Electric Double-Layer Capacitors Based on Multiwalled Carbon

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Electric Double-Layer Capacitors Based on Multi-Walled Carbon Nanotubes: Can Nanostructuring of the Nanotubes Enhance Performance? Anne-Riikka Rautio, Olli Pitkänen, Topias Järvinen, Ajaikumar Samikannu, Niina Halonen, Melinda Mohl, Jyri-Pekka Mikkola, and Krisztián Kordás J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512481b • Publication Date (Web): 27 Jan 2015 Downloaded from http://pubs.acs.org on February 3, 2015

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The Journal of Physical Chemistry

Electric Double-Layer Capacitors Based on Multi-Walled Carbon Nanotubes: Can Nanostructuring of the Nanotubes Enhance Performance? Anne-Riikka Rautio1,Olli Pitkänen1, Topias Järvinen1, Ajaikumar Samikannu2, Niina Halonen1, Melinda Mohl1, Jyri-Pekka Mikkola2,3, Krisztian Kordas1,∗ 1

Microelectronics and Materials Physics Laboratories, Department of Electrical Engineering, University of Oulu, P.O. Box 4500, FI-90014 Oulu, Finland 2

Technical Chemistry, Department of Chemistry, Chemical-Biological Center, Umeå University, SE90187, Umeå, Sweden

3

Laboratory of Industrial Chemistry and Reaction Engineering, Department of Chemical Engineering, Process Chemistry Centre Åbo Akademi University, Åbo FI-20500, Finland

Abstract Supercapacitors prepared from chemically modified and vacuum filtered buckypapers were studied. The aim was to evaluate how its pore structure impacts on the specific capacitance, energy and power density in different electrolytes. The specific capacitance varies in a linear fashion with the specific surface area for nanotubes modified by the means of catalytic, low-temperature partial catalytic oxidation using cobalt oxide nanoparticles decorating the nanotubes. In contrast, electrodes composed of nanotubes preactivated in CO2 demonstrated only a minor increase in their specific capacitance, despite the observed significant increase in specific surface area. The radically improved surface area was a result of emergence and deposition of soot on the nanotubes during the activation process, as revealed by transmission electron microscopy. Among six different types of electrode materials, the CoOx decorated materials proved to have the highest specific capacitance (~25 F/g in aqueous KOH and ~15 F/g in triethylsulfonium bis(trifluoromethylsulfonyl)imide ionic liquid). Thus, highly structured carbon nanotubes giving rise to energy and power storage densities comparable with commercial and other multiwalled carbon nanotube based electric double-layer capacitor devices were obtained.

Keywords: Supercapacitor, metal oxide, capacitor, carbon nanotube, specific surface area

∗

: Tel +358 40 8248763; E-mail: [email protected] (K. Kordas)

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1. Introduction In the recent years, considerable effort has been invested to implement carbon nanotubes (CNTs) in electrical devices related to energy applications such as batteries, fuel and solar cells and supercapacitors 1-5

. The good intrinsic electrical and thermal conductivity as well as reasonably large specific surface area

are the main reasons for the attention devoted to CNTs. Moreover, a number of procedures, such as chemical functionalization as well as activation, have been developed with the aim to further modify and enhance the physicochemical surface properties of these versatile carbon materials. Another great advantage of CNTs over many other materials is their feasibility to engineer macroscopic but nanostructured hierarchical assemblies and composites allowing for many different applications. Various types of supercapacitors have been prepared by using either MWCNTs or SWCNTs grown on different substrates (supported or free-standing, thin or thick film, patterned, non-patterned) containing a metal oxide (either by impregnation or catalysts residuals)

