Highly Stretchable and Self-Healable Supercapacitor with Reduced Graphene Oxide Based Fiber Springs Siliang Wang,† Nishuang Liu,*,† Jun Su,† Luying Li,† Fei Long,‡ Zhengguang Zou,‡ Xueliang Jiang,§ and Yihua Gao*,† †
Center for Nanoscale Characterization & Devices (CNCD), Wuhan National Laboratory for Optoelectronics (WNLO) & School of Physics, Huazhong University of Science and Technology (HUST), LuoyuRoad 1037, Wuhan 430074, People’s Republic of China ‡ School of Material Science & Engineering, Guangxi Nonferrous Metals Mineral and Materials, Collaborative Innovation Center, Guilin University of Technology, Jian’gan Road 12, Guilin, Guangxi 541004 People’s Republic of China § School of Material Science & Engineering, Wuhan Institute of Technology, Xiongchu Street 693, Wuhan 430073, People’s Republic of China S Supporting Information *
ABSTRACT: In large-scale applications of portable and wearable electronic devices, high-performance supercapacitors are important energy supply sources. However, since the reliability and stability of supercapacitors are generally destroyed by mechanical deformation and damage during practical applications, the stretchability and self-healability must be exploited for the supercapacitors. Preparing the highly stretchable and self-healable electrodes is still a challenge. Here, we report reduced graphene oxide fiber based springs as electrodes for stretchable and self-healable supercapacitors. The fiber springs (diameters of 295 μm) are thick enough to reconnect the broken electrodes accurately by visual inspection. By wrapping fiber springs with a self-healing polymer outer shell, a stretchable and self-healable supercapacitor is successfully realized. The supercapacitor has 82.4% capacitance retention after a large stretch (100%), and 54.2% capacitance retention after the third healing. This work gave an essential strategy for designing and fabricating stretchable and self-healable supercapacitors in next-generation multifunctional electronic devices. KEYWORDS: stretchable, self-healable, multifunctional, fiber springs, supercapacitor
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onto a stretchable substrate. However, the additional substrate adds weight and volume and consequently lowers the performance of the devices and makes further integration inconvenient. Furthermore, the stretchability and complex shape change are severely limited by the confined deformation of the underlying substrate. To prepare high-performance integrated functional devices with stretchability, it is necessary to develop intrinsically stretchable material structures and electrodes without additional substrates. Fiber materials play a critical role in our life and have been widely used as structure materials for supercapacitors with many advantages of being woven into lightweight, highly flexible, soft, and low-cost textiles.32−34 As a typical promising fiber material, graphene fibers have been widely assembled for their high electric and thermal conductivity, extraordinary
tretchable electronics devices have emerged as an important branch of modern electronics.1−8 They have the capacity to accommodate large levels of strain without obvious degradation in electronic properties and have great promising value in wearable electronics, biomedical devices, electronic paper displays, artificial skin incorporating sensors, and so on.9−20 Recently, the requirements of highly stretchable display panels, light-emitting diodes, strain sensor, and other stretchable devices have promoted the development of stretchable batteries and supercapacitors acting as essential energy storage components.21−25 Compared with batteries, supercapacitors, also called ultracapacitors or electrochemical capacitors, are promising for their obvious advantages of higher power densities, longer cycle lives, and easier fabricating processes.26,27 To date, many stretchable supercapacitors have been fabricated by creating wavy electrode materials on polymeric substrates, coating active materials on textiles, and wrapping conductive materials on elastic polymeric fibers.28−31 These above supercapacitors with mechanical robustness are made by a similar method of depositing electrode materials © 2017 American Chemical Society
Received: December 9, 2016 Accepted: January 23, 2017 Published: January 23, 2017 2066
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Figure 1. Schematic diagrams of the stretchable and self-healable mechanism, manufacturing process of PPy/RGO/MWCNT electrodes, and stretchable and self-healing supercapacitor. (A) RGO-based fiber wires were twisted into springs that can be stretched to a high percentage. Self-healable property arose from interfacial hydrogen bonding. (B) GO/MWCNT-based mixture solution was injected into pipes, followed by sealing at both ends. After GO was reduced to RGO at 90 °C in an oven, RGO/MWCNT composite fibers were prepared. A layer of PPy was electrodeposited on partially dried RGO/MWCNT composite fibers to achieve better electrochemical performances. The PPy/RGO/ MWCNT composite fibers were twisted into springs and assembled with solid electrolyte. The stretchable and self-healing PU shell was coated on the fibers. The PU shell ensures the stretchability of the supercapacitor and self-healing properties by reconnecting the broken fiber electrodes when they are brought together.
