CN117187867A - Heterostructure material, preparation method and application thereof - Google Patents

Abstract

The invention discloses a heterostructure material. The heterostructure material uses foam metal as a carrier, and forms a tricobalt tetroxide/iron cobalt phosphide composite material with a heterogeneous interface structure on the surface of the foam metal. The tricobalt tetroxide/iron cobalt phosphide composite material The phosphide composite material has obvious lattice stripes of cobalt tetroxide and iron-cobalt phosphide, and forms a corresponding heterogeneous interface. The heterostructure material has good catalytic performance and stability; the invention also provides a preparation of the heterostructure material Method, the preparation method uses a two-step hydrothermal method and a phosphating method to synthesize the heterostructure material. The raw materials used are low in cost, the equipment required is simple, suitable for large-scale production, and has certain economic feasibility. The invention also discloses an application of the above-mentioned heterostructure material in the field of electrocatalytic urea oxidation.

Description

A kind of heterostructure material, its preparation method and application

Technical field

The invention belongs to the technical field of new materials, and specifically relates to a heterostructure material, its preparation method and application.

Background technique

As an efficient, pollution-free and sustainable new energy, hydrogen energy is regarded as the clean energy with the greatest potential for development in the 21st century. It can help alleviate the energy crisis caused by the depletion of fossil fuels and the environmental crisis caused by the deteriorating ecological environment. With the advancement of the "dual carbon" policy, the market demand for hydrogen energy will explode. Therefore, it is crucial to develop green and efficient hydrogen production methods. Among the many ways to produce hydrogen, electrolysis of water to produce hydrogen is not only clean, environmentally friendly, and sustainable, but also can be coupled with renewable energy sources such as solar energy and wind energy. It is considered the most promising hydrogen production technology.

The traditional electrolytic water system includes the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode. Compared with HER, which transfers two electrons, OER is a complex process of multi-electron-proton coupling. The reaction kinetics is relatively slow, and additional energy is required to overcome the reaction energy barrier and speed up the reaction kinetics. Therefore, OER is one of the main factors in the high energy consumption of water electrolysis. one. There are currently two strategies to reduce energy consumption. One is to continue to develop efficient OER catalysts; the other is to actively seek alternative reactions, such as the oxidation reaction of small organic molecules such as urea, 5-hydroxymethylfurfural, benzyl alcohol and ethanol. to replace OER. In comparison, the above-mentioned small organic molecules are more easily oxidized, which can not only effectively save energy, but also play a role in treating wastewater or converting high value-added chemicals. Therefore, finding suitable alternative responses has received widespread attention in recent years. Among them, the urea oxidation reaction (UOR) under alkaline conditions is a simple and effective alternative reaction. The urea electrolytic cell composed of UOR and HER can reduce the theoretical voltage from 1.23V vs. RHE for water decomposition to 0.37V vs. RHE, can theoretically save ~70% energy. Furthermore, UOR is the core chemical reaction in direct urea fuel cells and urea-rich wastewater purification systems. Therefore, UOR can be used as an alternative reaction to OER in coupled electrolysis water systems to achieve the dual purposes of energy-saving hydrogen production and treatment of urea-containing wastewater.

Although UOR has a lower thermodynamic potential (0.37V vs. RHE), it is still a complex reaction of six electron transfer, which requires the design of highly active catalysts to speed up the reaction rate. At present, researchers have developed a large number of transition metal-based materials for catalyzing UOR, which is attributed to their ability to generate peroxides in situ under oxidative conditions, which can serve as catalytic active sites to enhance performance. Among them, there are relatively few studies on cobalt-based materials as UOR catalysts, especially cobalt-based oxides. The reason may be due to the semiconductor properties of cobalt-based oxides and the poisoning phenomenon during the catalytic process. Therefore, how to improve the conductivity of cobalt-based oxides is a key issue in the development of cobalt-based UOR catalysts; more importantly, improving the anti-toxicity properties of cobalt-based catalysts is an urgent problem that needs to be solved to enhance the UOR stability of cobalt-based materials.

Contents of the invention

In view of all or part of the deficiencies of the above-mentioned prior art, the purpose of the present invention is to:

One object of the present invention is to provide a heterostructure material, specifically a foam metal-loaded cobalt tetroxide/iron cobalt phosphide heterostructure material, in which the presence of iron cobalt phosphide can greatly improve the conductivity of cobalt tetroxide, and The strong electron interaction at the heterointerface between cobalt tetroxide and iron-cobalt phosphide reduces the poisoning effect of the UOR reaction intermediate on the catalyst, allowing the catalyst to maintain good catalytic performance in the long-term electrocatalytic urea oxidation reaction.

Another object of the present invention is to provide a method for preparing a heterostructure material. The preparation method synthesizes the heterostructure material through a two-step hydrothermal method and a phosphating method. The raw materials used are low in cost and the required equipment is It is simple, suitable for large-scale production, and has certain economic feasibility.

