Carbon-coated cobalt-based catalyst, preparation method thereof and application of carbon-coated cobalt-based catalyst in electrolyzed water

Abstract

The invention discloses a carbon-coated cobalt-based catalyst, a preparation method thereof and application of the carbon-coated cobalt-based catalyst in electrolyzed water. The preparation method of the carbon-coated cobalt-based catalyst comprises the following steps: S1, immersing foamed nickel into a solution composed of a molybdenum source, a cobalt source, urea, a fluorine source and water, and heating for hydrothermal reaction to obtain a foamed nickel supported Mo-Co LDH precursor; s2, carrying out annealing treatment on a product obtained in the step S1 under the protection of inert gas to obtain a Mo-Co LDH nanowire array supported by foamed nickel; s3, adding a product obtained in the step S2 into a 2-methylimidazole solution, and carrying out soaking treatment so as to load a metal organic framework material ZIF-67 on the Mo-Co LDH nanowire array supported by the foamed nickel; and S4, pyrolyzing the product obtained in the step S3 and a phosphorus source under the protection of inert gas to form the foamed nickel supported carbon-coated molybdenum-doped cobalt phosphide. The catalyst has excellent hydrogen evolution activity and stability, and the preparation method is environment-friendly, safe, low in energy consumption and suitable for large-scale preparation.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrocatalytic hydrogen production catalysts, and particularly relates to a carbon-coated cobalt-based catalyst, a preparation method thereof, and an application in electrolyzed water. Background Art

[0002] Electrolyzed water technology is considered an environmentally friendly alternative method, which can reduce the dependence on traditional fossil fuels and alleviate environmental problems. Large-scale industrial water electrolysis in alkaline media requires electrocatalysts that can drive high current densities for a long time. Nanomaterials based on platinum and iron / ruthenium oxides have always been regarded as benchmark materials for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) due to their favorable d-band centers, nearly zero hydrogen adsorption free energy, and remarkable electronic conductivity. However, their poor stability, high cost, and natural scarcity have hindered their large-scale application.

[0003] As is well known, platinum-based materials are the most active electrocatalysts for HER, but their high cost and scarcity have greatly hindered their large-scale application. Therefore, developing highly abundant and low-cost efficient HER electrocatalysts remains urgent and challenging. Currently, domestic research mainly focuses on non-noble metal catalysts, especially cobalt-based catalysts, which have similar empty d orbitals to noble metal catalysts, and are inexpensive and abundant in reserves, making them ideal substitutes for noble metal catalysts. In particular, cobalt phosphide with unique electronic properties, high electrocatalytic activity, and anti-corrosion characteristics has attracted a large amount of research attention and is considered an efficient and low-cost HER catalyst.

[0004] Cobalt-based electrocatalysts mainly include oxides, sulfides, hydroxides, carbides, and selenides, etc. The structure and composition of these substances have a significant impact on the electrocatalytic performance of the materials. However, existing cobalt-based catalysts still require relatively high hydrogen evolution and oxygen evolution overpotentials to reach industrial current densities of 500 and 1000 mA cm -2 respectively, and the catalytic activity needs to be further improved.

[0005] Incorporating electron-rich and electron-deficient heteroatoms with different atomic radii and electronegativities into the lattice of cobalt phosphide will cause subtle lattice distortion and redistribution of electron density, resulting in precise changes in the electronic structure, the d-band center approaching the Fermi level, favorable hydrogen adsorption Gibbs free energy, and enhanced intrinsic activity. At the same time, optimizing the electronic structure improves the adsorption energy of reaction intermediates, thereby enhancing the catalytic activity of the material. How to dope cobalt phosphide to improve its catalytic activity at high current densities in alkaline media still poses a huge challenge for current researchers. Summary of the Invention

[0006] The object of the present invention is to provide a carbon-coated cobalt-based catalyst, its preparation method and application in water electrolysis. Through the construction of a multi-layer structure and the heteroatom doping strategy, a three-dimensional CoP nanosphere doped with molybdenum supported on nickel foam (NF) is prepared, encapsulated in a nitrogen-doped carbon layer, realizing water electrolysis under simulated industrial conditions. This catalyst has excellent hydrogen evolution and oxygen evolution activities.

[0007] In a first aspect, the present invention provides a preparation method of a carbon-coated cobalt-based catalyst, comprising the following steps:

[0008] S1. Immerse the porous nickel foam into a solution composed of a molybdenum source, a cobalt source, urea, a fluorine source and water, and heat for hydrothermal reaction to obtain a Mo-Co LDH precursor supported on nickel foam;

[0009] S2. Anneal the product obtained in step S1 under the protection of an inert gas to obtain a Mo-Co LDH nanowire array supported on nickel foam;

[0010] S3. Add the product obtained in step S2 into a 2-methylimidazole solution for soaking treatment to load the metal-organic framework material ZIF-67 on the Mo-Co LDH nanowire array supported on the nickel foam;

[0011] S4. Pyrolyze the product obtained in step S3 and a phosphorus source together under the protection of an inert gas to form a carbon-coated molybdenum-doped cobalt phosphide supported on nickel foam, obtaining the carbon-coated cobalt-based catalyst.

