Carbon fiber paper for fuel cell and preparation method thereof

Abstract

The present invention relates to the field of fuel cell technology, and in particular, to a carbon fiber paper for fuel cells and a preparation method thereof. The carbon fiber paper for fuel cells comprises carbon fibers, 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. A carbonized substrate is obtained by placing a carbon fiber substrate in a phenolic resin impregnation solution for primary impregnation, drying, hot pressing and carbonization; and then placing the carbonized substrate in a secondary impregnation solution containing polyimide and / or asphalt for secondary impregnation, drying, hot pressing and curing, and graphitizing at 1500-2000°C. The present invention sequentially impregnates hard carbon and soft carbon on the outer side of the carbon fibers through secondary impregnation, and forms a glassy carbon-graphite carbon core-shell structure through carbonization and graphitization. The obtained carbon fiber paper has both high flexural strength and excellent electrical conductivity, and can reduce the graphitization temperature and energy consumption.

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 for the gas diffusion layer of fuel cells. Its preparation process mainly includes the steps of wet papermaking, resin impregnation, hot pressing and graphitization. First, a 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 and curing are performed. The linear resin molecules are transformed into a three-dimensional structure, which increases the bulk density, improves the mechanical strength, and enhances the dimensional stability. The cured carbon paper substrate is carbonized and graphitized (2000-3000℃) under the protection of an inert gas atmosphere. The three-dimensional resin is decomposed by heat, the carbon content is increased, 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 the 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 cost of increased energy costs, and the increase in graphitization temperature will also lead to reduced bending strength and increased brittleness, which is prone to damage during processing and use. The current carbon fiber paper is difficult to take into account both mechanical strength and conductive properties, and has high energy consumption.

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

[0005] The first objective of the present invention is to provide a carbon fiber paper for fuel cells that addresses the technical problem of existing fuel cell carbon fiber paper struggling to balance mechanical strength and electrical conductivity. By wrapping carbonized carbon fiber with glassy carbon, which is then carbonized with phenolic resin, and then with graphite carbon, forming a glassy carbon-graphite carbon core-shell structure, the resulting highly conductive carbon fiber paper exhibits both high flexural strength and excellent electrical conductivity, achieving a balanced balance of mechanical strength and electrical conductivity.

[0006] The second object 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. The inner layer of glassy carbon and the outer layer of graphite carbon work synergistically, greatly improving the electrical conductivity while maintaining high bending strength, and significantly reducing the graphitization temperature, 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 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 covered with a glassy carbon layer, and at least a portion of the outer surface of the glassy carbon layer is covered 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 pitch 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 impregnated substrate is hot pressed and carbonized to obtain a carbonized substrate;

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

[0015] S4. The secondary impregnation substrate is hot-pressed and cured, 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 2 wt%-40 wt%.

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

[0018] Preferably, the initial dipping 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 0.5 wt%-10 wt%.

[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 dipping time of the secondary dipping 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 minutes.

[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 fibers in the carbon fiber substrate through primary impregnation, hot pressing curing and carbonization to play a mechanical supporting role; then, through secondary impregnation, hot pressing curing and graphitization, a graphite carbon wrapping structure of the glassy carbon outer layer is formed, thereby improving the conductive properties of the carbon fiber paper and making the prepared carbon fiber paper have the advantages of high mechanical strength and good conductivity; the preparation process is simple, and on the basis of maintaining high flexural strength and excellent conductivity, the graphitization temperature is significantly reduced, thereby reducing energy consumption and production costs. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0030] Figure 1 Schematic diagram of the carbon fiber-glass carbon layer-graphite carbon layer structure in the carbon fiber paper provided by 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. Those who do not specify specific conditions in the embodiments are carried out according to normal conditions or the conditions recommended by the manufacturer. Those whose reagents or instruments are not specified by the manufacturer are 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 fuel cells, comprising carbon fibers, at least a portion of the outer surface of the carbon fibers being coated with a glassy carbon layer, and at least a portion of the outer surface of the glassy carbon layer being coated with a graphite carbon layer.

[0033] The present invention wraps a glassy carbon layer and a graphite carbon layer on the outside of the carbon fiber in sequence, 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 performance evaluation of carbon fiber paper includes two directions, namely, plane resistivity and vertical resistivity. When carbon fiber paper is used for the gas diffusion layer of a fuel cell, the plane of the fiber paper is in contact with the membrane electrode and the bipolar plate plane. Reducing the resistivity of the carbon fiber paper, especially the reduction of the plane resistivity, means lower energy loss and higher efficiency. Glassy carbon maintains a local body structure and has 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 positions of glassy carbon and graphite carbon have 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 highly conductive carbon fiber paper with higher plane resistivity, which is suitable for preparing high-performance fuel cell gas diffusion layers.

