Fabrication of CuO-decorated reduced graphene oxide nanosheets for supercapacitor applications

Fabrication of CuO-decorated reduced graphene oxide nanosheets for supercapacitor applications

Author’s Accepted Manuscript Fabrication of CuO-decorated reduced graphene oxide nanosheets for supercapacitor applications Ian Y.Y. Bu, Ray Huang ww...

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Author’s Accepted Manuscript Fabrication of CuO-decorated reduced graphene oxide nanosheets for supercapacitor applications Ian Y.Y. Bu, Ray Huang

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S0272-8842(16)31444-4 http://dx.doi.org/10.1016/j.ceramint.2016.08.136 CERI13588

To appear in: Ceramics International Received date: 13 July 2016 Revised date: 3 August 2016 Accepted date: 22 August 2016 Cite this article as: Ian Y.Y. Bu and Ray Huang, Fabrication of CuO-decorated reduced graphene oxide nanosheets for supercapacitor applications, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.08.136 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Fabrication of CuO-decorated reduced graphene oxide nanosheets for supercapacitor applications Ian Y.Y. Bu1*, Ray Huang2 1

Department of Greenergy, National University of Tainan, Taiwan

2

Department of Microelectronic Engineering, National Kaohsiung Marine University, Taiwan [email protected] [email protected] *

Corresponding author. Tel 886 972506900. Fax: +886 73645589.

Abstract Copper oxide(CuO) and CuO/reduced graphene oxide (rGO) films were prepared by using a hydrothermal method. The properties of the films were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS). The SEM imaging revealed that the CuO film consisted of 10nm diameter nanoparticles. The XRD pattern and XPS measurements confirmed the formation of the monoclinic phase of CuO. Electrochemical testing revealed that supercapacitors fabricated with CuO-rGO as electrode materials resulted in an four folds increase in capacitance compared to CuO-based supercapacitor due to the deduced graphene restacking and lower electrical resistance between the CuO nanoparticles.

Keywords: copper oxide; supercapacitor; graphene; reduced graphene oxide; restacking

Introduction Recently, with the rapid growth in demand for portable electronic devices and electric vehicles, there is increased interest in high-density energy storage systems[1, 2]. Supercapacitors, also known as electrochemical capacitors, are electrochemical energy storage with high power density, fast charging /discharging, long cycle life and stable performance[3, 4]. A key component determining the electrochemical performance of the supercapacitors is the electrode material[5]. Commonly used electrode materials include conducting polymers (polyanilines[6], polypyrrole [7]etc), transition metal oxides/hydroxide (nickel oxide[8, 9], cobalt oxide[10], mixed oxide compounds[11, 12], ruthenium oxide[13], manganese oxide, etc), and carbonaceous materials (activated carbon[14], carbon nanotube[15], graphene[16], carbon nanoonions[17], etc.). Among potential metal oxides proposed for supercapacitor applications, CuO has attracted significant interest due to its non-toxicity, earth abundance and low cost. Although CuO with various geometries has been investigated as a potential electrode material for supercapacitors[18, 19], it suffers from poor specific capacitance due to its limited surface area and low electrical conductivity[20]. To improve the performance of CuO-based supercapacitors, scholars have used compound mixture [20, 21]and different geometries CuO nanomaterial[19, 22]. One potential combination of compound mixture is CuO/graphene. Graphene is a single layer of carbon atoms arranged into a honeycomb crystal lattice that holds great potential for supercapacitor applications because of its excellent conductivity and high specific area[23, 24]. Generally, when used on its own, the potential of graphene is suppressed as it tends to restack during the exfoliation process and subsequent electrode precursor preparation procedure[25, 26]. This is supported by studies that confirmed that

