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Ionics

, Volume 23, Issue 5, pp 1267–1276 | Cite as

Green synthesis and electrochemical characterization of rGO–CuO nanocomposites for supercapacitor applications

  • Y. N. Sudhakar
  • H. Hemant
  • S. S. Nitinkumar
  • P. Poornesh
  • M. SelvakumarEmail author
Original Paper

Abstract

Reduced graphene oxide (rGO) were prepared from graphene oxide (GO) by using piperine as a green reducing agent extracted from Piper nigrum. The obtained rGO had few defects and lacked connectivity between the layers. To overcome these defects, copper oxide (CuO) nanoparticles were synthesized ultrasonically and nanocomposites of rGO–CuO were prepared. The conductivities of the rGO, CuO and rGO–CuO nanocomposites were determined by AC impedance spectroscopy in different electrolytes. Morphology, composition and electronic structure of CuO, rGO and rGO–CuO nanocomposites were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photon spectroscopy (XPS) and electrochemical techniques. Transmission electron microscopy (TEM) images portrait CuO as a fish caught in the net of rGO layers. The rGO–CuO nanocomposite exhibiting lower resistance and higher capacitance was used in fabrication of supercapacitor electrodes. The specific capacitance of the fabricated supercapacitor was found to be 137 Fg −1. The supercapacitor performance of the nanocomposite electrode is attributed to the synergistic effect of double-layer capacitance of rGO and redox capacitance of CuO nanoparticles.

Graphical abstract

Keywords

CuO Piperine rGO Supercapacitor 

Introduction

Chemically modified graphene has been studied in the context of potential applications in sensors, field-effect transistors, batteries and supercapacitors. Graphene is a 2D carbon compound having one-atom-thick planar sheet, which possesses high specific surface area. Graphene has excellent electrical, mechanical and thermal properties and high mobility for charge carriers. Hummer and Offeman developed a better oxidation method to synthesize graphene oxide which is widely used even today. One of the most important reactions of graphene oxide is its reduction. The product of this reaction has been given a various names depending on their structure and mode of preparation, such as reduced graphene oxide (rGO), chemically reduced graphene oxide and graphene. Both graphene and rGO are electrically insulating materials due to their disrupted sp2 bonding networks. The more common and one of the first reported reducing agent was hydrazine monohydrate. Later on, most strong reducing agents such as lithium aluminium hydride and sodium borohydride were used. Nevertheless, these reducing agents showed very strong reaction with water, and they are toxic as well as explosives. Hence, these reducing agents are not attractive option for reducing aqueous dispersions of graphene oxide (GO). As a consequence, there is need for further development and optimisation of eco-friendly, natural reducing agent for clean and effective reduction of GO [1]. Therefore, we have used eco-friendly reducing agent piperine, which is extracted from the naturally obtained black Piper nigrum. Piperine is recognized as natural antioxidant which completely stops or delays the process of oxidation. Piperine is a pungent alkaloid found in black pepper to the extent of 10% by weight and is known to be an amide. The other constituents being volatile oils (1.3%), starch (20–40%) and water (8–13%). Piperine can also quench free radicals and reactive oxygen species. The synthetic antioxidants usually are carcinogenic and toxic, but antioxidants present in the plants are non-toxic and environmental friendly [2]. Inspired by these properties of the naturally obtained piperine, we report a green route for chemical reduction of GO using piperine as a reducing agent [1, 3].

Copper oxide (CuO) is an important p-type metal oxide semiconductor with a narrow band gap (1.2 eV). Due to its unique physical and chemical properties, nano-CuO has attracted considerable attention for its diverse applications in optoelectronics devices and lithium batteries. Nano-CuO is also one of the most promising materials in the development of gas sensors due to its high specific surface area and good electrochemical activity [4]. Eco-friendly ultrasonication method was adopted to obtain nano-sized CuO particles.

The advantages of ultrasonication approach, like more uniform size distribution, higher surface area, faster reaction time and improved phase purity, over conventional methods in the synthesis of metal oxides are more promising [5, 6]. Ultrasonic irradiation significantly enhances the hydrolysis rate, and shock waves can induce morphological changes in metal oxides [7]. The physiological property depends strongly on the number of layers and the dispersion of the rGO layer. If the rGO is made to interact with nanoparticles, it could lead to well-defined hybrid nanocomposite and will show enhancement of super capacitive and conductivity properties. CuO acts as stabilizer against the aggregation of individual rGO sheets. Due to this, in the present work, CuO was introduced to rGO and nanocomposites were obtained. The synthesized nanocomposite materials were used to fabricate a symmetric cell and were evaluated as supercapacitors.

