Transition Metal Borides Research Articles

A Literature Survey of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) are a new class of oxygen evolution reaction (OER) electrocatalysts, which have become a relevant topic for electrocatalytic energy conversion and storage research. This is because of their earth abundance, high electrical conductivity, and resultant high OER performance. Furthermore, some X-ide electrocatalysts demonstrate various degrees of oxidation resistance under OER potentials due to their differences in chemical composition, crystal structure, and morphology. Interestingly, there are three possible post-OER electrocatalyst models: a fully oxidized oxide/(oxy)hydroxide material, a partially oxidized core@shell structure, and an unoxidized material.1 In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Recently, several topical reviews have been published. However, typically these literature reviews have provided limited conclusions targeted at only certain X-ides. Additionally, certain reviews omit key points, especially regarding the “catalytically active species” in X-ide OER electrocatalysts.In this work, we provide a comprehensive summary of (i) the experimental parameters (e.g., substrates, electrocatalyst loading amounts, electrocatalyst shapes, geometric overpotentials, Tafel slopes, etc.) and (ii) the details of the electrochemical stability tests and following post analyses used in all the X-ide OER electrocatalyst publications from 2013 (the birth of this research area) to 2022.2 In particular, based on the reported post-OER analytical results, we attempt to exhaustively classify all the X-ide electrocatalysts into four groups: no oxidation, partial oxidation, full oxidation, and unknown, which is relevant to the nature of the “catalytically active species” in X-ide OER electrocatalysts. After investigating the simpler X-ide systems, transition metal-based polyanion compounds/composites are also surveyed. Finally, we introduce the reported novel and special analytical techniques employed for observing X-ide reconstruction (i.e., oxidation and other phase transformations) during OER testing, which can be useful in selecting the analytical technique of maximum utility for the researchers’ purpose. Additionally, for each material sub-group (e.g., borides, carbides, nitrides, phosphides, sulfides, selenides, and tellurides), we also point out further challenges to be addressed and propose further questions yet to be answered, which may provide clues pointing to future research tasks. References B. R. Wygant, K. Kawashima, and C. B. Mullins, ACS Energy Lett., 3, 2956–2966 (2018).K. Kawashima et al., Chem. Rev., 123, 12795–13208 (2023).

Tuning Oxygen Evolving Catalytic Activity of a Precious Metal-Free Nickel Boride by a Cost-Effective Method

The production of hydrogen (H2) by electrochemical water splitting [Green Hydrogen (Green H2)], if driven by green electricity, has attracted considerable interest as an alternative sustainable energy carrier, e.g., in fuel cells for electricity or power and heat generation [1]. However, the lack of satisfactory and cheap electrocatalysts (ECs) to drive the vital electrochemical reactions of hydrogen evolution (HER) and oxygen evolution (OER) in water splitting is the biggest challenge for this technology.Currently, the most advanced electrocatalysts for the two half-reactions of water electrolysis (HER and OER) are noble metal-based materials [2]. Despite the excellent catalytic properties of these noble metal catalysts, their high cost, and scarcity make their commercial use both uneconomical and impractical in low temperature water electrolysis (LT-WE) systems at industrial scale for green H2 as a clean energy alternative to cheaper fossil fuels[3]. In addition to addressing this issue by the development of precious-material-based with a low-loading of precious metal elements, the development of desirable and high-efficiency electrocatalysts (ECs) based on earth-abundant metal elements materials is paramount. However, their catalytic activity is often limited by poor electron transfer, low conductivity, low stability, and small surface area of these materials. Thus, it is crucial to develop earth-abundant metal elements-based materials that are more cost-effective to reduce the high cost of active catalysts for water electrolysis. Although several low-cost materials have already been designed, the potential required for the practical application of water splitting is still very high compared to the theoretical potential (1.23 V), which means that much energy is consumed, which theoretically could be reduced. Due to their remarkable advanced charge transfer and durability for water oxidation in alkaline media, electrocatalysts derived from metal borides-based materials have received a lot of attention as new high-performance catalysts for water oxidation.[4] These properties stem from the multi-dimensional covalent bonding of the metalloid with the surrounding metal atoms.We present here an approach to tune the OER of transition metal borides (TMBs) using the cost-effective and less energy-consuming method that is nevertheless a highly efficient material for OER (Fig. 1). We have developed a low-cost, rational and easily scalable method to enhance the catalytic activity of nickel boride (Ni–B), which requires an annealing process to optimize the catalytic activity. However, this method may have limited applicability on a large scale as it requires a highly inert environment and high-temperature treatment of Ni–B catalysts. We also elucidate a structure-activity relationship using scale-bridging techniques such as TEM and electrochemical characterization. The developed based on non-noble electrocatalysts are prepared by a simple method of chemical reduction of transition metal elements with boron in aqueous solution. The production is a one-step process and the material does not require any further treatment, yet this material shows comparable activity to the state of the art (precious metal-based materials) for OER in alkaline solution at a selected current density. This improved catalytic performance is further corroborated by the microstructural investigations in the TEM. The nanoparticles obtained have an improved porous structure compared to Ni–B and thus provide more available sites for the surface reactions and hence for the catalytic performance of the material. Furthermore, it is shown that activation enables the morphological and structural changes, while some transition metal elements act as sacrificial elements to provide more accessible and stable sites for oxygen-forming centers. This paves the way for a better understanding of metal boride-derived electrocatalysts for water oxidation, a crucial chemical reaction in water electrolysis and other electrochemical energy technologies. Acknowledgements: This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 945422. We acknowledge the use of the DFG-funded Micro-and Nanoanalytics Facility (MNaF) at the University of Siegen (INST 221/131-1).

