Nickel Nanowire Oxidation and Its Effect on Platinum Galvanic Displacement and Methanol Oxidation
Extended Pt structures have previously been studied for oxygen reduction (ORR) activity and have been found to have specific activity and durability benefits to Pt nanoparticles.1-3 Although extended catalysts can produce high specific activities, they generally lack the surface area to produce high mass activity. Catalysts formed by galvanic displacement are poised to produce high mass activities for ORR by maintaining the high specific activities demonstrated by extended Pt surfaces and by allowing for the deposition of thin Pt layers.
Nickel (Ni) nanowires were previously used as a template for galvanic displacement, producing Pt coated Ni nanowires with an outer diameter of 200‒300 nm and a length of 100‒200 µm.4 Electrochemical measurements taken in rotating disk electrode (RDE) half-cells found that the Pt coated Ni nanowires produced surface areas as high as 91.3 m2 gPGM‒1 and mass ORR activities as high as 917 mA mgPGM‒1. These materials (Ni nanowires and Pt coated Ni nanowires) have been treated in oxygen at elevated temperatures. By heat treating in oxygen: the effects of an increasing Ni nanowire oxide layer on Pt displacement were examined; and durability losses (surface area and activity) of Pt coated Ni nanowires were reduced.
As-received Ni nanowires contain a thin oxide layer and a Ni metal core. Galvanic displacement of the untreated Ni nanowires was limited to less than 17 wt % Pt without the inclusion of additional acid to remove the oxide layer. Further heat treatment of the Ni nanowires with oxygen served to grow the oxide layer. At relatively low temperatures, oxide content near the nanowire surface increased; at higher temperatures, a NiO phase grew and consumed the metal nanowire core. Increasing the oxide layer served to limit and eventually shut off Pt displacement entirely.
Although Pt coated Ni nanowires produce high mass activities, losses (similar to Pt/HSC) were observed following RDE durability testing (30,000 potential cycles, 0.6‒1.0 V vs. RHE, similar protocol to DOE working group).5,6 While extended Pt structures have previously been found to offer durability benefits to Pt/HSC, the benefit is likely lessened by the presence of Ni metal. By heat treating Pt coated Ni nanowires in oxygen, the oxide layer thickness was increased thereby stabilizing the catalysts in potential cycling. Treated catalyst produced initial surface areas and activities that were lower than the untreated Pt coated Ni nanowires, presumably since Ni near the surface could not be removed electrochemically during break-in. With potential cycling, however, treatment of the nanowires dramatically improved retention of surface area and activity (Figure 1).
Following durability testing, treated Pt coated Ni nanowires produced a mass ORR activity of 680 mA mgPGM‒1, or a 9 % improvement of initial activity. Compared to conventional Pt electrocatalysts and fully displaced Pt nanotubes, Pt coated Ni nanowires appear to provide significant activity and durability advantages.
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