WO2021047687A2

WO2021047687A2 - Electrode and preparation method and use thereof

Info

Publication number: WO2021047687A2
Application number: PCT/CN2020/127682
Authority: WO
Inventors: Hongyuan Chen; Qite ZHAO; Junwei XUE
Original assignee: Magneto Special Anodes Suzhou Co Ltd
Current assignee: Magneto Special Anodes Suzhou Co Ltd
Priority date: 2019-09-10
Filing date: 2020-11-10
Publication date: 2021-03-18
Anticipated expiration: 2022-03-10
Also published as: EP4028580A2; JP2022548205A; KR20230088625A; EP4028580A4; WO2021047687A3; CA3149248A1; CN112553657B; CN112553657A

Abstract

The application discloses an electrode and a preparation method and use thereof. The electrode includes a substrate and a catalytic layer; the catalytic layer is positioned on the upper surface and/or the lower surface of the substrate, and the catalytic layer includes a plurality of catalytic layer units with different chemical compositions; an outermost catalytic layer unit in the catalytic layer that is away from the direction of the substrate is an oxide layer containing a first metal element; other catalytic layer units except the outermost catalytic layer unit are oxide composite layers containing a first metal element and a second metal element; the mole percentage of the first metal element in each layer of the catalytic layer units accounted for a total of metal elements in each layer of the catalytic layer units increases layer by layer along the direction away from the substrate. The electrode can be used as an insoluble anode in the electrometallurgy, can reduce the cell voltage to play an energy-saving role while having a long service life in the electrometallurgy process in which the electrolyte contains high concentration of fluoride ion, and the problems of environmental pollution like lead anode and influence on product purity will not occur.

Description

Electrode and Preparation Method and Use thereof

Technical Field

The application relates to, but is not limited to, the field of electrochemistry, in particular to, but is not limited to, an electrode and a preparation method and use thereof.

Background of the Related Art

Electrometallurgy is an important part of the electrochemical industry, which is mainly used for the electrolytic extraction and purification of metals from their solutions. In the electrolytic extraction process of metals such as electrolyzing metal zinc and manganese, an aqueous solution containing a certain concentration of fluoride ion is usually used as an electrolyte. This electrometallurgy process requires insoluble anodes to form a stable circuit and a continuous electrolytic current. Currently, the lead-based alloy electrodes mainly used are characterized by large weight, heavy pollution, and high energy consumption, etc. With the increasing environmental protection standards, it is urgent to find environmentally friendly substitutes.

An oxygen-evolution titanium electrode, as an environment-friendly insoluble anode, has been widely used in electrochemical industry, mainly focusing on electrochemical water treatment, metal element extraction, electroplating and other fine finishing processes. The oxygen-evolution titanium electrode is mainly composed of pure metal titanium or titanium alloy substrate and noble metal oxide catalytic layer on its surface. The substrate provides conductive and mechanical support. The catalytic layer can greatly reduce the oxygen-evolution potential in aqueous solution through its own redox process to achieve the effect of energy saving, while the anode has a long service life depending on its extremely low electrochemical consumption rate. The oxygen-evolution catalyst is mainly iridium oxide, which is mixed with tantalum or titanium oxide to make the coating denser to protect the substrate from quick passivation. Sometimes titanium or tantalum or an alloy or oxide of the two is also used as an intermediate layer interposed between the catalytic layer and the substrate to protect the substrate.

At present, the most widely used catalytic layer for the oxygen-evolution titanium electrode is iridium-tantalum composite oxide. This kind of coating usually has a long service life in purer acidic electrolyte. The oxygen-evolution overpotential of this electrode is significantly lower than that of lead alloy electrode commonly used in electrometallurgy (about 0.3V lower) , so it has a characteristic of more energy saving. In addition, it is light in weight and convenient to operate, and will not produce harmful substances to pollute the environment and affect the purity of the product. However, the aqueous electrolyte used for metal electrolytic extraction usually comes from mineral leaching and contains a certain amount of fluoride ions, which have a fatal corrosion effect on titanium electrodes. U.S. Patent Application No. US5407556 states that when the electrolyte contains 5 mg of fluoride ions per liter, the service life of the oxygen-evolution titanium oxide electrode will be drastically reduced to about 12%of the service life of an oxygen-evolution titanium oxide electrode under the condition without fluoride ions. The fluoride ion content in the electrolyte used in the electrolytic extraction of metals such as electrolyzing metal zinc and electrolyzing metal manganese is usually as high as 20 mg/L to 200 mg/L (even higher than 200 mg/L) . For this kind of application environment, if the existing lead electrode is replaced, a titanium electrode with a fluoride ion corrosion resistant coating is required.

