High energy density, robust and economical supercapacitor with poly(3,4-ethylenedioxythiophene)-CO2 activated rice husk derived carbon hybrid electrodes

Materials Today Energy 9 (2018) 137e153

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Materials Today Energy journal homepage: www.journals.elsevier.com/materials-today-energy/

High energy density, robust and economical supercapacitor with poly(3,4-ethylenedioxythiophene)-CO2 activated rice husk derived carbon hybrid electrodes Sathish Deshagani, K. Krushnamurty, Melepurath Deepa* Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, 502285, Sangareddy, Telangana, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 February 2018 Received in revised form 3 May 2018 Accepted 9 May 2018

High energy density but not at the expense of power density, large areal and gravimetric capacitances, durability, lightweight, ease of fabrication, low self-discharge rates, low cost, eco-friendly, non-hazardous components to enable scale-up and safe disposal are the desirable pre-requisites that supercapacitors are expected to satisfy so that they can bridge the gap between batteries and sole carbon based supercapacitors. These requirements are met in a hybrid based on poly(3,4-ethylenedioxythiophene) (PEDOT) fibers, a robust, inexpensive, easily processable conducting polymer, with an another remarkably cheap, CO2 activated carbon (CO2@C) derived from rice husk, a waste by-product of rice manufacturing, which is abundantly available (for two-thirds of the world's population consumes rice). The PEDOT-CO2@C hybrid based symmetric flexible supercapacitor delivers gravimetric and areal capacitances of 458 F g
1 (at 1 A g
1) and 850 mF cm
2 over a wide voltage window of 2.1 V, an equilibrated low leakage current of 0.14 mA, an exceptionally high energy density of 280 Wh kg
1 at a power density of ~1 kW kg
1, a low diffusion resistance of 3 U and a capacitance retention of ~98% over 5000 cycles. These performance metrics are significantly superior to that exhibited by symmetric cells of sole CO2@C or PEDOT electrodes. This study shows that the PEDOT-CO2@C hybrid overcomes the major limitations that a majority of the lab-level supercapacitors suffer from: low energy density and a processing methodology that is cost effective and scalable simultaneously. Three charged PEDOT-CO2@C hybrid based symmetric supercapacitors are connected in series. Using this assembly, red and yellow LEDs are illuminated, and a commercial glucometer is powered by replacing the 3 V battery with the hybrid supercapacitors. The latter demonstration opens up an exciting possibility of powering a range of micro-diagnostic devices with this easy to use, handle and dispose hybrid supercapacitors instead of toxic, difficult to dispose- batteries. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Supercapacitor Poly(3,4-ethylenedioxythiophene) Rice husk Activated carbon Energy density Glucometer

1. Introduction Energy storage solutions that can either complement or supplement rechargeable batteries are extremely relevant in today's age and time. Compared to batteries, supercapacitors can store and release charge at much faster rates than batteries, endure significantly more number of charge-discharge cycles without undergoing degradation (a million relative to a thousand for batteries), require almost no maintenance and are also safe to use. Supercapacitors can be used as stand-alone power sources, if a high

* Corresponding author. E-mail address: [email protected] (M. Deepa). https://doi.org/10.1016/j.mtener.2018.05.008 2468-6069/© 2018 Elsevier Ltd. All rights reserved.

power is required for a short time (e.g. regenerative braking in vehicles), or they can be used in combination with batteries, as in electric vehicles, and extend battery life. They can also be employed as memory backup for static random-access memory. Most commercial supercapacitors use activated carbons which offer capacitances in the range of 100e400 F g
1, a maximum voltage of 1 V (per cell), energy density less than ~5 Wh kg
1, and power densities in the range of 2e10 kW kg
1 [1,2]. Carbon nanostructures such as reduced graphene oxide (RGO) [3] and even its' composite with carbon nanotubes (CNTs) [4] due to their high effective surface areas and high electrical conductivities are also used extensively. In such cells, charge is accepted and delivered via a non-Faradaic mechanism, by an electrical double layer (EDL) formation, at the

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carbon nanostructure/electrolyte interface. Among recent reports, a supercapacitor based on hydrothermally activated graphene fiber fabric yielded an areal capacitance of 1060 mF cm
2 in a very thin thickness of ~150 mm and the capacitance was enhanced by using multiple layers to a record value of 7398 mF cm
2. However, the fabric preparation requires reduced pressure or hydrothermal conditions and a high speed shearing technique [5]. In a remarkable study, an asymmetric cell with cobalt oxide/hydroxide and activated carbon electrodes delivered a maximum energy density of 9.4 mWh cm
3, and this cell retained 97.4% of its' original capacitance after 5000 cycles [6]. In another interesting study, a cell with an electrode having a unique 3D mesoporous Ne, Se co-doped Carbon-CoSiO architecture combined with an activated carbon electrode delivered a capacitance of 352 mF cm
2 at a current density of 1.0 mA cm
2 [7]. The cell sustained 6000 cycles, without undergoing degradation. In another study of note, single layered mesoporous carbon sandwiched graphene nanosheets based supercapacitor delivered a high specific capacitance (SC) of 249 F g
1 at 1 A g
1, with a wide voltage window of 4 V and an energy density of 130 Wh kg
1 [8]. The method, however, involves several steps and many reagents. By adsorption of ionic liquid filled micelles onto graphene oxide (GO), a 3 V supercapacitor with a SC of 302 F g
1 was fabricated and the performance was found to be controlled by the ionic liquid content [9]. While at lab-scale, these methods are effective, but large scale production of supercapacitor grade CNTs or graphene or carbon at a low cost is a stumbling block for their commercialization. Electroactive redox materials can also function as supercapacitor electrodes, wherein they store and release charge by Faradaic processes, by undergoing reversible oxidation and reduction reactions [10]. Metal oxides (e.g., Fe3O4 [11] RuO2 [12] etc.) and conducting polymers (such as poly(aniline) or PANI [13], poly(pyrrole) or PPy [14] etc) are the most commonly used pseudocapacitive materials. Inorganic polyoxometalate nanocrystals containing metal-oxygen anionic clusters are also electrochemically active [15] and can also serve as potential candidates for supercapacitor electrodes. Conducting polymer based supercapacitors are attractive, for they can offer much higher energy densities (>50 Wh kg
1) and higher capacitances per unit weight (>400 F g
1) without compromising power density and cycle life. Surface modified nano-cellulose fibers with PPy delivered a SC of 127 F g
1 at a high current density of 33 A g
1, with a high mass loading of 9 mg cm
2 [16]. A supercapacitor with vapor phase polymerized PEDOT nanofibers deposited over hard carbon fiber paper showed a SC of 175 F g
1 and 94% capacitance retention after 1000 cycles [17]. Besides pristine conducting polymers and carbon nanostructures which have been used as supercapacitor electrodes, their composites show functional improvements, thus justifying the use of composites for supercapacitors. Poly(3,4propylenedioxypyrrole) (PProDOP) electropolymerized onto thin films of single walled carbon nanotubes (SWNTs), yielded a capacitance of 16.4 mF cm
2, compared to 8.1 mF cm
2 achieved for PProDOP alone [18]. A free standing PANI/SWNT hybrid film electrode delivered a high SC of 446 F g
1 and outstanding cycling stability, achieving 98% capacitance retention over 13,000 cycles [19]. However, the methods are either too complex to be scaled-up, or the components are expensive and non-abundant. The pre-requisites that a supercapacitor is expected to satisfy are: a wide working voltage, high areal and gravimetric SCs, an energy density greater than 50 Wh kg
1 without compromising power density, light weight electrodes to maximize SC per unit weight, low cost and easy processing to allow scale-up and a stable electrochemical cycling response. An electrode that satisfies almost all of the aforementioned requirements is a hybrid film of a conducting polymer with activated carbon derived from rice husk and

