CN116732814B

CN116732814B - Preparation method of multilayer gradient pore carbon paper

Info

Publication number: CN116732814B
Application number: CN202310788140.9A
Authority: CN
Inventors: 黄依可; 袁其栋; 方驰安; 李想; 周玥辰; 孔之奇; 郭大亮; 许银超
Original assignee: Zhejiang Lover Health Science and Technology Development Co Ltd
Current assignee: Zhejiang Lover Health Science and Technology Development Co Ltd
Priority date: 2023-06-30
Filing date: 2023-06-30
Publication date: 2024-09-20
Anticipated expiration: 2043-06-30
Also published as: CN116732814A

Abstract

The invention discloses a preparation method of multilayer gradient pore carbon paper, which comprises the following steps: preparing a carbon fiber slurry suspension; preparing a carbon fiber slurry suspension into a wet paper web through a wet sheet making process; stacking a plurality of wet paper webs, and drying the wet paper webs on a vulcanizing press to obtain carbon paper base paper; and (3) dipping, drying, hot pressing and preserving the carbon paper base paper to obtain the carbon paper. The invention can effectively improve the interlayer bonding effect of the carbon paper, reduce the pores and the resistivity of the carbon paper and improve the tensile strength of the carbon paper.

Description

Preparation method of multilayer gradient pore carbon paper

Technical Field

The invention relates to the field of carbon paper preparation, in particular to a preparation method of multilayer gradient pore carbon paper.

Background

The gradient pore carbon paper is a multi-layer carbon paper, has different pore characteristics among different layers, and can effectively improve the application performance of the gas diffusion layer. However, in the preparation of carbon paper with a gradient pore structure, the preparation method has an important influence on the performance of the carbon paper.

In the existing preparation process of the gradient pore carbon paper, especially for the preparation of the multilayer gradient pore carbon paper, the combination effect between layers is not ideal, more pores possibly exist, the density of the paper is smaller, and finally the performances of the carbon paper such as tensile strength and the like can be influenced. When the carbon paper is acted by external force, the carbon paper with poor bonding effect is easier to break due to uneven stress distribution.

Disclosure of Invention

The invention aims to provide a preparation method of multilayer gradient pore carbon paper. The invention can effectively improve the interlayer bonding effect of the carbon paper, reduce the pores and the resistivity of the carbon paper and improve the tensile strength of the carbon paper.

The technical scheme of the invention is as follows: the preparation method of the multilayer gradient pore carbon paper comprises the following steps:

s1: preparing a carbon fiber slurry suspension;

s2: preparing a carbon fiber slurry suspension into a wet paper web through a wet sheet making process;

S3: stacking a plurality of wet paper webs, and drying the wet paper webs on a vulcanizing press to obtain carbon paper base paper;

S4: and (3) dipping, drying, hot pressing and preserving the carbon paper base paper to obtain the carbon paper.

In the preparation method of the multilayer gradient pore carbon paper, the number of layers of the wet paper web is 2 or 3.

In the preparation method of the multilayer gradient pore carbon paper, the number of layers of the wet paper web is 3.

In the preparation method of the multilayer gradient pore carbon paper, the length of the carbon fiber of the first layer of wet paper web is 2-4mm, the carbon fiber of the second layer of wet paper web is a compound carbon fiber with the length of 2-8 mm, and the length of the carbon fiber of the third layer of wet paper web is 4-8mm.

In the preparation method of the multilayer gradient pore carbon paper, the length of the carbon fiber of the first layer of wet paper web is 4mm, the length of the carbon fiber of the second layer of wet paper web is a compound carbon fiber of 4mm and 8mm, and the length of the carbon fiber of the third layer of wet paper web is 8mm.

In the preparation method of the multilayer gradient pore carbon paper, the preparation of the carbon fiber slurry suspension comprises the following steps:

Adding Carbomer into deionized water, stirring until the Carbomer is completely dissolved, and preparing a Carbomer solution;

adding the carbon fiber and the PVA fiber into a Carbomer solution for mixing;

And adding NaOH into the mixed solution for thickening to obtain the carbon fiber slurry suspension.

