Bifunctional heterostructure catalyst, preparation method and application of bifunctional heterostructure catalyst in electrolyzed water

Abstract

The invention belongs to the technical field of electrolyzed water catalysts, and discloses a bifunctional heterojunction catalyst, a preparation method and application of the bifunctional heterojunction catalyst in electrolyzed water, the catalyst is characterized in that NiMoO4 nanosheets which grow on a foamed nickel conductive substrate in situ and have a clear heterogeneous interface structure coat Co3O4 nanowires to form a Co3O4 nanowire-coated NiMoO4 nanosheet array which is recorded as NiMoO4-coated Co3O4 / NF catalyst, and the NiMoO4-coated Co3O4 / NF catalyst is used for preparing the catalyst. The overall appearance of the catalyst is in the shape of a porous and wrinkled paper embroidered ball, and the heterogeneous interface structure of the catalyst enables the catalyst to have excellent OER / HER bifunctional catalytic activity under the alkaline condition. Through cooperation of a heterogeneous interface and a three-dimensional porous structure, the efficient hydrogen evolution and oxygen evolution activity is achieved in the water electrolysis process; the assembled full-battery electrolyzed water system can be subjected to overall water decomposition under low voltage, the performance is almost not attenuated under long-time operation, conversion and utilization of resources and energy are achieved, and the application prospect is wide.

Description

Technical Field

[0001] The present invention relates to the technical field of water electrolysis catalysts, and in particular to a bifunctional heterogeneous structure catalyst, a preparation method and application thereof in water electrolysis. Background Art

[0002] Since traditional fossil energy can no longer meet current consumption needs, efficient and clean new energy sources such as solar energy, wind energy, tidal energy, and hydrogen energy provide humans with better choices. Among them, hydrogen energy has been vigorously developed due to its advantages of being pollution-free and high in heat. Hydrogen production by water electrolysis is very promising among the many hydrogen production technologies currently available, and can produce high-purity, zero-emission green hydrogen. Water electrolysis is divided into two and a half reactions, namely the cathode hydrogen evolution reaction (HER) and the anode oxygen evolution reaction (OER). However, due to the limitations of the slow kinetics of HER and OER themselves, it is necessary to develop cost-effective and stable electrocatalysts that can efficiently and continuously convert water molecules into high-purity hydrogen and oxygen. In the prior art, given the differences in the catalytic mechanisms of HER and OER, different catalysts need to be designed for the two reactions to minimize the overpotential of water electrolysis. Traditionally, commercial HER catalysts are Pt / C, and OER catalysts are RuO2 and IrO2, both of which involve expensive and less stable precious metals. Therefore, water electrolysis technology currently still faces two key challenges: high overpotential caused by low electrode reaction activity and insufficient stability of electrode materials. In particular, the high cost and limited natural reserves of existing noble metal catalysts restrict their large-scale commercial application.

[0003] Therefore, the development of efficient, stable and low-cost non-precious metal-based electrocatalysts has become a current research hotspot, aiming to reduce the electrode overpotential to enhance the catalytic performance. However, there are significant differences in the optimal catalytic environments of OER and HER. Neutral and alkaline electrolytes are beneficial to OER, while acidic electrolytes are beneficial to HER. This mismatch in catalytic environment may affect the operating efficiency of the entire water electrolysis system, and existing catalysts are difficult to match the two catalytic environments to achieve dual-functional electrocatalysis.

[0004] In summary, the existing electrolytic water catalysts have problems such as high cost, low catalytic activity, and poor stability, which cannot meet the needs of industrial applications. In the process of designing and synthesizing bifunctional, high-performance electrocatalysts, there are many factors affecting the product components, ratios, microstructures, and preparation processes. In particular, it is necessary to make a comprehensive choice of multiple factors to construct a unique and efficient microstructure, that is, to overcome the constraints of catalytic environment mismatch through the coordination of multiple aspects; due to the many technical difficulties, the existing technology cannot solve these problems and synthesize bifunctional, high-performance electrocatalysts.

[0005] Therefore, adopting new technical concepts and overcoming problems such as catalytic environment mismatch from multiple aspects, designing and synthesizing OER / HER bifunctional electrocatalysts with high cost-effectiveness, high activity and stability, and enhancing bifunctional catalytic activity are of great significance for improving the overall efficiency of water electrolysis systems, reducing costs, meeting the needs of large-scale industrial applications, and promoting the development of the hydrogen energy economy. Summary of the invention

[0006] The purpose of the present invention is to provide a bifunctional heterostructure catalyst, a preparation method and its application in water electrolysis, aiming at the problems that the bifunctional catalyst for water electrolysis in the prior art must use precious metals and cannot overcome the mismatch of the catalytic environment, adopt a new technical concept, overcome the problems of catalytic environment mismatch from multiple aspects, design and synthesize an OER / HER bifunctional heterostructure catalyst with high cost performance, high activity and stability, and through the simultaneous improvement of components, proportions and processes, design and synthesize a unique microstructure and heterogeneous interface, a porous and wrinkled paper hydrangea-shaped NiMoO4@Co3O4 / NF with high cost performance, high activity and stability, and the catalyst has better catalytic activity and stability under alkaline conditions, improves the overall efficiency of the water electrolysis system, reduces costs, and can meet the needs of large-scale industrial applications.

