High-performance solid-state supercapacitors based on graphene-ZnO hybrid nanocomposites
© Li et al.; licensee Springer. 2013
Received: 17 September 2013
Accepted: 31 October 2013
Published: 12 November 2013
In this paper, we report a facile low-cost synthesis of the graphene-ZnO hybrid nanocomposites for solid-state supercapacitors. Structural analysis revealed a homogeneous distribution of ZnO nanorods that are inserted in graphene nanosheets, forming a sandwiched architecture. The material exhibited a high specific capacitance of 156 F g−1 at a scan rate of 5 mV.s−1. The fabricated solid-state supercapacitor device using these graphene-ZnO hybrid nanocomposites exhibits good supercapacitive performance and long-term cycle stability. The improved supercapacitance property of these materials could be ascribed to the increased conductivity of ZnO and better utilization of graphene. These results demonstrate the potential of the graphene-ZnO hybrid nanocomposites as an electrode in high-performance supercapacitors.
As a new class of energy storage device, supercapacitors, also known as electrochemical capacitors, has received considerable attention that can be used in hybrid electric vehicles, memory backup, and other emergency power supply devices due to their higher power density, superior cycle lifetime, and low maintenance cost. However, the energy density of supercapacitors is lower than batteries [1–6]. It is highly desirable to increase the energy density of supercapacitors to approach that of batteries, which could enable their use as primary power sources. Supercapacitors store electrical energy by two mechanisms [7, 8]: electrochemical double-layer capacitance (EDLC) and pseudocapacitance. In EDLC, the capacitance comes from the charge accumulated at the electrode-electrolyte interface. Carbon-based materials are widely used in EDLC electrode due to their high surface area and excellent electric conductivity. Compared to EDLCs, pseudocapacitors can provide much higher capacitance and energy density through Faradic reaction [6, 7]. Transition metal oxides and conducting polymers are the promising candidates because they can provide high energy density for pseudocapacitors. It has been found that carbon materials which combine with pseudocapacitive electrode materials can improve the capacitance of supercapacitors [8–10].
Graphene (Gr) is an atom-thick, two-dimensional (2D) material composed of a monolayer hexagonal sp2-hybridized carbon. Gr with the maximum surface area of 2,630 m2 g−1 and high intrinsic electrical conductivity is believed to be one of the most promising electrode materials for supercapacitors [11–14]. However, in practical applications, Gr nanosheets usually suffer from agglomeration or restacking due to strong van der Waals interactions [15–17], which leads to the loss of surface area and capacitance. Metal/metal oxide or metal hydroxide nanoparticles are currently introduced into the interlayer of Gr nanosheets to prevent agglomeration [18–21]. Transition metal oxides [22–25], which can contribute to pseudocapacitance such as RuO2, have been recognized as the best electrode materials for supercapacitors. However, their expensive nature and high toxicity severely limit their practical application in a large scale. Therefore, the development of low-cost and high-abundance metal oxide as an alternative is highly desirable [26–29]. ZnO is considered to be a promising material for supercapacitors due to its high specific energy density, low cost, non-toxicity, eco-friendliness, and abundant availability.
Very recently, Kim et al.  and Pan et al.  reported on reduced graphene oxide-ZnO nanocomposites for supercapacitor electrodes by microwave-assisted method, which exhibited a specific capacitance of 109 F g−1 at a scan rate of 2 mV s−1 and 146 F g−1 at a scan rate of 2 mV s−1, respectively. But only approximately 30 F g−1 at a scan rate of 100 mV s−1. A sandwiched nanoarchitecture of reduced graphene oxide/ZnO/deducted graphene oxide is fabricated by Huang et al.  using chemical vapor deposition method, which exhibited a specific capacitance of 51.6 F g−1 at a scan rate of 10 mV s−1. Additionally, graphene-ZnO nanocomposites synthesized by other method such as ultrasonic spray pyrolysis method and their electrochemical performance were reported [33, 34]. However, these materials were limited by a low specific capacitance and poor stability at higher scan rate or high current densities. An effective regulation of graphene-ZnO hybrid for high performance of supercapacitors is still challenging. On the other hand, the investigation of solid-state supercapacitors based on graphene-ZnO hybrid is very limited.
