Ionic‐Liquid‐Mediated Active‐Site Control of MoS2 for the Electrocatalytic Hydrogen Evolution Reaction

The layered crystal MoS₂ has been proposed as an alternative to noble metals as the electrocatalyst for the hydrogen evolution reaction (HER). However, the activity of this catalyst is limited by the number of available edge sites. It was previously shown that, by using an imidazolium ionic liquid as synthesis medium, nanometre‐size crystal layers of MoS₂ can be prepared which exhibit a very high number of active edge sites as well as a de‐layered morphology, both of which contribute to HER electrocatalytic activity. Herein, it is examined how to control these features synthetically by using a range of ionic liquids as synthesis media. Non‐coordinating ILs with a planar heterocyclic cation produced MoS₂ with the de‐layered morphology, which was subsequently shown to be highly advantageous for HER electrocatalytic activity. The results furthermore suggest that the crystallinity, and in turn the catalytic activity, of the MoS₂ layers can be improved by employing an IL with specific solvation properties. These results provide the basis for a synthetic strategy for increasing the HER electrocatalytic activity of MoS₂ by tuning its crystal properties, and thus improving its potential for use in hydrogen production technologies.     

High‐Resolution Electrochemical Mapping of the Hydrogen Evolution Reaction on Transition‐Metal Dichalcogenide Nanosheets

High‐resolution scanning electrochemical cell microscopy (SECCM) is used to image and quantitatively analyze the hydrogen evolution reaction (HER) catalytically active sites of 1H‐MoS₂ nanosheets, MoS₂, and WS₂ heteronanosheets. Using a 20 nm radius nanopipette and hopping mode scanning, the resolution of SECCM was beyond the optical microscopy limit and visualized a small triangular MoS₂ nanosheet with a side length of ca. 130 nm. The electrochemical cell provides local cyclic voltammograms with a nanoscale spatial resolution for visualizing HER active sites as electrochemical images. The HER activity difference of edge, terrace, and heterojunction of MoS₂ and WS₂ were revealed. The SECCM imaging directly visualized the relationship of HER activity and number of MoS₂ nanosheet layers and unveiled the heterogeneous aging state of MoS₂ nanosheets. SECCM can be used for improving local HER activities by producing sulfur vacancies using electrochemical reaction at the selected region.     

Dimeric [Mo2S12]2− Cluster: A Molecular Analogue of MoS2 Edges for Superior Hydrogen‐Evolution Electrocatalysis

Proton reduction is one of the most fundamental and important reactions in nature. MoS₂ edges have been identified as the active sites for hydrogen evolution reaction (HER) electrocatalysis. Designing molecular mimics of MoS₂ edge sites is an attractive strategy to understand the underlying catalytic mechanism of different edge sites and improve their activities. Herein we report a dimeric molecular analogue [Mo₂S₁₂]^2?, as the smallest unit possessing both the terminal and bridging disulfide ligands. Our electrochemical tests show that [Mo₂S₁₂]^2? is a superior heterogeneous HER catalyst under acidic conditions. Computations suggest that the bridging disulfide ligand of [Mo₂S₁₂]^2? exhibits a hydrogen adsorption free energy near zero (?0.05 eV). This work helps shed light on the rational design of HER catalysts and biomimetics of hydrogen‐evolving enzymes.     

Activation of MoS2 Basal Planes for Hydrogen Evolution by Zinc

Molybdenum disulfide (MoS₂) has been widely studied as a potential earth‐abundant electrocatalyst for the hydrogen‐evolution reaction (HER). Defect engineering and heteroelemental doping are effective methods to enhance the catalytic activity in the HER, so exploring an efficient route to simultaneously achieve in‐plane vacancy engineering and elemental doping of MoS₂ is necessary. In this study, Zinc, a low‐cost and moderately active metal, has been used to realize this strategy by generation of sulfur vacancies and zinc doping on MoS₂ in one step. Density functional theory calculations reveal that the zinc atoms not only lower the formation energy of S vacancies, but also help to decrease ΔG_H of S‐vacancy sites near the Zn atoms. At an optimal zinc‐reduced MoS₂ (Zn@MoS₂) example, the activated basal planes contribute to the HER activity with an overpotential of ?194 mV at 10 mA cm^?2 and a low Tafel slope of 78 mV/dec.     

