MBenesAn Emerging 2D Layered Material: Synthesis, DFT Studies, and Application as Electrocatalyst for Hydrogen Evolution Reaction: A Review

Abstract

MBenes are a relatively new addition to the family of 2-D layered materials and are still in its infancy. Their unique layered structure and remarkable propertiesincluding high electrical conductivity, magnetic properties, mechanical strength, surface functional groups, and abundant active siteshave established them as a rising star in the field of 2-D materials, and their potential use in a plethora of applications has been anticipated. Initially, in 2017, theoretical studies predicted the possible structures of MBenes (M _nB_2n‑2), and then experimental synthesis of certain MBenes confirmed their existence, paving the way for further research and development. In this review paper, we have discussed the fundamental aspects of MBenes, including their theoretically predicted crystal structures, stability, chemical compositions, and their synthesis from precursor MAB phases. The challenges currently hindering the progress of MBenes have also been mentioned. In this review, special emphasis is placed on the performance of various MBenes as electrocatalysts for the hydrogen evolution reaction (HER), along with the insights provided by theoretical studies.

1. Introduction

Two-dimensional (2D) materials have become one of the imperative topics in modern research due to their unique properties and broad applications in sensing, semiconductor, defense, energy storage, and energy conversion, biomedical and electronic technologies, etc. Since the isolation of monolayer graphene in 2004, intensive efforts have been devoted to exploring graphene-like layered systems, such as transition metal dichalcogenides (TMDs), phosphene, borophene, hexagonal boron nitride, MXenes, etc., which combine large surface area with unique physicochemical and electronic properties.

After the pioneering work by Gogotsi in 2011, an exciting class of 2-D materials, known as MXenes, has gained immense attention. MXenes are generally presented as M _n+1X _n T _x (where M = early transition metal (such as Ti, Se, Nb, Mo, Ta, V, etc.), X = carbon or/and nitrogen, T _x = surface terminations (e.g., –O, −OH, –F, –Cl, etc.), and n = 1–4. − MXenes are commonly synthesized from the corresponding MAX phases (where A is typically a group 13 or 14 element, e.g., Al, Si, P, S, Ga, Ge, As, In, Sn, etc.). In the MAX phase, M and X form strong covalently bonded M-X layers, and these M-X layers are separated by single-atom metallic layers of A. ,, Here, the bonding between M and A is relatively weak. The selective etching of an A layer from MAX phase yields MXenes with a unique layered structure. Owing to the exceptional features of MXenes, their crucial properties (e.g., electronic, optoelectronic, chemical, etc.) can be tuned by manipulating their chemical composition, interlayer spacing, morphologies, etc. − Currently, extensive exploration is going on to find the efficacy of the application of different MXenes in several fields, including sensors, energy storage devices, energy conversion, catalysis, antennas, wearable electronics, microwave shielding, Biomedical devices, etc. , These astonishing findings have initiated a quest to probe and explore another family of layered materials, which was designated as MBenes.

Two-dimensional transition metal borides (MBenes) have recently emerged as the boron analogue of MXenes, which were first predicted theoretically, and some of them were synthesized later. MBenes are layered transition metal borides derived from MAB phases. Monolayer or a few-layer of MBene sheets can be obtained by selective etching of the A-site atomic layers (A = Al, Ga, or In) from MAB phase crystals (i.e., ternary layered transition metal borides, where M denotes a transition metal). − Their wide structural and topological diversities originate from the multicentered bonding characteristics of boron, which impart a broad spectrum of properties to metal borides. The newly discovered MBenes exhibit exceptional stability, isotropic behavior, mechanical properties, etc. In addition, they demonstrate excellent catalytic, optical, and thermal properties. − A growing body of theoretical and experimental research is continuously validating their diverse applications, further establishing MBenes as a promising class of 2D materials. The progress of the development of MBenes with time has been depicted in Scheme .

1. Historical Timeline of MBenes .

^a Mo₂B₂: Reproduced with permission from ref . Copyright 2017 Royal Society of Chemistry. CrB: Reproduced with permission from ref . Copyright 2018 Elsevier. TiB: Reproduced from ref . CC BY 4.0. MnB₂: Reproduced from ref . CC BY 4.0. NiB: Reproduced from ref . Copyright 2021 American Chemical Society. Mo_4/3B₂: Reproduced with permission from ref . Copyright 2023 Royal Society of Chemistry. h-HfBO: Reproduced with permission from ref . Copyright 2023 Wiley-VCH. MoB: Reproduced from ref . Copyright 2024 American Chemical Society. Cu₈B₁₄: Reproduced from ref . CC BY-NC 4.0.

Although MBenes may initially seem like MXenes, where Boron (B) replaces C or N, their stoichiometric, stacking modes, and structural transformations are quite different from MXenes. MBenes offer superior oxidation resistance, metallic characteristics (e.g., electrical and thermal conductivities), larger surface area, etc. − These attributes make MBenes suitable candidates for efficient energy storage devices, sensors, electrocatalysts, photodetectors, etc. Some compositions of MBenes have already demonstrated their promise in next-generation electrochemical energy conversion and electronic applications. In this review, we have given an up-to-date account of the application of MBenes as an electrocatalyst for Hydrogen Evolution Reaction (HER).

