Hollow carbon nano tube loaded bimetallic heterogeneous phosphide electrocatalyst as well as preparation method and application thereof

Abstract

The invention provides a hollow carbon nanotube loaded bimetallic heterogeneous phosphide electrocatalyst as well as a preparation method and application thereof, and belongs to the technical field of electrocatalyst composite materials. According to the invention, adsorption and chelation of bimetal ions are realized through the strong coordination capability of an organic phosphorus source in the aromatic organic nitrogen source-organic phosphorus source copolymer with a hollow structure, and the hollow carbon nanotube loaded bimetal heterogeneous phosphide electrocatalyst is obtained through pyrolysis. Through a chemical anchoring-in-situ phase reconstruction strategy, the prepared hollow carbon nanotube loaded bimetal heterogeneous phosphide composite material has excellent ORR / OER bifunctional catalytic activity, and as a cathode of RZAB, the energy density and cycling stability of the RZAB can be effectively improved. The invention provides an efficient and stable solution for the cathode of the RZAB, and promotes the development of a renewable energy source technology.

Description

Technical Field

[0001] The present invention relates to the technical field of electrocatalyst composite materials, and particularly to a hollow carbon nanotube-supported bimetallic heterophosphate electrocatalyst, its preparation method and application. Background Art

[0002] In recent years, with the continuous climb of global energy demand and the urgent requirement of green transformation, the urgent need for efficient and stable renewable energy conversion and storage technologies has become increasingly prominent. The rechargeable zinc-air battery (RZAB) is gradually becoming a core technology with great potential in promoting portable electronic devices, electric vehicles and large-scale energy storage systems due to its excellent energy density, environmental protection characteristics and economic advantages. The theoretical energy density of RZAB is as high as about 1370 Wh / kg (excluding the mass of oxygen), and its semi-open structure, high safety and low cost of metallic zinc (only 1 / 20 of lithium) have attracted extensive attention from researchers at home and abroad. In RZAB, the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) constitute the energy conversion process. During discharge, the cathode catalyzes ORR, converting oxygen into water or hydroxide ions and releasing electrons; while during charging, OER occurs, and water or hydroxide ions are oxidized to generate oxygen.

[0003] However, RZAB still faces many technical obstacles in practical applications, including insufficient air cathode stability, slow charge-discharge reaction kinetics, low anodic coulombic efficiency, and the formation of zinc dendrites. Among them, the air cathode, as the core component of the battery reaction, directly affects the energy conversion efficiency and cycle life of the battery. However, compared with the anode, the exchange current density of the cathode during the ORR and OER processes is significantly lower. This is mainly because ORR and OER involve multi-step electron transfer, the intermediate conversion process is complex and the activation energy barrier is high, resulting in limited reaction rate and thus higher overpotential, which becomes a key factor restricting the performance improvement of RZAB. Although noble metal-based catalysts such as Pt / Pd and IrO2 / RuO2 are still the most efficient ORR or OER catalysts at present, their disadvantages such as poor bifunctional activity, poor long-term stability and high cost greatly limit their wide application in RZAB. Therefore, developing low-cost and high-performance ORR / OER bifunctional electrocatalysts is crucial for constructing an efficient RZAB system. This not only helps to improve the energy conversion efficiency and cycle stability of the battery, but also further promotes the rapid development of renewable energy technologies.

[0004] To improve the overall performance of RZAB, various types of non-platinum group metal (PGM)-based electrocatalysts, such as heteroatom-doped carbon materials, alloys, transition metal oxides, nitrides, phosphides, etc., have been successively developed, and these electrocatalysts have all shown good bifunctional activity and cycling stability. Among them, transition metal phosphides (MeP x ), such as FeP x , CoP x , NiP x , WP x and MoP x etc., due to the effective interaction between the lone pair electrons of P atoms and the d-band holes of transition metals, are conducive to regulating the d-band occupancy state of metal ions, forming a platinum-like d-band orbital structure, and optimizing the adsorption energy of key intermediates in catalytic reactions. In addition, MeP x can act as a Lewis base to interact with protons during the electrochemical breeding process, accelerating the electron transfer efficiency. Therefore, MeP x is regarded as a type of electrocatalyst with great application potential. For example, Huang et al. prepared a type of Co / Co2P heterojunction carbon nanotubes. The Schottky heterojunction effect of metal-semiconductor formed an internal electric field at the interface, effectively enhancing the electron transfer efficiency during the ORR / OER reaction process and realizing the synergistic improvement of the ORR / OER activity of the catalyst. This type of semiconductor-metal heterojunction is conducive to accelerating the electron transfer rate from metal to semiconductor, changing the electron cloud density, and shifting the d-band center of the heterojunction to a position favorable for adsorbing reaction intermediates. Yan et al. prepared a bimetallic Co-Fe phosphide hollow nanoblock, which showed extremely high conversion efficiency during the nitrate reduction reaction process. The research results showed that the synergistic effect of the bifunctional centers of nucleophilicity (Co) and electrophilicity (Fe) helped to lower the energy barrier of the rate-determining step and accelerate the catalytic reaction. Such bifunctional phosphide active sites with a tight heterostructure will contribute to the controllable synthesis of highly efficient ORR / OER electrocatalysts. The ORR and OER reactions are typical multi-electron transfer processes, involving multiple-step adsorption / desorption of reactants, intermediates, and products on the active sites of the catalyst. In addition, the ORR / OER process involves a three-phase interface composed of air / oxygen, electrolyte, and catalyst. Therefore, the mass transfer efficiency and the accessibility of active neutral sites are also the keys to determining the ORR / OER reaction efficiency of the catalyst.