11-17

6-10

and modified by various

kinds of activation procedures 18-22. Especially CNT composites with metal oxides and graphite have been reported to demonstrate high specific capacitances (Table 1). Supercapacitors can generally be divided in two sub-categories, depending on whether electrochemical oxidation of the electrode material takes place (usually referred as pseudo-capacitors) or not (electric double-layer capacitors). The functionality of the double-layer capacitors mainly depends on the surface area of electrodes since the surface charge is compensated by the counter ions of the electrolytes attracted by electrostatic forces. In order to lower the charge density and thus, the Coulomb repulsion occurring on the electrode and also in the electrolyte near to the interface, a plausible approach involves strategies aiming at increasing the specific surface area whereupon the electric charge storage capacity may be increased without increasing the weight of the device. On the other hand, the operation of pseudocapacitors is based on faradic processes that involve electrochemical reduction and oxidation of 2 ACS Paragon Plus Environment

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the electrodes. In this case, the capacitance of the device is primarily affected by the materials chemistry and mass. Further, the overall performance of the capacitors are also influenced by the conductivity of the electrode materials since in the charging/discharging cycles the rate of charging (i.e. the τ=RC time constant) and also the dissipated power in the form of Joule heat (i.e. P=RI2) are particularly important parameters besides the specific capacitance. Still, the surface/bulk textures of a material (pore size and distribution) are yet further aspects to be considered. The practical limit for surface area enhancement depends particularly on the electrode material, since highly structured conductors tend to reduce their conductivity and are also becoming prone to oxidation, mechanical degradation and adverse effects. Furthermore, good permeation of the porous structure with the electrolyte is a must and, when using organic or ionic liquid (IL) electrolytes with large ions, the pore size needs to be large enough to allow diffusion and penetration of the ions so that they actually can reach the surface. 18 Table 1 Double-layer supercapacitors based on CNTs, contact materials and electrolytes used in the measurements and measured specific capacitances. Electrode Non-patterned MWCNT film Patterned MWCNT film SWCNT film SWCNT film MWCNT MWCNT Graphene/MWCNT, 261m2/g MWCNT 4.8%MWCNT/Co3O4 50% MWCNT/Co3O4 5% MWCNT/Co3O4, 137 m2/g Cocatalyst/MWCNT, ~100 m2/g 10 %MWCNT/NiO, ~200 m2/g SWNT/MnO2 hybrid film GNs/SnO2-MWCNT In2O3 nanowire/SWCNT film MWCNT, ~131 m2/g MWCNT annealed in air at 540°C, ~167 m2/g MWCNT annealed in air at 650°C, ~175 m2/g CO2 and acid activated MWCNT, ~160 m2/g KOH activated MWCNT, 511 m2/g Acid activated CNT, ~280 m2/g Plasma etched MWCNT, ~400m2/g

Contact Inconel Inconel Free-standing Platinum Graphite foil Graphite foil Platinum foil Nickel gauze Nickel gauze Nickel gauze Nickel gauze Carbon paper Ni-foam Platinum Carbon cloth N.A. Aluminum foil Aluminum foil Aluminum foil Aluminum foil Acetylene black N.A. Gold

Electrolyte KOH KOH LiClO4 IL H2SO4 LiClO4 KOH KOH KOH KOH KOH H2SO4 KOH Na2SO4 KOH LiClO4 LiClO4 LiClO4 LiClO4 LiClO4 LiClO4 LiClO4 IL

C/m (F/g) 10 29 35 24 126 13 265 90 166 201 418 12 160 184 224 64 24 38 48 60 50 57 440

Ref. (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) 3

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In this paper, we studied how different kinds of chemical surface etching techniques (such as acidic oxidation, activation, catalytic oxidation with metal oxides and their combinations) can introduce nanosize defects in the wall structure of multi-walled carbon nanotubes. In turn, we use the nanostructured CNTs as electrodes of flexible electric double-layer capacitors and demonstrate that the specific capacitance of the devices is not necessarily scaling with the specific surface area.

