A method for the production of a self-supporting bifunctional electrode by the molten salt method

Abstract

The application discloses a method for preparing a self-supporting bifunctional electrode by a molten salt method and particularly relates to the technical field of electrocatalytic material preparation, and comprises the following steps: pretreating a foamed nickel, hydrothermally growing a precursor, performing molten salt assisted sulfuration and depositing an FeOOH active layer. The application realizes in-situ growth of a nickel-based precursor on the surface of a three-dimensional conductive substrate, converts the precursor into a metal sulfide layer with a porous structure, continuously extends an electron transmission path along the surface of the substrate, improves the conductivity of the electrode and the contact efficiency of an electrolyte, generates an amorphous metal oxyhydroxide active layer on the surface of the metal sulfide through chemical soaking, promotes the electron transmission process in the oxygen evolution reaction and the hydrogen evolution reaction, adjusts the growth process of the precursor and the deposition process of the active layer, maintains appropriate pore distribution and a covering state on the surface of the electrode, reduces the influence of surface accumulation on the flow of the electrolyte, and adopts a self-supporting structure form, thereby improving the structural stability during a long-time electrolysis process.

Description

Technical Field

[0001] This invention relates to the field of electrocatalytic material preparation technology, and in particular to a method for preparing a self-supporting bifunctional electrode using a molten salt method. Background Technology

[0002] Hydrogen production through water electrolysis is gradually becoming an important development direction in the field of new energy conversion and energy storage due to its advantages such as zero carbon emissions, high product purity, and the ability to operate in conjunction with renewable energy sources such as wind and solar power. In the water electrolysis process, both the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode involve multiple electron transfers and intermediate product adsorption processes. The oxygen evolution reaction, in particular, has a slower kinetic process and typically requires a higher driving voltage, which can easily lead to increased energy loss. Therefore, it is necessary to use bifunctional electrode materials with high catalytic activity and good electron transport capabilities to reduce the reaction overpotential.

[0003] Most existing electrocatalytic electrodes are prepared by coating powdered catalysts onto a conductive substrate. This process typically requires the addition of binders such as polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE) to fix the catalyst layer. However, these binders can cover some catalytic active sites and increase electron transport resistance. Furthermore, during prolonged electrolysis, the catalyst layer is prone to detachment or cracking, leading to decreased electrode stability. In addition, while some self-supporting electrodes use three-dimensional porous materials such as nickel foam as the conductive substrate, the adhesion of the active layer to the substrate surface is unstable. Local areas are prone to accumulation, blockage, or uneven coverage, affecting electrolyte diffusion and electron transport processes, making it difficult to simultaneously achieve performance in both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).

[0004] On the other hand, metal sulfides are widely used in electrocatalysis due to their high conductivity and abundant surface active sites. However, most existing sulfidation methods employ liquid-phase sulfidation processes, which are prone to problems such as uneven sulfur source diffusion, insufficient local sulfidation, and numerous crystal phase defects during the reaction. Furthermore, the liquid-phase environment can easily lead to precursor structure shrinkage, causing pore collapse and a decrease in specific surface area, thus affecting the contact efficiency between the electrode surface and the electrolyte. For the subsequent active layer deposition process, existing technologies typically employ direct coating methods, which can easily result in an excessively thick active layer or localized agglomeration, affecting electrolyte penetration and electron migration processes, ultimately limiting overall catalytic performance.

[0005] Therefore, there is a need for a method for preparing self-supporting bifunctional electrodes using the molten salt method, which can form a continuous conductive sulfide layer on the surface of a three-dimensional conductive substrate and improve the adhesion stability and electron transport capability of the active layer. Summary of the Invention

