Factors Affecting Spectroscopic State-of-Charge Measurement

Vanadium redox flow batteries (VRFBs) are an attractive technology for a variety of energy storage applications.^1,2 The catholyte and the anolyte in these batteries are circulated through the electrodes from reservoirs.  The active species in the catholyte are VO²⁺ and VO₂⁺ (i.e. V^IV and V^V) while the active species in the anolyte are V³⁺ and V²⁺.  VRFBs have a major advantage over other flow batteries in that cross-contamination due to transport through the separating membrane is effectively eliminated.³

Monitoring of the state of charge (SoC) is important for any battery system.  Additionally, in VRFBs, transfer of vanadium ions across the membrane⁴ and side reactions such as hydrogen formation⁵ at the negative electrode can result in SoC becoming unbalanced (e.g. more V^V on the positive side than V^II on the negative).  Therefore independent monitoring of SoC of both electrolytes is important for effective operation of VRFB technology.

Both overall SoC of a VRFB and individual SoCs of the positive and negative sides (determined by the V^III/V^II and V^IV/V^V ratios in the respective electrolytes) may be monitored in a number of ways using electrodes. However, these methods have drawbacks.⁶ Spectroscopic monitoring of SoC is independent of electrochemistry and offers the possibility of performing in-situ analysis.  V^II, V^III, V^IV, and V^V aqueous species have strong absorbance spectra in the visible region.^6-12 If the absorbance is a linear combination of that of the constituents UV-visible spectroscopy is a straightforward method of measuring the concentration and ratio of mixtures: e.g. for V^III-V^II mixtures or very dilute V^IV-V^V mixtures.^6-12

At higher concentrations, the absorbance of V^IV-V^V mixtures is a highly non-linear function of the mole fraction of V^IV. For V^IV-V^V mixtures, this non-linearity has been shown to be due to the formation of a complex between the V^IV and V^V species. ^6-8 Tang et al.¹⁰ and Liu et al.¹¹ addressed the problem of non-linearity by developing an empirical method of estimating SoC. However, the non-linearity can be quantitatively explained^6-8 allowing precise methods¹² of optical monitoring of SoC in VRFBs. Factors, such as the concentration of sulphate and vanadium, can significantly affect this non-linearity.^6,8 In this presentation, we present a detailed study of these factors in relation to spectroscopic SoC measurement. 

Acknowledgements

The authors acknowledge funding from Enterprise Ireland through Commercialisation Fund CF/2013/3303. The material in this research is partly based upon works supported by Science Foundation Ireland through the Charles Parson Initiative (CPI).

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