deposited over a flexible inexpensive carbon (C)-fabric substrate. Rice husk is abundant, biodegradable and generally regarded as a waste by-product of rice manufacturing. Upon carbonization of husk, to further increase the number of electrochemical active sites, an oversimplified treatment of carbon with a gaseous oxidizing agent like CO2 is performed at high temperature to yield a product labeled as CO2@C. This treatment activates carbon by the incorporation of oxygen containing functional groups, increases the ion-permeability, and allows the electrolyte to access the otherwise inaccessible crosssection of the active electrode. This method of activation is regarded to be eco-friendly since it does not produce waste-water. Among conducting polymers, poly(3,4-ethylenedioxythiophene) or PEDOT is chemically stable, electrically conductive in doped state, and undergoes facile reversible transitions between the oxidized and neutral states [20,21]. Further, PEDOT is easily economically processable in the form of thin films, and can operate efficiently under harsh conditions (deep charge/discharge cycles, over-ripples, temperature). CO2@C flakes in the hybrid prevents the polymer chains from excessive bundling, thus improving iondiffusion, and simultaneously, they also provide electrical interconnects between the polymer chains for facile propagation of electrons during charge-discharge. By employing the PEDOT fibersCO2@C hybrid as a supercapacitor electrode, enhanced and fast charge storage and release properties can be realized. Besides studying the structural and electrochemical characteristics of the hybrid, LED illumination demonstration is also performed by connecting three cells in series, thus illustrating its' practical applicability. We show that this PEDOT-CO2@C hybrid based symmetric cell also overcomes the main shortcoming of low energy density posed by commercial carbon-based supercapacitors. 2. Experimental section 2.1. Chemicals Sodium bis(2-ethylhexyl) sulfosuccinate (AOT), ferric chloride (FeCl3), EDOT (3,4-ethylenedioxythiophene), n-hexane, carbon black, poly(vinylidene fluoride) (PVdF, average MW: 5,34,000), Nmethyl pyrrolidone (NMP), lithium trifluoromethanesulfonate or triflate (LiCF3SO3), poly(methyl methacrylate) (PMMA, average MW: 9,96,000), propylene carbonate (PC), methanol, acetonitrile, GF/D membrane (1.2 mm thickness) were procured from Sigma Aldrich and used directly. Ultrapure water (resistivity ~18.2 MU cm) was obtained through a Millipore Direct-Q3 UV system. Carbon (C)-fabric of 0.2 mm thickness was purchased from Alibaba Pvt. Ltd. 2.2. Preparation of CO2@C Rice husk used in the present study was procured from a local rice mill. It was washed with distilled water and placed overnight in an oven at 90  C to remove the moisture content. The fibers obtained were subjected to carbonization in a furnace at 1173 K under N2 atmosphere for 6 h. Nitrogen gas flow rate was maintained at 100 mL min
1 and the heating rate was fixed at 10 K min
1. The resulting material was allowed to cool down to room temperature while continuously passing N2 gas through the sample. The black colored carbon product was collected and subjected to physical activation. Raw carbon was first compactly packed in a quartz tube between two thick layers of gun cotton on either side and the assembly was placed in a furnace. Gun cotton holds the carbon in place and prevents the its' expulsion during the passage of gas. The inlet of the quartz tube was flanked to a N2 generator and the furnace was switched on. To ensure the complete removal of

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oxygen, N2 gas was passed continuously through the raw carbon till the furnace temperature reached the required set value. The flow rate of N2 gas and the heating rate of furnace were kept constant at 10 mL min
1 and 10 K min
1 respectively. Once the required temperature of 973 K was reached, the N2 generator was switched off and CO2 gas was passed through the raw carbon for 2 h at 973 K. After completion of the treatment, the N2 generator was switched on and the sample was allowed to cool down to room temperature. The ensuing black carbon, activated by the passage of CO2 gas, is referred to as CO2@C. A schematic displaying the transformation of rice husk to CO2@C powder is shown in Scheme 1a. 2.3. Synthesis of PEDOT fibers PEDOT fibers were synthesized by using a previously reported method [22]. A reverse micro-emulsion was prepared by dissolving AOT (19.12 mmol) in n-hexane (70 mL). An aqueous solution (1 mL) of FeCl3 (10.0 mmol) was introduced to the above emulsion. The orange colored mixture was stirred for 5 min. The

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monomer, EDOT (3.52 mmol) was added to this mixture and stirred magnetically for 3 h. A bluish-black precipitate of PEDOT fibers was obtained and it was collected by filtration under suction. The product was washed with methanol and acetonitrile. It was dried under dynamic vacuum for 12 h at 80  C and a deep blue colored powder of PEDOT fibers was obtained. Scheme 1(b and c) shows cartoon-representations of PEDOT formation. 2.4. PEDOT-CO2@C hybrid and fabrication of cells PEDOT fibers (0.6 g) and CO2@C (0.2 g) were mixed in a 3:1 wt ratio and dry grinding was performed using a pestle and mortar for 3 h to yield the PEDOT-CO2@C hybrid. To this mixture, 0.1 g of PVdF and 0.1 g of carbon black were added and the grinding was continued for 1 h. A few drops of NMP were added to this dry mixture and by using a glass rod, a homogeneous slurry was prepared. The slurry was applied over C-fabric using doctor blading, and the coated C-fabric was heated at 70  C for 12 h in a vacuum oven to yield the PEDOT-CO2@C hybrid/C-fabric electrode. The

Scheme 1. (a) Photographs illustrating the conversion of rice husk to CO2@C. (b) Cartoon displaying the formation of PEDOT fibers, and (c) plausible structure for as-fabricated PEDOT (in doped state).

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geometric area coated with the hybrid material was 2.1 cm2. Slurries of pristine PEDOT fibers and sole CO2@C were also prepared separately by dry grinding 80 wt% or 0.8 g of the active material (PEDOT or CO2@C), 10 wt% or 0.1 g of PVdF and 10 wt% or 0.1 g of carbon black. Electrodes of PEDOT and CO2@C over C-fabric were prepared using the same method as the one employed for preparing the hybrid films. Symmetric cells of PEDOT-CO2@C hybrid were constructed by using two films of this hybrid facing each other and separated by a gel saturated porous glass microfiber GF/D separator. A gel electrolyte was prepared by dissolving 1 M LiCF3SO3 in PC at room temperature, followed by introduction of PMMA (12 wt%) into the liquid electrolyte. The mixture was continuously stirred for 6 h at 60e70  C, till the polymer dissolved, and yielded a viscous, colorless and transparent gel. The gel was cooled to room temperature and stored in a glove box. For cell fabrication, firstly, two to three drops of the gel electrolyte were applied over both the PEDOT-CO2@C hybrid/C-fabric electrodes, and the electrolyte was allowed to spread and seep through the electrodes for a few min. One to two drops of the same gel were applied on either side of the porous GF/D separator, and the separator was carefully sandwiched between the two electrodes. Any excess gel that oozed out, was removed using a glass rod, and the assembly was allowed to rest for 3e4 h. A cyanoacrylate ester based sealant was applied along the four sides of the cell, it was left undisturbed at room temperature for 1 h, prior to use. Symmetric cells of PEDOT/C-fabric and CO2@C/C-fabric electrodes were also fabricated in the same manner. 2.5. Instrumentation techniques X-ray diffraction patterns of the active materials were recorded on a PANalytical, X'PertPRO instrument with a Cu-Ka (l ¼ 1.5406 Å) radiation. Transmission electron microscopy (TEM, JEOL 2100 microscope) was performed on the samples (PEDOT fibers or CO2@C or PEDOT-CO2@C hybrid), by first sonicating 1 mg of the active material in 10 mL of acetone for 30 min. The sample was drop-cast over a carbon coated copper grid of 3.05 mm diameter and dried for 30 min for solvent evaporation, and then used. Fourier transform infrared spectra (FTIR) spectra were recorded for the active materials on a Bruker Alpha-P FTIR spectrometer with a quartz based sample holder. Surface morphology of the active materials was studied using a field emission scanning electron microscope (Carl Zeiss Supra 40 FE-SEM). Conducting-atomic force microscopy (CAFM) measurements were performed on the samples coated on stainless steel (SS) substrates using a microscope: Bruker (Veeco) Multimode 8 with ScanAsyst (Nanoscope 8.10 software). The commercial conducting probes are coated with Pt/Ir on front and back sides. The probe tip has a radius of 10 nm, a spring constant of 0.2 N cm
2, a current sensitivity of 1 nA V
1. For the measurement, a load force of 50 nN was maintained between the tip and the sample. The sample (PEDOT or CO2@C or PEDOT-CO2@C hybrid) was deposited as a thin layer over a SS substrate. This assembly was fixed on a SS disk with a carbon tape. A continuous strip of pin-hole free silver paste was used for taking contacts. The C-AFM cantilever scans the surface while in contact and both the topography and the current flowing through the sample are imaged as a function of distance simultaneously. Contact topography map is generated with a feedback loop that maintains a constant tip deflection and the current map is produced by measuring the current flow. A 50 mV bias was applied to the tip during imaging. Galvanostatic charge-discharge-, cyclic voltammetry (CV)-, self-discharge-, cell durability-, leakage current-, IeV measurements- and electrochemical impedance spectroscopy (EIS)- studies were performed on an Autolab PGSTAT 302N potentiostat-galvanostat-frequency response analyzer.

