Preparation method of phosphate radical intercalation NiCoFe-LDH catalyst and application of phosphate radical intercalation NiCoFe-LDH catalyst in water electrolysis hydrogen production

Abstract

The invention discloses a preparation method of a phosphate radical intercalation NiCoFe-LDH catalyst and application of the phosphate radical intercalation NiCoFe-LDH catalyst in hydrogen production through water electrolysis, and belongs to the technical field of hydrogen production through electrolysis. The method comprises the following steps: carrying out electrochemical adsorption and electrochemical oxidation treatment on a NiCoFe-LDH material in a phosphating solution, so as to obtain the phosphate radical intercalation NiCoFe-LDH catalyst. The phosphating solution is a sodium hypophosphite monohydrate solution. According to the preparation method, phosphate radicals are successfully introduced between LDH material layers in a manner of taking the phosphate radicals as a precursor for the first time and an electrochemical oxidation manner, so that the phosphate radical intercalation nickel-cobalt-iron-based hydrotalcite with a nanosheet structure is prepared; the material is stable in structure and low in overpotential in the water electrolysis hydrogen production process, stable operation can be conducted for 400 h when the current density is 400 mA. Cm <-2 >, and the potential retention rate reaches 95% or above; the preparation method is simple, the used reagents are low in cost and rich in source, and the catalyst is suitable for macroscopic preparation and has popularization and application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of hydrogen production by electrolysis, and particularly relates to a preparation method of a phosphate-intercalated NiCoFe-LDH catalyst and its application in hydrogen production by electrolyzing water. Background Art

[0002] To reduce carbon emissions and solve the severe energy and environmental problems, it is necessary to develop a new type of clean energy to replace traditional fossil fuels to meet the needs of the future green and clean transformation of energy. As a completely clean energy, hydrogen energy has attracted extensive attention from researchers in recent years. However, the "gray hydrogen" prepared by reforming traditional fossil fuels cannot achieve complete zero carbon emissions, and obtaining hydrogen energy by electrolyzing water is more in line with the concept of sustainable development. Hydrogen production by electrolyzing water consists of two half-reactions, namely the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode. OER involves a four-electron transfer process and usually requires more energy to drive the reaction compared to HER. To reduce the energy consumption of hydrogen production by electrolyzing water and lower the electricity cost, it is urgent to develop an OER catalyst with high catalytic activity and low cost.

[0003] Benefiting from the abundant catalytic active sites on the surface and between layers and the flexible compositional tunability, transition metal layered double hydroxides (LDHs) have received extensive attention in the field of OER catalysts. However, limited by their low intrinsic activity, the catalytic performance of LDH materials is low, and it is necessary to improve their catalytic activity, which is of great significance for efficient hydrogen production by electrolyzing water.

[0004] Anion addition can effectively improve the OER performance of LDHs. On the one hand, the anion group acts as a "proton acceptor" to alleviate the drastic decrease in the surface pH of LDH during the electrolysis process; on the other hand, the anion group can form coordination bonds with metal cations to alleviate the dissolution of metal components and greatly improve the operating stability of LDHs. In addition, the anion group can also regulate the surface charge structure of LDHs, alleviate the "electron-deficient" state on the LDH surface, reduce the adsorption strength of OER intermediates, and significantly improve the catalytic activity of LDHs. Currently, the widely used methods for anion intercalation are hydrothermal method and coprecipitation method. Both methods can directly insert anion groups into the interlayer structure of LDHs through ion exchange between LDH layers. However, the common defect of the two preparation methods is that the preparation period is relatively long, generally on the order of hours or days, and usually requires a high-temperature environment. The preparation conditions are not mild enough, which limits the rapid large-scale preparation of catalysts. Currently, there is no report on a new method for rapidly preparing anion-intercalated LDH materials. Summary of the Invention

[0005] The object of the present invention is to solve the problems that the traditional NiCoFe-LDH material has a relatively high overpotential for electrocatalytic OER, low stability, and the preparation process of inserting phosphate groups by hydrothermal / coprecipitation method is complex and has high preparation energy consumption, and to provide a preparation method of a phosphate-intercalated NiCoFe-LDH catalyst and its application in electrolytic water hydrogen production.

