Bromination Of Heterocyclic Poly(Ether Sulfone) Containing Dimethyl Moieties And Preparation Of Ion Exchange Membranes

Vanadium redox flow battery (VRB) is one of the most promising of energy storage technologies in energy storage systems. Its power and capacity can be lonely designed, meanwhile it has deep discharge capacity, long cycle life, low pollution emission, quick response, and so on. Nafion membranes have been used as separator for VRB, because of good chemical stability and high proton conductivity. Its disadvantages, such as high cost and lowly selective vanadium permeability, hinder the futher commercialization in VRB. It is necessary to study on alternative membranes to improve the ions selection and battery efficiency. Our groups developed a series of quaternized poly(phthalazinone ether) anion exchange membranes with chloromethylation and quaternization, which had low vanadium ions permeability and good battery performance. The carcinogenic chloromethyl methyl ether was used during chloromethylation, however, the process was non-environmentally friendly.To avoid the use of chloromethyl methyl ether, a series of heterocyclic poly (ether sulfone) containing dimethyl groups were synthesized via the copolymerization of4-(3,5-dimethyl-4-hydroxyphenyl)-2,3-phthalazin-l-one,4,4’-dichloro diphenyl sulfone and4-(4-hydroxyphenyl)-2,3-phthalazin-l-one. The intrinsic viscosity of DMPPES was0.91-1.21dL/g. DMPPES was brominated with N-bromosuccinimide to prepare brominated heterocyclic poly(ether sulfone)(BPPES). NMR analyse confirmed that bromomethyl group was successfully introduced into BPPES polymer. The molecular weight of BPPES was above4×104. BPPES had good solubility in CHC13, CH2C14,1-methyl-2-pyrrolidinone, and so on. The beginning degradation temperature of BPPES was260℃and the tensile strength of BPPES was above55MPa. The results showed BPPES had good thermal stabilities and mechanical properties.BPPES membranes were prepared by solution casting method, and they were aminated with trimethylamine to prepare quaternized heterocyclic poly(ether sulfone) anion exchange membranes(QBPPES). The range of ion exchange capacity (IEC) was1.00～1.58mmol/g. The beginning degradation temperature of QBPPES was180℃and the tensile strength of QBPPES was above50MPa. The properties of QBPPES anion exchange membranes were investigated, such as water uptake, swelling rate, area resistance and vanadium ion permeability of QBPPES. Water uptake and swelling rate were increased with the increase of IEC. Water uptake and swelling rate of QBPPES was10.6%～19.8%and2.0%～3.4%at25℃, respectively. However, area resistance decreased with the increase of IEC. The area resistance of QBPPES membranes was the range of2.18Ω·cm2～0.51Ω·cm2. Vanadium permeability coefficient was the range of9.91×10-7～0.79×10-7cm/min, which was lower than that of Nafion117membrane (Ks=1.19×10-4cm/min). The results indicated that QBPPES showed good vanadium resistance. The current efficiencies of cells with QBPPES were above97.5%, which was higher than that of Nafion117membrane at40mA/cm2current density. When the IEC of QBPPES was about1.42mmol/g, the efficiencies of cells with QBPPES were equal with that of Nafionl17membrane. The OCV of QBPPES (IEC=1.58mmol/g) was above1.0V for about188h, which was nearly4.3times longer than that of Nafion117(44h). The tensile strength of QBPPES membranes remained above45MPa after the membrane had been immersed in1.5mol/L VO2+solution for60days. The surface of QBPPES membrane did not appear significant change. Voltage and energy efficiency of QBPPES membrane only reduced2%～4%. The current efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) of QBPPES changed slightly during100cycles. The result showed that QBPPES membrane had good stability in VRB.The base membrane was prepared from BPPES-60(DS=0.59), then was aminated to prepare QBPPES anion exchange membrane. Effects of solvent evaporation time and heating temperature on the structure and properties of membrane were preliminarily investigated. QBPPES membrane became dense with an increase of solvent evaporation time or heating temperature, which led to high area resistance and low VE. Membrane structure could be adapted by changing solvent evaporation time and heating temperature to improve membrane performance.