stiffness and elasticity, and strong stability.35−38 Graphene− carbon nanotube composite fibers have been further proposed and fabricated to prevent stacking by the intersheet π−π interaction of graphene.39 However, when these graphene fiber based supercapacitors operate in practical applications, the electrode materials and substrates may suffer mechanical damage caused by deformation or accidental cutting. The reliability and stability of the supercapacitors were constrained by these failures, leading to whole scale breakdown of the electronic devices.40,41 Self-healing materials have the ability to heal damage partially or completely inflicted on them and restore mechanical and structural properties.42 So, self-healing materials can be used in supercapacitors to prevent the structural fractures and restore the structural integrity and electrical properties of the devices after mechanical damage.43−45 Unfortunately, conventional graphene-based fiber electrodes were thin, so it is unrealistic to reconnect the tiny broken fibers combined accurately by visual inspection. In order to acquire the restoration of electrochemical performance after damage, a good reconnection of the broken yarn (with thicker diameter) of the graphene-based fiber supercapacitor is essential. To date, no one has reported the work on reduced graphene oxide (RGO) fiber-based self-healable supercapacitor, not to mention further progress in integrated functional devices with self-healable capacity. Herein, we prepared spring-like RGO-based composite fibers. These fibers were thick, which allows the tiny broken fibers to be reconnected. The fibers could be stretched at high
percentages up to 300% with appropriate design. Stretchable carboxylated polyurethane (PU) that acted as a self-healing material was coated on spring-like fiber electrodes to ensure the stretchability and self-healing properties of the supercapacitor. The supercapacitor has 82.4% capacitance retention after a large stretch (100%), and 54.2% capacitance retention after the third healing. This research may provide an efficient way to make stretchable, self-healing, and robust electronic devices in the near future for multifunctional supercapacitors with an extremely long lifespan.
RESULTS AND DISCUSSION The stretchable and self-healable schematic diagram of the mechanism can be seen in the top part and bottom part of Figure 1A, respectively. RGO-based fiber wires consisting of polypyrrole (PPy)-decorated RGO/multiwalled carbon nanotubes (MWCNTs) were twisted into springs that can be greatly stretched. The self-healing property arose from interfacial hydrogen bonding of carboxylated PU. Figure 1B shows the preparation procedures of the stretchable and self-healable supercapacitor of PPy-decorated RGO/MWCNT fibers. At first, a mixture of a solution of GO, MWCNTs, sodium dodecyl sulfate (SDS), and vitamin C (VC) is injected into a polytetrafluoroethylene pipe, followed by sealing at both ends with polydimethylsiloxane (PDMS) to reduce the GO to RGO in the oven at 90 °C. During the reaction, as shrinkage of GO and pressure of gases came from the reduction of the functional groups on GO in the void space,46 the RGO/MWCNT 2067
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Figure 2. Microstructure of RGO-based fiber. (a,b) Side view and (c) cross-sectional SEM images of the RGO fiber, (d,e) side view and (f,g) cross-sectional SEM images of the RGO/MWCNT fiber with an initial weight ratio of 1:1 at low and high magnifications, respectively. SEM images of a (h) knotted and (i) twisted RGO/MWCNT fibers.
