A composite bipolar plate with a multi-scale conductive network and easy to thermoplastic mold

Abstract

The present invention relates to fuel cell technology, aiming to provide a composite bipolar plate with a multi-scale conductive network and easy thermoplastic molding. There is provided a composite bipolar plate with a multi-scale conductive network and easy thermoplastic molding, which is characterized in that the composite bipolar plate is obtained by premixing raw materials, melt extrusion, calendering and hot pressing of powdery polypropylene, graphite, conductive carbon black and ionic liquid. The present invention proposes to use ionic liquid in the preparation process of the bipolar plate, and achieve efficient compounding by effectively enhancing the melt fluidity; combined with the melt extrusion molding process, continuous production of the composite bipolar plate can be realized, and the production cost can be reduced; through the compounding of graphite and conductive carbon black and the use of conductive ionic liquid, a new conductive structure of three-dimensional solid conductive network + liquid conductive layer can be realized, and the use of ionic liquid can achieve a conductive enhancement effect that cannot be achieved by traditional plasticizers; the product structure is denser and more uniform, and has excellent repeatable processing performance.

Description

Technical Field

[0001] The present invention relates to the technical field of fuel cells, and particularly relates to a composite bipolar plate with a multi-scale conductive network and easy thermoplastic molding and a preparation method thereof. Background Art

[0002] A fuel cell is a device that directly converts chemical energy into electrical energy, and generates an electric current between two electrodes through an electrochemical reaction. The application scope of fuel cells is very wide, and fuel cells can be used in transportation, backup power supplies, portable power generation equipment, etc. Since it directly converts chemical energy into electrical energy, it is more efficient than traditional combustion power generation methods and has almost no pollutant emissions. Fuel cells play a crucial role in the new energy field, 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, etc.

[0003] The bipolar plate is an important component of the fuel cell, and its functions are mainly reflected in isolating and distributing oxidants and fuels, connecting the anode and cathode of the battery to conduct current, providing mechanical support to help maintain the overall stability of the battery components, and being designed with a structure having cooling channels to regulate the temperature inside the stack, etc. In order to meet the working requirements of the fuel cell, the bipolar plate must have excellent gas barrier properties, good electrical conductivity and corrosion resistance, good thermal conductivity, and mechanical properties. At the same time, at present, the weight and volume of the bipolar plate account for about 80% of the fuel cell, and the cost accounts for about 50% of the fuel cell. In actual application scenarios, it is necessary for the bipolar plate to be as thin and light as possible while ensuring various functional requirements, which is of great significance for improving the power density of the fuel cell and reducing the production cost of the fuel cell. High-performance materials and complex manufacturing processes will increase the cost of the bipolar plate, and developing cost-effective manufacturing methods is an important research direction.

[0004] Existing bipolar plates are usually composed of materials such as metals, composite materials, coatings, etc. When preparing, 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, which has good mechanical properties, less pollution, high production efficiency, moderate cost, and can be recycled. However, the electrical conductivity of such products is relatively low. Enhancing the electrical conductivity by increasing the content of the conductive filler will lead to problems such as difficult forming and processing of the composite material, reduced mechanical properties, and easy powdering. The main reason is that when filling with a high content of conductive filler, the melt lacks fluidity during the processing process, resulting in uneven dispersion of the filler.

[0005] Therefore, it is necessary to propose a new solution 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 with a multi-scale conductive network and easy to thermoform.

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

[0008] Provide a composite bipolar plate with a multi-scale conductive network and easy to thermoform, characterized in that the composite bipolar plate is prepared by premixing raw materials, melt extrusion, calendering and hot pressing of powdery polypropylene, graphite and conductive carbon black with ionic liquid; 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%.

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

[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 bis(trifluoromethanesulfonyl)imide salt ionic liquid, bis(2-ethylhexyl) phosphate ionic liquid or hexafluorophosphate ionic liquid, and their abbreviations are [cation][NTf2], [cation][DEHP], [cation][PF6] respectively; the cation is 1,3-dialkyl substituted imidazole or alkyltributylphosphine.

