Oxygen Reduction Reaction Activity of Nano-Flake Carbon-Deposited Pt75Ni25(111) Surfaces

Abstract

Oxygen reduction reaction (ORR) activity was investigated for nano-flake-like carbon-modified Pt75Ni25(111) surfaces. Surface cleaning through Ar+-sputtering and thermal annealing in an ultra-high vacuum (~ 10−8 Pa) resulted in a Pt-enriched topmost surface, i.e., a Pt(111)-skin on Pt75Ni25(111). Arc plasma deposition (APD) of graphite under 0.08 Pa N2 and in vacuum (~ 10−6 Pa) generated nitrogen-doped and non-doped nano-flake-like carbon on the Pt(111)-skin surfaces, respectively. For the latter, non-doped carbon-modified Pt(111)-skin, the area-specific initial ORR activity estimated in O2-saturated 0.1 M HClO4 decreased with increasing thickness of the deposited carbon. In contrast, the former, nitrogen-doped carbon with 2 and 6 A mass-thickness enhanced the ORR activity. The Pt 4f band energies for the nitrogen-doped Pt(111)-skin were measured by X-ray photoelectron spectroscopy (XPS) and showed the chemical shift to higher biding energy (~ 0.2 eV) compared with the corresponding bands for the non-doped and Pt(111)-skin surfaces. As for the electrochemical structural stability, a specific amount of the non-doped carbon species tends to suppress the degradation of the Pt(111)-skin under applying potential cycles. The results indicate that the surface modifications by the carbon hexagonal networks of the nano-flakes could be applicable to improve ORR performance of the practical Pt-M alloy catalysts.