Another object of the present invention is to disclose an application of the above heterostructure material in the field of electrocatalytic urea oxidation.

In order to achieve the above objects, the present invention provides the following technical solutions:

The invention provides a heterostructure material. The heterostructure material uses foam metal as a carrier, and forms a cobalt tetroxide/iron cobalt phosphide composite material with a heterogeneous interface structure on the surface of the foam metal. The cobalt tetroxide/iron cobalt phosphide composite material The phosphide composite material has obvious cobalt tetraoxide lattice stripes and iron-cobalt phosphide lattice stripes, and forms corresponding heterogeneous interfaces.

The beneficial effects of this technical solution are that, first, the metal foam serves as the base of the heterostructure material, and its surface has a rich three-dimensional porous structure, which increases the overall specific surface area of the catalyst and is conducive to mass transfer. More importantly, the metal foam For example, nickel foam has excellent electrical conductivity, and a composite material with a heterogeneous interface is grown in situ on its surface. The two are closely combined to facilitate electron transfer between the substrate and the catalyst, that is, the composite material. Secondly, the heterostructure material includes cobalt tetroxide/iron cobalt phosphide heterostructure. Compared with simple cobalt tetroxide or iron cobalt phosphide catalyst, the electron transfer of the cobalt tetroxide/iron cobalt phosphide heterostructure at the heterogeneous interface is better. It helps to adjust the electronic structure of the catalyst, thereby improving the conductivity of the cobalt-based oxide, while enhancing the intrinsic catalytic activity of the catalyst, thereby improving the catalytic performance. In addition, the formation of cobalt tetraoxide/iron cobalt phosphide heterogeneous interface can optimize the adsorption/desorption of reaction intermediates by cobalt-based materials in the urea oxidation reaction, and reduce the catalyst damage caused by excessive adsorption of intermediate products on the reaction active sites. Poisoning problem, thereby enhancing the long-term stability of UOR of cobalt tetroxide/iron cobalt phosphide heterostructure materials.

The foam metal is selected from nickel foam, cobalt foam, copper foam or aluminum foam. It should be noted that the heterostructure material of the present invention may be, for example, a nickel foam-supported tricobalt tetroxide/iron cobalt phosphide composite material.

The microscopic morphology of the cobalt tetroxide/iron cobalt phosphide composite material is nanocubes strung on nanowires, which is a composite morphology of nanocubes strung on nanowires, similar to the structure of "candied haws". The nanowires are vertically arranged on the surface of the foam metal, that is, the nanowires grow vertically on the surface of the foam metal. Compared with catalysts with simple nanowire or nanocube phase, the beneficial effects brought by this special composite morphology include the following: First, this morphology is conducive to further increasing the electrochemical active area and increasing the number of catalytic active sites. ; Secondly, this unique structure provides an optimized path for the diffusion of electrons and electroactive substances, which can accelerate charge transfer, reduce electron transfer resistance, and effectively promote mass transfer; most importantly, the electron transfer at the heterogeneous interface can be adjusted The electronic structure of the catalyst optimizes the adsorption/desorption of reactants and reaction intermediates at the catalytically active sites, reduces the reaction energy barrier, and thereby improves the intrinsic catalytic activity of the catalyst.

The invention also provides a method for preparing heterostructure materials, which includes the following steps:

S1: Add cobalt source and urea to the water and dissolve them to form a precursor solution;

S2: Using foam metal as a carrier, perform a hydrothermal reaction with the precursor solution to obtain the first intermediate;

S3: The first intermediate is subjected to a secondary hydrothermal reaction with potassium ferricyanide solution, and the reaction is carried out at a temperature above 30°C for 10-14 hours to obtain the second intermediate;

S4: Subject the second intermediate to phosphating treatment to obtain the heterostructure material.

The beneficial effect of this technical solution is to prepare heterostructure materials by combining a two-step hydrothermal method with a phosphating method. If the precursor solution and potassium ferricyanide solution are directly reacted with foam metal at the same time through a one-step hydrothermal method, Then the heterostructure material of the present invention cannot be obtained. When the first intermediate and potassium ferricyanide solution undergo a secondary hydrothermal reaction, the reaction time of the secondary hydrothermal reaction is controlled to 10-16h, preferably 10-14h, and the reaction temperature is controlled above 30°C. , there is hope to obtain cobalt tetroxide/iron cobalt phosphide heterostructure. If the reaction time is too short, only a small amount of phosphide will be produced and the heterostructure cannot be formed, and only a simple nanowire morphology can be formed; if the reaction time is too short, If it is too long, the nanowires will be etched away and cannot form a heterostructure, nor can they maintain the composite array structure, and can only form a simple nanocube morphology with chaotic arrangement. If the secondary hydrothermal reaction temperature is lower than 30°C, for example, if the secondary hydrothermal reaction is performed at room temperature, only a small amount of phosphide will be produced and the heterostructure cannot be formed. The reaction time and reaction temperature of the secondary hydrothermal reaction are It's important. The heterostructure material has a unique morphology and excellent UOR performance, and the preparation method is low-cost and suitable for large-scale industrial production; the preparation method can be used to synthesize the above-mentioned foam metal-loaded cobalt tetroxide/iron cobalt phosphide heterogeneous Structural materials.

