RSC Advances
View Article Online
COMMUNICATION
Published on 20 July 2015. Downloaded by McGill University on 13/03/2018 15:05:59.
Cite this: RSC Adv., 2015, 5, 63834
Received 8th June 2015
Accepted 20th July 2015
View Journal | View Issue
Self-assembled fullerene additives for boosting the
capacity of activated carbon electrodes in
supercapacitors†
Deepak Sridhar,a Kaushik Balakrishnan,b Tony J. Gnanaprakasa,c Srini Raghavanac
and Krishna Muralidharan*c
DOI: 10.1039/c5ra10903e
www.rsc.org/advances
Self-assembled fullerene additives at minor weight fractions (1 wt%)
are shown to improve the specific capacity of activated carbon electrode based supercapacitors significantly, while simultaneously
increasing the maximum power density. The integrated approach
developed in this work demonstrates the feasibility of using high
performance composite carbon electrodes in electrochemical energy
storage.
Electrochemical energy storage (EES) devices based on supercapacitors have gained considerable attention due to their
superior performance metrics1–4 (e.g. power density, charge/
discharge characteristics, long-term cyclic stability). In particular, supercapacitors employing carbon electrodes have made
signicant inroads towards commercialization, given the ease
of integration of a broad class of carbon electrodes with a variety
of electrolytes.5–13 Among the broad range of carbon structures,
activated carbon (AC) is the most widely employed class of
electrode materials,5–7 especially for electric double layer based
supercapacitors (EDLC). AC provides good chemical stability
and high surface area and can be prepared in large quantities in
an inexpensive fashion from petroleum8,9 and natural10,11 sources, thereby representing a major focus of research. However,
AC is characterized by unevenly distributed pores, as well as a
wide distribution of pore sizes, limiting electrolyte accessibility
as well as leading to an increase in series and interfacial resistance,14 which are detrimental to the device performance.12,13 To
alleviate these problems, composites of AC with carbon nanostructures such as graphene–AC13,15,16 and carbon nanotubes
(CNT)–AC17–19 have been successfully developed, resulting in
better control of porosity. Nevertheless, the more expensive
a
Department of Chemical and Environmental Engineering, University of Arizona,
Tucson, AZ 85721, USA
b
College of Optical Sciences, University of Arizona, Tucson, AZ 85721, USA
c
Department of Material, Science and Engineering, University of Arizona, Tucson, AZ
85721, USA. E-mail: krishna@email.arizona.edu
† Electronic supplementary
10.1039/c5ra10903e
information
63834 | RSC Adv., 2015, 5, 63834–63838
(ESI)
available.
See
carbon nanostructures are used in comparable proportions with
respect to AC to achieve notable performance improvement.
Additionally, conductive polymeric additives such as Naon,20
poly[2,5-benzimidazole],21
poly(3,4-ethylenedioxythiophene)
(PEDOT),22 poly(3-methylthiophene),23 poly[3-(4-uorophenyl)
thiophene],24 poly-aniline25 and many others have also been
examined.26–28 In AC-polymeric additive based electrodes, the
net resistance is shown to decrease though there is a simultaneous decrease in available surface area.21,26 Further, the polymeric additives oen show pseudocapacitance, but decrease the
cycle-life of the electrode as well the power density.29 On the other
hand, carbon electrodes that are AC-free and consisting of
graphene,30–32 graphene oxide (GO),33,34 chemically modied
graphene,35–37 reduced graphene oxide (r-GO),38–40 carbon
nanotubes (CNT)41–43 have been fabricated, but currently, such
electrodes have not yet widely replaced AC based electrodes in
commercial supercapacitor systems, due to the elaborate
fabrication process.
In this context, we demonstrate for the rst time, the ability
to signicantly improve the capacitive performance of AC-based
supercapacitors through the addition of fullerene selfassemblies (FSA),44 which constitute only a minor fraction of the
electrode-mass. In this work, the FSA are used in conjunction
with electrodes consisting of AC as well as Naon®, which
serves as a binder. The AC used in this work has a relatively low
specic area (750 m2 g 1).
