Enhanced brackish water desalination in capacitive deionization with composite Zn-BTC MOF-incorporated electrodes

Abstract

In this study, composite electrodes with metal–organic framework (MOF) for brackish water desalination via capacitive deionization (CDI) were developed. The electrodes contained activated carbon (AC), polyvinylidene fluoride (PVDF), and zinc-benzene tricarboxylic acid (Zn-BTC) MOF in varying proportions, improving their electrochemical performance. Among them, the E4 electrode with 6% Zn-BTC MOF exhibited the best performance in terms of CV and EIS analyses, with a specific capacity of 88 F g⁻¹ and low ion charge transfer resistance of 4.9 Ω. The E4 electrode showed a 46.7% increase in specific capacitance compared to the E1 electrode, which did not include the MOF. Physicochemical analyses, including XRD, FTIR, FESEM, BET, EDS, elemental mapping, and contact angle measurements, verified the superior properties of the E4 electrode compared to E1, showcasing successful MOF synthesis, desirable pore size, elemental and particle-size distribution of materials, and the superior hydrophilicity enhancement. By evaluating salt removal capacity (SRC) in various setups using an initially 100.0 mg L⁻¹ NaCl feed solution, the asymmetric arrangement of E1 and E4 electrodes outperformed symmetric arrangements, achieving a 21.1% increase in SRC to 6.3 mg g⁻¹. This study demonstrates the potential of MOF-incorporated electrodes for efficient CDI desalination processes.

Full text

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Enhanced brackish water
desalination in capacitive
deionization with composite
Zn‑BTC MOF‑incorporated
electrodes
Amirshahriar Ghorbanian
1, Soosan Rowshanzamir
1,2* & Foad Mehri
1
In this study, composite electrodes with metal–organic framework (MOF) for brackish water
desalination via capacitive deionization (CDI) were developed. The electrodes contained activated
carbon (AC), polyvinylidene uoride (PVDF), and zinc‑benzene tricarboxylic acid (Zn‑BTC) MOF in
varying proportions, improving their electrochemical performance. Among them, the E4 electrode
with 6% Zn‑BTC MOF exhibited the best performance in terms of CV and EIS analyses, with a specic
capacity of 88 F g−1 and low ion charge transfer resistance of 4.9 Ω. The E4 electrode showed a
46.7% increase in specic capacitance compared to the E1 electrode, which did not include the MOF.
Physicochemical analyses, including XRD, FTIR, FESEM, BET, EDS, elemental mapping, and contact
angle measurements, veried the superior properties of the E4 electrode compared to E1, showcasing
successful MOF synthesis, desirable pore size, elemental and particle‑size distribution of materials,
and the superior hydrophilicity enhancement. By evaluating salt removal capacity (SRC) in various
setups using an initially 100.0 mg L−1 NaCl feed solution, the asymmetric arrangement of E1 and E4
electrodes outperformed symmetric arrangements, achieving a 21.1% increase in SRC to 6.3 mg g−1.
This study demonstrates the potential of MOF‑incorporated electrodes for ecient CDI desalination
processes.
Keywords Capacitive deionization, Brackish water, Water desalination, Metal–organic framework, Electrode
e remarkable role of electrodes in enhancing the CDI technology is undeniable, and most attention was paid
to improve physical, chemical, and electrochemical properties of electrodes1–3. e most important factors
enhancing the SRC during CDI are high porosity and specic surface area (SSA), desirable pore size, particle-
size distribution, suitable wettability, high electrochemical performance, and physical and chemical stability of
electrodes1,4–6. For this purpose, the most common and widely used material is AC7. However, the utilization of
other types of carbonaceous materials, especially environmentally friendly ones (e.g., corn-stalk-based8,9 and
water hyacinth-based10 carbon aerogels) as well as conventional ones (e.g., mesoporous carbon11, graphene12,
and carbon nanotubes13), the addition of metal oxide nanoparticles (e.g., TiO214, SiO2, Al2O315, and ZnO16), and
conductive polymers (e.g., polyaniline17 and sulfonated polystyrene18) to the carbonaceous structures of the
electrodes were carried out to achieve a higher specic capacity, lower charge transfer resistance, more hydro-
philicity, and better wettability. is leads to improved electrochemical performance in the CDI process19–21. In
recent years, MOFs have emerged as a type of three-dimensional porous compounds composed of metal ions
linked to the organic ligands22,23. e incorporation of these materials in the electrodes of some electrochemical
systems (e.g., supercapacitors24 and electrical absorption processes25,26) is a good representation of their unique
properties such as porosity, high SSA, hydrophilicity, and desirable physical properties22,27–30. However, due to the
relatively poor electrical conductivity, high manufacturing cost, and limited direct applications in electrochemical
processes reported for most MOFs26,31,32, some researchers have fabricated MOF-carbon derived composites to
increase the electrical conductivity and porosity of these materials33. For instance, Li etal.34 fabricated carbon
OPEN
1Hydrogen & Fuel Cell Research Laboratory, School of Chemical, Petroleum and Gas Engineering, Iran University
of Science and Technology, Narmak, Tehran 16846-13114, Iran. 2Center of Excellence for Membrane Science and
Technology, Iran University of Science and Technology, Narmak, Tehran, Iran. *email: rowshanzamir@iust.ac.ir
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electrodes using Mg-MOF-74 as the precursor material for the CDI process. e resulting electrodes had a SRC
of 16.82mg g−1 for a feed solution containing 500mg L−1 of NaCl.
