Transition Metal Oxide Based Nanocomposites for High Performance Electrochemical Supercapacitor Applications

Abstract

The rising energy demand, as well as growing concerns regarding rising levels of environmental pollution and global warming from the usage of fossil fuels, has sparked intense research efforts into exploring the potential of energy generation from alternative renewable and sustainable energy sources and its subsequent storage. Supercapacitor devices have been explored due to their high-power performance, long life cycle and low maintenance costs. In the first part of this thesis, we present a direct hydrothermal synthesis of hydroxide/oxide nanocomposites of nickel grown on reduced graphene oxide (rGO). The materials were employed as a high-performance active layer for advanced supercapacitors. Ni(OH)2/rGO and NiO/rGO nanocomposites demonstrated a high specific capacitance of 1255.12 and 636.84 F g-1 at 10 mV s-1 obtained from CV curves while 1092.5 and 1070 F g-1 estimated from GCD at 1 A g-1, respectively. Subsequently, symmetric devices with a broad potential window of 1.4 V for both materials were also fabricated. Symmetric devices delivered the maximum specific capacitance of 115.71 and 80.28 F g-1 at 1 A g-1 with an energy density of 31. 5 Wh Kg-1 and 22 Wh kg-1 at a power density of 1.4 kW kg-1 which indicates the highrate capability of the devices. The devices have maximum capacity retention of 81% and 93%, when tested for 1000 discharge cycles at 3 A g-1 showing high cycle stability. The superlative performance of nanocomposites can be of the synergistic effects they exhibit, demonstrating that they are appealing electrode materials for supercapacitor applications. In the second part of this thesis, we demonstrate a successful synthesis of heteroatom (N,S) co-doped TiO2 decorated with Ag nanoparticles using a direct combustion approach. These materials serve as a high-performance active layer for advanced battery-type supercapacitors. Among all the investigated materials, N,S co-doped TiO2 nanoparticles and Ag/N,S co-doped TiO2 nanocomposite exhibited impressive specific capacitances of 360 and 480 F g-1 at a current density of 1 A g-1. Moreover, we employed Ag/N,S co-doped TiO2 nanocomposite as a symmetric supercapacitor from - -1 to +1 V. The symmetric electrode demonstrated a capacitance of 183 F g-1 at 3 A g- 1 with an energy density of 105 Wh kg-1 at a power density of 6 kW kg-1. One of the most promising aspects of this study was the high cyclic stability of the device, with capacity retention of 80% when tested for 3000 discharge cycles at 3 A g-1. The improved electrochemical performance of the Ag decorated N,S co-doped TiO2 iv structure confirms the synergistic behaviour of each component in the nanocomposite. These results indicate the remarkable durability of the device. Additionally, the materials used in this work are environmentally friendly, making them attractive candidates for battery-type supercapacitors. The third part of this thesis reports a newly developed, surfactant-free and scalable methodology for the synthesis of spherical nanoparticles of Ni(OH)2 and NiO, employing the polar, aprotic solvent, Dimethylformamide (DMF). The methodology was extended to the synthesis of the hybrid nanomaterial Ni(OH)2-MnO2 and, with the addition of SDBS, MnO2 nanoparticles. The nanostructures obtained were evaluated for their ability to be used as electrochemical battery-type supercapacitors. Of all electrode materials investigated in three-electrode configuration, surfactant-free NiO and α-Ni(OH)2-MnO2 exhibit the highest values of 1450 F g-1 and 1604.75 F g-1 at 1 A g-1, respectively. Subsequently, symmetric devices were fabricated and tested from 0 to 1 V for all synthesized materials, and their electrochemical performance was investigated. In symmetric devices where surfactant-free NiO and α-Ni(OH)2-MnO2 were employed as the active layers and showed specific capacitance of 340 F g-1 and 502 F g-1 at a current density of 1 A g-1 and energy densities of 47.21 Wh kg-1 and 69.72 Wh kg-1 at a power density of 1 kW kg-1 respectively, clearly demonstrating the high capabilities of the symmetric devices. The devices have a maximum capacity retention of 65% and 72% after 5000 cycles at 3 A g-1 demonstrating their high cycle stability. In the final part of this research dissertation, we present the successful synthesis of V2O5 nanoparticles, V2O5/GO, and V2O5/rGO using a newly developed colloidal method. The reaction parameters were carefully controlled using a single molecular precursor. The as-prepared V2O5 and its composites were evaluated for their potential in batterytype supercapacitors. All the materials exhibited excellent specific capacitance performance in three-electrode configurations, with V2O5/rGO demonstrating the highest value of 1190 F g-1 at 1 A g-1. These results highlight the high-rate capability of the fabricated device. This impressive performance of the device opens up new possibilities for the use of the above-mentioned materials for state-of-the-art storage systems. Additionally, the materials used in this work are environmentally friendly, making them attractive candidates for battery-type supercapacitors.