The ultraviolet-visible spectra of catholytes for vanadium flow batteries (VFBs) were measured and analyzed for a range of V IV :V V ratios and vanadium concentrations. Using a model of V 2 O 3 3+ in equilibrium with VO 2+ and VO 2 + , the spectra were characterized in terms of an excess absorbance parameter p and the molar extinction coefficients ε 4 and ε 5 of VO 2+ and VO 2 + , respectively. The results showed that p varies weakly with the vanadium concentration C and this variation was quantified relative to a reference concentration C r by means of a concentration coefficient φ r . Experimental data showed that plots of φ r versus Cφ r and plots of 1/φ r versus C are linear and, based on this linearity, φ r was expressed as a simple function of C in terms of its reference concentration C r and a single parameter M that is independent of the choice of C r . Standard spectra of p at a concentration C 0 = 1 mol dm −3 and of ε 4 and ε 5 were generated from which the spectrum of any catholyte may be simulated using the measured value of M in a governing equation. This enables determination of the state of charge for any VFB catholyte using absorbance measurements at a small number of wavelengths. The use of non-dispatchable power sources such as solar, wind and ocean energy is increasing.1 Due to the intermittency of these sources, their use is restricted unless there is a means of storing the energy they produce in periods of high availability for utilization in periods of limited availability. 2,3 There is considerable interest in flow batteries for storing energy from such sources and for other large and medium scale energy storage applications. 4,5 Vanadium flow batteries (VFB), 5-13 also known as vanadium redox flow batteries (VRFB or VRB), are particularly attractive because, in addition to having long cycle life, they are essentially immune to cross-contamination problems due to mass transfer across the membrane that can limit the service life of the electrolyte in other systems. 3,4,7,[14][15][16][17][18][19] This is because both the positive and negative sides of a VFB are based on vanadium species, eliminating the need for costly re-purification processes. 1,12 Typical cells have carbon felt electrodes; both cell design and the electrochemical behavior of electrodes are active areas of research. 20-31The cells can operate at coulombic efficiencies of over 90% 32,33 and their carbon electrodes have very good stability as long as the positive half-cell is not overcharged. 34,35 Accurate monitoring of state of charge (SoC) is intrinsically important for the reliability of energy storage systems, particularly large systems in critical applications. Furthermore, independent monitoring of the SoC of both electrolytes is important for effective operation of flow battery technology. For example in a VFB, transfer of vanadium ions across the membrane [36][37][38] and side reactions such as hydrogen formation 12,39-44 at the negative electrode can cause the battery to become unbalanced (e.g. more V V on the posi...

We showed that it is easy, in principle, to measure the state of charge (SoC) of VRFB negative electrolytes by ultraviolet-visible (UV-Vis) absorption spectroscopy because the spectra are a linear combination of the spectra of the components, VII and VIII. In contrast we showed, for a range of mole percentages of VV at several different concentrations of total vanadium, that the UV-Vis spectra of the positive electrolyte do not exhibit a similar linearity and that absorbance measurements at two different wavelengths are required to measure the SoC. All positive electrolytes showed an excess absorbance A ex which we have proposed to be proportional to the concentration of a 1:1 mixed-valence complex V2O3 3+ in equilibrium with VO2+ and VO2 +. We have defined a parameter p which directly relates A ex to the vanadium concentration C and the mole percentage of VV (i.e. the SoC). We showed that at any given wavelength, the value of p decreases somewhat with C. This is consistent with our model, when the variation of the V2O3 3+ equilibrium constant K c with C is small. We estimated approximate values for K c and for the excess molar extinction coefficient ε ex.

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). References H. Bindner, C. Ekman, O. Gehrke and F. Isleifsson, in Characterization of Vanadium Flow Battery, Risø Report (2011) M.J. Watt-Smith, P. Ridley, R.G.A. Wills, A.A. Shah and F.C. Walsh, J. Chem. Technol. Biotechnol. 88, 126 (2013) M. Rychcik and M. Skyllas-Kazacos, J. Power Sources 22, 59 (1988) J. Xi, Z. Wu, X. Qiu and L. Chen, J. Power Sources 166, 531 (2007) A.H. Whiteheadand M. Harrer, J. Power Sources 230, 271 (2013) D.N. Buckley, X. Gao, R.P. Lynch, N. Quill, and M.J. Leahy, J. Electrochem. Soc. 161 (4), A524 (2014) X. Gao, R.P. Lynch, M. Leahy and D. N. Buckley, ECS Transactions 45 (26), 25 (2013) P. Blanc, C. Madic and J.P. Launay, Inorg. Chem. 21, 2923 (1982) M. Skyllas-Kazacos and M. Kazacos, J. Power Sources 196, 8822 (2011) Z. Tang, D.S. Aaron, A.B. Papandrew and T.A. Zawodzinski, Jr., ECS Trans. 41 (23), 1 (2012) L. Liu, J. Xi, Z. Wu, W. Zhang, H. Zhou, W. Li and X. Qiu, J. Appl. Electrochem. 42(12), 1025 (2012) D.N. Buckley, X. Gao, R.P. Lynch, M.J. Leahy, A. Bourkeand G. Flynn, European Patent EP 13195315 (Application Date: 2 December 2013)