On the other hand, the combination of MOFs and conductive or carbonaceous materials such as AC is
known as a suitable solution to not only compensate for the MOFs’ poor conductivity29, but also bypass the time-
consuming and harsh procedures of Acid-washing and carbonization treatment23,35 involved in the fabrication
of MOF-derived materials and consequently reduce the manufacturing costs of these electrodes35. Wang etal.36
prepared ZIF-67 MOF/polypyrrol hybrid electrodes which yielded 11.34mg g−1 SRC from a 584mg L−1 NaCl
solution. e organic and metallic parts of these porous materials are combined with each other to makes their
use in electrochemical processes very attractive 36. In this regard, the type of metal and organic linker involved
in the MOF is signicant. For example, Benzene-1,3,5-tricarboxylic acid (H3BTC) with three functional groups
is a frequently used organic linker because of its more coordination mode with metal ions in all three directions
and consequently producing higher porosity than another linker37,38.
Regarding the eect of metal in MOF-based electrodes in CDI process, a few research studies on metals were
employed in MOFs such as Cobalt-MOF-based ZIF-6736,39,40, Chromium-based Metal–Organic Framework (Cr-
MOF)41, Cu-MOF29, and Mn-Fe-MOF-based27 electrodes. Xu etal.40 fabricated hybrid electrodes using ZIF-67
and CNTs which could achieve a SRC of 16.90mg g−1 in a 5mM NaCl solution. Feng etal.27 prepared hybrid
electrodes by combining Mn-Fe-MOF with holey graphene in the initial feed of 800mg L−1 NaCl, and the hybrid
electrodes led to a SRC of 39.6mg g−1. Zhang etal.39 were able to achieve 14.4mg g−1 SRC in a CDI process by
the preparation of nanopatterned ZIF-67 MOF electrodes using a feed solution of 5.0mM NaCl.
Until now, only a few cases have limited the direct use of unmodied or untreated MOFs in CDI electrodes
for water desalination. All of these works were performed in last few years, focusing on the use of metals such
as Fe, Co, Ni and Cu in the MOF structure in slightly larger quantities of MOFs27,29,36,39–43. is poses a cost
challenge for scaling up the process44. Among the dierent transition-metal ions involved in MOFs, Zn-BTC
MOF represents an attractive choice for MOF-based electrode materials. is is because Zn2+ can provide more
isostructural porous frameworks45, favorable electrical conductivity46, low cost, lack of toxicity, high chemical
stability in aqueous electrolytes, as well as high energy and charge density47,48. Moreover, the easily commercially
available H3BTC organic ligand facilitates the fabrication of a zinc-based MOF via a facile one-pot synthesis
using low-cost starting materials49, including zinc nitrate hexahydrate and H3BTC under solvothermal condi-
tions. Considering the desirable properties of Zn-BTC MOF, such as excellent water sorption abilities, good
stability, and performance over multiple sorption and desorption cycles in aqueous solutions50–52, as well as
favorable storage and thermal stability up to 200°C50–53, its performance in the CDI process has not been thor-
oughly investigated. To the best of our knowledge, the utilization of Zn-BTC MOF as well as the evaluation of
symmetric and asymmetric electrodes arrangements due to their desirable properties and high charge density is
being investigated for the rst time in a CDI system. In this study, the aim is to improve the physicochemical and
electrochemical properties of conventional carbon electrodes for brackish water desalination by using dierent
proportions of the synthesized Zn-BTC MOF and AC. erefore, a small percentage of Zn-BTC MOF (ranging
from 2 to 10wt%) was used as an additive to enhance electrode performance. Comprehensive physicochemi-
cal and electrochemical characterization tests were performed to analyze the synthesized MOF and fabricated
composite electrodes. Furthermore, the desalination performance of the electrodes was investigated in both
symmetric and asymmetric arrangements using a CDI cell.
Materials and methods
Materials
Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99%) and polyvinylidene uoride (PVDF, MW of 530,000) were
obtained from Sigma-Aldrich, Germany. Activated carbon powder (DARCO, BET surface area of 723 m2 g−1),
benzene-1,3,5-tricarboxylic acid (C9H6O6, 95%), N-methyl-2-pyrrolidone (NMP, 99.5%) and ethanol (99.8%)
were purchased from Merck Co., Germany. Graphite sheets (500.0 µm thickness) were supplied by Dongbang
Carbon Co., China. Carbon cloth was purchased from AvCarb Material Solutions, US. All chemicals were ana-
lytical reagent grade and used without further purication.