In this paper, we discuss the novel route of synthesis of rGO along with ultrasonication synthesis of CuO. Different ratios of the rGO–CuO nanocomposite were prepared, and electrochemical properties were characterized by cyclic voltammetry and AC impedance spectroscopy. Further, there are no reports on the synthesis of rGO using piperine. Therefore, synthesis of rGO, sonochemical-assisted precipitation synthesis of nano-CuO and preparation of rGO–CuO nanocomposite would improve the capacitance compared to other energy devices [8, 9, 10]. The results show that the nanocomposite material has much higher capacitance than the individual components. Combining the properties of both the components in a synergistic way, the nanocomposite shows an excellent rate capability and cycle performance, compared to the other individually fabricated electrodes.

Experimental

Extraction of piperine

Round bottom flask fitted with a Soxhlet extractor was placed with 30 g of finely powdered black pepper (commercially procured); 350 mL of 95% ethanol was added and refluxed the mixture for 8 h. The mixture was filtered and concentrated the filtrate to 25 mL by distillation. To the residue, 30 mL of warm 2-N ethanolic KOH solution was added. The warm mixture was stirred and filtered to remove any insoluble material. The solution was kept at 5–10 °C for about 1 week, and the crude piperine was filtered and washed with ethanol. Crude product was recrystallized from ethanol-ethyl acetate (10:1) mixture. Shiny yellow crystals of piperine were obtained after slow drying and melting point was determined [11]. The characterizations of piperine were carried out using Bruker 400 MHz NMR and Shimadzu 6400 FTIR.

Synthesis of graphene oxide

GO was synthesized using improved synthesis method [12]. A 9:1 mixture of concentrated H2SO4-H3PO4 (360:40 mL) was added to a mixture of graphite flakes (3.0 g, 1 wt. equiv., SP-I Bay carbon) and KMnO4 (18.0 g, 6 wt. equiv.). The reactants were then heated to 50 °C and stirred for 12 h. The reaction was cooled to room temperature and poured onto ice with 30% H2O2 (3 mL). The resultant material was washed in succession with 200 mL of water, 200 mL of 30% HCl and 200 mL of ethanol (twice). The resultant suspensions were filtered over a PTFE membrane with a 0.22-μm pore size. The filtered GO was dried under oven at 90 °C, which resulted in yellowish brown powder.

Reduction of GO to rGO

0.1 g of GO was dispersed in 50 mL of distilled water [13]. 0.1 g of eco-friendly reducing agent piperine (dissolved in alcohol) was added to the mixture and heated in microwave oven for 5 min. After the completion of reaction, the rGO was filtered and collected as a black powder. It was washed with distilled water several times to remove the excess of piperine and later dried in a vacuum oven at 80 °C.

Synthesis of CuO

CuO nanoparticles were synthesized by using an utrasonification-assisted precipitation followed by thermal treatment. In this process, 2.4 g of copper nitrate trihydrate (Cu(NO3)2.3H2O) was dissolved in 100 mL doubly distilled water. One hundred millilitres of 0.2-M NaOH solution was added, and the mixture was ultrasonicated for 2 h. The temperature of the reaction was adjusted to 60 °C in ultrasonic bath. The resulted dark brown precipitate was centrifuged and washed with double-distilled water, and then dried in oven at 80 °C for overnight [3].

Preparation of the rGO–CuO nanocomposite

Different ratios of rGO–CuO nanocomposites, i.e. 1:3, 1:2, 1:1, 2:1, 3:1, 4:1, 5:1 and 6:1 were prepared while keeping the total weight of composite mixture to 0.3 g. Each rGO–CuO nanocomposite was coated on stainless steel (1 cm2) electrode using n-methylpyrrolidone along with poly(vinylidenefluoride) as binder. A symmetrical supercapacitor cell was constructed using rGO–CuO nanocomposite-coated electrodes using polypropylene separator.

