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Research Papers

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

[+] Author and Article Information
Stefan Keller

Fraunhofer Institute for Solar Energy Systems
ISE, Hydrogen Technologies,
Department Fuel Cell Systems,
Heidenhofstrasse 2,
Freiburg 79110, Germany
e-mail: stefan.keller@ise.fraunhofer.de

Tansu Özel

Fraunhofer Institute for Solar Energy Systems
ISE, Hydrogen Technologies,
Department Fuel Cell Systems,
Heidenhofstrasse 2,
Freiburg 79110, Germany
e-mail: tansu.oezel@ise.fraunhofer.de

Anne-Christine Scherzer

Fraunhofer Institute for Solar Energy Systems
ISE, Hydrogen Technologies,
Department Fuel Cell Systems,
Heidenhofstrasse 2,
Freiburg 79110, Germany
e-mail: anne-christine.scherzer@ise.fraunhofer.de

Dietmar Gerteisen

Fraunhofer Institute for Solar Energy Systems
ISE, Hydrogen Technologies,
Department Fuel Cell Systems,
Heidenhofstrasse 2,
Freiburg 79110, Germany
e-mail: dietmar.gerteisen@ise.fraunhofer.de

Ulf Groos

Fraunhofer Institute for Solar Energy Systems
ISE, Hydrogen Technologies,
Department Fuel Cell Systems,
Heidenhofstrasse 2,
Freiburg 79110, Germany
e-mail: ulf.groos@ise.fraunhofer.de

Christopher Hebling

Fraunhofer Institute for Solar Energy Systems
ISE, Hydrogen Technologies,
Department Fuel Cell Systems,
Heidenhofstrasse 2,
Freiburg 79110, Germany
e-mail: christopher.hebling@ise.fraunhofer.de

Yiannos Manoli

Department of Microsystems
Engineering—IMTEK,
University of Freiburg,
Georges-Koehler-Allee 103,
Freiburg 79110, Germany
e-mail: ymanoli@imtek.uni-freiburg.de

1Corresponding author.

Manuscript received August 12, 2016; final manuscript received November 13, 2017; published online February 6, 2018. Assoc. Editor: Jan Van herle.

J. Electrochem. En. Conv. Stor. 15(2), 021002 (Feb 06, 2018) (10 pages) Paper No: JEECS-16-1107; doi: 10.1115/1.4038632 History: Received August 12, 2016; Revised November 13, 2017

## 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.

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## Figures

Fig. 1

Experimental data and fitted EEC. The EEC was used to extract the characteristic time constant of the low-frequency arc.

Fig. 2

Schematic segmentation with small segments at inlet and outlet and large segments at the center

Fig. 3

Experimental results of parameter variation on a 90-cell stack 9SSL. On the left-hand side is Nyquist plot, and on the right-hand side is ImZ plot. (a) Variation of cathode stoichiometry, (b) variation of pressure on anode and cathode, (c) variation of humidity on anode and cathode side, and (d) variation of temperature at constant dew point on cathode side.

Fig. 4

Pressure variation on a 90-cell stack 9SSL. ImZ plot and variation of τlow-f at (a) constant cathode stoichiometry and (b) constant cathode volume flow.

Fig. 5

Impedance spectra of locally resolved single cell at cathode stoichiometry of 3.0 compared to average stack spectrum of same geometry and operated under the same conditions. Frequency range is 5.000–0.5 Hz.

Fig. 6

Dependency of τlow-f on cathode stoichiometry for row 5–9 of segmented cell compared to stack average

Fig. 7

Variation of cathode stoichiometry at two different pressure levels (a) τlow-f over molar flow, (b) over volume flow, and (c) corrected by oxygen partial pressure

Fig. 8

Experimental results showing response of impedance spectra on varying excess airflow ratios for different stack platforms. On the left-hand side is Nyquist plot, and on the right-hand side is ImZ plot. (a) CEKA, (b) ASC, and (c) PM 200.

Fig. 9

Influence of cathode gas flow rate on τlow-f: (a) average volume flow per cell and (b) values normalized to the excess ratio of 1.7 (1.75 in case of CEKA)

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