IL36457A - An electrode,its production and its use as an oxygen anode - Google Patents

Description

ητυιο na ana»wni mis" Tx&n An electrode, Its production and its use as an oxygen anode DIAMOND SHAMROCK CORPORATION BACKGROUND OF THE INVENTION In the area of electrochemical reactions, those processes which employ electrodes functioning as "oxygen anodes" are of considerable commercial significance. Examples of some such processes include electrowinning, for example the aqueous electro-winning of antimony, cadmium, chromium, cobalt, copper, gallium, indium, manganese, thallium and zinc; water electrolysis^ metal plating and others. A variety of materials have been used for the fabrication of such anodes including graphite, platinum, platinized titanium, nickel, lead and lead alloys. These anodes are known to have various disadvantages which limit their application such as chemical reactivity, lack of dimensional stability, material cost, contamination of the product, sensitivity to impurities and others. While all of these problems are serious, the overriding disadvantage in all instances is the high oxygen overvoltage exhibited by the anodes. "Overvoltage" refers to the excess electrical potential over theoretical at which the desired element is discharged at the electrode surface.

Similar problems have plagued the chlor-alkali industry in its attempts to obtain a low chlorine overvoltage, dimension-ally stable anode for use in the production of chlorine and caustic. Thus, electrode development in this industry may be traced through the use of graphite and platinized titanium to the development of a recent composite type electrode which appears to be especially well suited for use in anodic applications.

These electrodes consist of a valve metal substrate and a coating which has variously been characterized as "mixed oxide", "mixed crystal", "solid solution" and "ceramic semi-conductor", but which is characterized for the most part by being based upon ί be successful to date have invariably been based upon the presence of the oxides in the crystalline form and for the most part in such a manner than an atom of, for example valve metal in the valve metal oxide crystal lattice is substituted with an atom of a non-valve metal. Such coatings hereinafter will be referred to as "solid solutions". Electrodes of this type have found almost immediate acceptance as chlorine anodes by reason of their excellent wear characteristics and extremely low chlorine overvoltages.

In view of the similarity in some respects between the problems relating to chlorine anodes and oxygen anodes, investigators have been led to attempt to employ the successful solid solution-type electrodes as oxygen anodes. In doing so it was observed that the oxygen overvoltages of some of the more widely used materials are as follow; lead Ο.85 volt, platinized titanium Ο.62 volt, graphite 0. 0 volt, nickel 0.37 volt and solid solution (ruthenium oxide-titanium oxide on titanium metal) 0.29 volt. These overvoltages are measured at 2 amperes per square inch in a IN NaOH solution at 80°C.

Thus it can readily be seen that the solid solution-type coating offers substantial advantage in terms of oxygen over-voltage which makes its use as an oxygen anode extremely appealing economically. However, it was soon found that such advantage could not be exploited for another reason., Whereas, when the solid solution-type electrode is employed as an anode for chlorine production, the chlorine overvoltage remains substantially constant for long periods of time in use, when the same electrode is employed as an oxygen anode, the oxygen overvoltage, while initially low, steadily increases until, if carried to an extreme, the anode passiyates completely, i.e., as the anode in sulfuric acid, the anode will passivate to such an extent as to render further operation uneconomic within approximately 30 hours at a current density of 1.0 ampere per square inch of anode surface area. Therefore, if the advantage of the extremely low oxygen overvoltage available with the solid solution-type electrodes is to be obtained, a method of preventing the passivation of this coating during use in the evolution of oxygen must be provided. Once it was found that apparently the method of passivation involves the slow diffusion of oxygen through the solid solution-type coating into the supporting valve metal, attention was directed to some method of preventing such diffusion.

STATEMENT OF THE INVENTION Therefore, it is an object of the present invention to provide a composite electrode and a method of producing same which electrode will exhibit improved resistance to passivation, particularly when employed as an oxygen anode.

A further object of the invention is to provide an improved electrolytic process involving the generation of oxygen at the anode, which process employs a passivation-resistant anode .