Characterization of rGO–CuO nanocomposites

The surface morphology images were taken using scanning electron microscope (SEM), ZEISS EVO18 special edition. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images were obtained from Tecnai 20 G2 (the Netherlands). XRD was obtained from Rigaku’s MiniFlex 600. XPS was determined using non-monochromatic Al Kα X-ray source (1486.6 eV) with pass energy of 50.0 eV for the general scan and 40 eV for the core level spectra of each element.

Nanocomposite-pasted stainless steel (304 qualities, 1 cm × 2 cm × 0.2 mm), platinum sheet and AgCl/Ag were used as the working, counter and the reference electrodes, respectively. The electrochemical properties of rGO–CuO nanocomposite electrodes were evaluated in different electrolytes namely 0.1 N sodium sulphate (Na2SO4), 0.1 N sulphuric acid (H2SO4), phosphoric acid (H3PO4) and 0.1-N 9:1 mixture of H2SO4-H3PO4. Electrochemical studies like cyclic voltammetry (CV), AC impedance and galvanostatic charge–discharge (GCD) were carried out using BioLogic SP-150.

Results and discussion

Morphology and structure

Scheme 1 shows oxidation and reduction of graphite using piperine as a reducing agent. Figure 1a shows the NMR peaks of the piperine; in this spectrum, the methylene hydrogens (−CH2–) of cyclo aliphatic ring containing nitrogen atom show chemical shift at δ 1.51–δ 1.58. The alkene protons show experimental chemical shift at δ 5.8–δ 6.7 which is shifted little downfield in comparison with the standard values of δ 4.6–δ 5.9. This may be due to the effect of carbonyl (C=O) group attached to the alkenic carbon. The compound shows the chemical shift value δ 6.65–δ 6.93 ppm for aromatic protons due to upfield shift. The deviation of chemical shifts of aromatic protons is due to the presence of methylene dioxy group (−O–CH2–O–) attached to the benzene ring [14].
Scheme 1

Oxidation and reduction of graphite by using piperine as reducing agent

Fig. 1

a NMR peaks of the piperine. b FTIR spectrum of piperine

Figure 1b shows the FTIR spectrum of piperine. The presence of aromatic C–H stretching was found in range of 3008 to 2935 cm−1, and peaks at 1581 and 1488 cm−1 are due to stretching of –C=C– of aromatic ring. The stretching of –CO–N– found to be at 1631 cm−1. For methylene dioxy group, asymmetric, symmetric –CH2– and aliphatic C–H stretching peaks were observed at 2935, 2854, 1441 and 1248 cm−1, respectively. Peak at 1026 cm−1 is due to =CO–C– symmetrical stretching. The delocalisation of –CO stretching frequency was found at 1550 cm−1; this stretching is very important in case of piperine to which confirms the structure [15]. In-plane bending of phenyl –C–H– can be observed at 1126 cm−1. Out of plane –C–H– bending of 1,2,4-trisubstituted phenyl (two adjacent hydrogen atoms) can be observed at 848, 827 and 798 cm−1, respectively. The melting point of piperine was found to be at 130 °C.

Figure 2 shows XRD pattern of the rGO, CuO and rGO–CuO. The reduction of GO using microwave treatment in presence of piperine produced rGO, which did not exhibit characteristic peak of graphite at 26°, while characteristic peak of rGO structure at 21° having interlayer spacing of 0.91 nm was observed, indicating increased spacing between the rGO layers. Further, rGO spectrum exhibits amorphous pattern indicating that the rGO sheets are loosely stacked, and it is different from the crystalline graphite. XRD investigation of CuO shows intense peak at 37.4°, along with 30.1°, 44.3°, 62.5° and 74.2°, which corresponds to (111), (110), (200), (220) and (311) planes, respectively. Whereas the peaks present at 2θ value 35.7°, 39.1°, 49.3°, 61.8°, 66.2° and 68.2° corresponding to (111), (200), (202), (113), (311) and (220) confirm presence of CuO phase. The CuO data matches with the original data in JCPDS card no. 48-1548. Remarkably, in XRD pattern of rGO–CuO nanocomposite, all the diffraction peaks were perfectly indexed to the monoclinic CuO, except the small peak at 2θ = 26.4° which can be related to small stacking of rGO sheets. The broad and relatively weak diffraction peak of rGO in the nanocomposite indicates lower face to face stacking due to hybridisation [16].
Fig. 2

