Fabrication of super-high energy density asymmetric supercapacitor prototype device employing NiCo2S4@f-MWCNT nanocomposite

Abstract

Materials with high porosity, high redox activity and rich electrochemically active sites are promising candidates for pseudocapacitance. Among the bimetallic sulphides of transition metals, nickel cobalt sulphide (NiCo2S4; NCS) is a promising pseudocapacitive material. NiCo2S4 (NCS) can be coupled with carbonaceous material such as functionalised multiwalled carbon nanotubes (NCS@f-MWCNT) to further enhance the electrochemical characteristics such as high charge-storage capacity, improved charge-discharge characteristics, stability and rate performance. In this line, bare NiCo2S4 (NCS) and functionalized multiwalled carbon nanotubes (f-MWCNT) loaded NiCo2S4 nanoparticles were synthesized by hydrothermal method by employing hexadecyltrimethylammonium bromide (CTAB) as surfactant. X-ray diffraction studies confirmed the formation of cubic phase of NiCo2S4 and transmission electron microscopic study revealed the formation of NCS@f-MWCNT nanocomposite. The XPS findings confirmed the co-existence of Ni3+, Ni2+, Co3+, and Co2+ species in both the NCS and NCS@f-MWCNT samples. Cyclic voltammetry analysis was performed to determine the respective impacts of the surface adsorption and diffusion-mediated processes on the charging/discharging kinetics. The incorporation of f-MWCNT into NCS led to improved overall charge storage kinetics, demonstrating a promising avenue for developing low-cost cathode materials for high-performance hybrid battery-type materials with both high power and energy densities. Bare NiCo2S4 showed a specific capacitance of ~899 Fg−1 at 1 Ag−1 with a capacitance retention of ~52 %. While NCS@f-MWCNT exhibited a high charge storage capacity of ~1360 Fg−1 with capacitance retention of 89 % at 10 Ag−1. An asymmetric coin cell devices were fabricated using NCS@f-MWCNT as a positive electrode and an activated carbon, reduced graphene oxide ((ASC2) or carbon fiber (ASC3) as negative electrodes. Among them, the NCS@f-MWCNT//AC (say ASC1) showed outstanding charge storage characteristics with a capacitance of ~109.9 Fg−1, energy density ~ 78.3 Whk g−1 and power density ~ 800 Wk g−1 at 1Ag−1. The presented analysis has demonstrated that a hybrid structure made of highly conductive materials like functionalized multiwall carbon nanotubes could be employed to synergistically enhance the electrochemical performance of NiCo2S4.