Nitrogen-rich microporous carbon materials for high-performance membrane capacitive deionization

Article abstract of DOI:10.1016/j.electacta.2019.04.172

Novel two-dimensional carbon nitride hexagonal-like sheets were fabricated from thermal condensation, which uses hexaazatriphenylene-based precursor prepared microporous and oxidation resistant carbons materials (HAT-CN). The results show that the as-prepared HAT-CN is the first electrode utilized in capacitive deionization, due to the rich heteroatom and border-group content, as well as the potential collaborative effect of C and N atoms, displaying the quick ion diffusion and strong charge transfer performance. Additionally, the capacitive deionization performance of HAT-CN is improved by the synergistic contribution of a large accessible surface area, large pore volume, and high graphitization. The electrode of HAT-CN was carbonized at 550 °C which display an excellent specific capacitance of 179.2 F g^?1 in 1-M NaCl solution at a scan rate of 5 mV s^?1. Subsequently, a high salt adsorption capacity of 24.66 mg g^?1 was attained for an applied voltage of 1.2 V in 500 mg L^?1 NaCl solution, showing good cyclic stability at 30 cycles. Considering those excellent properties of the prepared HAT-CN, the capacitive deionization electrode should be an ideal candidate for high-performance deionization application.

Full text of DOI:10.1016/j.electacta.2019.04.172

Accepted Manuscript
Nitrogen-rich microporous carbon materials for high-performance membrane
capacitive deionization
Danping Li, Xun-an Ning, Yue Huang, Shengchuan Li
PII:
S0013-4686(19)30878-3
https://doi.org/10.1016/j.electacta.2019.04.172
EA 34120
DOI:
Reference:
To appear in: Electrochimica Acta
Received Date: 25 February 2019
Revised Date: 15 April 2019
Accepted Date: 28 April 2019
Please cite this article as: D. Li, X.-a. Ning, Y. Huang, S. Li, Nitrogen-rich microporous carbon materials
for high-performance membrane capacitive deionization, Electrochimica Acta (2019), doi: https://
doi.org/10.1016/j.electacta.2019.04.172.
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ACCEPTED MANUSCRIPT
Nitrogen-Rich
Microporous
Carbon
Materials
for
high-performance membrane capacitive deionization
Danping Li, Xun-an Ning *, Yue Huang, Shengchuan Li
Guangzhou Key Laboratory of Environmental Catalysis and Pollution Control, School of
Environment Science and Engineering, Institute of Environmental Health and Pollution Control,
Guangdong University of Technology, Guangzhou 510006, China
*Corresponding author: Xun-an Ning, E-mail: ningxunan666@126.com, Tel/Faxꢀ(020)3932-2546
Abstract
Novel two-dimensional carbon nitride hexagonal-like sheets were fabricated from
thermal condensation, which uses hexaazatriphenylene-based precursor prepared
microporous and oxidation resistant carbons materials (HAT-CN). The results show
that the as-prepared HAT-CN is the first electrode utilized in capacitive deionization,
due to the rich heteroatom and border-group content, as well as the potential
collaborative effect of C and N atoms, displaying the quick ion diffusion and strong
charge transfer performance. Additionally, the capacitive deionization performance of
HAT-CN is improved by the synergistic contribution of a large accessible surface area,
large pore volume, and high graphitization. The electrode of HAT-CN was carbonized
at 550 ꢁ which display an excellent specific capacitance of 179.2 F g-1 in 1-M NaCl
solution at a scan rate of 5 mV s-1. Subsequently, a high salt adsorption capacity of
24.66 mg g-1 was attained for an applied voltage of 1.2 V in 500 mg L-1 NaCl solution,
showing good cyclic stability at 30 cycles. Considering those excellent properties of
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the prepared HAT-CN, the capacitive deionization electrode should be an ideal
candidate for high-performance deionization application.
Keywords: Two-dimensional, Nanoporous Carbon, Capacitive deionization,
Desalination
1. Introduction
Shortages in water resources and water security are two of the most important
problems threatening and hindering the rapid and sustained development of society,
exacerbated by unusable brackish water and the difficulty in regenerating domestic
and industrial sewage [1, 2]. Traditional desalination techniques such as membrane
distillation, ion-exchange resins, reverse osmosis, and electrodialysis are
energy-intensive and costly, including regeneration by environmentally unfriendly
means such as the use of an acid or base solution [3-8]. Thus, a novel water
desalination technique—capacitive deionization (CDI)—has been rapidly developed
because of its excellent performance, which includes high energy efficiency and water
recovery and low energy consumption and contamination [9]. CDI refers to the
removal of ions using capacitive adsorption. As the voltage is applied, the solution
containing the charged substance migrates toward the electrodes and is adsorbed,
forming double electric layers (EDLs) on the electrode-liquid interface [10, 11]. Then,
the charge is desorbed back into the solution through the cancelation of the electric
field or the reversal of the cell voltage. CDI could regenerate and restore during the
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adsorption and desorption processes [12, 13]. An example is membrane capacitive
deionization (MCDI), which integrates commercial ion-exchange membranes onto the
CDI devices. It has been demonstrated that the electroadsorption efficiency is
improved by greatly reducing the effect of the co-ion [14-16].
