End-group cross-linked sulfonated poly (arylene ether sulfone) (C-SPAES) membranes with varying degree of cross-linking density are prepared by in-situ casting and heating of thiol-terminated SPAES (SH-SPAES) solutions containing different amounts of multifunctional cross-linker such as acrylo-polyhedral oligomeric silsesquioxane (A-POSS) via thiol-ene click reaction. SH-SPAES is prepared by stepwise substitution reactions between phenoxide-terminated SPAES and end capping reagents such as decafluorobiphenyl followed by thiobisbenzethiol. The C-SPAES membranes exhibit vastly improved chemical and physical stability and mechanical strength compared with linear SPAES membrane due to effective cross-linked structure, and their proton conductivity values are higher than Nafion-212 in relative humidity range of 50-95% (at 90 degrees C) due to high degree of sulfonation (70 mol%) and ion exchange capacity (2.48 mequiv. g(-1)) of SH-SPAES. Under various operating conditions of proton exchange membrane fuel cell including intermediate temperature and low humidity conditions, membrane electrode assemblies employing the C-SPAES membranes with optimized cross-linker contents show outstanding cell performances (e.g., 1.08 W cm(-2) at 0.5 V under 90 degrees C and 50% RH with 150 kPa).