Asymmetric Supercapacitor

Electrochimica Acta 240 (2017) 146–154

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Hierarchical Polyaniline Spikes over Vegetable Oil derived Carbon Aerogel for Solid-State Symmetric/Asymmetric Supercapacitor Vikrant Sahu1, Ram Bhagat Marichi1, Gurmeet Singh* , Raj Kishore Sharma* Department of Chemistry, University of Delhi Delhi 110007, India

A R T I C L E I N F O

Article history: Received 14 January 2017 Received in revised form 9 March 2017 Accepted 9 April 2017 Available online 14 April 2017 Keywords: carbon aerogel polyaniline spikes bio-mass derived/vegetable oil solid-state symmetric/asymmetric supercapacitor

A B S T R A C T

Herein, we report an environment friendly methodology for the synthesis of carbon aerogel (CA) using bio-mass (mustard oil) as precursor. The synthesis process is highly reproducible and economical yet devoid of any sophisticated procedure and hazardous organic/inorganic chemicals. Exfoliated graphitic planes of carbon beads with enhanced inter-planer spacing are observed in synthesized CA with low bulk density (0.144 g cm
3) and high specific surface area (1032 m2 g
1). The CA comprises of meso/ microporosity with open pore microstructure. Symmetric supercapacitor cell (CACA) is fabricated to test the capacitive performance of CA in H2SO4-polyvinyl alcohol (H2SO4-PVA) gel as electrolyte. The CACA cell exhibits nearly symmetrical voltammograms up to working potential window of 2 V and impedance response demonstrates phase angle 80 indicating highly capacitive behavior. Polyaniline (PANI) spikes are further synthesized over CA using chemical oxidative polymerization route to introduce pseudocapacitive characteristics in addition to double layer capacitance. Solid-state symmetric (CAPANICA-PANI) and asymmetric (CACA-PANI) supercapacitor cells are fabricated to examine the electrochemical performance. Fabricated CACA-PANI supercapacitor cell exhibits remarkable high power density (7.9 kW kg
1) and high energy density (55.6 Wh kg
1). The CACA-PANI cell is capable to power a light-emitting diode (LED) bulb effectively for 3.5 min by assembling two supercapacitors (1 1 cm2). © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Ever-increasing worldwide depletion of fossil fuel leads to energy crisis as well as climate injuries which ultimately causes severe impact on public health and economy. To resolve these energy demand issues, energy storage and conversion devices such as batteries, fuel cells, photovoltaic cells, capacitors and supercapacitor cells have attracted much attention from last two decades. Fuel cells and batteries are considered as high-energy devices while the capacitors are the high-power devices. Supercapacitor cells, in addition to optimum energy storage as well as power delivery, have high rate performance with high operational working stability and high cycle life [1–3]. Electrochemical performance of a supercapacitor cell is primarily influenced by the porous morphology and ionic conductivity of electrode

* Corresponding authors. E-mail addresses: [email protected] (G. Singh), [email protected], [email protected] (R.K. Sharma). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.electacta.2017.04.058 0013-4686/© 2017 Elsevier Ltd. All rights reserved.

material as these govern the diffusion of electrolyte ions and its rate respectively. Carbon materials exist in variety of forms such as nanotube, activated carbon, aerogel, graphene, graphene nanoribbon, carbide-derived nanocomposites [4–8] etc. Among these, aerogels/ aerogel-like structures have shown outstanding characteristics such as 3-D structures with large open pores, very low density, more layer to layer distance (d-spacing) and high surface area [9– 11]. Owing to the unique features, carbon aerogels are expected to have potential applications in energy devices (fuel cell, supercapacitors, photovoltaic), sensors, actuators, catalysts, water treatment, optical applications [12–19] etc. Carbon-based aerogels are the promising electrode materials for supercapacitors due to micro/mesoporous 3-D morphology demonstrating outstanding electric double layer (EDL) performance and excellent mass transfer properties. Adequate thermal and electrical conductivity in ultralight weight structures (high volume/mass ratio) make them unique EDL supercapacitor materials for portable electronics. Usually, the organic and inorganic precursors are employed in synthesizing organic, inorganic and organic-inorganic hybrid aerogels. Carbon aerogels are typically synthesized from various organic precursors such as resorcinol–formaldehyde (RF) [20,21],