[0006] The main objective of this invention is to provide a method for preparing self-supporting bifunctional electrodes using the molten salt method, which can effectively solve the problems mentioned above in the background.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing a self-supporting bifunctional electrode using a molten salt method includes the following steps: S1. Nickel foam pretreatment: The three-dimensional porous metal substrate is degreased, pickled and dried to remove the surface oxide layer and expose the metal active sites. S2. Precursor hydrothermal growth: The treated metal substrate is placed in a precursor reaction solution containing metal salt, slow-release alkali source and crystal surface regulator to perform a hydrothermal reaction, so that a nickel-based crystalline precursor is grown in situ on the surface of the metal substrate. S3. Molten salt-assisted sulfidation: The metal substrate supporting the precursor and the sulfur-containing compound are respectively placed in different areas of the tubular reactor, and molten salt-assisted sulfidation is performed under a protective atmosphere to transform the precursor into a porous metal sulfide conductive framework. S4. FeOOH active layer deposition: The sulfided conductive skeleton is immersed in an iron-containing electrodeposition solution, and the conductive skeleton is used as the working electrode. Electrodeposition is performed for 150 to 1200 s at a current intensity of -30 to -100 mA to generate an amorphous metal oxide hydroxyl active layer on its surface in situ, thereby obtaining the anode of a self-supporting bifunctional electrode.

[0008] Preferably, the three-dimensional porous metal substrate includes one or more of the following: nickel foam, iron foam, copper foam, metal fiber felt, or porous metal mesh.

[0009] Preferably, the metal salt in the precursor reaction solution includes one or more of nickel nitrate, nickel chloride, and nickel sulfate; The slow-release alkaline source includes one or more of urea and hexamethylenetetramine. The crystal plane modifier includes one or more of ammonium fluoride, ammonium chloride, or citrate.

[0010] Preferably, the hydrothermal reaction temperature is 100℃~180℃, and the reaction time is 4h~12h; By adjusting the hydrothermal reaction time, the thickness and pore opening of the precursor nanosheets can be controlled, enabling the precursor to form a continuous cross-linked structure on the surface of the metal substrate.

[0011] Preferably, the sulfur-containing compound includes one or more of thiourea, thioacetamide, sulfur powder, or sodium sulfide; The molten salt-assisted sulfidation treatment temperature is 250℃~450℃, and the holding time is 0.5h~4h; During the sulfidation process, the diffusion of molten sulfur source causes the formation of a multi-level porous structure inside the precursor and increases the crystallinity of the metal sulfide phase.

[0012] Preferably, the conductive framework is immersed in an iron salt solution during the surface chemical deposition treatment; The iron salt includes one or more of ferric chloride, ferric nitrate, or ferric sulfate; During the immersion process, an amorphous FeOOH active layer is generated through local ion exchange and in-situ hydrolysis on the surface of the conductive framework.

[0013] Preferably, the amorphous metal oxyhydroxyl active layer covers the surface of the porous metal sulfide framework and forms a heterogeneous interface structure with the metal sulfide framework. The heterogeneous interface structure is used to enhance the interfacial electronic coupling capability and improve the charge transfer efficiency during the oxygen evolution reaction and hydrogen evolution reaction.

[0014] Preferably, the self-supporting bifunctional electrode simultaneously possesses catalytic functions for both oxygen evolution reaction and hydrogen evolution reaction in an alkaline electrolyte; At 1000 mA / cm 2 Under current density conditions, the overpotential of the oxygen evolution reaction is less than 500mV.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention transforms a nickel-based precursor into a porous metal sulfide layer by performing in-situ growth of a precursor on the surface of a three-dimensional conductive substrate and combining it with molten salt-assisted sulfidation treatment. The conductive path extends continuously along the substrate surface, making the electron transport process smoother. At the same time, the molten sulfur source performs gas-solid sulfidation under a protective atmosphere, which makes the sulfidation process more uniform, reduces local structural shrinkage and surface collapse, and allows the resulting metal sulfide to maintain a relatively complete pore morphology and a high surface contact area, thereby improving the electrode conductivity and electrolyte contact efficiency.

