Composite bipolar plate with multi-scale conductive network and easy to thermoform

Abstract

The invention relates to a fuel cell technology, and aims to provide a composite bipolar plate which is provided with a multi-scale conductive network and is easy to thermoform. The invention provides a composite bipolar plate with a multi-scale conductive network and easy thermoplastic molding, which is characterized in that the composite bipolar plate is prepared from powdery polypropylene, graphite, conductive carbon black and ionic liquid through raw material premixing, melt extrusion, calendaring and hot press molding. According to the invention, the ionic liquid is adopted in the preparation process of the bipolar plate, and efficient compounding is realized by effectively enhancing the melt flowability; a melt extrusion molding process is combined, so that continuous production of the composite bipolar plate can be realized, and the production cost is reduced; by compounding the graphite and the conductive carbon black and using the conductive ionic liquid, a novel conductive structure of a three-dimensional solid conductive network and a liquid conductive layer can be realized, and the conductive enhancement effect which cannot be realized by a traditional plasticizer can be realized by using the ionic liquid; the product is more compact and uniform in structure and has excellent repeatable processing performance.

Description

Technical Field

[0001] The present invention relates to the technical field of fuel cells, and in particular to a composite bipolar plate having a multi-scale conductive network and being easy to be thermoplastically formed, and a preparation method thereof. Background Art

[0002] A fuel cell is a device that converts chemical energy directly into electrical energy, generating an electric current between two electrodes through an electrochemical reaction. Fuel cells have a wide range of applications, from transportation to backup power supplies, portable power generation equipment, etc. Since they convert chemical energy directly into electrical energy, they are more efficient than traditional combustion power generation methods and emit almost no pollutants. Fuel cells play a vital role in the field of new energy, mainly reflected in high efficiency and environmental protection, storage and utilization of renewable energy (combined with wind energy, solar energy, etc.), flexible application scenarios, energy security, technological progress and economic development.

[0003] Bipolar plates are important components of fuel cells. Their main functions are to isolate and distribute oxidants and fuels, connect the anode and cathode of the battery to conduct current, provide mechanical support to help maintain the overall stability of the battery assembly, and be designed with a structure with cooling channels to regulate the internal temperature of the stack. In order to meet the working requirements of fuel cells, bipolar plates must have excellent gas barrier properties, good electrical conductivity and corrosion resistance, good thermal conductivity and mechanical properties. At the same time, the weight and volume of bipolar plates currently account for about 80% of fuel cells, and the cost accounts for about 50% of fuel cells. In actual application scenarios, bipolar plates need to be as light and thin as possible while ensuring various functional requirements. This is of great significance to improving the power density of fuel cells and reducing the production cost of fuel cells. High-performance materials and complex manufacturing processes will increase the cost of bipolar plates, and developing cost-effective manufacturing methods is an important research direction.

[0004] Existing bipolar plates are usually composed of metals, composite materials, coatings and other materials. During preparation, graphite is pre-mixed, melt-mixed and hot-pressed with conductive fillers such as thermoplastic resins and carbon black to obtain a composite bipolar plate with good mechanical properties, less pollution, high production efficiency, moderate cost, and recyclability. However, the conductivity of such products is relatively low. Increasing the content of conductive fillers to enhance conductivity will lead to problems such as difficulty in forming and processing of composite materials, reduced mechanical properties, and easy pulverization. The main reason for this is that when high-content conductive fillers are filled, the melt lacks fluidity during processing, resulting in uneven dispersion of the filler.

[0005] Therefore, it is necessary to propose new solutions to solve the above problems. Summary of the invention

[0006] The technical problem to be solved by the present invention is to overcome the deficiencies in the prior art and provide a composite bipolar plate having a multi-scale conductive network and being easy to thermoform.

[0007] To solve the technical problem, the solution of the present invention is:

[0008] Provided is a composite bipolar plate having a multi-scale conductive network and being easy to be thermoplastically formed, characterized in that the composite bipolar plate is prepared by premixing powdered polypropylene, graphite and conductive carbon black with an ionic liquid, melt extruding, calendering and hot pressing; based on 100 wt% of the total weight of the raw materials used, polypropylene accounts for 26-28 wt%, graphite accounts for 67-69 wt%, conductive carbon black accounts for 1-3 wt%, and the ionic liquid accounts for 2-4 wt%.

