| All vanadium redox flow battery?VRFB?is a kind of novel energy storage system which has many advantages such as flexible design,low cost,long life,high efficiency and environment-friendly,and so on.As one of the most important components of VRFB,membrane not only transports proton,but also prevents the vanadium ion cross-over in electrolytes.Generally,Nafion series membranes have been employed as commercial separators of VRFBs.However,high cost,low proton selectivities and high vanadium ion permeabilities of Nafion series membranes hamper their further large-scale commercial applications in VRFB.Therefore,novel membrane should be developed to meet the commercial requirement of VRFB application.Sulfonated polyimide?SPI?membranes have many merits such as reasonable proton conductivities,low vanadium ion permeabilities,good mechanical strength and membrane forming properties,outstanding thermal stabilities and low prices.Nevertheless,as same as most non-fluorinated sulfonated aromatic polymer membranes,the SPI membrane without structure design and optimization shows low chemical stability,which hampers its long-term application in VRFB.Research results show that:the chemical stabilities of branched polymer membranes are higher than those of linear polymer membranes.Besides,fluorine-containing polymer membranes have lower vanadium ion permeabilities and higher chemical stabilities compared to those of non-fluorinated polymer membranes.And ionic cross-linked structrue could further increase the vanadium ion resistance and chemical stability of membrane.Hence,in this thesis,a kind of non-sulfonated triamine with fluorine-containing brached structure was synthesized firstly,and a series of fluorine-containing branched SPI?Fb-SPI?membranes with different degrees of branching were prepared successfully.Then,a kind of non-sulfonated diamine with imidazole group in side-chain was synthesized,and a series of side-chain ionic cross-linked fluorine-containing branched SPI?sc-FbSPI?membranes with different degrees of sulfonation were prepared.The major contents of this thesis are as following:?1?Firstly,a kind of fluorine-containing branched triamine monomer 1,3,5-tris?2-trifluoromethyl-4-aminophenoxy?benzene?TFAPOB?was synthesized.Then a series of fluorine-containing branched SPI?Fb-SPI?membranes with different degrees of branching?DB:4%-12%?were prepared from 1,4,5,8-naphthalenetetra-carboxylic dianhydride?NTDA?,TFAPOB,4,4'-diamino-biphenyl-2,2'-disulfonic acid?BDSA?and 4,4'-oxydianiline?ODA?through polycondensation method.The chemical structure of TFAPOB and Fb-SPI membranes were characterized using FTIR,ATR-FTIR and 1H NMR spectra,and the surficial and cross-sectional morphologies as well as element distribution of Fb-SPI membranes were scanned using SEM-EDS.Also,studying the physico-chemical properties of Fb-SPI membranes.And these results demonstrate that the chemical stabilities of Fb-SPI membranes are higher than that of l-SPI membrane,and the vanadium ion permeabilities of Fb-SPI membranes(2.45-0.99×10-7 cm2 min-1)are much lower than that of Nafion 117membrane(17.1×10-7 cm2 min-1).In addition,the proton selectivities of Fb-SPI membranes(0.67-3.5×105 S min cm-3)are much higher than that of Nafion 117 membrane(0.34×105 S min cm-3).The self-discharge time?166 h?of VRFB with Fb-SPI-10 membrane which has the highest proton selectivity(3.5×105 S min cm-3)is much longer than that of VRFB with Nafion 117 membrane?97 h?.500-time cyclical charge-discharge test was measured at the current density of 40-80 mA cm-2.The results illustrate that the coulombic efficiencies?98.4-99.7%?and energy efficiencies?69.0-79.7%?of VRFB with Fb-SPI-10 membrane are higher than battery efficiencies of VRFB with Nafion 117 membrane at the same current densities,respectively.In addition,the capacity retention of VRFB with Fb-SPI-10membrane is higher than or comparable to the VRFB with Nafion 117 membrane.?2?To further increase the chemical stabilities and vanadium ion resistances of Fb-SPI membranes,a kind of non-sulfonated diamine?BIPBD?which has side-chain imidazole group was synthesized.A series of side-chain ionic cross-linked fluorine-containing branched SPI?sc-FbSPI?membranes with different degrees of sulfonation?DS:40%-70%?were fabricated from TFAPOB,BIPBD,BDSA and NTDA through polycondensation method.The chemical structure of BIPBD and its mid-products and sc-FbSPI membranes were measured by FTIR,ATR-FTIR and 1H NMR spectra.The surficial and cross-sectional morphologies of as-prepared membranes before and after VRFB cyclical test were characterized using SEM-EDS.Also,studying the physico-chemical properties of Fb-SPI membranes,and these results demonstrate that the vanadium ion permeabilities of sc-FbSPI membranes(0.36-3.87×10-7 cm2 min-1)are much lower than that of Nafion 115 membrane(27.43×10-7 cm2 min-1).In addition,the proton selectivities of sc-FbSPI membranes(0.89-4.20×105 S min cm-3)are much higher than that of Nafion 115 membrane(0.17×105 S min cm-3).The self-discharge time?232 h?of VRFB with sc-FbSPI-60 membrane which has the highest proton selectivity is much longer than the self-discharge time of VRFB with Nafion115 membrane?82 h?.The charge-discharge capacities and energies of VRFB with sc-FbSPI-60 membrane are higher than those of VRFB with Nafion 115 membrane at the same current densities.In addition,the cyclical charge-discharge tests of VRFB with sc-FbSPI-60and Nafion 115 membrane were conducted at current densities ranged from 60 to 80 mA cm-2.All these results show that the CE?97-99.5%?,EE?68.2-75.1%?and capacity retention?50.5-55.1%?of VRFB with sc-FbSPI-60 membrane are higher than those of VRFB wih Nafion 115 membrane. |
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