Progress in functional studies of transition metal borides

In recent years,transition metal borides(TMBs)have attracted much attention because they are considered as potential superhard materials and have more abundant crystal structures compared with traditional superhard materials.So far,however,no superhard materials have been found in TMBs.A large number of structures and potential new properties in TMBs are induced by the various hybridization ways of boron atoms and the high valence electrons of transition metals,which provide many possibilities for its application.And most TMBs have layered structures,which make TMBs have the potential to be a two-dimensional(2D)material.The 2D materials have novel properties,but the research on 2D TMBs is still nearly blank.In this paper,the research progress of TMBs is summarized involving structure,mechanical properties,and multifunctional properties.The strong covalent bonds of boron atoms in TMBs can form one-dimensional,twodimensional,and three-dimensional substructures,and the multiple electron transfer between transition metal and boron leads to a variety of chemical bonds in TMBs,which are the keys to obtain high hardness and multifunctional properties of TMBs.Further research on the multifunctional properties of TMBs,such as superconductors,catalysts,and high hardness ferromagnetic materials,is of great significance to the discovery of new multifunctional hard materials.

Metal phosphides and borides as the catalytic host of sulfur cathode for lithium–sulfur batteries

Lithium−sulfur batteries are one of the most competitive high-energy batteries due to their high theoretical energy density of _(2)600 W·h·kg^(−1).However,their commercialization is limited by poor cycle stability mainly due to the low intrinsic electrical conductivity of sulfur and its discharged products(Li_(2)S_(2)/Li_(2)S),the sluggish reaction kinetics of sulfur cathode,and the“shuttle effect”of soluble intermediate lithi-um polysulfides in ether-based electrolyte.To address these challenges,catalytic hosts have recently been introduced in sulfur cathodes to en-hance the conversion of soluble polysulfides to the final solid products and thus prevent the dissolution and loss of active-sulfur material.In this review,we summarize the recent progress on the use of metal phosphides and borides of different dimensions as the catalytic host of sulfur cathodes and demonstrate the catalytic conversion mechanism of sulfur cathodes with the help of metal phosphides and borides for high-en-ergy and long-life lithium-sulfur batteries.Finally,future outlooks are proposed on developing advanced catalytic host materials to improve battery performance.

Ultra-incompressible Ternary Diborides Re_(0.5)Ir_(0.5)B_2,Re_(0.5)Tc_(0.5)B_2,Os_(0.5)W_(0.5)B_2 and Os_(0.5)Ru_(0.5)B_2:Density Functional Calculation

In this paper, density functional computations have been applied to the structural, elastic and electronic properties of ternary transition metal diborides Re0.5Ir0.5B2, Re0.5Tc0.5B2, Os0.5W0.5B2 and Os0.5Ru0.5B2 in hexagonal （P63/mmc） and orthorhombic （Pmmn） structures with both local density approximation and generalized gradient approximation. LDA gives smaller lattice parameters and larger elastic moduli than GGA. Both results show that the hexagonal ones are more stable than orthorhombic ones except Os0.5Ru0.5B2. Moreover, the hexagonal structure has superior elastic property than orthorhombic one. Generally speaking, the calculated elastic moduli of Re0.5Ir0.5B2 and Os0.5Ru0.5B2 are smaller than those values of Re0.5Tc0.5B2 and Os0.5W0.5B2 within the same structure because of the filling of antibonding states. The relativistic effects result in weaker bonds of Tc-B （Ru-B） than those of Re-B （Os-B）. All the diborides are ultra-incompressible. Re0.5Tc0.5B2 has the largest shear modulus and it is a promising superhard diboride like Os0.5W0.5B2. The elastic properties are in high correlation with the bond strength. The shear moduli are more sensitive than the bulk moduli to the bond strength.

Prediction of scandium tetraboride from first-principles calculations:Crystal structures,phase stability,mechanical properties,and hardness

Using the evolutionary methodology for crystal structure prediction,we have predicted the orthorhombic Cmcm and Pnma phases for ScB_(4).The earlier proposed Cr B_(4)^(-),Fe B_(4)^(-),Mn B_(4)^(-),and Re P_(4)^(-)type structures for ScB_(4)are excluded.It is first discovered that the Cmcm phase transforms to the Pnma phase at about 18 GPa.Moreover,both phases are dynamically and mechanically stable.The large bulk modulus,shear modulus,and Young's modulus of the two phases make it an optimistic low compressible material.Moreover,the strong covalent bonding nature of ScB_(4)is confirmed by the ELF analysis.The strong covalent bonding contributes greatly to its stability.

Transition Metal Boride-Based Materials for Electrocatalytic Water Splitting

Electrocatalytic water splitting to produce hydrogen is an eco-friendly way to achieve sustainable utilization of renewable energy.The industrial application of water electrolysis, which is severely limited by slow kinetic reactions on electrode surfaces, requires the development of highly reactive, low-cost and stable electrocatalytic materials. Transition metal borides/borates have recently emerged as promising electrocatalytic materials for catalyzing hydrogen/oxygen evolution reactions(HER/OER) in inexpensive electrolyzers. However, so far, there has been little comprehensive summary of transition metal borides/borates. Here, this review provides the latest research progress on transition metal borides/borates for electrocatalytic water splitting. The structural characteristics of transition metal borides/borates and their synthesis methods in recent years are discussed. Then, the theoretical and experimental progress of transition metal borides including single-metal borides, multi-metal borides, borate derived and other nanocomposites containing boron(boron-doped nanocomposites/substrate with boron) in electrocatalytic reaction and the role of boron in regulating electrocatalytic performance are further emphasized. Finally, the potential challenges and future prospects of transition metal borides/borates in electrocatalysis are presented.

