Electrocatalytic composite material of MXene loaded copper phosphide nanoparticles and preparation method and application thereof

Abstract

The application discloses an electrocatalytic composite material of MXene loaded with copper phosphide nanoparticles as well as a preparation method and application thereof, and comprises the following steps: (1) dispersing MXene, a copper precursor, a phosphorus precursor and anhydrous sodium citrate in deionized water, continuously stirring until completely dissolved, adjusting pH by sodium hydroxide, and preparing a mixed solution; (2) under the environment of inhaled inert gas, adding sodium borohydride solution drop by drop into the mixed solution, and stirring intensively; when the reduction reaction is completed, centrifuging, washing and freeze-drying the obtained solid-liquid mixture, and finally obtaining P-Cu@MX composite material. The composite material is used as an electrocatalyst, is coated on the surface of a conductive material, forms a high-efficiency catalytic electrode, and constructs a reactor, so that the co-reduction of NO3 ‑ and CO2 into urea can be realized.

Description

Technical Field

[0001] This invention relates to the field of electrocatalytic composite materials technology, and in particular to an electrocatalytic composite material with MXene-supported copper phosphide nanoparticles, its preparation method and application. Background Technology

[0002] Nitrate (NO3) - NO3 is one of the most common pollutants in natural water bodies. Excessive NO3 in surface water - This can lead to localized eutrophication, thereby damaging the aquatic ecosystem, and excessive NO3 in drinking water... - It can cause serious diseases such as liver damage and methemoglobin syndrome. Steel mills and fertilizer plants, which are high-concentration NO3 plants, are particularly susceptible. - Wastewater from concentrated sources needs to be effectively treated to address NO3 at its source. - The issue of wastewater discharge is addressed to avoid pollution of natural water bodies.

[0003] Currently, regarding NO3-containing - Wastewater treatment methods mainly include biological denitrification, physical methods, and chemical reduction methods. Among these, the biological method is the most mature and widely used, which utilizes microorganisms to denitrify NO3- under anaerobic conditions. - Transformation and removal are possible, but the complex composition of industrial wastewater severely affects microbial activity, and NO3... - The removal process requires a large number of electron donors, significantly increasing operating costs and carbon emissions. Physical methods such as reverse osmosis, electrodialysis, and ion exchange, while simple to operate and highly efficient, only target NO3. - Spatial transfer occurred, resulting in high concentrations of NO3. - The wastewater requires further treatment. In contrast, chemical reduction methods utilize catalysts to reduce NO3. - The process reduces the substance to N2, NH3, etc., is simple to operate, and has high conversion efficiency. Electrochemical reduction utilizes electrons generated by the electric field on the cathode surface through the application of current to convert NO3 into N2, NH3, etc. - The reduced electron donor enables the reduction of NO3. - Degradation and transformation. Electrochemical reduction of NO3. - The method is highly efficient, controllable, and produces no secondary pollution in the removal of NO3 from industrial wastewater. - It shows great potential. In particular, in NO3 - Introducing CO2 during electroreduction can achieve co-reduction carbon-nitrogen coupling to synthesize small-molecule organic compounds such as urea and amides. For urea, the carbon-nitrogen coupling method effectively overcomes the problems of harsh reaction conditions, high energy consumption, and severe pollution associated with the currently used industrial Bosch-Meiser method, while simultaneously reducing NO3- concentration. -To promote the utilization of CO2 resources, alleviate the environmental crisis, and advance the goal of carbon neutrality.

[0004] The key to the carbon-nitrogen coupling synthesis of urea is the catalyst, and the d orbital energy level of copper (Cu) is related to NO3. - The lowest unoccupied molecule Similar to the (LUMO π) orbitals, Cu's open d orbitals facilitate electron transfer to the adsorbed NO3. - Transfer, overcoming NO3 - The high energy level of the LUMO π orbital hinders charge injection, making it difficult to inject NO3. - Reduced to NO2 - The reaction exhibits relatively fast reaction kinetics. Furthermore, Cu exhibits good reaction kinetics for the CO2 reduction intermediate. Its moderate adsorption capacity facilitates its reduction into various high-value carbon-containing chemicals, exhibiting unique performance advantages in CO2 reduction. Therefore, Cu can be used as a NO3-reducing agent. - An ideal catalyst for the co-reduction of carbon-nitrogen coupling to urea with CO2.

