Characteristic time constants derived from the low-frequency arc of impedance spectra of fuel cell stacks
Electrochemical impedance spectroscopy is used during operation of different polymer electrolyte membrane fuel cell (PEMFC) stack assemblies at various conditions with special interest given to the characteristic time constant tlow-f derived from the low-frequency arc of the spectra which is typically in the range of approximately 15-0.5 Hz. This was done by fitting an equivalent electrical circuit (EEC) consisting of one resistor and two RC-elements to the data. Parameter variation performed on a 90-cell stack assembly suggests that conditions leading to different air flow velocities in the flow channels affect tlow-f while other parameters like humidity influence the impedance spectrum, but not tlow-f. Comparison of the stoichiometry variation between short stack and locally resolved single cell shows similar results with the stack's time constant matching that of the cell's segments which are located off-center toward the outlet. However, a nonlinear dependency between gas flow velocity and tlow-f especially at low stoichiometric values is obvious. Results from stoichiometry variations at different pressure levels suggest that this could be attributed to the different steady-state oxygen partial pressures during the experiments. Comparison of the stoichiometry variation between different stack platforms result in similar dependencies of tlow-f on air flow rate with respect to a reference oxygen partial pressure regardless of size, flow field, geometry, or cell count of the stack. The time constant caused by oxygen diffusion through the gas diffusion layer (GDL), tGDL, was approximated and compared to tlow-f. While it was found that tlow-f ≫ tGDL at low stoichiometric values, tlow-f decreases toward tGDL at very high gas flow rates, suggesting that tGDL offsets tlow-f and becomes dominating if no oxygen concentration variation along the flow channel was present.