Functionalization of textile cotton fabric with reduced graphene oxide/MnO2/polyaniline based electrode for supercapacitor

In this work, a new cotton electrode has been synthesized by coating ternary materials of reduced graphene oxide (rGO), manganese dioxide (MnO2), and polyaniline (PANi) on textile cotton fabric. First, Graphene oxide was deposited on cotton fibers by a simple ‘dip and dry’ method and chemically reduced into rGO/cotton fabric. MnO2 nanoparticles were accumulated on rGO/cotton fabric by in situ chemical deposition method. PANi layer was coated on rGO/MnO2/cotton fabric by in situ oxidative polymerization technique. A thin PANi coating layer acts as a protective layer on rGO/MnO2/cotton fabric to restrain MnO2 nanoparticles and rGO from dissolution in H2SO4 acidic electrolyte. The specific surface area of cotton electrode was measured using the Brenauer-Emmett-Teller (BET) method. The cyclic voltammetry (CV) results show that the cotton electrode has good capacitive behavior. The ternary cotton electrode exhibits high specific capacitance values of 888 F g−1 and 252 F g−1 at a discharge current density of 1 A g−1 and 25 A g−1 in 1 M H2SO4 electrolyte solution. The high areal specific capacitance of 444 Fcm−2 was achieved for as-fabricated electrode. Also, the cotton electrode retains around 70% of specific capacitance after 3000 cycles at charge-discharge current density of 15 A g−1. The slow decrease in specific capacitance is observed with increased discharge current density which proves its excellent rate capability. These results of rGO/MnO2/PANi/cotton fabric electrode show that this can be an excellent electrode for supercapacitor in energy storage devices.

Briefly, a supercapacitor is an electrochemical device which can store high energy (electric charges) and release current density and capacitance within a short time interval. There are two categories of supercapacitors available: they are electric double layer capacitors (EDLC) and psuedocapacitors based on the charge storage mechanism. In EDLC, the storage of charge is achieved by separating electronic and ionic charges in the electrode and electrolyte interface. Alternatively, psuedocapacitors store charges by Faradaic reactions occur in the active materials of the electrodes. The carbon based materials are most common electrode for EDLCs. However, EDLCs are limited to low energy storage density. The conducting polymers and metal oxides can store greater amounts of energy in psuedocapacitors compared to an EDLC [14]. Many researchers are still working on the electrodes to enhance the electrochemical properties of the supercapacitors.
The functionalization of cotton fabric with graphene material can impart the electrical conductivity. The hydroxyl groups present on the cotton fiber surface provide active sites for functionalization with many additives which include graphene, graphene oxide, and carbon nanotubes [15][16][17]. The addition of graphene oxide onto cotton fabric leads to bind easily with the surface of fibers through interaction between the polar groups present on both cotton fiber and graphene oxide. There has been many research investigations reported on a flexible and light weight rGO/cotton fabric electrode based macroscopic supercapacitor. The electrode was fabricated by a combination of simple 'dipping and drying' method [18][19][20].
The transition metal oxides, MnO 2 , TiO 2 , and CuO [8,21] have been proved that they can be used as electrodes for supercapacitors. In particular, MnO 2 has its own advantages such eco-friendliness, low cost, high theoretical specific capacitance of 1230 mAhg −1 , and favorable cycling stability [7]. Xiao et al prepared carbon fabric composite electrode by chemically anchoring metal oxide nanoparticles (MnO 2 , SnO 2 , and RuO 2 )onto graphenenanosheets [8]. The prepared composite was coated over carbon fabric. The incorporation of MnO 2 nanoparticles improved the electrochemical performance. The cotton fabric surface contains exogenous groups which facilitates the uniform deposition rGOnanosheets and MnO 2 nanoparticles. Also, the uniform deposition prevents aggregation of rGO and MnO 2 nanoparticles [22,23].
In recent years, many investigations have been attempted on cotton based pseudocapacitors by coating conducting polymers, mainly polyaniline (PANi), polypyrrole (PPy) [24,25], and polythiophene [13]. Specifically, Polyaniline (PANi) has been extensively investigated in supercapacitor application because of its excellent theoretical specific capacitance of 2000 Fg −1 compared to polypyrrole. However, PANi has poor cycling stability which leads to rapid decrease of specific capacitance and resulting in short cycle life. Many research works have been carried out by the researchers to fabricate different PANi based composites by incorporating with carbon based nanomaterials and metal oxides for improving electrochemical properties such as specific capacitance and charge-discharge cycle stability of the electrode [24].
