Synthesis and Electrocatalytic Activity of Pt-Ni Nanowire

Abstract

Polymer electrolyte fuel cells (PEFCs) are known as sustainable devices and can produce the electricity with no emission of CO2 gas. The oxygen reduction reaction (ORR) occurs at the cathodes in PEFCs and platinum nanoparticles (NPs) are generally used as an ORR catalyst. Pt on the surface of the catalysts can be oxidatively dissolved under catalytic conditions and then their ORR activity decreases.[1, 2] Because alloying and nanostructuring are known to improve the tolerance of the catalyst to the oxidative dissolution[3], we have studied Pt-Ni nanowires (NWs) as an ORR catalyst.[4] NWs can be expected to show stronger interactions between the Pt surfaces and carbon supports than NPs.[5, 6] However, there is a lack of detailed researches on the durability of NWs.In this research, we synthesized Pt-Ni NWs, conducted an accelerating durability test (ADT) for the ORR activity and investigated the effect of the ADT on the electronic states of NWs by ex situ X-ray photoelectron spectroscopy (XPS) and in situ Pt L3-edge X-ray absorption spectroscopy (XAS). We also characterized NWs by scanning transmission microscopy (STEM), inductively coupled plasma mass spectrometry (ICP-MS) and powder X-ray diffraction (pXRD). Cyclic and linear sweep voltammograms of NWs showed that the mass activity of the NWs was higher than that of NPs. Interestingly, the mass activity of the NWs increases after the ADT whereas the activity of typical NPs decreases.[7] XPS peaks of the NW catalyst in the Pt 4f region were shifted to the higher binding energy after the ADT, compared to those before the ADT. This result suggests that the d-band center of the surface Pt of the NWs were downshifted after the ADT.[8] In situ Pt L3-edge XAS revealed that the potential dependence of the intensity of the NW catalyst in the white line region after the ADT is smaller than that of before the ADT. This result suggests that the formation of Pt oxide was suppressed after the ADT,[1] leading to the improvement of the mass activity.