Synthesis method of a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst

Abstract

The present invention provides a method for synthesizing a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst, which specifically adopts the following steps: preparing a suspension solution with a zinc salt, an N-containing organic linker, a surfactant and deionized water and stirring for a certain time; performing solid-liquid separation and washing on the suspension solution, placing the obtained reaction product in an organic solvent, and keeping warm for a certain time; performing solid-liquid separation on the organic solution, washing, drying, and calcining at a high temperature to obtain a product; configuring a solution with the product of the previous step, an iron source and an organic solvent and stirring for a certain time, and then performing solid-liquid separation and calcining at a high temperature to obtain a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst with an open structure. To overcome the disadvantages of traditional electrocatalytic materials such as irregular morphology, low active sites, low electron transfer rate, and uneven distribution of active centers. The present invention belongs to the technical field of sustainable energy.

Description

Technical Field

[0001] The present invention relates to a synthesis method of a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst with an open structure, belonging to the technical field of sustainable energy. Background Art

[0002] Fuel cells and metal-air batteries have attracted research interest due to their high specific energy, low cost, and low pollution. However, limited by the slow oxygen reduction reaction at the cathode of the battery, the energy conversion efficiency cannot be further improved. Currently, commercial Pt / C catalysts are hindered in their application in the above-mentioned devices due to their high price, poor stability, and methanol tolerance. Therefore, the development of non-precious metal-based materials with low cost, high catalytic activity, and good stability has become the current research goal. Among them, transition metal-based catalysts (M-N-C, M = Fe, Co, etc.) supported on nitrogen-doped carbon substrates exhibit high ORR catalytic activity and stability, becoming ideal catalysts to replace commercial Pt / C.

[0003] Zeolitic imidazolate framework materials (ZIFs) are metal-organic framework compounds assembled from metal cations (such as Zn 2+ , Co 2+ , etc.) and N-containing organic linkers (such as 2-methylimidazole), and are widely used as precursor templates and metal sources in energy storage and conversion materials. The nitrogen-doped carbon substrate formed after pyrolysis has a large specific surface area, which can accommodate a large number of ORR active sites. However, traditional ZIF-derived catalysts mostly exhibit the shape of rhombic dodecahedra and mainly have a microporous structure, which leads to slow reaction kinetics and requires further improvement of their ORR catalytic activity. Research shows that one-dimensional carbon substrates have an electron channel that conducts along the axis, which can quickly transport electrolytes to the exposed active sites, thereby improving the reaction kinetics. In addition, designing and preparing a carbon substrate with an open structure is also very important for the exposure of active sites. This patent proposes a simple method for preparing a rod-shaped ZIF-8-derived Fe-N-C oxygen reduction catalyst with a typical open structure. This catalyst is loaded with a large number of uniformly distributed Fe-Nx active sites, has a stable carbon structure, and good electrical conductivity. Compared with traditional rhombic dodecahedron ZIFs, the carbon nanorods formed after pyrolysis of one-dimensional rod-shaped ZIF-8 can significantly improve the electron transport efficiency. The results show that rod-shaped Fe-N-C can be used as an efficient oxygen reduction reaction catalyst, showing good ORR catalytic activity, cycle stability, and methanol tolerance in both alkaline and acidic solutions, and all performances exceed those of commercial Pt / C catalysts. The prepared rod-shaped Fe-N-C has broad application prospects in metal-air batteries and hydrogen-oxygen fuel cells. Its zinc-air battery as a cathode catalyst has a large power density and specific capacity. Summary of the Invention

[0004] The object of the present invention is to provide a synthesis method of a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst that is economical, environmentally friendly, simple and feasible, so as to overcome the disadvantages of traditional electrocatalytic materials such as irregular morphology, low active sites, low electron transfer rate, and uneven distribution of active centers.

[0005] To solve the above problems, the present invention provides a synthesis method of a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst, which is completed according to the following steps:

[0006] I. Under stirring conditions, 0.5-5 g of an organic complex and 2-20 mg of a surfactant are added to deionized water and stirred, and then 0.5-5 g of a zinc salt is added to the solution, and the reaction is continued to stir for 1-4 h to obtain a white suspension solution;

[0007] II. The white suspension solution prepared in step I is subjected to solid-liquid separation, and the separated white product is washed and placed in a mixed solvent of ethanol and N,N-dimethylformamide, and kept at 60-90 °C for 24-72 h;

[0008] III. The solution obtained in step II is successively subjected to solid-liquid separation and washing to obtain a white product;

[0009] IV. The white product obtained in step III is calcined at a high temperature of 900-1100 °C for 1-3 h under a protective atmosphere to obtain a black product;

[0010] V. An iron source and the black product obtained in step IV are added to an organic solvent and impregnated under stirring;

[0011] VI. The organic solution impregnated in step V is subjected to solid-liquid separation and drying to obtain a black reaction product;

[0012] VII. The black product obtained in step VI is calcined at a high temperature of 600-900 °C for 2-4 h under a protective atmosphere to obtain a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst.

