A CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material, its preparation method and application

Abstract

This invention provides a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material, its preparation method, and its applications. The preparation method includes: firstly, dispersing cobalt salt and organic ligands in an organic solvent to prepare ZIF-67; then, dispersing ZIF-67 in the organic solvent, introducing a molybdenum source, and preparing a CoMoO4-Co(OH)2 precursor via an oil bath reaction; and finally, preparing a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material by co-annealing the CoMoO4-Co(OH)2 precursor with NaH2PO2. The nanomaterial of this invention possesses a high specific surface area and high porosity, promoting interfacial reactions and accelerating ion transport and charge transfer. Furthermore, CoMoO4 enhances thermal stability, exhibiting excellent electrocatalytic activity and stability in alkaline media, and thus has significant industrial practical value.

Description

Technical Field

[0001] This invention relates to a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material, its preparation method and application, belonging to the fields of electrocatalytic water electrolysis and nanosynthesis technology. Background Technology

[0002] Hydrogen, due to its high calorific value and zero carbon emissions from combustion, is considered the most promising alternative to fossil fuels, and water electrolysis is the cleanest route for hydrogen production. Hydrogen production via water electrolysis consists of a transition metal electrochemical reaction (HER) at the cathode and an oxidation reaction (OER) at the anode. Although Pt, Pd, Ru, and Ir-based catalysts are generally the most active HER and OER electrocatalysts, their high cost and limited reserves restrict the large-scale production and application of precious metals. Therefore, developing alternative transition metal-based HER and OER electrocatalysts is crucial for optimizing hydrogen production processes.

[0003] Transition metal phosphides often exhibit resistance to H+ in acidic solutions. + The lower adsorption energy is a significant advantage. Furthermore, the high electronegativity of phosphorus (P) makes it easy to capture electrons from metal sites in terminating polymeric substances (TMPs), thereby increasing conductivity and facilitating rapid electron exchange between the electrocatalyst surface and H-adsorbed species. However, the strong adsorption effect of highly electronegative P on H-species undoubtedly makes it difficult for the product H2 to desorb from the TMP surface, thus increasing the reaction energy barrier and causing TMPs to often exhibit unsatisfactory activity and stability. Therefore, regulating the electronic structure of P sites in TMPs to optimize their adsorption energy for H-species is key to improving their alkaline HER performance. For example, the Chai team obtained Mo-CoP / MoO through a multi-step strategy involving anodic oxidation, hydrothermal reaction, phosphating, and fluorination encapsulation. x Nanoarrays were used to achieve ultra-low overpotential and high durability of HER under high current density. It was also found that Mo doping reduced the Gibbs free energy of the rate-determining step (PDS) of CoP water dissociation, thereby significantly improving the activity of alkaline HER. However, the modulation effect of Mo doping did not enable the application of CoP in electrocatalytic alkaline OER.

[0004] Transition metal oxides are inexpensive, readily available, environmentally friendly, and possess a strong affinity for oxygen-containing species and high stability in alkaline solutions. Novel spinel-based CoMoO4 can fully leverage the synergistic catalytic effect unique to the Co and Mo bimetallic sites. However, limited specific surface area, a small number of active sites, and low intrinsic conductivity restrict further improvements in its electrocatalytic performance. To address this issue, researchers have employed various methods, including vacancy engineering, heteroatom doping, and structural engineering, to enhance conductivity and expose more active sites. Among these, the rich interfaces created by heterostructures can modulate the electronic structure, further optimizing the adsorption energy during the electrocatalytic process.

[0005] Furthermore, constructing hollow structures is another effective strategy to improve electrocatalytic activity. Metal-organic frameworks (MOFs) possess structural advantages such as high specific surface area and high porosity, and are often used as template materials to prepare highly efficient water electrolysis catalysts. However, the electrocatalytic oxygen evolution performance of the aforementioned materials is unsatisfactory, and achieving excellent overall water electrolysis catalysts remains challenging. Chinese patent document CN113151854A discloses a multi-metal non-oxide electrocatalyst and its preparation method. This method involves preparing CoFeMo-LDH from a molybdate aqueous solution and a Co-Fe layered double hydroxide, and then phosphating or selenizing the CoFeMo-LDH to generate the electrocatalyst. This invention has cumbersome reaction steps and correspondingly high energy consumption. Therefore, designing highly efficient hydrogen evolution / oxygen evolution bifunctional catalysts is of great significance.

[0006] To address the aforementioned problems, this invention is proposed. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material, its preparation method, and its applications. This invention utilizes a metal-organic framework as a template, and employs MoO4... 2- A hollow polyhedral CoMoO4-CoP / NC heterostructure anchored on a N-doped carbon framework was prepared using a strategy involving hydrolysis, anion exchange, and co-annealing with NaH2PO2. The high specific surface area and porosity resulting from the hydrolysis and phosphating processes promoted interfacial reactions, accelerated ion transport and charge transfer, and the electron transfer at the heterostructure interface modulated the electronic structure of the electrocatalyst, thereby optimizing reaction kinetics. This resulted in excellent electrocatalyst water-splitting activity and stability, demonstrating potential for industrial applications. This invention prepares a hollow heterostructure composite material through a mild oil bath reaction and annealing reaction. The preparation method is simple and convenient, the product is environmentally friendly, and it exhibits excellent electrocatalytic water-splitting activity and stability, making it industrially promising.

