Introduction Pt-Ru on the carbon support has been regarded as a bimetallic catalyst with high CO tolerance for polymer electrolyte fuel cells (PEFCs). The addition of metal oxides such as SnO₂ into the carbon support has also been reported to further enhance the CO tolerance^1)-2). We have investigated the effect of valency of Sn in Sn-modified Pt-Ru catalyst on CO tolerance and HOR activity.

Experimental Sn in Sn-modified Pt-Ru catalysts was obtained by a rapid quenching method³⁾. Pt-Ru-Sn/C and Ru/SnO₂/C catalysts were treated in H₂ and He at 800 ^oC, respectively. The catalysts were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Hydrogen oxidation reaction (HOR) activities of the catalysts were determined by single cell tests.

Results and Discussion The TEM analysis, as shown in Fig. 1, indicates that the particle sizes of Pt-Ru/SnO₂/C and Pt-Ru-Sn/C are around 2.5 nm, which are slightly smaller than commercial Pt-Ru/C (TKK, TEC61E54). Since the particle sizes of SnO­­₂ are less than 2 nm, the interaction between Pt-Ru and SnO₂ is expected. Without the CO contamination, the terminal voltage of the cell using Pt-Ru-Sn/C is 752 mV at the current density of 0.2 A cm^-2, which is 18 mV lower than those using Pt-Ru/SnO₂/C and Pt-Ru/C (TKK, TEC61E54), as shown in Fig. 2. For Pt-Ru-Sn/C, the heat treatment in H₂­ at 800 ^oC possibly triggers the formation of the trimetallic Pt-Ru-Sn alloy by the reaction Pt-Ru + SnO₂ + 2H₂ → Pt-Ru-Sn + 2H₂O. HOR activities are suppressed by the formation of the trimetallic alloy. On the other hand, the terminal voltage of the cell using Pt-Ru/SnO₂/C is as high as 599 mV even in the presence of 1000-ppm CO contamination, which is 27 and 128 mV higher than those using Pt-Ru-Sn/C and Pt-Ru/C (TKK, TEC61E54), respectively. Modification with Sn clearly enhances the CO tolerance. From the above, it is concluded that Sn in the Pt-Ru-Sn alloy reduces the HOR activity, while SnO₂ does not. Both of metallic Sn in the Pt-Ru-Sn alloy and SnO₂ encourage the CO tolerance.

References

1) T. Takeguchi et al., Catal. Sci. Technol., 6, 3214-3219 (2016).

2) G. Wang et al., J. Electrochem. Soc., 156, B862-B869 (2009).

3) T. Takeguchi et al., J. Am. Chem. Soc., 134, No. 35, 14508-14512 (2012).

Acknowledgements

This research was partially supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan.