1. Introduction

Wearable technologies that can be worn by a person have become more popular in modern society[1,2]. They include smart phones, wearable electronics, sensors, and smart fashions[3-5]. One of the major challenging aspects in wearable technologies is power supplies of energy storage or conversion devices[6-10]. The power supply devices should exhibit the following features. (1) The energy devices can provide high energy and power performances with high safety. (2) They are able to be flexible or wearable and combine with wearable devices. In order to satisfy these requirements, supercapacitors have been developed extensively because of their high power density, fast charge/discharge rate, long cycle life, and simple power management. Along with high performance of supercapacitors, much attention has been also focused on the cell design and structure because the energy devices need to be installed within a limited area on human body[11,12]. Layered, planar, and cable designs have been mainly developed for wearable supercapacitor devices. Unlike layered and planar structures, the cable types of supercapacitors could enhance volumetric and area-capacitances and mechanical properties. In addition, the cable design enables to easily incorporate supercapacitors into textile-based cloths and wearable electronic devices[13-18].

Graphene-based materials have been widely explored as electrode materials for design and fabrication of supercapacitor devices[19-21]. The two-dimensional graphene sheets provide high surface area, high electrical conductivity, excellent mechanical strength, and high specific capacitance. The development of solution-based synthesis of graphene, which include graphene oxide (GO) and reduced graphene oxide (RGO) sheets, enabled that GO (or RGO) can be easily assembled into one-dimensional (1D) fibers and coated onto the surface of 1D current collector substrates[18]. These features enabled RGO-based supercapacitor cables to provide better electrochemical performances than other carbon materials of activated carbons and carbon nanotubes. However, the specific surface area of RGOs decreased usually during the chemical reduction of GO because of the π-π stacking between RGO sheets. One of the most promising approaches is construction of three-dimensional (3D) and porous structure of RGO electrodes to address the restacking and aggregation of RGO sheets[22,23]. Although self-assembly and template-assisted methods have been developed for 3D porous RGO materials, it is difficult to fabricate 3D porous frameworks on 1D current collector substrates because they usually require high temperature and pressure or structure guiding materials.

Herein, we report a straightforward and efficient method for preparing 3D porous wire electrodes of RGO-coated carbon thread (CT) based on microwave-activated process. The supercapacitor device was fabricated by combining two wire electrodes of microwave-treated RGO/CT (m-RGO/CT) coating with a polyvinyl alcohol-Na₂SO₄ gel electrolyte in the form of cable design. The microwave irradiation employed for wire electrodes enabled to convert GO into RGO without any reducing agents and activate RGO sheets into exfoliated and porous 2D sheets. The resulting supercapacitor cable is highly flexible and can be integrated into various shapes of devices or textiles. Various electrochemical techniques were performed on surpercapacitor cables, showing a high area-capacitance of 38.1 mF/cm, high rate capability of 88% at 500 mV/s, and good long-term stability (96.5% retention).

2. Experimental

2.1. Materials

Graphite powder (< 20 μm) and hydrazine were purchased from Aldrich (USA). Carbon cloth was purchased from Fuel Cell Earth (USA).

2.2. Synthesis m-RGO/CT electrode and preparation of supercapacitor cable

A piece of CT (> 10 cm) was extracted from carbon cloth and was cleaned by acetone, ethanol, and deionized (DI) water. CT sample was immersed in GO solution (5 mg/mL). GO was synthesized by previous report using the modified Hummers method[21]. GO-coated CT (GO/CT) was exposed to microwave irradiation for 20 s. After microwaving GO was converted into RGO, resulting in m-RGO/CT wires. As a control sample, RGO/CT was prepared by chemical reduction of GO/CT using hydrazine solution. A supercapacitor cable device was preparing by combining two electrodes of m-RGO/CT with a polyvinyl alcohol-Na₂SO₄ gel electrolyte in the form of cable design.