Zn‑BTC MOF synthesis
Zn-BTC MOF was prepared by a simple solvothermal method49 as depicted in Fig.S1. Initially, 1.8g of
Zn(NO3)2·6H2O and 0.6g of C9H6O6 each were dissolved in 30 mL of ethanol by constant stirring for 30min.
Subsequently, both solutions were mixed together and continuously stirred for another 60min. en, the mixture
was transferred to a 75mL Teon-lined stainless-steel autoclave at a rate of approximately 5°C per minute. e
reaction lasted for 14h at 130°C. Aerwards, the autoclave was cooled down to room temperature. e resultant
milky crystal precipitate of Zn-BTC MOF was centrifuged, washed several times with fresh ethanol and deion-
ized water, and dried in a vacuum oven at 80°C for 12h. e yield of the Zn-BTC MOF prepared at this stage,
compared to the metal salt used, was about 61.1%.
Composite electrode fabrication
Composite electrode fabrication consists of two stages as shown in Fig.S2: (1) the preparation of the electrode
ink and, (2) coating the resultant ink on a current collector. Ink preparation is the key step, so that it is essential
for the resulting ink to be homogeneous. In preparing the ink, six dierent electrode compositions (i.e., E1,
E2, E3, E4, E5, and E6), including three components (i.e., AC, PVDF, and Zn-BTC MOF), were investigated,
as observed in TableS1. For the preparation of each composite electrode, AC, PVDF, and Zn-BTC MOF were
weighted according to specied composition formula as indicated in TableS1. en NMP solvent was added to
each composition, and they were immersed in an ultrasonic bath for 20min. A completely homogeneous ink was
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prepared by placing it on an electromagnetic stirrer at ambient temperature for at least 12h, and consequently,
cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were conducted in a three-
electrode cell. e electrode with best performance in these electrochemical tests was then selected for further
characterization and desalination tests.
e specic capacitance and overall electrochemical resistance were measured using CV and EIS tests,
respectively25,54. For this purpose, CVs were conducted in 1.0M NaCl at a rate of 5.0mV s−1 [versus Ag/AgCl]
for potential range of −0.5 up to + 0.5V. EIS was also conducted for a frequency range of 700.0kHz to 1.0mHz,
with the alternating potential amplitude being 10.0mV around the open circuit potential. Each of the prepared
inks was coated on a glassy carbon (2.0mm diameter) as a working electrode. e counter electrode was a
3.0 cm2 rectangular platinum (Pt) plate, and the reference electrode was an Ag/AgCl electrode in saturated KCl.
e specic capacitance values (C) (F g−1) were determined using the I–V curve according to Eq.(1) 55:
where S is the area surrounded by the CV curve,
V
is the potential window (V), m is the mass of active material
on electrodes (g), and
ϑ
is the potential scan rate (V s−1).
e SRC (mg g−1), mean salt removal capacity (MSRC) (mg g−1 min−1), and salt removal eciency (SRE) (%)
were calculated based on the initial and nal concentrations of the feed solution using Eqs.(2), (3), and (4)56,57:
where Cf, C0, V, m, and t are the nal concentrations (mg L−1), initial concentrations (mg L−1), reservoir volume
(mL), electrode mass (g), and time (min), respectively.
Aer selecting the most suitable composition of electrode in terms of electrochemical performance by CV
and EIS tests, it is necessary to test the selected electrode in a CDI cell to evaluate its desalination performance.
e composite electrode fabrication process for desalination tests was as follows. First, for each desalination test,
the selected electrode composition (as the active layer of the electrode) was coated with a so brush onto two
circular pieces of carbon cloth (each 4.0cm in diameter) placed on the surface of two graphite sheets (each 8.0cm
in diameter) as anode and cathode; then the electrodes were completely dried in three steps by a vacuum oven; at
60°C for 3h, at 80°C for 2h and at 100°C for 1h. e electrodes were then removed from the oven and allowed
to cool to ambient temperature. e electrodes were rinsed with deionized water to remove contaminants. Finally,
the electrodes were placed in a vacuum oven at 100°C for 2h to dry completely. At last, the total dried mass of
the active layer on each composite electrode was 0.11g with 260µm in thickness. e CDI tests were conducted
in a batch setup at ambient temperature using a 50.0mL NaCl feed solution with an initial concentration of
100.0mg L−1 (with an initial electrical conductivity of 253.4µS cm−1), which is regarded as brackish water. e
NaCl solution conductivity was monitored with a conductivity meter during the test. e relationship between
conductivity and NaCl concentration was obtained by preparing a calibration curve before the experiments.
CDI experimental setup
CDI experiments were performed with a batch-mode setup. It contained a feed solution reservoir, a peristaltic
pump (Lab 2015, Shenchen Co., China), a Galvanostat/Potentiostat device (SP-150, Bio-Logic Science Instru-
ments SAS, France), a conductivity meter (EC-470 L, ISTEK Co., Korea), a pH meter (P25, ISTEK Co., Korea),
and a lab-made CDI unit cell. To investigate the performance of the electrodes, a CDI device (Fig.1) was con-
structed. is device consisted of two circular sheets of plexiglass for encasement, two composite electrodes
containing active material coated on carbon cloth xed on a circular graphite sheet as current collectors, and
separated by a nylon mesh spacer. Additionally, several silicone rubber gaskets were used for sealing.