2. Experimental 2.1 Sample preparation Pristine MWCNTs (~2.2 g, Aldrich, 724769, outer diameter of 6-9 nm and length of ~5 µm) were carboxyl functionalized by sonicating them in concentrated 600 mL of HNO3: H2SO4 mixture (1:3) for 5 h followed by centrifugation and filtration. The products were washed several times with deionized water in order to remove any residual acids, followed by drying overnight at 70°C. The acid treated MWCNTs (CNT-COOH) were decorated with cobalt oxide nanoparticles by the means of wet impregnation. First, CNT-COOHs (0.7 g) were sonicated in toluene (40 mL, Lab scan, A.R.) for 3 h to form a stable dispersion, followed by an addition of a suitable amount of cobalt acetylacetonate (Co(acac)3, Aldrich 98%) dissolved in toluene (80 mL). Consequently, the slurry containing the organometallic species and functionalized CNTs was stirred overnight, at room temperature. The solvent was evaporated under N2 flow at 65°C and the solids were dried overnight at 70°C. As the next step, the sample was calcined in air at 330°C for 5 min (heating rate of 5°C/min). The total mass loss recorded during the annealing process was 44 wt.%. The removal of cobalt nanoparticles from the nanotubes was accomplished by sonicating the annealed sample in 4 mol/L HCl for 15 min, followed by a 1 h reflux, filtration and rinsing with deionized water.


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The CO2 activation of pristine MWCNTs was carried out as described earlier

23

. First, the sample was

heated to the desired temperature under N2 flow followed by introduction of CO2. The temperature was kept constant (either 750°C or 800°C) for 5 h and thereafter the N2 atmosphere was reintroduced and the sample was cooled down. The electrodes made of thin CNT films were prepared by sonicating ~17 mg portions of the powder samples in DMF (FLUKA, >99%) for 2 h and then filtrated through PTFE membranes (Whatman, cat. no. 10411411). The top part of the dispersed sample was withdrawn with a Pasteur pipet and filtrated. Then, more DMF was added on top of the non-dispersed CNTs and the sample was again subjected to sonication. This procedure was repeated until all of the CNTs were dispersed and filtrated. Finally, the membranes were dried at room temperature and then cut with scalpel to smaller rectangular pieces, resulting in samples with an area typically around 0.6-0.8 cm2. Consequently, the area normalized mass of the as obtained films was ~1.5 mg/cm2. 2.2 Characterization of materials Structural properties of CNTs were determined with energy filtered transmission electron microscopy (EFTEM, Leo 912 Omega, acceleration voltage of 120 kV). Elemental contents of the materials were analyzed with field emission scanning electron microscopy (FESEM, Zeiss Ultra plus equipped with energy dispersive X-ray spectroscopy analyzer, EDX). The specific surface area, size and volume of pores were determined by using nitrogen physisorption (-196°C, Micromeritics ASAP 2020 Surface Analyzer). Further, Raman measurements (Horiba Jobin-Yvon Labram HR800 UV-VIS Μ-Raman) were performed to determine the quality of the samples from the ratio of sp3 and sp2 hybridized carbon atoms. The oxidation state of cobalt oxide was determined by X-ray diffraction (XRD, Bruker D8 Discover, Cu Kα –radiation) and X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD electron spectrometer, monochromated Al Kα source operated at 150 W, charge neutralizer).



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2.3 Capacitor measurements Electrochemical capacitors were assembled from the vacuum filtered MWCNT buckypaper electrode pairs (each supported on the PTFE filters) separated with a filter paper (Whatman 1, cat. no. 1001-047). Inconel 600® (thickness of 200 µm, Goodfellow) sheets with e-beam evaporated Ti (50 nm) and RFsputtered Au (450 nm) surface coatings were applied to render uniform electrical contacts with the nanotube films. The stacks were mechanically clamped with alligator clips and wetted with the electrolytes such as (i) a mixture of aqueous KOH (6 mol/L) and isopropanol with a ratio of 4:1 and (ii) triethylsulfonium bis(trifluoromethylsulfonyl)imide ionic liquid (TES-TFSI, IL, FLUKA, ≥99.0 %) mixed with ethanol (99.8 %). The role of the alcohols was to enhance the wetting of hydrophobic electrodes. The ionic liquid (IL) used was selected since it allows for a broad electrochemical window (2.4 V to 3.1 V) and good conductivity (5.5 mS/cm). The capacitance of the materials was assessed by cyclic voltammetry using voltage sweep rates of 0.05, 0.1, 0.25, 0.5 and 1.0 V/s, respectively (Princeton Applied Research VersaSTAT 3). The specific capacitance values were determined from the averages of the integrated current-time hysteresis curves (after transforming the variable V to t) normalized by the electrode mass =