[0007] In order to achieve the above object, the technical solution provided by the present invention is:

[0008] A bifunctional heterostructure catalyst is a NiMoO4 nanosheet-coated Co3O4 nanowire in situ grown on a nickel foam conductive substrate and having a clear heterogeneous interface structure to form a Co3O4 nanowire@NiMoO4 nanosheet array, which is denoted as a NiMoO4@Co3O4 / NF catalyst. The overall appearance of the catalyst is a porous, wrinkled paper hydrangea shape, and the heterogeneous interface structure enables the catalyst to have excellent OER / HER bifunctional catalytic activity under alkaline conditions; the catalyst is a nanocomposite material with a core-shell structure constructed by using a three-dimensional self-supporting porous conductive substrate nickel foam as a core and a transition metal oxide NiMoO4 nanosheet-coated Co3O4 nanowire layer as a shell; the Co3O4 nanowire layer on the conductive substrate has a typical nanowire morphology, constitutes an inner layer, is about 2μm long, and has a diameter of 50-100nm; the transition metal oxide NiMoO4 nanosheet layer has a typical nanosheet morphology, constitutes an outer layer, has an average diameter of 100-200nm, and a thickness of 15-25nm.

[0009] A preparation method of a bifunctional heterostructure catalyst, which uses nickel foam as a conductive substrate, generates a Co3O4 nanowire-loaded NiMoO4 nanosheet shell layer in situ on the nickel foam conductive substrate through a two-step hydrothermal method and subsequent heat treatment, and forms a heterogeneous interface structure of two transition metal oxides in the three-dimensional space of the conductive substrate. The overall shape is a porous paper hydrangea, which is tightly interwoven into a large spherical body by a plurality of small units, and the outer surface is covered with nano-scale irregular mesh bumps, pores and wrinkles, so as to prepare an OER / HER bifunctional heterostructure catalyst, which comprises the following steps:

[0010] S1: Preparation of nickel foam conductive substrate: pre-treating the nickel foam sheet to remove surface oxides to prepare the nickel foam conductive substrate;

[0011] The nickel foam sheet was immersed in 1.0M hydrochloric acid, ethanol and deionized water for ultrasonic treatment for 10 minutes, and the obtained nickel foam was washed with ethanol and water for 3-5 times respectively to serve as a nickel foam conductive substrate;

[0012] S2: Preparation of cobalt@nickel-molybdenum core-shell structure precursor: Take appropriate amounts of cobalt salt, urea and ammonium fluoride and dissolve them in water, ultrasonicate for 5-10 minutes to obtain a pink transparent aqueous solution A, immerse the nickel foam sheet in the solution and transfer it to a hydrothermal reactor for the first hydrothermal reaction to obtain a nickel foam product Co-OH@NF loaded with a cobalt precursor; wash the product several times and immerse it in a green transparent aqueous solution B containing nickel salt, molybdenum salt and urea, and transfer the whole to a reactor for a second hydrothermal reaction; after the reaction is completed, cool it to room temperature, take it out, wash it and dry it to obtain a nickel foam loaded with a cobalt@nickel-molybdenum precursor;

[0013] S3: Synthesis of catalyst: The nickel foam loaded with cobalt@nickel-molybdenum precursor was placed in a tubular furnace for calcination, the temperature was raised to 350°C and kept warm for one hour, and then cooled to room temperature to obtain the NiMoO4@Co3O4 / NF catalyst.

[0014] An application of the bifunctional heterogeneous structure catalyst in water electrolysis, the catalyst has dual catalytic functions of catalyzing hydrogen evolution reaction HER and oxygen evolution reaction OER under alkaline conditions, and can be assembled into a full battery system as anode and cathode for overall water decomposition. The catalyst catalyzes hydrogen evolution reaction under alkaline conditions to reach 10mAcm -2 The overpotential required for the current density is 121 mV, and the catalytic oxygen evolution reaction reaches 100 mA cm under alkaline conditions. -2 The overpotential required for the current density is 202mV. When the catalyst was used as a cathode and cathode in an electrolytic water splitting system under the same alkaline electrolyte for performance testing, it reached 10 and 100mAcm -2 The voltage required for the current density is 1.51V and 1.73V, and it can run stably for more than 120 hours.

[0015] Compared with the prior art, the present invention has at least the following beneficial effects:

[0016] 1. The bifunctional heterostructure catalyst, preparation method and application thereof provided by the present invention adopt a new technical concept, and through the simultaneous improvement of components, proportions and processes, a cheap and efficient heterostructure catalyst of Co3O4 nanowires loaded with NiMoO4 nanosheets is grown in situ using nickel foam with a three-dimensional porous structure as a conductive substrate. The overall design and synthesis of the OER / HER electrocatalyst NiMoO4@Co3O4 / NF with a unique overall appearance of a porous paper hydrangea-like microstructure and high cost performance, high activity and stability solves the problem that the bifunctional catalyst cannot overcome the mismatch of the catalytic environment, so that the catalyst exhibits excellent OER / HER catalytic activity and high stability in the alkaline water electrolysis system; the application of the catalyst in the OER and HER processes of catalytic water electrolysis can simplify the structure of the water electrolysis system as a whole, improve the overall efficiency of the water electrolysis system, and reduce the cost, and can promote the development of efficient and economical water electrolysis technology and promote the construction of a green hydrogen energy economy. Show broad potential and application prospects.

