A bifunctional catalyst of multiple heterojunctions and a preparation method thereof

Abstract

An embodiment of the present application discloses a bifunctional catalyst with multiple heterojunctions, comprising a NiCo-based precursor, a sulfidation layer, and an amorphous coating layer, wherein the sulfidation layer covers at least part of the surface of the NiCo-based precursor, and the amorphous coating layer wraps the sulfidation layer and is a nickel-manganese layered double hydroxide (NiMn LDH) layer. The catalyst has good hydrogen evolution reaction performance and urea oxidation performance. The present application also discloses a preparation method of the bifunctional catalyst with multiple heterojunctions.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of nanomaterials and electrocatalysis, and particularly relates to a multi-heterojunction catalyst. BACKGROUND

[0002] Ni-based compounds have excellent urea oxidation performance, and when ion doping is introduced to further enhance the urea oxidation performance of the Ni-based compounds, the ion doping is easy to cause chemical bond breakage and form vacancies, thereby affecting charge transport and d-orbital electron states. In addition, they do not have good hydrogen evolution reaction (HER) performance, and cannot meet the industrialization demand. Therefore, developing a catalyst for urea oxidation (UOR) and HER is crucial for the wide application of water electrolysis. SUMMARY

[0003] The present application aims to provide a multi-heterojunction bifunctional catalyst with good hydrogen evolution reaction performance and urea oxidation performance.

[0004] To achieve the above-mentioned purpose, one embodiment of the present application adopts the following technical solution:

[0005] A multi-heterojunction bifunctional catalyst comprises a NiCo-based precursor, a sulfidation layer, and an amorphous coating layer, the sulfidation layer covers at least part of the surface of the NiCo-based precursor, and the amorphous coating layer wraps the sulfidation layer, and the amorphous coating layer is a nickel-manganese layered double hydroxide (NiMn LDH) layer.

[0006] In the above technical solution, the catalyst comprises a NiCo-based precursor, a sulfidation layer, and an amorphous coating layer, the sulfidation layer covers at least part of the surface of the NiCo-based precursor, and the amorphous coating layer wraps the sulfidation layer, and the surface of the crystal-crystal sulfidation layer is loaded with amorphous NiMn LDH, and the three-dimensional hierarchical structure promotes the diffusion of the solution and the gas and increases the number of active sites. At the same time, the rich crystal-crystal and amorphous-crystal interfaces improve the surface electronic structure of the catalyst, which is conducive to the adsorption of intermediates generated in the HER and UOR processes, thereby enhancing the activity and kinetics of the catalyst for HER and UOR.

[0007] A preparation method of a multi-heterojunction bifunctional catalyst comprises the following steps:

[0008] Preparation of a NiCo-based precursor (NiCo-precursor / NF): mixing cobalt chloride hexahydrate (CoCl2·6H2O), nickel chloride hexahydrate (NiCl2·6H2O), and urea in a solvent to obtain a mixed solution, and placing the mixed solution and foam nickel (NF) in a reaction kettle for reaction;

[0009] Preparation of Ni3S2 / Co9S8 / NF: A Na2S·9H2O solution was prepared, and the NiCo-based precursor and the Na2S·9H2O solution were placed in a reaction kettle;

[0010] Preparation of NiMn LDH / Ni3S2 / Co9S8 / NF: NiCl2·6H2O and MnCl2·4H2O were dissolved in a solvent to prepare an electrodeposition solution, and electrodeposition was performed using a three-electrode system.