Global energy demand has risen sharply with population growth and the development of technologies, and has been projected to increase from 16 TW in 2010 to 30 TW by 2050. , Currently, ∼79.5% of energy comes from fossil fuels, while renewables contribute only ∼20.5%. Heavy reliance on fossil fuels accelerates their depletion and causes severe environmental challenges. This accentuates the urgency for clean, sustainable alternatives. Hence, global efforts have been focused on developing sustainable energy alternatives in line with SDG-7, which aims to ensure affordable and clean energy by 2030. −

Renewable sources such as solar, wind, and hydro power are promising, but their intermittency is one of the limitations. In this context, green hydrogen has gained prominence as a clean energy carrier, offering ∼2.5 times higher energy density than conventional fossil fuels (e.g., gasoline, kerosene, etc.). Among the available routes, hydrogen production via electrocatalytic water splitting is considered one of the most efficient and sustainable strategies for future energy portfolios. Thus, developing renewable, efficient, and clean hydrogen generation technology is essential for realizing a sustainable hydrogen economy.

MBenes emerge as a viable electrocatalyst for Hydrogen evolution via water splitting, owing to the presence of sufficient catalytically active sites, tunable layered structure for efficient charge migration, structural and chemical stability, etc. , Theoretical simulations suggest that its metallic/semimetallic electrical conductivity and low hydrogen absorption energy favor its activity as an electrocatalyst in the Hydrogen Evolution Reaction (HER).

In this review paper, we have captured the fundamental aspects of MBenes, including the synthesis of various MAB phases, the synthesis of MBenes from MAB phases, the structural stability of MAB phases, and the prediction of their stability using density functional theory (DFT) calculations. MBenes have shown tremendous potential to be used as an electrocatalyst or as an important component of an electrocatalyst. To the best of our knowledge, to date, no review paper is available which discusses the application of MBenes in the hydrogen evolution reaction (HER) as an electrocatalyst in detail. In this review paper, we have emphasized the HER activities of different MBenes in the light of the experimentally obtained performances and their understanding by theoretical studies.

2. Types and Structures of MAB Phases MBenes: DFT Studies

MBenes are a family of two-dimensional transition metal borides with the general chemical formulation of M _n B_2n–2, with structures of alternating stacks of transition metals (M), boron (B), and n = 2, 3, 4. ,, Several MBenes have been predicted using theoretical studies, and many of them have been synthesized experimentally (Figure ). By employing top-down techniques, layered MBenes are generally prepared from their corresponding MAB phases. In MAB ((M_1+n AB_2n )), mono or bilayers of Al are sandwiched between M-B layers. , To predict the possible members of MAB, several theoretical studies have been conducted.

1.

Metals that form borides (periodic table showing metals or semimetals that form metal borides highlighted in purple). The metals marked in orange are both predicted and experimentally realized as metal-boride-derived 2D nanostructures. The metals marked in red are theoretically predicted to exist as metal-boride-derived 2D nanostructures but are not yet confirmed experimentally. Reproduced with permission from ref . Copyright 2018 Wiley-VCH.

To date, extensive theoretical studies have been performed to elucidate the structures, stabilities, and electronic properties of MAB phases and MBenes. However, only a limited number of MBenes have been experimentally synthesized. Accordingly, this section discusses the crystal structures of various MAB phases and MBenes as predicted by density functional theory (DFT) calculations.

In 2022, using DFT calculations, Carlsson et al. estimated the phase stability of several MAB phases. They have calculated their formation energy (ΔE _f) and formation enthalpy (ΔH _cp) at 0 K. The formation energy (ΔE _f) was calculated according to eq

$${\mathrm{\Delta }E}_{\mathrm{f}}({\mathrm{M}}_x{\mathrm{A}}_y{\mathrm{B}}_z)=E({\mathrm{M}}_x{\mathrm{A}}_y{\mathrm{B}}_z)-x{E}_{\mathrm{M}}-y{E}_{\mathrm{A}}-z{E}_{\mathrm{B}}$$ 1

where E(M _x A _y B _z ) represents the total energy of the ternary compound, and E _i denotes the total energy of the constituent element i = {M, A, B}, in its ground-state crystal structure, respectively.

In another approach, an energy convex hull construction was employed to evaluate all possible linear combinations of competing M _x A _y B _z phases (ternary M–A–B material). The equilibrium simplex for the M _x A _y B _z phases was determined using eq

$$\min E_{\mathrm{cp}}(b^{\mathrm{M}},b^{\mathrm{A}},b^{\mathrm{B}})=\sum _i^n{x}_i{E}_i$$ 2

where b ^M, b ^A, and b ^B denote the amount of each element M, A, and B, and x _i and E _i denotes the amount and energy for a competing phase i. The minimized energy E _cp, represents the set of the most competing phases. The formation enthalpy, ΔH _cp (Convex hull construction) (meV/atom), which represents the difference between a specific phase and the set of most stable competing phases, can be calculated by using eq .

$$\mathrm{\Delta }{H}_{\mathrm{c}\mathrm{p}}({\mathrm{M}}_x{\mathrm{A}}_y{\mathrm{B}}_z)=E({\mathrm{M}}_x{\mathrm{A}}_y{\mathrm{B}}_z)-\min E_{\mathrm{c}\mathrm{p}}$$ 3

where E(M _x A _y B _z ) and E _cp are the calculated total energy of the M _x A _y B _z phase and a linear combination of the identified equilibrium simplex, respectively. For those M _x A _y B _z where ΔH_cp < 0, they were considered stable. Otherwise, when ΔH_cp > 0, the phases were not stable, or at best metastable.

