Method for improving mechanical strength of carbon fiber paper for proton exchange membrane fuel cell

Abstract

The invention discloses a method for improving mechanical strength of carbon fiber paper for a proton exchange membrane fuel cell, and relates to the technical field of proton exchange membrane fuel cells. The method comprises the following steps: ultrasonic impregnation: dissolving thermosetting phenolic resin in absolute ethyl alcohol to prepare impregnation liquid, completely impregnating carbon fiber raw paper in the impregnation liquid, putting a container in an ultrasonic instrument for ultrasonic impregnation treatment, and then taking out the carbon fiber raw paper for drying to obtain carbon fiber raw paper. The carbon fiber paper prepared by the method disclosed by the invention has the characteristics of relatively high mechanical strength, stable chemical performance and low resistivity.

Description

Technical Field

[0001] The invention relates to the technical field of proton exchange membrane fuel cells, and in particular to a method for improving the mechanical strength of carbon fiber paper used in proton exchange membrane fuel cells. Background Art

[0002] Proton exchange membrane fuel cells (PEMFCs) have been regarded as one of the candidates for the next generation of clean energy due to their low pollution and high energy conversion efficiency. However, the market demand for new energy products has determined that fuel cells have smaller volume or mass, lower manufacturing cost, and higher output power as their development goals. Fuel cells are mainly composed of bipolar plates, gas diffusion layers, catalyst layers, and proton exchange membranes. Among them, the gas diffusion layer is one of the core components of proton exchange membrane fuel cells, which plays a role in the transmission of reactants and products. It plays an irreplaceable role in fuel cells. The main functions of the gas diffusion layer in the battery also include: (1) supporting the catalyst layer; (2) transmitting the reaction gas to the catalyst layer for electrochemical reaction; (3) draining excess water out of the battery. Therefore, the ideal gas diffusion layer should have good air permeability, water permeability, electronic conductivity, certain mechanical strength, and good electrochemical stability.

[0003] Gas diffusion layer (GDL for short) refers to an electrode layer made of cellulose fibers or polymer nanomaterials as raw materials. The gas diffusion layer usually has micropores that allow gases such as hydrogen and oxygen to flow and disperse into the electrolyte membrane. It can not only provide gas transport during the electrochemical reaction, but also drive the liquid water on the surface of the catalyst layer to be quickly discharged to prevent the catalyst layer from being flooded. In other words, GDL has the function of transferring gas and liquid, and can also efficiently conduct heat and electricity, making the electrochemical reaction more efficient and stable. The gas diffusion layer is of great significance to the performance of the fuel cell membrane electrode.

[0004] The gas diffusion layer is mostly a two-layer structure, including a base layer obtained by hydrophobic treatment of carbon paper or carbon cloth, and a microporous layer prepared by coating, drying and sintering after mixing carbon particles, hydrophobic agent and solvent. Carbon fiber paper (CFPs) is widely used as a base material for gas diffusion layers due to its high conductivity, excellent corrosion resistance, air permeability and high mechanical strength. CFPs are usually made by combining carbon fibers (CFs) with thermosetting resins (such as phenolic resins) and then carbonizing at high temperatures. The microporous layer should have the characteristics of high porosity, large specific surface area, good mechanical strength, strong conductivity, etc. It is mostly prepared with smaller carbon materials, wherein the carbon materials include any one or more of acetylene black, carbon black, conductive graphite, carbon nanotubes, and graphene. The microporous layer slurry is prepared and the microporous layer slurry is applied to the surface of the carbonized substrate carbon fiber paper, and then dried and carbonized at high temperature to obtain a gas diffusion layer. The gas diffusion layer is an important component of the fuel cell. The efficiency and internal resistance of the battery are closely related to it. Therefore, improving the performance of the gas diffusion layer is an important way to develop fuel cells. The main problems of the existing gas diffusion layer are low strength, easy breakage, high resistance, poor gas diffusion performance and drainage performance. As the base layer used for the gas diffusion layer of the proton exchange membrane fuel cell, the mechanical strength of the carbon fiber paper is particularly important for the mechanical stability of the gas diffusion layer and the support in the fuel cell. At present, the effect of enhancing the mechanical strength is generally achieved by changing the composition of the carbon fiber paper, but this will lead to a complicated preparation process of the carbon fiber paper, so the added components are generally added to the original preparation process. However, for the realization of industrial production, the added components, whether it is reinforcing fiber or carbon nanotubes or carbon black, graphene and other highly conductive fillers, will greatly increase the complexity and cost of the preparation process. At the same time, the existing carbon fiber paper pursues low resistivity, high porosity and air permeability while ignoring the mechanical strength of the carbon fiber paper. After being assembled into a battery, since it cannot fully play the supporting role of the gas diffusion layer, long-term use can easily cause the battery structure to deform and collapse, resulting in low battery energy efficiency.

