Controllable Preparation And Supercapacitor Performances Of Interconnected Hollow Carbon Nano-onions

Supercapacitor is an electrochemical energy storage device with high power density and low maintenance cost,which bridge the gap between batteries and conventional dielectric capacitors.Compared with other energy storage materials,carbon nano-onions（CNOs）are considered as ideal materials for supercapacitor electrodes due to their unique structure,excellent stability and mechanical strength.However,conventional CNOs have some drawbacks,for instance,monodispersed and large-size structures will lead to high contact resistance between them and decrease capacitance;high graphitization also makes its specific surface area relatively small,which is not conducive to capacitance performance.Thus,it is of great significance to develop CNOs with high conductivity,small particle diameter and high specific surface area.In this thesis,we proposed a synthesis method of size controllable CNOs.Firstly,Fe₃O₄ nanoparticles with different particle sizes were prepared by thermal decomposition method as templates for CNOs synthesis.Then,interconnected hollow CNOs were successfully synthesized by carbonization,acid pickling and graphitization.Finally,the specific surface area of CNOs was increased by KOH chemical activation.The effects of graphite temperature,particle diameter and specific surface area on the electrochemical properties of CNOs were systematically investigated.CNOs electrodes were tested in a three-electrode configuration in 3M KOH electrolyte.The result shows that the unique interconnected structure enhances the conductivity of CNOs;the capacitance of CNOs prepared by a template with a size of 5.6 nm and graphitized at 1200°C is the largest,which is due to the hollow and small size structure increase specific surface area of CNOs,and the interconnected structure and graphitization increase the conductivity of CNOs.The specific capacitance of the CNOs electrode prepared under the optimal conditions can reach 182.1 F g^-1 at a current density of 1 A g^-1.After KOH chemical activation,the specific surface area of CNOs increases from505.5 m² g^-1 to 1186.8 m² g^-1,and the maximum capacitance is 282.6 F g^-1.After 50,000cycles at a current density of 2 A g^-1,the capacitance retention of the activated CNOs is97.1%.The results show that interconnected hollow small-size CNOs have great potential as active materials for supercapacitors,which shows an attractive development prospect.