Orthorhombic and hexagonal are common crystal systems of MAB phases. Carlsson et al. investigated 210 unique compositions (MAB, M₂AB₂, M₃AB₄, M₄AB₄, and M₄AB₆) and 420 MAB phases with hexagonal and orthorhombic space group symmetries. In these MAB phases, where M represents Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, and Co, and A is Al, Ga, and In. Space group symmetries corresponding to various MAB phases were (Cmcm) for MoAlB and WAlB, (Cmmm) for Cr₂AlB₂, Mn₂AlB_2, and Fe₂AlB₂, (Pmmm) for Cr₃AlB₄, and (Cmmm) for Cr₄AlB₆. The stability of seven experimentally known MAB phases was calculated, and also predicted Hf₂InB₂, Zr₂InB₂, and Mo₄AlB₄ phases to be hypothetically stable. In addition, 23 other phases were found to be close to thermodynamic stability. For the majority of the MAB phases, the hexagonal symmetry possesses the lowest energy when A = Ga and In, whereas for orthorhombic symmetry, the lowest energy is when A = Al. Similarly, M₄AB₄ systems prefer the orthorhombic symmetry. According to the formation energy (ΔE _f), out of 210 MAB phases were stable, whereas, as per ΔH_cp, only 10 phases were identified as thermodynamically stable. Hence, for predicting thermodynamic stability, formation enthalpy was considered as a better choice than formation energy. Crystal structure parameters, such as bond distances and bond angles between the constituent elements of different MAB phases, are listed in Table .

1. Atomic Separations between M–M Atoms on the Triangular Faces of the BM₆ Trigonal Prisms, Closest M–B Atoms, Intrachain B–B Distances, and Bonding Angles of the B–B Chains for Various MAB Phases and Corresponding Binaries.

sr. no	MAB	M–M (Å)	B–B (Å)	M–B (Å)	Al–B (Å)	B–B–B angle (deg)	ref
1	Cr2AlB2 (Cmmm)	2.71	1.76	2.19	2.28	113.0(1)
2	Cr4AlB4 (Immm)	2.71	1.73	2.17	2.19	116.0
3	CrB (Cmcm)	2.65	1.78	2.18	N/A	111.4
4	Mn2AlB2 (Cmmm)	2.74	1.75	2.16	2.28	113.1(4)
5	MnB (Pnma)	2.67	1.80	2.18	N/A	111.9
6	Fe2AlB2 (Cmmm)	2.72	1.74	2.15	2.28	113.9(4)
7	MoAlB (Cmcm)	2.93	1.81	2.35	2.29	117.4
8	β-MoB (Cmcm)	2.829	1.844	2.31	N/A	112.8(9)
9	WAlB (Cmcm)	2.944	1.838	2.35	2.28	115.1(9)
10	α-WB (I41/amd)	2.86	1.73	2.34	N/A	126.5
11	Cr3AlB4 (Pmmm)	2.826	1.735	2.14–2.19	2.24	116.4–121.8
12	Cr3B4 (Immm)	2.85	1.74	2.17–2.19	N/A	114.8
13	Cr4AlB6 (Cmmm)	2.81–2.96	1.7–1.75	2.15–2.28	2.23	116.2–121.9
14	Cr2B3 (Cmcm)	2.80–2.96	1.7–1.76	2.16–2.34	N/A	114.2–119.8

^a(Range of Angles due to Slight Bonding Angle Differences between B Atoms Occupying Different Crystallographic Sites) Adapted with Permission from Kota, S.; Sokol, M.; Barsoum, M. W. A Progress Report on the MAB Phases: Atomically Laminated, Ternary Transition Metal Borides. International Materials Reviews. 2020, 65 (4), 226–255 Copyright 2020, Sage Publications.

Dahlqvist et al. have employed first-principles studies to understand the chemical ordering upon metal alloying of M₂AlB₂ (M from groups 3 to 9) in orthorhombic and hexagonal symmetry. Fifteen stable phases with in-plane chemical ordering (i-MAB), and 16 disordered stable alloys have been identified and verified by synthesizing Mo_4/3Y_2/3AlB₂ and Mo_4/3Sc_2/3AlB₂. Miao et al. have investigated 140 M-A-B systems, where the thermodynamically stable MAB phases in 139 systems satisfy the compositions of M₂AB₂ (212), M₃AB₄ (314), and M₂AB (211). Among the 133 predicted h-MAB phases, 30 structures were thermodynamically stable, while 103 compounds were metastable. P₆₃/mmc (194), P6̅m2 (187), R3m (166), and R3̅m2 (160) are the space groups of h-MAB phases, and Cmmm (65) and Pmmm (47) for ort-MAB phases. The largest number of stable phase (13) and metastable (42) phases belong to M₂AB₂ composition, the space groups are P6̅m2 (187) or P ₆₃/mmc (194). Because of the significant anisotropic bonding, stable structure, and active chemical reactivity for A elements, etc., h-MAB phases could serve as better precursors for MBenes than the o-MAB phases. Figure a and b portray the crystal structures of different MAB phases and MBenes, respectively.