These and further objects of the present invention will become apparent to those skilled in the art from the specification and claims which follow.

It has now been found that the tendency for a solid solution-type electrode to passivate when oxygen is generated at its surface, may be substantially reduced, if not completely eliminated, by the provision of a barrier layer between the electrode comprises 1) an electrically-conductive supporting substrate] 2) a relatively thin, intermediate, electrically-conductive relatively oxygen-impermeable, barrier layer consisting essentially of a material selected from the group consisting of platinum-iridium alloys and oxides of cobalt, manganese palladium, lead and platinum and 3) a electrically-conductive, electrocatalytically-active, electrolyte-resistant outer coating consisting essentially of a solid solution of a valve metal oxide with at least one non-valve metal oxide. Such an electrode not only exhibits an extremely low initial oxygen overvoltage, but retains that low overvoltage through extended periods of use.

Furthermore, the wear-rate, that is, the physical loss of coating per unit time, is extremely low.

A further and unexpected advantage of the present invention may also be mentioned. While the various patents describe the provision of the solid solution-type coatings on a wide variety of substrates, it has heretofore been extremely difficult to apply a "true" solid solution-type coating to anything but a valve metal, especially titanium, substrate.

When, for example, one attempts to apply a ruthenium oxide-titanium oxide solid solution onto a steel substrate, a nonadherent, apparently amorphous, physical mixture of oxides is obtained which has little or no practical value as an electrode coating. In the chlorine industry this has been of little importance for the reason that titanium is in any event the preferred substrate material owing to its ability to heal itself if exposed to the corrosive cell environment unprotected by the solid solution coating. However, it is obvious that for other, less demanding, applications, a less costly substrate, such as apparently exhibits some sort of "catalytic** activity which** assures that the mixture of materials subsequently applied will form a true solid solution, regardless of the substrate.

The electrode the subject of the present invention is clearly distinguished over the prior art electrodes such as, for example, that disclosed in South African Patent Specification Ho. 68/00834· Thus the electrode disclosed in the South African patent in no way shows the provision of an intermediate barrier consisting of cobalt oxide or lead oxide* The invention is defined broadly as relating to an electrode. The word "electrode" is intended to refer to either anodes or cathodes as it will be apparent that the composite electrodes of the present invention will carry current in either capacity. Of course, since the primary advantage of the electrode is its resistance to passivation, which passivation usually occurs when oxygen Is generated at or near the electrode surface, most applications of the electrode will be as an anode, especially as an oxygen anode. It will be understood, however, that if, as is suspected at this time, the method of failure of a chlorine anode also relates to oxygen passivation, the composite electrodes of this invention will also act as chlorine anodes of extended life.

It should further be understood that the invention is independent of the mechanical configuration of the substrate and hence may take any shape which will allow the application of the intermediate and other coatings by the techniques generally described hereinbelow. Thus the electrodes may take the form of a wire, rod, cylinder, sheet and the like. Further, if the electrode is present in a sheet or plate form, it may be either solid or foraminous* Other configurations most useful in a particular application will be apparent to those skilled in the had been heretofore when a solid solution-type coating was contemplated. While valve metal substrates, particularly titanium, will still be preferred for many applications because of their ability to "heal" themselves under corrosive cell conditions should a defect in the coating arise, it is now possible to use most any material which has the desired combination of electrical conductivity and mechanical strength. Therefore graphite, steel, copper and the like are also quite useful for numerous applications of the present invention.

