Prior nonlinear optical studies of electrode:electrolyte interfaces have largely been based on SHG intensity measurements (see Gruen's and Nagy and Roy's pioneering work on nickel electrodes). These studies hark back to nonlinear electroreflectance studies from silver electrodes (please see Supplementary Information Note S1) . Recent approaches have focused on potential-of-zero charge quantifications via SHG amplitude and phase measurements on a platinum electrode . We now use optically transparent thin nickel nanolayers for which we quantify the Stern layer structure, the interfacial field, and the Stern layer energy density via Φ(0) !"! and c (2) .