Carbon fiber paper for fuel cell and preparation method thereof

Abstract

The invention relates to the technical field of fuel cells, in particular to carbon fiber paper for a fuel cell and a preparation method of the carbon fiber paper. The carbon fiber paper for the fuel cell comprises carbon fibers, at least part of the surface of the outer side of the carbon fibers is coated with a glassy carbon layer, and at least part of the surface of the outer side of the glassy carbon layer is coated with a graphite carbon layer. The preparation method comprises the following steps: placing a carbon fiber base material in a phenolic resin impregnation liquid, and carrying out primary impregnation, drying, hot-pressing curing and carbonization to obtain a carbonized base material; and placing the carbonized base material in a secondary impregnation liquid containing polyimide and / or asphalt for secondary impregnation, drying, hot-pressing and curing, and graphitizing at 1500-2000 DEG C to obtain the product. The hard carbon and the soft carbon are sequentially impregnated on the outer side of the carbon fiber through secondary impregnation, a glassy carbon-graphite carbon core-shell structure is formed through carbonization and graphitization, the obtained carbon fiber paper has high bending strength and excellent conductivity, the graphitization temperature can be reduced, and energy consumption can be reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of fuel cells, and in particular to a carbon fiber paper for fuel cells and a preparation method thereof. Background Art

[0002] Carbon fiber paper is often used as the base material of the gas diffusion layer of fuel cells. Its preparation process mainly includes wet papermaking, resin impregnation, hot pressing curing and graphitization. First, the carbon fiber substrate is obtained by wet papermaking, and the carbon fiber substrate is wet impregnated with a phenolic resin solution. After the solvent is dried, hot pressing curing is performed. The linear resin molecules are transformed into a three-dimensional structure, the bulk density is increased, the mechanical strength is improved, and the dimensional stability is enhanced; the cured carbon paper substrate is carbonized and graphitized (2000-3000℃) under the protection of an inert gas atmosphere. The bulk resin is decomposed by heat, the carbon content is increased, and the pore structure is improved, and finally a highly conductive carbon fiber paper is formed.

[0003] After carbonization, phenolic resin becomes glassy carbon. The conductivity of glassy carbon is worse than that of graphite carbon, resulting in poor conductivity of carbon fiber paper as a whole. Glassy carbon will form a relatively complete graphite conductive structure only under high temperature conditions above 2600°C. The improvement of conductivity comes at the expense of increased energy costs, and the increase in graphitization temperature will also lead to reduced bending strength, increased brittleness, and easy damage during processing and use. The current carbon fiber paper is difficult to take into account both mechanical strength and conductivity, and has high energy consumption.

[0004] In view of this, the present invention is proposed. Summary of the invention

[0005] The first purpose of the present invention is to provide a carbon fiber paper for fuel cells to solve the technical problem that the carbon fiber paper for fuel cells in the prior art is difficult to balance mechanical strength and conductivity. By wrapping glassy carbon carbonized with phenolic resin on the outside of the carbon fiber, and wrapping graphite carbon on the outside of the glassy carbon to form a glassy carbon-graphite carbon core-shell structure, the obtained high-conductivity carbon fiber paper has both high bending strength and excellent conductivity, and can balance mechanical strength and conductivity.

[0006] The second purpose of the present invention is to provide a method for preparing carbon fiber paper for fuel cells as described above, wherein a carbon fiber-glass carbon-graphite carbon coating structure is obtained by secondary impregnation and carbonization, wherein the inner layer of glass carbon and the outer layer of graphite carbon act synergistically, greatly improving the electrical conductivity while maintaining high bending strength, and the graphitization temperature is significantly reduced, thereby reducing energy consumption and production costs.

[0007] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are particularly adopted:

[0008] A carbon fiber paper for a fuel cell comprises carbon fibers, wherein at least a portion of the outer surface of the carbon fibers is coated with a glassy carbon layer, and at least a portion of the outer surface of the glassy carbon layer is coated with a graphite carbon layer.

[0009] Preferably, the glassy carbon layer is obtained by carbonizing phenolic resin in an inert atmosphere.

[0010] Preferably, the graphite carbon layer is obtained by carbonizing polyimide and / or asphalt in an inert atmosphere.

