Journal of Power Sources 124 (2003) 321–329
Investigation of electrochemical double-layer (ECDL) capacitors
electrodes based on carbon nanotubes and activated carbon materials
Ch. Emmenegger
a,∗
, Ph. Mauron
a
, P. Sudan
a
, P. Wenger
a
, V. Hermann
b
,
R. Gallay
b
, A. Züttel
a
a
Physics Institute, University of Fribourg, Pérolles, CH-1700 Fribourg, Switzerland
b
Montena Components, CH-1728 Rossens, Switzerland
Received 12 December 2002; received in revised form 14 April 2003; accepted 5 May 2003
Abstract
The carbon nanotubes (CNT) show promising electrochemical characteristics particularly for electrochemical energy storage. The
electrochemical double-layer (ECDL) capacitor is a new type of capacitor with features intermediate between those of a battery and a
conventional capacitor. ECDL capacitors have been made using various types of CNT and activated carbon (a-C) as electrode material.
The specific capacitance per surface area of the electrodes depends on the thickness and the specific surface area of the active material. The
CNT electrodes show a specific capacitance from 0.8 and 280 mF cm
−2
and8to16Fcm
−3
, respectively. Increasing the mass density also
helps to increase the capacitance. Commercially available activated carbon (a-C) electrodes were also tested in order to study their specific
capacitance as a function of their physical properties. The various a-C electrodes have specific capacitance per surface area ranging from
0.4 to 3.1 F cm
−2
and an average specific capacitance per volume of 40 Fcm
−3
due to their larger mass density.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Carbon nanotubes; Specific capacitance; ECDL capacitor; Activated carbon; Supercapacitors
1. Introduction
There have been considerable efforts to develop new
methods of efficient energy storage. This application re-
quires a high specific power from approximately 1–10 kW
kg
−1
[1], and a specific energy density of 0.5–10 Wh kg
−1
[2]. The most common storage system of electrical energy
is the battery. However, the cycle life and power (below
100Wkg
−1
) [1] limit the use of batteries for many mobile
applications. Batteries are useful for storage over long-time
application (>100 s), while conventional capacitors [3] are
useful for short-time storage (<0.01 s). Conventional capac-
itors provide a high power (>103 kW kg
−1
) and a long cycle
life, but with a small energy density (about 70 mW kg
−1
)
[1]. The capacitance C of the conventional capacitors is
given by Eq. (1):
C =
ε
o
ε
r
A
E
d
(1)
where A
E
is the geometric surface area of the electrode, ε
o
the permittivity of the vacuum, ε
r
the relative permittivity
of the dielectric material and d is the distance between the
∗
Corresponding author. Tel.: +41-26-300-9101; fax: +41-26-300-9747.
E-mail address: cemmenegger@asulab.ch (Ch. Emmenegger).
two electrodes. The amount of electrical energy (Wh kg
−1
)
stored in the polarised dielectric material [4] is given by
Eq. (2):
W =
1
2
CU
2
m
act
(2)
where U is the working voltage and m
act
is the amount of
active material. Conventional capacitors have the advantage
of high working voltage. However, their specific energy ca-
pacity is limited by the breakdown field (V m
−1
) of their
dielectric material. Overall, conventional capacitors do not
have a large enough energy storage capacity for applica-
tions where significant energy is needed. The electrochem-
ical double-layer capacitor (ECDL) [5] has been developed
for these cases which need a large energy density (Wh kg
−1
),
high power density (W kg
−1
) and long cycle life (>100,000).
The ECDL capacitor is a new type of capacitor offering new
features intermediate those of a battery and a conventional
capacitor [2].
The ECDL capacitor consists of two electrodes that are
immersed in an electrolyte with a separator between them
(Fig. 1, top). The electrode consists of a current collector in
contact with the active material. In ECDL capacitors, the en-
ergy storage arises mainly from the separation of electronic
0378-7753/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0378-7753(03)00590-1