A nickel cobalt oxide catalyst, a preparation method and application thereof
By preparing highly active and long-lived nickel-cobalt oxide catalysts, the problems of insufficient active sites and stability of nickel-cobalt based catalysts in the electrocatalytic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid were solved, and a highly efficient electrocatalytic conversion effect was achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- JILIN UNIVERSITY
- Filing Date
- 2023-12-29
- Publication Date
- 2026-05-29
AI Technical Summary
Existing nickel-cobalt-based catalysts for the electrocatalytic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid are limited by the number of available active sites and the stability of the electrocatalyst, resulting in low catalytic efficiency, especially at high current densities where applicable data are lacking.
A highly active and long-lived heterogeneous porous nickel-cobalt oxide catalyst was prepared by continuous electrodeposition, hydrothermal reaction and calcination. Using metal foam as a support, a mixed solution of nickel salt, cobalt salt, urea and ammonium fluoride was combined to carry out a hydrothermal reaction to form a nickel-cobalt oxide nanoflower array, which enhanced the mechanical stability and active sites of the catalyst.
The efficient conversion of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid was achieved under high current density. The catalyst has a large surface area and abundant mass transfer channels, which improves reaction kinetics and mechanical stability, and significantly enhances HMF conversion and FDCA yield.
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Figure CN117867530B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of electrocatalytic oxidation technology, and in particular to a method for preparing a nickel-cobalt oxide catalyst and its application in the electrocatalytic oxidation of 5-hydroxymethylfurfural to prepare 2,5-furandicarboxylic acid. Background Technology
[0002] Biomass energy is considered a sustainable and renewable non-fossil carbon resource and the most promising clean energy source to replace existing fossil fuels. 5-Hydroxymethylfurfural (HMF), a biomass-derived platform molecule, has been recognized as a multifunctional precursor for the synthesis of a range of valuable chemical substances. The oxidation of HMF to 2,5-furandicarboxylic acid (FDCA) has attracted widespread attention during HMF conversion. This is because FDCA, due to its molecular structure similar to aromatic terephthalic acid (TPA), has broad applications in polymer and pharmaceutical fields. Generally, FDCA can be obtained through direct stoichiometric oxidation of HMF, biocatalysis, and electrocatalysis. Electrocatalysis, as a cleaner and safer method for FDCA production, eliminates the need for high-temperature, high-pressure reaction conditions and the use of toxic reagents, improving operational safety and reaction economy. Furthermore, by controlling the potential and current, thermodynamic and kinetic parameters can be adjusted more precisely and conveniently.
[0003] To date, Pd, Au, and Pt-based nanomaterials have been considered excellent catalysts for the selective oxidation of alcohols to carboxyl groups, thus accelerating the kinetics of HMF to FDCA production. However, the rarity and high price of noble metal-based materials significantly hinder their large-scale industrial and commercial application. Therefore, exploring abundant metal-based catalysts with customizable nanostructures has been a very interesting research direction. In recent years, transition metal-based materials and their corresponding derivatives for the electrocatalytic oxidation of 5-hydroxymethylfurfural have been extensively studied. Among them, nickel-cobalt-based electrocatalysts have suitable surface atomic and electronic structures, and studies have shown that nickel-cobalt bimetallic compounds have a synergistic effect, with nickel species accelerating the kinetics of HMFOR and cobalt species lowering the onset potential of HMFOR, thus attracting widespread attention. Despite some progress, the efficiency of these nickel-cobalt-based catalysts in practical applications is often unsatisfactory, limited by the number of available active sites and / or the stability of the electrocatalyst. Furthermore, most studies focus on electrolysis under constant voltage or low current density, lacking electrolysis data under constant high current density suitable for industrial production. Summary of the Invention
[0004] This invention addresses the low catalytic efficiency of the aforementioned nickel-cobalt-based catalysts in the practical application of electrocatalytic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid, which is limited by the number of available active sites and the stability of the electrocatalyst. It proposes a strategy to prepare a highly active, long-life heterogeneous porous electrocatalyst through continuous electrodeposition, hydrothermal treatment, and calcination. This catalyst preparation process is simple and environmentally friendly, and can achieve efficient conversion of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid at high current densities.
