Characterisation of the ferrocene/ferrocenium ion redox couple as a model chemistry for non-aqueous redox flow battery research

The simple ferrocene/ferrocenium ion (Fc/FcBF(4)) redox couple was examined as a model chemistry for non-aqueous redox flow battery research. Its properties were fully characterised using voltammetry, flow-cell battery cycling, and UV-vis spectroscopy to validate flow-cell performance. Fc demonstrates facile kinetics and high stability of its oxidation states, making the Fc/FcBF(4) redox couple a useful low-cost model chemistry, despite its limited 0.16 M solubility in acetonitrile. By use of 'single redox couple cycling', in which only the Fc/FcBF(4) redox couple is battery cycled, the high capacity retention of Fc at 10 mM concentration was demonstrated; 80% capacity retention after 200 cycles (7.8 days). The mechanism for the capacity loss was investigated and diagnosed to occur via FcBF(4) decomposition in the electrolyte, which proceeds irrespective of battery cycling.