[0005] However, due to their high surface energy, Cu nanoparticles readily aggregate into larger particles, leading to a sharp decrease in active area and the number of active sites, thus limiting their catalytic performance. Furthermore, the electronic structure of the Cu surface is not sufficiently stable, and the adsorption of intermediates is outside the optimal window, resulting in poor catalytic activity and low CN coupling efficiency. In addition, pure Cu catalysts typically suffer from deactivation and cannot maintain high catalytic performance over long periods. Summary of the Invention

[0006] To address the aforementioned problems in existing technologies, this invention proposes an electrocatalytic composite material of MXene-supported copper phosphide nanoparticles, its preparation method, and its application. This invention uses MXene as a support material, and employs a wet chemical method to reduce Cu nanoparticles and grow them in situ on the MXene surface, while simultaneously modifying the Cu with phosphating to prepare a P-Cu@MX composite material. The high specific surface area of ​​MXene provides more attachment sites for Cu, effectively inhibiting Cu nanoparticle aggregation and exposing more active sites. Simultaneously, the introduction of phosphorus optimizes the electronic structure of the Cu surface, thereby improving the catalytic activity and stability of the Cu nanoparticles. Using this composite material as an electrocatalyst, a highly efficient catalytic electrode can be formed by coating it onto the surface of a conductive material, and a reactor can be constructed to achieve NO3- catalytic oxidation. - It co-reduces carbon and nitrogen couples with CO2 to form urea.

[0007] The technical solution of the present invention is as follows: The first objective of this invention is to provide a method for preparing an electrocatalytic composite material of MXene-supported copper phosphide nanoparticles, comprising the following steps: (1) Disperse MXene, copper precursor, phosphorus precursor and anhydrous sodium citrate in deionized water and stir continuously until completely dissolved. Adjust the pH with sodium hydroxide to obtain a mixed solution. (2) Under an inert gas atmosphere, sodium borohydride solution was added dropwise to the mixture and stirred vigorously. After the reduction reaction was completed, the resulting solid-liquid mixture was centrifuged, washed, and freeze-dried to finally obtain the P-Cu@MX composite material.

[0008] In one embodiment of the present invention, in step (1), the concentration of MXene in the mixture is 200~600 mg / L, the concentration of copper ions is 3~8 mmol / L, the phosphorus content is 2~8 mmol / L, and the concentration of anhydrous sodium citrate is 10~100 mmol / L.

[0009] In one embodiment of the present invention, in step (1), the copper precursor is one or more of copper sulfate pentahydrate, copper chloride dihydrate, and copper nitrate trihydrate; the phosphorus precursor is one or more of sodium hypophosphite monohydrate, ammonium hypophosphite, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0010] In one embodiment of the present invention, in step (1), the stirring speed is 200~300 rpm and the stirring time is 10~30 min.

[0011] In one embodiment of the present invention, in step (1), the pH is adjusted to 9-12.

[0012] In one embodiment of the present invention, in step (2), the inert gas is either nitrogen or argon, and the gas flow rate is 20~80 mL / min.

[0013] In one embodiment of the present invention, in step (2), the concentration of sodium borohydride solution is 1.0~2.0 mol / L, the dropping rate is 0.5~2.0 mL / min, and the dropping amount is 0.1~0.4 mol / L relative to the concentration of the mixed solution.

[0014] In one embodiment of the present invention, in step (2), the stirring speed is 400~600 rpm and the reduction reaction time is 1.5~4h.

[0015] In one embodiment of the present invention, in step (2), the centrifugal speed is 8000~12000 rpm.

[0016] In one embodiment of the present invention, in step (2), the washing is performed several times each with anhydrous ethanol and deionized water.

