CN111686812B - Ligand-activated transition metal layered dihydroxy compound, preparation method and application - Google Patents

Abstract

The invention relates to the technical field of new energy and electrocatalysis materials, and discloses a preparation method of a ligand-activated transition metal layered double hydroxide compound (TM LDH). The preparation method uses sodium borohydride to activate the hydroxyl ligand of TM LDH to become TM LDH activated by hydride, and can also use Cl ^‑ Negative ion, br ^‑ Negative ion, I ^‑ And further activating one or more of negative ions, N-containing negative ions, P-containing negative ions or S-containing negative ions. The invention also discloses the prepared ligand-activated TM LDH and application thereof, and a water-splitting anode and a water-splitting three-electrode system which take the ligand-activated TM LDH as a catalyst. The water decomposition anode is applied to the electrolysis of water, and compared with the traditional foamed nickel electrode, the water decomposition anode shows lower overpotential and Tafel slope, greatly improves the water decomposition capability and reduces the cost of hydrogen production by water decomposition.

Description

Ligand-activated transition metal layered dihydroxy compound, preparation method and use

technical field

The invention relates to the technical fields of new energy technology and electrocatalytic materials, in particular to a method for preparing a ligand-activated transition metal layered dihydroxy compound, the prepared ligand-activated transition metal layered dihydroxy compound and its application , and a water-splitting anode and a water-splitting three-electrode system comprising this ligand-activated transition metal layered bishydroxyl compound.

Background technique

The aggravation of global energy shortage and environmental pollution has made people's demand for clean and renewable energy more and more urgent. Hydrogen has the advantages of high energy density and non-polluting combustion products, and is considered to be one of the most ideal green energy sources that can replace fossil fuels. At present, the methods for producing hydrogen mainly include hydrogen production by steam reforming of natural gas, hydrogen production by methanol cracking and hydrogen production by water splitting. Among them, natural gas steam reforming hydrogen production and methanol cracking hydrogen production need to use natural gas or methanol fuel as raw material. More importantly, in addition to hydrogen, there are many impurities in the product, such as carbon dioxide, carbon monoxide, methane, etc. Hydrogen production by water splitting has the advantages of high efficiency, simple process, and high purity of reaction products. my country's first hydrogen energy field group standard "Proton Exchange Membrane Fuel Cell Vehicle Fuel Hydrogen" (T/CECA-G 0015-2017) has strict requirements on the purity of hydrogen, its volume fraction must be greater than or equal to 99.99%, and this purity Hydrogen can only be produced by water splitting.

Hydrogen production by water splitting is to split water by electricity or light to produce hydrogen. The chemical reaction formula is 2H ₂ 0→2H ₂ +O ₂ . The reaction process includes cathode hydrogen evolution reaction (hydrogen evolution reaction, HER) and anode oxygen evolution reaction (oxygenevolution reaction, OER). The reaction kinetics of the anode oxygen production reaction is very slow, which greatly limits the process of water splitting and hydrogen production. This has caused the current price of electrolyzed water to produce hydrogen to be higher than that of natural gas steam reforming and methanol cracking to produce hydrogen. Therefore, the development of efficient and cheap water splitting catalysts can lower the OER reaction energy barrier and reduce the energy consumption required for hydrogen production by water splitting. For the large-scale production of high-purity hydrogen and the development of new energy, it can promote the development of hydrogen vehicles and other fields, and alleviate environmental pollution. is crucial.

Layered double hydroxide (LDH) is composed of a positively charged main layer formed by metal ions and hydroxyl ligands and interlayer anions as charge compensation, wherein the metal ion species and molar ratio in the main layer are within a certain range Internally adjustable, interlayer anions can be changed by ion exchange. Transition metal-based layered double hydroxide (TM LDH) is an LDH in which the metal ions in the main layer are transition metal ions. In recent years, the application of TM LDH in the field of water splitting and hydrogen production has made great progress. The catalytic oxygen evolution activity and stability of TM LDH are close to those of noble metal oxygen evolution catalysts iridium oxide (IrO ₂ ) and ruthenium oxide (RuO ₂ ). However, non-noble metals such as transition metals are abundant on the earth, which makes non-noble metal water splitting catalysts such as TM LDH have greater application potential than noble metal catalysts.

TM LDH can be optimized to further improve the catalytic performance of TM LDH. Optimization is usually achieved by adjusting the type and molar ratio of the transition metal ion (see, for example, the SCI paper published by the inventor's research group in 2015: Cointake mediated formation of ultrathin nanosheets of transition metal LDH - An advanced electrocatalyst for oxygen evolution reaction, Chemical Communications 51(6), December 2014), or by changing the interlayer anion of TM LDH to achieve optimization (see, for example, the SCI paper published by the inventor's research group in 2014: A Strongly Coupled Graphene and FeNi Double Hydroxide Hybrid as an Excellent Electrocatalyst for the OxygenEvolution Reaction, Angewandte Chemie International Edition in English, 53(29), July 2014).

These optimizations can effectively regulate the electronic structure of transition metal ions that play a catalytic role, the conductivity of the catalyst, and the microscopic morphology such as the interlayer spacing and specific surface area of TM LDH, thus effectively optimizing the catalytic performance of transition metal-based water splitting catalysts. . However, limited by the atomic structure of layered bishydroxyl compounds, the optimization of the species of transition metal ions can only be selected among limited elements, and the optimization of the molar ratio of transition metal ions can only be carried out in a limited range. The transition metal ions for catalysis are far away, so the interlayer distance and the specific surface area of the water splitting catalyst can only be changed to a certain extent.

However, the modification of hydroxyl ligands attached to transition metal ions has not been reported so far.

Contents of the invention

The purpose of the present invention is to modify and activate the hydroxyl ligands of transition metal layered dihydroxyl compounds (TM LDH) to obtain ligand-activated TM LDH, which can be used as a catalyst for the oxygen generation reaction in the water splitting reaction , improve the performance of water splitting hydrogen production.