[0011] In the above technical solution, the NiMn LDH / Ni3S2 / Co9S8 / NF prepared by the above method has a crystal-crystal Ni3S2 / Co9S8 heterojunction and a surface loaded with amorphous NiMn LDH. The three-dimensional hierarchical structure promotes the diffusion of the solution and the gas, and increases the number of active sites. At the same time, the rich crystal-crystal and amorphous-crystal interfaces improve the surface electronic structure of the catalyst, which is beneficial to the adsorption of intermediates produced by the HER and UOR processes, thereby enhancing the activity and kinetics of the catalyst for HER and UOR. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 is an X-ray diffraction (XRD) pattern of the NiMn LDH / Ni3S2 / Co9S8 / NF obtained in Example 1;

[0013] Figure 2 is an SEM image of the NiCo-precursor / NF obtained in Example 1;

[0014] Figure 3 is an SEM image of the Ni3S2 / Co9S8 / NF obtained in Example 1;

[0015] Figure 4 is an SEM image of the NiMn LDH / Ni3S2 / Co9S8 / NF obtained in Example 1;

[0016] Figure 5 is a TEM image of the NiMn LDH / Ni3S2 / Co9S8 / NF obtained in Example 1;

[0017] Figure 6 is a HRTEM image of the NiMn LDH / Ni3S2 / Co9S8 / NF obtained in Example 1;

[0018] Figure 7 is a hydrogen evolution curve of the NiCo-precursor / NF, Ni3S2 / Co9S8 / NF, NiMn LDH / Ni3S2 / Co9S8 / NF obtained in Example 1, and NiMn LDH / NF obtained in Comparative Example 2 in a 1.0M KOH solution;

[0019] Figure 8 Electrochemical impedance spectroscopy (EIS) graphs of NiCo-precursor / NF, Ni3S2 / Co9S8 / NF, NiMn LDH / Ni3S2 / Co9S8 / NF obtained in Example 1 and NiMn LDH / NF obtained in Comparative Example 2;

[0020] Figure 9 Graphs of double-layer capacitance of NiCo-precursor / NF, Ni3S2 / Co9S8 / NF, NiMn LDH / Ni3S2 / Co9S8 / NF obtained in Example 1 and NiMn LDH / NF obtained in Comparative Example 2;

[0021] Figure 10 The urea oxidation curves of NiCo-precursor / NF, Ni3S2 / Co9S8 / NF, NiMn LDH / Ni3S2 / Co9S8 / NF obtained in Example 1 and NiMn LDH / NF obtained in Comparative Example 2 in a mixed solution of 1.0 M KOH and 0.33 M urea;

[0022] Figure 11 is the Tafel slope of NiCo-precursor / NF, Ni3S2 / Co9S8 / NF, NiMn LDH / Ni3S2 / Co9S8 / NF obtained in Example 1 and NiMn LDH / NF obtained in Comparative Example 2;

[0023] Figure 12 The performance comparison curves of the electrolytic cell assembled with the NiMn LDH / Ni3S2 / Co9S8 / NF obtained in Example 1 as cathode and anode for the complete decomposition of water in a 1.0 M KOH solution and the complete decomposition of urea in a mixed solution of 1.0 M KOH and 0.33 M urea are shown;

[0024] Figure 13 The performance comparison curves of the complete urea decomposition in a mixed solution of 1.0 M KOH and 0.33 M urea using the NiMn LDH / Ni3S2 / Co9S8 / NF obtained in Example 1 as the cathode and anode to assemble an electrolytic cell and an electrolytic cell assembled with Pt / C and RuO2;

[0025] Figure 14 The urea complete dissolution stability test curve of the electrolytic cell assembled with NiMn LDH / Ni3S2 / Co9S8 / NF obtained in Example 1 as cathode and anode in a mixed solution of 1.0 M KOH and 0.33 M Urea. [Specific implementation method]

[0026] The present application will be further described below with reference to the accompanying drawings and specific embodiments:

[0027] In order for those skilled in the art to better understand the technical solutions of the present application, the present application will be further described in detail below in combination with the drawings and specific embodiments. Obviously, the drawings in the following description are only some embodiments of the present application, and all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0028] A multi-heterojunction bifunctional catalyst includes a NiCo-based precursor, a sulfidation layer, and an amorphous coating layer. The sulfidation layer covers at least part of the surface of the NiCo-based precursor, and the amorphous coating layer wraps the sulfidation layer. The amorphous coating layer is a nickel-manganese layered double hydroxide (NiMn LDH) layer. The surface of the crystal-crystal sulfidation layer is loaded with amorphous NiMn LDH. The three-dimensional hierarchical structure promotes the diffusion of the solution and the gas, and increases the number of active sites. At the same time, the rich crystal-crystal and amorphous-crystal interfaces improve the surface electronic structure of the catalyst, which is beneficial to the adsorption of intermediates generated by the HER and UOR processes, thereby enhancing the activity and kinetics of the catalyst for HER and UOR. In an embodiment, the sulfidation layer is formed by sulfidizing the NiCo-based precursor with a sulfidation agent. The sulfidation agent is at least one of Na2S, thiourea, H2S, and sulfur powder. The NiCo-based precursor is Ni(OH)2-Co(OH)2 / NF. The diameter of the multi-heterojunction bifunctional catalyst is 55-80 nm.

[0029] A method for preparing a multi-heterojunction bifunctional catalyst includes:

[0030] Preparation of a NiCo-based precursor (NiCo-precursor / NF): mixing cobalt chloride hexahydrate (CoCl2·6H2O), nickel chloride hexahydrate (NiCl2·6H2O), and urea (Urea) in a solvent to obtain a mixed solution, and placing the mixed solution and foam nickel (NF) in a reaction kettle for reaction.

[0031] Preparation of Ni3S2 / Co9S8 / NF: configuring a Na2S·9H2O solution, and placing the NiCo-based precursor and the Na2S·9H2O solution in a reaction kettle.

[0032] Preparation of NiMn LDH / Ni3S2 / Co9S8 / NF: dissolving NiCl2·6H2O and MnCl2·4H2O in a solvent to prepare an electrodeposition solution, and performing electrodeposition using a three-electrode system.

[0033] The NiMn LDH / Ni3S2 / Co9S8 / NF, a crystal-crystal Ni3S2 / Co9S8 heterojunction, and amorphous NiMn LDH loaded on its surface, prepared using the aforementioned method, exhibit a three-dimensional hierarchical structure that facilitates solution and gas diffusion and increases the number of active sites. Furthermore, the abundant crystal-crystal and amorphous-crystal interfaces improve the catalyst's surface electronic structure, facilitating the adsorption of intermediates produced during the HER and UOR processes, thereby enhancing their catalytic activity and kinetics for HER and UOR.

[0034] In one embodiment, the molar ratio of cobalt chloride hexahydrate (CoCl 2 ·6H 2 O) to nickel chloride hexahydrate (NiCl 2 ·6H 2 O) is 2:1.

[0035] In one embodiment, in the step of preparing the NiCo-based precursor, the mixed solution and NF are placed in a reactor and reacted in an oven at 100° C. for 8 h to obtain a reactant, which is then rinsed alternately with deionized water and ethanol and dried in an oven at 60° C. for 4 h.

[0036] In one embodiment, in the step of preparing Ni3S2 / Co9S8 / NF, the NiCo-based precursor and the Na2S·9H2O solution were transferred to a reactor, heated in an oven at 100°C for 4 hours, and then cooled to room temperature. The reactor was then rinsed alternately with water and ethanol three times, and then dried in an oven at 60°C for 4 hours.

[0037] In one embodiment, the three-electrode system includes a graphite sheet as a counter electrode, Ag / AgCl as a reference electrode, and Ni3S2 / Co9S8 / NF as a working electrode.

[0038] In one embodiment, the electrodeposition solution temperature is 80° C., the deposition potential is −1.1 V vs. Ag / AgC, the electrodeposition time is 100 s, and after electrodeposition, the deposited product is alternately rinsed with H 2 O and ethanol and dried in a vacuum oven at 60° C. for 4 h.

[0039] In one embodiment, in the steps of preparing the NiCo-based precursor and preparing the Ni3S2 / Co9S8 / NF, the solvent is water.

[0040] In one embodiment, the molar ratio of NiCl2·6H2O:MnCl2·4H2O is 1.2:1.