[0006] The present invention uses NiCoFe-LDH / H2PO 2- as a precursor to in-situ transform into a PO4 3- intercalated NiCoFe-LDH material. This material maintains the original nanosheet-like structure of LDH, and at the same time obtains a larger interlayer spacing due to the insertion of phosphate groups between layers, exposing more active sites. The binding of phosphate groups to the metal active sites on the material surface regulates the surface electronic structure, optimizes the adsorption energy of OER intermediates, significantly reduces the OER overpotential, and significantly improves the catalytic performance and stability.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A preparation method of a phosphate-intercalated NiCoFe-LDH catalyst, the method is: subjecting the NiCoFe-LDH material to electrochemical adsorption and electrochemical oxidation treatment in a phosphating solution, and then obtaining the phosphate-intercalated NiCoFe-LDH catalyst.

[0009] Further, the phosphating solution is a sodium hypophosphite monohydrate solution.

[0010] Further, the electrochemical adsorption treatment is to use the NiCoFe-LDH material as a working electrode, and the treatment conditions are: in a three-electrode system at 20 - 25 °C, constant current adsorption is carried out at a current density of 200 - 300 mA·cm -2 for 1 - 5 minutes.

[0011] Further, the electrochemical oxidation treatment is to use the NiCoFe-LDH material adsorbed with H2PO 2- as a working electrode, and the treatment conditions are: in a three-electrode system at 20 - 25 °C, the NiCoFe-LDH / H2PO 2- material is placed in an alkaline solution and cyclically oxidized 45 - 55 times within a voltage range of 0.20 - 0.55 V.

[0012] Further, the method is specifically as follows:

[0013] (1) Synthesize the NiCoFe-LDH material by electrochemical deposition;

[0014] (2) Introduce H2PO 2- into the interlayer of NiCoFe-LDH by electrochemical adsorption, and synthesize H2PO2- / NiCoFe-LDH material;

[0015] (3) H2PO 2- Using the / NiCoFe-LDH material as a precursor, hypophosphite is completely oxidized to phosphate by electrochemical oxidation to obtain the intercalated NiCoFe-LDH material of PO4 3-

[0016] Furthermore, step (1) is specifically: mixing nickel salt, cobalt salt and iron salt evenly to obtain a mixed solution, soaking the nickel foam substrate in the mixed solution, in a three-electrode system, with the substrate as the working electrode, at 20 - 25 °C, a constant current deposition is carried out at a current density of -50 to -70 mA·cm -2 for 8 - 12 minutes for electrochemical deposition, and the reacted substrate is taken out to obtain the NiCoFe-LDH material.

[0017] In the present invention, the nickel foam is cleaned before use: to remove the oxide layer and residual organic matter on the surface of the nickel foam, the nickel foam is ultrasonically cleaned in dilute hydrochloric acid, acetone and ethanol respectively, and then the washed nickel foam is taken out, vacuum dried and stored.

[0018] Furthermore, the molar ratio of nickel element, cobalt element and iron element is 4:4:1.

[0019] Furthermore, the nickel salt is nickel chloride hexahydrate, the cobalt salt is cobalt chloride hexahydrate, and the iron salt is ferrous sulfate heptahydrate.

[0020] A phosphate intercalated NiCoFe-LDH catalyst prepared by the above preparation method.

[0021] Application of the above phosphate intercalated NiCoFe-LDH catalyst in alkaline water electrolysis for driving the oxygen evolution reaction at a low potential; the alkaline water is KOH solution or NaOH solution.

[0022] Compared with the prior art, the PO4 3- / NiCoFe-LDH material prepared by the present invention has achieved the following beneficial effects:

[0023] (1) The PO4 3- / NiCoFe-LDH material has excellent OER electrocatalytic performance. In 6M KOH solution, when the current density is 200 mA·cm -2 , the overpotential of PO4 3- / NiCoFe-LDH is only 250 mV. Under the room temperature environment condition of 25 °C, in 6M KOH solution, when the current density is 400 mA·cm -2 , it can operate stably for 400 h, and the potential retention rate is as high as 97.8%.