[0011] A method for preparing the carbon fiber paper for fuel cells as described above comprises the following steps:

[0012] S1. The carbon fiber substrate is placed in a phenolic resin impregnation solution for initial impregnation, and then dried to obtain a primary impregnated substrate;

[0013] S2. The primary impregnation substrate is subjected to hot pressing and carbonization to obtain a carbonized substrate;

[0014] S3. placing the carbonized substrate in a secondary impregnation solution for secondary impregnation, wherein the graphitized carbon precursor in the secondary impregnation solution comprises polyimide and / or asphalt, and drying after impregnation to obtain a secondary impregnation substrate;

[0015] S4. The secondary impregnation substrate is cured by hot pressing and graphitized at 1500-2000° C. to obtain the carbon fiber paper for fuel cells.

[0016] Preferably, the concentration of the phenolic resin in the phenolic resin impregnation solution is 2wt%-40wt%.

[0017] Preferably, the solvent of the phenolic resin impregnation solution includes methanol and / or ethanol.

[0018] Preferably, the initial impregnation time is 15-150s.

[0019] Preferably, the carbonization is carried out under an inert atmosphere, the carbonization temperature is 800-1000° C., and the carbonization time is 10-30 min.

[0020] Preferably, in the secondary impregnation solution, the concentration of the graphitized carbon precursor is 0.5wt%-10wt%.

[0021] Preferably, in the secondary impregnation solution, the graphitized carbon precursor is polyimide, and the solvent used includes at least one of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methylpyrrolidone.

[0022] Preferably, in the secondary impregnation solution, the graphitized carbon precursor is asphalt, and the solvent used includes ethanol and / or toluene.

[0023] Preferably, the secondary impregnation time is 15-150s.

[0024] Preferably, the secondary impregnation solution further contains a graphite nucleating agent, and the graphite nucleating agent includes at least one of graphene, nano-conductive carbon fiber, and graphite powder.

[0025] Preferably, in the secondary impregnation solution, the content of the graphite nucleating agent is 0.5%-5% of the mass of the graphitized carbon precursor.

[0026] Preferably, the graphitization is carried out under an inert atmosphere, and the graphitization time is 10-40 min.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The present invention wraps a glassy carbon layer on the carbon fiber in the carbon fiber substrate through primary impregnation, hot pressing curing and carbonization to play a mechanical supporting role; and then forms a graphite carbon wrapping structure of the glassy carbon outer layer through secondary impregnation, hot pressing curing and graphitization, thereby improving the conductive performance of the carbon fiber paper, so that the prepared carbon fiber paper has the advantages of high mechanical strength and good conductivity; the preparation process is simple, and on the basis of maintaining high bending strength and excellent conductivity, the graphitization temperature is significantly reduced, and the energy consumption and production cost are reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0030] Figure 1 A schematic diagram of the structure of carbon fiber-glass carbon layer-graphite carbon layer in the carbon fiber paper provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0031] The technical scheme of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments, but it will be understood by those skilled in the art that the following described embodiments are part of embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. If specific conditions are not specified in the embodiments, they are carried out according to normal conditions or conditions recommended by the manufacturer. If the manufacturer is not specified in the reagents or instruments used, they are all conventional products that can be purchased commercially.

[0032] like Figure 1 As shown, the first aspect of the present invention provides a carbon fiber paper for a fuel cell, comprising carbon fiber, at least a portion of the outer surface of the carbon fiber is coated with a glassy carbon layer, and at least a portion of the outer surface of the glassy carbon layer is coated with a graphite carbon layer.

[0033] The present invention sequentially wraps a glassy carbon layer and a graphite carbon layer on the outside of the carbon fiber, uses the glassy carbon layer to play a mechanical supporting role, and improves the conductivity of the carbon fiber paper through the graphite carbon layer, so that the carbon fiber paper has both high bending strength and excellent conductivity. The conductivity evaluation of the carbon fiber paper includes two directions, namely, the plane resistivity and the vertical resistivity. When the carbon fiber paper is used for the gas diffusion layer of the fuel cell, the plane of the fiber paper is in contact with the membrane electrode and the bipolar plate plane, and the resistivity of the carbon fiber paper is reduced, especially the reduction of the plane resistivity, which means lower energy loss and higher efficiency. Glassy carbon maintains a local body structure, with high mechanical strength, but relatively poor conductivity. The outer layer is wrapped with graphite carbon, which will greatly improve the conductivity of the material, especially the plane conductivity. The relative position of glassy carbon and graphite carbon has a significant effect on the plane resistivity of the carbon fiber paper. The carbon fiber-glassy carbon layer-graphite carbon layer core-shell structure provided by the present invention is conducive to obtaining a high-conductivity carbon fiber paper with higher plane resistivity, which is suitable for preparing a high-performance fuel cell gas diffusion layer.