Physicochemical/electrochemical characterizations
To evaluate Zn-BTC MOF crystal formation, X-ray diraction (XRD) test was performed at the 2Θ angle range
from 5 up to 90 degrees, using the Philips Xpert device (Netherlands) and Cu-Kalpha radiation source. In order
to investigate the chemical structure of H3BTC, Zn-BTC MOF, AC, and fabricated electrodes, Fourier transform
infrared (FTIR) tests were carried out using the ermo Electron Scientic Instruments LLC device (USA) in
the spectral range of 400–4000 cm−1. To study the morphology of the Zn-BTC MOF and the composite active
material of electrodes, eld emission scanning electron microscopy (FESEM) images with magnications of 1,
5, and 20µm were taken with a MIRA3 TESCAN device (Czech Republic), enabling a comprehensive analysis
of their structural characteristics. Energy dispersive X-ray spectroscopy (EDS) and Elemental Mapping tests
were conducted with a MIRA3 TESCAN device (Czech Republic) to further conrm the elemental composi-
tion and distribution within the materials. e particle size distribution of Zn-BTC MOF was also analyzed
and estimated using the ImageJ soware. e SSA and mean pore diameter of AC and Zn-BTC MOF were
calculated based on the adsorption–desorption isotherms of nitrogen gas at liquid nitrogen temperature by the
(1)
C
=
S
�Vmϑ
(2)
SRC
=

Cf−C0

×V
m
(3)
MSRC
=
SRC
t
(4)
SRE
=
C
f−
C
0
C0
×
100
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Brunauer–Emmett–Teller (BET) method using a Belsorp system (BEL Japan, Inc.). Contact angle (CA) tests
have been applied to investigate the hydrophilicity and wettability of the Zn-BTC MOF and electrodes with an
Integrated Exploitation System of Laboratory Equipment (IUST, Iran). A SP-150 Galvanostat/Potentiostat device
(Bio-Logic Science Instruments SAS, France) was used as a potential supplier in CV, EIS, and desalination tests.
Results and discussion
Zn‑BTC MOF physicochemical characterization
e XRD test was performed to conrm the construction of the Zn-BTC MOF. It is necessary to match the
spectrum obtained from the as-prepared MOF with the spectrum obtained from samples reported in previous
studies48,58,59. FigureS3a illustrates the XRD spectrum of the Zn-BTC MOF synthesized in this work and the XRD
spectrum of the samples synthesized by Osman etal.59. According to Fig.S3a, a highly intense peak at 2Θ = 10°,
and some minor peaks at 2Θ = 15.64°, 17.72°, and 26.16° are observed, conrming the successful construction
of the Zn-BTC MOF48,58,59.
FTIR is another test employed for investigating the Zn-BTC MOF structure, considering that the bonds in
the Zn-BTC MOF are formed by H3BTC organic ligand molecules60,61. e FTIR spectrum of the Zn-BTC MOF
and the spectrum of the H3BTC organic ligand were measured, as shown in Fig.S3b, which are described in
supplementary information.
e morphology of Zn-BTC MOF particles was investigated using FESEM images. FigureS4a shows images
at 1, 5 and 20μm magnications. e shapes of the Zn-BTC MOF particles are spherical and polyhedral. e
existence of two dierent shapes (spherical and polyhedral) with dierent sizes for the Zn-BTC MOF particles
can be caused by small variations in temperature during the synthesizing stage in the autoclave50,62,63.
Particle size distribution has been obtained from FESEM images using ImageJ soware. FigureS4b shows that
particles with a diameter between 30 and 500nm are most abundant. e presence of nano and micro-particles
in composite electrode structure can be inuential in two ways. e use of Zn-BTC MOF with nanometer-scale
dimensions can improve dispersion and uniformity in the electrode structure and thus enhance the overall
stability of structure. Conversely, coarser particles with micrometer-scale dimensions can aord more space
between AC particles in the electrode, and consequently leading to better diusion and greater access of ions to
the active sites within the electrode structure64,65.
EDS test is used to identify the type and quantity of elements and the elemental mapping test is used to
determine the quality of elemental distribution. e EDS result as shown in Fig.S5a, conrms the elemental
composition of the Zn-BTC MOF (i.e., C, O, Zn, and N). However, the additional peak observed belongs to
aluminum, which is caused by the aluminum surface of the sample holder63,66. Also, in Fig.S5b, the elemental
distribution of the MOF can be seen, which demonstrates the well distribution of all elements in the structure.
e SSA, pore size distribution, and pore volume of the Zn-BTC MOF were assessed using the BET test,
as well as nitrogen adsorption and desorption isotherms. ese analyses generated relevant graphs and tables,
which are presented in Fig.S6 and TableS2, respectively. According to the adsorption and desorption diagram
in Fig.S6a, the adsorption isotherm of this MOF is of the fourth type with a hysteresis loop of the third type17,67.
