A Film of Hydrophobic Cations Reshapes the Electric Double Layer at the Metal/Water Interface

In recent studies, it was observed that the addition of large hydrophobic cations, such as tetrabutylammonium (TBA⁺), into aqueous electrolytes enhances the rate of the hydrogen evolution reaction. This opens a promising perspective for optimizing electrochemical reactions via tuning of the electrolyte composition. We have constructed a THz ATR spectroelectrochemical cell that is able to unravel the underlying molecular details in the interfacial layer at the gold/water interface. We find spectroscopic evidence for the formation of a TBA⁺ rich film. Accompanying molecular dynamics simulations quantify the balance of electrostatics and hydrophobic solvation driving forces that dictates the TBA⁺ film formation. The composition of the film is strongly voltage-dependent, as confirmed by simulations. At positive potentials above 0.6 V versus Ag/AgCl, TBA⁺ only partially desorbs, causing the formation of a coadsorbed TBA⁺ and Cl^- layer and the partial rehydration of the surface. By our joint experimental-theoretical study, we disclose that a film of hydrophobic TBA⁺ cations has a major impact on the structure of the hydrogen bond network at the electrode-water interface. This allows rationalizing at the molecular level the ion-induced structural changes at the interface that tune the catalytic performances.