Fig. 6. Schematic HER mechanisms for graphene-covered non-noble metal catalysts.

a Proton-penetration-type mechanism. Protons penetrate through the graphene covering layers, and H₂ evolution occurs on the surface of the underlying non-noble metal substrate. b Graphene-surface-reaction-type mechanism. The H₂ evolution occurs on the outermost graphene layer by utilizing charge transfer from the underlying non-noble metal substrate. Colour code: carbon (white), nitrogen (red), proton (blue), nickel (cyan), molybdenum (purple), oxygen (light yellow), and hydrogen (dark yellow).