A carbon fiber substrate for fuel cells, a preparation method thereof, a carbon paper for fuel cells, and a preparation method thereof

Abstract

The present invention relates to the technical field of fuel cells, and in particular, to a carbon fiber substrate for fuel cells, a preparation method thereof, a green carbon paper, and a preparation method thereof. The preparation method of the carbon fiber substrate comprises the following steps: S1. Using MFC or a mixture of MFC and pre-oxidized silk pulp as reinforcing fibers, mixing the reinforcing fibers with water for beating to obtain a premixed slurry; S2. Mixing carbon fibers, the premixed slurry, and a dispersant uniformly to obtain a mixed slurry, and forming and drying the mixed slurry by a wet forming process to obtain the carbon fiber substrate. The present invention uses MFC or a mixture of MFC and pre-oxidized silk pulp as reinforcing fibers, and after beating, mixes them with carbon fibers and a dispersant for preparing the carbon fiber substrate, which can improve the dispersion of carbon fibers during the wet forming process, as well as the mechanical strength and conductivity of the carbon fiber substrate, and will not have a negative impact on the subsequent preparation of the green carbon paper.

Description

Technical Field

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

[0002] The working principle of a proton exchange membrane fuel cell (PEMFC) is that the fuel gas H2 reaches the anode catalyst layer through the flow channels provided by the gas diffusion layer, and an oxidation reaction occurs under the participation of the anode catalyst layer, generating protons H + and electrons. At the same time, O2 reaches the cathode catalyst layer after passing through the gas diffusion layer. Under the participation of the cathode catalyst, a cathode reduction reaction occurs with the protons passing through the proton exchange membrane and the electrons flowing back to the battery from the external circuit, generating water. The gas diffusion layer is an important component in the proton exchange membrane fuel cell, which plays a role in supporting the catalyst layer and providing channels for reaction gases and product water. At the same time, it also needs to have relatively good electrical conductivity and corrosion resistance under electrochemical reactions. Based on the requirements for the gas diffusion layer in terms of electrical conductivity, gas and water transmission, heat conduction, and high strength, the material currently used for the gas diffusion layer of fuel cells is carbon material. The carbon material has both relatively high electrical conductivity and strong anti-electrochemical corrosion performance and low cost, which are advantages that other materials do not have. However, the carbon material is relatively brittle and is not conducive to forming and processing, and it is also a relatively large difficulty to control its microstructure. Therefore, the research on the process method for preparing a gas diffusion layer with excellent performance using carbon materials has become the focus of research.

[0003] Carbon fiber substrates are the preferred materials for gas diffusion layers due to their mature manufacturing processes and stable performance. Currently, carbon fiber substrates for fuel cell gas diffusion layers are prepared by using carbon fibers or by mixing other fibers into carbon fibers through a wet forming method. Carbon fibers are generally fibers that have undergone high-temperature carbonization or even graphitization. Therefore, it is impossible to fibrillate and split the fibers during the beating stage like plant fibers or polymer fibers. Moreover, carbon fibers rarely contain non-carbon elements, have a smooth surface, low surface energy, few active groups, poor hydrophilicity, and are difficult to defibrate, which results in poor dispersion of carbon fibers in water and poor bonding ability between carbon fibers, thus leading to a decline in the mechanical properties of carbon fiber substrates for gas diffusion layers. Carbon fiber substrates for fuel cell gas diffusion layers must have appropriate mechanical strength for subsequent gas diffusion layer preparation processes, enabling the gas diffusion layer to independently support the catalyst layer and thus stabilizing the entire electrode structure. Currently, the mechanical strength of carbon fiber substrates is mainly enhanced by adding binders in the slurry or surface sizing. However, adding adhesives in the slurry due to their own characteristics will affect the dispersion effect of carbon fibers and the uniformity characteristics of carbon fiber substrates; the method of surface sizing to enhance carbon fiber substrates will result in poor impregnability, low resin sizing amount, and poor mechanical properties of raw carbon paper due to the presence of binders on the surface of carbon fiber substrates, thus affecting the assembly performance of gas diffusion layers. Therefore, it is of great significance to provide a preparation method for carbon fiber substrates for fuel cell gas diffusion layers and raw carbon paper with high physical strength, good conductivity, and good carbon fiber dispersion effect.

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

[0005] The first object of the present invention is to provide a preparation method for carbon fiber substrates for fuel cells, so as to solve the technical problems that the low physical strength of carbon fiber substrates for fuel cell gas diffusion layers leads to poor performance in gas diffusion layer preparation, and the traditional surface sizing method for enhancing carbon fiber substrates will result in a decline in the mechanical properties of raw carbon paper. The present invention uses microfibrillated cellulose (MFC) or a mixture of MFC and pre-oxidized pulp as reinforcing fibers, and after beating, it is mixed with carbon fibers and a dispersant for preparing carbon fiber substrates, which can improve the dispersion of carbon fibers during the wet forming process, as well as the mechanical strength and conductivity of carbon fiber substrates, and will not have a negative impact on the subsequent preparation of raw carbon paper.

[0006] The second object of the present invention is to provide a carbon fiber substrate for fuel cells, which is prepared by using the above-mentioned preparation method for carbon fiber substrates for fuel cells.

[0007] The third object of the present invention is to provide a preparation method of green carbon paper for fuel cells, which is obtained by impregnating a resin, drying, hot pressing and curing, and carbonizing and graphitizing the above-mentioned carbon fiber substrate. The green carbon paper prepared by the method of the present invention has high mechanical strength, low resistivity, and a simple preparation process, which is convenient for mass production.

[0008] The fourth object of the present invention is to provide a green carbon paper for fuel cells, which is prepared by the preparation method of the green carbon paper for fuel cells as described above.

