N/P Codoped Porous Carbon-Coated Graphene Nanohybrid as a High-Performance Electrode for Supercapacitors

Carbon nanomaterials have shown great promise for supercapacitors but are usually limited to their unsatisfactory energy densities. To address this issue, it requires rational design and tuning of the carbon composition, texture, and microstructure. Herein, we present a nanohybrid strategy for the preparation of nitrogen/phosphorus codoped porous carbon-coated graphene (KNPG) by conjunction of carbonization and activation of phytic acid on the graphene oxide in the presence of ethylenediamine. The as-synthesized KNPG is endowed with a unique three-dimensional (3D) nanohybrid architecture consisting of graphene layers sandwiched by porous carbon nanosheets, a hierarchically micro-/mesoporous structure, a high specific surface area (up to 596 m(2) g(-1)), and an efficient N/P codoping (3.6 atom % for N and 0.3 atom % for P). As a supercapacitor electrode, the KNPG shows a gravimetric capacitance of 201 F (200 F cm-3) at 0.5 A g(-1), and an excellent rate capability with a capacitance retention ratio of 75% at 20 A g(-1). Moreover, the obtained symmetric supercapacitor in 6 M KOH delivers a high gravimetric energy density of 9.10 W h kg(-1), a large volumetric energy density of 9.07 W h L-1, and a superior cycle stability of 84.6% retention after 20 000 cycles. The present study opens up new opportunities to couple graphene and N/P codoped carbon for high-performance supercapacitors.