g-C3N4 electrodes

g-C3N4 electrodes

Desalination 479 (2020) 114348 Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal Capacitive de...

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Desalination 479 (2020) 114348

Contents lists available at ScienceDirect

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

Capacitive deionization with MoS2/g-C3N4 electrodes a,b,c

Shichao Tian

, Xihui Zhang

a,b,c

, Zhenghua Zhang

b,c,⁎

T

a

Shenzhen Environmental Science and New Energy Technology Engineering Laboratory, Tsinghua-Berkeley Shenzhen Institute, Shenzhen 518055, PR China Institute of Environmental Engineering and Nano-Technology, Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, PR China Guangdong Provincial Engineering Research Center for Urban Water Recycling and Environmental Safety, Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, Guangdong, PR China

b c

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: Capacitive deionization Metal oxide/carbon electrode Molybdenum disulfide (MoS2) Graphitic carbon nitride (g-C3N4)

The issue of insufficient freshwater resources has become a worldwide problem, restricting social and economic development. As a promising technology, capacitive deionization (CDI) has been developed to address this problem. In this work, MoS2/g-C3N4 composite with a high supercapacitor performance was successfully synthesized and assembled as the CDI electrode material with its electrochemical properties deeply evaluated. The results demonstrated that the MoS2/g-C3N4-based electrode demonstrated a large specific capacitance of 118.3 F/g (at 1 A/g) and a remarkable rate capacitance retention of 52.16% (61.7 F/g) (at 10 A/g). Moreover, the MoS2/g-C3N4-based electrode exhibited obvious improvement in desalination performance with the maximum electrosorption capacity of 24.5 mg/g. The hierarchical architecture, enhanced conductivity, large surface area, and negative zeta potential of MoS2/g-C3N4 composite facilitated the fast diffusion of ions. The enhanced ions adsorption performance of the MoS2/g-C3N4-based electrode was evaluated based on a capacitive contribution of 85.26% and a diffusion-controlled contribution of 14.74%.

1. Introduction Recently, the lack of sufficient available water resources stimulates people's interest in developing efficient desalination technologies [1,2],

such as multi-stage flash, electrodialysis, and reverse osmosis. However, the applications of these techniques are hindered by some disadvantages, such as high energy consumption, increased cost and selfcontamination [3]. Therefore, it is urgent to develop a cost-effective

⁎ Corresponding author at: Institute of Environmental Engineering and Nano-Technology, Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, PR China. E-mail address: [email protected] (Z. Zhang).

https://doi.org/10.1016/j.desal.2020.114348 Received 1 October 2019; Received in revised form 21 January 2020; Accepted 21 January 2020 0011-9164/ © 2020 Elsevier B.V. All rights reserved.

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Zn2GeO4/g-C3N4 hybrids with a high reversible capacity and good rate capability [18]. Therefore, the synergistic effects between the carbon materials and metal oxides materials resulted in an excellent electrochemical performance of the hybrid materials. Molybdenum disulfide (MoS2) is attracting more and more attention for supercapacitors given its high theoretical specific capacitance [19]. The oxidation state of Mo atom in MoS2 can largely change from +2 to +6, important property for enhancing storage capability of pseudocapacitor materials [20]. In addition, the sandwich-type S-Mo-S layered structure of MoS2 would provide a high specific surface area, useful property for the EDL capacitance, whereas easy intercalation of electrolyte ions in MoS2 could occur given the weak van der Waals interactions between S-Mo-S layers [21]. However, the restacking of MoS2 sheets as well as the poor electrical conductivity hinder the application of MoS2 as a CDI electrode material [22]. Despite the efforts made so far to improve the CDI performance of MoS2, a clear understanding of the mechanism of ions removal is still lacking. Herein, a hybrid structure made of MoS2 and g-C3N4 was fabricated by a facile one-step hydrothermal process. The morphology, structure and composition of MoS2/g-C3N4 composite were investigated. Furthermore, the electrochemical properties of MoS2/g-C3N4-based electrode were evaluated. The results revealed that MoS2/g-C3N4-based electrode displayed a rather high electrosorption capacity of 24.16 mg/ g. Moreover, the mechanism of ion storage and improved properties of MoS2/g-C3N4-based electrode was proposed, as well.