For the existing fluoride ion corrosion resistant oxygen-evolution titanium electrode, the passivation of the valve-type metal substrate by fluoride ions is the main reason for the reduction of the electrolytic life of such coated titanium anode for electrolyte in the aqueous solution containing fluoride ions. In view of the environment with high concentration of fluoride ion in electrometallurgy, such electrodes cannot meet the requirements of long-life application.

Content of the Invention

The following is an overview of the subject matters described in detail herein. This summary is not intended to limit the scope of protection of the claims.

In order to overcome the defects of the prior art, the inventors of the application have carefully studied for many years to improve the fluoride ion corrosion resistance of the valve-type metal-substrated coated electrode for the electrolytic metal extraction process when the valve-type metal-substrated coated electrode is applied to an aqueous solution containing high concentration of fluoride ion.

The application provides an electrode, including a substrate and a catalytic layer; the catalytic layer being positioned on the upper surface and/or the lower surface of the substrate, and the catalytic layer including a plurality of catalytic layer units with different chemical compositions; wherein an outermost catalytic layer unit in the catalytic layer that is away from the direction of the substrate is an oxide layer containing a first metal element; other catalytic layer units except the outermost catalytic layer unit are oxide composite layers containing a first metal element and a second metal element; the first metal element is selected from one or more of iridium, platinum, rhodium, palladium and ruthenium, and the second metal element is selected from one or more of titanium, tantalum, niobium, tungsten and zirconium; and, the mole percentage of the first metal element in each layer of the catalytic layer units accounted for a total of metal elements in each layer of the catalytic layer units increases layer by layer along the direction away from the substrate, and the mole percentage of the first metal element in the outermost catalytic layer unit accounted for a total of metal elements in the outermost catalytic layer unit is 95%-100%.

In some embodiments, the molar content of the first metal element may be greater than the molar content of the second metal element in an innermost catalytic layer unit in the catalytic layer that is near the direction of the substrate.

In some embodiments, in the innermost catalytic layer unit, the mole percentage of the first metal element accounted for a total of metal elements in the innermost catalytic layer unit may be 65%to 95%, and may also be 80%.

In some embodiments, the outermost catalytic layer unit may be an oxide composite layer containing a first metal element and a second metal element.

In some embodiments, the loading capacity of the first metal element in each layer of the catalytic layer units may be greater than 3 g/m ², and may also be 6 g/m ² to 50 g/m ².

In some embodiments, the first metal element may be iridium and the second metal element may be tantalum.

In some embodiments, the catalytic layer may be two layers of the catalytic layer unit.

In some embodiments, the substrate may be a valve-type metal or an alloy of valve-type metals; the valve-type metal may be selected from one of titanium, tantalum, niobium, zirconium, hafnium, vanadium, molybdenum and tungsten; and the substrate may be titanium or titanium alloy.

In some embodiments, the electrode may include an intermediate protective layer between the substrate and the catalytic layer.

In some embodiments, the intermediate protective layer may be selected from the group consisting of following metals or alloys or oxides thereof: tantalum, titanium, tungsten, tin, antimony, niobium, zirconium, or combinations thereof.

The application also provides a method for preparing the electrode as described above, wherein the catalytic layer is formed by coating a precursor solution containing corresponding elements, drying and then sintering.

The application also provides use of the electrode as described above. The electrode may be used as an insoluble anode for electrometallurgy.

In some embodiments, electrometallurgy may include electrolyzing zinc and electrolyzing manganese in a sulfate electrolyte system.

In some embodiments, electrolyte in electrometallurgy may contain fluoride ions with a concentration of not less than 20 mg/L and not more than 500 mg/L.