[0016] 2. This invention generates an amorphous metal oxyhydroxyl active layer on the surface of a metal sulfide through chemical immersion, forming a stable interface region on the sulfide surface and promoting electron transport processes in the oxygen evolution reaction and hydrogen evolution reaction. Simultaneously, by adjusting the precursor growth process and the active layer deposition process, the electrode surface maintains an appropriate pore distribution and active layer coverage, reducing the impact of surface accumulation on electrolyte flow. The resulting electrode adopts a self-supporting structure, eliminating the need for additional binders and current collectors, reducing catalyst layer detachment, and improving structural stability during long-term electrolysis. Attached Figure Description

[0017] Figure 1 This is a bar graph showing the effect of different hydrothermal reaction times on the overpotential of the material according to the present invention. Detailed Implementation

[0018] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0019] A method for preparing a self-supporting bifunctional electrode using a molten salt method includes the following steps: S1. Nickel foam pretreatment: The three-dimensional porous metal substrate is degreased, pickled and dried to remove the surface oxide layer and expose the metal active sites. S2. Precursor hydrothermal growth: The treated metal substrate is placed in a precursor reaction solution containing metal salt, slow-release alkali source and crystal surface regulator to perform a hydrothermal reaction, so that a nickel-based crystalline precursor is grown in situ on the surface of the metal substrate. S3, Molten Salt Assisted Sulfation: The metal substrate supporting the precursor and the sulfur-containing compound are respectively placed in different areas of the tubular reactor, and molten salt assisted sulfation is performed under a protective atmosphere to transform the precursor into a porous metal sulfide conductive framework. S4, FeOOH active layer deposition: The sulfided conductive skeleton is immersed in a reaction solution containing iron ions to perform surface chemical deposition treatment, so that an amorphous metal oxide hydroxyl active layer is generated in situ on the surface of the conductive skeleton, forming the anode of a self-supporting bifunctional electrode.

[0020] Three-dimensional porous metal substrates include one or more of the following: nickel foam, iron foam, copper foam, metal fiber felt, or porous metal mesh.

[0021] The metal salts in the precursor reaction solution include one or more of nickel nitrate, nickel chloride, and nickel sulfate; Slow-release alkaline sources include one or more of urea and hexamethylenetetramine; Crystal facet modifiers include one or more of ammonium fluoride, ammonium chloride, or citrate.

[0022] The hydrothermal reaction temperature is 100℃~180℃, and the reaction time is 4h~12h; By adjusting the hydrothermal reaction time, the thickness and pore opening of the precursor nanosheets can be controlled, enabling the precursor to form a continuous cross-linked structure on the surface of the metal substrate.

[0023] Sulfur-containing compounds include one or more of thiourea, thioacetamide, sulfur powder, or sodium sulfide; The molten salt-assisted sulfidation treatment temperature is 250℃~450℃, and the holding time is 0.5h~4h; During the sulfidation process, the diffusion of molten sulfur source causes the formation of a multi-level porous structure inside the precursor and increases the crystallinity of the metal sulfide phase.

[0024] In the surface chemical deposition process, an iron salt solution is used to perform an immersion reaction on the conductive framework; Iron salts include one or more of ferric chloride, ferric nitrate, or ferric sulfate; During the immersion process, an amorphous FeOOH active layer is generated through local ion exchange and in-situ hydrolysis on the surface of the conductive framework.

[0025] An amorphous metal oxyhydroxyl active layer covers the surface of a porous metal sulfide framework and forms a heterogeneous interface structure with the metal sulfide framework. Heterointerface structures are used to enhance interfacial electronic coupling and improve charge transfer efficiency during oxygen evolution and hydrogen evolution reactions.

[0026] The self-supporting bifunctional electrode simultaneously possesses catalytic functions for both oxygen evolution reaction and hydrogen evolution reaction in alkaline electrolyte; At 1000 mA / cm 2 Under current density conditions, the overpotential of the oxygen evolution reaction is less than 500mV.

[0027] Example 1: A method for preparing a self-supporting bifunctional electrode using a molten salt method, comprising the following steps: S1, Nickel foam pretreatment: A 1cm×2cm piece of nickel foam was ultrasonically cleaned for 15 minutes each in acetone, anhydrous ethanol, and deionized water. After removal, it was immersed in 3mol / L hydrochloric acid solution for 10 minutes, then rinsed alternately with deionized water and anhydrous ethanol, and dried in a 60℃ oven.

