Monolayer MoS2 films supported by 3D nanoporous metals for high-efficiency electrocatalytic hydrogen production

Ball-milled MoS2 with graphene shows enhanced catalytic activity for hydrogen evolution reaction

The hydrogen evolution reaction (HER) is an important phenomenon in water splitting. Consequently, the development of an active, earth-abundant, and inexpensive HER catalyst is highly desired. MoS2 has drawn considerable interest as an HER catalyst because it is composed of non-precious metal and exhibits high catalytic activity in the nanosheet form. In this study, size-controlled MoS2 particles were synthesized by ball milling. The as-prepared samples exhibited significantly enhanced electrochemical and catalytic properties compared to those of pristine bulk MoS2. Furthermore, the HER activity improved further upon the introduction of graphene into the as-prepared ball-milled samples. In particular, the MoS2 sample ball-milled for 12 h mixed with graphene exhibited optimal performance, showing an overpotential (160 mV at 10 mA cm-2) that was ~ 335 mV lower than that of pristine bulk MoS2. The superior catalytic activity was ascribed to the exposed edge sites, sulfur vacancies, and 1T phase of MoS2, as well as the noteworthy fortifying effect of the electronically conductive flexible material, graphene. The results provide a promising strategy for its application as an efficient and stable HER catalyst.

Tuning Electrocatalytic Activities of Dealloyed Nanoporous Catalysts by Macroscopic Strain Engineering

It is technically challenging to quantitatively apply strains to tune catalysis because most heterogeneous catalysts are nanoparticles, and lattice strains can only be applied indirectly via core-shell structures or crystal defects. Herein, we report quantitative relations between macroscopic strains and hydrogen evolution reaction (HER) activities of dealloyed nanoporous gold (NPG) by directly applying macroscopic strains upon bulk NPG. It was found that macroscopic compressive strains lead to a decrease, while macroscopic tensile strains improve the HER activity of NPG, which is in line with the d-band center model. The overpotential and onset potential of HER display approximately a linear relation with applied macroscopic strains, revealing an ∼2.9 meV decrease of the binding energy per 0.1% lattice strains from compressive to tensile. The methodology with the high strain sensitivity of electrocatalysis, developed in this study, paves a new way to investigate the insights of strain-dependent electrocatalysis with high precision.

Chemical vapor deposition growth of graphene and other nanomaterials with 3D architectures towards electrocatalysis and secondary battery-related applications

Recently, two-dimensional (2D) layered materials, such as graphene and transition metal dichalcogenides (TMDCs), have garnered a lot of attention in energy storage/conversion-related fields due to their novel physical and chemical properties. Constructing flat graphene and TMDCs nanosheets into 3D architectures can significantly increase their exposed surface area and prevent the restacking of adjacent 2D layers, thus dramatically promoting their applications in various energy-related fields. Chemical Vapor Deposition (CVD) is a low-cost, facile, and scalable method, which has been widely employed to produce high-quality graphene and TMDCs nanosheets with 3D architectures. During the CVD process, the morphologies and properties of the 3D architectures of such 2D materials can be designed by selecting substrates with different compositions, stacking geometries, and micro-structures. In this review, we focus on the recent advances in the CVD synthesis of graphene, TMDCs, and their hybrids with 3D architectures on different 3D-structured substrates, as well as their applications in the electrocatalytic hydrogen evolution reaction (HER) and various secondary batteries. In addition, the challenges and future prospects for the CVD synthesis and energy-related applications of these unique layered materials will also be discussed.

Continuous strain tuning of oxygen evolution catalysts with anisotropic thermal expansion

Compressive strain, downshifting the d-band center of transition metal oxides, is an effective way to accelerate the sluggish kinetics of oxygen evolution reaction (OER) for water electrolysis. Here, we find that anisotropic thermal expansion can produce compressive strains of the IrO6 octahedron in Sr2IrO4 catalyst, thus downshifting its d-band center. Different from the previous strategies to create constant strains in the crystals, the thermal-triggered compressive strains can be real-timely tuned by varying temperature. As a result of the thermal strain accelerating OER kinetics, the Sr2IrO4 exhibits the nonlinear lnjo - T-1 (jo, exchange current density; T, absolute temperature) Arrhenius relationship, resulting from the thermally induced low-barrier electron transfer in the presence of thermal compressive strains. Our results verify that the thermal field can be utilized to manipulate the electronic states of Sr2IrO4 via thermal compressive strains downshifting the d-band center, significantly accelerating the OER kinetics, beyond the traditional thermal diffusion effects.

