A nitrogen-doped porous carbon fiber, a preparation method thereof, a carbon paper and applications thereof

Abstract

The present invention provides a nitrogen-doped porous carbon fiber and its preparation method, a carbon paper and its application, belonging to the technical field of carbon materials. The preparation method of the nitrogen-doped porous carbon fiber provided by the present invention includes the following steps: mixing carbon fiber, an organic polymer, boric acid and water, and performing a modification treatment to obtain a modified carbon fiber; annealing and calcining the modified short carbon fiber to obtain a porous carbon fiber; mixing the calcined carbon fiber with a nitrogen-containing compound and performing an activation treatment to obtain a nitrogen-doped porous carbon fiber. The carbon fiber prepared by the present invention has a high specific surface area and good dispersibility in water, and is very suitable for the preparation of carbon paper for fuel cell gas diffusion layers. In addition, the method provided by the present invention is simple to operate, low in cost, and has good industrial production prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of carbon materials, and particularly relates to a nitrogen-doped porous carbon fiber, a preparation method thereof, a carbon paper and an application thereof. Background Art

[0002] Carbon fiber has the advantages of high strength, high modulus, high conductivity and excellent corrosion resistance, and is a popular material for preparing the carbon paper substrate in fuel cells. The carbon paper for fuel cells has relatively high performance requirements. It not only needs to play a supporting role, but also needs to provide good gas diffusion channels and drainage channels for the electrode reaction. Among them, the performance of the carbon paper is mainly restricted by the properties of the carbon fiber itself, especially the dispersibility and specific surface area of the carbon fiber. Poor dispersibility of short carbon fibers will directly affect the performance of fuel cell electrodes and increase the contact resistance inside the electrodes. However, as a fiber material with a carbon content higher than 95%, carbon atoms in carbon fiber are mainly connected by covalent bonds. The surface of carbon fiber is smooth, the specific surface area is extremely low, and there are few active groups. It is difficult to be uniformly dispersed in an aqueous solution. The above properties cause carbon fibers to easily flocculate in water, thereby forming fiber flocs, which reduces the formation evenness of the base paper, and further leads to a low porosity and poor air permeability of the carbon fiber formed paper. At the same time, it also causes great difficulties for subsequent uniform coating of the catalyst.

[0003] Carbon fibers with a high specific surface area contribute to improving the porosity of the carbon paper. At the same time, carbon fibers with good dispersibility are not easy to form bundles, have a high degree of looseness, and the obtained carbon paper has good compactness, a flat surface and uniform pore distribution. Therefore, in order to improve the structural uniformity and air permeability of the carbon paper, it is often necessary to modify the carbon fiber to increase the specific surface area of the carbon fiber itself and improve the dispersibility.

[0004] Currently, the methods for preparing carbon fibers with a high specific surface area mainly include soft template method, hard template method, or adding a pore-forming agent (such as PMMA, alumina, ammonium chloride, etc.) to the spinning dope first, changing the composition of the carbon fiber precursor, and then forming pores by calcination and extending the surface structure to the carbon fiber. However, the above methods seriously affect the performance of the final carbon fiber, such as conductivity and tensile properties, and thus limit the application range of the carbon fiber. For finished carbon fiber filaments, there are also many studies on modification by gas treatment method (such as carbon dioxide), physical activation method or chemical activation method. Among them, the chemical activation method requires a large amount of strong acid and strong base activators to etch the surface of the carbon fiber, and the activation efficiency of the gas treatment method and the physical activation method is relatively low. Summary of the Invention

[0005] The purpose of the present invention is to provide a nitrogen-doped porous carbon fiber, a preparation method thereof, a carbon paper and an application thereof. The method provided by the present invention does not require the use of strong acids or strong bases, and the prepared nitrogen-doped porous carbon fiber has a relatively high specific surface area and good dispersibility in water.

