Figure 3: General schematic of redox active species adsorbing and optionally exchanging electrons at the graphene surface.

The black line represents the graphene basal plane (the smooth homogeneous graphene crystals) and the *-symbol a grain boundary, edge or other site which has a higher reactive capability. The electron exchange rate is much lower at the basal plane than at the reactive sites, however the vast majority of the graphene surface consists out of graphene basal. In each of the two pathways a series of steps are drawn, in the pathway on the left: (a), adsorption of a molecule at a reactive graphene site (b), electron exchange and adsorption for a certain average time (c) and desorption from the graphene surface (d). In the pathway on the right (e), adsorption of a molecule to the graphene basal plane (less reactive) (f), electron exchange for some of the adsorption species and adsorption for a certain average time (g), and desorption from the graphene surface (h).