Abstract
Binary nickel-cobalt hydroxides (denoted as Ni x Co 1-x (OH) 2 ) with advanced capacitive performances, such as the superior rate capability, energy efficiency, and capacitance rate-/cycle-retention, are synthesized by cathodic deposition. The effects of deposition variables are systematically investigated and discussed. The deposition current density has been found to determine the morphology of hydroxides whereby a small current density facilitates the formation of porous nanonetwork microstructures. Both pH and temperature of the precursor solution affect the Co/Ni ratio of hydroxides, which in turn influence the capacitive performances. Eventually, Ni 0.32 Co 0.68 (OH) 2 synthesized under our proposed deposition condition exhibits a specific capacitance value of ca. 1000 F g -1 at 5 mV s -1 , 100% capacitance cycle-retention over 1000 cycles, and 69% capacitance rate-retention (varying from 5 to 500 mV s -1 ). These superior capacitive performances make Ni 0.32 Co 0.68 (OH) 2 a promising material of the positive electrode for an asymmetric supercapacitor with an energy efficiency of 90% at 10 A g -1 . © 2013 Elsevier B.V. All rights reserved.