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Preparation and Characterization of Dopamine Modified Carbon Fiber Paper Composites for Gas Diffusion Layers

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26 July 2023

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27 July 2023

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Abstract
Carbon fibers (CFs) cannot be directly used for the preparation of CF paper because of their chemically inert nature. Herein, the surface of CF was modified via the spontaneous oxidative self-polymerization of dopamine. By taking full advantage of the spontaneous oxidation and self-polymerization properties of PD to maintain the maximum strength of CFs, a polydopamine-modified CF paper (PDA-CFP) with excellent performance was prepared using PD-modified CFs (PDA-CFs), which increased the proportion of hydrophilic functional groups on the surface of carbon fibers, increased the O/C ratio on the CF surface by 6 times, and improved the bond strength between the modified CF and the adhesive by making full use of the interaction force between polydopamine and PVA fibers. In this way, the primary properties of the CF paper was improved. Overall, the results showed that the dispersion of CF was considerably improved by the dopamine modification. In addition, the primary physical properties of PDA-CFP were better than those of virgin CF paper (CFP-0). PDA-CFP exhibited a maximum tensile strength of 2.04 kN/m2, a minimum resistivity of 0.06055 Ω·cm and a minimum porosity of 72.4%. The tightness could be increased by up to 12.1%.
Keywords: 
Subject: 
Chemistry and Materials Science  -   Paper, Wood and Textiles

1. Introduction

In recent years, the issue of energy scarcity has gained significant attention, leading to an increasing demand for green energy derived from environmentally-friendly resources. As a result, proton-exchange membrane fuel cells (PEMFCs) have emerged as a promising and prominent energy source due to their environmentally-friendly, efficient, and safe characteristics [1]. In PEMFCs, hydrogen and oxygen act as the main reactants, producing water as well as usable electrical energy and unused thermal energy as the primary by-products [2,3]. This property makes PEMFCs highly desirable as they result in minimal pollution and emissions in terms of raw materials and end products.
Typically, PEMFCs consist of two main components, i.e., the bipolar plate and the membrane electrode assembly (MEA) [4]. The gas diffusion layer (GDL), a crucial part of the MEA [5], functions as a support material within the fuel cell and plays a vital role in transferring generated water and heat during fuel cell operation. Additionally, the GDL is responsible for electron transport and carries the catalytic layer. To meet the demands of GDL fabrication, the raw material should possess high permeability, high porosity, low resistivity and excellent mechanical strength. Presently, candidate materials satisfying these criteria include carbon cloths, nonwoven fabrics, carbon black paper and carbon fiber papers (CFP). However, carbon cloths feature a prolonged production cycle and a complicated preparation process, resulting in higher application costs, and nonwoven fabrics exhibit poor durability, low working strength, limited stability as well as low safety. Furthermore, the limitations in the carbon black paper collection process and the preparation of raw materials lead to reduced porosity and inferior mechanical strength [6]. On the other hand, CFP, made from short carbon fibers (CFs), demonstrates excellent electrical conductivity, good mechanical strength, as well as outstanding mass and heat transfer properties while maintaining relatively lower raw material costs, making it a favorable choice as a raw material for fabricating the GDL.
Due to the unique production process of CFs, few reactive functional groups are present on the CF surface because of their excellent characteristics and production process [7]. Additionally, the typical “surface-core” structure of CFs results in their low surface energy [8]. The inertness of the CF surface affects their further development and utilization. However, the above-mentioned drawbacks can be overcome by the modification of CFs by means of methods including surface oxidation [9,10], rare-metal treatment [11,12], plasma treatment [13], chemical vapor deposition [14], electrodeposition [15] and surface grafting modification [16-18]. However, these techniques have many disadvantages, such as high costs, high hazards, deterioration of fiber strength after treatment and generation of contaminated waste liquids [19], making it necessary to develop a new and greener CF treatment method.
Dopamine, a class of catecholamines, is an endogenous nitrogen-containing organic compound with very rich amino group contents. Additionally, PD exhibits strong adhesion [20]. Owing to its distinct properties, the dopamine monomer is sensitive to oxygen and quickly undergoes a spontaneous oxidation reaction when exposed to oxygen, resulting in discoloration. The self-polymerization of dopamine can occur under alkaline conditions, and the rate of polymerization will be affected by the pH value [21,22].
Dopamine is oxidized to produce polydopamine (PDA), a green biomolecule that can be used in various applications by virtue of its excellent water dispersibility and stability. Besides, PDA can undergo the self-polymerization reaction on CFs by forming abundant covalent and noncovalent bonds with the CF surface [23]. These characteristics are extremely effective for the preparation of the CF paper, and the oxidation mechanism is shown in Figure 1.
Based on a greener method of modifying the CF surface with dopamine, the good adhesion property of PDA and the abundant phenolic hydroxyl groups capable of forming hydrogen bonds with hydroxyl groups and carboxyl groups on the surface of pretreated CFs was hereby used to prepare modified CFs. Additionally, the modified CFs were used for the preparation of CF paper. PDA was uniformly attached to the CF surface via dopamine modification and used as an intermediate to increase the CF–CF bonding ability and CF–polyvinyl alcohol (PVA) fiber bonding ability. The reactive groups on the surface of PD formed multiple hydrogen bonding and noncovalent bonding cross-linked networks with PVA fibers [24]. The hydrophilic functional groups greatly improved the dispersibility of CFs. Therefore, the CF paper prepared using modified CFs exhibited high strength and excellent uniformity. Besides, different sizing concentrations were hereby investigated, and the excellent performance of dopamine in preparing CF paper using CFs of different lengths was verified. The physical properties of CF paper were studied as well. Fabricating CF paper using polydopamine-modified CFs (PDA-CF) was found to exponentially increase the mechanical strength of CF paper while ensuring high porosity. Additionally, CF paper with excellent uniformity and high air permeability could be produced without adding any dispersant.

2. Materials and Methods

2.1. Material

3mm, 6mm polyacrylonitrile carbon fiber (Toray, Japan); PVA fiber (Shanghai Chenqi Chemical Co., Ltd.), dopamine hydrochloride 98%, tris (hydroxymethyl) aminomethane 99.9% (Shanghai Maclean Biochemical Technology Co., Ltd.), hydrochloric acid (Laiyang Economic and Technological Development Zone Fine Chemical Factory), anhydrous ethanol (Tianjin Fuyu Fine Chemical Co., Ltd.), sodium hydroxide (Sinopharm Chemical Reagent Co.).

