Preparation and application of heterogeneous element doped oxide composite catalyst

Abstract

The invention discloses preparation and application of a heterogeneous element doped oxide composite catalyst. The preparation method comprises the following steps: stirring an iron source, carbon nanotubes and triton in a solvent, adding a sodium borohydride solution at 2-10 DEG C, and rapidly stirring and uniformly mixing; aging the solution at 2-10 DEG C, collecting a sample, washing and drying; carrying out heat treatment to obtain a gamma-Fe2O3 precursor loaded on the carbon nano tube; stirring the gamma-Fe2O3 precursor loaded on the carbon nano tube and a heterogeneous element source in a solvent, evaporating to dryness and grinding; and performing heat treatment to obtain the composite catalyst. The gamma-Fe2O3 and carbon nanotube in-situ loaded composite material is used as a precursor, the structural stability of the substrate is effectively improved, meanwhile, the intrinsic catalytic activity of the gamma-Fe2O3 can be effectively improved through doping of multi-element heterogeneous elements, and compared with a fractional-step doping strategy, higher catalytic activity and higher utilization value are shown.

Description

Technical Field

[0001] This invention belongs to the fields of new materials technology and chemical synthesis technology, and relates to the preparation and application of a heterogeneous element-doped oxide composite catalyst; in particular, it relates to the preparation and application of a heterogeneous element-doped oxide composite catalyst with γ-Fe2O3 composite substrate as precursor. Background Technology

[0002] Since the beginning of the 21st century, people have paid increasing attention to energy security and environmental issues, leading to a growing demand for new green, safe, renewable energy sources and energy conversion devices, making their development and utilization urgent. Metal-air batteries, as a representative of new energy conversion devices, hold promise for widespread use due to their high energy density and environmental friendliness. A metal-air battery consists of an air electrode, an electrolyte, and a metal negative electrode. Its working principle involves oxygen undergoing a reduction reaction in the air electrode, combining with electrons generated by the oxidation reaction of the metal at the negative electrode. These electrons then travel through an external circuit to the positive electrode, outputting electrical energy to the outside of the system. The rate of the oxygen reduction reaction at the positive electrode is crucial to the efficiency of the metal-air battery. However, its slow reaction kinetics result in low efficiency, hindering the commercial development of metal-air batteries. Improving its reaction kinetics through the design of reasonable oxygen reduction catalysts is currently a major research direction. Platinum-based noble metal materials are currently the most commonly used commercial oxygen reduction catalysts, but their high price and low reserves limit large-scale application. Therefore, developing high-efficiency, low-cost, and stable non-noble metal oxygen reduction catalysts has extremely important scientific significance and practical value.

[0003] In practical applications and catalysis, spinel transition metal oxides, especially iron-based oxides, have been widely studied as non-noble metal oxygen reduction catalysts due to their low cost and potential to provide various active sites. Introducing heteroelemental elements into spinel oxides is an important strategy to improve their catalytic activity. Since the electronic interactions between multiple metal elements can induce synergistic effects, they play a crucial role in regulating the adsorption and desorption energies of reaction intermediates, thereby optimizing the reaction pathway. In the spinel structure, +2 and +3 valence metals occupy tetrahedral or octahedral sites, with octahedral sites considered superior active sites due to their greater electron-donating ability. Therefore, doping heteroelemental elements into octahedral sites is more beneficial for improving catalytic activity. However, currently, heteroelemental elements are mostly doped into the oxide lattice during synthesis on an oxide substrate. Because heteroelemental elements randomly occupy octahedral and tetrahedral sites when the substrate elements crystallize, this results in highly active heteroelemental elements being randomly doped into inactive tetrahedral sites, limiting their efficiency at octahedral sites. Due to the lack of directional induction, avoiding the simultaneous incorporation of heterogeneous elements into the octahedral sites of spinel while simultaneously incorporating them into inert tetrahedral sites is a challenge. Furthermore, current methods often use various metal salts as precursors, employing liquid-phase or solid-phase heat treatment processes, which frequently require high-temperature and high-pressure treatments. This not only consumes a lot of energy but also results in uncontrollable product phases and the potential for impurity phase formation. Therefore, enhancing oxygen reduction catalytic activity by incorporating heterogeneous elements into the highly active octahedral sites of spinel is a very effective method for improving catalytic activity. On the other hand, oxide catalysts generally have poor conductivity, necessitating structural optimization to improve their conductivity.

[0004] A method for inducing heteroelement doping into more active octahedrons using oxides as precursors is feasible, differing from previous strategies aimed at heteroelement doping during the synthesis of oxide substrates. However, achieving heteroelement doping based on oxide lattices as precursors is challenging for two main reasons. First, defect-free oxide crystals lack stable doping sites, making it difficult to replace substrate elements with heteroelemental elements or form interstitial heteroatoms by introducing dopant elements. Second, the inter-element incompatibility between the substrate and the heteroelement makes it difficult to maintain the stable presence of heteroelemental elements in the substrate crystal structure. Therefore, finding a preparation method using oxide substrates rich in stable heteroatom doping sites as precursors is a highly valuable endeavor.

