Materials Letters 152 (2015) 177–180
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A unique solar radiation exfoliated reduced graphene oxide/polyaniline nanofibers composite electrode material for supercapacitors Venkataramana Gedela, Sampath Kumar Puttapati, Charanadhar Nagavolu, Vadali Venkata Satya Siva Srikanth n School of Engineering Sciences and Technology (SEST), University of Hyderabad, Gachibowli, Hyderabad 500046, India
art ic l e i nf o
a b s t r a c t
Article history: Received 16 February 2015 Accepted 24 March 2015 Available online 2 April 2015
Solar radiation exfoliated reduced graphene oxide (SRGO) and polyaniline nanofibers (PANi NFs) containing composite is synthesized using the typical chemical oxidative polymerization. The composite exhibits an excellent specific mass capacitance of 655 F/g owing to an optimal chemical interaction between SRGO and PANi NFs in the composite. Specific mass capacitance of the composite remains almost constant at different current densities making it expedient as a working electrode material in supercapacitors. & 2015 Elsevier B.V. All rights reserved.
Keywords: Graphene Polyaniline nanofibers Polymeric composites Interfaces Supercapacitor
1. Introduction Any hybrid supercapacitor's performance depends mainly on its working electrode material [1,2]. In this context, various composite materials constituted by graphene (in modified forms namely graphene oxide (GO), reduced GO, functionalized GO and few-layered graphene) and polyaniline (PANi) (in different morphologies) have been recently developed [3–5]. It is observed that the processing of these composites is greatly influenced by the synthesis procedures of graphene (in modified forms) [3]. Owing to the innate nature of these procedures, it is more than often noticed that the graphene does not properly blend with PANi. In this work, processing and characteristics of a unique composite (constituted by solar radiation exfoliated reduced graphene oxide (SRGO) and polyaniline nanofibers (PANi NFs)) material exhibiting an excellent specific mass capacitance of 655 F/g are discussed.
2. Synthesis procedure As-synthesized GO was spread over a petri dish and sunlight was focussed on to GO using a converging lens leading to photothermal reduction and simultaneous exfoliation of GO (owing to rapid local heating of GO) to obtain SRGO. Synthesis procedures of GO and SRGO and other experimental details are included in n
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http://dx.doi.org/10.1016/j.matlet.2015.03.113 0167-577X/& 2015 Elsevier B.V. All rights reserved.
Supplementary data (SD). Polymerization of aniline in the presence of SRGO (Fig. S1, SD) was carried out by first taking 250 ml of aqueous HCl (1 M) in a 1 L round-bottom flask and adding 20 ml of vacuum distilled aniline to it; to this reaction mixture, 1 wt% of assynthesized SRGO (relative to the amount of aniline) was added and the resultant mixture was ultrasonicated for 1 h to disperse SRGO uniformly. The resultant mixture was then cooled to 0 1C using an ice bath. The polymerization was then initiated by rapidly adding (1:4 M ratio w.r.t. aniline) ammonium perdisulfate (APS) to the reaction mixture. During APS addition, temperature of the reaction mixture was maintained in 0–5 1C range. After APS addition, the reaction mixture was stirred for 4 h. Then the suspension was filtered to obtain a wet cake which was washed with water, then with methanol, and finally with diethyl ether. The washed dark green coloured cake was finally dried at 90 1C under vacuum for 48 h to obtain SRGO/PANi NFs composite (Fig. 1).
3. Results and discussion It is evident from secondary electron micrographs (Fig. 1(a) and (b)) that PANi NFs (diameter ¼20–50 nm) have nucleated and grown all over the SRGO surfaces. Such fibrous morphology was previously found advantageous [6]. Raman spectra of GO, SRGO and SRGO/PANi NFs composite are shown in Fig. 2. Welldocumented D and G bands (Fig. 2(a)) at 1334 and 1596 cm 1, respectively are identified in both GO and SRGO cases. The intensity ratio (ID/IG) in the case of SRGO is greater in comparison to that of GO. This indicates a decrease in the size of the in-plane
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Fig. 1. Secondary electron micrographs ((a) and (b)) of SGRO/PANi NFs composite.
