Foam-like Porous Structured Trimetal Electrocatalysts Exhibiting Superior Performance for Overall Water Splitting and Solid-Liquid Zinc-Air Batteries

Electrocatalysts through an interconnected porous structure that are highly durable, active, and affordable for industrial scale production are necessary for electricity conversion and storage devices with superior effectiveness. In the present study, we synthesized free-standing tri-metal oxide (FeNiCoO4) on top of an incredibly interconnected foam-like porous structure (FNCO) via a simple method. The enhanced FNCO-600 showed remarkable electrocatalytic activity and outstanding stability to the related half-cell responses with regard to oxygen reduction reaction (ORR = 0.757 V), oxygen evolution reaction (OER = 230 mV), and hydrogen evolution reaction (HER = 211 mV). Additionally, we looked into the overall efficiency of water splitting using the FNCO-600 catalyst, which exhibited exceptional longevity (70 h) and an impressive cell voltage (1.72 V). Furthermore, using FNCO-600 as the cathode, we created rechargeable solid-liquid electrolyte-based Zn-air batteries that demonstrated enhanced power densities of 21.8 mW cm-2 and 167.4 mW cm-2 with noteworthy durability. Finally, we showed how to synthesize and produce free-standing, foam-like porous structure catalysts on an industrial scale that provide excellent energy storage and conversion.

Quantum fundaments of catalysis: true electronic potential energy

Catalysis is a quantum phenomenon enthalpically driven by electronic correlations with many-particle effects in all of its branches, including electro-photo-catalysis and electron transfer. This means that only probability amplitudes provide a complete relationship between the state of catalysis and observations. Thus, in any atomic system material), competing space-time electronic interactions coexist to define its (related) properties such as stability, (super)conductivity, magnetism (spin-orbital ordering), chemisorption and catalysis. Catalysts, reactants, and chemisorbed and transition states have the possibility of optimizing quantum correlations to improve reaction kinetics. Active sites with closed-shell orbital configurations share a maximum number of spin-paired electrons, mainly optimizing coulombic attractions and covalency and defining weakly correlated closed-shell (WCCS) structures. However, in compositions with open-shell orbital configurations, at least, quantum spin exchange interactions (QSEIopenshells) arise, stabilising unpaired electrons in less covalent bonds and differentiating non-weakly (or strongly) correlated open-shell (NWCOS) systems. In NWCOS catalysts, electronic ground states can have bonds with diverse and rival spin-orbital orderings as well as ferro-, ferri- and multiple antiferro-magnetic textures, which deeply define their activities. Particularly in inter-atomic ferromagnetic (FM) bonds, the increase in relevance of non-classical quantum potentials can significantly optimize chemisorption energies, transition states (TSs), activation energies (overpotential) and spin-dependent electron transfer (conductivity), overall implying the need for explaining the thermodynamic and kinetic origin of catalysis from its true quantum electronic energy. To do so, we use the connection between the Born-Oppenheimer approximation and Virial theorem in the treatment of electronic kinetic and potential energies. Thus, the exact fundamental interactions that decompose TSs appear. The possibility of increasing the stabilization of TSs, due to quantum correlations on NWCO catalysts, opens the possibility of simultaneously reducing chemisorption enthalpies and activation barriers of reaction mechanisms, which implies the anticipation and explanation of positive deviations from the Brønsted-Evans-Polanyi principle.

Electrocatalytic Performance of 2D Monolayer WSeTe Janus Transition Metal Dichalcogenide for Highly Efficient H2 Evolution Reaction

Nowadays, the development of clean and green energy sources is the priority interest of research due to increasing global energy demand and extensive usage of fossil fuels, which create pollutants. Hydrogen has the highest energy density by weight among all chemical fuels. For the commercial-scale production of hydrogen, water electrolysis is the best method, which requires an efficient, cost-effective, and earth-abundant electrocatalyst. Recent studies have shown that the 2D Janus transition metal dichalcogenides (JTMDs) are promising materials for use as electrocatalysts and are highly effective for electrocatalytic H2 evolution reaction (HER). Here, we report a 2D monolayer WSeTe JTMD, which is highly effective toward HER. We have studied the electronic properties of 2D monolayer WSeTe JTMD using the periodic hybrid DFT-D method, and a direct electronic band gap of 2.39 eV was obtained. We have explored the HER pathways, mechanisms, and intermediates, including various transition state (TS) structures (Volmer TS, i.e., H*-migration TS, Heyrovsky TS, and Tafel TS) using a molecular cluster model of the subject JTMD noted as W10Se9Te12. The present calculations reveal that the 2D monolayer WSeTe JTMD is a potential electrocatalyst for HER. It has the lowest energy barriers for all the TSs among other TMDs. It has been shown that the Heyrovsky energy barrier (= 8.72 kcal mol-1) in the case of the Volmer-Heyrovsky mechanism is larger than the Tafel energy barrier (= 3.27 kcal mol-1) in the Volmer-Tafel mechanism. Hence, our present study suggests that the formation of H2 is energetically more favorable via the Volmer-Tafel mechanism. This study helps to shed light on the rational design of 2D single-layer JTMD, which is highly effective toward HER, and we expect that the present work can be further extended to other JTMDs to find out the improved electrocatalytic performance.

