Progress and prospects of pH-neutral aqueous organic redox flow batteries: Electrolytes and membranes

Aqueous organic redox flow batteries (AORFBs), which exploit the reversible electrochemical reactions of water-soluble organic electrolytes to store electricity, have emerged as an efficient electrochemical energy storage technology for the grid-scale integration of renewable electricity. pH-neutral AORFBs that feature high safety, low corrosivity, and environmental benignity are particularly promising, and their battery performance is significantly impacted by redox-active molecules and ion-exchange membranes (IEMs). Here, representative anolytes and catholytes engineered for use in pH-neutral AORFBs are outlined and summarized, as well as their side reactions that cause irreversible battery capacity fading. In addition, the recent achievements of IEMs for pH-neutral AORFBs are discussed, with a focus on the construction and tuning of ion transport channels. Finally, the critical challenges and potential research opportunities for developing practically relevant pH-neutral AORFBs are presented.