of tin and antimony oxides on the surface o~ the valve metal substrate. Typically such coatings may be formed by first physically and/or chemically cleaning the substrate? such as by degreasing and etching the surface in a suitable acid (such as oxalic or hydrochloric acid) or by sandblasting; then applying a solution of appropriate thermally decomposable compounds; drying; and heating in an oxidizing 15 atmosphere. The compounds that may be employed include any inorganic or organic salt or ester of tin and the antimony dopent which are thermally decomposable to their respective oxide forms, including their alkoxides, alkoxy halides, amines, and chlorides~ Typical salts include: antimony pentachloride, antimony trichloride, dibutyl tin dlchlor~de, stannic chlorlde and tin tetraethoxide. Suitable solvents include: amyl 20 alcohol, benzene~ butyl alcohol, ethyl alcohol, pentyl alcohol, propyl alcoholj toluene and other organic solvents as well as some inorganic solvents such as water.

, The solution o~ thermally decomposable compounds, containing salts of tin and antimony in the desired proportion, may be applied to the cleaned sur~ace of the valve metal substrate b~ brushing, dipping, rolling, spraying, or other suitable 25 mechanical or chemical methods. The coating is then dried by heating at about 100 degrees centigrade to 200 degrees centigrade to evaporate the solvent. This coating is then baked at a higher temperature such as 250 degrees centigrade to 800 degrees centigrade in an oxidizing atmosphere to convert the tin and antimony compounds to their respective oxides. This procedure is repeated as many times as necessary to 30 achieve a desired coating thickness or weight appropriate for the particular electrode to be manufactured. When porous titanium substrate is used, a desirable semi-conductive intermediate coating can be accomplished by sucking a solution of tin and antimony compounds through the substrate 2 to 6 times with baking between, and Eor titanium p1ate the desired thickness can be obtained by applying 2 to 6 coa~s of the tin anti-~/S mony compounds or by thermal decomposition of compounds. Alternatively, a desiredthickness of the semi-conductive intermediate coating can be built up by applying a number of layers with drying between applications such that the baking process to con~rert the tln and a~timony compounds to their respective oxides i~ p~æformed only Gnce at the end of a series of layering steps.
The topcoating of the electrode, of a transition metal oxide, selected from the group of chromium, manganese, iron, cobalt, nickel, molybdenum or tungsten, can be applied by several methods such as dipping, electroplatin~ or spraying- -The top coating can be layered in the same fashion as the inter-mediate coating to build up a thickness or wei~ht per unit area as desired for the 15 partlcular electrode. In the case of titanium mesh, one method ior applying a man~anese dioxide for instance is to electroplate manganese dioxide directly onto the coated electrode. Electroplating is a more convenient method of applying larger amounts of manganese dioxide in a shorter time, but the thermally decomposable compounds of the included transition metals may be painted or sprayed on the 20 electrode in a series of layers with a drylng period between each ~ayer and a brushin~off of any excess material present on the surface after drying. After the strip has been allowed to dry at room temperature it can then be baked for short periods of time at ~ `
an elevated temperature to transforrn the ~ransitlon metal compounds to their respective oxide forms. The top coating of an oxide of a transition metai may~
25 be applied by electroplating a transition metal oxide upon the surface of the intermediate coating. The top coating may be applied by electroplating in a bath containing a transition metal sulfate, drying the layer at room temperature, painting a solution containing the transition metal nitrafe over the electo-plated layer, baking the top coating to transform the transition metal to its : -. :, ~ . ' . ': , . ' ' ~7 Oxide form and repeating the process two to six times. Another ~;
substrate material consists of mesh covered with a porous ~;-material so as to obtain ~he structural integrity of the mesh and the surface area of the porous material. The coatings on this substrate can be applied by electrodeposition, by decompostion of thermally decomposable compounds.
Major uses of this type of electrode are expected to be in: the electrodeposition of metals from aqueous solutions ~ -; . , of metal salts, such as electrowlnning of antimony, cadmium, chromium, cobalt, copper, gallium, indium, manganese, nickei, ' ~~
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,t7 thallium, tin or zinc; production of hypochlorite; and inchlor-alkali cells for the pr~
duction of chlorine and caustic. Other possible uses include: cathodic protection o~
marine equipment, electrochemical generation of electrical power, electrolysis of -~
water and other aqueous soiutions, electrolytic cleaning, electrolytic production of metal powders, electro organic synthesis, and electroplating.

