Water oxidation catalyst as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of water electrolysis hydrogen production, and particularly relates to a water oxidation catalyst and a preparation method and application thereof. According to the method, a weak-acid heterogeneous soaking system is manufactured through the hydrolysis effect of metal cations in the hydrolyzable metal salt solution, a slow effect is achieved on the surface of the metal substrate, and the surface of the metal substrate can be partially etched while surface metal oxides are removed; the etched metal ions and the hydrolyzed metal ions are combined on the surface of the substrate to form an LDH catalyst structure, so that relatively high catalytic activity of the LDH catalyst structure is ensured; meanwhile, under the interface confinement effect, a compact transition layer structure is slowly formed on the interface of the metal substrate and the catalyst layer. The transition layer is used as a bridge between the metal substrate and the catalyst layer, has the same structure as LDH, is more compact in morphology, and completely covers the surface of the metal substrate, so that the LDH catalytic structure layer is firmly anchored on the surface of the metal substrate, and the OER catalyst has high activity and high stability under the condition of industrial current density.

Description

[0001] Cross - reference to related applications

[0002] This application claims the priority of a Chinese patent application with the application number 202410131060.0, titled "A Water Oxidation Catalyst and Its Preparation Method and Application", filed with the Chinese Patent Office on January 30, 2024. The entire content of which is incorporated herein by reference. Technical field

[0003] This application belongs to the technical field of hydrogen production by electrolyzing water, and specifically relates to a water oxidation catalyst and its preparation method and application. Background art

[0004] Hydrogen is a clean fuel with high energy density, wide sources, no pollution and storability, and is one of the key energy carriers to solve the crisis of traditional energy demand and environmental pollution problems under the background of "dual carbon". Compared with traditional fossil fuel - based hydrogen production (such as hydrogen production from coal, oil, etc., "gray hydrogen") and natural gas - based hydrogen production ("blue hydrogen"), hydrogen production by electrolyzing water with renewable energy (such as solar energy, wind energy, nuclear energy, etc., "green hydrogen") has the advantages of wide raw material sources, zero carbon emissions and renewability. The existing industrial low - temperature electrolytic water hydrogen production technologies mainly include alkaline water electrolysis cell hydrogen production (AWE) and proton exchange membrane water electrolysis hydrogen production (PEM - WE). AWE has low material cost, high stability and is easy to scale up industrially, but there are problems of hydrogen and oxygen inter - penetration, low operating efficiency, and corrosion problems are easy to occur when using high - concentration alkali; PEM - WE has a compact structure, high hydrogen production efficiency and short response time, but has high input costs, cannot get rid of the dependence on precious metals (such as Pt, etc.) and has limited hydrogen production capacity. In recent years, the newly developed anion exchange membrane water electrolysis (AEM - WE) combines the advantages of AWE and PEM - WE, that is, low material cost, compact structure, high hydrogen production efficiency, etc., and has great potential for the development of electrolytic water hydrogen production. However, AEM - WE still has a certain distance from large - scale industrial green hydrogen production, mainly because it still faces the problem that it is difficult to balance hydrogen production performance and stability at industrial - level current densities (such as ≥1000mA cm -2 )

[0005] The core parts of the electrolytic water hydrogen production reaction are the water oxidation (OER) catalyst and the hydrogen reduction (HER) catalyst. Among them, the reaction kinetics of the four-electron transfer OER catalytic process is slow, which restricts the entire electrolytic water reaction process more than the HER process. Therefore, the development of efficient and stable OER catalysts is particularly important for the green hydrogen project. Existing research shows that noble metal compounds represented by IrO2 and RuO2 exhibit excellent water oxidation activity, but it is difficult to popularize due to the high price of noble metals. Therefore, the development of non-noble metal OER catalysts has gradually become a research trend. However, current reports on non-noble metal OER catalysts pay more attention to the improvement of catalytic performance, and less attention is paid to long-term stability (such as >8000 h), especially the stability under industrial current density conditions and high-performance conditions.

[0006] Therefore, it is urgent to develop a preparation method that can firmly fix the catalyst structure on the surface of the metal substrate to meet the stable and efficient operation of the OER reaction under industrial current density. Summary of the Invention

[0007] Therefore, the technical problem to be solved by this application is to overcome the above-mentioned defects of the water oxidation catalyst in the existing electrolytic water hydrogen production process, so as to provide a water oxidation catalyst, its preparation method and application.

[0008] To this end, this application provides the following technical solutions:

[0009] This application provides a preparation method of a water oxidation catalyst, including the following steps:

[0010] S1, prepare a hydrolyzable metal salt solution, and adjust the pH of the hydrolyzable metal salt solution to 2.5 - 6.5;

[0011] S2, soak the metal substrate with the hydrolyzable metal salt solution;

[0012] S3, add an organic solvent to the system of step S2 to form a heterogeneous system, and react for 4 - 72 h.

[0013] Optionally, in step S1, the concentration of the hydrolyzable metal salt solution is 20 - 400 mmol / L.

[0014] Optionally, the hydrolyzable metal salt includes but is not limited to at least one of nickel chloride, nickel sulfate, nickel nitrate, iron sulfate, ferrous sulfate, iron chloride, ferrous chloride, vanadium trichloride, vanadyl sulfate, aluminum chloride, indium trichloride, cerium trichloride, cerium sulfate, bismuth trichloride.

[0015] Optionally, in step S2, the soaking temperature is 5 - 50 °C, and the soaking time is 0 - 5 h;

[0016] Optionally, the soaking time is 1-2 h.

[0017] Optionally, in step S3, the organic solvent includes but is not limited to at least one of acetone, methanol, ethanol, propanol, isopropanol, allyl alcohol, butanol, 2-butanol, tert-butanol, ethylene glycol, propylene glycol, tetrahydrofuran, dimethyl ether, ethyl acetate, N,N-dimethylformamide;

[0018] and / or, the volume ratio range of the organic solvent to the hydrolyzable salt solution is (0.5-9):1;

[0019] and / or, the metal substrate includes but is not limited to at least one of nickel foam, iron foam, nickel-iron foam, nickel mesh, iron mesh, nickel-iron mesh, nickel felt, iron felt, nickel-iron felt, nickel electrode plate, iron electrode plate. When the metal substrate contains only one metal element, in order to form a layered double metal compound (LDH) subsequently, the metal element in the hydrolyzable metal salt solution should be different from the metal component in the substrate.

[0020] Optionally, the reaction temperature in step S3 is 20-60 °C, and the reaction time is 10-72 h.

[0021] This application also provides a water oxidation catalyst prepared by the above preparation method.

[0022] This application also provides an application of the above water oxidation catalyst in hydrogen production by electrolyzing water.