graphene nanosheets (NSs) do not exhibit acceptable capacitance (intrinsic capacitance of graphene NSs was determined to be 21F/cm2[27] ) unless paired with a conducting polymer or transition metal oxide/hydroxide to form the supercapacitor. A CuO/graphene composite prepared via a lengthy, two-step electrostatic coprecipitation method has already been investigated for application in supercapacitors[18]. In terms of processing, it would be desirable to simplify the deposition process to a one-step process. In this paper, we synthesized a CuO/reduced graphene oxide (rGO) NS composite via a single-step hydrothermal method. The composite was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrochemical measurements. The electrochemical performance of CuO was compared to the CuO/rGO nanosheet nanocomposite electrodes under identical conditions to determine the benefits of graphene addition. Experimental All chemicals used in this study were of reagent grade, purchased from Sigma Aldrich and used without further purification. Graphene Graphene oxide (GO) was produced from graphite flakes by using a modified Hummer’s method[28]. First, 1.8g of graphite powder was added into concentrated sulfuric acid in a beaker (ice-cooled). Then 7g of KMnO4 was added into the graphite/sulphuric acid mixture and left to react for 3 hours. Then, 400ml of deionized (DI) water was introduced into the mixture, followed by the addition of H2O2, which resulted in the release of a large number of bubbles. The obtained mixture was bright

yellow, an indication of GO formation. Finally, the mixture was filtered and repeatedly washed with diluted hydrochloric acid to remove residual metal ions. Then the collected GO NSs were converted into rGO using a vacuum furnace set at 700oC, under s hydrogen flow of 5 sccm. CuO In a typical procedure, the substrate was introduced into a 0.05M copper nitrate solution in DI water. Then the prepared solution was sonicated in an ultrasonic bath for 10 minutes and then transferred into a Teflon-lined stainless steel autoclave, which was filled to 80% of its capacity (40ml). The autoclave was sealed and heated to 150oC for 2 hours and then cooled to room temperature. The deposited sample was collected and washed with water and ethanol and then dried at 80oC for 5 hours. CuO/rGO mixture To obtain a CuO/rGO mixture, the procedure used for CuO synthesis but with 30 mg of rGO added into the copper nitrate/water/urea mixture was used. Characterization For the electrochemical measurements, electrodes were prepared by mixing nmethylpyrrolidone with poly- vinylidiene fluoride (80:20) using a pestle and mortar. The effective surface area of the electrode was ~ 0.8cm2. The morphology of the deposited samples was investigated using SEM (FEI Quanta 400 F). The structure and crystal orientation was investigated using XRD (Siemens D5000) with Cu Kα radiation. SEM used to investigate the synthesized CuO composites. XPS was measured by using a Thermo Fisher Scientific Theta Probe with X-ray source using Al K = 1486.6 eV)

Electrochemical characterization of the samples was performed in a threecompartment cell using an electrochemical analyzer system (CH Instruments, Model 6627D) at room temperature with an Ag/AgCl electrode used as the reference electrode, and a piece of platinum foil used as the counter electrode. Cyclic voltammetry (CV) measurements were recorded in a 1.0 M Na2SO4 aqueous electrolyte in the range of -0.5 to 0.5 V with a scan rate of 10 mV/s. Charge-discharge measurements were performed using the same equipment. The specific capacitance of the fabricated supercapacitor was obtained using Eq. (1) based on the CV measurements or Eq. (2) based on the charge-discharge data.

(1)

(2)

where iaverage is the average current in amps (A), V is the scan rate, m is the mass of the electrode (0.0002 g), i is the constant current (A), t is the charge discharge time (s), and ΔV is the potential difference.

Results and discussion The SEM surface morphologies of synthesized CuO and CuO/rGO samples are shown in Figs. 1(a) and (b). From Fig. 1(a), it can be observed that the deposited CuO consisted of agglomerations of small particles (~10 nm) in a large interconnected structure. The deposited CuO is similar to the reported “cauliflower” structure [29]but with more interconnections between the large agglomerates. Figure 1(b) (CuO/rGO sample) also shows the formation of small particles but without further agglomeration. In addition, Fig. 1 (b) shows that the rGO surface was quite clean with only a few CuO nanoparticles (NPs) attached. Dai [30] found that the formation of metal

