A bimetallic oxygen reduction electrocatalyst and preparation method thereof

Abstract

The present invention discloses a bimetallic oxygen reduction electrocatalyst and a preparation method thereof. The catalyst substrate material is nitrogen-doped carbon nanofibers, and cobalt and erbium oxide are loaded on the substrate, wherein cobalt and erbium oxide are assembled to form a Mott-Schottky type heterojunction; the preparation method is as follows: (1) Dissolve polyvinylpyrrolidone in an organic solvent to form a polymer solution; (2) Dissolve cobalt salt and erbium salt in the polymer solution prepared in step (1); (3) Spin the solution prepared in step (2) to obtain a carbon nanofiber precursor; (4) Calcinate the product of step (3) at a high temperature under the protection of an inert gas to obtain the bimetallic oxygen reduction electrocatalyst; the catalyst forms a Mott-Schottky erbium oxide-cobalt heterojunction through erbium oxide and cobalt, thereby improving the oxygen reduction catalytic activity of the catalyst.

Description

Technical Field

[0001] The present invention relates to an oxygen reduction electrocatalyst and a preparation method thereof, and particularly to a bimetallic oxygen reduction electrocatalyst and a preparation method thereof. Background Art

[0002] Facing severe problems such as fossil energy consumption, environmental pollution, and energy shortage, the energy catalysis field is committed to the development of clean energy and the development of new renewable energy storage and conversion technologies. Among them, rechargeable metal-air batteries have received extensive attention due to their high energy density, low cost, and environmental friendliness. The rechargeable metal zinc-air battery system realizes the generation and storage of electrical energy through the redox reaction between the positive and negative electrodes. At the air cathode, the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) correspond to the discharge and charge processes respectively. However, the ORR and OER processes involve four-electron reactions, and their slow kinetic processes are an important factor restricting the energy conversion efficiency of metal zinc-air batteries.

[0003] Currently, the commonly used commercial oxygen reduction reaction catalyst is platinum-carbon. Although its oxygen reduction performance is excellent, its large-scale use is limited due to the scarce reserves, high cost, and insufficient long-term catalytic stability of the noble metal platinum itself. Therefore, the development of efficient and inexpensive bifunctional oxygen electrocatalysts is the key for rechargeable metal zinc-air batteries to move towards practical applications. Transition metal cobalt-based catalysts have rich reserves, low costs, and adjustable oxygen reduction performance. However, compared with the activity of commercial platinum-carbon catalysts, the catalytic activity of cobalt-based catalysts is relatively low, and there is still a large room for improvement. Therefore, it is necessary to further modify it to improve its intrinsic catalytic activity. Summary of the Invention

[0004] Object of the Invention: The first object of the present invention is to provide a bimetallic oxygen reduction electrocatalyst that improves the catalytic effect of Co-based oxygen reduction electrocatalysts; the second object of the present invention is to provide a preparation method of the catalyst.

[0005] Technical Solution: The bimetallic oxygen reduction electrocatalyst of the present invention has a nitrogen-doped carbon nanofiber as the substrate material, and cobalt and erbium oxide are loaded on the substrate, wherein cobalt and erbium oxide are assembled to form a Mott-Schottky type heterojunction.

[0006] Cobalt and erbium oxide are assembled to form a Mott-Schottky type heterojunction. The f-orbital electrons of erbium regulate the 3d electron state of metallic cobalt. Erbium oxide has a lower surface work function than metallic cobalt. When the two come into contact to form a heterojunction, electrons are transferred from the erbium oxide end to the cobalt end through the interface, generating a directional interface rectification effect until the Fermi level reaches a new balance, creating a blocking layer and a charge dissipation region, constructing a Schottky barrier, and generating directional interface rectification when converted to a bimetallic erbium-cobalt heterojunction with a mismatched work function, optimizing the surface electron state structure of the catalyst and enhancing the intrinsic catalytic activity of the catalyst.

[0007] Preferably, the molar ratio of Co to Er is 1.2:0.4 - 1.4. When the cobalt content in the catalyst is low, the catalytic performance decreases; when the cobalt content is too high, the heterojunction particles are prone to aggregation, which is not conducive to directional interface rectification and the overall catalytic performance decreases.

[0008] Preferably, the precursor of the nitrogen-doped carbon nanofibers is polyvinylpyrrolidone, and the mass ratio of polyvinylpyrrolidone to Co is 0.8 - 1.2:0.07.