[0006] To achieve the above-mentioned invention object, the present invention provides the following technical solutions:

[0007] The present invention provides a method for preparing nitrogen-doped porous carbon fiber, comprising the following steps:

[0008] Mix carbon fiber, organic polymer, boric acid and water, and carry out modification treatment to obtain modified carbon fiber;

[0009] Carry out annealing and calcination treatment on the modified carbon fiber to obtain porous carbon fiber;

[0010] Mix the porous carbon fiber with a nitrogen-containing compound, and carry out activation treatment to obtain nitrogen-doped porous carbon fiber.

[0011] Preferably, the length of the carbon fiber is 2-6 mm.

[0012] Preferably, the organic polymer includes one or more of polyvinyl alcohol, polyethylene glycol, poly(ethylene oxide), poly(propylene oxide), poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) and poly(ethylene oxide)-poly(propylene oxide) block copolymer; the mass ratio of the carbon fiber, organic polymer, boric acid and water is 2-10:2-5:0.1-0.75:100-300.

[0013] Preferably, the temperature of the modification treatment is 60-80 °C, and the time is 2-4 h.

[0014] Preferably, the temperature of the annealing and calcination treatment is 400-800 °C, the heat preservation time is 1-10 h; the heating rate for rising to the temperature of the annealing and calcination is 1-10 °C / min.

[0015] Preferably, the nitrogen-containing compound includes one or more of urea, ammonium chloride and melamine; the mass ratio of the porous carbon fiber to the nitrogen-containing compound is 0.5-3:1.

[0016] Preferably, the temperature of the activation treatment is 200-500 °C, the heat preservation time is 1-6 h; the heating rate for rising to the temperature of the activation treatment is 1-10 °C / min.

[0017] The present invention provides the nitrogen-doped porous carbon fiber obtained by the preparation method described in the above technical solution, and the specific surface area of the nitrogen-doped porous carbon fiber is 10-50 m 2 / g, and the pore diameter is 1-100 nm.

[0018] The present invention provides a carbon paper, and the preparation raw material includes the nitrogen-doped porous carbon fiber described in the above technical solution.

[0019] The present invention provides the application of the carbon paper described in the above technical solution in a fuel cell.

[0020] The present invention provides a method for preparing nitrogen-doped porous carbon fibers, comprising the following steps: mixing carbon fibers, an organic polymer, boric acid and water, and performing a modification treatment to obtain modified carbon fibers; performing an annealing and calcination treatment on the modified carbon fibers to obtain porous carbon fibers; mixing the calcined carbon fibers with a nitrogen-containing compound and performing an activation treatment to obtain nitrogen-doped porous carbon fibers. The method provided by the present invention does not require the use of strong acids or strong bases. By using an organic polymer and boric acid to modify the carbon fibers, the number of active groups on the surface of the carbon fibers is increased, the surface activity of the carbon fibers is enhanced, and further the dispersibility of the carbon fibers in water is improved; through the annealing and calcination treatment, carbon fibers with a porous structure are obtained, and the preparation of carbon fibers with a high specific surface area is successfully realized; meanwhile, through the activation treatment, nitrogen atoms in the nitrogen-containing compound are doped into the porous carbon fibers, further improving the hydrophilicity and dispersibility of the carbon fibers. The preparation method provided by the present invention is simple and convenient, the raw materials are easy to obtain and the price is low. By chemically treating the finished carbon fiber filaments, nitrogen-doped porous carbon fibers with an ultra-high specific surface area and high dispersibility in water can be obtained, which are very suitable for the preparation of carbon papers for fuel cell gas diffusion layers. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0022] Figure 1 Scanning electron microscope image of commercially available carbon fibers;

[0023] Figure 2 Scanning electron microscope image of the modified carbon fibers in Example 1 of the present invention;

[0024] Figure 3 Scanning electron microscope image of the porous carbon fibers in Example 1 of the present invention;

[0025] Figure 4 Photograph of the dispersion of the nitrogen-doped porous carbon fibers in Example 1 of the present invention and the desized carbon fibers in Comparative Example 1;

[0026] Figure 5 BET specific surface areas of the nitrogen-doped porous carbon fibers in Examples 1-2 of the present invention and the desized carbon fibers in Comparative Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] The present invention provides a method for preparing nitrogen-doped porous carbon fibers, comprising the following steps:

[0028] Mix carbon fiber, organic polymer, boric acid and water, and conduct a modification treatment to obtain modified carbon fiber;

[0029] Conduct an annealing and calcination treatment on the modified carbon fiber to obtain porous carbon fiber;

[0030] Mix the porous carbon fiber with a nitrogen-containing compound and conduct an activation treatment to obtain nitrogen-doped porous carbon fiber.

[0031] In the present invention, unless otherwise specified, the raw materials used are all commercially available products well-known to those skilled in the art.

[0032] In the present invention, carbon fiber, organic polymer, boric acid and water are mixed and subjected to a modification treatment to obtain modified carbon fiber. In the present invention, the length of the carbon fiber is preferably 1-10 mm, more preferably 2-8 mm, and even more preferably 3-6 mm; the diameter of the carbon fiber is preferably 7 mm. Using carbon fiber of the above length in the present invention can avoid phenomena such as flocculation, flying filaments, and wire jams of carbon fiber during the preparation of nitrogen-doped porous carbon fiber, ensure the uniformity of fiber length to the greatest extent, and at the same time endow the carbon fiber with good bundling property and processability. In the present invention, the carbon fiber is preferably PAN-based carbon fiber.

[0033] Before conducting the modification treatment, the present invention preferably immerses the carbon fiber in acetone, then washes and dries it in sequence to obtain desized carbon fiber. When the length of the carbon fiber is too long and does not meet the above range requirements, the present invention preferably cuts the carbon fiber and then immerses it in acetone. The present invention has no special limitation on the cutting method of the carbon fiber, and any cutting method well-known to those skilled in the art can be used. In the present invention, the dosage ratio of the carbon fiber to acetone is preferably 1-50 g: 200-2000 mL, more preferably 3-20 g: 300-1000 mL, and even more preferably 5 g: 400 mL. In the present invention, the temperature of the immersion treatment is preferably 60-80 °C, more preferably 70-80 °C; the time is preferably 4-8 h, more preferably 4-5 h. In the present invention, the immersion treatment is preferably carried out in a Soxhlet extractor. The present invention has no special limitation on the methods and conditions of the washing and drying, and any washing and drying methods well-known to those skilled in the art can be used. In the present invention, immersing the carbon fiber in acetone can fully remove impurities such as sizing agents and lubricants on the surface of the carbon fiber.

[0034] In the present invention, the organic polymer preferably comprises one or more of polyvinyl alcohol, polyethylene glycol, polyethylene oxide, polypropylene oxide, polyethylene oxide-polypropylene oxide-polyethylene oxide, and polyethylene oxide-polypropylene oxide block copolymer, more preferably comprises polyethylene oxide, polypropylene oxide, polyethylene oxide-polypropylene oxide-polyethylene oxide, or polyethylene oxide-polypropylene oxide block copolymer, and most preferably is polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer. In the present invention, the mass ratio of the carbon fiber, organic polymer, boric acid, and water is preferably 2-10:2-5:0.1-0.75:100-300, more preferably 4-7:2-4:0.2-0.5:150-250, and most preferably 5:2:0.25:200. In the present invention, the temperature of the modification treatment is preferably 60-80°C, more preferably 70-80°C; the time is preferably 2-4 h, and specifically can be 2 h, 3 h, or 4 h. The present invention modifies the carbon fiber by using an organic polymer and boric acid to increase the number of active groups on the surface of the carbon fiber, enhance the surface activity of the carbon fiber, and further improve the dispersibility of the carbon fiber in water.

[0035] After the modification treatment, the present invention preferably washes and dries the obtained modified product in sequence to obtain modified carbon fiber. In the present invention, the washing is preferably water washing, and the number of washing times is preferably 3-5 times. The present invention has no special limitation on the drying, and any drying method well known to those skilled in the art can be used.

