Abstract
Flexible cross-linked anion exchange membranes (AEMs) based on poly (p-phenylene oxide) grafted with N-spirocyclic quaternary ammonium cations were synthesized via UV-induced free-radical polymerization by using diallylpiperidinium chloride as an ionic monomer. Five membranes with ion exchange capacity (IEC) varying between 1.5 to 2.8 mmol Cl−·g−1 polymer were obtained and the correlation between IEC, water uptake, state of water in the membrane and ionic conductivity was studied. In the second part of this study, the influence of properties of four of these membranes on cell cycling stability and performance was investigated in an aqueous organic redox flow battery (AORFB) employing dimethyl viologen (MV) and N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride (TMA-TEMPO). The influence of membrane properties on cell cycling stability and performance was studied. At low-current density (20 mA·cm−2), the best capacity retention was obtained with lower IEC membranes for which the water uptake, freezable water and TMA-TEMPO and MV crossover are low. However, at a high current density (80 mA·cm−2), membrane resistance plays an important role and a membrane with moderate IEC, more precisely, moderate ion conductivity and water uptake was found to maintain the best overall cell performance. The results in this work contribute to the basic understanding of the relationship between membrane properties and cell performance, providing insights guiding the development of advanced membranes to improve the efficiency and power capability for AORFB systems.
Highlights
Redox flow batteries (RFBs) represent promising large-scale energy storage devices for many reasons, such as, being a source of independently scalable power and energy, allowing for modular production, are operational at ambient temperatures and are environmentally benign [1,2,3,4]
The micrographs support the results found in the resistance measurement and suggest a good mechanical and chemical stability of the employed anion exchange membranes (AEMs), benefiting from the noncorrosive nature of neutral electrolytes and the high chemical stability of the quaternary ammonium (QA) cations
Five AEMs based on PPO grafted with various amounts of poly (DAPCl) cations were fabricated via a rapid UV-irradiation method and four of them were tested in a TMA-TEMPO/MV-based aqueous organic redox flow battery (AORFB)
Summary
Redox flow batteries (RFBs) represent promising large-scale energy storage devices for many reasons, such as, being a source of independently scalable power and energy, allowing for modular production, are operational at ambient temperatures and are environmentally benign [1,2,3,4]. Organic redox flow batteries (ORFBs) have grown to become very promising candidates to fulfill the requirements of “green”, safe and sustainable energy storage [5]. The neutral pH value AORFB employing water-soluble dimethyl viologen (MV, N,N’-dimethyl-4,4 -bipyridinium dichloride) (anolyte) and TMA-TEMPO (N,N,N-2,2,6,6heptamethylpiperidinyl oxy-4-ammonium chloride) (catholyte) molecules has been demonstrated as a stable and high performing flow battery system [12]. A relatively high capacity of 54 Ah·L−1 giving a total energy density of 38 Wh·L−1 at a cell voltage of 1.4 V has been reported using TMA-TEMPO (2.3 M)/MV (2.4 M) as the active materials [12]. The relatively high energy density of this system is due to the high solubility of both active materials. The solubility of the organic active materials and, energy density can be further improved by modifying the functional group or tuning the composition of the supporting electrolytes [13,14]
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