[0012] The present invention further provides a preparation method of the foregoing composite bipolar plate, including the following steps:

[0013] (1) Weigh powdery polypropylene, graphite and conductive carbon black according to the weight percentage ratio; prepare ionic liquid and dilute it with an appropriate amount of ethanol to obtain an ionic liquid-ethanol mixture.

[0014] (2) After mixing polypropylene, graphite and conductive carbon black evenly, add the ionic liquid-ethanol mixture and stir mechanically; then carry out drying treatment to obtain a premix.

[0015] (3) The premix is subjected to melt mixing and extrusion treatment to obtain a melt, and the melt is calendered to obtain a composite sheet; the composite sheet is cut and heated and molded by die pressing to obtain a composite bipolar plate.

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

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

[0018] As a preferred embodiment of the present invention, in step (3), both the melt mixing and extrusion treatments are achieved using a twin-screw extruder, and its extrusion port is a flat channel with a heating module; during the melt mixing, the temperatures of the solid conveying zone, melting zone, and melt conveying zone of the twin-screw extruder are controlled at 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 step (3), during the hot press molding, the temperature is controlled at 220 °C, the pressure at 15 MPa, and the time at 20 min; after cooling to below 70 °C under the pressure holding condition, the pressure is released and the mold is removed to obtain the final product.

[0020] Description of the invention principle:

[0021] 1. Composite bipolar plates with a thermoplastic resin as the binder usually need to add a relatively high content of conductive fillers, which leads to problems such as difficult forming and processing of the composite material, reduced mechanical properties, and easy powdering. After long-term in-depth research, the applicant found that the lack of melt fluidity is the fundamental cause of the above problems. By adding a special plasticizer, it is possible to improve the melt fluidity while achieving the purpose of improving the conductivity of the bipolar plate product.

[0022] In order to improve the processing efficiency of the product and obtain composite properties, the present invention proposes an innovative solution, using a hydrophobic and highly stable ionic liquid (instead of a common single plasticizer) as the plasticizer during the product preparation process. Since ionic liquids have good conductivity and can also form an additional conductive network with the carbon-based conductive fillers in the product raw materials, thus playing a dual role of plasticization and improving conductivity, a composite bipolar plate with both good mechanical properties and conductive properties is obtained. The hydrophobic property can enable the bipolar plate to promote the rapid outflow of water (which is a product of the fuel cell) in the application scenario.

[0023] 2. In the present invention, the conductive filler uses a mixture of graphite and carbon black. The conductive carbon black with a smaller particle size compared to natural flake graphite is used to play a role in filling, intercalating, or overlapping, and can form new conductive bridges between the graphites, thereby enhancing the conductive effect.

[0024] 3. By using a combination of graphite (the main conductive material), conductive carbon black (the second conductive network), and conductive ionic liquid (the liquid conductive layer), the present invention can construct a new conductive structure of a three-dimensional solid conductive network + liquid conductive layer, achieving a conductive enhancement effect that cannot be achieved by traditional plasticizers.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

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

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

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

[0029] 4. Due to the good plasticization of thermoplastic resin by ionic liquid, the composite bipolar plates and cut-off scraps of the present invention can be recycled; based on this excellent reprocessing 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 It is a scanning electron microscope (SEM) image of the internal structure of the composite bipolar plate in Example 1 at different magnifications. DETAILED DESCRIPTION OF THE INVENTION

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

[0033] The First Part: The Implementation Scheme of the Present Invention

[0034] The composite bipolar plate with a multi-scale conductive network and easy thermoplastic molding described in the present invention is prepared by premixing raw materials, melt-extruding, calendering and hot-pressing powder-like thermoplastic polypropylene (PP) resin, graphite, conductive carbon black and ionic liquid; 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 alternative, the graphite is natural flake graphite with a particle size of 1000 mesh, and the particle size of the conductive carbon black is 30-45 nm. The ionic liquid is any one of bis(trifluoromethanesulfonyl)imide salt ([cation][NTf2]) ionic liquid, bis(2-ethylhexyl) phosphate ([cation][DEHP]) ionic liquid or hexafluorophosphate ([cation][PF6]) ionic liquid, and the cation in the ionic liquid is 1,3-dialkyl-substituted imidazole or alkyltributylphosphine. The ionic liquid is diluted with an appropriate amount of ethanol before use.