2.

(a) Crystal structures of considered MAB phases ((a–f) and non-MAB phases (g,h): (a) 111-type [Cmcm], (b) 212-type [Cmcm], (c) 314-type [Pmmm], (d) 416-type [Cmmm], (e) 322-type [Cmcm], (f) 414-type [Immm], (g) 512-type [I4/mcm], (h) 322-type [P2/m] and (i) 432-type [P4/mmm]). Reproduced from ref . CC BY-NC 3.0. (b) Structure and classification of MAB and related MBene phases by their crystal structures, including 212- 222-, 414-, 314-, 416-, CrB/β-MoB-, Ta₃B₄/Cr₃B₄-, and V₂B₃/Cr₄B₆-type structures. The M, A, B, and X atoms are shown in blue, purple, light yellow, and pink in individual unit cells, respectively. Reproduced with permission from ref . Copyright 2021 Wiley-VCH.

Based on the knowledge gained from the DFT calculations about the bond strengths of different elements that are present in various types of MBenes, and inspired by the widely used strategy of MXene synthesis by removal of Al layers from MAX phase, a similar approach was adopted by the researchers to synthesize MBenes from MAB precursor. ,, Edirisuriya et al. have predicted the correlation between the atomic number of A and the stability of MAB by ML models (namely, SVM, DNN, and RFR). The bond lengths of M-A increase when A changes from Al → Ga → In → Tl, and accordingly, the charge transfer from the A element decreases. Therefore, the exfoliation energy decreases as per this order. The same trend was observed for both orthorhombic and hexagonal MAB phases. They concluded that the exfoliation energy decreases when the A element changes from Al to Tl, owing to the weakening of M–A and B–A bonds.

MAB phases are non-van der Waals solids, where both interlayer and in-plane interactions involve strong chemical bonding, making selective removal of the A-layer without disrupting the MB framework challenging. To explore this, Miao et al. calculated interfacial separation energies of hexagonal (h-MAB) and orthorhombic (ort-MAB) phases. High-throughput screening identified 30 thermodynamically stable and 103 metastable h-MAB phases with stoichiometries M₂AB, M₂AB₂, and M₃AB₄. Of these, 81 phases were predicted to be etchable, yielding 20 possible h-MBenes, such as 2D M₂B, MB, and M₃B₄ (M = Ti, Zr, Hf, Sc, Ta, and Nb) structures. Compared to ort-MABs, h-MABs are more suitable precursors due to their stability, anisotropic bonding, and lower diffusion barriers for A atoms. Experimentally, Hf₂PbB, Hf₂InB₂, V₃PB₄, and HfBO (from Hf₂InB₂) were synthesized, with HfBO reported as the first h-MBene derived from a ternary h-MAB phase.

Orthogonal MBenes and hexagonal MBenes are two major classifications of MBenes. Figure shows the top and side views of orthorhombic and hexagonal structures of MBenes. Zigzag chains of boron are present in the orthorhombic structures, whereas in the hexagonal structures, Boron is in a flat honeycomb lattice. The initially discovered MBenes were orthorhombic Cr₂B₂, Mo₂B₂, W₂B₂, and Fe₂B_2, and their general formula was M₂B₂.

3.

Top views of (a) rectangular and (b) hexagonal 2D MB, M₂B₃, and M₃B₄ MBenes. (c,e,g) [(d,f,h)] Side views of rectangular (hexagonal) 2D MB, M₂B₃, and M₃B₄, respectively. a, b, and c directions are indicated by arrows. The structures are periodic in the ab plane and a vacuum space of 30 Å exists along the c axis. Reproduced with permission from ref . Copyright 2019 Royal Society of Chemistry.

To predict the structural stabilities of nonterminated MBenes and terminated MBenes (M = Sc, Ti, Zr, Hf, V, Nb, Ta, Mo, and W) with F, O, or OH groups, Khaledialidusti et al. determined their formation energies by conducting DFT calculations using GGA with PBE exchange–correlation. The formation energies of MBenes with terminal groups (T = F, O, and OH) were found to be large negative values. These values became less negative when the transition metal changed from Sc to W. For all the cases, B–B bonds were found to be stronger than M–B > M–M bonds. For terminated MBenes, strong bond formation occurs between the terminal groups and surface transition metals, and this bond is stronger than M–B or M–M. Hence, functionalized MBenes exhibit better mechanical stabilities than the pristine ones.

Arias-Camacho et al. calculated the spin-polarized band structure, density of states (DOS), and projected DOS (PDOS) of orthorhombic and hexagonal Cr₂B₂, Fe₂B₂, and Zr₂B₂. For pristine MBenes, no band gap was observed, and partially occupied bands crossed the Fermi level, confirming their metallic character. The B-p orbitals appeared deep in energy (−8 to −2 eV), and in orthorhombic phases, they were partially hybridized with TM-d orbitals. In both orthorhombic and hexagonal Cr₂B₂ and Fe₂B₂, the dominance of d orbitals of Cr and Fe was observed near the Fermi level, while in the PDOS of Zr₂B₂, near the Fermi level, the contribution of Zr-d is significantly higher than that of B-p orbital. The studies further explained the higher conductivity of orthorhombic Fe₂B₂ and Zr₂B₂, and this work also reported the dynamic stability of hex-Fe₂B₂ for the very first time.

