Oxygen Reduction Reaction Electrocatalytic Activity of SAD-Pt/GLAD-Cr Nanorods
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Oxygen Reduction Reaction Electrocatalytic Activity of SAD-Pt/GLAD-Cr Nanorods Wisam J. Khudhayer a*, Nancy Kariuki b, Deborah J. Myers b, Ali U. Shaikh c, and Tansel Karabacak d Departments of Systems Engineering, c Chemistry, d Applied Science, University of Arkansas at Little Rock, Little Rock AR, 72204, USA b Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL 60439-4837, USA
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ABSTRACT Nanorod arrays of chromium (Cr) were grown on glassy carbon (GC) electrodes by a dc magnetron sputtering glancing angle deposition (GLAD) technique. The Cr nanorods were used as low-cost, high surface area, metallic supports for a conformal layer of Pt thin film catalyst, as a potential low-loading electrocatalyst for the oxygen reduction reaction (ORR) in polymer electrolyte membrane (PEM) fuel cells. A dc magnetron sputtering small angle deposition (SAD) technique was utilized for a conformal coating of Pt on Cr nanorods. The ORR activity of SAD-Pt/GLAD-Cr electrodes was investigated using cyclic voltammetry (CV) and rotating-disk electrode (RDE) techniques in a 0.1 M HClO4 solution at room temperature. A reference sample consisting of GLAD Cr nanorods coated with a Pt thin film deposited at normal incidence (θ = 0o) was prepared and compared with the SAD-Pt/GLADCr nanorods. Compared to GLAD Cr nanorods coated with Pt thin film at θ = 0o, the SADPt/GLAD-Cr nanorod electrode exhibited higher ECSA and area-specific and mass-specific ORR activity. These results indicate that the growth of catalyst layer on the base-metal nanorods by the SAD technique provides a more conformal and possibly a nanostructured coating, significantly enhancing the catalyst utilization. INTRODUCTION The oxygen reduction reaction (ORR) at the cathode electrode of polymer electrolyte membrane (PEM) fuel cell is an important and well-studied electrochemical reaction because the slow kinetics of ORR causes it to be the dominant source of PEM voltage losses. 1 Typical cathode and anode electrocatalysts are platinum catalyst nanoparticles (3-5 nm in size) supported on carbon black. 2 In addition to the cost issue, this type of electrocatalyst faces other challenges related to the carbon support, summarized as follows: Oxidation of the carbon support causes catalyst loss, 3 the carbon support facilitates the formation of peroxide species that lead to degradation of the membrane polymer, 4 and carbon separates from the ionomer over time, leading to loss of catalyst utilization. 3 Therefore, extensive effort is currently underway to develop high-performance, durable, carbon-free, and low cost (low Pt loading) electrocatalyst materials. 1 For example, the 3M Company has demonstrated the improved durability and area-specific activity of nanostructured thin film Pt (NSTF Pt) electrocatalyst layers consisting of large-grained polycrystalline Pt thin film deposited on and encapsulating oriented crystalline whiskers of an organic pigment material.5 A potential disadvantage of their support material, however, is the decomposition of th
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