Synthesis method of hierarchically porous carbon nanofibers doped with aluminum fluoride for electrocatalysis

Abstract

The present invention relates to a method for synthesizing hierarchically porous carbon nanofibers doped with aluminum fluoride for electrocatalysis, belonging to the technical field of electrode materials. The preparation method comprises the following steps: (1) preparation of aluminum sol; (2) preparation of spinning solution; (3) preparation of primary fibers using electrostatic solution blowing technology; (4) obtaining pre-oxidized precursor nanofibers by pre-oxidizing the primary fibers; (5) obtaining hierarchically porous carbon nanofibers doped with aluminum fluoride by carbonizing the pre-oxidized precursor nanofibers. The present invention proposes a new strategy for simply constructing a hierarchically porous structure. The prepared hierarchically porous carbon nanofibers doped with aluminum fluoride have an ultra-high specific surface area due to the construction of the hierarchically porous structure, which can fully expose the in-situ grown active sites of aluminum fluoride. And during the carbonization process, fluorine atoms and nitrogen atoms are successfully inserted into the carbon matrix, optimizing the overall conductivity, adding more defects, accelerating the oxygen reduction reaction, and thus improving its electrocatalytic performance. The porous carbon nanofibers doped with aluminum oxide prepared by this method can be widely used in the field of electrocatalysis.

Description

Technical Field

[0001] The present invention relates to a method for synthesizing hierarchically porous carbon nanofibers doped with aluminum fluoride for electrocatalysis, belonging to the field of electrode material technology. Background Art

[0002] Due to the increasingly serious global environmental problems caused by the combustion of fossil fuels, in recent years, fuel cells that can efficiently generate electricity using clean and renewable fuels (such as H2, methanol, etc.) have received increasing attention. In a fuel cell, the fuel is oxidized at the anode, and the released electrons are transferred to the cathode through an external circuit, where O2 is reduced at the cathode. Current research shows that since the oxygen reduction reaction (ORR) rate at the cathode of the fuel cell is relatively slow compared to the anode, it has become a key factor limiting the overall discharge efficiency of the fuel cell. Therefore, a large amount of expensive Pt-based electrocatalysts are required. In traditional fuel cells, approximately 50% of the cost is used to purchase noble metal electrocatalysts, which greatly limits the industrialization process of fuel cells. Due to the scarcity and preciousness of platinum, finding abundant substitutes on Earth is one of the core tasks of fuel cell technology.

[0003] The study of the electrocatalytic properties of transition metal compounds (such as fluorides, sulfides, phosphides, selenides, etc.) has been gradually emphasized in the field of electrocatalysts for fuel cells, and its electrocatalytic mechanism is also being continuously revealed, becoming one of the hottest research topics at present and one of the most promising directions to replace noble metals and move towards commercialization. Among them, metal fluorides have excellent properties such as strong conductivity, large specific surface area, low reaction barrier, and strong hydrophilicity due to the high electronegativity (4.0) of fluorine. Fluorination of materials can also optimize the electronic structure and effectively improve the electrocatalytic performance of oxygen reduction. It is worth noting that the marginal substitution of fluorine prevents the oxidation of the significantly stable structure. Fluorine also increases the layer spacing and causes layer peeling, thus exposing more active centers. At the same time, the addition of fluorine may lead to atomic rearrangement to achieve precise atomic control. The above mechanism can effectively improve the number and activity of electrocatalytic sites, electron transfer ability, stability of the carbon skeleton, and control ability of the local chemical environment of fluorine-containing materials. Therefore, fluorides of transition metals have very good prospects in the field of electrocatalysts.

[0004] Transition metal aluminum is one of the most abundant metal elements on Earth. Therefore, it is inexpensive and widely used in our daily lives. Among them, aluminum fluoride is a very stable semiconductor that is insoluble in water, acids, and bases and has very stable properties. It can be used as an active site of an electrocatalyst. However, the conductivity of aluminum fluoride is poor and it cannot be directly applied to the electrocatalytic conversion process. Therefore, a carbon material precursor can be used for adsorption and calcination to obtain a highly conductive aluminum fluoride electrocatalyst. On the other hand, adding a highly conductive material to the fluorine-containing metal compound can also promote the electrocatalytic conversion process. In addition, aluminum fluoride can also have a certain degree of synergistic effect with carbon fibers and can be well used as an active site for electrocatalysis. After aluminum fluoride is incorporated into hierarchical porous carbon nanofibers, the overall electrochemical stability and activity of the carbon fibers will theoretically be improved to a certain extent.