Therefore, the first aspect of the present invention provides a kind of preparation method of the TM LDH of ligand activation, and this preparation method comprises the following steps:

(1) Make TM LDH evenly dispersed in polar solvents to become TM LDH suspension, the transition metal is any combination of divalent and trivalent transition metal ions;

(2) Add an appropriate amount of sodium borohydride for the activation reaction to the suspension in step (1) to carry out the activation reaction to obtain TM LDH activated by the ligand.

In a preferred embodiment of the first aspect of the present invention, the atomic ratio of the divalent/trivalent transition metal ion is between (3-10):1, more preferably between (4-8):1, still more Preferably between (5-6):1.

In a preferred embodiment of the first aspect of the present invention, the transition metal ion is a Ni(II)/Fe(III) combination, a Ni(II)/Mn(III) combination, a Ni(II)/Co(III) combination or Co(II)/Fe(III) combination, more preferably Ni(II)/Fe(III) combination.

In a preferred embodiment of the first aspect of the present invention, in step (1), the weight-to-volume ratio of the TM LDH to the polar solvent is (0.5-2) mg:1 ml, more preferably (0.8-1.8) mg : 1 ml, still more preferably (1.0-1.5) mg: 1 ml.

In a preferred embodiment of the first aspect of the present invention, in step (2), the amount of the activation reaction is such that the final concentration of the sodium borohydride in the suspension in step (1) is 0.001-0.01 M, more preferably , the final concentration is 0.003-0.008 M.

In a preferred embodiment of the first aspect of the present invention, the reaction temperature of the activation reaction is 20-80° C., and the reaction time is 0.5-10 hours. More preferably, the reaction temperature is 40-60° C., and the reaction time is 1-5 hours. Hour.

In a preferred embodiment of the first aspect of the present invention, the polar solvent is deionized water or ethanol or a mixed solvent thereof.

In a specific preferred embodiment of the first aspect of the present invention, the preparation method comprises the following specific steps:

(1) Ni(II)/Fe(III), Ni(II)/Mn(III), Ni(II)/Co(III) or Co(II) with an atomic ratio between (3-10):1 )/Fe(III) TM LDH is uniformly dispersed in deionized water or ethanol or their mixed solvents, the weight-to-volume ratio of the TMLDH to the polar solvent is (0.5-2) mg:1 ml, vibrated and stirred by ultrasonic Disperse evenly and become TM LDH suspension;

(2) Add sodium borohydride with a final concentration of 0.001-0.01 M to the suspension in step (1), stir and disperse evenly at a stirring speed of 400-1000 rpm, and then react at 20-80 °C and 700-1000 °C React at a stirring speed of rpm for 0.5-10 hours, after naturally cooling to room temperature, add an appropriate amount of deionized water or absolute ethanol as a washing solvent, centrifuge and wash three times at 6000-8000 rpm, each time for 5-10 minutes, and then vacuum dry After 3-6 hours, TM LDH activated by the hydride ligand was obtained.

In the above specific preferred embodiment of the first aspect of the present invention, the weight-to-volume ratio of the TM LDH to the polar solvent is more preferably (0.8-1.8) mg:1 ml, still more preferably (1.0-1.5) mg : 1 ml.

In the above specific preferred embodiment of the first aspect of the present invention, in step (2), the final concentration of the sodium borohydride is more preferably 0.003-0.008 M; the reaction temperature of the reaction is more preferably 40-60°C , the reaction time is more preferably 3-8 hours.

The second aspect of the present invention provides a kind of preparation method of the TM LDH of ligand activation, and this preparation method comprises the following steps:

(1) Make TM LDH evenly dispersed in polar solvents to become TM LDH suspension; the transition metal is any combination of divalent and trivalent transition metal ions;

(2) Add an appropriate amount of sodium borohydride for the activation reaction to the suspension in step (1), and add an appropriate amount of Cl ^- anions, Br ^- anions, I ^- anions, N-containing anions, P-containing anions or One or more combinations of S negative ions are used to carry out an activation reaction to obtain the ligand-activated TM LDH.

In a preferred embodiment of the second aspect of the present invention, the atomic ratio of the divalent/trivalent transition metal ions is between (3-10):1, more preferably between (4-8):1, still more Preferably between (5-6):1.

In a preferred embodiment of the second aspect of the present invention, the transition metal ion is a Ni(II)/Fe(III) combination, a Ni(II)/Mn(III) combination, a Ni(II)/Co(III) combination or Co(II)/Fe(III) combination, more preferably Ni(II)/Fe(III) combination.

In a preferred embodiment of the second aspect of the present invention, in step (1), the weight-to-volume ratio of the TM LDH to the polar solvent is (0.5-2) mg:1 ml, more preferably (0.8-1.8) mg : 1 ml, still more preferably (1.0-1.5) mg: 1 ml.

In a preferred embodiment of the second aspect of the present invention, in step (2), the amount of the activation reaction is such that the final concentration of the sodium borohydride in the suspension in step (1) is 0.001-0.01 M, more preferably , the final concentration is 0.003-0.008 M.

In a preferred embodiment of the second aspect of the present invention, in step (2), the activation reaction is applied in an amount such that the Cl ^- negative ion, Br ^- negative ion, I-negative ion, N ^- containing negative ion, P-containing negative ion or S-containing negative ion The total final concentration of one or more combinations in the suspension in step (1) is 0.1-10 M, more preferably, the total final concentration is 1-8 M, still more preferably, the total final concentration is 3 -6 M.

In a preferred embodiment of the second aspect of the present invention, the reaction temperature of the activation reaction is 20-80° C., and the reaction time is 0.5-10 hours. More preferably, the reaction temperature is 40-60° C., and the reaction time is 1-5 hours. Hour.

