NiFePS / Co-ZIF-67 electrocatalyst as well as preparation method and application thereof

Abstract

The invention provides a NiFePS / Co-ZIF-67 electrocatalyst as well as a preparation method and application of the NiFePS / Co-ZIF-67 electrocatalyst. The NiFePS / Co-ZIF-67 electrocatalyst is of a three-dimensional composite nanostructure formed by stacking NiFePS nanospheres on Co-ZIF-67 nanosheets grown on foamed nickel in situ. When the NiFePS / Co-ZIF-67 electrocatalyst is used as a difunctional electrocatalyst for full water splitting, the excellent difunctional full water splitting performance is shown, and the NiFePS / Co-ZIF-67 electrocatalyst has good catalytic stability under the conditions of HER, OER and full water splitting.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electrocatalytic water splitting, and particularly relates to a Co-ZIF-67 nanosheet stacked NiFePS nanosphere (NiFePS / Co-ZIF-67) electrocatalyst, a preparation method and application thereof. BACKGROUND

[0002] The use of fossil fuels (coal, oil, and natural gas) has led to a sharp increase in carbon dioxide (CO2) emissions, exacerbating global warming and environmental pollution. Electrochemical water splitting to produce hydrogen can convert renewable energy (such as wind and solar energy) into hydrogen through electrolysis, and the entire process does not produce greenhouse gases, which is an effective way to solve environmental problems caused by the dependence on traditional fossil fuels. Under the "double carbon target", it has become an important development direction. The core of electrochemical water splitting to produce hydrogen is an electrocatalyst. Most existing commercial electrocatalysts are noble metals such as platinum and ruthenium. Although these noble metals have better catalytic performance, they are expensive and difficult to meet the needs of large-scale applications. Therefore, it is of great practical significance to explore efficient and stable non-noble metal electrocatalysts.

[0003] Nickel-iron-based materials are the core system of non-noble metal water electrolysis catalysts, and have the potential to replace commercial noble metal catalysts due to their dual-metal synergistic effect, adjustable electronic structure, and low cost. In particular, they exhibit good overall water splitting performance in alkaline environments. However, their practical application still faces the following bottlenecks: there is still a gap between intrinsic activity and noble metal benchmarks, the synthesis path is relatively complex and not conducive to large-scale production, and the stability of long-term electrolysis under high current density is poor. Therefore, in order to optimize the electrochemical performance of nickel-iron-based catalysts and make them practical, targeted synthesis strategies still need to be designed. SUMMARY

[0004] In order to solve the problems of low water electrolysis activity and poor long-term electrolysis stability under high current density of the existing nickel-iron-based catalysts in the prior art, the application provides a NiFePS / Co-ZIF-67 electrocatalyst, a preparation method and application thereof.

[0005] The application is achieved by the following technical solutions: In a first aspect, the application provides a NiFePS / Co-ZIF-67 electrocatalyst with excellent catalytic performance and low cost. The catalyst is a three-dimensional composite nanostructure of Co-ZIF-67 nanosheet stacked NiFePS nanosphere grown in situ on a foam nickel. Specifically, the NiFePS / Co-ZIF-67 electrocatalyst comprises a foam nickel, the surface of the foam nickel is loaded with Co-ZIF-67 nanosheets, and the Co-ZIF-67 nanosheets are loaded with NiFePS nanospheres. The Ni in the NiFePS nanospheres comprises Ni 2+ and Ni3+ Two valence states, Fe contains Fe 2+ , Fe 3+ and Fe 0 Three valence states; Co in Co-ZIF-67 nanosheets contains Co 2+ and Co 3+ Two valence states.

[0006] There are Ni-S, Fe-O, Ni-O, Fe-S and other multi-metal bonds in the structure of the NiFePS / Co-ZIF-67 electrocatalyst, and there is a synergistic effect between the multi-metal bonds, so that the NiFePS / Co-ZIF-67 electrocatalyst has a large double-layer capacitance (45~60 mF·cm -2 ), can expose a large number of active sites, and can be applied to electrocatalytic water splitting.

[0007] In a second aspect, the present application provides a preparation method of a NiFePS / Co-ZIF-67 electrocatalyst, comprising the following steps: S1, immerse the foamed nickel in an impregnation solution to react to obtain foamed nickel loaded with Co-ZIF-67; wherein the impregnation solution is an aqueous solution of 2-methyl imidazole and cobalt nitrate; S2, use the foamed nickel loaded with Co-ZIF-67 as a working electrode to carry out an electrodeposition reaction in an electrolyte to obtain a NiFePS / Co-ZIF-67 electrocatalyst; wherein the components of the electrolyte include nickel chloride, iron chloride, thiourea, sodium hypophosphite, ammonium fluoride, potassium hydroxide and water.

[0008] In the present application, 2-methyl imidazole and cobalt nitrate react on the surface of foamed nickel to generate Co-ZIF-67 metal organic framework nanosheets, and foamed nickel loaded with Co-ZIF-67 is obtained; foamed nickel loaded with Co-ZIF-67 is used as a working electrode, nickel chloride provides nickel elements, iron chloride provides iron elements, thiourea provides sulfur elements, and sodium hypophosphite provides phosphorus elements, electrodeposition is carried out, and NiFePS nanospheres are deposited on the Co-ZIF-67 nanosheets.

[0009] Preferably, in S1, the foamed nickel is pretreated with a hydrochloric acid solution, and then washed and dried.

[0010] The acid treatment of foamed nickel with a hydrochloric acid solution can remove impurities or oxides on the surface of the foamed nickel to obtain clean foamed nickel, which provides a clean and impurity-free substrate for the subsequent deposition of Co-ZIF-67.

[0011] In some specific embodiments of the present application, the pretreatment step of the foamed nickel is as follows: (1) The foamed nickel is immersed in a 3.0 mol / L hydrochloric acid solution, and is left at room temperature for 20 minutes for acid treatment; (2) The foamed nickel after acid treatment is taken out and cleaned with water, and then is sequentially ultrasonically cleaned in acetone, anhydrous ethanol and water for 10 minutes.

