Capacitive deionization (CDI) is considered a promising technology for desalination of sea or brackish water. In this study, a ZnS/g-C3N4 composite was synthesized through a one-step high-temperature method and used as the main material to fabricate CDI electrodes. The results of SEM and TEM showed that spherical-like nanoparticles of ZnS were uniformly distributed on the g-C3N4 sheet. The g-C3N4 phase facilitates the ZnS particles precipitate and restrain their agglomeration, which contributes to a high specific surface area of ZnS. Furthermore, the electrochemical test results indicated that ZnS/g-C3N4 composite had a good capacitance characteristic, low resistance, and high electrochemical stability. Finally, the desalinization performance of the ZnS/g-C3N4 composite electrodes was tested in traditional mode and membrane capacitive deionization (MCDI) mode. The results showed that ZnS/g-C3N4//ZnS/g-C3N4 (MCDI) exhibited an optimal desalination capacity. The adsorption amount was 27.65, 50.26, and 65.34 mg/g for NaCl initial concentration of 200, 400, and 600 mg/L, respectively, with the voltage of 1.2 V and flow rate of 5 mL/min. Increasing initial concentration enhanced the conductivity and ion migration rate so as to increase the NaCl adsorption amount. ZnS/g-C3N4 composite can be used as potential electrode material for high performance of MCDI.

  • Capacitive deionization (CDI) has emerged as a promising approach to desalination.

  • ZnS/g-C3N4 composite can be used for CDI.

  • ZnS/g-C3N4 has a large specific surface area.

  • ZnS/g-C3N4 showed a high desalination capacity of 65.34 mg/g in membrane capacitance deionization (MCDI).

With the development of industry and the growth of population, the global demand for freshwater is rapidly increasing, and the original scarce freshwater resources are more tense. Since seawater and brackish water account for more than 96% of total water resources on the earth (Shiklomanov 2000), obtaining fresh water from seawater or micro-salt water has attracted considerable attention, which is supposed to be a feasible strategy to relieve the crisis of fresh water shortage. Traditional desalination technologies, such as reverse osmosis, multi-stage flash distillation, and electrodialysis processes have showed the disadvantages of excessive energy consumption, high cost, and low efficiency, which hinders their more widespread application (Mohamed et al. 2020). New desalination techniques that are energy efficient and cost-effective are explored all the time.

Capacitive deionization (CDI) technology has been a research hotspot in the area of seawater or brackish water desalination in recent years (Chung et al. 2020; Torkamanzadeh et al. 2020; Gong et al. 2021), which shows the advantages of low energy consumption, ecological friendliness, high separation, simple design, and easy operation (Yan et al. 2018). The technique is simple and easy to operate. Desalting can be carried out at low pressure and room temperature, and the applied voltage at both ends of the electrode generally does not exceed 1.6 V. Unlike reverse osmosis or distillation-based desalination systems, CDI does not need to be coupled to a high-pressure pump or heat source, allowing easy system scaling (Suss et al. 2015). Additionally, CDI is environmentally favorable because energy, rather than chemicals, is used to power the operation process. Therefore, it does not cause secondary pollution to water bodies, and is a clean and efficient desalination technology. This is mostly a physical process that allows CDI equipment to have a long service life and low maintenance costs (Porada et al. 2013). During the desalination process, saline ions are attracted and stored in the electrical double layer of charged electrodes, and released from the surface of discharged electrodes. Electrode material is crucial for the performance of CDI as its structure, porosity, surface and electrochemical properties greatly determine the ions adsorption efficiency (Peng et al. 2011; Dlugolecki & van der Wal 2013). Carbon materials, such as activated carbon, mesoporous carbon, carbon nanotubes, carbon aerogels, and graphene are mainly used for CDI electrode preparation because of the large surface area and wide pore size distribution (Huang et al. 2017; Leong et al. 2019). However, drawbacks of low conductivity, irregular pore shape and size distribution of these carbon-based electrodes limit the CDI desalination capacity (Sufiani et al. 2019). Therefore, exploring new electrode materials for high performance of the CDI system are necessary.

