N and O co-doped carbon electrode materials were synthesized by pyrolysis of Zeolitic Imidazolate Framework-8 (ZIF-8) at high temperature in air using a salt-sealing technique, aiming to solve the issue related with the low energy density of conventional carbon materials for supercapacitors. Compared with the conventional carbon materials prepared by direct pyrolysis of ZIF-8 in nitrogen atmosphere, the novel carbon materials prepared by this proposed technique have an open hollow structure with appropriate distribution of mesoporous and microporous, and higher specific surface area of 1589 m²·g^-1. The co-doping of heteroatoms N and O can improve the wettability of the material, while the open hollow structure is conducive to the diffusion of the electrolyte ions from both internal and external surfaces to the interior of the electrode material. Therefore, more solvated ions are adsorbed and desorbed in the pores, and the effective specific surface area is increased for the electrode material, therewith, more active sites of heteroatoms N and O can participate in the redox reaction, introducing higher Faraday capacitance. As a matter of course, the symmetrical supercapacitor assembled with the novel electrode material shows an energy density of 11 Wh·kg^-1 at power density of 250 W·kg^-1.