Sulfonated Polyimide Based Proton Conductive Membranes For All-Vanadium Redox Flow Battery Application

All vanadium redox flow battery(VRFB) is a new type of green secondary battery. Compared with other redox flow batteries, VRFB has many advantages such as adjustable capacity and power, nondestructive deep discharge with high current, easy to operate and maintenance, long cycle life, etc. Therefore, VRFB can be applied to load power supply system, large scale photovoltaic translating system, emergency power sources system, electric vehicle power supply, and so on.However, the large-scale commercial development of VRFB has been largely limited owing to the membrane materials. Up to now, 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 limit their large scale application. 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 good proton conductivity, good membrane forming property, excellent thermal stability and reasonable price, thus it is expected to replace the Nafion membrane and applied to VRFB. However, non designed pristine SPI membrane is easy to be oxidatively degraded in VRFB since the anode electrolyte has strong oxidability. The long-term stability of these pure SPI membranes is severely challenged.This thesis is aimed to fabricate a series of new type of oxidation resistant SPI/inorganic composite proton conductive membranes and well-designed pure SPI membranes for VRFB. As-prepared membranes show excellent vanadium ion resistance, good oxidative stability and reasonable proton conductivity, which is expected to provide some theoretical and technical support for the development of novel non-fluorinated membranes for VRFB application. Followings are the main contents in this work:(1) SPI was synthesized from 1,4,5,8-naphthalenetetracarboxylic dianhydride(NTDA), 4,4’-diamino-biphenyl 2,2’-disulphonic acid(BDSA) and 4,4’-oxydianiline(ODA) by polycondensation method. The sulfonated polyimide/inorganic composite membranes were prepared by introduction of Ti O2, Al OOH, Zr O2, s-Mo S2 into SPI membrane. The morphology of the composite membranes were examined by atomic force microscopy(AFM) and/or scanning electron microscopy(SEM); The micro-structure of composite membranes also were characterized by X-ray diffraction(XRD); The basic physico-chemical properties of composite membranes were determined such as water uptake, swelling ratio, ion exchange capacity, proton conductivity, vanadium ion permeability and oxidation stability, etc. The best addition amount of inorganic components including Ti O2, Al OOH, Zr O2 and s-Mo S2 is 5 wt %, 10 wt %, 15 wt % and 5 wt % respectively. Compared with the SPI membrane, the oxidative stability of the SPI/inorganic composite membranes is improved to some extent. The coulomb efficiency and the energy efficiency of VRFB with SPI/inorganic composite membranes are higher than that of Nafion 117 membrane. The open circuit voltages of SPI/Ti O2, SPI/Al OOH and SPI/s-Mo S2 membrane maintain at more than 0.8 V for about 160 h, 117 h and 193 h respectively, while that of Nafion 117 keeps only about 72 h. Besides, the VRFB with SPI/inorganic composite membranes have good cycle stability.(2) In order to improve the oxidative stability of SPI membranes, a series of SPI membranes with various non-sulfonated diamines were prepared from NTDA, BDSA and different non-sulfonated diamine monomers including 2-(4-aminophenyl)-5-aminobenzimidazole(APABI), 2,2-bis[4-(4-aminophenoxy) phenyl]propane(BAPP) and 4, 4-diaminodiphenylmethane(MDA). The research results show that: the BAPP-based SPI membrane has good oxidation stability; The SPI(BAPP) and Nafion 117 membranes in VRFB was also measured by carrying out 500 cycles with charge-discharge current densities ranged from 25 to 70 m A cm-2, the coulomb efficiency of the VRFB with SPI(BAPP) remains about 97%, which is always higher than that with Nafion 117(about 95%). At the same time, the energy efficiency of VRFB with SPI(BAPP)(about 68- 73%) is higher than that with Nafion 117(about 65- 71%) at all current densities. In addition, the comprehensive performance of VRFB with BAPP-based SPI membrane is better than other SPI membranes.