A bifunctional electrocatalyst for electrolytic water, its preparation method and application

Abstract

The present invention provides a bifunctional electrocatalytic water splitting catalyst, its preparation method and application. The 3D microflower Co3O4 / C@NiFeP catalyst comprises NiFeP nanosheet materials wrapping Co3O4 / C materials. The preparation method includes hydrothermally reacting Co3O4 / C, nickel salt, iron salt, urea and ammonium fluoride to grow hydroxides of Ni and Fe (NiFeLDH) on the surface of Co3O4 / C materials to obtain 3D flower-like Co3O4 / C@NiFeLDH; through low-temperature phosphidation, a 3D microflower Co3O4 / C@NiFeP metal oxide / phosphide heterostructure composite material is obtained. The electrocatalytic catalyst described in the present invention is an assembly of nanosheets with a large specific surface area, and has good oxygen evolution and hydrogen evolution catalytic activities. The preparation method of the present invention has the advantages of simple operation, low production cost and easy operation, etc.

Description

Technical Field

[0001] The present invention belongs to the field of catalyst preparation, and specifically relates to a bifunctional electrolytic water catalyst, a preparation method thereof, and an application thereof, specifically a Co3O4 / C@NiFeP bifunctional electrolytic water catalyst. Background Art

[0002] In recent years, with the rapid development of society, resource shortage and environmental pollution have become global crises. Therefore, for the sustainable development of mankind, developing green and clean renewable energy has become an urgent problem to be solved. Among them, hydrogen energy, as the most ideal alternative energy, has attracted people's attention. Hydrogen production by electrolyzing water is one of the most promising methods for large-scale production of renewable hydrogen energy. Electrolyzing water generates oxygen and hydrogen through the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode. However, due to the sluggish kinetic processes of the anode reaction and the cathode reaction, the large-scale application of the electrolytic water system is seriously hindered. In order to improve the catalytic reaction rate, it is necessary to develop efficient catalysts to accelerate the oxygen evolution reaction and the hydrogen evolution reaction to improve the energy conversion efficiency. Traditional oxygen evolution reaction catalysts are mainly noble metals ruthenium, iridium and their oxides, and hydrogen evolution reaction catalysts are mainly platinum-based noble metals. Due to their high prices, scarce sources, poor stability and other disadvantages, their large-scale commercial applications are limited. Therefore, developing non-noble metal oxygen evolution reaction and hydrogen evolution reaction bifunctional catalysts has important research value.

[0003] It has been reported in the literature that transition metal carbides, nitrides, sulfides, phosphides, etc. have good catalytic effects on both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Among them, transition metal oxides / phosphides derived from metal-organic frameworks (MOFs) have excellent catalytic activities. Due to the characteristics of ordered pore structures, high electrical conductivity and specific surface area, and stable dispersion of metals of MOF-derived carbon materials, they have been widely used in the catalytic field.

[0004] In order to obtain better performances of the oxygen evolution reaction and the hydrogen evolution reaction, one strategy is to regulate the morphology and microstructure of the catalyst material. Three-dimensional (3D) nanoflowers are a promising structure that can provide a large specific surface area and active sites.

[0005] Based on the above, using MOFs as the sacrificial template for metal oxide / phosphide materials, making them have rich active sites and fast electron transport characteristics, is of great significance for the oxygen evolution reaction and the hydrogen evolution reaction in water decomposition. However, common MOF-derived catalyst materials only have single oxygen evolution or hydrogen evolution catalytic reaction activities. Designing MOF-derived materials with bifunctional catalytic oxygen evolution and hydrogen evolution reaction activities has important application values. Summary of the Invention

[0006] Technical problem: To overcome the deficiencies in the prior art and address the defects in the existing preparation of MOFs-derived transition metal oxide / phosphide composites, the present invention provides a bimetallic phosphide NiFeP-rich electrocatalytic material with a metal oxide Co3O4 heterostructure wrapped therein and a preparation method therefor. Specifically, it is a MOFs-derived 3D microflower electrochemical catalyst and its preparation method and application. This method uses bio-renewable gallic acid as an organic ligand, avoiding problems such as the high cost and limited sources of traditional organic ligands. Using a cobalt-gallic acid metal-organic framework material as a metal oxide precursor, a metal oxide / phosphide composite is obtained through a hydrothermal reaction and low-temperature phosphidation treatment.

