Charge Storage in Defect Engineered Oxide Nanosheets
Date
2022-02
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Publisher
New York State College of Ceramics at Alfred University. Inamori School of Engineering.
Abstract
Layered δ-MnO2 (birnessite) exhibits a unique response to the reduction of Mn4+ to Mn3+, where the reduced Mn3+ Jahn-Teller distorted octahedron creates a 'surface Frenkel defect.' The reduced Mn3+O6 octahedron migrates out of the the plane of the sheet and creates a vacancy within the sheet. The charged defect content may be controlled during flocculation of exfoliated nanosheets by equilibrating the nanosheet suspension in a pH regulated environment. It has been shown earlier that charged defects in δ-MnO2 improve electrochemical properties by increasing capacitance beyond 300 F/g, decreasing charge transfer resistance by a factor of 10, and improving life cycle stability. Building on earlier work, the present study aims to understand the synthesis of layered MnO2 and Mn-containing layered perovskites and their chemomechanical response during charge and discharge. X-ray total scattering, X-ray absorption spectroscopy, and Raman spectroscopy were employed to probe the chemomechanical response during electrochemical cycling of MnO2 electrodes in a custom-built electrochemical cell.
While surface Frenkel defect content remained constant throughout electrochemical cycling, reversible reduction of Mn4+ to Mn3+ occurred, presumably within the sheet. MnO2 floccules equilibrated at a pH of 2 and 4 underwent 17% and 13% reduction from Mn4+ to Mn3+ upon K+ intercalation, respectively. Such reductions caused mechanical deformation of porous MnO2 electrodes and was found to be dependent on defect content and degree of restacking of the nanosheets. Floccules with a low degree of nanosheet restacking experienced lateral expansion upon K+ insertion of 0.7% and 0.5% for equilibration at a pH of 2 and 4, respectively, with no change in the interlayer (basal) direction spacing. However, MnO2 nanosheet floccules with a high degree of restacking experienced a 1.1% and 1.2% lateral expansion upon charging and a contraction in their basal direction of 1.7% and 0.7% for pH 2 and 4 samples, respectively. The interlayer contraction has been noted earlier, but largely eliminating the sheet-to-sheet stacking provides direct access to knowledge of the in-sheet chemomechanical response.
Microstructures of air-dried and freeze-dried floccules as well as floccules with varying degrees of surface Frenkel defect content were similar. Surface Frenkel defect content remained constant across processing variations such as drying conditions and degree of restacking when equilibrated to the same pH.
Description
Thesis completed in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Materials Science and Engineering at the Inamori School of Engineering, New York State College of Ceramics at Alfred University
Type
Thesis
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Keywords
Nanostructured materials