Fuel Cell (FC) durability and reliability are critical issues for introducing Polymer Electrolyte Fuel Cells (PEFC) into vehicle applications. For buses and cars, over 20000 and 6000 operating hours respectively are required [1-3]. To reach these target lifetimes, a considerable research effort is made in order to understand the physical and chemical phenomena involved in the PEFC through the development of diagnosis methodologies. Numerous papers deal with Electrochemical Impedance Spectroscopy (EIS) as a diagnostic tool for FC generators [4-7]. EIS is a powerful tool allowing distinguishing between the influences of different processes especially when the investigated electrochemical systems involve multiple-step reactions [8]. However, the use of EIS technology is costly and often strongly limited by the electrical specifications of the available spectrometers. Generally, commercial spectrometers have a compliance voltage of about 12V and a maximal current range of 50 A. These limitations are not compatible with the characterization of the large FC stacks which are needed to power vehicles. The paper presents a novel architecture of impedance spectrometer dedicated to the characterization and diagnosis of large FC stacks operated in galvanostatic mode. Experimental tests are first performed with a single cell and a three cell stack in order to compare the performance and accuracy of the instrument with a commercial impedance spectrometer (Zahner-IM6). Then, additional experiments are carried out on a stack composed of 20 cells. The new device allows impedance measurements of cells located in the middle of the stack, where common mode potentials are usually too high for commercial impedancemeters. Moreover, the impedance spectrometer can also acquire automatically and sequentially the impedance of different individual voltages in the stack with a synchronous measurement of the global stack voltage. This capability allows to distinct any singular cell behavior or a drift effect of operational parameters. The experimental characterization of the 20 cell stack will be done on a specific FC test bench. The study of various parameter sets (stack temperature and polarization current) is realized with the aim to highlight the different behaviors of the cells following their positions in the complete assembly. The tests are performed in the framework of the French DIAPASON research project which concerns the diagnosis of large PEFC stacks without any intrusive method.
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