Phenylene-Bridged Bispyridinium with High Capacity and Stability for Aqueous Flow Batteries

A rotating phenyl ring is introduced between the two pyridinium rings, namely, 1,1 '-bis[3-(trimethylamonium)propyl]-4,4 '-(1,4-phenylene)bispyridinium tetrachloride ((APBPy)Cl-4), to form a switchable conjugation. In this design, the conjugation is switched off in the oxidized state and the two pyridinium rings behave independently during the redox process, yielding a concomitant transfer of two electrons at the same potential and, thus, simplifying the battery management. The conjugation is switched on in the reduced state and the charge can be effectively delocalized, lowering the Lewis basicity and improving its chemical stability. By pairing 0.50 m (APBPy)Cl-4 with a 2,2,6,6-tetramethylpiperidin-1-yl oxyl derivative as the positive electrolyte, a flow battery delivers a high standard cell voltage of 1.730 V and a high specific capacity of 20.0 Ah L-1. The battery also shows an exceptionally high energy efficiency of 80.8% and a superior cycling stability at 80 mA cm(-2). This strategy proves itself a great success in engineering viologen as a two-electron storage mediator with high capacity and stability.

Automated summary

The introduction of a rotating phenyl ring to create a switchable conjugation system has proven successful in enhancing battery stability and capacity. By switching the conjugation off in the oxidized state and on in the reduced state, the battery demonstrates high energy efficiency and cycling stability.