In this research, new nanocomposite membranes were fabricated based on sulfonated poly(ether ether ketone)/ carbon quantum dot/graphitic carbon nitride nanosheet (SPEEK/CQD/g-C3N4). It was found that anchoring CQD onto g-C3N4 provides a new proton transport pathway in the design of proton exchange membranes for highperformance fuel cell applications. The uniform distribution of CQDs on the nanosheets and the interaction of hydrophilic groups such as the –NH2 and C–– O with the -SO3H of the polymer matrix creates a hydrogen bond transfer network and facilitates the protons transfer based on the Grotthuss mechanism. The incorporation of CQD/g-C3N4 in the polymer matrix has increased the proton conduction from 0.023 S/cm for SPEEK at 20 ◦C and 100 % R.H. to 0.052 S/cm for the SPEEK/CQD/g-C3N4-3 %wt. The proton conduction for the nanocomposite membrane containing 3 wt% CQD/g-C3N4 improved at higher temperatures (0.128 S/cm at 80 ◦C) and also, this membrane showed better thermal properties (maximum Tg at 176 ◦C). Compared to the SPEEK membrane, the nanocomposite membrane with 3 wt% CQD/g-C3N4 had a better single-cell direct methanol fuel cell (DMFC) test performance.

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