A comparison of single and double Co sites incorporated in N-doped graphene for the oxygen reduction reaction

Metal and nitrogen co-doped carbons (M-N/Cs) are promising alternatives to platinum-based catalysts for the oxygen reduction reaction (ORR). Here, density functional theory calculations are used to compare the ORR activity of Co single and double sites embedded in N-doped carbon. Two different models of a Co double site are investigated, one in which two single sites are stacked on top of each other and one which has two Co atoms next to each other in a single graphene sheet. For both it is found that the ORR can proceed via a dissociative mechanism that splits the *OOH intermediate into *O and *OH, but only for the double site in the graphene sheet does it result in a significant deviation from the scaling relations. The adsorption energies of the ORR intermediates are investigated using different implicit and explicit solvent models, showing some variation in the results. In particular, the addition of explicit water on the same side of the catalyst as the ORR intermediate can result in stabilisation due to hydrogen bonding, while an explicit water molecule adsorbed on the opposite side of the Co atom can have an effect due to the change in coordination which affects the splitting of the Co d orbitals.