Research On Sulfonated Poly(ether Ether Ketone) Based Ion Exchange Membrane For Vanadium Redox Flow Battery

The vanadium redox flow battery(VRFB) has various advantages such as long cycle-life, quick response, and low maintenance cost, leading to the wide application of VRFB in the field of energy storage. The perfluorinated cation membrane used in VRFB has some defects like high vanadium ion permeability and price, which has restricted the commercial application of VRFB. The research on high-efficiency, long cycle-life, and low-cost ion exchange membrane is the hot topic in the VRFB research field. In this dissertation, we study the modification of high degree of sulfonation(DS) sulfonated poly(ether ether ketone)(SPEEK) membrane, and explore the methods to enhance the performance and stability of SPEEK based ion exchange membrane. The main contents of this dissertation are listed as follows.We prepare SPEEK membrane by solution casting method, explore the influence of DS on the SPEEK membrane performance, and further investigate the characteristic of SPEEK membrane used in VRFB. As the results shown, when the DS is 67%, the SPEEK membrane has high ion selectivity, and the VRFB assembled with this membrane has good battery performance. Based on this SPEEK membrane, we study the influence of casting solvent on SPEEK membrane. As the results shown, the casting solvent of N,N-dimethylformamide(DMF) has better effect.We investigate the preparation method and structure characteristic of two composite membranes: SPEEK/poly(vinylidene fluoride)(SPEEK/PVDF) membrane, and SPEEK/poly(vinylidene fluoride-co-hexafluoropropylene)(SPEEK/P(VDF-HFP)) membrane. As the results shown, the SPEEK and PVDF exhibit a good compatibility. PVDF effectively inhibits the swelling of composite membrane, and the high chemical stability of PVDF improves the stability of composite membrane. Although SPEEK/P(VDF-HFP) composite membrane has phase separation structure, the SPEEK and P(VDF-HFP) have weak interaction, and the SPEEK/P(VDF-HFP) composite membrane has better chemical stability.We prepare mesoporous silica molecular sieve SBA-15 and SPEEK composite membrane. As the results shown, the-SO3 H group in SPEEK and-OH group in SBA-15 have hydrogen bond interaction, which effectively improves the thermal stability of composite membrane. When the SBA-15 mass ratio is 20%, the VRFB assembled with this composite membrane is operated at the current density of 60 m A cm-2. The battery coulombic efficiency and energy efficiency are 96.3% and 88.1% respectively, which are 4.1% and 7.1% respectively higher than those of the battery assembled with Nafion 117 membrane.By blending SPEEK and polyacrylonitrile(PAN), we prepare SPEEK/PAN blend membrane. As the results shown, the acidic-SO3 H group in SPEEK and basic-C≡N group in PAN have acid-base interaction. As the further results shown, with the increasing of PAN mass ratio, the acid-base interaction in blend membrane continuously increases, the water uptake and vanadium ion permeability of SPEEK/PAN blend membrane continuously decrease, and the ion selectivity of SPEEK/PAN blend membrane continuously increases. The VRFB assembled with S/PAN-20 membrane is operated at the current density of 80 mA cm-2. The battery coulombic efficiency and energy efficiency are 96.2% and 83.5% respectively, which are both 5.1% higher than those of the battery assembled with Nafion 117 membrane.