Field-Effect Tuned Adsorption Dynamics of VSe2 Nanosheets for Enhanced Hydrogen Evolution Reaction

Polarization-Switchable Electrochemistry of 2D Layered Bi2O2Se Bifunctional Microreactors by Ferroelectric Modulation

Ferroelectric catalysts are known for altering surface catalytic activities by changing the direction of their electric polarizations. This study demonstrates polarization-switchable electrochemistry using layered bismuth oxyselenide (L-Bi2O2Se) bifunctional microreactors through ferroelectric modulation. A selective-area ionic liquid gating is developed with precise control over the spatial distribution of the dipole orientation of L-Bi2O2Se. On-chip microreactors with upward polarization favor the oxygen evolution reaction, whereas those with downward polarization prefer the hydrogen evolution reaction. The microscopic origin behind polarization-switchable electrochemistry primarily stems from enhanced surface adsorption and reduced energy barriers for reactions, as examined by nanoscale scanning electrochemical cell microscopy. Integrating a pair of L-Bi2O2Se microreactors consisting of upward or downward polarizations demonstrates overall water splitting in a full-cell configuration based on a bifunctional catalyst. The ability to modulate surface polarizations on a single catalyst via ferroelectric polarization switching offers a pathway for designing catalysts for water splitting.

Stimulus of Work Function on Electron Transfer Process of Intermetallic Nickel-Antimonide Toward Bifunctional Electrocatalyst for Overall Water Splitting

Engineering the intermetallic nanostructures as an effective bifunctional electrocatalyst for hydrogen and oxygen evolution reactions (HER and OER) is of great interest in green hydrogen production. However, a few non-noble metals act as bifunctional electrocatalysts, exhibiting terrific HER and OER processes reported to date. Herein the intermetallic nickel-antimonide (Ni─Sb) dendritic nanostructure via cost-effective electro-co-deposition method is designed and their bifunctional electrocatalytic property toward HER and OER is unrevealed. The designed Ni─Sb delivers a superior bifunctional activity in 1 m KOH electrolyte, with a shallow overpotential of ≈119 mV at -10 mA for HER and ≈200 mV at 50 mA for OER. The mechanism behind the excellent bifunctional property of Ni─Sb is discussed via "interfacial descriptor" with the aid of Kelvin probe force microscopy (KPFM). This study reveals the rate of electrocatalytic reaction depends on the energy required for electron and proton transfer from the catalyst's surface. It is noteworthy that the assembled Ni─Sb-90 electrolyzer requires only a minuscule cell voltage of ≈1.46 V for water splitting, which is far superior to the art of commercial catalysts.

Interlayer Delocalized Electrons Activate Basal Plane for Ultrahigh-Current-Density Hydrogen Evolution

The basal plane of transition metal dichalcogenides (TMDCs) is inert for the hydrogen evolution reaction (HER) due to its low-efficiency charge transfer kinetics. We propose a strategy of filling the van der Waals (vdW) layer with delocalized electrons to enable vertical penetration of electrons from the collector to the adsorption intermediate vertically. Guided by density functional theory, we achieve this concept by incorporating Cu atoms into the interlayers of tantalum disulfide (TaS2). The delocalized electrons of d-orbitals of the interlayered Cu can constitute the charge transfer pathways in the vertical direction, thus overcoming the hopping migration through vdW gaps. The vertical conductivity of TaS2 increased by 2 orders of magnitude. The TaS2 basal plane HER activity was extracted with an on-chip microcell. Modified by the delocalized electrons, the current density increased by 20 times, reaching an ultrahigh value of 800 mA cm-2 at -0.4 V without iR compensation.

Spatially Confined Microcells: A Path toward TMD Catalyst Design

With the ability to maximize the exposure of nearly all active sites to reactions, two-dimensional transition metal dichalcogenide (TMD) has become a fascinating new class of materials for electrocatalysis. Recently, electrochemical microcells have been developed, and their unique spatial-confined capability enables understanding of catalytic behaviors at a single material level, significantly promoting this field. This Review provides an overview of the recent progress in microcell-based TMD electrocatalyst studies. We first introduced the structural characteristics of TMD materials and discussed their site engineering strategies for electrocatalysis. Later, we comprehensively described two distinct types of microcells: the window-confined on-chip electrochemical microcell (OCEM) and the droplet-confined scanning electrochemical cell microscopy (SECCM). Their setups, working principles, and instrumentation were elucidated in detail, respectively. Furthermore, we summarized recent advances of OCEM and SECCM obtained in TMD catalysts, such as active site identification and imaging, site monitoring, modulation of charge injection and transport, and electrostatic field gating. Finally, we discussed the current challenges and provided personal perspectives on electrochemical microcell research.