[0034] In some specific embodiments of the present invention, the glassy carbon layer is obtained by carbonizing a phenolic resin in an inert atmosphere.

[0035] In some specific embodiments of the present invention, the graphite carbon layer is obtained by carbonizing polyimide and / or asphalt in an inert atmosphere.

[0036] A second aspect of the present invention provides a method for preparing the carbon fiber paper for a fuel cell as described above, comprising the following steps:

[0037] S1. The carbon fiber substrate is placed in a phenolic resin impregnation solution for initial impregnation, and then dried to obtain a primary impregnated substrate;

[0038] S2. The primary impregnation substrate is hot pressed and carbonized to obtain a carbonized substrate;

[0039] S3. placing the carbonized substrate in a secondary impregnation solution for secondary impregnation, wherein the graphitized carbon precursor in the secondary impregnation solution comprises polyimide and / or asphalt, and drying after impregnation to obtain a secondary impregnation substrate;

[0040] S4. The secondary impregnation substrate is hot-pressed and cured, and graphitized at 1500-2000° C. to obtain the final carbon fiber paper for fuel cells.

[0041] The method of the present invention is to impregnate phenolic resin for the first time, and then perform low-temperature carbonization after curing, so that the phenolic resin is decomposed to release small molecules such as H2O, CO, CO2, CH4, etc., and the C content reaches more than 90%, forming a glassy carbon structure, improving conductivity and enhancing mechanical strength. In addition, by adopting low-temperature carbonization, the phenolic resin in the carbon fiber paper has residual oxygen-containing functional groups such as hydroxyl groups, which is equivalent to surface modification of the carbon fiber, reducing the surface energy, and facilitating the uniformity of subsequent impregnation; the graphitization precursor of the secondary impregnation is selected from soft carbon, which is easy to form a graphite structure after high-temperature treatment, and the carbon atoms are SP 2 Hybridization and regular conjugated π bond structure are conducive to the improvement of conductivity. The method of the present invention first impregnates phenolic resin hard carbon, then impregnates easily graphitized soft carbon, and sequentially wraps a glassy carbon layer and a graphite carbon layer on the outside of the carbon fiber, which can reduce the graphitization temperature, so that the glassy carbon maintains a high mechanical strength and plays a mechanical supporting role in the carbon fiber paper. The graphite carbon wrapped on the outermost side is used to improve the conductivity of the carbon fiber paper, so that the prepared carbon fiber paper has the advantages of high mechanical strength and high conductivity, especially in reducing the plane resistivity, and has a better effect.

[0042] In order to obtain higher electrical conductivity, traditional phenolic resin is used as impregnation resin and needs to be heat treated at a high temperature of 3000°C to complete graphitization, which consumes high energy and leads to decreased mechanical strength, brittle carbon paper, difficult processing and low yield. However, the present invention uses polyimide and / or asphalt as graphitized carbon precursor during secondary impregnation. The precursor is easier to graphitize and can significantly reduce the graphitization temperature. The graphitization process can be completed at 1500-2000°C to obtain better conductive properties. When used in conjunction with a graphite nucleating agent, the graphitization temperature can be further reduced. The reduction in graphitization temperature reduces energy consumption and manufacturing costs while maintaining the mechanical strength of the carbon paper.

[0043] In some embodiments, typically but not limiting, for example, the graphitization temperature in step S4 can be any value of 1500° C., 1600° C., 1700° C., 1800° C., 1900° C., 2000° C. or a range consisting of any two values.

[0044] In some specific embodiments of the present invention, the concentration of phenolic resin in the phenolic resin impregnation solution used is 2wt%-40wt%, for example, it can be any point value among 2wt%, 5wt%, 10wt%, 15wt%, 20wt%, 25wt%, 30wt%, 35wt%, 40wt% or a range value consisting of any two point values.

[0045] In some specific embodiments of the present invention, the solvent in the phenolic resin impregnation solution includes methanol and / or ethanol.