[0041] Example 1

[0042] In 30 mL of water, 0.095 g of CoCl2·6H2O, 0.048 g of NiCl2·6H2O, and 0.3 g of urea were added, and after being mixed uniformly, a mixed solution was obtained. The mixed solution and foamed nickel (NF) were transferred into a 50 mL reaction kettle, and a reactant was obtained by reacting in an oven at 100°C for 8 h. The reactant was washed with deionized water and ethanol alternately and was dried in an oven at 60°C for 4 h to obtain a NiCo-based precursor.

[0043] Preparation of Ni3S2 / Co9S8 / NF: A 6.7 mol / L Na2S·9H2O solution was prepared, and the above-prepared NiCo-based precursor and 36 mL of the Na2S·9H2O solution were transferred into a reaction kettle. After being heated in an oven at 100°C for 4 h and cooled to room temperature, the product was washed with water and ethanol alternately for three times, and was dried in an oven at 60°C for 4 h after washing.

[0044] Preparation of NiMn LDH / Ni3S2 / Co9S8 / NF: 1.2 mmol of NiCl2·6H2O and 1.0 mmol of MnCl2·4H2O were dissolved in 50 mL of water to prepare an electrodeposition solution. An electrodeposition was performed by using a three-electrode system, in which a graphite sheet was used as a counter electrode, an Ag / AgCl was used as a reference electrode, and the Ni3S2 / Co9S8 / NF was used as a working electrode. The deposition potential was -1.1 V vs. Ag / AgCl, the solution temperature was 80°C, and the deposition time was 100 s. After the electrodeposition, the deposition product was washed with H2O and ethanol alternately, and was dried in a vacuum oven at 60°C for 4 h.

[0045] Comparative Example 1

[0046] Foamed nickel (NF) (1×6 cm 2)10 minutes to remove surface oxides. The sublimed sulfur powder was then dissolved in 85% hydrazine hydrate to form a homogeneous solution of 0.4 mol / L (labeled as S-hydrazine hydrate solution). In a typical synthesis of Ni3S2 / NF, a piece of cleaned NF was immersed in 70 mL of the mixed solution, which contained 60 mL of distilled water and 10 mL of S-hydrazine hydrate solution, in a 100 mL Teflon-lined stainless steel autoclave. The autoclave was then placed in an oven and heated to 160 °C for 10 hours. After natural cooling to room temperature, Ni3S2 / NF was obtained. Second, Ni(OH)2was electrodeposited from 0.1 mol / L aqueous Ni(NO3)2solution at 70 °C for 45 minutes at a constant potential of -0.8 V in a three-compartment system. Ni3S2 / NF, saturated calomel electrode (SCE), and platinum wire were used as the working electrode (WE), reference electrode (RE), and counter electrode (CE), respectively. After washing with distilled water and ethanol three times and drying in a vacuum oven, a vermiculite-like Ni3S2-Ni(OH)2hybrid supported on foam nickel (labeled as v-Ni3S2-Ni(OH)2 / NF) was successfully prepared.

[0047] Comparative Example 2

[0048] Preparation of NiMn LDH / NF: 1.2 mmol of NiCl2·6H2O and 1.0 mmol of MnCl2·4H2O were dissolved in 50 mL of water to prepare an electrodeposition solution. Electrodeposition was carried out using a three-electrode system, in which a graphite sheet was used as the counter electrode, Ag / AgCl as the reference electrode, and NF as the working electrode. The deposition potential was -1.1 V vs. Ag / AgCl, the solution temperature was 80 °C, and the deposition time was 100 s. After electrodeposition, the deposition product was washed with H2O and ethanol alternately and was placed in a vacuum oven at 60 °C for drying for 4 h.

[0049] The specifications and manufacturers of the reagents used above are as follows:

[0050]

[0051] To verify the successful synthesis of the dual-functional catalyst with multiple heterojunctions of the present application, the structure thereof was characterized.

