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Aloe vera Derived Activated High Surface Area Carbon for Flexible and High Energy Supercapacitors Manickavasakam Karnan, Kaipannan Subramani, Sudhan Nagarajan, Nagarajan Ilayaraja, and Marappan Sathish ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10704 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 29, 2016

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ACS Applied Materials & Interfaces

Aloe vera Derived Activated High Surface Area Carbon for Flexible and High Energy Supercapacitors M. Karnana, K. Subramania,b, N. Sudhana,c, N. Ilayarajaa and M. Sathisha,b* a

Functional Materials Division, bAcademy of Scientific and Innovative Research (AcSIR), c

Centre for Education, CSIR-Central Electrochemical Research Institute, Karaikudi- 630003, Tamilnadu, India.

Corresponding authors:

[email protected]; [email protected]

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ABSTRACT The materials which possess high specific capacitance in device configuration with low cost are the essential criteria for viable application in supercapacitors. Herein, a flexible high energy supercapacitor device was fabricated using porous activated high surface area carbon derived from aloe vera leaf as a precursor. The aloe vera derived activated carbon showed mesoporous nature with high specific surface area of ~1890 m2/g. A high specific capacitance of 410 and 306 F/g was achieved in three electrode and symmetric two electrode system configurations in aqueous electrolyte, respectively. The fabricated all-solid-state device showed a high specific capacitance of 244 F/g with an energy density of 8.6 Wh/kg. In an ionic liquid electrolyte, the fabricated device showed a high specific capacitance of 126 F/g and a wide potential window up to 3V, which results in a high energy density of 40 Wh/kg. Further, it was observed that the activation temperature has significant role on the electrochemical performance, as the activated sample at 700 ºC showed best activity than the samples activate at 600 and 800 ºC. The electron microscopic images (FE-SEM and HR-TEM) confirmed the formation of pores by the chemical activation. A fabricated supercapacitor device in ionic liquid with 3 V could power up a red LED for 30 min upon charging for 20s. Also, it is shown that the operation voltage and capacitance of flexible all-solid-state symmetric supercapacitors fabricated using aloe vera derived activated carbon could be easily tuned by series and parallel combinations. The performance of fabricated supercapacitor devices using aloe vera derived activated carbon in allsolid-state and ionic liquid indicates their viable applications in flexible devices and energy storage. Keywords: Aloe vera; porous carbon; flexible supercapacitors; ionic liquid; bio-derived carbon


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INTRODUCTION The innovation of trustworthy, efficient and sustainable energy sources with free of pollution are the crucial solutions to compact with the increasing energy demand, exhaustion of fossil fuels, exploring major environmental and health concern.1 To assuage the effects of environmental concerns, a new advent of eco-friendly energy storage devices has to be placed in our existing energy storage technologies. In general, supercapacitor fills the energy gap between conventional capacitors and batteries.2 Based on the charge storage mechanism, supercapacitors are classified into two types (i) electric double layer capacitors (EDLCs) and (ii) pseudocapacitors.3,4 In EDLCs, a physical mechanism is driven by the accumulation of electrolyte ions at the electrode and electrolyte interface.5 In contrast, pseudocapacitors are based on faradaic process which deals with the fast surface redox reactions at the electrode-electrolyte interface.6 Generally, EDLC mechanism is experienced in carbon based materials and pseudocapacitance is experienced in metal oxides, conductive polymers due to the fast surface redox reactions.7,8 EDLCs based supercapacitors are commercially available due to their effective characteristics such as low cost, high power capability, long cyclability and nontoxicity.9–12 Generally, EDLCs consist of carbon based materials such as carbon nanotubes (CNT), activated carbon (AC), carbon aerogel and carbon nanofibres (CNF).5 Among these, activated carbon is extensively used as most admirable electrode materials for supercapacitors because of their superior characteristics like high surface area, abundance in nature, low cost, higher endurance, less corrosive and wide operating temperature.13–15 Recently, bio-derived activated carbon based materials have been employed as a high performance supercapacitor application due to their intrinsic and extrinsic characteristics such as high electronic conductivity, high surface area, low cost, affordable to large scale applications and easy synthesis methods.16,17 The selection of precursors, activation method and activation conditions will significantly alters the electrochemical performance of the bio-derived activated 3 ACS Paragon Plus Environment


