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REVIEW
The electrochemistry of activated carbonaceous materials:
past, present, and future
Malachi Noked & Abraham Soffer & Doron Aurbach
Received: 3 February 2011 /Revised: 28 February 2011 /Accepted: 3 March 2011 /Published online: 4 May 2011
#
Springer-Verlag 2011
Abstract Carbonaceous materials are widely used in
electrochemistry. All allotropic forms of carbons—graphite,
glassy carbon, amorphous carbon, fullerenes, nanotubes,
anddopeddiamond—are used as important electrode
materials in all fields of modern electrochemistry. Examples
include graphite and amorphous carbons as anode materials
in high-energy density rechargeable Li batteries, porous
carbon electrodes in sensors and fuel cells, nano-amorphous
carbon as a conducting agent in many kinds of composite
electrodes (e.g., cathodes based on intercalation inorganic
host materials for batteries), glassy carbon and doped
diamond a s stable r obust and high stability electrode
materials for all aspects of basic electrochemical studies,
and more. Amorphous carbons can be activated to form
very high specific surface area (yet stable) electrode
materials which can be used for electrostatic energy storage
and conversion [electrical double-layer capacitors (EDLC)]
and separation techniques based on electro-adsorption, such
as water desalination by capacitive de-ionization (CDI).
Apart from the many practical aspects of activated carbon
electrodes, there are many highly interesting and important
basic aspects related to their study, including transport
phenomena, molecular sieving behavior, correlation be-
tween electrochemical behavior and surface chemistr y, and
more. In this article, we review several important aspects
related to these electrode materials, in a time perspective
M. Noked
:
A. Soffer
:
D. Aurbach (*)
Department of Chemistry, Bar-Ilan University,
Ramat Gan 52900, Israel
e-mail: Doron.Aurbach@biu.ac.il
J Solid State Electrochem (2011) 15:1563–1578
DOI 10.1007/s10008-011-1411-y
(past, present, and future), with the emphasis on their
importance to EDLC devices and CDI processes.
Keywords Carbon electrodes
.
Activated carbons
.
EDLC
.
CDI
.
Adsorption phenomena
Introduction
Carbon, the sixth most abundant element in the universe,
has been known since ancient times. The three naturally
occurring allotropes of carbon known to exist are amor-
phous carbon (coal), graphite, and diamond. Beyond the
critical importance of carbon in organic chemistry, through-
out human history and especially in modern times, carbon
is highly important and is widely used as an element in its
various allotropic forms. Carbon is the source of the highest
strength fibers, one of the best lubricants (graphite), the
strongest crystal and hardest material (diamond), an
essentially noncrystalline product (vitreous carbon), one of
the best gas absorbers (activated charcoal), and one of the
best helium gas barriers (vitreous carbon) [1, 2]. A great
deal is yet to be learned, and more recently discovered
forms of carbon, such as the fullerene molecules [3], the
hexagonal polytypes of diamond [4], carbon nanotubes [5],
and graphene [6], are being studied extensively. Some of
the structures of vario us carbon allotropes are presented in
Fig. 1.
All the allotropic form s o f carbons—graphite, glassy
carbon, amorphous carbon, fullerenes, nanotubes, and
doped diamond—are used as important electrode materi-
als in all fields of modern electrochemistry. Examples
include graphite and amorphous carbons as anode
materials in high energy-density rechargeable Li batteries
porous carbon electrodes in sensors and fuel cells [7],
nano-amorphous carbon as a conducting agent in many
kinds of composite electrodes (e.g., cathodes based on
intercalation inorganic host materials for batteries), glassy
carbon and doped diamond as stable r obust and high
stability electrode materials for all aspects of basic
electrochemical studies, and many more. The unique
stability of carbon in various electrolyte solutions under
a wide range of potentials and temperatures, despite their
exposure to various reactive gases and ions in those
extremely reactive/corros ive c onditions, enables the uti li-
zation of co ndu ctive carbon materials (e.g., amorphous
carbon and graphite) for various electrochemical systems
and devices [7 ].
Amorphous carbon is highly abundant in the earth's crust
as coal. It was probably formed by the metamorphosis of
organic materials (mostly plants) in the course of old
geological thermal processes. Most organic materials can be
carbonized by anaerobic heat treatments. In fact, the
majority of carbon materials used today are derived from
carbon-rich organic precursors treated in inert atmospheres
at elevated temperatures (a process referred to as carbon-
ization). The ultimate properties of these carbons depend on
a number of critical factors, e.g., the carbon precursor, its
dominant aggregation phase during carbonization (i.e.,
solid, liquid, or gas), processing condi tions, and the
structural and textural features of the products. During
carbonization, a thermal decomposition of the carbon
precursors occurs. This pyrolisis eliminates all the volatile
materials including the heteroatoms, and therefore, the
majority of atoms left are carbon. The increasing proportion
of conjugated carbon atoms in t he sp
2
state during
carbonization progressively increases the conductivity of
a
bc
d
e
f
Fig. 1 Some of the structures of
various carbon allotropes. a
Amorphous carbon. b Diamond.
c Graphite. d Fullerene. e CNT.
f Graphene
1564 J Solid State Electrochem (2011) 15:1563–1578
the starting material as electrons associated with π-bonds
are delocalized and become available as charge carriers. It
is possible to design properties of carbonaceous materials
for electrochemical applications, by a judicious choice of
organic precursors (e.g., polymers, aromatic molecules,
organic molecules containing heteroatoms such as nitrogen)
and the carbonization param eters.
At the next stage, it is possible by mild oxidation to form
highly porous carbon (with specific surface area >2,000 m
2
/g)
with adjustable porosity in terms of pore size and pore
morphology (as discussed below). Activated carbons can be
used together with more novel forms of carbon, nanotubes,
or graphene sheets to form very interesting and important
composite electrodes. Thus, highly porous, activated carbons
can be considered among the most important electrode
materials in modern electrochemistry and therefore constitute
the main focus of this review.
Activated carbon electrodes: past and present
General description and discussion
The recent widespread use of conductive carbon materials
for e lectrochemical systems started in the nineteenth
century as replacement electrodes for co pper in Volta
batteries [7], and platinu m i n Gro ve ce ll s [ 8]. A significant
milestone in the history of carbonaceo us material as
candidates for electrochemical systems is the manufacture
of a rtifi cia l graphite by E.G. Acheson at the end of the
nineteenth century [9]. Since then, there has been rapid
growth in t he use of carbons in ele ctro chem ic al systems.
Amorphous carbons are unique electrically and thermally
conductive solids with adequate corrosion resistance, low
thermal expansion coefficients, low densities, low elastic-
ity, and low cost and can re adily be ob tai ned at hig h
purities. These properties put amorphous carbon at the top
of the list for electrochemical electrodes and conductive
additives. In addition, carbon material can be produced in
a variety of structures such as fibers, hollow fibers, thin
films, powders, large blocks, etc., from various precursors.