Compared with the prior art, the application has the beneficial effects that:

(1) by arranging the catalytic layer to include a plurality of catalytic layer units stacked from inside to outside and arranging the molar content of the first metal element to gradually increase outward, the catalytic layer unit at the inner side has oxides with a higher content of the second metal element, has better compactness, and can play a role of shielding fluoride ions and thus protecting a substrate;

(2) the catalytic layer unit at the outer side has oxides with a lower content of the second metal element, which is beneficial to slowing down the consumption of the first metal element and has a good electrocatalytic effect, and can effectively protect the catalytic layer unit at the inner side from being corroded by fluoride ions due to participating in oxygen evolution reaction prematurely; and

(3) when the electrode of the application is used as an insoluble anode in the electrometallurgy process in which the electrolyte contains high concentration of fluoride ion, it can reduce the cell voltage to play an energy-saving role while having a long service life, and in addition, the problems of environmental pollution like lead anode and influence on product purity will not occur.

Other features and advantages of the present invention will be set forth in the following description, and partly become apparent from the description, or be understood by implementing the invention. The purpose and other advantages of the application can be achieved and obtained by means of the structure specifically indicated in the description, claims and drawings.

Brief Description of Drawings

Drawings are for further understanding of the technical schemes of the application and constitute a part of the description, are used for explaining the technical schemes of the application in combination with the embodiments of the application, not for limiting the technical schemes of the invention.

Fig. 1 is a schematic diagram of an electrode structure according to an Example of the present application;

Fig. 2 is an X-ray photoelectron spectrum of the electrode prepared in the Example of the application; and

Fig. 3 is an X-ray photoelectron spectrum of an electrode prepared in Comparative Example of the application.

In the figures: 1. Substrate; 2. Catalytic layer; 3. Intermediate protective layer; 21. Second catalytic layer unit; and 22. First catalytic layer unit.

Detailed Description

In order to make the objective, technical scheme and advantages of this application more clear, examples of this application will be described in detail below with reference to the accompanying drawings. It should be noted that the embodiments in this application and the features in the embodiments can be combined with each other arbitrarily without conflict.

An example of the application provides an electrode, for example, as shown in Fig. 1, the electrode includes a substrate 1, an intermediate protective layer 3, a first catalytic layer unit 22 and a second catalytic layer unit 21 which are sequentially stacked from bottom to top, wherein the first catalytic layer unit 22 and the second catalytic layer unit 21 form a catalytic layer 2.

An intermediate protective layer 3 and a catalytic layer 2 may also be symmetrically arranged on both sides of the substrate 1; alternatively, an intermediate protective layer 3 and a catalytic layer 2 may be provided on one side of the substrate 1, and only the catalytic layer 2 may be provided on the other side of the substrate 1; alternatively, the intermediate protective layer 3 may not be provided on both sides of the substrate 1. The intermediate protective layer 3 may be an oxide layer, metal layer or and alloy layer consisting of one or more metals of tantalum, titanium, tungsten, tin, antimony, niobium and zirconium, and the intermediate protective layer 3 can slow down the trend of forming an insulating oxide layer and avoid increasing the oxygen evolution resistance.

The catalytic layer 2 may include a plurality of catalytic layer units, for example, it may be arranged to include two catalytic layer units, i.e., a first catalytic layer unit 22 and a second catalytic layer unit 21. The second catalytic layer unit 21 at the outer side has a lower fluoride ion corrosion rate while exerting the advantage of low oxygen-evolution potential, which can prolong the service life of the coating. The first catalytic layer unit 22 at the inner side has better compactness, which can prevent electrolyte and fluoride ions therein from contacting the titanium substrate, thus delaying passivation of the electrode. The catalytic layer 2 may also be provided to include three catalytic layer units, four catalytic layer units or more catalytic layer units. The number of units of the catalytic layer 2 on both sides of the substrate may be the same or different.

The unit loading capacity of the first metal element in each catalytic layer unit may be 10 g/m ² to 50 g/m ². This is because too low loading capacity will not be conducive for ensuring the life of the electrode, while too high loading capacity will lead to a too high electrode cost.

The first metal element may be iridium and the second metal element may be tantalum. Because iridium oxide has the characteristics of low oxygen-evolution potential and long service life compared with other metal oxides, the combination of tantalum oxide the iridium oxide will further improve the compactness of the catalytic layer and prevent electrolyte and harmful ions (such as fluoride ions, etc. ) therein from contacting the titanium substrate.

