Preparation of NiFe2O4 / CoMo LDH composite material and water electrolysis application

Abstract

The invention belongs to a composite heterostructure catalyst NiFe2O4 / CoMo LDH, wherein NiFe2O4 is used as a precursor, and a CoMo LDH nanosheet is grown on the precursor. The method is simple in steps and short in time consumption, and can be applied to oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). When the catalyst is used as an OER catalyst, only 335 mV overpotential is needed when the current density reaches 400 mA cm <-2 >, and the Tafel slope is as low as 68.19 mV dec-1. When the catalyst is used as a catalyst of HER, the current density of 400mA cm <-2 > can be reached only by 330mV overpotential. In addition, on the basis of an HER reaction or an OER reaction, excellent stability is achieved under the conditions of 50 mA cm <-2 > and 100 mA cm <-2 >. When the catalyst is used for a bifunctional catalyst, the volume ratio of O2 to H2 is about 1: 2 and is basically consistent with the theoretical gas production rate, and the Faraday efficiency is calculated to be 99.3%. The preparation method is low in cost, high in electrocatalytic activity, excellent in performance, simple and convenient to operate and good in stability, and has a wide application prospect when being constructed in the field of water electrolysis.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of new energy materials and electrocatalysis, and particularly relates to a preparation method of a NiFe2O4 / CoMo LDH composite material and application thereof in the field of water electrolysis. BACKGROUND

[0002] Since the 21st century, with the rapid growth of population and economy, the demand for energy of human beings is increasing. At present, coal, oil and natural gas are still the main energy sources of energy supply. However, the energy conversion of fuel is facing problems such as environmental pollution, greenhouse effect and energy depletion. Therefore, the development of clean, efficient and renewable energy is of great significance to promote the sustainable development of human society. At present, hydrogen (H2) is one of the most concerned clean energies, which has high energy density, and the only product after combustion is pollution-free water, which makes hydrogen an excellent energy carrier and a candidate for low-carbon energy systems. However, most of the hydrogen is currently produced by steam reforming of fossil fuels, which not only consumes fossil fuels, but also generates by-products that have a certain impact on the environment. The electrochemical water splitting technology provides a clean route for hydrogen production, and is a feasible method to realize the replacement of fossil fuels and realize the hydrogen economy cycle. In the process of water electrolysis, pure oxygen is produced at the anode, and high-purity oxygen has a wide range of applications in industries, medical treatment and aerospace. However, the voltage required to drive water electrolysis in practical application is higher than the theoretical value 1.23V. In order to obtain a lower decomposition voltage, noble metals such as platinum (Pt) and ruthenium-iridium oxides (RuO2, IrO2) are used as electrocatalysts, but the scarcity and poor durability of noble metals seriously hinder the large-scale application of water electrolysis technology. Therefore, it is necessary to develop efficient, low-cost and abundant non-noble metal electrocatalysts to replace noble metal-based catalysts.

[0003] Pure water accounts for only 2.5% of the total water in the world, and large-scale promotion will bring great pressure. Therefore, the availability of pure water resources is limited to maintain long-term electrochemical water splitting. In contrast, seawater accounts for about 96.5% of the total water supply in the world and is considered a huge energy treasure trove. If the abundant seawater is applied to water splitting, it has great potential. However, in the process of seawater splitting, the chemical composition of seawater is relatively complex, and there are also more microorganisms. In the reaction process, it will bury the active sites, causing electrode poisoning and reducing its stability. Chlorine electro-oxidation reaction (CER) will occur on the anode in seawater electrolysis, which will compete with the OER reaction on the anode, posing a major challenge to the water splitting process. In addition, there is less research on dual-function catalysts for seawater electrolysis. Therefore, designing a dual-function catalyst with high efficiency and high stability for electrolyzing seawater has great application potential. SUMMARY

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a NiFe2O4 / CoMo LDH composite material, its preparation method and application. The prepared material has the advantages of excellent catalytic performance, good stability and high Faraday efficiency.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] This invention provides a NiFe2O4 / CoMo LDH composite material, which achieves uniform distribution of NiFe2O4 / CoMo LDH heterostructures on the surface of a carbon felt substrate. The uniform distribution of CoMo LDH nanosheets grown on the NiFe2O4 nanosheets increases the interface of the heterostructure, exposes more active sites, thereby accelerating the reaction kinetics and promoting improved electrocatalytic performance.