[0017] 2. The bifunctional heterostructure catalyst successfully synthesized by the present invention uses a three-dimensional porous self-supporting conductive material nickel foam as a substrate, and constructs a typical self-supporting, binder-free catalyst through in-situ growth, which helps to improve the catalyst's strong mechanical strength and provide more active sites; this three-dimensional porous structure can more completely contact the electrolyte, so that hydrogen and oxygen bubbles can be quickly adsorbed and dissociated on the cobalt@nickel-molybdenum surface, promoting mass transfer and product release, so that the catalyst exhibits excellent electrocatalytic performance, and simultaneously improves the catalyst's catalytic activity, stability and cost-effectiveness, solving the problems of high cost, low catalytic activity, low stability, etc. of the bifunctional catalyst for water electrolysis in the prior art.

[0018] 3. Compared with independent metal oxide catalysts, the bifunctional heterostructure catalyst provided by the present invention has a clear interface formed between the outer NiMoO4 nanosheets and the inner Co3O4 nanowires due to the coupling between the two. The synergistic effect between the interfaces enables it to accelerate the electron transfer rate and regulate the electron transfer path. At the same time, the heterostructure characteristics at the interface can provide more active sites and help optimize the adsorption and dissociation of water molecules by the active sites through the interface effect, thereby improving the alkaline water electrolysis performance of the NiMoO4@Co3O4 / NF catalyst.

[0019] 4. The preparation method provided by the present invention uses nickel foam as the conductive substrate. The hydrothermal method and subsequent calcination method adopted are compact, simple to operate, highly efficient and environmentally friendly. The core is to utilize the interfacial synergistic effect of NiMoO4 nanosheets and inner-layer Co3O4 nanowires to achieve excellent hydrogen and oxygen evolution performance. The low-cost bifunctional water electrolysis catalyst prepared has better catalytic activity and stability than commercial Pt / C and RuO2, and can be suitable for industrial applications to realize the conversion and utilization of resources.

[0020] 5. The bifunctional heterostructure catalyst provided by the present invention has strong stability, can efficiently precipitate hydrogen and oxygen in alkaline electrolytes, and can simultaneously serve as a cathode and anode catalysts in the same electrolyte to construct a full battery system for overall water decomposition, and the performance is almost not attenuated after long-term operation. The core-shell heterostructure bifunctional electrocatalyst synthesized by the present invention has excellent alkaline OER / HER catalytic activity, which is of great significance for the further development and assembly of efficient and stable bifunctional electrocatalytic water electrolysis systems.

[0021] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific implementation modes. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 (a) SEM image and (b) TEM image of NiMoO4@Co3O4 / NF prepared in Example 1 of the present invention.

[0023] Figure 2 (a) High-resolution transmission electron microscopy image and (b) selected area electron diffraction image of NiMoO4@Co3O4 / NF prepared in Example 1 of the present invention.

[0024] Figure 3 (a) Linear sweep voltammetry test diagram of hydrogen evolution reaction (HER) of NiMoO4@Co3O4 / NF prepared in Example 1 of the present invention and Comparative Example 1 (Co3O4 / NF), Comparative Example 2 (NiMoO4 / NF) and Comparative Example 3 (NiMoO4@Co NWs / NF) under 1.0 M KOH alkaline conditions and (b) the corresponding Tafel slope.

[0025] Figure 4 (a) Linear sweep voltammetry test diagram of oxygen evolution reaction (OER) of NiMoO4@Co3O4 / NF prepared in Example 1 of the present invention and Comparative Example 1 (Co3O4 / NF), Comparative Example 2 (NiMoO4 / NF) and Comparative Example 3 (NiMoO4@Co NWs / NF) under 1.0 M KOH alkaline conditions and (b) the corresponding Tafel slope.

[0026] Figure 5(a) Full water splitting polarization curve diagram and (b) stability test diagram of the water electrolysis device assembled with NiMoO4@Co3O4 / NF prepared in Example 1 of the present invention as both anode and cathode. DETAILED DESCRIPTION

[0027] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments so that those skilled in the art can implement the invention with reference to the description.

[0028] It should be understood that the terms such as “having”, “including” and “comprising” used herein do not exclude the existence or addition of one or more other elements or combinations thereof.

[0029] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0030] Basic Example

[0031] The present embodiment provides a bifunctional heterostructure catalyst, which is a NiMoO4 nanosheet-coated Co3O4 nanowire in situ grown on a nickel foam conductive substrate and has a clear heterogeneous interface structure to form a Co3O4 nanowire@NiMoO4 nanosheet array, denoted as a NiMoO4@Co3O4 / NF catalyst, and its overall appearance is a porous, wrinkled paper hydrangea shape, and its heterogeneous interface structure enables the catalyst to have excellent OER / HER bifunctional catalytic activity under alkaline conditions; the catalyst is a nanocomposite material with a core-shell structure constructed with a three-dimensional self-supporting porous conductive substrate nickel foam as a core and a transition metal oxide NiMoO4 nanosheet-coated Co3O4 nanowire layer as a shell; the Co3O4 nanowire layer on the conductive substrate has a typical nanowire morphology, constituting an inner layer, with a length of about 2 μm and a diameter of 50-100 nm; the transition metal oxide NiMoO4 nanosheet layer has a typical nanosheet morphology, constituting an outer layer, with an average diameter of 100-200 nm and a thickness of 15-25 nm.

