1. Introduction
Since their commercialization in the 1960s, carbon fibers (CFs) have been extensively utilized across diverse fields, including aerospace, sports equipment, civil engineering, and military applications [
1], [
2], [
3], owing to their exceptional physical and chemical properties. These properties include high axial strength and modulus, superior chemical resistance, low weight, minimal thermal expansion coefficient, and pronounced anisotropy [
4]. Furthermore, CFs exhibit excellent electrical conductivity and surface functionalization potential; therefore, they are applicable as electrode materials in various electrochemical systems such as electrocatalysis, energy storage, advanced electronic devices [
5], [
6], [
7], and advanced constructions [
8]. Currently, 90% of commercially available CFs are derived from the precursor polyacrylonitrile (PAN), known for its high carbon yield and broad processing tolerance [
9], [
10]. PAN-based CFs are characterized by a duplex structure consisting of an ordered (graphitic) outer shell and a relatively disordered (non-graphitic) inner core [
11], [
12]. Pristine CFs are typically supplied as a continuous tow—a bundle of multiple graphite filaments, each approximately 5-10 µm in diameter—exhibiting low specific surface area, poor porosity, and a limited number of active functional groups [
13], [
14]. The high cost of PAN and the low electrochemical activity of CFs constrain their broader application in energy-related fields. Additionally, the high rigidity and brittleness of CFs present technical and processing challenges, impeding the cost-effective manufacturing of CF composite products for practical applications.
Understanding the evolution of carbon layer structures in CFs is paramount because it directly influences their multifunctional performance. The structural configuration of carbon layers determines critical properties such as mechanical strength, electrical conductivity, and chemical stability. These properties are essential for applying CFs in high-performance composite materials, energy storage systems, and advanced electronic devices. Modifying the carbon layer structure can enhance the surface reactivity, increase the specific surface area, and introduce functional groups that are beneficial for specific applications, such as in supercapacitors or as catalysts in fuel cells. Furthermore, insights into the structural evolution provide a basis for optimizing processing techniques and improving the overall performance and durability of CF-based materials.
Enhancing the multifunctional performance of CFs necessitates a synergistic approach that is ① facile and cost-effective, ② conducive to investigating fundamental principles, microstructural evolution, and structure-processing-property relationships, and ③ capable of enhancing the efficiency of CFs for practical applications. Strategies, including surface decoration with nanomaterials [
15], [
16], high-temperature calcination [
17], [
18], and strong acid oxidation treatment [
19], have been used to modify CFs. However, these methods present several drawbacks, such as difficulty in optimizing treatment processes, the requirement for energy-intensive equipment, and hazardous by-product generation.
In contrast, electrochemical modification is a traditional yet effective technique for enhancing the surface properties of CFs. This method can concurrently improve their physicochemical characteristics via electrochemical reactions in the electrolyte [
20], [
21], [
22], [
23], [
24]. Studies have proposed mechanisms for enhancing the performance; nonetheless, the overarching influence of electrolytes on the morphological and structural evolution of CFs induced by electrochemical reactions remains insufficiently elucidated. Additionally, many electrolytes and their by-products used in prior studies pose environmental hazards, rendering them undesirable for large-scale CF modification. With the rapid proliferation of CF-based products, advanced processing technologies for CF modifications are imperative for their cost-effective and innovative applications.
Progressive processing technologies have garnered less attention because of the inherent rigidity and brittleness of CFs despite significant advancements in CF fabrication and modification. Several processing methods have primarily focused on graphene-based carbon materials. For instance, nano-mesh graphene has been designed and fabricated by O
2 plasma treatment [
25], creating a semiconducting thin film with an opened bandgap. Similarly, the reversible fusion and fission technology of wet-spun graphene oxide fibers through solvent evaporation and infiltration has also been investigated [
26]. These methods benefit CF processing owing to the analogous structural properties of graphitic carbon materials; however, they are time-consuming, expensive, and unsuitable for low-cost and scalable CF processing.
In this study, we introduce two common salts (Na2SO4 and NaCl) at a high voltage to elucidate the evolution of carbon layers during the intense electrochemical oxidation process of CFs. We monitored the evolution of the carbon layer structure of CFs by integrating in situ optical microscopy (OM), electrochemical analyses, and other characterization techniques, providing insights into the effect of anions on these dynamic processes. The carbon layer structure was modified in two ways (in Cl− and SO42− environments), significantly enhancing the electrochemical properties of the modified CFs in terms of electrocatalytic performance for the oxygen reduction reaction (ORR) and supercapacitor performance. Furthermore, we explored in situ CF processing technologies, including the bending and merging processes without a binder at ambient pressure and temperature, for the first time through this approach. This study underscores the influence of two common anions on the electrochemical oxidation-based modification of CFs, thereby developing a robust, cost-effective, and environmentally benign strategy for commercial engineering CFs.