Nanocatalysis MoS2/rGO: An Efficient Electrocatalyst for the Hydrogen Evolution Reaction

In this study, a systematic investigation of MoS2 nanostructure growth on a SiO2 substrate was conducted using a two-stage process. Initially, a thin layer of Mo was grown through sputtering, followed by a sulfurization process employing the CVD technique. This two-stage process enables the control of diverse nanostructure formations of both MoS2 and MoO3 on SiO2 substrates, as well as the formation of bulk-like grain structures. Subsequently, the addition of reduced graphene oxide (rGO) was examined, resulting in MoS2/rGO(n), where graphene is uniformly deposited on the surface, exposing a higher number of active sites at the edges and consequently enhancing electroactivity in the HER. The influence of the synthesis time on the treated MoS2 and also MoS2/rGO(n) samples is evident in their excellent electrocatalytic performance with a low overpotential.

2D Metal-Free BSi5 with an Intrinsic Metallicity and Remarkable HER Activity

One of the most urgent and attractive topics in electrocatalytic water splitting is the exploration of high-performance and low-cost catalysts. Herein, we have proposed three fresh two-dimensional nanostructures (BSi5, BSi4, and BSi3) with inherent metallicity contributed by delocalized π electrons based on first-principles calculations. Their planar atoms arrangement, akin to graphene, is in favor of the availability of active atoms and H adsorption/deadsorption. Among them, the BSi5 monolayer shows the best HER activity, even superior to a commercial Pt catalyst. Moreover, its extraordinary HER activity can be maintained under high H coverage and large biaxial strain, mainly originating from the fact that B 2pz orbital electrons are responsible for the B-H interaction. Further analysis reveals that there appears to be a linear correlation between the magnitude of B 2pz DOS at the Fermi level and Gibbs free energy in both three proposed nanostructures and five hypothetical B-Si nanostructures. Our work represents a significant step forward toward the design of metal-free HER catalysts.

Transition metal doped WSi2N4monolayer for water splitting electrocatalysts: a first-principles study

High-performance water splitting electrocatalysts are urgently needed in the face of the environmental degradation and energy crisis. The first principles method was used in this study to systematically examine the electronic characteristics of transition metal (Sc, Ti, V, Cr, Mn, Fe, and Ru) doped WSi2N4(TM@WSi2N4) and its potential as oxygen evolution reaction (OER) catalysts. Our study shows that the doping of TM atoms significantly improves the catalytic performance of TM@WSi2N4, especially Fe@WSi2N4shows a low overpotential (ηOER= 470 mV). Interestingly, we found that integrated-crystal orbital Hamilton population and d-band center can be used as descriptors to explain the high catalytic activity of Fe@WSi2N4. Subsequently, Fe@WSi2N4exhibits the best hydrogen evolution reaction (HER) activity with a universal overpotential of 47 mV on N1sites. According to our research, Fe@WSi2N4offers a promising substitute for precious metals as a catalyst for overall water splitting with low OER and HER overpotentials.

Genetic descriptor search algorithm for predicting hydrogen adsorption free energy of 2D material

Transition metal dichalcogenides (TMDs) have emerged as a promising alternative to noble metals in the field of electrocatalysts for the hydrogen evolution reaction. However, previous attempts using machine learning to predict TMD properties, such as catalytic activity, have been shown to have limitations in their dependence on large amounts of training data and massive computations. Herein, we propose a genetic descriptor search that efficiently identifies a set of descriptors through a genetic algorithm, without requiring intensive calculations. We conducted both quantitative and qualitative experiments on a total of 70 TMDs to predict hydrogen adsorption free energy ([Formula: see text]) with the generated descriptors. The results demonstrate that the proposed method significantly outperformed the feature extraction methods that are currently widely used in machine learning applications.

Effect of MoSe2 nanoribbons with NW30 edge reconstructions on the electronic and catalytic properties by strain engineering

Monolayer transition metal dichalcogenides (TMDs), typical two-dimensional semiconductors, have been extensively studied for their extraordinary physical properties and utilized for nanoelectronics and optoelectronics. However, the finite samples and discontinuity in the synthesis process of TMD materials definitely induce defect edges in nanoribbons and greatly influence the device performance. Here, we systematically studied the atomic structures, energetic and mechanical stability, and electronic and catalytic properties of MoSe₂ nanoribbons on the basis of experiments. Clear benefits of ZZSe-Mo-NW30 edged nanoribbons were found to evidently increase the dynamic stability according to our first-principles calculations. Meanwhile, unsaturated Mo atoms at the edge sites induced local magnetic moments up to 0.54 μ_B and changed the chemical environments of adjacent Se atoms, which acted as active sites for the hydrogen evolution reaction (HER) with a lower onset potential of -0.04 eV. The external tensile strain on these nanoribbons can have negligible effects on the electronic and catalytic properties. The onset potential of the ZZSe-Mo-NW30 edged nanoribbons only changed 0.03 eV under critical tensile strain. The atomic-scale research of edge reconstructions in TMD materials provides new opportunities to modulate the synthesis mechanism for experiments and defect-engineering applications in electrochemical catalysts.

