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The vanadium redox flow battery high-performance electrolyte stability

Author: WuXueWen
Tutor: LiuSuQin
School: Central South University
Course: Applied Chemistry
Keywords: Vanadium redox flow battery Electrolyte Additive stability CTAB UV-visisble spectrum Electrochemical performance Vanadium(Ⅳ) ion species
CLC: TM912
Type: Master's thesis
Year: 2010
Downloads: 437
Quote: 2
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Abstract

In this paper, Preparation of high concentrated electrolyte, rapid analysis of electrolyte, CTAB as stabilizer to electrolyte were studied. The accurate information on vanadium(Ⅳ) species, made it possible to understand activity and reversibility of the electrochemical reaction of V(Ⅳ)/V(Ⅴ) positive electrolyte in the vanadium redox battery. The main points can be summarized as following.The electrolytes which composed of 1.6-4.0 mol·L-1 total vanadium concentration and 2.0-6.0 mol·L-1 sulphuric acid were prepared by mixing sulphuric acid and vanadium pentoxide at 235℃for 4 h. The method struggles against the limitation of low vanadium concentration of chemical method. The production that doesn’t adhibit impurity, featares high activity and low cost. The yield of electrolyte can be increased by scale-up.After UV-visible spectra study of various vanadium valence solutions, linearity region of V(Ⅲ)/V(Ⅳ) system was obtained. When V(Ⅱ)/V(Ⅲ) or V(Ⅳ)/V(Ⅴ) system is treated by standard V(Ⅳ) or V(Ⅲ) solution, we can achieve the analysis of being auto-sort and rapid analysis of all kinds of the electrolyte(including total vanadium concentration and the valence state analysis).Hexadecyl trimethyl ammonium bromide (CTAB) was used as the additive in electrolyte for Vanadium Redox flow Battery. Its stability and electrochemical performance were investigated by UV-visible Absorption Spectrophotometry, Scanning Electron Microscope (SEM), Square Wave Voltammetry(SWV), Cyclic Voltammetry(CV) and Examination of Stabilization. The results showed that the quaternary ammonium headgroups of CTAB micelles interacting with V(Ⅴ) ions in electrolyte prevents pentavalent vanadium from further polymerization which leads to a good suppression of the crystallization. The stable hemispherical particles forming at the graphite-liquid interface catalyze the redox reaction of V(Ⅳ)/V(Ⅴ), which is called Micellar catalysis. EIS and Charge-discharge tests showed that adding of CTAB makes charge transfer resistance much smaller, and doubles double-layer capacitance, so that the electrochemical reaction activity of the electrolyte improved. The result is consistent with CTAB micellar catalysis.The aquaoxovanadium(Ⅳ) ion in concentrated H2SO4 media was found to give soluble [VO(SO4)(H2O)4]·H2O group and its dimer, [VO(H2O)3]2(μ-SO4)2. Their formation mechanisms were investigated by UV-visible spectra, Raman spectroscope, X-ray diffraction, cyclic voltammetry and Electrochemical Impedance Spectrum. A new weak absorption spectrum in the visible region characterized by two transitions (671.5 and 820 nm) for [VO(SO4)(H2O)4]·H2O is recorded. Instead of an equatorial water oxygen in [VO(H2O)5]SO4 by a sulfate oxygen causes an increased deviation from the near centrosymmetry of the octahedral complexes. And the spectrum disappears because of the formed dimer. In contrast to the [VO(SO4)(H2O)4]·H2O, one more equatorial sulfate oxygen exists in the dimer. It shows symmetrical structure, which correlates very well with non-activity in UV-visible spectroscopy. [VO(SO4)(H2O)4]·H2O (Minasragrite, syn) is the main component of crystals from the supersaturated solution of VOSO4 in 1 M sulfuric acid, while [VO(H2O)3]2(μ-SO4)2 (Vanadyl Sulfate Hydrate) is in 12M sulphuric acid, which was worked out by X-ray diffraction. Raman spectra of the crystals show relative intensities of Raman bands at 1001.4 cm-1 in 12 M sulfuric acid doubled that at 972.2 cm-1 in 1 M sulphuric acid, which indicates the amount of V-O-S linkage in the complexes from 12 M sulphuric acid doubles that in 1 M sulphuric acid as well. A solution of vanadium(Ⅳ)(0.05 M) in 12 M H2SO4 exhibits a reversible redox behavior near 1.14 V (vs. SCE) on carbon paper electrode, it’s another evidence for the new soluble species of the aquaoxovanadium(Ⅳ) ion. The relationship between chemical species in solution and electrochemical activity of vanadium(Ⅳ) will lead to the optimum design of electrolyte performance for Vanadium Redox Flow Battery.

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