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phenolic–furfural [22,23], polymethylmethacrylate/polyacrylonitrile [24], poly (vinyl chloride) [25] etc. followed by pyrolysis. Pekala et al. have reported the first organic aerogel using RF condensation process [26,27]. Aerogel-like morphology can also be synthesized using silica, carbon materials, metal oxides and polymers as precursor [28]. Above reported synthesis approaches made use of expensive and environmental hazardous organic/ inorganic precursors. Several reports are available to cast off these expensive organic/ inorganic precursors. Among them, new and common synthetic approach is the direct conversion of renewable and sustainable natural resources such as crude plants [29,30], bagasse, coconut, peanut shells [31–34], cellulose [35], hemp fibers [36], watermelon [13], pollens, seaweeds, D-glucose, polysaccharides [37–41] etc. into carbon materials. An interesting approach to protect environment with clean and efficient energy generation/storage is the utilization of secondary non-biodegradable waste, examples are the utilization of hazardous diesel soot [42], bamboo-based industrial byproduct [43], scrap waste tires [44], cigarette filter [45]. There are scant reports on the utilization of carbon soots in energy storage however in the present article using chemical activation process, formation of the carbon powder aerogel with large surface area is reported for the first time. In this study, we used vegetable (mustard) oil [46] as precursor to synthesize an economic carbon aerogel (CA). The CA obtained exhibits micro and mesoporosity with high surface area (1032 m2 g
1) and low density value (0.144 g cm
3). The electrochemical performance of CA is studied by fabricating symmetric supercapacitor cell (CACA) using H2SO4-polyvinyl alcohol (H2SO4PVA) gel electrolyte. High energy storage capability can be achieved by synthesizing hybrid aerogel nanocomposites [47–50]. Therefore a nanocomposite of micro/mesoporous carbon aerogel with flexible and conducting polyaniline (PANI) is proposed to introduce pseudo-capacitance. Synergism in the proposed nanocomposite improved the cycling stability and mass transport limitation of polymers due to mass insertion/desertion during redox switching [51]. The polymer CA nanocomposite exhibits short relaxation time constant, fast and reversible adsorption/desorption of electrolyte ions in symmetric (CAPANICA-PANI) and asymmetric (CACA-PANI) supercapacitor cells at high current density. 2. Experimental Section 2.1. Synthesis of Carbon Aerogel The carbon material was obtained by burning the mustard oil in air. The synthesis route is illustrated in Fig. S1. The vegetable oil was kept in ordinary stainless steel lamp and burnt using cotton wick. For the collection of carbonaceous material, inverted petri dish was placed over lamp with the help of tripod stand at a height of two inches so that the flame of the lamp just touched the petri dish. The temperature of flame measured with the help of thermo couple was found to be 600 C. The carbon sample obtained was scratched out. So obtained sample was activated by heating carbon/KOH slurry  at 800 C under N2 ambient. In brief, 1.0 g of carbon and 7.0 g of KOH were taken in 50 mL round bottom flask. The minimum quantity of water was added to the above reaction mixture and magnetically stirred till the mixture turned into thick slurry-like appearance and kept uninterrupted for 24 hrs so that the KOH adsorbed thoroughly. The KOH soaked mixture was transferred to alumina-crucible and  kept at 110 C till the slurry like mass became solid. The crucible  was transferred into a tubular furnace programmed to 800 C at  10 C min
1 and hold for one hr. The black solid product was filtered and washed with distilled (DI) water till the pH of the filtrate

