Carbon spheres with hierarchical micro/mesopores for water desalination by capacitive deionization

Article abstract of DOI:10.1039/c6ta07616e

In this work, porous carbon spheres with hierarchical pores (denoted as hCSs) were fabricated via a sol-gel process using a surfactant-directing assembly strategy and investigated as capacitive deionization (CDI) electrode materials for the first time. By using transmission electron microscopy and N₂ adsorption/desorption analyses, a hierarchy of micropores and mesopores was demonstrated to be present in the hCSs. Based on the results of CDI measurements, the hCSs obtained at 800 °C (hCSs-800) displayed the best electrosorption performance of all the hCS samples tested: hCSs-800 exhibited a high electrosorption capacity of 15.8 mg g^-1 when the initial NaCl concentration was 500 mg L^-1. Furthermore, according to a Kim-Yoon plot analysis, hCSs-800 integrated the merits of both a high electrosorption capacity and fast electrosorption rate, indicating the superiority of these hCSs for CDI applications.

Full text of DOI:10.1039/c6ta07616e

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Carbon spheres with hierarchical micro/mesopores for water
desalination by capacitive deionization
Received 00th January 20xx,
Accepted 00th January 20xx
Xingtao Xu,‡ Hongmei Tang,‡ Miao Wang, Yong Liu, Yanjiang Li, Ting Lu and Likun Pan*
DOI: 10.1039/x0xx00000x
In this work, porous carbon spheres with hierarchical pores (denoted as hCSs) were fabricated via a sol–gel process with
surfactant-directing assembly strategy and invesiaged as capacitive deionization (CDI) electrode materials for the first time.
The hierarchical micro/mesopores of hCSs were demonstrated by characterizations from transmission electron microscopy
www.rsc.org/
and N
2
adsorption/desorption tests. The results of CDI measurements indicate that hCSs obtained at 800 °C (hCSs-800)
possesses the best electrosorption performance among all hCSs samples, and exhibits a high electrosorption capacity of
-
1
-1
1
5.8 mg g when the initial NaCl solution is 500 mg L . Further Kim-Yoon plot analysis proves that hCSs-800 integrates the
merits of both high electrosorption capacity and fast electrosorption rate, indicating the superioty of hCSs for CDI
applications.
the presence of overlapping effect caused by micropores,
which makes the ions very difficult to diffuse into the internal
pores, so the ion storage of mCSs decreases to a much less
Introduction
Capacitive deionization (CDI) is an emerging desalination
technology which has gained enormous interests in recent
years due to its several attracting features, such as low energy
consumption, high desalination efficiency and environmental
degree than those of porous carbons reported in similar
conditions by now. Notably, similar situation also happened in
3
6
the electrosorption process of microporous graphene sheets
until the emergence of mesoporous graphene sheets by Xu et
1
-10
friendliness.
ions by forming electrical double layer (EDL) on the surface of
Analogical to supercapacitor, CDI accumulates
3
al.
7
11-13
Taking this into consideration, an effective solution is to
porous carbons.
Therefore, the CDI performance of one
endow mCSs with abundant mesopores (2 to 50 nm pore size).
With the introduction of mesopores into mCSs, this type of CSs
would integrate mesopore (2 to 50 nm pore size) channels for
electrode is mainly dependent on the physical properties of
porous carbons, such as specific surface area, pore size
distribution, and so on. Currently, porous carbons such as
activated carbons, carbon nanofibers, carbon nanotubes,
graphene and their hybrids have been widely applied as CDI
3
7, 38
facilitating the electrolyte ion diffusion,
and micropores
(
<2 nm pore size) for providing abundant adsorbing sites for
3
9
1
4-29
ions. Such a hierarchical porous structure has also been
demonstrated to be the most important property for the
development of an advanced CDI electrode with high
electrode materials.
Despite the progresses achieved up to
date, the transfer of science into a realistic technology has still
yet to be completed. Exploring new porous carbon electrodes
should still be an urgent need for the development of CDI
technology.