[0005] Many studies have shown that carbon nanotubes with a hollow structure are an effective means to alleviate the mass transfer blockage at the three-phase interface. Therefore, developing a hollow-structured carbon-based transition metal phosphide catalyst that combines efficient electron transfer and rapid mass transfer will contribute to the controllable preparation of highly efficient and stable RZAB air cathodes. Summary of the Invention

[0006] Aiming at the deficiencies of the above prior art, the present invention provides a novel hollow carbon nanotube-supported bimetallic heterophosphate electrocatalyst with high performance, as well as an in-situ preparation method and application thereof.

[0007] To achieve the above object, the specific technical solutions of the present invention are as follows:

[0008] In the first aspect, the present invention provides a preparation method of a hollow carbon nanotube-supported bimetallic heterophosphate composite material, comprising the following steps:

[0009] (1) Mix an organic phosphorus source and an aromatic organic nitrogen source evenly in an acidic solution to obtain mixture I; adjust the pH value of the methyl orange (MO) solution to less than 1, add an initiator and mix evenly to obtain mixture II;

[0010] (2) Mix mixture I and mixture II for reaction. After the reaction is completed, filter the reaction solution by suction and wash it to obtain a copolymer;

[0011] (3) Disperse the copolymer in a solvent to obtain a dispersion liquid, add a metal cobalt source and a metal iron source to the dispersion liquid, mix evenly, and dry to obtain a precursor;

[0012] (4) Calcinate the precursor under an inert atmosphere to obtain a hollow carbon nanotube-supported bimetallic heterophosphate composite material.

[0013] The present invention addresses the key challenges of RZAB in energy conversion and storage, and provides a novel hollow-structured carbon nanotube-supported bimetallic heterophosphide oxygen electrocatalyst and a preparation method thereof. The reaction mechanism of the method of the present invention is as follows: Under acidic conditions, using rod-shaped micelles derived from methyl orange and an initiator as a soft template, an aromatic organic nitrogen source (such as aniline) and an organic phosphorus source (such as phytic acid) react under the action of the initiator and spontaneously coat the surface of the methyl orange micelles, forming a solid structure in which the aromatic organic nitrogen source-organic phosphorus source copolymer (such as aniline-phosphoric acid copolymer) coats the methyl orange micelles. Among them, the ammonium persulfate initiator on the rod-shaped methyl orange micelles induces the growth of the polyaromatic organic nitrogen source-organic phosphorus source nanotube precursor and determines the morphology and structure of the subsequent nanotubes. Next, the methyl orange micelles are removed by multiple washings, thereby forming a hollow-structured aromatic organic nitrogen source-organic phosphorus source copolymer. Subsequently, in a solution containing a metal source (such as a metal cobalt source and a metal iron source), the strong coordination ability of the organic phosphorus source in the copolymer is used to achieve the adsorption and chelation of metal ions. The hollow cavity structure is more conducive to the uniform adsorption of metal ions inside and outside the precursor. After pyrolyzing the precursor, a hollow-structured N, P co-doped carbon nanotube is finally formed, and the transition metal phosphide active sites are uniformly dispersed inside and outside the hollow-structured carbon nanotube. The hollow structure of the present invention provides a rich reaction interface. When used as an electrocatalyst in RZAB, it can promote the effective diffusion and transport of reactants, intermediates, and products inside the catalyst; by constructing a tight Fe2P-FeCoP heterointerface, the hydrophilicity of the catalyst is improved, and the surface energy of the catalyst is affected, thereby promoting the OH - contact with the active sites at the reaction interface, which helps the kinetic separation adsorption / dissociation of reaction intermediates and promotes the electrocatalytic reaction. The organic phosphorus source plays a crucial role in this complex preparation process. It realizes the in-situ phase reconstruction of multi-metal phosphide sites by forming stable coordination bonds with metal ions. The hollow carbon nanotube-supported transition metal phosphide composite material prepared by the present invention has excellent ORR / OER bifunctional activity and can be used as an air cathode catalyst in RZAB.

