Proton Accumulation near the Au Electrode/Acid Interface Probed by Electrochemical <i>In Situ</i> Infrared Spectroscopy

Electrochemical double layer (EDL) theory predicts that cations will accumulate in the EDL as the negative free excess charge increases on electrode surfaces. So far, it is not clear how the enrichment of protons affects the structure of interfacial water, the hydrogen bond network, and the kinetics of reactions such as hydrogen evolution reaction (HER). By taking the Au electrode/HClO4 interface as a model system, herein, we provide molecular evidence on the gathering of hydrated protons near the negatively charged electrode surface using electrochemical in situ infrared spectroscopy under attenuated total reflection configuration (ATR-FTIRS). To facilitate the assignment of IR bands observed at the interface, transmission IR spectra of bulk HClO4 solutions with various concentrations have also been recorded. Our results reveal that (i) there is a broad absorption continuum in the range from 1200 to 3000 cm–1, from the O–H stretching vibration of flanker water molecules directly connected to proton, in addition to the characteristic peak at ca. 1730–1 cm from the corresponding bending vibration; (ii) the higher the concentration of the acidic electrolyte or the more negative the applied potential, the stronger the absorption of the broad continuum at the Au/HClO4 interface. Such good correlation confirms the accumulation of protons near the negatively charged electrode surface. The impact of the enrichment of protons near the electrode surface on the interfacial structure of the water network and the kinetics of the HER are briefly discussed.