Adatom Doping of Transition Metals in ReSe2 Nanosheets for Enhanced Electrocatalytic Hydrogen Evolution Reaction

Boosting electrocatalytic hydrogen evolution via partial oxidation of rhenium through cobalt modification in nanoalloy structure

Although the theoretical electrocatalytic activity of rhenium (Re) for the hydrogen evolution reaction is comparable to that of platinum, the experimental performance of reported rhenium-based electrocatalysts remains unsatisfactory. Herein, we report a highly efficient and stable electrocatalyst composed of rhenium and cobalt (Co) nanoalloy embedded in nitrogen-doped carbon film (Re3Co2@NCF). The Re3Co2@NCF electrocatalyst exhibited remarkable hydrogen evolution performance, with an overpotential as low as 30 ± 3 mV to reach a current density of 10 mA cm-2. In addition, the Re3Co2@NCF demonstrated exceptional stability over several days at a current density of 150 mA cm-2. Theoretical calculations revealed that alloying cobalt with rhenium altered the electronic structure of the metals, causing partial oxidation of the superficial metal atoms. This modification provided a balance for various intermediates' adsorption and desorption, thereby boosting the intrinsic activity of rhenium for hydrogen evolution reaction. This work improves the electrocatalytic performance of rhenium to its theoretical activity, suggesting a promising future for rhenium-based electrocatalysts.

2D Transition Metal Chalcogenides (TMDs) for Electrocatalytic Hydrogen Evolution Reaction: A Review

Since the MoS2 synthesis, the family of two-dimensional transition metal chalcogenides (TMDs) have been intensively explored theoretically and experimentally. TMDs endowed with adjustable electronic, physical and chemical properties lead to increasing interest in the application of energy storage, molecule detection and catalysis. In the mini review, we present a forward-looking summary of 2D TMDs in hydrogen evolution electrocatalysis, including synthesis methods, hydrogen evolution performance, and optimization strategies. This review will deepen the fundamental understanding of the physical-chemical properties of TMDs with different phases and contribute unveil the universal principle among electronic configuration, atomic arrangement, physical and chemical property for the material design.

Engineered Two-Dimensional Transition Metal Dichalcogenides for Energy Conversion and Storage

Designing efficient and cost-effective materials is pivotal to solving the key scientific and technological challenges at the interface of energy, environment, and sustainability for achieving NetZero. Two-dimensional transition metal dichalcogenides (2D TMDs) represent a unique class of materials that have catered to a myriad of energy conversion and storage (ECS) applications. Their uniqueness arises from their ultra-thin nature, high fractions of atoms residing on surfaces, rich chemical compositions featuring diverse metals and chalcogens, and remarkable tunability across multiple length scales. Specifically, the rich electronic/electrical, optical, and thermal properties of 2D TMDs have been widely exploited for electrochemical energy conversion (e.g., electrocatalytic water splitting), and storage (e.g., anodes in alkali ion batteries and supercapacitors), photocatalysis, photovoltaic devices, and thermoelectric applications. Furthermore, their properties and performances can be greatly boosted by judicious structural and chemical tuning through phase, size, composition, defect, dopant, topological, and heterostructure engineering. The challenge, however, is to design and control such engineering levers, optimally and specifically, to maximize performance outcomes for targeted applications. In this review we discuss, highlight, and provide insights on the significant advancements and ongoing research directions in the design and engineering approaches of 2D TMDs for improving their performance and potential in ECS applications.

Synthesis and Electrocatalytic Applications of Layer-Structured Metal Chalcogenides Composites

Featured with the attractive properties such as large surface area, unique atomic layer thickness, excellent electronic conductivity, and superior catalytic activity, layered metal chalcogenides (LMCs) have received considerable research attention in electrocatalytic applications. In this review, the approaches developed to synthesize LMCs-based electrocatalysts are summarized. Recent progress in LMCs-based composites for electrochemical energy conversion applications including oxygen reduction reaction, carbon dioxide reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, overall water splitting, and nitrogen reduction reaction is reviewed, and the potential opportunities and practical obstacles for the development of LMCs-based composites as high-performing active substances for electrocatalytic applications are also discussed. This review may provide an inspiring guidance for developing high-performance LMCs for electrochemical energy conversion applications.

