Date of Award

Spring 1-1-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Charles B. Musgrave

Second Advisor

Steven M. George

Third Advisor

J. W. Medlin

Fourth Advisor

Sehee Lee

Fifth Advisor

Prashant Nagpal

Abstract

Of the many known stoichiometries and crystal phases of manganese oxide, a select few have been found to have remarkable properties for electrochemical charge storage. Among these are hollandite α-MnO2 and spinel λ-MnO2, which are studied in this work. α-Mn O2 has been observed to have a high electrochemical charge storage capacity with incredible rate capability and cycle lifetime in aqueous alkali salt electrolytes. In these conditions, this material also exhibits the unique property of pseudocapacitance, meaning that the electrochemical behavior mimics the properties of a parallel-plate dielectric capacitor. However, the underlying mechanism for charge storage in α-MnO2, and the origins of pseudocapacitance are generally not understood. Conversely, λ-MnO2 is a well-studied cathode for lithium ion batteries with a high capacity and rate for charge storage, which is known to degrade quickly, especially at elevated temperatures. In this work, we study both pseudocapacitive Mn O2 and λ-MnO2 using computational chemistry and experimental techniques. Computationally, the charge storage properties of these two materials are framed within a band-diagram and electronic structure framework to elucidate the e_ect of physical properties such as band gap, work function, point of zero charge, and pH on charge storage. Experimentally, thicknesses up to 200 nm of MnO grown by atomic layer deposition (ALD) are converted by electrochemical treatment into pseudocapacitive NaMn4O8 and spinel LiMn2O4. By studying the electrochemical properties of pseudocapacitive NaMn4O8 and spinel LiMn2O4 at varying thicknesses, a more detailed mechanistic understanding for charge storage in each material is established. This powerful combination of theoretical and experimental techniques provides a detailed picture of charge storage in these materials, which can be extended to other electrochemical charge storage materials.

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