Nickel phosphide-cobalt phosphide nano composite material as well as preparation method and application thereof

Abstract

The invention relates to a nickel phosphide-cobalt phosphide nano composite material as well as a preparation method and application thereof, and belongs to the technical field of catalysts. The preparation method of the nickel phosphide-cobalt phosphide nano composite material comprises the following steps: dissolving cobalt salt, ammonium fluoride and urea in a solvent I to obtain a precursor solution I, immersing hydrophilic carbon cloth into the precursor solution I, and performing hydrothermal reaction to obtain a precursor CoOHF; s, dissolving nickel salt, ammonium fluoride and urea into a solvent II to obtain a precursor solution II, immersing the precursor CoOHF into the precursor solution II, and performing hydrothermal reaction to obtain a precursor II; and S2, calcining a phosphorus source and the Ni-Co precursor in the step S2, driving phosphorus-containing gas generated by calcining the phosphorus source to the Ni-Co precursor under the action of reducing gas, and reacting to obtain the Ni-Co catalyst. The NiPx-CoPx disclosed by the invention is of a nanosheet-nanowire coexisting composite structure, and the heterostructure is beneficial to providing abundant active sites and enhancing the electron transmission capability and the catalytic interface stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalysts, and particularly relates to a nickel phosphide-cobalt phosphide nanocomposite material, a preparation method thereof, and an application thereof. Background Art

[0002] Transition metal phosphides (TMPs) nanomaterials have become efficient and promising electrocatalysts in the environmental field due to their tunable electronic structures, abundant earth reserves, and unique physical and chemical properties. Among them, phosphorus (P) in TMPs can not only act as a basic site for proton capture to promote hydrogen adsorption, but also regulate the electron density of metal atoms through electron supply, changing their adsorption properties, thereby enhancing catalytic activity. In addition, the electronegativity difference between metal (M) and P leads to electron transfer, further polarizing the M-P bond. And the metal sites of TMPs provide efficient charge transport channels, improving the conductivity of the catalyst. The electrocatalytic nitrate reduction reaction (NitRR) is a complex reaction involving the transfer of 8 electrons and 9 protons. The design of efficient catalysts usually requires good electron transport and proton coupling capabilities. Currently, during the process of electrochemically reducing nitrate to ammonia, side reactions may occur, resulting in poor selectivity for the final synthesized ammonia and low current density. Currently, during the process of electrochemically reducing nitrate to ammonia, mostly single metal phosphides are used as catalysts or simple bimetallic phosphide catalysts. However, the catalytic activity and stability of traditional single metal phosphides and simple bimetallic phosphide catalysts for the NitRR reaction are relatively low. Summary of the Invention

[0003] The first object of the present invention is to provide a nickel phosphide-cobalt phosphide nanocomposite material to solve the technical problems of relatively low catalytic activity and stability of traditional single metal phosphide catalysts and simple bimetallic phosphide catalysts in the NitRR reaction.

[0004] The second object of the present invention is to provide a preparation method of a nickel phosphide-cobalt phosphide nanocomposite material.

[0005] The third object of the present invention is to provide an application of a nickel phosphide-cobalt phosphide nanocomposite material.

[0006] In order to achieve the above objects, the technical solutions adopted by the present invention are as follows:

[0007] A preparation method of a nickel phosphide-cobalt phosphide nanocomposite material, comprising the following steps:

[0008] S1: Dissolve cobalt salt, ammonium fluoride, and urea in solvent I to obtain precursor solution I. Immerse hydrophilic carbon cloth in precursor solution I, and perform hydrothermal reaction to obtain precursor CoOHF;

[0009] S2: Dissolve nickel salt, ammonium fluoride, and urea in solvent two to obtain precursor solution two. Immerse the precursor CoOHF into precursor solution two, and after hydrothermal reaction, obtain the Ni-Co precursor.

[0010] S3: Calcinate the phosphorus source and the Ni-Co precursor described in S2. Under the action of a reducing gas, drive the phosphorus-containing gas generated by calcining the phosphorus source to the Ni-Co precursor, and obtain the product after reaction.

[0011] Further, the reducing gas described in S3 is a hydrogen-nitrogen mixed gas, and the volume ratio of hydrogen to nitrogen in the hydrogen-nitrogen mixed gas is 1:9.

[0012] Further, the calcination temperature described in S3 is 275 - 350 °C; the heating rate of the calcination is 1 - 5 °C / min; the calcination time is 0.5 - 2 h.

[0013] Further, the hydrothermal reaction temperature described in S1 and S2 is 120 - 180 °C, and the hydrothermal reaction time is 6 - 24 h.