[0046] In some specific embodiments of the present invention, in step S1, the method for preparing the carbon fiber substrate used comprises the following steps:

[0047] Take 2-15mm long PAN-based carbon fiber, use polyvinyl alcohol and polyacrylamide as dispersants, use water as solvent, disperse and slurry, control the sizing concentration at 0.00005%-0.0005%, wet process paper sheet forming, and dry to obtain carbon fiber substrate.

[0048] In some specific embodiments of the present invention, in step S1, the initial immersion time is 15-150s, for example, it can be any point value among 15s, 30s, 60s, 90s, 120s, 150s or a range value consisting of any two point values.

[0049] In some specific embodiments of the present invention, in step S2, carbonization is carried out under an inert atmosphere, and the carbonization temperature is 800-1000°C, for example, it can be any point value among 800°C, 850°C, 900°C, 950°C, 1000°C, or a range value consisting of any two point values; the carbonization time is 10-30min, for example, it can be any point value among 10min, 15min, 20min, 25min, 30min, or a range value consisting of any two point values.

[0050] Carbonization at 800-1000°C can not only decompose the phenolic resin to a carbon content of more than 90% to form a glassy carbon structure, but also leave oxygen-containing functional groups such as hydroxyl groups on the phenolic resin on the carbon fiber substrate, which is beneficial to the uniformity of the secondary impregnation.

[0051] In some specific embodiments of the present invention, the concentration of the graphitized carbon precursor in the secondary impregnation solution used is 0.5wt%-10wt%, for example, it can be any point value among 0.5wt%, 1wt%, 3wt%, 5wt%, 8wt%, 10wt% or a range value consisting of any two point values.

[0052] In some specific embodiments of the present invention, the graphitized carbon precursor in the secondary impregnation solution is polyimide, and the solvent of the secondary impregnation solution includes at least one of N,N-dimethylacetamide (DMAC), N,N-dimethylformamide (DMF), and N-methylpyrrolidone (NMP).

[0053] In some specific embodiments of the present invention, the graphitized carbon precursor in the secondary impregnation liquid is asphalt, for example, medium-high temperature coal asphalt or petroleum asphalt, and the solvent of the secondary impregnation liquid includes ethanol and / or toluene.

[0054] In some specific embodiments of the present invention, in step S3, the immersion time of the secondary immersion is 15-150 s. For example, it can be any point value among 15 s, 30 s, 60 s, 90 s, 120 s, 150 s or a range value consisting of any two point values.

[0055] In some specific embodiments of the present invention, in step S3, the secondary impregnation solution used further contains a graphite nucleating agent, and the graphite nucleating agent used includes at least one of graphene, nano-conductive carbon fiber (VGCF), and graphite powder.

[0056] The graphite nucleating agent itself is a graphite-like structure, which can act as a crystal nucleus in the graphitization crystallization process, play an inducing crystallization role, and accelerate the formation of a relatively complete graphitization structure. Under the same graphitization conditions, adding the graphite nucleating agent provided by the present invention will not have a negative impact on the mechanical strength of the carbon fiber paper; and after adding the graphite nucleating agent, better electrochemical performance can be obtained at a lower graphitization temperature, and the reduction of the graphitization temperature can significantly improve the flexural strength of the carbon fiber paper.

[0057] In some specific embodiments of the present invention, the content of the graphite nucleating agent in the secondary impregnation solution used is 0.5%-5% of the mass of the graphitized carbon precursor. For example, it can be any point value among 0.5%, 1%, 2%, 3%, 4%, 5%, or a range value composed of any two point values.

[0058] In some specific embodiments of the present invention, in step S4, graphitization is carried out under an inert atmosphere, and the graphitization time is 10-40 min, for example, it can be any point value among 10 min, 20 min, 30 min, 40 min, or a range value consisting of any two point values.

[0059] In some specific embodiments of the present invention, the inert atmospheres used in the carbonization process in step S2 and the graphitization process in step S4 are independently selected from nitrogen atmosphere and / or argon atmosphere.

[0060] The following is a detailed description of some embodiments of the present invention in conjunction with specific examples. The raw materials used in the examples can be purchased from the market unless otherwise specified.