[0005] The use of aluminum sol is different from the doping of aluminum salts in traditional electrospinning technology. Aluminum sol itself has a certain fiber-forming effect. Adding aluminum sol to the spinning precursor solution can, while adding aluminum elements, compensate for the overall strength of the fiber and greatly improve the stability and durability of the fiber. The mixed use of aluminum sol and spinning aids makes aluminum fluoride and carbon fibers fully bonded together, which greatly increases the contact area between the active site and carbon. The good electron conductivity of carbon can accelerate the electron transfer on the active site, thereby enhancing the overall electrocatalytic activity. Based on the above analysis, porous carbon fibers doped with aluminum fluoride have great potential in terms of stability and activity, and aluminum sol is cheaper and also has great industrial potential. Doping it into the precursor fiber, aluminum fluoride-doped hierarchical porous carbon fibers are obtained during subsequent carbonization. The hierarchical porous carbon material derived from doping aluminum fluoride into the precursor has excellent characteristics such as abundant active sites, a large number of ordered hierarchical porous structures, and good conductivity. Maintaining a good structure is an important factor in ensuring the high activity and high stability of the electrocatalyst. During the synthesis of porous carbon fibers, due to the addition of aluminum sol and the pore-forming agent PTFE, when the content of aluminum sol is too high, it will lead to a decrease in the spinnability of the spinning solution, reduce the spinning efficiency, and also cause the fiber diameter to increase, thereby reducing the active specific surface area and affecting the catalytic activity. When the content of the pore-forming agent is too high, the pore diameter will become larger, and the entire carbon fiber will be corroded during the carbonization process, resulting in serious honeycombing. Its porous structure becomes unstable during the electrocatalytic process, and the structural collapse will seriously affect the effective transmission of electrons in the carbon fiber. During the operation of the fuel cell, it will lead to a decrease in the reversibility of the battery. Therefore, the ratio of aluminum sol and PTFE is the key factor in spinning suitable aluminum fluoride-doped hierarchical porous carbon.

[0006] The aluminum fluoride-doped hierarchical porous carbon nanofibers produced by the method used in the present invention not only show good performance in electrocatalysis but also have great room for improvement in the future. Since it is only doped with single fluoride, more other active sites can continue to be doped based on it during subsequent use to further improve its electrocatalytic performance. Summary of the Invention

[0007] Aiming at the problems existing in the above-mentioned background technology, the purpose of the present invention is to propose a synthesis method of aluminum fluoride-doped hierarchical porous carbon nanofibers for electrocatalysis. First, aluminum sol is prepared by the method of condensation reflux. Secondly, after mixing the aluminum sol, PVP, PTFE and water in a certain proportion to prepare a spinning solution, the spinning solution is spun into nascent fibers by the electrostatic blowing-spinning technology (EBS). Subsequently, a pre-oxidation process of heating in air is used to stabilize the structure of the nascent fibers to form precursor fibers. At this time, the aluminum sol in the precursor fibers becomes alumina and a small amount of silica due to dehydration, and the precursor fibers are converted into carbon fibers during the subsequent carbonization process. During the carbonization process, PTFE will decompose into tetrafluoroethylene gas at high temperature. This fluorination atmosphere can easily fluorinate the alumina in the aluminum sol into aluminum fluoride and etch a small amount of silica in the aluminum sol to form small mesopores with a pore diameter of several nanometers. Due to the decomposition of PTFE, a large number of macropores with a diameter of dozens of nanometers will be formed in the fibers. Coupled with the small mesopores formed by etching a small amount of silica inside, a hierarchical porous structure will be formed. This method successfully synthesizes hierarchical porous carbon nanofibers doped with aluminum fluoride particles with hierarchical porous carbon nanofibers as the framework for the field of electrocatalysts. This method can facilitate the regulation of the morphology and pore diameter of carbon nanofibers, has a super high specific surface area, and this synthesis route has the advantages of simplicity, rapidity, high repeatability and stable structure.