3. Results and discussion 3.1. Morphological, electrical and structural features The FE-SEM images of PEDOT fibers, CO2@C, PEDOT-CO2@C hybrid are shown in Fig. 1. The micrographs of PEDOT fibers (Fig. 1a and b), show the presence of intertwined fibrillar structures of PEDOT, which are a few microns in length and their widths broadly lie in the range of 40e200 nm. They are straight, bent and curled, densely packed, and show no specific orientation. Activated carbon or CO2@C is composed of flakes of aggregated carbon, as can be judged from Fig. 1c and d. The flakes have irregular shapes, and their dimensions span several microns. The surface of the carbon flakes is not smooth, but it is textured, indirectly suggestive of the active sites created on the carbon surface by the CO2 activation process, for ion-adsorption from the electrolyte during chargedischarge. The images of the PEDOT-CO2@C hybrid (Fig. 1e and f) show the fibers of PEDOT interspersed with the flaky structures of CO2@C. In some regions, the fibers can be distinctly observed, but largely the morphology shows the mixing of the two components to be homogeneous, as the fibers are blended well with the flaky carbon structures. The TEM images of PEDOT, CO2@C and PEDOT-CO2@C hybrid are shown in Fig. 2. Bundles of PEDOT fibers are observed in the images of PEDOT (Fig. 2a and b). The fibers are mingling with each other, and the lengths vary from a few tens to a few hundred nanometers. The selected area electron diffraction (SAED) pattern of PEDOT (Fig. 2c) shows four discernible concentric diffuse rings with bright spots superimposed on them. While the fuzzy nature of the pattern indicates that the polymer is amorphous, but the spots arise from the lattice planes, thus indicating that PEDOT is semi-crystalline or partially polycrystalline. The spots on the rings correspond to interplanar spacings (d) of 0.27, 0.25, 0.18 and 0.13 nm. A representative TEM image of CO2@C (Fig. 2d) shows sheet or flake like structures of activated carbon. The flakes do not have any particular shape and the dimensions of the sheets extend up to approximately 3 microns. A high magnification image of the sheet displayed in Fig. 2e, shows that the sheets or flakes have corrugated wrinkly appearance similar to what is generally observed for RGO nanosheets. Such a structural feature is beneficial for attracting more number of ions during charge-discharge. A lattice scale image extracted from one such flake is shown in Fig. 2f. It exhibits lattice fringes with an interfringe separation of 0.35 nm, which matches with the graphitic inter-layer spacing of 0.34 nm, corresponding to the (002) plane. The PEDOT-CO2@C hybrid is characterized by the fiber like shapes of PEDOT superimposed over the sheet like structures of CO2@C, as can be judged from Fig. 2g. The contours of both the components of the hybrid are clearly visible. The lattice scale image shows fringes with inter-fringe distances of 0.35 and 0.27 nm, obtained from two different crystallites, and these arise from CO2@C and PEDOT respectively. This also indicates that the polymer and carbon flakes are in direct contact with each other which is useful for electron transfer between the two entities during the electrochemical charge-discharge process. The IeV characteristics of the three electrode materials: PEDOT fibers, CO2@C, and the PEDOT-CO2@C hybrid were recorded by compactly packing these materials into rectangular cavities of 0.5 cm2 area (a). The cavity walls were created by using insulating adhesive double sided tapes affixed on a stainless steel (SS) plate. The thickness (l) of the tape is 0.2 cm. After packing the material, a SS plate is affixed on top of the SS, and IeV plots were recorded (Fig. 3a). The IeV plots are linear in the
2 to þ2 V range for CO2@C, and the PEDOT-CO2@C hybrid, indicating an Ohmic conduction mechanism. However, for the pristine polymer, PEDOT, the plot is sigmoidal in the overall voltage range, which is characteristic of

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Fig. 1. FE-SEM images (a, b) PEDOT, (c, d) CO2@C and (e, f) PEDOT-CO2@C hybrid.

conducting polymers. A quasi-linear dependence over a short potential range of
2 to
0.4 V was observed. Therefore, from all the three linear dependences, the electrical conductivities (s) are calculated using the formula: s ¼ 1/R  l/a, where 1/R is equal to the slope (I/V) of the IeV profile. The conductivities are: 32, 8 and 13.6 mS cm
1 respectively for the CO2@C, PEDOT, and the PEDOTCO2@C hybrid. Among the three, CO2@C shows the highest electrical conductivity. It is composed of a network of carbon atoms, where the carbon atoms are sp2 hybridized, as in graphite and therefore have p-electrons in the unhybridized p-orbitals. These electrons are delocalized, which imparts electrically conducting properties to the CO2@C material. XRD pattern of CO2@C (Fig. 3b) confirms the same, for two broad peaks are observed at 2q ¼ 12 and 24.5 , followed by another weak peak at 44.1. These peaks correspond to inter-planar spacings (d) of 0.74, 0.36 and 0.205 nm. The first d-value matches with that of exfoliated graphite sheets

[23] this could be due to the presence of functional groups between the graphitic carbon layers. The second and third peaks are close to the d values of graphite (0.34 and 0.204 nm, powder diffraction file: 75e1621), corresponding to the (002) and (101) planes. The hybrid is characterized by an electrical conductivity intermediate to that of PEDOT and CO2@C, as anticipated. Conduction in pristine PEDOT (which exists in the oxidized form, in the as-fabricated state) is due to the delocalization of the radical cation (shown in Scheme 1c). Typically, one radical cation is delocalized over 3 to 4 monomer units in the polymer chain. As a consequence, PEDOT conducts electrons in the dedoped state. PEDOT and CO2@C follow different mechanisms for electrical conduction. The FTIR spectra of CO2@C, PEDOT and PEDOT-CO2@C hybrid are shown in Fig. 3c and d. CO2@C shows a peak at 755 cm
1 due to the CeC stretching mode, a very strong intense absorption peak at 1100 cm
1 due to CeO stretching, followed by a peak at 1561 cm
1

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Fig. 2. TEM images of: (a, b) PEDOT fibers and (c) the corresponding SAED pattern (the d-values have units of nm), (d, e) CO2@C and (f) the corresponding lattice scale image, (g) PEDOT-CO2@C hybrid, (h) the corresponding lattice scale image and (i, j) enlarged vies of the lattice fringes.

due to C]C stretching of the aromatic groups [24]. The latter is regarded to be characteristic of skeletal vibrations of the nonoxidized graphitic domains. This is in line with the XRD peak observed at d ¼ 0.36 nm. A small peak is observed at 1721 cm
1, due to the stretching vibrations of the C]O bonds of the carboxylate groups flanked to the CeC network of atoms. These peaks clearly show that CO2@C is composed of mixed networks of graphitic and amorphous carbon atoms. At the same time, many of the carbon atoms are functionalized by oxygen containing groups induced by the CO2 treatment. PEDOT exhibits distinct peaks at ~569 cm
1 (oxyethylene ring deformation), 674 cm
1 (CeSeC deformation), 962 cm
1 (symmetric CeSeC deformation), 1058 cm
1 (CeOeC deformation), 1320 cm
1 (CbeCb stretching) and 1519 cm
1 (asymmetric Ca ¼ Cb stretching). The same peaks are observed in the spectrum of the PEDOT-CO2@C hybrid. However, there are some differences in the intensities and slight positional variations, which are caused by van der Waal's and electrostatic interactions between the polymer (PEDOT) and carbon (CO2@C). The polymer backbone is constituted by intermittently positioned thiophene based radical

cations. The carbon network in CO2@C has eOH and eCOO functionalities, and these are attracted towards the oppositely (or positively) charged PEDOT backbone. In comparison to pristine PEDOT, an additional peak is observed at 746 cm
1, which arises from the CeC stretching vibration of the CeC bonds in CO2@C. This peak is red-shifted in comparison to CO2@C. The absorption peak which appears at 1058 cm
1 in pristine PEDOT is considerably broadened and is highly intense in the hybrid; the peak maximum is also shifted to 1070 cm
1. This could be due to overlapping contributions from the CeO stretching modes of the carboxylate groups in CO2@C and the CeOeC deformation in PEDOT. A shoulder peak is also observed at 1143 cm
1 in the hybrid, which is not observed for PEDOT. In comparison to CO2@C, PEDOT shows a 4-times lower conductivity, and the conductivity of the PEDOT-CO2@C hybrid is inbetween that of the two materials. This is useful during chargedischarge, for a good conductivity facilitates electron movement across the cross-section of the hybrid, which in turn allows a larger ion-uptake compared to the sole polymer cell. Electrical

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Fig. 3. (a) IeV characteristics of PEDOT, CO2@C and PEDOT-CO2@C hybrid; inset is an enlarged view for PEDOT. (b) XRD patterns of CO2@C and PEDOT-CO2@C hybrid. FTIR spectra of (c) CO2@C and (d) PEDOT and PEDOT-CO2@C hybrid.

conduction in oxidized (sole) PEDOT stems from the delocalization of a radical cation (implying an unpaired electron) over three to four monomer units along the polymer chain. This is not as effective in conducting electrons as compared to the mechanism in CO2@C. However, both the mechanisms contribute to electron conduction in the hybrid. The XRD pattern of the hybrid is almost similar to that of CO2@C. But the hybrid experiences slight downshifts to lower 2q values compared to CO2@C, thus implying that the interactions between the polymer and CO2@C, causes further spacing out of the interplanar distances in activated carbon. While lattice fringes and bright spots were observed for PEDOT in the TEM studies; in the XRD pattern no peak from PEDOT is seen and this could due to the fact the CO2@C tends to dominate. 3.2. Comparison of nanoscale current carrying capabilities To further asses the differences between the three materials, CAFM studies were performed. The surface topography maps and the corresponding 2D and 3D current maps of the CO2@C, PEDOT and PEDOT-CO2@C hybrid films deposited on stainless steel substrates are shown in Fig. 4. The topography maps are not particularly insightful, as the color variations in the maps only reflect the height differences, indicating that the films are granular. In contrast, the corresponding 2D current maps reveal significant and relevant information. The color scale on the right side of the current maps is interpreted as follows: the regions on the 2D and 3D maps in dark color represent the low current regions, and as the color shade gets lighter (in the maps) eventually culminating in stark white, the

