Densely Quaternized Fluorinated Poly(fluorenyl ether)s with Excellent Conductivity and Stability for Vanadium Redox Flow Batteries

Abstract

Cationic group distribution and elemental composition are two key factors determining the conductivity and stability of anion exchange membranes (AEMs) for vanadium redox flow batteries (VRFBs). Herein, fluorinated tetra-dimethylaminomethyl-poly­(fluorenyl ether)­s (TAPFE)­s were designed as the polymer precursors, which were reacted with 6-bromo-N,N,N-trimethylhexan-1-aminium bromide to introduce di-quaternary ammonium (DQA) containing side chains. The resultant DQA-TAPFEs with a rigid fluorinated backbone and flexible multi-cationic side chains exhibited distinct micro-phase separation as probed by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). DQA-TAPFE-20 with an ion exchange capacity (IEC) of 1.55 mmol g–1 exhibited a SO4 2– conductivity of 10.1 mS cm–1 at room temperature, much higher than that of a control AEM with an identical backbone but spaced out cationic groups, which had a similar IEC of 1.60 mmol g–1 but a SO4 2– conductivity of only 3.2 mS cm–1. Due to the Donnan repulsion effect, the DQA-TAPFEs exhibited significantly lower VO2+ permeability than Nafion 212. The VRFB assembled with DQA-TAPFE-20 achieved an energy efficiency of 80.4% at 80 mA cm–1 and a capacity retention rate of 82.9% after the 50th cycling test, both higher than those of the VRFB assembled with Nafion 212 and other AEMs in the literature. Therefore, the rationally designed DQA-TAPFEs are promising candidates for VRFB applications.

Efficiency and Oxidation Performance of Densely Flexible Side-Chain Piperidinium-Functionalized Anion Exchange Membranes for Vanadium Redox Flow Batteries

“…This is consistent with the oxidative stability of the quaternary ammonium structure in AEMs reported in other refs and . Combined with the in situ and ex situ testing results, it can be seen that PAES-8mPip- x had better oxidation stability than some reported AEMs for VRFB, , especially under in situ operation conditions. The separation of hydrophilic cations from the polymer skeleton of AEMs by densely flexible alkyl side chains can effectively reduce the charge induction between ionic groups and the polymer skeleton and better ensure the hydrophobic characteristics of the polymer skeleton, which plays a positive role in improving the oxidation stability of AEMs.…”

“…As shown in Figure 1 and Table S1, their SO 4 2− conductivity is between 9.6 and 18.1 mS cm −1 at 20 °C, which increases with the increase of IEC. Compared with some previously reported AEMs, including densely quaternized AEMs based on fluorinated poly(fluorenyl ether)s (DQA-TAPFE-20, 1.55 mmol g −1 , 10.1 mS cm −1 ) 35 and densely quaternized poly(arylene ether)s (QA-PAE-40, 2.32 mmol g −1 , 14.0 mS cm −1 ) 36 and the AEMs based on ether-free poly(p-phenylene)s (QPPP-3, 2.4 mmol g −1 , 3 mS cm −1 ), 37 PAES-8mPip-x exhibits higher or comparable SO 4 2− conductivity. This is probably attributed to the densely flexible side chains in the hydrophilic structural units of PAES-8mPipx, which further improves the mobility and aggregation of piperidinium cations, makes it more easy to form a more effective ion transport channel, and reduces the resistance of SO 4 2− conduction.…”

“…The SO 4 2– conductivity (σ) of AEMs was measured by electrochemical impedance spectroscopy according a reported method . The test was conducted by a four-point probe technique on an electrochemical station (Solartron 1287).…”

High-Performance Proton Exchange Membranes Based on Block Polybenzimidazole and Organic–Inorganic Fillers with a Low Acid Doping Level

“…Moreover, when the doping temperature rises above 110 °C, the conductivity of bPBI-2 exceeds that of bPBI, although the ADL of bPBI-2 is lower than that of bPBI. This shows that the proton conduction of bPBI-X not ≈75 ≈25 BrF6-PBI-b-pPBI 28 83 16 SPAEKS-60 30 21 ± 2 1 1 ± 3 PAEK-PIL0.6 31 33 ± 3 6 ± 1 OPBI-R1−6 34 ≈8 ≈26 DQA-TAPFE-15 57 ≈40 ≈15 N_U200−1 58 13.6 ± 0. only depends on the ADL but PWA-IL also plays a role in promoting proton conduction. 29 Figure 7b displays the conductivity of bPBI, bPBI-1, bPBI-2, and bPBI-3 tested at 170 °C after PA doping at different temperatures.…”

Preparation and Characterization of Non-N-Bonded Side-Chain Anion Exchange Membranes Based on Poly(2,6-dimethyl-1,4-phenylene oxide)

“…Since the beginning of the 21st century, the problems of traditional fossil energy depletion and environmental pollution caused by population growth and rapid social and economic development have become increasingly prominent. − Traditional fossil energy sources, such as coal, oil, and natural gas, are facing depletion due to their nonrenewability and human “overdraft” consumption. A fuel cell is an efficient and environmentally friendly energy technology that can convert chemical energy in hydrogen fuel and oxidizers into water and electricity through electrochemical reactions .…”