| Summary: | 碩士 === 國立聯合大學 === 化學工程學系碩士班 === 98 === Vanadium redox flow battery, called VRB, is one of the best candidates for electrical energy storage systems. This battery consists of polymer separator, porous carbon electrode, graphite bipolar plate and metal current collector. The electrolytes are circulating between the electrolytic single cell and storage tank. During the initial charge cycle, the V(IV) is oxidized into V(V) on the positive electrode and V(III) is reduced into V(II) on the negative electrode. Besides the vanadium redox reaction, a side reaction-water electrolysis may be taken place. The side reaction causes the cell’s charge/discharge efficiency loss. In order to improve this condition, we have to investigate the reactive kinetics that taken place on the electrodes. This study investigated the characteristics of a single cell and the influences of operating conditions on energy storage efficiency. Results indicated that the efficiency was depending on the charge/discharge current and electrolyte flow rate and electrolyte concentration. High efficiency could be obtained at high electrolyte flow rate and low current density and high electrolyte concentration. At low depth of charge/discharge, a efficiency of 80 % could be achieved. The water electrolysis interferes the kinetics of V(IV) redox reaction. By changing the pH value of the electrolyte, the potential of water electrolysis reaction could be shifted toward the negative direction. So the reduction current can be separated from the water electrolysis current. Vanadium reduction current can be observed in the negative potential region. All the electrochemical studies were carried out at a rotating electrode. The mass transfer rate of reactant was controlled by rotation disc electrode in different rotating speeds.
A linear relationship on the plot of 1 / I versus 1 /ω2 for vanadium redox reaction indicated that the vanadium redox reaction was first order with respected to vanadium. The rate constants of the redox reactions were also calculated and plotted as a function of electrode potential. From this plot, the Tafel slope and rate constant can also be evaluated. We found that the V(IV) / V(V) oxidation reaction is a first order reaction, the Tafel slope was 136 ?b 12 mV/decade. The V(IV) / V(III) reduction reaction was also an first order reaction with a Tafel slope of 64 ?b 7 mV/decade. The rate constant of V(IV) oxidation on Pt was around 0.24×10-3 ~ 1.47×10-3 cm/s。For Pt electrode covered with nano-tubes, the V(IV) / V(V) oxidation reaction was an first order reaction with a Tafel slope of 167 ?b 44 mV/decade. The rate constant of that reaction was around 0.09×10-3 ~ 0.48×10-3 cm/s。
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