[0024] (2) The present invention greatly simplifies the preparation process of PO4 3- / NiCoFe-LDH. The catalyst can be prepared only in two steps at room temperature by an electrochemical method, and has the potential for large-scale preparation. Compared with the commonly used industrial catalyst preparation method - thermal spraying, the preparation process is greatly simplified.

[0025] (3) The raw materials used in the present invention are all non-precious metal materials, which are inexpensive and abundant in reserves.

[0026] In summary, for the first time, the present invention successfully introduces phosphate into the interlayer of LDH material by using phosphite as a precursor and supplemented by electrochemical oxidation to prepare phosphate intercalated nickel-cobalt-iron-based layered double hydroxide with a nanosheet structure; this material has a stable structure and a low overpotential during the electrolysis of water to produce hydrogen. When the current density is 400 mA·cm -2 , it can operate stably for 400 h, and the potential retention rate reaches more than 95%; its preparation method is simple, the reagents used have low cost and rich sources, and it is suitable for the large-scale preparation of catalysts, having the prospect of popularization and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the Raman spectrum of the PO4 3- / NiCoFe-LDH catalyst prepared in Example 1 of the present invention.

[0028] Figure 2 is the SEM image of the PO4 3- / NiCoFe-LDH catalyst prepared in Example 1 of the present invention.

[0029] Figure 3 is the HRTEM image of the PO4 3- / NiCoFe-LDH catalyst prepared in Example 1 of the present invention.

[0030] Figure 4 is the XPS image of the PO4 3- / NiCoFe-LDH catalyst prepared in Example 1 of the present invention.

[0031] Figure 5 is the TEM image of the PO4 3- / NiCoFe-LDH catalyst prepared in Example 1 of the present invention.

[0032] Figure 6 is the current density-voltage relationship diagram of NiCoFe-LDH, NF, RuO2, PO4 3- / NiCoFe-LDH as an electrode for catalytic electrolysis of water to produce hydrogen prepared in Example 1 of the present invention.

[0033] Figure 7PO4 prepared in Example 1 of the present invention 3- Stability test chart of hydrogen production by electrolytic water catalyzed by PO4

[0034] Figure 8 PO4 prepared in Example 1 of the present invention 3- Tafel slope chart of the PO4

[0035] Figure 9 PO4 prepared in Example 1 of the present invention 3- Active specific surface area chart of PO4

[0036] Figure 10 PO4 prepared in Example 1 of the present invention 3- Faraday efficiency chart of hydrogen production by electrolytic water catalyzed by PO4

[0037] Figure 11 PO4 prepared in Example 1 of the present invention 3- Density of states chart of the PO4

[0038] Figure 12 PO4 prepared in Example 1 of the present invention 3- Adsorption energy curve chart of the PO4 Detailed implementation manners

[0039] The technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments, but are not limited thereto. Any modification or equivalent replacement of the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention shall be covered by the protection scope of the present invention.

[0040] Example 1:

[0041] A preparation method of a phosphate intercalated NiCoFe-LDH material, the material comprising a nickel foam substrate and a nickel-cobalt-iron-based hydrotalcite grown on the surface of the nickel foam substrate, comprising the following steps:

[0042] (1) Substrate cleaning: The nickel foam is successively placed in a hydrochloric acid aqueous solution with a concentration of 2M, acetone, and absolute ethanol and ultrasonically cleaned for 15 minutes respectively, and then the washed nickel foam is taken out, vacuum dried and stored;

[0043] (2) Preparation of NiCoFe-LDH material: Put 20 mmol of nickel chloride hexahydrate, 5 mmol of ferrous sulfate heptahydrate, 20 mmol of cobalt chloride hexahydrate, and 20 mmol of ammonium chloride into 100 mL of deionized water, stir for 20 minutes to obtain a mixed solution; transfer it to a single-compartment electrolytic cell, and use the washed nickel foam in step (1) as the working electrode, a carbon rod as the counter electrode, and an Ag / AgCl electrode as the reference electrode. Carry out electrochemical deposition at a constant current of -60 mA·cm -2 for 10 minutes at room temperature; after the reaction is completed, take out the nickel foam with the grown material, rinse the nickel foam with deionized water, dry it at 40 °C, and store it to obtain the NiCoFe-LDH material;