Three-dimensional bimodal pore-rich G/MXene sponge amalgamated with vanadium diselenide nanosheets as a high-performance electrode for electrochemical water-oxidation/reduction reactions

Exploring new strategies to design non-precious and efficient electrocatalysts can provide a solution for sluggish electrocatalytic kinetics and sustainable hydrogen energy. Transition metal selenides are potential contenders for bifunctional electrocatalysis owing to their unique layered structure, low band gap, and high intrinsic activities. However, insufficient access to active sites, lethargic water dissociation, and structural degradation of active materials during electrochemical reactions limit their activities, especially in alkaline media. In this article, we report a useful strategy to assemble vanadium diselenide (VSe2) into a 3D MXene/rGO-based sponge-like architecture (VSe2@G/MXe) using hydrothermal and freeze-drying approaches. The 3D hierarchical meso/macro-pore rich sponge-like morphology not only prevents aggregation of VSe2 nanosheets but also offers a kinetics-favorable framework and high robustness to the electrocatalyst. Synergistic coupling of VSe2 and a MXene/rGO matrix yields a heterostructure with a large specific surface area, high conductivity, and multi-dimensional anisotropic pore channels for uninterrupted mass transport and gas diffusion. Consequently, VSe2@G/MXe presented superior electrochemical activity for both the HER and OER compared to its counterparts (VSe2 and VSe2@G), in alkaline media. The overpotentials required to reach a cathodic and anodic current density of 10 mA cm-2 were 153 mV (Tafel slope = 84 mV dec-1) and 241 mV (Tafel slope = 87 mV dec-1), respectively. The Rct values at the open circuit voltage were as low as 9.1 Ω and 1.41 Ω for the HER and OER activity, respectively. Importantly, VSe2@G/MXe withstands a steady current output for a long 24 h operating time. Hence, this work presents a rational design for 3D microstructures with optimum characteristics for efficient bifunctional alkaline water-splitting.

What Makes On-Chip Microdevices Stand Out in Electrocatalysis?

Clean and sustainable energy conversion and storage through electrochemistry shows great promise as an alternative to traditional fuel or fossil-consumption energy systems. With regards to practical and high-efficient electrochemistry application, the rational design of active sites and the accurate description of mechanism remain a challenge. Toward this end, in this Perspective, a unique on-chip micro/nano device coupling nanofabrication and low-dimensional electrochemical materials is presented, in which material structure analysis, field-effect regulation, in situ monitoring, and simulation modeling are highlighted. The critical mechanisms that influence electrochemical response are discussed, and how on-chip micro/nano device distinguishes itself is emphasized. The key challenges and opportunities of on-chip electrochemical platforms are also provided through the Perspective.

The practice of reaction window in an electrocatalytic on-chip microcell

To enhance the efficiency of catalysis, it is crucial to comprehend the behavior of individual nanowires/nanosheets. A developed on-chip microcell facilitates this study by creating a reaction window that exposes the catalyst region of interest. However, this technology's potential application is limited due to frequently-observed variations in data between different cells. In this study, we identify a conductance problem in the reaction windows of non-metallic catalysts as the cause of this issue. We investigate this problem using in-situ electronic/electrochemical measurements and atom-thin nanosheets as model catalysts. Our findings show that a full-open window, which exposes the entire catalyst channel, allows for efficient modulation of conductance, which is ten times higher than a half-open window. This often-overlooked factor has the potential to significantly improve the conductivity of non-metallic catalysts during the reaction process. After examining tens of cells, we develop a vertical microcell strategy to eliminate the conductance issue and enhance measurement reproducibility. Our study offers guidelines for conducting reliable microcell measurements on non-metallic single nanowire/nanosheet catalysts.

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.

Au-Nanorods Supporting Pd and Pt Nanocatalysts for the Hydrogen Evolution Reaction: Pd Is Revealed to Be a Better Catalyst than Pt

Ordered thin films of Au nanorods (NRs) on Ti/Au/Si heterostructure substrates are electrodeposited in thin film aluminum oxide templates and, after template removal, serve as supports for Pd and Pt nanocatalysts. Based on previous work which showed a better electrocatalytic performance for layered Au/Pd nanostructures than monolithic Pd, electrodeposited 20 nm Pd discs on Au-NRs are first investigated in terms of their catalytic activity for the hydrogen evolution reaction (HER) and compared to monolithic 20 nm Pd and Pt discs. To further boost performance, the interfacial interaction area between the Au-NRs supports and the active metals (Pt and Pd) was increased via magnetron sputtering an extremely thin layer of Pt and Pd (20 nm overall sputtered thickness) on the Au-NRs after template removal. In this way, the whole NR surface (top and lateral) was covered with Pt and Pd nanoparticles, ensuring a maximum interfacial contact between the support and the active metal. The HER performance obtained was substantially higher than that of the other nanostructures. A Salient result of the present work, however, is the superior activity obtained for sputtered Pd on Au in comparison to that of sputtered Pt on Au. The results also show that increasing the Au-NR length translates in a strong increase in performance. Density functional theory calculations show that the interfacial electronic interactions between Au and Pd lead to suitable values of hydrogen adsorption energy on all possible sites, thus promoting faster (barrier-free diffusion) hydrogen adsorption and its recombination to H₂. A Volmer-Heyrovsky mechanism for HER is proposed, and a volcano plot is suggested based on the results of the Tafel plots and the calculated hydrogen adsorption energies.

Surface and near-surface engineering design of transition metal catalysts for promoting water splitting

Transition metal catalysts are widely used in the field of hydrogen production via water electrolysis. The surface state and near-surface environment of the catalysts greatly affect the efficiency of hydrogen production. Therefore, the rational design of surface engineering and near-surface engineering of transition metal catalysts can significantly improve the performance of water electrolysis. This review systematically introduces surface engineering strategies, including heteroatom doping, vacancy engineering, strain regulation, heterojunction effect, and surface reconstruction. These strategies optimize the surface electronic structure of the catalysts, expose more active sites, and promote the formation of highly active species, ultimately enhancing water electrolysis performance. Furthermore, near-surface engineering strategies, such as surface wettability, three-dimensional structure, high-curvature structure, external field assistance, and extra ion addition, are thoroughly discussed. These strategies expedite the mass transfer of reactants and gas products, improve the local chemical environment near the catalyst surface, and contribute toward achieving an industrial-level current density for overall water splitting. Finally, the key challenges faced by surface engineering and near-surface engineering of transition metal catalysts are highlighted and potential solutions are proposed. This review offers essential guidelines for the design and development of efficient transition metal catalysts for water electrolysis.