2.2. Preparation of PD-Modified CFs

CFs were heated at a high temperature of 800 ℃ for 2 h, dried and cooled. Then, the treated CFs were impregnated in a 15% sodium hydroxide solution for 0.5 h, washed and vacuum dried to obtain pretreated CFs (CF-1).
First, tris buffer was prepared [22]. Second, using the prepared tris buffer as the solvent, dopamine solutions with concentrations of 2 g/L, 5 g/L and 8 g/L were prepared. Then, CFs were completely submerged in the dopamine solution and fully washed with deionized water after 24 h of sizing, followed by vacuum drying. The product was named PDA-X-CF, where X represents the concentration of dopamine solution used to treat the CF, namely, 2, 5 or 8.

2.3. Preparation of the Modified CF Paper

Modified CFs were dispersed in deionized water to prepare a CF pulp. Then, the CF pulp was mixed with an 8% PVA fiber. Stirring and dispersion of PVA-CF pulp were continued at room temperature to obtain a CF paper pulp. The CF pulp was added to the sheeting machine for full foaming and dispersion. Foaming was continued until there were no CF bundles, and immediate filtration and drainage were then performed. The CF paper was removed from the copper mesh using Teflon as the cloth liner. Subsequently, the wet paper web was placed in an oven for multiple reverse side de-watering and drying. The dried CF paper was subjected to a 24-h moisture balance operation at a constant temperature and humidity. The CF paper was hot-pressed at 4 MPa and 170 °C for 4 min to obtain the final dopamine-modified CF paper (PDA-CFP). Finally, the physical properties of the CF paper and the optimal modification conditions were studied.
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Figure 2. Carbon fiber paper preparation flow chart.
Figure 2. Carbon fiber paper preparation flow chart.
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2.4. Characterization

Herein, the surface morphology and surface roughness of different CFs were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM), and the surface functional groups of different CFs were characterized using Fourier transform infrared spectroscopy (FTIR). Meanwhile, the chemical compositions of different CF surfaces were investigated using x-ray photoelectron spectroscopy (XPS). Raman spectra of different CFs used in this study were obtained by using a Raman spectrometer, and X-ray diffraction (XRD) patterns were obtained by using an X-ray diffractometer. The thermal stability of different CFs was explored via thermogravimetric analysis (TG), and mercuric pressure was used to obtain the thermal stability of various types of CFs. Besides, the data of porosity and pore size of carbon fiber paper were obtained by using the mercuric pressure method, and by using a universal tensile machine, an air permeability tester and a digital source meter, the physical properties of carbon fiber paper such as tensile strength, stress-strain, permeability and resistance were tested.

3. Results and Discussion

3.1. Surface Analysis of CF

As shown in Figure 3a, the surface of CF-0 was smooth, which was a protective agent wrapped on the surface of the carbon fiber by the manufacturer to protect the carbon fiber, and the glowing white spots were impurities on the CF-0 surface. As shown in Figure 3b, there were fewer impurities with obvious defective grooves and furrows on the surface of the CF-1, and this structure exposed more functional groups on the CF surface and facilitated subsequent dopamine sizing treatment. Figure 3c–e demonstrated the occurrence of dopamine polymerization on the CF surface. As the dopamine concentration increased, the cluster size increased gradually. When CFs were treated with a dopamine sizing concentration of 2 g/L, PDA was uniformly dispersed in small clusters on the CF surface, and the longitudinal grooves of CFs were filled. When carbon fibers were treated with a dopamine sizing concentration of 8 g/L, the surface of CFs was covered with large PDA clusters of flocculation, and the surface grooves of the CF were almost completely filled. The above images indicated that the sizing of CFs was complete after the dopamine self-polymerization reaction.
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Figure 3. (a) SEM image of the virgin CF (CF-0) surface; (b) SEM image of the CF-1; (c) SEM images of the PDA-2-CF; (d) SEM images of the PDA-5-CF; and (e) SEM images of the PDA-8-CF.
Figure 3. (a) SEM image of the virgin CF (CF-0) surface; (b) SEM image of the CF-1; (c) SEM images of the PDA-2-CF; (d) SEM images of the PDA-5-CF; and (e) SEM images of the PDA-8-CF.
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Figure 4. AFM topography of CF-0 (a), CF-1 (b), PDA-2-CF (c), PDA-5-CF (d) and PDA-8-CF (e) (2D).
Figure 4. AFM topography of CF-0 (a), CF-1 (b), PDA-2-CF (c), PDA-5-CF (d) and PDA-8-CF (e) (2D).
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Figure 5. AFM topography of cf-0 (a), CF-1 (b), PDA-2-CF (c), PDA-5-CF (d) and PDA-8-CF (e) (3D).
Figure 5. AFM topography of cf-0 (a), CF-1 (b), PDA-2-CF (c), PDA-5-CF (d) and PDA-8-CF (e) (3D).
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It can be observed from Table 1 that the Ra of CFs decreased by approximately 1.7 times after surface impurity removal through pretreatment, indicating that the surface smoothness of the carbon fibers was effectively improved after pretreatment, and most impurities were removed. As the dopamine sizing concentration increased, both Ra and Rq of CFs showed a significant increase. Compared to the Ra value (37.5 nm) and Rq value (29.8 nm) of CF-1, PDA-CF-8 exhibited an increased Ra value of 58.8 nm and an increased Rq value of 70.1 nm, respectively. Visual comparison of the images revealed that the pretreated carbon fiber surface became smoother with distinct grooves, while the dopamine-treated carbon fiber surface appeared rougher and uneven due to the filling of grooves. Moreover, higher dopamine sizing concentrations led to a substantial increase in surface roughness, presenting some noticeable sharp protrusions. These findings jointly demonstrated the successful loading of dopamine onto carbon fibers, which resulted in notable changes in the surface morphology of the carbon fibers [25].
As shown in Figure 6, a comprehensive analysis of the TG and DTG curves of carbon fibers revealed that CF-1 experienced a noticeable weight change starting at 750°C, while PDA-CF exhibited a significant weight change in the range of 580-610°C. The presence of 8g/L dopamine sizing concentration resulted in a higher accumulation of polydopamine on the carbon fiber surface, as observed in Figure 6, thus leading to a noticeable variation in the weight of PDA-8-CF with the increasing temperature. The significant weight loss observed in PDA-CF indicated the successful attachment of dopamine onto the carbon fiber surface. Despite the fact that the thermal decomposition temperature of dopamine on the carbon fiber surface was higher than that of the original dopamine (150-250°C) [26], it still decomposed at temperatures below 650°C, suggesting that the polydopamine on the carbon fiber surface did not interfere with the subsequent carbonization and graphitization processes of the fibers.

