METHOD FOR EXTENDING SERVICE LIFE OF A HYDROGEN-EVOLUTION ELECTRODE

Abstract: A method is provided for extending the service life of a hydrogen-evolution electrode having a metal oxide as a catalytic material by adding a component selected from a titanium, niobium, indium, cadmium or antimony component to the alkaline electrolyte during the course of the evolution of hydrogen. The method is suitable for use in the electrolysis of brine in chlor-alkali cells.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method for extending the service life of a hydrogen-evolution electrode which exhibits low hydrogen overvoltage. More particularly, the present invention is directed to a method for extending the service life of a hydrogen-­ evolution electrode having an active coating layer comprising a metal oxide as the active material on an electrically conductive substrate for use in electro­ lytic preparation of hydrogen in alkali electrolyte. In the field of electrolysis of brine and water, for a long period of time, many proposals concerning a hydrogen-evolution electrode have been made from view­ points of low hydrogen overvoltage and high stability. For example, in order to decrease the hydrogen over­ voltage of an electrode attempts have been made to increase the surface area of the electrode or to incor­ porate an electrode catalyst such as a transition metal, an alloy or an oxide of a transition metal into the electrode. Among the heretofore proposed electrodes, in which a metal, or an alloy is employed as the active material have reduced service life in that when elec­ trolysis is continuously effected using the electrode as a hydrogen-evolution electrode, the hydrogen overvoltage of the electrode increases with the lapse of time, and the activity of the electrode is lost. Recently, there have been proposed some methods in order to improve this advantage. US Patent 4,436,599 teaches a process for mak­ ing an iron activated cathode during the operation of a chloralkali cell which comprises adding to the cathode compartment an aqueous solution containing ferrite ions and passing an electric current through the cell. US Patent 4,105,516 discloses, that in order to reduce a cell voltage, a compound of a transition metal such as chromium, molibdenum, manganese, technetium, rhenium, iron, cobalt, ruthenium, rhodium, palladium, osmium and iridium is added to the catholyte solution, while an electrical current is being passed. This deposits the transition metal onto the cathode, and thereby maintaining a clean transition metal surface on the cathode during electrolysis. Further, there has been proposed a method for extending the life of metal cathodes by adding a reducing agent such as Na₂SO₃, urea and nickel to an electrolyte solution during the period of interrupted current flow to prevent the formation of chlorates (US Patents 4,358,353 and 4,055,476). In spite of these improvements of metal cathodes, further research and development have been continued on cathodes which have a metal oxide as the active material. For examples, US Patents 4,243,497, 4,394,231, 4,447,302 and 4,605,484 propose oxide cathodes characterized in a composition of oxides, comprising a surface structure and a preparation thereof. And US Patent 4,426,269 (PCT/GB79/00040) proposes recently, in order to prevent the decrease of the activity of an electrode which if left immersed in solution on an open circuit, a molybdenum, vanadium or tungsten compound is added to the electrolyte. In spite of these proposals, there remains a demand for an electrode which exhibits high stability for long period of time or the method for extending the life of the electrode for a practical industrial use because it is inevitable that the activity of the elec­ trode which has a metal oxide as the active material decreases substantially when the electrode is continu­ ously used in electrolysis for period of one year or more. SUMMARY OF THE INVENTION According to one aspect of the present invention there is provided a method for extending the service life of a hydrogen-evolution electrode comprising an electric­ ally conductive substrate having thereon a coating comprising a metal oxide in which said electrode is used in an alkaline electrolyte for hydrogen-evolution, comprising adding at least one component selected from titanium, niobium, indium, cadmium or antimony to the alkaline electrolyte during the course of the evolution of hydrogen. According to another aspect of the invention there is provided a method of electrolysing an electrolyte using a hydrogen-evolution electrode comprising an electrically conductive substrate having thereon a coating comprising a metal oxide wherein during said electrolysis at least one component selected from titanium, niobium, indium, cadmium or antimony is added to the electrolyte. The foregoing