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Review on carbon-based composite materials for
capacitive deionization
Yong Liu,†
a
Chunyang Nie,†
a
Xinjuan Liu,
b
Xingtao Xu,
a
Zhuo Sun
a
and Likun Pan*
a
The last five decades have witnessed the rapid development of capacitive deionization (CDI) as a novel,
low-cost and environment-friendly desalination technology. During the CDI process, salt ions are
sequestered by the porous electrodes once exposed to an electric field. These electrodes, acting as an
ion storage container, play a vital role during desalination. In this review, various carbon-based
composite electrode materials, including carbon–carbon composites, carbon–metal oxide composites,
carbon–polymer composites and carbon–polymer–metal oxide composites, are systematically
presented. Applications of these carbon-based composite materials for the removal of the salt ions
from solution are demonstrated and they exhibit improved CDI performances compared with pristine
carbon electrodes.
1 Introduction
With an increasing human population combined with the
tremendous exploitation of water resources for household
applications, industry and agriculture, water shortage has
emerged as a major issue in the 21st century.
1–3
Although there
are some approaches, including economic use and recycling of
water for human and animal consumption that can mitigate the
problem to some extent,
3
alternative sources of freshwater are
required to address its growing need.
It is reported that nearly 98% of the water resources on earth
are either seawater or brackish water. Therefore, desalination
has become an important approach to ensure an adequate
supply of freshwater.
4
In recent decades, the most widely used
processes in desalination plants are reverse osmosis (RO) and
thermal separations, including multistage ash distillation,
multi-effect distillation and mechanical vapor compression.
4
Forward osmosis is a promising new process used for seawater
desalination, while there is a long way to go before its practical
application.
5
However, when the salt concentration of water is
below 5000 ppm, it becomes more advantageous to remove the
relatively few salt molecules from the saline water than remove
the water molecules from the saline water through water-
permeable RO membranes or thermal evaporation. To
produce freshwater from low-salinity water, several desalination
technologies, including electrodialysis (ED),
6
ion concentration
polarization,
1
desalination batteries,
7
microbial desalination
Yong Liu received his Bachelor's
degrees in Materials Chemistry
from Inner Mongolia Normal
University in 2011. Now, he is
pursuing his PhD under the
supervision of Prof. Likun Pan in
East China Normal University.
His research interests are the
synthesis of nanomaterials and
their applications in capacitive
deionization and supercapacitors.
Chunyang Nie received her
Bachelor's and Master's degrees
in Physics from the East China
Normal University in 2010 and
2013. Then, she took up a PhD
scholarship to work under the
joint supervision of Dr Emma-
nuel Flahaut and Dr Marc
Monthioux. Her current
research project deals with the
use of CNTs as templates for the
synthesis of conned 1D
nanocrystals.
a
Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of
Education, Department of Physics, East China Normal University, Shanghai,
200062, China. E-mail: lkpan@phy.ecnu.edu.cn; Fax: +86 21 62234321; Tel: +86 21
62234132
b
Center for Coordination Bond and Electronic Engineering, College of Materials
Science and Engineering, China Jiliang University, Hangzhou 310018, China
†The authors contributed equally to this manuscript.
Cite this: RSC Adv.,2015,5,15205
Received 13th November 2014
Accepted 15th January 2015
DOI: 10.1039/c4ra14447c
www.rsc.org/advances
This journal is © The Royal Society of Chemistry 2015 RSC Adv.,2015,5, 15205–15225 | 15205
RSC Advances
REVIEW
cells
8
and capacitive deionization (CDI),
9
have been developed.
Among these technologies, CDI, also known as electrosorption
or capacitive desalination,
10–14
has attracted more and more
attention, in recent years, because of its environmental friend-
liness, no secondary pollution and reduced energy consump-
tion.
9,15–18
For example, when compared with RO, CDI can
operate with low (sub-osmotic) feed pressures and in theory it
requires no membrane components.
19
The pioneering work of CDI can be traced in the mid-1960s,
which was conducted by the Caudle and Johnson groups.
20,21
However, wide attention was not drawn to this area until the
1990s when Farmer et al.
22,23
developed a highly-efficient CDI
device based on a carbon aerogel electrode. Then, for the last
two decades, CDI has entered a stage of rapid development due
to the increasingly serious water scarcity.
The principle of CDI is based on imposing an external elec-
trostatic eld between the electrodes in order to force charged
ions to move towards the oppositely charged electrodes, as
shown in Fig. 1. The charged ions can be attracted within the
electrical double layer (EDL) formed between the bulk solution
and electrode interface. When the electrical eld is cut offor
even reversed, the absorbed ions can be released into the bulk
solution and the electrodes are regenerated and ready for the
next cycle. ED is regarded as the closest cousin of CDI because
both of them operate with a direct electrical voltage to drive ions
onto the electrodes. However, the operational voltage in the CDI
process is usually below 1.2 V, which is considerably lower than
that used in the ED process. In addition, no membranes are
involved in the CDI process. According to the work of Anderson
4
in which these two technologies were analyzed comparatively
from the aspect of energy consumption, ED cannot compete
with CDI even in high concentration saline water.
As reported by Humplik et al.,
3
there are some major chal-
lenges for the development of economical CDI systems, including
high energy consumption related to parasitic reactions at the
electrodes, inadequate electrical connectivity to the high surface
area electrodes and the limitations of the electrode materials and
geometries that lead to trade-offs between maximizing the
surface area and minimizing the distance for ionic electro-
migration. To achieve better insight into identifying the optimal
electrode geometries and operating conditions in future CDI
systems, many studies have been devoted to explore an appro-
priate EDL model within the CDI electrode, as the classical elec-
trokinetic theory cannot be applied. Additionally, studies have
started to address the ion transport process model by coupling a
macroscopic description of the CDI process with a modied EDL
model within the electrode.
24–26
Despite these theoretical devel-
opments, signicant research efforts have mainly been focused
on the utilization of various types of materials for CDI electrodes.
Generally, CDI electrodes are made of porous carbon mate-
rials with high specic surface area and superior conductivity,
both of which will be of great advantage in absorbing large
quantities of salt ions for desalination.
27
To date, high surface
area carbon electrodes in a variety of forms: carbon aerogels
(CAs),
10,23,28
activated carbon (AC),
29–33
including activated
carbon cloth (ACC)
34,35
and activated carbon bers (ACFs),
36–38
carbon nanotubes (CNTs),
16,39,40
mesoporous carbon (MC),
41–44
carbon nanobers (CNFs)
45
and graphene,
46–48
have been
reported as CDI electrode materials. However, some carbon
materials suffer from problems, such as high manufacturing
costs, poor wettability or poor mechanical stability.
Carbon based composite materials that take advantage of
the physical and chemical properties of carbon and another one
or even two different constituent materials became popular for
CDI electrodes recently because of the relatively low adsorption
capacity of single component carbon material, which limits CDI
from being used for scaling-up applications.
49,50
These addi-
tional components in the composite, typically carbon, metal
oxide or polymer, can enhance the electrosorption capacity of
pristine carbon materials by adjusting the intrinsic pore struc-
ture or improving the surface chemistry with functional groups,
wettability and zeta potential.
Fig. 1 Schematic diagram of CDI.
Prof. Zhuo Sun received his PhD
in 1995 at Lanzhou University.
His research interest includes
thin lm/nanostructred mate-
rials processing and applica-
tions, vacuum/plasma systems,
LED, FED and solar cells.
Prof. Likun Pan received his PhD
in 2005 at Nanyang Technolog-
ical University, Singapore. His
research interests include the
synthesis and properties of
nanomaterials and their appli-
cations in capacitive deioniza-
tion, photocatalysis, solar cell
and Li ion batteries. He has
published more than 150 jour-
nal articles with over 3000
citations.
15206 |RSC Adv.,2015,5, 15205–15225 This journal is © The Royal Society of Chemistry 2015
RSC Advances Review
In this review, we have summarized the carbon-based
composite electrode materials for CDI reported in the litera-
ture to date and categorized them into four types: carbon–
carbon composite, carbon–metal oxide composite, carbon–
polymer composite and carbon–polymer–metal oxide
composite materials. These composite materials will be dis-
cussed in detail in this manuscript.
2 Carbon–carbon composite
2.1 CNTs based composite
An enormous amount of research has been triggered since the
rst discovery of CNTs in 1991.
51
The unique structure of CNTs
comprising of a class of engineered nanoparticles (NPs)
composed of extensive sp
2
carbon atoms arranged in fused
benzene rings gives them remarkable mechanical, electrical,
optical and thermal properties, which are in turn utilized in
various applications, such as aerospace,
52
medical,
53
energy
storage devices
54,55
and water treatment technologies.
56–58
Fig. 2(a) and (b) show typical scanning electronic microscopy
(SEM) and transmission electronic microscopy (TEM) images of
CNTs. The curled structure of individual nanotubes can be
observed. The nanotubes are highly entangled with each other
as a result of van der Waals forces of attraction and form an
interconnecting network microstructure, which provides many
tunnels for the solution to enter and allows hydrated ions to
easily move onto the surface of the CNTs.
39
In the last ten years, CNTs have been successfully introduced
into CDI systems as the electrode material and received special
attention for their exceptional capacity in capacitive desalina-
tion.
16,39,40,60–64
In order to further enhance the desalination
performance, combining CNTs with other types of carbon
electrode materials to obtain composites that inherit the merits
of both materials is regarded as an efficient alternative.