Deformable Catalytic Material Derived from Mechanical Flexibility for Hydrogen Evolution Reaction

Deformable catalytic material with excellent flexible structure is a new type of catalyst that has been applied in various chemical reactions, especially electrocatalytic hydrogen evolution reaction (HER). In recent years, deformable catalysts for HER have made great progress and would become a research hotspot. The catalytic activities of deformable catalysts could be adjustable by the strain engineering and surface reconfiguration. The surface curvature of flexible catalytic materials is closely related to the electrocatalytic HER properties. Here, firstly, we systematically summarized self-adaptive catalytic performance of deformable catalysts and various micro-nanostructures evolution in catalytic HER process. Secondly, a series of strategies to design highly active catalysts based on the mechanical flexibility of low-dimensional nanomaterials were summarized. Last but not least, we presented the challenges and prospects of the study of flexible and deformable micro-nanostructures of electrocatalysts, which would further deepen the understanding of catalytic mechanisms of deformable HER catalyst.

Recent Advances in Engineering of 2D Materials-Based Heterostructures for Electrochemical Energy Conversion

2D materials, such as graphene, transition metal dichalcogenides, black phosphorus, layered double hydroxides, and MXene, have exhibited broad application prospects in electrochemical energy conversion due to their unique structures and electronic properties. Recently, the engineering of heterostructures based on 2D materials, including 2D/0D, 2D/1D, 2D/2D, and 2D/3D, has shown the potential to produce synergistic and heterointerface effects, overcoming the inherent restrictions of 2D materials and thus elevating the electrocatalytic performance to the next level. In this review, recent studies are systematically summarized on heterostructures based on 2D materials for advanced electrochemical energy conversion, including water splitting, CO2 reduction reaction, N2 reduction reaction, etc. Additionally, preparation methods are introduced and novel properties of various types of heterostructures based on 2D materials are discussed. Furthermore, the reaction principles and intrinsic mechanisms behind the excellent performance of these heterostructures are evaluated. Finally, insights are provided into the challenges and perspectives regarding the future engineering of heterostructures based on 2D materials for further advancements in electrochemical energy conversion.

Lattice-Strain Engineering for Heterogenous Electrocatalytic Oxygen Evolution Reaction

The energy efficiency of metal-air batteries and water-splitting techniques is severely constrained by multiple electronic transfers in the heterogenous oxygen evolution reaction (OER), and the high overpotential induced by the sluggish kinetics has become an uppermost scientific challenge. Numerous attempts are devoted to enabling high activity, selectivity, and stability via tailoring the surface physicochemical properties of nanocatalysts. Lattice-strain engineering as a cutting-edge method for tuning the electronic and geometric configuration of metal sites plays a pivotal role in regulating the interaction of catalytic surfaces with adsorbate molecules. By defining the d-band center as a descriptor of the structure-activity relationship, the individual contribution of strain effects within state-of-the-art electrocatalysts can be systematically elucidated in the OER optimization mechanism. In this review, the fundamentals of the OER and the advancements of strain-catalysts are showcased and the innovative trigger strategies are enumerated, with particular emphasis on the feedback mechanism between the precise regulation of lattice-strain and optimal activity. Subsequently, the modulation of electrocatalysts with various attributes is categorized and the impediments encountered in the practicalization of strained effect are discussed, ending with an outlook on future research directions for this burgeoning field.

Interfacially Engineered Nanoporous Cu/MnOx Hybrids for Highly Efficient Electrochemical Ammonia Synthesis via Nitrate Reduction

Electrochemical reduction of nitrate to ammonia (NH₃ ) not only offers a promising strategy for green NH₃ synthesis, but also addresses the environmental issues and balances the perturbed nitrogen cycle. However, current electrocatalytic nitrate reduction processes are still inefficient due to the lack of effective electrocatalysts. Here 3D nanoporous Cu/MnO_x hybrids are reported as efficient and durable electrocatalysts for nitrate reduction reaction, achieving the NH₃ yield rates of 5.53 and 29.3 mg h^-1 mg_cat. ^-1 with 98.2% and 86.2% Faradic efficiency in 0.1 m Na₂ SO₄ solution with 10 and 100 mm KNO₃ , respectively, which are higher than those obtained for most of the reported catalysts under similar conditions. Both the experimental results and density functional theory calculations reveal that the interface effect between Cu/MnO_x interface could reduce the free energy of rate determining step and suppress the hydrogen evolution reaction, leading to the enhanced catalytic activity and selectivity. This work provides an approach to design advanced materials for NH₃ production via electrochemical nitrate reduction.