[0014] Preferably, in step (1), the mass ratio of the organic phosphorus source to the aromatic organic nitrogen source is (3-9):1; the mass ratio of the organic phosphorus source, methyl orange, and the initiator is (3-9):1:(1-3).

[0015] Preferably, in step (1), the organic phosphorus source includes but is not limited to phytic acid.

[0016] Preferably, in step (1), the aromatic organic nitrogen source includes but is not limited to aniline.

[0017] Preferably, in step (1), the acidic solution includes but is not limited to sulfuric acid solution, and the concentration of the acidic solution is 0.05 - 2 mol / L.

[0018] Preferably, in step (1), the methyl orange solution is an aqueous methyl orange solution, and the concentration of the methyl orange solution is 0.1 - 0.5 mol / L.

[0019] Preferably, in step (1), hydrochloric acid is used to adjust the pH of the methyl orange solution.

[0020] Preferably, in step (1), the initiator includes but is not limited to ammonium persulfate.

[0021] Preferably, in step (2), the temperature of the reaction is 5 - 25 °C, and the time is 5 - 30 min.

[0022] Preferably, in step (3), the mass ratio of the copolymer, metal cobalt source, and metal iron source is (2 - 10):1:1.

[0023] Preferably, in step (3), the metal cobalt source includes but is not limited to cobalt chloride, and the metal iron source includes but is not limited to iron chloride.

[0024] Preferably, in step (3), the temperature of the mixing is 5 - 25 °C, and the time is 0.5 - 3 h.

[0025] Preferably, in step (3), the solvent includes but is not limited to ethanol.

[0026] Preferably, in step (4), the temperature of the calcination is 400 - 1000 °C, and the time is 6 - 10 h.

[0027] In a second aspect, the present invention provides a hollow carbon nanotube-supported bimetallic heterophosphide composite material prepared by the method described above.

[0028] In a third aspect, the present invention provides the application of the hollow carbon nanotube-supported bimetallic heterophosphide composite material in a zinc-air battery.

[0029] Compared with the prior art, the advantages of the present invention are as follows:

[0030] (1) The method provided by the present invention has a simple process and low cost, which is conducive to large-scale production;

[0031] (2) The design of the heterophosphide structure and the hollow carbon nanotubes in the present invention enhances the electron transfer efficiency and mass transfer efficiency, reduces the overpotential, and improves the reaction rates of oxygen reduction and oxygen evolution. The prepared hollow carbon nanotube-supported bimetallic heterophosphide composite material exhibits excellent electrocatalytic oxygen reduction and oxygen evolution performance;

[0032] When the hollow carbon nanotube-supported bimetallic heterophosphide composite material prepared by the present invention is used as the negative electrode material of a rechargeable metal-air battery, it exhibits a high energy density and excellent cycling performance. The experimental results show that the catalyst of the present invention can achieve long-term stable performance exceeding 1000 h. Brief Description of the Drawings

[0033] Figure 1 It is the preparation flow chart of the hollow structure carbon nanotube-supported bimetallic heterophosphide composite material of the present invention;

[0034] Figure 2 It is the SEM image of the composite material obtained in Example 1;

[0035] Figure 3 It is the HRTEM image of the composite material obtained in Example 1;

[0036] Figure 4 It is the HADDF-TEM image of the composite material obtained in Example 1;

[0037] Figure 5 It is the XRD pattern of the composite materials obtained in Example 1 and Comparative Examples 1-4;

[0038] Figure 6 It is the high-resolution X-ray photoelectron spectrum of the composite materials obtained in Example 1 and Comparative Examples 2-3;

[0039] Figure 7 It is the oxygen reduction performance curve of the composite materials obtained in Example 1 and Comparative Examples 1-4;

[0040] Figure 8 It is the oxygen evolution performance curve of the composite materials obtained in Example 1 and Comparative Examples 1-4;

[0041] Figure 9 It is the physical picture and open circuit voltage of the rechargeable zinc-air battery using the composite material obtained in Example 1;

[0042] Figure 10 It is the polarization and power density curves of the composite material obtained in Example 1;

[0043] Figure 11 It is the charge-discharge polarization curve of the composite material obtained in Example 1;