[0028] S2, precursor hydrothermal growth: Weigh 0.436 g of nickel nitrate hexahydrate, 0.450 g of urea and 0.111 g of ammonium fluoride and add them to 40 mL of deionized water. After stirring magnetically for 20 min, transfer the mixture to a 50 mL polytetrafluoroethylene-lined reactor. The treated nickel foam was placed at an angle on the inner lining side wall of the reactor, sealed, and then reacted at 120°C for 6 hours. After the reaction was completed, the nickel foam was naturally cooled to room temperature, removed and rinsed with deionized water, and then dried at 60°C to obtain nickel foam loaded with light green precursor.

[0029] S3, molten salt-assisted vulcanization: 2.0 g of thiourea was placed in the upstream region of the quartz tube, and the nickel foam carrying the precursor was placed in the downstream region of the quartz tube. After purging with argon gas for 20 minutes, the temperature was increased to 300℃ at a rate of 3℃ / min and held for 1 hour. After natural cooling, the sample was removed, rinsed with deionized water and anhydrous ethanol, and dried at 60°C to obtain the Ni3S2 / NF electrode.

[0030] S4, FeOOH active layer deposition: Weigh 0.675g of ferric chloride hexahydrate and add it to 50mL of deionized water to prepare the soaking solution; The Ni3S2 / NF electrode was immersed in the soaking solution and electrodeposited for 150 s at a current intensity of -30 mA. After removal, the electrode was rinsed with deionized water and dried at 60°C to obtain a Ni3S2@FeOOH self-supporting bifunctional electrode.

[0031] The electrode was tested and found to operate at 100 mA / cm. 2 The overpotential for the oxygen evolution reaction at the current density is 160 mV.

[0032] Example 2 is basically the same as Example 1, except that: The hydrothermal reaction time of the precursor was adjusted to 8h, 10h, 12h, and 14h, respectively; The holding time during the molten salt sulfidation stage has been adjusted to 1.5 hours.

[0033] The resulting electrode surface forms a plate-like sulfide structure, which accelerates the electrolyte wetting rate and the oxygen evolution reaction overpotential is 318mV.

[0034] The effect of different hydrothermal reaction times on the overpotential of materials, such as Figure 1 As shown, the performance is optimal when the reaction time is 10 hours.

[0035] Example 3 is basically the same as Example 1, except that: Ammonium chloride was used as the crystal form regulator in the precursor reaction solution; The hydrothermal reaction temperature was adjusted to 140℃; The electrodeposition times were adjusted to 300s, 600s, 900s, and 1200s at a current intensity of -30mA, respectively. The molten salt sulfidation temperature was adjusted to 320℃.

[0036] Electrode materials prepared with different electrodeposition times at 100 mA / cm 2 500mA / cm 2 1000mA / cm 2 The overpotentials of the oxygen evolution reaction at different current densities are shown in the table below: The effect of different electrodeposition times and current densities on the overpotential of the material was investigated, with the lowest overpotential observed when the electrodeposition time was 300 s.

[0037] Example 4 is basically the same as Example 1, except that: The three-dimensional porous metal substrate uses foamed iron; The sulfur-containing substance is thioacetamide; Ferric nitrate was used as the electrodeposition electrolyte during the FeOOH active layer deposition process, and electrodeposition was performed for 150 s at a current intensity of -30 mA. The resulting electrode maintains good structural stability and conductivity continuity during long-term electrolysis at 100 mA / cm². 2 The overpotential for the oxygen evolution reaction at the current density is 338 mV.

[0038] Example 5 is basically the same as Example 1, except that: The hydrothermal reaction time was adjusted to 10 hours. The molten salt sulfidation temperature was adjusted to 350℃; During the deposition of the FeOOH active layer, the electrodeposition time was adjusted to 1200 s at a current intensity of -30 mA. A thick FeOOH active layer forms on the surface of the resulting electrode, causing some pores to be covered and reducing the electrolyte transport capacity, which is lower than 100 mA / cm². 2 The overpotential for the oxygen evolution reaction at the current density is 347 mV.

[0039] Comparative Example 1 is basically the same as Example 1, except that: Instead of performing the molten salt-assisted sulfidation treatment in S3, FeOOH electrodeposition treatment was directly applied to the precursor electrode after hydrothermal growth (-30mA, 150s). The conductivity of the resulting electrode decreased significantly, at 100 mA / cm. 2 The overpotential for the oxygen evolution reaction at the current density is 412 mV.

