Redox flow batteries (RFBs) are promising for long-duration grid-scale sustainable energy storage. The ion-exchange membrane is a key component that determines energy efficiency and cycling stability. However, it remains challenging to develop membranes with high ionic conductivity and high selectivity toward redox-active electrolytes. We report the development of ion-conductive polymer membranes with record-breaking energy efficiency. By incorporating triptycene into poly(ether-ether-ketone) and controlled sulfonation, the resulting intrinsically microporous polymer membranes form highly interconnected water channels that facilitate transport of charge-balancing ions, particularly hydroxide anions. These microporous membranes showed high ionic conductivity without compromising the selectivity toward redox-active species. The membranes enabled excellent performance in alkaline aqueous organic and zinc-iron flow batteries, demonstrating long-term stability, high power density, and an operational current density up to 700 mA cm−2. The membranes also improved performance in neutral pH aqueous RFBs with high capacity utilization and retention, enhanced energy efficiency, and boosted power density.