[0052] The phase composition of the samples was characterized by X-ray powder diffractometer (XRD, Bruker D8 Discover Cu target). The morphology and lattice information of the samples were analyzed by scanning electron microscope (FESEM, Hitachi S-4800) and transmission electron microscope (TEM, JEM-2100). The UOR performance of the catalyst in 0.33 M urea and 1.0 M potassium hydroxide solution was tested by an electrochemical workstation (CHI760E). The linear sweep voltammetry (LSV) test rate was 5 mV·s-1. All potentials were corrected by the following formula:

[0053] E RHE =E Hg / HgO +0.098+0.0591×pH-IR

[0054] The overpotential was further compensated by combining I (current in LSV) and solution resistance (R). Electrochemical impedance spectroscopy (EIS) measurement was carried out at 1.326 V vs. RHE with a frequency of 100000-0.1 Hz. The double-layer capacitance (Cdl) was derived from the cyclic voltammogram (CV) curve between 0.986 V vs. RHE and 1.126 V vs. RHE. The current density difference at the intermediate potential was plotted against the scan rate, and the half of the linear slope was the C dl value.

[0055] Figure 1 X-ray diffraction (XRD) pattern of the NiMn LDH / Ni3S2 / Co9S8 / NF obtained in the present application. Figure 1 In the figure, the horizontal axis represents the X-ray diffraction angle (degree), and the vertical axis represents the X-ray diffraction intensity. The XRD patterns of the NiCo-precursor / NF, Ni3S2 / Co9S8 / NF and NiMn LDH / Ni3S2 / Co9S8 / NF are as follows: Figure 1The three peaks at 44.29°, 51.59°, 76.21° belong to the foamed nickel. In the XRD pattern of the NiCo-precursor / NF, the peak at 9.95° corresponds to the (001) crystal plane of Co(OH)2(Joint Committee on Powder Diffraction Standards (JCPDS) card No. 51-1731) and the (101), (012) crystal planes of Ni(OH)2(JCPDS No. 38-0715) at 33.66°, 39.65°. After sulfidation, the diffraction peaks at 21.62°, 31.01°, 37.76°, 38.46°, 50.02°, 55.23° are generated, which correspond to the (101), (110), (003), (021), (101), (110), (003), (021), (211), (122) crystal plane diffraction peaks of Ni3S2(JCPDS No. 44-1418) respectively, and there is also a peak at 29.67°, which corresponds to the (311) crystal plane of Co9S8(JCPDS No. 19-0364). It can be seen that after sulfidation of the NiCo-precursor / NF, the generated product is a composite of Ni3S2and Co9S8. After further electrodeposition treatment, there is an additional broadened peak near ~11.62°, which belongs to the NiMn LDH. Since the derived peak is steamed bun-shaped, it indicates that the prepared deposited NiMn LDH is amorphous.

[0056] Figures 2 to 4 are the scanning electron microscope (SEM) images of the NiCo-precursor / NF, Ni3S2 / Co9S8 / NF and NiMn LDH / Ni3S2 / Co9S8 / NF obtained by the implementation of the present application, respectively. Please refer to Figure 2 The NiCo-precursor / NF appears as a needle-shaped nanowire with a uniform surface, and the diameter of the nanowire is about 50 nm. Please refer to Figure 3 After sulfidation treatment, the generated Ni3S2 / Co9S8 / NF composite is still a needle-shaped nanowire, and the diameter of the nanoneedle is about 60 nm, and the tip and surface thereof are distributed with small particles, showing a rough morphology, indicating that the initial NiCo-precursor / NF is sulfidized by Na2S to generate new substances. Please refer to Figure 4, continue to deposit amorphous NiMn LDH on the surface of Ni3S2 / Co9S8 / NF composite, the nanowire surface wrapped many nanosheets, and the diameter of the nanosheet wrapped increased by about 10 nm. It can be seen that the deposited amorphous NiMn LDH is an ultra-thin nanosheet. After loading amorphous NiMn LDH, the nanowire becomes a core-shell type structure, and the core-shell nanowire is beneficial to the exposure of active sites, the diffusion of gas and solution, and the electron transfer on the surface of the catalyst, thereby expected to enhance the catalytic activity.

