Method for improving mechanical properties of carbon paper by aramid nanofiber modified phenolic resin

Abstract

The application discloses a method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofiber, which comprises the following steps: S1, preparing a 8-15wt% phenolic resin-ethanol solution, adding 0.5%-2.5% aramid nanofiber based on the mass of the phenolic resin-ethanol solution, and ultrasonic stirring and dispersing to obtain a mixed solution; S2, immersing the carbon paper base paper in the mixed solution of step 1 and performing ultrasonic treatment for 15-25 min, then taking out the carbon paper base paper and laying it flat to ensure the uniformity of resin penetration, and taking it as a prefabricated part; S3, drying the prefabricated part, and then performing compression molding by hot pressing in air at a temperature of 140-160 DEG C and a pressure of 2-5 MPa for 0.5-1.5 h; and S4, carbonizing the hot-pressed and cured carbon paper in a high-temperature tube furnace, taking out the finished product after heating at a rate of 8-12 DEG C / min, sintering at a temperature of 1000-1200 DEG C, and holding for 0.5-1.5 h. The application can enhance the mechanical properties of carbon paper, and the process is simple, and the practicality of carbon paper in fuel cells can be improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of carbon paper, and particularly relates to a method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofiber. BACKGROUND

[0002] Proton exchange membrane fuel cell (PEMFC) has the advantages of high efficiency, zero emission when using hydrogen as fuel, fast start, low noise, etc., and is a very promising energy conversion device for transportation and power generation. The membrane electrode assembly (MEA) that generates electricity is the core part of PEMFC. The MEA is usually composed of a gas diffusion layer, a catalyst layer and a membrane layer. The gas diffusion layer (GDL) is considered to be a key component of the MEA, which is usually composed of carbon paper and a microporous layer, and the anode and cathode catalyst layers are usually directly coated on the membrane to form a so-called catalyst-coated membrane (CCM). On each side of the CCM, the carbon paper has the functions of mass transfer, electron collection and heat transfer. More importantly, it provides strong structural support and protection for the CL and PEM during assembly and operation. Therefore, the carbon paper is required to have good mechanical strength, electrical conductivity, thermal conductivity, corrosion resistance, air permeability and water management capability.

[0003] The carbon paper (CP) is prepared by using short-cut carbon fibers as the main raw material, through wet papermaking, resin impregnation, hot pressing, carbonization and graphitization. The resin impregnation method refers to using a polymer binder such as phenolic resin to improve the mechanical strength and water resistance. The carbonization can significantly improve the electrical conductivity and structural stability of the CP. However, the phenolic resin film expands between the carbon fibers during the hot pressing process, which can cause uneven pore size and distribution of the carbon paper, and the defect of low porosity. More importantly, during the carbonization process, the organic components in the resin will also be decomposed and removed as the pyrolysis treatment temperature increases, although the graphite structure of the carbon fibers and the resin itself is gradually improved as the temperature increases, but the release and removal of part of the substances in the process make the fibers and the resin carbon between the fibers loose, and the binding force decreases, thereby negatively affecting the electrical conductivity and strength of the carbon paper. SUMMARY

[0004] The present application aims to provide a method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofiber. The present application can enhance the mechanical properties of the carbon paper, and the process is simple, which can improve the practicability of the carbon paper in fuel cells.

[0005] To solve the above technical problems, the technical solution provided by the present application is as follows: a method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofiber, comprising the following steps:

[0006] S1, prepare a 8-15wt% phenolic resin-ethanol solution, add 0.5%-2.5% aramid nanofiber by mass of the phenolic resin-ethanol solution, and ultrasonically stir and disperse to obtain a mixed solution;

[0007] S2, immerse the carbon paper base paper in the mixed solution of step 1 and perform ultrasonic treatment for 15-25min, then lay the carbon paper base paper after taking it out to ensure uniformity of resin penetration, and use it as a preform;

[0008] S3, dry the preform, and then perform compression molding by hot pressing in air at a temperature of 140-160℃ and a pressure of 2-5MPa for 0.5-1.5h through a sulfurization bed;

[0009] S4, carbonize the hot-pressed and cured carbon paper in a high-temperature tube furnace at a heating rate of 8-12℃ / min, a sintering temperature of 1000-1200℃, and a holding time of 0.5-1.5h, and then take out the product after cooling.

[0010] The method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofiber described above, in step 1, a 12wt% phenolic resin-ethanol solution is prepared, and 1.8% aramid nanofiber by mass of the phenolic resin-ethanol solution is added.

[0011] The method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofiber described above, in step S1, ultrasonic stirring is performed for 10min.

[0012] The method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofiber described above, in step S2, ultrasonic treatment is performed for 20min.

[0013] The method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofiber described above, in step S3, the preform is dried in an 80℃ oven for one hour to remove the solvent.

[0014] The method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofiber described above, in step S3, compression molding is performed by hot pressing in air at a temperature of 150℃ and a pressure of 4MPa for 1h through a sulfurization bed.

[0015] The method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofiber described above, in step S4, the heating rate is 10℃ / min, the sintering temperature is 1100℃, and the holding time is 1h.