Distance Synergy of MoS2‐Confined Rhodium Atoms for Highly Efficient Hydrogen Evolution

Perturbing the electronic structure of the MoS₂ basal plane by confining heteroatoms offers the opportunity to trigger in‐plane activity for the hydrogen evolution reaction (HER). The key challenge consists of inducing the optimum HER activity by controlling the type and distribution of confined atoms. A distance synergy of MoS₂‐confined single‐atom rhodium is presented, leading to an ultra‐high HER activity at the in‐plane S sites adjacent to the rhodium. By optimizing the distance between the confined Rh atoms, an ultra‐low overpotential of 67 mV is achieved at a current density of 10 mA cm^?2 in acidic solution. Experiments and first‐principles calculations demonstrate a unique distance synergy between the confined rhodium atoms in tuning the reactivity of neighboring in‐plane S atoms, which presents a volcanic trend with the inter‐rhodium distance. This study provides a new strategy to tailor the activity of MoS₂ surface via modulating the distance between confined single atoms.     

One‐pot Synthesis of CdS Nanocrystals Hybridized with Single‐Layer Transition‐Metal Dichalcogenide Nanosheets for Efficient Photocatalytic Hydrogen Evolution

Exploration of low‐cost and earth‐abundant photocatalysts for highly efficient solar photocatalytic water splitting is of great importance. Although transition‐metal dichalcogenides (TMDs) showed outstanding performance as co‐catalysts for the hydrogen evolution reaction (HER), designing TMD‐hybridized photocatalysts with abundant active sites for the HER still remains challenge. Here, a facile one‐pot wet‐chemical method is developed to prepare MS₂–CdS (M=W or Mo) nanohybrids. Surprisedly, in the obtained nanohybrids, single‐layer MS₂ nanosheets with lateral size of 4–10 nm selectively grow on the Cd‐rich (0001) surface of wurtzite CdS nanocrystals. These MS₂–CdS nanohybrids possess a large number of edge sites in the MS₂ layers, which are active sites for the HER. The photocatalytic performances of WS₂–CdS and MoS₂–CdS nanohybrids towards the HER under visible light irradiation (>420 nm) are about 16 and 12 times that of pure CdS, respectively. Importantly, the MS₂–CdS nanohybrids showed enhanced stability after a long‐time test (16 h), and 70 % of catalytic activity still remained.     

Size‐Dependent Enhancement of Electrocatalytic Oxygen‐Reduction and Hydrogen‐Evolution Performance of MoS2 Particles

MoS₂ particles with different size distributions were prepared by simple ultrasonication of bulk MoS₂ followed by gradient centrifugation. Relative to the inert microscale MoS₂, nanoscale MoS₂ showed significantly improved catalytic activity toward the oxygen‐reduction reaction (ORR) and hydrogen‐evolution reaction (HER). The decrease in particle size was accompanied by an increase in catalytic activity. Particles with a size of around 2 nm exhibited the best dual ORR and HER performance with a four‐electron ORR process and an HER onset potential of ?0.16 V versus the standard hydrogen electrode (SHE). This is the first investigation on the size‐dependent effect of the ORR activity of MoS₂, and a four‐electron transfer route was found. The exposed abundant Mo edges of the MoS₂ nanoparticles were proven to be responsible for the high ORR catalytic activity, whereas the origin of the improved HER activity of the nanoparticles was attributed to the plentiful exposed S edges. This newly discovered process provides a simple protocol to produce inexpensive highly active MoS₂ catalysts that could easily be scaled up. Hence, it opens up possibilities for wide applications of MoS₂ nanoparticles in the fields of energy conversion and storage.     

Tuning the Intrinsic Activity and Electrochemical Surface Area of MoS2 via Tiny Zn Doping: Toward an Efficient Hydrogen Evolution Reaction (HER) Catalyst

Molybdenum sulfide (MoS₂) is considered as an alternative material for commercial platinum catalysts for electrocatalytic hydrogen evolution reaction (HER). Improving the apparent HER activity of MoS₂ to a level comparable to that of Pt is an essential premise for the commercial use of MoS₂. In this work, a Zn-doping strategy is proposed to enhance the HER performance of MoS₂. It is shown that tiny Zn doping into MoS₂ leads to the enhancement of the electrochemical surface area, increases in proportion of HER active 1T phase in the material and formation of catalytic sites of higher intrinsic activity. These benefits result in a high-performance HER electrocatalyst with a low overpotential of 190 mV(@10 mA cm⁻²) and a low Tafel slope of 58 mV dec⁻¹. The origin for the excellent electrochemical performance of the doped MoS₂ is rationalized with both experimental and theoretical investigations.     