Synthesis, Characterization, and Magnetocaloric Properties of the Ternary Boride Fe2AlB2 for Caloric Applications.

The ternary transition metal boride Fe2AlB2 is a unique ferromagnetic "MAB" phase that demonstrates a sizable magnetocaloric effect near room temperature-a feature that renders this material suitable for magnetic heat pump devices (MHP), a promising alternative to conventional vapor compression technology. Here, we provide a comprehensive review of the material properties of Fe2AlB2 (magnetofunctional response, transport properties, and mechanical stability) and discuss alloy synthesis from the perspective of shaping these materials as porous active magnetic regenerators in MHPs. Salient aspects of the coupled magnetic and structural phase transitions are critically assessed to elucidate the fundamental origin of the functional response. The goal is to provide insight into strategies to tune the magnetofunctional response via elemental substitution and microstructure optimization. Finally, outstanding challenges that reduce the commercial viability of Fe2AlB2 are discussed, and opportunities for further developments in this field are identified.

Isothermal oxidation of bulk dense Fe2AlB2 and Mn2AlB2 phases in 700–1000 °C temperature range

Herein, the isothermal oxidation of Mn2AlB2 and Fe2AlB2 MAB phases in air was studied. When oxidized at 700 °C, the weight gain kinetics for Mn2AlB2 are sub-cubic up to 96 h as a result of the formation of a Mn2-xAlxO3 protective layer. Severe spallation of the oxides formed was observed at 800 °C and 900 °C, limiting its service temperature. The oxidation kinetics at 900 °C, for Fe2AlB2 become sub-parabolic beyond 60 h due to the formation of a passivating Αl4Β2Ο9 with excellent corner adherence. At both 800 °C and 1000 °C, however, the oxidation is rapid and severe. Therefore, both Mn2AlB2 and Fe2AlB2 may be used in practical application up to 700 °C and 900 °C, respectively. As far as we are aware, Fe2AlB2 is the second transition metal boride that is oxidation resistant at 900 °C; the first, MoAlB, is also a MAB phase.

Invited Article: The oxidation kinetics and mechanisms observed during ultra-high temperature oxidation of (HfZrTiTaNb)C and (HfZrTiTaNb)B2

Ultra-high temperature ceramics (UHTCs), most notably transition metal carbides and borides, exhibit melting temperatures exceeding 3000 °C, making them appropriate candidates to withstand the extreme temperatures (∼2000 °C) expected to occur at the leading edges of hypersonic vehicles. However, their propensity to react rapidly with oxygen limits their sustained application. The high entropy paradigm enables the exploration of novel UHTC compositions that may improve on the oxidation resistance of conventional refractory mono-carbides and -diborides. The oxidation kinetics of candidate high entropy group IV + V (HfZrTiTaNb)C and (HfZrTiTaNb)B2 materials were evaluated at 1500–1800 °C using Joule heating in one atmosphere 0.1%–1% oxygen/argon gas mixtures for times up to 15 min. Possible mechanisms based on the resulting complex time, temperature, and oxygen partial pressure dependencies are discussed. The carbides formed porous and intergranular oxides. Oxidation resistance was improved upon a continuous external scale formation. The diborides formed dense external scales and exhibited better oxidation resistance compared to the carbides. This improvement was attributed to the formation of liquid boria. Both compositions showed an unexpected reduction in material consumption at 1800 °C for all times tested, compared to results at lower temperatures. An in-depth analysis of the composition and morphology of the oxide scale and sub-surface regions for specimens tested at 1800 °C revealed that the formation of denser group IV-rich (Hf, Zr, Ti) oxides mitigated the formation of the otherwise detrimental liquid-forming group V (Ta, Nb) oxides, leading to the improved oxidation resistance.