[0012] Based on the above technical solutions, the doping of molybdenum with stronger electronegativity in the present invention brings about a redistribution of polarized electrons, thus causing the d-band center of CoP to shift and generating an appropriate hydrogen adsorption strength, effectively improving the water separation performance. Combining density functional theory (DFT), the doping of Mo produces a nearly thermoneutral hydrogen adsorption energy. In particular, the d-p hybridization of the Co-O bond reduces the reaction energy barrier of water dissociation. It is worth noting that in-situ characterization verifies that the CoP species undergoes a structural reconstruction to form an active CoOOH layer, thereby accelerating the reaction rate. In addition, the superhydrophilicity on the surface of CoP can also enable the electrolyte to thoroughly penetrate to the electrode surface and quickly release bubbles to refresh the active surface, thus achieving an excellent water separation effect at a large current density of up to 3000 mA cm -2 of.

[0013] The obtained carbon-coated cobalt-based catalyst is a carbon-coated molybdenum-doped cobalt phosphide nanoflower material, in which the molybdenum element exists in the form of substituting cobalt atoms in the cobalt phosphide lattice. The carbon layer is formed by the anchoring effect of the metal-organic framework and cobalt. The overall particle size is 10-15 microns. In the present invention, the active site of the catalyst is the molybdenum-cobalt coupling site. The three-dimensional nanoflower structure greatly increases the specific surface area and provides super hydrophilicity and gas hydrophobicity, making it suitable for working modes with high current and high rate. In addition, the carbon coating on the catalyst surface effectively protects the internal active sites and enhances the stability and corrosion resistance of the material. The obtained carbon-coated molybdenum-doped cobalt phosphide nanoflower catalyst has excellent hydrogen evolution activity and stability, does not contain precious metals, and the preparation method is environmentally friendly, safe and low in energy consumption, suitable for large-scale preparation.

[0014] In the preparation method of the above carbon-coated cobalt-based catalyst, further, the molybdenum source is at least one of ammonium heptamolybdate, ammonium molybdate, and sodium molybdate;

[0015] The mass of the molybdenum source does not exceed 30 wt% of the cobalt source, preferably 10 wt% - 30 wt%, such as 18%, 24%, 14%;

[0016] The cobalt source is at least one of cobalt chloride hexahydrate and cobalt nitrate hexahydrate;

[0017] The fluorine source is at least one of ammonium fluoride and sodium fluoride;

[0018] The feeding mass of the fluorine source does not exceed 150 wt% of the cobalt source, preferably 50 wt% - 100 wt%, such as 77%, 66%;

[0019] The feeding mass of the urea does not exceed 200 wt% of the cobalt source, preferably 150 wt% - 180 wt%, such as 158%;

[0020] The concentration of the cobalt source in the solution is 10 - 15 mg / mL, such as 12.6 mg / mL;

[0021] Each 2 cm * 3 cm * 0.1 cm of the porous nickel foam is immersed in 25 - 35 mL (such as 30 mL) of the solution composed of the molybdenum source, cobalt source, urea, fluorine source and water.

[0022] In step S1, the porous nickel foam (NF), molybdenum source, cobalt source, urea, and fluorine source form a layered double metal hydroxide precursor (LDH precursor) through a hydrothermal reaction. The addition of the porous nickel foam provides a rough surface, and compared with other carriers such as carbon cloth, it provides greater mechanical strength to adapt to the conditions of operation in a high-current environment; the fluorine source regulates the anion coordination in the LDH growth to obtain a set morphology (larger specific surface area and gas diffusion pathway).

[0023] As an example, before the addition, the nickel foam further includes the following pretreatment steps: NF is successively washed with dilute hydrochloric acid, deionized water, and ethanol, and then dried in vacuum. It can be understood that the hydrothermal reaction is carried out in a closed autoclave.

[0024] In the above preparation method of the carbon-coated cobalt-based catalyst, the temperature of the hydrothermal reaction is 120 - 160 °C, and the time is 8 - 12 hours. For example, it is heated at 120 °C for 10 hours in a sealed autoclave.

[0025] In the above preparation method of the carbon-coated cobalt-based catalyst, in the annealing treatment step, the heating rate is not higher than 2 °C / min, the annealing treatment temperature is 300 - 350 °C, the heat preservation time is 1 - 3 h, and it is cooled to room temperature at a cooling rate not higher than 5 °C / min. For example, it is heated to 350 °C or 300 °C at a heating rate of 2 °C / min, annealed for 1 hour, and cooled to room temperature at a cooling rate of 2 °C / min. As an example, the inert gas is nitrogen.