Nitrogen-Based Gas Molecule Adsorption on a ReSe2 Monolayer via Single-Atom Doping: A First-Principles Study

Two-dimensional materials have shown immense promise for gas-sensing applications due to their remarkable surface-to-volume ratios and tunable chemical properties. However, despite their potential, the utilization of ReSe2 as a gas-sensing material for nitrogen-containing molecules, including NO2, NO, and NH3, has remained unexplored. The choice of doping atoms in ReSe2 plays a pivotal role in enhancing the gas adsorption and gas-sensing capabilities. Herein, the adsorption properties of nitrogen-containing gas molecules on metal and non-metal single-atom (Au, Pt, Ni, P, and S)-doped ReSe2 monolayers have been evaluated systematically via ab initio calculations based on density functional theory. The findings strongly suggest that intrinsic ReSe2 has better selectivity toward NO2 than toward NO and NH3. Moreover, our results provide compelling evidence that all of the dopants, with the exception of S, significantly enhance both the adsorption strength and charge transfer between ReSe2 and the investigated molecules. Notably, P-decorated ReSe2 showed the highest adsorption energy for NO2 and NO (-1.93 and -1.52 eV, respectively) with charge transfer above 0.5e, while Ni-decorated ReSe2 exhibited the highest adsorption energy for NH3 (-0.76 eV). In addition, on the basis of transition theory, we found that only Au-ReSe2 and Ni-ReSe2 can serve as reusable chemiresisitve gas sensors for reliable detection of NO and NH3, respectively. Hence, our findings indicate that gas-sensing applications can be significantly improved by utilizing a single-atom-doped ReSe2 monolayer.

Electrocatalytic Activation in ReSe2-VSe2 Alloy Nanosheets to Boost Water-Splitting Hydrogen Evolution Reaction

It is challenging to control the electronic structure of 2D transition metal dichalcogenides (TMD) for extended applications in renewable energy devices. Here, ReSe2-VSe2 (Re1- xVxSe2) alloy nanosheets over the whole composition range via a colloidal reaction is synthesized. Increasing x makes the nanosheets more metallic and induces a 1T″-to-1T phase transition at x = 0.5-0.6. Compared to the MoSe2-VSe2 and WSe2-VSe2 alloy nanosheets, ReSe2 and VSe2 are mixed more homogeneously at the atomic scale. The alloy nanosheets at x = 0.1-0.7 exhibit an enhanced electrocatalytic activity toward acidic hydrogen evolution reaction (HER). In situ X-ray absorption fine structure measurements reveal that alloying caused the Re and V atoms to be synergically more active in the HER. Gibbs free energy (ΔGH*) and density of state calculations confirm that alloying and Se vacancies effectively activate the metal sites toward HER. The composition dependence of HER performance is explained by homogenous atomic mixing with the increased Se vacancies. The study provides a strategy for designing new TMD alloy nanosheets with enhanced catalytic activity.

Exceptionally large fracture strength and stretchability of 2D ReS2 and ReSe2

Two-dimensional rhenium disulfide (ReS2) and rhenium diselenide (ReSe2) have gained popularity due to their outstanding optoelectronic properties. However, their mechanical behavior has not been investigated experimentally and many of their mechanical parameters are still unexplored. Here we conducted atomic force microscopy (AFM) indentation experiments and extracted their Young's moduli and found that it is thickness-independent. In addition, we found that both materials are capable of sustaining large pretension. Importantly, fracture tests showed that these materials exhibit exceptionally large fracture strength (32.9 ± 2.4 GPa and 27.7 ± 3.9 GPa for ReS2 and ReSe2, respectively) and stretchability (up to 24.2% for ReS2 and 23.0% for ReSe2). Therefore, this study shows the superior mechanical properties of ReS2 and ReSe2. Thus, it will open the path for their future integration into advanced applications that will benefit from their outstanding mechanical durability and attractive optoelectronic properties, such as flexible photodetectors, stretchable photonic devices, and strain-engineered electronics.

