Synthesis of transition metal-bismuth-transition metal triatomic catalyst and application of transition metal-bismuth-transition metal triatomic catalyst in metal-air battery

Abstract

The invention belongs to the technical field of electro-catalysis, and particularly relates to synthesis of a transition metal-bismuth-transition metal triatomic catalyst and application of the transition metal-bismuth-transition metal triatomic catalyst in a metal-air battery. The transition metal-bismuth-transition metal triatomic catalyst provided by the invention is prepared by the following steps: firstly synthesizing a ZIF-8 precursor, then dissolving a transition metal precursor, a bismuth element precursor and the ZIF-8 in a mixed solution of methanol and nitric acid, and then performing high-temperature thermal cracking in an inert gas atmosphere. The transition metal-bismuth-transition metal three-atom catalyst has a rhombic dodecahedron structure and good conductivity, transition metal-bismuth-transition metal three atoms are uniformly distributed on a carbon frame substrate, active sites are dense, and the activity and stability of the catalyst are improved. Transition metal-bismuth-transition metal three-atom sites are loaded on a nitrogen-doped carbon framework, so that the transition metal coordination configuration and the electronic structure are regulated and controlled, and the catalytic activity and the stability are effectively improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrocatalysis, and particularly relates to the synthesis of a transition metal-bismuth-transition metal triatomic catalyst and its application in metal-air batteries. Background Art

[0002] Rechargeable zinc-air batteries (RZABs) are a promising energy storage technology and have attracted much attention due to their high theoretical energy density (1086 Wh·kg -1 ), low cost, environmental friendliness, and high safety. The positive electrode of a rechargeable zinc-air battery requires oxygen reduction reaction activity (ORR) for discharging and oxygen evolution reaction activity (OER) for charging. Compared with traditional noble metals and noble metal oxides (Pt, IrO2, RuO2), single-atom catalysts can utilize the quantum effect of non-noble metals to catalyze the ORR and OER reactions, thereby reducing costs and improving the performance of the battery. At present, it has been reported that rechargeable zinc-air batteries with noble metals and noble metal oxides as the positive electrode catalyst can only operate for 40-60 hours, and zinc-air batteries with single-atom iron as the catalyst can operate for about 200 hours. Currently, the relevant research on the single-atom catalyst positive electrode of rechargeable zinc-air batteries mainly focuses on transition metals such as Fe, Co, and Ni. However, transition metal single-atom catalysts face an important problem, that is, the catalytic performance is single and the stability is poor. This is because it is difficult for a single element to regulate the energy barrier of the catalytic reaction and requires the auxiliary regulation of other atoms. At the same time, most transition metal single atoms exist stably in the form of M-N4 structures on the carbon substrate. It is difficult for different metal atoms to form bonds between them, and they are limited due to insufficient coupling of active centers at a long distance, thus affecting the performance improvement of single-atom catalysts and greatly limiting the industrial application of RZABs.

[0003] Bismuth (Bi), as an element with a relatively large and relatively stable atomic number in the periodic table, has different bond lengths and coordination conditions from common transition metals. However, the solubility of Bi is not as easy as that of Fe, resulting in difficulty in fabricating it into a single-atom catalyst. Under the co-action of transition metals and Bi, Bi can destroy the common M-N4 structure of transition metals, and transition metals promote the stable binding of single-atom Bi on the substrate, thereby forming a triatomic catalyst of transition metal atom-bismuth atom-transition metal atom (M-Bi-M), effectively improving the performance of single-atom catalysts. Based on this, the present invention develops a multi-atom catalyst based on bismuth elements and transition metals to overcome the current dilemma of single-atom catalysts for rechargeable zinc-air batteries. Summary of the Invention

[0004] To overcome the deficiencies of the above-mentioned existing technologies, the present invention synthesizes a transition metal atom - bismuth atom - transition metal atom triatomic catalyst with bismuth as the core. The bismuth therein can effectively regulate the electronic structure of the single-atom catalyst, enhance the performance of the zinc-air battery, endow the battery with good stability, and has low raw material cost, making it a suitable positive electrode catalyst.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] The first aspect of the present invention provides a method for synthesizing a transition metal - bismuth - transition metal triatomic catalyst, comprising the following steps:

[0007] S1. Dissolve 2-methylimidazole and zinc nitrate in methanol, react at 50 - 70 °C for 20 - 30 hours, collect the solid product, and obtain ZIF-8 powder after drying.