[0026] In step S2, annealing the Mo-Co LDH precursor under the protection of an inert gas to form a Mo-Co LDH nanowire array can enhance the mechanical strength of the surface of the Mo-Co LDH precursor, facilitating the subsequent growth of ZIF-67 on the surface.

[0027] In the above preparation method of the carbon-coated cobalt-based catalyst, the mass ratio of the 2-methylimidazole to the cobalt source does not exceed 8:1, preferably (3 - 6):1, such as 4.3:1 or 5.3:1;

[0028] The concentration of the 2-methylimidazole solution is 0.8 - 2.4 g / mL (such as 1.64 g / mL), and the solvent is deionized water and ethanol with a volume ratio of 1:1;

[0029] The time of the soaking treatment is 12 - 24 h, such as soaking for 12 hours at room temperature.

[0030] In step S3, by forming a metal-organic framework material ZIF-67 on the Mo-Co LDH nanowire array and simultaneously forming a carbon coating layer during subsequent pyrolytic phosphating, the carbon coating effectively protects the internal active sites, enhancing the stability and corrosion resistance of the material.

[0031] In the above preparation method of the carbon-coated cobalt-based catalyst, the phosphorus source is at least one of sodium hypophosphite, sodium dihydrogen hypophosphite, ammonium dihydrogen hypophosphite, ammonium dihydrogen phosphate, and sodium dihydrogen phosphate;

[0032] The mass ratio of the phosphorus source to the cobalt source does not exceed 5:1, such as 1.3:1.

[0033] In the pyrolysis step, the heating rate is not higher than 2 °C / min, the pyrolysis temperature is 300 °C to 350 °C, and the pyrolysis time is 2 to 3 h. For example, at a heating rate of 2 °C / min, pyrolysis is carried out at 300 °C or 350 °C for 1 h. -1 For a heating rate of 2 °C / min, pyrolysis is carried out at 300 °C or 350 °C for 1 h.

[0034] In step S4, the pyrolysis-phosphidation coupling step can form a carbon coating layer while realizing the doping of phosphorus.

[0035] Second, the present invention provides a carbon-coated cobalt-based catalyst prepared by the method described in any one of the above.

[0036] Third, the present invention provides the application of the described carbon-coated cobalt-based catalyst in electrolytic water hydrogen evolution and / or oxygen evolution.

[0037] In the above application, the electrolytic water is carried out in an alkaline aqueous solution electrolytic cell or an alkaline anion exchange membrane electrolytic cell is used.

[0038] The present invention has the following beneficial effects:

[0039] (1) Through in-situ assembly and subsequent phosphidation, a carbon-coated molybdenum-doped cobalt phosphide nanoarray (Mo-CoP / NC) with a micron-scale three-dimensional nanoflower structure was prepared on nickel foam (NF), which has the characteristics of controllable synthesis and stable structural mechanical properties.

[0040] (2) The addition of porous nickel foam provides a rough surface, which provides greater mechanical strength to adapt to the operating conditions under high current compared to other carriers such as carbon cloth; the introduction of a fluorine source regulates the anion coordination in the growth of LDH to obtain a set morphology (larger specific surface area and gas diffusion pathway); the coordination of annealing and ZIF-67 is cleverly used to form a carbon coating layer on the surface to greatly extend the stable operating time under high current.

[0041] (3) Density functional theory (DFT) calculations show that doping with Mo will shift the d-band center of the Co site upward, resulting in a hydrogen adsorption energy close to zero, while the strong d-p hybridization of Co-O species helps to achieve an optimized Volmer step.

[0042] (4) Mo-CoP / NC can achieve HER and OER at a high current density of 3000 mA cm -2 In a 6.0 M KOH solution at 60 °C, a low voltage of only 1.87 V can provide a current density of 1000 mA cm -2 and has good stability within 100 hours. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a flow chart for preparing the carbon-coated molybdenum-doped cobalt phosphide catalyst of the present invention.

[0044] Figure 2 It is the SEM image of carbon-coated molybdenum-doped cobalt phosphide prepared in Specific Example 1 of the present invention.

[0045] Figure 3 It is the TEM image of carbon-coated molybdenum-doped cobalt phosphide prepared in Specific Example 1 of the present invention.

[0046] Figure 4 It is the contact angle test of carbon-coated molybdenum-doped cobalt phosphide prepared in Specific Example 1 of the present invention.

