ionic liquid–polyester resin

Composites: Part A 87 (2016) 256–262

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Multifunctional CuO nanowire embodied structural supercapacitor based on woven carbon fiber/ionic liquid–polyester resin Biplab K. Deka, Ankita Hazarika, Jisoo Kim, Young-Bin Park, Hyung Wook Park ⇑ Department of Mechanical Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, Ulsan 44919, Republic of Korea

a r t i c l e

i n f o

Article history: Received 30 December 2015 Received in revised form 29 April 2016 Accepted 4 May 2016 Available online 7 May 2016 Keywords: A. Carbon fibers A. Polymer-matrix composites (PMCs) B. Electrical properties E. Resin transfer molding (RTM)

a b s t r a c t Novel structural supercapacitors based on CuO nanowires and woven carbon fiber (WCF) has been developed for the first time employing vacuum assisted resin transfer molding (VARTM) process. The growth of CuO nanowires on WCF is an efficient process and can be used in structural capacitors which can trigger the electric vehicle industries toward a new direction. The specific surface area of the carbon fiber was enhanced by NaOH etching (41.36 m2 g
1) and by growing CuO nanowires (132.85 m2 g
1) on the surface of the WCF. The specific capacitance of the CuO–WCF based supercapacitor was 2.48 F g
1, compared with 0.16 F g
1 for the bare WCF-based supercapacitor. The usage of ionic liquid and lithium salt improved the capacitance to 5.40 and 6.75 F g
1 with lowest ESR and Rp values of 133 and 1240 X along with improving mechanical properties within an acceptable range. The energy and power densities were also increased up to 106.04 mW h kg
1 and 12.57 W kg
1. Thus, this study demonstrated that growing CuO nanowires on the surface of WCF is a novel approach to improve multifunctionality that could be exploited in diverse applications such as electric cars, unmanned aerial vehicles (UAVs), and portable electronic devices. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Carbon fiber-based polymer composites are used extensively in automotive, aerospace, and military applications because of their light weights, high specific strengths and stiffnesses, impressive thermal and electrical conductivities, and excellent corrosion resistances [1,2]. Currently, optimizing the specific properties in terms of weight and volume fractions is the primary goal of fiberreinforced composites. However, the interfacial interactions at the fiber–matrix interface are equally important and govern the overall performance of the composites [3]. The excellent interfacial interaction leads to well dissipation of applied stress to the composite that results improved specific properties. The enhancement of the fiber surface area is considered to be one of the most promising methods to improve the interfacial interaction as it enhanced the site of interaction with the matrix that results improved properties. Several methods have been used to accomplish this, e.g., KOH etching [4], nanofiber and nanotube growth [5], and surfactant grafting. Recently, different metal oxide nanowires or nanorods have been grown on the surface of carbon fibers; these have been extensively used in different applications [6]. These nanorods ⇑ Corresponding author. E-mail address: [email protected] (H.W. Park). http://dx.doi.org/10.1016/j.compositesa.2016.05.007 1359-835X/Ó 2016 Elsevier Ltd. All rights reserved.

increased the surface of the fiber and triggers the interaction of the fibers with the matrix. The multifunctional aspect of a carbon fiber-reinforced polymer composite relates to its structural and electrochemical energy storage functions. Such multifunctionality has great potential in mobile systems such as portable electronics, hybrid electric motor vehicles, and certain aerospace applications [7–9]. It is a challenging task to develop a multifunctional composite material that can simultaneously have outstanding structural and energy storage capabilities. Previous researchers developed multifunctional structural supercapacitors using carbon fiber electrodes [10]. Others have used woven carbon fiber (WCF) sizing with carbon nanotubes [11] and monolithic carbon aerogels [12], but WCF supercapacitors based on metal oxide nanowires have yet to be reported. A supercapacitor has a longer cycle life, higher power density, and better reversibility compared with a conventional battery [13]. Its energy density is as high as that of dielectric capacitors [14]. Among different metal-oxide nanowires, CuO is considered as one of the suitable materials due to its low cost, sufficient resources, chemical stability, and eco-friendly synthesis methods [15]. Owing to its large surface area and relatively high specific capacitance (674 mA h g
1), CuO is a deserving candidate to be used in electrochemical applications [16]. However, the development of the polymer matrix is a challenging task for the research-