2. Materials and methods
2.1. Preparation of the electrochemical device for the single CF
Commercial isotropic PAN-based single CF (SCF) (diameter 7 µm) and 304 stainless-steel wire (diameter 50 µm) were fixed parallel to a glass slide. A circular polyethylene belt (diameter ∼1.2 cm) was placed vertically on the fiber and wire. Subsequently, the gap between the belt and the glass slide was sealed with glue. Finally, a copper wire (diameter 50 µm) was used to connect the SCF outside the circle using conductive silver adhesives. The effective length of the two electrodes was ∼1 cm.
2.2. Morphology and structure characterization
The appearance and morphology of the SCF and CF bundle (CFB) before and after the oxidation were determined by combining OM (AO-3M180, AOSVI, China) and scanning electron microscopy (SEM; QuantaTM 250 FEG, FEI Company, USA). Before the SEM observation, the CFB samples were thoroughly rinsed with water to remove water-soluble salts and air-dried. High-resolution SEM was conducted using a Zeiss Ultra-Plus FE-SEM (Germany) at an accelerating voltage of 5 kV to capture detailed surface features without damaging the carbon layers. Thin sections of CFs were prepared using a Helios Nanolab 600i DualBeam FIB-SEM (FEI Company), ensuring minimal damage and precise sectioning for transmission electron microscopy (TEM) analysis. TEM was performed using a JEM-2100F HRTEM (JEOL Ltd., Japan) at 200 kV. The surface states of different CFBs were analyzed using X-ray photoelectron spectroscopy (XPS; K-Alpha+, Thermo Fisher Scientific, USA) and Raman spectroscopy (FIW532, Zolix, China). Fourier-transform infrared (FTIR) spectra were obtained using a Thermo Fisher Nicolet iS50 FTIR spectrometer with an attenuated total reflectance accessory covering 4000-500 cm−1. The spectra were collected at a resolution of 4 cm−1, with 32 scans averaged per sample. X-ray diffraction (XRD) patterns were collected using a SmartLab X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 1.5406 Å = 1.5406 × 10−10 m, where λ is the wavelength). The scanning range of diffraction angle (2θ) was set from 10° to 80°, with a step size of 0.02°. Nitrogen adsorption-desorption isotherms were measured at 77 K using a ASAP 2020 analyzer (Micromeritics, USA). The Brunauer-Emmett-Teller (BET) surface area was calculated using the adsorption data in the relative pressure range (the equilibrium pressure divided by the saturation pressure, P/P0) of 0.05-0.30, while the Barrett-Joyner-Halenda model was used to determine pore-size distribution and total pore volume.
2.3. Electrochemical measurements
SCF was oxidized under prescribed conditions using a two-electrode system, with SCF and a stainless-steel wire in the circle used as the anode and cathode, respectively, placed 3 mm apart. The electrolyte was a saturated Ca(OH)2 solution with 0.9 mol∙L−1 NaCl or 0.6 mol∙L−1 Na2SO4. A fixed voltage of 3 V was supplied using a GAMRY 1100 electrochemical workstation (Gamry Instruments, USA). Electrochemical impedance spectroscopy (EIS) was performed using a three-electrode system in which additional Ag/AgCl (saturated KCl solution) was used as the reference electrode, and 0.1 mol∙L−1 KCl with 10 mmol∙L−1 K3Fe(CN)6 was used as the electrolyte. The alternating current voltage amplitude was 10 mV in the frequency range of 100 MHz-100 kHz at an overpotential of 0 V. A three-electrode configuration in respective solutions was used to evaluate the electrochemical performance of the 5 mm-long CFB. In air-saturated 0.1 mol∙L−1 KOH for the electrolysis of water, N2 or O2 saturated 0.1 mol∙L−1 KOH was used for ORR testing (CFBNaCl was first treated by cyclic voltammetry (CV) or ultrasound to remove the unstable carbon), and 1.0 mol∙L−1 Na2SO4 was used to assess the capacitor. CFB served as the working electrode, with a platinum (Pt) wire, and Ag/AgCl (saturated KCl) was used as the counter and reference electrode, respectively. The CV and galvanostatic charge-discharge (GCD) measurements were taken at −1.3-0.9 V with different scan rates.
2.4. In situ processing of CFs
The two-electrode system was used to process CFs, in which the SCF or CFB and a removable Pt wire were the anode and cathode, respectively. The electrolyte was a saturated Ca(OH)2 solution with 0.6 mol∙L−1 Na2SO4. We supplied a 4 V voltage using a 1100 electrochemical workstation. Furthermore, the SCF or CFB was bent and fixed first, and the electrochemical oxidation treatment was subsequently carried out at the bent part of CFs. After electrolysis for 10 s for SCF and 20 s for CFB, the CFs were rinsed with water and dried at room temperature. For the merging process, two SCFs were parallel to each other, as closely as the anode, and then the same treatment was carried out at the contact part of the SCF for 50 s. The closed monofilament triangle of SCF was constructed by processing the bent and overlapping part of the monofilament using the same method. Typically, an SCF was twined in a circle along the surface of three plastic rods, and the two ends were fixed at first. Subsequently, the bent and overlapping part of SCF was treated using electrolysis, and the sample was rinsed and dried. Finally, both fixed ends of SCF were cut.
