indicates that the second-order nonlinear susceptibility becomes smaller in magnitude as the applied potential becomes more positive, and that it crosses zero to become negatively signed at +0.4 V (resp., 0.2 V) applied potential on the forward (resp., reverse) scan, just where the SHG intensities goes through their minima (Fig. ). At +0.8 V applied, 𝜒 (;) approaches -1.6 x 10 -22 x 10 -22 to -3.1 x 10 -22 x 10 -22 m 2 V -1 , about two times larger in magnitude, albeit oppositely signed, when compared to the value at OCP. Supplementary Fig. shows that the c (2) and F(0)tot estimates reproduce reasonably well over seven different electrodes, while Supplementary Fig. shows no apparent pH dependence of the c (2) vs applied potential response between pH 13 and 7, while the F(0)tot estimates increase to slightly over 1V at the highest positive potential applied, with a pH dependence of (-0.09 ± 0.04) V pH -1 , encompassing the theoretical 0.059 V pH -