Reliability studies of vanadium redox flow batteries: upper limit voltage effect

All-vanadium redox flow batteries (VRFBs) show promise as a long-duration energy storage (LDES) technology in grid applications. However, the continual performance fading over time poses a significant obstacle for VRFBs. This study systematically investigates the impact of increased upper limit voltage (1.6 V, 1.7 V, and 1.8 V) in the reliability and degradation of a scaled VRFB cell (49 cm²) over long-term testing (500⁺ cycles). The findings indicate that higher upper voltages significantly decrease capacity and voltage efficiencies. Although electrolyte remixing can restore the majority of the capacity, it only partially recovers voltage efficiency at 1.7 V and 1.8 V, suggesting substantial cell degradation. Analysis reveals that the overpotential increase induced degradation is mainly contributed by the anode during charging and the cathode during discharging. Increased upper voltage amplifies degradation, with the anode being more affected. As confirmed by electrochemical impedance spectroscopy (EIS) and polarization curves, elevated voltages lead to significant resistance increases, driven by charge transfer resistance (mostly from the anode). Moreover, the morphological, surficial, and electrochemical characterization results of cycled electrodes suggest that the degree and mode of degradation were contingent upon the cutoff voltage. For instance, the cathode experienced severe surface degradation at the maximal upper voltage of 1.8 V. This work highlights the importance of optimizing voltage limits to improve the lifetime of VRFBs and offers valuable insights into the development of predictive models through using accelerated stressor lifetime testing (ASLT) protocols for VRFBs.