Charge Storage by Electrochemical Reaction of Water Bilayers Absorbed on MoS2 Monolayers

From Bulk Molybdenum Disulfide (MoS2) to Suspensions of Exfoliated MoS2 in an Aqueous Medium and Their Applications

Two-dimensional (2D) layered materials like graphene, transition-metal dichalcogenides (TMDs), boron nitrides, etc., exhibit unique and fascinating properties, such as high surface-to-volume ratio, inherent mechanical flexibility and robustness, tunable bandgap, and high carrier mobility, which makes them an apt candidate for flexible electronics with low consumption of power. Because of these properties, they are in tremendous demand for advancement in energy, environmental, and biomedical sectors developed through various technologies. The production and scalability of these materials must be sustainable and ecofriendly to utilize these unique properties in the real world. Here, in this current review, we review molybdenum disulfide (MoS2 nanosheets) in detail, focusing on exfoliated MoS2 in water and the applicability of aqueous MoS2 suspensions in various fields. The exfoliation of MoS2 results in the formation of single or few-layered MoS2. Therefore, this Review focuses on the few layers of exfoliated MoS2 that have the additional properties of 2D layered materials and higher excellent compatibility for integration than existing conventional Si tools. Hence, a few layers of exfoliated MoS2 are widely explored in biosensing, gas sensing, catalysis, photodetectors, energy storage devices, a light-emitting diode (LED), adsorption, etc. This review covers the numerous methodologies to exfoliate MoS2, focusing on the various published methodologies to obtain nanosheets of MoS2 from water solutions and their use.

High-Ionic-Conductivity Sodium-Based Ionic Gel Polymer Electrolyte for High-Performance and Ultrastable Microsupercapacitors

Due to the lower cost and greater natural abundance of the sodium element on the earth than those of the lithium element, sodium-based ionic gel polymer electrolytes (IGPEs) are becoming a more cost-effective and popular material choice for portable and stationary energy solutions. The sodium-based IGPEs, however, appeared relatively inferior to their lithium-based counterparts for use in high-performance microsupercapacitors in terms of ionic conductivity and electrochemical stability. To tackle these issues, poly(ethylene glycol) diacrylate (PEGDA) with fast polymerization to build a polymer matrix and sodium perchlorate (NaClO₄) with high chemical stability and high thermal stability are employed to generate free ions for an ionic conducting phase with the support of tetramethylene glycol ether (G4) and 1-ethyl-3-methylimidazolium bis(triflouromethylsulfonyl)imide (EMIM-TFSI). It was found that the ionic conductivity (σ_dc) of this sodium-based IGPE reaches up to 0.54 mS/cm at room temperature. To manifest a high-conductivity sodium-based IGPE (SIGPE), a microsupercapacitor (MSC) with an area of 5 mm² is designed and fabricated on an interdigital reduced graphene oxide electrode. This MSC demonstrates prominent performance with a high power density of ∼2500 W/kg and a maximum energy density of ∼0.7 Wh/kg. Furthermore, after 20,000 cycles at an operating potential window from 0.0 to 1.0 V, it retains approximately 98.9% capacitance. An MSC array in 3 series × 3 parallels (3S × 3P) was successfully designed as a power source for a basic circuit with an LED. Therefore, we believe that our sodium-based IGPE microsupercapacitor holds its promising role as a solid-state energy source for high-performance and high-stability energy solutions.

Synchrotron radiation based X-ray techniques for analysis of cathodes in Li rechargeable batteries

Li-ion rechargeable batteries are promising systems for large-scale energy storage solutions. Understanding the electrochemical process in the cathodes of these batteries using suitable techniques is one of the crucial steps for developing them as next-generation energy storage devices. Due to the broad energy range, synchrotron X-ray techniques provide a better option for characterizing the cathodes compared to the conventional laboratory-scale characterization instruments. This work gives an overview of various synchrotron radiation techniques for analyzing cathodes of Li-rechargeable batteries by depicting instrumental details of X-ray diffraction, X-ray absorption spectroscopy, X-ray imaging, and X-ray near-edge fine structure-imaging. Analysis and simulation procedures to get appropriate information of structural order, local electronic/atomic structure, chemical phase mapping and pores in cathodes are discussed by taking examples of various cathode materials. Applications of these synchrotron techniques are also explored to investigate oxidation state, metal-oxygen hybridization, quantitative local atomic structure, Ni oxidation phase and pore distribution in Ni-rich layered oxide cathodes.

