For the widespread use of fuel cell vehicles, it is necessary to improve the performance and durability of the cathode catalyst of polymer electrolyte fuel cells (PEFCs). In particular, high-temperature operation of PEFCs is attracting attention for application to commercial vehicles for heavy-duty application. Operation at higher temperatures than the conventional one (60-80°C) is expected to enhance oxygen reduction kinetics and mass transfer and reduce specific adsorption of ionomers. Despite such advantages, due to the thermal effect, the oxide formation on Pt surface is expected to be enhanced and decrease the activity of the catalyst. However, the quantitative relationship between catalyst behavior and the electronic structure of Pt under high temperature operation has not yet been elucidated. Here, we developed an MEA cell for high temperature and high pressure conditions and analyzed the oxidation state and local structure of Pt on Pt/C catalysts at different temperatures by operando XAS measurements.

MEAs were prepared with Pd/C as anode catalyst, Pt/C as cathode catalyst. XAS measurements of the Pt L_III absorption edge were made at 80°C, 105°C, and 120°C with 100% H₂ flowing to the anode and Air flowing to the cathode, and spectra were collected by the transmission mode. The area values of the XANES spectra were calculated to compare the relationship of Pt oxide formation with operating temperature and the local structural model of Pt oxide at high temperature by EXAFS.

Comparison of operando XANES results of cathode catalyst Pt/C under conventional operating temperature (80°C), 105°C, and 120°C showed a marked difference, especially at 80°C and 120°C. In addition, the XANES area values increased as the operating temperature was increased. This is thought to be due to the fact that Pt oxide formation is accelerated at higher temperatures. Next, comparing the EXAFS analysis results at each temperature, the Pt-O bond peak is stronger at 120°C, indicating that Pt oxidation is more advanced than at 80°C. The 120°C results can be explained by a mixed model of β-PtO₂ and Pt metal. Pt-O bond coordination number analysis shows that β-PtO₂, in which oxygen is latent inside Pt, is formed more frequently at high temperatures. From this result, it was found that the structure of Pt oxide is different in high-temperature systems, and that catalyst design must take this into account.

Figure 1. Pt L_III-edge XANES spectra Area vs. Applied voltage for pure H₂ at the cathode of Pt/C. MEA temperature: blue circle is 80°C, black circle is 105°C, red circle is 120°C.

Figure 2. Pt L_III-edge EXAFS Pt-O bond coordination Number vs. Applied voltage for pure H₂ at the cathode of Pt/C. MEA temperature: blue triangle is 80°C, black triangle is 105°C, red triangle is 120°C.

Acknowledgement

This work was supported by a NEDO FC-Platform project commissioned by the New Energy and Industrial Technology Development Organization (NEDO)

References:

[1] Hideto I.; Koichi I.; Masashi M.; Yoshimi K.; Kazuo K.; Yasuhiko I., J. Am. Chem. Soc., 2009, 131 (17), 6293-6300.