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Author's Accepted Manuscript

High Rate Capacitive of Single- Walled Nanotube Katherine L. Van Aken, Carlos R. Pérez, Young- seok Oh, Majid Beidaghi, Yeon Joo Jeong, Mohammad F. Islam,

www.elsevier.com/nanoenergy

PII: S2211-2855(15)00244-X DOI: http://dx.doi.org/10.1016/j.nanoen.2015.05.028 Reference: NANOEN860

To appear in: Nano Energy

Received date: 16 March 2015 Revised date: 18 May 2015 Accepted date: 20 May 2015

Cite this article as: Katherine L. Van Aken, Carlos R. Pérez, Youngseok Oh, Majid Beidaghi, Yeon Joo Jeong, Mohammad F. Islam, Yury Gogotsi, High Rate Capacitive Performance of Single-Walled Aerogels, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2015.05.028

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. High Rate Capacitive Performance of Single-Walled Carbon Nanotube Aerogels

Katherine L. Van Aken a, Carlos R. Pérez a,b , Youngseok Oh c, Majid Beidaghi a, Yeon Joo Jeong c, Mohammad F. Islam c, Yury Gogotsi a,* a Department of and Engineering & A.J. Drexel Institute,

Drexel University, 3141 Chestnut Street, Philadelphia, PA, USA b Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut

Street, Philadelphia, PA 19104, USA c Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes

Avenue, Pittsburgh, PA 15213, USA

* Corresponding author. Tel.: +1 215 895 6446; fax: +1 215 895 1934; e-mail: [email protected].

1

Abstract

Single-walled carbon nanotube (SWCNT) aerogels produced by critical-point-drying of wet- precursors exhibit unique properties, such as high surface-area-to-volume and strength-to-weight ratios. They are free-standing, are binder-free, and can be scaled to thicknesses of more than

1 mm. Here, we examine the electric double layer capacitive behavior of these materials using a common room ionic electrolyte, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI). Electrochemical performance is assessed through galvanostatic cycling, cyclic voltammetry and impedance spectroscopy. Results indicate stable capacitive performance over 10000 cycles as well as an impressive performance at high charge and discharge rates, due to accessible pore networks and enhanced electronic and ionic conductivities of SWCNT aerogels. These materials can find applications in mechanically compressible and flexible devices with high power requirements.

1. Introduction

Electrochemical (ECs) are electrochemical energy storage devices that bridge the gap between batteries and dielectric or electrolytic capacitors. Most noticeable is their high power as they are used for high power applications requiring long cycle life [1,2].

Perhaps the most widely studied ECs are electric double-layer capacitors (EDLCs or ) which store charge within the double layer of ions and electrons at the electrode/electrolyte interface. High surface area carbon materials such as (AC)

[3], carbide-derived carbon (CDC) [4], carbon nanotubes (CNTs) [5], and [6] are typically used as electrode materials for EDLCs. Materials such as AC and CDC consist of

2 individual particles that contain porous networks, which provide a high specific surface area and therefore a high . It has been shown that the capacitance of EDLCs with AC or CDC based electrodes is maximized when the pore size is correctly tuned to the size of the ions [7,8].

However, in electrodes with very small micropores, the charge storage process becomes hindered by the slow diffusion of the ions through the porous network. Specifically with very large ions, the slow diffusion becomes a rate limiting factor for EDLCs based on porous carbon materials as electrodes [9,10].

Recently, carbon materials with open surface structures such as CNTs, onion-like

(OLCs) and graphene have attracted great interest as EC electrodes because of their large, open surface area that is completely exposed to the electrolyte ions. These materials have little or no internal and most of their surface area that is accessible to electrolyte ions is on the outside of the particles. [11] As a result, the charge storage in these materials is not limited by diffusion of ions into the pores, and EDLCs with open surface carbon based electrodes have demonstrated charging/discharging capability at very high rates [12]. In particular, CNT aerogels are expected to perform well as electrode materials for ECs due to their high specific surface area, electrical conductivity, low density, and excellent mechanical properties, features which make them promising for energy storage applications [13,14]. However, the specific surface area of these materials is much lower than that of porous carbons, and hence they show lower gravimetric and volumetric compared to that of porous carbons [15]. Furthermore, because of their lower specific capacitance (C sp ), the specific energy density (E sp ) of EDLCs with

CNT based electrodes is lower than that of porous carbon (Eq 1):

1 E= C V 2 , (1) sp2 sp

3 where V is the working potential [16].

