In the present study, carbon nanofibers containing multi-walled carbon nanotubes (MWCNTs) were fabricated using electrospinning and used as an electrode for structural supercapacitors. Electrospun polyacrylonitrile (PAN) nanofibers were converted to carbon nanofibers with stabilization and carbonization processes assisted by thermal treatment. The surface area of the electrospun carbon nanofiber electrodes was further increased several thousand times using an activation process. Multifunctional supercapacitors were fabricated using electrospun carbon nanofibers and a vacuum resin infusion technique. The specific capacitance was dramatically improved due to the decreased equivalent series resistance and increased surface area of the nanofiber electrodes. Moreover, the carbon nanofiber electrodes helped increase interlaminar fracture toughness of structural supercapacitors because it acted like ‘Velcro’ between the fabric interfaces. This work is a ‘proof-of-concept’ for a simple and effective method to produce an electrode for structural supercapacitors based on the electrospinning technique.
Research Article
Structural Energy Storage System Using Electrospun Carbon Nanofibers with Carbon Nanotubes
https://doi.org/10.21203/rs.3.rs-2750528/v1
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Multifunctional composites
Electrospinning
Carbon nanofibers
Energy materials
Electrospinning technology, producing polymer fibers with diameters in submicron range, has drawn attentions during the last decade because electrospinning provides a simple and cost-effective way. Various polymers can be electrospun with applications of electrospun fibers include protective clothing, filtration, tissue engineering scaffolds and drug delivery systems [1, 2]. Structural energy storage systems using CFRP, which have a load-bearing capacity and energy storage ability simultaneously, have been recently proposed [3, 4]. Carbon fiber-reinforced plastic (CFRP) has been used for structural materials because of their outstanding properties [5–7], because they have advantages such as light weight, high specific strength and stiffness, and high corrosion resistance. With concept of structural energy storage systems using CFRP, electrical energy storage (EES) devices can be load-bearing structures so that the overall weight of the system can be reduced [8, 9]. Electrical energy storage systems are divided into batteries and capacitors. Even though batteries have high energy density, their power density is low because of relatively slow redox kinetics [10]. Structural capacitors, on the other hand, have high power density. Structural capacitors consist of carbon fiber electrodes, a separator (which is usually a glass fiber mat), and a multifunctional matrix. One example of a capacitor with simple architecture is the dielectric capacitor that is generally composed of two carbon electrodes and a dielectric glass fiber separator sandwiched in-between. Carlson et al. made a series of structural capacitors with woven carbon fiber/epoxy laminated electrodes [11]. A variety of dielectric materials such as PA, PET, and PC films were employed as the separator. The key performance factor of capacitors was a separator thickness. Glass/epoxy prepregs could be used as a separator of multifunctional structural capacitors with metalized film electrodes [12].
However, dielectric structural capacitors have extremely low energy storage performance. In order to improve the energy storage efficiency of capacitors, the simple phenomenon of ion transfer between the electrode and electrolyte was utilized. Structural supercapacitors were developed by impregnating carbon fiber mat electrodes and a glass fiber mat separator with crosslinked poly (ethylene glycol) diglycidylether polymer electrolyte [13]. It exhibited a specific capacitance of 10.3 mF/cm3, which was much higher than that of structural dielectric capacitors due to the movement of the ionic liquid of the polymer electrolyte. Greenhalgh et al. used a T300 graphite fiber fabric for both structural reinforcement and as an electrode [14]. An epoxy-based resin with an ionic liquid and lithium salt was chosen for the multifunctional matrix. The results exhibited that the amount of ions adsorbed on the surface of the electrode is proportional to the energy storage efficiency, so an electrode with high specific surface area should be assured. However, the specific surface area of commercial carbon fiber is 0.2 ~ 0.5 m2/g, which is insufficient to adsorb the ions on the surface. Qian et al. fabricated structural supercapacitors with a carbon aerogel (CAG) modified fabric to increase the specific surface area up to around 740 m2/gCAG [15]. The incorporation of CAG enhanced the surface area, electrochemical performance, and the fiber-matrix interface. Metal nanowires such as CuO nanowire also can be grown on the surface of woven carbon fiber (WCF) to increase the specific surface area [16]. CuO-WCF-based supercapacitors displayed a maximum capacitance of 6.75 F/g with the assistance of an ionic liquid and lithium salt. However, these methods have drawbacks such as the process complexity, the low efficiency, and potential hazardous for the health and environment due to the chemical reagents.