The FSA considered in this work were synthesized based on a
previous investigation, where the ability to control the shape
and size of fullerene assemblies on graphene surfaces was
demonstrated.44 The procedure for obtaining FSA is as follows: a
solution consisting of C60 molecules dissolved in toluene (2 mg
ml 1) was sonicated for two hours and then drop-cast on copper
substrates, followed by a directed stream of N2 at 2 psi for one
minute to ensure the removal of toluene. This leads to the rapid
self-assembly of FSA within two minutes on copper substrates
(which serve as the current collector for the device). More details
on the synthesis procedure are given in the ESI.†
DOI:
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 20 July 2015. Downloaded by McGill University on 13/03/2018 15:05:59.
Communication
Scanning electron microscopy (SEM) images of FSA that
cover the underlying copper surface, are shown in Fig. 1 at
different magnications. The SEM images shown in Fig. 1
typies the surface coverage across the sample. The FSAs
resemble hollow rod-like structures, and their lengths range up
to 30 mm. Further, the morphologies of the FSA also vary
considerably with some FSA displaying visible cracks as evident
in the SEM images. The formation of these FSAs can be
understood on the basis of nucleation and growth mechanisms
as elucidated earlier by Gnanaprakasa et al.44 Specically,
surface imperfections in copper serve as nucleation sites for
adsorbed fullerene molecules, leading to formation of aggregates (i.e. nuclei) at the various surface nucleation sites.
Subsequently, during the growth phase, the aggregates grow
into larger self-assemblies as a result of diffusion of fullerene
(a–c) Scanning electron microscopy (SEM) images of the FSA on
copper obtained at different magnifications.
Fig. 1
This journal is © The Royal Society of Chemistry 2015
RSC Advances
molecules within the evaporating toluene lm. The resulting
open rod-like structures that vary in their size are in contrast to
uniform, prismatic nanorods that were formed on graphene on
copper substrates as shown by Gnanaprakasa et al.44
These differences are attributed to the different casting
techniques (drop-coating vs. dip-coating) as well as the nature of
the substrate (copper vs. graphene). In particular, graphene
grown on copper via chemical vapor deposition is characterized
by regular corrugations, which serve as periodic adsorption
sites for fullerene molecules. On the other hand, various
imperfections such as cold-rolling striations, kinks, jogs, steps
and terraces can all be found on copper surfaces. The respective
variations in the adsorption sites specic to the graphene (on
copper) surface as compared to the plain copper surface
underlies the variation in the size and morphology of the FSA on
copper as compared to graphene mediated FSA.
The fabrication and analysis protocols for preparing the
supercapacitors consist of three steps, namely: (i) electrode
preparation using AC, Naon® and FSA, (ii) supercapacitor
assembly using copper, aqueous KOH and cellulose lter sheets
(Whatman lter paper grade 1) as current collector, electrolyte
and separator respectively and (iii) electrochemical characterization of the supercapacitor performance. Detailed information on the above protocols is given in ESI.†
Two distinct sets of AC-electrodes were prepared in order to
examine the performance enhancement of AC based supercapacitors due to FSA addition. The rst set of electrodes were
devoid of fullerenes and served as the reference system. For
clarity and brevity, the samples devoid of FSA are labeled as EAC, while samples that incorporate FSA are labeled as E-ACC60.
Optical characterization showed that the amount of FSA
leading to a high coverage of the underlying copper substrate
corresponded to 1–1.2 wt% of the electrode mass of the
E-ACC60 systems. In this regard, we chose the lower wt%
composition, i.e. 1 wt% FSA for our studies. Also, the structural
integrity of the FSA structures was maintained even upon
coating with AC and Naon as shown in ESI (Fig. S3†).
The EDLC fabrication and testing was carried out using
established best practices;45 consequently, electrode thicknesses were ensured to be at least 100 mm and only symmetric
two electrode systems were considered. Cyclic voltammetry
(CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge (CD) were used to examine the
performance metrics of the two systems (E-AC and E-ACC60). In
addition to the specic capacitance, energy density (E) and
maximum power density (Pmax) were also calculated. The
equations employed for the calculations of the performance
metrics of the supercapacitors are detailed in ESI.† For each
system, a minimum of ten trials were carried out to ensure
reproducibility of the materials synthesis and the device fabrication. CV was carried between 0.5 to +0.5 V with scan rates
varying from 5–200 mV s 1. This operating window was chosen
aer a series of preliminary tests.
