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A review of electrode materials for electrochemical supercapacitors
Wang, Guoping; Zhang, Lei; Zhang, Jiujun
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This journal is
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Chem. Soc. Rev., 2012, 41, 797–828 797
Cite this:
Chem. Soc. Rev
., 2012, 41, 797–828
A review of electrode materials for electrochemical supercapacitors
Guoping Wang,*
ab
Lei Zhang*
b
and Jiujun Zhang
b
Received 4th March 2011
DOI: 10.1039/c1cs15060j
In this critical review, metal oxides-based materials for electrochemical supercapacitor (ES)
electrodes are reviewed in detail together with a brief review of carbon materials and conducting
polymers. Their advantages, disadvantages, and performance in ES electrodes are discussed
through extensive analysis of the literature, and new trends in material development are also
reviewed. Two important future research directions are indicated and summarized, based on
results published in the literature: the development of composite and nanostructured ES materials
to overcome the major challenge posed by the low energy density of ES (476 references).
1. Introduction
With the rapid development of the global economy, the depletion
of fossil fuels, and increasing environmental pollution, there is an
urgent need for efficient, clean, and sustainable sources of energy,
as well as new technologies associated with energy conversion
and storage.
In many application areas, some of the most effective and
practical technologies for electrochemical energy conversion
and storage are batteries, fuel cells, and electrochemical super-
capacitors (ES). In recent years, ES or ultracapacitors have
attracted significant attention, mainly due to their high power
density, long lifecycle, and bridging function for the power/
energy gap between traditional dielectric capacitors (which
have high power output) and batteries/fuel cells (which have
high energy storage).
1,2
The earliest ES patent was filed in 1957. However, not until
the 1990s did ES technology begin to draw some attention, in
the field of hybrid electric vehicles.
3
It was found that the main
function of an ES could be to boost the battery or fuel cell in a
hybrid electric vehicle to provide the necessary power for
a
College of Chemical Engineering, University of South China,
Hengyang 421001, China. E-mail: wgpcd@yahoo.com.cn;
Fax: +86 734 8282 375; Tel: +86 734 8282 667
b
Institute for Fuel Cell Innovation, National Research Council of
Canada, 4250 Wesbrook Mall, Vancouver, BC V6T 1W5, Canada.
E-mail: lei.zhang@nrc.gc.ca; Fax: +1 604 221 3001;
Tel: +1 604 221 3087
Guoping Wang
Dr Guoping Wang is an
associate professor at the
University of South China.
He joined the National
Research Council of Canada
Institute for Fuel Cell Innova-
tion as a visiting scholar in
2010. Dr Wang received his
BE and MEng from Sichuan
University and then his PhD
from Chinese Academy of
Science in 2005 in the field of
applied chemistry, under the
direction of Prof. Zuolong
Yu. Since 2002, Dr Wang
has been engaged in the
research in the field of electrochemistry. His research interests
focus on battery and supercapacitor materials, chemical engi-
neering and processes. He has published over twenty technical
papers and holds two Chinese patents.
Lei Zhang
Ms Lei Zhang is a Research
Council Officer at National
Research Council of Canada
Institute for Fuel Cell Innova-
tion. She received her first
MSc majoring in Materials
Chemistry from Wuhan
University, China, in 1993
and her second MSc in
Materials/Physical Chemistry
from Simon Fraser University,
Canada, in 2000. Ms Zhang’s
main research interests include
PEM fuel cell electrocatalysis,
catalyst layer/electrode struc-
ture, metal–air batteries and
supercapacitors. Ms Zhang is an adjunct professor of Federal
University of Maranhao, Brazil, and Zhengzhou University,
China, respectively. She is also an international advisory
member of the 7th IUPAC International Conference on Novel
materials and their Synthesis (NMS-VII) and an active member
of the Electrochemical Society and the International Society of
Electrochemistry.
Chem Soc Rev
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acceleration, with an additional function being to recuperate
brake energy.
4
Further developments have led to the recogni-
tion that ES can play an important role in complementing
batteries or fuel cells in their energy storage functions by
providing back-up power supplies to protect against power
disruptions. As a result, the US Department of Energy has
designated ES to be as important as batteries for future energy
storage systems.
5
Many other governments and enterprises
have also invested time and money into exploring, researching,
and developing ES technologies.
Recent years have yielded major progress in the theoretical
and practical research and development of ES, as evinced by a
large number of research articles and technical reports.
6–14
At the same time, the disadvantages of ES—including low
energy density and high production cost—have been identified
as major challenges for the furtherance of ES technologies.
To overcome the obstacle of low energy density, one of the
most intensive approaches is the development of new materials
for ES electrodes. Most popular today are carbon particle
materials, which have high surface areas for charge storage.