[0012] (3) The cleaned foamed nickel is vacuum dried to obtain clean foamed nickel.

[0013] Preferably, in S1, the reaction time is 2-6 hours.

[0014] Preferably, in S1, the mass ratio of 2-methylimidazole to cobalt nitrate in the immersion solution is (0.6-0.8):(0.7-0.9).

[0015] In some specific embodiments of the present application, the preparation method of the immersion solution is: (1) 2-methylimidazole is dissolved in water, and is fully stirred to obtain solution A; (2) Cobalt nitrate hexahydrate is dissolved in water, and is fully stirred to obtain solution B; (3) Solution A is poured into solution B, and is fully stirred to obtain the required immersion solution.

[0016] In some specific embodiments of the present application, in S1, after the reaction is completed, the foamed nickel is washed with deionized water and vacuum dried to obtain Co-ZIF-67 metal organic framework nanosheets (Co-ZIF-67) grown in situ on the foamed nickel.

[0017] Preferably, in S2, in the electrolyte, the proportions of nickel chloride, iron chloride, thiourea, sodium hypophosphite, ammonium fluoride, potassium hydroxide and water are (0.5-0.8) g:(0.2-0.4) g:(4.5-7.0) g:(0.5-0.8) g:(0.15-0.35) g:(0.1-0.3) g:30 mL.

[0018] Preferably, in S2, the electrodeposition reaction is carried out at a constant current of -0.01 to -0.25 A, more preferably at a constant current of -0.10 to -0.25 A.

[0019] Preferably, in S2, the electrodeposition reaction time is 100-400 seconds, for example, can be 100 seconds, 200 seconds, 300 seconds, 400 seconds, more preferably 300 seconds.

[0020] Preferably, in S2, a platinum sheet electrode is used as the counter electrode.

[0021] In a third aspect, the application provides application of the NiFePS / Co-ZIF-67 electrocatalyst as an electrode in electrolysis of water.

[0022] Preferably, the NiFePS / Co-ZIF-67 electrocatalyst is applied as an electrode in full electrolysis of water.

[0023] Compared with the prior art, the application has the following beneficial effects: The NiFePS / Co-ZIF-67 electrocatalyst provided by the application has the following advantages: (1) accelerated electron transfer rate: the NiFePS / Co-ZIF-67 electrocatalyst provided by the application uses high-conductivity nickel foam as a base material and high-conductivity Co-ZIF-67 as a precursor, and benefits from the synergistic effect of Ni, Fe and Co, so that the catalyst has a low charge transfer resistance (3.5 Ω and 1.42 Ω) in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) process, thereby accelerating the electron transfer rate. (2) a large number of exposed active sites: the electrocatalyst NiFePS / Co-ZIF-67 provided by the application has a three-dimensional composite nanostructure of Co-ZIF-67 nanosheets stacked NiFePS nanospheres, and benefits from the unique three-dimensional structure, the catalyst has a large double-layer capacitance (54.0 mF·cm -2 ), which can expose a large number of active sites. (3) excellent electrocatalytic water splitting performance: benefiting from the unique three-dimensional structure of nanosheets stacked nanospheres, the synergistic effect of multiple metals, and the selection of high-conductivity Co-ZIF-67 precursor, the electrocatalyst NiFePS / Co-ZIF-67 provided by the application can reach a current density of 10 mA·cm -2 under HER and OER under an overpotential of 64.1 mV and 263.8 mV in alkaline conditions, respectively. (4) excellent dual-function full water splitting performance: when the electrocatalyst NiFePS / Co-ZIF-67 provided by the application is used as a dual-function electrocatalyst for full water splitting, only a cell voltage of 1.51 V is required to drive a current density of 10 mA·cm -2 , which exhibits excellent dual-function full water splitting performance. (5) outstanding catalytic stability: under the conditions of HER, OER and full water splitting, the electrocatalyst NiFePS / Co-ZIF-67 provided by the application can catalyze the reaction at a large current density of 50 mA·cm -2 for 100 hours without significant performance decay, which confirms that it has good catalytic stability.

[0024] The NiFePS / Co-ZIF-67 electrocatalyst provided by the application has the following advantages: (1) the foam nickel is used as the base material, the Co-ZIF-67 precursor is grown in situ on the foam nickel, and then the NiFePS is electrodeposited, so that the foam nickel base material is firmly combined with the Co-ZIF-67, and the problem of easy falling off of the catalyst powder in the traditional electrocatalytic electrode is effectively solved; (2) low preparation cost: the preparation method of the electrocatalyst NiFePS / Co-ZIF-67 provided by the application is carried out at room temperature, is low in carbon and environmental protection, and uses cheap transition metals as raw materials, so that the cost of the catalyst is effectively reduced. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0026] Figure 1 is a scanning electron microscope image of the NiFePS / Co-ZIF-67 obtained in Example 1.

[0027] Figure 2 is a scanning electron microscope image of the Co-ZIF-67 obtained in Comparative Example 2.

[0028] Figure 3 is a scanning electron microscope image of the NiFePS / NF obtained in Comparative Example 3.

[0029] Figure 4 is a scanning electron microscope image of the NiFeS / Co-ZIF-67 obtained in Comparative Example 4.

[0030] Figure 5 is a scanning electron microscope image of the NiPS / Co-ZIF-67 obtained in Comparative Example 5.

[0031] Figure 6 is an element distribution map of the NiFePS / Co-ZIF-67 obtained in Example 1.

[0032] Figure 7 is an XRD test result of the NiFePS / Co-ZIF-67 obtained in Example 1 and the sample of the comparative example.

[0033] Figure 8 is a Ni 2p XPS fine spectrum of the sample obtained in Example 1 and Comparative Example 5.

[0034] Figure 9is the Co 2p XPS fine spectrum of the sample obtained from Example 1 and Comparative Example 5.

[0035] Figure 10 is the Fe 2p XPS fine spectrum of NiFePS / Co-ZIF-67 obtained from Example 1.