Orbital Occupancy and Spin Polarization: From Mechanistic Study to Rational Design of Transition Metal-Based Electrocatalysts toward Energy Applications

Over the past few decades, development of electrocatalysts for energy applications has extensively transitioned from trial-and-error methodologies to more rational and directed designs at the atomic levels via either nanogeometric optimization or modulating electronic properties of active sites. Regarding the modulation of electronic properties, nonprecious transition metal-based materials have been attracting large interest due to the capability of versatile tuning d-electron configurations expressed through the flexible orbital occupancy and various possible degrees of spin polarization. Herein, recent advances in tailoring electronic properties of the transition-metal atoms for intrinsically enhanced electrocatalytic performances are reviewed. We start with discussions on how orbital occupancy and spin polarization can govern the essential atomic level processes, including the transport of electron charge and spin in bulk, reactive species adsorption on the catalytic surface, and the electron transfer between catalytic centers and adsorbed species as well as reaction mechanisms. Subsequently, different techniques currently adopted in tuning electronic structures are discussed with particular emphasis on theoretical rationale and recent practical achievements. We also highlight the promises of the recently established computational design approaches in developing electrocatalysts for energy applications. Lastly, the discussion is concluded with perspectives on current challenges and future opportunities. We hope this review will present the beauty of the structure-activity relationships in catalysis sciences and contribute to advance the rational development of electrocatalysts for energy conversion applications.

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.

Theoretical Study on the High HER/OER Electrocatalytic Activities of 2D GeSi, SnSi, and SnGe Monolayers and Further Improvement by Imposing Biaxial Strain or Doping Heteroatoms

Under the DFT calculations, two-dimensional (2D) GeSi, SnSi, and SnGe monolayers, considered as the structural analogues of famous graphene, are confirmed to be dynamically, mechanically and thermodynamically stable, and all of them can also possess good conductivity. Furthermore, we systematically investigate their electrocatalytic activities in overall water splitting. The SnSi monolayer can show good HER catalytic activity, while the SnGe monolayer can display remarkable OER catalytic activity. In particular, the GeSi monolayer can even exhibit excellent bifunctional HER/OER electrocatalytic activities. In addition, applying the biaxial strain or doping heteroatoms (especially P atom) can be regarded as the effective strategies to further improve the HER activities of these three 2D monolayers. The doped GeSi and SnSi systems can usually exhibit higher HER activity than the doped SnGe systems. The correlative catalytic mechanisms are also analyzed. This work could open up a new avenue for the development of non-noble-metal-based HER/OER electrocatalysts.

Recent progress in 2D hybrid heterostructures from transition metal dichalcogenides and organic layers: properties and applications in energy and optoelectronics fields

Atomically thin transition metal dichalcogenides (TMDs) present extraordinary optoelectronic, electrochemical, and mechanical properties that have not been accessible in bulk semiconducting materials. Recently, a new research field, 2D hybrid heteromaterials, has emerged upon integrating TMDs with molecular systems, including organic molecules, polymers, metal-organic frameworks, and carbonaceous materials, that can tailor the TMD properties and exploit synergetic effects. TMD-based hybrid heterostructures can meet the demands of future optoelectronics, including supporting flexible, transparent, and ultrathin devices, and energy-based applications, offering high energy and power densities with long cycle lives. To realize such applications, it is necessary to understand the interactions between the hybrid components and to develop strategies for exploiting the distinct benefits of each component. Here, we provide an overview of the current understanding of the new phenomena and mechanisms involved in TMD/organic hybrids and potential applications harnessing such valuable materials in an insightful way. We highlight recent discoveries relating to multicomponent hybrid materials. Finally, we conclude this review by discussing challenges related to hybrid heteromaterials and presenting future directions and opportunities in this research field.

Investigation of the adsorption properties of gemcitabine anticancer drug with metal-doped boron nitride fullerenes as a drug-delivery carrier: a DFT study