In this work, we prepared a ternary composite based cotton fabric electrode material for the first time in the supercapacitor application which consists of reduced graphene oxide (rGO), manganese dioxide (MnO 2 ) and polyaniline (PANi). The electrochemical properties of developed cotton fabric electrode were studied. The addition of rGO and PANi improve the electrochemical properties of cotton fabric electrodes. Also, metal oxide coated rGO increases the capacitance and cycling stability of the electrode. The combination of these materials together increase electrochemical properties such as specific capacitance, charge-discharge cycle stability and also energy density of the cotton fabric electrode.

Synthesis of graphene oxide (GO)
Graphene oxide was synthesized from pure graphite flakes by a modified Hummer's method as reported in our previous work [40]. Briefly, 1.2 g of Graphite flakes and 2 g of NaNO 3 , were mixed with 50 ml of H 2 SO 4 in a volumetric flask (500 ml) kept in an ice bath with continuous stirring for 2 h. Then, 6 g of KMnO 4 was added very slowly for about 1 h due to exothermic oxidation reaction. The slow addition leads to intercalation of functional groups due to oxidation of graphene layer which resulted in formation of graphitic oxide. After the addition of KMnO 4 , the mixture was diluted by adding 100 ml of de-ionized water into the mixture. The reaction temperature was quickly increased to 90°C and the sample mixture was stirred continuously for 24 h. Then, the mixture became brownish paste like material. The increase in temperature led to the exfoliation of graphite oxide into graphene oxide. Then, 8 ml of H 2 O 2 (30%) was added slowly to the mixture to react completely with the excess KMnO 4 under stirring. After 10 min, a bright yellow solution was obtained and it was then kept without stirring for 4 h, where the particles settled at the bottom and remaining solution was poured. The resulting mixture was washed repeatedly with 5% HCl solution several times to remove the metal ions from the solution and decanted the upper liquid part. Then, the mixture was washed many times with de-ionized (DI) water until the solution's pH becomes neutral. Finally, the solution was filtered and the paste like material was dried in vacuum oven at 60°C for 12 h. The dried graphene oxide (GO) was ground into powder.

Fabrication of rGO/MnO 2 /PANi coated cotton fabrics
The textile cotton fabrics are highly flexible, low cost and commercially available for clothing. However, cotton cannot be used as flexible electrode due to its insulating and low electrochemical activity. Functionalization of cotton fabric with electrically conductive and pseudocapacitance materials is necessary to achieve its electrochemical performance [16].
The commercial cotton fabrics have number of impurities which include dirt, seed coat fragments, pesticides, chemical residues, metallic salts and immature fibers. Among the various pretreatments of cellulosic textile materials, only scouring (cleaning) employ an alkaline agent in concentrated solution. The scouring or boiling-off process permits the removal of certain impurities with which the fiber is associated. The white plain woven cotton fabrics (120 gm −2 ) was pretreated by dipping in NaOH (40 gl −1 ) aqueous solution at 80°C for 1 h. The GO suspension ink was prepared by dissolving 2 mg of GO powder in 150 mL of de-ionized water under ultra-sonication for 30 min. The 2 cm 2 with 0.02 cm thickness size of pretreated cotton fabric was dipped into a GO suspension ink and soaked for 30 min to coat GO on to the cotton fabric and then vacuum dried at 60°C for 1 h. The dip-coating process was repeated for several times to achieve more GO adsorption on cotton fabric. The obtained GO/cotton fabric was partially reduced into rGO/cotton fabric, by a chemical method using aqueous solution of NaBH 4 . The chemical reduction process was carried out by immersing GO/cotton fabric in an aqueous solution of NaBH 4 (0.1 M) for about 5 h under continuous stirring condition. The obtained rGO/ cotton fabric was washed with de-ionized water and then vacuum dried at 70°C for 6 h. The electrostatic interaction, van der Waals' force and hydrogen bonding between cotton fabric and partial rGO facilitates the uniform coat and adhesion forces between them.
