Factors Affecting Spectroscopic State-of-Charge Measurement
Authors: Quill Nathan , O'Mahony Marcus , Lynch Robert P. , Gao Xin , Oboroceanu Daniela , Lenihan Catherine , Petchsingh Cattleya , Ní Eidhin Deirdre , Buckley D. Noel 
Published: February 2020

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. VIV and VV) 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 VV on the positive side than VII 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 VIII/VII and VIV/VV 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. VII, VIII, VIV, and VV 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 VIII-VII mixtures or very dilute VIV-VV mixtures.6-12

At higher concentrations, the absorbance of VIV-VV mixtures is a highly non-linear function of the mole fraction of VIV. For VIV-VV mixtures, this non-linearity has been shown to be due to the formation of a complex between the VIV and VV 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 explained6-8 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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