Other than the structure of the carbon, electrolyte properties such as ionic conductivity, ion size, and working potential window also affect the capacitive performance of the supercapacitors

[17,18]. The conductivity influences the electronic resistance in the cell, the ion size affects the diffusion of ions through the pores, and the working potential window impacts the energy density. The capacitive performance of ECs using carbon materials with open surfaces has been studied with a variety of electrolytes, mostly aqueous [19] and organic [20,21]. Aqueous electrolytes have high ion mobility and lower resistance but have a narrow working potential window. Organic electrolytes, on the other hand, have larger working potential window but have poor conductivity. Another type of electrolyte that has recently attracted a lot of attention for supercapacitor applications is room temperature ionic (RTILs). Ionic liquids are generally solvent-free salts that are liquid at room temperature, and they are promising for energy storage since they are nonflammable, have a very low vapor , and are electrochemically stable at high potential (>4 V in some cases) [22,23]. RTILs can be designed to extend the active voltage window and the operating temperature range, making ECs deployable under more realistic performance conditions [12,24] and therefore, they have been tested thoroughly in combination with porous carbon electrodes for EDLC applications [25]. Due to their low ionic mobility and high viscosity [26], the charge-discharge rates of the porous carbon system is often limited. However, when open surface carbons such at CNTs are used as electrode materials, the performance of the ECs is less affected by the electrolyte’s limiting features, highlighting the benefits of using RTIL electrolytes [11].

In this paper we study high power supercapacitors using single-walled CNT (SWCNT) aerogel based electrodes and RTIL electrolyte operating at room temperature. The combination of the

4 high electronic conductivity [17] and the accessible surface area of the electrode with the extended voltage window of RTIL electrolyte results in a supercapacitor with very high power density, excellent rate cycling ability, and high energy density compared to conventional porous carbon electrode based supercapacitors.

2. Materials and Methods

CG100 SWCNTs (diameter: 0.9±0.2 nm, length: 300 nm to 2.3 µm) produced by the

CoMoCAT method were obtained from SouthWest Inc. and used as received to fabricate SWCNT aerogels. Methods for making SWCNT aerogels have been reported elsewhere [13,27,28]. Briefly, the SWCNTs were suspended in deionized water (Millipore) using sodium dodecylbenzene sulfonate (SDBS) surfactant (Acros Organics) at a SWCNT concentration of 1 mg/mL; the mass ratio of SWCNT:SDBS was 1:10 [29]. The solution was then tip-sonicated (Thermo Fisher 500) for 2 hours at 60 W; the of the dispersions were maintained at room temperature during sonication using a large water bath. The SWCNT dispersions were centrifuged at 21,000 g for 30 min (Beckman Coulter Allegra 25R) to sediment

SWCNT bundles. The supernatant, which contained mostly isolated nanotubes, was collected to fabricate SWCNT aerogels. The SWCNT concentration in the supernatant was determined using optical absorbance spectra (Varian Cary 5000 visible-near infrared spectrometer) with an extinction coefficient of 2.6 (absorbance·mL)/(mg·mm) at 930 nm and the Lambert-Beer equation [13]. The average SWCNT concentration in the supernatant was higher than

0.75 mg/mL. The absorption spectra from the supernatant showed sharp van Hove peaks, confirming SWCNTs were left intact after our purification and dispersion processes [13,27]. The solution was then further concentrated by evaporating water to a final concentration of

~20 mg/mL. The bubbles generated due to cavitation during the sonication were removed by

5 degassing the concentrated SWCNT solution in and the solution was then poured into rectangular molds. To avoid further drying, the SWCNT solution was left sealed with parafilm, and it formed a hydrogel within 12 hours [13,27,28]. After complete gelation of the solution, the

SDBS surfactant in the hydrogel was removed by soaking the gel first in water for 3 hours, then in 1 M at 50 °C for 20 min, and finally repeated rinsing with water until pH equilibrated to 7. The water was exchanged with by sequentially soaking the hydrogel in different concentrations of ethanol varying from 20-100% with the step size of 20% for 24 hours under ambient lab conditions. The sample was dried in a critical-point-dryer (CPD; Tousimis

Autosamdri-815) and created a SWCNT aerogel. The aerogel density was measured from the final mass and dimensions of the aerogels. The porous network within SWCNTs aerogels was imaged using scanning electron microscopy (SEM) (FEI Quanta 600) and transmission electron microscopy (TEM) (FEI Tecnai F20).

