| Vanadium redox flow battery(VRFB)is regarded as one of the most potential large-scale energy storage/conversion systems because of its flexible design,environment-friendliness,and long service life.As a key component of VRFB,the ion exchange membrane not only separates the positive and negative electrolytes but also transfers protons.At present,Nafion membranes have been widely used in VRFBs due to their excellent chemical stabilities and proton conductivities.However,the high price and serious vanadium ion permeability of Nafion membranes limit their commercialization in VRFBs.Therefore,it is urgent to develop novel ion exchange membranes to replace Nafion membranes.Polyimide(PI)based membrane is regarded as a potential candidate for VRFB application because of its easy membrane-forming,high vanadium ion resistance and reasonable price.However,the traditional PI-based membrane has obvious disadvantages such as low proton conductivity and poor chemical stability in the application of VRFB.Therefore,the molecular structures of PI-based membranes are designed,and a series of PI-based membranes are prepared by branching and crosslinking to address above problems in this thesis.The main contents are as follows:(1)A diamine monomer 2,2',3,3',5,5',6,6'-octafluoro-4,4'-bis-(4-aminophenoxy)-biphenyl(OFBAPB)is synthesized.Then,a series of branched polyfluoro sulfonated polyimide(BPFSPI-X-Y)membranes with different branching and sulfonation degrees are prepared by using OFBAPB,1,3,5-tris(2-trifluoromethyl-4-aminophenoxy)benzene(TFAPOB),4,4'-diamino-biphenyl-2,2'-disulfonic acid(BDSA)and 1,4,5,8-naphthalenetetra-carboxylic dianhydride(NTDA)as raw materials and regulating the contents of TFAPOB,OFBAPB and BDSA.The chemical structures and physico-chemical properties of BPFSPI membranes are characterized.The experimental results are as below:the proton conductivity of optimized BPFSPI-10-50 membrane is 2.94×10-2S cm-1,which is around three times as high as the commercially acceptable value.Besides,the proton selectivity of BPFSPI-10-50membrane(3.16×10~5S min cm-3)is about 7.5 times that of Nafion 212 membrane(0.42×10~5S min cm-3).In the VRFB efficiency tests(40-200 m A cm-2),the coulomb efficiencies(CEs)and energy efficiencies(EEs)of BPFSPI-10-50 membrane are higher than those of Nafion212 membrane(CE:94.2-98.9%vs 73.9-94.9%,EE:68.0-86.9%vs 67.7-77.9%).Moreover,the VRFB with BPFSPI-10-50 membrane has slower self-discharge rate and higher discharge capacity retention compared with Nafion 212 membrane.(2)The polyfluoro sulfonated polyimide(PFSPI)polymer is prepared by one-step polycondensation from OFBAPB,2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane(AHHFB),BDSA and NTDA.Then,the covalent cross-linked polyfluorosulfonated polyimide(PFSPI-PAA)membranes are prepared by using hydrophilic polymer polyacrylic acid(PAA)as a crosslinking agent.The chemical structure and physico-chemical properties of PFSPI-PAA membranes are characterized in detail.The experimental results are as the following:the optimized PFSPI-PAA-25 membrane(0.15?cm~2)has lower area resistance in contrast with Nafion 212 membrane(0.16?cm~2).Moreover,the vanadium ion permeability of PFSPI-PAA-25 membrane(5.90×10-9cm~2min-1)is also much lower than that of Nafion 212 membrane(7.53×10-7cm~2min-1).Meanwhile,the VRFB with PFSPI-PAA-25 membrane has higher CEs and EEs than those with Nafion 212 membrane at the current densities of 60-300 m A cm-2(CE:97.3-99.9%vs 90.9-96.5%;EE:90.7-73.6%vs84.7-72.0%).Besides,the peak power density of VRFB with PFSPI-PAA-25 membrane(451.4 m W cm-2)is also superior to that with Nafion 212 membrane(427.9 m W cm-2).Furthermore,the PFSPI-PAA-25 membrane shows excellent durability in 500 times cycling charge-discharge tests.(3)A tetraamine monomer 4,4',4'',4'''-((1H,3'H-[5,5'-bibenzo[d]imidazole]-2,2'-diylbis(benzene-5,1,3-triyl))tetrakis(oxy))tetraaniline(BIT)is synthesized.And then,a series of cross-linked polyimide(CrPI)membranes with different crosslinking degrees are prepared by using BIT,4,4'-diaminodiphenyl ether(ODA),2-(4-aminophenyl)-5-aminobenzimidazole(APABI)and NTDA as raw materials and adjusting the contents of BIT and ODA.A porous cross-linked polyimide(PCrPI-10)membrane is prepared by using?-cyclodextrin as a template.The experimental results are as below:the PCrPI-10 membrane has excellent vanadium ion resistance due to Donnan repulsion effect of imidazole groups.Meanwhile,the chemical stability of PCrPI-10 membrane is effectively enhanced by using imidazole groups instead of sulfonic acid groups.In the VRFB test,the PCrPI-10 membrane(CE:96.3-99.6%,EE:67.9-88.9%)has higher CEs and EEs than Nafion 212 membrane(CE:74.3-94.8%;EE:67.7-77.5%).Besides,the 2250 times charge-discharge cycles are performed for VRFB with PCrPI-10 membrane.During this process,the VRFB shows very stable CE,and the PCrPI-10 membrane maintains outstanding mechanical and chemical stabilities.Moreover,the excellent durability of PCrPI-10 membrane is reasonably explained by using density functional theory. |
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