To enable appropriate comparisons between the E-AC and EACC60 systems, we rst examine the respective CV curves
obtained at a scan rate of 20 mV s 1 as shown in Fig. 2a.
Analysis of the CV curves based on ten distinct samples showed
RSC Adv., 2015, 5, 63834–63838 | 63835
View Article Online
Published on 20 July 2015. Downloaded by McGill University on 13/03/2018 15:05:59.
RSC Advances
Communication
Comparison of EIS experimental plot along with the values
estimated from the model circuit (shown within the graph). The inset
depicts the enlarged image of the EIS plot at high frequency.
Fig. 3
Table 1 Energy density (E) and power density (Pmax) for E-AC and EACC60 electrode systems obtained from CV plots carried out at 20 mV
s 1
Fig. 2 (a) CV curves of E-AC and E-ACC60 at the scan rate of 20 mV
s 1. (b) Specific capacitances of E-AC and E-ACC60 at different scan
rates.
that the capacity was highly reproducible with the associated
standard deviation equaling (standard deviation?) 1 F g 1. As
evident from Fig. 2a, both curves demonstrate almost ideal
rectangular EDLC behavior with the specic capacitance of EACC60 (58 F g 1) being higher than that of E-AC (43 F g 1);
this improvement corresponds to approximately 35%
enhancement. This behavior is consistent over different scanrates as shown in Fig. 2b, the data for which were calculated
from the corresponding CV plots for E-AC and E-ACC60
respectively, as given in ESI (Fig. S4 & S5†).
The signicant increase in the specic capacitance corresponding to FSA addition can be attributed to the increase in
accessible surface area to the electrolyte ions arising due to the
morphology and spatial distribution of the FSA structures.
Specically, the rod-like FSA consist of fullerene molecules
arranged in a face center cubic (FCC) structure, with the lattice
parameter equaling 1.58 nm (see Experimental section, ESI†). In
addition to this underlying micro-porosity of the FSA, additional meso- and macro-porosity is obtained, corresponding to
the effective spacing between neighboring FSA that range
between few nm to microns as well as due to the open rod-like
structures of the FSA. As a result, an increase in the specic
capacitance is seen due to FSA incorporation.
In addition to investigating the effect of FSA on specic
capacitance, the importance of using FSA on the
63836 | RSC Adv., 2015, 5, 63834–63838
Electrode
E (W h kg 1)
Pmax kW kg
E-ACC60
E-AC
2.2
1.6
20.3
17.9
1
electrochemical series resistance and interfacial resistance and
thereby the power density of the supercapacitors was also
studied. To examine this effect, EIS experiments were conducted in the frequency range 10 mHz to 100 kHz and the
impedance results were analyzed using the GAMRY G750 soware. Specically, the Nyquist plot impedance data are reported
in Fig. 3. The respective impedance curves were tted to an
equivalent circuit (see Fig. S6, ESI†), consisting of (i) an electrochemical series resistance (ESR), (ii) a constant phase
element (CPE) representing the capacitance of the system, (iii)
an interfacial resistance (Ri) and a (iv) Warburg resistance (W). It
is to be noted that this circuit yielded excellent ts for both
systems as seen in Fig. S7 (ESI†). Importantly, both ESR as well
as Ri of the E-ACC60 system was found to be lower than that of
E-AC (Table S1, ESI†). The decrease in ESR can be understood in
terms of the role of FSA in improving contact between the
current collector and electrode. Further, noting that Ri is related
to the impedance at the electrode–electrolyte interface14 as well
as the transport of electrolyte ions within the electrode,46,47 the
decrease in Ri of the E-ACC60 system can be attributed to the
controlled increase in porosity of the electrode47,48 due to the
presence of FSA. Thus, the subtle addition of even 1 wt% FSA
increases the specic capacitance while reducing the ESR,
resulting in the twin enhancement of both energy density (E)
and maximum power density (Pmax) (see Table 1).