[0008] To achieve the above object, the present invention provides a synthesis method of aluminum fluoride-doped porous carbon nanofibers for electrocatalysis, which is characterized by including the following steps:

[0009] (1) Preparation of aluminum sol: First, aluminum chloride hexahydrate (AlCl3·6H2O) and aluminum powder are simultaneously dissolved in distilled water. After several hours of stirring, a clear aluminum sol is obtained after condensation reflux under certain temperature conditions. Subsequently, tetraethyl orthosilicate (Si(OC2H5)4) is incorporated into the above mixed solution, and then PVA is added as a spinning additive to finally obtain the aluminum sol.

[0010] (2) Preparation of nascent fibers: The aluminum sol prepared in step (1) is added to the spinning solution, and nascent fibers doped with aluminum sol are prepared in a subsequent electrostatic blowing-spinning device.

[0011] (3) Preparation of precursor fibers: The as - prepared primary fibers doped with aluminum sol in step (2) are pre - oxidized in an air atmosphere to obtain pre - oxidized precursor fibers.

[0012] (4) Preparation of aluminum fluoride - doped hierarchical porous carbon nanofibers: The pre - oxidized precursor nanofibers prepared in step (3) are carbonized under a N2 atmosphere with a certain programmed temperature rise to obtain aluminum fluoride - doped hierarchical porous carbon nanofibers.

[0013] The carbonization program is to rise to 600 °C at 5 °C per minute and then rise to 900 °C at 3 °C per minute and hold for 2 hours.

[0014] During the carbonization process, aluminum fluoride nanoparticles can be formed in one step and a hierarchical porous structure can be formed on the fibers.

[0015] Due to the above - mentioned technical solutions, the electrode material of the present invention has the following characteristics:

[0016] 1) After carbonization, the precursor fibers present a one - dimensional hierarchical porous carbon structure. One - dimensional materials have good electron conduction effects, and the stacking of a large number of one - dimensional porous carbon nanofibers to form a three - dimensional network - like interconnected structure can ensure the transfer of electrons.

[0017] 2) The hierarchical porous structure of aluminum fluoride - doped porous carbon nanofibers has a super high specific surface area, which can expose more active sites. The connection between aluminum fluoride and activated carbon can also produce a certain synergistic effect, thereby accelerating the reaction rate of electrocatalysis.

[0018] 3) When the aluminum fluoride - doped porous carbon nanofibers are prepared into an electrocatalyst, it can be found that its electrocatalytic performance is significantly improved compared with the undoped porous carbon nanofibers when tested using a rotating electrode device.

[0019] The above three characteristics make the prepared electrocatalyst have more excellent electrochemical performance and practicality in fuel cells.

[0020] The present invention relates to a synthesis method of aluminum fluoride - doped hierarchical porous carbon nanofibers for electrocatalysis, belonging to the field of electrode material technology. Description of the Drawings

[0021] Figure 1 is a schematic diagram of the preparation process of aluminum fluoride - doped hierarchical porous carbon nanofibers;

[0022] Figure 2 is the SEM image of the precursor fibers;

[0023] Figure 3 is the SEM image of the aluminum fluoride - doped hierarchical porous carbon nanofibers;

[0024] Figure 4 TEM image of hierarchically porous carbon nanofibers doped with aluminum fluoride;

[0025] Figure 5 XRD pattern of hierarchically porous carbon nanofibers doped with aluminum fluoride;

[0026] Figure 6 HR-TEM image of hierarchically porous carbon nanofibers doped with aluminum fluoride;

[0027] Figure 7 ORR-LSV test chart of the rotating electrode of hierarchically porous carbon nanofibers doped with aluminum fluoride in 0.1 mol / L KOH electrolyte;

[0028] Figure 8 Durability test chart of hierarchically porous carbon nanofibers doped with aluminum fluoride on the rotating electrode in 0.1 mol / L KOH electrolyte;

[0029] Figure 9 Methanol tolerance test chart of hierarchically porous carbon nanofibers doped with aluminum fluoride on the rotating electrode in 0.1 mol / L KOH electrolyte;

[0030] Figure 10 OER-LSV test chart of the rotating electrode of hierarchically porous carbon nanofibers doped with aluminum fluoride in 1 mol / L KOH electrolyte. Detailed implementation manners

[0031] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific implementation manners.

[0032] Example 1

[0033] (1) Preparation of aluminum sol: First, 4 g of aluminum chloride hexahydrate (AlCl3·6H2O) and 2 g of aluminum powder are simultaneously dissolved in 20 g of distilled water. After stirring for 2 hours, it is refluxed under high temperature conditions of 80 °C for about 8 hours, and 0.5 g of (Si(OC2H5)4) is added to the obtained colloid to obtain a clear aluminum sol.