The material of the substrate 1 may be titanium, the material of the intermediate protective layer 3 may be titanium tantalum oxide or its alloy, and the material of the first catalytic layer unit 22 may be iridium tantalum oxide; the material of the second catalytic layer unit 21 may be iridium oxide or iridium tantalum oxide; the molar content of iridium in the first catalytic layer unit 22 may account for 65%to 95%of the total molar content of iridium and tantalum, and the molar content of tantalum may account for 5%to 35%of the total molar content of iridium and tantalum; the molar content of iridium in the second catalytic layer unit 21 may account for 95%to 100%of the total molar content of iridium and tantalum, and the molar content of tantalum may account for 0%to 5%of the total molar content of iridium and tantalum. It can be ensured that the second catalytic layer unit 21 at the outer side has a low rate of noble metal consumption, while the first catalytic layer unit 22 at the inner side has a good protection effect on the titanium substrate.

The molar ratio of the second metal element of the catalytic layer unit located at the inner side is larger than the molar ratio of the second metal element of the outer layer, so that the compactness of the inner layer can be improved, thereby being beneficial to further improving the overall compactness of the catalytic layer unit and improving the shielding effect of fluoride ions.

Example 1

1) Preparation of a substrate

The surface of a 50×50 mm ² first-grade titanium sheet was oxidized and soaked in 3 wt%of oxalic acid aqueous solution at 90 ℃ for 15h to obtain a rough surface with a roughness Ra of 4 μm-7 μm. The rough surface was washed with deionized water and dried to obtain a metallic titanium substrate 1 suitable for coating a catalytic layer.

2) Preparation of an intermediate protective layer

0.2 mol/L tetra-n-butyl titanate and 0.05 mol/L tantalum ethoxide were dissolved in n-butanol, then 50 ml of hydrochloric acid with a concentration of 37%was added for acidification to prepare a precursor solution of an intermediate protective layer; the prepared precursor solution of the intermediate protective layer was coated on one side surface of the substrate 1 prepared in the step 1) , wherein the total loading capacity of the metal titanium and tantalum elements is 1 g/m ², then air dried, and sintered in an air atmosphere at 500 ℃; the above-mentioned procedures of coating, air drying and sintering were repeated once, and coating was carried out twice in total; the intermediate protective layer 3 was obtained by the twice coating, thereby obtaining the titanium tantalum oxide intermediate protective layer 3 with a molar ratio of titanium to tantalum elements of 4: 1.

3) Preparation of first catalytic layer unit

The metal elements contained in the first catalytic layer unit 22 were iridium and tantalum with the molar ratio of iridium to tantalum of 65: 35; the application method was as follows: 0.035 mol of tantalum ethoxide was dissolved in 200 ml of n-butanol solution and acidified with 50 ml of hydrochloric acid with a concentration of 37%; after that, 0.065 mol of chloroiridium acid crystal was dissolved in 200 ml of n-butanol and acidified with 50 ml of hydrochloric acid with a concentration of 37%; the two salt solutions were mixed according to a volume ratio of 1: 1 to obtain a precursor solution of the first catalytic layer unit with an element molar ratio of iridium to tantalum of 65: 35; the precursor solution of the first catalytic layer unit was coated on the surface of the intermediate protective layer 3 prepared in step 2) , wherein the coating amount of iridium was 1 g/m ² based on the weight of the metal element iridium, then dried, and sintered in an air atmosphere at 500 ℃; the above-mentioned procedures of coating, drying and sintering were carried out 9 times, and coating was carried out ten times in total; the first catalytic layer unit 22 was obtained by the above-mentioned ten times of coating, thereby obtaining the iridium tantalum oxide first catalytic layer unit 22 with a molar ratio of iridium to tantalum of 65: 35.

4) Preparation of second catalytic layer unit

The metal element contained in the second catalytic layer unit 21 was iridium; the application method was as follows: 0.065 mol of chloroiridium acid crystal was dissolved in 200 ml of n-butanol and acidified with 50 ml of hydrochloric acid with a concentration of 37%to prepare a precursor solution of the second catalytic layer unit; the precursor solution of the second catalytic layer unit was coated on the surface of the first catalytic layer unit 22, wherein the coating amount was determined based on the weight of the metal element iridium where the ratio of the weight of iridium to the coating area is 1g/m ², then dried, and sintered in an air atmosphere at 450 ℃; the above-mentioned coating, drying and sintering processes were repeated four times, and the coating was carried out five times in total, and the second catalytic layer unit 21 was obtained by the above-mentioned five times of coating, thereby preparing and obtaining the required electrode.