∆ ∙

, where I(t) is the

charging current averaged for 5 cycles, ∆U is the voltage difference between t2 and t1 moments upon charging/discharging and m is the mass of the electrode. The transient current measurement was made as follows: The capacitor was connected in series to a 3.3 Ω resistor and then the circuit was powered by using a signal generator (U= ±500 mV at 0.5 Hz, Agilent 33120A). From the potential drop on the resistor (measured with an oscilloscope, Tektronix TPS 2024B), the current in the circuit could be calculated by the Ohm’s law. The electrical measurements of devices with IL electrolyte were performed under inert atmosphere in a glove box.


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3. Results and discussion 3.1 Characterization of materials According to the XPS measurements, the main phase of the Co particles on the surface of MWCNTs was cobalt oxyhydroxide whereas the XRD results only hinted the existence of CoO phase. Hence, we define the Co particles as ‘CoOx’ materials. The transmission electron micrographs (Fig. 1) illustrate the structure of the nanotubes after being subjected to various chemical treatments. Soaking pristine MWCNTs in the ratio of 1:3 HNO3: H2SO4 resulted in insignificant changes (Fig. 1a-b) whereupon mainly amorphous carbon was removed from the surface. The increased oxygen concentration in the materials indicates the presence of carboxyl groups (Table 2). The specific surface area of the carboxyl functionalized sample was similar to that of the original nanotubes due to the loss of amorphous material along with the very short/thin nanotubes during the functionalization process (Table 2 and 3). More substantial changes in both appearance (broken tubes and large metal oxide particles) and specific surface area were observed when metal oxide nanoparticles were deposited on the surface and annealed in air (Fig. 1c and Tables 2 and 3). Notwithstanding, this result is not surprising since CNTs can undergo rapid oxidation even at very moderate temperatures in the presence of NiO, 24 PdO and CoOx nanoparticles. 2426

A subsequent acid treatment of CoOx/MWCNT powders removed the metal oxide particles thus leaving

behind hollow defects in the CNT structure that further increased their specific surface area (Fig. 1d). The changes observed for the CO2 activated CNTs were not completely obvious as the quality of the nanotube powder became inhomogeneous, particularly CNTs activated at 800°C where the specific surface area varied significantly within one single sample (Table 3). In some areas of the sample, CNTs with corrugated surface appear; while in other locations, graphitic structures and some amorphous carbon deposits were observed (Fig. 1e-f). The etched/corrugated CNT surface may explain the increase in the specific surface area; however the ratio of D and G-band obtained with Raman measurements (Fig. 1h 7 ACS Paragon Plus Environment


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and Table 2) was found rather similar for each sample analyzed in this work. The mass loss during the activation of CNTs increased with increasing temperature; at 750°C the total mass loss was ~7%, and at 800°C ~28% (Table 3).

Figure 1 TEM micrographs and Raman spectra of (a) pristine MWCNT, (b) MWCNT-COOH, (c) CoOx/MWCNT, (d) acid washed CoOx/MWCNT, (e) CNTs activated at 750°C, (f) and (g) CNTs activated at 800°C. Panel (h) displays the Raman spectra of the capacitor materials.

Table 2 Elemental contents of capacitor materials by FESEM-EDX and the ratios of D- and G-band according to Raman spectroscopy. Sample Pristine MWCNT MWCNT-COOH CoOx/MWCNT Acid washed CoOx/MWCNT Activated CNT at 750°C Activated CNT at 800°C

Elemental composition by EDX (wt.%) C 90.4±2.1 86.9±3.4 67.3±15.3 89.1±1.7 92.0±1.8 88.9±4.8