Sharma and Rangra investigated the electronic structures of transition-metal (TM) based AlB₂-type 2D hexagonal MBenes (e.g., VB₂, NbB₂, TaB₂, WB₂, TiB₂, FeB₂, ZrB₂, HfB₂) using DFT with GGA–PBE and HSE06 functionals in VASP. No Dirac cone at the Fermi level was observed for VB₂, NbB₂, TaB₂, and WB₂; instead, several band crossings at the Fermi level indicated their metallic nature. The electron-deficient graphene-like boron ring was stabilized by the addition of two electrons from the embedded TM atom. The calculations further identified TiB₂ and FeB₂ as semimetallic, while HfB₂ exhibited a direct band gap within PBE. Transport studies also revealed high average Fermi velocities (∼10⁵ ms^–1), comparable to graphene.

Ahmad et al. reported that both bare and T-terminated Mo₂BT₂ (T = O, S) monolayers possess metallic features, with thermodynamic stability. Similarly, by performing DFT studies, Wang et al. showed that monolayer CrB, FeB, and MnB could exhibit a stable average open-circuit voltage and an appreciable theoretical storage capacity, which hence could be superior to the other existing ordinary anode materials for Li-ion batteries. High-throughput DFT calculations combined with the nonequilibrium Green’s function (NEGF) method were used by Li et al. to screen M₂BT₂ (T = O, F, Cl, OH) to achieve ultralow-resistance contacts in transition metal dichalcogenides (TMDs) such as MoS₂, MoSe₂, WS₂, WSe₂, and MoTe₂, based on metal–semiconductor junctions at the quantum limit.

Hu et al. determined the binding energy of functionalized Cr₂B₂T₂ as an indicator of thermal stability and concluded that the order of stability is Cr₂B₂O₂ > Cr₂B₂Cl₂ > Cr₂B₂(OH)₂ > Cr₂B₂H₂. Moreover, high electrical conductivity with zero bandgap was also observed for both Cr₂B₂ and functionalized Cr₂B₂T₂.

3. Synthesis of MAB Phase and MBenes

3.1. Synthesis of MAB Phase

The synthesis of MAB phases was first reported in 1942, when Halla and Thury described layered transition metal borides, (MoAlB (Mo₇Al₆B₇)). , The first structurally resolved MAB phase, MoAlB, was synthesized in 1966 by Jeitschko. , In the same year, other ternary systems such as FeAlB, Mn₂AlB₂, and Fe₂AlB₂ were also reported. , Chaban and Kuz’ma subsequently synthesized Cr₂AlB₂ and Cr₃AlB₄ in 1973. , In a review, Ramezanzadeh et al. have discussed the common synthesis methodologies for the preparation of different MAB phases as depicted in Scheme . MAB phases are generally synthesized in the forms of (i) bulk (dens) ceramics, (ii) powders, (iii) single crystals (SC), and (iv) thin films. Ball-milling, Arc-melting, Aluminum flux, Self-propagating high-temperature synthesis, Hot-pressing, Spark plasma sintering, Molten salt-assisted method, Thermal explosion reaction, High-power pulsed magnetron sputtering, etc. are some of the techniques that were used to synthesize MAB. Examples of the synthesis of some MAB phases using these techniques are as follows:

• (i)Ball-milling: In this high-energy ball milling process, the first step is the grinding of the starting materials to make the pregrind particles into fine powder. The two main steps of this process are (i) the breaking phase (collision of particles between the tiny rigid balls (grinders)), and (ii) the welding phase. Kota et al. have synthesized Fe₂AlB₂ MAB phase by this method, where a mixture of commercial FeB and Al powder was ball-mixed at a rotation of ∼70 rpm in a 2:1.5 atomic ratio. The obtained mixture powders were uniaxially cold-pressed to 200 MPa and heated at 1273 K for 15 h in an alumina tube furnace under flowing Ar. The synthesis of several other MAB phases, such as Mn₂AlB₂, Mo₂AlB₂, Cr₂AlB₂, Cr₃AlB₄, and Ni₃ZnB₂ MAB phases, by this method were also reported. − Zhang et al. have synthesized phase-pure, crystalline Cr₂AlB₂ by first preparing CrB via a high-temperature reaction of Cr and B (1:1), followed by mixing CrB and Al powders (2:1.5 molar ratio) through wet ball milling. After drying and cold pressing, the pellets were annealed at 900 °C under Ar, yielding crystalline Cr₂AlB₂.
• (ii)Arc-melting (or vacuum arc melting) methods: Chai et al. have reported the synthesis of MAB phases (AlT₂B₂ (T = Mn, Fe, Cr), AlFe_2–x Mn _x B₂) by employing this technique. For the synthesis of AlFe_2–x Mn _x B₂, a mixture of Al, Fe, Mn, and B powders was first arc melted under vacuum (<10^–2 mbar) in a silica tube, followed by annealing at 1073 K for 1 week. The powder thus obtained was a mixture of target MAB phase along with a small amount of contamination of Al₁₃Fe₄ and MnB. After making a pellet with this mixture, it was sealed in a silica tube under vacuum and again heat-treated at 1073 K for another week. The resulting powder contained the desired MAB phase with a trace amount of Al₁₃Fe₄. , Using this technique, synthesis of a group of AlM₂B₂ MAB phases (where M = Fe, Mn, Cr, Ni, and Co) has been reported by Kádas et al.
• (iii)Aluminum flux method: In a review, Kanatzidis et al. explained the metal flux method and its application in the synthesis of intermetallic compounds. One of the early reports on the synthesis of MAB by this method is the synthesis of WAlB. A mixture of α-WB single crystals and Al metal as a flux was heated at 1500 °C for 10 h in an argon atmosphere, which resulted in the formation of pure orthorhombic WAlB. Recently, the preparation of Mn₂AlB₂ MAB phase has been reported by Roy et al. Ade and Hillebrecht reported the synthesis of Cr₂AlB₂, Cr₃AlB₄, and Cr₄AlB₆, and the growth of single crystals of ternary borides (Cr _x AlB _y , MoAlB, WAlB, Mn₂AlB₂, and Fe₂AlB₂) from liquid Al, and thus established the foundation for systematic design of MAB phases. Synthesis of several MAB phases (Fe₂AlB₂, Mn₂AlB₂, Cr₂AlB₂, MoAlB, and WAlB) via aluminothermic exchange reactions was reported by Merz et al. In this work, amorphous B, commercial grade Al, and MnO₂/ MoO₃/ Fe₂O₃/ Fe₃O₄/ Cr₂O₃/ WO₃ were first high-energy ball milled. Then, the synthesis was performed by heating the mixture at high temperature (e.g., 950 to 1200 °C, depending upon the composition of MAB) in a graphite crucible under Ar flow.
• (iv)Self-propagating high-temperature synthesis: Merz et al. reported the synthesis of Mn₂AlB₂ by induction-assisted self-propagating high-temperature synthesis (SHS), where the appropriate amount of Mn, Al, and amorphous B powder was first mixed using a high-energy ball mill, and then heated by a dual station benchtop induction furnace with Ar gas purging. They have investigated the influence of changes in different parameters, such as weight ratios of the starting materials, ball milling time, pressing pressure, etc. It has been proposed that the formation of Mn₂AlB₂ occurred in two stages preliminary SHS reaction, followed by an exothermic reaction. The plausible reaction mechanisms are (i) formation of MnB or other Mn–B and also Mn–Al compounds in the initial SHS reaction, and then the reaction between the initial intermediate phases, with each other and/or with residual Al. Alternatively, (ii) initial formation of a high temperature phase (MnB), which dissolved in Al and Mn₂AlB₂ Mab phase, directly precipitates. Potanin et al. used a self-propagating high-temperature synthesis method to prepare MoAlB lamellar grains 0.4 μm thick and ∼2–10 μm long. Compact samples were obtained by hot pressing of SHS powders. An induction furnace-assisted self-propagating high-temperature synthesis has been reported for the preparation of Mn₂AlB₂.
• (v)Hot-pressing: Several MAB phases, such as Fe₂AlB₂, Cr₂AlB₂, Cr₃AlB₄, and Mn₂AlB₂, etc. have been prepared through hot-pressing methods. ,, In the hot-pressing process, two plungers apply an axial load to compress the heated powders inside the die. Temperature and pressure are the main factors controlling the quality of the product. To prepare MoAlB, Kota et al. first ball-milled a mixture of MoB and Al, followed by heating the mixture at about 1200 °C in a cylindrical graphite die lined with graphite foil.
• (vi)Spark plasma sintering (SPS): The spark plasma sintering (SPS) method, a modified form of hot pressing, is a sintering technique that simultaneously applies uniaxial pressure and a high-intensity, low-voltage, pulsed direct current. In this process, the pressure rams also serve as electrodes connected to the DC power source, which supplies a direct current flow through the sample with high efficiency. Su et al. reported the synthesis of MoAlB MAB phase by this technique. Here, Al, Mo, and B powders were dry-mixed in a rotary machine and then placed in a cylindrical graphite die. This die was heated in a spark plasma sintering furnace for 10 min at a heating rate of 50 °C/min under 30 MPa in vacuum. (Mo, Cr)­AlB, (Mo,W)­AlB, and (Mo, W, Cr)­AlB, Mo_4/3Lu_2/3AlB₂ are some of the MAB phases that have been synthesized by this technique. ,
• (vii)Thermal explosion reaction (TER): It is one of the most efficient and cost-effective methods for MAB phase synthesis, characterized by an extremely high heating rate and completion within a few seconds; hence, it is often referred to as an “explosion” process. Although the TER method is similar to the SHS method, it involves the use of additional additives with different melting points (such as NaCl) along with M, A, and B. Using this TER method, Liang et al. synthesized Mn₂AlB₂ MAB. Here, pregrinned Al, Mn, B, and NaCl powders mixture was poured into a steel mold and a green body was prepared by applying a pressure of 100 MPa using a tablet press. Heating this green body in a quartz tube furnace at a temperature of 700 °C resulted in the formation of Mn₂AlB₂ MAB. Compared to arc-melting and hot-pressing, the TER method allows Mn₂AlB₂ formation under reduced temperature and reaction time conditions. ,
• (viii)High-power pulsed magnetron sputtering (HPPMS): This technique, also known as high power impulse magnetron sputtering (HIPIMS), is a physical vapor deposition process in which power is applied to the target in pulses with a low duty cycle (<10%) and frequency (<10 kHz), resulting in pulse target power densities of several kW cm^–2. This operating mode generates ultradense plasmas characterized by a high degree of ionization of sputtered atoms and an off-normal transport of ionized species relative to the target. These features enable the deposition of dense, smooth coatings even on complex-shaped substrates. Sahu et al. employed this method to deposit MoAlB thin films on Al₂O₃ (0001)-oriented substrates. The resulting films contained various compositional defects, including Mo₃Al₂B₄, Mo₄Al₃B₄, Mo₆Al₅B₆, and Al₃Mo phases, along with MoB MBenes.

