Heterostructure engineering of Co-doped MoS2 coupled with Mo2CTx MXene for enhanced hydrogen evolution in alkaline media

Materials for Electrocatalysis: Future Prospects in Energy Conversion

Electrocatalysts play a pivotal role in various energy conversion processes, such as water splitting, batteries, carbon dioxide reduction, and fuel cell reactions, by significantly reducing the energy barrier and enhancing reaction kinetics. This review highlights the potential of earth-abundant electrocatalysts, with a particular focus on their capabilities in critical electrochemical reactions, including oxygen evolution reaction, carbon dioxide reduction reaction, oxygen reduction reaction and hydrogen evolution reaction. Emphasis is also placed on bifunctional, trifunctional, and tetrafunctional performance, showcasing their adaptability and effectiveness across diverse energy applications. Exploration is done on a range of promising materials, including transition metal chalcogenides, MXenes, metal-organic frameworks, covalent organic frameworks, and layered double hydroxides. By examining their intrinsic properties, structural versatility, and surface engineering strategies, this review sheds light on the factors that govern their catalytic efficiency and stability. The integration of experimental advancements with theoretical insights provides a deeper understanding of mechanisms driving their catalytic activity. Additionally, we address the scalability, cost-effectiveness, and environmental impact of these materials, underlining their potential for large-scale deployment. By synthesizing recent progress and identifying challenges, this work delivers a roadmap for the model and application of multifunctional electrocatalysts, fostering innovations that align with the goals of sustainable energy systems.

Refractory Metal-Based MXenes: Cutting-Edge Preparation and Applications

Refractory metal-based MXenes refer to MXenes with M as a refractory metal. Due to their high conductivity, large specific surface area, multiple active sites, high photothermal conversion efficiency, adjustable surface groups, and controllable nanolayer spacing, they hold broad application prospects in various fields such as photoelectrocatalysis, biomedicine, water treatment, electromagnetic shielding, and sensors. The unique physical properties of refractory metal-based MXenes are related to their electronic and crystal structures. The interstitial layer causes the carbides to exhibit different behavior compared to the original metal. At the same time, different preparation methods have a great influence on the interlayer spacing and surface termination of refractory metal-based MXenes, thus affecting their performance. This review systematically summarizes the latest progress in the preparation methods and frontier applications of refractory metal-based MXenes, offering new insights for further development. Additionally, various characterization techniques and first-principles calculations are summarized, which are crucial for optimizing refractory metal-based MXenes for applications such as catalysis, energy storage, and sensors. In summary, the current challenges and future development prospects of refractory metal-based Mxenes are addressed, aiming to provide indispensable information for the intelligent design of 2D materials in the future.

Deep eutectic Solvents-Assisted synthesis of NiFe-LDHs/Mo2Ti2C3: A bifunctional electrocatalyst for overall electrochemical water splitting in alkaline media

The development of efficient and stable electrocatalysts is crucial for the advancement of green and clean hydrogen energy technologies. In this work, we synthesized a nanocomposite of nickel-iron layered double hydroxide/molybdenum titanium carbide (NiFe-LDHs/Mo2Ti2C3) using a deep eutectic solvent (DESs) by the solvothermal method. The formation of NiFe-LDHs/Mo2Ti2C3 nanocomposite was confirmed by various electron microscopic and spectroscopic techniques. The synthesized nanocomposite was investigated as a bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) under the alkaline condition. The NiFe-LDHs/Mo2Ti2C3-based electrodes exhibit small overpotentials of 204 and 306 mV for HER and OER at a current density of 10 mA cm-2. The anchor of NiFe-LDHs on the surface of Mo2Ti2C3 induces an interfacial synergistic effect, leading to a significantly improvement in electrochemical performance. Remarkably, the proposed NiFe-LDHs/Mo2Ti2C3 modified electrode demonstrates superior performance compared to many recently reported LDHs and MXenes-based electrocatalysts in an alkaline environment. Furthermore, a symmetrical two-electrode water splitting setup employing the NiFe-LDHs/Mo2Ti2C3 electrocatalyst requires an electrolysis voltage of 1.65 V to achieve a current density of 10 mA cm-2. The findings provide a new perspective on the rational design and synthesis of multifunctional electrocatalysts for electrochemical applications.