[0044] Figure 12 It is the cycling performance of the rechargeable zinc-air battery using the composite material obtained in Example 1. Detailed Embodiments

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

[0046] The present invention provides a method for preparing a hollow carbon nanotube-supported bimetallic heterophosphide composite, comprising the following steps:

[0047] (1) Mix an organic phosphorus source and an aromatic organic nitrogen source uniformly in an acidic solution to obtain mixture I; adjust the pH value of a methyl orange (MO) solution to less than 1, add an initiator and mix uniformly to obtain mixture II;

[0048] (2) Mix mixture I and mixture II and react at 5-25 °C for 5-30 min. After the reaction, filter the reaction solution by suction and wash it to obtain a copolymer;

[0049] (3) Disperse the copolymer in a solvent to obtain a dispersion, add a metal cobalt source and a metal iron source to the dispersion, mix at 5-25 °C for 0.5-3 h, and dry to obtain a precursor;

[0050] (4) Calcinate the precursor at 400-1000 °C in an inert atmosphere for 6-10 h to obtain a hollow carbon nanotube-supported bimetallic heterophosphide composite.

[0051] In some examples, the mass ratio of the organic phosphorus source to the aromatic organic nitrogen source is (3-9):1; the mass ratio of the organic phosphorus source, methyl orange, and initiator is (3-9):1:(1-3).

[0052] In some examples, the acidic solution includes, but is not limited to, a sulfuric acid solution, and the concentration of the acidic solution is 0.05-2 mol / L.

[0053] In some examples, the methyl orange solution is an aqueous methyl orange solution, and the concentration of the methyl orange solution is 0.1-0.5 mol / L.

[0054] In some examples, the mass ratio of the copolymer, metal cobalt source, and metal iron source is (2-10):1:1.

[0055] In the following specific embodiments, the organic phosphorus source is phytic acid; the aromatic organic nitrogen source is aniline; the initiator is ammonium persulfate; the solvent is ethanol; the metal cobalt source is cobalt chloride; the metal iron source is iron chloride.

[0056] Example 1

[0057] A composite material of hollow carbon nanotube-supported bimetallic heterophosphide, and the preparation method comprises the following steps:

[0058] (1) Weigh 548 μL of aniline and 3.5 mL of phytic acid (70 wt.%) and dissolve them in 60 mL of 0.5 mol / L sulfuric acid solution to obtain mixture I. Meanwhile, add 2 mmol of methyl orange to 60 mL of deionized water, continuously stir to form a suspension, then add 0.04 mL of concentrated hydrochloric acid thereto, adjust the pH value of the solution to less than 1, and add 1.25 g of ammonium persulfate, stir for 10 min to form a methyl orange micelle suspension. Drop the above methyl orange micelle suspension into mixture I, and initiate the polymerization reaction of aniline and phytic acid molecules on the methyl orange micelles at 15 °C for 30 min. Filter and wash the reacted turbid liquid, wash it with deionized water until the supernatant is dark green, wash it once with ethanol, and then dry the precipitate overnight at 60 °C. The obtained dark green product is labeled as H-PA-PANI.

[0059] (2) Disperse 0.2 g of H-PA-PANI in 20 mL of ethanol solution and magnetically stir for 30 min to obtain a dispersion. Subsequently, add 0.125 mmol of cobalt chloride and 0.125 mmol of iron chloride to the dispersion, fully stir at 15 °C for 2 h, and then dry the obtained mixture overnight at 60 °C to obtain a precursor. Under a nitrogen atmosphere, heat up to 400 °C at a heating rate of 2 °C / min, calcine the precursor for 2 h, then heat up to 900 °C at a heating rate of 5 °C / min, and continue to calcine for 5 h to obtain a composite material of hollow carbon nanotube-supported bimetallic heterophosphide.