[0036] Figure 11 is the O 1s XPS fine spectrum of the sample obtained from Example 1 and Comparative Example 5.

[0037] Figure 12 is the Raman spectrum of NiFePS / Co-ZIF-67 obtained from Example 1 and the Raman spectrum of the sample of Comparative Example.

[0038] Figure 13 is the Hydrogen evolution linear sweep voltammetry (LSV) polarization curve of Example 1, Comparative Examples 1-5 and commercial Pt / C electrode.

[0039] Figure 14 is the Hydrogen evolution overpotential histogram of the samples obtained from Example 1 and Comparative Examples 1-5.

[0040] Figure 15 is the HER Tafel slope plot of the samples obtained from Example 1 and Comparative Examples 1-5.

[0041] Figure 16 is the HER Nyquist plot of the samples obtained from Example 1 and Comparative Examples 1-5.

[0042] Figure 17 is the double layer capacitance of the samples obtained from Example 1 and Comparative Examples 1-5. C dl Figure.

[0043] Figure 18 is the HER polarization curve before and after CV cycling of the sample obtained from Example 1.

[0044] Figure 19 is the HER chronoamperometry plot of the sample obtained from Example 1.

[0045] Figure 20 is the Oxygen evolution LSV polarization curve of the samples obtained from Example 1 and Comparative Examples 1-5.

[0046] Figure 21 is the Oxygen evolution overpotential histogram of the samples obtained from Example 1 and Comparative Examples 1-5.

[0047] Figure 22 is the OER Tafel slope plot of the samples obtained from Example 1 and Comparative Examples 1-5.

[0048] Figure 23is the OER Nyquist plot of the sample obtained in Example 1 and Comparative Examples 1-5.

[0049] Figure 24 is the OER polarization plot before and after CV cycling of the sample obtained in Example 1.

[0050] Figure 25 is the OER chronoamperometry plot of the sample obtained in Example 1.

[0051] Figure 26 is the full water splitting polarization plot of the sample obtained in Example 1 and Comparative Examples 1-5.

[0052] Figure 27 is the full water splitting chronoamperometry plot of the sample obtained in Example 1.

[0053] Figure 28 is the hydrogen evolution LSV polarization plot of the sample obtained in Examples 1-4.

[0054] Figure 29 is the oxygen evolution LSV polarization plot of the sample obtained in Examples 1-4.

[0055] Figure 30 is the hydrogen evolution LSV polarization plot of the sample obtained in Example 1 and Examples 5-7.

[0056] Figure 31 is the oxygen evolution LSV polarization plot of the sample obtained in Example 1 and Examples 5-7. DETAILED DESCRIPTION

[0057] Other advantages and benefits of the present application will become apparent to those skilled in the art upon consideration of the disclosure herein or can be learned by practice of the application. The present application can be realized and achieved by means other than as specifically described herein and the present application is not limited to the specific embodiments described herein. Various modifications and changes can be made thereto without departing from the spirit and scope of the present application.

[0058] It should be understood that the following examples do not specifically mention the process equipment or devices, which are all conventional equipment or devices in the art.

[0059] It should be noted that the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products, or apparatuses. Furthermore, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not intended to limit the order of the method steps or define the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0060] Example 1 The NiFePS / Co-ZIF-67 electrocatalyst was prepared according to the preparation method proposed in this invention. The detailed steps are as follows: Step 1: Pretreatment of Nickel Foam (1) Cut the nickel foam into small pieces of 1 cm × 3 cm, and then immerse them in a 3.0 mol / L hydrochloric acid solution at room temperature for 20 minutes.

[0061] (2) Take out the acid-treated nickel foam and clean it with deionized water. Then, clean it with acetone, anhydrous ethanol and deionized water for 10 minutes each.

[0062] (3) After cleaning, the foamed nickel is placed in a vacuum drying oven and dried at 60 °C to obtain foamed nickel with a clean surface.

[0063] Step 2: The synthesis steps of the Co-ZIF-67 metal-organic framework are as follows: (1) Dissolve 0.728 g of 2-methylimidazole in 20 mL of deionized water and stir thoroughly to obtain solution A.

[0064] (2) Dissolve 0.821 g of cobalt nitrate hexahydrate in 20 mL of deionized water and stir thoroughly to obtain solution B.

[0065] (3) Pour solution A into solution B and stir thoroughly to obtain the required impregnation solution C.

[0066] (4) Immerse the pretreated nickel foam pieces in the impregnation solution C at room temperature for 4 hours.

[0067] (5) After the reaction is complete, the nickel foam sample is rinsed with deionized water and dried in a vacuum drying oven at 60 °C to obtain the Co-ZIF-67 metal-organic framework loaded on the nickel foam, which is denoted as Co-ZIF-67.

[0068] Step 3: The electrodeposition steps for NiFePS are as follows: (1) 0.642 g of nickel chloride hexahydrate, 0.243 g of iron chloride hexahydrate, 5.838 g of thiourea, 0.636 g of sodium hypophosphite monohydrate, 0.222 g of ammonium fluoride, and 0.2 g of potassium hydroxide were dissolved in 30 mL of deionized water and stirred uniformly to obtain an electrolyte.

[0069] (2) The Co-ZIF-67-loaded nickel foam was used as a working electrode, and a platinum sheet electrode was used as a counter electrode, which were placed in the electrolyte and deposited at a constant current of -0.20 A for 300 seconds.

[0070] (3) The sample after deposition was taken out of the electrolyte and cleaned with deionized water.

[0071] (4) The cleaned sample was placed in a vacuum drying oven at 60 °C for drying, thereby obtaining a NiFePS / Co-ZIF-67 electrocatalyst loaded on the nickel foam, which was recorded as NiFePS / Co-ZIF-67.

[0072] Example 2 The preparation process and steps of Example 2 were basically the same as those of Example 1, except that in the third step, the electrodeposition current was reduced to -0.01 A.

[0073] Example 3 The preparation process and steps of Example 3 were basically the same as those of Example 1, except that in the third step, the electrodeposition current was reduced to -0.10 A.