Figure1. Schematic diagram of CDI cell used in this work.
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is shows non-hard and plate-like meso and micro pores presented in its structure17,63,67. According to Fig.S6
and TableS2, an SSA of 34 m2 g−1, a pore volume of 0.096 cm3 g−1, and a Mean pore diameter of 11.54nm were
achieved using Burt–Joyner–Holland (BJH) method. It’s demonstrated that a mesoporous structure has better
performance than macro and microporous structures for adsorbing ions from the feed solution and forming
the electrical double layer (EDL)17,68,69.
According to Fig.S7, the CA of water with the tablet prepared from Zn-BTC MOF powder is 26.7 degrees,
which conrms the high wettability and hydrophilicity of this material30,69.
Electrodes physicochemical/electrochemical characterization
e electrochemical performance [i.e., Specic capacitance (F g−1) and Ion charge transfer resistance (Ω)] of the
six electrodes with dierent composition is indicated in Table1.
e adsorption potential of an electrode strongly hinges on its capacity to hold and retain ions within its
structure19,70. us, electrodes exhibiting higher specic capacitance values in the CV test are expected to dem-
onstrate superior adsorption performance19,71. According to Table1, the addition of up to 10% of the Zn-BTC
MOF results in a maximum increment of 46.7% in the specic capacitance of electrodes from E2 to E6 compared
to that of E1. A relatively sharp increase in ion charge transfer resistance was observed for MOF loadings greater
than 6%, probably due to decreased overall electrical conductivity and active surface of the electrode70,72,73. As a
result, the highest specic capacitance and minimum ion charge transfer resistance were obtained using the E4
electrode, which contained 6% of the Zn-BTC MOF. is outcome is likely related to the high hydrophilicity of
the Zn-BTC MOF as well as the proper pore size distribution of the electrodes69,74. e overall behavior of CV
and EIS test results indicates that the addition of a small quantity of Zn-BTC MOF and its optimization with
other materials in the composition of electrodes lead to the enhancement of synergistically characteristic of
electrode performance, which greatly impacts the specic capacitance and charge transfer kinetics of composite
electrodes70,74,75.
erefore, further examination was conducted only on E1 and E4 electrodes to better reveal the superior
performance of E4 as most appropriate electrode in CDI process. In the rst step, the graphs obtained from CV
and EIS characterizations were analyzed for E1 and E4 electrodes. Figure2a shows CV test behavior for E1 and
E4 electrodes. e CV curves of the electrodes have a quasi-rectangular shape and do not have peaks caused by
Faradaic reactions, which conrms the capacitive behavior of the electrodes due to the formation of the EDL17,76.
As a result, a greater surface area of the closed loop corresponds to a higher ion charge adsorption capacity of
the electrode17,25. Also, e presence of CV curves signies the reversible nature of the capacitive adsorption
performance of the electrodes17,25,77. e elevation of the current slope observed at the initial and nal stages
Table 1. Results of CV and EIS tests of the electrodes.
No. AC (wt%) PVDF (wt%) Zn-BTC MOF (wt%) Specic capacitance (F g−1) Ion charge transfer resistance (Ω)
E1 92 8 0 60 7.5
E2 90 8 2 74 6.3
E3 88 8 4 83 5.4
E4 86 8 6 88 4.9
E5 84 8 8 78 5.1
E6 82 8 10 67 5.6
Figure2. (a) e CV; and (b) EIS diagrams of E1 and E4 electrodes.
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of the CV curve for electrode E4, in contrast to E1, reects an enhanced hydrophilicity and reduced electrical
resistance of E4 compared to E169,76–78.
Figure2b shows the Nyquist curves obtained from the EIS test of E1 and E4 electrodes. As mentioned ear-
lier, the EIS test indicates electrochemical resistances, especially ion charge transfer resistance in electrodes. In
the EIS diagram, the vertical axis, which is imaginary resistance, is related to the capacitive resistance of the
electrode, and the horizontal axis, which is the real resistance, is related to the electrical resistance of the solu-
tion, the charge transfer resistance in the electrode, and the ion diusion resistance in the electrode17,25,72. e
rst intersection point of the curve with the horizontal axis indicates the electrical resistance of the electrolyte
solution17,76. Also, the semicircle range at high-frequency values in the plot reects the contact resistance of the
electrode/electrolyte, which aects the ecacy of ion transfer25,74,76. Both electrodes exhibit similar shapes and
trends in their respective plots. Upon the introduction of Zn-BTC MOF into the E4 electrode, a smaller half-
circle is observed as compared to the E1 electrode, indicative of lower ion charge transfer resistance within the
electrode structure and potentially better diusion of ions. However, the slope in the low-frequency region of
the plot reects the rate of ion diusion, which is found to be nearly equivalent for both electrodes17,25,73,74,76.
