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

The electrocatalyst layer of the polymer electrolyte fuel cells (PEFCs) has a complicated porous microstructure. Since the structure allows gas transport, proton 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 prepared model cells with the electrode area of 1cm². We then varied the Nafion ratio in the electrocatalyst layer to systematically modify the electrocatalyst microstructure. After model cell preparations, IV performance was measured and each electrode overvoltage was separated. We then observed electrocatalyst microstructure by the FIB-SEM technique to examine the correlations between the 3D microstructure and the cell performance. In addition, we analyzed cell performance and electrocatalyst microstructure of Pt/SnO₂-based electrocatalysts to examine the structural difference from the standard Pt/C electrocatalyst layer.

Figure 1 shows IV characteristics of several cells with different Nafion contents. In case Nafion ratio was 28wt.%, IV performance was the highest. In case Nafion ratio was less than 28wt.%, whilst IV performance was lower than that of 28wt.%, the cells can still generate high current densities. On the other hand, in case Nafion ratio was higher than 28wt.%, the cells can exhibit a lower cell voltage due to a much higher concentration overvoltage. This is because gas transport pathway in the electrocatalyst layer is filled with Nafion ionomer with increasing Nafion ratio. In order to examine the correlations between the cell performance and the microstructure, 3D microstructure of the electrocatalyst layers was systematically examined by FIB-SEM. As an example, Figure 2 shows 3D microstructure of the electrocatalyst layer with the Nafion ratio of 28wt.%. Similarly, we observed other Pt/C electrocatalyst samples with different Nafion contents. The Pt/SnO₂-based electrocatalyst layer was also analyzed for the comparison. Details on the FIB-SEM observation conditions and the microstructure-performance correlations are presented at the symposium.