[0032] A preparation method of a bifunctional heterostructure catalyst, which uses nickel foam as a conductive substrate, generates a Co3O4 nanowire-loaded NiMoO4 nanosheet shell layer in situ on the nickel foam conductive substrate through a two-step hydrothermal method and subsequent heat treatment, and forms a heterogeneous interface structure of two transition metal oxides in the three-dimensional space of the conductive substrate. The overall shape is a porous paper hydrangea, which is tightly interwoven into a large spherical body by a plurality of small units, and the outer surface is covered with nano-scale irregular mesh bumps, pores and wrinkles, so as to prepare an OER / HER bifunctional heterostructure catalyst, which comprises the following steps:

[0033] S1: Preparation of nickel foam conductive substrate

[0034] Pre-treating the nickel foam sheet to remove surface oxides to serve as a nickel foam conductive substrate;

[0035] Specifically, the nickel foam sheet is immersed in 1.0M hydrochloric acid, ethanol and deionized water for ultrasonic treatment for 10 minutes, and the obtained nickel foam is then washed with ethanol and water for 3-5 times respectively to serve as a nickel foam conductive substrate;

[0036] S2: Preparation of cobalt@nickel-molybdenum core-shell structure precursor: Take appropriate amounts of cobalt salt, urea and ammonium fluoride and dissolve them in water, ultrasonicate for 5-10 minutes to obtain a pink transparent aqueous solution A, immerse the nickel foam sheet in the solution and transfer it to a hydrothermal reactor for the first hydrothermal reaction to obtain a nickel foam product Co-OH@NF loaded with a cobalt precursor; wash the product several times and immerse it in a green transparent aqueous solution B containing nickel salt, molybdenum salt and urea, and transfer the whole to a reactor for a second hydrothermal reaction; after the reaction is completed, cool it to room temperature, take it out, wash it and dry it to obtain a nickel foam loaded with a cobalt@nickel-molybdenum precursor;

[0037] S3: Synthesis of catalyst: The nickel foam loaded with cobalt@nickel-molybdenum precursor was placed in a tubular furnace for calcination, the temperature was raised to 350°C and kept warm for one hour, and then cooled to room temperature to obtain the NiMoO4@Co3O4 / NF catalyst.

[0038] An application of the bifunctional heterostructure catalyst in water electrolysis, the catalyst has dual catalytic functions of catalyzing hydrogen evolution reaction HER and oxygen evolution reaction OER under alkaline conditions, and can be assembled into a full battery system as anode and cathode respectively for overall water decomposition.

[0039] The heterojunction catalyst for bifunctional water electrolysis provided by the embodiment of the present invention and its preparation method and application have the advantages that the constructed heterostructure catalyst is based on a three-dimensional skeleton nickel foam, and a transition metal oxide Co3O4 nanowire @ NiMoO4 nanosheet array (NiMoO4@Co3O4 / NF) is in situ grown on the substrate surface by a two-step hydrothermal method and subsequent heat treatment. Among them, the interfacial synergy between the Co3O4 nanowire and the NiMoO4 nanosheet can accelerate the electron transfer rate and provide abundant active sites. At the same time, the three-dimensional porous structure of the nickel foam substrate can accelerate the contact between the active components and the electrolyte, and realize the rapid adsorption and dissociation of water molecules on the catalyst surface. The catalyst has efficient hydrogen and oxygen evolution activity in the process of water electrolysis. The assembled full battery can perform overall water decomposition at low voltage, and the performance is almost attenuated under long-term operation, which can realize the conversion and utilization of resources and energy. The water electrolysis system assembled by the catalyst has excellent water electrolysis performance and broad application prospects.

[0040] Example 1

[0041] The bifunctional heterostructure catalyst, preparation method and application provided in this embodiment are refinements based on the aforementioned basic embodiments.

[0042] The bifunctional heterostructure catalyst provided in this embodiment is a catalyst that forms two transition metal oxide layers in situ on a three-dimensional conductive substrate. It uses a simple hydrothermal method and subsequent heat treatment to in situ generate a heterogeneous interface structure and a unique microstructure of Co3O4 nanowires loaded with NiMoO4 nanosheets on a nickel foam substrate. First, a uniform array of Co3O4 nanowires is in situ grown on a three-dimensional conductive substrate, and a uniform array of NiMoO4 nanosheets is in situ grown on the Co3O4 nanowires. A heterogeneous interface is formed between the Co3O4 nanowires and the NiMoO4 nanosheets. The heterogeneous interface structure is a component that significantly enhances the activity of the catalyst.

[0043] The method for preparing the bifunctional heterogeneous structure catalyst specifically comprises the following steps:

[0044] S1: Pre-treatment of nickel foam: Cut into pieces of 2×3 cm 2 The nickel foam sheet is then removed from the surface oxide; the nickel foam sheet is immersed in 1.0M hydrochloric acid, ethanol and deionized water for ultrasonic treatment for 10 minutes, and the obtained nickel foam is then washed with ethanol and water for 3-5 times respectively;

[0045] S2: Preparation of cobalt@nickel-molybdenum precursor: Take appropriate amounts of cobalt salt, urea and ammonium fluoride and dissolve them in water, and ultrasonicate for 5-10 minutes to obtain a pink transparent aqueous solution A;

[0046] The size is 2×3cm 2 The nickel foam was immersed in solution A, and then transferred to a 50 mL hydrothermal reactor for the first hydrothermal reaction, heated to 120° C. and kept warm for 5 hours, and then taken out after cooling to room temperature to obtain a nickel foam loaded with a cobalt precursor;