[0008] Terminology Explanation:

[0009] Room temperature, as is known in the art, refers to 25±5℃.

[0010] The technical solution of the present invention is as follows:

[0011] A CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material, wherein the microstructure of the composite material is as follows: CoMoO4-CoP nanoparticles are uniformly anchored on a hollow polyhedral N-doped carbon skeleton, and a heterostructure is formed between the CoMoO4 and CoP phases.

[0012] According to a preferred embodiment of the present invention, the CoMoO4-CoP nanoparticles have a particle size of 2-8 nm, and the hollow polyhedral N-doped carbon has a size of 250-500 nm.

[0013] According to the present invention, the preparation method of the above-mentioned CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material includes the following steps:

[0014] (1) Add the cobalt source solution to the organic ligand solution, stir evenly, and let it stand at room temperature to react; then centrifuge and wash to obtain metal-organic framework ZIF-67;

[0015] (2) Disperse the metal-organic framework ZIF-67 in an organic solvent, add an aqueous solution of molybdenum source, and react; after centrifugation, washing, and drying, obtain the CoMoO4-Co(OH)2 precursor;

[0016] (3) The CoMoO4-Co(OH)2 precursor was annealed with NaH2PO2 to obtain the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material.

[0017] According to a preferred embodiment of the present invention, the cobalt source in step (1) is cobalt nitrate hexahydrate, and the organic ligand is 2-methylimidazole; the molar ratio of the cobalt source to the organic ligand is 1:(6-9), more preferably 1:8; the present invention uses a higher proportion of 2-MIM to achieve rapid nucleation and slow growth of MOF, resulting in a ZIF-67 template with a specific smaller size, and the size effect leads to the derived electrocatalyst having a larger specific surface area and more exposed active sites, thereby achieving highly efficient electrocatalytic OWS.

[0018] According to a preferred embodiment of the present invention, the cobalt source solution in step (1) is obtained by dispersing a cobalt source in methanol, and the concentration of the cobalt source solution is 0.01-0.1 mol / L, preferably 0.05 mol / L; the organic ligand solution is obtained by dispersing an organic ligand in methanol, and the concentration of the organic ligand solution is 0.2-0.6 mol / L, preferably 0.4 mol / L.

[0019] According to a preferred embodiment of the present invention, the time for the room temperature static reaction in step (1) is 20-30 h, and more preferably 22-26 h.

[0020] According to a preferred embodiment of the present invention, the washing in step (1) is to use methanol for centrifugal washing 3-5 times.

[0021] According to the present invention, in step (1), the metal-organic framework ZIF-67 obtained by centrifugation is dispersed in methanol for later use, and the solvent is removed by centrifugation when it is used.

[0022] According to a preferred embodiment of the present invention, the organic solvent in step (2) is ethanol; the volume ratio of the organic solvent to the molar number of the metal-organic framework ZIF-67 is 60-100 mL: 1 mmol.

[0023] According to a preferred embodiment of the present invention, the molybdenum source in step (2) is sodium molybdate dihydrate, ammonium molybdate tetrahydrate, or ammonium phosphomolybdate; the concentration of the aqueous solution of the molybdenum source is 0.01-0.2 mmol / mL, more preferably 0.05-0.1 mmol / mL.

[0024] According to a preferred embodiment of the present invention, the molar ratio of the metal-organic framework ZIF-67 to the molybdenum source in step (2) is 1:1.2-4, and more preferably 1:2.5-3.

[0025] According to a preferred embodiment of the present invention, the temperature of the reaction in step (2) is 50-150°C, more preferably 80-90°C; and the reaction time is 0.5-4h, more preferably 1-2h.

[0026] According to a preferred embodiment of the present invention, the washing in step (2) is to wash by centrifugation with deionized water and ethanol 3-5 times in sequence; the drying is to vacuum dry at 60°C for 12-24 hours.

[0027] According to a preferred embodiment of the present invention, the mass ratio of the CoMoO4-Co(OH)2 precursor to NaH2PO2 in step (3) is 1:25-40.

[0028] According to a preferred embodiment of the present invention, the annealing temperature in step (3) is 280-330°C, more preferably 300°C, and the annealing time is 1-3 hours.

[0029] According to a preferred embodiment of the present invention, the annealing process in step (3) is as follows: under the protection of an inert gas, the CoMoO4-Co(OH)2 precursor and NaH2PO2 are placed downstream and upstream of the inert gas flow, respectively, and heated to the annealing temperature at a heating rate of 2-5℃ / min, and then held for heat treatment, and then naturally cooled to room temperature; the inert gas is argon.

[0030] A preferred embodiment of the present invention provides a method for preparing the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material, comprising the following specific steps:

[0031] (1) Cobalt nitrate hexahydrate and 2-methylimidazole were dispersed in methanol respectively. The resulting cobalt source solution was added to the organic ligand solution. After stirring at room temperature, the reaction was allowed to stand at room temperature for 24 hours. The resulting purple product was centrifuged to obtain ZIF-67, which was then redispersed in methanol for later use.

[0032] (2) After centrifuging the ZIF-67 methanol dispersion, it was redispersed in ethanol. Sodium molybdate dihydrate aqueous solution was added as a molybdenum source and poured into a three-necked flask. The reaction was carried out at 80℃ for 1 h. After the reaction was completed, the pink product was centrifuged and dried to obtain the CoMoO4-Co(OH)2 precursor.

