Nanostructured Tungsten Oxysulfide as an Efficient Electrocatalyst for Hydrogen Evolution Reaction

Fe7S8 coupled with VS4 heterogeneous interface engineering driven by FeV bimetallic MOFs: An efficient all-pH and durable hydrogen evolution

Rational design and preparation of a multiphase electrocatalyst for hydrogen evolution reaction (HER) has become a hot research topic, while applicable and pH versatility of vanadium tetrasulfide (VS4) and heptairon octasulfide (Fe7S8) composites have rarely been reported. Here, the facile topological sulfide self-template sacrifice method using FeV bimetallic MOFs is designed to obtain Fe7S8 coupled with VS4 heterostructures, enhancing the electron precipitation in the catalysts and attracts electrons to migrate. According to DFT simulations, the electronic coupling at the atomic orbital level and the modulation of interfacial electrons among various interfaces play a crucial role in enhancing the intermediate state process of the hydrogen evolution reaction (HER) across the entire pH range, promoting the optimal d-band centroid value (εd). Reassuringly, the prepared 3D Fe7S8/VS4 electrodes possessed excellent performances of η10 = 53 mV, η10 = 135 mV and η10 = 38 mV in a conventional three-electrode configuration in a 1 M KOH, 1 M Na2SO4, and 0.5 M H2SO4, and the stabilized currents can all be maintained for 48 h. This innovative design of in situ heterostructured materials constructed from dual transition metal sulfides provides inspiring ideas for the preparation of all-pH catalysts.

Conversion of Layered WS2 Crystals into Mixed-Domain Electrochemical Catalysts by Plasma-Assisted Surface Reconstruction

Electrocatalytic water splitting is crucial to generate clean hydrogen fuel, but implementation at an industrial scale remains limited due to dependence on expensive platinum (Pt)-based electrocatalysts. Here, an all-dry process to transform electrochemically inert bulk WS2 into a multidomain electrochemical catalyst that enables scalable and cost-effective implementation of the hydrogen evolution reaction (HER) in water electrolysis is reported. Direct dry transfer of WS2 flakes to a gold thin film deposited on a silicon substrate provides a general platform to produce the working electrodes for HER with tunable charge transfer resistance. By treating the mechanically exfoliated WS2 with sequential Ar-O2 plasma, mixed domains of WS2, WO3, and tungsten oxysulfide form on the surfaces of the flakes, which gives rise to a superior HER with much greater long-term stability and steady-state activity compared to Pt. Using density functional theory, ultraefficient atomic sites formed on the constituent nanodomains are identified, and the quantification of atomic-scale reactivities and resulting HER activities fully support the experimental observations.

Novel Heterostructure-Based CoFe and Cobalt Oxysulfide Nanocubes for Effective Bifunctional Electrocatalytic Water and Urea Oxidation

The development of effective oxygen evolution reaction (OER) and urea oxidation reaction (UOR) on heterostructure electrocatalysts with specific interfaces and characteristics provides a distinctive character. In this study, heterostructure nanocubes (NCs) comprising inner cobalt oxysulfide (CoOS) NCs and outer CoFe (CF) layered double hydroxide (LDH) are developed using a hydrothermal methodology. During the sulfidation process, the divalent sulfur ions (S2-) are released from the breakdown of the sulfur source and react with the Co-precursors on the surface leading to the transformation of CoOH nanorods into CoOS nanocubes. Further, X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analyses reveal that the interactions at the interface of the CF@CoOS NCs significantly altered the electronic structure, thus enhancing the electrocatalytic performance. The optimal catalysts exhibited effective OER and UOR activities, the attained potentials are 1.51 and 1.36 V. This remarkable performance is attributable to the induction of electron transfer from the CoFe LDH to CoOS, which reduces the energy barrier of the intermediates for the OER and UOR. Furthermore, an alkaline water and urea two-cell electrolyzer assembled using CF@CoOS-2 NCs and Pt/C as the anode and cathode requires a cell voltage of 1.63 and 1.56 V along with a durability performance.