[0007] Technical solution: To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] The first object of the present invention is to provide a highly efficient MOFs-derived 3D microflower transition metal oxide / phosphide bifunctional catalyst. Specifically, it is a bifunctional water electrolysis catalyst. This catalyst is Co3O4 / C@NiFeP, which is a metal oxide / phosphide heterostructure composite material doped with cobalt Co, nickel Ni, iron Fe, oxygen O, and phosphorus P. This catalyst is a composite material of NiFeP nanosheets wrapping Co3O4 / C nanoparticles and has a three-dimensional microflower structure.

[0009] Furthermore, the transition metal oxide / phosphide composite material is a three-dimensional microflower material doped with Co, Ni, Fe, N, and P.

[0010] Furthermore, this catalyst has a large specific surface area, and the specific surface area is 20 - 30 m 2 g -1 .

[0011] Furthermore, the thickness of the nanosheets of this catalyst is 20 - 40 nm.

[0012] The second object of the present invention is to provide a preparation method for a highly efficient MOFs-derived 3D microflower transition metal oxide / phosphide bifunctional catalyst. Specifically, it is a preparation method for a metal oxide / phosphide composite material used for electrocatalytic oxygen evolution reaction and hydrogen evolution reaction. Using a composite material formed by metal-organic framework Co-gallate and phytic acid (PA) as a precursor, and carrying out a solvothermal reaction with nickel nitrate, ferrous sulfate, urea, and ammonium fluoride, and then low-temperature phosphidation can obtain a MOFs-derived 3D microflower transition metal oxide / phosphide composite material, that is, Co3O4 / C@NiFeP. The specific steps are as follows:

[0013] (1) Preparation of Co3O4 / C@NiFeLDH material: Nickel salt, iron salt, urea, and ammonium fluoride are dissolved in water, Co3O4 / C is added, and after mixing evenly, hydrothermal reaction is carried out. After washing and drying, Co3O4 / C@NiFeLDH is obtained;

[0014] (2) Preparation of Co3O4 / C@NiFeP material: The Co3O4 / C@NiFeLDH is subjected to low-temperature phosphidation at 300 - 400 °C to obtain a metal oxide-phosphide Co3O4 / C@NiFeP heterostructure composite material.

[0015] Furthermore, the preparation method of Co3O4 / C in step (1) includes the following steps:

[0016] (1.1) Preparation of Co-gallate metal-organic framework: Cobalt chloride and gallic acid are dissolved in potassium hydroxide solution, and reflux condensation is carried out at 80 - 90 °C for 23 - 25 h. After washing with water and ethanol and drying at room temperature for 18 - 24 h, Co-gallate metal-organic framework is prepared;

[0017] (1.2) Preparation of Co3O4 / C material: The Co-gallate metal-organic framework is dispersed in absolute ethanol, phytic acid ethanol solution is added, and mechanical stirring is carried out for 15 - 20 min. After washing with ethanol and drying at room temperature, it is then placed in a muffle furnace for calcination and naturally cooled to room temperature to obtain Co3O4 / C material.

[0018] Furthermore, in step (1.1), the molar ratio of cobalt chloride to gallic acid is 1∶1 - 1∶2.

[0019] Furthermore, in step (1.1), after cobalt chloride and gallic acid are dissolved in potassium hydroxide solution, the concentration of cobalt chloride is 0.1 - 0.3 mol / L, and the concentration of gallic acid is 0.3 - 0.5 mol / L.

[0020] Furthermore, in step (1.1), the concentration of potassium hydroxide is 0.1 - 0.4 mol / L.

[0021] Even further, in step (1.1), the concentration of potassium hydroxide is 0.16 mol / L.

[0022] Furthermore, in step (1.1), the reaction temperature of the reflux condensation is 80 °C, and the reaction time is 24 h.

[0023] Furthermore, in step (1.2), the concentration of the phytic acid ethanol solution is 0.1 - 0.2 mol / L.

[0024] Further, in step (1.2), for every 0.2 g of Co-gallate metal-organic framework, 5 - 20 ml of phytic acid ethanol solution is added correspondingly.

[0025] Furthermore, in step (1.2), for every 0.2 g of Co-gallate metal-organic framework, 5 - 10 ml of phytic acid ethanol solution is added correspondingly.

[0026] Further, in step (1.2), the heating rate of the muffle furnace is 3 - 5 °C / min, the calcination temperature is 400 - 500 °C, and the calcination time is 2 - 3 h.

[0027] Further, in step (1), the nickel salt and iron salt include any one or a combination of more than one of nitrate, sulfate, or acetate.

[0028] Further, in step (1), the molar ratio of the nickel salt, iron salt, urea, and ammonium fluoride is 1∶(0.5 - 2)∶(5 - 20)∶(3 - 5).

[0029] Further, in step (1), based on 1 mmol of nickel salt, the corresponding mass of Co3O4 / C added is 20 - 40 mg.

[0030] Further, in step (1), the mixing evenly includes ultrasonic dispersion, and the ultrasonic dispersion time is 0.5 - 1 h.

[0031] Further, in step (1), the hydrothermal reaction temperature is 110 - 130 °C, and the hydrothermal reaction time is 10 - 12 h.

[0032] Further, in step (1), the washing is carried out with deionized water and ethanol.

[0033] Further, in step (1), the drying is carried out by vacuum drying at 55 - 60 °C for 10 - 12 h.

[0034] Further, in step (2), the low-temperature phosphating means placing the Co3O4 / C@NiFeLDH downstream of the porcelain boat and sodium hypophosphite upstream of the porcelain boat, putting the porcelain boat into a tube furnace and annealing at 3 - 5 °C / min -1 heating rate in a nitrogen atmosphere at 300 - 400 °C for 1 - 5 h.

[0035] Further, in step (2), the mass ratio of sodium hypophosphite to Co3O4 / C@NiFeLDH is 5∶1 - 20∶1.

[0036] Further, the phosphating temperature of the low-temperature phosphating is 350 °C.

[0037] The third objective of the present invention is to provide an application of a bio-based MOFs-derived 3D microflower transition metal oxide / phosphide bifunctional catalyst. The bifunctional catalyst is used as an electrolytic water catalyst for electrolytic water catalytic reactions, including catalyzing the oxygen evolution reaction and hydrogen evolution reaction of electrolytic water. This catalyst can efficiently catalyze the oxygen evolution reaction and hydrogen evolution reaction, and has a lower overpotential and faster reaction kinetics compared with ordinary catalysts, being closer to or even superior to commercial noble metal catalysts.

[0038] Beneficial effects: The present invention uses cobalt-gallic acid metal-organic framework as a precursor, and obtains a 3D microflower Co3O4 / C@NiFeP catalyst after a subsequent solvothermal reaction and low-temperature phosphidation treatment. The advantages of this method are:

[0039] (1) The raw materials required for synthesis are cheap and easily available;

[0040] (2) The process flow is simple and controllable;

[0041] (3) The prepared composite material has unique 3D structure, micron size, and large pore structure. Its unique flower-like structure is composed of nanosheets and has large channels, which is beneficial to the diffusion of electrolytes. Description of the drawings

[0042] Figure 1 XRD patterns of the composite materials obtained in Example 1, Comparative Example 2, and Comparative Example 3;

[0043] Figure 2 (a-b) Scanning electron micrographs of 3D microflower Co3O4 / C@NiFeP obtained in Example 1;

[0044] Figure 3 (a-b) Scanning electron micrographs of Co3O4 / C obtained in Comparative Example 2;

[0045] Figure 4 (a-b) Scanning electron micrographs of 3D microflower NiFeP obtained in Comparative Example 3;

[0046] Figure 5 Voltammogram (a) and Tafel slope curve (b) of the materials obtained in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 for the anodic oxygen evolution reaction of electrocatalytic water decomposition;

[0047] Figure 6 Voltammogram (a) and Tafel slope curve (b) of the materials obtained in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 for the cathodic hydrogen evolution reaction of electrocatalytic water decomposition;

[0048] Figure 7The voltammetry characteristic curve (a) and Tafel slope curve (b) of the materials obtained in Example 1, Example 2, and Example 3 for the oxygen evolution reaction at the anode of electrocatalytic water splitting;