Electrochemistry at Back-Gated, Ultrathin ZnO Electrodes: Field-Effect Modulation of Heterogeneous Electron Transfer Rate Constants by 30× with Enhanced Gate Capacitance

We report steady-state voltammetry of outer-sphere redox species at back-gated ultrathin ZnO working electrodes in order to determine electron transfer rate constants kET as a function of independently controlled gate bias, VG. We demonstrate that kET can be modulated as much as 30-fold by application of VG ≤ 8 V. The key to this demonstration was integrating the ultrathin (5 nm) ZnO on a high dielectric constant (k) insulator, HfO2 (30 nm), which was grown on a Pd metal gate. The high-k HfO2 dramatically decreased the required VG values and increased the gate-induced charge in ZnO compared to previous studies. Importantly, the enhanced gating power of the Pd/HfO2/ZnO stack meant it was possible to observe a nonmonotonic dependence of kET on VG, which reflects the inherent density of redox acceptor states in solution. This work adds to the growing body of literature demonstrating that electrochemical kinetics (i.e., rate constants and overpotentials) at ultrathin working electrodes can be tuned by VG, independent of the conventional electrochemical working electrode potential.

Recent advances in understanding and design of efficient hydrogen evolution electrocatalysts for water splitting: A comprehensive review

An unsustainable reliance on fossil fuels is the primary cause of the vast majority of greenhouse gas emissions, which in turn lead to climate change. Green hydrogen (H₂), which may be generated by electrolyzing water with renewable power sources, is a possible substitute for fossil fuels. On the other hand, the increasing intricacy of hydrogen evolution electrocatalysts that are presently being explored makes it more challenging to integrate catalytic theories, catalytic fabrication procedures, and characterization techniques. This review will initially present the thermodynamics, kinetics, and associated electrical and structural characteristics for HER electrocatalysts before highlighting design approaches for the electrocatalysts. Secondly, an in-depth discussion regarding the rational design, synthesis, mechanistic insight, and performance improvement of electrocatalysts is centered on both the intrinsic and extrinsic influences. Thirdly, the most recent technological advances in electrocatalytic water-splitting approaches are described. Finally, the difficulties and possibilities associated with generating extremely effective HER electrocatalysts for water-splitting applications are discussed.

Bifunctional Monolayer WSe2/Graphene Self-Stitching Heterojunction Microreactors for Efficient Overall Water Splitting in Neutral Medium

Developing efficient bifunctional electrocatalysts in neutral media to avoid the deterioration of electrodes or catalysts under harsh environments has become the ultimate goal in electrochemical water splitting. This work demonstrates the fabrication of an on-chip bifunctional two-dimensional (2D) monolayer (ML) WSe₂/graphene heterojunction microreactor for efficient overall water splitting in a neutral medium (pH = 7). Through the synergistic atomic growth of the metallic Cr dopant and graphene stitching contact on the 2D ML WSe₂, the bifunctional WSe₂/graphene heterojunction microreactor consisting of a full-cell configuration demonstrates excellent performance for overall water splitting in a neutral medium. Atomic doping of metallic Cr atoms onto the 2D ML WSe₂ effectively facilitates the charge transfer at the solid-liquid interface. In addition, the direct growth of the self-stitching graphene contact with the 2D WSe₂ catalyst largely reduces the contact resistance of the microreactor and further improves the overall water splitting efficiency. A significant reduction of the overpotential of nearly 1000 mV at 10 mA cm^-2 at the Cr-doped WSe₂/graphene heterojunction microreactor compared to the ML pristine WSe₂ counterpart is achieved. The bifunctional WSe₂/graphene self-stitching heterojunction microreactor is an ideal platform to investigate the fundamental mechanism of emerging bifunctional 2D catalysts for overall water splitting in a neutral medium.

Solvent-Induced Degradation of Electrochemically Exfoliated Vanadium Selenide Visualized by Electron Microscopy

Recently discovered two-dimensional ferromagnetic materials (2DFMs) have rapidly gained much interest in the fields of spintronics and computing, where they may prove powerful tools for miniaturizing devices such as magnetic tunnel junctions and spin-transfer torque memory bits. In addition, heterojunctions and twisted bilayer stacks of such materials may yield exotic spin textures. However, preparation of such devices is complicated by the air sensitivity of many 2DFMs. Here, we report details on the preparation of few-to-monolayer flakes of vanadium selenide (VSe2) using electrochemical exfoliation in propylene carbonate. We also present a detailed study of the effects of air on the structure and magnetic properties of bare and passivated VSe2 after different concentrations of surface passivation treatment. We characterized the microstructure of holes in the VSe2 flakes and the formation of new compounds arising from air exposure, solvent exposure during the exfoliating process, and deliberate electron beam irradiation (sculpting). We sculpt VSe2 flakes while retaining the 1T-VSe2 lattice structure, opening the door for top-down patterned high-resolution 2DFM nanostructures. Additionally, investigation of the magnetic response of nanosheets using magnetic force microscopy (MFM) showed that the oxidation-induced damage only affects the surface fields locally and does not quench large-scale magnetic signal. The findings of this study pave the way toward practical incorporation of 2D ferromagnetic materials in nanoelectronics.

