Platinum Nanorod Arrays with Preferred Morphological and Crystal Properties for Oxygen Reduction Reaction
Abstract:Vertically aligned platinum (Pt) nanorod arrays produced by glancing angle deposition technique (GLAD) have been investigated for their morphological, crystallographic, and electrochemical properties as a potential catalyst material for oxygen reduction reaction (ORR) in hydrogen polymer electrolyte membrane (PEM) fuel cells. These single-layer and single-crystal catalyst nanorods without any carbon support have been produced at lengths varying between 20–600 nm, which correspond to Pt loadings of 0.016–0.48 mg/cm2. GLAD nanorods have been grown on tilted azimuthally rotating substrates in a sputter deposition unit at an oblique angle of 85° as measured from the substrate normal. Electrodes of sputtered Pt thin films deposited at normal incidence and commercially available carbon supported Pt nanoparticles (Pt/C) were also prepared for comparison. The scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) results reveal that Pt nanorods are well-isolated, vertically aligned, and single-crystal. The single-crystal property allows enhanced electrochemical activity and reduced surface oxidation, while the isolated nature of the rods in lateral directions can provide a channeled porosity for effective transportation of oxygen. Possible crystal orientations at the sidewalls as well as at the tip-facets of Pt nanorods have also been investigated. Cyclic voltammetry (CV) results show that well-defined multiple peaks exist in the CV profiles of Pt nanorods, which are absent or weak in conventional Pt/C and Pt film electrodes. These multiple CV peaks are due to the electrochemically more active crystal orientations in Pt nanorods compared to those of Pt/C and Pt thin film. In addition, Pt nanorod electrocatalysts exhibit a more positive reduction peak potential (less overpotential) and greater stability against electrochemically-active surface area loss compared to Pt/C due to their decreased oxophilicity, single-crystal property, and the dominance of the preferred crystal orientation for ORR.
Document Type: Research Article
Publication date: November 1, 2011
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