[0034] (2) Weigh 20 g of the aluminum sol prepared in step (1), then add 0.1 g of polyvinyl alcohol (PVA, Mw = 150,000 g / mol) as a spinning aid, and add 10 g of water for dilution to obtain solution A; then add 1.5 g of polyvinylpyrrolidone (PVP, Mw = 1,300,000 g / mol) to 8 g of distilled water, stir for 2 hours, and slowly add 30 g of PTFE emulsion, and then continuously stir at room temperature to obtain solution B; finally, take 15 g of solution A and strongly stir it with solution B until a uniform mixed solution is formed to obtain the aluminum sol spinning solution C.

[0035] (3) Slowly add the aluminum sol spinning solution C prepared in step (2) into the electrostatic solution blowing device, where the extrusion speed of the spinning solution is 0.5 mL·min -1 , the inner diameter of the spinning needle is 1 mm, the spinning voltage is 40 kV, the air pressure is 0.2 Mpa, and the receiving distance is 0.8 m. Under the stretching of high-voltage static electricity and strong air flow, the spinning solution forms a liquid cone and is drawn into filaments, finally obtaining nanoscale primary fibers.

[0036] (4) Put the primary fibers prepared in step (3) into a blast drying oven, heat them to 240 °C at a heating rate of 2 °C in an air atmosphere and keep them at this temperature for 1 h to obtain pre-oxidized precursor nanofibers.

[0037] (5) Carbonize the pre-oxidized precursor nanofibers obtained in step (4) under a N2 atmosphere. The carbonization procedure is to rise to 600 °C at 3 °C per minute and then rise to 900 °C at 5 °C per minute and keep them at this temperature for 2 hours.

[0038] (6) Preparation parameters for the electrochemical test electrode: Disperse 4 mg of the hierarchical porous carbon nanofibers doped with aluminum fluoride into a mixed solution of 250 μL of DMF and 250 μL of deionized water, then add 50 μL of 5% Nafion solution, and ultrasonically disperse for 30 min. Drop 10 μL and load it onto a glassy carbon electrode and conduct a rotating electrode test.

[0039] Example 2

[0040] (1) Preparation of aluminum sol: First, dissolve 4 g of aluminum chloride hexahydrate (AlCl3·6H2O) and 2 g of aluminum powder simultaneously in 20 g of distilled water. After stirring for 2 hours, carry out condensation reflux at 80 °C for about 8 hours, and add 0.5 g of (Si(OC2H5)4) to the obtained colloid to obtain a clear aluminum sol.

[0041] (2) Weigh 20 g of the aluminum sol prepared in step (1), then add 0.1 g of polyvinyl alcohol (PVA, Mw = 150,000 g / mol) as a spinning aid, and add 10 g of water for dilution to obtain solution A; then add 1.5 g of polyvinylpyrrolidone (PVP, Mw = 1,300,000 g / mol) to 8 g of distilled water, stir for 2 hours, and slowly add 30 g of PTFE emulsion, and then continuously stir at room temperature to obtain solution B; finally, take 11.3 g of solution A and solution B and stir strongly until a uniform mixed solution is formed to obtain the aluminum sol spinning solution C.

[0042] (3) Slowly add the aluminum sol spinning solution C prepared in step (2) into the electrostatic solution blowing device, where the extrusion speed of the spinning solution is 0.5 mL·min -1, the inner diameter of the spinning needle is 1 mm, the spinning voltage is 40 kV, the air pressure is 0.2 Mpa, and the receiving distance is 0.8 m. Under the stretching of high-voltage static electricity and strong air flow, the spinning solution forms a liquid cone and is drawn into filaments, and finally nanofibers as-prepared are obtained.

[0043] (4) Put the nanofibers as-prepared in step (3) into a blast drying oven, heat them to 240 °C at a heating rate of 2 °C per minute in an air atmosphere and keep them at this temperature for 1 h to obtain pre-oxidized precursor nanofibers.

[0044] (5) Carbonize the pre-oxidized precursor nanofibers obtained in step (4) in an N2 atmosphere. The carbonization procedure is to heat from room temperature to 600 °C at a rate of 3 °C per minute and then to 900 °C at a rate of 5 °C per minute and keep them at this temperature for 2 h.