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.

Reinforcement of nanoporous lanthanum-doped zinc borate by vanadium selenide nanosheets for improved tribological activity

Nanoporous zinc borate (ZB) and 10% lanthanum-doped porous zinc borate (LZB) were synthesized to explore the role of porosity and doping in zinc borate during lubrication. HR-SEM, TEM, and HR-TEM authenticated nanoporous structures. The tribological properties of their blends with paraffin oil (PO) were compared by employing ASTM D4172 and ASTM D5183 norms on a four-ball tester. Vanadium selenide nanosheets (VSe2) were used to reinforce the structure of LZB for further advancement of the tribological properties. The superiority of the LZB/VSe2 over LZB and VSe2 nanosheets could be adjudged by tribological data. The porosity and lanthanum doping have yielded commendable tribological activity. The VSe2 nanosheets have strengthened the LZB matrix. The other constituent oxides of tribofilm from the LZB matrix, based on EDX analysis and XPS studies of the worn surface, ZnO, B2O3, La2O3, and V2O5, have abetted lubrication. The AFM and SEM investigations of wear track corroborated the tribological results.

Boosting the performance of single-atom catalysts via external electric field polarization

Single-atom catalysts represent a unique catalytic system with high atomic utilization and tunable reaction pathway. Despite current successes in their optimization and tailoring through structural and synthetic innovations, there is a lack of dynamic modulation approach for the single-atom catalysis. Inspired by the electrostatic interaction within specific natural enzymes, here we show the performance of model single-atom catalysts anchored on two-dimensional atomic crystals can be systematically and efficiently tuned by oriented external electric fields. Superior electrocatalytic performance have been achieved in single-atom catalysts under electrostatic modulations. Theoretical investigations suggest a universal "onsite electrostatic polarization" mechanism, in which electrostatic fields significantly polarize charge distributions at the single-atom sites and alter the kinetics of the rate determining steps, leading to boosted reaction performances. Such field-induced on-site polarization offers a unique strategy for simulating the catalytic processes in natural enzyme systems with quantitative, precise and dynamic external electric fields.

Rational Design of Better Hydrogen Evolution Electrocatalysts for Water Splitting: A Review

The excessive dependence on fossil fuels contributes to the majority of CO2 emissions, influencing on the climate change. One promising alternative to fossil fuels is green hydrogen, which can be produced through water electrolysis from renewable electricity. However, the variety and complexity of hydrogen evolution electrocatalysts currently studied increases the difficulty in the integration of catalytic theory, catalyst design and preparation, and characterization methods. Herein, this review first highlights design principles for hydrogen evolution reaction (HER) electrocatalysts, presenting the thermodynamics, kinetics, and related electronic and structural descriptors for HER. Second, the reasonable design, preparation, mechanistic understanding, and performance enhancement of electrocatalysts are deeply discussed based on intrinsic and extrinsic effects. Third, recent advancements in the electrocatalytic water splitting technology are further discussed briefly. Finally, the challenges and perspectives of the development of highly efficient hydrogen evolution electrocatalysts for water splitting are proposed.

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

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

Key Role of Lorentz Excitation in the Electromagnetic-Enhanced Hydrogen Evolution Reaction

Alternating magnetic fields (AMFs) are recently demonstrated as a promising strategy to promote the electrochemical catalytic reactions. However, the underlying mechanisms are still an open question. In this work, we systematically investigated the influence of AMFs on the hydrogen evolution reaction (HER) by using a Fe-Co-Ni-P-B magnetic catalyst. The HER catalytic efficiency is boosted significantly by AMFs, with 27% increase in current density at 20 mT. This is attributed to the enhancement of charge-transfer efficiency by Lorentz interaction with a minor contribution from the heating effect. The high magnetic permeability and skin effect of electromagnetic eddy current for the Fe-Co-Ni-P-B electrode can magnify the Lorentz effect. These findings clarify the mechanism of AMF-enhanced HER catalytic activities and open a door for designing a high-efficiency electrocatalysis system.

Controlled Synthesis of Ultrathin PtSe2 Nanosheets with Thickness-Tunable Electrical and Magnetoelectrical Properties

Thickness-dependent chemical and physical properties have gained tremendous interest since the emergence of two-dimensional (2D) materials. Despite attractive prospects, the thickness-controlled synthesis of ultrathin nanosheets remains an outstanding challenge. Here, a chemical vapor deposition (CVD) route is reported to controllably synthesize high-quality PtSe2 nanosheets with tunable thickness and explore their thickness-dependent electronic and magnetotransport properties. Raman spectroscopic studies demonstrate all Eg , A1 g , A2 u , and Eu modes are red shift in thicker nanosheets. Electrical measurements demonstrate the 1.7 nm thick nanosheet is a semiconductor with room temperature field-effect mobility of 66 cm2 V-1 s-1 and on/off ratio of 106 . The 2.3-3.8 nm thick nanosheets show slightly gated modulation with high field-effect mobility up to 324 cm2 V-1 s-1 at room-temperature. When the thickness is over 3.8 nm, the nanosheets show metallic behavior with conductivity and breakdown current density up to 6.8 × 105 S m-1 and 6.9 × 107 A cm-2 , respectively. Interestingly, magnetoresistance (MR) studies reveal magnetic orders exist in this intrinsically non-magnetic material system, as manifested by the thickness-dependent Kondo effect, where both metal to insulator transition and negative MR appear upon cooling. Together, these studies suggest that PtSe2 is an intriguing system for both developing novel functional electronics and conducting fundamental 2D magnetism study.