3
5, 40
electrosorption capacity and rate.
In general, hierarchical
porous carbons can be easily prepared via using silica, metal
4
1-44
oxides, surfactants, or other non-inorganics as templates.
However, to date only several kinds of hierarchical porous
Carbon spheres (CSs), as one kind of zero-dimensional
porous carbons, have emerged as a new class of promising
4
5-47
carbons have been reported in CDI field.
The synthesis of
materials for potential applications in energy storage, CO
0-33
2
3
CSs with hierarchical micro/mesopores (denoted as hCSs) has
rarely succeeded.
capture, and catalyst support.
Recently, the potential of
microporous CSs (mCSs) as CDI electrode material has been
3
demonstrated in our previous work. Although the resultant
4
To address this issue, herein hCSs were prepared via a sol–
gel process with surfactant-directing assembly by using
cationic surfactant cetyltrimethylammonium bromide (CTAB)
as a template, resorcinol-formaldehyde as a carbon source and
tetraethoxysilane (TEOS) as an assistant pore-forming agent.
Due to their hierarchical porous structure, high specific surface
areas and large pore volumes, hCSs exhibit a superior CDI
performance.
2
mCSs possessed a high specific surface area of 1321 m g ,
-1
-1
they suffered from a low capacity of only 5.81 mg g even in
the presence of a high operation voltage (1.6 V), which is far
3
below the theoretical prediction. This is mainly because of
5
Shanghai Key Laboratory of Magnetic Resonance, School of Physics and Materials
Science, East China Normal University, Shanghai 200062, China, Fax: +86 21
6
‡
2234321; Tel: +86 21 62234132; E-mail: lkpan@phy.ecnu.edu.cn
These two authors contributed equally to this work.
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outlet of the unit cell was used to monitor and measure the
Experimental
concentration variation. In our experiment, the electrosorption
-1
, mg g ) was defined as follows:
Materials Synthesis
capacity (
Γ
Resorcinol, formaldehyde (37 wt % solution), TEOS, CTAB,
ammonia aqueous solution (28 wt %) and ethanol were
obtained from Sinopharm Chemical Reagent Co., Ltd. (China).
(
C
0
−C ×V
m
e
)
(2)
Γ =
-
In a typical synthesis, resorcinol (0.30 g) and CTAB (0.30 g) where
C
0
e
and C are initial and final NaCl concentrations (mg L
1
were added to a mixed solution containing ammonia aqueous
solution (0.30 mL), ethanol (12 mL) and deionized water (30 of the electrodes (g).
mL), and stirred for 30 min. Then, TEOS (1.5 mL) and
Charge efficiency (
), V is the volume of NaCl solution (L) and m is the total mass
4
)
8
Λ
was calculated according to the
formaldehyde solution (0.42 mL) were added and stirred for 24 following equation:
h at room temperature, and subsequently heated at 100 °C for
Γ × F
Σ
(
3)
Λ =
2
4
h. The as-prepared products were collected by
centrifugation, washing and drying, and subsequently
carbonized at 600, 800 and 1000 °C for 3 h under N
-1
F is the Faraday constant (96485 C mol ), Γ
where
is the
-1
(charge, C g ) is
2
-1
electrosorption capacity (mol g ) and
Σ
atmosphere. Finally, hCSs were obtained by immersing
carbonized products into 10 wt % HF at room temperature for
obtained by integrating the corresponding current.
49
Kim-Yoon plot, which was firstly proposed by Yoon et al,
1
and further developed by Suss. et al, has been used to
evaluate the electrosorption performance of porous carbon
24 h with a subsequent washing and drying process. Notably
hCSs obtained at 600, 800 and 1000 °C are named as hCSs-600,
hCSs-800 and hCSs-1000, respectively.
