The crosslinked highly sulfonated polyphenylsulfone (SPPSU) membranes that consists of carbon nanodots (CNDs) was prepared as a proton exchange membrane for fuel cell applications. The crosslinked membranes were developed by annealing at 180 degrees C, in which the crosslinking occurred between the SPPSU and CNDs. The CNDs potential was explored in detail under various loadings (0 wt %, 1 wt %, 2 wt %, and 3 wt %). Upon annealing at 180 degrees C, the flexibilities and strength of the SPPSU-CNDs membranes improved. The proton conductivity of the crosslinked membrane was enhanced than that of pristine SPPSU membrane due to the crosslinking effect between SPPSU and CNDs. The highest conductivity, which was at 56.3 mS/cm was obtained when 3 wt % of CNDs was incorporated at 80 degrees C and 90% relative humidity (RH). The results indicated that the incorporation of CNDs in the SPPSU membrane by annealing at 180 degrees C, exhibited a proton conductive membrane in combination with superior dimensional stability, and proton conductivity suitable for fuel cell applications. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.