Silver Nanoparticles Embedded on Reduced Graphene Oxide@Copper Oxide Nanocomposite for High Performance Supercapacitor Applications

In this work, silver (Ag) decorated reduced graphene oxide (rGO) coated with ultrafine CuO nanosheets (Ag-rGO@CuO) was prepared by the combination of a microwave-assisted hydrothermal route and a chemical methodology. The prepared Ag-rGO@CuO was characterized for its morphological features by field emission scanning electron microscopy and transmission electron microscopy while the structural characterization was performed by X-ray diffraction and Raman spectroscopy. Energy-dispersive X-ray analysis was undertaken to confirm the elemental composition. The electrochemical performance of prepared samples was studied by cyclic voltammetry and galvanostatic charge-discharge in a 2M KOH electrolyte solution. The CuO nanosheets provided excellent electrical conductivity and the rGO sheets provided a large surface area with good mesoporosity that increases electron and ion mobility during the redox process. Furthermore, the highly conductive Ag nanoparticles upon the rGO@CuO surface further enhanced electrochemical performance by providing extra channels for charge conduction. The ternary Ag-rGO@CuO nanocomposite shows a very high specific capacitance of 612.5 to 210 Fg−1 compared against rGO@CuO which has a specific capacitance of 375 to 87.5 Fg−1 and the CuO nanosheets with a specific capacitance of 113.75 to 87.5 Fg−1 at current densities 0.5 and 7 Ag−1, respectively.


Introduction
The depletion of fossil fuel reserves alongside the increase in environmental pollution is driven by a continual rise in world energy consumption. In addressing these issues, researchers are engaged in the development of renewable and cleaner energy resources such as solar photovoltaics and wind power. However, a major problem with these particular energy resources lies in their intermittency; a solar cell depends on sunlight and a wind turbine requires favorable wind conditions [1]. Hence, these energy resources cannot fulfill the energy demands of our society and so energy storage becomes a necessity. In recent years, energy storage devices such as batteries, capacitors and supercapacitors which are efficient, low cost, easy to manufacture and environmentally friendly have appeared on the market [2]. Batteries have high energy density and low power density whereas capacitors have high power density but low energy density meaning these energy storage devices have practical limitations. Supercapacitors provide a bridge between batteries and capacitors, in that they can have both very high energy density and power density, as well as stability several analytical techniques. The prepared pure metal oxide and its nanocomposites were used to form an electrode that demonstrated stable and excellent performance in potential supercapacitor applications.

Synthesis of Electrode Materials
GO was synthesized from graphite flakes by a modified Hummer method. In this approach, 360 mL sulfuric acid (H 2 SO 4 ) and 40 mL phosphoric acid (H 3 PO 4 ) were poured into a 1 L glass beaker and mixed by magnetic stirring. Three grams of graphite flakes and potassium permanganate (18 g) were mixed ex-situ and added slowly to the acid mixture. The solution was stirred for 72 h at atmospheric temperature to obtain a dark green solution. This solution was poured over an ice cube of 600 mL water in a 2 L glass beaker. Hydrogen peroxide (H 2 O 2 ) was added until the dark green solution turned yellow. The prepared yellow solution was centrifuged with deionized (DI) water several times to obtain a yellow gel of GO. Figure 1 shows the schematic diagram for the preparation of pure CuO with its binary and ternary nanocomposites. specific capacity and rate capability. Synthesis of CuO, rGO@CuO and Ag-rGO@CuO w confirmed by several analytical techniques. The prepared pure metal oxide and its nan composites were used to form an electrode that demonstrated stable and excellent perfo mance in potential supercapacitor applications.

