Enhanced water retention and stable dynamic water behavior of sulfonated poly(ether ether ketone) membranes under low humidity by incorporating humidity responsive double-shelled hollow spheres

Many efforts have been devoted to maintaining the water content at a constant level in water retention proton exchange membranes, especially under a low humidity. It is necessary to develop a method for enhancing the water retention with a stable dynamic water behavior. Herein, humidity responsive double-shelled hollow spheres (DSHSs) were incorporated into a sulfonated poly(ether ether ketone) (SPEEK) matrix membrane to simultaneously enhance its water retention and improve its dynamic water behavior. The water retention of the DSHS incorporated SPEEK membrane was considerably increased compared to that of the SPEEK membrane due to the presence of a central cavity and gap between the inner and outer shell of the DSHSs. The humidity responsive DSHSs could rapidly adjust the water distribution within the SPEEK matrix and compensate the water evaporated under a low humidity, which is similar to the function of a vacuole in plant cells. As a result, the dynamic water behavior of the SPEEK/DSHS hybrid membranes was significantly improved. As a benefit from this, a continuous water network in the hybrid membranes could be maintained for a long time to enhance the proton conductivity of the hybrid membranes under a low humidity. A mechanism was proposed for the interpretation of these phenomena. The nanostructure of the hybrid membranes was studied by small-angle X-ray scattering (SAXS) and field emission scanning electron microscopy (FESEM).