Core-shell ferrocobalt bimetal oxygen reduction catalyst with universal pH as well as preparation method and application of core-shell ferrocobalt bimetal oxygen reduction catalyst

Abstract

The invention relates to the technical field of oxygen reduction reaction catalysts, and provides a pH universal core-shell ferrocobalt bimetal oxygen reduction catalyst and a preparation method and application thereof. The invention reports a strategy of a core-shell structure ZIF, namely, ZIF-8 (ZnCo-ZIF coated Fe / ZIF-8) containing ferric acetylacetonate is grown outside a prepared ZnCo-ZIF core, so that the bimetallic catalyst Co coated Fe-NC rich in defects is prepared on layered porous nitrogen carbon. The Co-coated Fe-NC catalyst prepared by the preparation method disclosed by the invention shows excellent ORR electrocatalytic activity in an electrolyte with universal pH (Potential of Hydrogen), especially in a neutral medium. The core-shell ferrocobalt bimetal oxygen reduction catalyst prepared by the invention opens up a new breakthrough point for assembling a zinc air battery, and shows huge application potential and value.

Description

Technical Field

[0001] The present invention relates to the technical field of oxygen reduction reaction catalysts, and in particular to a pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst, a preparation method thereof, and applications thereof. Background Art

[0002] The oxygen reduction reaction (ORR) plays a vital role in electrochemical energy conversion and storage technologies such as fuel cells and metal-air batteries. However, the slow four-electron transfer kinetics in the ORR process hinder the development of the above technologies, so there is an urgent need to find efficient ORR catalysts to improve the reaction efficiency. Initial research focused on platinum-based catalysts, but the high cost and limited reserves of precious metals greatly restrict their widespread application. At the same time, most research is devoted to the high catalytic activity of catalysts under acidic or alkaline conditions. Given the growing demand for new energy, there is an urgent need to develop efficient ORR catalysts under various pH conditions, especially neutral media, to adapt to the rapid development of technologies such as microbial / enzyme biofuel cells, seawater desalination and neutral zinc-air batteries.

[0003] MNC complexes have become a promising class of ORR catalysts due to their high cost-effectiveness, high catalytic activity and good stability. However, there are still challenges in preventing metal particles from aggregating and maintaining catalyst stability during the synthesis process. It is reported that zeolitic imidazolate framework material ZIF, which has the advantages of rich carbon and nitrogen ligands, high porosity and uniform spatial distribution of metal ions, is considered to be a good precursor for the preparation of MNC catalysts. ZIF-67 is composed of Co 2+ and 2-methylimidazole, by adding Zn 2+ , can replace a certain proportion of Co 2+ sites, thereby expanding the adjacent distance between Co atoms. However, direct carbonization of ZnCo-ZIF will lead to pore structure collapse, metal nanoparticle aggregation and excessive graphitization, which is not conducive to the mass transfer process and the exposure of active sites. ZIF-8 has a similar topological structure to ZIF-67, so the two can be epitaxially grown to form a core-shell structure ZnCo-ZIF@ZIF-8. The core-shell structure combines the advantages of the two components, such as improved stability, porosity, selectivity and catalytic performance, thereby achieving performance gains that exceed those of a single component. However, the synthesis of core-shell structures usually involves a complex process of precisely controlling the core size, shell thickness and interfacial compatibility. Therefore, to address the above challenges, the rational design of precursors based on ZnCo-ZIF@ZIF-8 can effectively inhibit the collapse of the pore structure, the agglomeration of metal nanoparticles and the excessive graphitization of carbon materials, thereby improving the catalytic activity. Summary of the Invention

[0004] This study addresses the issues of pore collapse, metal nanoparticle aggregation, and excessive graphitization caused by direct carbonization of ZnCo-ZIF precursors. A pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst (Co@Fe-NC) has been developed and designed. The synthesized Co@Fe-NC catalyst exhibits excellent ORR activity in pH-universal electrolytes, particularly under neutral conditions, surpassing commercial Pt / C. Furthermore, the Co@Fe-NC-based zinc-air battery exhibits high specific capacity and good charge-discharge durability.