Fig. 2. Raman spectra of (a) GO and SRGO and (b) PANi NFs and SRGO/PANi NFs composite.
sp2 domains and the removal of the oxygen functional groups from GO. The intensity of D band in the case of SRGO is higher in comparison to that of GO indicating that SRGO has defects and partially disordered structure. Raman spectrum of PANi NFs [6] (Fig. 2(b)) resembles that of PANi in emeraldine salt form (SD). Raman spectrum of SGRO/PANi NFs composite shows clear differences in comparison to that of PANi NFs (Fig. 2(b)). The spectrum reveals shift in wave numbers and change in intensity of typical Raman bands of PANi. C–H bending (at 1169 cm–1) in the quinoid rings of PANi is prominent in the case of composite. This implies that the presence of SRGO stabilizes the polaronic structure of PANi. In other words, PANi is more polaronic in the presence of SRGO. Similarly C¼ N stretching (at 1339 cm–1) associated with the quinoid rings (bipolarons) of PANi is also prominent in the case of composite. Other Raman bands at 608 (and 824 cm–1), 1245 cm–1, 1497 cm–1, 1566 cm–1 and 1618 cm–1 corresponding to out-ofplane C–H vibrations in the aromatic rings, C–N of the benzene diamine units, C–C plus C–N stretching, C¼C of the quinoid rings, and C–C of the benzenoid rings, respectively in PANi are also identified in the case of composite. The presence of the benzene rings in the polaronic structure could amplify π–π interaction with the graphitic walls (between the delocalized electrons of the SRGO and the aromatic rings of the PANi). During the synthesis as the monomer polymerizes over the SRGO walls, it stabilizes the above mentioned π–π interactions. Thus attained polaronic character allows the further formation of planar polymer chains (favouring the stacking of the chains over SRGO), and the predominance of benzenoid rings favours the π–π interaction with the SRGO. X-ray diffraction (XRD) analysis (please see SD) of the composite (Fig. S2, SD) clearly indicates the formation of SRGO/PANi NFs composite in which PANi NFs are in semi-crystalline state [6,7]. All in all, Raman
scattering and XRD analyses indicate the formation of the composite and a definite chemical interaction between the components constituting the composite. The analyses also clearly indicate that the interaction between the components is enhanced in comparison to a similar composite with thermally exfoliated reduced GO [7]. Brunauer–Emmett–Teller (BET) specific surface areas of GO, SRGO and SRGO/PANi NFs composite are 51.3, 107.5 and 197.5 m2/ g, respectively. Interestingly, the specific surface area of the SRGO/ PANi NFs is higher than that of pure SRGO indicating that SRGO is well-dispersed in the composite and it has inhibited the restacking of the graphene sheets. The composite is found to exhibit mesoporosity (with pore sizes in the range 10–50 nm) (Fig. S3, SD) which can play a major role in enhancing the performance of supercapacitor devices as it allows the fast diffusion of electrolytes and thereby improving electrolyte access to high interfacial area. Moreover, the low hysteresis in N2 adsorption/desorption isotherm (Fig. S3, SD) of composite indicates its capacity to store more charges when used as an electrode. Cyclic Voltammetry (CV) curves of SRGO/PANi NFs composite are shown in Fig. 3(a). The characteristics (superposition of SRGO and PANi NFs [6,7] characteristics) observed in the case of SRGO/ PANi NFs composite is similar to other composites [3,6]. Appearance of the pair of redox peaks and reversible charge–discharge behaviour [6,7] are evident (Fig. 3(a)). The peak current in the case of composite was higher than that of PANi NFs [6] and SRGO, indicating that SRGO has effectively increased the composite's capacitance [7]. The area under composite's CV curve (at same scan rate) is larger than that of GO, SRGO (Fig. S4, SD) and PANi NFs [6] implying a higher specific mass capacitance for the composite. Specific capacitance C S values of GO, SRGO and SRGO/PANi NFs composite at a scan rate of 5 mV/s are 36.2, 112.5 and 654.8 F/g,
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Fig. 3. (a) CV curve of SRGO/PANi NFs composite electrode and (b) comparison of specific capacitances of GO, SRGO and SRGO/PANi NFs composite.
