Defect-rich molybdenum disulfide as electrode for enhanced capacitive deionization from water
Graphical abstract
Introduction
Capacitive deionization (CDI) is a promising technology for desalination because of its competence at efficiency, cost, and environmental issues [[1], [2], [3]]. It is an electrosorption process that operates by adsorbing ions through electric double layer (EDL) formed at the interface of electrodes and a feed water stream [4,5]. When the electrodes are being charged, the sodium and chloride ions are firstly being attracted to approach the electrodes, and then adsorbed electrostatically in the EDL of the electrode materials. Once the electrodes are discharged, the adsorbed ions are released back to the effluent water stream. Based on the principle of EDL, the CDI efficiency is largely dependent on the specific surface area and electronic conductivity of the electrode materials [6]. Carbon materials, such as activated carbon [7,8], carbon aerogel [9], are conventional materials for the preparation of electrodes for CDI application due to their high surface and good electronic conductivity, however they offer a long diffusion path for the access of sodium ions to the EDL of electrode because of the small pore size. To bring a fast access of sodium ions into the electrode, 2D materials might be more suitable for being the CDI electrode material profited from their ultra-high surface-to-volume ratio.
Molybdenum disulfide (MoS2) is a typical layered transition-metal dichalcogenite, which consists of two hexagonal sheets of sulfur atoms and one intermediate hexagonal sheet of molybdenum atoms. The 2D MoS2 possesses huge specific surface area, providing an ideal platform for the physical and chemical reaction. Recent studies indicated that MoS2 exhibited outstanding performances and superior ability for various dyes and heavy metals removal due to the strong π-π interaction and S-heavy metal complexation [[10], [11], [12]]. However, MoS2 has not drawn widespread attention as a desalination material until it was revealed that MoS2 might be a promising candidate for desalination. Molecular dynamics simulations demonstrated that nanoporous 2D molybdenum disulfide would be an efficient filter membrane in sea water desalination because it could prevent salt ions and allow transport of water through nanopores, exhibiting high water transparency and a strong salt filtering capability [13,14]. However the above researches were based on theoretic calculation, the translation of applying MoS2 to a real water desalination was in 2017, where pure MoS2 or MoS2/carbon nanotube composite was used as electrodes [15,16]. Surprisingly, the experimental results indeed verified the excellent desalination capacity and good cycling stability of MoS2, which was even better than that of graphene [17,18]. In the investigation of desalination performance, Wang stated that the unique two-dimensional thin sheet structure enabled MoS2 a sound CDI electrode [15,16]. Capitalizing on the two-dimensional layered structure of MoS2, Presser further proposed that faradaic ion intercalation might be the main contributor for the efficient removal of Na+ and Cl− from a feed water [15,16]. These studies revealed the important role of two-dimensional structure in the desalination performance of MoS2, however no profound study has been performed on the surface property of MoS2 in its CDI behavior.
Thus in this study, an attempt was made to reveal the role and mechanism of surface properties on MoS2 in the CDI process through comparing the desalination performance of MoS2 and defect-rich MoS2. The MoS2 sample was first subjected to thermal treatment for its property modification and then characterized by high resolution transmission electron microscope (HRTEM) and atomic force microscope (AFM), while the desalination performance was investigated through the adsorption isotherm and kinetics experimental. The regenerability and stability of MoS2 electrode were also studied through the electrochemical and CDI cycle experiments. The object was to obtain a clear understanding in the role and influence mechanism of defects in the desalination process with MoS2 as electrodes, as well as to enhance the desalination performance of MoS2 and give a guidance for the preparation of MoS2 as electrode.
Section snippets
Materials and reagents
Hexaammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O), thiourea (CN2H4S), sodium chloride (NaCl),silver nitrate (AgNO3), potassium chromate (K2CrO4) and sodium hydroxide (NaOH) used in this study were purchased from the Sinopharm Chemical Reagent Co., Ltd. (China). Sulfuric acid (H2SO4) was supplied by Xinyang chemical reagent (China). All the reagents were of analytical grade. Titanium plate was purchased from Yongsheng Company, China, respectively. Milli-Q water (Millipore, Bedford,
Structure and morphology of molybdenum disulfide
XRD pattern of molybdenum disulfide is illustrated in Fig. S1. The typical diffraction at 2θ of 13.55°, 32.43°, 35.52°, 57.30° correspond to the primary diffractions of (002), (100), (103) and (110) planes of MoS2, respectively, which indicated the successful synthesis of MoS2. Raman spectra of molybdenum disulfide before and after thermal treatment are displayed in Fig. 2. Two characteristic peaks of molybdenum disulfide at 381.5 and 409.4 cm−1 were observed, which corresponded to the in-plane
Conclusions
MoS2 electrode with rich edge-defects on the surface has been facilely fabricated through a simple thermal treatment of MoS2 nanosheets. Defect-rich MoS2 exhibited an excellent CDI performance with a desalination capacity of 35 mg/g at 0.8 V in a low concentrated NaCl solution, which was around three times of that on MoS2 (12.8 mg/g). The enhanced CDI performance should be ascribed to the variation of the surface and electrochemical properties that emerged in the thermal treatment. Abundant
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
The financial supports for this work from the National Natural Science Foundation of China (51704220, 51674183 and 51704212), Natural Science Foundation of Hubei Province (2017CFB280), and China Postdoctoral Science Foundation (2016M600621) are gratefully acknowledged.
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