[0016] Compared with the prior art, the application firstly makes aramid nanofiber have good adhesion by taking it as a modified reinforcing agent of phenolic resin, then ensures the uniformity of resin penetration by impregnating and paving carbon paper, so as to obtain a prefabricated part, then ensures that the material is fully cured and cross-linked before carbonization by drying and compression molding of the prefabricated part, and finally obtains a finished product through carbonization. The carbon paper finished product prepared by the application has good mechanical properties, pore structure and distribution, and good improvement of electrical conductivity and air permeability. The performance of the carbon paper prepared by the application can improve the performance of a fuel cell, and the preparation process of the application is simple and convenient, and has good application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is the stability analysis of PF impregnation liquid with different ANF modification ratios (0, 0.6wt%, 1.2wt%, 1.8wt%, 2.4wt%); wherein, (a) is TSI stability, (b) is a real picture of ANF-PF dispersion impregnation liquid, (c) is ANF-PF-0.6, and (d) is a backscattering light intensity change trend chart of ANF-PF-1.8 suspension dispersion process;

[0018] Figure 2 (a) in the figure is the thermogravimetry of PF impregnation liquid with different ANF modification ratios (0, 0.6wt%, 1.2wt%, 1.8wt%, 2.4wt%); Figure 2 (b) in the figure is the corresponding infrared chart;

[0019] Figure 3 (a) in the figure is the Raman spectrum of PF matrix carbon with different ANF modification ratios (0 and 1.8wt%) at 700℃, Figure 3 (b) in the figure is the Raman spectrum of PF matrix carbon at 1100℃, Figure 3 (c) in the figure is the x-ray diffraction image of PF matrix carbon at 700℃, Figure 3 (d) in the figure is the x-ray diffraction image of PF matrix carbon at 1100℃;

[0020] Figure 4 is the SEM analysis of carbon paper with different ANF modification ratios (0, 0.6wt%, 1.2wt%, 1.8wt%, 2.4wt%); wherein, (a-e) are carbon paper surfaces, (a1-e1) are carbon fibers, and (a2-e2) are PF matrix carbons;

[0021] Figure 5 is the Raman spectrum of carbon paper with different ANF modification ratios (0 and 1.8wt%) at 700℃ (a) and 1100℃ (b) respectively;

[0022] Figure 6(a-c) are cross-sectional SEM images of ANF-PF / CP-0, ANF-PF / CP-1.2 and ANF-PF / CP-2.4; Figure 6 (d-g) are bulk density and thickness (d), electrical resistivity (e), tensile strength (f) and flexural strength (g) of carbon papers with different ANF modification ratios (0, 0.6wt%, 1.2wt%, 1.8wt%, 2.4wt%);

[0023] Figure 7 (a) are pore size distribution and (b) are air permeability and porosity of carbon papers with different ANF modification ratios (0, 0.6wt%, 1.2wt%, 1.8wt%, 2.4wt%);

[0024] Figure 8 are performance (a-c) and EIS (d-f) of PEMFC of carbon papers with different ANF modification ratios (0, 0.6wt%, 1.2wt%, 1.8wt%, 2.4wt%) at 20, 40 and 60 kgf.cm. DETAILED DESCRIPTION

[0025] The application will be further described below in connection with the embodiments and drawings, but not as the basis for limiting the application.

[0026] Example 1: A method for improving the mechanical properties of carbon paper by aramid nanofiber modified phenolic resin, raw materials are as follows: carbon paper raw paper, phenolic resin, ethanol (99%) and aramid nanofiber (purchased from commercial), including the following steps:

[0027] S1, prepare 8wt% phenolic resin-ethanol solution, add 0.5wt% aramid nanofiber (ANF) to the phenolic resin-ethanol solution, ultrasonic stirring and dispersing for 10 min, get the mixed solution, save it as adhesive (ANF-PF);

[0028] S2, immerse the carbon paper raw paper (CPRP) in the mixed solution of step 1 and perform ultrasonic treatment for 18 min, then lay the carbon paper raw paper after taking it out to ensure the uniformity of resin penetration, and use it as a preform;

[0029] S3, dry the preform in a 75℃ oven for one hour to remove the solvent, and the resulting sample is called Dry Carbon Paper Raw Paper (DCPRP), then compress it by hot pressing in a sulfurization bed at a temperature of 145℃ and a pressure of 5MPa for 0.8h in air to ensure sufficient curing and cross-linking of the adhesive before carbonization, which is called Hot Press Carbon Paper Raw Paper (HPCPRP);

[0030] S4, carbonize the hot-pressed and cured carbon paper in a high-temperature tube furnace, the heating rate is 8°C / min, the sintering temperature is 1050°C, the holding time is 0.8 hours, and the finished product is obtained after cooling.

[0031] Example 2: A method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofiber, raw materials are as follows: carbon paper raw paper, phenolic resin, ethanol (99%), and aramid nanofiber (purchased from commercial), including the following steps:

[0032] S1, prepare a 15wt% phenolic resin-ethanol solution, add 2.5wt% aramid nanofiber based on the mass of the phenolic resin-ethanol solution, ultrasonic stirring and dispersion for 10 min to obtain a mixed solution, which is saved as an adhesive (ANF-PF);

[0033] S2, immerse the carbon paper raw paper in the mixed solution of step 1 and perform ultrasonic treatment for 25 min, then lay the carbon paper raw paper flat to ensure uniform resin penetration, and use it as a preform;

[0034] S3, dry the preform in an 80°C oven for one hour to remove the solvent, and the resulting sample is called Dry Carbon Paper Raw Paper (DCPRP), then compress it by vulcanization bed at a temperature of 155°C and a pressure of 3MPa for 1.2h in air to ensure sufficient curing and cross-linking of the adhesive before carbonization, which is called Hot Press Carbon Paper Raw Paper (HPCPRP);

[0035] S4, carbonize the hot-pressed and cured carbon paper in a high-temperature tube furnace, the heating rate is 12°C / min, the sintering temperature is 1200°C, the holding time is 1.5 hours, and the finished product is obtained after cooling.