[0009] In order to achieve the above objects of the present invention, the following technical solutions are specifically adopted:

[0010] A preparation method of a carbon fiber substrate for fuel cells includes the following steps:

[0011] S1. Using MFC or a mixture of MFC and pre-oxidized silk pulp as reinforcing fibers, mixing the reinforcing fibers with water for beating to obtain a premixed slurry;

[0012] S2. Mixing carbon fibers, the premixed slurry and a dispersant evenly to obtain a mixed slurry, and forming and drying the mixed slurry by a wet forming process to obtain the carbon fiber substrate.

[0013] A carbon fiber substrate for fuel cells is prepared by the preparation method of the carbon fiber substrate for fuel cells as described above.

[0014] A preparation method of green carbon paper for fuel cells includes the following steps:

[0015] Using the above-mentioned carbon fiber substrate for fuel cells, impregnating with resin, drying, hot pressing and curing, and carbonizing and graphitizing to obtain the green carbon paper.

[0016] A green carbon paper for fuel cells is prepared by the preparation method of the green carbon paper for fuel cells as described above.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] (1) By adding MFC or a mixture of MFC and pre-oxidized silk pulp as reinforcing fibers to the slurry in the present invention, and utilizing the characteristic that more hydrogen bonds are exposed after beating of MFC, which is distributed between carbon fibers and builds a bridge between carbon fibers, the carbon fibers with poor binding ability are more tightly combined due to the existence of microfibrillated cellulose, and the pre-oxidized silk pulp can be used as a button for carbon fiber lapping, thereby improving the physical strength of the carbon fiber substrate.

[0019] (2) The surface of MFC has a negative charge consistent with that of carbon fiber. Due to the mutual repulsion of like charges, the fibers are well dispersed. Compared with the traditional method of reinforcing carbon fiber substrates, the substrates prepared by the method of the present invention have higher uniformity, a simple process flow, and high feasibility.

[0020] (3) After beating, MFC can form a denser network structure, which can serve as the supporting structure of the resin. And after beating, the specific surface area of MFC increases, exposing more hydrophilic groups on the surface of the carbon fiber substrate, enhancing the adsorption of the resin, with good subsequent impregnability, an increase in the resin sizing amount. Moreover, the larger specific surface area can provide more active sites and interfacial interactions with conductive substances. And the added MFC has a certain pore structure, increasing the adsorption of conductive substances, thereby improving the mechanical strength and conductivity of the carbon paper.

[0021] (4) The carbon fiber substrate prepared by the present invention has high mechanical strength and high uniformity, can meet the preparation requirements of the subsequent carbon paper, and the carbon paper obtained after impregnating with resin, curing, and carbonization graphitization treatment has good mechanical properties and high conductivity, and can meet the requirement standards of the gas diffusion layer of fuel cells. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0023] Figure 1 It is the microscope pictures of MFC before and after being treated under different conditions provided by the embodiments of the present invention. Among them, a is the unbeaten MFC (50X), b is the MFC after adding water and beating (500X), c is the MFC impregnated in the pretreatment solution, freeze-dried, and then added water and beaten (500X), d is the MFC beaten after being ultrasonically treated in water (100X), and e and f are the MFC ultrasonically treated in the pretreatment solution, freeze-dried, and then added water and beaten (e 200X, f 500X);

[0024] Figure 2 It is the microscope pictures of the carbon fiber substrates provided by different embodiments and comparative examples of the present invention. Among them, a is Comparative Example 1 (200X), b is Example 1 (200X), c is Example 5 (200X), d is Example 6 (200X), and e is Example 4 (200X);

[0025] Figure 3Microscope pictures of the carbon fiber substrates impregnated with pure resin in different embodiments and comparative examples of the present invention. Among them, a is Comparative Example 3 (200X), b is Example 7 (200X), and c is Example 9 (200X);

[0026] Figure 4 Microscope pictures of the carbon fiber substrates impregnated with resin containing conductive substances in different embodiments and comparative examples of the present invention. Among them, a is Comparative Example 4 (200X), and b is Example 10 (200X);

[0027] Figure 5 Microscope pictures of the green carbon paper prepared by impregnating the carbon fiber substrates with pure resin in different embodiments and comparative examples of the present invention. Among them, a is Comparative Example 3 (200X), b is Example 7 (200X), and c is Example 9 (500X);

[0028] Figure 6 Microscope pictures of the green carbon paper prepared by impregnating the carbon fiber substrates with resin containing conductive substances in different embodiments and comparative examples of the present invention. Among them, a is Comparative Example 4 (200X), and b is Example 10 (200X). Detailed Description of the Invention

[0029] The technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the following described embodiments are some embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be construed as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present invention. For those conditions not specified in the embodiments, they shall be carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments not specified by the manufacturer can all be obtained through commercial purchases and are conventional products.

[0030] The first aspect of the present invention provides a preparation method of a carbon fiber substrate for fuel cells, comprising the following steps:

[0031] S1. Using MFC or a mixture of MFC and pre-oxidized silk pulp as reinforcing fibers, mixing the reinforcing fibers with water for beating to obtain a premixed slurry;

[0032] S2. Mixing carbon fiber, the premixed slurry and a dispersant evenly to obtain a mixed slurry, and forming and drying the mixed slurry by a wet forming process to obtain the carbon fiber substrate.

