MXPA97005803A - Category for use on an electrolit cell - Google Patents

Abstract

A cathode, wherein the electrocatalytically active outer layer is of substantially uniform thickness and has contours, which at least substantially are equal to the contours of the substrate immediately underlying it. The electrode can be prepared by deposition of the electrocatalytically active outer layer by physical vapor deposition. The electrocatalytically active outer layer comprises (a) cerium and / or cerium oxide and at least one non-noble Group 8 metal, or (b) platinum and / or platinum and ruthenium and / or ruthenium oxide.

Description

CÁTODO FOR USE ON AN ELECTROLYTIC CELL DESCRIPTION OF THE INVENTION This invention relates to a cathode for use in an electrolytic cell, and in particular to a cathode, which has an over-assembly with a low hydrogen content, when used in the electrolysis of water or brine, for example, chloride solutions of aqueous alkali metal, and to a method for the preparation of the cathode. The voltage at which a solution can be electrolyzed at a given current density, develops and is influenced by a number of aspects, particularly the theoretical electrolytic voltage, the overvoltages at the anode and cathode, the resistance of the solution that is electrolyzed , the resistance of the diaphragm, if any, placed between the anode and the cathode and the resistance of the metallic conductors and their contact resistance. Since the cost of electrolysis is proportional to the voltage at which electrolysis is performed, and in view of the high cost of electrical power, it is desirable to reduce the voltage at which a solution is electrolyzed, at a voltage as low as possible. In the electrolysis of water and aqueous solutions, there is considerable scope to achieve such a reduction in the electrolyte voltage by reducing the hydrogen overvoltage at the cathode. There have been many proposals for means to achieve such a reduction in hydrogen overvoltage. For example, it is known that the overvoltage of hydrogen at a cathode that can be reduced by increasing the surface area of the cathode, for example, by etching the cathode surface with an acid, or by blasting with sand from the surface of the cathode. cathode, or coating the cathode surface with a mixture of metals, for example, with a mixture of nickel and aluminum, and selectively leaching one of the metals, e.g., aluminum, from the coating. Other methods for achieving an overvoltage cathode with a low hydrogen content, which have been described, involve coating the cathode surface with an electrocatalytically active material, which comprises a platinum Group metal and / or oxide thereof as it is mentioned in, for example, US 4,100,049 (wherein the coating is applied to an aqueous solution and then ignited), GB 1,511,719 (wherein the coating is applied through electrodeposition), Japanese Patent Publication Nos. 54090080 (in wherein the coating is applied through concretion coating), 54110983 (wherein the coating is applied as a dispersion), and 53100036 and EP 0,129,374 (wherein the coating is applied in the form of salts, which are then turned on). In EP 0,546,714 a cathode is described for use in an electrolytic cell, which has an overvoltage with a low hydrogen content when used in the electrolysis of water or aqueous solutions, and which does not depend on its effectiveness under the presence of a coating containing a metal of the Platinum Group, or oxide thereof. The cathode for use in an electrolytic cell described in EP 0,546,714, comprises a metal substrate and a coating thereon, having at least one outer layer comprising a cerium oxide and at least one non-noble Group 8 metal , wherein the cerium oxide provides at least 10%, and preferably at least 20%, in the X-ray diffraction analysis of the outer layer. It has surprisingly been found that cathodes can be prepared for use in electrolytic cells through physical vapor deposition (PVD) on a suitable substrate of a coating comprising (a) cerium and / or cerium oxide and a Group 8 metal not noble, or (b) platinum and / or platinum oxide and ruthenium and / or ruthenium oxide. In addition, it has been found that the durability of the cathode can be improved through subsequent heat treatment. The present invention provides an electrode and a method for the preparation thereof, which comprises (a) a metal substrate and a coating thereon comprising an outer layer of good electrocatalytic activity and of a uniform thickness that follows the contours of the substrate surface, and (b) when used in a cathode. In an electrolytic cell, where hydrogen is emitted to a cathode, it has an acceptable overvoltage and a high durability. According to the first aspect of the present invention, there is provided a cathode, which comprises a metal substrate and a coating thereon comprising an outer layer, which comprises an electrocatalytically active material, characterized in that (a) the outer layer it is of substantially uniform thickness and (b) the contours of the surface of the outer layer are at least substantially equal to the contours of the substrate immediately underlying it. In the cathode according to the present invention, the electrocatalytically active material comprises (a) cerium and / or cerium oxide and at least one non-noble Group 8 metal or (b) platinum and / or platinum oxide, and ruthenium and / or ruthenium oxide.