[0040] Comparative Example 2 is basically the same as Example 1, except that: In S4, FeOOH active layer deposition was not performed; only Ni3S2 / NF was used as the electrode. The number of active sites on the electrode surface is reduced at 100 mA / cm 2 The overpotential for the oxygen evolution reaction at the current density is 368 mV.

[0041] Comparative Example 3 is basically the same as Example 1, except that: The hydrothermal reaction time in S2 is adjusted to 2 hours; Subsequently, molten salt sulfidation and FeOOH electrodeposition (-30mA, 150s) were performed according to the conditions of Example 1. The resulting precursor has a discontinuous coating on the substrate surface, forming localized accumulation regions after vulcanization, at 100 mA / cm². 2 The overpotential for the oxygen evolution reaction at the current density is 389 mV.

[0042] Comparative Example 4 is basically the same as Example 1, except that: During the deposition of the FeOOH active layer, the electrodeposition time was adjusted to 1200 s at a current intensity of -30 mA. The resulting FeOOH layer is too thick, reducing the pores on the electrode surface and affecting electrolyte transport at 100 mA / cm². 2 The overpotential for the oxygen evolution reaction at the current density is 376 mV.

[0043] Specifically, oxygen evolution reaction performance tests were performed on the self-supporting bifunctional electrodes prepared in the above embodiments and comparative examples. The overpotential changes of the electrodes obtained under different process parameters were compared to verify the effects of molten salt-assisted sulfidation treatment, hydrothermal precursor growth process and FeOOH active layer deposition process on the catalytic performance of the electrodes.

[0044] Experimental procedure: The oxygen evolution reaction performance of each electrode was tested using a three-electrode system. The electrodes prepared according to the examples and comparative examples were used as working electrodes, Hg / HgO electrodes were used as reference electrodes, graphite rods were used as counter electrodes, and 1.0 mol / L KOH solution was used as the electrolyte. During the test, the polarization curve of the oxygen evolution reaction was measured using linear sweep voltammetry. The scan rate was set to 5 mV / s, and the values ​​of each electrode at 100 mA / cm² were recorded. 2 The overpotential of the oxygen evolution reaction corresponding to the current density was recorded. For the FeOOH deposition time optimization experiment, the overpotential changes under different current density conditions were further recorded to evaluate the effect of active layer thickness on the oxygen evolution performance at high current density.

[0045] The following are electrodes obtained in different embodiments and comparative examples at 100 mA / cm 2 Results of overpotential test for oxygen evolution reaction at current density.

[0046] As can be seen from the above, different process conditions have a significant impact on the oxygen evolution reaction performance of the self-supporting bifunctional electrode. The overpotentials of the electrodes obtained in Examples 1 and 3 are 160mV and 154mV, respectively, which are significantly lower than those of the comparative examples. This indicates that appropriate hydrothermal precursor growth, molten salt-assisted sulfidation treatment, and FeOOH active layer deposition are beneficial to improving the oxygen evolution reaction activity of the electrode. Comparative Example 1 did not undergo molten salt-assisted sulfidation treatment, and the overpotential increased to 412mV, indicating that the Ni3S2 conductive framework plays an important role in improving electron transport and catalytic performance. Comparative Example 2 did not have a FeOOH active layer deposited, and the overpotential was 368mV, indicating that the FeOOH active layer can increase the effective active sites. In Comparative Example 3, the overpotential increased due to insufficient precursor coverage caused by the short hydrothermal reaction time. In Comparative Example 4 and Example 5, the FeOOH deposition time was long, resulting in an excessively thick active layer that covered part of the pores, affecting electrolyte diffusion. In summary, molten salt-assisted sulfidation, hydrothermal precursor growth, and FeOOH active layer deposition need to be controlled in a coordinated manner. When the process parameters are appropriate, a lower oxygen evolution reaction overpotential can be obtained.