Electronic modulation of MOF-derived CoxMnyB nanosheet arrays toward efficient bifunctional electrocatalysts for water splitting

The design of robust and efficient bifunctional electrocatalysts for total water splitting is of paramount importance for the advancement of sustainable energy conversion technologies. To fully exploit the electrocatalytic capabilities of transition metal borides, we integrate manganese-incorporated cobalt boride (CoxMnyB) nanoarrays derived from metal-organic frameworks (MOFs). CoxMnyB nanosheet arrays exhibit pronounced electronic modulation, indicating superior electrochemical behavior. CoxMnyB can served as a binder-free catalytic electrode, which exhibits bifunctional electrocatalytic activity of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). CoxMnyB demonstrates an overpotential ∼ 277 mV under 10 mA cm−2 and a Tafel slope of 71 mV dec-1 for HER. It also exhibits optimised OER characteristics, with a potential of 1.54 V at a current density of 10 mA cm−2 and the lowest Tafel slope of 62 mV dec-1. CoxMnyB catalysts emerge as promising candidates for bifunctional electrocatalysts in overall water splitting, delivering remarkable and enduring electrocatalytic functionalities. The intricate mechanisms governing the electrocatalytic performance of the CoxMnyB bifunctional catalyst are systematically discussed in relation to nanostructure, conductive behavior, and the covalent bonding nature of metal-metalloid borides. Density functional theory (DFT) calculations reveal a reduced hydrogen binding energy at the catalyst surface and a weakened energy barrier of OOH*. This study suggests a favourable and feasible pathway for deploying metal borides in high-efficiency electrocatalysis and energy storage applications.

Research on the air cathode catalytic mechanism and performance of long life for air-breathing hybrid electrolyte lithium-air battery

Hybrid electrolyte lithium-air batteries offer a high energy density compared to other batteries. However, their internal redox efficiency is low when utilizing traditional catalysts, resulting in a shorter lifespan. Among the two-dimensional transition metal borides (MBenes) is two-dimensional molybdenum boride (2D-MoB), which has superior mechanical, electrochemical, and electrical conductivity qualities. This work discusses the hydrothermal preparation of 2D-MoB and its mixing with CNTs for in hybrid electrolyte lithium-air batteries. Density functional theory (DFT) was used to study the electrochemical characteristics of the compound as well as its redox process. The hydrothermally generated 2D-MoB exhibits more reactive active sites, a more significant specific surface area, and an evident layer structure. Using 2D-MoB as a catalyst for hybrid electrolyte lithium-air batteries can significantly extend their cycle life and lower their overpotential, according to ambient air’s constant-capacity charge/discharge tests. At a current of 0.1 mA (0.05 mA·cm−2), the battery life reached 3160 h, and its initial maximum discharge capacity was 9779 mAh·g−1. According to DFT calculations, the overpotential of 2D-MoB as a catalyst for lithium-air hybrid electrolyte batteries was 0.46 V, which was nearer the value observed in experiments. According to this study, 2D-MoB can be used as a cathode catalyst in lithium-air hybrid electrolyte batteries.

Covalent Transition Metal Borosilicides: Reaction Pathways in Molten Salts for Water Oxidation Electrocatalysis.

