Znmno3 catalyst of nitrogen-doped carbon nanotube and preparation method thereof

Abstract

The application discloses a preparation method of a ZnMnO3 catalyst doped with nitrogen-doped carbon nanotubes, which combines ZnMnO3 with the nitrogen-doped carbon nanotubes to change active sites and improve catalytic performance of the catalyst; in particular, the ZnMnO3 catalyst doped with the nitrogen-doped carbon nanotubes prepared by using a hydrothermal method has high catalytic activity, a high reduction peak potential and good stability, and the comprehensive performance of the catalyst is comparable to that of commercial Pt-C; and the catalyst is prepared into an aluminum-air battery, which is stable in constant-current discharge and has a high discharge voltage.

Description

Technical Field

[0001] This invention belongs to the field of oxygen reduction catalyst technology, specifically relating to nitrogen-doped carbon nanotube ZnMnO3 catalysts and their preparation methods. Background Technology

[0002] Aluminum-air batteries, as a green energy source, possess advantages such as high specific energy and zero pollution, making them a key focus of new energy research. However, they are constrained by various factors, such as catalyst activity and corrosion of the aluminum alloy anode plate. Therefore, developing a highly active and potent air electrode catalyst is essential. Current research indicates that precious metals are excellent air electrode catalysts, but their high cost limits their application. The industry tends to favor the use of inexpensive metal oxides and transition metal oxides as catalysts. Among these, Zn oxides and Mn oxides are highly favored due to their numerous advantages, including low cost, availability, non-toxicity, and low operating voltage. However, when used as catalysts, they also exhibit inherent limitations in practical applications, such as poor conductivity and low activity.

[0003] To realize the commercial application of aluminum-air batteries, researchers have been striving to find non-precious metal ORR catalysts. Transition metals (TMs) such as iron, cobalt, and nickel have been widely used as catalysts for ORR under alkaline conditions. Carbon-based materials have advantages such as good conductivity and low cost, but pure carbon-based materials lack catalytic activity. Therefore, constructing carbon-based materials to encapsulate transition metal composite oxide nanoparticles is a promising approach. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a nitrogen-doped carbon nanotube ZnMnO3 catalyst and its preparation method. By combining ZnMnO3 with N-doped carbon nanotubes, its active sites are altered, thereby improving its catalytic performance. This method is of great significance for preparing ZnMnO3@NC composite materials that can be used in aluminum-air batteries.

[0005] To achieve the above-mentioned technical objectives, the present invention is implemented through the following technical solution:

[0006] The preparation method of nitrogen-doped carbon nanotube ZnMnO3 catalyst includes the following steps:

[0007] S1: Dissolve MnC2O4 and ZnC2O4·2H2O in distilled water at a molar ratio of 1:1, and add MnC2O4·2H2O to the dissolved mixture. 2+ DCDA (dicyandiamide) and carbon nanotubes in a molar ratio of 1:1 were rapidly added to a 0.05–0.25 mol / L NaOH solution under magnetic stirring, and H2O2 was slowly added dropwise until no obvious reaction was observed. The mixed solution was then poured into the polytetrafluoroethylene liner of a hydrothermal reactor and kept at 130°C for 24 hours.

[0008] S2: After the reactor is cooled to 23℃~26℃, the suspension is allowed to stand and filtered. The precipitate is washed several times with anhydrous ethanol and deionized water. The precipitate is dried overnight in an 80℃ constant temperature drying oven to obtain the precursor. The precursor is calcined in a tube furnace under the protection of nitrogen atmosphere.

[0009] S3: After the tube furnace has cooled naturally to 23℃~26℃, the resulting black powder is centrifuged with deionized water until neutral, and then dried in an oven at 50℃ to obtain the ZnMnO3@CN-s catalyst.

[0010] Preferably, the tubular furnace in S2 operates at 5°C / min. -1 The temperature was increased to 500℃ at the same heating rate and held at this temperature for 1 hour. Then the temperature was increased to 900℃ at the same heating rate and held at this temperature for 4 hours.