[0005] In the prior art, most of the research focuses on modifying the surface matrix of carbon fiber or adding fillers to carbon fiber paper, but there is no report on improving the mechanical strength of carbon fiber paper. Summary of the invention

[0006] In order to solve the above-mentioned technical problems, the purpose of the present invention is to provide a method for improving the mechanical strength of carbon fiber paper for fuel cell gas diffusion layer. The carbon fiber paper prepared by the method of the present invention has strong mechanical strength, stable chemical properties and low resistivity.

[0007] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a method for improving the mechanical strength of carbon fiber paper for proton exchange membrane fuel cells, comprising ultrasonic impregnation, wherein the ultrasonic impregnation comprises: The carbon fiber base paper is completely placed in an impregnation liquid, and ultrasonicated in an ultrasonic instrument, and then taken out and dried to obtain a carbon fiber paper blank; wherein the impregnation liquid is prepared by dissolving a thermosetting phenolic resin in anhydrous ethanol.

[0008] Furthermore, according to the material-liquid ratio, the thermosetting phenolic resin: the anhydrous ethanol = 1 g: (4-6) mL; the thermosetting phenolic resin is an alcohol-soluble thermosetting phenolic resin.

[0009] Furthermore, the ultrasound is performed for 20 min to 40 min, and the drying temperature is 75°C to 85°C.

[0010] Furthermore, it also includes hot pressing curing and high temperature carbonization; wherein, The hot pressing curing comprises: firstly subjecting the carbon fiber paper blank to a first pressing curing, then increasing the pressure, and then subjecting the carbon fiber paper blank to a second pressing curing; The high temperature carbonization comprises: placing the carbon fiber paper blank after heat pressing and curing into a high temperature furnace, and performing high temperature carbonization in a nitrogen atmosphere to obtain carbon fiber paper.

[0011] Furthermore, the first compression molding curing has a pressure of 1 Mpa-2 Mpa, a temperature of 100 ℃-120 ℃, and a time of 25 min-35 min; the second compression molding curing has a pressure of 2.5 Mpa-3.5 Mpa, a temperature of 160 ℃-180 ℃, and a time of 1 h-1.5 h; the high-temperature carbonization has a carbonization temperature of 1450 ℃-1550 ℃ and a carbonization time of 1 h-1.5 h.

[0012] Furthermore, the carbon fiber base paper is prepared by: Pretreatment: soak the chopped carbon fiber in a surface treatment agent, take it out, and dry it; Prepare papermaking slurry, add the pretreated chopped carbon fibers into a dispersion, and ultrasonically stir to obtain papermaking slurry; wherein the dispersion is obtained by adding a dispersant and a binder into a solvent; Papermaking and forming: the papermaking slurry is passed through a paper sheet forming machine to wet-form a pure carbon fiber paper blank, and then a flat plate vulcanizer is used to shape and dry it to form a carbon fiber base paper.

[0013] Furthermore, in the pretreatment, the length of the chopped carbon fiber is 4 mm-6 mm; the surface treatment agent is a mixed solution of acetone, ethanol and water, wherein the mass ratio of acetone:ethanol:water=(0.5-1.5):(0.5-1.5):(15-25); the immersion time is 2 h-3 h.

[0014] Furthermore, in the preparation of papermaking pulp, The dispersant includes one or a mixture of polyethylene oxide, isodecanol polyoxyethylene ether, nonylphenol polyoxyethylene ether and polyethylene glycol p-isooctylphenyl ether; The binder includes one or a mixture of hydroxyethyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, anionic polyacrylamide and cationic polyacrylamide; The solvent is water; The dispersion contains 0.05%-0.15% of the dispersant by mass percentage; the mass ratio of the binder to the dispersant is 1: (4-6); the amount of the chopped carbon fiber added is 0.05 g-0.1 g per 100 g of the dispersion; The ultrasonic stirring time is 5 min-7 min.

[0015] Furthermore, the dispersant has a relative molecular weight of 750×10 4 -850×10 4 of polyethylene oxide.

[0016] Furthermore, in the papermaking forming, the shaping and drying using a flat vulcanizing machine is performed at 60°C-80°C for 10 min-20 min.

