Investigation And Modification Of PSf Based Anion Exchange Membranes For Vanadium Redox Flow Battery

All vanadium redox flow battery (VRFB), with the advantages of fast response, flexible design, and safety, can be used as a new kind of energy storage system in emergency power and renewable energy storage system. Anion exchange membranes, as a key component in VRFB, have attracted much attention due to its excellent vanadium preventing ability and low cost. The severe swelling ratio accelerates the degration of anion exchange membrane under the oxdative operating condition, which can lead to poor cell performances.In this thesis, a series of anion exchange membranes with N-methylimidazole and tetramethylethylenediamine (TMEDA) linked to PSf are fabricated by solvent evaporation method. The introduction of di-cationic side chain and cross-linking structure enhances ionic ineractions, which induces good micro-phase separation and excellent swelling resistance to get better cell performances.First of all, the homogeneous reaction of chloromethylated polysulfone and N-methylimidazole takes place to synthesize imidazolium-functionalized polysulfone (Im-PSf) at 80?, followed by a solution casting method to obtain Im-PSf memrbanes. Basic properties are investigated. Im-PSf membrane with an IEC of 1.12 mmol g-1 can prevent vanadium crossover efficiently, the vanadium permeability coefficient is only 2.834×10-12 cm2/min, tens of million of Nafion212. Attacked by the oxidative species, the Im-PSf membrane degrades after fully swelling. The discharge capacity decay per cycle during the 20-cycle test is 1.22% at the current density of 50 mA/cm2.Design of di-cationic side chain could promote hydrophilic-hydrophobic micro-phase separation and anti-swelling property. TMEDA-functionalized polysulfone membrane with single-cationic side chain is fabricated, followed by quaternization of CH3I and the free tertiary amine group of TMEDA to prepare I-TMEDA-PSf membranes. The micro-phase separation structure proved by SAXS enhances the swelling resistance, increases the ionic conductivity and improves the stability of membrane. In the cycling test, the VRFB assembled with I-TMEDA-PSf membrane exhibits excellent performance, the CE and EE is about 94.5% and 80% respectively during the whole test more than 300 hours, which are higher than Nafion212 (CE:87%, EE:79%). Moreover, the discharge capacity decay rate is 0.58% per cycle, smaller than that of Nafion212 (0.70% per cycle).At last, TMEDA is employed as both quaternization and cross-linking reagent to prepare cross-linking membranes (C-TMEDA-PSf). Membranes with greater cross-linking degree (DL) and ion exchange groups could be achieved at the same time by decreasing the amount of TMEDA. When the degree of chloromethylation is 78% and DL is 20%, the tensile stress reaches 24.9 MPa, water uptake and swelling ratio decrease to 14.4% and 1.4% respectively, which is suitable for VRFB. As for the cycling test under the constant current density of 50 mA/cm2 (more than 300 hours), the CE of membrane approaches 99%. After 80 cycles, the discharge capacity is about 62.6% of its initial value and the capacity decay rate per cycle is only 0.47%, which is smaller than I-TMEDA-PSf (0.58% per cycle) and Nafion 212 (0.70% per cycle).