Aqueous redox flow batteries (ARFB) represent a promising grid-scale energy storage technology, which can improve the utilization efficiency of renewable energy and resolve the contradiction between fast development of intermittent renewable energy and grid security. To overcome the drawbacks of traditional all-vanadium and Zn−Br2 RFBs, water-soluble organic redox molecules were developed to replace the inorganic materials for ARFB applications. Ferrocene (Fc) and its derivatives are widely used in electrochemical studies and 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. Paired with (NPr)2VCl4 anolyte, a 0.50 M C3-FcNCl/(NPr)2VCl4 AORFB delivered high battery performance, including perfect rate performance at current densities from 10 mA/cm2 to 50 mA/cm2, up to 91% EE and VE at 10 mA/cm2, and 99.9979% per cycle or 99.927% per day capacity retention in 500 charge/discharge cycles. Flow battery studies showed that the cycling stability of ferrocene catholytes is significantly improved with the enhancement of +I inductive effect of pendant functional groups. UV-Vis and gas chromatography (GC) studies confirmed the thermal- and photo-induced Cp- ligand dissociation decomposition pathways of the C1-FcNCl and C2-FcNCl catholytes. In contrast, in the cases of [C3-FcN]2+ species, +I inductive effect of 3-(trimethylammonium)propyl groups strengthens the coordination between FeII/FeIII center and the Cp- ligand which hinders the ligand-dissociation decomposition.

Mechanistic insights of ligand effects on the cycling stability of ferrocene catholytes in aqueous redox flow batteries

ACS Spring 2022

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