▎ 摘　　要

Ni-Co hydroxide nanoneedles embedded in graphene hydrogel are fabricated using an efficient two-step method and further explored as a binder-free electrode for high-performance asymmetric supercapacitors. First, freestanding graphene hydrogel is prepared via reduction in a mild condition. Second, Ni-Co hydroxide nanoneedles embedded in graphene hydrogel are obtained using a simple hydrothermal method. The existence of many hydrophilic functional groups in graphene hydrogel results in the generation of well-dispersed Ni-Co hydroxide nanoneedles throughout the graphene nanosheet. Moreover, the freestanding property of graphene hydrogel allows it to be used as a binder-free electrode, which can improve the energy density of the asymmetric supercapacitor without inhibiting its high power capabilities. The composite shows an excellent capacity of 544 C g(-1) at 2 A g(-1) in a three-electrode system. Moreover, the binder-free asymmetric supercapacitor achieves excellent performance with an energy density of 32.74Wh kg(-1) at a power density of 320 W kg(-1) and good cycling stability with 85% capacitance retention at a current density of 10 mA cm(-2) after 5000 cycles. Therefore, the Ni-Co hydroxide nanoneedles embedded in graphene hydrogel have immense potential as electrochemically active materials for the development of high-performance supercapacitors. (c) 2018 Elsevier Ltd. All rights reserved.