Ferrocene to functionalized ferrocene: a versatile redox-active electrolyte for high-performance aqueous and non-aqueous organic redox flow batteries

There is a significant need for large-scale energy storage systems for the current state of grid technology that can provide high reliability, minimal maintenance, environment-friendly, and high safety characteristics. In this regard, lithium-ion battery technology solved many challenges; however, the uneven abundance of lithium across the world triggers doubts about its practicability as a long-term large-scale energy storage solution. In the quest for new technology, redox flow batteries (RFBs) offer a great solution in terms of energy efficiency, specific capacity, and peak power density. To date, much focus has been devoted to the improvement of vanadium RFBs, whereas organic redox flow batteries (ORFBs) are the breakthrough from the recently developed RFBs. The ORFBs contain organic molecules as their electrolytes, which are dissolved in either aqueous or nonaqueous solvents. During the search for high-potential electrolyte molecules, ferrocene (Fc) emerged as the most promising organometallic molecule due to its high economic value, wide availability, less toxicity, and low environmental impact. Hence, extensive research and development efforts have been employed on optimizing the structure of the Fc-molecule to enhance ferrocene's solubility in various solvents. The purpose of this review article is to provide a concise and thorough overview of the evolution of various synthetic approaches and electrochemical measurements on ferrocene-based aqueous and non-aqueous RFBs. In the final section of this article, we have delved into the future perspective and outlook of functionalized ferrocene-based RFBs.