[0007] The present invention also provides a method for preparing the NiFe2O4 / CoMo LDH composite material, comprising the following steps:

[0008] (1) Preparation of NiFe2O4: Ferric nitrate nonahydrate, nickel nitrate hexahydrate, urea, and ammonium fluoride were dissolved in deionized water and stirred evenly. The treated carbon felt substrate was then immersed in the above solution and transferred together into a polytetrafluoroethylene reactor. The mixture was heated at 120-150℃ for 6-8 hours to obtain NiFe LDH. After drying at room temperature, the prepared NiFe LDH was calcined at high temperature in a tube furnace to obtain NiFe2O4.

[0009] (2) Preparation of NiFe2O4 / CoMo LDH: Weigh cobalt nitrate hexahydrate and sodium molybdate dihydrate and dissolve them in deionized water. Stir thoroughly and transfer the mixed solution and the NiFe2O4 obtained in step (1) to a high-pressure reactor. Heat at 100-150℃ for 4-6 hours. After naturally cooling to room temperature, rinse with deionized water and dry to obtain NiFe2O4 / CoMo LDH.

[0010] Furthermore, in step (2), the molar ratio of cobalt nitrate hexahydrate and sodium molybdate dihydrate solution is 1:1 to 1:4. A mixed solution with a molar ratio of 1:3 is preferred.

[0011] Furthermore, the carbon substrate is a highly flexible and highly conductive fabric carbon substrate such as carbon felt, carbon cloth, carbon paper, or carbon fiber.

[0012] The present invention also provides the performance of the NiFe2O4 / CoMo LDH composite material as a HER and OER catalyst in seawater.

[0013] The present invention also provides the Faraday efficiency of the NiFe2O4 / CoMo LDH composite material in seawater.

[0014] The present invention also provides the stability of the NiFe2O4 / CoMo LDH composite material in seawater.

[0015] Compared with the prior art, the beneficial effects of the present invention are:

[0016] 1. This invention uses carbon felt as a substrate, grows NiFe₂O₄ as a precursor via hydrothermal calcination, and then grows CoMo LDH via a secondary hydrothermal method, finally constructing a NiFe₂O₄ / CoMo LDH composite electrocatalyst. Its oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) exhibit high catalytic activity, good stability, and high Faradaic efficiency in the field of electrocatalysis. Therefore, this catalyst exhibits excellent OER and HER activities in 1M KOH. When the current density reaches 400 mA cm⁻¹... -2 When NiFe2O4 / CoMo LDH was used as a bifunctional electrode for bulk water / seawater pyrolysis at 400 mA cm⁻¹, -2 At current densities of [specific values ​​missing], the required voltages are 1.868 V and 1.971 V, respectively, which are superior to noble metal-based catalysts represented by Pt / ClRuO2. Furthermore, when used as a bifunctional catalyst in three different electrolytes, the overpotential remained almost unchanged after continuous testing for 50 hours.

[0017] 2. The carbon felt substrate used in this invention is widely used due to its three-dimensional porous structure, which facilitates electron transfer and gas transport, effectively improving the catalytic process. By directly loading active materials onto the carbon felt substrate to prepare the reaction electrode, the material clustering process is weakened while exposing more active sites, further improving the intrinsic activity and structural stability of the catalyst.

[0018] 3. The synergistic effect between the bimetals in this invention enables strong interactions between electrons, accelerates electron transfer, facilitates the distribution of active centers, and maintains good reaction kinetics. Attached Figure Description

[0019] Figure 1 SEM image of NiFe2O4 / CoMo LDH prepared in Example 1.

[0020] Figure 2 Example 1: TEM image of NiFe2O4 / CoMo LDH prepared.

[0021] Figure 3 Example 1: EDS image of NiFe2O4 / CoMo LDH was prepared.

[0022] Figure 4 Example 1: SEM images of NiFe2O4 / Co1Mo1 LDH, NiFe2O4 / Co1Mo2 LDH, NiFe2O4 / Co1Mo3 LDH and NiFe2O4 / Co1Mo4 LDH were prepared.

[0023] Figure 5 Example 1: OER performance diagrams of NiFe2O4 / CoMo LDH with different Co and Mo ratios.

