Towards optical monitoring of vanadium redox flow batteries (VRFBs): An investigation of the underlying spectroscopy

The UV-visible spectra of VIII-VII electrolytes in 3 mol dm?3 H2SO4 showed a linear relationship of absorbance to mole fraction but those of VIV-VV electrolytes did not. At all wavelengths, VIV-VV mixtures showed excess absorbance over that expected for a linear combination of the components. Plots of excess absorbance Aex against mole fraction f of VIV were found to be symmetrical parabolas that reach a peak at a mole fraction f = 0.5. Excess absorbance is attributed to a strongly absorbing 1:1 mixed-valence complex, V2O3 3+, in equilibrium with VO2+ and VO2 +. Based on this, an expression is derived that relates Aex to f and from this it is shown that when KcC < 1 (where Kc is the equilibrium constant and C is the total vanadium concentration), Aex should vary linearly with f(1 - f). Experimentally, plots of Aex versus f(1 - f) showed excellent linearity through the origin at all wavelengths, thus confirming the model. An excess molar extinction coefficient ?ex and an alternative excess absorbance parameter p are defined. Values of p and approximate values of Kc and ?ex were estimated; the spectrum of V2O3 3+ is plotted as its approximate molar extinction coefficient.