The prepared rGO/cotton fabric was immersed into a 250 ml flask containing 45 mL of 0.02 mol MnSO 4 aqueous solution and kept stirring for 15 min. Then, 30 ml of 0.02 mol KMnO 4 aqueous solution was added drop wise into the reaction mixture by stirring continuous for 6 h at room temperature. The color of rGO/cotton fabric turned from purple to brown which indicated the deposition of MnO 2 nano particles on the surface of rGO/cotton fabric. The obtained rGO/MnO 2 /cotton fabric (grayish blue) was then washed five times with de-ionized water to remove residual reactants and vacuum dried at 50°C for 4 h. The mass loading of MnO 2 was calculated from the difference in mass of rGO/cotton fabric and rGO/MnO 2 /cotton fabric electrodes.
Polyaniline was deposited onto the rGO/MnO 2 /cotton fabric by in situ chemical polymerization of aniline. In a typical process, rGO/MnO 2 cotton fabric electrode (2 cm 2 ) was immersed in 50 ml of de-ionized water and stirred the solution for 15 min to ensure that it was fully wet. Then, aniline (0.2 mol l −1 ) was added into the mixture containing 1 M HCl (10 ml) and was stirred for 2h. The oxidant, aqueous solution of ammonium persulfate (10 ml:0.2 M in 1 M HCl) was added to carry out the oxidative polymerization under continuous stirring. The in situ oxidative polymerization leads to coating of PANi on the rGO/MnO 2 cotton fabric electrode surface.
The reaction was carried out for 14 h and the color changed to bluish black and rGO/MnO 2 /PANi/cotton fabric was separated and washed with mixture of de-ionized water and ethanol for four times. The product was dried in a vacuum oven at 50°C for 10 h to obtain rGO/MnO 2 /PANi/cotton fabric electrode. The schematic synthesis roadmap of rGO/MnO2/PANi/cotton fabric electrode is shown in figure 1.
The change in color was identified which may be induced by the functionalization of cotton fabrics with rGO, MnO 2 nanoparticles, and PANi. Figure 2. shows the white LED lighted up by connecting the ternary coated cotton fabric electrode with electric voltage. The white LED connected to the prepared cotton electrode and electric voltage. LED light was glowing when electric current was passed through the cotton electrode.

Characterization
Fourier transform infrared (FTIR) spectroscopic results were obtained for different samples from a Perkin Elmer (Lambda 35) FTIR spectrometer with an ATR transmission mode in the wavelength range of 400-4000 cm −1 . Raman spectroscopy was conducted by Horiba JobinYvon t6400 instrument using a 532 nm laser source and in transmission mode in the wavelength range of 400-3000 cm −1 . The x-ray diffraction (XRD) data were obtained with Cu K α radiation (λ=0.1541 nm), an accelerating potential of 40 kV and 30 mA at a scanning rate of 0.5°/min on a Rigaku x-ray diffractometer. The Surface morphology of coated cotton fabrics with gold sputtered (5 nm) was investigated by Field Emission Scanning electron microscope (FESEM, Carl Zeiss Ultra 55) at accelerating voltage of 10 kV with energy and angle selective backscattered electron (EsB) detector. The elemental analysis was carried out by High-Angle Annular Dark-Field Scanning Transmission Electron Microscopic (HAADF-STEM) method using a JEOL 2100F microscope at 200 kV operating voltage. The specific surface area, pore volume and pore size of the electrode samples were measured by Brunauer-Emmett-Teller (BET) method with a BELSORP-mini II instrument. Electrochemical measurements, cyclic voltammetry (CV), Galvanostatic charge-discahrge cycle were studied by an Electrochemical Analyzer (Autolab-Ecochemie, Netherlands).   