[0060] The preparation flow chart of the composite material of hollow carbon nanotube-supported bimetallic heterophosphide in this example is as Figure 1As shown in the figure. Under acidic conditions, using methyl orange as an inducer, rod-shaped micelles derived from methyl orange and ammonium persulfate as a soft template, aniline and phytic acid molecules react under the action of an initiator and spontaneously coat the surface of the methyl orange micelles, forming a solid structure of aniline-phytic acid copolymer-coated methyl orange micelles. Among them, the ammonium persulfate initiator on the rod-shaped methyl orange micelles induces the growth of the polyaniline-phytic acid nanotube precursor and determines the morphology and structure of the subsequent nanotubes. Next, it is washed with water multiple times until neutral to remove the methyl orange micelles, thereby forming an aniline-phytic acid copolymer with a hollow structure. Subsequently, using ferric chloride and cobalt chloride as metal sources, through the strong coordination ability of the phytic acid component in the copolymer, the adsorption and chelation of metal ions are realized. The hollow cavity structure with a diameter of about 300 nm is more conducive to the uniform adsorption of metal ions inside and outside the precursor. After pyrolyzing the precursor, finally, N, P co-doped carbon nanotubes with a hollow structure are formed, and the active sites of Fe2P and Co2P are evenly dispersed inside and outside the hollow nanotubes. The TG-DSC analysis results of the precursor obtained by loading metal ions in the hollow aniline-phytic acid copolymer under an argon atmosphere show that an obvious exothermic peak appears in the precursor at about 541.7 °C, which can be attributed to the rapid thermal decomposition of the organic polymer. Phytic acid plays a crucial role in this complex preparation process. It realizes the fixation of Fe ions and Co ions by forming stable coordination bonds with metal ions, thereby realizing the in-situ phase reconstruction of Fe2P and Co2P sites. The composite material prepared in Example 1 of the present invention has excellent ORR / OER bifunctional activity and can be used as an air cathode catalyst in RZAB.

[0061] Characterize the microscopic morphology and structure of the composite material prepared in Example 1, and the results are as Figures 2 - 4 shown. From the Figure 2 SEM image, it can be seen that the composite material prepared in Example 1 of the present invention has a uniform hollow carbon nanotube structure. The composite material after loading metal ions well retains the morphology of the hollow carbon nanotubes, and the size of the hollow tubes is about 300 nm. From the Figure 3 HRTEM image, the close interfacial structure between the (1 1 1) crystal plane of Fe2P and the (0 0 2) crystal plane of Co2P can be clearly observed. Figure 4 The HADDF-TEM image shows that some Fe and Co elements are only distributed at the tips of the nanocrystals, while phosphorus elements are dispersed in the entire nanocrystal structure. This result further proves the unique structure of the biphasic Fe2P-CoFeP heterojunction loaded on the hollow carbon tubes.

[0062] Comparative Example 1

[0063] (1) Weigh 548 μL of aniline and 3.5 mL of phytic acid (70 wt.%) and dissolve them in a mixture I of 60 mL of 0.5 mol / L sulfuric acid solution. At the same time, add 2 mmol of methyl orange (MO) to 60 mL of deionized water. After continuously stirring to form a suspension, add 0.04 mL of concentrated hydrochloric acid to it, adjust the pH value of the solution to less than 1, and add 1.25 g of ammonium persulfate. Stir for 10 min to form a methyl orange micelle suspension. Drop the above methyl orange micelle suspension into mixture I, and initiate the polymerization reaction of aniline and phytic acid molecules on the methyl orange micelles at 15 °C for 30 min. Filter and wash the reacted turbid solution, wash it with deionized water until the supernatant is dark green, wash it once with ethanol, and then dry the precipitate overnight at 60 °C. The obtained dark green product is labeled as H-PA-PANI.

[0064] (2) Place H-PA-PANI under a nitrogen atmosphere, heat it at a heating rate of 2 °C / min to reach 400 °C, calcine it for 2 h, then heat it to 900 °C at a heating rate of 5 °C / min, and continue to calcine it for 5 h to obtain a hollow structure N, P-doped carbon nanotube composite material.

[0065] Comparative Example 2

[0066] (1) Weigh 548 μL of aniline and 3.5 mL of phytic acid (70 wt.%) and dissolve them in a mixture I of 60 mL of 0.5 mol / L sulfuric acid solution. At the same time, add 2 mmol of methyl orange (MO) to 60 mL of deionized water. After continuously stirring to form a suspension, add 0.04 mL of concentrated hydrochloric acid to it, adjust the pH value of the solution to less than 1, and add 1.25 g of ammonium persulfate. Stir for 10 min to form a methyl orange micelle suspension. Drop the above methyl orange micelle suspension into mixture I, and initiate the polymerization reaction of aniline and phytic acid molecules on the methyl orange micelles at 15 °C for 30 min. Filter and wash the reacted turbid solution, wash it with deionized water until the supernatant is dark green, wash it once with ethanol, and then dry the precipitate overnight at 60 °C. The obtained dark green product is labeled as H-PA-PANI.

[0067] (2) 0.2 g of H-PA-PANI was dispersed in 20 mL of ethanol solution and magnetically stirred for 30 min to obtain a dispersion. Subsequently, 0.25 mmol of ferric chloride was added to the dispersion, and after thoroughly stirring at 15 °C for 2 h, the resulting mixture was dried overnight at 60 °C to obtain a precursor. Under a nitrogen atmosphere, the temperature was raised at a heating rate of 2 °C / min to reach 400 °C, and the precursor was calcined for 2 h and then heated to 900 °C at 5 °C / min and continuously calcined for 5 h to obtain a hollow-structured carbon nanotube-supported transition metal phosphide composite material.