[0036] Example 3: A method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofiber, raw materials are as follows: carbon paper raw paper, phenolic resin, ethanol (99%), and aramid nanofiber (purchased from commercial), including the following steps:

[0037] S1, prepare a 12wt% phenolic resin-ethanol solution, add 1.8wt% aramid nanofiber based on the mass of the phenolic resin-ethanol solution, ultrasonic stirring and dispersion for 10 min to obtain a mixed solution, which is saved as an adhesive (ANF-PF);

[0038] S2, immerse the carbon paper raw paper (CPRP) in the mixed solution of step 1 and perform ultrasonic treatment for 20 min, then lay the carbon paper raw paper flat to ensure uniform resin penetration, and use it as a preform;

[0039] S3, drying the preform in an oven at 80℃ for 1 hour to remove the solvent, the resulting sample is called Dry Carbon Paper Raw Paper (DCPRP), then compression molding by hot press in air at 150℃ and 4MPa for 1 hour to ensure sufficient curing and cross-linking of the binder material before carbonization, called Hot Press Carbon Paper Raw Paper (HPCPRP);

[0040] S4, carbonization of the hot-pressed and cured carbon paper in a high-temperature tube furnace, the heating rate is 10℃ / min, the sintering temperature is 1100℃, and the holding time is 1 hour, and the finished product is obtained after cooling.

[0041] Test Example: Prepare a 12wt% phenol-formaldehyde resin (PF)-ethanol solution as a mixed solution, add different proportions (0, 0.6%, 1.2%, 1.8%, 2.4%) of aramid nanofiber (ANF), ultrasonic stirring for 10 min to ensure uniform dispersion in the mixed solution, and save it as an adhesive (ANF-PF). Then immerse the carbon paper raw paper (CPRP) in it and ultrasonic treat for 20 min and then lay it flat with a glass rod after taking it out to ensure uniform resin penetration, and use it as a preform. Then dry the preform in an oven at 80℃ for 1 hour to remove the solvent, the resulting sample is called Dry Carbon Paper Raw Paper (DCPRP). Then compression molding by hot press in air at 150℃ and 4MPa for 1 hour to ensure sufficient curing and cross-linking of the binder material before carbonization, called Hot Press Carbon Paper Raw Paper (HPCPRP).

[0042] Carbonization of the hot-pressed and cured carbon paper in a high-temperature tube furnace, the heating rate is 10℃ / min, the sintering temperature is 1100℃, and the holding time is 1 hour, and the finished product is obtained after cooling.

[0043] In this test example, the following tests were performed to test the performance and parameters of the corresponding steps and the obtained carbon paper.

[0044] 1.1, using TURBISCAN LAB multiple light scattering instrument to determine the stability of PF and different ANF modified PF impregnation solutions. Different ANF modified PF impregnation solutions are added to the transparent quartz sample cell of the instrument, and the sample cell is vertically placed in the isothermal chamber of the instrument. Set the test program, the test time is 30 min, and scan once every 25 s.

[0045] 1.2. Fourier transform infrared spectroscopy (FTIR) was used to test the impregnated paper samples and the hot-pressed paper samples respectively. By comparing the FTIR results of the two samples, the effect of boron infiltration on the chemical structure evolution during resin curing was characterized. Before testing, the samples were ground into powder and prepared using the KBr pellet method. The selected scan wavenumbers for testing were 4000–400 cm⁻¹. -1 The range was 32 scans, with a resolution of 2cm. -1 The dried PF, ANF, and ANF-PF were characterized using Fourier transform infrared spectroscopy (FTIR-8400S). The FTIR scanning wavenumber range was 4000-500 cm⁻¹. -1 .

[0046] 1.3 Thermogravimetric analysis (TGA) of the carbon paper was performed using a Netzsch TG209F3 Tarsus instrument (Germany). The sample was placed in an alumina crucible and heated from room temperature to 800°C at a rate of 10°C / min under nitrogen atmosphere. The nitrogen flow rate was 100 ml / min.

[0047] 1.4. The surface carbon structure of the PF matrix carbon was analyzed using Raman spectroscopy. The Raman spectrometer had a confocal lateral resolution of 1 μm and a measurement range of 400–4000 cm⁻¹. -1 The carbon content of the PF matrix was scanned separately. Analysis of the test results yielded the D peak (1360 cm⁻¹). -1 ) and G peak (1580cm) -1 The crystallinity of a sample is quantitatively characterized by the ratio of the peak areas of the two peaks, ID / IG.

[0048] 1.5. X-ray diffraction patterns of carbon paper and PF matrix carbon were recorded using an X-ray diffractometer (Bruker D8 Advance, Germany) with a 40 kV, 40 mA C Kα radiation source. Diffraction signals were measured at room temperature within the 2θ scattering range of 10–60° at a scan rate of 4° / min and a step size of 0.02°. Peak fitting was performed on the test results to calculate the crystallite parameters of carbon paper and PF matrix carbon.

[0049] The interlayer spacing d of the graphite microcrystals in the sample 002 Calculated using Bragg's formula:

[0050]

[0051] The thickness Lc of the graphite crystal layer in the sample was calculated using the Scherer equation:

[0052]

[0053] In the formula: λ is the incident X-ray wavelength, which is 0.154 nm in this experiment; β is the full width at half maximum (FWHM), in rad; θ is the diffraction angle of the 002 peak, in rad.

[0054] 1.6 The carbon paper was visualized under a SU1510 scanning electron microscope with an accelerating voltage of 10kV. The SEM images provided information on the morphology and fracture cross-section of the carbon paper.