[0009] As a preferred embodiment of the present invention, the graphite is natural flake graphite with a particle size of 1000 meshes.

[0010] As a preferred embodiment of the present invention, the particle size of the conductive carbon black is 30-45 nm.

[0011] As a preferred embodiment of the present invention, the ionic liquid is a bis(trifluoromethanesulfonyl)imide ionic liquid, a bis(2-ethylhexyl)phosphate ionic liquid or a hexafluorophosphate ionic liquid, and the abbreviations thereof are respectively represented by [cation] [NTf 2 ], [cationic] [DEHP], [cationic] [PF 6 ]; the cation is a 1,3-dialkyl substituted imidazole or an alkyl tributyl phosphine.

[0012] The present invention further provides a method for preparing the aforementioned composite bipolar plate, comprising the following steps:

[0013] (1) Weighing powdered polypropylene, graphite and conductive carbon black according to the weight percentage ratio; preparing an ionic liquid and diluting it with an appropriate amount of ethanol to obtain an ionic liquid-ethanol mixed solution;

[0014] (2) After the polypropylene, graphite and conductive carbon black are uniformly mixed, an ionic liquid-ethanol mixed solution is added and mechanically stirred; and then a premix is ​​obtained by drying;

[0015] (3) The premix is ​​melt-mixed and extruded to obtain a melt, and the melt is calendered to obtain a composite sheet; the composite sheet is cut and heated and molded to obtain a composite bipolar plate.

[0016] As a preferred embodiment of the present invention, in step (2), the three powders are mixed by a three-dimensional motion mixer, the speed is controlled to be 12 rpm, and the mixing time is 20 h; when the ionic liquid-ethanol mixture is added, a high-speed stirrer is used for stirring, the speed is controlled to be 32000 rpm, and the time is 10 min.

[0017] As a preferred embodiment of the present invention, in step (2), the drying temperature is 90° C. and the drying time is 12 h.

[0018] As a preferred embodiment of the present invention, in step (3), the melt mixing and extrusion treatment are both carried out by a twin-screw extruder, and the extrusion port is a flat channel with a heating module; during melt mixing, the temperatures of the solid conveying zone, the melting zone, and the melt conveying zone of the twin-screw extruder are controlled to be 180°C, 195°C, and 210°C, respectively, and the pressure at the extrusion port is 4-7 MPa.

[0019] As a preferred embodiment of the present invention, in the step (3), the temperature is controlled to be 220°C, the pressure is 15 MPa, and the time is 20 min during the heating compression molding; after the temperature is lowered to below 70°C under the pressure holding condition, the pressure is released and the mold is demolded to obtain the final product.

[0020] Description of the invention principle:

[0021] 1. Composite bipolar plates with thermoplastic resin as binder usually need to add a high content of conductive filler, which leads to the problems of difficult molding and processing of composite materials, reduced mechanical properties, and easy pulverization. After long-term and in-depth research, the applicant found that the lack of fluidity of the melt is the root cause of the above problems. By adding special plasticizers, the purpose of improving the conductivity of bipolar plate products can be achieved while improving the fluidity of the melt.

[0022] In order to improve the processing efficiency of the product and achieve composite performance, the present invention proposes an innovative solution, using a hydrophobic, highly stable ionic liquid (rather than an ordinary single plasticizer) as a plasticizer in the product preparation process. Since the ionic liquid has good conductivity, it can also form an additional conductive network with the carbon-based conductive filler in the product raw material, thereby playing a dual role of plasticization and improving conductivity, and obtaining a composite bipolar plate with good mechanical properties and conductive properties. The hydrophobic property allows the bipolar plate to promote the rapid outflow of water (a fuel cell product) in the application scenario.

[0023] 2. The conductive filler in the present invention uses a mixed material of graphite and carbon black, and uses conductive carbon black with a smaller particle size than natural flake graphite to play a filling, intercalation or overlapping role, which can form new conductive bridges between graphites and play a role in enhancing the conductive effect.

[0024] 3. The present invention can construct a new conductive structure of three-dimensional solid conductive network + liquid conductive layer through the compound use of graphite (main conductive material), conductive carbon black (second conductive network) and conductive ionic liquid (liquid conductive layer), achieving a conductive enhancement effect that traditional plasticizers cannot achieve.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1. The present invention proposes to use ionic liquids in the preparation process of bipolar plates to achieve efficient composite by effectively enhancing melt fluidity; combined with the melt extrusion molding process, continuous production of composite bipolar plates can be achieved, reducing production costs.

[0027] 2. During the product preparation process, a new conductive structure of three-dimensional solid conductive network + liquid conductive layer can be realized through the compounding of graphite and conductive carbon black and the use of conductive ionic liquid. The use of ionic liquid can achieve a conductive enhancement effect that traditional plasticizers cannot achieve.

[0028] 3. Since the traditional process of cooling, pelletizing and hot pressing the melt is not used, the composite bipolar plate structure of the present invention is more compact and uniform, and the mechanical and conductive properties are further guaranteed.

[0029] 4. Since the ionic liquid makes the thermoplastic resin well plasticized, the composite bipolar plate of the present invention and the cut scraps can be recycled; based on this excellent repeatable processing performance, better economic and environmental benefits can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic diagram of the preparation process of the composite bipolar plate in the present invention.

[0031] Figure 2 The following are scanning electron microscope (SEM) images of the internal structure of the composite bipolar plate in Example 1 at different magnifications. DETAILED DESCRIPTION

[0032] The present invention is further described below through specific implementation examples, but the present invention is not limited to the following embodiments.

[0033] Part I Implementation of the Invention

[0034] The composite bipolar plate with a multi-scale conductive network and easy to be thermoplastically formed described in the present invention is prepared by premixing powdered thermoplastic polypropylene (PP) resin, graphite and conductive carbon black with ionic liquid, melt extrusion, calendaring and hot pressing; based on 100 wt% of the total weight of the raw materials used, polypropylene accounts for 26-28 wt%, graphite accounts for 67-69 wt%, conductive carbon black accounts for 1-3 wt%, and ionic liquid accounts for 2-4 wt%.

[0035] As an optional solution, the graphite is natural flake graphite with a particle size of 1000 mesh, and the conductive carbon black particle size is 30-45 nm. The ionic liquid is bis(trifluoromethanesulfonyl)imide salt ([cation][NTf 2 ]) ionic liquids, bis(2-ethylhexyl) phosphate ([cation][DEHP]) ionic liquids or hexafluorophosphate ([cation][PF 6 ]) Any one of the ionic liquids, wherein the cation in the ionic liquid is 1,3-dialkyl substituted imidazole or alkyl tributyl phosphine. The ionic liquid is diluted with an appropriate amount of ethanol before use.

[0036] The present invention further provides a method for preparing the composite bipolar plate, comprising the following steps:

[0037] (1) Weighing powdered polypropylene, graphite and conductive carbon black according to the weight percentage ratio; preparing an ionic liquid and diluting it with an appropriate amount of ethanol to obtain an ionic liquid-ethanol mixed solution;

[0038] (2) After the polypropylene, graphite and conductive carbon black are uniformly mixed, an ionic liquid-ethanol mixed solution is added and mechanically stirred; and then a premix is ​​obtained by drying;

[0039] Among them, PP resin, graphite and conductive carbon black were mixed with a three-dimensional motion mixer, the speed was controlled at 12 rpm, and the mixing time was 20 h; when the ionic liquid-ethanol mixture was added, a high-speed stirrer was used for stirring, the speed was controlled at 32,000 rpm, and the time was 10 min. The temperature during the drying process was 90 ° C and the time was 12 h.

[0040] (3) The premix is ​​melt-mixed and extruded to obtain a melt, and then the melt is calendered to obtain a composite sheet; the composite sheet is cut and heated and molded to obtain the composite bipolar plate.