[0011] The beneficial effects of this invention are:

[0012] The preparation method and the nitrogen-doped carbon nanotube ZnMnO3 catalyst provided by this invention improve catalytic performance by combining ZnMnO3 with N-doped carbon nanotubes to change their active sites. In particular, the nitrogen-doped carbon nanotube ZnMnO3 catalyst prepared by the hydrothermal method of this invention has high catalytic activity, a high reduction peak position, good stability, and comprehensive performance comparable to commercial Pt-C. When used to prepare an aluminum-air battery, it exhibits stable constant current discharge and high discharge voltage. Attached Figure Description

[0013] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0014] Figure 1 These are XRD patterns of ZnMnO3@CN catalysts prepared by different methods in the embodiments of the present invention.

[0015] Figures 2(a)(b)(c) are SEM images of ZnMnO3@CN catalysts prepared by hydrothermal method, sol-gel method and coprecipitation method in the embodiments of the present invention; Figures 2(d)(e)(f) are TEM images of ZnMnO3@CN catalysts prepared by hydrothermal method, sol-gel method and coprecipitation method; and (g) is a partial TEM image of ZnMnO3@CN catalysts prepared by sol-gel method.

[0016] Figure 3 These are Raman spectra of ZnMnO3@CN catalysts prepared by different methods in the embodiments of the present invention;

[0017] Figure 4 (a) is the N2 adsorption / desorption isotherm of ZnMnO3@CN catalysts prepared by different methods in the embodiments of the present invention; (b) pore size distribution diagram of ZnMnO3@CN catalysts prepared by different methods;

[0018] Figures 5(a)(b)(c) are XPS full spectrum of ZnMnO3@CN catalysts prepared by hydrothermal method, sol-gel method and coprecipitation method in the embodiments of the present invention; (d)(e)(f)(g) XPS spectra of C1s, O1s, N1s and Mn2p of ZnMnO3@CN catalysts prepared by hydrothermal method, sol-gel method and coprecipitation method;

[0019] Figure 6 These are cyclic voltammograms of ZnMnO3@CN catalysts prepared by different methods in the embodiments of the present invention;

[0020] Figure 7 These are polarization curves of ZnMnO3@CN catalysts and Pt-C prepared by different methods in the embodiments of the present invention;

[0021] Figure 8 (a)(b)(c) are polarization curves of ZnMnO3@CN catalysts prepared by hydrothermal method, sol-gel method and co-precipitation method in the embodiments of the present invention at different rotation speeds; (d) is a KL curve of ZnMnO3@CN catalysts prepared by different methods;

[0022] Figure 9 (a)(b)(c) are the polarization curves of ZnMnO3@CN catalysts prepared by hydrothermal method, sol-gel method and coprecipitation method in the embodiments of the present invention before and after 200 cycles;

[0023] Figure 10 (a)(b)(c) are constant current discharge diagrams of ZnMnO3@CN catalysts prepared by hydrothermal method, sol-gel method and coprecipitation method in the embodiments of the present invention. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Example 1

[0026] S1: Dissolve 3.88g of MnC2O4 and 4.5g of ZnC2O4·2H2O in 100mL of distilled water. Add 2g of DCDA (dicyandiamide) and 0.2g of carbon nanotubes to the dissolved mixture. Under magnetic stirring, quickly add 0.1mol / L NaOH solution and slowly add 2mL of H2O2 dropwise. Then pour the mixed solution into the polytetrafluoroethylene liner of the hydrothermal reactor and keep it at 130℃ for 24 hours.

[0027] S2: After the reactor is cooled to 25°C, the suspension is allowed to stand and filtered. The precipitate is washed several times with anhydrous ethanol and deionized water. The precipitate is dried overnight in an 80°C constant temperature drying oven to obtain the precursor. The precursor is calcined in a tube furnace under the protection of nitrogen atmosphere.

[0028] S3: After the tube furnace has cooled naturally to room temperature, the resulting black powder is centrifuged with deionized water until neutral and dried in an oven at 50°C to obtain the ZnMnO3@CN-s catalyst.