[0005] The inventors' research group previously disclosed in CN 118831609 A a method for preparing and applying a manganese-doped oxide composite catalyst supported on the surface of carbon nanotubes, using commercially available γ-Fe₂O₃ as a doping precursor. The method involves adding γ-Fe₂O₃ precursor and carbon nanotubes to anhydrous ethanol to obtain a mixed solution; continuing to add manganese nitrate solution and heating and stirring until dry to obtain a black powder; placing the black powder in a crucible and heat-treating the crucible in a muffle furnace; after cooling, washing with alcohol and deionized water, and drying to obtain the manganese-doped γ-Fe₂O₃ composite catalyst supported on the surface of carbon nanotubes. The manganese-doped γ-Fe₂O₃ composite catalyst supported on the surface of carbon nanotubes uses γ-Fe₂O₃ as a precursor and consists of manganese-doped γ-Fe₂O₃ and carbon nanotubes, with the manganese-doped γ-Fe₂O₃ particles uniformly distributed on the carbon nanotubes. This preparation method requires relatively simple equipment, is easy to operate, has controllable conditions, high reproducibility, and low cost. However, previous preparation methods used a stepwise synthesis approach to achieve the composite of γ-Fe2O3 and carbon nanotubes. Due to the lack of sufficient binding energy between the two, the oxide particles are prone to agglomeration, resulting in structural failure and thus affecting the catalytic effect of the dopant elements. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the existing technology and provide a method for preparing and applying heteroelement-doped oxide composite catalysts. This invention selects γ-Fe₂O₃ with intrinsically vacant defects at octahedral sites and a composite material in situ supported by carbon nanotubes as the doping precursor. Simultaneously, heteroelement elements with similar radii and electron configurations to the substrate iron element are selected as the doping heteroelement. Multi-walled carbon nanotubes are then composited to improve the catalyst's conductivity. The proposed preparation process effectively improves the structural stability of the substrate, while the doping of multiple heteroelement elements effectively enhances the intrinsic catalytic activity of γ-Fe₂O₃. This method is simple, easy to operate, and highly reproducible. The heteroelement elements are uniformly distributed on the γ-Fe₂O₃, and the oxide particles are tightly bonded to the carbon nanotubes. The catalyst exhibits high catalytic activity, a simple process, high stability, and excellent prospects for energy catalysis applications.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] <First Aspect>

[0009] This invention provides a method for preparing a heterogeneous element-doped oxide composite catalyst, the method comprising the following steps:

[0010] S1. Stir the iron source, carbon nanotubes, and Triton in a solvent, then add sodium borohydride solution at 2-10℃ and stir quickly until homogeneous.

[0011] S2. After aging the solution at 2-10℃, collect the sample, wash, and dry it.

[0012] S3. Heat treatment yields γ-Fe2O3 precursor loaded on carbon nanotubes;

[0013] S4. Stir the γ-Fe2O3 precursor loaded with carbon nanotubes and the heterogeneous element source in a solvent, evaporate to dryness and grind.

[0014] S5. Heat treatment yields the composite catalyst.

[0015] The key to this invention lies in in-situ loading by adding carbon nanotubes during low-temperature liquid-phase reduction. Alternatively, S1 can be added to a heterogeneous element source solution (such as a nitric acid heterogeneous element solution) after low-temperature stirring and then dried to obtain the final product. However, since the precursor phase is not γ-Fe2O3, its lattice does not contain cation vacancies, thus failing to achieve the effect of inducing highly active heterogeneous elements to be doped into highly active octahedral sites.

[0016] The heteroelement-doped oxide composite catalytic material prepared by the method of this invention is composed of heteroelement-doped γ-Fe₂O₃ particles and carbon nanotubes, wherein the heteroelement-doped γ-Fe₂O₃ particles are uniformly distributed on the carbon nanotubes. The heteroelement-doped precursor is composed of γ-Fe₂O₃ loaded onto carbon nanotubes with intrinsically vacant defects at octahedral sites. In some embodiments, the γ-Fe₂O₃ precursor loaded onto carbon nanotubes is used as the heteroelement-doped substrate, a heteroelement source is added, and a one-step heat treatment is performed in a muffle furnace to uniformly distribute the heteroelement-doped γ-Fe₂O₃ particles on the carbon nanotubes.

[0017] As one embodiment of the present invention, in step S1, the iron source is selected from at least one of anhydrous ferric chloride, ferric chloride hexahydrate, and ferric nitrate.

[0018] In one embodiment of the present invention, in step S1, the solvent is selected from at least one of anhydrous ethanol and water.

[0019] In one embodiment of the present invention, in step S1, the stirring speed of rapid stirring is 600-2000 r / min, and the stirring time is 5-20 h.