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became neutral. The washed product was dried in vacuum oven at  100 C for 12 hrs. 2.2. Synthesis of CA-PANI Nanocomposite 2.0 mL of aniline and 14.0 mL of H2SO4 were added to 140 mL of ethanol (20%). To this solution, 700 mg CA was dispersed under vigorous stirring for 2 hrs. The above reaction mixture was kept in  ice bath to control the temperature at 0 C. Then ammonium persulfate solution (APS, 5.0 mL) was added drop wise while stirring. The product was filtered using 0.45 mm PTFE membrane and washed with DI water and ethanol. Product obtained was dried  at 70 C and is found to be 1.75 g. The content of polyaniline in CA: PANI composite is 1.5 times to that of CA and is abbreviated as CAPANI nanocomposite. 2.3. Fabrication of Solid-state Supercapacitor In order to fabricate the supercapacitor electrodes, 20 mg of CA and CA-PANI nanocomposite were separately dispersed in isopropyl alcohol under ultrasonication and spray deposited onto polished graphite sheets. Then H2SO4-PVA gel electrolyte was prepared as described in our earlier article [52] and used as gel electrolyte membrane. The solid-state symmetric as well as asymmetric supercapacitor cells were fabricated. In brief, the resulting gel membrane was spread onto 1 cm2 area of electrode  material and dried at room temperature (35–40 C) for 6 hrs to evaporate the excess water. Finally two spray coated electrodes were assembled together in face to face arrangement to get symmetric supercapacitor cell assembly (CACA or CA-PANICAPANI). In the same way, asymmetric supercapacitor cell (CACAPANI) was fabricated where CA was used as negative and CA-PANI was used as positive electrode. Charge storage is balanced (q+ = q
) with the help of q = Csp  DV  m equation [53], where Csp is the specific capacitance, DV is the operational potential window after iR drop and m is the total mass on two electrodes. The optimized mass loading ratio for negative to positive electrode is 2. 2.4. Structural and Electrochemical Characterization The crystalline structures of CA and CA-PANI nanocomposite were analyzed using powder X-ray diffraction (XRD, Bruker D8 advance X-ray diffractometer) and Raman spectroscopy (Renishaw Invia Reflex Microraman spectrometer, 514 nm laser). Specific surface area (SSA) was estimated from N2 adsorption/desorption measurements performed at
196  C and degassed at 300  C using Brunauer-Emmett-Teller (BET, Micromeritics ASAP 2020, USA) method. High resolution transmission electron microscopy (HRTEM, Phillips Technai T-300 microscope) and Scanning electron microscope with Energy-dispersive X-ray spectroscopy (SEM-EDX, Zeiss Ultra 55 microscope) were employed to study the microstructures. All electrochemical performances were evaluated in two electrode cell assembly using H2SO4-PVA gel electrolyte. Cyclic voltammograms (CVs) and electrochemical impedance spectroscopy (EIS) were carried out with the help of CHI 604D electrochemical analyzer. Electrical conductivity was measured by two-probe method using Keithley 6517 B electrometer. Galvanostatic charge/discharge (GCD) and cycling stability measurements were performed employing PARSTAT 4000. 3. Results and Discussion A brief synthetic procedure is sketched for the preparation of carbon aerogel in Scheme 1. Here concentric spheres corresponding to graphitic planes are observed in vegetable oil derived carbon and the outer graphitic planes of these concentric spheres get

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Scheme 1. Schematic for preparation of hierarchical polyaniline (PANI) spikes over carbon aerogel (CA).

Fig. 1. (a) X-ray diffractograms and (b) Raman spectra of CA and CA-PANI nanocomposite, Nitrogen adsorption/desorption isotherms of (c) CA and (d) CA-PANI (Inset show their respective average pore diameter with pore volume).

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ruptured upon chemical activation at high temperature treatment. This rupturing subsequently leads to exfoliation or projected graphitic planes those appear as carbon strands. In second step, while polymerizing aniline over CA, these carbon strands serve as the nucleation sites for the growth of PANI and consequently hierarchical spikes-like morphology of CA-PANI nanocomposite is observed. 3.1. Structural Characterization Two broad characteristic peaks centered at 2u value of 24 and  43 are observed in X-ray diffraction patterns of CA which can be indexed to (0 0 2) and (1 0 0) graphitic planes (Fig. 1a). The most  intense peak observed at 2u value of 24.5 exhibits enhanced dspacing (3.63 Å) than that of typical graphitic geometry (3.4 Å) [18]. The diffraction pattern of synthesized CA-PANI nanocomposite      shows five typical peaks at 8.7 , 14.9 , 20.5 , 25.3 , 27.1 corresponding to (0 0 1), (0 1 0), (0 1 1), (0 2 0), (2 0 0) planes   of PANI. Peaks at 20.5 and 25.3 are attributed to periodicity parallel and perpendicular to the PANI polymeric chain respectively [54]. Raman spectra of CA and CA-PANI demonstrate a set of broad peaks at 1350 and 1598 cm
1 corresponding to D and G band of CA (Fig. 1b) [55]. The PANI spikes show vibration stretching at 1594 and 1505 cm
1 which indicate the C¼C and C¼N stretching in quinonoid ring respectively. Peaks at 416, 514, 575 and 735 cm
1 denote the presence of phenazine-type units while the peak at 1380 cm
1 confirms the presence of substituted N-phenylphenazines. The stretching bands at 1334 cm
1 show the presence of 