Ultraefficient Electrocatalytic Hydrogen Evolution from Strain-Engineered, Multilayer MoS2

This paper reports an approach to repurpose low-cost, bulk multilayer MoS₂ for development of ultraefficient hydrogen evolution reaction (HER) catalysts over large areas (>cm²). We create working electrodes for use in HER by dry transfer of MoS₂ nano- and microflakes to gold thin films deposited on prestrained thermoplastic substrates. By relieving the prestrain at a macroscopic scale, a tunable level of tensile strain is developed in the MoS₂ and consequently results in a local phase transition as a result of spontaneously formed surface wrinkles. Using electrochemical impedance spectroscopy, we verified that electrochemical activation of the strained MoS₂ lowered the charge transfer resistance within the materials system, achieving HER activity comparable to platinum (Pt). Raman and X-ray photoelectron spectroscopy show that desulfurization in the multilayer MoS₂ was promoted by the phase transition; the combined effect of desulfurization and the lower charge resistance induced superior HER performance.

Strain Engineering: A Boosting Strategy for Photocatalysis

Whilst the photocatalytic technique is considered to be one of the most significant routes to address the energy crisis and global environmental challenges, the solar-to-chemical conversion efficiency is still far from satisfying practical industrial requirements, which can be traced to the suboptimal bandgap and electronic structure of photocatalysts. Strain engineering is a universal scheme that can finely tailor the bandgap and electronic structure of materials, hence supplying a novel avenue to boost their photocatalytic performance. Accordingly, to explore promising directions for certain breakthroughs in strained photocatalysts, an overview on the recent advances of strain engineering from the basics of strain effect, creations of strained materials, as well as characterizations and simulations of strain level is provided. Besides, the potential applications of strain engineering in photocatalysis are summarized, and a vision for the future controllable-electronic-structure photocatalysts by strain engineering is also given. Finally, perspectives on the challenges for future strain-promoted photocatalysis are discussed, placing emphasis on the creation and decoupling of strain effect, and the modification of theoretical frameworks.

Functionalization of 2D MoS2 Nanosheets with Various Metal and Metal Oxide Nanostructures: Their Properties and Application in Electrochemical Sensors

Two-dimensional transition metal dichalcogenides (2D TMDs) have gained considerable attention due to their distinctive properties and broad range of possible applications. One of the most widely studied transition metal dichalcogenides is molybdenum disulfide (MoS₂). The 2D MoS₂ nanosheets have unique and complementary properties to those of graphene, rendering them ideal electrode materials that could potentially lead to significant benefits in many electrochemical applications. These properties include tunable bandgaps, large surface areas, relatively high electron mobilities, and good optical and catalytic characteristics. Although the use of 2D MoS₂ nanosheets offers several advantages and excellent properties, surface functionalization of 2D MoS₂ is a potential route for further enhancing their properties and adding extra functionalities to the surface of the fabricated sensor. The functionalization of the material with various metal and metal oxide nanostructures has a significant impact on its overall electrochemical performance, improving various sensing parameters, such as selectivity, sensitivity, and stability. In this review, different methods of preparing 2D-layered MoS₂ nanomaterials, followed by different surface functionalization methods of these nanomaterials, are explored and discussed. Finally, the structure-properties relationship and electrochemical sensor applications over the last ten years are discussed. Emphasis is placed on the performance of 2D MoS₂ with respect to the performance of electrochemical sensors, thereby giving new insights into this unique material and providing a foundation for researchers of different disciplines who are interested in advancing the development of MoS₂-based sensors.

Engineering Phase Stability of Semimetallic MoS2 Monolayers for Sustainable Electrocatalytic Hydrogen Production

1T'-phase MoS₂ possesses excellent electrocatalytic performance, but due to the instability of the thermodynamic metastable phase, its actual electrocatalytic effect is seriously limited. Here, we report a wet-chemical synthesis strategy for constructing rGO/1T'-MoS₂/CeO₂ heterostructures to improve the phase stability of metastable 1T' phase MoS₂ monolayers. Importantly, the rGO/1T'-MoS₂/CeO₂ heterostructure exhibits excellent electrocatalytic hydrogen evolution reaction (HER) performance, which is much better than the 1T'-MoS₂ monolayers. The synergistic effects between CeO₂ nanoparticles (NPs) and 1T'-MoS₂ monolayers were systematically investigated. 1T'-MoS₂ monolayers combined with the cocatalyst of CeO₂ NPs can produce lattice strain and distortion on 1T'-MoS₂ monolayers, which can tune the energy band structure, charge transfer, and energy barriers of hydrogen atom adsorption (ΔE_H), leading to promotion of the phase activity and stability of 1T'-MoS₂ monolayers for hydrogen production. Our work offers a feasible method for the preparation of efficient HER electrocatalysts based on the engineering phase stability of metastable materials.

