EIS usually employs a frequency response analyzer (FRA) to
apply either a small AC voltage or current perturbation signal to a
cell, and measures its output signal for a wide frequency range. The
impedance is calculated by dividing the voltage by current, in the
form of a magnitude and phase angle, at each specific frequency.
Impedance spectroscopy has the ability to characterize many of
the electrical properties of materials and their interfaces with
electrodes. This ability has made the EIS technique widely used in
modeling and diagnostics of PEM fuel cells, where individual contributions
affecting cell performance can be distinguished by fitting
the impedance spectrum into parameters of an equivalent circuit
model. Electrical circuits with different configurations, components,
and degree of complexity have been proposed in the
literature.
The basic equivalent circuit often used to represent fuel cell
operation is the Randles circuit shown in Fig. 1, where Cdl is the
double layer capacitance of the catalyst surface, RHF describes the
movement within a conducting media and illustrates the sum of
contribution from contact resistance between components and
high frequency resistance of the cell components, and Rct is the
resistance that occurs when electrons transfer at the electrode/
electrolyte interface. Rct decreases when overpotential increases
due to the faster oxygen reaction rate. Kang et al. [5] noticed an
increase in Rct with increasing the degree of the reverse potential
fault in a fuel cell caused by fuel starvation.
Nyquist plots show the resistance versus reactance at multiple
frequencies, as obtained by the EIS measurements, Fig. 2. The high
frequency region of the impedance spectrum represents the high
frequency resistance whereas the low frequency region represents
the high frequency resistance and charge transfer resistance
respectively [6]. At high frequency, the Cdl is short circuited and the
RHF is measured. As frequency decreases, the impedance becomes a
combination of resistance and reactance from the capacitive
element. At low frequency, Cdl acts like a blocking diode and the
total resistance is equal to RHF and Rct.