[0057] Figure 5 is the transmission electron microscopy (TEM) diagram of the NiMn LDH / Ni3S2 / Co9S8 / NF obtained by the implementation of the present application. From Figure 5 It can be seen that the NiMn LDH / Ni3S2 / Co9S8 / NF microscopically is a needle-like nanowire; the needle tip surface is composed of many particles, and a layer of ultra-thin nanosheet is wrapped around the nanowire periphery, and the width of the nanosheet is about 10 nm, which is consistent with the SEM result. In addition, from Figure 6 It can be seen that there are clear lattice fringes in the nanowire, and no obvious lattice fringes are observed on the surface of the wrapped nanosheet, indicating that the prepared Ni3S2 / Co9S8 / NF composite nanowire is in a crystalline form, and the deposited NiMn LDH is in an amorphous form. While the IFFT conversion is carried out on the selected region inside the nanowire, the lattice fringe spacing of the blue dashed line selected area is 0.288 nm, which corresponds to the (110) crystal plane of Ni3S2. The lattice spacing corresponding to the yellow selected area is 0.293 nm, which corresponds to the (311) crystal plane of Co9S8. From the above, it can be seen that the prepared NiMn LDH / Ni3S2 / Co9S8 / NF composite is composed of amorphous NiMn LDH and crystalline Ni3S2 and Co9S8, and the three-phase material of NiMn LDH, Ni3S2 and Co9S8 forms a crystal-crystal and crystal-amorphous interface. The synergistic effect between them will be beneficial to accelerate the electrocatalytic reaction on the surface of the catalyst.

[0058] Figure 7The hydrogen evolution curves of NiCo-precursor / NF, Comparative Example 2 (NiMn LDH / NF), Ni3S2 / Co9S8 / NF and NiMn LDH / Ni3S2 / Co9S8 / NF in 1.0 mol / L KOH (1.0 M KOH) solution are shown. It can be seen that the performance of NiCo-precursor / NF is poorer than that of NiMn LDH / NF, Ni3S2 / Co9S8 / NF and NiMn LDH / Ni3S2 / Co9S8 / NF, while the hydrogen evolution performance of Ni3S2 / Co9S8 / NF formed after sulfidation is improved. In addition, the performance of pure NiMn LDH / NF is relatively good. When amorphous NiMn LDH is loaded on the surface of Ni3S2 / Co9S8 / NF, the performance reaches the best. Taking the overpotential at 10 mA·cm -2 Taking the overpotential at 10 mA·cm

[0059] The Tafel and overpotential of hydrogen evolution of Example 1 and Comparative Example 1 are shown in Table 1 below:

[0060] Table 1

[0061]

[0062] As can be seen from Table 1, the Tafel and overpotential of hydrogen evolution of NiMn LDH / Ni3S2 / Co9S8 / NF are both smaller than those of Ni3S2@Ni(OH)2 / NF, because a synergistic effect is generated between the crystalline Ni3S2 / Co9S8 and the amorphous NiMn LDH, which enhances the HER catalytic performance.

[0063] Figure 8 The electrochemical impedance diagrams of NiCo-precursor / NF, NiMn LDH / NF, Ni3S2 / Co9S8 / NF and NiMn LDH / Ni3S2 / Co9S8 / NF can reflect the charge transfer resistance of the catalyst. From the Figure 8It can be seen that the charge transfer resistance of NiMn LDH / Ni3S2 / Co9S8 / NF is the smallest, 1.81 Ω, which is smaller than 4.03 Ω of NiMn LDH / NF, 26.88 Ω of Ni3S2 / Co9S8 and 30.20 Ω of NiCo-precursor / NF. This is mainly because the NiMn LDH / Ni3S2 / Co9S8 / NF core-shell nanoneedle has excellent charge transport capacity, and the multiple heterojunctions formed enhance the transfer rate of interface electrons, thereby reducing the surface charge transfer resistance.

