Study On Carbon Nano Onion-based Supercapacitor For AC Filtering At Low Temperature

Aluminum electrolytic capacitors（AECs）are widely used in the field of AC filtering,but their large size,low capacity and high mass which greatly hinder the miniaturization and lightweight development of electronic equipment.Supercapacitor（SC）is a new type of energy storage device with high specific capacitance and small volume,which is promising to solve this problem.Conventional supercapacitor electrodes are mainly porous activated carbon,whose tortuous and narrow pores structure limits the diffusion rate of electrolyte ions,resulting in that the supercapacitor can not meet the high frequency response of AC filtering.In addition,polar exploration,aerospace and the other fields are placing higher demands on the AC filtering performance of supercapacitors at low temperatures.Therefore,it is necessary to develop AC filtering supercapacitors which can be charged and discharged rapidly at low temperatures,but unfortunately it is not known whether supercapacitors can be used for low-temperature AC filtering.Carbon nano-onions（CNOs）are carbon materials with a spherical structure surrounded by concentric graphite layers.The positive curvature of the surface facilitates the rapid migration of electrolyte ions at the electrode/electrolyte interface compared to other materials.By using carbon nano-onions as electrodes,supercapacitors have a faster adsorption/desorption of ions on the electrode surface during charging and discharging,which can improve the response rate of supercapacitor.Therefore,CNOs materials have a great potential in the field of AC filtering at low temperature.In this paper,hollow carbon nano-onions with small size and high curvature were prepared by chemical vapor deposition（CVD）and loaded onto the Ni foil surface by spin coating.After secondary chemical vapor deposition,a composite film electrode of carbon nano onion and graphene was prepared.The open structure of the carbon onion reduces the resistance to ion diffusion,the high temperature CVD treatment increases the conductivity of the electrode material,and the CVD-grown graphene tightly connects the electrode material to the Ni collector to reduce the interfacial resistance,thus achieving a fast response of the supercapacitor.The optimal parameters of carbon nano-onions-based supercapacitor for AC filtering was determined by tuning the spin-coating and CVD parameters using electrochemical impedance spectroscopy.It has the highest phase angle of 120 Hz（-85.8°）and the minute RC time constant of 0.098ms in aqueous electrolyte of 6 M KOH at room temperature,showing an ultra-fast charge-discharge rate.Supercapacitor in organic electrolyte of 1M EMIMBF₄/ACN also has a high phase angle of-82.3°and a voltage window of 3 V.Ultra-high scan rate of 2000V s^-1 can be achieved in both aqueous and organic electrolytes.Aqueous supercapacitors maintain nearly 100%coulomb efficiency and capacity retention after50,000 charges and discharges at the current density of 0.1 m A·cm^-2.By combining the low melting point（BMIMBF₄/ACN）low temperature electrolyte with the advantages of the carbon nano-onions outer surface structure,the supercapacitor maintains a phase angle with-50.8°and a high scan rate with 500 V s^-1at ultra-low temperature at-50°C,better than the commercial AEC.By designing and assembling the AC filtering circuit,we have demonstrated that both aqueous and organic electrolyte supercapacitors can successfully convert 60 Hz AC to smooth DC at room temperature.Most importantly,we have demonstrated for the first time that supercapacitors can also be used for AC filtering at temperatures as low as-50°C,which is a new application of CNOs-based supercapacitors in the field of low-temperature AC filtering.