and other features and advantages of the present invention will be apparent to those skilled in the art from the following detailed description taken together with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig 1 is a graph showing the relationship between the hydrogen overvoltage and the operation period of electrolysis of Example 1 and Comparative Example 1. Fig 2 is a graph showing the relationship between the degree of oxidation and the operation period of electrolysis of Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for extending the service life of a hydrogen-evolution electrode comprising an electrically conductive substrate having thereon a coating comprising a metal oxide as the active material which is used for hydrogen-evolution in an alkaline electrolyte, comprising adding at least one component selected from titanium, niobium, indium, cadmium or antimony to the alkaline electrolyte during the course of the evolution of hydrogen. In accordance with the present invention, the hydrogen overvoltage at the initial stage of electro­ lysis is substantially maintained for a long period. And the degree of oxidation of the electrode is hardly decreased so that the metal oxide as the active material in the electrode is hardly reduced and remains as an oxide even after the continuous operation of electro­ lysis. It is therefore possible to extend the service life of the electrode. The mechanism of such features are not clearly understood but it is believed that the component added to an alkaline electrolyte prevents, directly or indirectly the reduction of a metal oxide working as an active material because the component is observed in the coating of the electrode used in the present invention. These features are illustratively shown in Fig 1 and Fig 2 which are based on the results of Example 1 and Comparative Example 1. In a case of Example 1, by the addition of 1.6 x 10⁻²mmol of a titanium compound to an alkaline elec­ trolyte, the degree of oxidation decreases, slightly, about 10% after an operation of 10 months and the hydrogen overvoltage is hardly changed. On the other hand, in Comparative Example 1, the degree of oxidation is considerably decreased during the course of elec­ trolysis, all the metal oxide of the electrode is reduced to its metal after an operation of 10 months and the hydrogen overvoltage increases about 150 mV from the initial value. In the present invention, a hydrogen-evolution electrode means the electrode which comprises an elec­ trically conductive substrate having thereon a coating comprising a metal oxide, and shows a low hydrogen overvoltage when the electrode is used in evolution of hydrogen. As the metal oxide, there may be a single oxide, a mixture of two or more oxides, a double oxide and a solid solution or an amorphous oxide composed of two or more oxides. The metal oxide could also contain a small amount of the metal itself. Examples of the metal element which comprises the metal oxide, are iron, cobalt, nickel, manganese,chromium, molybdenum, tungsten, vanadium. niobium. tantalum. titanium, zirconium, copper, silver, gold, zinc, cadmium, aluminum, gallium, tin, ruthenium, rhodium, palladium, osmium, iridium and platinum. Preferred oxides are nickel oxide, cobalt oxide and an oxide composed of a combination of (i) a nickel and chromium component, (ii) a cobalt and chromium component, (iii) a nickel and titanium compo­ nent, (iv) a cobalt and titanium component or (v) a nickel, chromium and titanium component disclosed in Japanese Patent Appln. 225424/1985. More preferred are nickel oxide, an oxide composed of a nickel and chromium component in which the content of chromium is in the range of 0.5 to 20 atomic % and an oxide composed of a nickel, chromium and titanium component in which the contents of chromium and titanium are in the range of 0.5 to 40 atomic % and 0.1 to 10 atomic %, respectively (above the Japanese Patent Appln.). The atomic percent in the present invention is analyzed by the atomic absorption method by the following formula (1); wherein A M represents the number of the specific compo­ nent in the coating, and A T represents the total number of all of the components in the coating. The term "degree of oxidation" used herein is intended to indicate the value (%) of the formula wherein H₀ represents the height of a peak of the highest intensity X-ray diffraction line of a metal when the coating is analyzed by X-ray diffractometry, or in case the coating contains two or more metals, represents the sum of the heights of peaks of the highest intensity X-ray diffraction lines respectively of metals; H₁ represents the height of a peak of the highest intensity X-ray diffraction line of an oxide of said metal, or represents the sum of the heights of peaks of the highest intensity X-ray diffraction lines respectively of