[0044] (3) Preparation of H2PO 2- / NiCoFe-LDH material: Put 5 mmol of sodium hypophosphite monohydrate and 0.1 mol of potassium hydroxide into 100 mL of deionized water, stir for 10 minutes to obtain a mixed solution; transfer the mixed solution to a single-compartment electrolytic cell, and use the NiCoFe-LDH material obtained in step (2) as the working electrode, a platinum sheet as the counter electrode, and a Hg / HgO electrode as the reference electrode. Carry out electrochemical adsorption at a constant current of 250 mA·cm -2 for 1 minute at room temperature; after the reaction is completed, take out the material, rinse it with deionized water, dry it at 40 °C, and store it to obtain the H2PO 2- / NiCoFe-LDH material;

[0045] (4) Preparation of PO4 3- / NiCoFe-LDH material:

[0046] Put the H2PO 2- / NiCoFe-LDH material obtained in step (3) into a 1 M KOH aqueous solution, use cyclic voltammetry in a three-electrode system, and cycle oxidize 50 times in the voltage range of 0.2 - 0.55 V. After completion, take out the material, rinse it with deionized water, dry it at 40 °C, and store it to obtain the PO4 3- / NiCoFe-LDH material.

[0047] In this example, the prepared materials were characterized by Raman spectroscopy, SEM (scanning electron microscope), HRTEM (high-resolution transmission electron microscope), XPS (photoelectron spectrometer), and TEM (transmission electron microscope), and the results are as Figures 1-5 shown.

[0048] From Figure 1 the Raman spectrogram, it can be seen that: in addition to the characteristic peaks of metallic nickel, the characteristic peaks of NiCoFe-LDH were detected. In addition, at 1050 cm-1 The characteristic peak of phosphate was found nearby, proving that the phosphate group was successfully loaded into the LDH material.

[0049] From Figure 2 the SEM image, it can be seen that the morphology structure of the material is curly and wrinkled nanosheets.

[0050] From Figure 3 the HRTEM image, it can be seen that after the addition of phosphate, the interlayer spacing of the LDH material increases, and the crystal plane structure of NiCoFe-LDH is still retained, which is consistent with the results of Raman spectroscopy characterization.

[0051] From Figure 4 the XPS image, it can be seen that the material has a stable LDH structure, and it is again proved that PO4 can be successfully introduced into the interlayer of the LDH material by electrochemical oxidation. 3-

[0052] From Figure 5 the TEM image, it can be seen that three lattice spacings are observed on the surface of the nanosheets, which are 0.1669 nm, 0.2059 nm, and 0.3287 nm respectively, corresponding to layered hydroxide and phosphate. The selected area electron diffraction (SAED) pattern of the hydroxide shows clear diffraction rings of nickel, iron, cobalt-based hydroxide and phosphate.

[0053] Example 2:

[0054] This example involves the performance test of the PO4 3- / NiCoFe-LDH material prepared in Example 1, and its electrochemical performance is tested. The test method is as follows: The material is used as the working electrode, the Hg / HgO electrode is used as the reference electrode, and the graphite rod electrode is used as the counter electrode to form a three-electrode system. The electrolyte is an aqueous KOH solution with a concentration of 1 M. Test the current density-voltage relationship and the electrode stability when the PO4 3- / NiCoFe-LDH material catalyzes the OER reaction of electrolytic water, and test its Faraday efficiency and active specific surface area during the electrolysis process. The test results are as Figures 6-10 shown.

[0055] Taking NiCoFe-LDH, NF, and RuO2 materials as working electrodes to catalyze the OER of electrolytic water for comparative experiments, and test the current density and voltage relationship. The results are as Figure 6 shown. From Figure 6 ​It can be seen that the intercalation of phosphate groups can effectively improve the OER catalytic efficiency and reduce the OER overpotential. After the adsorption of phosphate groups, the surface *OH obtains electrons, resulting in the alleviation of the electron-deficient state of the catalytic sites, and then reducing the adsorption strength of reaction intermediates (*O→*OOH). Compared with pure NiCoFe-LDH, this material exhibits excellent OER catalytic activity.