[0033] (3) The CoMoO4-Co(OH)2 precursor prepared in step (2) was co-annealed with NaH2PO2. Under the protection of an inert atmosphere of argon, the CoMoO4-Co(OH)2 precursor and NaH2PO2 were placed downstream and upstream of the inert gas flow, respectively. The furnace temperature was raised to 300℃ at a heating rate of 5℃ / min and held for 2h. Then it was naturally cooled to room temperature to obtain the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material.

[0034] This invention also provides the application of the above-mentioned CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material in hydrogen evolution by water electrolysis, oxygen evolution by water electrolysis, or electrocatalytic complete water electrolysis.

[0035] The technical features and beneficial effects of this invention are as follows:

[0036] 1. This invention uses ZIF-67 as a self-sacrificing template, first utilizing MoO4 2- A synthetic strategy involving the preparation of CoMoO4 / Co(OH)2 precursors via hydrolysis and anion exchange, followed by phosphating with NaH2PO2 to obtain CoMoO4-CoP / NC, fully leverages the high intrinsic HER and OER activities of CoP and CoMoO4. This approach exhibits excellent OWS electrocatalytic performance in alkaline solutions, particularly the enhanced effect of the high electronegativity of P in CoP on H+. + The adsorption of these substances enhances the hydrogen evolution activity. The high specific surface area and high porosity resulting from the hydrolysis and phosphating processes promote interfacial reactions, accelerate ion transport and charge transfer, and the electron transfer at the heterogeneous interface modulates the electronic structure of the electrocatalyst, thereby optimizing the reaction kinetics. Therefore, the electrocatalyst possesses excellent overall water-splitting activity and stability, and has potential for industrial application.

[0037] 2. In the preparation of the composite material of the present invention, the amount of molybdenum source needs to be appropriate. An appropriate amount of molybdenum source does not significantly change the polyhedral morphology of CoP, while excessive molybdenum source leads to the agglomeration of metal particles in CoMoO4-CoP, and even the dodecahedral carbon framework structure exhibits fragmentation and collapse, losing effective anchoring of metal sites. Furthermore, the present invention requires phosphating under specific conditions to obtain a high-performance catalyst material. If the phosphating temperature is too low or the phosphating time is too short, incomplete phosphating will occur, thus reducing the electrocatalytic water splitting activity. If the phosphating temperature is too high or the phosphating time is too long, it will increase reaction energy consumption and reduce the catalytic activity of the material. In addition, if the ratio of NaH2PO2 to CoMoO4-Co(OH)2 precursor in the present invention is too low, it will also lead to incomplete phosphating, thus affecting the electrocatalytic activity. If this ratio is too high, it will also cause resource waste and reduce the catalytic activity of the material. Moreover, the present invention obtains a nitrogen-doped carbon framework-supported CoMoO4-CoP heterostructure through annealing treatment, without the need to introduce an additional carbon support, thus enhancing the conductivity and structural stability of the material.

[0038] 3. The composite material prepared by this invention achieves highly efficient alkaline HER, OER, and OWS multifunctional electrocatalytic processes; and possesses excellent HER, OER, and electrocatalytic water splitting activity and stability. Experiments show that the alkaline HER and OER overpotentials of the CoMoO4-CoP / NC of this invention are only 122 and 280 mV (@10 mA cm⁻¹), respectively. -2 And its stability is as high as 100h (@100mA cm). -2 It exceeds commercially available Pt / C and RuO2 by 20%. Meanwhile, the alkaline OWS battery requires only 1.62V (@10mA cm⁻¹). -2 This demonstrates promising application prospects for hydrogen production through water electrolysis. Attached Figure Description

[0039] Figure 1 Transmission electron microscope image of the ZIF-67 nanomaterial prepared in Example 1.

[0040] Figure 2 Transmission electron microscope image of the CoMoO4-Co(OH)2 precursor prepared in Example 1.

[0041] Figure 3 Transmission electron microscope image of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material prepared in Example 1.

[0042] Figure 4 Transmission electron microscope image of the CoP / NC hollow polyhedral nanomaterial prepared for Comparative Example 1.

[0043] Figure 5Transmission electron microscope image of CoMoO4 nanomaterials prepared for Comparative Example 2.

[0044] Figure 6 Transmission electron microscopy (TEM) image of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material prepared in Example 2.

[0045] Figure 7 Transmission electron microscopy (TEM) image of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material prepared for Comparative Example 3.

[0046] Figure 8 High-resolution transmission electron microscope image of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material prepared in Example 1.

[0047] Figure 9 Selected area electron diffraction (SEAD) image of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material prepared in Example 1.

[0048] Figure 10 The X-ray diffraction patterns are those of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material prepared in Example 1, the CoP / NC hollow nanomaterial prepared in Comparative Example 1, and the CoMoO4 nanomaterial prepared in Comparative Example 2.

[0049] Figure 11 The X-ray diffraction patterns are those of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite materials prepared in Examples 1-2 and Comparative Example 3.

[0050] Figure 12 The full X-ray photoelectron spectroscopy (XPS) spectra of CoMoO4-CoP / NC prepared in Example 1 and CoP / NC prepared in Comparative Example 1 are shown in (a). The fine XPS spectral Co 2p spectra of CoMoO4-CoP / NC prepared in Example 1 and CoP / NC prepared in Comparative Example 1 are shown in (b). The fine XPS spectral Mo 3d spectra of CoMoO4-CoP / NC prepared in Example 1 and CoMoO4 prepared in Comparative Example 2 are shown in (c). The fine XPS 2p spectra of CoMoO4-CoP / NC prepared in Example 1 and CoP / NC prepared in Comparative Example 1 are shown in (d).