[0047] The obtained nickel foam loaded with cobalt precursor (precursor of Co nanowires grown in situ on nickel foam) was washed three times with ethanol and water respectively, and dried at 60°C overnight;

[0048] 0.2910 g of nickel nitrate hexahydrate, 0.2420 g of sodium molybdate dihydrate and 0.3001 g of urea were dissolved in 30 mL of water and ultrasonically treated for 5 minutes to obtain a green transparent solution B;

[0049] The washed nickel foam loaded with the cobalt precursor was immersed in the green transparent solution B, and then transferred as a whole to a 50mL hydrothermal reactor, heated to 160°C and kept warm for 2 hours to carry out the second step of hydrothermal reaction to obtain a cobalt@nickel-molybdenum precursor, which was taken out after cooling to room temperature; the cobalt@nickel-molybdenum precursor loaded on the nickel foam was washed 3 times with ethanol and water respectively, and dried at 60°C overnight.

[0050] S3: Preparation of catalyst:

[0051] The dried nickel foam loaded with cobalt@nickel-molybdenum precursor was placed in a tube furnace and calcined at 100 mL min -1 The mixture was heated at 5℃min under a nitrogen flow of -1 The temperature was raised to 350 °C at a heating rate and kept for 1 hour, and then cooled to room temperature to obtain NiMoO4@Co3O4 / NF catalyst.

[0052] The bifunctional heterostructure catalyst prepared by the above method was used to assemble a bifunctional water electrolysis system. It was found that the NiMoO4@Co3O4 / NF catalyst had HER / OER bifunctional catalytic activity, and the hydrogen evolution reaction reached 10 mA cm under alkaline conditions. -2 The overpotential required for the current density is 121 mV, and the oxygen evolution reaction reaches 100 mA cm under alkaline conditions. -2 The overpotential required for the current density is 202 mV.

[0053] The NiMoO4@Co3O4 / NF catalyst has a bifunctional water electrolysis catalytic activity, and as a water electrolysis system assembled with anode and cathode in the same alkaline electrolyte, it can reach 10 and 100 mA cm -2 The voltage required for the current density is 1.51V and 1.73V, and it can operate stably for more than 120 hours.

[0054] The microstructure of the heterogeneous structure NiMoO4@Co3O4 / NF catalyst prepared in this example is shown in the attached figure. Figure 1 , 2 shown.

[0055] From the attached Figure 1From the scanning electron microscope and transmission electron microscope in the figure, it can be seen that the overall appearance of the bifunctional heterostructure catalyst is a porous, wrinkled paper hydrangea. A typical inner Co3O4 nanowire and outer NiMoO4 nanosheet heterojunction structure (nanowire@nanosheet array) coupled with each other is successfully generated on the three-dimensional nickel foam skeleton, and there is a clear interface between the two. Among them, the inner Co3O4 layer has a typical nanowire morphology, about 2μm long and 50-100nm in diameter, and the outer NiMoO4 layer has a typical nanosheet morphology, with an average size of 100-200nm and a thickness of 15-25nm. The array grows evenly on a three-dimensional nickel foam substrate. Attached Figure 2 The high-resolution transmission electron microscopy and selected area electron diffraction patterns in the figure show that the NiMoO4@Co3O4 / NF catalyst synthesized in the example has typical lattice stripes on both sides of the interface, and the inner layer is Co3O4 nanowires and the outer layer is NiMoO4 nanosheets, which further illustrates the formation of a heterogeneous structure.

[0056] The heterogeneous structure NiMoO4@Co3O4 / NF catalyst prepared in this example was tested for its point catalytic performance. All electrochemical tests were carried out on a CHI 760E electrochemical workstation test system, using 1.0 M KOH (pH = 14) as the electrolyte and an electrolytic cell assembled using a standard three-electrode system: the counter electrode and the reference electrode were composed of a graphite rod and a mercury / mercury oxide electrode, and the nanomaterials grown on the surface of the nickel foam were used as active materials. During the test, iR compensation was required for the hydrogen evolution reaction and the oxygen evolution reaction, and the potentials were converted to the reversible hydrogen electrode (RHE) as a reference according to the Nernst equation. The conversion formula for the electrode potential is: E (RHE) =E (Hg / HgO) +(0.059×pH+0.098)V.

[0057] Within the scope of the present invention, specific selections are made in other embodiments, and a similar two-step hydrothermal method and subsequent heat treatment are used to prepare a heterostructure NiMoO4@Co3O4 / NF catalyst with an outer layer of NiMoO4 nanosheets and an inner layer of Co3O4 nanowires on a three-dimensional porous nickel foam skeleton, and it can be used as a bifunctional water electrolysis catalyst under alkaline conditions. This embodiment develops a low-cost, efficient, and stable bifunctional heterostructure catalyst that does not use precious metals, which can achieve overall water decomposition. The bifunctional heterostructure catalyst provided in this embodiment can simultaneously promote the production of oxygen and hydrogen during water electrolysis. This integrated design can improve the efficiency of energy conversion, reduce energy loss, and play a key role in the production and storage of renewable energy. Transition metal oxides have the advantages of abundant reserves, low price, diverse structures, easy preparation, and superior physical and chemical properties, but because they are more suitable for adsorbing and activating oxygen molecules on the surface, and the characteristics of their electronic energy level structure, they often have better OER catalytic performance than HER. Therefore, the electronic structure of transition metal oxides can be regulated by alloying, doping, constructing composites, and hierarchical heterojunctions to increase the generation and precipitation rates of reaction intermediates and thus enhance the bifunctional catalytic activity.