, In order that those skilled in the art may more readily lmderstand the present invention and certain preferred aspects by which it may be carried into effect, the following specific examples are afforded.

A solution for the semi-conductive intermediate coating was prepared by mixing 30 milliliters of butyl alcohol~ 5 milliliters of hydrochloric acid ~HCl), 32 grams of antimony trichloride (SbC13), and 15.1 grams of stannic chloride penta-hydrate (SnC14 5H~O). A strip of clean titanium plate was immersed in hot HCl for lS 5~ hour to etch the surface. It was then washed with water and drled. The titanium was then coated twice by brushing with the alkoxy tin-antimony trichloride solution described above. The surface of the plate was dried Eor ten minutes in an oven at 125 degrees centigrade after applying each coating. The titanium was then baked at 480 de~rees centigrade for 7 ~ 1 minutes. The theoretical composition of the coatin~; thus prepared was 81.7 percent SnO2 and 18.3 percent antimony oxides (calculated as Sb2O3). The strip of titanium plate was then electroplated for ten minutes at 0.025 ampere per square lnch ( 4 milliamperes per square centimeter3 and at 80 ~ 85 degrees centigrade in a bath containing a mixture consisting of 150 grams of manganese sulfate and 25 grams of concentrated H2SO4 per liter. The strip was allowed ~o dry in air at room ~emperature. The strip was painted with a mixture consisting of equal volumes of isopropyl alcohol and a 50 percent aqueous solution of manganese nitrate, andbaked for ten minutes in an oven at 205 degrees centigrade. This electroplating,painting, and baking cycle was repeated two more times. An additional layer was electroplated as described above~ also including air drying at room temperature and a final bake at 205 degrees centigrade for ten minutes. During each of the above cycles~

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when the coatecl strip was removed from the oven, any excess coating was removed by brushing the strip under running water.
The anode, prepared as described aboYe, was installed and 1ested as an anode in a cell containing dilute sulfuric acid (150 grams of conc. H25Ol~/liter) main~
5 tained at a tempera~ure of about 50 degrees centigrade. The test was conducted at constant current densities of 1, 3 and 5 amperes per square inch (155, 465 and 775 milliamperes per square centimeter); the anode exhibited potentials of 1.45, 1.52 and 1.59 volts (versus a saturated calomel electrode), respectively.

E~XAMPLE 2 A strip of clean titanium plate was etched and ~hen two double coatings of conductire tin dioxide were applied by repeating the entire brush-dry-bake cycle described in Example 1. The baking temperature was 490 degrees centigrade instead of 480 degrees centigrade specified in Example 1. The strip of titanium was electro-~ plated for eight minutes at 0.025 ampere per square inch (39 milliamperes per square 15 centimeter) and at 80 to 85 degrees centigrade in a bath containing manganese sulfate ~150 grams per iiter) and concentra~ed sulfuric acid (25 grams per liter). The strip was then allowed to air dry at room temperature and was then baked for io minutes in an oven maintained at 205 degrees centigrade. This was repeated three timesO - ;~
The anode, prepared as described above, was installed and tested as an ~0 anode in a cell containing dilute sulfuric acid (150 grams per liter) at a temperature of about 50 degrees centigrade. The test was conduc~ed at current densities of l, 3 and 5 amperes per square inch (155, 465 and 775 milliamperes per square centimeter); the - anode exhibited potentials of 1.44, 1.50 and 1.55 volts, respectively. The weight oE the MnO2 coating was 0.075 gram, equivalent to about 29 grams per square meter.

25 - _AMPLIE 3 ~ strip of clean titanium plate, etched, coated with tin dioxide and plated with manganese dioxide as described in Example 2, was baked an additional 66 hours at 205 de~rees centigrade.

The anode, prepared as described above, was installed and tested as an _ g _ ,J~ rt7 anode in a cell containing dilute sulfuric acid (150 grams per liter) maintained at a temperature of about 50 degrees centigrade. The test was conducted at current densities of 1, 3 and 5 amperes per square inch (155, 465 and 775 milliamperes per square centimeter); the anode exhibited potentials of 1.43, 1.48 and 1.51 volts, re-spectively.

