Preparation method of an M-MOF nanomaterial with electrocatalytic activity

Abstract

The present invention provides a method for preparing an M-MOF nanomaterial with electrocatalytic activity, comprising: step S1: weighing a certain mass of NaOH or KOH and dissolving it in distilled water, stirring it fully and heating it to 95°C, adding a certain mass of FeCl3 aqueous solution and fulvic acid aqueous solution, then aging it at 95°C for a certain period of time, cooling it to room temperature, centrifuging it several times with distilled water and anhydrous ethanol, and then drying it in an oven to obtain a precursor; step S2: first fully grinding the precursor to obtain a uniform powder, and then roasting it in a N2 atmosphere in a tubular furnace; step S3: after the roasted precursor is cooled to room temperature, centrifuging it several times with distilled water and anhydrous ethanol, and finally drying it in an oven to obtain a black powdery solid product, which is marked as M-MOFs. The structure of the present invention is unique, so that its charge transfer ability is suitable for the electrocatalytic hydrogen production process, meets the needs of industrial development, and solves the problem of shortage of hydrogen energy.

Description

Technical Field

[0001] The present invention relates to the technical field of metal-organic frameworks, and more specifically to a preparation method of an M-MOF nanomaterial with electrocatalytic activity. Background Art

[0002] In the past century, the global economy has developed rapidly and the population has grown rapidly, resulting in an increasing global demand for energy. A large amount of non-renewable primary energy sources such as coal and oil have been consumed in large quantities, and the associated environmental problems have become increasingly serious. The use of traditional fossil fuels is accompanied by the generation of greenhouse gases such as CO 2 etc., which runs counter to the concept of environmentally friendly development. H 2 As an ideal and environmentally friendly new fuel energy source, it well solves this problem. After combustion, it not only does not produce greenhouse gases such as CO 2 etc., but also has a high energy after combustion. As a high-energy density energy source, its heat of combustion is nearly three times that of gasoline. Currently, the methods for preparing H 2 mainly include: hydrogen production from fossil fuels, biological hydrogen production, photocatalytic hydrogen production, and electrocatalytic hydrogen production, etc.

[0003] Electrocatalytic water splitting will be a very promising hydrogen production technology in the future. With the development of electrocatalytic technology, the role of the catalyst becomes more prominent. In the traditional catalytic field, noble metals have excellent catalytic performance, but the manufacturing cost is not directly proportional to the economic benefits they bring. The catalytic performance of ordinary metals cannot meet the actual production requirements. Therefore, the improvement of the catalytic performance of ordinary metals has become a new exploration path. As a new type of material for improving ordinary metals, MOFs materials have a high catalytic effect and have unique advantages and applications in the field of electrocatalysis.

[0004] Electrolysis of water is a clean and pollution-free H 2 preparation technology with a long history of development. Between 1789 and 1800, the phenomenon of electrolysis of water was discovered, and through a large number of experimental explorations, it was obtained that the products of electrolysis of water are H 2 and O 2 . According to the acidity and alkalinity of the electrolyte, electrolysis of water can be divided into three systems: alkaline, neutral, and acidic. The acidic environment is likely to generate strong acidic gases, which can easily damage the production equipment. Industrially, the alkaline electrolyte system is often used as the mainstream. Seawater, as a rich weak alkaline resource, is widely used in production.

[0005] An electrolytic cell takes a power source, electrodes, and an electrolyte as basic configurations. Figure 1 is a schematic diagram of electrolysis of water in an alkaline system. Under the condition of connecting an external power source, relying on the action of the electric field, hydrogen is generated at the cathode due to the gain of electrons. 2, the hydrogen evolution reaction (HER) occurred, and the oxygen evolution reaction occurred at the anode to generate O 2 (OER).