[0033] Microfibrillated cellulose (MFC) refers to a disordered network composed of a large number of linear nanoscale microfibers in an irregular manner. It is a cellulose derivative. MFC has characteristics such as a high specific surface area, high strength, and high crystallinity. Compared with ordinary cellulose, MFC has a larger specific surface area and exposes a large number of polar hydroxyl groups. During the preparation and processing of microfibrillated cellulose, the fiber surface carries a certain negative charge. After adding it to the slurry, due to the principle of mutual repulsion between ions with the same charge, it repels the negatively charged carbon fibers, enabling the fibers in the slurry to disperse from each other, thereby improving the uniformity of the slurry and simultaneously enhancing its physical properties. In addition, MFC has a large crystallinity, water retention value, specific surface area, stability, and dispersibility, which can improve the physical properties of paper. When preparing carbon fiber substrates for fuel cells, through the hydration between the hydroxyl groups of microfibrillated cellulose, the hydrogen bond binding effect between carbon fibers can be increased, improving the binding force of the fibers, and directly improving the physical strength of the carbon fiber substrate. This strength can meet the requirements of the impregnation and curing process.

[0034] In the present invention, by adding MFC or a mixture of MFC and pre-oxidized silk pulp as reinforcing fibers to the slurry, using the characteristic that more hydrogen bonds are exposed after the fibrillation of MFC during beating, it is distributed between carbon fibers and builds a bridge between carbon fibers. The carbon fibers with poor binding ability are more tightly bound due to the presence of microfibrillated cellulose, thereby improving the binding physical strength of the carbon fiber substrate. The pre-oxidized silk pulp can act as a button for carbon fiber lapping, binding the lapping of carbon fiber to carbon fiber and further enhancing its physical strength. The pre-oxidized silk pulp makes supplementary laps on the original structure, and after carbonization, the electrical conductivity of the pre-oxidized silk pulp increases, optimizing the conductive network of the green carbon paper and making the conductive performance of the green carbon paper better. The synergistic effect of MFC and pre-oxidized silk pulp can not only significantly improve the mechanical strength of the carbon fiber substrate and the green carbon paper, but also improve the electrical conductivity of the green carbon paper.

[0035] The purpose of beating the reinforcing fibers before mixing in the present invention is to make MFC form a more dense network structure, which serves as a supporting structure for the resin, increases the specific surface area of MFC to provide more active sites and interfacial interactions with conductive substances, and at the same time exposes more hydrophilic groups on the surface of the carbon fiber substrate, increasing its adsorption of conductive substances and resin, thereby improving the mechanical strength and electrical conductivity of the green carbon paper.

[0036] By Figure 1As can be seen from Figures a and b, the unbeaten MFC fibers exhibit a relatively complete, slender morphology, with a certain length and width, a relatively uniform length distribution, relatively independent fibers, no obvious interweaving and entanglement, a smooth fiber surface, no obvious damage and fracture marks, and a disordered fiber arrangement without a specific direction and pattern; while the beaten MFC shows a certain degree of fracture and refinement, the fiber surface appears worn and fuzzed, the surface becomes rough, and the specific surface area increases, which is caused by the mechanical force during the beating process. The fiber arrangement becomes more disordered, and the fibers have intertwined and wound together to form a more complex network structure.

[0037] In some specific embodiments of the present invention, in step S1, before preparing the premixed slurry, it further includes the step of performing a first pretreatment or a second pretreatment on the reinforcing fibers, that is, using the pretreated reinforcing fibers to prepare the premixed slurry;

[0038] The steps of the first pretreatment include: preparing a pretreatment solution containing a protonic acid, an organic solvent and water, immersing the reinforcing fibers in the pretreatment solution, and then performing solid-liquid separation and freeze-drying; or, placing the reinforcing fibers in the pretreatment solution and performing ultrasonic treatment, and after the ultrasonic treatment is completed, performing solid-liquid separation and freeze-drying;

[0039] The steps of the second pretreatment include: placing the reinforcing fibers in water and performing ultrasonic treatment, and after the ultrasonic treatment is completed, performing solid-liquid separation and freeze-drying.

[0040] Using the protonic acid in the pretreatment solution to perform protonic acid doping on the microfibrillated cellulose can protonate functional groups such as hydroxyl groups in the microfibrillated cellulose, and can increase the conductivity of the substrate; the microfibrillated cellulose after ultrasonic treatment is more refined after beating and has a stronger binding force with the carbon fiber. As Figure 1 can be seen from Figures b and c, the morphology of the MFC after beating only by protonic acid soaking treatment is not much different from that of the directly beaten MFC; while Figure 1 as can be seen from Figures d, e and f, the MFC ultrasonically activated in water or in the pretreatment solution shows a higher degree of fibrillation after beating, with more fiber filaments exposed, better fiber swelling degree, a rougher surface, and more fuzzing, bringing the fiber network of the MFC to the extreme state and making its application performance reach the best.

[0041] In the present invention, the protonation treatment can improve the conductivity of the substrate, the ultrasonic treatment can improve the fibrillation degree of the beaten MFC and improve its binding strength with the carbon fiber, and performing ultrasonic treatment in the pretreatment solution can achieve a better synergistic effect.

[0042] The organic solvent in the pretreatment solution of the present invention is mainly used to adjust the polarity and solubility of the reaction system to make it have good proton transfer ability.

[0043] In some preferred embodiments of the present invention, before preparing the premixed slurry, the reinforcing fiber is subjected to a first pretreatment, and the first pretreatment specifically includes the following steps: placing MFC in a pretreatment solution, ultrasonically activating for 40 - 60 min, then adding pre-oxidized silk pulp, and continuing ultrasonic treatment for 30 - 60 min, followed by solid-liquid separation and freeze-drying to obtain the pretreated reinforcing fiber.