current carrying capability of the film is enhanced proportionally. Thus the domains in lighter colors (green/yellow) and specifically white denote the high current zones. The 2D maps of both CO2@C and PEDOT-CO2@C hybrid films are predominantly characterized by white or high current regions, and the dark or low current domains are randomly distributed, and they are narrow (in terms of lateral dimensions). In comparison, the 2D map of the PEDOT film shows the film to be largely composed of low current or dark colored regions, and only a few white colored or high current streaks are observed in some regions. This is also reaffirmed from the 3D maps which bring out this difference between the CO2@C or PEDOTCO2@C hybrid and the PEDOT films distinctively. Akin to the 2D maps, in the 3D maps of the CO2@C or PEDOT-CO2@C hybrid films, the deep crevices in dark color are the low current regions, and the major regions are white. Hence, the maximum nanoscale electronic current which the CO2@C and PEDOT-CO2@C hybrid films can carry are ~13.6 and 9.2 nA respectively, and since the most of these film surfaces are observed to be in monochrome (or white), these two films are highly conducting. Contrasting this, is the 3D map of the PEDOT film, where the deep dark colored or low current crevices are dominant, and the white portions are almost invisible, thus illustrating that the outcome of IeV studies is in line with the CAFM results. While the maximum current is 3 nA, but the average current (carried by the bulk of the film surface) lies in the range of 0.9e1.3 nA. Another noticeable feature is that although the PEDOT content in the PEDOT-CO2@C hybrid is 3 times greater than that of CO2@C, but the current map of the hybrid resembles that of CO2@C instead of PEDOT. This is explained by the fact that in the hybrid, activated carbon prevents the polymer chains from aggregating,

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Fig. 4. Topography, 2D- and 3D- current maps of (aec) PEDOT, (def) CO2@C and (gei) PEDOT-CO2@C hybrid films, recorded by using C-AFM technique.

provides electrical interconnects, thus leading to uniform and large domains of high current. 3.3. Cyclic voltammetric and charge-discharge studies Cyclic voltammograms of symmetric cells of PEDOT, CO2@C, and the PEDOT-CO2@C hybrid, recorded at different scan rates of 5, 10, 20, 50, 100, 200 and 500 mV s
1 are shown in Fig. 5(aef). For each cell, the plots were recorded over two voltage windows of
0.1 V to þ1 V and
0.1 V to þ2 V. The CO2@C cell, when operated over the voltage windows of 1.1 and 2.1 V (Fig. 5a and b), shows featureless oblong leaf like shapes at low scan rates of 15 and 10 mV s
1. However, when the cells are operated at scan rates of 20, 50, and 100 mV s
1, the maximum current densities,

obtained in the cathodic and anodic branches increase as a function of scan rate. More importantly, the areas enclosed in the voltammograms are also larger compared to that observed at 5 and 10 mV s
1. Almost rectangular shapes with integrated areas (enclosed in the loops) of 3.5, 3.8 and 3.3 V mA cm
2 are obtained at 20, 50 and 100 mV s
1, compared to 1.3 and 2.3 V mA cm
2 obtained at 5 and 10 mV s
1. The integrated areas again shrink to 2.4 and 1.6 V mA cm
2 at 200 and 500 mV s
1. Similar maxima (in terms of integrated areas) are observed when the same cell is operated in the 2.2 V regime. The maximum areas are 7.8 and 7.7 V mA cm
2 obtained at 50 and 100 mV s
1, and the areas are lower at higher/lower scan rates. This shows that the cell is versatile, and can operate easily with high efficacy even when the voltage window is broadened.

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Fig. 5. Cyclic voltammograms of (a, b) CO2@C and (c, d) PEDOT and (e, f) PEDOT-CO2@C hybrid based symmetric cells recorded at different scan rates over voltage ranges of
0.1 to 1 V and
0.1 to 2 V respectively.

In the CO2@C cell, in the cathodic branch, the electrolyte cations (Liþ) are adsorbed on the active sites of carbon, which are the oxygen containing functional groups like COO
, eOH, adsorbed H2O and CeOeC linkages via electrostatic interactions, and also by the electrical double layer formation. During the anodic sweep, the Liþ ions are expunged back into the electrolyte, and the anions (CF3SO
3 ) are accumulated along the surface of the carbon flakes via non-Faradaic EDL. At intermediate scan rates of 20e100 mV s
1, the diffusion layer thickness, dx (as in: I ¼ nFADdC/dx), is optimum to maximize ion uptake or release, thus leading to the high capacitances. In the equation in parenthesis, I is current, n is the number of electrons involved in oxidation/reduction of PEDOT, A is the active

electrode area, D is the ion-diffusion coefficient and dC/dx is the concentration gradient. The magnitude of dx is inversely proportional to scan rate. The CV plots of the PEDOT based symmetric cells (Fig. 5c and d), also show nearly rectangular profiles at the intermediate scan rates of 50e200 mV s
1, especially over the voltage window of 2.1 V. The integrated areas are in the range of 7.5e8 V mA cm
2, when scan rates are varied from 50 to 200 mV s
1. In the 1.1 V voltage regime, the plots appear to be less capacitive. In PEDOT, Faradaic phenomena such as reduction of doped PEDOT to yield neutral PEDOT in the cathodic branch, which is accompanied by the release of anions (CF3SO
3 ) into the electrolyte and oxidation of dedoped PEDOT in the anodic half that

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allows uptake of anions, as per the following equation contribute predominantly to the observed capacitance.
(xþy)þ (PEDOT)xþ:(Cl/~SO3)
: x (Neutral) þ y(CF3SO3 ) $ (PEDOT)

(Cl/~SO3)
(1) x (CF3SO3)y (Oxidized) þ ye

The PEDOT-CO2@C hybrid based cell shows rectangular curves and the integrated area exhibits a peak value of 7.2 V mA cm
2 at 20 mV s
1 in the 2.1 V voltage regime (Fig. 5e and f). For the same hybrid cell, the integrated area is 5.3 V mA cm
2 at 5 mV s
1. It is two times greater than that achieved at the same scan rate for the PEDOT and CO2@C based cells in the same 2.1 V voltage domain, indicating that the hybrid is a better performer compared to its' pristine components, particularly at slow charging and discharging rates. The fill factor (FF) of a supercapacitor is the ratio of the area of the ideal geometric rectangle to the experimentally observed closed loop in the CV plot, at a given scan rate. The fill factors are surprisingly higher for pristine PEDOT and CO2@C based cells, ~66.3% at 50 mV s
1 for both cells, and the highest FF of 57.3% is achieved for the PEDOT-CO2@C hybrid cell at 20 mV s
1. The FF is higher for the PEDOT-CO2@C hybrid cell at the lowest scan rate of 5 mV s
1; it is 48.2%, as opposed to 30.7 and 31.7% at the same scan rate for PEDOT and CO2@C cells. This is also advantageous, for during galvanostatic charge-discharge, which is the preferred method for charging/discharging supercapacitors, the discharge

capacitance is found to be considerably larger for the hybrid compared to the polymer or carbon at low current densities. To also ascertain that C-fabric (i.e., the current collector) contributes minimally to the specific capacitance of any of the above cells, CV plots of two symmetric cells with (a) PEDOT-CO2@C hybrid/C-fabric based electrodes and (b) blank C-fabric based electrodes with the same electrolyte: 1 M LiCF3SO3/PC/12 wt% PMMA were recorded and these are shown in Fig. S1 (Supporting Information). The figure shows that C-fabric hardly contributes to the specific capacitance, for the current produced by the C-fabric based cell is of the order of a few microamperes, and in contrast to the hybrid based cell, this capacitive response is almost negligible. Galvanostatic charge-discharge characteristics recorded over the two voltage windows of 1.1 and 2.1 V, at different current densities varied from 1 to 5 A g
1 (in steps of 1 A g
1) are shown in Fig. 6(aef). Specific capacitances (SCs) were calculated using the equation provided below. SC ¼ I (current density, A g
1)  Dt (discharge time, s)/DV (voltage window, V)

(2)

Prior to comparing the cell performances, the reason for using PEDOT fibers based electrodes instead of conventional PEDOT particles is explained. A comparison of charge-discharge characteristics of symmetric cells of PEDOT particles versus PEDOT fibers, and their TEM images are shown in Supporting Information

Fig. 6. Galvanostatic charge-discharge characteristics of (a, d) CO2@C, (b, e) PEDOT and (c, f) PEDOT-CO2@C hybrid based symmetric cells recorded at different current densities over voltage ranges of
0.1 to 1 V and
0.1 to 2 V respectively. Rate capability profiles of the three cells over (g)
0.1e1 V and (h)
0.1e2 V, voltage windows. (i) Cycling stability of the three cells at 1 A g
1, over 5000 cycles.