[0014] Further, the cobalt salt described in S1 is one or more of cobalt nitrate hexahydrate, cobalt chloride, and cobalt sulfate, and the solvent one is water; the molar ratio of the cobalt salt to ammonium fluoride is 1:2 - 1:8; the molar ratio of the cobalt salt to urea is 1:2 - 1:8.

[0015] Further, the nickel salt is one or more of nickel nitrate hexahydrate, nickel chloride, and nickel sulfate, and the solvent two is water; the molar ratio of the nickel salt to ammonium fluoride is 1:2 - 1:8; the molar ratio of the nickel salt to urea is 1:2 - 1:8; the molar ratio of the nickel salt to the cobalt salt is 1:1 - 1:2.

[0016] Further, the preparation method of the hydrophilic carbon cloth described in S1 is: heat the carbon cloth under reflux in concentrated nitric acid, then alternately wash it with water and ethanol, and obtain it after drying; the temperature of the heating under reflux is 120 - 140 °C, and the time is 2 - 4 h.

[0017] Further, the phosphorus source described in S3 is sodium dihydrogen phosphate monohydrate, and 0.5 - 2 g of the phosphorus source is added per mmol of the nickel salt.

[0018] The nickel phosphide-cobalt phosphide nanocomposite material is prepared by using the above-mentioned preparation method of the nickel phosphide-cobalt phosphide nanocomposite material.

[0019] The application of the nickel phosphide-cobalt phosphide nanocomposite material in electrocatalytic ammonia synthesis and wastewater denitrification.

[0020] The beneficial effects of the present invention:

[0021] The present invention uses a two-step hydrothermal method to first synthesize a Ni-Co precursor and then perform low-temperature phosphidation treatment to successfully construct NiP x -CoP x nanosheet-nanowire composite heterostructure catalyst, which not only optimizes the electronic structure, realizes the reasonable distribution of Ni and Co components, but also enhances the utilization rate of active sites through the electronic reconstruction effect at the heterointerface, improving the deep reduction ability of NO3 - and effectively inhibiting the generation of by-products. Compared with traditional single-metal phosphide catalysts and simple bimetallic phosphide catalysts, NiP x -CoP x exhibits more excellent catalytic activity and stability in the NitRR reaction, providing new ideas for the material design of electrocatalytic ammonia synthesis and wastewater denitrification.

[0022] The NiP x -CoP x of the present invention presents a composite structure with coexisting nanosheets and nanowires. This heterostructure helps to provide abundant active sites and enhance the electron transport ability and catalytic interface stability. Description of the Drawings

[0023] Figure 1 SEM images of Example 1, Comparative Example 1, and Comparative Example 2, where (1a)-(1d) are for Comparative Example 1, (2a)-(2d) are for Comparative Example 2, and (3a)-(3d) are for Example 1;

[0024] Figure 2 High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and energy-dispersive X-ray spectroscopy (EDS) images of the nickel phosphide-cobalt phosphide nanocomposite of Example 1, where (a) is the HAADF-STEM image and (b)-(e) are the EDS images;

[0025] Figure 3 XRD patterns of Example 1, Comparative Example 1, and Comparative Example 2;

[0026] Figure 4 TEM images of the nickel phosphide-cobalt phosphide nanocomposite of Example 1;

[0027] Figure 5 XPS spectra of the nickel phosphide-cobalt phosphide nanocomposite of Example 1, where (a) is Ni 2p, (b) is Co2p; (c) is O1s; (d) is P 2p;

[0028] Figure 6 NitRR activities and performances in Example 1, Comparative Examples 1-3, where (a) is the linear sweep voltammetry curve, (b) is the NH3 selectivity, NH3 Faraday efficiency, NO3- Conversion rate diagram, (c) is the NH3 production rate and partial current density diagram, (d) is the NO3 - Production rate and Faraday efficiency diagram;

[0029] Figure 7 NitRR performance diagram of nickel phosphide-cobalt phosphide nanocomposite in Example 1 at different potentials, where (a) is the NH3 production rate and Faraday efficiency diagram; (b) is the NO2 - Production rate and Faraday efficiency diagram; (c) is the NH3 selectivity and NO3 - Conversion rate diagram;

[0030] Figure 8 For the nickel phosphide-cobalt phosphide nanocomposite in Example 1 at -1.05 V vs. SCE potential, NO3 - -N, NO2 - -N, NH3-N, NH2OH-N, N2H4-N concentration variation diagram with electrolysis time, where (a) is the NO3 - -N, NH3-N, N2H4-N concentration variation diagram with electrolysis time, (b) is the NO2 - -N, NH2OH-N, N2H4-N concentration variation diagram with electrolysis time, (c) is the NH3 selectivity, NH3 Faraday efficiency, NO3 - Conversion rate variation diagram with electrolysis time;