[0006] The following processes are the overall reaction equation of water electrolysis and the equations in different electrolytes:

[0007] Overall reaction equation: H 2 O → H 2 + 1 / 2O 2

[0008] In neutral or alkaline electrolytes: Cathode: 2H 2 O + 2e - → H 2 + 2OH -

[0009] Anode: 2OH - → H 2 O + 1 / 2O 2 + 2e -

[0010] In acidic electrolytes: Cathode: 2H + + 2e - → H 2

[0011] Anode: H 2 O → 2H + + 1 / 2O 2 + 2e -

[0012] Mechanism of electrocatalytic hydrogen production:

[0013] The reaction mechanism of HER is different in electrolytes with different acid-base properties, and it includes three elementary reaction steps: the Volmer reaction, the Heyrovsky reaction, and the Tafel reaction. The Volmer reaction is "electrochemical adsorption", and the Heyrovsky reaction generates hydrogen.

[0014] Metal-organic frameworks (MOFs), also known as porous coordination polymers (PCPs), have a certain crystal porous structure. They are self-assembled from multi-toothed organic bridging ligands and metal nodes, and have the advantages of extremely high specific surface area, porosity, and flexible structure. They have been widely explored and used in the fields of gas storage and separation, molecular sensing, optoelectronic materials, drug carriers, electrocatalysis, etc.

[0015] With the in-depth study of MOFs, the number of papers published on MOFs from 1999 to 2020 has shown an almost exponential growth. The development of MOFs presents a good trend, indicating that MOFs have great potential advantages in many fields.

[0016] MOFs materials combine inorganic metals and organic ligands. Since the first report of MOFs by Yaghi et al. in the 1990s, they have attracted extensive attention from researchers. The special topological structure, regular internal arrangement, and diverse pores of MOFs provide broad ideas and feasibility for MOFs materials with different functions. The metal nodes of MOFs can purposefully select catalytically active metal elements, such as Fe 2+ 、Ti 4+ 、Zr 4+ 、Al 3+ etc., to construct different types of catalytic materials. The highly ordered pores of MOFs are one of their most prominent features and have a uniform distribution. Theoretical calculations show that the upper limit of its specific surface area is about 1.46×104 m 2 ·g -1 . The extremely high porosity and specific surface area of MOFs maximize surface reactions by providing abundant active sites. In addition, the variable coordination modes and rich variety of organic ligands make the composition of MOFs extremely diverse. Therefore, by changing the type and quantity of organic ligands, the pore size and cage size in the MOF structure can be changed, and thus the specific surface area of MOFs can be changed. The large specific surface area and porosity not only increase the contact area between the reactant solution and the catalyst but also provide more active sites. Therefore, MOFs materials offer great possibilities for efficient electrocatalysis.

[0017] Electrocatalytic applications of metal-organic framework nanomaterials:

[0018] Hydrogen prepared by electrolysis is a clean energy source with high purity. In this process, a hydrogen evolution reaction occurs. However, due to the incomplete utilization of many energies, unnecessary energy waste occurs, the energy conversion rate and production capacity are low, and it cannot be applied to the actual hydrogen production field. Therefore, it is necessary to develop efficient electrocatalysts to reduce the energy consumption of the hydrogen production reaction. When MOFs are applied in the field of electrocatalysis, their electrocatalytic performance includes catalytic activity and chemical stability. Developing more suitable electrocatalysts for actual needs is the exploration goal of the scientific community and the production goal of the industrial community. MOFs materials will eventually occupy a prominent position in the field of electrocatalysis.

[0019] This paper takes the question of whether water electrolysis can efficiently catalyze hydrogen production as a starting point. Through the above analysis, it is concluded that electrocatalysts are the main factor affecting the efficiency of water electrolysis, and improving electrocatalysts is a feasible means to improve the efficiency of hydrogen production. Iron, as a metal resource with abundant reserves, is not only inexpensive, but also environmentally friendly, and is very suitable as a metal node for MOFs materials.