[0029] Preferably, the tubular furnace in S2 operates at 5°C / min. -1 The temperature was increased to 500℃ at the same heating rate and held at this temperature for 1 hour. Then the temperature was increased to 900℃ at the same heating rate and held at this temperature for 4 hours.

[0030] Example 2

[0031] Phase characterization of ZnMnO3@CN catalysts prepared by different methods

[0032] Figure (1) shows the XRD patterns of ZnMnO3@CN prepared by different methods;

[0033] Figure (1) shows the XRD patterns of different catalysts. It can be clearly seen that the ZnMnO3@CN-r, ZnMnO3@CN-g, and ZnMnO3@CN-s catalysts have a diffraction peak near 26°, which corresponds to the (002) crystal plane of graphite carbon. The diffraction peaks at 18.1°, 30.4°, 35.2°, 37.2°, 43.4°, 53.5°, 57.6°, and 63.3° correspond to the (111), (220), (311), and (220) crystal planes of cubic pure phase ZnMnO3. 22), (400), (422), (511) and (440) crystal planes (PDF#19-1461); it can be seen that the XRD patterns of ZnMnO3@CN prepared by different methods are roughly the same, and no impurity peaks appear, indicating that the crystallinity is good; among them, the peak of ZnMnO3@CN-r is relatively sharp and narrow compared with the other two, indicating that the sample prepared by this method has smaller crystal grains and smaller sample particles, which can better adhere to the outer wall of carbon nanotubes or be coated by carbon nanotubes and enter their inner wall, generating more active sites.

[0034] Morphology characterization of ZnMnO3@CN catalysts prepared by different methods

[0035] Figure (2) shows the SEM and TEM images of ZnMnO3@CN prepared by different methods;

[0036] As can be clearly seen from the SEM and TEM images in Figure (2), the carbon nanotube structure of the ZnMnO3@CN catalysts prepared by different methods is intact, and there is no obvious agglomeration of nanoparticles. In the ZnMnO3@CN catalyst prepared by the sol-gel method, some core-shell structures appeared. This may be because the carbon generated by the decomposition of added citric acid at high temperature fused with the original added carbon nanotubes to form an outer carbon shell, which encapsulates ZnMnO3 inside the shell, forming the core-shell structure shown in the figure. The TEM images show that the composite oxide nanoparticles are of different sizes but all successfully attached to the carbon nanotubes. Most of the composite oxide nanoparticles are attached to the outer wall of the carbon nanotubes, and some are wrapped into the inner wall by the carbon nanotubes. As can be seen from Figures d, e, and f, some nano-metal particles are attached to the outer walls of multiple carbon nanotubes.

[0037] Structural characterization of ZnMnO3@CN catalysts prepared by different methods

[0038] To further understand the microstructure of the carbon material in the catalyst, Raman spectroscopy was performed. Figure (3) shows the typical Raman spectra of ZnMnO3@CN catalysts prepared by different methods, with two peaks at 1350 cm⁻¹. -1 and 1580cm -1 The D and G bands, respectively, belong to nitrogen-doped carbon nanotubes. The D band is related to disordered structure and defects, while the G band indicates the degree of graphitization. The area ratio of the D to G bands (SD / SG) is used to qualitatively evaluate the degree of graphitization of carbon materials. The calculated SD / SG of ZnMnO3@CN catalyst prepared by sol-gel method is 1.35, that prepared by co-precipitation method is 1.38, and that prepared by hydrothermal method is 1.26. This indicates that the presence of more carbon defects in the prepared catalysts may be beneficial in providing a larger specific surface area and exposing more active sites.