Synthesis of Electrode Materials
GO was synthesized from graphite flakes by a modified Hummer method. In th approach, 360 mL sulfuric acid (H2SO4) and 40 mL phosphoric acid (H3PO4) were pour into a 1 L glass beaker and mixed by magnetic stirring. Three grams of graphite flakes an potassium permanganate (18 g) were mixed ex-situ and added slowly to the acid mixtu The solution was stirred for 72 h at atmospheric temperature to obtain a dark green sol tion. This solution was poured over an ice cube of 600 mL water in a 2 L glass beak Hydrogen peroxide (H2O2) was added until the dark green solution turned yellow. T prepared yellow solution was centrifuged with deionized (DI) water several times to o tain a yellow gel of GO. Figure 1 shows the schematic diagram for the preparation of pu CuO with its binary and ternary nanocomposites. Pure CuO nanosheets were synthesized by a microwave-assisted hydrotherm method. In this procedure, copper (II) nitrate trihydrate (CuN2O6.3H2O) and potassiu hydroxide (KOH) were taken in a 1:20 atomic ratio and dissolved in 200 mL of DI wat The solution was stirred for 30 min to obtain an ultra-fine homogenous solution. The soluti Pure CuO nanosheets were synthesized by a microwave-assisted hydrothermal method. In this procedure, copper (II) nitrate trihydrate (CuN 2 O 6 ·3H 2 O) and potassium hydroxide (KOH) were taken in a 1:20 atomic ratio and dissolved in 200 mL of DI water. The solution was stirred for 30 min to obtain an ultra-fine homogenous solution. The solution was placed in a microwave digester system with applied microwave power (750 W) for 10 min at 80 • C. The solution was cooled to room temperature and the resultant precipitate was collected. The precipitate dark brown color material was washed with DI water and absolute ethanol to remove the impurities. After further centrifugation, the dark brown product was annealed at 80 • C for 12 h to obtain CuO nanosheets.
To prepare rGO@CuO, a 2 mL GO solution at a concentration of 5 mg mL −1 was transferred dropwise onto 300 mg CuO nanosheets and mixed. The nanocomposite of GO@CuO was annealed at 400 • C for 4 h to reduce the GO into rGO and to get rGO@CuO composite.
To decorate the surface of rGO@CuO nanocomposite with Ag nanoparticles a chemical reduction method was employed. Solutions of 0.0010 M AgNO 3 (aq) and 0.0020 M NaBH 4 (aq) solution in DI water were separately prepared. A 90 mL portion of the sodium borohydride solution was added to a 500 mL Erlenmeyer flask and stirred for 20 min in an ice-cool atmosphere. 300 mg rGO@CuO of the nanocomposite was added and stirred for up to 10 min. Next, 30 mL of silver nitrate (AgNO 3 ) was added dropwise by a burette. The precipitated dark yellow material was collected and annealed at 80 • C for 12 h to get Ag-rGO@CuO.

Characterization Techniques
To identify the phase and crystallinity of the prepared samples, X-ray powder diffraction (Rigaku, Ultima IV XRD, Tokyo, Japan) was employed. Field emission scanning electron microscopy (JEOL, JSM-7600F, FESEM, Tokyo, Japan) was used to study the surface morphology and elemental compositions. Transmission electron microscopy (JEOL, JSM, ARM-200F, HRTEM, Tokyo, Japan) and Raman spectroscopy (DXR Raman Microscope, Thermo Scientific fitted with a DXR 532 nm laser, Madison, WI, USA) were employed to examine the size, shape, and chemical composition of the nanocomposites, respectively. The electrochemical behavior of the electrode was studied by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) using a Versa STAT 3 (AMETEK, Oak Ridge, TN, USA) electrochemical workstation.