[0036] The present invention further provides a preparation method of the composite bipolar plate, including the following steps:

[0037] (1) Weigh powder-like polypropylene, graphite and conductive carbon black according to the weight percentage ratio; prepare an ionic liquid and dilute it with an appropriate amount of ethanol to obtain an ionic liquid-ethanol mixture;

[0038] (2) After uniformly mixing polypropylene, graphite and conductive carbon black, add the ionic liquid-ethanol mixture and stir mechanically; then perform a drying treatment to obtain a premix;

[0039] Among them, a three-dimensional motion mixer is used to mix the PP resin, graphite and conductive carbon black, control the rotation speed at 12 rpm, and the mixing time at 20 h; when adding the ionic liquid-ethanol mixture, change to a high-speed mixer for stirring treatment, control the rotation speed at 32000 rpm, and the time at 10 min. The temperature during the drying treatment is 90 °C and the time is 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 by die pressing to obtain the composite bipolar plate.

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

[0042] Then, during the subsequent hot pressing and forming process, the pressure is further increased to make the composite sheet more compact. During hot pressing and forming, the temperature is controlled at 220 °C, the pressure at 15 MPa, and the time at 20 min; after cooling to below 70 °C under the pressure-holding condition, the pressure is released and the mold is removed to obtain the finally shaped composite bipolar plate. The forming surface of the mold should be coated with a mold release agent to prevent the product from sticking to the mold. In some implementation cases, a mold with a runner cavity can be selected for the hot pressing process. The processing procedures in this part can all directly adopt the existing well-known technologies, and the present invention will not elaborate further.

[0043] Second part: Implementation cases 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 to a three-dimensional motion mixer, and mix at a speed of 12 rpm for 20 hours to obtain a solid mixture; then continue to add a mixed solution of 0.04 kg of [P 4,4,4,14 [DEHP] and 40 mL of ethanol; among them, [P 4,4,4,14 [DEHP] is the abbreviation of trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate ionic liquid, and the contained cation is alkyltributylphosphonium. Then switch to a high-speed mixer and stir at a speed of 32000 rpm for 10 min; dry the mixture at 90 °C for 12 h to obtain a premix.

[0046] The total weight of the raw materials used is 100 wt%, among 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] as an ionic liquid accounts for 4 wt%.

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

[0048] (3) Immediately calender the sheet-like melt obtained in step (2), with the roll gap being 1 mm, the roll temperature being 200 °C, and the rolling linear speed being approximately 2 m / min. After the calendered sheet is air-cooled, collect it.

[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 a mold coated with a release agent; after reaching the set temperature, place the mold on the press, close the mold, and keep it warm 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 to 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: 0.28 kg of PP, 0.67 kg of flake graphite, 0.03 kg of conductive carbon black, and [P 4,4,4,14 [DEHP] 0.02 kg as the ionic liquid; the pressure at the extrusion outlet is 7 MPa, and other conditions are the same as those in Example 1. The finally obtained sample is 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 of PP, 0.67 kg of flake graphite, 0.03 kg of conductive carbon black, and [P 4,4,4,14 [DEHP] 0.03 kg as the ionic liquid; the pressure at the extrusion outlet is 6 MPa, and other conditions are the same as those in Example 1. The finally obtained sample is 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: 0.26 kg of PP, 0.69 kg of flake graphite, 0.01 kg of conductive carbon black, and 0.04 kg of [P 4,4,4,14 [DEHP] as the ionic liquid; the pressure at the extrusion die is 4 MPa, and other conditions are the same as those in Example 1. The finally obtained sample is 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: 0.26 kg of PP, 0.68 kg of flake graphite, 0.02 kg of conductive carbon black, and 0.04 kg of [P 4,4,4,14 [DEHP] as the ionic liquid; the pressure at the extrusion die is 5 MPa, and other conditions are the same as those in Example 1. The finally obtained sample is 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: 0.26 kg of PP, 0.67 kg of flake graphite, 0.03 kg of conductive carbon black, and 0.04 kg of [HMIm][NTf2] (the contained cation is 1-hexyl-3-methylimidazole) as the ionic liquid; the pressure at the extrusion die is 6 MPa, and other conditions are the same as those in Example 1. The finally obtained sample is abbreviated as 26% PP / 4% [HMIm][NTf2] / 67% Gr / 3% CB.

