Facile synthesis and strongly microstructure-dependent electrochemical properties of graphene/manganese dioxide composites for supercapacitors
© Zhang et al.; licensee Springer. 2014
Received: 28 July 2014
Accepted: 7 September 2014
Published: 13 September 2014
Graphene has attracted much attention since it was firstly stripped from graphite by two physicists in 2004, and the supercapacitor based on graphene has obtained wide attention and much investment as well. For practical applications of graphene-based supercapacitors, however, there are still many challenges to solve, for instance, to simplify the technological process, to lower the fabrication cost, and to improve the electrochemical performance. In this work, graphene/MnO2 composites are prepared by a microwave sintering method, and we report here a relatively simple method for the supercapacitor packaging, i.e., dipping Ni-foam into a graphene/MnO2 composite solution directly for a period of time to coat the active material on a current collector. It is found that the microwave reaction time has a significant effect on the microstructure of graphene/MnO2 composites, and consequently, the electrochemical properties of the supercapacitors based on graphene/MnO2 composites are strongly microstructure dependent. An appropriately longer microwave reaction time, namely, 15 min, facilitates a very dense and homogeneous microstructure of the graphene/MnO2 composites, and thus, excellent electrochemical performance is achieved in the supercapacitor device, including a high specific capacitance of 296 F/g and a high capacitance retention of 93% after 3,000 times of charging/discharging cycles.
81.05.ue; 78.67.Sc; 88.80.fh
In recent years, with the deterioration of the environment and the scarcity of natural resources, more and more researchers have turned their attention to the field of energy. Supercapacitor, serving as a novel energy storage device, is one of the mostly focused topics. The materials for supercapacitors that have been intensively studied so far can be divided into three groups: transition metal oxides, carbon materials, and conductive polymers . In the first group, MnO2 and RuO2 are two typical materials, and they are usually used for the fast and reversible redox reactions since the pseudocapacitance generated from the faradaic redox reactions is helpful for a remarkable increase in the specific capacitance of supercapacitors. However, metal oxides usually have a high electrical resistance, thereby leading to a low power density, and the high cost of RuO2 also limits its wide applications. For the second group, they are usually used in double-layer capacitors because of their electrochemical stability and high accessible surface area. For the third group, polyaniline and polypyrrole [2–5] for instance, they show high specific capacitance whether in aqueous or in nonaqueous electrolytes. However, the conductive polymers become unstable with extended lifetimes of charging/discharging. This may reduce severely the initial performance for supercapacitors . Due to the unique character in each supercapacitor material, it is significant for us to develop a composite material for supercapacitors, which takes advantage of both double-layer capacitors and the faradaic pseudocapacitors. Graphene/MnO2 composite is such an alternative material.
It is noteworthy that a simple method was used here for the supercapacitor packaging, i.e., dipping Ni-foam into a graphene/MnO2 composite solution directly for a period of time to coat the active material on a current collector. Moreover, the process-structure-property relationships were systematically investigated for the graphene/MnO2 composites. Interestingly, it was found that the microwave reaction time has a significant effect on the microstructure of graphene/MnO2 composites and the electrochemical properties of the supercapacitors based on graphene/MnO2 composites are therefore strongly microstructure dependent.
Synthesis: graphene and graphene/MnO2 composite
Graphite oxide (GO) was synthesized firstly from natural graphite according to a modified Hummers method . A GO solution that displays a brown dispersion was subsequently prepared. For purification, the mixture was successively washed with 5% HCl and deionized water for several times to completely remove residual salts and acids. Once the filter cake was dried in an electric thermostatic drying oven at 40°C, the graphene oxide powders were obtained. After that, the graphene oxide powder (640 mg) was dispersed in 600 ml deionized water and then sonicated until it was well distributed. Twenty milliliters hydrazine hydrate was added into the suspension, and the suspension was then kept at 90°C for 24 h [1, 23]. Finally, the suspension was filtered and washed several times with deionized water and alcohol, and then dried at 50°C for 12 h in a vacuum oven.
Graphene/MnO2 composites were prepared by a redox reaction between graphene and potassium permanganate under microwave irradiation . In the first step, 100 ml of graphene water suspension (1.65 mg/ml) was subjected to ultrasonic vibration for 1 h. Then KMnO4 powder (0.95 g) was added into the graphene suspension and stirred for about 10 min. Subsequently, the resulted suspension was heated using a household microwave oven (Midea, Foshan, China, 2,450 MHz, 700 W) for several minutes, and then cooled to room temperature naturally. Finally, the black deposit was filtered and washed several times with distilled water and alcohol, and then dried at 80°C for 6 h in a vacuum oven. Different microwave reaction times, namely, 5, 10, and 15 min, were selectively used to study its effect on the microstructure and the electrochemical properties of the graphene/MnO2 composites. Due to the lab condition restriction and, more importantly, in order to avoid the overflowing of the reaction solution, we did not extend the microwave reaction time further.