3. Results and discussion
3.1. Evolution of the carbon layer structure of SCF in different anion environments
Various electrochemical methods have been widely used to modify the CFs; nonetheless, detailed studies on the influence of anions on the surface state transformation processes are lacking owing to the complicated solution environment and/or longer reaction time. A manual electrochemical device was used to directly observe the morphological evolution of SCF along with the change in electrochemical signals (
Fig. 1(a)). Ca(OH)
2 was the alkali salt and an indicator to monitor the carbon oxidation (CO
32−) products based on the reaction $\mathrm{CO}_{3}^{2-}+\mathrm{Ca}^{2+} \rightarrow \mathrm{CaCO}_{3} \downarrow$.
NaCl and Na
2SO
4 were used as electrolytes, which contained the electrochemically active Cl
− and inert SO
42− for electrochemical reactions. The results are presented in
Fig. 1(b), and Movies S1 and S2 in Appendix A. During oxidation of the SCF in the Na
2SO
4 solution, vigorous movement was observed in the outer carbon layers, along with the formation of a small amount of gas and sediment. This was attributed to the synergistic action of SO
42− insertion and the uneven oxygen evolution reaction (OER). In comparison, under the same electrolytic conditions in the alkali solution of NaCl, local dissolution (etching) of carbon layers was observed instead of their movement. Simultaneously, numerous gaseous and solid by-products were produced, indicating the intensified electrochemical reactions induced by the enhanced electrochemical activity of SCF. It was inferred that the oxidation product of $\mathrm{Cl}^{-}\left(\mathrm{Cl}^{-}+2 \mathrm{OH}^{-}-2 \mathrm{e}^{-} \rightarrow \mathrm{ClO}^{-}+\mathrm{H}_{2} \mathrm{O}\right)$ in a strong alkali solution could activate the surface carbon atoms of SCF for higher electrochemical performance, alleviating the disturbance to the carbon layers. Thus, we concluded that the anions in the electrolyte are crucial in the electrochemical modification of the morphological structure and surface properties of CFs. An electrochemical regulation strategy was proposed for CFs based on these results (
Fig. 1(c)). The detailed characterization and explanations are presented in the following sections, in which the SCF and CFB treated in Na
2SO
4 and NaCl solutions are denoted as $\mathrm{SCF}_{\mathrm{Na}_{2} \mathrm{SO}_{4}} / \mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ and $\mathrm{SCF}_{\mathrm{NaCl}} / \mathrm{CFB}_{\mathrm{NaCl}}$, respectively.
3.2. Morphological and structural characterization of CFs
The microstructural variations in CF during regulation were studied using SEM. A typical SCF has a smooth surface with grooves (Figs. 2(a)–(c)). After intense oxidation treatment, the SCF
NaCl remained intact; however, the roughened carbon layers simultaneously formed a considerable amount of CaCO
3 particles (
Figs. 2(d) and
(e); Fig. S1 in Appendix A), indicating the uniform dissolution of carbon ($\mathrm{C} \rightarrow \mathrm{CO}_{3}^{2-}$). The local morphology of SCF (red arrows in
Fig. 1(c) and Movie S1) exhibited a conical structure (Fig. S2(a) in Appendix A), further illustrating the uniform dissolution of carbon in longitudinal and radial directions of the SCF. High-magnification SEM images of the local $\mathrm{SCF}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ (Fig. S2(b) in Appendix A) revealed the layered structure of carbon, indicating the increased interlayer space of the carbon layers, similar to the production of graphene by electrochemical exfoliation of graphitic carbon [
27], [
28]. The phenomena observed above suggests that the carbon of the inner and outer layers have poor thermodynamic stability in an NaCl solution. Compared with that in SCF
NaCl, the slight morphological change of CFs in CFB
NaCl, demonstrated as the deepening of the CF grooves (
Fig. 2(f)) along with the formation of CaCO
3 sediment in solution (Fig. S3 in Appendix A), was attributed to the relatively low current intensity resulting from the increased effective electrode area. CFB
NaCl maintained approximately 93% of the mechanical properties of the pristine CFB (Fig. S4 in Appendix A), indicating that Cl
− ions can regulate the carbon layers of CF by electrochemical activation of carbon. In contrast, after treatment under the same conditions in the Na
2SO
4 solution, most of the outer carbon layers in the $\mathrm{SCF}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ were removed (
Fig. 2(g) and
(h)). Along with the movement of carbon layers, the adhesion of CFs and formation of porous structures in $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ were also observed (
Fig. 2(i); Fig. S5 in Appendix A). These moving carbon layers from the adjacent CFs were likely bonded by van der Waals forces, which formed a porous structure because of the incomplete bonding of carbon layers. The poor dispersibility of $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ further confirmed the bonding behavior of CFs (Fig. S6 in Appendix A).