[0047] Figure 5 It is the hydrogen evolution performance test result of carbon-coated molybdenum-doped cobalt phosphide prepared in Specific Example 1 of the present invention. Among them, Figure 5 a is the LSV curve, Figure 5 b is the long-term stability test. Mo-CoP / NC-1 represents Comparative Example 3, Mo-CoP represents Comparative Example 2, CoP / NC represents Comparative Example 1, Mo-CoP / NC represents the carbon-coated molybdenum-doped cobalt phosphide prepared in Example 1, and Pt / C represents the commercial 20% Pt / C catalyst in Comparative Example 4.

[0048] Figure 6 It is the oxygen evolution performance test result of carbon-coated molybdenum-doped cobalt phosphide prepared in Specific Example 1 of the present invention. Among them, Figure 6 a is the LSV curve, Figure 6 b is the long-term stability test. Mo-CoP / NC-1 represents Comparative Example 3, Mo-CoP represents Comparative Example 2, CoP / NC represents Comparative Example 1, Mo-CoP / NC represents the carbon-coated molybdenum-doped cobalt phosphide prepared in Example 1, and RuO 2 represents the commercial RuO 2 catalyst in Comparative Example 5.

[0049] Figure 7 It is the overall water splitting test result of carbon-coated molybdenum-doped cobalt phosphide prepared in Specific Example 2 of the present invention. Specific Embodiments

[0050] The present invention will be further described in detail below in conjunction with specific embodiments. The provided embodiments are only for clarifying the present invention, rather than limiting the scope of the present invention. The following embodiments can be used as a guide for those of ordinary skill in the art to make further improvements and do not limit the present invention in any way.

[0051] The methods used in the following embodiments are all conventional methods unless otherwise specified, and are carried out according to the techniques or conditions described in the literature in this field or according to the product specifications. The materials, reagents, etc. used in the following embodiments can be obtained from commercial sources unless otherwise specified.

[0052] The nickel foam in the following examples was purchased from Cyber Electrochemical Materials Technology Co., Ltd., with a thickness of 1.0 mm, a surface density of 50 g / m 2 , and 110 PPI.

[0053] Example 1

[0054] Prepare a carbon-coated molybdenum-doped cobalt phosphide catalyst for high-current electrolysis of water according to the flow chart shown in Figure 1. The specific steps are as follows:

[0055] (1) Clean NF (3 cm long × 2 cm wide × 0.1 cm thick) successively with dilute hydrochloric acid, deionized water, and ethanol, and then dry it under vacuum. Mix 0.380 g of CoCl 2 ·6H 2 O, 0.07 g of (NH 4 ) 6 Mo 7 O 24 , 0.6 g of urea, and 0.296 g of NH 4 F into 30 mL of deionized water and stir for 5 minutes. Transfer the mixture solution to a 50 mL Teflon-lined stainless steel autoclave. Then, immerse a clean NF into the solution and seal the autoclave. After heating the autoclave at 120 °C for 10 hours, take out the prepared sample and wash it with deionized water to obtain a pink flaky material, the Mo-Co LDH precursor.

[0056] (2) Heat the product obtained in step (1) in an N 2 atmosphere at a heating rate of 2 °C / min -1 to 350 °C, hold for 1 hour for annealing treatment, and then cool to room temperature at a cooling rate of 2 °C / min -1 to obtain a black flaky material, the Mo-Co LDH nanowire array.

[0057] (3) Immerse the product obtained in step (2) into a homogeneous solution prepared from 1.64 g of 2-methylimidazole (2-MeIm), 5 mL of deionized water, and 5 mL of ethanol, and soak at room temperature for 12 hours. Finally, strings of ZIF-67 are formed on the Mo-Co LDH array to obtain a purple flaky material, Mo-Co LDH / ZIF-67.

[0058] (4) Place the product obtained in step (3) and 0.5 g of NaH 2 PO 2 powder separately in porcelain boats and place them at the upstream and downstream of a tube furnace. Heat in an N 2 atmosphere at a heating rate of 2 °C / min -1 to 300 °C, hold for 1 hour for pyrolysis, and then cool at a cooling rate of 2 °C / min -1The cooling rate was reduced to room temperature, and then black flaky carbon-coated molybdenum-doped cobalt phosphide Mo-CoP / NC was obtained.

