Capacity fade prediction for vanadium redox flow batteries during long-term operations

In this paper, a dynamic prediction model for electrolyte capacity fade in vanadium redox flow batteries (VRFBs) is proposed. The capacity fade characteristics of VRFBs were analyzed quantitatively from both microscopic ions crossover and macroscopic electrolyte volumetric change perspectives. The dynamic behavior of vanadium ions (V2+, V3+, VO2+, and VO+ 2), protons, and water molecules in VRFB half-cells and electrolyte tanks has been thoroughly explored. The parameters related to the capacity fade were obtained from a self-discharge test and formation charge processes. The accuracy of the proposed model was experimentally validated using a unit cell (25 cm2) in a long-term operation. The results reveal that after 200 continuous charge/discharge cycles (duration of 739,257 s) with a current density of 80 mA cm2, the model can accurately predict changes in electrolyte volume and capacity fade with errors of 0.632 mL and 0.0295 Ah, respectively. The electrolyte capacity fade under various current densities for long-term operations has also been predicted by the model and experimentally validated. Under all current densities applied to the VRFB, the discrepancies between simulation and experimental results for electrolyte volume change and capacity fade were <8.46% and 4.99%, respectively. The proposed model significantly improves the accessibility and reliability of accurate capacity fade prediction for VRFBs, enhancing the competitiveness of VRFBs in energy storage applications.