2.1.1 CNTs–AC composite. Shi et al.
65
fabricated CNTs–AC
composite electrodes and conducted a series of electrosorption
experiments using a ow-through capacitor apparatus (Fig. 3(a))
based on such composite electrodes. The commercial CNTs and
AC were mixed in different ratios and pressed using phenolic
resin as the binder. Aer carbonization at 850 CinanN
2
atmosphere, they were used as CDI electrodes with a size of
115 mm 75 mm. A 5000 mg L
1
NaCl solution was passed
through the apparatus at a ow rate of 20 mL min
1
, in which
the number of electrodes was 40. A voltage of 1.0 V was
imposed. The experimental results show that the composite
electrode containing 10 wt% CNTs exhibited the best perfor-
mance with greater than 90% removal efficiency (Fig. 3(b)) and
that the energy-consumption was reduced by about 67% when
compared with that of the AC electrode. As observed from
Fig. 3(c), a suitable amount of CNTs could bridge the macro-
pores, increase the number of mesopores and the specic
surface area of the composite. However, excessive CNTs
(>10 wt%) were not benecial for the electrosorption due to the
low specic surface area compared with that of AC. Further-
more, highly efficient regeneration of the composite electrodes
was easily achieved, as shown in Fig. 3(d).
2.1.2 CNTs–MC composite. Since Zou et al.
42
introduced
MC in the CDI eld, it has attracted considerable attention
because of its regular mesoporous arrangement, narrow pore
size distribution, high-specic surface area, chemical inertness
and high conductivity.
42
Recently, Zhang et al.
66
synthesized
CNTs–MC composite electrodes through an inorganic–organic
classic self-assembly route with sotemplates for CDI. In their
experiments, various quantities of CNTs were added in an
ethanol solution containing NaOH, phenol and formaldehyde
with triblock copolymer F127 as a template. The as-made CNTs–
copolymer mixture was calcined at 600 C for 4 h, and then
cooled down to room temperature under a protective N
2
atmo-
sphere. The TEM and SEM observations in Fig. 4(a) and (b) show
the 2D hexagonally ordered mesoporous channels and nano-
tubular morphology of the CNTs–MC composites. The CNTs
additive was inserted in the mesoporous structure to form a
conductive network, which could reduce the resistance of the
electrode although at the cost of its specic surface area. The
CDI experiments were conducted in a continuously recycling
system using 40 ppm NaCl solution as the feed solution. The
total solution volume was 35 mL and the mass of each electrode
was 1.2 g. Fig. 4(c) describes the electrosorption behavior of AC,
Fig. 2 SEM and TEM images of CNTs. (Reproduced with permission
from ref. 59.)
Fig. 3 (a) A schematic diagram of the flow-through capacitor appa-
ratus; (b) comparison of the removal characteristics of the CNTs–AC
composite electrodes; (c) SEM image of the composite electrode with
10 wt% CNTs; (d) removal and regeneration curve for the composite
electrode with 10 wt% CNTs. (Reproduced with permission from
ref. 65.)
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MC and the CNTs–MC composite with 10 wt% CNT. The elec-
trosorption capacity of the CNTs–MC was calculated to be
0.63 mg g
1
, which was considerably higher than those of MC
(0.54 mg g
1
) and AC (0.22 mg g
1
). The results proved that the
electrosorption capacity of MC could be enhanced with the
CNTs additive due to the formation of the conductive network
among the mesoporous structures.
Further experiments were carried out by same group
67
treating the CNTs–MC composites with KOH activation. It was
found that aer activation, new micropores were generated to
increase the surface area and extend the pore size and volume,
which was attributed to the interconnected micro/mesopores
being etched by KOH on the pristine mesoporous channels.
As shown in Fig. 4(d), the KOH activation improves the elec-
trosorption capacity of the CNTs–MC from 0.63 mg g
1
to
0.69 mg g
1
due to the well-connected micro/mesoporous
structure formed upon KOH activation, which facilitates ion
transportation and diffusion.
2.1.3 CNTs–CNFs composite. Another type of CNTs
composite electrode was developed by Pan et al.
69–72
using a low
temperature and low pressure chemical vapor deposition (CVD)
method in which acetylene was used as the carbon feedstock
and hydrogen as the carrier/dilution gas. The CNTs–CNFs
composite was directly grown onto a conductive layer as the CDI
electrode at a temperature of 550 C. This method can greatly
reduce the contact resistance and avoid the use of a binder,
which can result in the loss of specic surface area and simplify
the electrode preparation process. Fig. 5(a) and (b) show the
SEM and TEM images of the CNTs–CNFs lms. The diameters
of the CNTs and CNFs are about 15–50 nm and 80–130 nm,
respectively. The CNTs and CNFs are entangled and form a
continuous electroconducting network structure. Fig. 5(c)
demonstrates that most of the pores are smaller than 100 nm in
diameter and show two peak distributions of pore size at 4 and
30 nm, namely, the CNTs–CNFs composite lms are mainly
composed of mesopores (2–50 nm). Such a mesoporous
network structure ensures a low mass transfer and allows
hydrated ions to easily enter into the deep pores, which is
benecial for ion adsorption. The CDI experiments were con-
ducted in a continuously recycling system at a ow rate of 14 mL
min
1
using NaCl solution (40 mL) with an initial conductivity
of 100 mScm
1
. The CNTs–CNFs composite lm electrodes
exhibit a high electrosorption capacity (3.32 mg g
1
), which is
much better than those of conventional AC and MC electrodes,
and comparable to that of a CAs electrode (3.33 mg g
1
) when
the applied voltage was 1.2 V, although their specic surface
area was considerably smaller than these electrodes.
27,70
Furthermore, no decline in the desalination efficiency was
observed in the as-fabricated unit cell aer over 30 charge–
discharge experiments, demonstrating that the regeneration
was very efficient.
Aer that, several signicant aspects, such as the adsorption
isotherm, adsorption selectivity and adsorption kinetics were
investigated. It was found that the Langmuir isotherm could
describe the ion adsorption behavior inside the CDI, indicating
a monolayer coverage of ions on the electrode surface. The
kinetic and thermodynamic analyses indicated that the NaCl
adsorbed onto the CNTs–CNFs electrodes follow a pseudo-rst-
order kinetics model and was driven by a physisorption
process.
69,73,74
Further experiments on the ion removal selec-
tivity illustrated that the ion removal capacity of such meso-
porous CNTs–CNFs electrodes was strongly dependent on the
charge and radii of the ions. Taking cations, for example
(Fig. 5(d)), multivalent cations were preferentially adsorbed
from the aqueous solution while for cations with the same
Fig. 4 (a) TEM and (b) SEM images of the CNTs–MC composite (10
wt% CNTs); (c) comparison of the CDI profiles for the CNTs–MC
composite (10 wt% CNTs), MC and AC electrodes in a NaCl aqueous
solution. (Reproduced with permission from ref. 68.) (d) CDI perfor-
mance of the CNTs–MC electrodes before KOH activation (black line)
and after KOH activation (red lines) in a NaCl aqueous solution.
(Reproduced with permission from ref. 67.)
Fig. 5 (a) SEM and (b) TEM images of the CNTs–CNFs composite.
(Reproduced with permission from ref. 75.) (c) Pore size distribution of
the CNTs–CNFs composite. (Reproduced with permission from ref.
70.) (d) Electrosorption capacity and double layer capacitance of the
CNTs–CNFs composite electrodes in different cation solutions.
(Reproduced with permission from ref. 71.)
15208 |RSC Adv.,2015,5, 15205–15225 This journal is © The Royal Society of Chemistry 2015
RSC Advances Review
charge, the cations with a smaller hydrated radius would be
removed more effectively.
71
2.2 Graphene based composite
Graphene, an atom-thick two-dimensional (2D) carbon nano-
structure with novel physical and chemical properties, has
attracted extensive attention in many elds since 2004.
76–79
Chemical approaches via the reduction of graphite oxide (GO),
for the large-scale production of reduced graphene (RG) sheets,
have become a reality.
80
Theoretically, the surface area of a
single graphene layer can reach as high as 2630 m
2
g
1
and the
corresponding conductivity is 7200 S m
1
, substantially higher
than that of AC. These intriguing properties endow graphene to
be used as electrode material for energy storage devices and
CDI.
48,81
Pan et al.
46
rst introduced RG into CDI in 2009. The RG
was synthesized using the modied Hummers method and
hydrazine reduction. As shown in Fig. 6(a) and (b), the SEM and
TEM images of RG show that the corrugated and scrolled sheets
resemble a ower-shape and porous structure, which facilitates
the high storage capacity of ions from solution. The electro-
sorption capacity of RG was calculated to be 1.4–1.9 mg g
1
when the initial concentration of NaCl was 25 mg L
1
, which is
higher than that of AC under the same experimental condi-
tions.
46,47
However, an issue associated with the preparation of
RG via the oxidation and reduction of graphite is that incom-
plete chemical reduction or large aggregates are oen
observed.
82
Hence, the resulting graphitic regions are limited,
which is detrimental to carrier transport and conductivity due
to the disruption of the conjugated graphitic structure by
epoxide and hydroxyl groups on either side of the RG basal
plane.
83
Currently, a particularly attractive option is to design and
develop composites based on RG sheets to solve the above
problems.
84–89
Some attempts have applied RG based compos-
ites in the CDI eld, which are described in detail as follows:
2.2.1 RG–AC composite. Pan et al.
90
synthesized RG–AC
composites and studied their performance as CDI electrodes.
