Graphene/cotton composite fabrics as flexible electrode materials for electrochemical capacitors

Abstract

Graphene/cotton composite fabrics were successfully synthesized via a facile “dipping and drying” process followed by a NaBH₄ reduction method. The flexible 3D conductive network constructed by graphene sheets greatly enhances the conductivity of cotton fabrics. The morphology, structure and conductivity of the graphene/cotton composite fabrics were detailed characterized using scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FTIR) and a standard four-point probe method. Electrochemical measurements demonstrated that the graphene/cotton composite fabrics were used as flexible electrodes for electrochemical capacitors showing good capacitance, excellent mechanical flexibility, and good stability, which may lead to their future potential application in flexible and wearable electronic devices.

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1. Introduction

Developing a sustainable and renewable energy future has been one of the most important tasks for worldwide scientists and engineers in order to address the rapidly increasing global energy consumption coupled with the critical issue of climate change.^1–4 With increased renewable energy production, efficient energy storage systems are needed to make the best of the electricity generated from the renewable sources.⁵ Among the various energy storage systems, the electrochemical capacitor is one of the most promising energy-storage devices. Based on electric double-layer capacitance and pseudo-capacitance, electrochemical capacitors process great merits including fast charge–discharge, high power density, safe operation, great cycling stability and reliable cycling life.^6,7 Electrochemical capacitors have already showed high potentials in memory backup systems, portable consumer electronic products, hybrid electric vehicles, and industrial scale power and energy management.^7,8

Recently, flexible and wearable electronic devices that are suitable for arbitrary installation and applications have attracted great attention, which are desirable for developing lightweight, thin, and flexible portable/wearable electronics, such as rollup displays, electronic paper, and wearable systems for personal multi-media.^5,6,9–12 All these electronic applications require cheap, flexibility, light-weight, wearable power conversion and storage devices. However, the conventional electrochemical capacitors cannot effectively meet the requirements. As a type of flexible energy device, flexible electrochemical capacitors are extensively studied.^6,13,14 Cui et al., reported the synthesis of conductive cotton fabric using a SWNT ink method.^15,16 The electrochemical capacitor shows high specific capacitance (∼70–80 F g⁻¹ at 0.1 A g⁻¹) and good cycling stability. Qu et al., reported a electrochemical capacitor that is constructed using carbonized cotton mats as electrodes, which can be bent, rolled-up, and fully folded without loss of high-rate capacitive performance.⁵ Gogotsi et al., synthesized a flexible and lightweight fabric electrochemical capacitors electrode using a traditional printmaking technique (screen printing).⁹ Electrodes coated with activated carbon achieved a specific capacitance of 85 F g⁻¹. Yan et al., reported the synthesis of a nanocomposite electrode consisting of graphene sheets and cotton cloth using a “brush-coating and drying” technique.¹ The electrochemical capacitors showed a specific capacitance of 81.7 F g⁻¹. However, most reported methods for preparing flexible electrode materials consisted of the cotton fabrics and carbon materials always involve the use of organic surfactant or a high temperature treatment process, which inevitably influence the electrochemical performance or the practical production of flexible electrode materials.^2,9,15–17 Therefore, developing a facile synthesis route of flexible carbon/textile is still a great challenge.

In this work, we presents the synthesis of graphene/cotton composite fabrics via a facile “dipping and drying” process followed by NaBH₄ reduction method. The organic surfactant and high temperature treatment process are not needed in this synthesis method. The fabrication process is simple and scalable, similar to those widely used for dyeing fibers and fabrics in the textile industry. The graphene sheets can be firmly adsorbed on the cotton fibers after converting graphene oxide sheets. The graphene sheets coating makes these cotton fabrics conductive, with a sheet resistance of 560 Ω sq⁻¹. The electrochemical measurement results show that the graphene/cotton composite fabrics as electrodes have good capacitance and excellent mechanical flexibility. The combination of flexibility, electrical conductivity and electrochemical activity makes such graphene/cotton composite fabrics attractive for flexibly portable and wearable electronic devices.

2. Experimental section

2.1 Synthesis

2.1.1 Preparation of graphene oxide. Graphene oxide (GO) was prepared and purified according to the modified Hummer's method.⁴ In brief, 1.5 g of graphite powder was added into an aqueous solution containing 10 mL of 98% H₂SO₄, 1.25 g of K₂S₂O₈, and 1.25 g of P₂O₅. Then the solution was maintained at 80 °C for 4.5 h. The resulting preoxidized product was cleaned using double-distilled water and dried in a vacuum oven at 50 °C. After it was mixed with 60 mL of 98% H₂SO₄ and a slowly added 7.5 g of KMnO₄ at a temperature below 20 °C, then 125 mL of double-distilled water was added. After 2 h, an additional 200 mL of double-distilled water and 10 mL of 30% H₂O₂ were slowly added into the solution to completely react with the excess KMnO₄. After 10 min, a bright yellow solution was obtained. The resulting mixture was washed with diluted HCl aqueous (1/10 v/v) solution and double-distilled water. GO was obtained after drying in a vacuum oven at 40 °C.