A key parameter affecting the desalination efficiency of CDI is the performance of
the electrode material, which includes a high surface area and electric conductivity,
good wettability, and physicochemical stability [17]. Therefore, carbon materials
including activated carbon (AC) [18], carbon aerogel (CA) [20], carbon nanotubes
(CNTs) [19, 22], and grapheme [21, 22] have been investigated as the ideal candidates
for CDI electrodes, which is benefit by their excellent conductivity, high specific
surface area (SSA), and good chemical stability. However, the electrosorption
capacity didn’t achieve the expected performance due to the available surface area is
limited by the van der Waals forces within graphene flakes and unideal wettability of
AC and CNTs. [23-24] Also, the high manufacturing cost and complex preparation
process is also limited the further application as CDI electrode. [25]
Recently, the 2D crystalline C2N network— a structural analog of grapheme, as a
novel material, has been synthesized by Mahmood in 2015 [26]. C2N has regular
holes and a large electronic band gap (1.96 eV), which gives it better thermodynamic
stability and capacity than that of g-C3N4. Researchers are working to synthesize 2D
crystals with tuneable structures and properties, using a bottom-up approach [27-30].
C2N has been found to have broad energy and environmental applications in
nanoelectronics sensors, gas storage, and batteries, because of its high surface area,
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good crystallinity, and fast ion transport [31-33]. This is compared to the original
graphene structure, which is surrounded by 6 N atoms, each with a dangling bond in
C2N. The size of the hole allows the anchoring of a few interacting K+ or Cl- atoms.
Meanwhile, heteroatom doping can enhance the polarization of the structure and
increase specific binding sites, both working together to increase the enthalpies of
interaction between the guest and adsorbent species significantly, as compared to
carbon materials [34]. Relatively high nitrogen content can enhance hydrophilicity,
electric conductivity, and capacity [35, 36]. All these factors indicate that it will be
extremely interesting to investigate the feasibility of using C2N nanosheet as CDI
electrodes. All these highlights show that C2N nanosheet has great potential for
application in CDI.
In our work, the HAT precursor has been prepared by the simple reaction of
hexaketocyclohexane octahydrate and diaminomaleonitrile in hot acetic acid solution,
followed by carbonization and simple condensation to remove C2N2 species based on
π-electrocyclic rearrangements to prepare HAT-CN-X. The template, additional
nitrogen sources, and any complicated steps are not required. The electrochemical
behavior and MCDI performance of the HAT-CN-X electrode were examined in NaCl
aqueous solution. For comparison, an AC electrode was also investigated. As expected,
the HAT-CN-X electrode shows better capacitive and desalination performance than
AC electrodes. The results indicate that HAT-CN-X is a promising material for MCDI.
As far as we know, there have been no reports regarding the use of HAT-CN-X as an
electrode in MCDI.
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2. Experimental
2.1. Fabrication
Hexaketocyclohexane octahydrate, diaminomaleonitrile, acetic acid (AR, 99.5%),
nitric acid (AR, 65%), and acetonitrile (AR) were purchased from Sinopharm
Chemical Reagent Co., Ltd. All the reagents were used without further purification.
Activated charcoal (AC, SSABET ~1000 m2 g-1) was purchased from Shanghai Aladdin
biochemical technology Co., Ltd., CHN. Cation/anion exchange membranes
(LE-HoCM Grion 0011/1201) were supported by Hangzhou Grion Environment
Technology Co., Ltd., CHN.
The synthesis of hexaazatriphenylene-hexacarbonitrile (HAT) was carried out
according the procedure stated in the literature [37, 38]. Synthesis of HAT:
Hexaketocyclohexane octahydrate (8 g, 12.6 mmol) and diaminomaleonitrile (22 g,
100.8 mmol) were refluxed in AcOH (300 mL) for 2 h. The black suspension was
filtered off while hot and washed with hot AcOH (3 × 25 mL), resulting in a black
solid. The solid was suspended in 30 % HNO3 (60 mL) and heated at 100 °C for 3 h.
The hot dark brown suspension was poured into ice water (200 mL) and cooled
overnight. The suspension was filtered and the solid was refluxed in MeCN (500 mL)
for 2 h and was filtered. The filtrate was then evaporated in vacuo to yield an orange
solid (4.5 g, yield 50%).