Wood-Derived Freestanding Carbon-Based Electrode with Hierarchical Structure for Industrial-Level Hydrogen Production

The sustainable and scalable fabrication of low-cost, efficient, and durable electrocatalysts that operate well at industrial-level current density is urgently needed for large-scale implementation of the water splitting to produce hydrogen. In this work, an integrated carbon electrode is constructed by encapsulating Ni nanoparticles within N-doped carbonized wood framework (Ni@NCW). Such integrated electrode with hierarchically porous structure facilitates mass transfer process for hydrogen evolution reaction (HER). Ni@NCW electrode can be employed directly as a robust electrocatalyst for HER, which affords the industrial-level current density of 1000 mA cm-2 at low overpotential of 401 mV. The freestanding binder-free electrode exhibits extraordinary stability for 100 h. An anion exchange membrane water electrolysis (AEMWE) electrolyzer assembled with such freestanding carbon electrode requires only a lower cell voltage of 2.43 V to achieve ampere-level current of 4.0 A for hydrogen production without significant performance degradation. These advantages reveal the great potential of this strategy in designing cost-effective freestanding electrode with monometallic, bimetallic, or trimetallic species based on abundant natural wood resources for water splitting.

In Situ Preparation of Polyamine-Derived Ru Cluster@N-Doped Porous Carbon Nanoplates for Hydrogen Evolution over Wide pH Ranges

Efficient and low-cost electrocatalysts for the hydrogen evolution reaction (HER) are required for producing hydrogen energy through water splitting. Carbon materials as HER catalyst supports are explored widely since the strong metal-support interactions are generally believed to be active and stable toward HER. Herein, we report N-doped porous carbon materials as novel substrates to stabilize the cluster metal sites through the Ru(III) polyamine complexes, which play an important role not only in efficient electron transfer but also in the increasing utilization of metallic active sites. Meanwhile, due to the strong metal-support interactions driven by Ru(III) polyamine complexes, the obtained Ru cluster with a mass loading of 3% on N-doped porous carbon nanoplates (Ru cluster@NCs) exhibits robust stability for HER at a constant voltage, proving to be a promising candidate catalyst for HER. Density functional theory calculations further indicate that the Gibbs free energy (ΔG) of adsorbed H* of Ru cluster@NCs is much closer to zero compared to Ru@(10%)NCs and Pt/C(20%), thus Ru cluster@NCs facilitate the HER process.

Activating HfX2 (X = S, Se and Te) for the hydrogen evolution reaction by introducing defects: a first-principles study

An ideal catalyst should have a relative hydrogen adsorption Gibbs free energy (ΔGH) close to zero [J. K. Nørskov, et al., J. Electrochem. Soc., 2005, 152, J23]. However, most of the known catalysts cannot reach this standard. Based on first-principles calculations, we studied the hydrogen evolution reaction (HER) catalytic performance of pristine and defect (including vacancy and heteroatom doping) structures in terms of its ΔGH. We found that the ΔGH values of Co-doped HfS2 and P-doped HfSe2 are extremely close to zero, even closer than that of Pt (111), indicating that they are excellent catalysts. Moreover, we found that the source of the HER catalytic performance of Co-doped HfS2 is the reduction of electron accumulation of the active site S atom. Our work provides two potential ideal catalysts and provides guidance for the experimental group to search for suitable catalysts.

