Full Characterization of an Operating Fuel Cell Using High Energy X-Rays

The durability of proton exchange membrane fuel cells (PEMFC) is a major barrier to the commercialization of these systems for stationary and transportation power applications. The decrease in the performances is due to irreversible processes in the catalyst layers such as dissolution and contamination and reversible losses which are attributed to the evolution of the cell water distribution [1]. The mechanisms leading to the performance loss under PEMFC operation of individual components (catalyst, membrane, GDL) are well known, however, what is the connection between operating conditions and failure mechanism is still under debate. The degradation in real working conditions may be strongly accelerated and localized since all phenomena are highly inter-correlated and very dependent on the cell design and mode of operation. To provide a long-term stable cell operation, detailed knowledge about the underlying aging mechanisms and their correlation to fluid transport is essential. Here we show, for the first time, that it is possible to obtain simultaneously a space/time resolved information about the catalyst structure, water distribution and microstructural parameters in a running fuel cell using high energy X-ray diffraction techniques. The custom X-ray cell design (area of 5 cm2) allows us to reach current densities up to 4 A/cm2, therefore the performance of the most demanding application such as automotive fuel cells. We use advanced tomography imaging techniques coupled with the full Rietveld analysis of the diffraction (HE-XRD) patterns [2]. This allows us to distinguish (with micron resolution) different materials, their structure and to determine the water distribution as a function of the cell’s operating parameters (humidity, temperature, power output). For example the high resolution and large probed reciprocal space of HE-XRD patterns permit to determine the oxidation state of Pt cathode catalyst at different conditions. This information is then used to distinguish the potential distribution within the cell and to find the parts of the MEA where the Pt nanoparticles are either electrically disconnected from the carbon or they are not reachable by diffusing protons. Both phenomena would effectively lead to a lower performance of the cell. In another example we show that the parallel flowfield geometry of our cell and the defects in the GDL affect the water distribution within the membrane, catalyst layer and GDL. At 2 A/cm2 more water is generated close to the gas inlets of the cell and the defects in the GDL contain relatively less water. At lower current densities the water is much more homogeneously distributed within the MEA and the GDL defects do not play a major role in the water distribution. In conclusion we show that the high energy X-ray diffraction is an interesting non-invasive, relatively simple and robust technique which can be successfully used to study the fuel cell in operando conditions giving unprecedented insight into the working device. Ref.: [1] S. Cherevko et al, ChemElectroChem, 10, 2(2015), 1471–1478; [2] V. Rossi et al., Advanced Materials, 21(2009), 578-583. Fig. 1: (a) The top diagram shows the cross-section sketch of of the cathode side of fuel cell. The H 2 O, Pt and carbon cross-section maps obtained from the analysis of the HE-XRD patterns during the cell operation at 2 A/cm 2 are shown below. (b) The tomographic reconstructions obtained from HE-XRD measurements showing Pt distribution within catalyst layer for four different vertical positions and for three different electrochemical conditions. Figure 1

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