[0008] S2. Dissolve the transition metal salt and bismuth salt in a nitric acid - methanol mixed solution, then add it to the methanol solution of ZIF-8, and then stir and react at 50 - 70 °C for 10 - 15 hours. Collect the solid product and obtain transition metal - bismuth - ZIF after drying. Bismuth nitrate dissolved in the nitric acid - methanol solution can effectively disperse bismuth ions and does not destroy the rhombic dodecahedron structure of ZIF-8 after dilution. While organic reagents such as ethylene glycol can effectively disperse bismuth ions, it is difficult to maintain the three-dimensional structure of ZIF-8 during the drying process, resulting in structural collapse, reduced conductivity, decreased specific surface area, and reduced catalytic sites.

[0009] S3. Place the transition metal - bismuth - ZIF prepared in S2 under an inert gas atmosphere for high-temperature pyrolysis (calcination). The obtained product is ground, and the residual metal particles are removed by hydrochloric acid, and then high-temperature annealing treatment is carried out to finally obtain the transition metal - bismuth - transition metal triatomic catalyst.

[0010] Preferably, in S2, the bismuth salt includes bismuth nitrate, bismuth chloride, bismuth oxychloride, bismuth carbonate, and the transition metal salt includes iron salt, cobalt salt, nickel salt, copper salt, manganese salt, such as iron nitrate, etc.

[0011] Preferably, in S3, the high-temperature pyrolysis is first heated to 250 - 300 °C at a rate of 1 - 5 °C·min -1 and held for 1 - 3 hours, then heated to 900 - 1000 °C at a rate of 4 - 6 °C·min -1 and maintained for 2 - 4 hours.

[0012] Preferably, the method for removing the residual metal particles by hydrochloric acid in S3 is to stir-treat the product with 5 - 7 mol·L -1 hydrochloric acid at 70 - 90 °C for 20 - 30 hours.

[0013] Preferably, the high-temperature annealing treatment in S3 is annealing at 750 - 850 °C for 1 - 2 hours in an inert gas atmosphere.

[0014] Preferably, in S2, the mass ratio of iron nitrate to bismuth nitrate is 0.8 - 0.9:1 - 2.

[0015] Preferably, in the nitric acid - methanol mixed solution in S2, the concentration of nitric acid is 1 - 3 M, and the concentration of bismuth nitrate in the nitric acid - methanol mixed solution is 2 - 3 g / 10 mL.

[0016] Preferably, in S2, the volume ratio of the nitric acid - methanol mixed solution of iron nitrate and bismuth nitrate to the methanol solution of ZIF - 8 is 1 - 2:4 - 7, and the concentration of the methanol solution of ZIF - 8 is 2 - 3 g / 50 mL.

[0017] Preferably, in S1, the mass ratio of 2 - methylimidazole to zinc nitrate is 15 - 25:8 - 10.

[0018] The second aspect of the present invention provides a transition metal - bismuth - transition metal tri - atom catalyst prepared by the synthesis method described in the first aspect.

[0019] The third aspect of the present invention provides the application of the transition metal - bismuth - transition metal tri - atom catalyst described in the second aspect in a metal - air battery, characterized in that the transition metal - bismuth - transition metal tri - atom catalyst is used as the positive electrode catalyst of the metal - air battery.

[0020] Preferably, the metal - air battery is a zinc - air battery.