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

[0036] A second aspect of the present invention provides a method for preparing the carbon fiber paper for fuel cells 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 impregnated substrate is hot pressed and carbonized to obtain a carbonized substrate;

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

[0040] S4. The secondary impregnated 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 decomposes and releases 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 The hybridized, regular conjugated π bond structure is conducive to improving conductivity. The method of the present invention, which first impregnates phenolic resin hard carbon, then impregnates easily graphitized soft carbon, and then sequentially wraps a glassy carbon layer and a graphite carbon layer on the outside of the carbon fiber, can reduce the graphitization temperature, allowing the glassy carbon to maintain a high mechanical strength, providing mechanical support for the carbon fiber paper. The outermost graphite carbon is used to improve the conductivity of the carbon fiber paper, resulting in the obtained carbon fiber paper having the advantages of high mechanical strength and high conductivity, especially in reducing the plane resistivity.

[0042] In order to obtain higher electrical conductivity, traditional phenolic resin needs to be heat-treated at a high temperature of 3000°C to complete graphitization, which consumes high energy, reduces mechanical strength, makes carbon paper brittle, makes processing difficult, and has a low yield. However, the present invention uses polyimide and / or asphalt as graphitized carbon precursors during secondary impregnation. These precursors are easier to graphitize and can significantly reduce the graphitization temperature. The graphitization process can be completed at 1500-2000°C, obtaining better electrical conductivity. When used in combination 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 among 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 the 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 includes the following steps:

[0047] Take 2-15mm long PAN-based carbon fibers, use polyvinyl alcohol and polyacrylamide as dispersants, use water as solvent, disperse and slurry them, control the sizing concentration at 0.00005%-0.0005%, wet-form them into paper sheets, and dry them to obtain a 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 10-30 min, for example, it can be any point value among 10 min, 15 min, 20 min, 25 min, 30 min, or a range value consisting of any two point values.

[0050] Carbonization at 800-1000℃ 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 is 0.5wt%-10wt%, for example, it can be any value among 0.5wt%, 1wt%, 3wt%, 5wt%, 8wt%, 10wt% or a range value consisting of any two value points.

[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 solution is asphalt, for example, medium- and high-temperature coal asphalt or petroleum asphalt, and the solvent of the secondary impregnation solution 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 15-150s. 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.

[0055] In some specific embodiments of the present invention, in step S3, 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 (VGCF), and graphite powder.

[0056] The graphite nucleating agent itself has a graphite-like structure and can serve as a crystal nucleus during the graphitization crystallization process, inducing crystallization and accelerating the formation of a relatively complete graphitized structure. Under the same graphitization conditions, the addition of the graphite nucleating agent provided by the present invention does not negatively affect the mechanical strength of the carbon fiber paper. Furthermore, the addition of the graphite nucleating agent can achieve better electrochemical performance at a lower graphitization temperature. This lowering 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 is 0.5%-5% of the mass of the graphitized carbon precursor. For example, it can be any value among 0.5%, 1%, 2%, 3%, 4%, 5%, or a range value consisting of any two value points.

[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, 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 describes some embodiments of the present invention in detail with reference to 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 prepared, and polyvinyl alcohol and polyacrylamide (1:1) were used as dispersants and water was used as solvent. The dispersion and beating process was carried out for 20 minutes, and the sizing concentration was controlled at 0.0003%. The paper sheets were formed by wet method, and dried at 120°C for 20 minutes to obtain 30 g / m 2 Carbon fiber substrate.

[0062] Example 1

[0063] S1. A 15 wt% phenolic resin-methanol-ethanol solution (phenolic resin impregnation solution) was prepared using methanol and ethanol as solvents (volume ratio of 1:1). A carbon fiber substrate was immersed in the solution for 60 seconds. Excess phenolic resin was squeezed out and the substrate was dried at 120°C to obtain a primary impregnated substrate.

