For LA-GEIS measurements, a touchdown point interpolation method was used to calculate the magnitude of Rct from the impedance spectra (Fig. ). The current excitation ranged from 1 to 50 mA cm -2 , with 10 points per decade spacing in logarithmic spacing. At each current, the impedance spectrum was collected in a single scan with no averaging. Since the characteristic frequency of the semicircle increases as the resistance decreases, a variable frequency range was used to resolve the touchdown point while minimizing the number of data points. The frequency range varied from 1 -10 Hz at 1 mA cm -2 to 3 -15 Hz at 50 mA cm -2 . The rest time between impedance scans ranged from 1 min at 1 mA cm -2 to 10 min at 50 mA cm -2 . PEIS spectra were collected after each rest step to ensure stability. Before and after collecting the LA-GEIS dataset, which took ~ 2 h, the impedance measured with PEIS decreased by 0.4% (Fig. ) due to slight resistive heating.