Journal of Materials Science and Chemical Engineering, 2014, 2, 38-42
Published Online January 2014 (
Effective Modified Carbon Nanofibers as Electrodes for
Capacitive Deionization Process
Nasser A. M. Barakat1,2*, Ahmed G. El-Deen3, Khalil Abdelrazek Khalil4
1Organic Materials and Fiber Engineering Department, Chonbuk National University, Jeonju, South Korea
2Chemical Engineering Department, Faculty of Engineering, Minia University, El-Minia, Egypt
3BioNanosystem Department, Chonbuk National University, Jeonju, South Korea
4Mechanical Engineering Department, (NPST), King Saud University, Riyadh, Saudi Arabia
Email: *
Received November 2013
Carbon materials have the advantages of good electrical conductivity and excellent chemical stability, so many
carbon materials have been introduced as electrodes for the capacitive deionization (CDI) process. Due to the
low surface area compared to the other nanocarbonaceous materials, CNFs performance as electrode in the CDI
units is comparatively low. This problem has been overcome by preparing high surface area carbon nanofibers
and by creating numerous long pores on the nanofibers surface. The modified CNFs have been synthesized using
low cost, high yield and facile method; electrospinning technique. Stabilization and graphitization of electrospun
nanofiber mats composed of polyacrylonitrile (PAN) and poly (methyl methacrylate) (PMMA) leads form longi-
tudinal pores CNFs. The utilized characterizations indicated that the CNFs obtained from electrospun solution
having 50% PMMA have surface area of 181 m2/g which are more than the conventional CNFs. Accordingly,
these nanofibers revealed salt removal efficiency of ~90% and specific capacitance of 237 F/g.
Capacitive Deionization; Carbon Nanofibers; Electrospinning; Multi-Channels CNFs; Desalination
1. Introduction
In the last four decades, the number and capacities of
desalination units have increased dramatically; 45% Mul-
ti-Stage Flash (MSF) and 42% Reverse Osmosis (RO) of
world capacity [1]. Capacitive deionization (CDI) is a
promised desalination technology due to power saving,
low operation and maintenance costs [2], moreover no
chemicals are used in pre- or post-treatment processes; so
CDI is considered green and economic desalination tech-
nology [3]. Many carbon materials have been introduced
as electrodes for the CDI process such as carbon aerogels
[4,5], activated carbon (AC) [6,7], activated carbon fiber
(ACF) [8,9], carbon nanotubes (CNTs) [10], and Graphe-
ne [11-13]. Carbon nanofibres (CNFs) prepared by the
electrospinning process gradually attracted the attention
of most researchers because of the associated advantages
of the synthesizing technique and the obtained product.
The long axial ratio of the CNFs leads to decrease the
total surface area compared to the other nanostructural
carbonaceous materials, this might have negative influ-
ence upon utilizing CNFs as electrodes in the CDI process.
However, recently it was reported that the long axial ra-
tio provides advantage for the nanofibers over the nano-
particles in the electrons transfer-based processes as the
nanofibers reveal better performances [14]. In this study,
the surface area of the CNFs has been modified by pro-
ducing multi channeled CNFs. Typically, electrospinning
of binary polymer solution composed of PAN and poly
(methyl methacrylate) (PMMA) was achieved. Due to the
big difference in the thermal properties between the two
polymers as PMMA is completely eliminated during the
carbonization step [15], the physiochemical characteriza-
tion indicated that the surface area strongly increases
with increasing the PMMA content which positively af-
fects the performance as the nanofibers prepared from
PAN/PMMA solution having 50 wt% PMMA reveal high-
er ions electrosportion compared to the other formulations.
2. Experi men tal
2.1. Materials
Polyacrylonitrile (PAN, average Mw = 150000 g·mol 1)
*Corresponding author.
and poly(methyl methacrylate) (PMMA, average Mw =
120000 g·mol1) are of analytical grade and were pur-
chased from Sigma-Aldrich. N,N-dimethylformamide
(DMF 99.5% assay; SAMCHUN Pure Chemical Co.,
South Korea) without any further modifications was used
as a solvent..
2.2. Preparation of Multi-Channels Carbon
First, PAN and PMMA solutions (10 wt% in DMF) were
prepared individually in two glass bottles using ultra-
sonication for 2 h then heating with stirring in water bath
at 60˚C for 4 h. Electrospun solutions having different
PMMA:PAN ratios were prepared. Typically, PMMA
mass percentages with respect to PAN were 0, 25 and
50%. The bipolymer solutions were further ultrasconi-
cated for 0.5 h. Electrospinning of these solutions was
carried out at 20 kV and 18 cm distance between the col-
lector and the tip of the syringe. The oxidative stabiliza-
tion was carried out at 280˚C for 1 h with heating rate 5
deg/min and the carbonization was done at 1000˚C under
argon atmosphere for 5 h with a rate 5 deg/min.