[0064] To explore the reason for the high activity of the heterojunction catalyst, the Figure 9 It can be seen that the Cdl value of each catalyst in descending order is: NiCo-precursor / NF (4.3 mF·cm -2 ), Ni3S2 / Co9S8 / NF (5.3 mF·cm -2 ), NiMn LDH / NF (14.3 mF·cm -2 ), NiMn·LDH / Ni3S2 / Co9S8 / NF (42.9 mF·cm -2 ). The three-dimensional hierarchical nanowire has abundant active sites, and the amorphous NiMn LDH is further loaded on the crystal-crystal Ni3S2 / Co9S8 heterojunction. Due to the rich unsaturated sites and dangling bonds of the amorphous structure, the catalyst has more active sites, increases the electrochemical active area, and thus improves the performance of the catalyst.

[0065] Figure 10 The urea oxidation curves of NiCo-precursor / NF, NiMn LDH / NF, Ni3S2 / Co9S8 / NF and NiMn LDH / Ni3S2 / Co9S8 / NF and the NiMn LDH / NF obtained in Example 2 in a 1.0 M KOH and 0.33 mol / L (0.33 M) urea (Urea) mixed solution, from Figure 10 It can be seen that, compared with Ni3S2 / Co9S8 / NF and NiMn LDH / Ni3S2 / Co9S8 / NF, the performance of NiCo-pre / NF is poorer, while the performance of the Ni3S2 / Co9S8 / NF formed after sulfidation is improved. In addition, pure NiMn LDH / NF also shows good UOR catalytic performance. Importantly, when amorphous NiMn LDH is loaded on the surface of Ni3S2 / Co9S8 (NiMn LDH / Ni3S2 / Co9S8 / NF), the UOR performance is the best. At 100 mA·cm -2For example, the overpotential of NiCo-precursor / NF, NiMn LDH / NF, Ni3S2 / Co9S8 / NF and NiMn LDH / Ni3S2 / Co9S8 / NF is 1.657, 1.389, 1.365 and 1.343 V vs. RHE, respectively. The excellent UOR performance of NiMn LDH / Ni3S2 / Co9S8 / NF is mainly due to the amorphous-crystal and crystal-crystal interfaces in NiMn LDH and Ni3S2 / Co9S8, which change the electronic structure of the interface, reduce the phase transition energy barrier of NiOOH, and facilitate the breaking of C-N and N-H bonds in urea molecules, thereby enhancing the catalytic activity.

[0066] From Figure 11 It can be seen that the Tafel slope value of NiCo-precursor / NF is 130.8 mV·dec-1, while the Tafel value of Ni3S2 / Co9S8 / NF becomes 40.2 mV·dec-1, close to that of NiMn LDH / NF alone (33.6 mV·dec-1). When amorphous NiMn LDH is deposited on the surface of Ni3S2 / Co9S8 / NF, the Tafel slope is 27.1 mV·dec-1, mainly due to the interface synergistic effect between amorphous NiMn LDH and Ni3S2 / Co9S8, which enhances the catalytic kinetics of UOR. In addition, with the construction of heterojunction, the charge transfer resistance also shows a gradually decreasing trend, and the surface charge transfer of NiMn LDH / Ni3S2 / Co9S8 / NF is the fastest, with an Rct value of 2.1 Ω, which is smaller than that of NiCo-precursor / NF, NiMn LDH / NF and Ni3S2 / Co9S8 / NF. The unique three-dimensional hierarchical structure formed by nanoneedles and the rich heterojunction promote the transmission of electrons.

[0067] Since NiMn LDH / Ni3S2 / Co9S8 / NF has good HER and excellent UOR catalytic performance. It is assembled into a two-electrode water electrolysis cell as a bifunctional electrode. From Figure 12 It can be seen that, with the assistance of urea, the cell voltage of NiMn LDH / Ni3S2 / Co9S8 / NF·||·NiMn LDH / Ni3S2 / Co9S8 / NF for electrolysis of water is reduced, for example, at 100 mA·cm -2 , the cell voltage of urea-assisted water electrolysis is reduced by 279 mV, which means that the energy consumption is reduced, which shows that urea-assisted hydrogen evolution is an energy-saving method for hydrogen production. In addition, from Figure 13 It can be seen that, in the process of complete urea decomposition, the above-mentioned electrode has a lower cell voltage than Pt / C / NF·||·RuO2 / NF. For example, at 20 mA·cm -2The time slot voltage was 1.498 V, which was lower than 1.537 V of Pt / C / NF||RuO2 / NF. From Figure 14 It can be seen that NiMn LDH / Ni3S2 / Co9S8 / NF||NiMn LDH / Ni3S2 / Co9S8 / NF can be stably operated at 20 mA·cm-2 for 36 h when full urea is decomposed, and the final slot voltage only increases by 7.4%. -2 It can be seen that NiMn LDH / Ni3S2 / Co9S8 / NF||NiMn LDH / Ni3S2 / Co9S8 / NF can be stably operated at 20 mA·cm-2 for 36 h when full urea is decomposed, and the final slot voltage only increases by 7.4%.

