ORIGINAL PAPER MWCNT/activated-carbon freestanding sheets: a different approach to fabricate flexible electrodes for supercapacitors Rahmat Agung Susantyoko 1 & Fathima Parveen 1 & Ibrahim Mustafa 1 & Saif Almheiri 1 Received: 21 January 2018 /Revised: 27 April 2018 /Accepted: 30 April 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Wearable electronics require flexible supercapacitors with specially fabricated electrode materials, i.e., foldable and freestanding. Although activated carbon is the most used electrode’ s active material for aqueous supercapacitors, it is a challenge to pack the particulates into flexible electrodes. Typically, polytetrafluoroethylene binder and polymeric flexible substrate are used, rendering a large amount of inactive-material. Here, we successfully fabricated multiwalled carbon nanotube/activated-carbon (MWCNT- AC) freestanding sheet via a scalable surface-engineered tape-casting technique to be used as a flexible electrode for aqueous supercapacitors. Instead of focusing on improving MWCNTs as active materials, the sheets act as a conducting matrix that binds together the activated-carbon particulates. MWCNT-AC has a specific capacitance of 135.17 Fg −1 (123.9 Fg −1 after 1000 cycles) at 1 Ag −1 from − 0.8 to 0.2 V vs. Hg/HgO (in three-electrode cell). Keywords Activated carbon . Buckypapers . Flexible . Freestanding . MWCNTs Introduction The advent of wearable computing, human-computer interac- tion, and consumer electronics demands innovative hardware capable to support new requirements. A notable component to have in this regard is a flexible and wearable supercapacitor [1]. Dong et al. classified flexible electrodes into three catego- ries: fiber-like, paper-like, and three-dimensional porous flex- ible electrodes [2]. The paper-like flexible electrodes can be classified into flexible substrate supported electrodes or free- standing flexible electrodes (without a supporting substrate) [2]. The supporting substrate, such as nickel foam [3, 4], acts as an excellent current collector. However, the supporting sub- strate also acts as an inactive material which does not contribute to the supercapacitor energy storage mechanism, thus increasing the overall weight of a supercapacitor and decreasing its specific capacitance at the cell level. Hence, we focused on the second type of paper-like flexible elec- trodes which is freestanding flexible electrode. New binder materials have been investigated since com- mon binders for supercapacitors have electrically insulated property. In one approach, conducting polymers as binders have been proposed as one of the crucial element to attain robust freestanding substrate in terms of softness, flexibility, and conductivity [5–8]. In another approach, carbon-based nanomaterials such as reduced graphene oxide have been pro- posed as a multi-functional conductive binder for freestanding and flexible supercapacitor electrodes [9]. Herein, we pro- posed multiwall carbon nanotube as a conductive binder that enables freestanding and flexible supercapacitor electrodes. Table 1 shows a summary of the reported paper-like flexi- ble electrodes for supercapacitors [10–17]. Previous reports showed flexible supercapacitors can be achieved by having a freestanding electrode based on carbon cloth [10–12], carbon nanofiber [13, 14], carbonized cellulose nanofibrils [15], acti- vated reduced graphene oxide film [16], and multiwalled car- bon nanotube (MWCNT) sheets [17]. However, almost all the aforementioned references have electrode active materials made from exotic materials which have not been well mass- produced. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11581-018-2585-4) contains supplementary material, which is available to authorized users. * Rahmat Agung Susantyoko [email protected]; [email protected]* Saif Almheiri [email protected]; [email protected]1 Department of Mechanical Engineering, Khalifa University of Science and Technology, Masdar Institute, Masdar CityP.O. Box 54224Abu Dhabi, United Arab Emirates Ionics https://doi.org/10.1007/s11581-018-2585-4
Transcript
ORIGINAL PAPER
MWCNT/activated-carbon freestanding sheets: a different approachto fabricate flexible electrodes for supercapacitors
Rahmat Agung Susantyoko1& Fathima Parveen1
& Ibrahim Mustafa1 & Saif Almheiri1
Received: 21 January 2018 /Revised: 27 April 2018 /Accepted: 30 April 2018# Springer-Verlag GmbH Germany, part of Springer Nature 2018
AbstractWearable electronics require flexible supercapacitors with specially fabricated electrode materials, i.e., foldable and freestanding.Although activated carbon is the most used electrode’s active material for aqueous supercapacitors, it is a challenge to pack theparticulates into flexible electrodes. Typically, polytetrafluoroethylene binder and polymeric flexible substrate are used, renderinga large amount of inactive-material. Here, we successfully fabricated multiwalled carbon nanotube/activated-carbon (MWCNT-AC) freestanding sheet via a scalable surface-engineered tape-casting technique to be used as a flexible electrode for aqueoussupercapacitors. Instead of focusing on improving MWCNTs as active materials, the sheets act as a conducting matrix that bindstogether the activated-carbon particulates. MWCNT-AC has a specific capacitance of 135.17 Fg−1 (123.9 Fg−1 after 1000 cycles)at 1 Ag−1 from − 0.8 to 0.2 V vs. Hg/HgO (in three-electrode cell).