[0020] There are countless methods for synthesizing MOFs. Here we only list the common ones, such as the solvent evaporation method, which is to evenly mix the metal and ligand linker in the liquid phase without external energy supply, so that the solvent slowly evaporates to precipitate crystals, but this synthesis method is relatively time-consuming; the diffusion method, including the interface diffusion method and the vapor diffusion method, in which the interface diffusion method uses two liquids with a large density difference as solvents to dissolve organic ligands and metal salts respectively, and then uses the high-density solution as the bottom film and the low-density solution as the upper film, which are evenly spread and diffused at the interface between the two films to produce crystalline materials; the vapor diffusion method is to use a highly volatile poor solvent to diffuse into a good solvent containing metal ions and ligands, so as to reduce the solubility of the complex and then generate single crystals; this diffusion method has low yield and lacks selectivity. Summary of the invention

[0021] In order to solve the above problems, the present invention provides a method for preparing M-MOF nanomaterials with electrocatalytic activity, which greatly reduces the cost and is environmentally friendly.

[0022] To achieve the above object, the present invention provides the following technical solutions:

[0023] A method for preparing an M-MOF nanomaterial with electrocatalytic activity, characterized in that it comprises the following steps:

[0024] Step S1, preparation of precursor: weigh a certain amount of NaOH or KOH and dissolve it in distilled water, stir it thoroughly and heat it to 95°C, then add a certain amount of FeCl 3 aqueous solution and fulvic acid aqueous solution, then continuously aged at 95°C for a certain period of time, cooled to room temperature, centrifuged and washed with distilled water and anhydrous ethanol for multiple times, and then placed in an oven for drying to obtain a precursor;

[0025] Step S2, calcination: First, the precursor obtained in step S1 is fully ground to obtain a uniform powder, and then calcined in a tube furnace N 2 Firing in atmosphere;

[0026] Step S3, Preparation of the final product: After cooling the precursor calcined in Step S2 to room temperature, it was centrifugally washed multiple times with distilled water and absolute ethanol, and finally dried in an oven to obtain a solid product in the form of a black powder, labeled as M-MOFs.

[0027] Preferably, the aging time in Step S1 is 4 h.

[0028] Preferably, the drying temperature in the oven is 60 °C.

[0029] Preferably, the conditions of the calcination program in Step S2 are: the heating rate is 5 °C / min, and it is heated to 600 °C and maintained for 4 h.

[0030] Preferably, M = Fe, FeO.

[0031] Preferably, the precursor is a black solid with a metallic luster.

[0032] It can be seen from the above technical solutions that, compared with the prior art, the present invention has the following advantages:

[0033] Economically, it is simple to manufacture, inexpensive, easy to obtain, and widely used in many fields such as industry, agriculture, and the medical industry;

[0034] Structurally, in terms of the structure of carbon, it has a regular pore structure and has certain adsorption and complexation abilities. This ability enables it to attach to or wrap metal particles. As a transition metal, iron has a strong electron transfer ability due to its own structure and is suitable for catalytic materials;

[0035] In terms of size tunability, by adjusting the molecular weight of humic acid, humate particles with a size of several hundred nanometers can be obtained, which provides the possibility for constructing carbon skeletons of different sizes;

[0036] In terms of hydrophilicity, it contains hydrophilic functional groups (such as active groups like carboxyl, phenolic hydroxyl, and aromatic rings), and has metal ion complexation, ion adsorption, redox, and physiological activities, etc. Thus, it can improve the performance of composite with metal materials. Description of the Drawings

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained according to the provided drawings.

[0038] Figure 1The attached figure is the XRD pattern of the M-MOFs (M = Fe, FeO) material of the present invention before and after calcination.

[0039] Figure 2 The attached figure is the SEM image of the present invention.

[0040] Figure 3 The attached figure is the CV diagram of the M-MOFs (M = Fe, FeO) material of the present invention before and after calcination.

[0041] Figure 4 The attached figure is the linear voltammetry scanning curve of the precursor and the M-MOFs (M = Fe, FeO) material of the present invention under hydrogen evolution reactions at different scan rates.

[0042] Figure 5 The attached figure is the Tafel curve of the M-MOFs (M = Fe, FeO) material of the present invention before and after calcination.

[0043] Figure 6 The attached figure is the linear voltammetry scanning curve of the precursor and the M-MOFs (M = Fe, FeO) material of the present invention under oxygen evolution reactions at different scan rates.

