Synthesis Of Graphene And Its Nanocomposites As High Performance Electrode Material For Supercapacitors

The rapid development in the portable electronics and automobile industry lead to ever increasing demand for highly efficient renewable energy storage devices.Manganese dioxide and polyaniline(PANI)have been extensively explored as pseudocapacitive electrode materials for the supercapacitors,due to their high theoretical capacitance,low cost,and environmental friendly nature.However,the high internal resistance and low cyclic stability limit the electrochemical performance of these electrode materials.The performance of the electrode materials also suffers due to the addition of binders and conductive additives.To resolve these issues,current study aims to synthesize MnO2/PANI nanocomposites on graphene foam which serves as three dimensional binder free conducting scaffold.Additionally,the carbon nanotubes(CNTs)were employed in the Mn02/PANI nanocomposite to improve the internal conductivity through synergetic effect,subsequently boosting up the electrochemical performance.To accomplish research goals,graphene growth conditions were optimized for Cu and Ni catalyst substrates,and the effect of water vapors on the growth of graphene was studied.Then,the graphene grown on Ni foam was used to synthesize nanocomposites for electrochemical applications.The research outcomes are as follows.Graphene was synthesized by chemical vapor deposition(CVD)under the optimized growth conditions,and different factors affecting the growth of graphene were studied.Water vapors were used as a special tool to control the thickness and size of graphene islands on Cu foil.It was observed that graphene thickness increases from 2 to 25 layers with water vapors ranging from 0 to 2000 ppmv,introduced during the growth stage.The results suggested that water vapors play a dual role during growth of graphene,i.e.speeding up growth rate,subsequently resulting in multilayer graphene(MLG),and etching the graphene edges.Water(vapors)is a mild oxidizing agent and easy to control its concentration relative to oxygen containing molecular impurities in hydrogen gas.Thus water vapors were employed as oxygen containing molecular impurities in purified hydrogen gas to study etching behavior of hydrogen gas.A modified graphene growth mechanism was proposed,which implies that hydrogen does not etch graphene edges,but oxygen containing molecular impurities(i.e.,water vapors)present in hydrogen gas cause etching.The 3D graphene synthesized on nickel foam by CVD,which was used to synthesize graphene-MnO2-PANI(GM@PANI)nanocomposite,through a simple ultrasonication assisted hydrothermal and in situ polymerization process.As synthesized GM@PANI nanocomposite was used as a binder free electrode material for supercapacitors.The electrochemical analysis demonstrated a very interesting electrode activation phenomenon,i.e.increase in specific capacitance up to first 1500 cycles,which could be attributed to the activation of new active sites within the active electrode material,as a result of electrode/electrolyte interactions.The activated GM@PANI nanocomposite electrode exhibited maximum specific capacitance of 1369 F g-1 at 3 Ag-1 current density,which is significantly higher than the graphene oxide based MnO2/PANI nanocomposites reported in literature.The activated electrode demonstrated excellent rate capability over 70%at 15 A g-1 current density,and good cyclic stability by retaining more than 83%specific capacitance after 5000 cycles.The improved performance of the nanocomposite electrode could be attributed to the growth of MnO2 intercalated PANI nanorods like network resulting in reduced agglomeration,and 3D graphene as conducting scaffold.However,the results suggested that agglomeration of MnO2/PANI nanostructures still exist to significant level,which should be suppressed.In order to eliminate the agglomeration and improve the internal conductivity of MnO2/PANI nanostructures,CNTs were used to synthesize graphene-CNTs-MnO2-PANI(GCM@PANI)nanocomposite as binder free electrode material for supercapacitors.The results revealed uniform growth of spherical PANI nanoparticles,forming a layer over the surface of 3D graphene with CNTs/MnO2 nanostructures intercalated into or buried underneath the PANI nanoparticle layer.The agglomeration of MnO2/PANI nanostructures was successfully suppressed to negligible level.The activated GCM@PANI nanocomposite electrode exhibited astonishing specific capacitance of 3037 F g-1 at 8 A g-1 current density,excellent cyclic stability by retaining 83%specific capacitance after 12,000 cycles,and high rate capability with 84.6%capacitance retention at 20 A g-1 current density.The results analysis suggested that the astonishing electrochemical performance of the synthesized electrode material might be attributed to:(i)the increased surface area and porosity due to formation of PANI nanoparticle’s layer,(ii)suppression of the agglomeration to negligible level,(iii)the increased internal conductivity due to CNTs,(iv)the synergistic effect caused by CNTs/MnO2 nanostructures resulting in the fast charge transfer,and(v)the binder free nature of 3D graphene based nanocomposite electrode materials.