Atomic-Level Regulated 2D ReSe2 : A Universal Platform Boostin Photocatalysis

Solar hydrogen (H₂ ) generation via photocatalytic water splitting is practically promising, environmentally benign, and sustainably carbon neutral. It is important therefore to understand how to controllably engineer photocatalysts at the atomic level. In this work, atomic-level engineering of defected ReSe₂ nanosheets (NSs) is reported to significantly boost photocatalytic H₂ evolution on various semiconductor photocatalysts including TiO₂ , CdS, ZnIn₂ S₄ , and C₃ N₄ . Advanced characterizations, such as atomic-resolution aberration-corrected scanning transmission electron microscopy (AC-STEM), synchrotron-based X-ray absorption near edge structure (XANES), in situ X-ray photoelectron spectroscopy (XPS), transient-state surface photovoltage (SPV) spectroscopy, and transient-state photoluminescence (PL) spectroscopy, together with theoretical computations confirm that the strongly coupled ReSe₂ /TiO₂ interface and substantial atomic-level active sites of defected ReSe₂ NSs result in the significantly raised activity of ReSe₂ /TiO₂ . This work not only for the first time realizes the atomic-level engineering of ReSe₂ NSs as a versatile platform to significantly raise the activities on different photocatalysts, but, more importantly, underscores the immense importance of atomic-level synthesis and exploration on 2D materials for energy conversion and storage.

Scalable-doped Nanoporous 1T″ ReSe2 via a General Surface Co-Alloy Strategy

Reliable and controllable doping of 2D transition metal dichalcogenides is an efficient approach to tailor their physicochemical properties and expand their functional applications. However, precise control over dopant distribution and scalability of the process remains a challenge. Here, we report a general method to achieve scalable in situ doping of centimeter-sized bicontinuous nanoporous ReSe₂ films with transition metal atoms via surface coalloy growth. The distinct strains induced by the bending curvature of nanoporous structures and uniform dopants result in a local 1T' to 1T″ structure phase transition over nanoporous ReSe₂ films. The as-prepared nanoporous Ru-ReSe₂ with high 1T″ phase exhibits preferable electrochemical activity in hydrogen evolution reaction. The work demonstrates a unique and general approach to synthesize uniformly-doped transition metal dichalcogenides with 3D bicontinuous nanoporous structure, which can be scaled up to batch production for various applications.

Electrocatalytic activity of a β-Sb two-dimensional surface for the hydrogen evolution reaction

Hydrogen energy is considered to be one of the most promising clean energy sources. The development of highly active, low-cost catalysts, and good stability is essential for hydrogen production. Herein, the catalytic activity of a two-dimensional β-Sb surface doped with main-group elements (N, P, As, O, S, Se, and Te) for the hydrogen evolution reaction (HER) was investigated by density functional theory, and the catalytic activity of the β-Sb monolayer can be improved by doping group VIA atoms. The catalytic activity of Se@Sb and O@Sb structures at the doping concentration of 2.78% and the S@Sb structure at the doping concentration of 5.56% may be as good as the Pt(111) surface, while keeping energetically stable. In addition, the catalytic performance could be optimized under biaxial strain. Further analysis suggests that the activity is caused by hole states in the lone pair electrons, which are created by the group VIA atom dopants. And our work also reveals that the density of states at the Fermi level could be an appropriate descriptor of the hydrogenation Gibbs free energy. This work not only proposes a novel non-platinum HER catalyst but also provides physical foundations for further application on antimonene-based catalysts.

In Situ Growth of NiSe2-MoSe2 Heterostructures on Graphene Nanosheets as High-Performance Electrocatalyst for Hydrogen Evolution Reaction

Developing highly efficient and stable electrocatalysts for hydrogen evolution reaction (HER) is regarded as a crucial way to reduce energy loss in water splitting. Herein, NiSe2/MoSe2 heterostructures grown on graphene nanosheets (NiSe2-MoSe2 HTs/G) have been in situ synthesized by a simple hydrothermal reaction. As an electrocatalyst for HER, NiSe2-MoSe2 HTs/G delivers superior performance with a low Tafel slope of 65 mV dec−1, a small overpotential of 144 mV at 10 mA cm−2, and long-term stability up to 24 h. The superior performance for HER can be mainly ascribed to the synergistic effects of NiSe2-MoSe2 heterostructures, which can facilitate the rapid electron transfer from the electrode to the exposed MoSe2 edges to take part in the HER reaction, thus boosting the HER kinetics. Moreover, the graphene matrix with high conductivity can not only improve the overall conductivity of the composite but also greatly increase the exposed active sites, therefore further promoting the HER performance. This study provides a simple route for fabricating bimetallic selenides-based heterostructures on graphene as an efficient and stable electrocatalyst for HER.