In a preferred embodiment of the second aspect of the present invention, the polar solvent is deionized water or ethanol or a mixed solvent thereof.

In a specific preferred embodiment of the second aspect of the present invention, the preparation method comprises the following specific steps:

(1) Ni(II)/Fe(III), Ni(II)/Mn(III), Ni(II)/Co(III) or Co(II) with an atomic ratio between (3-10):1 )/Fe(III) TM LDH is uniformly dispersed in deionized water or ethanol or their mixed solvents, the weight-to-volume ratio of the TMLDH to the polar solvent is (0.5-2) mg:1 ml, vibrated and stirred by ultrasonic Disperse evenly and become TM LDH suspension;

(2) Add sodium borohydride with a final concentration of 0.001-0.01 M to the suspension in step (1), and add the Cl ^- anion, Br ^- anion, I ^- anion, A combination of one or more of N-containing negative ions, P-containing negative ions, or S-containing negative ions is stirred and dispersed evenly at a stirring speed of 400-1000 rpm, and then stirred at a reaction temperature of 20-80°C and a stirring speed of 700-1000 rpm React for 0.5-10 hours, after naturally cooling to room temperature, add an appropriate amount of deionized water or absolute ethanol as a washing solvent, centrifuge and wash three times at 6000-8000 rpm, each time for 5-10 minutes, and then vacuum dry for 3-6 hours , to obtain the ligand-activated TM LDH.

In the above specific preferred embodiment of the second aspect of the present invention, the weight-to-volume ratio of the TM LDH to the polar solvent is more preferably (0.8-1.8) mg:1 ml, still more preferably (1.0-1.5) mg : 1 ml.

In the above specific preferred embodiment of the second aspect of the present invention, in step (2), the final concentration of the sodium borohydride is more preferably 0.003-0.008 M; the Cl ^- negative ion, Br ^- negative ion, I ^- negative ion, The total final concentration of one or more combinations containing N negative ions, P negative ions or S negative ions is more preferably 1-8 M, and more preferably 3-6 M; the reaction temperature of the reaction is more preferably Preferably it is 40-60°C, and the reaction time is more preferably 3-8 hours.

In the second aspect of the present invention, N-containing negative ions, P-containing negative ions or S-containing negative ions are preferably NSC ^- , SCN ^- , CN ^- , NH ₃ , PO ₃ ^3- , PO ₄ ^3- , etc.

The third aspect of the present invention provides a ligand-activated TM LDH, which is prepared by the preparation method according to the first or second aspect of the present invention.

The ligand-activated TM LDH prepared according to the preparation method of the first aspect of the present invention is hydride ion (H ⁻ ) activated TM LDH.

The ligand-activated TM LDH prepared according to the preparation method of the second aspect of the present invention is an anion of chloride (Cl ^- ), an anion of bromide (Br ^- ), an anion of iodide (I ^- ), an anion containing nitrogen (N), an anion containing phosphorus ( P) TM LDH activated by one or more combinations of negative ions and sulfur-containing (S) negative ions.

A fourth aspect of the present invention provides the use of the ligand-activated TM LDH of the third aspect of the present invention as a water-splitting anode oxygen-generating catalyst.

The fifth aspect of the present invention provides a water-splitting anode, the water-splitting anode includes foamed nickel, carbon cloth or iron substrate and the composition of the third aspect of the present invention coated on the foamed nickel, carbon cloth or iron substrate. In vivo activated TM LDH. This ligand-activated TM LDH acts as an anode oxygen evolution catalyst for water splitting.

The water-splitting anode can be prepared as follows. Take 1-2 mg of the TM LDH catalyst activated by the ligand, put it in 1 ml of absolute ethanol, and disperse it with strong ultrasonic to obtain a catalyst dispersion. Take 200-500 µL of prepared catalyst dispersion and 4-8% PTFE aqueous solution by volume ratio (1-3): 1, mix evenly, and ultrasonically disperse. Spread the resulting mixed dispersion evenly on nickel foam, carbon cloth or iron substrate, and bake in an oven at 40-70ºC for 20-40 minutes.

The sixth aspect of the present invention provides a water-splitting three-electrode system, including the water-splitting anode according to the fifth aspect of the present invention, Pt wire as a counter electrode, Ag/AgCl as a reference electrode, and 0.5-1.5 M Potassium hydroxide or sodium hydroxide aqueous solution. Preferably, the electrolyte is 1 M potassium hydroxide or sodium hydroxide aqueous solution.

Beneficial effects of the present invention:

The preparation method of the ligand-activated TM LDH of the present invention can solve the problem of difficult electron transfer between transition metal ions and unsatisfactory transition metal ion d electron arrangement by activating the hydroxyl ligand directly connected to the transition metal ion In this case, the electronic arrangement of catalytically active atoms can be optimized, the coordination environment of transition metal ions can be effectively changed, and the synergistic interaction between metal ions can be enhanced.

The ligand-activated TM LDH prepared by the preparation method of the present invention can be used as an anode catalyst for water splitting to realize efficient and stable oxygen production and improve the hydrogen production performance of the catalyst by splitting water. The water-splitting anode prepared with the ligand-activated TM LDH is applied to the electrolysis of water. Compared with the traditional nickel foam (NF) electrode, it shows a lower overpotential and Tafel slope, which greatly improves the ability of water splitting. Reduce the cost of water splitting to produce hydrogen.

