A multi-parameter analysis of iron/iron redox flow batteries: effects of operating conditions on electrochemical performance

Abstract

Iron/iron redox flow batteries (IRFBs) are emerging as a cost-effective alternative to traditional energy storage systems. This study investigates the impact of key operational characteristics, specifically examining how various parameters influence efficiency, stability, and capacity retention. IRFB systems with a volume of 60 mL per tank (20.25 Ah L⁻¹) demonstrated superior capacity utilization, achieving a coulombic efficiency (CE) of up to 95% and an energy efficiency (EE) of 61% over 25 charge/discharge cycles. In contrast, systems with lower capacity utilization in larger electrolyte volumes (5.67 Ah L⁻¹) required more charge/discharge cycles to reach the optimal pH-induced kinetic benefits due to increased proton content. Extended charging durations of up to 8 hours facilitated complete redox conversion, enhancing CE, EE, and voltage efficiency (VE). Brief rest intervals of 5 to 10 minutes supported stable discharge capacity retention and energy efficiency throughout the cycles, while more extended rest periods (e.g., 60 minutes) were associated with diminished performance, possibly due to ionic resistance buildup in the membrane or system imbalances occurring during extended idle times. Charge cutoff voltages between 1.6 and 1.65 V provided an optimal compromise between suppressing side reactions, enhancing capacity retention, and improving efficiency. The constant current–constant voltage (CCCV) method yielded better voltage efficiencies than the constant current (CC) approach when sustaining long-term cycling. Additionally, integrating a recombination cell minimized hydrogen-related losses, enhancing operational stability. These findings provide valuable insights for optimizing the operation of IRFBs in energy storage applications.