[0024] Figure 6 Example 1: HER performance diagrams of NiFe2O4 / CoMo LDH with different Co and Mo ratios.

[0025] Figure 7 Example 1: OER performance LSV curves of NiFe2O4 / CoMo LDH prepared and control sample.

[0026] Figure 8 Tafel slope diagrams of NiFe2O4 / CoMo LDH and control sample prepared in Example 1.

[0027] Figure 9 Impedance diagrams of the NiFe2O4 / CoMo LDH prepared in Example 1 and the control sample.

[0028] Figure 10 Stability diagram of NiFe2O4 / CoMo LDH prepared in Example 1.

[0029] Figure 11 Example 1: HER performance LSV curves of NiFe2O4 / CoMo LDH prepared and the control sample.

[0030] Figure 12 Tafel slope diagrams of NiFe2O4 / CoMo LDH and control sample prepared in Example 1.

[0031] Figure 13 Impedance diagrams of the NiFe2O4 / CoMo LDH prepared in Example 1 and the control sample.

[0032] Figure 14 Stability diagram of NiFe2O4 / CoMo LDH prepared in Example 1.

[0033] Figure 15 Example 1: Faraday efficiency calculation diagram of NiFe2O4 / CoMo LDH prepared in Example 1. Detailed Implementation

[0034] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and preferred embodiments.

[0035] Example 1: A method for preparing a NiFe2O4 / CoMo LDH composite material, comprising the following steps:

[0036] (1): 1.74 g of ferric nitrate nonahydrate, 0.81 g of nickel nitrate hexahydrate, 0.60 g of urea, and 0.19 g of ammonium fluoride were dissolved in 50 mL of DI and stirred. The mixed solution and pretreated CF were then placed in a polytetrafluoroethylene high-pressure reactor and heated at 120 °C for 6 h to obtain NiFe LDH. Finally, the mixture was washed alternately with DI and ethanol and dried in an oven for 12 h. The NiFeLDH precursor was placed in a ceramic boat and then calcined at high temperature in an air atmosphere in a tube furnace to further oxidize it to NiFe2O4. The calcination temperature was 300 °C, the holding time was 30 min, and the heating and cooling rates were both 5 °C / min. -1 .

[0037] (2): Dissolve 0.29 g of cobalt nitrate hexahydrate and 0.73 g of sodium molybdate dihydrate in 60 mL of DI and stir. After stirring, add the NiFe2O4 prepared above and transfer them together into a 100 mL polytetrafluoroethylene (PTFE) high-pressure reactor, and heat at 140 °C for 4 h. Then wash alternately with DI and ethanol and dry in an oven for 12 h to obtain NiFe2O4 / CoMo LDH. Figures 1-2 The images show SEM and TEM images of the NiFe₂O₄ / CoMo LDH nanosheets. The SEM image reveals that the NiFe₂O₄ / CoMo LDH nanosheet array grows uniformly on the carbon fiber surface, and the nanosheet array remains intact. This indicates that the morphology did not change after calcination; a layer of CoMo LDH nanosheets still surrounds the NiFe₂O₄, and the sample exhibits a nanostructure with abundant heterogeneous interfaces. The TEM image shows that the NiFe₂O₄ / CoMo LDH has a layered heterogeneous structure, with CoMo nanosheets tightly wrapping around the NiFe₂O₄ nanosheets, and the two are mutually supportive and connected. This not only improves the stability of the catalyst but also exposes more active sites, thereby promoting the electrolyte permeation process, facilitating gas diffusion, and further enhancing electrocatalytic performance. Figure 3 The image shows the EDS diagram of NiFe2O4 / CoMo LDH, which reveals the presence and uniform distribution of Ni, Fe, Co, Mo, C, and O elements. Co and Mo elements are mainly distributed on the outside, while Ni and Fe elements are distributed on the inside, indicating that the NiFe2O4 / CoMo LDH heterostructure has been successfully prepared.

[0038] Example 2: By changing the molar ratio of the solution in step 2 of Example 1, while keeping other conditions the same as in Example 1, the molar ratio of cobalt nitrate hexahydrate to sodium molybdate dihydrate was 1:4, resulting in thick and dense CoMo LDH nanosheets.