[0061] All the examples and comparative examples used the same carbon fiber substrate. The manufacturing process of the carbon fiber substrate was as follows: PAN-based carbon fibers with a length of 2-15 mm were taken, polyvinyl alcohol and polyacrylamide (1:1) were used as dispersants, water was used as a solvent, and the dispersion and beating were performed for 20 minutes. The sizing concentration was controlled at 0.0003%, and the paper sheets were formed by a wet method. After drying at 120°C for 20 minutes, 30 g / m 2 Carbon fiber substrate.

[0062] Example 1

[0063] S1. Using methanol and ethanol as solvents (volume ratio 1:1), phenolic resin is prepared into a 15wt% phenolic-methanol-ethanol solution (phenolic resin impregnation solution), and the carbon fiber substrate is immersed in the above solution for 60s, and the excess phenolic resin is squeezed out and then dried at 120°C to obtain a primary impregnation substrate;

[0064] S2. The primary impregnation substrate was hot-pressed and cured using a flat press, the pressure was set to 2t, and hot-pressed at 160°C for 30min to obtain a cured carbon fiber paper; under nitrogen atmosphere protection, carbonized at 1000°C for 30min to obtain a carbonized substrate;

[0065] S3. The carbonized substrate was immersed in a 3 wt % polyimide resin-DMAC solution (secondary impregnation solution), taken out after 60 s, and the excess resin was squeezed out and dried at 150 ° C to obtain a secondary impregnation substrate;

[0066] S4. The secondary impregnation substrate was hot-pressed and cured using a flat plate press with the pressure set to 2t, hot-pressed at 190°C for 30min, and graphitized at 1800°C for 30min under nitrogen atmosphere to obtain carbon fiber paper for fuel cells.

[0067] Example 2

[0068] S1. Using methanol and ethanol as solvents (volume ratio 1:1), phenolic resin was prepared into a 15wt% phenolic-methanol-ethanol solution (phenolic resin impregnation solution), and the carbon fiber substrate was immersed in the above solution for 60s, and the excess phenolic was squeezed out and dried at 120°C to obtain a primary impregnation substrate;

[0069] S2. The primary impregnation substrate was hot-pressed and cured using a flat press, the pressure was set to 2t, and hot-pressed at 160°C for 30min to obtain a cured carbon fiber paper; under nitrogen atmosphere protection, carbonized at 1000°C for 30min to obtain a carbonized substrate;

[0070] S3. A secondary impregnation solution is prepared, graphene is selected as a graphite nucleating agent, and the graphite nucleating agent is added to a 3wt% polyimide resin-DMAC solution, the graphite nucleating agent is 1wt% of the polyimide resin content, and a high shear disperser is used to stir the secondary impregnation solution, and the carbonized substrate is placed in the secondary impregnation solution and immersed for 60s. After taking it out, the excess resin is squeezed out and dried at 150°C to obtain a secondary impregnation substrate;

[0071] S4. The secondary impregnation substrate was hot-pressed and cured using a flat plate press with the pressure set to 2t, hot-pressed at 190°C for 30min, and graphitized at 1800°C for 30min under nitrogen atmosphere to obtain carbon fiber paper for fuel cells.

[0072] Example 3

[0073] Example 3 is similar to Example 2, with the only difference being that the graphitization temperature in step S4 is adjusted to 1600° C., and the other conditions are the same as those in Example 2.

[0074] Example 4

[0075] Example 4 is similar to Example 2, with the only difference being that VGCF is used as a graphite nucleating agent in this example, and the graphene in Example 2 is replaced with an equal amount of VGCF, and the other conditions are the same as those in Example 2.

[0076] Example 5

[0077] S1. Using methanol and ethanol as solvents (volume ratio 1:1), phenolic resin is prepared into a 15wt% phenolic-methanol-ethanol solution (phenolic resin impregnation solution), and the carbon fiber substrate is immersed in the above solution for 60s, and the excess phenolic resin is squeezed out and then dried at 120°C to obtain a primary impregnation substrate;

[0078] S2. The primary impregnation substrate was hot-pressed and cured using a flat press, the pressure was set to 2t, and hot-pressed at 160°C for 30min to obtain a cured carbon fiber paper; under nitrogen atmosphere protection, carbonized at 1000°C for 30min to obtain a carbonized substrate;

