Factors affecting spectroscopic state-of-charge measurements of positive and negative electrolytes in vanadium redox flow batteries

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 V₂O₃³⁺ 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 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 V₂O₃³⁺ equilibrium constant K_c with C is small. We estimated approximate values for K_c and for the excess molar extinction coefficient ε_ex.