the metal oxides in case the coating contains oxide of two or more metals. In the present invention, the degree of oxida­ tion is preferably more than 20%, more preferably more than 30%. When the degree is less than 20%, the activity of the metal portion of the coating of the electrode becomes easily low during the course of elec­ trolysis, causing the activity of the electrode as a whole to be decreased and it is meaningless and diffi­ cult to extend the service life of the electrode. A component to be added into an alkaline elec­ trolyte during operation in order to extend the service life of an electrode, is one component selected from a titanium, niobium, indium, cadmium, or antimony compo­ nent. Among them, a titanium component is preferable. If the component is soluble in an alkaline electrolyte, it is used unrestrictly as all forms of the element. Preferable example is a form of metal, a metal oxide and a metal ion. An amount of the component in the elec­ trolyte is preferably in the range of 1 x 10⁻³mmol to the saturation solubility of the component per one liter of the electrolyte. When the concentration is small; less than 1 x 10⁻³mmol, improvement of service life may be obtained but is not as good. The addition of the component to the electro­ lyte has to be conducted during the course of working of electrode and hydrogen-evolution. When the electrode is dipping into a solution containing the component, and then is used in the electrolysis without the addition of the component, the metal oxide of the electrode is reduced to a metal so that effect of service life is not obtained. The addition of the component may be conducted continuously or intermittently when the electrode is in a working state. In a case of an intermittent addition, it is possible to add the component at an interval of several hours, several days or one or two months. As illustrated above, a hydrogen-evolution electrode having a metal oxide as an active material in the present invention is not limited by the method of preparing the electrode. For example , as a method of preparing a coating containing an active metal oxide may be a method comprising applying onto the substrate a homogeneous solution containing a salt of a metal, followed by baking in an oxygen-containing atmosphere; a method comprising melt-spraying, such as plasma spraying and flame spraying, a powder of a metal or a compound of a metal capable of forming such an oxide; and a method comprising electroplating and/or chemical plating of the substrate in a homogeneous solution containing a salt of a metal, followed by oxidative-calcination in an oxygen-containing atmosphere. Of these methods, the method comprising melt-­ spraying is most preferable, for it ensures complete coating with a predetermined composition and gives an electrode having a high activity which can be utilized for a prolonged period of time. Illustratively stated, according to this method, the operations of melting of the powder and solidification and coating formation of the melted material on the substrate can be accomplished instantaneously. Hence, a non-stoichiometric composi­ tion tends to be formed. This is believed to be the reason why an electrode coating having a high activity can be obtained by melt-spraying. Moreover, a uniform composition consisting of a plurality of components may be obtained with certinty by the use of relatively easy and secure techniques, such as mixing and granulation. By melt-spraying such a uniform composition, it is possible to obtain the desirable electrode coating freely. Therefore, the melt-spraying method is one of the most suitable methods for the purpose of the present invention, which is to provide a hydrogen-evolution electrode, with a coating of a metal oxide thereon, having a high activity and long life. With respect to the melt-spraying method, it is important to improve the affinity between the active material of the electrode and the material giving reduction resistance thereto so that they may fully exhibit their respective functions. In the method of melt-spraying, the powder of a metal or a compound, is preferably granulated as described in U.S. Patent 4,605,484. The preferred thickness of the coating of electrode is 10 to 300 µm. Where the thickness of the coating is less than 10 µm, there cannot be obtained an electrode exhibiting a satisfactorily lowered hydrogen overvoltage. On the other hand, the increase in thick­ ness of the coating to more than 300 µm is not advantageous from an economical viewpoint because even if the coating thickness is more than 300 µm, the hydrogen overvoltage does not exceed a certain value and the increase of thickness of the coating to more than 300 µm only causes the cost for the coating to be increased without any proportional advantage. It is preferable to subject an electrically