We utilized density functional theory (DFT), to calculate energy barriers for various reaction steps leading to the 4e^- water formation and the 2e^-formation on graphene catalysts. We examined the Eley-Rideal reactions for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells, determining the energy pathways for associative electrocatalytic mechanism generated by stepwise addition of hydrogen.

Our binding energy calculations show why ORR occurs only on the graphene edges because the O₂ and OOH intermediates do not bind on the basal plane surface. We predict an onset potential for ORR to produce 2e^- products on armchair graphene edges to be 0.65 V, which compares very well to the experimental onset potential of 0.68 eV¹. We find that the onset potential for ORR to produce 4e^- products on armchair graphene is also 0.65 V. In comparison, we find that the zigzag graphene edges formed bonds to the ORR intermediates that were too strong, requiring an onset potential of 0.14 V to produce 4e^- ORR products. This result shows that the preferred site for ORR is the armchair edge.

These PBE calculations were carried out to determine energy barriers as a function of applied potential² using the a new CANDLE method to simulate the effect of water solvation³. We found the rate determining step (RDS) to form the 2e^- product is OO + H -> OOH, with an electronic energy barrier of 0.78 eV at a potential of 0.65 V. We found that the RDS to form the 4e^-product is the reaction, O + H -> OH, with a barrier of 1.35 eV at a potential of 0.65 V. Figure 1 shows the compares the ORR mechanism for the 2e- and 4e- reaction. Figure 2 shows the calculated Tafel Plot of the preferred 2e^- reaction using the Butler-Volmer equation using current densities calculated from potential dependent barriers.

References:

1. S. Malkhandi, P. Trinh, A. K. Manohar, K. C. Jayachandrababu, A. Kindler, G. K. Prakash and S. R. Narayanan, J Electrochem Soc 160(9), F943-F952 (2013).

2. H. Xiao, T. Cheng, W. A. Goddard and R. Sundararaman, J Am Chem Soc 138(2), 483-486 (2016).

3. R. Sundararaman and W. A. Goddard, J Chem Phys 142(6), 064107 (2015).