A number of techniques for determining the state of vanadium electrolytes have been proposed including absorbance measurements and conductivity measurements. [4-7] However, all techniques face a significant challenge, primarily due to the transport properties of the ionic membrane separating the two half-cells. Ideally, a proton exhange membrane would transport only H+ ions between the two half-cells and complete the electrochemical circuit. However, it is well known that significant amounts of both vanadium ions and water molecules will also be transported. This leads to a gradual change in the total vanadium and sulphate concentration in each half-cell.  Since both the solution absorbance and conductivity vary with vanadium and sulphate concentration, any technique based on determining a “calibration curve” that relates SoC to either conductivity  or absorbance  will become less and less accurate as the battery is cycled.
To make these techniques viable during long-term cycling, accurate models of the variation of the absorbance and conductivity of vanadium battery electrolytes with SoC, vanadium concentration and sulphate concentration are required. We have recently reported detailed analyses of the absorbance of VFB catholytes. [4,5] To date however, there have been no reports of attempts to accurately characterise the conductivity of these solutions as a function of the various solution components. In this report, we will present the results of a careful study on the variation of the conductivity of VFB catholytes with VIV concentration, VV concentration and sulphate concentration. It will be shown that, in general, the solution is well behaved, and that the variation of conductivity with most parameters is close to linear over a reasonable range. It will also be shown that varying the sulphate concentration in the range of 4-5 mol dm-3 (typical of commercial VFBs) has little effect on the measured conductivity. From this data, an accurate analytical model of the conductivity of VFB catholytes is developed which has four variables; the concentration of VIV, the concentration of VV, the concentration of sulphate and the temperature of the electrolyte. It will be shown that the model predicts the measured conductivity to within <2% over wide ranges of VFB catholyte compositions and temperatures. This work represents an important step in the accurate determination of the SoC of a VFB during long term cycling using conductivity measurements.
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