Bifunctional Electrocatalyst with 0D/2D Heterostructure for Highly Efficient Hydrogen and Oxygen Generation

A novel MoS₂ quantum dots/CoSe₂ nanosheet (MoS₂ QDs/CoSe₂) hybrid with 0D/2D heterostructure has been developed. The CoSe₂ nanosheets (NSs) enable an excellent oxygen evolution reaction (OER) activity with increasing vacancy configuration on one hand, while the MoS₂ QDs serve as an eminent hydrogen evolution reaction (HER) catalyst on the other. By integrating MoS₂ QDs and CoSe₂ NSs, the hybrid exhibits excellent electrocatalytic performances in HER and OER. The unique 0D/2D hetero‐interface increases the exposed active sites and facilitates electron transfer, thereby boosting the electrocatalytic activity. Relatively low overpotentials of 82 mV and 280 mV are required to drive the current density of 10 mA/cm² for HER and OER, with corresponding Tafel slopes of 69 and 75 mV/dec, respectively. As such, this work provides an efficient yet simple approach to construct bifunctional electrocatalysts with enhanced activity and stability.     

3D Carbon Foam Supported Edge-Rich N-Doped MoS2 Nanoflakes for Enhanced Electrocatalytic Hydrogen Evolution

Molybdenum disulfide (MoS₂) is one of the most promising alternatives to the Pt-based electrocatalysts for the hydrogen evolution reaction (HER). However, its performance is currently limited by insufficient active edge sites and poor electron transport. Hence, enormous efforts have been devoted to constructing more active edge sites and improving conductivity to obtain enhanced electrocatalytic performance. Herein, the 3D carbon foam (denoted as CF) supported edge-rich N-doped MoS₂ nanoflakes were successfully fabricated by using the commercially available polyurethane foam (PU) as the 3D substrate and PMo₁₂O₄₀³⁻ clusters (denoted as PMo₁₂) as the Mo source through redox polymerization, followed by sulfurization. Owing to the uniform distribution of nanoscale Mo sources and 3D carbon foam substrate, the as-prepared MoS₂-CF composite possessed well-exposed active edge sites and enhanced electrical conductivity. Systematic investigation demonstrated that the MoS₂-CF composite showed high HER performance with a low overpotential of 92 mV in 1.0 m KOH and 155 mV in 0.5 m H₂SO₄ at a current density of 10 mA cm⁻². This work offers a new pathway for the rational design of MoS₂-based HER electrocatalysts.     

Self‐Optimization of the Active Site of Molybdenum Disulfide by an Irreversible Phase Transition during Photocatalytic Hydrogen Evolution

The metallic 1T‐MoS₂ has attracted considerable attention as an effective catalyst for hydrogen evolution reactions (HERs). However, the fundamental mechanism about the catalytic activity of 1T‐MoS₂ and the associated phase evolution remain elusive and controversial. Herein, we prepared the most stable 1T‐MoS₂ by hydrothermal exfoliation of MoS₂ nanosheets vertically rooted into rigid one‐dimensional TiO₂ nanofibers. The 1T‐MoS₂ can keep highly stable over one year, presenting an ideal model system for investigating the HER catalytic activities as a function of the phase evolution. Both experimental studies and theoretical calculations suggest that 1T phase can be irreversibly transformed into a more active 1T′ phase as true active sites in photocatalytic HERs, resulting in a “catalytic site self‐optimization”. Hydrogen atom adsorption is the major driving force for this phase transition.     

High-Resolution Electrochemical Mapping of the Hydrogen Evolution Reaction on Transition-Metal Dichalcogenide Nanosheets

High-resolution scanning electrochemical cell microscopy (SECCM) is used to image and quantitatively analyze the hydrogen evolution reaction (HER) catalytically active sites of 1H-MoS₂ nanosheets, MoS₂, and WS₂ heteronanosheets. Using a 20 nm radius nanopipette and hopping mode scanning, the resolution of SECCM was beyond the optical microscopy limit and visualized a small triangular MoS₂ nanosheet with a side length of ca. 130 nm. The electrochemical cell provides local cyclic voltammograms with a nanoscale spatial resolution for visualizing HER active sites as electrochemical images. The HER activity difference of edge, terrace, and heterojunction of MoS₂ and WS₂ were revealed. The SECCM imaging directly visualized the relationship of HER activity and number of MoS₂ nanosheet layers and unveiled the heterogeneous aging state of MoS₂ nanosheets. SECCM can be used for improving local HER activities by producing sulfur vacancies using electrochemical reaction at the selected region.     