[0045] (6) Preparation parameters for the electrochemical test electrode: Disperse 4 mg of the hierarchical porous carbon nanofibers doped with aluminum fluoride into a mixed solution of 250 μL of DMF and 250 μL of deionized water, then add 50 μL of 5% Nafion solution and ultrasonically disperse for 30 min. Drop 10 μL of the dispersion onto a glassy carbon electrode and perform a rotating electrode test.

[0046] Example 3

[0047] (1) Preparation of aluminum sol: First, dissolve 4 g of aluminum chloride hexahydrate (AlCl3·6H2O) and 2 g of aluminum powder in 20 g of distilled water at the same time. After stirring for 2 h, condense and reflux at 80 °C for about 8 h, and add 0.5 g of (Si(OC2H5)4) to the obtained colloid to obtain a clear aluminum sol.

[0048] (2) Weigh 20 g of the aluminum sol prepared in step (1), then add 0.1 g of polyvinyl alcohol (PVA, Mw = 150,000 g / mol) as a spinning aid and add 10 g of water for dilution to obtain solution A; then add 1.5 g of polyvinylpyrrolidone (PVP, Mw = 1,300,000 g / mol) to 8 g of distilled water, stir for 2 h, then slowly add 30 g of PTFE emulsion, and then continuously stir at room temperature to obtain solution B; finally, take 7.5 g of solution A and strongly stir it with solution B until a uniform mixed solution is formed to obtain aluminum sol spinning solution C.

[0049] (3) Slowly add the aluminum sol spinning solution C prepared in step (2) into an electrostatic solution blowing device, where the extrusion speed of the spinning solution is 0.5 mL·min -1, the inner diameter of the spinning needle is 1 mm, the spinning voltage is 40 kV, the air pressure is 0.2 Mpa, and the receiving distance is 0.8 m. Under the stretching of high-voltage static electricity and strong air flow, the spinning solution forms a liquid cone and is drawn into filaments, and finally nanofibers as as-spun fibers are obtained.

[0050] (4) Put the as-spun fibers prepared in step (3) into a blast drying oven, heat them to 240 °C at a heating rate of 2 °C in an air atmosphere and keep them at this temperature for 1 h to obtain pre-oxidized precursor nanofibers.

[0051] (5) Carbonize the pre-oxidized precursor nanofibers obtained in step (4) under a N2 atmosphere. The carbonization program is to increase the temperature from room temperature to 600 °C at a rate of 3 °C per minute and then increase the temperature to 900 °C at a rate of 5 °C per minute and keep it at this temperature for 2 hours.

[0052] (6) Preparation parameters for the electrochemical test electrode: Disperse 4 mg of fluorinated aluminum-doped hierarchical porous carbon nanofibers into a mixed solution of 250 μL of DMF and 250 μL of deionized water, then add 50 μL of 5% Nafion solution and ultrasonically disperse for 30 min. Drop 10 μL of the dispersion onto a glassy carbon electrode and perform a rotating electrode test.

[0053] Example 4

[0054] (1) Preparation of aluminum sol: First, dissolve 4 g of aluminum chloride hexahydrate (AlCl3·6H2O) and 2 g of aluminum powder in 20 g of distilled water at the same time. After stirring for 2 hours, condense and reflux at 80 °C for about 8 hours, and add 0.5 g of (Si(OC2H5)4) to the obtained colloid to obtain a clear aluminum sol.

[0055] (2) Weigh 20 g of the aluminum sol prepared in step (1), then add 0.1 g of polyvinyl alcohol (PVA, Mw = 150,000 g / mol) as a spinning aid, and add 10 g of water for dilution to obtain solution A; then add 1.5 g of polyvinylpyrrolidone (PVP, Mw = 1,300,000 g / mol) to 8 g of distilled water, stir for 2 hours, then slowly add 30 g of PTFE emulsion, and then continuously stir at room temperature to obtain solution B; finally, take 5.6 g of solution A and vigorously stir it with solution B until a uniform mixed solution is formed to obtain aluminum sol spinning solution C.