[0017] In one embodiment of the present invention, in step (2), the freeze-drying temperature is -40~-70℃ and the time is 18~30 h.

[0018] The second objective of this invention is to provide an electrocatalytic composite material of MXene-supported copper phosphide nanoparticles prepared by the above-described method.

[0019] A third objective of this invention is to provide an application of the above-mentioned MXene-supported copper phosphide nanoparticle electrocatalytic composite material for fabricating a catalytic electrode and for the co-reduction of nitrate and carbon dioxide to synthesize urea.

[0020] In one embodiment of the present invention, the electrocatalytic composite material of MXene-loaded copper phosphide nanoparticles is coated on the surface of a conductive material as a catalytic electrode, and a three-electrode electrocatalytic reactor is constructed to perform nitrate and carbon dioxide co-reduction carbon-nitrogen coupling to urea in a mixed electrolyte containing saturated carbon dioxide.

[0021] In one embodiment of the present invention, the conductive material is one of carbon cloth, carbon paper, carbon felt, and carbon fiber brush.

[0022] In one embodiment of the present invention, the loading amount of the electrocatalytic composite material of MXene-supported copper phosphide nanoparticles in the catalytic electrode is 0.05~1.2 mg / cm³. 2 .

[0023] In one embodiment of the present invention, in the three-electrode electrocatalytic reactor, the counter electrode is one of a platinum sheet, graphite, or carbon fiber brush, the reference electrode is one of a saturated calomel electrode or an Ag / AgCl electrode, and the reactor configuration is one of an H-type or a flow tank.

[0024] In one embodiment of the present invention, the flow rate of carbon dioxide introduced into the electrolyte is 30~80 mL / min, the concentration of potassium bicarbonate is 0.1 mol / L, the concentration of potassium nitrate is 0.01~1 mol / L, and the applied cathode potential is -0.5~-1.0 V (vs. RHE).

[0025] Beneficial effects: (1) The present invention uses MXene as a carrier to provide a larger attachment area for copper nanoparticles, effectively improve the dispersion and stability of copper nanoparticles, and increase the number of exposed copper active sites.

[0026] (2) The present invention uses non-metallic phosphorus to regulate the electronic structure of copper surface, optimize its d-band center, and improve the catalytic activity of copper.

[0027] (3) The preparation method of the present invention is simple to operate, the conditions are easy to control, the equipment requirements are low, and it has a wide range of application prospects.

[0028] (4) This invention combines the properties of MXene and phosphorus to optimize the electronic structure of the copper surface while dispersing copper nanoparticles, thereby improving its electrosynthesis performance of urea. Experiments show that using the P-Cu@MX composite material as an electrocatalyst for NO3 synthesis... - Urea was synthesized by co-reduction with CO2, with a yield of 65.59 ± 3.00 mmol / h. -1 g cat -1 The Faraday efficiency is 21.16% ± 0.91%, which is better than most Cu-based nanoparticle catalysts currently available. Attached Figure Description

[0029] Figure 1 This is an X-ray diffraction (XRD) pattern of the P-Cu@MX composite material prepared in Example 1 of the present invention.

[0030] Figure 2 shows scanning electron microscope (SEM) images of the P-Cu@MX / CC electrode (a) prepared in Example 1 of the present invention and the Cu / CC electrode (b) prepared in Comparative Example 2.

[0031] Figure 3 shows transmission electron microscopy (TEM) images of the P-Cu@MX composite material (a) prepared in Example 1 of the present invention and the Cu nanoparticles (b) prepared in Comparative Example 2, as well as the average particle size and particle size distribution of the nanoparticles in the materials.

[0032] Figure 4 The P-Cu@MX / CC electrode prepared in Example 1, the Cu@MX / CC electrode prepared in Comparative Example 1, and the Cu / CC electrode prepared in Comparative Example 2 were subjected to NO3. - Linear sweep voltammetry (LSV) curves in an electrolyte containing CO2.

[0033] Figure 5 The P-Cu@MX / CC electrode prepared in Example 1 of this invention is used in NO3 - Linear sweep voltammetry (LSV) curves in electrolytes containing CO2 alone or in combination with it.