Anticancer-drug delivery is now becoming a challenging approach for researchers as it allows controlled drug delivery near cancerous cells with minimized generic collection and the avoidance of secondary side effects. Hence in this work, the applications of nanostructures as anticancer drug-delivery carriers were widely investigated to target cancerous tissues. Based on DFT calculations, we investigated the transition metal-doped boron nitride nanostructure as a drug-delivery agent for the gemcitabine drug utilizing the B3LYP/6-31G (d, p) level of theory. In this research, the adsorption energy and electronic parameters of gemcitabine on the interaction with the metal-doped BN nanostructures were studied. It has been observed that metal doping significantly enhances the drug-delivery properties of BN nanostructures. Among the investigated nanostructures, Ni–BN has been found to be the most prominent nanostructure to transport gemcitabine with an elevated value of adsorption energy in both the gas phase (−45.79) and water media (−32.46). The interaction between gemcitabine and BN nanostructures was confirmed through frontier molecular orbitals and stabilization energy analysis. The fractional charge transfer, MEP, NCI, and NBO analyses exposed the charge transfer from drug molecule to the BN nanostructures. Transition density maps and UV-VIS spectra were also plotted to investigate the excited-state properties of the designed complexes. Thus, the present study provides an in-depth interaction mechanism of the gemcitabine drug with BN, which reveals that metal-doped BN nanostructures can be a favorable drug-delivery vehicle for the gemcitabine anticancer drug.

Anticancer-drug delivery is now becoming a challenging approach for researchers as it allows controlled drug delivery near cancerous cells with minimized generic collection and the avoidance of secondary side effects.

Oxidation and Degradation of WS2 Monolayers Grown by NaCl-Assisted Chemical Vapor Deposition: Mechanism and Prevention

The preservation of two-dimensional WS₂ in the environment is a concern for researchers. In addition to water vapor and oxygen, the latest research points out that degradation is directly related to light absorption. Based on the selection rules of nonlinear optics, two-photon absorption is dipole forbidden in the exciton 1s states, but second-harmonic generation (SHG) is allowed with virtual transitions. According to this mechanism, we proved that SHG is an optical detection method with non-photooxidative damage and energy characteristics. With this detection method, we can explore the oxidation and degradation mechanisms of WS₂ grown by NaCl-assisted chemical vapor deposition in its original state. The WS₂ monolayers that use NaCl to assist in growth have undergone different degradation processes, starting to oxidize from random positions in the triangular flake. We use a photocatalytic reaction to explain the photo-induced degradation mechanism with sulfur vacancies. It was further found that WS₂ grown with NaCl assistance is hydrolyzed in a dark and high-humidity environment, which does not occur in pure WS₂. Finally, we demonstrated that changing the direction of the sapphire substrate relative to the gas flow direction to grow NaCl-assisted WS₂ can greatly improve its stability in the ambient atmosphere, even when exposed to light. The optimal geometric structures and ground state energies are investigated by the density functional theory-based calculations. According to the orientation and symmetry of NaCl-assisted WS₂, we can expect that it will have a better growth quality when the gas flow direction is perpendicular to the [112̄0] direction of the sapphire substrate. This contributes to the nucleation and subsequent growth of NaCl-assisted WS₂. This research provides a more stable optical inspection method than other established methods and greatly improves the operational stability of NaCl-assisted WS₂ under environmental conditions.

Advancing the extended roles of 3D transition metal based heterostructures with copious active sites for electrocatalytic water splitting

The replacement of noble metals with alternative electrocatalysts is highly demanded for water splitting. From the exploration of 3D -transition metal based heterostructures, engineering at the nano-level brought more enhancements in active sites with reduced overpotentials for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). However, recent developments in 3D transition metal based heterostructures like direct growth on external substrates (Ni foam, Cu foam) gave highly impressive activities and stabilities. Research needs to be focused on how the active sites can be enhanced further with 3D heterostructures of transition metals by studying them with various counterparts like hydroxides, layered double hydroxides and phosphides for empowering both OER and HER applications. This perspective covers the way to enlarge the utilization of 3D heterostructures successfully in terms of reduced overpotentials, highly exposed active sites, increased electrical conductivity, porosity and high-rate activity. From the various approaches of growth of transition metal based 3D heterostructures, it is easy to fine tune the active sites to have a viable production of hydrogen with less applied energy input. Overall, this perspective outlines a direction to increase the number of active sites on 3D transition metal based heterostructures by growing on 3D foams for enhanced water splitting applications.

Multilevel Theoretical Screening of Novel Two-Dimensional MA2Z4 Family for Hydrogen Evolution

The synthesis of 2D MoSi₂N₄ marked the birth of a new family of 2D MA₂Z₄ materials, whose potential applications are expected to sweep the field of nanoscale devices and catalysis in the future. In this work, we propose a multilevel screening workflow to systematically explore the mechanic stabilities, electronic properties, and hydrogen evolution performances of the 2D MA₂Z₄ family, among which seven stable, metallic, and highly active 2D MA₂Z₄ monolayers (2H-α-VGe₂N₄, 2H-α-NbGe₂N₄, 2H-α-TaGe₂N₄, 2H-α-NbSi₂N₄, 2H-β-VGe₂N₄, 2H-β-NbGe₂N₄, and 2H-β-TiGe₂P₄) are predicted as promising hydrogen evolution reaction (HER) catalysts with near-zero hydrogen adsorption free energy (ΔG_H). The lowest unoccupied state energy (E_LUS) of the MA₂Z₄ basal plane is identified as a concise descriptor to influence the electron filling and eventually determine its ΔG_H. The criteria of -6.0 < E_LUS < -5.6 eV is confirmed as the HER active window to explore novel HER catalysts. In particular, group-VI-terminated MA₂Z₄ basal planes have a higher E_LUS (> -5.6 eV), which leads to weak H adsorption and poor HER activities accordingly.