[0051] Figure 13Hydrogen evolution performance tests were conducted on the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material prepared in Example 1, the CoP / NC hollow polyhedral nanomaterial prepared in Comparative Example 1, the CoMoO4 nanomaterial prepared in Comparative Example 2, and a commercially available Pt / C nanomaterial with a Pt content of 20 wt%. The results included: (a) linear voltammetry curves; (b) Tafel curves; (c) double-layer fitting curves; (d) AC impedance diagrams; and (e) linear polarization curves of CoMoO4-CoP / NC before and after different cyclic voltammetry scans. (f) Hydrogen evolution performance tests were performed at 100 mA / cm². -2 Vt curves for electrolysis of CoMoO4-CoP / NC and 20% Pt / C.

[0052] Figure 14 The oxygen evolution performance of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material prepared in Example 1, the CoP / NC hollow polyhedral nanomaterial prepared in Comparative Example 1, the CoMoO4 nanomaterial prepared in Comparative Example 2, and RuO2 was tested. (a) Linear voltammetry curves, (b) Tafel curves, (c) double-layer fitting curves, (d) AC impedance diagrams, with the inset showing the equivalent circuit diagram; (e) linear polarization curves of CoMoO4-CoP / NC before and after different cyclic voltammetry scans, and (f) at 100 mA cm⁻¹. -2 Vt curves for electrolysis of CoMoO4-CoP / NC and 20% Pt / C.

[0053] Figure 15 The graph shows a comparison of the linear polarization curves of hydrogen evolution (a) and oxygen evolution (b) of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite materials prepared in Examples 1-6. The horizontal axis represents the standard hydrogen electrode potential relative to the Ag / AgCl reference electrode.

[0054] Figure 16 The graph shows the linear polarization curves of hydrogen evolution (a) and oxygen evolution (b) for the CoP / NC hollow polyhedral nanomaterial prepared in Comparative Example 1, the CoMoO4 nanomaterial prepared in Comparative Example 2, and the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite materials prepared in Comparative Examples 3-7. The horizontal axis represents the standard hydrogen electrode potential relative to the Ag / AgCl reference electrode.

[0055] Figure 17Linear polarization curves of the total water splitting of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material obtained in Example 1, the CoP / NC hollow polyhedral nanomaterial prepared in Comparative Example 1, the CoMoO4 nanomaterial prepared in Comparative Example 2, and commercially available Pt / C and RuO2 with a Pt content of 20 wt% were obtained. (a) The CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material obtained in Example 1 was subjected to a total water splitting curve at 10 mA cm⁻¹. -2 The Vt curve below. Detailed Implementation

[0056] The present invention will now be described in detail with reference to specific embodiments and accompanying drawings, but the present invention is not limited thereto.

[0057] Unless otherwise specified, the experimental methods used in the examples are conventional methods. Unless otherwise specified, all materials and reagents used in the examples are commercially available.

[0058] Example 1

[0059] A method for preparing a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material includes the following steps:

[0060] (1) 5 mmol (1.455 g) Co(NO3)2·6H2O and 40 mmol (3.28 g) 2-methylimidazole were dispersed in 100 mL of methanol and ultrasonically dispersed into a homogeneous solution. The Co(NO3)2·6H2O methanol solution was poured into the 2-methylimidazole methanol solution and stirred for 10 min. The resulting mixed solution was allowed to stand at room temperature for 24 h. After centrifugation, the purple product was centrifuged three times with methanol to remove impurities, and ZIF-67 was obtained. It was then redispersed in 200 mL of methanol for later use. The molar amount of ZIF-67 obtained was 5 mmol, calculated based on the complete reaction of the cobalt source.

[0061] (2) Take 20 mL of a methanol solution of ZIF-67 (0.5 mmol of ZIF-67), centrifuge, wash twice with ethanol, and redisperse in 40 mL of ethanol. Weigh 1.45 mmol (0.352 g) of Na2MoO4·2H2O and disperse it in 20 mL of deionized water. Add the resulting Na2MoO4·2H2O aqueous solution to the prepared ethanol dispersion of ZIF-67. Transfer the resulting mixed solution to a 100 mL three-necked flask and react at 80 °C for 1 h. The solution changes from dark purple to light pink. After centrifugation, wash the resulting light pink product three times each with deionized water and ethanol. Dry the resulting solid in a vacuum drying oven at 60 °C for 12 h to obtain the CoMoO4-Co(OH)2 precursor.

[0062] (3) 20 mg of CoMoO4-Co(OH)2 precursor was placed in a ceramic boat and placed downstream of the argon flow direction in a tube furnace. 600 mg of NaH2PO2 was placed in the ceramic boat and placed upstream of the argon flow direction. Under the protection of an inert Ar atmosphere, the furnace temperature was raised to 300 °C at a heating rate of 5 °C / min and held for 2 h. After that, it was naturally cooled to room temperature, and the black product was collected to obtain the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material, denoted as CoMoO4-CoP / NC.

[0063] Example 2

[0064] The preparation method of a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material is as described in Example 1, except that in step (2), 0.75 mmol (0.182 g) of Na2MoO4·2H2O is weighed and dispersed in 20 mL of deionized water, and the other conditions are the same as in Example 1. The resulting sample is denoted as CoMoO4-CoP / NC-0.1825.

