Stabilizing and Activating Active Sites: 1T-MoS2 Supported Pd Single Atoms for Efficient Hydrogen Evolution Reaction

Metallic 1T-MoS2 with high intrinsic electronic conductivity performs Pt-like catalytic activity for hydrogen evolution reaction (HER). However, obtaining pure 1T-MoS2 is challenging due to its high formation energy and metastable properties. Herein, an in situ SO4 2--anchoring strategy is reported to synthesize a thin layer of 1T-MoS2 loaded on commercial carbon. Single Pd atoms, constituting a substantial loading of 7.2 wt%, are then immobilized on the 1T-phase MoS2 via Pd─S bonds to modulate the electronic structure and ensure a stable active phase. The resulting Pd1/1T-MoS2/C catalyst exhibits superior HER performance, featuring a low overpotential of 53 mV at the current density of 10 mA cm-2, a small Tafel slope of 37 mV dec-1, and minimal charge transfer resistance in alkaline electrolyte. Moreover, the catalyst also demonstrates efficacy in acid and neutral electrolytes. Atomic structural characterization and theoretical calculations reveal that the high activity of Pd1/1T-MoS2/C is attributed to the near-zero hydrogen adsorption energy of the activated sulfur sites on the two adjacent shells of atomic Pd.

Molecular Connectors Boosting the Performance of MoS2 Cathodes in Zinc-Ion Batteries

Zinc-ion batteries (ZIBs) are promising energy storage systems due to high energy density, low-cost, and abundant availability of zinc as a raw material. However, the greatest challenge in ZIBs research is lack of suitable cathode materials that can reversibly intercalate Zn2+ ions. 2D layered materials, especially MoS2-based, attract tremendous interest due to large surface area and ability to intercalate/deintercalate ions. Unfortunately, pristine MoS2 obtained by traditional protocols such as chemical exfoliation or hydrothermal/solvothermal methods exhibits limited electronic conductivity and poor chemical stability upon charge/discharge cycling. Here, a novel molecular strategy to boost the electrochemical performance of MoS2 cathode materials for aqueous ZIBs is reported. The use of dithiolated conjugated molecular pillars, that is, 4,4'-biphenyldithiols, enables to heal defects and crosslink the MoS2 nanosheets, yielding covalently bridged networks (MoS2-SH2) with improved ionic and electronic conductivity and electrochemical performance. In particular, MoS2-SH2 electrodes display high specific capacity of 271.3 mAh g-1 at 0.1 A g-1, high energy density of 279 Wh kg-1, and high power density of 12.3 kW kg-1. With its outstanding rate capability (capacity of 148.1 mAh g-1 at 10 A g-1) and stability (capacity of 179 mAh g-1 after 1000 cycles), MoS2-SH2 electrodes outperform other MoS2-based electrodes in ZIBs.

Solution-Processable and Printable Two-Dimensional Transition Metal Dichalcogenide Inks

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with layered crystal structures have been attracting enormous research interest for their atomic thickness, mechanical flexibility, and excellent electronic/optoelectronic properties for applications in diverse technological areas. Solution-processable 2D TMD inks are promising for large-scale production of functional thin films at an affordable cost, using high-throughput solution-based processing techniques such as printing and roll-to-roll fabrications. This paper provides a comprehensive review of the chemical synthesis of solution-processable and printable 2D TMD ink materials and the subsequent assembly into thin films for diverse applications. We start with the chemical principles and protocols of various synthesis methods for 2D TMD nanosheet crystals in the solution phase. The solution-based techniques for depositing ink materials into solid-state thin films are discussed. Then, we review the applications of these solution-processable thin films in diverse technological areas including electronics, optoelectronics, and others. To conclude, a summary of the key scientific/technical challenges and future research opportunities of solution-processable TMD inks is provided.

Effect of cobalt doping on the enhanced energy storage performance of 2D vanadium diselenide: experimental and theoretical investigations

Vanadium Diselenide (VSe₂) is a prominent candidate in the 2D transition metal dichalcogenides family for energy storage applications. Herein, we report the experimental and theoretical investigations on the effect of cobalt doping in 1T-VSe₂. The energy storage performance in terms of specific capacitance, stability and energy and power density is studied. It is observed that 3% Co doped VSe₂exhibits better energy storage performance as compared to other concentrations, with a specific capacitance of ∼193 F g^-1in a two-electrode symmetric configuration. First-principles Density Functional Theory based simulations support the experimental findings by suggesting an enhanced quantum capacitance value after the Co doping in the 1T-VSe₂. By making use of the advantages of the specific electrode materials, a solid state asymmetric supercapacitor (SASC) is also assembled with MoS₂as the negative electrode. The assembled Co-VSe₂//MoS₂SASC device shows excellent energy storage performance with a maximum energy density of 33.36 Wh kg^-1and a maximum power density of 5148 W kg^-1with a cyclic stability of 90% after 5000 galvano static charge discharge cycles.