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ers as the mechanical properties and ionic conductivity of a polymer electrolyte have so far been inversely proportional [17]. Although conventional carbon fiber polymer composite can achieve highest degree of mechanical properties, the electrochemical performances always showed lowest values. However, previous research has shown improvement in both the parameters simultaneously by using some non-structural polymer like polyethylene-oxide (PEO) and diglycidyl ether [18]. The ionic movement in the supercapacitor was widely improved by incorporating the ionic liquid to the matrix [19]. Addition of some extra amount of cationic salt has catalyzed the ionic transport process [20]. However, the critical problem is that the matrix became compliant with an increasing proportion of ionic liquid and can’t give sufficient structural properties. Thus, it is a challenging task to develop solid polymer electrolyte (SPE) which can give sufficient ionic conductivity along with desirable mechanical properties [21]. In this study, CuO nanowires were grown on WCF and novel multifunctional structural supercapacitors were developed using vacuum-assisted resin transfer molding (VARTM). These results have not been reported by any other researchers until now and it is a quite efficient process compared to the previous reported work of structural supercapacitors [10,11]. Moreover, VARTM is a simple, reliable, economic, and time-efficient process. A new SPE synthesized from an unsaturated polyester resin, an ionic liquid, and a lithium salt had better load-bearing capacity than a traditional SPE and was successfully formed into a supercapacitor using VARTM.

temperature of 100 °C and atmospheric pressure. After eight hours, the samples were washed with distilled water to remove any unreacted particles and dried at room temperature. The entire process of growing CuO nanowires is described as follows [22]:

C6 H12 N4 þ 6H2 O $ 6HCHO þ 4NH3 NH3 þ H2 O $

NHþ4




þ OH

ð1Þ ð2Þ

2OH
þ Cu2þ $ CuðOHÞ2

ð3Þ

CuðOHÞ2 $ CuO þ H2 O

ð4Þ

2.3. Preparation of the CuO nanowire–WCF/polyester composite capacitor

Structural supercapacitors samples were prepared by two sheets of WCF (T-300, density 1.76 g cm
3, 3K plain weave, Amoco Corporation, Chicago, IL, USA) electrodes sandwiched with one woven glass fiber (thickness  0.2 mm, plain weave, weight  200 g m
2) supplied by JMC Corporation, Korea. The surface area of the WCF electrodes was enhanced by growing CuO nanowires on them. These nanowires were prepared from copper nitrate tetrahydrate (Zn(NO3)23H2O) and hexamethylene tetramine (C6H12N4) purchased from Sigma–Aldrich (St. Louis, MO, USA). The unsaturated polyester resin (PES) (LSP-8020B) and methyl ethyl ketone peroxide (MEKP) crosslinker used for the matrix development were obtained from CCP Composites (Jeollabuk-do, Korea) and ARKEMA (Kunsan city, Korea), respectively. The ionic liquid 1-ethyl-3methylimidazolium tetrafluoroborate (EMIMBF4; reagent-grade; C–TRI, Chuncheon city, Korea) and the lithium salt lithium trifluoromethanesulfonate (LiTf) (Sigma–Aldrich, Seoul, Korea) were used to increase the ionic conductivity of the matrix. Other reagents and chemicals were of analytical grade.

