Development of an UV-Visible spectroelectrochemical method to correlate State of Charge (SOC) and Open Circuit Voltage (OCV) in a Vanadium-Vanadium Redox Flow Battery

Vanadium-Vanadium Redox Flow Battery (VRFB) is a type of Redox Flow Battery (RFB) which uses the redox couples VO2+/VO2+ (cathode side) and V3+/V2+ (anode side) in a concentrated solution of H2SO4, with a cell voltage of 1.26 V at room temperature [1-2]. Since vanadium ions show different colours depending on their oxidation state, the UV-Vis spectroscopy is a useful technique to study all the systems which involve them. Due to the high concentration both of the vanadium ions and of sulphuric acid, the number of the species and their equilibriums vary with the operative conditions and the cell’s State of Charge. This produces deviations from linearity in the absorbance vs. concentration diagram and as consequence it is very difficult choose a specific wavelength which has a linear trend with the concentration of defined species. In this context, a model to correlate the State of Charge (SOC) and the Open Circuit Voltage (OCV) by employing both the electrochemical and the spectral techniques have been developed. In a typical experiment a certain amount of charge has been applied to the cell, then the current flow is interrupted to register both the OCV and the spectrum in equilibrium conditions. The sequence is repeated for the required number of times to charge/discharge the cell completely. From the obtained results, it is possible to assume that a more accurate correlation among OCV, Absorbance and SOC should be achievable, being known pure species spectra, applying methods of Pattern Recognition to the entire spectrum [3]. References: 1) Z. Yang, J. Zhang, M. C. W. Kintner-Meyer, X. Lu, D. Choi, J. P. Lemmon, J. Liu, Chem. Rev., 2011, 111, 3577-3613. 2) S. Hamelet, T. Tzedakis, J.-B. Leriche, S. Sailler, D. Larcher, P.-L. Taberna, P. Simon, J.-M. Tarascon, Journal of The Electrochemical Society, 2012, 159 (8), A1360-A1367. 3) A. Herrero, S. Zamponi, R. Marassi, P. Conti, M.C. Ortiz, L.A. Sarabia, Chemometrics and Intelligent Laboratory Systems, 2002, 61, 63-74.