[0056] (3) Slowly add the aluminum sol spinning solution C prepared in step (2) into an electrostatic solution blowing device, where the extrusion speed of the spinning solution is 0.5 mL·min -1, the inner diameter of the spinning needle is 1 mm, the spinning voltage is 40 kV, the air pressure is 0.2 Mpa, and the receiving distance is 0.8 m. Under the stretching of high-voltage static electricity and strong air flow, the spinning solution forms a liquid cone and is drawn into filaments, and finally nanofibers as-prepared are obtained.

[0057] (4) Put the nanofibers as-prepared in step (3) into a blast drying oven, heat it to 240 °C at a heating rate of 2 °C in an air atmosphere and keep it warm for 1 h to obtain pre-oxidized precursor nanofibers.

[0058] (5) Carbonize the pre-oxidized precursor nanofibers obtained in step (4) in an N2 atmosphere. The carbonization procedure is to increase the temperature from room temperature to 600 °C at a rate of 3 °C per minute and then increase the temperature to 900 °C at a rate of 5 °C per minute and keep it warm for 2 hours.

[0059] (6) Preparation parameters of the electrochemical test electrode: Disperse 4 mg of fluorinated aluminum-doped hierarchical porous carbon nanofibers into a mixed solution of 250 μL of DMF and 250 μL of deionized water, then add 50 μL of 5% Nafion solution and ultrasonically disperse for 30 min. Drop 10 μL and load it onto a glassy carbon electrode and perform a rotating electrode test.

[0060] Example 5

[0061] (1) Preparation of aluminum sol: First, dissolve 4 g of aluminum chloride hexahydrate (AlCl3·6H2O) and 2 g of aluminum powder in 20 g of distilled water at the same time. After stirring for 2 hours, condense and reflux at 80 °C for about 8 hours, and add 0.5 g of (Si(OC2H5)4) to the obtained colloid to obtain a clear aluminum sol.

[0062] (2) Weigh 20 g of the aluminum sol prepared in step (1), then add 0.1 g of polyvinyl alcohol (PVA, Mw = 150,000 g / mol) as a spinning aid, and add 10 g of water for dilution to obtain solution A; then add 1.5 g of polyvinylpyrrolidone (PVP, Mw = 1,300,000 g / mol) to 8 g of distilled water, stir for 2 hours, then slowly add 30 g of PTFE emulsion, and then continuously stir at room temperature to obtain solution B; finally, take 4.5 g of solution A and strongly stir it with solution B until a uniform mixed solution is formed to obtain aluminum sol spinning solution C.

[0063] (3) Slowly add the aluminum sol spinning solution C prepared in step (2) into an electrostatic solution blowing device, where the extrusion speed of the spinning solution is 0.5 mL·min -1, the inner diameter of the spinning needle is 1 mm, the spinning voltage is 40 kV, the air pressure is 0.2 Mpa, and the receiving distance is 0.8 m. Under the stretching of high-voltage static electricity and strong air flow, the spinning solution forms a liquid cone and is drawn into filaments, and finally nanofibers of the nascent fibers are obtained.

[0064] (4) Put the nascent fibers prepared in step (3) into a blast drying oven, heat it to 240 °C at a heating rate of 2 °C in an air atmosphere and keep it for 1 h to obtain pre-oxidized precursor nanofibers.

[0065] (5) Carbonize the pre-oxidized precursor nanofibers obtained in step (4) in an N2 atmosphere. The carbonization program is to rise to 600 °C at 3 °C per minute and then rise to 900 °C at 5 °C per minute and keep it for 2 hours.

[0066] (6) Preparation parameters of the electrochemistry test electrode: Disperse 4 mg of aluminum fluoride-doped hierarchical porous carbon nanofibers into a mixed solution of 250 μL of DMF and 250 μL of deionized water, then add 50 μL of 5% Nafion solution and ultrasonically disperse for 30 min. Add 10 μL and load it onto a glassy carbon electrode and perform a rotating electrode test.

[0067] Performance test:

[0068] A synthesis method of aluminum fluoride-doped hierarchical porous carbon nanofibers for electrocatalysis proposed in this application is characterized in that a hierarchical porous carbon nanofiber structure is successfully constructed by a simple method, and aluminum fluoride nanoparticles are in-situ grown in the fibers. The hierarchical porous carbon nanofibers prepared by this method have a very high specific surface area, which is beneficial to fully expose the active sites and accelerate the electrocatalytic reaction. And its stability exceeds that of the current commercial Pt / C electrocatalyst, and it has good cycle stability.

