High-performance Coaxial Electrospinning Of CNT Hybrid SPEEK Fiber Membrane For Vanadium Flow Battery

The membrane is the core component of the vanadium redox flow battery.It plays a role in transferring protons and blocking vanadium ions during battery operation,which determines the energy efficiency and long-term stability of the battery.Maintaining high energy efficiency and reducing capacity decay are the huge challenges faced by vanadium redox flow batteries.It requires the membrane to have both high proton conductivity and low vanadium permeability.Compared with the commercial perfluorosulfonic acid membrane,non-fluorinated poly?ether ether ketone?membrane?SPEEK?has the characteristics of high vanadium resistance,easy modification,and low cost.In the earlier of this group,the uniaxial electrospinning method was proposed to achieve higher energy efficiency by constructing a long-range ordered proton pathway.However,due to the influence of swelling ratio,the capacity retention of long-term cycles still needs to be improved.This paper proposes a method for fiberizing organic-inorganic coaxial electrospinning.The degree of low sulfonation anti-swelling SPEEK is placed in the core layer of fiber,and the degree of high sulfonation with high-proton conductivity SPEEK is placed in the shell layer of fiber;Introducing carboxylated multi-walled carbon nanotubes?MWCNT?into the fiber to form a hydrogen bond network structure with sulfonate groups,to transfer protons and block vanadium ions,improve the protons selectivity of the membrane,improve energy efficiency and reduce capacity deacy.First,coaxial electrospinning method is proposed to reduce the swelling ratio while constructing nanofiber network channels.When the SPEEK electrospinning solution with sulfonation degree of 54%is used in both the inner and outer layer of fibers,the swelling ratio of the coaxial electrospun membrane?Co-spun?is reduced by 41.3%compared to the uniaxial electrospun membrane?Uni-spun?,and the proton conductivity increased by 13.9%,and area resistance decreased by 10.8%.At current density of 100 mA cm^-2,the energy efficiency?83.7%?of the coaxial electrospun membrane is higher than that of the uniaxial electrospun membrane?83.0%?.Then,using the designability of the coaxial electrospinning method,the SPEEK material with degree of low sulfonation is placed in the core layer of the fiber to form anti-swelling support;the SPEEK material with degree of high sulfonation is placed in the shell layer of the fiber to form efficient proton pathway.In the long-term cycles test,the membrane with degree of sulfonation of SPEEK in shell layer of 65%has the best battery performance,and the energy efficiency exceeds 86%.Finally,molecular simulations show that the MWCNT and sulfonic acid groups form a hydrogen bond to improve the selectivity of the membrane for protons.The doping position of MWCNT in the coaxial fiber was optimized,and the basic performance and battery performance of MWCNT placed in the core layer and shell layer were compared.In the long cycle test,the SPEEK sulfonation degree of the shell is 65%,and the MWCNT has the best battery performance when placed in the fiber shell layer?energy efficiency:87.5%,capacity attenuation:0.34%/cycle?,better than Nafion115 membrane with the best performance of commercial batteries?energy efficiency:83.4%,capacity attenuation:0.37%/cycle?.