The synthesis of HAT-CN-X was prepared using HAT (1 g) for carbonization in a
tubular furnace at different temperatures for 1 h under N2 gas flow. The heating ramp
was set up to be 2 °C min-1 from 30 to 100 °C, and 5 °C min-1 from 100 °C to 450,
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550, 700 or 800 °C. The described resulting materials are labeled as HAT-CN-X,
where CN stands for the element type after carbonization and X refers to the synthesis
temperature.
2.2. Characterization
The products were characterized by using a scanning electron microscope (SEM,
JEOL JEM-700F, JP) equipped with an energy dispersive X-ray spectrometer (EDS)
and a transmission electron microscope (TEM, JEOL, JEM- 200CX, JP). The
crystalline structures of the samples were identified by powder X-ray diffraction
(PXRD, Bruker, D8, GER) using a Cu-Kα radiation source at 40 kV and 40 mA.
Fourier transform infrared (FTIR) spectroscopy was performed with a Perkin-Elmer
Spectrum GX at a range of 4000–500 cm-1. Raman spectra (HORIBA, LabRam HR
Evolution, FR) were recorded at a laser of 532 nm. X-ray photoelectron spectroscopy
(XPS) data was recorded on a Thermo Fisher Scientific K-Alpha system equipped
with a twin anode Al-Kα (1486.6 eV) X-ray source. C/H/N elemental analysis (EA)
was accomplished as combustion analysis using a Vario Micro device. Nitrogen
adsorption/desorption isotherms were measured with an ASAP 2460 (Micromeritics,
USA) at 77 K. Before the measurements, all samples were degassed 20 h at 493 K
under vacuum. The specific surface areas (SSAs) and the pore volumes were
calculated using the multi-point Brunauer-Emmett-Teller (BET) model. The pore size
distributions were derived from the desorption branches of the isotherms using the
nonlocal density functional theory model (NLDFT). The surface wettability of the
samples was investigated by a drop-shape analysis system (DingSheng, JY-82B, CN).
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Thermogravimetric analysis (TG) was performed on a TG209 (NETZSCH, GER)
under nitrogen atmosphere with a 5 °C min-1 heating rate.
2.3 Electrochemical capacitance evaluation
The electrochemical properties were determined by an electrochemical
workstation (CHI 660E, Chinstruments, CN). The test uses a three-electrode system in
a 1.0-M NaCl solution, which includes a HAT-CN-X electrode, a piece of platinum
plate electrode (1 × 1 cm2), and an Ag/AgCl (saturated 1.0-M KCl) electrode, which
serves as the working, counter, and reference electrodes, respectively. The working
electrode was prepared by mixing the active component, acetylene black, and poly
(vinyl alcohol) in a ratio of 8:1:1 wt.% to form homogeneous slurries, then coated
onto a titanium mesh and dried at 110 °C in a vacuum oven for 1 h. For comparison,
AC and HAT-CN-X electrodes were prepared under the same experimental conditions
as the working electrodes.
The electrochemical performance of the electrodes was evaluated by cyclic
voltammetry (CV) and galvanostatic charge-discharge measurements (GCD) in a
potential ranging from 0 to 1.0 V. Electrochemical impedance spectroscopy (EIS) was
conducted with an amplitude of 5.0 mV and a frequency range from 100 kHz to
0.01 Hz. The specific capacitances (Cs) were evaluated by the following formula with
the CV curves:
ꢃꢄꢅ
ꢀꢁ = ꢂ
(1)
ꢆꢇꢈ
where Cs (F g-1) is the specific capacitance, I (A) is the response current density, U (V)
is the potential window, υ (V s-1) is the potential scan rate, and m is the mass of the
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electrode material.
2.4 CDI experiments
The deionization performance of electrodes was investigated in a CDI device
apparatus, as shown in Figure S1. The electrode parameters included a mass of about
200 mg, size of 30 × 30 mm2, and thickness of 0.2 mm. In each experiment, the
analytical pure NaCl solution (500, 700, and 1000 mg L-1) was continuously pumped
from a peristaltic pump into the cell, and the effluent returned to the unit cell with a
flow rate of 22 mL min-1. The volume of the solution was maintained at 80 mL at
298 K. The voltage was set within a range of 1.2, 1.4, and 1.8 V. The salt adsorption
capacity (SAC) was obtained as the following formula:
SAC = ꢉ ꢋꢉꢌ × V
(2)
ꢊ
ꢇ
where SAC (mg g-1) is the salt electrosorption capacity (mg g-1), Co and Ct (mg L-1)
are the initial and final concentrations of supply water, V (L) is the volume of the
solution, and m (mg) is the mass of the electrode material.