[0013] In the foregoing method, the stirring speed of the stirring is 500-800 rpm. In step I, the organic complex and the surfactant are added to 80 mL of deionized water and stirred for 5-20 min; the organic complex in step I is 2-methylimidazole; the surfactant in step I is cetyltrimethylammonium bromide; the zinc salt in step I is zinc nitrate hexahydrate.

[0014] In the foregoing method, the specific operation of the solid-liquid separation in step two is as follows: The suspension solution obtained in step one is separated by high-speed centrifugation to obtain a solid precipitate; the specific operation of the washing is as follows: Wash with deionized water 1-4 times, and then wash with the organic solvent N,N-dimethylformamide 1-3 times to obtain a washed solid precipitate; the mixed solvent is a mixed solution of N,N-dimethylformamide and absolute ethanol, with a volume ratio of 1:1 - 4:1; the specific operation of the heat preservation is as follows: After the separated white product is washed and placed in the mixed solvent of ethanol and N,N-dimethylformamide, the mixed organic solution is sealed in a glass bottle, placed in a forced-air drying oven, and heat-preserved at 60-90°C for 24-72 h.

[0015] In the foregoing method, the specific operation of the solid-liquid separation in step three is as follows: The mixed organic solution obtained in step two is separated by filtration to obtain a solid precipitate; the specific operation of the washing is as follows: Wash with absolute ethanol 1-4 times to obtain a washed solid precipitate.

[0016] In the foregoing method, the protective atmosphere in step four is argon protection; the heating furnace used is a tubular furnace, and the heating rate is 5-20°C / min.

[0017] In the foregoing method, 2-20 mg of iron source and 100 mg of the black product obtained in step four are added to an organic solvent, and impregnated with stirring for 1-3 h. The iron source is iron(III) nitrate nonahydrate; the organic solvent in step five is isopropanol.

[0018] In the foregoing method, the specific operation of the solid-liquid separation in step six is as follows: The organic solution obtained in step five is separated by filtration to obtain a solid precipitate; the specific operation of the drying is as follows: Place the solid precipitate in the air and dry at room temperature for 1-3 h.

[0019] In the foregoing method, the protective atmosphere in step seven is argon protection; the heating furnace used is a tubular furnace, and the heating rate is 5-20°C / min.

[0020] The present invention innovatively prepares a method for a rod-shaped ZIF-8-derived Fe-N-C oxygen reduction catalyst, which catalyst has dense and dispersed Fe-Nx active sites and a stable carbon structure. The results show that Fe-N-C, as a non-precious metal electrocatalyst for the oxygen reduction reaction (ORR) in metal-air batteries, has broad application prospects. It exhibits considerable electrocatalytic activity, good methanol tolerance, and cyclic stability in both alkaline and acidic solutions, and these properties exceed those of the Pt / C catalyst. The open circuit potential (OCP) of a zinc-air battery (ZAB) with Fe-N-C as the cathode catalyst is as high as 1.46 V, achieving a maximum power density of up to 185 mW cm -2 and 818 mAh g-1 Ultra-high specific capacity.

[0021] Compared with the prior art, the advantages of the present invention are as follows: First, the present invention synthesizes a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst for the first time. The one-dimensional carbon nanorod structure is more conducive to electron transfer and can form more and evenly distributed active sites, overcoming the disadvantages of traditional electrocatalytic materials such as irregular morphology, low active sites, low electron transfer rate, and uneven distribution of active centers. Compared with various three-dimensional materials, the rod-shaped structure with an ideal mesoporous structure can achieve fast oxygen reduction reaction kinetics due to its efficient electron channels with a one-dimensional structure. It has excellent oxygen reduction electrocatalytic performance in both alkaline and acidic environments, and the metal-air electrode assembled with it has a high power density and specific capacity. It can be a strong candidate for oxygen reduction reaction catalysts and cathode materials of zinc-air batteries; Second, the synthesis process of the present invention is simple, with high yield, controllable morphology, low cost, and environmentally friendly, and can be used for industrial production. Description of the Drawings

[0022] Figure 1 is the SEM image of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment magnified 5,000 times; Figure 2 is the SEM image of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment magnified 10,000 times; Figure 3 is the SEM image of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment magnified 30,000 times; Figure 4 is the low-magnification TEM image of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment; Figure 5 is the high-resolution TEM (HRTEM) image of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment; Figure 6 is the selected area electron diffraction pattern (SAED) of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment; Figure 7 is the aberration-corrected scanning transmission electron microscope (AC-STEM) image of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment; Figure 8 is the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment at Figure 7 The corresponding electron energy loss spectrum (EELS); Figure 9 is the high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) image of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment; Figure 10 is the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment at Figure 9 The corresponding elemental mapping.