The graphite phase carbon nitride (g-C3N4) is a novel non-metallic semiconductor material with the merit of environment friendly large specific surface area, good electrical conductivity, etc. (Zheng et al. 2012). g-C3N4 has caused broad interest in the scientific community as photocatalyst. Also, it is begun to be used as electrode active materials in recent years due to its good conductivity and hydrophilicity. For example, Wang et al. (2018) used g-C3N4 as electrode materials for CDI, showing an excellent salt adsorption capacity and cyclic stability. Kavil et al. (2018) prepared nano-g-C3N4/MnO2 and g-C3N4/SnO2 composites for supercapacitor applications, and the g-C3N4 improved the specific surface area and promoted the electron transport at the electrode-electrolyte interface. Thiagarajan et al. (2020) prepared NiMoO4/g-C3N4 for supercapacitors, which showed a large specific capacitance (501 F/g) and high stability, maintaining a capacity of up to 91.8% even after 2,000 charge and discharge cycles. In a word, g-C3N4 as electrode materials could improve the conductivity, hydrophilicity, and ion transmission of the electrodes.

Metal sulfides (MSs) are a kind of potential electrode materials in the metal matrix. MSs tend to cause sulfur defects in the formation process, thus forming a large number of active sites, which facilitate the catalytic reaction (Shiraz et al. 2021). Zinc sulfide (ZnS) has attracted extensive attention due to its advantages such as high capacity and density, rich sulfur and zinc resources, low cost and less pollution, and has become a cathode material for high performance of energy storage (Guo et al. 2018; Xie et al. 2018). In addition, ZnS is an important semiconductor material with wide energy gap (3.5–3.8 eV) and high specific capacitance (926.3 mAh/g) (Du et al. 2017; Xu et al. 2018). However, disadvantages of ZnS, such as poor conductivity and volume expansion have been found as well during its application. Recombination with other materials is supposed to be a way to make up for the deficiency. For example, Sarma et al. (2015) synthesized a flower-shaped ZnS/TiO2 nanotubes composite electrode, showing high specific capacitance and stability under large charging conditions.

In this study, g-C3N4 was used as the carrier to support ZnS nanoparticles, and ZnS/g-C3N4 composite was prepared by a high-temperature calcination method. The morphology and structure of the composite was analyzed, and the electrode was prepared. The electrochemical characteristics of the composite and the desalination performance were studied. The results show that ZnS can form a spherical particle structure on g-C3N4 lamellar layer, which weakens the stacking effect between the carbon nitrification lamellar layer and improves the capacitance of the material, thus improving the capacitive deionization performance of the electrode. The power supply voltage, inflow velocity and initial concentration of NaCl solution all have an effect on the desalting capacity of the capacitor. Through different CDI and MCDI mode tests, ZnS/g-C3N4//ZnS/g-C3N4 (MCDI) maximum adsorption capacity is 27.65 mg/g.

Preparation of g-C3N4

Thiourea was chosen as the experimental precursor. To ensure a well-mixed product, 5 g of thiourea was weighed in a mortar and ground for 30 min. After that, it was placed in a quartz boat and then into a tubular furnace (SK-G08123K, Tianjin Zhonghuan Electric Furnace Co., Ltd, China). It was heated to 550 °C at 10 °C/min in a nitrogen atmosphere for 2 h. The light yellow g-C3N4 powder was successfully prepared after being cooled to room temperature.

Preparation of ZnS/g-C3N4

The precursor of choice was thiourea. To create a homogenous mixture, 1 g of zinc acetate and 5 g of thiourea were weighed and mortared for 30 min. The quartz boat of the tubular furnace should then be used. It was heated to 550 °C under nitrogen environment at a rate of 10 °C/min and maintained for 2 h. To obtain orange ZnS/g-C3N4 powder, the mixture was cooled to room temperature.

Preparation of ZnS/g-C3N4 electrode

First, ZnS/g-C3N4 complex, acetylene black, and polyvinylidene fluoride were ground evenly in a mortar with a mass ratio of 8:1:1. Acetylene black was used as conductive agent and polyvinylidene fluoride was used as adhesive. Then drop N-methyl pyrrolidone as a solvent, stirring for more than half an hour, to obtain an uniform slurry. Using graphite paper as fluid collector, the electrode paste was coated on graphite paper by a simple coating method, and the electrode was obtained by drying it in a drying oven at 60 °C for 12 h.

