Electrochemical Study of a Novel High Performance Supercapacitor Based on MnO2/Nitrogen-Doped Graphene Nanocomposite

Supercapacitors as energy storage devices have advantages in comparison to the batteries such as higher power densities, fast charging/discharging rates, and excellent cycling stabilities.Supercapacitors are used particularly for the applications involving in high power electronics, electrical utilities, transportation, medical electronics and military defense systems [1]. MnO2 as a supercapacitor electrode has attracted many attentions due to its relatively low cost, and beingenvironmental friendly as neutral aqueous electrolytes [2]. However, the low electrical conductivity (10−5-10−6 S cm−1) and low electrochemical kinetic of MnO2 limit its charge storage and thus its commercial usage. The electrical conductivity of MnO2 could be improved byincorporating various types of carbonaceous compounds such as graphite, graphene oxide,graphene, carbon nanotube, mesoporous carbon, and carbon black by various methods [1-3]. Nitrogen doped reduced graphene oxide (NRGO) can increase the conductivity of metal oxidebasedsupercapacitors. In the present study, a simple method for preparation of MnO2/NRGO asthe supercapacitor electrodes is introduced using ultrasonic vibration. Applying ultrasonic waves is a simple method for the synthesis of nanostructures [23]. The process, known as cavitation, isbased on incorporating bubbles in the liquid, where they grow up and finally collapse. Thebursting of bubbles results in very high local temperature and pressure leading to reactions in the solution [4]. The structure and morphology of MnO2/NRGO nanocomposites are characterized by X-ray diffraction, X-ray photoemission spectroscopy (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. The electrochemical supercapacitive performance of the nanocomposite was investigated by cyclic voltammetry (CV), FFT Continuous cyclic voltammetry (CCV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy (EIS) methods. The supercapacitive performance of the RGO, NRGO, pure MnO2, MnO2/RGO andMnO2/NRGO electrodes were studied by CV method, using a three-electrode system using Ag/AgCl as the reference and platinum foil as the counter-electrode. Fig. 1a illustrations thetypical CV curves of the MnO2, RGO/MnO2 and NRGO/MnO2 electrodes were measured at 50 mV s-1 in 0.5 M Na2SO4. The results show that the CV curves of the RGO/MnO2 and NRGO/MnO2 electrodes are symmetrical respect to the zero-current line and a rapid current change around the potential reversal at the end each scan. In fact, existing a quasi-rectangular shapes and symmetric I–V responses indicates the ideal pseudocapacitive behavior of thematerials. The measured currents of the MnO2 and MnO2/RGO electrodes, under the same conditions, were smaller than the measured current for the MnO2/NRGO electrode. However, for the case MnO2/NRGO electrode, the current enhancement could be the results of synergistic effect for the combination MnO2 and NRGO, which cause an improvement of the electronicconductivity of the nanocomposite. These results also, shows the calculated SC values for the MnO2, MnO2/RGO, and MnO2/NRGO samples are equal 298, 437 and 522 F g-1 at scan rate 2 mV s-1, respectively. Fig. 1b shows the CV curves of the MnO2/NRGO electrode at scan rates of 5, 10, 20, 30, 40 and 50 mV s-1. As shown on the figure, there is not a notable change in the rectangular shape with scan rate, which is an indication of a fast and reversible process at the electrode surface. The CCV technique could be considered the best tools for examination of the monitoring in theCVs and charge storage of a capacitor during that time [2, 4]. In this method, under a long-termpotential cycling, the stability of the electrodes are evaluated. The calculated SC as a function of time are presented in Fig. 1c. As shown in Fig. 1c, after applying 4000 CVs, the value of SCdecrease 17.9%, 7.0% and 3.7%for the MnO2, MnO2/RGO and MnO2/NRGO electrodes, respectively. The above results prove that the MnO2/NRGO electrode are highly stable during potential cycling test compared to MnO2 and MnO2/RGO electrodes. Fig. 1d shows threedimensional (3D) CCVs, which was performed at a scan rate of 200 mV s-1. In these two 3Dplots,the changes in the CVs over time are more noticeable.

کلیدواژه ها:

Supercapacitor ، Nanocomposite ، N-doped Graphene ، MnO2 nanoparticles ، Sonochemistry