In the preparation method of the multilayer gradient pore carbon paper, the mass concentration of the Carbomer solution is 0.5wt%.

In the preparation method of the multilayer gradient pore carbon paper, the wet sheet making process comprises the following steps of:

Pouring the carbon fiber slurry suspension into a paper sheet former, adding excessive NaOH, homogenizing for 1-3min, and dispersing the carbon fiber slurry suspension;

And opening a water drain valve of the paper sheet former to enable carbon fibers in the carbon fiber slurry suspension to freely settle on a copper net so as to form a wet paper web.

In the aforementioned preparation method of the multilayer gradient pore carbon paper, in S3, the drying temperature is 150 ℃, and the drying pressure is 1MPa.

In the preparation method of the multilayer gradient pore carbon paper, the steps of impregnating, drying, hot pressing and heat preserving the carbon paper base paper specifically comprise:

Immersing carbon paper base paper in a 10wt% phenolic resin solution for 30min;

Drying the impregnated carbon paper base paper in a ventilation environment, and then drying the impregnated carbon paper base paper in a forced air drying oven at 80 ℃ for 40min;

placing the dried carbon paper base paper on a flat vulcanizing machine, and hot-pressing for 40min at 150 ℃ and 10 MPa;

and (3) placing the dried carbon paper base paper in an atmosphere furnace, introducing nitrogen for protection, raising the temperature to 1500 ℃ at a speed of 30 ℃/min, and preserving the temperature for 30min to obtain the carbon paper.

Compared with the prior art, the invention has the following beneficial effects:

In the process of preparing the multilayer gradient pore carbon paper, firstly, wet paper webs are prepared through a wet sheet making process, then, a plurality of wet paper webs are overlapped to prepare multilayer carbon paper base paper, and finally, the carbon paper base paper is prepared into a carbon paper finished product. The multi-layer gradient pore carbon paper prepared by the method has the advantages that the combination between the multi-layer gradient pore carbon paper and the layers is tighter, the tensile strength of a carbon paper finished product is larger, the resistivity is smaller, and the thickness of the carbon paper prepared by the method is smaller compared with the interlayer combination mode of superposing the multi-layer base paper. In summary, the preparation method provided by the invention can effectively improve the interlayer bonding effect of the carbon paper, reduce the pores and the resistivity of the carbon paper and improve the tensile strength of the carbon paper.

Drawings

FIG. 1 is a flow chart of the present invention;

FIG. 2 shows the results of the carbon paper and binarization treatment for different combinations;

FIG. 3 is a cross section of a carbon paper prepared by different bonding methods and the result of binarization treatment;

FIG. 4 is a graph showing the comparison of properties of carbon papers prepared in various combinations;

FIG. 5 is a graph showing the results of a single-ply carbon paper and binarization treatment for different length fibers;

FIG. 6 is a graph showing the results of a bi-layer carbon paper and binarization treatment for different lengths of fiber;

FIG. 7 is a graph showing the results of a three-layer carbon paper and binarization treatment for different length fibers;

FIG. 8 is a cross-section of a carbon paper made from different length fibers and the result of a binarization process;

FIG. 9 is a graph showing the performance comparison of multi-layer carbon papers made with different lengths of fibers.

Detailed Description

The invention is further illustrated by the following figures and examples, which are not intended to be limiting.

Examples: a preparation method of multilayer gradient pore carbon paper, as shown in figure 1, comprises the following steps:

S1: preparing a carbon fiber slurry suspension, comprising the steps of: adding Carbomer (namely polyacrylic resin) into deionized water, stirring until the Carbomer (namely polyacrylic resin) is completely dissolved, and preparing a Carbomer solution, wherein the mass concentration of the Carbomer solution is 0.4-0.6wt%, and preferably, the mass concentration of the Carbomer solution is 0.5wt%; adding the carbon fiber and the PVA fiber into a Carbomer solution for mixing; and adding NaOH into the mixed solution for thickening to obtain the carbon fiber slurry suspension.