But in spite of these large specific surface areas, the charges
physically stored on the carbon particles in porous electrode
layers are unfortunately limited. ES of this kind, called electro-
static or electrical double-layer supercapacitors (EDLS), have
a limited specific capacitance (measured in Faradays per gram
of the electrode material) and a low ES energy density.
Advanced approaches to increase the ES energy density are
to hybridize the electrode materials by adding electro-
chemically active materials to a carbon-particle-based ES
electrode layer or to completely replace the carbon materials
with electrochemically active materials. ES with electro-
chemically active materials as electrodes are called faradaic
supercapacitors (FS). It has been demonstrated that faradaic
or hybrid double-layer supercapacitors can yield much higher
specific capacitance and ES energy density than EDLS.
15
Regarding advanced ES materials, metal oxides such as
ruthenium oxides and manganese oxides are considered the
most promising materials for the next generation of ES.
Therefore, in this review we pay particular attention to metal
oxides and their applications in ES electrodes. First, however,
we provide some introductory background on ES, which we
hope will facilitate our review and analysis of the literature.
Finally, we will discuss the direction that future research in ES
might be expected to take.
2. Fundamentals and applications of ES
2.1 Two types of ES
An ES is a charge-storage device similar to batteries in design
and manufacturing. As shown in Fig. 1, an ES consists of two
electrodes, an electrolyte, and a separator that electrically
isolates the two electrodes. The most important component
in an ES is the electrode material. In general, the ES’s
electrodes are fabricated from nanoscale materials that have
high surface area and high porosity. It can be seen from Fig. 1
that charges can be stored and separated at the interface
between the conductive solid particles (such as carbon particles
or metal oxide particles) and the electrolyte. This interface can
be treated as a capacitor with an electrical double-layer
capacitance, which can be expressed as eqn (1):
C ¼
Ae
4pd
ð1Þ
where A is the area of the electrode surface, which for a super-
capacitor should be the active surface of the electrode porous
Fig. 1 Principles of a single-cell double-layer capacitor and illustration
of the potential drop at the electrode/electrolyte interface.
3
(Reprinted
from ref. 3 with permission from Elsevier.)
Jiujun Zhang
Dr Jiujun Zhang is a Senior
Research Officer and PEM
Catalysis Core Competency
Leader at the National
Research Council of Canada
Institute for Fuel Cell Innova-
tion (NRC-IFCI). Dr Zhang
received his BS and MSc in
Electrochemistry from Peking
University in 1982 and 1985,
respectively, and his PhD in
Electrochemistry from Wuhan
University in 1988. After
completing his PhD, he was
an associate professor at the
Huazhong Normal University
for two years. Starting in 1990, he carried out three terms of
postdoctoral research at the California Institute of Technology,
York University, and the University of British Columbia.
Dr Zhang has over twenty-eight years of R&D experience in
theoretical and applied electrochemistry. He holds several
adjunct professorships, and is an active member of the Electro-
chemical Society, the International Society of Electrochemistry,
and the American Chemical Society.
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Chem. Soc. Rev., 2012, 41, 797–828 799
layer; e is the medium (electrolyte) dielectric constant, which
will be equal to 1 for a vacuum and larger than 1 for all other
materials, including gases; and d is the effective thickness of
the electrical double layer.
As described in the Introduction, two types of ES exist. One
is the EDLS, in which the electrode material, such as carbon
particles, is not electrochemically active. In other words, there
is no electrochemical reaction on the electrode material during
the ES charging and discharging processes, and pure physical
charge accumulation occurs at the electrode/electrolyte
interface. The other type is the FS, in which the electrode
material is electrochemically active, e.g. metal oxides, which
can directly store charges during the charging and discharging
processes.
15–17
2.1.1 Electrostatic supercapacitors (EDLS). The capaci-
tance of the electrode/interface in an electrostatic or EDLS
is associated with an electrode-potential-dependent accumula-
tion of electrostatic charge at the interface. The mechanism of
surface electrode charge generation includes surface dissocia-
tion as well as ion adsorption from both the electrolyte and
crystal lattice defects.
5
These processes operate solely on the
electrostatic accumulation of surface charge. As shown in
Fig. 1, this electrical double-layer capacitance comes from
electrode material particles, such as at the interface between
the carbon particles and electrolyte, where an excess or a
deficit of electric charges is accumulated on the electrode
surfaces, and electrolyte ions with counterbalancing charge
are built up on the electrolyte side in order to meet electro-
neutrality. During the process of charging, the electrons travel
from the negative electrode to the positive electrode through
an external load. Within the electrolyte, cations move towards
the negative electrode while anions move towards the positive
electrode. During discharge, the reverse processes take place.
