Platinum Nanorod Arrays with Preferred Morphological and Crystal Properties for Oxygen Reduction Reaction

摘要

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/cm~2. 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.