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.