Date of Degree
Access restricted until 07/03/2019
PhD (Doctor of Philosophy)
Shaw, Scott K.
First Committee Member
Larsen, Sarah C.
Second Committee Member
Third Committee Member
Margulis, Claudio J.
Fourth Committee Member
Stone, Elizabeth A.
The electrochemical double layer (EDL) at the solid–liquid interface is the near surface region where important electrochemical processes (e.g., electrodeposition, corrosion, and heterogeneous catalysis) take place. Subtle changes in the electrode surface material/topography and the nature of the fluid medium can drastically alter interactions between liquid molecules and the solid surface. A better understanding of this interfacial region can help advance numerous applied fields, such as battery technologies, solar cells, double layer capacitors, and carbon dioxide capture/conversion.
Ionic liquids (IL) are an emerging class of solvents that could replace traditional aqueous/non-aqueous solvents due to their advantageous physiochemical properties (e.g., wide solvent window, high thermal stability, and excellent solvating power). However, our understanding of the near-surface structure of ILs in the EDL is still being developed. This thesis focuses on the fundamental electrochemical behavior of ILs to help understand its interfacial behavior in three main areas: 1) the nature of capacitance-potential relationships in neat ILs, 2) the role of ‘user-defined’ experimental variables on capacitive electrochemical measurements, and 3) the impact of IL + water mixtures on experimental data.
The general shape of capacitance-potential curves can suggest at the broad architecture of the EDL region. Fundamental capacitive studies of the IL EDL show a wide range of results, even for similar electrochemical systems. Theoretical predictions suggest the capacitance-potential curve should exhibit bell- or camel-shaped curvature depending on the nature of the IL. Experimental observations have demonstrated several functional shapes such as U-shaped, bell-shaped, camel-shaped, and relatively featureless responses. Much of the work in this thesis starkly contrasts theoretical expectations by demonstrating capacitive behavior that is analogous to high temperature molten salts and dilute aqueous electrolytes with metallic and non-metallic electrode materials. However, our systematic studies of a model IL electrochemical system reveal that there are several ‘user-defined’ experimental variables (i.e. potential scan direction, data acquisition protocol, experimental technique, and potential range probed) which in some instances can significantly impact the resulting capacitance curvature. Some of these variables are often overlooked in the literature and our efforts are aimed at uniting the scientific community in this area to help better compare and understand results. An additional experimental variable of importance is the sorption of water into ILs, which is nearly impossible to prevent due to their hygroscopic nature. The presence of water is known to have a significant effect on the resulting mixtures’ bulk and interfacial properties. While the interaction between ILs and water can significantly vary depending on the nature of the IL, this thesis demonstrates that within small quantities (e.g., < 5000 ppm) of sorbed water there are only minor changes in spectroscopic and electrochemical responses. Collectively, the work outlined in this thesis helps the scientific community better understand electrochemical measurements in IL solvents by examining key analytical variables associated with capacitive measurements.
The fundamental electrochemical studies described in this thesis demonstrate that the solid-liquid interface for IL solvents is response to even subtle changes in surface chemistries. These governing interfacial properties have ramifications in myriad applications from energy storage to lubrication.
Capacitance, Electrochemical Double Layer, Electrochemistry, Hysteresis, Ionic Liquid
xxi, 295 pages
Includes bibliographical references (pages 273-295).
Copyright © 2018 Anthony Joseph Lucio
Lucio, Anthony Joseph. "The electrochemical double layer in ionic liquids." PhD (Doctor of Philosophy) thesis, University of Iowa, 2018.