S2: preparing a carbon fiber slurry suspension into a wet paper web through a wet sheet making process; the wet sheet making process comprises the following steps: pouring the carbon fiber slurry suspension into a paper sheet former, adding excessive NaOH, homogenizing for 1-3min, and dispersing the carbon fiber slurry suspension; and opening a water drain valve of the paper sheet former to enable carbon fibers in the carbon fiber slurry suspension to freely settle on a copper net so as to form a wet paper web.

S3: and superposing a plurality of wet paper webs, and drying the paper webs on a vulcanizing press at a temperature of 150 ℃ and under a pressure of 1MPa to obtain the carbon paper base paper.

S4: the carbon paper base paper is impregnated, dried, hot pressed and heat preserved to prepare the carbon paper, which specifically comprises the following steps: immersing carbon paper base paper in a 10wt% phenolic resin solution for 30min; drying the impregnated carbon paper base paper in a ventilation environment, and then drying the impregnated carbon paper base paper in a forced air drying oven at 80 ℃ for 40min; placing the dried carbon paper base paper on a flat vulcanizing machine, and hot-pressing for 40min at 150 ℃ and 10 MPa; and (3) placing the dried carbon paper base paper in an atmosphere furnace, introducing nitrogen for protection, raising the temperature to 1500 ℃ at a speed of 30 ℃/min, and preserving the temperature for 30min to obtain the carbon paper.

In the above-described operation steps, when the carbon paper is produced, the interlayer bonding method of the base paper of the multi-layered carbon paper adopts a form in which a plurality of wet paper webs are superimposed. In addition, the method also comprises a mode of superposing a plurality of carbon paper base papers, and the specific process comprises the steps of firstly preparing the wet paper web according to the preparation mode of the wet paper web, then placing the single-layer wet paper web on a flat vulcanizing machine, drying at 150 ℃ and under the condition of 1MPa, stripping to obtain the carbon paper base papers, finally immersing the multi-layer carbon paper base papers in a 10wt% phenolic resin solution for 30min, superposing, and then carrying out impregnation, drying, hot pressing and heat preservation to obtain the carbon paper. In this example, the preparation of double-layer carbon paper and triple-layer carbon paper is described as an example, and the materials and the combination manner of the preparation of double-layer carbon paper and triple-layer carbon paper are shown in tables 1 and 2, respectively.

Table 1 double layer carbon paper material table

Numbering device	Length and amount of first layer carbon fiber	Length and amount of carbon fiber of the second layer	Bonding mode
				DL1	2Mm carbon fiber 1g	4Mm carbon fiber 1g	Wet paper web stacking
DL2	2Mm carbon fiber 1g	8Mm carbon fiber 1g	Wet paper web stacking
				DL3	4Mm carbon fiber 1g	8Mm carbon fiber 1g	Wet paper web stacking
DL4	4Mm carbon fiber 1g	8Mm carbon fiber 1g	Raw paper superposition

Table 2 three-layer carbon paper material table

The gradient pore carbon paper made by different interlayer combination modes is tested, DL3, TL3, DL4 and TL4 in the table are taken as examples, as shown in fig. 2, the front and back sides of each group of carbon paper are photographed by using a video microscope of the come card, and binarization treatment is carried out, so that the pore characteristics of different sides are analyzed. Wherein (a) and (b) of DL3 are two sides of double-layer gradient pore carbon paper prepared by a wet paper web superposition method, and (a) and (b) of DL4 are two sides of double-layer gradient pore carbon paper obtained by superposing base paper. TL3 (a) and (b) are two sides of three-layer gradient pore carbon paper prepared by stacking wet paper webs, respectively, and TL4 (a) and (b) are two sides of three-layer gradient pore carbon paper obtained by stacking base paper. In addition, (c) and (d) of each group are results obtained by subjecting the groups (a) and (b) to binarization treatment, respectively, in which black portions are positions where fibers are stacked and white is a position where pores are formed.

