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. 2021 Jul 26;6(30):19578–19585. doi: 10.1021/acsomega.1c02091

CNT–rGO Hydrogel-Integrated Fabric Composite Synthesized via an Interfacial Gelation Process for Wearable Supercapacitor Electrodes

Seok Hun Kang †,‡,§,∥, Gil Yong Lee †,‡,§, Joonwon Lim †,‡,§,⊥,*, Sang Ouk Kim †,‡,§
PMCID: PMC8340110  PMID: 34368544

Abstract

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We demonstrate a flexible and stretchable supercapacitor assembled via straightforward interfacial gelation of reduced graphene oxide (rGO) with carbon nanotube (CNT) on a stretchable fabric surface. The difference between the redox potential of aqueous graphene oxide (GO) dispersion, prepared using a modified Hummers' method, and of a solid Zn plate, which was used as an external stimulus, induces a spontaneous reduction of GO flakes forming porous CNT–rGO hydrogel at the liquid–solid interface. With the aid of Zn, a macroporous and flexible CNT–rGO hydrogel was fabricated on a stretchable fabric platform using a facile fabrication method, and the CNT–rGO fabric composite was assembled into a supercapacitor to demonstrate its feasibility as a wearable electrode. The porous structure of the as-formed CNT–rGO fabric composite allows excellent electrolyte accessibility and ion transport that result in a fast charge/discharge rate up to 100 mV/s and a large areal capacity of 10.13 mF/cm2 at a discharge rate of 0.5 mA (0.1 mA/cm2). The inclusion of one-dimensional CNT as conductive bridges allows an excellent capacity retention of 95.2% after complete folding of the electrode and a capacity retention of 93.3% after 1000 bending cycles. Additional stretching test displayed a high capacity retention of 90.0% even at an applied strain as high as 50%, overcoming previous limitations of brittle graphene-based electrodes. This low-cost, lightweight, easy to synthesize, stretchable supercapacitor holds promise for next-generation wearable electronics and energy storage applications.

Introduction

Smart devices, which include all portable electronic devices capable of computing, have been dominating the electronics market for the past two decades owing to their convenience and utility.1−4 Among these devices, smartphones are the epitome of the current generation, capable of high-degree computation and are light and thin, making them portable and easily accessible. With the advancement of technology, a higher degree of portability and integration of electronics into our daily lives is demanded.5−7 Recently, coupled with the advancements in the Internet of Things, virtual or augmented reality, and robotics, this trend opened doors to wearable electronics for human connectedness, which includes all smart electronic devices that are designed to be light, thin, and conform to the shape of the human body.8 For devices to be wearable, they must be designed to be flexible and stretchable to accommodate various motions of the human body. It has been reported in previous literature that the human skin can experience tensile strain as high as 55% during the motion of walking.9 To accommodate such motion, electronics must endure numerous cycles of bending and stretching to be utilized in wearable technology.10−12

Diverse low-dimensional nanomaterials have been exploited for next-generation energy storage/conversion materials.13−18 Carbon nanomaterials, such as graphene and carbon nanotubes (CNTs), are promising low-dimensional materials for flexible or stretchable electrodes for energy storage devices owing to their many superior properties, including high surface area, conductivity, mechanical strength, and chemical stability.19−23 Especially, graphene in the oxidized form can be prepared at a low cost via the modified Hummers' method from pristine graphite and is solution-processable, making it scalable for large-scale applications.24,25 However, graphene oxide (GO) and its derivatives are generally very stiff, owing to the strong interlayer bonding, making them unsuitable for stretchable electronics.26 On the other hand, CNT exhibits excellent conductivity and flexibility owing to the one-dimensional structure with a high aspect ratio.27−29 Recent research demonstrates a stretchable supercapacitor fabricated based on buckled single-wall carbon nanotube (SWCNT) films that possess over 100% stretchability, highlighting the stretchable property of CNTs.30−32 However, the low surface area of CNTs critically limits the overall energy capacity of the CNT-based energy storage device.