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carbon due to the inimitable surface area, pore size and electrical conductivity.18 To date numerous biomaterials such as firwood,19 grape fruit peel,20 sugarcane bagasse,21,22coffee beans,23 corn grains,24 sea weed25 and rice husk26 have been successfully converted into activated porous carbon and their performance in supercapacitor have been studied. It was observed that the type of chemical activation process involved in the preparation plays a vital role on the formation of activated carbon frame work and their porosity.27 The bio-derived activated carbon synthesis is an efficient way to prepare low cost nanoporous carbon materials by controlling the chemical activation rather than complex methods of porous carbon preparation. Till now various biomass/bio-waste materials has been extensively studied to ameliorate the energy storage materials prospect with low cost.28 Recently, Wenhua Yu et al., reported a hierarchical porous carbon derived from algae for high-capacity supercapacitors and battery anodes.29 Based on the results, bio-derived activated carbon based materials have attained a significant attention to enhance the electrochemical performance of the supercapacitor device. The energy density of the supercapacitor mainly depends on the specific capacitance and cell voltage. In aqueous based electrolytes, the cell voltage is limited due to the decomposition of water at 1.23 V, organic and ionic liquid based electrolytes have higher potential window that will improve the energy density of the supercapacitor significanlty.30–33 Energy storage devices are the key factor in the miniaturization of portable, flexible34 and foldable electronic gadgets. Thus, various attempts have been focused on the development of flexible energy storage devices such as paper-like, screen printed and all-solid-state devices.35–37 Gao et al. reported a graphene based solid-state flexible device fabrication such as cellulose nanofiber-graphene based flexible supercapacitors with a specific capacitance of 207 F/g at scan rate of 5 mV/s.38 Chen et al. reported MnO2-modified hierarchical graphene fibre electrochemical supercapacitor.39 Xu et al. reported that flexible all-solid-state supercapacitors with a high gravimetric specific capacitance of 80-200 F/g.40 Solid-state device using bio-derived activated carbon materials has a much easier 4 ACS Paragon Plus Environment

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fabrication method than graphene based solid-state devices41 and their low cost makes them a potential candidate in the development of economically viable energy storage prospect. Here, we demonstrate a facile route to prepare nanoporous carbon from aloe vera plant leaf (Barbadensis Miller) and their potential application in electrochemical supercapacitors. Aloe vera is a miracle plant with wonders possessing many applications in medicinal, antibacterial and cosmetics. To the best of our knowledge, for the first time we report the activated carbon from Aloe vera leaf and explored their potential applications in aqueous, ionic liquids and all-solid-state supercapacitors. The aloe vera derived activated carbon shows a high specific surface area of ~1890 m2/g with high specific capacitance of 410 F g-1 at 0.5 A g-1 current density in a three electrode system. In ionic liquids, it exhibits a specific capacitance of 126 F/g and a wide cell voltage of 3V with a high energy density and power density of 40 Wh kg-1 and 150 W kg-1, respectively. In addition, the fabricated all-solid-state symmetric supercapacitors device using PVA/H2SO4 solid gel electrolyte exhibits a high specific capacitance of 244 F/g and highly flexible in nature. Also, it is showed that the cell voltage and the capacitance of the all-solid-state device have been enhanced by parallel and series combination of electrodes for their viable applications in energy storage. A detailed literature report for the bio-derived and nitrogen doped activated carbon based supercapacitors is listed in Table S1. EXPERIMENTAL SECTION Materials Aloe vera plant leaf was collected from urban land, activated conducting carbon (mesoporous graphitized carbon black, 99.95%), PVDF (Poly Vinylidene Fluoride) and Poly (vinylalcohol) ([-CH2CHOH-]n, Mw ~125,000) were procured from Sigma-Aldrich, India. Potassium Hydroxide (KOH), Ethanol (CH3CH2OH, 99%), N-Methyl-2-pyrrolidone (NMP), were purchased from E-Merck. Sulfuric acid (H2SO4, 98%) and hydrochloric acid (HCl, 30%) were



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purchased from Rankem, India. Graphite foil (0.13 mm thick, 99.8%) and stainless steel mesh (0.127 mm thick) purchased from Alfa Aesar, India. Synthesis of activated porous carbon and characterization The raw aloe vera was cut into small pieces followed by washing with DI water for several times, and it was dried in direct sun light for 48 h followed by hot air oven at 80 °C for 24 h. The dried materials were powdered well using an agate mortar followed by carbonization at 400 °C for 3 h under Argon (Ar) atmosphere. The carbon obtained from carbonization of aloe vera was subjected to KOH chemical activation process. In a typical activation process, the KOH pellets and aloe vera carbon materials were mixed together in a weight ratio of 3:1 and heat treated at 700 °C for 1 h in Ar atmosphere at 5 °C/min heating rate. To optimize the activation temperature, the samples were also heat treated at 600, 700 and 800 °C. Subsequently, the resulting activated carbon was washed thoroughly with DI water until the pH of the filtrate becomes neutral followed by ethanol wash and dried for 5 h in a hot air oven at 60 °C. The just carbonized sample at 400 °C before the KOH activation process, and the KOH activated samples at 600, 700 and 800 °C were denoted as AVB-400, AV-600, AV-700 and AV-800 in the following discussions, respectively. The crystalline nature of aloe vera derived carbon materials were analysed using powder X-ray diffraction (PXRD) measurements by X-ray Diffractometer (BRUKER D8 ADVANCE) with Cu Kα radiation (α=1.5418 Å). The surface morphology of aloe vera derived carbon sample was examined by Field emission scanning electron microscopy with an accelerating voltage of 30 kV (FE-SEM; Carl Zeiss AG, Supra 55VP). Fourier transform infrared (FT-IR) spectra were measured using (Bruker, TENSOR 27 spectrometer) KBr pellet technique ranging from 400 to 4000 cm-1. Thermogravimetric analysis (TGA) was carried out in TGA/DTA analyser (SDT Q 600) to find out the material decomposition temperature ranging from room temperature to 1000 °C in an air atmosphere. Raman spectroscopic measurements were done using RENISHAW I via


