A study on energy-storage materials for supercapacitor applications

Abstract

Supercapacitors have attracted extremely high interest from scientists and engineers due to their broad applications in portable electronics, electric vehicles and flexible systems. They have remarkable properties compared to batteries such as fast charging and discharging, high power density, long cycling life and safe operation. Electrode materials and structure could greatly affect the performance of a supercapacitor. In this research, various methods are investigated to enhance the supercapacitors performance. The study began on MnCo₂S₄-NiCo(OH)₂ core-shell nanocomposite for high-performance solid-state supercapacitor applications. Transition metal sulfides, for example CoSx, NiSx and MnSx, possessing high electrochemical performance for supercapacitors have attracted great attention in recent years. A ternary metal sulphide nanostructure MnCo₂S₄ was designed and fabricated to combine the advantages of CoSx and MnSx for supercapacitors. MnCo₂S₄ nanostructure can serve as porous platforms for loading a large amount of additional active materials, enabling fast electron conduction and ion diffusion. To further improve the electrochemical performance, Ni-Co hydroxide nanosheet was deposited on the surface of MnCo₂S₄ to form a hierarchical core-shell structure. Core-shell structure is a promising electrode configuration due to its rich redox reaction sites and sufficient contact area between electrode and electrolyte. The electrode based on the MnCo₂S₄@NiCo(OH)₂ core-shell nanocomposite showed excellent areal capacitance and the symmetric supercapacitor fabricated with this electrode exhibited high energy density, power density and good rate capability. With a view to promoting the electrochemical behavior, a ternary metal sulphide was then used as a shell material and grown on the surface of MnCo₂S₄ by a simple electrochemical deposition to form a MnCo₂S₄@Ni-Co-S (MCS@NCS) core-shell nanostructure. Due to the synergistic effect of core and shell materials, the electrochemical performance of the core-shell hybrid electrode could be enhanced significantly. The as-prepared MCS@NCS core-shell hybrid electrode displayed ultrahigh areal capacitance (10.14 F/cm² at a current density of 1 mA/cm²). The morphology, crystallinity, specific surface area and porosity of the electrode materials were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction and Brunauer-Emmett-Teller N₂ adsorption/desorption measurement system. Moreover, an asymmetric supercapacitor using Ni foam@MCS@NCS as the positive electrode and Ni foam@activated carbon as the negative electrode was fabricated with 6 M KOH as the electrolyte. The fabricated asymmetric supercapacitor showed large operating voltage (1.65 V), excellent energy density (7.26 Wh/m²) and durability, revealing its great potential applications as an efficient energy storage device. Flexible supercapacitors have attracted significant attention in recent years due to their potential application in miniaturized, portable, flexible and wearable electronics. In view of this, flexible solid-state supercapacitors using paper-based electrodes for energy storage were investigated. A soaking and electrodeposition method was introduced to coat carbon nanotubes (CNTs) and nickel cobalt sulphide (NCS) on a filter paper to prepare flexible paper-based electrodes. Paper was chosen as the substrate due to its notable features including wide availability, lightweight, flexibility, low cost, recyclability and environmental benignancy. These Paper-CNT-NCS hybrid electrodes and the solid-state electrolyte were assembled together to form the supercapacitor. By combining the advantages of CNT and NCS, the as-fabricated solid-state flexible supercapacitor demonstrated great potential as an energy storage component for flexible electronic applications. Finally, for the realization of low-cost, scalable and printable energy storage devices, interdigitated planar structure of flexible and solid-state symmetric supercapacitors with active electrode material consisting of Mn-Co deposited by inkjet printing was developed. The interdigitated structure with finger electrodes has many advantages including large surface area, short migration distance of ions and small internal resistance. The correlation between the dimensions of the interdigitated electrodes and the electrochemical performance of the supercapacitors was studied. Compared to the conventional synthesis methods such as electrochemical deposition and hydrothermal treatment, inkjet printing method allows more large-scale and rapid production for industrial applications.