[0023] Optionally, the current density of hydrogen production by electrolyzing water is 500-4000 mA cm -2 . This application is particularly applicable to hydrogen production by electrolyzing water with an industrial current density of 1000-4000 mA cm -2 for hydrogen production by electrolyzing water.

[0024] Optionally, it is applicable to the electrolysis of an alkaline water system. The alkaline water system includes but is not limited to alkaline water, alkaline seawater, alkaline mineral water, etc.

[0025] This application provides a water oxidation catalyst, which includes a metal substrate and a catalyst layer. A transition layer attached to the surface of the metal substrate is further included between the metal substrate and the catalyst layer. The catalyst layer and the transition layer have the same lamellar structure, and the morphology of the transition layer is denser than that of the catalyst layer.

[0026] It should be noted that the "lamellar structure" in this application means that the main structures of the catalyst layer and the transition layer are lamellar structures. These lamellar structures in the transition layer are closely packed, and more delicate structures such as petal-shaped, honeycomb-shaped, cluster-shaped, and bundle-shaped can be formed between these lamellar structures in the catalyst layer.

[0027] It should also be noted that the meaning of "dense" is that the area of the lamellar structure is smaller and the density of the arrangement of the lamellar structure is greater.

[0028] It should also be noted that the smaller the gap between the transition layer and the metal substrate, the more suitable the water oxidation catalyst is for stable and efficient operation at industrial current density. The reason is as follows: The reason why the water oxidation catalyst of the present application can achieve stable and efficient operation at industrial current density is that the water oxidation catalyst of the present application includes a dense transition layer, and the dense transition layer firmly anchors the catalytic layer on the surface of the metal substrate, that is, the existence of the transition layer enables the water oxidation catalyst of the present application to have good mechanical stability. Therefore, the water oxidation catalyst of the present application can withstand a higher industrial current density. Thus, the smaller the gap between the transition layer and the metal substrate, the stronger the binding between the catalyst and the metal substrate, and the more capable the water oxidation catalyst is of stable and efficient operation at industrial current density.

[0029] In some embodiments, there is no gap at most positions between the substrate and the transition layer, where most positions are more than 80% of the positions.

[0030] Optionally, the thickness of the transition layer is 0.2 - 4 μm;

[0031] Optionally, the thickness of the transition layer is 0.2 - 3 μm;

[0032] Optionally, the thickness of the transition layer is 1 - 3 μm;

[0033] Optionally, the thickness of the transition layer is 2 - 3 μm.

[0034] It should be noted that in the present application, the thickness of the transition layer in the water oxidation catalyst is not exactly the same at each position. Some places have a larger thickness, and some places have a smaller thickness. Among them, the thickness of the transition layer in the water oxidation catalyst is related to the position of the metal reaction sites in the metal substrate. For example, when the metal substrate is nickel foam, since nickel foam has a honeycomb structure, the nickel reaction sites in the nickel foam face in all directions. When the orientation of the nickel reaction sites in the nickel foam is the same as the direction of gravity, since the hydrolyzable salt solution is located above the metal substrate, the reaction contact surface of this part of the nickel reaction sites in the same direction as gravity is smaller, the etching rate of this part of the nickel reaction sites in the same direction as gravity is slower, and the catalyst layer formed at the position of this part of the nickel reaction sites in the same direction as gravity is thinner. Therefore, the thickness of the transition layer is smaller. On the contrary, when the orientation of the nickel reaction sites in the nickel foam is opposite to the direction of gravity, the thickness of the transition layer is larger.

[0035] It should also be noted that the thickness of the transition layer refers to the thickness range where the thickness is located at most positions of the transition layer. For example, the thickness of the transition layer refers to the range composed of the thickness at least exceeding 50% of the positions of the transition layer. In most cases, the thickness of the transition layer refers to the range composed of the thickness at least exceeding 60% of the positions of the transition layer. That is, the thickness range where the thickness is located at most positions of the transition layer can generally be used as the thickness of the transition layer, but it does not exclude the situation where the thickness at individual positions is lower or higher than the thickness range of the transition layer.

[0036] It should be noted that in the water oxidation catalyst of the present application, the measurement method of the thickness of the transition layer includes the following steps:

[0037] S11, obtain a cross-sectional scanning electron micrograph of the water oxidation catalyst to be measured (illustrated by Figure 19 as an example);

[0038] S12, as Figure 23 shown, determine the boundary line 4 between the metal substrate and the transition layer according to the position where the morphological structures of the metal substrate 3 and the transition layer 2 change significantly, and determine the demarcation line 5 between the catalyst layer 1 and the transition layer 2 according to the position where the density of the lamellar structure changes significantly;

[0039] S13, give a trend line 6 according to the boundary line 4, and measure the distance between the trend line 6 and the demarcation line 5 along the vertical direction of the trend line at the position to be measured, which is the thickness of the transition layer at the position to be measured; Figure 24 is another schematic diagram of the thickness test of the transition layer in the water oxidation catalyst. For the part that significantly deviates from the trend line in the figure (the part circled by the circle in the figure), it will be discarded during the process of determining the trend line 6 according to the boundary line 4;

[0040] S14, select several different positions and measure the thickness according to the method in step S13. The range composed of the thickness at least exceeding 50% of the positions is the thickness of the transition layer of the water oxidation catalyst.

[0041] Optionally, the metal substrate includes at least one of nickel foam, iron foam, nickel-iron foam, nickel mesh, iron mesh, nickel-iron mesh, nickel felt, iron felt, nickel-iron felt, nickel electrode plate, and iron electrode plate;

[0042] And / or, the compositions of the catalyst layer and the transition layer are both layered double metal compounds.

[0043] Optionally, the metal substrate is nickel foam or iron foam;

[0044] The compositions of the catalyst layer and the transition layer are at least one of nickel-iron layered double metal compound, nickel-vanadium layered double metal compound, nickel-aluminum layered double metal compound, and nickel-cerium layered double metal compound.

[0045] The reaction principle of the present application is as follows:

[0046] After the metal salt undergoes hydrolysis in water in step S1, a large amount of hydrogen ions are generated. In steps S2 and S3, these hydrogen ions come into contact with the metal substrate, first etching away the loose metal oxides on the surface; as time goes by, part of the surface metal will also be slowly etched away. In addition to cleaning the surface of the metal substrate, the etching process also releases a certain amount of substrate metal ions (such as Ni 2+ , Fe 2+ , Fe 3+ , etc.). These released ions can be used as raw materials for the subsequent formation of layered double metal hydroxides (LDH). This mild etching method avoids problems such as environmental pollution, damage to the metal substrate structure, and low utilization rate of metal ion sources that may be caused by conventional pickling of metal substrates. The etched metal ions can be used as raw materials for forming LDH catalysts in subsequent heterogeneous systems; while the ions participating in the hydrolysis reaction are usually metal ions with higher valence states (such as +3 valence), which can also participate in the subsequent formation process of LDH catalysts while completing the hydrolysis reaction.