[0044] Blending MFC with pre-oxidized silk pulp after ultrasonic activation in the pretreatment solution shows that the pre-oxidized silk pulp loading of the prepared carbon fiber substrate is larger, which can better optimize the conductive network structure of the carbonized paper, resulting in better electrical conductivity of the carbonized paper. The present invention improves the electrical conductivity of the carbon fiber substrate by adding pretreated microfibrillated cellulose and pre-oxidized silk pulp to the slurry. This is because the hydroxylation of pretreated microfibrillated cellulose is protonated, increasing the conductivity of the substrate; and further improving the physical strength of the carbon fiber substrate. One reason is that the microfibrillated cellulose after ultrasonic treatment becomes finer after beating and has a stronger binding force with the carbon fiber. The other reason is that the pre-oxidized silk pulp with a certain particle size and length becomes the "button" at the lap joints of the carbon fibers.

[0045] In some embodiments, typically but not restrictively, for example, before adding the pre-oxidized silk pulp, the ultrasonic activation time of MFC in the pretreatment solution can be any value among 40 min, 45 min, 50 min, 55 min, 60 min or a range value composed of any two of these values; after adding the pre-oxidized silk pulp, the time for continuing ultrasonic treatment can be any value among 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min or a range value composed of any two of these values.

[0046] In some specific embodiments of the present invention, the ultrasonic power of the first pretreatment and the second pretreatment is 250 - 300 W. For example, it can be independently selected from any value among 250 W, 260 W, 270 W, 280 W, 290 W, 300 W or a range value composed of any two of these values; the treatment temperature is 30 - 50 °C. For example, it can be independently selected from any value among 30 °C, 35 °C, 40 °C, 45 °C, 50 °C or a range value composed of any two of these values.

[0047] In some specific embodiments of the present invention, the concentration of H + in the pretreatment solution is 0.15 - 2 mol / L. For example, it can be 0.15 mol / L, 0.5 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L or a range value composed of any two of these values; the amount of protonic acid is controlled by the concentration of H + in the pretreatment solution.

[0048] In some specific embodiments of the present invention, the protonic acid used includes hydrochloric acid and / or sulfuric acid.

[0049] In some specific embodiments of the present invention, the organic solvent used includes ethanol or acetone, and its main function is to adjust the polarity and solubility of the system and improve the proton transfer ability.

[0050] In some specific embodiments of the present invention, in the pretreatment liquid, the mass ratio of the organic solvent to water is 1:1 - 3:1. For example, it can be any value among 1:1, 1.5:1, 2:1, 2.5:1, 3:1 or any range value composed of any two of these values; to avoid impurities, as an example, the water used is pure water.

[0051] In some specific embodiments of the present invention, in step S1, the rotational speed of beating is 5 - 30 m / s (linear velocity). For example, it can be any value among 5 m / s, 10 m / s, 15 m / s, 20 m / s, 25 m / s, 30 m / s or any range value composed of any two of these values; the beating time is 5 - 20 min. For example, it can be any value among 5 min, 10 min, 15 min, 20 min or any range value composed of any two of these values. The beating treatment can break and refine the MFC, and the fibers show surface wear and fuzzing, making the surface rough, increasing the specific surface area, causing the fibers to intertwine and entangle with each other, forming a complex network structure, and improving the bonding strength with the carbon fiber.

[0052] In some specific embodiments of the present invention, before step S2, it further includes a step of performing a third pretreatment on the carbon fiber, that is, using the carbon fiber after the third pretreatment to mix with the premixed slurry and the dispersant to prepare a mixed slurry. The third pretreatment includes the following steps: performing air pre-oxidation treatment on the carbon fiber in an air atmosphere, and the temperature of the air pre-oxidation treatment is from 300 to 600 °C. For example, it can be any value among 300 °C, 400 °C, 500 °C, 600 °C or any range value composed of any two of these values; the treatment time is 30 - 150 min. For example, it can be any value among 30 min, 60 min, 90 min, 120 min, 150 min or any range value composed of any two of these values.

[0053] In some specific embodiments of the present invention, in step S2, the stirring rotational speed during the mixing process is 5 - 30 m / s (linear velocity). For example, it can be any value among 5 m / s, 10 m / s, 15 m / s, 20 m / s, 25 m / s, 30 m / s or any range value composed of any two of these values; the mixing time is 5 - 30 min. For example, it can be any value among 5 min, 10 min, 20 min, 30 min or any range value composed of any two of these values.

[0054] In some specific embodiments of the present invention, in step S2, the mixed slurry is prepared under the conditions of 20 - 30 °C and a relative humidity of 20% - 50%.

[0055] In some specific embodiments of the present invention, in step S2, in the mixed slurry, the mass concentration of carbon fiber is 0.1% - 0.5%. For example, it can be any value among 0.1%, 0.2%, 0.3%, 0.4%, 0.5% or a range value composed of any two of these values.

[0056] In some specific embodiments of the present invention, in step S2, the mass percentage of MFC in carbon fiber is 5% - 10%. For example, it can be any value among 5%, 6%, 7%, 8%, 9%, 10% or a range value composed of any two of these values. If the amount of MFC is too small, the strengthening effect is not good; if the amount is too large, the conductivity of the carbon fiber substrate will decrease. Therefore, it is necessary to reasonably control the dosage range of MFC.

[0057] In some specific embodiments of the present invention, in step S2, the mass percentage of pre - oxidized silk pulp in carbon fiber is 0% - 10%. For example, it can be any value among 0%, 2%, 4%, 6%, 8%, 10% or a range value composed of any two of these values.

[0058] In some specific embodiments of the present invention, in step S2, in the mixed slurry, the mass concentration of the dispersant is 0.1% - 0.5%. For example, it can be any value among 0.1%, 0.2%, 0.3%, 0.4%, 0.5% or a range value composed of any two of these values.

