The oxygen reduction reaction on graphene from quantum mechanics: Comparing armchair and zigzag carbon edges

Using density functional theory (PBE-D2 flavor), we report the mechanism for the oxygen reduction reaction (ORR) on graphene sheets. We find that ORR starts with OO chemisorbing onto the carbon edges, rather than the basal plane face, which is not energetically favorable. The carbon edges were described as one-dimensional periodic graphene ribbons with both armchair and zigzag edges. We calculated the binding energies of the ORR products (OO, OOH, O, OH, HOH, HOOH) for the zigzag and armchair edges, examining both the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER), to understand how OO is reduced. For the armchair edge, we calculate an onset potential of 0.55 V vs reversible hydrogen electrode (RHE), which corresponds to -0.22 V at pH 13 in agreement with experiments. We find that the rate-determining step (RDS) to form peroxide (a 2e^- process) is hydrogenation of adsorbed OO with a barrier of 0.92 eV. The process to make water (a 4e^- process) was found to be unfavorable at the onset potential but becomes more favorable at lower potentials. Thus, undoped carbon catalysts prefer the 2e^- mechanism to form peroxide, rather than the 4e^- process to form water, which agrees with experiment. The predictions open the route for experimental studies to improve the sluggish ORR on carbon catalysts.