The cathode according to the present invention offers the advantages of an increased surface area for a given mass of electrocatalytically active material and the most efficient use thereof to obtain its minimum thickness. When the outer layer of the coating on the cathode according to the present invention contains cerium and / or cerium oxide, the possibility that it can contain one or more other metals of the lanthanide series, for example, is not excluded. the same lanthanum, which is something of the cerium that can be replaced by one or more of the other metals of the lanthanides. However, where this other metal of the lanthanide series is present in the outer layer, it must provide less than 2% w / w thereof and the cerium must be present as the largest amount of the total metal of the lanthanide series, including cerium. Where the outer layer of the coating on the cathode according to the present invention comprises cerium and / or cerium oxide and a non-noble Group 8 metal, the non-noble Group 8 metal can be iron, cobalt or preferably nickel . Typically, the outer layer of the coating comprises a cerium intermetallic compound and a non-noble Group 8 metal, particularly nickel. One should be aware of certain previous descriptions, where the use of intermetallic compounds as overvoltage cathode coatings with a low hydrogen content has been described, for example, Doklady Akad Nauk SSSR 1984, Vol. 276, No. 6, ppl424 -1426; Proceedings of a Symposium on Electrochemical Engineering in the Chlor-alkali and Chlorate Industries, The Electrochemical Society, 1988, pl84-194; Journal of Applied Electrochemistry, Vol. 14/1984, ppl07-115; and EP 0,089,141. When the outer layer of the coating on the cathode according to the present invention contains platinum and / or platinum oxide and ruthenium and / or ruthenium oxide, it should contain 5-90 mol% platinum and preferably 10-80 mol% of ruthenium. The cathode substrate according to the present invention may comprise a ferrous metal, a film-forming metal or an alloy thereof, having similar properties thereto, for example, titanium, or preferably nickel or an alloy thereof having properties similar to it. However, it is usually preferred that the cathode substrate be made of another material having an outer face of nickel or a nickel alloy. For example, the cathode may comprise a core of another metal, for example, steel or copper, and an outer face of nickel or nickel alloy. Preferably, the substrate comprises nickel or nickel alloy, such substrate must be resistant to corrosion in an electrolytic cell, where an aqueous alkali chloride solution is electrolyzed and the cathodes according to the present invention comprising a nickel substrate or nickel alloy have a long-term overvoltage performance with a low hydrogen content. The cathode substrate according to the present invention can have any desired structure. For example, it may be in the form of a plate, which may be open, for example, the cathode may be a perforated plate, or it may be in the form of an expanded metal, or it may be woven or non-woven. The cathode does not necessarily have the shape of a plate. In this way, it can take the form of a plurality of so-called cathode ends between which the anode of the electrolytic cell can be placed. In the cathode according to the present invention, the defined coating may be in direct contact with the surface of the substrate. However, the possibility that the defined coating can be applied to an intermediate coating of another material on the surface of the substrate is not excluded. Such an intermediate coating can be, for example, a porous nickel coating. However, the invention will be described below, with reference to a cathode where such intermediate coating is not present.

According to a further aspect of the present invention, there is provided a method for the preparation of an electrode according to the first aspect of the present invention, the method comprises the steps of: (A) depositing the outer layer of the coating on the substrate through physical vapor deposition (PVD); and (B) heating the product of Step A as long as the electrocatalytically active material comprises cerium and / or cerium oxide, the heating is carried out in an atmosphere without oxidation. Examples of PVD include, among others, cathodic radiofrequency (RF) sublimation, cathodic sublimation ion deposition, arc evaporation, electron beam evaporation, de-sublimation, reactive PVD, etc., or combinations thereof. It will be appreciated that when evaporation techniques are used in the same evaporation chamber in the PVD system, separate targets may be used, for example, a cerium target and a nickel target, instead of;, or in addition to, a Cerium / nickel intermetallic objective. By "objective" is meant the material that is vaporized to produce a vapor for deposition on the substrate in the PVD system.