2.1.2 Preparation of GO/cotton composite fabrics. GO suspension with a concentration of 2 mg mL⁻¹ was prepared by dispersing GO powder in double-distilled water by a sonication process for 30 min.

The commercial cotton fabrics were cleaned by marinating in a boiled 1.0 mol L⁻¹ NaOH aqueous solution for 1 h, washed thoroughly with double-distilled water, and then dried at 120 °C in an oven. The cotton fabrics were dipped into the prepared GO suspension, soaked for 30 min at room temperature and then dried in a vacuum oven at 50 °C for 2 h. Because of the strong adsorption, the fabric was quickly coated by GO. The coating process was repeated twenty times in order to increase GO adsorption.

2.1.3 Preparation of graphene/cotton composite fabrics. The GO/cotton composite fabrics were immersed in 100 mL of 0.5 mol L⁻¹ NaBH₄ aqueous solution. The mixture was kept at room temperature for 12 h under stirring. The resulting fabrics were washed with double-distilled water 3 times and dried in an oven at 105 °C.

2.2 Materials characterization

The microscopic features of the samples were characterized by SEM (Hitachi S-4800 and JEOL JSM-5600LV). The fourier transform IR (FTIR) spectra was recorded on a Thermo Fisher Nicolet 6700 spectrometer. The mechanical property tests of GO/cotton composite fabrics and graphene/cotton composite fabrics were conducted with a YG(B) 033D tearing tester. The sheet resistance of the composite fabrics was measured by using a standard four-point probe method (4 Probes Tech, RTS-9).

2.3 Electrochemical test

Electrochemical test experiments were carried out using a three-electrode system, in which platinum wire and saturated calomel electrode were used as the counter and reference electrodes in an electrolyte solution of 1.0 mol L⁻¹ Na₂SO₄, respectively. The electrochemical characteristics were evaluated by cyclic voltammetry (CV) using a CHI 600E electrochemical analyser and galvanostatic charge–discharge (GCD) using a CHI 760D electrochemical workstation at room temperature. The specific capacitance was calculated from the CV and GCD curves based on the following eqn (1) and (2), respectively:¹Here, I is the constant discharging current (A g⁻¹); m is the total mass of graphene coated on cotton fabric (g); ν is the scan rate (mV s⁻¹); ΔV is the potential window during the discharge process after IR drop (V); Δt is the discharge time.

3. Results and discussion

The synthesis of graphene/cotton composite fabrics follows the steps described in Scheme 1. The cotton fabrics were immersed in GO suspension. GO was adsorbed on the cotton fibers due to the electrostatic interaction, van der Waals' force and hydrogen bond between the oxygen-containing functional groups on GO and cotton fibers, forming the GO/cotton composite fabrics.^2,18 Then, GO on the cotton fibers were converted to graphene sheets using NaBH₄ reduction method. The graphene/cotton composite fabrics with high conductivity and specific surface area are desirable for flexible electrode materials for electrochemical capacitors.

Scheme 1 Schematic representation of the preparation of graphene/cotton composite fabrics.

Fig. 1a shows the typical SEM image of GO with a size of several micrometers, revealing a wavy and wrinkled structure, as a result of deformation upon the exfoliation and restacking process. As shown in Fig. 1b, the morphology and size of graphene sheets are similar to that of GO after being reduced in 0.5 mol L⁻¹ NaBH₄ aqueous solution. Fig. 1c shows the XRD patterns of graphite, GO and graphene sheets. The raw graphite shows a very strong (002) peak at about 26°.¹⁹ The most intensive peak of GO at about 10° corresponds to the (001) reflection, and the interlayer spacing (0.87 nm) is much larger than that of pristine graphite (0.34 nm) due to the introduction of oxygen-containing functional groups on the graphite surface.^19–22 After being reduced in aqueous NaBH₄ solution, a gradual change in the XRD pattern achieves a randomly ordered carbonaceous layered solid, with basal spacing of 0.34 nm, indicating that the bulk of the oxygen-containing functional groups are removed from GO.²² This result clearly revels the successful transformation from GO to graphene. In the FTIR spectrum of GO (Fig. 1d), the peaks at 3430 and 1400 cm⁻¹ are assigned to the stretching vibration of O–H.²³ The peaks at 1716, 1631, 1225 and 1056 correspond to the stretching vibrations of C=O, C=C, C–OH and C–O–C, respectively. After NaBH₄ reduction, these oxygen-containing functional groups derived from the intensive oxidation were reduced significantly. The new peak at 1564 cm⁻¹ is ascribed to the skeletal vibration of the graphene sheets.^24,25 This result clearly revels the successful transformation from GO to graphene sheets.

