Preparation method and application of cobalt / nitrogen co-doped multi-section nano-chain carbon fiber oxygen electrode

Abstract

The invention relates to a preparation method and application of a cobalt / nitrogen co-doped multi-section nano-chain carbon fiber oxygen electrode, and the preparation method comprises the following steps: (1) dissolving an organic ligand and a cobalt salt in an organic solvent, uniformly stirring and standing, and then centrifuging, washing and drying to obtain ZIF-67 powder; (2) dissolving a high-molecular polymer and ZIF-67 powder in a solvent, and uniformly stirring to obtain an electrostatic spinning solution; (3) performing electrostatic spinning on the electrostatic spinning solution to obtain a fiber membrane; and (4) pre-oxidizing the fiber membrane at a medium-low temperature in an air atmosphere, and then transferring the fiber membrane to a nitrogen atmosphere for high-temperature calcination to obtain the cobalt / nitrogen co-doped multi-section nano-chain carbon fiber oxygen electrode. According to the invention, the distance of the ZIF-67 cluster on the single fiber is accurately controlled, the electron transmission path and the active site distribution are optimized, the breakthrough of the bifunctional catalytic performance is realized, and the application prospect is good.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of electrochemical catalytic composite materials, and particularly relates to a preparation method and application of a cobalt / nitrogen (Co / N) co-doped multi-joint nanochain carbon fiber oxygen electrode. BACKGROUND

[0002] Metal-air batteries have attracted much attention due to their high theoretical energy density, low cost and high safety. However, the key reactions of the cathode: oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), involve slow kinetics, leading to low round-trip efficiency, which seriously restricts their practical application. So far, platinum (Pt) and iridium (Ir) based catalysts are considered as the most advanced ORR and OER catalytic materials, respectively. However, their high cost, catalytic single and poor stability limit the large-scale application and commercialization of metal-air batteries. Therefore, it is crucial to develop high-performance, strong-stability and low-cost non-noble metal-based bifunctional catalysts to replace noble metal-based catalysts.

[0003] Metal-organic frameworks (MOFs) are considered as precursor materials for the preparation of heteroatom-doped porous carbon with rich N-C / M-N-C active sites due to their morphological diversity, adjustable porosity and high specific surface area. However, the high-temperature carbonization of MOF materials has problems such as easy agglomeration of metal nanoparticles and poor mechanical stability, which leads to the failure to effectively expose active sites and form efficient electron transfer channels, thereby affecting the practical application of the catalyst. In addition, during the preparation of air electrodes, the introduction of polymer binders not only covers the number of active sites, but also its degradation over time can lead to the shedding of the catalyst, further affecting the performance of the battery.

[0004] Electrospinning technology can directly embed MOF nanoparticles into one-dimensional (1D) fiber structures, achieving one-step construction of independent self-supporting oxygen electrodes, while avoiding the dependence on binders for traditional electrode preparation. Embedding MOF into nanofibers can effectively solve the problem of incoherent electron transport network caused by MOF particle agglomeration, and expose more active sites by adjusting the porosity. However, embedding MOF into electrospun fibers still has challenges such as uneven distribution of active sites inside the fiber material and poor overall conductivity. Therefore, how to precisely control the number and distribution of active sites on a single fiber to realize high-efficiency bifunctional catalysts for the development of metal-air batteries is of great significance. SUMMARY

[0005] The purpose of the present application is to provide a preparation method and application of a cobalt / nitrogen co-doped multi-joint nanochain carbon fiber oxygen electrode, to solve the problems of low active site utilization and limited electron transport caused by disordered stacking when embedding MOF into nanofiber matrix, thereby improving the performance of the catalyst and prolonging the service life of the battery.

[0006] To solve the above problems, the present application provides a preparation method of a cobalt / nitrogen co-doped multi-section nanochain carbon fiber oxygen electrode, comprising the following steps: (1) Dissolve an organic ligand and a cobalt salt in an organic solvent, stir uniformly and stand, then centrifuge, wash and dry to obtain ZIF-67 powder; (2) Dissolve a high molecular polymer and the ZIF-67 powder obtained in step (1) in a solvent, stir uniformly to obtain an electrospinning solution; (3) Electrospin the electrospinning solution in step (2) to obtain a fiber membrane; (4) Pre-oxidize the fiber membrane in step (3) at low temperature in an air atmosphere, then transfer to a nitrogen atmosphere for high-temperature calcination to obtain a multi-section nanochain carbon fiber oxygen electrode with cobalt / nitrogen co-doping.

