Nitrogen and fluorine co-doped graphite nanofibers as high durable oxygen reduction catalyst in acidic media for polymer electrolyte fuel cells

Nitrogen and fluorine co-doped graphite nanofibers (N-F/GNFs) were synthesized using melamine and ammonium fluoride as precursors, respectively, for use as metal-free catalysts for oxygen reduction reaction (ORR). The N-F/GNF catalyst undergoes structural transformation resulting in wrinkled graphene structures with many open-edge sites when F is doped into N/GNF. The developed catalyst has no effect on CH3OH or CO, which makes it highly desirable as a metal-free electrocatalyst for the ORR. The developed catalyst was subjected to 20,000 repeated potential cycles, no degradation of ORR activity was observed in acidic media. X-ray photoelectron spectroscopy analysis of the N-F/GNF catalyst reveals the presence of active pyridine and graphitic type N with highly active semi-ionic C-F bond in the graphitic structure. First-principles density functional theory calculations further support the formation of graphene structures from the GNF with the interlayer distance increasing from 3.9 to 5.1 Å. The Bader charge analysis predicts the site specificity of F doping in N-doped GNF. This N-F/GNF catalyst delivers a peak power density of 165 mW cm−2 at a load current density of 850 mA cm−2 in a H2/O2 polymer electrolyte fuel cell, as a nonmetallic electrocatalyst in acidic media.