Unveiling the propagation mechanisms of local defects in PEMFCs, by measurement and exploitation of local current density distribution – LOCALI

First, LOCALI has developed dedicated instrumentation for the local measurement of current density, potential, electro-active surface, etc.: well-instrumented segmented cells and magnetic field measurement were the core competences to achieve these objectives. The second challenge of LOCALI has been to characterize how the local and global performance of MEAs is impacted by making MEAs with defects, so-called adapted pathological MEAs or thanks to specific operating conditions (flooding, reagent depletion, ...). Our objective was, on the one hand, to identify the source of the heterogeneities and, on the other hand, to locate the degraded zones within a stack. Finally, LOCALI has made it possible to follow, during ageing, how an initial and controlled defect propagates during operation. Particular attention has been paid to two points: (i) how a defect in one of the MEA materials, in our case a lack of active layer at the anode, influences the local degradation of its neighboring materials; (ii) how this defect propagates spatially 'at the MEA scale' (e.g. from the input to the output regions) or at the stack scale, i.e. from the cell to its neighbors.

The results provided the first evidence of defect propagation within an MEA and from cell to cell in a stack scale. At the segmented cell scale, this was evidenced by a loss of electroactive surface area of the anode and significant thinning of the membrane in the direction of H2 and O2 flow. At the stack scale, permeation current measurements taken during the tests showed an increase in permeation (and thus a loss of functional properties) of the cells close to the defect. Based on these results, a degradation propagation mechanism was proposed for one type of defect.

This project enabled the consortium to acquire new skills in the field of fuel cell characterisation and to produce new knowledge on the mechanisms of defect propagation, which could be used in the PEMFC95 and DURASYS-PAC (PEPR-H2) projects involving the LOCALI partners. On the other hand, only the influence of a few types of defects could be investigated in the LOCALI project. It could therefore be relevant to continue with other defects to complete the propagation mechanisms initiated by this study.

This project also led to the drafting of multi-partner publications in high-impact journals and participation in national and international conferences on the main results of the project. A more general article was also published on the CNRS INSIS department news website (https://www.insis.cnrs.fr/fr/cnrsinfo/piles-combustible-des-defauts-dans-les-electrodes-peuvent-se-propager-dautres-composants).

Reliability and durability are key considerations to successfully deploy Proton Exchange Membrane Fuel Cells (PEMFCs). Since the link between materials defects and performances at the scales of the Membrane Electrode Assembly (MEA) and the stack is now well documented, LOCALI shall provide information about the propagation of these defects to other materials or to other locations in the stack. LOCALI aims to improve the existing systems and will ultimately provide effective tools to control their mass-production, the quality of the stacks and their diagnosis for on-site maintenance (stationary) or for on-board (transportation) applications. To these goals, the study focuses on three main axes, developed for PEMFCs (but which can easily be implemented for E-PEM).
Firstly, LOCALI will develop instrumentation dedicated to local current density measurement and local electrochemical impedance spectroscopy: well-instrumented segmented cells and magnetic fields measurement are the core competences to these goals. The second challenge of LOCALI is, by using tailored defective MEAs or thanks to specific operating conditions (flooding, reagent exhaustion, ...) to characterize how local and overall performances of the MEA are affected, and to identify the signatures of the various anomalies. Our target is to identify the source of the heterogeneities as well as to locate degraded areas inside a stack. Finally, LOCALI will enable to track, during ageing, how the initial and controlled defects do propagate upon operation. A particular attention will be paid on two points: (i) does a defect in the one material of the MEA (e.g. a hole in the PEM) influence the local degradation of its neighboring materials (e.g. the catalyst layer); (ii) does the defect propagate spatially, and if so, does it happen only at the MEA scale (e.g. from the inlet to the outlet regions) or at the stack scale (i.e. from the defective cell to its neighboring ones).