All-Soluble All-Iron Aqueous Redox Flow Batteries: Towards Sustainable Energy Storage

All-iron aqueous redox flow batteries (AI-ARFBs) are attractive for large-scale energy storage due to their low cost, abundant raw materials, and the safety and environmental friendliness of using water as the solvent. However, traditional deposition-type AI-ARFBs suffer from limitations in charge and discharge depth due to the coupling of energy and power. In contrast, all-soluble AI-ARFBs (ASAI-ARFBs), which feature fully soluble iron species throughout charge and discharge cycles, achieve the decoupling of energy and power, thus overcoming the limitations and improving operational scalability. Despite their benefits, challenges remain in redox species solubility, electrolyte stability, electrode reactivity, membrane selectivity and capacity decay mechanism. This review provides a comprehensive overview of current research on ASAI-ARFBs, focusing on the needs for robust electrolytes, advanced electrode structures, durable membrane materials and in-situ characterisation techniques to address these challenges and enhance their performance. Additionally, this review highlights the importance of integrating renewable energy technologies with ASAI-ARFBs to boost their commercialisation potential.