Use of facile mechanochemical method to functionalize carbon nanofibers with nanostructured polyaniline and their electrochemical capacitance
© Du et al; licensee Springer. 2012
Received: 20 September 2011
Accepted: 8 February 2012
Published: 8 February 2012
A facile approach to functionalize carbon nanofibers [CNFs] with nanostructured polyaniline was developed via in situ mechanochemical polymerization of polyaniline in the presence of chemically treated CNFs. The nanostructured polyaniline grafting on the CNF was mainly in a form of branched nanofibers as well as rough nanolayers. The good dispersibility and processability of the hybrid nanocomposite could be attributed to its overall nanostructure which enhanced its accessibility to the electrolyte. The mechanochemical oxidation polymerization was believed to be related to the strong Lewis acid characteristic of FeCl3 and the Lewis base characteristic of aniline. The growth mechanism of the hierarchical structured nanofibers was also discussed. After functionalization with the nanostructured polyaniline, the hybrid polyaniline/CNF composite showed an enhanced specific capacitance, which might be related to its hierarchical nanostructure and the interaction between the aromatic polyaniline molecules and the CNFs.
Keywordscarbon nanofiber conducting polymer nanocomposite polyaniline capacitance mechanochemistry
As a conducting polymer, polyaniline [PANI] has attracted much attention in recent years due to its potential applications in various hi-tech areas, for example, electrochemical displays, sensors, catalysis, capacitors, anti-corrosion coatings, electromagnetic shielding, and secondary batteries [1, 2]. Many methods to prepare PANI have been developed [3–10], including interfacial polymerization, templates, and surfactant-assisted strategies, as well as mechanochemical methods. Since the mechanochemical methods have also been applied to disperse nanofillers in engineering polymers [11, 12], it is worthwhile to extend this facile technique to modify functional conductive nanoparticles with PANI nanomaterials as the combination of PANI with electrically conductive nanoparticles (such as carbon nanotubes and graphite nanosheets) has been recently demonstrated to be a promising approach to improve their electronic or electrochemical performance [13–21].
As a one-dimensional carbon nanomaterial, carbon nanofibers [CNFs] are much easier to produce in large scale and are cheaper than the well-known carbon nanotubes [CNTs]. It is expected that the corresponding PANI/CNF composites have a wider range of useful properties and hence more promising commercial applications. However, a study on the functionalization of these cheap carbon nanomaterials with PANI is limited compared with the many investigations on PANI/CNT composites. In this paper, we described a simple route to the modification of CNF via in situ mechanochemical polymerization of PANI in the presence of chemically treated CNFs. It was found that the resultant composite was easy to disperse in ethanol and that the dispersion had very good stability, which is believed to be related to the novel hierarchical nanostructure formed during in situ mechanochemical polymerization. Moreover, the introduction of PANI greatly enhanced the electrochemical specific capacitance of CNFs.
CNFs (Pyrograf Products Inc., Cerdaville, OH, USA) were first treated with nitric acid to remove the metal impurities in the product. CNF modified with a PANI sample was prepared using the following procedure. In a typical process, 1 g aniline (98%; Sigma-Aldrich, New South Wales, Australia) and 0.2 g treated CNF powder were mixed and hand ground for 1 min in a 250 mL glass mortar in a glove box, and 5 g FeCl3 powder was then added in several portions in 10 min and mixed together with further grinding. After grinding for another 10 min, the product was collected and purified by washing with water and ethanol. A small portion of the wet product was then dispersed in 10 mL ethanol. The stability of the dispersion was studied, and some of its drops were also transferred to the copper grids for transmission electron microscopy [TEM] analysis. Pure PANI was also prepared without CNFs using the same mentioned procedure.
Characterization and measurements
X-ray diffraction [XRD] patterns were obtained using an X-ray diffractometer (Siemens 5000, Siemens AG, Munich, Germany) with Ni-filtered Cu Kα radiation. Scanning electron microscopy [SEM] (Zeiss ULTRA plus, Carl Zeiss AG, Oberkochen, Germany) and TEM (Philips CM12, Philips, Eindhoven, The Netherlands) were used to examine the morphologies of the prepared samples. Fourier transform infrared [FT-IR] and UV-visible [vis] spectrum of the products were recorded on a Varian FT-IR spectrometer (Varian Inc., Palo Alto, CA, USA) and a Cary 5-UV-vis spectrometer (Agilent Technologies Inc., Santa Clara, CA, USA), respectively. Differential scanning calorimetry [DSC] data were obtained with a TA modulated DSC 2920 instrument at a heating rate of 20°C/min in nitrogen. Weight loss temperatures of the products were measured with a TA thermogravimetric analyzer (TGA 2950, TA Instruments, New Castle, DE, USA) at a heating rate of 20°C/min in N2 atmosphere. A CHI1202A Electrochemical Analyzer (CH Instruments, Austin, TX, USA) and a three-electrode electrochemical cell were used for electrochemical measurements. The sample was dispersed in 1 mL ethanol and sonicated for 20 min to prepare a suspension; 5 μL of such solution was added to the surface of the glass carbon electrode (Φ = 3 mm), followed by a drop of 5 wt.% Nafion solution to fix the material on the surface of the electrode. Prior to electrochemical measurements, the material-covered electrode was soaked in a 1-M H2SO4 solution for 2 min. The counter electrode was a Pt foil, and the reference electrode was a saturated calomel electrode.