[0009] Preferably, the nitrogen-doped carbon nanofibers are one-dimensional nanofibers. This structure not only endows the catalyst with excellent mechanical stability, but also its one-dimensional long-chain structure helps to provide a larger specific surface area in space, fully exposing more active sites for loading heterojunction particles.

[0010] The preparation method of the bimetallic oxygen reduction electrocatalyst of the present invention is characterized by comprising the following steps:

[0011] (1) Dissolve polyvinylpyrrolidone in an organic solvent to form a polymer solution;

[0012] (2) Dissolve cobalt salt and erbium salt in the polymer solution prepared in step (1);

[0013] (3) Spin the solution prepared in step (2) to obtain a carbon nanofiber precursor;

[0014] (4) Calcinate the product of step (3) at a high temperature under the protection of an inert gas to obtain the bimetallic oxygen reduction electrocatalyst.

[0015] In step (1), polyvinylpyrrolidone is the precursor of the carbon source and nitrogen source. Its nitrogen-containing functional groups have a multi-directional grasping function and can respectively anchor metallic cobalt and metallic erbium to form a homogeneous polymer solution precursor.

[0016] Preferably, in step (1), the organic solvent is a mixed solution of N,N-dimethylformamide and absolute ethanol. Further preferably, the volume ratio of N,N-dimethylformamide to absolute ethanol is 1:1.

[0017] Preferably, in step (2), the cobalt salt is CoCl2, Co(NO3)2, CoSO4 or Co(OH)2; the erbium salt is Er(NO3)3, ErCl3 or Er2(SO4)3.

[0018] In step (3), the spinning method is: loading the solution prepared in (2) into a metal head syringe, and collecting the carbon nanofiber precursor under a positive and negative high electric field.

[0019] Preferably, in step (4), the high-temperature calcination temperature is 600-1000°C, the calcination time is 3-5 hours, and the programmed temperature rise rate is 1-10°C / min. The oxygen-containing elements of the solvent molecules are retained in the carbon nanofiber precursor. Under the high-temperature calcination environment, because the rare earth element erbium has a stronger affinity for oxygen than the transition metal cobalt, it thermodynamically competes for the oxygen element, thereby greatly avoiding the oxidation of the cobalt element, making it easier to produce a reduced state of cobalt element. The metal erbium salt is converted into erbium oxide, and the metal cobalt salt is reduced to a cobalt element component, and the two form a Mott-Schottky heterojunction structure.

[0020] Invention Mechanism: This invention synthesizes an erbium-cobalt-containing carbon nanofiber precursor using a cobalt salt as the cobalt source, a rare earth erbium salt as the rare earth metal source, and polyvinylpyrrolidone as the polymer carbon and nitrogen source. When N,N-dimethylformamide and anhydrous ethanol are used as solvents, polyvinylpyrrolidone dissolves well in the organic solvent mixture to form a homogeneous, transparent polymer solution. The nitrogen-containing functional groups of polyvinylpyrrolidone anchor the cobalt and erbium sources, creating crosslinking and coordination effects within the solution. Ultimately, the erbium-cobalt-containing polymer precursor, loaded onto one-dimensional nanofibers with uniform morphology, is collected through spinning. This precursor is then annealed at high temperature under an inert gas atmosphere, thermally reducing the cobalt source to elemental particles. The rare earth erbium source, in the form of erbium oxide, forms a Mott-Schottky heterojunction with the cobalt metal, forming nanoparticles loaded onto the nitrogen-doped carbon nanofibers. This results in a rare earth-transition metal Mott-Schottky heterojunction erbium oxide-cobalt oxygen reduction electrocatalyst.

[0021] The rare earth erbium element in the bimetallic catalyst of the present invention possesses unique 4f orbital electrons, which can form a gradient orbital coupling electronic structure with the 3d orbital electrons of the transition metal cobalt through the 2p oxygen ligand. This optimizes the overall electronic state of the catalyst and enhances the intrinsic catalytic activity through current reforming at the heterojunction interface. The Mott-Schottky erbium oxide-cobalt heterojunction oxygen reduction catalyst, supported by one-dimensional nitrogen-doped carbon nanofibers, has uniform size and controllable morphology, and possesses a typical one-dimensional nanofiber structure. This structure not only provides the catalyst with excellent mechanical stability, but also its one-dimensional long chain structure helps to provide a larger specific surface area in space, fully exposing more active sites on the supported heterojunction particles.