oxide/hydroxide on rGO is strongly influenced by the degree of oxidation, with little crystal growth on the high-quality graphene surface. This would indicate that our rGO was of good quality, sufficiently reduced with a limited number of oxygen sites for CuO growth. It is expected that such a high-quality rGO could provide the high conductivity that has been lacking in CuO. XRD was used for investigating the structure of the synthesized samples. Figure 2 shows the XRD patterns of CuO and CuO/rGO samples. For the CuO sample, the XRD pattern can be indexed to the monoclinic phase of CuO (JCPDS card no. 481548). Closer examination of the XRD pattern indicated some unreacted copper nitrate from the precursor solution indexed at 27.4°[31]. For the CuO/rGO sample, the peak at about 2θ = 26.4° corresponds to the (0 0 2) reflection of the stacked GO sheets with a d-spacing of 3.4 Å, much wider than that of natural graphite (3.3 Å) due to the insertion of oxygen-containing groups between the GO sheets. XPS analysis was performed to determine the chemical bonding of the CuO/rGO sample. Figure 3(a) shows the full survey spectrum of the CuO/rGO sample, which indicates the presence of Cu, O, C, and N without obvious impurities on the surface. From the deconvoluted C 1s spectra in Fig. 3(b), peaks belonging to C=C at 284.39 eV, C-O at 285.60 eV and C=C-O at 290.24 eV can be observed. These values are in good agreement with previously reported values[32, 33]. The presence of oxygencontaining carbon species suggests either an incomplete reduction process of rGO or bonding between rGO and CuO. Both mechanisms may occur simultaneously, as complete removal of function groups on GO is difficult and Cu is very reactive towards oxygen. Figure 3(c) shows the high-resolution O 1s spectrum for CuO/rGO, with peaks centered at around 531.90 eV, again in good agreement with previous report[18]. The distinctive peaks at binding energy of 531.90 eV can be attributed to

the oxygen in CuO and Cu(OH)2, respectively. The XPS Cu 2p core-level spectrum is shown in Fig. 3(d). The peaks at 934.80 and 954.20 eV are assigned to the binding energies of Cu 2p3/2 and Cu 2p1/2, respectively, indicating the presence of Cu2+ on the sample. The 20-eV gap between these levels is consistent with the standard spectrum of CuO. Figure 3(d) also shows two additional shake-up peaks at 943.29 and 963.00 eV, respectively, which are positioned at higher binding energies compared to those of the main peaks, suggesting the presence of an unfilled Cu 3d shell and thus further confirming the existence of Cu2+ in the sample. The XPS data indicate high oxygen incorporation on the CuO surface, which can lead to higher electrochemical activity due to the additional surface states. Figure 4 a) and b) shows CV measurements of the CuO and CuO/rGO samples scanned at various rates (10, 20, 40, and 100 mV·s-1). It can be seen that the current under the curve slowly increased with scan rate. This means that the voltammetric current is directly proportional to the CV scan rate, indicating ideally capacitive behaviour. The electrochemical tests of the CuO and CuO/rGO electrodes showed a pair of redox peak, indicating the combination of electric double-layer capacitance (EDLC) and pesudocapacitor behaviour. The pesudocapacitor behaviour originated from the conversion between different copper oxidation states in the CuO NPs. The surface and electrode charge storage mechanisms are proposed as follows: The first mechanism is based on the surface absorption/desorption of Na ions on the surface (Equation 3). ⇔

Where A can be either Na or H ions.

))

(3)

The charge storage can also occur as a result of intercalation or de-intercalation of Na+/H+ ions in the electrode (redox reaction). The reduction occurs as negative voltage is swept across the CuO electrode, which resulted in reduction of Cu2+ ions to Cu+ (Equation 4). Conversely, the Cu+ ion oxidizes as a positive voltage sweep across the electrode. ⇔

(4) ⇔

(5)

The capacitance values were determined by extracting the integrated charge from the CV curves. Figure 4(c) shows the variation of specific capacitance with scan rate. Note that the CuO/rGO sample exhibited a maximum specific capacitance of 80 F·g-1 at a scan rate of 10 mV·s-1. Previous studies on the supercapacitive properties of CuO thin films reported specific capacitance values of 36 to 43 F·g-1 [34, 35]. The high specific capacitance obtained in the present study is probably due to the CuO NPs facilitating fast ionic transfer. Figure 4 c) shows the variation of specific capacitance of the CuO and CuO/rGO supercapacitors with different scan rates, which indicates a decrease in capacitance with increasing scan rate, which is due to the presence of inaccessible inner active sites that cannot sustain the redox transitions due to the diffusion effect of ions in the electrode. It can be seen from Fig. 4c) that the CuO/rGO sample outperforms the CuO-based supercapacitors. The specific capacitance of CuO/rGO (80 F·g−1) was more than four times that of CuO (20 F·g−1) at a current density of 2 A·g−1. The enhanced capacitive