[0017] Compared with the prior art, the present invention has at least the following beneficial effects: The method for improving the mechanical strength of carbon fiber paper for gas diffusion layer of proton exchange membrane fuel cell proposed in the present invention is different from the traditional method of impregnating carbon fiber paper with phenolic resin. The present invention adopts ultrasonic impregnation to impregnate and cure the carbon fiber base paper with phenolic resin, and finally prepares carbon fiber paper with high mechanical strength, low resistivity and low corrosion current that is suitable for gas diffusion layer of fuel cell.

[0018] The present invention optimizes the carbon fiber paper preparation process, is simple to operate, saves the preparation cost of high mechanical performance carbon fiber paper, and provides an important technical basis for improving the mechanical strength of the gas diffusion layer, maintaining the stable operation of the fuel cell, and producing high mechanical performance carbon fiber paper. The method of the present invention is applicable to all carbon fiber impregnation schemes, and the method is simple and low in cost, providing important method innovation and technical support for the research on improving the mechanical properties of carbon fiber paper and optimizing the preparation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0020] Figure 1 This is a graph of measured data of compressive stress and strain of carbon fiber paper prepared by different impregnation methods.

[0021] Figure 2 This is a graph of measured resistivity data of carbon fiber paper prepared by different impregnation methods.

[0022] Figure 3 It is the measured data of electrochemical corrosion of carbon fiber paper prepared by different impregnation methods. DETAILED DESCRIPTION

[0023] The technical scheme in the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiment of the present invention. Obviously, the described embodiment is only a part of the embodiment of the present invention, not all of the embodiments. Based on the embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0024] According to a first aspect of an embodiment of the present invention, there is provided a method for improving the mechanical strength of carbon fiber paper for proton exchange membrane fuel cells, the method comprising the following steps: Ultrasonic impregnation: Thermosetting phenolic resin is dissolved in anhydrous ethanol to prepare an impregnation solution, the carbon fiber base paper is completely immersed in the impregnation solution, the container is placed in an ultrasonic instrument for ultrasonic impregnation treatment, and then the carbon fiber base paper is taken out and dried to obtain a carbon fiber paper blank.

[0025] In view of the problem of low mechanical strength of carbon fiber paper in the prior art, the present invention proposes a preparation method for improving the mechanical strength of carbon fiber paper for gas diffusion layer of proton exchange membrane fuel cell. No additional components are added in the general preparation process. In the process of impregnating phenolic resin, the impregnation method is changed to ultrasonic impregnation, thereby improving the impregnation effect of phenolic resin in carbon fiber paper and the wettability of phenolic resin and carbon fiber. The present invention achieves the purpose of enhancing the mechanical strength of carbon fiber paper, reducing resistivity and enhancing corrosion resistance by ultrasonic impregnation of phenolic resin.

[0026] In some feasible implementations, according to the material-liquid ratio, thermosetting phenolic resin: anhydrous ethanol = 1 g: (4-6) mL.

[0027] Specifically, thermosetting phenolic resin can increase the density of carbon fiber paper. By adding thermosetting phenolic resin, the carbon fiber base paper can be densified and strengthened. If the concentration of thermosetting phenolic resin is too low, it cannot fully bond the carbon fiber network, and cannot play a role in bonding and stabilizing the carbon fiber paper after carbonization; although within a certain range, as the concentration of thermosetting phenolic resin increases, the density of carbon fiber paper increases, and the mechanical strength also increases, but if the concentration of thermosetting phenolic resin is too high, it will cover the carbon fiber surface and block the carbon fiber pores after hot pressing, affecting the conductivity and porosity of the carbon fiber paper.

[0028] In some feasible embodiments, the thermosetting phenolic resin is an alcohol-soluble thermosetting phenolic resin.

[0029] Specifically, the water-soluble phenolic resin has a relatively large surface tension of water molecules, which leads to a slow penetration rate during the impregnation process, and it is impossible to penetrate into the carbon paper, making the impregnation effect uneven. The carbon fiber paper impregnated with the water-soluble phenolic resin has a low tensile strength, and the compression strain is too large. The structure of the carbon paper is not stable enough, and it is easy to deform in practical applications. The water-soluble phenolic resin can affect the polycondensation reaction speed of the phenolic resin due to the presence of water molecules. It needs to be completely cured at a higher temperature and a longer time and has poor fluidity. Therefore, at the same curing time and temperature, the phenolic resin in the carbon fiber paper impregnated with the water-soluble phenolic resin fails to be completely cured, and cannot be completely evenly distributed in the carbon fiber paper under the same curing pressure. Therefore, compared with the carbon paper impregnated with the alcohol-soluble phenolic resin that can be completely cured and has good fluidity and wettability, its resistance is higher and the corrosion resistance is poor and unstable.

