Fig. 4. Universal correlation between the HER activity and the proton penetration through the graphene layers.

Graphene-covered Ni sheet, NiNP, and NiMoNP with various numbers of graphene layers were employed. Colour code: carbon (white), nitrogen (red), nickel atom (cyan), nickel sheet (grey) and molybdenum (purple). Error bars show the fluctuations in the measured signals. The current density was normalized by the window area for the X-axis and by the total surface area of the catalysts for the Y-axis. Note: a 10 nA proton current is equivalent to a 10 mA cm⁻² proton current density.