Application of two-dimensional layered materials in surface-enhanced Raman spectroscopy (SERS)

2D materials are promising SERS substrates. Seven feasible strategies to improve the SERS performance of 2D substrate materials are summarized. The prospect of future progress in SERS and possible challenges of 2D layered materials are put forwarded.

Nano-dendrite structured cobalt phosphide based hybrid supercapacitor with high energy storage and cycling stability

The supercapacitors possessing high energy storage and long serving period have strategic significance to solve the energy crisis issues. Herein, fluffy nano-dendrite structured cobalt phosphide (CoP) is grown on carbon cloth through simple hydrothermal and electrodeposition treatments (CoP/C-HE). Benefit from its excellent electrical conductivity and special structure, CoP/C-HE manifests a high specific capacity of 461.4 C g^-1at 1 A g^-1. Meanwhile, the capacity retention remains 92.8% over 10 000 cycles at 5 A g^-1, proving the superior cycling stability. The phase conversion of Co₂P during the activation process also contributes to the improved performance. The assembled two-electrode asymmetric supercapacitor demonstrates excellent performance in terms of energy density (42.4 W h kg^-1at a power density of 800.0 W kg^-1) and cycling stability (86.3% retention over 5000 cycles at 5 A g^-1), which is superior to many reported cobalt-based supercapacitors. Our work promotes the potential of transition metal phosphides for the applications in supercapacitors.

Phase-Transition Mo1-xVxSe2 Alloy Nanosheets with Rich V-Se Vacancies and Their Enhanced Catalytic Performance of Hydrogen Evolution Reaction

Alloys of transition-metal dichalcogenide can display distinctive phase evolution because of their two-dimensional structures. Herein, we report the colloidal synthesis of Mo_1-xV_xSe₂ alloy nanosheets with full composition tuning. Alloying led to a phase transition at x = 0.7 from the semiconducting 2H phase MoSe₂ to the metallic 1T phase VSe₂. It also produced significant V and Se vacancies, which became the richest in the 2H phase at x = 0.3-0.5. Extensive spin-polarized density functional theory calculations consistently predicted the 2H-1T phase transition at x = 0.7, in agreement with the experimental results. The vacancy formation energy also supports the formation of V and Se vacancies. Alloying in the 2H phase enhanced the electrocatalytic performance toward hydrogen evolution reaction (HER) at x = 0.3 (in 0.5 M H₂SO₄) or 0.4 (in 1 M KOH). The Gibbs free energy along the HER pathway indicates that this maximum performance is due to the highest concentration of active V and Se vacancy sites.

Substitutional Vanadium Sulfide Nanodispersed in MoS2 Film for Pt-Scalable Catalyst

Among transition metal dichalcogenides (TMdCs) as alternatives for Pt-based catalysts, metallic-TMdCs catalysts have highly reactive basal-plane but are unstable. Meanwhile, chemically stable semiconducting-TMdCs show limiting catalytic activity due to their inactive basal-plane. Here, metallic vanadium sulfide (VSn ) nanodispersed in a semiconducting MoS2 film (V-MoS2 ) is proposed as an efficient catalyst. During synthesis, vanadium atoms are substituted into hexagonal monolayer MoS2 to form randomly distributed VSn units. The V-MoS2 film on a Cu electrode exhibits Pt-scalable catalytic performance; current density of 1000 mA cm-2 at 0.6 V and overpotential of -0.08 V at a current density of 10 mA cm-2 with excellent cycle stability for hydrogen-evolution-reaction (HER). The high intrinsic HER performance of V-MoS2 is explained by the efficient electron transfer from the Cu electrode to chalcogen vacancies near vanadium sites with optimal Gibbs free energy (-0.02 eV). This study provides insight into ways to engineer TMdCs at the atomic-level to boost intrinsic catalytic activity for hydrogen evolution.

Activating a Two-Dimensional PtSe2 Basal Plane for the Hydrogen Evolution Reaction through the Simultaneous Generation of Atomic Vacancies and Pt Clusters

Two-dimensional (2D) PtSe₂ has emerged as a promising ultrathin electrocatalyst due to its excellent catalytic activity and conductivity. However, the PtSe₂ basal plane is inert for the hydrogen evolution reaction (HER), which greatly limits its electrocatalytic performance. Here, in light of theoretical calculations, we designed a facile approach for activating the 2D PtSe₂ basal plane for the HER by simultaneously introducing atomic vacancies of Se, Pt, and Pt clusters through a mild Ar plasma treatment. We tracked changes in the structures and catalytic performance of PtSe₂ by combining microscopic imaging, spectroscopic mapping, and electrochemical measurements in microcells. The highest performance of the activated PtSe₂ basal plane that we obtained was superior to those of other 2D transition metal dichalcogenide-based electrocatalysts measured in microcells in terms of the overpotential, the Tafel slope, and the exchange current density. This study demonstrates the great potential of activated 2D PtSe₂ as an ultrathin catalyst for the HER and provides new insights on the rational design of 2D electrocatalysts.