Material characterization

The microstructure and morphology of the samples were investigated by scanning electron microscopy (SEM, Zeiss MERLIN Compact) and transmission electron microscopy (TEM, FEITecnai G2 F30). X-ray diffractometer (XRD, Rigaku Ultima IV) was used to analyze the crystal structure of the material. Fourier transform infrared spectrometer (FTIR) was used to analyze the structure of the materials, mainly used to determine the surface functional groups of the samples. Nitrogen adsorption and desorption isotherms were measured at 77 K. Before the measurement, the samples were degassed at 463 K for 6 h in vacuum using the McAsap2460 instrument. The Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface area and pore volume. A Barrett–Joyner–Halenda (BJH) model was used to derive the pore size distribution from the adsorption branch of the isotherm. The water contact angle was measured using JY-82C Kruss DSA on the powder sample after compression. Titration of ultrapure water was done automatically using equipment containing 1.6 μL of water droplets.

Electrochemical measurement

In this experiment, electrochemical workstation (CHI660E, Chenhua, China) was used to test the electrochemical performance of samples with a three-electrode system. In 1-M NaCl aqueous solution, a three-electrode system was used at room temperature. g-C3N4 and ZnS/g-C3N4 composites were used as working electrodes. Before the electrochemical performance test, the working electrodes were immersed in NaCl for 12 h to fully soak the electrolyte solution. Platinum (Pt) electrode and saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively.

The potentials measured by cyclic voltammetry (CV) ranged from −0.2 to 0.8 V at different scanning rates (5, 10, 20, 50, and 100 mv/s). The specific capacitance is obtained from Equation (1). Electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 10−2–105 Hz at ac voltage near equilibrium potential (0 V). The test range of constant current charge-discharge (GCD) is −0.2–0.8 V, and the curves of constant current charge–discharge under different current densities (0.2, 0.5, 0.8, 1, 2 A/g) are tested simultaneously.
formula
(1)
where C is the specific capacitance (F/g), V is the voltage (V), I is the response current (A), V is the scanning rate (V/s), and m refers to the mass of the active substance at the electrode (g).
The specific discharge capacity of the electrode during constant current charge-discharge measurement is calculated according to Equation (2):
formula
(2)
where Cg is the capacitance (F/g), Im is the current density (A/g), △t is discharge time (s), △V is the discharge potential change (V) without iR drop.

Electrosorption measurement

This CDI experiment was conducted using a self-assembled MCDI module, the operation of which is depicted in Figure 1(a) and the structure schematic diagram in Figure 1(b). The desalination experiments were carried out in 50 mL NaCl solution in an intermittent manner. A conductivity meter, a water storage device, a peristaltic pump, a CDI cell, and a DC stabilized power supply are all part of the system. The self-assembled MCDI module was made up of a Plexiglas spacer, a rubber septum spacer, a pair of parallel electrodes, and a non-woven septum. The electrodes in this experiment were 5cm × 5 cm in size, and a pair of anion and cation exchange membranes were placed in front of the electrode. HoAM G-1204 anion exchange membrane was placed in front of the positive electrode and HoCM G-0014 cation exchange membrane was placed in front of the negative electrode.
Figure 1

The schematic diagram of capacitor deionizing process (a) and MCDI cell structure (b).

Figure 1

The schematic diagram of capacitor deionizing process (a) and MCDI cell structure (b).

Close modal

The CDI cell operates in flow mode, which means that two electrodes are parallel to each other with a gap between them, allowing brine to flow between them. The NaCl solution was directly driven up and down by a peristaltic pump (BT100-2J, Chinese long pump). The solution entered the device through the lower port and exited through the upper port. The electric field was powered by a constant voltage DC power supply (VICTOR 3003A, China). A conductivity meter was used to continuously record the change in conductivity of the NaCl solution (DDSJ-308F, INESA Scientific Instrument Co. Ltd, China). The concentration of NaCl solution was calculated using the measured conductivity and a standard conductivity-concentration curve.