Performance test

a) The surface of the electrode prepared above was characterized by X-ray photoelectron spectroscopy, as shown in Fig. 2, it can be seen that there is no peak of tantalum.

b) The electrode prepared above was cut into three samples with a size of 25×25 mm ², and electrometallurgy was carried out in an electrolytic zinc system: the electrolyte contains 150 g/L sulfuric acid, 60 g/L zinc ion, 7 g/L divalent manganese ion, 20 mg/L industrial bovine bone glue and 0.221 g/L sodium fluoride (converted into 100 mg/L of fluoride ion concentration) , the test temperature was 43 ℃, the current density was 500 A/m ², and the electrode deactivation was determined by setting the cell voltage to rise by 0.5 V (based on the sensitivity of electrometallurgy to cell voltage) .

The average cumulative overcharge of the three samples to deactivation was 12.3 MAh/m ², and the residual iridium content of the noble metal-containing catalytic layer (including the first catalytic layer unit and the second catalytic layer unit) to deactivation amounted to 7 g/m ², i.e. the consumption rate of iridium element was 0.65 g/MAh/m ².

Example 2

Preparation of the electrode: in step 2) , the intermediate protective layer was prepared as a titanium tantalum oxide layer with a molar ratio of titanium to tantalum of 4: 1 using a magnetron sputtering method; except for the above, the other steps were the same as the corresponding steps of Example 1.

Performance test: the same electrometallurgy test b) was conducted as in Example 1, the average accumulated overcharge of the three samples to the deactivation was 12.7 MAh/m ², and the residual amount of iridium element in the catalytic layer to the deactivation was 7.5 g/m ², i.e. the consumption rate of iridium element was 0.59 g/MAh/m ².

Example 3

Preparation of the electrode: in step 1) , a rough surface with a roughness Ra of 4 μm～7 μm was obtained by soaking the surface in 30 wt%sulfuric acid aqueous solution at 85 ℃ for 1.5 hours; in step 2) , a tantalum oxide intermediate protective layer was applied, wherein the loading capacity of each layer of tantalum element is 1.0 g/m ², and the intermediate protective layer was coated three times; in step 3) the first catalytic layer unit was coated seven times; in step 4) the second catalytic layer unit was coated eight times; except for the above, the other steps were the same as the corresponding steps of Example 1.

Performance test: the same electrometallurgy test b) was conducted as in Example 1, the average accumulated overcharge of the three samples to the deactivation was 12.6 MAh/m ², and the residual amount of iridium element in the catalytic layer to the deactivation was 8.3 g/m ², i.e. the consumption rate of iridium element was 0.53 g/MAh/m ².

Example 4

Preparation of the electrode: in step 3) , the metal elements contained in the first catalytic layer unit were iridium and tantalum with the molar ratio of the iridium to tantalum elements of 80: 20; except for the above, the other steps were the same as the corresponding steps of Example 1.

Performance test: the same electrometallurgy test b) was conducted as in Example 1, the average accumulated overcharge of the three samples to the deactivation was 15.9 MAh/m ², and the residual amount of iridium element in the catalytic layer to the deactivation was 6.7 g/m ², i.e. the consumption rate of iridium element was 0.52 g/MAh/m ².

Example 5

Preparation of the electrode: in step 1) , a rough surface with a roughness Ra of 7 μm～9 μm was obtained by sand blasting with brown corundum and then soaking in 3 wt%oxalic acid aqueous solution at 90 ℃ for 1 hour; in step 2) a titanium tantalum oxide intermediate protective layer with a molar ratio of titanium to tantalum of 3: 2 was applied, wherein the total loading capacity of the titanium and tantalum elements is 0.5 g/m ², and the intermediate protective layer was coated twice; in step 3) the metal elements contained in the first catalytic layer unit were iridium and tantalum with the molar ratio of iridium to tantalum of 80: 20; except for the above, the other steps were the same as the corresponding steps of Example 1.

Performance test: the same electrometallurgy test b) was conducted as in Example 1, the average accumulated overcharge of the three samples to the deactivation was 16.6 MAh/m ², and the residual amount of iridium element in the catalytic layer to the deactivation was 5.7 g/m ², i.e. the consumption rate of iridium element was 0.56 g/MAh/m ².