[0069] Figure 1 It is a schematic diagram of the preparation process for growing aluminum fluoride nanoparticles on hierarchical porous carbon nanofibers. The technology of aluminum fluoride-doped hierarchical porous carbon nanofibers in this patent application is first prepared by electrospinning and blowing technology, pre-oxidation process, and carbonization process. After pre-oxidation, the precursor fibers will have better stability, and can better maintain the morphology in the subsequent carbonization process, ensure the integrity of the porous structure and prevent collapse. Finally, add ethanol and Nafion solution to the aluminum fluoride-doped hierarchical porous carbon nanofibers to prepare electrode materials for ORR and OER reactions.

[0070] Figure 2 It is the SEM image of the precursor fibers. First, it can be seen that the honeycomb-like porous carbon nanofibers are not adhered, and the fiber diameters are relatively uniform; after pre-oxidation, due to the shrinkage of polyvinylpyrrolidone, the PTFE nanoparticles protrude.

[0071] Figure 3 SEM image of hierarchically porous carbon nanofibers doped with aluminum fluoride. It can be found that the fibers are uniform in thickness after carbonization, indicating that the pore size distribution is also relatively uniform, and the average diameter of the fibers is about 200 nm.

[0072] Figure 4 TEM image of hierarchically porous carbon nanofibers doped with aluminum fluoride. It can be found from the transmission electron microscope image that small pores of about 15 nm are uniformly distributed inside the porous carbon, successfully forming a hierarchically porous structure.

[0073] Figure 5 XRD pattern of hierarchically porous carbon nanofibers doped with aluminum fluoride. It can be seen from the XRD pattern that aluminum fluoride nanoparticles have been successfully grown on the hierarchically porous carbon nanofibers.

[0074] Figure 6 HR-TEM image of hierarchically porous carbon nanofibers doped with aluminum fluoride. The lattice spacings of the crystals grown on the fibers are observed to be 0.201 nm and 0.352 nm under high-magnification transmission electron microscopy, corresponding to the (202) crystal plane and (012) crystal plane of the aluminum fluoride standard card in XRD, respectively, proving that aluminum fluoride nanoparticles have been successfully grown on the porous carbon nanofibers.

[0075] Figure 7Rotating electrode test diagram (LSV) of aluminum fluoride-doped hierarchical porous carbon nanofibers in 0.1 mol / L KOH electrolyte. This test is used to evaluate the ORR activity of electrocatalysts. Examples 1-5 are implementation schemes with different doping ratios of aluminum sol, and the prepared aluminum fluoride-doped hierarchical porous carbon nanofibers correspond to AlF3@PCNFs-1, AlF3@PCNFs-2, AlF3@PCNFs-3, AlF3@PCNFs-4, and AlF3@PCNFs-5 in the test diagram respectively. The results show that the group with the best performance in this application is Example 3, denoted as AlF3@PCNFs-3. The initial potential of the aluminum fluoride-doped hierarchical porous carbon nanofibers (AlF3@PCNFs-3) is 0.91 V, and the half-wave potential is 0.865 V, which are higher than the initial potential (0.81 V) and half-wave potential (0.75 V) of ordinary porous carbon nanofibers, and also higher than the initial potential (0.90 V) and half-wave potential (0.86 V) of commercial Pt / C; different doping ratios of aluminum sol have a great influence on the electrocatalytic performance. This is because aluminum fluoride has poor conductivity. When the content of aluminum fluoride is relatively high, it will affect the overall conductivity and is not conducive to the progress of the catalytic reaction. In summary, a reasonable doping ratio of aluminum sol can effectively improve the electrocatalytic activity of aluminum fluoride-doped hierarchical porous carbon nanofibers, and this performance has significantly better ORR electrocatalytic performance compared with ordinary porous carbon, indicating that the doping of aluminum fluoride effectively enhances the electrocatalytic activity. Finally, the ORR catalytic activity of the aluminum fluoride-doped hierarchical porous carbon nanofibers prepared in Example 3 is comparable to that of commercial Pt / C electrocatalyst.

[0076] Figure 8 Durability test diagram of aluminum fluoride-doped hierarchical porous carbon nanofibers on a rotating electrode in 0.1 mol / L KOH electrolyte. After 2000 CV cycles, the LSV value of AlF3@PCNFs-3 remains basically unchanged, which proves its good stability.