149

C  N+ segment of delocalized polaronic form. The existence of partial double bond () results in delocalization of charge over PANI rings. Peaks at 1220, 1166 and 781 cm
1 are observed due to ring deformation (in-plane), C-H bending (in-plane) and imine deformation respectively. Depending upon pore sizes, IUPAC classify porous materials into three categories, (i) macropores ( > 50 nm) (ii) mesopores (2– 50 nm) and (iii) micropores (< 2 nm) and therefore BET isotherms can be classified into six types [56]. The BET N2 adsorption/ desorption isotherm shown in Fig. 1c for CA matches with type II and IV isotherms that indicates the presence of micro and mesoporosity with monolayer-multilayer adsorption. CA sample exhibits significantly large BET surface area of value 1032 m2 g
1. The total pore volume is found to be 1.21 cm3 g
1 with 1.08 cm3 g
1 (89%) mesopore and 0.12 cm3 g
1 (10%) micropore volumes. The majority of mesopores in aerogel texture directs the potential electrode material for EDL supercapacitor [57] because large pore diameter ( > 2 nm) and pore volume are essentially important parameters for high accessibility of electrolyte ions through porous channels. On contrary, the BET surface area decreases to 12 m2 g
1 in case of CA-PANI nanocomposite, Fig. 1d. Fig. 2a,b exhibits HRTEM micrographs of raw carbonaceous material obtained from vegetable oil. The carbonaceous material somewhat appears like a chain of interconnected beads. TEM micrographs of CA reveal the similar morphology as typically found in powder carbon aerogel synthesized from resorcinolformaldehyde resin [58]. High magnification micrographs display connected beads-like structure with planes giving the wavy/ herringbone appearance in spherical form (Fig. 2c). The size of

Fig. 2. HRTEM micrographs of carbon material (a,b), CA (c,d) and CA-PANI (e,f) nanocomposite.

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Fig. 3. SEM micrographs of (a,b,c) CA and (d,e,f) CA-PANI nanocomposite.

carbon beads are in the range of 50 nm. Further magnification reveals that surface of carbon beads in aerogel is not smooth due to rupturing of graphitic planes (Fig. 2d). Therefore the ruptured outer planes of CA are seen as projected fibers. These projected planes act as nucleation site for the polymerization of aniline as illustrated in Scheme 1. Results indicate that the CA is completely enwrapped by PANI (Fig. 2e) with protruded spike like morphology. Polymerized aniline (Polyaniline, PANI) covered the spike and acquired the fibrous shape with length ranging from 15–20 nm (Fig. 2f). Fig. 3a-c shows SEM images of CA. At low magnification, CA demonstrates morphology as observed in case of aerogel obtained from resorcinol-formaldehyde resin [59]. Highly porous morphology of CA as observed in Fig. 3a, further supports large BET surface area which plays important role in electrochemical accessibility of electrolyte ions. The 50–60 nm globular spherules of carbon constitute sponge-like porous channels (Fig. 3b). At high magnification SEM micrographs in agreement with the TEM results suggest that the spherule surface is not smooth due to the projecting graphitic strands (Fig. 3c). Fig. S2 and Fig. 3d-f demonstrate the SEM-EDX elemental mapping and SEM micrographs of CA-PANI composite respectively. EDX images (Fig. S2) shows the uniform distribution of desired elements i.e. C, O and N throughout sample. The CA-PANI composite is mainly composed of carbon component (66 wt%). Presence of nitrogen (13.86 wt%) and oxygen (17.33 wt%) can be attributed to polymerized aniline and KOH activated CA respectively. Sulfur present in minor proportion might be introduced during polymerization of aniline using sulfuric acid and APS as synthesizing reagent. CA covered with PANI in lower magnification (Fig. 3d) shows sponge like