[0065] Example 3

[0066] The preparation method of a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material is as described in Example 1, except that in step (3), the temperature is kept at 300℃ for 1h, and the other conditions are the same as in Example 1. The obtained sample is recorded as CoMoO4-CoP / NC-1h.

[0067] Example 4

[0068] The preparation method of a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material is as described in Example 1, except that in step (3), the temperature is kept at 300℃ for 3h, and the other conditions are the same as in Example 1. The resulting sample is denoted as CoMoO4-CoP / NC-3h.

[0069] Example 5

[0070] A method for preparing a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material is as described in Example 1, except that: in step (2), 1.45 mmol (1.79 g) of ammonium molybdate tetrahydrate ((NH4)6Mo7O) is weighed. 24 Disperse 4H2O) into 20 mL of deionized water, and the remaining conditions are the same as in Example 1. The resulting sample is denoted as CoMoO4-CoP / NC-ammonium molybdate.

[0071] Example 6

[0072] A method for preparing a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material is as described in Example 1, except that: in step (2), 1.45 mmol (2.75 g) of ammonium phosphomolybdate ((NH4)3PMo) is weighed. 12 O 40 Disperse it in 20 mL of deionized water, and the other conditions are the same as in Example 1. The resulting sample is denoted as CoMoO4-CoP / NC-ammonium phosphomolybdate.

[0073] Comparative Example 1

[0074] A method for preparing CoP / NC hollow polyhedral nanomaterials is as described in Example 1, except that in step (2), 20 mL of ZIF-67 methanol solution is taken, centrifuged, washed twice with ethanol, and then redispersed in 40 mL of ethanol. The solution is transferred to a 100 mL three-necked flask and reacted at 80 °C for 1 h, during which the solution changes from dark purple to light pink. After centrifugation, the resulting light pink product is washed three times each with deionized water and ethanol, and dried in a vacuum drying oven at 60 °C for 24 h to obtain the Co(OH)2 precursor. The remaining conditions are the same as in Example 1, and the CoP / NC hollow polyhedral nanomaterials are obtained, denoted as CoP / NC.

[0075] Comparative Example 2

[0076] A method for preparing CoMoO4 nanomaterials includes the following steps:

[0077] 1 mmol (291.03 mg) of Co(NO3)2·6H2O and 1 mmol (241.95 mg) of Na2MoO4·2H2O were dispersed in 15 mL of deionized water. After stirring at room temperature for 20 minutes, the mixture was transferred to a 50 mL reactor and placed in an oven at 160 °C for 6 h. The obtained Co-Mo precursor was washed with deionized water and ethanol by centrifugation to remove unreacted impurities, and then dried in a vacuum drying oven at 60 °C for 12 h. Finally, the dried product was annealed in air at a heating rate of 5 °C / min to 350 °C for 2 h to obtain CoMoO4 nanomaterials, denoted as CoMoO4.

[0078] Comparative Example 3

[0079] The preparation method of a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material is as described in Example 1, except that: in step (2), 3 mmol (0.726 g) of Na2MoO4·2H2O is weighed and dispersed in 20 mL of deionized water, and the other conditions are the same as in Example 1. The resulting sample is denoted as CoMoO4-CoP / NC-0.7258.

[0080] Comparative Example 4

[0081] The preparation method of a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material is as described in Example 1, except that: in step (3), the mass of NaH2PO4 is 400mg, and the other conditions are the same as in Example 1, which is denoted as CoMoO4-CoP / NC-400mg.

[0082] Comparative Example 5

[0083] The preparation method of a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material is as described in Example 1, except that: in step (3), the mass of NaH2PO4 is 900mg, and the other conditions are the same as in Example 1, which is denoted as CoMoO4-CoP / NC-900mg.

[0084] Comparative Example 6

[0085] The preparation method of a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material is as described in Example 1, except that in step (3), the temperature is raised to 250℃, and the other conditions are the same as in Example 1, which is denoted as CoMoO4-CoP / NC-250℃.

[0086] Comparative Example 7

[0087] The preparation method of a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material is as described in Example 1, except that in step (3), the temperature is raised to 350℃, and the other conditions are the same as in Example 1, which is denoted as CoMoO4-CoP / NC-350℃.

[0088] Experimental Example 1

[0089] The ZIF-67, CoMoO4-Co(OH)2 precursor, and CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material obtained in Example 1, the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material prepared in Example 2 and Comparative Example 3, the CoP / NC hollow polyhedral nanomaterial prepared in Comparative Example 1, and the CoMoO4 nanomaterial prepared in Comparative Example 2 were characterized and tested as follows:

[0090] Transmission electron microscopy: First, the transmission electron microscopy image of the ZIF-67 precursor obtained in Example 1 is as follows. Figure 1 As shown, ZIF-67 has a dodecahedral polyhedral structure with a smooth surface and a size of approximately 300–500 nm. A transmission electron microscope image of the CoMoO4-Co(OH)2 precursor obtained in Example 1 is shown below. Figure 2As shown, the morphology of CoMoO4-Co(OH)2 transforms into a hollow polyhedral cubic cage assembled from smooth-surfaced thin sheets, with a size of approximately 300–500 nm. This structure offers advantages such as increased specific surface area, improved utilization of active sites, shortened ion transport paths, and enhanced conductivity. A transmission electron microscope image of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material obtained in Example 1 is shown below. Figure 3 As shown, CoMoO4-CoP consists of rough-surfaced particles uniformly dispersed and anchored within a hollow polyhedral nitrogen-doped carbon framework, largely retaining the original morphology and structural advantages of the precursor, indicating that CoMoO4-CoP / NC also possesses excellent thermal stability. A transmission electron microscope image of the CoP / NC hollow polyhedral nanomaterial prepared in Comparative Example 1 is shown below. Figure 4 As shown, the carbon framework structure of CoP is severely collapsed, and the CoP particles are significantly aggregated, confirming that CoMoO4 in the composite phase improves the thermal stability of the material. Transmission electron microscopy images of the CoMoO4 nanomaterials prepared in Comparative Example 2 are shown below. Figure 5 As shown, without the presence of CoP, CoMoO4 nanorods would grow to a length of approximately 1 μm, which would significantly reduce electrocatalytic activity due to their larger size and smaller specific surface area. The transmission electron microscope image of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material prepared in Example 2 is shown below. Figure 6 As shown, its morphology can maintain the original polyhedral morphology of MOF, but the surrounding derivation of sheet-like structures is less than that of the heterostructure nanomaterial prepared in Example 1. This indicates that fewer active sites are provided, thus affecting the electrocatalytic activity. The transmission electron microscope of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material prepared in Comparative Example 3 is shown in Figure 3. Figure 7 As shown, it was found that when the amount of molybdenum source added was too high, the resulting composite material could not fully maintain the polyhedral structure of the MOF and structural collapse occurred.

[0091] High-resolution transmission electron microscopy: High-resolution transmission electron microscopy images of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material prepared in Example 1 are shown below. Figure 8 As shown, the interplanar spacings of the prepared CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material are 0.198, 0.231, 0.254, 0.279, 0.283 nm and 0.221 nm, respectively, which belong to the (112), (201), (200), (002) and (011) crystal planes of CoP and the (400) crystal plane of CoMoO4. This is consistent with the XRD results, and there is a clear heterostructure between CoMoO4 and CoP, indicating that a CoMoO4 / CoP heterostructure has been obtained.

[0092] Selected area electron diffraction (SEG) images: The selected area electron diffraction (SEG) images of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material prepared in Example 1 are shown below. Figure 9 As shown, the image shows diffraction rings corresponding to the (101) and (102) crystal planes of CoP and the (021) crystal plane of CoMoO4, further confirming the coexistence of CoMoO4 and CoP.

[0093] X-ray diffraction pattern: Figure 10 The X-ray diffraction patterns are shown for the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material prepared in Example 1, the CoP / NC hollow polyhedral nanomaterial prepared in Comparative Example 1, and the CoMoO4 nanomaterial prepared in Comparative Example 2. Besides the diffraction peaks at 31.60°, 36.31°, 48.13°, and 56.03° corresponding to the (011), (102), (211), and (020) crystal planes of the orthorhombic CoP phase, a diffraction peak corresponding to the (-132) crystal plane of the monoclinic α-CoMoO4 also appears at 38.30°, indicating that a CoMoO4-CoP composite phase was formed in Example 1 after annealing. Furthermore, the phase composition of CoP / NC and CoMoO4 also proves that only pure phases were obtained in Comparative Examples 1-2. Figure 11 The X-ray diffraction patterns of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite materials prepared in Examples 1-2 and Comparative Example 3 are shown. The comparison revealed that only the CoP and CoMoO4 phases were obtained, and no other diffraction peaks appeared. This indicates that the introduction of different molybdenum contents does not change the crystal structure.

[0094] X-ray photoelectron spectroscopy: Figure 12 (a) XPS full spectrum of CoMoO4-CoP / NC and CoP / NC prepared in Example 1. The characteristic peaks at 231 and 417 eV in CoMoO4-CoP / NC, which belong to the Mo 3d and Mo 3p orbitals, are considered to be the successful introduction of Mo. Figure 12 (b) is the fine spectrum of Co 2p, where the peaks at 779.60 and 782.58 eV in CoMoO4-CoP / NC correspond to Co 3+ and Co 2+ Furthermore, the introduction of Mo will increase Co. 3+ The ratio of [specific valence] and the average valence state of Co cause the Co site to be in an electron-deficient state. Figure 12 (c) is a fine spectrum of Mo 3d, in Mo 3d 5 / 2 and Mo 3d 3 / 2 Within the orbit, besides Mo located at 230.47 and 234.05 eV... 6+ In addition to the fitted peak, there are also tiny Mo values ​​at 232.30 and 235.81 eV. 4+The fitted peak shows that the introduction of Mo reduces the Mo content in CoMoO4. 6+ The ratio of α and β and the average valence state of α make the α sites in an electron-rich state. Figure 12 (d) shows the fine spectrum of P 2p, revealing that the P species in both materials are mainly in the form of PO4. 3- The PO bond exists in the middle, and Mo plays a role in inhibiting the oxidation of the CoP surface.