A strip of clean titanium plate, etched and coated with tin dioxide as described in Example 2, was electroplated for 24 minutes at 0.025 ampere per square inch ( 4 milliamperes per Square centimeter) and at 80 to 85 degrees centigrade in a bath containing manganese sulfate (150 ~rams per liter) and concentrated sulfuric acid (25 grams per liter). The -weight of the MnO2 coating was 0.083 gram,~ equivalent to about 3~ grams per~square meter. Thls plate was not baked after electroplating in the manganese sulfate-sulfuric acid bath.
- ~ The anode, prepared as described above~ was~ tested as an anode as de-, scribed in Examples 2 and 3. Passivation occurred and no readings of potential could be made. This test shows that a titanium plate containing a l~lnO2 coating over tin dioxide~requires baking, as described in Examples 2 and 3, so that it may exhibit a useful li~e.

A strip of clean titanium plate was etched and coated with three dbuble coatings of tln dioxide uslng the rnethod described in Example 1 except that the baking temperature after applying each double coating was 560 degrees centigrade instead of 490 degrees centigrade as specified in Example 1.
The strip of titanium plate was then electrbplated for 20 minutes at 0.0166 ampere per square inch (1.~ milliamperes per square centimeter) and at 90 to 95 degrees centigrade in a bath containing manganese sul~ate ~150 grams per liter~ and concentrated sulfuric acld (25 ~rams per liter). The strip was then allowed to dry in air at room temperature and was thén painted with a mixture consisting of equal volumes of isopropyl alcohol and of a 50 percent aqueous solution of manganese nitrate and then baked for ten minutes in an oven at a temperature of 205 degrees centigrade.
This electroplating-painting-baking cycle was repeated two more times. Additional coatings of MnO2 were applied to the plate using three electroplating-painting-bakin~
cycles under the conditions specified in the previous paragraph with the exception that the electroplating period was increased to 30 minutes during each cycle. The weight of the MnO2 coatings applied thus far was 0.524 gram, equivalent to about 135 grams per square meter. Additional coatings of MnO2 were applied to the plate using five electropla~ing-painting-baking cycles under the conditions of the preceding paragraph with the exception that the current was increased to 0.15 ampere per square inch (23 milliamperes per square centimeter). The total electroplating time for all the cycles specified in thls Example was five hours.
The titanium strip, prepared as described above, was tested as an anode in a cell containing 150 grams per liter of concentrated sulfuric acid maintained at a temperature of about 50 degrees centigrade. The anode exhibited potentials of 1.48, 1.56 and 1.62 volts at current densities of 1, 3 and 5 amperes per square inch (155, 465 and 775 milliamperes per square centimeter), respectively.

FxAMpLE 6 A strip of porous titanium was etched and coated with two double coatings of tin dioxide using the method described in Example 1 except that the strip was baked at 500 degrees centigrade for 20 minutes instead of 490 degrees centigrade for seven minutes. The coated titanium strip was then dipped into a mixture consisting of 20 milliliters water, 5 milliters isopropyl alcohol and 5 ml. manganese nitrate (50 percent aqueous solution~. The strip was allowed to dry in air at room temperature and was then baked for 30 minutes in an oven maintained at 205 degrees centigrade. This dipping-baking process was repeated four times. The weight of the MnO2 coating was about 50 grams per square foot (540 grams per square meter).
The titanium strip, prepared as described above, was tested as an anode9 as described in Exam?le 1. The area of the anode was 2.4 square inches (15.4~ square centimeters) including the front, back and edges~ The anode exhibited potentials of 1.419 1052 and 1.59 volts at current densities of 0.25, 1.0 and 3.0 amperes per square . - 11 -inch 139, 155 and 465 milliamperes per square centimeter), respectively.

A strip of porous titanium was etched and coated with two double coatings of tin dioxide as described in Exarnple 6. Coatings of MnO2 were then applied by 5 electroplating and dipping. The strip was electroplated at room temperature for 20 minutes using a current of 0.03 ampere per square inch ( 4.7 milliamperes per square centimeterj in a bath containing manganese sulfate (150 grams per liter) and concentrated sulfuric acid (25 grams per li~er). The strip was allowed to dry in air at room temperature. It was then dipped into a mixture consisting of 20 millillters 10 wa~er, 5 milliliters isopropyl alcohol and 5 milliliters manganese nitrate~(50 percent aqueous solution) and then baked in an oven at 205 degrees centigrade for 30 minutes.
This plating-dipping-baking cycle was repeated three more times to increase the thickness of the MnO2 coating.
The titanium strip, prepared as described ;above, was tested as an anode as 15 described in Examples 1 and 6. The anode exhibited potentials of 1.41, 1.~7 and 1.54 : :.
volts at current densities of 0.259 1.0 and 3.0 amperes per square inch (39, 155 and 465 milliamperes per square centlmeter), respechvely.