[0059] The SEM image of the Mo-CoP / NC catalyst prepared in this example is shown in Figure 2 , Mo-CoP / NC consists of uniformly distributed tiny nano-flowers and nano-arrays grown on the surface. The diameter of the spheres is about 10.9 microns. This nano-microsphere morphology provides a large specific surface area and gas diffusion channels for subsequent electrocatalytic reactions. HRTEM( Figure 3 ) shows that there is a carbon layer on the surface of the nano-arrays of Mo-CoP / NC. Mo-CoP is protected by the surface carbon layer, which greatly enhances its long-term stability and protection ability against chloride ions. In addition, from Figure 3 , lattice fringes of 2.48 nm and 2.15 nm can be observed, corresponding to the CoP(111) crystal plane and Co 2 P(202) crystal plane, respectively. Then, we carried out water and underwater bubble contact angle (CA) tests( Figure 4 ) to monitor the super-hydrophilicity and gas-phobicity of Mo-CoP / NC. The water contact angles of Mo-CoP / NC are 143.5° and 0°, respectively, indicating that Mo-CoP / NC has super-hydrophilicity, which is beneficial to the close contact between the electrolyte aqueous solution and the solid electrode. During the OER process, the oxygen desorption ability on the surface of the electrocatalyst was preliminarily determined through the underwater bubble CA test. The significant gas-phobicity enables the bubbles to quickly escape from the surface of the electrocatalyst, promoting the reconstruction and exposure of active sites, thus accelerating the reaction kinetics. The combination of the super-wettability and gas-phobicity of Mo-CoP / NC promotes the diffusion of reactants, the replenishment of local electrolytes, and the detachment of bubbles, which is crucial for efficient industrial electrocatalysis at an amperometric current density.

[0060] Example 2

[0061] In this example, the Mo-CoP / NC electrocatalyst was prepared by a method similar to that of Example 1. Different from Example 1, NH 4 F in step (1) was replaced with 0.252 g of sodium fluoride. The remaining steps were the same as those in Example 1, and the Mo-CoP / NC electrocatalyst was obtained.

[0062] Example 3

[0063] In this example, the Mo-CoP / NC electrocatalyst was prepared by a method similar to that of Example 1. Different from Example 1, 0.093 g of (NH 4 ) 6 Mo 7 O 24 was added in step (1). The remaining steps were the same as those in Example 1, and the Mo-CoP / NC electrocatalyst was obtained.

[0064] Example 4

[0065] In this example, the Mo-CoP / NC electrocatalyst was prepared in a similar method to Example 1. The difference from Example 1 is that 0.056 g of (NH 4 ) 6 Mo 7 O 24 was added in step (1). The remaining steps were the same as those in Example 1, and the Mo-CoP / NC electrocatalyst was obtained.

[0066] Example 5

[0067] In this example, the Mo-CoP / NC electrocatalyst was prepared in a similar method to Example 1. The difference from Example 1 is that 0.047 g of (NH 4 ) 6 Mo 7 O 24 was added in step (1). The remaining steps were the same as those in Example 1, and the Mo-CoP / NC electrocatalyst was obtained.

[0068] Example 6

[0069] In this example, the Mo-CoP / NC electrocatalyst was prepared in a similar method to Example 1. The difference from Example 1 is that the annealing temperature in step (2) was 300 °C. The remaining steps were the same as those in Example 1, and the Mo-CoP / NC electrocatalyst was obtained.

[0070] Example 7

[0071] In this example, the Mo-CoP / NC electrocatalyst was prepared in a similar method to Example 1. The difference from Example 1 is that 2 g of 2-methylimidazole was added in step (3). The remaining steps were the same as those in Example 1, and the Mo-CoP / NC electrocatalyst was obtained.

[0072] Example 8

[0073] In this example, the Mo-CoP / NC electrocatalyst was prepared in a similar method to Example 1. The difference from Example 1 is that it was heated at 350 °C in step (4). The remaining steps were the same as those in Example 1, and the Mo-CoP / NC electrocatalyst was obtained.

[0074] Comparative Example 1

[0075] The preparation steps were the same as those in Example 1, except that no molybdenum source was added. The specific steps were as follows:

[0076] (1) NF (3 cm in length × 2 cm in width × 0.1 cm in thickness) was successively cleaned with dilute hydrochloric acid, deionized water, and ethanol, and then vacuum dried. 0.380 g of CoCl 2 ·6H2 O, 0.6 g of urea and 0.296 g of NH 4 F were mixed into 30 mL of deionized water and stirred for 5 minutes. The mixture solution was transferred to a 50 mL Teflon-lined stainless steel autoclave. Then, a clean NF was immersed in the solution and the autoclave was sealed. After the autoclave was heated at 120 °C for 10 hours, the prepared sample was taken out and washed with deionized water to obtain Co(OH) 2 precursor.

[0077] (2) The product obtained in step (1) was heated to 350 °C at a heating rate of 2 °C min 2 in an N -1 atmosphere, held for 1 hour for annealing treatment, and then cooled to room temperature at a cooling rate of 2 °C min -1 to obtain Co(OH) 2 nanowire arrays.