[0058] The bifunctional heterostructure catalyst provided in this embodiment has a heterostructure composed of two or more different materials, which are combined to form a special structure at the interface, and the electron transfer and water adsorption and dissociation capabilities at the interface are usually better than those of a single material. This interfacial effect can effectively enhance the stability and activity of the active site by adjusting the structure and chemical environment of the catalyst surface. At the same time, the transmission and distribution of electrons can be adjusted to help optimize the electronic structure of the catalyst. The energy band structure and electronic state of different materials can also form an optimized pair of electronic energy levels in the heterostructure, which is conducive to promoting the formation and transfer of HER and OER reaction intermediates. In addition, the surface of the heterojunction catalyst usually has more abundant accessible active sites and excellent stability, and these advantages significantly improve its catalytic performance in the electrolysis of water reaction. Therefore, it is of great significance to use transition metal oxides to construct heterojunction electrocatalysts for bifunctional HER / OER in this embodiment.

[0059] Comparative Example 1

[0060] The preparation method of in-situ growth of Co3O4 nanowires on three-dimensional nickel foam is as follows:

[0061] 0.8730 g of cobalt nitrate hexahydrate, 1.5015 g of urea and 0.3704 g of ammonium fluoride were dissolved in 35 mL of water and subjected to ultrasonic treatment for 5 minutes to obtain a pink transparent solution A.

[0062] The size is 2×3cm2 The nickel foam was immersed in solution A, and then transferred as a whole to a 50 mL hydrothermal reactor, heated to 120 ° C and kept warm for 5 hours, and then taken out after cooling to room temperature.

[0063] The obtained Co nanowire precursor grown in situ on nickel foam was washed three times with ethanol and water respectively, and dried at 60° C. overnight.

[0064] The dried cobalt nanowire precursor loaded on nickel foam was placed in a tube furnace at 100 mL min -1 Under a nitrogen flow, at 5 °C min -1 The temperature was raised to 350°C at a heating rate and kept at that temperature for 1 hour, and then cooled to room temperature to obtain the final product Co3O4 / NF catalyst.

[0065] Comparative Example 2

[0066] The preparation method of in-situ growth of NiMoO4 nanosheets on three-dimensional nickel foam is as follows:

[0067] 0.2910 g of nickel nitrate hexahydrate, 0.2420 g of sodium molybdate dihydrate and 0.3001 g of urea were dissolved in 30 mL of water and subjected to ultrasonic treatment for 5 minutes to obtain a green transparent solution B.

[0068] The size is 2×3cm 2 The nickel foam was immersed in solution B, and then transferred as a whole to a 50 mL hydrothermal reactor, heated to 160 ° C and kept warm for 2 hours, and then taken out after cooling to room temperature.

[0069] The obtained nickel-molybdenum precursor supported on nickel foam was washed three times with ethanol and water respectively, and dried at 60° C. overnight.

[0070] The dried nickel-molybdenum precursor loaded on nickel foam was placed in a tube furnace at 100 mL min -1 Under a nitrogen flow, at 5 °C min -1 The temperature was raised to 350°C at a heating rate and kept at that temperature for 1 hour, and then cooled to room temperature to obtain the final product NiMoO4 / NF catalyst.

[0071] Comparative Example 3

[0072] The preparation method of in-situ growth of NiMoO4@Co NWs / NF catalyst on three-dimensional nickel foam is as follows:

[0073] 0.8730 g of cobalt nitrate hexahydrate, 1.5015 g of urea and 0.3704 g of ammonium fluoride were dissolved in 35 mL of water and subjected to ultrasonic treatment for 5 minutes to obtain a pink transparent solution A.

[0074] The size is 2×3cm 2The nickel foam was immersed in solution A, and then transferred as a whole to a 50 mL hydrothermal reactor, heated to 120 ° C and kept warm for 5 hours, and then taken out after cooling to room temperature.

[0075] The obtained Co nanowire precursor grown in situ on nickel foam was washed three times with ethanol and water respectively, and dried at 60° C. overnight.

[0076] 0.2910 g of nickel nitrate hexahydrate, 0.2420 g of sodium molybdate dihydrate and 0.3001 g of urea were dissolved in 30 mL of water and subjected to ultrasonic treatment for 5 minutes to obtain a green transparent solution B.

[0077] The obtained Co nanowire precursor grown in situ on nickel foam was immersed in solution B, and then transferred as a whole to a 50 mL hydrothermal reactor, heated to 160° C. and kept warm for 2 hours, and then taken out after cooling to room temperature.

[0078] The obtained cobalt@nickel-molybdenum precursor loaded on nickel foam was washed three times with ethanol and water respectively, dried at 60°C overnight, and taken out after cooling to room temperature to obtain the final product NiMoO4@Co NWs / NF.