In this work, graphene, which possesses a large surface area, has been synergistically combined with highly conductive, flexible, and stretchable CNT to fabricate a conductive textile for wearable application via a straightforward interfacial gelation principle. The spontaneous gelation technique that uses Zn as an external stimulus is used to fabricate a highly porous reduced graphene oxide (rGO) framework with well-dispersed CNTs on the surface of a stretchable spandex fabric. This CNT–rGO-coated fabric composite (CNT–rGO@F) possesses high conductivity and porosity suitable for energy storage applications. Using two CNT–rGO@F as electrodes, a stretchable supercapacitor was assembled to demonstrate the potential use of CNT–rGO@F conductive textile in stretchable energy storage applications. The as-formed supercapacitor demonstrated electrochemical performance clearly indicative of capacitive behavior that was retained even after severe bending and stretching cycles.

Results and Discussion

Using the Zn-induced graphene gelation method, a conductive CNT–rGO hydrogel was selectively coated on a spandex fabric to fabricate the CNT–rGO–fabric composite (CNT–rGO@F). Spandex fabric provides a mechanically stable and stretchable platform for the conductive rGO framework to be utilized in stretchable and flexible energy storage devices (Figures S1 and S2). Figure 1a shows the schematic illustration of the synthesis of CNT–rGO@F composite. A solid Zn plate is first prepared in the desired dimension. Then, a spandex fabric of a larger dimension is slightly stretched (prestrain ∼10%) over the Zn substrate and fixed in place. The fixed system is immersed in a weakly acidic (HCl volume concentration of 0.1%) CNT–GO solution to carry out Zn-induced graphene gelation. Owing to the difference in the standard reduction potential between GO and Zn, GO spontaneously reduces at the surface of the Zn substrate by the following chemical reaction.19

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Figure 1.

Figure 1

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Synthesis of the CNT–rGO@F composite. (a) Schematic diagram of the CNT–rGO@F synthesis. (b) Optical image of the as-synthesized CNT–rGO@F composite. (c–g) Scanning electron microscopy (SEM) images of the freeze-dried CNT–rGO@F composite.

Graphene gelation is not hindered by the presence of the spandex fabric on the Zn plate due to large macropores in the fabric. Microscale GO platelets undergo layer-by-layer deposition within the large pores in the fabric, and with sufficient gelation time, the resultant hydrogel becomes thick enough to cover the entire fabric with a CNT–rGO network. During the gelation process, GO is spontaneously reduced to rGO due to the reduction potential difference between GO and the Zn plate.25,33 After the gelation is complete, the Zn plate is removed using 1M HCl solution, and CNT–rGO@F is immersed in 1M HCl solution for 1 h to completely remove remnant Zn impurities. Finally, CNT–rGO@F is cleansed with deionized water to remove HCl. Synthesized CNT–rGO@F is shown by scanning electron microscopy (SEM) in Figure 1b–g. The novel protocol to combine the stretchable fabric and CNT–rGO using the Zn plate enables site-specific gelation on the fabric, as shown in Figure 1b. After sufficient gelation time, a thick CNT–rGO hydrogel that can fully coat the entire fabric could be synthesized (Figure 1c and d). As seen in the cross-sectional SEM image, the CNT–rGO gel structure possesses high porosity throughout the entire CNT–rGO network (Figure 1e). A highly magnified SEM image of the CNT–rGO gel structure clearly shows uniformly dispersed one-dimensional (1D) CNTs inside the porous two-dimensional (2D) rGO gel structure (Figure 1f). The CNT and rGO platelets not only fill the empty voids within the fabric but also coat individual fibers of the fabric through van der Waals force (Figure 1g). Using this method, a large area of conductive textile can be fabricated in one simple step. In contrast to fiber-shaped supercapacitor electrodes, in which individual fibers are coated one at a time with a porous graphene framework, the synthesized CNT–rGO@F covers a large area of textile with a porous CNT–rGO framework in a simple immersion step. This simplicity makes the entire process efficient in terms of time and cost, resulting in high scalability.