[0020] In one embodiment of the present invention, in step S1, the concentrations of iron source, carbon nanotubes, Triton, and sodium borohydride in the mixture formed by stirring are 0.1 to 40 g / L, respectively.

[0021] In one embodiment of the present invention, in step S2, the collected sample is ultrasonically cleaned and dried multiple times with deionized water and anhydrous ethanol. The ultrasonic cleaning time is 20–60 min; the drying temperature is 40–100 °C, and the drying time is 6–24 h. In some embodiments, the drying temperature is 30 °C–80 °C.

[0022] In one embodiment of the present invention, in step S3, the temperature of the heat treatment is 150-450°C and the reaction time is 1-8 hours.

[0023] As one embodiment of the present invention, in step S4, the heterogeneous element source is selected from nitrate heterogeneous elements, sulfate heterogeneous elements, or chloride heterogeneous elements.

[0024] In one embodiment of the present invention, in step S4, the solvent is selected from at least one of anhydrous ethanol, water, and isopropanol.

[0025] In one embodiment of the present invention, in step S4, the stirring speed is 600-2000 r / min and the stirring time is 0.5-5 h.

[0026] In one embodiment of the present invention, in step S4, the concentrations of the precursor and the heterogeneous element source in the mixture formed by stirring are 0.1 to 40 g / L, respectively.

[0027] In one embodiment of the present invention, in step S4, the evaporation temperature is 30°C to 80°C.

[0028] In one embodiment of the present invention, in step S5, the temperature of the heat treatment is 150-450°C and the reaction time is 1-10 hours.

[0029] In one embodiment of the present invention, in step S1, the iron source is anhydrous ferric chloride with a purity of 80.0%–99.999%. The solvent is anhydrous ethanol with a purity of 80.0%–99.7%. The carbon nanotubes have a purity of 80.0%–99.7%. The Triton nanotubes have a purity of 80.0%–99.7%. The sodium borohydride has a purity of 80.0%–99.7%.

[0030] In one embodiment of the present invention, in step S4, the solvent is anhydrous ethanol with a purity of 80.0-99.7%; the heteroelement salt has a purity of 40-99.7%.

[0031] In some implementation examples, the preparation of heteroelement-doped oxide composite catalytic materials includes the following steps:

[0032] (1) Weigh out anhydrous ferric chloride, carbon nanotubes and Triton, stir them in a solvent, add sodium borohydride solution at 2-10℃ and stir quickly to mix evenly.

[0033] (2) After aging the product obtained in (1) in a refrigerator (2-10℃), the sample is collected, washed, and dried.

[0034] (3) The product obtained in (2) was heat-treated in a muffle furnace to obtain a γ-Fe2O3 substrate loaded with carbon nanotubes;

[0035] (4) Weigh the product obtained in (3) loaded on carbon nanotubes γ-Fe2O3 and nitric acid heterogeneous element solution, stir in anhydrous ethanol, evaporate to dryness and grind evenly.

[0036] (5) Place the product obtained in (4) into a crucible and place the crucible into a muffle furnace for one-step heat treatment to obtain a heterogeneous element-doped oxide composite catalyst with γ-Fe2O3 as the precursor.

[0037] (6) The powder prepared in (5) is mixed with carbon powder, isopropanol and Nafion solution and ultrasonically mixed to obtain a slurry. The slurry is then dropped onto a rotating disk electrode and dried to obtain an electrode for later use.

[0038] <Second aspect>

[0039] The present invention also provides an electrode for an oxygen reduction reaction, wherein the electrode contains a heterogeneous element-doped oxide composite catalyst prepared by the aforementioned method.

[0040] As one embodiment of the present invention, the preparation of the electrode includes: mixing the heterogeneous element-doped oxide composite catalyst with carbon powder, isopropanol, and Nafion solution, ultrasonically obtaining a slurry, and dropping it onto a rotating disk electrode and drying it to obtain an electrode for later use.

[0041] In one embodiment of the present invention, the composite catalyst has a purity of 80.0%–99.7%. The carbon powder has a purity of 80.0%–99.7%. The isopropanol has a purity of 80.0%–99.7%. The Nafion has a purity of 2%–99.7%.

[0042] In one embodiment of the present invention, the amount of composite catalyst and carbon powder in the slurry is 0.1-10 kg / L, and the concentrations of isopropanol and Nafion are 0.2-8 mol / L, respectively.

[0043] As one embodiment of the present invention, the drying temperature is 40-100℃ and the time is 6-24h.

[0044] <Third aspect>

[0045] The present invention also provides a three-electrode system for oxygen reduction reaction, wherein the working electrode of the three-electrode system contains a heterogeneous element-doped oxide composite catalyst prepared by the aforementioned method, a carbon rod is used as the counter electrode, a saturated calomel electrode is used as the reference electrode, and a (0.1 mol / L) KOH solution is used as the electrolyte.

[0046] In some embodiments, the heterogeneous element-doped oxide composite catalyst of the present invention, with γ-Fe2O3 supported on carbon nanotubes as a precursor, is preferably used as the test electrode for the oxygen reduction reaction as described in step (6) above.