[0039] Figure (4) shows the N2 adsorption / desorption isotherms and pore size distribution of ZnMnO3@CN catalysts prepared by different methods;

[0040] As shown in Figure (4)a, when N2 adsorption-desorption experiments were conducted at 77K, the isotherm shapes of all samples were basically the same, and they could be classified as Type IV isotherms according to the IUPAC classification. 0The presence of a hysteresis loop within the range of 0.4–1 indicates that the prepared catalyst has a mesoporous structure; the specific surface area of ​​the ZnMnO3@CN catalyst prepared by the sol-gel method is 87.15 m². 2 ·g -1 The specific surface area of ​​the ZnMnO3@CN catalyst prepared by the hydrothermal method was found to be 93.42 m². 2 ·g -1 The ZnMnO3@CN catalyst prepared by the co-precipitation method had a specific surface area of ​​83.59 m². 2 ·g -1 The pore volumes are 0.312 cm³. 3 ·g -1 0.309cm 3 ·g -1 and 0.303cm 3 ·g -1 As can be seen from pore size distribution diagram b, the pore size distribution of the prepared catalyst is mainly concentrated around 1.5 nm, 3.5 nm, 5 nm and 10.4 nm. In summary, ZnMnO3@CN prepared by hydrothermal method has a larger specific surface area, which can provide a wider site for catalytic reaction; the larger pore volume is beneficial to the transport and diffusion of electrolyte and ions.

[0041] The elemental composition was determined by XPS analysis, as shown in Figure (5). Full spectra a, b, and c clearly show that only C, N, O, Zn, and Mn elements are present in the ZnMnO3@CN catalysts prepared by different methods. Table 1 summarizes the specific content of each element in the catalyst. The C1s spectrum was divided into three peaks using Avantage software (Figure d): CC (284.6 eV), CN (285.4 eV), and CO (286.5 eV), and their specific contents are summarized in Table 2. The peaks at 532.4 eV and 533.3 eV in the N1s spectrum indicate the presence of C=O and CO bonds (Figure e); the high-resolution N1s spectrum (Figure f) shows four nitrogen atoms at 398.2 eV, 400.1 eV, 401.5 eV, and 403.2 eV, corresponding to pyridine nitrogen, pyrrole nitrogen, graphitic nitrogen, and oxidized nitrogen, respectively. The specific contents of these four nitrogen atoms are summarized in Table 3; the peaks at 642.8 eV and 653.2 eV in the Mn2p spectrum (Figure g) indicate the presence of Mn2p bonds. 3+ and Mn 4+ The existence of.

[0042] Table 1. Elemental composition information of the catalyst

[0043]

[0044] Table 2 Percentage of C1s in the catalyst

[0045]

[0046] Table 3. Specific content of four types of nitrogen in the total elemental composition of the catalyst.

[0047]

[0048] Previous studies have shown that the main active center for oxygen adsorption is the carbon atom next to pyridine nitrogen, which is the initial step of the oxygen reduction reaction. Since pyrrole nitrogen is located at the edge of the carbon layer, it is usually accompanied by many structural defects, which also provides a large number of active sites. Meanwhile, graphitic nitrogen is thought to promote the adsorption of oxygen atoms by extracting electrons from adjacent carbon atoms, which plays a key role in the electrocatalytic activity of ORR. By comparison, it was found that the ZnMnO3@NC catalyst prepared by hydrothermal method has a large amount of pyridine nitrogen and pyrrole nitrogen, which may improve its ORR performance.

[0049] Example 3

[0050] Catalytic performance study of ZnMnO3@CN catalysts prepared by different methods

[0051] The concentration was determined by CV in an O2-saturated 0.1M KOH solution at 50 mV·s. -1 The ORR activity of the catalyst was evaluated by scanning rate; cyclic voltammetry diagrams of ZnMnO3@CN catalysts prepared by hydrothermal method, sol-gel method and coprecipitation method were obtained, as shown in Figure (6).

[0052] As shown in Figure (6), all catalysts exhibited obvious oxygen reduction peaks under O2 saturation, indicating their potential high activity for ORR; and the ZnMnO3@CN catalyst prepared by hydrothermal method showed a high current peak at a relatively positive 0.62V, indicating its excellent ORR activity.

[0053] After conducting CV studies, RDE experiments were performed to elucidate the ORR performance of all catalysts; polarization curves of ZnMnO3@CN catalysts prepared by hydrothermal method, sol-gel method, coprecipitation method and Pt-C were obtained, as shown in Figure (7).

