Computational Study of Oxygen Reduction Reaction on Co-Functionalized Porphyrin-like Graphene.

The application of proton exchange membrane fuel cells (PEMFCs), using an abundant, efficient and clean energy source of hydrogen, is highly limited by its current requirement of precious Pt-based electrode materials. An alternative to solve this problem is to replace them by transition metal/nitrogen doped carbon-based materials (TM-N/C). The performance of the new materials is not as good as that of Pt-based materials but very close, and their price is much lower. In this study, the first-principles density functional theory (DFT) calculations are employed to study the true nature of active sites and the effect of varied N atoms number to the oxygen reduction reaction (ORR) on Co-N/C materials. OOH dissociation and O-OH reduction is also investigated as elemental reactions of ORR. The transition state and active energy of OOH dissociation is obtained by climbing-image nudged elastic band (Cl-NEB) method. The possible active site is assumed planar Co-Nx clusters embedded in graphene. Adsorption energy calculation shows that the lack of surrounding N atoms strengthens the adsorption of all ORR species on Co-Nx clusters and leads to a remarkably strong adsorption on their bridge site. Further investigation suggests that such strong adsorption on bridge site could decrease the active energy required by OOH dissociation and change the limiting elemental reaction. After the analyses of the free energy landscapes of 4e- and 2e- ORR on all possible CoNx clusters, one of them shows a strong possibility acting as the active site of 4e- ORR. The active energy of OOH dissociation is 0.78eV and the limiting potential is 0.42V, which is very close to the property of Pt. Besides, 2e- ORR research gives a limiting potential of 0.47V, suggesting an acceptable selectivity of the cluster as a 4e- ORR catalytic site