100 μm were stacked, which is consistent with cross-sectional SEM images in Figure 2c. After MWCNTs were added to RGO, the specific surface area was increased and the diameter was increased to 295 μm (Figure 2d,e). The cross-sectional SEM images of RGO/MWCNT fibers with an initial weight ratio of 1:1 are shown in Figure 2f,g. The fiber shows the porous structure. With increasing weight ratio of MWCNTs, the diameter of a RGO/MWCNT fiber was increased from 253 to 334 μm (Figure S3a,c) and the fiber did not change the porous structure (Figure S3b,d). Figure 2h,i shows the SEM images of the RGO/MWCNT fiber that was tied into knots and twisted into spring without obvious damage to the structure, which demonstrated the good flexibility and robustness. The mechanical properties originated from two aspects, the high mechanical flexibility of MWCNTs and welldispersed MWCNTs within intersheets of graphene preventing stacking.51,52 The X-ray energy dispersion spectrum (EDS) analysis of the RGO/MWCNT fiber is shown in Figure S4, in which the C signal came from the RGO/MWCNT composite, and the O signal came from the insufficiently reduced GO. In order to improve the electrochemical performances of the fiber, a thin PPy film was wrapped around the surface of the RGO/MWCNT fiber. Various electrodeposition times were performed to optimize the electrochemical performances. After deposition, the thickness of the fiber shows no obvious change (Figure S5a,c,e), but the specific surface areas were decreased along with the electrodeposition time (Figure S5b,d,f). Figure S6a shows the EDS of PPy/RGO/MWCNTs, in which the N signal came from PPy. Figure S6b shows the Raman spectra of RGO/MWCNTs, PPy, and PPy/RGO/MWCNT composites. The presence of typical bands (D, G, 2D) is associated with the RGO/MWCNT nanostructure. The bands at 936 and 1084 cm−1 correspond to the dication (bipolaron) structure. The
composite tends to be columnar in the center of pipes without contacting the inner tube surface. After completely reacting, the RGO/MWCNT composite fibers were taken out to allow them to dry naturally. Then, a thin layer of PPy was electrodeposited on the partially dried composite fibers to improve the conductivity and electrochemical performances. To fabricate stretchable and self-healing supercapacitors, the PPy/RGO/ MWCNT fibers were twisted into springs and wrapped with poly(vinyl alcohol)−phosphoric acid (PVA-H3PO4) gel that serves as the solid electrolyte and a separator. At last, the stretchable, self-healing, and mechanically strong carboxylated PU that is greatly compatible with the textile industry was selected to coat the outermost layer of the supercapacitor as a protection shell and make the device stretchable and selfhealable. Hydrogen bond acceptors and donors in abundance in the supramolecular network of carboxylated PU give rise to selfhealable properties.47−49 Moreover, PVA gel is a type of selfhealable material to some degree.50 In addition, thick PPy/ RGO/MWCNT springs were beneficial for accurately reconnecting the tiny broken fibers by visual inspection. Once the spring electrodes were broken, as long as the separated parts are placed at suitable places, self-healing carboxylated PU can guide the reconnection process, leading to the restoration of the electrical properties. To prepare the RGO and RGO/MWCNT composite fiber, the mixture solution of GO (lateral size = 0.5−3 μm, thickness = 0.9 nm, and C/O ratio = 1.824/1, Figure S1a,b) and MWCNTs with various weight ratios and uniform dispersion (Figure S2a,b) was injected into pipes. After the reaction was complete and dried, the RGO and RGO/MWCNT composite fiber can be obtained. Figure 2a,b shows the side view SEM images of RGO fiber at low and high magnifications, respectively. The RGO sheets in the fiber with a diameter of 2068
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Figure 3. Electrochemical performances of RGO, RGO/MWCNTs, and PPy/RGO/MWCNT fiber electrodes. (a) Cyclic voltammograms at the scan rate of 0.05 V s−1. (b) Volumetric capacitances vs scan rate. Cyclic voltammograms with scan rate (c) from 0.001 to 0.02 V s−1 and (d) from 0.05 to 0.2 V s−1. (e) Galvanostatic discharge curves at various current densities and (f) Nyquist plot of the PPy/RGO/MWCNT fiber electrode.
