Eco-friendly facile synthesis of glucose–derived microporous carbon spheres electrodes with enhanced performance for water capacitive deionization

Desalination 477 (2020) 114278

Contents lists available at ScienceDirect

Desalination journal homepage: www.elsevier.com/locate/desal

Eco-friendly facile synthesis of glucose–derived microporous carbon spheres electrodes with enhanced performance for water capacitive deionization

T

⁎

Shaimaa K. Mohameda, , Mahmoud Abuelhamda, Nageh K. Allamb, Ahmed Shahata,d, ⁎ Mohamed Ramadanb, Hassan M.A. Hassana,c, a

Department of Chemistry, Faculty of Science, Suez University, Suez, Egypt Energy Materials Laboratory (EML), School of Sciences and Engineering, The American University in Cairo, New Cairo, 11835, Egypt c Chemistry Department, College of Science, Jouf University, PO Box 2014, Sakaka, Saudi Arabia d Department of Chemistry, College of Science and Art, King Abdulaziz University, Rabigh 21911, Saudi Arabia b

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Desalination Capacitive deionization Carbon spheres Electrosorption capacity Carbon electrode

Water desalination via capacitive deionization technique (CDI) is deeply affected by the properties of electrode materials. In this work, microporous carbon spheres (CSs) were fabricated by simple green eco-friendly process with low cost materials where glucose is converted to carbon spheres via polymerization reactions through hydrothermal treatment followed by carbonization. X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), Field Emission Scanning Electron Microscopy (FESEM) and Nitrogen adsorption-desorption studies have characterized the obtained CSs. Measurements of Cyclic Voltammetry (CV), Galvanostatic Charge and Discharge (GCD), and Potentio-Electrochemical Impedance Spectroscopy (PEIS) were used to investigate the electrochemical behavior. The desalination performance and the effect of different carbonization temperature on the chemical, structural and surface properties were studied. It was found that the CS800 electrode possessed large surface area and high electrosorption capacity (10.3 mg g−1) which suggest its use as a promising candidate for CDI.

⁎

Corresponding author. E-mail addresses: [email protected] (S.K. Mohamed), [email protected] (H.M.A. Hassan).

https://doi.org/10.1016/j.desal.2019.114278 Received 11 October 2019; Received in revised form 28 November 2019; Accepted 9 December 2019 0011-9164/ © 2019 Elsevier B.V. All rights reserved.

Desalination 477 (2020) 114278

S.K. Mohamed, et al.

1. Introduction

carbon source with subsequent thermal treatment exhibiting 5.81 mg g−1 electrosorption ability. Carbon hollow spheres (CHS), produced through a modified Stober process, were reported by Li et al. [46]. A sol–gel method was used to produce porous carbon spheres with hierarchical pores (hCSs) applying resorcinol-formaldehyde as a carbon source with a surfactant-directing assembly technique and subsequent HF treatment to extract the template [47]. Although, good electrosorption capacities were reported in the previous studies but some drawbacks are revealed such as long complicated fabrication process and using of environmentally harmful chemicals; HF, sulphuric acid, ammonia, etc. Here, we report the fabrication of microporous carbon spheres (CSs) derived from glucose as the carbon source via hydrothermal treatment followed by carbonization at different temperatures. The synthesis process is simple, green, template-free, and eco-friendly without any use of chemical additives. The performance of the fabricated CSs in CDI is studied and the impact of different carbonization temperature on the chemical, structural and surface features is reported.

The lack of clean, fresh, potable water is considered one of the most pervasive problems afflicting people worldwide. It is expected to grow worse in the coming decades as a result of increasing population growth, population density, and industrial activities along with increscent global water scarcity. Desalination techniques of seawater and saline aquifers have emerged as a viable solution to this indecipherable problem [1–3]. Commercial desalination techniques are categorized as thermal based technologies; including multistage flash distillation (MSF) [4], Multi-effect evaporation and vapor Compression, or membrane based technologies such as reverse osmosis (RO) [3] and electro dialysis (ED) [5]. Nonetheless, these techniques suffer from serious drawbacks, likes high energy usage, increased cost, great salt content by-product, and environmentally harmful secondary wastes. Capacitive deionization (CDI) is considered as a low energy consumption, environment-friendly and low-cost alternative among desalination technologies [6–10]. CDI is low pressure and non-membrane desalination technique that based on electro sorption property achieved by ions adsorption at the electric double layer (EDL) generated at the surface of porous electrodes by implementing an electric potential and when the charge potential is removed, later desorption of the ions from the electrodes occur [11–13]. The CDI performance basically based on the features of electrodes substance including the composition and the physical features of the electrode substance such as a large surface area, suitable porous structure for smooth electro sorption, ample electric conductance for electron transfer, wettability and durability [6] . Because of their complex shapes, big surface area, good conductance and regulated pore distribution, carbon-based materials [14,15] are therefore commonly used as CDI electrode materials. To date, carbon electrodes of big surface area in a set of forms: activated carbons and activated carbon textile [16–19], aerogel carbon [20], ordered mesoporous carbons [21], carbon spheres [22], hierarchical porous carbons [23–26], carbon nanotubes [27], graphene [28–33] and their composites [34–37] were reported as CDI electrode materials by different research groups. Carbon spheres are of particular interest as one type of carbon porous materials that exhibit regular geometries and high specific surface areas. In addition, the distribution of pores and form of porosity can be regulated through a reasonable development process by different synthesis parameters [38]. Carbon spheres have been widely used in numerous applications including adsorbents [39,40], catalyst supports [41], lithium ion batteries [42], and super capacitors [43,44]. Many research groups have recently been using carbon spheres as CDI electrode materials. Liu et al. [45] manufactured porous carbon spheres (PCSs) using a microwave-assisted method applying sucrose as

2. Material and methods 2.1. Materials D-Glucose (Analytical Grade) and Polyvinylidene fluoride (PVD) powder were purchased from Merck and Alfa Aeser, respectively where Ethanol (≥99.8%(GC), absolute) and N, N-Dimethyl Formamide (DMF) (99.7) were acquired from Loba Chemie. Carbon black was obtained from Capot Corporation, USA. All materials were used with no further treatment. Graphite sheets for the fabrication of electrodes were purchased from XRD Graphite, China.

