(569e) Fabrication of Porous Carbon Nanofibers with Adjustable Pore Sizes As Electrodes for Supercapacitors

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

Chau Tran
and Vibha Kalra

Department
of Chemical and Biological Engineering

Drexel
University, Philadelphia, PA 19104

We
report a facile method for obtaining extremely high surface area and uniformly
porous carbon nanofibers for supercapacitor application.  As a first step, blends
of polyacrylontritrle (PAN) and a sacrificial polymer in dimethyl formamide
(DMF) have been electrospun into non-woven nanofiber mats with diameters in the
range of 200-300 nm.  Fast evaporation of solvent (~200 nl/s) and high
elongational flow rate (~10⁵ s^-1) during electrospinning
allowed us to prevent phase separation and develop a co-continuous morphology
of PAN and the sacrificial polymer in the nanofibers. As a second step,
electrospun nanofiber mats were subjected to stabilization and carbonization
processes to obtain porous carbon nanofibers (CNFs) as PAN converted to carbon
and the sacrificial polymer decomposed out to create intra-fiber pores. Unlike
other studies so far, we chose the sacrificial polymer possessing a high decomposition
temperature and chain rigidity which prevented fibers from shrinking and
collapsing during the PAN stabilization process at ~280 ^oC. Here we
demonstrated that using Nafion as a sacrificial polymer allowed us to obtain
CNFs (Figure. 1) with specific surface area of up to 1600 m²/g
without any activation process. We exhibit the tunability of thepore sizes within
 CNFs by varing material composition. Furthermore, the non-woven fiber mats of
CNFs enabled the construction of supercapcitor without the addition of
polymeric binding agents that add dead mass and reduce the overall specific
capacitance and conductivity of the electrode. Because of unique porous
structure, electrochemical measurements showed a specific capacitance up to 210
F/g (Figure. 2) in 1 M H₂SO4 at a high cyclic voltammetry scan rate
of 100 mV/s. This value is indeed much higher than nanofelts of
carbide-derived carbon (~100 F/g) at same scan rate and
comparable with activated carbon nanofibers ( up to 220
F/g) at an extremely low scan rate of 5 mV/s. This,
we believe, is owing to the presence of a large fraction of
meso-pores (2-4 nm) in our materials compared to activated
carbons(pores <2 nm), which leads to an increase in the accessible carbon
surface thereby improving specific capacitance even at high scan rates.

In
this study, we utilized a variety of different techniques to characterize CNFs.
The external morphology of fibers was observed under scanning electron
microscopy (SEM) while the internal morphology, both cross sections and
longitudal sections of fibers, was investigated using tranmission electron
microscopy (TEM). Unique interconnected pore network throughout the entire
fibers was observed. The nitrogen sorption isotherms were used to measure the
surface area, pore size distribution and total pore volume of CNFs. Extremely
high surface area with a majority of mesopores in the range of 2-4 nm was
found. Also, the electrochemical performance of CNFs was conducted with cyclic
voltammetry at different scan rates, galvanostatic charge-discharge
measurements at different current densities, and electrochemical impedance
spectroscopy (EIS) in 1 M H2SO4. Electrochemical characterization of these
materials in organic electrolytes is currently underway.  

                  Figure
1- Uniform porous carbon nanofibers obtained after carbonization

Figure 2- Cyclic Voltammetry of porous CNFs