GO was prepared according to the modied Hummers method
by oxidizing graphite powder using a strong oxidizing agent
(concentrated nitric acid and concentrated sulfuric acid)
46
and
AC was oxidized in nitric acid. GO was mixed with oxidized AC,
and then reduced using hydrazine. A “plane-to-point”(RG to
AC) conducting network structure in the RG–AC composite, as
illustrated in the Fig. 7(a), was proposed. It can be clearly seen
that AC is wrapped in large exfoliated RG layers and the RG
sheets bridge the gaps between the AC particles. Such a “plane-
to-point”porous network structure not only improves the elec-
trical conductivity of the composite but also increases the
proportion of mesopores, which contributes to the enhanced
electrosorption capacity of the RG–AC electrode when
compared with the AC electrode, as shown in Fig. 7(a). The
electrosorption capacity was calculated by CDI experiments in a
continuously recycling system at a ow rate of 25 mL min
1
.
NaCl solution with an initial conductivity of 50 mScm
1
was
employed as the feed solution and a direct voltage of 1.0 to 2.0 V
was applied. However, the rate constant of electrosorption
kinetics for the composite electrode was lower than that found
for the AC electrode, as shown in Fig. 8(b). The possible
mechanism of ion transfer in the AC and RG–AC electrodes is
presented in Fig. 7(b). In RG–AC electrode, the “plane-to-point”
conducting network is benecial for decreasing the aggregation
of AC particles, and therefore enhances the pore availability. In
this case, the ions can be transported inside the RG–AC elec-
trode, resulting in a low rate constant. As for the AC electrode,
due to the presence of strong aggregation among the AC parti-
cles and a large number of micropores, the ions cannot easily
enter into the inside pores, resulting in a high rate constant.
2.2.2 RG–MC composite. Zhang et al.
91
fabricated a RG–MC
composite using an inorganic–organic classic self-assembly
route with sotemplates for CDI. RG nanosheets were
produced by the thermal exfoliation of GO and the preparation
procedure of RG–MC is similar to that of the CNTs–MC
composite described in Section 2.1.2. The RG–MC composite
exhibits mesoporous and paper-like structures, as shown in
Fig. 9(a). The well-dispersed RG nanosheets work as a conduc-
tive carrier to support the well-arranged mesoporous channels
on the surface. The surface area and conductivity are enhanced,
ensuring a low energy cost during the CDI process. The CDI
experiments were carried out in a continuously recycling system
in NaCl aqueous solution with an initial conductivity of
89.5 mScm
1
at an applied voltage of 2.0 V. The total volume
and ow rate of the solution were 40 mL and 25 mL min
1
,
respectively. The mass and size of the electrode was 1.0 g and
Fig. 6 (a) SEM and (b) TEM images of RG. (Reproduced with permis-
sion from ref. 46.)
Fig. 7 Schematic diagrams for (a) the synthesis of the RG–AC
composite and (b) the proposed electrosorption mechanism of ions
onto AC and the RG–AC electrodes. (Reproduced with permission
from ref. 90.)
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70 mm 80 mm, respectively. The electrosorption capacity of
the RG–MC (5 wt% RG) and MC electrodes were 0.73 and 0.59
mg g
1
, respectively, and both of them are considerably higher
than that of the AC electrode, as shown in Fig. 9(b). The
enhanced desalination performance of the RG–MC composite
can be attributed to the improved electric conductivity, high
and controllable specic surface area, as well as a uniform pore
size distribution.
2.2.3 RG–CNTs composite. Zhang et al.
84
rst reported the
fabrication of RG–CNTs composites with different proportions
of CNTs for CDI. In their work, GO was synthesized using the
Hummers method.
92
Then, various masses of CNTs were added
to the GO solution in an ice bath. The RG–CNTs were produced
using thermal exfoliation of the GO–CNTs at 300 C for 10 min
under an air atmosphere. From the TEM image shown in
Fig. 10(a), it can be seen that the tortuous CNTs are uniformly
dispersed onto the thin wrinkled RG surface to form a porous,
crumpled and loose architecture. The addition of CNTs into the
composites not only increases the specic surface area by
reducing the agglomeration of RG and improves the conduc-
tivity but also results in narrow pore sizes and an increased
proportion of mesopores. The CDI experiments were carried out
in a continuously recycling system using the RG–CNTs elec-
trodes (10 wt% CNTs) in NaCl aqueous solution with an initial
conductivity of 57 mScm
1
at an applied voltage of 2.0 V.
The total volume and ow rate of the solution were 35 mL
and 25 mL min
1
, respectively. As shown in Fig. 10(b),
the composite shows a higher electrosorption capacity
(1.41 mg g
1
) when compared with pure RG (1.10 mg g
1
) and
AC (0.99 mg g
1
) due to its high surface area and good
conductive network formed between the CNTs and RG.
Recently, another two papers on the use of RG–CNTs
composites as CDI electrodes were published by Li et al.
93
and
Zou et al.
94
In these two papers, the composites were synthe-
sized by reducing a mixture of GO solution and acid oxidized
CNTs solution using hydrazine with different mixing ratios of
RG to CNTs. When compared with pure RG or CNTs, the
RG–CNTs exhibited a higher electrosorption capacity and faster
desalination. The signicant improvement in desalination
performance was mainly ascribed to the existence of CNTs,
which acted as wires connecting the large RG sheets together
and lling the vacancies, as well as reducing the aggregation of
RG. The conducting network structure formed could serve as
fast electronic and ionic conducting channels.
87,95
A type of RG–CNTs hybrid aerogel was also studied by Zhang
et al.
96
as a CDI electrode. The synthetic procedure is described
as follows: pre-dispersed GO sheets and multi-walled CNTs were
mixed homogenously. Then, vitamin C was added as a reducing
agent for the reduction of GO to RG.
97
The RG–CNTs hydrogel
precursors were obtained by heating the mixtures, and then
drying them in supercritical CO
2
to obtain the aerogel. The
porous structure of the RG–CNTs hybrid aerogels was revealed
by the SEM images shown in Fig. 11. A three-dimensional
network structure composed of hierarchical pores can be
observed. Randomly oriented CNTs and wrinkled RG sheets
were well dispersed in the monolithic aerogel matrix with a
number of mesopores. Such a hierarchically porous structure is
favorable for mass transfer and can reduce the transport limi-
tations.
98
The resulting RG–CNTs hybrid aerogel shows an
Fig. 8 (a) Electrosorption capacity of the AC and RG–AC (20 wt% RG)
electrodes at different voltages in NaCl solution with an initial
conductivity of 50 mScm
1
; (b) rate constants of the electrosorption
kinetics for AC and RG–AC (20 wt% RG) electrodes at different volt-
ages. (Reproduced with permission from ref. 90.)
Fig. 9 TEM image of (a) the RG–MC composite with 5 wt% RG; (b) a
comparison of the CDI curves for the AC, MC and RG–MC (5 wt% RG)
electrodes in a NaCl aqueous solution. (Reproduced with permission
from ref. 91.)
Fig. 10 (a) TEM image of the RG–CNTs composite with 10 wt% CNTs;
(b) comparison of the CDI profiles for the RG–CNTs (10 wt% CNTs), RG
and AC electrodes in an NaCl aqueous solution. (Reproduced with
permission from ref. 84.)
Fig. 11 (a) Low-magnification and (b) high-magnification SEM images
of the RG–CNTs hybrid aerogels. The arrows in (b) are an indication of
the CNTs. (Reproduced with permission from ref. 96.)
15210 |RSC Adv.,2015,5, 15205–15225 This journal is © The Royal Society of Chemistry 2015
RSC Advances Review
extremely high electrosorption capacity (633.3 mg g
1
) in NaCl
solution with an initial concentration up to 35 g L
1
measured
in a continuously recycling system at an applied voltage of 1.0 V,
which is considerably better than the reported maximum
capacity among various electrode materials for CDI reported to
date. The superior desalination performance was attributed to
several factors, including the hierarchically porous structure of
the hybrid aerogel,
98
the functional groups attached to the
surface of the RG sheets and/or CNTs,
99
the high conductivity
resulting from the interconnection among RG and CNTs, as well
as the large surface areas of these hybrid aerogels.
2.3 Other carbon–carbon composite
2.3.1 CAs–AC composite. Kohli et al.
100
synthesized meso-
porous CAs–microporous AC composite electrodes for CDI
applications. The CAs were prepared by supercritically drying
and carbonizing the gel of resorcinol and furfural in iso-
propanol, and AC was commercial product. The CDI experiment
was carried out in 1000 mg L
1
NaCl solution at a voltage of
1.2 V. The solution volume and the electrode size were 100 mL
and 10 cm 10 cm, respectively. A highest salt removal capacity
of 17 mg g
1
was observed for the composite electrode with CAs
and AC in the ratio of 75 : 25, which was higher than those
found for pure CAs (13 mg g
1
) and AC (7 mg g
1
). The excellent
CDI performance of the composite electrode was ascribed to the
combination of the mesopores in CAs and the high surface area
of AC. A regeneration efficiency of 92% for the composite
electrode was obtained for a number of adsorption and
desorption cycles.
2.3.2 ACFs–carbon black (CB) composite. Qiu et al.
101
proposed a new strategy of fabricating hierarchical electrospun
ACFs webs with tuned micro-, meso- and macro-porous struc-
ture by incorporating 5 wt% CB in a polyacrylonitrile precursor
solution. In their experiments, the as-made ACFs–CB composite
webs activated at 900 Cinowing CO
2
(ACF/CB900) were
robust and exible. From the SEM images shown in Fig. 12(a)
and (b), it can be observed that the agglomerated CB has effect
on the smooth cylindrical shape of the bers in the composite
webs. The TEM image (Fig. 12(c)) further conrms that the CB
particles were embedded in the carbon bers (CFs) matrix and
some pores were formed in both the bers and in the junctions
between the bers and CB. The incorporation of CB in the
composite increases the electrical conductivity by 3 times and
the mesopore proportion by 32% in comparison to pristine
ACF900 webs without any CB. During electrosorption, the NaCl
solution was continuously pumped into the cell at a rate of
5 mL min
1
. The applied potential and initial NaCl concentra-
tion was 1.6 V and 90 mg L
1
, respectively. The electrosorption
capacity of the ACFs and ACFs–CB web electrodes activated at
different temperatures were compared, as shown in Fig. 12(d).
