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

• Ni–Co binary hydroxide systems with controllable morphologies were fabricated.
• Different morphologies lead to distinct supercapacitive behaviours.
• The performance degradation at high mass loadings can be greatly mitigated.
• Asymmetric cells composed of hydroxides and graphene showed excellent performance.

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.

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