[0055] 1.7 Weigh the carbon paper using an analytical balance with an accuracy of 0.0001g, measure the thickness using a thickness gauge, measure the length and width using a ruler with an accuracy of 0.001m, and finally calculate the bulk density of the carbon paper using the mass-volume method.

[0056] 1.8. The mechanical properties of the carbon paper were tested using a general-purpose mechanical testing machine (KYD-2000NS, China). Tensile strength was tested according to ASTM D828 standard. The carbon paper was cut into rectangular pieces (70×10mm). 2 The tensile rate was 10 mm / min. Bending strength was measured using a three-point bending test, with a span of 30 mm and a descent rate of 0.5 mm / min. Five repeated measurements were performed for each test.

[0057] 1.9 The surface resistivity of paper samples was tested using a four-probe method to investigate the effect of ANF-modified carbon paper on surface resistivity. The testing procedure and principle are as follows: Four probes arranged in a straight line were pressed vertically onto the surface of the sample under a certain pressure. A current was passed between the two outermost probes, generating a certain voltage between the two middle probes. This voltage was measured, and the surface resistivity of the sample was calculated using the following formula.

[0058] P=V / I×F(D / S)×F(d / S)×d×Fsp;

[0059] Where ρ is the surface resistivity in mΩ·cm; D is the sample diameter in mm; S is the average probe spacing in mm (1.59 mm in this experiment); d is the sample thickness in mm; Fsp is the probe spacing correction factor (1.00 in this experiment); F(D / S) is the sample diameter correction factor (4.532 in this experiment); F(D / S) is the sample thickness correction factor (1 in this experiment); V is the voltage between the two middle probes in mV; and I is the current between the two outermost probes in mA. Two parallel samples were tested in each group, and 10 points were randomly selected from each sample for testing. The average value was then taken.

[0060] 1.10. X-ray photoelectron spectroscopy (XPS) was used to scan the surface of the carbon paper samples. The chemical structure evolution of the modified carbon paper was determined by analyzing the surface chemical composition. Test conditions: The X-ray excitation source was an AlKα monochromator target X-ray source with a power of 180W; the spot size irradiated on the sample surface was 700μm.

[0061] 1.11. The internal porosity of the carbon paper was measured using a mercury porosimeter (Micromeritics AutoPore V9620, USA). The interconnected porosity and pore size distribution (PSD) of the sample were determined in detail using the mercury porosimetry method. This reflects the ease with which internal gases can penetrate and be expelled from the sample.

[0062] 1.12. The air permeability of the carbon paper base paper was analyzed using an air permeability tester (FX3000). The base paper was placed on the stage of the air permeability tester, and the test area was selected as 20 cm². 2 The test pressure was set to 400 Pa, and the analysis was performed according to the instrument's operating instructions. Data was recorded after the instrument readings stabilized. At least five points were tested for air permeability for each sample, and the average value was taken to obtain the air permeability of that sample.

[0063] 1.13. Fuel Cell Performance Analysis: Using CP as the gas diffusion layer in a proton exchange membrane fuel cell, it needs to be cut into sections with an area of ​​2×2 cm². 2 The blocks were then used in 3×3cm format. 2 A three-layer membrane electrode assembly was formed using a commercially available Johnson Matthey (HISPEC 9100) catalytic coated membrane (CCM). The CCM thickness was 40 μm. The catalyst Pt / C anode loading was 0.12 mg / cm³. 2 The cathode concentration is 0.48 mg / cm². 2 This invention primarily investigates the influence of carbon paper conductive carbon materials, without subjecting the carbon paper to microporous layer treatment. No hot-pressing treatment was performed on the CCM and carbon paper; instead, torques of 20, 40, and 60 kgf.m were used during fuel cell assembly to ensure tight contact between the layers.

[0064] The MEA was conducted within a fuel cell testing system. The activation area was 4 cm². 2The bipolar plates were serpentine flow-channel graphite plates made of highly compacted graphite. The gas flow rate at the anode (H2) was 200 cc / min (humidifier temperature maintained at 70°C), and the gas flow rate at the cathode (O2) was 300 cc / min (humidifier temperature maintained at 40°C). The battery temperature was set to 60°C during the test. The single cell was activated according to the procedure described in the reference before measurement. The polarization behavior of the single cell was recorded using a programmable DC electronic load (Faith Instruments). Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical analyzer (chi660E, Shanghai Chenhua Instruments Co., Ltd.), with a frequency range of 1 kHz to 0.1 Hz and an AC voltage amplitude of 5 mV.

[0065] The results and analysis are as follows:

[0066] 2.1 Dispersion effect of aramid nanofiber modified phenolic resin impregnation solution;

[0067] To investigate the effect of ANF modification on the dispersion of PF impregnation solutions, stability analyses were performed on PF impregnation solutions with different ANF modification ratios (0.6 wt%, 1.2 wt%, 1.8 wt%, and 2.4 wt%). Figure 1 As shown. The TSI stability index reflects the cumulative change in backscattered light intensity over time; the smaller the value, the stronger the system stability. From Figure 1 As shown in Figure a, as the ANF modification ratio increased from 0.6 wt% to 1.8 wt%, the TSI stability index of the PF impregnation solution decreased from 29.19 to 0.79, a decrease of 97.29%. This indicates that the stability of ANF in PF increases with the increase of the ANF modification ratio. Furthermore, from... Figure 1 As shown in c and d, as time increases from 0 to 30 min, when the ANF modification ratio is 1.8 wt%, the PF impregnation solution exhibits high stability. However, when the ANF modification ratio is 0.6 wt%, the tendency for ANF to aggregate at the bottom of the sample cell becomes more pronounced within 30 minutes, making it unable to maintain stability in the PF impregnation solution. Figure 1 b also shows that ANF precipitation occurs in the PF impregnation solution with an ANF modification ratio of 0.6 wt%, while the impregnation solution with an ANF modification ratio of 1.8 wt% exhibits higher stability. The reason for this phenomenon may be the mutual repulsion between the polar groups in ANF and the polar groups in PF, as well as the bridging effect between ANF nanoparticles, thereby enhancing the high stability of ANF in the PF impregnation solution.