Fabrication of Electrodes and Electrochemical Measurements
A chemically cleaned nickel foam was used to fabricate the electrode. For this, a 1 cm × 1 cm area of nickel foam was coated with slurry of the active material. The slurry of active material comprising 80 wt. % of the Ag-rGO@CuO nanocomposite, 10 wt. % of activated carbon (AC), and 10 wt.% of polyvinylidene fluoride (PVdF) was mixed in anhydrous 1-methyl-2-pyrrolidinone (NMP) and stirred homogeneously at ambient temperature for 12 h. The nickel foam was coated with the slurry and annealed at 90 • C for 12 h to dry. The loading of the active material was~1.5 mg. The same method was used to prepare the working electrode for pure CuO nanosheets and the rGO@CuO nanocomposite. Electrochemical measurements on the electrode were performed using a three-electrode system in an aqueous 2M potassium hydroxide (KOH) electrolyte ( Figure S1). In the assembled half-cell Ag/AgCl (3M KCL), platinum, and fabricated electrode served as reference electrode, counter electrode, and working electrode, respectively. Cyclic voltammetry (CV) was performed at different scan rates over a 0 to 0.5 V potential window. Galvanostatic charge-discharge (GCD) was measured at different current loads and across the same potential window.

X-ray Diffraction Studies
X-ray powder diffraction (XRD) was employed for phase identification and structural analysis of the prepared samples. Figure 2 shows the XRD pattern for pure CuO nanosheets, rGO@CuO, and Ag-rGO@CuO nanocomposites. In the pure CuO nanosheet pattern high-intensity peaks were located at 32. The absence of a peak at~26 2θ confirms the successful coating of rGO and the absence of any graphitic impurities [34]. The intensity peak of CuO nanosheets decreased in the Ag-rGO@CuO nanocomposite due to the introduction of Ag and rGO which cover some area of CuO sheets as well as due to the interactions between Ag, rGO and CuO. Similar results were also reported by Javed et al. [35], where peak intensity of Co 3 O 4 decreased after the incorporation of MWCNT and Ag. Figure S2 shows the XRD pattern of few layered and multilayered GO and rGO. 61.35°, 66.17°, 67.80° and 75.05°, and corresponded to the planes (110), (002), (111), (20 (020), (202), (11−3), (022), (113) and (004), respectively with a monoclinic phase [DB C No. 01-080-1917]. The diffraction peaks of rGO@CuO nanocomposites are located at same diffraction angle and plane as the CuO nanosheets. Additional peaks were also served in the Ag-rGO@CuO sample at 38.04°, 43.16° and 64.17° attributed to the (1 (200), and (220) planes of the FCC phase of silver [DB Card No. 00-00-1167], respectiv The absence of a peak at ~26 2θ confirms the successful coating of rGO and the absenc any graphitic impurities [34]. The intensity peak of CuO nanosheets decreased in the rGO@CuO nanocomposite due to the introduction of Ag and rGO which cover some a of CuO sheets as well as due to the interactions between Ag, rGO and CuO. Similar res were also reported by Javed et al. [35], where peak intensity of Co3O4 decreased after incorporation of MWCNT and Ag.

SEM, EDX and TEM Characterization
SEM images of the CuO nanosheets, rGO@CuO, Ag-rGO@CuO and EDX of the rGO@CuO nanocomposite are shown in Figure 3. Pure CuO shows ultrathin sheet varying dimensions ranging from ~50-700 nm in Figure 3a. The sheets across some gions are well stacked together while at other places sheets with fibrillar-like structu are arranged in a haphazard manner, which may be due to breakage or exfoliation larger sheets [36]. The rGO@CuO nanocomposite in Figure 3b shows a similar morp ogy to CuO and the sheets of rGO are not clearly observed. This might be attributed range of reasons such as their similar morphology to the thin sheets of CuO, rGO sh covering CuO, or some rGO sheets that might be deeply buried inside the CuO sh stacks. In the case of Ag-rGO@CuO, from Figure 3c, a large number of small Ag parti in clusters of varying sizes can be seen covering the sheets and stacked between loos packed sheets. EDX analysis of the Ag-rGO@CuO nanocomposite in Figure 3d shows o the elemental peaks of C, Cu, O and Ag, which suggests that the nanocomposite is from impurities and supports the efficacy of the synthesis method (Table 1). Furtherm

