Separation and Purification Technology 256 (2021) 117771
Contents lists available at ScienceDirect
Separation and Purification Technology
journal homepage: www.elsevier.com/locate/seppur
Highly efficient water desalination by capacitive deionization on
biomass-derived porous carbon nanoflakes
Ting Lu a, b, Yong Liu c, Xingtao Xu d, e, *, Likun Pan b, Asma A. Alothman f, Joe Shapter g,
Yong Wang a, *, Yusuke Yamauchi g, h, i, *
a
Department of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, PR China
Shanghai Key Laboratory of Magnetic Resonance, School of Physics and Materials Science, East China Normal University, Shanghai 200062, PR China
School of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao, Shandong 266042, PR China
d
College of Hydrology and Water Resources, Hohai University, Nanjing 210098, PR China
e
International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
f
Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia
g
Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane, QLD 4072, Australia
h
School of Chemical Engineering, Faculty of Engineering, Architecture and Information Technology, The University of Queensland, Brisbane, QLD 4072, Australia
i
Department of Plant & Environmental New Resources, Kyung Hee University, 1732 Deogyeong-daero Giheung-gu, Yongin-si, Gyeonggi-do 446-701, South Korea
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Water desalination
Capacitive deionization
Porous carbon nanoflakes
Electrosorption
Biomass materials
Capacitive deionization (CDI) works by using the electrical double layer on various materials including nano­
porous carbons to separate ions from saline water. To help realize industrial application, there has been an
increasing interest in the exploration of carbon materials from low cost, eco-friendly and abundant biomass for
CDI to align with the demands of sustainable development strategies. Herein we report pyrolysis of xylose with
KHCO3 to prepare hierarchically porous carbon nanoflakes which display a satisfactory salt adsorption capacity
of 16.29 mg g− 1. This novel strategy can design highly efficient carbon materials from naturally-developed
biomass materials with its low preparation cost, environmentally friendliness and superior desalination perfor­
mance. Our xylose-derived hierarchically porous carbon nanoflakes are promising for potential industrial
application for CDI.
1. Introduction
Due to industrialization, population growth, environmental pollu­
tion and overexploitation of groundwater, a shortage of fresh water has
become one of the most severe global challenges of modern society
[1–5]. Seawater desalination methods, such as electrodialysis, multistage flash and reverse osmosis, have been explored to alleviate this
problem [6–8]. Unfortunately, drawbacks like high energy use, sec­
ondary pollution, poor efficiency and high cost hinder the application of
these conventional desalination methods in future [9]. Recently, the use
of capacitive deionization (CDI) with its low energy loss, environment
friendliness and simple operation has emerged as a promising alterna­
tive for salt removal [10–13].
CDI is generally carried out with a low voltage (usually below 1.23
V) applied between two electrodes that could adsorb counter ions from
the saline water based on the electrical double layer (EDL) principle
[14–17]. Recently, many efforts have been made to promote the
development of CDI using electrode materials engineering, theoretical
model analysis, cell architecture design, etc. [18–24]. For example, a
pore size distribution model that was proposed by Porada et al. in 2013
[25] has been considered an effective tool for the prediction of desali­
nation performance of electrode materials. Carbons with large specific
surface area (SSA), good conductivity and remarkable stability,
including carbon nanotubes, graphene, templated carbons, metal­
− organic frameworks-derived carbons and their hybrids, are undoubt­
edly the most widely studied electrode materials in CDI and other
electrochemical applications [26–36]. However, these carbons always
suffer from drawbacks such as expensive precursors, complicated or
* Corresponding authors at: International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba,
Ibaraki 305-0044, Japan (X. Xu). Department of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444,
PR China (Y. Wang). Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane, QLD 4072, Australia (Y.
Yamauchi).
E-mail addresses: Xu.Xingtao@nims.go.jp (X. Xu), yongwang@shu.edu.cn (Y. Wang), y.yamauchi@uq.edu.au (Y. Yamauchi).
https://doi.org/10.1016/j.seppur.2020.117771
Received 30 May 2020; Received in revised form 3 September 2020; Accepted 17 September 2020
Available online 23 September 2020
1383-5866/© 2020 Elsevier B.V. All rights reserved.
T. Lu et al.
Separation and Purification Technology 256 (2021) 117771
Fig. 1. FESEM images of (a, b) CM and (c, d) PCN6.
time consuming synthesis procedures, etc. [37–40] resulting in the dif­
ficulty for large-scale production. Therefore, exploring sustainable, costeffective and eco-friendly carbons for CDI is highly desired.