Exfoliation of 2D materials by saponin in water: Aerogel adsorption / photodegradation organic dye

The biggest challenge for the paint industry is to clean the contaminated waste dye solution before it released into the water or to reuse it to create new paint and to protect the water from environmental pollution. Here in this work, exfoliating layered transition metal dichalcogenide materials prepare to the exfoliated 2D materials thin sheets in water with the assistance of natural saponin. Then, the three-dimensional (3D) MoS₂-aerogel composite was synthesized by using greenway exfoliated two-dimensional (2D) MoS₂ thin sheets to form MoS₂-aerogel composite. The prepared 3D MoS₂-aerogel composite demonstrates excellent 94% methylene blue (MB) dye adsorption ability over 5 min. Moreover, the adsorbed MB of the MoS₂-aerogel shows ∼80% dye degradation activity in the presence of visible light. Therefore, these synthesized 3D MoS₂-aerogel composite could be an excellent candidate for photocatalytic applications in the future.

Quicker and More Zn2+ Storage Predominantly from the Interface

Aqueous zinc-ion batteries are highly desirable for large-scale energy storage because of their low cost and high-level safety. However, achieving high energy and high power densities simultaneously is challenging. Herein, a VO_x sub-nanometer cluster/reduced graphene oxide (rGO) cathode material composed of interfacial VOC bonds is artificially constructed. Therein, a new mechanism is revealed, where Zn²⁺ ions are predominantly stored at the interface between VO_x and rGO, which causes anomalous valence changes compared to conventional mechanisms and exploits the storage ability of non-energy-storing active yet highly conductive rGO. Further, this interface-dominated storage triggers decoupled transport of electrons/Zn²⁺ ions, and the reversible destruction/reconstruction allows the interface to store more ions than the bulk. Finally, an ultrahigh rate capability (174.4 mAh g^-1 at 100 A g^-1 , i.e., capacity retention of 39.4% for a 1000-fold increase in current density) and a high capacity (443 mAh g^-1 at 100 mA g^-1 , exceeding the theoretical capacities of each interfacial component) are achieved. Such interface-dominated storage is an exciting way to build high-energy- and high-power-density devices.

Strain engineering of two-dimensional multilayered heterostructures for beyond-lithium-based rechargeable batteries

Beyond-lithium-ion batteries are promising candidates for high-energy-density, low-cost and large-scale energy storage applications. However, the main challenge lies in the development of suitable electrode materials. Here, we demonstrate a new type of zero-strain cathode for reversible intercalation of beyond-Li+ ions (Na+, K+, Zn2+, Al3+) through interface strain engineering of a 2D multilayered VOPO4-graphene heterostructure. In-situ characterization and theoretical calculations reveal a reversible intercalation mechanism of cations in the 2D multilayered heterostructure with a negligible volume change. When applied as cathodes in K+-ion batteries, we achieve a high specific capacity of 160 mA h g-1 and a large energy density of ~570 W h kg-1, presenting the best reported performance to date. Moreover, the as-prepared 2D multilayered heterostructure can also be extended as cathodes for high-performance Na+, Zn2+, and Al3+-ion batteries. This work heralds a promising strategy to utilize strain engineering of 2D materials for advanced energy storage applications.

In situ monitoring of the electrochemically induced phase transition of thermodynamically metastable 1T-MoS2 at nanoscale

1T-MoS2 is widely used in the hydrogen evolution reaction (HER) due to its abundant active sites and good conductivity. However, 1T-MoS2 is thermodynamically metastable due to the distorted crystal structure. Recently, researchers have detected the J1 and A1g Raman peaks after the HER process and confirmed that the 2H-1T phase possesses good stability. Therefore, continuous HER is likely to transform 1T-MoS2 into a stable 2H-1T mixed phase. The in situ characterization of 1T-MoS2 individual nanosheets in the HER process is important to understand the intrinsic electrocatalytic behaviour at confined nanoscale, which has rarely been investigated. Herein, we built an individual 1T-MoS2 nanosheet micro-nano device by the intercalation of N-butyllithium into 2H-MoS2. Then, the device was kept at an overpotential (η) of 450 mV, which was much lower than the onset potential, for 20 minutes to ensure continuous HER. Through this electrochemical treatment, we successfully obtained a mixed phase of 2H-1T and monitored the electrochemical phase transition by in situ Raman mapping and atomic force microscopy (AFM). The HER performance of the 2H-1T phase was superior to that of 1T-MoS2 and 2H-MoS2. Additionally, computational simulations demonstrated that the 2H-1T phase exhibited optimal hydrogen adsorption energy. The presented work displays the excellent catalysis of the mixed phase obtained by the electrochemical phase transition, which provides new directions for improving the catalytic activity of TMDs.