Surface engineering strategies for MoS2 towards electrochemical hydrogen evolution

Water splitting driven by renewable energy sources is an environmentally friendly and sustainable way to produce hydrogen as an ideal energy source in the future. Electrocatalysts can promote the water splitting performance at the both ends. Therefore, the development of cost-effective, high-performance electrocatalysis is a key factor in promoting water decomposition and renewable energy conversion. Among candidates, layered molybdenum disulfide (MoS 2 ) is considered as a most promising electrocatalyst to replace Pt for hydrogen evolution reaction (HER). Surface atomic engineering and interface engineering can induce new physicochemical properties for MoS 2 to greatly enhance HER activity. In this report, we summarize the latest improvement strategies and research progress to improve the catalytic activity of MoS 2 -based material catalysts through the surface and interface atomic and molecular engineering, thus effectively improving HER process. In addition, some unsolved problems in the large-scale application of modified MoS 2 catalyst are also discussed.

Challenges of modeling nanostructured materials for photocatalytic water splitting

Understanding the water splitting mechanism in photocatalysis is a rewarding goal as it will allow producing clean fuel for a sustainable life in the future. However, identifying the photocatalytic mechanisms by modeling photoactive nanoparticles requires sophisticated computational techniques based on multiscale modeling. In this review, we will survey the strengths and drawbacks of currently available theoretical methods at different length and accuracy scales. Understanding the surface-active site through Density Functional Theory (DFT) using new, more accurate exchange-correlation functionals plays a key role for surface engineering. Larger scale dynamics of the catalyst/electrolyte interface can be treated with Molecular Dynamics albeit there is a need for more generalizations of force fields. Monte Carlo and Continuum Modeling techniques are so far not the prominent path for modeling water splitting but interest is growing due to the lower computational cost and the feasibility to compare the modeling outcome directly to experimental data. The future challenges in modeling complex nano-photocatalysts involve combining different methods in a hierarchical way so that resources are spent wisely at each length scale, as well as accounting for excited states chemistry that is important for photocatalysis, a path that will bring devices closer to the theoretical limit of photocatalytic efficiency.

The Effect of the 3D Nanoarchitecture and Ni-Promotion on the Hydrogen Evolution Reaction in MoS2 /Reduced GO Aerogel Hybrid Microspheres Produced by a Simple One-Pot Electrospraying Procedure

The transition toward renewable energy sources requires low-cost, efficient, and durable electrocatalysts for green H₂ production. Herein, an easy and highly scalable method to prepare MoS₂ nanoparticles embedded in 3D partially reduced (pr) graphene oxide (GO) aerogel microspheres (MoS₂ /prGOAMs) with controlled morphology and composition is described. Given their peculiar center-diverging mesoporous structure, which allows easy access to the active sites and optimal mass transport, and their efficient electron transfer facilitated by the intimate contact between the MoS₂ and the 3D connected highly conductive pr-GO sheets, these materials exhibit a remarkable electrocatalytic activity in the hydrogen evolution reaction (HER). Ni atoms, either as single Ni atoms or NiO aggregates are then introduced in the MoS₂ /prGOAMs hybrids, to facilitate water dissociation, which is the slowest step in alkaline HER, producing a bifunctional catalyst. After optimization, Ni-promoted MoS₂ /prGOAMs obtained at 500 °C reach a remarkable η₁₀ (overpotential at 10 mA cm^-2 ) of 160 mV in 1 m KOH and 174 mV in 0.5 m H₂ SO₄ . Moreover, after chronopotentiometry tests (15 h) at a current density of 10 mA cm^-2 , the η₁₀ value improves to 147 mV in alkaline conditions, indicating an exceptional stability.

Recent advances in cobalt-based catalysts for efficient electrochemical hydrogen evolution: a review

Hydrogen (H2) is a new type of renewable energy that can meet people's growing energy needs and is environmentally friendly. In order to improve the industrial application prospect and electrochemical...

Manipulation on active electronic states of metastable phase β-NiMoO4 for large current density hydrogen evolution

Non-noble transition metal oxides are abundant in nature. However, they are widely regarded as catalytically inert for hydrogen evolution reaction (HER) due to their scarce active electronic states near the Fermi-level. How to largely improve the HER activity of these kinds of materials remains a great challenge. Herein, as a proof-of-concept, we design a non-solvent strategy to achieve phosphate substitution and the subsequent crystal phase stabilization of metastable β-NiMoO₄. Phosphate substitution is proved to be imperative for the stabilization and activation of β-NiMoO₄, which can efficiently generate the active electronic states and promote the intrinsic HER activity. As a result, phosphate substituted β-NiMoO₄ exhibits the optimal hydrogen adsorption free energy (−0.046 eV) and ultralow overpotential of −23 mV at 10 mA cm⁻² in 1 M KOH for HER. Especially, it maintains long-term stability for 200 h at the large current density of 1000 mA cm⁻² with an overpotential of only −210 mV. This work provides a route for activating transition metal oxides for HER by stabilizing the metastable phase with abundant active electronic states.