The charge–discharge characteristics of the different systems
were measured at current densities varying from 0.1 to 0.5 A g 1
(Fig. S8, ESI†). The capacity measured by the CD measurements
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 20 July 2015. Downloaded by McGill University on 13/03/2018 15:05:59.
Communication
Fig. 4 Cyclic stability of E-ACC60 electrode at 1 A g 1 over 5000
cycles (inset-CD plot for the E-ACC60 system at 0.3 A g 1).
at different rates was found to be similar to those measured
using CV. Inset of Fig. 4 shows CD curves of the E-ACC60 system
at 0.3 A g 1. Further, the cyclic stability of both the systems was
also tested using the CD experiments and both systems showed
excellent cyclic stability. Specically, both E-AC and E-ACC60
retained 93% of their capacity over 5000 cycles.
To further probe the effect of the FSA, two related studies
were undertaken. In the rst study, the effect of FSA wt% was
examined. It was seen that the specic capacitance increased up
to 1 wt%, while the respective specic capacitances at 1 and
1.2 wt% were found to be very similar; beyond 1.2 wt%, the
specic capacitance slightly decreased (Fig. S9 ESI†). This
observation points to the fact that once the surface coverage is
high, the addition of more fullerene molecules does not facilitate the further formation of the rod-like extended FSA structures, which is reected in the plateauing/slight decrease in the
specic capacitance beyond 1.2 wt%.
Next, electrodes that had similar mass and composition as
that of the 1 wt% E-ACC60 electrodes were prepared and tested.
An important distinction regarding these electrodes is the fact
that an equivalent mass (1 wt%) of individual fullerene molecules were simply dispersed within the AC–Naon mixture
which was then cast on the copper substrates. Using CV, the
specic capacitance of this system consisting of symmetric
electrodes with dispersed fullerenes was evaluated as a function
of scan-rate; interestingly, the specic capacitance of this
system was always considerably lower than that of the E-AC
systems (40 F g 1). This observation unequivocally highlights
the role of the FSA on enhancement of AC based supercapacitors. Specically, the rod-like, porous FSA provide higher
accessible specic area to the electrolytes and improved contact
with the current collector, thus enhancing the specic capacitance and other performance metrics such as energy density
and maximum power density.
To put our work in context, it is worth discussing other
investigations that have reported on fullerene based supercapacitor electrodes. As compared to graphene/GO/r-GO-based
and CNT-based supercapacitors, the literature on fullerene as
This journal is © The Royal Society of Chemistry 2015
RSC Advances
additives is sparse. Relevant literature on fullerene based EDL
supercapacitors include Ma et al. (2014)49 who used fullerenes
(10% by weight) in conjunction with GO, while Okajima et al.
(2005)50 employed AC with fullerenes, with the fullerene fraction
ranging up to 30% by weight. In both studies, fullerene molecules were simply dispersed within the electrode matrix and no
effort was made to either characterize or control the possible
self-assemblies of fullerenes.
Clearly, the distinguishing feature of this work is the fact
that we have, for the rst time, successfully incorporated
fullerene self-assemblies as key constituents in supercapacitors,
resulting in a marked impact on performance even at very low
weight fractions (1–1.2 wt%). While we have examined FSA in
conjunction with relatively low specic area AC, FSA should lead
to similar performance enhancement even when they are
incorporated with other carbon systems. Further, the procedure
developed for electrode synthesis in this work is very simple and
straightforward and can be integrated within standard fabrication processes to allow scalable production of high performance low-cost carbon composite electrodes for energy storage
applications.
In conclusion, we have demonstrated a simple, facile
procedure for improving the performance of AC-based supercapacitors. Specically, by employing fullerene self-assemblies
in conjunction with low-cost activated carbon mixtures, a
specic capacitance enhancement of 35% is readily achieved. In
addition, other performance metrics such as energy density and
maximum performance density have signicantly improved
along with very good cyclic stability.
Acknowledgements
This work was supported by grants from the Renewable Energy
Network at the University of Arizona and GPSC Research-Project
Grant Award # RSRCH 103FY'15.