[0047] An electrode prepared using a heterogeneous element-doped oxide composite catalyst with γ-Fe2O3 supported on carbon nanotubes as the precursor was used as the working electrode, a carbon rod as the counter electrode, a saturated calomel electrode as the reference electrode, and a 0.1 mol / L KOH solution as the electrolyte to form a three-electrode system for oxygen reduction reaction.

[0048] The mechanism of action of this invention lies in the fact that γ-Fe₂O₃ has an inverse spinel structure with a high vacancy content of approximately 11% iron vacancies at octahedral sites. These vacancies provide necessary doping sites for the introduction of heterogeneous elements and reduce the binding energy of substrate elements. Therefore, this facilitates the migration of heterogeneous elements within the substrate lattice and preferential doping at octahedral sites. Furthermore, based on the similar ionic radii and electron arrangements of iron and heterogeneous elements, the heterogeneous elements are used as suitable heterogeneous dopants, which helps the dopant heterogeneous elements to exist stably at the original iron sites in the γ-Fe₂O₃ lattice, avoiding significant internal lattice stress caused by significant differences in element size and mismatch in the number of coordinating elements. In addition, the difference in adsorption energy between the heterogeneous elements and iron on the oxygen reduction reaction intermediate may lead to beneficial synergistic effects. Furthermore, to overcome the inherent conductivity limitations of γ-Fe₂O₃, highly conductive carbon nanotubes are integrated with the catalyst. The synergistic promotion of these two aspects contributes to improving the performance of the catalyst. More importantly, the in-situ composite of γ-Fe₂O₃ and carbon nanotubes as a doped precursor effectively improves the structural stability of the catalyst compared to distributed loading and doping. Simultaneously, the multi-element co-doping effectively coordinates the electronic structure of the catalyst, enhancing its intrinsic catalytic activity.

[0049] Compared with the prior art, the present invention has the following advantages:

[0050] (1) The oxygen reduction half-reaction catalytic material provided by the present invention has a uniform morphology and is composed of heterogeneous element doped γ-Fe2O3 and carbon nanotubes. The γ-Fe2O3 that is in situ composite with carbon nanotubes has higher structural stability, which is more conducive to the exposure of active sites and the increase of conductivity, thereby promoting the improvement of electrochemical performance.

[0051] (2) The preparation method proposed in this invention requires simple equipment, is easy to operate, has controllable conditions, high repeatability, and low preparation cost, making it suitable for large-scale factory production.

[0052] (3) γ-Fe₂O₃ containing octahedral vacancies loaded on carbon nanotubes is used as a precursor to induce the doped heteroelement to enter the octahedral positions of the anti-spinel, thereby improving utilization efficiency. Carbon nanotubes can provide one-dimensional conductive network channels. Therefore, the obtained heteroelement-doped oxide composite catalyst with γ-Fe₂O₃ loaded on carbon nanotubes as a precursor has high conductivity, good intrinsic catalytic activity, and superior reaction kinetics. At the same time, the bonding force between the heteroelement-doped γ-Fe₂O₃ and carbon nanotubes is strong, reducing contact resistance and making the structure more stable. Compared with distributed composites and doping, it effectively reduces the aggregation of nanoparticles, while multi-element doping improves the catalytic activity of the catalyst. Thanks to these advantages, the electrode exhibits excellent oxygen reduction activity and stability in alkaline solutions, and has broad application prospects in metal-air batteries and other fields. Attached Figure Description

[0053] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0054] Figure 1 Aberration-corrected electron microscopy image of the heteroelement-doped oxide composite catalyst prepared by the present invention, which uses carbon nanotube-supported γ-Fe2O3 as a precursor;

[0055] Figure 2 These are transmission electron microscopy (TEM) images and high-resolution TEM images of the heteroelement-doped oxide composite catalyst prepared by the present invention, which uses carbon nanotube-supported γ-Fe2O3 as a precursor.

[0056] Figure 3 The image shows the elemental distribution of the heterogeneous element-doped oxide composite catalyst prepared by the present invention, which uses carbon nanotube-supported γ-Fe2O3 as a precursor.

[0057] Figure 4 The XRD curves of the heteroelement-doped oxide composite catalyst with carbon nanotube-supported γ-Fe2O3 as precursor prepared in this invention are shown.

[0058] Figure 5 The LSV diagram shows the oxygen reduction of the heterogeneous element-doped oxide composite catalyst prepared in this invention with carbon nanotube-supported γ-Fe2O3 as the precursor in 0.1M KOH electrolyte.

[0059] Figure 6 The figure shows the long-term oxygen reduction stability test results of the heterogeneous element-doped oxide composite catalyst with carbon nanotube-supported γ-Fe2O3 as precursor prepared in this invention in 0.1M KOH electrolyte.

