Active site manipulation in MoS2 cluster electrocatalysts by transition metal doping

Atomic-Level Engineering of Transition Metal Dichalcogenides for Enhanced Hydrogen Evolution Reaction

2D transition metal dichalcogenides (2D-TMDs) have attracted considerable attention due to their characteristic layered structures, which provide abundant accessible surface sites. Significant research efforts are dedicated to designing nanostructures and regulating electron properties to enhance the catalytic performance of the hydrogen evolution reaction (HER) of TMDs. However, elucidating the HER mechanism, particularly the role of active sites, remains challenging owing to the complex surface and electronic structures introduced by nanoscale modification. Recent advances have focused on achieving efficient HER catalysis through atomic-level control of TMD surface structures and precise identification of the coordination environment of active sites. Atomic-level engineering of TMDs, including incorporating or removing specific atoms onto the basal surfaces or within the interlayer via advanced synthetic approaches, has emerged as a promising strategy. These modifications optimize the adsorption/desorption energy of H, increase the density of active sites, and create synergetic active sites by arranging atoms in controlled configuration, in single-atomic modified TMDs (SA-TMDs) catalysts. Further, the insights of a notable increase of HER performance in SA-TMDs are discussed in detail when compared to both their pure and conventionally doped counterparts. This review aims to advance the understanding of atomic-level catalysis and provides a basis for developing next-generation materials for energy applications.

Electrocatalytic biomass upgrading coupled with hydrogen evolution and CO2 reduction

Clean energy production and CO2 utilization have attracted increasing interest. Electrocatalysis represents an effective way to produce green hydrogen from water and reduce CO2 to valuable compounds. However, for either the hydrogen evolution reaction (HER) or the CO2 reduction reaction (CO2RR), the reaction efficiency is significantly limited by the slow kinetics of the oxygen evolution reaction (OER) at the anode, which consumes most of the input energy. Therefore, great efforts have been made to replace the OER with organic oxidation reactions at the anode to decrease the reaction energy barrier. Biomass has an advantage of broad source, and when it is employed as an OER alternative in the anode oxidation reactions, not only can the reduction reaction efficiency at the cathode including the HER and CO2RR be enhanced but high-value chemicals can also be obtained, representing an attractive OER alternative. This review comprehensively summarizes the recent achievements in electrocatalytic biomass upgrading coupled with the HER and CO2RR, cataloged based on the type of biomass. The design of electrocatalysts for such coupled reaction systems is discussed. Finally, the challenges and perspectives in the field of this energy-saving and value-added coupling system are provided to inspire more efforts in pushing forward the development of this field.

Doping-Induced Enhancement of Hydrogen Evolution at MoS2 Electrodes

Rate theory and DFT calculations of hydrogen evolution reaction (HER) on MoS2 with Co, Ni and Pt impurities show the significance of dihydrogen (H2*) complex where both hydrogen atoms are interacting with the surface. Stabilization of such a complex affects the competing Volmer-Heyrovsky (direct H2 release) and Volmer-Tafel (H2* intermediate) pathways. The resulting evolution proceeds with a very small overpotential for all dopants ( η ${\eta }$ =0.1 to 0.2 V) at 25 % edge substitution, significantly reduced from the already low η ${\eta }$ =0.27 V for the undoped edge. At full edge substitution, Co-MoS2 remains highly active ( η ${\eta }$ =0.18 V) while Ni- and Pt-MoS2 are deactivated ( η ${\eta }$ =0.4 to 0.5 V) due to unfavorable interaction with H2*. Instead of the single S-vacancy, the site of intrinsic activity in the basal plane was found to be the undercoordinated central Mo-atom in threefold S-vacancy configurations, enabling hydrogen evolution with η ${\eta }$ =0.52 V via a H2* intermediate. The impurity atoms interact favorably with the intrinsic sulfur vacancies on the basal plane, stabilizing but simultaneously deactivating the triple vacancy configuration. The calculated shifts in overpotential are consistent with reported measurements, and the dependence on doping level may explain variations in experimental observations.

Unraveling Hydrogen Adsorption on Transition Metal-Doped [Mo3S13]2- Clusters: Insights from Density Functional Theory Calculations

Molecular and dissociative hydrogen adsorption of transition metal (TM)-doped [Mo3S13]2- atomic clusters were investigated using density functional theory calculations. The introduced TM dopants form stable bonds with S atoms, preserving the geometric structure. The S-TM-S bridging bond emerges as the most stable configuration. The preferred adsorption sites were found to be influenced by various factors, such as the relative electronegativity, coordination number, and charge of the TM atom. Notably, the presence of these TM atoms remarkably improved the hydrogen adsorption activity. The dissociation of a single hydrogen molecule on TM[Mo3S13]2- clusters (TM = Sc, Cr, Mn, Fe, Co, and Ni) is thermodynamically and kinetically favorable compared to their bare counterparts. The extent of favorability monotonically depends on the TM impurity, with a maximum activation barrier energy ranging from 0.62 to 1.58 eV, lower than that of the bare cluster (1.69 eV). Findings provide insights for experimental research on hydrogen adsorption using TM-doped molybdenum sulfide nanoclusters, with potential applications in the field of hydrogen energy.

