where |Yn| is the magnitude of the response signal at each harmonic frequency . The THD versus frequency data, which is calculated up to 7 th harmonic in the EC-LAB software, is shown for LA-GEIS measurements with different current excitations (Fig. ). THD is always low (< 1%) at high frequencies (e.g., > 1 kHz) where ohmic resistance and double-layer capacitance, which are both linear processes, dominate the voltage response . For low excitation currents, THD remains small even at low frequencies because the interfacial kinetics are maintained within the linear region. For high excitation currents, THD increases sharply with decreasing frequency in the intermediate frequence range, where the voltage response shifts from mostly capacitive to mostly faradaic . THD plateaus at low frequencies, and as a function of current, to values of ~ 10%. In general, a THD of < 1% is considered sufficiently linear, while a THD of > 3% can be considered problematic . To ensure linearity conditions for LA-GEIS data analysis, the current range used to fit the kinetic data was limited to ~ 10 mA cm -2 RMS to maintain THD < 2%.