Impact of discharge cut-off voltage on alizarin-based redox flow batteries

To compete in the large-scale energy storage market, aqueous organic redox flow batteries (AORFBs) must retain 80% of their capacity over 10–20 years, requiring exceptional long-term stability (>99.995% capacity retention per day, assuming one cycle per day). While efforts have focused on enhancing the chemical stability of redox-active compounds, less attention has been given to optimizing cycling conditions, particularly charge and discharge voltage limits during galvanostatic cycling. This study investigates how operational parameters affect capacity fade using a system composed of alizarin as the negolyte and ferrocyanide as the posolyte. This system presents a paradox: lower discharge cut-off voltages improve stability, yet cyclic voltammetry reveals a degradation product under the same conditions. A design of experiments approach was used to evaluate capacity decrease across various conditions. The results highlight the dominant influence of discharge cut-off voltage on stability, with a lesser but notable effect from charge cut-off voltage. In contrast, the electrolyte flow rate had no significant impact within the tested range. In situ monitoring with reference electrodes helped identify the mechanistic origin of the improved stability under deep discharge, offering insights for the rational design of more durable AORFB systems. • Deep discharge led to degradation but paradoxically enhanced battery stability. • A design of experiments was applied for the first time to AORFBs. • The impact of cell voltage limits on capacity retention was clearly demonstrated. • Potential monitoring revealed alizarin oxidation during deep discharge. • Results provided an explanation for alizarin paradoxical behaviour.