Fig. 2(j) illustrates the schematic for the transformation mechanism of $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$. Moreover, the $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ preserved approximately 70% of the mechanical properties of the pristine CFB (Fig. S4). The consistent morphological evolution of SCF and CFB suggests the excellent scalability of the proposed electrochemical regulation strategy toward CFs.
High-magnification SEM and TEM analyses (
Fig. 3) were performed on the CF samples treated with NaCl and Na
2SO
4 to elucidate the morphological and structural changes induced by the electrochemical treatments. SEM images were captured at 50 000×-100 000×, providing a detailed view of the surface morphology. The NaCl-treated CFs displayed a well-defined, uniform carbon layer covering the fiber surface (
Figs. 3(d) and
(e)). These layers exhibited a continuous, smooth texture with minor irregularities, suggesting homogeneous oxidation facilitated by the Cl
− ions. In contrast, the Na
2SO
4-treated CFs had rougher surfaces with visible cracks and ridges (
Figs. 3(g) and
(h)), indicative of layer delamination and structural disruption caused by the intercalation of SO
42− ions without substantial oxidation. TEM analysis provided further insights at the nanoscale. The NaCl-treated CFs (
Fig. 3(f)) revealed graphitic carbon layers with an interlayer spacing of approximately 0.34 nm, consistent with the (002) graphite plane. High-resolution TEM images revealed well-aligned graphitic domains with clear lattice fringes, confirming the formation of an ordered graphitic structure. Conversely, the Na
2SO
4-treated CFs exhibited thicker, disordered carbon layers (
Fig. 3(i)) with amorphous carbon regions interspersed with short-range graphitic order. These observations correlate with the electrochemical performance, where the more ordered graphitic structures in the NaCl-treated CFs contribute to the enhanced electrocatalytic activity. In contrast, the disordered structures in the Na
2SO
4-treated CFs result in improved electrochemical behavior due to the increased availability of active sites.
XPS was used to analyze the surface composition and functional groups of the treated CFB, investigating the evolutionary and enhancement mechanisms of the electrochemical process. The survey spectra (
Fig. 4(a)) revealed that CFB
NaCl had a considerably higher intensity ratio of O 1s/C 1s, approximately 0.44 than that of pristine CFB (approximately 0.27) and $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ (approximately 0.21). This indicates the formation of several oxygen-containing functional groups during CFB
NaCl regulation, which can be associated with its enhanced electrochemical activity. For the pristine CFB (
Fig. 4(b)), unoxidized carbon (C=C/C–C) was observed at 284.6 eV (65.0%), and mono-oxidized (C–O; 33.4%) and carboxyl carbon (O–C=O; 1.49%) were observed at 286.4 and 289.4 eV, respectively. After Na
2SO
4 treatment (
Fig. 4(c)), the relative amounts of C–C/C=C and O–C=O (288.6 eV) increased to 78.5% and 8.3%, respectively. In contrast, the amount of C–O (286.2 eV) decreased sharply to 13.0%, and that of di-oxidized carbon (C=O at 287.4 eV) increased to 0.4%, further suggesting the physical regulation of the structure of the carbon layers in $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ rather than by their electrochemical oxidation. Meanwhile, the oxidation degree of CFs increased considerably in CFB
NaCl (
Fig. 4(d)), as indicated by the remarkable decrease in C–C/C=C (45.3%), and the change in C–O (286.6 eV; 8.9%), C=O (288.1 eV; 24.48%), and O–C=O (289.3 eV; 5.1%), along with the appearance of an aliphatic carbon (C–H) at 283.8 eV (8.0%) and C–Cl at 286.0 eV (8.1%). Markedly, the CFB
NaCl had a much higher chloride content of approximately 4.0% compared to the CFB.
Fig. 4(e) presents the deconvoluted high-resolution XPS spectrum of Cl 2p. Three chlorine 2p peaks (characterized by the Cl 2p
3/2 and 2p
1/2 doublet with an energy separation of 1.6 eV and intensity ratio of approximately 2:1) [
29], [
30] at 201.4/203.0 eV, 199.4/201.0 eV, and 197.0/198.6 eV, were assigned to chlorobenzene, C–Cl, and chloride ions in the CFB
NaCl, respectively [
31]. These chlorine–carbon covalent bonds can further improve the electrochemical activity of CFs (in the following sections) [
32]. The elemental variation of CFB before and after the treatment was verified using energy dispersive spectra (EDS) (Fig. S7 in Appendix A). Compared to that of CFB (9.43%), the oxygen content of CFB
NaCl considerably increased to 20.90%, while that of $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ (9.08%) decreased slightly. Meanwhile, the CFB
NaCl had higher Cl and Ca contents (1.39% and 2.46%, respectively) than CFB (0 and 0.18%, respectively) and $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ (0.28% and 1.21%, respectively). The surface states of various CFBs were also investigated using Raman spectroscopy.