[0059] In some specific embodiments of the present invention, in step S2, the dispersant includes at least one of polysorbate, polyurethane, and hydroxypropyl cellulose.

[0060] The second aspect of the present invention provides a carbon fiber substrate for fuel cells, which is prepared by using the preparation method of the carbon fiber substrate for fuel cells described in any one of the foregoing embodiments. The carbon fiber substrate provided by the present invention has good uniformity and high mechanical strength, can meet the preparation requirements of subsequent carbon paper, and compared with traditional strengthening methods, improves the conductivity of the carbon fiber substrate and carbon paper.

[0061] The third aspect of the present invention provides a preparation method of carbon paper for fuel cells, including the following steps:

[0062] Using the carbon fiber substrate described in the foregoing embodiments, through impregnation with resin, drying, hot - press curing, and carbonization and graphitization treatment, carbon paper is obtained.

[0063] In some specific embodiments of the present invention, the impregnating resin may be a phenolic resin or a phenolic resin containing a conductive substance. As an example, the conductive substance may be graphite powder with a particle size of 0.5 - 30 μm. In some embodiments, the phenolic resin accounts for 15wt% - 25wt% of the impregnating solution; if a conductive substance is contained, the conductive substance accounts for 5wt% - 20% of the impregnating solution.

[0064] In some specific embodiments of the present invention, the pressure for hot pressing and curing is 2.0 - 4.0 MPa. For example, it can be any value among 2.0 MPa, 2.5 MPa, 3.0 MPa, 3.5 MPa, 4 MPa or a range value composed of any two of these values; the temperature is 120 - 200 °C. For example, it can be any value among 120 °C, 150 °C, 180 °C, 200 °C or a range value composed of any two of these values; the time is 2 - 10 min. For example, it can be any value among 2 min, 4 min, 6 min, 8 min, 10 min or a range value composed of any two of these values.

[0065] In some specific embodiments of the present invention, the temperature for carbonization and graphitization treatment is 1600 - 2200 °C. For example, it can be any value among 1600 °C, 1700 °C, 1900 °C, 2200 °C or a range value composed of any two of these values; the treatment time is 60 - 120 min.

[0066] The fourth aspect of the present invention provides a carbon paper for fuel cells, which is prepared by using the preparation method of the carbon paper for fuel cells described in any one of the foregoing embodiments. Compared with the carbon paper obtained by the traditional method, the carbon paper provided by the present invention has significantly improved tensile strength and flexural strength, and significantly reduced planar resistivity and vertical resistivity, and both mechanical strength and conductivity have been improved.

[0067] The following will describe in detail some embodiments of the present invention with reference to specific examples. The raw material substances used in the examples can be obtained through commercial purchase without special instructions.

[0068] Example 1

[0069] This example provides a carbon fiber substrate for fuel cells, and the preparation steps are as follows:

[0070] S1. Put MFC into water and carry out premixing and beating under the condition that the stirring linear velocity is 20 m / s for 10 min to obtain a premixed slurry;

[0071] Carbon fiber pretreatment: Air-oxidize the pre-prepared short carbon fibers in a muffle furnace. The length of the short carbon fibers is 6 mm, the treatment temperature is 500 °C, and the treatment time is 120 min;

[0072] S2. Under the conditions of a temperature of 25 °C and a relative humidity of 40%, mix the pretreated carbon fiber, the premixed slurry, and polysorbate. Under the condition of a stirring linear velocity of 10 m / s, stir and mix for 5 min to uniformly mix the slurry, obtaining a mixed slurry in which the carbon fiber and microfibrillated cellulose are uniformly dispersed. Among them, the addition amount of polysorbate is 0.1 wt% of the mixed slurry, the addition amount of carbon fiber is 0.1 wt% of the mixed slurry, and the addition amount of MFC is 5% of the mass of the carbon fiber;

[0073] Form the above mixed slurry by a wet forming process and obtain a carbon fiber substrate for fuel cells after drying.

[0074] Example 2

[0075] The preparation method of Example 2 is similar to that of Example 1, with the only difference being that the addition amount of MFC is 10% of the mass of the carbon fiber, and the other conditions are the same as those of Example 1.

[0076] Example 3

[0077] This example provides a carbon fiber substrate for fuel cells, and the preparation steps are as follows:

[0078] S1. Put MFC and pre-oxidized silk pulp into water and perform premixing and beating under the condition of a stirring linear velocity of 20 m / s for 10 min to obtain a premixed slurry;

[0079] Carbon fiber pretreatment: Perform air oxidation treatment on the prepared short-cut carbon fiber in a muffle furnace. The length of the short-cut carbon fiber is 6 mm, the treatment temperature is 500 °C, and the treatment time is 120 min;

[0080] S2. Under the conditions of a temperature of 25 °C and a relative humidity of 40%, mix the pretreated carbon fiber, the premixed slurry, and polysorbate. Under the condition of a stirring linear velocity of 10 m / s, stir and mix for 5 min to uniformly mix the slurry, obtaining a mixed slurry in which the carbon fiber, microfibrillated cellulose, and pre-oxidized silk pulp are uniformly dispersed. Among them, the addition amount of polysorbate is 0.1 wt% of the mixed slurry, the addition amount of carbon fiber is 0.1 wt% of the mixed slurry, the addition amount of MFC is 5% of the mass of the carbon fiber, and the addition amount of pre-oxidized silk pulp is 8% of the mass of the carbon fiber;

[0081] Form the above mixed slurry by a wet forming process and obtain a carbon fiber substrate for fuel cells after drying.

