Amorphous Ni–Co binary hydroxide with super-long cycle life and ultrahigh rate capability as asymmetric supercapacitors

Abstract

Ni–Co binary hydroxide (Ni x Co1−x (OH)2) with nanostructure is prepared by one-step electrochemical deposition process with de-ionized water as electrolyte. The molar ratio of Ni/Co for Ni x Co1−x (OH)2 can be accurately controlled via changing the composition of the alloy target. A series of typical hydroxides are synthesized with Ni/Co molar ratios of 1:2, 1:3, 1:4, 1:6, 6:1, 4:1, 3:1, 2:1 and 1:1. The electrochemical performances of Ni x Co1−x (OH)2 exhibit remarkable improvement in rate capability and cycling stability compared to monometallic hydroxide. Electrochemical test results reveal that Ni4/5Co1/5(OH)2 delivers the maximum specific capacitance of 2425 F g−1, while Ni0.5Co0.5(OH)2 exhibits ultrahigh rate capability (a 14% capacity decrease after a 100-fold increase in scan rate and 7% capacity decrease after a 40-fold increase in current density) and super-long cycle life (no capacitance loss after 50 000 cycles). Especially, the Ni0.5Co0.5(OH)2//AC supercapacitor exhibits a super-long cycle life with a 2% capacitance loss after 100 000 cycles, which is quite better than that of crystalline devices.