The mass of the web electrode was about 0.2 g. It was found that
the electrosorption capacity of the ACFs–CB electrodes
increased by 84–116% in comparison to the ACFs electrode
without CB, depending on the activation temperature. The
composite electrodes also exhibited good regeneration ability
over charge–discharge cycles.
2.3.3 ACFs–CNFs composite. Pan et al.
102
fabricated ACFs–
CNFs composite electrodes by depositing CNFs onto commer-
cial ACFs lms via a CVD method using acetylene and hydrogen
mixture gas for different times. From the SEM and TEM images
shown in Fig. 13(a)–(c), it can be observed that good contact
between the ACFs and CNFs was formed and the average
diameters of CNFs were about 20–40 nm. The electrosorption
experiments were conducted in a continuously recycling system
using NaCl solution with an initial conductivity of 50 mScm
1
at
aow rate of 40 mL min
1
. A direct voltage of 1.2 V was applied
and the electrode size was 80 mm 80 mm. Fig. 13(d) displays
the desalination performance of the ACFs and different ACFs–
CNFs composite electrodes. The results show that due to its
Fig. 12 SEM images of (a) ACF/CB900 and (b) ACF900; (c) TEM image
of ACF/CB900; (d) electrosorption capacity of the ACFs and ACFs–CB
web electrodes activated at different temperatures. (Reproduced with
permission from ref. 101.)
Fig. 13 SEM images of (a) ACFs and (b) ACFs/CNFs-1 h; (c) TEM image
of ACFs/CNFs-1 h; (d) electrosorption process for the different elec-
trodes in NaCl solution. Initial 30 min: physical absorption; subsequent
120 min: electrosorption; final 30 min: desorption. The inset in (b) is
the corresponding magnified image. (Reproduced with permission
from ref. 102.)
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optimized mesoporous structure and wettability, the ACFs–
CNFs composite electrode with CNFs deposited in 1 h (ACFs/
CNFs-1 h) exhibits the best desalination performance with a
maximum electrosorption capacity of 17.19 mg g
1
, which is
considerably higher than that for the ACFs (1.85 mg g
1
). The
good regeneration of the composite electrodes was also realized
over charge–discharge cycle testing.
3 Carbon–metal oxide composite
Carbon and metal oxides are two hotspots as electrode mate-
rials for supercapacitor applications.
103,104
The combination of
these two types of materials improves the specic capacitance
largely due to their synergistic effects.
105,106
Because of the
similar principle between supercapacitor and CDI, such
carbon–metal oxide composites have also been explored in CDI
applications.
107,108
The metal oxide on carbon electrodes may
contribute to the CDI desalination efficiency with their suitable
physicochemical properties such as high hydrophilicity to
increase the wettability of the electrode, inhibition of physical
adsorption of ion species affecting the electrochemical
adsorption/desorption during CDI operation,
107,108
or alteration
of the surface zeta-potential on the carbon electrode to increase
the ion removal rate.
109
However, an important issue noticed
was that some metal oxides, typically ZnO, are not stable in
acidic or basic solutions. The dissolution of such metal oxides
will release additional ions, which can interfere with the
experimental data by affecting the solutions conductivity.
3.1 AC–TiO
2
composite
Seo et al.
108,110
rst reported the utilization of ACC by TiO
2
modication for CDI. The modied electrode was prepared
using the reaction between ACC and alkoxide by immersing
pure ACC in anhydrous ethanol solution containing titanium
butoxide. TiO
2
-modied ACC with different loadings were
obtained by changing the concentration of titanium butoxide.
No noticeable difference in the SEM images of ACC and Ti
(0.81)/ACC (6.4 wt% TiO
2
) was observed, as shown in Fig. 14(a)
and (b), which indicated the high dispersion of TiO
2
on the
surface of ACC. During the CDI experiments, the initial
concentration of NaCl solution and applied voltage were
0.1 mM and 1.0 V, respectively. The effective electrode area was
4.5 cm 4.5 cm. The CDI performance of ACC and the modied
ACC are depicted in Fig. 14(c). When compared with pure ACC,
the Ti (1.05)/ACC (8.4 wt% TiO
2
) shows a reduced physical
adsorption capacity but increased electrosorption capacity. The
considerable decrease in physical adsorption was attributed to
the lower surface area of Ti (1.05)/ACC (1890 m
2
g
1
) than that
of ACC (1980 m
2
g
1
), as well as the introduction of metal atoms
through the reaction between metal alkoxide molecules and the
polar groups of ACC because the polar groups worked as
adsorption sites for physical adsorption. On the other hand, the
incorporation of metal atoms on the surface of ACC enhanced
its positive potential, and therefore improved the electro-
sorption capacity. In addition, TiO
2
was a well-known material
to be partially reduced in reduction environments
110
and the
conversion of the electric properties of titanium atoms due to
the charged electric eld was also responsible for the signicant
rise in electrosorption capacity. The absorbed amount of NaCl
gradually increased with the loading amount of TiO
2
and Ti
(1.05)/ACC ranks the best, as shown in Fig. 14(d). The Ti (1.05)/
ACC electrode also exhibited good regeneration ability by
charge–discharge cycle testing.
Recently, Chang et al.
107
reported an AC electrode modied
with TiO
2
for CDI. The AC loaded with TiO
2
(AC-TiO
2
) electrode
was fabricated as follows: rst, a mixture of HNO
3
, anhydrous
ethanol, polyethylene glycol and distilled water in a certain ratio
was slowly added to an anhydrous ethanol solution containing
titanium butoxide. Then, the obtained TiO
2
solution was mixed
with pretreated AC powder and evaporated at room tempera-
ture. The carbonization of the electrode was achieved at 850 C
for 2 h in an N
2
atmosphere to improve the pore structure and
hydrophilicity. From the SEM images shown in Fig. 15(a) and
(b), it can be observed that a lot of occulent substance appears
on the surface of the AC-TiO
2
electrode, which could be the
accumulation of TiO
2
during the high-temperature annealing
process. A series of desalination experiments were carried out
using 20 mL of NaCl solution with an initial concentration of
500 mg L
1
at an operating voltage of 1.2 V. As observed in
Fig. 15(c), the AC-TiO
2
electrode exhibits a reduced physical
adsorption capacity and increased electrosorption capacity
when compared with the AC electrode, which is similar to the
result reported by Seo et al.
110
The reversibility of the AC-TiO
2
electrode was also investigated and the results are displayed in
Fig. 15(d), which reveal that by applying a reverse voltage of
1.2 V for 10 min and 0 V for 20 min the regeneration could be
easily accomplished.
Fig. 14 SEM images of (a) ACC and (b) Ti (0.81)/ACC; (c) adsorption
profiles of NaCl on the ACC and Ti (1.05)/ACC electrodes; (d) physical
adsorption and electric field adsorption of the modified ACC elec-
trodes. (Reproduced with permission from ref. 110.)
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RSC Advances Review
More recently, a microwave-assisted ionothermal approach
was used to synthesize an AC-TiO
2
composite as a CDI electrode
material.
111
When compared with a conventional sol–gel
method that involves high temperature and long post-
calcination time, the microwave assisted ionothermal method
has emerged as a new and fast route for the production of
inorganic crystalline materials and their composites under
ambient conditions.
112,113
In this work, a controlled sol–gel
reaction step and a crystallization step induced by
[Bmin]
+
[BF
4
]
ionic liquid (IL) were involved. In the rst step, a
determined amount of AC was dispersed in isopropyl alcohol
(IPA), and then mixed with titanium tetraisopropoxide, IL and
deionized water. Aer that, the mixture was stirred under
microwave irradiation for 30 min. To crystallize the composite,
IL and deionized water were added to the IPA solution con-
taining the product from the rst step and the obtained mixture
was stirred under microwave irradiation for 60 min. The CDI
experiments were carried out in a continuously recycling system
at a ow rate of 12 mL min
1
. 150 mL of a 100 mg L
1
NaCl
solution and a working voltage of 1.2 V were used. The AC-TiO
2
composite electrode achieved a maximum electrosorption
capacity of 8.05 0.34 mg g
1
, which is considerably higher
than that of the pure AC electrode (5.43 0.91 mg g
1
). This
result was in accordance to the other reports.
107,110
In another recent work,
114
a highly durable TiO
2
coated
carbon electrode was fabricated using the sol–gel spraying of a
titanium butoxide precursor onto an AC electrode. In the CDI
experiment, the 10 mM NaCl solution was supplied to the CDI
unit cell by a peristaltic pump at a ow rate of 10 mL min
1
and a 1.2 V voltage was applied. The TiO
2
coated electrode
showed approximately two times higher electrosorption
capacity (17 mg g
1
) than the carbon electrode itself (9 mg g
1
),
although the specic capacitance of the TiO
2
coated electrode
was not signicantly different from that of the carbon electrode.
This enhanced desalination performance was attributed to the
facile accessibility of water and ions from bulk solution to the
electrode surface because of the high wettability of the TiO
2
coated electrode.
