Ion electrosorption in nanoporous carbons

Christian Prehal

Research output: ThesisDoctoral Thesis

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A fundamental understanding of the mechanisms controlling ion charge storage and transport in confinement of carbon nanopores is essential to improve the performance of supercapacitors or devices for capacitive desalination. A key to verify detailed predictions of atomistic simulations is information obtained from experimental data about the spatial distribution of ions and solvent molecules within the nanopores. In the framework of the present thesis in situ small angle X-ray scattering (in situ SAXS) was established as an experimental method to study ion rearrangements during charging and discharging of an in situ supercapacitor cell, using aqueous electrolytes and nanoporous carbons as electrodes. The high X-ray flux of the Austrian SAXS beamline at the Synchrotron radiation source ELETTRA (Trieste, Italy) enables time resolutions in the sub-second regime. A detailed analysis of the X-ray transmission (XRT) signal allowed a quantification of the global cation and anion concentration change as a function of the applied cell voltage. During charging, only the balance between counter-ion and co-ion concentration is disturbed, where the total ion concentration remains constant. A first attempt to analyze the SAXS data revealed also local ion rearrangements within the nanopore structure, by interpreting correlation length parameters as a function of electrode charge and considering a so-called two-phase model. However, the complexity induced by the multiphase character of the system makes a detailed interpretation of the in situ SAXS data and understanding of the ongoing physical processes a challenging task. For that reason a novel data analysis strategy was developed, involving atomistic Monte Carlo simulations to obtain equilibrium ion positions for each electrode charge within structural models of the nanoporous carbons. Carbon models and ion concentration are determined by experimental SAXS and XRT data. Subsequent Fourier transformation of the simulation box yields simulated SAXS curves which can in turn be compared with the experimental in situ SAXS measurements as a function of the electrode charge. This approach allowed the precise prediction of location (quantified by a degree of confinement) and desolvation of ions within the carbon nanopore confinement. As a main result, charge was found to be stored most effectively in sites of the carbon structure with highest possible geometrical confinement, accompanied with partial desolvation of ions. Moreover length-scale dependent ion kinetics was found by investigating in situ SAXS data applying cyclic voltammetry with different scan rates.
Translated title of the contributionIonen-Elektrosorption in nanoporösen Kohlenstoffen
Original languageEnglish
  • Presser, Volker, Assessor B (external), External person
  • Paris, Oskar, Assessor A (internal)
Publication statusPublished - 2017

Bibliographical note

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  • in situ SAXS
  • supercapacitors
  • nanopores
  • energy storage
  • Synchrotron
  • ion electrosorption

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