S. Deshagani et al. / Materials Today Energy 9 (2018) 137e153

(Fig. S2). The comparison clearly shows that the cell with PEDOT particles delivers a significantly lowered specific capacitance of 85.6 F g
1, compared to a capacitance of 135.8 F g
1 achieved for PEDOT fibers at the same current density of 1 A g
1. The higher effective surface area of the fibers and the better electrical conduction permitted by the fibers is responsible for the superior performance of the fibers relative to the particles. The fibrillar shapes also allow fast electron conduction, due to their one dimensional structures and an inherent high conductivity of 8 mS cm
1. In the PEDOT-CO2@C hybrid, the polymer fibers are in intimate contact with the activated carbon flakes/sheets, as has been confirmed from TEM studies. Thus, the fibers not only permit fast electron transfer between the polymer fibers but also from the polymer to the carbon flakes. The fibers tend to bundle up when used alone, but in the PEDOT-CO2@C hybrid, they are prevented from aggregated by the carbon flakes, and the carbon flakes are also restricted from agglomerating by the carbon flakes. This synergy comes to the fore in the hybrid, and thus the hybrid delivers a higher capacitance and energy density compared to sole CO2C flakes and sole PEDOT fibers based symmetric supercapacitors. This is discussed below. The charge-discharge curves of all the three symmetric cells with PEDOT, CO2@C, and the PEDOT-CO2@C hybrid electrodes have almost triangular shapes in the 1.1 V voltage window. But when the charging voltage is raised to 2 V, the profiles are not triangular. In the charge cycle, the CO2@C symmetric cell shows a monotonic rapid increase until ~1.7 V, but thereafter, the increase till 2.1 V is slow. This behavior is increasingly pronounced at applied current densities of 1 and 2 A g
1, and is found to be common to all the three cells. In the initial region of the oxidation process: till ~1.5e1.7 V, all the easily accessible redox sites (in PEDOT) and the non-Faradaic active sites (in CO2@C) get occupied by the CF3SO
3 anions from the electrolyte. Hence, at higher voltages >1.7 V, anions have to diffuse deeper through the material, to reach the less accessible but available active sites. As a result, the charging times are prolonged. In the discharge cycle, a sharp Ohmic drop in voltage is observed (by ~0.1 V), and the voltage decay rate to the limiting potential of
0.1 V is much faster than the charging rate. It is perceivable, that discharging is faster than charging due to an additional factor: which is the back emf (electromotive force). The back emf also acts in the same direction, as the applied current, which now has a reverse (reductive) polarity. This emf provides an extra driving force to the anions to eject or the cations or the Liþ ions to enter, and thus the discharging occurs at a faster rate. For the CO2C@C symmetric cell, SC shows a systematic increase from 32 to 101 F g
1 and from 90 to 162 F g
1, with decreasing current density (5e1 A g
1) in the 1.1 V and 2.1 V voltage windows respectively. The PEDOT based cell delivers SCs that vary from 74 to 135 F g
1 and 159e199 F g
1 over the 1.1 and 2.1 V ranges, when the current density is lowered from 5 to 1 A g
1. The comparison of rate capability profiles (SC versus current density) are shown in Fig. 6g and h. These values in both the potential ranges are higher than that observed for the CO2@C cell. The hybrid based cell has a higher polymer content compared to the CO2@C proportion; they are mixed in a 3:1 ratio by weight. The benefits of both the materials, the higher electrical conductivity of CO2@C, its' larger effective surface area, relative to pristine PEDOT, and the ability of PEDOT to uptake and liberate more ions compared to CO2@C, possibly due to the Faradaic charge storage mechanism, are thus bestowed to the hybrid based cell. The Brunauer-EmmettTeller (BET) surface areas are found to be 424, 4 and 89 m2 g
1 for CO2@C, PEDOT PEDOT-CO2@C hybrid materials. Despite the low effective surface area, PEDOT outperforms CO2@C, simply due to its ability to undergo efficient redox reactions. The double layer charge accumulation plays an inconsequential role in the charge storage

147

and release processes in pristine PEDOT. Consequently, the hybrid based cell delivers enhanced SCs over the two voltage windows (2.1 and 1.1 V), in comparison to PEDOT and CO2@C based cells. The PEDOT-CO2@C hybrid based cell, over the 2.1 V voltage window, delivers a SC of 458 F g
1 (at 1 A g
1), and even at a high current density of at 5 A g
1, a SC of 182 F g
1 is retained. Similarly, the SC is 240 F g
1 at 1 A g
1 and it reduces to 127 F g
1 at 5 A g
1, over the 1.1 V voltage window. The hybrid allows better utilization of the active material compared to PEDOT or CO2@C. The electrical conduction properties of CO2@C allow more number of electrons to access the cross-section of the hybrid during discharge (from þ2 to
0.1 V), and this in turn increases the number of Liþ ions which are attracted from the electrolyte to the hybrid, thus increasing the capacitance. Further, in the pristine CO2@C cell, despite the electrical conductive pathways available to the electrons, the capacitance is limited (a) by the aggregation of the carbon flakes, which hinders ion-movement, and perhaps renders some regions to be inaccessible by the ions and (b) the intrinsic mechanism of charge storage, which is non Faradic and adsorptive in nature. The higher working voltage of 2 V, is better utilized by the hybrid cell compared to the cells based on its' components. This among other factors, is largely controlled by the inherent electrochemical reactions of the material [25]. Therefore, compared to the pristine PEDOT based cell, in the hybrid cell, the polymer fibers are inhibited from agglomeration by the CO2@C flakes, and therefore the electrode becomes more amenable for undergoing the redox reactions of PEDOT during charge and discharge, thus resulting in a higher capacitance. Previously, composites of RGO with PANI, PPy and PEDOT exhibited SCs of 361, 248 and 108 F g
1 at the same current density of 0.3 A g
1 [26]. It has also been shown in the past that SC values which are typically reported per unit mass of the active material can be misleading [18,27]. For instance, in the present study, the active material weight by summing the weights at each electrode in each cell (by excluding the weight of PVdF and carbon black), lies in the range of 3.8e4.2 mg. Authors in a report on polymeric dioxypyrrole based supercapacitors [18], particularly emphasized on the importance of areal capacitance in assessing and optimizing electrodes. Areal capacitance is defined as the capacitance over a unit geometric area of the electrode substrate (C-fabric). They also stressed that the areal capacitance magnitude reflects the accessibility and efficiency with which the mass of an active material is used, when applied over a given substrate. In this context, here too, the areal capacitances (ACs) were calculated, by using the equation: AC (mF cm
2) ¼ discharge time (s)  applied current (mA)/area (cm2)  DV (V). The highest obtained areal capacitance is 850 mF cm
2 for the PEDOT-CO2@C hybrid based cell, over the 2.1 V voltage window. The parlance per unit weight is 458 F g
1. Similarly, the maximum values of areal capacitances for the CO2@C and PEDOT based cells are: 320 and 363 mF cm
2. For a poly(3,4propylenedioxypyrrole) (PEDOP) film on a sticky single walled carbon nanotubes (SWNTs) based substrate, the AC was 16.4 mF cm
2 [18]. A graphite/PEDOT/MnO2 nanowires bilayer assembly gave an areal capacitance of 316 mF cm
2 at 10 mV s
1, and a SC of 196 F g
1 at 0.5 A g
1 [28]. A transparent 1 cm2 PEDOT:poly(styrene sulfonate) or PSS film based supercapacitor gave a capacitance of 1 mF [29]. In another study, macroporous graphene/ graphene paper/PANI gave an AC of 538 mF cm
2 [13] Our values, especially for the PEDOT-CO2@C hybrid based supercapacitor cell are comparable or superior to the literature values. 3.4. Durability studies and Ragone plots Cycling stability of the three symmetric cells with PEDOT, CO2@C, and the PEDOT-CO2@C hybrid is compared in Fig. 6i. The SCs

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for the cells are plotted as a function of cycle number, over 5000 charge-discharge cycles operated at a fixed current density of 1 A g
1. Table 1 provides a comparison of some of the key parameters of the symmetric supercapacitors based on different electrodes. The CO2@C, and the PEDOT-CO2@C hybrid cells show good durability characteristics, for the capacitance fade with cycling is almost insignificant for the hybrid cell, and nominal for the CO2@C cell. With cycling, 92.5 and 98.2% capacitances are retained after 5000 cycles for CO2@C, and the PEDOT-CO2@C hybrid, indicating that the CO2@C and the PEDOT-CO2@C hybrid cells can be used for practical applications. If the application requires a high SC, the PEDOT-CO2@C hybrid cell can be used, and if low SC suffices, then the CO2@C cell can be utilized. The PEDOT based cell retains only 78.7% of its' original capacitance. This is due to the fact that the polymer swells and shrinks during charge-discharge, and it tends to delaminate from the current collector, causing a drastic SC fade. The presence of CO2@C in the hybrid prevents this peeling off of the active material from the C-fabric, for it accommodates the strain experienced by the polymer during cycling, and thus the high SC of the hybrid cell is preserved. It must be noted that the Coulombic efficiencies for the cells are lower over the 2.1 V voltage range particularly at 1 A g
1, but despite the longer charging time, the cycling stability is not severely affected. This is due to the fact that charging upto ~1.75 V (for the hybrid based cell) is acquired in a rapid ramping manner, but the charging is extremely slow and monotonic for the remaining 0.25 V, which impacts the Coulombic efficiency adversely. However, despite this, the cycling stability is not severely affected for ~98% of the initial capacitance is retained for the hybrid after 5000 charge-discharge cycles, thus indicating that the prolonged charging time does not jeopardize the durability behavior of the cell. Further, the Coulombic efficiencies are high and acceptable in the 1.1 V voltage windows, which is the widely used range in most of the reported supercapacitors. In an earlier study, porous graphene carbon-based supercapacitors exhibited a reversible SC of ~300 F g
1 and an energy storage performance of 67 Wh kg
1 [30]. Almost 99% capacitance retention was achieved after 5000 cycles [30]. The SC of dyes/ rGO@cellulose fiber paper supercapacitor remained at 160 F g
1 (~90.9%) after 5000 charge-discharge cycles [31]. 3D printed graphene oxide-graphene nanoplatelets-SiO2-2 electrodes gave a SC of ~75 F g
1, and they were found to be extremely stable, with no loss of capacitance after 10,000 consecutive charge and discharge cycles [32]. Hydrothermally activated graphene fiber fabrics (HAGFFs) based supercapacitor showed a capacitance of 244 F g
1 at 0.1 A g
1, and the capacitance was found to stabilize only after a few thousand cycles [5]. Cross-linked nitrogen rich graphene nanosheets exhibited a SC of 201 F g
1 at 0.05 Ag
1 in a 1 M H2SO4 electrolyte, and an excellent retention rate of 96.2% of the initial capacitance after 10,000 cycles at a current density of 5 A g
1 [33]. Furthermore, cells based on GO/ionic liquid nanodroplets [9], activated carbon [34], PEDOT nanofibers [17], RGO-PEDOT [26] show 92% (after 5000