[0023] Figure 11 is the XRD pattern of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment; Figure 12 is the Raman spectrum of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment detected by a Raman spectrometer; Figure 13 is the nitrogen adsorption-desorption isotherm (BET) and pore size distribution curve of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment; Figure 14 is the composition and chemical state of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment studied by X-ray photoelectron spectroscopy (XPS); Figure 15 is to further study the site and structure of single-atom iron in the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment using X-ray absorption fine structure (XAFS).

[0024] Figure 16 is the linear sweep voltammetry curve (LSV) of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the comparative sample under alkaline conditions (0.1 mol / L KOH); Figure 17 is the Tafel plot of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the comparative sample under alkaline conditions (0.1 mol / L KOH); Figure 18 is the J of the catalyst at 0.85 V of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the comparative sample under alkaline conditions (0.1 mol / L KOH) K (kinetic current density) and TOF (turnover frequency) histograms; Figure 19 is the polarization curve at different rotation speeds and the K-L fitting line at different potentials of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment under alkaline conditions (0.1 mol / L KOH); Figure 20 is the number of transferred electrons and peroxide yield in the ORR process calculated based on the RRED curve of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the comparative sample under alkaline conditions (0.1 mol / L KOH); Figure 21 is the stability test (0.5 V) of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the comparative sample under alkaline conditions (0.1 mol / L KOH); Figure 22 is the methanol cross-tolerance test of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the comparative sample under alkaline conditions (0.1 mol / L KOH). Figure 25 is the J of the catalyst at 0.85 V of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the comparative sample under acidic conditions (0.1 mol / L HClO 4 )K (Kinetic current density) and TOF (turnover frequency) histograms; Figure 26 are the polarization curves of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment under acidic conditions (0.1 mol / L HClO 4 ) and the K-L fitting lines at different rotation speeds; Figure 27 are the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample under acidic conditions (0.1 mol / L HClO 4 ) calculating the number of transferred electrons and peroxide yield during the ORR process based on the RRED curve; Figure 28 are the stability tests (0.5 V) of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample under acidic conditions (0.1 mol / L HClO 4 ); Figure 29 are the methanol cross-tolerance tests carried out on the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample under acidic conditions (0.1 mol / L HClO 4 ).

[0025] Figure 30 is the open circuit potential diagram (OCP) of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample used as the air electrode of a zinc-air battery; Figure 31 is the discharge polarization curve and the corresponding power density curve of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample used as the air electrode of a zinc-air battery; Figure 32 is the constant current discharge curve of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample used as the air electrode of a zinc-air battery at different current densities; Figure 33 is the constant current discharge specific capacity curve of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample used as the air electrode of a zinc-air battery at a current density of 20 mA cm -2 . Detailed implementation mode

[0026] To make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0027] Embodiment

[0028] This embodiment provides a synthesis method of a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst, which is specifically completed according to the following steps:

[0029] 1. Add the organic complex and surfactant to deionized water and stir for 5 - 20 min at a stirring speed of 500 - 800 rpm, then add the zinc salt to the solution and continue to react for 1 - 4 h at a stirring speed of 500 - 800 rpm to obtain a white suspension (forming one-dimensional rod-shaped ZIF-L); 2. First, perform solid-liquid separation on the white suspension prepared in step 1, wash the separated white product and place it in a mixed solvent of ethanol and N,N-dimethylformamide, and react at 60 - 90 °C for 24 - 72 h (forming one-dimensional rod-shaped ZIF-8); 3. Perform solid-liquid separation and washing on the mixed organic solution in step 2 in sequence; 4. Calcinate the white product obtained in step 3 at 900 - 1100 °C for 1 - 3 h under a protective atmosphere to obtain a black reaction product (forming one-dimensional carbon nanorods after calcination); 5. Add the black product obtained in step 4 and the iron source to an organic solvent and impregnate for 1 - 3 h at a stirring speed of 500 - 800 rpm; 6. Perform solid-liquid separation and drying on the organic solution in step 5 to obtain a black reaction product; 7. Calcinate the black product obtained in step 6 at 600 - 900 °C for 2 - 4 h under a protective atmosphere to obtain a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst.

