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

Abstract

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 FeIII 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.