Proton conductivity improvement of sulfonated poly(ether ether ketone) nanocomposite membranes with sulfonated halloysite nanotubes prepared via dopamine-initiated atom transfer radical polymerization

To achieve high proton conductivity, the nanocomposite proton exchange membranes (PEMs) require a good dispersion of inorganic nano-fillers that bear proton transport facilitating functional groups. In this work, we demonstrate that a facile approach based on dopamine-initiated atom transfer radical polymerization (ATRP) can be employed to prepare sulfonated halloysite nanotubes (SHNTs). Polystyrene sulfonic acid (PSSA) was successfully grafted on both inner and outer surfaces of nanotubes, which was confirmed by the transmission electron microscopy analysis. The kinetics of PSSA grafting reaction reflected the living nature of ATRP process. The presence of sulfonic acid groups on the surface of SHNTs significantly improved the compatibility and interfacial adhesion between the nano-fillers and sulfonated poly(ether ether ketone) (SPEEK) matrix, resulting in the uniform and void-free morphology of the nanocomposite membranes. Adding SHNTs into SPEEK matrix also enhanced the phase separation which enabled to form larger hydrophilic domains and improved the proton transport. In addition, the grafted PSSA also provided additional pathways for proton conduction both inside the lumens and along the outer surfaces of nanotubes. Therefore, the enhancement in proton conductivity and the reduction in the activation energy for proton transport were achieved for the SPEEK/SHNT nanocomposite membranes. Compared to that of SPEEK control membrane, the proton conductivity of SPEEK/SHNT membrane with 15 wt% SHNTs was 54% higher at fully hydrated state and more than 2 orders of magnitude higher at 47% and 59% relative humidity (RH).