Iodine-Functionalized rGO/Graphite Felt Interfaces for Enhanced Mass Transport and Charge-Transfer Kinetics in Vanadium Redox Flow Batteries

High-power operation of vanadium redox flow batteries (VRFBs) is still limited by sluggish interfacial charge transfer, insufficient electrolyte wettability, and concentration polarization within porous carbon electrodes. Herein, an iodine-functionalized reduced graphene oxide/graphite felt electrode (GF@rGO-V) was prepared through hydroiodic acid vapor treatment to regulate the solid-liquid interface of graphite felt while retaining its three-dimensional porous framework. Structural and surface analyses indicate that HI vapor treatment produces a less compact iodine-containing rGO interface with improved electrolyte accessibility. Electrochemical measurements further reveal enhanced apparent ion transport and reduced charge-transfer resistance toward both VO2+/VO2+ and V2+/V3+ redox couples. Temperature-dependent impedance analysis shows that the apparent activation energy for the positive reaction decreases from 40.2 kJ mol-1 on pristine graphite felt to 23.7 kJ mol-1 on GF@rGO-V, suggesting a more favorable interfacial kinetic environment. As a result, the assembled VRFB delivers an energy efficiency of 82.2% at 400 mA cm-2, a peak power density of 1257.6 mW cm-2, and stable cycling over 1500 cycles at 400 mA cm-2. This work demonstrates an iodine-assisted interfacial engineering strategy for improving mass transport and charge-transfer kinetics in high-power aqueous flow batteries.