2.2. Synthesis of porous carbon spheres (CSs) In 60 ml of demineralized water, 4.0 g of glucose was dissolved and the solution was moved to an autoclave of 70 ml of Teflon-lined stainless steel. The autoclave was enclosed and heated in an electric furnace where the temperature was slowly raised to 200 °C in 40 min then maintained at 200 °C for 8 h after which it was kept to cool down to ambient temperature. The obtained dark products were gathered by filtration, rinsed many times with demineralized water and ethanol and then dried at 80 °C overnight. The CSs was calcined in a tube furnace at 700,800 and 900 °C for 4 h under nitrogen gas atmosphere with a heating rate of 5° per minute. The carbon spheres were denoted CSx (where x = 700, 800, or 900) depending on the carbonization temperature. The synthesis procedure is summarized in Fig.1.

Fig. 1. The synthesis process of CSs using different carbonization temperature. 2

Desalination 477 (2020) 114278

S.K. Mohamed, et al.

2.3. Fabrication of CDI electrodes

2.6. Desalination testing

A blend of the fabricated carbon spheres, carbon black and PVD with a weight ratio of 70: 20: 10 as active materials, conducting agent and binder respectively, were blended in (DMF) to get homogeneous slurry. Finally, the mixture is poured on a graphite sheet (10 cm * 10 cm) and dried in vacuum oven at 80 °C overnight. The electrode mass charge was 300 mg.

The CDI behavior of the fabricated Carbon spheres electrodes have been examined in a bench scale flow-by type CDI cell by batch-mode experiments, which performed using symmetric electrodes. In all experiments, the temperature of solution and volume were held at 298 K and 100 ml respectively and the solution flow rate was managed by the peristaltic pump at 40 mL min−1 by Cole–Parmer Masterflex L/S 77800-60 with 07528-10 pump head to push the saline solution. NaCl Stock solutions of concentrations (100 and 250) ppm were prepared to emphasize the impact of concentration on CDI efficiency with nitrogen bubbling as a pre-treatment to reduce the dissolved oxygen content in the solution. Also, different cell voltages (1.2, 1.4 and 1.6) Volts were applied by Biologic SP-150 potentiostat to study the effect of changing the cell voltage as it is another operator that affects the CDI performance. During the experiment, conductivity meter (HANNA HI 2030) continuously monitored the change in conductivity of the solution. The cell was first cleaned with deionized water prior to the experiments began, then the NaCl solution flowed through the cell during each experiment without applying voltage until the process reached equilibrium. The potential for salt adsorption (SAC) has been determined using the following Eq. (3)

2.4. Characterization Panalytical Empyrean X-Ray Diffractometer (XRD) used copper Cu Kα radiation (30 mA, 40 kV) to detect and identify the phases. Raman measurements are conducted on a 532 nm laser beam wavelength Raman microscope (Pro Raman-L Analyzer). Field emission scanning electron microscopy was performed by (Zeiss FESEM Ultra 60) operated with 5 kV. Micromeritics ASAP-2020 was performed to obtain nitrogen adsorption - desorption isotherms at −196 °C, where the samples were degassed first for 3 h at 100 °C to remove any humidity or contaminants. To evaluate the values of surface area, the BrunauerEmmett-Teller (BET) method was employed. In addition, the Pore size distribution was calculated from the nitrogen isotherm adsorption branch via the Barrett-Joyner-Halenda (BJH) approach. The surface area and volume of micropore were analyzed applying the t-plot method. The Carbon spheres functional groups were investigated using Fourier transform infrared spectroscopy (FTIR); the measurements were conducted using a BRUKER Vertex 70 FTIR spectrometer. High-resolution transmission electron microscope (HRTEM) images were taken by JEOL JEM-2100 Electron Microscope. Energy dispersive X-ray elemental analyses were conducted on JCM-6000 plus.

SAC =

(Co − Cf )V (3)

m

where the volume of the saline solution is denoted by V (L) while m (g) is the whole mass of CSs and binder in the electrodes. Co and Cf represent the original and final NaCl concentrations (ppm), respectively. The solution concentration has been estimated by implementing a previously committed calibration Table from the conductivity measurements (μS/cm). 3. Results and discussion

2.5. Electrochemical behavior 3.1. Chemical, structural, morphological and surface features of CSs The materials CSs-700,800&900 were tested in order to determine its electrochemical properties using a Biologic SP-150 potentiostat as following: The cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) were performed using a potential window between −0.5 and 0.5 V. Potentio-Electrochemical Impedance Spectroscopy (PEIS) has been operated with a sinusoidal perturbation of 10 mV amplitude at the open circuit potential through the frequency spectrum 0.01 Hz to 100 kHz. All the electrochemical experiments were done in a traditional three-electrode system with 1 M NaCl aqueous solution as the electrolyte. The working electrodes were produced by blending the fabricated carbon spheres, carbon black and PVD with a weight ratio of 70: 20: 10 as active materials, conducting agent and binder, respectively. Then the mixture was applied into titanium sheet of (1 cm * 1 cm) and heated at 80 °C overnight. The electrode mass charge was 1.5 mg. The produced CSs electrodes were employed as the working cell electrode while Ag/ AgCl and Pt electrodes served as reference and counter electrodes, respectively. The specific capacitance of the produced electrodes was measured from CV and GCD based on respectively Eqs. (1) and (2) [48].

1 νm

CGCD =

∫ V1 dV

(1)

I × ∆t (m × ∆V )

(2)

(002)

Intensity (a.u.)

Ccv =

Fig. 2 displays the XRD patterns of the prepared carbon spheres at different carbonization temperature. All samples exhibited two wide diffraction peaks appeared at 2θ value of 24° and 44°which were related to the features graphitic carbon (002) and (100) diffractions, respectively [22,50]. The broadening of peaks is an indication of low crystallinity and high amount of structural disorder. Raman spectra were applied to investigate information about the nature of chemical structure of the prepared carbon spheres. As illustrated in Fig. 3, all CS samples displayed D and G bands at 1350 and 1590 cm−1, respectively, corresponding to both sp3‑carbon and sp2‑carbon [9,47]. The ratio of D-band to G-band (ID/IG) representing defective or disordered carbon matrix degree was measured and presented in Table 1. Clearly, with the raise in the carbonization temperature, ID/IG value increases, implying the increase of disorders or

(100)

10

where (Ccv) are the specific capacitances from cyclic voltammetry, (CGCD) are the specific capacitances from galvanstatic charge/discharge (GCD), ʋ is scan rate, V is the potential window, (m) are the mass of carbon material, I is the current density and Δt is discharge time [49].