In this type of ES, no charge transfers across the electrode/
electrolyte interface, and no net ion exchanges occur between
the electrode and the electrolyte. This implies that the electro-
lyte concentration remains constant during the charging and
discharging processes. In this way, energy is stored in the
double-layer interface.
If the two electrode surfaces can be expressed as E
S1
and
E
S2
, an anion as A
, a cation as C
+
, and the electrode/
electrolyte interface as //, the electrochemical processes for
charging and discharging can be expressed as eqn (2)–(5).
18,19
On one electrode (say, a positive one):
E
S1
þ A
!
charging
E
þ
S1
==A
þ e
ð2Þ
E
þ
S1
==A
þ e
!
discharging
E
S1
þ A
ð3Þ
On the other electrode (say, a negative one):
E
S2
þ C
þ
þ e
!
charging
E
S2
==C
þ
ð4Þ
E
S2
==C
þ
!
discharging
E
S2
þ C
þ
þ e
ð5Þ
And the overall charging and discharging process can be
expressed as eqn (6) and (7):
E
S1
þ E
S2
þ A
þ C
þ
!
charging
E
þ
S1
==A
þ E
S2
==C
þ
ð6Þ
E
þ
S1
==A
þ E
S2
==C
þ
!
discharging
E
S1
þ E
S2
þ A
þ C
þ
ð7Þ
2.1.2 Faradaic supercapacitors (FS). Faradaic super-
capacitors (FS) or pseudocapacitors are different from electro-
static or EDLS. When a potential is applied to a FS, fast and
reversible faradaic reactions (redox reactions) take place on the
electrode materials and involve the passage of charge across the
double layer, similar to the charging and discharging processes
that occur in batteries, resulting in faradaic current passing
through the supercapacitor cell. Materials undergoing such
redox reactions include conducting polymers and several metal
oxides, including RuO
2
,MnO
2
,andCo
3
O
4
.
15,20–22
Three types
of faradaic processes occur at FS electrodes: reversible adsorp-
tion (for example, adsorption of hydrogen on the surface of
platinum or gold), redox reactions of transition metal oxides
(e.g. RuO
2
), and reversible electrochemical doping–dedoping in
conductive polymer based electrodes.
15
It has been demonstrated that these faradaic electrochemical
processes not only extend the working voltage but also
increase the specific capacitance of the supercapacitors.
23
Since
the electrochemical processes occur both on the surface and in
the bulk near the surface of the solid electrode, a FS exhibits
far larger capacitance values and energy density than an
EDLS. As reported by Conway et al.,
24
the capacitance of a
FS can be 10–100 times higher than the electrostatic capaci-
tance of an EDLS. However, a FS usually suffers from
relatively lower power density than an EDLS because faradaic
processes are normally slower than nonfaradaic processes.
25
Moreover, because redox reactions occur at the electrode, a FS
often lacks stability during cycling, similar to batteries.
It is worth mentioning that hybrid ES with an asymmetrical
electrode configuration ( e.g. one electrode consists of electro-
static carbon material while the other consists of faradaic
capacitance material) have been extensively studied recently to
capitalize on both electrode materials’ advantages in improving
overall cell voltage, energy, and power densities.
14,26,27
In this
kind of hybrid supercapacitor, both electrical double-layer
capacitance and faradaic capacitance mechanisms occur simul-
taneously, but one of them plays a greater role. In both
mechanisms, large surface area, appropriate pore-size distri-
bution, and high conductivity are essential properties of the
electrode materials to achieve large capacitance, as will be
discussed in a later section.
2.2 ES capacitance, voltage, power, and energy density
Fig. 1 shows that the entire cell can be treated as two
capacitors in series. If the capacitances of the two electrodes,
i.e. positive and negative, can be expressed as C
p
and C
n
,
respectively, the overall capacitance (C
T
) of the entire cell can
be expressed as eqn (8):
11
1
C
T
¼
1
C
p
þ
1
C
n
ð8Þ
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If the two electrodes are the same, namely, C
p
= C
n
, the
overall capacitance C
T
would be half of either one’s capaci-
tance, the corresponding ES is called a symmetric ES. In the
case of C
p
a C
n
(the anode and the cathode have two different
electrode materials, the corresponding ES is called an asym-
metric ES), C
T
is mainly dominated by the one with smaller
capacitance. In general, the capacitance and stored charge
essentially depend on the electrode material used.
When the ES is charged, a voltage (V) will build up across
the two electrodes. The theoretical (or maximum) energy (E)
and power densities (P) of this ES can be expressed as
eqn (9) and (10):
7,8
E ¼
1
2
CV
2
¼
QV
2
ð9Þ
P ¼
1
4R
S
V
2
ð10Þ
where Q denotes the stored total charges in the ES and R
S
stands for the equivalent inner resistance of the ES. From
these two equations, it can be seen that V, C and R
S
are three
important variables determining the ES’s performance.