Successful reduction of GO into rGO was confirmed using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis (Figure 2). In the Raman spectra in Figure 2a, the G and D peaks appear at 1594 and 1352 cm–1 for GO, whereas they appear at 1582 and 1352 cm–1 for CNT–rGO gel, respectively. The downshifted G peak position from 1594 to 1582 cm–1, which moves more closely to the position for the pristine graphene (∼1580 cm–1), indicates an enlarged graphitic area after the gelation process. The D/G peak intensity ratio (ID/IG) is commonly used to evaluate the reduction of GO into rGO. The ID/IG increased from 0.88 to 1.27 as GO was reduced to rGO. The increase of ID/IG is due to the fact that the average size of the sp2 domains was decreased upon reduction of the exfoliated GO and new graphitic domains were created that are smaller in size to the ones present in GO before reduction but more numerous in number.34 C 1s XPS spectra of the GO and CNT–rGO gel reconfirm the successful reduction of GO (Figure 2b). The three peaks shown for GO represent the graphitic structure (C–C/C=C at 284.6 eV), hydroxyl/epoxy groups (C–O at 286.7 eV), and carbonyl group (O–C=O at 288.5 eV). In contrast to the XPS spectrum of GO, the XPS spectrum of the CNT–rGO gel shows a significant reduction of peak intensities for oxygen functional groups, indicating a high-level reduction of GO after gelation.25,35 Similarly, Fourier transform infrared spectroscopy (FT-IR) spectra of GO, rGO, and CNT–rGO implies the removal of oxygen functional groups after the reductive gelation process (Figure S3).36,37

Figure 2.

Figure 2

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Raman spectroscopy (a) and XPS (b) of a GO thin film prepared using the drop-casting method and CNT–rGO@F prepared using the Zn-induced graphene gelation method.

CNT–rGO@F displays electrically conductive behavior due to the successful reduction of GO into rGO. A higher GO concentration speeds up the gelation process and thus leads to a greater thickness after a specific gelation time (Figure 3a). A thicker hydrogel is more conductive; however, when the gel becomes too thick, conductivity loss at applied strain becomes greater (Figure S4). The activity of GO dispersion is an influencing factor for gelation kinetics and the final pore structure of the rGO hydrogel (Figure 3b). When HCl concentration becomes too high, the rate of layer-by-layer deposition increases due to the accelerated electron supply from the Zn plate to the GO platelets, causing the resultant rGO hydrogel to become mechanically unstable. The final optimized CNT–rGO@F displays a linear electrical resistance of 1.90 kohm/cm (Figure 3a) and a conductivity retention of 2.53 (Figure 3c). Conductivity retention is calculated by the following equation.

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Figure 3.

Figure 3

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Electrical resistance and conductivity retention of CNT–rGO@F composites. (a) Change of the electrical resistance of CNT–rGO@F with GO concentration for the gelation process. (b) Change of the electrical resistance of CNT–rGO@F with HCl concentration for the gelation process. (c) Conductivity retention at 50% tensile strain of CNT–rGO@F for various CNT concentrations. (d) Conductivity retention of rGO@F and CNT–rGO@F during one cycle of stretching up to 50% tensile strain and releasing.

Figure 3d clearly shows the enhanced effect of the CNT–rGO@F conductivity due to the introduction of CNT. The CNT–rGO@F displays enhanced electrical properties in terms of both conductivity and conductivity retention. Even at a high elongation condition, carbon nanomaterials remain attached to the fiber surface due to the π–π interaction between the conjugated structure on the carbon nanoparticles and the benzene rings on the fiber polymers, as reported in the previous literature.38 Notably, the CNT–rGO@F exhibits very little conductivity loss even at an applied tensile strain of 50%, as shown in Figure 3d. An electrical resistance of 1.14 kohm/cm was observed for CNT–rGO@F stretched to 50% of its initial length. Conductivity retention calculated for CNT–rGO@F was 1.34, in contrast to 2.53 for rGO@F, showing ∼2 times enhancement in strain fault tolerance. It is interesting to note that conductivity is recovered when the strain is released for both rGO@F and CNT–rGO@F. This recovery behavior indicates that a tensile strain up to 50% does not result in a permanent loss of electrical contact and suggests that conductivity can be maintained after repeated cycles of stretching.

