The significance of charge and discharge current densities in the performance of vanadium redox flow battery

In this study, the effects of charge current density (CDChg), discharge current density (CDDchg), and the simultaneous change of both have been investigated on the performance parameters of the vanadium redox flow battery (VRFB). In addition, the crossover and ohmic polarization have been studied from a mechanism point of view to understand how the current density affects the characteristics of VRFB. Generally, increasing CDChg, CDDchg, or both of them simultaneously, leads to a decrease in the parameters of charge and discharge capacities, charge and discharge energies, as well as voltage efficiency. While the discharge current is constant, the energy efficiency does not change significantly with increasing CDChg. However, as CDDchg or both of CDChg and CDDchg increase, the energy efficiency decreases. In addition, increasing the current density generally increases the coulombic efficiency. Increasing CDChg has no major impact on the discharge voltage. Similarly, increasing CDDchg does not change the charge voltage significantly. From a mechanism view, low currents cause more crossover of vanadium ions because there are more opportunities for ions to diffuse across the membrane, which lowers the coulombic efficiency. The high charging current causes a reduction in the crossover of vanadium ions because there is not enough time for more diffusion of vanadium ions. On the other hand, because of the high current, electrons transfer more quickly while there are not enough vanadium species to react with all the electrons. This leads to a high ohmic polarization, which causes a reduction in the capacity and voltage efficiency.