Ni-Co hydroxide nanoneedles embedded in graphene hydrogel as a binder-free electrode for high-performance asymmetric supercapacitor

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 32.74 Wh 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. The paper reported the Ni-Co hydroxide nanoneedles embedded in graphene hydrogel using facile two-step method. The asymmetric supercapacitor prepared by using the nanocomposite shows excellent energy density and cycling stability. [Display omitted] •Ni-Co hydroxide nanoneedles embedded in 3D graphene hydrogel were prepared by an efficient two-step method.•The nanocomposite of Ni-Co hydroxide/graphene hydrogel was used as a binder-free supercapacitor electrode.•The nanocomposite shows a high capacity of 544 C g−1 at 2 A g−1 in a three-electrode system.•The asymmetric supercapacitor prepared by using the nanocomposite shows excellent power/energy density and cycling stability.