Results and discussion
The main reason for good dispersibility of these novel functionalized CNF composites is believed to come from their overall nanostructured morphology. Both components, the PANI and the CNFs modified with the PANI nanostructures, become stable in dispersion because of their nanometric sizes and the stereo-hindrance effect of the nanostructured PANI. Ethanol dispersions of PANI dendritic nanofibers are very stable due to their highly branched nanostructure as reported recently ; hence, it is believed that the mechanism of stabilization for the CNFs functionalized with PANI is the same, and the CNFs behave like scaffolds for PANI. Also, the rapid formation of homogeneous and stable dispersions with ultrasonic irradiation may contribute to the good segregation of the material to its individual nanostructure. The direct preparation and stability of the dispersions represent a significant advancement in processing PANI/CNF composites. All the obtained results show that the nanostructure of PANI has lead to enhanced processability of these composites. In a previous work , PANI/graphite nanosheet composites were synthesized, and fine polyaniline particles adhered on the surfaces of graphite nanosheets. Due to the large size of the graphite nanosheet (up to 20 μm in diameter), the dispersion of the composites is very unstable and ready to precipitate. This indicates the small size of the CNFs also contributes to the stability of the dispersions, as shown in Figure 8.
where I is the response current (amperes); ΔV is the potential window (volts); v is the potential scan rate (volts per second), and m is the mass of electrode materials (grams). According to Figure 9, PANI/CNF has a specific capacitance of 139.8 F/g, which is less than that of PANI/MWCNT (approximately 190 F/g with the same PANI loading) at the same scanning rate . However, the specific capacitance of the PANI/CNF composite is much higher than that of the CNF (22 F/g as calculated from Figure 9), thus highlighting the remarkable enhancement of the capacitance of CNFs by modification of PANI via the mechanochemical polymerization method. The enhancement (approximately six times) of the capacitance of CNF by the modification of PANI is comparable to that of MWCNTs modified by in situ polymerization of PANI . The cyclic stability of the composite was also studied. As shown in Figure S3 in Additional file 1, the current (for instance, at 0.45 V) still retains approximately 93% of the original value after 500 cycles, indicating a good electrochemical stability of the composite. Considering the lower price and availability of CNFs, they could find more promising commercial applications.
At present, the growth mechanism of mechanochemical polymerization of polyaniline nanostructures remains unclear, and further investigations are required. Some may argue that polymerization did not take place during the mechanical processing but occurred after the samples were purified with water. However, this is unlikely the case since it is convenient to monitor the process of polymerization by the obvious appearance change of the mixture. The reaction proceeded so quickly that the loose mixture of the powders of CNF and aniline would change to a hard block in just 1 min when all the FeCl3 was added in one portion. In contrast, when aniline was polymerized with FeCl3 as an oxidation agent in water, no polyaniline products precipitated in such a short time, and even the color of the solution did not change. All these indicated that the mechanochemical reaction took place easily and could be attributed to the strong Lewis acid characteristic of FeCl3  and the Lewis base characteristic of aniline, respectively. Although water was used to purify the product and could provide another possible reaction system for the polymerization, the washing time (several minutes) was obviously too short for the polymerization in the solution-based method.
On the formation of branched fibrous PANI, we believe that the mechanism is likely to be related to both the mechanochemical oxidation polymerization process and the linear nature of the PANI macromolecule chains . It is known that the final structure of the products depends on the rates of nucleation and growth during the process of the reaction. At the start of the reaction, the aniline monomer is easily absorbed on the surface of treated CNFs through hydrogen bonding. When mixing with FeCl3, both the layered structure and strong Lewis acid characteristics of FeCl3 benefit the mechanochemical reaction with aniline as the NH2 group in aniline can easily coordinate to the iron ions with one free orbital through the free electron pair of the nitrogen atoms and complete the electron exchange . Mechanical grinding not only causes new surfaces for the oxidation polymerization, but also promotes interfacial diffusion of the reactants and CNFs. Moreover, the diffusion of solid reactant particles at ambient temperature is often short-range during the mechanochemical reaction and is favorable for the formation of short polymer nanofibers. In addition, the in situ mechanochemical polymerization process may allow the polymerization to have a more site-selective interaction with the CNFs, resulting in a more effective electron delocalization and enhancing the electrochemical activity of the products. According to the conventional nucleation and growth theory , the PANI coating and the nanofibers that initially formed on the surfaces of CNFs serve as nucleation sites and surface active sites for further growth of the polymer shell and secondary fibers. Subsequent growth of PANI on such preformed rough polymer shells and nanofibers may lead to the formation of dendrites around the CNFs (as shown in Figure 7) since the heterogeneous growth of available nuclei is energetically more favorable than the formation of new nuclei .
A facile process to functionalize carbon nanofiber with nanostructured polyaniline was developed via a simple mechanochemical in situ polymerization method. TEM examinations confirmed the grafting (and/or coating) of novel hierarchical nanostructured polyaniline onto carbon nanofibers. The resultant hybrid composites showed good dispersibility, and their dispersion had good stability which benefited their processability. Electrochemical tests also showed that the electrochemical specific capacitance of the PANI-functionalized CNFs was much larger than that of the CNFs.
differential scanning calorimetry
Fourier transform infrared
scanning electron microscopy
transmission electron microscopy
X-ray diffraction patterns.
XD acknowledges the award of the University of Sydney Bridging Fellowship. HL thanks the University of Sydney for the support of this project through the University of Sydney Bridging Support Grant 2011.
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