Fig. 1 SEM images of (a) GO and (b) graphene sheets. (c) XRD pattern of graphite, GO and graphene sheets. (d) FTIR spectra of GO and graphene sheets. The inset in (b) is the TEM image of graphene sheets.

Fig. 2a shows the typical optical image of white cotton fabrics. After immersing in GO suspension and drying in a vacuum oven, GO was firmly absorbed on the cotton fabrics. The colour of cotton fabrics changed from white to brown after 20 dipping–drying cycles (see Fig. 2b). The loading of GO is around 9.3 wt%. As shown in Fig. 2c, the colour of GO/cotton composite fabrics further changed from brown to black when immersing in aqueous NaBH₄ solution. It reveals the GO on the cotton fabrics is successfully converted to graphene using NaBH₄ reduction method. Fig. 2d shows the formed graphene/cotton composite fabrics are flexible enough to be bent.

Fig. 2 The optical images of (a) cotton fabrics, (b) GO/cotton composite fabrics and (c and d) graphene/cotton composite fabrics.

FTIR spectra of cotton fabrics, GO/cotton composite fabrics and graphene/cotton composite fabrics have been presented in Fig. 3. The spectrum of cotton fabrics show some peaks at around 3340, 2900, 1648, 1428 and 1057 cm⁻¹, which are attributed to the OH stretching, CH stretching, OH of water absorbed from cellulose, CH₂ symmetric bending and C–O stretching, respectively.²⁶ Compared to cotton fabrics, the spectrum of GO/cotton composite fabrics shows a new peak at around 1632 cm⁻¹ is attributed to the carbon skeleton vibration of graphene. After converting GO to graphene, the peak at 1648 cm⁻¹ is almost absent. The appearance of peak at around 1568 cm⁻¹ is attributed to C=C skeletal vibration of graphene.^27,28

Fig. 3 FTIR spectra of cotton farbics, GO/cotton composite fabrics and graphene/cotton composite fabrics.

The sheet resistances of cotton fabrics, GO/cotton composite fabrics, and graphene/cotton composite fabrics were measured using a standard four point-probe method. Cotton fabrics and GO/cotton composite fabrics are insulated. Fig. 4a shows the effect of number of dipping–drying cycle on the sheet resistance of graphene/cotton composite fabrics. After coating graphene sheets, the cotton fabrics become conductive. The sheet resistance greatly decreased with increasing the number of dipping–drying cycle. After 20 dipping–drying cycles, the sheet resistance decreased to 611 Ω sq⁻¹. It indicates increasing the number of dipping–drying cycle can significantly improve the loading of graphene sheets, which facilitates the formation of 3D conductive network on the cotton fabrics. Fig. 4b shows the effect of NaBH₄ concentration on the sheet resistance of graphene/cotton composite fabrics. The sheet resistance significantly decreased with increasing the NaBH₄ concentration from 0.15 to 0.3 mol L⁻¹. When the NaBH₄ concentration is higher than 0.3 mol L⁻¹, the sheet resistance of graphene/cotton composite fabrics tends to be stable. The lowest sheet resistance was measured to be 560 Ω sq⁻¹ at a NaBH₄ concentration of 0.5 mol L⁻¹. Some reported results have shown that high concentration of reducing regent can more effectively remove oxygen-functional groups of GO for its conversion into conductive graphene sheets.²⁹ After completely removing the oxygen-functional groups, the excess NaBH₄ has no significant effect on further improving the conductivity of graphene/cotton composite fabrics. The effect of reduction time on sheet resistance was also studied. As shown in Fig. 4c, the sheet resistance significantly decreased with increasing the reaction time from 2 to 6 h, indication a rapid removal of the oxygen-functional groups of GO during the first stage of reduction. However, further increasing the reaction time above 6 h did not have significantly effect on the sheet resistance, indicating most oxygen-function groups of GO were removed completely. For further studying the effect of dipping and drying cycle and the NaBH₄ reduction process on the mechanical properties of cotton fabrics, the mechanical properties of GO/cotton and graphene/cotton composite fabrics were tested. Fig. 4d shows tensile strength of cotton fabrics, GO/cotton composite fabrics and graphene/cotton composite fabrics. The tensile strength of cotton fabrics was determined to be 1169 cN. The coating of GO and graphene sheets on the cotton fabrics decreased the tensile strength of cotton fabrics. It may due to the dipping and drying process that wreaks destruction on the structure of cotton fibers.