XRD patterns of the rGO, CuO and rGO–CuO

The morphology of the rGO, CuO and rGO/CuO was characterized using TEM, SEM and selected area electron diffraction (SAED) and are shown in Fig. 3a–d, respectively. The pure rGO (Fig. 3a) obtained in current process is found to be similar to rGO reported by Sudhakar et al. [10] having rippling structure, folding and scrolling nature. The SAED pattern from this rGO discloses that the rGO materials are hexagonal rings of different intensities and spot sizes on the same circle confirming the random orientation of graphene layers within the graphene sheets [17]. Figure 3b shows nano-leave-like structure of CuO. The CuO leaf substrate shows a three-dimensional open network structure, which would provide a high volumetric surface area and good mass transport property for electrolyte diffusion. In the TEM images of rGO–CuO nanocomposite (Fig. 3c), the CuO as nano-leaves with uniform size of about 100 nm are found to be well distributed within rGO. Scheme 2 shows the CuO embedded in rGO sheets revealing like a fish caught in the net and it would diffuse or vibrated locally along the sheets of rGO in the electrolyte during charging and discharging process. SEM image of rGO–CuO (Fig. 3d) shows interesting uniform blend of rGO and CuO nano-powder. This is due to existence of chemisorption and Vander-Waals interactions between oxygen-containing defect sites of the CuO and pristine regions of the rGO. In addition, oxygen functional groups located at the surface of rGO effectively hinder diffusion and movement of CuO grains. The SAED pattern of nano-leaves of CuO shows 4 nm and matches well with the result of XRD, indicating CuO is uniformly dispersed in rGO [18, 19, 20].
Fig. 3

TEM and SAED of rGO (a), CuO (b) and rGO–CuO (c); d SEM image of rGO–CuO

Scheme 2

Illustration of CuO as fish caught in the net of rGO

Figure 4a shows C1s XPS spectrum of rGO–CuO nanocomposite after deconvolution. The peaks specify C–C, C–O, C=O and O–C=O groups of rGO which are observed at 284.5, 285.2, 286.5 and 287.8 eV, respectively. In Fig. 4b, the O1s XPS spectrum of rGO–CuO nanocomposite shows peaks at 529.47, 530.99, 532.5 and 534.15 eV from Cu–O, O–O, OH and H2O, respectively. In Fig. 4c, it shows that the XPS survey of rGO–CuO nanocomposite displays small peaks at 945.3 eV of Cu 2p3/2 and 965.0 eV of Cu 2p1/2. It has difference of 20 eV which matches with the standard value of monoclinic structure of CuO [20]. The XPS indicates that the surface mainly consists of rGO due to predominate C element and probably CuO nanoparticles are embedded within rGO sheets.
Fig. 4

XPS spectrum of rGO–CuO nanocomposite C1s (a) and Ols (b); c XPS survey of rGO–CuO

Electrochemical studies of rGO–CuO nanocomposite electrodes

Figure 5 shows the Nyquist plots of 1:3, 1:2, 1:1, 2:1, 3:1, 4:1, 5:1 and 6:1 rGO–CuO nanocomposite mixtures. The 3:1 (rGO–CuO) nanocomposites showed better performance compared to the other electrode ratios. However, the Nyquist plot patterns of all the electrodes are same and exhibit higher charge transfer resistance R ct value in the case of electrodes with higher content of CuO. Two distinct characteristic regions of impedance spectra are observed with increase in frequency. The frequency value at the end of semicircle region, i.e. R ct values of 1:3, 1:2, 1:1, 2:1, 3:1, 4:1, 5:1 and 6:1 rGO–CuO electrodes were found to be about 1200, 820, 670, 550, 510, 515, 540 and 546 Ω, respectively. The 3:1 composition exhibited the lower resistance and the highest capacitance, it is clear that CuO powder has reduced the internal resistance of the nanocomposite at a certain composition beyond which increase in either of rGO or CuO has enhanced the resistance in the composite. When rGO content in the composite was more, the synergic effect between rGO and CuO is reduced due to lack of connectivity between rGO layers. Hence, this rGO/CuO composite certainly enhanced the conducting property of rGO.
Fig. 5