desalination technology with the properties of no secondary pollution and low energy consumption. Capacitive deionization (CDI) as a promising technology is widely applied to achieve water desalination. In principle, when saline solution passes through one or more pairs of parallel electrodes, the electric field will facilitate the adsorption of the positive and negative ions to the surface of the negative and positive electrodes, respectively [4]. Meanwhile, reversing and/or short circuiting the electrodes can regenerate the electrodes when the ions adsorption reaches saturation [5]. Comparing with other desalination technologies, CDI has several merits, such as use of inexpensive membrane materials, lack of chemical reagents for regeneration of electrodes, low operating voltage and energy consumption, and absence of secondary pollution [6]. High capacitance and fast mass transfer of ions in and out of electrodes are the critical properties ensuring the CDI performance [7]. Until now, many carbon materials (e.g. activated carbons, mesoporous carbons, carbon nanotubes, carbon aerogels, and graphene) have been wildly used as CDI electrode materials [8]. However, the aforementioned materials exhibit limited NaCl adsorption capacity with a range of 10–15 mg/g due to their disordered pore arrangement, the existence of micropores and the overlapping effect of the electric double layer (EDL) [9,10]. A highly promising approach is the use of novel electrode materials, other than carbons, which are favorable for the Faradaic reactions. This method allows the replacement of ion electrosorption by surface redox reactions or ion intercalation [11]. Metal oxide/carbon composite electrodes for CDI process are proposed to overcome the disadvantages of currently used carbon electrodes. Such composite materials have the main advantage of using the functionality of both carbon materials and metal oxides, which results in improved surface area, electron transfer ability, and ion adsorption capacity [12]. Graphitic carbon nitride (g-C3N4) with sp2 hybridized carbon is a two dimensional (2D) porous nitrogen-substituted graphite framework [13]. Owing to its rich nitrogen content, unique electronic structure, chemical and thermal stability, and environmentally acceptable character, g-C3N4 has the potential to provide multifunctional properties in catalysis and energy conversion [14]. However, the electrochemical applications of g-C3N4 are limited mainly due to its inherent low electrical conductivity and low surface area [15]. Many researchers have synthesized hybrid electrode materials by combining metal oxides with g-C3N4 with the purpose to improve the electrochemical properties. For example, Guo et al. fabricated a NiCo2S4/gC3N4-based electrode, which demonstrated a large capacitance of 1557 F/g (at 1 A/g) [16]. Dong et al. synthesized a [email protected](OH)2based electrode, which effectively accommodated the electrolyte ions and promoted an efficient electron transport [17]. Li et al. prepared

2. Experimental section 2.1. Chemicals Urea (CH4N2O), ammonium molybdate ((NH4)6Mo7O24·4H2O), thiourea (CH4N2S), sodium hydroxide (NaOH), sulfuric acid (H2SO4), sodium sulfate (Na2SO4), anhydrous ethanol (CH3CH2OH), polytetrafluoroethylene (PTFE), and carbon black were supported by the Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). 2.2. Fabrication of MoS2/g-C3N4 composite The MoS2/g-C3N4 composite was synthesized through a hydrothermal method. Typically, (NH4)6Mo7O24·4H2O (0.93 g) and CH4N2S (1.71 g) were dissolved into 50 mL Milli-Q water to form a clear solution. Then, g-C3N4 (0.5 g) was dosed to the above solution and subjected to ultrasounds for 45 min. Afterwards, the solution was transfer to a Teflon-lined stainless steel autoclave followed by a hydrothermal treatment at 220 °C for 24 h. At the end of the treatment, the solid

Scheme 1. Preparation of MoS2/g-C3N4 composite. 2

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Fig. 1. SEM analysis of different materials. (a) g-C3N4, (b) MoS2, (c) MoS2/g-C3N4 composite. Elemental mapping of (d) C, (e) N, (f) Mo, (g) S for MoS2/g-C3N4 composite. (h) EDS spectrum of MoS2/g-C3N4 composite.

3

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product was recovered by centrifugation and then washed three times with Milli-Q water and anhydrous ethanol. Finally, the solid was dried at 60 °C for 24 h in a vacuum drying oven [23]. The fabrication process is demonstrated in Scheme 1. Herein, prior to the synthesis of the MoS2/g-C3N4 composite, the g-C3N4 was first obtained by a hydrothermal treatment of the urea precursor (Step a) [24]. Meanwhile, pure MoS2 was also prepared without the addition of g-C3N4 under the same conditions (Step b).

L), respectively; V, m, i, F and M are the volume of the NaCl solution (L), total mass of the two electrodes (0.395 g), current in the adsorption process (A), 96,485C/mol (Faraday constant), and 58.5 g/mol (molar mass of NaCl), respectively; VCell is the voltage applied and σ can be obtained by integrating the electrical current (Ie) passing through the cell over the experimental period.

