The Investigation On The Oxidation Stability Of The Non-fluorinated Membrane In Vanadium Flow Battery

The membrane is a key material for vanadium flow batteries(VFBs).Its cost,reliability and properties directly affect the overall performance of VFBs.At present,the poor ion selectivity and high price of the widely used perfluorosulfonic acid ion exchange membranes have limited the industrialization of VFBs to a certain extent.Non-fluorine ion exchange membranes have the advantages of high selectivity and low cost.However,they have poor oxidation stability in the VFB system.In this thesis,the degradation mechanism of non-fluorine ion exchange membranes in VFBs was investigated.A porous membrane with high selectivity,high ion conductivity and low cost was prepared via tailored structural design.The main conclusions in this paper are as follows:(1)The degradation products of sulfonated polyetheretherketone(SPEEK,a model compound)cation membrane was investigated using ex-situ and in-situ methods in VFBs.The structures of the degradation products were confirmed,and the degradation mechanism of this cation exchange membrane was clarified.(2)In the non-fluorine ion-exchange membrane,the introduction of ion-exchange group changes the distribution of electron cloud of polymer backbone,leading to its oxidative degradation.In addition,protonation will also accelerate the oxidative degradation of the membrane.Therefore,avoiding the introduction of protonated elements is expected to improve its oxidation stability.(3)The novel blended porous membrane with the core-shell structure composed of PVDF-HFP/PVP is designed and prepared.The membrane processes high ionic conductivity due to its porous structure and the easily protonated amine groups which facilitate the conduction of carriers in the membrane.The small pore size distribution and the repulsive effect of anion-exchange groups on cation ions can effectively prevent the interpenetration of vanadium ions and endow the membrane high ion selectivity.Compared with the commercial Nafion115 membrane,the VBFs with the PVDF-HFP/PVP blended membrane treated with VO₂⁺ demonstrate higher efficiencies at the current density of 80 mA cm^-2.This work may further promote commercialization of VFBs.