[0044] Figure 7 The attached figure is the electrochemical impedance spectroscopy curve of the precursor and the M-MOFs (M = Fe, FeO) material of the present invention. Detailed implementation manners

[0045] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the attached figures in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0046] Embodiment:

[0047] 1 Materials and methods

[0048] 1.1 Experimental drugs and experimental instruments

[0049] 1.1.1 Experimental drugs

[0050] The main drugs and reagents used in the experiment are shown in Table 1 below.

[0051] Table 1 Main drugs and reagents used in the experiment

[0052]

[0053] 1.1.2 Experimental instruments

[0054] The main instruments used in the experiment are shown in Table 2.

[0055] Table 2 Main instruments used in the experiment

[0056]

[0057] 1.2 Preparation and electrocatalytic performance of M-MOFs nanomaterials

[0058] 1.2.1 Preparation of M-MOFs (M=Fe, FeO) nanomaterial precursors

[0059] Accurately weigh 12.00 g of NaOH and dissolve it in 100 mL of distilled water. Stir thoroughly and heat to 95 °C. Quickly add 50 mL of 0.4 mol / LFeCl 3 The aqueous solution and 50 mL of 0.02 g / mL fulvic acid aqueous solution were aged at 95 °C for 4 h. After cooling to room temperature, the product was centrifuged and washed several times with distilled water and anhydrous ethanol, and finally dried in an oven at 60 °C to obtain a black solid precursor with a metallic luster.

[0060] 1.2.2 Calcination of M-MOFs (M=Fe, FeO) nanomaterials

[0061] The precursor was first ground thoroughly to obtain uniform powder, and then heated in a tube furnace. 2 The samples were calcined in a molten-salt atmosphere at a heating rate of 5°C / min to 600°C for 4 hours. After cooling to room temperature, the calcined samples were taken out and washed by centrifugation with distilled water and anhydrous ethanol for multiple times, and finally dried in an oven to obtain a black powdery solid product, which was labeled as M-MOFs.

[0062] 1.3 Sample characterization test

[0063] X-ray diffraction (XRD) can be used to identify the crystal structure of a substance or other structures with a specific order. The setting parameters of the X-ray diffractometer are: scanning range 10°-80°, angle increment 0.05°, sampling time 0.5s, voltage 40kV, current 30mA.

[0064] Scanning electron microscope (SEM) is a morphology observation tool between transmission electron microscope and optical microscope. ZEISS Gemini SEM 300 scanning electron microscope was used to photograph the sample morphology. The acceleration voltage was 3 kV during the morphology photography.

[0065] 1.4 Electrochemical performance test of samples

[0066] 1.4.1 Preparation of the working electrode

[0067] The current standard method for fabricating electrodes is drop casting. Generally, an organic reagent Nafion is added during electrode preparation to improve the adhesion between the sample layer and the conductive substrate. Generally speaking, most catalysts are in powder form. Due to its convenience and applicability, the drop casting method is often widely used in the preparation of catalyst electrodes.

[0068] Preparation of the working electrode: (1) Prepare the slurry for the dropping electrode. Weigh an appropriate amount of the sample and place it in a bullet-shaped centrifuge tube. Add about 1 mL of absolute ethanol and an appropriate amount of Nafion solution, and ultrasonicate for 30 min to obtain the slurry for the dropping electrode. (2) Polish the working electrode. First, dissolve Al 2 O 3 polishing powder with ultrapure water, and polish it with uniform force in the shape of an "8", polishing for 3 min each time. After polishing, ultrasonicate it with ultrapure water 2 - 3 times (not exceeding 10 s each time) to wash it clean. (3) Drop the electrode. Use a pipette to transfer 3 μL of the slurry containing the catalyst and evenly spread it on the surface of the glassy carbon electrode (D = 3 mm, S = 0.07065 cm 2 ), and dry it with an infrared lamp (do not place it too close to prevent cracking caused by excessive temperature after the catalyst is dried). After drying, drop 0.5% Nafion dilution solution. If the dropping is not successful at one time, ultrasonicate the catalyst on the electrode surface with ethanol, dry it, and continue dropping.