3.2. Dispersibility of CFs

Herein, CFs were prepared as 0.075% slurry and ultrasonically dispersed for 10 minutes, and placed in a white-bottom culture dish for 3 min to study the CF dispersion. The poor dispersion of the virgin CF resulted in the poor stability after dispersion in water without dispersant (no significance as a control group). Therefore, the CF-1 was chosen as a blank control sample. As shown in Figure 7a, CF-1 still dispersed poorly in water, in which there was pronounced flocculation of CFs. Figure 7b–d shows the dispersion of CFs after the three dopamine sizing treatments, respectively. Compared with the CF-1, the CFs after dopamine modification exhibited excellent dispersibility in water, presenting almost no flocculation phenomenon, and could be uniformly dispersed in water. The abundant hydroxyl and amino groups in PD greatly improved the polarity of the surface of the CF and the hydrophilic property of the CF. Besides, the content of acidic functional groups on the CF surface increased after the modification. These functional groups considerably increased the negative charge on the CF surface after ionization in solutions, which in turn increased the repulsive force between CFs. Under the multiple effects, the dispersibility of CFs was greatly improved. The high sizing concentration caused the PD on the CF surface to react with the PD on the other CF surface. Additionally, the re-flocculation between fibers led to poor dispersion.

3.3. Performance Testing of CF Paper

Conventional CF-based paper requires the addition of papermaking agents such as dispersants and surfactants, and tedious steps such as the dilution of papermaking additives are required. However, modified CFs can overcome the disadvantages of CFs such as difficult dispersion. Meanwhile, PD has bonding properties, which can significantly reduce the amount of adhesive used [27]. Herein, the CF paper made from CF-0 was named CFP-0; the CF paper made from pretreated CF was named CFP-1; and the CF paper made from dopamine-modified CF was named PDA-X-CFP (X represents different sizing concentrations).

3.3.1. Homogeneity of the CF Paper

The uniformity of the CF paper critically affects its mass transfer capacity [28]. The degree of flocculation and dispersion of CFs can be determined by comparing the CF paper uniformity. The dispersion degree of CFs directly affects the contact resistance of the fiber mass, while the uniformity of the CF dispersion can improve the strength of CF paper and facilitate the formation of uniform voids, exerting a considerable impact on the subsequent impregnation and curing of CF paper and carbonization [29,30]. CFP-0 exhibits a loose structure due to the inert nature of CF-0, resulting in uneven fiber dispersion and weak bonding between fibers. In contrast, pretreatment removes impurities from CF surface, enhancing dispersibility, reducing agglomeration of CF bundles, and improving light transmission and uniformity. Dopamine-treated CFs demonstrate excellent dispersibility, which leads to strong fiber bonding and uniformity after hot pressing. CF paper prepared with 3 mm CFs contains a percentage of large holes compared to CF paper prepared with 6 mm CFs, attributed to short fiber length and fewer cross-linking points. Under higher pressure during hot pressing, bonded fibers may fracture, affecting the macroscopic structure and light transmission of CF paper. However, the high intertwining of 6mm fibers ensures minimal changes in the macroscopic structure even if breakage occurs between fibers under pressure.

3.3.2. Tightness and Tensile Strength Performance of the CF Paper

The thickness of the CF paper directly affects its water transport behavior [27], with an excessive thickness increasing water saturation in the fuel cell and prolonging the time required to reach the water breakthrough point and the stable transport state of liquid water [31]. However, thin CF paper can address the issue, but has a typically low mechanical strength. Enhancing the tightness between CFs can improve the paper strength and extend its service life. Tightness is indicative of the bonding level between CFs. Figure 9a demonstrates that dopamine-modified CF paper (6 mm) exhibits a minimum tightness 2.67% higher than CFP-0, reaching 2969.697 g/m3 compared to CFP-0’s 2475.248 g/m3, a 12.12% increase. Besides, dopamine modifications introduce cross-linked structures and molecular bridges at fiber contact points. Dopamine molecules undergo self-polymerization to form dopamine polymer layers through condensation reactions. These layers create cross-linked bridges between fibers, increasing the number and strength of connections and resulting in a higher degree of tight fiber stacking.
The strength of the CF paper is an important parameter when the paper is used as a GDL in practical applications [32]. Practically, the CF paper is subjected to different degrees of force during transportation, loading and use after preparation. Therefore, the mechanical strength of the CF paper is a guarantee of its longevity and performance as a substrate layer in GDLs in PEMFCs [33]. The tensile strength of the CF paper after various treatments is depicted in Figure 9b. Notably, irrespective of the CF length, the pretreated CF paper exhibited superior tensile strength compared to CFP-0. Specifically, for 3 mm CF, the tensile strengths at different dopamine concentrations (low to high) were 1.87 kN·m-1, 1.26 kN·m-1 and 1.49 kN·m-1, respectively, which were 5.1, 3.8 and 4 times higher than the 0.32 kN·m-1 of CFP-0. Similarly, for 6 mm CF, the tensile strengths at different dopamine concentrations (low to high) were 2.04 kN·m-1, 1.52 kN·m-1 and 1.6 kN·m-1, respectively, which were 4.1, 2.8 and 3 times higher than CFP-0’s 0.4 kN·m-1.
The poor dispersion performance of CF-0 could be attributed to the presence of negative charges on the CF surface, which led to repulsive forces between the fibers and resulted in their clustering during dispersion. Moreover, the initial dispersion of CFs was challenging to maintain after re-flocculation, leading to poor paper uniformity and weak bonding between CF clusters.
In contrast, the pretreatment effectively removed most of the impurities from the CF surface and increased the bonding force between CFs by enhancing the connections between exposed functional groups. The subsequent sizing treatment involved dopamine forming hydrogen bonds with PVA fibers, and the phenolic and amine functional groups in dopamine molecules underwent chemical reactions with the surface functional groups of carbon fibers, forming chemical bonds or physical adsorption. This chemical cross-linking enhanced the connections between fibers, consequently elevating the overall strength of the CF paper. Furthermore, the surface of the dopamine-modified carbon fiber exhibited certain adhesive properties and could interact with other fibers through physical adsorption, further promoting mutual adhesion and enhancing the overall strength of the paper, ultimately leading to a significant increase in the tensile strength of PDA-CFP.