The intermediate barrier layer has been said to be selected from the group consisting of platinum-iridium alloys and oxides of cobalt, manganese, palladium, lead and platinum. These materials may be applied in relatively thin layers, that is, as low as 0.1 micron, to form an electrically-conductive layer which appears to prevent the diffusion of oxygen through the relatively porous outer coating to the underlying substrate. On this basis, any barrier layer meeting the other criteria herein, especially electroconductivity, which is less permeable to oxygen diffusion than the covering solid solution-type layer, will theoretically result in improved resistance to passivation. Cobalt, manganese, lead and palladium oxides may be provided directly, and in the proper crystalline form, on the electrically-conductive substrate by electrolytic deposition using techniques well-known to those skilled in the art. Platinum oxide is not deposited directly but rather a metallic coating of platinum is first electrodeposited, followed by a brief heat treatment which appears to convert a substantial portion of the platinum to the oxide form. It appears at this time that the heat treatment is critical since it has been found that the solid solution coating s d he eth ds the same reason. The effect of the heat treatment i converting the platinum to the proper form may be evidenced by the distinct color change from the original metallic finish. The platinum-iridium alloys appear to remain substantially in the metallic form even following thermal application of the solid solution. These alloys generally, but not necessarily, contain 20-50 iridium, typically 30$, and are applied by any of the known methods, such as thermochemical deposition from mixed salt solution, which result in an adherent, relatively non-porous layer.

It is the solid solution-type outer coating which gives the composite electrode its ability to catalyze a number of electrochemical reactions at remarkably low overvoltages . This coating consists of a solid solution of a valve metal oxide with at least one non-valve metal oxide. The term "valve metal" in this context has its usual significance, that is, it relates to metals such as titanium, tantalum, zirconium, niobium and the like. The non-valve metal oxide is chosen to be such that the desired solid solution is formed, that is, it must have the proper crystal size to mesh with the crystal lattice of the valve metal oxide, usually by substitution of one atom of non-valve metal for one atom of valve metal, thus providing electrical conductivity in a normal non-conductive material. Furthermore, it must provide the desired electrocatalytic activity. Especially suitable non-valve metal oxides useful in the practice of the present invention including platinum, palladium, iridium, ruthenium, rhodium, osmium, molybdenum, tin, tungsten, vanadium, chromium, rhenium, manganese and the like.

In order that those skilled in the art may more readily understand the resent invention and certain referred methods EXAMPLE 1 A piece of 0.016 inch thick solid titanium sheet (A.S.T.M. B 2β5 58T Grade 2) is degreased with acetone and etched 10 minutes at 90°C. in 20$ HC1. This sheet is made the cathode in a 2$ solution of chloroplatinic acid in a 2.0 N HC1. Platinum is deposited for 10 minutes at room temperature and a current density of 6.2 amperes per square foot (a.s.f.). The electro-deposited platinum metal-coated titanium substrate thus obtained is then heated in air for 7 minutes at 450°C. A solution consisting of 1 gram RuCl^'XHgO (0.4 gram Ru metal), 6.2 ml. n-butyl alcohol, 3.0 ml. tetrabutyl orthotitanate, and 0.4 ml. 3 $ HC1 is then painted onto the sample surface, and the sample is heated in air at 450°C. for 7 minutes. This painting and heating cycle is repeated five more times to bring the total number of coats to six. This electrode when employed as the anode is a I N NaOH solution at 8o°C. and an applied current density of 2.0 amperes per square inch (a.s.i.), exhibits an oxygen overvoltage of 0.29 volts.

When the electrode prepared above is operated as an anode in a 100 grams per liter aqueous solution of sulfuric acid at 20°c. and a current density of 4 a.s.i., it continues to generate oxygen for 110 hours. An electrode prepared in the same manner but without the intermediate heat-treated platinum barrier layer, passivates in about 1 hour,* Thus it will be seen that the barrier layer very significantly extends the useful life of a solid solution-type electrode. Furthermore it should be realized that the life of the anode at 4 a.s.i. is equivalent to several months of operation at normal commercial current densities of from 30-40 a.s.f.