Table 1 Salient electrochemical properties of symmetric supercapacitors. Parameter
1


1

SC (F g ) DV: 2.1 V, 1 A g AC (mF cm
2) DV: 2.1 V SC (F g
1) DV: 1.1 V, 1 A g
1 Energy density (Wh kg
1) (2.1 V) Power density (kW kg
1) (2.1 V) Leakage current (mA) Self-discharge from 2 V in 100 min (V) SC (F g
1) DV: 2.1 V, 1 A g
1, after 5000 cycles

CO2@C

PEDOT

PEDOT-CO2@C

162 320 101 99 1 0.012 0V 150

199 363 135 122 1 0.14 0.26 V 157

458 850 240 281 1 0.13 0.36 V 445

cycles), 90% (after 10,000 cycles), 94% (after 1000 cycles) and 88% (after 1000 cycles) capacitance retention with repetitive cycling. The capacity retention trends in the above-described works, lo9o … … … where more than 95% of the initial capacitances are preserved, are comparable to the endurance performance of the PEDOTCO2@C hybrid based cell. Ragone plots of energy density (E) versus power density (P) for the three symmetric cells based on PEDOT, CO2@C, and the PEDOTCO2@C hybrid are shown in Fig. 7a and b. They were calculated using the formulas below: E (Wh kg
1) ¼ C  DV2  1000/2  3600

(3)

P (W kg
1) ¼ 3600  E/Dt

(4)

The power density of all the three symmetrical cells varies from 0.5 to 2.7 kW kg
1 and 1.0e5.2 kW kg
1 when current density is raised from 1 to 5 A g
1, over the voltage windows of 1.1 and 2.1 V respectively. Power density is largely invariant as a function of material type and only depends on (i) the voltage window of cell operation and (ii) applied current density. On the other hand, energy density is dependent on the material type. For the pristine CO2@C based symmetrical supercapacitor cell, energy density decreases from 17 to 5.4 Wh kg
1 when current density is increased from 1 to 5 A g
1, over the 1.1 V voltage range. The energy density magnitudes vary from 22.8 to 12.4 Wh kg
1 and 41.9 to 21.3 Wh kg
1 over the same voltage window and same current density regime for PEDOT and PEDOT-CO2@C hybrid based symmetrical supercapacitors. For the CO2@C and PEDOT based symmetric supercapacitor cells, energy densities lie in the range of 99.4 Wh kg
1 to 54.1 Wh kg
1 and 121.9 Wh kg
1 to 97.7 Wh kg
1 over the 2.1 V voltage region, and when current density is increased from 1 to 5 A g
1. Energy density increases to 280.8 Wh kg
1 at 1 A g
1 (or at 1 kW kg
1) over the 2.1 V voltage region and it is 111.6 Wh kg
1 at 5 A g
1 (or at 5.2 kW kg
1) for the PEDOT-CO2@C hybrid based cell. These are the highest energy densities achieved in this study. This behavior specifically at low current density of 1 A g
1 is akin to that of a hybrid supercapacitor. Typically for hybrid supercapacitors, energy densities lie in the range of few tens of Wh kg
1, and therefore they are in-between those of electrical double layer capacitors (~10 Wh kg
1), and batteries (100e800 Wh kg
1). In the past, for a single layered mesoporous carbon sandwiched graphene nanosheets based cell, a high power density of 16.4 kW kg
1 was obtained at 20 A g
1. In the same work, authors obtained an energy density of ~130 Wh kg
1, at a power density of less than 1 kW kg
1 [8]. In another study, for a PPynanocellulose based cell, gravimetric energy and power densities of 4 Wh kg
1 and 3.5 kW kg
1 were achieved [16]. For a PEDOP/ sticky SWNTs based supercapacitor, power and energy densities of 828 W kg
1 and 4.6 Wh kg
1 were obtained [18]. While our values of energy density are extremely high, our power densities are comparable to some of the literature values. Such a high energy density range (111e281 Wh kg
1), as observed for the PEDOTCO2@C hybrid based cell, is extremely advantageous for applications, where a stable power is required for a longer time. 3.5. Self-discharge, leakage current and impedance studies Another two aspects of a supercapacitor that should be considered for its practical deployment are the self-discharge rate and leakage current; these are often not studied in literature. Selfdischarge is defined as the voltage drop of a charged cell after a duration of time without a load. In a previous report on a PEDOT/ ionic liquid based supercapacitor [20] during the self-discharge, an initial 30% drop in the cell voltage occurred during the first 50 min,

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149

Fig. 7. Ragone plots of PEDOT, CO2@C and PEDOT-CO2@C hybrid based symmetric cells: (a)
0.1e1 V and (b)
0.1e2 V. Self-discharge plots of the same cells: (c) from 1 V and (d) from 2 V. (e) Leakage current versus time for the same cells, when fully charged to 2 V. (f) Nyquist plots for the same symmetric cells, before cycling and after 5000 cycles.

followed by a slower, more linear decay. In the present case, when the three cells were allowed to discharge spontaneously under zero external current (Fig. 7c), from ~1 V, the PEDOT-CO2@C hybrid cell discharges to ~0.26 V, 74% of its' original voltage in 100 min. The PEDOT cell discharges to ~0.19 V in the same time, which amounts to 81% of the original charging voltage. The CO2@C based cell showed an anomalous behavior; the cell voltage decreases

gradually to 0.17 V in 47 min, and then it abruptly drops to 0 V. A similar response is registered for this cell when it is discharged from 2 V (Fig. 7d), the cell voltage drops to ~0.5 V in 34.4 min, and then it shows a sharp drop to 0 V. For the remaining two cells of PEDOT-CO2@C hybrid and PEDOT, 82 and 87% decline in voltages is observed in 100 min, to ~0.36 and 0.26 V respectively. The only reason for the sudden drop in voltage to 0 V for the CO2@C cell, is

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perhaps the charge storage mechanism. Here, charge storage is solely governed by the electrical double layer formation, unlike the other two cells, where both pseudocapacitive and EDL mechanisms prevail. Further, self-discharge rate is controlled by the leakage current and charge redistribution. The charge redistribution corresponds to the displacement of charges from an easily accessible charged region of the electrode with a low access time constant, to a more difficult one, with a longer time constant. This mode is probably active in the PEDOT based cells. Leakage current is the current required to maintain a constant voltage, and it is also directly proportional to cell capacitance. Here, from the fully charged state of 2 V, when the leakage current is measured for the three cells as a function of time (Fig. 7e), the initial magnitudes are 4.8  10
4, 4.37  10
5 and 0.01 A for the CO2@C, PEDOT, and the PEDOT-CO2@C hybrid cells. It is the highest for the hybrid, which is in line with the observed highest SC. It plummets non-linearly but

steeply for the CO2@C cell, and reaches a value of 1.2  10
8 A in ~11 min. The response is inexplicable. For the PEDOT, and the PEDOT-CO2@C hybrid based cells, the initial decline is rapid, but it shows a stable value in the range of 0.13e0.15 mA after ~60 min. In comparison, for a PEDOT based supercapacitor, the equilibrium leakage current when it was continuously charged to 1 V, was 2 mA [20]. To study the effect of (a) material type (PEDOT or CO2@C or PEDOT-CO2@C hybrid) and (b) repeated cycling on the cell performance, Nyquist plots were recorded for the three symmetric cells before and after they were cycled 5000 times. The plots were recorded under an ac amplitude of 10 mV applied over a frequency range of 1 MHz to 0.1 Hz (with a 0 V dc bias). Fig. 7f shows the Z00 versus Z0 curves for the cells. All the three cells show a distorted semicircle in the high frequency domain, which is followed by an inclined straight line. The high frequency intercept on the real axis

Fig. 8. (a) Charge-discharge characteristics of symmetric PEDOT-CO2@C hybrid based: 1 cell, and 3 cells connected in series, at 1 A g
1. (b) Photographs of 6-PEDOT-CO2@C hybrid electrodes deposited on C-fabric used for fabricating the 3 cells and illumination of red and yellow LEDs by a fully charged 3-cell assembly. (c) CV plots of symmetric PEDOT-CO2@C hybrid based: 1 cell, and 3 cells connected in series. (d) Photographs of a bent PEDOT-CO2@C hybrid based cell, charged, and then undergoing self-discharge. (e) CV curves for a bent PEDOT-CO2@C hybrid based cell at a sweep rate of 100 mV s
1 over 2 voltage ranges.