In the past decades, low-cost and widely available biomass and their
derivatives have been widely utilized as the precursors for large-scale
production of carbon materials [41–45]. The derived carbon materials
possess several unique advantages such as low production cost and
naturally developed structure, as well as flexibility in controlling the
composition, structure and morphology [46–49]. Porous carbon nano­
flakes (including nanosheets) (PCNs), in particular, have received
considerable attention owing to their unique 2D conductive skeletons,
which endow them with tunable porous structures/morphologies and
short diffusion paths of electrolyte ions [50]. For instance, nitrogendoped graphene-like layered carbons from carbonization of glucose,
fructose or 5-HMF exhibited superior performances in epoxidation re­
actions [51]. Nanocarbon flakes prepared via carbonization of starch
followed by KOH activation exhibited a good CDI performance [52].
Xylose is a valuable biomass derivative produced from lignocellulosic
biomass via hydrolysis. Although there have been many attempts to
convert xylose to economically favorable materials [53–55], relatively
less attention has been paid to their conversion to PCNs for a CDI
electrode.
In this work, a series of PCNs were synthesized via the carbonization
of xylose with the presence of KHCO3 as an activator. The PCNs display a
3D framework constructed by carbon nanoflakes with hierarchically
porous structure, which promotes effective contact between the elec­
trode and electrolyte. When evaluated as electrode materials for CDI,
these PCNs show a high salt adsorption capacity (SAC) of 16.29 mg g− 1.
residual KHCO3 until no gas was produced. The suspension was vacuum
filtrated, washed with deionized water three times, and dried at 60 ◦ C
for 12 h. The black powder was annealed again at 700 ◦ C for 2 h in a N2
atmosphere to stabilize the structure and decompose the residual xylose.
According to the mass ratios of xylose to KHCO3, the obtained samples
are labeled PCN2, PCN4, PCN6, and PCN8, respectively. For compari­
son, carbon material directly derived from xylose without KHCO3 was
also prepared under the same conditions, which is tagged as CM.
2.2. Characterization and electrochemical measurements
The morphologies of the samples were examined by a field emission
scanning electron microscope (FESEM, HITACHI S4800). The porous
characteristics were investigated by nitrogen adsorption–desorption
isotherm at 77 K using a Micromeritics ASAP2020. A powder X-ray
diffractometer (XRD, Rigaku, Japan) was used to measure the structure
with Cu K radiation (λ = 0.15418 nm) from 10◦ to 80◦ . Raman spectra
were obtained using a Raman spectrometer (HARIBOR, HR-800). Elec­
trochemical experiments were carried out using an electrochemical
workstation (CHI760D). The electrochemical impedance spectroscopy
(EIS), cyclic voltammetry (CV) and charge–discharge tests were
measured in a three-electrode system in 1 M NaCl solution, where an
Ag/AgCl electrode was used as reference electrode and a platinum foil
served as counter electrode. A 2 × 2 cm2 electrode loading with sample
was used as working electrode. The specific capacitance (Cs, F g− 1) is
calculated from the following equation:
∫
IdU
Cs =
(1)
2νmΔU
2. Experimental
where I is the current (A), ν is the scan rate (V s− 1), m is the mass of
electrode and ΔU is the voltage window (V)
2.1. Fabrication of PCNs
2.3. CDI experiments
An activator-assisted pyrolysis process was carried out to prepare
PCNs utilizing xylose as carbon resource and KHCO3 as activator. Firstly,
the mixtures of xylose and KHCO3 with mass ratios of 1:2, 1:4, 1:6, and
1:8, respectively, were mixed and ground for 30 min. After that, the
mixture was transferred into a tube furnace and heated at 450 ◦ C for 3 h
under a N2 atmosphere. HCl solution (6 M) was used to remove the
A system based on CDI module, peristaltic pump and conductivity
meter was used in the CDI experiments. The CDI electrodes were pre­
pared according to our previous work [56]. The concentration, volume
and flow speed of NaCl solution are 1000 mg L− 1, 20 mL and 25 mL
min− 1, respectively. The voltage applied on the electrodes is 1.2 V. The
2
T. Lu et al.
Separation and Purification Technology 256 (2021) 117771
3. Results and discussion
The morphology and structure of the bulk CM and PCN samples were
first investigated by FESEM (Fig. 1). In comparison to the bulk CM
(Fig. 1a, b), with increasing amounts of KHCO3, the size of flakes in the
xylose-derived carbon decreases revealing etching effect of KHCO3
(Fig. 1c, d and S1). In particular, the PCN6 shows a 3D framework
structure connected by carbon nanoflakes (Fig. 1c, d). Previous reports
have suggested that KHCO3 decomposes into K2CO3 as well as with CO2
and H2O at low temperature, which may dilate and etch the bulk
structure to produce the carbon nanoflakes [57]. However, further
increasing the mass ratio of KHCO3 and xylose to 8:1, provides excessive
KHCO3 that will lead to the excessive etching of the well-developed
interconnected flakes to generate smaller scattered flakes, and de­
stroys the 3D framework structure, as observed in Fig. S1e, f.