[0056] From Figure 7 It can be seen that after 400 h of electrolysis, the potential retention rate of the material is as high as 97.8%, indicating that the insertion of phosphate groups significantly increases the operating stability of the NiCoFe-LDH material.

[0057] From Figures 8-9 It can be seen that this material has excellent reaction kinetics and active specific surface area.

[0058] From Figure 10 It can be seen that during the long-term electrolysis process, the Faraday efficiency of this material for electrolysis is close to 100%.

[0059] From Figures 11-12 It can be seen that this material has a high density of states near the Fermi level, reflecting its excellent electrocatalytic activity. At the same time, the formation of the important reaction intermediate *OOH is the rate-limiting step of this reaction. The free energy barrier of NiCoFe-LDH is about 1.01 eV, while that of PO4 3- / NiCoFe-LDH has been reduced to 0.84 eV, thereby reducing the overpotential by 0.17 eV.

Claims

1. A preparation method of a phosphate-intercalated NiCoFe-LDH catalyst, characterized in that: The method is as follows: The NiCoFe-LDH material is subjected to electrochemical adsorption and electrochemical oxidation treatment in a phosphating solution, and the phosphate-intercalated NiCoFe-LDH catalyst is obtained.

2. The preparation method of a phosphate intercalated NiCoFe-LDH catalyst according to claim 1, characterized in that: The phosphating solution is a sodium hypophosphite monohydrate solution.

3. The preparation method of a phosphate intercalated NiCoFe-LDH catalyst according to claim 1, characterized in that: The electrochemically adsorption treatment uses the NiCoFe-LDH material as the working electrode, and the treatment conditions are as follows: in a three-electrode system, at a constant current density of 200-300 mA·cm -2 under a constant current adsorption for 1-5 minutes at 20-25 °C.

4. The preparation method of a phosphate intercalated NiCoFe-LDH catalyst according to claim 1, characterized in that: The electrochemically oxidative treatment is to use H2PO 2- The adsorbed NiCoFe-LDH material is used as the working electrode, and the treatment conditions are as follows: in a three-electrode system, at 20 - 25 °C, within the voltage range of 0.20 - 0.55 V, the NiCoFe-LDH / H2PO 2- material is placed in an alkaline solution and cyclically oxidized 45 to 55 times.

5. The preparation method of the phosphate intercalated NiCoFe-LDH catalyst according to any one of claims 1 to 4, characterized in that: The method specifically is: (1) Synthesize the NiCoFe-LDH material by electrochemical deposition; (2) Introduce H2PO 2- into the interlayer of NiCoFe-LDH by electrochemical adsorption to synthesize H2PO 2- / NiCoFe-LDH material; (3) Take H2PO 2- / NiCoFe-LDH material as the precursor, and completely oxidize hypophosphite to phosphate by electrochemical oxidation to obtain the PO4 3- intercalated NiCoFe-LDH material.

6. The preparation method of a phosphate intercalated NiCoFe-LDH catalyst according to claim 5, wherein: The specific steps of step (1) are as follows: Mix nickel salt, cobalt salt and iron salt evenly to obtain a mixed solution, immerse the nickel foam substrate in the mixed solution, and under a three-electrode system, the substrate serves as the working electrode, and at a temperature of 20-25 °C, a constant current deposition is carried out at a current density of -50 to -70 mA·cm -2 for 8-12 minutes, and the substrate after the reaction is taken out to obtain the NiCoFe-LDH material.

7. The preparation method of a phosphate intercalated NiCoFe-LDH catalyst according to claim 6, characterized in that: The molar ratio of nickel element, cobalt element and iron element is 4:4:

1.

8. The preparation method of a phosphate intercalated NiCoFe-LDH catalyst according to claim 6, characterized in that: The nickel salt is nickel chloride hexahydrate, the cobalt salt is cobalt chloride hexahydrate, and the iron salt is ferrous sulfate heptahydrate.

9. A phosphate-intercalated NiCoFe-LDH catalyst prepared by the preparation method according to any one of claims 1-8.

10. Use of the phosphate intercalated NiCoFe-LDH catalyst according to claim 9 in alkaline water electrolysis, characterized in that: It is used for oxygen evolution reaction driven by low potential; the alkaline water is KOH solution or NaOH solution.

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