Layered MOF supported on 2D delaminated MXene (Mo2Ti2C3) nanosheets boosted water splitting

Metal organic frameworks (MOFs) have a porous structure, high surface area, and high charge transfer, and they have been regarded as model electrocatalysts. Optimization of the electrocatalytic activity of MOFs is challenging, and an effective strategy for this optimization is the construction of a well-defined interfacial bond bridge. In this work, an in situ approach of composite synthesis is reported for MOF (CoNiNH2BDC) with MXenes (Mo2Ti2C3), as the electrocatalytic properties of MOF can be greatly enhanced with the incorporation of the conductive material MXene. The prepared composite material was characterized thoroughly using XRD, XPS, FESEM, EDX, TEM, and BET. The synergistic effect of both components of this composite material resulted in enhanced conductivity and the number of active sites, which led to enhanced electrocatalytic performance. The CoNiNH2BDC MOF with different ratios of Mo2Ti2C3 MXene were synthesized, and the resulting materials were evaluated for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activities. It was observed that the MOFMX3 attained a 10 mA cm-2 current density at 1.44 V for OER and -0.037 V for HER (vs. RHE), and lower values of Tafel slopes of 44.8 mV dec-1 for OER and 45 mV dec-1 for HER in 0.1 M KOH were achieved. The higher double layer capacitance (C dl) values lead to high electrochemical surface area (ECSA) values. Lower Tafel slope values for MOFMX3 show that the presence of MXene nanosheets in the hybrid provides support to the layered and porous configuration of MOF, and the chances of the interaction of electrolyte to the catalytically active sites are significantly enhanced. This work highlights the idea of growing bimetallic MOFs on Mo2Ti2C3 MXene using an interdiffusion reaction strategy and opens up an avenue for designing highly electrocatalytic systems.

Two-Dimensional MXene-Based Electrocatalysts: Challenges and Opportunities

MXene, regarded as cutting-edge two-dimensional (2D) materials, have been widely explored in various applications due to their remarkable flexibility, high specific surface area, good mechanical strength, and interesting electrical conductivity. Recently, 2D MXene has served as a ideal platform for the design and development of electrocatalysts with high activity, selectivity, and stability. This review article provides a detailed description of the structural engineering of MXene-based electrocatalysts and summarizes the uses of 2D MXene in hydrogen evolution reactions, nitrogen reduction reactions, oxygen evolution reactions, oxygen reduction reactions, and methanol/ethanol oxidation. Then, key issues and prospects for 2D MXene as a next-generation platform in fundamental research and real-world electrocatalysis applications are discussed. Emphasis will be given to material design and enhancement techniques. Finally, future research directions are suggested to improve the efficiency of MXene-based electrocatalysts.

MAX Phase (Nb4AlC3) For Electrocatalysis Applications

In search for novel materials to replace noble metal-based electrocatalysts in electrochemical energy conversion and storage devices, special attention is given to a distinct class of materials, MAX phase that combines advantages of ceramic and metallic properties. Herein, Nb4AlC3 MAX phase is prepared by a solid-state mixing reaction and characterized morphologically and structurally by transmission and scanning electron microscopy with energy-dispersive X-ray spectroscopy, nitrogen-sorption, X-ray diffraction analysis, X-ray photoelectron and Raman spectroscopy. Electrochemical performance of Nb4AlC3 in terms of capacitance as well as for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is evaluated in different electrolytes. The specific capacitance Cs of 66.4, 55.0, and 46.0 F g-1 at 5 mV s-1 is determined for acidic, neutral and alkaline medium, respectively. Continuous cycling reveals high capacitance retention in three electrolyte media; moreover, increase of capacitance is observed in acidic and neutral media. The electrochemical impedance spectroscopy showed a low charge transfer resistance of 64.76 Ω cm2 that resulted in better performance for HER in acidic medium (Tafel slope of 60 mV dec-1). In alkaline media, the charge storage value in the double layer is 360 mF cm-2 (0.7 V versus reversible hydrogen electrode) and the best ORR performance of the Nb4AlC3 is achieved in this medium (Tafel slope of 126 mV dec-1).