band located at 968 cm−1 is ascribed to ring deformations, and the band located at 1382 cm−1 is assigned to the ring stretching mode of PPy. The band with π-conjugated structure located at 1578 cm−1, with the CαCα′ stretching (inter-ring) at 1248 cm−1. Simultaneously displaying 936, 968, and 1084 cm−1 and a 2D band in the Raman spectrum of the composite further revealed the coexistence of PPy, RGO, and MWCNTs.53−56 To evaluate the electrochemical performance of the fiber based on the PPy/RGO/MWCNT hybrid structure, a three-electrode configuration was used in the electrochemical measurements. Figure 3a shows the cyclic voltammograms (CV) curves of RGO, RGO/MWCNT, and PPy/RGO/MWCNT fibers. The RGO/MWCNT fiber electrode exhibited a current density substantially larger than that of the RGO fiber electrode at the scan rate of 0.05 V s−1. This was due to the good electrochemical performance of MWCNTs and restrained stacking of RGO. RGO/MWCNT hybrid fibers with various weight ratios were compared to better optimize the electrochemical performance. Figure S7a−d shows the CV curves of RGO and RGO/MWCNTs with initial weight ratios of 2:1, 1:1, and 1:1.5. All of the RGO/MWCNT fiber electrodes exhibited a current density larger than that of a single RGO fiber electrode, and the current density of RGO/MWCNT fiber electrodes with various ratios varied little (Figure S7e). Because of the well-restrained stacking and similar porous structure indicating sufficient ion accessibility, the volumetric capacitances of the fiber electrodes with various ratios varied little (Figure S7f). Thicker fiber electrodes mean larger total capacitances. The RGO/MWCNT fiber with a ratio of 1:1 has a low capacitance decrease with scan rate, and the diameter is thick enough to reconnect accurately by visual inspection; therefore, we chose the fiber with a ratio of 1:1 to set up the next experiments. The PPy/RGO/MWCNT fiber electrode with 1.5 min PPy electrodeposition showed a more ideal capacitive behavior with more rectangular CV curves than did
RGO and RGO/MWCNT fiber electrodes (Figure 3a). The volumetric capacitance increased from 11.4 F cm−3 (RGO) and 10.8 F cm−3 (RGO/MWCNTs) to 25.9 F cm−3 (PPy/RGO/ MWCNTs) at the scan rate of 0.01 V s−1. Even at 0.1 V s−1, the volumetric capacitance of the PPy/RGO/MWCNT fiber was still 18.2 F cm−3 higher than that of the RGO fiber (2.9 F cm−3) and RGO/MWCNT fiber (6.5 F cm−3) (Figure 3b). Figure S8a−d shows the CV and galvanostatic discharge (GCD) curves of PPy/RGO/MWCNT fibers with 1 and 2 min PPy electrodeposition time. Since PPy is more electrochemically active than RGO/MWCNTs, appropriate electrodeposition of PPy will increase the capacitance, but over-electrodeposition of PPy decreases the capacitance for the lower specific surface. The fiber with 1.5 min PPy electrodeposition showed higher volumetric capacitances and lower capacitance decrease with scan rate and current density (Figure S8e,f), so we chose the fiber with 1.5 min electrodeposition to perform the next experiments. Figure 3c,d presents CV curves of the PPy/RGO/ MWCNT fibers with scan rates from 0.001 to 0.2 V s−1. As expected, the electrode showed rectangular CV curves, indicating ideal capacitive behavior. To further evaluate the electrochemical performances of the PPy/RGO/MWCNT fibers, GCD measurements were performed (Figure 3e). The Coulombic efficiency was as high as 90% at the charge− discharge current density of 0.2928 A cm−3. Figure 3f was the electrochemical impedance spectrum (EIS) of the PPy/RGO/ MWCNT fiber. The intercept of the plot at the X-axis represents the equivalent series resistance (ESR), which determines the charge−discharge rate of the fiber.57,58 The ESR of the fiber was 0.7 Ω, which was much lower than that in the previous work, indicating the fast ion transport at the active material−electrolyte interface.59,60 These electrochemical properties of the fiber demonstrated that the PPy-decorated RGO/ MWCNT fiber was an ideal candidate as an electrode for the supercapacitor. 2069
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Figure 4. Strethable and self-healable properties of PPy/RGO/MWCNT fibers. (a) Photographs of the fiber, (b) twisted into a spring and stretched to 300%. (c,e,g) Photographs of the fiber electrode within a circuit with an LED before breaking, after breaking, and after healing. (d,f,h) Electrical resistance of the fiber electrode with a length of 1 cm before breaking, after breaking, and after healing; insets show the enlarged photographs of the fiber electrode. The unit of the resistance displayed on the multimeter is Ω.