Example 6

Preparation of the electrode: in step 1) , a rough surface with a roughness Ra of 11 μm～13 μm was obtained by sand blasting and then soaking in boiling 37 wt%hydrochloric acid for 1 hour; in step 2) , the intermediate protective layer was coated three times; in step 3) , the metal elements contained in the first catalytic layer unit were iridium and tantalum with the molar ratio of iridium to tantalum of 80: 20, and the first catalytic layer unit was coated five times; in step 4) , the second catalytic layer unit was coated ten times; except for the above, the other steps were the same as the corresponding steps of Example 1.

Performance test: the same electrometallurgy test b) was conducted as in Example 1, the average accumulated overcharge of the three samples to the deactivation was 17.2 MAh/m ², and the residual amount of iridium element in the catalytic layer to the deactivation was 6.1 g/m ², i.e. the consumption rate of iridium element was 0.52 g/MAh/m ².

Example 7

Preparation of the electrode: in step 1) , a rough surface with a roughness Ra of 9 μm～11 μm was obtained by sand blasting and then soaking in boiling 37 wt%hydrochloric acid for 1 hour; in step 2) , the intermediate protective layer was coated three times; in step 3) , the metal elements contained in the first catalytic layer unit were iridium and tantalum with the molar ratio of iridium to tantalum of 80: 20, and the first catalytic layer unit was coated five times; in step 4) , the metal elements contained in the second catalytic layer unit were iridium and tantalum with the molar ratio of the iridium to tantalum elements of 90: 10; in step 5) , a third catalytic layer unit was prepared, the metal element contained in the third catalytic layer unit was iridium, and the third catalytic layer unit was coated five times. Except for the above, the other steps were the same as the corresponding steps of Example 1.

Performance test: the same electrometallurgy test b) was conducted as in Example 1, the average accumulated overcharge of the three samples to the deactivation was 16.9 MAh/m ², and the residual amount of iridium element in the catalytic layer to the deactivation was 6.2 g/m ², i.e. the consumption rate of iridium element was 0.52 g/MAh/m ².

Example 8

Preparation of the electrode: in step 1) , after the surface of a 50×50 mm ² first-grade titanium sheet was oxidized, a rough surface with a roughness Ra of 5 μm～8 μm was obtained by sand blasting and then soaking in boiling 37 wt%hydrochloric acid for 1 hour; in step 2) , the intermediate protective layer was coated three times; in step 3) , the metal elements contained in the first catalytic layer were iridium and tantalum with the molar ratio of iridium to tantalum of 65: 35, and the first catalytic layer unit was coated seven times; in step 4) , the metal elements contained in the second catalytic layer were iridium and tantalum with the molar ratio of iridium to tantalum of 95: 5, and the second catalytic layer unit was coated eight times; except for the above, the other steps were the same as the corresponding steps of Example 1.

Performance test: the same electrometallurgy test b) was conducted as in Example 1, the average accumulated overcharge of the three samples to the deactivation was 12.1 MAh/m ², and the residual amount of iridium element in the catalytic layer to the deactivation was 4.9 g/m ², i.e. the consumption rate of iridium element was 0.83 g/MAh/m ².

Comparative Example 1

1) Preparation of a substrate

A 50×50 mm ² first-grade titanium sheet was treated by sand blasting with brown corundum until the surface roughness reaches Ra of 9 μm～11 μm, and soaked in a 30 wt%sulfuric acid aqueous solution at 95 ℃ for 1.5 hours, the sheet was taken out, cleaned with deionized water, and dried to obtain a metal titanium substrate 1 suitable for coating a catalytic layer.

2) Preparation of an intermediate protective layer

0.035 mol tantalum ethoxide was dissolved in 200 mL n-butanol solution, then 50 ml of hydrochloric acid with a concentration of 37%was added for acidification, to prepare a precursor solution of an intermediate protective layer; the prepared precursor solution of the intermediate protective layer was coated on the surface of the substrate 1 prepared in the step 1) , wherein the loading capacity of the metal element tantalum is 1 g/m ², then air dried, and sintered in an air atmosphere at 500 ℃; the above-mentioned procedures of coating, air drying and sintering were repeated once, and coating was carried out twice in total; the intermediate protective layer 3 was obtained by the twice coating, thereby obtaining the tantalum oxide intermediate protective layer 3.