The two-layer gradient pore and three-layer gradient pore carbon paper prepared by the two methods has obvious difference in pore distribution on the front and back sides. This means that, in the case of satisfying the good uniformity of each group of base papers, the groups of carbon papers have differences in pore characteristics in the thickness direction, and the carbon papers are gradient pore carbon papers. In addition, the carbon paper prepared by stacking the base paper exhibits a more loose structure with the same number of layers. Wherein DL4 (a) shows a non-uniform pore distribution, a part of the positions are fiber-dense, and another part of the positions have a large number of larger pores. The pore size distribution and pore distribution in (b) of DL4 are relatively uniform, and the number of pores is greater than that of DL3 on both sides. A large number of larger voids are present in TL4 (a), and the size and distribution of voids in TL4 (b) is relatively uniform.

The cross section of the carbon paper was photographed using a Leica video microscope and subjected to binarization processing, and the result is shown in FIG. 3. The DL3 and TL3 groups are respectively double-layer gradient pore carbon paper and three-layer gradient pore carbon paper prepared by stacking wet paper webs. The DL4 and TL4 groups are respectively double-layer gradient pore carbon paper and three-layer gradient pore carbon paper which are prepared by stacking base paper. Wherein (a) is a photograph of a cross section, and (b) is a result obtained after the binarization treatment, and the white position is a position where a pore exists.

The multi-layer gradient pore carbon paper prepared by stacking the wet paper web has more compact interlayer bonding compared with the multi-layer gradient pore prepared by stacking the base paper. As shown in DL4 (a) and (b), there are a large number of macropores between the layers. In addition, the characteristics exhibited by each layer of the double-layer carbon paper in the thickness direction do not exhibit a significant difference in the thickness direction. The three-layer gradient pore carbon paper TL3 exhibits a significant pore variation in the thickness direction. This is because the multilayer base paper produced by stacking single-layer base paper has a worse layer-to-layer direct bond and more porosity than the base paper produced by stacking wet paper webs directly. Although this portion of the void will be filled with phenolic resin after impregnation with resin. However, after subsequent carbonization, the resin devolatilizes, cracks and finally carbonizes to form carbides, which have a large number of pores compared to the carbon fiber-composed part.

The bulk density, tensile index, porosity, air permeability, thickness and resistivity of the carbon paper were analyzed and the results are shown in fig. 4. In fig. 4, the bulk densities of four of DL3, DL4, TL3 and TL4 are close, and DL4 and TL4 produced from the base paper overlay are slightly larger than DL3 and TL3 produced from the wet paper web overlay. However, after the carbon paper is prepared, the bulk densities of DL3 and TL3 are greater than those of DL4 and TL4, respectively. Wherein, the bulk density of the DL3 carbon paper is 0.24g/cm ³, which is 30.33 percent higher than that of the base paper, and the bulk density increment is far more than 20.39 percent of that of DL 4. The volume density of the TL3 carbon paper is 0.25g/cm ³, which is increased by 35.48 percent compared with the original paper, and the volume density increment is far greater than 24.19 percent of TL4. This is because the multilayer base paper produced by stacking single-layer base paper has worse layer-to-layer direct bonding and more porosity than the base paper produced by stacking wet paper web directly, so that DL4 and TL4 have a higher bulk density than DL3 and TL3 having the same number of layers. Although this portion of the pores will be filled with phenolic resin after impregnation with resin, after subsequent carbonization, the resin devolatilizes, cracks and finally carbonizes to form carbide and carbon fiber components having a much smaller density than the larger number of pores. In addition, larger pores are also detrimental to the adsorption of the phenolic resin. Therefore, the base paper produced by the wet web stacking exhibits a higher bulk density increase after carbonization.