In Step A of the method according to the present invention, the chamber in the PVD system can be charged with oxygen or ozone and / or an inert gas. When an inert gas is present in the chamber, it is preferably argon. It will be appreciated that when the target in the PVD system is metallic and when it is desired to deposit an oxide, for example, cerium oxide, platinum oxide or ruthenium oxide, an oxidation atmosphere is used in the PVD system. The specific conditions used in Step A of the method according to the present invention can be found by any person skilled in the art through simple experimentation. For example, the pressure in the deposition chamber may be in the range of 10"2 to 10" 10 atmospheres. When the objective in the PVD system in Step A of the method according to the present invention is an intermetallic compound containing cerium for the preparation of a cathode as claimed in claim 3, it will be appreciated that it must contain at least one non-noble Group 8 metal, that is, at least one of iron, cobalt and nickel, as well as cerium. Intermetallic compounds containing cobalt and / or nickel, particularly nickel, are preferred. The intermetallic cerium-containing compound, if used, may contain one or more additional metals to the cerium and a non-noble Group 8 metal, but these other metals, when present, will generally be present in a proportion not greater than 2%. The intermetallic compound containing cerium, when used, may have the empirical formula CeM ?, wherein M is at least one non-noble Group 8 metal, x is on the scale of about 1 to 5, and wherein part of the cerium can be replaced by one or more of the other metals of the lanthanides as described above. When used, an intermetallic compound containing cerium as target in the PVD system in Step A of the method according to the present invention, this may be a net intermetallic compound, for example, CeNio, or a mixture of intermetallic compounds, example, CeNio and Ce2Ni7 or an intimate mixture of a metal powder, preferably Ni, with an intermetallic, for example, Ce2Ni7, to form, for example, in a notional form CeNi22 / or a cerium / nickel alloy containing CeNi phases? , where x is 1-5. When an intermetallic compound containing zero is used as an objective in the PVD system in Step A of the method according to the present invention, the concentration of the cerium therein is typically not greater than about 50% w / w and so It is generally preferred that it be not less than about 10% w / w.

When platinum and ruthenium metals are used as targets in the PVD system in Step A of the method according to the present invention, these can be present, for example as a bed or a mixed disk. In Step B of the method according to the present invention, the temperature at which the product of Step A is heated is preferably about 300 ° C and less than 1000 ° C, and most preferably about 500 ° C. . The product of Step A is preferably heated for less than 8 hours and more than 0.5 hours, and more preferably for at least one hour. The typical heating rate is between 1 ° C and 50 ° C per minute, and is preferably in the range of 10-20 ° C / minute. As examples of non-oxidizing atmospheres, which may be used in Step B of the method according to the present invention, there may be mentioned, among others, a vacuum, a reduction gas, for example hydrogen, or preferably an inert gas , for example, argon, or mixtures thereof, for example, by heating in argon, followed by a vacuum treatment at elevated temperature. When the outer layer at the cathode according to the present invention comprises platinum and / or platinum oxide and ruthenium and / or ruthenium oxide, heating in Step B is typically carried out in air.

The precise temperature that will be used in the Step B of the method according to the present invention depends at least to some degree on the precise method by which the outer layer of the coating is deposited in Step A. The mechanical properties and the chemical / physical composition of the outer layer of the The coating on the durable electrode according to the present invention depends, inter alia, on the hardness, the heating rate and the temperature used in Step B. The cathode according to the present invention can be a monopolar electrode or can be be part of a dipolar electrode. The cathode according to the present invention is suitable for use in an electrolytic cell comprising an anode, or a plurality of anodes, a cathode, or a plurality of cathodes, and optionally a spacer placed between each anode and adjacent cathode. The separator, when present, may be a diaphragm permeable to the porous electrolyte, or it may be a hydraulically impermeable cation permeable selective membrane. The anode in the electrolytic cell can be metallic, and the nature of the metal will depend on the nature of the electrolyte that will be electrolyzed in the electrolytic cell. A preferred metal is a film-forming metal, particularly when an aqueous solution of an alkali metal chloride is going to be electrolyzed in the cell. The structure of the cathode, and of the electrolytic cell where the cathode is going to be used, will vary depending on the nature of the electrolytic process, which will be done using the cathode. However, since the inventive aspect of the present invention does not reside in the nature of the electrolytic cell or the cathode, there is no need for the cell or the cathode to be described in more detail. Suitable types and structures of electrolytic cell and cathode can be selected from the prior art, depending on the nature of the electrolytic process that will be carried out in the cell. The cathode may have, for example, a foraminate structure, as in a woven or non-woven mesh, or as a mesh formed by slitting and expanding a metal sheet or alloy thereof, although other electrode structures may be used. Prior to the deposition of the coating on the substrate in the method according to the present invention, the substrate can be subjected to treatments, which are known in the art. For example, the surface of the substrate may be roughened, for example, by blasting sand, in order to improve the adhesion of the subsequently-applied coating, and in order to increase the actual surface area of the substrate. The surface of the substrate can also be cleaned and chemically etched, for example, by contacting the substrate with an acid, for example, an aqueous solution of hydrochloric acid, and the acid-treated substrate can then be washed, for example with water, and dried. The present invention is further illustrated with reference to the accompanying drawing, which shows, by way of example only, a micrograph of an electrode according to the present invention, which can be prepared by the method according to the present invention. In the drawing: Figure 1 is a micrograph of an electrode, which can be prepared in Example 1. In Figure 1, (1) is the electrode coating, (2) is the electrode substrate and (3) is the base on which the electrode was mounted to prepare the micrograph. In Figure 1, it can be seen that the electrode coating (1) is of a uniform thickness and that the outline of the surface thereof is substantially equal to the outline of the substrate immediately underlying it (2). The present invention is further illustrated with reference to the following examples.

