Material Effects on the Electrocapacitive Performance for the Energy-storage Electrode with Nickel Cobalt Oxide Core/shell Nanostructures

Core/shell nanostructures of metal oxides and sulfides are intensively studied as the energy-storage materials. Most researchers focus on a single core/shell system and optimize the morphology of nanomaterials to enhance the electrochemical performance, but limited reports studying the effect of the material category in the same morphology on the performance of the core/shell electrode. In this work, multiple core/shell systems based on the similar sheets-on-sheets morphology are made using the same NiCo2O4 core and different shells of NiSx, NiOx, NiMoxOy and MnOx. By minimizing the effects of the morphology for the core/shell structure, the material category is verified to play important roles on the electrochemical performances of the electrodes. The NiCo2O4/NiSx, NiCo2O4/NiOx, and NiCo2O4/NiMoxOy core/shell electrodes present larger specific capacitance (C-F) but an unexpected smaller C-F value is obtained for the NiCo2O4/MnOx core/shell electrode as compared with that for the single NiCo2O4 nanosheet electrode, suggesting the depositing of the shell to enhance the surface area for conducting more Faradic reactions is not always helpful on improving the electrocapacitive performance of the electrode. The highest C-F value of 6.12 F cm(-2) is obtained for the NiCo2O4/NiSx electrode evaluated by using the galvanic charge/discharge curves at the current density of 1 mA cm(-2), due to the superior intrinsic property of NiSx other than its high surface area of the sheets-on-sheets morphology since the structure variation is almost excluded in the work. The asymmetric supercapacitor (ASC) based on the NiCo2O4/NiSx positive electrode is also made with the C-F value of 28.6 F g(-1) based on the whole mass of the ASC at the current density of 0.5 A g(-1) as well as the maximum energy density of 12.83 Wh kg(-1) at a power density of 80 W kg(-1). (C) 2017 Elsevier Ltd. All rights reserved.