[0005] The objective of this invention is achieved through the following technical solution:
[0006] A method for preparing a nickel-cobalt oxide catalyst includes the following steps:
[0007] (1) Foam metal is selected as the carrier; the foam metal is one of foam nickel, foam copper, foam iron, and foam cobalt;
[0008] (2) Prepare a mixed solution of nickel salt and ammonium salt as the electrodeposition solution, use the carrier as the cathode and the nickel plate as the anode for electrodeposition, and clean the sample with deionized water after electrodeposition; the concentration of nickel salt is 0.05-0.5 mol / L and the concentration of ammonium salt is 1-5 mol / L.
[0009] (3) Prepare a mixed solution of nickel salt, cobalt salt, urea and ammonium fluoride, stir for 30-120 min, transfer the electrodeposited sample and solution to a high-pressure reactor, seal it and carry out a hydrothermal reaction at 100-150℃ for 5-12 h; the concentration of nickel salt in the mixed solution is 1-10 mmol / L, the concentration of cobalt salt is 1-10 mmol / L, the concentration of urea is 15-60 mmol / L and the concentration of ammonium fluoride is 6-24 mmol / L;
[0010] (4) After the hydrothermal reaction is complete, take out the sample, wash it and vacuum dry it.
[0011] (5) The dried sample was quickly transferred to a tube furnace for calcination at a temperature of 200-600℃ for 1-5 hours. After calcination, the sample was removed after the tube furnace cooled to room temperature to obtain the target electrocatalyst.
[0012] Preferably, the nickel salt in step (2) is selected from one or more of nickel chloride, nickel nitrate, nickel sulfate, and nickel acetate, and the ammonium salt is selected from one or more of ammonium sulfate, ammonium chloride, and ammonium nitrate.
[0013] Preferably, the electrodeposition in step (2) is performed using constant current electrodeposition with a current density of -0.5 to -5 A / cm³. 2 The electrodeposition time is 100–800 s.
[0014] Preferably, the nickel salt in step (3) is selected from one or more of nickel chloride, nickel nitrate, nickel sulfate, and nickel acetate, and the cobalt salt is selected from one or more of cobalt nitrate, cobalt chloride, cobalt sulfate, and cobalt acetate.
[0015] Preferably, the stirring time in step (3) is 30 to 60 minutes.
[0016] Preferably, the vacuum drying temperature in step (4) is 30 to 100°C and the drying time is 6 to 15 hours.
[0017] Preferably, the calcination in step (5) is carried out in argon or air.
[0018] The application of the nickel-cobalt oxide catalyst in the electrocatalytic oxidation of 5-hydroxymethylfurfural to prepare 2,5-furandicarboxylic acid is as follows:
[0019] Nickel-cobalt oxide catalyst was used as the working electrode, platinum sheet as the counter electrode, Hg / HgO as the reference electrode, and alkaline solution as the electrolyte. A substrate containing 5-hydroxymethylfurfural was added for electrolysis to catalytically oxidize 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid. Electrolysis was carried out using constant current or constant voltage.
[0020] Preferably, the alkaline solution is a 1 mol / L potassium hydroxide or sodium hydroxide solution, and the substrate concentration is 10–100 mmol / L.
[0021] Preferably, the electrode potential for constant voltage electrolysis is 1.3–1.5 V vs. RHE, and the current density for constant current electrolysis is 10–300 mA / cm². 2 .
[0022] The beneficial effects achieved by this invention are as follows:
[0023] (1) The catalyst provided by the present invention is synthesized by continuous electrodeposition, hydrothermal and calcination. Its synthesis process has the advantages of being green and simple and having wide applicability.
[0024] (2) The numerous three-dimensional open macroporous structures on the surface of the electrocatalyst and the closely arranged nanoflower array formed by nickel cobalt oxide nanosheets increase the surface area of the reaction interface, providing abundant active sites and a large number of mass transfer channels for the reaction, thereby improving the electrocatalytic activity.
[0025] (3) The electrocatalyst itself has good conductivity and its layered porous structure can effectively contact the electrolyte, which not only enhances mass transfer, but also accelerates the transfer of interfacial charge, improves reaction kinetics, and further enhances the catalytic activity for HMFOR.
