Application of graphene-based materials in electrochemical energy storage devices and investigation of electroactive species in all vanadium redox flow cells

The main global energy challenge is to reduce our dependence from fossil fuels. The main reason is that combustion of fossil fuels, that are limited, to produce energy generates CO2, that is the prime responsible of climate change. In order to reduce the use of fossil fuels, two main strategies are to be considered. One strategy consists in shifting electricity production from burning fuel to sustainable energy sources; the other one in developing electric vehicles, decreasing the use of oil for vehicles propulsion. For both strategies, electrical energy storage systems, that store energy from available resources and subsequently exploit that energy when needed, are critically important. Being the renewable energy supply not continuous but intermittent, electrical energy storage systems are needed to store the excess of energy produced during the peak production and to assure quality power level during the peak demand. Hence, the development of electrical storage systems with long term stability and prolonged cycle life is of great interest. Furthermore, to reduce the use of oil, conventional internal combustion engines needs to be replaced by electric ones. In this respect, an electric energy storage system should enable the vehicles to drive for long distances with a reasonable speed. For this reasons high energy and high power density with acceptable safety are the requirements for energy storage system in the automotive field. Different typologies of electrical energy storage systems can be listed: lithium-ion batteries, redox flow cells, lithium-air, super capacitors. They are suitable for several applications, accordingly to their different features. The research work presented in this thesis deals with three different types of electrical energy storage systems, i.e. lithium-ion batteries, electrochemical double layer capacitors and flow cells. More in details, investigation of graphene and graphene based composite as electrode materials for rechargeable lithium ion batteries (LIBs) and electrochemical double layer capacitors (EDLCs), as well as a spectroscopic method for determining the state of charge (SOC) of flow cells, will be presented. In the first part of my PhD thesis, graphene and metal-graphene composites are investigated as anodic materials for lithium-ion batteries. Graphene (RGO) was either synthesized in our labs or purchased from companies and characterized first morphologically, by X-ray and SEM analysis, and then electrochemically. Synthesis of Sn-graphene, Sb-graphene, Sn-Sb-graphene composites were also developed using microwave-assisted polyol procedure and polyacrylic acid (PAA) as surfactant. Their morphological and electrochemical characterization is presented. The second part of the present dissertation describes the results obtained during the six months period spent as a visiting PhD student at the ZSW (Zentrum fa'¼r Sonnen-Energie und Wasserstoff-Forschung) in Ulm, Germany, under the supervision of Dr. Margret Wohlfahrt-Mehrens and Dr. Sonia Dsoke. The work deals with the study of graphene based electrodes for EDLC electrodes. Samples of graphene nano sheets with different morphologies were used as active material. Graphene nano sheets were also mixed together with activated carbon in different percentages and tested as electrodes in order to enhance specific capacitance and overall conductivity. In the last part of the thesis, a preliminary study on redox flow cell system is presented. The redox couple VO2+/VO2+ was investigated by cycling voltammetry with different working electrodes as well as by voltabsorptometry.