The porosity and morphology of amorphous carbon can
be shaped via mild burn-off processes using oxidants. This
procedure, usually called activation, forms highly porous
materials whose surface area is in the range of several
hundreds to more than 2,000 m
2
/g. These pores may
occupy a large portion (30–80%) of the total volume of
the activated carbon material. Due to their associated large
internal surface area and high pore volume, the adsorption
capacitance of activated carbon materials is substantial,
more than 20 mol/kg, thus enabling the use of activated
carbons in separa tion and purification processes for gaseous
and liquid phases. Porous carbon materials with various
pore sizes and pore structures can be produced via several
different synthetic routes [10–17].
The pores in activated carbons are divided into three
groups: micropores with diameters less than 2 nm, meso-
pores with diameters between 2 and 50 nm, and macropores
with diameters greater than 50 nm [18]. Microporo us
activated carbons can be synthesized through the activation
process, usually consisting of a partial burn-off of the
amorphous carbon under mild oxidation conditions (e.g.,
CO
2
or KOH under elevated temperatures, or nitric acid in
ambient conditions). Ordered microporous carbon materials
can be produced using microporous zeolites as templates
for polymerization followed by carbonization and etching
of the aluminosilicate from the surface using HF [19–26].
Mesoporous carbons with a disordered pore structure can be
obtained by various methods, including catalytic activation
using metal species [27–30], carbonization of carbonize-able
polymer/pyrolize-able polymer blends or copolymer [31–33]
(e.g., PAN/PMMA or PAN/Hexamine), carbonization of
organic aerogels (e.g., resorcinol-formaldehyde), or template
synthesis using silica nanoparticles. Ordered mesoporous
carbons with various pore structures have been synthesized
using mesoporous silica materials as templates. Non-
graphitizing carbons (e.g., carbons which cannot be converted
to graphite by further high temperature treatment) are
produced from materials such as biomass (e.g., wood, nut
shells, etc.), non-fusing coals, and/or different thermosetting
polymers (e.g., polyvinylidene chloride, PVDC, etc.). These
carbon precursors remain in the solid phase during carbon-
ization, and the resulting limited mobility of the formed
crystallites leads to the formation of a rigid amorphous
structure that consists of randomly oriented graphene layers.
The loss of volatile compounds and the retention of a rigid,
complex molecular structure during the carbonization of
many non-graphitizable carbons can lead to a highly porous
structure without the need for further activation. Fractal
porous morphology can be obtained by multistage carbon
treatment based on a sequence of several of the above-
mentioned synthesis routes.
An important consideration related to activated carbons
(ACs) as adsorption materials is the difference between
their average pore size and the size of the species whose
adsorption is required: pores that are smaller than a critical
molecule or ion size cannot adsorb these species [34–38].
Thus, controlling the size of the pores in ACs is essential
for achieving specific adsorption by molecular sieving.
Unlike molecular sieves based on zeolites which are highly
porous crystalline materials characterized by a three-
dimensional pore network with pores of precisely defined
dimensions [39, 40], the carbon molecular sieves are
amorphous solids with a pore-size distribution that needs
to be characterized before they can be used for separation.
Among the porous solids, the selectivity a chieved by
J Solid State Electrochem (2011) 15:1563–1578 1565
zeolites and carbon ad sorbents is the best reported, even
among molecular probes of similar size. An interesting
development in carbon molecular sieves is their application
as stereoselective membranes for gas separation. Unlike
zeolite molecular sieve membranes, carbon membranes for
gas separation could be produce d free of defects. Therefore,
the motivation for developing these carbon allotropes as
membranes has intensified [10, 41].
Many e fforts have been mad e, using sophisticated,
sometimes multistage, techniques, to adjust the pore size
to fall between the sizes of the molecules that have to be
separated. Pore-size calibrated carbon molecular sieves can
be made by controlled activation [42, 43] or controlled
closure, using carbon chemical vapor deposition [ 14 , 15].
The calibration of molecular sieving carbon electrodes was
carried out by monitoring adsorption kinetics in the gaseous
phase of probe molecules having a known dimension.
These calibrated carbon molecular sieves have been
demonstrated as tools for estimating the effective dimen-
sions of different ionic species in solutions. The molecular
sieving effect of CMS is also used in the fiel d of water
purification, separation from organic pollutants, and sepa-
ration by means of the selective adsorption of organic
molecules in aqueous and non-aqueous solutions. In
general, selectivity in the ad sorption of different species
originates from one or more of the following factors:
1. Differences in the solvation energy of the adsorbates in
the fluid can relate to a difference in their ease of
adsorption.
2. Adsorption stereoselectivity: the size of the pores is
adjusted by different techniques to be close to the size
of the adsorbed species. Therefore, only speci es that are
smaller than the pores can be adsorbed and sized.
3. Adsorption of planar or flat-shaped molecules: these
are selectively adsorbed into slit-shaped pores as long
as the planar molecule is thinner than the slit-shaped
pore.
4. Different functional groups on the AC surfa ce can also
induce specific adsorption. Hence, the most common
methods to achieve adsorption stereoselectivity involve
the control of the pore size and structure and the surface
properties of the carbons.
The techniques used for developing ACs with sharp pore-
size distribution functions ma y include control of the activation
procedure of the carbons, formation of surface functional
groups on the activated carbon, template carbon synthesis,
extraction of metals from carbides, dehydrohalogenation of
halogen-containing polymers, carbon ization of copo lyme r, and
chemical vapor deposition (CVD) of surface layers.
Generally speaking, electrochem ical processes may be
divided into two main ca tegories: Faradaic (red-ox)
processes in which charge transfer across interfaces occurs,
and capacitive processes (adsorption/desorption phenome-
na), which occur within the EDL of a surface. Electrode
red-ox processes can occur only when the applied potential
exceeds a certain barrier beyond which an electrochemical
reaction becomes thermodynamically favorable. Depending
on the electro-active species presen t, each electrochemical
system has a potential range between the cathodic and
anodic potential barriers (for reduction and oxidation
processes, respectively) in which only capacitive processes
can take place. This potential range is termed the electro-
chemical window, or the EDL region of the system. Most of
the efforts in electrochemistry have been devoted over the
years to the study of red-ox reactions, their exploitation, e.g.,
for the storage of energy, or their prevention, e.g., when
corrosion phenomena are involved. However, in the last years,
many efforts have also been devoted to understanding and
utilizing the non-Faradaic, electrostatic electrochemistry,
whichissorelevanttohighlyporouselectrodes.
It is well established that the interface between electron-
ically conducting solid surfaces and electrolyte solutions
can be described by the EDL phenomenon. The configu-
ration of this inte rfacial double layer includes either
electron surface excess or deficiency at the electrode's side
of the interface, leading to an accumulation of ions of
opposite charge (to that of the electrode), and depletion of
ions of the same charge sign at the solution side of the
interface. Altogether, the interface between electrodes and
electrolyte solutions may be considered as a parallel plate
capacitor in which a layer of adsorbed solvent molecules
are the dielectric separator between the plates [44]. In
Helmholtz's view of the double-layer region, the attracted
ions are assumed to approach the electrode surface at a
distance limited by the size of the solvated ion: the overall
result is two layers of charge and a linear potential drop that
is confined to this region only. In a later model put forward
by Gouy and Chapman, it was [45, 46] suggested that the
electrostatic interactions near the interface are perturbed by
the Brownian motio n of the so lution species, thereby
leading to a charge distribution in a solution layer that
may be a few nanometers thick (known as the diffuse
layer). Hence, the potential drop at the solution side is not
linear but rather exponential along the diffuse layer. A
further understanding of the complicated situation at
electrified interfaces lead to the presen tation of the famous
Stern model, which describes the EDL in terms of two
capacitive components: the inner layer (which behaves like
the Helmholtz model), which is in contact with the
electrode, includes ions that are adsorbed onto the electrode
surface due to high electrostatic interactions, and the
outer, diffuse double layer, which behaves according to the
Gouy–Chapman model.