[0060] Figure 7The image shows the morphology of the heterogeneous element-doped oxide composite catalyst prepared in this invention, with carbon nanotube-supported γ-Fe2O3 as the precursor, after stability testing in 0.1M KOH electrolyte. Detailed Implementation

[0061] The present invention will be described in detail below with reference to specific embodiments and corresponding accompanying drawings. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0062] This invention uses a composite material of γ-Fe2O3 and carbon nanotubes in situ as a precursor, which effectively improves the structural stability of the substrate. At the same time, the doping of multiple heterogeneous elements can effectively improve the intrinsic catalytic activity of γ-Fe2O3, showing higher catalytic activity than mono-Mn doping, and thus has higher utilization value.

[0063] Example 1

[0064] (1) Weigh ferric chloride, carbon nanotubes and Triton at a concentration of 0.1 g / L, sonicate in ultrapure water and stir for 40 min; stir for 30 min.

[0065] (2) Weigh 0.1 g / L of sodium borohydride solution at 4℃ and quickly add it to the solution in step (1). Stir and mix for 2 min, place it in a refrigerator at 4℃ for 2 h, ultrasonically clean it several times in ultrapure water and anhydrous ethanol, and dry it. The ultrasonic time is 40 min; the drying temperature is 60℃ and the drying time is 12 h.

[0066] (3) Place the product obtained in step (2) into a crucible, place the crucible in a muffle furnace, and keep it at 300°C for 5 hours;

[0067] (4) The product obtained in step (3) was washed with deionized water and alcohol and dried at 60°C for 12 hours to obtain a carbon nanotube-supported γ-Fe2O3 vacancy-controlled iron-based catalyst.

[0068] (5) Weigh γ-Fe2O3 loaded with carbon nanotubes at a concentration of 0.1 g / L, ultrasonically clean it several times in ultrapure water or anhydrous ethanol, and dry it; the ultrasonic time is 40 min; the drying temperature is 60 ℃ and the drying time is 12 h.

[0069] (6) Weigh out γ-Fe2O3 loaded with carbon nanotubes and 0.1 g / L manganese nitrate and cobalt nitrate solution (ratio 1:1) and stir magnetically in anhydrous ethanol at 1200 r / min for 10 h at room temperature until they are mixed evenly. Continue stirring at 80 °C until dry and grind evenly.

[0070] (7) Place the product obtained in step (6) into a crucible, place the crucible in a muffle furnace, and keep it at 300°C for 5 hours to obtain a heterogeneous element-doped oxide composite catalyst.

[0071] (8) Mix 10 mg of the product obtained in step (7) with 10 mg of carbon powder, 0.965 ml of isopropanol and 0.035 ml of Nafion solution, and sonicate to obtain a slurry. Drop the slurry onto a rotating disk electrode and let it dry to obtain a test electrode.

[0072] Figure 1 The image shown is a spherical aberration electron microscope image of the heteroelement-doped oxide composite catalyst prepared in this invention, which uses carbon nanotube-supported γ-Fe2O3 as a precursor. It shows that the heteroelement is doped at octahedral sites in γ-Fe2O3.

[0073] Figure 2 This is a high-resolution transmission electron microscope (TEM) image of the heteroelement-doped oxide composite catalyst prepared in this invention, using carbon nanotube-supported γ-Fe₂O₃ as a precursor. The image shows that the material has a one-dimensional nanostructure, with the heteroelement-doped oxide particles tightly bonded to the carbon nanotubes, which is beneficial for improving electrochemical performance.

[0074] Figure 3 The image shows the elemental distribution of the heterogeneous element-doped oxide composite catalyst prepared in this invention, which uses γ-Fe2O3 supported on carbon nanotubes as a precursor. It shows that the elements are uniformly distributed in the sample.

[0075] Figure 4 The XRD spectrum of the heteroelement-doped oxide composite catalyst with γ-Fe2O3 supported on carbon nanotubes as the precursor prepared in this invention shows the presence of γ-Fe2O3 and carbon nanotube phases, but no heteroelement phase. The heteroelement is doped into the lattice of γ-Fe2O3 without forming an impurity phase.

[0076] Figure 5 The image shows the LSV of the heteroelement-doped oxide composite catalyst with carbon nanotube-supported γ-Fe2O3 as the precursor prepared in this invention during oxygen reduction in 0.1M KOH electrolyte. The heteroelement-doped oxide composite catalyst with γ-Fe2O3 as the precursor can increase the oxygen reduction half-wave potential to 0.86V, which can effectively improve the energy conversion efficiency.

[0077] Figure 6The figure shows the long-term oxygen reduction stability test results of the heteroelement-doped oxide composite catalyst with carbon nanotube-supported γ-Fe2O3 as the precursor prepared in this invention in 0.1M KOH electrolyte. The material can still maintain good stability after 10,000 CV scans in an alkaline environment, indicating that it has good stability.