Nyquist plots of rGO–CuO in ratios 1:3, 1:2, 1:1, 2:1, 3:1, 4:1, 5:1 and 6:1. (inset: high-frequency region)

Influence of different electrolytes on rGO–CuO nanocomposite electrode

The presence of redox potential due to CuO is a factor of consideration for achieving higher capacitance; therefore, 3:1 rGO–CuO nanocomposite electrode was tested in 0.1-M concentration of four different electrolytes, namely Na2SO4, H3PO4, H2SO4 and (9:1) H2SO4:H3PO4. Figure 6a, b shows CV and Nyquist plots of different electrolytes obtained for testing of 3:1 rGO–CuO nanocomposite electrode. In Fig. 6a, redox peaks and size of current window varied with different electrolytes. Specific capacitances were calculated from CVs at scan rate of 5 mV s−1 for Na2SO4, H3PO4, H2SO4 and (9:1) H2SO4:H3PO4 and found to be 151, 153, 185 and 224 Fg−1, respectively. Figure 6b shows the R ct values due to the combined effect of movement and redox reaction of ions. This plays an important role in optimizing the electrolyte suitable for further studies. So, considering the AC impedance studies, R ct values of the electrodes calculated from the Nyquist plots were 1260 1120, 600 and 110 Ω for the electrodes in the different electrolytes. The increase in resistivity from PO4 3− to SO4 2− can be explained by the fact that common ion effect is present in the structural orientation of rGO [21]. In phosphoric acid mixture due to the presence of common ion (SO4 2− and PO4 3−), the dissociation of the electrolyte is reduced; nevertheless, the remaining free SO4 2− and PO4 3− ions can efficiently get accessed in the rGO–CuO nanocomposite matrix to form double layer as well as show pseudo capacitance behaviour in electrodes. Hence, these electrodes resulted in high specific capacitance. In optimum electrolyte concentration, the ion transport within the electrode is easier, leading to an effective building-up of double layer. However, if the electrolyte concentration is too high, the ion activity will reduce due to poor dissociation and lack of supporting medium.
Fig. 6

a CVs of 3:1 rGO–CuO tested in different electrolytes; b Nyquist plots of 3:1 rGO–CuO tested in different electrolytes

The optimized concentration was used for testing rGO–CuO at different scan rates. Figure 7a shows CV response for the optimized concentration of 0.1 M H2SO4:H3PO4 (9:1) at different scan rates. The CV loop was close to a rectangular shape at higher scan rates, which indicates double-layer characteristics, and at lower scan rate, combined effect of double layer and redox potentials was observed; hence, large change in enveloped area of CV is not observed with decrease in scan rate. As predicted, the rectangular area increased with increasing the scan rate [22]. The electrolyte studies reveal that the acid mixture (9:1) is suitable for supercapacitors
Fig. 7

a CV responses of composite in 0.1 M H2SO4:H3PO4 (9:1) electrolyte at different scan rates; b CV patterns of the rGO and CuO electrodes; c Nyquist plots of rGO and CuO electrodes

Comparison of electrochemical properties of rGO, CuO and rGO–CuO single electrodes

The CV patterns of the rGO and CuO are shown in Fig. 7b. The electrochemical performance of rGO and CuO were evaluated using CV and impedance spectroscopy using 0.1 M H2SO4:H3PO4 (9:1). The specific capacitance values of rGO and CuO were found to be 105 and 98 F g−1 at the 5 mV s−1, respectively. The corresponding Nyquist plots are shown in Fig. 7c, and its R ct values were 840 and 340 Ω, respectively. The CV loop of rGO was close to a rectangular shape due to the occurrence of double layer at electrode/electrolyte interface region, which is desirable for double-layer supercapacitor. The comparison of CuO with rGO shows redox behaviour as seen with semicircle region, while rGO shows larger Warburg impedance, i.e. diffusional impedance for the diffusion layer of infinite thickness showing a double-layer characteristic feature of rGO.