[0060] Example 7

[0061] 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 [HMIm][PF6] (the contained cation is 1-hexyl-3-methylimidazole) as the ionic liquid; the pressure at the extrusion die is 7 MPa, and other conditions are the same as those in Example 1. The finally obtained sample is abbreviated as 26% PP / 4% [HMIm][PF6] / 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 dipentyl phthalate (DAP) as a traditional plasticizer; other conditions are the same as in Example 1, and the finally obtained 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 to a three-dimensional motion mixer, mix at a speed of 12 rpm for 20 hours to obtain a solid mixture. Then use a high-speed mixer to stir at a speed of 32000 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%, among 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) Set the temperatures of the first zone (solid conveying zone), the second zone (melting zone), and the third zone (melt conveying zone) of the twin-screw extruder to 180, 195, and 210 °C respectively, set the feeding speed to 40 rpm and the screw speed to 50 rpm; set the die temperature at the discharge port to 200 °C. After reaching the set temperature, add the premix to the feeding port of the twin-screw extruder for melt mixing and extrusion. It is found that it is impossible to extrude in the form of flakes, and the pressure measured at the extrusion port reaches about 20 MPa, and the extrusion speed is very slow. Therefore, only the sample bar can be directly extruded and granulated and then hot-pressed.

[0068] (3) Granulate the sample bar obtained in step (2); set the press temperature to 220 °C, put the obtained particles into a mold coated with a release agent; after reaching the set temperature (220 °C), place the mold on the press, close the mold, keep it warm 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 to 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 to a three-dimensional motion mixer, and mix at a speed of 12 rpm for 20 hours to obtain a solid mixture. Then use a high-speed mixer to stir at a speed of 32000 rpm for 10 min; dry the mixture at 90 °C for 12 h to obtain a premix.

[0071] The total weight of the raw materials used is 100 wt%, among which, PP as a thermoplastic resin accounts for 30 wt%, and graphite accounts for 70 wt%. Conductive carbon black and ionic liquid are not added in this comparative example.

[0072] (2)Set the temperatures of the first zone (solid conveying zone), the second zone (melting zone), and the third zone (melt conveying zone) of the twin-screw extruder to 180, 195, and 210 °C respectively, set the feeding speed to 40 rpm, and the screw speed to 50 rpm; set the die temperature at the discharge port to 200 °C. After reaching the set temperature, add the premix to the feeding port of the twin-screw extruder for melt mixing and extrusion. It is found that it cannot be extruded in the form of flakes, and the pressure measured at the extrusion port also reaches about 20 MPa, and the extrusion speed is very slow. Therefore, only the sample bar can be directly extruded and granulated before hot pressing.

[0073] (3)Granulate the sample bar obtained in step (2); set the press temperature to 220 °C, put the obtained particles 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 warm 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 to obtain the composite bipolar plate. The obtained sample is abbreviated as 30%PP / 70%Gr.

[0074] Performance test and summary:

[0075] Use an RTS-9 type double-electrical measurement four-probe tester to test the in-plane electrical conductivity (σ) of the bipolar plate products in each example and comparative example. The calculation formula of the conductivity meter is shown in Formula 1. Prepare 3 samples for each example and comparative example, randomly select more than 20 points for each sample to test as the conductivity of the sample, and record the average value of the 3 samples. Before testing the conductivity, polish the surface of the sample with 400-mesh and 1000-mesh sandpaper in turn, and blow and sweep to remove the surface powder.

