Improving the energy density of carbon-based supercapacitors is one of the most urgent demands for developing high-power energy supplies, which in general requires delicate engineering of the carbon composition and textures. By pre-functionalization of graphene nanosheets and successive one-step (NH4)(3)PO4 activation, we prepared a type of nitrogen and phosphorus co-doped graphene (NPG) with high specific surface areas, hierarchical pore structures as well as tunable N and P contents. The as obtained NPG shows high specific capacitances of 219 F g(-1) (123 F cm(-3)) at 0.25 A g(-1) and 175 F g(-1) (98 F cm(-3)) at 10 A g(-1), respectively. Accordingly, the NPG-based symmetrical supercapacitor device, working at a potential window of 1.3 V, could deliver an enhanced energy density of 8.2 Wh kg(-1) (4.6 Wh L-1) at a power density of 162 W kg(-1) (91 W L-1), which still retains 6.7 Wh kg(-1) at 6.5 kW kg(-1). In particular, under a current density of 5 A g(-1), the device endows an 86% capacitance retention of initial after 20,000 cycles, displaying superior cycle stability. Our results imply the feasibility of NPG as a promising candidate for high-performance supercapacitors. (C) 2017 Elsevier B.V. All rights reserved.