Design and manufacturing of dimensionally controlled graphene based hybrid structures by core-shell electrospinning for energy storage systems

In the first part of study, two-dimensional (2D) graphene oxide sheets were converted into three forms of fibers, foam, and spheres by using different carrying polymers through one-step core-shell electrospraying/electrospinning technique. In this work, graphene-based foam was produced for the first time by utilizing core-shell electrospraying technology instead of available chemical vapor deposition techniques. Electrospraying/electrospinning prevents the aggregations and crumbling of graphene sheets by constructing interconnected framework and provides homogeneous dispersion of graphene sheets in polymer solution under electric field. The proper polymer concentration and solution viscosity were determined by using Mark-Houwink-Sakurada equation. Morphology and dimension of the structures were controlled by tailoring solution and process parameters. Moreover, hollowness of the fabricated 3D graphene-based spheres was altered by changing the core solvent during process. In the second part, platinum (Pt) decorated graphene-based spheres, foam, and fibers were prepared as electrodes via core-shell electrospinning/electrospraying technique followed by reduction and carbonization process. The effect of morphology and dimension of graphene-based carbon electrodes on the electrochemical behavior of electrodes were investigated by cyclic voltammetry and galvanostatic charge-discharge methods. Polyacrylonitrile (PAN) polymer was selected as a carrier to increase the interconnections in graphene network and carbon content. Among three different electrodes, Pt supported 3D graphene-based spheres exhibited the highest specific capacitance of 118 F/g at a scan rate of 1 mV/s as well as good cyclic stability owing to its unique structure and small size of Pt particles. On the other hand, Pt-decorated graphene-based fiber showed lowest specific capacitance of 8 F/g at a scan rate of 1 mV/s. In the last part, a novel and hierarchical hybrid electrode was constructed by the addition of manganese oxide and polyaniline (PANI) into the fiber structure to further improve the electrochemical performance of graphene-based fibers. Manganese oxide with its high theoretical specific capacitance and low cost was integrated to the fiber structure during electrospinning. Whereby, at the last step of process, to enhance the electrical conductivity of electrodes, PANI was deposited on the surface of fibers through in-situ polymerization of aniline monomer. In order to fully understand the effect of graphene on the structure and electrochemical performance of electrodes, two types of graphene including thermally exfoliated graphene oxide (TEGO) and graphene nanoplatelet (GNP) were selected based on the number of graphene layers. Among two fabricated electrodes with different graphene sources, GNP/PANI/manganese oxide carbon fibers showed the highest specific capacitance of 454 F/g at a scan rate of 1 mV/s. The mentioned electrode exhibited a high cycling stability whereby only 11% of capacitance lost after 1000 cycles of charging-discharging. High oxygen functional groups of GNP is believed to enhance the interfacial interactions between electrode components by providing an exfoliated structure. This study especially brings a new insight into the fabrication of high-performance hybrid electrodes for energy storage devices.