Electrospinning is a widely used method to produce continuous nanometer-sized fibers by applying an electric field. It is an efficient and effective way to fabricate various sizes of nanofibers with diameters ranging from 10 to 1000 nm. Because carbon nanofibers could be fabricated using polyacrylonitrile (PAN) through an electrospinning technique, this technique have been applied to various energy storage studies [17–19]. In the present work, carbon nanofibers were fabricated through the electrospinning process and used as an electrode material in structural supercapacitors to increase their specific surface area. To further increase electrical conductivity, electrospun carbon nanofibers containing carbon nanotubes (CNTs) were fabricated. Structural supercapacitors with various electrode nanofibers were manufactured through the vacuum resin infusion technique. A charge–discharge experiment was conducted to investigate the effect of electrospun carbon nanofiber electrodes on the energy storage ability of the structural supercapacitors. Tensile and Mode Ⅰ fracture toughness tests were also performed to confirm the load-bearing capacities of the structural supercapacitors. Based on these properties, the multi-functionality of the structural supercapacitors with electrospun carbon nanofiber electrodes was evaluated.
2.1. Materials
PAN (average Mw 150,000), poly (ethylene glycol) diglycidyl ether (PEGDGE, average Mn 500), triethylenetetramine (TETA, technical grade, 60%), and 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide (EMITFSI, ≥ 98%) were purchased from Sigma-Aldrich, Korea. N, N-dimethylformamide (DMF, purity 99.5%) was purchased from DaeJung Chemicals & Metals, Korea. The multi-walled carbon nanotubes (MWCNT, 95+ % C purity & -COOH surface-treated, Nanocyl, Belgium) had a mean diameter of 1 nm and length of 1.5 µm. Plain carbon (C-120, 200 g/m2, Muhan Composites, Korea) and glass fabric (108, 77 g/m2, Muhan Composites, Korea) were used as a reinforcement and separator of the structural supercapacitors, respectively.
2.2. Fabrication of electrospun carbon nanofiber
In this study, PAN, which is widely used as a precursor for producing carbon fiber, was used to fabricate carbon nanofiber. First, PAN with 10 wt.(weight) % was dissolved in DMF using a stirrer at 60 ⁰C for 4 h to prepare an electrospinning solution. In order to fabricate a nanofiber mat containing carbon nanotubes, MWCNTs were added to the solvent and dispersed for 3 h using a tip-type ultrasonicator (VC505, 500W ultrasonic processor, Sonics & Materials Inc., US). MWCNTs were uniformly dispersed in the solvent because the carboxyl surface treatment improves dispersion into the polymer matrix [20]. The amount of MWCNTs added to the solution was fixed at 1 wt.% with respect to the mass of the polymer resin. The solutions were spun with an electrospinning apparatus (ESR 100D, NanoNC, Korea) at a 1 ml/h flow rate, with a 12 kV applied voltage, and 15 cm distance between the drum collector and needle. The rotating speed of the drum collecting plate was kept as low as 300 rpm to prevent the breakage of the electrospun nanofibers. Electrospun carbon nanofiber mat was fabricated with approximately average diameter of 200 ~ 300 nm.
