Mass transport properties of catalyst layers (CLs) for proton exchange membrane fuel cells (PEMFCs) strongly depends on its structure. The ionomer is a proton-carrier material in the CLs. On the other hand, the gas transfer is prevented by the ionomer coated on a catalyst. Ionomer distribution in the CLs, therefore, have gathered much attention. Some papers have suggested methods to analyze the ionomer distribution [1-4]. However, quantitative validation is not enough so far. In addition, these methods especially focused on a microscopic local ionomer distribution. From the point of view of mass transfer, overall through-plane ionomer distribution is important as well as the microscopic distribution. In this study, a novel visualization method of the overall through-plane ionomer distribution in the CLs by using a two-stage ion-beam processing is suggested.

The first stage of the two-stage ion-beam processing is the formation of flat and smooth cross-section by a broad ion beam. The second stage is the selective removal of the ionomer in the catalyst layer by a focused ion beam. Scanning ion microscopic (SIM) images were obtained after the first and second stage. Smoothing filter was applied on the obtained SIM images. The SIM image after the second stage was then conducted threshold by Otsu method and a carbon distribution map was obtained. A pores distribution map is obtained from a combined image of the SIM image after the first stage and the carbon distribution map. The threshold was determined by a p-tile method with ionomer to carbon ratio (I/C). Ionomer distribution was extracted from the carbon and pores distribution maps.

The ionomer distribution of the catalyst layer with I/C 2.0 was analyzed as shown in Figure 1. A catalyst coated membrane was fabricated by a doctor-blading and decal-transfer method [5]. Lower side in the figures is the interface with a polymer electrolyte membrane (PEM) and upper edge is the bottom in the fabrication process before decal-transferring. The through-plane ionomer distribution is not uniform. Thin ionomer layer was formed at the interface with the PEM. This can be because of deposition of ionomer during the drying process.

Acknowledgments

This work was supported by JSPS KAKENHI Grant Number 15H03932.

References

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