2.3. Characterization

Scanning electron microscopy (SEM) images were obtained using a field emission scanning electron microscope (S-4800). Fourier transform infrared (FT-IR) spectra were collected on a JASCO FT-IR 4,600. Each spectrum was recorded from 4,000 to 400 /cm. Electrochemical characterization was performed using a a VersaSTAT 4 (Princeton Applied Research). All electrochemical measurements were performed at room temperature, and the obtained data were within the error range of ±1%. A conventional three-electrochemical system was constructed with working electrodes of m-RGO/CT, an Ag/AgCl (3 M KCl) reference electrode, and a platinum wire counter electrode. The electrolyte was 1M Na₂SO₄ solution. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge (GCD) measurements were tested. The specific line capacitance (C_L) of electrode materials was calculated from the discharge curve of GCD according to the equation[21,23]: C_L = IΔt/ΔU

where, I is current density (A/g), U is the potential window (V), and t is the time (s). The energy (E) and power (P) densities of supercapacitor cable devices were calculated using the following formulas[ 21,23]: E = 1/2C_F(ΔV)² and P = E/Δt

where, C_F is the cell capacitance (F/cm) and V is the cell voltage (V).

3. Results and Discussion

The experimental procedure of supercapacitor cable based on three-dimensional RGO-coated carbon thread using microwaves is illustrated in Figure 1a. Carbon thread was obtained by extracting carbon cloth. The CT is a flexible wire composed of multiple carbon microfibrils bundled together (Figure 1b). Prior to prepare 3D RGO-CT, CT was coated with GO sheets via simple immersion in a GO solution and drying process (Figure 1c). GO sheets are strongly interacted with CT surface by van der Waals interactions. GO-coated CT was exposed to microwave irradiation in a commercial microwave oven (2.45 GHz and 700 W) during 20 s. GO sheets readily absorbs microwave radiation and accelerated high temperature, and thus microwave-reducing GO into RGO. Moreover, The microwave irradiation efficiently exfoliated and activated RGO sheets with high surface area and porous structure. SEM image of Figure 1d shows the microstructure of m-RGO/CT. Obviously, exfoliated RGO sheets were coated uniformly onto the surface of CT. The cross-linked networks of RGO sheets created large pore structure. And, microwave activation formed small-sized pore structure on RGO sheets. Pores were generated randomly and their size varied from macro- to nanometers. Longer irradiation times (> 20 s) may cut the carbon thread by microwave sparking or arcing. This microwave-activated process is a fast and simple method for preparing 3D porous RGO-CT electrode materials. After combining two electrodes with a polyvinyl alcohol (PVA)-gel electrolyte, a cable-type of supercapacitor device was completed (Figure 1e). To investigate the effect of activated RGO on supercapacitor performances, we also prepared chemically modified GO-coated CT (RGO/CT) by chemical reduction of GO (using hydrazine) without microwave irradiation.

The conversion of GO into RGO using microwave irradiation was demonstrated by FT-IR spectroscopy (Figure 2). GO shows strong absorption peaks relating to the oxygenated groups. A prominent peak of νO-H at 3,374 cm^-1 is due to the stretching vibration of the hydroxyl groups located on the plane of GO. Carboxyl groups at the edges of GO sheets were observed at a peak of ν_C=O (stretching vibration mode and 1,727 cm^-1). In addition, the epoxy ν_C-O stretching vibration appeared at a peak of 1090 cm^-1. After microwave exposure to GO, successful deoxygenation was observed at m-RGO/CT sample. This indicates that microwave irradiation reduces oxygenated groups within a few seconds.

Three-electrode system in electrochemical analysis provided electrochemical properties of m-RGO/CT as electrode materials for supercapacitor applications. The m-RGO/CT, Pt wire, and Ag/AgCl were used as working, counter, and reference electrodes, respectively. 1 M Na₂SO₄ was used as electrolyte. Control samples of a-CT and RGO/CT electrodes were also tested. Cyclic voltammetry (CV) measurements were performed in the potential rangeof 0-1 V (vs. Ag/AgCl) at various scan rates of 10-500 mV/s. All electrodes showed near rectangular shapes of CV curves, indicating an ideal electrical double-layer capacitance behavior. When comparing CV curves measured at a scan rate of 50 mV/s, CV area of m-RGO/CT increased dramatically compared to CT, which is indicative of substantially increased specific capacitance.

