A transition metal-rare earth composite two-component oxygen reduction electrocatalyst and its preparation method

Abstract

This invention discloses a transition metal-rare earth composite two-component oxygen reduction electrocatalyst and its preparation method. The substrate material of the catalyst is two-dimensional nitrogen-doped carbon nanosheets, on which cobalt ions and terbium oxide are loaded. Cobalt is atomically positioned and anchored by lattice oxygen on the surface of terbium oxide, establishing a localized terbium-oxygen-cobalt structural unit with gradient orbital coupling. The preparation method is as follows: (1) Melamine and sucrose are dissolved in water, and then cobalt salt and terbium salt are added to form a mixed solution; (2) The mixed solution prepared in step (1) is subjected to a hydrothermal reaction; (3) The product of step (2) is calcined at high temperature under inert gas protection to obtain the transition metal-rare earth composite two-component oxygen reduction electrocatalyst. The catalyst exhibits excellent oxygen reduction catalytic activity by combining terbium oxide and cobalt to form an atomically supported cobalt-terbium oxide complex.

Description

Technical Field

[0001] This invention relates to an oxygen reduction electrocatalyst and its preparation method, and particularly to a transition metal-rare earth composite two-component oxygen reduction electrocatalyst and its preparation method. Background Technology

[0002] Currently, rapid economic growth has led to serious problems such as excessive consumption of traditional fossil fuels and environmental pollution. To address these issues, many scholars are continuously dedicated to developing storage and conversion technologies for clean energy and new renewable energy sources. Among these, rechargeable alkaline metal-air batteries, especially alkaline zinc-air batteries, have attracted considerable attention due to their high energy density, low cost, and environmental friendliness. For rechargeable zinc-air battery systems, the main focus is on designing catalysts for the air electrode to catalyze the oxygen electrocatalytic reaction. Specifically, on the air cathode, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) correspond to the discharge and charge processes, respectively. However, both ORR and OER processes involve four-electron reactions, which exhibit slow kinetics, thus severely limiting the energy conversion efficiency of alkaline zinc-air batteries.

[0003] Currently, platinum-carbon is the commercially available catalyst for oxygen reduction (OR). While platinum-carbon exhibits superior OR performance, its large-scale use is limited by the scarcity, high cost, and insufficient long-range stability of the precious metal platinum. Therefore, developing efficient and inexpensive bifunctional oxygen electrocatalysts remains crucial for the practical application of rechargeable zinc-air batteries. In contrast to the precious metal platinum, transition metal cobalt-based catalysts possess advantages such as abundant reserves, low cost, and tunable OR performance. Commonly used cobalt-based catalysts include cobalt-based alloys, cobalt-based oxides, cobalt-based phosphides, cobalt-based nitrides, cobalt-based hydroxides, and cobalt-based double-layer hydroxides. However, compared to commercial platinum-carbon catalysts, the catalytic activity of cobalt-based catalysts is still relatively low, making it difficult to meet the practical requirements of metal-air batteries. However, their variable valence electron structure allows for variable OR electrocatalytic activity, thus showing significant potential for further catalytic activity improvement. Summary of the Invention

[0004] Objectives of the Invention: The first objective of this invention is to provide a rare earth-transition metal composite two-component oxygen reduction electrocatalyst that improves the catalytic effect of cobalt-based oxygen reduction electrocatalysts; the second objective of this invention is to provide a method for preparing the aforementioned rare earth-transition metal composite two-component oxygen reduction electrocatalyst.

[0005] Technical solution: The transition metal-rare earth composite two-component oxygen reduction electrocatalyst of the present invention has a substrate material of two-dimensional nitrogen-doped carbon nanosheets, on which cobalt ions and terbium oxide are loaded. The cobalt is atomic in size and is anchored by the lattice oxygen on the surface of terbium oxide, thus establishing a localized terbium-oxygen-cobalt structural unit with gradient orbital coupling.

[0006] Terbium-oxygen-cobalt structural units with gradient orbital coupling serve as highly active sites for the oxygen reduction reaction, promoting its smooth progress.

