Morphology-controlled Growth of Manganese Oxide Electrodes
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Morphology-controlled Growth of Manganese Oxide Electrodes Banafsheh Babakhani 1 and Douglas G. Ivey 1 1 Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2V4 ABSTRACT Manganese oxide electrodes are synthesized by anodic deposition on Au coated Si substrates from acetate-containing aqueous solutions. By changing the deposition parameters including deposition current density, electrolyte composition, pH and temperature, a series of nanocrystalline manganese oxide electrodes with various morphologies (non–uniform continuous coatings, rod–like structures, aggregated rods and thin sheets) is obtained. Detailed microstructural characterization of as-deposited electrodes is conducted using scanning electron microscopy (SEM). Electrochemical analysis using cyclic voltammetry showed that manganese oxide electrodes with rod–like and thin sheet morphology exhibit enhanced electrochemical performance by improving manganese oxide utilization. The highest specific capacitance (~230 F g−1) and capacitance retention rates (~88%) are obtained for manganese oxide thin sheets after 250 cycles in 0.5M Na2SO4 at 20 mV s−1. INTRODUCTION The growing interest in electrochemical supercapacitors has arisen from their potential application as electrical storage devices operating in parallel with batteries to provide high power densities [1]. The surface morphology of electrode materials for supercapacitor applications, which depends on the preparation method and processing parameters, significantly affects their capacitive behavior. Various structures, such as nanowires [2], nanorods [3], nanotubes [4] and nanosheets [5], have been reported to exhibit outstanding charge storage properties because they have large surface areas for charge storage and fast redox reactions. Among different transition metal oxides, manganese oxide has attracted considerable attention due to its relatively high capacitance and low cost. Many investigations have attributed the capacity of manganese dioxides to their structural, morphological and compositional characteristics [6–11]. Electrodeposition of manganese dioxide in aqueous solutions is achieved through the oxidation of Mn2+ [12–14]. Mn2+ + H2O
MnO2 + 4H+ + 2e–
[1]
Morphology–controlled growth can generally be achieved by controlling the deposition parameters. Manganese oxide electrodes with different surface morphologies can be obtained under different electrodeposition conditions [6,8-10]. However, there still has been little done to clarify the growth mechanisms of manganese oxide electrodes and the relationship between the synthesis parameters and morphological, structural and electrochemical properties of the electrodes. One of the authors [15] has shown that the morphology of manganese oxides is primarily determined by the supersaturation ratio, which affects the reaction kinetics in aqueous solutions [15]. This paper reports on the synthesis and growth mechanisms of manganese oxide electrodes prepared from manganese acetate–containing aqueous so
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