[0047] Due to the different solubilities of metal compounds in water and organic solvents, the addition of organic solvents in S3 leads to the precipitation of a large number of nanoparticles. These nanoparticles are evenly dispersed in the liquid phase and are easily adsorbed on the surface of the metal substrate (such as nickel foam, etc.), thereby reducing the adsorption energy of the metal surface. Due to the acidic characteristics of the liquid phase system (caused by metal ion hydrolysis), acid etching on the surface of the metal substrate is still occurring, resulting in a significant increase in the metal ion concentration at the solid-liquid interface. These etched ions and the metal ions in the liquid grow gradually to form an LDH catalytic structure with the nanoparticles adsorbed on the metal surface as the core (or seed). As the catalytic structure grows slowly, the surface of the metal substrate is gradually covered, and the acid etching gradually weakens (the rate slows down but still occurs). At this time, metal ions are only enriched at the junction between the metal substrate surface and the catalytic structure; under the interfacial confinement effect, a dense transition layer is gradually formed at the junction over time (with relatively low growth kinetics). When the transition layer completely covers the metal substrate, the etching process stops, the surrounding metal ion concentration decreases, and the growth of the catalytic structure and the transition layer stops, ending the catalyst growth process. Since the growth of the transition layer is accompanied by the etching of the metal on the substrate surface, when the catalyst stops growing, the metal surface has been completely covered by the transition layer. Due to the existence of the dense transition layer, the LDH catalytic layer with a good three-dimensional structure is firmly anchored on the surface of the metal substrate. The catalyst obtained in this application has both high OER activity and high stability under industrial-grade current density conditions. Moreover, due to the complete coverage of the metal substrate surface by the transition layer, it can effectively protect the metal substrate from the erosion of harmful ions (such as Cl - [[ID=X]], etc.), enabling the catalyst to be applied not only to alkaline water electrolysis for hydrogen production but also to alkaline seawater electrolysis for hydrogen production and alkaline mineral water electrolysis for hydrogen production.

[0048] The technical solution of this application has the following advantages:

[0049] The preparation method of the water oxidation catalyst provided by this application includes the following steps: S1, prepare a hydrolyzable metal salt solution and adjust the pH of the hydrolyzable metal salt solution to 2.5 - 6.5; S2, soak a metal substrate with the hydrolyzable metal salt solution; S3, add an organic solvent to the system in step S2 to form a heterogeneous system and react for 4 - 72 h. This application uses the hydrolysis of metal cations in the hydrolyzable metal salt solution to create a weakly acidic heterogeneous soaking system, which slowly acts on the surface of the metal substrate (such as nickel foam, iron foam, etc.). While removing the surface metal oxides, it also partially etches the surface of the metal substrate; these etched metal ions combine with the hydrolyzed metal ions on the substrate surface to form an LDH catalyst structure, ensuring its high catalytic activity; at the same time, under the interfacial confinement effect, a dense transition layer structure is slowly formed (with a lower growth kinetics) at the interface between the metal substrate and the catalyst layer. This transition layer serves as a bridge between the metal substrate and the catalyst layer. Its structure is the same as that of LDH, but its morphology is denser and it completely covers the surface of the metal substrate, thereby realizing the firm anchoring of the LDH catalytic structure layer on the surface of the metal substrate. The outer layer (catalyst layer) of the obtained 3D self-supporting catalyst is responsible for the high activity of OER, the metal substrate is responsible for support and electron transfer, and the intermediate dense transition layer is responsible for firmly anchoring the catalyst layer on the surface of the metal substrate, thereby achieving high activity and high stability of OER water oxidation catalysis under industrial-grade current density conditions. In addition, this application uses the weak acid effect of cation hydrolysis to etch the surface of the metal substrate (without adding or adding only a very small amount of acid to adjust the pH value), avoiding environmental pollution that may be caused by conventional pickling and other means; the cations etched on the surface of the metal substrate participate in the formation of the catalyst layer, avoiding the addition of an external substrate metal ion source and saving costs; this method has low raw material costs, mild treatment conditions, simple operation, high repeatability, high OER activity, strong OER stability, and is easy to scale up for large-scale applications.

[0050] The preparation method of the water oxidation catalyst provided by this application can improve the thickness of the transition layer by further optimizing the reaction time, and can further improve the operating stability of the catalyst under industrial-grade current density.

[0051] The water oxidation catalyst provided by the present application, the water oxidation catalyst includes a metal substrate and a catalyst layer. A transition layer attached to the surface of the metal substrate is further included between the metal substrate and the catalyst layer. Both the catalyst layer and the transition layer are in a sheet structure, and the morphology of the transition layer is denser than that of the catalyst layer. The surface of the metal substrate of the water oxidation catalyst has been completely covered by the transition layer. Due to the existence of the transition layer, and the catalyst layer and the transition layer have the same sheet structure, the LDH catalyst layer with a good three-dimensional structure can be firmly anchored on the surface of the metal substrate. Since the morphology of the transition layer is denser than that of the catalyst layer, this multi-level structure can enable the catalyst to be distributed in depth on the surface of the metal substrate, which is beneficial to the mass transfer process during the catalytic process. The transition layer is responsible for providing high OER activity, and the metal substrate is responsible for support and electron transfer. The catalyst obtained in the present application has both high OER activity and high stability under industrial-level current density conditions. And due to the complete coverage of the surface of the metal substrate by the transition layer, the metal substrate can be effectively protected from the erosion of harmful ions (such as Cl - etc.), so that the catalyst has an anti-corrosion function. In addition to being used for alkaline water hydrogen production, it can also be applied to alkaline seawater hydrogen production and alkaline mineral water hydrogen production, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings required to be used in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0053] Figure 1 It is a spherical aberration electron microscope (STEM) image of the nanoparticles in the heterogeneous system formed in Example 1;

[0054] Figure 2 It is a scanning electron microscope (SEM) image of the catalyst on the surface of the nickel foam substrate obtained in Example 1;

[0055] Figure 3 It is a growth process diagram of the LDH catalytic layer and the dense transition layer on the surface of the nickel foam substrate in Example 2;

[0056] Figure 4 It is a photo of the catalyst on the large-sized (20 cm × 20 cm) nickel foam substrate provided in Example 3;