Using the fabricated CNT–rGO@F conductive textile as electrodes, a wearable supercapacitor was assembled using the sandwich setup commonly used for a wearable supercapacitor assembly, as shown in the Supporting Information Figure S5. Poly(vinyl alcohol) (PVA) gel electrolyte was used as the electrolyte for this system. A clean spandex fabric was first immersed in the gel electrolyte and used as the stretchable separator for this system. CNT-rGC@F textiles were immersed in the PVA gel electrolyte for more than 4 h in a vacuum pump desiccator for efficient solvent exchange (water molecules in the hydrogel replaced by the PVA gel electrolyte). A Pt foil was used as the electrical contact and a customized tensile stretching machine was used to test the electrochemical performance of the supercapacitor while applying tensile strain.

Figure 4 presents various electrochemical measurements that show clear capacitive behavior of the as-assembled supercapacitor. Figure 4a shows the cyclic voltammograms (CVs) of the CNT–rGO@F supercapacitors, displaying an almost rectangular shape clearly indicative of an electrical double layer capacitor with a negligible resistive element in the electrodes up to a scan rate of 100 mV/s. The triangular shapes of the galvanostatic charge–discharge curves in Figure 4b indicate the low internal resistance of the supercapacitor. The areal capacitance calculated using the discharge curve after the IR drop at a current value of 0.5 mA (0.1 mA/cm2) was 10.13 mF/cm2. The Ragone plot in Figure S6 compares the areal power and energy densities of the CNT–rGO gel capacitor with other stretchable electrochemical capacitors reported thus far. The maximal areal energy density and power density were calculated to be 1.41 μWh/cm2 and 0.061 mW/cm2 at 0.1 mA/cm2, respectively. The electrochemical impedance analysis was conducted from 10 mHz to 100 kHz with a 10 mV sinusoidal voltage input. Ideal supercapacitors should display the impedance spectrum perpendicular to the real axis at a low frequency in the Nyquist plot.39,40 The quasi-vertical line at low frequencies in the impedance analysis indicates that the CNT–rGO@F-based supercapacitor efficiently stores the given electrical energy as chemical energy by forming an electrical double layer, i.e., the adsorption of charged ions, rather than the redox reaction. Solution resistance (Rs) and charge transfer resistance (Rct) were estimated with a simplified Randles cell consisting of Rs, Rct, and Cdl (double layer capacitance), depicted in the inset image of Figure 4c. Rs estimated from the intercept of high-frequency impedance spectrum with real axis (inset of Figure 4c) was measured to be 2.94 Ω. The low Rs value originates from the fluent ion transport via a hydrated open porous structure of the CNT–rGO framework.41Rct resulting from the redox reactions of residual oxygen functional groups in CNT–rGO hydrogels was determined to be 2.3 Ω from the radius of the semicircle in the high-frequency region.

Figure 4.

Figure 4

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Capacitive behavior of the CNT–rGO@F-based supercapacitor. (a) Cyclic voltammograms for various scan rates of 10, 50, and 100 mV/s. (b) Charge–discharge curves for various current densities of 0.1, 0.2, and 0.4 mA/cm2. (c) Electrochemical impedance analysis of the CNT–rGO@F-based supercapacitor.

For the CNT–rGO@F-based supercapacitor to be utilized in wearable electronics, the device should demonstrate flexibility and high bending fatigue. Figure 5a presents the capacitive performances of the CNT–rGO@F supercapacitor bent to different bending radii. The bending radius of 18 mm indicates a nearly flat setup, while a bending radius of 1.4 mm represents a supercapacitor that was almost completely folded. It can be seen that even when the supercapacitor is almost completely folded, the capacitive performance remains nearly unchanged, showing a capacity retention of 95.2% (Figure 5b). Bending fatigue was measured by repeating the bending cycles up to 1000 times, with each cycle bending the supercapacitor to a bending radius of 1.4 mm. After the first bending cycle, a capacitance loss of 10% was observed, possibly due to a permanent structural damage that occurred at the bending point, where the bending strain is the highest. However, subsequent capacitance loss after the first bending cycle was marginal. Total capacitance loss after 1000 cycles of folding and unfolding remained at 14%, as shown in Figure 5c. Putting aside the loss associated with plastic deformation from the first bending cycle, the capacitance was retained to 93.3% even after 999 cycles of repeated folding and unfolding, demonstrating high bending fatigue.