3) Preparation of a catalytic layer

The metal elements contained in the catalytic layer 2 were iridium and tantalum with the molar ratio of iridium to tantalum of 80: 20; the application method was as follows: 0.035mol of tantalum ethoxide was dissolved in 200 ml of n-butanol solution and acidified with 50 ml of hydrochloric acid with a concentration of 37%; after that, 0.065 mol of chloroiridium acid crystal was dissolved in 200 ml of n-butanol and acidified with 50 ml of hydrochloric acid with a concentration of 37%; the two salt solutions were mixed according to a volume ratio of 1: 1 to obtain a precursor solution of the catalytic layer with an element molar ratio of iridium to tantalum of 80: 20; the precursor solution of the catalytic layer was coated on the surface of the intermediate protective layer 3 prepared in step 2) , wherein the coating amount of iridium was 1 g/m ² based on the weight of the metal element iridium, then dried, and sintered in an air atmosphere at 450 ℃; the above-mentioned procedures of coating, drying and sintering were carried out 9 times, and coating was carried out ten times in total; the catalytic layer was obtained by the above ten times of coating, thereby obtaining the iridium tantalum oxide catalytic layer with a molar ratio of iridium to tantalum of 80: 20.

Performance test

a) The surface of the electrode prepared above was characterized by X-ray photoelectron spectroscopy, as shown in Fig. 3, it can be seen that there are obvious peaks in each characteristic binding energy position of tantalum, i.e. there is only one catalytic layer.

b) The electrode prepared above was cut into three samples with a size of 25×25 mm ², and electrometallurgy was carried out in an electrolytic zinc system: the electrolyte contained 150 g/L sulfuric acid, 60 g/L zinc ion, 7 g/L divalent manganese ion, 20 mg/L industrial bovine bone glue and 0.221 g/L sodium fluoride (converted into 100 mg/L of fluoride ion concentration) , the test temperature was 43 ℃, the current density was 500 A/m ², and the electrode deactivation was determined by setting the cell voltage to raise by 0.5V (based on the sensitivity of electrometallurgy to cell voltage) .

The average cumulative overcharge of the three samples to deactivation was 6.4 MAh/m ², and the residual iridium content of the noble metal-containing catalytic layer to deactivation amounted to 2 g/m ², i.e. the consumption rate of iridium element was 2.03 g/MAh/m ².

Comparative Example 2

Preparation of electrode: in step 1) , a rough surface with a roughness Ra of 7 μm～9 μm was obtained by sand blasting with brown corundum and then soaking in 7 wt%oxalic acid aqueous solution at 95 ℃ for 1 hour; in step 2) , a titanium tantalum oxide intermediate protective layer with a molar ratio of titanium to tantalum of 4: 1 was applied, and the intermediate protective layer was coated three times (the preparation of the precursor solution of the titanium -tantalum intermediate protective layer was the same as that of step 2) of Example 1) ; in step 3) , an iridium tantalum oxide catalytic layer with a molar ratio of iridium to tantalum of 65: 35 was applied, and the catalytic layer was coated 15 times; except for the above, the other steps were the same as the corresponding steps of Example 1.

Performance test: the same electrometallurgy test b) was conducted as in Comparative Example 1, the average accumulated overcharge of the three samples to the deactivation was 5.1 MAh/m ², and the residual amount of iridium element in the catalytic layer to the deactivation was 9 g/m ², i.e. the consumption rate of iridium element was 1.17 g/MAh/m ².

Comparative Example 3

The preparation of the electrode: in step 1) , after a 50×50 mm ² first-grade titanium sheet was oxidized, a rough surface with a roughness Ra of 4 μm～7 μm was obtained by soaking in 7 wt%oxalic acid aqueous solution at 95 ℃ for 1 hour and then soaking in the same solution at 80 ℃ for 15 hours; in step 2) , a titanium tantalum oxide intermediate protective layer with a molar ratio of titanium to tantalum elements of 3: 2 was applied; in step 3) , an iridium oxide catalytic layer was applied, wherein the coating amount of iridium was 1.5 g/m ² based on the weight of the metal element iridium; except for the above, the other steps were the same as those of Comparative Example 1.

Performance test: the same electrometallurgy test b) was conducted as in Comparative Example 1, the average accumulated overcharge of the three samples to the deactivation was 7.8 MAh/m ², and the residual amount of iridium element in the catalytic layer to the deactivation was 12 g/m ², i.e. the consumption rate of iridium element was 0.38 g/MAh/m ².

The specific performance test results of the Examples and Comparative Examples of this application are shown in Table 1.