[0078] Figure 7 The image shows the morphology of the heteroelement-doped oxide composite catalyst with γ-Fe₂O₃ as the precursor prepared in this invention after stability testing in 0.1 MkOH electrolyte. This indicates that the composite structure possesses excellent structural stability.

[0079] Example 2

[0080] (1) Weigh ferric chloride, carbon nanotubes and Triton at a concentration of 0.1 g / L, sonicate in ultrapure water and stir for 40 min; stir for 30 min.

[0081] (2) Weigh 0.1 g / L of sodium borohydride solution at 4℃ and quickly add it to the solution in step (1). Stir and mix for 2 min, place it in a refrigerator at 4℃ for 2 h, ultrasonically clean it several times in ultrapure water and anhydrous ethanol, and dry it. The ultrasonic time is 40 min; the drying temperature is 60℃ and the drying time is 12 h.

[0082] (3) Place the product obtained in step (2) into a crucible, place the crucible in a muffle furnace, and keep it at 300°C for 5 hours;

[0083] (4) The product obtained in step (3) was washed with deionized water and alcohol and dried at 60°C for 12 hours to obtain a carbon nanotube-supported γ-Fe2O3 vacancy-controlled iron-based catalyst.

[0084] (5) Weigh γ-Fe2O3 loaded with carbon nanotubes at a concentration of 0.1 g / L, ultrasonically clean it several times in ultrapure water or anhydrous ethanol, and dry it; the ultrasonic time is 40 min; the drying temperature is 60 ℃ and the drying time is 12 h.

[0085] (6) Weigh the carbon nanotube-loaded γ-Fe2O3 and 0.1 g / L manganese nitrate solution and stir magnetically at 1200 r / min for 10 h at room temperature in anhydrous ethanol until they are evenly mixed. Continue stirring at 80 °C until dry and grind evenly.

[0086] (7) Place the product obtained in step (6) into a crucible, place the crucible in a muffle furnace, and keep it at 300°C for 5 hours to obtain a heterogeneous element-doped oxide composite catalyst.

[0087] (8) Mix 10 mg of the product obtained in step (7) with 10 mg of carbon powder, 0.965 ml of isopropanol and 0.035 ml of Nafion solution, and sonicate to obtain a slurry. Drop the slurry onto a rotating disk electrode and let it dry to obtain a test electrode.

[0088] Characterization of its morphology and structure using aberration-corrected electron microscopy, transmission electron microscopy, and XRD revealed the successful fabrication of a heteroelement-doped oxide composite catalyst with carbon nanotube-supported γ-Fe₂O₃ as the precursor. The heteroelement-doped oxide composite catalyst prepared in this example, using carbon nanotube-supported γ-Fe₂O₃ as the precursor, is composed of γ-Fe₂O₃ and carbon nanotubes. The heteroelement-doped oxide particles are uniformly distributed on the carbon nanotubes, exhibiting a near-one-dimensional structure.

[0089] In this embodiment, the heteroelement-doped oxide composite catalyst with γ-Fe2O3 as the precursor was tested for oxygen reduction reaction activity in 0.1 MkOH electrolyte. The results showed that the oxygen reduction half-wave potential of the heteroelement-doped oxide composite catalyst with γ-Fe2O3 as the precursor is about 0.845 V, which can effectively improve the energy conversion efficiency.

[0090] Example 3

[0091] (1) Weigh ferric chloride, carbon nanotubes and Triton at a concentration of 0.1 g / L, sonicate in ultrapure water and stir for 40 min; stir for 30 min.

[0092] (2) Weigh 0.1 g / L of sodium borohydride solution at 4℃ and quickly add it to the solution in step (1). Stir and mix for 2 min, place it in a refrigerator at 4℃ for 2 h, ultrasonically clean it several times in ultrapure water and anhydrous ethanol, and dry it. The ultrasonic time is 40 min; the drying temperature is 60℃ and the drying time is 12 h.

[0093] (3) Place the product obtained in step (2) into a crucible, place the crucible in a muffle furnace, and keep it at 300°C for 5 hours;

[0094] (4) The product obtained in step (3) was washed with deionized water and alcohol and dried at 60°C for 12 hours to obtain a carbon nanotube-supported γ-Fe2O3 vacancy-controlled iron-based catalyst.

[0095] (5) Weigh γ-Fe2O3 loaded with carbon nanotubes at a concentration of 0.1 g / L, ultrasonically clean it several times in ultrapure water or anhydrous ethanol, and dry it; the ultrasonic time is 40 min; the drying temperature is 60 ℃ and the drying time is 12 h.

[0096] (6) Weigh the carbon nanotube-loaded γ-Fe2O3 and 0.1 g / L cobalt nitrate solution and stir magnetically at 1200 r / min for 10 h at room temperature in anhydrous ethanol until they are evenly mixed. Continue stirring at 80 °C until dry and grind evenly.

[0097] (7) Place the product obtained in step (6) into a crucible, place the crucible in a muffle furnace, and keep it at 300°C for 5 hours to obtain a heterogeneous element-doped oxide composite catalyst.