[0041] Among them, melt mixing and extrusion processing are both achieved by a twin-screw extruder, and its extrusion port is a flat channel with a heating module to form a flat melt of a certain thickness. The resulting flat melt will be calendered by a calender to obtain a flat composite sheet with uniform thickness. During melt mixing, the temperatures of the solid conveying zone, melting zone, and melt conveying zone of the twin-screw extruder are controlled to be 180°C, 195°C, and 210°C, respectively, and the pressure at the extrusion port is 4~7 MPa.

[0042] Then, in the subsequent heating and compression molding process, the pressure is further increased to make the composite sheet more compact. During heating and compression molding, the temperature is controlled to be 220°C, the pressure is 15 MPa, and the time is 20 min; after cooling to below 70°C under the pressure holding condition, the pressure is released and the mold is demolded to obtain the final composite bipolar plate. The mold forming surface should be coated with a release agent to prevent the product from sticking to the mold. In some implementation cases, a mold with a flow channel cavity can be selected for the hot pressing process. This part of the processing process can directly adopt existing known technologies, and the present invention will not repeat them.

[0043] Part II Implementation Case and Test Analysis

[0044] Example 1

[0045] (1) Weigh 0.26 kg of PP, 0.67 kg of flake graphite, and 0.03 kg of conductive carbon black, respectively, add them into a three-dimensional motion mixer, and mix at 12 rpm for 20 hours to obtain a solid mixture; then continue to add 0.04 kg [P 4,4,4,14 ][DEHP] and 40 mL of ethanol; 4,4,4,14 [DEHP] is the abbreviation of trihexyltetradecylphosphine bis(2-ethylhexyl)phosphate ionic liquid, and the cation contained is alkyl tributylphosphine. Then a high-speed stirrer was used to stir at a speed of 32000 rpm for 10 min; the mixture was dried at 90 °C for 12 h to obtain a premix.

[0046] The total weight of the raw materials used is 100 wt%, of which PP as a thermoplastic resin accounts for 26 wt%, flake graphite accounts for 67 wt%, conductive carbon black accounts for 3 wt%, and [P 4,4,4,14 ][DEHP] accounts for 4 wt%.

[0047] (2) The temperatures of the first zone (solid conveying zone), second zone (melting zone), and third zone (melt conveying zone) of the twin-screw extruder were set to 180, 195, and 210 °C, respectively; the feeding rate was set to 40 rpm, and the screw speed was set to 50 rpm; the die temperature of the discharge port was set to 200 °C, and the outlet height was set to 2 mm. After reaching the set temperature, the premix was added to the feeding port of the twin-screw extruder for melt mixing and extrusion. The pressure at the extrusion port was 5 MPa to obtain a flake melt.

[0048] (3) The sheet melt obtained in step (2) is rapidly calendered with a roller gap of 1 mm, a roller temperature of 200 °C, and a rolling line speed of about 2 m / min. The calendered sheet is collected after air cooling.

[0049] (4) Set the press temperature to 220 °C, cut the sheet obtained in step (3) into the size of the mold cavity and place it in the mold coated with a release agent; after reaching the set temperature, place the mold on the press, close the mold, and keep it at 220 °C for 20 min; then hot press at 220 °C and 15 MPa for 20 min; after the press temperature drops below 70 °C, demold, and obtain the composite bipolar plate. The obtained sample is abbreviated as 26%PP / 4%[P 4,4,4,14 ][DEHP] / 67%Gr / 3%CB.

[0050] Example 2

[0051] The difference from Example 1 is that the weighed raw materials include: PP 0.28 kg, flake graphite 0.67 kg, conductive carbon black 0.03 kg, as the ionic liquid [P 4,4,4,14 ][DEHP] 0.02 kg; the pressure at the extrusion port was 7 MPa, and the other conditions were the same as those in Example 1. The final sample was abbreviated as 28% PP / 2% [P 4,4,4,14 ][DEHP] / 67%Gr / 3%CB.

[0052] Example 3

[0053] The difference from Example 1 is that the weighed raw materials include: 0.27 kg PP, 0.67 kg flake graphite, 0.03 kg conductive carbon black, and [P 4,4,4,14 ][DEHP]0.03 kg; the pressure at the extrusion port was 6 MPa, and the other conditions were the same as those in Example 1. The final sample was abbreviated as 27% PP / 3% [P 4,4,4,14 ][DEHP] / 67%Gr / 3%CB.

