High-energy Density And Highpower Density Supercapacitor Based On Nitrogen-doped Interconnected Hollow Carbon Nano-onions

Supercapacitor,as a new type of electrochemical energy storage device,has the advantages of high-power density and long cycle life.However,its practical application is limited because it often has a low energy density.Although it is of great significance to improve the energy density of supercapacitor without sacrificing the power density,the realization of this goal still has great challenges.Carbon nano-onions(CNOs)are quasi-zero-dimensional carbon nanomaterial with a multilayer concentric spherical graphite structure.Because it has a high positive curvature surface structure,which allows the ions adsorption/desorption process to proceed rapidly,resulting in high power performances under high rate charge and discharge regulation.However,the synthesized carbon nano-onions at present are mainly monodisperse solid structures,with a low utilization rate of the carbon layer and no interconnected conductive skeletons.As a result,the overall conductivity of the carbon nano-onions particle film is low,and the capacitance and energy density are difficult to improve.In the present study,Fe₃O₄ nanoparticles were successfully prepared by thermal decomposition.With the dense arrangement of metal oxide nanoparticles,an interconnected hollow carbon nano-onion skeleton was synthesized.Then,nitrogen-doped interconnected hollow carbon nano-onions(N-IHCNOs)were synthesized by using urea as a nitrogen source through the hydrothermal method.Further tests confirm that the material has the following attractive performance advantages.First,the interconnected conductive skeleton has excellent conductivity;second,its hollow structure can provide charge storage space and improve gravimetric specific capacitances;third,nitrogen-doping improves the chemical properties of carbon nano-onions'surface.Based on the structural advantages of N-IHCNOs,the effects of doping ratio and annealing temperature on the electrochemical performance of N-IHCNOs-based electrodes were systematically studied.Furthermore,the supercapacitor devices based on the above materials were successfully assembled.In addition,the effect of electrolyte types on the supercapacitive performance of N-IHCNOs materials and supercapacitor devices has also been systematically studied.The results show that in 3 M KOH and 1 M EMIMBF4/ACN electrolyte,with nitrogen-doped,the energy densities of N-IHCNOs are increased by 49.3%and 45.2%,respectively,without loss of power density.Because the mesoporous area of N-IHCNOs is as high as 82.9%,and the mesopores between carbon shells are highly interconnected,N-IHCNOs have high adaptability in ionic liquid electrolytes.The symmetrical supercapacitor assembled with pure EMIMBF₄ ionic liquid electrolyte has an excellent supercapacitive performance.To be specific,it has an energy density of 168.9 Wh kg^-1at a power density of 2 k W kg^-1;and maintains an energy density of 45.2 Wh kg^-1 even at an ultrahigh-power density of 400 k W kg^-1,as well as a superior cycling performance(98.2%of the capacitance retention after 50,000 cycles).As the electrode materials with high-energy densities and high-power densities,N-IHCNOs have a simple preparation process and show a new solution to improve the energy densities of supercapacitors and bring wide application prospects.