The classical EDL theory presented above was developed
for planar surfaces. Significant modifications of this theory
1566 J Solid State Electrochem (2011) 15:1563–1578
are necessary to account for the diffuse layer formed in a
meso- and micropores interface between a solid porous
electrode, and the electrolyte solution.
Extensive work has b een devoted over the years to EDL
phenomena related to porous electrodes. Evidence was
found that EDLs exist within the porous carbon as long as
the pores are sufficiently large enough to accommodate
hydrated ions [41, 47–49].
Theassociatedlargesurfaceareaofporouscarbon
materials, together with their good electrical conductivity,
enables the use of amorphous carbon elec tro-adsorption ,
namely the potential-induced adsorption/desorption of
ions at the EDL within the pores. Figure 2 represents the
EDL formation, its equivalent electrical circuit analog, and
the potential profile on a porous carbon electrode in an
electrolyt e solut ion. Due to the high internal surf ace area ,
very high specific capacitance can be realized (<100 Fg
−1
carbon depending on the level of porosity and the nature
of the electrolyte solution) [50]. This high specific
capacity can be e xploited for e nergy storage in EDL
capacitors, as discussed below, and for the removal of ions
from saline water, i.e., water desalination by capacitive
deionization (CDI).
Both the energy storage density and the water desalting
capacity related to acti vated carbon electrodes increase with
increase in their specific surface area. Hence, there is strong
incentive to develop smaller and more extensive pores [51].
However, the reduction of the pore size is limited by the
size of the adsorb ate (i.e., it must be larger). Also,
decreasing the pore size slows down the kinetics of the
electro-adsorption. Therefore, the optimization of the pore
system of adsorbing carbons entails a tradeoff between
capacity and kinetics. Another inherent tradeoff between
the parameters of the processes of electro-ad sorption is the
extent of burn-off (activation) which increases the surface
area at the expense of the structural integrity, mechanical
stability, and electrical conductivity of the carbon electrode.
In addition to the use of amorphous carbon materials for
electro-adsorption applications, three additional graphitic allo-
tropes of carbon have been investigated more recently, namely
carbon nanotubes (CNT) [52, 53], fullerenes, and graphenes
[54]. These forms can be utilized as the sole active electrode
material, or incorporated as additives to the formulation of the
active material of electro-adsorption electrodes.
Due to their unique morphology and extended graphitic
layers [52–55], CNTs and graphene have both been
ø
ø
E
ø
S
χ
Hel.
Diffuse
Layer
+
Cathode
Anode
Fig. 2 Cartoon representation
of the EDL formation, its
equivalent electrical circuit
analog, and the potential profile
on a porous carbon electrode in
an electrolyte solution
J Solid State Electrochem (2011) 15:1563–1578 1567
explored as candidate electrode materials which may
overcome the drawbacks of conventional carbon materials
(e.g., hindered electrical conductivity and m echanical
strength). CNTs (especially multiwall) may be characterized
by exceptional conduction and mechanical properties which
allow them to be used directly as three-dimensional
supports for active materials. Additionally, it was demon-
strated that the open mesoporous network formed by the
entanglement of nanotubes allows the ions to diffuse easily
to the active surface of the composite components. The two
latter properties (high conductivity and mesoporous struc-
ture) may be useful in lowering the equivalent series
resistance (ESR) and consequently increasing the power
of the device.
The surface area of a single graphene sheet is 2,630 m
2
/g
[56], substantially higher than values deriv ed from BET
surface area measurements of activated carbons used in
current EDL capacitors. The surface accessibility of
adsorbates to individual graphene sheets does not depend
on the distribution of pores in a solid matrix in order to
reach large surface area (as is the case for activated
carbons); rather, every graphene sheet can be utilized for
electro-adsorption of ions (to both sides) in various
electrolyte solutions. Hence, electrodes comprising gra-
phene sheets may enable the full utilization of their high
surface area for capacitive interactions or red-ox reactions,
while benefiting from the good electrical conductivity and
excellent mechanical strength.
Surface groups on carbonaceous materials
Elementary carbon after alkali and alkaline earth metals is
probably one of the strongest reducing agents in chemistry
[1–3, 7]. At ambient temperatures, its oxidation is kineti-
cally hindered; therefore, it is apparently chemically stable.
However, at high temperatures, i.e., above 400 °C, its
reducing ability becomes very effective. For instance, most
of metals (except alkaline, alkaline earth, Al, B, and
precious metals) are produced from their ores by high-
temperature reduction processes with coal. Interestingly,
even prior to total burn-off of carbon to CO and CO
2
,
oxygen is chemibound to the surface of carbonaceous
materials, thus forming surface oxygen groups (SOG). The
adsorption properties of activated carbons may be highly
influenced by the presence of surface groups and their
nature [57]. Their immediate influence is the existence of
surface dipoles whose negative poles are facing the solution
side. Such surface dipoles shift the PZC of the electrodes to
more positive potentials, so that cation electro-adsorption
becomes preferable to anion electro-adsorption within the
electrochemical window of the EDL [7, 57]. Practically, all
of these carbon–oxygen surface groups can be removed
upon heating the carbon in anaerobic (or vacuum) con-
ditions to 1,000 °C. It is noteworthy that an opposite effect
exists when an activated carbon is treated with a reducing
agent such as hydrogen above 550 °C (creating C–H groups
with the positive poles facing the solution side). Such
surface groups (C–H) shift the PZC to more negative
values. The oxygen-containing surface groups on activated
carbons can be divided into three types, depending on the
nature of the C–O bonds:
1. Chemically fixed groups (e.g., carbonyl) which are
degassed as CO, only upon heating the carbon to above
800 °C. These groups are electrochemically inactive,
and their main effect is a shift of the PZC.
2. Carboxylic type groups which provide surface acidity.
These are degassed at above 400 °C mostly as CO
2
.
3. Surface groups with electrochemical red-ox activity such
as quinone/hydroquinone moieties [7]. Such surface
groups are easily recognized by the electrochemical
response of the carbon electrodes (e.g., a couple of
reversible peaks in voltammetri c measurements). It
should be noted that these red-ox sites are active only
under acidic conditions (solutions of low pH).
Electro-adsorption into porous carbon electrodes
The quantification and characterization of the electro-
adsorption capabilities of porous carbon electrodes, in
either separation processes or in energy storage devices,
should be divided into both thermodynamic and kinetics
aspects. The capacitance of electro-adsorption devices is
reported either per weight or per volume, depending on the
particular application being considered. Such capacitance
values represent the possible energy density relevant to a
device from a thermodynamic point of view. However, for
many applications, the power density should also be taken
into account, as it relates to the kinetics of the electro-
adsorption processes.
