Fabrication of 3D flower-like MoS2/graphene composite as high-performance electrode for capacitive deionization
Graphical abstract
Introduction
The shortage of freshwater has been one of the most urgent issues in the world for human being in the 21st century. Numerous methods, such as distillation, ion exchange, electrodeionization, ion exchange, reverse osmosis and capacitive deionization (CDI) have been proposed to make freshwater industrially [1]. Among them, CDI has attracted worldwide attention due to its advantages of safety, low-cost, low voltages (<2 V), no secondary pollution, high level of automation and low energy consumption, etc. [2,3]. CDI is an emerging method for desalination, which removes charged ions from aqueous solution according to the principle of an electric double-layer (EDL) [4]. Anions and cations in seawater are adsorbed on the anode and cathode respectively during the charge process, and energy is stored simultaneously. When the electrodes are reversed, the ions are desorbed from electrodes to solution, and energy is released synchronously. During the discharge process, the electrodes are regenerated without any secondary pollution [5]. Emphatically, it has been confirmed that electrode materials played an important role in the electrosorptive capacity of CDI [[6], [7], [8]].
Currently, abundant work has been done to develop efficient electrode materials for CDI [[9], [10], [11]]. Activated carbon, carbon fibers, carbon aerogel, carbon cloth and graphene and its composites have been used as electrode materials for CDI attributed to their great specific surface area, good conductivity and excellent chemical stability [5,11,12]. However, the presence of micropores, overlapping effect and disordered pore arrangement of carbonaceous materials lead to the decrease of their CDI efficiency [2]. MoS2 is a typical layered transition metal sulfide which is composed of three atom layers: a Mo layer sandwiched between two S layers. Previous studies have found that MoS2 had been extensively applied in electronics, mechanical, optoelectronics, energy conversion and storage, etc. due to its unique different physical and chemical properties [[13], [14], [15], [16]]. Emphatically, chemically exfoliated MoS2 (ce-MoS2) was also used as electrode material for CDI by Xing et al., and found that the ce-MoS2 nanosheets had a good cycling stability, high ion quality removal capacity of 8.81 mg/g [2]. In addition, Jia et al. applied defect-rich MoS2 as electrode material for CDI, and obtained that the defect-rich MoS2 sheets hold a much great desalination capacity and excellent regenerability and stability [17]. However, researchers have confirmed the low electronic conductivity of MoS2 became to a barrier for the energy storage applications, and the combination of carbonaceous materials and MoS2 could overcome the deficiencies [16,18,19]. Hence, most of previous publications were assembled MoS2 with graphene, and centered on the enhanced performances of lithium storage [20], electromagnetic [21], NO2 sensor [22], supercapacitor [[23], [24], [25]], Hg(II) scavenging [26], etc. of the resulting composites. It can be inferred that the desalination capacity of carbonaceous materials and MoS2 composites may be also improved. However, to the best of our knowledge, no profound study has been performed on the CDI behavior of MoS2/graphene composites.
In the study, 3D flower-like MoS2/graphene composites were fabricated by one-step hydrothermal method and used as electrode materials for CDI. The effects of solvent on the micro-structure, electrochemical performance and desalination capacity of the composites were first systematically investigated, and the desalination capacities of the 3D flower-like MoS2/rGO composites were systematically investigated (adsorption isotherm and kinetics experiments were used to evaluate the desalination properties of the MoS2/graphene composites.). The results showed that the MoS2/graphene composite prepared from the solvent with volume ratio of water and ethanol 2:1, demonstrated a pronounced improvement of desalination capacity with good cycling stability and high ion removal capacity (the maximum experimental desalination capacity of 16.82 mg/g). It indicated that the composite had a great potential application in electrode materials for CDI device.
Section snippets
Materials synthesis
Graphite oxide (GrO) was synthesized by a modified Hummers method as reported previously [27,28]. Analytical reagents of (NH4)6Mo7O24·4H2O, NH2CSNH2, NaCl, Na2SO4 and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Graphene oxide was achieved after the following steps. Firstly, 0.125 g of GrO powder was placed in a beaker containing desired volume of H2O (18.25 MΩ·cm), and then peeled off using Sonics ultrasonic processor (800 W, 20 kHz) with 40% amplitude for 10 min to
Characterization of MoS2/rGO composites
Fig. 1 shows the XRD patterns of MoS2/rGO composites with various volume ratios of water and ethanol. As shown in Fig. 1, three peaks located at 2θ = 13.9°, 33.4° and 59.1° in the XRD pattern of MSG were attributed to the (002), (100) and (110) planes of MoS2 (JCPDS 37-1492) [30], which indicates that MoS2 is successfully loaded on the surface of rGO. In addition, the (002) peak shifted from 2θ = 13.9° to 9.3° after adding various volume ratios of ethanol in the solvents. As listed in Table 1,
Conclusion
In the work, 3D flower-like MoS2/rGO composites were prepared with various volume ratios of water and ethanol as solvents by one-step hydrothermal method, and then used as electrode materials for CDI. The MoS2/rGO composite prepared from the solvent with the volume ratio of water and ethanol 2:1 exhibited a superior CDI performance with a maximum desalination capacity of 16.82 mg/g at 1.0 V in 200 mg/L NaCl solution. The greatest desalination capacity and electrochemical performance of the
Declaration of competing interest
There are no conflicts of interest to declare.
Acknowledgments
Support from the National Natural Science Foundation of China (project No. 51804275 and U1704252) is gratefully acknowledged. Moreover, we also thank the China Postdoctoral Science Foundation (No. 2018M632811 and 2019T120638), Scientific Research Start-up Project of Zhengzhou University (No. 32210793), Key Scientific Research Project Plan of Henan Colleges and Universities (No. 19A45001), Science and Technology Project of Henan Province (192102310246), and Modern Analysis and Computing Centre
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