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Materials based on reduced graphene oxides for supercapacitor devices

Materials based on carbon are important because of its low price and abundance, many potential applications in various fields of science and engineering. Graphene is carbon based material consisting of atomic layer of graphite, organized into a hexagonal lattice in which carbon atoms are connected using sp2 hybridisation resulting in unique electrical properties of zero bandgap semiconductor with both holes and electrons as charge carriers. Its excellent electrical and mechanical properties combined with optical and thermal ones, make it the one of the most investigated material in recent years. One of the possible applications of graphene is based on its high surface area which together with high conductivity opens up the possibility for its application as active electrode components in supercapacitors. Till today a lot of efforts have been made to create the best method for providing graphene in bulk quantities. Most frequently used methods were mechanical or thermal exfoliation of bulk graphite, chemical vapor deposition or epitaxial growth. However, until now only chemical2 synthetic methods proved capable of production of graphene with relatively good properties in bulk quantities. In chemical methods first step is oxidation and exfoliation of graphite into graphene oxide (GO), product that contains high quantities of different oxygen functionalities, followed by its reduction into the compound usually termed reduced graphene oxide (rGO). However, rGO are far away from theoretical predictions due to agglomeration of rGO sheets and also due to residual defects from oxidation step which highly disrupts the conductivity of such materials and consequently limits its application properties. The aim of this work was the synthesis of rGO starting from different precursors and using different experimental conditions in order to determine the optimal conditions for production of rGO with the best capacitive properties. Two precursors were used: natural graphite flakes and synthetic graphite. Both materials were subject to the oxidation and exfoliation by Staudenmaier method followed by treatment with either sodium borohydrate2 or hydrazine hydrate as reducing agents. Obtained GO and rGO were characterized using SEM, AFM, XPS and UV spectroscopy. Supercapacitive properties of resulting materials were investigated by cyclic voltammetry. In addition, the influence of rGOs on the pseudocapacitive responses of pseudocapacitive materials such as MnO2 and polypyrrole was also investigated. Using cyclic voltammetry method it was shown that GO layer on Pt support had poor conductivity and that it was not electrochemically active, while rGO had much better conductivity and reversible capacitive response due to the charging/discharging process at electrode/electrolyte interface. Capacitive values of rGO varied from 10-100 F g-1 depending on choice of graphite precursor in the oxidation step, the type of reducing agents as well as on the pH value and reaction temperature during synthesis.

graphene materials, supercapacitors. MnO2, polypyrrole

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