An FIB-SEM Study on Correlations between PEFC Electrocatalyst Microstructure and Cell Performance

Abstract

The electrocatalyst layer of the polymer electrolyte fuel cells (PEFCs) has a complicated porous microstructure. Since the structure allows gas, proton, and electron transport, and water management, cell performance depends strongly on their 3-dimensional microstructure. Understanding on the correlations between the electrocatalyst microstructure and the cell performance is therefore essential in improving the PEFC electrochemical performance. The purpose of this study is thus to clarify these correlations using focused-ion-beam coupled scanning electron microscopy (FIB-SEM). We used the standard electrocatalyst (TEC10E50E) and varied the Nafion-to-electrocatalyst ratio (hereafter the Nafion ratio) for the electrocatalyst layer because the Nafion ratio affects the electrocatalyst microstructure and the cell performance[1],[2]. After model cell preparations, current-voltage (IV) performance was measured and thus each electrode overvoltage contribution was separated. We then considered FIB-SEM working condition and method of image processing to observe electrocatalyst microstructure by the FIB-SEM technique. In particular, because it was difficult to distinguish between solid and pore in SEM images due to the complicated porous microstructure, we solved this problem by separating solid and pore objectively by auto-image thresholding technique[3]. Then, we examined the correlations between the 3D microstructure and the cell performance. According to the results of IV characteristics of several cells with different Nafion contents, in case Nafion ratio was 28 wt.%, IV performance was the highest. In case Nafion ratio was less than 28 wt.%, whilst IV performance was lower than that of 28 wt.%, the cells can still generate high current densities. On the other hand, in case Nafion ratio was higher than 28 wt.%, the cells can exhibit a lower cell voltage due to a much higher concentration overvoltage. In order to examine the correlations between the 3D microstructure and the cell performance, we observed electrocatalyst microstructure by the FIB-SEM. Figure 1 describes the observation process. According to the results of 3D analysis of samples with different Nafion contents, we could confirm that gas transport pathway in the electrocatalyst layer was filled with Nafion ionomer with increasing Nafion ratio. Details on the FIB-SEM observation conditions and the microstructure-performance correlations are presented at the symposium. (1) S. Jeon, J. Lee, G.M. Rios, H.-J. Kim, S.-Y. Lee, E Cho, Intl. J. Hydrogen Energy 35 (2010) 9678-86.(2) T. Suzuki, S. Tsushima, S. Hirai, Intl. J. Hydrogen Energy 36 (2011) 12361-12369.(3) J. Bernsen, 8th ICPR (1986) 1251-1255. Figure 1