[0068] 2.2 Performance of aramid nanofiber modified phenolic resin impregnation solution;

[0069] The thermogravimetric properties of PF impregnation solutions with different ANF modification ratios (0.6wt%, 1.2wt%, 1.8wt%, and 2.4wt%) are shown in [reference needed]. Figure 2 As shown. From Figure 2 As shown in Figure a, as the ANF modification ratio increased from 0 to 2.4 wt%, the char residue of PF after high-temperature pyrolysis gradually decreased from 59.60% to 55.53%, with the char residue of ANF-modified PF consistently lower than that of PF. This indicates that the char residue of the PF impregnation solution decreases with increasing ANF modification ratio. This is likely because ANF has a relatively low char residue. Figure 2 As can be seen from b, the modified PF impregnation solution molecular chain contains a large number of amide bonds, as evidenced by the NH bending vibration peak at 1530 cm⁻¹, the C=O stretching vibration peaks at 820 cm⁻¹ and 1629 cm⁻¹, and the peak at 1330 cm⁻¹. -1 The CN stretching vibration peaks indicate the presence of amide bonds in the molecular chain, suggesting that ANF was successfully doped into PF impregnation.

[0070] 2.3 XRD Raman spectroscopy;

[0071] The carbon structures of the PF matrix with different ANF modification ratios (0 and 1.8 wt%) are shown in the figure. Figure 3 As shown. Two sharp peaks appear approximately at 1350 and 1580 cm⁻¹ in the spectrum. -1 These can be respectively assigned to defects in the D band, the structure of disordered and amorphous carbon, and the graphitic structure of ideal carbon in the G band.

[0072] [47,48]. For example... Figure 3 As shown in a and b, compared with the unmodified phenolic resin PF matrix carbon, at 700℃, the ID / IG ratio of the phenolic resin PF matrix carbon with an ANF modification ratio of 1.8wt% decreased from 2.74 to 2.61, and at 1100℃, the ID / IG ratio decreased from 2.16 to 2.02. This indicates that the amorphous carbon content of the PF matrix carbon gradually decreased, the carbon structure gradually became more regular and ordered, and the crystalline carbon content increased. This may be because the carbonized ANF itself has a high degree of crystallinity []. In addition, as the carbonization temperature increased from 700℃ to 1100℃, the D peak and G peak of the PF matrix carbon gradually separated, the full width at half maximum (FWHM) narrowed, the peak shape became sharper, and the IG / ID values ​​of both increased, indicating that the surface carbon structure of both was more ordered.

[0073] from Figure 3As shown in c and d, at 700℃, compared with unmodified PF matrix carbon, the intensity of the (002) diffraction peak at 26°(2θ) is significantly increased in PF matrix carbon with an ANF addition ratio of 1.8wt%. When the carbonization temperature increases from 700℃ to 1100℃, the intensity of the 002 diffraction peak of PF matrix carbon increases, indicating a greater amount of carbon deposited on the surface. However, the 002 diffraction peak of PF matrix carbon with an ANF addition ratio of 1.8wt% becomes broader. This may be due to the increase in the crystal size distribution range and the increase in crystal defects of PF matrix carbon at high temperatures caused by the addition of ANF, indicating that ANF was successfully doped into PF matrix carbon, which is consistent with the XPS results.

[0074] 2.4. Influence of aramid nanofibers on the microstructure of modified carbon paper;

[0075] 2.4.1 SEM;

[0076] To investigate the effect of ANF modification on the interfacial bonding performance between PF matrix carbon and carbon fiber prepared by PF impregnation solution, SEM surface micromorphology analysis was performed on PF impregnated carbon paper with different ANF modification ratios (0, 0.6wt%, 1.2wt%, 1.8wt%, and 2.4wt%). Figure 4 As shown. From Figure 4 (a1-d1) shows that as the ANF modification ratio increases from 0 to 1.8 wt%, the PF matrix carbon gradually forms a coating on the carbon fibers, thereby improving the interfacial bonding. In the unmodified PF-impregnated CP, the PF matrix carbon and carbon fibers are severely decoupled, with obvious interfacial separation, and the PF resin carbon has difficulty adhering to the smooth fiber surface. This may be because the nanofiber network structure of ANF increases the specific surface area, greatly increasing the contact area with PF. Furthermore, the polar groups on ANF can interact with the phenolic hydroxyl and hydroxymethyl groups of PF, enhancing the interfacial interaction between ANF and PF. When the ANF modification ratio is 2.4 wt%, the PF matrix carbon attached to the carbon fibers exhibits an irregular, rough surface, which may weaken the interfacial bonding between the PF matrix carbon and the carbon fibers.