comparison to PANi NFs and SRGO cases. This implies that the electrode constituted by the composite shows superior (in comparison to PANi NFs and SRGO cases) capacitive behaviour representing good ion diffusion in the electrode structure. Noteworthy observation is that the composite electrode exhibits a better capacity tendency than PANi NFs electrode. It is evident that the addition of SRGO enhances the conductivity and improves charge transfer performance of SRGO/PANi NFs composite electrode. At the same time, the anchored PANi NFs prohibit aggregation of graphene sheets and the loose-formed open structure facilitates fast electron transfer between the active material and the charge collector. These observations are consistent with the high specific mass capacitance exhibited by the composite constituted by SRGO and PANi NFs. Fig. 4. Nyquist plots of GO, SRGO and SRGO/PANi NFs composite electrodes.
4. Conclusions respectively. C S values at different scan rates (Fig. 3(b)) clearly show that composite offers specific capacitance values above 654.8 F/g even at higher scan rates. CV curve of composite exhibited a large rectangular area mainly due to its chemical bonding nature and ordered morphology. The large-scale π–π conjugations between PANi NFs and SRGO (as indicated by Raman scattering analysis) facilitated charge transfer which enhanced the electrochemical performance. Galvanostatic charge/discharge curves (Fig. S5, SD) of the samples exhibited a quasi-triangular shape implying an ideal capacitor character. First discharge specific mass capacitance (based on the galvanostat) C sp values of GO, SRGO and SRGO/PANi NFs composite are 49.7, 116.9 and 632.8 F/g, respectively. The shape (clearly a superposition of the shapes of the charge/discharge curves of GO, SRGO structures and PANi NFs [6]) of the charge/discharge curve of the composite clearly indicates that its enhanced capacitance is a resultant of electrical double layer capacitor (EDLC) offered by SRGO and Faradaic pseudocapacitance offered by PANi NFs. To understand the conduction mechanisms, electrochemical impedance spectra of SRGO, PANi NFs, and SRGO/PANi NFs composite electrodes were obtained. Fig. 4 shows that the resulting Nyquist plots exhibit two distinct regimes that include an arc in the high frequency regime and a sloped line in the low frequency regime in the case of PANi NFs and composite. The arc corresponds to the charge transfer resistance (Rct) caused by the Faradaic reactions and the double-layer capacitance at the contact interface between electrode and electrolyte solution. Diameter of the semi-circular arc is Rct. However PANi NFs and composite electrodes showed incomplete semicircles. Nonetheless it is clear from Fig. 4 that Rct for composite electrode is the lowest implying that the composite exhibits highest conductivity (or lowest internal resistance). In the low-frequency regime of the Nyquist plots, the sloped line in the case of composite has much lesser slope in
The above presented results and discussion show that by making a miniscule, yet an effective change of using SRGO instead of thermally exfoliated reduced GO in the processing of the PANi composite, the specific mass capacitance values could be almost doubled (in comparison to a similar composite [6]). Since the same strategy can be applied in processing other polymer matrix composites, this work paves a way for designing novel graphene–polymer composites for a wide variety of other applications.
Acknowledgements GVR extends his gratitude to CSIR, India for the financial support through senior research fellowship. PSK and VVSSS thank UGC, India for its financial support through major research scheme (F. 41-993/2012 (SR)).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2015.03.113. References [1] Béguin F, Frackowiak E. Supercapacitors: materials, systems and applications. 1st ed. #. Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA; 2013. [2] Yu A, Chabot V, Zhang J. Electrochemical supercapacitors for energy storage and delivery: fundamentals and applications. 1st ed. #. Florida: CRC Press; 2013. [3] Srikanth VVSS, Gedela VR, Puttapati SK. Perspectives on state-of-the-art carbon nanotube/polyaniline and graphene/polyaniline composites for hybrid supercapacitor electrodes. Adv Carbon 2015. http://dx.doi.org/10.1166/ac.2014.1014.
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