Pre‐Loaded Functional Ion Pair in Hollow‐Sphere Channels for Enhanced Hydroxide Ion Conduction in Alkaline Flow Battery Membranes

Abstract Ion conductive membranes for energy conversion and storage require both stable operation and efficient power generation. However, the energy efficiency (EE) of energy devices assembled with membranes tends to drop dramatically at elevated current densities due to still high membrane resistance. To address this challenge, a novel functional ion pair ([EMIm][PF 6 ])‐mediated hollow carbon sphere (HCS) composite membrane is developed and its exceptional performance in alkaline flow batteries is demonstrated. The PF 6 − further promotes the penetration of water molecules through the pore entrance of HCS by ion‐dipole interaction. And the EMIm + in the cavity of HCS weakens the interaction between water molecules and facilitates hopping transport of OH − at water by Grotthuss mechanism. As a consequence, the developed membrane achieves 10% enhancement in average EE at 200 mA cm −2 for 700 cycles in alkaline zinc–iron flow batteries while effectively mitigating zinc dendrites formation. When applied to alkaline organic flow batteries, it maintains 7.5% increase in average EE at 140 mA cm −2 for 600 cycles with enhanced discharge capacity and capacity retention.