Heterointerface engineering of cobalt molybdenum suboxide for overall water splitting

Highly active and earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are of great significance for sustainable hydrogen generation through alkaline water electrolysis. Here, with an aim to enhance the bifunctional electrocatalytic activity of cobalt molybdate towards overall water splitting, we demonstrate a simple method involving the modulation of the cobalt to molybdenum ratio and creation of phase-modulated heterointerfaces. Samples with varying Co/Mo molar ratios are obtained via a microwave-assisted synthesis method using appropriate starting precursors. The synthesis conditions are modified to create a heterointerface involving multiple phases of cobalt molybdenum suboxides (CoO/CoMoO3/Co2Mo3O8) supported on Ni foam (NF). Detailed electrochemical studies reveal that modulating the composition and hence the interface can tweak the bifunctional electrocatalytic activity of the material for HER and OER and thus improve the overall water splitting efficiency with very high durability over 500 h. To further evaluate the practical applicability of the studied catalyst in water splitting, an alkaline electrolyser is fabricated with the optimized cobalt molybdenum suboxide material (CMO-1.25) as a bifunctional electrocatalyst. A current density of 220 mA cm-2 @1.6 V and 670 mA cm-2 @1.8 V was obtained, and the device showed very good long-term durability.

Dual CoxSy-Modified Tungsten Disulfide Double-Heterojunction Electrocatalyst for Efficient Hydrogen Evolution in All-pH Media

The rational design and preparation of a heterogeneous electrocatalyst for hydrogen evolution reaction (HER) has become a research hotspot, while applicable and pH-universal tungsten disulfide (WS₂)-based hybrid composites are rarely reported. Herein, we propose a novel hybrid catalyst (WS₂/Co₉S₈/Co₄S₃) comprising two heterojunctions of WS₂/Co₄S₃ and WS₂/Co₉S₈, which grow on the porous skeleton of Co, N-codoped carbon (Co/NC) flexibly applicable to all-pH electrolytes. The effect of double heterogeneous coupling on HER activity is explored as the highly flexible heterojunction is conducive to tune the activity of the catalyst, and the synergistic interaction of the double heterojunctions is maximized by adjusting the proportion of heterojunction components. Theoretical calculations show that both WS₂/Co₉S₈ and WS₂/Co₄S₃ heterojunctions have a Gibbs free energy of H reaction (|ΔG_H*|) close to 0.0 eV and a facile decomposition water barrier. As collective synergy of dual Co_xS_y-modified WS₂ double heterojunction, WS₂/Co₉S₈/Co₄S₃ greatly enhances HER activity compared to bare Co₉S₈/Co₄S₃ or single heterojunction (WS₂/Co₉S₈) in all-pH media. Besides, we have elucidated the unique HER mechanism of the double heterojunction to decompose H₂O and confirm its excellent activity under alkaline and neutral conditions. Thus, this work provides new insights into WS₂-based hybrid materials potentially applied to sustainable energy.

Highly Selective Room Temperature Detection of NH3 and NOx Using Oxygen-Deficient W18O49-Supported WS2 Heterojunctions

In this paper, we reported the controlled synthesis of tungsten disulfide/reduced tungsten oxide (WS₂/W₁₈O₄₉) heterojunctions for highly efficient room temperature NO_x and ammonia (NH₃) sensors. X-ray diffraction analysis revealed the formation of the oxygen-deficient W₁₈O₄₉ phase along with WS₂. Field-emission scanning electron microscopy and transmission electron microscopy displayed the formation of WS₂ flakes over W₁₈O₄₉ nanorods. X-ray photoelectron spectroscopy showed the presence of tungsten in W⁴⁺, W⁵⁺, and W⁶⁺ oxidation states corresponding to WS₂ and W₁₈O₄₉, respectively. The WS₂/W₁₈O₄₉ heterojunction sensor exhibited sub-ppm level sensitivity to NO_x and NH₃ at room temperature. The heterojunction sensor detected 0.6 ppm NO_x and 0.5 ppm NH₃, with a corresponding response of 7.1 and 3.8%, respectively. The limit of detection of the sensor was calculated to be 0.05 and 0.17 ppm for NH₃ and NO_x, respectively. The cyclic stability test showed that the sensor exhibited high stability even after 24 cycles for the detection of NH₃ and 14 cycles for NO_x. Compared to pristine WO₃ and WS₂, the WS₂/W₁₈O₄₉ heterojunction showed high selectivity toward NO_x and NH₃. The results could be useful for the development of room temperature NO_x and NH₃ sensors.

