Preparation method of fluorine-doped Mo2C nanocatalyst on nitrogen-carbon nanosheets

Abstract

The present invention relates to a method for preparing a fluorine-doped Mo₂C nanocatalyst on a nitrogen-carbon nanosheet. This method uses a simple two-step pyrolysis method. First, two molybdenum sources are introduced, and MoO₃: ammonium molybdate = 1: (2-2.5). Then, ammonium fluoride (NH₄F) and molybdenum-dopamine (Mo-PDA) are uniformly mixed by solid-phase grinding. After calcination, the anion fluorine is doped into molybdenum carbide and modifies it, and then the fluorine-doped Mo₂C nanocatalyst on the nitrogen-carbon nanosheet can be obtained. The catalyst obtained by the present invention has higher catalytic activity, cyclic stability and hydrogen production efficiency in the reaction of electrolyzing water under alkaline conditions, and has the advantages of simple synthesis process, simple operation and short synthesis cycle.

Description

Technical Field

[0001] The present invention relates to the fields of catalyst preparation and the sustainable development of the environment and energy, and particularly to a method for preparing a fluorine-doped Mo2C nanocatalyst on a nitrogen-carbon nanosheet. Background Art

[0002] As a green and renewable energy source, hydrogen energy will become a promising alternative to traditional fossil fuels in future energy demands. Hydrogen production by electrolysis of water driven by solar or wind energy technologies is considered a sustainable method. To reduce the overpotential and save electrical energy, it is inevitable to introduce electrocatalysts with high activity and durability.

[0003] During electrocatalytic hydrogen evolution, an externally applied voltage much greater than 1.23 V is required to overcome the energy barrier difference between reaction intermediates and products, resulting in energy waste. Therefore, noble metals such as Pt need to be used as catalysts to reduce the overpotential. Due to its low natural reserves and high price, Pt is difficult to be widely applied. Mo2C has received extensive attention due to its excellent metallic conductivity, good HER catalytic activity, and low cost. The Pt-like electronic state structure of Mo2C originates from the hybridization between the d orbit of Mo and the s and p orbits of carbon, enabling Mo2C to have a stable hydrogen evolution performance close to that of platinum under alkaline conditions. A large number of research works have been devoted to enhancing its HER activity through morphology control, high refractive index plane modulation, transition metal cation doping, and heterostructure engineering. However, the electrocatalytic activity of the above-mentioned Mo2C-based catalysts for HER in alkaline media is still significantly lower than that of the benchmark Pt catalyst. Recently, anion engineering has emerged as an effective strategy to regulate the electronic structure of transition metal compounds (TMCs), thereby optimizing the adsorption / desorption energy of intermediates. In addition to regulating the electronic structure of TMCs, this strategy has many other advantages, such as increasing active sites and possibly improving electrical conductivity. Specifically in the research of Mo2C, in addition to the above problems, there are also: (1) existing research rarely studies Mo2C materials through anion engineering, and the performance is far from that of Pt-based catalysts; (2) for anion-doped nanocatalysts, due to their small size and high surface energy, they are prone to agglomeration during the preparation process, resulting in a reduction in metal active sites and thus a decrease in the catalytic activity of the catalyst.

[0004] In summary, it is very necessary to find a simple and effective anion doping method to synthesize inexpensive, highly efficient, and well-dispersed nanocatalysts for reducing the cost of catalysts and improving the efficiency of hydrogen production by electrolysis of water in alkaline solutions. Summary of the Invention

[0005] The present invention mainly aims at the disadvantages existing in the electrolytic water hydrogen production reaction in an alkaline solution, such as harsh reaction conditions, uneven dispersion of nanoparticles, and low catalyst activity, and provides a preparation method of a fluorine-doped Mo2C nanocatalyst on a nitrogen-carbon nanosheet. This method uses a simple two-step pyrolysis method. First, two molybdenum sources are introduced, and the ratio of MoO3: ammonium molybdate is 1:(2-2.5). Then, ammonium fluoride (NH4F) and molybdenum-dopamine (Mo-PDA) are uniformly mixed by solid-phase grinding. After calcination, the anion fluorine is doped into molybdenum carbide and modified, and the fluorine-doped Mo2C nanocatalyst on the nitrogen-carbon nanosheet can be obtained. The catalyst obtained by the present invention has higher catalytic activity, cyclic stability and hydrogen production efficiency in the electrolytic water reaction under alkaline conditions, and has the advantages of simple synthesis process, easy operation and short synthesis cycle.

