Obtaining carbon nanofibers from electrospinning PAN copolymers for application as supercapacitors

ABSTRACT

Currently, there is a demand in the search for materials with high power density, long life cycle and low environmental impact that are mostly used fot the supercapacitors production, which are considered promising energy power for electronic systems. The most promising types of materials for this application are those based on carbon, due to their flexibility, surface area and good electrochemical stability. For the production of carbonaceous materials, the main precursor used is polyacrylonitrile (PAN) and its variations from the addition of monomers, such as methyl acrylate (MA). Generally, these materials are used in forms of blankets or yarns produced from spinning processes and later carbonized to generate the carbonaceous material. One of the spinning processes that has been widely studied for obtaining carbon nanofibers is the electrospinning process. Therefore, this paper presents the results obtained from the carbonization of electrospun blankets manufactured from solutions of polyacrylonitrile homopolymer (PANH) and poly (acrylonitrile-co-6 % methyl acrylate) (PAN6MA) combined with dimethylformamide in order to obtain material for use in supercapacitors. The polymers used were analyzed by DSC to obtain the values related to cyclization, by that were possible to observe a better thermal stability related to the samples of PAN6MA, with a temperature range of 240-312 ºC and maximum temperature of the exothermic peak of 292 °C. The electrospun mats were oxidized at 235°C per 5 min and carbonized at 90 °C per 5 min. Both pre-carbonization and post-carbonization mats were analyzed by SEM, showing fibers with smooth surface, randomly dispersion and nanometric size, with values of diameter of approximately 219 nm for PANH carbon nanofibers (NfcPANH) and 185 nm for PAN6MA carbon nanofibers (NfcPAN6MA). The carbonized mats were analyzed by RAMAN technique and it was possible to obtain the data related to the carbon structure present in the samples, where the obtained values showed that the mats manufactured from PAN6MA presented the lower value of degree of crystallinity measured from the relationship between bands D and G known as factor I_D/I_G, the value obtained being equal to 1.06 for NfcPAN6MA and 1.24 for NfcPANH, this shows a higher concentration of crystalline graphitic structures when compared with the samples produced from PANH. To obtain the capacitive characteristic, the load/discharge curves were surveyed by chronopotentiometric analysis, where it was possible to observe the best characteristics related to the specific capacity and low current cycling stability applied to the PAN - co- 6% methyl acrylate sample with specific capacity values of 270.9 F/g, energy density of 30.0 Wh/kg and power density of 153.5 W/kg. The results obtained demonstrated that the material presented potential for the proposed application.

Keywords
Electrospinning; Carbonization; Supercapacitor; PAN copolymers