materials. In detail, the electrosorption capacity at
t
t min (Γ ,
-1
Characterization
mg g ) of the electrode materials was calculated according to
the following equation:
The surface morphology and structure of hCSs were investigated by
scanning electron microscopy (SEM, JEOL JSM-LV5610) and
transmission electron microscopy (TEM, CM200). Pore properties of
(
C
0
−C ×V
m
t
)
(4)
Γt =
where C
electrosorption rate (v
calculated according to the following equation:
t
are the concentration at t min. The corresponding mean
2
hCSs were investigated by N physical adsorption measurement
-1
-1
t
, mg g min ) of the electrode materials was
conducted at 77 K on ASAP 2010 Accelerated Surface Area and
Porosimetry System (Micrometitics, Norcross, GA). Disorders and
graphitization degrees of hCSs were investigated by Raman spectra
Γt
(5)
vt =
(
RenishawinVia microscope) with a He-Ne laser (633 nm) used as
the light source for excitation. Electrochemical performances where
Autolab PGSTAT 302N electrochemical workstation) of hCSs were
investigated in three-electrode system by using cyclic
t
is the time of electrosorption process.
t
(
a
Results and discussion
voltammetry (CV) measurements in 1 M NaCl solution. A standard
calomel electrode and a platinum foil were used as reference Fig.
1
shows SEM images of hCSs-800. Notably, the
electrode and counter electrode, respectively. The specific morphologies of hCSs-600 and hCSs-1000 (not shown here) are
-1
capacitance (C, F g ) can be obtained from CV curves using the similar with that of hCSs-800. As shown in Fig. 1a, hCSs-800
following equation:
exhibits uniform spherical structures with rough surfaces. The
particle size of hCSs-800 is around 300 nm. Further
observation by enlarged SEM image (Fig. 1b) proves the
porous surfaces of hCSs-800. Additionally, the pores within
hCSs-800 were investigated by TEM measurements. As shown
in Fig. 2a and b, enormous pores can be clearly seen from
hCSs-800, due to the removal of template. Such a porous
~
C = i /(v × m)
(1)
~
-1
is the scan rate (V s )
where
and
CDI tests
i
is the average current (A),
v
m
is the mass of electrodes (g).
Electrode preparation: Each CDI electrode was prepared by structure is beneficial for providing more sites for ion
pasting a mixture of the sample with polymeric binder, accommodation.
polyvinylpyrrolidone (PVP)/polyvinyl butyral (PVB) onto
graphite paper. In detail, each electrode was composed of the
The pore properties of hCSs samples were investigated by
2
adsorption/desorption isotherms. As shown in Fig. 3a, all
N
sample, 1.5 wt.% PVP and 6.0 wt.% PVB. The mixtures were hCSs samples show hysteresis loops with a combination of
pressed onto graphite papers and dried in vacuum oven at 60 type I and type IV, indicating a hierarchically porous structure
2
0
°
C overnight. Additionally, the mass of each electrode is
∼
65 composed of micropores and mesopores. The corresponding
specific surface areas, pore volumes and mean pore diameters
mg.
Batch-mode CDI tests: All experiments were carried out in NaCl are listed in Table 1. It can be seen that hCSs-800 exhibits a
-1
2
-1
solution (
∼
100 mg L ) with a volume of 20 mL, and the high specific surface area of 1529 m g and a large pore
solution temperature and flow rate were kept at 298 K and 27 volume of 1.73 cm g , higher than those of hCSs-600 (1351
3
-1
-1
2
-1
3
-1
2
-1
3
-1
mL min , respectively. A direct voltage of 1.2 V was applied on m g ; 1.64 cm g ) and hCSs-1000 (1281 m g ; 1.46 cm g ).
the opposite electrodes and an ion conductivity meter at the Furthermore, the hierarchically porous structures of hCSs
2
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samples were proved by Barrett-Joyner-Halenda (BJH) pore
size distribution curves (Fig. 3b). Obviously, all hCSs samples
display hierarchical micro/mesopore structures. As reported
5
0, 51
by previous works,
mesopores are favourable for
enhancing mass transport and ion accommodation, and
micropores are beneficial to ion accommodation. Moreover,
due to the introduction of mesopores in hCSs, more micopores
can be accessible for ion accommodation. Therefore, hCSs with
micro/mesopore structures should be effective for CDI
application.