3.3.3. Porosity and Bulk Density of the CF Paper

Porosity can significantly determine the performance of the GDL [34]. A higher porosity leads to stronger drainage in the GDL substrate layer, a longer service life, a higher fuel cell energy conversion efficiency and better fuel cell performance [35]. Herein, the untreated CFs had a poor dispersion uniformity, with a variation coefficient of 0.65. The porosity of the CF paper at the fiber aggregation was low, and the pore size was small (Figure 10a). By contrast, the low tightness of fiber binding and larger pores at the sparse CF resulted in high porosity, and the random distribution of the five measurement locations led to the large variation coefficient of the porosity. The variation coefficient of the modified CF paper was 0.2, while that of the modified CF paper decreased by three-fold. The porosity of CFP-0 with different fiber lengths ranged from 94% to 95%, indicating poor dispersion of the original carbon fiber. After pretreatment, the dispersion improved, leading to a decrease in porosity from over 90% to 80%. Dopamine treatment further enhanced dispersion, resulting in a more noticeable decrease in porosity. The carbon fiber paper prepared with 6 mm carbon fiber showed a less pronounced effect on porosity, maintaining it at approximately 80%, which indicated the greater impact of dopamine treatment on 3 mm carbon fiber. High porosity in carbon fiber paper was preferable for subsequent processing with conductive carbon materials. The dopamine modified layer formed a polymer cover layer on the surface of the carbon fiber, which would fill the microscopic pores between the carbon fibers. The dopamine polymer in solution could form a colloidal substance, thereby forming a dense cover layer with the drying process and thus reducing the overall porosity of the carbon fiber paper. However, reducing the porosity might bring other advantages, such as improved paper compactness, mechanical strength, electrical conductivity, etc.
The volume density of carbon fiber paper mattered considerably, as it reflected its compactness and the presence of smaller pores and channels, which were beneficial for gas molecule collision and transfer and could enhance its electronic conductivity [32]. As shown in Figure 10b, the volume density of 6mm CF paper was higher than that of 3mm CF paper, which might be attributed to the better fiber alignment, stronger fiber cross-linking and reduced inter-fiber gaps in carbon fiber paper made from longer fibers. Furthermore, increasing the carbon fiber sizing concentration led to a gradual increase in the volume density of the carbon fiber paper. The volume densities of CFP-0 prepared from 3mm and 6mm carbon fibers were 0.125g·ml-1 and 0.152g·ml-1, respectively. After dopamine treatment, due to the chemical modification of the carbon fiber paper by dopamine, the volume density reached a maximum of 1.68g·ml-1 for 3mm CF paper and 1.7g·ml-1 for 6mm CF paper. The formation of a dense polydopamine coating on the carbon fiber surface filled micro-pores and fine gaps, resulting in a more compact overall structure and higher volume density for the carbon fiber paper.

3.3.4. Contingency and Stresses of the CF Paper

As shown in Figure 11a, both the strain and stress of the CF paper showed significant changes compared to CFP-0, with a more pronounced effect observed for dopamine-treated longer carbon fibers. Regarding strain, the carbon fiber paper prepared with 3mm carbon fibers presented a maximum strain of 9.73% after treatment, which was 2.3 times higher than the 4.06% of CFP-0. For the carbon fiber paper prepared with 6mm carbon fibers, the strain of CFP-0 was 3.81%, which featured a 2.2 times increase to a minimum of 8.48% after dopamine treatment. The maximum strain could reach 15.07%, showing a 3.9 times increase compared to CFP-0 [36]. Similarly, as depicted in the Figure 11b, the stress of PDA-CFP was significantly higher than CFP-0. CFP-0 showed stresses of 1.34MPa and 2.16MPa, while the maximum stress increased to 4.04MPa and 7.80MPa after treatment, which was a nearly four-fold increase. The removal of impurities from the fibers during pulping facilitated compaction during paper preparation, resulting in a denser paper structure, which, to some extent, could enhance the stress-strain behavior of the carbon fibers. Moreover, the self-assembled polymer layer of dopamine on the carbon fiber surface enhanced the adhesion of the fibers with other components in the paper, allowing the fibers to better withstand stress. Additionally, dopamine modification might alter the chemical properties of the carbon fiber surface, such as increasing surface oxygen content and functional groups, thereby strengthening the interaction between fibers and other components, which contributed to the improvement of paper strength and stiffness. Consequently, dopamine modification greatly enhanced the stress-strain performance of the carbon fiber paper.

3.3.5. Resistivity and Permeability of the CF Paper

As shown in Figure 12a, the high resistivity of CFP-0 and the uneven distribution of resistivity at different sites (high resistivity coefficient of variation) were primarily attributed to the uneven fiber dispersion, more fiber contact points at the flocculation, low resistivity in most of the contact area, less fiber in other void parts, loose structure and high resistivity due to few electrons transport channels [27,37]. The introduction of dopamine modification led to a reduction in the resistivity and resistivity coefficient of the variation of CFP-1 compared to CFP-0. The CFs exhibited a uniform dispersion after dopamine treatment, resulting in minimal variation in resistivity at different sites and a small resistivity variation coefficient. The resistivity reduction trend of the CF paper prepared with both lengths was similar to that of CFP-0, but with significantly improved performance. For 3mm length fibers, with increasing dopamine concentration, the resistivity of PDA-CFP-2, PDA-CFP-5, and PDA-CFP-6 were measured as 0.1584Ω·cm-1, 0.11215Ω·cm-1, and 0.13536Ω·cm-1, respectively, compared to CFP-1’s 0.17615Ω·cm-1, indicating a reduction in resistivity of 10.07%, 36.33%, and 21.16%, respectively. Meanwhile, for 6mm length fibers, with increasing dopamine concentration, the resistivity of PDA-CFP-2, PDA-CFP-5, and PDA-CFP-6 were measured as 0.8157Ω·cm-1, 0.6055Ω·cm-1 and 0.6457Ω·cm-1, respectively, compared to CFP-1’s 0.8795Ω·cm-1, suggesting a resistivity reduction of 7.25%, 31.15%, and 26.58%, respectively. Clearly, as the dopamine concentration increased, the resistivity initially decreased and then increased, with the optimal resistivity observed at the 5g/L dopamine concentration for both 3mm and 6mm length fibers. The decrease in resistivity could be attributed to the increased functional group content on the surface of the carbon fiber after dopamine modification. The functional groups in the dopamine molecule chemically reacted with the carbon fiber surface, forming chemical bonds with a higher oxygen content and a polymer layer with a conjugated structure. This covering layer enhanced the electrical conductivity of the carbon fibers by providing more active sites for charge transfer and electron transport. Additionally, the increased roughness and surface activity of the carbon fiber surface induced by dopamine further contributed to the efficiency of electron transport, and resulted in improved electrical conductivity of the dopamine-modified CF paper compared to CFP-0. While higher concentrations of dopamine might lead to overgrowth or aggregation of polymer layers, potentially affecting conductivity, the overall enhancement of electrical conductivity achieved through dopamine modification outweighed any negative effects, proving dopamine modification as an effective strategy for improving the electrical conductivity of the CF paper.
As the base layer of the GDL, the CF paper should possess high air permeability to enhance the transfer efficiency of different systems and improve fuel cell performance [38]. It can be observed from Figure 12b that the air permeability of CF paper prepared using dopamine-modified carbon fiber showed a significant change. The air permeability of CFP-0 prepared with 3mm and 6mm length carbon fiber was 4230 I·m-2·s-1 and 4670 I·m-2·s-1, respectively. However, with the increase of dopamine concentration, the air permeability initially decreased and then increased. At a sizing concentration of 5g/L, the air permeability for the 3mm and 6mm CF paper was measured to be 3861 I·m-2·s-1 and 4290 I·m-2·s-1, respectively, which was 8.72% and 8.13% lower than that of CFP-0, respectively. Subsequently, the air permeability increased again, which could be attributed to the following reasons: at lower dopamine concentrations, the polymerization reaction of dopamine molecules on the surface of carbon fibers was relatively limited, resulting in a thin modified layer and decreased porosity. However, as the concentration increased, the cohesive effect of dopamine polymers saturated, causing polymer particles to aggregate and form larger gel structures, which, in turn, created larger pore spaces on the fiber surface and led to an increase in porosity.