[0006] To achieve the above object, the present invention is implemented according to the following technical scheme:

[0007] A preparation method of a fluorine-doped Mo2C nanocatalyst on a nitrogen-carbon nanosheet, comprising the following steps:

[0008] Step 1: Ammonium molybdate tetrahydrate is dissolved in a mixed solution, transferred to an autoclave, kept at 200 °C - 250 °C for 15 - 20 h, cooled and then centrifuged to collect, washed with water and ethanol, and dried overnight to obtain MoO3;

[0009] Among them, in the mixed solution, the volume ratio of nitric acid to ultrapure water is 1:(3 - 7), and 0.5 - 2.0 g of ammonium molybdate tetrahydrate is added to every 40 mL of the mixed solution;

[0010] Step 2: The MoO3 obtained in the previous step is added to ultrapure water and ultrasonicated for 15 - 30 min to obtain a uniformly dispersed milky white solution A;

[0011] Among them, 80 - 120 mg of MoO3 is added to every 70 mL of ultrapure water;

[0012] Step 3: Ammonium molybdate and hydrochloric acid dopamine are dissolved in solution A to form a wine-red solution B, and stirred for 40 - 60 min;

[0013] Among them, 160 - 240 mg of ammonium molybdate and 80 - 120 mg of hydrochloric acid dopamine are added to every 70 mL of solution A; the mass ratio of MoO3: ammonium molybdate is 1:(2 - 2.5);

[0014] Step 4: Anhydrous ethanol is poured into solution B, stirred for 5 - 10 min, then NH3·H2O is added, and stirred at room temperature for 2 - 4 h. After centrifugal separation, the precipitate is collected, washed by ethanol centrifugation, and vacuum dried overnight to obtain a Mo-dopamine precursor (Mo-PDA);

[0015] Among them, 40 - 60 mL of absolute ethanol and 0.4 - 0.6 mL of NH₃·H₂O are added to every 70 mL of solution B;

[0016] Step Five: Mix and grind the obtained Mo - PDA and NH₄F for 3 - 8 min, then heat up to 800 - 900 °C and keep the temperature for 5 - 6 h to burn into a fine black powder sample (F - Mo₂C / NC).

[0017] Among them, the mass ratio of Mo - PDA:NH₄F = 100:30 - 90;

[0018] The mass concentration of nitric acid in Step One is 60% - 75%;

[0019] In Step Three, the mass concentration of NH₃·H₂O is 28% - 30%;

[0020] In Step Four, the rotation speed for centrifugal separation and centrifugal washing is 8000 - 12000 rpm;

[0021] In Step Five, the heating rate is 2 - 3 °C / min.

[0022] The substantial features of the present invention are:

[0023] In the present invention, NH₄F is used as a fluorine source to dope - modify Mo₂C. The addition of fluorine elements can effectively change the electronic structure of molybdenum carbide, which is beneficial to optimizing the adsorption energy of hydrogen / water and maintaining good dispersibility, thereby greatly improving the activity of the catalyst, making the catalyst have higher catalytic activity and cyclic stability, and improving the hydrogen production efficiency.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] The present invention synthesizes a fluorine - doped Mo₂C nanocatalyst on nitrogen - carbon nanosheets by a two - step pyrolysis method, which has the advantages of short synthesis time, simple operation, high catalytic activity and cyclic stability, etc. Moreover, it significantly improves the dispersibility of Mo₂C nanoparticles on NC nanosheets, reduces the size of metal nanoparticles and optimizes the electronic structure of Mo₂C nanoparticles. The synthesized F - Mo₂C / NC catalyst is used for catalyzing the decomposition of 1 M KOH solution to produce hydrogen. Under the condition of no surfactant, it has extremely high catalytic activity ( Figure 5 (a) and (b) prove the high catalytic activity), 100% hydrogen selectivity (Faraday efficiency is shown in Figure 6 ) and good cyclic stability ( Figure 5 (c) and (d) prove the good cyclic stability). As an ideal HER electrocatalyst, F - Mo₂C / NC has a current density of 10 mA cm -2The overpotential (η) is 65 mV and the Tafel slope is 58 mV dec -1 . The catalytic performance is much higher than that of most of the catalysts reported so far. For example, Mo2C / CXG (overpotential is 170 mV, Tafel slope is 264 mVdec -1 ), MNG-40 (overpotential is 185 mV, Tafel slope is 78 mV dec -1 ), Ni-MoS2 / RGO-5 (overpotential is 398 mV, Tafel slope is 140 mV dec -1 ), etc. Moreover, no noble metal is added to F-Mo2C / NC, effectively reducing the cost of the catalyst and accelerating the application of alkaline water splitting for hydrogen production in the actual production process. All in all, the present invention synthesizes a fluorine-doped Mo2C nanocatalyst on nitrogen-carbon nanosheets by a simple two-step pyrolysis method. The synthesized catalyst has small particle size, good dispersibility, and good hydrogen adsorption and desorption energies, enabling the catalyst to expose more active sites, having higher catalytic activity compared to most catalysts for alkaline water splitting for hydrogen production, and having good cycle stability. The development of this catalyst has opened up a new way for realizing safe, stable, efficient, and inexpensive catalysts. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram for the preparation of the F-Mo2C / NC catalyst in Example 1;

[0027] Figure 2 X-ray diffraction patterns of the catalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2;

[0028] Figure 3 X-ray photoelectron spectroscopy of Mo 3d in the F-Mo2C / NC catalyst in Example 1 and Mo2C / NC in Comparative Example 1;

[0029] Figure 4 Transmission electron micrograph of the F-Mo2C / NC catalyst in Example 1;

[0030] Figure 5 Performance test graphs of the catalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 for alkaline water splitting for hydrogen production, where: Figure 5 (a) LSV; Figure 5 (b) Tafel; Figure 5 (c) i-t; Figure 5 (d) Performance graph of LSV before and after 3000 CV cycles;

[0031] Figure 6 The Faraday efficiency of Example 1 for HER is 170 mV at 1 M KOH for 60 min. Detailed implementation mode

[0032] The present invention will be further described below in conjunction with specific embodiments. The illustrative embodiments and explanations of this invention are used to explain the present invention, but do not limit the present invention.

[0033] Embodiment 1

[0034] As Figure 1 shown, a preparation method of a fluorine-doped Mo2C nanocatalyst on nitrogen-carbon nanosheets includes the following steps:

[0035] Step 1: 1.4 g of ammonium molybdate tetrahydrate is dissolved in a 40 mL mixed solution of nitric acid with a mass fraction of 65% and ultrapure water. The volume ratio of nitric acid to ultrapure water is 1:5. Transfer it to an autoclave (70 mL), keep it at 200 °C for 20 h, cool it, collect by centrifugation, wash it 3 times with ethanol, and dry it overnight at 70 °C to obtain MoO3;

[0036] Step 2: Add 100 mg of the prepared MoO3 to a 150 mL glass bottle containing 70 mL of ultrapure water, and ultrasonicate for 30 min to obtain a uniformly dispersed milky white solution;

[0037] Step 3: Completely dissolve 200 mg of ammonium molybdate and 100 mg of dopamine hydrochloride in the above solution to form a Mo-dopamine complex (Mo-PDA), obtaining an orange-red solution, and stir for 40 min;