3. Results and discussion
3.1 Characteristics
The synthesis process of preparation of HAT and HAT-CN-X is shown in
Scheme 1, based on previous studies in which synthesis occurred through the thermal
pyrolysis
polymerization
of
hexaketocyclohexane
octahydrate
and
diaminomaleonitrile, and condensation in N2 atmosphere [34].
The phase morphology of HAT and HAT-CN-X is characterized by SEM and
TEM images, as shown in Figure S2 and Figure 1. Clearly, the shape of HAT and
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HAT-CN-X appear as some accumulation of small regular hexagons and a simply
smooth surface in Figure S2 (a–d). Therefore, the macrostructure, caused by the
pre-assembly of HAT, can be transferred to HAT-CN-X after carbonization, and the
regular shape also showed good mechanical stability. As the carbonization
temperature increases, some uniform cracks (5–10 ꢀm in size) begin to form in the
plates, which indicates that their structures are made up of small primary slabs,
randomly arranged. Furthermore, the evenly porous nanostructures of HAT-CN-450,
HAT-CN-550, and HAT-CN-700 are investigated in TEM (Figure 1(e–h)) images.
These structures provide more ion-accessible space and significantly increased
electrosorption capacitance. However, the structure of HAT-CN-800 appears stacking
and interpenetrated with each other in Figure 1(h). There are signs that a structural
rearrangement after carbonization changes an inert carbon to a nearly graphitic carbon
phase without heteroatoms, following a temperature rise.
The higher nitrogen content was indicated in the as-prepared HAT-CN-X materials
at lower carbonization temperatures, characterized by C/H/N EA, with the XPS and
EDS mapping spectrometer results shown in Table 1. Clearly, the N elemental content
measured by EA (31.5 at.%) is approach by XPS (29.3 at.%) and further demonstrated
in the EDS (36.1 at.%) examination. All of the results show that HAT-CN-550 has a
nearly perfect C2N-type stoichiometry, as previously reported [27, 39]. Moreover,
EDS mapping also shows that carbon and nitrogen elements are evenly distributed, as
shown in Figure 1(a-d). Therefore, the governed condensation mechanism can
guarantee that the stoichiometric composition, calculated for HAT-CN, is transferred
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to the C2N-type products.
XPS was used to identify the composition and functionality characteristics of C and
N atoms in the materials, as shown in Figure S3. The N 1s signal (Figure 3) shown
exists in cyano (~398 eV), pyrazine (~399 eV), quaternary nitrogen atoms (~400 eV),
and oxidized nitrogen (~402 eV) groups after deconvolution. [34] The carbonization
temperature can be regulated to control the amount of these groups in the as-prepared
products. The increase in temperature is the primary reason for the reduction in N,
characterized by the absence of cyano and pyrazine groups, as their content decreased
continuously from 10.84 % to 5.00 % and 14.61 % to 9.57 %, respectively. On the
contrary, the content of quaternary nitrogen group increases with higher temperature
from 4.75 % to 9.83 % because of its higher binding energy. Previous studies have
shown that carbon materials with high N-heteroatom content mainly introduced from
cyano and pyrazine groups have good wettability and charge transfer ability, and
therefore have better capacitive deionization performance. [34, 35]
The C1s spectrum (Figure S4) exists in graphitic C=C carbons (284 eV),
sp2-hybridized nitrogen atoms (285 eV and 286 eV), oxidized carbon atoms (289–290
eV), and oxidized nitrogen (~402 eV) groups after deconvolution. With the increase in
temperature, the single peaks are transferred to lower binding energies, exhibiting a
reduction in the positive working potential and less-inert properties of the electrons.
Eventually, the inertia is lost again at very high temperatures. However, the content of
graphitic C=C is increased at higher temperatures from 6.95 % to 22.12 %.
Additionally, the controllable carbon electronic structure and the potential synergistic
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effect of C and N atoms would be advantageous to the capacitive and deionization
performance.
Figure S5 and Figure 2(a) shows the Raman spectrum of HAT and HAT-CN-X,
which is indicated by so-called disordered sp3 carbon (D-) and crystallized graphitic
sp2 carbon (G-), near 1350 cm-1 and 1595 cm-1, respectively. With the increase in
carbonization temperature, the ID/IG ratio increases from 0.96 to 1.21, owing to the
opportune adjustability of the carbon structure in six-rings and infrequently replaced
N atoms, that is, the continuous formation of the primitive carbon phase. Therefore,
material with higher carbon symmetry and electrical conductivity was prepared at
higher temperatures.
The FT-IR spectra of the HAT and HAT-CN-450/550/700/800 electrodes are shown
in Figure 2(b). All five samples show the appearance of the bands at 1599, 1459, 1333,
and 1221 cm–1, which indicate stretching vibrations of aromatic CN bonds in
condensed N-containing hybridization. The absorption bands at 1333 and 1599 cm–1
may correspond to the stretching vibrations of the C–N and C=N bonds, respectively.
