| Popis: |
The global energy demand has escalated during last few decades due to the population growth, urbanization and industrialization. On the other side, rapid and continuous exploitation of non-renewable fossil fuels have raised the concerns on environmental degradation. This necessitates developing efficient energy storage systems besides energy production to help match the supply and demand. Carbon materials have been extensively studied as electrodes for energy storage applications. However, carbon materials used as electrodes in commercial energy storage systems at present are derived from non-renewable fossil fuels. Thus, there is need to explore alternative sustainable carbon precursors. This will help reduce the sole dependency on nonrenewable fossil fuels and also contribute in creating healthy environment. Biomass are rich in carbon content, and are low cost, easily available, abundant, renewable and environmentally-friendly. Food waste is an inevitable by-product produced in different stages of the food supply chain. Fruit peels waste as bio-waste are neglected reusable resource which offer a big potential for employing it as precursor for the production of value-added products such as porous carbons. Therefore, comprehensive study is needed to evaluate the potential of employing this underutilized sustainable resource for preparation of value-added useable products such as porous carbon. Further investigations are also needed to evaluate the electrochemical performance of these carbons as electrodes for energy storage applications, and development of high performance energy storage devices. This has motivated this study to explore fruit peels waste as precursor materials for energy storage applications such as vanadium redox flow battery (VRB) and high energy supercapacitors. High surface area activated carbons (ACs) were synthesized by carbonization followed by chemical activation from the five screened precursors, and the assynthesized ACs were subjected to extensive characterizations. Two AC samples synthesized from orange peels waste and lychee peels waste were further screened for evaluating them as electrode materials for energy storage applications. The orange peel-derived ACs modified graphite electrodes demonstrate improved electrocatalytic activity in both positive and negative side redox couples compared with the pristine graphite electrode. The improved electrode activities are attributed to the synergistic effects of high surface area of the ACs and rich oxygen functionalities on the surface of ACs. The results suggest the OP-ACs to be promising electrodes for VRB applications and can be incorporated into making conducting plastics electrode to lower the VRB cell stack weight and cost. The lithium ion capacitors (LICs) fabricated using OP-AC2 with pre-lithiated graphite (LiC6) delivered specific energy of ~106 Wh kg–1 . A systematic improvement from the specific energy of ~8 to 106 Wh kg–1 is noted from aqueous to LIC assembly. The other porous carbon synthesized from lychee peel waste (LP-AC) with surface area of 1601 m2 g -1 was evaluated as electrode material for supercapacitors in symmetric (LP-AC||LP-AC) and asymmetric (Gr/PANI||LP-AC) cell configurations. Performance tests showed that the asymmetric cell assembly could deliver specific capacitance of ~141 F g-1 at 0.5 A g-1 and specific energy to ~50 Wh kg-1 as compared to the specific capacitance of 105 F g-1 at a current density of 0.25 A g–1 and specific energy of 9.4 Wh kg-1 for the symmetric cell configuration. The asymmetric cell also showed good cycling stability with ~88% of the initial specific capacitance retained after 5000 cycles. To sum up, this study successfully demonstrated the feasibility of converting selected fruit peels waste to value-added porous carbons as promising electrodes for energy storage applications. For the first time, this study evaluated performance of the OP-AC and the T-OP-AC modified electrodes for VRB applications. The fabrication of whole LIC assembly using OP-AC to deliver high energy was also reported for the first time. For the first time, the porous AC synthesized from a new precursor, i.e. lychee peel, was reported as pseudocapacitive electrode for aqueous supercapacitors. The utilization of this renewable resource contributes not only in waste management but also generates value-added products for promising applications, and contribute in reducing sole dependency on fossil fuels for energy. Doctor of Philosophy (CEE) |
