Bifunctional mechanism of N, P co-doped graphene for catalyzing oxygen reduction and evolution reactions.

The development of bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is highly desirable for fuel cells and rechargeable metal-air batteries. Till now, it is still challenging to achieve both efficient activities on a single commercial noble-metal catalyst. Recently, N, P co-doped graphene has shown good bifunctional evidence. However, the atomic-scale understanding of the bifunctional mechanism is still lacking. Here, we show that the N and P atoms prefer to bond with each other, forming embedded N-P clusters in graphene. The catalytic performances of the N-P clusters are sensitive to their geometries, especially the N:P ratios. The N:P ratio of ∼2 is optimal for OER, while ∼3 is optimal for ORR. Through evaluating the ORR/OER potential gaps, we found that the N-P cluster designated as NC 2PC 1 shows both the high performances of ORR and OER, responsible for the unique bifunctionality in the N, P co-doped graphene.