[0038] Step 4: Pour 40 mL of absolute ethanol into the above orange-red solution, stir for 5 min, the solution turns wine red and turbid. After adding 0.4 mL of NH3·H2O (28% - 30%), stir at room temperature for 2 h, centrifuge and separate, collect the wine red precipitate, wash it three times by ethanol centrifugation, with a centrifugation speed of 9500 rpm each time, and vacuum dry it overnight at 60 °C to obtain a Mo-PDA precursor;

[0039] Step 5: Grind 100 mg of Mo-PDA and 60 mg of NH4F by solid-phase grinding for 5 min to make them evenly mixed, heat it to 850 °C at 2 °C / min, keep it at this temperature for 5 h, and sinter it into a fine black powder sample (F-Mo2C / NC).

[0040] 2. Sample detection

[0041] (1) Wash the above-prepared F-Mo2C / NC catalyst, vacuum dry it, and grind it into fine powder; Refer to Figure 2From the X-ray powder diffraction (XRD) results, it can be seen that the F-Mo2C / NC catalyst has a good crystal structure. The diffraction peaks of Mo2C can be clearly seen, and after fluoride ion doping, the diffraction peaks become sharper and shift towards smaller angles. This is because the radius of F is larger than that of C atoms, indicating that fluoride ions have been successfully doped into the molybdenum carbide lattice.

[0042] (2) Wash the as-prepared F-Mo2C / NC catalyst and then dry it under vacuum; refer to Figure 3 , The peak-fitting results of X-ray photoelectron spectroscopy (XPS) show that compared with Mo2C / NC, the Mo 2+ 3d peak (228.5 eV) of F-Mo2C / NC shifts significantly towards a lower binding energy direction. At the same time, the intensity of the Mo 2+ sub-peaks (231.8 eV and 228.5 eV) increases significantly, indicating that the valence state of Mo in F-Mo2C / NC is significantly higher than that in Mo2C / NC, which means that the electron density around Mo atoms increases after F doping.

[0043] (3) Dilute the as-prepared F-Mo2C / NC catalyst with ethanol, drop it on a carbon support film, and dry it; refer to Figure 4 , The transmission electron microscope (TEM) results show that the F-Mo2C / NC sample has a small particle size (~4.38 nm) and uniform dispersion.

[0044] 3. Catalytic hydrogen production from alkaline water

[0045] Ultrasonically disperse 10 mg of the catalyst in 300 μL of absolute ethanol, 680 μL of deionized water, and 20 μL of Nafion solution (5 wt%) for 30 min to form a uniform ink. Cast 10 μL of the solution on a glassy carbon electrode (GCE) with a diameter of 3 mm. After drying the GCE at ambient temperature, the loading is 1.4 mg cm -2 . A carbon rod is used as the counter electrode, and a saturated calomel electrode (SCE) is used as the reference electrode. The measured potential is converted to the response of the potential to the reversible hydrogen electrode (RHE) according to E vs RHE = E vs SCE + E° SCE + 0.059pH, where E° SCE is 0.2412 V. CV activation is performed for at least 6 cycles before the evaluation process. This time, under normal temperature reaction conditions, the F-Mo2C / NC catalyst electrolyzes water to produce hydrogen under alkaline conditions, and the hydrogen production amount (μmol) versus time (min) graph is as shown in Figure 6As shown, the amount of gas generated by the catalytic alkaline water hydrogen production can reach 300 μmol within 60 min, and the conversion rate reaches 100%. At a scanning rate of 5 mV / s, a complete linear sweep voltammogram was obtained. After fitting, a Tafel curve graph can be obtained. The double-layer capacitance (Cdl) with a scanning rate of 20 - 100 mV / s in the non-Faraday region was calculated using cyclic voltammograms (CV). The CV curve was measured in the potential range from 0.08 to 0.18 V (vs. RHE) without redox processes. The relationship between the current density and the scanning rate was obtained from the CV curve. The long-term stability test was carried out at continuous potentials using chronoamperometry. All electrochemical measurements were evaluated using a CHI760E (instruments, China) workstation in a standardized three-electrode system.

