Electrocatalysis on Edge-Rich Spiral WS2 for Hydrogen Evolution

Enhancing hydrogen evolution reaction activity through defects and strain engineering in monolayer MoS2

Molybdenum disulfide (MoS2) has recently emerged as a promising electrocatalyst for the hydrogen evolution reaction (HER). However, the poor in-plane electrical conductivity and inert basal plane activity pose major challenges in realizing its practical application. Herein, we demonstrate a new approach to induce biaxial strain into CVD-grown MoS2 monolayers by draping it over an array of patterned gold nanopillar arrays (AuNAs) as an efficient strategy to enhance its HER activity. We vary the magnitude of applied strain by changing the inter-pillar spacing, and its effect on the HER activity is investigated. To capitalize on the synergistic effect of improved ΔG H via strain engineering and leverage basal plane activation by introduction of sulphur vacancies, we further exposed the strained MoS2 monolayers to oxygen plasma treatment to create S-vacancies. The strained MoS2 on AuNAs with optimal inter-pillar spacing is exposed to oxygen plasma treatment for different durations, and we study its electrocatalytic activity towards the HER using on-chip microcell devices. The strained and vacancy-rich monolayer MoS2 draped on AuNAs with a 0.5 μm inter-pillar spacing and exposed to plasma for 50 s (S0.5μmV50-MoS2) is shown to exhibit remarkable improvement in HER activity, with an overpotential of 53 mV in 0.5 M H2SO4. Thus, the synergistic creation of additional vacancy defects, along with strain-induced active sites, results in enhancement in HER performance of CVD-grown monolayer MoS2. The present study provides a highly promising route for engineering 2D electrocatalysts towards efficient hydrogen evolution.

Insights into the pH effect on hydrogen electrocatalysis

Hydrogen electrocatalytic reactions, including the hydrogen evolution reaction (HER) and the hydrogen oxidation reaction (HOR), play a crucial role in a wide range of energy conversion and storage technologies. However, the HER and HOR display anomalous non-Nernstian pH dependent kinetics, showing two to three orders of magnitude sluggish kinetics in alkaline media compared to that in acidic media. Fundamental understanding of the origins of the intrinsic pH effect has attracted substantial interest from the electrocatalysis community. More critically, a fundamental molecular level understanding of this effect is still debatable, but is essential for developing active, stable, and affordable fuel cells and water electrolysis technologies. Against this backdrop, in this review, we provide a comprehensive overview of the intrinsic pH effect on hydrogen electrocatalysis, covering the experimental observations, underlying principles, and strategies for catalyst design. We discuss the strengths and shortcomings of various activity descriptors, including hydrogen binding energy (HBE) theory, bifunctional theory, potential of zero free charge (pzfc) theory, 2B theory and other theories, across different electrolytes and catalyst surfaces, and outline their interrelations where possible. Additionally, we highlight the design principles and research progress in improving the alkaline HER/HOR kinetics by catalyst design and electrolyte optimization employing the aforementioned theories. Finally, the remaining controversies about the pH effects on HER/HOR kinetics as well as the challenges and possible research directions in this field are also put forward. This review aims to provide researchers with a comprehensive understanding of the intrinsic pH effect and inspire the development of more cost-effective and durable alkaline water electrolyzers (AWEs) and anion exchange membrane fuel cells (AMFCs) for a sustainable energy future.

Flexible tungsten disulfide superstructure engineering for efficient alkaline hydrogen evolution in anion exchange membrane water electrolysers

Anion exchange membrane (AEM) water electrolysis employing non-precious metal electrocatalysts is a promising strategy for achieving sustainable hydrogen production. However, it still suffers from many challenges, including sluggish alkaline hydrogen evolution reaction (HER) kinetics, insufficient activity and limited lifetime of non-precious metal electrocatalysts for ampere-level-current-density alkaline HER. Here, we report an efficient alkaline HER strategy at industrial-level current density wherein a flexible WS2 superstructure is designed to serve as the cathode catalyst for AEM water electrolysis. The superstructure features bond-free van der Waals interaction among the low Young's modulus nanosheets to ensure excellent mechanical flexibility, as well as a stepped edge defect structure of nanosheets to realize high catalytic activity and a favorable reaction interface micro-environment. The unique flexible WS2 superstructure can effectively withstand the impact of high-density gas-liquid exchanges and facilitate mass transfer, endowing excellent long-term durability under industrial-scale current density. An AEM electrolyser containing this catalyst at the cathode exhibits a cell voltage of 1.70 V to deliver a constant catalytic current density of 1 A cm-2 over 1000 h with a negligible decay rate of 9.67 μV h-1.