2.3. The characterizations of MoS2/g-C3N4 composite

3.1. Structure, morphology and composition

The morphology, structure and composition of the synthesized products were investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) (Zeiss, LEO1530VP), transmission electron microscopy (TEM) (JEOL, JEM-2100F), X-ray diffraction (XRD) (Rigaku, D8 ADVANCE), X-ray photoelectron spectroscopy (XPS) (Kratos, AXIS ULTRA DLD, Al Kα radiation) using C 1 s (Binding Energy = 284.8 eV) as a reference, Raman spectra (Renishaw Instruments, England) and Fourier transform infrared spectroscopy (FTIR) (Tensor 27, Bruker, German). The specific surface area and pore size were calculated based on the N2 physisorption isotherms using the Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) model, respectively (The experiments were carried out on a Micomeritics Analyzer, ASAP-2010). Zeta potential was studied on a Zetasizer instrument (Malvern Instruments Ltd., UK).

3.1.1. SEM-EDS analysis The morphology and composition of obtained powders (g-C3N4, MoS2, and MoS2/g-C3N4 composite) were analyzed by SEM-EDS. As demonstrated in Fig. 1a, layered g-C3N4 has many thin nanosheets with a porous structure and size diameter of about 100–200 nm. The pure MoS2 particles display a flowerlike nanostructure consisting of thin nanosheets and size diameter of about 200–300 nm (Fig. 1b). The rippled nanosheets are assembled in a loose porous architecture, which allows avoiding the disordered stacking of MoS2 layers [28]. The MoS2/ g-C3N4 composite obtained by the hydrothermal treatment displays a more loose microstructure with the MoS2 nanoflowers coated onto the g-C3N4 nanosheets without obvious agglomeration (Fig. 1c). Besides, the porous structures also provide numerous channels for electrolyte transport [29]. The intimate contact between MoS2 and 2D g-C3N4 nanosheets and the controllable porous nanostructure are both beneficial for the CDI performance. The corresponding elemental mapping shows that the C, N, S, and Mo elements distribute homogeneously throughout the whole MoS2/g-C3N4 composite (Figs. 1d-g). Hence, the MoS2 was successfully coated onto the g-C3N4 surface (Fig. 1h).

3. Results and discussion

2.4. Electrochemical analysis Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) were tested on a CHI 690E electrochemical workstation, in which a three-electrode system including counter electrode (Pt wire), reference electrode (Ag/AgCl electrode) and working electrode (prepared electrode) was used. The specific capacitance (Cg, F/g) was assessed from the CV curves using Eq. (1) [25].

Cg =

I×t m × ∆V

3.1.2. XRD analysis The XRD analysis of pure g-C3N4, MoS2, and MoS2/g-C3N4 composite are illustrated in Fig. 2. For pure g-C3N4, the strong peak at 2θ of 27.4° and the weak peak at 13.1° are attributed to the (002) and (100) planes, respectively [30]. For pure MoS2, the diffraction peaks displayed at 2θ of 14.2°, 33.2°, 40.2°, 49.7°, and 57.3° belong to the (002), (100), (103), (105), and (110) planes, respectively [31]. Moreover, obvious diffraction peaks corresponding to g-C3N4 and MoS2 were observed for MoS2/g-C3N4 composite, indicating the formation of MoS2/gC3N4 composite.

(1)

where I, t, m and ΔV are the discharge current (A), discharge time (s), mass of the active material (g), and potential change during the discharge process (V), respectively. 2.5. Analysis of desalination performance

3.1.3. XPS analysis The XPS spectra of MoS2/g-C3N4 composite (Fig. 3a) displays

The synthesized materials were assembled as the CDI electrodes and their CDI performance (Fig. S1) was evaluated under different applied voltages (0.8, 1.0, 1.2, 1.4, 1.6, and 1.8 V) with a NaCl aqueous solution (0.15 L) of different concentrations (20, 50, 70, 100, 150, 200, and 250 mg/L). The flow velocity of NaCl solution is 80 mL/min and the initial conductivity was measured online and recorded every 60 s by a conductivity meter (Mettler Toledo 7 compact conductivity). More details were described in one of our previous publications [26]. The salt adsorption capacity (SAC, Γ, mg/g), charge consumed (ζ, C/g), charge efficiency (Λ), total charge (σ) and energy consumption (E, Wh/m3) were analyzed by Eqs. (2), (3), (4), (5) and (6), respectively [27].

Γ = (C0 − Ct ) × V / m

ζ=

(∫ i dt)/m

Λ = (Γ × F )/(M × ζ ) σ=

∫0

t

Ie (t) ∗ dt

E = (Vcell × σ )/3600 (t × V )

(2) (3) (4) (5) (6) Fig. 2. XRD analysis of g-C3N4, MoS2, and MoS2/g-C3N4.

where C0 and Ct are the influent and effluent NaCl concentrations (mg/ 4

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(a)

C 1s

Mo 3d

(b)

N 1s O 1s

In ten sity (a.u .)