3.4. Mechanistic Analysis of Dopamine Modified Carbon Fiber

The carbon fiber surface modification process using dopamine involved impregnating the carbon fiber with dopamine and allowing the self-polymerization of dopamine on the carbon fiber surface to form polydopamine. This polydopamine contained abundant amino and hydroxyl functional groups, and numerous hydrogen bonds with the functional groups were formed on the carbon fiber surface. As a result, the surface of the carbon fiber became more hydrophilic, and the number of active groups increased, greatly enhancing its dispersibility. The combined effects of the chemical reactions and self-polymerization of dopamine resulted in a solid polymer coating on the carbon fiber surface, which further created a strong cross-linking structure and covalent bonding between the carbon fibers, leading to improved bonding strength among the fibers. This modification significantly enhanced the mechanical strength and stability of the carbon fiber paper while also improving its electrical conductivity and surface activity. Overall, the use of dopamine as a surface modifier effectively improved the performance of carbon fiber paper in various applications.
Figure 14a shows that the CF surface primarily comprised carbon (284.82 eV) and oxygen (532.48 eV), and the nitrogen (399.8 eV) content was rather insignificant [39]. The appearance of the nitrogen peak was attributed to the fact that some impurities were not completely removed during the pretreatment of the CFs. After dopamine polymerization, the elemental ratio on the CF surface changed considerably, accompanied by the increased nitrogen (399.8 eV) content. The O/C ratio on the CF surface became larger with increasing dopamine concentration. The O/C ratio was proportional to the amount of oxygen-containing functional groups present on the CF surface [40]. The change in the O/C ratio of the CF surface with the dopamine concentration confirmed the successful polymerization of dopamine on the PDA-CF surface.
Figure 14b shows the comparison between the FTIR spectra of pretreated CFs and sized CFs. Considering the –OH stretching vibration on the CF surface, the FTIR spectra of all CFs exhibited peaks near 3450 cm−1. The peaks in the FTIR spectra of the dopamine-modified CFs underwent redshift in varying degrees because of the presence of –NH2 on the surface of PDA-CF (–NH2 has stretching vibration at 3300–3500cm−1) [41]. The FTIR spectra of all CFs showed peaks at 1655 cm−1, 2850cm−1 and 2930cm−1, corresponding to the presence of C=C, –CH2 and –CH3 stretching vibrations of the CF structure [42], respectively. Additionally, the intensity of the –CH– stretching vibration peak at 2860cm−1 increased considerably, which could be explained the hydrogen bonding between dopamine and CF molecules and the dopamine-induced C=C to C–C transition of some CFs during the oxidative self-polymerization reaction. Besides, the FTIR spectra of CFs after sizing treatment presented distinct vibrational peaks at 1400cm−1 and 430cm−1 because of the superposition of the bending vibration in the –OH plane of PDA-CF (1410cm−1) and the C–N (1430cm−1) stretching vibration. The change in CF after sizing at 880cm−1 [43]. A sharp peak at 880 cm−1 was visually observed in the FTIR spectrum, which corresponded to the stretching vibration caused by a large amount of primary amine (–NH2) in PD. Therefore, dopamine successfully underwent oxidative self-polymerization on the CF surface. The presence of different degrees of the induction effect and the conjugation effect of dopamine during oxidation resulted in the FTIR peaks of all functional groups in Figure 14b shifting to different degrees.
It can be clearly observed from Figure 15a–d that the carbon fibers treated with dopamine showed new C-N peaks after C1s fitting, and the C-O peak as well as the O=C-O peak area of the carbon fibers were significantly changed. This was attributed to the fact that the dopamine molecule contained functional groups such as hydroxyl (OH) and amine (NH2) groups, which could chemically react with the functional groups on the surface of carbon fibers to form C-O bonds and C-N bonds. For example, hydroxyl functional groups could undergo esterification or condensation reactions with carboxylic acid or carbonyl functional groups on the surface of carbon fiber to form C-O bonds; amine functional groups could undergo condensation reactions with acidic functional groups on the surface of carbon fiber to form C-N bonds; and some surface active centers might be formed during the adsorption of dopamine on the surface of carbon fiber, which had certain catalytic effects and promoted the formation of C-O bonds, C=O bond and C-N bond formation. Besides, the dopamine molecule itself had an oxidative nature and could oxidize with the functional groups on the surface of carbon fibers to form C=O bonds. This oxidation reaction might involve the reaction of the aldehyde group (CHO) or ketone group (C=O) in the dopamine molecule with the functional groups on the surface of the carbon fiber.
As shown in Figure 15e–h, the CF had a high carbon content. The first-order Raman spectrum of the original CFs exhibited two typical peaks at 1330–1340 cm−1 (D-band) and 1590–1600 cm−1 (G-band), which corresponded to the degree of carbon amorphization and graphitization, respectively. Meanwhile, the integrated areas of the D-band and G-band were calculated separately [44], and the degree of graphitization of CFs was subsequently measured using the area ratio (ID/IG). Raman spectra were subjected to split-peak fitting processing using the Lorentzian function to obtain parameters such as the position, half-peak width, and peak intensity of the D-band and G-band characteristic peaks. By calculating the peak fitted integral area ratio of the samples, it was found that the ID/IG of CF-1 was 1.03, while the ID/IG after dopamine modification was 0.72(PDA-CF-2), 0.74(PDA-CF-5) and 0.71(PDA-CF-8), respectively. Additionally, the area ratio presented a significant decreasing trend, and the area ratio significantly decreased, probably because that after dopamine modification, the aliphatic structure of the CF was formed because of the reaction between the carboxyl group on the CF and the amino group on PD. Besides, after dopamine modification, the organic groups on the surface of the CF were reduced. Dopamine could change the molecular orbitals of carbon atoms in CFs from sp3 orbitals to sp2, thereby improving the graphitization of CFs to some extent. In addition, the aromatic ring structure in dopamine interacted with the aromatic structure on the carbon fiber surface by π-π stacking, which promoted the formation of sp2 hybridized carbon structure on the carbon fiber surface. This stacking action could also increase the aromatic carbon content on the carbon fiber surface, and the functional groups in dopamine (such as amino and phenolic groups) underwent chemisorption reactions with the functional groups on the carbon fiber surface to form chemical bonds. This chemisorption could further introduce new sp2 hybridized carbon structures, and the stacking effect as well as various effects such as chemical reactions led to a significant increase in the sp2 degree of carbon fibers [23,24]. Therefore, the intensity of the G-peak of the modified CF increased in the spectra, which also yielded a significant increase in the peak of the –CH– stretching vibration at 2860 cm−1 in the FTIR spectra (Figure 14b). Additionally, the D-band peak exhibited material defects. As the content of PD on the CF surface increased, the degree of surface defects decreased, and the intensity of the D-peak decreased correspondingly.
As shown in Figure 15i, the characteristic diffraction peaks of the untreated CFs were observed at 2θ = 25.9° and 43.1° in the XRD pattern, and the diffraction peaks corresponding to (002) and (010) of PDA-CF (Figure 15j–l) with different sizing concentrations were in the same position as the two diffraction peaks of CF-1, demonstrating PD as an amorphous polymer. The sized dopamine could undergo oxidative self-polymerization on the CF surface and protect the CF crystal structure without changing the properties of the CFs.

