Facile electrodeposition of 3D concentration-gradient Ni–Co hydroxide nanostructures on nickel foam as high performance electrodes for asymmetric supercapacitors

Novel three-dimensional (3D) concentration-gradient Ni–Co hydroxide nanostructures (3DCGNC) have been directly grown on nickel foam by a facile stepwise electrochemical deposition method and intensively investigated as binder- and conductor-free electrode for supercapacitors. Based on a threeelectrode electrochemical characterization technique, the obtained 3DCGNC electrodes demonstrated a high specific capacitance of 1,760 F·g~(–1) and a remarkable rate capability whereby more than 62.5% capacitance was retained when the current density was raised from 1 to 100 A·g~(–1). More importantly, asymmetric supercapacitors were assembled by using the obtained 3DCGNC as the cathode and Ketjenblack as a conventional activated carbon anode. The fabricated asymmetric supercapacitors exhibited very promising electrochemical performances with an excellent combination of high energy density of 103.0 Wh·kg~(–1) at a power density of 3.0 kW·kg~(–1), and excellent rate capability—energy densities of about 70.4 and 26.0 Wh·kg~(–1) were achieved when the average power densities were increased to 26.2 and 133.4 kW·kg~(–1), respectively. Moreover, an extremely stable cycling life with only 2.7% capacitance loss after 20,000 cycles at a current density of 5 A·g~(–1) was achieved, which compares very well with the traditional doublelayer supercapacitors.