-* EXAMPLE 2 A piece of .060 inch expanded titanium mesh is pre-treated as in Example 1 and made the anode in a solution containing 291 grams Co(ΝΟ^^'βΐ^Ο. Cobalt oxide is deposited for 10 minutes at a temperature of 6o°C. and a current density of 4.8 a.s.f. Six coats of the ruthenium-titanium solution are applied as in Example 1. Oxygen overvoltage is again measured at Ο.29 volts and 4 hours is required for the anode to passivate EXAMPLE 3 A piece of .016 inch titanium sheet is pretreated as in Example 1 and made the anode in a solution containing 100 grams per liter MnSOj. and 20 grams per liter H2SO . MnOg is deposited for 10 minutes at a temperature of 80°C. and a current density of 7.2 a.s.f. Six coats of the foregoing ruthenium-titanium solution are again applied. An oxygen overvoltage of 0.29 is noted and the electrode is employed under the conditions of Example 1 without noticeable passivation after more than 88 hours.

EXAMPLE 4 A piece of .060 expanded titanium is pretreated as in Example 1 and made the anode in a solution containing 3.3 grams per liter palladium nitrate. Palladium oxide is deposited for 1 hour at a temperature of 50°C. and a current density of 2.4 a.s.f. After application of the solid solution-type coating as before, an electrode is obtained which exhibits an, oxygen overvoltage of 0.29 and a "life" of 37 hours at 3 a.s.i.

EXAMPLE 5 A piece of unimpregnated Union Carbide graphite Grade YAV is ground down to expose a fresh surface. Mn02 is then deposited on this surface as in Example 33 and six coats of the solid solution coating are applied as in Example 1. This sample again gave the low oxygen overvoltage of Ο.29. The anode is operated for 16 hours without change, it being apparent that passivation will not occur absent the film-forming metal substrate .

EXAMPLE 6 A piece of .016 inch titanium sheet is pretreated as in Example 1 and made the anode in a solution containing 300 grams per liter P^NO-^g* 2 grams per liter Cu(N03)2*H20, and 1 gram per liter non-ionic wetting agent. An undetermined amount of lead dioxide is deposited. Six coats of the titanium-ruthenium solution are applied as before, with the exception that the bake temperature is reduced to 300°C. owing to the low decomposition temperature of lead dioxide. X-ray analysis establishes the presence of the usual solid solution structure and a 12 hour test demonstrates that no passivation occurs.

EXAMPLE 7 A titanium metal sheet is provided, by thermochemical deposition, with a 70$ platinum - 30$ iridium alloy layer amounting to 4.5 milligrams per square inch. Six applications of the solid solution coating are then made as in Example 1.

The resultant electrode continues to operate for 305 hours While the invention has been described with reference to certain specific embodiments thereof, these examples are intended to be illustrative only and the invention should not be so limited since changes may be made therein which are sti-11 within the intended scope of the appended claims.

Claims (2)

1. 36457/2 WE CLAIM: 1. An electrode comprising: a) an electrically-conductive supporting substrate; b) a relatively thin, intermediate, electrically-conductive, relatively oxygen-impermeable, barrier layer consisting essentially of one oxide of the group consisting of oxides of cobalt and lead and c)an electrically-conductive, electrocatalytically | active, electrolyte-resistant, solid solution-type outer i coating consisting at least one valve metal oxide and at least one oxide of metal selected from the group consisting of platinum, palladium, iridium, ruthenium, rhodium, osmium, ; molybdenum, tin, tungsten, vanadium, chromium, rhenium and manganese.

2. An electrode of Claim 1 wherein the outer coating is a solid solution of titanium dioxide and ruthenium oxide, j 36457/2 6. In a method of conducting an electrochemical reaction wherein oxygen is liberated at the anode, the improvement which comprises using as said anode a composite electrode comprising: a) an electrically-conductive supporting substrate, b) a relatively thin, intermediate, electrically-conductive, relatively oxygen-impermeable, barrier layer consisting essentially of one oxide of the group consisting of oxides of cobalt and lead and c) an electrically-conductive, electrocatalytically-active, electrolyte resistant, solid solution-type outer coating consisting at least one valve metal oxide and at least one oxide of a metal selected from the group consisting of platinum, palladium, iridium, ruthenium, rhodium, osmium, molybdenum, tin, tungsten, vanadium, chromium rhenium and man ar.ese. For the Applicants DR. By