Encapsulated RuP2-RuS2 nanoheterostructure with regulated interfacial charge redistribution for synergistically boosting hydrogen evolution electrocatalysis

Exploring cost-effective electrocatalysts with suitable hydrogen binding strength and rational micro/nano-architecture towards the hydrogen evolution reaction (HER) is crucial for energy technologies, yet remains a tough challenge. Herein we present the first instance of a nanoscale RuP₂-RuS₂ heterostructure encapsulated in N, P, and S co-doped porous carbon nanosheets (RuP₂-RuS₂/NPS-C) for boosting the HER. The synthesis involves the construction of a 2D core-shell structured precursor in which Ru³⁺-functionalized g-C₃N₄ is wrapped by poly(cyclotriphosphazene-co-4,4'-sulfonyldiphenol) followed by pyrolysis. In this nanocomposite, the unique architecture with a highly dispersed embedded RuP₂-RuS₂ nanoheterostructure guarantees not only full exposure of the active sites with enhanced robustness but also smooth mass/charge transfer. More significantly, the experimental results and theoretical calculations reveal that coupling RuP₂ with RuS₂ to construct a heterointerface can induce charge redistribution, giving rise to optimized hydrogen adsorption energy for substantially accelerating the HER. This work provides a novel strategy to engineer high-performance Ru-based electrocatalysts by elegantly modulating the micro-/nano-architecture and interface coupling effect.

Janus MoPC Monolayer with Superior Electrocatalytic Performance for the Hydrogen Evolution Reaction

Designing the earth's abundant and high-performance electrocatalysts, which possess high stability, excellent electrical conductivity, inherent active sites, and catalytic activity identical with Pt, is challenging but crucial for the hydrogen evolution reaction (HER). By first-principles structure search simulations, we identify a new two-dimensional (2D) MoPC material with the Janus structure as a promising catalyst. This novel 2D monolayer has superior stability and metallic conductivity. Especially, it exhibits a remarkable HER catalytic activity, where all of the constituent atoms, including Mo, P, and C, can uniformly act as active sites in view of the near-zero ΔG_H* value. Its active site density counts up to 1.46 × 10¹⁵ site/cm², larger than that of many reported materials and even comparable to Pt. The excellent HER catalytic activity can also be maintained at a very high H coverage with or without external strain. The MoPC monolayer can produce H₂ spontaneously through the favorable Volmer-Heyrovsky pathway. The detailed catalytic mechanism behind the high HER activity has been also analyzed. Our work provides a feasible action for the experimental synthesis of excellent HER catalysts.

Anion-exchange membrane water electrolyzers and fuel cells

The key components, working management, and operating techniques of anion-exchange membrane water electrolyzers and fuel cells are reviewed for the first time.

Increasing Oxygen Vacancy of CeO2 Nanocrystals by Ni Doping and reduced Graphene Oxides Decoration towards the Electrocatalytic Hydrogen Evolution

The oxygen vacancy (VO) engineering is proved to be an effective approach for improving the hydrogen evolution reaction (HER) performance of low-cost metal oxides electrocatalysts. Cerium dioxide (CeO2), one of...

2D Layered Material Alloys: Synthesis and Application in Electronic and Optoelectronic Devices

2D layered materials (2DLMs) have come under the limelight of scientific and engineering research and broke new ground across a broad range of disciplines in the past decade. Nevertheless, the members of stoichiometric 2DLMs are relatively limited. This renders them incompetent to fulfill the multitudinous scenarios across the breadth of electronic and optoelectronic applications since the characteristics exhibited by a specific material are relatively monotonous and limited. Inspiringly, alloying of 2DLMs can markedly broaden the 2D family through composition modulation and it has ushered a whole new research domain: 2DLM alloy nano-electronics and nano-optoelectronics. This review begins with a comprehensive survey on synthetic technologies for the production of 2DLM alloys, which include chemical vapor transport, chemical vapor deposition, pulsed-laser deposition, and molecular beam epitaxy, spanning their development, as well as, advantages and disadvantages. Then, the up-to-date advances of 2DLM alloys in electronic devices are summarized. Subsequently, the up-to-date advances of 2DLM alloys in optoelectronic devices are summarized. In the end, the ongoing challenges of this emerging field are highlighted and the future opportunities are envisioned, which aim to navigate the coming exploration and fully exert the pivotal role of 2DLMs toward the next generation of electronic and optoelectronic devices.