Ligand Modulation of Active Sites to Promote Cobalt-Doped 1T-MoS2 Electrocatalytic Hydrogen Evolution in Alkaline Media

Highly efficient hydrogen evolution reaction (HER) electrocatalyst will determine the mass distributions of hydrogen-powered clean technologies, while still faces grand challenges. In this work, a synergistic ligand modulation plus Co doping strategy is applied to 1T−MoS₂ catalyst via CoMo-metal-organic frameworks precursors, boosting the HER catalytic activity and durability of 1T−MoS₂. Confirmed by Cs corrected transmission electron microscope and X-ray absorption spectroscopy, the polydentate 1,2-bis(4-pyridyl)ethane ligand can stably link with two-dimensional 1T−MoS₂ layers through cobalt sites to expand interlayer spacing of MoS₂ (Co−1T−MoS₂-bpe), which promotes active site exposure, accelerates water dissociation, and optimizes the adsorption and desorption of H in alkaline HER processes. Theoretical calculations indicate the promotions in the electronic structure of 1T−MoS₂ originate in the formation of three-dimensional metal-organic constructs by linking π-conjugated ligand, which weakens the hybridization between Mo-3d and S-2p orbitals, and in turn makes S-2p orbital more suitable for hybridization with H-1s orbital. Therefore, Co−1T−MoS₂-bpe exhibits excellent stability and exceedingly low overpotential for alkaline HER (118 mV at 10 mA cm⁻²). In addition, integrated into an anion-exchange membrane water electrolyzer, Co−1T−MoS₂-bpe is much superior to the Pt/C catalyst at the large current densities. This study provides a feasible ligand modulation strategy for designs of two-dimensional catalysts.     

Heterostructures of NiFe LDH hierarchically assembled on MoS2 nanosheets as high-efficiency electrocatalysts for overall water splitting

Typically, rational interfacial engineering can effectively modify the adsorption energy of active hydrogen molecules to improve water splitting efficiency. NiFe layered double hydroxide (NiFe LDH) composite, an efficient oxygen evolution reaction (OER) catalyst, suffers from slow hydrogen evolution reaction (HER) kinetics, restricting its application for overall water splitting. Herein, we construct the hierarchical MoS₂/NiFe LDH nanosheets with a heterogeneous interface used for HER and OER. Benefiting the hierarchical heterogeneous interface optimized hydrogen Gibbs free energy, tens of exposed active sites, rapid mass- and charge-transfer processes, the MoS₂/NiFe LDH displays a highly efficient synergistic electrocatalytic effect. The MoS₂/NiFe LDH electrode in 1 mol/L KOH exhibits excellent HER activity, only 98 mV overpotential at 10 mA/cm². Significantly, when it assembled as anode and cathode for overall water splitting, only 1.61 V cell voltage was required to achieve 10 mA/cm² with excellent durability (50 h).     

Gold Nanoclusters Grown on MoS2 Nanosheets by Pulsed Laser Deposition: An Enhanced Hydrogen Evolution Reaction

Au nanoparticles were decorated on a 2H MoS₂ surface to form an Au/MoS₂ composite by pulse laser deposition. Improved HER activity of Au/MoS₂ is evidenced by a positively shifted overpotential (−77 mV) at a current density of −10 mA cm⁻² compared with pure MoS₂ nanosheets. Experimental evidence shows that the interface between Au and MoS₂ provides more sites to combine protons to form an active H atom. The density functional theory calculations found that new Au active sites on the Au and MoS₂ interface with improved conductivity of the whole system are essential for enhancing HER activity of Au/MoS₂.     

Electrochemically exfoliated Ni-doped MoS2 nanosheets for highly efficient hydrogen evolution and Zn-H2O battery

Thanks to tunable physical and chemical properties, two-dimensional (2D) materials have received intensive interest, endowing their excellent electrocatalytic performances for applications in energy conversion. However, their catalytic activities are largely determined by poor adsorption energy and limited active edge sites. Herein, a one-step electrochemical exfoliation strategy was developed to fabricate 2D Ni-doped MoS₂ nanosheets (Ni-EX-MoS₂) with a lateral size of ～500 nm and thickness of ～3.5 nm. Profiting from high electrical conductivity and abundant exposing active sites, Ni-EX-MoS₂ catalyst displayed an admirable performance for electrochemical hydrogen evolution reaction (HER) with a low overpotential of 145 mV at 10 mA/cm² as well as a small Tafel slope of 89 mV/dec in alkaline media, which are superior to those of the most reported MoS₂-based electrocatalysts. The formed Ni species with tuning electronic structure played a crucial role as primary active center of Ni-EX-MoS₂, as well as the forming stable 1T/2H phase MoS₂ interface demonstrated a synergistic effect on electrocatalytic HER performance. Further, Ni-EX-MoS₂ was employed as a cathode electrode for alkaline Zn-H₂O battery, which displayed a high power density of 3.3 mW/cm² with excellent stability. This work will provide a simple and effective guideline for design of electrochemically exfoliated transition metal-doped MoS₂ nanosheets to inspire their practical applications in energy catalytic and storage.     