20

30 40 50 60 2 Theta (degree)

CS700 CS800 CS900

70

80

Fig. 2. XRD patterns of the prepared carbon spheres. 3

Desalination 477 (2020) 114278

S.K. Mohamed, et al.

CS900 CS800 CS700

Intensity (a.u.)

D

1200

Table 2 The Surface and textural properties of carbon spheres.

G

1300 1400 1500 1600 Raman shift ( cm-1)

Fig. 3. Raman spectra obtained for the prepared carbon spheres. Table 1 The calculated ID/IG ratios and elemental composition of the prepared carbon spheres. CS700

CS800

CS900

ID/IG Carbon mass% Oxygen mass%

0.52 98.91 1.09

0.70 99.31 0.69

0.71 99.49 0.51

CS700 CS800 CS900 CS-As grown

Transmittance (a.u.) 4000

3000 2000 1000 Wavenumber , cm-1

Fig. 4. FTIR spectra of the prepared carbon spheres.

defects in the carbon framework [51] which can cause improvement for the charge transfer in the electrosorption process [52]. FT-IR spectra of the fabricated electrodes are shown in Fig.4. The spectrum of the as grown carbon spheres (before calcination) contained the following bands: broad absorption peak from 3000 cm−1 to 3600 cm−1 attributed to stretching vibrations of OeH (hydroxyl or carboxyl), small band at 2900 cm−1 corresponded to stretching vibrations of aliphatic CeH, bands at about 1700 cm−1 and 1600 cm−1

120

Sin (m2/g)

Sex (m2/g)

Vp total (cm3/g)

Vmicro (cm3/g)

Average pore diameter (nm)

CS700 CS800 CS900

271.5 361.1 211.7

238.7 315.6 185.1

32.7 45.4 26.6

0.1240 0.1740 0.1057

0.1098 0.1425 0.0850

1.82 1.93 1.98

0.020

CS700 CS800 CS900

CS700 CS800 CS900

(b) Pore Volume (cm³/g)

Quantity Adsorbed (cm³/g STP)

140

(a)

SBET (m2/g)

assigned to C]O and C]C vibrations respectively, bands between 1000 and 1500 cm−1 corresponded to C–OH stretching and OeH bending vibrations and finally bands between 900 and 500 cm−1 are related to CeH bending vibrations. The obtained results revealed the occurrence of aromatization and polymerization processes during the hydrothermal treatment of glucose which is in accordance with the works of Sevilla and Fuertes [53] and Li et al. [54]. When the as grown carbon spheres were calcined at 700 °C, the bands at 2900 cm−1, 1700 cm−1 and, 1000–1500 cm−1 region disappeared, suggesting further removing of the groups containing oxygen. On the other hand, the carbonization at 800 or 900 °C led to complete defunctionalization of the surface as indicated by the absence of any bands in the obtained spectra. The N2 adsorption–desorption isotherms for the fabricated CSs (Fig. 5. a) demonstrated a typical type I isotherm, according to IUPAC classification, revealing the microporous nature of the sphere's structure. The adsorption of high amount of nitrogen in the (P/P0) range lower than 0.01 may be attributed to the existence of a lot of micropores while the nearly horizontal plateau in the (P/P0) range of 0.1–0.98 accompanied by well-defined knee indicate a monomodal distribution of micropores [55]. The pore size distribution curves (Fig.5.b) revealed the uniform mono-modal distribution of pore size in the micropore region. The number of pores with a microporous features decreased with the increase of temperature of carbonization to 900 °C. The surface and textural properties of the prepared CSs are summoned in Table 2. Inspection of Table 2 revealed the following: (i) The specific surface area was increased by raising carbonization temperature from 700 to 800 °C which may be attributed to complete carbonization of the sample [44], in accordance with the results from FTIR or due to increased number of defects generated [46], as emphasized by Raman spectra; (ii) Further raising of carbonization temperature to 900 °C led to lowering in the value of SBET as a result of sintering and collapse of the pore structure; (iii) CS800 possessed the highest total pore volume while the microporous pore volume percentages for all prepared CSs were > 80% from total pore volume which again confirms the microporous nature of the prepared CSs; (iv) The calculated average pore diameters were increased by carbonization temperature. The morphology and the size of the fabricated CSs were emphasized using SEM. From SEM images shown in Fig. 6, one can observe that the CSs are well dispersed and have well-defined uniform spherical

1700

Sample

Sample

100 80 60

0.015

0.010

0.005

40 0.000

0.0

0.2

0.4

0.6

0.8

1.0

1

2

5

10

50

Average Diameter (nm)

Relative Pressure (P/Po)

Fig. 5. (a) Nitrogen sorption isotherms for the prepared carbon spheres, (b) BJH pore size distributions. 4

Desalination 477 (2020) 114278

S.K. Mohamed, et al.