[0030] In this embodiment, a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst is synthesized for the first time. The one-dimensional carbon nanorod structure is more conducive to electron transfer, and can form more and evenly distributed active sites, overcoming the disadvantages of traditional electrocatalytic materials such as irregular morphology, low active sites, low electron transfer rate, and uneven distribution of active centers. Compared with various three-dimensional materials, the rod-shaped structure with an ideal mesoporous structure can achieve fast oxygen reduction reaction kinetics due to its one-dimensional high-efficiency electron channel. It has excellent oxygen reduction electrocatalytic performance in both alkaline and acidic environments, and the metal-air electrode assembled with it has a high power density and specific capacity. It can be a strong candidate for oxygen reduction reaction catalysts and zinc-air battery cathode materials.

[0031] The synthesis process of this embodiment is simple, with high yield, controllable morphology, low cost, and green environmental protection, and can be used for industrial production.

[0032] Specific Embodiment 2: The difference between this embodiment and Specific Embodiment 1 is that: the N-containing organic linker described in step 1 is 2-methylimidazole; the surfactant described in step 1 is cetyltrimethylammonium bromide; the zinc salt described in step 1 is zinc nitrate hexahydrate. Others are the same as Specific Embodiment 1.

[0033] Specific Embodiment 3: The difference between this embodiment and one of Specific Embodiments 1 or 2 is that: the specific operation of the solid-liquid separation described in step 2 is as follows: Use high-speed centrifugation to separate the suspension obtained in step 1 to obtain a solid precipitate. Others are the same as Specific Embodiment 1 or 2.

[0034] Specific Embodiment 4: The difference between this embodiment and any one of Embodiments 1 to 3 is as follows: The specific operation of the washing in Step 2 is as follows: Wash with deionized water 1 to 4 times, and then wash with the organic solvent N,N-dimethylformamide 1 to 3 times to obtain the washed solid precipitate. Others are the same as those in Embodiments 1 to 3.

[0035] Specific Embodiment 5: The difference between this embodiment and any one of Embodiments 1 to 4 is as follows: The mixed organic solvent described in Step 2 is a mixed solution of N,N-dimethylformamide and absolute ethanol (volume ratio 1:1 - 4:1). Others are the same as those in Embodiments 1 to 4.

[0036] Specific Embodiment 6: The difference between this embodiment and any one of Embodiments 1 to 5 is as follows: The specific operation of the heat preservation in Step 2 is as follows: Seal the mixed organic solution in a glass bottle, place it in a forced-air drying oven, and keep it at 60 - 90 °C for 24 - 72 h. Others are the same as those in Embodiments 1 to 5.

[0037] Specific Embodiment 7: The difference between this embodiment and any one of Embodiments 1 to 6 is as follows: The specific operation of the solid-liquid separation in Step 3 is as follows: Separate the mixed organic solution obtained in Step 2 by filtration to obtain the solid precipitate. Others are the same as those in Embodiments 1 to 6.

[0038] Specific Embodiment 8: The difference between this embodiment and any one of Embodiments 1 to 7 is as follows: The specific operation of the washing in Step 3 is as follows: Wash with absolute ethanol 1 to 4 times to obtain the washed solid precipitate. Others are the same as those in Embodiments 1 to 7.

[0039] Specific Embodiment 9: The difference between this embodiment and any one of Embodiments 1 to 8 is as follows: The protective atmosphere described in Step 4 is argon protection; the heating furnace used is a tubular furnace, and the heating rate is 5 - 20 °C / min. Others are the same as those in Embodiments 1 to 8.

[0040] Specific Embodiment 10: The difference between this embodiment and any one of Embodiments 1 to 9 is as follows: The iron source described in Step 5 is ferric nitrate nonahydrate; the organic solvent described in Step 5 is isopropyl alcohol. Others are the same as those in Embodiments 1 to 9.

[0041] Specific Embodiment 11: The difference between this embodiment and any one of Embodiments 1 to 10 is as follows: The specific operation of the solid-liquid separation in Step 6 is as follows: Separate the organic solution obtained in Step 5 by filtration to obtain the solid precipitate. Others are the same as those in Embodiments 1 to 10.

[0042] Embodiment Twelve: The difference between this embodiment and any one of Embodiments One to Eleven is as follows: The specific operation of drying in Step Six is as follows: Place the solid precipitate in the air and dry it at room temperature for 1 - 3 h. Others are the same as those in Embodiments One to Eleven.

