For O-GEIS measurements, the harmonic distortions are negligible. Due to the smallamplitude excitation (≤ 2.5 mA cm -2 ), the amplitude of the sinusoidal voltage response is limited to < 25mV (Fig. ). Only the even harmonics are generated above the noise level, which arise from a slight asymmetry of the voltage response about the direct current bias due to the non-linear kinetics (Fig. ). THD for all O-GEIS measurements is maintained at <1%, which is considered exemplary (Fig. ) . Notably, THD is sensitive to the excitation current, and there can be tradeoffs between reducing THD and increasing the signal-to-noise ratio. At low applied current densities, the excitation amplitude can be increased to ~ 25% of the applied current to achieve better signal-to-noise, while at high applied current densities, the excitation can be as low as < 5%