[0079] See attached Figure 3 , which shows the HER electrocatalytic performance of NiMoO4@Co3O4 / NF and comparative samples in alkaline medium. Figure 3 The LSV polarization curve of (a) shows that NiMoO4@Co3O4 / NF has a -2 The overpotential of the current density is 121 mV, which is much lower than that of NiMoO4@Co NWs / NF, NiMoO4 / NF and Co3O4 / NF, indicating the good HER performance of the catalyst. Figure 3 (b) shows the corresponding Tafel curve. The Tafel slope of NiMoO4@Co3O4 / NF is 94.14 mV dec. -1 , indicating that it follows the Volmer-Heyrovsky process. Figure 4 (a) shows the LSV curve of oxygen evolution reaction in alkaline medium, which can reach 100 mA cm at an overpotential of 202 mV. -2 The high current density shows that the catalyst has good OER performance, especially at high current. Figure 4 (b) shows the corresponding Tafel curve, and the Tafel slope of the catalyst is 58.49 mVdec -1 The NiMoO4@Co3O4 / NF catalyst was used as the cathode and anode to construct an overall water splitting system in an alkaline medium. Figure 5 The polarization curves in (a) show that the catalyst can reach 10 and 100 mA cm-2 The voltage required for the current density is 1.51V and 1.73V, and the Figure 5 (b) It shows that it can run stably for more than 120 hours.

[0080] The above embodiment of the present invention focuses on the use of new technical concepts to prepare a heterostructure catalyst for bifunctional water electrolysis without the need for precious metals. The catalyst is based on a three-dimensional porous conductive skeleton nickel foam, and in situ grown transition metal oxide nanocomposite NiMoO4 nanosheets coated with a heterostructure of Co3O4 nanowires (NiMoO4@Co3O4 / NF). The microstructure of the catalyst Co3O4 nanowires and NiMoO4 nanosheets is obvious, the crystallinity is good, the heterogeneous interface is clear, and the synergistic effect between the interfaces enables it to accelerate the electron transmission rate and regulate the electron transfer path. At the same time, the heterostructure at the interface can provide more abundant active sites, so that the catalyst exhibits excellent HER / OER catalytic activity under alkaline conditions, and the overall water decomposition system assembled with the catalyst as the positive and negative poles in an alkaline medium also exhibits the characteristics of low voltage and high stability. The heterojunction catalyst NiMoO4@Co3O4 / NF designed by the present invention for bifunctional water electrolysis is simple to operate, low cost, superior performance, good stability, and realizes full utilization of resources and energy.

[0081] The preparation method provided by the present invention is to composite two transition metal oxides on a three-dimensional conductive substrate to form a heterostructure catalyst through a simple two-step hydrothermal and subsequent heat treatment. The catalyst has excellent HER / OER bifunctional catalytic activity under alkaline conditions, specifically represented by NiMoO4@Co3O4 / NF.

[0082] Electrochemical tests were performed using 1.0 M KOH (pH = 14) as the electrolyte. The heterojunction NiMoO4@Co3O4 / NF catalyst designed in the present invention can be used for HER / OER catalytic activity in bifunctional water electrolysis, and the hydrogen evolution reaction can reach 10 mA cm under alkaline conditions. -2 The overpotential required for the current density is 121 mV, which is better than Co3O4 / NF (175 mV), NiMoO4@Co3O4 / NF (158 mV), and NiMoO4@CoNWs / NF (130 mV). The oxygen evolution reaction can reach 100 mA cm under alkaline conditions. -2 The overpotential required for the current density is 202mV, which is better than Co3O4 / NF (341mV), NiMoO4@Co3O4 / NF (425mV), and NiMoO4@CoNWs / NF (261mV). At the same time, the catalyst is used as a water electrolysis system assembled with anode and cathode in the same alkaline electrolyte, reaching 10 and 100mAcm -2The voltage required for the current density is 1.51V and 1.73V, and it can operate stably for 120 hours, which can be used as an effective alternative to the water electrolysis device assembled with commercial catalysts Pt / C and RuO2.

[0083] In summary, the bifunctional heterostructure catalyst and its preparation method and application provided by the embodiment of the present invention have the advantages that the constructed heterojunction catalyst has a unique microstructure and significantly improves the bifunctional catalytic activity of the catalyst; specifically, a three-dimensional skeleton nickel foam is used as a substrate, and a transition metal oxide Co3O4 nanowire @ NiMoO4 nanosheet array (NiMoO4@Co3O4 / NF) is in situ grown on the substrate surface by a two-step hydrothermal method and subsequent heat treatment. The interfacial synergy between the Co3O4 nanowire and the NiMoO4 nanosheet can accelerate the electron transfer rate and provide abundant active sites. At the same time, the three-dimensional porous structure of the nickel foam substrate can accelerate the contact between the active components and the electrolyte, and realize the rapid adsorption and dissociation of water molecules on the catalyst surface. The catalyst has efficient hydrogen and oxygen evolution activity in the process of water electrolysis. The assembled full battery can perform overall water decomposition at low voltage, and the performance is almost attenuated under long-term operation, which can realize the conversion and utilization of resources and energy; the electrolysis water system assembled by the catalyst has excellent water electrolysis performance and broad application prospects.

[0084] It should be particularly pointed out that within the scope of the components, ratios and process parameters recorded in the present invention, other technical solutions obtained by specific selection can achieve the technical effects of the present invention, so they are no longer listed one by one. At the same time, other catalyst technical solutions obtained by using the components, ratios, preparation methods and applications equivalent to those recorded in the present invention are all included in the protection scope of the present invention.

[0085] In the description of the present invention, the description of the terms "one embodiment", "some embodiments", "specific embodiments", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In the present invention, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0086] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A bifunctional heterostructure catalyst, characterized in that: It is a NiMoO4 nanosheet-coated Co3O4 nanowire grown in situ on a nickel foam conductive substrate with a clear heterogeneous interface structure, forming a Co3O4 nanowire@NiMoO4 nanosheet array, denoted as NiMoO4@Co3O4 / NF catalyst. Its overall appearance is a porous, wrinkled paper hydrangea shape. Its heterogeneous interface structure enables the catalyst to have excellent OER / HER bifunctional catalytic activity under alkaline conditions.

