Mechanisms of capacity decay in iron‑chromium flow batteries: unraveling the role of electrolyte imbalance

The imbalance in iron‑chromium mixed electrolytes is a major cause of capacity decay in iron‑chromium flow batteries (ICFBs). This study proposes an electrolyte state model that examines the impact of molar and valent imbalances on the theoretical capacity of ICFBs using molar-based formulations. The model correlates the extent of valent imbalance with the capacity of an iron ion concentration cell (IICC), a specialized operational mode of ICFBs. In this mode, valent imbalance worsens primarily due to oxygen intrusion, which motivates us to quantify the O 2 intrusion rate by leveraging the capacity evolution of the IICC. During normal operation, electrolyte imbalance also arises from the parasitic hydrogen evolution reaction (HER) and the crossover of energy-bearing species. These factors are investigated by analyzing gas-liquid flow data from ultrasonic bubble sensors and species concentration data from imbalanced electrolyte samples, obtained via ICP-OES. The combination of these methodologies enables the quantitative decoupling of key contributors to capacity decay in ICFBs and subsequently facilitates the formulation of a contributor decomposition approach for monitoring the electrolyte's state of health (SOH). Additionally, a second SOH monitoring method is proposed via the integration of signals from in situ Ag/AgCl reference electrodes. Grounded in the electrolyte's compositional characteristics, this approach is validated against the former one. These dual monitoring strategies demonstrate that the addition of either bismuth or indium mitigates capacity decay induced by the parasitic HER. With the introduction of these catalysts, O 2 intrusion emerges as a critical factor that induces capacity decay in ICFBs.