morphology as that of CA. At higher magnification, increased grain size depicts that PANI is coated uniformly over the CA (Fig. 3e). Small pointed projections over the surface in Fig. 3f are PANI spikes coated over CA. 3.2. Electrochemical Characterization Meso/microprous carbon aerogel is used to fabricate solid-state supercapacitor cell (CACA) and electrochemical properties are studied using H2SO4-PVA gel electrolyte membrane (Fig. 4) as well as aqueous electrolyte (1 M H2SO4, Fig. S3 and S4 in supporting information). Since the symmetry in shape of CVs plots is maintained in both, positive as well as in negative CV scan indicating that aerogel is capable of changing both the polarizations reversibly, Fig. 4a. The specific capacitance (Cm) values were calculated using CVs and GCD curves according to equations as given in S5 [60]. The Cm values obtained in
1 to 0 V,
0.5 to 0.5 V and 0 to 1 V potential windows are 111.5, 115 and 117.5 F g
1 respectively at 5 mV s
1. The overall Cm value for 2 V (from
1 to 1 V) is found to be 277 F g
1. The consistency in rectangle-like shape is observed as the scan rate increases without any significant distortion in regular shape, demonstrating the stable CA electrode material (Fig. 4b). 115, 113.5, 91.3, 77, 67.5, 60.5 F g
1 are the observed cell capacitances at different scan rates ranging from 5– 100 mV s
1. Further GCD characteristics of CACA supercapacitor cell are also carried out in both, positive as well as negative polarizations employing potential window of 1 V (Fig. 4c). The specific capacitance value found for CACA supercapacitor cell is 102 F g
1 at 0.5 A g
1.

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Fig. 4. CV curves of CACA symmetric cell (a) in different potential windows up to 2.0 V, (b) in potential window of 1.0 V at different scan rates and (c) its comparative galvanostatic charge/discharge curves in different potential windows.

The CV of CA-PANICA-PANI symmetric cell is studied in H2SO4PVA gel electrolyte (Fig. 5a) demonstrating fairly high charge storage capacity. With increase in scan rate from 5 to 100 mV s
1, the specific capacitance is found to decrease from 398.5 to 178.5 F g
1. Fig. 5b demonstrates the GCD characteristics of symmetric cell, CA-PANICA-PANI, indicating dominant pseudocapacitive characteristics. The specific capacitance (Cm) values are calculated and found to be 221.5, 210.6, 191.7, 130.5, 71 F g
1 at various current densities varies from 1.2 to 5 A g
1. Fig. 6 displays the electrochemical characteristics of asfabricated solid state asymmetric cell employing CA as negative and CA-PANI nanocomposite as positive electrode material. The CV curves retain its symmetry even at operating potential window of 2 V, demonstrating capacitive behavior with retaining its electrochemical stability (Fig. 6a). Fig. 6b presents the galvanostatic charge/discharge characteristics of solid-state asymmetric supercapacitor cell. CACA-PANI cell exhibits highest capacitance of 64.5 F g
1 at current density of 2.8 A g
1 and decreases to 41.8 F g
1 as the current density increased to a high value (14.3 A g
1). Good capacitance retention (65%) even at high current density (14.3 A g
1) and consistent symmetry in GCD curves at different current

densities demonstrate the stability and reversibility of CACA-PANI supercapacitor cell. The EIS responses of CACA, CA-PANICA-PANI and CACA-PANI supercapacitor cells, performed in frequency range of 10 mHz to 100 kHz at 5 mV, are summarized in Table 1. Fig. 7a demonstrates the Nyquist plot with inset exhibiting high frequency region. Since no semicircle at high frequency region is observed which might be due to low faradaic resistance in CA or CA-PANI electrode materials [61]. The equivalent circuit used for impedance data analysis is also given in inset where Rs and Rct are the solution resistance and charge transfer resistance. W, Cdl and CF are the Warburg resistance, double layer and pseudo capacitance respectively. The Bode and Nyquist plots of above mentioned supercapacitor cells exhibit phase angle close to 90 demonstrating nearly ideal capacitive behavior (Fig. 7a,b). The relaxation time constant (to) and phase angle obtained for CACA, CA-PANICA-PANI and CACAPANI cells are summarized in Table 1. In CA solid state cell, only the disrupted graphitic planes are in contact with the polymer electrolyte that leads to the hindered electronic conduction pathways. However, in case of CA-PANI the space between ruptured and normal graphitic planes is filled up by PANI which

Fig. 5. (a) Cyclic voltammograms and (b) Galvanostatic charge/discharge characteristics of CA-PANICA-PANI symmetric cell.