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

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

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

[0067] Example 2

[0068] S1. A 15 wt% phenolic resin-methanol-ethanol solution (phenolic resin impregnation solution) was prepared using methanol and ethanol as solvents (volume ratio of 1:1). The carbon fiber substrate was immersed in the solution for 60 seconds. The excess phenolic resin was squeezed out and the substrate was dried at 120°C to obtain a primary impregnated substrate.

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

[0070] S3. A secondary impregnation solution was prepared, wherein graphene was selected as a graphite nucleating agent. The graphite nucleating agent was added to a 3wt% polyimide resin-DMAC solution, and the graphite nucleating agent was 1wt% of the polyimide resin content. The secondary impregnation solution was stirred evenly using a high shear disperser, and the carbonized substrate was placed in the secondary impregnation solution and immersed for 60s. After removal, the excess resin was squeezed out and dried at 150°C to obtain a secondary impregnated substrate.

[0071] S4. The secondary impregnated substrate was hot-pressed and cured using a flat plate press with a pressure of 2t at 190°C for 30 min. The substrate was then graphitized at 1800°C for 30 min under a nitrogen atmosphere to produce 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 the graphite nucleating agent in this example, and the graphene in Example 2 is replaced with an equal amount of VGCF. The other conditions are the same as those in Example 2.

[0076] Example 5

[0077] S1. A 15 wt% phenolic resin-methanol-ethanol solution (phenolic resin impregnation solution) was prepared using methanol and ethanol as solvents (volume ratio of 1:1). A carbon fiber substrate was immersed in the solution for 60 seconds. Excess phenolic resin was squeezed out and the substrate was dried at 120°C to obtain a primary impregnated substrate.

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

[0079] S3. A secondary impregnation solution was prepared by grinding the asphalt into a powder, passing it through a 100-mesh sieve, and adding an ethanol solvent to prepare a 10 wt% asphalt-ethanol impregnation solution. Graphene was selected as a graphite nucleating agent, and the amount of the graphite nucleating agent was 1 wt% of the asphalt content. The graphite nucleating agent was added to the above 10 wt% asphalt-ethanol impregnation solution and stirred uniformly to obtain a secondary impregnation solution. The carbonized substrate was placed in the secondary impregnation solution and immersed for 60 s. After removal, the excess asphalt was squeezed out and dried at 150 ° C to obtain a secondary impregnation substrate.

[0080] S4. The secondary impregnated substrate was hot-pressed and cured using a flat plate press with a pressure of 2t at 190°C for 30 min. The substrate was then graphitized at 1800°C for 30 min under a nitrogen atmosphere to produce 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 the graphite nucleating agent in this example, and the graphene in Example 5 is replaced with an equal amount of VGCF. The other conditions are the same as those in Example 5.

[0083] Comparative Example 1

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

[0085] S1. Using methanol and ethanol as solvents (volume ratio of 1:1), a 15 wt% phenolic resin-methanol-ethanol solution was prepared as an impregnation solution. A carbon fiber substrate was immersed in the impregnation solution for 60 seconds. Excess phenolic resin was squeezed out and the substrate was dried at 120°C to obtain a primary impregnated substrate.

[0086] S2. The primary impregnated substrate was hot-pressed and cured using a flatbed press at 2t pressure and 160°C for 30 min to obtain cured carbon fiber paper. The substrate was then graphitized at 1800°C for 60 min under a nitrogen atmosphere to produce 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 one-time impregnation, the specific steps are as follows:

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

[0092] S2. The primary impregnated substrate was hot-pressed and cured using a flatbed press at 2t pressure and 190°C for 30 min to obtain cured carbon fiber paper. The carbon fiber paper for fuel cells was then graphitized at 1800°C for 60 min under a nitrogen atmosphere.

[0093] Comparative Example 4

[0094] The specific steps for secondary impregnation of phenolic resin are as follows:

[0095] S1. A 15 wt% phenolic resin-methanol-ethanol solution (phenolic resin impregnation solution) was prepared using methanol and ethanol as solvents (volume ratio of 1:1). A carbon fiber substrate was immersed in the solution for 60 seconds. Excess phenolic resin was squeezed out and the substrate was dried at 120°C to obtain a primary impregnated substrate.

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

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

[0098] S4. The secondary impregnated substrate was hot-pressed and cured using a flat plate press at 160°C and 2t pressure for 30 min. The substrate was then graphitized at 1800°C for 30 min under a nitrogen atmosphere to produce 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 polyimide resin was prepared as the primary impregnation solution, and the carbon fiber substrate was immersed in the solution for 60 s. After squeezing out the excess polyimide resin, the substrate was dried at 150 ° C to obtain a primary impregnated substrate.

