Differentiating between the effects of nitrogen plasma and hydrothermal treatment on electrospun carbon fibers used as supercapacitor electrodes

Nitrogen-doped electrospun carbon fibers were prepared for use as binder-free supercapacitor electrodes. We varied the surface modification approaches and investigated how these processes influenced the surface chemistry and capacitive properties of the nitrogen-doped carbon fibers. The nitrogen content on the fiber surface increased considerably when hydrothermal treatment was performed using urea or tetra-2-pyridinylpyrazine as the nitrogen source. However, hydrothermal treatment caused substantial decreases in the specific surface area and pore volume, which offset the pseudocapacitance contributed by pyridinic and pyrrolic nitrogen and resulted in a reduction in the specific capacitance. By contrast, surface modification with nitrogen plasma resulted in a small decrease in the nitrogen content but a substantial increase in the oxygen content on the fiber surface. Furthermore, the amounts of pyridinic and pyrrolic nitrogen decreased. However, the large amount of quinone oxygen enhanced the pseudocapacitance and compensated for the decreased contributions of pyridinic and pyrrolic nitrogen to the pseudocapacitance. Consequently, the capacitive performance of nitrogen-plasma-treated carbon fibers was more favorable than that of nitrogen-doped carbon fibers treated with hydrothermal processes.