Defect-engineered three-dimensional vanadium diselenide microflowers/nanosheets on carbon cloth by chemical vapor deposition for high-performance hydrogen evolution reaction

In the past decades, defect engineering has become an effective strategy to significantly improve the hydrogen evolution reaction (HER) efficiency of electrocatalysts. In this work, a facile chemical vapor deposition (CVD) method is firstly adopted to demonstrate defect engineering in high-efficiency HER electrocatalysts of vanadium diselenide nanostructures. For practical applications, the conductive substrate of carbon cloth (CC) is selected as the growth substrate. By using a four-time CVD method, uniform three-dimensional microflowers with defect-rich small nanosheets on the surface are prepared directly on the CC substrate, displaying a stable HER performance with a low Tafel slope value of 125 mV dec^-1and low overpotential voltage of 295 mV at a current density of 10 mA cm^-2in alkaline electrolyte. Based on the results of x-ray photoelectron spectra and density functional theory calculations, the impressive HER performance originates from the Se vacancy-related active sites of small nanosheets, while the microflower/nanosheet homoepitaxy structure facilitates the carrier flow between the active sites and conductive substrate. All the results present a new route to achieve defect engineering using the facile CVD technique, and pave a novel way to prepare high-activity layered electrocatalysts directly on a conductive substrate.

Recent Advances in 2D Group VB Transition Metal Chalcogenides

2D materials have received considerable research interest owing to their abundant material systems and remarkable properties. Among them, 2D group VB transition metal chalcogenides (GVTMCs) stand out as emerging 2D metallic materials and significantly broaden the research scope of 2D materials. 2D GVTMCs have great advantages in electrical transport, 2D magnetism, charge density wave, sensing, catalysis, and charge storage, making them attractive in the fields of functional devices and energy chemistry. In this review, the recent progress of 2D GVTMCs is summarized systematically from fundamental properties, growth methodologies to potential applications. The challenges and prospects are also discussed for future research in this field.

Single WTe2 Sheet-Based Electrocatalytic Microdevice for Directly Detecting Enhanced Activity of Doped Electronegative Anions

The high electrical conductivity of 1T'-WTe₂ deserves particular attention and may show a high potential for hydrogen evolution reaction (HER) catalysis. However, the actual activity certainly does not match expectations, and the inferior HER activity is actually still ambiguous at the atomic level. Unraveling the underlying HER behaviors of 1T'-WTe₂ will give rise to a new family of HER catalysts. Our structural analysis reveals that the inferior activity could result from insufficient charge density around the Te site and blocked adsorption channel at the W site, which cause too weak hydrogen adsorption. Herein, we fabricated a single WTe₂ sheet-based electrocatalytic microdevice for directly extracting enhanced HER activity of doped electronegative F atoms. The overpotential at -10 mA cm^-2 reduced to 0.27 V after F doping compared to 0.45 V for the original state. In situ electrochemical measurement and electrical tests on a single sheet indicate that doped F can regulate surface charge and hydrogen adsorption behavior. Furthermore, the theory simulation uncovers that the smaller atomic radius of F contributes to an empty coordination environment; meanwhile, strong electronegativity induces hydrogen adsorption. Thus, the ΔG_H^* at W sites around the doped F is as low as 0.18 eV. Synergistically modulating the charge properties and opening steric hindrance provides a new pathway to rationally construct electrocatalysts and beyond.

2D Transition Metal Dichalcogenides: Design, Modulation, and Challenges in Electrocatalysis

Hydrogen has been deemed as an ideal substitute fuel to fossil energy because of its renewability and the highest energy density among all chemical fuels. One of the most economical, ecofriendly, and high-performance ways of hydrogen production is electrochemical water splitting. Recently, 2D transition metal dichalcogenides (also known as 2D TMDs) showed their utilization potentiality as cost-effective hydrogen evolution reaction (HER) catalysts in water electrolysis. Herein, recent representative research efforts and systematic progress made in 2D TMDs are reviewed, and future opportunities and challenges are discussed. Furthermore, general methods of synthesizing 2D TMDs materials are introduced in detail and the advantages and disadvantages for some specific methods are provided. This explanation includes several important regulation strategies of creating more active sites, heteroatoms doping, phase engineering, construction of heterostructures, and synergistic modulation which are capable of optimizing the electrical conductivity, exposure to the catalytic active sites, and reaction energy barrier of the electrode material to boost the HER kinetics. In the last section, the current obstacles and future chances for the development of 2D TMDs electrocatalysts are proposed to provide insight into and valuable guidelines for fabricating effective HER electrocatalysts.

Photocatalysis Enhanced by External Fields

The efficient conversion of solar energy by means of photocatalysis shows huge potential to relieve the ongoing energy crisis and increasing environmental pollution. However, unsatisfactory conversion efficiency still hinders its practical application. The introduction of external fields can remarkably enhance the photocatalytic performance of semiconductors from the inside out. This review focuses on recent advances in the application of diverse external fields, including microwaves, mechanical stress, temperature gradient, electric field, magnetic field, and coupled fields, to boost photocatalytic reactions, for applications in, for example, contaminant degradation, water splitting, CO₂ reduction, and bacterial inactivation. The relevant reinforcement mechanisms of photoabsorption, the transport and separation of photoinduced charges, and adsorption of reagents by the external fields are highlighted. Finally, the challenges and outlook for the development of external-field-enhanced photocatalysis are presented.

