Green synthesis and electrochemical characterization of rGO–CuO nanocomposites for supercapacitor applications
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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.
KeywordsCuO Piperine rGO Supercapacitor
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 . 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 . 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 . 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 . 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.
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 . 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 . 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 . 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 .
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
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 . 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.
Electrochemical studies of rGO–CuO nanocomposite electrodes
Influence of different electrolytes on rGO–CuO nanocomposite electrode
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 . 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.
Specific capacitance (Fg−1)
In situ polymerization.
Spin coating technique
Hydrothermal-assisted redox reaction
CuO template + PPy
In situ chemical polymerization
CuO-poly(acrylic) acid/CNT hybrid
Spin coating technique
Piperine + ultrasonication
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 . 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.
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.
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.
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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