[0047] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a self-supporting bifunctional electrode using a molten salt method, characterized in that, Includes the following steps: S1. Nickel foam pretreatment: The three-dimensional porous metal substrate is degreased, pickled and dried to remove the surface oxide layer and expose the metal active sites. S2. Precursor hydrothermal growth: The treated metal substrate is placed in a precursor reaction solution containing metal salt, slow-release alkali source and crystal surface regulator to perform a hydrothermal reaction, so that a nickel-based crystalline precursor is grown in situ on the surface of the metal substrate. S3. Molten salt-assisted sulfidation: The metal substrate supporting the precursor and the sulfur-containing compound are respectively placed in different areas of the tubular reactor, and molten salt-assisted sulfidation is performed under a protective atmosphere to transform the precursor into a porous metal sulfide conductive framework. S4. FeOOH active layer deposition: The sulfided conductive skeleton is immersed in an iron-containing electrodeposition solution, and the conductive skeleton is used as the working electrode. Electrodeposition is performed for 150 to 1200 s at a current intensity of -30 to -100 mA to generate an amorphous metal oxide hydroxyl active layer on its surface in situ, thereby obtaining the anode of a self-supporting bifunctional electrode.

2. The method for preparing a self-supporting bifunctional electrode using the molten salt method according to claim 1, characterized in that: The three-dimensional porous metal substrate includes one or more of the following: nickel foam, iron foam, copper foam, metal fiber felt, or porous metal mesh.

3. The method for preparing a self-supporting bifunctional electrode using the molten salt method according to claim 1, characterized in that: The metal salt in the precursor reaction solution includes one or more of nickel nitrate, nickel chloride, and nickel sulfate. The slow-release alkaline source includes one or more of urea and hexamethylenetetramine. The crystal plane modifier includes one or more of ammonium fluoride, ammonium chloride, or citrate.

4. The method for preparing a self-supporting bifunctional electrode using the molten salt method according to claim 1, characterized in that: The hydrothermal reaction temperature is 100℃~180℃, and the reaction time is 4h~12h; By adjusting the hydrothermal reaction time, the thickness and pore opening of the precursor nanosheets can be controlled, enabling the precursor to form a continuous cross-linked structure on the surface of the metal substrate.

5. The method for preparing a self-supporting bifunctional electrode using the molten salt method according to claim 1, characterized in that: The sulfur-containing compound includes one or more of thiourea, thioacetamide, sulfur powder, or sodium sulfide; The molten salt-assisted sulfidation treatment temperature is 250℃~450℃, and the holding time is 0.5h~4h; During the sulfidation process, the diffusion of molten sulfur source causes the formation of a multi-level porous structure inside the precursor and increases the crystallinity of the metal sulfide phase.

6. The method for preparing a self-supporting bifunctional electrode using the molten salt method according to claim 1, characterized in that: In the surface chemical deposition treatment, an iron salt solution is used to perform an immersion reaction on the conductive framework. The iron salt includes one or more of ferric chloride, ferric nitrate, or ferric sulfate; During the immersion process, an amorphous FeOOH active layer is generated through local ion exchange and in-situ hydrolysis on the surface of the conductive framework.

7. The method for preparing a self-supporting bifunctional electrode using the molten salt method according to claim 1, characterized in that: The amorphous metal oxy hydroxyl active layer covers the surface of the porous metal sulfide framework and forms a heterogeneous interface structure with the metal sulfide framework. The heterogeneous interface structure is used to enhance the interfacial electronic coupling capability and improve the charge transfer efficiency during the oxygen evolution reaction and hydrogen evolution reaction.

8. The method for preparing a self-supporting bifunctional electrode using the molten salt method according to claim 1, characterized in that: The self-supporting bifunctional electrode simultaneously possesses catalytic functions for both oxygen evolution reaction and hydrogen evolution reaction in alkaline electrolyte; At 1000 mA / cm 2 Under current density conditions, the overpotential of the oxygen evolution reaction is less than 500mV.

9. A method for preparing a self-supporting bifunctional electrode using the molten salt method according to claim 1, characterized in that: The method also includes the fabrication of a self-supporting bifunctional electrode cathode, as detailed below: The sulfided conductive framework is immersed in an electrodeposition solution containing molybdenum or cobalt salts, and the conductive framework is used as the working electrode to electrodeposit for 150 to 1200 s at a current intensity of -30 to -100 mA, so that an amorphous metal oxide hydroxyl active layer is generated in situ on its surface.

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