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Fabrication of porous carbon nanofibers with adjustable pore sizes and their composites as electrodes for supercapacitors
Dissertation   Open access

Fabrication of porous carbon nanofibers with adjustable pore sizes and their composites as electrodes for supercapacitors

Chau Duc Tran
Doctor of Philosophy (Ph.D.), Drexel University
Jun 2014
DOI:
https://doi.org/10.17918/etd-7299
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Abstract

Carbon nanofibers Electrospinning Supercapacitors Chemical Engineering Molecular Dynamics
Porous carbon nanofibers have been fabricated via pyrolysis/carbonization of electrospun nanofiber mats of blends of polyacrylonitrile (PAN)/Nafion. While PAN serves as the carbon precursor, Nafion serves as a sacrificial polymer forming intra-fiber pores within carbon nanofibers resulting in specific surface area of up to 1600 m2 g-1. Porous CNFs were incorporated as freestanding electrodes in electric double layer capacitors (EDLCs) without any additive/binder. Electrochemical measurements show large specific capacitances of 210 F g-1 and 60 F cm-3 in 1 M H₂SO4. We achieved an energy density of 4 Wh kg-1 at a high power density of 20 kW kg-1. The hierarchical pore structure in porous CNFs allowed faster diffusion kinetics resulting in high power. To further improve energy density, the samples were activated using potassium hydroxide (KOH) to increase specific surface area to over 2200 m2 g-1 and tested in ionic liquid electrolyte, 1-ethyl-3-methylimidazolium bis(trifluoromethane sufonyl)imide. The materials exhibit a specific capacitance of ~190 F g-1 with a 4 V voltage window resulting in an energy density of 75 Wh kg-1. In-operando spectroelectrochemistry studies were conducted in collaboration with Prof. Yossef Elabd to better understand the effect of activation on ion transport dynamics. To further enhance charge storage and volumetric performance, we incorporated pseudocapacitive polyanilene into porous CNFs with the aim to integrate the benefits of EDLCs (high power, cyclability) and pseudocapacitors (high energy density). By using galvanostatic electropolymerization, we achieved a thin conformal coating of polyaniline on porous CNFs. The combination of two different storage mechanisms resulted in excellent specific capacitance of 365 F g-1, 140 F cm-3 and 1.7 F cm-2. We integrated our experimental work with molecular dynamics (MD) simulations to understand the effect of elongational flow, a key characteristic of electrospinning, on assembly within phase separating polymer blends. This work was motivated by the successful prevention of phase separation within electrospun nanofibers of immiscible blends of Nafion and PAN, which enabled us to successfully develop porous carbon nanofibers. The MD results reveal that high elongational flow rate leads to prevention of phase separation and formation of a disordered morphology as also seen in our electrospun nanofibers.

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