Nanocomposite Proton Exchange Membranes Based On Lacunary Silicotungstic Acid

Proton exchange membrane is one of the core components of proton exchange membrane water electrolysis and proton exchange membrane fuel cells.It mainly plays the role of separating the cathode and anode and conducting protons.Perfluorosulfonic acid membrane is the most widely used proton exchange membrane with excellent chemical stability and proton conductivity,but its high price and sharp decline in performance under high temperature and low humidity conditions hinder its further development.Fluorine-free proton exchange membrane has the advantages of low price and excellent mechanical properties,so it can be used as a substitute for perfluorosulfonic acid membrane;however,its proton conductivity is usually low,so it is necessary to modify the membrane to improve its performance.Silicotungstic acid(HSiW)is a Keggin-type heteropoly acid,which can be used to modify the proton exchange membrane and improve the proton conductivity due to its excellent proton ionization ability;however,HSiW is easily dissolved in water and will be lost when incorporated into the membrane,resulting in unstable properties of the composite membrane.In order to solve the problem of the loss of HSiW in the composite membrane,this paper firstly prepared lacunary silicotungstic acid(SiW11)by controlling the reaction conditions.Compared with HSiW,SiW11 lacks a WO6 octahedral lattice,and the solubility of SiW11 in water is greatly reduced.The SPEEK/SiW11 composite proton exchange membrane was prepared by mixing SiW11 into sulfonated poly(ether ether ketone)(SPEEK).The proton conductivity of SPEEK/SiW11-20%composite membrane was 1.66 times as much as the pure SPEEK membrane.The SiW11 in the composite membrane no longer lost after 4 days in water.The total loss was less than 3.7%of the total amount added.During the five-month test,the proton conductivity of the composite membrane remained basically unchanged,proving that the composite membrane has good stability.In order to solve the problem of a small amount of SiW11 loss in composite membranes,this paper took advantage of the fact that dopamine can self-polymerize in weakly alkaline aqueous solution,coated polydopamine on the surface of the Halloysite nanotubes to obtain DHNTs,which were mixed with SiW11 and SPEEK to form membranes.There were basic amino groups on the surface of DHNTs,which can form acid-base pair interaction with SiW11 and immobilize SiW11.Meanwhile,one-dimensional DHNTs reduced the curvature of proton transport,formed continuous proton conduction channels,and improving the proton conduction capability;simultaneously,the proton conductivity of the SPEEK/DHNTs/SiW11(9/1/4)composite membrane was 1.66 times as much as the pure SPEEK membrane.In order to further improve the proton conductivity,covalent organic frameworks(COFs)with sulfonic acid and amino groups were bonded with DHNTs by hydrothermal method to obtain DHNTs@COF,which was added to SPEEK.The sulfonic acid groups on the surface of COF can effectively improve the proton conductivity of the composite membranes,meanwhile the proton conductivity of SPEEK/DHNTs@COF-2%composite membrane was 1.4 times as much as the pure SPEEK membrane.The addition of SiW11 on the basis of the composite membrane,with a large number of basic imino groups on DHNTs@COF,facilitated the immobilization and dispersion of SiW11,while the proton conductivity of the composite membrane was further improved,in which the proton conductivity of the SPEEK/DHNTs@COF/SiW11(24/1/12)composite membrane is 1.85 times as much as the pure SPEEK membrane.