Constructing WS2/WO3-x heterostructured electrocatalyst enriched with oxygen vacancies for accelerated hydrogen evolution reaction

H2 produced through hydrogen evolution reaction (HER) is a shining star in the field of clean energy. Significant efforts have been dedicated to develop efficient and stable electrocatalysts to reduce the energy barrier and accelerate the kinetics of Hydrogen evolution reaction (HER) under various environments. Herein, we propose a strategy to accelerate the kinetics of HER under acid and alkaline environments by combining heterostructure engineering with defect engineering. We have successfully synthesized a series of WS2/WO3-x heterostructured catalysts, accompanied with substantial oxygen vacancies using a two-step synthesis method. With the partially sulfurization of WO3-x, the heterojunction interface of WS2 and WO3-x was formed along with the appearance of oxygen vacancies, which can facilitate the migration of electrons. The heterostructured catalyst enriched with oxygen vacancies (defined as WS2/WO3-x-2) demonstrates superior HER performance in acidic and alkaline electrolytes. At a current density of 10 mA cm-2, the WS2/WO3-x-2 heterostructured catalyst manifests an overpotential of 120 mV in the acidic electrolytes and a slightly higher overpotential of 150 mV in an alkaline environment. The overpotentials offer an improvement compared to reported W-based catalysts in terms of HER performance. This work provides guiding significance on the design of heterostructured catalysts with promising performance for HER in acidic and alkaline environments.

Interlayer Biatomic Pair Bridging the van der Waals Gap for 100% Activation of 2D Layered Material

2D layered materials are regarded as prospective catalyst candidates due to their advantageous atomic exposure ratio. Nevertheless, the predominant population of atoms residing on the basal plane with saturated coordination, exhibit inert behavior, while a mere fraction of atoms located at the periphery display reactivity. Here, a novel approach is reported to attain complete atom activation in 2D layered materials through the construction of an interlayer biatomic pair bridge. The atoms in question have been strategically optimized to achieve a highly favorable state for the adsorption of intermediates. This optimization results from the introduction of new gap states around the Fermi level. Moreover, the presence of the interlayer bridge facilitates the electron transfer across the van der Waals gap, thereby enhancing the reaction kinetics. The hydrogen evolution reaction exhibits an impressive ultrahigh current density of 2000 mA cm-2 at 397 mV, surpassing the pristine MoS2 by approximately two orders of magnitude (26 mA cm-2 at 397 mV). This study provides new insights for enhancing the efficacy of 2D layered catalysts.

Computational screening of layered metal chalcogenide materials for HER electrocatalysts, and its synergy with experiments

Layered materials have emerged as attractive candidates in our search for abundant, inexpensive and efficient hydrogen evolution reaction (HER) catalysts, due to larger specific area these offer. Among these, transition metal dichalcogenides have been studied extensively, while ternary transition metal tri-chalcogenides have emerged as promising candidates recently. Computational screening has emerged as a powerful tool to identify the promising materials out of an initial set for specific applications, and has been employed for identifying HER catalysts also. This article presents a comprehensive review of how computational screening studies based on density functional calculations have successfully identified the promising materials among the layered transition metal di- and tri-chalcogenides. Synergy of these computational studies with experiments is also reviewed. It is argued that experimental verification of the materials, predicted to be efficient catalysts but not yet tested, will enlarge the list of materials that hold promise to replace expensive platinum, and will help ushering in the much awaited hydrogen economy.

Copious Dislocations Defect in Amorphous/Crystalline/Amorphous Sandwiched Structure P-NiMoO4 Electrocatalyst toward Enhanced Hydrogen Evolution Reaction

The design and synthesis of efficient, inexpensive, and long-term stable heterostructured electrocatalysts with high-density dislocations for hydrogen evolution reaction in alkaline media and seawater are still a great challenge. An amorphous/crystalline/amorphous sandwiched structure with abundant dislocations were synthesized through thermal phosphidation strategies. The dislocations play an important role in the hydrogen evolution reactions. Copious dislocation defects, combined with cracks, and the synergistic interfacial effect between crystalline phase and amorphous phase regulate the electronic structure of electrocatalyst, provide more active sites, and thus endow the electrocatalysts with excellent catalytic activity under alkaline water and seawater. The overpotentials of P-NiMoO4 at 10 mA/cm2 in 1 M KOH aqueous solution and seawater are 45 and 75 mV, respectively. Additionally, the P-NiMoO4 electrocatalyst exhibits long-term stability over 100 h. This study provides a simple approach for synthesizing amorphous/crystalline/amorphous sandwiched non-noble-metal electrocatalysts with abundant dislocations for hydrogen evolution reaction.