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

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

Promoting heterogeneous catalysis beyond catalyst design

Despite the indisputable success of conventional approaches to manipulate the performance of heterogeneous catalysts by tuning the composition and structure of active sites, future research on catalysis engineering will likely go beyond the catalyst itself. Recently, several auxiliary promotion methods, either promoting the activity of reagents or enabling optimized adsorbate-catalyst interactions, have been proven as viable strategies to enhance catalytic reactions. Those auxiliary promotion methods range from electric/magnetic fields and electric potentials to mechanic stress, significantly altering the properties of reagent molecules and/or the surface characteristics of nanostructured catalysts. Apart from static enhancement effects, they in principle also allow for spatially and temporally variable modifications of catalyst surfaces. While some of those methods have been demonstrated, some are only theoretically predicted, opening exciting avenues for future experimental advances. Besides fundamental descriptions and comparisons of each activation method, in this perspective we plan to provide examples for the applications of those techniques for a variety of catalytic reactions as diverse as N₂ and CO₂ hydrogenation as well as electrochemical water splitting. Finally, we provide a unifying view and guidelines for future research into the use of promotion methods, generating deeper understanding of the complex dynamics on the nanoparticle surface under auxiliary promotion and the expansion of auxiliary techniques to different sustainability-related reactions.

Reversing Interfacial Catalysis of Ambipolar WSe2 Single Crystal

An improved understanding of the origin of the electrocatalytic activity is of importance to the rational design of highly efficient electrocatalysts for the hydrogen evolution reaction. Here, an ambipolar single-crystal tungsten diselenide (WSe2) semiconductor is employed as a model system where the conductance and carrier of WSe2 can be individually tuned by external electric fields. The field-tuned electrochemical microcell is fabricated based on the single-crystal WSe2 and the catalytic activity of the WSe2 microcell is measured versus the external electric field. Results show that WSe2 with electrons serving as the dominant carrier yields much higher activity than WSe2 with holes serving as the dominant carrier even both systems exhibit similar conductance. The catalytic activity enhancement can be characterized by the Tafel slope decrease from 138 to 104 mV per decade, while the electron area concentration increases from 0.64 × 1012 to 1.72 × 1012 cm-2. To further understand the underlying mechanism, the Gibbs free energy and charge distribution for adsorbed hydrogen on WSe2 versus the area charge concentration is systematically computed, which is in line with experiments. This comprehensive study not only sheds light on the mechanism underlying the electrocatalysis processes, but also offers a strategy to achieve higher electrocatalytic activity.

Generation of Ni3S2 nanorod arrays with high-density bridging S22- by introducing a small amount of Na3VO4·12H2O for superior hydrogen evolution reaction

Bridging S22- moieties have been demonstrated to be highly active sites existing in metal polysulfides for the hydrogen evolution reaction (HER), thus the incorporation of high-density bridging S22- into a Ni3S2 material to improve its electrocatalytic HER performance is highly desirable and challenging. Herein, we report a novel Ni3S2 nanorod array decorated with (020)-oriented VS4 nanocrystals grown on nickel foam (Shig-NS-rod/NF) via a simple and facile solvothermal method. Results show that the in situ incorporation of VS4 not only triggers the formation of such a nanorod array structure, but also contributes to the uniform grafting of high-density and high catalytically active bridging S22- sites on the interface between Ni3S2 and VS4 for enhanced HER activity, and also promotes the absorption ability of OH- radicals and thus accelerates the HER Volmer step in alkaline media. As expected, the resultant Shig-NS-rod/NF material exhibits impressive catalytic performance toward the HER, with a much lower overpotential of 137 mV at 10 mA cm-2 and a long-term durability for at least 22 h, and is superior to Ni3S2 nanorod arrays with low-density bridging S22- (Slow-NS-rod/NF) and NS-film/NF counterparts (without VS4), even outperforming the NF-supported 20% Pt/C at a large current density of over 120 mA cm-2. Our findings put forward fresh insight into the rational design of highly efficient electrocatalysts toward the HER for green hydrogen fuel production.

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.

Boosting Alkaline Hydrogen Evolution Electrocatalysis over Metallic Nickel Sites through Synergistic Coupling with Vanadium Sesquioxide

For renewable and sustainable energy, developing cut-price and high-efficiency electrocatalysts for the hydrogen evolution reaction (HER) by alkaline water electrolysis is of paramount importance. In this study, a compound electrocatalyst composed of nickel-vanadium sesquioxide nanoparticles supported on porous nickel foam (Ni-V₂ O₃ /NF) is found to exhibit electrocatalytic performance towards HER that is superior to that of the commercial Pt/C catalyst, with nearly zero onset overpotential, an extremely low overpotential of 25 mV to obtain a current density of -10 mA cm^-2 , a Tafel slope of 58 mV dec^-1 , and a good durability for 24 h in 1.0 m KOH. Theoretical calculations reveal that the presence of V₂ O₃ optimizes the electronic structure of active Ni components and continuously accelerates the dissociation of water molecules, which in turn improves the HER kinetics. The present work will advance the development of highly efficient nanocomposite electrocatalysts for alkaline water electrocatalysis.

Is Charge-Transfer Doping Possible at the Interfaces of Monolayer VSe2 with MoO3 and K?

Being a metallic transition-metal dichalcogenide, monolayer vanadium diselenide (VSe₂) exhibits many novel properties, such as charge density waves and magnetism. Its interfaces with other materials can potentially be used in device applications as well as for manipulating its intrinsic properties. Here, we present a scanning tunneling microscopy and synchrotron-based X-ray photoemission spectroscopy study of the surface charge-transfer doping using efficient electron-withdrawing and electron-donating materials, that is, molybdenum trioxide (MoO₃) and potassium (K), on the molecular beam epitaxy-grown monolayer VSe₂ on highly oriented pyrolytic graphite (HOPG). We demonstrate that monolayer VSe₂ is immune to MoO₃- and K-doping effects. However, at the monolayer edges where the local chemical reactivity is higher because of Se deficiency, MoO₃ is seen to react with VSe₂ to form molybdenum dioxide (MoO₂) and vanadium dioxide (VO₂). Compared to the obvious charge-transfer doping effects of MoO₃ and K on HOPG, the electronic structure of monolayer VSe₂ is barely perturbed. This is attributed to the large density of states at the Fermi level of monolayer VSe₂ carrying the metallic character. This work provides new insights into the chemical and electronic properties of monolayer VSe₂, important for future VSe₂-based electronic device design.