[0031] Figure 9 Performance diagram of the nickel phosphide-cobalt phosphide nanocomposite in Example 1 at -1.05 V vs. SCE potential for 11 consecutive cycles, where (a) is the NH3 production rate diagram for 11 cycles, (b) is the NH3 Faraday efficiency and i-t curve diagram for 11 cycles;

[0032] Figure 10 For the nickel phosphide-cobalt phosphide nanocomposite in Example 1 at -1.05 V vs. SCE potential with different NO3 - -N concentrations of NitRR performance diagram, where (a) is the NH3 production rate and Faraday efficiency variation diagram with NO3 - -N concentration, (b) is the by-product generation amount variation diagram with NO3 - -N concentration;

[0033] Figure 11 CV test diagrams and double-layer capacitance fitting calculation diagrams of Example 1, Comparative Example 1, and Comparative Example 2 at different scan rates, where (a) is NiP x -CoP x , (b) is CoP x , (c) is NiP x , (d) is the double-layer capacitance fitting calculation diagram;

[0034] Figure 12 EIS spectra of Example 1, Comparative Example 1, and Comparative Example 2 at -1.05 V vs. Ag / AgCl. Detailed implementation manners

[0035] The present invention will be further described below in conjunction with the embodiments of the present invention and the accompanying drawings.

[0036] All the electrochemical tests of the present invention were carried out on a CHI 600E electrochemical workstation (Shanghai Chenhua). A sealed H-type membrane-exchangeable electrolytic cell was used as the reaction device, separated by a Nafion 117 proton exchange membrane in the middle. Before use, the proton exchange membrane was treated with 5 wt% H2O2 solution at 80 °C for 1 h, washed with deionized water for 20 min, treated with 0.5 M H2SO4 solution at 80 °C for 1 h, and finally rinsed and soaked with deionized water for 30 min. A standard three-electrode system was adopted. The reference electrode was a saturated calomel electrode (filled with saturated KCl solution), the counter electrode was a platinum sheet electrode of 1*1 cm 2 and the working electrode was the composite material prepared by the present invention of 1*1.5 cm 2 . It was put into the electrolyte solution to ensure that the immersed geometric area was 1 cm 2 . The cathode electrolyte for NitRR test was 35 mL of 0.5 M K2SO4 and 200 ppm of KNO3-N (1 ppm = 1 μg mL -1 ), and the anode electrolyte was 35 mL of 0.5 M K2SO4. K2SO4 was only used as a supporting electrolyte to play a conductive role. Before the experiment, high-purity argon gas was continuously introduced into the system for 30 min to exclude N2 pollution in the air and dissolved oxygen in the solution. Subsequently, the resistance of the system was measured to ensure that the initial state of the system was the same before each experiment. Then, the current-potential curve (LSV curve) was scanned at a sweep rate of 10 mV·s -1 until it was stable (the LSV curves were basically coincident for three consecutive times) to activate the electrode. Finally, a constant potential electrolysis (i-t) test was carried out, and the test potential range was -0.9 to -1.2 V vs. SCE, and the electrolysis time was 2 h.

[0037] Example 1

[0038] The preparation method of the nickel phosphide-cobalt phosphide nanocomposite material in Example 1 includes the following steps:

[0039] S1: Immerse a 3*4 cm 2 carbon cloth in concentrated nitric acid and reflux at 120 °C for 2 h to improve the hydrophilicity of the carbon cloth. Subsequently, it was ultrasonically washed with deionized water for 10 min, then ultrasonically washed with absolute ethanol for 10 min, washed alternately with water and ethanol three times, and dried at 60 °C to obtain a hydrophilic carbon cloth;

[0040] S2: Dissolve 4 mmol of cobalt nitrate hexahydrate (Co(NO3)2·6H2O), 20 mmol of ammonium fluoride (NH4F), and 20 mmol of urea ((NH2)2CO) in 50 mL of deionized water, and magnetically stir until the solution is clear and transparent to obtain precursor solution one. Transfer precursor solution one to a 100 mL polytetrafluoroethylene inner liner, and immerse a 3*4 cm 2 hydrophilic carbon cloth into precursor solution one. After loading into the autoclave, hydrothermal react at 120 °C for 12 h to obtain precursor CoOHF. Wash the precursor CoOHF three times each with deionized water and absolute ethanol, and dry at 80 °C for 6 h for later use;

[0041] S3: Dissolve 4 mmol of nickel nitrate hexahydrate (Ni(NO3)2·6H2O), 20 mmol of NH4F, and 20 mmol of (NH2)2CO in 50 mL of deionized water, and magnetically stir until the solution is clear and transparent to obtain precursor solution two. Transfer precursor solution two to a 100 mL polytetrafluoroethylene inner liner, and immerse 3*4 cm 2 CoOHF into precursor solution two. After loading into the autoclave, hydrothermal react at 120 °C for 12 h to obtain the Ni-Co precursor.