SEM, EDX and TEM Characterization
SEM images of the CuO nanosheets, rGO@CuO, Ag-rGO@CuO and EDX of the Ag-rGO@CuO nanocomposite are shown in Figure 3. Pure CuO shows ultrathin sheets of varying dimensions ranging from~50-700 nm in Figure 3a. The sheets across some regions are well stacked together while at other places sheets with fibrillar-like structures are arranged in a haphazard manner, which may be due to breakage or exfoliation of larger sheets [36]. The rGO@CuO nanocomposite in Figure 3b shows a similar morphology to CuO and the sheets of rGO are not clearly observed. This might be attributed to a range of reasons such as their similar morphology to the thin sheets of CuO, rGO sheets covering CuO, or some rGO sheets that might be deeply buried inside the CuO sheet stacks. In the case of Ag-rGO@CuO, from Figure 3c, a large number of small Ag particles in clusters of varying sizes can be seen covering the sheets and stacked between loosely-packed sheets. EDX analysis of the Ag-rGO@CuO nanocomposite in Figure 3d shows only the elemental peaks of C, Cu, O and Ag, which suggests that the nanocomposite is free from impurities and supports the efficacy of the synthesis method (Table 1)     Transmission electron microscopy was employed to analyze the size and shape of Ag-rGO@CuO nanocomposite at different magnifications and the micrographs are shown in Figure 4. Large clusters of Ag nanoparticles, CuO and rGO sheets of dimensions below 100 nm were observed. Figure 5a represents the low magnification image of Ag-rGO@CuO nanocomposite, which indicates that the CuO nanosheets were grafted by the rGO sheets. At low magnification in Figure 5b, it can be seen that the Ag nanoparticles decorate the rGO@CuO nanocomposite surface. At high magnification, in Figure 5c, the Ag nanoparticles are shown to be 20-50 nm in size. Transmission electron microscopy was employed to analyze the size and shape of Ag-rGO@CuO nanocomposite at different magnifications and the micrographs are shown in Figure 4. Large clusters of Ag nanoparticles, CuO and rGO sheets of dimensions below 100 nm were observed. Figure 5a represents the low magnification image of Ag-rGO@CuO nanocomposite, which indicates that the CuO nanosheets were grafted by the rGO sheets. At low magnification in Figure 5b, it can be seen that the Ag nanoparticles decorate the rGO@CuO nanocomposite surface. At high magnification, in Figure 5c, the Ag nanoparticles are shown to be 20-50 nm in size.

Raman Spectrum
Raman spectra of the CuO nanosheets and their nanocomposites are shown in Figure  6. All the samples show two common Raman peaks located at 284 and 620 cm −1 , which are attributed to the Ag and Bg 2 modes of the monoclinic CuO structure. The slight shifting of peaks with respect to bulk CuO may be due to different nanoscale structure and morphology [37]. Similar shifting was also observed by Murthy and Venugopalan in their na-

Raman Spectrum
Raman spectra of the CuO nanosheets and their nanocomposites are shown in Figure 6. All the samples show two common Raman peaks located at 284 and 620 cm −1 , which are attributed to the A g and B g 2 modes of the monoclinic CuO structure. The slight shifting of peaks with respect to bulk CuO may be due to different nanoscale structure and morphology [37]. Similar shifting was also observed by Murthy and Venugopalan in their nanosized CuO [38]. In the rGO@CuO composite, due to GN, there are two prominent peaks at 1340 and 1593 cm −1 called D and G bands, respectively. The D band represents defects or disorder created by the attachment of functional groups containing oxygen and the G band denotes graphitization arising from first-order scattering of the E 2g mode [39]. Similarly, the Ag-rGO@CuO composite shows the same characteristic D and G bands as in the rGO@CuO composite but with peaks at different positions: 1352 and 1587 cm −1 respectively. The inset of Figure 5 shows the Raman spectrum for GO with peaks at 1360 and 1602 cm −1 for D and G band, respectively. The intensity ratio of the D and G bands (I D /I G ) in GO, rGO@CuO, and Ag-rGO@CuO were determined to be 0.90, 1.03, and 1.06, respectively. The ratio of (I D /I G ) present in rGO@CuO, and Ag-rGO@CuO is greater than GO which confirms the reduction of GO to rGO. Similar results were also reported by Mehti et al. [40], where the ratio (I D /I G ) rGO in rGO-ZnO nanocomposite was found to be greater than of GO and was interpreted to predict the reduction of GO into rGO during microwave heating. Apart from this, the NaBH 4 used in reduction of AgNO 3 has also been reported to reduce GO into rGO by Shin et al. [41] The higher values of I D /I G in rGO@CuO than GO reveal that the number of defects is increased during the reduction of GO into rGO. Further increase in the intensity ratio in Ag-rGO@CuO might be due to an increase in the number of defects after incorporation of Ag nanoparticles on the surface as well as between the sheets. In the Ag-rGO@CuO spectrum, the D and G band intensities were higher than those for rGO@CuO because the Raman signal was increased by surface-enhanced Raman scattering (SERS) [42,43].

