LAUSR

Abstract Aqueous organic redox flow battery (AORFB) using viologens suffers from poor cycling life especially at high concentrations due to the dimerization of viologen radical cations. Here, we report a molecular spectator approach, exploiting α-cyclodextrin (α-CD) with suitable cavity size as the spectator, to weaken the intermolecular interaction of viologen radicals and suppress dimerization in AORFBs. UV–vis spectroscopy reveals that α-CD effectively suppresses the dimerization of viologen radicals. Applying α-CD as the spectator, a symmetric static cell (0.1 M EV-0.01 M α-CD) demonstrated a significantly lower capacity decay rate of 0.075% per day (20 days, 100% capacity utilization) compared to the sole EV electrolyte (0.31% per day). Asymmetric flow cells (1.0 M EV-0.1 M α-CD) coupling with a ferrocene derivative demonstrated stable cycling over 500 cycles during 26 days at 80% capacity utilization. This work provides a facile and effective strategy to stabilize the viologen radicals for AORFBs applications.