Factors Affecting Spectroscopic State-of-Charge Measurements of Positive and Negative Electrolytes in Vanadium Redox Flow Batteries

Abstract

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, V II and V III . In contrast we showed, for a range of mole percentages of V V 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 V 2 O 3 3+ in equilibrium with VO 2+ and VO 2 + . We have defined a parameter p which directly relates A ex to the vanadium concentration C and the mole percentage of V V (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 V 2 O 3 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 .