Electrochemical Capacitive Performance Analysis
The electrochemical performance of the prepared electrodes was examined by CV and GCD methods using three-electrode cells in a freshly-prepared 2M solution of KOH

Electrochemical Capacitive Performance Analysis
The electrochemical performance of the prepared electrodes was examined by CV and GCD methods using three-electrode cells in a freshly-prepared 2M solution of KOH electrolyte. CV is a very important tool to measure the reduction-oxidation (redox) behavior as well as the capacitive nature of the electrode material. Figure 7a-c represents the cyclic voltammetry curves corresponding to pure CuO nanosheets, rGO@CuO, and Ag-rGO@CuO at different scan rates (10-50 mVs −1 ) and Figure 8a shows the comparative CV curve of all the prepared three electrodes at a fixed scan rate of 50 mVs −1 in a potential range of 0.0-0.5 V. The CV curve of each electrode shows the pair of anodic and cathodic redox peaks that appear during electrochemical reactions. The presence of two visible peaks (redox peaks) confirms that all the samples display pseudocapacitive behavior in contrast with an EDLC where the CV profile takes a rectangular form with no obvious redox peaks. In a CuO-based system, oxidation and reduction of electrons between the electrode and electrolyte takes place by faradaic redox reactions as follows: In the CV curve of the rGO@CuO composite (Figure 7b) both the anodic and cathodic peak curves appear to be higher than those for the CuO sheets because the introduction of rGO sheets into CuO significantly reduces CuO nanosheet aggregation and creates more active sites on the electrode surface for redox reactions to take place. These extra rGO@CuO was found to be 689 Fg −1 which is much higher than that for rGO@CuO (511 Fg −1 ) and pure CuO nanosheets (202 Fg −1 ). Galvanostatic charge-discharge (GCD) was performed to investigate the rate capability of the different electrode materials. The individual GCD curves were recorded at different current densities within a potential difference of 0.4 V as shown in Figure 9a-c. Figure 8b shows the GCD curves of all fabricated electrodes at a fixed current load of 0.5 Ag −1 . The specific capacitance of the pure CuO sheets (Figure 9a) was 113.75, 112.50, 100, 100 and 87.50 Fg −1 at current loads of 0.5, 1, 2, 5 and 7 Ag −1 , respectively. Similarly, the specific capacitances of the binary rGO@CuO ( Figure 9b) and the ternary Ag-rGO@CuO nanocomposite ( Figure 9c) were 375, 370, 265, 237.5, 87.50 Fg −1 and were 612.5, 605, 405, 300, 210 Fg −1 at current loads of 0.5, 1, 2, 5, 7 Ag −1 , respectively. The Ag-rGO@CuO nanocomposite electrode shows excellent capacitive performance in comparison to rGO@CuO and pure CuO nanosheets. This is due to the addition of rGO which provides more active sites or conducting paths, which increases electron and ion mobility, limiting unwanted aggregation within the CuO and rGO sheets. The silver nanoparticles on the surface of rGO@CuO provide extended channels for electron transfer during the redox process. The specific capacitive value of CuO nanosheets and its nanocomposites was calculated from the GCD curves using the following equation: where Csp is the specific capacitance (Fg −1 ), I is the charge-discharge current (A), dt is the discharge time (s), m denotes the mass (g) of the active material loaded onto the Ni foam and dv represents the voltage difference between the upper and lower potential. The redox signal of all electrodes is attributed to the oxidation of Cu + to Cu 2+ and the reduction of Cu 2+ to Cu + [44]. In Figure 7a the pure CuO nanosheets show asymmetric redox peaks at different scan rates but with a slight shifting observed at a higher scanning rate indicating poor electron mobility at the electrode-electrolyte interface [45].