Field Effect Modulation of Electrocatalytic Hydrogen Evolution at Back-Gated Two-Dimensional MoS2 Electrodes

Electrocatalytic activity for hydrogen evolution at monolayer MoS₂ electrodes can be enhanced by the application of an electric field normal to the electrode plane. The electric field is produced by a gate electrode lying underneath the MoS₂ and separated from it by a dielectric. Application of a voltage to the back-side gate electrode while sweeping the MoS₂ electrochemical potential in a conventional manner in 0.5 M H₂SO₄ results in up to a 140 mV reduction in overpotential for hydrogen evolution at current densities of 50 mA/cm². Tafel analysis indicates that the exchange current density is correspondingly improved by a factor of four to 0.1 mA/cm² as gate voltage is increased. Density functional theory calculations support a mechanism in which the higher hydrogen evolution activity is caused by gate-induced increase in the electronic charge on Mo metal centers adjacent to the S vacancies (the active sites), leading to enhanced Mo-H bond strengths. Overall, our findings indicate that the back-gated working electrode architecture is a convenient and versatile platform for investigating the connection between tunable electronic charge at active sites and overpotential for electrocatalytic processes on ultrathin electrode materials.

Role of Sulfur Vacancies and Undercoordinated Mo Regions in MoS2 Nanosheets toward the Evolution of Hydrogen

Low-dimensional materials have been examined as electrocatalysts for the hydrogen evolution reaction (HER). Among them, two-dimensional transition metal dichalcogenides (2D-TMDs) such as MoS₂ have been identified as potential candidates. However, the performance of TMDs toward HER in both acidic and basic media remains inferior to that of noble metals such as Pt and its alloys. This calls for investigating the influence of controlled defect engineering of 2D TMDs on their performance toward hydrogen production. Here, we explored the HER activity from defective multilayered MoS₂ over a large range of surface S vacancy concentrations up to 90%. Amorphous MoS₂ and 2H MoS₂ with ultrarich S vacancies demonstrated the highest HER performance in acid and basic electrolytes, respectively. We also report that the HER performance from multilayered MoS₂ can be divided into two domains corresponding to "point defects" at low concentrations of surface S vacancies (Stage 1) and large regions of undercoordinated Mo atoms for high concentrations of surface S vacancies (Stage 2). The highest performance is obtained for Stage 2 in the presence of undercoordinated Mo atoms with a TOF of ∼2 s^-1 at an overpotential of 160 mV in 0.1 M KOH which compares favorably to the best results in the literature. Overall, our work provides deeper insight on the HER mechanism from defected MoS₂ and provides guidance for the development of defect-engineered TMD-based electrocatalysts.

Evidence of Spin Frustration in a Vanadium Diselenide Monolayer Magnet

Monolayer VSe₂ , featuring both charge density wave and magnetism phenomena, represents a unique van der Waals magnet in the family of metallic 2D transition-metal dichalcogenides (2D-TMDs). Herein, by means of in situ microscopy and spectroscopic techniques, including scanning tunneling microscopy/spectroscopy, synchrotron X-ray and angle-resolved photoemission, and X-ray absorption, direct spectroscopic signatures are established, that identify the metallic 1T-phase and vanadium 3d¹ electronic configuration in monolayer VSe₂ grown on graphite by molecular-beam epitaxy. Element-specific X-ray magnetic circular dichroism, complemented with magnetic susceptibility measurements, further reveals monolayer VSe₂ as a frustrated magnet, with its spins exhibiting subtle correlations, albeit in the absence of a long-range magnetic order down to 2 K and up to a 7 T magnetic field. This observation is attributed to the relative stability of the ferromagnetic and antiferromagnetic ground states, arising from its atomic-scale structural features, such as rotational disorders and edges. The results of this study extend the current understanding of metallic 2D-TMDs in the search for exotic low-dimensional quantum phenomena, and stimulate further theoretical and experimental studies on van der Waals monolayer magnets.

Hot electron-driven electrocatalytic hydrogen evolution reaction on metal-semiconductor nanodiode electrodes

Hot electrons generated on metal catalysts influence atomic and molecular processes, leading to hot electron-driven catalytic reactions. Here, we show the acceleration of electrocatalytic hydrogen evolution caused by internal injection of hot electrons on Pt/Si metal–semiconductor electrodes. When a forward bias voltage is applied to the Pt/Si contact, hot electrons are injected. The excess energy of these electrons allows them to reach the Pt/electrolyte interface and reduce the adsorbed hydrogen ions to form H₂ (2H⁺ + 2e⁻→H₂). We show that the onset potential of the hydrogen evolution reaction shifts positively by 160 mV while the cathodic current exhibits an 8-fold increase in the presence of hot electrons. The effect disappears when the thickness of the Pt film exceeds the mean free path of the hot electrons. The concept of a hot electron-driven reaction can lead to the development of a novel mechanism for controlling reactivity at liquid–solid interfaces.