Evaluation of the onset voltage of water adsorption on Pt(111) surface using density functional theory/implicit model calculations

By applying density functional theory (DFT)/implicit model calculations we demonstrate that the minimum voltage required for adsorption of water molecules on Pt(111) surface could be evaluated in a good agreement with experiment (similar to 0.4 V vs. SHE (standard hydrogen electrode)). We also show that reliance on a computationally determined potential of SHE with respect to vacuum level is more appropriate than using the experimental range of values. Although the value of this computed SHE potential differs substantially from the experiment (3.3 V versus 4.4-4.8 Vas found experimentally) the calculated voltage of water adsorption is in a very close agreement with experimental measurements due to error cancelation in DFT calculations of the energy differences (i.e. water on the Pt(111) surface versus water in solution). We attribute this error to inherent inaccuracy of DFT in evaluation of the materials work functions. We also show that upon hydration of a Pt surface there exists a peculiar non-monotonic change of a potential upon charging of a slab in contrast to monotonic relation fora clean Pt surface, which we explain by modification of water binding mechanism with changing of a charge state of Pt surface.