2.3. Characterizations
The surface morphology was studied by a JEOL JSM-
5900 scanning electron microscope (JEOLLtd., Japan)
and field-emission scanning electron microscope (FE-
SEM Hitachi S-7400, Japan). High resolution image and
selected area electron diffraction patterns were obtained
with transmission electron microscope (TEM, JEOL
JEM-2010, Japan) operated at 200 kV. Cyclic voltam-
metry measurement was carried out using different con-
centrations (0.1, 0.5 and 1 M) NaCl solutions and the
sweep potential rang was adjusted from 0.4 to 0.6 V in
electrochemical cell with thr e e-electrode system: plati-
num wire as counter electrode, Ag/AgCl as reference
electrode and the prepared materials as working electrode.
This system was controlled using VersaStat4 potentiostat
device with through in VersaStudio software program.
3. Results and Discussion
Simplicity of the electrospinning process, the diversity of
the electrospinnable materials, and the unique features of
the obtained electrospun nanofibers provide especial in-
terest for both of the technique and the resultant products.
The past decades have witnessed tremendous progress in
the development of the electrospinning process to widen
the applications of the obtained products. Most of the
reported functional inorganic nanofibers have been pre-
pared using the electrospinning techniques. Good mor-
phology nanofibers were obtained from electrospinning
of PMMA/PAN polymer blends. Figure 1 displays the
Figure 1. SEM image for the PAN/PMMA electrospun nan-
electrospun nanofibers obtained from the prepared
PMMA/PAN solutions, as shown in the figure good
morphology nanofibers have been obtained; continuous
and beads-free nanofibers are observed. It is noteworthy
mentioning that the diameters frequency chart has been
estimated. From the obtained data, the average diameters
have been estimated to be 519, 335 and 226 nm for the
electrospun nanofiber mats prepared from the solutions
containing 0, 25 and 50 wt% PMMA with respect to
PAN, respectively.
Figure 2 shows the FE SEM images after stabilization
and graphitization processes, the corresponding average
diameters have been estimated by the same methodology;
the results indicated that the average diameters are 382,
220 and 160 nm for the sintered nanofibers prepared
from the solutions containing 0, 25 and 50 wt% PMMA
with respect to PAN, respectively. Interestingly, the nan-
ofibers synthesized from the last formulation have many
channels on the surface. This finding clearly appears in
the FE SEM and TEM images. Formation of the ob-
served channels can be explained from the nature of the
PMMA/PAN solution. This polymer-blend solution has
phase separation which results in sea-islands feature oc-
curs due to the intrinsic properties (e.g., interfacial ten-
sion, viscosity, elasticity) of the two polymers [16].
Therefore, the prepared solutions can be considered as
stable emulsion-like polymer-blend solutions. The con-
tinuous phase consisted of PAN solution and the homo-
geneously dispersed phase consisted of PMMA solution.
It is noteworthy mentioning that the viscosity and surface
tension of the PMMA/PAN solutions were intensively
studied by other authors [16]. The obtained results indi-
cated that the viscosity increased when the volume frac-
tion of PAN in the blend solution increased. The surface
tension of the PAN solution (5.195 mNm1) was found to
be higher than that of the PMMA solution (8.843 mNm1).
Thus, the surface tension of the constituent polymers in
Figyre 2. FE SEM images for the graphitized PAN/PMMA
electrospun nanofibers; PMAA = 50%.
the blend solution is the most important factor in deter-
mining the surface morphology of the electrospun or-
ganic nanofibers. As a result, the low-surface-tension
polymer (PAN) occupies the continuous phase of the
solution while the high-sur fa ce -tension polymer (PMMA)
forms the discontinuous phase.
C yclic voltammetry (CV) measurements are often ap-
plied as effective tools to characterize the CDI perform-
ance of electrodes. Figure 3 depicts CV curves of the
CNFs, reduced graphene oxide (rGO) and multi-channels
carbon nanofibers MC-CNFs 25 %, and MC-CNFs 50%
electrodes at sweep rates 10, 50 and 100 mV·s1 in 0.1,
0.5, and 1 M NaCl solution. As shown in the figure, the
MC-CNFs 50% clearly retain a rectangular shape over a
wide range of applied voltage. Obviously, typical rec-
tangular profile implies excellent electrochemical double-
layer capacitance and also indicates highly reversible and
reliable electrosorption charge-discharge process [17].
