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.
In
this work, we propose a sulfonated poly (ether ether ketone)
(SPEEK) composite proton-conductive membrane based on a 3-(1-hydro-imidazolium-3-yl)-propane-1-sulfonate
(Him-pS) additive to break through the trade-off between conductivity
and selectivity of a vanadium redox flow battery (VRFB). Specifically,
Him-pS enables an oriented distribution of the SPEEK matrix to construct
highly conductive proton nanochannels throughout the membrane arising
from the noncovalent interaction. Moreover, the “acid–base
pair” effect from an imidazolium group and a sulfonic group
further facilitates the proton transport through the nanochannels.
Meanwhile, the structure of the acid–base pair is further confirmed
based on density functional theory calculations. Material and electrochemical
characterizations indicate that the nanochannels with a size of 16.5
nm are vertically distributed across the membrane, which not only
accelerate proton conductivity (31.54 mS cm–1) but
also enhance the vanadium-ion selectivity (39.9 × 103 S min cm–3). Benefiting from such oriented proton-conductive
nanochannels in the membrane, the cell delivers an excellent Coulombic
efficiency (CE, ≈ 98.8%) and energy efficiency (EE, ≈
78.5%) at 300 mA cm–2. More significantly, the cell
maintains a stable energy efficiency over 600 charge–discharge
cycles with only a 5.18% decay. Accordingly, this work provides a
promising fabrication strategy for a high-performance membrane of
VRFB.
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