[0078] (3) The product obtained in step (2) was immersed in a homogeneous solution prepared from 1.64 g of 2-methylimidazole (2-MeIm), 5 mL of deionized water and 5 mL of ethanol, and soaked at room temperature for 12 hours. Finally, strings of ZIF-67 were formed on the Mo-Co LDH arrays to obtain Co(OH) 2 / ZIF-67.

[0079] (4) The product obtained in step (3) and 0.5 g of NaH 2 PO 2 powder were respectively placed in porcelain boats at the upstream and downstream of a tube furnace, and heated to 300 °C at a heating rate of 2 °C min 2 in an N -1 atmosphere, held for 1 hour for pyrolysis, and then cooled to room temperature at a cooling rate of 2 °C min -1 to obtain CoP / NC.

[0080] Comparative Example 2

[0081] The preparation steps were the same as those in Example 1, except that carbon coating was not used and a molybdenum source was added. The specific steps were as follows:

[0082] (1) NF (3 cm in length × 2 cm in width × 0.1 cm in thickness) was successively washed with dilute hydrochloric acid, deionized water and ethanol, and then dried under vacuum. 0.380 g of CoCl 2 ·6H 2 O, 0.07 g of (NH 4 ) 6 Mo 7 O 24 、0.6 g of urea and 0.296 g of NH 4Mix 30 mL of deionized water with F, and stir for 5 minutes. Transfer the mixture solution to a 50 mL Teflon-lined stainless steel autoclave. Then, immerse a clean NF into the solution and seal the autoclave. After heating the autoclave at 120 °C for 10 hours, take out the prepared sample and wash it with deionized water to obtain the Mo-Co LDH precursor.

[0083] (2) Put the product obtained in (1) and 0.5 g of NaH 2 PO 2 powders into porcelain boats respectively and place them at the upstream and downstream of a tube furnace. Heat them to 300 °C at a heating rate of 2 °C min 2 in an N -1 atmosphere, hold for 1 hour for pyrolysis, and then cool to room temperature at a cooling rate of 2 °C min -1 to obtain Mo-CoP.

[0084] Comparative Example 3

[0085] The preparation steps are the same as those in Example 1, except that no fluorine source is added. The specific steps are as follows:

[0086] (1) Wash NF (3 cm in length × 2 cm in width × 0.1 cm in thickness) successively with dilute hydrochloric acid, deionized water and ethanol, and then dry it in vacuum. Mix 0.380 g of CoCl 2 ·6H 2 O, 0.07 g of (NH 4 ) 6 Mo 7 O 24 and 0.6 g of urea in 30 mL of deionized water, and stir for 5 minutes. Transfer the mixture solution to a 50 mL Teflon-lined stainless steel autoclave. Then, immerse a clean NF into the solution and seal the autoclave. After heating the autoclave at 120 °C for 10 hours, take out the prepared sample and wash it with deionized water to obtain the Mo-Co LDH-1 precursor.

[0087] (2) Anneal the product obtained in step (1) at 350 °C in an N 2 atmosphere at a heating rate of 2 °C min -1 to 350 °C, hold for 1 hour, and then cool to room temperature at a cooling rate of 2 °C min -1 to obtain the Mo-Co LDH-1 nanowire array.

[0088] (3) Immerse the product obtained in step (2) into a homogeneous solution prepared from 1.64 g of 2-methylimidazole (2-MeIm), 5 mL of deionized water, and 5 mL of ethanol, and soak for 12 hours at room temperature. Finally, strings of ZIF-67 are formed on the Mo-Co LDH array to obtain Mo-Co LDH-1 / ZIF-67.

[0089] (4) Place the product obtained in step (3) and 0.5 g of NaH 2 PO 2 powder into porcelain boats and place them at the upstream and downstream of a tubular furnace. Heat to 300 °C at a heating rate of 2 °C min 2 and hold for 1 hour for pyrolysis. Subsequently, cool to room temperature at a cooling rate of 2 °C min -1 to obtain Mo-CoP / NC-1. -1

[0090] Comparative Example 4

[0091] Commercial 20% Pt / C catalyst.

[0092] Comparative Example 5

[0093] Commercial RuO 2 catalyst.

[0094] Test Example

[0095] Cut the catalyst materials in the above examples and comparative examples into 1 cm * 1 cm electrodes, and conduct electrochemical performance tests on them.

[0096] Hydrogen evolution performance test: The electrolyte is 1 M KOH, the mercury / mercuric oxide electrode is the reference electrode, and the graphite rod electrode is the counter electrode. In the test, first run 40 cycles by cyclic voltammetry (scanning potential from -0.9 V to -1.6 V, scan rate 0.1 V / s) to stabilize the catalyst performance, and then perform linear voltammetry scanning (scanning potential from -0.9 V to -3 V, scan rate 0.005 V / s). All overpotentials are converted to overpotentials relative to the standard hydrogen electrode, and the conversion formula is E (RHE) = E (Hg / HgO) + 0.098 + 0.0592 * pH.