[0049] Figure 8 The voltammetry characteristic curve (a) and Tafel slope curve (b) of the materials obtained in Example 1, Example 2, and Example 3 for the hydrogen evolution reaction at the cathode of electrocatalytic water splitting. Detailed implementation manners

[0050] The present invention provides a preparation method and application of a MOFs-derived 3D microflower Co3O4 / C@NiFeP electrocatalyst, that is, a transition metal oxide / phosphide composite material and its preparation method and application, belonging to the technical fields of energy and material preparation. The present invention uses Co-gallate as a sacrificial template, prepares metal oxides by calcination in an air atmosphere, introduces nickel and iron transition metals during the solvothermal process, and successfully prepares Co3O4 / C@NiFeP by low-temperature phosphidation. The preparation method of the present invention has the advantages of simple and easy preparation process and green and environmental protection process. Using the composite material formed by metal-organic framework Co-gallate and phytic acid (PA) as a precursor, and carrying out a solvothermal reaction with nickel salt, iron salt, urea, and ammonium fluoride, and low-temperature phosphidation can obtain a MOFs-derived 3D microflower transition metal oxide / phosphide composite material, that is, Co3O4 / C@NiFeP, which specifically includes the following steps:

[0051] 1) Dissolve nickel nitrate, ferrous sulfate, urea, and ammonium fluoride in deionized water, add Co3O4 / C to the solution, ultrasonically disperse evenly, carry out a hydrothermal reaction at 110 - 130 °C for 10 - 12 h, wash with deionized water and ethanol, and vacuum dry at 55 - 60 °C for 10 - 12 h to obtain a MOFs-derived 3D microflower Co3O4 / C@NiFeLDH material; among them, the preparation method of Co3O4 / C is as follows:

[0052] 1.1) Add cobalt chloride and gallic acid to a potassium hydroxide solution, stir to dissolve, then carry out a reflux condensation at 80 - 90 °C for 23 - 25 h, wash with deionized water and ethanol, and dry at room temperature for 18 - 24 h;

[0053] 1.2) Disperse the Co-gallate in step 1.1) in absolute ethanol, then add a phytic acid ethanol solution, mechanically stir for 15 - 20 min, wash with ethanol, dry at room temperature, then put it into a muffle furnace for calcination and naturally cool to room temperature to obtain a Co3O4 / C material;

[0054] 2) Place the Co3O4 / C@NiFeLDH obtained in step 1) downstream of the porcelain boat, and sodium hypophosphite upstream of the porcelain boat. Place the porcelain boat in a tubular furnace and under a nitrogen atmosphere at 3 - 5 °C min-1 The heating rate was annealed at 300 - 400 °C for 2 h to obtain the 3D microflower Co3O4 / C@NiFeP composite material.

[0055] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention. All raw materials used in the embodiments are obtained through commercial channels.

[0056] Example 1: Co3O4 / C@NiFeP-350

[0057] (1) Weigh 0.145 g of Ni(NO3)2·6H2O (0.5 mmol), 0.139 g of FeSO4·7H2O (0.5 mmol), 0.3 g of urea (5 mmol) and 0.0741 g of NH4F (2 mmol), add them to 30 mL of deionized water, stir for 30 min. After weighing 30 mg of Co3O4 / C in step (2) and ultrasonicating it evenly, then transfer the above solution to a 50 mL reaction kettle, heat it to 120 °C and keep it warm for 12 h. After cooling to room temperature, wash the centrifuged precipitate twice with deionized water and once with absolute ethanol, and dry it overnight in a vacuum at 60 °C to obtain the Co3O4 / C@NiFeLDH composite material. Among them, the preparation method of Co3O4 / C is as follows:

[0058] (1.1) Weigh 4.76 g (20 mmol) of cobalt chloride (CoCl2·6H2O) and 6.8 g (40 mmol) of gallic acid (C7H6O5), dissolve them in 100 mL of potassium hydroxide (0.16 mol / L) aqueous solution, mechanically stir for 30 minutes until dissolved, then transfer to an oil bath, continuously stir at 80 °C, reflux for 24 h, centrifuge, wash 3 times with deionized water and absolute ethanol respectively, and air dry at room temperature for 20 h to obtain the cobalt-gallate (Co-gallate) metal-organic framework.