The properties of transition metal borides and silicides are intimately linked to the covalent character of the chemical bonds within their crystal structures. Bringing boron and silicon together within metal borosilicides can then engender different competing covalent networks and complex charge distributions. This situation results in unique structures and atomic environments, which can impact charge transport and catalytic properties. Metal borosilicides, however, hold the status of unusual exotic species, difficult to synthesize and with poor knowledge of their properties. Our strategy consists of developing a redox pathway to synthesize transition metal borosilicides in inorganic molten salts as high-temperature solvents. By studying the formation of Ni6Si2B, Co4.75Si2B, Fe5SiB2, and Mn5SiB2 with in situ X-ray diffraction, we highlight how new reaction routes, maintaining covalent structural building blocks, draw a general scheme of their formation. This pathway is driven by the covalence of the chemical bonds within the boron coordination framework. Next, we demonstrate high efficiency for water oxidation electrocatalysis, especially for Ni6Si2B. We ascribe the strongly increased resistance to corrosion, high stability, and electrocatalytic activity of the Ni6Si2B-derived material to three factors: (1) the two entangled boron and silicon covalent networks; (2) the ability to codope with boron and silicon an in situ generated catalytic layer; and (3) a rare electron enrichment of the transition metal by back-donation from boron atoms, previously unknown within this compound family. With this work, we then unveil a new chemical dimension for Earth-abundant water oxidation electrocatalysts by bringing to light a new family of materials.

Formation of Delocalized Linear M-B-M Covalent Bonds: A Combined Experimental and Theoretical Study of BM2(CO)8+ (M = Co, Rh, Ir) Complexes.

Investigations of transition-metal boride clusters not only lead to novel structures but also provide important information about the metal-boron bonds that are critical to understanding the properties of boride materials. The geometric structures and bonding features of heteronuclear boron-containing transition metal carbonyl cluster cations BM(CO)6+ and BM2(CO)8+ (M = Co, Rh, and Ir) are studied by a combination of the infrared photodissociation spectroscopy and density functional calculations at B3LYP/def2-TZVP level. The completely coordinated BM2(CO)8+ complexes are characterized as a sandwich structure composed of two staggered M(CO)4 fragments and a boron cation, featuring a D3d symmetry and 1Eg electronic ground state as well as metal-anchored carbonyls in an end-on manner. In conjunction with theoretical calculations, multifold metal-boron-metal bonding interactions in BM2(CO)8+ complexes involving the filled d orbitals of the metals and the empty p orbitals of the boron cation were unveiled, namely, one σ-type M-B-M bond and two π-type M-B-M bonds. Accordingly, the BM2(CO)8+ complexes can be described as a linear conjugated (OC)4M═B═M(CO)4 skeleton with a formal B-M bond index of 1.5. The three delocalized d-p-d covalent bonds render compensation for the electron deficiency of the cationic boron center and endow both metal centers with the favorable 18-electron structure, thus contributing much to the overall structural stability of the BM2(CO)8+ cations. As a comparison, the saturated BRh(CO)6+ and BIr(CO)6+ complexes are determined to be a doublet Cs-symmetry structure with an unbridged (OC)2B-M(CO)4 pattern, involving a two-center σ-type (OC)2B → M(CO)4+ dative single bond along with a weak covalent B-M half bond. This work offers important insight into the structure and bonding of late transition metal boride carbonyl cluster cations.

Laser powder bed fusion additive manufacturing of Ti6Al4V-matrix composite with outstanding hardness/strength: Microstructural evolution and performance enhancement mechanisms

Incorporation of transition-metal boride with excellent mechanical performance as a reinforcing phase, represents a promising approach to address the challenge of insufficient hardness/strength in Ti6Al4V alloys. Despite the common challenge of achieving homogeneous distribution and refinement of TMB, this study leveraged a novel strategy involving laser-induced in-situ synthesis and a greatly strengthened laser-molten-pool effect, to additively manufacture highly-dense Ti6Al4V-matrix composites reinforced with mono-dispersed TiB nanowires. This was achieved through selective laser melting route of laser powder bed fusion. Remarkably, the unique microstructural morphology of these composites characterized by the uniform dispersion of high-aspect-ratio reinforcing phases, boron dissolution into α-Ti grain boundaries of the Ti6Al4V matrices, and unified crystallographic coordination of gradient transitional layers, contributed to their superior microhardness and compressive strength compared to the counterparts even with the higher contents of reinforcing phases. This achievement was attributed to the synergistic effects of various morphological mechanisms including second-phase, grain refinement, and stress-transfer reinforcing/strengthening. This research provided new insights into the precise control of microstructural evolution in metal-matrix composites and the enhancement of their overall mechanical performance.

