DFT study of stabilization effects on N-doped graphene for ORR catalysis

Noble metal free catalysts, such as N-doped graphene, have drawn a lot of attention as a promising replacement for platinum in low temperature fuel cells. Computational prediction of catalytic activity requires accurate description of the oxygen reduction reaction (ORR) intermediates adsorption energies. Two stabilizing effects, immanently present in experimental ORR setups with basal plane N-doped graphene catalyst, are studied systematically by means of density functional theory. Distant nitrogen with no adsorbates on neighboring carbon atoms selectively stabilizes *O and *O-2 adsorbates. Water solvation stabilizes all ORR intermediates, having a greater impact on *O and *O-2, than on *OH and *OOH, in contrast to metal and oxide catalysts. Synergistic stabilization of *O caused by both effects reaches remarkably a high value of 1.5 eV for nitrogen concentrations above 4.2% N. Such a strong effect is explained by a high reactivity of *O and *O-2, which possess empty O(sp) states. At 6.25% N, the reaction environment is found to comprise *O and free nitrogen spectators. Finally, strong *O solvation is found to be present in a broader class of systems, comprising all materials where the ORR occurs on a 2nd row element. Including at least a single explicit water layer is paramount to achieve the correct description of the ORR intermediates adsorption energies on these materials.