Behind the Synergistic Effect Observed on Phosphorus Nitrogen Codoped Graphene during the Oxygen Reduction Reaction

Ab initio calculations are performed to investigate how the simultaneous introduction of phosphorus and nitrogen into graphene modifies the availability and spatial distribution of catalytic active sites for an oxygen reduction reaction (ORR). A phosphoryl group (R-3-P=0) is selected as a representative for the phosphorus doping, and the ORR is studied under alkaline conditions where a 4e(-) mechanism is used to determine the limiting step and overpotential (eta(ORR)) along the entire graphene surface. A scanning procedure is used to construct eta(ORR) maps for pristine-, N-, P-, and diverse PN codoped graphenes. The results indicate that a single N (P) atom activates up to 17 (3) C atoms, while the simultaneous introduction of P and N activates up to 55 C atoms equivalent to 57% of the surface. Additionally, PN codoped graphenes reveals that the relative location of both dopants has significant effects on the ORR performance, where a P N separation distance of at least 4 angstrom minimizes the localization of electronic states on the neighboring C atoms and improves the quantity and distribution of active sites. The results shows the importance of designing synthesis procedures to control the dopant concentration and spatial distribution to maximize the number of active sites. Furthermore, the eta(ORR) maps reveal features that could be obtained by scanning tunneling microscopy allowing us to experimentally identify and possibly quantify the catalytic active sites on carbon-based materials.