FTIR results of AC, Zn-BTC MOF, E1, and E4 electrodes are shown in Fig.3. According to that, the AC spec-
trum shows peaks at 1060 cm−1, 1635 cm−1, and 2820 cm−1 to 3633 cm−1 related to C–O, C=C, and O–H bonds,
respectively17. Additionally, peaks at 474 cm−1, 624 cm−1, and 890 cm−1 are caused by C–C=O, C–C–C, and C–H
bonds in the AC structure79. e spectrum of the E1 electrode is very similar to AC’s spectrum, and due to the
small amounts of PVDF and the overlap of a number of PVDF and activated carbon indicator peaks, no apparent
dierence is observed in the spectrum of the E1 electrode and AC. In general, the peaks at 470 cm−1, 621 cm−1,
and 1064 cm−1 in the E1 electrode spectrum, in addition to being related to C–C=O, C–C–C, and C–O bonds,
can also indicate the presence of CF2 bonds79,80. Also, the peaks at 1458 cm−1 and 2970 cm−1 conrm the presence
of CH2 bonds79. In the E4 electrode spectrum, the eect of increasing the Zn-BTC MOF on the mixture of AC
and PVDF is observed. According to this spectrum, the weak peak at 717 cm−1 is due to the presence of a Zn–O
bond48,81, and the peak ranging from 1480 to 1596 cm−1 equally belongs to the carboxyl group of the benzene
ring due to the presence of a C=O bond46,48,81.
e FESEM images of the E1 and E4 electrodes are shown in Fig.S8a and b, respectively, at three magnica-
tions of 1, 5, and 20μm. e good pore distribution and particle dispersity of compositions in the E4 electrode
can be clearly observed, revealing the eect of MOF on the structure of the electrode, as compared to the E1
electrode.
e EDS results of the E1 and E4 electrodes are shown in Fig.S9a and b, respectively, as well as the elemental
mapping images of both electrodes in Figs.S10 and S11, which are described in supplementary information.
Enhancing the hydrophilicity and wettability of the electrode surface can promote more ecient diusion
of ions within the electrode matrix17,30. erefore, more pores participate in the ion adsorption process17. In
addition, more active electrode surface is available to ions73. In Fig.4, the CA of water with E1 and E4 electrodes
can be seen. e CA of E1 and E4 electrodes is 108.3 and 52.4 degrees, respectively. PVDF binder and AC are
both hydrophobic materials that generally make electrodes more hydrophobic17,29. Given the prominently high
hydrophilicity exhibited by the Zn-BTC MOF, the observed rise in hydrophilicity of the E4 compared to the
E1 electrode is consistent with prior studies29,30,73. e magnitude of the observed increment in hydrophilicity
can result in a concomitant elevation in the total quantity and rate of ionic diusion into the porous structure
of the electrode17,69,73. Consequently, a broader and more stable EDL is established on the active surface of the
electrode17,29,73.
According to the adsorption and desorption diagram (Fig.S12a), the adsorption isotherm of AC is of the
fourth type with a hysteresis loop of the third type, indicating the presence of non-hard, plate-like meso and
Figure3. FTIR spectrum of AC, E1 electrode, Zn-BTC MOF and E4 electrode.
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macropores in its structure22,47,54. Additionally, Fig.S12 and TableS3 show an SSA of 723 m2 g−1, a pore volume
of 0.364 cm3 g−1, and a Mean pore diameter of 3.10nm that were achieved using BJH method. Materials with
mesoporous structures exhibit superior performance compared to those with macro and microporous structures
in terms of ion adsorption and the formation of an EDL within the material, eectively accommodating ions
and facilitating ion diusion17,68,69.
Electrodes desalination performance
In the CDI cell, the desalination process was executed using three dierent electrode arrangements: SymE1 (sym-
metric arrangement, with E1 used as both anode and cathode), SymE4 (symmetric arrangement, with E4 used as
both anode and cathode), and Asym (asymmetric arrangement, with E4 used as anode and E1 as cathode). ese
arrangements were selected for two main reasons: (1) to study and compare the eect of adding Zn-BTC MOF
to electrodes in a symmetric arrangement on the increase in SRC, and (2) to investigate the potential impact of
the positive charge density of Zn-BTC MOF in the anode on both the electrical eld force induced by externally
applied voltage and the interaction forces of ions with electrodes in an asymmetric arrangement. Each desalina-
tion test was repeated three times, from a NaCl feed solution with an initial concentration of 100.0mg L−1 and
an initial electrical conductivity of 253.4µS cm−1. It is important to determine the appropriate conditions for the
CDI process to achieve the best performance. erefore, a suitable applied potential dierence was determined.
Insucient applied voltage leads to a reduced formation of a suitable EDL, causing a decrease in the adsorption
capacity of the electrode17,70. Excessive applied voltage can trigger Faradaic reactions or electrolysis of water,
compromising the accuracy and stability of the electrode–electrolyte system17,82,83. erefore, determining the
optimal voltage for CDI cells is of particular importance.
