One-Pot Fabrication of Layered α-Phase Nickel-Cobalt Hydroxides as Advanced Electrode Materials for Pseudocapacitors

A novel and facile one-step method for growing alpha-phase nickel-cobalt hydroxides is developed. The use of 1-methyl-2-pyrrolidone (NMP) in the mixed solvent plays the decisive role in the precipitation of metal ions and the formation of alpha-phase hydroxides. The synthetic method is robust enough to synthesize Ni-Co hydroxides with different stoichiometric ratios of Ni and Co, and can be extended to the fabrication of Ni0.67Zn0.33 hydroxide in the alpha phase. More encouragingly, the method employed can be further used to grow layered nickel-cobalt hydroxides on Ni foam. As the electroactive materials for pseudocapacitors, the Ni-Co hydroxides deliver an ultrahigh specific capacitance (1337.4 Fg(-1) at 1 Ag-1 for Ni0.67Co0.33 hydroxide) with improved rate capability and cycling stability. Moreover, a greatly increased specific capacitance (2248.9 and 2286.7 Fg(-1) at 1 Ag-1 for Ni0.5Co0.5 and Ni0.67Co0.33 hydroxides, respectively) with superior cycling stability is obtained after growth on Ni foam.