| Summary: | 碩士 === 國立聯合大學 === 化學工程學系碩士班 === 100 === Vanadium redox flow battery (VRB) is a novel electrical energy storage system. The battery uses V(II)/V(III) and V(IV)/V(V) redox couple as the electrolyte in the negative and positive compartment, respectively. During VRB charging, the electrolyte composition is changed and water electrolysis is also taking place. The cell voltage loss is mainly due to the sluggish reaction of these redox reactions, it also affects the overall VRB energy storage efficiency. In order to improve this situation, we investigate the reaction kinetics on the carbon electrode. We study the kinetic of V(IV)/(V) and V(II)/V(III) redox reactions, respectively. We use rotating disk electrode (RDE) to measure the polarization curves of V(IV)/(V) and V(II)/V(III) redox reactions in different electrolyte compositions. The linear sweep voltammetry (LSV) and cyclic voltammetry (CV) are used during the measurement.
The reaction rate constants of these redox reactions are calculated from measured data and limiting current equations. On Carbon electrode, the rate constants of V(IV) oxidation is much slower than the rate constant of V(II) oxidation, they are 7.89 × 10-14 ~4.38 × 10-7 cm s-1 for V(IV) oxidation and are 5.52 × 10-3 ~ 2.81 × 101 cm s-1 for V(II) oxidation. The mainly voltage loss is due to the V(IV)/(V) in the electrolytes on positive electrode, and the reactions on Pt electrode is faster than the reactions on carbon electrode. We also found that the Stoke radius of V(IV)/V(V) ion is larger than the Stoke radius of V(II)/V(III). This provides a reasoning of lower reaction constant of V(IV)/V(V) ion pair.
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