[0022] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages: (1) The catalyst forms a Mott-Schottky erbium oxide-cobalt heterojunction through erbium oxide and cobalt, improving the oxygen reduction catalytic activity of the catalyst; (2) The catalyst has a typical one-dimensional nanofiber structure with uniform size and regular shape. The surface of its unique one-dimensional long-chain structure is fully assembled with a metal erbium cobalt Mott-Schottky structure, and at the same time, it forms a secondary interface current reforming in contact with the non-metal substrate, which is beneficial to effectively expose the active sites and exhibits high catalytic activity and stability for ORR; (3) The preparation method is simple and efficient; (4) The catalyst can be well applied to the positive electrode of a rechargeable metal zinc-air battery and has broad application prospects in the future energy storage industry. Description of the Drawings

[0023] Figure 1 SEM image of the catalyst prepared in Example 3;

[0024] Figure 2 HRTEM image of the catalyst prepared in Example 3;

[0025] Figure 3 Enlarged HRTEM image of the catalyst prepared in Example 3;

[0026] Figure 4 XRD pattern of the catalyst prepared in Example 3;

[0027] Figure 5 XPS spectrum of the catalyst prepared in Example 3;

[0028] Figure 6 Schematic diagram of the Mott-Schottky heterojunction interface rectification of the bimetallic oxygen reduction electrocatalyst of the present invention;

[0029] Figure 7 Oxygen reduction performance graph of the catalysts prepared in Examples 1-5;

[0030] Figure 8 Oxygen reduction performance comparison graph of the catalysts prepared in Example 3 and Comparative Examples 1-2 and commercial Pt / C. Detailed Description of the Invention

[0031] The technical solution of the present invention will be further described below in conjunction with the embodiments.

[0032] Example 1

[0033] The preparation method of the bimetallic oxygen reduction electrocatalyst of the present invention includes the following steps:

[0034] (1) Weigh 1 g of polyvinylpyrrolidone and dissolve it in 6 mL of absolute ethanol and 6 mL of N,N-dimethylformamide, and stir to form a polymer precursor solution. Weigh 1.2 mmol of Co(NO3)2 and 0.4 mmol of Er(NO3)3 and dissolve them in the above precursor solution. Aspirate the resulting pink precursor solution with a syringe equipped with a metal needle, and perform electrospinning under an electric field of 18 kV to collect the carbon nanofiber precursor on the negative electrode collecting plate;

[0035] (2) Heat-treat the precursor prepared in step (1) in a nitrogen atmosphere at a programmed heating rate of 3 °C / min to 800 °C and hold for 3 h at this temperature. After cooling to room temperature, the final product is obtained.

[0036] Example 2

[0037] On the basis of Example 1, change the dosage of Er(NO3)3 to 0.6 mmol, and keep the other conditions unchanged.

[0038] Example 3

[0039] On the basis of Example 1, change the dosage of Er(NO3)3 to 0.8 mmol, and keep the other conditions unchanged.

[0040] Example 4

[0041] On the basis of Example 1, change the dosage of Er(NO3)3 to 1.2 mmol, and keep the other conditions unchanged.

[0042] Example 5

[0043] On the basis of Example 1, change the dosage of Er(NO3)3 to 1.4 mmol, and keep the other conditions unchanged.

[0044] Example 6

[0045] On the basis of Example 1, change the dosage of polyvinylpyrrolidone to 0.8 g, and keep the other conditions unchanged.

[0046] Example 7

[0047] On the basis of Example 1, change the dosage of polyvinylpyrrolidone to 1.2 g, and keep the other conditions unchanged.

[0048] Example 8

[0049] (1) Weigh 1 g of polyvinylpyrrolidone and dissolve it in 6 mL of absolute ethanol and 6 mL of N,N-dimethylformamide, and stir to form a polymer precursor solution. Weigh 1.2 mmol of CoCl2 and 0.8 mmol of ErCl3 and dissolve them in the above precursor solution. Aspirate the resulting pink precursor solution with a syringe equipped with a metal needle and perform electrospinning under an electric field of 18 kV, and collect the carbon nanofiber precursor on the negative electrode collecting plate;

[0050] (2) Heat-treat the precursor prepared in step (1) under a nitrogen atmosphere at a programmed heating rate of 5 °C / min to 600 °C and hold at this temperature for 3 h, and then cool to room temperature to obtain the final product.