The capacitor samples were prepared using vacuum-assisted resin transfer molding (VARTM) [23]. In this process, the CuO nanowire–embedded WCF and WGF samples were placed inside the vacuum chamber. Basically, the structural supercapacitor consists of two activated carbon based electrodes with high specific surface area and the glass fiber layer as the electron insulation. However, the ions can conduct easily from one side to other (Fig. 1). The interface of the electrodes and electrolytes accumulates the charges and the energy stored in the capacitor is highly related to the available surface area of the electrodes, ion size and concentration and the stability of the electrolyte during oxidation and reduction reactions. Each sample was prepared at about (70  70) cm2 in dimensions and the average of three samples has been reported. Two copper wires were attached to the WCF electrodes using a conductive epoxy glue prior to the VARTM. One releasing medium followed by one resin transporting medium were placed immediately above each ply. A pressure-resistant plastic film was finally attached with adhesive tape. An inlet and outlet were provided to enable vacuum and resin flow to the chamber. The outlet was connected to vacuum pump to create ca. 60 kPa of pressure inside the chamber. This reduced pressure assisted resin flow into the chamber. After sufficient resin had been inlet, the chamber was sealed for 48 h to allow the samples to cure. The sample configurations and loading percentages of the ionic liquid and the lithium salt are given in Table 1. In case of samples 1, 2, and 3, 100% (w/w) of polyester resin was used as electrolyte for the supercapacitors. To prepare the ionic liquid and polyester resin based polymer electrolyte (sample 4), 20% (w/w) of EMIMBF4 was mixed with 80% (w/w) of polyester resin. The mixture was stirred thoroughly to become homogeneous and infused to the VARTM chamber. For the electrolyte preparation of sample 5, 10% (w/w) of lithium salt was initially mixed with 20% of ionic liquid. The stirring was continued until all salt is dissolved in the ionic liquid. This salt and ionic liquid mixture were subsequently added to 70% of polyester resin. After completion of mixing process, it was used as electrolyte and inserted to the VARTM system.

2.2. Hydrothermal synthesis of CuO nanowires on WCF

3. Results and discussion

The surface areas of the WCF electrodes were increased by growing CuO nanowires on them using a hydrothermal process. In this method, sufficient hexamethylenetetramine (HMTA) and copper nitrate tetrahydrate solution precursors were mixed at a 1:1 M ratio. For the preparation of 60 mM of growth solution, 60 mM each of HMTA and copper nitrate were separately dissolved in distilled water at room temperature over 10 min. The two solutions were then combined and stirred for another 30 min at room temperature. The pH of the final growth solution was maintained at 6–8. The precursor solution was transferred to a stainless steel autoclave and the WCF electrode samples were immersed in the solution. The autoclave was kept inside heating oven at a fixed

3.1. Surface area and morphology of the CuO nanowire–WCF electrodes

2. Materials and methods 2.1. Materials

The specific surface area and pore size distribution of the WCF electrodes were measured using a Physisorption Analyzer (ASAP2020 Analysis, Micromeritics, Norcross, GA, USA) in a pure nitrogen atmosphere at 77 K following the Brunauer, Emmett and Teller (BET) method. The BET surface area significantly increased (to ca. 41 m2 g
1) after treatment of the fibers with a strong base (Table 2). Shirshova et al. previously reported the enhancement of the BET surface area of carbon fibers using various processes [24]. However, this is the first report of improving the surface area

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Fig. 1. Schematic diagrams of the WCF based structural supercapacitor with its structural components. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1 Supercapacitors formulation and codes along with the loading percentage. Sl no.

Sample

Sample code

1 2

Woven carbon fiber/glass fiber/polyester resin (100%) NaOH activated-woven carbon fiber/glass fiber/ polyester resin (100%) CuO nanowire–woven carbon fiber/glass fiber/ polyester resin (100%) CuO nanowire–woven carbon fiber/glass fiber/ionic liquid (20%)/polyester resin (80%) CuO nanowire–woven carbon fiber/glass fiber/ionic liquid (20%)/lithium salt (10%)/polyester resin (70%)

WCF/GF/PES ac-WCF/GF/ PES CuO–WCF/ GF/PES CuO–WCF/ GF/IL/PES CuO–WCF/ GF/IL/LiS/PES

3 4 5

Table 2 BET surface area determination of WCF, NaOH activated WCF, and CuO nanowire grown WCF samples. Sample

BET surface area (m2/g)

Pore volume (cm3/g)

Pore size (nm)

WCF ac-WCF CuO–WCF

0.589 ± 0.002 41.365 ± 0.004 132.852 ± 0.001

0.0128 0.849 0.726

86.6 63.2 65.8

through the growth of CuO nanowires and applying these novel materials in structural supercapacitors. Decorating with CuO nanowires enhanced the surface area to ca. 132 m2 g
1. The pore volume increased from 0.0128 cm3 g
1 for the untreated WCF to 0.849 cm3 g
1 after the fibers were treated with base, compared with 0.726 cm3 g
1 for the CuO-nanowire–decorated WCF. The weight percent of CuO nanowires grown on woven carbon fiber was calculated from Thermogravimetric analysis (TGA, TA Instruments, USA) and its result was around 14%. Our previous research has shown that the surface area of carbon fiber increases with increasing the weight percent of CuO grown [6]. However, in this study, one concentration of the nanowire has been considered.