SiOC functionalization of MoS2 as a means to improve stability as sodium-ion battery anode

The development of feasible, scalable, and environmentally-safe electrode materials that provide stable cycling performance are critical for success of beyond lithium rechargeable batteries and supercapacitors. With respect to the sodium-ion battery (SIB) anodes constituting of transition metal dichalcogenides such as molybdenum disulfide (MoS₂), poor cycle stability and fast capacity degradation, due to low electronic conductivity and dissolution of chemical species in the electrolyte, hinders use of these promising layered materials as SIB anodes. Herein we report chemical functionalization in MoS₂ nanosheets with polymer-derived silicon oxycarbide or SiOC with the aim to preserve MoS₂ from dissolution in the SIB organic electrolyte, without compromising its role in sodiation and desodiation processes. Our results suggest that a MoS₂-SiOC composite electrode is effective in bringing improved cycle stability to sodium-ion cycling over neat MoS₂ even after 100 cycles.

Recent progress of TMD nanomaterials: phase transitions and applications

Transition metal dichalcogenides (TMDs) show wide ranges of electronic properties ranging from semiconducting, semi-metallic to metallic due to their remarkable structural differences. To obtain 2D TMDs with specific properties, it is extremely important to develop particular strategies to obtain specific phase structures. Phase engineering is a traditional method to achieve transformation from one phase to another controllably. Control of such transformations enables the control of properties and access to a range of properties, otherwise inaccessible. Then extraordinary structural, electronic and optical properties lead to a broad range of potential applications. In this review, we introduce the various electronic properties of 2D TMDs and their polymorphs, and strategies and mechanisms for phase transitions, and phase transition kinetics. Moreover, the potential applications of 2D TMDs in energy storage and conversion, including electro/photocatalysts, batteries/supercapacitors and electronic devices, are also discussed. Finally, opportunities and challenges are highlighted. This review may further promote the development of TMD phase engineering and shed light on other two-dimensional materials of fundamental interest and with potential ranges of applications.

Enhanced electrochemical performances of ZnCo2O4@CoMoO4 core–shell structures with long cycling stabilities

Developing electrode materials with high specific capacitance and excellent stability for energy storage is necessary to solve energy shortage issues.

Insights into the Crystallinity of Layer-Structured Transition Metal Dichalcogenides on Potassium Ion Battery Performance: A Case Study of Molybdenum Disulfide

Layer-structured transition metal dichalcogenides (LS-TMDs) are being heavily studied in K-ion batteries (KIBs) owing to their structural uniqueness and interesting electrochemical mechanisms. Synthetic methods are designed primarily focusing on high capacities. The achieved performance is often the collective results of several contributing factors. It is important to decouple the factors and understand their functions individually. This work presents a study focusing on an individual factor, crystallinity, by taking MoS₂ as a demonstrator. The performance of low and high-crystallized MoS₂ is compared to show the function of crystallinity is dependent on the electrochemical mechanism. Lower crystallinity can alleviate diffusional limitation in 0.5-3.0 V, where intercalation reaction takes charge in storing K-ions. Higher crystallinity can ensure the structural stability of the MoS₂ layers and promote surface charge storage in 0.01-3.0 V, where conversion reaction mainly contributes. The low-crystallized MoS₂ exhibits an intercalation capacity (118 mAh g^-1 ), good cyclability (85% over 100 cycles), and great rate capability (41 mAh g^-1 at 2 A g^-1 ), and the high-crystallized MoS₂ delivers a high capacity of 330 mAh g^-1 at 1 A g^-1 and retains 161 mAh g^-1 at 20 A g^-1 , being one of the best among the reported LS-TMDs in KIBs.

A Pseudolayered MoS2 as Li-Ion Intercalation Host with Enhanced Rate Capability and Durability

As a popular strategy, interlayer expansion significantly improves the Li-ion diffusion kinetics in the MoS₂ host, while the large interlayer spacing weakens the van der Waals force between MoS₂ monolayers, thus harming its structural stability. Here, an oxygen-incorporated MoS₂ (O-MoS₂ )/graphene composite as a self-supported intercalation host of Li-ion is prepared. The composite delivers a specific capacity of 80 mAh g^-1 in only 36 s at a mass loading of 1 mg cm^-2 , and it can be cycled 3000 times (over 91% capacity retention) with a 5 mg cm^-2 loading at 2 A g^-1 . The O-MoS₂ exhibits a dominant 1T phase with an expanded layer spacing of 10.15 Å, leading to better Li-ion intercalation kinetics compared with pristine MoS₂ . Furthermore, ex situ X-ray diffraction tests indicate that O-MoS₂ sustains a stable structure in cycling compared with the gradual collapse of pristine MoS₂ , which suffers from excessive lattice breathing. Density functional theory calculations suggest that the MoO_x (OH)_y pillars in O-MoS₂ interlayers not only expand the layer spacing, but also tense the MoS₂ layers to avoid exfoliation in cycling. Therefore, the O-MoS₂ shows a pseudolayered structure, leading to remarkable durability besides the outstanding rate capability as a Li-ion intercalation host.

