Figure 4: DFT and Monte Carlo simulations.
From: Direct observation of the oxygenated species during oxygen reduction on a platinum fuel cell cathode
(a) Calculated coverages of adsorbed oxygen, OH and water as a function of computed electrochemical potential, USHE, obtained from DFT and Monte Carlo simulations. The regions marked are separated by the emergence of O(ad) at 0.85 V. (b) Representative snapshots of the surface at 0.8 and 0.95 V from the Monte Carlo simulations exemplify the two regimes shown in a, with H2O(ad) (red and white circles), hydrated OH(ad) (blue and white circles), non-hydrated OH(ad) (cyan and white circles) and O(ad) (green circles) shown on a Pt (grey circles) surface. The adsorbate layer between 0.85 and 1 V consists of co-existing localized domains of both hydrated and non-hydrated OH, which we have marked in the corresponding schematic. (c) Free-energy diagrams for the ORR, based on DFT calculations for both hydrated and non-hydrated pathways, at a potential of USHE=0.9 V. The change in Gibbs free energy, ΔG, along the reaction coordinate shows that the thermodynamically limiting step in the hydrated phase is the removal of adsorbed OH. In the non-hydrated phase, the limiting step is the activation of O2 as OOH(ad).