On-chip electrocatalytic microdevices

On-chip electrocatalytic microdevices (OCEMs) are an emerging electrochemical platform specialized for investigating nanocatalysts at the microscopic level. The OCEM platform allows high-precision electrochemical measurements at the individual nanomaterial level and, more importantly, offers unique perspectives inaccessible with conventional electrochemical methods. This protocol describes the critical concepts, experimental standardization, operational principles and data analysis of OCEMs. Specifically, standard protocols for the measurement of the electrocatalytic hydrogen evolution reaction of individual 2D nanosheets are introduced with data validation, interpretation and benchmarking. A series of factors (e.g., the exposed area of material, the choice of passivation layer and current leakage) that could have effects on the accuracy and reliability of measurement are discussed. In addition, as an example of the high adaptability of OCEMs, the protocol for in situ electrical transport measurement is detailed. We believe that this protocol will promote the general adoption of the OCEM platform and inspire further development in the near future. This protocol requires essential knowledge in chemical synthesis, device fabrication and electrochemistry.

Metallic Metastable Hybrid 1T'/1T Phase Triggered Co,PSnS2 Nanosheets for High Efficiency Trifunctional Electrocatalyst

The development of trifunctional electrocatalyst for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) with deeply understanding the mechanism to enhance the electrochemical performance is still a challenging task. In this work, the distorted metastable hybrid-phase induced 1T'/1T Co,PSnS₂ nanosheets on carbon cloth (1T'/1T Co,PSnS₂ @CC) is prepared and examined. The density functional theoretical (DFT) calculation suggests that the distorted 1T'/1T Co,PSnS₂ can provide excellent conductivity and strong hydrogen adsorption ability. The electronic structure tuning and enhancement mechanism of electrochemical performance are investigated and discussed. The optimal 1T'/1T Co,PSnS₂ @CC catalyst exhibits low overpotential of ≈94 and 219.7 mV at 10 mA cm^-2 for HER and OER, respectively. Remarkably, the catalyst exhibits exceptional ORR activity with small onset potential value (≈0.94 V) and half-wave potential (≈0.87 V). Most significantly, the 1T'/1T Co,PSnS₂ ||Co,PSnS₂ electrolyzer required small cell voltages of ≈1.53, 1.70, and 1.82 V at 10, 100, and 400 mA cm^-2 , respectively, which are better than those of state-of-the-art Pt-C||RuO₂ (≈1.56 and 1.84 V at 10 and 100 mA cm^-2 ). The present study suggests a new approach for the preparation of large-scalable, high performance hierarchical 3D next-generation trifunctional electrocatalysts.

Etching dopant elements to construct active-site-rich Mo2C for the hydrogen evolution reaction

We propose a strategy to etch dopants to construct Mo₂C with more unsaturated coordination of Mo atoms and lattice distortion for enhanced catalytic activity. It is more effective than doping and etching pure Mo₂C and provides a novel strategy for the preparation of catalysts with high catalytic activity.

Recent Advances in Defect-Engineered Transition Metal Dichalcogenides for Enhanced Electrocatalytic Hydrogen Evolution: Perfecting Imperfections

Switching to renewable, carbon-neutral sources of energy is urgent and critical for climate change mitigation. Despite how hydrogen production by electrolyzing water can enable renewable energy storage, current technologies unfortunately require rare and expensive platinum group metal electrocatalysts, which limit their economic viability. Transition metal dichalcogenides (TMDs) are low-cost, earth-abundant materials that possess the potential to replace platinum as the hydrogen evolution catalyst for water electrolysis, but so far, pristine TMDs are plagued by poor catalytic performances. Defect engineering is an attractive approach to enhance the catalytic efficiency of TMDs and is not subjected to the limitations of other approaches like phase engineering and surface structure engineering. In this minireview, we discuss the recent progress made in defect-engineered TMDs as efficient, robust, and low-cost catalysts for water splitting. The roles of chalcogen atomic defects in engineering TMDs for improvements to the hydrogen evolution reaction (HER) are summarized. Finally, we highlight our perspectives on the challenges and opportunities of defect engineering in TMDs for electrocatalytic water splitting. We hope to provide inspirations for designing the state-of-the-art catalysts for future breakthroughs in the electrocatalytic HER.