For the SymE1 arrangement, the results of desalination of a NaCl feed solution at voltages of 1.2V and 1.6V,
and a ow rate of 20mL min−1, are depicted in Fig.S13a. It should be noted that due to the intense changes
in the pH of the solution at a voltage of 2.0V and the occurrence of Faradaic reactions17,82,83, the deionization
process was stopped at this voltage, and therefore its results are not presented. According to Fig.S13a and some
pre-tests in dierent voltages in all three dierent arrangements of electrodes, the best voltage was determined
to be 1.6V. As can be seen at the voltage of 1.6V, the electrical conductivity of the feed solution has decreased to
a greater extent in a period of 30min, which means more desalination. e SRC at 1.2 and 1.6V was equal to 2.4
and 5.2mg g−1, respectively, while the SRE was measured at 8.4% and 18.1%, respectively. is clearly indicates
the direct eect of the electrical eld force induced by the applied voltage on the amount of salt adsorption by
the CDI cell25. Furthermore, the absence of gas bubbles and lack of intense pH changes suggests that Faradaic
reactions or water electrolysis did not occur17,82,83.
In another test to determine the appropriate ow rate of feed solution at 1.6V, the SRC at ow rates of 10, 20,
and 30mL min−1 resulting in corresponding values of 4.7, 5.2, and 4.1mg g−1, respectively, as shown in Fig.S13b.
e corresponding SRE values for these ow rates were found to be 16.3%, 18.1%, and 14.3%, respectively. Based
on the results from Fig.S13b and pre-tests using dierent ow rates and electrode arrangements, the optimal
ow rate was determined to be 20mL min−1. is outcome can be attributed to the eective diusion of ions and
the establishment of a stable EDL in the porous electrode structure, facilitated by adequate time84,85. erefore,
the assessment of desalination process were performed at the voltage of 1.6V and the ow rate of 20mL min−1.
e results of desalination in all three arrangements (SymE1, SymE4, and Asym) are shown in Fig.S13c. e
SRC within 30 min of the desalination process for SymE1, SymE4, and Asym arrangements is equal to 5.2, 6.0,
and 6.3mg g−1, respectively, which are equivalent to 18.1%, 20.8%, and 21.9% of SRE, respectively. As expected,
the SymE4 has more desalination than the SymE1 arrangement. e high hydrophilicity of the Zn-BTC MOF,
coupled with the greater specic capacitance and lower ion charge transfer resistance of the E4 electrode com-
pared to E1, results in faster and more ecient ion diusion and a more stable formation of the EDL within the
electrode structure17,73. Additionally, the Asym arrangement exhibits more desalination eciency than the SymE4
arrangement. e reason behind this phenomenon refers to the electrostatic interactions from the electrical eld
induced by the applied voltage on the electrodes and the charge of zinc ions (Zn2+) present within the Zn-BTC
MOF structure29,60. e presence of Zn2+ ions in MOF structure creates positively charged sites that have a higher
charge density than –COO– groups in the structure29,53,86. e anode, being the positively charged electrode,
exhibits a distinct behavior because of the incorporation of Zn-BTC MOF along with the electric force eect of
the externally applied voltage eld29. e presence of Zn2+ ions within the MOF structure exerts an attractive
Figure4. CA of E1 and E4 electrodes.
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electrostatic force on the anions, thereby leading to enhanced attraction and separation of a larger number of
anions from the passing solution comprising Na+ and Cl− ions within the CDI cell85. is consequently leads to
the formation of a stable EDL within the porous structure of the electrode53,60,87,88. e schematic mechanism
involved in the Asym arrangement in the CDI system is depicted in Fig.5.
Furthermore, it can be inferred that the inclusion of Zn-BTC MOF within the cathode material of the
SymE4 arrangement improves the electrode specic capacitance and concurrently reduces the ion diusion
resistance86,89. However, the presence of Zn2+ ion sites within the cathode structure could potentially decreases
its performance due to the consequent repulsion between Na+ and Zn2+ cations. Consequently, a decrease in
cation adsorption can lower the performance of the SymE4 in comparison to the Asym arrangement29,53,89. e
results of SRC and SRE of all three arrangements, considering the possible error for each arrangement, are shown
in Fig.S13d and e, respectively.
e results of a complete CDI process cycle (adsorption and desorption) are given in Fig.6a. e desorp-
tion stage that removes ions from the electrodes (electrode regeneration) is carried out at 0.0V and a ow
rate of 20mL min−1. It can be seen that in the SymE1 arrangement, the electrodes are fully regenerated faster.
Aer 15min from the desorption stage, the feed solution electrical conductivity returns to its initial value. In
the SymE4 arrangement, it takes 25min for the electrodes to be completely regenerated and for the electrical
conductivity of the feed solution to return to its initial value. is may be due to the increased adsorption of
ions during the adsorption stage, leading to a prolonged time interval for their subsequent removal29,60,87. Addi-
tionally, the proposed arrangement exhibits a higher degree of stability of EDL compared to that of the SymE1
arrangement29,86,89.