2. Schematic Presentation of Various Processing Methods for MAB Phase Preparation .

^a Reproduced from ref . CC BY-NC-ND 4.0.

In some cases, a high-temperature solid-state method was employed, where a common tube furnace was used. Hanner and co-workers have expanded the family of layered ternary transition metal borides (e.g., Cr₂AlB₂, Fe₂AlB₂, MoAlB) by studying their crystal structures and thermal expansion coefficients. They have employed a two-step route protocol. First, (Mn_1–x Cr _x )B solid solutions were prepared by heat treating a mixture of Mn, Cr, and B_c. Then MAB was prepared by reacting (Mn_1–x Cr _x )B with Al at 1050 °C for 5 h.

Ti₂InB₂ was synthesized by Wang et al. from a mixture of Ti, In, and B powder through a solid-state reaction, by heating the mixture in a quartz tube sealed with Ar gas, and they observed that the yield was relatively low. To improve the yield, they performed the synthesis at various experimental conditions and heating at 1100 °C for 36 h under optimum conditions. This was followed by the reaction of the latter with Al to form the corresponding MAB phase. Several MAB phases that have been synthesized so far are listed in Table .

2. Examples of Some of the Synthesised MAB Phases.

sr. no	synthesized MAB phase	ref
1	Mo7Al6B7
2	Mo4/3Y2/3AlB2
3	Mo4/3Sc2/3AlB2
4	Fe2AlB2
5	WAlB
6	Mn2AlB2
7	Mo4MnSiB2
8	CrMn4SiB2
9	Mo0.8Mn4.2SiB2
10	Fe(MoB)2
11	Ta4CrSiB2
12	Ta4VSiB
13	Ta4MoSiB2
14	Cr2AlB2

3.2. Synthesis of MBenes

The strategies that are developed for MXene synthesis from MAX phases have also been applied to prepare MBenes from MAB phases. In this process, selective removal of the “A” group element is critical, while preserving the strong M–B bonding framework. Within MAB phases, Al–Al bonds are relatively weak and can be preferentially removed. , For 2D materials, several etching techniques, including chemical etching, reactive sputter etching (RSE), ion-assisted plasma etching, reactive gas etching, atomic layer etching, metal-assisted splitting, and laser etching, have been reported. The choice of etching method strongly influences product characteristics such as morphology, conductivity, mechanical, thermal properties, etc. To date, chemical etching remains the most widely adopted route for MBene synthesis, typically employing HF, HCl, LiF + HCl, or NaOH as etchants. The scheme presented in Scheme illustrates different deintercalation strategies for removing the “A” layer from MAB. The etching process using HF can be summarized by the equations stated in eqs –.

$${\mathrm{M}}_{1+\mathit{n}}{\mathrm{A}\mathrm{B}}_n+\mathrm{H}\mathrm{F}\to {\mathrm{A}\mathrm{F}}_3+\frac{1}{5}{\mathrm{H}}_2+{\mathrm{M}}_{1+\mathit{n}}{\mathrm{B}}_n$$ 4

$${\mathrm{M}}_{\mathit{n}+1}{\mathrm{B}}_n+{2\mathrm{H}}_2\mathrm{O}\to {\mathrm{M}}_{\mathit{n}+1}{\mathrm{B}}_n(\mathrm{O}\mathrm{H})+{\mathrm{H}}_2$$ 5

$${\mathrm{M}}_{\mathit{n}+1}{\mathrm{B}}_n+2\mathrm{H}\mathrm{F}\to {\mathrm{M}}_{\mathit{n}+1}{\mathrm{B}}_n{\mathrm{F}}_2+{\mathrm{H}}_2$$ 6

3. Schematic Illustration of Various Synthesis Routes for MBenes Synthesis.

^a Reproduced with permission from ref . Copyright 2022 Wiley-VCH.