Boosting Hydrogen Evolution Reaction Activity of Amorphous Molybdenum Sulfide Under High Currents Via Preferential Electron Filling Induced by Tungsten Doping

The lack of highly efficient, durable, and cost-effective electrocatalysts for the hydrogen evolution reaction (HER) working at high current densities poses a significant challenge for the large-scale implementation of hydrogen production from renewable energy. Herein, amorphous molybdenum tungsten sulfide/nitrogen-doped reduced graphene oxide nanocomposites (a-MoWS_x /N-RGO) are synthesized by plasma treatment for use as high-performance HER catalysts. By adjusting the plasma treatment duration and chemical composition, an optimal a-MoWS_x /N-RGO catalyst is obtained, which exhibits a low overpotential of 348 mV at a current density of 1000 mA cm^-2 and almost no decay after 24 h of working at this current density, outperforming commercial platinum/carbon (Pt/C) and previously reported heteroatom-doped MoS₂ -based catalysts. Based on density functional theory (DFT) calculations, it is found that with a reasonable tungsten doping level, the catalytic active site (2S^{2 -} ) shows excellent catalytic performance working at high current densities because extra electrons preferentially fill at 2S^{2 -} . The introduction of tungsten tends to lower the electronic structure energy, resulting in a closer-to-zero positive Δ G H ∗ $\Delta {G}_{{{\rm{H}}}^{\rm{*}}}$ . Excessive tungsten introduction, however, can lead to structural damage and a worse HER performance under high current densities. The work provides a route towards rationally designing high-performance catalysts for the HER at industrial-level currents using earth-abundant elements.

Atomic-Scale Laminated Structure of O-Doped WS2 and Carbon Layers with Highly Enhanced Ion Transfer for Fast-Charging Lithium-Ion Batteries

WS₂ anode materials show huge potential for fast-charging lithium-ion batteries (LIBs) due to the naturally good 2D diffusion pathways but suffer from large Li⁺ diffusion barrier energy and poor intrinsic electrical conductivity. Here, a defect-rich atomic-scale laminated structure of WS₂ and C (D-WS₂ -C) with O doping and enlarged interlayer distance from 0.62 to 1.06 nm of WS₂ is first fabricated, which is assembled into micron-sized spheres to prepare WS₂ /C composite microspheres. D-WS₂ -C with maximized molecular layer contact area between WS₂ and carbon and large interlayer spacing greatly enhances the electrical conductivity of WS₂ and reduces Li-ion diffusion energy barrier, confirmed by density functional theory calculations. Besides, the unique D-WS₂ -C enables the formation of vast superfine W nanoparticles (1-2 nm) during the conversation reaction, resulting in the construction of a space charge zone on W surface. Based on these characteristics of D-WS₂ -C, the prepared WS₂ /C composite microspheres show superior fast-charging capability with a high capacity of 647.8 mAh g^-1 at 20 C in half cells. For full cells, a high-energy density of 100.9 Wh kg^-1 is achieved at a charge time of only 8.5 min at 5 C, representing the best fast-charging performances in WS₂ -based anode materials to date.

Screening of transition metal single-atom catalysts supported by a WS2 monolayer for electrocatalytic nitrogen reduction reaction: insights from activity trend and descriptor