4. Conclusions

Herein, by modifying CFs of different lengths and preparing the corresponding CF papers, the effect of dopamine-modified CFs on CF paper was studied. A green CF modification method was obtained from the experimental data analysis, and the CF modified by this method prepared carbon fiber paper with uniform fiber dispersion, low resistivity and high permeability. The tensile strength of the CF paper prepared using modified PDA-CF was four times higher, and the modified CFs could improve the paper uniformity of the CF paper. The close contact between the fibers reduced the electron transfer resistance. The resistivity of the CF paper after dopamine modification could reach as low as 0.06055 Ω·cm−1, which could be reduced by up to approximately 37.03% relative to the virgin CF paper resistivity. Additionally, dopamine oxidation was observed on the CF surface. PD attached to the CF had little effect on the CF permeability, and compared with the virgin CF paper, the maximum reduction of permeability was only 8.17%. Besides, the use of dopamine-modified CFs in the preparation of carbon fiber paper greatly expanded the application of modified carbon fibers, and the successfully prepared PDA-CFP could be used as a good substrate layer for GDL and further loaded with other conductive substances to produce excellent GDL.

Author Contributions

Writing—original draft preparation, validation, writing—review and editing, Jiahua Ma; Conceptualization, methodology, project administration, Chuanshan Zhao; software, validation, formal analysis, Xiangyu Chen, Xiaoshuai Sun. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The QUTJBZ Program (No. 2022JBZ01-05) financially supported funding this article.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Kallem, P.; Yanar, N.; Choi, H. Nanofiber-Based Proton Exchange Membranes: Development of Aligned Electrospun Nanofibers for Polymer Electrolyte Fuel Cell Applications. ACS Sustain. Chem. Eng. 2019, 7, 1808–1825. [Google Scholar] [CrossRef]
  2. Liu, C.Y.; Sung, C.C. A review of the performance and analysis of proton exchange membrane fuel cell membrane electrode assemblies. J. Power Sources 2012, 220, 348–353. [Google Scholar] [CrossRef]
  3. Ye, D.H.; Zhan, Z.G. A review on the sealing structures of membrane electrode assembly of proton exchange membrane fuel cells. J. Power Sources 2013, 231, 285–292. [Google Scholar] [CrossRef]
  4. Liang, P.; Qiu, D.K.; Peng, L.F.; Yi, P.Y.; Lai, X.M.; Ni, J. Structure failure of the sealing in the assembly process for proton exchange membrane fuel cells. Int. J. Hydrog. Energy 2017, 42, 10217–10227. [Google Scholar] [CrossRef]
  5. Sadeghifar, H. In-plane and through-plane local and average Nusselt numbers in fibrous porous materials with different fiber layer temperatures: Gas diffusion layers for fuel cells. J. Power Sources 2016, 325, 311–321. [Google Scholar] [CrossRef]
  6. Yang, W.F.; Zhu, L.J.; Wang, S.L.; Yin, Z.Q.; Xiao, L.S.; Shao, Q.S.; Jung, J.C.Y.; Sui, P.C. Investigation of fabrication of gas diffusion substrate for proton exchange membrane fuel cells. Int. J. Hydrog. Energy 2022, 47, 35423–35436. [Google Scholar] [CrossRef]
  7. Corujeira-Gallo, S.; Dong, H.S. Effect of microstructure on the plasma surface treatment of carbon fibres. J. Compos Mater. 2017, 51, 3239–3256. [Google Scholar] [CrossRef]
  8. Guo, X.S.; Cheng, Y.X.; Fan, Z.; Feng, Z.H.; He, L.L.; Liu, R.G.; Xu, J. New insights into orientation distribution of high strength polyacrylonitrile-based carbon fibers with skin-core structure. Carbon 2016, 109, 444–452. [Google Scholar] [CrossRef]
  9. Jiang, J.J.; Yao, X.M.; Xu, C.M.; Su, Y.; Zhou, L.C.; Deng, C. Influence of electrochemical oxidation of carbon fiber on the mechanical properties of carbon fiber/graphene oxide/epoxy composites. Compos. Pt. A-Appl. Sci. Manuf. 2017, 95, 248–256. [Google Scholar] [CrossRef]
  10. Kruppke, I.; Scheffler, C.; Simon, F.; Hund, R.D.; Cherif, C. Surface Treatment of Carbon Fibers by Oxy-Fluorination. Materials 2019, 12, 18. [Google Scholar] [CrossRef]
  11. Xu, Z.W.; Huang, Y.D.; Song, Y.J.; Zhang, C.H.; Li, L. Surface characteristics of rare earth treated carbon fibers and interfacial properties of composites. J. Rare Earths 2007, 25, 462–468. [Google Scholar] [CrossRef]
  12. Xu, Z.W.; Huang, Y.D.; Zhang, C.H.; Chen, G. Influence of rare earth treatment on interfacial properties of carbon fiber/epoxy composites. Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 2007, 444, 170–177. [Google Scholar] [CrossRef]
  13. Moosburger-Will, J.; Lachner, E.; Loffler, M.; Kunzmann, C.; Greisel, M.; Ruhland, K.; Horn, S. Adhesion of carbon fibers to amine hardened epoxy resin: Influence of ammonia plasma functionalization of carbon fibers. Appl. Surf. Sci. 2018, 453, 141–152. [Google Scholar] [CrossRef]
  14. Zheng, L.B.; Wang, Y.X.; Qin, J.J.; Wang, X.H.; Lu, R.J.; Qu, C.; Wang, C.G. Scalable manufacturing of carbon nanotubes on continuous carbon fibers surface from chemical vapor deposition. Vacuum 2018, 152, 84–90. [Google Scholar] [CrossRef]
  15. Rohini, R.; Bose, S. Electrodeposited carbon fiber and epoxy based sandwich architectures suppress electromagnetic radiation by absorption. Compos. Pt. B-Eng. 2019, 161, 578–585. [Google Scholar] [CrossRef]