In the Asym arrangement, even aer 30min of the desorption stage, the electrodes have not been fully
regenerated, and the electrical conductivity of the feed solution has not reached the initial value. is phenom-
enon can arise from several reasons. Firstly, the enhanced ionic adsorption in the adsorption stage can lead
to a subsequent elongation in the ion removal time29,60,87. Secondly, the formation of a more stable EDL in the
proposed arrangement outperforms that of both the SymE1 and SymE4 arrangements29,86,89. Last but not least,
the anode comprises the Zn-BTC MOF in which the positive charge density is comparatively higher than the
negative charge density53,86,87. e anions that accumulate in the EDL of the anode tend to retain their position
even aer the applied voltage is discontinued. Since the performance and eciency of the anode and cathode
electrodes during deionization processes are interconnected, the behavior of each electrode signicantly aects
the other29,89. is eect is triggered for the cations adsorbed at the cathode as well. Consequently, both SymE1
and SymE4, showed the lower ion removal eciency and electrode regeneration29,60,86–88.
Figure6b shows CDI Ragone plots for the three arrangements of electrodes. e CDI Ragone plots of both the
SymE4 and Asym arrangements shi towards the upper right region compared to SymE1, indicating that they
have a higher desalination capacity and desalination rate, possibly due to their increased accessible surface area
and mesopores, as well as improved hydrophilicity29,57,90. As mentioned before, because of the eects of charge
density in the anode and Zn2+ electrostatic force of the Asym arrangement, which favor ion diusion in the pores
of the electrode matrix, it demonstrates both a higher desalination capacity and desalination rate compared to the
other two arrangements. is indicates the eect of Zn-BTC MOF on capacitive behavior and ion charge trans-
fer kinetics29,57,75,89. Figure6c shows the cyclic adsorption/desorption experiments of the representative Asym
arrangement of electrodes in a NaCl feed solution with a starting concentration of 100.0mg L−1. It is noted that
the electrodes showed approximately a 2.9% decay in SRC aer 50 cycles, proving its good cycling performance.
Table2 provides the details of the desalination process for all the three arrangements. e process was
conducted at a voltage of 1.6V and a ow rate of 20mL min−1. e initial feed solution contained 100.0mg L−1
NaCl. e details are provided for one cycle of the process. e table demonstrates that incorporating a small
quantity of Zn-BTC MOF into composite electrodes, in combination with an asymmetric electrode arrangement,
signicantly increases SRC. is is evident from the 15.3% and 21.1% increase in SRC for SymE4 and Asym
arrangements, respectively, when compared to symE1.
Figure5. Schematic mechanism involved in the Asym arrangement in CDI system.
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Conclusion
Incorporation of a small amount of Zn-BTC MOF into the carbon electrodes enhanced the electrochemical and
desalination performance. Although the SymE4 and particularly Asym arrangements exhibited weaker perfor-
mance during the desorption stage, this issue could be resolved either by increasing the ow rate or by applying
a reverse voltage for a short duration. Additionally, the electrode mass was periodically measured throughout
the experiments, and the lack of any signicant mass variation indicated their favorable stability and the absence
of noticeable faradaic reactions during the desalination processes.
MOFs, like conventional additives to carbon electrodes, enhances the performance, but MOFs typically exhibit
higher SSA and increased active sites. e incorporation of MOF particles with variable sizes ranging from the
nanometer to micrometer scale, coupled with the high hydrophilicity of MOFs, enhances MOF particle distri-
bution and uniformity, and also ion diusion within the electrode structure. is improved dispersion within
the AC matrix then promotes better ion accessibility to the electrode porous structure. Zn-BTC MOF exhibits
superior desalination performance in asymmetrical arrangement when compared to symmetrical arrangements
in CDI systems. is is due to the higher density of positive charge relative to negative charge. Hence, this work
demonstrated that composite MOF-incorporated electrodes could be of signicant interest for future research
aiming at enhancing the performance of CDI systems.
Considering the lack of substantial research on investigating pH changes in adsorption/desorption cycles and
their eect on the electrical conductivity of solutions, the importance of studying this aspect in future research
is highly signicant. Additionally, prioritizing future research should involve examining the optimization con-
ditions of material composition percentages and the selection of metallic nodes and organic ligands for MOF
Figure6. (a) e results of one cycle of adsorption and desorption process for all the three arrangements of
electrodes. (b) CDI Ragone plots of all the three arrangements of electrodes. (c) e adsorption and desorption
cycling stability test of Asym arrangement of electrodes for 50 cycles. Experimental conditions: a voltage of 1.6V
and a ow rate of 20mL min−1.
Table 2. Details of the desalination process in all the three arrangements.
Arrangement Mass of electrode active layer
(g) Voltage (V) Flow rate (mL min−1)NaCl Initial feed
concentration (mg L−1) SRC (mg g−1) SRE (%)
SymE1
0.22 1.6 20 100.0
5.2 18.1
SymE4 6.0 20.8
Asym 6.3 21.9
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synthesis. It’s crucial to address potential challenges associated with scaling up MOF-incorporated electrode
production and ensuring its long-term stability in CDI systems. Moreover, investigating the possibility of using
the CDI process independently for water desalination or coupling it with other desalination methods like reverse
osmosis warrants further exploration.
Data availability
e datasets used and/or analysed during the current study available from the corresponding author on reason-
able request.
Received: 3 March 2024; Accepted: 26 June 2024
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Author contributions
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