US3917525A - Electrode for electrochemical reactions - Google Patents

Abstract

An electrode for electrochemical reactions comprising a substrate of a film forming or barrier metal covered with a cobaltite of at least two rare earth metals, one of the rare earth metals having a high atomic number and the other having a lower atomic number.

Description

United States Patent. [191 Bouy etal. Nov. 4, 1975 ELECTRODE FOR ELECTROCHEMICAL [56] Reference; Cited REACTIONS UNITEDSTATES PATENTS [75] Inventors: Pierre Bouy, Enghien-les-Bains; Guy 3,329,594 7/1967 Anthony et al. 204/95 Cheradame, Pont-de-Claix, both f 3,801,490 4/1974 Welch 204/290 F France 3,804,740 4/1974 Welch 204/290 R [73] -Assignee: Rhone-Progil, Courbevoie, France FOREIGN PATENTS OR APPLICATIONS Filed Ju y 5 1 4 7,204,743 6/1973 Netherlands 204/290 F [21] Appl. N0.: 486,051 Primary Examiner-F. C. Edmundson [30] Foreign Application Priority Data [57] ABSTMCT July 20 I 973 France 73 26694 An electrode for electrochemical reactlons comprising a substrate of a film forming or barrier metal covered with a cobaltite of at least two rare earth metals, one '3 ggiff of the rare earth metals having a high atomic number [58] Field 0 204/290 1: 290 F 291 and the having a ammi" number" 5 Claims, No Drawings ELECTRODE FOR ELECTROCHEMICAL REACTIONS BACKGROUND OF THE INVENTION The present invention concerns a new electrode which can be used in electrolytic cells serving for the production of chlorine, caustic soda or chlorates. The cells serving for the production of chlorine or caustic soda are either diaphragm cells or mercury cells. The chlorates are produced in a cell whose structure is similar to that of the diaphragm cells but which nevertheless has no diaphragm.

The electrodes previously generally employed as anodes in electrolytic cells were frequently made of graphite. Their use has always entailed certain disadvantages resulting from their wear which causes an increase in the voltage necessary for the proper operation of the electrolysis cell as the result of the wear which increases in the distance between anodes and cathodes and the contamination of the electrolyte.

More recently it has been attempted to develop anodes from a metal having good resistance to corrosion by the electrolyte which metal is covered with an electrochemically active precious metal, the resulting composite then being subjected to a treatment which favors activation. These anodes are dimensionally stable and do not have the above-mentioned drawbacks. For anodes of this type it has been proposed to employ a core of zirconium, zirconium-titanium alloy, tantalum or niobium covered with platinum. There has also been proposed an anode of titanium covered with platinum. Titanium, like the other core metals mentioned above, being a film forming or barrier metal capable of forming a film or barrier layer of oxide in the electrolysis solutions to protect its surface from corrosion at the places where the platinum is porous.

Also, electrodes have been produced of one of these film forming or barrier metals or alloys capable of forming a film or barrier layer, covered with an oxide of precious metal or with mixtures of oxides of precious and non-precious metals.

As an electrode covering or coating there has also been proposed an electrolytic deposit of cobalt oxide, the electrocatalytic properties of which are very close to those of the precious metals, their alloys or their compounds. It is also known that deposits of saline oxide, cobalt oxide (C 0 have properties very close to those of the precious metals. However, none of these compounds of cobalt can be used in solid form or as deposit in industrial practice as a result of the lack of stability of their electrocatalytic properties. As a matter of fact, these compounds when used as anodes, rapidly become electrically insulating and oppose the passage of the current, thus producing a resistance which leads to prohibitive overvoltages.

It has recently been suggested that these drawbacks could be avoided by means of an electrode formed of a substrate or titanium or other similar film forming or barrier metal covered with a thin film of an electroconductive coating, of a metal of the platinum group, for instance, on which an outer layer or surface of perovskite is applied. The perovskite is an oxygenated compound of two different metals which is well known in the literature and may be represented by the empirical formula:

. a special X-ray diffraction diagram.

These cobaltites have a relatively high electric conductivity which varies with the temperature, the rare earth metal playing an important role in the mechanism of conduction.