Fig. 4 The effects of (a) dipping–drying process, (b) NaBH₄ concentration and (c) NaBH₄ reduction reaction time on the sheet resistance of graphene/cotton composite fabrics. (d) The tensile strength of cotton fabrics, GO/cotton composite fabrics and graphene/cotton composite fabrics.

Fig. 5a and b shows the SEM images of cotton fabrics. It is clear that the cotton fabrics display a typically hierarchical pore network structure composed of micron-sized cotton fibers. As shown in Fig. 5c, graphene sheets coat the cotton fibers thoroughly and firmly after immersing in GO suspension for 5 dipping–drying cycles and subsequently being reduced by NaBH₄. High-magnification SEM image in Fig. 5d clearly shows the wavy and wrinkled structure of graphene sheets on the cotton fibers. Increasing the number of dipping–drying cycle has not significant effect on the morphologies of graphene/cotton composite fabrics and dispersion state of graphene sheets on the cotton fibers (see Fig. 5e–h).

Fig. 5 SEM images of (a and b) cotton fabrics. SEM images of graphene/cotton composite fabrics with different dipping–drying cycles: (c and d) 5 cycles, (e and f) 10 cycles and (g and h) 20 cycles.

The large specific surface area and high conductivity of graphene sheets on the cotton fabrics make the grahene/cotton composite fabrics a promising flexible electrode material for electrochemical capacitors. As shown in Fig. 6a, the graphene/cotton composite fabrics as electrodes show the CV curves at various voltage windows from 0.0–0.2 to 0.0–1.0 V at a scan rate of 5 mV s⁻¹. These CV curves exhibit nearly rectangular CV curves, showing ideal capacitive behavior. The specific capacitance increased with increase of the voltage windows. The specific capacitance was determined to be 40 F g⁻¹ within 0.0–1.0 voltage window. Fig. 6b shows the CV curves of the composite electrode at scan rates up to 200 mV s⁻¹ within 0.0–1.0 V voltage window. It can be seen that these curves have nearly ideal rectangular shape, indicating superior electrochemical capacitor behavior even at such high scan rate. Fig. 6c shows the specific capacitance decrease with increasing the scan rate.

Fig. 6 CV curves of graphene/cotton composite fabrics with different (a) voltage windows, (b) scan rates, and (d) bending times. (c) Specific capacitances at various scan rates. (e) Galvanostatic charge–discharge curves at different current densities. (e) Capacitance retention of graphene/cotton composite fabrics during cycling tests, the inset in (f) is the typical galvanostatic charge–discharge cures.

To further study the effect of mechanical bending on the capacitive characteristics, the graphene/cotton composite electrode was bent from 0° to 180°, and back to the initial state. Fig. 6d shows the CV curves of graphene/cotton composite electrode retain rectangular shapes and remain almost unchanged at a scan rate of 5 mV s⁻¹ after 100 times of bending. The specific capacitance remained stable during the bending process. This result demonstrates the composite electrode has excellent mechanical flexibility, implying that the synthesized graphene/cotton composite fabrics can be applied for textile flexible and wearable electronic devices. As shown in Fig. 6e, the galvanostatic charge–discharge curves of the graphene/cotton composite electrode present nearly triangular shapes at current densities of 0.51, 1.1 and 3.4 A g⁻¹, demonstrating an ideal electric double-layer capacitor behaviour. These results indicate the high conductivity and high specific surface area of graphene on the cotton fibers, and the hierarchical pore network structure of the composite fabrics greatly facilitate the fast transportation of electrolyte ions within the electrode materials. Galvanostatic charge–discharge test during 1000 cycles for the graphene/cotton composite electrode was carried out at a current density of 0.85 A g⁻¹. Fig. 6f and inset show the galvanostatic charge–discharge curves maintained nearly unchanged during 1000 cycles. The specific capacitance slightly decreases and then retains 90% of its initial capacitance, suggesting a good electrochemical cycling stability.

4. Conclusions

In summary, graphene/cotton composite fabrics were synthesized by using a facile “dipping and drying” process followed by NaBH₄ reduction method. The graphene sheets firmly adsorbed on the cotton fabrics have large specific surface area and high conductivity. It facilitates the formation of 3D conductive network on the cotton fabrics. The graphene/cotton composite fabrics were used as a flexible electrode for electrochemical capacitors, showing good capacitance performances. The graphene/cotton composite fabrics can be bent without significant loss of capacitive performance. It is believed that our strategy could be applied for the preparation of flexible and foldable electrochemical capacitor devices in future.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51402048, 51473032), the Fundamental Research Funds for the Central Universities, DHU Distinguished Young Professor Program, Shanghai Municipality Research Projects (13520720100), Shanghai Qianren Program and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

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