Controllable synthesis of N-doped hollow, yolk-shell and solid carbon spheres via template-free method

N-doped carbon spheres with different inner structure have attracted increasing attention due to their low density and high electrical conductivity in supercapacitors. Here, we reported controllable synthesis of hollow, yolk-shell and solid structure carbon spheres via template-free assembly method. Ethanediamine was used as nitrogen source to realize in-situ nitrogen doping and as basic catalyst to drive resorcinol-formaldehyde (RF) resin polymerization. The polymerization degree of internal RF resin tremendously depended on polymerization time, resulting in different levels of heterogeneity of the whole RF resin sphere. This fact made the structural adjustment of carbon spheres from hollow, to yolk-shell and even to solid structure after dissolution of low-molecular-weight RF resin by organic solvents and annealing treatment. As electrode material for supercapacitor, carbon spheres with yolk-shell structure exhibited excellent performance with capacitance of 242 F g⁻¹ at the current density of 1 A g⁻¹ and outstanding cycling life stability (97.3% after 5000 cycles), which may be attributed to its unique structure, high specific surface area (1263 m² g⁻¹) and pore volume (0.68 cm³ g⁻¹). The strategy of making full use of the difference of polymerization degree of resin provides a new idea for the structural engineering of functional carbon materials.