Non-noble transition metal oxides are common yet typically poor hydrogen evolution catalysts due to scarce active electronic states. This work provides a route for achieving hydrogen evolution at high current densities by stabilizing a metastable NiMoO₄ phase with abundant active electronic states.

The Role of Surface Curvature in Electrocatalysts

Excessive consumption of fossil fuels has caused unavoidable environmental problems. The development of renewable and clean alternatives is essential for the sustainable and green development of human society. Electrocatalysts are most important parts in these energy-related devices. Recently, scientists found that the surface curvature of electrocatalysts could play an important role for the improvement of catalytic performance and the optimization of intrinsic catalytic activity during electrocatalytic process. The role of surface curvature in electrocatalysts is still under investigating. In this minireview, we summarized the latest progress of electrocatalysts with different surface curvatures and their applications in energy-related applications. This review mainly involves the strategies for preparation of electrocatalysts with different surface curvatures, three typical electrocatalysts with different surface curvatures (curled surface, onion-like structure, and spiral structure), and the potential mechanisms that surface curvature in electrocatalysts affects activities.

Recent developments in 2D transition metal dichalcogenides: phase transition and applications of the (quasi-)metallic phases

The advent of two-dimensional transition metal dichalcogenides (2D-TMDs) has led to an extensive amount of interest amongst scientists and engineers alike and an intensive amount of research has brought about major breakthroughs in the electronic and optical properties of 2D materials. This in turn has generated considerable interest in novel device applications. With the polymorphic structural features of 2D-TMDs, this class of materials can exhibit both semiconducting and metallic (quasi-metallic) properties in their respective phases. This polymorphic property further increases the interest in 2D-TMDs both in fundamental research and for their potential utilization in novel high-performance device applications. In this review, we highlight the unique structural properties of few-layer and monolayer TMDs in the metallic 1T- and quasi-metallic 1T'-phases, and how these phases dictate their electronic and optical properties. An overview of the semiconducting-to-(quasi)-metallic phase transition of 2D-TMD systems will be covered along with a discussion on the phase transition mechanisms. The current development in the applications of (quasi)-metallic 2D-TMDs will be presented ranging from high-performance electronic and optoelectronic devices to energy storage, catalysis, piezoelectric and thermoelectric devices, and topological insulator and neuromorphic computing applications. We conclude our review by highlighting the challenges confronting the utilization of TMD-based systems and projecting the future developmental trends with an outlook of the progress needed to propel this exciting field forward.

One-dimensional, space-confined, solid-phase growth of the Cu9S5@MoS2 core-shell heterostructure for electrocatalytic hydrogen evolution

Binary transition metal chalcogenide core-shell nanocrystals are considered the most promising nonprecious metal catalysts for large-scale industrial hydrogen production. Herein, we report a one-dimensional, space-confined, solid-phase strategy for the growth of a Cu₉S₅@MoS₂ core-shell heterostructure by combining electrospinning and chemical vapor deposition methods. The Cu₉S₅@MoS₂ core-shell nanocrystals were synthesized in situ on carbon nanofibers (Cu₉S₅@MoS₂/CNFs) by an S vapor graphitization process. Tuning of the MoS₂ shell numbers can be controlled by changing the mass ratio of the Cu and Mo precursors. We experimentally determined the effects of the thickness of the MoS₂ shell on the electrocatalytic activity for the hydrogen evolution reaction (HER) in acidic and alkaline solutions. When the mass ratio is 3:1, the Cu₉S₅@MoS₂/CNFs show the fewest MoS₂ shells with just 1-2 layers each and exhibit the best HER performance with small overpotentials of 116 mV and 114 mV in acidic and alkaline solutions, respectively, at a current density of 10 mA cm^-2. The core shell structures, with their unique Cu-S-Mo nanointerfaces, could enhance the electron transfer and surface area, thus increasing the performance of the HER. This work provides a facile method to design unique core shell assemblies in one-dimensional nanostructures.

Rational strain engineering of single-atom ruthenium on nanoporous MoS2 for highly efficient hydrogen evolution

Maximizing the catalytic activity of single-atom catalysts is vital for the application of single-atom catalysts in industrial water-alkali electrolyzers, yet the modulation of the catalytic properties of single-atom catalysts remains challenging. Here, we construct strain-tunable sulphur vacancies around single-atom Ru sites for accelerating the alkaline hydrogen evolution reaction of single-atom Ru sites based on a nanoporous MoS₂-based Ru single-atom catalyst. By altering the strain of this system, the synergistic effect between sulphur vacancies and Ru sites is amplified, thus changing the catalytic behavior of active sites, namely, the increased reactant density in strained sulphur vacancies and the accelerated hydrogen evolution reaction process on Ru sites. The resulting catalyst delivers an overpotential of 30 mV at a current density of 10 mA cm^-2, a Tafel slope of 31 mV dec^-1, and a long catalytic lifetime. This work provides an effective strategy to improve the activities of single-atom modified transition metal dichalcogenides catalysts by precise strain engineering.

