Comparing the oxygen reduction reaction on armchair and zigzag edges from quantum mechanics

Abstract

We utilized d. functional theory (DFT), to calc. energy barriers for various reaction steps leading to the 4e- water formation and the 2e- peroxide formation on graphene catalysts. We examd. the Eley-Rideal reactions for oxygen redn. reaction (ORR) in proton exchange membrane fuel cells, detg. the energy pathways for associative electrocatalytic mechanism generated by stepwise addn. of hydrogen. Our binding energy calcns. show why ORR occurs only on the graphene edges because the O_2 and OOH intermediates do not bind on the basal plane surface. We predict an onset potential for ORR to produce 2e- products on armchair grapheme edges to be 0.65 V, which compares very well to the exptl. onset potential of 0.68 eV. 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. We found the rate detg. step (RDS) to form the 2e- product is OO + H a 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 a OH, with a barrier of 1.35 eV at a potential of 0.65 V.