EXAMPLES 1-2 These Examples illustrate cathodes according to the present invention. General Method A sheet of nickel was cleaned with acetone after it was sand blasting with 60/80 alumina sand. The sheet was mounted on a stainless steel plate (maintained with a nickel sheet mask) and deposited in the PVD system, which was allowed to pump during the night. The pressure in the PVD chamber was adjusted to 10 ~ 2 by controlling the flow of argon. A CeNi powder target was presubliminated for 2.5 hours at an incident RF power of 500W before use. The target record was removed and the powder target sublimed for 60 hours, whereby a coating with a nominal thickness of 10 microns was obtained on the nickel substrate. In Example 2, the cathode removed from the PVD chamber was subjected to a heat treatment in argon at 500 ° C for 1 hour. The cathodes prepared in Examples 1 and 2 were tested under conditions described in EP 0,546,714. The results of the tests are shown in the Table.

TABLE Example No. Voltage saving on Voltanickel saving with je blasting over nickel sand / mV after 6 blasting at 3kAm-2 sand / mV after 1 short 1 261 177 2 277 264 Nickel hydrogen overvoltage with blasting sand was taken as 350 mV From the Table, it can be seen that the cathode of Example 1 has an overpotential with a low hydrogen content, while the cathode of Example 2 has both an overpotential with a low hydrogen content and good durability.

Claims (15)

1. A cathode which comprises a metallic substrate and a coating thereon, comprising an outer layer which comprises an electrocatalytically active material characterized in that (a) the outer layer is of substantially uniform thickness and (b) the outline of the surface of the The outer layer is at least substantially the same as the contour of the immediately underlying substrate.

2. The cathode in accordance with the claim 1, characterized in that the electrocatalytically active material comprises cerium and / or cerium oxide and at least one non-noble Group 8 metal.

3. The cathode in accordance with the claim 2, characterized in that the electrocatalytically active material comprises a cerium intermetallic compound and a non-noble Group 8 metal.

4. The cathode according to claim 2 or 3, characterized in that the non-noble Group 8 metal is nickel.

5. The cathode according to claim 1, characterized in that the electrocatalytically active material comprises platinum and / or platinum oxide and ruthenium and / or ruthenium oxide.

6. The cathode according to claim 5, characterized in that the electrocatalytically active material comprises 5-90 mol% platinum and 10-80 mol% ruthenium.

7. A method for the preparation of a cathode according to claim 1, characterized in that the method comprises the steps of: (A) depositing the outer coating layer on the substrate by physical vapor deposition (PVD); and (B) heating the product of Step A with the proviso that when the electrocatalytically active material comprises cerium and / or cerium oxide, the heating is carried out in a non-oxidizing atmosphere.

8. The method according to claim 7, characterized in that the target in the PVD system comprises a cerium intermetallic compound and a non-noble Group 8 metal.

9. The method according to claim 8, characterized in that the non-noble Group 8 metal is nickel.

10. The method in accordance with the claim 7, characterized in that the PVD used in Step A is electrodeposited by RF cathodic sublimation.

11. The method according to claim 7, characterized in that the PVD in Step A is carried out in an atmosphere comprising argon.

12. The method according to claim 7, characterized in that the non-oxidizing atmosphere used in Step B comprises argon.

13. A method according to claim 7, characterized in that in Step B the product of Step A is heated to a temperature between 300 ° C and 1000 ° C.

14. The electrolytic cell, wherein at least one cathode is a cathode in accordance with claim 1, and / or prepared by the method according to claim 7.

15. The process for preparing the electrolysis of water or an aqueous solution, carried out in an electrolytic cell according to claim 14.