A few important parameters related to the electrodes'
physiochemical properties eventually determine the power
and energy density of devices compr ising them.
The ratio between pore and ion size
In the case of volume-based optimization, the goal is to
attain as large a surface area per volume as possible. This
requires as great a subdivision of the solid as possible,
resulting in smaller pores. In other words, an increasingly
larger surface area can be packed into a unit volume of a
porous electrode as the pores are smaller. It is important to
emphasize that the relationship between the electrode pore
size and the ionic dimension of the solution species is very
important for electro-adsorption performances of activated
carbon electrodes [58]. It has been demonstrated by many
1568 J Solid State Electrochem (2011) 15:1563–1578
studies that upon charging the EDL of porous electrodes,
the ions enter into pores whose size conforms to the
hydrated state of the ions (this may be denoted as the
effective ion size). In the case of pores that are significantly
larger than the effective ion size, the specific EDL
differential capacity of the porous electrodes (Fg
−1
or F/cc)
is proportional to the overall pore surface.
As stated earlier, there is a lower limit to the desirable size
of the pores, which corresponds to their accessibility by the
ions from the solution bulk outside the pores. As the average
pore size approaches the size of the ion, the electro-adsorption
kinetics becomes hindered. When the average pore size is
smaller than the smallest ions size in solution, no ion electro-
adsorption into the pores can take place. As is the case with
adsorption from the gas phase, an ideal situation can be
attained when the average electrode pore size is slightly larger
than the ion size, enabling ion interactions with the pore walls.
Thus, an optimal capacity can be achieved with no adverse
effects on the electro-adsorption and desorption kinetics.
The effe ctive ion size plays a most important role in
determining transport phenomena in both bulk electro lyte
solutions and in the course of electro-adso rption processes. In
fact, consideration of the effective ion size goes back to the
early corrections for finite-size charge carriers in the Debye–
Huckel theory for po int charge electrolytes. Many studies have
been carried out to determine optimal porous structures of
activated c arbon elec trodes in order to achieve optimal electro-
adsorption performances. Gogotsi et al. [59]demonstratedan
anomalous increase in EDLC using new, sophisticated carbon
materials with well-tuned porous morphologies and tuning the
pores to be slightly smaller than the solvated ion, hence
enabling distortion of the hydration shell.
In order to understand the tradeoff between kinetics and
thermodynamics, we recently reported on a comparison
among various carbon materials with different porous
morphologies, indicating the feasibility and beneficial use of
hierarchical, fractal morphologies of porous structures within
activated carbon electrode [me rate]. We also recently
demonstrated an effective utilization of in situ electrochemical
quartz crystal microbalance (EQCM) measurements as a new
tool for investigating electro-adsorption phenomena. These
studies revealed the nature of ionic fluxes during electro-
adsorption and d esorption processes and how they are
affected by the electrode micro/mesoporous structural param-
eters and by unique ion–pore interactions [60].
Efforts have been devoted to understanding the impedance
behavior of activated carbon electrodes in electro-adsorption
processes. It was demonstrated that the shape, size, and
volume of the pores, as well as the pore size distribution, all
have serious effects on the impedance of these systems, and
therefore on the kinetics of electro-adsorption [50, 61, 62].
Additionally, it was demonstrated that the presence of
heteroatoms (such as nitrogen atoms) in carbon electrodes'
structure may have significant influence on the nature of
electro-adsorption processes. The presence of surface groups
on carbon electrodes also has a pronounced influence on
their electro-adsorption processes [50].
The electron conductivity of porous activated carbon
electrodes
Upon discussing and analyzing electro-adsorption processes
into activated carbon electrodes, most attention is focused on
the solution side and on the transport phenomena related to
ions. Although amorphous carbons are very good electron-
conducting materials, as they are more activated (i.e., more
porous), their electrical conductivity may become worse
because the cross sections for electron tran sfer in their bulk
structure become narrower [50]. The correlation between
activation and electrical conductivity of porous carbons and
the impact of electro-adsorption on the electrical conductivity
of activated carbon electrodes has been studied and reported
[7, 63]. For instance, it was found that annealing processes of
activated carbon at high temperatures under inert atmosphere
increase their electronic conductivity rema rkably, not at the
expense of the capacity of electro-adsorption processes into
them [7, 50]. Also, when highly porous active mass have to
be used (in order to obtain maximal capacity), whose porosity
is at the expense of electronic conductivity, it is possible to
use composite electrodes that contain in addition to the active
mass, additives such as graphite, carbon black, or CNTs
(maybe even graphene sheets) that maintain good enough
electrical integrity for the entire composite electrode. The
advantage of CNTs and graphene sheets as conductive
additive relies on the fact that these materials have high
electrical conductivity, very good electrochemic al stability,
high intrinsic surface area, and high aspect ratios, which
enables good electrical contact with particulate active mass.
Recently, it was demonstrated that CNTs or graphene sheets
can be incorporated into a carbon precursor (e.g., a polymer) for
activated carbon electrodes [64–68
]. Then, carbonization and
further activation of the amorphous carbon matrix so formed,
leaving the CNT or graphene components unaffec ted. This
results in the formation of highly robust and electronically
conducting activated carbon electrodes with improved me-
chanical and electrical properties (despite the highly porous
structure). In recent years, we see more and more publications
reporting on the use of CNTs and graphene sheets as stand-
alone active electrode materials for EDLC applications.
On the relation between EDL charging and the electronic
conductivity of activated carbon electrodes
The EDL charging of activated carbon electrodes may affect
their electrical conductivity over wide potential ranges [63,
68, 69] (see for instance Figs. 10 and 16 in [70]). Normally,
J Solid State Electrochem (2011) 15:1563–1578 1569
in the absence of sieving effects, the electrical conduc-
tivity of activated carbon electrodes shows a parabolic
dependence on the electrode potential with a minimum at
the PZC. This is bec aus e electro-adsorp tion intensifi es
accumulation of charge carriers with the opposite sign, at
the electrode (solid) side of the EDL. The minimum in
electrical conductivi ty measured usually at electrodes'
PZC is because at that potential electro-adsorption (and
hence its effect on the electrode side of the EDL) is
minimal. Hence, electrical conduct ivity measurements of
activated carbon electrodes as a function of potential,
during electro-adsorption processes can serve as a method
for determining their PZC. See further detailed discussions
on these phenomena in [63].
EDL applications
The high EDL capacitance of activated carbon electrodes
has led to the successful development of commercial
carbon-based electrical double-layer capacitors (EDLCs).
These capacitors are electrochemical energy storage device s
that have higher energy densities than electrolytic capaci-
tors and h igher power densities than rechargeable batteries.
Because only electrostatic interactions exist between the
electrodes and the solution species, with no involvement of
red-ox reactions, the electro-ad sorption/desorption cycles of
EDLC systems are highly reversible, with practically no
capacity fading. Typical voltammetric and the chrono-
potentiometric responses of activated carbon electrodes in
electrolyte solutions are presented in Fig. 3a, b.