[0043] Embodiment Thirteen: The difference between this embodiment and any one of Embodiments One to Twelve is as follows: The protective atmosphere in Step Seven is argon protection; the heating furnace used is a tube furnace, and the heating rate is 5 - 20 °C / min. Others are the same as those in Embodiments One to Twelve.

[0044] The following tests are used to verify the effects of the present invention:

[0045] A method for synthesizing a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst is specifically completed according to the following steps:

[0046] 1. Under a stirring speed of 500 - 800 rpm, add 1 - 3 g of 2 - methylimidazole and 15 - 25 mg of cetyltrimethylammonium bromide to 60 - 100 mL of deionized water and stir for 10 min. Then add 1 - 4 mL of an aqueous zinc nitrate hexahydrate solution (concentration 1.0 - 4.0 mol / L) to the solution and continue to stir for 1 - 4 h under a stirring speed of 500 - 800 rpm to obtain a white suspension (forming one - dimensional rod - shaped ZIF - L, which is an original step); 2. First, centrifuge the white suspension prepared in step 1 at high speed. Wash the separated product 1 - 4 times with deionized water and then 1 - 3 times with an organic solvent of N,N - dimethylformamide. Place the washed white solid precipitate in a mixed solution of N,N - dimethylformamide and absolute ethanol (volume ratio 1:1 - 4:1). Seal the mixed organic solution in a glass bottle, place it in a forced - air drying oven, and keep it at 60 - 90 °C for 24 - 72 h (forming one - dimensional rod - shaped ZIF - 8, which is an original step); 3. Use the filtration method to separate the mixed solution in step 2 to obtain a white solid precipitate. Wash it 1 - 4 times with absolute ethanol to get the washed white solid precipitate; 4. Calcine the product obtained in step 3 in a tubular furnace under an argon - protected atmosphere at a high temperature of 900 - 1100 °C for 1 - 3 h with a heating rate of 5 - 20 °C / min to obtain a black reaction product (after calcination, one - dimensional carbon nanorods are formed, which is an original step and the core point of this patent); 5. Add 50 - 200 mg of the product obtained in step 4 and 5 - 20 mg of ferric nitrate nonahydrate to 20 - 50 mL of isopropanol solvent and stir for 1 - 3 h under a stirring speed of 500 - 800 rpm (a commonly used step for preparing catalysts currently); 6. Filter and separate the mixed solution in step 5, place the obtained black solid precipitate in the air, and dry it at room temperature for 1 - 3 h to obtain a reaction product; 7. Calcine the product obtained in step 6 in a tubular furnace under an argon - protected atmosphere at a high temperature of 600 - 900 °C for 2 - 4 h with a heating rate of 5 - 20 °C / min to obtain a rod - shaped Fe - N - C oxygen reduction reaction electrocatalyst.

[0047] The rod - shaped Fe - N - C oxygen reduction reaction electrocatalyst prepared in this experiment was observed by scanning electron microscopy and transmission electron microscopy, as Figures 1-3 shown. Figure 1 is the SEM image of the rod - shaped Fe - N - C oxygen reduction reaction electrocatalyst prepared in this experiment magnified 5000 times, Figure 2 is the SEM image of the rod - shaped Fe - N - C oxygen reduction reaction electrocatalyst prepared in this experiment magnified 10000 times, Figure 3 is the SEM image of the rod - shaped Fe - N - C oxygen reduction reaction electrocatalyst prepared in this experiment magnified 30000 times. Through Figure 1 、 Figure 2 and Figure 3It can be seen that the morphology of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment consists of regular rods with a length range between 2 - 3 μm. Figure 4 is a low-magnification TEM image of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment, showing complete carbon nanorods with some semi-skeletal vacancies on their surfaces; Figure 5 is a high-resolution TEM (HRTEM) image of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment, showing partial graphitization of the carbon nanorods; Figure 6 is a selected area electron diffraction pattern (SAED) of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment, showing two different diffraction rings that are associated with the (100) and (002) planes of graphite carbon; Figure 7 is a spherical aberration corrected scanning transmission electron microscope (AC-STEM) image of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment, showing some bright spots of Fe atoms distributed in the carbon matrix, but no concentrated bright particles are observed, which also indicates that the heavy element Fe is uniformly distributed in the entire carbon nanorod in the form of single atoms rather than clusters; Figure 8 is the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment at Figure 7 The corresponding electron energy loss spectrum (EELS) shows an obvious peak for the Fe element, and the signal of Fe (≈707 eV) is detected, confirming the existence of Fe single atoms; Figure 9 is a high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) image of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment, showing a large number of pores less than 5 nm uniformly distributed in the carbon matrix; Figure 10 is the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment at Figure 9 The corresponding elemental mapping.

