Transmission Electron Microscopic Observation of Both Ionomer and Pt Distribution and Their Effects on Cathode Performance for Polymer Electrolyte Fuel Cells
Authors: Kawamoto Masako , Hommura Satoru , Kakinuma Katsuyoshi , Uchida Makoto 
Published: February 2020

In the electrode of a polymer electrolyte fuel cell (PEFC), the distribution of perfluorosulfonic ionomer on the Pt catalyst supported on carbon black (CB) significantly affects the fuel cell performance. The observation of the distribution of the ionomer and Pt catalyst on the carbon support are important for the improvement of PEFC performance. Recently, the direct observation of the ionomer with transmission electron microscopy (TEM) without damage by the electron beam has become possible due to technological improvements.In this study, four types of cathode catalyst inks were prepared in which the added amount of the perfluorosulfonic ionomer, prepared by AGC, was varied with respect to the CB (I/C ratio; I, weight of fluoro-ionomer; C, weight of graphitized CB (GCB)). The membrane electrode assemblies (MEAs) were made from catalyst layers (CLs), which were decals printed with these inks by the hot-press method. The catalyst powders collected from the cathodes were placed on a TEM grid without pretreatment. The ionomer distribution on GCB-supported Pt catalysts was observed with a high resolution TEM (Hitachi HT7700) with a low acceleration voltage (80 kV).1 The pore distribution of each sample was measured by use of N2 absorption analysis. The current-voltage (I-V) performances of the MEAs were also evaluated, and the effects of the ionomer distribution on the cell performance were investigated. A TEM image of the catalyst collected from the cathode (I/C = 0.8) is shown in Fig. 1a. The amorphous structure of the ionomer is able to be clearly observed on the surfaces of both the GCB and the Pt nanoparticles with the low acceleration voltage, high resolution TEM. A TEM image of the electrode catalyst of I/C = 1.6 sample is shown in Fig. 1b. The amorphous areas increased with increasing ionomer content and were observed more frequently. Agglomerations of Pt particles were also observed in some areas. In a previous study, Pt agglomeration and re-deposition due to sulfonate groups included in the high IEC ionomer were reported.2 These results suggested that the increased acid concentration of the ionomer around the Pt nanoparticles can cause severe Pt agglomeration. The mobilities of both ionomer and Pt particles might be influenced by the surface of the CB, and those on GCB would be expected to be larger than those on CB due to the smooth surface of graphite. The I-V performance of the MEAs is shown in Fig. 2. The performance increased with increasing I/C from 0.4 to 0.8 and 1.2 and then decreased for 1.6. Improvements in current density were considered to be due to increases in the number of proton-conductive pathways due to the ionomer in the CLs. The pore distribution of various the I/C samples were measured by use of N2 absorption analysis. The pore volumes of both nanopores of 2-5 nm and primary pores of 5-10 nm for the I/C = 1.6 cathode were decreased. The performance decrease for I/C = 1.6 is considered to be due to a decrease in the number of oxygen transport paths due to occupation of the pores of the carbon support with the ionomer.

References

(1) Y.-C. Park, H. Tokiwa, K. Kakinuma, M. Watanabe, M. Uchida, J. Power Sources,
315, 179 (2016).
(2) R. Shimizu, Y.-C. Park, K. Kakinuma, A. Iiyama, M. Uchida, J. Electrochem. Soc., 165 (6) F3063-F3071 (2018)

Figure 1