Transport phenomena associated with capacity loss of all-vanadium redox flow battery

ABSTRACT This study investigates transport of different species through the Nafion® 115 membrane in an all vanadium redox flow battery to understand transport phenomena associate with capacity loss. We consider several driving forces related to transport of vanadium ions, proton, and water molecules and examine the variations in ion concentration during charge-discharge, and long-term operation. First, variations in ion concentration are analyzed during the 3rd and the 300th charge-discharge cycles to compare the ion transport process at between early stage and late stage. The capacity loss is closely related to the self-discharge reaction caused by diffusion through the membrane, and proton transport is another important factor of capacity loss since the imbalance in proton concentration can accelerate water transport through the membrane. Furthermore, variations in solution volume and ion concentration are examined during long-term charge-discharge cycles. Since ions accumulate on the positive electrode due to self-discharge reaction and repeated electrochemical reaction, water molecules transport to the positive electrode side. Finally, the relationship between the changes in solution volume and capacity loss is examined, and an empirical equation is suggested for the prediction of capacity loss from changes in the solution volume during long-term operation. Through the results, we can predict the lifetime of VFB system by simple measurement.