The WCF morphology was studied by scanning electron microscopy (SEM; Nanonova 230; FEI, Hillsboro, OR, USA). Fig. 2A shows SEM images of the bare WCF sample. The etching process carried out by NaOH treatment generated mesopores on the surface, which increased the surface area and the specific capacitance of the electrode (Fig. 2B). The surface area and specific capacitance were further enhanced after decorating the WCF surface with CuO nanowires (Fig. 2C). The nanowires not only enhanced the electrochemical properties but also increased contact with the multifunctional matrix. This improved surface contact should mitigate matrix-dominated failure processes, which is an issue of great concern for structural electrical applications. The hierarchical microstructure of mesoporous ionic liquid/polyester matrix is presented by micrograph 2D. The presence of ionic liquid droplets segregates in the continuous polyester affluent portion of the matrix. Inside the segregates, there are some bi-continuous networks connected to each other forming some spherical knots. The micropores and gaps inside these knots are primarily responsible for the ionic conductivity of the matrix [19]. 3.2. Electrochemical analysis The specific capacitance (C) of a composite sample was measured by cyclic voltammetry (NuVant Systems, Crown Point, IN, USA). A sample was tested in a three-electrode system consisting of a silver–silver chloride reference electrode and a platinum counter electrode in a 3 M KCl solution. A composite sample was connected with copper wire to function as the working electrode. The electrochemical response was recorded using different sweep rates of 1, 5, 30, 60, and 100 mV s
1 within a voltage window of
0.5 to +0.5 V. The specific capacitance was calculated by the following equation [25]:

C¼

1

mv ðV c
V a Þ

Z

Vc

Va

Iv dV

ð5Þ

B.K. Deka et al. / Composites: Part A 87 (2016) 256–262

259

Fig. 2. SEM images of (A) WCF, (B) NaOH activated WCF, (C) CuO grown WCF samples and (D) microstructure of PES/IL/LiS matrix.

where C is the specific capacitance (F g
1), m is the active mass of the electrodes (g), v is the scan rate (mV s
1), Iv is the voltammetric current (A), and (Vc
Va) is the sweep potential range (V). Fig. 3A shows the cyclic voltammograms of the WCF/PES composites at a scan rate of 10 mV s
1. The curves had rectangular shapes, similar to that of WCF [26]. As expected, the untreated WCF/PES composite had a very small specific area and capacitance during charging and discharging, indicating ideal pseudocapacitive behavior. The WCF surface-activated by NaOH had a larger specific area (41.37 m2 g
1) and specific capacitance (1.17 F g
1) (Fig. 4A) than bare WCF (specific area ca. 0.59 m2 g
1, Csp ca. 0.15 F g
1). The

capacitance was further enhanced when CuO nanowires were grown on the surface of the WCF (Csp ca. 2.48 F g
1). Previous research reveals that CuO nanowires provide the reasonable improvement of the surface area as well as high specific capacity and outstanding mechanical properties in the composites [27]. Moreover, the high wettability provided by the CuO nanowires increased the intimate contact of the electrodes with the electrolyte, which boosted the electrochemical performance. Qian et al. [12] applied a continuous carbon aerogel to a carbon fiber fabric and thereby enhanced the surface area and the electrochemical properties by almost 100-fold. In the work reported here, addition

Fig. 3. Cyclic voltammograms of (A) woven carbon fiber (WCF) and WCF based polyester composites at 10 mV/s scan rate and (B) CuO–WCF/IL/LiS/PES composite at different scan rates. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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in the device. The energy and power densities of the supercapacitor have been calculated from the following equations [10],

1 2 CU 2 int U2 P ¼ int 4Rs

E¼

ð6Þ ð7Þ R

p where U int ¼ Rp þR U appl . s

Both energy and power densities have been significantly improved after the electrodes modified by CuO nanowires growing and using ionic liquid along with lithium salt in the polyester resin matrix and presented in Table 3. 3.3. Tensile properties of the composites

