Interest in redox flow batteries (RFBs) has increased because of their excellent performance in energy storage and a potential for addressing the solution to the issues of discontinuity and intermittency of renewable energy sources such as wind or solar. Aqueous RFBs (ARFBs) draw much more attention for the reason that, in general, ARFB devices possess advantages such as lower electrolyte resistance, lower costs, improved safety, and improved environmental friendliness, when compared to nonaqueous RFBs. More recent researches focus on the study of ferrocene-based ARFBs because some of the ferrocene derivatives possess stable and reversible redox reactions and they are highly soluble in water.
Here, we present two new ferrocene compounds with, respectively, propyl and butyl zwitterionic side chains: 3-((ferrocenyl)methyldimethylammonio)-1-propanesulfonate (Fc3) and 4-((ferrocen-yl)methyldimethylammonio)-1-butanesulfonate (Fc4). These compounds are highly soluble in water (0.66 M for Fc3 and 2.01 M for Fc4). When paired with anthraquinone-2,7-disulfonate (AQDS) as the anolyte, these zwitterionic ferrocenes exhibit excellent performance under neutral aqueous conditions. The high concentration Fc4/AQDS RFB has voltage and energy efficiencies of about 88%, while the Fc3/AQDS RFB is about 84% when the cycling become stable. We also have an important observation in stability between Fc3 and Fc4, in which Fc4 shows higher stability than Fc3. The capacity drop for Fc4 is about ∼1% per day. Density functional theory calculations revealed striking differences in the conformational properties between Fc3 and Fc4, with Fc4 retaining a linear structure of the side chain in solution, while Fc3 favored both linear and curved geometries.

Sanjay Gaire

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