[0007] The nitrogen-doped carbon nanosheets are two-dimensional nanosheets. This structure not only gives the catalyst excellent mechanical stability, but also helps to provide a larger surface area in space, which is conducive to the smooth progress of mass transfer and can fully expose more active sites.

[0008] Preferably, the molar ratio of cobalt to terbium is 0.4–1.0:1.0. When the cobalt content in the catalyst is low, the catalytic performance is insufficient; when the cobalt content is high, the particles tend to aggregate, leading to a decrease in overall catalytic performance.

[0009] Preferably, the precursors for the two-dimensional nitrogen-doped carbon nanosheets are melamine and sucrose.

[0010] Preferably, the mass ratio of melamine to sucrose is 2.5:2.0 to 3.0.

[0011] The preparation method of the transition metal-rare earth composite two-component oxygen reduction electrocatalyst of the present invention is characterized by comprising the following steps:

[0012] (1) Dissolve melamine and sucrose in water, and then add cobalt salt and terbium salt to form a mixed solution;

[0013] (2) The mixture solution prepared in step (1) is subjected to hydrothermal reaction to obtain a carbon-containing precursor of anchored metal components;

[0014] (3) The precursor obtained in step (2) is calcined at high temperature under inert gas protection to obtain the transition metal-rare earth composite two-component oxygen reduction electrocatalyst.

[0015] In step (2), melamine and sucrose, two small molecules, can be electrostatically interacted and oriented to form carbon nanoprecursors through electrostatic potential penetration. The exposed nitrogen- and oxygen-containing functional groups can be used to anchor the metal cations cobalt and terbium, forming a preliminary carbonized nanostructure during the hydrothermal process, in which the metal cations are effectively anchored in the carbon nanostructure.

[0016] Preferably, in step (2), the temperature of the hydrothermal reaction is 180-200°C and the reaction time is 8-10 hours.

[0017] In step (3), the oxygen-containing elements of sucrose molecules and solvent molecules can be retained in the carbon nanotube precursor. Under high-temperature calcination, rare earth terbium has stronger oxygen affinity than transition metal cobalt, and thermodynamically competes for oxygen, thus greatly avoiding the aggregation of cobalt and the formation of cobalt-based oxides. It is easier to generate reduced state and atomically dispersed cobalt. Metal terbium salt is converted into terbium oxide. Atomically dispersed cobalt is anchored by lattice oxygen on the surface of terbium oxide, establishing a localized terbium-oxygen-cobalt structural unit with gradient orbital coupling, which serves as a highly active site for oxygen reduction reaction and promotes the smooth progress of oxygen reduction reaction.

[0018] Preferably, in step (3), the high-temperature calcination temperature is 700-1000℃, the heating rate is 1-10℃ / min, and the calcination time is 3-5 hours.

[0019] Preferably, the cobalt salt is CoCl2, Co(NO3)2, CoSO4 or Co(OH)2.

[0020] Preferably, the terbium salt is Tb(NO3)3, TbCl3, or Tb2(SO4)3.

[0021] Mechanism of Invention: This invention uses transition metal cobalt salts as the cobalt source, rare earth terbium salts as the rare earth metal source, and melamine and sucrose as the nitrogen and carbon sources, respectively, to synthesize a carbon nanotube precursor containing cobalt and terbium components. When melamine and sucrose are used as precursors for small-molecule carbon nanomaterials and water is used as the solvent, the electrostatic interaction between the two small molecules allows for in-situ directional assembly through electrostatic potential penetration to form the carbon nanotube precursor. The exposed nitrogen- and oxygen-containing functional groups can be used to anchor the metal cations cobalt and terbium, forming a preliminary carbonized nanostructure during hydrothermal processing, in which the metal cations are effectively anchored within the carbon nanostructure. This precursor is then subjected to high-temperature annealing in an inert gas atmosphere. The rare earth terbium source exists in the form of terbium oxide, and the exposed lattice oxygen anchors the transition metal atomic-level cobalt, forming an atomic-level cobalt-supported terbium oxide composite. This composite is loaded onto the surface of two-dimensional carbon nanofibers, ultimately yielding a rare earth-transition metal composite two-component oxygen reduction electrocatalyst.

