Chemical modulation of redox polymers for high-performance aqueous redox flow batteries

Poly(isobutylene-alt-maleic anhydride) (PIMA) was chemically engineered into a redox-active polymer electrolyte via an amidation reaction using 3-picolylamine, resulting in PPIMA. The subsequent quaternization using acetic anhydride, 4-(chloromethyl)benzonitrile, and 3-propane sultone generated modified versions, including PAPIMA, PCNPIMA, and PSPIMA, respectively. These modifications further modulate the polarity, electron density, and ion-transport pathways in aqueous redox-flow batteries (RFBs). FTIR,¹H NMR, and TGA verified the chemical structures of the transformed polymers. The quantitative integration yielded high substitution levels such as PPIMA ≈100%, PSPIMA ≈96%, PCNPIMA ≈83%, and PAPIMA ≈55% per repeat unit. Electrochemical behavior was assessed through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in 1.0 M KOH, and the charging-discharging profiles of these polymeric electrolytes (anolytes) were validated in a 5.06 cm²laboratory RF cell containing a separator of Nafion® N424, electrodes of activated graphite felts, and K₄[Fe(CN)₆] as a catholyte. Under galvanostatic testing, PPIMA delivered the highest discharge capacity, 119 mAh g⁻¹, while PSPIMA and PCNPIMA each reached ∼102 mAh g^−1,and PAPIMA achieved 96 mAh g⁻¹. PAPIMA and PPIMA exhibited high coulombic efficiencies, approaching 100% at low current density and remaining stable up to 80 mA. All polymers exhibited robust cycling over 500 cycles at 100 mA. The results trend maps directly onto substituent electronics and sterics: pyridyl groups enhance delocalization and site accessibility (PPIMA), sulfonates boost hydrophilicity but add steric drag (PSPIMA), nitriles withdraw electron density (PCNPIMA), and acetylation moderates redox activity (PAPIMA). Moreover, to gain deeper insight into the electronic and steric effects of substituent modifications on the electrochemical behavior, we performed density functional theory (DFT) calculations using Materials Studio DMol³. These results establish rational backbone/pendant engineering of PIMA as a viable route to durable, high-efficiency aqueous polymer electrolytes for RFBs.