Impact of Magnesium Chloride Additives on All‐Iron Redox Flow Battery Performance

ABSTRACT Addressing the urgent need for large‐scale energy storage from grid‐connected intermittent renewable sources like wind and solar power, this study focuses on reducing the hydrogen evolution reaction (HER) at the anode of an all‐iron redox flow battery. We demonstrate that adding magnesium chloride (MgCl 2 ) creates a synergistic inhibitory mechanism under acidic chloride aqueous electrolyte solution conditions (pH −1.0 to 0.0). Electrochemical tests show that Mg 2+ influences the kinetics of the Fe 2+ /Fe redox couple, improving reaction reversibility and slowing the charge transfer kinetics of the HER. Microscopic analysis confirms that MgCl 2 enhances iron deposition, resulting in deposits with a preferred (211) crystallographic orientation. The optimized battery system showed exceptional stability over 10,000 charge/discharge cycles, maintaining an average coulombic efficiency of 99% during the last 7000 cycles. This work offers an innovative approach for designing high‐stability flow batteries by synergistically controlling electrolyte composition, electrode interfaces, and deposition behavior.