In order to stabilize the nanofiber mats, the as-spun nanofiber mats were heated at a rate of 5°C/min up to 280°C, and maintained for 1 h. The stabilized nanofiber mats were carbonized in a N2 atmosphere after heating at a rate of 5°C/min to 1000°C, which was maintained for 1 h. To increase the specific surface area of the electrospun carbon nanofibers, a physical activation process was performed using CO2 gas at a flow rate of 600 mL/min at 1000°C for 30 min. Figure 1 shows the schematic of the overall process for the preparation of electrospun carbon nanofibers.
2.3. Characterization of electrospun carbon nanofiber mat
FE-SEM (Hitachi SU5000, Japan) and HR-TEM (JEM-3010, JEOL, Japan) were used to observe the morphology of the electrospun nanofiber and MWCNTs inside the fiber. The microstructure of the electrospun carbon nanofiber was examined using X-ray diffraction (XRD, D8-Advance) with a scanning range of 2θ = 10 ~ 40° using Cu Kα radiation. Nitrogen adsorption/desorption isotherms were obtained using BELSORP-max (MicrotracBEL) with pure N2 at 77 K. The specific surface area and pore size distribution of the electrospun carbon nanofiber were obtained based on the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods.
2.4. Fabrication of structural supercapacitors with an electrospun nanofiber mat
The laminate consisted of two carbon fabrics as a backbone, two electrospun carbon nanofiber mat electrodes, and two glass fabric separators. Two carbon fabric backbones on which an electrospun carbon nanofiber mat was laid were sandwiched between two layers of glass fabric separators, therefore the laying sequence consists of CF/electrospun CF/GF/electrospun CF/CF. Copper tape was connected to the electrodes as a current collector. The laminate was placed on a Teflon-coated aluminum mold, and the resin mixture for the multifunctional matrix was injected into the mold using the vacuum resin infusion method. The resin mixture composed of 82.6 wt.% of PEGDGE-based epoxy, 7.4 wt.% of TETA, and 10 wt.% of EMITFSI was mixed using a stirring bar for 5 min. Before the resin injection, the resin mixture was degassed to avoid void formation in the composite. The curing process of the resin-impregnated specimen was conducted at a temperature of 80°C for 24 h.
2.5. Electrochemical and mechanical characterization of structural supercapacitors
In order to characterize the electrochemical properties of structural supercapacitors with an electrospun nanofiber mat, charge–discharge experiments were conducted using a potentiostat/galvanostat instrument (VersaSTAT 3, AMETEK, US). In this experiment, the equivalent circuit model of the supercapacitor was used to calculate the electrical capacitance of the structural supercapacitor fabricated by the vacuum resin infusion method. The charge–discharge response current, I(t), was measured by applying a 1 V step voltage for a 60 s, and the measured current curves were fitted by the transient current response model derived from the equivalent circuit of the supercapacitor,
$$I\left(t\right)=\frac{{U}_{applied}}{{R}_{s}}{e}^{(-t/\tau )}+\frac{{U}_{applied}}{{R}_{s}+{R}_{p}}(1-{e}^{(-t/\tau )})$$
1
where Rs, Rp, and τ are the equivalent series resistance (ESR), parallel resistance, and time constant, respectively. Through the equation and parameters, the capacitance of the structural supercapacitor, Csp, could be determined as:
$${C}_{sp}=\frac{\tau ({R}_{s}+{R}_{p})}{{R}_{s}{R}_{p}}$$
2
The tensile properties and interlaminar fracture toughness were investigated by tensile and double cantilever beam (DCB) testing according to ASTM D882 and D5528 standards, respectively.
The layup of the specimens for tensile testing was same as that of the structural supercapacitors, which sandwiched two layers of glass fabric between two layers of carbon fabrics. Tensile test specimens were prepared using the following dimensions: width, b = 20 mm, length, l = 80 mm, and thickness, t = 1 mm.