3.2 AC–ZnO composite
Myint and Dutta
115
reported a ZnO nanorods (NRs) modied
ACC electrode via a nanoparticle seeding process and hydro-
thermal process for CDI applications. Different ACC–ZnO
composite samples were synthesized using pretreatment and
changing the concentration and pH value of the precursor
solutions, as well as their growth time, and their morphologies
examined by SEM (Fig. 16). It was found that the experimental
conditions, such as pretreatment, pH of the precursor solutions
and growth time, affect the surface morphology of the
composites. A series of desalination experiments using the
ACC–ZnO electrodes (area 8.41 cm
2
) were conducted with a feed
solution of 100 ppm NaCl solution and a ow rate of 2 mL
min
1
under an imposed potential of 1.2 V. Fig. 17 shows the
desalination and regeneration performance of the electrodes. It
was found that the desalination efficiency of the composite
electrodes was improved when compared with pure ACC elec-
trode. For the electrodes obtained with pretreatment, a
maximum desalination efficiency of 34% was achieved when
the ZnO growth time was 10 h and for the electrodes fabricated
under controlled pH conditions the desalination efficiency
improves upon increasing the growth time, while the regener-
ation efficiency was considerably reduced. In their subsequent
work,
116
Dutta et al. further studied the electrodes consisting of
ZnO micro/nanostructures (NPs, NRs, microsheets and
Fig. 15 SEM images of (a) AC and (b) AC-TiO
2
; (c) desalination and
desorption curves for the different electrodes; (d) electrosorption and
desorption cycles for the AC-TiO
2
electrode. (Reproduced with
permission from ref. 107.)
Fig. 16 SEM images of the as-grown ZnO NRs on ACC using a
hydrothermal method: growth time (a) 10 hours (no pre-treatment of
ACC or pH control during growth); (b) 10 hours and (c) 15 hours (the
hydrothermal process was conducted with pre-treatment of the
seeded ACC substrates by dipping then into a 20 mM precursor
solution); (d) 10 hours (using the hydrothermal method by adjusting
the pH to 6.8 to 6.9 at the initial state). (Reproduced with permission
from ref. 115.)
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microspheres) hydrothermally grown on ACC for CDI applica-
tions. The morphology of the ZnO structures was found to
signicantly affect the salt removal efficiency of the composite
electrodes. A highest salt removal capacity of 8.5 mg g
1
was
achieved for both the ZnO microsheets graed ACC electrode
and ZnO NRs graed ACC electrode, which was considerably
higher than those for pure ACC (5.8 mg g
1
) and other ZnO
structure coated ACC electrodes, as shown in Fig. 18.
3.3 Nanoporous carbon (NC)–MnO
2
composite
Zou et al.
117
developed a novel NC–MnO
2
composite electrode
for CDI applications. The NC was prepared by the replication of
a porous silica template in a solution composed of triblock
polymer P
123
poly(ethyleneglycol)–poly(propyleneglycol)–poly-
(ethyleneglycol) ethanol and sulphuric acid with subsequent
carbonization at 850 C. The silica template was nally removed
using HF. A co-precipitation method that involved a redox
reaction in an aqueous solution containing KMnO
4
, MnSO
4
,
NaOH and NC was exploited to synthesize the NC–MnO
2
composite. During preparation, two types of silica with pore
sizes of 4 nm and 2.7 nm were adopted and the obtained NC and
NC–MnO
2
samples were denoted as C
A
,C
B
,C
A-Mn
and C
B-Mn
,
respectively. A disordered but cross-linked porous structure was
observed from pure NC and needle-shaped MnO
2
was observed
on the surface of the NC in sample C
A-Mn
, as shown in Fig. 19(a)
and (b). The pore size of the silica template was determined to
have a large effect on the porous structure of the NC–MnO
2
composites. C
A-Mn
possessed more mesopores but a lower
surface area than C
B-Mn
. It was noted that the MnO
2
particles
could be formed on both the internal and external surfaces of
the porous carbon matrix. For C
A-Mn
, the MnO
2
was deposited
on the internal surface, and therefore blocked the pores in the
carbon matrix, while the main porous structure remained.
Therefore, C
A-Mn
exhibited a decreased surface area when
compared with C
A
. In the case of C
B-Mn
, the external deposition
of MnO
2
leads to a rise in the surface area in comparison with
C
B
. The CDI performance of the C
A-Mn
,C
B-Mn
and AC electrodes
was investigated using a ow through system at a ow rate of
20 mL min
1
in 50 mL of NaCl solution with an initial
conductivity of 50 mScm
1
. The total mass of the electrode
materials and applied voltage were 2 g and 1.2 V, respectively.
The salt removal capacity of C
A-Mn
(0.99 mg g
1
) and C
B-Mn
(0.95
mg g
1
) were considerably higher than that of AC (0.32 mg g
1
).
The enhanced CDI performance could be attributed to the
following reasons: (i) the amount of mesopores, which play a
vital role in the electrosorption of ions in the NC–MnO
2
composites was considerably higher than that in AC; (ii) two
commonly accepted charge storage mechanisms of MnO
2
including the adsorption/desorption process of protons or
alkaline cations on the MnO
2
surface and intercalation/
deintercalation of protons or alkaline cations accompanied by
the Faradic reaction greatly contributed to the electrosorption.
Although a Faradic reaction occurred on the surface of the NC–
MnO
2
composites, their full regeneration could still be ach-
ieved, as shown in Fig. 19(c) and (d), indicating the chemical
stability of the composites.
3.4 RG–TiO
2
composite
Tang et al.
50
presented a novel, simple and versatile method to
fabricate 3D RG–TiO
2
composites using TiCl
3
and GO as the
precursors by hydrothermal treatment and subsequent freeze-
drying. The SEM and TEM images shown in Fig. 20(a) and (b)
clearly reveal the well-dened 3D open pore nature of the RG–
Fig. 17 Desalination and regeneration efficiency of the electrodes
obtained (a) with pretreatment and (b) under controlled pH conditions.
(Reproduced with permission from ref. 115.)
Fig. 18 Adsorbed salt on the different types of electrodes during the
desalination process in 100 ppm NaCl solution under an applied
potential of 1.2 V. (Reproduced with permission from ref. 116.)
Fig. 19 TEM images of (a) C
A
and (b) C
A-Mn
; adsorption/desorption
behavior of (c) C
A-Mn
and AC and (d) C
B-Mn
and AC. (Reproduced with
permission from ref. 117.)
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RSC Advances Review
TiO
2
product and that a large amount of TiO
2
NPs were
uniformly distributed on the RG sheets. The CDI performance
of the RG–TiO
2
composite was investigated in a continuously
recycling system at a ow rate of 30 mL min
1
using 60 mL of
500 mg L
1
NaCl solution at an applied voltage of 1.2 V. As
shown in Fig. 20(c), the electrosorption capacity of the
composite was 15.1 mg g
1
, which is 1.5 and 12.6 times of those
of pure RG (9.9 mg g
1
) and commercial AC (1.2 mg g
1
),
respectively. While, at 6000 mg L
1
, the corresponding capacity
is 24.2 mg g
1
for RG–TiO
2
, which is 1.6 and 7.3 times of that of
RG (15.4 mg g
1
) and AC (3.3 mg g
1
), respectively. The good
reversibility of the RG–TiO
2
composite was conrmed by per-
forming the desalination and regeneration process over
10 cycles (Fig. 20(d)).
3.5 RG–MnO
2
composite
Barakat et al.
118
reported a rapid, cost-effective and eco-friendly
method for preparing RG–MnO
2
composites with control over
their morphology by the addition of MnSO
4
, ammonium per-
sulfate and piperdine during the one pot synthesis of RG.
Ammonium persulfate was used as an efficient oxidizing agent
for graphite and MnSO
4
simultaneously with a microwave
exfoliation to intercalate the formed MnO
2
among the RG
sheets. The shape and insertion of MnO
2
nanostructures (NPs
and NRs) among the RG sheets could be managed during the
fabrication process by adjusting the microwave irradiation. The
SEM and TEM images shown in Fig. 21(a) and (c) display the
sandwich morphology of the intercalated MnO
2
NPs among the
RG sheets with good and uniform distribution. Fig. 21(b) and
(d) show the SEM and TEM images of the MnO
2
NRs@RG.
Conversion of MnO
2
NPs into NRs was achieved by elongation
of the microwave irradiation treatment time, which resulted in
sintering the synthesized NPs to form the 1D structure. The CDI
experiments were conducted in a continuously recycling system
using 50 mL of NaCl solution with an initial conductivity of
100 mScm
1
at an applied voltage of 1.2 V. As shown in Fig. 22,
the synthesized MnO
2
NRs@RG reveals excellent results with a
high electrosorption capacity (5.01 mg g
1
), high salt removal
efficiency (93%) and distinguished recyclability when
compared to MnO
2
NPs@RG, pristine RG and AC. This
tremendous improvement in the salt removal was attributed to
the ease of surface accessibility for ion adsorption from the
saline solution to the electrode surface due to the unique
morphology in addition to the electrochemical activity of MnO
2
.
Fig. 20 (a) SEM and (b) TEM images of the RG–TiO
2
composite; (c) the
desalination efficiency of AC, RG and RG–TiO
2
in 500 mg L
1
NaCl
solution at different times; (d) electrosorption and regeneration cycles
of RG–TiO
2
in 500 mg L
1
NaCl solution. (Reproduced with permis-
sion from ref. 50.)