In S4, the phosphating treatment includes: placing the excess phosphorus source and the second intermediate in different porcelain boats and placing them together in a tube furnace. The phosphorus source is placed upstream of the gas and raised to 250-450°C. Cool to room temperature naturally. The phosphorus source may be, but is not limited to, sodium hypophosphite monohydrate. The phosphating treatment temperature is further preferably 350°C. If the phosphating temperature is too low, the phosphorus source cannot be fully decomposed to release phosphine gas (the truly effective phosphating agent), and sufficient phosphating cannot be achieved; If the temperature is too high, the substrate, that is, the carrier, such as nickel foam, cannot withstand the high temperature.

In S1, the cobalt source can be cobalt chloride hexahydrate or cobalt nitrate hexahydrate, etc. The molar ratio of the cobalt source to urea is 1:2-8, and 1:5 can be further selected. The concentrations of the cobalt source and urea are generally relatively large to ensure the formation of sufficient first intermediates (cobalt-based nanowires).

In S2, the reaction temperature of the hydrothermal reaction is 40-100°C, and the reaction time is 6-18h. Furthermore, in S2, the reaction temperature of the hydrothermal reaction is preferably 80°C and the reaction time is 12 hours. The temperature of the hydrothermal reaction affects the morphology of the heterostructure. When the temperature is 80°C, it is beneficial to the subsequent formation of a special composite morphology of nanocubes strung on nanowires.

In S3, the reaction temperature of the secondary hydrothermal reaction is 40-100°C, and the reaction time is 12-14h. Further, the reaction temperature of the secondary hydrothermal reaction is 60°C, and the reaction time is 12 hours. The beneficial effect of this technical solution is that the foam metal-loaded cobalt tetroxide/iron cobalt phosphide composite material synthesized under the reaction temperature and reaction time has a suitable composite array structure of nanocubes and nanowires, and the nanowires will not be completely Etched away, it has a better nanomorphology, thereby ensuring its excellent catalytic performance; and the obtained foam metal-supported cobalt tetroxide/iron cobalt phosphide composite material has stable performance and can exert a catalytic effect for a long time.

In S2, the product of the hydrothermal reaction is washed and dried to obtain the first intermediate; in S3, the product of the secondary hydrothermal reaction is washed and dried to obtain the second intermediate; the drying method in S2 and S3 is : Dry in a drying oven at 50-100℃ for 10-15h. Under this drying temperature and drying time condition, most of the water in the product obtained after the two-step hydrothermal treatment can be effectively removed without causing obvious surface oxidation, thereby avoiding affecting the subsequent high-temperature phosphating process. The drying box may preferably be a vacuum drying box to further prevent oxidation.

Before S2, the foam metal can also be cleaned in advance. The method of cleaning the foam metal, such as nickel foam, can be: selecting an organic solvent and/or a low-concentration acid to soak and/or ultrasonic cleaning the nickel foam. The organic solvent can be, for example, For acetone and acid, 0.1-3mol/L hydrochloric acid can be selected. Acetone has certain fat solubility and water solubility, and can effectively remove oil stains on the surface of nickel foam sheets. Hydrochloric acid within this concentration range can remove oxides on the surface of nickel foam sheets. The step of pretreating the nickel foam sheet can make the composite material to be prepared more uniformly loaded on the surface of the nickel foam sheet and prevent the active material from collapsing or falling off. Of course, you can also choose other reagents to pretreat the nickel foam sheets, such as ethanol, dilute sulfuric acid, etc. The cleaning step may further include the process of rinsing away the residual reagent on the foamed nickel sheet with water and drying it. The drying method may be natural blow-drying at room temperature or drying in an oven. In order to prevent the nickel foam sheets from being oxidized during the drying process, a vacuum oven is preferred. The specific cleaning method can be: ultrasonic cleaning with ethanol, acetone, 0.1-3mol/L hydrochloric acid for 15-30 minutes, and finally ultrasonic cleaning with water for 20-30 minutes, changing the water 6-8 times in between.

The invention also discloses an application of the heterostructure material in the above technical solution in the field of electrocatalytic urea oxidation. Specifically, it can be used as a catalyst in an electrocatalytic urea oxidation reaction under alkaline conditions, and the heterostructure material serves as an anode of an electrolytic cell to catalyze the urea oxidation reaction under alkaline conditions. The formation of cobalt tetroxide/iron cobalt phosphide heterogeneous interface in the heterostructure material can effectively optimize the catalytic performance of the catalyst.