Description of drawings

Figure 1 is a scanning electron micrograph (SEM) and transmission of NiFe LDH (A, C, D) and H-NiFe LDH (B, E, F) after activation according to Example 1 of the present invention before sodium borohydride activation Electron micrograph (TEM), where (A, B) are SEM images, (C, E) are TEM images, (D, F) are high-resolution TEM (HRTEM) images;

Fig. 2 is the XRD image of NiFe LDH before sodium borohydride activation and H-NiFe LDH after activation according to embodiment 1 of the present invention;

Fig. 3 is the ² H magic angle spin nuclear magnetic resonance (MAS NMR) figure of H-NiFe LDH after sodium borohydride activation according to embodiment 1 of the present invention;

4 is an electrochemical impedance diagram of H-NiFe LDH at an overpotential of 300 mV after sodium borohydride activation according to Example 1 of the present invention _, compared with NiFe LDH and H-Ni(OH) ;

Fig. 5 is according to the test example NiFe LDH of the present invention as the structural representation of the three-electrode system of water electrolysis anode catalyst, and wherein 1 represents working electrode, 2 represents reference electrode, and 3 represents counter electrode;

Fig. 6 is according to the catalysis hydrogen production polarization curve comparative figure of test example foam nickel, NiFe LDH and H-NiFe LDH of the present invention;

Fig. 7 is according to the catalytic oxygen generation Tafel curve comparative figure of test example foam nickel, NiFe LDH and H-NiFe LDH of the present invention;

Fig. 8 is the chronopotentometric test graph of H-NiFe LDH under the current density of 10, 20 and 50 mA cm ^-2 according to the test example of the present invention, in which the inset is the H-NiFe LDH exceeding the current density of 50 mA cm ^-2 12-hour long-time chronopotential test chart;

Fig. 9 is a comparison diagram of the polarization curves of catalytic hydrogen production of NCS-NiFe LDH and NiFe LDH according to the test example of the present invention.

Detailed ways

The present invention will be further described in detail below through specific embodiments and in conjunction with the accompanying drawings.

Transition metal layered dihydroxyl compounds (TM LDHs) have been extensively studied as one of the most efficient oxygen evolution reaction (OER) catalysts. However, the effect of the hydroxyl (-OH) ligand of LDH on OER catalysis has not been reported so far. According to the rules of superexchange interaction, electrons can be transferred from one transition metal ion to the adjacent transition metal ion through the non-magnetic anion, so the inventors believe that the electron transfer inside the TM LDH during the OER process may be affected by the direct interaction with the transition metal ion. The effect of octahedrally complexed hydroxyl ligands, thereby affecting the catalytic performance of TM LDH. Since the hydride ion (H ⁻ ) is small in size and has a strong complexing ability, the inventors believe that it can easily react with a proton to form a molecule of hydrogen, thereby affecting the hydroxyl ligand of TM LDH. To confirm this hypothesis, the inventors performed DFT calculations using sodium borohydride (NaBH ₄ ) as a hydride source and the most effective OER catalyst, NiFe LDH, as a typical TM LDH model. The results show that the hydride ion of sodium borohydride will acquire a proton from the hydroxyl ligand of NiFe LDH to generate a molecule of H ₂ , and at the same time, a strong BO σ bond will be formed. Since B has an empty p orbital, O has a lone pair of electrons, and the BO bond has a strong tendency to form a BO double bond, the hydride ions can easily migrate to the metal center to form a transition metal-hydride bond (Fe-H or Ni- H) with the formation of NaBOH ₂ , which can further form NaBO ₂ .

Since hydride ions are strong-field ligands, the energy of the antibonding d-orbitals of complexed transition metal ions can be easily enhanced. Both Fe(III) and Ni(II) d orbitals are occupied, making the hydride-treated NiFe LDH unstable. Therefore, to lower the energy of the system, an electron is transferred from the antibonding orbital of Ni(II) to the bonding orbital of Fe(III) via a hydride bridge via a superexchange interaction, thereby forming Ni(III) and Fe(II ). The inventors calculated the partial charge densities at the Fermi level of NiFeLDH and H-NiFe LDH in order to understand the influence of hydride ions, and found that higher charge densities appear in the region near activated transition metal ions, indicating that by substituting TM Hydroxyl (-OH) ligands of LDH, transition metal ions can be activated. However, the generated transition metal-hydride ions are close to each other and can easily undergo reductive elimination reactions to generate hydrogen gas and reduce transition metals, leading to the formation of metal nanoparticle-decorated LDHs. To avoid the reduction of transition metals, the inventors greatly reduced the concentration of sodium borohydride to 1 mM to obtain a low concentration of transition metal-hydrides, while the hydroxyl ligands of TM LDH can still be replaced by hydrides.

In short, after in-depth research, the present invention was inspired by the super-exchange interaction, and found that the activation of the hydroxyl ligand of TM LDH with a low concentration of sodium borohydride can affect electron transfer, adjust the coordination status of transition metal ions, and affect its Surface physicochemical properties, thereby affecting the catalytic performance of TM LDH. Taking NiFe LDH as an example, the activation of hydroxyl ligands causes electron transfer from low-oxidation state Ni(II) to high-oxidation state Fe(III) via hydrogen ion bridge through superexchange interactions, resulting in OER-active Ni(III) , leading to the enhanced OER performance of NiFe LDH, and the Ni(III) concentration is directly proportional to the OER performance. The _NiFe4 units with hydroxyl ligands and hydrogen ion ligands in the activated NiFe LDH are catalytically active centers, where Ni sites serve as absorption/desorption sites, while neighboring Fe ions accumulate electrons through superexchange interactions. In addition to NiFe LDH, the present inventors have found that TM LDHs such as NiMn LDH, NiCo LDH, and CoFe LDH also have the same effect. Moreover, in addition to hydride ions activating the hydroxyl ligands of TM LDH, the Cl ^- anion, Br ^- anion, I ^- anion, N-containing anion, P-containing anion or S-containing anion can be further used to activate the hydroxyl group of TM LDH. The ligand is activated.