Specific surface area (SSA), pore diameter (2 r), pore volume ( V), and the pore diameter distribution (d V/d r) were measured through nitrogen and desorption at 77 K using a surface area analyzer (Micromeritics Gemini VII 2390) and the Brunauer-Emmet-Teller (BET) theory [30]. The pore diameter, pore volume and pore diameter distribution were calculated using the density functional theory (DFT) from the desorption branch of the isotherms.

The RTIL chosen for this study was 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI), as it is the most studied ionic liquid for EDLC applications. The ratio of the two ion sizes is small, and the electrolyte stable between -2.2 V and

2.2 V [31,32]. The supercapacitor electrodes were tested using a conventional sandwich-type cell, with identical symmetric electrodes [33]. Each cell was assembled and tested in an argon- filled glove-box. A VMP3 potentiostat/galvanostat (Biologic, USA) was used for cyclic

6 voltammetry (CV), galvanostatic cycling (GC), and electrochemical impedance spectroscopy

(EIS) measurements. The anode and cathode were made of the aerogel, and the potential across the entire symmetric cell was controlled using the EC-Lab software. For comparison, we used both YP-50F AC (Kuraray Chemical Company, Japan) and CDC synthesized at 600 °C with a surface area of 1500 m2/g, a material known to perform well as a supercapacitor electrode

[9,16,18].

3. Results and Discussion

3.1 Aerogel material

Photographs of free-standing aerogels of various shapes and sizes are shown in Figure 1a.

High resolution SEM images of cross-sections of aerogel show that individual nanotubes form an isotropic porous network with an open surface structure (Figure 1b), which is further confirmed by low-resolution TEM images (Figure 1c). This is very different than the porous, high surface area structure seen in SEM images of the AC[34] and CDC[35] materials used for comparison.

Furthermore, TEM imaging of a cross-section of SWCNT aerogels (Figure 1c) shows minimal residual surfactant and SWCNT walls are clearly visible. Energy dispersive X-ray spectroscopy

(EDX) analysis shows <1 wt% of sodium and sulfur, which are components of the surfactant used to disperse SWCNT, further indicating that the aerogels are essentially surfactant-free [13]; note, quantitative analysis limit of EDX was below 1 wt%. NIR spectra from aqueous dispersions created from powdered SWCNT aerogels clearly show numerous

(Figure. 2a) and are similar to the spectra obtained from SWCNT suspensions before hydrogel formation, confirming that SWCNTs remain well-preserved through the aerogel fabrication process [13]. The radial breathing modes and the peak intensity ratio of D-band to G-band are

7 similar for nanotube powder prior to any processing and from aerogels, also confirming structural integrity of nanotubes in the aerogels [13]. The pore diameter distribution (d V/d r) shows that the aerogels have mostly mesopores, i.e., pore diameters in the range of 2-50 nm,

with few micropores, i.e., pore diameters less than 2 nm and macropores, i.e., pore diameters

greater than 50 nm (Figure. 2b) [13]. The measured specific surface area of the aerogel was

900 m2/g [13].

3.2 Electrochemical performance

CV was used to measure and compare the electrochemical performance of supercapacitor cells assembled with SWCNT aerogel electrodes and CDC electrodes. At a low scan rate of

0.005 V/s, CV curves of the cell with CDC electrodes exhibit a rectangular shape, characteristic of good double layer performance (Figure 3a). However, as the scan rate is increased to 0.05 and

0.1 V/s, the effect of series resistance in cell behavior becomes more prominent, as evidenced by the rounding of the curves at regions of scan inversion. At scan rates higher than 0.1 V/s CDC cells show a completely resistive behavior as evidenced by the non-rectangular CV curve. Figure