Intensity (a.u.)

N-C=N

S 2p

800

700

600

500

400

300

200

100

C-C

300 298 296 294 292 290 288 286 284 282 280 278

Binding Energy (eV)

Binding Energy (eV) (c)

(d)

Mo 3d3/2

Intensity (a.u.)

Intensity (a.u.)

N-C=N

N-(C)3

412

408

404

400

396

392

388

Mo 3d5/2

S 2s

242 240 238 236 234 232 230 228 226 224

Binding Energy (eV)

Binding Energy (eV)

(e)

Intensity (a.u.)

S 2p1/2

S 2p3/2

176 174 172 170 168 166 164 162 160 158 156

Binding Energy (eV) Fig. 3. (a) XPS spectra of MoS2/g-C3N4 composite and (b) high-resolution C 1s, (c) N 1s, (d) Mo 3d, and (e) S 2p spectra.

facilitated the charge transfer ability and wettability, which are beneficial for improving the CDI performance [35]. The Mo 3d region is illustrated in Fig. 3d. The 232.8 and 229.6 eV peaks belong to Mo 3d3/2 and Mo 3d5/2 of MoS2 (Mo4+), respectively [36]. Moreover, a new 235.2 eV peak displayed corresponds to Mo6+ 3d5/2, suggesting the oxidation of MoS2 in the hybrid [37]. The S 2p spectrum of MoS2 (Fig. 3e) displays peaks at 162.1 and 163.5 eV, which belong to S 2p3/2 and S 2p1/2, respectively [38].

binding energies at 288.1, 398.9, 229.2, and 162.1 eV, which are characteristic to C, N, Mo, and S elements, respectively [32]. The highresolution C 1 s spectrum (Fig. 3b) displays two main peaks at 278.8 and 284.6 eV, corresponding to the sp2-hybridized carbon (N-C=N) and the standard carbon (C-C bonds), respectively [33]. The N 1s signal (Fig. 3c) was deconvoluted in two peaks at 399.1 and 401.2 eV, assigning to the C-N=C and N-(C3) groups, respectively [34]. It was reported that the heteroatoms (especially N) in the carbon materials 5

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Fig. 4. TEM analysis of MoS2/g-C3N4 composite.

100

0.24

(a)

(b)

g-C3N4

80

0.20

V a /c m 3 (S T P ) g -1

MoS2 MoS2/g-C3N4

MoS2 MoS2/g-C3N4

0.16

d V p /d r p

60

g-C3N4

0.12

40

0.08

20 0.04 0 0.00 0.0

0.2

0.4

0.6

0.8

1.0

0

Relative pressure (P/P0)

2

4

6

8

10

12

14 16

18

20

Pore Diameter (nm)

Fig. 5. (a) Isotherms of nitrogen adsorption and desorption and (b) BJH adsorption pore size distribution of g-C3N4, MoS2, and MoS2/g-C3N4.

FTIR spectra of three prepared samples, i.e., pure g-C3N4, MoS2, and MoS2/g-C3N4 are depicted in Fig. S2a. For the pure MoS2, the peak at 593 cm−1 was assigned to the vibrational modes of Mo-S. The broad band displayed at 3100–3300 cm−1, indicating the residual N-H groups and O-H bands and the generated hydrogen bonds network, was related to the adsorbed H2O molecules and uncondensed amino groups. The broad vibration band (1243–1636 cm−1) was associated with the polycondensation structure of g-C3N4, the vibration characteristics of the heptazine heterocyclic ring units and s-triazine ring units. Notably, the intensities of all these bands are reduced in the spectrum of MoS2/gC3N4, which proves the interactions between these functional groups to generate the hybrid material [41].

3.1.4. TEM, FTIR and Raman spectroscopies Further information on the microstructure of MoS2/g-C3N4 composite can be observed from TEM images. As shown in Fig. 4, the both phases, i.e., g-C3N4 and MoS2, are in intimate contact and MoS2 nanosheets are firmly anchored on the porous g-C3N4. The high-resolution TEM image (Fig. 4d) presents well defined lattice fringes with an interplanar spacing of 0.62 nm indexed as (002) plane of MoS2. A multilayer film made of about 10–20 layers of MoS2 has been grown on the carbon surface and no obvious crystal fringes were observed for g-C3N4 possibly as a result of its amorphous structure [39]. The unique structure of MoS2/g-C3N4 composite would facilitate the electron transfer and ion diffusion during the CDI process [40]. 6