[0057] Figure 5 It is a uniformity distribution diagram of the catalyst obtained in Example 4: (a) a photo of the 5 cm × 5 cm catalyst, (b) a comparison of the linear scanning performance curves at different points (25 °C, 1 M KOH);

[0058] Figure 6 is the stability curve of the nickel foam-based catalyst in Example 4 in 1 M KOH electrolyte (25 °C, 1000 mA cm -2 );

[0059] Figure 7 is the linear sweep performance curve of the nickel foam-based catalyst obtained in Example 4 in alkaline simulated seawater and real seawater containing 1 M KOH (25 °C);

[0060] Figure 8 is the stability curve of the nickel foam-based catalyst obtained in Example 4 in alkaline seawater electrolyte containing 1 M KOH (25 °C, 1000 mA cm -2 );

[0061] Figure 9 is the cross-sectional scanning electron microscopy (SEM) image of the iron foam-based catalyst obtained in Example 8;

[0062] Figure 10 is the surface scanning electron microscopy (SEM) image of the iron foam-based catalyst obtained in Example 8;

[0063] Figure 11 is the linear sweep performance curve of the iron foam-based catalyst obtained in Example 8 (25 °C, 1 M KOH);

[0064] Figure 12 is the linear sweep performance curve of the catalyst obtained in Example 10 (25 °C, 1 M KOH);

[0065] Figure 13 is the cross-sectional scanning electron microscopy (SEM) image of the catalyst obtained in Comparative Example 1;

[0066] Figure 14 is the stability curve of the catalyst obtained in Comparative Example 1 in 1 M KOH electrolyte (25 °C, 1000 mA cm -2 );

[0067] Figure 15 is the stability curve of the catalyst obtained in Comparative Example 1 in alkaline seawater electrolyte of 1 M KOH (25 °C, 1000 mA cm -2 );

[0068] Figure 16 is the cross-sectional scanning electron microscopy (SEM) image of the catalyst obtained in Comparative Example 3;

[0069] Figure 17 is the cross-sectional scanning electron microscopy (SEM) image of the catalyst obtained in Comparative Example 5;

[0070] Figure 18 is the cross-sectional scanning electron microscopy (SEM) image of the catalyst obtained in Example 3;

[0071] Figure 19 It is the cross-sectional scanning electron microscope (SEM) image of the catalyst obtained in Example 4;

[0072] Figure 20 It is the cross-sectional scanning electron microscope (SEM) image of the catalyst obtained in Example 5;

[0073] Figure 21 It is the cross-sectional scanning electron microscope (SEM) image of the catalyst obtained in Example 7;

[0074] Figure 22 It is the cross-sectional scanning electron microscope (SEM) image of the catalyst obtained in Example 9;

[0075] Figure 23 It is a schematic diagram for testing the thickness of the transition layer in the water oxidation catalyst;

[0076] Figure 24 It is another schematic diagram for testing the thickness of the transition layer in the water oxidation catalyst;

[0077] Reference numerals:

[0078] 1. Catalyst layer; 2. Transition layer; 3. Metal substrate; 4. Boundary line; 5. Dividing line; 6. Trend line. Detailed implementation manners

[0079] The following embodiments are provided to better further understand the present application, which are not limited to the described optimal implementation manner, and do not constitute a limitation to the content and protection scope of the present application. Any product identical or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with those of other existing technologies falls within the protection scope of the present application.

[0080] For those not specifying specific experimental steps or conditions in the embodiments, the operations or conditions of the conventional experimental steps described in the literature in this field can be followed. For the reagents or instruments not specifying the manufacturer, they are all conventional reagent products that can be obtained through commercial purchase.

[0081] It should be noted that in the embodiments of the present application, the metal substrate, the transition layer, and the catalyst layer are distinguished by SEM images of the water oxidation catalyst profile. Among them, in the SEM image of the profile, the metal substrate is located at the bottom of the image, and its morphology is significantly different from the flaky morphologies of the transition layer and the catalyst layer. The transition layer is located between the catalyst layer and the metal substrate. The position where the morphological structure changes significantly is used as the boundary line between the transition layer and the metal substrate. For example, when the metal substrate is nickel foam, it can be seen that the metal substrate has a porous structure and the transition layer has a lamellar structure. The position where the density of the flaky morphology changes significantly is used as the boundary line between the transition layer and the catalyst layer. In the following examples and comparative examples, the thickness of the transition layer refers to the range of the thickness at the position where the transition layer exceeds 50%, which is determined according to the thickness of the transition layer measured at multiple different positions in the SEM image.

[0082] Example 1

[0083] This example provides a water oxidation catalyst, and its preparation steps and operating parameters are as follows:

[0084] S1: Preparation of weak acidic hydrolysis solution: Dissolve a certain amount of easily hydrolyzable ferrous chloride metal salt in water, stir to form a solution with a metal ion concentration of 100 mol / L, and adjust the pH to 3.5 with sulfuric acid;

[0085] S2: Etching of the surface oxide of the metal substrate: Immerse nickel foam (Suzhou Christie Company, 80 ppi, the same below, size 2 cm × 2 cm) into the weak acidic solution in S1 and let it stand at 30 °C for 4 h;

[0086] S3: In-situ growth of the catalyst: Under ultrasonic stirring conditions, slowly add ethanol to the system in S2, where the volume ratio of ethanol to the ferrous chloride solution is 3:1 to form a heterogeneous liquid phase system, let it stand at 40 °C for 24 h, take it out, rinse it with water, and dry it.

[0087] Among them, the meaning of the heterogeneous liquid phase system is: a liquid phase system (suspension, emulsion) containing insoluble solid particles (including nano- or micron-sized particles). The heterogeneous liquid phase system is formed by the solubility difference of metal inorganic salts in different solvents. For example, FeCl2 is soluble in water but insoluble in ethanol. Therefore, under stirring conditions, adding ethanol to an aqueous solution of FeCl2 can produce many uniform but insoluble particles, and the overall forms a heterogeneous liquid phase system.

[0088] Figure 1After adding ethanol in step S3 of this example, the TEM image of the insoluble nanoparticles in the liquid phase system shows that these nanoparticles are amorphous and have a uniform size distribution (nanoscale); these particles are easily adsorbed on the surface of the metal substrate in the liquid phase environment to reduce its surface energy, and then serve as the core (or seed) to bypass the nucleation process that the LDH new phase formation and growth originally need to experience, assisting in the rapid formation of the LDH structure. Figure 2 The SEM image of the catalyst obtained in this example shows that the catalyst is mainly composed of closely packed lamellae at the bottom (i.e., the transition layer) and the upper secondary lamellar structure (the catalyst layer). This multi-level structure can enable the catalyst to be distributed in depth on the surface of the metal substrate, which is beneficial to the mass transfer process during the catalytic process.