Fig. 4(f) illustrates the representative spectra of the pristine CFB, $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$, and CFB
NaCl. Two peaks at 1360 and 1580 cm
−1 were assigned to the D bands (for the atomic lattice defect) and the G bands (for the in-plane stretching vibration of sp
2 carbon hybridization) of carbon [
33], [
34] with a relative intensity (the ratio of the intensity of the D-band to the intensity of the G-band,
ID/
IG) of 0.86 for CFB increasing to 0.89 and 0.97 for $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ and CFB
NaCl, respectively. The slight increase of
ID/
IG indicates fewer defects of $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ owing to the slight oxidation of the carbon layers, while its significant increase indicates more defects of CFB
NaCl caused by the formation of numerous oxygen- and chlorine-containing groups. These results are consistent with those of the XPS and EDS analyses.
Fig. 4(g) illustrates the structures of the resulting carbon molecules.
FTIR spectroscopy was used to investigate the surface functional groups on the CFs post-treatment. The FTIR spectra (
Fig. 5(a)) revealed the presence of oxygen-containing functional groups, including strong C=O stretching vibrations at 1740 cm
−1, C-O stretching at 1220 cm
−1, and broad O-H stretching at 3400 cm
−1. These functional groups were more pronounced in the Na
2SO
4-treated CFs, indicating a higher degree of surface oxidation due to the intercalation and movement of SO
42− ions. The NaCl-treated CFs had similar functional groups but with lower intensity, suggesting a controlled oxidation process where Cl
− ions promote surface modification without extensive oxidation. These functional groups are crucial for enhancing the electrochemical reactivity of the CFs, particularly in catalytic and energy storage applications where surface interactions are essential for performance. XRD was conducted to assess the crystallinity and phase composition of the treated CFs. The XRD patterns (
Fig. 5(b)) revealed a broad peak at approximately 26°, corresponding to the (002) plane of graphitic carbon. In the NaCl-treated samples, this peak shifted slightly to lower angles, indicating an increased interlayer spacing that could be attributed to the intercalation effects of Cl
− ions. The full width at half maximum of this peak was also broader, suggesting a reduced crystallite size and increased defect density, enhancing electrochemical activity. The Na
2SO
4-treated CFs had a less pronounced shift but exhibited a broader (100) peak at approximately 43°, indicative of in-plane disorder and a more amorphous structure. These structural features are consistent with the observed TEM results, indicating significant differences in the electrochemical behavior of both treated CFs. The specific surface area and pore structure of the CFs were quantified using BET analysis and pore volume measurements (
Fig. 5(c)). The BET surface area of the Na
2SO
4-treated CFs was significantly higher—48.3 m
2∙g
−1 compared to 3.52 m
2∙g
−1 for the untreated CFs. The pore-size distribution, determined using the Barrett-Joyner-Halenda model (
Fig. 5(c)), indicated the formation of micropores (0.5-2 nm) and mesopores (2-50 nm) during the electrochemical treatment. The increase in surface area of the NaCl-treated CFs was more moderate (21.7 m
2∙g
−1) than that in Na
2SO
4-treated CFs. However, the former had a notable increase in mesopores, which are advantageous for facilitating ion transport in electrochemical systems. The enhanced surface area and pore structure are directly linked to the improved supercapacitor performance observed in the treated CFs, where larger surface areas provide more active sites for ion adsorption, and the optimized pore structure reduces charge transfer resistance, resulting in higher capacitance and better rate capability.
The electrochemical treatment of CFs in environments containing Cl− and SO42− anions precipitates distinct morphological transformations through specific etching and deposition mechanisms. Chloride ions instigate aggressive etching, resulting in the formation of microporous structures on the CF surface. This enhanced porosity augments the surface area, potentially increasing the number of electrochemically active sites. The electrochemical reaction facilitated by Cl− anions is as follows:
Conversely, SO42− engenders a more uniform oxidation process, promoting the formation of oxygen-containing functional groups. These functional groups significantly enhance the electrochemical properties of CFs by improving their wettability and electrochemical activity. The corresponding electrochemical reaction involving SO42− anions can be described as follows:
Recent studies corroborate these findings. For instance, Cl
− etching substantially enhances the surface area of CFs by creating microporous structures. Similarly, the uniform oxidation in SO
42− environments increases oxygen functionalities, thereby improving the electrochemical performance of CFs [
33], [
34], [
35]. By elucidating these electrochemical mechanisms, our study furnishes a robust scientific foundation for the observed morphological transformations, underscoring the pivotal role of anion-specific interactions in modulating the structural and electrochemical attributes of CFs.
3.3. Evolution of electrochemical behavior of CFs
Along with chronoamperometry, the electrochemical signals of SCF during oxidation were monitored in real time.
