Mechanistic insights of cycling stability of ferrocene catholytes in aqueous redox flow batteries

Abstract

Water soluble ferrocene (Fc) derivatives are promising cathode materials for aqueous organic redox flow batteries (AORFBs) towards scalable energy storage. However, their structure–performance relationship and degradation mechanism in aqueous electrolytes remain unclear. Herein, physicochemical and electrochemical properties, battery performance, and degradation mechanisms of three Fc catholytes, (ferrocenylmethyl)trimethylammonium chloride (C₁-FcNCl), (2-ferrocenyl-ethyl)trimethylammonium chloride (C₂-FcNCl), and (3-ferrocenyl-propyl)trimethylammonium chloride (C₃-FcNCl) in pH neutral aqueous electrolytes were systemically investigated. UV-Vis and gas chromatography (GC) studies confirmed the thermal and photolytic C_x-Cp⁻ ligand dissociation decomposition pathways of both discharged and charged states of C₁-FcNCl and C₂-FcNCl catholytes. In contrast, in the case of the C₃-FcNCl catholyte, the electron-donating 3-(trimethylammonium)propyl group strengthens the coordination between the C₃-Cp⁻ ligand and the Fe³⁺ or Fe²⁺ center and thus mitigates the ligand-dissociation degradation. Consistently, the Fc electrolytes displayed cycling stability in both half-cell and full-cell flow batteries in the order of C₁-FcNCl < C₂-FcNCl < C₃-FcNCl.