Fluorine-Free MXenes: Recent Advances, Synthesis Strategies, and Mechanisms

MXenes, an exceptional class of 2D materials, possess high conductivity, adaptable surface chemistry, mechanical strength, and tunable bandgaps, making them attractive for diverse applications. Unlocking the potential of MXenes requires precise control over synthesis methods and surface functionality. Conventionally, fluorine-based etchants are used in MXenes synthesis, posing both environmental concerns and alterations to surface properties, along with the introduction of certain defects. This prompts the exploration of innovative fluorine-free strategies for MXenes synthesis. This review focuses on environmentally friendly, fluorine-free techniques for MXene synthesis, emphasizing mechanisms and recent breakthroughs in alternative etching strategies. The comprehensive coverage includes electrochemical etching, Lewis acid-driven molten salt etching, alkaline/hydrothermal techniques, chemical vapor deposition (CVD), and recent innovative methods. Fluorine-free MXenes synthesis yields terminations such as ─O, ─OH, ─Cl, etc., influencing surface chemistry and improving their properties. The presence of ─OH groups in NaOH etched MXenes boosts their energy storage, while ─Cl functionality from Lewis acidic salts optimizes electrochemical performance. Fluorine-free methods mitigate adverse effects of ─F terminations on MXene conductivity, improving electronic properties and broadening their applications. In addition to traditional approaches, this review delves into novel fluorine-free methods for tailoring MXenes properties. It comprehensively addresses challenges, opportunities, and future perspectives in fluorine-free MXenes.

Progress in the Synthesis Process and Electrocatalytic Application of MXene Materials

With their rich surface chemistry, high electrical conductivity, variable bandgap, and thermal stability, 2D materials have been developed for effective electrochemical energy conversion systems over the past decade. Due to the diversity brought about by the use of transition metals and C/N pairings, the 2D material MXene has found excellent applications in many fields. Among the various applications, many breakthroughs have been made in electrocatalytic applications. Nevertheless, related studies on topics such as the factors affecting the material properties and safer and greener preparation methods have not been reported in detail. Therefore, in this paper, we review the relevant preparation methods of MXene and the safer, more environmentally friendly preparation techniques in detail, and summarize the progress of research on MXene-based materials as highly efficient electrocatalysts in the electrocatalytic field of hydrogen precipitation reaction, nitrogen reduction reaction, oxygen precipitation reaction, oxygen reduction reaction, and carbon dioxide reduction reaction. We also discuss the technology related to MXene materials for hydrogen storage. The main challenges and opportunities for MXene-based materials, which constitute a platform for next-generation electrocatalysis in basic research and practical applications, are highlighted. This paper aims to promote the further development of MXenes and related materials for electrocatalytic applications.

Temperature, pressure, and humidity SAW sensor based on coplanar integrated LGS

This paper presents a surface acoustic wave (SAW) sensor based on coplanar integrated Langasite (LGS) that is fabricated using wet etching, high-temperature bonding, and ion beam etching (IBE) processes. The miniaturized multiparameter temperature‒pressure-humidity (TPH) sensor used the MXene@MoS2@Go (MMG) composite to widen the humidity detection range and improve the humidity sensitivity, including a fast response time (3.18 s) and recovery time (0.94 s). The TPH sensor was shown to operate steadily between 25-700 °C, 0-700 kPa, and 10-98% RH. Coupling issues among multiple parameters in complex environments were addressed by decoupling the Δf-temperature coupling factor to improve the accuracy. Therefore, this work can be applied to simultaneous measurements of several environmental parameters in challenging conditions.

