Cation-Surface Interactions During Electrocatalytic Hydrogen Evolution Probed by Surface X‑ray Diffraction.

The electrocatalytic production of hydrogen is pivotal for the sustainable generation of hydrogen fuel and hydrogen as reducing agent. Metal cations in alkaline electrolytes can facilitate or significantly impede this reaction, depending on electrolyte pH, catalyst surface, cation identity, and concentration. Still, the underlying mechanisms governing these effects remain elusive, in part due to the lack of direct characterization of how cations interact with the electrocatalyst surface and interfacial water. Here, using Surface X-ray Diffraction combined with ab initio Molecular Dynamics, we elucidate the formation of a cation layer at the electrochemical interface during hydrogen evolution on a hexagonally reconstructed Au(100) model catalyst under varying pH, potential, cation concentration, and electrolyte composition. Specular Crystal Truncation Rod measurements show that as the potential becomes more negative, Cs+ cations increase in surface coverage and move progressively closer to the interface, revealing that the positions of the inner- and outer-Helmholtz planes continuously shift with potential. Slightly smaller cation-to-surface distances and higher coverages are observed in alkaline media compared to acidic environments. Ab initio Molecular Dynamics simulations reveal that the Cs+ ions move into the first water layer as they approach the surface at more negative potentials. No water molecules can then reside between the ions and the Au surface, leading to a broken solvation symmetry and an opening angle. This opening is initially established by an orientation and slight distortion of the solvation shell, while at shorter ion-surface distances the ion sheds ∼10% of its solvation, enabling a closer approach to the surface. Finally, in Li+/Cs+ electrolyte mixtures at pH 3, we observe the preferential accumulation of Cs+ at the electrochemical interface, albeit in a more disordered fashion compared to the Li+-free situation. This is expected to enhance interfacial mobility and influence electrocatalytic activity beyond traditional "structure making/breaking" descriptions.