Figure 5.

Figure 5

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Electrochemical characterization under bending and stretching. (a) Cyclic voltammetry measurements of the CNT–rGO@F-based supercapacitor bent to various bending radii. (b) Normalized capacitance of the supercapacitor as a function of bend radius. (c) Cyclic voltammetry measurements of the CNT–rGO@F supercapacitor after multiple cycles of bending and unbending (inset) and normalized capacitance of the supercapacitor as a function of bending cycles. (d) Cyclic voltammetry measurements of the CNT–rGO@F supercapacitor stretched to various strain levels. (e) Normalized capacitance of the supercapacitor as a function of applied strain. (f) Cyclic voltammetry measurements of the CNT–rGO@F supercapacitor after multiple cycles of stretching and releasing (inset) and normalized capacitance of the supercapacitor as a function of stretching cycles.

Human skin can experience tensile strain up to 55% during the motion of walking.9 For wearable devices to conform to the shape of the human body and stay functional during active motion, they should also accommodate such tensile strain. To test the feasibility of the CNT–rGO@F conductive textile as stretchable supercapacitor electrodes, capacitive performances of the assembled supercapacitor were observed for various tensile strains. A customized tensile stretching machine was used to apply various strains to the supercapacitor during electrochemical measurements (Figure S7). Performance degradation after the first stretching cycle was more obvious than degradation from bending. Despite our efforts to maximize the conductivity retention of a CNT–rGO@F textile at high applied strain by incorporating mechanically flexible and stretchable CNTs, slight conductivity loss at a high strain level was inevitable. When the CNT–rGO@F supercapacitor was stretched, the overall resistance of the device increased accordingly and permanent damage to the rGO framework was sustained, leading to more resistive behavior and loss of capacitance. The capacitance of the CNT–rGO@F supercapacitor was decreased to 33.6% of its original value after the first stretching cycle stretched up to 50% of its original length. However, similar to the bending analysis, subsequent stretching cycles after the first cycle showed a much improved capacitance retention. Normalizing the capacitance by the capacitance after the first stretching cycle (33.6% value), subsequent stretching up to 50% strain resulted in a capacitance retention of 90.0% (Figure 5d, e), which was almost completely recovered when the strain was released.

Although some capacitance loss at high strain is inevitable, wearable devices do not remain stretched at all times. Stretching occurs only during active bodily motion, which means when our body is at rest, the applied strain on the device would be released. Since the CNT–rGO@F textile shows recovery of electrical conductivity when the strain is released, the CNT–rGO@F supercapacitor was also expected to show similar recovery behavior. The CV curves for supercapacitor after repeated cycles of stretching to 50% strain are shown in Figure 5f. It can be seen that although the capacitance gradually decreases with repeated stretching cycles, capacitive behavior is fairly maintained even after 1000 stretching cycles. Capacitance was retained at 90.4% after 1000 stretching cycles relative to the capacitance of the supercapacitor after the first stretching cycle. Interestingly, the recovery of capacitance did not happen immediately. This slow recovery behavior of a textile supercapacitor was also reported in previous work by Jost et al.42 In his work, the test was performed to determine whether or not continued degradation would occur from mechanical deformation by testing the device after a long resting time. After 6 h of rest, the fabric supercapacitor regained some capacitance. It was suggested that this recovery behavior is due to the slow contraction of the textile supercapacitor back to its original dimensions. Repeated stretching and releasing motion up to 2000 cycles resulted in a capacity retention of 75.0%. Taking account of the recovery behavior and the fact that wearable devices are not stretched at all times, this textile supercapacitor shows promise as a next-generation wearable energy storage system.