Remote Plasma-Induced Synthesis of Self-Assembled MoS2/Carbon Nanowall Nanocomposites and Their Application as High-Performance Active Materials for Supercapacitors

The objective of this study is to investigate the synthesis and influence of MoS2 on carbon nanowalls (CNWs) as supercapacitor electrodes. The synthesis of MoS2 on CNW was achieved by the introduction of hydrogen remote plasma from ammonium tetrathiomolybdate (ATTM) without deterioration of the CNWs. The topographical surface structures and electrochemical characteristics of the MoS2–CNW composite electrodes were explored using two ATTM-dispersed organic solvents—acetonitrile and dimethylformamide (DMF). In this study, CNW and MoS2 were synthesized using an electron cyclotron resonance plasma. However, hydrogen radicals, which transform ATTM into MoS2, were provided in the form of a remote plasma source. The electrochemical performances of MoS2–CNW hybrid electrodes with various morphologies—depending on the solvent and ATTM concentration—were evaluated using a three-electrode system. The results revealed that the morphology of the synthesized MoS2 was influenced by the organic solvent used and affected both the electrochemical performance and topographical characteristics. Notably, considerable enhancement of the specific capacitance was observed for the MoS2 with open top edges synthesized from DMF. These encouraging results may motivate additional research on hybrid supercapacitor electrodes and the rapid synthesis of MoS2 and other transition metal dichalcogenides.

Highly improved supercapacitance properties of MnFe2O4 nanoparticles by MoS2 nanosheets

Manganese ferrite (MnFe₂O₄) nanoparticles were synthesized via a hydrothermal method and combined with exfoliated MoS₂ nanosheets, and the nanocomposite was studied as a supercapacitor. X-ray diffractometry and Raman spectroscopy confirmed the crystalline structures and structural characteristics of the nanocomposite. Transmission electron microscopy images showed the uniform size distribution of MnFe₂O₄ nanoparticles (~ 13 nm) on few-layer MoS₂ nanosheets. UV-visible absorption photospectrometry indicated a decrease in the bandgap of MnFe₂O₄ by MoS₂, resulting in a higher conductivity that is suitable for capacitance. Electrochemical tests showed that the incorporation of MoS₂ nanosheets largely increased the specific capacitance of MnFe₂O₄ from 600 to 2093 F/g (with the corresponding energy density and power density of 46.51 Wh/kg and 213.64 W/kg, respectively) at 1 A/g, and led to better charge-discharge cycling stability. We also demonstrated a real-world application of the MnFe₂O₄/MoS₂ nanocomposite in a two-cell asymmetric supercapacitor setup. A density functional theory study was also performed on the MnFe₂O₄/MoS₂ interface to analyze how a MoS₂ monolayer can enhance the electronic properties of MnFe₂O₄ towards a higher specific capacitance.

Energy storage performance of 2D MoS2 and carbon nanotube heterojunctions in symmetric and asymmetric configuration

Excellent cyclic stability and fast charge/discharge capacity demonstrated by supercapacitors foster research interest into new electrode materials with 100% cycle life and high specific capacitance. We report an improvement in the electrochemical performance of MoS₂/multiwalled carbon nanotubes (MWCNT) nanohybrid and intensively explored its performance in symmetric and asymmetric supercapacitor (ASC) assembly. The symmetric assembly of MoS₂/MWCNT exhibits capacitance of around 274.63 F g^-1 at 2 A g^-1 with higher specific energy/power outputs (20.70 Wh kg^-1/1.49 kW kg^-1) as compared to the supercapacitor based on pristine MoS₂ (5.82 Wh kg^-1/1.07 kW kg^-1). On the other hand, a unique all-carbon-based ASC assembled with MoS₂/MWCNT and VSe₂/MWCNT composite with K₂SO₄ as electrolyte delivers the highest energy density of 32.18 Wh kg^-1 at a power density of 1.121 kW kg^-1 with exceptional cycling stability and excellent retention of about 98.43% even after 5000 cycles. These outstanding results demonstrate the excellent electrochemical properties of both symmetric and asymmetric systems with high energy density and performance, which further enable them to be a potential candidate for supercapacitor applications.

MoS2/cellulose paper coupled with SnS2 quantum dots as 2D/0D electrode for high-performance flexible supercapacitor

The effective incorporation of novel and highly conductive hybrid functional nanomaterials onto flexible and porous substrates is extremely desirable to develop flexible supercapacitors.

Enhanced Electrochemical Performance of Self-Assembled Nanoflowers of MoS2 Nanosheets as Supercapacitor Electrode Materials

As supercapacitor electrode materials, their structures, including specific surface area, instability, and interconnection, determine the electrochemical performances (specific capacitance, cycle stability, and rate performance). In this study, 1T-MoS₂ nanosheets were self-assembled into nanoflowers via a one-pot facile hydrothermal reaction. The nanoflowers retain the excellent electrical conductive performance and the feature of inherent high specific surface area of the nanosheets. For the sheets are interconnected to each other in flower structure, the structure is more stable and the charges are more easily transferred. Thus, compared to the nanosheet electrode, the nanoflower electrode shows the remarkable advantage when used as the electrode of the energy-storage device, whether it is 1T phase or 2H phase in KCl or in KOH. When measured at 0.5 A g^-1 in KOH electrolyte, the MoS₂ nanoflower electrode exhibits a high specific capacitance of 1120 F g^-1. At the same time, when cycling 2000 times at a current density of 10 A g^-1, the capacitance retention ratio can reach up to 96%.