[0007] Preferably, the mass ratio of the organic ligand to the cobalt salt in step (1) is 4-40:1; and the particle size of the ZIF-67 powder is 250-300 nm.

[0008] Preferably, the organic ligand in step (1) includes but is not limited to any one of 2-methylimidazole, terephthalic acid and trimesic acid; the cobalt salt includes any one of cobalt nitrate, cobalt chloride, cobalt sulfate and cobalt acetate; and the organic solvent includes but is not limited to any one of ethanol and methanol.

[0009] Preferably, the high molecular polymer in step (2) includes but is not limited to polyacrylonitrile (PAN) or polyvinylpyrrolidone (PVP); and the solvent includes but is not limited to any one of N-N dimethylformamide (DMF) or dimethylacetamide (DMAc).

[0010] Preferably, by adjusting the mass ratio of the high molecular polymer to the ZIF-67 in the electrospinning solution in step (2) to be 1:0.5-2.5, the spacing of the ZIF-67 clusters on the single fiber of the multi-section nanochain carbon fiber oxygen electrode in step (4) is controlled to be 400 nm-2 um.

[0011] More preferably, the spacing of the ZIF-67 clusters on the single fiber of the multi-section nanochain carbon fiber oxygen electrode in step (4) is 450-550 nm.

[0012] Preferably, in the spinning process in step (3), the spinning voltage is 12-20 kV, the feeding speed is 0.6-1.5 mL / h, and the distance from the spinning needle to the receiving plate is 12-20 cm.

[0013] Preferably, the pre-oxidation temperature in the step (4) is 250-300 ℃, and the time is 60-180 min; the calcination temperature is 700-1000 ℃, and the time is 90-180 min.

[0014] The application provides a cobalt / nitrogen co-doped multi-joint nanochain carbon fiber oxygen electrode prepared according to the preparation method.

[0015] The application also provides application of the cobalt / nitrogen co-doped multi-joint nanochain carbon fiber oxygen electrode to a metal-air battery, in particular to preparation of a good cathode air electrode.

[0016] Preferably, the metal-air battery includes any one of a magnesium-air battery, a zinc-air battery or an aluminum-air battery.

[0017] Beneficial effects

[0018] (1) The application precisely constructs a conductive network with a multi-joint nanochain structure by compounding MOFs and one-dimensional nanofibers, and controls the ZIF-67 cluster spacing to 500±50 nm, which effectively inhibits cobalt particle sintering and optimizes mass transfer, simultaneously constructs an electronic tunneling network between cobalt clusters, realizes high-efficiency ORR and OER dual-function catalytic activity, and solves the problems of single-function dominance and low dual-function synergistic efficiency of traditional catalysts.

[0019] (2) The application realizes sufficient exposure of active sites by optimizing the design of micropore-mesopore-macropore multi-level channels, optimizes the mass transfer path and significantly improves the electron transfer rate, thereby overcoming the problems of mass transfer limitation caused by poor continuity of the conductive network and micropore blockage of traditional MOF derived carbon materials.

[0020] (3) The unique multi-joint nanochain carbon fiber oxygen electrode in the application can realize binder-free electrode preparation, avoids the introduction of inert substances, and is applied to a zinc-air battery, and exhibits high power density and long-term cycle stability. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 SEM images of cobalt / nitrogen co-doped multi-joint nanochain carbon fiber oxygen electrodes with different spacings, Example 1 (a), Example 2 (b), Example 4 (c), Example 5 (d).

[0022] Figure 2 SEM images of ZIF-67 synthesized in Example 3 (a) and cobalt / nitrogen co-doped multi-joint nanochain carbon fiber oxygen electrode Co / Co-N X SEM image of C@CNF-1.5 (b).

[0023] Figure 3 Co / Co-N in Example 3X Nitrogen adsorption-desorption curve of C@CNF-1.5.

[0024] Figure 4 Co / Co-N with different intervals in Examples 1-5 X Comparison chart of ORR performance of C@CNF oxygen electrode.