There are less intense peaks of terminal cyano and pyrazine groups at 2242 cm–1 with
increasing synthetic temperature [40]. The results were further confirmed by Raman
and XRD spectrum analysis.
X-ray diffraction (XRD) patterns of the as-prepared HAT and HAT-CN-X materials
are further investigated. Figure 2(c) revealed that HAT has a crystalline structure by
self-assembly because of its strong supramolecular aggregation, making it a hopeful
material for the preparation of inert carbons. After condensation, the absence of
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crystalline inorganic impurities led to the formation of an amorphous structure with
increasing temperature. The results shown an absence of characteristic peaks of
graphitic stacking (2θ, 26) at high temperature, which may also be the reason for the
formation of high specific surface-area carbon. TG analysis of HAT (Figure S6) also
confirmed that it decomposes completely below 800 °C. Raman spectra further
confirms this result. Nevertheless, the increasing peak intensity (2θ, 26) indicates that
the reemergence of graphitic phase stacking at higher carbonization temperatures is
consistent with the TEM results.
Figure 4 shows the contact angle of the as-prepared materials, utilizing water
droplets on the surfaces. Their values are summarized in Table 1, showing an
excellent surface hydrophilicity for all the as-prepared materials. Meanwhile, all of
the values are almost same and both small. The favorable hydrophilicity of
HAT-CN-X significantly contributes to its electrosorption performance because of the
high nitrogen content and polar groups on its surfaces such as –OH and –C≡N.
N2 physisorption isotherms were used to characterize the specific surface area (SSA)
and pore size distribution, suggesting type-I isotherms in Figure 2(d). A large amount
of N2 is shown to absorb at p/p0 < 0.1 with no further adsorption at p/p0 = 0.1–0.9,
indicating the almost unique microporous properties of HAT-CN. Meanwhile, the
inorganic porogen-free, as-prepared samples provide high SSA in the range of 600–
800 m2 g-1, shown in Table 1. The conspicuous micropores have representative
geometric shapes and angles of a polycondensed framework, which is condensed from
the original nonporous HAT crystals. After condensation, considerable, accessible
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microporosity is created by a good arrangement of HAT. In addition, the micropore
enlarged with increasing synthesis temperature provided an increase in micropore
volume. Finally, the adsorption capacity at p/p0 > 0.9 came from the adsorption on the
thin flake surface and within the hierarchical cavity of a wafer. In a previous report,
higher SSA of micropores introduced more active sites to adsorb ions [41]. Clearly,
HAT-CN-550 exhibits a highly microporous structure, which is essential to its
capacitive and deionization property.
3.2 Electrosorption experiments
The electroadsorption behavior of as-prepared samples was analyzed by cyclic
voltammetry (CV) measurements. Figure 5(a) shows the CV test for all formulated
electrode materials. Clearly, no oxidation/reduction peaks were observed in all the
formulations under the selected potential range, indicating that the ions were adsorbed
on the electrode surface by forming EDLs due to coulomb interaction rather than
electrochemical reaction [41]. Owing to the intrinsic resistance and polarization effect
of the electrode little deviation from the standard rectangular structure is evident.
However, the CV curve of the as-prepared material also shows an approximately
rectangular shape, suggesting that reverse potential scanning can be achieved to
rapidly charge the platform, that is, salt ions have an excellent ability to adsorb/desorb
from the electrodes quickly.
According to the calculations from the CV curve for a sweep speed of 5 mV s-1, the
specific capacitance (Cs) is 52.25 F g-1 for HAT-CN-450, 179.20 F g-1 for
HAT-CN-550, 162.54 F g-1 for HAT-CN-700, and 99.86 F g-1 for HAT-CN-800.
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Clearly, the HAT-CN-550 electrode has the best Cs, as greater graphitization leads to
good conductivity and low internal resistance. However, the continuous pore size and
relatively integrated structure within flakes supply many more active sites. That is, a
more exposed surface can ensure the easier migration of salt ions across the interface
between solution and material. [41]
In Figure 5(b), the CV test of HAT-CN-550 was investigated at various scan rates
from 1–50 mV s-1, showing no oxidation/reduction reaction due to the rectangular
nature of the shape of the CV curves. This suggests that the salt ion can move in or
out quickly within the electrode, that is, Na+ and Cl- can adsorb/desorb from
HAT-CN-550 electrode. Compared to low scan rates (1–10 mV s-1), the higher scan
rates (50 mV s-1) show a CV curve shaped like a piece of leaves, exhibiting a degree
of distortion from the rectangle. This indicates the less available SSA in the internal
pores of the electrode due to insufficient time for the salt ions to move from solution
to materials and accumulate. On the contrary, the ions have sufficient time to diffuse
and accumulate in almost all obtainable SSA, producing a preferable capacitive
behavior at a slower sweep rate.
