Computational analysis of water coverage formation on Pt(111) surface under humid conditions with the effect of applied voltage

Abstract

DFT calculations with implicit treatment of solvation effects are applied to the study of water coverage buildup and formation of dehydrogenated species (*OH and *O) on the surface of Pt electrode in response to the applied voltage. Using our previous work on initial step of water adsorption on Pt(111) at low voltage (0.5 V vs. standard hydrogen electrode (SHE)) as a starting point, we analyze subsequent stages of water coverage formation when higher potential is applied. Our study reveals that adsorption of the second water next to the initially adsorbed is less favorable than formation of a surface water plus hydroxyl species (*H₂O + *OH) in agreement with experimental measurements. We explain a high stability of *H₂O + *OH units by introduction of low energy p-states, localized on oxygen of *OH due to bonding to a nearby *H₂O. Our calculations also demonstrate that *OH species acts as a nucleation center for adsorption of subsequent water molecules, giving rise to formation of intermediate coverage of 0.33 ML of *H₂O on Pt(111). Subsequent scenarios lead to formation of particularly stable periodic structure of 0.33 ML (monolayer) of *H₂O and 0.33 ML of *OH, previously observed in experimental works. Higher voltages up to 1 V are predicted to be needed either for formation of surface *O species or further increase of the coverage of surface hydroxyls also in a good agreement with experimental observations. Overall, our work demonstrates that employed computational framework can be used for accurate prediction of electrochemical reactions on electrodes under applied voltage.