conductive substrate to a pre-treatment prior to melt-­ spraying. The pre-treatment consists in degreasing and grinding the surface of substrate. By the pre-­ treatment, the stains on the surface of substrate are removed and the surface of substrate is appropriately coarsened, thereby enabling strong bonding between the substrate and the melt-sprayed coating to be obtained. With respect to the method of pre-treatment, there is not any particular restriction. Usually, there may be employed grinding by an acid-etching, a blast finishing (for example, grit blasting, shot blasting, sand blast­ ing or liquid honing), an electrolytic grinding or the like in combination with degreasing by means of an organic liquid, a surfactant, vapor, calcination or the like. Explanation will now be made of an electrically conductive substrate to be used in the present inven­ tion. The electrically conductive substrate of electrode should be sufficiently resistant to an elec­ trolytic solution not only at a potential of the sub­ strate during the electrolysis but also at a potential of the substrate at the time when the electrolysis is not effected. The surface of a substrate having an active, porous coating thereon has a potential which is noble as compared with the potential on the surface of the coating even during a period of time in which hydrogen is evolved from the surface of the coating of the electrode. Therefore, it is not unusual that the potential at the surface of the substrate is noble as compared with the dissolution-deposition equilibrium potential of iron. For this reason, when iron is used as the substrate of electrode, the iron is corroded and dissolved out from the surface of the substrate. As a result, the electrolytic solution and the electrode coating are contaminated and, in an extreme case, the coating of electrode is caused to be exfoliated and comes off from the surface of electrode so that the activity of the electrode is greatly decreased. As examples of the material which has an anticorrosive property sufficient for use as the substrate of elec­ trode of the present invention and is commercially easily available, there may be mentioned, for example, nickel, a nickel alloy, an austenite type stainless steel and a ferrite type stainless steel. Of the above-mentioned materials, nickel, a nickel alloy and an austenite type stainless steel are preferred, and nickel and a nickel alloy are particularly preferred. Besides, those which each are composed of an electrically con­ ductive substrate having on its surface a non-pinhole coating or nickel, a nickel alloy, an austenite type stainless steel or a ferrite type stainless steel may also preferably be used as the substrate of electrode. Such a non-pinhole and anticorrosive coating may be obtained by known techniques, for example, electro­ plating, electroless plating, melt-plating, rolling, pressure-adhesion by explosion, clothing of metal, vapor deposition, ionization plating and the like. The preferred shape of the substrate of elec­ trode is of such a structure that hydrogen gas generated during the electrolysis is smoothly released so that a superfluous voltage loss due to the current-shielding by the hydrogen gas may be avoided and that the effective surface area for electrolysis is large so that the current is hardly concentrated. The substrate having such a shape as mentioned above may be made of a wire screen having a suitable wire diameter and spacing between the mutually adjacent wires, a perforated metal having a suitable thickness, size of opening and pitch of opening arrangement, an expanded metal having suit­ able lengths of long axis and short axis, or the like. The present invention can be effectively applied for the hydrogen-evolution electrode used as a cathode in the electrolysis of sodium chloride by the ion exchange membrane process or the diaphragm process, electrolysis of alkali metal halides other than sodium chloride, electrolysis of water, electrolysis of Glauber's salt and the like. It is preferred that the electrolytic solution in contact with the electrode of the present invention be alkaline. The type of elec­ trolytic cell to be used together with the electrode in the present invention may be either monopolar arrange­ ment or bipolar arrangement. When the electrode of the present invention is used in the electrolysis of water, it may be used as a bipolar electrode. The present invention will be further illus­ trated in more detail with reference to the following Examples which should not be construed to be limiting the scope of the present invention. In the Examples, various measurements were done as follows. The content of each of the components in the coating of an electrode is determined by the ICAP (Inductively Coupled Argon Plasma Emission Spectro­ phtometry) method as follows. One part by weight