2b shows the CV curves of the cells assembled with SWCNT aerogel electrodes. In this case, even at high scan rates of up to 1 V/s CV curves show a distinct rectangular shape. At scan rates of 2 V/s and above, the CV curves show their resistive contribution but overall remain very capacitive at these very high rates (up to 10 times the rate of the CDC device). This difference in capacitive behavior of the cells is summarized in Figure 4a, where the specific capacitances are plotted at different scan rates and are also compared with specific capacitance data of similar cells assembled with activated carbon (AC) electrodes. Values for specific capacitance in were calculated from the integral of the discharge half cycles of CVs according to equation 2:

8

∫I dV C = 2 (2) sp vVm

where Csp is the specific capacitance of one electrode normalized by mass (F/g), I is the current

of the discharge curve integrated with respect to the potential V (A), v is the charge-discharge

rate of the scan (V/s), V is the potential window for the scan (V), and m is the mass of one

electrode (g).

The CDC films, which were fabricated for optimized performance with EMI-TFSI, have a higher specific capacitance at low scan rates, which is expected based on their pore structure and higher specific surface area. At a scan rate near ~0.04 V/s, the capacitance for both systems is equal and the two curves intersect. Beyond this low scan rate, CDC’s specific capacitance drops sharply to almost zero at a rate of 1 V/s. The capacitance of the SWCNT aerogel device, however, remains consistent until about 1 V/s, which supports the high rate performance expected for open surface carbon materials. As another porous carbon with an even higher surface area, the AC electrode yields the highest capacitance of the three materials at low scan rates (Figure 4). However, it follows the trend of CDC electrodes, dropping significantly at around 0.2 V/s, falling below the

SWCNT electrode curve. AC, which is used in commercial EDLC devices, is known for a higher capacitance but here we see evidence of its lackluster performance at high rates, suggesting that if we are to use IL electrolytes, these SWCNT structures are much better for high power applications than commercial ACs.

GC tests also confirm this vast difference in rate performance for the two different electrode materials, which can be observed in the vertical drop in voltage at the beginning of the discharge, also known as IR drop. Since this potential drop is a function of the resistance (R) and current

(I), and at low current (0.1, 0.5, 1.0 A/g) the potential drop is <0.1 V, we can conclude

9 that the CDC electrodes exhibit a low resistance (Figure 2c). At 5 A/g the drop increases to more than 0.5 V which is large enough that the total capacitance is negatively affected. As before, the same galvanostatic data is shown for the SWCNT aerogel electrode in Figure 3d. Again the drop is <0.1 V, but this time up to a high current density of 30 A/g. Even at this very high rate, the potential drop remains at 0.25 V and does not reach 0.5 V before a current density of 60 A/g. The specific capacitances of the electrodes calculated from the slope of the discharge curves is plotted for both systems as a function of rate from the galvanostatic results in Figure 4b. Values for specific capacitance from GC were calculated using the slope of the discharge curve, according to equation 3:

I C = (3) sp (dV / dt ) m where Csp is the specific capacitance of one electrode normalized by mass (F/g), I is the current of the discharge curve (A), dV/dt is the slope of the discharge curve, and m is the mass of one

electrode (g). The specific capacitance of the CDC electrodes again drops immediately with

increasing rate while the SWCNT aerogel system maintains most of its capacitance through

30 A/g which is consistent with the results from CV. The rate performance of up to 1 V/s in CV

or 30 A/g in Galvanostatic cycling tests can be attributed to the open surface structure, which

supports quick charge-discharge rates. These rates are significantly higher than charge and

discharge rates of typical supercapacitors made with conventional materials, which are reported

to charge at rates on the order of 20 mV/s and 2 A/g [36].