Fig. 4. Specific capacitance of (A) different WCF based samples at 10 mV s
1 scan rate (black line) and (B) CuO–WCF/GF/IL/LiS/PES composite (blue line) at different scan rates. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of an ionic liquid-based electrolyte increased the specific capacitance of the CuO–WCF/PES composite to ca. 5.40 F g
1. An ionic liquid provides a channel for ion movement inside a composite due to formation of microporous structure inside the matrix as illustrated by Fig. 2D and hence, it significantly improved the capacitance of the supercapacitor [28]. The capacitance of the CuO–WCF composite increased still further when the lithium salt was added to the polyester matrix (i.e., to Csp ca. 6.75 F g
1). Quantitative analysis of the LiTf salt revealed that one-third of the Tf anions formed coordination bonds with Li cations. The remaining two-thirds of the anions remained free in the matrix. In contrast, two-thirds of the BF4 anions of the EMIMBF4 ionic liquid were coordinately bound with the Li cations. The anions remaining free in both cases were expected to facilitate the ionic conductivity of the composite [29]. The cyclic voltammetry test was performed at different scan rates to establish the rate capability of the composite samples (Fig. 3B). At low scan rates, the composite samples displayed rectangularly-shaped voltammograms, indicating ideal capacity. As the scan rate increased, the voltammetric current that passed through the samples also increased and the shape of the curve deviated from its rectangular shape because of polarizations associated with micropore blocking, ionic resistance, and ion depletion [30]. The specific capacitances of the samples at different scan rates can be estimated from Fig. 4B; the value for the CuO–WCF/ IL/LiS/PES composite at 100 mV s
1 was only 7.87% of that measured at 1 mV s
1 (Fig. 4B). At high scan rates, the availability of ions near the electrode surfaces changed and the diffusion coefficient of ions decreased, driving the capacitances to lower values. The equivalent series resistance (ESR) and polarization resistance (Rp) were determined from electrochemical impedance spectroscopy (EIS) by using the same instrument (NuVant Systems, Crown Point, IN, USA) and presented in Table 3. This table has revealed that ESR as well as Rp values were decreased gradually as the CuO nanowires were grown on WCF electrodes along with ionic liquid and lithium salt added to the polyester resin. The ESR of a supercapacitor depends on many factors, such as electrical resistance of the electrodes and presence of active sites on it, ionic resistance provide by the electrolyte and the existence of interfacial resistance between the electrodes and electrolyte [31]. CuO nanowires grown on carbon fiber electrodes enhanced the surface area as well as active sites and the complex of ionic liquids and lithium salt increased conductivity between electrodes and electrolyte. These low ESR and Rp values are very much important for the higher performance of the supercapacitor. The low Rp values of some supercapacitor samples may be obtained by the presence of leakage

Tensile testing was carried out according to the standard ASTM D3039 using an Instron 5982 universal testing machine equipped with a 10 kN load cell. Five specimens of each sample were tested at a displacement rate of 2 mm min
1. The total volume fraction (Vf), tensile strength and modulus values of bare WCF and CuO– WCF composites with ionic liquid and lithium salt are presented in Table 3. As expected, the bare WCF composite sample had the lowest strength and modulus because of the absence of significant interfacial interaction between the carbon fibers and the resin. Increasing the surface area of the carbon fibers through treatment with NaOH improved the interfacial interaction with the resin and thereby increased the strength (by 56.6%) and modulus (by 49.2%) of the composite. The CuO nanowire–decorated WCF samples exhibited the greatest improvement in strength (80.6%) and modulus (77.1%). This behavior was similar to that reported for carbon fiber/polymer composites where the carbon fibers were decorated with metal oxide nanowires [32]. The growth of CuO nanowires on the fiber surface facilitated load sharing and resulted in a higher energy required to break the composite. Moreover, the increased total volume fraction of WCF due to growth of CuO nanowire on the fiber surface felicities the effective load sharing with the fiber, hence required higher energy to break the composite. Furthermore, the hydroxyl, carboxyl, and carbonyl functional groups on the surface of the carbon fiber formed ionic bonds with the CuO nanowires and also bonded with the ester groups of the resin. This extra bonding sequence increased the strength and modulus of the composites. The ionic liquid incorporated into the resin occupied some interfacial sites within the composites and reduced the interfacial bonding area [33]. Therefore, a marked decline in the mechanical properties (strength to 73.6%, modulus to 71.2%) was observed. The number of interfacial interaction sites was further decreased by the addition of the lithium salt: the strength reduced to 68.2% and the modulus to 66.1%. The stress–strain curves for the composites are shown in Fig. 5A. The elasticities of the composites increased after activation of the fiber surface or after growth of the CuO nanowires, which suggests better accommodation of an imposed strain. 3.4. In-plane shear testing In-plane shear tensile testing was done according the standard ASTM D3518/D3518M-13 using the same instrument and displacement speed as used for the tensile testing. This test is typically used to determine the in-plane uniaxial tensile stress–strain response of ±45° composite materials reinforced by highmodulus continuous fibers or woven fabrics. Five samples were tested and the average values are reported. The shear strength was calculated according to Eq. (8) [22];

