Characteristic Time Constants Derived From the Low-Frequency Arc of Impedance Spectra of Fuel Cell Stacks

Abstract

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 τlow-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 τlow-f while other parameters like humidity influence the impedance spectrum, but not τlow-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 τlow-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 τlow-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), τGDL, was approximated and compared to τlow-f. While it was found that τlow-f ≫ τGDL at low stoichiometric values, τlow-f decreases toward τGDL at very high gas flow rates, suggesting that τGDL offsets τlow-f and becomes dominating if no oxygen concentration variation along the flow channel was present.

Analysis of gas transport phenomena in a polymer electrolyte fuel cell by electrochemical pressure impedance spectroscopy

“…Experimental work has been carried out in a test bench previously described [5,23]. The fuel cell was a single 100 cm 2 cell (UbzM, Ulm, Germany).…”

“…The complex impedance calculated by equivalent electric circuit models, involves a resistance and a time constant indicating on the frequency range for this loop. Because of the possible contribution of Knudsen diffusion, diffusion coefficients of gases in the catalytic layer (CL), the microporous layer (MPL) and the macroporous substrate (MPS) are to differ from each other [5]. Time constants for gas diffusion often defined as the square of the layer thickness divided by the diffusion coefficient, are also to be very different.…”

“…First, EIS data for frequency below 10-100 mHz are often scattered because of the insufficient accuracy in the impedance calculation from the current and cell voltage fluctuations, then making investigations of mHz frequencies nearly impossible. Secondly, the single loop covering all mass transfer phenomena is characterized by a single time constant, usually in the range 100 ms -1 s as reported for instance in [5], rendering difficult the separate analysis of transfer phenomena.…”

Investigation of the Effects of Fuel Cells on V-Q & V-P Characteristics

“…This section is aimed to introduced a usable algorithm for dynamic simulation of the mentioned fuel cells. The given algorithm is of 4 stages which are explained deliberately in the following [40,41].…”

Polymer Electrolyte Fuel Cell Degradation Mechanisms and Their Diagnosis by Frequency Response Analysis Methods: A Review

“…Many physically-based or EC models have been developed in order to fit the low frequency impedance features and to evaluate transport parameters as for example oxygen diffusivity and internal flow rate velocity to diagnose flooding states [87][88][89][90][91].…”