of the coating of an elec­ trode is mixed with 50 parts by weight of a flux (a mixture of 2 parts by weight of sodium peroxide and one part by weight of sodium carbonate) and the resulting mixture is melted. A predetermined amount of hot water and aqueous 50% sulfuric acid are added to the resulting mixture to obtain a homogeneous solution. The obtained solution is analyzed by a ICAP-575-Mark II (manufactured by Nippon Jarrell-Ash Co., Ltd., Japan). Example 1 and Comparative Example 1 100 Parts by weight of powder nickel oxide (NiO) were added to an aqueous solution consisting of 100 parts by weight of water, 2.25 parts by weight of gum arabic as a binder, 0.7 part by weight of sodium carboxymethyl cellulose as a dispersant, 0.001 part by weight of sodium lauryl sulfate as a surfactant and 0.1 part by weight of phenol as an antiseptic agent. The resulting mixture was vigorously stirred to obtain a homogeneous suspension. The particle diameters of the nickel oxide showed 0.2 to 2 µm by the analysis of an electron photomicrograph. The thus obtained suspension was dried and granulated by means of a spray dryer type granulation machine to obtain a spherical granules having a diameter of 5 to 50 µm and containing less than 0.1% of water. A 5 cm x 5 cm nickel wire screen (wire dia­ meter, 0.7 mm; 12 mesh) was degreased with trichlene, and then blasted by means of Al₂O₃. The blasted wire screen (substrate) was melt spray coated on each side thereof with the above-prepared granules by plasma spraying as indicated below. Feeding rate of plasma gas of argon and nitrogen: 1 m³ (at normal state)/hr and 0.8 m³ (at normal state)/hr, respectively Distance between substrate and spray gun (spray distance): 10 cm Angle of the plasma flame relative to the face of the substrate: 90° The thickness of the coating on the front of the wire screen was 100 µm and on the back was 50 µm. The analysis by x-ray diffractometry showed that the coating of the electrode contained nickel oxide (NiO) and metal nickel (Ni). There was provided an electrical cell which was partitioned by a cation exchange membrane having a carboxylic acid layer and a sulfonic acid layer into a cathode chamber and an anode chamber accommodating therein an anode made of a titanium expanded metal having thereon a coating composed of ruthenium oxide, titanium oxide and zirconium oxide. In the cell, the above described nickel-oxide coated electrode was used as cathode with its front side facing the carboxylic acid layer of the membrane. Brine having a NaCl concentration of 175 g/l was supplied to theanode chamber. 30% aqueous sodium hydroxide solution containing titanium at 1.6 x 10⁻² mmol per on liter of the solution by the addition of a titanium sulfate was supplied in the cathode chamber. Electrolysis was continuously conducted at a current density of 40 A/dm² and at 90°C. The hydrogen overvoltage was measured by the current interrupt method and in a manner in which a Luggin capillary of Teflon was connected to a reference electrode (Hg/HgO). The degree of oxidation was measured by calculation from the height of the peak attributed to crystal face (012) with respect to NiO and the height of the peak attributed to crystal face (111) of Ni by X-ray diffraction. In order to show the effect of the example, the Comparative example was conducted by the same manner with the Example 1 except that the addition of a titanium sulfate was not conducted. The results are shown in the Table 1. As shown in Table 1, in Example 1, although the degree of oxidation decreased about 20% after 10 months, the hydrogen overvoltage changed only small degree. In Comparative Example 1, the drop of the oxidation degree was very large, and all of the nickel oxide was reduced to a nickel metal after 10 months. The hydrogen overvoltage increased about 150 mV in comparison with at the initial stage. Examples 2 to 5 The same procedures with Example 1 were con­ ducted emloying some different amounts of titanium content. The results are shown in Table 2. In example 5, although a white turbidity was observed in the catholyte since the amount of the titanium component was more than the saturation solubility, the other results were good. Examples 6 to 8 and Comparative Examples 2 to 4 Three kinds of electrode as described below were prepared in substantially the same manner as described in Example 1 employing the following compounds as the raw materials of the coating; Raw material (a) nickel oxide powder (NiO) having particle diameter of 0.2 to 2 µm (b) chromium oxide powder (Cr₂O₃) having particle diameter of 0.5 to 3 µm (c) titanium oxide powder (TiO₂) having particle diameter of 1.0 to 10 µm Electrodes In Example 6: a nickel oxide-chromium coated electrode having 5 mol% of chromium in the coating In Example 7: a nickel oxide-chromium coated electrode having 3 mol% of chromium