sm12 ¼

Fm ; 2bd

ð8Þ

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B.K. Deka et al. / Composites: Part A 87 (2016) 256–262 Table 3 Mechanical and electrochemical performances of supercapacitors. Sample no.

WCF/GF/PES ac-WCF/GF/PES CuO–WCF/GF/PES CuO–WCF/GF/IL/PES CuO–WCF/GF/IL/LiS/ PES

WCF Vf (vol%)

39.4 40.1 45.6 44.8 44.5

(±0.59) (±0.46) (±0.84) (±0.73) (±0.61)

Tensile

In-plane shear

ESR

Rp

Energy density
1

Strength (MPa)

Modulus (GPa)

Strength (MPa)

Modulus (GPa)

(X)

(X)

(mW h kg

149.640 (±4.489) 234.275 (±7.028) 270.275 (±8.105) 259.816 (±7.794) 251.758 (±7.553)

11.817 (±0.368) 17.638 (±0.469) 20.912 (±0.627) 20.183 (±0.315) 19.616 (±0.267)

57.914 (±1.158) 89.578 (±1.792) 102.439 (±2.049) 95.529 (±1.911) 92.464 (±1.849)

11.937 (±0.302) 18.072 (±0.386) 21.584 (±0.261) 19.682 (±0.615) 18.927 (±0.182)

730 724 370 330 130

1240 1150 986 888 674

27.63 36.86 42.15 93.75 106.04

)

Power density (W kg
1) 1.03 2.52 4.14 10.39 12.57

Fig. 5. (A) Tensile and (B) in-plane shear stress–strain curves of WCF and CuO–WCF/GF/IL/LiS/PES composites. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

where F m [N] is the maximum in-plane load, sm 12 [MPa] is the inplane shear strength, b [mm] is the specimen width, and d [mm] is the specimen thickness. Table 3 depicts the in-plane shear strength of bare WCF and surface modified WCF/polyester composites. It is evident that the shear strength of the supercapacitor increased with NaOH activation (by 54.6%) or CuO nanowire decoration (by 76.9%) of the WCF. The activation process or nanowire growth increased the surface area of the WCF, which improved wetting and enhanced the interaction of the carbon fiber with the matrix. The stress–strain curves (Fig. 5B) show better elongation at a given tensile stress for all of the modified composites, compared with the WCF/PES control. This behavior is attributed to larger specific areas and reaction of the hydroxyl, carboxyl, and carbonyl functional groups of the carbon fiber surfaces with the ester linkages of the polyester resin. Furthermore, the Cu2+ ions likely formed strong ionic bonds with the carboxylic functional groups of the carbon fiber. However, the shear strength and strain decreased after incorporating the ionic liquid (EMIMBF4) (to 64.9%) and lithium salt (LiTf) (to 59.6%) to the composite. The ionic liquid and the salt interfered with the interfacial bonding of the composite by blocking the interaction sites, as mentioned earlier. These results clearly indicate that although the surface activation of WCF by the base treatment and growth of nanowires enhanced the mechanical performance, the ionic liquid and salt were unable to adequately transfer the load from the matrix to fiber, thereby degrading the mechanical properties.