In the CV curve of the rGO@CuO composite (Figure 7b) both the anodic and cathodic peak curves appear to be higher than those for the CuO sheets because the introduction of rGO sheets into CuO significantly reduces CuO nanosheet aggregation and creates more active sites on the electrode surface for redox reactions to take place. These extra active sites improve the conductivity and capacitance behavior of rGO@CuO by providing new conducting paths for electron transfer. Therefore, the capacitive behavior of rGO@CuO is increased in comparison with pure CuO electrode material. A nanocomposite based on CuO and GN has been reported to show high performance as an anode material for lithiumion batteries [46]. In the case of Ag-rGO@CuO Figure 7c, the redox peak current is higher than in both pure CuO and the rGO@CuO nanocomposite due to the surface decoration by Ag nanoparticles. As outlined previously, the excellent electrical conductivity of Ag provides more channels for electron transfer during the redox process. The results from the Ag-rGO@CuO nanocomposite demonstrate that this type of material could be an emerging candidate in supercapacitor applications [47]. The CV curves for the pure CuO ultra-fine nanosheets, the rGO@CuO binary and the Ag-rGO@CuO ternary composite at fixed scan rate (50 mVs −1 ) over the potential range 0.0-0.5 V are shown in Figure 8a. Out of all three electrode materials investigated, the ternary nanocomposite Ag-rGO@CuO showed the best electrochemical performance as determined from the high electrochemical surface area enclosed by its CV curve. The specific capacitance of pure CuO and its nanocomposites was calculated by the following formula: where Q s represents the specific capacity (Fg −1 ), V denotes the scan rate (Vs −1 ), m is the mass of active electrode material in grams loaded on Ni foam, and the integrated area represents the anodic peak area under the CV curve. The specific capacitance of Ag-rGO@CuO was found to be 689 Fg −1 which is much higher than that for rGO@CuO (511 Fg −1 ) and pure CuO nanosheets (202 Fg −1 ). Galvanostatic charge-discharge (GCD) was performed to investigate the rate capability of the different electrode materials. The individual GCD curves were recorded at different current densities within a potential difference of 0.4 V as shown in Figure 9a-c. Figure 8b shows the GCD curves of all fabricated electrodes at a fixed current load of 0.5 Ag −1 . The specific capacitance of the pure CuO sheets (Figure 9a) was 113.75, 112.50, 100, 100 and 87.50 Fg −1 at current loads of 0.5, 1, 2, 5 and 7 Ag −1 , respectively. Similarly, the specific capacitances of the binary rGO@CuO ( Figure 9b) and the ternary Ag-rGO@CuO nanocomposite ( Figure 9c) were 375, 370, 265, 237.5, 87.50 Fg −1 and were 612. 5,605,405,300,210 Fg −1 at current loads of 0.5, 1, 2, 5, 7 Ag −1 , respectively. The Ag-rGO@CuO nanocomposite electrode shows excellent capacitive performance in comparison to rGO@CuO and pure CuO nanosheets. This is due to the addition of rGO which provides more active sites or conducting paths, which increases electron and ion mobility, limiting unwanted aggregation within the CuO and rGO sheets. The silver nanoparticles on the surface of rGO@CuO provide extended channels for electron transfer during the redox process. The specific capacitive value of CuO nanosheets and its nanocomposites was calculated from the GCD curves using the following equation: where C sp is the specific capacitance (Fg −1 ), I is the charge-discharge current (A), dt is the discharge time (s), m denotes the mass (g) of the active material loaded onto