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

[0103] S3. Using the phenolic resin impregnation solution in Example 1 as the secondary impregnation solution, the carbonized substrate was immersed in the secondary impregnation solution for 60s, extruded and dried at 120 ° C to obtain a secondary impregnation substrate;

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

[0105] Comparative Example 6

[0106] Comparative Example 6 is similar to Example 1, with the only difference being 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 (the test method was in accordance with GB / T 20042.7-2014 Proton Exchange Membrane Fuel Cell Part 7: Carbon Paper Characteristics Test Method). 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 conductive performance. The carbon fiber paper obtained by the method of the present invention has significantly improved conductivity while maintaining its mechanical strength, compared with the traditional carbon fiber paper formed by only impregnating phenolic resin and then hot pressing, curing, carbonization, and graphitization.

[0113] Comparative Example 2 is only impregnated with phenolic resin hard carbon. After high-temperature treatment at 2600°C, although the conductivity is improved compared with Comparative Example 1, 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 requirements. Compared with Comparative Example 3, Example 1 has a decreased conductivity, but a significantly improved mechanical strength. The bending strength is nearly 5 times that of Comparative Example 3, and the overall performance is better.

[0115] Compared with Comparative Example 5, Example 1 has a slightly improved flexural strength, a reduced vertical resistivity and a reduced planar resistivity, especially a significant reduction in the planar resistivity, and better overall performance, indicating that the impregnation sequence has a greater influence on the conductivity of the carbon fiber paper, and the core-shell structure of the glassy carbon-graphite carbon is more complete, which can better exert 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 Examples 2 and 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 Examples 2 and 4 are further reduced than those of Example 1, indicating that the addition of the graphite nucleating agent is conducive 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 while maintaining high flexural strength, thereby obtaining better comprehensive performance, so that the prepared carbon fiber paper has both mechanical strength and conductivity.

[0118] The flexural strength of Example 3 reached 35.1 MPa, which is much higher than that of other samples, and the electrical conductivity is better than that of Example 1, indicating that the addition of graphite nucleating agent accelerates the graphitization process. 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 using asphalt instead of polyimide can also form a core-shell structure to form 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 using specific embodiments, it should be appreciated that the above embodiments are merely intended to illustrate the technical solutions of the present invention rather than to limit them. 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 deviate from the essence of the corresponding technical solutions within 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 fuel cells, characterized in that: The carbon fiber comprises a glassy carbon layer on at least a portion of its outer surface, and a graphite carbon layer on at least a portion of its outer surface; The glassy carbon layer is obtained by carbonizing phenolic resin in an inert atmosphere; The graphite carbon layer is obtained by carbonizing polyimide in an inert atmosphere; The method for preparing the carbon fiber paper for fuel cells comprises the following steps: S1. After the carbon fiber, dispersant and water are dispersed and beaten, a carbon fiber substrate is obtained by a wet papermaking process, and 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 impregnated substrate is hot pressed and carbonized to obtain a carbonized substrate; S3. The carbonized substrate is placed in a secondary impregnation solution for secondary impregnation, wherein the graphitized carbon precursor in the secondary impregnation solution comprises a polyimide, and the secondary impregnation solution further comprises a graphite nucleating agent. After impregnation, the substrate is dried to obtain a secondary impregnation substrate; S4. The secondary impregnation substrate is hot-pressed and cured, and graphitized at 1500-2000° C. to obtain the carbon fiber paper for fuel cells.

2. The carbon fiber paper for fuel cells according to claim 1, characterized in that: In step S1, at least one of the following features is included: (1) The concentration of the 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.

3. The carbon fiber paper for fuel cells according to claim 1, wherein: The carbonization is carried out under an inert atmosphere, the carbonization temperature is 800-1000° C., and the carbonization time is 10-30 minutes.

4. The carbon fiber paper for fuel cells according to claim 1, wherein: 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) The immersion time of the secondary immersion is 15-150s.

5. The carbon fiber paper for fuel cells according to claim 1, wherein: The graphite nucleating agent includes at least one of graphene, nano-conductive carbon fiber, and graphite powder.

6. The carbon fiber paper for fuel cells according to claim 1, 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.

7. The carbon fiber paper for fuel cells according to claim 1, wherein: The graphitization is carried out under an inert atmosphere, and the graphitization time is 10-40 minutes.

Images