Photo-assisted Zn-air battery-driven self-powered aptasensor based on the 2D/2D Schottky heterojunction of cadmium-doped molybdenum disulfide and Ti3C2Tx nanosheets for the sensitive detection of penicillin G

A novel photocatalyzed Zn-air battery-driven (ZAB)-based aptasensor has been manufactured using the two dimensional (2D)/2D Schottky heterojunction as photocathode and Zn plate as photoanode. It was then employed to sensitively and selectively detect penicillin G (PG) in the complex environment. The 2D/2D Schottky heterojunction was established by the in situ growth of cadmium-doped molybdenum disulfide nanosheets (Cd-MoS₂ NSs) around Ti₃C₂T_x NSs (denoted as Cd-MoS₂@Ti₃C₂T_x) by using phosphomolybdic acid (PMo₁₂) as precursor, thioacetamide as sulfur source, and Cd(NO₃)₂ as a doping agent through the hydrothermal method. The gained Cd-MoS₂@Ti₃C₂T_x heterojunction possessed contact interface, hierarchical structure, and plenty of sulfur and oxygen vacancies, thus showing the enhanced separation ability of photocarriers and electron transfer. Due to the enhanced UV-vis light adsorption ability, high photoelectric conversion efficiency, and exposed catalytic active sites, the constructed photocatalyzed ZAB displayed a boosted output voltage of 1.43 V under UV-vis light irradiation. The developed ZAB-driven self-powered aptasensor demonstrated an ultralow detection limit of 0.06 fg mL^-1 within a PG concentration ranged from 1.0 fg mL^-1 to 0.1 ng mL^-1, as deduced from the power density-current curves, along with high specificity, good stability and promising reproducibility, as well as excellent regeneration ability and wide applicability. The present work provided an alternative analysis method for the sensitive analysis of antibiotics based on the portable photocatalyzed ZAB-driven self-powered aptasensor.

Tremella-Like Ni-NiO with O-Vacancy Heterostructure Nanosheets Grown In Situ on MXenes for Highly Efficient Hydrogen and Oxygen Evolution

Electronic modulation via heterostructures or vacancies has been recently regarded as an effective strategy to improve electrocatalytic activity by optimizing the adsorption free energies of hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) active intermediates during the reaction. Herein, tremella-like Ni-NiO with O-vacancy heterostructure nanosheets grown in situ on Ti₃C₂T_x MXenes (Ni-NiO/Ti₃C₂T_x MXene) are fabricated via a facile strategy. Benefitting from the heterointerfaces between Ni and NiO, the synergetic coupling effects of MXenes and Ni-NiO heterostructures, the O-vacancies, and the unique architecture, the as-prepared Ni-NiO/Ti₃C₂T_x MXene showed superior activity toward the HER and OER in alkaline electrolyte, only requiring overpotentials of 72 mV for the HER and 248 mV for the OER to offer 10 mA cm^-2. Density functional theory (DFT) calculations revealed that Ni-NiO with O-vacancies can effectively increase the electron density around the Fermi level and modulate the Gibbs free energies of the intermediates during catalytic reactions, thus accelerating the reaction kinetics.

Tunable Structured MXenes With Modulated Atomic Environments: A Powerful New Platform for Electrocatalytic Energy Conversion

Owing to their rich surface chemistry, high conductivity, tunable bandgap, and thermal stability, structured 2D transition-metal carbides, nitrides, and carbonitrides (MXenes) with modulated atomic environments have emerged as efficient electrochemical energy conversion systems in the past decade. Herein, the most recent advances in the engineering of tunable structured MXenes as a powerful new platform for electrocatalytic energy conversion are comprehensively summarized. First, the state-of-the-art synthetic and processing methods, tunable nanostructures, electronic properties, and modulation principles of engineering MXene-derived nanoarchitectures are focused on. The current breakthroughs in the design of catalytic centers, atomic environments, and the corresponding structure-performance correlations, including termination engineering, heteroatom doping, defect engineering, heterojunctions, and alloying, are discussed. Furthermore, representative electrocatalytic applications of structured MXenes in energy conversion systems are also summarized. Finally, the challenges in and prospects for constructing MXene-based electrocatalytic materials are also discussed. This review provides a leading-edge understanding of the engineering of various MXene-based electrocatalysts and offers theoretical and experimental guidance for prospective studies, thereby promoting the practical applications of tunable structured MXenes in electrocatalytic energy conversion systems.