[0098] (8) Mix 10 mg of the product obtained in step (7) with 10 mg of carbon powder, 0.965 ml of isopropanol and 0.035 ml of Nafion solution, and sonicate to obtain a slurry. Drop the slurry onto a rotating disk electrode and let it dry to obtain a test electrode.

[0099] Characterization of its morphology and structure using aberration-corrected electron microscopy, transmission electron microscopy, and XRD revealed the successful fabrication of a heteroelement-doped oxide composite catalyst with carbon nanotube-supported γ-Fe₂O₃ as the precursor. The heteroelement-doped oxide composite catalyst prepared in this example, using carbon nanotube-supported γ-Fe₂O₃ as the precursor, is composed of γ-Fe₂O₃ and carbon nanotubes. The heteroelement-doped oxide particles are uniformly distributed on the carbon nanotubes, exhibiting a near-one-dimensional structure.

[0100] In this embodiment, the heteroelement-doped oxide composite catalyst with γ-Fe2O3 as the precursor was tested for oxygen reduction reaction activity in 0.1 MkOH electrolyte. The results showed that the oxygen reduction half-wave potential of the heteroelement-doped oxide composite catalyst with γ-Fe2O3 as the precursor was about 0.832 V, which can effectively improve the energy conversion efficiency.

[0101] Comparative Example 1

[0102] (1) Weigh out commercial α-Fe2O3 with a concentration of 0.1 g / L, ultrasonically clean it several times in ultrapure water and anhydrous ethanol, and dry it; the ultrasonic time is 40 min; the drying temperature is 60 ℃ and the drying time is 12 h;

[0103] (2) Weigh 0.1 g / L of cleaned commercial α-Fe2O3, 0.1 g / L of carbon nanotubes, and 0.1 g / L of manganese nitrate solution and stir magnetically at 1200 r / min for 10 h in anhydrous ethanol at room temperature until they are evenly mixed. Continue stirring at 80 °C until dry and grind evenly.

[0104] (3) Place the product obtained in step (2) into a crucible, place the crucible in a muffle furnace, and keep it at 400°C for 5 hours;

[0105] (4) The product obtained in step (3) was washed with deionized water and alcohol and dried at 60°C for 12 hours to obtain a heterogeneous element-doped oxide composite catalyst with α-Fe2O3 as the precursor.

[0106] (5) Mix 10 mg of the product obtained in step 4 with 10 mg of carbon powder, 0.965 ml of isopropanol and 0.035 ml of Nafion solution and sonicate to obtain a slurry. Drop the slurry onto a rotating disk electrode and let it dry to obtain a test electrode.

[0107] Characterization of its morphology and structure using spherical aberration electron microscopy, transmission electron microscopy, and XRD revealed the successful fabrication of a heteroelement-doped oxide composite catalyst with α-Fe₂O₃ as the precursor. The heteroelement-doped oxide composite catalyst prepared in this example consists of α-Fe₂O₃ and carbon nanotubes, with the heteroelement-doped oxide particles uniformly distributed on the carbon nanotubes, exhibiting a near-one-dimensional structure.

[0108] In this comparative example, the heteroelement-doped oxide composite catalyst prepared with α-Fe2O3 as the precursor was tested for oxygen reduction reaction activity in 0.1 MkOH electrolyte. The results showed that the oxygen reduction half-wave potential of the heteroelement-doped oxide composite catalyst with α-Fe2O3 as the precursor was about 0.70 V. This demonstrates that the catalyst prepared using vacancy-free α-Fe2O3 as the precursor has a significantly lower catalytic efficiency than the catalyst prepared using vacancy-containing γ-Fe2O3 as the precursor.

[0109] Comparative Example 2

[0110] (1) Weigh out commercial γ-Fe2O3 with a concentration of 0.1 g / L, ultrasonically clean it several times in ultrapure water and anhydrous ethanol, and dry it; the ultrasonic time is 40 min; the drying temperature is 60 ℃ and the drying time is 12 h;

[0111] (2) Weigh 0.1 g / L of cleaned commercial γ-Fe2O3, 0.1 g / L of carbon nanotubes, and 0.1 g / L of cobalt nitrate solution and stir magnetically at 1200 r / min for 10 h in anhydrous ethanol at room temperature until they are evenly mixed. Continue stirring at 80 °C until dry and grind evenly.

[0112] (3) Place the product obtained in step (2) into a crucible, place the crucible in a muffle furnace, and keep it at 300°C for 5 hours;

[0113] (4) The product obtained in step (3) was washed with deionized water and alcohol and dried at 60°C for 12 hours to obtain a heterogeneous element-doped oxide composite catalyst with α-Fe2O3 as the precursor.

[0114] (5) Mix 10 mg of the product obtained in step 4 with 10 mg of carbon powder, 0.965 ml of isopropanol and 0.035 ml of Nafion solution and sonicate to obtain a slurry. Drop the slurry onto a rotating disk electrode and let it dry to obtain a test electrode.

