Nanocomposite membranes are synthesized by the combination of sulfonated poly(ether ether ketone) (SPEEK), poly(ethylene glycol) (PEG), and 3-aminopropyl triethoxysilane (APTEOS)-modified CaTiO3. From thermogravimetric analysis (TGA) and mechanical and Arrhenius studies, it is seen that the covalent cross-linking of PEG with SPEEK exhibits a lower weight loss (91.84 wt %) at 120 degrees C than SPEEK (88.19 wt %). Moreover, higher activation energy (E-a) and tensile stress values are obtained in the order of 26.23 kJ/mol and 23.32 MPa, respectively. The APTEOS-modified CaTiO3 nanoparticles impregnated into the SPEEK/PEG membrane cause the acid-base proton hopping mechanism (HSO3/NH2) and induce a more compact structure. The nanocomposite membranes possess exceptional proton conductivity, thermal, oxidative, and physicochemical properties than SPEEK. Further, 6 wt % functionalized CaTiO3 in the SPEEK/PEG membrane manifests the highest values of proton conductivity, current density, and power density at 80 degrees C in 100% RH of 65.32 mS cm(-1), 403 mA cm(-2), and 90 mW cm(-2) with an open-circuit voltage (OCV) of 0.90 V, respectively. This membrane retains 95.37 wt % original weight during 3 h of the Fenton test.