2D MXene Nanomaterials as Electrocatalysts for Hydrogen Evolution Reaction (HER): A Review

MXenes, a novel family of 2D transition metal carbide, nitride and carbonitride materials, have been gaining tremendous interest in recent days as potential electrocatalysts for various electrochemical reactions, including hydrogen evolution reaction (HER). MXenes are characterized by their etchable metal layers, excellent structural stability, versatility for heteroatoms doping, excellent electronic conductivity, unique surface functional groups and admirable surface area, suitable for the role of electrocatalyst/support in electrochemical reactions, such as HER. In this review article, we summarized recent developments in MXene-based electrocatalysts synthesis and HER performance in terms of the theoretical and experimental point of view. We systematically evaluated the superiority of the MXene-based catalysts over traditional Pt/C catalysts in terms of HER kinetics, Tafel slope, overpotential and stability, both in acidic and alkaline electrolytic environments. We also pointed out the motives behind the electro catalytic enhancements, the effect of synthesis conditions, heteroatom doping, the effect of surface terminations on the electrocatalytic active sites of various MXenes families. At the end, various possible approaches were recommended for a deeper understanding of the active sites and catalytic improvement of MXenes catalysts for HER.

Magnesium-regulated oxygen vacancies of cobalt-nickel layered double hydroxide nanosheets for ultrahigh performance asymmetric supercapacitors

Rational design of layered double hydroxide (LDH) electrodes is of great significance for high-performance supercapacitors (SCs). Herein, ultrathin cobalt-nickel-magnesium layered double hydroxide (CoNiMg-LDH) nanosheets with plentiful oxygen vacancies are synthesized via sacrificial magnesium-based replacement reaction at room temperature. Self-doping and mild reduction of magnesium can significantly increase the concentration of oxygen vacancies in CoNiMg-LDH, which promotes the electrochemical charge transfer efficiency and enhances the adsorption ability of electrolytes. Density functional theory (DFT) calculations also indicate that Mg²⁺ doping can decrease the formation energy of oxygen vacancies in CoNiMg-LDH nanosheets, which increases the concentration of oxygen vacancies. Thus, the assembled asymmetric supercapacitor CoNiMg-LDH//Actived Carbon accomplishes a superior capacity of ∼ 333 C g^-1 (208 F g^-1) at 1 A g^-1 and presents a gravimetric energy density of 73.9 Wh kg^-1 at 0.8 kW kg^-1. It presents only 13% capacity loss at 20 A g^-1 after 5000 cycles. This discovery emphasizes the positive role of magnesium in regulating oxygen vacancies to improve the performance of supercapacitors, which should be beneficial for extending the scope of superior SCs active materials.

Achieving ultra-dispersed 1T-Co-MoS2@HMCS via space-confined engineering for highly efficient hydrogen evolution in the universal pH range

A multi-level spatial confinement strategy is proposed for the fabrication of ultra-dispersed 1T-Co-MoS2 nanoclusters, which exhibit remarkable electrocatalytic activity and durability for HER.

Recent advances in MoS2-based materials for electrocatalysis

The increasing energy demand and related environmental issues have drawn great attention of the world, thus necessitating the development of sustainable technologies to preserve the ecosystems for future generations. Electrocatalysts...

Recent Status and Challenges in Multifunctional Electrocatalysis Based on 2D MXenes

Due to their chemical and electrical characteristics, such as metallic conductivity, redox-activity in transition metals, high hydrophilicity, and adjustable surface properties, MXenes are emerging as important contributors to oxygen reduction...