[0025] Figure 5 Co / Co-N prepared in Example 3 X Comparison chart of OER performance of C@CNF-1.5 oxygen electrode and commercial Ir / C catalyst.

[0026] Figure 6 Co / Co-N prepared in Example 3 X Power density chart of zinc-air battery assembled by C@CNF-1.5 oxygen electrode.

[0027] Figure 7 Co / Co-N prepared in Example 3 X Charge-discharge cycle performance chart of zinc-air battery assembled by C@CNF-1.5 oxygen electrode. DETAILED DESCRIPTION

[0028] The application will be further described below in connection with specific embodiments. It should be understood that these embodiments are only used to illustrate the application and not used to limit the scope of the application. Furthermore, it should be understood that those skilled in the art can make various modifications or changes to the application after reading the content taught by the application, and these equivalent forms also fall within the scope defined by the appended claims.

[0029] The micro-morphology of the product of the embodiments of the application is tested by a scanning electron microscope (SEM, Hitachi S 4800). The specific surface area is tested by Autosorb-IQ (Quantachrome, USA). The battery power density and electrochemical performance LSV curve are tested by a CHI760 series electrochemical workstation of Chenhua. The charge-discharge performance is tested on a CT2001A blue battery test system.

[0030] Example 1

[0031] This embodiment provides a cobalt / nitrogen co-doped multi-joint nanochain carbon fiber oxygen electrode, and a preparation method thereof includes the following steps: (1) Preparation of ZIF-67: a certain amount of 2-methylimidazole is dissolved in 200 mL of methanol, stirred uniformly, and then a methanol solution containing a certain amount of cobalt nitrate is added to the above solution (2-methylimidazole: cobalt nitrate = 40:1 by mass ratio), mixed and stirred uniformly, and then left to stand for 24 h, washed with anhydrous ethanol three times, and dried at 60°C overnight to obtain ZIF-67 powder; (2) Electrospinning solution preparation: a certain amount of polyacrylonitrile (PAN) was dissolved in 10 mL DMF and stirred at 60°C overnight, and then the ZIF-67 powder prepared in step (1) was slowly added to the above solution (PAN:ZIF-67 = 1:0.5 mass ratio), and stirring was continued for 6 h to obtain an electrospinning solution; (3) Electrospinning: the electrospinning solution prepared in step (2) was injected into a 10 mL syringe for spinning, and the spinning process parameters were as follows: voltage 18-20 kV; feed rate 0.9-1.1 mL / h; distance between spinning needle and receiving plate 12-15 cm, to obtain a fiber membrane; (4) Pre-oxidation and carbonization: the fiber membrane in step (3) was placed in a muffle furnace and heated to 280°C at 1°C / min under air atmosphere for 2 h, and then cooled to room temperature to obtain a pretreated fiber membrane; then it was placed in a tube furnace and heated to 800°C at 5°C / min under inert atmosphere for 2 h, and then cooled to room temperature to obtain a multi-joint nanochain carbon fiber oxygen electrode, denoted as Co / Co-N X -C@CNF-0.5, the SEM image of which is shown in Figure 1 a, and the ZIF-67 cluster spacing is 2 um.

[0032] Example 2

[0033] This example provides a cobalt / nitrogen co-doped multi-joint nanochain carbon fiber oxygen electrode, and the preparation method is the same as that of Example 1, except that the mass ratio of PAN:ZIF-67 in step (2) is 1:1. A multi-joint nanochain carbon fiber oxygen electrode is prepared, denoted as Co / Co-N X -C@CNF-1, the SEM image of which is shown in Figure 1 b, and the ZIF-67 cluster spacing is 1 um.

[0034] Example 3

[0035] This example provides a cobalt / nitrogen co-doped multi-joint nanochain carbon fiber oxygen electrode, and the preparation method is the same as that of Example 1, except that the mass ratio of PAN:ZIF-67 in step (2) is 1:1.5. A multi-joint nanochain carbon fiber oxygen electrode is prepared, denoted as Co / Co-N X -C@CNF-1.5, the SEM image of which is shown in Figure 2 b, and the ZIF-67 cluster spacing is 500 nm.