[0026] (4) The interface profile of nickel cobalt oxide nanosheets and Ni / NF is very consistent, indicating that the surface active phase is tightly attached to the substrate, thereby effectively inhibiting the aggregation of electrocatalyst and improving the mechanical stability of the electrode. Attached Figure Description
[0027] Figure 1 The images show the SEM images of Ni / NF at 50 μm and 5 μm in Example 2.
[0028] Figure 2 Ni3CoO (Example 1) x SEM images of / Ni / NF at 50μm and 5μm;
[0029] Figure 3 Ni3CoO (Example 3) x SEM image of / NF at 2μm;
[0030] Figure 4 Ni3CoO (Example 1) x LSV plot of / Ni / NF before and after addition of 100 mmol / L HMF;
[0031] Figure 5 The LSV diagrams for Ni / NF in Example 2 before and after the addition of 100 mmol / L HMF are shown.
[0032] Figure 6 Ni3CoO (Example 3) x LSV plot of / NF before and after adding 100 mmol / L HMF;
[0033] Figure 7 Ni3CoO (Example 1) x Graph showing the change in concentration of Ni / NF over time during electrolysis;
[0034] Figure 8 SEM images of the electrodes after electrolysis in Examples 1 (a and c) and 3 (b and d). Detailed Implementation
[0035] The technical solution of the present invention will be further described in detail below through specific examples and in conjunction with the accompanying drawings.
[0036] Example 1. Ni3CoO x Preparation of / Ni / NF
[0037] 1) Cut into 1×1.5cm pieces 2Nickel foam (NF) was ultrasonically cleaned for 10 minutes each with acetone, 3 mol / L HCl, and deionized water. Then, a mixed solution containing 0.1 mol / L NiCl₂ and 2 mol / L NH₄Cl was prepared as the electrodeposition solution. The nickel foam was used as the cathode, the nickel plate as the anode, the electrodeposition time was 500 s, and the current density was -1 A / cm². 2 The electrodeposited sample was cleaned with deionized water to obtain Ni / NF.
[0038] 2) A mixed aqueous solution containing 3 mmol / L Ni(NO3)2·6H2O, 1 mmol / L Co(NO3)2·6H2O, 60 mmol / L urea, and 24 mmol / L NH4F was vigorously stirred at room temperature for 30 minutes. The solution was then transferred to a reaction vessel, and the Ni / NF obtained above was placed inside the vessel. After sealing, the mixture was kept at 120°C for 12 hours to obtain Ni3CoO. x / Ni / NF precursor. The hydrothermally treated sample was washed with deionized water and placed in a vacuum drying oven at 60°C for 8 hours.
[0039] 3) The dried sample was quickly transferred to a tube furnace for calcination in an argon atmosphere. The time required for the tube furnace to rise from room temperature to 350°C was 1 hour, followed by calcination at 350°C for 2 hours. After calcination, the sample was removed after the tube furnace temperature had cooled to room temperature, yielding Ni3CoO. x / Ni / NF.
[0040] Figure 2 Ni3CoO x The SEM image of the Ni / NF electrode shows that nanosheets have grown on the Ni / NF, indicating that Ni3CoO x Successfully grown on Ni / NF. Ni3CoO x The nanosheets exhibit a very good fit with the Ni / NF interface profile, displaying a nanoflower array. This indicates that Ni3CoO x The nanosheets are tightly attached to Ni / NF, which enhances the mechanical stability of the electrode.
[0041] The Ni3CoO prepared in Example 1 x The catalytic performance of the / Ni / NF electrode was tested using the following method:
[0042] A three-electrode system was used at room temperature, Ni3CoO x / Ni / NF is used as the working electrode (effective geometric area is 1×1cm). 2A platinum sheet (1 cm × 1.5 cm) was used as the counter electrode, and Hg / HgO (1 mol / L KOH) was used as the reference electrode. Linear sweep voltammetry was performed in a diaphragm-free electrolytic cell, and a scan rate of 5 mV / s was selected to obtain the linear sweep voltammetric curve. Figure 4 The LSV results show that the current density can reach 400 mA / cm². 2 .