The dimensions of specimens for DCB testing was: thickness, t = 3 mm, length, l = 200 mm, and width, d = 20 mm. The initial crack length, a0, was 50 mm; it was made by inserting a Teflon film at the initial point of the specimens. The hinges were bonded to both sides of the surfaces with an acrylic adhesive and cured for 24 h at the room temperature. The DCB specimens were loaded onto a universal testing machine (LLOYD LR 50K, AMETEK, US) using a load cell of 500 N capacity at a rate of 5 mm/min and the strain energy release rate, GI, was calculated using by the modified beam theory (MBT) in the ASTM standard [21]:
$${G}_{I}=\frac{3P\delta }{2b(a+\left|\varDelta \right|)}$$
3
where P is the applied load, δ is the load point displacement, b is the specimen width, and a is the delamination length of the DCB specimens. Δ was determined experimentally by generating a least-squares plot of the cube root of compliance (C1/3) as a function of delamination length. The compliance, C, is the ratio of the load point displacement to the applied load, δ/P.
3.1. Characterization of electrospun carbon nanofibers
Electrospun carbon nanofibers could be fabricated from PAN precursor solutions through the electrospinning method as shown in Fig. 1. Electrospun PAN nanofibers must be thermally-treated in order to convert to carbon nanofiber structures. In the heat treatment, the stabilization process, which is generally referred to as a cyclization and dehydrogenation reaction, was conducted at about 300°C in an air environment to ensure thermal resistance to the electrospun PAN nanofibers [22]. After the stabilization process, carbonization was carried out at a high temperature of 1000°C to remove non-carbon components before converting them to carbon fibers having a carbon content of 95% or more [23, 24]. To further improve the specific surface area of the electrospun carbon nanofibers, a physical activation process with CO2 gas at high temperature was used. Carbon activation makes small, low-volume micro-pores that increase the surface area useful for many applications like chemical adsorption and energy storage [25, 26].
When the electrospun nanofiber was fabricated using a 10 wt.% PAN precursor solution with high viscosity, it was confirmed that the continuous shape of the fiber was produced without any aggregation of fibers due to the high viscosity shown in Fig. 2. During the heat treatment process, the diameters of nanofiber in the activated carbon nanofiber mats were reduced, because of elimination of the non-carbon elements of PAN (Fig. 3). In order to further increase the electrical conductivity of electrospun carbon nanofibers, MWCNTs were dispersed into the PAN polymer. In this study, the interaction between DMF and MWCNTs was increased by using MWCNTs surface-treated with carboxyl groups, which improved the dispersion of MWCNTs in the electrospinning solution [20]. The use of MWCNTs in the electrospinning process not only improved electrical conductivity, but also further increased the specific surface area of the fibers during the activation process [27]. In order to prevent aggregation of the MWCNTs, their addition was fixed at 1 wt.%, which prevented high viscosity or particle aggregation effects. The electrical conductivity of the solution in which MWCNTs were dispersed was increased; therefore, the diameter of PAN-MWCNT electrospun nanofibers decreased. In addition, the reduction of diameter after the carbonization process was also observed.
Figure 4 shows TEM images of the PAN-MWCNT electrospun nanofibers; MWCNTs were in the nanofibers and aligned in the longitudinal direction of the fibers. Dror et al. theoretically proved that electrospinning could help to align the CNTs inside the electrospun fibers along the longitudinal direction [28]. Considering the difference in diameter of MWCNT and PAN nanofibers ranged from tens to hundreds of times, it could be considered that several strands of MWCNTs were aligned together in the nanofibers.
XRD was performed to investigate the macromolecular and crystalline structure of carbon nanofibers produced by electrospinning. As shown in Fig. 5 (a), PAN nanofibers prepared by the electrospinning method showed peaks at 2θ = 17° and 28 ~ 29°, which correspond to the dominant crystal facets: (100) and (110) facets of crystal structures of PAN, respectively [29]. In the case of the heat-treated specimens, however, it can be seen that a very broad peak appears near 2θ = 25 ~ 26° which is attributed to the crystallographic planar structure of graphite [29, 30]. For the PAN-MWCNT electrospun nanofiber, it was shown that relatively higher height of the peak was observed because MWCNTs act like a nucleus, the crystal structure of graphite was formed around the nuclei [31]. According to Bragg’s and Scherrer’s equations, the average interplanar spacing (d(002)), and crystallite size parameter (Lc) could be determined based on the XRD graphs as:
\({d}_{\left(002\right)}=\frac{\lambda }{2sin\theta }\) (Bragg’s Equation) (4)
\({L}_{c}=\frac{0.9\lambda }{\beta cos\theta }\) (Scherrer’s Equation) (5)
where θ is the scattering angle, λ is the wavelength of the X-ray, and β is the width of the diffraction peak measured at half its height.