[0046] Comparative Example 1 (without adding NH4F)

[0047] 1.4 g of ammonium molybdate tetrahydrate was dissolved in a 40 mL mixed solution of 65% HNO3 and ultrapure water, with a volume ratio of 65% HNO3: ultrapure water of 1:5. It was transferred to an autoclave (70 mL), kept at 200 °C for 20 h, cooled and then centrifuged to collect. It was washed several times with water and ethanol and dried overnight at 70 °C to obtain MoO3; 100 mg of the prepared MoO3 was added to a glass bottle containing 70 mL of ultrapure water and sonicated to obtain a uniformly dispersed milky white solution; 200 mg of ammonium molybdate and 100 mg of dopamine hydrochloride were completely dissolved in the above solution to form a Mo-dopamine complex, obtaining an orange-red solution, and stirred for 40 min; 40 mL of absolute ethanol was poured into the above orange-red solution and stirred for 5 min. The solution turned wine-red and turbid. After adding 0.4 mL of NH3·H2O (28% - 30%), it was stirred at room temperature for 2 h, and the wine-red precipitate was collected by centrifugation, and centrifugally washed three times with ethanol at a centrifugal speed of 9500 rpm / min for 5 min, and vacuum dried overnight at 60 °C; 100 mg of Mo-PDA was placed in a tube furnace and heated to 850 °C at 2 °C / min and kept for 5 h to burn into a fine black powder sample (Mo2C / NC).

[0048] 10 mg of the Mo2C / NC catalyst was ultrasonically dispersed in 300 μL of absolute ethanol, 680 μL of ultrapure water and 20 μL of Nafion solution (5 wt%) for 30 min to form a uniform ink. 10 μL of the solution was cast on a glassy carbon electrode (GCE) with a diameter of 3 mm. After drying the GCE at ambient temperature, the loading amount was 1.4 mg cm -2 . The carbon rod was used as the counter electrode, and the saturated calomel electrode (SCE) was used as the reference electrode. In 1 M KOH solution, its overpotential at 10 mA cm -2 was 164 mV, and the Tafel slope was 146 mV dec -1 .

[0049] Comparative Example 2 (without adding molybdenum source and NH4F)

[0050] 100 mg of dopamine hydrochloride was completely dissolved in 70 mL of ultrapure water and stirred for 40 min; 40 mL of absolute ethanol was poured into the above solution and stirred for 5 min; after adding 0.4 mL of NH3·H2O (28% - 30%), it was stirred at room temperature for 2 h, and the black precipitate was collected by centrifugation, washed three times with ethanol by centrifugation, the centrifugation speed was 9500 rpm / min, maintained for 5 min, and vacuum dried at 60 °C overnight; a black powder sample (NC) was obtained.

[0051] 10 mg of the NC catalyst was ultrasonically dispersed in 300 μL of absolute ethanol, 68 μL of ultrapure water and 20 μL of Nafion solution (5 wt%) for 30 min to form a uniform ink. 10 μL of the solution was cast on a glassy carbon electrode (GCE) with a diameter of 3 mm. After drying the GCE at ambient temperature, the loading amount was 1.4 mg cm -2 。A carbon rod was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. Its overpotential at 10 mA cm -2 was 700 mV and the Tafel slope was 239 mV dec -1 。

[0052] Figure 2 XRD patterns of the catalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2. It can be seen from the figure that Mo2C nanoparticles formed a crystal structure.

[0053] Through the performance tests of Example 1 and Comparative Examples 1 and 2, it can be seen that the F-Mo2C / NC catalyst prepared by the present invention has excellent catalytic activity and hydrogen production efficiency in an alkaline solution (1 M KOH). The good performance may be that the doping of anion F changes the electronic structure of Mo2C nanoparticles, which is beneficial to optimizing the adsorption energy of hydrogen / water and maintaining good dispersion, thus greatly improving the activity of the catalyst, making the catalyst have higher catalytic activity and cycle stability, and improving the hydrogen production efficiency.