The electrocatalytic power of these cobaltite compounds is not necessarily related to the perovskite structure, since there are numerous compounds having this structure, such as, for instance, LaCrO lanthanum chromite, which are without it. However, it is necessary in the case of the rare earth cobaltites to obtain the perovskite structure which alone seems to withstand corrosion in slightly acid medium. It has been noted that this corrosion is smaller the more acid the character of the rare earth used. The compound LaCoO lanthanum cobaltite, for instance, although having remarkable electrocatalytic properties, is entirely unsuitable to constitute an anode for an electrolysis cell as a result of the ease with which it passes into solution in slightly acid chlorinated medium. This defect decreases when the lanthanum is replaced by a rare earth of higher atomic number. One succeeds in this way in considerably improving the resistance to chemical and electrochemical corrosion by using rare earths of higher and higher atomic number, as will be shown below:

The compounds LaCoO PrCoO NdCoO and GdCoO are prepared from an intimate mixture of the oxides of the stated. elements, which is calcined at 1200C. for 15 hours. The series of compounds thus prepared is analyzed by X-ray diffraction and is found in each case to be solely of the perovskite structure. The chemical resistivity in acid medium of these mixed oxides is then measured as follows:

To 1 gram of the powder of each compound there are added 200 ml. of O. IN hydrochloric acid. The attack is allowed to continue for 1 hour in the cold. After filtration, the cobalt and the rare earth present in the filtrate are determined. The following table sets forth the corrosion of the compounds in terms of percentage, that is to say, the ratio of the total mass of the metal elements present in the solution to the total mass of the metal elements present in 1 gram of cobaltite.

Corrosion.

LaCoO 35% PrCoO 9.6% NdCoO 5.7% GdCoO 4.3%

LaCoO PrCoO NdCoO GdCoO about about about about Time 1 hr. 30 hr. 400 hr. 500 hr.

There is thus noted the good correlation between the electrolysis life and the corrosion in acid medium.

However, one is limited in the use of the heavy rare earth metals as electrodes in electrolysis cells by the tendency which these rare earths have to give in whole or in part a mixed solid phase, Co (TR) ,O which is more or less rich in cobalt and known by crystallographs under the phase designation CTl O A range of existence of the different crystalline phases has been established and is described, for instance, on page of the book by F. S. Galasso, Structure Properties of Perovskite-Type Compounds, Pergamon Press, 1969. This limitation is very disturbing, since the cobalt oxide phase, rare earth metal oxide of the structure CTI O being readily soluble in acids, is unsuitable for the desired use in electrolysis. This particular behavior of the rare earths of high atomic number is explained by crystallographic considerations utilizing ionic rays.

It is, accordingly, an object of the present invention to provide electrodes for electrochemical reactions which do not have the shortcomings of the prior art.

It is also an object of the present invention to provide an electrode for an electrolytic cell which employs a cobaltite of perovskite structure, which electrode has improved properties.

It is a further object of the present invention to provide electrodes for electrolytic cells, which electrodes have excellent resistance to corrosion.

Further objects will be apparent to those skilled in the art from the present description.

GENERAL DESCRIPTION OF THE INVENTION It has now surprisingly been discovered that these drawbacks of the prior art can be eliminated by means of employing new rare-earth or rare earth metal cobaltite compounds comprising at least two rare earth metals, one or more of these rare earth metals having a high atomic number of at least about 65 and not resulting in a compound of perovskite structure when they are combined alone with the cobalt. Another of the rare earth metals having an atomic number below about 65. This new rare-earth cobaltite compound has a special X-ray diffraction pattern and a characteristic perovskite structure. This diffraction pattern and structure are fully described in the literature. For instance, in Chapter 5 of the book, Diffraction Procedures, by Klug and Alexander, John Wiley and Sons (1954), see pages 235 to 318.

The new electrodes in accordance with the invention comprise a substrate of a film forming or barrier metal covered with a cobaltite compound described above which forms the surface of the electrode. This compound has the general formula in which Ln represents a rare earth metal of high atomic number, such as at least about 65, Ln a rare 4 earth metal of lower atomic number, such as below about 65, and x is a number between 0.001 and 0.999, and preferably between about 0.05 and 0.3.

The new cobaltite compounds in accordance with the invention have a substantially higher resistance to acid corrosion than the known rare earth metal cobaltites, while having the same characteristics of conductivity and the same electrocatalytic properties.

The rare earth metals which can be used are those listed in the Periodic Table of the Elements. Those of high atomic number comprise terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. The rare earths of lower atomic number comprise lanthanum, cerium, praseodymium, neodymium, samarium, europium and gadolinium.

The substrate, or core, of the electrode is advantageously formed of film forming or barrier metal, that is to say, of metal forming a passivating layer of oxide which permits the passage of current only in the direction towards the cathode. These film forming metals are well known and include, for example, titanium, tantalum, tungsten, hafnium, zirconium, aluminum, niobium and their alloys. Graphite can also be used and is intended to be included in the term film forming metal as used herein. The substrates may be solid pieces or thin, non-perforated plates. They may also be of perforated plates or metal gauze. Their shape is desirably that customarily employed for the anodes of electrolysis cells.