Electrospun Cobalt-Incorporated MOF-5 Microfibers as a Promising Electrocatalyst for OER in Alkaline Media

Metal-organic framework (MOF)-based materials have attracted attention in recent times owing to their remarkable properties such as regulatable pore size, high specific surface area, and elasticity in their network topology, geometry, dimension, and chemical functionality. It is believed that the incorporation of a MOF network into a fibrous matrix results in the improvement of the electrocatalytic properties of the material. Herein, we have synthesized a Co-incorporated MOF-5-based fibrous material by a simple wet-chemical method, followed by an electrospinning (ES) process. The as-prepared Co-incorporated MOF-5 microfibers were employed as an electrocatalyst for the oxygen evolution reaction (OER) in 1 M KOH electrolyte. The catalyst demands a lower overpotential of 240 mV to attain a current density of 10 mA/cm² with a lower Tafel slope value of 120 mV/dec along with a charge transfer resistance value of 2.9 Ω from electron impedance spectroscopy (EIS) analysis. From these results, it has been understood that the incorporation of Co metal into the MOF-5 microfibrous network has significantly improved the OER performance, which made them a potential entrant in other energy-related applications also.

Identifying Metallic Transition-Metal Dichalcogenides for Hydrogen Evolution through Multilevel High-Throughput Calculations and Machine Learning

High-performance electrocatalysts not only exhibit high catalytic activity but also have sufficient thermodynamic stability and electronic conductivity. Although metallic 1T-phase MoS₂ and WS₂ have been successfully identified to have high activity for hydrogen evolution reaction, designing more extensive metallic transition-metal dichalcogenides (TMDs) faces a large challenge because of the lack of a full understanding of electronic and composition attributes related to catalytic activity. In this work, we carried out systematic high-throughput calculation screening for all possible existing two-dimensional TMD (2D-TMD) materials to obtain high-performance hydrogen evolution reaction (HER) electrocatalysts by using a few important criteria, such as zero band gap, highest thermodynamic stability among available phases, low vacancy formation energy, and approximately zero hydrogen adsorption energy. A series of materials-perfect monolayer VS₂ and NiS₂, transition-metal ion vacancy (TM-vacancy) ZrTe₂ and PdTe₂, chalcogenide ion vacancy (X-vacancy) MnS₂, CrSe₂, TiTe₂, and VSe₂-have been identified to have catalytic activity comparable with that of Pt(111). More importantly, electronic structural analysis indicates active electrons induced by defects are mostly delocalized in the nearest-neighbor and next-nearest neighbor range, rather than a single-atom active site. Combined with the machine learning method, the HER-catalytic activity of metallic phase 2D-TMD materials can be described quantitatively with local electronegativity (0.195·LEf + 0.205·LEs) and valence electron number (Vtmx), where the descriptor is ΔG_H* = 0.093 - (0.195·LEf + 0.205·LEs) - 0.15·Vtmx.

Mitrofanovite, Layered Platinum Telluride, for Active Hydrogen Evolution

Two-dimensional (2D) layered catalysts have been considered as a class of ideal catalysts for hydrogen evolution reaction (HER) because of their abundant active sites with almost zero Gibbs energy change for hydrogen adsorption. Despite the promising performance, the design of stable and economic electrochemical catalyst based on 2D materials remains to be resolved for industrial-scale hydrogen production. Here, we report layered platinum tellurides, mitrofanovite Pt₃Te₄, which serves as an efficient and stable catalyst for HER with an overpotential of 39.6 mV and a Tafel slope of 32.7 mV/dec together with a high current density exceeding 7000 mA/cm². Pt₃Te₄ was synthesized as nanocrystals on a metallic molybdenum ditelluride (MoTe₂) template by a rapid electrochemical method. X-ray diffraction and high-resolution transmission microscopy revealed that the Pt₃Te₄ nanocrystals have a unique layered structure with repeated monolayer units of PtTe and PtTe₂. Theoretical calculations exhibit that Pt₃Te₄ with numerous edges shows near-zero Gibbs free-energy change of hydrogen adsorption, which shows the excellent HER performance as well as the extremely large exchange current density for massive hydrogen production.