Specifically, the heterostructure material serves as an anode electrode to catalyze the alkaline urea electro-oxidation reaction. The material uses foam metal as the load base, so it can be used directly as an electrode without the need to add additional binders (such as Nafion, etc.) during the electrode preparation process. ), while simplifying the electrode preparation process, it also avoids the decrease in the electron transfer rate during the reaction caused by the binder, and the drastic change in the catalytic effect that may be caused by the shedding of the binder during the long-term reaction. Therefore, this difference The direct use of structural materials as electrodes is beneficial to the simultaneous improvement of conductivity and stability.

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

(1) The micromorphology of the heterostructure provided by the present invention can be manifested as a composite morphology of nanocubes strung on nanowires. The nanowires grow vertically on the surface of the foam metal, which is beneficial to increasing the electrochemical active area. , promote mass transfer, accelerate charge transfer, and improve the catalytic activity of the catalyst.

(2) Cobalt tetroxide/iron cobalt phosphide forms a large number of heterogeneous interfaces in heterostructure materials. The strong electronic interaction between the two substances adjusts the electronic structure of the catalyst and optimizes the adsorption/desorption of reaction intermediates on the catalyst surface. Reduce the problem of catalyst poisoning caused by the difficulty of desorption of intermediate products, thereby enhancing catalytic stability.

(3) The method used in the present invention to prepare cobalt tetroxide/iron cobalt phosphide heterostructure materials is a two-step hydrothermal method and high-temperature phosphating. The preparation method is simple and easy to implement, and the required equipment and raw materials are abundant, low-priced and easy to obtain. It is easy to control costs in industrial production and is suitable for large-scale production.

(4) The present invention discloses the application of the heterostructure material prepared above in catalyzing the electrooxidation reaction of urea under alkaline conditions. The tricobalt tetroxide/iron cobalt phosphide heterogeneous composite material has good catalytic performance and is suitable for use as a catalyst under alkaline conditions. It is a catalyst for the electrooxidation reaction of urea; and the foam metal-supported tricobalt tetroxide/iron cobalt phosphide composite material has stable performance and no significant decline in catalytic performance during the long-term reaction process, meeting the demand for catalyst stability in industrial production.

Description of the drawings

In order to more clearly illustrate the technical solutions in the specific embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings without exerting creative efforts.

Figure 1 is a flow chart of the preparation method of heterostructure materials provided by the present invention;

Figure 2 is the SEM image and TEM image of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF prepared in Example 1; where a is the SEM of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF under the 1 μm scale bar. Figure; b is the TEM image of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF under the 100nm scale bar;

Figure 3 is the HRTEM image and Mapping image of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF prepared in Example 1; where a is the HRTEM image of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF at a scale of 5 nm. Figure; b is the element distribution Mapping diagram of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF under the 500nm scale bar;

Figure 4 is the XRD pattern of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF prepared in Example 1;

Figure 5 shows the relationship between Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF prepared in Example 1, Co ₃ O ₄ @NF prepared in Comparative Example 6, and Fe _0.3 Co _0.7 P@NF prepared in Comparative Example 4. XPS picture; among them, a is the XPS narrow scan comparison picture of Co 2p in Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF and Co ₃ O ₄ @NF; b is the XPS narrow scan comparison picture of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF The XPS narrow scan comparison picture of Fe 2p in Fe _0.3 Co _0.7 P@NF; c is the XPS narrow scan comparison picture of P 2p; d is the XPS narrow scan comparison picture of O1s;

Figure 6 is an SEM image of the product prepared in Comparative Example 1;

Figure 7 is an SEM image of the product prepared in Example 2;

Figure 8 is an SEM image of the product prepared in Example 3;

Figure 9 is an SEM image of the product prepared in Comparative Example 2;

Figure 10 is an SEM image of the product prepared in Comparative Example 3;

Figure 11 is an SEM image of the product prepared in Comparative Example 4;

Figure 12 is an XRD comparison chart of Co ₃ O ₄ @NF prepared in Comparative Example 6 and Fe _0.3 Co _0.7 P@NF prepared in Comparative Example 4; where a is the XRD pattern of Co ₃ O ₄ @NF; b is XRD pattern of Fe _0.3 Co _0.7 P@NF;

Figure 13 shows Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF prepared in Example 1, Co ₃ O ₄ @NF prepared in Comparative Example 6, and Fe _0.3 Co _0.7 P@NF prepared in Comparative Example 4 at 1M. The polarization curve measured at a scanning speed of 5mV/s in KOH+0.33M urea electrolyte (a in Figure 13); b in Figure 13 is the Co ₃ O ₄ /Fe _0.3 prepared in Example 1 Comparison of polarization curves of Co _0.7 P@NF for catalytic UOR and OER;

Figure 14 is a current-time curve of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF prepared in Example 1 when the voltage is 1.36V vs. RHE.

Detailed ways

The technical solutions in the specific embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention. The steps in the following embodiments do not correspond one-to-one with the content of the invention.