[0054] Example 4

[0055] The difference from Example 1 is that the weighed raw materials include: PP 0.26 kg, flake graphite 0.69 kg, conductive carbon black 0.01 kg, as the ionic liquid [P 4,4,4,14 ][DEHP]0.04 kg; the pressure at the extrusion port was 4 MPa, and the other conditions were the same as those in Example 1. The final sample was abbreviated as 26% PP / 4% [P 4,4,4,14 ][DEHP] / 69%Gr / 1%CB.

[0056] Example 5

[0057] The difference from Example 1 is that the weighed raw materials include: PP 0.26 kg, flake graphite 0.68 kg, conductive carbon black 0.02 kg, as the ionic liquid [P 4,4,4,14 ][DEHP]0.04 kg; the pressure at the extrusion port was 5 MPa, and the other conditions were the same as those in Example 1. The final sample was abbreviated as 26% PP / 4% [P 4,4,4,14 ][DEHP] / 68%Gr / 2%CB.

[0058] Example 6

[0059] The difference from Example 1 is that the weighed raw materials include: PP 0.26 kg, flake graphite 0.67 kg, conductive carbon black 0.03 kg, [HMIm][NTf 2 ]0.04 kg (the cation contained was 1-hexyl-3-methylimidazole); the pressure at the extrusion port was 6 MPa, and the other conditions were the same as those in Example 1. The final sample was abbreviated as 26%PP / 4%[HMIm][NTf 2 ] / 67%Gr / 3%CB.

[0060] Example 7

[0061] The difference from Example 1 is that the weighed raw materials include: PP 0.26 kg, flake graphite 0.67 kg, conductive carbon black 0.03 kg, [HMIm][PF 6 ] 0.04 kg (the cation contained is 1-hexyl-3-methylimidazole); the pressure at the extrusion port is 7 MPa, and the other conditions are the same as those in Example 1. The final sample is abbreviated as 26%PP / 4%[HMIm][PF 6 ] / 67%Gr / 3%CB.

[0062] Comparative Example 1

[0063] The difference from Example 1 is that the weighed raw materials include: 0.26 kg of PP, 0.67 kg of flake graphite, 0.03 kg of conductive carbon black, and 0.04 kg of diamyl phthalate (DAP) as a traditional plasticizer; other conditions are the same as those in Example 1, and the final sample is abbreviated as 26% PP / 4% DAP / 67% Gr / 3% CB. In this comparative example, the ionic liquid is replaced with a traditional plasticizer (the main component is DAP).

[0064] Comparative Example 2

[0065] (1) Weigh 0.30 kg of PP, 0.67 kg of flake graphite, and 0.03 kg of conductive carbon black respectively, add them into a three-dimensional motion mixer, and mix them at 12 rpm for 20 hours to obtain a solid mixture. Use a high-speed stirrer to stir at 32,000 rpm for 10 min; dry the mixture at 90 °C for 12 h to obtain a premix.

[0066] The total weight of the raw materials used is 100 wt %, of which PP as a thermoplastic resin accounts for 30 wt %, graphite accounts for 67 wt %, and conductive carbon black accounts for 3 wt %. No ionic liquid is added in this comparative example.

[0067] (2) The temperatures of the first zone (solid conveying zone), second zone (melting zone), and third zone (melt conveying zone) of the twin-screw extruder were set to 180, 195, and 210 °C, respectively, the feeding speed was set to 40 rpm, and the screw speed was set to 50 rpm; the die temperature of the discharge port was set to 200 °C. After reaching the set temperature, the premix was added to the feeding port of the twin-screw extruder for melt mixing and extrusion. It was found that it was impossible to extrude in the form of sheets. The pressure measured at the extrusion port reached about 20 MPa, and the extrusion speed was very slow. Therefore, the specimen could only be extruded directly, and then pelletized and hot pressed.