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capacitive ability for the composite electrode material. GCD curves of the MoS2/g-C3N4-based electrode at different current densities are displayed in Fig. 6d. The triangular shape of GCD cycles with high symmetry and almost a linear slope indicates the EDL capacitive behavior, which is in line with the CV curves [49]. The results of specific capacitance of the MoS2/g-C3N4-based electrode are illustrated in Fig. 6e. The MoS2/g-C3N4 composite displays higher values of the specific capacitance compared with g-C3N4 and MoS2 irrespective of the current density. This achievement is ascribed to the enhanced electrochemical properties. MoS2/g-C3N4 exhibits a large specific capacitance of 118.3 F/g (1 A/g) and a remarkable rate capacitance retention (52.16%) (61.7 F/g, 10 A/g) [50]. The Nyquist curves of the g-C3N4, MoS2, and MoS2/g-C3N4-based electrodes are similar in shape (Fig. 6f). The radius of the small semicircle displayed at high frequency region shows the charge transfer resistance (Rct) at the interface of electrode/electrolyte. The x-intercept of the curves indicates the bulk resistance (Rs), including the contact resistance of electrode material/current collector. A steep line at low frequency region follows a 45° Warburg line in the mid-frequency region, showing the EDL capacitive behavior (Cdl) [51]. For the MoS2/gC3N4 composite-based electrode, the semicircle is smaller than for the gC3N4 and MoS2-based electrodes at high frequency, pointing out that the MoS2/g-C3N4 composite-based electrode has a good electrical conductivity with a smaller charge transfer resistance. In addition, the slope of the curve corresponding to the MoS2/g-C3N4 composite-based electrode is larger than that of g-C3N4 and MoS2, suggesting faster ion diffusion. In light of the above discussed results, it can be stated that the MoS2/g-C3N4 can facilitate the charge transfer with the reduced contact resistance due to the ordered nanobelt architecture (Fig. 4).

The Raman spectroscopy was performed to study the vibrational properties of g-C3N4, MoS2, and MoS2/g-C3N4. As showed in Fig. S2b, two new Raman bands at 378 and 407 cm−1 for MoS2/g-C3N4 composite were assigned to the typical vibration modes of E2g1 and A1g of 2H-MoS2, respectively [42]. In light of the results of physico-chemical characterization discussed above, it can be stated that 2H-MoS2 was successfully decorated onto the g-C3N4 surface. 3.1.5. Surface area and pore size distribution The textural properties of MoS2/g-C3N4 composite were evaluated. As demonstrated in Fig. 5, the N2 adsorption isotherms of all powders are of type IV based on IUPAC classification, indicating the existence of both micro- and mesopores (Fig. 5a) in line with the pore size distributions (Fig. 5b). The BET specific surface area and the total pore volume of MoS2/g-C3N4 composite are 174.26 m2/g and 0.187 cm3/g, respectively, which are larger than those of g-C3N4 (32.81 m2/g and 0.069 cm3/g) and MoS2 (67.61 m2/g and 0.066 cm3/g). MoS2 attached to the surface of g-C3N4 could hinder the agglomeration of g-C3N4 nanosheets with the formation of a structure with thinner nanosheets and smaller size. Clearly, MoS2/g-C3N4 composite exhibits a highly porous structure, which is beneficial for its capacitive behavior [43]. In addition, MoS2/g-C3N4 composite with a large surface area and pore volume would facilitate its CDI performance because of higher amount of the adsorption sites for ion anchoring [44]. 3.1.6. Zeta potential analysis Zeta potential is an important factor to measure the effective surface charge associated with the electric double layer [45]. As shown in Table 1, all three samples are negatively charged under the experimental conditions (250 mg/L NaCl solution, pH = 6.62) and the MoS2/ g-C3N4 composite has the highest value of −40.7 mV. The highest negative charge of the MoS2/g-C3N4 composite will attract more positively charged sodium ions in the EDL [46]. Meanwhile, the increased surface area and pore volume of MoS2/g-C3N4 composite (Fig. 5) would also facilitate ion anchoring on the surface of MoS2/g-C3N4 composite by offering more adsorption sites compared to g-C3N4 and MoS2 alone [44]. As such, the increased zeta potential of the MoS2/g-C3N4 composite would enhance the EDL capacitance and CDI performance of the MoS2/g-C3N4 electrodes [46,47].