[0089] In Example 1, in the S3 system, a transition layer is generated, and the thickness of the transition layer is 1 - 2 μm. The obtained catalyst is used for the stability test of alkaline water (1 M KOH) oxidation at an industrial current density (1000 mA cm -2 ), and it can operate stably for more than 4500 h.

[0090] Example 2

[0091] This example provides a water oxidation catalyst, and its preparation steps and operating parameters are as follows:

[0092] S1: Preparation of weak acidic hydrolysis solution: Dissolve a certain amount of easily hydrolyzed ferrous chloride metal salt in water, stir to form a solution with a metal ion concentration of 200 mol / L, and adjust the pH to 3.5 with hydrochloric acid.

[0093] S2: Etching of the metal substrate surface oxide: Immerse 8 pieces of nickel foam (size 2 cm × 2 cm) into the S1 weak acidic solution and leave it standing at 30 °C for 4 h.

[0094] S3: In-situ growth of the catalyst: Under ultrasonic stirring conditions, slowly add ethanol to the S2 system, where the volume ratio of ethanol to the ferrous chloride solution is 3:1 to form a heterogeneous liquid phase system. Leave it standing at 25 °C, and take out one piece at the 1st h, 2nd h, 4th h, 8th h, 16th h, 24th h, 36th h, and 72nd h of soaking respectively. After taking out, rinse it with water and dry it.

[0095] Figure 3SEM cross-sectional view of the catalyst obtained in this example. As can be seen from the figure, an obvious lamellar structure (catalyst layer) can be obtained by soaking in the S3 system for 1 h. As the time prolongs, the lamellar structure grows. When the soaking time in step S3 reaches 4 h or more, a dense transition layer is generated between the catalyst layer and the nickel foam substrate (the part between the dotted lines in the figure is the transition layer, the lamellar structure with hydrophobic morphology above the dotted line is the catalyst layer, and the lower part of the dotted line is the metal substrate without lamellar structure), with a thickness of about 0.2 - 0.5 μm, and the thickness gradually increases with the prolonging of time (when the soaking time is 8 h, the thickness of the transition layer is 0.5 - 0.8 μm; when the soaking time is 16 h, the thickness of the transition layer is 1 - 2 μm; when the soaking time is 24 h, the thickness of the transition layer is 1.5 - 2.5 μm); while when the time prolongs to 36 h or more, the thickness of the transition layer no longer increases and finally remains at 2 - 3 μm. The catalyst obtained by reacting for 36 h is used for the stability test of alkaline water (1 M KOH) oxidation at an industrial current density (1000 mA cm -2 ) and can operate stably for more than 6800 h.

[0096] Example 3

[0097] This example provides a water oxidation catalyst, and its preparation steps and operating parameters are as follows:

[0098] S1: Preparation of weak acidic hydrolysis solution: Dissolve a certain amount of easily hydrolyzed ferrous chloride metal salt in water, stir to form a solution with a metal ion concentration of 30 mol / L, and adjust the pH to 6.5 with hydrochloric acid;

[0099] S2: Etching of the oxide on the metal substrate surface: Immerse nickel foam (size 20 cm × 20 cm) into the S1 weak acidic solution and let it stand at 5 °C for 0.1 h;

[0100] S3: In-situ growth of the catalyst: Under ultrasonic stirring conditions, slowly add ethanol to the S2 system, where the volume ratio of ethanol to the ferrous chloride solution is 0.5:1 to form a heterogeneous liquid phase system, let it stand at 20 °C for 6 h, take it out, rinse it with water, and dry it.

[0101] As Figure 4 shown, the obtained catalyst grows well on the surface of the nickel foam substrate, indicating that even in a relatively low growth environment (such as low metal ion concentration, high pH value, and short soaking time), the catalyst can still grow relatively uniformly on the surface of the foam metal with a large size (20 cm × 20 cm).

[0102] From Figure 5It can be seen that in this embodiment, when the size of the metal substrate is relatively large, the potential difference at different points is extremely small. It can be seen that in the water oxidation catalyst prepared in this embodiment, the performance differences at various locations are small. Thus, during the actual industrial use process, the catalytic effects at various locations on the water oxidation catalyst are similar, and the difference between the effective area where the water oxidation catalyst participates in the reaction and the area of the water oxidation catalyst is extremely small. That is, during the actual industrial use process, the proportion of the part of the water oxidation catalyst participating in the reaction is extremely high, which can greatly improve the industrial production efficiency. In Example 3, in the S3 system, a transition layer is generated, as Figure 18 shown, and the thickness of the transition layer is 0.2 - 0.3 μm. The obtained catalyst was used for the stability test of alkaline water (1 M KOH) oxidation at an industrial-grade current density (1000 mA cm -2 ), and it could operate stably for more than 1500 h.

[0103] The metal substrate in this method can be replaced with a metal plate of a larger size. In this way, a transition layer and a catalyst layer can be grown on the surface of the metal plate to achieve the protection of the metal plate and avoid the corrosion of the metal plate.

[0104] Example 4

[0105] This embodiment provides a water oxidation catalyst, and its preparation steps and operating parameters are as follows:

[0106] S1: Preparation of a weakly acidic hydrolysis solution: Dissolve a certain amount of hydrolyzable ferrous chloride metal salt in water, stir to form a solution with a metal ion concentration of 400 mol / L, and adjust the pH to 2.6 with hydrochloric acid;

[0107] S2: Etching of the oxide on the metal substrate surface: Immerse the nickel foam (5 cm × 5 cm) into the S1 weakly acidic solution and let it stand at 50 °C for 5 h;

[0108] S3: In-situ growth of the catalyst: Under ultrasonic stirring conditions, slowly add ethanol to the S2 system, where the volume ratio of ethanol to the ferrous chloride solution is 8:1 to form a heterogeneous liquid phase system. Let it stand at 60 °C for 70 h, take it out, rinse it with water, and dry it.

[0109] In Example 4, in the S3 system, a transition layer is generated, as Figure 19 shown, and the thickness of the transition layer is 3 - 4 μm.