[0051] Example 9

[0052] (1) Weigh 1 g of polyvinylpyrrolidone and dissolve it in 6 mL of absolute ethanol and 6 mL of N,N-dimethylformamide, and stir to form a polymer precursor solution. Weigh 1.2 mmol of CoSO4 and 0.4 mmol of Er2(SO4)3 and dissolve them in the above precursor solution. Aspirate the resulting pink precursor solution with a syringe equipped with a metal needle and perform electrospinning under an electric field of 18 kV, and collect the carbon nanofiber precursor on the negative electrode collecting plate;

[0053] (2) Heat-treat the precursor prepared in step (1) under a nitrogen atmosphere at a programmed heating rate of 5 °C / min to 1000 °C and hold at this temperature for 3 h, and then cool to room temperature to obtain the final product.

[0054] Comparative Example 1

[0055] On the basis of Example 3, without adding rare earth erbium salt, and other conditions remain unchanged.

[0056] Comparative Example 2

[0057] On the basis of Example 3, without adding rare earth cobalt salt, and other conditions remain unchanged.

[0058] Structure Characterization

[0059] The catalyst prepared in Example 3 was physically characterized by TEM, XRD and XPS.

[0060] From Figure 1 the SEM image, it can be seen that the prepared catalyst has a typical one-dimensional fibrous long-range nanostructure.

[0061] From Figure 2 the HRTEM image and Figure 3 the enlarged HRTEM image, it can be seen that the particles supported on the surface of the nanofibers clearly form a Mott-Schottky heterojunction interface containing bimetallic erbium cobalt.

[0062] FromFigure 4 It can be seen from the XRD pattern of that numerous diffraction peaks of the catalyst are in complete agreement with the standard cards (Co JCPDS: 15-0806) and Er2O3 (JCPDS: 43-1007), enabling the controllable formation of the bimetallic erbium-cobalt component.

[0063] From Figure 5 It can be seen from the XPS spectrum of that, compared with the single-component catalyst, in the erbium-cobalt oxide catalyst forming a Mott-Schottky heterojunction, the binding energy of the 2p spectrum of component Co shifts towards a higher binding energy direction, indicating that the Co site gains electrons, while the binding energy of the Er 4d spectrum shifts towards a higher binding energy direction, indicating that the Er site loses electrons. This shows that in the catalyst, the formation of the Mott-Schottky heterojunction between bimetallic erbium and cobalt is used for directional interfacial charge rectification, inducing the directional movement of electrons.

[0064] Figure 6 This is the schematic diagram of the Mott-Schottky heterojunction interface rectification of the bimetallic oxygen reduction electrocatalyst of the present invention. The two ends forming the Mott-Schottky heterojunction are Co(111) and Er2O3(222) respectively. When they come into contact, due to the different work functions of the Co(111) and Er2O3(222) surfaces, it is easy to cause the redistribution of interfacial charges. According to the theoretical calculation results, Er2O3(222) has a lower work function and is a semiconductor itself. Therefore, electrons flow from the Er2O3(222) end to the Co(111) end. When the Fermi level reaches a new balance, a charge depletion region is generated, and the rectification effect is thus manifested.

[0065] Performance Test

[0066] Oxygen Reduction Reaction (ORR) Activity Test: Using the catalysts prepared in Examples 1-7 and Comparative Examples 1-2 and commercial Pt / C as reference catalysts, the ORR activity of different catalysts in alkaline media was investigated.

[0067] Test Method: Weigh 5 mg of the catalyst and dissolve it in 200 μL of absolute ethanol, 700 μL of deionized water, and 100 μL of Nafion to form a catalyst solution. Pipette 20 μL of the catalyst solution and smear it on the surface of the rotating ring-disk electrode, and dry it at a constant temperature of 40 °C to form a catalyst film. Alkaline oxygen reduction tests were carried out using a three-electrode system. The selected solutions were 0.1 M KOH solution saturated with N2 and 0.1 M KOH solution saturated with O2. First, cyclic voltammetry tests were carried out in the N2-saturated solution for activation, and then rotating ring-disk electrode reaction tests were carried out in the O2-saturated solution. The rotation speed was selected as 1600 rpm, and O2 was continuously introduced to impact the electrode surface during the test. The test results are as Figure 7 shown.