[0039] Example 3: The molar ratio of cobalt nitrate hexahydrate and sodium molybdate dihydrate in step (2) of Example 1 was changed to 1:3, while other conditions remained the same as in Example 1, to obtain the NiFe2O4 / Co1Mo3 LDH. The results were obtained by comparing SEM images (…). Figure 4 In (a) of Example 1, when NiFe2O4 / Co1Mo1 LDH is used, irregular sheet-like materials are formed on the periphery of NiFe2O4, and there is obvious adhesion between the sheet-like structures, resulting in poor separation. Changing the molar ratio in step (2) of Example 1 to 1:3, the NiFe2O4 / Co1Mo3 LDH is obtained. Figure 4 (b)). CoMo LDH nanosheets grow uniformly and vertically on the NiFe2O4 surface, and there is a clear and complete layered structure between the nanosheets. This was confirmed by LSV testing. Figures 5-6 Different molar ratios affect the material's properties. When the molar ratio of the two solutions is 1:3, the material, whether OER or HER, exhibits performance at 10 or 400 mA cm⁻¹. -2 The overpotential is minimized at the given current density.

[0040] Example 4: The NiFe2O4 / Co1Mo3 LDH composite material prepared in Example 1 was used as a catalyst for the OER reaction. The main test steps are as follows:

[0041] (1) Electrochemical tests were performed using a three-electrode system. In 1M KOH solution, NiFe2O4 / Co1Mo3LDH was used as the working electrode, a platinum sheet as the counter electrode, and a mercury / mercury oxide electrode as the reference electrode.

[0042] (2) Figure 7 The image shows the performance of the OER. Before testing, the catalyst was subjected to 40 cycles of cyclic voltammetry (CV) to better activate it. For linear sweep voltammetry (LSV), the voltage window was -0.1 to 0.7 V, and the scan rate was 5 mV / s. -1 All LSV tests were performed with 95% iR compensation. The LSV of different catalysts was calculated at 10 mA cm⁻¹. -2 The overpotential required for each catalyst at a given current density. A lower overpotential indicates better electrocatalytic performance in the OER reaction.

[0043] (3) The Tafel slope is calculated using the formula η = a + b lgI, combined with the LSV curve through linear transformation. The lower the Tafel slope, the faster the oxygen evolution reaction rate. Figure 8 Tafel slope images for NiFe2O4 / Co1Mo3 LDH and the control sample. From Figure 8 It can be concluded that, compared to the control sample, the NiFe2O4 / Co1Mo3 LDH has the smallest Tafel slope (68.19 mV dec). -1 This indicates that it possesses minimal reaction kinetics.

[0044] (4) Figure 9 Impedance images of NiFe2O4 / Co1Mo3 LDH and a control sample are shown. The impedance was selected with a voltage of 0.2V, a frequency range of 0.01-100kHz, and an amplitude of 0.005V. From Figure 8 The results show that the NiFe2O4 / Co1Mo3 LDH has the lowest resistance, indicating that the structure of NiFe2O4 / Co1Mo3 LDH is conducive to the rapid transfer of electrons / ions.

[0045] (5) Figure 10 Stability testing of NiFe2O4 / Co1Mo3 LDH was conducted. After 50 hours at 50 and 100 mA cm⁻¹, the stability was determined. -2 Under these conditions, the overpotential remains almost unchanged, indicating excellent durability.

[0046] Example 5: The NiFe2O4 / Co1Mo3 LDH composite material prepared in Example 1 was used as a catalyst for the HER reaction. The main test steps are as follows:

[0047] (1) The electrolyte and parameters set in this test are the same as in Example 5.

[0048] (2) Figure 11 The image shows the HER performance of the electrocatalyst. Based on the LSV performance test results, compared to other comparative samples, the NiFe2O4 / Co1Mo3 LDH composite material exhibits the lowest overpotential (η) under both low and high current conditions. 400 =330mV), indicating that it has good HER activity.

[0049] (3) Figure 12 This is the Tafel slope diagram for the catalyst. A smaller Tafel slope indicates a faster reaction kinetic rate. Figure 12 Tafel slope images for NiFe2O4 / Co1Mo3 LDH and the control sample. From Figure 12 From this, it can be concluded that the Tafel slope of NiFe2O4 / Co1Mo3 LDH is the smallest (78.51mV dec). -1This indicates that it possesses minimal reaction kinetics.