3D-printed MoS2/Ni electrodes with excellent electro-catalytic performance and long-term stability for dechlorination of florfenicol

Here, we report the production of 3D-printed MoS2/Ni electrodes (3D-MoS2/Ni) with long-term stability and excellent performance by the selective laser melting (SLM) technique. As a cathode, the obtained 3D-MoS2/Ni could maintain a degradation rate above 94.0% for florfenicol (FLO) when repeatedly used 50 times in water. We also found that the removal rate of FLO by 3D-MoS2/Ni was about 12 times higher than that of 3D-printed pure Ni (3D-Ni), attributed to the improved accessibility of H*. In addition, the electrochemical characterization results showed that the electrochemically active surface area of the 3D-MoS2/Ni electrode is about 3-fold higher than that of the 3D-Ni electrode while the electrical resistance is 4 times lower. Based on tert-butanol suppression, electron paramagnetic resonance and triple quadrupole mass spectrometer experiments, a "dual path" mechanism and possible degradation pathway for the dechlorination of FLO by 3D-MoS2/Ni were proposed. Furthermore, we also investigated the impacts of the cathode potential and the initial pH of the solution on the degradation of FLO. Overall, this study reveals that the SLM 3D printing technique is a promising approach for the rapid fabrication of high-stability metal electrodes, which could have broad application in the control of water contaminants in the environmental field.

Electrocatalytic study of the hydrogen evolution reaction on MoS2/BP and MoSSe/BP in acidic media

Molecular hydrogen (H2) production by the electrochemical hydrogen evolution reaction (HER) is being actively explored for non-precious metal-based electrocatalysts that are earth-abundant and low cost like MoS2. Although it is acid-stable, its applicability is limited by catalytically inactive basal planes, poor electrical transport and inefficient charge transfer at the interface. Therefore, the present work examines its bilayer van der Waals heterostructure (vdW HTS). The second constituent monolayer boron phosphide (BP) is advantageous as an electrode material owing to its chemical stability in both oxygen and water environments. Here, we have performed first-principles based calculations under the framework of density functional theory (DFT) for the HER in an electrochemical double layer model with the BP monolayer, MoS2/BP and MoSSe/BP vdW HTSs. The climbing image nudged elastic band method (CI-NEB) has been employed to determine the minimum energy pathways for Tafel and Heyrovsky reactions. The calculations reveal that the Tafel reaction shows no reaction barrier. Thereafter, for the Heyrovsky reaction, we obtained a low reaction barrier in the vdW HTSs as compared to that in the BP monolayer. Subsequently, we have observed no significant difference in the reaction profile of MoS2/BP and MoSSe/BP vdW HTSs in the case of 2 × 2 supercell configuration. However, in the case of 3 × 3 and 4 × 4 configurations, MoSSe/BP shows a feasible Heyrovsky reaction with no reaction barrier. The coverages with 1/4H+ concentration (conc.) deduced high coverage with low conc. and low coverage with high conc. to be apt for the HER via the Heyrovsky reaction path. Finally, on observing the activation barrier of the Heyrovsky pathway along with that of second H adsorption at the surface, the Heyrovsky path is expected to be favoured.

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.

meta-Position synergistic effect induced by Ni-Mo co-doped WSe2 to enhance the hydrogen evolution reaction

Transition metal dichalcogenides have been the most attractive two-dimensional layered materials for electrocatalytic hydrogen evolution due to their unique structure and multi-phase electronic states. However, the enhancement of the WSe₂ electrocatalytic hydrogen evolution reaction (HER) performance by bimetal co-doping has been rarely reported. Herein, the NiMo-WSe₂ catalyst has been synthesized by a one-step hydrothermal reaction, with lower overpotentials of 177 and 188 mV at a current density of 10 mA cm^-2 in 0.5 M H₂SO₄ and 1 M KOH, respectively. The large specific surface area and thinner edge morphology provide more active sites for hydrogen production, thereby significantly improving the charge transfer kinetics. Density functional theory calculation results show that under acidic conditions the ΔG_H* values of NiMo-WSe₂ with different structures and hydrogen adsorption sites are also different, when the hydrogen adsorption site was located at the top of the Se-Ni bond, the meta NiMo-WSe₂ has a ΔG_H* value (-0.04 eV) that is closest to 0. Meanwhile, NiMo-WSe₂ (meta) also has a minimum of ΔG_H* under alkaline conditions. DOS confirmed that Ni doping has a large impact on the electronic states at the WSe₂ Fermi level, while NiMo co-doping greatly reduces the potential energy barrier of the HER reaction, jointly increasing the current density, and thus improving the HER performance.

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.

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.

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.