Example 1

This embodiment provides a heterostructure material and a preparation method thereof (for the preparation method, please refer to Figure 1). The heterostructure material in this embodiment, that is, the preparation method of cobalt tetroxide/iron cobalt phosphide loaded with nickel foam is:

(1) Ultrasonically clean nickel foam sheets of a specific shape and size with ethanol and acetone for 15 minutes in sequence to remove organic matter and other impurities on the surface, and then ultrasonically clean them with 3 mol/L hydrochloric acid solution for 15 minutes to remove metal oxides, etc. on the surface To remove impurities, use a large amount of secondary water to ultrasonic for 20 minutes, change the water 6-8 times in the middle, and rinse until the cleaning liquid is neutral to ensure a completely clean metal nickel surface.

(2) Primary hydrothermal: Dissolve 0.285g CoCl ₂ ·6H ₂ O (cobalt chloride hexahydrate) and 0.36g urea (the molar ratio of the two is 1:5) in 15ml of secondary water, and fully dissolve to form the precursor Solution; transfer the precursor solution to the reaction kettle, add pretreated nickel foam (using nickel foam as a carrier) to perform a hydrothermal reaction. The temperature of the hydrothermal reaction is 80°C, maintained for 12h, and naturally cooled to After room temperature, wash with water and dry in a vacuum drying oven at 75°C for 10 hours to obtain the first intermediate for later use. The concentrations of the cobalt source and urea are generally relatively large to ensure the formation of sufficient first intermediates (cobalt-based nanowires).

(3) Secondary hydrothermal: Weigh 0.12g potassium ferricyanide and dissolve it in 15ml of water to form a uniform solution and transfer it to the reaction kettle. Place the first intermediate obtained in (2) into the reaction kettle. Secondary hydrothermal reaction, the temperature of the secondary hydrothermal reaction is 60°C, maintained for 12h, naturally cooled to room temperature, washed with water, and dried in a vacuum drying oven at 75°C for 10h to obtain the second intermediate for later use.

(4) Phosphating treatment: Place the second intermediate obtained in (3) and 1g Na ₂ HPO ₂ ·H ₂ O (1g, to ensure sufficient phosphating) in different porcelain boats, and place them together in the tube In a furnace, Na ₂ HPO ₂ ·H ₂ O is placed upstream of the gas, and is raised to 350°C at a heating rate of 5°C/min, maintained for 5 hours, and finally cooled to room temperature naturally to obtain the heterostructure material.

The product prepared in this example is a Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF heterostructure material, that is, using nickel foam as a carrier, cobalt tetroxide/iron cobalt phosphorus with a heterogeneous interface structure is formed on the surface of the nickel foam. compound composite material, in this embodiment specifically a Co ₃ O ₄ /Fe _0.3 Co _0.7 P composite material. The Co ₃ O ₄ /Fe _0.3 Co _0.7 P composite material has obvious Co ₃ O ₄ lattice stripes and Fe _0.3 Co _0.7 P lattice stripes and form corresponding heterogeneous interfaces. The microscopic morphology of the Co ₃ O ₄ /Fe _0.3 Co _0.7 P composite material is nanocubes strung on nanowires, and the nanowires are vertically arranged on the surface of the nickel foam.

Example 2

The only difference between Example 2 and Example 1 is that the conditions for a hydrothermal reaction (corresponding to the hydrothermal reaction in S2) are to carry out the reaction at 60°C and maintain it for 12 hours. The prepared product is a composite material with irregular morphology.

Example 3

The only difference between Example 3 and Example 1 is that the conditions for a hydrothermal reaction (corresponding to the hydrothermal reaction in S2) are to carry out the reaction at 100°C and maintain it for 12 hours. The prepared product is a composite material with irregular morphology.

Example 4

The only difference between Example 4 and Example 1 is that the conditions for the secondary hydrothermal reaction (corresponding to the secondary hydrothermal reaction in S3) are to carry out the reaction at 90°C and maintain it for 12 hours. The prepared product is a composite material with irregular morphology.

Comparative example 1

The only difference between Comparative Example 1 and Example 1 is that nickel foam is not added in all steps, and the obtained product is a composite material without substrate support.

Comparative example 2

The only difference between Comparative Example 2 and Example 1 is that the conditions for the secondary hydrothermal reaction (corresponding to the secondary hydrothermal reaction in S3) are to carry out the reaction at 60°C and maintain it for 6 hours. The prepared product is a composite material containing a small amount of phosphide.

Comparative example 3

The only difference between Comparative Example 3 and Example 1 is that the conditions for the secondary hydrothermal reaction (corresponding to the secondary hydrothermal reaction in S3) are to carry out the reaction at 60°C and maintain it for 9 hours. The prepared product is a composite material containing a small amount of phosphide.