The advent of wearable computing, human-computer interac-tion, and consumer electronics demands innovative hardwarecapable to support new requirements. A notable component tohave in this regard is a flexible and wearable supercapacitor[1]. Dong et al. classified flexible electrodes into three catego-ries: fiber-like, paper-like, and three-dimensional porous flex-ible electrodes [2]. The paper-like flexible electrodes can beclassified into flexible substrate supported electrodes or free-standing flexible electrodes (without a supporting substrate)[2]. The supporting substrate, such as nickel foam [3, 4], actsas an excellent current collector. However, the supporting sub-strate also acts as an inactive material which does not
contribute to the supercapacitor energy storage mechanism,thus increasing the overall weight of a supercapacitor anddecreasing its specific capacitance at the cell level. Hence,we focused on the second type of paper-like flexible elec-trodes which is freestanding flexible electrode.
New binder materials have been investigated since com-mon binders for supercapacitors have electrically insulatedproperty. In one approach, conducting polymers as bindershave been proposed as one of the crucial element to attainrobust freestanding substrate in terms of softness, flexibility,and conductivity [5–8]. In another approach, carbon-basednanomaterials such as reduced graphene oxide have been pro-posed as a multi-functional conductive binder for freestandingand flexible supercapacitor electrodes [9]. Herein, we pro-posed multiwall carbon nanotube as a conductive binder thatenables freestanding and flexible supercapacitor electrodes.
Table 1 shows a summary of the reported paper-like flexi-ble electrodes for supercapacitors [10–17]. Previous reportsshowed flexible supercapacitors can be achieved by having afreestanding electrode based on carbon cloth [10–12], carbonnanofiber [13, 14], carbonized cellulose nanofibrils [15], acti-vated reduced graphene oxide film [16], and multiwalled car-bon nanotube (MWCNT) sheets [17]. However, almost all theaforementioned references have electrode active materialsmade from exotic materials which have not been well mass-produced.
Electronic supplementary material The online version of this article(https://doi.org/10.1007/s11581-018-2585-4) contains supplementarymaterial, which is available to authorized users.
1 Department of Mechanical Engineering, Khalifa University ofScience and Technology, Masdar Institute, Masdar CityP.O. Box54224Abu Dhabi, United Arab Emirates
Industrially, activated carbon is the most common electrode’sactive material for supercapacitors [18]. Recent report demon-strated a facile synthesis of nitrogen-doped and hierarchical po-rous carbons with a high surface area of 2412 m2 g−1 forsupercapacitor [19]. The activated carbon is also important asthe negative electrodes for asymmetric supercapacitor configura-tion [11, 12]. However, it is a challenge to pack the spherical-shaped or particulate-shaped activated carbon into flexible elec-trodes for flexible supercapacitors. Typically, activated carbon,conducting graphite, and polytetrafluoroethylene binder are coat-ed on a polymeric flexible substrate or ametal foam (e.g., nickel),rendering a large amount of inactive-material or dead weight. Apromising approach is to create a composite freestanding sheet ofMWCNT/activated carbon (MWCNT-AC). Xu et al. demon-strated the development of freestanding MWCNT-AC sheetsusing the filtration method [17]. However, the electrochemicalcharacterization of theMWCNT-AC sheets in reference [17] wasperformed by using nickel foam as the current collector; thus,their configuration cannot be considered a truly freestandingMWCNT-AC electrode.