[0077] Figure 9 Methanol tolerance test diagram of aluminum fluoride-doped hierarchical porous carbon nanofibers on a rotating electrode in 0.1 mol / L KOH electrolyte. It is found that the current of the AlF3@PCNFs electrocatalyst hardly changes after adding methanol, while that of Pt / C decreases to 60% of the original, indicating that the AlF3@PCNFs electrocatalyst has good methanol tolerance.

[0078] Figure 10Rotating electrode test chart of aluminum fluoride-doped hierarchical porous carbon nanofibers in 1 mol / L KOH electrolyte. This test is used to evaluate the OER activity of electrocatalysts. It can be seen from the test chart that the aluminum fluoride-doped hierarchical porous carbon nanofibers (AlF3@PCNFs-3) prepared in Example 3 have the lowest overpotential (310 mV) among these cases, far higher than that of ordinary porous carbon (520 mV), and slightly lower than the overpotential of commercial OER electrocatalyst Ir / C (260 mV). This can prove that it also has certain OER performance.

[0079] Example 3 is the best example of this application. Examples 1, 2, 4, and 5 show performance characteristics slightly lower than those of Example 3 after the above performance tests. Therefore, the optimal doping ratio of aluminum sol is initially determined. Therefore, in summary, the characteristics of a bifunctional electrocatalyst made of aluminum fluoride-doped hierarchical porous carbon nanofibers proposed in this application are that it uses thermally decomposable PTFE as a pore-forming agent, and tetrafluoroethylene generated by thermal decomposition can etch silica in aluminum sol to generate secondary mesopores and form a hierarchical porous structure. In addition, the formation of nanofiber pores occurs synchronously with the metal fluorination process, which greatly simplifies the production steps and enables the aluminum fluoride-doped hierarchical porous carbon nanofibers to be completed in one step during the carbonization process. In the final performance test, the aluminum fluoride-doped hierarchical porous carbon nanofibers obtained good ORR catalytic activity and relatively good OER electrocatalytic activity, with certain bifunctional characteristics. Coupled with aluminum sol, as an industrial-grade raw material, being inexpensive and simple to prepare and obtain, this electrocatalyst has great industrial potential in the field of electrocatalysis of fuel cells.

Claims

1. A method for synthesizing hierarchically porous carbon nanofibers doped with aluminum fluoride for electrocatalysis, characterized in that, It includes the following steps: (1) Preparation of aluminum sol: First, dissolve 4 g of aluminum chloride hexahydrate and 2 g of aluminum powder in 20 g of distilled water simultaneously. After stirring for 2 hours, carry out condensation reflux at 80 °C for 8 hours, and add 0.5 g of Si(OC2H5)4 to the obtained colloid to obtain a clear aluminum sol; (2) Weigh 20 g of the aluminum sol prepared in step (1), then add 0.1 g of polyvinyl alcohol, Mw = 150,000 g / mol as an additive, and add 10 g of water for dilution to obtain solution A; Then add 1.5 g of polyvinylpyrrolidone, Mw = 1,300,000 g / mol, to 8 g of distilled water, stir for 2 hours, then slowly add 30 g of PTFE emulsion, and then continuously stir at room temperature to obtain solution B; Finally, take 7.5 g of solution A and vigorously stir it with solution B until a uniform mixed solution is formed to obtain aluminum sol spinning solution C; (3) Slowly add the aluminum sol spinning solution C prepared in step (2) into the electrostatic solution blowing device, where the extrusion speed of the spinning solution is 0.5 mL·min -1 , the inner diameter of the spinning needle is 1 mm, the spinning voltage is 40 kV, the air flow pressure is 0.2 MPa, and the receiving distance is 0.8 m; under the stretching of high-voltage static electricity and strong air flow, the spinning solution forms a liquid cone and is drawn into filaments, and finally nano-scale primary fibers are obtained; (4) Put the nascent fiber prepared in step (3) into a blast drying oven, heat it to 240 °C at a heating rate of 2 °C per minute in an air atmosphere and keep it warm for 1 h to obtain a pre-oxidized precursor nanofiber; (5) Carbonize the pre-oxidized precursor nanofiber obtained in step (4) under an N2 atmosphere. The carbonization procedure is to rise to 600 °C at 3 °C per minute and then rise to 900 °C at 5 °C per minute and keep it warm for 2 hours.

Images