[0115] Characterization of its morphology and structure using aberration-corrected electron microscopy, transmission electron microscopy, and XRD revealed the successful fabrication of a heteroelement-doped oxide composite catalyst with γ-Fe₂O₃ as the precursor. The heteroelement-doped oxide composite catalyst prepared in this example consists of γ-Fe₂O₃ and carbon nanotubes, with the heteroelement-doped oxide particles uniformly distributed on the carbon nanotubes, exhibiting a near-one-dimensional structure.

[0116] In this comparative example, the heteroelement-doped oxide composite catalyst with γ-Fe2O3 as the precursor was tested for oxygen reduction reaction activity in 0.1 MkOH electrolyte. The oxygen reduction half-wave potential of the heteroelement-doped oxide composite catalyst with γ-Fe2O3 as the precursor was about 0.76 V, which proves that the catalyst prepared by the stepwise method has a significantly lower catalytic efficiency than the catalyst prepared by using in-situ supported γ-Fe2O3 precursor.

[0117] The heteroelement-doped oxide composite catalyst with γ-Fe2O3 as the precursor and its preparation method disclosed in this invention can be implemented by those skilled in the art by appropriately modifying the conditions and routes, etc., based on the content of this document. Although the methods and preparation techniques of this invention have been described through preferred embodiments, those skilled in the art can obviously modify or recombine the methods and technical routes described herein without departing from the content, spirit, and scope of this invention to achieve the final preparation technique. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art, and they are all considered to be included in the spirit, scope, and content of this invention.

Claims

1. A method for preparing a heterogeneous element-doped oxide composite catalyst, characterized by, The method specifically comprises the following steps: S1, stirring the iron source, carbon nanotubes and Triton in a solvent, adding a sodium borohydride solution at 2-10°C and rapidly stirring to mix uniformly; S2, placing the solution at 2-10°C for aging, collecting the sample, washing and drying; S3, heat treatment to obtain a γ-Fe2O3 precursor loaded on the carbon nanotubes; S4, stirring the γ-Fe2O3 precursor loaded on the carbon nanotubes and a hetero-element source in a solvent, evaporating dry and grinding; S5, heat treatment to obtain the composite catalyst.

2. The method of claim 1, wherein the method is characterized by: In step S1, the iron source is at least one selected from the group consisting of anhydrous ferric chloride, ferric chloride hexahydrate and ferric nitrate; and / or, the solvent is at least one selected from the group consisting of anhydrous ethanol and water; and / or, the stirring speed of the rapid stirring is 600-2000 r / min, and the stirring time is 5-20 h; and / or, the concentrations of the iron source, carbon nanotubes, Triton and sodium borohydride in the mixture formed by stirring are 0.1-40 g / L respectively.

3. The method of claim 1, wherein the method is characterized by: In step S2, the collected sample is cleaned by ultrasonic waves with deionized water and anhydrous ethanol for multiple times, and dried; the ultrasonic time is 20-60 min; the drying temperature is 40-100°C, and the drying time is 6-24 h.

4. The method of claim 1, wherein the method is characterized by: In step S3, the heat treatment temperature is 150-450°C, and the reaction time is 1-8 h.

5. The method of claim 1, wherein the method further comprises the step of: In step S4, the hetero-element source is selected from the group consisting of nitrate, sulfate and chloride; ​ and / or, the solvent is at least one selected from the group consisting of anhydrous ethanol, water and isopropanol; and / or, the stirring speed is 600-2000 r / min, and the stirring time is 0.5-5 h; and / or, the concentrations of the precursor and the hetero-element source in the mixture formed by stirring are 0.1-40 g / L respectively; and / or, the evaporation dry temperature is 30-80°C.

6. The method of claim 1, wherein the method further comprises the step of: In step S5, the heat treatment temperature is 150-450°C, and the reaction time is 1-10 h. ​ 7. An electrode for oxygen reduction reaction, characterized by, The electrode contains the hetero-element doped oxide composite catalyst prepared by the method in any one of claims 1-6.

8. The electrode for oxygen reduction reaction according to claim 7, wherein The preparation of the electrode comprises: mixing the hetero-element doped oxide composite catalyst, carbon powder, isopropanol and Nafion solution to obtain a slurry by ultrasonic waves, dropping the slurry on a rotating disc electrode to dry and obtaining the electrode for standby.

9. The electrode for oxygen reduction reaction according to claim 8, wherein The amounts of the composite catalyst and the carbon powder in the slurry are 0.1-10 kg / L respectively, and the concentrations of isopropanol and Nafion are 0.2-8 mol / L respectively.

10. An oxygen reduction reaction three-electrode system, characterized in that, In a three-electrode system, the working electrode contains the hetero-element doped oxide composite catalyst prepared by the method in any one of claims 1-6, the three-electrode system uses a carbon rod as the counter electrode, a saturated calomel electrode as the reference electrode and a KOH solution as the electrolyte.

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