MWCNT-modified MXene as cost-effective efficient bifunctional catalyst for overall water splitting

Utilization of cost-effective, bifunctional, and efficient electrocatalysts for complete water splitting is desirable for sustainable clean hydrogen energy.

Facile fabrication of conductive MoS2 thin films by sonication in hot water and evaluation of their electrocatalytic performance in the hydrogen evolution reaction

Molybdenum disulfide (MoS₂) has long been used in catalysis and is a promising material for energy conversion devices. In order to utilize MoS₂ in electrocatalytic applications, it needs to be sufficiently conductive. Even though a metallic 1T phase of MoS₂ exists, its exfoliation process is expensive and difficult to scale because it involves hazardous materials and procedures, limiting its practical applications. We have previously reported an efficient and environmentally friendly procedure to exfoliate conductive MoS₂ via sonication in very dilute aqueous hydrogen peroxide. Here, we report a new way of exfoliating heavily doped conductive MoS₂ by sonication in pure water at 60 °C without additives. Conductivity measurements, Raman spectroscopy and X-ray photoelectron spectroscopy demonstrate that controlling the sonication time and temperature lead to the generation of small quantities of hydrogen peroxide in the water that interact with MoS₂ to form a small amount of sub-stoichiometric MoO_3-y . This impurity acts as a dopant and is responsible for the increase in conductivity of the MoS₂ films without compromising their structural integrity. We also evaluate the performance of the doped MoS₂ films as electrocatalysts in the hydrogen evolution reaction. We elucidate the mechanistic origin of the catalytic properties of these materials which may be of future use to develop a family of electrocatalysts based on doped MoS₂.

Hierarchical MXene/transition metal chalcogenide heterostructures for electrochemical energy storage and conversion

MXenes have gained rapidly increasing attention owing to their two-dimensional (2D) layered structures and unique mechanical and physicochemical properties. However, MXenes have some intrinsic limitations (e.g., the restacking tendency of the 2D structure) that hinder their practical applications. Transition metal chalcogenide (TMC) materials such as SnS, NiS, MoS₂, FeS₂, and NiSe₂ have attracted much interest for energy storage and conversion by virture of their earth-abundance, low costs, moderate overpotentials, and unique layered structures. Nonetheless, the intrinsic poor electronic conductivity and huge volume change of TMC materials during the alkali metal-ion intercalation/deintercalation process cause fast capacity fading and poor-rate and poor-cycling performances. Constructing heterostructures based on metallic conductive MXenes and highly electrochemically active TMCs is a promising and effective strategy to solve these problems and enhance the electrochemical performances. This review highlights and discusses the recent research development of MXenes and hierarchical MXene/TMC heterostructures, with a focus on the synthesis strategies, surface/heterointerface engineering, and potential applications for lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, supercapacitors, electrocatalysis, and photocatalysis. The critical challenges and perspectives of the future development of MXenes and hierarchical MXene/TMC heterostructures for electrochemical energy storage and conversion are forecasted.

CoS2 needle arrays induced a local pseudo-acidic environment for alkaline hydrogen evolution

The alkaline electrocatalytic hydrogen evolution reaction (HER) is a potential way to realize industrial hydrogen production. However, the sluggish process of H₂O dissociation, as well as the accumulation of OH^- around the active sites, seriously limit the alkaline HER performance. In this work, we developed a unique CoS₂ needle array grown on a carbon cloth (NAs@C) electrode as an alkaline HER catalyst. Finite-element simulations revealed that CoS₂ needle arrays (NAs) induce stronger local electric field (LEF) than CoS₂ disordered needles (DNs). This LEF can greatly repel the local OH^- around the active sites, and then promote the forward H₂O dissociation process. The local pH changes of the electrode surface confirmed the lower OH^- concentration and stronger local pseudo-acidic environment of NAs@C compared to those of DNs@C. As a result, the NAs@C catalyst exhibited a low HER overpotential of 121 mV at a current density of 10 mA cm^-2 in 1 M KOH, with the Tafel slope of 59.87 mV dec^-1. This work provides a new insight into nanoneedle arrays for the alkaline HER by electric field-promoted H₂O dissociation.

