Highly Selective Anion Exchange Membrane Based on Quaternized Poly(triphenyl piperidine) for the Vanadium Redox Flow Battery

Vanadium redox flow batteries (VRFBs) have attracted great attention recently owing to the increasing supply of intermittent renewable energies. However, VRFBs usually suffer from serious vanadium ion crossover and high cost when perfluorinated membranes are employed as the separator. In this study, a highly selective anion exchange membrane (AEM) is synthesized from the aryl ether-free poly(terphenyl piperidine) (PTP). Using 3-chloro-2-hydroxypropyltrimethyl ammonium chloride (CHPTMA-Cl) as the quaternization reagent, not only are the piperidinium cations formed in the PTP main chain, but also the side-chain quaternary ammonium cation and hydroxyl group are introduced into the PTP backbone. Compared with pure PTP-TFA and methyl quaternized PTP (PTP-Me) membranes, the obtained hydroxypropyltrimethyl ammonium grafted poly(terphenyl piperidinium) (PTP-CHPTMA) membrane exhibits high H+ permeability (1.82 x 10(-5) cm(2) min(-1)) and low area resistance (0.35 Omega cm(2)) mainly due to the presence of the hydrophilic hydroxyl group. Owing to the electrostatic repulsion effect of main-chain piperidinium and side-chain quaternary ammonium cations to vanadium ions, the PTP-CHPTMA membrane achieves a low vanadium ion permeability (1.21 x 10(-8) cm(2) min(-1)). Consequently, the PTP-CHPTMA membrane reaches 2 orders of magnitude higher ion selectivity than Nafion 115. The assembled single VRFB with PTP-CHPTMA possesses high Coulombic efficiencies of close to 100% at 60-160 mA cm(-2) and higher energy efficiencies than the cell with Nafion 115. The self-discharge duration of the cell with PTP-CHPTMA (381 h) is nearly 4.5 times longer than that of Nafion 115 (86 h). Meanwhile, the VRFB based on PTP-CHPTMA displays excellent cycle stability and discharge capacity retention over 580 charge-discharge cycles at 100 mA cm(-2).