Fig. 21 SEM images of (a) MnO
2
NPs@RG and (b) MnO
2
NRs@RG; TEM
images of (c) MnO
2
NPs@RG and (d) MnO
2
NRs@RG. (Reproduced
with permission from ref. 118.)
Fig. 22 CDI performance of the various electrode materials in NaCl
solution at 1.2 V. (A) RG, (B) MnO
2
NPs@RG, (C) MnO
2
NRs@RG and (D)
AC. The inset depicts the regeneration profiles for the RG and MnO
2
NRs@RG electrodes. (Reproduced with permission from ref. 118.)
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4 Carbon–polymer composite
Similar to metal oxides, a conducting polymer can contribute
the pseudo-capacitance, which can largely enhance the capaci-
tive performance of carbon materials for supercapacitor or CDI
applications.
119–121
Furthermore, on one hand, when these
polymers are combined with carbon materials the functional
groups, typically oxygen- and nitrogen-containing groups, in
these polymers exhibit a high affinity to metal ions by chelation
or redox reactions,
122,123
which is advantageous to the electro-
sorption of metal ions. On the other hand, the charging and
discharging processes of conducting polymers are accompanied
by ion exchange between the polymer and the electrolyte that
occurs in the interior of the electrode (not just on the surface),
124
which can improve the adsorption capacity.
4.1 Carbon–chitosan (CS) composite
CS as an inexpensive, widely used, hydrophilic and non-toxic
polysaccharide biopolymer exhibits a high specicity towards
metal ions due to its abundant number of amino and hydroxyl
functional groups.
125,126
Therefore, CS was utilized to modify the
carbon electrodes for electrosorption of ions.
4.1.1 ACFs–CS composite. Huang and Su
127
reported the
modication of ACFs cloth with CS and their application in the
removal of Cu
2+
ions from wastewater by adsorption/
electrosorption. The ACFs–CS composite was obtained via an
immersion of ACFs in CS solution with a CS content of about
3.0 wt%. A conventional 3D electrode static state system was
adopted in the adsorption/electrosorption investigation at both
open circuit and 0.3 V bias potential in Cu(NO
3
)
2
solution. The
initial concentration of the Cu
2+
ions in the solution ranged
from 40 to 200 mg L
1
. A piece of ACFs cloth connected by a
titanium plate acted as the working electrode and the initial pH
of the test solution was adjusted to 4.0. The results revealed that
the ACFs–CS composite possessed a higher Cu
2+
ion adsorption
and electrosorption capacity when compared with ACFs cloth.
The reason was attributed to the ion-exchange mechanism of
Cu
2+
ions with H
+
ions protonating the amine end groups of CS.
Moreover, the existing carboxyl and hydroxyl functional groups
on the surface of the composite could also promote the
adsorption of inorganic ions in solution via an ion-exchange
reaction.
4.1.2 CNTs–CS composite. Pan et al.
59
proposed a CNTs–CS
composite electrode for the electrosorption of Cu
2+
ions. The
composite electrode was prepared by rst reacting HNO
3
-
oxidized CNTs with thionyl chloride, and then covalently
graing CS onto the surface of the CNTs. From the SEM images
of the CNTs and CNTs–CS shown in Fig. 23(a) and (b), it can be
observed that the CNTs–CS displays a more compact surface in
which wire-like CNTs are dispersed in the CS matrix. The TEM
images show that the CNTs in the CNTs–CS composite are
considerably shorter than in the pristine CNTs due to treating
the CNTs with HNO
3
.
128
The shorter CNTs might contribute to
the higher specic surface area of CNTs–CS (52.7 m
2
g
1
) than
that found with pristine CNTs (44.5 m
2
g
1
), which was bene-
cial to the removal of Cu
2+
. The CS functionalization of CNTs
was found to lower the point of zero charge and improve the
surface hydrophilicity, which could enhance the electrostatic
adsorption of Cu
2+
, and therefore increase the removal effi-
ciency. The electrosorption experiments were carried out in a
continuously recycling system at a ow rate of 40 mL min
1
using CuCl
2
solutions with an initial conductivity of 50 mScm
1
.
A direct voltage of 1.2 V was applied. The variation in solution
conductivity with time was recorded (Fig. 23(c)) and it was clear
that no obvious physical adsorption was observed for both
CNTs and CNTs–CS. The CNTs–CS presents an 85% Cu
2+
removal ratio, which is 25% higher than that of the CNTs (60%).
The low Cu content (0.29 at%) on the CNTs–CS aer the
experiment obtained by X-ray photoelectron spectroscopy indi-
cated that electrodeposition did not occur remarkably due to
the intrinsic resistance of the electrodes that consumes some
voltage and electrosorption was mainly responsible for the
removal of Cu
2+
. The kinetics of the electrosorption process for
CNTs and CNTs–CS studied using a pseudo-rst-order kinetics
equation (Fig. 23(d)) shows that when compared with CNTs, the
CNTs–CS exhibit a higher kinetic rate constant, which is bene-
cial to the removal of Cu
2+
. The stability of the CNTs–CS
electrode was also veried aer 30 charge–discharge cycles
without any apparent decay in the removal efficiency of Cu
2+
.
4.2 Carbon–RF composite
Hao and co-workers
129,130
proposed a RG and resol (RG–RF)
composite as the CDI electrode material for the removal of FeCl
3
and NaCl. The procedure for synthesizing the RG–RF composite
is described as follows: resorcinol, formaldehyde and sodium
carbonate catalyst in a ratio of 200 : 400 : 1 were slowly added to
a GO solution. Then, the solution was transferred to a Teon-
lined autoclave and maintained at 85 C for 3 days. The
mixture was reduced by calcination at 900 C under an N
2
atmosphere, resulting in the RG–RF composite. Fig. 24 shows
the TEM and SEM images of GO and the RG–RF composite. It
Fig. 23 SEM images of (a) CNTs and (b) CNTs–CS. The insets are their
TEM images. (c) Variation of solution conductivity with time and (d)
linear plot of the pseudo-first-order kinetic equation for the electro-
sorption of Cu
2+
by CNTs and CNTs–CS. (Reproduced with permission
from ref. 59.)
15216 |RSC Adv.,2015,5, 15205–15225 This journal is © The Royal Society of Chemistry 2015
RSC Advances Review
can be observed that the RF entangles with RG to form a good
network structure. The introduction of RF largely enhances the
specic surface area. The electrosorption experiments were
conducted in a continuously recycling system using RG–FR, RG
and AC electrodes. The mass and size of the electrodes were 1.5 g
and 70 mm 140 mm, respectively. The FeCl
3
and NaCl solu-
tions with a volume of 200 mL were employed as the feed solu-
tion, respectively, and the ow rate was 20 mL min
1
. As shown
in Fig. 25(a) and (b), the electrosorption capacity of the RG–RF
electrode was 3.47 mg g
1
in 40 mg L
1
FeCl
3
solution and
2.135 mg g
1
in 40 mg L
1
NaCl solution at an electrical voltage
of 2.0 V, higher than those of the AC (1.41 and 0.612 mg g
1
) and
RG (2.46 and 1.510 mg g
1
) electrodes due to the presence of RF,
which is benecial in restricting the aggregation of RG, resulting
in a high specic surface area. As for the AC electrode, despite its
higher specic surface area than that of RG–RF, its larger
micropore volume andsmaller average pore size is not benecial
for electrosorption and results in a low electrosorption capacity.
4.3 Carbon–polyaniline (PANI) composite
Zou et al.
119
synthesized single-walled CNTs (SWCNTs) and PANI
composites by in situ polymerization of aniline in a CNTs
suspension. From the TEM images shown in Fig. 26(a) and (b),
it can be clearly observed that PANI polymerizes along the
SWCNTs, with the SWCNTs as the core wrapped in a layer of
PANI. Packing of PANI outside the SWCNTs causes an increase
in mesopore volume, which is benecial to CDI performance.
The CDI tests were carried out in a continuously recycling
system using NaCl solution with an initial conductivity of
100 mScm
1
at a ow rate of 20 mL min
1
. A direct voltage of
1.2 V was applied. The mass and size of the electrodes were
1.34 g and 70 mm 140 mm, respectively. The CNTs–PANI
composite electrode demonstrates a higher electrosorption
capacity and better recyclability than those of the CNTs elec-
trode, as shown in Fig. 26(c) and (d). The p–pconjugated
interaction between the PANI backbone and the graphite-like
structure of the CNTs, which facilitate ion transport, should
also contribute to the enhanced CDI performance.
4.4 Carbon-ion exchange polymer composite
In addition to the electrode materials, some other factors
during the electrosorption process, such as the co-ion expulsion
effect, can signicantly inuence CDI performance.
131
When an
electric potential is applied between the electrodes, counter-ion
adsorption and co-ion expulsion effects happen simulta-
neously, which leads to a poor charge efficiency. This problem
becomes worse with an increasing concentration.
132
Therefore,
coulomb inefficiency has become a major problem that limits
CDI applications. Andelman et al.
133
proposed an effective
method by introducing a charge barrier membrane into CDI,
called membrane capacitive deionization (MCDI), which has
been studied by several groups in recent years.
63,131,134–144
The
salt removal of the MCDI system was found to be largely
improved and could reach even 50% higher than that of the
CDI system.
131,135,141,145
However, an issue associated with the
introduction of the ion exchange membranes should be noted.
The weak contact adhesion between the electrodes and the ion-
Fig. 24 TEM images of (a) GO and (b) RG–RF; SEM images of (c) GO
and (d) RG–RF. (Reproduced with permission from ref. 130.)