Electron Density Modulation of MoO2/Ni to Produce Superior Hydrogen Evolution and Oxidation Activities

Hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) have aroused great interest, but the high price of platinum group metals (PGMs) limits their development. The electronic reconstruction at the interface of a heterostructure is a promising strategy to enhance their catalytic performance. Here, MoO₂/Ni heterostructure was synthesized to provide effective HER in an alkaline electrolyte and exhibit excellent HOR performance. Theoretical and experimental analyses prove that the electron density around the Ni atom is reduced. The electron density modulation optimizes the hydrogen adsorption and hydroxide adsorption free energy, which can effectively improve the activity of both HER and HOR. Accordingly, the prepared MoO₂/Ni@NF catalyst reveals robust HER activity (η₁₀ = 50.48 mV) and HOR activity (j₀ = ∼1.21 mA cm^-2). This work demonstrates an effective method to design heterostructure interfaces and tailor the surface electronic structure to improve HER/HOR performance.

MXene derivatives: synthesis and applications in energy convention and storage

Transition metal carbides or nitrides (MXene) have shown promising applications in energy convention and storage (ECS), owing to their high conductivity and adjustable surface functional groups. In the past several years, many MXene derivatives with different structures have been successfully prepared and their impressive performance demonstrated in ECS. This review summarizes the progress in the synthesis of MXene and typical Ti3C2T x MXene derivatives with different morphologies, including 0D quantum dots, 1D nanoribbons, 2D nanosheets and 3D nanoflowers. The mechanisms involved and their performance in photocatalysis, electrocatalysis and rechargeable batteries are also discussed. Furthermore, the challenges of MXene derivatives in ECS are also proposed.

Interface chemistry of two-dimensional heterostructures - fundamentals to applications

Two-dimensional heterostructures (2D HSs) have emerged as a new class of materials where dissimilar 2D materials are combined to synergise their advantages and alleviate shortcomings. Such a combination of dissimilar components into 2D HSs offers fascinating properties and intriguing functionalities attributed to the newly formed heterointerface of constituent components. Understanding the nature of the surface and the complex heterointerface of HSs at the atomic level is crucial for realising the desired properties, designing innovative 2D HSs, and ultimately unlocking their full potential for practical applications. Therefore, this review provides the recent progress in the field of 2D HSs with a focus on the discussion of the fundamentals and the chemistry of heterointerfaces based on van der Waals (vdW) and covalent interactions. It also explains the challenges associated with the scalable synthesis and introduces possible methodologies to produce large quantities with good control over the heterointerface. Subsequently, it highlights the specialised characterisation techniques to reveal the heterointerface formation, chemistry and nature. Afterwards, we give an overview of the role of 2D HSs in various emerging applications, particularly in high-power batteries, bifunctional catalysts, electronics, and sensors. In the end, we present conclusions with the possible solutions to the associated challenges with the heterointerfaces and potential opportunities that can be adopted for innovative applications.

Promoting effect of MXenes on 1T/2H–MoSe2 for hydrogen evolution

The 1T/2H–MoSe₂/Ti₃C₂ composites integrated via a facile hydrothermal method exhibit an optimal overpotential of 150 mV at 10 mA cm⁻² in 1 M KOH, indicating that Ti₃C₂ is an ideal conductive support for building highly efficient electrocatalysts.

