Electrochemical and in situ studies of the insertion of alkali metals in single -wall carbon nanotubes

Agnes Sandrine Claye, University of Pennsylvania

Abstract

In this thesis, we used electrochemistry and in-situ techniques to investigate the mechanism for alkali insertion in single wall carbon nanotubes (SWNT) and the properties of these doped compounds. SWNT were reversibly doped electrochemically with Li and K, up to stoichiometries of Li1.27C6 and KC24. No first order phase transition vs. composition was observed, distinguishing this new carbon guest-host system from graphite, C60 and polyacetylene. Ion insertion and removal proceed on a wide range of potentials, yielding steep voltage profiles characteristic of electrochemical capacitors. In-situ X-ray diffraction showed that alkali insertion in SWNT does not yield crystalline superlattices, but creates irreversible structural disorder in the rope lattice. We explain the absence of ordered phase in doped SWNT by the polydispersity in tube diameters and symmetries within a rope. The effect of alkali doping on the electronic properties of SWNT was investigated using in-situ conductivity, in-situ electron spin resonance and in-situ Raman scattering. All three methods showed a reversible charge transfer between the dopants and the host, similar to that in graphite intercalation compounds. The conductivity and spin susceptibility of SWNT increase monotonically and reversibly with alkali concentration, and the SWNT tangential phonon mode is downshifted in frequency upon doping. These phenomena are reversible upon de-doping, showing that SWNT constitute the newest generation of synthetic metals, analogous to GICs, fullerides and polyacetylene. Finally, electrochemical impedance spectroscopy was used to study the diffusion kinetics of alkalis in SWNT. An equivalent circuit was developed to model the complex impedance of SWNT electrodes. Three processes with different time constants were identified: charge transfer across the electrode/electrolyte interface, diffusion through mats of bulk SWNT, and diffusion inside SWNT ropes. The diffusion kinetics of alkalis in SWNT were found to be similar to those in polyacetylene films. Based on these results, we propose an inhomogeneous structural model for alkali-doped SWNT, in which alkalis decorate the external and internal surfaces of the ropes. We believe that alkali-doped SWNT consist of small fully doped domains embedded into undoped or very lightly doped regions. As doping proceeds the number and/or size of these saturated domains increases until saturation.

Subject Area

Materials science

Recommended Citation

Claye, Agnes Sandrine, "Electrochemical and in situ studies of the insertion of alkali metals in single -wall carbon nanotubes" (2000). Dissertations available from ProQuest. AAI9976408.
https://repository.upenn.edu/dissertations/AAI9976408

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