Nanoporous Ni nanoparticles (NPs) have been successfully prepared by a facile two-step process, hydrogen plasma-metal reaction (HPMR) and chemical dealloying. Via the subsequent passivation at room temperature, an oxide layer is generated outside the Ni NPs as the result of surface oxidation. The passivated Ni@NiO NPs of 48 nm exhibit a highly porous structure with pore volume of 0.812 cm(3)/g. The NPs are subsequently annealed at different temperatures to adjust the content of Ni and NiO. With the increase of the annealing temperature from 423 to 573 K, the particle size increases from 54 to 61 nm and the pore volume decreases from 0.432 to 0.02 cm(3)/g. The specific capacitance (SC) of the Ni@NiO NPs decreases with the increasing annealing temperature, and the passivated sample with a large pore volume and low resistance has the highest SC of 641 F/g at a charge-discharge current density of 1 A/g. The passivated sample also shows an energy density of 25.95 W h/kg, a power density of 2.7 kW/kg, and good stable specific capacitance retention of 73% after 1000 cycles. The charge-discharge mechanism is discussed in terms of pore architecture and core@shell structure. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.