[0043] At 100mA / cm 2 Constant current electrolysis was performed at a current density controlled by a galvanometer. An H-type electrolytic cell was used, with the anode and cathode chambers separated by a Nafion 117 membrane. 25 mL of 1.0 mol / L KOH solution and 1M KOH solution containing 100 mmol / L HMF were used as the electrolytes for the cathode and anode chambers, respectively. The prepared Ni3CoO2 was then electrolyzed. x The / Ni / NF electrode is placed in the anode chamber of the electrolytic cell as the working electrode, and a platinum sheet is placed in the cathode chamber as the counter electrode. The change in concentration of the substances over time during the reaction is as follows: Figure 7 As shown. After determining the theoretical charge, the electrolyte was qualitatively and quantitatively analyzed using liquid chromatography. The HMF conversion rate was 99.5%, the FDCA yield was 93.2%, the FE yield was 97.3%, and the selectivity was 93.8%.
[0044] Ni3CoO after 60 hours of continuous electrolysis x SEM images of / Ni / NF, such as Figure 8 As shown in a and c, the surface-active phase remains intact on Ni / NF, indicating that Ni3CoO x / Ni / NF exhibits good mechanical stability.
[0045] Example 2. Preparation of Ni / NF
[0046] Compared to Example 1, the preparation of the Ni / NF electrode omits the hydrothermal treatment step, while the rest of the preparation process is the same as in Example 1.
[0047] SEM images of the prepared Ni / NF are shown below. Figure 1 As shown, the Ni / NF exhibits a three-dimensional macroporous structure generated by hydrogen bubbles during the preparation process, with numerous micropores existing between the interconnected nickel particles. This structure significantly increases the surface area of the active phase and the number of active sites.
[0048] The catalytic performance of the Ni / NF electrode prepared in Example 2 was tested using the following methods:
[0049] A three-electrode system was used at room temperature, with Ni / NF as the working electrode (effective geometric area 1×1 cm). 2A platinum sheet (1 cm × 1.5 cm) was used as the counter electrode, and Hg / HgO (1 mol / L KOH) was used as the reference electrode. Linear sweep voltammetry was performed in a diaphragm-free electrolytic cell, and a scan rate of 5 mV / s was selected to obtain the linear sweep voltammetric curve. Figure 5 The LSV results show that the current density can reach 199 mA / cm². 2 .
[0050] Constant-voltage electrolysis was performed at an electrode potential of 1.43 V vs. RHE. An H-type electrolytic cell was used, with the anode and cathode chambers separated by a Nafion 117 membrane. 25 mL of 1.0 mol / L KOH solution and 1 mol / L KOH solution containing 10 mmol / L HMF were used as the electrolytes for the cathode and anode chambers, respectively. The prepared Ni / NF was placed in the anode chamber as the working electrode, Hg / HgO as the reference electrode, and a platinum sheet was placed in the cathode chamber as the counter electrode. After determining the theoretical charge, the electrolyte was qualitatively and quantitatively analyzed by liquid chromatography. The HMF conversion was close to 100%, the FE was 95%, and the yield was 94%.
[0051] Example 3. Ni3CoO x / NF preparation
[0052] Compared with Example 1, Ni3CoO x The / NF electrode preparation process omits the electrodeposition step, but otherwise remains the same as in Example 1.
[0053] Prepared Ni3CoO x SEM image of / NF as shown Figure 3 As shown, Ni3CoO is directly grown on a nickel foam substrate. x The nanosheet array is laid out in a flat pattern.
[0054] The Ni3CoO prepared in Example 3 x The catalytic performance of the / NF electrode was tested. The specific method is as follows:
[0055] A three-electrode system was used at room temperature, Ni3CoO x / NF is used as the working electrode (effective geometric area is 1×1cm). 2 A platinum sheet (1 cm × 1.5 cm) was used as the counter electrode, and Hg / HgO (1 mol / L KOH) was used as the reference electrode. Linear sweep voltammetry was performed in a diaphragm-free electrolytic cell, and a scan rate of 5 mV / s was selected to obtain the linear sweep voltammetric curve. Figure 6 The LSV results show that the current density can reach 209 mA / cm². 2 .
