Start date :31/01/2020
End date :31/01/2020
Location : salle des thèses (C002) de l'ENSEEIHT - 2 rue Charles Camichel, 31000, Toulouse
Doctoral School: GEET
FCs are submitted to ageing and various operating conditions leading to several failures or abnormal operation modes. Hence, there is a need to develop tools ded-icated to the diagnosis and fuel cell ageing monitoring. One of reliable approaches used for the FC SoH monitoring is based on parametric identification of a model through experimental data.
Widely used for the FC characterization, the polarization curve (V-I) and the Electrochemical Impedance Spectroscopy (EIS) coupled with a model describing the involved phenomena may provide further information about the FC SoH. Two models were thus developed: a quasi-static model whose parameters are identified from the polarization curve and a dynamic one identified from EIS data. The need to develop a dynamic model whose formulation may vary over time “without a priori” has been reported in this thesis.
The original approach of this thesis is to consider conjointly both characteriza-tions during all the proposed analysis process. This global strategy ensures the sep-aration of the different fuel cell phenomena in the quasi-static and dynamic do-mains by introducing into each parametrization process (one for the quasi-static model and one for the dynamic model) parameters and/or laws stemming from the other part. The global process starting from the a priori knowledge until the identi-fication of the models parameters was developed during the chapters of this thesis. In addition to the good reproduction of experimental data and the separation of the losses in both static and dynamic domains, the method makes it possible to monitor the FC SoH via the evolution of models parameters.
The fact to take into account the coupling between quasi-static and dynamic models revealed the notion of a “residual impedance”. This impedance makes it possible to overcome the recurrent experimental observation made by the daily users of EIS: there is a not-clearly explained difference between the low frequency resistance of the EIS and the slope of the polarization curve for a given current density. Theoretically the two quantities have to tend towards the same value. In others words, a part of the impedance spectra is not clearly and easily exploitable to characterize fuel cell performance. This topic has been discussed in the literature in the last years. An attempt to explain physico-chemical phenomena related to this impedance is also a part of objectives of this thesis.
From an experimental point of view, before applying this method to ageing monitoring, it was indeed necessary to “calibrate” it regarding its relative complex-ity. In this way, experiments with a single cell with different sets of internal com-ponents (different membrane thicknesses and different platinum loadings in the Active Layer (AL)) were achieved and analyzed by applying the proposed method. Therefore, the method was evaluated in the framework of three ageing campaigns carried out with three 1 kW PEM stacks.