[0030] In some feasible implementations, the carbon fiber base paper is completely immersed in the impregnation liquid, the container is placed in an ultrasonic instrument for ultrasonic impregnation treatment for 20 min-40 min, and then the carbon fiber base paper is taken out and dried at 75°C-85°C.

[0031] Specifically, ultrasonic impregnation mainly makes the thermosetting phenolic resin evenly distributed in the carbon fiber paper and tightly bonded to the carbon fiber. If the ultrasonic time is too short, the thermosetting phenolic resin is not evenly distributed in the carbon fiber paper and is not tightly bonded to the carbon fiber, and the effect of improving the mechanical strength cannot be achieved; the longer the ultrasonic time, the more evenly the thermosetting phenolic resin is distributed in the carbon fiber paper, the tighter it is bonded to the carbon fiber, and the more it can improve the mechanical strength of the carbon fiber paper, but if the ultrasonic time is too long, the treatment effect has no obvious growth change, but increases the preparation time and cost.

[0032] In some feasible embodiments, hot pressing curing and high temperature carbonization are also included; Hot pressing curing: The carbon fiber paper blank is first pressed and cured on a flat vulcanizer, then the pressure is increased and then pressed and cured for the second time; High temperature carbonization: The carbon fiber paper blank after hot pressing and curing is placed in a high temperature furnace and subjected to high temperature carbonization in a nitrogen atmosphere to obtain carbon fiber paper.

[0033] In the embodiment of the present invention, the phenolic resin and the carbon fiber structure are fully cross-linked by hot pressing curing, thereby improving the mechanical strength of the carbon fiber paper. After high-temperature carbonization treatment, the phenolic resin loaded on the carbon fiber overlap is completely carbonized, which enhances the conductivity of the overall carbon fiber network, and the phenolic resin in the pores is pyrolyzed and escaped, exposing more pores, thereby improving the overall porosity and mass transfer capacity of the carbon paper.

[0034] In some feasible embodiments, in the hot pressing curing step, the first compression molding curing is performed at 1 Mpa-2 Mpa and 100 ℃-120 ℃ for 25 min-35 min; the pressure is increased to 2.5 Mpa-3.5 Mpa; and the second compression molding curing is performed at 2.5 Mpa-3.5 Mpa and 160 ℃-180 ℃ for 1 h-1.5 h.

[0035] In the embodiment of the present invention, the first molding curing is used to pre-cure the phenolic resin, reduce the fluidity of the phenolic resin inside the carbon fiber paper, and enhance the structural stability. By increasing the pressure and the second molding curing, the bonding force between the phenolic resin and the carbon fiber is further enhanced, and the mechanical strength of the carbon fiber paper is improved. The first molding curing is cured at 1 Mpa-2Mpa, 100 ℃-120 ℃ for 25 min-35 min, and the second molding curing is cured at 2.5 Mpa-3.5 Mpa, 160 ℃-180 ℃ for 1 h-1.5 h. If the pressure is too low, the internal network of the carbon fiber paper cannot be fully bonded to the phenolic resin, and the overall structure of the carbon fiber paper cannot be stabilized; if the pressure is too high, the pore structure of the carbon fiber will collapse; the temperature range is 100-180 ℃, the temperature is too low to completely cure the phenolic resin, and the temperature is too high to pyrolyze the phenolic resin, and the carbon fiber paper structure cannot be cured; if the time is too short, the phenolic resin and the carbon fiber are not fully bonded, and if the time is too long, the high pore structure collapses and the phenolic resin pyrolyzes.

[0036] In some feasible embodiments, in the high temperature carbonization step, the high temperature carbonization is performed by heating at 1450° C.-1550° C. for 1 h-1.5 h.

[0037] Specifically, the carbonization temperature and time ensure the carbonization degree of the phenolic resin, which directly affects the conductivity of the carbon fiber paper. If the temperature is too high, the phenolic resin will pyrolyze too quickly and fail to play the role of bonding and curing. If the temperature is too low, the phenolic resin cannot be completely carbonized, which reduces the conductivity and mechanical strength of the carbon fiber paper.

[0038] In some feasible implementations, the method for preparing carbon fiber base paper comprises the following steps: Carbon fiber surface pretreatment: Soak the chopped carbon fiber in the surface treatment agent, take it out and dry it; Preparing papermaking slurry: adding a dispersant and a binder into a solvent to obtain a dispersion, then adding the pretreated chopped carbon fibers, and ultrasonically stirring to obtain papermaking slurry; Papermaking and forming: The papermaking pulp is passed through a paper sheet forming machine to be wet-processed into pure carbon fiber paper blanks, which are then shaped and dried using a flat vulcanizer to form carbon fiber base paper.