For further investigation, the relationship between specific capacitances and sweep
rate of HAT-CN-550 electrode is shown in Figure 5(c). Trends of the specific
capacitance show an increase, while the sweep rate decreased from 50 mV s-1 to 1 mV
s-1, while the specific capacitances for 50, 10, 5, 1 mV s-1 are 87.76 F g-1, 168.43 F g-1,
179.20 F g-1 and 221.28 F g-1, respectively.
In Figure 5(d), the influence of ion concentration on the electrical adsorption
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property of the HAT-CN-550 electrode was studied with a potential window of 0 to
1.0 V in 0.5-, 1.0-, and 1.5-M NaCl solutions at a scan rate of 5 mV s-1. Apparently,
the higher concentration of the NaCl solution has larger areas of CV curves,
suggesting that salty ion has a better effect of accumulation and adsorption on
electrode surface at a higher concentration of NaCl solution, resulting in higher
electronic conductivity and more ion contents in EDLs region. Hence, HAT-CN-550
has good electrosorption properties in CDI process, and also has great advantages in
seawater desalination.
For further study, the stability and invertibility effect of HAT-CN-550 electrode, the
galvanostatic charge-discharge curves (GCD) is tested at a current density of 200 mA
g-1 with a potential window of 0 to 1.0 V in 1.0-M NaCl solution, as shown in Figure
6(a). HAT-CN-X electrode is appearance a symmetric triangle feature of ideal
capacitor behaviors. It is worth noting that the curves are not an absolute symmetric
triangle, which may be the influence of a small pseudo-capacitance emanating from
rich-nitrogen content [43]. It also can be found that the HAT-CN-550 electrode has a
longer charge/discharge time, that its result is the same as the CV results, suggesting it
has higher specific capacitances. In Figure 6(b), it is investigated the GCD curves of
HAT-CN-550 electrode at a current density of 100–500 mA g-1. Clearly, at the
beginning of the discharge process appears, the internal resistance drops. A lower
gradient means a lower total drag, suggesting HAT-CN-550 electrode has low internal
resistance and is suitable for CDI. Furthermore, invertibility and stability are
investigated by GCD in 1.0-M NaCl solution at a current density of 200 mA g-1. In
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Figure 6(c), it can be found that there is no charge and discharge attenuation after 600
cycles, showing the HAT-CN-550 electrode has the long life for CDI applications due
to key features, such as good stability and cyclability performance.
To investigate the electrochemical impedance spectra, it is tested in 1.0-M NaCl
solution, as shown in Figure 6(d). The high frequency corresponds to the limiting
process of charge transfer and the connection of charge transfer resistance and the
EDLs at the contact interface between the electrode and the electrolyte
solution. Clearly, a small semicircle at high frequencies can be found, that is, the
smaller the charge transfer resistance is due to the smaller semicircle. Meanwhile, it is
displayed that higher carbonization temperature will lead to lower charge transfer
resistance, which improves the capacitive performance. But all of the as-prepared
electrodes have a small interface contact resistance, except of HAT-CN-450 electrode.
3.3 MCDI performance
The MCDI performance was investigated in a NaCl aqueous solution at a certain
voltage and flow rate by batch mode. The MCDI cell fabricated by symmetrical
electrodes consist of the as-prepared electrodes, in contrast to the commercially
activated carbon (AC) electrodes.
As shown in Figure 7(a), all the as-prepared electrodes cell have a sharp decrease
with 10 min at voltage of 1.2 V in initial concentration of 500 mg L-1, suggesting that
the electrodes have an excellent ability to absorb salt ions onto the relatively charged
electrodes quickly. With increasing absorption time, conductivity decreases gradually
and then is closed to electrosorption equilibrium when the curves flattens out at 60
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min. As shown in Figure 7(a), the salt adsorption capacitance (SAC) values of
HAT-CN-450, HAT-CN-550, HAT-CN-700, and HAT-CN-800 electrode are 22.79
mg g-1, 24.66 mg g-1, 19.13 mg g-1, and 17.50 mg g-1, respectively. Compared to the
as-prepared sample, the AC electrode has the lowest desalination capacitance (11.14
mg g-1). The salt adsorption rate (SAR) is also one of the factors affecting MCDI
performance. [44] It can be combined with SAC to form a MCDI Ragone diagram, as
shown in Figure 7(b). Clearly, the HAT-CN-550 situated in the upside and right of the
CDI Ragone figure indicates the highest SAC, owing to the high micro-porous
content and perfect ratio of nitrogen content, including excellent electrical
conductivity and a rich vary boundary structure. The AC electrode has the lowest
SAC, because of its low micro-porous contributing SSA. Although HAT-CN-700 and
HAT-CN-800 electrodes have lower impedance than HAT-CN-550, the impedance of
HAT-CN-550 is also small. Moreover, the excellent wettability of HAT-CN-550
contributes to adequate contact between the electrodes and solution, which is one of
the key factors for MCDI test. In short, the acquired results explicitly demonstrate the
importance of the micro-pore amount and the ratio of nitrogen content in improving
the desalination performance of an electro-active material.