The lack of intimate bonding from layer to layer also has a significant impact on its tensile strength. Although the tensile strength of the multi-ply base paper prepared by laminating the base papers is greater, the trend is completely reversed after carbonization. The tensile strength of DL3 was 0.42kN/m, the tensile strength of DL4 was only 0.15kN/m, the tensile strength of TL3 was 0.41kN/m, and the tensile strength of TL4 was only 0.14kN/m. The tensile strength of the multi-layer carbon paper prepared by directly superposing the wet paper web is far better than that of the multi-layer carbon paper prepared by directly superposing the base paper. This phenomenon exists mainly due to the following reasons: firstly, the strength of the multi-layer carbon paper is greatly influenced by the interface between the layers, and the multi-layer base paper obtained by stacking base paper is weaker in combination between the layers after carbonization, so that when the carbon paper is subjected to external force, the stress distribution is uneven and the carbon paper is easier to break. Secondly, when the resin is impregnated, a large amount of resin adsorbed by the larger pores between the layers of the multilayer base paper of DL4 and TL4 can volatilize and decompose at high temperature, and bubbles generated in the process can cause more cracking and air holes to the multilayer carbon paper tape, so that the strength of the carbon paper is weaker.

After carbonization, the porosity of the base papers of DL3 and TL3 was reduced from 92.55% and 92.91% to 92.79% and 92.50%, respectively. Whereas the porosities of DL4 and TL4 decreased from 92.28% and 92.50% to 92.11% and 92.23%, respectively. The porosity variation amplitude of the carbon paper base paper obtained by directly superposing the base paper is obviously smaller than that of the carbon paper base paper prepared by superposing a wet paper web. This is because a large amount of resin adsorbed by the non-close bonding between the layers leaves a large number of pores after carbonization, so that the porosity of the carbon paper becomes large. For this reason, unlike other base papers, which have reduced air permeability after carbonization, the air permeability of the base papers of DL4 and TL4 increases from 1080L/(m ² s) and 750L/(m ² s) to 1100L/(m ² s) and 1080L/(m ² s) after carbonization.

Because of the lack of layer-to-layer bonding, DL4 and TL4, whether base or carbon paper, are of a greater thickness than DL3 and TL3 of the same number of layers. And the thickness difference caused by different superposition modes can be further amplified along with the increase of the layer number. The thickness of the TL4 carbon paper is 290 μm and 7% greater than the thickness of TL3 which is 275 μm. The thickness of DL4 carbon paper is 290 μm, which is 1% larger than the thickness of DL3 which is 286 μm. Both gradient pore carbon papers DL4 and TL4 prepared by superposition of base papers exhibit greater resistivity. The gradient pore carbon paper TL3 prepared by wet web stacking exhibited the lowest resistivity in each group, which was 19.53mΩ·cm.

Although the improvement of porosity and gas permeability of the carbon paper is beneficial to the gas-liquid transmission of the final gas diffusion layer, the interface effect caused by poor interlayer bonding can lead to poorer tensile strength and higher resistivity, and is not beneficial to the continuous and stable operation and good conductivity of the gas diffusion layer. In summary, different interlayer bonding methods have a great influence on the strength performance and the conductivity performance of the carbon paper. The gradient pore carbon paper layers prepared by stacking the wet paper webs are more tightly combined, show better tensile strength and resistivity, and have smaller thickness. The tensile strength of the double-layer gradient pore carbon paper and the three-layer gradient pore carbon paper is respectively 0.42kN/m and 0.41kN/m, which are far greater than that of the carbon paper overlapped by the base paper. The resistivity of the three-layer gradient pore carbon paper superimposed by the wet paper web was the smallest for each group, which was only 19.53mΩ·cm.

For the multi-layer gradient pore carbon paper prepared by stacking the wet paper webs, the difference of fiber lengths also affects the performance of the carbon paper, in this embodiment, three kinds of carbon fibers of different lengths of 2mm, 4mm and 8mm are used to prepare single-layer carbon papers SL1, SL2 and SL3, double-layer gradient pore carbon papers DL1, DL2 and DL3, and three-layer gradient pore carbon papers TL1, TL2 and TL3. The materials of the single-layer carbon paper are shown in table 3, and the preparation method comprises the steps of placing a single wet paper web on a flat vulcanizing machine, drying at 150 ℃ and under 1MPa, stripping to obtain carbon paper base paper, and preparing the carbon paper base paper into carbon paper.