[0036] Figure 2 uniformly sized ZIF-67 material prepared in step (1) of Example 3 is shown in Figure 2b indicates that the MOF is successfully embedded into the fiber matrix by direct electrospinning technology, and the distribution of the MOF on the single fiber is further regulated by adjusting the addition amount of the MOF, with the distance between the MOFs being 500 nm being optimal, which can reduce the disorder degree of the electrospun fiber, effectively improve the mass transfer process and realize fast electron transfer.

[0037] The Co / Co-N X The nitrogen adsorption-desorption curve of the C@CNF-1.5 is shown in Figure 3 The specific surface area (SSA) of the Co / Co-Nx-C@CNF-1.5 material is as high as 338.4 m 2 ·g -1 , and the average pore size is 3.8 nm, achieving a favorable balance between active site exposure and efficient mass transfer. The synergistic effect of the optimized specific surface area and mesoporous structure significantly reduces the charge transfer impedance, thereby directly improving the electrochemical performance of the zinc-air battery.

[0038] Example 4

[0039] This example provides a cobalt / nitrogen co-doped multi-joint nanochain carbon fiber oxygen electrode, and the preparation method is the same as that of Example 1, except that the mass ratio of PAN:ZIF-67 in step (2) is 1:2. A multi-joint nanochain carbon fiber oxygen electrode is prepared, which is denoted as Co / Co-N X -C@CNF-2, and the SEM image thereof is shown in Figure 1 c, and the ZIF-67 cluster spacing is 450 nm.

[0040] Example 5

[0041] This example provides a cobalt / nitrogen co-doped multi-joint nanochain carbon fiber oxygen electrode, and the preparation method is the same as that of Example 1, except that the mass ratio of PAN:ZIF-67 in step (2) is 1:2. A multi-joint nanochain carbon fiber oxygen electrode is prepared, which is denoted as Co / Co-N X -C@CNF-2.5, and the SEM image thereof is shown in Figure 1 d, and the ZIF-67 clusters are severely agglomerated.

[0042] Example 6

[0043] 5 mg of the Co / Co-N X -C@CNF prepared in Examples 1-5 and commercial Pt / C powder were dissolved in a solution composed of isopropanol and 5% Nafion (1:0.8% by volume), and ultrasonicated for 30 min to form a uniform slurry, which was then drop-casted on the glassy carbon surface of a rotating disc electrode (loading amount of 0.2 mg / cm 2), and after natural drying, it was used as a working electrode, together with a platinum wire counter electrode and a saturated calomel reference electrode to form a three-electrode system, and a potassium hydroxide solution was used as an electrolyte, and oxygen was introduced into the solution for 30 min, and a CHI760 series electrochemical workstation of Chenhua Company was used for polarization curve testing.

[0044] The test results are shown in Table 1. Figure 4 As shown in Table 1, under the condition of adding different contents of ZIF-67, the half-wave potentials of ORR are 0.808 V, 0.822 V, 0.845 V, 0.833 V, and 0.838 V, respectively, and the limiting current densities are 4.77 mA / cm 2 , 4.85 mA / cm 2 , 5.23 mA / cm 2 , 5.05 mA / cm 2 , and 4.99 mA / cm 2 , respectively. It can be seen that the Co / Co-N X -C@CNF-1.5 catalyst in Example 3 is superior to the other four, and is comparable to the performance of the commercial Pt / C catalyst (half-wave potential: 0.847 V, limiting current density: 5.29 mA / cm 2 ), indicating that the ORR performance of the catalyst material corresponding to the PAN:ZIF-67=1:1.5 mass ratio is optimal. Figure 5 Figure 2 is an OER performance comparison diagram of Example 3 and commercial Ir / C. X As can be seen from the figure, the Co / Co-N 2 -C@CNF-1.5 has a potential of 1.66 V at a current density of 10 mA / cm X , which is very close to the commercial Ir / C, so the Co / Co-N X -C@CNF-1.5 has a dual-function characteristic.

[0045] Further, the Co / Co-N X -C@CNF-1.5 prepared in Example 3 was directly used as a cathode, a zinc plate was used as an anode, and 6M KOH was used as an electrolyte to assemble a zinc-air battery, and an electrochemical workstation and a battery test system were used to perform power density testing and charge-discharge testing, respectively.

