Sulfonated Polyimide Based Proton Exchange Membranes For All-vanadium Redox Flow Battery Application

All vanadium redox flow battery (VRB) is a novel environmentfriendly battery. Compared with other redox flow batteries, VRB hasmany advantages such as high energy efficiency, deep dischargecapacity, fast response time, long cycle life, low cost and low pollution.Therefore, it has a wide range of applications such as solar/windturbine generator, re mote area power syste ms, e mergency back-upusage and uninterruptable power sources (UPS), load leveling at powerplants, and so on.However, the large-scale commercial development of VRB hasbeen largely limited owing to the membrane materials. Up to now,Nafion me mbranes (Dupont Co., USA) have been widely used in theVRB system owing to their high proton conductivities and excellentche mical stabilities. Nevertheless, Nafion me mbranes also reveal suc hdisadvantages as high cost and high permeability of vanadium ions.Therefore, it is necessary to develop novel proton exchange me mbraneswith good cell perfor mance and low cost.A series of sulfonated polyi mide (SPI) proton exchangeme mbranes with different sulfonation degrees and sulfonatedpolyimide/chitosan (SPI/CS) co mposite me mbranes were prepared inthis work, which have low per meability of vanadium ion, reasonableproton conductivity and low cost. It’s worth noting that the single cellwith SPI/CS comp osite me mbrane shows excellent comprehensiveperformance. Thus, SPI based me mb ranes are very promising for VRBapplication. Followings are the main contents in this work:(1) A series of sulfonated polyimides were synthesized inm-cresol from4,4’-oxydianiline (ODA),4,4’-diamino-biphenyl2,2’-disulphonic acid (BDSA) and1,4,5,8-naphthalenetetracarboxylicdianhydride (NTDA) in the presence of triethyla mine and benzoic acid.And the sulfonation degree (SD) was controlled through the ratio ofBDSA to ODA. The effects of several factors on the yield wereinvestigated and optimized in detail. The solubilities of SPI-NEt3were investigated, and results shown that they are dissolved completely inm-cresol.(2) A series of SPI me mbranes with different SDs were prepared bycasting method. The effects of membrane preparation process werestudied and opti mized. And the physico-che mical properties of SPIme mbranes were investigated such as water uptake, ion exchangecapacity, proton conductivity and vanadium ion permeability, and so on.Co mpared with Nafion117me mbrane, SPI membranes have excellentvanadium ion barrier property. And the proton conductivities of SPI-40，SPI-50， SPI-60and SPI-80me mbranes are in the order of×10-2S cm-1,which are quite potential for VRB application. However, the weightloss of SPI me mbranes in V(Ⅴ) solution is higher than Nafion117me mbrane.(3) In order to promote the oxidative stability of SPI me mbranes,the novel sulfonated polyimide/chitosan (SPI/CS) compositeme mbranes were prepared through an immers ion and self-assemblymethod. The effect of the immersion ti me with chitosan solution onphysico-che mical properties of the SPI/CS membranes was studied.Co mpared with the SPI-50me mbrane, the oxidative stability, protonconductivity and vanadi um ion barrier property of the SPI/CSme mbranes are improved significantl y. And the performance of VRBsingle cell with SPI/CS and Nafion117me mbranes was measured andcompared with each other. The re sults show that both the coul ombefficiency and the energy efficiency for all SPI/CS compo siteme mbranes are higher than those for Na fion117me mbrane. Especially,the coulomb efficiency, the voltage efficiency and the energy efficiencyfor SPI/CS-24me mbrane achieve as high as97.8%,90.6%and88.6%,respectively.