[0021] It has been proven by research in the present invention that the RZABs using the transition metal - bismuth - transition metal tri - atom catalyst of the present invention can reach a peak power density of 237 mW·cm -2 , the discharge voltage is as high as 1.04 V at 50 mA·cm -2 , and it can stably operate for 720 hours (2160 charge - discharge cycles) at 10 mA·cm -2 . Meanwhile, the flexible quasi - solid - state RZABs based on the transition metal - bismuth - transition metal tri - atom catalyst of the present invention can maintain the charge - discharge voltage when bent at 90° and 180° at 2 mA·cm -2 . It shows that the transition metal - bismuth - transition metal tri - atom catalyst with bifunctional oxygen catalytic activity prepared in the present invention can be applied in the positive electrode of rechargeable zinc - air batteries and the positive electrode of fuel cells.

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] The present invention discloses a method for synthesizing a transition metal-bismuth-transition metal triatomic catalyst. First, a ZIF-8 precursor is synthesized, and then a transition metal precursor (such as Fe(NO3)3, Co(NO3)3) and a bismuth element precursor (Bi(NO3)3) are dissolved in a mixed solution of methanol and nitric acid together with ZIF-8, and then high-temperature pyrolysis is carried out under an inert gas atmosphere to obtain the product. The transition metal-bismuth-transition metal triatomic catalyst synthesized by the present invention has a rhombic dodecahedron structure and good electrical conductivity. The transition metal-bismuth-transition metal triatomic atoms are uniformly distributed on the carbon framework substrate, and the active sites are dense, which is beneficial to improving the activity and stability of the catalyst. By loading the transition metal-bismuth-transition metal triatomic sites on the nitrogen-doped carbon framework, the present invention regulates the coordination configuration and electronic structure of the transition metal, effectively improving the catalytic activity and stability.

[0024] Specifically, the present invention has the following advantages: (1) Bismuth atoms can adjust the configuration of transition metal single atoms to triatomic atoms; (2) The M-Bi-M structure in the transition metal-bismuth-transition metal triatomic catalyst has good ORR / OER catalytic activity; (3) The dodecahedron structure formed by the pyrolysis of the ZIF-8 precursor provides good electrical conductivity and active sites, enhancing the activity and stability of single atoms; (4) Through p-d orbital coupling, bismuth changes the d orbital energy level of the transition metal and adjusts its binding strength to oxygen-containing intermediates. Description of the Drawings

[0025] Figure 1 are the transmission electron microscope images (a) and the triatomic site map (b) of the iron-bismuth-iron triatomic catalyst.

[0026] Figure 2 is the energy spectrum of the iron-bismuth-iron triatomic catalyst.

[0027] Figure 3 are the linear sweep voltammograms of the iron-bismuth-iron triatomic catalyst and the unit catalyst in an oxygen-saturated 0.1 M potassium hydroxide electrolyte (a) and the linear sweep voltammograms of the iron-bismuth-iron triatomic catalyst and the unit catalyst in 1 M potassium hydroxide electrolyte (b).

[0028] Figure 4 are the performance tests of a rechargeable zinc-air battery based on the iron-bismuth-iron triatomic catalyst; (a) discharge polarization curve and power density; (b) rate performance test; (c) cycle stability test.

[0029] Figure 5 is the cycle stability of the flexible quasi-solid-state RZABs based on the iron-bismuth-iron triatomic catalyst at different bending angles. Detailed Embodiments

[0030] The specific embodiments of the present invention will be further described below. It should be noted here that the description of these embodiments is used to help understand the present invention, but does not constitute a limitation on the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0031] The experimental methods in the following examples are all conventional methods unless otherwise specified. The test materials used in the following examples are all commercially available through conventional channels unless otherwise specified.

[0032] Example 1: Preparation of iron-bismuth-iron triatomic catalyst

[0033] (1) Dissolve 50 g of 2-methylimidazole (2-MeIm) in 250 mL of methanol to obtain solution A; dissolve 22.5 g of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) in 250 mL of methanol to obtain solution B. Then, under continuous stirring, slowly pour solution A into solution B. Subsequently, heat the mixture to 60 °C and stir the reaction at this temperature for 24 hours. After the reaction, centrifuge the obtained suspension (remove the supernatant) and collect the solid product. Finally, place the solid in a vacuum oven and vacuum dry it at 60 °C for 24 hours. The obtained white powder is the synthesized ZIF-8 material.