After the PPy/RGO/MWCNT fiber (Figure 4a) was twisted into a spring, it could be stretched from 1 to 4 cm, stretched 300% (Figure 4b and Movie S1), which means the fiber spring could be potentially used in stretchable electronics devices. After the PPy/RGO/MWCNT fiber was wrapped with the solid electrolyte and the PU self-healing shell, the self-healing capacity of the fiber electrode with regard to the electrical conductivity was investigated. The self-healing fiber was used as a conducting wire to power a common commercial lightemitting diode (LED). The LED lighted up successfully at a voltage of 3.0 V and was immediately extinguished after being cut with a knife. After the two halves of the bisected electrodes were brought into contact by a gentle pressure, the LED lighted up again with a brightness similar to that of the original intact fiber electrode (Figure 4c,e,g and Movie S2). The magnified photographs of the process of self-healing fiber electrodes before being cut and after healing are shown in Figure 4d,f,h. A multimeter was used to trace the resistance of the electrode (Figure 4d,f,h). Compared to that of the original electrode, the resistance slightly increased from 63.9 to 67.8 Ω. Such outstanding healable electrical conductivity should be a result from the PU shell. Two-electrode configuration was used to evaluate the electrochemical performance of the supercapacitor composed of PU-wrapped PPy/RGO/MWCNT fiber springs. A variety of electrochemical measurements have been carried out. Figure 5a,b shows the CV curves of the devices within a potential window of 0.8 V at scan rates from 0.001 to 0.1 V s−1. The
curves have good symmetrical rectangular shape and large enclosed areas (Figure 5a). To further evaluate the electrochemical performances of the device, GCD measurements were performed (Figure 5c). Good linear potential−time profiles were achieved at different current densities that varied from 0.0183 to 0.2928 A cm−3, demonstrating a good capacitance performance of the devices. The volumetric capacitance was evaluated by the above CV and GCD results (Figure 5d,e). The supercapacitor showed a high energy density of 0.94 mWh cm−3 at a power density of 7.32 mW cm−3. Even at a power density of 117.12 mW cm−3, the energy density was still as high as 0.36 mWh cm−3 (Figure 5f). Figure 5g shows the EIS of the device. The ESR value of 55.2 Ω, nearly vertical profile at low frequency and the absence of a semicircle at high frequency, suggested the good electrochemical properties.61 To meet specific energy and power needs for practical applications, three supercapacitors were assembled in series and then parallel. The three supercapacitors connected in series exhibited a 2.4 V voltage window with similar discharge time, and those connected in parallel exhibited an increased capacitance of 3fold. The results roughly obey the basic rule of series and parallel connections of capacitors (Figure 5h,i). As a proof-of-concept, the electrochemical performances of stretchable and self-healable supercapacitors based on two parallel fiber springs wrapped with gel electrolyte and PU were investigated. The supercapacitor can be easily stretched up to 100% (Figure 6a). The CV curves had little shrinkage of enclosed area when the supercapacitor was stretched from 0 to 2070
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Figure 5. Electrochemical measurements of the as-prepared supercapacitor. Cyclic voltammograms of the supercapacitor with a scan rate from (a) 0.001 to 0.01 V s−1 and (b) from 0.02 to 0.1 V s−1. (c) Galvanostatic discharge curves at various current densities, (d) volumetric capacitances vs scan rate, (e) volumetric capacitances vs current density, (f) energy and power density plot, and (g) Nyquist plot of the supercapacitor. (h) Cyclic voltammograms and (i) galvanostatic discharge curves of a single supercapacitor and a group of three supercapacitors connected in series or parallel.
