The observation of membrane-electrodes assemblies (MEAs) in cross-section is a basic tool for their morphological characterization, and the study of changes occurring in the different layers by operation in a fuel cell. But the preparation of a well-defined and representative cross-section plane in the MEAs is difficult due to a layered structure with very different mechanical properties among the layers, including a Nafion membrane, two catalyst layers, and two gas diffusion layers. For this reason, the preparation of samples for cross-sectional observation requires appropriate cutting techniques able to leave a cross-sectional plane showing the morphology of each layer with minor changes, ie. avoiding uneven surfaces, mixing of layers, porosity and morphology alterations, and/or delamination. It is possible that no single method may accomplish with all requirements, so different cross sectioning procedures must be used to have a complete characterization. With this aim, different preparation methods for cross-section analysis of MEAs have been tested in order to make a comparative analysis, showing advantages and disadvantages for each of the methods.

The preparation procedures analyzed are cutting with sharp-edge, laser-cutting, embedded-mechanical polishing, and focused ion beam (FIB). Results are shown in the figure. Cutting with a sharp edge (Fig. a) is a quick and easy method that may provide good cross-sectional observation and morphology preservation if an appropriate tool is used and careful handling. However, the probability for a successful preparation is not high due to frequent intermixing of layers, delamination, and uneven surface. Laser-cutting with a CO₂ laser (Fig. b)) occurs by burning and vaporization of the material, leaving a plane with a clear damage at a micrometric scale. Especially Nafion membranes and carbon clothes showed severe alteration of the phases in the cutting plane, so this method cannot be used for a micrometric study of the morphology of the layers. Mechanical polishing of an embedded MEA a resin (Fig.c) provides, on the other hand, better preserved layers and more reproducible results; the morphology of powder layers, like the catalyst layer and microporous layer, may be altered because of the rearrangement of particles during the polishing process. Better preservation of multilayered samples can be obtained with the FIB technique (Fig.d), although the observation profile cannot be extended beyond a few millimeters. The FIB cross-cutting technique, that may be combined with a cross section polishing with argon gas, appears to better preserve the morphology of the layers and in a reproducible way.

Acknowledgements.

This work was supported by the Ministerio de Economía y Competitividad of Spain, and Fondo Europeo de Desarrollo Regional (FEDER), Project E-LIG-E, ENE2015-70417-P (MINECO/FEDER).