shear strength [34]. This work instead used the specific capacitance as a function of shear modulus for the same purpose. To find the ideal performance value, the lower limit of the capacitance and modulus were connected and the intersection point was considered as the ideal point; this location is indicated by an asterisk (⁄) in the figure [35]. The composite approached the ideal point as its multifunctionality increased. Growing CuO nanowires on the WCF surface improved the multifunctionality to a satisfactory level. The high wettability and improved surface area of the carbon fiber after decorating with the CuO nanowires enhanced the inter-

3.5. Multifunctionality Fig. 6 presents the multifunctionality of the CuO–WCF-based structural supercapacitors. Previous researchers assessed multifunctionality by measuring the energy density as a function of

Fig. 6. Multifunctionality (modulus vs. specific capacitance) of various WCF/PES composites. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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facial contact of the fiber to the resin, which increased the multifunctionality of the composite. Although incorporation of the EMIMBF4 ionic liquid and the LiTf salt improved the specific capacitance, the mechanical performance was considered unsatisfactory. Further research is planned to improve the balance of specific capacitance and shear modulus toward the ideal value. 4. Conclusions Structural supercapacitors based on CuO-nanowire–decorated WCF were designed and fabricated using VARTM, an effective and eco-friendly process. The surface area of the WCF was increased by treatment with 4 M NaOH solution. Growing CuO nanowires on the surface of the WCF also improved the specific surface area. BET analysis revealed that the NaOH treatment increased the surface area to ca. 41.4 m2 g
1 while the CuOnanowire–decorated WCF had an area of ca. 132.9 m2 g
1. These areas are much greater than that of bare WCF (0.6 m2 g
1). Scanning electron microscopy was used to study the NaOH etching and the growth of the CuO nanowires. The specific capacitances of the new supercapacitors were characterized by cyclic voltammetry, which revealed that the CuO decoration had increased the capacitance to 2.48 F g
1. Adding the EMIMBF4 ionic liquid and the LiTf salt further increased the specific capacitance to 5.40 F g
1 and 6.75 F g
1, respectively. However, the tensile and in-plane shear strengths suffered. The ESR and Rp values drop to 130 and 674 X starting from 730 and 1240 X. The ionic liquid and lithium salt based nanowires grown samples showed up to 106.04 mW h kg
1 of energy density and 12.57 W kg
1 of power density values. Hence, the CuO nanowire decoration substantially increased the multifunctionality of the supercapacitors. However, it remains necessary to develop a multifunctional matrix that can provide good electrical properties without compromising the mechanical performance. Overall, eco-friendly CuO-nanowire/ WCF structural supercapacitors were developed cost-effectively using VARTM. Related supercapacitors are expected to find uses in applications such as energy storage in UAVs, aerospace, and electric vehicles. Acknowledgements This work was supported by the 2016 Research Fund (1.160005.01) of UNIST (Ulsan National Institute of Science and Technology) and the Technology Innovation Program (10053248, Development of Manufacturing System for CFRP (Carbon Fiber Reinforced Plastics) Machining) funded by the Ministry of Trade, industry & Energy (MOTIE, Korea). References [1] Botelhoa EC, Silvac RA, Pardinia LC, Rezend MC. A review on the development and properties of continuous fiber/epoxy/aluminum hybrid composites for aircraft structures. Mater Res 2006;9(3):247–56. [2] Guo M, Yi X, Liu G, Liu L. Simultaneously increasing the electrical conductivity and fracture toughness of carbon–fiber composites by using silver nanowiresloaded interleaves. Compos Sci Technol 2014;97:27–33. [3] He H, Wang J, Li K, Wang J, Gu J. Mixed resin and carbon fibres surface treatment for preparation of carbon fibres composites with good interfacial bonding strength. Mater Des 2010;31(10):4631–7. [4] Yoon SH, Lim S, Song Y, Ota Y, Qiao W, Tanaka A, et al. KOH activation of carbon nanofibers. Carbon 2004;42(8–9):1723–9. [5] Miranda AN, Pardini LC, Santos CAM, Vieira R. Evaluation of carbon fiber composites modified by in situ incorporation of carbon nanofibers. Mater Res 2011;14(4):560–3. [6] Deka BK, Kong K, Seo J, Kim D, Park YB, Park HW. Controlled growth of CuO nanowires on woven carbon fibers and effects on the mechanical properties of woven carbon fiber/polyester composites. Composites Part A 2015;69:56–63.

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