Spectroscopic Measurement of State of Charge in Vanadium Flow Batteries with an Analytical Model of V^IV-V^VAbsorbance

Abstract

The ultraviolet-visible spectra of catholytes for vanadium flow batteries (VFBs) were measured and analyzed for a range of VIV:VV ratios and vanadium concentrations. Using a model of V2O33+ in equilibrium with VO2+ and VO2+, the spectra were characterized in terms of an excess absorbance parameter p and the molar extinction coefficients ɛ4 and ɛ5 of VO2+ and VO2+, respectively. The results showed that p varies weakly with the vanadium concentration C and this variation was quantified relative to a reference concentration Cr by means of a concentration coefficient ϕr. Experimental data showed that plots of ϕr versus Cϕr and plots of 1/ϕr versus C are linear and, based on this linearity, ϕr was expressed as a simple function of C in terms of its reference concentration Cr and a single parameter M that is independent of the choice of Cr. Standard spectra of p at a concentration C0 = 1 mol dm−3 and of ɛ4 and ɛ5 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.