Fig. 6. FESEM images of the fabricated carbon spheres at different carbonization temperature: (a, b) 700; (c, d) 800 and (e, f) 900 °C.

rings in the selected region electron diffraction (SAED) pattern revealed the amorphous existence of the CS800. The influence of the temperature of carbonization on the wettability of CSs was studied by water contact angle experiments and illustrated by Fig.8. As expected, the contact angle was increased with increasing carbonization temperature due to full carbonization of the CSs surfaces which is in accordance with the FTIR measurements. Better wettability is noticed for CS700 by performing dynamic contact angle measurements (not shown here). However, the hydrophilicity is not the only factor to govern the performance of electrodes in the CDI studies.

morphology with smooth surfaces. Besides, CSs exhibited average diameter ranging from 670 to 1000 nm. The nature of CSs porosity surfaces can be proved from the enlarged SEM images (Fig.6 b). It can be noted that raising the calcination temperature led to reduction in the average diameter of the spheres. The energy dispersive X-rays' analysis of the prepared CSs was performed and the calculated element's mass percent was presented in Table 1. Carbon is the main component of the CSs and very small amount of oxygen is also found. As expected, the amount of oxygen decreased as the calcination temperature was increased due to complete carbonization of the samples which further confirmed the results obtained from both FT-IR and Raman studies. TEM and HRTEM images were obtained for one selected sample (CS800) in order to investigate its surface and morphological characteristics and were given in Fig.7. Worm-like channels and homogeneously dispersed micropores can be seen in the carbon surface which emphasized the results obtained from nitrogen adsorption–desorption isotherm. The appearance of somber diffraction

3.2. Electrochemical behavior of the fabricated CSs electrodes The cyclic voltammograms of the fabricated CSs electrodes over the potential window of −0.5 to 0.5 V vs. Ag/AgCl at various rates of scan from 5 to 100 mV s−1 are given in Fig.9. At low scan rate, all the CVs reveal rectangular and symmetrical shape suggesting a typical electrical 5

Desalination 477 (2020) 114278

S.K. Mohamed, et al.

Fig. 7. (a) TEM and (b) HRTEM images of CS800. Inset is the (SAED) pattern.

1.2

5 mV/s 20 mV/s 70 mV/s

10 mV/s 50 mV/s 100 mV/s

0.6 0.0 -0.6 -1.2

CS700

5 mV/s 20 mV/s 70 mV/s

-0.4 -0.2 0.0 0.2 0.4 Potential (V) vs. Ag/AgCl

0.6

3

10 mV/s 50 mV/s 100 mV/s

2 0

-0.6

2

5 mV/s 20 mV/s 70 mV/s

10 mV/s 50 mV/s 100 mV/s

1 0

-1

-2

CS900

-2

CS800

-4

-1.8 -0.6

4

Current density A g-1

1.8

Current density A g-1

Current density A g-1

Fig. 8. Water contact angle measurements for CSs.

-0.4 -0.2 0.0 0.2 0.4 Potential (V) vs. Ag/AgCl

0.6

-3 -0.6

-0.4 -0.2 0.0 0.2 0.4 Potential (V) vs. Ag/AgCl

0.6

Current density A g-1

4 3 2

CS700 CS800 CS900

(a)

1 0 -1 -2 -3 -0.6

-0.4 -0.2 0.0 0.2 0.4 Potential (V) vs. Ag/AgCl

Spacific capacitance (F g-1)

Fig. 9. Cyclic voltammograms of the fabricated CSs electrodes over the potential window of −0.5 to 0.5 V vs. Ag/AgCl at different scan rates.

0.6

80

CS700 CS800 CS900

(b) 60

53 %

40

51 %

20

30 % Rate of capability

0 0

20

40 60 80 Scan rate (mV s-1)

100

Fig. 10. (a) Comparison between the CVs of all CSs electrodes over the potential window of −0.5 to 0.5 V vs. Ag/AgCl at scan rate of 50 mV s−1. (b) the calculated specific capacitances from cyclic voltammetry of the fabricated CSs electrodes at different scan rates and the corresponding rate of capability.

The declines in capacitance with increasing scanning rate can be due to the adsorption and diffusion difficulties of ions on the surface of electrode at higher scanning rate [56]. Fig. 10a compares the cyclic voltammograms of the fabricated CSs electrodes at scanning rate of 50 mV s−1. It is obvious that the CVs of

double-layer capacitive behavior and the ease of adsorption/desorption of Na+ and Cl− ions at/from micropores of CSs. At higher scan rates, CS800 and CS900 samples retained their rectangular shape and their ideal capacitive behavior while CS700 sample showed more deviation, indicating that CS700 has higher internal resistance than other samples. 6

Desalination 477 (2020) 114278

S.K. Mohamed, et al.

300

50

-Z"/Ohm

-Z"/Ohm

(b)

40

(a)

200

30 20 10 0

100

0

50

CS700 CS800 CS900

0 0

100

100

Z’/Ohm

200

300

Z’/Ohm

0.4 A g-1 0.6 A g-1 0.8 A g-1 1.0 A g-1 1.2 A g-1 1.4 A g-1 1.6 A g-1 2.0 A g-1

CS700 0.3 0.0 -0.3

0.6

0.4 A g-1 0.6 A g-1 0.8 A g-1 1.0 A g-1 1.2 A g-1 1.4 A g-1 1.6 A g-1 2.0 A g-1

CS800

0.3 0.0

-0.3

-0.6 10

20

30 40 Time (s)

50

60

CS900

0.3 0.0

0.4 A g-1 0.6 A g-1 0.8 A g-1 1.0 A g-1 1.2 A g-1 1.4 A g-1 1.6 A g-1 2.0 A g-1

-0.3

-0.6

0

0.6

Voltage (V) vs. Ag/AgCl

0.6

Voltage (V) vs. Ag/AgCl

Voltage (V) vs. Ag/AgCl

Fig. 11. (a) Potentio-Electrochemical impedance spectroscopy (PEIS) spectra of the fabricated CSs electrodes. (b) The proposed equivalent circuit diagram.

0

50

100 150 Time (s)

200

250

-0.6 0

20

40

60 80 Time (s)

100

0.6

spacific capacitance (F g-1)

Voltage (V) vs. Ag/AgCl

Fig. 12. The GCD curves of the fabricated CSs electrodes at various current densities.

(a)

0.4

CS700 CS800 CS900

0.2 0.0 -0.2 -0.4 -0.6 0

20

40 60 Time (s)

80

100

60

(b)

50

CS700 CS800 CS900

40 30 20 10 0 0.4

0.8 1.2 1.6 Current denisty (A g-1)

2.0

Fig. 13. (a) GCD curves of the fabricated CSs electrodes at current density of 1 A g−1, (b) specific capacitance at different current densities calculated from GCD.

