Coupling Surface Coverage and Electrostatic Effects on the Interfacial Adlayer–Water Structure of Hydrogenated Single-Crystal Platinum Electrodes

Atomically flat, single-crystal solid-liquid interfaces attract considerable interest through their electrochemical relevance and well-defined structure facilitating controlled atomistic characterization. Yet, crucial details especially regarding the nanoscale adlayer-water dynamics remain uncertain. Here, the influence of adsorbate coverage on the interfacial structure and solvent relaxation on hydrogenated Pt(111) is examined by extensive density functional molecular dynamics simulations. Pronounced water dynamics is observed with increasing hydrogen coverage, for which an interpretation based on displacement of specifically co-adsorbed water and strong screening of the electrostatic interaction across the interface is proposed. However, the magnitude of the solvent fluctuations is argued to be partly overestimated by the employed RPBE-D3 exchange-correlation functional, which impedes water chemisorption and charge transfer to sparsely hydrogenated platinum. This manifests as overestimated equilibrium electrode potentials compared to experimental adsorption isotherms, which are conversely well reproduced by static calculations invoking the computational hydrogen electrode formalism. By coupling the interfacial structure with electrostatic properties, our work underscores the profound importance of functional choice as well as the persisting value and comparable precision of carefully employed static approximations in electrochemical simulations.