[0069] 1.4.2 Cyclic Voltammetry (CV)

[0070] (1) The cyclic voltammetry method uses a classical three - electrode working system. A linear alternating voltage is applied across the working electrode and the auxiliary electrode, and then multiple cyclic scans are performed to record the current and potential information of the redox reaction. By analyzing the recorded waveform data, it is possible to observe whether the substance reaction is reversible, as well as information such as the peak potential and peak current of the reactant. An electrochemical workstation of model CHI - 760E is used. First, connect the three - electrode system. Rinse the electrolytic cell 2 - 3 times. The electrolyte is 1 mol / L KOH solution. The green wire is connected to the dried electrode (glassy carbon electrode) prepared by the drop - coating method. The red wire is connected to the counter electrode (platinum electrode or graphite electrode, the platinum electrode is used in this experiment). The white wire is connected to the reference electrode (mercuric oxide / mercury electrode or calomel electrode, the mercuric oxide electrode is used in this experiment). The black wire does not need to be connected. The platinum electrode can be a platinum sheet electrode or a platinum wire electrode. Then turn on the power of the electrochemical workstation, open the CHI - 760E software, select the control item, click open circuit potential, accurately record the value, input it into IR compensation---testE(V) in the control item, then click Test, obtain the value in Resistence(ohm) on the left, then multiply it by 0.8 (or 0.75, 0.8 is used in this experiment), input it into Resistence(ohm) on the right, check the two items Always and Manual, and also check in front of IR compensation for Next Run. Every time the test item is changed, come back to check in front of IR compensation for Next Run. This will also be mentioned later.

[0071] (2) CV scan in this experiment: Enter the T option, select the Cyclic Votammetry Parameters item. Under alkaline conditions (mercuric oxide electrode): Set the initial potential of the voltage scan range to -1V, the upper limit potential to 0.2V, the lower limit potential to -1V, and the end potential to 0.2V. (Under acidic conditions, a calomel electrode is used: the initial potential of the voltage scan range is set to -0.2V, the upper limit potential to 1V, the lower limit potential to 1V, and the end potential to -0.2V. Alkaline conditions are used in this experiment.) Select the sweepsegments (number of scan segments) item and set it to 200 segments. Set the scan rates to 10mV / s, 20mV / s, 30mV / s, 50mV / s, and 100mV / s respectively for testing. For the Sensitivity item, select 1.e-004 (or 003, 002, but not 005, 006. Sensitivity of 1.e-004 is used in this experiment). After setting the experimental parameters, click Start to conduct the test. Wait until the CV images of two cycles roughly overlap, which is the stable state, then stop the scan. Change to different scan speeds and scan again.

[0072] 1.4.3 Linear Sweep Voltammetry (LSV)

[0073] (1) This is a relatively basic and widely used voltammetric testing technique. An electrolytic cell is composed of a classical three-electrode working system. By electrolyzing a dilute solution of the analyte, analysis is carried out based on the obtained current-potential curve.

[0074] (2) LSV scan in this experiment: After connecting the three - electrode system and stabilizing through cyclic voltammetry scanning, enter the T item, select the linear sweep voltammetry item. Under alkaline conditions (mercuric oxide electrode): The initial potential of the voltage scanning range for the oxygen evolution reaction is set to 0 V, and the lower limit potential is set to 1.4 V. Under acidic conditions (calomel electrode): The initial potential of the voltage scanning range for the oxygen evolution reaction is set to 0.8 V, and the lower limit potential is set to 1.6 V. This experiment uses alkaline conditions. The scanning rates are set to 10 mV / s, 20 mV / s, 30 mV / s, 50 mV / s, and 100 mV / s respectively for testing. For the Sensitivity item, select 1.e - 003 (or 002, but not 005, 006. This experiment uses a sensitivity of 1.e - 003). Check the box after the IR compensation for Next Run item before clicking start. Set the experimental parameters and click start to conduct the test. After waiting for the LSV images of the first and second scans to roughly overlap, stop the scan, change to a different scanning speed, and scan again. The initial potential of the voltage scanning range for the hydrogen evolution reaction is set to - 0.8 V, and the lower limit potential is set to - 1.6 V. Under acidic conditions (calomel electrode): The initial potential of the voltage scanning range for the hydrogen evolution reaction is set to 0 V, and the lower limit potential is set to 1 V. This experiment uses alkaline conditions. The scanning rates are set to 10 mV / s, 20 mV / s, 30 mV / s, 50 mV / s, and 100 mV / s respectively for testing. For the Sensitivity item, select 1.e - 003 (or 002, but not 005, 006. This experiment uses a sensitivity of 1.e - 003). Check the box after the IR compensation for Next Run item before clicking start. Set the experimental parameters and click start to conduct the test. After waiting for the LSV images of the first and second scans to roughly overlap, stop the scan, change to a different scanning speed, and scan again.