[0034] Figure 6 The P-Cu@MX / CC electrode prepared in Example 1, the Cu@MX / CC electrode prepared in Comparative Example 1, and the Cu / CC electrode prepared in Comparative Example 2 were subjected to NO3 treatment. - Urea yield and Faraday efficiency graph for the co-reduction synthesis of urea with CO2.

[0035] Figure 7To test the NO3 reaction of the P-Cu@MX / CC electrode prepared in Example 1 of this invention under different cathode potentials. - Urea yield and Faraday efficiency graph for the co-reduction synthesis of urea with CO2.

[0036] Figure 8 The graph shows the changes in urea yield and Faraday efficiency of the P-Cu@MX / CC electrode prepared in Example 1 of this invention during 15 consecutive urea synthesis experiments. Detailed Implementation

[0037] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0038] Example 1 (1) Weigh 162.58 mg of anhydrous sodium citrate, 78.59 mg of copper sulfate pentahydrate, 34.23 mg of sodium hypophosphite monohydrate and 30 mg of MXene and add them to 63 mL of deionized water. Disperse by sonication for 30 min, adjust the pH to 12 with sodium hydroxide solution with a concentration of 1 mol / L, then introduce nitrogen gas and stir continuously at 300 rpm for 30 min.

[0039] (2) Weigh 37.83 mg of sodium borohydride and dissolve it in 5 mL of deionized water to prepare a 2 M sodium borohydride solution. Add it dropwise to the mixture in step (1) at a rate of 1 mL / min. Stir continuously at 500 rpm for 2 h. Nitrogen gas is continuously introduced during the reduction process at a flow rate of 30 mL / min.

[0040] (3) The solid-liquid mixture after reduction was placed in a high-speed centrifuge and centrifuged at 8000 rpm. Then it was washed twice with anhydrous ethanol and deionized water respectively. The obtained solid catalyst material was placed in a vacuum freeze dryer at -50 ℃ and dried for 24 h to finally obtain P-Cu@MX composite material.

[0041] (4) 0.2 mg of the obtained P-Cu@MX composite material was dispersed in a mixture of 200 μL anhydrous ethanol and 20 μL Nafion 117, and then uniformly drop-coated onto a 1×2 cm² substrate. 2 The carbon cloth surface has a loading of 0.1 mg / cm². 2 P-Cu@MX / CC electrodes were obtained.

[0042] Example 2 Same as Example 1, except that in step (1), the amount of copper sulfate pentahydrate and sodium hypophosphite monohydrate added is changed so that the mass ratio of P to Cu is 3:3, 1.5:4.5, 1.2:4.8 and 1:5 respectively.

[0043] Example 3 Same as Example 1, except that in step (4), the loading of the composite material on the carbon cloth surface is changed to 0.05, 0.075, 0.15, 0.2, and 0.3 mg / cm. 2 .

[0044] Comparative Example 1 (1) Weigh 129.03 mg of anhydrous sodium citrate, 78.59 mg of copper sulfate pentahydrate, and 20 mg of MXene and dissolve them in 50 mL of deionized water. Disperse the solution by sonication for 30 min, add 1 M NaOH to adjust the pH to 12, and then purge with nitrogen and stir continuously for 30 min. Add 5 mL of 2 M freshly prepared sodium borohydride solution dropwise for reduction, and reduce for 2 h.

[0045] (2) After the reaction was completed, the catalyst was centrifuged at 8000 r / min using a high-speed centrifuge. Then it was thoroughly washed with anhydrous ethanol and deionized water and dried overnight in a vacuum freeze dryer at -50 ℃ to obtain Cu@MX catalyst.

[0046] (3) Add 0.2 mg of the prepared Cu@MX catalyst powder to a mixture of 200 μL anhydrous ethanol and 20 μL Nafion 117, sonicate for 30 min to disperse it, and then uniformly drop it onto a 1×2 cm layer. 2 The carbon cloth surface was dried in an oven at 60 ℃ to obtain a Cu@MX / CC electrode.

