Triboelectric nanogenerators (TENGs) are one of the most promising energy sources for self-powered electronic devices in the near future. Improving the dielectrics with high tribo-potential is a primary requirement to increase the output performance of TENGs. In this study, spring supported TENGs consisting of polyvinylpyrrolidone/ethyl-cellulose (PVP/EC) nanofibers and various carbon-doped polyacrylonitrile (PAN) nanofibers as positive and negative dielectric layers, respectively, were fabricated. According to the experimental results, reduced graphene oxide (rGO) and carbon nanotube (CNT) which were grafted to PAN matrix, both increased surface charge density and enhanced the output voltage of the TENGs. On the other hand, carbon black (CB) reduced the tribo-potential of PAN as a negative dielectric layer. As the best result, a 40 × 40 mm2 TENG constructed of PVP/EC and 3 wt% CNT doped PAN nanofibers demonstrates high triboelectric characteristics with a charge capacity of 260 nC (under 0.022 μF capacitive load), a maximum peak output voltage of 960 V (under a 70 MΩ load resistance), and a maximum peak power density of 14.6 W/m2 (under a 14.6 MΩ load resistance). In other words, the addition of 3 wt% CNT to PAN increased the charge amount by 136%, and the maximum peak power density by 125%. This work presents an effective way to take advantage of the coupling effect of carbon additive and nanofiber structure to significantly enhance the output performance of TENGs.