Synthesis of Grain-like MoS2 for High-Performance Sodium-Ion Batteries

MoS₂ is a promising anode material for sodium-ion batteries (SIBs) due to its attractive theoretical capacity and low cost. MoS₂ generally presents a sheet-like structure based on its (002) lattice plane; however, such a structure tends to result in agglomeration and stacking of the sheets that cannot accommodate volume expansion, resulting in poor cyclability. Herein, grain-like MoS₂ particulates (G-MoS₂ ) are synthesized by sulfiding MoO₃ in highly concentrated sulfur vapor, which results in epitaxial growth of MoS₂ in (002), (100), and (110) lattice planes, with the product consisting of MoS₂ particulates of about 300 nm coated with few-layered MoS₂ nanosheets. The unique G-MoS₂ architecture ensures good dispersion and sufficient distance to accommodate volume expansion during sodiation/desodiation, which effectively prevents stacking of MoS₂ , maintaining structural stability. When employed as the working electrode for SIB, G-MoS₂ delivers a high reversible capacity of 324 mAh g^-1 at 0.5 A g^-1 , retaining 312 mAh g^-1 over 300 cycles with an average coulombic efficiency of 99.8 %. Even when G-MoS₂ is cycled at a high current density (2.0 A g^-1 ), the retained capacity is 175 mAh g^-1 after 400 cycles. Comparison with literature reveals that these capacities are among the more promising reversible values reported for pure MoS₂ .

Sodium storage in a promising MoS2-carbon anode: elucidating structural and interfacial transitions in the intercalation process and conversion reactions

Sodium-ion batteries and capacitors are considered as low-cost energy storage devices, compared to Li-ion counterparts. However, most anodes for sodium-ion devices show sluggish kinetics and poor structural stability caused by the large radius (1.02 Å) of Na+. One candidate anode is MoS2, a 2D atomic layered material with a large interlayer spacing of 6.2 Å, that can take up and release Na+via two working principles: a two-electron intercalation process and a four-electron conversion reaction. Herein, we report a facile method to synthesize a MoS2-amorphous carbon (MoS2-AC) nanocomposite and further study the effect of the two working principles on the structure, interphase, and charge storage properties of MoS2-AC. The two-electron intercalation reaction enables the MoS2-AC electrode to have a higher rate capability and superior stability than that via the four-electron Na+ conversion reaction. This favorable Na+ charge storage performance of MoS2-AC via the two-electron intercalation process is attributed to its pseudocapacitive behavior, a stable solid electrolyte interphase and robust stability of the structure, which enables us to fabricate a sodium-ion capacitor that can deliver high energy density at a high rate. This work underscores the potential importance of realizing fast Na+ charge storage via an intercalation process as a strategy for the fabrication of high-performance sodium-ion capacitors and batteries.

Pseudocapacitive Sodium Storage by Ferroelectric Sn2 P2 S6 with Layered Nanostructure

Sodium ion batteries (SIB) are considered promising alternative candidates for lithium ion batteries (LIB) because of the wide availability and low cost of sodium, therefore the development of alternative sodium storage materials with comparable performance to LIB is urgently desired. The sodium ions with larger sizes resist intercalation or alloying because of slow reaction kinetics. Most pseudocapacitive sodium storage materials are based on subtle nanomaterial engineering, which is difficult for large-scale production. Here, ferroelectric Sn₂ P₂ S₆ with layered nanostructure is developed as sodium ion storage material. The ferroelectricity-enhanced pseudocapacitance of sodium ion in the interlayer spacing makes the electrochemical reaction easier and faster, endowing the Sn₂ P₂ S₆ electrode with excellent rate capability and cycle stability. Furthermore, the facile solid state reaction synthesis and common electrode fabrication make the Sn₂ P₂ S₆ that becomes a promising anode material of SIB.

MoS2 nanosheets with expanded interlayer spacing for enhanced sodium storage

The interlayer spacing of MoS₂ is largely expanded to fully exploit the van der Waals gaps for Na-ion storage.