Figs. 6(a) and
(b) present the relevant current (
I)-time (
t) curves of SCF
NaCl/ $\mathrm{SCF}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ and CFB
NaCl/ $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$. The SCF
NaCl exhibited a much higher current than $\mathrm{SCF}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$, supporting the higher electrochemical activity of SCF
NaCl. Moreover, both curves gradually peaked during initial oxidation, demonstrating the improved electrochemical performance due to the surface activation of SCF, with the current of $\mathrm{SCF}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ increasing by approximately 3.6 times, while that of SCF
NaCl by approximately 5.8 times. However, both curves declined rapidly in the later treatment stage, which could be attributed to the broken SCF. In comparison, the multistep transition process and longer duration of SCF
NaCl yielded a more moderate degradation than that of $\mathrm{SCF}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ under extreme oxidation. Thus, the total loaded charge on SCF
NaCl was nearly seven times more than that on $\mathrm{SCF}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ (
Fig. 6(a)). The high charge-loading capacity and electrochemical stability of these CFs in strong alkali solutions and high-chloride environments demonstrate the ability for multifunctional construction and electrode materials for marine buildings [
35], [
36], [
37]. Similar electrochemical behaviors were observed during CFB regulation.
Fig. 6(b) illustrates the
I-
t curves of $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ and CFB
NaCl, where both current curves peak rapidly in the initial treatment phase because of the surface activation of CFs. Gradually, the current of CFB
NaCl decreased slightly; contrastingly, that of $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ decreased sharply. After 1 h, the current of CFB
NaCl was almost four times that of $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$, and the current retention of CFB
NaCl was > 95% of the maximum value, whereas $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ retained < 50% of the maximum current. The oxidation reaction (
) under high applied voltage might have contributed to the current of CFB
NaCl. However, Cl
2 was not released in the alkaline solution (
) during this process [
38], [
39] (Fig. S8 in Appendix A). The more stable open circuit potential and shorter equilibrium time of CFB
NaCl compared to that of $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ indicate that the solution with Cl
− has higher wettability than that of SO
42− on the surface of CF (Fig. S9 in Appendix A). The Tafel analysis (
Fig. 6(c)) revealed a relatively higher corrosion potential of the CFB
NaCl, suggesting that Cl
− ions are more permeable to the surface of CFB than SO
42−.
Fig. 6(c) presents the corresponding electrochemical reactions during these regulation processes. These observations correspond to the morphological changes described above.
EIS was performed to assess the evolutionary electrochemical activity of SCF during regulation. At the early treatment stage and with the immersing time extension, the capacitive arc in Nyquist plots of SCF
NaCl gradually decreased, and the Bode plots presented a single-phase angel peak (
Pphase) (
Fig. 6(d); Fig. S10(a) in Appendix A). With increasing electrolysis time, the capacitive arc of SCF
NaCl (
Fig. 6(e)) decreases significantly, demonstrating a notable improvement in electrochemical activity during the regulation process. In the later stages of electrolysis, the reduction in the capacitive arc further suggests enhanced surface activity of SCF. The Bode plot of SCF
NaCl (
Fig. 6(f)) indicates that, during electrolysis, the
Pphase position remains relatively stable, while the phase angle decreases progressively. This change is attributed to the gradual exposure and activation of the inner carbon layers. The Nyquist plot of $\mathrm{SCF}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ (
Fig. 6(g); Fig. S10(b) in Appendix A) remains relatively stable, displaying two distinct time constants (
Pphase) located at both high and low frequencies. This behavior is governed by the combined effects of the sizing agent layer and the CF surface. During electrolysis, the capacitive arc of $\mathrm{SCF}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ (
Fig. 6(h); Fig. S10(e) in Appendix A) also decreases significantly, indicating an increase in electrochemical activity over time. The reduction in the capacitive arc, especially in the later stages of electrolysis, can be attributed to the movement of carbon layers and an increase in surface area. The Bode plot of $\mathrm{SCF}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ (
Fig. 6(i)) shows a substantial shift in the
Pphase towards higher frequencies while maintaining stable phase angles. This shift is associated with the exposure of the inner carbon layers, reflecting the evolving electrochemical properties of $\mathrm{SCF}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$. The equivalent circuits (Fig. S10(c) in Appendix A) were used to fit the EIS data of SCF
NaCl and $\mathrm{SCF}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$. The impedance value of SCF
NaCl was an order of magnitude lower than that of $\mathrm{SCF}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$, suggesting that the Cl
− solution has a superior wettability on the SCF surface, leading to a high contact area of the latter in the solution, thereby decreasing its charge transfer resistance. The equivalent circuit in Fig. S10(d) in Appendix A with a one-time constant was used to fit the EIS data during $\mathrm{SCF}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ regulation, where
Rct and CPE
dl represent the resistance and constant phase element of the SCF, respectively. The second equivalent circuit with Warburg impedance (
Zw) and the third equivalent circuit with an additional time constant corresponding to the inner carbon layers were adopted to simulate the EIS data during the early and late SCF
NaCl regulation. However, the capacitive arcs progressively increased sharply for both SCFs; this is attributable to their breaking. Overall, the EIS data variation corresponds to the morphological evolution and electrochemical properties of CFs (
Fig. 6(j)).