100% (Figure 6b). At a stretch of 100%, the device retained 82.4% capacitance retention (Figure 6c). After being cut and healed, the capacitance of the device was still maintained at a high level. Figure 6d shows the CV curves of the device before being cut and after the sever was healed, where a largely rectangular shape remains even after the third healing. During the cutting/healing cycles, the device unavoidably experienced vigorous manipulation, resulting in misalignment between the broken electrodes and then slightly deviated curves. The capacitance retention ratio is about 54.2% after the third healing (Figure S9a). The GCD results (Figure 6e), consistent with Figure 6d, show good restoration of the supercapacitor. For better understanding, the EIS of the stretchable and selfhealable supercapacitor was investigated. The ESRs of the capacitors before cutting and after the first, second, and third healing are 55.2, 56.2, 61.9, and 81.4 Ω, respectively. Because of good reconnection of the electrode, the first and second impedance spectra nearly mixed together (Figure 6f). Three thousand cycle GCD tests were carried out. The device before cutting had a 9.6% decay, and the device after the third healing had a 16% decay (Figure S9b). The stretchable and selfhealable supercapacitor was used to drive a photodetector of perovskite nanowires, as illustrated in Figure 6g. Before cutting and after healing (Figure 6h), the on/off ratio only had a slight
decay (Figure 6i), which demonstrated the good restoration of the supercapacitor’s function. These performances mean that the stretchable and self-healable supercapacior had practical application in robust electronic devices.
CONCLUSION We obtained a supercapacitor that integrates stretchable and self-healable functions. The supercapacitor was designed and fabricated with two parallel fiber springs wrapped with gel electrolyte and PU. The stretchable property comes from the stretchable fiber springs and PU; meanwhile, the outstanding mechanically self-healing performance originates from carboxylated PU shell. Both the stretchable and self-healing properties are expected to be crucial for practical applications. The supercapacitor has 82.4% capacitance retention after a large stretch (100%), and 54.2% capacitance retention after the third healing. Even after 3000 GCD cycles, the device with third healing retained 84% capacitance without obvious degradation. The stretchable and self-healable supercapacitor has an easy preparation and the fewest number of components. The successful preparation of stretchable and self-healable supercapacitors may provide a way to design and fabricate multifunctional supercapacitors and other next-generation multifunctional electronics devices, even a solution to expand 2071
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Figure 6. Electrochemical measurements and application for as-prepared stretchable and self-healing supercapacitors. (a) Photographs, (b) cyclic voltammogram curves, and (c) evolutions of specific capacitance of the supercapacitor before and after stretching to 100%. (d) Cyclic voltammogram curves, (e) galvonostatic charge−discharge measurements, and (f) Nyquist plots of the supercapacitor before healing and after several self-healings. (g) Illustration of the supercapacitor driving a photodetector of perovskite nanowires. (h) Photographs of the supercapacitor before and after self-healing. (i) Photocurrent dependence on time of the photodetector under illumination of on/off states driven by the original and self-healing supercapacitor after a healing cycle; red corresponds to the self-healing supercapacitor and black to the original. electrolyte, and dried under vacuum at room temperature. The springs were fixed in parallel, coated with another layer of electrolyte, and dried under vacuum. Finally, the PU was coated on the supercapacitor and dried in air. Characterization. The lateral size and thickness of GO were tested with an atomic force microscope (AFM, SPM 9700). The C/O ratio of GO was tested with XPS (AXIS-ULTRA DLD-600W). The dispersion of MWCNTs in GO solution was observed with transmission electron microscopy (TEM, FEI Titan G2 60-300). The morphologies of RGO, RGO/MWCNT, and PPy/RGO/ MWCNT fibers were observed with a scanning electron microscope (SEM, FEI Nova Nano-SEM 450). Raman spectroscopy was recorded using a LabRAM HR800 spectrometer with 514 nm laser light (HORIBA Jobin Yvon). Electrochemical measurements including GCD, CV, and EIS (100 kHz to 0.01 Hz) were tested with an electrochemical workstation (CHI 660E). A three-electrode system, a platinum electrode, and Ag/AgCl were used as a counter electrode and reference electrode, with a 4 M LiCl solution serving as an electrolyte, to test the electrochemical performance of individual electrodes. The performances of the supercapacitors were measured using a twoelectrode configuration. The performance of the photodetector was tested by switching a portable white lamp between “on” and “off” states. All of the measurements were carried out in ambient conditions at room temperature.
the lifetime of future stretchable electronics devices, meeting the requirement of sustainability.
METHODS Reagents and Materials. GO was prepared by oxidation of natural graphite powder using a modified Hummers’ method, PVA (molecular weight = 146 000−186 000, 99+% hydrolyzed), H3PO4, pyrrole, MWCNTs (Beijing Boyu Gaoke New Material Technology Co., Ltd., diameter =
