N-doped hollow carbon spheres with controllable shell numbers for high-performance electrical double-layer capacitors

Currently, hollow carbon spheres (HCS)-based supercapacitors are gained tremendous interest due to their excellent electrochemical property . However, the inferior structural stability and unsatisfactory space utilization restrict the applications for high power devices. Herein, a kind of novel N-doped HCS (N-HCS) with controllable shell numbers is constructed by layer-by-layer assembly of polydopamine (PDA) and polypyrrole (PPy) layers on carboxyl modified polystyrene (PS–COOH) spheres. Compared with single-shelled N-HCS, the optimize N-HCS with 4 shells (N-HCS-4) exhibits improved specific surface area from 43.9 m 2  g −1 to 1110.2 m 2  g −1 , which provides a large accessible surface area for the electrolyte, thereby facilitating the ion diffusion and electron transportation. Meanwhile, the doping heteroatom endows N-HCS with the additional pseudocapacitance . Very interestingly, the significant electrochemical properties of N-HCS at the varied coating sequence and number of PDA and PPy layers indicate the formation of the optimal microstructure, which could match with the electrolyte ion size for accelerating the capacitive response. The assemble asymmetrical supercapacitor displays a high energy density of 41.18 Wh kg −1 at a power density of 250 W kg −1 , and excellent cycling stability of 99.1% over 10,000 cycles due to the improved structural stability from the synergistic effect in different shells.