A Study on Carbon-based Hybrid Supercapacitor Electrodes: Design, Fabrication, and Testing

Abstract

In recent years due to the lack of fossil fuels, population growth and the development of portable electronic devices, it is essential to design a novel energy conversion and storage devices. Various energy storage devices are commercially available, among them supercapacitors (SCs) have been given special attention because of their high energy and power density, longer cycle-life (> 100000), fast charging-discharging process, a wide range of operating temperatures and high efficiency. These outstanding properties of SC makes a wide range of potential applications, such as in-memory backup systems, hybrid electric vehicles, LED drivers, energy management fields, industrial power supplies, etc. The SCs bridge energy and power gap between the conventional capacitor and battery. However, low energy density is the main concern for SC. Hence, active research is going on in search of novel electrode materials and new designs to improve energy density without losing its power density. SCs are classified based on their electrode materials, such as electrical double-layer capacitors (EDLC) in which mostly carbon materials are used as electrodes. The second one is pseudocapacitors (PCs) that combine transition metal oxides and conductive polymers as electrode material. Another class is the hybrid capacitor (HC), which combines carbon and metal oxide/conductive polymer for electrode material. Carbon materials (carbon nanotubes, graphene, activated carbon etc.) are given much attention to SCs owing to their unique physical and chemical properties like high specific surface area, good electrical conductivity, developed porous structure, environmentally friendly, non-toxic, good thermal and mechanical stability. However, carbon-based SC's, energy density is the limitation for the realization of commercial applications as per the energy demand globally. The metal oxide and conductive polymers involve reversible redox reactions during their charging-discharging time which results in electrodes accumulating more electrolyte ions. Hence, the performance of carbon-based SC could be further improved by integrating it with metal oxides or conductive polymers. In this thesis, the fabrications of various carbon composite electrodes have been investigated for SC application, where the proper optimization of carbon composite has been carried out. Furthermore, the presence of metal oxides in the carbon composite for SC application was also investigated. The symmetric SC devices were also fabricated for practical application. The novelty of this research work is the cost-effective synthesis of the electrode materials, new design of the device, and the accomplishment of outstanding properties. The selected carbon materials chosen are carbon nanotubes (multiwalled carbon nanotubes (MWCNTs)), few-layer graphene (FLGR), mixed phase carbon material (MPCM) and activated carbon (AC). Though there are various metal oxides, ZnFe2O4 (ZFO) was chosen and incorporated in different carbon structures to make SC electrodes. The electrochemical properties of the electrode materials were investigated using cyclic voltammetry (CV), Galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). It has been observed that carbon-based SC results good storage performance, i.e., the maximum specific capacitance obtained by the MWCNTs-5/AC electrode was 222 F/g at 1 Ag-1 and 239 F/g at 5 mV/s. The functionalized MWCNTs/AC electrode has a capacitance of 372 F/g at 60 Ag-1 and 395 F/g at 5 mV/s. The cyclic stability test of the fabricated electrodes has been investigated and the MWCNTs-5/AC retained a maximum capacitance of 90%, whereas functionalized MWCNTs/AC retained 79% over 10,000 cycles. The fabricated 10 wt. % of FLGR in AC (FLGR-10/AC) results excellent SC properties, and the maximum obtained specific capacitance was 176 Fg-1 at 1 Ag-1 and 180 Fg-1 at 5 mV/s. The fabricated FLGR-30/MPCM showed a maximum specific capacitance of 283 F g-1 at 1 A g-1 and 282 F g-1 at 5 mV/s. The fabricated electrodes have been further subjected to a cyclic stability test for 10,000 cycles. The FLGR-30/MPCM retained a maximum capacitance of 93%, whereas FLGR-10/AC retained 91%. The performances of carbon-based SC have been further improved by incorporating ZFO. The maximum specific capacitance of the ZFO-10/MWCNTs/AC electrode was 613 F g-1 at 5 mV/s and 609 F g-1 at 1 Ag-1. In a two-electrode symmetric device, ZFO-10/MWCNTs/AC electrode based device has shown the highest specific capacitance i.e. 323 Fg-1 at 1 Ag-1 and 313 Fg-1 at 5 mV/s. The energy density and power density of the ZFO-10/MWCNTs/AC device were evaluated following standard relation. The maximum obtained energy density is 16.15 Wh/kg and a power density of 6000 W/kg among the other two devices. Moreover, ZFO-10/MWCNTs/AC device shows outstanding cyclic stability performance i.e. 97 % capacitance has been retained over 10,000 cycles. Therefore, the presence of ZFO in carbon composites enhances storage performances and offers excellent cyclic stability. The finding suggests that the prepared carbon-based hybrid electrodes have performed excellent storage performance and cycle stability. The obtained energy and power density of carbon-based hybrid SC might contribute to the current challenges towards the energy and power-efficient SC device requirement.