[0048] The rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment was detected using an X-ray diffractometer, Figure 11 is the XRD pattern of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment. It can be seen that the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment is an amorphous carbon phase. Figure 12 is the Raman spectrum of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst detected by a Raman spectrometer. It can be seen that there is a D peak at 1350 cm -1 and a G peak at 1580 cm -1 corresponding to disordered carbon and sp 2 type carbon respectively. The I of Fe-N-CD / I G (~0.93), indicating its low crystallinity and rich defects, which is highly consistent with the XRD results. The pore structure characteristics and specific surface area of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment were analyzed using nitrogen adsorption-desorption isotherms (BET) ( Figure 13 ). Fe-N-C shows a typical type I isotherm curve, indicating that there are both micropores and mesopores in Fe-N-C. The pore size distribution obtained from the isotherm ( Figure 13 inset in ) shows that the Fe-N-C catalyst has a large number of mesopores with a size distribution of 1-5 nm. The Fe-N-C carbon nanorod substrate is rich in micropores and mesopores, with a specific surface area as high as 1155.77 m 2 g -1 , which can load a large number of oxygen reduction reaction active sites. The composition and chemical state of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment were studied by X-ray photoelectron spectroscopy (XPS). The XPS full scan spectrum confirmed the presence of Fe, N, C, and O elements ( Figure 14 a). The high-resolution C1s spectrum of Fe-N-C ( Figure 14 b) has four peaks, corresponding to the characteristic peaks of C-C (284.8 eV), C-N (286.1 eV), C=O (287 eV), and C-O (287.8 eV), respectively. The N content in Fe-N-C is about 6%. The N 1s spectrum ( Figure 14 c) shows the coexistence of four nitrogen species, pyridine-N (398.6 eV), Fe-N (399.8 eV), graphitic-N (401.0 eV), and oxide-N (402.5 eV). It can be intuitively seen that the pyridine-N content of the Fe-N-C sample is the highest, about 40%, and these N sites can also stabilize highly active Fe atoms during the annealing process. The high-resolution Fe 2p ( Figure 14 d) has three peaks, and the two strongest peaks are located at 710.7 eV and 723.6 eV, belonging to the 2p 2+ and 2p 3 / 2 of Fe 1 / 2 orbitals, indicating that the valence state of Fe ions is basically +2. Although Fe-N-C was synthesized using iron salts (Fe(NO 3 )) as the iron source, the Fe 3 species peak is extremely small. During the high-temperature calcination process, in-situ carbon (carbothermal reduction) reduces Fe 3+ to Fe 3+ (Fe-Nx) and Fe 2+ (Fe / Fe 0 C). Theoretical calculations and experimental results show that Fe 3 C). Theoretical calculations and experimental results show that Fe 2+The existence is conducive to the formation of Fe-Nx, and Fe-Nx species are generally considered to be the main active sites for ORR. Pyridine-N can coordinate with Fe atoms to form Fe-Nx groups, and its formation is beneficial to improving the electrical conductivity and ORR catalytic activity of the catalyst. X-ray absorption fine structure (XAFS) was used to further study the sites and structures of single-atom iron in the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment. Figure 15 a shows Fe foil and Fe 2 O 3 The Fe K-edge X-ray absorption near-edge structure (XANES) spectra of the reference Fe-N-C. The near-edge absorption energy of Fe-N-C is between that of Fe foil and Fe 2 O 3 This indicates that the valence state of isolated single Fe atoms shows a positive charge. Through Fourier transform (FT) processing, Figure 15 The extended X-ray absorption fine structure (EXAFS) spectrum of Fe-N-C in b shows only one main peak at Although no Fe-Fe coordination is observed in the spectrum This indicates that single-dispersed iron atoms are fixed by N. The EXAFS fitting results are as shown in Figure 15 c, and the quantitative structural configuration of Fe atoms in the Fe-N-C nanostructure is obtained. The coordination number of the central Fe atom is about 4, and the average length of the Fe-N bond is 1.99, indicating that the isolated Fe atoms in Fe-C-N are coordinated by four N atoms supported by hollow CN. An atomic coordination model was established to show this Fe-N 4 structure ( Figure 15 inset in c). Wavelet transform (WT) has strong resolution in both k and R spaces and can be used to separate backscattering atoms ( Figure 15 d-15f). The contour map of Fe-N-C ( Figure 15 d) shows only one intensity maximum of about This is attributed to the contribution of Fe-N. From the above results, it can be seen that Fe atoms isolated by N atoms are dispersed and stabilized on the N-C support.