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laser Raman microscope with He–Ne laser (wavelength λ = 633 nm) to understand the chemical nature of the prepared carbon. The C, H, N and S content in the aloe vera derived carbon was estimated using CHNS analysis (elementar vario EL III). The elemental composition of the activated carbon materials were studied using X-ray photoelectron spectroscopy (XPS) with Mg Kα (1253.6 eV) as X-ray source (Thermo Scientific, MULTILAB 2000). The surface area of the AV-700 was measured using surface area and pore size analyser (Quantachrome, NOVA 3200e). The dispersion of activated carbon and particle size were studied using high-resolution transmission electron microscopy (HR-TEM, 200 kV, Tecnai G2 TF20). Electrochemical measurement The electrochemical performance of AVB-400, AV-600, AV-700 and AV-800 was studied using Cyclic Voltammetry (CV) and galvanostatic charge-discharge (CD) analysis. The working electrode was prepared by mixing 80:15:5 weight ratio of activated carbon from aloe vera, superp-carbon black and polytetrafluroethylene (PTFE), respectively. The resulting working electrode paste (~2-3 mg) pressed over a current collector (stainless steel mesh). The electrochemical behaviour of activated aloe vera carbon materials (AV-600, AV-700 and AV-800) were studied in three and two electrode configuration in aqueous 1M H2SO4 electrolyte using electrochemical work station (BioLogic SP-300). For the three electrode system, activated aloe vera carbon, Hg/Hg2SO4 and a piece of Pt foil were used as working, reference and counter electrodes, respectively. The CV and CD experiments were carried out from -0.8 to 0.4 V vs Hg/Hg2SO4 at different scan rates (10 to 50 mV/s) and current densities (0.5 to 10 A g-1), respectively. From the charge-discharge profile, the specific capacitance of the electrode was calculated using the following equation.

C =

× ∆

(1)

∆ ×



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Where, Csp is the specific capacitance (F/g), i is the current density (A), ∆t is the discharge time (s), ∆v is the working voltage of the electrode materials during discharge process, and m is the active mass of the electrode materials. To understand the various resistance factors experienced in the electrode-electrolyte interface and charge transfer process, electrochemical impedance spectroscopy (EIS) analysis was carried out for the AV-600, AV-700 and AV-800 electrode materials at the frequency range of 100 kHz to 10 mHz with an 5 mV bias volatge.

In order to study the potential applications of activated aloe vera carbon materials in supercapacitors, the model supercapacitor device is fabricated in non-aqueous electrolyte (ionic electrolyte), aqueous (1M H2SO4) and aqueous gel electrolyte (1M H2SO4/PVA). A Swagelok type cell was fabricated for the electrochemical measurements in ionic liquids and coin-like (1 cm in diameter) electrodes obtained from thin self-standing working electrode paste was placed in both sides. Two thin Pt disks and whatmann filter paper were used as current collectors and separator, respectively. The performances of the above devices were studied in the potential range of 0 to 3 V at various scan rates (10 to 50 mV/s). Similarly, all-solid-state supercapacitor device was fabricated using gel electrolyte (1M H2SO4/PVA) placed between the electrode materials coated on flexible carbon sheets current collectors. The specific capacitance, energy density and power density was calculated using the following equation:17,42

=

(2)

= 4

(3)

(4)

=

(5)

=


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Where, Ct is total specific capacitance (F g-1), Csp is the specific capacitance of the electrode materials (F g-1), I is the discharge current (A), ∆t is the discharge time (sec), M is the total mass of the active material on both electrodes (g), ∆V is cell voltage (V), E is the energy density (Wh kg-1) and P is the power density (W kg-1).

RESULTS AND DISCUSSION Activation of carbon materials particularly bio-derived carbon plays vital role on the chemical nature and physical properties. Both, physical and chemical activation methods are reported for the preparation of extremely porous carbon materials.13 However, it was observed that most of the chemical activation methods are superior owing to high specific surface area, low cost, high yield and facile process.43 The preparation of activated porous carbon from aloe vera has two steps (i) carbonization and (ii) chemical activation. Usually, the carbonization was carried out at inert atmosphere and the chemical activation was carried out by means of mixing the carbonized carbon with activating agents like KOH, ZnCl2, NaOH and H3PO4. The schematic preparation of activated porous carbon from aloe vera is shown in Scheme 1.

