Origin of improved electrochemical activity of β-MnO₂ nanorods: effect of the Mn valence in the precursor on the crystal structure and electrode activity of manganates
Date
2009
Authors
Kim, I.Y.
Ha, H.W.
Kim, T.W.
Paik, Y.
Choy, J.H.
Hwang, S.J.
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Journal article
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Journal of Physical Chemistry C, 2009; 113(51):21274-21282
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Abstract
1D nanorods/nanowires of manganese oxides with different crystal structures and morphologies were prepared and characterized to understand the influence of the Mn valence in the solid-state precursor on the electrochemical activity of these nanomaterials and to elucidate the mechanism responsible for the excellent activity of β-MnO₂ nanorods as well. According to powder X-ray diffraction analyses, treating manganese oxide precursors that have an oxidation state of ≤+3 with persulfate ions under hydrothermal conditions yields manganese oxides with the β-MnO₂ structure. In contrast, the use of a LiMn₂O⁴ precursor with a higher Mn valence leads to the formation of the α-MnO₂-structured manganese oxide. Electron microscopic studies clearly show a 1D nanorod-type morphology for the β-MnO₂ material, whereas a 1D nanowire-type morphology with a higher aspect ratio is observed for the α-MnO₂ material. The diameter of the β-MnO₂ nanorods decreases as the Mn valence in the precursors becomes smaller. According to electrochemical measurements, the formation of nanorods dramatically improves the electrode performance of the β-MnO₂ phase. This compares with a relatively weak performance enhancement for the α- and δ-MnO₂ phases upon the nanowire formation. The optimum electrode property results from the smaller β-MnO₂ nanorods prepared with the MnO precursor. ⁷Li magic angle spinning nuclear magnetic resonance spectroscopy clearly demonstrates that Li+ ions in the lithiated β-MnO₂ phase are adsorbed mainly on the sample surface. On the basis of this finding, we attribute the improved electrode performance of the β-MnO₂ nanorods to their expanded surface area.
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Data source: Supporting information, http://pubs.acs.org/doi/suppl/10.1021/jp908556h
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Copyright 2009 American Chemical Society