Electrochemical and impedance analyses of supercapacitive energy sorage systems

Energy storage systems are just as important as energy production in maintaining the energy supply-demand balance. Supercapacitors stand out among other energy storage systems due to their high power densities and long cycle lives. However, in order to meet the increasing energy demands of modern technology, the energy densities of the supercapacitors need to be increased. The performance of the supercapacitors are mainly determined by the properties of the materials they contain. In addition to the chemical properties, physical features such as pore structure, morphology, crystal structure, internal resistance and conductivity define the performance. In this thesis; a nanocomposite was obtained from a liquid phase (aqueous) exfoliated graphene and MnO2 that was synthesized via environment-friendly, cheap and simple precipitation method in the presence of a template. Pluronic triblock copolymer (PEO 20-PPO 70-PEO 20, P123) was not only utilized as a parogen and carbon source, but also as a graphite exfoliation agent. Graphene, with large surface area and high electrical conductivity, contributed to electrical double layer capacitance of the composite, while pseodocapacitive MnO2 improved the overall capacitance and energy density. The soft template (P123) aided in the formation of nanopore structure which facilitates ion diffusion and increased pore volume. The synthesized materials were fully characterized by; X-ray diffractometry, Raman spectroscopy, thermogravimetric analyses, X-ray photoelectron spectroscopy, surface area and pore size analyses, scanning electron microscopy and transmission electron microscopy. Electrochemical performances were examined by cyclic voltammetry, galvanostatic charge/discharge tests and AC electrochemical impedance spectroscopy. The composite electrode which was studied in 1 M Na2SO4 aqueous electrolyte and in the potential range of 0-1,2 V (Ag/AgCl sat. KCl) exhibited a high specific capacitance of 587,4 F g-1 at 1 A g-1. The impedance behavior of the electrode was modelled with modified Kang's circuit and a very low internal resistance (24,17 Ω) was recorded. The MnO2/Graphene//MnO2/Graphene symmetric supercapacitor delivered 2 Wh kg-1 (33 W kg-1) energy at a current rate of 0,1 A g-1. The MnO2/Graphene//Fe3O4/Carbon and MnO2/Graphene//Active Carbon asymmetric supercapacitors delivered energy densities of 21,10 Wh kg-1 (106,14 W kg-1) and 24,8 Wh kg-1 (103,78 W kg-1), respectively, at current rate of 0,1 A g-1. The nanocomposite obtained through a scalable and practical methodology is expected to make a significant contribution to the energy storage applications.