3.4. Enhanced electrochemical performance of CF
Various electrochemical performances of CFB
NaCl and $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ were tested to assess the electrochemical activity of the regulated CFs. Preliminarily, the anode polarization current in 0.1 mol∙L
−1 KOH remarkably increased from the pristine CFB to $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ and then to CFB
NaCl (
Fig. 7(a)), demonstrating the enhanced electrocatalytic performance of CFB. Given the formation of C–Cl covalent bonds, the electrocatalytic performance of CFB
NaCl toward ORR has been investigated [
33]. CFB
NaCl exhibited a significantly enhanced electrocatalytic activity toward ORR compared to pristine CFB, where the CFB
NaCl had a high ORR current density with a well-defined cathodic peak at −0.31 V under O
2-saturated conditions (
Fig. 7(b)). The initial ORR potential of CFB
NaCl (−0.13 V) was approximately 100 mV less than that of the pristine CFB (−0.23 V) and 130 mV higher than that of the Pt electrode (0 V). The electrocatalytic durability of the CFB
NaCl electrode was satisfactory for ORR (
Fig. 7(c)). This electrocatalytic behavior of CFB
NaCl was consistent with that of chlorine-doped carbon nanomaterials but with a higher electrocatalytic current density [
32], [
40]. Meanwhile, $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ exhibited minimal electrocatalytic activity in terms of ORR. However, the enveloped area of the corresponding CV curve considerably increased (Fig. S11 in Appendix A), implying a substantial increase in the performance of the CFB capacitor, possibly because of the formation of the porous structure of $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ during regulation. CV, GCD, and cycling stability tests were performed using a three-electrode system in a 1.0 mol∙L
−1 Na
2SO
4 solution to further evaluate the performance of $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ as a supercapacitor.
Fig. 7(d) illustrates the CV curves of the pristine CFB and $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ electrodes at a scan rate of 100 mV∙s
−1 in the potential range of −1.3–0.9 V vs Ag/AgCl. Oxygen or hydrogen evolution was not evident during this test, indicating their wide ranges of potential stability. Compared with the pristine CFB, $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ exhibited a non-rectangular CV curve with a much larger enveloped area and pseudocapacitive properties, indicating capacitance increases by orders of magnitude primarily because of the electrochemical double-layer capacitor and the oxygen-containing functional groups on the surface (pseudocapacitor) [
41]. The much larger integral area indicated a remarkably large surface area of $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ for the electrochemical double-layer capacitor with considerable charge accumulation. The specific capacitance of $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ (
Figs. 7(e) and
(f)) reached 57.50 F∙g
−1 at 1.0 A∙g
−1, which is 478 times higher than that of the pristine CFB electrode (0.12 F∙g
−1). This performance surpassed that of most previously reported CFs (Table S1 in Appendix A). The superior specific capacity of $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ can be attributed to the formation of porous structures and oxygen functional groups during oxidation. These functional groups provide additional pseudocapacitance [
42], and improve the wettability [
41], potential windows [
43], and accessible electroactive surface of CFs [
44]. Upon testing the CV curve of $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ at different scan rates (10–200 mV∙s
−1;
Fig. 7(g)), all CV curves had a quasi-rectangular shape, indicatingtheexcellent reversibility and rate capability of the $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ electrode. Moreover, at different current densities, the GCD curves had a quasi-symmetric triangular shape with a slight deviation from the ideal isosceles triangle, which is related to the pseudocapacitance of the oxygen-containing functional groups [
45], [
46]. At a current density of up to 16 A∙g
−1, the $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ electrode also displayed a superior capability with 70% (40.52 F∙g
−1) capacity retention. The near-vertical slopes of the Nyquist plots in mid- and low-frequency regions and the absence of a semicircle in the high-frequency region indicated that the $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ electrodes demonstrate capacitive-like behavior, characterized by good electrical conductivity and efficient ion diffusion (
Fig. 7(h)) [
47]. After 10 000 cycles of repetitive GCD at a current density of 10 A∙g
−1, the specific capacitance of $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ retained 90% of its initial capacitance with a slight decrease in the slope of the Nyquist plots, indicating the excellent cycling stability of the $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ electrode (
Fig. 7(i)). These results demonstrate that $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ is suitable as an electrode material for CF-based supercapacitors [
17]. It is promising for developing flexible and portable electronic devices [
48], [
49] and functional construction with energy storage functions [
50], [
51].
Fig. 7(j) illustrates the enhanced electrochemical performance of CFs. The implications for the electrochemical performance of CFs are profound based on the electrochemical treatment and subsequent morphological modifications detailed in our study. Specifically, the structural alterations induced by electrochemical oxidation, including enhanced surface area and the incorporation of functional groups, enhance ORR activity and supercapacitor performance. Introducing microporous structures increases the active surface area available for electrochemical reactions, facilitating improved ORR catalytic activity by providing additional reaction sites. Experimental results substantiate these improvements, revealing higher current densities (up to 5 mA∙cm
−1) and reduced overpotentials (decreased by 100 mV) for CFs treated during ORR testing. Moreover, supercapacitor characterization demonstrates a notable increase in specific capacitance, measuring 300 F∙g
−1 for $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ compared to untreated counterparts, underscoring the efficacy of our electrochemical approach in enhancing CF performance across diverse electrochemical applications.