[0082] Example 4

[0083] This example provides a carbon fiber substrate for fuel cells, and the preparation steps are as follows:

[0084] S1. Enhancement fiber pretreatment: Prepare a pretreatment solution with ethanol, hydrochloric acid, and pure water. The concentration of hydrochloric acid in the pretreatment solution is 1 mol / L, and the mass ratio of ethanol to pure water is 1:1. Place MFC in the pretreatment solution, and ultrasonically activate it for 40 min at 30 °C with an ultrasonic power of 250 W. Then add pre-oxidized pulp, and continue to ultrasonically treat it for 60 min under the same conditions. Then, filter and freeze-dry to obtain the pretreated enhancement fiber.

[0085] Beating: Put the pretreated enhancement fiber into water, and perform premixing beating under the condition that the stirring linear velocity is 20 m / s for 10 min to obtain a premixed slurry.

[0086] Carbon fiber pretreatment: Oxidize the prepared short-cut carbon fiber in a muffle furnace. The length of the short-cut carbon fiber is 6 mm, the treatment temperature is 500 °C, and the treatment time is 120 min.

[0087] S2. Under the conditions of a temperature of 25 °C and a relative humidity of 40%, mix the pretreated carbon fiber, the premixed slurry, and polysorbate. Under the condition that the stirring linear velocity is 10 m / s, stir and mix for 5 min to uniformly mix the slurry, obtaining a mixed slurry in which carbon fiber, microfibrillated cellulose, and pre-oxidized pulp are uniformly dispersed. Among them, the addition amount of polysorbate is 0.1 wt% of the mixed slurry, the addition amount of carbon fiber is 0.1 wt% of the mixed slurry, the addition amount of MFC is 5% of the mass of carbon fiber, and the addition amount of pre-oxidized pulp is 8% of the mass of carbon fiber.

[0088] Form the above mixed slurry by a wet forming process, and obtain a carbon fiber substrate for fuel cells after drying.

[0089] Example 5

[0090] The preparation method of Example 5 is similar to that of Example 4, and the only difference is that when pretreating the enhancement fiber, ultrasonic treatment is not applied, and only soak it in the pretreatment solution, then filter and freeze-dry to obtain the pretreated enhancement fiber; the remaining preparation conditions are the same as those of Example 4.

[0091] Example 6

[0092] The preparation method of Example 6 is similar to that of Example 4, and the only difference is that when pretreating the enhancement fiber, the pretreatment solution is replaced with pure water, and the other conditions are the same as those of Example 4.

[0093] Example 7

[0094] This example provides a green carbon paper for fuel cells, and the preparation steps are as follows:

[0095] The carbon fiber substrate prepared in Example 1 was impregnated with phenolic resin. The concentration of phenolic resin in the impregnating solution was 25 wt%. After drying, it was hot-pressed and cured at 3.0 MPa and 150 °C for 10 min, and then carbonized and graphitized at 1900 °C for 60 min under the protection of nitrogen atmosphere to obtain green carbon paper.

[0096] Example 8

[0097] This example provides a green carbon paper for fuel cells, which is prepared by using the carbon fiber substrate prepared in Example 3, and the preparation conditions of the green carbon paper are the same as those in Example 7.

[0098] Example 9

[0099] This example provides a green carbon paper for fuel cells, which is prepared by using the carbon fiber substrate prepared in Example 4, and the preparation conditions of the green carbon paper are the same as those in Example 7.

[0100] Example 10

[0101] This example provides a green carbon paper for fuel cells, which uses the carbon fiber substrate prepared in Example 4 and is impregnated with a phenolic resin impregnating solution containing graphite powder. The concentration of phenolic resin is 25 wt%, the concentration of graphite powder is 10%, and the particle size of graphite powder is 10 μm; the remaining process conditions are the same as those in Example 9.

[0102] Example 11

[0103] This example provides a green carbon paper for fuel cells, which is prepared by using the carbon fiber substrate prepared in Example 5, and the preparation conditions of the green carbon paper are the same as those in Example 7.

[0104] Example 12

[0105] This example provides a green carbon paper for fuel cells, which is prepared by using the carbon fiber substrate prepared in Example 6, and the preparation conditions of the green carbon paper are the same as those in Example 7.

[0106] Comparative Example 1

[0107] This comparative example provides a carbon fiber substrate for fuel cells. The difference from Example 1 is that MFC premixed slurry is not added. The specific preparation steps are as follows:

[0108] S1. Carbon fiber pretreatment: The prepared short-cut carbon fibers were subjected to air oxidation treatment in a muffle furnace. The length of the short-cut carbon fibers was 6 mm, the treatment temperature was 500 °C, and the treatment time was 120 min;

[0109] S2. Under the conditions of a temperature of 25°C and a relative humidity of 40%, the pretreated carbon fiber and polysorbate are mixed. Under the condition of a stirring linear velocity of 10 m / s, the mixture is stirred for 5 min to uniformly mix the slurry, obtaining a mixed slurry with uniformly dispersed carbon fibers. Among them, the addition amount of polysorbate is 0.1 wt% of the mixed slurry, and the addition amount of carbon fiber is 0.1 wt% of the mixed slurry;

[0110] The above-mentioned mixed slurry is formed by a wet forming process and dried to obtain a carbon fiber substrate for fuel cells.

[0111] Comparative Example 2

[0112] After preparing the carbon fiber substrate according to the method in Comparative Example 1, its surface is sized for strengthening. The sizing process is as follows: Dextrin is made into a slurry by adding water, coated on the carbon fiber substrate, and dried; the sizing amount is 3.0 g / m 2 .

[0113] Comparative Example 3

[0114] This example provides a green carbon paper for fuel cells, which is prepared by using the carbon fiber substrate in Comparative Example 1, and the preparation conditions of the green carbon paper are the same as those in Example 7.