[0042] S3: Place 2 g of sodium hypophosphite monohydrate (NaH2PO2·H2O) and a 1.5*4 cm 2 Ni-Co precursor into two magnetic boats and place them at the upstream and downstream of a tube furnace. Pass a 10% hydrogen-nitrogen mixed gas, and calcine at 300 °C for 2 h at a heating and cooling rate of 2 °C / min to obtain a nickel phosphide-cobalt phosphide nanocomposite. The nickel phosphide-cobalt phosphide nanocomposite of Example 1 is named NiP x -CoP x . The volume ratio of hydrogen to nitrogen in the 10% hydrogen-nitrogen mixed gas is 1:9.

[0043] Comparative Example 1

[0044] The preparation method of cobalt phosphide in Comparative Example 1 is as follows:

[0045] S1: Immerse a 3*4 cm 2 carbon cloth in concentrated nitric acid and reflux at 120 °C for 2 h to improve the hydrophilicity of the carbon cloth. Then, ultrasonically wash with deionized water for 10 min, and then ultrasonically wash with absolute ethanol for 10 min. Alternately wash with water and ethanol three times and then dry at 60 °C to obtain the hydrophilic carbon cloth;

[0046] S2: Dissolve 4 mmol of Co(NO3)2·6H2O, 20 mmol of NH4F, and 20 mmol of (NH2)2CO in 50 mL of deionized water, and stir magnetically until the solution is clear and transparent to obtain a precursor solution. Transfer the precursor solution to a 100 mL Teflon liner, immerse a 3*4 cm 2 hydrophilic carbon cloth into the precursor solution, load it into the autoclave, and hydrothermally react at 120 °C for 12 h to obtain the precursor CoOHF. Wash the precursor three times each with deionized water and absolute ethanol, and dry it at 80 °C for 6 h.

[0047] S3: Put 2 g of NaH2PO2·H2O and the dried CoOHF into two boats and place them at the upstream and downstream of the tube furnace respectively. Pass a 10% hydrogen-nitrogen mixed gas, and calcine at 300 °C for 2 h at a heating rate of 2 °C / min to obtain the product. The cobalt phosphide of Comparative Example 1 is named CoP x . The volume ratio of hydrogen to nitrogen in the 10% hydrogen-nitrogen mixed gas is 1:9.

[0048] Comparative Example 2

[0049] The preparation method of nickel phosphide in Comparative Example 2 is as follows:

[0050] S1: Immerse a 3*4 cm 2 carbon cloth in concentrated nitric acid and reflux at 120 °C for 2 h to improve the hydrophilicity of the carbon cloth. Then, ultrasonically wash it with deionized water for 10 min, and then ultrasonically wash it with absolute ethanol for 10 min. After washing alternately with water and ethanol three times, dry it at 60 °C to obtain a hydrophilic carbon cloth;

[0051] S2: Dissolve 4 mmol of Ni(NO3)2·6H2O, 20 mmol of NH4F, and 20 mmol of (NH2)2CO in 50 mL of deionized water, and stir magnetically until the solution is clear and transparent to obtain a precursor solution. Transfer the precursor solution to a 100 mL Teflon liner, immerse a 3*4 cm 2 hydrophilic carbon cloth into the precursor solution, load it into the autoclave, and hydrothermally react at 120 °C for 12 h to obtain a precursor;

[0052] S3: Put 2 g of NaH2PO2·H2O and a 1.5*4 cm 2 precursor into two boats and place them at the upstream and downstream of the tube furnace respectively. Pass a 10% hydrogen-nitrogen mixed gas, and calcine at 300 °C for 2 h at a heating rate of 2 °C / min to obtain the product. The nickel phosphide of Comparative Example 2 is named NiP x . The volume ratio of hydrogen to nitrogen in the 10% hydrogen-nitrogen mixed gas is 1:9.

[0053] Comparative Example 3

[0054] The preparation method of the nickel phosphide-cobalt phosphide nanocomposite of Comparative Example 3 includes the following steps:

[0055] S1: Dissolve 2 mmol of Co(NO3)2·6H2O, 2 mmol of Ni(NO3)2·6H2O, 20 mmol of NH4F, and 20 mmol of (NH2)2CO in 50 mL of deionized water, and magnetically stir until the solution is clear and transparent to obtain a precursor solution. Transfer the precursor solution to a 100 mL polytetrafluoroethylene inner liner, and immerse a 3*4 cm 2 hydrophilic carbon cloth into the precursor solution. After loading the autoclave, hydrothermal treatment is carried out at 120 °C for 12 h to obtain a precursor; the preparation method of the hydrophilic carbon cloth is the same as that in Example 1.