Large‐scale Two‐dimensional MoSx Catalyst Prepared under Mild Conditions for Enhancing Electrocatalytic Hydrogen Evolution Reaction

As an electrocatalyst with abundant resources and great potential, molybdenum disulfide is regarded as one of the most likely alternatives to expensive noble‐metals catalysts. However, it is still a challenge to achieve large scale production of few‐layer MoS₂ with enhancing activity of electrocatalytic hydrogen reaction at ambient conditions. Herein, we developed a simple environmentally friendly two‐step method, which included intercalation reaction and a subsequent electrochemical reduction reaction for mass preparation of defect‐rich desulfurized MoS_x (D?MoS_x) nanosheets with plentiful sulfur vacancies. The ratio of sulfur‐molybdenum atoms can be adjusted from 2 : 1 to 1.4 : 1 by regulating the desulfurization voltage. It was found that the HER catalytic activity of the D?MoS_x was enhanced compared with that of pristine MoS₂ (P?MoS₂), the current density of D?MoS_x (desulfurization at ?1.0 V) at ?0.3 V versus RHE was about 169% of the P?MoS₂, and the Tafel slope decreased to 136 mV dec^?1. This method can be widely applied to large‐scale preparation of other two‐dimensional materials.     

Controlled Assembly of Porphyrin‐MoS2 Composite Nanosheets for Enhanced Photoelectrochemical Performance

As a promising non‐precious metal photoelectrochemical (PEC) catalyst, MoS₂ exhibits high electrocatalytic activity and stability, while the weak light absorption efficiency and low photoresponse current limit its practical application. Herein, a facile co‐assembly approach is proposed to construct porphyrin‐MoS₂ composite photoelectrocatalysts. The as‐prepared photoelectrocatalysts show a significantly enhanced photocurrent response as high as 16 μA cm^?2, which is about 2 times higher than that of bare MoS₂. Furthermore, the obtained porphyrin‐MoS₂ catalysts exhibit excellent durability when tested for 23000 s, thus providing a useful strategy for the design of highly efficient dye‐sensitized PEC catalysts.     

Synergistic Doping and Intercalation: Realizing Deep Phase Modulation on MoS2 Arrays for High‐Efficiency Hydrogen Evolution Reaction

A synergistic N doping plus PO₄^3? intercalation strategy is used to induce high conversion (ca. 41 %) of 2H‐MoS₂ into 1T‐MoS₂, which is much higher than single N doping (ca. 28 %) or single PO₄^3? intercalation (ca. 10 %). A scattering mechanism is proposed to illustrate the synergistic phase transformation from the 2H to the 1T phase, which was confirmed by synchrotron radiation and spherical aberration TEM. To further enhance reaction kinetics, the designed (N,PO₄^3?)‐MoS₂ nanosheets are combined with conductive vertical graphene (VG) skeleton forming binder‐free arrays for high‐efficiency hydrogen evolution reaction (HER). Owing to the decreased band gap, lower d‐band center, and smaller hydrogen adsorption/desorption energy, the designed (N,PO₄^3?)‐MoS₂/VG electrode shows excellent HER performance with a lower Tafel slope and overpotential than N‐MoS₂/VG, PO₄^3?‐MoS₂/VG counterparts, and other Mo‐base catalysts in the literature.     

Selenium‐Enriched Nickel Selenide Nanosheets as a Robust Electrocatalyst for Hydrogen Generation

To address the urgent need for clean and sustainable energy, the rapid development of hydrogen‐based technologies has started to revolutionize the use of earth‐abundant noble‐metal‐free catalysts for the hydrogen evolution reaction (HER). Like the active sites of hydrogenases, the cation sites of pyrite‐type transition‐metal dichalcogenides have been suggested to be active in the HER. Herein, we synthesized electrodes based on a Se‐enriched NiSe₂ nanosheet array and explored the relationship between the anion sites and the improved hydrogen evolution activity through theoretical and experimental studies. The free energy for atomic hydrogen adsorption is much lower on the Se sites (0.13 eV) than on the Ni sites (0.87 eV). Notably, this electrode benefits from remarkable kinetic properties, with a small overpotential of 117 mV at 10 mA cm^?2, a low Tafel slope of 32 mV per decade, and excellent stability. Control experiments showed that the efficient conversion of H⁺ into H₂ is due to the presence of an excess of selenium in the NiSe₂ nanosheet surface.