Electrochemical and Computational Study of Oxygen Reduction Reaction on Nonprecious Transition Metal/Nitrogen Doped Carbon Nanofibers in Acid Medium

In this study, we performed both electro-chemical measurements, such as the rotating disk electrode and the rotating ring-disk electrode techniques, and density functional theory (DFT) calculations to investigate oxygen reduction reaction (ORR) on nonprecious transition metal/nitrogen doped carbon nanofiber (Fe/N/C and Co/N/C) catalysts in acid medium. The nanofiber catalysts were synthesized by electrospinning and subsequent heat treatment procedures. Our electrochemical measurements showed that the pyrolyzed Fe/N/C catalyst possesses higher activity for ORB. than the pyrolyzed Co/N/C catalyst and could promote four-electron (4e(-)) ORR In comparison, O-2 electroreduction was found to proceed mainly with 2e(-) pathway on the Co/N/C catalyst. To gain insights into underlying catalytic mechanisms, we calculated the adsorption energies of all possible chemical species and the activation energies for O-O bond dissociation reactions involved in ORR on the FeN4 and CoN4 active sites embedded in graphene. Our DFT calculations predicted that the ORR. could happen through 4e(-) associative pathway on the FeN4 site, whereas it might end with a 2e(-) pathway on the CoN4 site due to high activation energy for O-O bond splitting and extremely weak adsorption of H2O2 on the CoN4 site. Complementary experimental and theoretical results suggest that the FeN4 and CoN4 dusters might be the main active sites for promoting ORR on the transition metal/nitrogen doped carbon nanofiber catalysts in acidic medium.