PVD growth of spiral pyramid-shaped WS2 on SiO2/Si driven by screw dislocations

Atomically thin layered transition metal dichalcogenides (TMDs), such as MoS₂ and WS₂, have been getting much attention recently due to their interesting electronic and optoelectronic properties. Especially, spiral TMDs provide a variety of candidates for examining the light-matter interaction resulting from the broken inversion symmetry, as well as the potential new utilization in functional optoelectronic, electromagnetic and nanoelectronics devices. To realize their potential device applications, it is desirable to achieve controlled growth of these layered nanomaterials with a tunable stacking. Here, we demonstrate the Physical Vapor Deposition (PVD) growth of spiral pyramid-shaped WS₂ with ∼200  μ m in size and the interesting optical properties via AFM and Raman spectroscopy. By controlling the precursors concentration and changing the initial nucleation rates in PVD growth, WS₂ in different nanoarchitectures can be obtained. We discuss the growth mechanism for these spiral-patterned WS₂ nanostructures based on the screw dislocations. This study provides a simple, scalable approach of screw dislocation-driven (SDD) growth of distinct TMD nanostructures with varying morphologies, and stacking.

Spatially Dependent Electronic Structures and Excitons in a Marginally Twisted Moiré Superlattice of Spiral WS2

Twisted two-dimensional transition metal dichalcogenide (TMD) moiré superlattices provide an additional degree of freedom to engineer electronic and optical properties. Nevertheless, controllable synthesis of marginally twisted homo TMD moiré superlattices is still a challenge. Here, physical vapor deposition grown spiral WS₂ nanosheets are demonstrated to be a marginally twisted moiré superlattice using scanning tunneling microscopy and spectroscopy. Periodic moiré superlattices are found on the third layer (3L) and 4L of the spiral WS₂ nanosheet owing to the marginally twisted alignment between two neighboring layers, resulting in a highly localized flat band near the valence band maximum. Their bandgap depends on atomic stacking configurations, which gives a good interpretation for split moiré excitons using photoluminescence at 77 K. This work can benefit the development of twisted homo TMD moiré superlattices and could promote the profound research of twisted TMDs in the prospective field, such as strongly correlated physics and twistronics.

A colloidal route to semiconducting tungsten disulfide nanosheets with monolayer thickness

Transition metal dichalcogenides (TMDs) are a class of materials that have been extensively studied in the last decade, with molybdenum disulfide (MoS₂) being the main protagonist. Typically, the interesting TMD properties, e.g. a direct band gap transition, or broken inversion symmetry, are only present in monolayer thick TMDs, and in the absence of strong lateral confinement, we require different materials or alloys thereof when we want to obtain TMDs with varying (direct) band gap energies. With this in mind, tungsten disulfide (WS₂) is emerging as a direct competitor of MoS₂ due to its similar properties but larger band gap energy. While several colloidal strategies have been reported for the synthesis of WS₂, the synthesis of monolayer WS₂ and detailed studies on the effect of synthesis parameters on the synthesis outcome have remained elusive. In this work we therefore focused on a colloidal synthesis method for monolayer WS₂ using a design of experiment (DOE) approach. After optimization, we obtained nanosheets with a band gap transition consistent with the expected value for a monolayer. The thickness was further confirmed by Raman spectroscopy. While we could identify two temperature ranges where we could obtain a monolayer, sample characterization by XPS spectroscopy revealed the presence of different ratios of the metallic phase, with the sample synthesized at lower temperature displaying a lower concentration of the metallic phase.

Recent advances in metallic transition metal dichalcogenides as electrocatalysts for hydrogen evolution reaction

Layered metallic transition metal dichalcogenides (MTMDs) exhibit distinctive electrical and catalytic properties to drive basal plane activity, and, therefore, they have emerged as promising alternative electrocatalysts for sustainable hydrogen evolution reactions (HERs). A key challenge for realizing MTMDs-based electrocatalysts is the controllable and scalable synthesis of high-quality MTMDs and the development of engineering strategies that allow tuning their electronic structures. However, the lack of a method for the direct synthesis of MTMDs retaining the structural stability limits optimizing the structural design for the next generation of robust electrocatalysts. In this review, we highlight recent advances in the synthesis of MTMDs comprising groups VB and VIB and various routes for structural engineering to enhance the HER catalytic performance. Furthermore, we provide insight into the potential future directions and the development of MTMDs with high durability as electrocatalysts to generate green hydrogen through water-splitting technology.