Functionalization of graphite into graphene oxide was confirmed by Raman spectroscopy. Figure 4(a) shows Raman spectra of graphite and graphene oxide and figure 4(b) shows for GO/Cotton, rGO/cotton, rGO/MnO 2 /cotton, and rGO/MnO 2 /PANi cotton. The GO exhibited a defect D-band due to carbon disorder at 1351.1 cm −1 and a peak appeared at 1602.7 cm −1 corresponds to shifted graphitic G-band due to sp 2 -bonded carbon as compared with graphite (1573cm −1 ). A broad shifted 2G-band observed due to phonon double resonance at 2765 cm −1 for graphite and this peak was disappeared in GO. The intensity ratio of the I D /I G was high compared to GO and other samples (GO/Cotton, rGO/cotton, rGO/MnO 2 /cotton, and rGO/MnO 2 /PANi cotton. Figure 4  The peak intensity ratio observed from rGO/cotton is calculated to be 1.06 which is slightly higher than GO/cotton i.e., 1.027. The increase in I D /I G ratio from 1.027 to 1.060 confirmed the reduction of GO to rGO. It can be attributed to a decrease in the average size of the sp 2 domains due to reduction of GO and also an increase in the fraction of graphene edges. After functionalization of MnO 2 with rGO, the peaks at 1347.06 cm −1 and 1597.47cm −1 for rGO shifted to 1341.82 cm −1 and 1607.6 cm −1 and in the case of rGO-MnO 2 also the intensity ratio has changed from 1.06 to 1.025 due to the suppression of vibrating species of MnO 2 surpasses the rGO band through stokes effect. This confirms the bonding of MnO 2 functionalization over the rGO surface and at the same time it can be inferred that the incorporation and intercalation of MnO 2 molecules intern to facilitates the π-π conjugation in PANi which in turn leads to the composite with good conductivity. X-ray diffraction patterns of GO, rGO/cotton, rGO/MnO 2 /cotton, and rGO/MnO 2 /PANi/cotton are shown in figure 5 (a). The XRD pattern of MnO 2 ( figure 5 (b)) was similar to that of figure (a) with additional peaks at 14.65 and 16.37 which indicate that there is an in situ growth of MnO 2 layer on the rGO/cotton surface. The diffraction peaks at 15.17 and 16.86 of 2θ value in the XRD pattern of rGO/MnO 2 /PANi/cotton also confirm that PANi covered the cotton fibers completely. The XRD pattern of MnO 2 nano particles prepared by in situ deposition method is provided in figure 5(b). It shows sharp peaks 2θ at 37.17°and 66.29°, which correspond to crystalline α-MnO 2 . Also, the appearance of broad peaks may be due to the partial presence of amorphous α-MnO 2 .          PANi was coated on rGO/MnO 2 /cotton through in situ polymerization and PANi was covered the rGO/MnO 2 coated cotton fibers as thin layer, as shown in figures 8(c), (d). Because the thick coating layers would block the diffusion of electrolyte ions to rGO and MnO 2 layers which results to low capacitance of the electrode. Finally, SEM images confirmed the transformation of morphology from rGO/cotton fabric to ternary sandwich structure of rGO/MnO 2 /PANi/cotton fabrics.
Energy-dispersive x-ray (EDX) spectroscopy was used to detect the composition of rGO/MnO 2 /PANi/cotton fabric by elemental mapping technique. Figure 9    The overall electrochemical performance of the synthesized electrocatalyst are directly associated with specific surface area and pore diameter of the cotton samples. The N 2 adsorption and desorption isotherms of rGO/MnO 2 /PANi/cotton fabric as shown in figure 10(a) results indicates that type -IV with H3 hysteresis loop curves between (0−1 p/p o ) relative pressure, which indicates the presence of mesopores and macropores features in samples. The pore size distribution of the rGO/MnO 2 /PANi/cotton fabric was evaluated by Barret-Joyner-Halenda model (BJH) as shown in figure 10(b) [41].
The result shows that the pore size distribution was mesoporous, and macroporous existed in rGO/MnO 2 /PANi/cotton and it shows that BET surface area was 26 m 2 g −1 . This results indicates the polyaniline coated on the MnO 2 and graphene sheet. This architecture facilitates the transportation of ions and electrons in the matrix easily and thus enhances the electrochemical performances of the cotton electrode.

