Hydrogen Evolution Intermediate in Two-Dimensionally Confined Space between Graphene and Au(111)

Graphene-covered Au(111), Gr/Au(111), shows much better hydrogen evolution activity than bare Au(111) or graphene in alkaline electrolytes. This is due to a new state of hydrogen confined between graphene and Au(111). According to cyclic voltammetry, electrochemical Raman spectroscopy, and scanning tunneling microscopy measurement, when Gr/Au(111) is biased to a potential lower than the H₂O/H₂ electrode potential, hydrogen deposits into the space between Au(111) and graphene. The Gibbs energy and entropy of hydrogen deposition are measured using temperature-controlled cyclic voltammetry. Statistical mechanics calculation suggests that hydrogen forms a two-dimensional atomic gas in the two-dimensional space between Au(111) and graphene, which serves as a space-confined reactor, while the hydrogen atoms inside it serve as the reaction intermediate. The edge of graphene is the active site for hydrogen evolution because hydrogen evolution is significantly better on partially covered Gr/Au(111) than on fully covered Gr/Au(111).