Defective Manganese and Vanadium Oxide Nanosheets for Electrochemical Supercapacitors

Date

2018-08

Journal Title

Journal ISSN

Volume Title

Publisher

New York State College of Ceramics at Alfred University. Inamori School of Engineering.

Abstract

Electrochemical supercapacitors, which can provide higher power density than batteries and higher energy density than electrostatic capacitors, have received great attention in recent years as promising alternative energy storage devices. δ-MnO2 and VO2 nanosheets are considered as most promising electrode materials, due to their low cost, environmental friendliness, and large capacitance. Especially, their layered structures can provide high-speed pathways for the intercalation of protons or alkali cations during electrochemical cycling, which leads to excellent charge storage capability. While a lot of effort has been devoted to improve the electrode performance by microstructure control or load the samples on conducting materials, new intrinsic approaches are urgently needed to realize more and faster charge storage. in this study, a new strategy has been proposed to increase the capacitance of δ-MnO2 and VO2 nanosheets by intentionally creating defects (such as cation vacancies) in their lattice structures. δ-MnO2 nanosheets are prepared by exfoliating the parent crystals, and then flocculated under carefully controlled experimental conditions. The obtained 3-D porous assemblies with 150 m2/g specific surface area are equilibrated in varied pH, in order to create charged defect pairs we term 'surface Frenkel Defects.' The XANES data demonstrates an increase of the Mn3+/Mn4+ ration with decreasing pH equilibration values. The X-ray scattering and PDF analysis shows that the Mn surface Frenkel defect content reaches 26.5% for the nanosheet assemblies equilibrated at pH = 2 and 19.9% for the pH = 4 sample, indicating that equilibration at lower pH leads to the formation of more Mn vacancies in the reassembled δ-MnO2 nanostructures. The electrochemical results show that the specific capacitance increased from about 200 F/g (pH = 4) to over 300 F/g (pH = 2) by intentional introduction of ~30% surface Frenkel defects, while at the same time the charge transfer resistance decreased from ~15 Ω to ~3 Ω, indicating direct correlation of Mn cation defects with specific capacitance. The alkali cation intercalation mechanism has also been investigated through in-situ X-ray PDF and XANES measurements. The in-situ XRD and PDF data show reversible expansion/contraction of the nanosheet layers upon charge/discharge, as well as unchanged interlayer spacing during cycling. The in-situ XANES data exhibits a reversible shift of the absorption edge to lower energies for different pH equilibrated samples when decreasing the applied potentials, indicating the reduction of Mn4+ to Mn3+ and confirms that the Faradaic redox reaction is the main charge storage mechanism in the defective MnO2 nanosheet system. A slower reduction of Mn for the MnO2 nanosheets with higher defect content has been observed when comparing the oxidation state derived from XANES and that calculated from cyclic voltammetry, emphasizing the important role of defects in the charge storage process without affecting the Mn oxidation states. The pure and Mn-doped VO2(B) nanosheets have been prepared by hydorthermal methods. Mn incorporation leads to porous and more open structures, which facilitates sodium ion intercalation and thereby greatly improves its charge storage performance. In general, this work provides a new way to the design next generation electrochemical supercapacitors through controlling the defect structures of layered transition metal oxide nanosheets.

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

Keywords

Nanostructured materials, Supercapacitors

Citation

DOI