Nafion, a perfluorosulfonic acid (PFSA) polymer, is a vital electrolyte that extensively contributes to the commercialization of polymer electrolyte fuel cells (PEFCs). However, adopting Nafion in high-temperature (HT) PEFCs (HT-PEFCs) is still a great challenge. Herein, we present a potential additive (carbon nitride-gadolinium zirconium oxide (gC(3)N(4)-Gd2Zr2O7)) to simultaneously extend the power density and lifetime of Nafion in HT-PEFCs. Owing to the porous nature, radical-scavenging ability, and thermal stability of gC(3)N(4)-Gd2Zr2O7, the Nafion/gC(3)N(4)-Gd2Zr2O7 membrane exhibited high water uptake, oxidative stability, and thermomechanical stability compared to the unmodified Nafion membrane. At 100 degrees C under 30% RH, the maximum power density of Nafion/gC(3)N(4)-Gd2Zr2O7 was 504 mW cm(-2) at a load current density of 804 mA cm(-2), which is 1.9 and 2.3 fold higher than that of commercial Nafion-212 and pristine Nafion membranes. Moreover, Nafion/gC(3)N(4)-Gd2Zr2O7 displays an open-circuit voltage (OCV) decay of 0.1 mV h(-1) during 515 h of continuous operation, which is significantly lower than that of the Nafion membrane during the 300 h durability operation (2.8 mV h(-1)). When increasing the operating temperature to 125 degrees C under 15% RH, the Nafion/gC(3)N(4)-Gd2Zr2O7 based PEFC largely outperforms the Nafion-212 and pristine Nafion-based PEFCs, achieving an output of 380 mW cm(-2), one of the most competitive HT-PEFC power densities reported for Nafion-based membranes. Post durability tests further ensure the superior stability of Nafion/gC(3)N(4)-Gd2Zr2O7 during HT-PEFC operation. The present study provides deep insight into developing and understanding of advanced Nafion composite membranes applicable for HT-PEFCs.