[0068] (3) Pellet the strips obtained in step (2); set the press temperature to 220 °C, and place the obtained pellets into a mold coated with a release agent; after reaching the set temperature (220 °C), place the mold on the press, close the mold, and keep it at 220 °C for 20 min; then hot press at 220 °C and 15 MPa for 20 min; demold after the press temperature drops below 70 °C, and obtain a composite bipolar plate. The obtained sample is abbreviated as 30%PP / 67%Gr / 3%CB.

[0069] Comparative Example 3

[0070] (1) Weigh 0.30 kg of PP and 0.70 kg of flake graphite respectively, add them into a three-dimensional motion mixer, and mix them at 12 rpm for 20 hours to obtain a solid mixture. Use a high-speed stirrer to stir at 32,000 rpm for 10 minutes; dry the mixture at 90 °C for 12 hours to obtain a premix.

[0071] The total weight of the raw materials used is 100 wt %, of which PP as a thermoplastic resin accounts for 30 wt % and graphite accounts for 70 wt %. In this comparative example, no conductive carbon black and ionic liquid are added.

[0072] (2) The temperatures of the first zone (solid conveying zone), second zone (melting zone), and third zone (melt conveying zone) of the twin-screw extruder were set to 180, 195, and 210 °C, respectively, the feeding speed was set to 40 rpm, and the screw speed was set to 50 rpm; the die temperature of the discharge port was set to 200 °C. After reaching the set temperature, the premix was added to the feeding port of the twin-screw extruder for melt mixing and extrusion. It was found that it was impossible to extrude in the form of sheets, and the pressure measured at the extrusion port also reached about 20 MPa, and the extrusion speed was very slow. Therefore, the specimen could only be extruded directly, and then pelletized and hot pressed.

[0073] (3) Pellet the strips obtained in step (2); set the press temperature to 220 °C, and place the obtained pellets into a mold coated with a release agent; after reaching the set temperature (220 °C), place the mold on the press, close the mold, and keep it at 220 °C for 20 min; then hot press at 220 °C and 15 MPa for 20 min; demold after the press temperature drops below 70 °C, and obtain a composite bipolar plate. The obtained sample is abbreviated as 30%PP / 70%Gr.

[0074] Performance test and summary:

[0075] The in-plane electrical conductivity (σ) of the bipolar plate products in each embodiment and comparative example was tested using an RTS-9 dual-electrical four-probe tester. The conductivity meter calculation formula is shown in Formula 1. Three samples were prepared for each embodiment and comparative example, and more than 20 points of each sample were randomly selected for testing as the conductivity of the sample. The average value of the three samples was recorded. Before testing the conductivity, the sample surface was polished with 400-mesh and 1000-mesh sandpapers in turn, and the surface powder was purged to remove it.

[0076] (1)

[0077] Where I is the current between probes 1 and 4, V is the voltage between probes 2 and 3, d is the sample thickness, l 13 , l 24are the distances between probes 1 and 3, and between probes 2 and 4, respectively.

[0078] The bending strength of the bipolar plate products in each embodiment and comparative example was tested using a KJ-1065 universal testing machine. The test was conducted in accordance with GB / T 9341-2008. The sample size was 60 mm×5 mm×2 mm, and the test speed was 1 mm / min. -1 All tests were conducted at room temperature, and 5 samples were selected for each embodiment and comparative example for testing, and the average value was recorded. The results of electrical conductivity and flexural strength are shown in Table 1.

[0079] Table 1 Performance of the composite bipolar plates of Examples 1 to 7 and Comparative Examples 1 to 3

[0080] ;