[0076] (1)

[0077] Among them, 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 24They are the distances between Probe 1 and Probe 3, and between Probe 2 and Probe 4 respectively.

[0078] The bending strength of the bipolar plate products in each example and comparative example was tested using a KJ-1065 universal testing machine, and the test was carried out according to the standard of 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 carried out at room temperature. 5 samples were selected for testing in each example and comparative example, and the average value was recorded. The conductivity and bending strength results are shown in Table 1.

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

[0080] ;

[0081] As can be seen from Table 1, generally speaking, the composite bipolar plates containing plasticizer and conductive ionic liquid prepared in Examples 1-7 showed relatively high conductivity and bending strength. This is attributed to the fact that the ionic liquid can effectively enhance the melt fluidity, achieve efficient compounding, thereby increasing the conductivity of the bipolar plate, while maintaining relatively high mechanical strength. It should be noted that the use of traditional DAP plasticizer and ionic liquid plasticizer will both cause a certain degree of decline in the bending performance. By controlling the content of the plasticizer, the bending performance of the composite bipolar plate can meet the application requirements. From Comparative Example 2 and Comparative Example 3, it can be seen 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 carbon black can form new conductive bridges between the flake graphite. By comparing Example 1 with Comparative Examples 1 and 2, it can be seen that both traditional plasticizer and ionic liquid plasticizer can improve the conductivity of the bipolar plate; compared with the bipolar plate using traditional DAP plasticizer, the conductivity of the bipolar plate using ionic liquid plasticizer increased by 42%, and the bending strength also increased 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 an idea and method for a composite bipolar plate with a multi-scale conductive network and easy thermoplastic molding and its preparation method. There are many specific methods and ways to implement this technical solution. The above description is only the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention. Each component not clearly defined in this embodiment can be realized by existing technologies.

Claims

1. A composite bipolar plate with a multi-scale conductive network and easy to thermoform, characterized in that, The composite bipolar plate is prepared by premixing raw materials, melt-extruding, calendering and hot-pressing powdery polypropylene, graphite, conductive carbon black and ionic liquid; 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%. The ionic liquid used as a plasticizer is bis(trifluoromethanesulfonyl)imide salt ionic liquid, bis(2-ethylhexyl) phosphate ionic liquid or hexafluorophosphate ionic liquid, and their abbreviations are [cation][NTf2], [cation][DEHP], [cation][PF6] respectively; the cation is 1,3-dialkyl-substituted imidazole or alkyltributylphosphine.

2. The composite bipolar plate according to claim 1, wherein, The graphite is natural flake graphite with a particle size of 1000 mesh.

3. The composite bipolar plate according to claim 1, wherein The particle size of the conductive carbon black is 30-45 nm.

4. The preparation method of the composite bipolar plate according to claim 1, characterized in that, It includes the following steps: (1) Weigh powdery polypropylene, graphite and conductive carbon black according to the weight percentage; prepare ionic liquid and dilute it with an appropriate amount of ethanol to obtain an ionic liquid-ethanol mixture. (2) After mixing polypropylene, graphite and conductive carbon black evenly, add the ionic liquid-ethanol mixture and carry out mechanical stirring; then carry out drying treatment to obtain a premix. (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 by die pressing to obtain a composite bipolar plate.

5. The method according to claim 4, wherein In step (2), a three-dimensional motion mixer is used to mix the three powders, the rotation speed is controlled at 12 rpm, and the mixing time is 20 h; when adding the ionic liquid-ethanol mixture, a high-speed mixer is used for stirring, the rotation speed is controlled at 32000 rpm, and the time is 10 min.

6. The method according to claim 4, wherein In step (2), the temperature during drying treatment is 90 °C and the time is 12 h.

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

8. The method according to claim 4, characterized in that, In step (3), the temperature is controlled at 220 °C, the pressure is 15 MPa, and the time is 20 min during heating and die pressing; after cooling to below 70 °C under the pressure-holding condition, the pressure is released and the mold is removed to obtain the final product.

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