[0075] 1.4.4 AC Impedance Method (AC Impedance)

[0076] (1) The AC impedance method is also known as Electrochemical Impedance Spectroscopy (EIS): This is to measure the three - electrode system using perturbation signals of various frequencies and not - too - large amplitudes, and obtain the variation relationship between the electrochemical impedance and the frequency of the perturbation signal from it.

[0077] (2) EIS test in this experiment: After connecting the three-electrode system and stabilizing through cyclic voltammetry scanning, enter item T, select A.C. Impedance, set the initial voltage to -0.6V, the low frequency to 0.01Hz, the high frequency to 100000Hz, and the Amplitude to 5mV. After setting the experimental parameters, click Start, wait for the test to end, and save the data.

[0078] 2 Results and Analysis

[0079] 2.1 Characterization Results and Analysis

[0080] 2.1.1 X-ray Diffraction of M-MOFs (M = Fe, FeO) Materials

[0081] In the present invention Figure 1 The XRD patterns of M-MOFs (M = Fe, FeO) materials before and after calcination are shown. This figure is obtained by analyzing the data measured by XRD using Jade software and plotting with Origin. It can be seen from the figure that the precursor does not have a crystalline structure, while the calcined M-MOFs (M = Fe, FeO) materials have a good crystalline structure; the characteristic peak at the crystal plane (110) is relatively sharp and coincides with the peak of Fe, proving that the sample contains Fe; there are no other impurity peaks in the figure, only the characteristic peaks of Fe and FeO. The characteristic peaks at the crystal planes (111), (200), (220), (311), and (222) are relatively sharp and coincide with the peak of FeO, proving that the sample contains FeO. This indicates that Fe and FeO have been incorporated into MOFs after calcination.

[0082] 2.1.2 Scanning Electron Microscope

[0083] Below Figure 2 In (a) and (b) below are the SEM images of the precursor. It can be seen that the precursor has a two-dimensional layered structure and has a very small particle diameter of about 20nm, which is very evenly distributed and highly consistent, indicating that iron is evenly distributed on the carbon layer. Below Figure 2 (c) and (d) below are the SEM images of M-MOFs (M = Fe, FeO) materials. It can be seen that the carbon particles evenly wrap the Fe and FeO particles, and the wrapped particles evenly exhibit an "ant nest type" distribution, and it has a three-dimensional porous spherical composite three-dimensional structure. This three-dimensional structure gives it a high porosity, which can increase the contact area between the catalytic material and the electrolyte interface, thereby improving the catalytic performance. The reason why the catalytic performance of M-MOFs (M = Fe, FeO) materials is higher than that of the precursor is partly due to the fact that its three-dimensional structure is superior to the two-dimensional structure.