Single-Atom Engineering to Ignite 2D Transition Metal Dichalcogenide Based Catalysis: Fundamentals, Progress, and Beyond

Single-atom catalysis has been recognized as a pivotal milestone in the development history of heterogeneous catalysis by virtue of its superior catalytic performance, ultrahigh atomic utilization, and well-defined structure. Beyond single-atom protrusions, two more motifs of single-atom substitutions and single-atom vacancies along with synergistic single-atom motif assemblies have been progressively developed to enrich the single-atom family. On the other hand, besides traditional carbon material based substrates, a wide variety of 2D transitional metal dichalcogenides (TMDs) have been emerging as a promising platform for single-atom catalysis owing to their diverse elemental compositions, variable crystal structures, flexible electronic structures, and intrinsic activities toward many catalytic reactions. Such substantial expansion of both single-atom motifs and substrates provides an enriched toolbox to further optimize the geometric and electronic structures for pushing the performance limit. Concomitantly, higher requirements have been put forward for synthetic and characterization techniques with related technical bottlenecks being continuously conquered. Furthermore, this burgeoning single-atom catalyst (SAC) system has triggered serial scientific issues about their changeable single atom-2D substrate interaction, ambiguous synergistic effects of various atomic assemblies, as well as dynamic structure-performance correlations, all of which necessitate further clarification and comprehensive summary. In this context, this Review aims to summarize and critically discuss the single-atom engineering development in the whole field of 2D TMD based catalysis covering their evolution history, synthetic methodologies, characterization techniques, catalytic applications, and dynamic structure-performance correlations. In situ characterization techniques are highlighted regarding their critical roles in real-time detection of SAC reconstruction and reaction pathway evolution, thus shedding light on lifetime dynamic structure-performance correlations which lay a solid theoretical foundation for the whole catalytic field, especially for SACs.

Photosensitive electrocatalysts based on Ni-WS2nanohybrids for hydrogen evolution reaction

Efficient hydrogen evolution by electrolysis plays an indispensable role for hydrogen fuel generation in green energy devices. In order to implement high-performance electrocatalytic activity, it is usually necessary to design economically viable, effective and stable electrocatalysts to reduce activation potential barriers. Herein, we report the photosensitive Ni-WS₂nanohybrids for enhanced electrocatalytic hydrogen evolution reaction (HER). Optimisation of chemical composition in catalysts has resulted in the rapid water electrolysis which was further promoted by illumination of 532 nm light. Obvious HER has been achieved at over potential of as low as -210 mV versus RHE without and -190 mV versus RHE (at -10 mA cm^-2) with illumination. Being a photosensitive electrocatalysts, Ni-WS₂Nanohybrids have demonstrated stable time-resolved photoresponse with photocurrent of 12.7 mA cm^-2at -250 mV V versus RHE as well as self-powered photodetection with current 0.68 mA cm^-2. Finally, HER with improvement under visible light illumination has shown considerable development in clean energy generation by using renewable energy sources.

Ab Initio Study of CO2 Activation on Pristine and Fe-Decorated WS2 Nanoflakes

There is an intense race by the scientific community to identify materials with potential applications for the conversion of carbon dioxide (CO₂) into new products. To extend the range of possibilities and explore new effects, in this work, we employ density functional theory calculations to investigate the presence of edge effects in the adsorption and activation of CO₂ on pristine and Fe-decorated (WS₂)₁₆ nanoflakes. We found that Fe has an energetic preference for hollow sites on pristine nanoflakes, binding with at least two two-fold edge S atoms and one or two three-fold core S atoms. Fe adsorption on the bridge sites occurs only at the edges, which is accompanied by the breaking of W-S bonds in most cases (higher energy configurations). CO₂ activates on (WS₂)₁₆ with an OCO angle of about 129° only at higher energy configurations, while CO₂ binds via a physisorption mechanism, linear structure, in the lowest energy configuration. For CO₂ on Fe/(WS₂)₁₆, the activation occurs at lower energies only by the direct interaction of CO₂ with Fe sites located near to the nanoflake edges, which clearly indicates the enhancement of the catalytic activity of (WS₂)₁₆ nanoflakes by Fe decoration. Thus, our study indicates that decorating WS₂ nanoflakes with TM atoms could be an interesting strategy to explore alternative catalysts based on two-dimensional materials.

