Synthesis of a novel Magnetite/Nitrogen-doped Reduced Graphene Oxide nanocomposite as High Performance Supercapacitor

In today's world, electrochemical supercapacitors (ESs) could be considered as one of the most important energy storage devices for a larger number of power portable electronics and electricalvehicles. They can be classified into two major groups based on the charge-storage mechanismsin them, electrical double-layer capacitors (EDLCs) and pseudocapacitors [1-2].The pseudocapacitors, could have an individual capacitance due to their electrochemically activematerials through rapid and faradic redox reaction. Numerous materials are used in constructionof them, such as transition metal oxides, and conductive polymers. Indeed, these materials have shown a higher capacity than those carbon-based materials. A number of metal oxides suchRuO2, NiO, MnO2, Co3O4, and Fe3O4 have found to be very suitable for making pseudocapacitors [3]. But the high internal resistance and lower the cyclic stability of metal oxides could be the main reason for the limitation in their use in commercial ESs. There are many attempt to solve thisproblem, such as application of the Metal oxides with carbon-based materials as nanocomposite, for examples; exfoliated graphite, mesoporous carbon, carbon black, carbon nanotubes, and graphene oxide [4].In the present study, a simple method for preparation of nanostructured Fe3O4/NRGO as the ES electrodes is introduced using ultrasonic vibration. Applying ultrasonic waves is a simple methodfor the synthesis of nanostructures. The pseudocapacitive behaviors of the Fe3O4 and Fe3O4/NRGO composite electrodes were examined by cyclic voltammetry (CV), Continuous cyclic voltammetry (CCV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy (EIS) in a 0.5 M Na2SO4 electrolyte. The supercapacitive performance of the NRGO, Fe3O4, Fe3O4/RGO and Fe3O4/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 the typical CV curves of the NRGO,Fe3O4, Fe3O4/RGO and Fe3O4/NRGO electrodes were measured at 50 mV s-1 in 0.5 M Na2SO4. The results show that the CV curves of the Fe3O4/RGO and Fe3O4/NRGO 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–Vresponses indicates the ideal pseudocapacitive behavior of the materials. The measured currents of the Fe3O4 and Fe3O4/RGO electrodes, under the same conditions, were smaller than the measured current for the Fe3O4/NRGO electrode. However, for the case Fe3O4/NRGO electrode,the current enhancement could be the results of synergistic effect for the combination Fe3O4andNRGO, which cause an improvement of the electronic conductivity of the nanocomposite. These results also, shows the calculated specific capacitance values for the Fe3O4, Fe3O4/RGO, andFe3O4/NRGO electrodes are equal 143, 271 and 355 F g-1 at scan rate 2 mV s-1, respectively. Fig. 1b shows the CV curves of the Fe3O4/NRGO electrode at different scan rates. 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. In order to gain further understanding about the supercapacitive performance, the galvanostatic charge/discharge of the Fe3O4, Fe3O4/RGO, and Fe3O4/NRGO electrodes are measured at the potential range of –0.8–0.2 V at charge/discharge current density of 2.0 A g-1 (Fig. 1c). The electrodes have stable electrochemical properties in the 0.5 M Na2SO4 electrolyte where thecapacitor voltage is varied linearly with the time during both charging and discharging. Almostno voltage drop was observed during the discharge process, giving high values of charge/discharge efficiency. Fig. 1d shows the charge/discharge of the Fe3O4/NRGO electrode at different current densities. A good linear variation of potential versus time was observed for allthe curves, which is another typical characteristic of an ideal capacitor. The small iR drop indicates a conductive characteristic of a composite material. This is related to the formation of ahighly conductive network by adding NRGO, which facilitates the interface contact between the electrolyte and the Fe3O4 of the composite electrode. Therefore, the internal resistance within the composite electrode itself is reduced and good electronically conducting pathways are provided for protonic and electronic transportation during the rapid charge/discharge process. The shape ofthe charge/discharge curves shows agreement with the result of the CV curves

کلیدواژه ها:

Supercapacitor ، Nanocomposite ، N-doped Graphene ، Fe3O4 nanoparticles