Electrochemical Capacitors
Utilizing
Low Surface Area Carbon Fiber
Stephen M. Lipka
Florida Atlantic University
ABSTRACT
The performance of electrochemical capacitors
containing different commercial carbon fibers is reviewed.
High specific capacitance (ca. 300 F/ is obtained with
low surface area carbon fiber (< 1 m /g) using a
proprietary activation process. Capacitance is primarily
achieved through pseudocapacitance resulting from
surface functional groups. The performance of these
devices is dependent on the type of carbon fiber, its
carbon content, aspect ratio and microstructure. These
devices can achieve high cycle life (ca. 100k) without
significant loss in capacitance.
CARBON POWDER.
FIBER, FOAM (AEROGELS)
OR MESOBEADS
4'
ELECTROLYTE
SEPARATOR
INTRODUCTION
SUPERC.WACITOR
Electrochemical double-layer capacitors (EDLCs)
or supercapacitors are devices that develop their capacitive
behavior either through charge separation at the
electrode/electrolyte interface (electric double-layer) or
pseudocapacitance resulting from charge transfer
reactions. Most double-layer capacitors use high surface
area carbonaceous materials to achieve high capacitance.
These materials can have specific surface areas of several
Author's Current Address:
Florida Atlantic University, Department of Ocean Engineering, 777 Glades Road,
Boca Raton, FL 33431.
Based on a presentation at the 1996 Battery Conference.
0885-89851971$10.00 0 1997 IEEE
IEEE AES Systems Magazine, Jiily 1997
I-zv
Fig. 1. Schematic of Electrochemical Capacitor
thousand m2 per gram. Pseudocapacitative behavior in
carbons can be achieved through the attachment of a
variety of surface functional groups using either thermal,
chemical or electrochemical treatments.
Typical carbons that have been investigated for use
in electrochemical capacitors (ECs) include glassy carbon
[l],powders [2], fibers [3,4], aerogels [5],films 161 and
foams prepared from the pyrolysis of polymeric
precursors. These materials can achieve high specific
surface areas either through the synthesis or activation
processes or both. A schematnc representation of an
electrochemical capacitor unit cell is shown in Figure 1.
27
Typically, it is composed of two porous electrodes
separated by a thin, porous nonconductive separator.
The electrolyte may be either aqueous (e.g., H2so4)
or nonaqueous (e.g., TEABF4 in propylene carbonate).
ECs containing aqueous electrolytes arc limited to 1V
per cell while devices containing organic electrolytes
can achieve 3 to 4 V.
Some applications for ECs include power and
memory backup, load-leveling, pulse power and
microactuators. Electrochemical capacitors have relatively
low energy density (1-10 Wh/kg) and very high power
density (1000-2000 Wkg) compared to batteries [7]. In
addition, charge/discharge cycles > 100k with coulombic
efficiencies up to 95% can be obtained readily [7].
Nominal charge/discharge times for ECs range from
1 to 30 s.
In this paper, we present some of the basic
characteristics of electrochemical capacitors containing
IOWsurface area carbon fiber. The performance of
ECs containing carbon fiber prepared from different
precursor materials and having various degrees of
graphitization are compared.
Table 1. Carbon Fiber Properties
Fiber df
Carbon SurfaceArea
Type (pm) Content (%o)
(m2/g)
dooz
Lc
(13,
PitchA 10
Pitch B 10
Pitch C 11
99
99
97
0.40
0.35
0.70
3.40 169
3.48 139
3.44 24
PANA
PANB
92
94
0.45
<1
3.44 15
3.54 14
7
8
3.0
i
r
II
-
2.0
+PITCH
-PITCH
-PITCH
-PAN
e
A 1
1
B
C
A
0
E
c
0
v
1.0
Carbon fiber in the form of continuous tow was
obtained from several commercial vendors. Three of the
carbon fiber samples were prepared from mesophase pitch
(PITCH A, B, C ) while two other fiber samples were
prepared from polyacrylonitrile (PAN) precursor (PAN A,
B). The specific surface area of the as-received carbon
fiber samples was less than 1 m2 per gram [SI. Powder
x-ray diffraction (XRD) was used to determine the degree
of crystallinity of the as-received carbon fiber.
The carbon fibers were activated using a proprietary
process developed in our laboratory. After activation, the
fibers were chopped by hand to lengths averaging
approximately 0.5 cm. In some instances, the carbon fibers
were chopped to shorter lengths ranging from 100 to 300
pm. Once chopped to the desired length, the carbon fibers
were assembled into cells using the same procedure
described previously [9]. Each unit cell contained two
carbon electrodes 1.9 cm in diameter and 0.05 cm thick.
Each electrode contained 220 mg of mass which included
both the electrolyte (38 wgt % H2SO4) and carbon fiber. It
was assumed that one-half of the electrode mass was
composed of active material. Each electrode was separated
by a single layer of nonconductive porous polymer.
