Towards Optical Monitoring of Vanadium Redox Flow Batteries (VRFBs): An Investigation of the Underlying Spectroscopy

The UV-visible spectra of V III -V II electrolytes in 3 mol dm −3 H 2 SO 4 showed a linear relationship of absorbance to mole fraction but those of V IV -V V electrolytes did not. At all wavelengths, V IV -V V mixtures showed excess absorbance over that expected for a linear combination of the components. Plots of excess absorbance A ex against mole fraction f of V IV 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, V 2 O 3 3+ , in equilibrium with VO 2+ and VO 2 + . Based on this, an expression is derived that relates A ex to f and from this it is shown that when K c C < 1 (where K c is the equilibrium constant and C is the total vanadium concentration), A ex should vary linearly with f (1 – f ). Experimentally, plots of A ex versus f (1 – f ) showed excellent linearity through the origin at all wavelengths, thus confirming the model. An excess molar extinction coefficient ɛ ex and an alternative excess absorbance parameter p are defined. Values of p and approximate values of K c and ɛ ex were estimated; the spectrum of V 2 O 3 3+ is plotted as its approximate molar extinction coefficient.