[0110] As Figure 5 shown, the obtained catalyst grows more uniformly on the nickel foam, and the linear sweep curves measured at at least three positions thereon almost coincide, indicating that the catalytic activities at multiple points of the catalyst are uniform. The obtained catalyst was used for the stability test of alkaline water (1 M KOH) oxidation at an industrial-grade current density (1000 mA cm -2 ), as Figure 6As shown, during the test exceeding 8760 h (one year), no obvious increase in electric potential was observed, indicating that the material has excellent catalytic stability in alkaline water. The catalyst was used for hydrogen production by electrolyzing simulated alkaline seawater and real alkaline seawater, as Figure 7 shown, in 1 M KOH + 0.5 M NaCl simulated seawater (with the same chloride ion concentration as seawater) and 1 M KOH + real seawater, the catalyst exhibited excellent catalytic activity, and the overpotentials at 1000 mA cm -2 were 203 mV and 218 mV respectively, exceeding the vast majority of existing reports. When applied to real alkaline seawater electrolysis, it can stably operate for more than 2000 h at an industrial current density of 1000 mA cm -2 (see Figure 8 , according to its stable operation trend, it is expected to reach a stable operation duration similar to that of alkaline water). The excellent seawater electrolysis stability mainly stems from two aspects: First, the dense transition layer completely coats the metal substrate, protecting it from corrosive chloride ions; Second, the LDH structure of the catalyst has an anion exchange ability. After absorbing some chloride ions, it can repel more chloride ions in seawater as a co-ion to protect the catalyst.

[0111] Example 5

[0112] This example provides a water oxidation catalyst, and its preparation steps and operating parameters are as follows:

[0113] S1: Preparation of weak acidic hydrolysis solution: Dissolve a certain amount of vanadium trichloride metal salt that is easy to hydrolyze in water, stir to form a solution with a concentration of 200 mol / L, and adjust the pH to 3.5 with sulfuric acid;

[0114] S2: Etching of the metal substrate surface oxide: Immerse the nickel foam (2 cm × 2 cm) into the S1 weak acidic solution and let it stand at ³⁰ °C for 4 h;

[0115] S3: In-situ growth of the catalyst: Under ultrasonic stirring conditions, slowly add ethanol to the S2 system, where the volume ratio of ethanol to vanadium trichloride solution is 3:1 to form a heterogeneous liquid phase system, let it stand at 40 °C for 24 h, take it out, rinse it with water, and dry it.

[0116] In Example 5, in the S3 system, a transition layer is generated, as Figure 20 shown, and the thickness of the transition layer is 0.5 - 1 μm. The obtained catalyst was used for the stability test of alkaline real seawater oxidation (containing 1 M KOH, 25 °C) at an industrial current density (1000 mA cm -2 ), and it can stably operate for 1200 h; the obtained catalyst was used for an industrial current density (1000 mA cm -2)Stability test of alkaline water oxidation (1M KOH), can stably operate for more than 2100h.

[0117] Example 6

[0118] This example provides a water oxidation catalyst, and its preparation steps and operating parameters are as follows:

[0119] S1: Preparation of weak acidic hydrolysis solution: Dissolve a certain amount of hydrolyzable aluminum chloride metal salt in water, stir to form a solution with a concentration of 200 mol / L, and adjust the pH to 3.5 with sulfuric acid;

[0120] S2: Etching of the metal substrate surface oxide: Immerse nickel foam (2 cm × 2 cm) into the S1 weak acidic solution and let it stand at 30 °C for 4 h;

[0121] S3: In-situ growth of the catalyst: Under ultrasonic stirring conditions, slowly add ethanol to the S2 system, where the volume ratio of ethanol to aluminum chloride solution is 3:1, to form a heterogeneous liquid phase system, let it stand at 40 °C for 24 h, take it out, wash it with water, and dry it.

[0122] In Example 6, in the S3 system, a transition layer is generated, and the thickness of the transition layer is 0.2 - 0.5 μm. The obtained catalyst is used for the stability test of alkaline real seawater oxidation (containing 1M KOH, 25 °C) at an industrial current density (1000 mA cm -2 ) and can stably operate for 1000 h; the obtained catalyst is used for the stability test of alkaline water (1M KOH) oxidation at an industrial current density (1000 mA cm -2 ) and can stably operate for more than 1200 h.

[0123] Example 7

[0124] This example provides a water oxidation catalyst, and its preparation steps and operating parameters are as follows:

[0125] S1: Preparation of weak acidic hydrolysis solution: Dissolve a certain amount of hydrolyzable cerium sulfate metal salt in water, stir to form a solution with a concentration of 200 mol / L, and adjust the pH to 3.5 with sulfuric acid;

[0126] S2: Etching of the metal substrate surface oxide: Immerse nickel foam (2 cm × 2 cm) into the S1 weak acidic solution and let it stand at 30 °C for 4 h;

[0127] S3: In-situ growth of the catalyst: Under ultrasonic stirring conditions, slowly add ethanol to the S2 system, where the volume ratio of ethanol to cerium sulfate solution is 3:1, to form a heterogeneous liquid phase system, let it stand at 40 °C for 24 h, take it out, wash it with water, and dry it.

[0128] In Example 7, in the S3 system, a transition layer is generated, such as Figure 21 shown, and the thickness of the transition layer is 1 - 2 μm. The obtained catalyst is used for the stability test of alkaline real seawater oxidation (containing 1 M KOH, 25 °C) under industrial-grade current density (1000 mA cm -2 ), and it can operate stably for 1000 h; the obtained catalyst is used for the stability test of alkaline water (1 M KOH) oxidation under industrial-grade current density (1000 mA cm -2 ), and it can operate stably for more than 3200 h.

[0129] Example 8

[0130] This example provides a water oxidation catalyst, and its preparation steps and operating parameters are as follows:

[0131] S1: Preparation of weakly acidic hydrolysis solution: Dissolve a certain amount of hydrolyzable ferrous sulfate and nickel nitrate metal salts in water, stir to form a solution with a concentration of 200 mol / L, the molar ratio of iron element to nickel element is 1:3.5, and adjust the pH to 3.5 with sulfuric acid;

[0132] S2: Etching of the metal substrate surface oxide: Immerse the iron foam (2 cm × 2 cm) into the S1 weakly acidic solution and let it stand at 30 °C for 4 h;

[0133] S3: In-situ growth of the catalyst: Under ultrasonic stirring conditions, slowly add ethanol to the S2 system, where the volume ratio of ethanol to ferrous sulfate and nickel nitrate solution is 3:1 to form a heterogeneous liquid phase system, let it stand at 40 °C for 24 h, take it out, rinse it with water, and dry it.

[0134] In Example 8, in the S3 system, a transition layer is generated, and the thickness of the transition layer is 1 - 2 μm. As Figure 9 shown in the cross-sectional SEM image, the obtained catalyst contains a three-layer structure: an upper lamellar structure, a middle transition layer structure, and a bottom iron foam structure. As Figure 10 shown in the top-view SEM image, the water oxidation catalyst is also composed of a bottom closely packed lamellar layer (transition layer) and a large petal secondary lamellar structure (catalyst layer). Perform a linear sweep curve test on it, as Figure 11 shown, and the overpotential is 240 mV at 1000 mA cm -2 .

