Functioning Water-Insoluble Ferrocenes for Aqueous Organic Flow Battery via Host-Guest Inclusion

Ferrocene (Fc) is one of the very limited organic catholyte options for aqueous organic flow batteries (AOFBs), a potential electrochemical energy storage solution to the intermittency of renewable electricity. Commercially available Fc derivatives are barely soluble in water, while existing methods for making water-soluble Fc derivatives by appending hydrophilic or charged moieties are tedious and time-consuming, with low yields. Here, a strategy was developed based on host-guest inclusion to acquire water-soluble Fc-based catholytes by simply mixing Fc derivatives with β-cyclodextrins (β-CDs) in water. Factors determining the stability and the electrochemical behavior of the inclusion complexes were identified. When adopted in a neutral pH AOFB, the origin of capacity loss was identified to be a chemical degradation caused by the nucleophilic attack on the center Fe^III atom of the oxidized Fc derivatives. By limiting the state of charge, a low capacity fade rate of 0.0073 % h^-1 (or 0.0020 % per cycle) was achieved. The proposed strategy may be extended to other families of electrochemically active water-insoluble organic compounds, bringing more electrolyte options for practical AOFB applications.