It has been found that the value of the ionic rays of the component rare earth metals of the cobaltite compound is important, and that it is not possible to combine merely any rare earth metals in any proportion. Thus if one uses a rare earth having an ionic radius as small as that of erbium, it is necessary to introduce a rather large proportion of a rare earth metal having a rather high ionic radius such as that of neodymium.

Of course, the rare earth cobaltite need not be limited to two rare earths, but may comprise three rare earths or even more, the essential factor being the retention of the perovskite structure from one or more rare earth metals leading to this structure with one or more rare earths not leading to it.

These new compounds may be prepared like all the other cobaltites or perovskite structure by processes well known to the man skilled in the art. That is to say, thermolyzable organic or inorganic salts, oxides or hydroxides of the different elements are mixed, coprecipitated and cocrystallized. Then after the drying and crushing operations, the powder obtained, whether or not compacted, is calcined at a temperature between about 900 and about 1500C. for a period of time which may vary from 2 hours to 72 hours. In general, the perovskite compounds which can be used for the electrodes of the invention may be prepared by any of the processes described in the literature. For example, by the process described in the journal American Mineralogist, Vol. 39 (l), 1954.

SPECIFIC DISCLOSURE OF THE INVENTION In order to disclose more clearly the nature of the present invention, the following examples illustrating the invention are given. It should be understood, however, that this is done solely by way of example and is intended neither to delineate the scope of the invention nor limit the ambit of the appended claims. In the examples which follow, and throughout this specification, the quantities of material are expressed in terms of parts by weight, unless otherwise specified.

EXAMPLE 1 Compounds are prepared of the general formula Gd e Tb Coo in which x is the quantity of Gd ions in the gadolinium cobaltite which are replaced by terbium ions.

These compounds are prepared from an intimate mixture of gadolinium, terbium and cobalt oxides the quantities of which, as a function of x, are summarized in Table 1 below:

Table l X 0121 0 Tb,O-, Cobalt oxide content (grams) (grams) 71% (grams) The mixtures of oxides are compresed under a pressure of tons into the form of pellets and then 'calcined at 1200C. for hours. The calcined pellets are then crushed into fine form.

The resulting series of compounds thus prepared is analyzed by X-ray diffraction for identification of the phases. Table 2, below, summarizes the results obtained:

Table 2 x 0 perovskite structure x 0.05 perovskite structure x 0.1 perovskite structure x 0.2 perovskite structure very little C-Tl O structure x 0.3 perovskite structure abundant C-Tl 0 structure x 0.5 perovskite structure very abundant C-Tl O structure x 1 very little perovskite structure c-rl o, structure The chemical resistivity in acid medium of these mixed oxides is then measured as described above. Table 3, below, summarizes the results obtained:

Table 3 Gd ,,Tb,CoO: x Corrosion 9999. LIILQN O It is thus noted that the compounds of the general formula Gd Tb,CoO have minimum corrosion for the highest possible quantity of terbium, which leads to the only true perovskite structure, that is to say, for x 6 then crushed until the size of the grains is less than 10 microns. The black powder thus obtained has a characteristic X-ray diffraction pattern of the perovskite structure of the cobaltites.

The cobaltite thus prepared is then deposited on a titanium plate of 10 mm. width by 30 mm. length and 1 mm. thickness which has been previously cleaned by sanding, washed with distilled water. and dried.

A suspension of the cobaltite is prepared in the following manner: To 1 gram of powder there is added 1 gram of hydrated cobalt nitrate hexahydrate, 1 ml. of water and 1 ml. of isopropyl alcohol. The paste obtained is agitated vigorously until homogeneous suepension is obtained, the agitation being maintained during the production of the deposit. A layer of the suspension of the cobaltite is applied on the surface of the titanium plate by brush. After drying for 5 min. in an oven at 100C, the resulting electrode is kept for 10 min. in a furnace at a temperature of 400C. while it is swept by air. This operation is repeated 20 times. The amount of product deposited is 40 mg./cm The deposit on the electrode consists of cobaltite and 20% cobalt oxide.

The electrode thus prepared is placed in an electrolysis cell for the manufacture of chlorine and caustic soda, in which the electrolyte is a solution of 300 grams per liter of sodium chloride maintained at 80C. and a pH of 4. A current such as to produce an anodic current density of 25 amperes per square decimeter is then passed into the cell; the anodic oxidation voltage of the chloride ions is l millivolts when referred to a saturated calomel electrode. After 1000 hours of electrolysis, the anode potential remains unchanged.

