Perfect capacity retention of all-vanadium redox flow battery using Nafion/polyaniline composite membranes

Crossover of vanadium ions through proton conducting membranes in all-vanadium redox flow batteries (VRFBs) causes considerable engineering problems and deteriorates VRFB performance by reducing capacity retention over cycles. In this study, we have succeeded in fabrication of multi-layered polyaniline/Nafion (PANI-Nafion) composite membranes that exhibit a very low vanadium ion permeability and, therefore, can significantly improve VRFB performance by preserving electric capacity almost perfectly. The multi-layered PANI-Nafion membrane is fabricated by sequentially coating a PANI layer followed by Nafion layer on Nafion 115 membrane using a dip-coating method. The fabricated membrane outperforms pristine Nafion 115 membrane in terms of VRFB performance by exhibiting a very high coulombic efficiency of ∼ 99 % and an energy efficiency of ∼ 91 % at 50 mA cm−2. Particularly, it shows a remarkable improvement in durability and capacity retention with a 100 % discharge capacity retention over 100 cycles of charge/discharge operation as compared with that of pristine N115 (63 %). In-depth analyses have been conducted to characterize the PANI-Nafion composite membrane and to understand the mechanism of how it enhances the performance of VRFB. This study has shown a possibility to make VRFBs having a perfect capacity retention using PANI-coated composite membranes, which may help expedite full-fledged commercialization of VRFBs.