[0056] At 100mA / cm 2 Constant current electrolysis was performed at a specific current density. After determining the theoretical charge, the electrolyte was qualitatively and quantitatively analyzed using liquid chromatography. The HMF conversion rate reached 98%, the FDCA yield was 89.1%, the FE yield was 85.5%, and the selectivity was 90.5%.
[0057] Ni3CoO after 60 hours of continuous electrolysis x SEM image of / NF, such as Figure 8 As shown in b and d, Ni3CoO is supported on a nickel foam substrate. x The severe detachment of the active phase indicates that Ni3CoO x The mechanical stability of the / NF electrode is poor.
[0058] Example 4. Ni2Co2O x / CF preparation
[0059] Cut into 1×1.5cm pieces 2 Copper foam (CF) was ultrasonically cleaned for 10 minutes each with acetone, 3 mol / L HCl, and deionized water. A mixed aqueous solution containing 2 mmol / L Ni(NO3)2·6H2O, 2 mmol / L Co(NO3)2·6H2O, 60 mmol / L urea, and 24 mmol / L NH4F was vigorously stirred at room temperature for 30 minutes. The solution was then transferred to a reaction vessel, and the cleaned CF was placed inside. The vessel was sealed and maintained at 120°C for 10 hours to obtain Ni2Co2O. x / CF precursor. The hydrothermally treated sample was washed with deionized water and placed in a vacuum drying oven at 60℃ for 8 hours. The dried sample was then rapidly transferred to a tube furnace for calcination in an air atmosphere. The time required for the tube furnace to rise from room temperature to 350℃ was 1 hour, followed by calcination at 350℃ for 2 hours. After calcination, the tube furnace temperature was allowed to drop to room temperature before the sample was removed, yielding Ni₂Co₂O. x / CF.
[0060] The Ni2Co2O prepared in Example 4 x The catalytic performance of the / CF electrode was tested. The specific method is as follows:
[0061] A three-electrode system, Ni2Co2O, was used at room temperature. x / CF is used as the working electrode (effective geometric area is 1×1cm). 2 A platinum sheet (1 cm × 1.5 cm) was used as the counter electrode, and Hg / HgO (1 mol / L KOH) was used as the reference electrode. Linear sweep voltammetry was performed in a diaphragm-free electrolytic cell, with a scan rate of 5 mV / s selected to obtain the linear sweep voltammetric curve. The current density could reach 160 mA / cm².2 .
[0062] At 100mA / cm 2 Constant current electrolysis was performed at a current density of [value missing]. After determining the theoretical charge, the electrolyte was qualitatively and quantitatively analyzed by liquid chromatography. The HMF conversion rate reached 98%, FDCA yield was 78%, FE yield was 80%, and selectivity was 79%.
[0063] Example 5. Ni2Co2O x Preparation of / Ni / CF
[0064] Cut into 1×1.5cm pieces 2 The copper foam was ultrasonically cleaned for 10 minutes each with acetone, 3 mol / L HCl, and deionized water. Then, a mixed solution containing 0.2 mol / L NiCl₂ and 3 mol / L NH₄Cl was prepared as the electrodeposition solution. The copper foam was used as the cathode, the nickel plate as the anode, the electrodeposition time was 200 s, and the current density was -1.5 A / cm². 2 The electrodeposited sample was cleaned with deionized water to obtain Ni / CF. Subsequent hydrothermal and calcination processes were then performed to prepare Ni₂Co₂O. x Consistent with / CF.
[0065] The Ni2Co2O prepared in Example 5 x The catalytic performance of the / Ni / CF electrode was tested. The specific method is as follows:
[0066] A three-electrode system was used at room temperature. Ni₂Co₂O x / Ni / CF was used as the working electrode (effective geometric area is 1×1cm). 2 A platinum sheet (1 cm × 1.5 cm) was used as the counter electrode, and Hg / HgO (1 mol / L KOH) was used as the reference electrode. Linear sweep voltammetry was performed in a diaphragm-free electrolytic cell, with a scan rate of 5 mV / s selected to obtain the linear sweep voltammetric curve. The current density could reach 350 mA / cm². 2 .