[0022] From the perspective of catalytic reaction, cobalt supported on the terbium oxide surface is in an ionic state, and its 3d valence band exhibits localization. Furthermore, benefiting from the synergistic effect of the gradient orbital coupling between the terbium ion's 4f state, the lattice oxygen's 2p state, and the cobalt ion's 3d state, the cobalt ion's 3d band is positioned at a relatively suitable energy level. In addition, the localized 3d state can generate synergistic σ and π bonds when interacting with oxygen-containing intermediates in the oxygen reduction reaction. These antibonded σ and π bonds are closer to the Fermi level. Therefore, during the electrocatalytic reaction, the binding of cobalt ion sites to oxygen-containing intermediates can be flexibly controlled, thereby promoting the smooth occurrence of the oxygen reduction reaction and enhancing the intrinsic catalytic activity.

[0023] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) The catalyst forms an atomically supported cobalt-terbium oxide complex by combining terbium oxide and cobalt, exhibiting excellent oxygen reduction catalytic activity; (2) The catalyst has a typical two-dimensional nanosheet structure with uniform size and regular shape. Its unique two-dimensional structure surface can fully support the atomically supported cobalt-terbium oxide complex, which is conducive to the mass transfer process and the full exposure of surface active sites, as well as the guarantee of conductivity, 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 rechargeable zinc-air batteries and has broad application prospects in the future energy storage industry. Attached Figure Description

[0024] Figure 1 SEM image of the catalyst prepared in Example 2;

[0025] Figure 2 HRTEM image of the catalyst prepared in Example 2;

[0026] Figure 3 Aberration-corrected electron micrograph of the catalyst prepared in Example 2;

[0027] Figure 4 The XRD pattern of the catalyst prepared in Example 2;

[0028] Figure 5 XPS valence band structure of the catalyst prepared in Example 2;

[0029] Figure 6 Comparison of oxygen reduction performance between Example 2, Comparative Examples 1-2, and the catalyst prepared from commercial Pt / C;

[0030] Figure 7 The graph shows the zinc-air cell performance of the catalyst prepared in Example 2. Detailed Implementation

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

[0032] Example 1

[0033] The transition metal-rare earth composite two-component oxygen reduction electrocatalyst of the present invention is prepared by the following steps:

[0034] (1) Weigh 2.5g of melamine and 2.5g of sucrose and dissolve them in 50mL of deionized water while stirring continuously to form a mixture solution. Weigh 0.4mmol of Co(NO3)2 and 1.0mmol of Tb(NO3)3 and dissolve them in the above mixture solution.

[0035] (2) A hydrothermal reaction was carried out, with the temperature controlled at 200℃ and the reaction time at 10h. After the reaction was completed, the resulting turbid liquid was filtered, washed three times with ethanol and deionized water respectively, and the resulting powder was dried in a constant temperature oven at 40℃.

[0036] (3) The carbon-containing precursor with loaded metal components obtained in step (2) is heat-treated in a nitrogen atmosphere by heating to 800°C at a programmed temperature of 3°C / min, and held at this temperature for 3 hours. After cooling to room temperature, the final product is obtained.

[0037] Example 2

[0038] Based on Example 1, the amount of Co(NO3)2 was changed to 0.6 mmol, while the other conditions remained unchanged.

[0039] Example 3

[0040] Based on Example 1, the amount of Co(NO3)3 was changed to 0.8 mmol, while the other conditions remained unchanged.

[0041] Example 4

[0042] Based on Example 1, the amount of Co(NO3)3 was changed to 1.0 mmol, while the other conditions remained unchanged.

[0043] Example 5

[0044] Based on Example 1, the amount of sucrose was changed to 2.0g, while the other conditions remained the same.

[0045] Example 7

[0046] Based on Example 1, the amount of sucrose was changed to 3.0g, while the other conditions remained the same.

[0047] Example 8

[0048] (1) Weigh 2.5g of melamine and 2.5g of sucrose and dissolve them in 50mL of deionized water and stir continuously to form a mixture solution. Weigh 0.4mmol of CoCl2 and 1.0mmol of Tb(NO3)3 and dissolve them in the above mixture solution. Then carry out a hydrothermal reaction, control the temperature at 180℃ and the reaction time at 10h.