Table 3 single layer carbon paper material table

Numbering device	Length and amount of carbon fiber	Bonding mode
			SL1	2Mm carbon fiber 2g	Wet paper web stacking
SL2	4Mm carbon fiber 2g	Wet paper web stacking
			SL3	8Mm carbon fiber 2g	Wet paper web stacking

The come video microscope was used to take pictures of the front and back sides of each group of single-layer carbon papers, and binarization was performed, and the results are shown in fig. 5. The front and back sides of each group of single-layer carbon paper respectively show similar pore characteristics, wherein the pores in SL1 are unevenly distributed and mostly have larger pores. The SL2 and SL3 pore locations are uniformly distributed, and the pore sizes are also uniformly distributed. SL1 prepared from 2mm carbon fiber showed a larger number of larger size pores observed on the surface than SL2 prepared from 4mm and SL3 prepared from 8mm, and the distribution of these pores was more concentrated. This is because, after carbonization, the polyvinyl alcohol fibers and the phenolic resin that originally provided strength to the carbon paper are carbonized, and at this time, the strength of the carbon paper is mainly provided by the carbon material itself. Carbon paper made from shorter fibers has weaker interaction forces between the fibers. Carbon paper is prone to fracture when subjected to force. In the carbonization process, phenolic resin and polyvinyl alcohol fibers are thermally decomposed at high temperature to generate gas, and the porosity and air permeability of base paper prepared from shorter fibers are poor, so that thermal stress can be generated on carbon paper when the gas cannot be released in time. In addition, phenolic resins and polyvinyl alcohol fibers shrink when heated. Therefore, the carbon paper prepared from the shorter carbon fibers is more easily affected by thermal stress and shrinkage in the high-temperature carbonization process, so that larger pores and cracks appear in the carbon paper, and the pores are distributed in a concentrated manner.

Photographs of the front and back sides of each group of double-layered gradient pore carbon paper and three-layered gradient pore carbon paper were taken using a Leica video microscope, and binarized, and the results are shown in FIGS. 6 and 7. The front photographs are shown in (a) and (b), and the binarization results are shown in (c) and (d). DL1 and TL1 prepared using 2mm and 4mm both present large pores and uneven pore distribution. And likewise DL2 and TL2 with short fibers added do not show maldistribution of pores. Both the front and back sides of the SL2 and SL3 groups of carbon papers showed a certain difference. The (a) plane of SL2 is larger in pore size than the (b) plane. The (a) plane of SL3 has a similar size distribution of pores as compared to the (b) plane, but the number of pores is relatively small. TL2 and TL3 exhibit a large difference between the opposite sides. DL2 is (a) more dense, with smaller pore numbers and pore sizes than the (b) plane. The pores of the (b) plane of DL2 are also larger than the other groups of pores while being uniformly distributed. The (a) plane of DL3 is more porous than the (b) plane, and the pore size and the number of pores are larger.

Cross-sectional views of each group of carbon papers were photographed using a video microscope of the come card, and binarized, and the results are shown in fig. 8. The single-layer carbon papers SL1, SL2, and SL3 have a relatively even pore distribution in the thickness direction, and the pore pairs extend in the planar direction. There are a large number of larger voids in SL1, and a large number of voids in SL2 and SL3, but both are smaller in size. DL1 is smaller in thickness than SL1, but there are also a large number of voids of larger area therein, and the sites where the layers are bonded behave loosely, almost all voids of larger size. TL1 shows a gradient difference, and from (b) it can be seen that the pore size and number become progressively larger from top to bottom. DL2 and DL3 show a distinct gradient difference, and DL2 is clearly divided into two parts, the upper part of the carbon paper is more tightly represented in the thickness direction, and the lower part of the carbon paper has more pores. TL2 does not exhibit a significant three-layer difference, but it can be observed that there are more and larger pores in the lower part of the carbon paper. The uppermost part of DL3 exhibits a more dense structure, while the lower part exhibits similar pore characteristics, with a certain number of larger pores. TL3 shows a similar situation, with the upper part of the carbon paper being more dense than the lower part, with a smaller number of pores and smaller pore size.

