Efficient and durable vanadium flow batteries enabled by high-performance fluorinated poly(aryl piperidinium) membranes

Abstract

Ion exchange membranes (IEMs) have been extensively investigated as diaphragm materials for vanadium flow batteries (VFBs). However, current IEMs made of polymers still encounter challenges in ion selectivity (trade-off between ionic conductivity and vanadium resistance) and long-term stability (mechanical durability and chemical stability). Herein, fluorinated poly(aryl piperidinium) (PFDP) membranes without ether bonds are proposed to address these dilemmas. Microphase separation channels enriched with quaternary ammonium groups allow acceleration of ion transport while effectively rejecting vanadium ions. The ether-free polymer backbone and fluorinated structure can synergistically improve the stability of the IEM. The results indicate that the prepared PFDP membranes possess both excellent ion selectivity and stability, such as area resistance up to 0.17 Ω cm², low vanadium permeability (<1.4 × 10⁻⁸ cm² min⁻¹) and dimensional swelling (swelling ratio < 8% in the electrolyte), high tensile strength (∼60 MPa) and chemical stability (>28 days in the antioxidant test). Benefiting from these advantages, the single-cell assembled with PFDP-90 has a high efficiency at 120 mA cm⁻², especially the excellent energy efficiency (up to 84.3%), as well as shows stable cycling over 1000 cycles at 120 mA cm⁻². This work provides new insights into advanced diaphragm materials at the molecular level.