Symmetric domain segmentation in WS2flakes: correlating spatially resolved photoluminescence, conductance with valley polarization

The incidence of intra-flake heterogeneity of spectroscopic and electrical properties in chemical vapour deposited (CVD) WS₂flakes is explored in a multi-physics investigation via spatially resolved spectroscopic maps correlated with electrical, electronic and mechanical properties. The investigation demonstrates that the three-fold symmetric segregation of spectroscopic response, in topographically uniform WS₂flakes are accompanied by commensurate segmentation of electronic properties e.g. local carrier density and the differences in the mechanics of tip-sample interactions, evidenced via scanning probe microscopy phase maps. Overall, the differences are understood to originate from point defects, namely sulfur vacancies within the flake along with a dominant role played by the substrate. While evolution of the multi-physics maps upon sulfur annealing elucidates the role played by sulfur vacancy, substrate-induced effects are investigated by contrasting data from WS₂flake on Si and Au surfaces. Local charge depletion induced by the nature of the sample-substrate junction in case of WS₂on Au is seen to invert the electrical response with comprehensible effects on their spectroscopic properties. Finally, the role of these optoelectronic properties in preserving valley polarization that affects valleytronic applications in WS₂flakes, is investigated via circular polarization discriminated photoluminescence experiments. The study provides a thorough understanding of spatial heterogeneity in optoelectronic properties of WS₂and other transition metal chalcogenides, which are critical for device fabrication and potential applications.

Tailoring the Vertical and Planar Growth of 2D WS2 Thin Films Using Pulsed Laser Deposition for Enhanced Gas Sensing Properties

In this study, pulsed laser deposition has been utilized for the controllable synthesis of WS₂ thin films with growth orientation ranging from vertically to horizontally aligned layers, and the effect of growth parameters has been investigated. The growth of thin films on SiO₂ substrates at three different pressures (30, 50, and 70 mTorr) and three different temperatures (400, 500, and 600 °C) has been studied. Detailed characterizations carried out on the as-grown layers clearly show the formation of the 2H-WS₂ phase and its morphological evolution with deposition conditions. Atomic force microscopy and cross-sectional transmission electron microscopy have been used to deduce the growth mechanism of the vertical and planar films with different deposition parameters. The samples grown with a combination of lower temperatures and higher pressures exhibit a vertical flake-like growth with a flake thickness of ∼2-5 nm. However, at higher temperatures and lower pressures, the film growth is observed to be rather planar. The gas sensing parameters and the underlying mechanism have been observed to be quite different for vertically and horizontally grown layers. The vertical layers showed a selective response toward NO₂ gas at room temperature (RT) with a limit of detection less than 50 ppb. In comparison, a very subdued and poor gas sensing response was recorded for the planar film at RT. A large specific area and abundance of active edge sites along with the flat basal plane present in the vertically grown layers seem to be responsible for efficient gas sensing toward NO₂.

Unveiling the Electrocatalytic Activity of 1T'-MoSe2 on Lithium-Polysulfide Conversion Reactions

The dissolution of intermediate lithium polysulfides (LiPS) into an electrolyte and their shuttling between the electrodes have been the primary bottlenecks for the commercialization of high-energy density lithium-sulfur (Li-S) batteries. While several two-dimensional (2D) materials have been deployed in recent years to mitigate these issues, their activity is strictly restricted to their edge-plane-based active sites. Herein, for the first time, we have explored a phase transformation phenomenon in a 2D material to enhance the number of active sites and electrocatalytic activity toward LiPS redox reactions. Detailed theoretical calculations demonstrate that phase transformation from the 2H to 1T' phase in a MoSe₂ material activates the basal planes that allow for LiPS adsorption. The corresponding transformation mechanism and LiPS adsorption capabilities of the as-formed 1T'-MoSe₂ were elucidated experimentally using microscopic and spectroscopic techniques. Further, the electrochemical evaluation of phase-transformed MoSe₂ revealed its strong electrocatalytic activity toward LiPS reduction and their oxidation reactions. The 1T'-MoSe₂-based cathode hosts for sulfur later provide a superior cycling performance of over 250 cycles with a capacity loss of only 0.15% per cycle along with an excellent Coulombic efficiency of 99.6%.

Metallic phase WSe2 nanoscrolls for the hydrogen evolution reaction

Nanostructured metastable metallic phase transition-metal dichalcogenides (TMDs) have attracted tremendous attention due to their promising practical applications in the hydrogen evolution reaction (HER).