[0068] Comparative Example 3

[0069] (1) 548 μL of aniline and 3.5 mL of phytic acid (70 wt.%) were weighed and dissolved in 60 mL of 0.5 mol / L sulfuric acid solution to form Mixture I. Meanwhile, 2 mmol of methyl orange (MO) was added to 60 mL of deionized water, and after continuously stirring to form a suspension, 0.04 mL of concentrated hydrochloric acid was added thereto, the pH value of the solution was adjusted to less than 1, and 1.25 g of ammonium persulfate was added and stirred for 10 min to form a methyl orange micelle suspension. The above methyl orange micelle suspension was dropped into Mixture I, and aniline and phytic acid molecules were polymerized on the methyl orange micelles at 15 °C for 30 min. The reaction mixture was filtered and washed, washed with deionized water until the supernatant was dark green, washed once with ethanol, and then the precipitate was dried overnight at 60 °C. The obtained dark green product was labeled as H-PA-PANI.

[0070] (2) 0.2 g of H-PA-PANI was dispersed in 20 mL of ethanol solution and magnetically stirred for 30 min to obtain a dispersion. Subsequently, 0.25 mmol of cobalt chloride was added to the dispersion, and after thoroughly stirring at 15 °C for 2 h, the resulting mixture was dried overnight at 60 °C to obtain a precursor. Under a nitrogen atmosphere, the temperature was raised at a heating rate of 2 °C / min to reach 400 °C, and the precursor was calcined for 2 h and then heated to 900 °C at 5 °C / min and continuously calcined for 5 h to obtain a hollow-structured carbon nanotube-supported transition metal phosphide composite material.

[0071] Comparative Example 4

[0072] (1) Weigh 548 μL of aniline and 3.5 mL of phytic acid (70 wt.%) and dissolve them in 60 mL of 0.5 mol / L sulfuric acid solution to obtain mixture I. Add 0.04 mL of concentrated hydrochloric acid to it, adjust the pH value of the solution to less than 1, and add 1.25 g of ammonium persulfate. Initiate the polymerization reaction of aniline and phytic acid molecules at 15 °C for 30 min. Filter the turbid solution after the reaction by suction filtration, and dry the precipitate overnight at 60 °C to obtain the copolymer.

[0073] (2) Disperse 0.2 g of the copolymer in 20 mL of ethanol solution and stir magnetically for 30 min to obtain a dispersion. Subsequently, add 0.125 mmol of cobalt chloride and 0.125 mmol of iron chloride to the dispersion. After stirring thoroughly at 15 °C for 2 h, dry the resulting mixture overnight at 60 °C to obtain the precursor. Under a nitrogen atmosphere, heat it at a heating rate of 2 °C / min to reach 400 °C, calcine the precursor for 2 h, then heat it to 900 °C at a rate of 5 °C / min and continue to calcine for 5 h to obtain the N, P-doped porous carbon-supported bimetallic composite material.

[0074] Figure 5 Fig. shows the XRD patterns of the composite materials obtained in Example 1 and Comparative Examples 1-4. It can be seen from the figure that the "hump peak" of amorphous carbon appears at 24° in the composite materials prepared in Example 1 and Comparative Examples 1-3, which belongs to the (0 0 2) crystal plane of graphite carbon, while the amorphous carbon "hump peak" in Comparative Example 4 is not obvious. In addition, the broad peak appearing at about 45° in the composite material of Comparative Example 1 belongs to the (1 0 1) crystal plane of graphite carbon. The composite material obtained by calcining the precursor loaded only with cobalt ions in Comparative Example 3 has significant characteristic diffraction peaks of orthorhombic Co2P (JCPDS PDF#89-3030), and the diffraction peaks at 40.7°, 43.3°, 44.0° and 48.7° can be attributed to the (1 1 2), (2 1 1), (1 0 3) and (0 1 3) crystal planes respectively. At the same time, the characteristic diffraction peaks appearing at 40.2°, 44.2° and 47.3° in the composite material obtained by calcining the precursor loaded only with iron ions in Comparative Example 2 correspond to the (1 1 1), (2 0 1) and (2 1 0) crystal planes of hexagonal Fe2P respectively. It should be noted that in the composite material of Example 1, due to the formation of a heterostructure, the characteristic diffraction peaks of Fe2P and Co2P show obvious shifts, and there is a certain peak shift in Comparative Example 4 but it is not obvious.

