Controllable Preparation And Electrochemical Performance Study Of Carbon Nano Onions

Energy shortage is a global problem that needs to be solved urgently.The drastic reduction of fossil fuel reserves forces people to develop and utilize cleaner renewable energy,and the performance of energy storage equipment is a key of determining energy efficiency.Supercapacitors,as a new type of energy storage equipment between batteries and capacitors,are widely used in various fields due to their excellent rate capability,power density and safety.The quality of electrode materials fundamentally determines the performance of supercapacitors,and carbon nano onion?CNOs?,as a zero dimensional nano carbon material,with the great electrical conductivity and chemical stability due to their high degree of graphitization and nano spherical structure,which have shown brilliant application value in the field of supercapacitor electrode materials.Because of the limitation of energy storage mechanism and voltage window,the aqueous-electrolyte system supercapacitor is limited in the problem of energy density.CNOs have been restricted in many fields because of its uncontrollable morphology and strict preparation requirements.In order to solve these problems,CNOs were prepared by chemical vapor deposition with metal catalyst,and the optimum preparation process was explored.The CNOs/MnO₂ composite material with best electrochemical performance was prepared by hydrothermal method,and the influence of reaction conditions on the structure and properties of the material was studied systematically.This paper studied the effect of CNOs as the conductive agent on the performance of activated carbon supercapacitors in aqueous-electrolyte systems and the improvement of the performance of asymmetric supercapacitors.The results are as follows:?1?The catalytic effect of pure Ni catalyst that prepared by reduction for pyrolysis of methane is better than Fe-Ni compound catalyst.When the reaction temperature is 800?,the reaction time of 5 h,methane flow rate for 300 sccm,it is effective to obtain high purity,particle size in50-100 nm,high degree of graphitization of the spherical CNOs with the productivity of 2.37 g·g^-1.?2?In the preparation of CNOs/MnO₂ and the study of its electrochemical properties,the influences of the hydrothermal time,the ratio of raw materials,the hydrothermal temperature and CNOs dosage on the material morphology and comprehensive properties were investigated.When the hydrothermal time is 2 h,CNOs/MnO₂ products has both?-and?-crystal forms,and the crystallinity increased.The increase of raw material ratio n?KMnO₄?:n?MnSO₄?will change the crystal phase of the material.The CNOs/MnO₂ synthesized at a ratio of3:1 is a mixed state of the layered?-MnO₂ and amorphous structure.Hydrothermal temperature rise could decrease the crystallinity,when temperature is higher than 120?,the crystal structure of pure?-MnO₂gradually transformed into a multiphase crystal doped with?-MnO₂;too low dosage of CNOs may reduce the purity of the complex in the product,and too high dosage may aggravate crystal aggregation.Under the optimal reaction conditions,the CNOs/MnO₂ had a maximum specific capacitance of 248.8 F·g^-11 at the current density of 0.5 A·g^-1.When the current density changes from 0.5 to 10 A·g^-1,the specific capacitance retention rate is still 61.6%.After 2000 cycles of charge and discharge,the capacity retention rate can reach 98.4%.?3?Compared with acetylene black conductive agent,CNOs as a conductive agent for activated carbon electrodes are more effective in aqueous electrolytes.AC/CNOs-Na₂SO₄ initial specific capacitance is up to 148.14 F·g^-1,10000 charge and discharge cycles capacitance retention rate is 105%.And specific capacitance at 5 A·g^-11 maintained at the low currentdensityof91.1%.Byassembling AC/CNOs//Na₂SO₄//CNOs/MnO₂asymmetricsupercapacitors,the working window in the aqueous electrolyte is expanded to 2.0 V,while having an initial specific capacitance of 49.4 F·g^-1,when the power density is at 188.3 W·kg^-1,the energy density is as high as 20.92 Wh·kg^-1,which provides an idea for the improvement of specific capacitance and energy density of water-based supercapacitors.