Benzidine Derivatives: A Class of High Redox Potential Molecules for Aqueous Organic Flow Batteries.

The development of water-soluble redox-active molecules with high potentials is one of the effective ways to enhance the energy density of aqueous organic flow batteries (AOFBs). Herein, a series of promising N-substituted benzidine analogues as water-soluble catholyte candidates with controllable redox potentials (0.78 ~1.01 V vs. standard hydrogen electrode (SHE)) were obtained by the molecular engineering of aqueous irreversible benzidines. Theoretical calculations reveal that the redox potentials of these benzidine derivatives in acidic solution are determined by their electronic structure and alkalinity. Among these benzidine derivatives, N,N,N',N'-Tetraethylbenzidine (TEB) shows both high redox potential (0.82 V vs. SHE) and good solubility (1.1 M). Pairing with H4[Si(W3O10)4] anolyte, the cell displayed discharge capacity retention of 99.4% per cycle and a high coulombic efficiency (CE) of ~100% over 1200 cycles. The stable discharge capacity of 41.8 Ah L-1 was achieved at the 1.0 M TEB catholyte with a CE of 97.2% and energy efficiency (EE) of 91.2%, demonstrating that N-substituted benzidines could be promising for AOFBs.