The electrocatalytic nitrogen reduction reaction (NRR), as an alternative green technology to the Haber-Bosch process, can efficiently synthesize ammonia under ambient conditions and has a reduced carbon footprint. Here we systematically investigate the NRR activity and selectivity of transition metal (TM) single-atom catalyst (SAC) anchored WS₂ monolayers (TM@WS₂) by means of first-principles calculations and microkinetic modeling. The construction of the reaction activity trend and the identification of an activity descriptor, namely *N₂H adsorption energy, facilitate the efficient screening and rational design of SACs with high activity. Manipulating the adsorption strength of the pivotal *N₂H intermediate is a potential strategy for enhancing NRR activity. Utilizing the limiting potential difference of NRR and the hydrogen evolution reaction (HER) as a selectivity descriptor, we screen three SACs with excellent activity and selectivity toward NRR, i.e., Re@WS₂, Os@WS₂ and Ir@WS₂ with favorable limiting potentials of -0.44 V, -0.38 V and -0.69 V. By using the explicit H₉O₄⁺ model, the kinetic barriers of the rate-determining steps (0.47 eV-1.15 eV) of the solvated proton transfer on the screened SACs are found to be moderate, indicative of a kinetically feasible process. Microkinetic modeling shows that the turnover frequencies of N₂ reduction to NH₃ on Re@WS₂, Os@WS₂ and Ir@WS₂ are 1.52 × 10⁵, 8.21 × 10² and 4.17 × 10^-4 per s per site at 400 K, achieving fast reaction rates. The coexistence of empty and occupied 5d orbitals of candidate SACs is beneficial for σ donation and π* backdonation, endowing them with extraordinary N₂ adsorption and activation. Moreover, the screened SACs possess good dispersity and thermodynamic stability. Our work provides a promising solution for the efficient screening and rational design of high-performance electrocatalysts toward the NRR.

Recent Developments of Cathode Materials for Thermal Batteries

Big progress has been made in batteries based on an intercalation mechanism in the last 20 years, but limited capacity in batteries hinders their further increase in energy density. The demand for more energy intensity makes research communities turn to conversion-type batteries. Thermal batteries are a special kind of conversion-type battery, which are thermally activated primary batteries composed mainly of cathode, anode, separator (electrolyte), and heating mass. Such kinds of battery employ an internal pyrotechnic source to make the battery stack reach its operating temperature. Thermal batteries have a long history of research and usage in military fields because of their high specific capacity, high specific energy, high thermal stability, long shelf life, and fast activation. These experiences and knowledge are of vital importance for the development of conversion-type batteries. This review provides a comprehensive account of recent studies on cathode materials. The paper covers the preparation, characterization of various cathode materials, and the performance test of thermal batteries. These advances have significant implications for the development of high-performance, low-cost, and mass production conversion-type batteries in the near future.

Liquid metals: an ideal platform for the synthesis of two-dimensional materials

The surfaces of liquid metals can serve as a platform to synthesise two-dimensional materials. By exploiting the self-limiting Cabrera-Mott oxidation reaction that takes place at the surface of liquid metals exposed to ambient air, an ultrathin oxide layer can be synthesised and isolated. Several synthesis approaches based on this phenomenon have been developed in recent years, resulting in a diverse family of functional 2D materials that covers a significant fraction of the periodic table. These straightforward and inherently scalable techniques may enable the fabrication of novel devices and thus harbour significant application potential. This review provides a brief introduction to liquid metals and their alloys, followed by detailed guidance on each developed synthesis technique, post-growth processing methods, integration processes, as well as potential applications of the developed materials.

[NiFe]-(Oxy)Sulfides Derived from NiFe2O4 for the Alkaline Hydrogen Evolution Reaction

The development of noble-metal-free electrocatalysts is regarded as a key factor for realizing industrial-scale hydrogen production powered by renewable energy sources. Inspired by nature, which uses Fe- and Ni-containing enzymes for efficient hydrogen generation, Fe/Ni-containing chalcogenides, such as oxides and sulfides, received increasing attention as promising electrocatalysts to produce hydrogen. We herein present a novel synthetic procedure for mixed Fe/Ni (oxy)sulfide materials by the controlled (partial) sulfidation of NiFe2O4 (NFO) nanoparticles in H2S-containing atmospheres. The variation in H2S concentration and the temperature allows for a precise control of stoichiometry and phase composition. The obtained sulfidized materials (NFS) catalyze the hydrogen evolution reaction (HER) with increased activity in comparison to NFO, up to −10 and −100 mA cm−2 at an overpotential of approx. 250 and 450 mV, respectively.