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Fig. 6. (a) Comparative CV study of asymmetric (CACA-PANI) cell in different operating potential windows at 25 mV s
1 (b) its GCD curves at various current densities.

Table 1 Electrochemical response of fabricated symmetric/asymmetric supercapacitor cells in H2SO4-PVA gel electrolyte. Cell Notation

ESR (V)

Rct (V)

to (sec)

Phase Angle (degree)

CACA CA-PANICA-PANI CACA-PANI

4.9 2.6 2.1

0.2 0.4 0.3

8.35 1.5 0.5

80 82.5 85

improves the over all charge conduction. Therefore, low electrochemical series resistance (ESR) values for CA-PANICAPANI and CACA-PANI cells are observed than CACA cell. Interfacial charge transfer resistance (Rct) increases from CACA to CA-PANICAPANI cell since the conductivity of sample decreases from CA to CAPANI resulting in increased Rct value [62]. The relaxation time constant (to) values decrease from CACA to CACA-PANI cells showing fast recharging characteristics which ultimately indicates the fast ion diffusion in asymmetric cells compared to symmetric cells [53]. Two-probe method as illustrated by Biswal et al. [63] is used to measure the current voltage (I–V) characteristics of CA,

CA-PANI composite recorded at room temperature (Fig. 7c). CA shows ohmic behavior with electronic conductivity 8.62 S m
1 while CA-PANI shows decreased electronic conductivity (36 mS m
1). All the three fabricated supercapacitor cells, CACA, CA-PANICAPANI and CACA-PANI, exhibit excellent capacitance retention having value of 99.2%, 95%, 98% of its initial value after 3000 charge/discharge cycles at current density of 5.0, 5.0, 4.2 A g
1 respectively (Fig. 8a). The superior charge/discharge cycling stability is attributed to highly porous microstructure of carbon aerogel that permits the diffusion of electrolytic ions more efficiently. Ragone plots (power density vs energy density) of symmetric and asymmetric supercapacitor cells are shown in Fig. 8b. The power density (P in W kg
1) and energy density (E in Wh kg
1) were calculated at different current densities using equations as reported by Marichi et al. [64]. For CA-PANICA-PANI supercapacitor cell, the energy density decreases from 25 to 14.7 Wh kg
1 as the power density increases from 461 to 1018 W kg
1. While the CACA-PANI supercapacitor cell exhibits seven times enhanced power density (7.9 kW kg
1) even at high energy density

Fig. 7. (a) Nyquist plots and (b) frequency response of CACA, CA-PANICA-PANI and CACA-PANI supercapacitor cells, (c) I–V plots of CA and CA-PANI.

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Fig. 8. (a) Cycling performance of CACA, CA-PANICA-PANI and CACA-PANI supercapacitor cells at current density of 5.0, 5.0, 4.2 A g
1 respectively, and (b) their Ragone plots, (c) image of two supercapacitor cells connected in series and (d) single cell to power a 3 V LED bulb.

(24.4 Wh kg
1). For CACA supercapacitor cell, the maximum power (230.8 W kg
1) as well as maximum energy density (12.4 Wh kg
1) obtained are very low compared to CACA-PANI supercapacitor cell. To check the practical application, two asymmetric cells are connected in series and charged for only 15 sec which can effectively lighten up a light-emitting diode (LED, 3 V, 5 mm diameter) bulb for 3.5 min. The nominal current flowing through circuit is estimated 15 mA. The same LED can also be powered even with single 1 1 cm2 supecapacitor cell that demonstrates the potential application of fabricated supercapacitor cell, Fig. 8c,d. 4. Conclusions We demonstrate a green synthesis of carbon aerogel employing vegetable oil as precursor. The synthesized aerogel affords light weight and high surface carbon material which ultimately results in high electric double layer charge storage characteristics. Further polyaniline spikes synthesized over CA enhance the total charge storage capability. The asymmetric supercapacitor cell composed of CA as negative and CA-PANI nanocomposite as positive electrode shows significantly high power as well as energy density compared to the symmetric supercapacitor cells. Acknowledgements The authors gratefully acknowledge University of Delhi for supporting the research through R&D grant (2015-2016). Authors also acknowledge the financial support (SR/S1/PC-31/2010) from Science and Engineering Board, New Delhi. One of the authors, Vikrant Sahu acknowledges CSIR, India for Senior Research Fellowship award. Ram Bhagat Marichi also acknowledges UGC,

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