  16. Naito, K. Effect of Hybrid Surface Modifications on Tensile Properties of Polyacrylonitrile- and Pitch-Based Carbon Fibers. J. Mater. Eng. Perform. 2016, 25, 2074–2083. [Google Scholar] [CrossRef]
  17. Yu, W.Q.; Zhang, H.B.; Lan, A.; Yang, S.H.; Zhang, J.J.; Wang, H.C.; Zhou, Z.; Zhou, Y.M.; Zhao, J.H.; Jiang, Z.H. Enhanced bioactivity and osteogenic property of carbon fiber reinforced polyetheretherketone composites modified with amino groups. Colloid Surf. B-Biointerfaces 2020, 193, 9. [Google Scholar] [CrossRef]
  18. Tian, X.L.; Han, S.; Zhuang, Q.X.; Bian, H.G.; Li, S.M.; Zhang, C.Q.; Wang, C.S.; Han, W.W. Surface Modification of Staple Carbon Fiber by Dopamine to Reinforce Natural Latex Composite. Polymers 2020, 12, 13. [Google Scholar] [CrossRef]
  19. Xing, Y.; Deng, S.Y.; Feng, S.L.; Wang, Q.Q.; Hou, Y.P. Selective oxidation of carbon to enhance both tensile strength and interfacial adhesion of carbon fiber. J. Adhes. 2020, 96, 873–882. [Google Scholar] [CrossRef]
  20. Huan, X.H.; Shi, K.; Yan, J.Q.; Lin, S.; Li, Y.J.; Jia, X.L.; Yang, X.P. High performance epoxy composites prepared using recycled short carbon fiber with enhanced dispersibility and interfacial bonding through polydopamine surface-modification. Compos. Pt. B-Eng. 2020, 193, 14. [Google Scholar] [CrossRef]
  21. Chen, T.P.; Liu, T.C.; Su, T.L.; Liang, J.F. Self-Polymerization of Dopamine in Acidic Environments without Oxygen. Langmuir 2017, 33, 5863–5871. [Google Scholar] [CrossRef] [PubMed]
  22. Della Vecchia, N.F.; Luchini, A.; Napolitano, A.; D’Errico, G.; Vitiello, G.; Szekely, N.; d’Ischia, M.; Paduano, L. Tris Buffer Modulates Polydopamine Growth, Aggregation, and Paramagnetic Properties. Langmuir 2014, 30, 9811–9818. [Google Scholar] [CrossRef] [PubMed]
  23. Szabo, L.; Imanishi, S.; Hirose, D.; Tsukegi, T.; Wada, N.; Takahashi, K. Mussel-Inspired Design of a Carbon Fiber-Cellulosic Polymer Interface toward Engineered Biobased Carbon Fiber-Reinforced Composites. ACS Omega 2020, 5, 27072–27082. [Google Scholar] [CrossRef] [PubMed]
  24. Wang, T.Y.; Qiblawey, H.; Judd, S.; Benamor, A.; Nasser, M.S.; Mohammadian, A. Fabrication of high flux nanofiltration membrane via hydrogen bonding based co-deposition of polydopamine with poly(vinyl alcohol). J. Membr. Sci. 2018, 552, 222–233. [Google Scholar] [CrossRef]
  25. Wu, Q.; Razzak, A.; Bai, H.H.; Deng, H.; Ye, Z.Y.; Zhu, J.F. Dopamine concentration-dependent surface modification for gaining carbon fiber composites with enhanced interfacial adhesion. Compos. Commun. 2022, 29, 7. [Google Scholar] [CrossRef]
  26. Chen, Z.H.; Du, K.K.; Yin, H.N.; Zhang, D.Y.; Gao, J.; Song, W.; Zhang, S.B. Surface modification of bamboo fiber with dopamine associated by laccase for poly(3-hydroxylbutyrate) biocomposites. J. Clean Prod. 2023, 389, 10. [Google Scholar] [CrossRef]
  27. Heo, Y.J.; Park, M.; Kang, W.S.; Rhee, K.Y.; Park, S.J. Preparation and characterization of carbon black/pitch-based carbon fiber paper composites for gas diffusion layers. Compos. Pt. B-Eng. 2019, 159, 362–368. [Google Scholar] [CrossRef]
  28. Perdoch, W.; Cao, Z.R.; Florczak, P.; Markiewicz, R.; Jarek, M.; Olejnik, K.; Mazela, B. Influence of Nanocellulose Structure on Paper Reinforcement. Molecules 2022, 27, 16. [Google Scholar] [CrossRef]
  29. Qiu, D.K.; Peng, L.F.; Yi, P.Y.; Lai, X.M. A micro contact model for electrical contact resistance prediction between roughness surface and carbon fiber paper. Int. J. Mech. Sci. 2017, 124, 37–47. [Google Scholar] [CrossRef]
  30. Zhou, P.P.; Zhu, P.H.; Chen, G.; Liu, Y.; Kuang, Y.D.; Liu, Y.Y.; Fang, Z.Q. A study on the transmission haze and mechanical properties of highly transparent paper with different fiber species. Cellulose 2018, 25, 2051–2061. [Google Scholar] [CrossRef]
  31. Philips, M.F.; Gruter, G.J.M.; Koper, M.T.M.; Schouten, K.J.P. Production of Gas Diffusion Layers with Tunable Characteristics. ACS Omega 2022, 9. [Google Scholar] [CrossRef] [PubMed]
  32. Lian, B.B.; Chen, L.; Liu, M.R.; Fang, T.; Hu, J.; Li, H.L. The quantitative analysis for the formation of carbon fiber paper and its influencing factors. J. Mater. Sci. 2020, 55, 6566–6580. [Google Scholar] [CrossRef]
  33. Chen, Y.Q.; Jiang, C.; Cho, C.D. An experimental investigation of three-dimensional mechanical characteristics of gas diffusion layers in proton electrolyte membrane fuel cells. J. Solid State Electrochem. 2019, 23, 2021–2030. [Google Scholar] [CrossRef]
  34. Xia, L.C.; Ni, M.; He, Q.J.; Xu, Q.D.; Cheng, C. Optimization of gas diffusion layer in high temperature PEMFC with the focuses on thickness and porosity. Appl. Energy 2021, 300, 12. [Google Scholar] [CrossRef]
  35. Gao, Y.; Montana, A.; Chen, F.X. Evaluation of porosity and thickness on effective diffusivity in gas diffusion layer. J. Power Sources 2017, 342, 252–265. [Google Scholar] [CrossRef]
  36. Shi, S.S.; Guo, X.; Chen, B.Z.; Sun, Z. A logarithmic-type constitutive model for carbon fiber papers considering Hertz contact effect. J. Eng. Fiber Fabr. 2019, 14, 10. [Google Scholar] [CrossRef]
  37. Liu, Y.W.; Wu, S.Y.; Qin, Y.Z.; Zhang, M.F.; Liu, X.; Zhang, J.F.; Yin, Y. Mass transport and performance of proton exchange membrane fuel cell considering the influence of porosity distribution of gas diffusion layer. Int. J. Green Energy 2022, 19, 1503–1511. [Google Scholar] [CrossRef]
