it's only -0.076 V (SHE). At pH=13, the equilibrium potential for R5 in RHE is 0.69 V, while the transition from 2e -to 4e -also occurs around 0.7 V (RHE). This is not a coincidence. The free energy surface for reaction for HOO* formation (R1) is quite flat, with both the barrier and the free energy change for R1 being small (Fig. ). As the electrode potential is raised, R1 would soon become energetically unfavorable. The turning point should be none other than the equilibrium potential for R5, above which R5 is reversed, and so are the related elementary steps, R1 and R3. Yet, while the 2e -pathway is henceforth closed due to thermodynamics, the dissociation channel (R2) is now opened up, since its barrier of 0.48 eV is accessible at room temperature. ORR on Au(100) now becomes a 4e -process, and the calculated barrier for O-O* cleavage is in reasonable agreement with the experimentally measured value of 0.6 eV for ORR. The dissociation of O-O* bond is followed by the formation of O* and OH*, and eventually by the desorption of OH*. While the production of OH* is known to be promoted by the presence of water, OH* desorption could be a key factor in determining the ORR current. Again, it's instructive to compare the relevant steps on Pt(111) and Au(100). Hydrogenation of O* could go through two possible channels, either