Comparative example 4

The only difference between Comparative Example 4 and Example 1 is that the conditions for the secondary hydrothermal reaction (corresponding to the secondary hydrothermal reaction in S3) are to carry out the reaction at 60°C and maintain it for 18 hours. The prepared product is a completely phosphated material, namely Fe _0.3 Co _0.7 P@ NF , which is iron cobalt phosphide supported by nickel foam.

Comparative example 5

The only difference between Comparative Example 5 and Example 1 is that the conditions for the secondary hydrothermal reaction (corresponding to the secondary hydrothermal reaction in S3) are to carry out the reaction at room temperature and maintain it for 12 hours. The prepared product is a composite material containing a small amount of phosphide.

Comparative example 6

The difference between Comparative Example 6 and Example 1 is that the first intermediate was calcined in an air atmosphere at a heating rate of 3°C/min, maintained at 350°C for 2 hours, and then naturally cooled to room temperature to obtain Co ₃ O ₄ @NF That is, nickel foam loaded cobalt tetroxide.

The main preparation conditions involved in the above-mentioned Examples 1-4 and Comparative Examples 1-6 are summarized in Table 1 below:

Summary of relevant preparation conditions in Table 1 Examples and Comparative Examples

1. Characterization of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF

Using the Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF prepared in Example 1 as a test sample, scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and elemental It was characterized by distribution map (Mapping), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The Co ₃ O ₄ @NF prepared in Comparative Example 6 and the Fe _0.3 Co _0.7 P@NF synthesized in Comparative Example 4 were used as control samples. XPS was used to compare Co ₃ O ₄ @NF and Fe _0.3 Co _0.7 P@NF. The electron binding energies of the elements contained are compared and tested.

The characterization results are described and analyzed in detail below with reference to the attached figures:

a in Figure 2 and b in Figure 2 are respectively the SEM image and TEM image of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF prepared in Example 1. As shown in a in Figure 2, combined with two-step water Thermal and high-temperature phosphating methods grew a large number of nanocubes on the base nickel foam and regularly stacked them into the shape of nanorods. Further TEM characterization (b in Figure 2) found that a nanowire passed through the interior of these nanocubes. They are connected to each other to form a special "candied haws"-like structure. This structure further increases the electrochemical active area of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF, and is conducive to exposing more active sites and improving the intrinsic activity of the catalyst. In addition, the rich porous structure formed between the nanorods is conducive to the transmission of reaction materials, provides sufficient reactants for the catalytic reaction, and helps the catalytic reaction to proceed.

a in Figure 3 and b in Figure 3 are respectively the HRTEM image and element mapping image of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF prepared in Example 1. From a in Figure 3, we can see clear lattice stripes attributed to Co ₃ O ₄ and Fe _0.3 Co _0.7 P respectively, where, Corresponds to the (331) crystal plane of cubic crystal system Co ₃ O ₄ (the dotted box in the upper left corner is a partial enlargement), and/> Corresponds to the (011) crystal plane of the orthorhombic Fe _0.3 Co _0.7 P (the dotted box in the upper right corner is a partial enlargement), and the figure shows two types of Co ₃ O ₄ (331) and Fe _0.3 Co _0.7 P (011) The lattice fringes have obviously different directions, proving the formation of a heterogeneous interface (the dotted curve in the middle of a in Figure 3). b in Figure 3 is the element distribution Mapping diagram of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF. It can be seen from the figure that the four elements Co, Fe, O, and P are evenly distributed on the entire nanorod. The elemental composition of the catalyst was further confirmed. Figure 4 is the XRD pattern of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF. Comparison with the standard XRD pattern shows that the test sample shows both Co ₃ O ₄ (JCPDS: 42-1467) and Fe _0.3 Co _0.7 The characteristic diffraction peak of P (JCPDS: 29-0497) indicates the formation of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF heterostructure material.

In order to further illustrate the electronic structure of the Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF heterostructure material, Co ₃ O ₄ @NF prepared in Comparative Example 6 and Fe _0.3 Co _0.7 P@NF synthesized in Comparative Example 4 were used. As a control sample, the XPS spectra of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF, Co ₃ O ₄ @NF and Fe _0.3 Co _0.7 P@NF were comparatively analyzed. As can be seen from a in Figure 5, both Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF and Co ₃ O ₄ @NF have the characteristic peaks of Co 2p, among which there are two peaks located around 783eV and 798eV. They belong to Co 2p3/2 and Co 2p1/2 orbitals respectively. Compared with Co ₃ O ₄ @NF, the electron binding energy of Co 2p3/2 orbital in Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF has been shifted positively by 0.5eV. In b in Figure 5, the characteristic peaks of Fe 2p appear in both Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF and Fe _0.3 Co _0.7 P@NF. The two peaks located near 713eV and 724eV correspond to Fe 2p3/2 and Fe 2p1/2 orbitals. Compared with Fe _0.3 Co _0.7 P@NF, the electron binding energy of Fe 2p3/2 orbital in Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF has been negatively shifted by 0.4eV. Combined with the positive shift of the electron binding energy of Co mentioned above, it can be seen that there is a strong electronic interaction between Co ₃ O ₄ and Fe _0.3 Co _0.7 P in the heterostructure. This electron transfer can adjust the electronic structure of heterostructure materials and adjust the adsorption/desorption of reaction intermediates by heterogeneous materials, thereby enhancing their catalytic performance. c and d in Figure 5 are narrow scan images of P 2p and O1s respectively in Example 1, indicating the existence and respective existence states of the two elements P and O.

