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Huff, Laura A.
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article
Synthesis of Manganese Oxide Microspheres by Ultrasonic Spray Pyrolysis and Their Application as Supercapacitors
Abstract
<p>Manganese oxide (MnO<sub>2</sub>) microspheres are prepared using an ultrasonic spray pyrolysis (USP) process. A mixed solution of potassium permanganate and hydrochloric acid is nebulized into microsized droplets, which are then carried by air flow through a furnace tube. Each microdroplet serves as one microreactor and produces one microsphere. Upon heating, KMnO<sub>4</sub> is decomposed into MnO<sub>2</sub> microspheres; this synthetic process can easily be scaled up. Characterization of the MnO<sub>2</sub> microspheres by scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectra is described. Different morphologies of MnO<sub>2</sub> microspheres can be controlled by tuning the precursor concentrations (and ratios) and furnace temperatures. Microspheres synthesized at 150 °C give amorphous MnO<sub>2</sub> while synthesis at 500 °C yields crystalline α-MnO<sub>2</sub>. The electrochemical properties investigated by cyclic voltammetry give specific capacitance as high as 320 F g<sup>-1</sup>, demonstrating promising properties as supercapacitors. In addition, these microspheres can be directly sprayed on conductive substrates, such as carbon fiber paper, and may have useful applications as a supercapacitor electrode coating. The supercapacitive properties of MnO<sub>2</sub> microspheres at higher charge and discharge rates can be improved by increasing the surface area coverage or coating them with a thin layer of conductive polymer.</p>