Fig. 14. (a) Electrosorption paths for CS800 electrode at 100 ppm NaCl solution and different voltage CS800, (b) comparison for electrosorption path for all fabricated electrodes at 1.6 V and 250 ppm.

capacitance than that of both CS700 and CS900 accompanied with higher capability rate. For example, at 5 mV s−1 the specific capacitance of CS800 is 71 F g−1, which is 1.7 times more than that of CS900 (43 F g−1) and double that of CS700 (35 F g−1) which may be related to specific greater surface area of CS800 sample than other samples. It is

the CS800 electrode shows much larger IV areas as compared with both CS700 and CS900, signifying a higher specific capacitance of CS800 electrode. A comparison between the calculated specific capacitances from CV of the fabricated CSs electrodes at various scanning rates is shown in Fig. 10b. At all scanning rates, CS800 possessed the higher 7

Desalination 477 (2020) 114278

S.K. Mohamed, et al.

SAC at 100 ppm

SAC mg g-1

8 6 4

1.4

1.6

5.8

5.2 4.2

3.3

1.2

4.7

4

4.7 3.3 3.7

2 0 CS 700

CS 800

SAC at 250 ppm

12

SAC mg g-1

10 8

8.2

CS 900 1.2

1.4

1.6

10.3

9.3

8.7 7.7

7.2

6.3

6

7.0

5.2

4 2 0 CS 700

CS 800

CS 900

Fig. 15. The calculated SAC values for the fabricated CSs electrodes at different voltages using 100 and 250 ppm NaCl solution.

well established fact that increasing the surface area of porous carbon materials leads to higher capability for the gathering of charges in the electrode/electrolyte interface resulting in greater gravimetric capacitance [57]. Potentio-Electrochemical Impedance Spectroscopy (PEIS) analysis was conducted to determine the electrical resistance and capacitive attitude for the fabricated electrodes [58]. Nyquist plots of the fabricated CSs electrodes are displayed at Fig. 11a. At low frequencies, all prepared electrodes displayed the nearly vertical line, indicating ideal capacitive behavior. While at great frequencies, the appearance of the small semicircle pointed to the charge transfer resistance (Rct) occurring at the interface between electrolyte and electrode [59] that is considered a remarkable variable dominates the adsorption/desorption rate. The small diameter of the semicircle corresponds to smaller (Rct) value and greater charge/discharge rate. Compared to that, CS800 and CS900 have smaller semicircle diameter than CS700 indicating that CS700 possess the highest internal resistance compared to the other electrode [60]. Furthermore, the proposed equivalent circuit diagram used to analyze PEIS spectra is depicted in Fig.11b. Accordingly, the values of Rct were calculated as follows: CS700 has Rct of 59.43 Ohm which is higher 6 times than both CS800 (9.617 Ohm) and CS900 (10.97 Ohm). The obtained values confirmed the smallest charge transfer resistance of CS800. The GCD plots of the fabricated electrodes at different current densities in 1 M NaCl were displayed in Fig. 12. Obviously, all electrodes had almost symmetrical GCD curves with a typical triangular shape, suggesting the formation of EDL. The observed linear V-t plot suggested fast IeV response behavior during the entire electrosorption process with favorable electrochemical reversibility. It is found that the time of discharging of CS800 is the longest among the three electrodes as revealed by the comparison of GCD curves of the fabricated electrodes at 1.0 A g−1 in Fig. 13a; which also supports the greatest specific

Table 3 The calculated SAC of the fabricated CS800 electrode verses other reported carbon based electrodes. Materials

Initial NaCl concentration ppm

SAC mg.g−1

Reference

mCSs800 CHS AC PCS1000 N-HMCSs N-HPC PCP1200 CS 800

500 250 250 500 250 250 250 250

6.3 6.5 4 5.8 13.1 10.4 10.1 10.3

[47] [46] [46] [45] [22] [26] [59] This study

Fig. 16. Recycling electrosorption experiment for CS800 electrode at 1.2 V with 100 ppm NaCl solution.

8

Desalination 477 (2020) 114278

S.K. Mohamed, et al.

Fig. 17. The CDI Ragone plots for CS800 at different cell voltage (a), and different salt concentrations (b).

regeneration process with good cycling stability over 20 adsorption−desorption cycles. The CDI Ragone plots for CS800 at different cell voltage and different concentrations of salt were constructed and presented at Fig. 17. The plot moved more to the up and right direction with increasing the voltage indicating the enhancement of both the SAC and SAR. This can be illustrated by the fact that greater cell voltage can trigger strong electrostatic forces which resulted in thicker EDL [62]. Low cell voltage, on the other hand, may result in inadequate EDL formation and decrease in both SAC and ASAR. Increasing the salt concentration switched the plot toward the right, upper region of the plot as an indication of the enhancement of both the SAC and SAR. This behavior is common in CDI and the reasons for this can be outlined by the two following factors: (i) the compaction of the EDL as a result of raising of charge concentration in the aqueous solution which led to enhancement of the SAC. (ii) The SAR was enhanced as a result of increasing the conductivity of the aqueous solution with subsequent fast transport of ions to the micropores of the electrode [63].

capacitance of CS800. Moreover, the values of specific capacitances computed from GCD analysis at different current densities are summarized in Fig. 13b which reveals the noticeable lowering in specific capacitance values with raising the current density, which is a common behavior at high current densities [61]. At all current densities, the specific capacitance of CS800 is bigger than that of both CS700 and CS900 in accordance with CV analysis results. Based on the obtained electrochemical performance results, it can be concluded that CS800 possessed the best electrochemical performance between the fabricated electrodes and it is expected to show better performance in CDI. 3.3. Desalination performance The Electrosorption paths for CS800 electrode at salt concentration of 100 ppm and different voltage (1.2, 1.4 and 1.6 V) are shown in Fig. 14a. When voltage was adopted across the electrodes during ion adsorption, the salt adsorption capacity (SAC) increased while the average salt adsorption rate (ASAR) decreased until equilibrium was reached indicating for the maximum adsorption and full occupation for active sites. Then, the conductivity increased again during ion desorption by short-circuiting until reaching the initial conductivity. Fig. 14b compares the electrosorption paths for all the fabricated CSs electrodes at 1.6 V and 250 ppm. The conductance decreased quickly at the first 10 min with the highest ASRA, revealing fast ions adsorption on the majority of electrode active sites. After that, the adsorption rate dropped until equilibrium was reached. Upon applying of zero voltage, the conductivity rapidly increased at the first 20 min and continued to increase in a slower rate until the initial conductivity was reached indicating a complete desorption of Na+ and Cl− ions from the active sites and successful regeneration of the electrode. Furthermore, it was found that CS800 has the lowest conductivity equilibrium point at the end of electrosorption process after 60 min of charging, while CS700 came in the second place and CS900 was the last rank. The calculated values of SAC are summarized in Fig. 15 which confirms better desalination performance of CS800 electrode over other fabricated electrodes in this study. This result may be justified by the existence of great amount of defects or disorders in the framework of microporous carbon sphere, possession of the largest surface area and highest specific capacitance as confirmed earlier by Raman, N2 adsorption/desorption and electrochemical analyses. Moreover, as concluded from Table 3, the obtained SAC is reasonably high compared with many reported carbon based electrodes. This fact along with the facile eco-friendly synthesis procedure of the fabricated CSs may support its application as a successful CDI water desalination nominee. The stability and the regeneration/recycling efficiency of the fabricated CS800 electrode was examined by the potential on/off experiments and presented in Fig. 16 which displayed a complete