[0047] Comparative Example 2 (1) Weigh 129.03 mg of anhydrous sodium citrate and 78.59 mg of copper sulfate pentahydrate and dissolve them in 50 mL of deionized water. Disperse the solution by sonication for 30 min, add 1 M NaOH to adjust the pH to 12, and then purge with nitrogen and stir continuously for 30 min. Add 5 mL of 2 M freshly prepared sodium borohydride dropwise for reduction, and reduce for 2 h.

[0048] (2) After the reaction was completed, the catalyst was centrifuged at 8000 r / min using a high-speed centrifuge. Then it was thoroughly washed with anhydrous ethanol and deionized water and dried overnight in a vacuum freeze dryer at -50 ℃ to obtain nano-Cu catalyst.

[0049] (3) Add 0.2 mg of the prepared nano-Cu catalyst powder to a mixture of 200 μL anhydrous ethanol and 20 μL Nafion 117, sonicate for 30 min to disperse it, and then uniformly drop it onto a 1×2 cm layer. 2 The carbon cloth surface is dried in an oven at 60 ℃ to obtain a Cu / CC electrode.

[0050] Test example: (1) The P-Cu@MX composite material prepared in Example 1 was characterized by XRD using a Bruker D8 advance X-ray diffractometer. The results are as follows: Figure 1 As shown.

[0051] Depend on Figure 1 As can be seen, in the XRD pattern of the P-Cu@MX composite material prepared in Example 1, diffraction peaks of Cu(111), Cu(200), and Cu(220) crystal planes were observed, as well as characteristic peaks of Ti(002), Ti(004), Ti(006), and Ti(111) crystal planes in MXene, and diffraction peaks of Cu3P(112) crystal plane. This indicates that Cu nanoparticles were successfully phosphated and loaded onto MXene. Furthermore, the diffraction peaks of the Cu(111) crystal plane in P-Cu@MX showed a negative shift compared to Cu@MX and Cu nanoparticles, indicating that Cu and MXene interacted, and that P doping entered the Cu lattice, increasing the lattice spacing of the Cu(111) crystal plane. This is beneficial for better adsorption of CO2 and NO3. - .

[0052] (2) The P-Cu@MX / CC electrode prepared in Example 1 was characterized by SEM using a Hitachi SU8600 scanning electron microscope. Figure 2a Nanoparticles can be observed distributed on the surface and between layers of MXene, indicating that the phosphated Cu nanoparticles were successfully loaded and dispersed on MXene. Figure 2b In Comparative Example 2, the Cu / CC electrode prepared without MXene and without phosphating exhibited severe agglomeration, indicating that MXene provides sufficient attachment sites for Cu nanoparticles and effectively inhibits their agglomeration.

[0053] (3) The P-Cu@MX composite material prepared in Example 1 and the Cu nanoparticles prepared in Comparative Example 2 were characterized by TEM using a FEI Talos F200X transmission electron microscope, and the average particle size of the Cu nanoparticles was analyzed. Figure 3a It can be seen that only the lattice fringes of the Cu(111) crystal plane are observed on the P-Cu@MX composite material, while Figure 3bIn the comparative example, lattice fringes of Cu(111) and Cu(200) crystal planes were observed in the Cu nanoparticles. The Cu(111) crystal plane not only suppresses the hydrogen evolution side reaction but also promotes the generation of nitrogen-containing intermediates for CN coupling. The increased proportion of Cu(111) crystal planes in the Cu@MX composite material indicates that MXene and P can simultaneously optimize the structure of Cu and improve its catalytic activity. Furthermore, the average particle size of Cu nanoparticles in the P-Cu@MX composite material is 4.89±1.63 nm, while the average particle size of Cu nanoparticles in Comparative Example 2 is 7.92±2.62 nm, indicating that MXene provides more active sites for Cu nanoparticles, inhibits their aggregation, and exposes more active sites.