[0084] 2.2 Electrochemical Performance Test Results and Analysis

[0085] 2.2.1 Cyclic Voltammetry Scanning Curves

[0086] Figure 3 The CV diagrams before and after calcination of M-MOFs (M = Fe, FeO) materials are shown. This figure is obtained by analyzing the data measured by chi760e using Excel software and plotting with Origin. These figures are the cyclic voltammetry curves of the precursor and M-MOFs (M = Fe, FeO) materials at a scan rate of 10 mV / s. It can be seen from the figure that the precursor has no obvious redox peaks, and it can be speculated that the catalytic performance of the precursor is not very good, which will be further explained in the LSV; for the M-MOFs (M = Fe, FeO) materials obtained by calcination, it can be clearly seen that there is a pair of redox peaks, indicating the existence of a reversible process, proving that this electrode is a typical redox electrode (also known as a Faraday electrode). The redox peaks in the M-MOFs (M = Fe, FeO) materials are very wide, and it can be speculated that the encapsulated structure of the M-MOFs (M = Fe, FeO) materials results in relatively strong interfacial charge transfer at the interface between the active substance and the electrolyte, thus obtaining a wider redox peak. This further illustrates the feasibility of M-MOFs (M = Fe, FeO) materials as high-performance catalytic materials.

[0087] 2.2.2 Linear Sweep Voltammetry Curves and Tafel Curves under Hydrogen Evolution

[0088] Figure 4 (a) and (b) show the LSV diagrams of M-MOFs (M = Fe, FeO) materials before and after calcination. This figure is obtained by analyzing the data measured by CHI-760E using Excel software and plotting with Origin. Figure 4 (a) and (b) are both linear sweep voltammetry curves under hydrogen evolution reactions at different scan rates. It can be seen from the figure that at a scan rate of 10 mV / s, both the precursor and M-MOFs (M = Fe, FeO) materials have the best electrocatalytic hydrogen evolution performance. When the current density is 10 mA cm -2 , the overpotential of the precursor is 541.65 mV, and the overpotential of the M-MOFs (M = Fe, FeO) materials is only 147.6 mV, proving that the M-MOFs (M = Fe, FeO) materials obtained after calcination have more excellent electrocatalytic hydrogen evolution performance than the precursor. Its excellent HER performance is not only because the material itself has strong conductivity, which improves the charge transfer rate between the electrolyte and the working electrode, but also because of the synergistic effect between Fe and FeO and the unique chemical structure of this catalyst.

[0089] Figure 5The Tafel curves of M-MOFs (M = Fe, FeO) materials before and after calcination are shown. This figure is obtained by analyzing the data measured by CHI-760E using Excel software and plotting with Origin. The Tafel slope is an important evaluation criterion in the kinetics of electrocatalytic hydrogen evolution. It can be seen from the figure that the M-MOFs (M = Fe, FeO) materials obtained by calcination exhibit smaller Tafel slopes, which are 177.5 mV / dec and 191.0 mV / dec respectively. This result further proves that the HER process of M-MOFs (M = Fe, FeO) materials proceeds fastest, indicating that the M-MOFs (M = Fe, FeO) materials obtained by calcination have higher electrocatalytic performance.

[0090] Figure 6 (a) and (b) show the LSV diagrams of M-MOFs (M = Fe, FeO) materials before and after calcination. This figure is obtained by analyzing the data measured by CHI-760E using Excel software and plotting with Origin. Figure 6 (a) and (b) are both linear voltammetry scanning curves under oxygen evolution reactions at different scan rates. It can be seen from the figure that at a scan rate of 100 mV / s, both the precursor and M-MOFs (M = Fe, FeO) materials have the best electrocatalytic oxygen evolution performance. When the current density is 10 mA cm -2 , the overpotential of the precursor is 2.555 V, and the overpotential of the M-MOFs (M = Fe, FeO) materials is 2.403 V. Although the overpotentials of both are relatively large, the oxygen evolution performance of the M-MOFs (M = Fe, FeO) materials after calcination is still better than that of the precursor, which further shows that the materials obtained by calcination are more excellent.

[0091] 2.2.3 AC impedance analysis

[0092] The following Figure 7 describes the electrochemical impedance spectra of the precursor and M-MOFs (M = Fe, FeO) materials. It can be concluded from this that they have a very small semicircle in the high-frequency range. The M-MOFs (M = Fe, FeO) materials have lower electrochemical impedance, indicating that the active substances have a high mass transfer rate. This is because the encapsulated Fe and FeO particles increase the interfacial contact area, thereby increasing the active specific surface area. The extremely small AC impedance of the M-MOFs (M = Fe, FeO) materials further shows that it is suitable for the field of catalytic hydrogen production.

