
330 Fuel Cells
8.2.2.7 Time and Frequency Domains
In EIS, the data are represented in two domains, the time domain and the fre-
quency domain. In the time domain, signals are represented as signal ampli-
tude versus time. In the frequency domain, the data are plotted as amplitude
versus frequency. The Fourier transform and inverse Fourier transform are
used to switch between the domains. In EIS systems, lower frequency data
are usually measured in the time domain. An FFT (fast Fourier transform
as done in a digital computer) is used to convert the current signal into the
frequency domain.
Modern EIS analysis uses a computer to nd the model p