[0056] S2: Place 2 g of NaH2PO2·H2O and a 1.5*4 cm 2 precursor into two magnetic boats and place them at the upstream and downstream of a tube furnace. Introduce a 10% hydrogen-nitrogen mixed gas, and calcine at 300 °C for 2 h at a heating rate of 2 °C / min to obtain the product. The cobalt nickel phosphide of Comparative Example 3 is named CoNiP x . The volume ratio of hydrogen to nitrogen in the 10% hydrogen-nitrogen mixed gas is 1:9.

[0057] From Figure 1 it can be seen that CoP x is based on nanowires, and the surface is transformed into a uniform nanosheet structure. The flaky morphology is evenly distributed and arranged in layers. The thickness of a single sheet is relatively thin, and the edges are clear. NiP x presents an obvious nanoflower-like structure. The "petals" are stacked by many thin sheets, and there are micropores and wrinkles on the surface. NiP x -CoP x shows a composite structure in which nanowires and nanosheets coexist. From Figure 2 the left side of a, it can be seen that after ultrasonic dispersion, NiP x -CoP x still maintains the morphology of nanosheets covering nanowires, Figure 2 and the right side area of a are all nanosheets detached from the nanowires. From Figure 2 b- Figure 2 e, it can be seen that Co, Ni, and P elements are evenly distributed in NiP x -CoP x . However, only Ni and P exist in the nanosheets detached from the nanowires, and the presence of O is related to surface adsorbed oxygen. Combining Figure 1 and Figure 2 it can be seen that NiP x -CoP x contains NiP x and CoP x existing in the structures of nanosheets and nanowires respectively.

[0058] From Figure 3 it can be seen that CoP x exhibits four distinct diffraction peaks at diffraction angles of 31.48°, 46.22°, 48.28°, and 56.26°, corresponding to the (011), (112), (202), and (212) crystal planes of CoP (JCPDS 01-089-4862), respectively. Characteristic diffraction peaks corresponding to the (002), (200), (012), and (-113) crystal planes of CoP2 (JCPDS 04-012-7038) are observed at diffraction angles of 34.72°, 35.48°, 38.7°, and 51.72°. NiP x Four characteristic diffraction peaks can be observed at diffraction angles of 40.63°, 44.53°, 47.33°, and 53.99°, corresponding to the (111), (201), (210), and (002) crystal planes of Ni2P (JCPDS 04-006-5974), respectively. Four diffraction peaks attributed to the (212), (204), (006), and (214) crystal planes of Ni5P4 (JCPDS 04-007-3964) are observed at 43.8°, 45.08°, 49.68°, and 52.84°. Ni5P4 exhibits the best HER performance among nickel phosphide species. NiP x -CoP x It can be seen that it simultaneously exhibits the characteristic diffraction peaks of both CoP x and NiP x , and the crystallinity of the NiP x part is relatively strong.

[0059] From Figure 4 it can be seen that NiP x -CoP x exhibits a distinct morphology of nanosheets attached to nanowires. The interplanar spacings of 0.516 nm and 0.218 nm correspond to the (101) and (211) crystal planes of Ni5P4, respectively. The interplanar spacings of 0.293 nm and 0.291 nm correspond to the (110) crystal plane of Ni2P. The interplanar spacings of 0.204 nm and 0.197 nm correspond to the (102) crystal plane of CoP2. The interplanar spacings of 0.231 nm and 0.254 nm correspond to the (201) and (200) crystal planes of CoP, respectively. Additionally, HRTEM also reveals a small amount of amorphous regions. The XRD and TEM results indicate that the as-prepared NiP x -CoP x is mainly composed of four phases: Ni5P4, Ni2P, CoP2, and CoP.