Two-Dimensional MoS2: Structural Properties, Synthesis Methods, and Regulation Strategies toward Oxygen Reduction

Compared with three-dimensional (3D) and other materials, two-dimensional (2D) materials with unique properties such as high specific surface area, structurally adjustable band structure, and electromagnetic properties have attracted wide attention. In recent years, great progress has been made for 2D MoS₂ in the field of electrocatalysis, and its exposed unsaturated edges are considered to be active sites of electrocatalytic reactions. In this review, we focus on the latest progress of 2D MoS₂ in the oxygen reduction reaction (ORR) that has not received much attention. First, the basic properties of 2D MoS₂ and its advantages in the ORR are introduced. Then, the synthesis methods of 2D MoS₂ are summarized, and specific strategies for optimizing the performance of 2D MoS₂ in ORRs, and the challenges and opportunities faced are discussed. Finally, the future of the 2D MoS₂-based ORR catalysts is explored.

Engineering of Amorphous Structures and Sulfur Defects into Ultrathin FeS Nanosheets to Achieve Superior Electrocatalytic Alkaline Oxygen Evolution

Integration of amorphous structures and anion defects into ultrathin 2D materials has been identified as an effective strategy for boosting the electrocatalytic performance. However, the in-depth understanding of the relationship among the amorphous structure, vacancy defect, and catalytic activity is still obscure. Herein, a facile strategy was proposed to prepare ultrathin and amorphous Mo-FeS nanosheets (NSs) with abundant sulfur defects. Benefited from the ultrathin, amorphous nanostructure, and synergy effect of Mo-doping and sulfur defect, the Mo-FeS NSs manifested excellent electrocatalytic activity toward oxygen evolution reaction (OER) in alkaline medium, as shown by an ultralow overpotential of 210 mV at 10 mA cm^-2, a Tafel slope of 50 mV dec^-1, and retaining such good catalytic stability over 30 h. The efficient catalytic performance for Mo-FeS NSs is superior to the commercial IrO₂ and most reported top-performing electrocatalysts. Density functional theory calculations revealed that the accelerated electron/mass transfer over the oxygen-containing intermediates can be attributed to the amorphous structure and sulfur-rich defects caused by structural reconfiguration. Furthermore, the S vacancies could enhance the activity of its neighboring Fe-active sites, which was also beneficial to their OER kinetics. This work integrated both amorphous structures and sulfur vacancies into ultrathin 2D NSs and further systematically evaluated the OER performance, providing new insights for the design of amorphous-layered electrocatalysts.

Defect Engineering of Molybdenum-Based Materials for Electrocatalysis

Molybdenum-based electrocatalysts have been widely applied in electrochemical energy conversion reactions. The essential roles of defects, including doping, vacancies, grain boundaries, and dislocations in improving various electrocatalytic performances have been reported. This review describes the latest development of defect engineering in molybdenum-based materials for hydrogen evolution, oxygen reduction, oxygen evolution, and nitrogen reduction reactions. The types of defects, preparation methods, characterization techniques, and applications of molybdenum-based defect materials are elucidated. Finally, challenges and future research directions for these types of materials are also discussed.

General Synthesis of Nanoporous 2D Metal Compounds with 3D Bicontinous Structure

Although 2D layered metal compounds are widely exploited using various techniques such as exfoliation and vapor-phase-assisted growth, it is still challenging to construct the 2D materials in a 3D configuration with preservation of the unique physicochemical properties of the metal compounds. Herein, a general synthetic strategy is reported for a wide variety of 2D (atomic-scale thickness) metal compounds with 3D bicontinous nanoporous structure. 19 binary compounds including sulfides, selenides, tellurides, carbides, and nitrides, and five alloyed compounds, are successfully prepared via a surface alloy strategy, which are readily created by using a recyclable nanoporous gold assisted chemical vapor deposition process. These 3D nanoporous metal compounds with preserved 2D physicochemical properties, tunable pore sizes, and compositions for electrocatalytic applications, show excellent catalytic performance in the electrochemical N₂ reduction reaction. This work opens up a promising avenue for fundamental studies and potential applications of a wide variety of nanoporous metal compounds.

