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Abstract
The novel Vanadium Bromide (V/Br) redox flow cell employs a V (III)/V (II) couple in the negative half-cell and a Br/Br2 couple in the positive half-cell, with hydrobromic acid and hydrochloric acid as the supporting electrolyte. This study evaluated the chemical and electrochemical properties of the electrolytes and assessed experimental and commercial membranes for use in the V/Br flow cell.
A number of techniques were employed to characterise the composition of the V/Br flow cell electrolytes. During charge, the conductivity of the positive half-cell electrolyte increased, whilst the density and viscosity increased. The reverse was observed for the negative half-cell. The UV-visible spectra of the electrolytes showed characteristic peak wavelengths of the vanadium oxidation states and provided and insight into the halogenated species forming during the operation of the V/Br flow cell. The electrochemical properties of the electrolytes were also examined using cyclic voltammetry. NMR studies examined the relationships between the 35CI and 79Br nuclei in the presence of halide and paramagnetic vanadium ions. It was established that the SOC and performance of the V/Br flow cell can be measured by changes in slllectral chemical shifts and line widths.
Small-scale cycling experiments were conducted to evaluate the performance of ion exchange membranes in the V/Br redox flow cell. Of the membranes evaluated, a number were not suitable for use due to high membrane resistances or low chemical stability. The perfluorinated Nafion® and Gore Select® ion exchange membranes proved to be the most chemically inert and showed low resistances. The Gore Select® membranes did however exhibit blistering during extended cycling. The chemical stability and cycling performance of the HiporeTM microporous separator showed promise for future studies to optimise the selectivity and ion exchange capacity of the membrane.
Tests of membrane ion exchange capacity, diffusivity and conductivity mirrored the properties displayed in the cell cycling experiments. Results suggested that the structural characteristics of the membrane (including functionality and crosslinking) greatly influenced membrane properties and performance. Tests of long term stability showed a negative change in membrane properties. These changes did not however reflect measured changes during cell cycling experiments.