3.3. CDI performance Fig. 7a shows the CDI performance of different electrodes in the presence of ~500 μS/cm NaCl at 1.6 V. For the MoS2/g-C3N4 compositebased electrode, there was a dramatic decrease until reaching adsorption saturation with the conductivity reduced from 500 to 372.67 μS/ cm. By contrast, the conductivity for g-C3N4 and MoS2-based electrodes in NaCl solution reduced from 500 to 467.15 and 421.46 μS/cm, respectively. The SAC values of g-C3N4, MoS2, and MoS2/g-C3N4 composite-based electrodes were 6.25, 14.91, and 24.18 mg/g, respectively (Fig. S3). In order to further evaluate the CDI performance of different electrodes, the effect of applied voltage on the CDI performance was investigated. As observed in Fig. 7b, with the increase of the applied voltage (≤1.6 V), SAC of all electrodes increased, which was attributed to the strong electrostatic forces associated with the high voltage applied. Moreover, the SAC (Fig. 7b) and charge efficiency (Fig. 7c) of MoS2/g-C3N4-based electrode were larger than those of g-C3N4 and MoS2-based electrodes at any applied voltage. However, the charge efficiency reduced with the applied voltage, which is ascribed to co-ions expulsion and side reactions [52]. As such, an optimal electrical voltage of 1.6 V was used for further desalination experiments given the desalination performance and energy efficiency. At the optimum conditions for desalination process (VCell is 1.6 V), the calculated energy consumption of MoS2/g-C3N4 electrodes is 50.498 Wh/m3. In addition, no water splitting was observed during the experiment even though the optimal electrical voltage of 1.6 V is higher than the standard electrode potential of 1.23 V for water splitting, which is due to the compensation of the circuit system resistance [53]. In addition, the SAC of MoS2/g-C3N4 composite electrode increased with the initial NaCl concentration (Fig. 7d). The results are explained by easy formation of the EDL and avoiding EDL overlapping at high NaCl concentration [54]. However, the electrosorption capacity slightly reduced when further increased the NaCl concentration (250 mg/L) as a result of co-ions expulsion with hindering the migration of ions into the electrode pores [55]. The adsorption mechanism was evaluated based

3.2. Electrochemical analysis The CV curves of the electrodes made of g-C3N4, MoS2 and MoS2/gC3N4 composite are demonstrated in Fig. 6a. The CVs exhibit quasi rectangular mirror-image responses without obvious redox peaks. These results indicate a good EDL capacitive behavior of electrodes, and they are in line with previously reported data [48]. Besides, the integral area inside the CV curve of MoS2/g-C3N4-based electrode was substantially larger than those of the other two electrodes (g-C3N4 and MoS2). By increasing the scan rate, the rectangular CV shape of MoS2/gC3N4 electrode did not change even at 200 mV/s (Fig. 6b), indicating that the MoS2/g-C3N4 electrode has a robust rate capability and supercapacitor behavior. The GCD curves of the g-C3N4, MoS2 and MoS2/g-C3N4 compositebased electrodes were recorded at 8 A/g as depicted in Fig. 6c. The GCD curves of pure g-C3N4, MoS2, and MoS2/g-C3N4-based electrodes were recorded for longer discharge time. They indicate high conductivity and Table 1 Zeta Potential of g-C3N4, MoS2, and MoS2/g-C3N4 materials (250 mg/L NaCl solution, pH = 6.62). NaCl solution (250 mg/L) Material Zeta potential (mV)

g-C3N4 −26.9

MoS2 −37.0

MoS2/g-C3N4 −40.7

7

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0.004

0.008

(a)

(b)

0.006 0.004

C urrent (A )

C u rrent (A )

0.002

0.000

0.002 0.000 -0.002 -0.004

5 mV/s 10 mV/s 20 mV/s 50 mV/s 100 mV/s 200 mV/s

-0.002

g-C3N4 MoS2 -0.004

-0.006 -0.008

MoS2/g-C3N4 0.0

0.2

0.4

0.6

0.8

-0.010 0.0

1.0

0.2

Potencial (V vs. Ag/AgCl)

0.8

0.6

0.8

1.0

(d)

1.0

g-C3N4 MoS2 MoS2/g-C3N4

0.6 0.4 0.2

Potential (V vs. Ag/AgCl)

P o ten tia l (V v s. A g /A g C l)

1.0 (c)

0.4

Potencial (V vs. Ag/AgCl) 1 A/g 2 A/g 5 A/g 8 A/g 10 A/g

0.8

0.6

0.4

0.2

0.0

0.0 0

50

100

150

200

250

-100

300

0

100

200

300

400

500

600

700

800

900

Time (s)

Time (s) 140

80 g-C3N4 MoS2

70

100

MoS2/g-C3N4

60

-Z '' (oh m )

S p ecific ca p a cita n ce (F /g )

(e) 120

80 60

50 40 30

40

20

20

10

0

g-C3N4 MoS2 MoS2/g-C3N4

(f)