S. Deshagani et al. / Materials Today Energy 9 (2018) 137e153

is the electrolyte resistance (RS), and upon extrapolating the low frequency straight line data onto the abscissa, this intercept is labeled as Ri. By using the relation: RS ¼ 3 (Ri e RS), RS, the iondiffusion resistance is calculated. The magnitudes of RS are 4.4, 3 and 3 U for the PEDOT, CO2@C and PEDOT-CO2@C hybrid based cells respectively. The values are equal for CO2@C and its' hybrid with PEDOT, but higher for the CO2@C cell. The high electrical conductivity of CO2@C allows ease of ion-transport across the CO2@C and the PEDOT-CO2@C hybrid as well. After cycling, the RS increase to 6.4, 3.8 and 3.9 U for the PEDOT, CO2@C and PEDOT-CO2@C hybrid cells respectively. This is due to the fact that with repeated ion insertion and extraction during charge-discharge, there is volume expansion and contraction, which causes a loss of the inter-particle contact thus affecting the ion-conducting pathways. 3.6. Practical uses of hybrid based supercapacitor To assess the ability of the PEDOT-CO2@C hybrid based symmetric supercapacitor for practical applications, 3 cells based on the hybrid were connected in series, and charged from
0.1 V to Vmax at a constant current density of 1 A g
1, which was found to be 4.5 V (Fig. 8a). The photographs of the 6-PEDOT-CO2@C hybrid electrodes used for fabricating the 3 symmetric cells and one such cell are shown in Fig. 8b. For the sake of comparison, one such cell was also charged to 1.5 V at the same current density. The chargedischarge plots are shown in Fig. 8a. The charge discharge profiles are observed to have curved but triangular shapes and show with negligible Ohmic drop, thus ratifying the robustness, in particular, of the 3-hybrid cells (connected in series). The discharge capacitances were calculated using Equation (1), and they are estimated to be 273 and 78 F g
1 for 1-hybrid cell and 3-hybrid cells connected in series respectively. This value for 3-cells is lower, because, when the cells are connected in series, the net voltage is the algebraic sum of the voltages of the individual cells (3  1.5 V ¼ 4.5 V) and the capacitance, theoretically should be one-third of the individual cell capacitance. The latter is found to be only slightly lower than the expected value of 91 F g
1. The loss could be due to a small imbalance in the weights of the active materials across the 3-cells, and also due to resistances at the contacts. Cyclic voltammograms were also recorded for the two systems (individual and 3-hybrid cells in series), at a fixed scan rate

151

of 50 mV s
1 (Fig. 8c), over the same voltage ranges of
0.1 to 1.5 V and
0.1 to 4.5 V respectively. The CV plot for the three cells connected in series, also has a quasi-rectangular shape, indicating that multiple cells in series retain the expected profile. The 3hybrid cells (connected in series) were fully charged to 4.5 V, and then connected to a red LED. The red LED lights up, and its illuminance decreased slowly as a function of time, due to voltage drop. A yellow LED was also illuminated, using the same 3-cells. To further affirm that the symmetric PEDOT-CO2@C hybrid based cell continues to function when bent, one cell was bent and it was charged to ~1.08 V (Fig. 8d) allowed to discharge on its own. The voltage dropped to 0.95 V in 30 s. Moreover, CV plots were also recorded for the bent cell between the voltage ranges of
0.1 to 1 V and
0.1 to 2 V, at the same scan rate of 100 mV s
1 and compared (Fig. 8e). It is observed that the shape of the curve is no longer rectangular, but is leaf-like with tapered ends. From the curves, it is apparent that the charge storage capacity of the bent hybrid cell diminishes in comparison to the flat cell, nonetheless, the performance does not reduce by a large extent. Another successful demonstration that we were able to implement using this hybrid supercapacitor was the powering of a commercial glucometer, and a clear illustration is provided in Fig. 9. People check their blood glucose levels by using a glucometer. This is accomplished by dropping a drop of blood sample over a test strip which is inserted into the glucometer. The glucometer display, is powered by a 3 V coin cell (Lithium battery). The battery is fixed on the backside of this device in a cavity, and once the strip with the sample is detected by the glucometer, it displays the blood glucose level. Here, we dispensed a drop of an aqueous solution of glucose on the test strip (Fig. 9a). We removed the coin cell from the glucometer, and then connected the positive and negative terminals of the glucometer with the corresponding terminals of the 3 PEDOT-CO2@C hybrid based supercapacitors, already connected in series, and charged to ~3.8 V, as can be seen in Fig. 9b. The strip with the glucose solution was inserted into the crevice provided in the glucometer (Fig. 9b). The glucometer immediately showed a reading of 101 mg dL
1 (Fig. 9b). A mirror was placed behind the glucometer to prove that the cavity of the coin cell is empty, and during operation, the glucometer is solely powered by the hybrid supercapacitors (Fig. 9c). This demonstration shows that the hybrid supercapacitor is able to power a commercially available diagnostic

Fig. 9. (a) An analysis strip with a drop of glucose solution. (b) A glucometer connected to 3 charged PEDOT-CO2@C hybrid supercapacitors shows the glucose level. (c) An enlarged view of the rear-side of the glucometer as seen in a mirror with “no 3 V battery”, when it is operated with the hybrid supercapacitors.

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electronic device (the glucometer), just like a commercial Lithium based battery does. Compared to a battery, the hybrid supercapacitor is very safe to use and safe to dispose as well. These demonstrations indubitably confirm the potential that the PEDOTCO2@C hybrid has for practical applications, especially for powering electronic devices, which require power for a short time. 7. Conclusions The major bottlenecks in supercapacitor materials and research, namely, low energy density, (typically less than 10 Wh kg
1), and a wide voltage window of operation, are addressed to some extent in the present work by the development and study of symmetric supercapacitors based on a hybrid of PEDOT and rice husk derived carbon activated by CO2. Furthermore, a few reports in literature that furnish solutions for the above two, generally entail very complex processing of electroactive materials and require costly precursors. This drawback is also overcome here for rice husk is plentiful, cheap, and can be converted to activated carbon by a procedure that is easily scalable, and EDOT, is also commercially available. The PEDOT-CO2@C hybrid based symmetric supercapacitor delivers capacitances of 458 F g
1 (per unit weight) at 1 A g
1, and 850 mF cm
2 (per unit area) over a voltage window of 2.1 V, which are significantly greater than that delivered by sole CO2@C and PEDOT based supercapacitors. Energy density of 281 Wh kg
1 (at a power density of 1 kW kg
1) is achieved by the hybrid based supercapacitors, and this value is close to the energy density shown by some Lithium ion batteries. Low ion-diffusion resistance, stable cycling life over 5000 cycles, with minimal capacity fade, slow self-discharge rate, and a low leakage current also advocate the usefulness of this material for high energy density supercapacitors. Two practical demonstrations: (i) red and yellow LED lighting accomplished by the use of three charged hybrid supercapacitors connected in series and (ii) the powering or the activation of the display of a commercial glucometer by replacing the Lithium battery of the glucometer with the same 3 hybrid supercapacitors show the promise this energy storage material has for practical applications. The ability of this hybrid based supercapacitor to serve as a safe to use, easy to dispose and a relatively non-toxic substitute for Lithium based battery which is generally used for powering devices like the glucometer here provide irrefutable validation for taking it to the next level, i.e., for developing a wide range of environmentally friendly micro-analytical electronic devices for consumer applications. Acknowledgements Financial support from the Department of Science & Technology (Project: India-UK Center for education and research in clean energy (IUCERCE), Grant no. DST/RCUK/JVCCE/2015/04 (1) (G)) is gratefully acknowledged. S.D. is thankful to University Grants Commission (UGC) for the grant of junior research fellowship. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.mtener.2018.05.008. References [1] Y. Shao, M.F. El-Kady, C.W. Lin, G. Zhu, K.L. Marsh, J.Y. Hwang, Q. Zhang, Y. Li, H. Wang, R.B. Kaner, 3D freeze-casting of cellular graphene films for ultrahighpower-density supercapacitors, Adv. Mater. 28 (2016) 6719e6726. [2] L. Weinstein, R. Dash, Supercapacitor carbons: have exotic carbons failed? Mater. Today 16 (2013) 356e357.