[0074] Example 4 The preparation process and steps of Example 4 were basically the same as those of Example 1, except that in the third step, the electrodeposition current was increased to -0.25 A.

[0075] Example 5 The preparation process and steps of Example 5 were basically the same as those of Example 1, except that in the third step, the electrodeposition time was shortened to 100 seconds.

[0076] Example 6 The preparation process and steps of Example 6 were basically the same as those of Example 1, except that in the third step, the electrodeposition time was shortened to 200 seconds.

[0077] Example 7 The preparation process and steps of Example 7 were basically the same as those of Example 1, except that in the third step, the electrodeposition time was extended to 400 seconds.

[0078] Comparative Example 1 The pretreated surface-cleaned nickel foam was directly used as the electrode for electrolysis of water as Comparative Example 1, denoted as NF.

[0079] Comparative Example 2 The difference from Example 1 is that the third step was not performed, and the Co-ZIF-67 metal organic framework loaded on the nickel foam was directly used as the electrode for electrolysis of water as Comparative Example 2, denoted as Co-ZIF-67.

[0080] Comparative Example 3 The difference from Example 1 is that the second step was not performed, and the sample obtained by electrodeposition of NiFePS nanoparticles directly on the nickel foam was used as the electrode for electrolysis of water as Comparative Example 3, denoted as NiFePS / NF.

[0081] Comparative Example 4 The difference from Example 1 is that no sodium hypophosphite monohydrate was added to the electrolyte in the third step, and the comparative sample without phosphorus was obtained and used as the electrode for electrolysis of water as Comparative Example 4, denoted as NiFeS / Co-ZIF-67.

[0082] Comparative Example 5 The difference from Example 1 is that no ferric chloride hexahydrate was added to the electrolyte in the third step, and the comparative sample without iron was obtained and used as the electrode for electrolysis of water as Comparative Example 5, denoted as NiPS / Co-ZIF-67.

[0083] Figure 1 is a scanning electron microscope image of the NiFePS / Co-ZIF-67 obtained in Example 1. It can be seen that the electrodeposited NiFePS nanospheres are uniformly loaded on the Co-ZIF-67, forming a unique three-dimensional structure of small nanospheres stacked on large nanosheets. This three-dimensional structure has a large active area, can provide sufficient active sites for the reaction, and is more conducive to the removal of gas products during the reaction, thereby improving the catalytic efficiency of the active sites. Figure 2 is a scanning electron microscope image of the Co-ZIF-67 obtained in Comparative Example 2. It can be seen that the leaf-shaped Co-ZIF-67 nanosheets with a length of about 10 μm are irregularly stacked on the surface of the nickel foam. This three-dimensional structure not only increases the contact area of the electrode with the electrolyte, but also provides sufficient space for the growth of subsequent materials. In addition, thanks to the selection of the nickel foam base material and reasonable reaction conditions, there is no obvious agglomeration phenomenon in the Co-ZIF-67, which means that the catalyst has a large specific surface area. Figure 3 is a scanning electron microscope image of the NiFePS / NF obtained in Comparative Example 3. It can be seen that the NiFePS is stacked on the nickel foam in the form of nanospheres with a size of about 1 μm, and the surface of the nanospheres is covered with many wrinkles. Figure 4 and Figure 5The SEM images of NiFeS / Co-ZIF-67 and NiPS / Co-ZIF-67 obtained from Comparative Example 4 and Comparative Example 5, respectively, can be seen that in NiFeS / Co-ZIF-67, NiFeS grows in the form of numerous small nanospheres on the surface of Co-ZIF-67, and the original nanosheet structure of Co-ZIF-67 is no longer visible, because the loading amount of NiFeS is too much, and a large number of nanospheres are accumulated on the surface of Co-ZIF-67, so that it loses the original sheet morphology and the advantage of unique high specific surface area; while in NiPS / Co-ZIF-67, NiPS particles are densely wrapped on the surface of Co-ZIF-67. Figure 6 The EDS element distribution map of NiFePS / Co-ZIF-67 obtained from Example 1 can be seen that Ni, Fe, Co, P, S and O elements are uniformly distributed on the surface of the catalyst, and O element is also detected on the surface of the sample due to air oxidation.

[0084] Figure 7 The XRD test results of NiFePS / Co-ZIF-67 obtained from Example 1 and other several comparative examples can be seen that Co-ZIF-67 exhibits strong diffraction peaks at 7.34°, 10.44°, 12.76°, 14.72°, 17.01°, 18.02°, 21.78°, 23.49°, 29.65°, 30.92° and 32.77°, respectively, corresponding to the standard Co-ZIF-67 (011), (002), (112), (022), (013), (222), (114), (233), (044), (244) and (235) multiple crystal faces, proving the successful preparation of Co-ZIF-67. NiPS / Co-ZIF-67, NiFeS / Co-ZIF-67 and NiFePS / Co-ZIF-67 materials have no obvious characteristic peaks, and they show a broad peak in the range of 25°, indicating that these substances mainly exist in amorphous state. This can be attributed to the slow diffusion rate of ions at low temperature, which is not conducive to the ordered growth of crystals, and the large current density selected during preparation, which can cause rapid deposition of ions during the reaction, without enough time for the ions to arrange in order on the electrode surface to form a well-structured crystal, ultimately resulting in no detection of corresponding crystal diffraction peaks in the XRD results. Some studies have shown that the introduction of amorphous structure can increase the number of active sites of the catalytic reaction, and can enhance the performance of the catalyst for water electrolysis.