[0054] As shown in Figure (7), based on the initial potential (E) onset ) and half-wave potential (E 1 / 2 Compared with other methods, the ZnMnO3@CN catalyst prepared by the hydrothermal method exhibits higher catalytic activity, indicating that the ZnMnO3@CN catalyst prepared by the hydrothermal method demonstrates superior ORR catalytic activity. Furthermore, compared with commercial Pt-C catalysts, the onset potential (E0) of the ZnMnO3@CN catalyst prepared by the hydrothermal method is significantly higher. onset ) and half-wave potential (E) 1 / 2The ORR performance of the ZnMnO3@CN catalyst prepared by the hydrothermal method is higher than that of Pt-C, but its limiting current density is relatively low. The excellent ORR performance of the ZnMnO3@CN catalyst may be due to the unique structure of the nitrogen-doped carbon nanotubes that encapsulate Mn nanoparticles, which facilitates the directional diffusion of electrons from the nitrogen-doped carbon nanotubes to the metal center.

[0055] The Tafel slope was obtained from the ORR polarization curve to evaluate the kinetic properties of the catalyst; the Tafel slope of the ZnMnO3@CN catalyst prepared by the hydrothermal method was 54 mV / dec. -1 The Tafel slope of the ZnMnO3@CN catalyst prepared by the sol-gel method is 59 mV.dec -1 The Tafel slope of the ZnMnO3@CN catalyst prepared by coprecipitation was 63 mV.dec -1 The ZnMnO3@CN catalyst prepared by the hydrothermal method has a lower Tafel slope, indicating that it has a superior electron transfer capability. The excellent ORR performance of the ZnMnO3@CN catalyst prepared by the hydrothermal method is comparable to that of commercial Pt-C catalysts. By comparing the limiting current density of different catalysts at 0.4V, it can be found that the limiting current density of the ZnMnO3@CN catalyst prepared by the hydrothermal method at 0.4V (vs RHE) is significantly higher than that of the ZnMnO3@CN catalyst prepared by the sol-gel method and the coprecipitation method, but relatively lower than that of the commercial Pt / C catalyst.

[0056] Figure (8) shows the ORR polarization curves of ZnMnO3@CN catalysts prepared by different methods in 0.1 M KOH solution at rotation speeds of 400 rpm, 600 rpm, 800 rpm, 1000 rpm, 1200 rpm, 1400 rpm, 1600 rpm, 1800 rpm, and 2000 rpm. It can be seen that the diffusion limiting current density increases with increasing rotation speed because the higher the rotation speed, the thinner the oxygen diffusion layer between the bulk electrolyte and the electrode surface, i.e., the smaller the mass transfer resistance. The KL plot further obtained the ORR kinetic information of ZnMnO3@CN catalysts prepared by different methods at the selected potentials. -1 and ω -1 / 2 The good linear relationship between the two values ​​indicates that the ORR catalysis of these catalysts is a first-order reaction kinetic relative to the dissolved oxygen concentration. Furthermore, the average electron transfer number n of each catalyst at different potentials (0.3–0.6 V vs RHE) was calculated using polarization curves obtained from different rotation speeds. The results show that the average n values ​​for the ZnMnO3@CN catalysts prepared by the hydrothermal method, sol-gel method, co-precipitation method, and Pt-C method are 3.91, 3.78, 3.71, and 3.97, respectively, further confirming that the ZnMnO3@CN catalyst prepared by the hydrothermal method is close to 4e. -The oxygen reduction pathway.

[0057] Besides catalytic activity, stability is another important indicator in catalyst applications. After 10 mV·s... -1 After 200 consecutive CV cycles, the polarization curve shown in Figure (9) was obtained.

[0058] After 10mV·s -1 After 200 consecutive CV cycles, the diffusion current density of ZnMnO3@CN catalysts prepared by different methods showed only a small decrease, and the half-wave potential decreased only slightly, close to 2mV; in this respect, ZnMnO3@CN catalysts prepared by different methods all showed excellent stability.