[0081] It can be seen from Table 1 that, in general, the composite bipolar plates containing plasticizer and conductive ionic liquids prepared in Examples 1 to 7 exhibit higher electrical conductivity and flexural strength. This is attributed to the fact that ionic liquids can effectively enhance melt fluidity and achieve efficient composites, thereby increasing the conductivity of the bipolar plate while maintaining higher mechanical strength. It should be pointed out that the use of traditional DAP plasticizers and ionic liquid plasticizers will cause a certain degree of decrease in flexural resistance. By controlling the content of plasticizers, the flexural resistance of the composite bipolar plate can meet application requirements. It can be seen from Comparative Examples 2 and 3 that the addition of the second conductive filler carbon black can significantly improve the conductivity of the bipolar plate. Figure 2 It can be seen that the added conductive carbon black can form new conductive bridges between flake graphite. By comparing Example 1 with Comparative Examples 1 and 2, it can be seen that both traditional plasticizers and ionic liquid plasticizers can improve the conductivity of bipolar plates; and compared with bipolar plates using traditional DAP plasticizers, the conductivity of bipolar plates using ionic liquid plasticizers is increased by 42%, and the bending strength is also improved to a certain extent. Through the compounding of carbon-based conductive fillers and the use of conductive ionic liquids, a new conductive structure of three-dimensional solid conductive network + liquid conductive layer can be realized, achieving a conductive enhancement effect that cannot be achieved by traditional plasticizers.

[0082] The present invention provides a composite bipolar plate having a multi-scale conductive network and being easy to thermoform and a method for preparing the same. There are many methods and approaches to implement the technical solution. The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention, and these improvements and modifications should also be regarded as the scope of protection of the present invention. All components not specified in this embodiment can be implemented by existing technologies.

Claims

1. A composite bipolar plate having a multi-scale conductive network and being easy to thermoform, characterized in that: The composite bipolar plate is prepared by premixing powdered polypropylene, graphite and conductive carbon black with ionic liquid, melt extrusion, calendering and hot pressing; based on 100 wt% of the total weight of the raw materials used, polypropylene accounts for 26-28 wt%, graphite accounts for 67-69 wt%, conductive carbon black accounts for 1-3 wt%, and ionic liquid accounts for 2-4 wt%.

2. The composite bipolar plate according to claim 1, characterized in that: The graphite is natural flake graphite with a particle size of 1000 meshes.

3. The composite bipolar plate according to claim 1, characterized in that: The conductive carbon black particle size is 30-45 nm.

4. The composite bipolar plate according to claim 1, characterized in that: The ionic liquid is a bis(trifluoromethanesulfonyl)imide salt ionic liquid, a bis(2-ethylhexyl)phosphate ionic liquid or a hexafluorophosphate ionic liquid, and the abbreviations thereof are respectively represented by [cation][NTf2], [cation][DEHP], [cation][PF6]; the cation is a 1,3-dialkyl substituted imidazole or an alkyl tributyl phosphine.

5. The method for preparing the composite bipolar plate according to claim 1, characterized in that: The following steps are involved: (1) Weighing powdered polypropylene, graphite and conductive carbon black according to the weight percentage ratio; preparing an ionic liquid and diluting it with an appropriate amount of ethanol to obtain an ionic liquid-ethanol mixed solution; (2) After the polypropylene, graphite and conductive carbon black are uniformly mixed, an ionic liquid-ethanol mixed solution is added and mechanically stirred; and then a premix is ​​obtained by drying; (3) The premix is ​​melt-mixed and extruded to obtain a melt, and the melt is calendered to obtain a composite sheet; the composite sheet is cut and heated and molded to obtain a composite bipolar plate.

6. The method according to claim 5, characterized in that In the step (2), the three powders are mixed by a three-dimensional motion mixer, the speed is controlled at 12 rpm, and the mixing time is 20 h; when the ionic liquid-ethanol mixture is added, a high-speed stirrer is used for stirring, the speed is controlled at 32000 rpm, and the time is 10 min.

7. The method according to claim 5, characterized in that In the step (2), the temperature during the drying process is 90° C. and the time is 12 h.

8. The method according to claim 5, characterized in that In the step (3), the melt mixing and extrusion treatment are both carried out by a twin-screw extruder, whose extrusion port is a flat channel with a heating module; during melt mixing, the temperatures of the solid conveying zone, the melting zone, and the melt conveying zone of the twin-screw extruder are controlled to be 180°C, 195°C, and 210°C, respectively, and the pressure at the extrusion port is 4-7 MPa.

9. The method according to claim 5, characterized in that In the step (3), the temperature is controlled to be 220°C, the pressure is 15 MPa, and the time is 20 min during the heating compression molding; after the temperature is lowered to below 70°C under the pressure holding condition, the pressure is released and the mold is demolded to obtain the final product.

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