Supporting Information
Hierarchical Nanohybrids with Porous CNT-networks Decorated Crumpled Graphene
Balls for Supercapacitors
By Shun Mao1‡, Zhenhai Wen1‡, Zheng Bo2, Jingbo Chang1, Xingkang Huang1, and Junhong
Chen1*
1
Department of Mechanical Engineering, University of Wisconsin-Milwaukee, 3200 North
Cramer Street, Milwaukee, WI 53211, USA
2
State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering,
Department of Energy Engineering, Zhejiang University, 38 Zheda Road, Hangzhou,
Zhejiang Province, 310027, P. R. China
Figure S1. Schematic illustration of the experimental setup for preparation of CGB/Fe3O4
hybrids by rapid compression of GO sheets in evaporating aerosol droplets and simultaneous
chemical reactions for growth of Fe3O4 nanocrystals on the CGB surface. The CGB/Fe3O4
hybrids were collected with a filter membrane at the downstream of the tube furnace.
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Figure S2. (a) SEM image of the GO sheets. (b) XRD spectrum of the CGB/Fe3O4 hybrids.
Inset shows the SEM image of the CGB/Fe3O4 hybrids. The XRD spectrum shows a good
crystalline structure of Fe3O4 as evidenced by sharp diffraction peaks corresponding with
(220), (311), (222), (400), (422), (511), and (440) crystal facets from cubic Fe3O4. The broad
diffraction peak from CGB at 23° is also evidenced, indicating that the interlayer spacing of
the CGB is 3.7 Å, which is smaller than that of GO (8.32 Å, 10.6°) but larger than that of
graphite (3.36 Å, 26.5°). (c) The high degree of crystallinity of the CGB and CNT in the pCNTn/CGB hybrids were verified by Raman spectroscopy. Raman spectrum of the hybrids
shows a strong G band at ~1598 cm-1, a strong D band at ~1327 cm-1, and a very small 2D
band at ∼2619 cm-1. The G band is assigned to the first-order scattering of the E2g phonon
from sp2 carbon; while the D band is from the defects and vacancies in the CGB and CNT.
The D/G ratio of the p-CNTn/CGB hybrids is ∼1.03, suggesting the presence of defects. (d)
Nitrogen adsorption/desorption isotherms of p-CNTn/CGB were tested and the isotherm
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curves of p-CNTn/CGB show type IV isotherms with a distinct hysteresis loop at a relative
pressure P/P0 ranging from 0.4 to 1. The measured BET surface area of the p-CNTn/CGB is
263.0 m2/g with a pore volume and average pore size of 0.28 cm3/g and 4.7 nm, respectively.
Figure S3. Galvanostatic charge/discharge curves of (a) CGB and (b) RGO cells under
different current densities. (c) and (d) The IR drop plots of CGB and RGO cells at different
discharge current densities.
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Figure S4. (a) Specific capacitance of the p-CNTn/CGB cell as a function of loading mass of
the active materials on each electrode at current densities of 0.325, 0.65, and 3.25 A/g. (b)
Specific area capacitance of the p-CNTn/CGB cells as a function of loadings of the active
materials on each electrode at current densities of 0.325, 0.65, and 3.25 A/g. (c) Specific area
capacitance of the p-CNTn/CGB, CGB, and RGO cells at various discharge current densities.
The loadings are 0.15, 0.14, and 0.24 mg for the p-CNTn/CGB, CGB, and RGO cells,
respectively. (d) Specific volume capacitance of the p-CNTn/CGB cells as a function of
loadings of the active materials on each electrode at current densities of 0.325, 0.65, and 3.25
A/g. The volume of each electrode was calculated based on the volume of the Ni foam
(diameter of 1.2 cm and thickness of 0.24 mm after compression). Under the current loading
level, it is difficult to estimate the volume of the active materials; therefore, the volume of the
Ni foam (much larger than the actual volume of active materials) was used for the specific
volume capacitance calculation.
We believe the use of the Ni foam volume is more
appropriate for the cell evaluation since it represents the true volume of the cell. The specific
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volume capacitance of the p-CNTn/CGB cells were in the range of 0.81-9.59 F/cm3 with
various loadings and discharge currents and increased with the increase of loading and
decrease of charge/discharge current.