In summary, when the uniformity of the base paper is good and the carbon distribution in the carbon paper is uniform, the pores on the front and back sides of DL2, TL2, DL3 and TL3 are uniformly distributed, and the number of pores and the size of the pores are obviously different. In addition, the cross sections of the four groups of carbon papers show a gradient change in the pore in the thickness direction. Therefore, when carbon fibers with different lengths and longer carbon fibers are contained in the carbon fibers, the gradient pore carbon paper with uniform pore distribution and gradient change of plane pore characteristics can be prepared by a mode of stacking the wet paper web.

The bulk density, tensile strength, porosity, air permeability, thickness and resistivity of each set of paper samples were analyzed, and the results are shown in fig. 9. The bulk density of each set of paper patterns was compared to find that the bulk density of each set of base paper decreased as the number of superimposed layers of the wet web increased. But shows the opposite trend after the carbon paper is produced. The bulk densities of the two-layer gradient pore carbon paper DL3 and the three-layer gradient pore carbon paper TL3 prepared using 4mm and 8mm were 0.24g/cm3 and 0.25g/cm3, respectively. The bulk densities of the two-layer gradient pore carbon paper DL1 and the three-layer gradient pore carbon paper TL1 prepared using 2mm and 4mm were only 0.231g/cm3 and 0.25g/cm3, respectively. In the case of the number of superimposed layers of the wet paper web, the bulk densities of DL3 and TL3 using longer carbon fibers are both greater than DL1 and TL1. In addition, when carbon fibers of the same length are used, the bulk density of the three-layer gradient pore carbon paper TL1, TL2 and TL3 is 0.25 to 0.25g/cm3, and the bulk density of the two-layer gradient pore carbon paper DL1, DL2 and DL3 is 0.23 to 0.24g/cm3. Three-layer gradient pore carbon paper exhibits greater bulk density.

Comparing the bulk density increases for each set of carbon papers, it was found that SL1, DL1 and TL1 prepared using shorter fibers all exhibited lower bulk density increases with the same number of superimposed layers of wet paper web. With the same fiber length, the bulk density increment of the double-layer gradient pore carbon paper is smaller than that of the single-layer carbon paper with the increase of the number of layers of the laminated wet paper web, but the bulk density increment of TL3 is increased to 35.49 percent and is larger than 34.14 percent of SL3 with the continuous increase of the number of layers.

The porosity of the base paper was found to be substantially between 91.55% and 93.25% as compared to the porosity of each set of paper patterns, and the porosity of the carbon paper base paper prepared using longer fibers was higher. After the carbon paper is prepared, the porosity change is completely opposite. With the same number of superimposed layers of wet paper web, the carbon paper prepared using longer fibers showed smaller porosity, with a minimum of 88% for SL3, and nearly 90.47% and 90.20% for DL3 and TL3, respectively. Similar phenomena can also be found when the surfaces of the various groups of carbon papers are observed by photographs as previously, carbon papers made using longer fibers tend to exhibit fewer and smaller pores on the surfaces.

The air permeability of each group of paper patterns was compared, and it was found that the air permeability of the base paper steadily increased with the increase of the fiber length, but the air permeability of the carbon paper changed greatly with the change of the number of superimposed layers of the wet paper web and the fiber length. For single-layer carbon paper, the air permeability of the carbon paper becomes worse with increasing fiber length, and the air permeability of SL3 is only 700L/(m ² s). For the multi-layered gradient pore carbon paper, DL2 and TL2 prepared using 2mm and 8mm carbon fibers exhibit greater air permeability when the number of superimposed layers of the wet paper web is the same, and the air permeability thereof is 1150L/(m ² s) and 1230L/(m ² s), respectively. While DL1 and TL1 prepared using 2mm and 4mm carbon fibers can observe unevenly distributed larger voids in fig. 5 and 6, the two sets of carbon papers are inferior in air permeability, which is only 811L/(m ² s) and 900L/(m ² s). This means that a uniform distribution of pores is advantageous to some extent for enhanced breathability.

