Abstract
Binary nickel-cobalt oxy-hydroxides (denoted as NCOH) with tunable crystal phases are prepared from an atomic mixture of Ni(OH) 2 + Co(OH) 2 in the 1:2 ratio (denoted as (Ni/2Co)(OH) 2 ) through a microwave-assisted hydrothermal annealing (MAHA) method. The (Ni/2Co)(OH) 2 powders are synthesized through a sol-gel process from an aqueous solution containing NiCl 2 ·6H 2 O and CoCl 2 ·6H 2 O. The MAHA process is demonstrated to be a novel and promising method for transforming/tuning the crystal phase and size of NCOH at relatively low annealing temperatures in short time. The MAHA temperature and time are varied to transform the crystalline phases from a hydroxide mixture to NiCo 2 O 4 nanocrystals, which can be quantified by a crystalline index (denoted as CI) value obtained from X-ray diffraction (XRD) patterns. The higher content and larger size of NiCo 2 O 4 nanocrystals are, the longer electrochemical activation time is needed. The rate of phase transformation can be enhanced by adding microwave adsorbents, graphene oxide (GO), to obtain a NiCo 2 O 4 /RGO composite with reduced crystal size (RGO: reduced GO). The variation in the crystalline index and the effect of the potential window on the specific capacitance of NCOH powders are systematically compared and discussed in this work. © 2013 Elsevier B.V. All rights reserved.