[0068] From Figure 7It can be seen that the half-wave potentials of the catalysts prepared in Examples 1 to 5 are 0.78 V, 0.79 V, 0.835 V, 0.80 V, and 0.78 V, respectively, and the limiting currents are 4.2 mA cm -2 ,4.5mA cm -2 ,5.5mA cm -2 ,5.6mA cm -2 ,5.4mA cm -2 . By changing the amount of different rare earth erbium salts, different oxygen reduction performance control results are presented. The oxygen reduction performance of the catalyst shows a trend of first increasing and then decreasing with the increase of erbium oxide content. This is because, at low loading, due to the insufficient electronic regulation and rectification effect of rare earth erbium oxide on the cobalt site, the auxiliary catalytic ability is weak, which is reflected in the lower limiting current density; under the condition of high rare earth erbium oxide presence, since it does not act as an active site itself, the excessive presence will lead to the occupation and obstruction of the active site itself, because it also shows a weak auxiliary catalytic ability, which is reflected in the higher improvement of the limiting current.

[0069] The half-wave potentials of the catalysts prepared in Comparative Examples 1 and 2 and the commercial Pt / C catalyst were 0.67 V, 0.77 V, 0.835 V, and 0.84 V, respectively; 3 mA cm -2 , the limiting current is 4 mA cm -2 ,5.5mA cm -2 ,5.8mA cm -2 The ORR activities of the catalysts prepared in Examples 1 to 5 in alkaline electrolyte were superior to those of the catalysts prepared in Comparative Examples 1 and 2 and close to those of commercial Pt / C catalysts. In particular, the catalyst prepared in Example 3 showed the best effect, which was mainly attributed to the secondary interfacial charge reforming induced by its unique assembled Mott-Schottky heterojunction and the electronic structure optimization effect of rare earth f electrons on transition metal d electrons.

Claims

1. A bimetallic oxygen reduction electrocatalyst, characterized in that, The substrate material is nitrogen-doped carbon nanofibers, on which cobalt and erbium oxide are loaded, and cobalt and erbium oxide are assembled to form a Mott-Schottky type heterojunction; the precursor of the nitrogen-doped carbon nanofibers is polyvinylpyrrolidone, and the mass ratio of polyvinylpyrrolidone to cobalt is 0.8~1.2:0.07; The electrocatalyst is prepared by the following method: (1) Dissolve polyvinylpyrrolidone in an organic solvent to form a polymer solution; (2) Dissolve cobalt salt and erbium salt in the polymer solution prepared in step (1); (3) Spin the solution prepared in step (2) to obtain a carbon nanofiber precursor; (4) Calcinate the product of step (3) at a high temperature under the protection of an inert gas to obtain the double-metal oxygen reduction electrocatalyst; In step (2), the molar ratio of cobalt in the cobalt salt to erbium in the erbium salt is 1.2:0.4~1.

4.

2. The bimetallic oxygen reduction electrocatalyst according to claim 1, wherein The nitrogen-doped carbon nanofibers are one-dimensional nanofibers.

3. A method for preparing the bimetallic oxygen reduction electrocatalyst according to claim 1 or 2, characterized in that, It includes the following steps: (1) Dissolve polyvinylpyrrolidone in an organic solvent to form a polymer solution; (2) Dissolve cobalt salt and erbium salt in the polymer solution prepared in step (1); (3) Spin the solution prepared in step (2) to obtain a carbon nanofiber precursor; (4) Calcinate the product of step (3) at a high temperature under the protection of an inert gas to obtain the double-metal oxygen reduction electrocatalyst.

4. The preparation method of the bimetallic oxygen reduction electrocatalyst according to claim 3, wherein, In step (4), the temperature of the high-temperature calcination is 600~1000 °C, and the calcination time is 3~5 hours.

5. The preparation method of the bimetallic oxygen reduction electrocatalyst according to claim 3, wherein In step (1), the organic solvent is a mixed solution of N,N-dimethylformamide and absolute ethanol.

6. The preparation method of the bimetallic oxygen reduction electrocatalyst according to claim 3, wherein In step (2), the cobalt salt is CoCl2, Co(NO3)2, CoSO4 or Co(OH)2.

7. The preparation method of the bimetallic oxygen reduction electrocatalyst according to claim 3, wherein In step (2), the erbium salt is Er(NO3)3, ErCl3 or Er2(SO4)3.

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