2. Research on factors influencing the formation and morphology of heterogeneous structures

(1) Influence of carrier (substrate): Figure 6 is an SEM image of the product prepared in Comparative Example 1. As shown in Figure 6, when no substrate is added, the product obtained has a heterogeneous structure, but the morphology is irregular. The composite material does not have a special morphology of nanocubes strung on nanowires. The presence or absence of a substrate such as nickel foam will affect the product morphology.

(2) Effect of primary hydrothermal temperature: Figure 7 is an SEM image of the product prepared in Example 2, and Figure 8 is an SEM image of the product prepared in Example 3. As shown in Figures 7 and 8, when the primary hydrothermal temperature is low or high, the resulting product has a heterogeneous structure, but is a composite material with irregular morphology, that is, there is no formation of nanocubes strung on nanowires. Special shape. That is, when the primary hydrothermal temperature is inappropriate, composite materials with regular morphology and composite matrix arrangement cannot be obtained. The primary hydrothermal temperature has a great influence on the product morphology.

(3) Effect of secondary hydrothermal time: Figure 9 is the SEM image of the product prepared in Comparative Example 2, Figure 10 is the SEM image of the product prepared in Comparative Example 3, and Figure 11 is the SEM image of the product prepared in Comparative Example 4 SEM image of the product. The time of secondary hydrothermal heating has a direct impact on the product. If the secondary hydrothermal time is too short, such as 6 hours in Comparative Example 2 and 9 hours in Comparative Example 3, and the secondary hydrothermal time is not enough, the heterostructure cannot be obtained, that is, only A small amount of phosphide cannot form a complete heterostructure, and too little phosphide cannot be detected by XRD characterization. It can be seen from Figure 9 that no nanocube structure is formed, and in Figure 10 there is no obvious morphology of nanocubes strung on nanowires. If the secondary hydrothermal time is too long, such as 18 hours in Comparative Example 4, the heterostructure cannot be obtained, and the product is a completely phosphated material, that is, Fe _0.3 Co _0.7 P@NF. It can be seen from Figure 11 that no obvious nanowire structure is formed and the arrangement is irregular. As shown in Figure 9-11, by adjusting the time of the secondary hydrothermal etching (6h, 9h and 18h), that is, Comparative Examples 2, 3, and 4, products with insufficient and excessive secondary etching were obtained respectively. For products containing a small amount of phosphide and completely phosphated products, heterogeneous structures cannot be obtained. After testing, it can be seen that the secondary hydrothermal time is conducive to the formation of heterostructures when the secondary hydrothermal time is 10-14h, especially 12-14h.

(4) Effect of secondary hydrothermal temperature: The temperature of secondary hydrothermal also has a certain impact on the product. Secondary hydrothermal treatment at room temperature cannot form a heterogeneous structure. In Example 4, the secondary hydrothermal temperature reached 90°C. Although heterogeneous structures can be obtained, the morphology is irregular and no special "candied haws" structure is formed. After testing, it can be seen that if the secondary hydrothermal temperature is higher than 30°C, especially higher than 40°C, it is expected to form a heterogeneous structure. At the same time, if the secondary hydrothermal temperature is below 80°C, it is expected to obtain a special "candied haws" structure. Appearance.

Effect of subsequent high-temperature treatment: a in Figure 12 represents a single-component Co ₃ O ₄ @NF (JCPDS : 42-1467); In addition, we extended the secondary hydrothermal reaction time to 18h (i.e. Comparative Example 4), and the remaining steps were the same as in Example 1 to obtain a single component Fe _0.3 Co _0.7 P@NF (Figure 12 b, JCPDS: 29-0497), indicating that secondary hydrothermal is actually an etching process. When the reaction time is long enough, the nanowire template can be completely etched, and a single phosphide can be obtained after subsequent phosphating. If the nanowire is completely etched, the special composite morphology cannot be formed and becomes a simple nanocube morphology. Therefore, if the heterogeneous interface is not formed, the conductivity and catalytic performance cannot be improved. More importantly, since the heterogeneous interface is not formed, the catalyst poisoning phenomenon cannot be solved and stability cannot be achieved. Even if a heterogeneous interface is formed, without the composite morphology of nanocubes strung on nanowires, it is impossible to obtain an optimized diffusion path for electrons and electroactive substances, accelerate charge transfer, and reduce electron transfer resistance. The above experiments prove the importance of controlling reaction time and reaction temperature on product components, morphology and structure. Optimizing reaction time and reaction temperature is helpful to form heterostructure materials.