In an electrospun carbon nanofiber made using PAN-MWCNT nanofiber, the interplanar spacing was greatly reduced, and the size of the crystal structure increased as shown in Fig. 5 (b). This means that in specimens containing MWCNTs, the structure of the graphite layer becomes denser and the crystals develop and form a more ordered structure [29]. Therefore, as mentioned above, MWCNTs act as nuclei that promote the development of graphite layers.
Generally, the carbon fiber fabrics used in fiber-reinforced composites could not achieve a sufficient energy storage performance due to the very low specific surface area as shown in Table 1. However, the specific surface area of electrospun CNF and CNF-MWCNT were increased about 50 times as compared with carbon fiber fabrics, and that means electrospinning could be one of the most effective ways to solve the low surface area problem. Through the activation process, the micropores structures were developed by gasification of the pore walls and the specific surface area increased by about several thousand times than that of non-activated carbon nanofibers. The generation of micropores on the surface of the carbon nanofibers resulted in a decrease in the mean pore diameter. The amount of micropores on the surface of the activated carbon nanofibers (ACNF) in which MWCNTs (ACNF-MWCNT) were embedded increased; therefore, the specific surface area of ACNF-MWCNT was found to be twice as high as that of ACNF.
Table 1 Surface properties of as-received CF fabrics and electrospun CNF and ACNF with MWCNTs.
3.2. Electrical and mechanical properties of structural supercapacitors with electrospun carbon nanofibers
Using the electrospun carbon nanofiber mat, structural supercapacitors were fabricated by vacuum resin infusion method as shown in Fig. 6. The carbon fiber fabric was used as a backbone which could guarantee good mechanical properties, and electrospun carbon nanofiber was laid beneath the fabric as an electrode of the supercapacitor. Two layers of glass fiber fabric were used as a separator that could prevent the contact of the electrodes. In order to increase the ionic conductivity of the multifunctional matrix for the structural supercapacitors, 10 wt% of ionic liquid in which a salt was dissolved was added to the epoxy resin. PEGDGE-based epoxy, which is a relatively soft epoxy that can help to move the ions in the capacitors, was used as a base material of the multifunctional matrix [13, 32].
The electrochemical properties of the structural supercapacitors fabricated with electrospun carbon nanofiber were investigated using cyclic voltammetry and charge–discharge measurements. Transient current behavior was measured as a voltage of 1 V was applied for 60 s through the structural supercapacitors. Based on the equivalent circuit of the supercapacitors, the transient response could be fitted using Eq. (1), and the capacitance determined using the properties derived from equation Rs, Rp, and τ. Rs, also referred to as the equivalent series resistance (ESR), was caused by the electrical resistance of the electrodes and the ionic resistance of the electrolyte. Rp is generated by the electrical resistance of the two electrodes constituting the supercapacitor or electrical contact in an electrochemical reaction. Since Rp is connected to the capacitor in parallel, it causes leakage current in the charging process [15, 33].
