DFT Simulation of Edge Halogenated Graphene Nanosheets As an ORR Catalyst in Fuel Cells

Abstract

Edge halogenated graphene-nanosheets1have recently been discovered to be a low cost Oxygen Reduction Reaction (ORR) catalysts for alkaline fuel cells. Better understanding of ORR on these catalysts is needed to understand: i) why these doped catalysts form water in the preferred 4e-ORR mechanism over the peroxide formation found in undoped graphene. ii) how to improve these catalysts, so that ORR can occur in the preferred acidic environment. We used the PBE flavor density functional theory (DFT) with the SeqQUEST code2to calculate the binding energies, onset potential, and reaction barriers for ORR using either bromine, chlorine or iodine as the hetero-atom dopants on edge halogenated graphene-nanosheets. We examined both the zig-zag and armchair edges and find that the ORR intermediates bind to the zig-zag edges more strongly than the armchair. We find that the ORR intermediate species HOO does not bind to the basal plane. Thus, the ORR occurs on the graphene edges and not on the basal plane surface. We also find that the halide doped graphene zig-zag and armchair edges lead to a skewed geometry due to nonbonded interactions of the halogen with each other on the adjacent edge sites. Although the ORR intermediate species bind more strongly on the zigzag edges, our results predict that ORR occurs in the armchair edges based on the predicted onset potential calculation that agree well with experiments (Cl: 0.95 V, Br: 0.70 V, I: 0.72 V). We used nudged elastic band (NEB) calculations in conjunction with the CANDLE water solvation method3 to predict the potential dependent barriers4of the ORR. The dissociation of O2 in the second step of the ORR indicates that halogen doped graphene favor the 4e- pathway. In addition, the barrier to form –OH is much lower on the halide graphene edges than undoped graphene. Based on the calculated barriers, we conclude that iodine-doped graphene has the lowest thresholds water formation (0.56 eV at 0.72 V), which we find to be the rate determine step (RDS). This is in agreement with ORR activity of experimental results indicating iodine doped graphene performs better than bromine-doped, chlorine-doped, and undoped1.