EXAMPLe 8 A strip of porous titanium was etched and coated with MnO2 as described 20 in ~xample 6 except that no coating of tin dioxlde was applied. The weight of the MnO2 coating was about 55 grams per square foot (600 grams per square meter). ;
The titanium strip, prepared as described above, was tested as an anode as described in Example 6. The anode exhibited potentials of 1.62, 1.95 and 2027 volts at -current densities of 0.25, 1.0 and 3.0 amperes per square inch (39, 155 and 465 25 milliamperes per square centimeter), respectively.
E~y comparing these results with the test results of the anode containing an intermediate conductive tin dioxide layer lsee Example 6), it is apparent that the anode with the conductive tin dioxide layer has lower potentials (.21, .43 and .68 volts difference) when tested at 0.25, 1.0, 3.0 amperes per square inch (39, 155 and 465 30 milliamperes per square centimeter), respectively~

A strip of porous titanium was etched and coated with conducti~e tin dioxide using the method described in Example 1 except that vacuum was used to pull the alkoxy tin-antimony trichloride solution through the strip each time that it was 5 applied thereby producing a more uniform coating. The following conditions in preparing this electrode were also different from those specified in Example 1: drying time at 125 degrees centigrade was 20 minutes, baking time was 30 minutes, baking temperature was 500 degrees centigrade, and two rnore tin dioxide conductive coatings were applied by repeating the coat-dry-bake cycle described above.
10~ The strip of titanium plate was coated with 50 percent aqueous man~anese nitrate solution; vacuum was then applied to puil the solution through the pores. The coating-vacuum cycle was repeated one time, then ~he strip was baked at 200 degrees centigrade for 30 minutes. The above procedure for preparmg the MnO2 coating was repeated five times to increase the thickness of the MnO2 layer.
15The anode, prepared as described above, was installed and~tested as an anode in a cell containing~ 15û grams of concentrated sulfuric acld per liter of solution.
The cell temperature was maintained at 50 degrees centigrade throughout the test The anode exhiblted potèntlals of 1.41~, 1.45~and~l.52 volts at current densities of 0.4~
1.0 and 3.0 amperes per square inch (62, 155 and 465 milliamperes per sq4are 20 centimeter), respectively.

, An anode was prepared as described in Example 9 except that no conduc-tive tin dioxide coating was applied; the procedure used in Example 9 to apply that coating was, therefore, omitted. However, the r~AnOz coating was applied in the nor-25 mal manner, as described in Example 9.
The anode, prepared as described above, was tested as described in Example 9. The anode exhibited potentials of 1.43, 1.54 and 1.78 volts at current densities of 0.1f, 1.0 and 3.0 amperes per square inch (62, 155 and 465 milliamperes per square centimeter), respectively. By comparing the test results of the anodes prepared in Examples 9 and 10, it is apparent that the anode containing the conductive tin dioxide coating exhibited lower voltages, (.02, .09, .26 volts difference) at 0.4, 1~0 and 3.0 amperes per square inch (62, 155 and 465 milliamperes per square centimeter), respectively. This lowering of voltage is particularly striking at high current densities which are economically desirable in an industrial process.
, A strip of clean titanium plate was etched and then the semi-conductive intermediate ~ coating of oxides was applied as described in xample I except that the baking~temperature waa 600 degrees centigrade. The coated titaniurn strip was then painted with a 50 percent aqueous solution of manganese nitrate and fired at approximately 300 degrees centigrade. This process was repeated until approximately 14.4 grams per square foot (155 grams per square meter) of manganese dioxide were present on the strip.
The titanium strip, prepared as described above, was tes~ed as an anode, as described in Example 1. The area of the anode was approximately 12 square inches(77.4 square centimeters) and exhibited potentials of 1.38, 1.42 and 1.43 volts at cur-rent densities of 1.0, 3.0 and 5.0 amperes per square inch (1559465 and 775 milliamperes per square centimeter), respectively.

