Growth of Ni-Co binary hydroxide on a reduced graphene oxide surface by a successive ionic layer adsorption and reaction (SILAR) method for high performance asymmetric supercapacitor electrodes

A simple, additive-free, cost-effective and scalable successive ionic layer adsorption and reaction (SILAR) method is reported to prepare nickel-cobalt binary hydroxide (Ni-Co-BH) on a reduced graphene oxide (RGO) directing template over a macro-porous conductive nickel foam substrate. This green technique is not only considered as fundamental research interest, but also describes the commercial applications of supercapacitors to reduce the electrode fabrication cost. Three different Ni-Co-BH-G (Ni-Co-BH/RGO) composites are synthesised by tailoring the nickel-cobalt ratios. The flower-like 3D framework of Ni-Co-BH-G provides a porous nano-structure to facilitate the charge transfer and ion diffusion. The cathodic peak current density vs. square root of the scan rate slope values of cyclic voltammetry are consistent with specific capacitance (SC) retention (vs. current density) from charge-discharge curves and the diffusion time constant of the Nyquist plot of the Ni-Co-BH-G composites. Taking the advantage of 3D conductive mesoporous open framework, the Ni-Co-BH-G has provided an excellent SC of 2130 F g^-1 at 2 A g^-1. An asymmetric supercapacitor device is designed with the optimized Ni-Co-BH-G as the positive electrode and concentrated HNO₃ treated conducting carbon cloth (CCN) as the negative electrode. An excellent energy density of ∼92 W h kg^-1 and a high power density of ∼7.0 kW kg^-1 with lifetime stability up to 10 000 charge-discharge cycles (capacitance retention ∼ 80%) are provided by the asymmetric device. Four asymmetric devices have been assembled in series, which provided ∼5.6 V charge-discharge potential. The assembled system has powered a 5 V light-emitting diode (LED) successfully.