In this report, a simple and facile synthesis route is developed to prepare graphene-ZnO hybrid as an electrode material for supercapacitors using one-step hydrothermal technique. Initially, graphene oxide (GO) was synthesized using the well-known modified Hummer's method. ZnO nanorods are inserted between the graphene nanosheets layer-by-layer rather than simply decorated on the surface of graphene during GO hydrothermal reduction process. This strategy provides a novel method for the preparation of highly active materials (ZnO nanorods) directly grown on Gr surface that avoids the restacking of Gr sheets, which show high specific capacitance even at higher scan rate and excellent long-term cycle stability applied in a all solid-state supercapacitor device. Such high electrochemical properties provide important prospects for graphene-ZnO hybrid to be widely used as electrode material in supercapacitor.
Graphite powder was purchased from Sigma Aldrich (St. Louis, MO, USA). All other reagents were commercially available and analytic grade and were used directly without any purification. Double-distilled water was used throughout the experiments.
Synthesis of graphene oxide
Graphite oxide was prepared from natural graphite powder through a modified Hummers method . One gram of graphite powder, 1.1 g sodium nitrate, and 46 ml sulfuric acid were mixed and stirred for 10 min. Then, 3.0 g potassium permanganate was added slowly and temperature maintained below 20°C. DI water was added slowly and the temperature was raised to 90°C. The solution turned bright yellow when 3.0 ml of hydrogen peroxide (30%) was added. The mixture was filtered while warm and washed with warm DI water. Then GO was subjected to dialysis to completely remove metal ions and acids. Finally, the product was dried in air at room temperature.
Synthesis of ZnO nanorods
Pure ZnO nanorods were synthesized by hydrothermal method. In a typical experiment, 100 mg of Zn(NO3)2 was first dispersed into 30 ml deionized water. Then, 15 μl of hydrazine hydrate was added drop by drop under stirring, followed by ultrasonication for 30 min. Then the solution was transferred to a 50 ml of Teflon-lined autoclave and heated at 160°C for 12 h. Finally, the ZnO nanostructures were collected after washing and centrifugation.
Synthesis of the graphene-ZnO hybrid nanostructure
As-synthesized GO (50 mg) was dispersed in 100 ml double-distilled water; the dispersion was brown in color. The dispersed GO was exfoliated, using sonication for 1 h, and then 20 mg Zn(NO3)2 and 10 μl hydrazine hydrate were added into the abovementioned solution under ultrasonication. After hydrothermal reaction at 160°C for 12 h, the graphene-ZnO nanocomposites were synthesized and collected through washing, centrifugation, and drying.
The microstructure morphologies and crystal structures of the as-synthesized pure ZnO, pristine graphene, and graphene-ZnO nanocomposites were characterized using field-emission scanning electron microscope (FESEM, Quanta 250 FEG; FEI, Hillsboro, OR, USA), X-ray diffraction (XRD, D8 ADVANCE, Bruker, Billerica, MA, USA) with Cu-Kα radiation (λ = 0.154178 nm), transmission electron microscopy (TEM) (JEM2010-HR, JEOL, Akishima, Tokyo, Japan), and laser micro-Raman spectrometry (Renishaw inVia, Gloucestershire, UK). Energy dispersive spectrometer (EDS) mapping analysis was used to analyze the element distribution of the as-synthesized nanocomposites. Inductively coupled plasma atomic emission spectroscopy (ICP, SPECTRO, Birmingham, UK) was used to analyze the loading of ZnO on graphene. The electrochemical measurements were carried out on a CHI 660D electrochemical workstation (Chenhua, Shanghai, China) at room temperature.
Preparation of electrodes and electrochemical characterization
The working electrode was prepared as follows: approximately 10 mg of as-synthesized material was first mixed with polytetrafluoroethylene (60 wt.% water suspension; Sigma-Aldrich, St. Louis, MO, USA) in a ratio of 100:1 by weight and then dispersed in ethanol. The suspension was drop-dried into a 1 cm × 1 cm Ni foam (2-mm thick) at 80°C. The sample loaded foam was compressed before measurement.