The highest specific capacitance of 224 Fg−1 was noted for rGO–CuO (3:1) nanocomposite electrode. It may be noted that the specific capacitance obtained here is higher than the values of nanocomposites reported in the literature [19, 20, 21, 22]. The higher value of specific capacitance shown by rGO–CuO (3:1) nanocomposite electrode compared to that of CuO electrode indicates the enhanced ionic conductivity of the nanocomposite. The increase in specific capacitance of the nanocomposite is due to the combined effect of faradic capacitance of the metal oxide and double-layer capacitance of the rGO [22]. The high surface area of the nano-leaf CuO serves as a good support to the rGO to facilitate the electrochemical process leading to higher capacitance. The impedance studies show that the R ct value decreased in the presence of CuO in between rGO; this may be inferred due to synergic effect of pseudocapacitance and electrochemical double-layer capacitance of rGO–CuO nanocomposite.

Supercapacitor properties

The CV plot of the supercapacitor fabricated using rGO–CuO nanocomposite is shown in Fig. 8a. The synergetic effect of both pseudocapacitance and electrochemical double-layer capacitance was observed in the CV pattern. With increase in the scan rate, there is less distortion in the CV pattern and scan rate properties. This suggests that high-charge transfer kinetics exists at the electrode/electrolyte interface. The specific capacitance (C S) values of the supercapacitor were calculated using the following equation:
$$ {C}_s\kern0.5em =\kern0.5em \frac{2A}{\varDelta V\kern0.5em \times \kern0.5em v\kern0.5em \times \kern0.5em m}, $$
(1)
Fig. 8

Supercapacitor properties of rGO–CuO. a CV; b Nyquist plot; c imaginary part (reactive power) |Q|/|S|% versus real part (active power) |P|/|S|%

where A is the integral area of the CV loop, ΔV is the potential window, v is the scan rate and m is the mass of materials at each electrode. A factor 2 is used because the series capacitance formed in two electrode system. The maximum value of specific capacitance obtained is 137 F g−1 at 5 mV s−1. Comparisons of similar composites have been shown in Table 1. This shows that among all other techniques and specially CuO/rGO combination, the present work showed to have the highest specific conductance. The Nyquist plot of supercapacitor is shown in Fig. 8b. Semicircle arc can be seen at higher frequency region, indicating the intrinsic resistance of the electrode material and electrolyte, which is the trend of the ideal pseudocapacitive behaviour and agrees with the CV results. Warburg impedance observed at higher frequencies shows that the present nanocomposites (rGO–CuO) has diffusive property and hence, low resistance for ionic conductivity. The semicircle in the high-frequency region implies low intrinsic internal resistance due to improved hydrophilicity of the rGO–CuO materials, resulting in enhanced wettability and facilitating rapid electrolyte ion transport within the spacing of the graphene layers and metal oxide. In addition, the ionic diffusion in the middle frequency region is achieved very fast, and a more vertical line seen in the low-frequency region indicates that the supercapacitor having nanocomposites has diffusive property and hence, lower resistance for ionic conductivity, thereby exhibiting higher ionic conductivity and electrochemical performance.
Table 1

Supercapacitor performances

Material

Preparation

Specific capacitance (Fg−1)

Reference

CuO/polyaniline

In situ polymerization.

185

24

CuO/rGO

Hydrothermal method

97

25

CuO/poly(acrylic)acid

Spin coating technique

136

26

CuO cauliflowers

Potentiodyanamic mode

179

27

Cu2O/CuO/RGO

Hydrothermal-assisted redox reaction

173

28

CuO template + PPy

In situ chemical polymerization

225

29

CuO-poly(acrylic) acid/CNT hybrid

Spin coating technique

258

30

Nanoflower CuO/rGO

Piperine + ultrasonication

137

Present work

Figure 8c shows normalized reactive power |Q|/|S|% and active power |P|/|S|% of the complex power versus frequency plot of the supercapacitor. The detailed calculation is presented in the literature [23]. The normalized reactive power reactive |Q|/|S|% increases with decrease in frequency, and a maximum power was reached and the supercapacitor behaves like a pure capacitor. At the same time, when |P|/|S|% is 100% at higher frequencies, the supercapacitor behaves like a pure resistor, i.e. power is dispersed into the system and then |P|/|S|% decreases as frequency increases. The crossing of two plots appears when |P| = |Q| at the time constant τ 0. The calculated time constant was found to be equal to 0.8 ms and hence, indicates that the present system can be efficiently used at low frequencies.