[0135] The obtained catalyst is used for the stability test of alkaline real seawater oxidation (containing 1 M KOH, 25 °C) under industrial-grade current density (1000 mA cm -2 ), and it can operate stably for 1500 h; the obtained catalyst is used for industrial-grade current density (1000 mA cm -2)Stability test of oxidation in alkaline water (1 M KOH), and it can operate stably for more than 2500 h.

[0136] Example 9

[0137] This example provides a water oxidation catalyst, and its preparation steps and operating parameters are as follows:

[0138] S1: Preparation of weakly acidic hydrolysis solution: Dissolve a certain amount of easily hydrolyzable ferrous sulfate metal salt in water, stir to form a solution with a concentration of 200 mol / L, and adjust the pH to 3.5 with sulfuric acid;

[0139] S2: Etching of metal substrate surface oxide: Immerse nickel foam (2 cm × 2 cm) into the weakly acidic solution of S1 and let it stand at 30 °C for 4 h;

[0140] S3: In-situ growth of catalyst: Under ultrasonic stirring conditions, slowly add isopropyl alcohol to the system of S2. Among them, the volume ratio of isopropyl alcohol to ferrous sulfate solution is 3:1 to form a heterogeneous liquid phase system. Let it stand at 40 °C for 24 h, take it out, rinse it with water, and dry it.

[0141] In Example 9, in the system of S3, a transition layer is generated, as Figure 22 shown. The thickness of the transition layer is 2 - 3 μm. The obtained catalyst is used for the stability test of oxidation in alkaline real seawater (containing 1 M KOH, 25 °C) under industrial-grade current density (1000 mA cm -2 ), and it can operate stably for 1800 h; the obtained catalyst is used for the stability test of oxidation in alkaline water (1 M KOH) under industrial-grade current density (1000 mA cm -2 ), and it can operate stably for more than 5500 h.

[0142] Example 10

[0143] This example provides a water oxidation catalyst, and its preparation steps and operating parameters are as follows:

[0144] S1: Preparation of weakly acidic hydrolysis solution: Dissolve a certain amount of easily hydrolyzable ferrous sulfate metal salt in water, stir to form a solution with a concentration of 200 mol / L, and adjust the pH to 3.5 with sulfuric acid;

[0145] S2: In-situ growth of catalyst: Under ultrasonic stirring conditions, slowly add ethanol to the system of S1. Among them, the volume ratio of ethanol to ferrous sulfate solution is 3:1 to form a heterogeneous liquid phase system that submerges the metal substrate. Immerse nickel foam (2 cm × 2 cm) into it, let it stand at 40 °C for 36 h, take it out, rinse it with water, and dry it.

[0146] In Example 10, a transition layer is formed in the S3 system, and the thickness of the transition layer is 0.5-1.5 μm. The obtained catalyst is subjected to a linear scanning curve test in 1M KOH alkaline aqueous electrolyte (25°C). Figure 12 As shown, at 1000mA cm -2 The overpotential was 225 mV. The obtained catalyst was used for industrial-grade current density (1000 mA cm -2 ) Stability test of alkaline water oxidation (1M KOH), stable operation for more than 2500h.

[0147] Comparative Example 1

[0148] This comparative example provides a water oxidation catalyst, the preparation steps and operating parameters of which are as follows:

[0149] S1: Acid pickling pretreatment of metal foam: nickel foam (2 cm × 2 cm) was ultrasonically washed in 2 M hydrochloric acid for 30 min, and then cleaned with anhydrous ethanol, acetone and deionized water in sequence;

[0150] S2: Preparation of metal salt suspension: nickel nitrate was dissolved in ethanol and ferrous sulfate was dissolved in deionized water in a volume ratio of ethanol to water of 2:1. The mixture was thoroughly mixed to form a metal salt suspension with a metal ion concentration of 200 mol / L.

[0151] S3: In-situ growth of catalyst: soak the pretreated metal foam in a metal salt suspension at 30°C for 24 hours, take it out and rinse it repeatedly with anhydrous ethanol and deionized water for 2-5 times, and then dry it for use.

[0152] In step S2 of this comparative example, the metal ion source required for the growth of the LDH structure is added to enable the catalytic layer to grow rapidly; and the pH value of the system is not adjusted, so there is almost no or only weak etching of the metal substrate surface, and it is impossible to form a transition layer under the effect of interface confinement. Figure 13 As shown, the catalytic layer structure of the comparative catalyst is similar to the catalyst layer obtained in the present application, consisting of a multi-level lamellar structure, but no obvious transition layer is found, and there is a small gap between the catalytic layer and the metal substrate.

[0153] The stability of the obtained catalyst was tested in 1M KOH alkaline aqueous electrolyte (25°C). Figure 14 As shown, at 1000mA cm -2 The potential increased by about 400 mV around 1000 h, indicating that the catalytic activity gradually decreased with time.

[0154] The stability of the obtained catalyst was tested in 1M KOH alkaline seawater electrolyte (25°C). Figure 15 As shown, at 1000mA cm-2 The potential increase is about 1000 mV in the next about 5 h, indicating that the catalyst can hardly operate stably in alkaline seawater.

[0155] Comparative Example 2

[0156] This comparative example provides a water oxidation catalyst. Compared with Example 2, the only difference is that the reaction time in step S3 is 2 h.

[0157] The obtained catalyst was stable in an alkaline seawater electrolyte of 1 M KOH (25 °C, 1000 mA cm -2 ), and operated stably for 3 h, and then the potential increased rapidly. It can be Figure 3 seen that the catalyst growth time of 2 h cannot produce a transition layer. Therefore, although the catalyst has more catalytic lamellar structures, due to the lack of protection of the transition layer, it cannot be applied in alkaline seawater.

[0158] Comparative Example 3

[0159] This comparative example provides a water oxidation catalyst. Compared with Example 1, the only difference is that no organic solvent is added in step S3.

[0160] The obtained catalyst was stable in an alkaline seawater electrolyte of 1 M KOH (25 °C, 1000 mA cm -2 ), and operated stably for about 3 h, and then the potential increased rapidly. The main function of the organic solvent is to induce the generation of nanoparticles. In an aqueous solution system lacking nanoparticles, the obtained catalyst transition layer is not significant (as Figure 16 shown), so it does not have the function of stable catalysis in alkaline seawater.

[0161] Comparative Example 4

[0162] This comparative example provides a water oxidation catalyst. Compared with Example 1, the only difference is that the non-hydrolyzable metal salt NaCl is used.

[0163] It is hardly possible to observe the attachment of catalyst structures on the surface of the obtained catalyst. In an alkaline seawater electrolyte of 1 M KOH (25 °C, 1000 mA cm -2 ), it operates stably for less than 1 h. It shows that non-hydrolyzable metal salts cannot meet the etching of the metal substrate and the generation of catalyst structures, and are difficult to be used for hydrogen production by seawater electrolysis.