Pristine MWCNTs were reported to have a limited specificcapacitance because of the limited real active surface area[20]. Surface modification ofMWCNTs using mild oxidation,activation, and/or unzipping the MWCNTs structure weredone to increase the specific capacitance of MWCNTs [20,21]. However, we used a different approach in this work;instead of using the MWCNTs as active materials, we inves-tigated the use of pristine MWCNTs as a conducting matrixthat binds the activated-carbon particulates for flexible aque-ous supercapacitor.
It is important to note that the performance characteristics(specific capacitance, energy density, power density, etc.)shown in Table 1 were not obtained while considering themass of carbon cloth or nickel foam current collector, binder,and carbon conductor. The mass of current collector, binder,and carbon conductor should be considered for practicalsupercapacitors. If the mass of the current collector was takeninto consideration, their reported performance characteristicswill be much smaller than what is listed in Table 1. In contrast,being a freestanding sheet, the MWCNT-AC does not neednickel foam or carbon cloth current collector, binder, and car-bon conductor.
Chemical vapor deposition (CVD) [22–25] and filtration[26–28] are the most commonly used methods to prepare free-standing MWCNT composite electrodes. These techniquesare associated with high capital and operating costs and longprocessing time and require energy-intensive equipment.Recently, Susantyoko et al. have developed a surface-engineered tape-casting fabrication technique to produceMWCNT-based freestanding sheets; the technique is charac-terized by being low-cost, high-throughput, scalable, large-scale, and capable of roll-to-roll processing [29]. In the con-ventional casting method, MWCNTs or MWCNTcomposites
are cast onto a metal current collector and always tend to stickto the substrate and almost impossible to peel as a perfectsheet. In contrast, surface-engineered tape-casting fabricationtechnique allows easy separation of the active material fromthe supporting substrate to produce mechanically strong free-standing sheets. The report [29] showed that easy detachmentof MWCNTcomposites from a substrate can be accomplishedif the following criteria are met: (a) enough difference betweenactive material and substrate surface energies, and (b) castingon a substrate with micro-pyramid pore structure morphology.Herein, we report for the first time the fabrication ofMWCNT/activated-carbon freestanding sheet using thesurface-engineered tape-casting process to be used as flexibleelectrodes in aqueous supercapacitors, whereby the electro-chemical characterization was carried out without using anickel-foam current collector.
Materials and methods
Fabrication of MWCNT/activated-carbon freestandingsheets
MWCNT flakes, product code ANS-ECF-01-000-PEG01,was obtained from Applied NanoStructured Solutions, LLC(USA), a spin-off company of the Lockheed MartinCorporation [30, 31]. The surface area of MWCNT sheetwas 275.5 m2 g−1 [29]. Activated carbon (TF-B520) was ob-tained from MTI Corporation. The activated-carbon powderhas a high surface area of 2001.038 m2 g−1 (see BResults anddiscussion^); in agreement to the specification of 2000 ±100 m2 g−1 in the datasheet. De-ionized water was producedusing a Purite Select Fusion Deionised Water PurificationSystem. Ethanol (product code 34870; purity ≥ 99.8%) wasprocured from Sigma-Aldrich.
MWCNT/activated-carbon freestanding sheet (MWCNT-AC) was prepared by using a surface-engineered tape-castingprocedure as follows: First, 400 mg of MWCNT flakes,1600 mg activated carbon, 10 ml ethanol, and 10 ml de-ionizedwater were mixed inside a mortar for 2 min. Afterwards, themixture was transferred to a beaker and 90 ml de-ionized waterand 90 ml ethanol were added. Simultaneous sonication andstirring were performed using VCX 750 Ultrasonic Processor(Sonic, USA) and advanced hotplate stirrer (VWR, USA) atroom temperature. The process parameters were 40% amplitude(30–33W; > 18,000 J) for 10 min, with 1000 rpm stirring for thefirst 2 min and 1600 rpm for the remaining 8 min. The dispersedslurry was then degassed using a vacuum oven (VD 53,BINDER GmbH). Casting was performed manually at roomtemperature using a micrometer adjustable film applicator (EQ-Se-KTQ-150, MTI Corporation, USA) with a doctor-blade gapof 5 mm on the matt-side of a copper foil (EQ-bccf-9u, MTICorporation, USA). The cast film was then dried in a forced
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convection oven (FD 53, BINDER GmbH) at 120 °C for 1 h.The dried film was then easily separated/detached from the cop-per substrate, resulting in MWCNT/activated-carbon freestand-ing sheet.