3. Application of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF as urea oxidation electrode

The Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF prepared in Example 1 was used as the test sample, the Co ₃ O ₄ @NF prepared in Comparative Example 6 and the Fe _0.3 Co _0.7 P@ synthesized in Comparative Example 4 were used. NF was used as a control sample to test the catalytic performance of the three as urea electro-oxidation electrodes under alkaline conditions.

a in Figure 13 is the scanning speed of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF, Co ₃ O ₄ @NF and Fe _0.3 Co _0.7 P@NF in 1M KOH+0.33M urea solution at a scanning speed of 5mV/s. Plot of measured polarization curves. Compared with Co ₃ O ₄ @NF and Fe _0.3 Co _0.7 P@NF, Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF exhibits a lower onset potential and higher current density under the same conditions, indicating that Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF has more excellent UOR catalytic activity. From the above comparison results, it can be seen that the formation of the heterogeneous interface between Co ₃ O ₄ /Fe _0.3 Co _0.7 P enhances its UOR performance. The present invention further tested and compared the UOR and OER performance of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF. As shown in b in Figure 13, at the same current density, the potential required to catalyze UOR is much lower than OER. , indicating that the present invention can achieve the effect of energy saving.

Figure 14 is the current-time curve of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF at a voltage of 1.36V vs. RHE. As can be seen from Figure 14, under constant voltage, the current of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF does not significantly decay after a long period of continuous electrolysis reaction, indicating that Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF has good durability and stability. On the one hand, this stability comes from the stability of Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF itself. On the other hand, Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF does not require the addition of additional binders as a working electrode. , eliminating the instability caused by the adhesive falling off during long-term testing. Therefore, Co ₃ O ₄ /Fe _0.3 Co _0.7 P@NF can be used as a urea oxidation electrode under alkaline conditions for a long time, meeting the industry's requirements for catalyst stability.

The description of the above embodiments is only used to help understand the method and core idea of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the scope of the claims of the present invention.

Claims (10)

1. A heterostructure material, characterized in that the heterostructure material uses foam metal as a carrier, and forms a cobalt tetroxide/iron cobalt phosphide composite material with a heterogeneous interface structure on the surface of the foam metal, and the cobalt tetroxide /Iron-cobalt phosphide composite material has obvious cobalt tetraoxide lattice stripes and iron-cobalt phosphide lattice stripes, and forms corresponding heterogeneous interfaces. 2. The heterostructure material according to claim 1, characterized in that the foam metal is selected from nickel foam, cobalt foam, copper foam or aluminum foam. 3. The heterostructure material according to claim 1, characterized in that the microscopic morphology of the cobalt tetroxide/iron cobalt phosphide composite material is nanocubes strung on nanowires, and the nanowires are vertically arranged and formed on The foam metal surface. 4. A method for preparing heterostructure materials, characterized by comprising the following steps: S1: Add cobalt source and urea to the water and dissolve them to form a precursor solution; S2: Using foam metal as a carrier, perform a hydrothermal reaction with the precursor solution to obtain the first intermediate; S3: The first intermediate is subjected to a secondary hydrothermal reaction with potassium ferricyanide solution, and the reaction is carried out at a temperature above 30°C for 10-14 hours to obtain the second intermediate; S4: Subject the second intermediate to phosphating treatment to obtain the heterostructure material. 5. The preparation method according to claim 4, characterized in that, in S4, the phosphating treatment includes: placing the excess phosphorus source and the second intermediate in different porcelain boats, and placing them together in a tube furnace , the phosphorus source is placed upstream of the gas, raised to 250-450°C, and finally cooled to room temperature naturally. 6. The preparation method according to claim 4, characterized in that, in S1, the cobalt source is cobalt chloride hexahydrate or cobalt nitrate hexahydrate, and the molar ratio of the cobalt source to urea is 1:2-8 ; In S2, the reaction temperature of the hydrothermal reaction is 40-100°C, and the reaction time is 6-18h. 7. The preparation method according to claim 4, characterized in that in S3, the reaction temperature of the secondary hydrothermal reaction is 40-100°C, and the reaction time is 12-14h. 8. The preparation method according to claim 4, characterized in that, in S2, the product of the hydrothermal reaction is washed and dried to obtain the first intermediate; in S3, the product of the secondary hydrothermal reaction is washed and dried. Obtain the second intermediate; the drying method in S2 and S3 is: drying in a drying oven at 50-100°C for 10-15h. 9. Application of the heterostructure material according to any one of claims 1 to 3, or the heterostructure material prepared by the preparation method according to any one of claims 4 to 8, in the field of electrocatalytic urea oxidation. 10. The application according to claim 9, characterized in that the heterostructure material is used as an anode of an electrolytic cell to catalyze the urea oxidation reaction under alkaline conditions.