EXAMPL~ 12 Three strips of clean titanium plate were etched and then the semi-conduc-tive intermediate coating of tin and antimony oxides ~.as applied according to E~xample 1 until each o~ the three strips had between 0.012 grams and 0.014 grams weight gain of tin and antimony compounds. The area of each strip was approximately 4 square inches (25.8 square centimeters). Strip A was then electroplated with manganese dioxide for three hours to obtain a weight gain of approximately 18.9 ~rams per square foot (203 grams per square meter) of manganese dioxide. Strip B was electroplated in one-half hour intervals and baked for 20 minutes at approximately 220 degrees centigrade between each half hour of electroplating, a total of five times to obtain approximately 14.5 grams per square foot (155 grams per square meter) of Srj' ~''~' ~",,y manganese dioxide on the surface of strip B. Strip C was first electroplated for one-half hour and ~hen coated with a thermally decomposable manganese nitrate and baked for twenty minutes at approximately 220 degrees centigrade. This process was repeated five times to obtain a weight gain of approxirnately 15.8 grams per square 5 foot ~170 grams per square meter) of manganese dioxide onto the surface of strip C.
The resultant strips A, B and C prepared as described above were tested an anodes in a cell containing 150 grams per liter of concentrated sulfuric acid main-tained at a temperature of approximately 50 degrees centigrade. Strip A when sub-jected to a current density of approximately 0.5 amperes per square inch (77.5 mil-10 liamperes per square centimeter) developed a serious flaking off of the coatings. StripB exhibited a potential of 1.41, 1.45 and 1.57 volts at current densities of 0.5, 1.0 and 3.0 amperes per square inch (77.5, 155 and 465 milliamperes per square centimeter), respectively. There was a ~lakmg off of the coating at the bottom edge of strip B
during this process. Strip C exhibited potentials of 1.41, 1.43 and 1.50 volts at current densities of 0.5, 1.0 ancJ 3.0 amperes per square inch (77.5, 155 and 465 milliamperes per square centimeter), respectively.

A strip of porous tltaniurn having a surface area of approximately 7 square Inches (45 square centimeters) was coated with a solution of tin and antimony com-pounds by use o~ a vacuum to suck the solution through the porous material~ The solution consisted of 5.27 grams of stannous sulfate, 2.63 grams of antirnony trichlor-ide, 10 milliliters of hydrochloric acid, and 20 rnilliliters of butyl alcohol. ~his was done four times with the baking of one-half hour at approxlmately 500 degrees centi-grade between each pass through the porous titanium material. A 50 percent aqueous solution of manganese nitrate was passed through the material in the same fashion with a baking between each pass of 45 to 60 minutes at approximately 200 degrees centigrade until a weight gain in the range of 3.36 to 3.56 grams o-f manganese dioxide is contained therein.
The strip of porous titanium prepared as described above was tested as an anode, as described in Example 1. The anode exhibited potentials of 1.~4, 1.49, 1.51, r7. ~ , 1.54 volts at current densities of 0.25, 0.5, 0.75, and 1.0 t39, 77.5, 116 and 155 milliamperes per square centimeter)S respectively. l~ife tests of this anode have revealed that the anode is in good working order after over 2,000 hours of continuous use.
, - 5 EX~MPLE 14 A strip of porous titamum was coated with tin/antimony compounds Sy sucking through the material with a vacuum, a solution of tin and antimony compounds .
as described in Example 13. This procedure was repeated four times with baking between each pass of one hour at approximately 490 degrees centigrade. A solution of 10 50 percent aqueous manganese nitrate was also sucked through the coated porous . .
titanium strip with a vacuum four times with a 40 to 50 minute baking at 210 degrees ~ -centigrade after each application.
The porous titanium strip prepared as above-described was tested an an anode as described in Example i~ The anode exhibited a potential of 1.49 volts at a ~:..
curren~ density of 0.5 amperes per square inch (77.5 milliamperes per square centi~
, meter). This electrode remains in good condition after over 2,000 hours of continuous use thus showing a good life time.

EXAZ~/lPLE 1 5 A solution for the semi-conductive intermediate coating was prepared by ~
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20 mixing 30 milliliters of butyl alcohol, 6 milliliters of concentrated sulphuric acid (H2so4?, 1.1 grams of antimony trichloride SSbC13) and 9.7 grams of stannic chloride~
pentahydrate lSnC14 5H2O). A strip of clean titanium mesh was irnmersed in hot s;~
HCl for Yz hour ~o etch the surface. It was then rinsed with distilled water and dried.
Three coats of the alkoxy tin antimony trichloride solution were applied to the titan- ;
. . .- .:

2~ ium plate by brushing. After each application of solution the plate was dried for 30 minutes at 120C followed by baking at 600C for 30 minutes. The strip was then ~;
painted with a 50 percent aqueous solution of cobaltvus nitrate and baked at 235C for 30 minutes. Twelve coats of the cobaltous nitrate solution were applied. The anode, prepared as described above, was installed and tested in a cell containing a solution of 7 . :
300 gpl NaCl adjusted to a pH less than 1 and maintained at a temperature of about 75 ~ ~-degrees centigrade. At applied current densities of 0.15, 0.45 and 0.75 arnps/cm2 the anode exhibited potentials of 1.14, 1.17 ancl 1.21 volts vs. SC ~corrected for iR
solution resistance), respectively.