Electrochemical impedance spectroscopy provides further information about the two materials

including the resistance of the device and the time constant for charging and discharging. In

Figure 5a, the standard Nyquist plots can be used to reveal the resistive behavior of

10 supercapacitors at high frequencies where the curves cross the x-axis and the capacitive behavior at low frequencies, where the points become a straight and almost vertical line [37]. Seen more clearly in the inset, the CDC device curve is much less vertical and has a larger semicircle than that of the SWCNT aerogel. The shape of the curves at lower frequencies can be explained by

Figures 5b and 5c, which show the relationship of real and imaginary capacitances to frequency for both materials, given by [37]

−Im[ Z ] Re[] C = (4) 2π f | Z | 2

Re[ Z ] Im[] C = (5) 2π f | Z | 2

The capacitance of the device is given by the maximum point of the real capacitance, which for the CDC is more than 20 times higher than the SWCNT, which is consistent with the high surface area and greater mass loading of the porous CDC. The imaginary part reaches a maximum at a specific frequency f0 which is used to determine the time constant τ0 = 1/ f0. This relaxation time is much lower for the SWCNT aerogel, at about 0.3 seconds, while the CDC has a time constant on the order of 30 seconds. These results confirm the earlier conclusions made from CV and GC data that the SWCNT can charge and discharge over much shorter times and is potentially a good candidate for high frequency applications.

3.3 Long-term stability

While it is clear that the device made with SWCNT aerogel electrodes is able to charge and discharge quickly, it is also necessary for an energy storage device to repeat this over many cycles. Galvanostatic cycling at 6.75 A/g over 10,000 repeated cycles was used to test the lifetime stability of the aerogel’s capacitance over a 2.5 V voltage window. Figure 6 shows that

11 after a few cycles in which the capacitance settles at ~46 F/g, the device retains 92% of the initial capacitance after 10,000 charge and discharge cycles, a very promising retention rate for EDLCs, which are to be highly cycled in applications. Additionally, the resistance remains at about

2.5 Ω·cm 2 and does not increase with cycling.

Using an ionic liquid electrolyte with a novel electrode material, we have shown that the resulting system shows very high rate performance and long-term stability compared to previously studied materials such as CDC and AC. Because of their mechanical properties, the

SWCNT aerogels are especially suited for use as a backbone for flexible electrode materials.

They can also potentially be scaled into thick, 3-D electrodes, which would allow for the progression away from the demonstrated thin-film performance.

4. Conclusions

Electrochemical performance and capacitive behavior of SWCNT aerogel electrodes

were investigated in supercapacitors with an ionic liquid electrolyte, EMI-TFSI. The open

surface structure of the SWCNT aerogel leads to enhanced rate performance and power

capability over other commonly used electrode materials such as CDC and AC. Compared to

these porous materials, the SWCNT aerogel electrodes are less resistive in addition to having

unique mechanical properties such as flexibility. Additionally, when compared to devices made

with CNT or other similar materials, the reported SWCNT aerogel shows higher power and

energy densities than other supercapacitor reports (Fig. 6b). When analyzed for lifetime

performance, the material also shows < 10% decrease in capacitance after 10,000 cycles, which

makes it even better for application requiring high power and many repeated cycles.

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5. Acknowledgements

Electrochemical testing and characterization at Drexel was supported as part of the Fluid

Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research

Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy

Sciences. Material synthesis at Carnegie Mellon was supported by the National Science

Foundation through grants CMMI-1335417 (M.F.I.).

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[46] Z. Jiang, A. Al-Zubaidi, S. Kawasaki, Unusual increase in the electric double-layer capacitance with charge–discharge cycles of nitrogen doped single-walled carbon nanotubes, Mater. Express. 4 (2014) 331–336.

Figure 1 (a) Images of SWCNT aerogels in various sizes and shapes. (b) SEM and (c) TEM images of an aerogel cross-section showing networks of mostly isolated and entangled nanotubes.

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Figure 2 (a) NIR fluorescence spectra of aqueous dispersions of powdered SWCNT aerogel. (b) Pore diameter distribution (d V/d r) versus pore diameter (2 r) of the SWCNT aerogels, calculated from the desorption branch of the isotherm using density functional theory (DFT) methods. BET measurements were done on SWCNT aerogels of density 13 mg/mL.

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Figure 3. Electrochemical characteristics of EDLCs with CDC and SWCNT aerogels as electrodes in EMI-TFSI RTIL electrolyte. (a-b) Cyclic voltammograms and (c-d) galvanostatic charge/discharge of cells with CDC (a, c) and SWCNT aerogel (b, d) electrodes.

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Figure 4. Summary of cyclic voltammogram (a) and galvanostatic (b) results for SWCNT aerogels, CDC, and AC electrode materials.