Fig. 3 (a) N
2
adsorption/desorption isotherms and (b) BJH pore size
distribution curves of hCSs samples.
Table 1 Specific surface areas, pore volumes, mean pore diameters
and I /I values of hCSs samples.
D
G
Fig. 1 SEM images of hCSs-800.
Sample
Specific
Pore
Mean pore
diameter
(nm)
D G
I /I
surface area
volume
2
-1
3
(cm g )
-1
(
m g )
1351
1529
1281
hCSs-600
hCSs-800
hCSs-1000
1.64
4.9
1.02
1.73
4.5
4
0.85
0.76
.6
1.46
Fig. 2 TEM images of hCSs-800.
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Fig. 4 Raman spectra of hCSs samples.
3 demonstrate that hCSs-800 shows the best electrosorption
Raman spectra were used to investigate the local structure performance.
of hCSs. As shown in Fig. 4, all hCSs samples show D and G
-
1
bands centered at
~
1349 and
corresponding to sp -carbon and sp -carbon in carbon matrix.
The intensity of D-band to G-band (I /I ) is representative for
~1598 cm , respectively,
3
2
D
G
3
4
defective or disordered degree in carbon matrix, and listed in
Table 1. Obviously, with the increase in carbonization
D G
temperature, I /I value increases, indicating the increase of
defects or disorders in hCSs, which is believed to favour ion
3
accommodation during the electrosorption process.
4
CV tests were used to investigate the electrochemical
performances of hCSs samples before electrosorption tests. As
shown in Fig. 4, all CV curves display relatively distorted
rectangular shapes. Moreover, hCSs-800 exhibits a largest
current density among all samples, indicating the largest
capacitance. The specific capacitances of hCSs samples
calculated according to equation (1) are listed in Table 2. It can
-1
Fig. 5 NaCl concentration variations for hCSs samples.
be seen that hCSs-800 shows a high capacitance of 263.6 F g ,
-
1
Table 3 Electrosorption capacities of hCSs samples in NaCl solution with an
which is much higher than those of hCSs-600 (216.8 F g ) and
-1
-1
initial concentration of ∼100 mg L .
hCSs-1000 (175.6 F g ), possibly due to its higher specific
surface area and larger pore volume, which could provide
more sites for the formation of EDL. Considering the
correlation between capacitance and electrosorption capacity,
it should be believed that hCSs-800 should have the best
electrosorption performance.
Sample
hCSs-600
hCSs-800
8.4
hCSs-1000
6.7
Electrosorption
5
.6
-1
capacity (mg g )
Fig. 4 CV curves of hCSs samples in 1 M NaCl solution at a scan rate of
-
mV s .
1
2
Table 2 Specific capacitances of hCSs samples.
Sample
Specific
-
capacitance (F g )
hCSs-600
16.8
hCSs-800
263.6
hCSs-1000
175.6
2
1
To figure out the electrosorption performances of hCSs
electrodes, batch-mode CDI experiments were carried out in NaCl
-1
solution with an initial concentration of ∼100 mg L . The applied
voltage was 1.2 V and the current variations were recorded
simultaneously and independently at each experiment. Fig. 5
shows the NaCl concentration variations for hCSs samples. It can
be seen that once the electric field is imposed, a sharp decrease of
NaCl concentration can be observed and it reaches equilibrium
after 45 min. The electrosorption capacities of hCSs shown in Table
Fig. 6 (a) SEM images and (b)
