Investigating the Electric Double-Layer Structures between a Pt Electrode and Water/Acetonitrile Hybrid Electrolytes

Controlling the reactivity of water at electrocatalytic interfaces is a critical challenge in many electrocatalytic reactions. Its reactivity can be adjusted by altering the composition of hybrid aqueous/organic electrolytes. To advance this approach, it is essential to understand how the structure of the electrode/hybrid electrolyte interface depends upon the electrode potential. This understanding is largely lacking. Herein, using surface-enhanced infrared absorption spectroscopy (SEIRAS), we probed the interfaces formed between a Pt electrode and acetonitrile/water mixtures containing 0.1 M LiClO₄ or (butyl₄N)ClO₄. In the presence of Li⁺ and with decreasing potential, crystalline LiOH deposits on the electrode in electrolytes with a low water content (∼1% by weight) due to the low solubility of this salt in acetonitrile, blocking the active sites of the hydrogen evolution reaction (HER). In the presence of butyl₄N⁺, the surface becomes more hydrophobic with decreasing potential. Notably, butyl₄N⁺ ions form an irreversibly physisorbed adlayer solely in electrolytes with a high water content. Despite the formation of the adlayer, the electrode remains active for the HER.