Nanocomposites of Ni(OH)2/Reduced Graphene Oxides with Controllable Composition, Size, and Morphology: Performance Variations as Pseudocapacitor Electrodes

Although Ni(OH)2 is a promising electrode material for electrochemical capacitors on account of its high theoretical specific capacitance and low cost, its application has been hindered by the low measured specific capacitance and poor cycle stability often associated with low specific surface area and poor electrical conductivity. This study aims to develop an amorphous-precursor method to anchor Ni(OH)2 nanocrystals onto conductive graphene sheets. We demonstrate the ability to control the composition, size, and morphology of Ni(OH)2 nanocrystals on graphene sheets by combination with an Ostwald ripening process, which has allowed investigation on how such factors influence their performance if used as pseudocapacitor electrodes. It was found that the specific capacitance increased and then decreased as the ratio of reduced graphene oxide (RGO) to Ni(OH)2 increased; the sample that contained 5.2 wt?% of RGO achieved the highest specific capacitance of 1804.0 F?g-1 (based on the total composite mass) and 1902.9 F?g-1 (based on the mass of Ni(OH)2), close to the theoretical value of Ni(OH)2 (2082 F?g-1). With a given ratio of RGO to Ni(OH)2, the specific capacitance decreased as the Ni(OH)2 nanocrystal size increased. However, the cycling performance increased as the stability of Ni(OH)2 in the nanocomposites increased; the highest was achieved for the most stable Ni(OH)2 hexagonal nanoplates on RGO. In all, with appropriate optimization of the parameters of the materials, a good pseudocapacitor performance was achieved for the Ni(OH)2/RGO hybrid system in terms of both the specific capacitance and the cycling stability.