Defect Engineering of van der Waals Solids for Electrocatalytic Hydrogen Evolution

Van der Waals solids with tunable band gaps and interfacial properties have been regarded as a class of promising active materials for electrocatalytic hydrogen evolution reaction (HER). However, due to the anisotropic features, their basal planes are usually electrochemically inert, only a few unsaturated edge atoms could serve as active centers to actuate H₂ generation. Hence, material utilization and productivity efficiency are insufficient for practical applications. Recently, diverse defects have been confirmed to enable tailoring atomic configurations and electronic properties of van der Waals solids, thus triggering their superior catalytic activity of in-plane atoms while introducing high amount of new active sites. In this minireview, we summarize the state-of-the-art progress of defect engineering in van der Waals solids for HER, focusing in particular on their advantages in material modification and corresponding catalytic mechanisms. We also propose the challenges and perspectives of these catalytic materials in terms of both experimental synthesis and fundamental understanding of the defect structures.

Electrochemical Compression Technologies for High-Pressure Hydrogen: Current Status, Challenges and Perspective

Abstract

Hydrogen is an ideal energy carrier in future applications due to clean byproducts and high efficiency. However, many challenges remain in the application of hydrogen, including hydrogen production, delivery, storage and conversion. In terms of hydrogen storage, two compression modes (mechanical and non-mechanical compressors) are generally used to increase volume density in which mechanical compressors with several classifications including reciprocating piston compressors, hydrogen diaphragm compressors and ionic liquid compressors produce significant noise and vibration and are expensive and inefficient. Alternatively, non-mechanical compressors are faced with issues involving large-volume requirements, slow reaction kinetics and the need for special thermal control systems, all of which limit large-scale development. As a result, modular, safe, inexpensive and efficient methods for hydrogen storage are urgently needed. And because electrochemical hydrogen compressors (EHCs) are modular, highly efficient and possess hydrogen purification functions with no moving parts, they are becoming increasingly prominent. Based on all of this and for the first time, this review will provide an overview of various hydrogen compression technologies and discuss corresponding structures, principles, advantages and limitations. This review will also comprehensively present the recent progress and existing issues of EHCs and future hydrogen compression techniques as well as corresponding containment membranes, catalysts, gas diffusion layers and flow fields. Furthermore, engineering perspectives are discussed to further enhance the performance of EHCs in terms of the thermal management, water management and the testing protocol of EHC stacks. Overall, the deeper understanding of potential relationships between performance and component design in EHCs as presented in this review can guide the future development of anticipated EHCs.

Graphic Abstract

'Template-free' hierarchical MoS2 foam as a sustainable 'green' scavenger of heavy metals and bacteria in point of use water purification

Molybdenum disulfide (MoS₂), with its unique optical and electrical properties, has been explored for a variety of applications in the recent past. Still, its capabilities in point-of-use heavy metal ion removal remain to be explored. Herein, for the first time using a facile approach, we fabricated three-dimensional (3D) MoS₂ foam from exfoliated single to few-layered MoS₂ sheets for the selective exclusion of heavy metals and stringent bactericidal response. This foam was able to exclude 99.9% of Pb(ii) and 98.7% of As(iii) instantaneously and reduced more than 98% of bacteria E. coli. Moreover, the foam exhibits selective toxicity towards bacterial cells while showing no observable toxicity towards mammalian cells. The foam can be recycled and reused for at least five cycles under accelerated conditions and thus can be used for a promising non-cytotoxic, facile, and environmentally benign process for inline water remediation to remove heavy metal ions from the feed and as a potential antibacterial agent.

Improved photocatalytic performance of metal-organic frameworks for CO2 conversion by ligand modification

Here we demonstrate that the utilization of 2,4,6-tris(4-pyridyl)pyridine (tpy) for metal-organic framework modification can greatly improve the photocatalytic performance for CO₂ reduction. The electron-donating nature of tpy enables the charge transfer effect, which induces strong CO₂ binding affinity, facilitates *COOH formation and promotes CO₂-to-CO conversion.

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.

2D molybdenum disulphide nanosheets incorporated with single heteroatoms for the electrochemical hydrogen evolution reaction

2D nanosheets give enhanced surface area to volume ratios in particle morphology and they can also provide defined surface sites to disperse foreign atoms. Placing atoms of catalytic interest on 2D nanosheets as Single Atom Catalysts (SAC) represents one of the novel approaches due to their unique but tunable electronic and steric characteristics. Here in this mini-review, we particularly highlight some recent and important developments on heteroatom doped MoS₂ nanosheets (SAC-MoS₂) as catalysts for the electrochemical hydrogen evolution reaction (HER) from water, which could lead to opening up to a flagship of important renewable technologies in future. It is shown that the nature of dopants, doping positions and the polytypes of MoS₂ nanosheets are the determining factors in the overall catalytic abilities of these functionalised nanosheets. This may serve to obtain atomic models which lead to further understanding of the 'metal-support interaction' in catalysis.

