At each PSU angle, we collect the SHG signal at 100 ms acquisition time for 5, 10, or 20 seconds, so it takes as little as a minute to record one fringe and reset the PSU motor position. We employ an applied voltage staircase in 100 mV steps that parks at a given voltage for the time required to record three fringes, of which we employ the third for fitting. We then obtain the signal (i.e., electrode:electrolyte interface) amplitude and phase from a trigonometric fit function detailed in Supporting Information Section S2.