Scheme 1. Schematic representation for the preparation of activated porous carbon from aloe vera plant leaf. The XRD patterns of AV-600, AV-700 and AV-800 shown in Figure 1a clearly indicates a broad peak around 23° and 43° corresponds to (002) and (100) plane, respectively. The observed 9 ACS Paragon Plus Environment


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broad lines indicates that the activated porous carbons are moderately crystalline in nature. The XRD lines of AV-700 are slightly intense than other two samples, and there is no significant difference between AV-600 and AV-800. The carbonization followed by activation process severely modifies the surface nature of the carbon materials by altering the surface functional moieties. To understand the surface properties of AV-600, AV-700 and AV-800, FT-IR spectrum was recorded (Figure 1b). The obtained spectra for all the three activated carbons clearly indicate the existence of oxygen containing functional groups on their surface. The characteristic peak observed around 3459 cm-1 for AV-600 (Figure 1b (i)) was attributed due to O-H stretching of the surface hydroxyl groups and it was observed that it shifts to slightly lower wavenumber of 3448 cm-1 and 3440 cm-1 for AV-700 (Figure 1b (ii)) and AV-800 (Figure 1b (iii)), respectively. The C-H stretching was observed around 2900 cm-1 and the characteristic peak observed around 1629 cm-1 is assigned to the characteristic oxygen containing carbonyl C=O stretching. It is worthy to note here that the relative intensities of peaks corresponding to oxygen containing groups in AV700 and AV-800 samples are significantly reduced compared to AV-600 sample. This may be attributed to reduction of oxygen containing functional groups and high degree of graphitization at high temperature treatment. The elemental composition in AV-600, AV-700 and AV-800 samples was analysed using CHNS analysis; the results confirmed that there is no significant amount of hetero atoms such as nitrogen and sulphur in the sample. CHNS data for all samples are shown in Table S2. Raman spectroscopy is a powerful analytical tool to estimate the degree of functionalization in carbon based materials.44 To understand the nature of functionalization by the carbonization and activation processes Raman spectra of AVB-400 and AV-700 are compared in Figure 1c. Generally, the intensity ratio of D band (ID) and G band (IG) demonstrate the degree of graphitization as well as the defective site on the carbon network. Both samples show a characteristic D and G bands at 1347 and 1596 cm-1, respectively. The ID/IG ratio of AV-700 is 10 ACS Paragon Plus Environment

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significantly smaller than AVB-400 that indicates the high temperature activation process enhanced the degree of graphitization by removal of defects and reduction of functional groups, it is in good agreement with the earlier FT-IR observations.

Figure 1. (a) XRD pattern and (b) FT-IR spectrum of (i) AV-600, (ii) AV-700 and (iii) AV-800 (c) Raman spectrum of (i) AVB-400 and (ii) AV-700, (d) TGA profile, (e) BET-Adsorptiondesorption profile of AV-700 and (f) pore size distribution of AV -700.



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The TGA profiles of AV-600, AV-700 and AV-800 exhibit (Figure 1d) a weight loss of around 15%, 10% and 20% below 150 °C due to removal of surface adsorbed water and moisture, respectively. The AV-600 sample showed a major weight loss between 450-550 °C due to the complete decomposition of carbon in air. Whereas, the AV-700 and AV-800 samples showed the complete decomposition profile at slightly higher potential range (550-650 °C) compared to AV600. This may be attributed to high degree of graphitization or formation of structurally uniform carbon atoms at high temperature heat treatment. The complete decomposition without ant any residuals indicates that the samples contains only carbon and there is no other metal/metal oxide impurities. To investigate the porosity and surface area of the activated samples, BET nitrogen adsorption-desorption isotherm were carried out (Figure 1e). From the BET isotherm, a very high BET surface area of 1890 m2 g-1 with pore volume of 0.167 cm3/g was observed for AV-700. The BET adsorption-desorption curves shows a type-IV isotherm with mesoporous in nature. The formation of high surface area after activation indicates that the KOH activation significantly introduce porosity in the carbon materials, which are essential for electrolyte to access the entire surface for efficient charge storage. The pore size distribution profile of AV-700 possesses mesoporous in the pore diameter range of 3.8 and 12.5 nm. (Figure 1f) Due to the mesoporous the specific surface area is enhanced significantly and the mesoporous allows the electrolyte ions to pass through, which will enhance the specific capacitance, energy density and rate capability. XPS is a characteristic tool to evaluate the electrode materials functionalities and their oxidation states. The survey spectrum of AV-700 materials shows set of high intensity peaks corresponding to the C and O atoms (Figure S1a). It could be clearly seen that there are no peaks corresponding to the existing of sulphur and phosphorus. However, a trace amount of N could be seen clearly in the survey spectrum. Thus, the region from 396 to 406 eV is scanned further, even after several scan there is no significant amount of nitrogen doping was observed in the AV-700 (Figure S1b). This clearly indicates that only trace amount of N exists in the AV-700 sample 12 ACS Paragon Plus Environment