Thus, the inventors developed a method for preparing ligand-activated TM LDH, the obtained ligand-activated TMLDH and its use, and a water-splitting anode and water-splitting three electrodes comprising the ligand-activated TM LDH system, as described above in the "Summary of the Invention" section. Wherein, the inventors found that in the preparation method of the ligand-activated TM LDH of the present invention, the concentration of sodium borohydride, reaction temperature and reaction time are crucial. When the concentration of sodium borohydride is high, the transition metal ions will be reduced to the corresponding metal nanoparticles and reduce the catalytic effect. If the water bath temperature is too low or the reaction time is too short, ligand activation is difficult or incomplete, and the catalysis is not ideal; if the water bath temperature is too high or the reaction time is too long, the reduction is excessive, and the catalysis is not ideal.

The invention is illustrated below by means of non-limiting examples.

Embodiment 1: the synthesis and characterization of the NiFe LDH catalyst (H-NiFe LDH) of hydride ion activation

1.1 Synthesis of NiFe LDH

Synthesis of NiFe LDH nanopowder by hydrothermal method, the specific operation is as follows. Mix 0.725 ml of 1 M nickel chloride (NiCl ₂ ) aqueous solution and 0.145 ml of 1 M ferric chloride (FeCl ₃ ) aqueous solution with 70.8 ml of deionized water in a beaker. Then, under magnetic stirring, 5.6 ml of 0.5 M urea aqueous solution and 2 ml of 0.01 M trisodium citrate aqueous solution were added to the beaker. Then the resulting mixed solution was transferred into a 100 ml polytetrafluoroethylene-lined stainless steel pressure cooker, sealed and subjected to hydrothermal reaction in an oven at 150 ºC for 24 h. After the reaction, the powder was collected by centrifugation at 7500 rpm for 10 min, then washed several times with deionized water and high-purity ethanol, and then dried in an oven at 50 ºC overnight to obtain NiFe LDH nanopowder, which is Ni(II)/Fe( III) Transition metal layered bishydroxy compounds.

1.2 Synthesis of H-NiFe LDH catalyst

Add 1 mg of the above-prepared NiFe LDH powder into 10 ml of deionized water in a 15 ml sealed bottle, and disperse evenly by ultrasonic vibration at a vibration frequency of 50 KHz at a stirring speed of 800 rpm to prepare a NiFe LDH suspension. Add 1.9 mg NaBH ₄ into the sealed bottle, stir and disperse evenly at a stirring speed of 500 rpm, and obtain a reaction solution with a final concentration of NaBH ₄ of 0.005 M. The sealed bottle was placed in a water bath at 50°C and reacted for 2 hours at a stirring speed of 800 rpm. After the reaction, cool down to room temperature naturally, add 5 ml of deionized water to the beaker to dilute the reactant, transfer the diluted solution to a 10 ml centrifuge tube, and centrifuge and wash at 7000 rpm for 10 minutes. Discard the supernatant, and perform centrifugation and washing twice in the same manner. The final precipitate was vacuum-dried at -100 KPa relative to the atmospheric pressure for 5 hours to obtain the H-NiFe LDH catalyst.

1.3 Characterization of NiFe LDH (H-NiFe LDH) catalysts activated by hydride ions

Figure 1 shows the scanning electron micrographs (SEM) and transmission electron micrographs of NiFe LDHs (A, C, D) and H-NiFe LDHs (B, E, F) after activation of this example before _NaBH4 activation. Images (TEM), where (A, B) are SEM images, (C, E) are TEM images, and (D, F) are high-resolution TEM (HRTEM). It can be seen that the nanostructure of NiFe LDH is maintained after NaBH treatment.

Figure 2 shows the XRD images of NiFe LDH before NaBH ₄ activation and H-NiFe LDH after activation in this example, showing the same characteristic diffraction peaks, indicating that the crystal phase is unchanged and the crystallinity is high.

Figure 3 shows the ² H magic-angle spin nuclear magnetic resonance (MAS NMR) pattern of H-NiFe LDH after activation by NaBH in _this example, showing the formation of HM bonds, indicating that the activation of hydroxyl ligands in H-NiFe LDH is related to replace.

Figure 4 shows the electrochemical impedance diagram of H-NiFe LDH at an overpotential of 300 mV after activation by NaBH ₄ in this example, for comparison with NiFe LDH and H-Ni(OH) ₂ . It can be seen from Fig. 4 that the H-NiFe LDH catalyst has the lowest impedance after activation by _NaBH4 , indicating that the catalyst has the best electronic conductivity.

Among them, H-Ni(OH) ₂ used for comparison was synthesized as follows. Ni(OH) ₂ is firstly synthesized by hydrothermal method, and the specific operation is as follows. 0.87 ml of 1 M nickel chloride (NiCl ₂ ) aqueous solution was mixed with 70.8 ml of deionized water in a beaker. Then, under magnetic stirring, 5.6 ml of 0.5 M urea aqueous solution and 2 ml of 0.01 M trisodium citrate aqueous solution were added to the beaker. Then the resulting mixed solution was transferred into a 100 ml polytetrafluoroethylene-lined stainless steel pressure cooker, sealed and subjected to hydrothermal reaction in an oven at 150 ºC for 24 h. After the reaction, the powder was collected by centrifugation at 7500 rpm for 10 minutes, then washed several times with deionized water and high-purity ethanol, and then dried in a 50ºC oven overnight to obtain Ni(OH) ₂ . Then, prepare H—Ni(OH) ₂ in a similar manner to 1.2.