Metal-organic frameworks derived carbon-incorporated cobalt/dicobalt phosphide microspheres as Mott-Schottky electrocatalyst for efficient and stable hydrogen evolution reaction in wide-pH environment

Cobalt phosphides, as low cost and abundant non-noble materials for hydrogen evolution reaction (HER), are always constrained by their inferior charge transfer and sluggish intrinsic electrocatalytic kinetics. In this work, carbon-incorporated Co/Co₂P microspheres (Co/Co₂P@C) as a novel Mott-Schottky catalyst were synthesized successfully via carbonization and gradual phosphorization of Co based metal-organic frameworks. The unique merits, including Mott-Schottky effect at the interface formed between metal Co and semiconductor Co₂P, the incorporated carbon-layer on the surface and the spherical structure endow Co/Co₂P@C with favorable electrical conductivity, preferable kinetics and long-term stability when it was evaluated as electrocatalyst for HER in wide-pH range. As a result, the Co/Co₂P@C with the optimized phosphorization degree delivers a benchmark current density of 10 mA cm^-2 at the low overpotential of 192 and 158 mV in acidic and alkaline electrolytes, respectively, with a remarkable stability (CV cycling for 3000 cycles and continuous electrolysis at the overpotential of 200 mV for 48 h). Therefore, the as-designed Co/Co₂P@C should be one of the most promising catalysts for HER application.

Recent Modification Strategies of MoS2 for Enhanced Electrocatalytic Hydrogen Evolution

Molybdenum disulfide (MoS₂) has been recognized as one of the most promising catalysts to replace Pt for hydrogen evolution reaction (HER) electrocatalysis because of the elemental abundance, excellent catalytic potential, and stability. However, its HER efficiency is still below that of Pt. Recent research advances have revealed that the modification of pristine MoS₂ is a very effective approach to boost its HER performance, including improving the intrinsic activity of sites, increasing the number of edges, and enhancing the electrical conductivity. In this review, we focus on the recent progress on the modification strategies of MoS₂ for enhanced electrocatalytic hydrogen evolution. Moreover, some urgent challenges in this field are also discussed to realize the large-scale application of the modified-MoS₂ catalysts in industry.

Improved charge injection of edge aligned MoS2/MoO2 hybrid nanosheets for highly robust and efficient electrocatalysis of H2 production

Molybdenum disulfide (MoS2) can be an efficient electro-catalyst for the hydrogen evolution reaction (HER) as an alternative to precious metals, but significant efforts are still needed to further improve its efficiency. Among various approaches, the formation of edge aligned MoS2 on an electrically conductive support is highly promising for cost-effective H2 production. Nevertheless, catalysis is highly impeded by the poor charge transport between the electrode materials and also between the multilayers of MoS2. This research presents a strategy to improve the HER catalysis by binding layers of metallic molybdenum dioxide (MoO2) and MoS2 to form hybrid MoS2/MoO2 nanosheets (attached and cross-linked to each other). Taking advantage of the hybrid structure and the mechanical strength of the carbon cloth, a catalyst with outstanding catalytic performance in the HER is demonstrated. This work shows not only a strategy to efficiently improve the electrochemical process, but also the preparation of a highly efficient catalyst for constant and robust H2 production.

Van der Waals interfacial reconstruction in monolayer transition-metal dichalcogenides and gold heterojunctions

The structures and properties of van der Waals (vdW) heterojunctions between semiconducting two-dimensional transition-metal dichalcogenides (2D TMDs) and conductive metals, such as gold, significantly influence the performances of 2D-TMD based electronic devices. Chemical vapor deposition is one of the most promising approaches for large-scale synthesis and fabrication of 2D TMD electronics with naturally formed TMD/metal vdW interfaces. However, the structure and chemistry of the vdW interfaces are less known. Here we report the interfacial reconstruction between TMD monolayers and gold substrates. The participation of sulfur leads to the reconstruction of Au {001} surface with the formation of a metastable Au4S4 interfacial phase which is stabilized by the top MoS2 and WS2 monolayers. Moreover, the enhanced vdW interaction between the reconstructed Au4S4 interfacial phase and TMD monolayers results in the transition from n-type TMD-Au Schottky contact to p-type one with reduced energy barrier height.

An in situ grown lanthanum sulfide/molybdenum sulfide hybrid catalyst for electrochemical hydrogen evolution

An La₂S₃–MoS₂ catalyst with expanded interlayer spacing and engineered nano-interfaces was facilely synthesized, demonstrating enhanced catalytic activity for electrochemical hydrogen evolution.

Inserting Co and P into MoS2 photocathodes: enhancing hydrogen evolution reaction catalytic performance by activating edges and basal planes with sulfur vacancies

The production of hydrogen using solar energy via a photoelectrochemical system is an effective technique for meeting present clean energy needs.