[0005] The present invention is achieved through the following technical solution: synthesizing ZnCo-ZIF bimetallic nanocrystals. Then, epitaxially growing a thin layer of Fe-containing ZIF-8 on the ZnCo-ZIF nanocrystals to obtain ZnCo-ZIF@Fe / ZIF-8. Finally, calcining the resulting catalyst in a protective gas atmosphere yields a Co@Fe-NC catalyst.

[0006] The preparation method of the above-mentioned pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst comprises the following steps:

[0007] 1) Dissolve Co(NO3)2·6H2O and Zn(NO3)2·6H2O (Zn and Co at different molar ratios) in methanol. Dissolve 2-methylimidazole in an equal volume of methanol.

[0008] 2) mixing the two solutions obtained in step 1) and magnetically stirring at room temperature for a certain period of time;

[0009] 3) centrifuging the solution obtained in step 2), washing with methanol, and drying to obtain ZnCo-ZIF powder;

[0010] 4) dispersing the ZnCo-ZIF powder obtained in step 3) in methanol and ultrasonicating for a certain period of time, and then pouring the methanol solution containing Fe(acac)3 into the ZnCo-ZIF solution under vigorous magnetic stirring;

[0011] 5) adding a methanol solution containing 2-methylimidazole to the mixture solution obtained in step 4), and stirring at room temperature for a certain period of time;

[0012] 6) adding a methanol solution containing Zn(NO3)2·6H2O dropwise to the solution obtained in step 5) to grow a second layer of ZnCo-ZIF@Fe / ZIF-8;

[0013] 7) centrifuging the solution obtained in step 6), washing with methanol, and drying to obtain ZnCo-ZIF@Fe / ZIF-8 powder;

[0014] 8) pyrolyzing the ZnCo-ZIF@Fe / ZIF-8 powder obtained in step 7) under protective gas;

[0015] Furthermore, in the above-mentioned method for preparing a core-shell ZIF-derived cobalt-iron bimetallic catalyst, the molar ratios of Zn to Co in step 1) are 2:1, 3:1, 4:1, 6:1, and 9:1, respectively.

[0016] Furthermore, in the above-mentioned method for preparing a pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst, the amount of methanol used in step 1) is 80 mL.

[0017] Furthermore, in the preparation method of the above-mentioned pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst, the stirring time in step 2) is 24 hours.

[0018] Furthermore, in the preparation method of the above-mentioned pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst, the drying temperature in step 3) is 60° C. and the drying time is 8 hours.

[0019] Furthermore, in the preparation method of the above-mentioned pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst, the amount of ZnCo-ZIF used in step 4) is 200 mg, the amount of Fe(acac)3 used is 0.2472 g, and the amount of methanol used is 50 mL.

[0020] Furthermore, in the above-mentioned method for preparing a pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst, the ultrasonication in step 4) should be performed at room temperature for 0.5 h.

[0021] Furthermore, in the above-mentioned method for preparing a pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst, in step 5), the amount of 2-methylimidazole used is 0.462 g, and the amount of methanol used is 20 mL.

[0022] Furthermore, in the above-mentioned method for preparing a pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst, in step 6), the amount of Zn(NO3)2·6H2O used is 0.418 g, and the amount of methanol used is 20 mL.

[0023] Furthermore, in the preparation method of the above-mentioned pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst, the drying temperature in step 7) is 60° C. and the drying time is 8 hours.

[0024] Furthermore, in the preparation method of the above-mentioned pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst, in step 8), the protective gas is argon, the pyrolysis temperature is 900°C, and the heating rate is 5°C min -1 , the pyrolysis time is 2h.

[0025] The beneficial effects of the present invention are:

[0026] The present invention uses a core-shell method to prepare the Co and Fe bimetallic catalyst Co@Fe-NC. The Co@Fe-NC catalyst exhibits excellent ORR performance at different pH levels, which may be attributed to the synergistic effect between the metals, the core-shell structure that minimizes the aggregation of metal nanoparticles, and the excellent electron / proton transport capacity brought about by its abundant defects and hierarchical porous characteristics. Co@Fe-NC exhibits a direct four-electron transfer pathway for oxygen reduction and has good stability in neutral, alkaline, and acidic media. Alkaline zinc-air batteries with Co@Fe-NC as the cathode have excellent performance, with an open circuit voltage of up to 1.50V and a peak power density of up to 126.9mW cm -2 In addition, neutral zinc-air batteries also exhibited excellent performance. The research results of this invention provide a new approach for preparing transition metal nitrogen-doped carbon materials for high-performance metal-air batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 SEM (a) and TEM (b) images of the Co@Fe-NC catalyst prepared in Example 3.