Dual-Regulation of Defect Sites and Vertical Conduction by Spiral Domain for Electrocatalytic Hydrogen Evolution

Insufficient active sites and weak vertical conduction are the intrinsic factors that restrict the electrocatalytic HER for transition-metal dichalcogenides. As a prototype, we proposed a model of spiral MoTe₂ to optimize collectively the above issues. The conductive atomic force microscopy of an individual spiral reveals that the retentive vertical conduction irrespective of layer thickness benefits from the connected screw dislocation lines between interlayers. Theoretical calculations uncover that the regions near the edge step of the spiral structures more easily form Te vacancies and have lower ΔG_H * as extra active sites. A single spiral MoTe₂ -based on-chip microcell was fabricated to extract HER activity and achieved an ultrahigh current density of 3000 mA cm^-2 at an overpotential of 0.4 V, which is about two orders of magnitude higher than the exfoliated counterpart. Profoundly, this unusual spiral model will initiate a new pathway for triggering other inert catalytic reactions.

Finding the catalytically active sites on the layered tri-chalcogenide compounds CoPS3 and NiPS3 for hydrogen evolution reaction

Electronic structure calculations based on density functional theory are used to identify the catalytically active sites for the hydrogen evolution reaction on single layers of the two transition metal tri-chalcogenide compounds CoPS₃ and NiPS₃. Some of the under-coordinated P and S atoms at the edges are found to act as the active sites, the details of which depend on the coverage of H on the electrode. Overpotentials along the two possible pathways for HER are also estimated for the two materials. These findings not only resolve an apparent discrepancy between published experimental results and our earlier calculations, but also provide insights which can be used to enhance catalytic efficiency of these materials further.

Recent Advances on Transition Metal Dichalcogenides for Electrochemical Energy Conversion

Transition metal dichalcogenides (TMDCs) hold great promise for electrochemical energy conversion technologies in view of their unique structural features associated with the layered structure and ultrathin thickness. Because the inert basal plane accounts for the majority of a TMDC's bulk, activation of the basal plane sites is necessary to fully exploit the intrinsic potential of TMDCs. Here, recent advances on TMDCs-based hybrids/composites with greatly enhanced electrochemical activity are reviewed. After a summary of the synthesis of TMDCs with different sizes and morphologies, comprehensive in-plane activation strategies are described in detail, mainly including in-plane-modification-induced phase transformation, surface-layer modulation, and interlayer modification/coupling. Simultaneously, the underlying mechanisms for improved electrochemical activities are highlighted. Finally, the strategic evaluation on further research directions of TMDCs in-plane activation is featured. This work would shed some light on future design trends of TMDCs-based functional materials for electrochemical energy-related applications.

Defect Engineering of Two-Dimensional Transition-Metal Dichalcogenides: Applications, Challenges, and Opportunities

Atomic defects, being the most prevalent zero-dimensional topological defects, are ubiquitous in a wide range of 2D transition-metal dichalcogenides (TMDs). They could be intrinsic, formed during the initial sample growth, or created by postprocessing. Despite the majority of TMDs being largely unaffected after losing chalcogen atoms in the outermost layer, a spectrum of properties, including optical, electrical, and chemical properties, can be significantly modulated, and potentially invoke applicable functionalities utilized in many applications. Hence, controlling chalcogen atomic defects provides an alternative avenue for engineering a wide range of physical and chemical properties of 2D TMDs. In this article, we review recent progress on the role of chalcogen atomic defects in engineering 2D TMDs, with a particular focus on device performance improvements. Various approaches for creating chalcogen atomic defects including nonstoichiometric synthesis and postgrowth treatment, together with their characterization and interpretation are systematically overviewed. The tailoring of optical, electrical, and magnetic properties, along with the device performance enhancement in electronic, optoelectronic, chemical sensing, biomedical, and catalytic activity are discussed in detail. Postformation dynamic evolution and repair of chalcogen atomic defects are also introduced. Finally, we offer our perspective on the challenges and opportunities in this field.

Defect engineering and characterization of active sites for efficient electrocatalysis

Electrocatalysis plays a decisive role in various energy-related applications. Engineering the active sites of electrocatalysts is an important aspect to promote their catalytic performance. In particular, defect engineering provides a feasible and efficient approach to improve the intrinsic activities and increase the number of active sites in electrocatalysts. In this review, recent investigations on defect engineering of a wide range of electrocatalysts such as metal-free carbon materials, transition metal oxides, transition metal dichalcogenides and metal-organic frameworks (MOFs) will be summarized. Different defect creation strategies will be outlined, for example, heteroatom doping and removal, plasma irradiation, hydrogenation, amorphization, phase transition and reduction treatment. In addition, we will overview the commonly used advanced characterization techniques that could confirm the existence and identify the detailed structures, types and concentration of defects in electrocatalysts. The defect characterization tools are beneficial for gaining an in-depth understanding of defects on electrocatalysis and thus could reveal the structure-performance relationship. Finally, the major challenges and future development directions on defect engineering of electrocatalysts will be discussed.