[0095] Experimental Example 2

[0096] Hydrogen evolution and oxygen evolution performance test

[0097] The hydrogen evolution and oxygen evolution performance of this invention was tested as follows:

[0098] The working electrode was prepared by dispersing 5 mg of catalyst sample in 500 μL of ethanol, simultaneously adding 20 μL of Nafion solution, and then sonicating for 1 h to form a homogeneous slurry. Then, 100 μL of the slurry was dropwise added to pretreated carbon paper, with a loading concentration of 1 mg cm⁻¹. -2 The catalyst samples were: CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite materials prepared in Examples 1-6, CoP / NC hollow polyhedral nanomaterials prepared in Comparative Example 1, CoMoO4 nanomaterials prepared in Comparative Example 2, commercially available Pt / C and RuO2 with a Pt content of 20wt%, and CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite materials prepared in Comparative Examples 3-7.

[0099] The polarization voltammetry (LSV) and cyclic voltammetry (CV) curves of this invention were tested on a CHI 660 electrochemical workstation. The electrolyte was a 1.0 mol / L KOH solution, which was purged with nitrogen for 30 min to remove oxygen. A graphite electrode and Ag / AgCl (3 mol / L KCl) were used as the counter electrode and reference electrode, respectively. The catalyst was supported on carbon paper (1*1 cm²). 2 And the area immersed in the electrolyte is 1 cm². 2 The scan rate in the LSV curve was set to 5 mV·s. -1 The iR compensation resistor can be obtained from the AC impedance test results.

[0100] In the AC impedance test of this invention, the DC voltage parameters are set to -0.236 / 1.564V (relative to the Ag / AgCl electrode), the frequency is from 100000Hz to 0.01Hz, and the amplitude is 0.005V.

[0101] The Tafel plot of this invention can be obtained by using an iR-compensated polarity curve, with the selected range being the interval between hydrogen evolution and oxygen evolution. Then, the Tafel slope is calculated to evaluate the kinetic performance of the catalyst in electrocatalytic hydrogen production.

[0102] The double-layer capacitor of the present invention was tested in the non-Radida region, and the CV curves (5-40 mVs) at different scan rates were obtained. -1 The interval is 5mV s -1 )get.

[0103] The hydrogen evolution and oxygen evolution properties of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material prepared in Example 1, the CoP / NC hollow polyhedral nanomaterial prepared in Comparative Example 1, the CoMoO4 nanomaterial prepared in Comparative Example 2, the commercially available Pt / C with a Pt content of 20 wt%, and RuO2 are as follows: Figure 13 and Figure 14 As shown.

[0104] Figure 13 (a) and Figure 14 (a) Linear polarization curves (LSV) of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material prepared in Example 1. As shown in the figure, the basic HER and OER overpotentials of CoMoO4-CoP / NC are only 122 and 280 mV (@10 mA cm⁻¹), respectively. -2 Furthermore, the hydrogen evolution and oxygen evolution properties of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material are superior to those of CoP / NC and CoMoO4.

[0105] Depend on Figure 13 (b) and 14(b) show that the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material has a small Tafel slope for hydrogen and oxygen evolution, with Tafel slopes of 68.22 mV dedc. -1 With 52.16mV dec -1 .

[0106] The double-layer capacitance of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material obtained in Example 1 is as follows: Figure 13 As shown in (c) and 14(c), the C of CoMoO4-CoP / NC dl The values ​​reached as high as 164.39 mF cm. -2 With 160.35mFcm -2 The specific surface area is much higher than that of CoP / NC and CoMoO4, and CoMoO4-CoP / NC has the largest ECSA, which is conducive to the full exposure of active sites and synergistic catalytic reactions among multiple sites, thanks to its high specific surface area.

[0107] The AC impedance image of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material obtained in Example 1 is shown below. Figure 13As shown in (d) and 14(d), it can be seen from the figures that the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material has a small electron transfer resistance, which is 4.58Ω and 4.56Ω, respectively.

[0108] In the stability test, the linear polarization curves of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material obtained in Example 1 before and after 1000, 3000, and 5000 CV cycles in the hydrogen evolution and oxygen evolution regions are as follows: Figure 13 As shown in (e) and 14(e), the almost overlapping linear polarization curves before and after cycling demonstrate that the electrocatalyst of this invention has excellent cycling stability for hydrogen evolution and oxygen evolution.

[0109] The CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material obtained in Example 1 was tested at 100 mA cm⁻¹. -2 The following constant current stability test is performed as follows: Figure 13 As shown in (f) and 14(f), the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material retains 93.29% and 78% of its activity after continuous electrolysis for 100 h, indicating good stability in hydrogen evolution reaction and oxygen evolution reaction.

[0110] Figure 15 This is a comparison of the linear polarization curves (a) and (b) of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite materials prepared in Examples 1-6. Figure 16 The graph shows a comparison of the linear polarization curves (a) and (b) of hydrogen evolution and oxygen evolution for the CoP / NC hollow polyhedral nanomaterial prepared in Comparative Example 1, the CoMoO4 nanomaterial prepared in Comparative Example 2, and the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite materials prepared in Comparative Examples 3-7. Figure 15 and Figure 16 It can be seen that the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material prepared in Example 1 of the present invention has high catalytic performance, which indicates that the raw material ratio in Example 1 of the present invention is optimal.