[0077] Depend on Figure 4 (a2-e2) indicates that when the ANF modification ratio is less than 0.6%, no fine ANF fibers were found on the surface of the PF matrix carbon, but a large number of PF matrix carbon fractures were observed. Figure 4 (a) When the modification ratio is greater than 1.2 wt%, obvious ANF fiber distribution is found inside the PF matrix carbon, which helps to inhibit the breakage of PF matrix carbon and thus enhance the interfacial bonding strength between ANF and PF, playing a decisive role in improving the strength performance of carbon paper. However, when the ANF modification ratio reaches 2.4 wt%, it is found that ANF is unevenly distributed on the surface of PF matrix carbon and makes the surface of PF matrix carbon rough.

[0078] 2.4.2, XRD, XPS;

[0079] To further investigate the effect of ANF modification on the surface composition of CP, XPS analysis was performed on PF-impregnated carbon paper with different ANF modification ratios (0 and 1.8 wt%). The high-resolution XPS spectra of the C1s and O1s peaks are shown below. Figure 5 As shown in AD. Carbon materials typically consist of two types of carbon atoms: basal sp2 and defect sp3. The content of sp3 can reflect the defect level. High-resolution C1s spectra show two peaks at 284.8 and 285.9 eV. Figure 5 a) and b) represent different carbon bonds, corresponding to trigonal carbon (sp2 center) and tetrahedral carbon (sp3 center) [22, 26, nitrogen-containing, oxygen-containing]. High-resolution C1s spectra of unmodified CP and CP modified with 1.8 wt% ANF show that the sp3 content increases in CP modified with 1.8 wt% ANF compared to unmodified CP. This is because the surface of the carbon paper with 1.8 wt% ANF modification has modified PF matrix carbon adhering to it, resulting in successful ANF doping into the PF matrix carbon (this can also be seen from electron microscopy).

[0080] In addition, such as Figure 5 As shown in c and d, the high-resolution XPS spectrum shows four peaks (531.6, 532.6, 533.8, and 535.0 eV). The peak at 531.6 eV represents the carboxyl group (COO-) and the oxygen-carbon double bond (O=CO) in the carboxylate. The peak at 532.6 eV corresponds to the hydroxyl (C-OH) and carbonyl (C=O) functional groups. The peaks at 533.8 eV and 535.0 eV represent the oxygen single bonds in esters and carboxylic acids (O=CO) and chemisorbed oxygen or water (HOH), respectively. The high-resolution XPS spectrum of the O1s peak shows that the content of C=O and -OH groups on the surface of CP modified with 1.8 wt% ANF is significantly higher than that of unmodified CP. The C=O and -OH functional groups can increase the cross-linking and interaction forces within the carbon paper, thereby improving the mechanical properties of the carbon paper.

[0081] 2.5 Performance analysis of aramid nanofiber modified carbon paper;

[0082] 2.5.1 Analysis of the cross-section, bulk density, resistivity and mechanical properties of carbon paper;

[0083] The thickness, bulk density, resistivity, and mechanical properties of PF-impregnated carbon paper with different ANF modification ratios (0, 0.6 wt%, 1.2 wt%, 1.8 wt%, 2.4 wt%) are shown in [reference needed]. Figure 6 As shown. From Figure 6As shown in Figure d, as the ANF modification ratio increased from 0 to 2.4 wt%, the thickness of the carbon paper gradually decreased from 0.190 mm to 0.159 mm, a decrease of 16.32%. This demonstrates that the thickness of the carbon paper gradually decreases with increasing ANF modification ratio. Furthermore, as the ANF modification ratio increased from 0 to 2.4 wt%, the bulk density of the carbon paper increased from 0.32 g·cc. -1 Increase to 0.45g·cc -1 This represents an increase of 40.63%. It is evident that the bulk density of the carbon paper gradually increases with the increase of the ANF modification ratio. This may be because a higher ANF modification ratio leads to stronger adhesion between the PF matrix carbon and the carbon fibers, resulting in a tighter bond between the carbon fibers and the PF matrix carbon, thus reducing the thickness of the carbon paper and increasing its bulk density. This can also be further explained by the cross-sectional morphology. Figure 6 (ac)).

[0084] from Figure 6 As shown in Figure e, as the ANF modification ratio increases from 0 to 2.4 wt%, the resistivity of the carbon paper decreases from 4.25 mΩ·cm to 2.98 mΩ·cm, a decrease of 29.88%. This indicates that the resistivity of the carbon paper exhibits a decreasing trend with increasing ANF modification ratio. The possible reason for this phenomenon is that with increasing ANF modification ratio, the thickness of the carbon paper decreases, the carbon fiber content per unit volume increases, and the fiber network structure becomes denser, thus ensuring multiple electron transport pathways and reducing the resistivity of the carbon paper.

[0085] from Figure 6 As shown in f and g, as the ANF modification ratio increases from 0 to 1.8 wt%, the tensile strength and flexural strength of the carbon paper increase from 6.98 MPa and 240.54 MPa to 14.28 MPa and 531.61 MPa, respectively, representing increases of 104.58% and 121%. When the ANF modification ratio continues to increase to 2.4 wt%, the tensile strength and flexural strength of the carbon paper decrease to 12.15 MPa and 471.23 MPa, respectively. Therefore, the tensile strength and flexural strength of the carbon paper exhibit a pattern of first increasing and then decreasing with the increase of the ANF modification ratio. This phenomenon is caused by two main factors. First, the nanoscale characteristics of ANF provide a larger surface area for contact with PF. As a pressure transmission medium, when ANF is subjected to load, cracks deflect to the ANF nanoparticles, transforming large cracks into multiple smaller cracks. This increases the crack propagation direction and avoids stress concentration, thereby effectively improving the mechanical properties of hot-pressed carbon paper. Second, the numerous polar functional groups in ANF provide strong interactions at the interface between ANF and PF, ensuring that ANF is uniformly dispersed in the PF impregnation solution. After carbonization, the PF matrix carbon is uniformly coated on the carbon fibers. Figure 4d1) Improves the interfacial bonding strength of carbon paper. However, when the ANF modification ratio continues to increase to 2.4 wt%, the higher ANF content causes ANF to aggregate in the PF impregnation solution, resulting in poor ANF dispersion in PF. These aggregated ANFs lead to stress concentration inside the PF matrix carbon, thereby reducing the mechanical properties of the carbon paper.

