Modification And Theoretical Study On Polypropylene Membranes For All Vanadium Redox Flow Battery Applications

The vanadium redox flow batteries?VRFBs?as one of the most ideal choices for large-scale energy storage devices have been developed rapidly in recent years.VRFBs have shown a wide range of applications,including,but not limit to,power quality optimization,auxiliary storage for renewable energy sources?solar,wind,and etc.?,and backup power.The main reasons is that they have high efficiency,long cycle life,reasonable cost and other characteristics.Global demand for VRBs is increasing year by year,but its commercial applications have been limited due to the high cost of the key materials,especially the ions exchange membranes?IEMs?.Compared with other membrane materials,polypropylene membrane has obvious cost and industrial advantages because it has a complete generation technology in China and already widely used in lithium batteries.This paper focuses on the experimental and theoretical studies on the preparation of polypropylene membranes with suitable low cost and high performance for the VRFBs applications.The main contents are as follows:For the first time,the polypropylene/silicon dioxide?PP/Si O₂?nanofiltration membranes were successfully prepared by using the in situ hydrolysis method.PP/SiO₂nanofiltration membrane shows good ion selectivity and good cycle performance in vanadium redox flow battery compared with Nafion115 membrane.The battery assembled with PP/SiO₂ nanofiltration membrane exhibit good columbic efficiency?CE?and energy efficiency?EE?under 50 mA/cm² current density.After 40 cycles,the CE and EE maintain above 94.5%and 75.4%,respectively.The cost of PP/SiO₂ nanofiltration membrane is only1/20 of Nafion115 membrane and its preparation process is very environmental-friendly.This membrane shows highly commercial application prospects for the vanadium redox flow batteries.Compared the C-H bond breaking energies of polypropylene membrane and polypropylene ion exchange membranes using density functional theory calculation.The calculation results show that the introduction of anion exchange membrane group is expected to strengthen the stability of polypropylene membrane in VRFB.For the first time a three dimensional?3D?volume calculation model based on the total electrostatic potential?ESP?analysis and DFT calculations is applied to study the volume differences between hydrated multivalent vanadium ions(V²⁺,V³⁺,VO²⁺and VO₂⁺)and charge-balancing ions(H₃O⁺,SO₄^2-and HSO₄^-)encountered in the electrolyte solutions of vanadium redox flow batteries?VRB?.The calculated results indicate that radii of all charge-balancing ions are less than3.01?and of all hydrated multivalent vanadium ions are greater than 3.78?.The results of our calculations also suggest that polypropylene cation exchange membranes with pore sizes ranging from 3.98 to 7.56?and polypropylene anion exchange membranes with pore sizes ranging from 6.02 to 7.56?are suitable for the VRB application.These computational results agree very well with reported experimental results and provide valuable guidance for the selection and design of membranes on the molecular level for VRB applications.