[0085] In order to determine the types and chemical states of elements contained in the catalyst and judge the influence of the addition of Fe element on the electronic structure of the catalyst surface, the samples NiPS / Co-ZIF-67 and NiFePS / Co-ZIF-67 were subjected to XPS characterization test. Figure 8XPS fine spectrum of nickel element, the characteristic peaks of NiFePS / Co-ZIF-67 at 855.7 and 873.4 eV, 857.2 and 875.2 eV are the spin-orbit splitting peaks of Ni 2+ and Ni 3+ , respectively, which proves that Ni exists in different valence states in the sample. While in NiPS / Co-ZIF-67, the characteristic peaks at 855.6 and 873.2 eV indicate that Ni mainly exists in the form of divalent state in the sample. The above test results can prove that the change of Ni valence state in NiFePS / Co-ZIF-67 can be attributed to the interaction between multi-metal atoms, that is, the addition of iron atoms changes the electronic structure of Ni, thereby inducing the appearance of Ni 3+ in the sample. Some studies have shown that Ni 3+ has stronger OH 2+ adsorption capacity and stronger electrocatalytic activity than Ni - , so the presence of Ni 3+ in the synthesized NiFePS / ZIF-67 helps to improve its electrocatalytic ability. In addition, the coexistence of multi-valence Ni will cause the decrease of Ni 2p electron density, thereby making the d-band center rise, which in turn optimizes the hydrogen adsorption free energy of the catalyst, and finally realizes the improvement of the water electrolysis activity. The peak values of 861.7 and 880.0 eV of the sample NiFePS / Co-ZIF-67 belong to the satellite peaks of Ni 2p, which are consistent with the characteristics of the prepared nickel-based electrocatalysts in existing researches. Figure 9 XPS fine spectrum of cobalt element, the characteristic peaks of NiFePS / Co-ZIF-67 at 781.2 eV and 796.7 eV are the spin-orbit peaks of Co 2+ , and the peak values of 785.8 eV and 803.5 eV are the corresponding satellite peaks; in addition, a characteristic peak at binding energy 775.4 eV is observed, which proves the existence of trivalent cobalt. Considering that the electrocatalytic decomposition of water is a typical electron exchange related redox reaction, the coexistence of multi-valence Ni and Co can enhance the reaction activity of the catalyst. Compared with NiPS / Co-ZIF-67, the positive shifts of 0.2 and 0.1 eV in the spin-orbit splitting of Co 2+ 2p 3 / 2 and Co 2+ 2p 1 / 2 of NiFePS / Co-ZIF-67, respectively, again prove that the addition of iron atoms can realize the adjustment of the electronic structure of the catalyst surface. Figure 10 XPS fine spectrum of iron element, NiFePS / Co-ZIF-67 exhibits obvious characteristic peaks at 713.8 and 725.3 eV, corresponding to Fe 3+ 2p 3 / 2 and Fe3+ 2p 1 / 2 A peak was also observed at 707.7 eV, confirming the presence of Fe in the catalyst. 0 A satellite peak for Fe was identified at 718.7 eV, consistent with existing reports on iron-based catalysts. XPS results indicate that the electronic structure of NiFePS / ZIF-67 is relatively complex, with all metal elements existing in multiple valence states. More importantly, XPS results demonstrate that the addition of Fe can, to some extent, modulate the electronic structure of the catalyst, enabling it to perform more efficient electrocatalytic water splitting. Figure 11 The XPS fine spectrum of oxygen shows that oxygen exists in the sample bonded to the metal.

[0086] The Raman spectroscopy results of the sample are as follows: Figure 12 As shown, no obvious characteristic peaks were observed in the nickel foam substrate, indicating its high purity and clean surface. Co-ZIF-67, on the other hand, exhibited more peaks, at 186 and 662 cm⁻¹. -1 The Raman peaks observed at 425 and 1173 cm⁻¹ correspond to the oxides of Co, which is consistent with the XRD results. -1 The characteristic peaks at 260 and 464 cm⁻¹ indicate the presence of Co-ZIF-6. -1 The characteristic peaks at these locations are related to the vibrations of N-Co-N and Ni-CN chemical bonds, respectively. The characteristic peaks of the NiFeS / Co-ZIF-67 sample are located at 287 cm⁻¹. -1 341 cm -1 and 531 cm -1 These correspond to Fe-O, Ni-S, and Fe-S chemical bonds, respectively. NiPS / Co-ZIF-67 at 502 cm⁻¹ -1 and 964 cm -1 Characteristic peaks are observed at 964 and 1074 cm⁻¹, which are associated with nickel sulfides. -1 The characteristic peaks observed can be attributed to metal oxide bonds (Ni-O and Co-O) and carbon-oxygen bonds (CO). The characteristic peaks are most pronounced in the NiFePS / Co-ZIF-67 sample, with peaks at 221 and 341 cm⁻¹. -1 The characteristic peak at 282 cm⁻¹ corresponds to Ni-S. -1 The characteristic peaks at these locations are related to Fe-O. (449 and 976 cm⁻¹) -1 The peak at 550 cm⁻¹ belongs to Ni-O, while the peak at 550 cm⁻¹ belongs to Ni-O. -1 The peak at that location is related to Fe-S. Raman spectroscopy further confirmed the chemical composition and structure of the catalyst, especially the presence of polymetallic bonds in NiFePS / Co-ZIF-67.

[0087] Figure 13For the polarization curves of the linear sweep voltammetry (LSV) of the samples obtained in Example 1 and other comparative examples, for the convenience of comparison, the overpotentials required for them to reach the current density of 10, 50, 100 mA·cm -2 -2 were drawn into columnar charts for comparison, as shown in Figure 14 It can be seen that NiFePS / Co-ZIF-67 exhibits the best HER performance, and only requires overpotentials of 64.1, 108.2, and 128.7 mV to reach the current density of 10, 50, and 100 mA·cm -2 -2, respectively. The overpotentials required for the series of comparative sample NF, Co-ZIF-67, NiFeS / Co-ZIF-67, NiPS / Co-ZIF-67, NiFePS / NF, and commercial Pt / C to drive the current density of 10 mA·cm -2 -2 are 311.6, 266.3, 79.2, 61.0, 111.4, and 54.5 mV, respectively. NiPS / Co-ZIF-67, Pt / C, and NiFePS / Co-ZIF-67 all have good HER activity at low current density, but the overpotential required by NiFePS / Co-ZIF-67 is obviously lower at high current density. In addition, the hydrogen evolution activity of Co-ZIF-67 is general, but the HER performance is obviously improved after being compounded with other active materials. The hydrogen evolution performance of NiFePS / NF directly deposited on the nickel foam is far inferior to that of NiFePS / Co-ZIF-67, which can be attributed to the synergistic effect of Co-ZIF-67 and the increase in active area provided by the metal organic framework as a substrate material. It is worth noting that the multi-metallic NiFePS / Co-ZIF-67 exhibits better hydrogen evolution performance than the bimetallic NiFeS / Co-ZIF-67 and NiPS / Co-ZIF-67, which can be explained as the synergistic effect of multi-metallic is conducive to the regulation of the electronic structure of the catalyst surface, so that the sample has better hydrogen adsorption free energy, thereby reducing the energy barrier to be overcome in the reaction process. Although the HER performance of the optimal sample NiFePS / Co-ZIF-67 still lags behind that of the platinum carbon electrode at low current density, it exhibits more excellent activity at high current density. Considering the overpotential required by the sample at each current density, NiFePS / Co-ZIF-67 has the best HER activity.