Active Basal Plane Catalytic Activity via Interfacial Engineering for a Finely Tunable Conducting Polymer/MoS2 Hydrogen Evolution Reaction Multilayer Structure

The fixation of the catalyst interface is an important consideration for the design of practical applications. However, the electronic structure of MoS₂ is sensitive to its embedding environment, and the catalytic performance of MoS₂ catalysts may be altered significantly by the type of binding agents and interfacial structure. Interfacial engineering is an effective method for designing efficient catalysts, arising from the close contact between different components, which facilitates charge transfer and strong electronic interactions. Here, we have developed a layer-by-layer (LbL) strategy for the preparation of interfacial MoS₂-based catalyst structures with two types of conducting polymers on various substrates. We demonstrate how the assembled partners in the LbL structure can significantly impact the electronic structures in MoS₂. As the number of bilayers grows, using polypyrrole as a binder remarkably increases the catalytic efficacy as compared to using polyaniline. On the one hand, the ratio of S₂^2- (or S^2-), which is related to the remaining active hydrogen evolution reaction (HER) species, is further increased. On the other hand, density functional theory calculations indicate that the interfacial charge transport from the conducting polymers to MoS₂ may boost the HER activity of the interfacial structure of the conducting polymer/MoS₂ by decreasing the adsorption free energy of the intermediate H* at the S sites in the basal plane of MoS₂. The optimized catalytic efficacy of the (conducting polymer/MoS₂)_n assembly peaks is obtained with 16 assembly cycles. In preparing interfacial catalytic structures, the LbL-based strategy exhibits several key advantages, including the flexibility of choosing assembly partners, the ability to fine-tune the structures with precision at the nanometer scale, and planar homogeneity at the centimeter scale. We expect that this LbL-based catalyst immobilization strategy will contribute to the fundamental understanding of the scalability and control of highly efficient electrocatalysts at the interface for practical applications.

Non-stoichiometric molybdenum sulfide clusters and their reactions with the hydrogen molecule

The empty bridge site of Mo–Mo in non-stoichiometric molybdenum sulfide clusters may act a bridge for H atom transfer and be beneficial for hydrogen evolution reaction.

Surface selectivity of Ni3S2 toward hydrogen evolution reaction: a first-principles study

Exploring materials with high catalytic performance toward hydrogen evolution reaction (HER) is of importance for the development of clean hydrogen energy, and their surface structure is essential for this function. In this study, using density functional theory (DFT), we reported a comprehensive study on the phase stability, surface structures, electronic properties and HER catalytic properties of the low-index surfaces of Ni3S2, including the (0001), (101[combining macron]0), (101[combining macron]1), (112[combining macron]0) and (112[combining macron]1) planes with different terminations. Our calculated results demonstrate that S-rich surfaces and several stoichiometric surfaces of Ni3S2 are thermodynamically stable, including (0001)A, (101[combining macron]0)A, (112[combining macron]0)C, (101[combining macron]0)C, (101[combining macron]0)B and (112[combining macron]1)A surfaces. Among the six stable surface structures, the (0001)A, (101[combining macron]0)B and (101[combining macron]0)C surfaces of Ni3S2 are indispensable for high HER performance because of their high catalytic activity, suitable potential and high thermodynamic stability. The calculated changes of Gibbs free energy (ΔGH*) of the Top S2 site on (0001)A, Hollow Ni2S3S4 site on (101[combining macron]0)C, and Bridge Ni1Ni3 site and Hollow Ni2S1S2 site on (101[combining macron]0)B are -0.143, 0.122, 0.012, and -0.112 eV, respectively, comparable with or even better than those of Pt(111) (-0.07 eV). In addition, the possible Volmer-Heyrovsky and Volmer-Tafel processes on the considered surfaces are also investigated. When the overpotential is in the range of 0 to 300 mV, the density of active sites on the (101[combining macron]0)B surface of Ni3S2 is found to be the highest. This work provides significant insights on the surface selectivity of Ni3S2 toward HER and provides a route to optimize the performance of Ni3S2 with exposed surfaces.

Enhanced Electrocatalytic Activity of Stainless Steel Substrate by Nickel Sulfides for Efficient Hydrogen Evolution

Water splitting is one of the efficient ways to produce hydrogen with zero carbon dioxide emission. Thus far, Pt has been regarded as a highly reactive catalyst for the hydrogen evolution reaction (HER); however, the high cost and rarity of Pt significantly hinder its commercial use. Herein, we successfully developed an HER catalyst composed of NiSx (x = 1 or 2) on stainless steel (NiSx/SUS) using electrodeposition and sulfurization techniques. Notably, the electrochemical active surface area(ECSA) of NiSx/SUS was improved more than two orders of magnitude, resulting in a considerable improvement in the electrochemical charge transfer and HER activity in comparison with stainless steel (SUS). The long-term HER examination by linear scan voltammetry (LSV) confirmed that NiSx/SUS was stable up to 2000 cycles.

Carbon Nanohorn-Based Electrocatalysts for Energy Conversion

In the context of even more growing energy demands, the investigation of alternative environmentally friendly solutions, like fuel cells, is essential. Given their outstanding properties, carbon nanohorns (CNHs) have come forth as promising electrocatalysts within the nanocarbon family. Carbon nanohorns are conical nanostructures made of sp2 carbon sheets that form aggregated superstructures during their synthesis. They require no metal catalyst during their preparation and they are inexpensively produced in industrial quantities, affording a favorable candidate for electrocatalytic reactions. The aim of this article is to provide a comprehensive overview regarding CNHs in the field of electrocatalysis and especially, in oxygen reduction, methanol oxidation, and hydrogen evolution, as well as oxygen evolution from water splitting, underlining the progress made so far, and pointing out the areas where significant improvement can be achieved.