In this experiment, NaCl solutions of various concentrations were prepared using ultrapure water with conductivity values below 3 μS/cm. At a steady temperature, the conductivity and salt solution concentration have a positive relationship. Supplementary material, Figure S1 depicts the connection between conductivity and the concentration of NaCl solution. By using linear fitting, it was possible to derive the function relationship between solution concentration and conductivity value as y = 1.9375x + 19.226 with correlation coefficient R2 = 0.9997, which is a satisfactory fit. It was discovered that, in the solution concentration range of 0–2,000 mg/L, the concentration of NaCl solution showed a good linear association with the conductivity. The results of the solution's measured conductivity can be used to determine how much desalination is required given changes in the solution's concentration.

The initial concentration of NaCl was 200 mg/L and the flow rate was 5 mL/min. The desalination performance of g-C3N4//g-C3N4(CDI), g-C3N4//g-C3N4(MCDI), ZnS/g-C3N4//ZnS/g-C3N4(CDI), ZnS/g-C3N4//ZnS/g-C3N4 (MCDI) was tested at a voltage of 1.2 V. The desorption was performed by short-circuit (voltage 0 V), and the desorption time was 3,500 s. MCDI tests the desalting performance at different flow rates, voltages and initial concentrations are discussed below. Calculate the desalting amount according Equation (3).
formula
(3)
where Q is the amount of desalinization, represents the mass of solution ions adsorbed by active substance per gram (mg/g), C0 and Ce are the initial concentration and adsorption saturation concentration of solution (mg/L) respectively, Vt is the volume of solution (ml), and m is the mass of active substance at the electrode (g).

Structure and composition of ZnS/g-C3N4

The morphology of g-C3N4 and ZnS/g-C3N4 composites was characterized by SEM and TEM. Supplementary material, Figure S2(a) and (b) shows that the pure g-C3N4 has a porous and layered stack structure with smooth surfaces. From the SEM images of ZnS/g-C3N4 composites (Supplementary material, Figure S2(c) and (d)), it can be observed that spherical-like nanoparticles of ZnS were uniformly distributed on the g-C3N4 sheet, and this was further verified by TEM images (Supplementary material, Figure S2(e) and (f)). The g-C3N4 phase facilitates the ZnS particles precipitate and restrain their agglomeration, which contributes to a high specific surface area of ZnS. Besides, the porous structure of the composites could be beneficial to the ions diffusion during the desalination process.

Supplementary material (Figure S3) shows the XRD patterns of pure g-C3N4 and ZnS/g-C3N4 composites. For g-C3N4, a weak peak at 13.1° and a strong peak at 27.4° are observed, which correspond to (100) and (002) planes of g-C3N4 crystal, respectively (JCPDS No. 87-1526) (Liao et al. 2012). For ZnS/g-C3N4 composites, there are four additional diffraction peaks at approximately 28.5°, 33.1°, 47.5°, and 56.3°, corresponding to (111), (200), (220), and (311) planes of the hexagonal crystal of ZnS (JCPDS No. 05-0566). The observed diffraction peaks of both g-C3N4 and ZnS confirmed the successful synthesis of the ZnS/g-C3N4 composite.

The FTIR spectra of g-C3N4 and ZnS/g-C3N4 composites are shown in Supplementary material, Figure S4. On the g-C3N4 curve, the wide absorption peak at 1,243–1,636 cm−1 represents the specific stretching mode of C-N heterocyclic ring, and the peak at 808 cm−1 may be attributed to the classical stretching mode of triazine units (Zhang et al. 2013). In addition, broadband in the range of 3,000–3,500 cm−1 shows the elastic oscillation modes of water molecules on the N–H and O–H outer surfaces. The ZnS/g-C3N4 composite has similar spectra with pure g-C3N4, indicating that ZnS has no significant effect on the structure of g-C3N4.