[0088] Figure 15The HER Tafel slope plots of the samples obtained in Example 1 and Comparative Examples 1-5 are shown. It can be seen that NiFePS / Co-ZIF-67 has the lowest Tafel slope value of 45.2 mV / dec, meaning that its reaction path is optimal and the overpotential required for the current density to increase by 10 times is the smallest. It is also proved that the NiFePS / Co-ZIF-67 catalyzes the hydrogen evolution reaction to follow the Volmer-Heyrovsky mechanism, in which the Heyrovsky step is the rate-determining step. The Tafel slopes of the samples NF, Co-ZIF-67, NiFeS / Co-ZIF-67, NiPS / Co-ZIF-67 and NiFePS / NF are 166.7, 115.8, 124.0, 94.6 and 96.2 mV / dec, respectively, which again confirms that NiFePS / Co-ZIF-67 has the optimal HER activity. To further evaluate the charge transfer rate of the catalyst during the hydrogen evolution reaction, the samples were subjected to an alternating current impedance test, and the results are shown in FIG. 6, and the relevant parameters were fitted based on the circuit model in the figure. The minimum charge transfer resistance of NiFePS / Co-ZIF-67 is 3.50 Ω, which is lower than that of NF (7.40 Ω), Co-ZIF-67 (3.90 Ω), NiFeS / Co-ZIF-67 (4.31 Ω), NiPS / Co-ZIF-67 (4.82 Ω) and NiFePS / NF (3.68 Ω), indicating that NiFePS / Co-ZIF-67 has the fastest electron transfer rate during the hydrogen evolution reaction. Thanks to the selection of the highly conductive nickel foam substrate, all the prepared samples have a small charge transfer resistance value. The NiFePS / Co-ZIF-67 containing three metal components has a smaller charge transfer resistance value than the bimetallic catalyst, which can be explained as the synergistic effect of multiple metals on the improvement of the conductivity and activity of the catalyst. In addition, compared with NiFePS / NF, the NiFePS / Co-ZIF-67 composite material has a better charge transfer resistance, which can be explained as the improvement of the charge transfer rate by the heterostructure of the latter. Figure 16 R ct R ct

[0089] Electrochemically active surface area ECSA is commonly used to quantitatively evaluate the number of surface active sites and reaction efficiency of the catalyst, and is one of the key indicators for judging the hydrogen evolution performance of the catalyst. ECSA and double-layer capacitance C dl ECSA = C dl / C s C s ​​​​​is the specific capacitance of double-layer per unit area, used to normalize the measured double-layer capacitance, generally taken as 40 μF·cm -2 . Therefore, by measuring the size of the catalyst sample C dl , the electrochemical active area can be evaluated. Figure 17 is the double-layer capacitance of the samples obtained in Example 1 and Comparative Examples 1-5 C dl Figure. NiFePS / Co-ZIF-67 has the largest double-layer capacitance C dl value (54.0 mF·cm -2 ), which is much higher than that of NF (0.5 mF·cm -2 ), Co-ZIF-67 (1.40 mF·cm -2 ), NiFeS / Co-ZIF-67 (23.2 mF·cm -2 ), NiPS / Co-ZIF-67 (12.8 mF·cm -2 ), and NiFePS / NF (2.2 mF·cm -2 ), confirming that it has excellent charge storage capacity and a larger number of active sites. Notably, the materials prepared based on Co-ZIF-67 have larger active areas, especially the C dl value of NiFePS / Co-ZIF-67 is tens of times higher than that of NiFePS / NF, which can be attributed to the higher specific surface area and porous structure framework of Co-ZIF-67, which is conducive to the construction of nanostructures during electrodeposition, thereby increasing the active area. The NiFePS / Co-ZIF-67 with three metal components has a higher catalytic active area than the bimetallic component, which can be explained by the fact that different electrolyte components during preparation ultimately result in different morphological structures of the synthesized materials.

[0090] To further study the performance decay of the samples over time, the stability test was performed on the NiFePS / Co-ZIF-67 sample, and the test results are shown in Figure 18 and Figure 19 . Among them Figure 18 is the HER polarization curve of NiFePS / Co-ZIF-67 before and after 2000 cycles of CV, and the two polarization curves almost coincide, confirming that the catalyst has excellent stability. Figure 19 is the chronoamperometry test result graph of NiFePS / Co-ZIF-67. It can be seen that the sample continuously catalyzes the reaction for 100 hours at a large current density of-50 mA·cm -2 and there is no obvious rise in overpotential, again indicating that it has excellent HER stability.

[0091] The electrochemical test results not only show that the NiFePS / Co-ZIF-67 obtained in Example 1 has excellent HER performance, but also confirm that the high catalytic activity of the sample is attributed to the following points: the high specific surface area of the metal organic framework as a precursor and the appropriate electrodeposition parameters make the sample have a larger active area, which helps to improve the catalytic performance. The synergistic effect of multiple metals on the regulation of the electronic structure of the catalyst surface further optimizes the HER performance of the sample. The selection of the high-conductivity substrate material, nickel foam, is conducive to the charge transfer in the reaction process.