Table 1 Test results of samples from different Examples and Comparative Examples

As can be seen from Table 1, compared with the electrodes of the single catalytic layer of Comparative Examples 1-3, Examples 1-8 of the application adopt multiple catalytic layer units to form the catalytic layer, effectively combining the respective characteristics of the outer catalytic layer and the inner catalytic layer, thus significantly improving the service life of the coating in fluoride-containing aqueous solution.

The service life of Examples 1-7 is longer than that of Example 8. It can be seen that the proportion of the second metal element that is not resistant to fluoride ion corrosion in the surface catalytic layer unit can be reduced as much as possible, the content of the first metal element in the outermost catalytic layer unit can be further increased, the fluoride ion corrosion resistance of the coating can be improved, and the service life of the electrode can be further improved.

The service life of Examples 4-7 is significantly longer than that of Examples 1-3 and 8. It can be seen that further increasing the content of the first metal element in the inner catalytic layer unit is beneficial to improve the service life of the electrode.

While the embodiments disclosed in the application are as above, the foregoing contents merely are embodiments employed for easy to understand the application, and are not intended to limit the application. A person skilled in the art can make any modification and change to the forms and details of the embodiments without departing from the spirit and scope of the application, but the patent protection scope of the application shall subject to the scope defined by the appended claims.

Claims

An electrode comprising a substrate and a catalytic layer;

the catalytic layer being positioned on the upper surface and/or the lower surface of the substrate, and the catalytic layer comprising a plurality of catalytic layer units with different chemical compositions;

wherein an outermost catalytic layer unit in the catalytic layer that is away from the direction of the substrate is an oxide layer containing a first metal element; other catalytic layer units except the outermost catalytic layer unit are oxide composite layers containing a first metal element and a second metal element; the first metal element is selected from one or more of iridium, platinum, rhodium, palladium and ruthenium, and the second metal element is selected from one or more of titanium, tantalum, niobium, tungsten and zirconium; and

the mole percentage of the first metal element in each layer of the catalytic layer units accounted for a total of metal elements in each layer of the catalytic layer units increases layer by layer along the direction away from the substrate, and the mole percentage of the first metal element in the outermost catalytic layer unit accounted for a total of metal elements in the outermost catalytic layer unit is 95%-100%.
The electrode according to claim 1, wherein the molar content of the first metal element is greater than the molar content of the second metal element in an innermost catalytic layer unit in the catalytic layer that is near the direction of the substrate.
The electrode according to claim 2, wherein the mole percentage of the first metal element accounted for a total of metal elements in the innermost catalytic layer unit, is 65%to 95%, preferably 80%.
The electrode according to any one of claims 1-3, wherein the outermost catalytic layer unit is an oxide composite layer containing the first metal element and the second metal element.
The electrode according to any one of claims 1-4, wherein the loading capacity of the first metal element in each layer of the catalytic layer units is greater than 3 g/m ², preferably 6 g/m ² to 50 g/m ².
The electrode according to any one of claims 1-5, wherein the first metal element is iridium and the second metal element is tantalum.
The electrode according to any one of claims 1-6, wherein the catalytic layer is two layers of the catalytic layer unit.
The electrode according to any one of claims 1-7, wherein the substrate is a valve-type metal or an alloy of valve-type metals; preferably, the valve-type metal is selected from one of titanium, tantalum, niobium, zirconium, hafnium, vanadium, molybdenum and tungsten; preferably, the substrate is titanium or titanium alloy.
The electrode according to any one of claims 1-8, wherein the electrode further comprises an intermediate protective layer between the substrate and the catalytic layer.
The electrode according to claim 9, wherein the intermediate protective layer is selected from the group consisting of following metals or alloys or oxides thereof: tantalum, titanium, tungsten, tin, antimony, niobium, zirconium, or combinations thereof.
A method for preparing an electrode according to any one of claims 1-10, wherein the catalytic layer is formed by coating a precursor solution containing corresponding elements, drying, and then sintering.
Use of an electrode according to any one of claims 1-10, as an insoluble anode for electrometallurgy.
The use according to claim 12, wherein the electrometallurgy comprises electrolyzing zinc and electrolyzing manganese in a sulfate electrolyte system.
The use according to claim 12 or 13, wherein an electrolyte in the electrometallurgy contains fluoride ions with a concentration of not less than 20 mg/L and not more than 500 mg/L.