[0049] (2) The precursor obtained in step (1) was heat-treated in a nitrogen atmosphere at a programmed temperature of 5°C / min to 700°C and held at that temperature for 5 hours. After cooling to room temperature, the final product was obtained as in Example 1.

[0050] Example 9

[0051] (1) Weigh 2.5g of melamine and 2.5g of sucrose and dissolve them in 50mL of deionized water and stir continuously to form a mixture solution. Weigh 0.4mmol of CoSO4 and 1.0mmol of Tb(NO3)3 and dissolve them in the above mixture solution. Then carry out a hydrothermal reaction, control the temperature at 200℃ and the reaction time at 8h.

[0052] (2) The precursor obtained in step (1) was heat-treated in a nitrogen atmosphere at a temperature of 5°C / min to 1000°C and held at that temperature for 3 hours. After cooling to room temperature, the final product was obtained as in Example 1.

[0053] Comparative Example 1

[0054] Based on Example 2, terbium salts of rare earth metals were not added, while other conditions remained unchanged.

[0055] Comparative Example 2

[0056] Based on Example 2, without adding transition metal cobalt salt, and with other conditions remaining unchanged.

[0057] Structural characterization

[0058] The physical structure of the catalyst prepared in Example 2 was characterized, and the results are as follows: Figures 1-5 As shown.

[0059] Depend on Figure 1 The SEM images show that the prepared catalyst has a typical two-dimensional nanosheet structure.

[0060] Depend on Figure 2 The HRTEM images show that the surface of the two-dimensional nitrogen-doped carbon nanosheets is uniformly loaded with nanoparticles formed by the bimetallic composite, and there is no aggregation phenomenon.

[0061] Depend on Figure 3The aberration-corrected electron microscopy images show that the formed particles are mainly composed of transition metal cobalt loaded in atomic form on the surface of rare earth oxide terbium oxide, thus forming rare earth-transition metal composite two-component nanoparticles.

[0062] Depend on Figure 4 The XRD pattern shows that the numerous diffraction peaks of the catalyst are in perfect agreement with the standard card (Co JCPDS: 15-0806) and Tb2O3 (JCPDS: 23-1418), confirming the presence of Tb2O3, while the Co peak indicates the presence of some Co nanoparticles.

[0063] Depend on Figure 5 The XPS valence band structure reveals that, compared to single-component catalysts, the rare-earth-transition-metal composite bicomponent catalyst exhibits regulated electronic structure properties. Specifically, compared to the valence band structure of Co, the composite catalyst displays a localized Co 3d band; compared to the valence band structure of Tb₂O₃, its 4f valence state shows a broadening and negative shift trend. This change in valence electronic structure is consistent with density functional theory calculations, confirming the effective electronic interaction between Co and Tb₂O₃ and demonstrating the unique electronic properties of the constructed cobalt-oxygen-terbium sites with gradient orbitals.

[0064] Performance testing

[0065] Oxygen reduction reaction (ORR) activity test: The ORR activity of different catalysts in alkaline media was investigated using the catalysts prepared in Examples 1-9 and Comparative Examples 1-2 and commercial Pt / C as reference catalysts.

[0066] Test method: 5 mg of catalyst was dissolved in 200 μL of anhydrous ethanol, 700 μL of deionized water, and 100 μL of Nafion to form a catalyst solution. 20 μL of this catalyst solution was transferred and coated onto the surface of a rotating ring electrode, then dried at 40°C to form a catalyst film. A three-electrode system was used for alkaline oxygen reduction testing. The solutions selected were 0.1 M KOH solution saturated with N2 and 0.1 M KOH solution saturated with O2. First, cyclic voltammetry was performed in the N2-saturated solution for activation. Then, a rotating ring electrode reaction test was performed in the O2-saturated solution at a rotation speed of 1600 rpm, while O2 was continuously introduced to impact the electrode surface during the test. Test results are as follows: Figure 6 and 7 As shown.