4. Conclusion In this work, well dispersed uniform microporous carbon spheres with smooth surfaces were prepared from glucose via polymerization reactions through hydrothermal treatment followed by carbonization at different temperatures. EDL properties of prepared carbon spheres were investigated and specific capacitances were calculated for all prepared carbon spheres. CS800 has the greatest specific capacitance (71 F g−1), the lowest internal resistance and the highest surface area (361 m2/g) among the other fabricated electrodes. All previous parameters led to the superiority of CS800 as it possess the highest salt adsorption capacity (10.3 mg g−1) at 1.6 V. In addition to that, CS800 has good regeneration performance, suggesting that CS800 should be a worthy candidate electrode material for CDI application. Moreover, these results will open avenues for the synthesis of low cost, green and ecofriendly materials for CDI applications. CRediT authorship contribution statement Shaimaa K. Mohamed:Conceptualization, Visualization, Supervision, Writing original draft.Mahmoud Abuelhamd:Investigation.Nageh K. Allam:Conceptualization, Resources.Ahmed Shahat:Writing - review & editing.Mohamed Ramadan:Investigation.Hassan M.A. Hassan:Supervision. Declaration of competing interest The authors declare that they have no known competing financial 9

Desalination 477 (2020) 114278

S.K. Mohamed, et al.

interests or personal relationships that could have appeared to influence the work reported in this paper.

[26]

Acknowledgements [27]

We are thankful to Suez University in which this work was done. The authors thank Dr. Rehab El-Maghraby for performing the contact angle measurements at the Enhanced Recovery Oil laboratory/Oil and green chemistry research center; which is funded by STDF under project number: 12395.

[28] [29] [30]

References [31] [1] A. Subramani, J.G. Jacangelo, Emerging desalination technologies for water treatment: a critical review, Water Res. 75 (2015) 164–187. [2] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Mariñas, A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301–310. [3] L.F. Greenlee, D.F. Lawler, B.D. Freeman, B. Marrot, P. Moulin, Reverse osmosis desalination: water sources, technology, and today’s challenges, Water Res. 43 (2009) 2317–2348. [4] C. Koroneos, A. Dompros, G. Roumbas, Renewable energy driven desalination systems modelling, J. Clean. Prod. 15 (2007) 449–464. [5] S. Al-Amshawee, M.Y.B.M. Yunus, A.A.M. Azoddein, D.G. Hassell, I.H. Dakhil, H.A. Hasan, Electrodialysis desalination for water and wastewater: a review, Chem. Eng. J. 380 (2020) 122231. [6] S. Porada, R. Zhao, A. van der Wal, V. Presser, P.M. Biesheuvel, Review on the science and technology of water desalination by capacitive deionization, Prog. Mater. Sci. 58 (2013) 1388–1442. [7] Y.J. Kim, J.H. Choi, Improvement of desalination efficiency in capacitive deionization using a carbon electrode coated with an ion-exchange polymer, Water Res. 44 (2010) 990–996. [8] Y.-J. Kim, J.-H. Choi, Enhanced desalination efficiency in capacitive deionization with an ion-selective membrane, Sep. Purif. Technol. 71 (2010) 70–75. [9] R.T. Mayes, C. Tsouris, J.O. Kiggans Jr, S.M. Mahurin, D.W. DePaoli, S. Dai, Hierarchical ordered mesoporous carbon from phloroglucinol-glyoxal and its application in capacitive deionization of brackish water, J. Mater. Chem. 20 (2010). [10] G. Divyapriya, K.K. Vijayakumar, I. Nambi, Development of a novel graphene/ Co3O4 composite for hybrid capacitive deionization system, Desalination 451 (2019) 102–110. [11] Y. Liu, C. Nie, X. Liu, X. Xu, Z. Sun, L. Pan, Review on carbon-based composite materials for capacitive deionization, RSC Adv. 5 (2015) 15205–15225. [12] I. Villar, S. Roldan, V. Ruiz, M. Granda, C. Blanco, R. Menéndez, R. Santamaría, Capacitive deionization of NaCl solutions with modified activated carbon electrodes†, Energy Fuel 24 (2010) 3329–3333. [13] J. Han, L. Shi, T. Yan, J. Zhang, D. Zhang, Removal of ions from saline water using N, P co-doped 3D hierarchical carbon architectures via capacitive deionization, Environmental Science: Nano 5 (2018) 2337–2345. [14] W. Zhang, X. Jiang, X. Wang, Y.V. Kaneti, Y. Chen, J. Liu, J.-S. Jiang, Y. Yamauchi, M. Hu, Spontaneous weaving of graphitic carbon networks synthesized by pyrolysis of ZIF-67 crystals, Angew. Chem. Int. Ed. 56 (2017) 8435–8440. [15] N.L. Torad, Y. Li, S. Ishihara, K. Ariga, Y. Kamachi, H.-Y. Lian, H. Hamoudi, Y. Sakka, W. Chaikittisilp, K.C.W. Wu, Y. Yamauchi, MOF-derived nanoporous carbon as intracellular drug delivery carriers, Chem. Lett. 43 (2014) 717–719. [16] K.-H. Park, D.-H. Kwak, Electrosorption and electrochemical properties of activated-carbon sheet electrode for capacitive deionization, J. Electroanal. Chem. 732 (2014) 66–73. [17] J.-H. Choi, Fabrication of a carbon electrode using activated carbon powder and application to the capacitive deionization process, Sep. Purif. Technol. 70 (2010) 362–366. [18] M.-W. Ryoo, G. Seo, Improvement in capacitive deionization function of activated carbon cloth by titania modification, Water Res. 37 (2003) 1527–1534. [19] H.-J. Oh, J.-H. Lee, H.-J. Ahn, Y. Jeong, Y.-J. Kim, C.-S. Chi, Nanoporous activated carbon cloth for capacitive deionization of aqueous solution, Thin Solid Films 515 (2006) 220–225. [20] C. Zhang, X. Wang, H. Wang, X. Wu, J. Shen, A positive-negative alternate adsorption effect for capacitive deionization in nano-porous carbon aerogel electrodes to enhance desalination capacity, Desalination 458 (2019) 45–53. [21] C. Tsouris, R. Mayes, J. Kiggans, K. Sharma, S. Yiacoumi, D. DePaoli, S. Dai, Mesoporous carbon for capacitive deionization of saline water, Environ Sci Technol 45 (2011) 10243–10249. [22] Y. Li, J. Qi, J. Li, J. Shen, Y. Liu, X. Sun, J. Shen, W. Han, L. Wang, Nitrogen-doped hollow mesoporous carbon spheres for efficient water desalination by capacitive deionization, ACS Sustain. Chem. Eng. 5 (2017) 6635–6644. [23] L. Chao, Z. Liu, G. Zhang, X. Song, X. Lei, M. Noyong, U. Simon, Z. Chang, X. Sun, Enhancement of capacitive deionization capacity of hierarchical porous carbon, J. Mater. Chem. A 3 (2015) 12730–12737. [24] H. Wang, T. Yan, L. Shi, G. Chen, J. Zhang, D. Zhang, Creating nitrogen-doped hollow multiyolk@Shell carbon as high performance electrodes for flow-through deionization capacitors, ACS Sustain. Chem. Eng. 5 (2017) 3329–3338. [25] S. Zhao, T. Yan, H. Wang, J. Zhang, L. Shi, D. Zhang, Creating 3D hierarchical carbon architectures with micro-, meso-, and macropores via a simple self-blowing