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which is in good agreement with the CHNS analysis. Similarly, the C 1s and O 1s lines are analysed at higher magnification, the observed C 1s and O1s lines are deconvoluted and fitted based on well-known Gaussian fitting method as shown in Figure S1 (c and d). The deconvoluted C 1s spectrum of AV-700 (Figure S1c) confirms the presence of C=C (C1), C-O-C/C-N (C2), COH (C3) and C=O (C4) groups due to the existence of line at the bonding energies of 284.8, 285.4, 286.2 and 289.4 eV respectively.16 The deconvoluted O 1s spectrum of AV-700 (Figure S1d) reveals the presence of -OH (O1), C-O (O2), -C=O (O3) and absorbed H2O (C4) at the bonding energy of 531.7, 532.5, 533.5 and 535.8 eV respectively.20 Form the XPS analysis, it is conform that AV-700 electrode material has mainly contains oxygen functionalities with trace amount of nitrogen containing functional groups. The formation of pores by the activation process is confirmed by comparing the FE-SEM images of carbonized and activated samples. The FE-SEM images of AVB-400 (Figure 2(a-b)) shows the formation of micron size particles without any pores over the surface, whereas the FESEM images of AV-700 (Figure 2(c-d)) evidently confirms the arrangement of interconnected mesoporous throughout the sample. The above observation clearly confirms that the KOH activation of aloe vera carbon resulted the formation of pores over the carbon materials. Though, the nanometre size pores are not seen in FE-SEM images and it was obviously confirmed using HR-TEM analysis. The HR-TEM images of AV-700 clearly showed the micro and nanopores over the surface of carbon framework (Figure 2(e-f)). It is worthy to note here that the formation of H2O and CO2 products during the KOH activation process contributes to the development of the pores through the gasification of carbon.43,45 The resulted KOH activated porous carbon is the result of various synergistic, comprehensive actions such as chemical activation, physical activation, and carbon lattice expansion. Also, it is believed that in addition to the amount of KOH, activation temperature, the nature of the carbon source and its unique reaction with activating agent has key role on the success of activation process.45,46 Thus, the activation of 13 ACS Paragon Plus Environment


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various bio-carbon sources need not to show similar pore structure, surface area and catalytic activity.

Figure 2. FE-SEM images of AVB-400 (a-b) and AV-700 (c-d); HR-TEM images of AV-700 (e-f)


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Figure 3. The CV profile of (a) AVB-400, AV-600, AV-700 and AV 800 at 50 mV/s, (b) AV-700 electrode at different scan rates, (c) CD profile of AVB-400, AV-600, AV-700 and AV 800 at 0.5 A/g current density, (d) CD profile of AV-700 electrodes at different current density, (e) specific capacitance as a function of current densities for AV-600, AV-700 and AV-800 electrode and (f) specific capacitance as a function of cycle number for AV-700 electrode at 10 A/g current density.



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The electrochemical supercapacitor performance of activated aloe vera was studied in aqueous, non-aqueous and solid gel electrolytes. In aqueous system, 1M H2SO4 was used as an electrolyte and the electrochemical studies were investigated in both three and two electrode configurations. For three electrode configuration, the activated aloe vera was used as a working electrode, Hg/Hg2SO4 as a reference electrode and the platinum mesh as a counter electrode. The CV and CD studies were evaluated at various scan rates and current densities, respectively. Figure 3a shows the CV profile of AVB-400, AV-600, AV-700 and AV-800 samples in a potential range of -0.8 to 0.4 V vs. Hg/Hg2SO4. It can be obviously seen that the activated aloe vera shows the typical rectangular shape characteristics EDLCs like behaviour with slight pseudocapacitive contribution due to the influence of surface hetero atom functionalities. The CV and CD curve for AVB-400 at different scan rates and different current densities are shown in Figure S2. The area under the curve of AV-700 is larger than the other AV materials (Figure 3a), indicating that the capacitance behaviour of AV-700 is superior over other electrodes. The CV profile of AV-700 at different scan rates show similar rectangular shape profile that indicate the good rate performance of the electrode (Figure 3b). The CD profile of AVB-400, AV-600, AV-700 and AV-800 samples show the nonlinear discharge profile attributes to the presence of pseudocapacitive behaviour are shown in Figure 3c. From the CD profile at 0.5 A/g current density, a high specific capacitance of 410 F/g was calcualted for the AV-700 electrode (Figure 3d). When the current density was increased from 0.5 to 1, 2, 3, 4 and 5 A/g, the specific capacitance decreased progressively from 410 to 347, 300, 276, 257 and 252 F/g, respectively (Figure 3e) and it shows a 62% high initial capacitance retention at high current density of 5 A/g. The specific capacitance values calculated for AV-600, AV-700 and AV-800 electrode at different current densities are shown in Table S3 and the CV and CD curve for AV-600 and AV-800 were shown in Figure S3. The long term stability of electrode materials is a crucial parameter for their potential applications in practical supercapacitors. Thus, the cyclic 16 ACS Paragon Plus Environment