[0059] (1.2) Weigh 0.2 g of the Co-gallate metal-organic framework in step (1.1) and dissolve it in 30 mL of absolute ethanol. Then add 10 mL of phytic acid ethanol solution (0.1 mol / L), stir the mixed solution under magnetic stirring for 15 min, collect the product by centrifugation, wash it three times with ethanol, and then fully air dry at room temperature to obtain Co-gallate / PA. Weigh Co-gallate / PA (0.2 g) and then heat it to 450 °C at a heating rate of 3 °C / min in a muffle furnace and keep it warm for 2 h to obtain the product, denoted as Co3O4 / C.

[0060] (2) Weigh 30 mg of Co3O4 / C@NiFeLDH obtained in step (1) and place it downstream of the porcelain boat, and weigh 0.6 g of sodium hypophosphite and place it upstream of the porcelain boat. Then put the porcelain boat into a tube furnace and anneal it at 350 °C for 2 h at a heating rate of 3 °C / min under a nitrogen atmosphere to obtain a metal phosphide Co3O4 / C@NiFeP composite material. -1

[0061] Figure 1 (c) The XRD pattern obtained in Example 1 shows that in the Co3O4 / C@NiFeP sample, the characteristic peaks at 40.7°, 44.5°, 47.3°, and 54.2° correspond to the (111), (201), (210), and (300) crystal planes of Ni2P (JCPDS No. 03-0953), indicating the presence of Ni2P in the sample. The characteristic peaks at 17.5°, 30.4°, 31.3°, 35.3°, 40.2°, 44.2°, and 47.2° correspond to the characteristic peaks of Fe2P. Among them, 40.2°, 44.2°, and 47.4° correspond to the (111), (201), and (210) crystal planes of the Fe2P standard card (JCPDS No. 27-1171).

[0062] Figure 2 The SEM image can clearly show the 3D microflower structure. Each flower-like structure is composed of many nanosheets and large pores can be seen, which is beneficial to the diffusion of the electrolyte.

[0063] In this example, the catalytic activities of the obtained catalyst for efficient electrocatalytic water splitting in the oxygen evolution reaction and hydrogen evolution reaction were tested. The standard three-electrode system was used as the test condition, with a 3 mm glassy carbon electrode as the working electrode, a saturated Ag / AgCl electrode as the reference electrode, and a platinum wire as the counter electrode. A 1 mol / L -1 KOH solution was used as the electrolyte, and the test instrument was a Shanghai Chenhua 660E electrochemical workstation. 5 mg of the sample catalyst was dispersed in a mixed solution of 500 μL of absolute ethanol, 490 μL of deionized water, and 20 μL of Nafion (5%). 6 μL of the prepared catalyst dispersion was evenly dropped on the surface of the polished glassy carbon electrode in two times. At room temperature of 25 °C, its cyclic voltammetry curve and Tafel slope were tested. The cyclic voltammetry curves of the obtained catalyst for efficient electrocatalytic water splitting in the oxygen evolution reaction and hydrogen evolution reaction are as shown in Figure 5 (a) and as shown in Figure 6 (a), the solid line d. The Tafel slope is as shown in Figure 5 (b) and as shown in Figure 6 (b), the solid line d. For the specific result analysis, please refer to the subsequent analysis of the example results. The overpotential (η) is used to evaluate the overall activity of the target electrocatalyst, usually at a specified current density of 10 mA / cm 2The corresponding overpotential is used to compare the electrocatalytic activities of different catalysts. The overpotential (η / mV) at a current density of 10 mA cm -2 can be calculated from the cyclic voltammogram. By converting the current density to the logarithm of 10 as the x-axis and the overpotential as the y-axis to obtain the polarization curve, we can get the Tafel plot of the target catalyst, which shows the dependence of the steady-state current density (j) on the overpotential (η). Fitting through the Tafel equation: η = a + b logj, where b is the Tafel slope and a is a constant determined by the exchange current density j0 and the Tafel slope. Therefore, according to the Tafel equation, we can extract two important kinetic parameters. A so-called Tafel slope, which is usually related to the electrochemistry reaction mechanism, represents the rate at which the current density increases as the overpotential increases. That is, a smaller Tafel slope indicates that a much larger current density is achieved with a much smaller change in overpotential, showing fast electrocatalytic reaction kinetics. Therefore, the smaller the overpotential, the smaller the Tafel slope, and the higher the electrocatalytic activity.