[0115] Comparative Example 4

[0116] This example provides a green carbon paper for fuel cells, which is prepared by using the carbon fiber substrate in Comparative Example 1, and the preparation conditions of the green carbon paper are the same as those in Example 10.

[0117] Comparative Example 5

[0118] This example provides a green carbon paper for fuel cells, which is prepared by using the sized and strengthened carbon fiber substrate in Comparative Example 2, and the preparation conditions of the green carbon paper are the same as those in Example 7.

[0119] Test Example

[0120] (1) Microscopic observation was carried out on the carbon fiber substrates prepared in Examples 1, 4, 5, 6 and Comparative Example 1, as Figure 2As shown in the figure, for the carbon fiber substrate prepared without adding MFC in Comparative Example 1, the carbon fibers are relatively distinct from each other, and there is only the mutual overlap of carbon fibers (Figure a); for the carbon fiber substrate prepared with MFC added in Example 1, there is an approximately transparent MFC fiber network connecting between carbon fibers (Figure b); there is no significant difference between the carbon fiber substrate prepared by only soaking and pretreating the reinforcing fibers with a pretreatment solution containing a protonic acid in Example 5 and the carbon fiber substrate directly beaten in Example 1 (Figure b and Figure c); for the carbon fiber substrate prepared with MFC after ultrasonic activation treatment in Example 6 and Example 4, the formed MFC fiber network slightly increases, and there are relatively more transparent MFC network connections in the carbon fiber substrate formed by combining the two methods (Figure d and Figure e). This is because there are a large number of hydroxyl groups on the surface of the MFC molecular chain after beating, increasing the reaction activity and enhancing the mutual connection with carbon fibers. Ultrasonic activation intensifies this phenomenon, further increasing the mechanical strength of the carbon fiber substrate.

[0121] (2) Observe the sizing amount of the impregnated and dried carbon fiber substrates in Examples 7, 9, 10 and Comparative Examples 3, 4 under a microscope, as Figure 3 and Figure 4 shown. Compared with the carbon fiber substrate without adding MFC, the sizing amount of the carbon fiber substrate with MFC added is relatively more, and the sizing amount of the carbon fiber substrate with pretreated MFC and pre-oxidized pulp added is even more, and correspondingly, more conductive substances are adsorbed. This is because the complex network structure of MFC and the increase in its surface active groups improve the adsorption effect on resins and conductive substances.

[0122] (3) Observe the green carbon paper obtained in Examples 7, 9, 10 and Comparative Examples 3, 4 under a microscope, as Figure 5 and Figure 6 shown. Similarly, compared with the green carbon paper without adding MFC, the green carbon paper with MFC has more residual carbon of resin, and the green carbon paper with pretreated MFC has even more residual carbon, and correspondingly, the residual carbon of the resin containing conductive substances also increases.

[0123] (4) Test the mechanical strength and resistivity of the carbon fiber substrates prepared in Examples 1 - 6 and Comparative Examples 1, 2. The test method is determined according to the national standard GB / T20042.7 - 2014: "Proton Exchange Membrane Fuel Cells - Part 7: Test Methods for the Characteristics of Carbon Paper", and the formation uniformity test of the carbon fiber substrate is determined according to the enterprise standard Q / CZSO13 - 1997 "ZDD - 1 Paper Formation Tester". The test results are shown in Table 1.

[0124] Table 1

[0125]

[0126] As can be seen from the data in Table 1, the evenness of the carbon fiber substrate after adding MFC is improved; the tensile strength of the carbon fiber substrate without adding MFC in Comparative Example 1 is only 0.25 MPa, and the tensile strengths of the carbon fiber substrates after adding MFC in Example 1 and Example 2 are 5.12 MPa and 10.34 MPa respectively. When the addition amount accounts for 5% of the carbon fiber, the strength effect of surface sizing in Comparative Example 2 can be achieved, and the same trend is observed for the flexural strength; in terms of electrical properties, the resistivity of the carbon fiber substrate with MFC added in Example 1 is significantly lower than that of the carbon fiber substrate with surface sizing in Comparative Example 2. Among them, the in-plane resistivity is reduced by about 61.47%, and the vertical resistivity is reduced by about 70.32%.

[0127] From the data in Examples 4-6, it can be seen that the carbon fiber substrates prepared after pretreatment of the reinforcing fibers have higher evenness, and different pretreatment methods will affect the properties of the carbon fiber substrates. Using ultrasonic activation can make MFC more refined, with more exposed hydroxyl groups, increasing the connection between them and with the carbon fibers. Adding pre-oxidized silk pulp can bind the lap joints of carbon fibers to carbon fibers, and the more refined MFC will adhere to more pre-oxidized silk pulp, further improving its physical strength; using a pretreatment solution containing a protonic acid to protonate MFC can protonate the hydroxyl groups in the cellulose molecules, thereby improving the electrical conductivity of the carbon fiber substrate; in Example 4, after ultrasonic treatment of MFC and pre-oxidized silk pulp in a pretreatment solution containing a protonic acid, the prepared carbon fiber substrate has the best effect. Compared with the carbon fiber substrate prepared by adding untreated MFC in Example 1, the in-plane resistivity is reduced by 44.50%, and the vertical resistivity is reduced by 16.61%.

[0128] (5)The mechanical strength and resistivity of the green carbon paper prepared in Examples 7-12 and Comparative Examples 3, 4, and 5 were tested. The test method was determined in accordance with the national standard GB / T 20042.7-2014: "Proton Exchange Membrane Fuel Cells - Part 7: Test Methods for the Characteristics of Carbon Paper". The test results are shown in Table 2.

