Nanocomposite membranes based on sulfonated poly (ether ether ketone) (SPEEK) and sulfonated core-shell TiO2 nanoparticles were prepared. TiO2 nanoparticles were sulfonated by redox polymerization method by using sodium styrene sulfonate (SSA) and 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) monomers. The resultant hybrid nanoparticles (PAMPS-gTiO(2) and PSSA-g-TiO2) were introduced to SPEEK with a sulfonation degree of 68%. Grafting of sulfonated polymers onto TiO2 nanoparticles enhanced the content of proton transport sites in the membrane, leading to an increase in proton conductivity and power density. Besides, the mechanical and dimensional stabilities of the nanocomposite membranes were also improved compared with pure SPEEK membrane. The maximum power density for membranes containing 7.5 wt% of PAMPS-gTiO(2) and PSSA-g-TiO2 nanoparticles at 80 degrees C obtained 283 mW cm(-2) and 245 mW cm(-2), respectively. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.