SSA, which could be measured by nitrogen adsorption–desorption
isotherms, is a key parameter for evaluating carbon materials and plays
an important role in determining the CDI performance of the prepared
carbon materials [58,59]. As seen in Fig. 2, it is obvious that all samples
exhibit rich microporous structure revealed by type-I isotherms. The
corresponding SSAs, pore volumes (Vpore) and mean pore sizes of the
samples are listed in Table 1. It is obvious that the SSA and Vpore of PCNs
are improved significantly compared to CM, and PCN6 shows the
highest SSA of 408.1 m2 g− 1 and largest Vpore of 0.278 cm3 g− 1. What’s
more, PCN6 and PCN8 exhibit enlarged mean pore sizes, possibly due to
the enhanced etching effect of higher amounts of KHCO3. Besides, the
mean pore size of PCN8 is a little higher than that of PCN6, which is
ascribed to the pore expansion by etching of excessive KHCO3.
The Raman spectra displayed in Fig. 3a show that the CM and PCNs
exhibit two broad peaks, namely the D-band at ~ 1344 cm− 1 and G-band
at ~ 1590 cm− 1, revealing the disordered sp3-carbon structure and inplane sp2-carbon vibrations of graphitic carbon [59], respectively. The
relative intensity ratio of D-band to G-band (ID/IG) is used to charac­
terize the degree of defects or disordered structures in carbon materials
[32]. It is obvious that PCNs possess higher ID/IG values than CM,
indicating that abundant defects or disordered structures have been
generated during the carbonization of xylose with KHCO3 [57]. More­
over, with the increase of additive KHCO3, more defective structures
could be generated due to the etching of the increasing additive KHCO3,
consequently resulting in relatively higher ID/IG values. The structures
of the samples were further tested by XRD and the corresponding pat­
terns are shown in Fig. 3b. All samples exhibit two broad peaks at
around ~ 26◦ and ~ 44◦ , corresponding to (002) and (101) reflections of
carbon materials, respectively [60].
Electrochemical analysis of CM and PCNs was carried out in a threeelectrode model by using 1 M NaCl solution as the electrolyte, an Ag/
Fig. 2. N2 adsorption–desorption isotherms of CM and PCNs.
Table 1
SSAs, pore volumes and mean pore sizes of CM and PCNs.
2
− 1
SSA (m g )
Vpore (cm3 g− 1)
Mean pore size (nm)
CM
PCN2
PCN4
PCN6
PCN8
102.6
0.058
2.29
266.1
0.149
2.23
354.1
0.196
2.22
408.1
0.278
2.72
390.4
0.273
2.80
SAC (mg g− 1) and mean salt adsorption rate (MSAR, mg g−
min were calculated from the following equations:
SAC =
V(C0 − Ct )
m
MSAR =
SAC
t
1
min− 1) at t
(2)
(3)
where, V (L) is the volume of NaCl solution, C0 and Ct (mg L− 1) are
concentrations of the NaCl solution at the initial and time t (min), and m
(g) is the total mass of electrodes.
Fig. 3. (a) Raman spectra and (b) XRD patterns of CM and PCNs.
3
T. Lu et al.
Separation and Purification Technology 256 (2021) 117771
Fig. 4. (a) CV curves and (b) Nyquist plots of CM and PCNs.
AgCl electrode as the reference electrode and a platinum foil as the
counter electrode. All samples exhibit rectangular-like CV curves at 2
mV s− 1 from − 0.5 – 0.5 V, and little distortions were observed for the CV
curves of PCNs (Fig. 4a), possibly due to the effect of oxygen-containing
groups [61]. The corresponding specific capacitances listed in Table 2
reveal that all PCNs exhibit improved capacitances compared to CM
sample, and PCN6 has the largest capacitance of 187.6 F g− 1 among all
samples, indicating the best capacitive performance. The discharge
Table 2
Capacitive properties of CM and PCNs.
Cs/F g−
Rct/Ω
1
CM
PCN2
PCN4
PCN6
PCN8
93.4
1.34
141.8
0.97
169.2
0.89
187.6
0.84
176.3
0.94
Fig. 5. (a) Conductivity variation profiles of NaCl solution and (b) corresponding SAC variations of CM and PCNs.
Fig. 6. (a) SAC variations and (b) corresponding CDI Ragone plots for PCN6 in NaCl solutions with varying concentrations.
4
T. Lu et al.
Separation and Purification Technology 256 (2021) 117771
Fig. 7. (a) Cycling SAC retention ratio for PCN6 in NaCl solution under an open condition. High-resolution C 1s XPS spectra of PCN6 electrode before (b) and after (c)
long-term cycling tests.
curves at 1 A g− 1 shown in Fig. S2 further prove the best capacitive
property of PCN6 among all the samples. The superior capacitive per­
formance of PCNs is ascribed to the higher SSAs, novel 2D nanoflake
structure, and increased defective structures, providing more sites for
ions accommodation [62]. In particular, the PCN6 sample possesses the
highest SSA, which thereby gives rise to the highest capacitance [58].
