The Preparation And Modification Of The Non-fluorinated Membaranes For Vanadium Redox Flow Battery

Development of renewable energy sources to fossil fuels is an international urgent issue to solve the worldwide energy problems, which extremely spurs the scientific and technological endeavour for energy storage realm. The Vanadium redox flow battery?VRFB? a kind of large-scale energy storage technique, has attracted extensive academic and increasingly commercial interest due to its outstanding advantages such as long cycle life, flexible design, deep discharge, rapid response, and safety. It has a big potenial for the turbine generators, smart grids, remote area power systems and emergency back-up applications. As a key component of VRFB, the ion exchange membrane, which directly influences the efficiencies and performance of the VRFB. The Nafion series membrane, as a kind of perfluorinated sulfonic acid polymers, have been widely used in VRFB system due to its high proton conductivity and superior chemical stability. However, problems like high vanadium ion permeability and cost are inevitable for Nafion based VRFB system to encounter.Consequently, the kinds of no-perfluorinated membranes with low cost, high ion selectivety and long life are great needed. In this article, a series of highly branched sulfonated poly?fluorenyl ether ketone sulfone?s?HSPAEK? are synthesized by direct polycondensation reactions. Besides, sulfonated poly?ether ether ketone??SPEEK? are conducted various modifications. To evaluate their integrated performance and VRFB system applicability, the resultant composite membranes are investigated by various characterization tests.Rationally designed highly branched?DB value of 8%? nonfluorinated sulfonated poly?fluorenyl ether ketone sulfone??HSPAEK? was successfully synthesized by polycondensation with sulfonated 4'4-difluorobenzophenone, bisphenol fluorene, 1,3,5-tris?4-?4-fluorophenylsulfonyl? phenyl? benzene and 4'4-difluorodiphenyl sulfone. The HSPAEK shows good solubility and could be cast into transparent, flexible, and homogeneous membranes. The SEM and EDX investigation shows that the HSPAEK membrane is compact and smooth without visible holes. The HSPAEK membrane exhibits low swelling ratio, reasonable water uptake, good IEC and proton conductivity. Besides, HSPAEK membrane has low VO²⁺ permeability, high ion selectivity and long self-discharge time. The single cell performance of VRFB assembled with HSPAEK membrane possesses higher cell efficiency and superior cycling stability compared with the commercial Nafion 117 membrane. Therefore, the low-cost HSPAEK membranes have a very promising prospect of substituting Nafion membrane for the VRFB application.Three kinds of sulfonated poly?ether ether ketone??SPEEK?/nano oxides?Al₂O₃, SiO₂, and TiO₂? composite membranes are fabricated for vanadium redox flow battery?VRFB? application. The composite membranes with 5 wt. % of Al₂O₃, SiO₂ and TiO₂?S/A-5%, S/S-5%, and S/T-5%? exhibit excellent cell performance in VRFB. Incorporation of nano oxides?Al₂O₃, SiO₂, and TiO₂? in SPEEK membrane improves in the thermal, mechanical and chemical stabilities due to the hydrogen bonds interaction between SPEEK matrix and nano oxides. The energy efficiencies?EEs? of composite membranes are higher than that of Nafion 117 membrane, owing to the good balance between proton conductivity and vanadium ion permeability. The discharge capacity retention of composite membranes with S/A-5%?53.3%?, S/S-5%?50.8%?, and S/T-5%?39.5%? greatly overwhelms the Nafion 117 membrane?24.7%? after 200 cycles, indicating their good stability in VRFB system. These low-cost SPEEK/nano oxides composite membranes exhibit great potential for the application in VRFB.PDA coating is adopted for the surface modification of SiO₂, and then add to the SPEEK membrane. From the TEM, it could be found that a dense layer about 2-3nm is uniformly after modification adhered to SiO₂ after modification. The energy efficiencies?EEs? of composite membranes are higher than SPEEK membrane, but the discharge capacity retention of composite membranes is lower than S/S-5% after 100 cycles.