Investigation of operating conditions for 200 kW fuel cell system based on electrochemical impedance spectroscopy

To meet the increasing power demand of the application, a 200 kW fuel cell system with an alternating current impedance measuring system was installed on the test bench. The equivalent circuit model (ECM) and the essential equations for distribution of relaxation time (DRT) was been presented. The basic performance and operation parameters were tested using polarization curves. The impedance of the fuel cell was measured in the steady-state variable load condition, and the basic impedance loss of the stack was understood via ECM and DRT analyses. The impedance measurement, ECM, and DRT analyses were conducted by varying the cathode stoichiometric ratio, stack coolant temperature, and shutdown purge time, respectively. Results revealed that the total impedance of the stack decreased with the gradual increase in current density, and the relaxation time function $\gamma ln\left(\tau \right)$$\gamma ln\left(\tau \right)$of the fuel cell was approximately 600–620 mΩ cm². When the cathode stoichiometric ratio increased, mass transfer resistance significantly increased, while ohmic resistance (R_ohm), effective charge transfer resistance (R_act), and anode activation impedance and the cathode proton transport impedance (R_a) slightly changed. Moreover, the peak P₁ was significantly decreased, while the peaks P₂, P₃, and P₄ were unchanged. The overall impedance of the stack did not vary notably with the increase in the coolant outlet temperature of the stack at low-power operation, while a certain decline could be found at medium and high-power operation. With the increasing purging time, the R_ohm significantly increased from 46.1 to 59.7 mΩ cm² due to water content decreasing in the proton exchange membrane during purging. Additionally, R_act increased as the current decreased gradually, and the temperature and humidity in the stack dropped as well, leading to an increase in charge transfer resistance.