[0111] Experimental Example 3

[0112] The test method for total water hydrolysis is as follows:

[0113] In a typical two-electrode system, the CoMoO4-CoP / NC prepared in Example 1 serves as both the cathode and anode. The working electrode is prepared using the same method as the three-electrode system described above, with 100 μL of slurry still dropped onto the CP (ensuring a loading of 1 mg cm⁻¹). -2For comparison, CoP / NC prepared in Comparative Example 1 was used as both cathode and anode, as was CoMoO4 prepared in Comparative Example 2. Equal amounts of 20% Pt / C and RuO2 slurries were also drop-coated onto CP as cathode and anode, respectively. The electrolyte used for electrochemical testing was a 1.0 mol / L KOH aqueous solution, which required pre-purging with nitrogen for 30 minutes to remove oxygen. The carbon paper used needed to be pre-treated with a mixture of concentrated sulfuric acid and concentrated nitric acid to remove oxides and impurities, and the actual surface area immersed in the electrolyte was 1 cm². 2 Two-electrode tests for complete water splitting were conducted within a potential range of 0–2 V, with the scan rate of the linear polarization curve set to 5 mV·s. -1 .

[0114] The results of the total water electrolysis performance test are as follows:

[0115] The water-splitting properties of the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material obtained in Example 1, the CoP / NC hollow polyhedral nanomaterial prepared in Comparative Example 1, the CoMoO4 nanomaterial prepared in Comparative Example 2, and commercially available Pt / C and RuO2 with a Pt content of 20 wt% are as follows: Figure 17 As shown, the assembled two-electrode CoMoO4-CoP / NC||CoMoO4-CoP / NC only requires an applied potential of 1.62V to achieve a 10mA cm-wave amplitude. -2 The current density of CoMoO4-CoP / NC is 150 mV and 210 mV lower than that required by CoP / NC and CoMoO4-CoMoO4, respectively, indicating excellent catalytic activity for water electrolysis. Meanwhile, CoMoO4-CoP / NC exhibits excellent catalytic activity at 20 mA cm⁻¹. -2 After continuous electrolysis for 30 hours, the potential changed by only 0.06V, demonstrating good stability in alkaline water hydrolysis.

[0116] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art can make changes or modifications to the disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material, characterized in that, The microstructure of the composite material is as follows: CoMoO4-CoP nanoparticles are uniformly anchored on a hollow polyhedral N-doped carbon framework, and a heterostructure is formed between the CoMoO4 and CoP phases. The preparation method of the above-mentioned CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material includes the following steps: (1) The cobalt source solution was added to the organic ligand solution, stirred evenly, and allowed to stand at room temperature for reaction; then centrifuged and washed to obtain the metal-organic framework ZIF-67. (2) Disperse the metal-organic framework ZIF-67 in an organic solvent, add an aqueous solution of molybdenum source, and react; after centrifugation, washing, and drying, obtain the CoMoO4-Co(OH)2 precursor; the molar ratio of the metal-organic framework ZIF-67 to the molybdenum source is 1:1.2-4. (3) The CoMoO4-Co(OH)2 precursor and NaH2PO2 are annealed to obtain a CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon skeleton composite material; the mass ratio of the CoMoO4-Co(OH)2 precursor to NaH2PO2 is 1:25-40; the annealing temperature is 280-330℃ and the annealing time is 1-3h; the annealing steps are as follows: under the protection of inert gas, the CoMoO4-Co(OH)2 precursor and NaH2PO2 are placed downstream and upstream of the inert gas flow, respectively, and heated to the annealing temperature at a heating rate of 2-5℃ / min, and then held for heat treatment, and then naturally cooled to room temperature; the inert gas is argon.

2. The method for preparing the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material according to claim 1, characterized in that, The cobalt source in step (1) is cobalt nitrate hexahydrate, the organic ligand is 2-methylimidazole, and the molar ratio of the cobalt source to the organic ligand is 1:(6-9); The cobalt source solution is obtained by dispersing a cobalt source in methanol, and the concentration of the cobalt source solution is 0.01-0.1 mol / L; the organic ligand solution is obtained by dispersing an organic ligand in methanol, and the concentration of the organic ligand solution is 0.2-0.6 mol / L.

3. The method for preparing the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material according to claim 1, characterized in that, In step (1), the molar ratio of cobalt source to organic ligand is 1:8; the concentration of cobalt source solution is 0.05 mol / L; and the concentration of organic ligand solution is 0.4 mol / L.

4. The method for preparing the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material according to claim 1, characterized in that, The reaction time at room temperature in step (1) is 20-30 hours, and the washing is performed by centrifugation with methanol 3-5 times.

5. The method for preparing the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material according to claim 1, characterized in that, The organic solvent in step (2) is ethanol; the volume ratio of the organic solvent to the molar number of the metal-organic framework ZIF-67 is 60-100 mL: 1 mmol; the molybdenum source is sodium molybdate dihydrate, ammonium molybdate tetrahydrate or ammonium phosphomolybdate, and the concentration of the aqueous solution of the molybdenum source is 0.01-0.2 mmol / mL.

6. The method for preparing the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material according to claim 1, characterized in that, The molar ratio of the metal-organic framework ZIF-67 to the molybdenum source in step (2) is 1:2.5-3.

7. The method for preparing the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material according to claim 1, characterized in that, The reaction temperature in step (2) is 50-150℃, the reaction time is 0.5-4h, the washing is centrifugation washing with deionized water and ethanol 3-5 times in sequence, and the drying is vacuum drying at 60℃ for 12-24h.

8. The method for preparing the CoMoO4-CoP heterojunction / hollow polyhedral N-doped carbon framework composite material according to claim 1, characterized in that, The reaction temperature in step (2) is 80-90℃, and the reaction time is 1-2h.

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