[0097] Oxygen evolution performance test: The electrolyte is 1 M KOH, the mercury / mercuric oxide electrode is the reference electrode, and the graphite rod electrode is the counter electrode. In the test, first run 40 cycles by cyclic voltammetry (scanning potential from 0 V to 3 V, scan rate 0.1 V / s) to stabilize the catalyst performance, and then perform linear voltammetry scanning (scanning potential from 0 V to 5 V, scan rate 0.005 V / s). All overpotentials are converted to overpotentials relative to the standard hydrogen electrode, and the conversion formula is E (RHE) = E(Hg / HgO) +0.098 + 0.0592 * pH - 1.23。

[0098] Stability test: Steady-state constant current polarization tests were carried out at constant current densities of 500 mA / cm 2 (oxygen evolution) and -500 mA / cm 2 (hydrogen evolution) for 100 h to obtain the stability performance curves of oxygen evolution / hydrogen evolution.

[0099] The experimental results are shown in Tables 1 - 2.

[0100] Table 1. Electrochemical performance test results (hydrogen evolution)

[0101]

[0102]

[0103] Table 2. Electrochemical performance test results (oxygen evolution)

[0104]

[0105] Figure 5 are the hydrogen evolution performance test results of the carbon-coated molybdenum-doped cobalt phosphide prepared in Example 1. As Figure 5 can be seen from a, Mo-CoP / NC requires overpotentials of 177 mV and 198 mV respectively to reach current densities of 500 and 1000 mA cm -2 . These values are better than those of Mo-CoP (183 and 207 mV), CoP / NC (225 and 289 mV), Mo-CoP / NC-1 (279 and 403 mV), and commercial Pt / C (247 and 308 mV), indicating that molybdenum doping and carbon coating improve the hydrogen evolution reaction kinetics, thus greatly enhancing the HER performance. Figure 6 are the oxygen evolution performance test results of the carbon-coated molybdenum-doped cobalt phosphide prepared in Example 1. As Figure 6 can be seen from a, Mo-CoP / NC requires overpotentials of 352 and 384 mV respectively to reach current densities of 500 and 1000 mA cm -2 , which is better than those of Mo-CoP (363 and 390 mV), CoP / NC (428 and 492 mV), Mo-CoP / NC-1 (458 and 547 mV), and commercial RuO 2 (449 and 576 mV). The extremely low overpotentials for hydrogen evolution and oxygen evolution reflect that the catalyst material of the present invention can greatly reduce the reaction energy barrier and energy consumption in the large-current electrolysis of water reaction.

[0106] At 500 mA cm -2Chronopotentiometry measurements (E-t) of Mo-CoP / NC at industrial current densities verified its superiority and reliable prospects in industrial-scale water electrolysis. Figure 5 b, 6b). After 100 h of durability testing, Mo-CoP / NC showed only a slight potential drop in both HER and OER, indicating its strong long-term electrochemical stability.

[0107] Figure 7 This is the overall water splitting test result of carbon-coated molybdenum-doped cobalt phosphide prepared in Example 2. Figure 7 As can be seen, the assembled Mo-CoP / NC electrolyzer was initially tested in 1.0 M KOH solution and could provide high current densities of 500, 1000, 2000, and 3000 mA cm -2 at cell voltages of 1.80, 1.87, 1.92, and 1.93 V, respectively, which is better than that of RuO 2 ||Pt / C (2.02, 2.30, 2.54, and 2.61 V).

[0108] Compared with the catalyst sample shown in Comparative Example 1, the catalyst in Example 1 added a molybdenum source. From the comparison results of Example 1 and Comparative Example 1 in Tables 1 and 2, it can be seen that the hydrogen production efficiency and oxygen production efficiency of Example 1 were greatly improved. This is because the doping of molybdenum species improved the ability of cobalt phosphide materials to dissociate water, optimized the hydrogen adsorption free energy, enhanced the carrier migration ability, and improved the reaction kinetics.

[0109] Compared with the catalyst sample shown in Comparative Example 2, the catalyst in Example 1 was provided with carbon coating. From the comparison results of Example 1 and Comparative Example 2 in Tables 1 and 2, it can be seen that the hydrogen production efficiency and oxygen production efficiency of Example 1 were improved. This is because annealing the Mo-Co LDH precursor under inert gas protection to form the metal-organic framework material ZIF-67 can form a carbon coating layer during pyrolysis. The carbon layer coating is formed by the anchoring effect of the metal-organic framework and cobalt, effectively protecting the internal active sites and enhancing the catalytic activity, stability, and corrosion resistance of the material.

