Hydrophobic Solvation-Driven Stabilization of the Fluorenone Radical for the Anolyte of All-Organic Flow Batteries under Benign pH Conditions

Fluorenone (FL) is a promising anolyte candidate for aqueous organic redox flow batteries (AORFBs), but its reduction is accompanied by protonation-induced degradation. Here, we demonstrated that the radical anion of fluorenone (FL–·) can be stabilized without alkalization by forming a hydrophobic solvation environment using highly concentrated 1-butyl-1-methylpyrrolidinium chloride (BMPyrCl). A water-soluble FL derivative enables systematic investigation of redox behavior across electrolyte conditions. Electrochemical and spectroscopic measurements and molecular dynamics simulations revealed that increasing BMPyrCl concentration induces a water-in-molecular-salt state, which expels water from the solvation shell of FL–· and suppresses its protonation, while the concentrated LiTFSI-based water-in-salt electrolyte cannot make the FL–· environment hydrophobic. When paired with a TEMPO-based catholyte, the resulting AORFB delivered stable cycling performance, with a significantly reduced capacity fade, and the cell achieved a voltage of 1.64 V, representing a notably high value for AORFBs employing organic electrolytes as both anolyte and catholyte. These results highlight that hydrophobic solvation design is a critical enabler of high-voltage, stable aqueous organic redox electrolyte-based energy storage systems.