The N2 adsorption–desorption isotherms and pore size distribution of g-C3N4 and ZnS/g-C3N4 composites are shown in Figure 2. According to the International Union of Pure and Applied Chemistry's (IUPAC) physical adsorption isotherm classification, ZnS/g-C3N4 is a typical type IV adsorption curve with a ‘S’ shape and a clear H3 hysteresis loop between relative pressures of 0.4 and 0.6, as shown in Figure 2(a). The ZnS/g-C3N4 composite appears to have a mesoporous structure and is relatively irregular. The pore size distribution of the ZnS/g-C3N4 composite is primarily at 3–4 nm, as shown in Figure 2(b). This is consistent with the isotherm, and the sample does contain pores in the mesoporous region. Mesopores are very important for ion transfer and contribute to ion transport and adsorption properties, as well as improving desalination capacity. The specific surface area, pore volume, and pore size of the materials are shown in Table 1. The specific surface areas of g-C3N4 and ZnS/g-C3N4 are 6.91 and 10.36 m2/g, respectively. The spacing effect of the ZnS particles between the g-C3N4 layers allows more ions to interact with the material, resulting in improved desalination performance.
Table 1

Characterization of specific surface area, pore volume, and pore size of g-C3N4 and ZnS/g-C3N4 composites

SampleSBET (m2/g)D (nm)Vtotal (m3/g)
g-C3N4 6.91 21.16 0.036 
ZnS/g-C3N4 10.36 14.85 0.037 
SampleSBET (m2/g)D (nm)Vtotal (m3/g)
g-C3N4 6.91 21.16 0.036 
ZnS/g-C3N4 10.36 14.85 0.037 

Note: SBET: special surface area calculated from the BET method; D: average pore diameter; Vtotal: total pore volume.

Figure 2

N2 adsorption–desorption isotherms (a) and BJH pore size distribution of g-C3N4 and ZnS/g-C3N4 composites (b).

Figure 2

N2 adsorption–desorption isotherms (a) and BJH pore size distribution of g-C3N4 and ZnS/g-C3N4 composites (b).

Close modal

The performance of CDI is significantly influenced by the electrode material's wettability, which can be determined using contact angle testing. Supplementary material, Figure S5 illustrates the contact angles of g-C3N4 and ZnS/g-C3N4 with water. The contact angle is typically less than 90° for hydrophilic materials and more than 90° for hydrophobic ones. Contact angles of 51.37° for g-C3N4 and 49.55° for ZnS/g-C3N4 show that both substances are hydrophilic. Furthermore, the composite was successfully constructed, increasing its wettability, as evidenced by the decrease in the ZnS/g-C3N4 contact angle. The performance of the desalination process is improved by encouraging contact between the NaCl solution and the electrode.

Electrochemical performance of ZnS/g-C3N4

The electrochemical properties of g-C3N4 and ZnS/g-C3N4 were tested in a three-electrode system using 1-M NaCl solution as electrolyte. As shown in Figure 3(a) and 3(b), CV curves of g-C3N4 and ZnS/g-C3N4 electrodes show good rectangular shapes without redox peaks, indicating that the electrodes have good EDL capacitive characteristics. The rectangular area of ZnS/g-C3N4 electrode increases with the increasing scanning rate. The specific capacitance was calculated the specific capacitance of g-C3N4 and ZnS/g-C3N4 are 24.66 and 30.27 F/g calculated according to Equation (1) at the scanning rate of 5 mV/s. As shown in Figure 3(c), with the increase of scanning rate, the specific capacitance of ZnS/g-C3N4 is always higher than that of g-C3N4, indicating a better capacitance characteristic.
Figure 3

CV curves of the g-C3N4 (a) and ZnS/g-C3N4 (b) composite electrode at various scan rates (a), specific capacitances (c), and Nyquist impedance plots (d) for the g-C3N4 and ZnS/g-C3N4 composite electrodes.

Figure 3

CV curves of the g-C3N4 (a) and ZnS/g-C3N4 (b) composite electrode at various scan rates (a), specific capacitances (c), and Nyquist impedance plots (d) for the g-C3N4 and ZnS/g-C3N4 composite electrodes.

