Identification of crossover flux in VRFB cells during battery cycling

When developing dynamical models of vanadium redox flow batteries (VRFBs), it is important to find a trade-off between simplicity, convenience, and model accuracy. Crossover can contribute significantly to the dynamics of such batteries, especially when the number of charge-discharge cycles is large. In this work, we propose a crossover flux modeling approach that takes into account the membrane properties associated with its preparation and operation, when detailed information about its physical characteristics is not available. The proposed approach showed good agreement with the experimental data over 25 cycles with an average error of less than 2 %. A detailed analysis of the contribution of different crossover components revealed that the main influence on the observed capacity drop is related to the diffusion component, which dominates over migration and convection across all cycles, presenting more than 60 % of the losses. In addition, the results showed that migration and convection "mitigate" the influence of diffusion during long-term cycling, thereby reducing the capacity drop. As a result, the proposed approach can be used to analyze the effects of different types of crossover on the capacity decay, which provides researchers and engineers with important information for improving the design and operating conditions of VRFBs to ensure their reliable and fail-safe operation under long-term cycling.