[0046] Figure 6 Figure 3 is a power density diagram of the Co / Co-N 2 -C@CNF-1.5 prepared in Example 3 as an air electrode. Figure 7 As can be seen from Figure 3, the maximum power density reaches 156.71 mW / cm XThe cycle performance diagram of C@CNF-1.5 as an air electrode for long-time charge and discharge is shown in the figure. 2 The stable cycle can reach more than 60 h, and the charge and discharge voltage difference is only 0.9 V, and the voltage difference has no obvious change after more than 300 cycles, indicating that the Co / Co-N X The zinc-air battery assembled by the self-supporting electrode of C@CNF has excellent cycle stability and can be applied to small current equipment such as wearable devices.

[0047] The application uses an electrostatic spinning nanofiber membrane as a carrier, directly embeds ZIF-67 into a nanofiber matrix, and then obtains a cobalt / nitrogen-doped carbon fiber oxygen electrode through pre-oxidation and carbonization. Among them, the Co / N active sites derived from ZIF-67 are connected through carbon fibers to form a multi-section nanochain structure, and by adding different proportions of ZIF-67 powder, the spacing of ZIF-67 clusters on a single fiber is accurately controlled, the overlapping and aggregation of active sites are effectively avoided, and a continuous and efficient conductive network structure is formed. This unique structure can improve the utilization rate of active sites, optimize the mass transfer path, and thus significantly enhance the overall electrocatalytic performance. The metal-air battery assembled by using it has high power density and long-term stability. The application provides an effective electrospinning strategy for developing high-efficiency dual-function electrocatalysts and their application in metal-air batteries, and has good application prospect.

Claims

1. A method for preparing a cobalt / nitrogen co-doped multi-segmented carbon nanochain oxygen electrode, comprising the following steps: (1) Dissolve the organic ligand and cobalt salt in an organic solvent, stir evenly and let stand, then centrifuge, wash and dry to obtain ZIF-67 powder; (2) Dissolve the polymer and the ZIF-67 powder obtained in step (1) in a solvent, stir evenly, and obtain an electrospinning solution; (3) Electrospin the electrospinning solution in step (2) to obtain a fiber membrane; (4) The fiber membrane in step (3) is pre-oxidized at low temperature in air atmosphere, and then transferred to nitrogen atmosphere for high temperature calcination to obtain a multi-segment carbon fiber oxygen electrode with cobalt / nitrogen co-doped nanochains.

2. The preparation method according to claim 1, characterized in that, In step (1), the organic ligand includes any one of 2-methylimidazole, terephthalic acid, and trimesic acid; the cobalt salt includes any one of cobalt nitrate, cobalt chloride, cobalt sulfate, and cobalt acetate; and the organic solvent includes any one of ethanol and methanol.

3. The preparation method according to claim 1, characterized in that, In step (1), the mass ratio of organic ligand to cobalt salt is 4~40:1; the particle size of ZIF-67 powder is 250~300 nm.

4. The preparation method according to claim 1, characterized in that, The polymer in step (2) includes either polyacrylonitrile or polyvinylpyrrolidone; the solvent includes either N,N dimethylformamide or dimethylacetamide.

5. The preparation method according to claim 1, characterized in that, By adjusting the mass ratio of polymer and ZIF-67 in the electrospinning solution in step (2) to 1:0.5~2.5, the spacing of ZIF-67 clusters on a single fiber of the multi-segmented nanochain carbon fiber oxygen electrode in step (4) is controlled to be 400 nm~2 μm.

6. The preparation method according to claim 1, characterized in that, In step (3) of the spinning process, the spinning voltage is 12~20 kV, the feeding speed is 0.6~1.5 mL / h, and the distance from the spinning needle to the receiving plate is 12~20 cm.

7. The preparation method according to claim 1, characterized in that, In step (4), the pre-oxidation temperature is 250~300 ℃ and the time is 60~180 min; the calcination temperature is 700~1000 ℃ and the time is 90~180 min.

8. A cobalt / nitrogen co-doped multi-segmented carbon nanochain oxygen electrode prepared by the preparation method according to claim 1.

9. The application of a cobalt / nitrogen co-doped multi-segmented carbon nanochain oxygen electrode as described in claim 8 in a metal-air battery.

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