[0050] (4) Figure 13 Impedance images of NiFe2O4 / Co1Mo3 LDH and a control sample. From Figure 13 The results show that the NiFe2O4 / Co1Mo3 LDH has the lowest resistance, indicating that the heterostructure of NiFe2O4 / Co1Mo3 LDH has a faster redox reaction rate, which is conducive to rapid electron transfer.

[0051] (5) Figure 14 Stability testing of NiFe2O4 / Co1Mo3 LDH was conducted. It was found that the stability was maintained at 50 and 100 mA cm⁻¹. -2 As time progresses, the curve shows almost no fluctuations, indicating excellent stability.

[0052] Example 6: Using the NiFe2O4 / Co1Mo3 LDH composite material prepared in Example 1 as a bifunctional catalyst, the hydrogen and oxygen generated during the experiment were collected by the downward water displacement method to calculate its Faraday efficiency. The main test steps are as follows:

[0053] Figure 15 For the assembled NiFe2O4 / CoMo LDH electrode pair at 100 mA cm -2 The actual hydrogen and oxygen production is obtained and then the Faraday efficiency is calculated by comparing it with the theoretical hydrogen and oxygen production.

[0054] In summary, a bifunctional catalyst, NiFe₂O₄ / CoMoLDH, for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) was prepared. When used as a catalyst for OER, this material exhibits good performance at 400 mA cm⁻¹. 2 Only a 335mV overpotential is required at this current density. When used in the HER reaction, at 400mA cm⁻¹... 2 At a current density of only 330 mV, an overpotential of only 330 mV is required. Furthermore, as a bifunctional catalyst, the material exhibits an O2 to H2 volume ratio of approximately 1:2, which is consistent with the theoretical gas production, leading to a calculated overall Faraday efficiency of 99.3%.

[0055] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a NiFe2O4 / CoMo LDH composite material, characterized in that: A uniformly distributed array of NiFe2O4 / CoMo LDH nanosheets, with a thickness of approximately 20–30 nm, is grown on a carbon substrate, comprising the following steps: (1) Preparation of NiFe2O4: Nickel nitrate hexahydrate, ferric nitrate nonahydrate, urea, and ammonium fluoride were dissolved separately in deionized water and stirred evenly. The treated carbon felt substrate was then immersed in the above solution and transferred together into a polytetrafluoroethylene reactor. The mixture was heated at 120-150℃ for 6-8 hours to obtain NiFe LDH. After drying at room temperature, the prepared NiFe LDH was calcined at high temperature in a tube furnace to obtain NiFe2O4. (2) Preparation of NiFe2O4 / CoMo LDH: Weigh cobalt nitrate hexahydrate and sodium molybdate dihydrate and dissolve them in deionized water. Stir thoroughly and transfer the mixed solution and the NiFe2O4 obtained in step (1) to a high-pressure reactor. Heat at 100-150℃ for 4-6 hours. After naturally cooling to room temperature, rinse with deionized water and dry to obtain NiFe2O4 / CoMo LDH.

2. The method for preparing the NiFe2O4 / CoMo LDH composite material according to claim 1, characterized in that: In step (1), the mass concentration of the aqueous solution of nickel nitrate hexahydrate is 0.03-0.3 g / mL, the mass concentration of the aqueous solution of ferric nitrate nonahydrate is 0.01-0.05 g / mL, the mass concentration of the aqueous solution of ammonium fluoride is 0.003-0.009 g / mL, and the mass concentration of the aqueous solution of urea is 0.012-0.018 g / mL.

3. The method for preparing the NiFe2O4 / CoMo LDH composite material according to claim 1, characterized in that: The mass concentration of the cobalt nitrate hexahydrate aqueous solution in step (2) is 0.005-0.05 g / mL, and the mass concentration of the sodium molybdate dihydrate aqueous solution is 0.01-0.1 g / mL.

4. The method for preparing the NiFe2O4 / CoMo LDH composite material according to claim 1, characterized in that: The carbon substrate is carbon felt, carbon cloth, carbon paper, or carbon fiber.

5. The application of the NiFe2O4 / CoMo LDH composite material according to claim 1 as an OER catalyst in water electrolysis.

6. The application of the NiFe2O4 / CoMo LDH composite material as described in claim 1 as a HER catalyst in water electrolysis.

7. The application of the NiFe2O4 / CoMo LDH composite material according to claim 1 as a bifunctional catalyst in water electrolysis.

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