In-situ NMR Study of Molecular and Ionic Processes inside Carbon Nanopores

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  • March 19, 2019
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  • Luo, Zhixiang
    • Affiliation: College of Arts and Sciences, Department of Physics and Astronomy
Abstract
  • Interactions of simple ions with water and interfaces play critical roles in many electrochemical and biological processes. They are especially significant in nanoconfined regions and have a profound impact in many applications, for instance nanofluidics and supercapacitors. This dissertation employs a nuclear magnetic resonance (NMR) technique to study their influence on the ionic processes inside carbon nanopores. To characterize the carbon micropore structure, a convenient NMR method is established by taking a 1H magic angle spinning (MAS) spectrum of the adsorbed water. A density functional theory (DFT) computation of the nucleus-independent chemical shift (NICS) yields a quantitative relationship between the NICS values and the micropore sizes. The carbon micropore size and distribution are derived from the chemical shift and the spectrum lineshape. For aqueous electrolytes inside uncharged carbon nanopores, the measurement of ion concentrations reveals a substantial electroneutrality breakdown. The specific ion effects and ion-ion correlations are shown to play crucial roles in determining the degree of electroneutrality breakdown. The importance of those interactions is further revealed by the asymmetric and nonlinear responses of ion concentrations to the charging of the confining carbon walls. Such information is obtained with a carbon supercapacitor built into the NMR probe. The NMR observations are validated by a numerical calculation of the ion distribution in the nanopores using the generalized Poisson-Boltzmann (PB) equation, demonstrating that the nonelectrostatic interfacial interactions can indeed dominate the electrostatic interactions and lead to the breakdown of electroneutrality inside nanoconfined regions. Interfacial ion hydration is an essential part of the specific ion effects. Using in-situ 23Na and 19F NMR on carbon supercapacitors with different carbon pore sizes, I provide a molecular-scale understanding of the permeation and dehydration of ions in voltage-gated carbon nanopores.
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  • In Copyright
Advisor
  • Warren, Warren
  • Branca, Rosa Tamara
  • McNeil, Laurie
  • Evans, Charles
  • Washburn, Sean
  • Wu, Yue
  • Gubbins, Keith
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2015
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  • Chapel Hill, NC
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