284122 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
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