The crystallographic structures of the graphene/MnO2 composites were measured by X-ray diffraction (XRD; DX-1000, Dandong Fangyuan Instrument Co., Ltd., Dandong, China) using Cu Kα radiation (λ = 0.154056 nm). The microstructure was characterized by scanning electron microscopy (SEM; Hitachi S-4800, Hitachi, Ltd., Chiyoda-ku, Japan, operated at 30 kV) and transmission electron microscopy (TEM; JEOL JEM-2100 F, JEOL, Ltd., Akishima-shi, Japan, operated at 200 kV). Note that the samples were dispersed in alcohol and dropped on a holey copper grid for TEM observations. The electrochemical measurements (cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy) were measured by using an electrochemical station (CHI 660E, CH Instruments, Inc., Austin, TX, USA) with a two-electrode system, which consists of two identical working electrodes in 6 M KOH alkaline electrolytes.
The fabrication of working electrodes was carried out in the following way. Briefly, the materials, including graphene/MnO2 composite, carbon black, and polytetrafluoroethylene (PTFE), were mixed in a mass ratio of 75:20:5 and dispersed in ethanol . Then the nickel foam substrate in the form of small rounds was dipped directly into the as-prepared suspension for several minutes and subsequently dried at 80°C for 12 h in a vacuum oven. Since the nickel foam is easy to be oxidized and its surface may contain oil, the nickel foam was cleaned sequentially by acetone, deionized water, diluted hydrochloric acid, deionized water, and ethanol before it was used. Taking out the dried Ni-foam with active material coated and exerting 10-MPa pressure by a table press, we got the electrode slices for use. After dipping the electrode slices into a 6 M KOH alkaline electrolyte solution for 24 h, we assembled the button-type supercapacitor for tests.
Results and discussion
To characterize the electrochemical behavior of the button-type supercapacitor based on graphene/MnO2 composites, we have carried out cyclic voltammetry (CV), galvanostatic charge/discharge (CD), and electrochemical impedance spectroscopy (EIS) tests using a two-electrode system. To avoid complex steps, we adopted a simple method of dipping directly the as-prepared Ni-foam into the suspension of graphene/MnO2 composite, carbon black, and PTFE to prepare electrode slices. This method is simple to operate, and the active materials are not easy to fall off once they have been coated on Ni-foam. Furthermore, the active materials are relatively well distributed. However, there is also a disadvantage, that is, it is hard to control the mass of the active materials. Hence, we cannot get the difference in specific capacitance directly just by looking at the CV and CD diagrams since the mass of the active materials is different in those three samples. Nonetheless, the mass of the active materials can be calculated out by comparatively weighing the Ni-foam before and after coating the active materials, and the specific capacitance can be therefore obtained for each supercapacitor device.
where I is the change/discharge current, t is the discharging time, m is the mass of the active material of two electrodes, and V is the voltage window after the deduction of the IR drop. Accordingly, as the microwave reaction time increases from 5 to 15 min, the specific capacitance at a charging current of 2 mA reaches 246, 260, and 296 F/g, respectively. As aforementioned, though the mass of the active material in each device is different, we can get the specific capacitance values similarly through calculating the data in CV curves. The enhancement of specific capacitance with increasing the microwave reaction time is believed to stem from the improved microstructure, as mentioned above. The results obtained in this work are comparable to or even higher than those reported in the literature for similar graphene-based material systems, where the specific capacitance was reported to be about 200 to 300 F/g [30–32].
High-quality graphene/MnO2 composites have been prepared by a relatively simple microwave sintering method, and a simple method, i.e., dipping Ni-foam into a graphene/MnO2 composite solution directly for a period of time to coat the active material on a current collector, has been proposed for the supercapacitor packaging. It is demonstrated that the microwave reaction time has a significant effect on the microstructure of graphene/MnO2 composites and the electrochemical properties of the supercapacitors based on graphene/MnO2 composites are strongly microstructure dependent. An appropriately longer microwave reaction time, namely, 15 min, facilitates a very dense and homogeneous microstructure of the graphene/MnO2 composites, and thus, excellent electrochemical performance has been achieved in the supercapacitor device based on the graphene/MnO2 composite, comprising a high specific capacitance of 296 F/g and a high capacitance retention of 93% after 3,000 times of charging/discharging cycles. The results obtained in this work pave the way for facile synthesis and optimization of graphene-based composite materials for practical supercapacitor applications.
CZ is a postgraduate student at Sichuan University (SCU). XZ has a PhD degree and is a professor at SCU. ZW and PS have a master's degree. YR is a postgraduate student at SCU. JZ (Jiliang) has a PhD degree and is a professor at SCU. JZ (Jianguo) has a PhD degree and is a professor and head of a school at SCU. DX is a professor and head of a research laboratory at SCU.
This work was financially supported by the Ministry of Science and Technology of China (MOST) (Grant No. 2013CB934700).
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