To further investigate, the influence of applied voltages ranging from 1.2 to 1.8 V,
and concentrations of NaCl solution between 500 mg L-1 to 1000 mg L-1 on MCDI
performance of the HAT-CN-550 electrode is shown in Figure 7(c). The MCDI
behaviors of the HAT-CN-550 electrode were studied at varying test applied voltages
from 1.2 to 1.8 V with a flow rate of 22 mL min-1. The curves in the figure dropped
17

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sharply within 10 min and reach a stable state within 60 min. Increasing the applied
voltage, the SAC is increased from 24.66 to 33.17 mg g-1, indicating that the higher
potential can produce a stronger electrostatic interaction to bring about a larger
removal of NaCl. Meanwhile, MCDI Ragone plots of the HAT-CN-550 electrode
performance at applied potentials varying from 1.2 to 1.8 V are exhibited in Figure
7(d). The results reveal that the curves approach more upside and to the right with
increase in applied potential, suggesting a higher SAC and SAR increase with the
enhancement of the applied potentials. It is noting that no obvious bubbles were found,
while the applied potentials were set to 1.8 V, suggesting that no electrolytic reaction
of water took place, even at an overvoltage higher than 1.23 V [45] because of the
effect on membrane resistance and intrinsic resistance of electrode. [46]
Additionally, the effect of different NaCl concentration on MCDI performance was
investigated at the applied potential of 1.2 V. The SAC of HAT-CN-550 electrode in
NaCl concentration of 500, 700 and 800 mg L-1 is 24.66, 26.43, and 30.79 mg g-1,
respectively. Clearly, the curves increase with increasing NaCl concentration, and the
plots show an increase to the right, revealing a higher adsorption mass of NaCl and
removal rate due to higher conductivity makes it easy to form EDLs, with a faster
transportation speed of the salt ions to the electrode spacing channel when the NaCl
concentration in the solution is higher. However, when the increase in ions in the
solution is much greater than the adsorption amount, the species of ions in the pore is
saturated, leading to a decrease in removal efficiency.
Finally, the effect of the regeneration stability of the HAT-CN-550 electrode was
18

ACCEPTED MANUSCRIPT
investigated with a charge voltage of 1.2 V, revealing its ability to absorb, and in a
short circuit, to desorb, as shown in Figure 8. The deionization process completes
quickly after the charge voltage was applied. Then the short circuit of the cell has a
quick desorption performance during the discharge process, suggesting that the
HAT-CN-550 electrode has excellent regeneration. Additionally, no apparent
decrease in solution conductivity was observed after 30 regeneration cycles, which
reveals an excellent regeneration property. Moreover, the electrosorption capacities
performance are higher than those for other Nitrogen-doped electrode materials
reported in previous work as listed in Table 2. Therefore, the HAT-CN-550 electrode
has excellent deionization performance in CDI.
4. Conclusions
In conclusion, a novel hexagon-like carbon framework was prepared by
condensation of HAT through in situ synthesis and used it as an electrode in
capacitive deionization. Results indicate that HAT-CN has a large specific surface area,
rich micro-pore structure, and excellent conductivity, which leads to high MCDI
performance, contributed by the furnishing of plenty of space and active sites for
charge storage. Additionally, HAT-CN has a high nitrogen content, good wettability,
and low impedance, which all lead to the fast transport of salt ions. Meanwhile, the
HAT-CN electrode, as the heteroatom-based carbon material, reveals favorable
regeneration performance. Thus, HAT-CN is a promising electrode material suitable
for MCDI application and alkaline treatment.
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Acknowledgements
This work was supported by the Local Innovative and Research Teams Project of
Guangdong Pearl River Talents Program [grant number 2017BT01Z032]; the Special
Applied Technology Research and Development of Guangdong Province (major
project) [grant number 2015B020235013]; the Science and Technology Plan of
Guangzhou [grant number 201607010330]; and the Natural Science Foundation of
China [Grant No. 21577027].
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Table
Table 1 Summary of characterization results as well as experimental carbon yield of the
HAT-CN-X materials.
Table 2 Comparison of electrosorption capacities of Nitrogen-doped electrode materials from this
study and reported in previous work.
