First-principles identification of site dependent activity of graphene based electrocatalyst

Abstract

Graphene based metal free electrocatalysts have been considered as potential candidate for efficient oxygen reduction reaction (ORR) in fuel cell systems. Using density functional theory we investigated the site dependent ORR activity of nitrogen doped and divacancy (DV_555-777) graphene structures considering free energy calculations and correlate with the occupancy of pz(π) electrons. The dioxygen adsorption strength differs for each carbon site in case of N doped and DV_555-777 graphene system. Range of overpotential and on-set potential has been estimated for same system. Among the sites and systems considered, the C2′ site of 2N doped graphene system is most active towards oxygen reduction with lowest overpotential. We have found that the DV_555-777 is more active than the pure system. In this case also we have observed a range of overpotential considering various C sites. We have been estimated the occupancy of pz(π) electrons of each C site of different systems. The occupancy of pz(π) electrons of C atoms increase near the dopant site due to back-donation mechanism in N doped graphene system. A strong correlation has been identified in-between activity of each C site with the occupancy of pz(π) electrons of corresponding site. Overall we concluded that each C site of N doped and defective graphene has different on-set potential and pz(π) electrons play a major role to define the activity.