References
1 V. V. N. Obreja, Phys. E, 2008, 40, 2596–2605.
2 L. L. Zhang and X. S. Zhao, Chem. Soc. Rev., 2009, 38, 2520–
2531.
3 M. Sevilla and R. Mokaya, Energy Environ. Sci., 2014, 7, 1250–
1280.
4 A. Davies and A. Yu, Can. J. Chem. Eng., 2011, 89, 1342–1357.
5 E. Frackowiak and F. Béguin, Carbon, 2001, 39, 937–950.
6 T. Chen and L. Dai, Mater. Today, 2013, 16, 272–280.
7 A. G. Pandolfo and A. F. Hollenkamp, J. Power Sources, 2006,
157, 11–27.
8 M. Wu, Q. Zha, J. Qiu, X. Han, Y. Guo, Z. Li, A. Yuan and
X. Sun, Fuel, 2005, 84, 1992–1997.
9 T. Kawano, M. Kubota, M. S. Onyango, F. Watanabe and
H. Matsuda, Appl. Therm. Eng., 2008, 28, 865–871.
10 M. J. Prauchner and F. Rodrı́guez-Reinoso, Microporous
Mesoporous Mater., 2012, 152, 163–171.
11 A. L. Cazetta, A. M. M. Vargas, E. M. Nogami, M. H. Kunita,
M. R. Guilherme, A. C. Martins, T. L. Silva, J. C. G. Moraes
and V. C. Almeida, Chem. Eng. J., 2011, 174, 117–125.
RSC Adv., 2015, 5, 63834–63838 | 63837
View Article Online
Published on 20 July 2015. Downloaded by McGill University on 13/03/2018 15:05:59.
RSC Advances
12 K. H. An, W. S. Kim, Y. S. Park, Y. C. Choi, S. M. Lee,
D. C. Chung, D. J. Bae, S. C. Lim and Y. H. Lee, Adv.
Mater., 2001, 13, 497–500.
13 Y. Chen, X. Zhang, H. Zhang, X. Sun, D. Zhang and Y. Ma,
RSC Adv., 2012, 2, 7747.
14 J. Kang, J. Wen, S. H. Jayaram, A. Yu and X. Wang,
Electrochim. Acta, 2014, 115, 587–598.
15 C. Zheng, X. Zhou, H. Cao, G. Wang and Z. Liu, J. Power
Sources, 2014, 258, 290–296.
16 S. Yu, Y. Li and N. Pan, RSC Adv., 2014, 4, 48758–48764.
17 G. Xu, C. Zheng, Q. Zhang, J. Huang, M. Zhao, J. Nie, X. Wang
and F. Wei, Nano Res., 2011, 4, 870–881.
18 P.-L. Taberna, G. Chevallier, P. Simon, D. Plée and T. Aubert,
Mater. Res. Bull., 2006, 41, 478–484.
19 B. N. M. Dolah, M. A. R. Othman, M. Deraman, N. H. Basri,
R. Farma, I. A. Talib and M. M. Ishak, J. Phys.: Conf. Ser.,
2013, 431, 012015.
20 F. Lufrano, P. Staiti and M. Minutoli, J. Electrochem. Soc.,
2004, 151, A64.
21 R. S. Hastak, P. Sivaraman, D. D. Potphode, K. Shashidhara
and A. B. Samui, Electrochim. Acta, 2012, 59, 296–303.
22 T. S. Sonia, P. A. Mini, R. Nandhini, K. Sujith, B. Avinash,
S. V. Nair and K. R. V. Subramanian, Bull. Mater. Sci., 2013,
36, 547–551.
23 A. Balducci, U. Bardi, S. Caporali, M. Mastragostino and
F. Soavi, Electrochem. Commun., 2004, 6, 566–570.
24 A. Laforgue, P. Simon, J. F. Fauvarque, M. Mastragostino,
F. Soavi, J. F. Sarrau, P. Lailler, M. Conte, E. Rossi and
S. Saguatti, J. Electrochem. Soc., 2003, 150, A645.
25 Q. Wang, J. Li, F. Gao, W. Li, K. Wu and X. Wang, New Res.
Carbon Mater., 2008, 23, 275–280.