In order to increase ES’s energy density and power density,
one has to put effort in increasing the values of both V and C
or reducing the value of R
S
. Here the value of the ES voltage
(V) is dependent on the materials used for the electrode and
electrolyte (e.g. when carbon is used as the electrode material
for aqueous electrolytes, the cell voltage or supercapacitor
voltage window is about 1 V, while in organic electrolytes the
cell voltage is in the range of 3–3.5 V), whereas the operating
voltage is determined by the electrolyte’s stability window.
From both eqn (9) and (10), it can be seen that both energy
and power densities are proportional to the square of voltage,
therefore, increasing the voltage may be more effective than
increasing capacitance or reducing inner resistance in terms of
raising the ES’s energy and power densities. To increase the
ES’s cell voltage within the electrolyte’s stability window,
selecting the type of electrode materials and optimizing
electrode structures can achieve high cell voltages.
Furthermore, eqn (10) indicates that the larger the cell
internal resistance, the lower the power density will be.
Therefore, in order to increase the power density of ES,
reducing cell’s internal resistance, a sum of electrode and
electrolyte resistances, should be the major focus. In general,
ES’s inner resistance is much smaller than that of batteries due
to the rapid combination of positive and negative charges
(even in a faradaic-type ES, the redox processes involving
electron and ion transfers are also very fast), the power density
of an ES is normally much higher than in batteries. Even so,
reducing inner resistance can always benefit the ES’s perfor-
mance in terms of power density improvement.
Eqn (9) indicates that the energy density of ES is proportional
to its capacitance, meaning that the higher the capacitance, the
higher the energy density will be. Therefore, increasing the
capacitance is an effective way to improve energy density.
This can be achieved by improving the specific capacitance
of electrode materials as well as optimizing electrode layer
structures. In order to increase overall cell capacitance,
both electrode capacitances have to be increased. Therefore,
developing electrode materials should be one of the key
approaches in ES research and development.
In evaluating an electrode material for ES, another gener-
ally used definition is the specific capacitance (C
s
), with a unit
of Faraday per gram (F g
1
), which can be expressed as
eqn (11):
C
s
¼
C
i
W
ð11Þ
where W is the weight in grams of the electrode material in the
electrode layer, and C
i
is the electrode capacitance (anode or
cathode). Note that this specific capacitance is the intrinsic
capacitance of the material. A higher specific capacitance
does not necessarily mean that this material will be a better
ES electrode material, because electrode capacitance is also
strongly dependent on the electrode layer structure and the
electron and ion transfers within the layer. For example, a
material that forms an extremely thin film on the electrode
surface can yield a huge specific capacitance value because of
its very low weight. However, when using this high capacitance
material to construct a thick layer and thereby achieve high
electrode capacitance for energy storage, the electrode capa-
citance may not be as high as expected; this will be discussed
further in later sections. Nonetheless, the concept of specific
capacitance has been adopted as an important parameter in
evaluating an ES material.
2.3 Electrolyte
As shown in Fig. 1, besides the two electrodes, the electrolyte,
which resides inside the separator as well as inside the active
material layers, is also one of the most important ES compo-
nents. The requirements for an electrolyte in ES include: wide
voltage window, high electrochemical stability, high ionic
concentration and low solvated ionic radius, low resistivity,
low viscosity, low volatility, low toxicity, low cost as well as
availability at high purity.
The electrolyte used in an ES can be classified into three
types: (1) aqueous electrolyte, (2) organic electrolyte, and (3)
ionic liquids (ILs).
(1) Aqueous electrolyte. Compared with organic electrolytes,
aqueous electrolytes (such as H
2
SO
4
, KOH, Na
2
SO
4
and
NH
4
Cl aqueous solution and so on) can provide a higher
ionic concentration and lower resistance. ES containing
aqueous electrolyte may display higher capacitance and higher
power than those with organic electrolytes, probably due to
higher ionic concentration and smaller ionic radius. In addi-
tion, aqueous electrolytes can be prepared and utilized without
stringently controlling the preparing processes and conditions,
while organic ones need strict processes and conditions to
obtain ultra-pure electrolytes.
Unfortunately, a large disadvantage of aqueous electro-
lytes is their small voltage window as low as about 1.2 V,
much lower than those of organic electrolytes. According to
eqn (9) and (10), it can be seen that aqueous electrolytes have a
large limitation in terms of improving both energy and power
densities due to their narrow voltage window. This is the
reason why organic electrolytes are often recommended.
(2) Organic electrolyte. Compared to aqueous electrolytes,
organic electrolytes can provide a voltage window as high as 3.5 V.
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