Physical characterizations
Scanning electron microscopy (SEM) (Quanta 250, FEI—Thermo Fisher Scientific) was used to determine the morphol-ogy of theMWCNT-AC freestanding sheet. The samples wereput on top of a copper double-sided tape on an aluminum stub.The samples were not coated with a palladium/gold layer.Raman spectroscopy was performed using a Witec Alpha300RAS with 532 nm excitation wavelength.
In-plane sheet resistance was measured using a LakeShore7607 Hall Measurement System with Van der Pauw configura-tion at room temperature. The tape-cast MWCNT-AC freestand-ing sheets were cut into 2 cm× 2 cm and its thickness was mea-sured using a vernier caliper. A small amount of silver paste wasdropped and dried on every corner of the cut sample. Prior to theresistivity measurement, contact formation was done to decreasethe contact resistance between probes and samples. At least threedata points of resistivity were taken, and then averaged. Theelectrical conductivity was calculated according to σ ¼ 1
ρ, where
ρ is the electrical resistivity (Ω cm) and σ is the electrical con-ductivity (S cm−1).
To calculate the density of MWCNT-AC freestandingsheet, the volume and mass were obtained. Tape-castMWCNT-AC freestanding sheet was punched with a diameterof 1.2 cm. The thickness was measured using a vernier caliper.The volume (cm3) was then calculated according to the equa-
tion: volume ¼ π d2
� �2t, where d= diameter (cm) and t= thick-ness of the sheet (cm). The mass loading was measured usinga precision balance (Mettler Toledo MS105DU Semi-MicroAnalytical Balance) with a readability of 10−5 g. The density(g cm−3) was calculated using the equation: density ¼ mass
volume.Quantachrome NOVA 2000e gas sorption systemwas used
to generate N2 adsorption-desorption isotherm of the sample.Prior to the operation, the sample was first degassed at 150 °Cfor 6 h. Multi-point Brunauer-Emmett-Teller (BET) techniqueover the 0–0.2 P/Po linear range was used to measure the BETspecific surface area. The specific surface area was taken asthe average of the calculated BET values acquired from theadsorption and desorption isotherms. Pore analysis was donefrom the N2 adsorption-desorption isotherm using the Barrett–Joyner–Halenda (BJH) methods.
Electrochemical characterizations
For electrochemical characterizations, the MWCNT-AC free-standing sheets were cut into a rectangular shape. One side ofthe rectangular sheet was connected to the potentiostat while
the other side was waxed to have a total exposed area to theelectrolyte of typically 5 mm × 3 mm. The mass of the ex-posed part was calculated from the density and the exposedvolume of the sample. The electrolyte was 6 M KOH (Sigma-Aldrich). A three-electrode cell was setup using a glass cellvial on a C3 cell stand (EF-1085, Bioanalytical Systems,USA) with working, counter, and reference electrodes ofMWCNT-AC sheet, platinum coil (MW-1033, BioanalyticalSystems, USA) and Hg/HgO (Radiometer Analytical XR400,1 M KOH), respectively. We chose Hg/HgO as a referenceelectrode instead of Ag/AgCl because Hg/HgO is more stablein alkaline conditions while Ag/AgCl has a very limited life-time in alkaline electrolytes (more suitable for acidic environ-ment). Hg/HgO electrode has a potential of − 0.054 V vs. Ag/AgCl. Cyclic voltammetry (CV) was performed usingAutolab PGSTAT302N (Metrohm Autolab) with the ultra-fast sampling ADC10M module to generate a linear cyclicvoltammetry. The lower and upper voltage limits were − 0.8and 0.2 V vs. Hg/HgO, respectively. Cyclic voltammogramswere acquired with iR compensation enabled. Constant-current charge discharge (CCCD) was performed at variousrates (1, 2, 3, and 5 Ag−1) with a potential window of − 0.8 to0.2 V vs. Hg/HgO. The specific capacitance was determinedby normalizing the capacitance by the mass of the exposed/non-waxed MWCNT-AC sheet that was immersed in electro-lyte. Specific capacitance (Fg−1) was calculated from CCCDusing the following equation: Specific capacitance ¼ C ¼It
ΔV ∙m where I is the discharge current (A), t is the dischargetime (s), ΔV is the voltage drop upon discharging (V)—withIR drop excluded—andm is the mass of exposed electrode (g)of (MWCNT + activated carbon).