[0060] From Figure 5 a, it can be seen that NiP x-CoP x and NiP x both exhibit characteristic peaks of Ni species in the oxidized state, located at approximately 856.75 eV, 858.83 eV (2p 3 / 2 ), and 874.31 eV, 876.43 eV (2p 1 / 2 ), and are accompanied by satellite peaks. In NiP x -CoP x , two deconvoluted peaks located at binding energies of approximately 853.38 eV (2p 3 / 2 ) and 870.62 eV (2p 1 / 2 ) are attributed to the Ni-P species (Niδ + ). Compared with NiP x , the binding energy of the Ni-P characteristic peak in NiP x -CoP x shifts positively by 0.1 eV, indicating that the interaction between nickel phosphide and cobalt phosphide leads to a change in the electronic structure. Figure 5 As can be seen from b, two sets of peaks at binding energies of approximately 780.92 eV, 782.55 eV (2p 3 / 2 ) and 795.61 eV, 797.44 eV (2p 1 / 2 ) correspond to Co(II) in the oxidized state, and two satellite peaks of each are present. Compared with CoP x , the Co 2p peaks of NiP x -CoP x shift overall towards lower binding energies, and the electron cloud density around the Co atom increases. Combining the shifts of the Ni 2p and Co 2p peaks indicates that there is a charge redistribution at the interface between nickel phosphide and cobalt phosphide, with electrons transferring from Ni to Co. Figure 5 As can be seen from c, after deconvolution, the peak at a binding energy of approximately 531.5 eV is attributed to the P-O species in the phosphate, and the peak at 533.1 eV is attributed to the adsorbed oxygen (O ads ) species, while there is almost no metal-oxygen chemical bond (M-O). As can be seen from Figure 5 d, NiP x -CoP x , NiP x , and CoP x all exhibit characteristic peaks of the P-O species at a binding energy of approximately 134 eV, corresponding to the O 1s results. Moreover, characteristic peaks of the M-P (M = Ni, Co) chemical bond are also observed at binding energies of approximately 129.42 eV (2p 3 / 2 ) and 130.38 eV (2p 1 / 2 ). The intensity of the characteristic peak at M-P in NiP x -CoP x is significantly higher than that of the single phosphide, indicating that the M-P characteristic peak intensity in NiP x -CoPx Cobalt phosphide and nickel phosphide coexist on the surface. In addition, NiP x -CoP x The P 2P peak of x , NiP x and CoP x shows an overall negative shift of about 0.4 eV compared to NiP x and CoP

[0061] From Figure 6 a, it can be seen that in the electrolyte of 0.5 M K2SO4 and 200 ppm KNO3-N, NiP x -CoP x exhibits a higher negative current density at the same potential, indicating that it may have the best NitRR activity. CoP x and CoNiP x have similar NitRR activities, while NiP x shows a larger current density than CoP x and CoNiP x in the potential range of -0.8 V to -1.05 V vs. SCE. However, at a more negative potential (< -1.05 V vs. SCE), obvious bubbles are generated on the surface of NiP x -CoP x , resulting in a slowdown in the increasing trend of the current density of NiP x and lower than that of CoP x and CoNiP x , indicating that HER gradually dominates. From Figure 6 b, after 2 h of electrolysis, NiP x -CoP x has a NO3 - conversion rate of 81.64%, an NH3 Faraday efficiency of 97.78%, and an NH3 selectivity of 99.95%. The NO3 - conversion rate, NH3 Faraday efficiency, and NH3 selectivity all show the best performance, indicating that it can efficiently catalyze the deep reduction of NO3 - to NH. Figure 6 b further shows the NH3 yields and the corresponding partial current densities of Example 1 and Comparative Examples 1-3. NiP x -CoP x has the highest NH3 yield at -1.05 V vs. SCE, exceeding 0.2 mmol h -1cm -2 At the same time, the NH3 current density is also high, which is 43.89 mA cm -2 , indicating that NiP x -CoP x The NitRR activity is stronger and can more effectively promote NO3 - The electron reduction process of CoNiP x The NH3 production rate and current density of the metal phosphide are lower than those of single metal phosphide, and have not even reached that of NiP. x -CoP x The performance is half of that of NitRR, indicating that the bimetallic phosphides prepared by the one-step hydrothermal rephosphating method are poor in electron transport and active site regulation, and the simple Ni-Co combination cannot significantly improve the catalytic effect of NitRR. Figure 6 d analyzed NO2 - The yield and Faradaic efficiency of NiP x -CoP x NO2 - The yield was the lowest, 2 μmmol h -1 cm -2 , the Faradaic efficiency is also the lowest (<5‰), indicating that NiP x -CoP x NO2 in the catalytic process - The accumulation of NiP is less, which is consistent with its high NH3 selectivity. x -CoP x The synergistic effect of the components can optimize the electronic structure of the catalyst, improve the effectiveness of the catalytic active sites, and accelerate the removal of NO3 - The deep reduction process can achieve efficient and highly selective NH3 synthesis.