[0093] 3 Conclusions and discussions

[0094] Metal-organic framework complexes have many advantages such as low cost, environmental friendliness, high specific surface area, and high porosity. The main advantages of using iron salts and humic acid as the basic raw materials in this experiment are as follows: Economically, they are simple to manufacture, inexpensive, easily accessible, and widely used in many fields such as industry, agriculture, and the medical industry; Structurally, in terms of the carbon structure, they have a regular pore structure and certain adsorption and complexation abilities, which enable them to attach to or encapsulate metal particles. As a transition metal, iron has a strong electron transfer ability due to its own structure and is suitable for catalytic materials; In terms of size tunability, by adjusting the molecular weight of humic acid, humate particles with a size of several hundred nanometers can be obtained, providing the possibility to form carbon skeletons of different sizes; In terms of hydrophilicity, they contain hydrophilic functional groups (such as carboxyl groups, phenolic hydroxyl groups, aromatic rings and other active groups), and have metal ion complexation, ion adsorption, redox and physiological activities, etc., which can improve the performance of composite with metal materials.

[0095] In this experiment, the precursor was first obtained by the solvothermal method, and the M-MOFs (M = Fe, FeO) material was obtained after calcination. Its morphology and structure were characterized by X-ray diffraction technology and scanning electron microscopy, which proved that it contained Fe and FeO and had a wrapped structure; Finally, its electrochemical performance was tested, the cyclic voltammogram was analyzed, and its reversibility was determined. After analyzing the hydrogen evolution reaction, the feasibility and high efficiency of its electrocatalytic hydrogen production were proved; Its electrochemical impedance spectrum shows that it has a high mass transfer rate. Generally speaking, the unique structure of the M-MOFs (M = Fe, FeO) nanomaterial makes its charge transfer ability suitable for the electrocatalytic hydrogen production process, which can meet the needs of industrial development and thus solve the problem of shortage of hydrogen energy.

[0096] In this specification, each embodiment is described in a progressive manner. The key point of each embodiment is to illustrate the differences from other embodiments. For the same or similar parts among the embodiments, reference can be made to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and reference can be made to the description in the method part for the relevant parts.

[0097] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing M-MOF nanomaterials with electrocatalytic activity, It is characterized in that The following steps are involved: Step S1, preparation of precursor: weigh a certain amount of NaOH or KOH and dissolve it in distilled water, stir it thoroughly and heat it to 95°C, then add a certain amount of FeCl 3 aqueous solution and fulvic acid aqueous solution, then continuously aged at 95°C for a certain period of time, cooled to room temperature, centrifuged and washed with distilled water and anhydrous ethanol for multiple times, and then placed in an oven for drying to obtain a precursor; Step S2, calcination: First, the precursor obtained in step S1 is fully ground to obtain a uniform powder, and then calcined in a tube furnace N 2 Firing in atmosphere; Step S3, preparation of the final product: After the precursor calcined in step S2 is cooled to room temperature, it is centrifugally washed multiple times with distilled water and anhydrous ethanol, and finally placed in an oven for drying to obtain a black powdery solid product, which is labeled as M-MOFs.

2. A method for preparing an M-MOF nanomaterial with electrocatalytic activity according to claim 1, It is characterized in that The aging time in step S1 is 4 hours.

3. A method for preparing an M-MOF nanomaterial with electrocatalytic activity according to claim 1, It is characterized in that The drying temperature in the oven is 60°C.

4. A method for preparing an M-MOF nanomaterial with electrocatalytic activity according to claim 1, It is characterized in that The conditions of the calcination procedure in step S2 are: a heating rate of 5°C / min, heating to 600°C and maintaining for 4 hours.

5. A method for preparing an M-MOF nanomaterial with electrocatalytic activity according to claim 1, It is characterized in that The M=Fe, FeO.

6. A method for preparing an M-MOF nanomaterial with electrocatalytic activity according to claim 1, It is characterized in that The precursor is a black solid with metallic luster.

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