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 × 10–5 cm2 min–1) and low area resistance
(0.35 Ω cm2) 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 × 10–8 cm2 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.
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