1. Introduction

Considerable efforts have been made for FCEV commercialization, where both performance and durability have to be achieved¹.SnO₂-supported electrocatalysts with carbon backbone have a potential to achieve both high catalytic activity and high cycling durability^2-3, but it is necessary to further optimize electrochemical performance up to high current densities for such alternative electrocatalysts in the future. The objective of this study is to examine cell performance at high current densities by varying electrocatalyst layer fabrication conditions and selecting suitable gas diffusion layer and sealant thickness for the MEAs.

2. Experimental

Membrane-electrode-assemblies (MEAs) were fabricated using a Pt/C standard catalyst (Tanaka Kikinzoku Kogyo, TEC10E50E) for both electrodes, and cell performance up to 2.5 A/cm² were examined by varying sealant thickness and gas diffusion layers on the cathode side. Using mesoporous carbon (MC) as a support backbone and SnO₂ as a support surface layer on the cathode side, cells with Pt/Sn_0.92Nb_0.08O₂/MC electrocatalysts were fabricated. Cell performance and overvoltages up to high current densities were evaluated by varying ionomer ratio and heat treatment temperature in electrocatalyst layer fabrication. Cells prepared under optimized conditions were fabricated and subjected to a 60,000-start-stop cycle tests.

3. Results and discussion

The cell with the standard Pt/C catalyst exhibited the lowest concentration overvoltage and the highest performance when the GDL was 22BB (SGL Carbon), the GDL thickness was 225μm, and the sealant thickness was 100 µm on the cathode side, as shown in Fig. 1 (measured at full humidification at 80°C). When the 22BB GDL with microporous layer (MPL) is used, the cell performance was found to be improved by compressing GDL with a thinner sealant.

For cells with the alternative SnO₂-based catalyst, cell performance reached a maximum when the ionomer was 8 wt.% of the catalyst, when the GDL was 22BB, and when the sealant thickness was 100μm. Furthermore, as shown in Figs. 2 and 3, the activation overvoltage was minimized, and cell performance was maximized when the heat treatment temperature of tin oxide loading was 600°C. Under the optimized conditions, the ECSA retention after 60,000 start-stop voltage cycles was 45% as shown in Fig. 4. This confirms that even when MC is used as the catalyst support backbone, the cycle durability is improved by applying tin oxide.

In the future, we aim to further improve the catalytic activity by modifying parameters such as heat treatment temperature and loadings in catalyst preparation to reduce activation overvoltage. In addition, to suppress the increase in concentration overvoltage, we have to improve water management. Based on these efforts, latest results of cell performance will be reported in this presentation.

Acknowledgments

A part of this study was supported by the New Energy and Industrial Technology Development Organization (NEDO).

References

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