Equivalent series resistance (Ω) of three-electrode cell con-figuration was calculated from the analysis of IR drop duringthe initial stage of discharging curve from CCCD test as fol-
lows: ESR ¼ ΔV IR drop
ΔI ¼ ΔV IR drop
2I where ΔVIR drop is the voltageof the IR drop (V) and ΔI is the current of the voltage drop (A)[32].
Results and discussion
The MWCNT-AC freestanding sheet can be easily fabricatedusing tape-casting process. Figure 1 shows that the MWCNT-AC sheet is freestanding, i.e., no additional flexible substrateis needed. This is beneficial to minimize inactive-material ordead weight in the supercapacitor. The spring-like nature andflexibility of MWCNT-AC to a certain extend (curvature ra-dius of 2.2 mm) is shown in Fig. 1 and Video 1 in theSupplementary Material. The MWCNT-AC freestandingsheet has an electronic conductivity of 12.42 S cm−1 for anuncompressed sheet with a density of 0.687 g cm−3. The elec-tronic conductivity of MWCNT-AC freestanding sheet is one
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order of magnitude lower than electronic conductivity of un-compressed MWCNT sheet [33] of 120 S cm−1, due to thehigh mass loading (80 wt%) of the activated carbon in theMWCNT-AC freestanding sheet.
We investigated the morphologies of the as-received activat-ed-carbon (AC) particulates and MWCNT-AC freestandingsheet using SEM. The activated carbon consisted of particulatestructures (Fig. 2a, b) in a powder formwith amedian andmeandiameter of 5.36 and 5.61 μm, respectively (Fig. 2c). The
activated-carbon powder has a high surface area of2001.038 m2 g−1 as shown in Fig. 2d. Figure 3a–c shows themorphology of MWCNT-AC freestanding sheet where the ACparticulates were encapsulated within the entangled networksof the MWCNTs carbon strands, bridging the AC particulateswithin the MWCNTs carbonaceous matrix. Figure 3d showsthe energy-dispersive X-ray spectroscopy (EDS) of MWCNT-AC freestanding sheet that confirms that the MWCNT-ACsamples are 97.83 wt% C element, 1.74 wt% O presumably
Fig. 1 The mechanical strengthof the surface-engineered tape-cast MWCNT-AC freestandingsheet as a flexible electrode isdemonstrated by bending thesheet at various curvature levelsusing a tweezer
Fig. 2 Scanning electron microscopy images of activated-carbon particulates probed at (a) × 500 and (b) × 5000 magnification. (c) Particle sizedistribution and (d) N2 adsorption-desorption isotherm of activated-carbon particulates
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from the activated carbon, 0.11 wt% Fe from the catalyst in theMWCNT, and 0.32 wt% Cu from the SEM substrate.
The different types of carbonaceous elements in the samplewere distinguished using Raman spectroscopy. Figure 4shows the Raman spectra comparison of the precursors ofMWCNT flake and activated-carbon powder as well as thetape-cast MWCNT-AC sheet. The MWCNT flake has a rela-tively sharp D and G bands at 1340.3 and 1579.4 cm−1 as wellas 2D band at 2665.6 cm−1 [34]. The high intensity of D bandof MWCNT flake is attributed to the defects from highlycross-linked and branching of the MWCNT strands [29, 31].The activated carbon also has characteristic D and G bands(referred to as D1 and G1 bands), but additional bands of D2and G2 formed the broad shoulders of D1 and G1 spectra [35].These D2 and G2 spectra did not exist in the Raman spectra ofMWCNT flake. The activated carbon did not have 2D band of2665.6 cm−1. The Raman spectra of MWCNT-AC sheet wasthe addition of Raman of its precursors as shown in Fig. 4.MWCNT-AC sheet has D, G band, and 2D bands at 1342.8,1579.4, and 2663.5 cm−1, as well as broad shoulder at D2 and
Fig. 3 Scanning electron microscopy images of a MWCNT-AC freestanding sheet probed at (a) × 500, (b) × 5000 and (c) × 50,000 magnification. (d)Energy-dispersive X-ray spectroscopy of MWCNT-AC freestanding sheet
G2 bands. This shows that both activated carbon andMWCNTwere present in the sample and they did not signif-icantly change in terms of the chemical bonds after tape-casting to form freestanding sheets.