Embodiment 2: the synthesis of the NiMn LDH catalyst (H-NiMn LDH) of hydrogen anion activation

2.1 Synthesis of NiMn LDH

Synthesis of NiMn LDH nanopowder by hydrothermal method, the specific operation is as follows. Mix 0.435 ml of 1 M nickel chloride (NiCl ₂ ) aqueous solution and 0.145 ml of 1 M manganese chloride (MnCl ₂ ) aqueous solution with 70 ml of deionized water in a beaker. Then, under magnetic stirring, 4.48 ml of 0.5 M urea aqueous solution and 1.6 ml of 0.01 M trisodium citrate aqueous solution were added to the beaker. Then the resulting mixed solution was transferred into a 100 ml polytetrafluoroethylene-lined stainless steel pressure cooker, sealed and subjected to hydrothermal reaction in an oven at 150 ºC for 24 h. After the reaction, the powder was collected by centrifugation at 7500 rpm for 10 min, then washed several times with deionized water and high-purity ethanol, and then dried in an oven at 50 ºC overnight to obtain NiMn LDH nanopowder, which is Ni(II)/Mn( III) Transition metal layered bishydroxy compounds.

2.2 Synthesis of H-NiMn LDH catalyst

Add 0.5 mg of the above-prepared NiMn LDH powder into 1 ml of deionized water in a 15 ml sealed bottle, and disperse evenly by ultrasonic vibration with an amplitude of 50 KHz and a stirring speed of 800 rpm to prepare a NiMn LDH suspension. Add 3.8 mg NaBH ₄ into the sealed bottle, stir and disperse evenly at a stirring speed of 500 rpm, and obtain a reaction solution with a final concentration of NaBH ₄ of 0.001 M. The sealed bottle was placed in a water bath at 80°C, and reacted at a stirring speed of 800 rpm for 10 hours. After the reaction, cool down to room temperature naturally, add 5 ml of deionized water to the beaker to dilute the reactant, transfer the diluted solution to a 20 ml centrifuge tube, and centrifuge and wash at 7000 rpm for 10 minutes. Discard the supernatant, and perform centrifugation and washing twice in the same manner. The final precipitate was vacuum-dried at -100 KPa relative to the atmospheric pressure for 5 hours to obtain the H-NiMn LDH catalyst.

Embodiment 3: the synthesis of the NiCo LDH catalyst (H-NiCo LDH) of hydrogen anion activation

3.1 Synthesis of NiCo LDH

Synthesis of NiCo LDH nanopowders by hydrothermal method, the specific operation is as follows. 1.16 ml of 1 M nickel chloride (NiCl ₂ ) aqueous solution and 0.145 ml of 1 M cobalt chloride (CoCl ₂ ) aqueous solution were mixed with 70 ml of deionized water in a beaker. Then, under magnetic stirring, 10.1 ml of 0.5 M urea aqueous solution and 1.1 ml of 0.01 M trisodium citrate aqueous solution were added to the beaker. Then the resulting mixed solution was transferred into a 100 ml Teflon-lined stainless steel autoclave, sealed and subjected to hydrothermal reaction in an oven at 150 ºC for 24 h. After the reaction, the powder was collected by centrifugation at 7500 rpm for 10 min, then washed several times with deionized water and high-purity ethanol, and then dried in an oven at 50 ºC overnight to obtain NiCo LDH nanopowder, which is Ni(II)/Co( III) Transition metal layered bishydroxy compounds.

3.2 Synthesis of H-NiCo LDH catalyst

Add 1.5 mg of the NiCo LDH powder prepared above to 10 ml of absolute ethanol in a 15 ml sealed bottle, and disperse evenly by ultrasonic vibration with an amplitude of 50 KHz and a stirring speed of 800 rpm to prepare a NiCo LDH suspension . Add 3 mg NaBH ₄ into the sealed bottle, stir and disperse evenly at a stirring speed of 500 rpm, and obtain a reaction solution with a final concentration of NaBH ₄ of 0.008 M. The sealed bottle was placed in a 30°C water bath, and reacted at a stirring speed of 800 rpm for 5 hours. After the reaction, cool down to room temperature naturally, add 5 ml of absolute ethanol to the beaker to dilute the reactant, transfer the diluted solution to a 20 ml centrifuge tube, and centrifuge and wash at 7000 rpm for 10 minutes. Discard the supernatant, and perform centrifugation and washing twice in the same manner. The final precipitate was vacuum-dried at -100 KPa relative to the atmospheric pressure for 5 hours to obtain the H-NiCo LDH catalyst.

Embodiment 4: the synthesis of the CoFe LDH catalyst (H-CoFe LDH) of hydrogen anion activation

4.1 Synthesis of CoFe LDH

Synthesis of CoFe LDH nanopowder by hydrothermal method, the specific operation is as follows. Mix 0.145 ml of 1 M cobalt chloride (CoCl ₂ ) aqueous solution and 1.45 ml of 1 M ferric chloride (FeCl ₃ ) aqueous solution with 70 ml of deionized water in a beaker. Then, under magnetic stirring, 11.2 ml of 0.5 M urea aqueous solution and 4 ml of 0.01 M trisodium citrate aqueous solution were added to the beaker. Then the resulting mixed solution was transferred into a 100 ml Teflon-lined stainless steel autoclave, sealed and subjected to hydrothermal reaction in an oven at 150 ºC for 24 h. After the reaction, the powder was collected by centrifugation at 7500 rpm for 10 min, then washed several times with deionized water and high-purity ethanol, and then dried in an oven at 50 ºC overnight to obtain a CoFe LDH nanopowder, which is Co(II)/Fe( III) Transition metal layered bishydroxy compounds.

4.2 Synthesis of H-CoFe LDH catalyst

Add 2 mg of the CoFe LDH powder prepared above to 10 ml of absolute ethanol in a 15 ml sealed bottle, and disperse evenly by ultrasonic vibration with an amplitude of 50 KHz and a stirring speed of 800 rpm to prepare a CoFe LDH suspension . Add 3.8 mg NaBH ₄ into the sealed bottle, stir and disperse evenly at a stirring speed of 500 rpm, and obtain a reaction solution with a final concentration of NaBH ₄ of 0.01 M. The sealed bottle was placed in a 60°C water bath, and reacted for 0.5 hour at a stirring speed of 800 rpm. After the reaction, cool down to room temperature naturally, add 5 ml of absolute ethanol to the beaker to dilute the reactant, transfer the diluted solution to a 20 ml centrifuge tube, and centrifuge and wash at 7000 rpm for 10 minutes. Discard the supernatant, and perform centrifugation and washing twice in the same manner. The final precipitate was vacuum-dried at -100 KPa relative to the atmospheric pressure for 5 hours to obtain the H-CoFe LDH catalyst.