Three-cell devices were fabricated for each type of carbon
fiber by stacking each of the unit cells in series.
Electrochemical impedance spectroscopy (EIS) was
used to evaluate the performance of the double-layer
devices. A 5 mV sinusoidal waveform ranging in frequency
from 500 kHz to 1 mHz was applied to each three-cell
device that was biased at 0.01 and 3 V. The capacitance of
the device was measured at 1 mHz for each bias voltage.
28
0.0
0.0
2.0
1.0
3.0
2' ( o h m s )
Fig. 2. Nyquist Plots for Carbon Fiber Capacitors
Biased at 0.01 V (0 Cycles)
Cycle tests were also conducted on selected
electrochemical capacitors in order to determine their
ability to store charge after continuous charge and
discharge. Capacitors were charged to 3 V for 30 s at 200
mA. The capacitors were then discharged through a fixed
resistor for 30 s to 1.5V. On average, the current during
discharge was 80 mA. After completion of a specified
number of cycles, impedance measurements were
conducted in order to measure the capacitance and
equivalent series resistance (ESR) of the device.
RESULTS AND DISCUSSION
Results from powder XRD are shown in Table 1
along with some of the physical properties of the
as-received carbon fibers. Lc, the crystallite size along the
c-axis direction, was estimated from the Scherrer equation
[lo]. In general, the pitch fibers were more graphite than
the PAN-based fiber as evidenced by the L values and the
sharp, intense (002) XRD peaks. As expected, the pitch
fiber had a higher carbon content (or greater carbon yield)
IEEEAES Systems Magazine, July 1997
2.5
Table 2. Performance of Carbon Fiber
Electrochemical Capacitors
i 1
2
Fiber Type Specific Capacitance Volumetric Capacitance
(Fk)*
(F/C@)#
1.5
-E
O
‘
1
0.5
Pitch A
Pitch B
Pitch C
@O.OlV
275
310
190
@3V
78
130
76
@O.O%V
216
244
149
@3V
61
102
60
PAN A
PAN B
60
47
8.6
47
37
6.8
-
-
* Capacitance per gram of carbon for single electrode
# Capacitance per sinRle electrode volume
0
0
0.5
1.5
1
2
2.5
,
2.0
2 ,uhm
than the PAN-based fiber materials. do02 spacings were
comparable for all the fibers except for the PAN B fiber
which had the largest c-spacing. The data show that the
pitch A fiber had the highest degree of structural order
followed by pitch B, pitch C, PAW A and PAN B which
was the least ordered carbon fiber. After activation, there
was only a two-fold increase in the specific surface area of
the carbon fibers [S, 91 which means that the fiber still
maintained a very low surface area (< 2 m2/g).
Nyquist plots for freshly assembled carbon fiber
double-layer capacitors biased at 0.01 V are shown in
Figure 2, on previous page. As can be seen, devices
containing pitch A and B fibers displayed superior
capacitive behavior when compared to devices containing
less ordered carbon fiber (pitch C , PAN A and PAN B). In
general, charge saturation (frequency at which the Nyquist
curve starts to become vertical) for the pitch A and pitch B
devices occurred at ca. 12 mHz. ESR values (high
frequency intercept with the real axis) for devices
containing fiber having lower carbon content (pitch C,
PAN A and PAN B) were also higher when compared to
the more highly ordered carbon fiber materials. Similar
behavior was observed when the devices were biased at 3
V. In some cases (e.g., for specific carbon fibers), the
response of the double-layer device was faster in the
charged state (3 V) than in the discharged state. The
electrochemical performance for each of the carbon fiber
devices is summarized in Table 2.
The data in Table 2 indicate that the best
performing capacitors were those containing more
crystalline (or graphite) carbon fiber and that, in general,
the more graphite the carbon the higher the capacitance
for the device. As seen, the highest capacitance was
achieved for the pitch B capacitor which has less structural
order than the pitch A fiber. This behavior has been
attributed to the greater degree of disruption in the
radially oriented graphene planes that has been observed
IEEE AES Systems Magazine, July 1997
,
1.0
I
Fig. 3. Nyquist Plots for Bitch B Capacitor Showing
the Effect of Fiber Length on Performance
I
a/..
?I
U
I
0.0
f
?
I
t
‘
0
I
I
,
75000
100000
’
25000
50000
0.0
NUMBER OF C Y C L E S
Fig. 4. Capacitance and ESR vs. Cycle Nwnber for Capacitor Containing Pitch B Carbon Fiber (3V Bias)
to occur for pitch B carbon fiber after activation when
compared to pitch A fiber [8]. The activation process
causes some of the carbon fibers to “split” radially thus
exposing a considerable number of graphene planes to the
electrolyte. The data in Table 2 also demonstrate that the
capacitance is voltage-dependent, that is, capacitance is
greater for a device in the uncharged or 0 V condition.
This behavior has been shown to exist in other
carbonaceous materials [11].
