Vanadium redox flow batteries for large-scale energy storage:electrochemistry, efficiency and spectroscoptic monitoring

Abstract

This thesis describes characterizations of a vanadium redox flow battery (VRFB) in electrochemical and spectroscopic aspects. The cycling performance of a cell was characterized. To study coulombic efficiency of the cell, a series of charging and discharging experiments was carried out at 0.1 A. It was found that the current efficiencies of the positive and negative half-cells were imbalanced; low current efficiency at the negative electrode resulted in degradation of the overall performance of the cell. Further charging and discharging experiments indicated that the efficiency imbalance is mainly due to hydrogen evolution at the negative electrode rather than the transfer of vanadium ions across the membrane. To study the voltage behaviour of a VRFB, a full charging and discharging cycle was performed at 0.2 A. It was found that the majority of the voltage loss was at the negative electrode, suggesting that the kinetics of the VII/VIII reaction at the carbon felt electrode is slower than that of the VIV/VV reaction at the positive electrode. At the negative electrode of a VRFB, hydrogen evolution occurs not only during the reduction of VIII to VII but also during the reduction of VIV to VIII. It was found that the reduction of VIV to VIII requires a large overpotential. Consequently the potential is below that for hydrogen evolution, and hydrogen gas was produced. It was shown that the rate of hydrogen evolution greatly enhanced by the presence of VIV (VO2+). A mechanism involving a VII reactive intermediate for this enhanced hydrogen evolution was proposed. Based on from this mechanism, a quantitative model was developed which shows good agreement with experiment. To develop a method for monitoring the state of charge of a VRFB, absorption spectra of the electrolyte were studied using an ultraviolet visible (UV-Vis) spectrophotometer. For spectra of VIV/VIII and VII/VIII mixtures, it is found that the UV-Vis absorption spectra are linear combinations of those of the constituent. However, for spectra of VV/VIV mixtures, it is found a 1:1 complex forms, complicating the absorbance spectrum. The absorbance characteristics of this complex were analysed and the effect of its presence on spectrometric analysis of the catholyte is discussed.