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46] [47]

[48]

[49]

[50]

[51]

[52]

[53]

10

strategy for a flow-through deionization capacitor, ACS Appl. Mater. Interfaces 8 (2016) 18027–18035. Y. Li, I. Hussain, J. Qi, C. Liu, J. Li, J. Shen, X. Sun, W. Han, L. Wang, N-doped hierarchical porous carbon derived from hypercrosslinked diblock copolymer for capacitive deionization, Sep. Purif. Technol. 165 (2016) 190–198. L. Wang, M. Wang, Z.-H. Huang, T. Cui, X. Gui, F. Kang, K. Wang, D. Wu, Capacitive deionization of NaCl solutions using carbon nanotube sponge electrodes, J. Mater. Chem. 21 (2011). X. Xu, L. Pan, Y. Liu, T. Lu, Z. Sun, D.H. Chua, Facile synthesis of novel graphene sponge for high performance capacitive deionization, Sci. Rep. 5 (2015) 8458. P. Liu, T. Yan, L. Shi, H.S. Park, X. Chen, Z. Zhao, D. Zhang, Graphene-based materials for capacitive deionization, J. Mater. Chem. A 5 (2017) 13907–13943. H. Wang, T. Yan, P. Liu, G. Chen, L. Shi, J. Zhang, Q. Zhong, D. Zhang, In situ creating interconnected pores across 3D graphene architectures and their application as high performance electrodes for flow-through deionization capacitors, J. Mater. Chem. A 4 (2016) 4908–4919. Z. Li, B. Song, Z. Wu, Z. Lin, Y. Yao, K.-S. Moon, C.P. Wong, 3D porous graphene with ultrahigh surface area for microscale capacitive deionization, Nano Energy 11 (2015) 711–718. H. Duan, T. Yan, G. Chen, J. Zhang, L. Shi, D. Zhang, A facile strategy for the fast construction of porous graphene frameworks and their enhanced electrosorption performance, Chem Commun (Camb) 53 (2017) 7465–7468. T. Yan, J. Liu, H. Lei, L. Shi, Z. An, H.S. Park, D. Zhang, Capacitive deionization of saline water using sandwich-like nitrogen-doped graphene composites via a selfassembling strategy, Environmental Science: Nano 5 (2018) 2722–2730. Z. Peng, D. Zhang, L. Shi, T. Yan, High performance ordered mesoporous carbon/ carbon nanotube composite electrodes for capacitive deionization, J. Mater. Chem. 22 (2012). M. Ramadan, H.M.A. Hassan, A. Shahat, R.F.M. Elshaarawy, N.K. Allam, Ultrahigh performance of novel energy-efficient capacitive deionization electrodes based on 3D nanotubular composites, New J. Chem. 42 (2018) 3560–3567. H. Li, Z.Y. Leong, W. Shi, J. Zhang, T. Chen, H.Y. Yang, Hydrothermally synthesized graphene and Fe3O4 nanocomposites for high performance capacitive deionization, RSC Adv. 6 (2016) 11967–11972. J. Han, T. Yan, J. Shen, L. Shi, J. Zhang, D. Zhang, Capacitive deionization of saline water by using MoS2–graphene hybrid electrodes with high volumetric adsorption capacity, Environmental Science & Technology 53 (2019) 12668–12676. J. Liu, N.P. Wickramaratne, S.Z. Qiao, M. Jaroniec, Molecular-based design and emerging applications of nanoporous carbon spheres, Nat. Mater. 14 (2015) 763–774. L. Guo, X. Cui, Y. Li, Q. He, L. Zhang, W. Bu, J. Shi, Hollow mesoporous carbon spheres with magnetic cores and their performance as separable bilirubin adsorbents, Chem. Asian J. 4 (2009) 1480–1485. L. Guo, L. Zhang, J. Zhang, J. Zhou, Q. He, S. Zeng, X. Cui, J. Shi, Hollow mesoporous carbon spheres–an excellent bilirubin adsorbent, Chem Commun (Camb) (2009) 6071–6073. F.P. Hu, Z. Wang, Y. Li, C. Li, X. Zhang, P.K. Shen, Improved performance of Pd electrocatalyst supported on ultrahigh surface area hollow carbon spheres for direct alcohol fuel cells, J. Power Sources 177 (2008) 61–66. S. Yang, X. Feng, L. Zhi, Q. Cao, J. Maier, K. Mullen, Nanographene-constructed hollow carbon spheres and their favorable electroactivity with respect to lithium storage, Adv. Mater. 22 (2010) 838–842. J. Han, G. Xu, B. Ding, J. Pan, H. Dou, D.R. MacFarlane, Porous nitrogen-doped hollow carbon spheres derived from polyaniline for high performance supercapacitors, J. Mater. Chem. A 2 (2014) 5352–5357. X. Liu, L. Zhou, Y. Zhao, L. Bian, X. Feng, Q. Pu, Hollow, spherical nitrogen-rich porous carbon shells obtained from a porous organic framework for the supercapacitor, ACS Appl. Mater. Interfaces 5 (2013) 10280–10287. Y. Liu, L. Pan, T. Chen, X. Xu, T. Lu, Z. Sun, D.H.C. Chua, Porous carbon spheres via microwave-assisted synthesis for capacitive deionization, Electrochim. Acta 151 (2015) 489–496. H. Li, S. Liang, M. Gao, J. Li, Uniform carbon hollow sphere for highly efficient electrosorption, J. Porous. Mater. 23 (2016) 1575–1580. X. Xu, H. Tang, M. Wang, Y. Liu, Y. Li, T. Lu, L. Pan, Carbon spheres with hierarchical micro/mesopores for water desalination by capacitive deionization, J. Mater. Chem. A 4 (2016) 16094–16100. M. Ramadan, A.M. Abdellah, S.G. Mohamed, N.K. Allam, 3D interconnected binderfree electrospun MnO@C nanofibers for supercapacitor devices, Sci. Rep. 8 (2018) 7988. N. Ahmed, B.A. Ali, M. Ramadan, N.K. Allam, Three-dimensional interconnected binder-free Mn–Ni–S nanosheets for high performance asymmetric supercapacitor devices with exceptional cyclic stability, ACS Applied Energy Materials 2 (2019) 3717–3725. F. Su, C.K. Poh, J.S. Chen, G. Xu, D. Wang, Q. Li, J. Lin, X.W. Lou, Nitrogen-containing microporous carbon nanospheres with improved capacitive properties, Energy Environ. Sci. 4 (2011) 717–724. C. Young, R.R. Salunkhe, J. Tang, C.-C. Hu, M. Shahabuddin, E. Yanmaz, M.S.A. Hossain, J.H. Kim, Y. Yamauchi, Zeolitic imidazolate framework (ZIF-8) derived nanoporous carbon: the effect of carbonization temperature on the supercapacitor performance in an aqueous electrolyte, Phys. Chem. Chem. Phys. 18 (2016) 29308–29315. C. Nie, L. Pan, H. Li, T. Chen, T. Lu, Z. Sun, Electrophoretic deposition of carbon nanotubes film electrodes for capacitive deionization, J. Electroanal. Chem. 666 (2012) 85–88. M. Sevilla, A.B. Fuertes, Chemical and structural properties of carbonaceous products obtained by hydrothermal carbonization of saccharides, Chemistry 15 (2009)