Electrochemical properties
The electrochemical properties were studied experimentally by a three-electrode cell system which consisted of 1 M H 2 SO 4 aqueous solution as electrolyte, a Pt counter electrode, rGO/MnO 2 /PANi cotton fabric working electrode, and an Ag/AgCl reference electrode. The Cyclic voltammetry (CV) and Galvanostatic chargedischarge cycle were studied for the developed cotton fabric electrodes. Figure 11(a) shows CV curves of rGO/cotton, rGO/MnO 2 /cotton rGO/PANi/cotton, and rGO/MnO 2 /PANi/cotton fabric electrodes in the range of -0.2-0.8 V at a scan rate of 20 mV s −1 . The rGO/cotton electrode does not show any rectangular shape CV curve. This may be due to the presence of functional groups such as -OH, -COOH, C-O-C in graphene oxide which were not removed completely in rGO. The rGO/MnO 2 /cotton fabric electrode also does not show rectangular CV curve. This can be explained by the following reversible redox reaction mechanism The charge storage of MnO 2 coated cotton electrode in aqueous H 2 SO 4 electrolyte is caused by the intercalation of proton during reduction (Mn 4+ is reduced to Mn 3+ ) and de/intercalation upon oxidation (Mn 3+ is oxidized to Mn 4+ ) in the electrode. The rGO/MnO 2 /PANi/cotton electrode has pseudo-capacitance with largest capacitive current. This may be due to the redox reaction mechanism of PANi with two possible transitions such as leucoemeraldine-emaraldine and emeraldine-pernigraniline transition [42] as shown in figure 12. Compared with rGO/PANi/Cotton electrode, the difference in peak potential of rGO/MnO 2 /PANi/cotton electrode is decreased and this may be due to the redox reactions occur more reversibly. In figure 11 (a), the closed area of hybrid of rGO/MnO 2 /PANi components coated on cotton substrate is larger than that of rGO/cotton, rGO-MnO 2 /cotton, and rGO-PANi/cotton electrode. This proved that the capacitive performance of rGO/MnO 2 /PANi/cotton electrode is the best among the other electrodes. Figure 11(b) shows the Galvanostatic charge-discharge curves of rGO/cotton, rGO/MnO 2 /cotton, and rGO/MnO 2 /PANI/cotton electrodes at the discharge current density of 1.0 A g −1 in the potential range between −0.2 and +0.8 V versus Ag/AgCl. It can be seen that all the electrodes show an asymmetric curves of charge-discharge cycles, which indicate the electrodes are pseudocapacitors.
The specific capacitance is calculated by equation (2) = D D C spec I t m V 2 ( ) Where, C spec is the specific capacitance (F g −1 ), 'I' is the charge-discharge current density (A g −1 ), ′Δt′ is the discharge time (s), 'm' is the mass of active material in the working electrode (g), and ¢D ¢ V is the potential window (V). The C spec of developed cotton electrodes was calculated. The C spec of rGO/MnO 2 /PANi/cotton electrode is 888 F g −1 at 1.0 A g −1 current density, which is much higher compared to (136 F g −1 ) of rGO and (523 F g −1 ) of rGo/MnO 2 . The specific capacitance of rGO/MnO 2 /cotton electrode is improved because of the contribution of pseudocapacitive behavior from MnO 2 . Further, the addition of PANi coating layer to rGO/MnO 2 /cotton drastically enhance the capacitance. This may be due to the synergic effect of PANi with rGO and MnO 2 . It can be explained that a thin PANi coating layer acts as a protective layer on rGO/MnO 2 /cotton fabric to restrain MnO 2 nanoparticles and rGO from dissolution in H 2 SO 4 acidic electrolyte. Figure 13(a) shows CV curves of rGO/MnO 2 /PANi/cotton electrode at scan rates ranging from 5 mV s −1 to 100 mV s −1 . There is some redox peaks observed due to the pseudo capacitance by the presence of MnO 2 and PANi. It is observed that the cathodic peaks shift to positive side and the anodic peaks shift to negative side when the scan rate increases from 5 to 100 mV s −1 , due to the resistance of the electrode material. The Galvanostatic charge-discharge curves of rGO/MnO 2 /PANi/cotton electrode at different current ranges between 1.0 and 5 A g −1 is shown in figure 13(b).
