Nanocomposite Proton Exchange Membranes Incorporating Phosphotungstic Acid Anchored on Imidazole-Functionalized Halloysite Nanotubes

In this work, sulfonated poly(ether ether ketone) (SPEEK) nanocomposite membranes were prepared with high loading of phosphotungstic acid (HPW) anchored on imidazole-functionalized halloysite nanotubes (mHNTs), which were synthesized via bio-inspired dopamine self-polymerization and dopamine-initiated atom transfer radical polymerization (ATRP) process. The SPEEK/mHNTs/HPW nanocomposite membranes demonstrated dense and uniform morphology where both mHNTs and HPW were well dispersed. Due to the strong acid-base interactions between HPW and imidazole groups of mHNTs, HPW was successfully immobilized on the surfaces of high aspect-ratio mHNTs, resulting in more active sites and shorter distance for proton hopping. Membrane proton conductivity was greatly improved and the activation energy for the proton transfer was significantly reduced. The proton conductivity of the SPEEK/mHNTs/HPW nanocomposite membrane with 42.9 wt% HPW was 82% higher than that of SPEEK control membrane, and remained almost constant for more than 1000 h under immersion in liquid water.