The harmonics can be measured by calculating the discrete Fourier transform of the voltage versus time data from EIS, which is done automatically by many modern potentiostats (Fig. ). In the case of half-wave symmetry (i.e., current-voltage relation is symmetric about the origin), only odd harmonics are generated; this condition is achieved by the physical symmetry of symmetric cells, even if the kinetics are asymmetric (Fig. ) . At high excitation currents (e.g., 100 mA cm - 2 ), the magnitude of the harmonics becomes quite significant relative to the linear response, and harmonic responses above the noise level can observed up to the 19 th harmonic (Fig. ). Overall, measuring the interfacial kinetics at high current densities with EIS requires either quantitatively accounting for these harmonic distortions, or ensuring that they are negligible; in this work, the latter approach is taken because it is significantly simpler.