The effect of fiber length on the performance of a
carbon fiber capacitor is shown in Figure 3 for a device
containing pitch B fiber. The data are shown for the device
biased at 0.01 and 3 V and containing two different fiber
lengths. The data labeled “chopped” refers to a capacitor
containing fibers having lengths ranging from 100 to 300
mm while the data that is not labeled was obtained for a
capacitor containing fibers having an average length of 0.5
cm. The device containing the longer length fiber
saturated at about 12 mHz. After chopping the fiber to
shorter lengths, the device then saturated at 24 mHz. In
29
other words, by merely reducing the aspect ratio of the
fiber, the device becomes faster.
The performance of a capacitor containing pitch B
fiber after 100k cycles is shown in Figure 4, previous page.
The data were collected at the 3 V bias and the
capacitance was measured at 1 mHz. The data show a 14%
loss in capacitance after 100k cycles while data collected at
0.01 V bias showed an 11%loss. Also shown is the ESR
which steadily increased with cycling. This is due to the
loss of electrolyte resulting evaporation and leakage. This
was a common occurrence for these devices’since they
were not hermetically sealed. Coulombic efficiencies for
the pitch-based carbon fiber capacitors were usually
greater than 98%.
The performance of electrochemical capacitors
prepared from low surface area carbon fiber show great
promise. This work has demonstrated that high surface
area, highly porous carbons are not required in order to
achieve high capacitance. The performance of devices
prepared from carbon fiber is greatly dependent on the
type of carbon fiber used, its precursor, structural order
and aspect ratio.
OWLEDGMENT
The financial assistance of Idaho National
Engineering Laboratory, Tim Murphy, Project manager, is
gratefully acknowledged. The fabrication of
electrochemical capacitors by Dr. John R. Miller of JME,
Inc., is also gratefully acknowledged.
REFERENCES
[I] M. Sullivan, R. Kotz and 0. Haas, 1996,
in Electrochemical Capacitorsll995,
F.M. Delnick and M. Tomkiewicz, Editors,
PV 95-29,
p. 198,
The Electrochemical Society Proceedings Series,
Pennington, NJ.
[2] S. Evans, 1996,
J. Electrochemical Society,
113,
p. 165.
[3] N. Nawa, T. Nogami and H. Mikawa, 1994,
J. Electrochemical Society,
131 (6),
p. 1457.
[4] X. Chu and K. Kinoshita, 1996,
in Electrochemical Capacitors,
F.M. Delnick and M. Tomkiewicz, Editors,
PV 95-29,
p. 235,
The Electrochemical Society Proceedings Series,
Pennington, NJ.
[5] S.T. Mayer, R.W. Pekala and J.L. Kaschmitter, 1993,
J. Electrochemical Society,
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[6] M.X. Tan and M.A. Bankert, October 6-11, 1996,
Abstract No. 697 presented at the 190th Meeting of the
Electrochemical Society,
San Antonio, TX.
[7] A.F. Burke and T.C. Murphy, 1995,
in Materials for Electrochemical Energy Storage and Conversion
Batteries, Capacitors and Fuel Cells,
D.H. Doughty, B. Vyas, T. Takamura and J.R. Huff, Editors,
p. 375,
Materials Research Society Symposium, Volume 393,
Materials Research Society,
Pittsburgh, PA.
[8] S.M. Lipka, October 6-11, 1996,
Abstract No. 699 presented at the 190th Meeting of the
Electrochemical Society,
San Antonio, TX.
[9] S.M. Lipka, December 6-8,1993,
“Applicationof Carbon Fiber Materialsfor Double-Layer
Capacitors,
Proceedings of the Third International Seminar on Double-Layer
Capacitors and Similar Energy Storage Devices,
Deerfield Beach, FL.
I’
[lo] L.V. Azaroff, 1968,
Elements of X-ray Crystallography,
McGraw-Hill,
New York.
[ 111 F.M. Delnick, D. Ingersoll and D. Firsich, December 6-8, 1993,
“DoubleLayer Capacitance of Carbon Foam Electrodes,”
Proceedings of the Third International Seminar on Double-Layer
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Stephen M. Lipka is an Associate Professor in the Ocean Engineering Department a t Florida Atlantic
University. He received a B.S. in Materials Engineering from Wilkes College in Wilkes-Barre, PA and a M.S. and
Ph.D. in Materials Science from the University of Virginia. Dr. Lipka worked in industry as a Research
Electrochemist a t the Chemical Research Division of Amencan Cyanamid in Stamford, CT and a s a Principal
Scientist a t Physical Sciences, Inc., in Andover, MA, before joining the Faculty a t Florida Atlantic University in
1989 as a n Assistant Professor. His major research emphases are in energy storage devices such as batteries,
electrochemical capacitors and fuel cells with special interest in materials synthesis and carbon chemistry.
30
IEEE AES Systems Magazine, July 1997