[0034] (2) Synthesis of iron-bismuth precursor (FeBi-ZIF): Dissolve 2.02 g of iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O) and 2.43 g of bismuth(III) nitrate pentahydrate (Bi(NO3)3·5H2O) in 10 mL of a 2 M nitric acid-methanol mixed solution (concentration calculated based on methanol as the solvent), and ultrasonically treat it for 2 hours to fully disperse it to obtain solution C. Take another 2.5 g of ZIF-8 powder and disperse it in 50 mL of methanol, and ultrasonically treat it for 2 hours to ensure uniform dispersion to obtain a ZIF-8 dispersion. Then mix 1.0 mL of solution C with the above ZIF-8 dispersion, heat it to 60 °C under continuous stirring and keep the temperature constant for 12 hours. After the reaction, centrifuge to collect the solid product. Dry the obtained solid in a vacuum oven at 60 °C for 24 hours to finally obtain FeBi-ZIF.

[0035] (3) Synthesis of iron-bismuth-iron triatomic catalyst (Fe2BiN5 / C): Place the above-prepared FeBi-ZIF in a quartz boat and calcine it using a two-step heating program: First, heat it to 280 °C at a heating rate of 2 °C / min -1 and keep it at this temperature for 2 hours; then further heat it to 920 °C at a rate of 5 °C / min -1 and maintain it for 3 hours. Grind the calcined powder and use 6 mol·L at 80 °C-1 The hydrochloric acid was stirred for 24 hours to remove residual metal particles. After the sample was dried, it was annealed at 800 °C for 1 hour in an argon atmosphere, and finally the Fe2BiN5 / C material was obtained.

[0036] As Figure 1 shown in the phase-contrast corrected transmission electron microscopy image, in the Fe2BiN5 / C synthesized in this example, iron atoms and bismuth atoms are uniformly distributed on the carbon framework substrate in the form of triatomic atoms.

[0037] As Figure 2 shown in the energy spectrum diagram, it can be clearly seen that iron and bismuth elements are uniformly distributed on the carbon framework substrate, thus forming an efficient catalytic active site of iron-bismuth-iron triatomic atoms with a synergistic effect.

[0038] Example 2: Linear sweep voltammetry test of the iron-bismuth-iron triatomic catalyst Fe2BiN5 / C

[0039] A three-electrode configuration was selected for the linear sweep voltammetry test. 5 mg of the catalyst Fe2BiN5 / C was ultrasonically dispersed in a solution prepared from isopropanol, water, and Nafion at a volume ratio of 0.485:0.485:0.30, and then 13.74 μL was taken and dropped on a glassy carbon electrode with a diameter of 5 mm as the working electrode; the reference electrode was Hg / HgO, and the counter electrode was graphite; the tests were carried out in 0.1 M potassium hydroxide and 1 M potassium hydroxide electrolytes respectively, and the scanning rate was 5 mV·s -1 .

[0040] Figure 3 is the activity diagram of the iron-bismuth-iron triatomic catalyst Fe2BiN5 / C evaluated by linear sweep voltammetry. During the whole measurement process, the absolute values of the hydroxide oxidation current and oxygen reduction current of the iron-bismuth-iron triatomic catalyst are both greater than those of commercial platinum-carbon, Fe–N / C, and Bi–N / C unit catalysts. This Fe-Bi-Fe catalyst not only has good stable oxygen reduction catalytic (ORR) performance, with a half-wave potential of 0.918 V (vs. RHE); but also has good stable oxygen evolution catalytic (OER) performance, and the current density reaches 10 mA·cm at a bias voltage of 1.475 V (vs. RHE) -2 . This indicates that the iron-bismuth-iron triatomic catalyst exhibits significantly higher activity than commercial catalysts and unit catalysts. Among them, the preparation method of the Fe–N / C unit catalyst is basically the same as that of Fe2BiN5 / C, the difference is that: Bi(NO3)3·5H2O was not used in step (2). The preparation method of the Bi–N / C unit catalyst is also basically the same as that of Fe2BiN5 / C, the difference is that: Fe(NO3)3·9H2O was not used in step (2).