[0049] The oxygen reduction reaction catalytic performance of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment was detected under alkaline conditions (0.1 mol / L KOH) and acidic conditions (0.1 mol / L HClO 4 ) using a three-electrode system in an electrochemical workstation. 20% commercial platinum-carbon (Pt / C) and carbon nanorods without an iron source were used as comparative samples, and the detection results are shown in the figure. Figure 16This is the linear sweep voltammetry (LSV) curve of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample under alkaline conditions (0.1 mol / L KOH). It can be seen that Fe-N-C has the highest half-wave potential (E 1 / 2 = 0.91 V) and limiting current density (J L = 6.0 mA cm -2 ); Figure 17 This is the Tafel plot of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample under alkaline conditions (0.1 mol / L KOH). Fe-N-C has the smallest Tafel slope, indicating its good ORR electrocatalytic activity; Figure 18 This is the histogram of the J K (kinetic current density) and TOF (turnover frequency) of the catalyst at 0.85 V for the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample under alkaline conditions (0.1 mol / L KOH); Figure 19 This is the polarization curve at different rotation speeds and the K-L fitting line at different potentials of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment under alkaline conditions (0.1 mol / L KOH). It can be seen that Fe-N-C basically follows the four-electron transfer route; Figure 20 This is the calculation of the number of transferred electrons and peroxide yield during the ORR process based on the RRED curve for the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample under alkaline conditions (0.1 mol / L KOH). The number of transferred electrons of Fe-N-C is above 3.95, and the H 2 O 2 yield is less than 1%; Figure 21 This is the stability test (0.5 V) of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample under alkaline conditions (0.1 mol / L KOH). The stability of Fe-N-C is stronger than that of Pt / C; Figure 22 This is the methanol cross-tolerance test of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample under alkaline conditions (0.1 mol / L KOH). Fe-N-C has better methanol tolerance. Figure 23 This is the linear sweep voltammetry (LSV) curve of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample under acidic conditions (0.1 mol / L HClO 4 ). It can be seen that Fe-N-C has the highest half-wave potential (E 1 / 2 = 0.80 V) and limiting current density (J L = 6.7 mA cm -2 ); Figure 24is the Tafel plot of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample under acidic conditions (0.1 mol / L HClO 4 ). Fe-N-C has the smallest Tafel slope, indicating its good ORR electrocatalytic activity; Figure 25 is the histogram of the catalyst's J 4 (kinetic current density) and TOF (turnover frequency) at 0.85 V of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample under acidic conditions (0.1 mol / L HClO K ); Figure 26 are the polarization curves at different rotation speeds and the K-L fitting lines at different potentials of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment under acidic conditions (0.1 mol / L HClO 4 ). It can be seen that Fe-N-C basically follows the four-electron transfer route; Figure 27 is the calculation of the number of transferred electrons and the peroxide yield during the ORR process based on the RRED curve of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample under acidic conditions (0.1 mol / L HClO 4 ). The number of transferred electrons of Fe-N-C is above 3.8, and the H 2 O 2 yield is less than 5%; Figure 28 is the stability test (0.5 V) of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample under acidic conditions (0.1 mol / L HClO 4 ). The stability of Fe-N-C is stronger than that of Pt / C; Figure 29 is the methanol cross-tolerance test of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample under acidic conditions (0.1 mol / L HClO 4 ). Fe-N-C has better methanol tolerance;

[0050] The rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment was assembled as an air cathode catalyst in a zinc-air battery and detected using an electrochemical workstation and a blue electrochemical battery test system, with a Zn sheet used as the anode. As a comparison, a zinc-air battery with Pt / C as the battery catalyst was also tested under the same conditions. The test results are as Figures 30-33 shown. Figure 30 is the open circuit potential diagram (OCP) of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the reference sample as the air electrode of the zinc-air battery. The open circuit potential of Fe-N-C is 1.46 V, and this value remains stable during the long-term operation of the battery, showing good stability; Figure 31The polarization curves and corresponding power density curves of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the comparative samples as the air electrode of the zinc-air battery are shown. The maximum power density of Fe-N-C is 185 mW cm -2 , significantly exceeding that of Pt / C (145 mW cm -2 ); Figure 32 The constant current discharge curves of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the comparative samples as the air electrode of the zinc-air battery at different current densities are shown. The discharge voltage of the Fe-N-C-based zinc-air battery is significantly higher than that of the Pt / C-based zinc-air battery at different current densities. When the current density decreases to 2 mA cm -2 , the discharge voltage can almost recover to the initial value, indicating that Fe-N-C has excellent and stable discharge ability; Figure 33 The constant current discharge specific capacity curves of the rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst prepared in this experiment and the comparative samples as the air electrode of the zinc-air battery at a current density of 20 mA cm -2 are shown. Fe-N-C always outputs a higher discharge voltage at a current density of 20 mA cm -2 . Fe-N-C enables the assembled battery to have a high specific capacity of 818 mA h g Zn -1 , which is better than 795 mA h g Zn -1 of Pt / C.

