Kang Yu1 2 Fan Yang3 Jian Xie3 Paulo Ferreira1 2

1, University of Texas at Austin, Austin, Texas, United States
2, International Iberian Nanotechnology Laboratory, Braga, , Portugal
3, Indiana University/Purdue University, Indianapolis, Indiana, United States

Proton Exchange Membrane Fuel Cells (PEMFCs) have gained increasing attention due to their high energy/power density, high efficiency. The sluggish oxygen reduction reaction (ORR), occurred in the catalyst layer in the membrane electrode assembly (MEA), is the major bottleneck for the FEMFC performance. The ORR is strongly depends on the nanostructure of the catalyst layer, in particular, the ionomer/catalysts/carbon interface in the catalyst layer. Thus, it is imperative to determine the nanostructure of the interface in terms of the ionomer coverage, thickness of the ionomer film over the catalyst nanoparticles.
In this context, Aberration Corrected-Scanning Transmission Electron Microscope (ac-STEM) coupled with Electron Energy Loss Spectroscopy (EELS) are used to study the nanostructure of the interface. Most of the previous research was focused on observing the presence of fluorine, which is a distinct element in the perfluorosulfonated ionomer. However, the use of fluorine as the marker may not be as accurate as one expected as there is a significant F loss under electron beam [1].
In this work, EELS analysis of Vulcan XC72 (carbon) and nafion (ionomer) is employed to distinguish between these two kinds of amorphous carbon.. A single spectrum of both Vulcan XC72 and Nafion, with an energy resolution of 0.3 eV, is obtained in ac-STEM coupled with a monochromator. Decomposition of the carbon K-edge in Nafion and Vulcan XC72 showed that the ratio of intensities between σ edge (at 287ev) and π edge (295ev) is significantly different, as well as the energy loss near edge structure (ELNES) of the σ edge. Moreover, an extra peak at 292ev is shown in the spectrum of Nafion, which is primarily attributed to the carbonyl group in Nafion .
To prepare electron transparent MEAs, a partial embedded method on ultramicrotome is applied to avoid resin penetration. Spectrum images on the catalyst layers are obtained under the same condition for obtaining single spectrums for Nafion and Vulcan XC72. Principal component analysis (PCA) and multiple linear least square (MLLS) fitting is then applied to electrode spectrum images. The Nafion and carbon particle distribution are clearly observed and distinguished. Meanwhile, the spectrum images of the catalyst layer from the fluorine K-edge is also obtained, which shows good agreement with the results from the carbon K-edge but with a lower signal-to-noise ratio.
In summary, STEM-EELS characterization of the carbon K-edge provides a novel method to determine the interplay between the ionomer/carbon/catalysts distribution on the catalyst layers of PEMFCs. As the cross section for carbon K-edge is normally higher than F-edge in EELS, this new method also provides insight into detecting low perfluorosulfonated ionomer contents in the PEMFCs.

[1] D A Cullen, R Koestner, R S Kukreja, Z Y Liu, S Minko, O Trotsenko, A Tokarev, L Guetaz, H M Meyer III, C M Parish, K L More, J. Electrochem Soc 161, F1111 (2014)