Figure 5a shows the cyclic voltammograms of MWCNT-AC freestanding sheet at various scan rates. Cyclic voltamm-etry ofMWCNT-AC freestanding sheet was tested at a voltagerange of − 0.8 to 0.2 V vs Hg/HgO which is within the elec-trolyte potential window. There were no faradaic peaks in anyscan rate indicating that the supercapacitor was an electricdouble-layer capacitor. The equivalent series resistance(ESR) of 54.5 Ω was calculated from the CCCD curve inFig. 5b when 1 Ag−1 current density was applied. MWCNT-AC freestanding sheet has a specific capacitance of135.17 Fg−1 at 1 Ag−1 current density and − 0.8 to 0.2 V vs.Hg/HgO potential window. At higher current densities of 2, 3,
and 5 Ag−1, theMWCNT-AC freestanding sheet has a specificcapacitance of 121.53, 112.85, and 103.85 Fg−1, respectively.Endurance test showed that the MWCNT-AC freestandingsheet could be cycled for 1000 cycles, with a specific capac-itance of 123.9 Fg−1 at 1 Ag−1 current density.
Figure 6a, b shows that the activated carbon played animportant role in the charge-discharge storage mechanism.When the freestanding sheet was fabricated without the infu-sion of activated carbon, the specific capacitance droppedfrom 135.17 Fg−1 (MWCNT-AC sheet) to 18.21 Fg−1
(MWCNT sheet), see Fig. 6b. This is attributed to the higherBET specific surface area of MWCNT-AC sheet of1620.590 m2 g−1 as compared with the MWCNT sheet (with-out activated-carbon infusion) of 275.5 m2 g−1 [29], whichwere calculated by analyzing the N2 adsorption and desorp-tion isotherms using the multi-point method. International
Fig. 5 Three-electrode cell: aCyclic voltammogram ofMWCNT-AC sheet at 10, 30, 50,70, and 100 mVs−1. b Charge-discharge curves of MWCNT-ACsheet at current densities of 1, 2, 3,and 5 Ag−1. c Rate capability ofMWCNT-AC sheetsupercapacitor at current densitiesof 1, 2, 3, and 5 Ag−1. d Charge-discharge curves of MWCNT-ACsheet at 1st, 100th, 500th, and1000th cycle. e Cycling test ofMWCNT-AC sheet at 1 Ag−1 for1000 cycles
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Union of Pure and Applied Chemistry (IUPAC) defined mi-cro-, meso-, and macro-porous materials to have pore diame-ters of < 2 nm, 2 to 50 nm, and > 5 nm, respectively [36]. Theisotherm graph (Fig. 6c) shows that our material conformed toa chemisorption response of type I and type IV, indicating thatour material contained a micro-porous and a meso-porousrange of porosities. Recall that we used NOVA 2000e ma-chine, which is not designed for accurate analysis of pore radiiless than 1 nm. Thus, we analyzed the meso-porous range(Fig. 6d) using the BJH method, which indicated that a poredistribution between 1 and 200 nm, with the most recurrentsize to be equal to ∼ 14.8 Å (1.4 nm). The role of theMWCNTs was to enable the electrical conductivity and flex-ibility of the MWCNT-AC sheet. Thus, the MWCNTs andactivated-carbon composite is a promising combination forflexible electrodes applicable for aqueous supercapacitors.
Conclusions
We demonstrated that the surface-engineered tape-cast free-standing MWCNT/activated-carbon sheet has an excellentsupercapacitor performance with a specific capacitance of135.17 Fg−1 at 1 Ag−1 current density and − 0.8 to 0.2 V vs.Hg/HgO potential window. The MWCNT-AC freestandingsheet maintained a specific capacitance of 103.85 Fg−1 at a highrate of 5 Ag−1. These interesting results justify the possibility toresearch the freestanding MWCNT-based electrodes fabricated
by tape-casting for different types of supercapacitors such asasymmetric supercapacitor, ionic-liquid supercapacitor, deep-eutectic-solvent supercapacitor, etc.
Acknowledgments The authors acknowledge the support of AppliedNanoStructured Solutions LLC, a Lockheed Martin Company, for pro-viding the MWCNT flakes. We thank Dr. Giovanni Palmisano for the useof the gas sorption system for specific surface area and pore analysis.
Compliance with ethical standards
Competing interests The authors declare that they have no competinginterests.
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