[0064] Comparative Example 1: Blank group

[0065] Using commercially available ruthenium oxide as the electrode catalytic material, the oxygen evolution reaction of water electrolysis is carried out. The cyclic voltammogram is as shown by the solid line a in Figure 5 (a), and the Tafel slope is as shown by the solid line a in Figure 5 (b). For the specific result analysis, please refer to the result analysis of the subsequent examples.

[0066] Using commercially available platinum-carbon catalyst as the electrode catalytic material, the hydrogen evolution reaction of water electrolysis is carried out. The cyclic voltammogram is as shown by the solid line a in Figure 6 (a), and the Tafel slope is as shown by the solid line a in Figure 6 (b). For the specific result analysis, please refer to the result analysis of the subsequent examples.

[0067] Comparative Example 2: Co3O4 / C

[0068] Synthesized by the same method as in steps (1.1) and (1.2) of Example 1 to obtain Co3O4 / C. The XRD test results are as shown in Figure 1 (a). The cyclic voltammograms for the oxygen evolution reaction and hydrogen evolution reaction of the obtained electrocatalytic water splitting are respectively as shown by the solid line b in Figure 5 (a) and 6(a), and the Tafel slopes are as shown by the solid line b in Figure 5 (b) and 6(b). For the specific result analysis, please refer to the result analysis of the subsequent examples.

[0069] Comparative Example 3: NiFeP

[0070] (1) Weigh 0.145 g of Ni(NO3)2·6H2O (0.5 mmol), 0.139 g of FeSO4·7H2O (0.5 mmol), 0.3 g of urea (5 mmol) and 0.0741 g of NH4F (2 mmol), add them to 30 mL of deionized water, stir for 30 min, transfer the above solution to a 50 mL reaction kettle, heat to 120 °C and keep warm for 12 h. After cooling to room temperature, wash the precipitate collected by centrifugation twice with deionized water and once with absolute ethanol, and dry it overnight in vacuum at 60 °C to obtain NiFe LDH.

[0071] (2) Weigh 30 mg of Co3O4 / C@NiFe LDH in step (1) and place it downstream of the porcelain boat, and 0.6 g of sodium hypophosphite upstream of the porcelain boat. Put the porcelain boat into a tube furnace and anneal it at 350 °C for 2 h at a heating rate of 3 °C / min in a nitrogen atmosphere to obtain the metal phosphide NiFeP composite material. The XRD test results are as shown in -1 (b). The cyclic voltammograms of the obtained oxygen evolution reaction and hydrogen evolution reaction for electrocatalytic water splitting are respectively as shown in Figure 1 (a), the solid line c in 6(a), and the Tafel slope is as shown in Figure 5 (b), the solid line c in 6(b). For the specific result analysis, please refer to the result analysis of the subsequent examples. Figure 5 (b), the solid line c in 6(b). For the specific result analysis, please refer to the result analysis of the subsequent examples.

[0072] Example 2: Co3O4 / C@NiFeP-300

[0073] Change the calcination temperature in step (4) of Example 1 to 300 °C, and the rest is the same as in Example 1. Obtain Co3O4 / C@NiFeP-300. The cyclic voltammograms of the obtained oxygen evolution reaction and hydrogen evolution reaction for electrocatalytic water splitting are respectively as shown in Figure 7 (a), the solid line a in 8(a). For the specific result analysis, please refer to the result analysis of the subsequent examples.

[0074] Example 3: Co3O4 / C@NiFeP-400

[0075] Change the calcination temperature in step (4) of Example 1 to 400 °C, and the rest is the same as in Example 1. Obtain Co3O4 / C@NiFeP-400. The cyclic voltammograms of the obtained oxygen evolution reaction and hydrogen evolution reaction for electrocatalytic water splitting are respectively as shown in Figure 7 (a), the solid line c in 8(a). For the specific result analysis, please refer to the result analysis of the subsequent examples.