The electro-adsorption of ions from salt-c ontaining
aqueous solutions onto activated carbon electrodes is also
the basis of water desalination by capacitive deionization
(CDI) processes. In fact, impressive electro-adsorption
capacities (1–2.5 mE/g), compa rable to the capaci ty of ion
exchangers, can be achieved in CDI processes. Short
historical descriptions of these applications, the state-of-
the-art (present) devices and developments, and some
future insights are presented below.
EDLC (called also super- or ultracapacitors), past
and present
The most common and important electrochemical devices
for energy storage and conversion devices are batteries. In
fact, batteries can be considered as the most important
application (and success) of modern e lectrochemistry.
Especially impressive in this respect is Li ion battery
technology that conquers more and more markets and is
currently at the focus of R&D into electric vehicles (EV).
EV applications expose the limitations of rechargeable
batteries in terms of cycle life, safety features, and p ower
density (all resulting from the fact that batteries involve
complicated red-ox reactions, in which three bulks (electrodes
and electrolyte solution) and two interfaces have to work
simultaneously with no side reactions).
These limitations have focused attention on EDLCs as
complementary device s for energy storage and conversion
[63–69]. The most effective way to judge and compare
energy storage and conversion devices is by the so-called
Ragone charts, in which the power density of various
devices is plotted as a function of energy density (see an
example in Fig. 4). As demonstrated in Fig. 4, EDLCs may
have much lower energy density (by orders of magnitudes)
than advanced rechargeable batteries due to the lower
energy that can be involved in capacitive, electrostatic
interactions, compared to red-ox reactions. However, the
power density of EDLCs is generally much higher than that
of advanced batteries, and in addition, their cycle-ability is
much longer as well, because electrostatic interactions are
very fast and are not supposed to affect the electrodes'
structure at all. Hence, there are many designs of power
sources, coupling batteries and EDLCs, in which the energy
should be provided by the battery and high power by the
capacitor. Consequently, we see in recent years accelerated
R&D related to EDLCs and their components and also
emergence of commercial EDLCs.
ECs have been studied for many years. The first patents
date back to 1957, in which a capacitor based on high
surface area carbon was described by Becker [71]. In 1969,
the first attempts to market such devices were undertaken
by SOHIO. However, it was only in the 1990s that EDLCs
became important in the context of hybrid electric vehicles.
A first DOE (US) EDLC development program was
initiated in 1989. In parallel, many research groups around
the world have struggled to develop novel carbon electrode
materials for EDLCs with optimized physicochemical
properties from a variety of carbon precursors using various
carbonization and activation procedures. Matching opti-
mized electrolyte solutions to EDLCs is also highly
important [70, 72–76]. In aqueous solution s, it was possible
to obtain very high capacity in electro-adsorption processes
into activated carbon electrodes (up to 350 F/g, corresponding
to 80–90 mAh/g, in acidic solutions). However, since the
energy density of EDLC is proportional to E
2
(the cell
voltage), there is a strong incentive to develop non-aqueous
EDLCs to which potential up to 3 V can be applied and even
EDLCs with ionic liquids, whose electrochemical window
may reach 4 V (see further discussion below). Using non-
aqueous solutions means one can achieve a much lower
EDLcapacity(e.g.,upto150F/gcomparedto>300F/g
obtainable in aqueous solutions) and also slower kinetics.
However, in cases where the energy density of the EDLCs is
important, non-aqueous systems are better due to the high
voltage, despite the lower EDL capacity.
1570 J Solid State Electrochem (2011) 15:1563–1578
Hence, the tradeoff between the energy density and the
electro-adsorption time constants of the device (e.g., higher
specific surface area means higher capacitance, but lower
electrode conductivity, non-aqueous solutions means higher
voltage but lower adsorption kinetics) is an important
design considera tion. The e lectrochemical stability of
carbon electrodes is unique. Theoretically, activated carbon
electrodes may reach electrochemical windows wider than
5 V (slightly affected the presence of surface groups)
limited only by the stability of the electrolytic medium.
Although the electrochemical window of water is 1.23 V,
the practical operational voltage of aqueous EDLCs should
be below 1 V, in order to avoid parasitic reactions,
especially at elevated temperatures. Non-aqueous solutions
for EDLC applications should possess high ionic conduc-
tivity in order to maintain a reasonable rate capability for
these devices (always lower compared to aqueous syst ems).
Relevant solvents are alkyl carbonates, esters, and acetoni-
trile (ACN). Since in the presence of metallic cations these
solvents are reduced to form insoluble species that can
precipitate and block the electrodes pores, tetra-alkyl
ammonium-based electrolytes are used (e.g., tetraethyl
ab
ø
t
Capacitor
ECs
Battery
-200
-150
-100
-50
0
50
100
150
200
250
00.511.522.533.5
IL1/AN25%
organic
acidic
E(V)
C(F/g)
Ideal capacitor
EDL capacitor
Zimg
Zreal
ESR
Distribution
resistance
c
[Ω]
[Ω]
Fig. 3 Typical responses to electrochemical measurements. a Cyclic
voltammetric in aqueous, organic, and ionic liquids electrolyt e
solutions Reused with permission from Elzbieta Frackowiak, Grzegorz
Lota, and Juliusz Pernak, Applied Physics Letters, 2005, 86, 164104.
Copyright 2005, American Institute of Physics. b Chrono-potentiometric
discharge profile of different devices. c Electrochemical impedance
spectroscopy responses (Nyquist plot) of activated carbon electrodes in
electrolyte solution
J Solid State Electrochem (2011) 15:1563–1578 1571
ammonium tetrafluoroborate, TEABF
4
)[70, 73–76]. EDLCs
loaded with alkyl carbonates or ACN and electrolyte such as
TEABF
4
can work in a practical potential domain up to
2.5 V. The voltammetric response presented in Fig. 3.shows
a compa rison between the electroc hemical window of
activated carbon electrodes in various electrolyte solutions.
A further increase in EDLC voltage range can be
achieved using i onic liquids (ILs) [72]. ILs based on
derivatives of imidazolium, piperidini um, and pyrrolidi-
nium cations (with anions such a s BF
4
-
,(CF
3
SO
2
)
2
N
-
)
may demonstrate electrochemical windows wider than 5 V.
They have high thermal stability and negligible vapor
pressure which implies good safety features [77–79].
However, they are often characterized by a high viscosity
which complicates the wetting of the carbon materi als,
especially those with a highl y de vel op ed m i cr opo ro si ty
(which means relatively l ow rate capability). The conduc-
tivity of ILs is also lower than common non-aqueous
electrolyte solutions. Hence, since the ions in non-aqueous
systems are more voluminous than those i n aqueous
systems, the po rosi ty of a ctiv ate d carbon electrodes for
non-aqueou s EDLCs has to be designed and adjuste d
accordingly.