Nitrogen-Doped Cobalt Diselenide with Cubic Phase Maintained for Enhanced Alkaline Hydrogen Evolution

The introduction of heteroatoms is one of the most important ways to modulate the intrinsic electronic structure of electrocatalysts to improve their catalytic activity. However, for transition metal chalcogenides with highly symmetric crystal structure (HS-TMC), the introduction of heteroatoms, especially those with large atomic radius, often induces large lattice distortion and vacancy defects, which may lead to structural phase transition of doped materials or structural phase reconstruction during the catalytic reaction. Such unpredictable situations will make it difficult to explore the connection between the intrinsic electronic structure of doped catalysts and catalytic activity. Herein, taking thermodynamically stable cubic CoSe₂ phase as an example, we demonstrate that nitrogen incorporation can effectively regulate the intrinsic electronic structure of HS-TMC with structural phase stability and thus promote its electrocatalytic activity for the hydrogen evolution activity (HER). In contrast, the introduction of phosphorus can lead to structural phase transition from cubic CoSe₂ to orthorhombic phase, and the structural phase of phosphorus-doped CoSe₂ is unstable for HER.

Doping regulation in transition metal compounds for electrocatalysis

In electrocatalysis, doping regulation has been considered as an effective method to modulate the active sites of catalysts, providing a powerful means for creating a large variety of highly efficient catalysts for various reactions. Of particular interest, there has been growing research concerning the doping of two-dimensional transition-metal compounds (TMCs) to optimize their electrocatalytic performance. Despite the previous achievements, mechanistic insights of doping regulation in TMCs for electrocatalysis are still lacking. Herein, we provide a systematic overview of doping regulation in TMCs in terms of background, preparation, impacts on physicochemical properties, and typical applications including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO₂ reduction reaction, and N₂ reduction reaction. Notably, we bridge the understanding between the doping regulation of catalysts and their catalytic activities via focusing on the physicochemical properties of catalysts from the aspects of vacancy concentrations, phase transformation, surface wettability, electrical conductivity, electronic band structure, local charge distribution, tunable adsorption strength, and multiple adsorption configurations. We also discuss the existing challenges and future perspectives in this promising field.

Achieving high hydrogen evolution reaction activity of a Mo2C monolayer

Two-dimensional Mo2C materials (1T and 2H phases) have emerged as promising electrocatalysts for the hydrogen evolution reaction (HER) due to their low cost, inherent metallicity, and high stability. Unfortunately, the catalytic activity of Mo2C is lower than that of Pt, and it needs to be substantially improved for practical applications. It is necessary and urgent to consider the effect of synergetic interactions among defects, functions, and strain on the HER activity. In this study, the geometric structures, electronic properties, and the HER activity of the Mo2C monolayer, with vacancy defects (i.e. Mo and C), oxygen functionalization, and strain, are studied by using first-principles calculations. According to our results, the combination of Mo vacancies, which can be obtained under C-rich conditions, and oxygen functionalization is the most effective way to improve the HER activity of 1T- and 2H-Mo2C. Considering the abundant active sites and optimal Gibbs free energy of hydrogen adsorption, the 1T phase we obtained shows excellent HER activity even at high H coverage and improves the utilization of active sites, for which the HER activity is comparable to that of Pt. This can be attributed to the fact that oxygen atoms gain more electrons from Mo2C, which weakens the strength of the O-H bond. Our work provides not only an opportunity to better understand the catalytic mechanism, but also a guide to achieving high HER activity of a Mo2C monolayer.