Local potential measurement through reference electrodes in vanadium redox flow batteries: Evaluation of overpotentials and electrolytes imbalance

Vanadium redox flow battery performance is actually hindered by increased overpotentials at high current, due to poor electrochemical activity of the most commonly used carbon electrodes and to electrolyte distribution, implying local mass transport limiting conditions. Moreover, vanadium cross-contamination leads to coulombic efficiency reduction and uncontrolled electrolytes imbalance. This work presents the application of through-plate reference electrodes at inlet and outlet of both positive and negative electrodes. The utility of the electrodes potential measurement is firstly demonstrated in the identification of a relation between electrolytes potential and the corresponding state of charge. Subsequently, local overpotentials and impedance spectra at both electrodes are evaluated, evidencing that the negative electrode is kinetically dominated and presents high overpotential even at low current, while the positive exhibits mass transport effects at high current, especially at cell outlet. Finally, during cycling operation with fixed capacity reference electrode measurements permit to monitor electrolytes imbalance induced by cross-contamination, that mainly affects negative electrolyte. Moreover, additional insights into electrodes potential loss during cycling are provided.