[0054] (4) LSV tests were performed using the P-Cu@MX / CC electrode prepared in Example 1, the Cu@MX / CC electrode prepared in Comparative Example 1, and the Cu / CC electrode prepared in Comparative Example 2, all using a CHI 660E electrochemical workstation. Figure 4 It can be seen that the current density at the P-Cu@MX / CC electrode is greater than that at the Cu@MX / CC and Cu / CC electrodes, indicating that the P-Cu@MX / CC electrode has higher electrocatalytic activity. MXene improves the dispersibility of Cu nanoparticles, thereby exposing more active sites, and phosphating modification further improves the catalytic activity of the catalyst by optimizing the electronic structure.

[0055] The P-Cu@MX / CC prepared in Example 1 was subjected to LSV testing. Figure 5 The P-Cu@MX / CC prepared in Example 1 of this invention is in NO3 - LSV curves of P-Cu@MX / CC in electrolytes containing CO2 alone or in coexistence. The figure shows that, within the voltage range of 1.1 V (vs. RHE) to -0.6 V (vs. RHE), the LSV curves of P-Cu@MX / CC in NO3... - The current density under conditions of coexistence with CO2 is significantly greater than the current density when either is present alone, indicating that P-Cu@MX / CC exhibits good NO3-coating properties. - Co-reduction ability with CO2.

[0056] (5) The electrode prepared according to the example or comparative example is used as the working electrode, and the saturated calomel electrode is used as the reference electrode, 1×1 cm 2 A platinum sheet electrode was used as the counter electrode, and an H-type reactor with an effective volume of 100 mL for the anode and cathode chambers was used to construct the urea electrosynthesis apparatus. A mixture of 0.1 M potassium bicarbonate and 0.1 M potassium nitrate was prepared as the electrolyte, and carbon dioxide was continuously introduced into the cathode at a flow rate of 30 mL / min. The urea electrosynthesis experiment was conducted by applying a cathode potential.

[0057] Figure 6The urea yield and Faradaic efficiency of the P-Cu@MX / CC electrodes prepared in Example 1, the Cu@MX / CC electrodes prepared in Comparative Example 1, and the Cu / CC electrodes prepared in Comparative Example 2 were tested after 2 h of reaction at -0.7, -0.9, and -0.9 V (vs. RHE), respectively. As shown in the figure, the urea yield and Faradaic efficiency of the P-Cu@MX / CC electrode are significantly higher than those of the Cu@MX / CC and Cu / CC electrodes.

[0058] Figure 7 The urea yield and Faradaic efficiency of the P-Cu@MX / CC electrode prepared in Example 1 were tested after 2 h of reaction at different cathode potentials. As shown in the figure, the urea yield of the P-Cu@MX / CC electrode was consistently above 40 mmol / h in the range of -0.5 V (vs. RHE) to -1.0 V (vs. RHE). -1 g cat -1 The above results were obtained, and the urea yield reached a maximum of 65.59 ± 3.00 mmol / h at a cathode potential of -0.7 V (vs. RHE). -1 g cat -1 The corresponding Faraday efficiency is 21.16% ± 0.91%. Therefore, the P-Cu@MX composite material can be a highly efficient NO3-emitting agent. - A catalyst for the co-reduction of CN with CO2 to form urea.

[0059] Using the P-Cu@MX / CC prepared in Example 1 of this invention, 15 consecutive urea electrosynthesis experiments were conducted under the above electrolyte conditions. The tested urea yield and Faraday efficiency are as follows: Figure 8 As shown in the figure. The results indicate that the urea electrosynthesis performance of the P-Cu@MX / CC electrode did not decrease significantly after 15 cycles, demonstrating its excellent catalytic stability.

[0060] In summary, the P-Cu@MX composite material prepared in Example 1 of this invention was successfully prepared by a simple wet chemical reduction method and applied to NO3. - CN was co-reduced with CO2 to synthesize urea. MXene, as a support, significantly inhibited the aggregation of Cu nanoparticles, while P further enhanced its electrocatalytic activity through regulation. The P-Cu@MX composite material exhibited excellent urea electrosynthesis performance and stability, and showed good performance in NO3--coal oxidation. - It has great potential for co-reduction synthesis of urea with CO2.