[0129] Table 2

[0130]

[0131] As can be seen from the data in Table 2, the physical strength of the carbon paper prepared by adding unpretreated MFC in Example 7 and the carbon paper prepared by adding unpretreated MFC and PAN pulp in Example 8 has been significantly improved compared with the carbon papers in Comparative Example 3 and Comparative Example 5. This is mainly attributed to the increase in the sizing amount. Compared with Comparative Example 3, the in-plane resistivity in Example 7 decreased by 5.26%, and the through-plane resistivity decreased by 5.91%. That is, adding MFC can not only significantly improve the mechanical strength of the carbon fiber substrate and the carbon paper, but also improve the conductivity of the carbon paper. In Example 8, after adding PAN pulp, the mechanical strength and conductivity of the carbon paper were further improved. Compared with Comparative Example 3, the in-plane resistivity in Example 8 decreased by 12.56%, and the through-plane resistivity decreased by 14.01%. This is because the conductivity of PAN improved after carbonization, optimizing the conductive network of the carbon paper, and the loading amount of PAN pulp was more after ultrasonic activation, resulting in better conductivity.

[0132] Comparing Example 11 and Example 8, it can be seen that protonating the reinforcing fibers can improve the conductivity of the carbon paper. Comparing Example 12 and Example 8, it can be seen that ultrasonic treatment of the reinforcing fibers can significantly improve the tensile strength and flexural strength of the carbon paper. This is mainly attributed to more MFC retained in the carbon fiber substrate network and a higher resin sizing amount. Comparing Example 9 and Example 8, it can be seen that ultrasonic treatment of MFC and PAN pulp in a pretreatment solution containing a protonic acid can improve both the mechanical strength and conductivity of the carbon paper. Impregnating the phenolic resin containing graphite powder in Example 10 can further improve the conductivity of the carbon paper on the basis of ensuring high mechanical strength. The carbon paper in Example 10 had a 60.90% decrease in in-plane resistivity and a 56.26% decrease in through-plane resistivity compared with the surface-sized carbon paper in Comparative Example 5. The carbon paper in Example 10 had a 38.95% decrease in in-plane resistivity and a 31.30% decrease in through-plane resistivity compared with the carbon paper in Example 8.

[0133] Although the present invention has been illustrated and described with reference to specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Those of ordinary skill in the art should understand that without departing from the spirit and scope of the present invention, the technical solutions described in the foregoing embodiments can be modified, or some or all of the technical features can be equivalently replaced. These modifications or replacements do not cause the essence of the corresponding technical solutions to 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 preparation method of a carbon fiber substrate for a fuel cell, characterized in that, It includes the following steps: S1. Using MFC or a mixture of MFC and PAN pulp as reinforcing fibers, mixing the reinforcing fibers with water for beating to obtain a premixed slurry; S2. Mixing carbon fibers, the premixed slurry and a dispersant to obtain a mixed slurry, forming the mixed slurry by a wet forming process and then drying it to obtain the carbon fiber substrate; In step S1, before preparing the premixed slurry, it further includes a step of performing a first pretreatment on the reinforcing fibers; The step of the first pretreatment includes: preparing a pretreatment solution containing a protonic acid, an organic solvent and water, and immersing the reinforcing fibers in the pretreatment solution; Or, placing the reinforcing fibers in the pretreatment solution and performing ultrasonic treatment; The protonic acid includes hydrochloric acid and / or sulfuric acid; the organic solvent includes ethanol and / or acetone; in the pretreatment liquid, the concentration of H + is 0.15 - 2 mol / L, and the mass ratio of the organic solvent to the water is 1:1 - 3:

1.

2. The preparation method of the carbon fiber base material for fuel cells according to claim 1, wherein The first pretreatment specifically includes the following steps: placing the MFC in the pretreatment solution, ultrasonically activating for 40 - 60 min, then adding the PAN pulp, continuing ultrasonic treatment for 30 - 60 min, performing solid-liquid separation and freeze-drying to obtain the pretreated reinforcing fibers.

3. The preparation method of the carbon fiber substrate for fuel cells according to claim 1, characterized in that, In step S1, the rotational speed of the beating is 5 - 30 m / s, and the beating time is 5 - 20 min.

4. The preparation method of the carbon fiber substrate for fuel cells according to claim 1, characterized in that, Before step S2, it further includes a step of performing a third pretreatment on the carbon fibers. The third pretreatment includes: performing air pre-oxidation treatment on the carbon fibers in an air atmosphere, the temperature of the air pre-oxidation treatment is 300 - 600 °C, and the treatment time is 30 - 150 min.

5. The preparation method of the carbon fiber substrate for fuel cells according to claim 1, characterized in that, In step S2, it includes at least one of the following features: (1) The stirring rotational speed of the mixing is 5 - 30 m / s, and the mixing time is 5 - 30 min; (2) In the mixed slurry, the mass concentration of the carbon fibers is 0.1% - 0.5%; (3) The mass percentage of the MFC in the carbon fibers is 5% - 10%; (4) The mass percentage of the PAN pulp in the carbon fibers is 0% - 10%; (5) In the mixed slurry, the mass concentration of the dispersant is 0.1% - 0.5%; (6) The dispersant includes at least one of polysorbate, polyurethane, and hydroxypropyl cellulose.

6. A carbon fiber substrate for a fuel cell, characterized in that, It is prepared by using the preparation method of the carbon fiber substrate for fuel cell according to any one of claims 1 - 5.

7. A preparation method of carbon paper for fuel cells, characterized in that, It includes the following steps: Using the carbon fiber substrate for fuel cell according to claim 6, through impregnating with resin, drying, hot pressing and curing, and carbonization and graphitization treatment to obtain the green carbon paper.

8. A carbon paper for fuel cells, characterized in that, It is prepared by using the preparation method of the green carbon paper for fuel cell according to claim 7.

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