Synthesis and radiation damage tolerance of Mo0.75W0.25AlB solid solution for nuclear fusion reactor applications

MAB phases (M = transition metal, A = aluminium (Al), B = boron (B)), a new family of nano-laminated ternary transition metal borides with an excellent combination of functional and structural properties, are currently being investigated as potential radiation shielding materials in nuclear fusion reactors. Here, we report the fabrication of a solid solution MAB phase Mo0.75W0.25AlB (Mo - molybdenum, W - tungsten), in the form of dense and high purity pellets, synthesised for the first time using a reactive hot-pressing method. The shielding and irradiation damage properties of the material were also evaluated. Monte Carlo N-Particle (MCNP) calculations showed that W doping in the MoAlB phase is likely to enhance the shielding capacity against neutrons and gamma rays of different energies. The experimental damage tolerance of Mo0.75W0.25AlB to silicon ion (Si+) irradiation indicated that this solid solution compound has a high resistance to crack formation and other radiation damage effects compared to that of pure MoAlB. Thermal annealing of the amorphous Mo0.75W0.25AlB layer formed by irradiation showed that the damaged structure easily recrystallizes at high temperatures, completely returning to the virgin state. The results show that this solid solution should be considered as a promising candidate material for radiation shielding components.

Density Functional Theory Investigation on the Nitrogen Reduction Mechanism in Two-Dimensional Transition-Metal Boride with Ordered Metal Vacancies.

The creation of ordered collective vacancies in experiment proves challenging within a two-dimensional lattice, resulting in a limited understanding of their impact on catalyst performance. Motivated by the successful experimental synthesis of monolayer molybdenum borides with precisely ordered metal vacancies [Zhou et al. Science 2021, 373, 801-805] through dealloying, the nitrogen reduction reaction (NRR) in monolayer borides was systematically investigated to elucidate the influence of such ordered metal vacancies on catalytic reactions and the underlying mechanisms. The results reveal that the N-containing intermediates tend to dissociate, facilitating the NRR process with reduced UL. The emergence of ordered metal vacancies modulates the electronic properties of the catalyst and partially facilitates the decomposition of N-containing intermediates. However, the UL for NRR in Mo4/3B2 and W4/3B2 exhibits a significant increase. The compromised electrochemical performance is explained through the development of a simple electronic descriptor of the d-p band center (ΔdM-pB). Among these materials, Mo4/3Sc2/3B2 exhibits the most superior catalytic activity with a UL of -0.5 V and favorable NRR selectivity over the HER. Our results provide mechanistic insights into the role of ordered metal vacancies in transition-metal boride for the NRR and highlight a novel avenue toward the rational design of superior NRR catalysts.

Multilayer Fluorine-Free MoBTx MBene with Hydrophilic Structural-Modulating for the Fabrication of a Low-Resistance and High-Resolution Humidity Sensor.

2D transition metal borides (MBenes) with abundant surface terminals hold great promise in molecular sensing applications. However, MBenes from etching with fluorine-containing reagents present inert -fluorine groups on the surface, which hinders their sensing capability. Herein, the multilayer fluorine-free MoBTx MBene (where Tx represents O, OH, and Cl) with hydrophilic structure is prepared by a hydrothermal-assisted hydrochloric acid etching strategy based on guidance from the first-principle calculations. Significantly, the fluorine-free MoBTx-based humidity sensor is fabricated and demonstrates low resistance and excellent humidity performance, achieving a response of 90% to 98%RH and a high resolution of 1%RH at room temperature. By combining the experimental results with the first-principles calculations, the interactions between MoBTx and H2O, including the adsorption and intercalation of H2O, are understood first in depth. Finally, the portable humidity early warning system for real-time monitoring and early warning of infant enuresis and back sweating illustrates its potential for humidity sensing applications. This work not only provides guidance for preparation of fluorine-free MBenes, but also contributes to advancing their exploration in sensing applications.

3D transition metal boride monolithic electrode for industrial hectoampere-level current anion exchange membrane water electrolysis

High performance, cost-efficient anion exchange membrane water electrolysis (AEMWE) is of great current interest for industrial-level hydrogen production. However, the lack of active and robust catalytic electrode severely impedes the development of this technology. Herein, a versatile strategy of 3D hierarchical porous monolithic electrode enabling industrial hectoampere-level current AEMWE is successfully explored for the first time. By a facile electroless plating technique coupled with corrosion engineering process, a series of low-cost and highly active 3D transition metal boride (etched-TMB, TM=Ni, Co, NiP, NiMo, CoP, CoMo, CoNi) catalytic electrodes have been prepared. A distinctive hierarchically structured etched-NiPB@MS alloy monolithic electrode exhibits a superior bifunctional hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalytic activity and large-current stability, which derive from enhanced intrinsic activity, sufficient electrochemical active sites, mechanical stability as well as efficient gas/liquid transport pathways. An AEMWE electrolyzer with 10×10 cm2 etched-NiPB@MS as both anode and cathode works efficiently at large current of 100 A (1 A cm−2) and reaches a H2 production rate of 41.78 L h−1, verifying its huge potential for industrial hydrogen production. This study paves out a new approach for high-efficient catalytic electrode and industrial-level current AEMWE.