[0039] The carbon fiber base paper prepared by the method of the embodiment of the present invention has a high carbon fiber content, good dispersibility, and certain mechanical strength.

[0040] In some feasible embodiments, in the carbon fiber surface pretreatment step: the length of the chopped carbon fiber is 4 mm-6 mm; the surface treatment agent is a mixed solution of acetone, ethanol and water, and the mass ratio of acetone: ethanol: water = (0.5-1.5): (0.5-1.5): (15-25); the immersion time is 2-3 h.

[0041] Specifically, the appropriate carbon fiber length can make the base paper have better mechanical properties, more stable structure, more conducive to conductivity, and the carbon paper has a certain pore structure. The surface glue of the carbon fiber can be removed by surface pretreatment, which is conducive to its better dispersion. If the immersion time is too short, the surface treatment effect cannot be achieved, and if it is too long, the surface of the carbon fiber will be damaged and its performance will be affected.

[0042] In some feasible embodiments, in the step of preparing papermaking slurry: the solvent is water; the dispersant includes one or more of polyethylene oxide, isodecanol polyoxyethylene ether, nonylphenol polyoxyethylene ether and polyethylene glycol p-isooctylphenyl ether; the binder includes one or more of hydroxyethyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, anionic polyacrylamide and cationic polyacrylamide; the dispersion contains 0.05%-0.15% dispersant by mass percentage; the mass ratio of binder: dispersant = 1: (4-6); the amount of chopped carbon fiber added is 0.05g-0.1g of chopped carbon fiber per 100g of dispersion. Ultrasonic stirring for 5 min-7 min. Preferably, the dispersant has a relative molecular weight of 750×10 4 -850×10 4 of polyethylene oxide.

[0043] In the embodiment of the present invention, a certain amount of dispersant is added to the solvent water to increase the dispersibility of water on the carbon fiber. At the same time, a suitable binder and dispersant are selected and used in combination to improve the dispersion stability of the carbon fiber in the dispersion liquid, making it more conducive to molding.

[0044] In some feasible implementations, in the papermaking and forming step, the shaping and drying using a flat vulcanizing machine is performed at 60° C.-80° C. for 10 min-20 min.

[0045] Example 1 Effect of impregnation method on improving the mechanical strength of carbon fiber paper for proton exchange membrane fuel cells 1. The method is as follows Step 1: Carbon fiber surface pretreatment: After soaking the chopped carbon fibers with a length of about 5 mm in the surface treatment agent for 2 h, the carbon fibers were taken out, the liquid was drained, and the carbon fibers were dried at 60 °C.

[0046] The surface treatment agent is made by mixing acetone, ethanol and water. The mass ratio is acetone: ethanol: water = 1:1:20.

[0047] Step 2: Preparation of papermaking pulp: The average relative molecular weight of the dispersant is 800×10 4 The polyethylene oxide and the binder polyvinyl alcohol are added into water and stirred evenly to obtain a dispersion. The dispersion contains 0.10% polyethylene oxide by mass. The mass ratio of polyethylene oxide to polyvinyl alcohol is 5:1.

[0048] 0.07 g of chopped carbon fiber was added to every 100 g of the dispersion, and the chopped carbon fiber pretreated in step 1 was added to the dispersion, and ultrasonically stirred for 5 min to obtain a papermaking slurry; Step 3: Papermaking and forming: The papermaking pulp was made into pure carbon fiber paper blanks by a wet method through a paper sheet forming machine using the Cathay rapid papermaking method. Subsequently, a flat vulcanizer was used to set and dry the paper at 70°C for 15 minutes to make carbon fiber base paper.

[0049] Step 4: Dipping method: According to the material-liquid ratio, alcohol-soluble thermosetting phenolic resin: anhydrous ethanol = 1 g: 5 mL, the alcohol-soluble thermosetting phenolic resin is dissolved in anhydrous ethanol to prepare an impregnation solution.

[0050] 1. The present invention - ultrasonic impregnation: The carbon fiber base paper was completely immersed in the impregnation liquid, and the container was placed in an ultrasonic instrument for ultrasonic impregnation treatment for 30 minutes. Then, the carbon fiber base paper was taken out and dried at 80°C to obtain a carbon fiber paper blank.

