Polymer electrolyte fuel cells (PEFCs) are clean, sustainable devices and can produce electricity without the emission of CO₂ gas. The oxygen reduction reaction (ORR) catalysts are required for the cathode in PEFCs because the ORR is sluggish. Pt shows the highest ORR activity in the metals. The ORR activity of Pt has been improved by alloying with 3d transition metals such as Ni,¹⁾ and Pt–Ni alloys have been used for the cathode in PEFCs. The ORR activity was also improved by using nitrogen-doped carbon (NC) as supports for PGM nanoparticles.²⁾ This is because platinum–nitrogen interaction finely tunes the electronic structure of catalyst surface.

Herein, we studied Pt–Ni nanoframe³⁾ supported on NC (Pt–Ni NF/NC) or non-doped carbon black (Pt–Ni NF/C) for the ORR.⁴⁾ In order to clarify support effects on the Pt–Ni NF, we performed in situ Pt L₃-edege X-ray absorption fine structure (XAFS) measurements of Pt–Ni NF/NC and Pt–Ni NF/C. The ORR activity of Pt­–Ni NF/NC was higher than that of Pt­–Ni NF/C. In situ Pt L₃-edge XAFS spectroscopy of Pt­–Ni NF/NC showed almost no potential dependence of the intensity of the white line region, whereas Pt­–Ni NF/C showed some changes. This result suggests that the formation of Pt surface oxide was suppressed for Pt­–Ni NF/NC under electrochemical conditions, leading to the improvement of the ORR activity.

Acknowledgments　
This work was supported by NEDO.

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