[0092] Figure 20 The oxygen evolution LSV polarization curve graphs of the samples obtained in Example 1 and Comparative Examples 1-5 are shown. In order to observe the differences in the overpotential required by the samples at different current densities, the overpotential histograms of the samples at current densities of 10, 50 and 100 mA·cm -2 -2 are shown as follows. Figure 21 It can be seen that the overpotentials required by NF, Co-ZIF-67, NiFeS / Co-ZIF-67, NiPS / Co-ZIF-67, NiFePS / NF and NiFePS / Co-ZIF-67 to reach a current density of 10 mA·cm -2 -2 are 431.3, 368.3, 261.4, 321.1, 248.9 and 263.8 mV, respectively. The OER performances of the samples NiFeS / Co-ZIF-67, NiFePS / NF and NiFePS / Co-ZIF-67 at low current densities are almost the same, while the performance of NiFePS / Co-ZIF-67 is slightly better at high current densities. The OER activity of NiPS / Co-ZIF-67 is relatively poor compared with other electrodeposited samples, which can be explained as the addition of Fe element can improve the OER activity of the catalyst through the synergistic effect of multiple metals. It can be seen that the samples NiFeS / Co-ZIF-67, NiFePS / NF and NiFePS / Co-ZIF-67 all exhibit good OER activity.

[0093] In order to further study the advantages and disadvantages of the kinetics of the catalyst in the OER reaction process, the Tafel slope test was performed on the samples, and the Tafel curves obtained are shown as follows. Figure 22As shown, the Tafel slopes of samples NF, Co-ZIF-67, NiFeS / Co-ZIF-67, NiPS / Co-ZIF-67, NiFePS / NF, and NiFePS / Co-ZIF-67 are 213.7, 93.7, 48.0, 75.6, 46.3, and 47.8 mV / dec, respectively. This indicates that NiFeS / Co-ZIF-67, NiFePS / NF, and NiFePS / Co-ZIF-67 all possess low Tafel values, confirming a faster reaction pathway and a more significant increase in the driven current density response overpotential. Furthermore, the Tafel slope test results agree well with the oxygen evolution polarization curve test results, both demonstrating that the aforementioned three samples, NiFeS / Co-ZIF-67, NiFePS / NF, and NiFePS / Co-ZIF-67, possess excellent OER performance.

[0094] To further investigate the charge transfer impedance and electron transport capability of the sample during the OER process, electrochemical impedance spectroscopy was performed, and the resulting Nyquist curve is shown below. Figure 23 As shown in the figure, the charge transfer impedance value can be obtained by fitting the parameters of the sample impedance diagram based on the equivalent circuit model shown in the figure. R ct This is used to explain the differences in OER performance of the samples from a reaction kinetic perspective. Among them, NiFePS / Co-ZIF-67... R ct The lowest (1.42 Ω), lower than NF (5.69 Ω), Co-ZIF-67 (3.49 Ω), NiFeS / Co-ZIF-67 (2.86 Ω), NiPS / Co-ZIF-67 (6.03 Ω), and NiFePS / NF (2.63 Ω), demonstrates that it possesses the lowest interfacial charge transfer resistance during the reaction process. The precursor Co-ZIF-67 exhibits lower... R ct The value indicates good conductivity, proving that it can accelerate the electron transport rate during the reaction process as a precursor. Furthermore, the NiPS / Co-ZIF-67 sample exhibits a relatively large conductivity value compared to other iron-containing samples. R ct The value further demonstrates that the addition of Fe can improve conductivity through multi-metal synergy, thereby accelerating the charge transfer process in the OER reaction and optimizing the OER performance of the catalyst.

[0095] To further investigate the performance degradation of the samples over a long period of OER, stability tests were conducted on the NiFePS / Co-ZIF-67 samples. The test results are as follows: Figure 24 and Figure 25 As shown. Among them. Figure 24The OER polarization curves of NiFePS / Co-ZIF-67 before and after 2000 cycles of CV are shown in the figure. It can be seen that the two polarization curves are almost coincident at low current density, but the performance after cycling has declined at high current density, which can be attributed to the shedding of part of the active material on the surface of the catalyst at large current density. Figure 25 The results of the chronoamperometric test of the sample are shown in the figure. It can be seen that there is no obvious rise in overpotential for the OER reaction of the sample at a large current density of-50 mA·cm -2 for 100 hours, which proves that the sample has good OER stability.

[0096] The OER electrochemical performance test results prove that NiFePS / Co-ZIF-67 has good catalytic activity. The experimental results prove that the addition of Fe element can well optimize the OER performance of the sample. The samples containing iron, NiFeS / Co-ZIF-67, NiFePS / NF and NiFePS / Co-ZIF-67, all have good oxygen evolution activity, and NiFePS / Co-ZIF-67 has good charge transfer rate and reaction stability.

[0097] To study the performance of the sample as a bifunctional catalyst for water electrolysis, full water splitting LSV test was carried out in a two-electrode system, and the obtained polarization curve is shown in Figure 26 . It can be seen that NiFePS / Co-ZIF-67 exhibits excellent full water splitting performance, and only a voltage of 1.51 V is needed to produce a current density of 10 mA·cm -2 , which is lower than that of NF (1.79 V), Co-ZIF-67 (1.72 V), NiFeS / Co-ZIF-67 (1.56 V), NiPS / Co-ZIF-67 (1.61 V) and NiFePS / NF (1.57 V). Generally speaking, the HER performance and OER performance of the sample will affect its full water splitting performance, and OER as a complex four-electron process is the rate-limiting step in the full water splitting process. Therefore, thanks to the good OER activity, NiFeS / Co-ZIF-67, NiFePS / NF and NiFePS / Co-ZIF-67 all show good full water splitting performance. Among the three, NiFePS / Co-ZIF-67 has better HER performance, so it has the best reaction activity when it is used to form a two-electrode full water splitting system. To further evaluate the full water splitting performance of NiFePS / Co-ZIF-67, chronoamperometric method was used for reaction stability test, and the results are shown in Figure 27 . It can be seen that the sample can stably full water splitting at a large current density of 50 mA·cm -2 for 100 hours without obvious rise in overpotential, which proves that it has good catalytic stability.