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capability of AV-700 electrode was studied for 10,000 cycles (Figure 3f) at 10A/g current density indicating and it was observed that a 98 % of intital capacitance was retained over 10,000 cycles. To the best of our knowledge, this is the highest stability value reported at a high current density of 10 A/g for biomass derived activated carbon materials.17,20 The above observation clearly validates that the aloe vera derived activated carbon holds excellent electrochemical stability and good electrochemical performance in the three electrode system.

Figure 4. (a) CV and (b) CD profile of AV-700 electrode materials in two electrode system using aqueous 1M H2SO4 electrolyte with different scan rate and current density respectively; (c) specific capacitance as a function of current densities for AV-700 electrode; (d) specific capacitance as a function of cycle number at 20 A/g current density and Ragone plot for AV-700 electrode materials in two electrode system using aqueous 1M H2SO4 electrolyte (inset).



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The typical Nyquist plot of AV-600, AV-700 and AV-800 electrode shows a semicircle in high-frequency region and a linear curve in low-frequency region (Figure S4). The obtained EIS was fitted with an appropriate equivalent circuit model (Figure S4 inset) and the physical components such as solution resistance (Rs), charge transfer resistance (Rct) and double layer capacitance (CDL). All the electrode materials shows an equal solution resistance of Rs ~0.4 Ω and Rct varies of 6.9, 3.2 and 2.3 Ω for AV-600, AV-700 and AV-800, respectively. Further,the vertical line in the low-frequency region exhibits the domination of the capacitance behavior at the electrode-electrolyte interface.20 To evaluate the potential application of AV-700 in aqueoues electrochemical supercapacitors, a symmetric two electrode device was fabaricated. The fabaricated device with the symmertic electodes was tested in the potential range of 0 to 1.2 V in 1 M H2SO4 electrolyte. The CV profiles of AV-700 at different scan rates shows rectangular shape that indicates the ideal capacitive behaviour of electrode materials (Figure 4a). The CD profile of AV-700 electrode at different current densities (0.5 to 20 A/g) clearly shows the maximum specific capacitance of 306 F/g at 0.5 A/g (Figure 4b). When the current density was increased to 1, 2, 3, 4, 5, 10 and 20 A/g the specific capacitance decreased to 293, 280, 270, 254, 250, 236 and 212 F/g, respectively (Figure 4c). To study the electrochemical stability of the electrodes in the aqueous electrolyte, charge-discharge cycling was carried out for 20,000 cycles at 20 A/g current density (Figure 4d). It could be calrly seen that a 83% of intital capacitance retention was observed at the end of 20,000 cycles. The Ragone plot for the AV-700 electrode in aqueous system shows that the energy density of the AV-700 is much higher and close to the batteries. (Figure 4d inset). The maximum energy density obtained for AV-700 in aqueous system is ~15 Wh/kg at a power density of 300 W/kg. When the power density was raised to 12045 W/kg the energy density still remains 10.6 Wh/kg, which is significantly better compared to the other supercapacitors device fabricated using bio-derived carbon. For instance, recently Suying Bai et.al., reported that 18 ACS Paragon Plus Environment