[3] J.J. Yoo, K. Balakrishnan, J. Huang, V. Meunier, B.G. Sumpter, A. Srivastava, M. Conway, A.L. Mohana Reddy, J. Yu, R. Vajtai, P.M. Ajayan, Ultrathin planar graphene supercapacitors, Nano Lett. 11 (2011) 1423e1427. [4] B.S. Mao, Z. Wen, Z. Bo, J. Chang, X. Huang, J. Chen, Hierarchical nanohybrids with porous CNT-networks decorated crumpled graphene balls for supercapacitors, ACS Appl. Mater. Interfaces 6 (2014) 9881e9889. [5] Z. Li, T. Huang, W. Gao, Z. Xu, D. Chang, C. Zhang, C. Gao, Hydrothermally activated graphene fiber fabrics for textile electrodes of supercapacitors, ACS Nano 11 (2017) 11056e11065. [6] H. Pang, X. Li, Q. Zhao, H. Xue, W.Y. Lai, Z. Hu, W. Huang, One-pot synthesis of heterogeneous Co3O4-nanocube/Co(OH)2-nanosheet hybrids for highperformance flexible asymmetric all-solid-state supercapacitors, Nano Energy 35 (2017) 138e145. [7] X. Li, S. Ding, X. Xiao, J. Shao, J. Wei, H. Pang, Y. Yu, N, S co-doped 3D mesoporous carboneCo3Si2O5(OH)4 architectures for high-performance flexible pseudo-solid-state supercapacitors, J. Mater. Chem. A 5 (2017) 12774e12781. [8] Y. Liu, H. Zhang, H. Song, O. Noonan, C. Liang, X. Huang, C. Yu, Single-layered mesoporous carbon sandwiched graphene nanosheets for high performance ionic liquid supercapacitors, J. Phys. Chem. C 121 (2017) 23947e23954. [9] Z. She, D. Ghosh, M.A. Pope, Decorating graphene oxide with ionic liquid nanodroplets: an approach leading to energy-dense, high-voltage supercapacitors, ACS Nano 11 (2017) 10077e10087. [10] R. Mukkabla, M. Deepa, A. Kumar, Poly(3,4-ethylenedioxypyrrole) enwrapped Bi2S3 nanoflowers for rigid and flexible supercapacitors, Electrochim. Acta 164 (2015) 171e181. [11] B.N. Reddy, S. Deshagani, M. Deepa, P. Ghosal, Effective pseudocapacitive charge storage/release by hybrids of poly(3,4-ethylenedioxypyrrole) with Fe3O4 nanostructures or Co3O4 nanorods, Chem. Eng. J. 334 (2018) 1328e1340. [12] J. Zang, S.J. Bao, C.M. Li, H. Bian, X. Cui, Q. Bao, C.Q. Sun, J. Guo, K. Lian, Wellaligned cone-shaped nanostructure of polypyrrole/RuO2 and its electrochemical supercapacitor, J. Phys. Chem. C 112 (2008) 14843e14847. [13] M.A. Memon, W. Bai, J. Sun, M. Imran, S.N. Phulpoto, S. Yan, Y. Huang, J. Geng, Conjunction of conducting polymer nanostructures with macroporous structured graphene thin films for high-performance flexible supercapacitors, ACS Appl. Mater. Interfaces 8 (2016) 11711e11719. [14] J. Lee, H. Jeong, R. Lassarote Lavall, A. Busnaina, Y. Kim, Y.J. Jung, H. Lee, Polypyrrole films with micro/nanosphere shapes for electrodes of highperformance supercapacitors, ACS Appl. Mater. Interfaces 9 (2017) 33203e33211. [15] X. Li, H. Xue, H. Pang, Facile synthesis and shape evolution of well-defined phosphotungstic acid potassium nanocrystals as a highly efficient visiblelight-driven photocatalyst, Nanoscale 9 (2017) 216e222. [16] Z. Wang, D.O. Carlsson, P. Tammela, K. Hua, P. Zhang, L. Nyholm, M. Strømme, Surface modified nanocellulose fibers yield conducting polymer-based flexible supercapacitors with enhanced capacitances, ACS Nano 9 (2015) 7563e7571. [17] J.M. D'Arcy, M.F. El-Kady, P.P. Khine, L. Zhang, S.H. Lee, N.R. Davis, D.S. Liu, M.T. Yeung, S.Y. Kim, C.L. Turner, A.T. Lech, Vapor-phase polymerization of nanofibrillar poly(3,4-ethylenedioxythiophene) for supercapacitors, ACS Nano 8 (2014) 1500e1510. [18] M. Ertas, R.M. Walczak, R.K. Das, A.G. Rinzler, J.R. Reynolds, Supercapacitors based on polymeric dioxypyrroles and single walled carbon nanotubes, Chem. Mater. 24 (2012) 433e443. [19] F. Liu, S. Luo, D. Liu, W. Chen, Y. Huang, L. Dong, L. Wang, Facile processing of free-standing polyaniline/SWCNT film as an integrated electrode for flexible supercapacitor application, ACS Appl. Mater. Interfaces 9 (2017) 33791e33801. € [20] A.M. Osterholm, D.E. Shen, A.L. Dyer, J.R. Reynolds, Optimization of PEDOT films in ionic liquid supercapacitors: demonstration as a power source for polymer electrochromic devices, ACS Appl. Mater. Interfaces 5 (2013) 13432e13440. [21] A.P. Saxena, M. Deepa, A.G. Joshi, S. Bhandari, A.K. Srivastava, Poly(3,4-ethylenedioxythiophene)-ionic liquid functionalized graphene/reduced graphene oxide nanostructures: improved conduction and electrochromism, ACS Appl. Mater. Interfaces 3 (2011) 1115e1126. [22] X. Zhang, J.S. Lee, G.S. Lee, D.K. Cha, M.J. Kim, D.J. Yang, S.K. Manohar, Chemical synthesis of PEDOT nanotubes, Macromolecules 39 (2006) 470e472. [23] W. Gao, L.B. Alemany, L. Ci, P.M. Ajayan, New insights into the structure and reduction of graphite oxide, Nat. Chem. 1 (2009) 403e408. [24] H. Yi, Q. Yu, X. Tang, P. Ning, L. Yang, Z. Ye, J. Song, Phosphine adsorption removal from yellow phosphorus tail gas over CuO-ZnO- La2O3/activated carbon, Ind. Eng. Chem. Res. 50 (2011) 3960e3965. [25] X. Xia, D. Chao, Z. Fan, C. Guan, X. Cao, H. Zhang, H.J. Fan, A new type of porous graphite foams and their integrated composites with oxide/polymer core/ shell nanowires for supercapacitors: structural design, fabrication, and full supercapacitor demonstrations, Nano Lett. 14 (2014) 1651e1658. [26] J. Zhang, X.S. Zhao, Conducting polymers directly coated on reduced graphene oxide sheets as high-performance supercapacitor electrodes, J. Phys. Chem. C 116 (2012) 5420e5426. [27] M. Hughes, G.Z. Chen, M.S.P. Shaffer, D.J. Fray, A.H. Windle, Electrochemical capacitance of a nanoporous composite of carbon nanotubes and polypyrrole, Chem. Mater. 14 (2002) 1610e1613. [28] P. Tang, L. Han, L. Zhang, Facile synthesis of graphite/PEDOT/MnO2 composites on commercial supercapacitor separator membranes as flexible and high-

S. Deshagani et al. / Materials Today Energy 9 (2018) 137e153 performance supercapacitor electrodes, ACS Appl. Mater. Interfaces 6 (2014) 10506e10515. [29] T.M. Higgins, J.N. Coleman, Avoiding resistance limitations in highperformance transparent supercapacitor electrodes based on large-area, high-conductivity PEDOT: PSS films, ACS Appl. Mater. Interfaces 7 (2015) 16495e16506. [30] J. Huang, J. Wang, C. Wang, H. Zhang, C. Lu, J. Wang, Hierarchical porous graphene carbon-based supercapacitors, Chem. Mater. 27 (2015) 2107e2113. [31] D. Yu, H. Wang, J. Yang, Z. Niu, H. Lu, Y. Yang, L. Cheng, L. Guo, Dye wastewater cleanup by graphene composite paper for tailorable supercapacitors, ACS Appl. Mater. Interfaces 9 (2017) 21298e21306.

153

[32] C. Zhu, T. Liu, F. Qian, T. Yong-Jin Han, E.B. Duoss, J.D. Kuntz, C.M. Spadaccini, M.A. Worsley, Y. Li, Supercapacitors based on three-dimensional hierarchical graphene aerogels with periodic macropores, Nano Lett. 16 (2016) 3448e3456. [33] Y. Wen, T.E. Rufford, D. Hulicova-Jurcakova, X. Zhu, L. Wang, Structure control of nitrogen-rich graphene nanosheets using hydrothermal treatment and formaldehyde polymerization for supercapacitors, ACS Appl. Mater. Interfaces 8 (2016) 18051e18059. [34] W. Ma, S. Chen, S. Yang, W. Chen, W. Weng, M. Zhu, Bottom-up fabrication of activated carbon fiber for all-solid-state supercapacitor with excellent electrochemical performance, ACS Appl. Mater. Interfaces 8 (2016) 14622e14627.