“Experimental development of pressure modulation electrochemical impedance spectroscopy for the characterization of transport phenomena in a membrane fuel cell ”

Composition of the jury :

Rapporteurs :
Prof. Marie-Cécile PÉRA (Professor), FEMTO-ST, University of Franche-Comté.
Dr. Jonathan DESEURE (MdC- HDR), LEPMI, Grenoble Alpes University.
Examiners :
Prof. Wolfgang BESSLER (Professor), University of Offenburg, Germany.
Dr. Armelle RINGUEDÉ (Director of research – CNRS), Chimie ParisTech PSL, Paris.
Dr. Caroline BONNET (MdC : co-director of thesis) LRGP, University of Lorraine.
Dr. François LAPICQUE (Research director: thesis supervisor) LRGP, University of Lorraine.
Guests :
Prof. Stéphane RAËL (Professor GREEN), University of Lorraine.
Dr. Joël PAUCHET (Research engineer), CEA-LITEN, Grenoble

Proton exchange membrane fuel cells (PEMFCs) have mainly drawn the attention of the transport sector towards them as they promise a much cleaner fuel option than fossil fuels. For the automobile industry, PEMFC operation at high current density is of particular interest as it allows to obtain high power density. Although a PEMFC can sustain the high current density operation, it heavily relies on the fuel and oxidant’s uninterrupted supply to respective electrodes transported through the gas diffusion layer (GDL) to the catalytic site to perform optimally.
At high current density, the performance of PEMFCs is governed by various coupled transport processes, and it can decline because of poor transport phenomena. These limitations in the transport phenomena increase with the aging of the membrane electrode assembly (MEA) and pose problems such as increased diffusion control of oxygen due to the risk of flooding and uneven distribution of the gases.
EIS is a frequency response analysis (FRA) technique that uses transfer functions between electrical variables to study the system’s dynamics typically in the range 10 kHz to 100 mHz. EIS can be used to decouple certain performance losses such as ohmic losses, charge transfer, and kinetic losses at high and moderate frequencies to identify the source of degradation, and to some extent, mass transport limitations in the lower-frequency region.
However, the mass transport processes (such as gas transport and liquid water transport) have time constants with a comparable order of magnitude (over one second), and using electrical variables to analyze them can lead to observation of coupled or combined processes. Therefore, spectral analysis of an EIS response can lead to poor decoupling and interpretation of transport processes causing misleading conclusions about performance limitations because of poor transport phenomena.
These ambiguities have set the wheels in motion to search for novel diagnostic techniques based on the transfer function between a non-electrical and an electrical variable to decouple and interpret transport processes. This work is dedicated to experimental development and validation of such a diagnostic technique called electrochemical pressure impedance spectroscopy (EPIS), which uses gas pressure as the non-electric variable. In this work, the transfer function between the cathode back-pressure and the cell voltage was analyzed at a constant current density to characterize the transport processes in the fuel cell. EPIS was performed by applying fluctuations of the gas pressure at the fuel cell cathode outlet in the frequency range 1 mHz – 1 Hz to gaze into transport processes.

An experimental bench was designed and set up for reliable EPIS measurements on a single UBzM FC. Conditions and constraints of pressure modulation in the test bench were thoroughly examined, leading to a standard operation protocol. EPIS was tested in various operating conditions specifically designed to decouple the two principal transport processes in a fuel cell i.e., (1) gas transport and (2) liquid water transport and its control over gas transport.

EPIS has shown significant sensitivity towards gas transport in dry conditions, both in terms of impedance modulus and phase shift, especially over 100 mhz. In addition, in flooding conditions, variations in the impedance modulus were observed in the lower frequency region, below 100 mhz. The phase shift behavior in flooding conditions is slightly unclear and would need more investigation. Besides, these obtained results with dry and flooding conditions tend to evidence the control of transport phenomena in the fuel cell by EPIS. The work explored the potential of EPIS as a complementary tool to EIS in the low-frequency region as it can evidence the transport phenomena and can decouple them from each other.