[0076] Result analysis of examples:

[0077] First, in Example 1, an electrocatalyst of a MOFs-derived 3D microflower transition metal oxide / phosphide composite material was obtained. The size of the generated 3D microflowers was about 5 μm, and the thickness of the nanosheets was 20 - 40 nm, as shown in Table 1 below.

[0078] Table 1 Test data of the examples

[0079] Example 1 Example 2 Example 3 3D Micron Flower Size (um) 4-6 4-6 4-6 Thickness (nm) 20-40 20-40 20-40 <![CDATA[Specific surface area (m 2 g -1 )]]> 25.4 20.2 22.6

[0080] Secondly, according to Figures 5 - 8 the data, the overpotential data of the examples and comparative examples are statistically shown in Table 2 below.

[0081] Table 2 Test data of the examples and comparative examples

[0082] Overpotential / mV Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Oxygen Evolution Reaction Electrocatalysis 281 296 299 310 372 314 Hydrogen Evolution Reaction Electrocatalysis 182 286 350 58 390 304

[0083] Regarding the electrocatalytic performance of the oxygen evolution reaction, as Figure 5 (a) shows, compared with the original Co-gallate / PA carbonization product of Comparative Example 2, the nickel-iron bimetallic phosphide of Comparative Example 3, and the commercially available ruthenium oxide of the commercial noble metal catalyst of Comparative Example 1, Example 1 has better electrocatalytic performance for the oxygen evolution reaction. The overpotential of the oxygen evolution reaction electrocatalytic performance at 10 mA cm -2 is 281 mV (solid line d), and the overpotentials of Comparative Example 1 (solid line a), Comparative Example 2 (solid line b), and Comparative Example 3 (solid line c) are 310 mV, 372 mV, and 314 mV respectively. As Figure 5 (b) shows, the Tafel slope of Example 1 is 41.7 mV dec - -1 (solid line d), and the Tafel slopes of Comparative Example 1 (solid line a), Comparative Example 2 (solid line b), and Comparative Example 3 (solid line c) are 79.2 mV dec -1 , 82.6 mV dec -1 , and 68.4 mV dec -1 .

[0084] According to the comparison of Examples 1, 2, and 3, it can be seen that when different low-temperature phosphating is used, it has a great influence on the performance of the finally obtained products. The polarization curves of the materials obtained in Examples 1, 2, and 3 as the oxygen evolution reaction of the electrocatalytic water decomposition anode are shown in Figure 7 (a). The overpotentials at 10 mA cm -2 are 281 mV, 296 mV, and 299 mV respectively, and the Tafel slopes are shown in Figure 7 (b), which are 58.1 mV dec -1 , 41.7 mV dec -1 , and 62.4 mV dec -1, both are lower than the overpotential and Tafel slope of the comparative example.

[0085] Regarding the electrocatalytic performance of the hydrogen evolution reaction, such as Figure 6 (a) shows that the overpotential of Example 1 at 10 mA cm -2 is 182 mV (solid line d), and the overpotentials of Comparative Example 1 (solid line a), Comparative Example 2 (solid line b), and Comparative Example 3 (solid line c) are 58 mV, 390 mV, and 304 mV respectively. As Figure 6 (b) shows, the Tafel slope of Example 1 is 84 mV dec -1 (solid line d), and the Tafel slopes of Comparative Example 1 (solid line a), Comparative Example 2 (solid line b), and Comparative Example 3 (solid line c) are 128.8 mV dec -1 , 87 mV dec -1 , 125.1 mV dec -1 . It can be seen that the target product in Example 1 prepared above has a low overpotential under the same environment, exhibits faster catalytic reaction kinetics, and good catalytic activity.

[0086] As Figure 8 (a) shows in the polarization curve of the electrocatalytic water splitting cathode reaction, the overpotentials of the hydrogen evolution reaction at 10 mA cm -2 are 182 mV, 286 mV, and 350 mV respectively, and the Tafel slopes are as Figure 8 (b) shows, which are 105.6 mV dec -1 , 87.0 mV dec -1 , and 124.4 mV dec -1 . It can be seen that the target product in Example 1 prepared has a low overpotential under the same environment and good catalytic activity.