[0062] from Figure 7 It can be seen from a that the yield and Faraday efficiency of NH3 first increase and then decrease with the increase of potential, reaching the maximum at -1.10V~-1.05Vvs.SCE, indicating that NiP x -CoP x The NitRR catalytic activity of the catalyst was the best, with the highest NH3 Faradaic efficiency of 97.78% at -1.05 V vs. SCE. Figure 7 b shows the main by-product NO2 - The yield and Faradaic efficiency of NO2 change with potential. It can be seen that at a relatively negative potential of -1.20V to -1.05V vs. SCE, NO2 - The yield of NiP is low, indicating that x -CoP x In this potential range, NO3 can be effectively catalyzed -Reduced to NH3 instead of remaining at NO2 - Intermediate state. However, when the potential is further increased (> -1 V vs. SCE), the yield of NO2 - significantly increases, and the Faradaic efficiency of NO2 - also correspondingly increases, indicating that the reduction ability of NiP x -CoP x is insufficient at relatively positive potentials. This further demonstrates that at lower overpotentials, the kinetics of NitRR are restricted and cannot effectively drive the deep transfer of electrons, ultimately resulting in the accumulation of NO2 - . As shown in Figure 7 c, within the entire selected potential range, the selectivity of NH3 remains above 91%, and the conversion rate of NO3 - shows a volcano-shaped trend. Combining with Figure 7 a, it indicates that at more negative potentials, due to the HER competition, part of the electrons flow to HER, resulting in a low Faradaic efficiency of NH3. Therefore, when the working potential of NiP x -CoP x is -1.05 V vs. SCE, efficient and highly selective removal of NO3 - can be achieved, and the generation of by-products is minimized.

[0063] Figure 8 Shows the NitRR performance of NiP x -CoP x at -1.05 V vs. SCE. As the electrolysis time increases, the NO3 - -N concentration gradually decreases, the NH3-N gradually increases, and the concentrations of by-products NO2 - -N, NH2OH-N, and N2H4-N always remain at relatively low levels. During the entire electrolysis process, the conversion rate of NO3 - steadily increases, the Faradaic efficiency of NH3 remains at a high level (> 96%), and the selectivity of NH3 is close to 100%, indicating that NiP x -CoP x can maintain stable and efficient catalytic performance during a long-term electrolysis process. In addition, when the electrolysis time is 2 h, the increasing rate of NH3-N significantly slows down.

[0064] Performed 11 consecutive cyclic tests on NiP x -CoP x at -1.05 V vs. SCE. Each cycle was electrolyzed for 2 h, and fresh electrolyte was replaced. As shown in Figure 9 a, after 11 cycles, the NH3 yield still maintained at 0.20 - 0.22 mmol h -1 cm -2around, indicating NiP x -CoP x exhibits good catalytic activity and stability during long-term electrolysis without obvious activity decay. As can be seen from Figure 9 b, the potentiostatic electrolysis curves for each cycle are basically consistent, and the Faradaic efficiency of NH3 always remains above 97%, indicating that the electron utilization efficiency of the catalyst still maintains a high level after multiple cycles and there is no obvious by-product competition.

[0065] As can be seen from Figure 10 a, when electrolyzing for 2 h at the potential of NH3 - 1.05 V vs. SCE, the yield of NH3 increases significantly with the increase of NO3 - -N concentration. At lower concentrations (50 - 500 ppm), the yield of NH3 shows a linear increase, indicating that in this range, the active sites of NiP x -CoP x are sufficient and the conversion of NO3 - is driven by the reactant concentration. However, when the NO3 - -N concentration increases to above 1400 ppm (1 M KNO3), the growth trend of the NH3 yield gradually slows down, probably because the surface of NiP x -CoP x reaches saturation and the active sites are fully occupied, and further increasing the NO3 - concentration will not significantly improve the conversion rate. In addition, the Faradaic efficiency of NH3 remains at a high level (>90%) in the range of 200 - 2000 ppm, indicating that NiP x -CoP x has a high electron utilization efficiency for NO3 - and can effectively promote the deep conversion of NO3 - to NH3. However, at 2000 ppm, the Faradaic efficiency of NH3 decreases slightly. As can be seen from Figure 10 b, this is because at high NO3 - concentrations, the surface loading of NiP x -CoP x is too high, electron transport is limited, and side reactions are enhanced, resulting in a part of the electrons flowing to other paths, such as NO3 - →NO2 - . The ammonia production performance can be optimized by increasing the area of NiP x -CoP x or raising the reaction potential in a high-concentration electrolyte. Generally speaking, the NiP x -CoP x catalyst exhibits excellent NitRR performance and is widely applicable to NO3 - -N with different concentration ranges.

[0066] From Figure 11 it can be seen that the electrochemically active surface area (ECSA) value is positively correlated with the double-layer capacitance (C dl ). The ECSA magnitudes of Example 1, Comparative Example 1, and Comparative Example 2 follow the order: NiP x -CoP x > CoP x > NiP x . NiP x -CoP x , due to its larger ECSA, can provide more active sites, promote the adsorption and reduction of NO3 - , and improve the catalytic activity, which explains the higher NH3 production rate and NO3 x -CoP x exhibits in NitRR and the NO3 - removal rate.