[0079] S3. Prepare a secondary impregnation solution, grind the asphalt into powder, pass it through a 100-mesh sieve, add ethanol solvent to prepare a 10wt% asphalt-ethanol impregnation solution, select graphene as a graphite nucleating agent, the amount of the graphite nucleating agent is 1wt% of the asphalt content, add the graphite nucleating agent to the above 10wt% asphalt-ethanol impregnation solution and stir evenly to obtain a secondary impregnation solution, place the carbonized substrate in the secondary impregnation solution and immerse it for 60s, take it out and squeeze out the excess asphalt, and dry it at 150°C to obtain a secondary impregnation substrate;

[0080] S4. The secondary impregnation substrate was hot-pressed and cured using a flat plate press with the pressure set to 2t, hot-pressed at 190°C for 30min, and graphitized at 1800°C for 30min under nitrogen atmosphere to obtain carbon fiber paper for fuel cells.

[0081] Example 6

[0082] Example 6 is similar to Example 5, with the only difference being that VGCF is used as a graphite nucleating agent in this example, and the graphene in Example 5 is replaced with an equal amount of VGCF, and the other conditions are the same as those in Example 5.

[0083] Comparative Example 1

[0084] The specific steps of one-time impregnation of phenolic resin are as follows:

[0085] S1. Using methanol and ethanol as solvents (volume ratio 1:1), phenolic resin is prepared into a 15wt% phenolic-methanol-ethanol solution as an impregnation liquid, and the carbon fiber substrate is immersed in the above impregnation liquid for 60s, and then the excess phenolic resin is squeezed out and dried at 120°C to obtain a primary impregnation substrate;

[0086] S2. Use a flat press to hot-press and cure the primary impregnated substrate, set the pressure to 2t, and hot-press at 160°C for 30 minutes to obtain cured carbon fiber paper; under the protection of a nitrogen atmosphere, graphitize at 1800°C for 60 minutes to obtain carbon fiber paper for fuel cells.

[0087] Comparative Example 2

[0088] Comparative Example 2 is similar to Comparative Example 1, except that the graphitization temperature in step S4 is adjusted to 2600° C., and the other conditions are the same as those in Comparative Example 1.

[0089] Comparative Example 3

[0090] Polyimide is impregnated once, and the specific steps are as follows:

[0091] S1. A 3 wt % DMAC solution of a polyimide resin is prepared as an impregnation liquid, and the carbon fiber substrate is immersed in the impregnation liquid for 60 s, and the excess polyimide resin is squeezed out and then dried at 150 ° C to obtain a primary impregnation substrate;

[0092] S2. Use a flat press to heat-press and cure the primary impregnation substrate, set the pressure to 2t, and hot-press at 190°C for 30 minutes to obtain cured carbon fiber paper; under the protection of a nitrogen atmosphere, graphitize at 1800°C for 60 minutes to obtain carbon fiber paper for fuel cells.

[0093] Comparative Example 4

[0094] Phenolic resin secondary impregnation, the specific steps are as follows:

[0095] S1. Using methanol and ethanol as solvents (volume ratio 1:1), phenolic resin is prepared into a 15wt% phenolic-methanol-ethanol solution (phenolic resin impregnation solution), and the carbon fiber substrate is immersed in the above solution for 60s, and the excess phenolic resin is squeezed out and then dried at 120°C to obtain a primary impregnation substrate;

[0096] S2. The primary impregnation substrate was hot-pressed and cured using a flat press, the pressure was set to 2t, and hot-pressed at 160°C for 30min to obtain a cured carbon fiber paper; under nitrogen atmosphere protection, carbonized at 1000°C for 30min to obtain a carbonized substrate;

[0097] S3. The carbonized substrate is placed in a phenolic resin impregnation solution (same as step S1) and immersed for 60s, and then extruded and dried at 120°C to obtain a secondary impregnated substrate;

[0098] S4. The secondary impregnation substrate was hot-pressed and cured using a flat plate press at 160° C. and 2t pressure for 30 min, and graphitized at 1800° C. for 30 min under nitrogen atmosphere protection to obtain carbon fiber paper for fuel cells.