0

1

2

5

8

10

4

Current density (A/g)

8

12

16

20

24

28

32

Z' (ohm)

Fig. 6. (a) CV curves of g-C3N4, MoS2, and MoS2/g-C3N4-based electrodes at 100 mV/s. (b) CV curves of MoS2/g-C3N4-based electrodes at 5, 10, 20, 50, 100, and 200 mV/s. (c) GCD curves of g-C3N4, MoS2. and MoS2/g-C3N4-based electrodes at current density of 8 A/g. (d) GCD curves of MoS2/g-C3N4-based electrode at 1, 2, 5, 8, and 10 A/g. (e) Specific capacitances of g-C3N4, MoS2, and MoS2/g-C3N4-based electrodes at 1, 2, 5, 8, and 10 A/g. (f) EIS curves of g-C3N4, MoS2, and MoS2/gC3N4-based electrodes.

has a well-fitting of experimental data and a SACmax of 24.5 mg/g. To better understand the advantages of g-C3N4/MoS2 composite as electrode material, the results obtained herein for this material were compared with those reported in recent years for various CDI electrode materials. The results summarized in Table 2 highlight the superior salt adsorption capacity of g-C3N4/MoS2 composite compared with the most CDI electrode materials reported so far under similar experimental

on the Langmuir adsorption model and Eq. (7)

K [NaCl] SAC = SACmax 1 + K [NaCl]

(7)

where SACmax (mg/g) is the maximum electrosorption capacity and K (L/mg) is the equilibrium constant of Langmuir fitting. As illustrated in Fig. 7d, the electrosorption data of MoS2/g-C3N4-based electrode were fitted with the Langmuir model. The MoS2/g-C3N4 composite electrode 8

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C o n d u c tiv ity ( S /c m )

(a) 500

g-C3N4 MoS2

480

MoS2/g-C3N4

E lectro so rp tio n C a p a city (m g /g )

520

460 440 420 400 380

25 (b)

g-C3N4 MoS2

20

MoS2/g-C3N4

15

10

5

0

360

0

10

20

30

40

50

0.8 V

60

1.0 V

Time (min)

1.2 V

1.4 V

1.6 V

1.8 V

Applied Voltage (V)

0.9

(c)

C h a rg e E fficien cy

0.8

E le c tr o so r p tio n C a p a c ity (m g /g )

g-C3N4 MoS2 MoS2/g-C3N4

0.7

0.6

0.5

0.4

(d)

25 20

SACmax = 24.5 mg/g

15 10 5

MoS2/g-C3N4 experimental data MoS2/g-C3N4 fitting plot

0 0.8

1.0

1.2

1.4

1.6

0

1.8

50

100

150

200

250

Concentration (mg/L)

Applied Voltage (V)

Fig. 7. (a) CDI performance of g-C3N4, MoS2, and MoS2/g-C3N4-based electrodes (at 1.6 V). (b) Electrosorption capacity of g-C3N4, MoS2, and MoS2/g-C3N4-based electrodes at different applied voltages (initial conductivity of 500 μs/cm). (c) Charge efficiency of g-C3N4, MoS2, and MoS2/g-C3N4-based electrodes at different applied voltages from 0.4 to 2.0 V (initial conductivity of 500 μs/cm). (d) Electrosorption isotherm of MoS2/g-C3N4-based electrode with Langmuir fitting (at 1.6 V).

curves were described with Eq. (8).

Table 2 Electrosorption capacity of CDI electrode materials.

(8)

i = av b

Electrode material

Initial NaCl concentration (mg/L)

Applied voltage (V)

SAC (mg/g)

ACs [56] Graphene [57] g-C3N4 [58] MoS2 [46] [email protected] [59] NMCs-800 [60] [email protected] [61] NGCPs [62] Ti3C2-MXene [63] GNS-2 [27] MoS2/g-C3N4 (This work)

500 295 500 2900 500 584 40 500 300 500 250

1.2 1.5 1.4 1.2 1.5 1.2 1.6 1.4 1.2 1.2 1.6

9.40 9.60 4.90 4.41 24.30 20.63 5.09 17.73 15.00 18.70 24.16

where i and v are the current and sweep speed, respectively, a and b are the constant. The current is diffusion-controlled (b = 0.5) or capacitorcontrolled (b = 1.0) [63]. As such, the CDI mechanism can be analyzed by quantitatively defining the respective contribution of diffusioncontrolled process and capacitor-controlled process by Eq. (9).

