Promotion or suppression of hydrogen evolution activity? The competition between sodium cations and quaternary ammonium ions at the metal/water interface

Quaternary ammonium salts (QASs) are frequently utilized to modulate the structure of the cathodic electric double layer in processes such as water electrolysis and hydrogenation reactions. However, literature reports have shown that QASs can both suppress and promote hydrogen evolution activity, yet the underlying mechanisms remain incompletely understood. In this study, we experimentally observed that the presence of QASs alone accelerates hydrogen evolution compared to NaOH solutions. Conversely, when the QAS is combined with Na⁺ or H⁺, it inhibits hydrogen evolution. <i>Ab initio</i> molecular dynamics simulation and surface-enhanced infrared absorption spectroscopy results indicate that Na⁺ ions disrupt the hydrogen bond network at the interface, leading to a disorder in the water chain arrangement. In contrast, QASs enhance the hydrogen bond network, thereby facilitating the hydrogen evolution reaction. However, coexistence of Na⁺ and QASs leads to hydration competition, creating gaps in the hydrogen bond network near the surface and impeding hydrogen transport. These findings enhance our understanding of QASs in hydrogen evolution and guide future interface modulation strategies.