[0041] Example 3: Performance Test of Rechargeable Zinc-Air Battery Based on Iron-Bismuth-Iron Tri-Atom Catalyst

[0042] Using Fe2BiN5 / C, an iron-bismuth-iron tri-atom catalyst, as the positive electrode catalyst, with a hydrophobic carbon paper as the current collector for the positive electrode. After preparing the catalyst solution according to the method in Example 2, it was coated on the carbon paper with a surface mass density of 5 mg·cm -2 . At the same time, a 0.5 mm Zn sheet was used as the negative electrode metal, and the battery performance was tested in a mixed electrolyte of 6 M potassium hydroxide and 0.2 M zinc acetate.

[0043] Figure 4 a shows the discharge polarization curve and corresponding power density of Fe2BiN5 / C at a scan rate of 5 mV·s -1 . It can be seen that the iron-bismuth-iron tri-atom catalyst Fe2BiN5 / C has a higher peak power density compared to the iron single-atom unit catalyst Fe–N / C and the commercial catalyst (1:1 wt% mixed Pt / C + RuO2). Figure 4 b shows the rate performance test results of Fe2BiN5 / C at 1–50 mA·cm -2 . It can be seen that the discharge voltage of the iron-bismuth-iron tri-atom catalyst Fe2BiN5 / C is higher than that of the commercial catalyst Pt / C + RuO2 and the single-atom catalyst Fe–N / C. Figure 4 c shows the cyclic stability test of the battery. It can be seen that the battery using the iron-bismuth-iron tri-atom catalyst Fe2BiN5 / C can be stably cycled for 720 hours, far exceeding 160 hours of the iron single-atom Fe–N / C and 80 hours of the commercial catalyst Pt / C + RuO2. It can be seen that the RZABs using the iron-bismuth-iron tri-atom catalyst as the positive electrode can reach a peak power density of 237 mW·cm -2 during discharge, with a discharge voltage as high as 1.04 V at 50 mA·cm -2 and can stably operate for 720 hours (2160 charge-discharge cycles) at 10 mA·cm -2 . The above results indicate that the zinc-air battery composed of the iron-bismuth-iron tri-atom catalyst Fe2BiN5 / C has better discharge power density, discharge rate performance, and cyclic stability.

[0044] Example 4: Flexibility Test of Quasi-Solid-State Zinc-Air Battery with Iron-Bismuth-Iron Tri-Atom Catalyst

[0045] Prepare the catalyst solution according to the method in Example 2, with a surface mass density of 5 mg·cm -2The areal mass density is coated on the carbon cloth as the positive electrode; at the same time, a zinc foil with a thickness of 0.1 mm is used as the negative electrode, and a quasi-solid electrolyte polyacrylamide-sodium citrate (PAM-SC) organic hydrogel is used as the electrolyte. The battery is assembled according to the structure of negative electrode - quasi-solid electrolyte - positive electrode. The electrolyte is synthesized by polymerization: 3 g of acrylamide (AM), 2.5 g of sodium citrate (SC), 100 mg of N’N-methylenebisacrylamide (MBA) and 17 mL of deionized water are mixed evenly, and then 3 mL of 50 mg·mL -1 potassium persulfate (KPS) is added. The mixed solution is allowed to stand and heat-insulate at 50 °C for 12 hours, and after cooling, it is soaked in a mixed solution of 6 M potassium hydroxide and 0.2 M zinc acetate for 3 days to obtain the PAM-SC organic hydrogel quasi-solid electrolyte. Figure 5 is the flexibility test of the quasi-solid-state zinc-air battery with an iron-bismuth-iron triatomic catalyst, showing that it can still maintain a stable charge-discharge voltage after being bent at 90° and 180° at a current density of 2 mA·cm -2 This indicates that the quasi-solid-state zinc-air battery with an iron-bismuth-iron triatomic catalyst has good flexibility and stability.

