| Summary: | 碩士 === 國立臺灣科技大學 === 材料科學與工程系 === 107 === Vanadium redox flow battery (VRFB) is one of the most promising large-scale energy storage system. However, high production cost and low energy efficiency still limit its practicability. Therefore, the development of low-cost and high-activity electrocatalyst is essential to enhance the performance of VRFB. In this study, low-cost niobium oxide and graphene oxide are used as reactants to synthesis NbO2/NbC/GO. The niobium oxide is uniformly dispersed on the surface of graphene oxide by refluxing and hydrothermal method, followed by annealing the sample with varies temperature ranges of 450, 850 1050 °C in a high temperature tubular furnace. At the annealing temperature of 1050 °C, niobium pentoxide is reduced to niobium dioxide due to the carbothermal reduction reaction between niobium oxide and graphene oxide, and simultaneously reacts at to form niobium carbide. Moreover, the surface oxygen functional groups of graphene oxide will be decomposed at high temperature to form reduced graphene oxide. However, in this study we use carbothermal reduction method to retain the large amount of surface oxygen functional groups on graphene oxide after high temperature annealing, which act as the electrochemical active sites for the vanadium ion redox reactions. In addition, a high content of oxygen vacancies is generated during the reduction of niobium pentoxide, which can effectively adsorb the oxygen functional group to form active sites, and the presence of niobium carbide increases conductivity of the electrocatalyst.
In order to show the synergistic effects of NbO2/NbC/GO, various electrochemical measurements are performed to compare with commercially available niobium oxide, niobium carbide, and graphene oxide. Among all samples, NbO2/NbC/GO exhibits the best electrochemical activity toward VO2+/VO2+ and V2+/V3+ redox reactions. Finally, the charge-discharge test further demonstrates the battery using NbO2/NbC/GO modified graphite felt (GF) as the electrode exhibits the energy efficiency of 81.9 % and 76.5 % at the current densities of 40 and 80 mA cm−2, respectively. The NbO2/NbC/GO modified GF demonstrates an excellent electrochemical activity compared to untreated and pretreated GF. In addition, after 100 cycles of charge-discharge stability test at a current density of 80 mA cm−2, there is no significant decay in energy efficiency. The NbO2/NbC/GO as an electrocatalyst not only possesses outstanding electrochemical activity but great stability in strong acid electrolyte.
Keywords: Vanadium redox flow battery; electrocatalytic activity; carbothermal reaction; electrode surface modification
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