i (V ) = k1 v 1 / 2 + k2 v

(9)

where i(V), k1v , k2v and v are the current at a fixed voltage, the diffusion-controlled current, the capacitance-controlled current, and the scan rate, respectively. Besides, k1 and k2 vary at different applied voltages [64]. The value of b (0.836) for the MoS2/g-C3N4-based electrode indicates that both diffusion-controlled process and capacitorcontrolled process are the contributors for electrosorption (Fig. 8a) with the contributions of 85.26% (capacitance surface-controlled process) and 14.74% (diffusion-controlled process) (Fig. 8b). Therefore, it is demonstrated that MoS2/g-C3N4-based electrode was helpful for ion intercalation, which resulted in an increased CDI performance. 1/2

conditions. The SAC of this work (24.16 mg/g) was 1.86 times comparing with the average value (12.98 mg/g) of the selected ten excellent results for other electrodes. 3.4. Mechanism of enhanced CDI performance for MoS2/g-C3N4 composite electrodes

3.5. The stability of MoS2/g-C3N4-based electrode

The relationship between peak currents (i) and sweep rates (v) of CV

For CDI applications, the recycle stability of electrodes is a very 9

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1.4

(a)

0.008

1.2

0.004 1.0 0.8

Slope = 0.836 0.6

C urrent (A )

L og (peak current, A )

(b)

0.006

0.4

0.002 0.000 -0.002 -0.004 -0.006

0.2

-0.008

0.0

Capacitive contribution, 85.26%

-0.010 0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

0.0

0.2

0.4

0.6

0.8

1.0

Potencial (V vs. Ag/AgCl)

Log (Scan rate, mV/s)

Fig. 8. (a) The values of b in the equation of i = avb and (b) quantitative analysis of the respective contribution of diffusion-controlled process and capacitorcontrolled process of the CV curve for the MoS2/g-C3N4-based electrode (scan rate of 200 mV/s).

conductivity compared to the MoS2 and g-C3N4-based electrodes. The MoS2/g-C3N4-based electrode exhibited an enhanced electrosorption capacity with a high SAC value of 24.16 mg/g at 1.6 V. This performance was superior to that of g-C3N4 (8.51 mg/g) and MoS2 (13.31 mg/ g). The cycle adsorption-desorption tests demonstrated that the MoS2/ g-C3N4-based electrode is highly stable for at least 10 cycles. The improved desalination performance of the MoS2/g-C3N4-based electrode originates from two different contributions, i.e., capacitive (85.26%) and diffusion-controlled (14.74%) contributions. According to the results obtained in this work, the MoS2/g-C3N4-based electrode showed its promising future for CDI application.

520

C o n d u c tiv ity ( S /c m )

500

1st

10 th

480 460 440 420 400

Nomenclature

380

BET BJH CDI EDL EDS EIS GCD SAC SACmax TEM XPS

360 0

200

400

600

800

1000

1200

Time (min) Fig. 9. Cycling stability of MoS2/g-C3N4-based electrode (at 1.6 V).

important parameter. Therefore, the stability tests of MoS2/g-C3N4 composite electrode were also conducted. As shown in Fig. 9, in the CDI process, the ions in the feed water were stored within the EDL so the conductivity of solution was decreased. The regeneration test for the MoS2/g-C3N4-based electrode was carried out by reversing the voltage when the saturation of the ions adsorption was reached. So, the conductivity of solution increased again and finally recovered to the initial conductivity as a result of the desorption of the adsorbed ions. There was no obvious decrease of SAC for MoS2/g-C3N4–based electrode after 10 cycles (Fig. S4), demonstrating the good stability of the prepared electrode in consecutive electrosorption–desorption cycling.

Brunauer–Emmett–Teller Barrett–Joyner–Halenda Capacitive deionization Electric double layer Energy-dispersive X-ray spectroscopy Electrochemical impedance spectroscopy Galvanostatic charge/discharge Salt adsorption capacity Maximum electrosorption capacity Transmission electron microscopy X-ray photoelectron spectroscopy

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 4. Conclusions This research was supported by the National Natural Science Foundation of China (51708325) and the Development and Reform Commission of Shenzhen Municipality (urban water recycling and environment safety program).

In summary, the MoS2/g-C3N4 composite was successfully fabricated via a facile hydrothermal method. The morphology, structure, texture, as well as surface composition and charge of synthesized MoS2/ g-C3N4 composite were investigated by SEM-EDS, TEM, XRD, N2 physisorption, FTIR, Raman, and XPS spectroscopies, and zeta potential. The results of N2 physisorption revealed a MoS2/g-C3N4 composite with a large specific area and suitable porous structure for the CDI desalination. Additionally, the electrochemical analysis of MoS2/g-C3N4based electrode indicated the improvement of specific capacitance and

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.desal.2020.114348. 10

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