Nickel cobalt oxide (NiCo2O4) particles with a diamond-shaped hexahedral porous sheet structure are successfully synthesized by a facile hydrothermal method, followed by calcination in one step. NiCo2O4-I and NiCo2O4-II particles are prepared using the same method with different contents of urea (CO(NH2)(2)) and ammonium fluoride (NH4F). The different morphologies of the NiCo2O4-I and NiCo2O4-II particles illustrate that CO(NH2)(2) and NH4F play an important role in crystal growth. To verify the influence of NH4F and CO(NH2)(2) on the morphology of the NiCo2O4 particles, the theory of crystal growth morphology is analyzed. The electrochemical measurements show that NiCo2O4 particles exhibit a high specific capacitance. At a current density of 1.0 mA cm(-2), the mass specific capacitances of the NiCo2O4-I and NiCo2O4-II electrodes are 690.75 and 1710.9 F g(-1), respectively, in a 6 M KOH aqueous electrolyte. The specific capacitances of the NiCo2O4-I and NiCo2O4-II electrodes remain similar to 95.95% and similar to 70.58% of the initial capacitance values after 5000 cycles, respectively. According to the two-electrode test, the NiCo2O4-II//AC asymmetric electrodes exhibited an ultrahigh energy density of 64.67 Wh kg(-1) at the power density of 12 kW kg(-1), demonstrating its excellent application potential as an electrode material for supercapacitors. (C) 2017 Elsevier B.V. All rights reserved.