N-doped porous carbon capsules with tunable porosity for high-performance supercapacitors

A procedure for the fabrication of N-doped hollow carbon spheres with a high rate capability for supercapacitors has been developed. The approach is based on a nanocasting method and the use of a nitrogen-rich compound (pyrrole) as carbon precursor. The carbon particles thus produced combine a large BET surface area (~1500 m g) with a porosity made up of mesopores of ~ 4 nm and a high nitrogen content (~ 6 wt %) and a capsule morphology which entails short ion diffusion paths derived from the shell morphology (thickness ~ 60 nm). The porous properties of these hollow particles can be enhanced by means of an additional activation step with KOH. The activation process does not alter the hollow structure or spherical morphology, but strongly modifies the pore structure from a mesoporous network to a microporous one. The N-doped carbon capsules were tested in aqueous and organic electrolytes. In an aqueous medium (1 M H2SO4), the mesoporous carbon capsules offer the best performance due to the pseudocapacitive contribution of the N-groups, exhibiting a specific capacitance of ~ 240 F g at 0.1 A g and a capacitance retention as high as 72 % at 80 A g. On the contrary, in an organic electrolyte 2 (1 M TEABF4/AN), where the charge storage mechanism is based on the formation of the electric double-layer, the microporous capsules perform better due to the larger specific surface area. Thus, the microporous carbon capsules display a specific capacitance of up to 141 F g at 0.1 A g and an outstanding capacitance retention of 93 % for an ultra-high discharge current density of 100 A g.