Figure 3a shows CV curves for bare CT and m-RGO/CT electordes measured at a scan rate of 50 mV/s. Both samples showed a rectangular shape of CVs, indicating an ideal electrical double layer capacitance behavior. The m-RGO/CT had a higher CV area than bare CT, which is indicative of a higher capacitance of m-RGO/CT due to the m-RGO. As the scan rates were increased, CV areas were also increased and showed rectangular shapes (Figure 3b). The specific capacitance values of m-RGO/CT electrodes obtained from CV curves were plotted with the scan rates (Figure 3c). The highest area-capacitance of 38.1 mF/cm for m-RGO/CT electrode was obtained at a scan rate of 10 mV/s. This value is higher than that of RGO/CT (27 mF/cm). The maximum capacitance of m-RGO/CT was slightly decreased as the scan rates were increased and reached to 33.7 mF/cm with 88% retention of its original value. The capacitance retention of 88% is higher than that RGO/CT electrode (64% retention). The enhanced capacitance and rate capability of m-RGO/CT was attributed to the enhanced ion transfer through 3D porous pathways even at high charge/discharge process. Long-term stability is another key factor for development of supercapacitor devices. Cycling performance was carried out using CV measurement. Figure 3d shows capacitance retention measured at a scan rate of 50 mV/s over 10,000 cycles. During the charge/discharge processes, the initial capacitance was almost retained. After 10,000 cycles, m-RGO/CT electrode had 99% retention of the initial capacitance, indicating an excellent cycle life. Electrochemical impedance spectroscopy was performed to demonstrate the superior performance of m-RGO/CT compared to RGO/CT. Figure 4 shows Nyquist plots of m-RGO/CT and RGO/CT electrodes. The m-RGO/CT exhibits smaller charge transfer resistance and more vertical impedance line than those of RGO/CT. This result indicates that the pore structure of m-RGO facilitates charge transfer and ion diffusion behavior at the electrode/electrolyte interface. Cable-type of supercapacitor device was fabricated by integrating two m-RGO/CT wire electrodes into a single unit using a gel electrolyte of PVA-Na₂SO₄. Two m-RGO/CTelectrodes were tightly incorporated in a gel electrolyte without any holes and cracks. The supercapacitor cables are high flexible, and thus can be easily subjected to various shapes and geometries (Figure 5a). Figure 5b shows CV curves of supercapacitor cables measured at 50 mV/s. A rectangular shape of CV indicates electric double-layer capacitance. The same CV curve was observed even at mechanically wrapped state (Figure 5b). The galvanostatic charge/dischage (GCD) was also tested at different current densities of 0.2 - 1 mA/cm (Figures 5c and 5d). Symetric charge and discharge curves were consistent with CV curves. Based on GCD curves and linear equation, the supercapacitor cable had a maximum area-capacitance of 16.5 mF/cm at 0.2 mA/cm. As the current density increased, the maximum capacitance of 16.5 mF/cm decreased to 11.3 mF/cm measured at 1 mA/cm with a rention of 68% of the initial capacitance. This high area-capacitance and high rate capability were attributed to the open pore structure and high electrical conductivity of 3D graphene frameworks. The supercapacitor cables were further tested at an applied constant current density of 1 mA/cm over 10,000 cycles under mechanically normal and wrapped states (Figure 5e). During the charge/discharge processes, 96.5% of its original capacitance was maintained, indicating excellent long-term stability. Acheiving high energy density of supercapacitors is another aspect for development of all-solid-sate supercapacitor devices. The symmetric and linear curves of GCD allows us to calculate energy and power densities based on the total area of both electrodes. The obtained energy and power densities were displayed in a Ragone plot of Figure 6. As the energy densities were decreased, the power densities were increased. The highest energy density of 56.5 mWh/cm was obtained at a power density of 1.6 W/cm.

4. Conclusion

Highly flexible solid-state supercapacitor cables were fabricated by closely assembling two 3D porous RGO-CT electrodes in a parallel orientation using a gel electrolyte of PVA-Na₂SO₄. 3D porous RGO frameworks onto the surface of carbon thread were formed by coating with GO and subsequent microwave exposing. The high surface area and porous structure of RGO-CT electrodes are favorable for fast and efficient electron and ion transfer during electrochemical reactions. The resultant supercapacitor cables exhibited a high specific capacitance, high rate capability, and long cycle life. Furthermore, high energy (56.5 mWh/cm) and power (2.29 W/cm) densities achieved when measuring galvanostatic charge/discharge curves. Along with excellent electrochemical performances, the supercapacitor cable is flexible and tunable to wearable electronics and textiles, and thus could meet with the requirements of wearable technologies.