The dependence of areal and volumetric specific capacitances on current density range of 1-25 A g −1 ) of rGO/ MnO2/PANi/cotton electrode with the size of 2 cm 2 area and 0.05 cm thickness was then studied ( figure 14). The maximal areal and volumetric specific capacitance of 444.0 F cm −2 and 403.6 F cm −3 at current density of 1 A g −1 decreased to 125.0 F cm −2 and 114.5 F cm −3 at current density of 25 A g −1 respectively. This shows that an increase in current density decreases the specific capacitance gradually of rGO/MnO 2 /PANi/cotton electrode.
The main objective of the Electrochemical Impedance Spectroscopic studies is to evaluate ion diffusions in the electrode and electrolyte interface. The electrochemical impedance and resistance of the electrode material can be represented by Nyquist plot ( figure 15). It is the sum of real (Z′), and imaginary (Z″) components which represent the resistance and capacitance of the electrode, respectively. The shape of Nyquist plot includes a semicircle region lying on the Z′-axis followed by a straight line. The semicircle region represents the electrontransfer-limited process and the straight line region corresponds to the diffusional-limited electron-transfer process.
From Nyquist plots (figures 15(a)-(c)), rGO/cotton shows a semicircle at high frequency region which is followed by a straight line at low frequency region. The rGO/MnO 2 /cotton also forms a semicircle at low frequency and this may be due to the addition of MnO 2 . It can be observed that the rGO/MnO 2 /PANi/cotton fabric electrode displays a semicircle at high frequency region and a more vertical straight line at low frequency region compared to rGO/Cotton and rGO/MnO 2 /cotton electrodes. This indicates that rGO/MnO 2 /PANi/cotton electrode has low Faradaic charge transfer resistances and a faster ion (H + ) diffusion rate which leads the material to have better capacitive behavior. The resistance of rGO/cotton, rGO/MnO 2 /cotton, and rGO/MnO 2 /PANi/cotton were measured to be 100.0 Ω, 26.0 Ω, and 6.0 Ω respectively. There was a decrease in resistance observed due to the addition of MnO 2 and PANi to rGO/cotton electrode.
Galvanostatic charge/discharge curve ( figure 16(a)) of cotton electrode was recorded at a discharge current density of 2 A g −1 for 12000 seconds. Figure 16(b) shows the specific capacitance of rGO/MnO 2 /PANi/cotton electrode at different discharge currents ranging between 1 and 25 A g −1 . It can be seen that the specific capacitance decreases from 888 F g −1 to 252 F g −1 with increasing current density (1.0 A g −1 to 25.0 A g −1 ).  The comparison between as fabricated electrode and the other electrode materials for supercapacitors reported in literature is shown in table 2. The obtained specific capacitance of 888 F g −1 at a current density of 1 A g −1 is higher than the reported rGO/cotton based electrode in literature [31]. Also, the long-cycle stability of supercapacitor is an important requirement for energy storage applications. Figure 16(c) shows the curve of specific capacitance versus cycle number. The rGO/MnO 2 /PANi/cotton electrode retains around 70% after 3000 cycles at a high discharge current of 15 A g −1 . This shows that the ternary materials coated cotton electrode has higher cycling stability even at high discharge current density. Therefore, the rGO/MnO 2 /PANi/cotton electrode can be an excellent electrode material for supercapacitors in energy storage application.

Conclusion
In this work, the unique ternary materials rGO/MnO 2 /PANi coated cotton electrodes have been successfully fabricated by step-wise synthesis procedure. The electrochemical studies of rGO/MnO 2 /PANi/cotton electrode confirm that PANi layer can protect rGO and MnO 2 particle on cotton surface and also increase the specific capacitance of the electrode. The fabricated rGO/MnO 2 /PANi/cotton fabric as working electrode was tested in a three electrode electrochemical cell with 1 M H 2 SO 4 electrolyte for energy storage application. For the rGO/MnO 2 /PANi/cotton electrode, the maximum specific capacitance value of 888 F g −1 and minimum of 250 F g −1 was achieved at the current density of 1.0 A g −1 and 25 A g −1 respectively. It retained around 70% of initial specific capacitance after 3000 charge-discharge cycles at high discharge current of 15 A g −1 , which demonstrates the ternary materials coated cotton electrode with excellent specific capacitance and good cycle stability. This proves that rGO/MnO 2 /PANi/cotton fabric will be the suitable electrode material for supercapacitor in energy storage applications.