Rapid synthesis of two-dimensional MoB MBene anodes for high-performance sodium-ion batteries

Two-dimensional (2D) transition metal borides (MBenes) have emerged as a rising star and hold great potential promise for catalysis and metal ion batteries owing to a well-defined layered structure and excellent electrical conductivity. Unlike well-studied graphene, perovskite and MXene materials in various fields, the research about MBene is still in its infancy. The inadequate exploration of efficient etching methods impedes their further study. Herein, we put forward an efficient microwave-assisted hydrothermal alkaline solution etching strategy for exfoliating MoAlB MAB phase into 2D MoB MBenes with a well accordion-like structure, which displays a remarkable electrochemical performance in sodium ion batteries (SIBs) with a reversible specific capacity of 196.5 mAh g–1 at the current density of 50 mA g–1, and 138.6 mAh g–1 after 500 cycles at the current density of 0.5 A g–1. The underlying mechanism toward excellent electrochemical performance are revealed by comprehensive theoretical simulations. This work proves that MBene is a competitive candidate as the next generation anode of sodium ion batteries.

Mesoporous amorphous non-noble metals as versatile substrates for high loading and uniform dispersion of Pt-group single atoms.

Atomically dispersed Pt-group metals are promising as nanocatalysts because of their unique geometric structures and ultrahigh atomic utilization. However, loading isolated Pt-group metals in single-atom alloys (SAAs) with distinctive bimetallic sites is challenging. In this study, we present amorphous mesoporous Ni boride (Ni-B) as an ideal substrate to uniformly disperse Pt atoms with tunable loadings (1.7 to 12.2 wt %). The effect of the morphology, composition, and crystal phase of the Ni-B host on the growth and dispersion of Pt atoms is discussed. The resulting amorphous Pt-Ni-B mesoporous nanospheres exhibit superior electrocatalytic H2 evolution performance in acidic media. This strategy holds the potential to synthesize a diverse library of mesoporous amorphous Pt-group SAAs, by leveraging functional amorphous nanostructured 3d transition-metal borides as substrates, thereby proposing a comprehensive strategy to control atomically dispersed Pt-group metals.

Ultralow lattice thermal conductivity in type-I Dirac MBene TiB2

MBenes, the emergent novel two-dimensional family of transition metal borides have recently attracted remarkable attention. Transport studies of such two-dimensional structures are very rare and are of sparking interest. In this paper Using Boltzmann transport theory with ab-initio inputs from density functional theory, we examined the transport in TiB2 MBene system, which is highly dependent on number of layers. We have shown that the addition of an extra layer (as in bilayer BL) destroys the formation of type-I Dirac state by introducing the positional change and tilt to the Dirac cones, thereby imparting the type-II Weyl metallic character in contrast to Dirac-semimetallic character in monolayer ML. Such non-trivial electronic ordering significantly impacts the transport behavior. We further show that the anisotropic room temperature lattice thermal conductivity κ L for ML (BL) is observed to be 0.41 (0.52) and 2.00 (2.04) W m−1 K−1 for x and y directions, respectively, while the high temperature κ L (ML 0.13 W m−1 K−1 and BL 0.21 W m−1 K−1 at 900 K in x direction) achieves ultralow values. Our analysis reveals that such values are attributed to enhanced anharmonic phonon scattering, enhanced weighted phase space and co-existence of electronic and phononic Dirac states. We have further calculated the electronic transport coefficients for TiB2 MBene, where the layer dependent competing behavior is observed at lower temperatures. Our results further unravels the layer dependent thermoelectric performance, where ML is shown to have promising room-temperature thermoelectric figure of merit (ZT) as 1.71 compared to 0.38 for BL.

Nanometabolomics Elucidated Biological Prospective of Mo4/3B2-x Nanosheets: Toward Metabolic Reprogramming of Amino Acid Metabolism.