Manipulating Interfaces of Electrocatalysts Down to Atomic Scales: Fundamentals, Strategies, and Electrocatalytic Applications

Raising electrocatalysis by rationally devising catalysts plays a core role in almost all renewable energy conversion and storage systems. The principal catalytic properties can be controlled and improved well by manipulation of interfaces, ascribed to the interactions among different components/players at the interfaces. In particular, manipulating interfaces down to atomic scales is becoming increasingly attractive, not only because those atoms at around the interface are the key players during electrocatalysis, but also, understandings on the atomic level electrocatalysis allow one to gain deep insights into the reaction mechanism. With the feature down-sizing to atomic scales, there is a timely need to redefine the interfaces, as some of them have gone beyond the conventionally perceived interfacial concept. In this overview, the key active players participating in the interfacial manipulation of electrocatalysts are examined, from a new angle of "atomic interface," including those individual atoms, defects, and their interactions, together with the essential characterization techniques for them. The specific approaches and pathways to engineer better atomic interfaces are investigated, and thus to enable the unique electrocatalysis for targeted applications. Looking beyond recent progress, the challenges and prospects of the atomic level interfacial engineering are also briefly visited.

Single-Atom and Dual-Atom Electrocatalysts Derived from Metal Organic Frameworks: Current Progress and Perspectives

Single-atom catalysts (SACs) have attracted increasing research interests owing to their unique electronic structures, quantum size effects and maximum utilization rate of atoms. Metal organic frameworks (MOFs) are good candidates to prepare SACs owing to the atomically dispersed metal nodes in MOFs and abundant N and C species to stabilize the single atoms. In addition, the distance of adjacent metal atoms can be turned by adjusting the size of ligands and adding volatile metal centers to promote the formation of isolated metal atoms. Moreover, the diverse metal centers in MOFs can promote the preparation of dual-atom catalysts (DACs) to improve the metal loading and optimize the electronic structures of the catalysts. The applications of MOFs derived SACs and DACs for electrocatalysis, including oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, carbon dioxide reduction reaction and nitrogen reduction reaction are systematically summarized in this Review. The corresponding synthesis strategies, atomic structures and electrocatalytic performances of the catalysts are discussed to provide a deep understanding of MOFs-based atomic electrocatalysts. The catalytic mechanisms of the catalysts are presented, and the crucial challenges and perspectives are proposed to promote further design and applications of atomic electrocatalysts.

Ion-biosorption induced core–shell Fe2P@carbon nanoparticles decorated on N, P co-doped carbon materials for the oxygen evolution reaction

This work employs bacteria as precursors and induces a cost-effective biosorption strategy to obtain Fe₂P@carbon nanoparticles decorated on N and P co-doped carbon (Fe₂P@CNPs/NPC) materials.

Scalable and low-cost fabrication of hydrophobic PVDF/WS2 porous membrane for highly efficient solar steam generation

Solar steam generation based on the light-to-heat conversion via photothermal materials has been considered as one of emerged technologies for utilizing solar energy to produce clean water. Here, a hydrophobic PVDF/WS₂ porous membrane for highly efficient solar steam generation was prepared by a scalable and low-cost method. The WS₂ photothermal materials were fabricated through a simple ball milling, and then a non-solvent induced phase inversion method was used to fabricate the porous PVDF/WS₂ membrane. The PVDF/WS₂ evaporator could absorb the sunlight of 90.58% from UV to NIR region due to the multiscattering of the porous structure and the synergistic effect of WS₂ and seawater. Moreover, the PVDF/WS₂ evaporator exhibits the hydrophobic properties. Taking the advantages mentioned above, our evaporator could manifest the evaporation rate of 4.15 kgm^-2h^-1 with the solar thermal efficiency of 94.2% under 3 sun irradiation, as well as an outstanding durability upon continuous running. Also, the evaporator shows both the excellent seawater desalination and sewage treatment ability. Outdoor experiments illustrate that the evaporator has practical applications under a natural sunlight condition. The numerous advantages of our PVDF/WS₂ evaporator, including the high solar-thermal efficiency, the outstanding durability, and the simple and scalable manufacture process, may provide a potential photothermal material for the commercial solar desalination application and wastewater treatment.