Surface Functionalization of WS2 Nanosheets with Alkyl Chains for Enhancement of Dispersion Stability and Tribological Properties

Tungsten disulfide (WS₂) exhibits intriguing tribological properties and has been explored as an excellent lubricious material in thin-film and solid lubricants. However, the poor dispersibility of WS₂ has been a major challenge for its utilization in liquid lubricant applications. Herein, a top-down integrated approach is presented to synthesize oxygenated WS₂ (WS₂-O) nanosheets via strong acid-mediated oxidation and ultrasound-assisted exfoliation. The ultrathin sheets of WS₂-O, comprising 4-7 molecular lamellae, exhibit oxygen/hydroxyl functionalities. The organosilanes having variable surface-active leaving groups (chloro and ethoxy) are covalently grafted, targeting the hydroxyl/oxygen functionalities on the surface of WS₂-O nanosheets. The grafting of organosilanes is governed by the reactivity of chloro and ethoxy leaving groups. The DFT calculations further support the covalent interaction between the WS₂-O nanosheets and organosilanes. The alkyl chain-functionalized WS₂-O nanosheets displayed excellent dispersibility in mineral lube base oil. A minute dose of chemically functionalized-WS₂ (0.2 mg.mL^-1) notably enhanced the tribological properties of mineral lube oil by reducing the friction coefficient (52%) and wear volume (79%) for a steel tribopair. Raman analysis of worn surfaces revealed WS₂-derived lubricious thin film formation. The improved tribological properties are attributed to ultralow thickness, stable dispersion, and low shear strength of chemically functionalized WS₂ nanosheets, along with protective thin film formation over the contact interfaces of a steel tribopair. The present work opens a new avenue toward exploiting low-dimensional nanosheets for minimizing energy losses due to high friction.

A cobalt-doped WS2/WO3 nanocomposite electrocatalyst for the hydrogen evolution reaction in acidic and alkaline media

A simple preparation method of doped nanoflowers produces improved catalytic activity due to the complex structure of the phases and compositions.

Synthesis, structure, and magnetic properties of the quaternary oxysulfides Ln 5V3O7S6 (Ln = La, Ce)

Two rare earth oxysulfides Ln 5V3O7S6 (Ln = La, Ce) have been synthesized and their structures determined. The two isostructural compounds crystallize in the orthorhombic space group Pmmn (no. 59). The structure features one-dimensional edge-sharing VS4O2 octahedron chains parallel to the b axis. The bonding between V and S/O is covalent, and between Ln 3+ and the rest of the matrix ionic. Magnetic susceptibility measurement revealed that V is in a mixed valence state of V3+ and V4+. Its magnetic behavior follows the Curie-Weiss law.

Revisiting trends in the exchange current for hydrogen evolution

Nørskov and collaborators proposed a simple kinetic model to explain the volcano relation for hydrogen evolution reaction. Our new model decreases the discrepancy between calculated and experimental exchange current density values.

Addressable surface engineering for N-doped WS2 nanosheet arrays with abundant active sites and the optimal local electronic structure for enhanced hydrogen evolution reaction

The precise control over the geometric and electronic structures of active materials on flexible substrates is of great importance to address the current challenges in optimizing and developing high-performance flexible devices for energy conversion and storage. In this work, an addressable surface was demonstrated to engineer structurally controllable active nanomaterials for electrocatalytic hydrogen evolution. The nanostructures of WS2/MOF/metal hydroxide/oxide with different formation energy barriers electrodes could be tuned by modifying the ratio of O/C and the concentration of carbon defects at the surface of carbon cloth. The morphological structure of the vertical WS2 nanosheets that are favorable to electrocatalysis was found to be highly related to the addressable surface of carbon cloth though heterogeneous nucleation and the interactions between the monomers and surface functional groups. Moreover, the electronic structure of WS2 was further modified with N doping (N-WS2) to deliver an addressable surface for the reaction species involved in the electrocatalytic hydrogen evolution reaction (HER), and the resultant N-WS2 exhibited enhanced HER activity compared with the original WS2. The systematic experimental research and electronic-structure density functional theory (DFT) calculations demonstrated the interesting features of the N dopant: (i) the strong hybridization of the p orbital of dopant N with d orbital of W and p orbital of S atoms (W d-S p-N p hybridization) close to the Fermi level can disperse the conducting charges, thus leading to an improved conductivity across the basal plane of WS2 nanosheets; (ii) the local electron transfer from W to N atoms provides the local charge, thus promoting the H adsorption process in the HER for N-WS2. Our research can be expected to offer new perspectives in the precise construction of highly reactive nanostructures toward high-efficiency and highly stable flexible devices for energy conversion and storage.