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)Cl4 ), 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)Cl4 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.
Keywords: aqueous redox flow batteries; charge delocalization; conjugation; pyridinium.
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