performance is due to the combination of EDLC from rGO NSs and pseudocapacitance from CuO NPs. The charging and discharging performance of the prepared electrodes was evaluated using chronopotentiometric measurements. The galvanostatic charge-discharge curves are shown in Fig. 5. For the CuO/rGO sample, the charge-discharge curve does not exhibits equilateral triangular shape, indicating the involvement of faradic reaction process[36]. It can be also observed from Fig. 5 that both of the CuO and CuO/rGO electrode exhibited a potential drop (IR drop). The IR drop originated from the internal resistance of the electrode (electrical resistance and solution resistance). From the chronopotentiometric measurements, the specific capacitance can be determined using Eq. (2). The specific capacitance for CuO/rGO (78.72 F·g−1) is higher than that for CuO (13.68 F·g−1) and consistent with the CV data. Therefore, the improvement in capacitance performance for the CuO/rGO sample can be attributed to the rGO NSs providing an improved electrical connection between the CuO NPs, facilitating the electron transfer process. This theory is further supported by the SEM analysis, which shows well-dispersed CuO NPs on the surface of rGO NSs. Attachment of the CuO NPs on the rGO NSs effectively prevents the CuO from agglomerating and rGO from restacking, thus improving ion transport. In order to understand the enhanced electrochemical performance in CuO/rGO electrodes compared to bare CuO, AC impedance measurements were performed and plotted in Fig. 6. The Nyquist plot of supercapacitors with CuO and CuO/rGO electrodes shows straight lines in the low-frequency region and an arc in the highfrequency region. This high-frequency loop is related to the resistance between rGO NSs for the CuO/rGO electrodes . The magnitude of equivalent series resistance (ESR)

can be extracted from the x-intercept of the Nyquist plot in Fig. 6 for the electrodes. The ESR values for CuO and CuO/rGO electrodes are 13.69 and 2.56 Ω, respectively. The difference in ESR can be attributed to the different conductance values of the electrode materials. Basically, the high conductivity of rGO resulted in a significant drop in charge resistance between the CuO NPs and hence reduced ESR. The semicircle in the high-frequency range represent the charge transfer resistance. It can be observed from Fig. 6 that the CuO/rGO electrode exhibit smaller diameter than pure CuO in the Nyquist plot, indicating lower charge transfer resistance. Generally, increased impedance in the lower frequency region of the Nyquist plot suggests either an increased ion diffusion path or impeded ion movement[18]. From Fig. 6, it can be observed that the CuO/rGO electrode possesses a more vertical (lower impedance) line in the lower frequency region than the pure CuO sample. This is an indication of reduced charge density in the electrolyte, which resulted in lower resistance of ion transfer and hence higher capacitance. Conclusion This study fabricated supercapacitors using CuO and CuO/rGO as electrode materials, respectively, and found that an increase of around 400% in capacitance can be achieved by employing a rGO NS between CuO NPs. The improvement of the supercapacitor electrodes is due to the prevention of the graphene restacking process by the introduction of CuO and reduced electrical resistance between CuO NPs. Acknowledgement

The authors would like to thank the National Science Council of Taiwan for providing financial support under grant NSC 103-2221-E-024-019 and 104-2221-E-024 -022. The financial support from National University of Tainan is also greatly appreciated.

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Figure 1 SEM image of a) hydrothermally synthesized CuO and b) CuO/RGO mixture (scale bar 5m)

a)

Figure 2 XRD pattern for the CuO and CuO/RGO composite

b)

Figure 3 XPS analysis a) general survey b) C1s spectra c) O1s speatra and d) Cu2p spectra of the CuO/rGO sample

Figure 4 a) Cyclic voltammograms of CuO/rGO electrode at different scanning rate, b) Cyclic voltammograms of CuO electrode at different scanning rate and c) extracted specific capacitance of the CuO and CuO/rGO electrode

Figure 5 Galvanostatic charge-discharge curve of CuO and CuO/rGO electrode at 1mAcm-2 current density

Figure 6 Nyquist plots of A) CuO and b) CuO/rGO electrode