[0067] The EIS spectra of Example 1, Comparative Example 1, and Comparative Example 2 were tested at -1.05 V vs. Ag / AgCl. From Figure 12 it can be seen that the Nyquist plot of NiP x -CoP x has the smallest radius, indicating that NiP x -CoP x has the fastest charge transfer rate. The simulated EIS equivalent circuit diagram consists of three components: charge transfer resistance (R ct ), solution resistance (R s ), and constant phase element (CPE). The charge transfer resistances R ct of Example 1, Comparative Example 1, and Comparative Example 2 follow the order: NiP x -CoP x (11.62 Ω) < CoP x (30.47 Ω) < NiP x (31.73 Ω). The EIS analysis results show that NiP x -CoP x has the lowest R ct , indicating its optimal charge transport ability, which can promote the rapid transfer of electrons at the catalyst / electrolyte interface and improve the kinetic performance of the NitRR reaction.

Claims

1. Preparation method of nickel phosphide-cobalt phosphide nanocomposite, characterized in that, It includes the following steps: S1: Dissolve cobalt salt, ammonium fluoride, and urea in solvent 1 to obtain precursor solution 1. Immerse hydrophilic carbon cloth in precursor solution 1, and after hydrothermal reaction, obtain precursor CoOHF; S2: Dissolve nickel salt, ammonium fluoride, and urea in solvent 2 to obtain precursor solution 2. Immerse precursor CoOHF in precursor solution 2, and after hydrothermal reaction, obtain Ni-Co precursor; S3: Calcinate the phosphorus source and the Ni-Co precursor described in S2. Under the action of reducing gas, drive the phosphorus-containing gas generated by calcining the phosphorus source to the Ni-Co precursor, and obtain the product after reaction.

2. The preparation method of the nickel phosphide-cobalt phosphide nanocomposite according to claim 1, characterized in that, The reducing gas described in S3 is a hydrogen-nitrogen mixed gas, and the volume ratio of hydrogen to nitrogen in the hydrogen-nitrogen mixed gas is 1:

9.

3. The preparation method of the nickel phosphide-cobalt phosphide nanocomposite according to claim 1, wherein, The calcination temperature described in S3 is 275 - 350 °C; the heating rate of the calcination is 1 - 5 °C / min; the calcination time is 0.5 - 2 h.

4. The preparation method of the nickel phosphide-cobalt phosphide nanocomposite according to claim 1, wherein, The hydrothermal reaction temperature described in S1 and S2 is 120 - 180 °C, and the hydrothermal reaction time is 6 - 24 h.

5. The preparation method of the nickel phosphide-cobalt phosphide nanocomposite according to claim 1, characterized in that, The cobalt salt described in S1 is one or more of cobalt nitrate hexahydrate, cobalt chloride, and cobalt sulfate, and the solvent 1 is water; the molar ratio of the cobalt salt to ammonium fluoride is 1:2 - 1:8; the molar ratio of the cobalt salt to urea is 1:2 - 1:

8.

6. The preparation method of the nickel phosphide-cobalt phosphide nanocomposite according to claim 1, wherein, The nickel salt is one or more of nickel nitrate hexahydrate, nickel chloride, and nickel sulfate, and the solvent 2 is water; the molar ratio of the nickel salt to ammonium fluoride is 1:2 - 1:8; the molar ratio of the nickel salt to urea is 1:2 - 1:8; the molar ratio of the nickel salt to the cobalt salt is 1:1 - 1:

2.

7. The preparation method of the nickel phosphide-cobalt phosphide nanocomposite according to claim 1, wherein The preparation method of the hydrophilic carbon cloth described in S1 is: Heat the carbon cloth under reflux in concentrated nitric acid, then alternately wash it with water and ethanol, and obtain it after drying; the temperature of the heating under reflux is 120 - 140 °C, and the time is 2 - 4 h.

8. The preparation method of the nickel phosphide-cobalt phosphide nanocomposite according to claim 1, wherein, The phosphorus source described in S3 is sodium dihydrogen phosphate monohydrate, and 0.5 - 2 g of the phosphorus source is added per mmol of the nickel salt.

9. Nickel phosphide-cobalt phosphide nanocomposite, characterized in that, It is prepared by using the preparation method of the nickel phosphide-cobalt phosphide nanocomposite material described in any one of claims 1 - 8.

10. The application of the nickel phosphide-cobalt phosphide nanocomposite material as described in claim 9 in electrocatalytic ammonia synthesis and wastewater denitrification.

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