The decrease of it between 1.5 < L < 3, Figure , can be shown to result from hindered mass transport of S2O8 2-and Ru(NH3)6 3+ into the gap region caused by the proximity of the substrate electrode to the tip. Figure shows a simplified schematic of the gap between the tip and substrate electrode, including a hypothetical boundary (i.e., dotted green line) across which the fluxes (mol/cm 2 s) and local transport rates (mol/s) of S2O8 2-and Ru(NH3)6 3+ were calculated using FEM simulations. The rate of mass transport of S2O8 the gap is ~ 100x smaller than that for S2O8 2-, indicating that the current decrease between 1.5 < L < 3 is primarily due to the lower concentration of S2O8 2-resulting from hindered mass transport coupled with its continuous consumption via eq. 2.