[0067] Depend on Figure 6 It was found that the half-wave potentials of the catalysts prepared in Example 2 and Comparative Examples 1-2 were 0.85V, 0.79V, and 0.69V, respectively, and the limiting currents were 5.8mA / cm², respectively. -2 4.6mA cm -22.2mA cm -2 By coupling the transition metal cobalt and the rare earth oxide terbium, the oxygen reduction performance of the coupled catalysts differed from that of the individual cobalt-based catalysts and terbium oxide catalysts. The coupled catalysts exhibited higher oxygen reduction performance than the individual catalysts. Since the cobalt group, as the main active site, possessed higher oxygen reduction activity than terbium oxide, and the oxygen reduction catalytic performance was further enhanced after combining terbium oxide with cobalt, it is evident that terbium oxide plays a catalytic assisting role for cobalt. The results of the tuning showed that the oxygen reduction performance of Example 2 even matched that of commercial platinum-carbon catalysts, demonstrating its superior performance.

[0068] Benefiting from the excellent oxygen reduction catalytic performance of Example 2, Figure 7 The results are from zinc-air battery tests. Example 2 exhibits a higher open-circuit voltage of 1.52V, compared to 1.42V for a commercially available platinum-carbon catalyst. Furthermore, Example 2 also demonstrates a higher power density of 166 mW / cm². -2 Significantly due to the 130mW cm⁻¹ of commercially available platinum-carbon -2 The results show that rare earth-transition metal composite bimetallic components also have promising value in practical applications of zinc-air batteries.

Claims

1. A transition metal-rare earth complexed two-component oxygen reduction electrocatalyst characterized in that, The base material of the catalyst is two-dimensional nitrogen-doped carbon nanosheet, and cobalt ions and terbium oxide are loaded on the base, wherein the cobalt is atom level, and is anchored by the lattice oxygen on the surface of the terbium oxide, to establish a localized terbium-oxygen-cobalt structural unit with gradient orbital coupling.

2. The transition metal-rare earth complexed two-component oxygen reduction electrocatalyst according to claim 1, wherein The molar ratio of the cobalt to the terbium is 0.4-1.0:1.

0.

3. The transition metal-rare earth complexed bi-component oxygen reduction electrocatalyst according to claim 1, wherein The precursor of the two-dimensional nitrogen-doped carbon nanosheet is melamine and sucrose.

4. The transition metal-rare earth complexed two-component oxygen reduction electrocatalyst according to claim 3, wherein The mass ratio of the melamine to the sucrose is 2.5:2.0-3.

0.

5. A method for preparing the transition metal-rare earth complexed two-component oxygen reduction electrocatalyst as claimed in any one of claims 1 to 4, characterized by, The method comprises the following steps: (1) dissolving melamine and sucrose in water, and then adding a mixture solution of cobalt salt and terbium salt; (2) performing hydrothermal reaction on the mixture solution prepared in step (1); (3) calcining the product of step (2) under inert gas protection at high temperature to obtain the transition metal-rare earth composite two-component oxygen reduction electrocatalyst.

6. The method for preparing a transition metal-rare earth complexed bi-component oxygen reduction electrocatalyst according to claim 5, characterized in that, In step (2), the temperature of the hydrothermal reaction is 180-200 DEG C.

7. The preparation method of the transition metal-rare earth composite two-component oxygen reduction electrocatalyst according to claim 5, characterized in that, In step (3), the temperature of the high-temperature calcination is 700-1000 DEG C, and the calcination time is 3-5 hours.

8. The preparation method of the transition metal-rare earth composite two-component oxygen reduction electrocatalyst according to claim 5, wherein the cobalt salt is CoCl2, Co (NO3) 2, CoSO4 or Co (OH) 2.

9. The preparation method of the transition metal-rare earth composite two-component oxygen reduction electrocatalyst according to claim 5, wherein the terbium salt is Tb (NO3) 3, TbCl3 or Tb2 (SO4) 3.

10. The preparation method of the transition metal-rare earth composite two-component oxygen reduction electrocatalyst according to claim 7, wherein the heating rate of the high-temperature calcination is 1-10 DEG C / min.

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