in the coating in Exmaple 8: a nickel oxide-chromium titanium coated electrode having 20 mol% of chromium and 2 mol% of titanium in the coating On the other hand, a spherical sintering material having diameter of 5 to 10 mm was prepared by a heat treatment of titanium oxide under the condition of 600°C and 4 hours. By employing the electrode obtained above, electrolysis was conducted as the same manner with Example 1 except that the concentration of titanium ion in the catholyte was controlled to about 5 x 10⁻²mmol by dipping and dissolving the sintering material in the catholyte. In comparative examples, electrolysis was conducted without using the sintering material. The results are shown in Table 3. By X-ray diffractometry analysis, the coating of the electrode used in the electrolysis for 20 months was found to be composed of NiO and Ni. and X-ray diffraction peaks to be attributed to chromium and titanium components were not observed. Examples 9 to 12 and Comparative Examples 5 to 24 Electrolysis was conducted in substantially the same manner described in Example 1 except that a compound of respectively niobium, indium, cadmium or antimony was used instead of a titanium compound, in Examples 9 to 12. In comparison Examples 5 to 24, the many compounds described in Table 4 were used instead of a titanium compound. The compound was added to the catholyte as an acidic solution of a chloride, oxide, nitrate or fluoride to give a concentration of 1.6 x 10⁻²mmol/l in the catholyte. The results are shown in Table 4. Examples 9 to 12 show that Oxidation degrees decrease only 20 to 25% and that hydrogen overvoltages change little. Comparative Examples 5 to 24 show that large amounts of nickel oxide were reduced to nickel metal and that hydrogen overvoltages increase about 30 to 40mV after 5 months. Example 13 The electrode prepared by the method of Example 1 was reduced under a hydrogen atomsphere at 300°C for 4 hours. The obtained electrode having oxidation degree of 2% below was oxidized in an air at 500°C for 3 hours. Substantially the same electrolysis as in Example 1 was repeated to effect electrolysis for evaluation of the performance of the electrode. The results are shown in Table 5. Example 14 The electrode was prepared by the same manner with Example 1 except that a nickel powder (Ni ≧ 99%, ø = 4 ∼ 7µ) was used for a melt-spray instead of a nickel oxide granule. The electrode obtained was reduced under a hydrogen atomsphere at 300°C for 4 hours to obtained a reduced electrode having oxidation degree of 0%. The reduced electrode was oxidized in an air at 500°C for 4 hours. Substantially the same electrolysis as in Example 1 was conducted to evaluate the obtained electrode. The results are shown in Table 5. Comparative Examples 23 and 24 Substantially the same electrolyses as in Examples 13 and 14 were repeated without the addition of a titanium using the same electrodes prepared in Examples 13 and 14. The results are shown in Table 5.

Claims: 1. A method for extending the service life of a hydrogen-evolution electrode comprising an electrically conductive substrate having thereon a coating comprising a metal oxide in which said electrode is used in an alkaline electrolyte for hydrogen-evolution, comprising adding at least one component selected from titanium, niobium, indium, cadmium or antimony to the alkaline electrolyte during the course of the evolution of hydrogen. 2. A method of claim 1 wherein following the addition, the concentration of the component in the alkaline electrolyte is in the range of from 1 x 10⁻³mmol to the saturation solubility of the component. 3. A method of claim 1 wherein the component is a titanium. 4. A method of claim 1 wherein the component is a metal compound. 5. A method of claim 1 wherein the metal oxide comprises nickel oxide. 6. A method of claim 1 wherein the metal oxide comprises a nickel and chromium component, in which the content of chromium is in the range of 0.5 to 20 atomic %. 7. A method of claim 1 wherein the metal oxide comprises a nickel, chromium and titanium component in which the content of chromium and titanium is in the range of 0.5 to 40 atomic % and 0.1 to 10 atomic %, respectively. 8. A method of claim 1 wherein the degree of oxidation of the electrode is more than 20%. 9. A method of electrolysing an electrolyte using a hydrogen-evolution electrode comprising an electrically conductive substrate having thereon a coating comprising a metal oxide wherein during the electrolysis at least one component selected from titanium, niobium, indium, cadmium or antimony is added to the electrolyte. 10. A method according to claim 8 wherein the electrolyte is brine.

Inventors: KASHIWADA, AKIO, NOAKI, YASUHIDE
Applicants: ASAHI KASEI KOGYO KABUSHIKI KAISHA
Application year: 1986
Main classification: C25B1
IPC codes: C25B1, C25B15