High temperature superconductor Na_(2)B_(2)H stabilized by hydrogen intercalation under ambient pressure

Hydrogenated metal borides have attracted much attention due to their potential high-temperature superconductivity.Here,we propose a new strategy for hydrogen intercalation tuning the stability and superconductivity of the boron honeycomb sublattice,and predict an unprecedented layered compound Na_(2)B_(2)H,which hosts excellent superconductivity.Strikingly,the superconducting transition temperature(Tc)of Na_(2)B_(2)H reaches 42 K at ambient pressure.The Tcvalue can be further increase to 63 K under 5%biaxial tensile strain.The excellent superconductivity originates from the strong electron-phonon coupling between theσ-bonding bands near the Fermi level and the B-B stretching optical E modes.The interstitial electron localization and crystal orbitals of the H-intercalated Na ion layer well match the boron honeycomb lattice and act as a chemical template to stabilize the B layer.Furthermore,the introduction of hydrogen tuned the Fermi level,and the coupling vibration of Na and H ions effectively enhanced the dynamic stability of the structure.Na_(2)B_(2)H represents a new family of layered high-temperature superconductors,and the strategy of stabilizing the honeycomb boron sublattice via chemical template hosts great potential for application to more layered compounds.

First demonstration of possible two-dimensional MBene CrB derived from MAB phase Cr_2AlB_2

In this communication, the possibility for the preparation of two-dimensional MBene CrB from MAB phase Cr2AlB2 is demonstrated for the first time. Herein M is a transition metal, A is a group IIIAor IVA element and B is boron; MAB phases are layered transition metal ternary borides, MBene is used to emphasize the loss of A group element from the parent MAB phases and to highlight the 2 D nature. The possible 2 D CrB nano sheets are prepared by selectively etching out Al layers from Cr2AlB-2 by immersing the Cr2AlB2 powders in dilute HCl solution at room temperature. The as-prepared 2 D CrB nano sheets are examined using X-ray diffraction and scanning electron microscopy and energy dispersive X-ray spectroscopy（EDS）. In addition, different structure models are built to explain the observed phenomena.The discovery in this work opens a door to the synthesis of a large number of new 2 D MBenes from MAB phases.

Deintercalation of Al from MoAlB by molten salt etching to achieve a Mo_(2)AlB_(2) compound and 2D MoB nanosheets

Two-dimensional(2D)MoB metal borides(MoB MBene)have attracted much attention due to their fascinating properties and functional applications.So far,work on the synthesis of 2D MoB nanosheets by acid or alkaline etching of MoAlB has not been very successful.It has been proposed that the 2D MoB MBene may be fabricated by chemical etching of a Mo_(2)AlB_(2) precursor,but further investigations were not performed possibly due to the difficult preparation of the metastable Mo_(2)AlB_(2) compound at high temperatures by solid-state reactions.Here,we report on the successful synthesis of the Mo_(2)AlB_(2) compound and 2D MoB nanosheets by the deintercalation of Al from MoAlB through a ZnCl_(2) molten salt etching approach at relatively low temperatures.The influence of etching temperature,etching time,and starting mixtures on the formation of desirable phases have been investigated.A pure Mo_(2)AlB_(2) compound was synthesized at temperatures below 600℃,while the 2D MoB MBene nanosheets were obtained at 700℃through the molten salt etching of MoAlB.In addition,the present work further confirms that the MoB MBene can be prepared by etching the as-synthesized Mo_(2)AlB_(2) precursor in LiF–HCl solution.Our work demonstrates that the molten salt etching is an effective method to prepare 2D MoB MBene.

Nickel-iron borate coated nickel-iron boride hybrid for highly stable and active oxygen evolution electrocatalysis

The development of efficient and cost-effective electrocatalysts toward anodic oxygen evolution reaction(OER) is crucial for the commercial application of electrochemical water splitting.As the most promising electrocatalysts,the OER performances of nickel-iron-based materials can be further improved by introducing metalloid elements to modify their electron structures.Herein,we developed an efficient hybrid electrocatalyst with nickel-iron boride(NiFeB) as core and amorphous nickel-iron borate(NiFeBi)as shell(NiFeB@NiFeBi) via a simple aqueous reduction.The obtained NiFeB@NiFeBi exhibits a small overpotential of 237 mV at 10 mA/cm^2 and Tafel slope of 57.65 mV/dec in 1.0 mol/L KOH,outperforming most of the documented precious-metal-free based electrocatalysts.Benefiting from the in situ formed amorphous NiFeBi layer,it shows excellent stability toward the oxygen evolution reaction(OER).These findings might provide a new way to design advanced precious-metal-free electrocatalysts for OER and the application of electrochemical water splitting.