Figure 7 shows the transient current response and capacitance of the structural supercapacitors with electrospun nanofibers. As shown in Table 2, a lower equivalent series resistance was exhibited in the structural supercapacitors fabricated with electrospun carbon nanofiber mat. A number of factors could affect ESR, such as the surface area, the electrical conductivity, and the interfacial resistance of the electrode; in this case, ESR decreased because of the increased active surface area with electrospun carbon nanofibers [15]. The lower ESR values contributed to an increase of about 4-fold of the specific capacitances of the structural supercapacitors where the electrospun carbon nanofiber structure was inserted. However, the MWCNTs could not affect the specific capacitance and energy storage ability of the supercapacitors with CNF-MWCNT electrodes. Due to the micropores created by the activation process, ESR further decreased in the supercapacitors made by activated electrospun carbon nanofiber electrodes because of the increased ionic accessibility of the electrode. Apart from the reduced series resistance, the parallel resistance for activated carbon nanofiber electrode also decreased. For ideal supercapacitors, there should be nearly zero values of Rs and infinite Rp in order to store electrical energy without any leakage. However, for the activated electrospun carbon nanofiber electrode, the value of Rp became lower than that of a non-activated electrode. This means that there was a short-circuit in the structural supercapacitors where the ACNF electrode was applied. This may be caused by the small-sized particles released from the carbon nanofiber electrodes during the vacuum resin infusion process, which could create a bypass for the current and lead to a short-circuit with the electrode on the opposite side due to migration or convection through the separator. In fact, it could be observed in transient current measurement for charge–discharge methods. The current graph was floated from the bottom while the supercapacitors were charged with a 1 V voltage (60 ~ 120 s). That was the current stored in the electrode during charging leaked through the bypass formed by isolated carbon particles from the electrode. As a result, although the ACNF and ACNF-MWCNT could improve the specific capacitance and energy density caused by low series resistances, low parallel resistances led to a decrease of specific power density of the structural supercapacitors, as shown in Fig. 8.
In order to investigate the effect of electrospun carbon nanofibers on mechanical properties, tensile testing was performed with an electrospun carbon nanofiber mat which was inserted between the interfaces of the carbon and glass fiber fabrics as it was done with the structural supercapacitors. The tensile testing was conducted with 10 samples under each condition, and the results are shown with calculated error bars. Figure 9 shows the result of tensile testing of multifunctional supercapacitors according to the ASTM D882 Standard. The tensile modulus and strength were not significantly affected even after electrospun carbon nanofiber structures were inserted. Because tensile properties are fiber-dominant, the insertion of electrospun nanofibers could not have an effect on the tensile strength and modulus because of its low mechanical properties. However, the insertion of carbon nanofiber electrodes containing MWCNTs slightly improved the tensile modulus and strength compared to specimens that did not contain MWCNTs.
To examine the effect of electrospun carbon nanofibers on the interfacial mechanical properties, which are matrix-dominant, a DCB test was conducted in accordance with ASTM D5528. Figure 10 show the load–displacement and delamination resistance curves for non-interleaved and interleaved DCB specimens. The critical load, Pc, and fracture toughness, GIc, of the specimens with electrospun carbon nanofiber mats inserted into them were higher than that of the non-inserted specimen. This is because carbon nanofibers inserted between carbon fiber fabrics cause interlocking effects such as “hooks and loops” in Velcro, and improve interfacial strength. In addition, the insertion of the electrospun carbon nanofiber has an effect of preventing stress concentration caused by the difference of mechanical properties between the carbon fiber and matrix [34]. The critical load and interlaminar fracture toughness were decreased for the specimens in which carbon nanofiber mat was inserted. The micropores of the carbon nanofiber that were developed during the activation process act as defects, and thus it is difficult to achieve a sufficient interlocking effect.