Charge–discharge studies were carried at three different current densities 1, 2 and 3 mA cm−1 and are shown in Fig. 9. The specific capacitance was derived from the charge–discharge curve using the following equation:
$$ {C}_g=\frac{I\kern0.5em \times \kern0.5em \varDelta t}{\varDelta V\kern0.5em \times \kern0.5em m}, $$
(2)
Fig. 9

Charge–discharge studies at different current densities

where I is the applied discharge current, Δt is the discharged time after IR drop, ΔV is the discharge potential window after IR drop and m is the mass of the single-electrode materials. The specific capacitance from charge–discharge studies calculated for supercapacitor was 68 F g−1. Figure 9 shows voltage drop due to ohmic resistance, and good stability was exhibited at low current density. This implies that when the rGO/CuO layers are adjusted to the size of ions, the pore volume become saturated by electrolyte species, limiting the maximum operating voltage and the energy storage in the supercapacitor. However, with an increase in current density, the voltage drop decreased, representing that a sufficient amount of ions was accommodated, and during discharge, intercalation/insertion and trapping of ions were not observed. Supercapacitor performance was therefore optimized with respect to wettability, based to a certain extent on the texture/structure of the electrode material.

The energy (E) and power (P) densities and equivalent series resistance (ESR) were calculated from the following equations:
$$ E\kern0.5em =\kern0.5em \frac{C\kern0.5em \times \kern0.5em \Delta {V}^2}{2}\kern0.5em \times \kern0.5em \frac{1000}{3600} $$
(3)
$$ P\kern0.5em =\kern0.5em \frac{I\kern0.5em \times \kern0.5em \varDelta V}{2\kern0.5em \times \kern0.5em m}\kern0.5em \times \kern0.5em 1000 $$
(4)
$$ ESR\kern0.5em =\kern0.5em \frac{iR_{\mathrm{drop}}}{2\kern0.5em \times \kern0.5em I}, $$
(5)

where ESR is equivalent series resistance (Ω). iR drop (V) is defined as the electrical potential difference between the two ends of a conducting phase during charging to discharging. Due to the porous structure of the rGO–CuO layer, there will be minimal loss of capacitance at different scan rates and inability of large ions to move out completely from the layers during discharge cycle. These values further support that the metabolic trend of the energy density and the power density are the same with those of the impedance spectroscopy measurements. A maximum E of 14 W h kg−1 and P of 12 kW kg −1 were obtained. The ESR value was 7.14 Ω. The high value of the P is well suited for surge-power delivery applications. This may be due to CuO nanoparticle embedded in the rGO layers which enhances the ionic diffusion and difference in charge carrying ability of rGO–CuO nanocomposite and led to both pseudocapacitive and electrochemical double-layer capacitive properties. Therefore, rGO–CuO nanocomposite is a promising candidate for use in supercapacitor application.

Conclusion

In summary, rGO from GO was synthesized by using naturally extracted piperine which acts as green reducing agent. The prepared piperine was confirmed by NMR and FTIR studies. Due to the defects and lacking of connectivity between the layers in rGO, CuO nanoparticles were embedded in rGO. rGO, CuO and rGO/CuO nanocomposites were characterized by XRD, XPS, SEM and TEM. Different ratios of rGO–CuO (1:1 to 6:1) were prepared to study the interaction between rGO and CuO. The rGO–CuO nanocomposite with composition (3:1) found to have lower resistance and higher specific capacitance compared to rGO and CuO electrodes. The electrolyte studies reveal that the acid mixture (9:1) is suitable for supercapacitors. The symmetrical supercapacitor fabricated exhibits C S of 137 F g−1, and C g was found to be stable during charge–discharge cycling. The high performance of the supercapacitor is attributed to the enhanced interaction between the adaptive electrodes. The present work shows that rGO–CuO nanocomposite emerged as a promising material for supercapacitor applications.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2016

Authors and Affiliations

  • Y. N. Sudhakar
    • 1
  • H. Hemant
    • 2
  • S. S. Nitinkumar
    • 2
  • P. Poornesh
    • 3
  • M. Selvakumar
    • 2
    Email author
  1. 1.Department of ChemistryChrist UniversityBengaluruIndia
  2. 2.Department of Chemistry, Manipal Institute of TechnologyManipal UniversityManipalIndia
  3. 3.Department of PhysicsManipal Institute of Technology, Manipal UniversiryManipalIndia

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