General Colloidal Synthesis of Transition-Metal Disulfide Nanomaterials as Electrocatalysts for Hydrogen Evolution Reaction

The material-efficient monolayers of transition-metal dichalcogenides (TMDs) are a promising class of ultrathin nanomaterials with properties ranging from insulating through semiconducting to metallic, opening a wide variety of their potential applications from catalysis and energy storage to optoelectronics, spintronics, and valleytronics. In particular, TMDs have a great potential as emerging inexpensive alternatives to noble metal-based catalysts in electrochemical hydrogen evolution. Herein, we report a straightforward, low-cost, and general colloidal synthesis of various 2D transition-metal disulfide nanomaterials, such as MoS₂, WS₂, NiS_x, FeS_x, and VS₂, in the absence of organic ligands. This new preparation route provides many benefits including relatively mild reaction conditions, high reproducibility, high yields, easy upscaling, no post-thermal annealing/treatment steps to enhance the catalytic activity, and, finally, especially for molybdenum disulfide nanosheets, high activity in the hydrogen evolution reaction. To underline the universal application of the synthesis, we prepared mixed Co_xMo_1-xS₂ nanosheets in one step to optimize the catalytic activity of pure undoped MoS₂, which resulted in an enhanced hydrogen evolution reaction performance characterized by onset potentials as low as 134 mV and small Tafel slopes of 55 mV/dec.

Recent progress of TMD nanomaterials: phase transitions and applications

Transition metal dichalcogenides (TMDs) show wide ranges of electronic properties ranging from semiconducting, semi-metallic to metallic due to their remarkable structural differences. To obtain 2D TMDs with specific properties, it is extremely important to develop particular strategies to obtain specific phase structures. Phase engineering is a traditional method to achieve transformation from one phase to another controllably. Control of such transformations enables the control of properties and access to a range of properties, otherwise inaccessible. Then extraordinary structural, electronic and optical properties lead to a broad range of potential applications. In this review, we introduce the various electronic properties of 2D TMDs and their polymorphs, and strategies and mechanisms for phase transitions, and phase transition kinetics. Moreover, the potential applications of 2D TMDs in energy storage and conversion, including electro/photocatalysts, batteries/supercapacitors and electronic devices, are also discussed. Finally, opportunities and challenges are highlighted. This review may further promote the development of TMD phase engineering and shed light on other two-dimensional materials of fundamental interest and with potential ranges of applications.

A unique pentagonal network structure of the NiS2 monolayer with high stability and a tunable bandgap

The NiS₂ monolayer with an intriguing pentagonal ring network is stable up to 500 K based on density functional theory calculations.

Hydrogen evolution reaction from bare and surface-functionalized few-layered MoS2 nanosheets in acidic and alkaline electrolytes

Hydrogen is considered as an ideal and sustainable energy carrier because of its high energy density and carbon-free combustion. Electrochemical water splitting is the only solution for uninterrupted, scalable, and sustainable production of hydrogen without carbon emission. However, a large-scale hydrogen production through electrochemical water splitting depends on the availability of earth-abundant electrocatalysts and a suitable electrolyte medium. In this article, we demonstrate that hydrogen evolution reaction (HER) performance of electrocatalytic materials can be controlled by their surface functionalization and selection of a suitable electrolyte solution. Here, we report syntheses of few-layered MoS₂ nanosheets, NiO nanoparticles (NPs), and multiwalled carbon nanotubes (MWCNTs) using scalable production methods from earth-abundant materials. Magnetic measurements of as-produced electrocatalyst materials demonstrate that MoS₂ nanoflakes are diamagnetic, whereas surface-functionalized MoS₂ and its composite with carbon nanotubes have strong ferromagnetism. The HER performance of the few-layered pristine MoS₂ nanoflakes, MoS₂/NiO NPs, and MoS₂/NiO NPs/MWCNT nanocomposite electrocatalysts are studied in acidic and alkaline media. For bare MoS₂, the values of overpotential (η₁₀) in alkaline and acidic media are 0.45 and 0.54 V, respectively. Similarly, the values of current density at 0.5 V overpotential are 27 and 6.2 mA/cm² in alkaline and acidic media, respectively. The surface functionalization acts adversely in the both alkaline and acidic media. MoS₂ nanosheets functionalized with NiO NPs also demonstrated excellent performance for oxygen evolution reaction with anodic current of ~60 mA/cm² and Tafel slope of 78 mVdec^-1 in alkaline medium.