3.5. In situ processing of CFs via the regulation method
CF-based products have been widely used in various fields owing to their high strength and lightweight. Manufacturing technology accounts for > 50% of the total cost of these products. The development and application of low-cost processing technologies will significantly improve the application efficiency of these materials. Currently, the expensive molding technologies and the machining processes in preparing CF composite materials are costly, especially for different shape requirements. Meanwhile, no feasible
in situ processing technology exists for microscopic CFs, probably because of their graphitic structure. The simple and universal processing technology of graphitic CFs was proposed based on the revealed regulation effect of anions on carbon layer structure during the electrochemical process. The etching technology of CFs and the graphitic carbon materials can be designed through the electrochemical method in Cl
−/OH
− solution in future studies. Meanwhile, through the same electrochemical process in SO
42−/OH
− solution, two representative processing modes of CFs were demonstrated. The CF bending in macro and micro scales was achieved through rapid perturbation of the bent graphitic carbon layers. The detailed machining process and the corresponding mechanism are described in
Fig. 8(a). The tension of these bent CFs was relieved by moving carbon layers under the intense electrochemical processes. After the treatment, the SCF with a 90° bending and the CFB with a 180° bending were prepared (
Figs. 8(b) and
(c); Movies S3 and S4 in Appendix A, respectively). Conversely, the bent CFB samples, after being treated by immersion, returned to the pristine shape when the external bending force was removed (
Fig. 8(c)), further confirming the influence of SO
42− on the structure of carbon layers during electrochemical regulation. Meanwhile, the microscale fusion of graphitic CFs was also achieved through the
in situ movement and combination of the outer carbon layers of two adjacent SCFs (
Fig. 8(d)).
Fig. 8(e) illustrates the resulting sample. From the local SEM images (inset of
Fig. 8(e)), both CFs bonded uniformly. Combined with these two proposed processing modes of CF, a closed loop of SCF was constructed (
Figs. 8(f) and
(g); Movie S5 in Appendix A), verifying the processing ability of the method toward the rigid and microscopic CF. Thus, this strategy can process graphitic CFs
in situ without a binder at normal temperature and pressure. Considering the cost, operability, energy consumption, and environmental friendliness, this strategy provides a promising technology to process commercial CFs, which will aid in the manufacture of products and the design and exploitation of novel microelectronic devices.
Future studies on this method should focus on optimizing the regulation, investigating the regulation mechanisms, and testing various electrolysis anions. Additionally, they should enhance the processing effect and novel regulation routes of the carbon layer structure and improve the regulation techniques. This should be followed by scale-up through the grass-root industrial CF-based products.
4. Conclusions
In this study, a straightforward and cost-effective anion-assisted electrochemical strategy was proposed for regulating carbon layer structure to enhance the multifunctional properties of CFs and facilitate their in situ processing. Two model anions, namely the electrochemically active Cl− and inert SO42−, alongside an in situ monitoring strategy, were used to elucidate the underlying regulation mechanisms. During the intense electrochemical oxidation of CFs in an alkali solution, Cl− significantly boosts the electrochemical activity of carbon layers by orchestrating their homogeneous oxidation, while SO42− exacerbates layer movement through intercalation rather than direct participation in electrochemical reactions. This diversity in carbon layer variation in distinct anion environments enables precise modulation of CFs’ electrochemical performance and interfacial properties.
Specifically, CFBNaCl demonstrates exceptional electrocatalytic activity towards the ORR, surpassing Pt electrode overpotentials by approximately 130 mV. In contrast, $\mathrm{CFB}_{\mathrm{Na}_{2} \mathrm{SO}_{4}}$ exhibits approximately 480-fold enhancement in supercapacitor performance, demonstrating outstanding rate capability and long-term stability, highlighting its promise for energy storage applications. Furthermore, our strategy achieves a significant milestone by enabling in situ processing of graphitic CFs under ambient temperature and pressure conditions for the first time, revealing their inherent flexibility and bonding capabilities.
Overall, this study presents a robust, environmentally friendly, and economically viable electrochemical regulation strategy for modifying and processing commercial CFs. Our method aligns with sustainable manufacturing practices by reducing energy consumption and employing non-toxic electrolytes, establishing a solid basis for the scalable production and application of CFs in various advanced materials and industrial applications.
CRediT authorship contribution statement
Chun Pei: Writing - original draft, Visualization, Formal analysis. Hongtao Yu: Writing - review & editing, Supervision, Methodology, Data curation. Ji-Hua Zhu: Writing - review & editing, Funding acquisition, Conceptualization. Feng Xing: Writing - review & editing, Supervision, Project administration, Conceptualization.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to thank the funding support from the Key-Area Research and Development Program of Guangdong Province (2019B111107002), the National Natural Science Foundation of China (52478266 and 52108231), the Basic and Applied Basic Research Fund of Guangdong Province (2023A1515012150 and 2023A1515012409), and the Shenzhen Science and Technology Innovation Program (20220810140230001 and 20220810160453001).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.eng.2024.09.017.
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