Mo4/3B2-x nanosheets are newly developed, and 2D transition metal borides (MBene) were reported in 2021, but there is no report on their further applications and modification; hence, this article sheds light on the significance of potential biological prospects for future biomedical applications. Therefore, elucidation of the biocompatibility, biotoxicology, and bioactivity of Mo4/3B2-x nanosheets has been an urgent need to be fulfilled. Nanometabolomics (also referred as nanomaterials-based metabolomics) was first proposed and utilized in our previous work, which specialized in interpreting nanomaterials-induced metabolic reprogramming through aqueous metabolomics and lipidomics approach. Hence, nanometabolomics could be considered as a novel concept combining nanoscience and metabolomics to provide bioinformation on nanomaterials' biomedical applications. In this work, the safe range of concentration (<50 mg/L) with good biosafety toward human umbilical vein endothelial cells (HUVECs) was discovered. The low concentration (5 mg/L) and high concentration (50 mg/L) of Mo4/3B2-x nanosheets were utilized for the in vitro Mo4/3B2-x-cell interaction. Nanometabolomics has elucidated the biological prospective of Mo4/3B2-x nanosheets via monitoring its biocompatibility and metabolic shift of HUVECs. The results revealed that 50 mg/L Mo4/3B2-x nanosheets could lead to a stronger alteration of amino acid metabolism with disturbance of the corresponding amino acid-related pathways (including amino acid metabolism, amino acid degradation, fatty acid biosynthesis, and lipid biosynthesis and metabolism). These interesting results were closely involved with the oxidative stress and production of excess ROS. This work could be regarded as a pathbreaking study on Mo4/3B2-x nanosheets at a biological level, which also designates their further biochemical, medical, and industrial application and development based on nanometabolomics bioinformation.

Computational exploration of two-dimensional vacancy-free boridene sheet and its derivatives: high stabilities and the promise for hydrogen evolution reaction

The recent synthesis of a two-dimensional (2D) MBene sheet, referred to as the boridene sheet (Mo4B6Tz), has ignited considerable interest in exploring 2D transition metal borides. Boridene has an ordered arrangement of metal vacancies, which are pivotal to its stability. Employing first-principles calculations, we explored the stable phases, electronic properties and catalytic abilities of boridene with different vacancy concentrations (Vm). Our results demonstrate that Vm significantly influences the cohesive energies of boridene sheets. Phonon spectrum and ab initio molecular dynamics simulations reveal the high stability of the vacancy-free boridene Mo6B6T6 (T = O, -OH), underscoring their potential for experimental realization. Substituting Mo atoms with Nb, Ta, or W enhances the structural stability of boridene sheets, leading to the identification of four stable variants: Nb6B6F6, Ta6B6F6, Ta6B6O6, and W6B6O6. These boridene sheets exhibit metallic behavior, with five structures displaying near-zero Gibbs free energy for hydrogen atom adsorption, indicating their potential as catalysts for the hydrogen evolution reaction. The uncovering of vacancy-free boridenes and their 2D derivatives greatly broadens the scope of the MBene family.

Theoretical study on catalytic performance of Ti2B monolayer as cathodes of Li-O2 and Na-O2 batteries promoted by surface functionalization

Alkali metal–oxygen batteries with high theoretical energy density are regarded as one of the promising next-generation batteries. The suitable catalyst is the key to realize the application of alkali metal–oxygen batteries. In this study, two-dimensional (2D) Ti2B monolayer with extremely high Young’s modulus (1024.7 N·m−1) was investigated as cathode catalysts of Li-O2 and Na-O2 batteries by density functional theory (DFT) calculations. The electrocatalytic performance can be further improved by surface functionalization, and the catalytic activities based on total overpotentials are in order: Ti2BF2 (1.244/1.131 V) < Ti2BCl2 (1.383/1.247 V) ≈ Ti2BS2 (1.373/1.256 V) < Ti2B (9.514/9.191 V) for both Li-O2 and Na-O2 batteries. Furthermore, there is a volcanic relationship between overpotentials and the binding energies of peroxides and substrates. The energy evolution of four-electron steps on these materials reveals that surface functionalization regulated catalytic activities by balancing the adsorption strength of alkali metal and oxygen, and the disproportionation side reactions on surface functionalized Ti2B monolayers are thermodynamically unfavorable. Besides, the densities of states (DOSs) and alkali metal migration behaviors of these materials indicate their good electronic and ionic conductivities. Our study of functionalized Ti2B monolayer cathode catalysts may shed light on the application of 2D transition metal borides to alkali metal–oxygen batteries.