2H-MoS2 on Mo2CTx MXene Nanohybrid for Efficient and Durable Electrocatalytic Hydrogen Evolution

The development of highly efficient and durable earth-abundant hydrogen evolution reaction (HER) catalysts is crucial for the extensive implementation of the hydrogen economy. Members of the 2D MXenes family, particularly Mo₂CT_x, have recently been identified as promising HER catalysts. However, their inherent oxidative instability in air and aqueous electrolyte solutions is hindering their widespread use. Herein, we present a simple and scalable method to circumvent adventitious oxidation in Mo₂CT_x MXenes via in situ sulfidation to form a Mo₂CT_x/2H-MoS₂ nanohybrid. The intimate epitaxial coupling at the Mo₂CT_x/2H-MoS₂ nanohybrid interface afforded superior HER activities, requiring only 119 or 182 mV overpotential to yield -10 or -100 mA cm^-2_geom current densities, respectively. Density functional theory calculations reveal strongest interfacial adhesion was found within the nanohybrid structure as compared to the physisorbed nanohybrid, and the possibility to tune the HER overpotential through manipulating the extent of MXene sulfidation. Critically, the presence of 2H-MoS₂ suppresses further oxidation of the MXene layer, enabling the nanohybrid to sustain industrially relevant current densities of over -450 mA cm^-2_geom with exceptional durability. Less than 30 mV overpotential degradation was observed after 10 continuous days of electrolysis at a fixed -10 mA cm^-2_geom current density or 100,000 successive cyclic voltammetry cycles. The exceptional HER durability of the Mo₂CT_x/2H-MoS₂ nanohybrid presents a major step forward to realize practical implementation of MXenes as noble metal free catalysts for broad-based applications in water splitting and energy conversion.

Topochemical synthesis of low-dimensional nanomaterials

Over the past several decades, nanomaterials have been extensively studied owing to having a series of unique physical and chemical properties that exceed those of conventional bulk materials. Researchers have developed a lot of strategies for the synthesis of low-dimensional nanomaterials. Among them, topochemical synthesis has attracted increasing attention because it can provide more new nanomaterials by improving and upgrading inexpensive and accessible nanomaterials. In this review, we summarize and analyze many existing topochemical synthesis methods, including selective etching, liquid phase reactions, high-temperature atmosphere reactions, electrochemically assisted methods, etc. The future direction of topochemical synthesis is also proposed.

Rational Design of Two-Dimensional Transition Metal Carbide/Nitride (MXene) Hybrids and Nanocomposites for Catalytic Energy Storage and Conversion

Electro-, photo-, and photoelectrocatalysis play a critical role toward the realization of a sustainable energy economy. They facilitate numerous redox reactions in energy storage and conversion systems, enabling the production of chemical feedstock and clean fuels from abundant resources like water, carbon dioxide, and nitrogen. One major obstacle for their large-scale implementation is the scarcity of cost-effective, durable, and efficient catalysts. A family of two-dimensional transition metal carbides, nitrides, and carbonitrides (MXenes) has recently emerged as promising earth-abundant candidates for large-area catalytic energy storage and conversion due to their unique properties of hydrophilicity, high metallic conductivity, and ease of production by solution processing. To take full advantage of these desirable properties, MXenes have been combined with other materials to form MXene hybrids with significantly enhanced catalytic performances beyond the sum of their individual components. MXene hybridization tunes the electronic structure toward optimal binding of redox active species to improve intrinsic activity while increasing the density and accessibility of active sites. This review outlines recent strategies in the design of MXene hybrids for industrially relevant electrocatalytic, photocatalytic, and photoelectrocatalytic applications such as water splitting, metal-air/sulfur batteries, carbon dioxide reduction, and nitrogen reduction. By clarifying the roles of individual material components in the MXene hybrids, we provide design strategies to synergistically couple MXenes with associated materials for highly efficient and durable catalytic applications. We conclude by highlighting key gaps in the current understanding of MXene hybrids to guide future MXene hybrid designs in catalytic energy storage and conversion applications.

2D organ-like molybdenum carbide (MXene) coupled with MoS2 nanoflowers enhances the catalytic activity in the hydrogen evolution reaction

Our work introduces an emerging route for the synthesis of MoS₂ nanoflowers decorating organ-like Mo₂CT_x MXene. This effective synthesis strategy of MoS₂@Mo₂CT_x nanohybrid structure can shed some light on energy-related applications.