O 8.5±1.9 12.1±3.9 15.8±4.3 10.1±1.7 6.1±1.9 9.8±4.6

Co 15.1±10.2 -

Al 0.6±0.5 0.5±0.2 0.6±0.4 0.3±0.1 1.3±0.7 0.7±0.5

Fe 0.5±0.3 0.6±0.4 0.7±0.4 0.4±0.1 0.6±0.2 0.5±0.2

Raman D and G band ratios by Intensity Area 1.16 1.37 1.11 1.48 1.20 1.38 1.22 1.41 1.06 1.29 1.10 1.30

Table 3 Specific surface area, average volume and average size of the pores in capacitor materials. Sample Pristine MWCNT MWCNT-COOH CoOx/MWCNT Acid washed CoOx/MWCNT Activated CNT at 750°C Activated CNT at 800°C

Specific surface area (m2/g) BET 255.4±3.5 266.4±14.6 302.4±10.2 354.7±5.7 324.2±5.0 416.3±92.9

Pore volume (cm3/g) 1.2±0.5 0.9±0.3 0.9±0.3 1.0±0.2 1.0±0.2 2.3±1.6

Pore size (nm) 18.0±7.3 13.0±3.7 11.2±4.0 11.5±3.0 12.2±2.2 21.2±10.1


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3.2 Capacitor measurements The C-V curves for each sample in KOH follow the ideal shape (Fig. 2a)

6,7

over a wide range of scan

rates (Fig. 2b). In the TES-TFSI electrolyte however, the curves appear somewhat butterfly-shaped (Fig. 2c) bearing resemblance to those observed earlier.

8,22

The irregular curve shape can be explained by the

different orientation of the non-spherical symmetric ions at different electrode bias. Molecular dynamics calculations

27

have shown that non-spherical ions of various ILs can orient parallel or perpendicular to

the surface of CNTs depending on the surface charge density (σ). When surface charge density σ = 0.5 e/nm2 was applied, it alone already induced a significant effect on the orientation, thus influencing the apparent thickness of the Helmholtz layer. Since the surface charge densities on the nanotubes used in our experiments are in the same range (up to 0.5 e/nm2 at 2 V), the voltage dependent variation of specific capacitance is a plausible source of the perturbed C-V curves assessed in the IL. Although one would expect pseudocapacitance curves for the electrodes build of MWCNT-COOH and CoOx/MWCNT - due to the presence of surface functional groups and metal oxide nanoparticles - no obvious redox reactions were detected.

28

Evidently, the number of surface groups and CoOx were not

large enough to have any influence on the shape of the C-V hysteresis curves, albeit by increasing the relative surface area some contribution to the capacitance could be observed. 11,14 Due to the electrolysis of water, already at ~1.0 V,

29,30

the electrochemical window of aqueous electrolytes are rather narrow

and thus, result in a limited energy density for such devices. For the IL used in our experiments, the allowed voltage range is higher (from -2.4 V up to 3.1 V) but, nevertheless, in the anodic scan we observed an onset of an oxidation peak after ~2 V. This was most probably caused by the oxidation of ethanol traces being present (ethanol was added to the electrolyte to enable better wetting of the CNT electrodes).



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Figure 2 a) Cyclic voltammetry curves of EDLCs made of CNT films in KOH electrolyte: pristine (CNT/pristine, red), carboxyl functionalized (CNT-COOH, dark green), CoOx decorated (CNT-CoOx, magenta), CoOx decorated and acid washed (CNT-CoOx/acid, black), CO2 activated at 750°C (CNT/Act.750°C, blue) and at 800°C (CNT/Act.800°C (light green). b) C-V curves of symmetric CoOx/MWCNT electrodes measured at different charge/discharge rates in KOH electrolyte. c) C-V curves of symmetric CNT/pristine (red), CNT-CoOx/acid (black), CO2 activated at 750°C (CNT/Act.750°C, blue) and at 800°C (CNT/Act.800°C (light green) CNT thin film in TES-TFSI ionic liquid electrolyte measured from -2 to 2 V. (Note: invalid data points caused by the acquisition software are masked.)