[0099] Comparative Example 5

[0100] Change the impregnation order of phenolic resin and polyimide, the specific steps are as follows:

[0101] S1. A 3 wt % DMAC solution of a polyimide resin is prepared as a primary impregnation solution, and a carbon fiber substrate is immersed in the solution for 60 s, and then the excess polyimide resin is squeezed out and dried at 150° C. to obtain a primary impregnation substrate;

[0102] S2. The primary impregnation substrate was hot-pressed and cured using a flat press, the pressure was set to 2t, and hot-pressed at 190°C for 30min to obtain a cured carbon fiber paper; under nitrogen atmosphere protection, carbonized at 1000°C for 30min to obtain a carbonized substrate;

[0103] S3. The phenolic resin impregnation solution in Example 1 is used as the secondary impregnation solution, and the carbonized substrate is immersed in the secondary impregnation solution for 60s, and then dried at 120°C after extrusion to obtain a secondary impregnation substrate;

[0104] S4. The secondary impregnation substrate was hot-pressed and cured using a flat plate press at 160°C and 2t pressure for 30 minutes. Finally, the sample was placed under nitrogen atmosphere and graphitized at 1800°C for 30 minutes to obtain carbon fiber paper for fuel cells.

[0105] Comparative Example 6

[0106] Comparative Example 6 is similar to Example 1, except that the graphitization temperature in step S4 is adjusted to 2600° C., and the other conditions are the same as those in Example 1.

[0107] Test example

[0108] The flexural strength, plane resistivity and vertical resistivity of the carbon fiber paper for fuel cells prepared in each embodiment and each comparative example were tested respectively (the test method was in accordance with GB / T 20042.7-2014 Proton Exchange Membrane Fuel Cell Part 7: Carbon Paper Property Test Method), and the test results are shown in Table 1.

[0109] Table 1

[0110]

[0111]

[0112] It can be seen from the data in Table 1 that the flexural strength of Example 1 is basically the same as that of Comparative Example 1, indicating that the glassy carbon still plays a supporting role; the conductivity of Example 1 is significantly improved compared with Comparative Examples 1 and 4, indicating that the secondary impregnation of polyimide forms a graphite carbon layer on the surface of the glassy carbon, thereby improving the conductivity. The carbon fiber paper obtained by the method of the present invention, compared with the traditional carbon fiber paper formed by only impregnating phenolic resin through hot pressing, curing, carbonization, and graphitization, has significantly improved conductivity while maintaining its mechanical strength.

[0113] Comparative Example 2 is only impregnated with phenolic resin hard carbon. After high-temperature treatment at 2600°C, the conductive performance is improved compared with Comparative Example 1, but the bending strength is greatly reduced, which is only about 50% of that of Comparative Example 1. It is difficult to take into account both high mechanical strength and high conductivity. Compared with Comparative Example 2, Example 1 greatly reduces the carbonization temperature, saves energy costs, and enhances the bending strength. The conductive performance is basically the same, and the overall performance is better.

[0114] Comparative Example 3 is only impregnated with polyimide soft carbon. Although the obtained carbon fiber paper has excellent conductivity, its bending strength is very low and it is difficult to meet production needs. Compared with Comparative Example 3, Example 1 has decreased conductivity, but significantly improved mechanical strength. Its bending strength is nearly 5 times that of Comparative Example 3, and its comprehensive performance is better.

[0115] Compared with Comparative Example 5, Example 1 has a slightly improved flexural strength, and both the vertical resistivity and the plane resistivity are reduced, especially the plane resistivity is significantly reduced, and the overall performance is better, indicating that the impregnation order has a great influence on the conductivity of carbon fiber paper, and the core-shell structure of glassy carbon-graphite carbon is more complete, which can better play the synergistic effect of the two.

[0116] Compared with Example 1, Comparative Example 6 has improved conductivity, but due to the high graphitization temperature, energy consumption is significantly increased, resulting in increased production costs. In addition, due to the graphitization of the phenolic resin, it loses strength, resulting in a significant decrease in bending strength. It is easy to be damaged during the processing process, and the overall performance is poor.

[0117] The flexural strength of Example 2 and Example 4 is basically the same as that of Example 1, indicating that the addition of the two graphite nucleating agents has no negative effect on the flexural strength. At the same heat treatment temperature, the plane resistivity and vertical resistivity of Example 2 and Example 4 are further reduced than those of Example 1, indicating that the addition of the graphite nucleating agent is beneficial to the formation and improvement of the graphite structure. By adding the graphite nucleating agent to the secondary impregnation solution, the conductivity can be further improved on the basis of maintaining high flexural strength, and better comprehensive performance can be obtained, so that the prepared carbon fiber paper has both mechanical strength and conductivity.