[0110] Compared with the catalyst sample shown in Comparative Example 3, the catalyst in Example 1 added a fluorine source to the precursor. From the comparison results of Example 1 and Comparative Example 3 in Tables 1 and 2, it can be seen that the hydrogen production efficiency and oxygen production efficiency of Example 1 were improved. This is because the introduction of the fluorine source regulated the anion coordination in the growth of LDH to obtain a larger specific surface area and gas diffusion pathway of the final product by controlling the morphology of the precursor, thereby improving the catalytic performance.

[0111] From the comparison results with Comparative Examples 4-5, it can be seen that the catalytic activities of hydrogen evolution and oxygen evolution catalyzed by the present invention are much higher than those of current commercial catalysts. While improving the catalytic activity, the present invention greatly reduces the raw material cost.

[0112] The present invention has been described in detail above. For those skilled in the art, within the scope of not departing from the gist and scope of the present invention, the present invention can be implemented within a relatively wide range under equivalent parameters, concentrations and conditions. Although specific embodiments of the present invention are given, it should be understood that the present invention can be further improved. In short, according to the principle of the present invention, this application intends to cover any changes, uses or improvements of the present invention, including those that deviate from the scope disclosed in this application and are made with conventional techniques known in the art.

Claims

1. A method for preparing a carbon-coated cobalt-based catalyst, characterized in that: The steps include: S1, immersing the nickel foam in a solution consisting of a molybdenum source, a cobalt source, urea, a fluorine source and water, heating and performing a hydrothermal reaction to obtain a Mo-Co LDH precursor supported by the nickel foam; S2, annealing the product obtained in step S1 under the protection of an inert gas to obtain a Mo-CoLDH nanowire array supported by nickel foam; S3, adding the product obtained in step S2 into a 2-methylimidazole solution for immersion treatment, so as to load the metal organic framework material ZIF-67 on the Mo-Co LDH nanowire array supported by the nickel foam; S4, pyrolyzing the product obtained in step S3 and a phosphorus source together under the protection of an inert gas to form carbon-coated molybdenum-doped cobalt phosphide supported by nickel foam, thereby obtaining the carbon-coated cobalt-based catalyst.

2. The method for preparing a carbon-coated cobalt-based catalyst according to claim 1, characterized in that: The molybdenum source is at least one of ammonium heptamolybdate, ammonium molybdate and sodium molybdate; The mass of the molybdenum source does not exceed 30wt% of the cobalt source; The cobalt source is at least one of cobalt chloride hexahydrate and cobalt nitrate hexahydrate; The fluorine source is at least one of ammonium fluoride and sodium fluoride; The mass of the fluorine source does not exceed 150wt% of the cobalt source; The mass of the urea does not exceed 200wt% of the cobalt source; The concentration of the cobalt source in the solution is 10-15 mg / mL.

3. The method for preparing a carbon-coated cobalt-based catalyst according to any one of claims 1 to 2, characterized in that: The temperature of the hydrothermal reaction is 120-160° C. and the time is 8-12 hours.

4. The method for preparing a carbon-coated cobalt-based catalyst according to any one of claims 1 to 3, characterized in that: In the annealing step, the heating rate is no higher than 2°C / min, the annealing temperature is 300°C to 350°C, the holding time is 1 to 3 hours, and the temperature is lowered to room temperature at a cooling rate no higher than 5°C / min.

5. The method for preparing a carbon-coated cobalt-based catalyst according to any one of claims 1 to 4, characterized in that: The mass ratio of the dimethylimidazole to the cobalt source is no more than 8:1; The concentration of the 2-methylimidazole solution is 0.8-2.4 g / mL, and the solvent is deionized water and ethanol in a volume ratio of 1:1; The soaking treatment time is 12 to 24 hours.

6. The method for preparing a carbon-coated cobalt-based catalyst according to any one of claims 1 to 5, characterized in that: The phosphorus source is at least one of sodium hypophosphite, sodium dihydrogen hypophosphite, ammonium dihydrogen hypophosphite, ammonium dihydrogen phosphate, and sodium dihydrogen phosphate; The mass ratio of the phosphorus source to the cobalt source is no more than 5:

1.

7. The method for preparing a carbon-coated cobalt-based catalyst according to any one of claims 1 to 6, characterized in that: In the pyrolysis step, the heating rate is no higher than 2°C / min, the pyrolysis temperature is 300°C to 350°C, and the pyrolysis time is 2 to 3 hours.

8. A carbon-coated cobalt-based catalyst prepared by the method according to any one of claims 1 to 7.

9. Use of the carbon-coated cobalt-based catalyst according to claim 8 in hydrogen and / or oxygen evolution by electrolysis of water.

10. The use according to claim 9, characterized in that: The water electrolysis is carried out in an alkaline aqueous solution electrolyzer or an alkaline anion exchange membrane electrolyzer.

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