The electrochemical measurements including cyclic voltammograms (CVs), galvanostatic charge/discharge, and electrochemical impedance spectroscopy were performed in a three-electrode setup: a Ni foam coated with the active materials serving as the working electrode, a platinum foil electrode, and a saturated calomel electrode (SCE) serving as the counter and reference electrodes, respectively.
Fabrication of solid-state supercapacitors
The device was assembled by two pieces of graphene-ZnO electrodes with a separator (Whatman 8-μm filter paper) sandwiched in between and polyvinyl alcohol (PVA)-gelled as a solid electrolyte. The PVA-gel electrolyte was made by following method. 600 mg PVA was mixed with 5 ml Milli-Q water (Millipore Corp., Billerica, MA, USA). The mixture was heated at 80°C under stirring for 30 min and then cooled naturally. Then approximately 10 ml of 0.5 M NaNO3 was added to the mixture and stirred for 30 min. The graphene-ZnO hybrid materials were collected on a Teflon membrane (0.2-m pore size) by vacuum filtration and then pressed onto the carbon-coated Al current collector. The graphene-ZnO electrodes and a separator were sandwiched together in a stainless steel cell for the fully assembled two-electrode cell device.
Results and discussion
The long cycle life of the supercapacitors is an important parameter for their practical application. The cycle stability of the graphene-ZnO hybrid electrode was further evaluated by repeating the CV measurements between 0 and 1.0 V at a scan rate of 100 mV s−1 for 5,000 cycles. Figure 6d shows the capacitance retention ratio as a function of cycle number. The capacitance of graphene-ZnO hybrid electrode retained 94% of its initial capacitor after 5,000 cycles (Figure 6d), which demonstrates excellent electrochemical stability. From these results, we concluded that the graphene-ZnO hybrid electrode materials showed a higher specific capacitance, significantly improved energy density, and excellent cycling performance.
The better electrochemical performance of the as-prepared graphene-ZnO electrode can be attributed to the following aspects: On the one hand, Gr sheets in the hybrid structure can act as a conducting agent, which greatly improves the electrical conductivity of the hybrid structure. On the other hand, the small size of the ZnO nanorods uniformly dispersed between the Gr sheets can effectively prevent the agglomeration and restacking of the Gr nanosheets, resulting in an EDLC for the overall specific capacitance. At the same time, Gr nanosheet with a large surface area in the hybrid structure not only provided double-layer capacitance to the overall energy storage but also effectively inhibited the aggregation of ZnO nanorods, resulting in fast electron transfer throughout the entire electrode matrix as well as an overall improvement in the electrochemical performance. Moreover, the nanometer-sized smaller ZnO rods facilitate faster charge–discharge rates, because the faradaic reaction replaced the diffusion-controlled Na+ ion intercalation process which usually occurs at the ZnO surface . Therefore, the supercapacitive performance of graphene-ZnO hybrid based supercapacitor is significant improved.
In summary, the graphene-ZnO hybrid nanostructure as an electrode material for solid-state supercapacitors was successfully synthesized using one-step hydrothermal method. The surface morphology, microstructure, composition, and capacitive behaviors of the as-prepared materials were well investigated. SEM and TEM images revealed the uniform distribution of ZnO nanorods on the Gr sheet substrate. In comparison with the specific capacitance of ZnO and pristine Gr electrode, the specific capacitance of graphene-ZnO hybrid electrode (156 F g−1 at a scan rate of 5 mV s−1) is significantly improved. Moreover, the material exhibited excellent electrochemical stability. The improved supercapacitance performance of the graphene-ZnO hybrid was mainly attributed to the pseudocapacitance of the ZnO phase and the intrinsic double-layer capacitance of the Gr sheets. The low price, abundant resources, and environmental friendliness of ZnO may render their nanocomposites a promising candidate for practical applications.
The authors are grateful for support from the National Natural Science and Henan Province United Foundation of China (no. U1204601 and no. 51072063), Natural Science Foundation of Henan Province (no. 122300410298), Natural Science Foundation of Education Department of Henan Province (no. 13A480365), and PhD Foundation of Zhengzhou University of Light Industry (no. 2010 BSJJ 029).
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