[0118] The flexural strength of Example 3 reaches 35.1 MPa, which is much higher than that of other samples, and the conductivity is better than that of Example 1, indicating that the addition of graphite nucleating agent accelerates the process of graphitization. A better graphitization effect can be achieved by using a lower heat treatment temperature of 1600°C. A lower graphitization temperature is conducive to improving mechanical strength and reducing energy costs. Compared with Example 1, the mechanical strength and conductivity are improved while lowering the graphitization temperature, achieving a better overall effect.

[0119] Compared with Examples 2 and 4, the mechanical strength and conductivity of Examples 5 and 6 are basically the same, indicating that the use of asphalt instead of polyimide can also form a core-shell structure to form a high-strength, high-conductivity carbon fiber paper.

[0120] From the above data, it can be seen that the carbon fiber paper prepared by the method of the present invention has a low graphitization temperature, can reduce energy consumption and production costs, and the prepared carbon fiber paper has the advantages of high mechanical strength and good conductivity.

[0121] Although the present invention has been illustrated and described with specific embodiments, it should be appreciated that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents without departing from the spirit and scope of the present invention. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention. Therefore, this means that all such replacements and modifications within the scope of the present invention are included in the appended claims.

Claims

1. A carbon fiber paper for a fuel cell, characterized in that: The invention comprises carbon fiber, at least a part of the outer surface of the carbon fiber is covered with a glassy carbon layer, and at least a part of the outer surface of the glassy carbon layer is covered with a graphite carbon layer.

2. The carbon fiber paper for fuel cell according to claim 1, characterized in that: The glassy carbon layer is obtained by carbonizing phenolic resin in an inert atmosphere.

3. The carbon fiber paper for fuel cell according to claim 1, characterized in that: The graphite carbon layer is obtained by carbonizing polyimide and / or asphalt in an inert atmosphere.

4. The method for preparing carbon fiber paper for fuel cells according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1. The carbon fiber substrate is placed in a phenolic resin impregnation solution for initial impregnation, and then dried to obtain a primary impregnated substrate; S2. The primary impregnation substrate is subjected to hot pressing and carbonization to obtain a carbonized substrate; S3. placing the carbonized substrate in a secondary impregnation solution for secondary impregnation, wherein the graphitized carbon precursor in the secondary impregnation solution comprises polyimide and / or asphalt, and drying after impregnation to obtain a secondary impregnation substrate; S4. The secondary impregnation substrate is cured by hot pressing and graphitized at 1500-2000° C. to obtain the carbon fiber paper for fuel cells.

5. The method for preparing carbon fiber paper for fuel cell according to claim 4, characterized in that: In step S1, at least one of the following features is included: (1) The concentration of phenolic resin in the phenolic resin impregnation solution is 2wt%-40wt%; (2) The solvent of the phenolic resin impregnation solution includes methanol and / or ethanol; (3) The time of the initial immersion is 15-150s.

6. The method for preparing carbon fiber paper for fuel cell according to claim 4, characterized in that: The carbonization is carried out under an inert atmosphere, the carbonization temperature is 800-1000° C., and the carbonization time is 10-30 min.

7. The method for preparing carbon fiber paper for fuel cell according to claim 4, characterized in that: In step S3, at least one of the following features is included: (1) In the secondary impregnation solution, the concentration of the graphitized carbon precursor is 0.5wt%-10wt%; (2) In the secondary impregnation solution, the graphitized carbon precursor is polyimide, and the solvent used includes at least one of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methylpyrrolidone; (3) In the secondary impregnation solution, the graphitized carbon precursor is asphalt, and the solvent used includes ethanol and / or toluene; (4) The secondary impregnation time is 15-150s.

8. The method for preparing carbon fiber paper for fuel cell according to claim 4, characterized in that: The secondary impregnation solution also contains a graphite nucleating agent, and the graphite nucleating agent includes at least one of graphene, nano-conductive carbon fiber, and graphite powder.

9. The method for preparing carbon fiber paper for fuel cells according to claim 8, characterized in that: In the secondary impregnation solution, the content of the graphite nucleating agent is 0.5%-5% of the mass of the graphitized carbon precursor.

10. The method for preparing carbon fiber paper for fuel cell according to claim 4, characterized in that: The graphitization is carried out under an inert atmosphere, and the graphitization time is 10-40 minutes.

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