[0087] The above are only the preferred embodiments of the present invention. It should be noted that: for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A preparation method of a bifunctional electrolyzed water catalyst, characterized in that, It includes the following steps: (1) Prepare Co3O4 / C@NiFeLDH material: Dissolve nickel salt, iron salt, urea, and ammonium fluoride in water, add Co3O4 / C, mix evenly, carry out hydrothermal reaction, wash, and dry to obtain Co3O4 / C@NiFeLDH; (2) Prepare Co3O4 / C@NiFeP material: Carry out low-temperature phosphating of the Co3O4 / C@NiFeLDH at 300-400 °C to obtain a metal oxide-phosphide Co3O4 / C@NiFeP heterostructure composite material; In step (1), the preparation method of the Co3O4 / C includes the following steps: (1.1) Prepare Co-gallate metal-organic framework: Dissolve cobalt chloride and gallic acid in potassium hydroxide solution, carry out condensation reflux at 80-90 °C for 23-25 h, wash with water and ethanol, and dry at room temperature for 18-24 h to obtain Co-gallate metal-organic framework; (1.2) Prepare Co3O4 / C material: Disperse the Co-gallate metal-organic framework in absolute ethanol, add phytic acid ethanol solution, mechanically stir for 15-20 min, wash with ethanol, dry at room temperature, then put it into a muffle furnace for calcination and naturally cool to room temperature to obtain Co3O4 / C material.

2. The preparation method according to claim 1, wherein In step (1.1), the molar ratio of cobalt chloride to gallic acid is 1:1-1:

2. After cobalt chloride and gallic acid are dissolved in potassium hydroxide solution, the concentration of cobalt chloride is 0.1-0.3 mol / L, and the concentration of gallic acid is 0.3-0.5 mol / L.

3. The preparation method according to claim 1, wherein In step (1.1), the concentration of potassium hydroxide is 0.1-0.4 mol / L.

4. The preparation method according to claim 1, wherein, In step (1.1), the reaction temperature of the condensation reflux is 80 °C, and the reaction time is 24 h.

5. The preparation method according to claim 1, characterized in that, In step (1.2), the concentration of the phytic acid ethanol solution is 0.1-0.2 mol / L, and 5-20 ml of phytic acid ethanol solution is added for every 0.2 g of Co-gallate metal-organic framework.

6. The preparation method according to claim 1, characterized in that, In step (1.2), the heating rate of the muffle furnace is 3-5 °C / min, the calcination temperature is 400-500 °C, and the calcination time is 2-3 h.

7. The preparation method according to claim 1, characterized in that In step (1), the nickel salt and iron salt include any one or more combinations of nitrates, sulfates, or acetates.

8. The preparation method according to claim 7, characterized in that, The molar ratio of nickel salt, iron salt, urea, and ammonium fluoride is 1:(0.5-2):(5-20):(3-5).

9. The preparation method according to claim 1, wherein In step (1), based on 1 mmol of nickel salt, the corresponding mass of Co3O4 / C added is 20-40 mg.

10. The preparation method according to claim 1, wherein, In step (1), the mixing evenly includes ultrasonic dispersion evenly, and the ultrasonic dispersion time is 0.5-1 h.

11. The preparation method according to claim 1, characterized in that, In step (1), the temperature of the hydrothermal reaction is 110-130 °C, and the hydrothermal time is 10-12 h.

12. According to the preparation method described in claim 1, characterized in that, In step (1), the washing is with deionized water and ethanol.

13. The preparation method according to claim 1, characterized in that, In step (1), the drying is vacuum drying at 55-60 °C for 10-12 h.

14. The preparation method according to claim 1, wherein In step (2), the low-temperature phosphating means placing the Co3O4 / C@NiFeLDH downstream of the porcelain boat and sodium hypophosphite upstream of the porcelain boat, putting the porcelain boat into a tube furnace and annealing it at 300-400 °C for 1-5 h at a heating rate of 3-5 °C·min -1 .

15. The preparation method according to claim 14, characterized in that, In step (2), the mass ratio of sodium hypophosphite to Co3O4 / C@NiFeLDH is 5:1 to 20:1.

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