PECVD Syntheis Of Vertically Oriented Graphene And Its Electrochemical Performance In Supercapacitors

As environmental friendly energy storage devices,supercapacitors have high power density,fast charging/discharging and long cycle life.Therefore,supercapacitors have been applied in many fields,such as new-energy vehicles,solar and wind power generations.Porous carbon materials are generally used as active materials in traditional electrochemical double-layer capacitors?EDLCs?.Vertically oriented graphene?VG?with unique nanosheets has great advantages in specific surface area and electrical conductivity.In this paper,VG with good surface morphology was prepared by plasma enhanced chemical vapor deposition?PECVD?and its performance in EDLCs was investigated.VG was then composited with manganese dioxide nanowire,a typical pseudocapacitor material to serve as electrode materials of supercapacitors,and the composite electrochemical performance was studied.In PECVD preparation of VG,radio frequency generator was used as plasma source,and methane was injected as carbon source.VG was directly deposited on current collectors.Effects of synthesis conditions on the morphology of VG were studied,especially growth time,temperature,RF power,flow ratio of precursors and substrates.VG was characterized by scanning electron microscope?SEM?and transmission electron microscope?TEM?.The optimal condition in PECVD was selected to obtain non-stack uniform transparent thin graphene nanosheets.The as-prepared VG was few-layer graphene with proper nanosheets size and spacing,which showed good morphology features.VGs deposited on four different substrates,platinum,nickel,nickel foam and conductive glass were utilized as electrode materials for supercapacitors.The conductive substrates acted as current collectors.Electrochemical performance of as-prepared supercapacitors was tested in a three-electrode configuration.Capacitive performance was evaluated by cyclic voltammetry,galvanostatic charge-discharge and electrochemical impedance spectroscopy tests.Test results indicated that VG deposited on platinum has both the best morphology and most excellent capacitive performance.VG prepared under optimal conditions showed high specific areal capacity of 2.435 mF/cm² at the scan rate of 2 mV/s and high efficiency of charging/discharging with good cyclic performance.After5000 cycles,100.14%of the capacity was maintained and no obvious capacity fading was observed.VG with long cycle life indicated an ideal active material for EDLCs.Manganese dioxide nanowire was deposited on as-prepared VG/Pt electrode by electrodeposition to acquire composite electrode for supercapacitors.Electrochemical test results of MnO₂/VG/Pt composite electrode indicated that VG played a significant role in reducing charge transfer resistance,lifting charging/discharging efficiency and promoting cyclic performance.MnO₂/VG composite prepared at appropriate ratio showed good overall electrochemical properties with high specific capacity of 17.415 mF/cm² at the scan rate of 2 mV/s and retention rate of 89.3%after 5000 charging/discharging cycles.With the methods of PECVD and electrodeposition,high performance MnO₂/VG composite electrode material for supercapacitors was obtained.Studies shows that VG with vertically standing nanosheets is an ideal active material for EDLCs.VG behaves good conductivity and high charging/discharging efficiency.In further composite with manganese dioxide nanowire as composite electrode material for supercapacitors,VG shows significant effects of reducing internal resistance and improving cyclic performance.Composite electrode materials with high specific capacity,fast charging/discharging,high energy density and power density was synthesized in this paper for application of supercapacitors.