[0075] Figure 6 Fig. shows the high-resolution X-ray photoelectron spectra of the composite materials obtained in Example 1 and Comparative Examples 2-3. It can be seen from the figure that Fe-P 2p 1 / 2 and Fe-P 2p3 / 2 The positions of the peaks are 719.7 / 707.1 eV and 719.8 / 706.9 eV respectively. Among them, the binding energy of the Fe-P bond in the composite material of Example 1 shifted to higher energy by 0.1 / 0.2 eV, which further confirmed the successful construction of the stable bimetallic phosphide of the present invention. In the Fe 2p spectrum of the composite material of Comparative Example 2, the positions of the Fe-O bond and the satellite peak were also observed to be 709.7 / 723.6 eV and 714.3 / 728 eV respectively, which were inconsistent with the positions of the two characteristic peaks in Example 1 (711.2 / 724.9 eV and 719.8 / 729.5 eV). The shift of the binding energy of Fe 2p to lower energy indicated the enrichment state of electrons. This was because the electronegativity of iron was higher than that of cobalt, resulting in partial transfer of the electron cloud density of cobalt to iron to balance the electronegativity of the system. In the composite materials of Example 1 and Comparative Example 3, Co2p 3 / 2 and Co 2p 1 / 2 appeared at 778.3 / 788.8 eV and 778.7 / 790.4 eV respectively, while the Co-P peak appeared at 778.3 / 788.8 eV and 778.7 / 790.4 eV, which also confirmed the successful construction of the stable bimetallic phosphide of the present invention.

[0076] Using a rotating disk electrode (RDE) as the working electrode, in an oxygen-saturated 0.1 M KOH solution, the ORR and OER electrocatalytic performances of the composite materials in Example 1 and Comparative Examples 1-4 were evaluated through a standard three-electrode system. The results are as Figure 7 、 8 shown. As can be seen from Figure 7 , when the composite material of Example 1 was used as an ORR electrocatalyst, its onset potential (E onset ) was 0.97 V, the half-wave potential (E 1 / 2 ) was 0.83 V, and the limiting current density was -4.50 mA cm -2。The composite material of Comparative Example 1 exhibited a smaller half-wave potential, indicating poor oxygen reduction activity; when the composite material of Comparative Example 2 was used as an ORR electrocatalyst, its onset potential was 1.04 V and the half-wave potential was 0.79 V; when the composite material of Comparative Example 3 was used as an ORR electrocatalyst, its onset potential was 1.03 V and the half-wave potential was 0.81 V; when the composite material of Comparative Example 4 was used as an ORR electrocatalyst, its onset potential was 1.04 V and the half-wave potential was 0.79 V. In addition, it can be seen from the figure that the catalytic activity of the composite material of Example 1 was close to that of the 20 wt% platinum-carbon catalyst, indicating that the composite material of Example 1 had obvious advantages in terms of ORR electrocatalytic efficiency, further confirming the remarkable effect of the hollow-structured carbon nanotube-supported bimetallic phosphide heterojunction composite material of the present invention in optimizing ORR performance.

[0077] The OER electrocatalytic activities of the composite materials in Example 1 and Comparative Examples 1-4 and commercial ruthenium oxide in 1.0 M KOH solution. From Figure 8 it can be seen that when the composite material of Example 1 was used as an OER electrocatalyst, it showed excellent activity, and its low OER overpotential (j 10 ) reached a current density of 10 mA cm -2 at 1.66 V. In contrast, the composite material of Comparative Example 1 showed a larger overpotential at a current density of 10 mA cm -2 , indicating that it had almost no electrocatalytic oxygen evolution performance; the low OER overpotential of the composite material of Comparative Example 2 was 1.77 V; the low OER overpotential of the composite material of Comparative Example 3 was 1.69 V, and the low OER overpotential of the composite material of Comparative Example 4 was 1.69 V. The low OER overpotential of commercial ruthenium oxide was 1.58 V. The above results further confirmed that the formation of the heterojunction in Example 1 could effectively accelerate the OER reaction kinetics.

[0078] A flow-type secondary zinc-air battery (RZAB) was constructed using the composite material in Example 1 as the air cathode, KOH solution as the electrolyte, and zinc foil as the anode. The physical diagram and open-circuit voltage (OCV) are as Figure 9 shown, and its open-circuit voltage was as high as 1.55 V. The polarization and power density curves of the RZAB are as Figure 10 shown. The figure shows that the RZAB assembled based on the composite material of Example 1 achieved a peak power density of 145 mW cm -2 at a current density of 5 mAcm -2 , which was much higher than 142 mW cm -2 of the batteries based on commercial platinum-carbon and ruthenium oxide. The charge-discharge polarization curves of the RZAB are as Figure 11As shown in the figure, it can be seen that the RZAB assembled from the composite material based on Example 1 has a narrower charge-discharge voltage difference compared to the RZAB assembled from platinum-carbon and ruthenium oxide, indicating better charge-discharge efficiency.