Furthermore, MC-CNFs 50% nanofibers showed prece-
dence at high concentration NaCl solutions with high
electrosorption capacity which indicates hard saturation
Figure 3. Cyclic voltammetry results for the prepared ma-
terials at different NaCl concentration and various sweep
of the internal mesoporous and systematic channel struc-
ture. Another important finding can be observed from
Figure 3 is that the performances of the investigated
materials are linearly related in the estimated surface
areas which support our original target about improving
the surface area of the CNFs. It is noteworthy mentioning
that the MC-CNFs 50% nanofibers reveal the best per-
formance at all the scan rates and for all the investigated
NaCl solution concentrations. As aforementioned in the
experimental section, the specific capacitances have been
estimated from I-V cycles. Figures 3(c) and (d) display
the cyclic voltammetery measurements for the prepared
materials at various scan rates and 0.5 M NaCl solution.
Graphene has been introduced as extremely promising
electrode for capacitive deionization [11-13]. Graphene
is an intriguing 2D monolayer carbon sheet whose dis-
tinct properties make it very promising in desalination
applications. Since the first report of graphene produc-
tion by mechanical exfoliation method, many techniques
broadly classified as top down and bottom up syntheses
have been developed. Among the various reported
methods for producing graphene, chemical reduction of
graphite oxide (GO), or chemically converted graphene,
provides not only an established, low-cost and scalable
approach, but also a highly flexible method for the
chemical fictionalization of graphene materials [18,19].
Therefore, in this study, this chemical rout was invoked.
An important property of GO, brought about by the hy-
drophilic nature of the oxygenated graphene layers, is its
easy exfoliation in aqueous media. As a result, GO read-
ily forms stable colloidal suspensions of thin sheets in
water [20]. After a suitable ultrasonic treatment, such
exfoliation can produce stable dispersions of very thin
graphene oxide sheets in water [18]. Therefore, colloidal
suspension of the prepared GO prepared with the aid of
ultrasound was clear, homogenous, and stable. It is
known that with increasing the scan rate the specific ca-
pacitance dramatically decreases. As shown in Figure 4,
the corresponding specific capacitance of the introduced
MC-CNFs 50% is higher than the other carbonaceous
materials. Typically, at 10 mV/s scan rate, the specific
capacitances are 235, 188, 162 and 63 F/g for the MC-
CNFs 50%, rGO, MC-CNFs 25% and CNFs, respec-
tively. Another interesting finding is that the introduced
materials display good specific capacitance stability at
the high scan rates which is a good feature for the carbo-
naceous materials making them adequate to be utilized as
electrodes in the CDI units. It is noteworthy mentioning
that the electrosorption capacitance is generally high at
lower scan rates because that the diffusion of ions from
the solution could gain more available access to the elec-
trode surface which leads to more surface adsorp-
tion/desorption of ions [21]. However, at high scan rates,
the effective inner surface adsorption of ions would be
010 20 30 40 50 60 70
50wt PMMA
25wt PMMA
Pure CNFs
0wt PMMA
Specific Capacitance (F/g)
PMMA Contant (wt%)
Figure 4. Effect of PMMA content on the specific capaci-
tance of the introduced CNFs.
reduced accordingly. The desalination performance be-
havior of the synthesized materials was determined by
using the CDI cell in a NaCl aqueous solution that has an
initial conductivity of ~73 µS/cm. Obviously, rapid de-
crease in the conductivity in early stage reflects distinct
change in the NaCl concentration in CDI cell which in-
dicates the capability of the investigated material towards
ionic adsorption/desorption process. The desalination
performance curve illustrates the salt removal efficiency
(η) of the MC-CNFs 50% has a great proportion reach to
89.04% compared to CNFs, MC-CNFs 25% and rGO
which have desalination efficiencies of 56.57%, 73.68%,
82.89%, respectively. This improvement in salt removal
can be attributed to the propitious internal pore structure
which facilitates the ions adsorption from the solution to
channel punctures on electrode. Furthermore, the MC-
CNFs 50% have the highest electrosorptive capacity
(2.21 mg/g) compared to the other formulations because
of created channels on the surface of introduced modified
carbon nanofibers.
4. Conclusion
Polyacrylonitrile (PAN) and poly (methyl methacrylate)
(PMMA) in DMF form stable polymer blend solution.
Electrospinning of the PAN/PMMA biopolymer solution
leads to produce smooth and beads-free nanofibers. Be-
cause of the complete thermal decomposition of PMMA
within the temperature range of the graphitization pro-
cess, the electrospun mats containing 50 wt% PMMA
with respect to PAN reveals carbon nanofibers having
many channels on the surface. The proposed modified
CNFs have adsorption and desorption ability of ions salt
more than reduced graphene oxide and normal CNFs. In
addition, the introduced CNFs exhibit superiority in spe-
cific capacitance value. Therefore, the modified carbon
nanofibers can be successfully utilized as electrodes in
capacitive deionization units.
Acknowled gements
This work was financially supported by the National Plan
for Science & Technology (NPST), King Saud Univer-
sity Project No. 11-NAN1460-02.
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