| All vanadium redox flow battery(VRFB) is a type of novel green flow battery, whose redox couple consists of a single metal ion(vanadium ion). VRFB has many advantages such as adjustable capacity and power, non destructive deep discharge at high current, easy for operation and maintenance, long cycle life, etc. Usually, VRFB can be applied to load power supply system, large scale photovoltaic translating system, wind energy sources system, electric vehicle power supply, and so on.So far, VRFB has been still at the early stage of commercial application, and the membrane is one of the limitations of VRFB's large-scale development. Currently, Nafion membranes(Dupont Co., USA) have been widely used in the VRFB system because of their high proton conductivity and excellent chemical stability. But the high vanadium ion permeability, high cost and water migration still limit their large scale applications. Therefore, it is of marked research significance and commercial value to develop novel non-fluorinated proton conductive membranes with low cost, reduced vanadium permeability and improved comprehensive battery performance.Sulfonated polyimide(SPI) has acceptable proton conductivity, good membrane forming property, excellent thermal stability and reasonable price, thus it is expected to replace Nafion membrane and apply in VRFB. However, non optimized pristine SPI membrane is not endurable enough in VRFB since the positive electrolyte is strongly acidic and oxidative. Therefore, the long-term stability of the pure SPI membrane is severely challenged.This thesis is aimed to design and fabricate a series of new type of oxidation resistant SPI membranes for VRFB. As-prepared membranes will show excellent vanadium ion resistance, good chemical stability and reasonable proton conductivity, which is expected to provide some theoretical and technical supports for the development of novel non-fluorinated membranes for VRFB application. Followings are the main contents in this thesis:(1) SPI(ODA)-PDMS was synthesized from 1,4,5,8-naphthalenetetracarboxylic dianhydride(NTDA), 4,4'-diamino-biphenyl 2,2'-disulphonic acid(BDSA), 4,4'-oxydianiline(ODA) and ?,?-diaminopropyl polydimethylsiloxane(PDMS) by the polycondensation method. When the content of PDMS was changed to be 3%, 5%, 7% and 9%, a series of proton conductive membranes named as SPI(ODA)-PDMS-3, SPI(ODA)-PDMS-5, SPI(ODA)-PDMS-7 and SPI(ODA)-PDMS-9 were obtained respectively. The structures of prepared membranes were characterized by FT-IR, and the morphologies were examined by SEM. The basic physico-chemical properties of membranes were determined such as water uptake, swelling ratio, ion exchange capacity, proton conductivity, vanadium ion permeability and chemical stability, etc. The results showed that: Compared with the SPI membrane without PDMS segment, the chemical stabilities of all SPI(ODA)-PDMS membranes were improved to some extent. The optimum PDMS content was 5%. The selectivity of SPI(ODA)-PDMS-5 membrane achieved the highest(13.7×104 S min cm-3), which was four times as much as that of Nafion 117 membrane. Therefore, the SPI(ODA)-PDMS-5 membrane was chosen to be applied in VRFB. During the 100-time cyclic charge-discharge testing at 25-70 m A cm-2, the coulomb efficiency of the VRFB with the SPI(ODA)-PDMS-5 membrane was higher than 97%, which was slightly higher than that of Nafion 117 membrane. The energy efficiency of the VRFB with the SPI(ODA)-PDMS-5 membrane was 67-82%, which was higher than that of Nafion 117 membrane. These results verified that the SPI(ODA)-PDMS-5 membrane had good cyclic VRFB performance.Besides, in order to further improve the chemical stability of membranes, the non-sulfonated diamine monomer(ODA) was changed as 4,4'-(4,4'-isopropylidenediphenyl-1,1'-diyldioxy) dianiline(BAPP), and the SPI(BAPP)-PDMS membrane was prepared and shown better chemical stability than the SPI membrane without PDMS segment. The SPI(BAPP)-PDMS membrane had the proton selectivity as 5 times high as Nafion 117 membrane. The coulomb efficiency and energy efficiency of VRFB with SPI( BAPP)-PDMS membrane are about 5% higher than those of Nafion 117 membrane. The open circuit voltage of SPI( BAPP)-PDMS membrane maintained above 1.3 V for about 550 h, while that of Nafion 117 kept only about 65 h. Besides, the VRFB with SPI(BAPP)-PDMS membrane had good cyclic stability.(2) 1,3,5-tris(4-nitrophenoxy)benzene(TNPOB) was synthesized from phloroglucinol and 4-chloronitro-benzene. Then TNPOB was reduced by Fe Cl3·6H2O and hydrazine hydrate, nitro groups were transferred to aminos completely, and 1,3,5-tris(4-aminophenoxy) benzene(TAPOB) was synthesized successfully.The ‘Y' shape branched sulfonated polyimide membranes(b SPIs) were synthesized and prepared from NTDA, BDSA, ODA and TAPOB by the polycondensation method. The degree of sulfonation was fixed as 50%. The content of TAPOB was changed to be 4%, 6%, 8%, 10% and 12%, and a series of branched proton conductive membranes named as b SPI-4, b SPI-6, b SPI-8, b SPI-10 and b SPI-12 were synthesized and prepared respectively. The structures of membranes were characterized by FT-IR and NMR. The morphologies of membranes were examined by SEM. The basic physico-chemical properties of membranes were determined such as water uptake, swelling ratio, ion exchange capacity, proton conductivity, vanadium ion permeability and chemical stability, etc. Results shown that the b SPI-8 membrane had high mechanical strength, good chemical stability and the highest proton selectivity. Therefore, the b SPI-8 membrane was selected as the optimum branched proton conductive membrane.To further explore the impact of the degree of sulfonation of the branched SPI membrane, the degree of branching was fixed as 8%, and the content of sulfonated diamine(BDSA) was changed to be 30%, 40%, 50%, 60% and 70%, and the b SPI-30, b SPI-40, b SPI-50, b SPI-60, b SPI-70 membranes were synthesized and prepared respectively. The structures of membranes were characterized by FT-IR and NMR. The morphologies of membranes were examined by SEM. The basic physico-chemical properties of membranes were determined such as water uptake, swelling ratio, ion exchange capacity, proton conductivity, vanadium ion permeability and oxidation stability, etc. Research results indicated that the b SPI membrane with the sulfonation degree of 50% had the highest proton selectivity.According to above-obtained results, the b SPI membrane with the degree of branching of 8% and the degree of sulfonation of 50% was determined as the optimum one, thus it was applied to VRFB system. When the current density was 50-120 m A cm-2, 800 charge-discharge cycles were conducted. The coulomb efficiency(about 97%) and energy efficiency(68-73%) of VRFB with the b SPI-8 membrane were higher than those of Nafion 117 membrane(coulomb efficiency: 95%, energy efficiency: 65-71%). Moreover, the VRFB with the b SPI-8(b SPI-50 namely) membrane had similar capacity retention to that with Nafion 117 at low current densities, and the VRFB with the b SPI-8(b SPI-50) membrane had slightly higher capacity retention to that with Nafion 117 at high current densities. These results demonstrated that the as-prepared b SPI-8(b SPI-50 namely) membrane had great potential in VRFB application. |
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