Iron Sulfide Materials: Catalysts for Electrochemical Hydrogen Evolution

The chemical challenge of economically splitting water into molecular hydrogen and oxygen requires continuous development of more efficient, less-toxic, and cheaper catalyst materials. This review article highlights the potential of iron sulfide-based nanomaterials as electrocatalysts for water-splitting and predominantly as catalysts for the hydrogen evolution reaction (HER). Besides new synthetic techniques leading to phase-pure iron sulfide nano objects and thin-films, the article reviews three new material classes: (a) FeS2-TiO2 hybrid structures; (b) iron sulfide-2D carbon support composites; and (c) metal-doped (e.g., cobalt and nickel) iron sulfide materials. In recent years, immense progress has been made in the development of these materials, which exhibit enormous potential as hydrogen evolution catalysts and may represent a genuine alternative to more traditional, noble metal-based catalysts. First developments in this comparably new research area are summarized in this article and discussed together with theoretical studies on hydrogen evolution reactions involving iron sulfide electrocatalysts.

Morphology-Controlled Metal Sulfides and Phosphides for Electrochemical Water Splitting

Because H₂ is considered a promising clean energy source, water electrolysis has attracted great interest in related research and technology. Noble-metal-based catalysts are used as electrode materials in water electrolyzers, but their high cost and low abundance have impeded them from being used in practical areas. Recently, metal sulfides and phosphides based on earth-abundant transition metals have emerged as promising candidates for efficient water-splitting catalysts. Most studies have focused on adjusting the composition of the metal sulfides and phosphides to enhance the catalytic performance. However, morphology control of catalysts, including faceted and hollow structures, is much less explored for these systems because of difficulties in the synthesis, which requires a deep understanding of the nanocrystal growth process. Herein, representative synthetic methods for morphology-controlled metal sulfides and phosphides are introduced to provide insights into these methodologies. The electrolytic performance of morphology-controlled metal sulfide- and phosphide-based nanocatalysts with enhanced surface area and intrinsically high catalytic activity is also summarized and the future research directions for this promising catalyst group is discussed.

Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond

This review describes an in-depth overview and knowledge on the variety of synthetic strategies for forming metal sulfides and their potential use to achieve effective hydrogen generation and beyond.

Tungsten-inert gas welding electrodes as low-cost, green and pH-universal electrocatalysts for the hydrogen evolution reaction

Lanthanated tungsten electrodes were shown to be green, durable, low-cost, pH-universal and efficient electrocatalysts for the hydrogen evolution reaction.

Promoting proton coupled electron transfer in redox catalysts through molecular design

Mini-review on using the secondary coordination sphere to facilitate multi-electron, multi-proton catalysis.

Theoretical design of a series of 2D TM–C3N4 and TM–C3N4@graphene (TM = V, Nb and Ta) nanostructures with highly efficient catalytic activity for the hydrogen evolution reaction

Coupled with high structural stability and metallic conductivity, all of the new composite systems TM–C₃N₄ and TM–C₃N₄@graphene (TM = V, Nb and Ta) can possess considerably high catalytic activity for the hydrogen evolution reaction.

Stable and Efficient Nitrogen-Containing Carbon-Based Electrocatalysts for Reactions in Energy-Conversion Systems

High activity and stability are crucial for the practical use of electrocatalysts in fuel cells, metal-air batteries, and water electrolysis, including the oxygen reduction reaction, hydrogen evolution reaction, oxygen evolution reaction, and oxidation reactions of formic acid and alcohols. Electrocatalysts based on nitrogen-containing carbon (N-C) materials show promise in catalyzing these reactions; however, there is no systematic review of strategies for the engineering of active and stable N-C-based electrocatalysts. Herein, a comprehensive comparison of recently reported N-C-based electrocatalysts regarding both electrocatalytic activity and long-term stability is presented. In the first part of this review, the relationships between the electrocatalytic reactions and selection of the element to modify the N-C-based materials are discussed. Afterwards, synthesis methods for N-C-based electrocatalysts are summarized, and strategies for the synthesis of highly stable N-C-based electrocatalysts are presented. Multiple tables containing data on crucial parameters for both electrocatalytic activity and stability are displayed in this review. Finally, constructing M-N_x moieties is proposed as the most promising engineering strategy for stable N-C-based electrocatalysts.

Nitrogen-Doped CoP Electrocatalysts for Coupled Hydrogen Evolution and Sulfur Generation with Low Energy Consumption

Hydrogen production is the key step for the future hydrogen economy. As a promising H₂ production route, electrolysis of water suffers from high overpotentials and high energy consumption. This study proposes an N-doped CoP as the novel and effective electrocatalyst for hydrogen evolution reaction (HER) and constructs a coupled system for simultaneous hydrogen and sulfur production. Nitrogen doping lowers the d-band of CoP and weakens the H adsorption on the surface of CoP because of the strong electronegativity of nitrogen as compared to phosphorus. The H adsorption that is close to thermos-neutral states enables the effective electrolysis of the HER. Only -42 mV is required to drive a current density of -10 mA cm^-2 for the HER. The oxygen evolution reaction in the anode is replaced by the oxidation reaction of Fe²⁺ , which is regenerated by a coupled H₂ S absorption reaction. The coupled system can significantly reduce the energy consumption of the HER and recover useful sulfur sources.