In the past decades, supercapacitors have attracted great attentions due to their high power density, long life cycle, and fast recharge capability. In general, the electrode materials for supercapacitors are divided into two categories on the basis of the energy storage mechanism: electrical double layer capacitors (EDLCs) and pseudocapacitors (PCs). However, PCs exhibit much larger capacitance values and energy density than EDLCs due to their fast and reversible redox reactions of the electrochemically active electrode materials.
Mixed transition-metal oxides (MTMOs), such as single-phase ternary metal oxides with two different metal cations, typically in a spinel structure (donated as AxB3-xO4, A, B = Co, Ni, Zn, Mn, Fe, ie.), have captured much attention as promising electrode materials for supercapacitors. Among the MTMOs, compared with NiO and Co3O4, the spinel nickel cobaltite (NiCo2O4) exhibits better electrical conductivity and higher electrochemical activity. However, the relatively weak conductivity and small specific surface area make the capacity greatly lower than the theoretical value. Therefore, numerous efforts have been made to optimize the supercapacitors performance of NiCo2O4 via various methods, including control of microstructures, crystallinity, and electrical conductivity.
Herein we have designed two different NiCo2O4/carbon composites as electrode materials for high performance supercapacitor: (1) The CNS/NiCo2O4 core-shell structural sub-microspheres, in which the CNS act as a core and NiCo2O4 coated on the CNS surface, exhibited a high specific capacitance and excellent cycling stabilities at high current density. (2) The sandwich-like NiCo2O4/rGO/NiO heterostructure composite on nickel foam, wherein the components were assembled into a uniform structure and each component could partially retain its individual traits to improve electrochemical properties. The above rational combination of NiCo2O4 and carbon material can provide a synergistic effect for supercapacitors to enhance the electrochemical performance. The conductive carbon material not only facilitate the electron transport, but also effectively prevent the NiCo2O4 agglomeration and ensure the full utilization of the electroactive materials, which lead to the high supercapacitor performance of the composites.
This work was supported by the Shenzhen Science and Technology Innovation Committee 2017 basic research (free exploration) project of Shenzhen City of China (no. JCYJ20170303170542173).