EXAMPl E 16 ~
A strip of titanium mesh with an approximately 0.033cm layer of porous titan- . ;
ium on both sides was coated with tin oxide and antimony oxide as described in Exampl~ 15. ~elve coats of a 50% aqueous solution of cobaltous nltrate werebrushed onto the strip and after each coating application the strip was baked at 235C for 30 minutes. The weight of the cobalt oxide coating was about 280 g/m2. The anode, prepared as described above, was ins~alled and tested in a cell containing dilute sul-phuric acid (150 g/l of conc. H2S04) maintained at a temperature of about 50C. At applied current densities of 0.15, 0.45 and 0.75~ amps/cm2 the anode exhibited ;
potentials of 1.41, 1.45 and 1.49 volts vs. SCE (correc~ed ~or iR solution resistance~, respectively. `

Thus it should be apparent from the fore~oing description of the preferred embodiments that the composition hereindescribed ac:complishes the objects of the invention and solves the problems that attendant to such electrode compositions for use in electrolytic cells for electrochemical production.

'.'~' ~, .

.

Claims (11)

WHAT IS CLAIMED IS:

1. An electrode for use in an electrolytic cell comprising a valve metal substrate selected from the group of aluminum, molybdenum, niobium, tantalum, titanium, tungsten, zirconium and alloys thereof; on the surface of said valve metal substrate a semi-conductive intermediate coating of tin and antimony compounds applied and converted to their respective oxides, said semi-conductive inter-mediate coating of tin and antimony compounds having between 0.1 and 30 weight percent of antimony compounds; and on the surface of said semi-conductive intermediate coating, a top coating consisting of oxides of transition metals selected from the group of chromium, iron, cobalt, nickel, molybdenum, or tungsten.

2. An electrode according to claim 1 wherein said valve metal substrate is titanium.

3. An electrode according to claim 2 wherein said valve metal substrate is porous titanium.

4.An electrode according to claim 1 wherein said semi-conductive inter-mediate coating of tin and antimony compounds has an amount of antimony compounds within the preferred range of 3 to 15 weight percent.

5. A method for manufacture of an electrode for use in an electrolytic cell comprising the steps of: selecting a valve metal substrate from the group of aluminum, molybdenum, niobium, tantalum, titanium, tungsten, zirconium or alloys thereof; applying to said valve metal substrate a semi-conductive intermediate coat-ing of thermally decomposable compounds of tin and antimony containing 0.1 to 30 weight percent antimony, drying said semi-conductive intermediate coating; baking said semi-conductive intermediate coating in an oxidizing atmosphere at an elevated temperature to transform the tin and antimony compounds to their respective oxides;
and applying on the surface of said semi-conductive intermediate coating a top coating consisting of oxides of transition metals selected from the group chromium, iron, cobalt, nickel, molybdenum or tungsten.

6. A method according to claim 5 wherein said semi-conductive inter-mediate coating is applied in a series of layers, each being dried before subsequent application of the next layer, and being baked at the conclusion thereof to their re-spective oxides.

7. A method according to claim 5 wherein the top coating is applied by painting a transition metal nitrate in a series of layers, dried, and baked to its oxide form.

8. A method according to claim 5 wherein the top coating of a oxide of a transition metal is applied by electroplating a transition metal oxide upon the surface of the semi-conductive intermediate coating.

9. A method according to claim 5 wherein the top coating is applied by electroplating in a bath containing a transition metal sulfate; drying the layer at room temperature; painting a solution containing the transition metal nitrate over the electroplated layer; baking the top coating to transform the transition metal to its oxide form; and repeating this process two to six times.

10. A method according to claim 5 wherein porous titanium is selected, and the method for applying the semi-conductive intermediate coating is by sucking a solution through the substrate using a vacuum.

11. A method according to claim 10 wherein the method for applying the top coating of the oxide of a transition metal is by sucking a transition metal nitrate solution through the substrate using a vacuum and baking the top coating to transform the transition metal to its oxide form.