[0061] The embodiments provided above are not intended to limit the scope of the invention, nor are the described steps intended to limit the order of execution. Any obvious modifications made to the invention by those skilled in the art based on existing common knowledge also fall within the scope of protection defined by the claims.

Claims

1. A method for preparing an electrocatalytic composite material of MXene-supported copper phosphide nanoparticles, characterized in that, Includes the following steps: (1) Disperse MXene, copper precursor, phosphorus precursor and anhydrous sodium citrate in deionized water and stir continuously until completely dissolved. Adjust the pH with sodium hydroxide to obtain a mixed solution. (2) Under an inert gas atmosphere, sodium borohydride solution was added dropwise to the mixture and stirred vigorously. After the reduction reaction was completed, the resulting solid-liquid mixture was centrifuged, washed, and freeze-dried to finally obtain the P-Cu@MX composite material.

2. The preparation method according to claim 1, characterized in that, In step (1), the concentration of MXene in the mixture is 200~600 mg / L, the concentration of copper ions is 3~8 mmol / L, the phosphorus content is 2~8 mmol / L, and the concentration of anhydrous sodium citrate is 10~100 mmol / L.

3. The preparation method according to claim 1, characterized in that, In step (1), the copper precursor is one or more of copper sulfate pentahydrate, copper chloride dihydrate, and copper nitrate trihydrate; the phosphorus precursor is one or more of sodium hypophosphite monohydrate, ammonium hypophosphite, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

4. The preparation method according to claim 1, characterized in that, In step (1), the stirring speed is 200~300 rpm and the stirring time is 10~30 min; the pH is adjusted to 9~12.

5. The preparation method according to claim 1, characterized in that, In step (2), the inert gas is either nitrogen or argon, and the gas flow rate is 20~80 mL / min.

6. The preparation method according to claim 1, characterized in that, In step (2), the concentration of sodium borohydride solution is 1.0~2.0 mol / L, the dropping rate is 0.5~2.0 mL / min, the dropping amount is 0.1~0.4 mol / L relative to the concentration of the mixed solution; the stirring speed is 400~600 rpm, and the reduction reaction time is 1.5~4h.

7. The preparation method according to claim 1, characterized in that, In step (2), the centrifugation speed is 8000~12000 rpm; the washing is performed several times with anhydrous ethanol and deionized water respectively; the freeze-drying temperature is -40~-70℃ and the time is 18~30 h.

8. An electrocatalytic composite material of MXene-supported copper phosphide nanoparticles prepared by the preparation method according to any one of claims 1-7.

9. The application of the electrocatalytic composite material of MXene-supported copper phosphide nanoparticles as described in claim 8, characterized in that, Used to fabricate catalytic electrodes and to perform carbon-nitrogen coupling synthesis of urea via the co-reduction of nitrate and carbon dioxide.

10. The application according to claim 9, characterized in that, The electrocatalytic composite material of MXene-loaded copper phosphide nanoparticles was coated on the surface of a conductive material as a catalytic electrode, and a three-electrode electrocatalytic reactor was constructed. Urea was synthesized by the co-reduction of nitrate and carbon dioxide in a mixed electrolyte containing saturated carbon dioxide. The conductive material is one of the following: carbon cloth, carbon paper, carbon felt, or carbon fiber brush; The loading of MXene-supported copper phosphide nanoparticles in the electrocatalytic electrode ranged from 0.05 to 1.2 mg / cm³. 2 ; In a three-electrode electrocatalytic reactor, the counter electrode is one of a platinum sheet, graphite, or carbon fiber brush, the reference electrode is one of a saturated calomel electrode or an Ag / AgCl electrode, and the reactor configuration is one of an H-type or a flow tank. The flow rate of carbon dioxide into the electrolyte is 30~80 mL / min, the concentration of potassium bicarbonate is 0.1 mol / L, the concentration of potassium nitrate is 0.01~1 mol / L, and the applied cathode potential is -0.5~-1.0 V (vs. RHE).

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