Close modal

The EIS test was employed to evaluate the electrical resistance and results are shown in Figure 3(d). ZnS/g-C3N4 composite electrode showed a smaller charge transfer resistance. In addition, the slope in the low frequency part of the ZnS/g-C3N4 composite electrode is higher than that of g-C3N4, suggesting better ion diffusion.

GCD curves of g-C3N4 and ZnS/g-C3N4 electrodes were measured at different current densities (0.2, 0.5, 0.8, 1, 2 A/g). As shown in Figure 4(a), compared with g-C3N4 electrode, the curve of ZnS/g-C3N4 electrode presents a longer discharge time, indicating a better conductivity. In addition, the GCD curve of ZnS/g-C3N4 electrode has a symmetrical and triangular shape at different current densities, suggesting the capacitance behavior of EDL, which is consistent with the CV curve. The corresponding specific capacitance at different current densities is shown in Figure 4(b). The ZnS/g-C3N4 electrode has a higher value than the g-C3N4 electrode at any current density.
Figure 4

GCD curves (a) and specific capacitances (b) of the g-C3N4 and ZnS/g-C3N4 electrodes at different current densities.

Figure 4

GCD curves (a) and specific capacitances (b) of the g-C3N4 and ZnS/g-C3N4 electrodes at different current densities.

Close modal

Desalination performance of ZnS/g-C3N4 composite electrodes

Table 2 depicts the electrodes' desalination performance in various modes with an initial concentration of 200 mg/L NaCl solution, a flow rate of 5 mL/min, and a voltage of 1.2 V. The adsorption capacities of the g-C3N4//g-C3N4 (CDI), ZnS/g-C3N4//ZnS/g-C3N4 (CDI), g-C3N4//g-C3N4 (MCDI) and ZnS/g-C3N4///ZnS/g-C3N4 (MCDI) processes had adsorption capacities of 4.61, 8.38, 11.05, and 27.65 mg/g, respectively. The best desalination performance was achieved with the ZnS/g-C3N4 composite electrode in MCDI mode. It is clear that the use of membranes increases the electrode's adsorption capacity. The use of a membrane during the adsorption process can suppress the co-ion effect and allow the adsorbed ions to reach the adsorption maximum, which can adsorb more ions than CDI cell (Broseus et al. 2009). During desorption, the ion concentration on the surface of the electrode plate of the MCDI cell is higher than that of the CDI cell. At the same time, a large number of ions in the CDI cell's solution suffer from the disadvantage of attraction to the side electrode, causing the ion desorption of the MCDI cell's electrode to be greater than that of the CDI cell. As a result, given the effect of membrane addition on treatment capacity and desalination, the MCDI mode is appropriate.

Table 2

The electrosorption capacity of electrodes in different CDI modes

CDI modeCapacity (mg/g)
g-C3N4//g-C3N4 (CDI) 4.61 
ZnS/g-C3N4//ZnS/g-C3N4 (CDI) 8.38 
g-C3N4//g-C3N4 (MCDI) 11.05 
ZnS/g-C3N4//ZnS/g-C3N4 (MCDI) 27.65 
CDI modeCapacity (mg/g)
g-C3N4//g-C3N4 (CDI) 4.61 
ZnS/g-C3N4//ZnS/g-C3N4 (CDI) 8.38 
g-C3N4//g-C3N4 (MCDI) 11.05 
ZnS/g-C3N4//ZnS/g-C3N4 (MCDI) 27.65 

To further optimize the desalination performance, different voltages were applied to the ZnS/g-C3N4 electrode in MCDI mode. The amount of energy consumed during MCDI is directly proportional to the output voltage. At an initial NaCl concentration of 200 mg/L and a flow rate of 5 mL/min, the adsorption capacity gradually increased with the applied voltage (0.6–1.2 V), as shown in Figure 5(a) and 5(b). The electroabsorption capacity was 27.65 mg/g when the voltage was 1.2 V, and the desalination effect was relatively good. The electric field between the plates gets stronger as the plate voltage gets higher, which speeds up the electrodes' desalination process. Therefore, MCDI may produce a higher desalination rate and realize better desalination results by increasing the electrode plate voltage as much as feasible without going above the electrode bilayer capacity load (Jande & Kim 2014; Li et al. 2019). As a result, in this experiment, the voltage of 1.2 V is advantageous for the MCDI desalination.
Figure 5

Conductivity change during MCDI using ZnS/g-C3N4 electrodes under different voltages (a) and corresponding electrosorption capacities (b), and under flow rates (c), and corresponding electrosorption capacities (d).