[0028] Figure 2 This is the XRD pattern of the Co@Fe-NC catalyst prepared in Example 3.

[0029] Figure 3 (a) N2 adsorption / desorption isotherm diagram and (b) pore size distribution diagram of the Co@Fe-NC catalyst prepared in Example 3.

[0030] Figure 4 CV curves of the Co@Fe-NC catalysts prepared in Example 3 under neutral conditions. Figures (ae) are Co@Fe-NC-2, Co@Fe-NC-3, Co@Fe-NC-4, Co@Fe-NC-6, and Co@Fe-NC-9, respectively.

[0031] Figure 5 This is the LSV curve of the Co@Fe-NC catalyst prepared in Example 3 under neutral conditions.

[0032] Figure 6 This is a graph showing the H2O2 yield and electron transfer number obtained under neutral conditions for the Co@Fe-NC catalyst prepared in Example 3.

[0033] Figure 7 LSV curves of the Co@Fe-NC catalyst prepared in Example 3 under alkaline (a) and acidic (b) conditions.

[0034] Figure 8Stability test of the Co@Fe-NC catalyst prepared in Example 3 under different pH conditions. (a), (c) and (e) are accelerated durability experiments under neutral, alkaline and acidic conditions, respectively. (b), (d) and (f) are chronoamperometric response tests under neutral, alkaline and acidic conditions, respectively.

[0035] Figure 9 Open circuit curves (a) and discharge power density (b) of alkaline and neutral zinc-air batteries assembled with the Co@Fe-NC catalyst prepared in Example 3. Specific implementation methods

[0036] Example 1 Synthesis of ZnCo-ZIF

[0037] Co(NO₃)₂·6H₂O and Zn(NO₃)₂·6H₂O (6 mmol total) (Zn:Co molar ratios of 2:1, 3:1, 4:1, 6:1, and 9:1, respectively) were dissolved in 80 mL of methanol. 2-Methylimidazole (1.848 g) was dissolved in an equal volume of methanol, and the two solutions were mixed with stirring. After the reaction was continued at room temperature for 24 hours, the resulting ZnCo-ZIF precursors (named ZnCo-ZIF-2, ZnCo-ZIF-3, ZnCo-ZIF-4, ZnCo-ZIF-6, and ZnCo-ZIF-9) were collected by centrifugation, washed twice with methanol, and finally dried at 60°C for 8 hours.

[0038] Example 2 Synthesis of ZnCo-ZIF@Fe / ZIF-8

[0039] 200 mg of the ZnCo-ZIF precursor obtained in Example 1 was dispersed in 50 mL of methanol, and then 50 mL of a methanol solution (containing 0.2472 g of Fe(acac)3) was poured into the ZnCo-ZIF solution under vigorous magnetic stirring. Then, 20 mL of a methanol solution containing 0.462 g of 2-methylimidazole was injected into the above mixture solution and stirred continuously at room temperature overnight. Then, 20 mL of a methanol solution containing 0.418 g of Zn(NO3)2·6H2O was added dropwise to the above solution to grow the second layer of ZnCo-ZIF@Fe / ZIF-8. Finally, the prepared ZnCo-ZIF@Fe / ZIF-8 was collected by centrifugation for 3 minutes and washed with methanol several times. After the precipitate was dried in vacuum at 60 °C for 8 h, ZnCo-ZIF@Fe / ZIF-8 powders were obtained (named ZnCo-ZIF-2@Fe / ZIF-8, ZnCo-ZIF-3@Fe / ZIF-8, ZnCo-ZIF-4@Fe / ZIF-8, ZnCo-ZIF-6@Fe / ZIF-8, and ZnCo-ZIF-9@Fe / ZIF-8, respectively).