Defect Engineering in Metastable Phases of Transition-Metal Dichalcogenides for Electrochemical Applications

Metastable metallic phases of transition-metal dichalcogenide (TMD) nanomaterials have displayed excellent performance and emerged as promising candidates for sustainable energy sources low-cost storage and conversion because of their two-dimensional (2D) layered structures and extraordinary physicochemical properties. In order to broaden the range of potential applications, defect engineering is applied to the metastable phases of TMDs for further improvement of their catalytic and electronic properties. According to some recent studies, effective introduction of defects without perturbing the interior conductivity contributes to the development of metastable TMDs. This review provides deep insights into recent progress in electrochemistry using defect engineering in the metastable phases of TMDs. After introducing the structures of metastable phases and methods for defect construction, significant developments in catalysis and energy storage applications are discussed to elucidate structure-function relationships. Key challenges and future directions for defect engineering in the metastable phases of TMDs are also highlighted in the conclusions.

One-Pot Synthesis of W2C/WS2 Hybrid Nanostructures for Improved Hydrogen Evolution Reactions and Supercapacitors

Tungsten sulfide (WS2) and tungsten carbide (W2C) are materialized as the auspicious candidates for various electrochemical applications, owing to their plentiful active edge sites and better conductivity. In this work, the integration of W2C and WS2 was performed by using a simple chemical reaction to form W2C/WS2 hybrid as a proficient electrode for hydrogen evolution and supercapacitors. For the first time, a W2C/WS2 hybrid was engaged as a supercapacitor electrode and explored an incredible specific capacitance of ~1018 F g-1 at 1 A g-1 with the outstanding robustness. Furthermore, the constructed symmetric supercapacitor using W2C/WS2 possessed an energy density of 45.5 Wh kg-1 at 0.5 kW kg-1 power density. For hydrogen evolution, the W2C/WS2 hybrid produced the low overpotentials of 133 and 105 mV at 10 mA cm-2 with the small Tafel slopes of 70 and 84 mV dec-1 in acidic and alkaline media, respectively, proving their outstanding interfaced electrocatalytic characteristics. The engineered W2C/WS2-based electrode offered the high-performance for electrochemical energy applications.

Hydrogen-Etched Bifunctional Sulfur-Defect-Rich ReS2 /CC Electrocatalyst for Highly Efficient HER and OER

The design on synthesizing a sturdy, low-cost, clean, and sustainable electrocatalyst, as well as achieving high performance with low overpotential and good durability toward water splitting, is fairly vital in environmental and energy-related subject. Herein, for the first time the growth of sulfur (S) defect engineered self-supporting array electrode composed of metallic Re and ReS₂ nanosheets on carbon cloth (referred as Re/ReS₂ /CC) via a facile hydrothermal method and the following thermal treatment with H₂ /N₂ flow is reported. It is expected that, for example, the as-prepared Re/ReS₂ -7H/CC for the electrocatalytic hydrogen evolution reaction (HER) under acidic medium affords a quite low overpotential of 42 mV to achieve a current density of 10 mA cm^-2 and a very small Tafel slope of 36 mV decade^-1 , which are comparable to some of the promising HER catalysts. Furthermore, in the two-electrode system, a small cell voltage of 1.30 V is recorded under alkaline condition. Characterizations and density functional theory results expound that the introduced S defects in Re/ReS₂ -7H/CC can offer abundant active sites to advantageously capture electron, enhance the electron transport capacity, and weaken the adsorption free energy of H* at the active sites, being responsible for its superior electrocatalytic performance.

Designing Champion Nanostructures of Tungsten Dichalcogenides for Electrocatalytic Hydrogen Evolution

Fine-tuning strain and vacancies in 2H-phase transition-metal dichalcogenides, although extremely challenging, is crucial for activating the inert basal plane for boosting the hydrogen evolution reaction (HER). Here, atomically curved 2H-WS₂ nanosheets with precisely tunable strain and sulfur vacancies (S-vacancies) along with rich edge sites are synthesized via a one-step approach by harnessing geometric constraints. The approach is based on the confined epitaxy growth of WS₂ in ordered mesoporous graphene derived from nanocrystal superlattices. The spherical curvature imposed by the graphitic mesopores enables the generation of uniform strain and S-vacancies in the as-grown WS₂ nanosheets, and simultaneous manipulation of these two key parameters can be realized by simply adjusting the pore size. In addition, the formation of unique mesoporous WS₂ @graphene van der Waals heterostructures ensures the ready access of active sites. Fine-tuning the WS₂ layer number, strain, and S-vacancies enables arguably the best-performing HER 2H-WS₂ electrocatalysts ever reported. Density functional theory calculations indicate that compared with strain, S-vacancies play a more critical role in enhancing the HER activity.

On-chip electrocatalytic microdevice: an emerging platform for expanding the insight into electrochemical processes

This comprehensive summary of on-chip electrocatalytic microdevices will expand the insight into electrochemical processes, ranging from dynamic exploration to performance optimization.