Figure S5. SEM images of the LCNT/CGB hybrids produced with different CNT growth
conditions: thermal annealing of the CGB/Fe3O4 hybrids in a mixed Ar and H2 (flow rate 3:2)
atmosphere at 700 °C for 30 minutes and then with a H2 and C2H2 (flow rate 5:1) atmosphere
for three minutes. The produced CNTs have a larger diameter (~100 nm) and are longer (2-10
μm) than the p-CNTn/CGB samples with one minute CNT growth. The supercapacitor
performance of the LCNT/CGB hybrid was investigated by CVs and galvanostatic
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charge/discharge tests.
The specific capacitances were plotted with different discharge
currents. The results show that the cell had a specific capacitance of 124.7 F/g at a current
density of 0.28 A/g and the capacitance decreased with increasing discharge current and
dropped to 58.7 F/g (53% drop) when the current density increased to 28 A/g. The results
show that the CGB-LCNT cell also had a much better rate capability than the cells fabricated
with RGO and CGBs. Although the specific capacitance of the LCNT/CGB cell is smaller
than that of the p-CNTn/CGB cell, which is due to the decreased specific surface from the
increased amount of CNTs, the decoration of CNT-networks on the CGBs has shown its
advantages in enhancing the specific capacitance and rate capabilities of graphene-based
active materials, indicating the role of contact in designing high performance supercapacitors.
These results indicate that CNTs could greatly improve the rate capability of CGB-based
supercapacitor devices while hybrids with small CNTs show the best performance since small
CNTs can largely preserve the high specific surface area of CGBs. The content of CNTs in
the hybrids for achieving the best performance is estimated to be 33-55% based on a rough
estimate from the size/number of CNTs and the size of CGBs.
Capacitor performance calculations
The specific capacitance of the p-CNTn/CGB cell was calculated from the CV and
galvanostatic charge/discharge results. From the CV test, the specific capacitance of single
electrode (Ccv, unit: F/g) was calculated by the following equation:
,
(1)
where m is the mass of the single electrode (unit: g), ν is the potential scan rate (unit: V/s),
I(V) is the measured current (unit: A), and ∆V is the potential range (unit: V), respectively.
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From the galvanostatic charge/discharge test, the specific capacitance of single electrode (Cg,
unit: F/g) was calculated by the following equation:
∆
∆
,
(2)
where i is the discharge current (unit: A), ∆t is the discharge time (unit: s), and ∆U is the
voltage drop upon discharging (unit: V), respectively.
The specific area capacitance of single electrode (Ca, unit: F/cm2) was calculated by the
following equation:
∆
∆
,
(3)
where a is the surface area of the single electrode (unit: cm2).
The specific volume capacitance of single electrode (Ca, unit: F/cm3) was calculated by the
following equation:
∆
∆
,
(4)
where b is the volume of the single electrode (unit: cm3).
The energy density (E, unit: Wh/kg) and power density (P, unit: W/kg) of the capacitor were
calculated by the following equations:
Δ ,
(5)
.
(6)
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Summary of supercapacitor performance of graphene-based active materials
Table S1. Supercapacitor performance of graphene-based active materials.
Active materials
p-CNTn/CGB
(This work)
Flat graphene/CGB
[s1]
Specific
capacitance
(F/g)
162.3
106.1
CV
scan
rate
(mV/s)
10
1000
178.6
20
202.0
96.7
181
108.6
104
100
175
160.7
1000
150
62
26.2
5
100
Graphene/CNT [s2]
Vertically-oriented
graphene/graphene
film [s3]
B, N-doped
graphene [s4]
B-doped graphene
[s5]
N-doped graphene
[s6]
Chemically
modified graphene
[s7]
Specific
capacitance
(F/g)
107
200
Loading of
active
materials
(mg/cm2)
Electrolyte
0.14-1.15
6 M KOH
20
5 M KOH
6 M KOH
6 M KOH
1 M H2SO4
200
140
282
165
100
Charge/
discharge
current
density (A/g)
0.325
65
0.1
2.5
0.2
1
1
0.1
5
1
33
1.18-1.97
6 M KOH
1
6 M KOH
3.75
5.5 M
KOH
20
40
Ultrathin vertical
graphene [s8]
247.3
0.176
Activated graphene
[s9]
Graphene hydrogel
[s10]
200
166
184-222
117-165
0.7
5.7
1
100
1.6×10-3
3.2-5.1
Polymer
gel
electrolyte
Organic
electrolyte
5 M KOH
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