2. The bifunctional heterostructure catalyst according to claim 1, characterized in that: The catalyst is a nanocomposite material with a core-shell structure, which is constructed by using a three-dimensional self-supporting porous conductive substrate nickel foam as a core and a transition metal oxide NiMoO4 nanosheet coated Co3O4 nanowire layer as a shell.

3. The bifunctional heterostructure catalyst according to claim 1, characterized in that: The Co3O4 nanowire layer on the conductive substrate has a typical nanowire morphology, constitutes an inner layer, is about 2 μm long, and has a diameter of 50-100 nm; the transition metal oxide NiMoO4 nanosheet layer has a typical nanosheet morphology, constitutes an outer layer, has an average diameter of 100-200 nm, and a thickness of 15-25 nm.

4. A method for preparing a bifunctional heterostructure catalyst, characterized in that: It uses nickel foam as the conductive substrate, and through a two-step hydrothermal method and subsequent heat treatment, in situ generates Co3O4 nanowire-loaded NiMoO4 nanosheet shells on the nickel foam conductive substrate, forming a heterogeneous interface structure of two transition metal oxides in the three-dimensional space of the conductive substrate. The overall shape is a porous paper hydrangea, which is tightly interwoven into a large spherical body by multiple small units. The outer surface is covered with nano-scale irregular mesh bumps, pores and wrinkles, and an OER / HER bifunctional heterostructure catalyst is prepared.

5. The method for preparing a bifunctional heterostructure catalyst according to claim 4, characterized in that: It includes the following steps: S1: Preparation of nickel foam conductive substrate Pre-treating the nickel foam sheet to remove surface oxides to serve as a nickel foam conductive substrate; The nickel foam sheet was immersed in 1.0M hydrochloric acid, ethanol and deionized water for ultrasonic treatment for 10 minutes, and the obtained nickel foam was washed with ethanol and water for 3-5 times respectively to serve as a nickel foam conductive substrate; S2: Preparation of cobalt@nickel-molybdenum core-shell structure precursor: Take appropriate amounts of cobalt salt, urea and ammonium fluoride and dissolve them in water, ultrasonicate for 5-10 minutes to obtain a pink transparent aqueous solution A, immerse the nickel foam sheet in the solution and transfer it to a hydrothermal reactor for the first hydrothermal reaction to obtain a nickel foam product Co-OH@NF loaded with a cobalt precursor; wash the product several times and immerse it in a green transparent aqueous solution B containing nickel salt, molybdenum salt and urea, and transfer the whole to a reactor for a second hydrothermal reaction; after the reaction is completed, cool it to room temperature, take it out, wash it and dry it to obtain a nickel foam loaded with a cobalt@nickel-molybdenum precursor; S3: Preparation of catalyst: The nickel foam loaded with cobalt@nickel-molybdenum precursor was placed in a tubular furnace for calcination, the temperature was raised to 350°C and kept warm for one hour, and then cooled to room temperature to obtain NiMoO4@Co3O4 / NF catalyst.

6. The method for preparing a bifunctional heterostructure catalyst according to claim 5, characterized in that: Step S1: Preparation of nickel foam conductive substrate, specifically cutting nickel foam sheet into pieces of size 2×3 cm 2 The nickel foam sheet was immersed in 1.0M hydrochloric acid, ethanol and deionized water for ultrasonic treatment for 10 minutes respectively. The obtained nickel foam was then washed with ethanol and water for 3-5 times respectively to remove the surface oxide, thereby serving as the nickel foam conductive substrate.

7. The method for preparing a bifunctional heterostructure catalyst according to claim 5, characterized in that: The pink transparent aqueous solution A of step S2 is obtained by dissolving 0.8730 g of cobalt nitrate hexahydrate, 1.5015 g of urea and 0.3704 g of ammonium fluoride in 35 mL of water and ultrasonically treating for 5 minutes; The green transparent aqueous solution B is specifically prepared by dissolving 0.2910 g of nickel nitrate hexahydrate, 0.2420 g of sodium molybdate dihydrate and 0.3001 g of urea in 30 mL of water and subjecting the solution to ultrasonic treatment for 5 minutes.

8. Use of the bifunctional heterostructure catalyst according to any one of claims 1 to 3 in electrolysis of water, characterized in that: This catalyst has dual catalytic functions of catalyzing the hydrogen evolution reaction HER and the oxygen evolution reaction OER under alkaline conditions, and can be assembled into a full battery system as anode and cathode respectively for overall water decomposition.

9. The use according to claim 8, characterized in that: The catalyst catalyzed the hydrogen evolution reaction under alkaline conditions and reached 10 mA cm -2 The overpotential required for the current density is 121 mV, and the catalytic oxygen evolution reaction reaches 100 mA cm under alkaline conditions. -2 The overpotential required for the current density is 202 mV.

10. The use according to claim 8, characterized in that: When the catalyst was used as a positive and negative electrode in a water electrolysis system assembled under the same alkaline electrolyte for performance testing, it reached 10 and 100 mA cm -2 The voltage required for the current density is 1.51V and 1.73V, and it can run stably for more than 120 hours.

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