Origin of Large Onset Overpotential and Sluggish Kinetics of the Oxygen Reduction Reaction on Pt(111) Surface

The origin of the large onset overpotential and sluggish kinetics of the oxygen reduction reaction (ORR) on Pt has been a challenging problem in electrochemistry, which hinders the optimization of the cathode catalyst in fuel cells. This is largely due to the lack of a clear mechanistic view of the ORR on Pt, despite extensive studies during the past years. It remains uncertain whether the 4-electron oxygen reduction starts through the direct adsorption and dissociation of O2 (dissociation pathway) or proton-coupled adsorption of O2 (association pathway); and the exact nature of the rate-determining step (rds) is still elusive. In this study, the ORR mechanism is investigated theoretically by emphasizing the importance of various oxygenated adsorbates in the reaction kinetics. The adsorption of oxygenated species may affect ORR kinetics by modulating both the reaction energetics and the number of reactive sites. We first investigate the oxygenation processes of Pt(111) surface using combined density functional theory (DFT) calculations and Monte Carlo simulation, which allows the surface phase structures of oxygenated adsorbates at different potentials to be determined. The results show that the conversion between the adsorbed hydroxyl groups (OH*) and water molecules (H2O*) in the (Ö3´Ö3)R30°-structured OH*-H2O* network, and the phase transition between the OH*-H2O* network and the (Ö3´Ö3)-structured O* adsorption phase, can reasonably describe the cyclic voltammetric responses of Pt(111) electrode. Based on this, it is concluded that there are mainly two types of oxygenated surface phases at potentials of ORR relevance (below ca. 1.1 V), namely, the (Ö3´Ö3)R30°-patterned OH*-H2O* networks and the Ö3´Ö3-structured O* phase. The reaction energy calculations indicated that the adsorption of O2, which is the initial step of the ORR, is thermodynamically much more difficult to take place on the surface covered with the ordered O* adsorption phase than that on the surface bears the OH*-H2O* co-adsorption network. This suggests that the latter provides the main active sites for the ORR below ca. 1.07 V. The large onset overpotential of the ORR thus can be attributed to the full occupation of the strong spectator phase of O* at potentials above ca. 1.07 V. Besides, the potential-dependent Tafel slopes can be explained in terms of the coverage variation of this spectator phase with potential. By investigating the energetics of various possible reaction channels for O2 adsorption, we identify a new type of proton/electron-transfer (PET) coupled O2 adsorption reaction as the initial and also the activity-determining step in the ORR, in which the O2 adsorption is accompanied by a PET to an OH* in the OH*-H2O* network, rather than to the O2 itself to form an end-on OOH as commonly believed. That is, (1) The O2 molecule adsorbs on surface in a t-b-t configuration with one O atom occupying the central Pt atom inside the OH*-H2O* hexagonal structure and the other one displaces an OH* in the hexagonal structure. The present study thus reveals the distinct roles of various oxygenated adsorbates in the ORR. The adsorbed oxygen atoms, O*s, act mainly as surface spectators for the ORR; while OH*s and H2O*s on the one hand act as spectators to block the surface sites, but on the other hand provide reaction sites by forming hexagonal networks, with some OH*s and H2O*s participating into the reaction as intermediates and/or PET mediators. The sluggish ORR kinetics even after the onset potential should be due to the limited reaction sites on the surface, on which OH*, H2O* and O* occupy the most of surface sites (>2/3 ML).