Conclusions

We have demonstrated a stretchable conductive textile for wearable supercapacitors based on the synergistic properties of graphene, CNTs, and stretchable spandex fabric. A unique Zn-induced interfacial gelation principle was used as a novel approach to integrate a highly porous conductive carbon framework with a large area of woven fabric in one facile fabrication step. The porous structure of the conductive CNT–rGO framework, one-dimensional structure of the conductive CNT, and stretchable spandex platform create a synergistic effect that results in a stretchable, conductive textile with a large surface area suitable for stretchable energy storage applications. A CNT–rGO textile-based stretchable supercapacitor showed an energy storage capability of 10.13 mF/cm2 and retained the capacitive performance after multiple bending and stretching cycles. This low cost, lightweight, easy to synthesize, highly scalable, and stretchable smart textile holds great promise for next-generation wearable electronics and energy storage applications.

Experimental Section

Interfacial Gelation of rGO with CNTs

Aqueous GO dispersion (3 mg/mL) was prepared from graphite (Bay Carbon) following a modified Hummers' method. A stretchable and nonconductive spandex textile made of polyurethane and polyester copolymer was provided by KOLON Corporation. Fabric(spandex)/zinc templates (Alfa Aesar) were immersed in the GO dispersions with a predetermined amount of HCl (10–3 to 10–2 M) for the desired gelation time. Surface-grown rGO hydrogel was thoroughly washed with deionized water to remove unreacted physisorbed GO flakes. The resultant rGO hydrogels were immersed in a 5% HCl solution for enough time to separate the hydrogels from the zinc template. The free-standing hydrogel was additionally immersed in a 5% HCl solution for 2 h to remove residual zinc impurities. The purified gel was kept in deionized water for 2 h to remove acidic residues. For CNT–rGO hydrogel, sodium dodecylbenzenesulfonate (SDBS) surfactant was used to uniformly disperse CNTs in the GO solution. The heterogeneous mixture was thoroughly mixed with a vortex mixer and 1 h of additional sonication. As-prepared mixture dispersion was employed for interfacial gelation under identical conditions with pure GO dispersion.

Materials Characterization

The morphology of the prepared gels was characterized by a field emission scanning electron microscope (SEM) Hitachi S-4800 (Hitachi). XPS analyses were carried out with K-Alpha (Thermo VG Scientific, Inc.). Raman spectroscopy was performed with a ARAMIS (HORIBA Jobin Yvon) dispersive Raman spectrometer using a 514 nm green laser beam. Fourier transform infrared spectroscopy (FT-IR) was performed with Nicolet iS50 (Thermo Fisher Scientific Instrument) spectrometer. All materials characterizations were carried out with freeze-dried CNT–rGO@F aerogel samples.

Electrochemical Characterization

All electrochemical analyses were carried out with VSP (BioLogics). Using the fabricated rGC@F conductive textile as electrodes, the stretchable supercapacitor was assembled using the setup commonly used for stretchable supercapacitor assembly in the previous literature. The poly(vinyl alcohol) (PVA) gel electrolyte that was used as the electrolyte was synthesized by mixing PVA with H2SO4 acid and deionized water using a ratio of 1: 1 g: 10 mL, by mechanical stirring on a hot plate at 90 °C. A clean spandex fabric was first immersed in the gel electrolyte and used as the stretchable separator for this system. CNT–rGO@F textiles were immersed in the PVA gel electrolyte for 4 h. By sandwiching two CNT–rGO@F electrodes and a fabric separator together, the stretchable textile supercapacitor was assembled. A Pt foil was used as the current collector and a customized tensile stretching machine was used to test the electrochemical performance of the supercapacitor while applying tensile strain.

Acknowledgments

This work was supported by a grant from Kyung Hee University in 2020 (KHU-20201240).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c02091.

  • SEM images; FT-IR; Ragone plot; and optical images (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c02091_si_001.pdf (1,007.7KB, pdf)

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