A comprehensive experimental and modelling approach for the evaluation of cross-over fluxes in Vanadium Redox Flow Battery

Vanadium cross-over is a critical issue in Vanadium Redox Flow Battery consisting in a complex interplay of different mechanisms of which a complete comprehension has not been reached yet. Due to the complexity of the involved phenomena, several models have been developed in literature to investigate vanadium cross-over. However, the conventional approaches for model calibration present a limited set of experiments for the validation preventing a complete understanding of cross-over phenomena. In this work a new and comprehensive approach is proposed. It is based on charge-discharge cycles with fixed exchanged capacity, able to isolate the capacity loss induced by cross-over fluxes, and on the measure of the self-discharge of the single electrolyte solutions by exploiting through-plate reference electrodes. Moreover, a 1D physically-based model of the operation of the battery is developed and calibrated on the data of electrolyte imbalance during charge-discharge cycles at three different current densities to obtain model parameters able to accurately describe the involved physics in different operating conditions. The model is then exploited to investigate the main vanadium transport mechanisms through the membrane and to evaluate the influence of the current density on the vanadium cross-over fluxes, net vanadium transport and self-discharge rate of the electrolyte.