[0051] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for synthesizing a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst, characterized in that, this method is completed according to the following steps: I. Under stirring conditions, add 0.5 - 5 g of organic complex and 2 - 20 mg of surfactant to deionized water and stir, then add 0.5 - 5 g of zinc salt to the solution, and continue stirring and reacting for 1 - 4 h to obtain a white suspension solution; II. Perform solid-liquid separation on the white suspension solution prepared in step I, wash the separated white product and place it in a mixed solvent of ethanol and N,N-dimethylformamide, and keep it warm at 60 - 90 °C for 24 - 72 h; III. Perform solid-liquid separation and washing on the solution obtained in step II in sequence to obtain a white product; IV. Calcinate the white product obtained in step III at 900 - 1100 °C in a protective atmosphere for 1 - 3 h to obtain a black product; V. Add an iron source and the black product obtained in step IV to an organic solvent and impregnate under stirring; VI. Perform solid-liquid separation and drying on the organic solution impregnated in step V to obtain a black reaction product; VII. Calcinate the black product obtained in step VI at 600 - 900 °C in a protective atmosphere for 2 - 4 h to obtain a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst.

2. The method for synthesizing a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst according to claim 1, characterized in that: The stirring speed of the stirring is 500 - 800 rpm. In step I, the organic complex and the surfactant are added to 80 mL of deionized water and stirred for 5 - 20 min.

3. The method for synthesizing a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst according to claim 1, characterized in that: In step V, add 2 - 20 mg of iron source and 100 mg of the black product obtained in step IV to an organic solvent and impregnate under stirring for 1 - 3 h.

4. The method for synthesizing a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst according to claim 1, characterized in that: The organic complex described in step I is 2-methylimidazole; the surfactant described in step I is cetyltrimethylammonium bromide; the zinc salt described in step I is zinc nitrate hexahydrate.

5. The method for synthesizing a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst according to claim 1, characterized in that, The specific operation of the solid-liquid separation described in step II is as follows: Use a high-speed centrifugation method to separate the suspension solution obtained in step I to obtain a solid precipitate.

6. The method for synthesizing a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst according to claim 1, characterized in that, The specific operation of the washing described in step II is as follows: Wash with deionized water 1 - 4 times, and then wash with N,N-dimethylformamide organic solvent 1 - 3 times to obtain a washed solid precipitate.

7. The method for synthesizing a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst according to claim 1, characterized in that: The mixed solvent described in step II is a mixed solution of N,N-dimethylformamide and absolute ethanol, and the volume ratio is 1:1 - 4:

1.

8. The synthesis method of a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst according to claim 1, characterized in that, the specific operation of heat preservation in step two is as follows: after the separated white product is washed and placed in a mixed solvent of ethanol and N,N-dimethylformamide, the mixed organic solution is sealed in a glass bottle, placed in a blast drying oven, and heat-preserved at 60-90 °C for 24-72 h.

9. The synthesis method of a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst according to claim 1, characterized in that, the specific operation of solid-liquid separation in step three is as follows: the mixed organic solution obtained in step two is separated by filtration to obtain a solid precipitate.

10. The synthesis method of a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst according to claim 1, characterized in that, the specific operation of washing in step three is as follows: wash with absolute ethanol 1-4 times to obtain a washed solid precipitate.

11. The synthesis method of a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst according to claim 1, characterized in that, the protective atmosphere in step four is argon protection; the heating furnace used is a tubular furnace, and the heating rate is 5-20 °C / min.

12. The synthesis method of a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst according to claim 1, characterized in that, the iron source in step five is ferric nitrate nonahydrate; the organic solvent in step five is isopropanol.

13. The synthesis method of a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst according to claim 1, characterized in that, the specific operation of solid-liquid separation in step six is as follows: the organic solution obtained in step five is separated by filtration to obtain a solid precipitate.

14. The synthesis method of a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst according to claim 1, characterized in that, the specific operation of drying in step six is as follows: place the solid precipitate in the air and dry at room temperature for 1-3 h.

15. The synthesis method of a rod-shaped Fe-N-C oxygen reduction reaction electrocatalyst according to claim 1, characterized in that, the protective atmosphere in step seven is argon protection; the heating furnace used is a tubular furnace, and the heating rate is 5-20 °C / min.

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