[0051] 2. Comparative Example 1 - Ordinary impregnation: The carbon fiber base paper was completely immersed in the impregnation liquid for 30 min, and then the carbon fiber base paper was taken out and dried at 80 °C to obtain a carbon fiber paper blank.

[0052] 3. Comparative Example 2 - Vacuum impregnation: The carbon fiber base paper was placed in a Buchner funnel, and the impregnation liquid was allowed to enter the carbon fiber base paper by negative pressure filtration. The impregnation was carried out for 30 min, and then the carbon fiber base paper was taken out and dried at 80 °C to obtain a carbon fiber paper blank.

[0053] Step 5: Hot pressing and curing: The carbon fiber paper blank was first pressed and cured on a flat vulcanizer at 1 MPa and 110 °C for 30 min, then the pressure was increased to 3.0 MPa, and then the second press curing was carried out at 3.0 MPa and 170 °C for 1 h.

[0054] Step 6: High temperature carbonization: The carbon fiber paper blank after hot pressing and curing in step 5 is placed in a high-temperature furnace and carbonized at 1500°C for 1 h in a nitrogen atmosphere to obtain carbon fiber paper.

[0055] (II) Determination of mechanical strength of carbon fiber paper: The mechanical strength of carbon fiber paper is determined by the tensile strength and compressive strength, because it needs to be installed in a fixture when running in a proton exchange membrane fuel cell. Both can be tested by a universal testing machine for tensile strength and compressive strength. At the same time, in addition to the most basic mechanical properties, the performance evaluation of carbon fiber paper also includes basic electrochemical parameters and corrosion resistance, so it is necessary to use a four-probe resistance meter to measure the resistivity of carbon fiber paper and use it test to characterize its corrosion resistance.

[0056] In order to avoid the failure of impregnation due to the large weight loss of phenolic resin after high temperature carbonization, the tensile stress test and compressive stress test of carbon fiber paper were carried out after hot pressing curing, and the resistivity and it curve after hot pressing were measured. The results are shown in Table 1 and Figure 1-Figure 3 .

[0057] Table 1

[0058] As can be seen from Table 1, compared with traditional ordinary impregnation, vacuum impregnation can improve the mechanical properties of carbon fiber paper, but it is still weaker than ultrasonic impregnation. The main reason is that the negative pressure effect during vacuum impregnation causes the phenolic resin to precipitate, resulting in uneven impregnation distribution, and the phenolic resin has poor wettability with the carbon fiber surface and is not tightly bonded. Compared with ordinary impregnation and vacuum impregnation, the ultrasonic impregnation provided by the present invention can make the phenolic resin load larger and more tightly bonded with the carbon fiber. Ultrasonic treatment improves the wettability of the phenolic resin to the carbon fiber surface, making the phenolic resin more evenly distributed in the carbon fiber paper, thereby greatly improving the mechanical strength of the carbon fiber paper. Figure 1It can be more intuitively reflected that the compression rate of carbon fiber paper is greatly reduced after ultrasonic impregnation, which means that in actual fuel cells, the ultrasonically impregnated carbon fiber paper has a smaller compression amount in the fuel cell stack, and the structure is more stable and not easy to deform.

[0059] The resistance and corrosion resistance test results are as follows Figure 2 , Figure 3 Due to the negative pressure effect, vacuum impregnation causes phenolic resin to precipitate, resulting in uneven impregnation distribution, and the phenolic resin has poor wettability with the carbon fiber surface and is not tightly bonded. Therefore, compared with ultrasonic impregnation, vacuum impregnation has higher resistivity and higher corrosion current density, and its corrosion resistance is unstable. Since the phenolic resin impregnated by ultrasonic impregnation is more tightly bonded to the carbon fiber and more evenly distributed, the carbon fiber paper has a smaller current density and better stability at the same corrosion voltage.

[0060] In summary, compared with the traditional method of immersing the impregnated resin and impregnating the resin by vacuum filtration, the present invention achieves the purpose of enhancing the mechanical strength of the carbon fiber paper, reducing the resistivity and enhancing the corrosion resistance by ultrasonic impregnation of the resin.

[0061] Example 2 Effect of ultrasonic immersion time on improving the mechanical strength of carbon fiber paper for proton exchange membrane fuel cells 1. The method is as follows Step 1: Carbon fiber surface pretreatment: same as Example 1.

[0062] Step 2: Prepare papermaking pulp: same as Example 1.

[0063] Step 3: papermaking and forming: same as in Example 1.

[0064] Step 4: Dipping method: According to the material-liquid ratio, alcohol-soluble thermosetting phenolic resin: anhydrous ethanol = 1 g: 5 mL, the alcohol-soluble thermosetting phenolic resin is dissolved in anhydrous ethanol to prepare an impregnation solution.