Table 1 Summary of characterization results as well as experimental carbon yield of the HAT-CN-X materials.
SSABET
[m2 g-1]
Contact
angle[º]
ID/
IG
yeild
[%]
C [at.%]
EDS
N [at.%]
EDS
37.1
O [at.%]
XPS EA EDS
T[K]
EA
XPS
62.9
68.4
66.5
81.1
EA
XPS
1.8
2.3
3.2
3.6
450
55.2
60.5
42.9
31.5
29.1
21.4
35.3
29.3
30.3
15.3
-
-
-
-
2.4
2.4
2.5
3.5
595.5
771.2
830.9
701.3
41.9
32.8
25.3
28.9
-
91.5
550 49. 5 61.0
36.1
0.96 62.5
1.07 38.4
1.21 25.5
700
800
47.7
56.5
63.6
67.9
34.9
28.6

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Table 2 Comparison of electrosorption capacities of Nitrogen-doped electrode materials from this study and
reported in previous work.
Applied Initial concentration/ Electrosorption
Raw materials and preparation procedure
Ref.
47
voltage conductivity of salt
capacity
Nitrogen-doped graphene sponge fabricated via
directly freeze-drying graphene oxide solution
followed by annealing in NH3 atmosphere
500 mg L-1
1.2 V
21.0 mg g-1
NaCl solution
Nitrogen-doped carbon nanorods prepared from
naturally based nanocrystalline cellulose through
freeze drying with subsequent thermal treatment
under an ammonia atmosphere at different
temperatures
500 mg L-1
1.2 V
17.62 mg g-1
48
NaCl solution
Nitrogen-doped porous hollow carbon spheres
(N-PHCS) prepared by using polystyrene spheres
as hard templates and dopamine hydrochloride as
carbon and nitrogen sources by carbonization at
different temperatures
500 mg L-1
1.4 V
12.95 mg g-1
49
50
NaCl solution
Facile dual doping strategy via carbonization of
covalent organic frameworks to prepare
hierarchically porous carbon spheres and
carbonization at different temperatures
500 mg L-1
1.2 V
18.5 mg g-1
NaCl solution
Two-dimensional
boron
carbon
nitride
new
nanosheets were fabricated using
a
500 mg L-1
1.4 V
approach, which uses g-C3N4 as both the
template and the nitrogen source, boric acid as
the boron source and a subsequent pyrolysis
process
13.6 mg g-1
25
19
NaCl solution
Metal-organic-framework (ZIF-67) derived
carbon polyhedron and carbon nanotube
hybrids(HCN)
1000 µS cm-1
1.4 V
7.08 mg g-1
NaCl solution
Hexaazatriphenylene-based precursor prepared
microporous and oxidation resistant carbons
materials
1200 µS cm-1
1.2
our
24.66 mg g-1
NaCl solution
work

ACCEPTED MANUSCRIPT
Figure
Scheme 1 Condensation reaction process
Figure 1 TEM images of (a) HAT-CN-450; (b) HAT-CN-550; (c) HAT-CN-700 and (d)
HAT-CN-800.
Figure 2 (a) Raman survey spectra of HAT-CN-450/550/700 /800; (b) FT-IR survey spectra of
HAT-CN-450/550/700 /800; (c) PXRD measurements of HAT and HAT-CN-450/550/700 /800; (d)
N2 physisorption isotherms measurement (the inset represents Pore size distribution) of
HAT-CN-450/550/700/800.
Figure 3 Fitted N1s XPS line scans of (a) HAT-CN-450; (b) HAT-CN-550; (c) HAT-CN-700 and
(d) HAT-CN-800.
Figure 4 Contact Angle measurement of (a) HAT-CN-450, (b) HAT-CN-550, (c) HAT-CN-700,
and (d) HAT-CN-800.
Figure 5 (a) CV curves of the electrodes at a potential sweet rate of 5 mV s-1; (b) CV curves of
the HAT-CN-550 at different scan rates; (c) the specific capacitance of the electrodes at 1-50 mV
s-1; (d) CV curves of the HAT-CN-550 at different concentration of NaCl.
Figure 6 (a) the GCD curves of the HAT-CN-450/550/700/800 electrodes; (b) the GCD at
various current density of HAT-CN-450; (c) continuous GCD curves of theHAT-CN-550
electrodes with a current density of 200 mA g-1; (d) the Nyquist plots (the inset represents the
details of the Nyquist plots) in 1 M NaCl solution.
Figure 7 (a) the conductivity variation with deionization time (the inset represents the SAC with
different material) and (b) CDI Ragone plots of HAT-CN-450/550/700/800 and AC electrodes in a
500 mg L-1 NaCl solution at a cell voltage of 1.2 V with a flow rate of 22 mL min-1; CDI