Embodiment 5: the synthesis of the NiFe LDH catalyst (Cl-NiFe LDH) of chloride anion activation

5.1 Synthesis of NiFe LDH

Synthesize NiFe LDH nanopowder by 1.1 of embodiment 1.

5.2 Synthesis of NiFe LDH catalyst activated by chloride anions

Add 1 mg of the NiFe LDH powder prepared above to 10 ml of deionized water in a 15 ml sealed bottle, and disperse evenly by ultrasonic vibration with an amplitude of 50 KHz and a stirring speed of 800 rpm to prepare a NiFe LDH suspension. Add 1.9 mg NaBH ₄ and 584 mg sodium chloride to the sealed bottle, stir and disperse evenly at a stirring speed of 500 rpm to obtain a reaction solution with a final concentration of NaBH ₄ of 0.005 M and a final concentration of Cl- of 1 M. The sealed bottle was placed in a 50°C water bath, and reacted for 5 hours at a stirring speed of 800 rpm. After the reaction, cool down to room temperature naturally, add 5 ml of deionized water to the beaker to dilute the reactant, transfer the diluted solution to a 20 ml centrifuge tube, and centrifuge and wash at 7000 rpm for 10 minutes. Discard the supernatant, and perform centrifugation and washing twice in the same manner. The final precipitate was vacuum-dried at -100 KPa relative to the atmospheric pressure for 5 hours to obtain a Cl-NiFe LDH catalyst.

Embodiment 6: the synthesis of the NiFe LDH catalyst (Br-NiFe LDH) of bromide anion activation

The present embodiment is carried out in the same manner as in Example 5, but replaces sodium chloride with sodium bromide, so that the final concentration of Br in the reaction solution is 1 M, and finally a Br-NiFe LDH catalyst is obtained.

Embodiment 7: the synthesis of the NiFe LDH catalyst (I-NiFe LDH) of iodide anion activation

The present embodiment is carried out in the same manner as in Example 5, but replaces sodium chloride with sodium iodide, so that the final concentration of I- in the reaction solution is 1 M, and finally the I-NiFe LDH catalyst is obtained.

Embodiment 8: the synthesis of the NiFe LDH catalyst (N-NiFe LDH) of negative ion activation

The present embodiment is carried out in the same manner as in Example 5, but potassium isothiocyanate (NCS) is used instead of sodium chloride, so that the final concentration of N in the reaction solution is 1 M, and finally the N-NiFe LDH catalyst is obtained.

Embodiment 9: the synthesis of the NiFe LDH catalyst (P-NiFe LDH) of phosphorus anion activation

The present embodiment is carried out in the same manner as in Example 5, but sodium phosphate is used instead of sodium chloride, so that the final concentration of P in the reaction solution is 1 M, and finally the P-NiFe LDH catalyst is obtained.

Embodiment 10: the synthesis of the NiFe LDH catalyst (S-NiFe LDH) of sulfide anion activation

The present embodiment is carried out in the same manner as in Example 5, but potassium thiocyanate is used instead of sodium chloride, so that the final concentration of S in the reaction solution is 1 M, and finally the S-NiFe LDH catalyst is obtained.

Embodiment 11: the preparation of catalyst/nickel foam electrode

Nickel foam was immersed in 1 M HCl solution for 10 minutes to remove surface oxides, then washed several times with deionized water and ethanol, and dried in an oven at 60 ºC for later use. 1 mg of the H-NiFe LDH catalyst synthesized in Example 1 was ultrasonically dispersed in 1 ml of ethanol for 2 hours to obtain a catalyst dispersion. Take 500 uL of the prepared catalyst dispersion and 5% PTFE aqueous solution in a volume ratio of 2:1, mix evenly, and ultrasonically disperse. The resulting mixed dispersion was evenly spread on a piece of nickel foam (1 cm x 1 cm), and baked in an oven at 60°C for 30 minutes to obtain the H-NiFe LDH catalyst/nickel foam electrode.

In a similar manner, the H-NiFe LDH catalyst prepared in Example 1 can be used to prepare electrodes on carbon cloth or iron substrates. Similarly, the catalysts prepared in Examples 2-10 can be used to prepare electrodes on nickel foam, carbon cloth or iron substrates in a similar manner.

Test example: Water splitting catalytic performance of H-NiFe LDH catalyst

Using the H-NiFe LDH catalyst/nickel foam electrode prepared in Example 11 as the water splitting anode (working electrode), Pt wire as the counter electrode, Ag/AgCl as the reference electrode, and 1 M potassium hydroxide aqueous solution as the electrolyte, construct The three-electrode system for water splitting is shown in Figure 5, wherein reference numeral 1 denotes a working electrode, 2 denotes a reference electrode, and 3 denotes a counter electrode. The NiFe LDH catalyst/nickel foam electrode was prepared according to the method of Example 11, and a water splitting three-electrode system using the NiFe LDH catalyst/nickel foam electrode as the working electrode was constructed in the same manner as in this test example. In addition, a three-electrode system for water splitting using nickel foam electrodes as working electrodes was constructed in the same manner as in this test example.

