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Glancing Angle Deposited Platinum Nanorod Arrays for Oxygen Reduction Reaction Wisam J. Khudhayer1, Nancy Kariuki2, Deborah Myers2, Ali Shaikh3, and Tansel Karabacak4 1

Department of Applied Science, Engineering Science and Systems, University of Arkansas at Little Rock, AR, 72204, USA 2

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL 60439-4837, USA 3

Department of Chemistry, University of Arkansas at Little Rock, AR, 72204, USA

4

Department of Applied Science, University of Arkansas at Little Rock, AR, 72204, USA

ABSTRACT In this work, we investigated the electrocatalytic oxygen reduction reaction (ORR) activity of vertically aligned, single-layer, carbon-free, and single crystal Pt nanorod arrays utilizing cyclic voltammetry (CV) and rotating-disk electrode (RDE) techniques. A glancing angle deposition (GLAD) technique was used to fabricate 200 nm long Pt nanorods, which corresponds to Pt loading of 0.16 mg/cm2, on glassy carbon (GC) electrode at a glancing angle of 85o as measured from the substrate normal. An electrode comprised of conventional carbon-supported Pt nanoparticles (Pt/C) was also prepared for comparison with the electrocatalytic ORR activity and stability of Pt nanorods. CV results showed that the Pt nanorod electrocatalyst exhibits a more positive oxide reduction peak potential compared to Pt/C, indicating that GLAD Pt nanorods are less oxophilic. In addition, a series of CV cycles in acidic electrolyte revealed that Pt nanorods are significantly more stable against electrochemically-active surface area loss than Pt/C. Moreover, room temperature RDE results demonstrated that GLAD Pt nanorods exhibit higher area-specific ORR activity than Pt/C. The enhanced electrocatalytic ORR activity of Pt nanorods is attributed to their larger crystallite size, single-crystal property, and the dominance of (110) crystal planes on the large surface area nanorods sidewalls, which has been found to be the most active plane for ORR. However, the Pt nanorods showed lower mass specific activity than the Pt/C electrocatalyst due to the large diameter of the Pt nanorods. INTRODUCTION Polymer electrolyte membrane (PEM) fuel cells are electrochemical energy conversion devices that have attracted great interest as a primary power source for electric vehicles due to their relatively low operating temperature, high efficiency, and low emissions [1-3]. However, the high cost of the noble metal (e.g., Pt) electrocatalysts used in PEM fuel cells is one of the major barriers to their commercial viability. Compared to anodic hydrogen oxidation reaction (HOR), the oxygen reduction reaction (ORR) at the cathode is a sluggish reaction and typically the major contributor to the efficiency loss in an operating PEM fuel cell [1-4]. Typical cathode and anode electrocatalysts are comprised of layers of platinum catalyst nanoparticles (3-5 nm in

size) supported on carbon black [3]. In addition to the cost issue, this type of electrocatalyst faces other challenges related to the carbon support