To be able to monitor the charging/discharging current with good time resolution, a resistor of 3.3 Ω was connected to the capacitor in series, and then the circuit was powered by rectangular voltage pulses (U=±500 mV at 0.5 Hz). From the temporal change of the measured voltage drop on the series resistor, we get the current in the circuit by Ohm’s law as I(t)=U(t)/3.3 Ω. The value of the measured initial current Io was then used to calculate the actual series resistance of the device as Rc=(0.5 V/I0)-3.3 Ω giving 41.5 Ω and 39.8 Ω for the capacitors made of pristine and CoOx decorated nanotubes, respectively. The as obtained RC time constants (τ = 0.51 s for pristine and τ =1.00 s for CoOx/CNT, respectively, were in reasonable agreement with those obtained from fitting the exponential decay curves on the experimental data (τpristine= 0.74 s and τCoOx/CNT=1.48 s, Fig. 3).

Figure 3 (a) A schematic drawing for the circuit applied to measure the charging/discharging current and transient current slopes. (b) Current versus time plot for the CoOx/CNT supercapacitor. (c) Linear fit (intercepting the origin) 10 ACS Paragon Plus Environment

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of specific capacitance vs. specific surface area of different capacitor materials, excluding CNTs activated at 750°C and 800°C. Surface area slopes: 65 ± 10 mF/m2 in aqueous KOH and 42 ± 1 mF/m2 in TES-TFSI ionic liquid.

Further, the energy efficiencies of the pristine and CoOx/CNT capacitors were evaluated by calculating

the dissipated energy i.e. the Joule heat = and the energy needed to load the devices

= 0.5 $ % & resulting in an efficiency of ~33% for both types of devices. The specific capacitance of the electrodes increases proportionally with increasing specific surface area (Table 4 and Fig. 3c) of the different kinds of electrode materials. The capacitors containing the metal oxide decorated nanotube electrodes as well as the decorated and subsequently acid washed nanotubes show ~2-fold increase of the specific capacitance in reference to the non-decorated and carboxyl functionalized devices, respectively. The role of CoOx in the samples was limited to the localized catalytic oxidative etching during the synthesis phase, thus contributing to the increased specific surface area and capacitance. The CoOx nanoparticles do not seem to undergo electrochemical reduction and oxidation (i.e. switching between oxidation states Co2+ and Co3+)

11,31,32

as we may conclude from the

lack of oxidation/reduction peaks in the C-V curves, both when applying KOH and TES-TFSI electrolytes. Accordingly, the CoOx/MWCNT composite was behaving as an electric double-layer capacitor and not as a pseudo-capacitor. This was also supported by the very similar specific capacitance obtained for the electrodes made of acid washed CoOx/MWCNTs. Table 4 Specific capacitance and energy density of capacitors in KOH (at 0.5 V) and ionic liquid (triethylsulfonium bis(trifluoromethylsulfonyl)imide) (at 2.0 V) electrolytes. Sample Pristine MWCNT MWCNT-COOH CoOx/MWCNT Acid washed CoOx/MWCNT

Specific capacitance in KOH (F/g) 16.6 8.5 24.9 25.0

Energy density in KOH (Wh/kg) 0.58 0.29 0.86 0.87

Specific capacitance in IL (F/g) 10.4 n.m. n.m. 15.0

Energy density in IL (Wh/kg) 5.79 n.m. n.m. 8.32



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Motivated by the relatively good linear relationship between the increasing specific capacitance and specific surface area of the nanostructured CNTs, we also explored the possibility to apply nanotubes activated in CO2 as supercapacitor electrodes. Despite the significant increase of the specific surface area after the activation of CNTs (in CO2 at 750°C or 800°C, respectively), the specific capacitance of the electrodes were practically identical to the devices without having the CO2 treatment. This was surprising, especially in the light of the earlier measurements performed with nanotubes that were nanostructured by catalytic oxidation. The TEM studies of the surface of CO2 activated CNT samples reveal a carbonaceous deposit (soot) and also an appearance of additional 2D particles most likely due to the disproportionation of carbon monoxide (2 CO → C + CO2) formed during the etching step (C + CO2 → 2 CO), the reversible Boudouard reaction. The etching of the nanotubes and the formation of new carbon allomorphs results in increased overall surface area as assessed by N2 physisorption studies (BET method: 324 m2/g for CNT/Act.750°C and 416 m2/g for CNT/Act.800°C); however, even a small amount of the deposited soot can introduce electrically poor interfaces with the electrolytes and thus, can induce a relatively limited capacitive behavior (14.6 F/g for CNT/Act.750°C and 11.3 F/g for CNT/Act.800°C in KOH) deviating from the linear data of Fig. 3c. The capacitance values obtained when applying the ionic liquid electrolyte were typically lower than those of the corresponding samples measured in KOH solution. Considering the size of ions (3.3 Å for hydrated K+ and 3.0 Å for OH-

