Structurally Modulated Graphitic Carbon Nanofiber and Heteroatom (N,F) Engineering toward Metal-Free ORR Electrocatalysts for Polymer Electrolyte Membrane Fuel Cells.

The present study designates the heteroatom (N,F)-doped various graphitic carbon nanofibers (GNFs) viz. GNF-linear segmented platelets, antlers, herringbone type, and their structural deformations from pristine fiber with many open-edge active centers as metal-free, cost-effective electrocatalysts for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs). Introduction of heteroatoms to GNF frameworks enlarges the lattice spacing between graphene platelets and leads to structural modulation. The developed GNF/N-F catalysts show excellent ORR activity with insensitivity to CHOH and demonstrated outstanding electrochemical potential cycling stability of 10,000 cycles with well-retained ORR kinetics without much loss in the activity. X-ray photoelectron spectroscopy investigation of GNF/N-F catalysts explicitly shows the highly active forms of N (pyridinic, pyrrolic, and graphitic-N) and semi-ionic, ionic C-F of F in the catalysts. The deep-rooted synergistic effect among N and F atoms creates more active centers entrenched with extensive C-C bond polarization and larger charge delocalization with larger spin density differences accomplished in GNF/N-F catalysts. Wide open-edge cavities, opened tips, and many extensively accessible facets collectively enhance the ORR activity of the GNF-H/N-F catalyst. The present study provides a deep insight into the understanding of advanced metal-free electrocatalysts for efficient ORR in PEMFCs and metal-air batteries.