Research On Rational Use And Structure-Performance Relationship Of Nafion Membrane For Vanadium Redox Flow Batteries

All vanadium flow battery?VFB?become the most promising large-scale energy storage technology because of its design flexibility,long cycle life and unlimited capacity.Nafion membranes are now the most widely used ion exchange membrane for vanadium flow battery owing to the high chemical stability.Nowadays,what type of Nafion membranes should be selected and how to pretreat these Nafion membranes have become critical issues,which directly affect the performance and cost of VFB.In this paper,the commercial Nafion series membrane was studied,and the relationship between the macroscopic morphology,microstructure and battery performance of Nafion membrane was scientifically related,which clarified the use strategy of Nafion membrane in VFB.Firstly,a batch of commercial Nafion membranes with thicknesses of 50?m?Nafion112?,88?m?Nafion 1135?,125?m?Nafion 115?and 175?m?Nafion 117?are selected to study the effect of Nafion membrane structure on the VFB performance.The self-discharge process,battery efficiency,electrolyte utilization and long-cycle cycling stability of VFB cells based on various Nafion membranes are compared comprehensively.Owing to the good balance between vanadium ions permeability and membrane resistance,Nafion 115 membrane presents the highest energy efficiency and electrolytes utilization at current density from 120 to 240 mA cm^-2.Secondly,we select the Nafion 115 membrane to study the effect of the pretreatment process?As received,Wet,Boiled and Boiled&Dried?on the performance of the VFB.The microstructures and proton transfer properties of Nafion films with different pretreatment are analyzed,and the self-discharge process,battery efficiency,electrolyte utilization and long-term cycle stability were compared.The battery capacity decay mechanism and the electrolyte imbalance process of various pretreatment Nafion 115membranes are also discussed.The Boiled Nafion 115 membrane with small area resistance is suitable for the benchmark membrane.The Wet treatment is simple operation and the VFB with Wet Nafion 115 membrane has high battery efficiency,good battery cycle stability.The Wet pretreatment processes for Nafion 115 membrane more suitable for the actual use of vanadium flow battery.The last,we established a wide temperature?-20-50 ^oC?ressarch platform for VFB single cell evaluation.A series of typical cation exchange membranes and anion exchange membrane are studied in terms of their VFB efficiencies,long-term cycling stability and capacity fading mechanism in a temperature range of-20-50 ^oC.In addition,the ideal ion exchange membranes for all vanadium flow cells under different temperature conditions are discussed and presented.The Nafion 115 membrane has a slower capacity decay in the long period of wide temperature battery test.Considering battery efficiency and capacity retention.Nafion 115 is suitable for use at room temperature and short cycle at high temperature,while Nafion 212 is more suitable for use at low temperatures.In summary,the macrostructure and microstructure of Nafion 115 is appropriate,Wet pretreatment is simple;the Wet Nafion 115 membrane of VRB has great battery stability and slow capacity decay at wide temperature which is the best choice for the all-climate vanadium flow battery.