Fig. 25 Comparative results of RGO–RF, RGO and AC electrodes in
terms of electrosorption performance in (a) 40 mg L
1
FeCl
3
solution
and (b) 40 mg L
1
NaCl solution. (Reproduced with permission from
ref. 129 and 130.)
Fig. 26 (a) Low magnification and (b) high magnification TEM images
of the CNTs–PANI composite; (c) CDI performance and (d) adsorp-
tion/desorption cycles for the CNTs–PANI composite. (Reproduced
with permission from ref. 119.)
This journal is © The Royal Society of Chemistry 2015 RSC Adv.,2015,5, 15205–15225 | 15217
Review RSC Advances
exchange membranes might produce contact resistance and
increase the bulk resistivity of the entire unit, which would
increase energy consumption and decrease ion adsorption. To
overcome this problem, ion exchange polymers were proposed
to replace the ion exchange membranes in MCDI
devices.
140,142,146,147
Kim and Choi
140,142
rst proposed a novel carbon electrode
coated with a cation-exchange polymer for CDI. The composite
electrode was prepared by casting a polymer solution containing
10 wt% poly(vinyl alcohol) and 30 wt% sulfosuccinic acid (SSA)
onto an AC electrode and heating the electrode at 130 Cfor1h.
The SSA acted as a crosslinking agent and provided ion-exchange
functional groups. The SEM images in Fig. 27(a) and (b) show
that the ion-exchange polymer is wellcoated on top of the carbon
electrode. The prepared composite electrode integrates both the
advantages of high capacitance from the carbon electrode and
the permselectivity of the ion-exchange polymer. According to
electrochemical impedance spectroscopy measurements, it was
found that the equivalent series resistance for the coated elec-
trode was higher than that of uncoated electrode but the resis-
tance of the coating layer was relatively low when compared to
commercial ion-exchange membranes. The CDI experiments
were performed in a ow-through system with a feed NaCl
solution of 200 mg L
1
. The size of the electrodes was 100 mm
100 mm. Desalination curves for the CDI cell and MCDI cell (one
uncoated and one coated-carbon electrode) at a voltage of 1.2 V
are shown in Fig. 27(c). It can be seen that the desalination
efficiency can be enhanced using a carbon electrode coated with
an ion-exchange polymer. When compared with the CDI cell, the
desalination efficiency and current efficiency of the MCDI cell
are enhanced by about 27–56% and 69–95%, respectively relying
on the operating conditions. In the MCDI cell, anions in the bulk
solution cannot penetrate through the cation-exchange layer and
desorption of the absorbed cations was restrained, which
thereby improved the CDI performance. The selectivity of the
coated electrode was further veried by conducting the desali-
nation experiment in the MCDI cell at a potential of 1.5 V, in
which the coated electrode acted as the anode. As seen from
Fig. 27(d), few ions are electroabsorbed and desorbed in the
MCDI cell due to anion rejection by the cation-exchange layer of
the coated electrode, which demonstrates that the coated cation-
exchange layer functions well to selectively transport ions.
Pan et al.
146
reported a CNTs–polyacrylic acid (PAA)
composite lm electrode for CDI applications. The CNTs–PAA
composite lm electrode was fabricated via an electrophoretic
deposition method in PAA solution containing the CNTs. The
PAA served as the cation-exchange polymer and matrix to
incorporate the CNTs. From the SEM images of the CNTs and
CNTs–PAA shown in Fig. 28(a) and (b), it can be seen that the
density of CNTs in the CNTs–PAA composite was lower than
that in the pure CNTs indicating that the CNTs are well
dispersed in the PAA polymer matrix. Three cells, CNTs–PAA cell
assembled with one CNTs electrode and one CNTs–PAA
composite electrode, CNTs cell consisting of two CNTs elec-
trodes and a modied CNTs cell with introducing a cation-
exchange membrane (CNTs–CEM cell), were tested in a
continuously recycling system at a ow rate of 40 mL min
1
.
The effective area of the electrodes was 8 cm 8 cm. NaCl
solution with an initial conductivity of 50 mScm
1
was used as
feed solution and a direct voltage of 1.2 V was applied. The
result is shown in Fig. 28(c). Among the three cells, the CNTs–
PAA cell exhibits the highest NaCl removal of 83%, 51% higher
than that of the CNTs cell (32%) and 12% higher than that of
the CNTs–CEM cell (71%), which is similar to the other
report.
140
Furthermore, the cation selectivity of the CNTs–PAA
composite lm electrode was conrmed by the observation of
no obvious electrosorption when the CNTs–PAA cell was oper-
ated under a voltage of 1.2 V, as shown in Fig. 28(d).
Fig. 27 SEM images of (a) uncoated carbon electrodes and (b) carbon
electrodes coated with an ion-exchange polymer; the conductivity
transient of effluent measured at a cell potential of (c) 1.2 V with a flow
rate of 20 mL min
1
and (d) 1.5 V with a flow rate of 20 mL min
1
.
Solid line: MCDI cell, dashed line: CDI cell. (Reproduced with
permission from ref. 140.)
Fig. 28 SEM images of (a) CNTs and (b) CNTs–PAA composite elec-
trodes; (c) electrosorption process for the CNTs cell, CNTs–PAA cell
and CNTs–CEM cell; (d) electrosorption behavior for the CNTs–PAA
cell under an applied voltage of 1.2 V. (Reproduced with permission
from ref. 146.)
15218 |RSC Adv.,2015,5, 15205–15225 This journal is © The Royal Society of Chemistry 2015
RSC Advances Review
In the above work, the enhanced desalination efficiency was
attributed to the selective transport of cations from bulk solu-
tion to the electrode surface due to the presence of the cation-
exchange polymer. However, the research on the anion-
exchange polymer is far from enough compared with its
cation partner. Therefore, in view of their practical applications,
investigations on the MCDI cell employing both anion and
cation exchange polymers to replace the anion and cation
exchange membranes are necessary.
Moon et al.
147
achieved some progress on this issue. They
studied an advanced MCDI (A-MCDI) by adhering both the
cation exchanger and anion exchanger (sulfonation and ami-
nation of bromomethylated poly(2,6-dimethyl-1,4-phenylene
oxide) (BPPO)) onto the surface of carbon cloth electrodes.
The electrodes were prepared in two steps: spraying the BPPO
slurry onto the surface of carbon cloth, and then immersing the
electrodes in sulfuric acid solution or trimethylamine solution
to complete the sulfonation or amination reaction. From the
SEM images shown in Fig. 29(a) and (b), it was found that the
clean surface and the pores on the CFs of the original carbon
cloth are lled with the polymer. The desalination experiments
using CDI, MCDI and A-MCDI cells were carried out in a
continuously recycling system at a ow rate of 4 mL min
1
.
100 mg L
1
NaCl solution was used as the feed solution and a
direct voltage of 1.8 V was applied. As shown in Fig. 29(c), it is
clearly observed that the A-MCDI cell shows the highest
removal efficiency of 83.4% but only a little enhancement
(4.3%) is achieved when compared with the CDI device.
Different from the results reported by other groups,
135,141,145
the
MCDI cell (9.23%) exhibits a considerably lower desalination
efficiency than the CDI cell (79.1%). Moon et al. attributed this
to the contact resistance and membrane resistance. The good
stability of the A-MCDI cell was veried using a cyclic desali-
nation test (Fig. 29(d)).
Recently, Pan et al.
144
further developed a modied MCDI
(m-MCDI) device based anion and cation exchange polymers.
Polyethyleneimine (PEI) and dimethyldiallyl ammonium chlo-
ride (DMDAAC) were used as the cation and anion exchange
polymers, respectively, and incorporated into the CNTs elec-
trodes using a casting method. From the SEM images in
Fig. 30(a)–(c), it can be observed that the CNTs–PEI and CNTs–
DMDAAC electrodes display compact surfaces compared to the
CNTs electrode and the wire-like CNTs are dispersed in the PEI
or DMDAAC matrix. The CDI experiments were carried out in a
continuously recycling system at a ow rate of 50 mL min
1
.
The size of all the electrodes was 8 cm 8 cm and the average
mass of the electrodes was 905 mg. The m-MCDI unit cell based
on both anion and cation exchange polymers exhibits a high
NaCl removal of 93%, much higher than that of the conven-
tional CDI cell with the CNTs electrode (25%) or the MCDI cell
with commercial anion and cation exchange membranes (74%)
in a certain experiment at 1.2 V and for an initial conductivity of
50 mScm
1
, as shown in Fig. 30(d). The electrosorption behavior
of the m-MCDI and MCDI cells in NaCl solution with an initial
conductivity of 1000 mScm
1
at a voltage of 2 V was also
investigated and their electrosorption capacity were 9.3 and
6.67 mg g
1
, respectively. The large improvement in desalina-
tion performance was mainly due to the reduced co-ion expul-
sion effect by introducing the ion exchange polymers and the
better contact adhesion between the ion exchange polymers and
electrodes than that found between commercial ion exchange
membranes and electrodes.
Despite the progress to date, there are still two problems that
need to be solved: (i) when a polymer is mixed too deeply into
the electrode, due to differential swelling of the two materials, it
may lead to fracturing of the carbon pores and also these
separate materials still have a resistive component that is
unfavourable for the CDI process, and (ii) no matter if the MCDI
is based on membranes or polymers, there still exists a
concentrated solution trapped behind the membranes/
polymers, as found in ED. In order to solve these problems,
Fig. 29 SEM images of (a) the bare carbon cloth surface and (b) the
embedded carbon electrode surface; (c) variations in the ion
conductivity of the CDI, MCDI and A-MCDI during their operation; (d)
continuous mode operation of the A-MCDI system by repeated
adsorption and desorption cycles. (Reproduced with permission from
ref. 147.)