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symmetric supercapacitor possess an maximum energy density of 8.9 Wh/kg and maximum power density of 10000 W/kg for pumpkin-derived porous carbon for supercapacitors with high performance.16 Similarly, Ru-Juan Mo et.al., reported that the maximum energy density of 11.39 Wh/kg was achieved for activated carbon derived from nitrogen rich watermelon rind for highperformance supercapacitors.17 However, the practical application of the aqueous supercapacitors is limited due to their low cell voltage. In order to enhance the energy density of the supercapacitors either the specific capacitance or the cell voltage has to be improved.47 There are number of attempts have been reported to increase the energy density by selective appropriate active electrode materials such as porous carbon, metal oxides/polymers and composite materials.48 The most frequently used aqueous electrolytes have its own drawback that the working potential cannot be raised beyond the water decomposition potential. There are several attempts in increasing the potential or cell voltage by designing asymmetric cathode and anode materials, but only limited success was obtained.4,49,50 Recently, it was shown that the non-aqueous electrolytes such as ionic liquids are stable up to 5 V and possess lot of potential applications in energy storage devices.30,51,52 Therefore, the AV-700 electrode performance in 1-ethyl-3-methyl imidazolium tetra fluro borate [EMIM][BF4] electrolyte has been studied. Figure 5a shows the CV curve of AV-700 symmetric cell with different scan rates ranging from 10 to 50 mV/s and it confirmed that the AV-700 electrode are stable up to 3 V. The CD profile of AV-700 electrode at different current densities ranging from 0.1 to 0.5 A/g (Figure 5b) clearly indicates a high specific capacitance of 126 F/g at 0.1 A/g. The obtained specific capacitance value of AV-700 is significantly high compared to the recent literature. The cycling performance of AV-700 electrode in [EMIM][BF4] electrolyte evaluated at 0.1 A/g current density and it can be clearly seen that a 83% of initial specific capacitance was retained after 500 charge-discharge cycles (Figure 5c). The significant increment in the cell voltage and the specific capacitance in ionic liquid electrolyte enhance the energy and 19 ACS Paragon Plus Environment


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power density of the device, significantly. The Ragone plot for the AV-700 electrode in ionic liquids shows that the energy density of the AV-700 is close to batteries with high power density (Figure 5d). The maximum energy density obtained for AV-700 symmetric cell is about 40 Wh/kg, which is progressively higher than the symmetric cells fabricated in aqueous electrolyte. Figure 5d (inset) shows the fabricated Swagelok type symmetric supercapacitor device using AV700 electrodes. The time required for complete charging is less than 20s at a low current rate of 0.5 A/g and it could power up a red LED for 30 min. (Figure 6 and Video S1 in ESI)

Figure 5. (a) Cyclic voltammetry profile of AV-700 symmetric two electrode cell in EMIM borate (b) CD profile of AV-700 symmetric two electrode cell at different current densities (c) Cyclability profile of AV-700 symmetric two electrode cell for 500 cycles at 0.1 A/g current density in EMIM borate (d) Ragone plot of AV-700 electrode and the photograph of AV-700 symmetric two electrode cell in EMIM borate powered red LED (inset). 20 ACS Paragon Plus Environment

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Figure 6. Fabricated symmetric supercapacitor device using AV-700 in EMIM borate ionic liquid for powering red LED as a function of time.

A modern society needs flexible and bendable energy storage devices for application in flexible and foldable electronic devices35,53. Thus, various attempts are in progress to make a flexible and bendable supercapacitors and it was observed that the electrolyte and current collectors are the key components in the development of supercapacitor devices. Here, we developed all solid state flexible supercapacitors using activated AV-700 electrode materials with PVA/H2SO4 based solid state electrolyte. The fabricated flexible all-solid-sate symmetry supercapacitors (FSSC) device was examined using CV and CD measurements. Figure 7a reveals the CV profile for single device shows a characteristics rectangular shape profile due to EDLCs behaviour in the potential range of 0 to 1 V. Even at high scan rate, the CV curves retain their shape without any notable distortion. The CD curves measured at various current densities (0.5 to 5 A/g) shows that the fabricated single device delivers a high specific capacitance of 244 F/g at 0.5 A/g (Figure 7b). When, the current densitiy was increased to 1, 2, 3, 4 and 5 A/g, the specific capacitance decreased progressively to 232, 184, 168, 141 and 120 F/g, due to the slow diffusion of electrolyte at the electrode surface, respectively. The specific capacitance as a function of current density clearly shows that even at high current density (5A/g) the AV-700 electrode retains 21 ACS Paragon Plus Environment


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49% of initial capacitance. The specific capacitance values calculated for FSSC device with two, three and four devices series connection at different current densities were shown in Table S4. To increase the cell voltage for practical applications, the FSSC devices are connected in series to attain 4 V. The four cells connected in series system delivered a specific capacitance of 40 F/g at 0.25 A/g in shown in Figure 7c. The CV and CD profiles of two, three and four FSSC devices connected in series are shown in Figure 7(d) and Figure 7(e).

Figure 7. (a) CV profile and (b) CD profile of fabricated single cell. (c) CD profile of four cells connected in series system, (d) CV profile for fabricated device at 50 mV/s scan rate in series combination (e) CD profile for a fabricated cell in series connection, (f) CV profile of fabricated device at 50 mV/s scan rate in parallel combination (g) CD profile of fabricated cell in parallel connection. (h) Cyclability of FSSC cell for 5000 cycles at current density of 5 A/g.