[0068] It should be noted that the above examples are only used to illustrate the present application and not to limit the technical solutions described in the present application. Although the present application has been described in detail with reference to the above examples, those skilled in the art should understand that the skilled in the art can still modify or equivalently replace the present application, and all technical solutions and improvements which do not deviate from the spirit and scope of the present application should be covered within the scope of the claims of the present application.

Claims

1. A bifunctional catalyst with multiple heterojunctions, comprising a NiCo-based precursor, a sulfidation layer, and an amorphous coating layer, wherein the sulfidation layer covers at least part of the surface of the NiCo-based precursor, and the amorphous coating layer is a NiMn LDH layer covering the sulfidation layer, and the NiCo-based precursor is Ni (OH) 2-Co (OH) 2 / NF.

2. The bifunctional catalyst of multiple heterojunctions according to claim 1, wherein, The sulfidation layer is formed by sulfidation of the NiCo-based precursor using at least one of Na 2 S, thiourea, H 2 S, and sulfur powder.

3. The bifunctional catalyst of multiple heterojunctions according to claim 1, wherein, The diameter of the catalyst is 55-80 nm. 4.A method for preparing the bifunctional catalyst with multiple heterojunctions according to any one of claims 1 to 3, comprising: Preparation of the NiCo-based precursor (NiCo-precursor / NF): mixing cobalt chloride hexahydrate (CoCl 2 ·6H 2 O), nickel chloride hexahydrate (NiCl 2 ·6H 2 O), and urea in a solvent to obtain a mixed solution, and placing the mixed solution and foam nickel (NF) in a reaction kettle for reaction; Preparation of Ni 3 S 2 / Co 9 S 8 / NF: preparing a Na 2 S·9H 2 O solution, and placing the NiCo-based precursor and the Na 2 S·9H 2 O solution in a reaction kettle for reaction; Preparation of NiMn LDH / Ni 3 S 2 / Co 9 S 8 / NF: dissolving NiCl 2 ·6H 2 O and MnCl 2 ·4H 2 O in a solvent to prepare an electrodeposition solution, and performing electrodeposition using a three-electrode system.

5. The preparation method according to claim 4, characterized in that The molar ratio of cobalt chloride hexahydrate (CoCl 2 ·6H 2 O) to nickel chloride hexahydrate (NiCl 2 ·6H 2 O) is 2:

1.

6. The preparation method according to claim 4, characterized in that The three-electrode system uses a graphite sheet as the counter electrode, Ag / AgCl as the reference electrode, and Ni 3 S 2 / Co 9 S 8 / NF as the working electrode.

7. The preparation method according to claim 4, characterized in that The temperature of the electrodeposition solution is 80 ℃, the deposition potential is -1.1 V vs. Ag / AgC, the electrodeposition time is 100 s, and after electrodeposition, the deposition product is washed with H 2 O and ethanol alternately, and then placed in a 60 ℃ vacuum oven for drying for 4 h.

8. The preparation method according to claim 4, characterized in that In the preparation of the NiCo-based precursor and the preparation of Ni 3 S 2 / Co 9 S 8 / NF, the solvent is water.

9. The method of any one of claims 4-8, wherein, The molar ratio of NiCl 2 ·6H 2 O to MnCl 2 ·4H 2 O is 1.2:1.

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