The EIS of all samples provides detailed analysis on electrochemical
properties (Fig. 4b). All Nyquist plots display a quasi-semicircle at highfrequency range whose diameter reflects the charge transfer resistance
(Rct) at the interfaces of active material/current collector and the elec­
trode/electrolyte, as well as an inclined line at low-frequency range
which reflects ion diffusion in the electrolyte [63]. The values of Rct
listed in Table 2 are obtained from fitting of the data to an equivalent
electric circuit (inset of Fig. 4b), suggesting that PCN6 exhibits the
lowest Rct value among all samples, indicating the best charge transfer
ability. In addition, all PCN electrodes possess steeper line gradients
than CM electrode corresponding to the faster ion diffusion rate.
The desalination performances of CM and PCNs electrodes at 1.2 V
were tested in a NaCl solution (1000 mg L− 1). As shown in Fig. 5a, once
the voltage is applied on the system, the ions in the solution would move
to oppositely charged electrodes and be absorbed within the pores of the
active electrode material, leading to a dramatically decreased conduc­
tivity. Fig. 5b indicates that the SAC values of PCNs increase gradually
with the time, and reach a maximum value of 16.29 mg g− 1 at 30 min for
PCN6, much higher than the maximum SAC value observed for CM
(3.34 mg g− 1). Even compared with previously reported carbon mate­
rials (Table S1), PCN6 exhibits a superior CDI performance, which
should be ascribed to the interconnected nanoflake structure with large
accessible SSAs providing more adsorption sites for ion accommodation,
shortened ion diffusion pathway for mass transport and good electron
transfer characteristic.
To further evaluate the CDI performance of PCN6, the SAC variations
in NaCl solutions with varying concentration from 100 to 1000 mg L− 1
were carried out (Fig. 6a). It is obvious that higher NaCl concentrations
usually lead to higher SAC values. The corresponding Ragone plots
presented in Fig. 6b indicate that, with the increase in NaCl concen­
tration, the Ragone plots shift relatively to higher SAC and MSAR values
indicating higher desalination capacity and rate [64]. Furthermore, we
evaluated the cycling desalination stability of PCN6 in saline water with
an initial concentration of 1000 mg L− 1 under an open condition over
100 cycles (Fig. 7a). It is found that in the first 25 cycles, the PCN6-based
CDI cell exhibits a relatively high SAC retention ratio. However, after
prolonged cycles, the SAC retention ratio tends to significantly decrease,
possibly due to the reduction of dissolved oxygen to H2O2 that oxidizes
carbon electrode with the flow of solution [65,66]. We further provided
the change of C 1s X-ray photoelectron spectroscopy (XPS) spectra of the
PCN6 electrode before (Fig. 7b) and after (Fig. 7c) long-term cycling
– O groups after cycling tests,
tests, clearly showing the increase of O–C–
indicating the significant oxidization of carbon electrodes after longterm cycling process [67].
4. Conclusion
In this work, PCNs have been synthesized by carbonization of xylose
in the presence of KHCO3. Thanks to the rich microporous structure and
large SSA, PCN6 exhibits a high SAC of 16.29 mg g− 1, which proves the
practicality of our elaborately designed PCNs. There is little doubt that
the low preparation cost, environmentally friendliness and superior
desalination performance will be of great interest to industry.
Author statement
T. Lu, X. Xu, Y. Wang and Y. Yamauchi designed the project. T. Lu,
and Y. Liu conducted the experimental process and collected the
research data. T. Lu, Y. Liu and X. Xu wrote the manuscript draft. L. Pan,
A. A. Alothman, and J. Shapter gave sustainable discussions and guid­
ance during the experimental and writing process. X. Xu, J. Shapter, Y.
Wang and Y. Yamauchi revised and edited the manuscript. All authors
have reviewed the manuscript and agreed to the publication.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
This work was jointly supported by the National Natural Science
Foundation of China (51909066), the Fundamental Research Funds for
the Central Universities (B200202034), the “JSPS Postdoctoral Fellow­
ship for Overseas Researchers” (ID No. P20338), and the ERATO-FS
“Yamauchi’s Materials Space-tectonics Project” supported by The Japan
Science and Technology Agency (JST). This work was also supported by
the Researchers Supporting Project (RSP-2020/243), King Saud Uni­
versity, Riyadh, Saudi Arabia. This work was partially performed at the
Queensland node of the Australian National Fabrication Facility, a
company established under the National Collaborative Research
5
T. Lu et al.
Separation and Purification Technology 256 (2021) 117771
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.seppur.2020.117771.
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