[0036] After obtaining the modified carbon fiber, the present invention subjects the modified carbon fiber to annealing calcination treatment to obtain porous carbon fiber. In the present invention, the temperature of the annealing calcination treatment is preferably 400-800°C, more preferably 450-600°C, and most preferably 500°C; the heat preservation time is preferably 1-10 h, more preferably 3-6 h, and most preferably 4 h; the heating rate for heating to the temperature of the annealing calcination is preferably 1-10°C / min, more preferably 2-5°C / min, and most preferably 3°C / min. In the present invention, the annealing calcination treatment is preferably carried out in a protective atmosphere, and the protective atmosphere preferably comprises nitrogen or argon, and most preferably is nitrogen. The present invention obtains carbon fiber with a porous structure through annealing calcination treatment, and successfully realizes the preparation of carbon fiber with a high specific surface area. The present invention preferably carries out the annealing calcination treatment under the above conditions, which can make the surface of the originally smooth carbon fiber present a microporous structure, greatly improving the specific surface area of the carbon fiber; if the temperature of the annealing calcination treatment is too high or the heating rate is too fast, the cross-linked structure on the surface of the carbon fiber will collapse or damage the single-filament tensile strength of the carbon fiber itself. In the examples of the present invention, specifically, the modified carbon fiber is placed in a crucible and covered, and the crucible is placed in a plasma furnace for annealing calcination treatment.

[0037] After obtaining the porous carbon fiber, the present invention mixes the porous carbon fiber with a nitrogen-containing compound and performs an activation treatment to obtain a nitrogen-doped porous carbon fiber. In the present invention, the nitrogen-containing compound preferably includes one or more of urea, ammonium chloride, and melamine, and more preferably melamine; the mass ratio of the porous carbon fiber to the nitrogen-containing compound is preferably 0.5 to 3:1, and more preferably 1.5:1. The temperature of the activation treatment in the present invention is preferably 200 to 500 °C, further preferably 200 to 400 °C, and more preferably 200 to 300 °C; the heat preservation time is 1 to 6 h, further preferably 2 to 4 h, and more preferably 2 to 3 h; the heating rate for heating to the temperature of the activation treatment is preferably 1 to 10 °C / min, further preferably 3 to 5 °C / min, and more preferably 3 to 4 °C / min; the activation treatment is preferably carried out in a protective atmosphere, and the protective atmosphere preferably includes nitrogen or argon, and more preferably nitrogen. In the examples of the present invention, specifically, the porous carbon fiber is placed in a crucible, and a nitrogen-containing compound is added in a tubular furnace for activation treatment. Through the activation treatment in the present invention, nitrogen atoms in the nitrogen-containing compound are doped into the porous carbon fiber, increasing the number of active groups on the fiber surface and further improving the hydrophilicity and dispersibility of the carbon fiber.

[0038] The present invention also provides a nitrogen-doped porous carbon fiber prepared by the preparation method described in the above technical solution. In the present invention, the specific surface area of the nitrogen-doped porous carbon fiber is 10 to 50 m 2 / g, and the pore size is 1 to 100 nm.

[0039] The present invention also provides a carbon paper, and the preparation raw materials include a nitrogen-doped porous carbon fiber, and the nitrogen-doped porous carbon fiber is prepared by the preparation method described in the above technical solution or the nitrogen-doped porous carbon fiber described in the above technical solution.

[0040] The present invention also provides an application of the carbon paper described in the above technical solution in a fuel cell. The present invention has no special limitation on the application, and the application of the carbon paper well-known to those skilled in the art in a fuel cell can be adopted.

[0041] In order to further illustrate the present invention, the nitrogen-doped porous carbon fiber provided by the present invention, its preparation method, the carbon paper and its application will be described in detail below with reference to the drawings and examples, but they cannot be understood as limiting the protection scope of the present invention.

[0042] The carbon fiber used in the examples is Toray PAN-based carbon fiber from Japan, with the model T300(3k).

[0043] All the reagents used in the examples are commercially available.

[0044] Example 1

[0045] The PAN-based carbon fibers were cut into 3 mm in length. 5 g of the cut PAN-based carbon fibers were placed in a Soxhlet extractor, and then 400 mL of acetone was added to the Soxhlet extractor. The mixture was treated under reflux (80 °C) for 4 h. After the reflux ended, the obtained desized carbon fibers were washed thoroughly and dried; then 2 g of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer, 0.25 g of boric acid and 200 g of all the desized carbon fibers obtained in the previous step were mixed and subjected to modification treatment at 80 °C for 2 h. Then, they were washed with water and dried in sequence to obtain modified carbon fibers.

