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

[Display omitted] •Dip-coated Polyaniline/Nafion composite membranes (PN115) are developed for VRFB.•The composite PN115 membranes effectively prohibit crossover of vanadium ions.•Significant impact of PN115 membranes on VRFB performance is demonstrated.•Stability of PN115 membranes is enhanced by further Nafion layer coating.•A VRFB with Nafion-PN115 membranes exhibits perfect capacity retention (100%). 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.