Supramolecular Viologen–Cyclodextrin Electrolytes for Aqueous Organic Redox Flow Batteries

Along with a primary modification of redox active materials, an additional introduction of secondary noncovalent interactions can synergistically enhance bulk properties of electrolytes for redox flow batteries. Herein, we highlight the host–guest complex formation between tailored viologens and highly water soluble (2-hydroxypropyl)-β-cyclodextrin as a key electrolyte interaction to modulate relevant electrochemical properties of aqueous redox flow batteries (AORFBs). The cyclodextrin-modified AORFB anolytes demonstrated a complex interrelation of molecular structure and inherent binding activity as well as bulk electrochemical stability of the anolyte. The screening of different combinations of viologen substituents in the presence of cyclodextrin enabled an electrochemically stable AORFB performance for more than 500 cycles with a temporary capacity fade rate of 0.26%/day at high energy (>70%) and Coulombic (>99.7%) efficiencies. A selective interplay of supporting electrolytes and engineered redox active materials is a promising strategy for enhanced energy characteristics of AORFB electrolytes.