[0046] All the modified carbon fibers obtained in the previous step were placed in a crucible and covered. The crucible was put into a plasma furnace. Under a nitrogen atmosphere, the temperature of the plasma furnace was raised to 500 °C at a rate of 3 °C / min, and annealing calcination treatment was carried out for 4 h while maintaining the temperature. Then, it was cooled to obtain porous carbon fibers.

[0047] All the porous carbon fibers obtained in the previous step were placed in a crucible and covered. The crucible was put into a tube furnace, and 2 g of melamine powder was taken and placed in the tube furnace; under a nitrogen atmosphere, the temperature of the tube furnace was raised to 200 °C at a rate of 3 °C / min, and activation treatment was carried out for 2 h while maintaining the temperature to obtain nitrogen-doped porous carbon fibers.

[0048] Figure 1 is the scanning electron microscope image of commercially available carbon fibers. It can be seen from Figure 1 that there is an obvious sizing agent layer on the surface of the carbon fibers.

[0049] Figure 2 is the scanning electron microscope image of the modified carbon fibers in Example 1 of the present invention. It can be seen from Figure 2 that the surface of the modified carbon fibers is smooth, and there are only a few grooves distributed along the fiber axis, and the specific surface area is small.

[0050] Figure 3 is the scanning electron microscope image of the porous carbon fibers in Example 1 of the present invention. It can be seen from Figure 3 that after annealing calcination, the surface of the originally smooth modified carbon fibers presents a microporous structure, and the pore size is between 1 and 100 nm, greatly improving the specific surface area of the carbon fibers.

[0051] Example 2

[0052] The PAN-based carbon fibers are cut into short pieces of 3 mm, and 5 g of the cut PAN-based carbon fibers are placed in a Soxhlet extractor. Then, 400 mL of acetone is added to the Soxhlet extractor, and the mixture is treated under reflux (80 °C) for 4 h. After the reflux ends, the obtained desized carbon fibers are washed thoroughly and dried. Then, 2 g of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer, 0.25 g of boric acid, and 200 g of all the desized carbon fibers obtained in the previous step are mixed and subjected to a modification treatment at 80 °C for 4 h. Then, they are washed with water and dried in sequence to obtain modified carbon fibers.

[0053] All the modified carbon fibers obtained in the previous step are placed in a crucible and covered. The crucible is put into a plasma furnace. Under a nitrogen atmosphere, the temperature of the plasma furnace is raised to 500 °C at a rate of 3 °C / min and kept at this temperature for 4 h for annealing calcination treatment. Then, it is cooled to obtain porous carbon fibers.

[0054] All the porous carbon fibers obtained in the previous step are placed in a crucible and covered. The crucible is put into a tube furnace, and 2 g of melamine powder is taken and placed in the tube furnace. Under a nitrogen atmosphere, the temperature of the tube furnace is raised to 200 °C at a rate of 3 °C / min and kept at this temperature for 2 h for activation treatment to obtain nitrogen-doped porous carbon fibers.

[0055] Comparative Example 1

[0056] The commercially available PAN-based carbon fibers of model T300 are cut into short pieces of 3 mm, and 5 g of the cut PAN-based carbon fibers are placed in a Soxhlet extractor. Then, 400 mL of acetone is added to the Soxhlet extractor, and the mixture is treated under reflux (80 °C) for 4 h. After the reflux ends, the carbon fibers after the immersion treatment are washed thoroughly and dried to obtain desized carbon fibers.

