Nitrogen doped and hierarchically porous carbons derived from chitosan hydrogel via rapid microwave carbonization for high-performance supercapacitors

We have demonstrated a rapid and facile approach for preparing nitrogen doped and hierarchically porous carbons (NHPCs) using renewable chitosan as precursor via microwave carbonization in only 10 min. In microwave carbonization, ZnCl₂ impregnated in chitosan hydrogel serves as microwave absorber, chemical activation agent and porogen. The porous textures of NHPCs can be modulated by altering the concentration of ZnCl₂ in chitosan hydrogel. The optimal NHPC possesses a high nitrogen content of 9.4 wt% along with high specific surface area of 1170 m² g⁻¹, pore volume of 1.32 cm³ g⁻¹ and mesopore ratio of 78%. Electrochemical performance suggests that a high specific capacitance of 435 F g⁻¹ is achieved at 0.2 A g⁻¹ in three-electrode system with 6 M KOH as electrolyte. The as-assembled symmetrical supercapacitor exhibits a high energy density of 9.4 Wh kg⁻¹ at the power density of 50 W kg⁻¹ and remarkable retention ratio of 79.8% at a high power density of 2500 W kg⁻¹ in 6 M KOH. Moreover, the supercapacitor displays a long cycle life with the capacitance retention of nearly 100% over 10000 cycles. Conversion of chitosan hydrogel to NHPCs via rapid microwave carbonization represents a promising route for preparing carbon materials for high-performance supercapacitors.