Morphology controlled high performance supercapacitor behaviour of the Ni-Co binary hydroxide system

The morphology evolution of the Ni-Co binary hydroxides was studied varying from nanosheets, to nanoplate-nanospheres, to nanorods and to a nanoparticle geometry by simply controlling the Co:Ni ratio in the initial reactant. High capacitances of 1030 F g^-1 and 804 F g^-1 can be achieved in the 1-D nanorod morphology at mass loading of 1 mg cm^-2 and 2.8 mg cm^-2 at a current density of 3 A g^-1, respectively. To demonstrate its practical application, the binary hydroxide electrode was coupled with chemically-reduced graphene (CG) forming an asymmetric supercapacitor in order to improve the potential window and thus energy density. The asymmetric supercapacitor delivers a high energy density of 26.3 Wh kg ^-1 at the power density of 320 W kg^-1. The approach of controlling morphology and crystallinity of the binary system for optimizing supercapacitive performance may be applied developing other promising multiply metal hydroxide/oxide systems for supercapacitor applications.