Carbon materials for EDLCs, past and present
Starting with carbon black (CB), this is a group of materials
with nearly spherical particles of sub-micrometric and even
nano-metric size [7]. The main stages of carbon black
formation in the thermal decomposition or partial oxidation
of their hydrocarbon precursors are the formation of
polyaromatic macromolecules in the vapor phase, followed
by their nucleation into droplets and final conversion into
carbon particles. The key properties of carbon black are
considered to be fineness (primary particle size), structure
(aggregate size/shape), porosity, and surface chemistry. The
morphology and physiochemical properties of those par-
ticles\particle agglomerates vary with feedstock and man-
ufacturing conditions, and they are u sually classified
according to their method of preparation or intende d
application. The surface-area (BET) of carbon blacks
covers a wide r ange, i.e., from <10 to greater than
1,500 m
2
/g [7, 80]. The porous net of this material is
generally considered to be more accessible than other forms
of high surface-area carbon. EDLC electrodes have been
produced from high surface-area carbon black (using a
chemical binder such as polyvinyl difluoride, PVdF) with
specific capacitances of up to 250 F/g, corresponding to
double-layer capacitances in the range of 10–16 μFcm
−2
.
The main draw back of CB as an active mass in EDLC
electrodes is their [2, 81, 82] relatively small particle size,
which requires the use of a large amount of binder in order
to compose mechanically stable electrodes. Such electrodes
may suffer from low electrical conductivity and volumetric
capacitance.
Pekala et al. synthesized carbon aerogels from the carbon-
ization of organic aerogels based on a resorcinol–formaldehyde
Specific Power (Wkg )
Specific ener
g
y (Whk
g
)
10
5
10
4
10
3
10
10
2
11010
1
10
2
10
2
10
3
1
Fig. 4 Ragone charts, in which
the power density of various
devices is plotted as a function
of energy density; inserted
numbers are for remarkable
results reported for ECs (numbers
are in correlation with the
references)
1572 J Solid State Electrochem (2011) 15:1563–1578
gel [83–85]. Later studies presented the synthesis of carbon
aerogel from various other organic aerogels (e.g., phenol-
furfural, phenol-resorcinol-formaldehyde, melamine-
formaldehyde, polyurethanes, polyureas, and polyvinyl chlo-
ride) according to the same procedures [83–93]. Generally, the
porous organic aerogel is prepared via polymerization of the
monomers into cross-linked polymer clusters forming wet
gels. These gels are then dried under conditions that prevent
the collapse of the porous structure (e.g., CO
2
supercritical
drying, solvent exchange and controlled evaporation). The
aerogels are then carbonized at elevated temperatures. Carbon
aerogels can be prepared in the form of monoliths, powders,
microspheres, and thin film composites.
Electrochemical studies on carbon aerogels have
reported that their specific capacity is correlated with their
true mesoporous surface-area rather than with their total
BET surface-area. The corresponding double-layer charge
storage capacity of carbon aerogels is typically 18 μFcm
−2
.
Their surface area can be increased by activation of the
carbon aerogel, thus forming activated carbon electrodes
with fractal porous structures composed both of meso- and
micropores [94].
Glassy carbon (GC), also referred to as vitreous carbon,
is a typically hard solid material [7]. Its production consists
of the thermal degradation of selected organic polymers
similar to those used for the carbon aerogel [7, 95]. The
resin is treated in three stages and cured very slowly, then
carbonized and heated to elevated temperatures (typically
1,800 °C). Glassy carbo n has a very low electrical
resistivity (3–8×10
−4
Ωcm) [96] and is therefore particu-
larly suitable for high-power EDLCs that require a low
internal resistance [7, 97]. Another attractive feature of
glassy carbon is the possibility to produce it as free-standing
films, thin sheets, or powders. During the special carboniza-
tion processes that form glassy carbons, some internal,
isolated porosity can be developed within the carbon. Such
pores in GC can be opened by thermal oxidation processes
(activation) to produce a material with a high specific surface
area that is well suited for use as an EDLC electrode material.
Volumetric surface areas of ∼800 m
2
cm
−3
and double-layer
capacitances of ∼20 μFcm
−2
have been achieved for
thermally oxidized glassy carbons [97–101].
Templated porous carbon material can also be utilized
for electroadsorption applications [102, 103]. The general
template synthetic procedure for porous carbons includes
the following steps: (1) preparation of the carbon precursor/
inorganic template composite, (2) carbonization, and (3)
etching out the inorganic template. Various inorganic
materials, including silica nanoparticles (silica sol), zeolites,
anodic alumina membranes, and mesoporous silica materi-
als have been used as templates [102–107]. The carboni za-
tion and subsequent removal of the templates generate
porous carbons with interconnected pore structures and
relatively uniform pore sizes. Various carbon structures
with well-controlled micropores, mesopores, and/or macro-
pores produced from different types of te mplates and
various template carbons have been studied for EDLC
applications. A functionalized microporous carbon material
was obtained by using zeolite Y as a template, and the
resulting carbon material possessed a high gravimetric
capacitance of about 340 F/g in aqueous electrolyte with
good cyclability (over 10,000 cycles) [108]. Yamada's
group has also synthesize d ordered porous carbons con-
taining meso/macro/micropores with large surface areas
using a colloidal-crystal templating technique. A high EDL
capacitance of 200–350 F/g was achieved in an acidic
electrolyte solution [109].
Another important self-supporting carbon material is
activated carbon fibers (ACFs). The initial processing steps
of most carbon fibers involve stabilization (heating organic
fiber precursors, e.g., polyacrylonitrile, Rayon, cellulose,
phenolic resins, and pitch-based materials), in air to
temperatures up to 300 °C to render the fibers thermosetting
via cross-linking, followed by heating (inert atmosphere) to
temperatures <1,000°C to convert the stabilized fibers to
carbon. ACFs with very high associated surface areas (up to
2,500 m
2
/g) are now commercially available in many
forms, such as tow (bundles), chopped fiber, mat, felt,
cloth, and thread. The majority of the pores in ACFs are
micropores [1–3, 110]; hence, some hindrance in the rates
of electro-adsorption may be expec ted. This kinetic
limitation is compensated, however, by the thin fiber
dimensions, which means that the porosity of ACFs is
largely situated at the surface of the fibers and thereby
provides good accessibility of solution species to the active
sites. Additionally, it was shown that both the pore diameter
and pore length can be more readily controlled in ACFs,
compared to regular activated carbon materials [1–3, 7,
111]. These features, together with the ease of preparation
of electrodes comprising such fibers, make ACFs a very
attractive electrode material (high adsorption capacities and
rates). On the other hand, the cost of ACF products is
generally higher than that of activated carbon powders.
Some non-graphitized carbons were also tested for EDLC
applications. Among this group of carbons, however, few
works with outstanding results were published, especially
with carbonized PVDC [111, 112].
One of the most important features, unique to PVDC-based
carbons, is that carbonization of this polymer produces
carbons with high surface area, up to >800 m
2
/g. Remarkable
capacitance values of 350–400 F/g were obtained for those
carbons, even without activation of the carbon [111, 112].
CNTs and nanofibers are also interesting and maybe
promising candidates as electrode material for EDLC and
other energy-storage devices. Single-walled (SWNT) and
multi-walled nanotubes (MWNTs) have been studied as
J Solid State Electrochem (2011) 15:1563–1578 1573
electrode materials in both aqueous and non-aqueous electro-
lyte solutions by many groups worldwide. Generally, the
specific capacitance of unmodified CNTs has been shown to
be highly dependent on their morphology and purity. The
values of their specific capacitance vary typically from 15 to
80 F/g for pure CNTs without amorphous carbon as additive.
Specific capacitance of CNTs can be increased to 130 F/g by
subsequent oxidative treatment (e.g., with nitric acid) which
modifies the surface texture of the CNTs and introduces
additional surface functionality, which contributes to the
capacitance via pseudo-capacitance behavior of the function-
alized surface of the CNTs [95].