[0086] 2.5.2 Analysis of the pore structure of carbon paper;

[0087] PSD is an important physical property of carbon paper, especially in water management for fuel cells. The pore size distribution, permeability, and porosity of carbon paper with different ANF modification ratios (0, 0.6 wt%, 1.2 wt%, 1.8 wt%, 2.4 wt%) are shown in [reference needed]. Figure 7 As shown. From Figure 7 As shown in Figure a, the pore size distribution of unmodified carbon paper is entirely within the range of ≥40μm, while the pore size distribution of ANF-modified carbon paper is ≤40μm. This demonstrates that ANF modification has a significant impact on the pore size distribution of carbon paper. Furthermore, as the ANF modification ratio increases from 0 to 1.8%, the pore size distributions in the ranges of <5μm, 5-10μm, 10-20μm, 20-30μm, and 30-40μm gradually increase. However, when the modification ratio continues to increase to 2.4%, the pore size distributions in these ranges show a significant decrease again. This phenomenon may be caused by the breakage of methylene bridges in PF during high-temperature pyrolysis, releasing CO2 and CH4, forming macropores. However, the uniform dispersion of ANF in PF inhibits the drastic shrinkage of PF, thus resulting in pores smaller than 40μm.

[0088] like Figure 7 As shown in b, as the ANF modification ratio increases from 0 to 1.8 wt%, the air permeability increases from 1760 L·m⁻²·s⁻¹. -1 Increased to 2520 L·m⁻²·s -1 The porosity increased from 73.57% to 85.94%, representing increases of 43.18% and 16.81%, respectively. When the ANF modification ratio was 2.4 wt%, the air permeability and porosity decreased to 2210 L·m⁻². -2 ·s -1 The permeability and porosity of the carbon paper both initially increased and then decreased with increasing ANF modification ratio, as shown by the results of 82.68%. This is because, during the carbonization process, the permeability and porosity of the carbon paper are affected by both the release of non-carbon elements and the shrinkage of the PF matrix carbon. The non-carbon elements released during the thermal decomposition of the PF matrix carbon increase the open pores, thereby increasing permeability and porosity. Secondly, the pyrolysis residue of ANF-modified PF is low; with increasing ANF addition, more non-carbon elements are released from the PF matrix carbon.

[0089] 2.6 Polarization curves and EIS analysis of aramid nanofiber modified carbon paper;

[0090] The polarization curves, power densities, and electrochemical impedance spectroscopy (EIS) spectra of fuel cells assembled with PF-impregnated carbon paper at different ANF modification ratios (0, 0.6 wt%, 1.2 wt%, 1.8 wt%, and 2.4 wt%) under different assembly torques (20 kgf·cm, 40 kgf·cm, and 60 kgf·cm) are shown in the figure. Figure 8 As can be seen in (a), when the battery assembly torque is 20 kgf·cm, as the ANF modification ratio increases from 0 to 1.8 wt%, the limiting current density of the fuel cell increases from 0.80 A·cm. -2 Increased to 1.31 A·cm -2 The limiting power density is from 0.25 W·cm³. -2 Increased to 0.43 W·cm -2 These figures represent increases of 63.75% and 72%, respectively; when the ANF modification ratio is 2.4 wt%, the limiting current density and limiting power density decrease to 1.24 A·cm⁻¹, respectively. -2 and 0.41 W·cm -2 It is evident that the limiting current density and limiting power density of carbon paper-assembled fuel cells exhibit a pattern of first increasing and then decreasing with increasing ANF modification ratio. This is due to two factors. Firstly, the modified carbon paper exhibits good interfacial bonding between the PF matrix carbon and the carbon fiber, ensuring multiple electron transport pathways, reducing the resistivity of the carbon paper, and thus lowering contact resistance, which helps improve PEMFC output performance. Secondly, the modified carbon paper possesses high permeability and a pore structure consisting of micropores (<5μm) and macropores. Macropores provide effective gas diffusion and water removal, while micropores facilitate water transport from the catalyst layer to the carbon paper, thereby reducing mass transfer impedance. However, when the ANF modification ratio is 2.4wt%, the performance of the PEMFC decreases because excessive PF matrix carbon leads to decreased permeability and porosity, causing mass transfer problems. Figure 7 b).

[0091] from Figure 8 As can be seen from b, when the battery assembly torque is 40 kgf·cm, as the ANF modification ratio increases from 0 to 1.8 wt%, the limiting current density of the fuel cell increases from 1.2 A·cm⁻¹. -2 Increased to 1.69 A·cm -2 The limiting power density is from 0.35 W·cm³. -2 Increased to 0.49 W·cm -2 The current density and power density increased by 40.83% and 40%, respectively; when the ANF modification ratio was 2.4 wt%, the limiting current density and limiting power density decreased to 1.57 A·cm⁻¹. -2and 0.43 W·cm -2 It can be seen that the limiting current density and limiting power density of carbon paper assembled fuel cells both show a change pattern of first increasing and then decreasing with the increase of ANF modification ratio.