[0057] 0.25 g of the nitrogen-doped porous carbon fibers prepared in Example 1 and 0.25 g of the desized carbon fibers prepared in Comparative Example 1 are taken respectively. 400 g of water is added to the nitrogen-doped porous carbon fibers and the desized carbon fibers respectively to obtain mixture 1 and mixture 2. The mixture 1 and mixture 2 are placed in an ultrasonic machine and ultrasonicated for 2 h under the frequency condition of 25 kHz. After the ultrasonication is completed, stirring is started, and the mixture 1 and 2 are stirred at a rotation speed of 1500 rpm for 2 h to disperse them. The photos of the fiber dispersion conditions after ultrasonication and stirring are as Figure 4 shown (the left side is the nitrogen-doped porous carbon fibers prepared in Example 1, and the right side is the desized carbon fibers prepared in Comparative Example 1). From Figure 4It can be seen that the nitrogen-doped porous carbon fibers prepared in Example 1 are uniformly dispersed in water without bundling or flocculation, and no obvious sedimentation is found after dispersion. However, the desized carbon fibers in Comparative Example 1 are difficult to be uniformly dispersed in water, and a large amount of fiber flocs can be seen with the naked eye, and they quickly settle to the bottom of the beaker due to gravity. This is because the surface of the carbon fibers after modification, calcination and activation treatment has high roughness, a large specific surface area, high surface chemical activity and good hydrophilicity, and it is not easy for the carbon fibers to form flocs, thus improving the dispersibility of the nitrogen-doped porous carbon fibers in water.

[0058] The BET tests were respectively carried out on the desized carbon fibers prepared in Comparative Example 1, the nitrogen-doped porous carbon fibers prepared in Example 1 and Example 2, and the test results are as Figure 5 shown. It can be Figure 5 seen that the specific surface area of the desized carbon fibers prepared in Comparative Example 1 is 0.3 m 2 / g, the specific surface area of the nitrogen-doped porous carbon fibers prepared in Example 1 is 15.3 m 2 / g, and the pore size range is between 1 and 100 nm; the specific surface area of the nitrogen-doped porous carbon fibers prepared in Example 2 is 30 m 2 / g. This is because the nitrogen-doped porous carbon fibers prepared in Example 1 and Example 2 have a microporous structure on the surface, thus increasing the specific surface area of the carbon fibers.

[0059] Although the above embodiments have made a detailed description of the present invention, they are only a part of the embodiments of the present invention, not all embodiments. People can also obtain other embodiments according to these embodiments without creative efforts, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A preparation method of nitrogen-doped porous carbon fiber, comprising the following steps: Mix carbon fiber, organic polymer, boric acid and water, and carry out modification treatment to obtain modified carbon fiber; The carbon fiber is PAN-based carbon fiber; the organic polymer is polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer; Carry out annealing and calcination treatment on the modified carbon fiber to obtain porous carbon fiber; Mix the porous carbon fiber with a nitrogen-containing compound and carry out activation treatment to obtain nitrogen-doped porous carbon fiber; the nitrogen-containing compound includes one or more of urea, ammonium chloride and melamine.

2. The preparation method according to claim 1, characterized in that the length of the carbon fiber is 2-6 mm.

3. The preparation method according to claim 1, characterized in that the mass ratio of the carbon fiber, organic polymer, boric acid and water is 2-10: 2-5: 0.1-0.75: 100-300.

4. The preparation method according to claim 1 or 3, characterized in that the temperature of the modification treatment is 60-80 °C and the time is 2-4 h.

5. The preparation method according to claim 1, characterized in that the temperature of the annealing and calcination treatment is 400-800 °C, the heat preservation time is 1-10 h; the heating rate to the temperature of the annealing and calcination is 1-10 °C / min.

6. The preparation method according to claim 1, characterized in that the mass ratio of the porous carbon fiber to the nitrogen-containing compound is 0.5-3:

1.

7. The preparation method according to claim 1 or 6, characterized in that the temperature of the activation treatment is 200-500 °C, the heat preservation time is 1-6 h; the heating rate to the temperature of the activation treatment is 1-10 °C / min.

8. The nitrogen-doped porous carbon fiber prepared by the preparation method according to any one of claims 1 to 7 has a specific surface area of 10 to 50 m 2 / g and a pore diameter of 1 to 100 nm.

9. A carbon paper, the preparation raw material includes the nitrogen-doped porous carbon fiber described in claim 8.

10. Application of the carbon paper described in claim 9 in a fuel cell.

Images