The thickness of the base paper was found to increase significantly with increasing carbon fiber length by comparing the thickness of each set of patterns. For single layer carbon paper, SL3 prepared using 8mm carbon fiber exhibited a maximum thickness of 370 μm. For multi-layered gradient pore carbon paper, when the wet paper web is stacked in two layers, the carbon paper has a greater thickness, DL2 and DL3 having thicknesses of 292 μm and 286 μm, respectively, and greater than those of TL2 and TL 3. By combining the bulk density and bulk density increment of each group of carbon paper, it can be found that the multi-layer gradient pore carbon paper using longer fibers has a thickness and bulk density larger than those of single-layer carbon paper and multi-layer gradient pore carbon paper prepared by using shorter fibers, which means that the pore structure with gradient change is designed in the thickness direction, which is beneficial to densification of the carbon paper.

As can be seen by comparing the resistivity of the paper patterns of the respective groups, the resistivity of the base paper increases with increasing fiber length. And after the carbon paper is prepared, the resistivity of each group of carbon paper is not too different. Wherein TL2 and TL3 have relatively low resistivity of 23.47mΩ cm and 19.53mΩ cm, respectively.

Such variations in the sets of base papers occur because the bond between the layers of the base papers made by the wet-web overlay is less tight than the direct wet-formed single layer base papers, resulting in more porosity and greater thickness and more prone to cracking when subjected to external forces. As the number of layers of the laminated moistened web increases, the tensile strength decreases, the air permeability increases, and the resistivity increases.

Such variation occurs for each group of carbon papers because the multi-layered gradient pore carbon papers exhibit good densification effects and have greater bulk densities after increasing the fiber length. Thus exhibiting better strength and conductivity. Each layer in the gradient pore carbon paper has different pore characteristics, so that the gradient pore carbon paper can improve the air permeability by utilizing the Bernoulli effect. The bernoulli effect refers to the variation in pressure caused by the variation in flow rate of a gas in a channel. The gas flow rate is now faster as the air passes through the larger pore levels, and hence the pressure of the gas is lower here, which promotes easier passage of the gas. When the gas reaches the level of the smaller pores, the gas flow rate will slow down as the channels narrow, at which time the pressure of the gas will become greater, and the high pressure of the gas will facilitate the gas to overcome the narrowing of the channels, forcing it through the level of the smaller pores. In addition, the multi-layer gradient pore carbon paper also shows more uniform pore distribution. The gradient pore carbon papers DL2, DL3, TL2 and TL3 exhibit better air permeability.

The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the scope of the claims of the present invention should be included in the scope of the present invention.

Claims

1. A preparation method of multilayer gradient pore carbon paper is characterized by comprising the following steps: the method comprises the following steps:

s1: preparing a carbon fiber slurry suspension;

S3: stacking the multi-layer wet paper webs, and drying the multi-layer wet paper webs on a vulcanizing press to obtain carbon paper base paper;

S4: dipping, drying, hot pressing and preserving the carbon paper base paper to prepare carbon paper;

The number of layers of the wet paper web is 3; the length of the carbon fiber of the first wet paper web is 4mm, and the length of the carbon fiber of the first wet paper web is 0.6g; the length of the carbon fiber of the second wet paper web is 4mm and the length of the carbon fiber of the second wet paper web are 8mm, and the length of the carbon fiber of the second wet paper web is 0.35g; the length of the carbon fiber of the third wet paper web is 8mm, and the length of the carbon fiber of the third wet paper web is 0.6g;

the preparing of the carbon fiber slurry suspension comprises the following steps:

adding the carbon fiber and the PVA fiber into a Carbomer solution for mixing;

adding NaOH into the mixed solution for thickening to obtain carbon fiber slurry suspension;

the mass concentration of the Carbomer solution is 0.5wt%;

the wet sheet making process comprises the following steps:

2. The method for preparing the multi-layer gradient pore carbon paper according to claim 1, wherein the method comprises the following steps: in S3, the drying temperature is 150 ℃ and the drying pressure is 1MPa.

3. The method for preparing the multi-layer gradient pore carbon paper according to claim 1, wherein the method comprises the following steps: the carbon paper base paper is impregnated, dried, hot pressed and heat-preserved, and the method specifically comprises the following steps:

Immersing carbon paper base paper in a 10wt% phenolic resin solution for 30min;