26 G. Wang, L. Zhang and J. Zhang, Chem. Soc. Rev., 2012, 41,
797.
27 G. A. Snook, P. Kao and A. S. Best, J. Power Sources, 2011, 196,
1–12.
28 W. Zhang, A. Dehghani-Sanij and R. Blackburn, J. Mater. Sci.,
2007, 42, 3408–3418.
29 M. Mastragostino, C. Arbizzani, L. Meneghello and
R. Paraventi, Adv. Mater., 1996, 8, 331–334.
30 C. Liu, Z. Yu, D. Neff, A. Zhamu and B. Z. Jang, Nano Lett.,
2010, 10, 4863–4868.
31 Y. Bin Tan and J.-M. Lee, J. Mater. Chem. A, 2013, 1, 14814.
63838 | RSC Adv., 2015, 5, 63834–63838
Communication
32 C. Xu, B. Xu, Y. Gu, Z. Xiong, J. Sun and X. S. Zhao, Energy
Environ. Sci., 2013, 6, 1388.
33 B. Xu, S. Yue, Z. Sui, X. Zhang, S. Hou, G. Cao and Y. Yang,
Energy Environ. Sci., 2011, 4, 2826–2830.
34 N. Li, S. Tang, Y. Dai and X. Meng, J. Mater. Sci., 2014, 49,
2802–2809.
35 A. K. Mishra and S. Ramaprabhu, J. Phys. Chem. C, 2011, 115,
14006–14013.
36 T. Y. Kim, H. W. Lee, M. Stoller, D. R. Dreyer,
C. W. Bielawski, R. S. Ruoff and K. S. Suh, ACS Nano, 2011,
5, 436–442.
37 J. Liu, J. Tang and J. J. Gooding, J. Mater. Chem., 2012, 22,
12435–12452.
38 Y. Chen, X. Zhang, D. Zhang, P. Yu and Y. Ma, Carbon, 2011,
49, 573–580.
39 B. Zhao, P. Liu, Y. Jiang, D. Pan, H. Tao, J. Song, T. Fang and
W. Xu, J. Power Sources, 2012, 198, 423–427.
40 X.-Y. Peng, X.-X. Liu, D. Diamond and K. T. Lau, Carbon,
2011, 49, 3488–3496.
41 M. Endo, Y. J. Kim, T. Chino, O. Shinya, Y. Matsuzawa,
H. Suezaki, K. Tantrakarn and M. S. Dresselhaus, Appl.
Phys. A: Mater. Sci. Process., 2006, 82, 559–565.
42 B. Kim, H. Chung and W. Kim, Nanotechnology, 2012, 23,
155401.
43 W. Lu, L. Qu, K. Henry and L. Dai, J. Power Sources, 2009, 189,
1270–1277.
44 T. J. Gnanaprakasa, D. Sridhar, W. J. Beck, K. Runge,
B. G. Potter, T. J. Zega, P. A. Deymier, S. Raghavan and
K. Muralidharan, Chem. Commun., 2015, 51, 1858–1861.
45 M. D. Stoller and R. S. Ruoff, Energy Environ. Sci., 2010, 3,
1294.
46 X. Xia, Q. Ma, S. Yi, H. Chen, H. Liu, Y. Chen and L. Yang,
Mater. Chem. Phys., 2014, 148, 631–638.
47 K. Tõnurist, I. Vaas, T. Thomberg, A. Jänes, H. Kurig,
T. Romann and E. Lust, Electrochim. Acta, 2014, 119, 72–77.
48 D. Wu, X. Chen, S. Lu, Y. Liang, F. Xu and R. Fu, Microporous
Mesoporous Mater., 2010, 131, 261–264.
49 J. Ma, Q. Guo, H.-L. Gao and X. Qin, Fullerenes, Nanotubes,
Carbon Nanostruct., 2014, 23, 477–482.
50 K. Okajima, A. Ikeda, K. Kamoshita and M. Sudoh,
Electrochim. Acta, 2005, 51, 972–977.
This journal is © The Royal Society of Chemistry 2015
Deepak Sridhar