2
N adsorption/desorption isotherm of mCSs-
8
00. Inset of (b) is corresponding BJH pore size distribution curve.
In order to further highlight the superiority of hCSs, mCSs were
5
prepared as described in the literature with a carbonization
process at 800 °C (denoted as mCSs-800). The morphology and
2
pore property of mCSs-800 were investigated by SEM and N
2
4
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adsorption/desorption isotherm, respectively. As shown in Fig. hCSs-800
ARTICLE
6
a, mCSs-800 displays uniform spherical structures with
smooth surfaces. The particle size of mCSs-800 is around 500
nm. The characterization using N adsorption/desorption
isotherm shows that mCSs-800 displays a type-I hysteresis loop can be ascribed to following reasons: (i) hCSs-800 exhibits a
(in this
work)
1.73
1529
15.8
0.78
2
The superior electrosorption performance of hCSs-800
2
-1
2
-1
(Fig. 6b) with a BET specific surface area of 656 m g and a higher specific surface area of 1529 m g and a larger pore
3
-1
3
-1
pore volume of 0.32 cm g . Obviously, these values are much volume of 1.73 cm g compared with other kinds of CSs-
lower than those of hCSs-800. Furthermore, BJH pore size based materials (Table 4). As reported previously, such a high
distribution curve shown in the inset of Fig. 6b proves the specific surface area and pore volume (1529 m g and 1.73
2
-1
3
-1
cm g ) can provide more sites for ions storage, thus
35
microporous structure of mCSs-800.
The obtained mCSs-800 was applied for constructing CDI improving the electrosorption performance. (ii) hCSs-800
electrodes with a similar process with hCSs-based CDI electrodes. possesses a hierarchical porous structure. This type of
The CDI performances of hCSs-800 and mCSs-800 were compared in hierarchically porous structure integrates mesopore (2 to 50
-1
a NaCl solution of ∼500 mg L at an applied voltage of 1.2 V. The nm pore size) channels for facilitating the electrolyte ion
3
7, 38
corresponding electrosorption capacities and charge efficiencies are diffusion,
and micropores (<2 nm pore size) for providing
39
shown in Fig. 6. Clearly, hCSs-800 exhibits a high electrosorption abundant adsorbing sites for ions. Therefore, the hierarchical
capacity of 15.8 mg g and charge efficiency of 0.78, much higher micro/mesoporous structure of hCSs-800 is believed to not
than those of mCSs-800 (6.3 mg g and 0.56). In addition, these only accelerate the mass transfer, but also help to release more
values (electrosorption capacity and charge efficiency) also surpass accessible surface area for ion accommodation.
-1
-1
those of other CSs-based CDI electrodes (Table 4), and hierarchical
-1 45, 46, 53
porous carbons-based CDI electrodes (0.7-9.1 mg g )
reported previously.
Kim-Yoon plot model was used to further evaluate the CDI
performances of hCSs-800 and mCSs-800. As shown in Fig. 8, the
Kim-Yoon plot of hCSs-800 shifts towards the upper and right
region compared with that of mCSs-800, indicating that hCSs-800
integrates the merits of both high deionization capacity and
fast deionization rate.
Fig. 7 Electrosorption capacities and charge efficiencies of mCSs and hCSs in
-1
a NaCl solution of ∼500 mg L .
Table 4 Comparison of electrosorption capacities and charge efficiencies for
-1
hCSs and other CSs-based CDI electrodes in a NaCl solution of ∼500 mg L .
Fig. 8 Kim-Yoon plots of mCSs-800 and hCSs-800.
Specific
Pore
Electrosorption
Charge
Sample
surface area
volume
-1
capacity (mg g )
efficiency
2
-1
3
-1
(
m g )
(cm g )
3
4
mCSs
0.59
1321
5.8
0.40
0.49
N-doped
0
0
.79
.67
-
1640
809
13.7
54
mCSs
Hollow
13.2
7.2
-
-
5
5
CSs
Hollow
618
5
6
CSs
N-doped
hollow
0.70
512
656
10.3
6.3
0.52
0.56
5
7
CSs
mCSs-
8
00 (in
this
Fig. 9 Cycle stability of hCSs-800-based CDI unit cell in NaCl solution of
-1
00 mg L .
0
.32
5
work)
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1
1
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2
3
, 13418-13425.
-
1
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Journal of Materials Chemistry A
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DOI: 10.1039/C6TA07616E
Graphical Contents Entry
Carbon spheres with hierarchical micro/mesopores were prepared via a sol-gel
process with surfactant-directing assembly strategy and applied for capacitive
deionization.