Benzidine Derivatives as Electroactive Materials for Aqueous Organic Redox Flow Batteries

This paper presents a theoretical and experimental evaluation of benzidine derivatives as electroactive molecules for organic redox flow batteries. These redox indicators are novel electroactive materials that can perform multielectron transfers in aqueous media. We performed the synthesis, electrochemical characterization, and theoretical study of the dimer of sodium 4-diphenylamine sulfonate, a benzidine derivative with high water solubility properties. The Pourbaix diagram of the dimer shows a bielectronic process at highly acidic pH values (≤ 0.9) and two single-electron transfers in a pH range from 0 to 9. The dimer was prepared in situ and tested on a neutral electrochemical flow cell as a stability diagnostic. To improve cell performance, we calculate and calibrate, with experimental data, the Pourbaix diagrams of benzidine derivatives using different substitution patterns and functional groups. A screening process allowed the selection of those derivatives with a bielectronic process in the entire pH window or at acidic/neutral pH values. Given the redox potential difference, they can be potential catholytes or anolytes in a flow cell. The couples formed with the final candidates can generate a theoretical cell voltage of 0.60 V at pH 0 and up to 0.68 V at pH 7. These candidate molecules could be viable as electroactive materials for a full-organic redox flow battery.