[0054] Example 2

[0055] Other steps were the same as in Example 1, except that the doping amount of F was changed to 30 mg; the obtained catalyst still had a crystal structure and its performance was close to that of Example 1.

[0056] Example 3

[0057] Other steps were the same as in Example 1, except that no fluorine source was added, that is, Mo2C / NC;

[0058] The obtained catalyst still has a crystal structure, and its electrocatalytic hydrogen production performance is as Figure 5 , the overpotential and Tafel slope have both increased. This may be due to the agglomeration of Mo2C nanoparticles, which obliterates some active sites, resulting in a decrease in catalytic activity.

[0059] Example 4

[0060] Other steps are the same as those in Example 1, except that no fluorine source (NH4F) and molybdenum source (MoO3 and ammonium molybdate) are added, that is, NC; the obtained catalyst still does not have a crystal structure and hardly has electrocatalytic activity, because it has no fixed active sites.

[0061] The technical solution of the present invention is not limited to the limitations of the above specific embodiments. Any technical deformation made according to the technical solution of the present invention falls within the protection scope of the present invention.

[0062] Matters not covered in the present invention are well-known technologies.

Claims

1. A preparation method of a fluorine-doped Mo2C nanocatalyst on a nitrogen-carbon nanosheet, characterized in that the method comprises the following steps: Step 1: Ammonium molybdate tetrahydrate is dissolved in a mixed solution, transferred to an autoclave, maintained at 200 - 250 °C for 15 - 20 h, cooled, centrifuged to collect, washed with water and ethanol, and dried overnight to obtain MoO3; Among them, in the mixed solution, the volume ratio of nitric acid to ultrapure water is 1:(3 - 7), and 0.5 - 2.0 g of ammonium molybdate tetrahydrate is added to every 40 mL of the mixed solution; Step 2: The MoO3 obtained in the previous step is added to ultrapure water and sonicated for 15 - 30 min to obtain a uniformly dispersed milky white solution A; Among them, 80 - 120 mg of MoO3 is added to every 70 mL of ultrapure water; Step 3: Ammonium molybdate and dopamine hydrochloride are dissolved in solution A to form an orange-red solution B, and stirred for 40 - 60 min; Among them, 160 - 240 mg of ammonium molybdate and 80 - 120 mg of dopamine hydrochloride are added to every 70 mL of solution A, and the mass ratio of MoO3: ammonium molybdate = 1:(2 - 2.5); Step 4: Absolute ethanol is poured into solution B, stirred for 5 - 10 min, then NH3·H2O is added, and stirred at room temperature for 2 - 4 h. After centrifugal separation, the precipitate is collected, centrifugally washed with ethanol, and vacuum dried overnight to obtain a Mo-dopamine precursor (Mo-PDA); Among them, 40 - 60 mL of absolute ethanol and 0.4 - 0.6 mL of NH3·H2O are added to every 70 mL of solution B; Step 5: The Mo-PDA obtained in the previous step is mixed and ground with NH4F for 3 - 8 min, then heated to 800 - 900 °C, maintained for 5 - 6 h, and fired into a fine black powder sample (F-Mo2C / NC); Among them, the mass ratio of Mo-PDA:NH4F = 100:(30 - 90).

2. The preparation method of the fluorine-doped Mo2C nanocatalyst on the nitrogen-carbon nanosheet according to claim 1, characterized in that the mass concentration of nitric acid in step 1 is 60% - 75%.

3. The preparation method of the fluorine-doped Mo2C nanocatalyst on the nitrogen-carbon nanosheet according to claim 1, characterized in that In step 3, the mass concentration of NH3·H2O is 28% - 30%.

4. The preparation method of the fluorine-doped Mo2C nanocatalyst on the nitrogen-carbon nanosheet according to claim 1, characterized in that In step 4, the rotation speed of centrifugal separation and centrifugal washing is 8000 - 12000 rpm.

5. The preparation method of the fluorine-doped Mo2C nanocatalyst on the nitrogen-carbon nanosheet according to claim 1, characterized in that In step 5, the heating rate is 2 - 3 °C / min.

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