  38. Tamayol, A.; Bahrami, M. Asme. IN-PLANE GAS PERMEABILITY OF PROTON EXCHANGE MEMBRANE FUEL CELL GAS DIFFUSION LAYERS. In Proceedings of the 3rd Joint US-European Fluids Engineering Division Summer Meeting, Montreal, CANADA, Aug 01-05, 2010; pp. 1241–1248. [Google Scholar]
  39. Lindsay, B.; Abel, M.L.; Watts, J.F. A study of electrochemically treated PAN based carbon fibres by IGC and XPS. Carbon 2007, 45, 2433–2444. [Google Scholar] [CrossRef]
  40. Xu, N.; Li, Y.Z.; Zheng, T.; Xiao, L.; Liu, Y.Y.; Chen, S.H.; Zhang, D.X. A mussel-inspired strategy for CNT/carbon fiber reinforced epoxy composite by hierarchical surface modification. Colloid Surf. A-Physicochem. Eng. Asp. 2022, 635, 10. [Google Scholar] [CrossRef]
  41. Szabo, L.; Milotskyi, R.; Tsukegi, T.; Wada, N.; Takahashi, K. Quantitative analysis of native reactive functional groups on carbon fiber surface: An electrochemical approach. Appl. Surf. Sci. 2019, 494, 315–325. [Google Scholar] [CrossRef]
  42. Deng, J.Y.; Xu, L.; Liu, J.H.; Peng, J.H.; Han, Z.H.; Shen, Z.G.; Guo, S.H. Efficient method of recycling carbon fiber from the waste of carbon fiber reinforced polymer composites. Polym. Degrad. Stabil. 2020, 182, 9. [Google Scholar] [CrossRef]
  43. Sun, N.; Zhu, B.; Gao, X.P.; Qiao, K.; Zhang, Y.; Wang, B.M.; Fan, J.Y.; Yu, K.; Liu, C.H.; Li, C.S.; et al. Improved the interfacial characteristics of carbon fiber/polyamide 6 composites by synthesizing polydopamine rapidly on the carbon fiber surface with ultrasound-assisted. Compos. Sci. Technol. 2023, 234, 11. [Google Scholar] [CrossRef]
  44. Yang, W.W.; Cai, W.Q.; Zhou, J.X.; Dang, C.X.; Peng, X.; Chen, Y.T.; Wei, X.C.; Bo, S.L.; Liang, S.L.; Luo, Z.J. Mussel-inspired MgAl-LDH/carbon fiber film modified by polydopamine for highly efficient removal of Pb2+. J. Environ. Chem. Eng. 2021, 9, 9. [Google Scholar] [CrossRef]
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Figure 1. Dopamine oxidation mechanism process.
Figure 1. Dopamine oxidation mechanism process.
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Figure 6. (a) TG curves of CF-1and PDA-CF; and (b) DTG curves of CF-1and PDA-CF.
Figure 6. (a) TG curves of CF-1and PDA-CF; and (b) DTG curves of CF-1and PDA-CF.
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Figure 7. (a) Dispersions of CF-1; (b) Dispersions of PDA-CF-2; (c) Dispersions of PDA-CF-5; and (d) Dispersions of PDA-CF-8.
Figure 7. (a) Dispersions of CF-1; (b) Dispersions of PDA-CF-2; (c) Dispersions of PDA-CF-5; and (d) Dispersions of PDA-CF-8.
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Figure 8. Light transmission renderings of CFP-0 (a), CFP-1 (b), PDA-2-CFP (c), PDA-5-CFP (d) and PDA-8-CFP (e) with a fiber length of 3mm, and light transmission renderings of CFP-0 (f), CFP-1 (g), PDA-2-CFP (h), PDA-5-CFP (i) and PDA-8-CFP (j) with a fiber length of 6mm.
Figure 8. Light transmission renderings of CFP-0 (a), CFP-1 (b), PDA-2-CFP (c), PDA-5-CFP (d) and PDA-8-CFP (e) with a fiber length of 3mm, and light transmission renderings of CFP-0 (f), CFP-1 (g), PDA-2-CFP (h), PDA-5-CFP (i) and PDA-8-CFP (j) with a fiber length of 6mm.
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Figure 9. (a) Tightness of different CFPs; and (b) Tensile strength of different carbon fiber papers.
Figure 9. (a) Tightness of different CFPs; and (b) Tensile strength of different carbon fiber papers.
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Figure 10. (a). Porosity of different CFPs; and (b) Bulk density of different CFPs.
Figure 10. (a). Porosity of different CFPs; and (b) Bulk density of different CFPs.
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Figure 11. (a) Contingency of different CFP; and (b) Stresses of different CFPs.
Figure 11. (a) Contingency of different CFP; and (b) Stresses of different CFPs.
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Figure 12. (a) Resistivity of different CFPs; and (b) Air permeability of different CFPs.
Figure 12. (a) Resistivity of different CFPs; and (b) Air permeability of different CFPs.
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Figure 13. Schematic diagram of dopamine modified carbon fiber process.
Figure 13. Schematic diagram of dopamine modified carbon fiber process.
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Figure 14. (a) Ratio of O/C contents of CFs after different treatments; and (b) Combined FTIR spectrum of different treated CFs.
Figure 14. (a) Ratio of O/C contents of CFs after different treatments; and (b) Combined FTIR spectrum of different treated CFs.
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Figure 15. C1s XPS peak fit of CF-1 (a), PDA-2-CF (b), PDA-5-CF (c) and PDA-8-CF (d). Raman spectrum of the CF-1 (e), PDA-2-CF (f), PDA-5-CF (g) and PDA-8-CF (h). (i) XRD pattern of the CF-1 (i), PDA-2-CF (j), PDA-5-CF (k) and PDA-8-CF (l).
Figure 15. C1s XPS peak fit of CF-1 (a), PDA-2-CF (b), PDA-5-CF (c) and PDA-8-CF (d). Raman spectrum of the CF-1 (e), PDA-2-CF (f), PDA-5-CF (g) and PDA-8-CF (h). (i) XRD pattern of the CF-1 (i), PDA-2-CF (j), PDA-5-CF (k) and PDA-8-CF (l).
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Table 1. Ra and Rq of CFs.
Table 1. Ra and Rq of CFs.
Sample Ra Rq
CF-0 39.5nm 53.3nm
CF-1PDA-2-CFPDA-5-CFPDA-8-CF 37.5nm41.2 nm53.9 nm58.8 nm 29.8nm54.7 nm68.3 nm70.1 nm
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