Mechanism insight into the oxygen reduction reaction on dual FeN2 embedded graphene for proton exchange membrane fuel cells

Abstract

The design of non-noble heteroatom-doped graphene electro-catalysts for the oxygen reduction reaction (ORR) is beneficial to the commercial application of fuel cells. Since a higher concentration of dispersed FeN₂ moieties is more beneficial to the activity of the oxygen reduction reaction, the electronic properties and oxygen reduction performance of several possible dual FeN₂ moieties embedded in graphene (Fe₂N₄/G) have been investigated by using density functional theory calculations (DFT) in this work. It is found that the series of dual FeN₂ moiety embedded graphene is more stable than single FeN₂ moiety embedded graphene, and the catalyst Fe₂N₄-1/G shows the most outstanding stability among these geometries. The magnetic moments for dual FeN₂ catalysts are smaller than that of an isolated FeN₂ catalyst, and the central symmetric geometry of dual FeN₂ is beneficial to reducing the magnetism of Fe atoms. Moreover, the adsorption trend for these oxygen-containing intermediates adsorbed on these active centers is almost consistent with the increasing order of H₂O < O₂ < OOH < OH < O. The oxygen reduction reaction process on these active centers is thermally downhill except for Fe₂N₄-2/G and Fe₂N₄-3/G, all of which promote a single-site 4e⁻ ORR process on a pure FeN₂ moiety through the reaction path O₂ → O₂* → OOH* → O* → OH* → H₂O. It is found that these four reduction steps except for the second step of *OOH reduction could be the thermodynamic rate-determining step (RDS). The free energies of *O₂, *OOH, *O and *OH on these active sites have an almost linear scaling relation with different slope values according to their adsorption properties, and the linear scaling relations of *O₂vs. *OH, *O vs. *OH and *OOH vs. *OH have slope values close to 1, 2 and 1, respectively. The adsorption properties of OH have a strong relationship with the d-band center, the highest potential and Mulliken charge. Finally, the catalyst Fe₂N₄-1/G shows the most outstanding stability with a cohesive energy of −11.76 eV and activity with the highest potential of 0.73 V among these geometries.