| With fossil supplies increasingly exhausted and the environment highly deteriorated by their consumption, renewable energy sources such as solar energy and wind energy have been developed. However, these energies have instability and discontinuous interrupt due to the climate change. So the energy conservation technologies have drawn great attention. Compared to the other energy conservation technologies, all vanadium redox flow battery (VRB) has been accepted as a very promising energy conversion device due to its green safety, fast response, deep charge-discharge ability, easily designed capacity, high energy efficiency, low cost, and no limit in geographical position. As a key component in VRB, the separator is vital to restrain the VRB's performance and its large-scale application. In this paper, the research work was focused on the preparation and properties of the membranes with high ionic conductivity and selectivity, long life, and low price. The main points in this research can be summarized as follows.The area resistance (AR) for several commercial ionic exchange membranes (IEM) was determined by electrochemical impedance spectroscopy (EIS). The results showed Nafion membrane had the least AR value in all samples, in the domestic membranes AR values of heterogeneous membranes were markedly higher than homogeneous ones, and AR values of cationic exchange membranes (CEM) were lower than that of anionic exchange membranes (AEM). The AEM had a significant reduction in crossover of vanadium ions compared with the CEM due to the Donnan effect of AEM. Correspondingly, the AEM had a lower water transfer than the CEM. Cell test indicated that Nafion exhibited a superior charge-discharge performance and a higher capacity. For the domestic membranes, the VRB with the AEM showed a better charge-discharge performance and higher capacity than that of CEM at the lower current density, and at higher current density the cell with CEM exhibited a superior charge-discharge performance to that of AEM. The chemical stability tests showed the Poly(aryl ether)s AEM (DF-a) possessed more chemical stability and was more suitable for VRB than the other domestic IEM.A Nafion/TiO2hybrid membrane was fabricated by a hydrothermal method. The primary properties of the hybrid membrane were measured and compared with the Nafion membrane. The results of Scanning Electron Microscope (SEM), Energy Dispersive X-Ray Spectroscopy (EDS) and X-Ray Diffraction (XRD) of the hybrid membrane revealed that the TiO2phase was formed in the bulk of the prepared membrane. Thermo Gravimetric Analysis (TG) showed that hydrothermal modification had almost no effect on the thermal property of the hybrid membrane. The Nafion/TiO2hybrid membrane had a dramatic reduction in crossover of vanadium ions compared with the Nafion membrane (6.72×10-6vs.2.26×10-5cm2·min-1). The columbic efficiency (CE) and energy efficiency EE of the VRB with the hybrid membrane were88.8%and71.5%at60mA·cm-2, respectively, while those of the VRB with Nafion membrane were86.3%and69.7%at the same current density.Sulfonated poly(phthalazinone ether sulfone)(SPPES) suitable for ionic exchange membrane fabrication was prepared by sulfonating PPES with fuming sulfuric acid at40℃for4-6h. By testing the sulfonation degree (SD), intrinsic viscosity and solubility of SPPES, the results showed that sulfonated polymers had higher intrinsic viscosities and excellent solubility in most polar solvents. IR analysis revealed that the-SO3H group was successfully attached to SPPES backbone. DSC and TG results showed that SPPES exhibited higher Tg than that of PPES, and Td at the first weight loss of SPPES was about300℃. The SPPES membrane (SP-02) showed a dramatic reduction in crossover of vanadium ions across the membrane compared with that of the Nafion membrane (1.24×10-7vs.22.63×10-7cm2·min-1). Cell tests identified that VRB with the SPPES membrane exhibited a lower self-discharge rate, higher CE (92.82%) and EE (67.58%) compared with the Nafion system. Furthermore, cycling tests indicated the single cell with SPPES membrane exhibited a stable performance during100cycles.A SPPEK/WO3membrane was fabricated by a hydrothermal method to improve its performance in VRFB. SEM of the composite membrane revealed that WO3phase was well formed in the bulk of the SPPEK membrane. Compared with Nafion membrane, the hybrid membrane showed a dramatic reduction in crossover of vanadium ions across the membrane and a higher selectivity (5.81×104vs.0.22x104minScm-3). Cell tests identified that the VRB with the SPPEK/WO3hybrid membrane presented a higher coulombic efficiency (98.07%vs.92.81%) and energy efficiency (78.60%vs.76.19%) compared with the Nafion system. Cycling and chemical stability tests indicated that the hybrid membrane had enough stability to be applied in VRB system.A SPPEK/TPA composite membrane was prepared by blending SPPEK and tungstophosphoric acid (TPA). SEM and XRD showed that TPA had excellent compatibility with SPPEK in the homogeneous membrane. With TPA content increasing, the composite membrane exhibited lower IEC, greater water uptake, lower swelling ratio, unchangeable tensile strength, higher ionic conductivity, lower selectivity, and lower permeability of vanadium ions which was still superior to Nafion membrane. In terms of vanadium ion valence, from quintavalent to trivalence, the permeability of the vanadium ion for all membrane samples increased correspondingly, especially for Nafion membrane. The static and flow cell tests showed SPPEK-TPA-17membrane exhibited a lower self-discharge rate, and higher CE and EE. Cycling and chemical stability tests indicated that the SPPEK/TPA composite membrane had enough stability to be applied in VRB system.
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