EXAMPLE 2 In accordance with the procedure of Example 1, compounds are prepared of the general formula Gd ,Dy CoO from gadolinium, dysprosium and cobalt oxides, the quantities of which, as a function of x, are summarized in Table 4, below:

An analysis of the powders by X-ray diffraction confirms the results obtained in Example 1 and shows an evolution for x increasing from O to 1 of the perovskite structure towards the structure C-T1 O The chemical resistivity of these mixed oxides in 0.1N hydrochloric acid medium is then measured, the results being summarized in Table 5, below:

Table 5 9 9. moan-O Corrosion It is thus found that the compounds of general formula Gd Dy CoO have a minimum corrosion for the highest possible quantity of dysprosium, which con- 1 fers upon the product the only completely perovskite structure, that is to say, for x equal 0.1.

An electrode is prepared with a surface of Gd Dy C on a titanium plate in accordance with the pro- 1 cedure of Example 1. This electrode is used as electrolysis anode for the manufacture of chlorine. For a brine of 300 grams per liter at 80C. and a pH of 4, there is obtained a strong liberation of chlorine with a current density of 25 amperes per square decimeter, under a voltage of 100 millivolts when referred to a saturated calomel electrode. After a prolonged period of electrolysis, the anode potential remains unchanged.

EXAMPLE 3 In accordance with the procedure of Example 1, compounds of the general formula Nd( ,Tb CoO are prepared from neodymium, terbium and cobalt oxides the quantities of which, as a function of x, are summarized in Table 6, below:

Table 6 x Nd O Tb.,O Cobalt oxide content (grams) (grams) 71% (grams) An analysis of the powders by X-ray diffraction confirms the results obtained in Examples 1 and 2 and shows an evolution for x increasing from 0 to 1 of the perovskite structure towards the CTl O structure.

The chemical resistivity of these mixed oxides is then measured in an 0.1N hydrochloric acid medium, the results of which are summarized in Table 7, below:

T able '7con1inued It is thus found that the compounds of general formula Nd Tb CoO present minimum corrosion for the highest possible amount of terbium, which confers 0 upon the product the only completely perovskite structure, that is to say, for x equal 0.2.

An electrode is prepared having a surface of Nd Tb CoO on a plate of titanium by means of an organic or inorganic binder in accordance with a procedure substantially the same as that of Example 1. This electrode is used as electrolysis anode for the manufacture of chlorine. For a brine of 300 grams per liter at 80C. and a pH of 4, there is obtained a strong liberation of chlorine at a current density of 25 amperes per square decimeter under a voltage of 1100 millivolts against a saturated calomel electrode. After a prolonged time of electrolysis, the anode potential remains unchanged.

As will be apparent to those skilled in the art from the foregoing disclosure, cobaltites of other rare earth metals may be employed in the foregoing examples.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

1. An electrode for electrochemical reactions, comprising a substrate covered with a compound having a perovskite structure, characterized by the fact that the substrate is of a film forming metal and the compound of perovskite structure is a cobaltite of rare earths having the general formula Ln Ln ,CoO in which Ln has an atomic number of at least about and Ln has an atomic number below about 65, wherein x is between 0.001 and 0.999.

2. An electrode according to claim 1, in which x is between about 0.05 and 0.3.

3. An electrode according to claim 1, in which the rare earth of Ln has a high atomic number and is a member selected from the class consisting of terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.

4. An electrode according to claim 1, in which Ln is a member selected from the class consisting of lanthanum, cerium, praseodymium, neodymium, Samarium, europium and gadolinium.

5. An electrode according to claim 1, in which the film forming metal substrate is a member selected from the class consisting of titanium, tantalum, tungsten, hafnium, zirconium, aluminum, niobium and their alloys.

Claims (5)

1. AN ELECTRODE FOR ELECTROCHMECIAL REACTIONS, COMPRISING A SUBSTRATE COVERED WITH A COMPOUND HAVING A PEROVSKITE STRUCTURE, CHARACTERIZED BY THE FACT THAT THE SUBSTRATE IS OF A FILM FORMING METAL ANDTHE COMPOUND OF PEROVSKITE STRUCTURE IS A COBALTITE OF RARE EARTHS HAVING THE GENERAL FORMULA LNXLN'' (1-X)CO03 IN WHICH LN HAS AN ATOMIC NUMBER OF AT LEAST ABOUT 65 AND LN'' HAS AN ATOMIC NUMBER BELOW ABOUT 65, WHEREIN X IS BETWEEN 0.001 AND 0.999.

2. An electrode according to claim 1, in which x is between about 0.05 and 0.3.

3. An electrode according to claim 1, in which the rare earth of Ln has a high atomic number and is a member selected from the class consisting of terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.

4. An electrode according to claim 1, in which Ln'' is a member selected from the class consisting of lanthanum, cerium, praseodymium, neodymium, samarium, europium and gadolinium.

5. An electrode according to claim 1, in which the film forming metal substrate is a member selected from the class consisting of titanium, tantalum, tungsten, hafnium, zirconium, aluminum, niobium and their alloys.