Desalination 477 (2020) 114278

S.K. Mohamed, et al.

[59] Y. Liu, X. Xu, M. Wang, T. Lu, Z. Sun, L. Pan, Metal-organic framework-derived porous carbon polyhedra for highly efficient capacitive deionization, Chem Commun (Camb) 51 (2015) 12020–12023. [60] N. Ahmed, M. Ramadan, W.M.A. El Rouby, A.A. Farghali, N.K. Allam, Non-precious co-catalysts boost the performance of TiO2 hierarchical hollow mesoporous spheres in solar fuel cells, Int. J. Hydrog. Energy 43 (2018) 21219–21230. [61] F.M. Ismail, M. Ramadan, A.M. Abdellah, I. Ismail, N.K. Allam, Mesoporous spinel manganese zinc ferrite for high-performance supercapacitors, J. Electroanal. Chem. 817 (2018) 111–117. [62] Y. Zhao, X.-m. Hu, B.-h. Jiang, L. Li, Optimization of the operational parameters for desalination with response surface methodology during a capacitive deionization process, Desalination 336 (2014) 64–71. [63] T. Kim, J. Yoon, CDI ragone plot as a functional tool to evaluate desalination performance in capacitive deionization, RSC Adv. 5 (2015) 1456–1461.

4195–4203. [54] M. Li, W. Li, S. Liu, Hydrothermal synthesis, characterization, and KOH activation of carbon spheres from glucose, Carbohydr. Res. 346 (2011) 999–1004. [55] J. Chen, Z. Lang, Q. Xu, B. Hu, J. Fu, Z. Chen, J. Zhang, Facile preparation of monodisperse carbon spheres: template-free construction and their hydrogen storage properties, ACS Sustain. Chem. Eng. 1 (2013) 1063–1068. [56] M. Samir, N. Ahmed, M. Ramadan, N.K. Allam, Electrospun mesoporous Mn–V–O@C nanofibers for high-performance asymmetric supercapacitor devices with high stability, ACS Sustain. Chem. Eng. 7 (2019) 13471–13480. [57] G. Wang, C. Pan, L. Wang, Q. Dong, C. Yu, Z. Zhao, J. Qiu, Activated carbon nanofiber webs made by electrospinning for capacitive deionization, Electrochim. Acta 69 (2012) 65–70. [58] I.H. Abdullah, N. Ahmed, M.A. Mohamed, F.M.A. Ragab, M.T.A. Abdel-Wareth, N.K. Allam, An engineered nanocomposite for sensitive and selective detection of mercury in environmental water samples, Anal. Methods 10 (2018) 2526–2535.

11