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Figure 5. Frequency response of EDLCs with CDC and SWCNT aerogels as electrodes. (a) Nyquist plot of EDLCs with both CDC and SWCNT aerogel electrodes. Real and imaginary capacitance for cells with (b) CDC and (c) SWCNT aerogel based electrodes.

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Figure 6. (a) Stability of SWCNT aerogels as EDLC electrodes. Galvanostatic results for the SWCNT sample’s capacitance (black) and resistance (blue) over 10,000 cycles. Device is cycled to 2.5 V at a rate of 6.75 A/g. (b) Ragone plot of CNT-based electrochemical energy storage devices including batteries and supercapacitors. [38–46]

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Keywords: SWCNT Aerogel; Supercapacitor; Ionic liquid; Electrochemistry

Graphical Abstract

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Katherine L. Van Aken received her BS in Physics in 2012 from Haverford College. Katie is currently a 3 rd year Ph. D. candidate at Drexel University in the department of Materials Science and Engineering, studying under the advisement of Dr. Yury Gogotsi. Her research focuses on electrolytes for carbon-based supercapacitors for energy storage, specifically ionic liquids. Katie is the president of the Drexel MRS chapter and is highly involved in the department’s outreach opportunities.

Carlos R. Pérez obtained his Ph.D. in Material Science Engineering from the Drexel University in 2013, working under the supervision of Prof. Gogotsi. Since then, he has been appointed postdoctoral research fellow in the School of Engineering and Applied Science of the University of Pennsylvania, also in Philadelphia. His research focus includes carbon nanomaterials for energy applications, and the use of scanning probe techniques (AFM, SIMM) for the study of the interfaces between these materials and others (, ceramics, etc).

Youngseok Oh obtained his Ph.D. in Nanotechnology from Sungkyunkwan University in South Korea in 2010. He then worked with Prof. Islam as a postdoctoral associate. He is currently a senior researcher at the Korea Institute of Material Science (KIMS).

Yoon Joo Jeong is a 4 th year Ph.D. student in the Material Science Engineering department at Carnegie Mellon University working with Prof. Islam. She is currently investigating properties of carbon nanotube networks and composites fabricated from such networks.

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Majid Beidaghi is a Research Associate at A. J. Drexel Nanomaterials Institute and the Department of Material Science and Engineering at Drexel University. He received his PhD in Materials Science and Engineering from Florida International University in 2012. His research interests are in the field of electrochemical energy storage with the main focus on the synthesis of multifunctional materials and design and fabrication of electrochemical energy storage devices such as electrochemical capacitors and batteries. Mohammad F. Islam is a faculty in the Materials Science and Engineering Department at Carnegie Mellon University. He received his Ph.D. in Physics from Lehigh University and a postdoctoral training at the University of Pennsylvania. His research group employs both soft- and nanomaterial approaches to engineer multifunctional materials with tailored optical, electrical, thermal and mechanical properties. These unique materials have diverse applications in areas such as , fuel cells, supercapacitors, drug delivery vessels, scaffolds for , etc.

Yury Gogotsi is Distinguished University Professor and Trustee Chair of Materials Science and Engineering at Drexel University. He is also the founding Director of the A.J. Drexel Nanomaterials Institute and Associate Editor of ACS Nano. He has a Ph.D. in Physical Chemistry from Kiev Polytechnic, D.Sc. in Materials Engineering from Ukrainian Academy of Sciences and Dr.h.c. degree from Paul Sabatier University, Toulouse, France. He works on nanostructured carbons and two-dimensional carbides for energy related and biomedical applications. He has co-authored 2 books, more than 450 journal papers and obtained more than 50 patents. He has received numerous national and international awards for his research, was recognized as Highly Cited Researcher by Thomson-Reuters in 2013 and elected a Fellow of AAAS, MRS, ECS and ACerS and a member of the World Academy of Ceramics.

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Highlights

• High-surface area single-walled carbon nanotube aerogel electrodes were synthesized • Supercapacitors were assembled using a room temperature ionic liquid electrolyte • Highly accessible surface leads to impressive rate performance up to 1 V/s. • Supercapacitors show capacitive stability over 10000 cycles. • Aerogels can be used as supercapacitor electrodes for high power applications.

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