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Summary In this study, the performance of alkaline aqueous organic redox flow battery (AORFB) using an isomeric mixture of 1,2‐naphthoquinone‐4‐sulfonic acid sodium salt and 2‐hydroxy‐1,4‐naphthoquinone (NQSO) and potassium ferrocyanide (FeCN) as active materials dissolved in potassium hydroxide (KOH) is enhanced with replacing commercial Nafion membrane with new crosslinked sulfonated polyphenylsulfone (crosslinked‐SES) membrane. The optimal ratio of sulfonate and sulfone groups of the crosslinked‐SES membrane is determined to balance between mechanical strength and ionic conductivity by the handling of curing time. With that, ionic conductivity and mechanical strength depending on the amount of sulfonate and sulfone are well balanced. The thickness of membrane is also optimized. Such optimized crosslinked‐SES (crosslinked‐SES24) membrane induces better mechanical stability and ionic conductivity than Nafion 211 (Young's modulus and in‐plane conductivity of the potassium form of crosslinked‐SES24 and Nafion 211 are 1108 and 214 MPa, and 20‐23 and 7.8 mS·cm−1) with lower cost. In tests of AORFB using crosslinked‐SES24 that is operated at a high current density of 200 mA·cm−2 over 1000 cycles, the AORFB shows stable coulombic, voltage and energy efficiencies of 99.7, 65.3, and 65.0%, while its capacity loss rate is as low as 0.01% per cycle.

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