[0059] Example 4

[0060] Aluminum-air battery performance study

[0061] ZnMnO3@CN catalyst and ZnMnO3 catalyst prepared by different methods were used to prepare air electrodes. The discharge curves were tested using a battery testing system. Pure aluminum was used as the anode material in the battery, and the experimentally prepared air electrode was used as the cathode. The electrolyte was 6 mol·L⁻¹. -1 The NaOH solution was used and circulated using a peristaltic pump at an oxygen flow rate of 60 mL / min. -1 The entire electrochemical test was conducted in an electrolytic cell at room temperature; the potential curve obtained from the constant current test is shown in Figure (10).

[0062] It can be seen that the discharge voltage of the battery decreases with increasing current density. During constant current discharge, when the load current is too large and the current cannot meet the load requirements, the voltage will be reduced to compensate for the insufficient current, thus the voltage will drop. Therefore, the discharge voltage decreases with increasing current density. The discharge voltage of ZnMnO3@CN catalysts prepared by various methods is significantly improved. In particular, the discharge voltage of ZnMnO3@CN catalysts prepared by hydrothermal method decreases the least with increasing current density. The discharge voltage is relatively stable at high current densities, which is consistent with the results obtained by CV and LSV. Small fluctuations also appear at high current densities. Obviously, due to the different methods used, the particle size of ZnMnO3 prepared is different. As shown by SEM and TEM, some ZnMnO3 is attached to the outer wall of carbon nanotubes and some is attached to the inner wall of carbon nanotubes, which causes small fluctuations at high current densities.

[0063] The average discharge voltages of samples prepared by different methods at different current densities are statistically summarized in Table 4.

[0064] The energy released on the air electrode during discharge was calculated using formula (1). The total energy released by batteries prepared using ZnMnO3@CN catalysts prepared by the hydrothermal method, sol-gel method, and co-precipitation method was calculated to be 55.59 mWh·cm⁻¹. -2 48.07 mWh·cm -2 52.09 mWh·cm -2 It can be seen that the ZnMnO3@CN catalyst prepared by the hydrothermal method releases the highest energy.

[0065]

[0066] Where the discharge current density J n The unit is mA·cm -2 Discharge voltage V n The unit is V, and the discharge time is T. n The unit is h;

[0067] To more intuitively illustrate the influence of different preparation methods on the performance of ZnMnO3, the best-performing samples prepared above were compared, resulting in Table 5.

[0068] Table 5 clearly shows that the samples prepared by the hydrothermal method have higher reduction potential, limiting current density, lower Tafel slope, and the highest electron transfer number.

[0069] Table 4. Average discharge voltage (unit: V) of samples prepared by different methods at different current densities.

[0070]

[0071] Table 5 Comparison of sample performance prepared by different methods

[0072]

Claims

1. A method for preparing a nitrogen-doped carbon nanotube ZnMnO3 catalyst, characterized in that, Includes the following steps: S1: Dissolve MnC2O4 and ZnC2O4·2H2O in distilled water at a molar ratio of 1:1, and add MnC2O4·2H2O to the dissolved mixture. 2+ DCDA (dicyandiamide) and carbon nanotubes in a molar ratio of 1:1 were rapidly added to a 0.05–0.25 mol / L NaOH solution under magnetic stirring, and H2O2 was slowly added dropwise until no obvious reaction was observed. The mixed solution was then poured into the polytetrafluoroethylene liner of a hydrothermal reactor and kept at 130°C for 24 hours. S2: After the reactor is cooled to 23℃~26℃, the suspension is allowed to stand and filtered. The precipitate is washed several times with anhydrous ethanol and deionized water. The precipitate is dried overnight in an 80℃ constant temperature drying oven to obtain the precursor. The precursor is calcined in a tube furnace under the protection of nitrogen atmosphere. S3: After the tube furnace has cooled naturally to 23℃~26℃, the resulting black powder is centrifuged with deionized water until neutral, and then dried in an oven at 50℃ to obtain the ZnMnO3@CN-s catalyst.

2. The method for preparing the nitrogen-doped carbon nanotube ZnMnO3 catalyst according to claim 1, characterized in that, The S2 tubular furnace operates at 5°C / min. -1 The temperature was increased to 500℃ at the same heating rate and held at this temperature for 1 hour. Then the temperature was increased to 900℃ at the same heating rate and held at this temperature for 4 hours.

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