[0065] The carbon fiber base paper was completely immersed in the impregnation liquid, and the container was placed in an ultrasonic instrument for ultrasonic impregnation treatment for 10 min, 20 min, 30 min, 40 min and 50 min, respectively. Then the carbon fiber base paper was taken out and dried at 80 ℃ to obtain carbon fiber paper blanks with different ultrasonic impregnation times.

[0066] Step 5: Hot pressing and curing: same as in Example 1.

[0067] Step 6: High temperature carbonization: same as Example 1.

[0068] (II) Determination of mechanical strength of carbon fiber paper: In order to avoid the failure of impregnation due to the large weight loss of phenolic resin after high-temperature carbonization, the tensile stress test and compressive stress test of carbon fiber paper were carried out after hot pressing curing, and the resistivity and it curve after hot pressing were measured. The results are shown in Table 2.

[0069] Table 2

[0070] As can be seen from Table 2, with the increase of impregnation time, the tensile strength of carbon fiber paper gradually increases, and begins to decrease and stabilize after more than 30 min, mainly because the impregnation time is too long and the ultrasonic treatment causes the pores in the internal structure of the carbon fiber paper to collapse. The compressive strain increases after reaching the optimum value for the same reason.

[0071] The resistance and corrosion resistance test results show that the carbon fiber paper blank prepared by ultrasound for 30 minutes has a smaller current density and better stability under the same corrosion voltage because the ultrasonically impregnated phenolic resin is more tightly combined with the carbon fiber and distributed more evenly. In addition, the carbon fiber paper blank prepared by ultrasound for 30 minutes has a smaller resistivity in comparison because the ultrasonic impregnation makes the phenolic resin evenly and tightly combined with the carbon fiber and does not block the internal pores of the carbon fiber, increasing the electron transfer path so that most of the electrons are still transferred along the carbon fiber network.

[0072] Therefore, in the present invention, preferably, the ultrasonic immersion treatment is performed for 20 min to 40 min, and more preferably, the ultrasonic immersion treatment is performed for 30 min.

[0073] Example 3 Effect of resin on improving the mechanical strength of carbon fiber paper for proton exchange membrane fuel cells 1. The method is as follows Step 1: Carbon fiber surface pretreatment: same as Example 1.

[0074] Step 2: Prepare papermaking pulp: same as Example 1.

[0075] Step 3: papermaking and forming: same as in Example 1.

[0076] Step 4: Ultrasonic immersion: According to the material-liquid ratio: alcohol-soluble thermosetting phenolic resin: anhydrous ethanol = 1g: 5mL; water-soluble thermosetting phenolic resin: water = 1g: 5mL, the impregnation solutions were prepared respectively. The carbon fiber base paper was completely immersed in the two phenolic resin impregnation solutions, and the container was placed in an ultrasonic instrument for ultrasonic impregnation treatment for 30 minutes respectively. Then the carbon fiber base paper was taken out and dried at 80℃ to obtain a carbon fiber paper blank.

[0077] Step 5: Hot pressing and curing: same as in Example 1.

[0078] Step 6: High temperature carbonization: same as Example 1.

[0079] (II) Determination of mechanical strength of carbon fiber paper: In order to avoid the failure of impregnation due to the large weight loss of phenolic resin after high-temperature carbonization, the carbon paper was subjected to tensile stress test and compressive stress test after hot pressing curing. The results are shown in Table 3.

[0080] Table 3

[0081] As can be seen from Table 3, compared with the impregnation effect of alcohol-soluble phenolic resin, the tensile strength of carbon fiber paper impregnated with water-soluble phenolic resin is lower, and the compressive strain is too large, indicating that the structure of carbon paper is not stable enough and is prone to deformation in practical applications. The main reason is that the surface tension of water molecules is relatively large, and the wettability on the surface of carbon fiber paper is not as good as that of alcohol-soluble phenolic resin, resulting in a slow penetration speed during the impregnation process, unable to penetrate into the interior of the carbon paper, and making the impregnation effect uneven.

[0082] Therefore, the present invention prefers alcohol-soluble thermosetting phenolic resin.

[0083] It is easy for those skilled in the art to understand that, under the premise of no conflict, the above-mentioned advantageous methods can be freely combined and superimposed. The above are only preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application. The above are only preferred implementation methods of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and variations can be made without departing from the technical principles of the present application, and these improvements and variations should also be regarded as the protection scope of the present application.