33

vs. 3.7 Å for triethylsulfonium

34

and 4.4 Å

35

for bis(trifluoromethylsulfonyl)imide)

participating in the formation of the Helmholtz layer at the electrolyte-electrode interface, the ~40% lowered capacitance values in ionic liquid are understandable. On the other hand, as the electrochemical window of the applied TES-TFSI electrolyte is about 4 times wider than that of aqueous electrolytes, the overall enabled energy and power densities are approximately an order of magnitude higher (0.9 vs. 8.3 Wh/kg as well as 1.3 and 10.3 kW/kg as evaluated for acid washed CoOx/MWCNTs electrodes) and comparable with the performance of commercially available ultracapacitors 36-38. 12 ACS Paragon Plus Environment

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To have a brief estimate on the reliability of the capacitor structures, cyclic voltammetry curves of a capacitor made from the acid washed CoxO/MWCNT was measured for more than 1500 cycles. The device did not show any particular sign of deactivation, other than a decrease of the charging current after about 15 minutes due to the open measurement set-up causing the electrolyte to gradually evaporate. When performing the measurements in a nearly saturated vapor of the electrolyte, the C-V curves remained rather stable (for over 1000 cycles) suggesting that the devices integrated in hermetic packages would be functional for extended number of charging/discharging cycles also in practical applications.

4. Conclusions In this work, flexible, electric double-layer capacitors made of chemically modified and vacuum filtered MWCNT thin film stacks were studied with the goal to evaluate how the pore structure is influencing the specific capacitance and energy density of the devices. The specific surface area and the associated specific capacitance followed a linear correlation for nanotubes modified via catalytic partial oxidation over cobalt oxide nanoparticles. On the other hand, experiments with CO2 activated CNTs revealed that a high surface area itself cannot guarantee a high specific capacitance. The plausible reason for this was the observed soot formation that compromises the good electric double layer at the solid-electrolyte interface. Among the six different kinds of CNT based electrode materials, the CoOx decorated as well as the decorated followed by acid washing, highly structured nanotube devices exhibited the highest specific capacitances (~25 F/g in aqueous KOH and ~15 F/g in triethylsulfonium bis(trifluoromethylsulfonyl)imide room temperature ionic liquid, respectively). Consequently, the corresponding energy and power storage densities of the capacitors residing in the ionic liquid electrolyte (~8 Wh/kg and ~10 kW/kg, respectively) compete with commercial and other MWCNT based supercapacitor devices.



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Acknowledgements The authors would like to thank Andrey Shchukarev (Umeå Univ.) and Jouko Virkkala (Univ. Oulu) for assisting the XPS and nitrogen physisorption measurements, respectively. A.-R. Rautio is grateful for the post-graduate position and for the personal grants received from the Graduate School in Electronics, Telecommunications and Automation and Tauno Tönning and Emil Aaltonen foundations. The work was supported by Tekes (projects Imphona and AutoSys), by the Academy of Finland (project OPTIFU) and by the European Union FP7 program (Napep). This work is also a part of the activities of the Process Chemistry Centre (Åbo Akademi University), the Infotech Oulu and the Advanced Materials Graduate Programmes (University of Oulu). The Kempe Foundations as well as the Bio4Energy and the Artificial Leaf projects are acknowledged.

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Electric Double-Layer Capacitors Based on Multiwalled Carbon

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