Readily Accessible M-Ferrocenyl-Phenyl Sulfonate as Novel Cathodic Electrolyte for Aqueous Organic Redox Flow Batteries

Ferrocene derivatives are amongst the most promising electroactive organic electrolytes. The bottleneck problems of their application in aqueous redox flow batteries are their poor solubility and lower potential as well as the complexity of the modification methods to solve these problems. In this study, a benzenesulfonic acid group is easily introduced into the ferrocene structure by a mature diazotization reaction, and the synthesized sodium m-phenylferrocene sulfonate BASFc is used as the novel cathodic electroactive electrolyte for AORFB. The hydrophilicity and the electron-absorbing effect of the introduced benzenesulfonic group can effectively improve the water solubility and redox potential of ferrocene. Moreover, the introduction of phenyl extends the conjugated structure of ferrocene and increases its structural dimension, which may be conducive to reducing its membrane permeability and improving the structural stability to some extent. The physical structure and the electrochemical properties of BASFc are studied systematically; the feasibility of its application as a cathodic electrolyte in AORFBs is verified by assembling the half-cell and full-cell. The results verify the good electrochemical reaction kinetics of BASFc in acid electrolyte and the corresponding AORFB shows satisfactory efficiency and stability.