[0164] Comparative Example 5

[0165] This comparative example provides a water oxidation catalyst. Compared with Example 1, the only difference is that the step of adjusting the pH is not included in step S1.

[0166] The obtained catalyst has a relatively small lamellar structure with an overall thickness of only 1-2 μm. Although a dense transition layer can be observed, there is a significant peeling phenomenon between the transition layer and the nickel foam substrate, and the transition layer does not closely adhere to the metal substrate. Therefore, the catalyst layer cannot be firmly anchored on the surface of the metal substrate (see Figure 17 ), which may be related to the relatively high pH of the immersion system. The obtained catalyst was tested for stability in an alkaline aqueous electrolyte of 1 M KOH (25 °C). At 1000 mA cm -2 , the potential increase was approximately 300 mV after about 500 h; while in an alkaline seawater electrolyte of 1 M KOH (25 °C, 1000 mA cm -2 ), the potential increased rapidly after stable operation for 5 h. It shows that adjusting the mother liquor to a reasonable pH range is crucial for the etching of the metal substrate and the formation of the catalyst. Without a reasonable weakly acidic mother liquor environment, the formation of the catalyst (morphology, structure, and the degree of adhesion to the substrate) is inhibited to varying degrees, which is not conducive to its stable electrocatalysis in alkaline water and alkaline seawater.

[0167] For the convenience of comparing data, some parameters and test results of the water oxidation catalysts provided in the examples and comparative examples are summarized as follows:

[0168] Table 1

[0169]

[0170]

[0171]

[0172] Note: " / " in the table represents that this test was not carried out.

[0173] From the test data in the above table, it can be seen that the increase in the thickness of the transition layer can significantly improve the stable operation time of the catalyst. This may be because the transition layer can play a certain anti-corrosion function, thus prolonging the service life of the catalyst. From the comparison between the examples and the comparative examples, it can be seen that when the transition layer closely adheres to the surface of the metal substrate, it can firmly anchor the catalyst layer on the surface of the metal substrate and significantly extend the stable operation duration of the catalyst. If the thickness of the transition layer is almost zero or the adhesion between the transition layer and the metal substrate is not tight, the attachment of the catalyst layer on the metal substrate is very unstable, and it cannot play an anti-corrosion function, and it can hardly operate stably in real alkaline seawater.

[0174] Obviously, the above embodiments are merely examples given for clear illustration and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to exhaustively list all implementation manners here. And the obvious changes or modifications derived therefrom still fall within the protection scope of the present invention.

Claims

1. A preparation method of a water oxidation catalyst, characterized in that, It includes the following steps: S1. Prepare a hydrolyzable metal salt solution and adjust the pH of the hydrolyzable metal salt solution to 2.5 - 6.5; S2. Immerse a metal substrate in the hydrolyzable metal salt solution; S3. Add an organic solvent to the system in step S2 to form a heterogeneous system and react for 4 - 72 h.

2. The preparation method of the water oxidation catalyst according to claim 1, wherein In step S1, the concentration of the hydrolyzable metal salt solution is 20 - 400 mmol / L.

3. The preparation method of the water oxidation catalyst according to claim 1, wherein, The hydrolyzable metal salt includes at least one of nickel chloride, nickel sulfate, nickel nitrate, ferric sulfate, ferrous sulfate, ferric chloride, ferrous chloride, vanadium trichloride, vanadyl sulfate, aluminum chloride, indium trichloride, cerium trichloride, cerium sulfate, bismuth trichloride.

4. The preparation method of the water oxidation catalyst according to claim 1, wherein In step S2, the immersion temperature is 5 - 50 °C and the immersion time is 0 - 5 h; Optionally, the immersion time is 1 - 2 h.

5. The preparation method of the water oxidation catalyst according to claim 1, wherein, In step S3, the organic solvent includes at least one of acetone, methanol, ethanol, propanol, isopropanol, allyl alcohol, butanol, 2 - butanol, tert - butanol, ethylene glycol, propylene glycol, tetrahydrofuran, dimethyl ether, ethyl acetate, N,N - dimethylformamide; and / or, the volume ratio range of the organic solvent to the hydrolyzable salt solution is (0.5 - 9):1; and / or, the metal substrate includes at least one of nickel foam, iron foam, nickel - iron foam, nickel mesh, iron mesh, nickel - iron mesh, nickel felt, iron felt, nickel - iron felt, nickel electrode plate, iron electrode plate.

6. The preparation method of the water oxidation catalyst according to any one of claims 1-5, characterized in that, The preferred reaction temperature in step S3 is 20 - 60 °C and the preferred reaction time is 10 - 72 h.

7. A water oxidation catalyst prepared by the preparation method according to any one of claims 1 - 6.

8. An application of the water oxidation catalyst according to claim 7 in hydrogen production by electrolysis of water.

9. The application according to claim 8, wherein The current density of hydrogen production by electrolyzing water is 500 - 4000 mA cm -2 .

10. The application according to claim 8 or 9, characterized in that, It is applicable to the electrolysis of an alkaline water system; Optionally, the alkaline water system includes alkaline water, alkaline seawater or alkaline mineral water.

11. A water oxidation catalyst, characterized in that, The water oxidation catalyst includes a metal substrate and a catalyst layer. There is also a transition layer attached to the surface of the metal substrate between the metal substrate and the catalyst layer. The catalyst layer and the transition layer have the same layered structure in composition, and the morphology of the transition layer is denser than that of the catalyst layer.

12. The water oxidation catalyst according to claim 11, wherein, The thickness of the transition layer is 0.2 - 4 μm; Optionally, the thickness of the transition layer is 0.2 - 3 μm; Optionally, the thickness of the transition layer is 1 - 3 μm; Optionally, the thickness of the transition layer is 2 - 3 μm.

13. The water oxidation catalyst according to claim 11 or 12, characterized in that, The metal substrate includes at least one of nickel foam, iron foam, nickel - iron foam, nickel mesh, iron mesh, nickel - iron mesh, nickel felt, iron felt, nickel - iron felt, nickel electrode plate, iron electrode plate; and / or, the compositions of both the catalyst layer and the transition layer are layered double - metal compounds.

14. The water oxidation catalyst according to claim 13, wherein The metal substrate is nickel foam or iron foam; The compositions of the catalyst layer and the transition layer are at least one of nickel - iron layered double - metal compounds, nickel - vanadium layered double - metal compounds, nickel - aluminum layered double - metal compounds, nickel - cerium layered double - metal compounds.

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