[0046] The above has described the embodiments of the present invention in detail, but the present invention is not limited to the described embodiments. For those skilled in the art, without departing from the principle and spirit of the present invention, various changes, modifications, substitutions and variations made to these embodiments still fall within the protection scope of the present invention.

Claims

1. A method for synthesizing a transition metal-bismuth-transition metal triatomic catalyst, characterized in that, It includes the following steps: S1. Dissolve 2-methylimidazole and zinc nitrate in methanol, react at 50-70 °C for 20-30 hours, collect the solid product, and obtain ZIF-8 powder after drying; S2. Dissolve the transition metal salt and bismuth salt in a nitric acid-methanol mixed solution, then add it to the methanol solution of ZIF-8, and then stir and react at 50-70 °C for 10-15 hours, collect the solid product, and obtain transition metal-bismuth-ZIF after drying; S3. Place the transition metal-bismuth-ZIF prepared in S2 under an inert gas atmosphere for high-temperature pyrolysis, grind the obtained product, remove the residual metal particles with hydrochloric acid, and then perform high-temperature annealing treatment to finally obtain a transition metal-bismuth-transition metal triatomic catalyst.

2. The synthesis method of a transition metal-bismuth-transition metal triatomic catalyst according to claim 1, wherein In S2, the bismuth salt includes bismuth nitrate, bismuth chloride, bismuth oxyhydroxide, bismuth carbonate, and the transition metal salt includes iron salt, cobalt salt, nickel salt, copper salt, manganese salt.

3. The synthesis method of a transition metal-bismuth-transition metal triatomic catalyst according to claim 1, characterized in that, The high-temperature pyrolysis described in S3 is first heated at 1-5 °C·min -1 to 250-300 °C and held for 1-3 hours, and then heated at 4-6 °C·min -1 to 900-1000 °C and maintained for 2-4 hours.

4. The synthesis method of a transition metal-bismuth-transition metal triatomic catalyst according to claim 1, characterized in that The method of using hydrochloric acid to remove residual metal particles in S3 is to stir the product with 5 - 7 mol·L hydrochloric acid at 70 - 90 °C for 20 - 30 hours. -1 ​ 5. The synthesis method of a transition metal-bismuth-transition metal triatomic catalyst according to claim 1, characterized in that, The high-temperature annealing treatment in S3 is to anneal at 750-850 °C for 1-2 hours under an inert gas atmosphere.

6. The synthesis method of a transition metal-bismuth-transition metal triatomic catalyst according to claim 1, characterized in that, In S2, the mass ratio of iron nitrate to bismuth nitrate is 0.8-0.9:1-2.

7. A method for synthesizing a transition metal-bismuth-transition metal triatomic catalyst according to claim 1, characterized in that, In the nitric acid-methanol mixed solution of S2, the concentration of nitric acid is 1-3 M, and the concentration of bismuth nitrate in the nitric acid-methanol mixed solution is 2-3 g / 10 mL.

8. The synthesis method of a transition metal-bismuth-transition metal triatomic catalyst according to claim 1, characterized in that, In S2, the volume ratio of the nitric acid-methanol mixed solution of iron nitrate and bismuth nitrate to the methanol solution of ZIF-8 is 1-2:4-7, and the concentration of the methanol solution of ZIF-8 is 2-3 g / 50 mL.

9. A transition metal-bismuth-transition metal triatomic catalyst prepared by the synthesis method according to any one of claims 1-8.

10. Use of the transition metal-bismuth-transition metal triatomic catalyst according to claim 9 in a metal-air battery, characterized in that, The transition metal-bismuth-transition metal triatomic catalyst is used as a positive electrode catalyst of a metal-air battery.

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