the Ni foam and dv represents the voltage difference between the upper and lower potential. From Figure 10a the ternary Ag-rGO@CuO nanocomposite shows a very high specific capacitance (612.5 Fg −1 ) at a current load of 0.5 (Ag −1 ) as compared to rGO@CuO (375 Fg −1 ) and pure CuO nanosheets (113.75 Fg −1 ). Again, this enhancement in Ag-rGO@CuO is due to the incorporation of rGO and Ag nanoparticles which create more electrochem- From Figure 10a the ternary Ag-rGO@CuO nanocomposite shows a very high specific capacitance (612.5 Fg −1 ) at a current load of 0.5 (Ag −1 ) as compared to rGO@CuO (375 Fg −1 ) and pure CuO nanosheets (113.75 Fg −1 ). Again, this enhancement in Ag-rGO@CuO is due to the incorporation of rGO and Ag nanoparticles which create more electrochemically active sites between the electrolyte and electrode. 9. (a) GCD curve of pure CuO nanosheets, (b) rGO@CuO and (c) Ag-rGO@CuO nanocomposites at different From Figure 10a the ternary Ag-rGO@CuO nanocomposite shows a ver cific capacitance (612.5 Fg −1 ) at a current load of 0.5 (Ag −1 ) as compared to rGO Fg −1 ) and pure CuO nanosheets (113.75 Fg −1 ). Again, this enhancement in Ag is due to the incorporation of rGO and Ag nanoparticles which create more el ically active sites between the electrolyte and electrode.  In addition to studying the capacitive behavior of the prepared electrodes by CV and GCD, the charge-discharge cycling stability was examined. To check the cycle stability, 3000 charge-discharge cycles were performed and the results are shown in Figure 10b.
The Ag-rGO@CuO nanocomposite shows excellent cycle stability with up to 92% of the initial capacitance retained. Under similar test conditions the CuO nanosheets retained up to 60% of the initial capacitance. The high capacitance retention in the Ag-rGO@CuO nanocomposite is once more due to the presence of rGO which provides conducting paths for electron flow, and the incorporation of Ag nanoparticles which increases electron mobility and the creation of more active sites during the charge-discharge process. In comparison, the CuO electrode has fewer active sites due to the aggregation of nanosheets, which reduces the overall electron mobility. The capacitance of the prepared ternary nanocomposite (Ag-rGO@CuO) is compared against other reported pure CuO and CuO based nanocomposites in Table 2.

Conclusions
In this study, pure CuO nanosheets, a binary composite of rGO@CuO and a ternary composite of Ag-rGO@CuO were synthesized by a microwave-assisted hydrothermal and chemical reduction method. The prepared electrode materials were characterized by different analytical techniques to confirm their formation and decoration of Ag nanoparticles upon nanosheets of rGO and CuO. Electrochemical studies showed that the ternary nanocomposite (Ag-rGO@CuO) exhibited a high specific capacitance of 612 Fg −1 at 0.5 Ag −1 which was higher than that found for pure CuO nanosheets and the rGO@CuO composite. The high capacitive value of the ternary composite was obtained by incorporating rGO thin sheets into ultrafine CuO sheets which provided a conductive platform within the rGO and CuO sheets. Furthermore, the Ag nanoparticles also created new conducting channels which further facilitated ion conduction. The Ag-rGO@CuO composite exhibited very good cycle stability after 3000 cycles with 92% capacitance retention. Thus, an electrode material based on Ag-rGO@CuO ternary nanocomposite may open a gateway to fabricate high-performance electrochemical storage devices.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/ma14175032/s1, Figure S1: Schematic diagram of the electrochemical cell, Figure S2: XRD pattern of few layered and multi layered GO and rGO.