Study On Membrane Materials For Zn/4-HO-TEMPO Redox Flow Batteries

Redox flow battery is an electrochemical energy storage technology,suiting for large-scale energy storage.Zn/4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl free radical（4-HO-TEMPO）hybrid flow battery is one of the representatives of cost-efficient,low-toxic and high-voltage flow battery techniques,in which organic molecule 4-HO-TEMPO replaces traditional inorganic Br₂ as the redox active material for cathode.However,the current Zn/4-HO-TEMPO cells usually use relatively expensive perfluorosulfonic acid cation exchange membranes（CEM）.To reduce the cost of membranes,polyvinylidene fluoride（PVDF）porous membranes（PPM）were prepared by controlling the crystallization in this thesis.Comparing the performance of PPMS with CEMS,the membrane fouling phenomenon and the cell capacity decay mechanism were further investigated.Besides,a non-fluorinated ion-exchange membrane was prepared to achieve high energy efficiency and low capacity decay rate.In this thesis,PPMs were synthesized with controllable pore parameters method.PVDF powder and nucleating agent sodium allyl sulfonate（SAS）were dissolved in organic solvent dimethyl sulfoxide（DMSO）.The PPM was obtained by solution casting method,and the membrane pore size was controlled by adjusting the solvent evaporation temperature and time.With high evaporation temperature,PVDF spherical crystals fuse faster and the membranes are denser.At a typical evaporation temperature,with longer evaporation time,the spherical crystals keep fusing until the pores reduce to nanometer size.The diffusivity experiments and the conductivity results show that the smaller pores are the stronger barrier to 4-HO-TEMPO,leading to the lower diffusivity of 4-HO-TEMPO.Meanwhile,the smaller pores have influence on the conduction of other ions and the conductivity of the membrane is lower.Next,PPMs suitable for Zn/4-HO-TEMPO redox flow batteries were preferably selected,and the suitability of CEMs for Zn/4-HO-TEMPO cells was compared.The PPMs prepared in different conditions were applied to the Zn/4-HO-TEMPO cell.Considering the Coulombic efficiencies and voltage efficiencies of the cell,the best performance of the membrane with evaporation time of 375 min at 80 C was selected for the rate test and long-term cycling tests.The PPMs prepared in this thesis were applied to the Zn/4-HO-TEMPO cell.Compared with the cell using the CEM,the cell with the PPM exhibits comparable Coulombic efficiency,higher voltage efficiency,higher charge and discharge capacity at a current density of 20-100 m A cm^-2.In the long-term cycles,the PPM cell possesses more stable energy efficiency and voltage efficiency,while the CEM cell shows a rapid decrease in voltage efficiency and energy efficiency after 72cycles,but the PPM cell have a slightly lower capacity retention rate.On the other hand,this thesis observed the fouling phenomenon in the Zn/4-HO-TEMPO batteries with CEMs.The commercial ion-exchange membrane shows a voltage delay in the discharge process where the voltage dropped and then rose.The membrane fouling of CEMs is attributed to the variation of resistance during the charging-discharging process caused by 4-HO-TEMPO⁺,while PVDF porous membrane prepared in this thesis does not show membrane fouling phenomenon.4-HO-TEMPO and 4-HO-TEMPO⁺diffusivity experiments prove that diffusion ascribed to the decrease of the concentration of positive active material during the battery operation.This is the main reason for the decrease in the concentration of the positive active material during battery operation,which is the cause of the battery capacity decay.However,the capacity of cell with PPMs will rapidly decay.A non-fluorinated ion exchange membrane was further developed using for Zn/4-OH-TEMPO redox flow battery in this thesis to improve this problem.The effect of the dry or wet state of the membrane on the performance of the battery was investigated.The Coulombic efficiency of the battery using the dry membrane is higher than the wet membrane.The capacity retention rate is 35.3%and the decay rate is 0.37%h^-1 after 300 cycles,achieving the goal of long life and low capacity decay rate.It is also demonstrated that changing the charge cutoff voltage does not have a significant effect on the battery capacity decay.In addition,no fouling phenomenon was observed in non-fluorinated ion exchange membranes.This thesis suggests that the fouling of ion exchange membranes by organic molecules has an important relationship with the membrane material,and it is possible to overcome the membrane contamination problem in organic liquid streams through the regulation of the membrane material.