Kay Hyeok An et al. [113] have investigated the key
factors determining the performance of EDLC using
SWCNT electrodes. By heat treatment of SWCNT electro-
des, they managed to reduce the CNT-electrode resistance,
demonstrating a maximal specific capacitance of 180 F/g
with a large power density of 20 kWkg
−1
at an energy
density of 6.5 Whkg
−1
(1 in Fig. 4).
Niu et al. [114] produced catalytically grown MWNTs,
subsequently treated with nitric acid and formed into
electrodes that consisted of freestanding mats of
entangled nanotubes with an increased surface-area of
430 m
2
g
−1
. The specific capacitance of the nanotube mat
electrodes in sulfuric acid was determined to be 102 F/g
(at 1 Hz), and this corresponds to a double-layer capacity
of 24.2 μFcm
−2
. The same cell also had an estimated
power density of >8 kWkg
−1
.
Chunsheng Du and coworkers, using an electrophoretic
deposition technique, succeeded in fabricating CNT thin
films. The supercapacitors built from such thin film
electrodes have a very small equivalent series resistance
and a high specific power density (over 20 kWkg
−1
)[115].
By activation of MWCNTs with potassium hydroxide,
Frackowiak et al. managed to increase the BET surface-area
of MWNTs from an initial value of 430 to 1,035 m
2
g
−1
.
While the product still maintained a high degree of
mesoporosity, the activation process also introduced con-
siderable mic roporosity to that active mass. The specific
capacitance of the material was 90 F/g (8.7 μFcm
−2
)in
alkaline media [116].
Composite carbon material synthesized by the carbon-
ization of polymer/CNTs composites are also addressed as
active material for EDLC. An interesting CNT–aerogel
composite material was synthesized by uniformly dispers-
ing a carbon aerogel throughout the CNT host matrix
without destroying the integrity or reducing the aspect ratio
of the CNT. A high specific surface area of 1,059 m
2
g
−1
and extremely high specific capacitance of 524 F/g were
obtained, but at the expense of a tedious preparation
pathway [ 117].
Activated polyacrylonitrile (PAN)/carbon nanotube
(CNT) composite film-based electrodes have been prepared
by chemical activation with potassium hydroxi de for
electrochemical capacitors. A maximum value of specific
capacitance of 302 F/g was achieved for the samples
activated at 800 °C. Energy density for PAN/CNT 80/20
sample when tested with ionic liquid/organic electrolyte
system was as high as 22 Whkg
−1
[118] (2 in Fig. 4).
A novel composite of single-walled carbon nanotubes
(20 wt.%) as scaffolding for single-walled carbon nano-
horns (80 wt.%) was recently presented as a supercapacitor
electrode with a high maximum power rating (1 GW/kg,
396 kW/l) exceeding power performances of any other
electrodes [119]. The high power capability of these
electrodes was attributed to the unique meso–macropore
structure of the material engineered. These novel composite
electrodes also exhibited durable operation (6.5% decline in
capacitance o ver 100,000 cycles) as a result of their
monolithic chemical composition and mechanical stability.
CNTs directly grown on conductive substrates can also
be considered as very interesting self-standing electrode
materials for EDLC applications. Indeed, there are recent
publications on CNTs grown on graphite and aluminum
foils tested as monolithic electrodes for EDLC applications
with promising performances [120, 121].
Graphene is the name given to a two-dimensional sheet
of sp
2
-hybridized carbon. Long-range π-conjugation in
graphene sheets yields extraordinary thermal, mechanical,
and electrical properties [6], which have long been the
subject of many theoretical studies and more recently
became an exciting area for experimentalists and various
application including EDLC.
Rao et al. have fabricated graphenes, prepared by three
different methods, as electrodes for electrochemical super-
capacitors [122]. The samples prepared by exfoliation of
graphitic oxide and by the transformation of nanodiamond
exhibit high specific capacitance in aqeous H
2
SO
4
, with
values reaching up to 117 F/g. Using IL-based electrolyte
solution, it was possible to operate grapheme-based EDLC
at 3.5 V demonstrating energy density close to 32 Whkg
−1
.
There are other recent publications [123 , 124]on
graphene-based EDLC, and it seems that extensive work
in this direction is in progress. In a recent pioneering work,
chemically modified graphene (CMG) was also tested as
active mass in electrodes for EDLCs [125].
Desalination by CDI, past and present
Currently, most of the electrochemical studies related to
activated carbons are connected to the field of energy
storage and conversion, mostly as EDLC electrodes but
also for batteries and fuel cells electrodes (being the c urrent
collectors and suppor t of the catalysts necessary to conduct
effective oxygen electro-reduction and fuel electro-oxidation).
1574 J Solid State Electrochem (2011) 15:1563–1578
However, another potentially highly important use of
activated carbon electrodes electro-adsorption capability
is for water desalination (so-called capacitive deioniza-
tion—CDI). A CDI cell is principally composed of two
activated carbon electrodes between or through which the
saline wat er flows. When the cell is charged, ions are
removed by electro-adsorption onto the electrodes, result-
ing in a diluted aqueous solution as the product.
Subsequent discharging (shorting) the cell regenerates the
electrodes via desorption of the ions, resulting in a flow of
concentrated (waste) solution [126].
At first glance, this operation looks simple, involving
purely capacitive interactions of ions which are electro-
adsorbed/desorbed into (and out of) the porous electrodes.
However, CDI processes may be highly complicated due to
the breakdown of the permselectivity of the electrodes:
application of potentials leads to simultaneous adsorption of
counter ions and desorption of co-ions [127]. The latter
process may significantly reduce the charge efficiency of
electrochemical desalination processes by CDI. The extent
of this problem depends on the electrode structure and the
potentials applied to the cells. For instance, it is possible to
considerably increase the charge efficiency of CDI processes
by discharging the CDI cells to certain low positive potentials
but not to zero (i.e., avoid full shorting) [128].
Depending on impurities in solutions, functional groups
on the electrodes (leading to shifts in their PZC), and the
potential applied to CDI cells, they may behave asymmet-
rically even if their electrodes were initially identical [7].
Hence, there is a general challenge in R&D of CDI
processes: increasing to maximum the charge efficiency of
water desalination with a minimal compromise on the
capacity of salt removal per cycle. Meeting this challenge
requires appropriate electrode design (porosity, surface
groups) and judicious application of potential on CDI cells.
The early studies of CDI date back to the electrochemical
demineralization work of the Caudle and Johnson groups in
the late 1960s and early 1970s [129]. The introduction of the
concept of CDI and the use of porous carbon electrodes for
water desalination were first performed by Caudle et al.