Table 2 Electrochemical properties of structural supercapacitors with different electrode materials
3.3. Multifunctionality
Structural supercapacitors should carry a mechanical load whilst simultaneously storing electrical energy. In order to determine whether both performance criteria are met simultaneously, an appropriate standard should be present. O’Brien et al. suggested an equation that provides a fundamental design requirement for mass-saving multifunctional designs [12]. Based on this equation, there is criteria for energy storage efficiency and structural efficiency of a conventional system. However, because determining the criteria is vague and controversial, the multifunctionality of the overall system can be calculated differently depending on the criteria. Therefore, Fig. 11 presents mechanical properties and energy density to verify only the tendency of the multifunctionality for each case. In the graphs, moving from the bottom left to the top right in the graph could be determined to be a multifunctional supercapacitor. According to Fig. 11, the structural supercapacitor fabricated using activated carbon nanofiber with MWCNTs was the most multifunctional structure when tensile modulus was used as a criterion of mechanical properties. These trends were observed in Table 3. A CNF-MWCNT-based structural supercapacitor had an even higher tensile modulus than an ACNF-MWCNT electrode, but it had an insufficient energy density to store electrical energy. In the case of interlaminar fracture toughness, ACNF-MWCNT electrode-based supercapacitors could also be determined to be a multifunctional composite. Therefore, there is the possibility of application of carbon nanofibers as an electrode in multifunctional composites fabricated using a simple and convenient method. This work is ‘proof-of-concept’ of structural supercapacitors, and shows the most effective way to improve the multifunctionality of the composites.
In this study, the carbon nanofibers containing CNTs were fabricated using electrospinning and used as electrodes for multifunctional supercapacitors. Structural supercapacitors with various electrode nanofibers were manufactured through the vacuum resin infusion technique. A charge–discharge experiment and Tensile and Mode Ⅰ fracture toughness tests was conducted to investigate the effect of electrospun carbon nanofiber electrodes on the energy storage ability and to confirm the load-bearing capacities of the structural supercapacitors, respectively. The multi-functionality of the structural supercapacitors with electrospun carbon nanofiber electrodes was evaluated. The main conclusions of the study are as follows:
• Electrospun PAN nanofibers were successfully produced and converted to carbon nanofibers through a stabilization and carbonization process using heat treatment. In addition, an activation process with CO2 was also performed to further increase the specific surface area of the electrodes. Through the stabilization, carbonization and activation process, the specific surface area could be improved several thousand times.
• MWCNTs could increase the electrical conductivity of the PAN solution, thus helping to decrease the diameter of the electrospun nanofiber. In addition, MWCNTs promoted the formation of a graphite crystal structure and helped to decompose the surface of electrospun carbon nanofibers during the activation process.
• Structural supercapacitors were fabricated using vacuum resin infusion of electrospun carbon nanofiber electrodes. Carbon fiber fabric was used as a backbone in order to guarantee mechanical properties.
• A charge–discharge experiment was conducted to characterize the energy storage ability of the structural supercapacitor. The specific capacitance was increased when electrospun carbon nanofiber electrodes were used because of the increased specific surface area and decreased ESR. However, Rp also decreased because small-sized particles were released from the carbon nanofiber electrodes, causing a short-circuit when activated carbon nanofiber electrodes were used.
• In order to evaluate the mechanical properties of structural supercapacitors, a tensile and Mode Ⅰ test were conducted. Although the carbon nanofibers could not affect the tensile strength of the structural supercapacitors, the interlaminar fracture toughness of the structural supercapacitors was improved by more than 20%.
• Based on the mechanical and electrical properties, the multifunctionality of the structural supercapacitors could be verified. According to the results, the structural supercapacitor fabricated by ACNF-MWCNT was found to be the most multifunctional structure. It was a worthwhile ‘proof-of-concept’ of structural supercapacitors produced using an electrospinning technique, which is a simple and effective way to produce electrodes having a high specific surface area.
Acknowledgments
This work was supported by the Research Fund (PNK6650) of the Korea Institute of Materials Science (KIMS)
Funding
This study is financially supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (20213000000020, Development of core equipment and evaluation technology for construction of subsea power grid for offshore wind farm).
Conflict of interest
The authors declare no competing interests.
Authors’ contributions
All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by J.J. Jung and G.H. Lee. D.S. Lee and S.W. Choi wrote the manuscript. W.I. Lee and M.K. Um gave the meaningful advice in theoretical analysis and writing the manuscript.
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No competing interests reported.