The three three-electrode systems for water splitting constructed above were tested on EC-lab (Bio-Logic) and Chi electrochemical workstation (Shanghai Chenhua) to verify the catalytic performance of the H-NiFe LDH catalyst of the present invention for water splitting. Figure 6 shows the comparison of the polarization curves of the catalytic hydrogen production of nickel foam, NiFe LDH and H-NiFe LDH. It can be seen from Figure 6 that the activated H-NiFeLDH catalyst has the lowest overpotential, indicating that the energy consumed to achieve the same current density is the lowest. Figure 7 shows the comparison of the catalytic oxygen evolution Tafel curves of nickel foam, NiFe LDH and H-NiFe LDH. It can be seen from Fig. 7 that the H-NiFe LDH catalyst after ligand activation has the lowest Tafel slope, which represents the fastest oxygen evolution reaction rate.

The stability of H-NiFe LDH was tested by chronopotentiometry. Figure 8 is a graph showing the chronopotentiometry of H-NiFeLDH at current densities of 10, 20 and 50 mA cm ^{- 2} , where the inset is the long-term Chronopotential test diagram. It can be seen from Figure 8 that the catalyst after ligand activation has better stability and has industrial application prospects.

In addition, the NCS-NiFe LDH catalyst/nickel foam electrode prepared by Example 8 was also used to prepare the NCS-NiFe LDH catalyst/nickel foam electrode in the manner of Example 10, and the H-NiFe LDH catalyst/nickel foam electrode prepared in Example 11 was prepared according to the present invention. The operations of the test cases are compared. Figure 9 shows the comparison of the polarization curves of catalytic hydrogen production of NCS-NiFe LDH and NiFe LDH. It can be seen from Figure 9 that, similar to H-NiFe LDH, the activated NCS-NiFe LDH catalyst has the lowest overpotential, indicating that the energy consumed to achieve the same current density is the lowest.

The above uses specific examples to illustrate the present invention, which are only used to help understand the present invention, and are not intended to limit the present invention. Those skilled in the technical field to which the present invention belongs can also make some simple deduction, deformation or replacement based on the concept of the present invention. These deduction, deformation or replacement schemes also fall within the scope of the claims of the present invention.

Claims (7)

1. A preparation method of a ligand-activated transition metal layered dihydroxy compound, characterized in that, comprising the steps: (1) Uniformly dispersing the transition metal layered bishydroxyl compound in a polar solvent to form a transition metal layered bishydroxyl compound suspension, the transition metal being any combination of divalent and trivalent transition metal ions; The atomic ratio of divalent/trivalent transition metal ions is between (3-10):1; the transition metal ions are Ni(II)/Fe(III) combination, Ni(II)/Mn(III) combination, Ni(II)/Co(III) combination or Co(II)/Fe(III) combination; (2) Add an appropriate amount of sodium borohydride for the activation reaction to the suspension in step (1), and add an appropriate amount of Cl ^- anions, Br ^- anions, I ^- anions, N-containing anions, P-containing anions or The combination of one or more kinds of negative ions containing S is subjected to an activation reaction to obtain the ligand-activated transition metal layered bishydroxyl compound. 2. The preparation method according to claim 1, characterized in that, in step (1), the weight-to-volume ratio of the transition metal layered dihydroxy compound to the polar solvent is (0.5-2) mg:1 mL; in step (2), the applicable amount of the activation reaction makes the final concentration of the sodium borohydride in the suspension of step (1) be 0.001-0.01 M, the Cl ^- negative ion, Br ^- negative ion, I ^- The total final concentration of one or more combinations of negative ions, N-containing negative ions, P-containing negative ions, or S-containing negative ions in the suspension in step (1) is 0.1-10 M, and the reaction temperature of the activation reaction The temperature is 20-80°C, and the reaction time is 0.5-10 hours. 3. preparation method according to claim 1, is characterized in that, comprises the following concrete steps: (1) Ni(II)/Fe(III), Ni(II)/Mn(III), Ni(II)/Co(III) or Co(II) with an atomic ratio between (3-10):1 )/Fe(III) transition metal layered bishydroxyl compound is uniformly dispersed in deionized water or ethanol or their mixed solvents, and the weight-to-volume ratio of the transition metal layered bishydroxyl compound to the polar solvent is (0.5 -2) mg: 1 mL, uniformly dispersed by ultrasonic vibration and stirring to become a suspension of transition metal layered bishydroxyl compound; (2) Add sodium borohydride with a final concentration of 0.001-0.01 M to the suspension in step (1), and add the Cl ^- anion, Br ^- anion, I ^- anion at a total final concentration of 0.1-10 M , a combination of one or more of N-containing anions, P-containing anions, or S-containing anions, stirred and dispersed evenly at a stirring speed of 400-1000 rpm, and then at a reaction temperature of 20-80°C and a stirring speed of 700-1000rpm React for 0.5-10 hours, after naturally cooling to room temperature, add an appropriate amount of deionized water or absolute ethanol as a washing solvent, centrifuge and wash three times at 6000-8000 rpm, each time for 5-10 minutes, and then vacuum dry for 3-6 hours , to obtain the ligand-activated transition metal layered bishydroxyl compound. 4. A ligand-activated transition metal layered dihydroxy compound, characterized in that, the ligand-activated transition metal layered dihydroxy compound is prepared according to the preparation method described in any one of claims 1-3 have to. 5. The use of the transition metal layered dihydroxy compound activated by the ligand according to claim 4 as an anode oxygen generation catalyst for water splitting. 6. A water splitting anode, is characterized in that, comprises nickel foam, carbon cloth or iron substrate and is coated on the ligand activation according to claim 4 on described nickel foam, carbon cloth or iron substrate Transition metal layered bishydroxyl compounds. 7. A water-splitting three-electrode system, comprising the water-splitting anode according to claim 6, the Pt wire as the counter electrode, the Ag/AgCl as the reference electrode and the 0.5-1.5 M potassium hydroxide as the electrolyte or Aqueous sodium hydroxide solution.

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