[130]. Later, Johnson et al. studied CDI as a reversible
process. Funds were dedicated to their work, for the
theoretical understanding of the basis of CDI by parametric
studies of the electro-adsorption from flowing solution on
various carbon electrode materials. On the basis of this
research, a comprehensive theoretical analysis of ion
adsorption into porous electrodes was published by Johnson
and Newman [129, 131]. Nevertheless, due to stability
limitations of the relevant carbon electrodes, mainly the
anode side (i.e., the positive electrodes), the research was
postponed [132].
During the late 1970s and the early 1980s, only Soffer
and Ore n were investigating the field of CDI. They studied
fundamental aspects of electro-adsorption phenomena on
various electrodes, and also established and modeled new
mode of solution flow inside CDI cells, which they referred
to as electrochemical parametric pumping [126]. It was
only in the mid 1990s that a new version of CDI device was
established. Farmer and coworkers at Lawrence Livemore
National Labs utilized new high surface area, high
conductivity carbon conducting carbon formed as aerogel
electrodes for water desalination by CDI [133]. A recent
comprehensive review, with description of fundamental
aspects regarding electro-adsorption processes, and various
flow modes in CDI reactors, has been presented by Y. Oren
[132]. It is beyond the scope of this article to describe all of
these aspects. Our focus is mainly on the various porous
carbon materials used in this field.
Since the early work of the Caudle and Johnson groups,
substantial efforts have been made to develop new carbon
materials, which will be suitable for effective electro-
adsorption processes in CDI reactors. Although the main
physical phenomenon in both CDI and EDLC is electro-
adsorption; the conditions under which this process is
involved in CDI cells are significantly more complex
because the solution in the latter process is flowing through
the cell. Thereby, contaminants such as dissolved oxygen,
biological, and fouling agents are continuously introduced
into the cell and affect the electrodes. Red-ox reactions of
oxygen and water that may accidentally occur in CDI cells
lead to corrosion of the activated carbon electrodes. The
majority of current CDI electrode materials are high-surface
carbons in a variety of forms. As for EDLC applications,
the basic requirements of the electrodes are high available
specific surface area, optimized electric conductivity, and
high electrochemical stability. A simple electrode for the
electro-adso rption of charged species from aqu eous
solutions in CDI reactors was fabricated using pressed
activated carbon granules or ordered mesoporous carbon
microbead s synthesized by a modified sol–gel process
[128, 134].
Carbon aerogels were also used in CDI processes [135–
138]. Other electrode materials include activated carbon
cloth [139], carbon sheets, activated carbon cloth modified
by titania, carbon felt, carbon black, sintered activated
carbon, carbon nanotubes, and carbon nanofibers [132, 140].
Recent work demonstrated the feasibility of selective
desalination using carbon molecular sieve electrodes,
produced by CVD treatment of activated carbon cloth
[94, 110]. We also st ruggled with possible limitations to
the stability of activated carbon electrodes in prolonged
periodic CDI processes and explored ways to stabilize
them by pretreatments, judicious application of potential
to CDI cells and maintenance of the electrodes which
avoids the need for their replacement after long service
periods [141].
J Solid State Electrochem (2011) 15:1563–1578 1575
Activated carbon electrodes: future and concluding
remarks
Activated carbon electrodes, due to their high specific
surface area, unique stability, and good electrical conduc-
tivity, can be utilized as electro-adsorption electrodes for
electrostatic energy storage and conversion (e.g., EDLCs),
and for separation processes by electroadsorption, such as
water desalination by CDI.
In addition to their relevant physicochemical properties,
carbon materials can be produced at very reasonable costs
and can be obtained in various forms with reasonably
adaptable porosity and surface functionality.
In the ongoing efforts for R&D of sustainable and
renewable energy sources (e.g., solar, wind), we suffer from
a lack of suitable technologies for energy storage (the so-
called load leveling application). Here, the most important
properties are not energy density but rather superb stability,
very prolonged cycle life, and the use of safe and abundant
materials (since large devices for storing huge amount of
energy are needed). EDLC technology may be found to be
very suitable for such applications due to the impressive
cyclability, the abundance of carbonaceous materials on
earth, and the fact that carbons are safe and non-toxic
electrode materials (no adverse envir onmental aspects). The
high power capabilities of EDLCs make them attractive
also as elements in mobile power sources (e.g., for EV
applications). However, EDLCs have much lower energy
density than batteries, and their low energy density is in
most cases the factor that determines the feasibility of their
use for high powe r applications. A major future challenge
related to EDLCs and to R&D of their components is to
maximize the energy density of these devices, with a
minimal compr omise on powe r density. Some rece nt
advances have involved the engineering of nanoporous
carbons with tunable pore sizes to fit the size of ions in
selected electrolytes solutions, incorporating and/or modi-
fying carbon nanotubes and graphene sheets as critical
components in EDLCs and the use of organic and IL-based
electrolyte solutions for higher operating voltages.
Nano-architecture of electrodes can lead to further
improvements in power delivery of EDLC devices. This is
also an important future direction related to carbon
electrochemistry. There are so many carbon-based active
materials developed so far as well as so many electrolyte
solutions available. Hence, we need to develop judicious
and effective tools (theoretical and analytical) that will
provide guidance for appropriate and effective selection of
materials and fit them in an optimal way to the desired uses.
It is important to intensify theoretical work related to
porous electrodes and electro-adsorption processes. Re-
markable progress was demonstrated recently in a better
understanding of the intimate pore–ion interaction using
EQCM. This direction deserves more effort, especially in
taking into account viscoelastic effects by using this tool
not just in its gravimetric mode but rather working also in
the frequency domain (i.e., impedance-type measurements
and analysis). In recent years, we see more and more
attempts to develop composite electrodes that comprise
both activated carbons and electro-active materials which
possess surface red-ox behavior (e.g., nano particles of
transition metal oxides or electronically conductive poly-
mers). It is challenging to develop such compo site electro-
des that can close the gap between EDLC and battery
technologies in terms of energy density, but NOT at the
expense of superb cycle life and high power densi ty which
are the main advantages of EDLCs. In parallel, there is a
great challenge in R&D of novel electrolyte solutions,
especially ionic liquids and their blends that can provide the
widest electrochemical window and yet posses better ionic
conductivity and better wet porous structures than existing
currently tested ILs. Improved electrochemical capacitors
can be used in rechargeable power sources for many kinds
of portable devices. They can be integrated into smart
clothing, sensors, electronics, and drug delivery systems. In
some instances, they will replace batteries , but in many
cases, they will either complement batteries, increasing
their efficiency and lifetime, or serve as energy solutions
where an extremely large number of cycles, long lifetime,
and high power uptake and delivery are required.
Another field in which activated carbon electrodes can
be very important is separation processes. Electrochemical
water desal ination (by CDI processes) can be very suitable
for brackish water. It is possible to develop highly selective
water purification processes, in which special contaminants
can be remo ved. There are future challenges in electrode
design for water purification processes and the engineering
of effective integral systems for such processes which
include optimized cells, switching, sensing, and monitoring
devices.
Finally, it is important to note that this review is devoted
mostly to activated carbon electrodes with an emphasis on
two main applications. However, the electrochemistry of
carbonaceous materials is a much broader field that relates
to batteries, fuel cells, sensors, electro-catalysis in general,
electro-synthesis, and many more.
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