Transient Nyquist Diagram for Fuel Cells

Abstract

Transient Nyquist diagrams were calculated by significantly changing the load current of polymer electrolyte fuel cells (PEFCs) while a perturbation current of a constant amplitude of ±2 A (±0.04 A cm−2) was simultaneously applied to the load current. The frequencies used for the calculation ranged from approx. 0.38 Hz to approx. 10 kHz, the number of noise-reduced impedance points was 255, and the measurement time was approx. 2.6 s. Fast Fourier Transform (FFT) electrochemical impedance spectroscopy (EIS) was used to calculate the impedance, and the ergodic hypothesis was used for noise reduction. Instead of a time-averaged impedance, this method uses an ensemble average of impedances with different frequencies (quasi-ensemble-median). The load current was changed in the form of a step or ramp, or in a form equivalent to the US06 drive cycle. The obtained transient Nyquist diagrams were arc-shaped, similar to those obtained by the usual FRA method, and altering RH did not change their shape. For the step response and US06 drive cycle, a local increase in impedance was observed when the load current increased sharply. This is believed to be caused by an increase in gas diffusion resistance or charge transfer resistance.