Figure 5

Conductivity change during MCDI using ZnS/g-C3N4 electrodes under different voltages (a) and corresponding electrosorption capacities (b), and under flow rates (c), and corresponding electrosorption capacities (d).

Close modal

To investigate the desalination performance of ZnS/g-C3N4 composites at different inlet flow rates. At an initial NaCl concentration of 200 mg/L and an applied voltage of 1.2 V, different inlet flow rates (5, 10, 15, and 20 mL/min) were chosen for this experiment. Figure 5(c) and (d) shows the variation of the conductivity of the NaCl solution with time, as well as the calculated electrosorption capacity of the electrode after reaching adsorption equilibrium. The adsorption capacity decreases gradually as the inlet flow rate increases. The electrosorption capacity was 27.65 mg/g at a flow rate of 5 mL/min, and the desalination effect was relatively good. This could be due to the short retention time at high flow rates, which could reduce ion adsorption (Tang et al. 2015). Increased solution inlet flow rate may result in a shorter retention time of the raw aqueous solution in the flow channel, and the ions migrating to the electrode surface may not have been adequately adsorbed and stored in the double electric layer (Agartan et al. 2019). Alternatively, the ions adsorbed on the electrode surface are less than the impact force applied and flow out of the desalination unit with the solution, reducing the electrode adsorption capacity.

Figure 6 depicts the MCDI desalination of the ZnS/g-C3N4 composite electrode with varying initial NaCl concentration. At initial NaCl concentrations of 200, 400, and 600 mg/L, the adsorption amounts were 27.65, 50.26, and 65.34 mg/g, respectively, at an operating voltage of 1.2 V and a flow rate of 5 mL/min. The desalination amount increased with the initial NaCl concentration, first rising and then reaching equilibrium. As the initial concentration increases, it leads to an increase in the bilayer capacitance and more salt ions come in contact with the active sites on the electrode material. Moreover, higher ion concentrations indicate a lower Ohmic loss of the solution, leading to a higher effective voltage between the electrodes and a subsequent increase in electrode adsorption (Zhao et al. 2013; Sakar et al. 2017). Furthermore, the conductivity of the solution increases, and the ion migration rate is subsequently accelerated, improving the electrode's desalination capacity.
Figure 6

Conductivity change during MCDI using ZnS/g-C3N4 electrodes in different initial NaCl concentrations (a) and corresponding electrosorption capacities (b).

Figure 6

Conductivity change during MCDI using ZnS/g-C3N4 electrodes in different initial NaCl concentrations (a) and corresponding electrosorption capacities (b).

Close modal

In this study, a ZnS/g-C3N4 composite was successfully prepared by a one-step high-temperature calcination method and used as the main active material for electrodes in CDI desalination. SEM and TEM images revealed that spherical ZnS particles were evenly distributed on the layered structure of porous g-C3N4. N2 adsorption–desorption tests and pore size distribution analysis demonstrated the ZnS/g-C3N4 composite had a larger specific surface area and suitable porous structure compared to g-C3N4 alone. Furthermore, ZnS/g-C3N4 exhibited favorable specific capacitance characteristics, relatively low resistance and high electrochemical stability. Notably, the ZnS/g-C3N4//ZnS/g-C3N4 (MCDI) electrodes showed a high adsorption capacity of 27.65 mg/g with an applied voltage of 1.2 V, a flow rate of 5 mL/min, and an initial NaCl concentration of 200 mg/L. When the initial concentration of NaCl increased to 400 and 600 mg/L, the adsorption capacity reached 50.26 and 65.34 mg/g, respectively. In summary, the ZnS/g-C3N4 composite electrodes could enable efficient desalination in MCDI systems.

The research is financially supported by the fundamental research program of Shanxi Province (No. 20210302123051).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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