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Similarly, the two, three and four FSSC devices are connected in parallel to get the maximum specific capacitance (Figure 7 (f&g)). The four FSSC device connected in series shows a 86% capacitance retention for 5000 cycles at a high current density of 5 A/g (Figure 7h). The CV and CD of FSSC device with two and three cells connected in series were shown in Figure S5. The photographic images of fabricated FSSC device are shown in Figure 8(a-d) with flexible, bendable and thin in nature. Figure 8e shows the CV profile of fabricated FSSC device at different bending angles. Even at higher bending angle of 150° and back to flat position, the rectangular shape of the CV curves remains same which clearly confirms the mechanical flexibility of electrodes without any specific capacitance loss and maintains good electrical contact between the current collector and electrode materials. From the CD profile (Figure S6) it was observed that there is no significant loss in specific capacitance values, which further confirmed the mechanical stability and bending ability of the device. The above results clearly demonstrate that the fabricated supercapacitor device using AV-700 electrodes are promising for smart energy storage applications. From the above studies, it is confirmed that the AV-700 electrode materials are promising for the fabrication of FSSC device. The energy density and power density of the energy storage devices are the key factors to distinguish them from the reported supercapacitors, batteries and fuel cells. The typical Ragone plot of fabricated FSSC device with two, three and four electrodes connected in series at different current densities are shown in Figure 8f. Remarkably, the fabricated FSSC device with four cells connected in series will exhibits a high energy and power density of 22.4 Wh/kg and 2037 W/kg, respectively. The calculated energy density value is three times higher than single FSSC device. The energy and power density of FSSC device with cells in series system were shown in Table S5. This clearly confirms that the fabricated supercapacitor device with four electrodes in series serves as promising for energy storage applications. Due to the low cost electrode materials and flexible nature, fabrication of large scale supercapacitor 23 ACS Paragon Plus Environment


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device using AV-700 electrode materials in non-aqueous and aqueous solid-state electrolytes is highly warranted. And the high energy and power densities associated with the AV-700 electrodes is promising for large scale energy storage device for applications in electric, heavy electric vehicles and flexible electronics.

Figure 8. Photographic images of FSSC device with (a) flexible, (b) bendable, (c) smart device (d) four cells connected in series to glow the blue LED, (e) CV profiles of FSSC device fabricated with AV-700 electrode at different bending angles and (f) Ragone plot for AV-700 fabricated FSSC devices.

CONCLUSIONS Aloe vera derived activated carbon electrodes has been showed for their application in supercapacitors using aqueous, non-aqueous and all-solid-state device configurations. The chemical activation of aloe vera derived carbon materials with KOH showed a maximum specific capacitance of 410 F/g at 0.5 A/g, 126 F/g at 0.1 A/g and 244 F/g at 0.5 A/g current density in aqueous, non-aqueous and all-solid-state configuration, respectively. The high performance of 24 ACS Paragon Plus Environment

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aloe vera derived activated carbon was attributed to the micro and meso porous that was confirmed using FE-SEM and HR-TEM analysis. The symmetric supercapacitor device fabricated using this activated carbon electrodes in ionic liquid could power up red LED for more than 20 min upon charging for less than 20s. The energy density obtained for ionic liquid electrolyte and four FSSC devices connected in series combinations is about 40 and 22 Wh/kg, respectively which is significantly higher for the bio-derived carbon. The high performance by the aloe vera derived activated carbon promises its viable application in supercapacitor. The use of bio-carbon derived from aloe vera plants with high performance in supercapacitors will significantly reduce the cost of the energy storage device for practical applications. ASSOCIATED CONTENT Supporting Information The supporting information containing the comparison table of bio-derived activated carbon, CHNS data, XPS analysis of AV-700, CV & CD profile of AVB-400, AV-600 and AV-800 electrode materials, CV &CD profile of FSSC device with two and three cells connected in series combinations, CD profile of AV-700 FSSC device with different bending angle and the red LED power up by the fabricated supercapacitor device video are available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxx.

Acknowledgements The authors thank the Science & Engineering Research Board, Department of Science and Technology (GAP 07/14, DST-SERB), India for financial support. Mr K. Subramani (IF131153) thanks DST for INSPIRE Fellowship. The authors thank the Central Instrumentation Facility, CECRI, Karaikudi. Reference (1)

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Table of contents Aloe vera Derived Activated High Surface Area Carbon for Flexible and High Energy Supercapacitors M. Karnan, K. Subramani, N. Sudhan, N. Ilayaraja and M. Sathish* a

Functional Materials Division, b Academy of Scientific and Innovative Research (AcSIR), c

Centre for Education, CSIR-Central Electrochemical Research Institute, Karaikudi- 630003, Tamilnadu, India.

A flexible and all solid state supercapacitors was fabricated using aloe vera derived activated high surface area carbon. It shows high specific capacitance of 244 and 126 F/g with a high energy density 8 and 40 Wh kg-1 in all solid state and ionic liquids, respectively. With the wide cell voltage of 3V in ionic liquid, it could power up a LED for 30 min upon charging for 20s.


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