[0067] At 100mA / cm 2 Constant current electrolysis was performed at a current density of [value missing]. After determining the theoretical charge, the electrolyte was qualitatively and quantitatively analyzed by liquid chromatography. The HMF conversion rate reached 99%, FDCA yield was 85%, FE yield was 88%, and selectivity was 86%.
[0068] The specific embodiments described above illustrate the technical solution and beneficial effects of the present invention in detail. It should be understood that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.
Claims
1. A method for preparing a nickel-cobalt oxide catalyst, characterized in that, The method includes the following steps: (1) Foam metal is selected as the carrier; the foam metal is one of foam nickel, foam copper, foam iron, and foam cobalt; (2) A mixed solution of nickel salt and ammonium salt was prepared as the electrodeposition solution. The carrier was used as the cathode and the nickel plate as the anode for electrodeposition. After electrodeposition, the sample was cleaned with deionized water to form a nickel layer on the surface of the foam metal. The concentration of nickel salt was 0.05~0.5 mol / L and the concentration of ammonium salt was 1~5 mol / L. The electrodeposition was performed using constant current electrodeposition with a current density of -0.5~-5 A / cm. 2 The electrodeposition time is 100~800 s; (3) Prepare a mixed solution of nickel salt, cobalt salt, urea and ammonium fluoride, stir for 30-120 min, transfer the electrodeposited sample and solution to a high-pressure reactor, seal it and carry out a hydrothermal reaction at 100-150 ℃ for 5-12 h; the concentration of nickel salt in the mixed solution is 1-10 mmol / L, the concentration of cobalt salt is 1-10 mmol / L, the concentration of urea is 15-60 mmol / L and the concentration of ammonium fluoride is 6-24 mmol / L; (4) After the hydrothermal reaction is complete, remove the sample, wash it and vacuum dry it; (5) The dried sample was quickly transferred to a tube furnace for calcination at a temperature of 200-600 °C for 1-5 h. After calcination, the sample was removed after the tube furnace cooled to room temperature to obtain the target electrocatalyst.
2. The method for preparing the nickel-cobalt oxide catalyst according to claim 1, characterized in that, The nickel salt mentioned in step (2) is selected from one or more of nickel chloride, nickel nitrate, nickel sulfate, and nickel acetate, and the ammonium salt is selected from one or more of ammonium sulfate, ammonium chloride, and ammonium nitrate.
3. The method for preparing the nickel-cobalt oxide catalyst according to claim 1, characterized in that, The nickel salt mentioned in step (3) is selected from one or more of nickel chloride, nickel nitrate, nickel sulfate, and nickel acetate, and the cobalt salt is selected from one or more of cobalt nitrate, cobalt chloride, cobalt sulfate, and cobalt acetate.
4. The method for preparing the nickel-cobalt oxide catalyst according to claim 1, characterized in that, The vacuum drying temperature in step (4) is 30~100 ℃ and the drying time is 6~15 h.
5. The method for preparing the nickel-cobalt oxide catalyst according to claim 1, characterized in that, The calcination described in step (5) is carried out in argon or air.
6. A nickel-cobalt oxide catalyst prepared by the method described in any one of claims 1 to 5.
7. The application of the nickel-cobalt oxide catalyst described in claim 6 for the electrocatalytic oxidation of 5-hydroxymethylfurfural to prepare 2,5-furandicarboxylic acid, wherein the specific method is as follows: A nickel-cobalt oxide catalyst was used as the working electrode, a platinum sheet as the counter electrode, Hg / HgO as the reference electrode, and an alkaline solution as the electrolyte. A substrate containing 5-hydroxymethylfurfural was added for electrolysis to catalytically oxidize 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid. Electrolysis was performed using a constant current or constant voltage method.
8. The application according to claim 7, characterized in that, The alkaline solution is a 1 mol / L potassium hydroxide or sodium hydroxide solution, and the concentration of 5-hydroxymethylfurfural in the substrate is 10~100 mmol / L.
9. The application according to claim 7 or 8, characterized in that, The electrode potential for constant voltage electrolysis is 1.3~1.5 V vs. RHE, and the current density for constant current electrolysis is 10~300 mA / cm². 2 .