Electrochemical Impedance Spectroscopy Study Of PVDF Membranes In Different Environments

Membrane separation method is one of the most widely used water treatment methods because of its economy,efficiency and convenience.The development of separation membrane with higher retention performance and greater water flux requires a deeper understanding of its structure.Electrochemical impedance spectroscopy(EIS)is an in-situ non-destructive method which can provide more information of dynamic information and interface structure than other traditional methods,so it has been widely used in material research field.In this dissertation,polyvinylidene fluoride(PVDF)membrane and PVDF/graphene composite membranes were prepared by phase conversion method,and the electrochemical impedance spectroscopy were used to characterize the nanostructure of the membranes:(1)Improving the EIS detecting efficiency with optimizing the measuring parameters.A series of PVDF membranes with different porosity were prepared by changing the porogen content.With the increase of PEG content,the porosity gradually increased from 29.5% to 39%,and the membrane impedance decreased from 22235? to 1001?.The correlation between the characteristic frequency in the Bode diagram and porosity was successfully found through theoretical calculation and experimental data,it is found that the porosity of separation membranes can be rapidly evaluated by the characteristic frequencies,and the efficiency of electrochemical impedance spectroscopy(EIS)can be improved by optimizing the range of test frequencies.(2)Corona treatment is a kind of membrane surface modification method which can be used in industrial production.However,long corona treated time would lead to the aging of membrane materials themselves.The effect of corona treated time on the aging behavior of PVDF membranes was investigated by EIS.PVDF membranes were treated by using corona for different time under 5 k V.The EIS and XRD results shows that high-voltage treatment of PVDF will lead to part of the alpha phase transforming into beta phase at initial treatment,while longer corona treatment would lead to the destruction of the pore structure of PVDF membrane,in addition EIS can also detect the phase transition.(3)The stability of the pores in the PVDF/graphene composite membrane at different temperatures was evaluated by EIS and SEM.It was found that the presence of graphene improved the thermal stability of pore structure in PVDF/graphene composite membrane.The PVDF/graphene composite membrane with 1.5 % graphene content showed the best thermal conductivity and the best pore stability.This can be attributed to the excellent thermal conductivity and mechanical property of graphene.