Claims

1. A method for improving the mechanical strength of carbon fiber paper for proton exchange membrane fuel cells, characterized in that: Comprising ultrasonic impregnation, the ultrasonic impregnation comprising: The carbon fiber base paper is completely placed in an impregnation liquid, and ultrasonicated in an ultrasonic instrument, and then taken out and dried to obtain a carbon fiber paper blank; wherein the impregnation liquid is prepared by dissolving a thermosetting phenolic resin in anhydrous ethanol.

2. The method for improving the mechanical strength of carbon fiber paper for proton exchange membrane fuel cells according to claim 1, characterized in that: According to the material-liquid ratio, the thermosetting phenolic resin: the anhydrous ethanol = 1 g: (4-6) mL; the thermosetting phenolic resin is an alcohol-soluble thermosetting phenolic resin.

3. The method for improving the mechanical strength of carbon fiber paper for proton exchange membrane fuel cells according to claim 2, characterized in that: The ultrasonic treatment time is 20 min-40 min; the drying temperature is 75°C-85°C.

4. A method for improving the mechanical strength of carbon fiber paper for proton exchange membrane fuel cells according to claim 3, characterized in that: It also includes hot pressing curing and high temperature carbonization; among which, The hot pressing curing comprises: firstly subjecting the carbon fiber paper blank to a first pressing curing, then increasing the pressure, and then subjecting the carbon fiber paper blank to a second pressing curing; The high temperature carbonization comprises: placing the carbon fiber paper blank after heat pressing and curing into a high temperature furnace, and performing high temperature carbonization in a nitrogen atmosphere to obtain carbon fiber paper.

5. A method for improving the mechanical strength of carbon fiber paper for proton exchange membrane fuel cells according to claim 4, characterized in that: The first molding curing has a pressure of 1 Mpa-2 Mpa, a temperature of 100°C-120°C, and a time of 25 min-35 min; The second molding curing has a pressure of 2.5 Mpa-3.5 Mpa, a temperature of 160°C-180°C, and a time of 1 h-1.5 h; The high temperature carbonization has a carbonization temperature of 1450°C-1550°C and a carbonization time of 1 h-1.5 h.

6. A method for improving the mechanical strength of carbon fiber paper for proton exchange membrane fuel cells according to any one of claims 1 to 5, characterized in that: The carbon fiber base paper, the preparation method thereof comprises: Pretreatment: soak the chopped carbon fiber in a surface treatment agent, take it out, and dry it; Prepare papermaking slurry, add the pretreated chopped carbon fibers into a dispersion, and ultrasonically stir to obtain papermaking slurry; wherein the dispersion is obtained by adding a dispersant and a binder into a solvent; Papermaking and forming: the papermaking slurry is passed through a paper sheet forming machine to wet-form a pure carbon fiber paper blank, and then a flat plate vulcanizer is used to shape and dry it to form a carbon fiber base paper.

7. A method for improving the mechanical strength of carbon fiber paper for proton exchange membrane fuel cells according to claim 6, characterized in that: In the pre-processing, The length of the chopped carbon fiber is 4 mm-6 mm; The surface treatment agent is a mixed solution of acetone, ethanol and water, wherein the mass ratio of acetone:ethanol:water=(0.5-1.5):(0.5-1.5):(15-25); The soaking time is 2 h-3 h.

8. The method for improving the mechanical strength of carbon fiber paper for proton exchange membrane fuel cells according to claim 6, characterized in that: In the preparation of papermaking pulp, The dispersant includes one or a mixture of polyethylene oxide, isodecanol polyoxyethylene ether, nonylphenol polyoxyethylene ether and polyethylene glycol p-isooctylphenyl ether; The binder includes one or a mixture of hydroxyethyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, anionic polyacrylamide and cationic polyacrylamide; The solvent is water; The dispersion contains 0.05%-0.15% of the dispersant by mass percentage; the mass ratio of the binder to the dispersant is 1: (4-6); the amount of the chopped carbon fiber added is 0.05 g-0.1 g per 100 g of the dispersion; The ultrasonic stirring time is 5 min-7 min.

9. A method for improving the mechanical strength of carbon fiber paper for proton exchange membrane fuel cells according to claim 8, characterized in that: The dispersant has a relative molecular weight of 750×10 4 -850×10 4 of polyethylene oxide.

10. The method for improving the mechanical strength of carbon fiber paper for proton exchange membrane fuel cells according to claim 6, characterized in that: In the papermaking forming, the shaping and drying using a flat vulcanizing machine is performed at 60°C-80°C for 10 min-20 min.

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