Fig. 30 SEM images of the (a) CNTs, (b) CNTs–PEI and (c) CNTs–
DMDAAC electrodes; (d) electrosorption process for the CDI, MCDI,
m-MCDI cells. (Reproduced with permission from ref. 144.)
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Review RSC Advances
Andelman et al.
132
developed ionic group derivatized nano-
porous carbon electrodes. In their methodology the problem of
charge efficiency was settled by modifying the electrode mate-
rial itself, which could prevent the solution from being trapped.
The ionic group derivatized nanoporous carbon electrodes were
prepared according to the following steps: two activated carbon
electrodes were separately soaked in sodium dodecyl sulfate
and hexadecyltrimethylammonium aqueous solutions for one
month, and then washed in water. The CDI tests were carried
out using a 0.1 M NaCl feed solution at a ow rate of 20 mL
min
1
. A direct voltage of 1.2 V was applied. As shown in Fig. 31,
the charge efficiency of the ow through capacitor cell based on
ionic group derivatized nanoporous carbon electrodes charged
in the positive polarity was 70% with a product solution elec-
trosorption capacity of 10.5 mg g
1
, which is considerably
higher than those of the scientic control capacitor without the
attached ionic groups.
5 Carbon–polymer–metal oxide
composite
Although the MCDI technology integrating the ion-exchange
polymer and carbon electrode into one structure is considered
to deliver economical desalination performance, the further
reduction of the electrical resistance of the polymer coating
layer while maintaining the ion selectivity of the electrodes is
important for improving the desalination efficiency. Lee and
Choi developed a composite electrode by coating a mixture of
metal-oxide (TiO
2
) NPs and ion-exchange polymers (sulfonated
polystyrene) onto the AC electrode surface.
148
The TiO
2
could be
effective at maintaining the ion selectivity of the electrodes
because their surfaces were charged, while the electrical resis-
tance of the coating layer was thought to be reduced by the
formation of miscellaneous pores among the TiO
2
NPs. The
optimal content of TiO
2
in the carbon composite electrodes, in
terms of electrical resistance and ion selectivity, was found to be
approximately 10 wt%. The desalination performance of the
composite electrodes (10 cm 10 cm) was investigated in a
200 mg L
1
NaCl solution at a ow rate of 20 mL min
1
and a
potential of 1.2 V, and the desalination efficiency of the
composite carbon electrodes was improved by approximately
30% over that of an unmodied carbon electrode.
In summary, we have introduced various carbon-based
composite electrodes. In addition to these above-mentioned
materials, other composites, such as CNTs–polystyrene
sodium sulfonate–MnO
2
,
149
RG–PANI,
150
GO–CNFs,
151
CFs–SiO
2
/
g-Al
2
O
3
,
152
CFs–RG,
153
and CAs–silica
154
have also been studied
for CDI or MCDI applications. Table 1 lists the electrochemical
and electrosorption performances of different carbon-based
electrodes reported in the literature. As seen, the electro-
sorption performance of these materials is highly related to
both the electrochemical factors and the initial concentration,
which is in accordance with the ndings in ref. 132. The elec-
trosorption capacity varies from 0.13 to 17 mg g
1
, and most
of the composite materials exhibit superior desalination
performance when compared to their single component carbon
materials.
6 Prospects
CDI, as a rising star, has attracted more and more attention in
recent years due to its energy- and cost-saving advantages over
traditional water treatment technologies in the application of
desalination. However, there are some issues that should be
taken into account, otherwise, it will limit the applications of
CDI. One of the most key issues is related to the electrode
materials. During CDI operation, the electrosorption capacity is
closely associated with the accessibility, stability, permeability
and surface properties. It is important to note that in the
carbon–metal oxide composites, the stability of the metal oxide
in the aqueous solution under potential is critical for electrode
performance. Furthermore, water electrolysis and other para-
sitic redox reactions (such as the reduction of oxygen dissolved
in the water, oxidation of the carbon surface and oxidation of
Cl
ions to chlorine gas) may take place more easily when using
the carbon–metal oxide composite materials because the metal
oxide may change the potential of zero charge of the electrode to
be close to the potential of the parasitic redox reactions.
Therefore, the possible contamination caused by the metal
oxides eroded into the puried water should be the main
concern. Moreover, the MCDI system with high charge effi-
ciency and desalination efficiency is promising for practical
applications of CDI. However, the introduction of ion-exchange
membranes can result in high resistance, high cost and
expansion of the cell dimensions. Integrating ion-exchange
polymers with carbon electrodes can help to alleviate these
problems. Hence, more studies on this issue are required in
Fig. 31 Electrosorption process for the capacitor based on an ionic
group derivatized nanoporous carbon electrode charged in the posi-
tive polarity and scientific control capacitor (Reproduced with
permission from ref. 132.)
15220 |RSC Adv.,2015,5, 15205–15225 This journal is © The Royal Society of Chemistry 2015
RSC Advances Review
order to perfect the replacement of ion-exchange membranes by
carbon–polymer composite electrodes.
Another problem faced by CDI technology is that although
the composite electrode materials were proposed to improve the
salt removal by CDI, the treatment of highly concentrated
salt water such as seawater is still difficult. This is because of
the low charge efficiency
132,134
caused by the inaccessible
pore volume and the co-ion expulsion effect. In addition, this
Table 1 Comparison of the electrochemical and electrosorption performance among different carbon-based electrode materials reported in
the literature
Electrode material
Electrochemical
performance Electrosorption performance (in NaCl solution)
Ref.
Specic capacitance
(F g
1
)/scan rate (mV s
1
)
Applied voltage
(V)
Initial concentration
(mg L
1
)
Electrosorption
capacity (mg g
1
) Charge efficiency
CAs —1.2 50 1.4 —23
1.2 500 2.9 —
1.2 58.5 1.34 —161
—1.2 140 4.51 —
AC —1.5 200 3.70 —141
—1.2 292 10.90 0.85 26
—1.4 1170 13.00
—1.2 292 10.50 —162
—1.2 292 6.90 0.85 15
—1.4 292 8.40
169/1 1.2 25 0.25 —42
108/1 1.2 50 0.27 —163
—1.2 100 6.10 —33
—1.2 200 8.00 —
—1.2 500 9.72 —
—1.2 1000 10.80 —
—1.2 1500 11.00 —
—1.2 2000 11.76 —
—1.2 60 0.13 —108
CNTs —1.2 3000 1.7 —62
—2.0 23 1.3 —61
—1.2 60 0.7 37
—1.2 500 2.57
—1.2 1000 3.71 40
—1.2 1500 4.76 —
—1.2 2000 5.24 —
CNFs —1.2 60 3.2 —37
228/2 1.6 95 4.6 —38
MC 251/1 1.2 25 0.68 —42
192/1 0.8 50 0.93 —163
CNTs–MC —1.2 40 0.63 —65
132.6/10 1.2 40 0.69 —66
CNTs–CNFs —1.2 50 3.32 —27 and 70
RG–AC 181/1 1.2 500 2.94 0.24 90
RG–MC 89.6/1 2.0 45 0.73 —91
RG@MC sphere 43.2/10 1.6 34 2.3 —164
RG–CNTs 175/1 2.0 27 1.41 —68
220/5 2.0 770 26.42 —165
311.1/10 1.6 50 0.88 —93
RG–CNTs aerogel —1.6 4000 79.4 —96
1.0 35 000 633.3 —
ACF–CNFs 29.2/10 1.2 25 17.19 —102
ACF–CB —1.6 90 9.13 —101
AC-TiO
2
84.7/10 1.2 100 8.05 0.34 —111
104/2 1.2 584.4 17 —114
AC–ZnO 95/1 1.2 100 8.5 —116
AC–MnO
2
77.2/10 1.2 25 0.99 —149
RG–TiO
2
119.7/100 1.2 500 15.1 —50
1.2 6000 24.2 —
RG–RF 135.7/10 2.0 40 2.14 —129
CNTs–PEI–DMDAAC 52.6/5 2.0 1000 9.3 0.70 144
Polarized porous carbon 20.5/—1.2 5850 10.5 0.70 132
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Review RSC Advances
problem becomes worse with an increasing concentration.
Recently, a new form of CDI structure, which employs a ow
electrode, named as FCDI, has been proposed to realize the
desalination of seawater.
155–157
This particular design uses sus-
pended carbon electrode instead of a traditional xed electrode,
which allows the ions to ow through a ow path between the
ion-exchange membrane and the current collector, with the salt
water running through a spacer. This design makes sure that
the carbon particles are fully exposed to the salt ions to x the
problem of low charge efficiency in a certain degree. Prelimi-
nary results have shown that FCDI is a highly efficient design
even in high salinity. However, further exploration on suitable
ow electrodes, especially the composite with high electro-
sorption capacity should be a key in the future study of FCDI.
As a versatile technology CDI can also be used for the
extraction of heavy metal ions or uoride, nitrate and phos-
phate from aquatic resources. A new feature of CDI is the
selective removal of specic ions by composite electrodes
showing specic adsorption for heavy metal ions or calcium
ions can be possibly fabricated in the future.
158–160
In the fore-
seeable future, selective adsorption of target ions should be a
concern in CDI research.
In summary, CDI is a promising desalination technology and
shows a bright future despite its related challenges. With a
continuous search for solutions to these challenges, CDI or its
advanced versions may be considered as key to the worldwide
water shortage in the future.
Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (no. 21276087) is gratefully acknowledged.
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