Covalent/ionic co-crosslinking constructing ultra-densely functionalized ether-free poly(biphenylene piperidinium) amphoteric membranes for vanadium redox flow batteries

Abstract A novel covalently/ionically co-crosslinked poly(biphenylene piperidinium) amphoteric membrane was designed for improved stability and efficiencies in vanadium redox flow batteries (VRFB). The amphoteric-side-chain structure provides an ultrahigh functionalization degree (4.93 mmol·g − 1) and unobstructed ion channels, which endowed the membrane with an excellent ion conduction capacity comparable to Nafion 212 (area resistance: 0.23–0.29 Ω·cm2). Surpassing traditional sulfonated membranes and single ionic crosslinked membranes, even at high level functionalization, the covalently/ionically co-crosslinking structures guaranteed the membrane a low swelling ratio (10.1%) and VO2+ permeability of 0.79 × 10−8 cm2·s − 1. Moreover, due to the lack of sensitive aryl ether bonds, these membranes showed very good chemical stability, with a slight mass loss of 3.43% (nearly 5.5 times lower than SPEEK) after soaking in vanadium oxygen species for 30 days. On this basis, the membrane exhibited excellent battery performance. The Coulombic, voltage, and energy efficiencies of a battery using the above membrane reached 98.12, 87.27, and 85.98% at 120 mA·cm−2 current density and no obvious decline occurred after 400 cycles. The self-discharge time of battery up to 136 h. These observations indicated that this membrane appeared very promising for use in VRFB applications.