[0040] Example 3 Synthesis of Cobalt-Iron Bimetallic Catalyst Co@Fe-NC Derived from Core-Shell ZIF

[0041] The ZnCo-ZIF@Fe / ZIF-8 prepared in Example 2 was heated at 5°C min -1 The temperature was raised at a rate of 900 ° C for 2 h. After cooling naturally to room temperature, the prepared samples Co@Fe-NC (named Co@Fe-NC-2, Co@Fe-NC-3, Co@Fe-NC-4, Co@Fe-NC-6, and Co@Fe-NC-9) were obtained.

[0042] Example 4 Characterization of Core-Shell ZIF-Derived Cobalt-Iron Bimetallic Catalyst Co@Fe-NC

[0043] The Co@Fe-NC catalyst prepared in Example 3 was characterized by SEM, TEM, XRD, and N2 adsorption / desorption isotherms. Figure 1 、 Figure 2 and Figure 3 As shown. Figure 1 The SEM and TEM images show that the Co@Fe-NC-4 catalyst maintains a rhombic dodecahedral structure with a uniform size distribution and slightly shrunken edges. Meanwhile, the surface of the Co@Fe-NC-4 catalyst is rough and covered with a thin shell. Figure 2 The XRD spectrum of the Co@Fe-NC catalyst shows two broad peaks at 26° and 44°, corresponding to the (002) and (101) planes of graphitic carbon, respectively, indicating an amorphous carbon structure. The sharp diffraction peaks at 44.3°, 51.5°, and 75.7° correspond to the face-centered cubic (fcc) structure of metallic Co (111), Co (200), and Co (220) crystal planes. Figure 3 The N2 adsorption / desorption isotherm of the Co@Fe-NC catalyst shows a typical type IV curve and an H4 hysteresis loop, indicating the presence of a mesoporous structure. Specifically, the Co@Fe-NC catalyst has a hierarchical porous structure rich in micropores and mesopores, which can provide uniform transport channels for ORR-related species at the electrode-electrolyte interface and expose more potential catalytic active sites, thereby promoting the ORR process.

[0044] Example 5 Oxygen reduction catalytic activity test of Co@Fe-NC catalyst under neutral conditions

[0045] Methods: The ORR activities of the Co@Fe-NC catalyst prepared in Example 3 and the control catalyst were evaluated by rotating disk electrode measurement in 0.1 M PBS (pH 7.0) solution saturated with nitrogen and oxygen.

[0046] from Figure 4 It can be seen from the CV curves that compared with other control catalysts with different Co contents, Co@Fe-NC-4 shows a more positive reduction peak (0.714 V) in O2-saturated PBS solution, indicating that it has more effective catalytic activity for oxygen reduction in neutral solution. Figure 5 The linear sweep voltammetry (LSV) test shows that the half-wave potential (E 1 / 2 ) is 0.756V, and the limiting current density (J L ) is 5.7 mA cm -2 , which is significantly better than the control catalyst, indicating that it has better ORR activity. Figure 6 The oxygen reduction activity of Co@Fe-NC-4 and Pt / C catalysts was tested in 0.1 M PBS using a rotating ring-disk technique. The H₂O₂ yield of Co@Fe-NC-4 was less than 12%, with an electron transfer number of 3.76-3.83. These results indicate that the ORR process catalyzed by Co@Fe-NC-4 favors a four-electron transfer pathway, with an efficiency comparable to that of commercial Pt / C.

[0047] Example 6 Oxygen reduction catalytic activity test of Co@Fe-NC catalyst under acidic and alkaline conditions

[0048] Method: Referring to Example 5, the Co@Fe-NC catalyst prepared in Example 3 and commercial Pt / C were subjected to LSV tests in oxygen-saturated 0.1 M KOH and 0.5 M H2SO4 solutions.

[0049] from Figure 7 It can be found that the E of Co@Fe-NC-4 catalyst in 0.1M KOH solution 1 / 2 The value is 0.827V, and the E 1 / 2 The value is 0.674 V, and its ORR activity is comparable to that of commercial Pt / C catalyst.

[0050] Example 7 Stability test of Co@Fe-NC catalyst under different pH conditions

[0051] from Figure 8 It can be seen that the ORR performance of Co@Fe-NC-4 does not decrease significantly after 3k consecutive CV cycles and 15h It operation, which means that it has good stability in neutral, alkaline and acidic solutions.