[0079] The long-term stability of the composite material of Example 1 and the platinum-carbon / ruthenium oxide cathode was further evaluated by constant current discharge-charge measurements at 5 mA cm -2 . The results are as Figure 12 shown. The results show that the initial voltage difference between the discharge and charge processes of the RZAB based on the composite material of Example 1 was calculated to be 0.91 V. After 500 and 1000 h of cyclic testing, its voltage difference increased slightly, while the platinum-carbon / ruthenium oxide-based RZAB showed poor durability within 220 h.

[0080] In summary, through the strong coordination ability of the organic phosphorus source in the hollow-structured aromatic organic nitrogen source-organic phosphorus source copolymer of the present invention, the adsorption and chelation of bimetallic ions are realized, and a bimetallic heterophosphide electrocatalyst supported on hollow carbon nanotubes is obtained by pyrolysis. Through the chemical anchoring-in-situ phase reconstruction strategy, the composite material prepared has excellent ORR / OER bifunctional catalytic activity. As the cathode of RZAB, it can effectively improve the energy density and cycle stability of RZAB, and has good application prospects.

[0081] The above specific embodiments have described the implementation of the present invention in detail. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the claims and technical concept of the present invention, various simple modifications and changes can be made to the technical solutions of the present invention, and these simple variants all belong to the protection scope of the present invention.

Claims

1. A preparation method of a hollow carbon nanotube-supported bimetallic heterophosphide composite material, characterized in that, It includes the following steps: (1) Mix an organic phosphorus source and an aromatic organic nitrogen source evenly in an acidic solution to obtain mixture I; adjust the pH value of the methyl orange solution to less than 1, add an initiator and mix evenly to obtain mixture II; (2) Mix mixture I and mixture II for reaction. After the reaction ends, filter the reaction solution by suction and wash it to obtain a copolymer; (3) Disperse the copolymer in a solvent to obtain a dispersion liquid, add a metal cobalt source and a metal iron source to the dispersion liquid, mix evenly, and dry to obtain a precursor; (4) Calcinate the precursor in an inert atmosphere to obtain a hollow carbon nanotube-supported bimetallic heterophosphide composite material.

2. The preparation method of a hollow carbon nanotube-supported bimetallic heterophosphate composite material according to claim 1, characterized in that, In step (1), the mass ratio of the organic phosphorus source to the aromatic organic nitrogen source is (3-9):1; the mass ratio of the organic phosphorus source, methyl orange, and initiator is (3-9):1:(1-3).

3. The preparation method of a bimetallic heterophosphate composite material supported by hollow carbon nanotubes according to claim 1, characterized in that, In step (3), the mass ratio of the copolymer, metal cobalt source, and metal iron source is (2-10):1:

1.

4. The preparation method of a bimetallic heterophosphate composite material supported by hollow carbon nanotubes according to claim 1, characterized in that In step (1), the organic phosphorus source includes but is not limited to phytic acid, and the aromatic organic nitrogen source includes but is not limited to aniline.

5. The preparation method of a bimetallic heterophosphate composite material supported on hollow carbon nanotubes according to claim 1, characterized in that In step (1), the acidic solution includes but is not limited to a sulfuric acid solution, and the concentration of the acidic solution is 0.05-2 mol / L.

6. The preparation method of a bimetallic heterophosphate composite material supported on hollow carbon nanotubes according to claim 1, characterized in that, In step (2), the temperature of the reaction is 5-25 °C, and the time is 5-30 min.

7. The preparation method of a bimetallic heterophosphate composite material supported on hollow carbon nanotubes according to claim 1, characterized in that, In step (3), the metal cobalt source includes but is not limited to cobalt chloride, and the metal iron source includes but is not limited to iron chloride.

8. The preparation method of a bimetallic heterophosphate composite material supported by hollow carbon nanotubes according to claim 1, wherein, In step (4), the temperature of the calcination is 400-1000 °C, and the time is 6-10 h.

9. A hollow carbon nanotube-supported bimetallic heterophosphide composite material prepared by the method according to any one of claims 1-8.

10. Application of the hollow carbon nanotube-supported bimetallic heterophosphide composite material according to claim 9 in a zinc-air battery.

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