[0092] from Figure 8 As can be seen from Figure c, when the battery assembly torque is 60 kgf·cm, as the ANF modification ratio increases from 0 to 1.8 wt%, the limiting current density of the fuel cell increases from 1.15 A·cm⁻¹. -2 Increased to 1.76 A·cm -2 The limiting power density is 0.39 W·cm³. -2 Increased to 0.55 W·cm -2 The performance was improved by 53.04% and 41.03% respectively; when the ANF modification ratio was 2.4 wt%, the limiting current density and limiting power density decreased to 1.68 A·cm⁻¹. -2 and 0.53 W·cm -2 It can be seen that the limiting current density and limiting power density of carbon paper assembled fuel cells both show a change pattern of first increasing and then decreasing with the increase of ANF modification ratio.

[0093] When the battery assembly torque is 20, 40, and 60 kgf·cm, the limiting power density of PEMFC with an ANF modification ratio of 1.8 wt% reaches 0.43 W·cm⁻¹ as the assembly torque increases. -2 0.55W·cm -2 and 0.49 W·cm -2 The output power density shows a gradual upward trend. This is partly because as the assembly torque increases, the contact resistance gradually decreases, leading to a gradual increase in output power density. Secondly, as the assembly torque continues to increase, the high mechanical strength of the carbon paper and its good interfacial bonding with the PF matrix carbon effectively maintain the pore structure between the PF matrix carbon and the fibers, preventing collapse and preserving good air permeability. Furthermore, the increased assembly torque also shortens the material transport path, resulting in increased battery output performance.

[0094] In addition, to further understand the functions of different CPs, a 700A·cm battery was used. -2 EIS was measured during operation. Nyquist graph as shown. Figure 8As shown in the figure, under different assembly torques of 20, 40, and 60 kgf·cm, the curvature of the EIS curve gradually decreases as the ANF modification ratio increases from 0 to 1.8 wt%, while the curvature increases when the ANF modification ratio reaches 2.4 wt%. This indicates that CP with an appropriate ANF content exhibits better water management capabilities with the addition of ANF. Firstly, this is due to the high permeability and porosity of the modified carbon paper, which is beneficial for reactant transport. Secondly, the modified carbon paper has a more reasonable pore size distribution, possessing both macropores and micropores, while unmodified carbon paper only has macropores, thus exhibiting better mass transport performance. However, when the ANF modification ratio reaches 2.4 wt%, the mass transfer impedance increases due to the decrease in permeability and porosity. Furthermore, as the battery assembly torque increases from 20 kgf·cm to 60 kgf·cm, the intercept of the EIS curve shifts to the left, indicating a decrease in the ohmic impedance of the membrane electrode. This is because PEM exhibits higher proton conductivity under high torque. Furthermore, the radius of the EIS curve gradually decreases, indicating a significant reduction in mass transfer impedance. This is because the carbon paper is subjected to intense compression, shortening the mass transport path and thus reducing mass transfer impedance. It may also be due to the excellent interfacial bonding between the carbon fiber and the PF matrix carbon, which reduces pore collapse and prevents severe damage to the pore structure, resulting in maintained good mass transport performance.

[0095] In summary, this invention first uses aramid nanofibers as a modifier and reinforcing agent for phenolic resin, giving it good adhesive properties. Then, carbon paper is impregnated and laid flat to ensure uniform resin penetration, resulting in a preform. The preform is then dried and compressed to ensure sufficient curing and cross-linking of the bonding material before carbonization. Finally, carbonization yields the finished product. The carbon paper prepared by this invention exhibits excellent mechanical properties, pore structure and distribution, and significantly improved electrical conductivity and permeability. The carbon paper prepared by this invention can enhance the performance of fuel cells, and the preparation process is simple and convenient, showing promising application prospects.

Claims

1. A method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofibers, characterized in that: Includes the following steps: S1. Prepare a 12wt% phenolic resin-ethanol solution, add 1.8% by mass of aramid nanofibers to the phenolic resin-ethanol solution, and disperse by ultrasonic stirring to obtain a mixed solution. S2. Immerse the carbon paper base paper in the mixed solution of step 1 and sonicate it for 15-25 minutes. After taking out the carbon paper base paper, lay it flat to ensure uniform resin penetration and use it as a preform. S3. Dry the preforms, and then compress them by hot pressing in air at a temperature of 140-160℃ and a pressure of 2-5 MPa for 0.5-1.5 hours using a vulcanizing bed. S4. The hot-pressed carbon paper is carbonized in a high-temperature tube furnace at a heating rate of 8-12°C / min, a sintering temperature of 1000-1200°C, and a holding time of 0.5-1.5 hours. After cooling, the finished product is obtained.

2. The method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofibers according to claim 1, characterized in that: In step S1, ultrasonic stirring is performed for 10 minutes.

3. The method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofibers according to claim 1, characterized in that: In step S2, the ultrasound treatment lasts for 20 minutes.

4. The method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofibers according to claim 1, characterized in that: In step S3, the preform is dried in an oven at 80°C for one hour to remove the solvent.

5. The method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofibers according to claim 1, characterized in that: In step S3, compression molding is performed by hot pressing in air at a temperature of 150°C and a pressure of 4 MPa for 1 hour using a vulcanizing bed.

6. The method for improving the mechanical properties of carbon paper by modifying phenolic resin with aramid nanofibers according to claim 1, characterized in that: In step S4, the heating rate is 10 °C / min, the sintering temperature is 1100 °C, and the holding time is 1 hour.

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