[0098] The four NiFePS / Co-ZIF-67 samples obtained from Examples 1-4 were subjected to hydrogen evolution LSV polarization test, and the results are shown in Figure 28 . When the catalytic HER reaction reached a current density of 10 mA·cm -2 , the overpotential required for the NiFePS / Co-ZIF-67 samples deposited at-0.01, -0.10, -0.20 and-0.25 A current was 180.4, 90.3, 64.1 and 75.5 mV, respectively, indicating that the NiFePS / Co-ZIF-67 prepared at a deposition current of-0.20 A had the optimal hydrogen evolution activity. The four NiFePS / Co-ZIF-67 samples obtained from Examples 1-4 were subjected to oxygen evolution LSV polarization test, and the results are shown in Figure 29 . The overpotential required for the NiFePS / Co-ZIF-67 samples deposited at-0.01, -0.10, -0.20 and-0.25 A current to reach a current density of 10 mA·cm -2 was 316.3, 268.3, 263.8 and 213.2 mV, respectively, indicating that the NiFePS / Co-ZIF-67 prepared at a deposition current of-0.25 A had the optimal OER activity at low current density, followed by the samples prepared at deposition currents of-0.10 A and-0.20 A.

[0099] The four NiFePS / Co-ZIF-67 samples obtained from Examples 1 and 5-7 were subjected to hydrogen evolution LSV polarization test, and the results are shown in Figure 30 . The overpotential required for the NiFePS / Co-ZIF-67 samples deposited at-0.20 A current for 100 s, 200 s, 300 s and 400 s to reach a current density of 10 mA·cm -2 was 108.3 mV, 84.3 mV, 64.1 mV and 128.4 mV, respectively, indicating that the NiFePS / Co-ZIF-67 sample synthesized by electrodeposition for 300 s had the optimal HER performance. The four NiFePS / Co-ZIF-67 samples obtained from Examples 1 and 5-7 were subjected to oxygen evolution LSV polarization test, and the results are shown in Figure 31 . To reach a current density of 10 mA·cm -2 , the overpotential required for the NiFePS / Co-ZIF-67 samples deposited for 100 s, 200 s, 300 s and 400 s was 255.0, 255.0, 263.8 and 266.0 mV, respectively. It can be seen that the OER performance of the samples did not differ much, but the NiFePS / Co-ZIF-67 sample deposited for 300 s at high current density had better OER performance.

[0100] The above examples and comparative examples are only used to illustrate the technical solutions of the present application but not to limit the present application. Although the present application has been described in detail with reference to the above examples, ordinary skilled person in the art can make modifications or equivalent replacements to the specific implementation of the present application without departing from the spirit and scope of the present application. Any modification or equivalent replacement made without departing from the spirit and scope of the present application shall fall within the protection scope of the claims of the present application.

Claims

1. A NiFePS / Co-ZIF-67 electrocatalyst, characterized in that, The Co-ZIF-67 nanosheet is loaded with NiFePS nanospheres on the surface of the foam nickel; Ni in the NiFePS nanospheres comprises Ni 2+ and Ni 3+ Two valence states, Fe comprises Fe 2+ , Fe 3+ and Fe 0 Three valence states; Co in the Co-ZIF-67 nanosheet comprises Co 2+ and Co 3+ Two valence states.

2. The method for preparing the NiFePS / Co-ZIF-67 electrocatalyst according to claim 1, characterized in that, The method comprises the following steps: S1, immerse the nickel foam in an impregnation solution to react, and obtain Co-ZIF-67 loaded nickel foam; wherein the impregnation solution is an aqueous solution of 2-methylimidazole and cobalt nitrate; S2, use the Co-ZIF-67 loaded nickel foam as a working electrode to carry out electrodeposition reaction in an electrolyte, and obtain a NiFePS / Co-ZIF-67 electrocatalyst; wherein the components of the electrolyte include nickel chloride, iron chloride, thiourea, sodium hypophosphite, ammonium fluoride, potassium hydroxide and water.

3. The method for preparing the NiFePS / Co-ZIF-67 electrocatalyst according to claim 2, characterized in that, In S1, the nickel foam is pretreated with hydrochloric acid solution, and then washed and dried.

4. The method for preparing the NiFePS / Co-ZIF-67 electrocatalyst according to claim 2, characterized in that, In S1, the reaction time is 2-6 hours.

5. The method of claim 2, wherein the NiFePS / Co-ZIF-67 electrocatalyst is prepared by the steps of: In S1, the mass ratio of 2-methylimidazole to cobalt nitrate in the impregnation solution is (0.6-0.8):(0.7-0.9).

6. The method for preparing the NiFePS / Co-ZIF-67 electrocatalyst according to claim 2, characterized in that, In S2, the ratio of nickel chloride, iron chloride, thiourea, sodium hypophosphite, ammonium fluoride, potassium hydroxide and water in the electrolyte is (0.5-0.8) g:(0.2-0.4) g:(4.5-7.0) g:(0.5-0.8) g:(0.15-0.35) g:(0.1-0.3) g:30 mL.

7. The method for preparing the NiFePS / Co-ZIF-67 electrocatalyst according to claim 2, characterized in that, In S2, the electrodeposition reaction is carried out at a constant current of-0.01 to-0.25 A.

8. The method for preparing the NiFePS / Co-ZIF-67 electrocatalyst according to claim 2, characterized in that, In S2, the electrodeposition reaction time is 100-400 seconds.

9. The NiFePS / Co-ZIF-67 electrocatalyst of claim 1 is used as an electrode for electrolysis of water.

10. The NiFePS / Co-ZIF-67 electrocatalyst of claim 1 is used as an electrode for all-electrolysis of water.

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