[0052] Example 8 Performance test of alkaline and neutral zinc-air batteries assembled with Co@Fe-NC catalyst

[0053] Methods: Alkaline and neutral zinc-air batteries were constructed using the Co@Fe-NC-4 catalyst-supported composite carbon paper prepared in Example 3 as the air cathode and Zn foil as the anode. The open-circuit voltage (OCV) of the batteries was measured using an electrochemical workstation (IT), and the discharge curve of the zinc-air batteries was measured using a LSV curve.

[0054] exist Figure 9 As can be seen in the figure, the alkaline zinc-air battery based on Co@Fe-NC-4 shows good performance, such as an open circuit voltage of up to 1.50 V and a high current density of 167.2 mA cm -2 The peak power density is 126.9 mW cm -2 , surpassing the alkaline zinc-air battery based on Pt / C (1.48 V and 132.4 mA cm -2 101.9 mW cm -2 The assembled neutral zinc-air battery also has a high open circuit voltage of 1.37V and an excellent peak power density (94.3mA cm -2 53.4 mW cm -2 ) and a charge-discharge durability of up to 120 h, which is superior to that of Pt / C-based batteries (1.35 V and 95.7 mA cm -2 29.5 mW cm -2 These results demonstrate that Co@Fe-NC-4 exhibits excellent ORR activity in neutral and alkaline Zn-air batteries.

Claims

1. A pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst, characterized in that: The defect-rich bimetallic catalyst Co@Fe-NC was obtained by calcining ZIF-8 containing iron acetylacetonate grown outside the ZnCo-ZIF core.

2. The method for preparing a pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst according to claim 1, characterized in that: The method comprises the following steps: Step 1: First, Co(NO3)2·6H2O and Zn(NO3)2·6H2O were dissolved in methanol, and 2-methylimidazole was dissolved in an equal volume of methanol. The two solutions were mixed under stirring and reacted at room temperature for a certain time. The obtained ZnCo-ZIF product was collected by centrifugation, washed with methanol, and dried to obtain a ZnCo-ZIF precursor; Step 2: The ZnCo-ZIF precursor is dispersed in methanol and ultrasonicated for a certain time. Then, a methanol solution containing Fe(acac)3 is poured into the ZnCo-ZIF solution under vigorous magnetic stirring. A methanol solution containing 2-methylimidazole is injected into the above mixture solution, and the reaction is continuously stirred at room temperature for a certain time. Then, a methanol solution containing Zn(NO3)2·6H2O is added dropwise to the above solution to grow a second layer of ZnCo-ZIF@Fe / ZIF-8. Finally, the prepared ZnCo-ZIF@Fe / ZIF-8 is collected by centrifugation, washed multiple times, and dried to obtain ZnCo-ZIF@Fe / ZIF-8 powder. Step 3: The prepared ZnCo-ZIF@Fe / ZIF-8 was pyrolyzed under protective gas and naturally cooled to room temperature to obtain the prepared sample Co@Fe-NC.

3. The method for preparing a pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst according to claim 2, characterized in that: The molar ratio of Zn to Co in step 1 is (2-9):

1.

4. The method for preparing a pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst according to claim 2, wherein: The reaction time in step 1 is 24 hours under magnetic stirring at room temperature.

5. The method for preparing a pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst according to claim 2, characterized in that: The drying in step 1 is performed at 60° C. for 8 h, and the drying in step 2 is performed at 60° C. for 8 h.

6. The method for preparing a pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst according to claim 2, characterized in that: The ultrasonic treatment in step 2 should be carried out at room temperature for 0.5 h.

7. The method for preparing a pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst according to claim 2, characterized in that: The reaction time in step 2 should be 12 h under magnetic stirring at room temperature.

8. The method for preparing a pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst according to claim 2, wherein: The protective gas in step 3 should be argon.

9. The method for preparing a pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst according to claim 2, characterized in that: The pyrolysis in step 3 was performed at 5°C min -1 The heating rate was 900 °C for 2 h.

10. Use of the pH-universal core-shell cobalt-iron bimetallic oxygen reduction catalyst according to claim 1 in zinc-air batteries.

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