Size-dependent phase stability in transition metal dichalcogenide nanoparticles controlled by metal substrates

Nanomaterials based on MoS₂ and related transition metal dichalcogenides (TMDCs) are remarkably versatile; MoS₂ nanoparticles are proven catalysts for processes such as hydrodesulphurization and the hydrogen evolution reaction, and transition metal dichalcogenides in general have recently emerged as novel 2D components for nanoscale electronics and optoelectronics. The properties of such materials are intimately related to their structure and dimensionality. For example, only the edges exposed by MoS₂ nanoparticles (NPs) are catalytically active, and extended MoS₂ systems show different character (direct or indirect gap semiconducting, or metallic) depending on their thickness and crystallographic phase. In this work, we show how particle size and interaction with a metal surface affect the stability and properties of different MoS₂ NPs and the resulting phase diagrams. By means of calculations based on the Density Functional Theory (DFT), we address how support interactions affect MoS₂ nanoparticles of varying size, composition, and structure. We demonstrate that interaction with Au modifies the relative stability of the different nanoparticle types so that edge terminations and crystallographic phases that are metastable for free-standing nanoparticles and monolayers are expressed in the supported system. These support-effects are strongly size-dependent due to the mismatch between Au and MoS₂ lattices, which explains experimentally observed transitions in the structural phases for supported MoS₂ NPs. Accounting for vdW interactions and the contraction of the Au(111) surface underneath the MoS₂ is further found to be necessary for quantitatively reproducing experimental results. We finally find that support-induced effects on the stability of nanoparticle structures are general to TMDC nanoparticles on metal surfaces, which we demonstrate also for MoS₂ on Au(111), WS₂ on Au(111), and WS₂ on Ag(111). This work demonstrates how the properties of nanostructured transition metal dichalcogenides and similar layered systems can be modified by the choice of supporting metal.

An in situ phosphorization strategy towards doped Co2P scaffolded within echinus-like carbon for overall water splitting

Eco-environmental synthesis of non-expensive electrocatalysts such as transition-metal phosphides (TMPs) is critical to advancing renewable hydrogen fuel. TMP nanostructures prepared typically by introducing additional conventional phosphorus sources are suggested as promising durable and low-cost electrocatalysts. Herein, an eco-efficient guest/host precursor-based synthesis route is demonstrated to prepare doped Co₂P scaffolded within echinus-like carbon ((M_0.2Co_0.8)₂P@C, M = Fe and Ni) as electrocatalysts for overall water splitting. (Fe_0.2Co_0.8)₂P@C is derived by directly pyrolyzing a precursor of sodium dodecyl phosphate-intercalated CoFe-layered double hydroxide (CoFe-LDH), without introducing any additional phosphorus source. Electrocatalytic testing shows that (Fe_0.2Co_0.8)₂P@C requires overpotentials of 290 and 130 mV at a current density of 10 mA cm^-2 for oxygen and hydrogen evolution reactions (OER and HER) in an alkaline electrolyte, respectively. Furthermore, a different (Ni_0.2Co_0.8)₂P@C composite, obtained only by altering a NiCo-LDH host, exhibits better electrocatalytic activities than those of Fe-doped (Fe_0.2Co_0.8)₂P@C. In particular, the (No_0.2Co_0.8)₂P@C||(Ni_0.2Co_0.8)₂P@C electrolyzer affords a current density of 10 mA cm^-2 at a decent voltage of 1.62 V for overall water splitting. Electron energy-loss spectroscopy (EELS) observations show the oxyhydroxide layer formed on the surface, and density functional theory (DFT) calculations reveal that Fe-/Ni-doping lowers the Gibbs free energy barrier for the OER and the HER, both underpinning the enhancements.

Principles of Operation of a Rotating Wall Mass Analyzer for Preparative Mass Spectrometry

In this contribution, we describe the principles of operation of a rotating wall mass analyzer (RWMA), a mass-dispersive device for preparative mass spectrometry. Ions of different m/z are spatially separated by RWMA and deposited onto ring-shaped areas of distinct radii on a surface. We use a combination of an analytical equation for predicting the radius of the deposition ring and SIMION simulations to understand how to optimize the experimental conditions for the separation of multicomponent mixtures. The results of these simulations are compared with the experimental data. We introduce a universal mass calibration procedure, based on a series of polyacrylamide ions, which is subsequently used to predict the deposition radii of unknown analytes. The calibration is independent of the polarity, kinetic energy, and charge state of the ion as demonstrated by assigning m/z values of different analytes including multiply charged ubiquitin ions. We demonstrate that mass resolution of the RWMA is affected by the width and kinetic energy distribution of the ion beam. The best mass resolution obtained in this study is m/Δm = ∼20. Preparative mass spectrometry using RWMA provides the advantages of simplicity, compactness, and low fabrication cost, which are particularly promising for the development of miniaturized instrumentation. The results presented in this work can be readily adapted to preparative separation of a variety of charged species of interest to the broad scientific community.