Surface Enhanced Raman Spectroscopy and Cyclic Voltammetry Studies of Ni-rich Oxide Nanowires as Electrode Materials
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Surface Enhanced Raman Spectroscopy and Cyclic Voltammetry Studies of Ni-rich Oxide Nanowires as Electrode Materials Que Anh S. Nguyen, Yash V. Bhargava, Shawn A. Thorne, Tzipi Cohen-Hyams, and Thomas M. Devine Materials Science & Engineering, University of California, Berkeley, CA, 94720 ABSTRACT A novel electrode material for Ni metal hydride batteries, composed of electrochemically synthesized, Ni-rich oxide nanowires, was investigated in this study. The nanowires, grown on Alloy 600 in a high-temperature, high-pressure aqueous electrolyte, are typically 25nm in diameter and 5-10um long. Cyclic voltammetry, with surface enhanced Raman spectroscopy (SERS) suggests that anodic polarization of the wires converts Ni2+ species to Ni3+, present as γ−NiOOH, the chemical phase involved in the charge/discharge reaction in Ni-MH batteries. The reversibility of the reaction was confirmed via multiple polarization cycles and SEM imaging. In addition, the current density and polarization behavior of the nanowires in 0.01M KOH solution is compared to that of planar NiO and Alloy 600. Results suggest that the nanowires, due to their higher surface area, achieves at least 10 times the current density (for a given apparent area) of the non-nanowire samples. INTRODUCTION Nanowire and nanotube structures hold great promise for use in device applications due to their unique electronic, optical, and magnetic properties [1]. Recently, nanowires have been utilized in chemical and biological sensors [2-4], lasers [5], integrated circuits [6], and solar cells [7]. In particular, the use of nanowire materials in battery devices is very appealing. Because of the high surface area available for electrochemical reactions to occur, there is potential for great increases in device efficiency when nanomaterials are utilized. In studies involving Ni metal hydride (Ni-MH) batteries, it has been reported that increasing the specific surface area of Ni(OH)2 provides a high density of active reaction sites and promote the material’s interaction with the surrounding electrolyte [8-9]. In addition, it has been shown that as the electrode’s crystallite size decreases, nickel utilization increases, leading to better charge/discharge cycling behavior [10]. In this work we present a novel Ni-rich oxide nanowire material, synthesized via electrochemical means. The wires, grown directly onto a metal substrate, offer the possibility of a high performing and cost effective electrode material. Surface enhanced Raman spectroscopy, along with cyclic voltammetry analysis were performed on the wires to examine their stability and electrochemical performance. EXPERIMENTAL DETAILS Nanowire synthesis involved the electrochemical oxidation of Alloy 600 (Ni-15%Cr8%Fe) in an aqueous solution containing 2ppm Li and 1200ppm B. The oxidation was carried out in a Ti autoclave at 288oC and 1500psi, where the high-pressure environment was required to prevent the solution from vaporizing at the growth temperature. A series of reducing and oxidizing potential was
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