The probable reactions occurring during the etching process, done by using NaOH, are given in eqs and .

$${\mathrm{M}}_{1+\mathit{n}}{\mathrm{A}\mathrm{B}}_n+\mathrm{O}\mathrm{H}+5{\mathrm{H}}_2\mathrm{O}\to {\mathrm{M}}_{\mathit{n}+1}{\mathrm{B}}_n{(\mathrm{O}\mathrm{H})}_2+{\mathrm{A}(\mathrm{O}\mathrm{H})}_4+\frac{5}{2}{\mathrm{H}}_2$$ 7

$${\mathrm{M}}_{1+\mathit{n}}{\mathrm{A}\mathrm{B}}_n+\mathrm{O}\mathrm{H}+5{\mathrm{H}}_2\mathrm{O}\to {\mathrm{M}}_{\mathit{n}+1}{\mathrm{B}}_n{\mathrm{O}}_2+{\mathrm{A}(\mathrm{O}\mathrm{H})}_4+\frac{7}{2}{\mathrm{H}}_2$$ 8

One of the earliest reports on MBene synthesis via chemical etching was published by Alameda et al. in 2017. They demonstrated that etching MoAlB single crystals with 10% NaOH for 24 h caused partial removal of Al, and then 25% urea intercalation expanded the interlayer spacing (Figure a). XRD confirmed the retention of the parent structure, while SEM-EDS revealed decreased Al content. The etching produced 50–300 nm MoAlB slabs, which are much thinner than bulk MoAlB, and generated metastable Mo–Al–B intergrowth phases along with 2D MoB monolayers. Increasing Al deintercalation also induced stacking faults, within which four intergrowth phases (Mo₂AlB₂, Mo₃Al₂B₃, Mo₄Al₃B₄, Mo₆Al₅B₆) were identified, with Mo₂AlB₂ as the most probable precursor for MoB. Later, Zhou et al. have synthesized the single-layer molybdenum borides (e.g., Mo_4/3B_2–x T _z ) by selectively etching Al/Y/Sc from (Mo_2/3Y_1/3)₂AlB₂ and (Mo_2/3Sc_1/3)₂AlB₂, while Wang et al. have reported 2D TiB₂ from Ti₂InB₂ via acid etching. Zhang et al. achieved CrB MBenes by HCl etching of Cr₂AlB₂, through the reaction given in eq .

$${\mathrm{C}\mathrm{r}}_2{\mathrm{A}\mathrm{l}\mathrm{B}}_2+3\mathrm{H}\mathrm{C}\mathrm{l}\to {\mathrm{C}\mathrm{r}}_2{\mathrm{B}}_2+{\mathrm{A}\mathrm{l}\mathrm{C}\mathrm{l}}_3+\frac{3}{2}{\mathrm{H}}_2(\mathrm{g})$$ 9

4.

(a) SEM images showing etched MoAlB single crystals after (a) treatment with 10% NaOH, (b) HER catalysis and fragmentation, and (c) treatment with 10% NaOH followed by intercalation with 25% urea. Reproduced from ref . Copyright 2017 American Chemical Society. (b) Microstructure of Cr₂AlB₂ powders before (a) and after (b) and (c) treatment in diluted HCl solution. Reproduced with permission from ref . Copyright 2018 Elsevier.

XRD showed the appearance of new peaks at 2θ = 14.26° and 21.06°, attributed to 2D-CrB nanosheets, which is consistent with SEM observations of layer exfoliation (Figure b). Investigations on the effect of etching time during the synthesis of CrB MBene from Cr₂AlB₂ using HCl revealed that etching beyond 2 h led to Cr₂O₅ formation due to oxidation. A major limitation of acidic etchants (HF, HCl, LiF + HCl) is the partial corrosion of the boride layer; for instance, HF etching of MoAlB caused surface coating and dissolution, particularly along the [001] direction. To overcome these drawbacks, alkaline solutions such as NaOH have been explored. However, NaOH etching often results in only partial Al removal. Xiong et al. have demonstrated that hydrothermal treatment of MoAlB in 25 wt % NaOH at 150 °C for 24 h produced MoB with an accordion-like layered structure. Similarly, Jin et al. have employed microwave-assisted chemical etching to synthesize manganese boride, followed by coordination-induced exfoliation to obtain Bianchored single manganese boride nanosheets (MBBNs), which were successfully applied in multimodal imaging-guided photothermal cancer therapy.

Bhaskar et al. reported MBene synthesis from non-MAB precursors, performing room-temperature topochemical deintercalation of Li from layered LiNiB polymorphs by exposure to air, water, or dilute HCl, resulting in Li_∼0.5NiB. They proposed a “zip-lock” mechanism, where condensation of [NiB] layers into double/triple fragments produced Li­[NiB]₂ and Li­[NiB]₃ units, alternating with Li layers in the Li_∼0.5NiB structure. Extending this approach, Chen et al. synthesized a new Zr-based MBene by exfoliating ZrB₂.

Ouladsmane et al. employed a reaction of MoCl₅ and MgB₂ as reactants with HCl, ethanol, and deionized water to produce layered MoB nanosheets through a straightforward solid-state synthesis method under an argon (Ar) atmosphere at a carefully controlled 650 °C temperature for 24 h. The reactions occurring during this process are stated in eq , where the MoB was obtained as a final product with MgCl as the byproduct.

$${\mathrm{M}\mathrm{g}\mathrm{B}}_2+{\mathrm{M}\mathrm{o}\mathrm{C}\mathrm{l}}_5\to \mathrm{M}\mathrm{o}\mathrm{B}+{\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{l}}_2(\mathrm{l})$$ 10

The diffraction peaks in the XRD spectra indicated the formation of a single-phase MoB with a tetragonal crystal structure of the layered MoB nanosheets. Guo et al. developed a molten salt-assisted strategy for the controllable synthesis of metal diborides, including TiB₂, VB₂, CrB₂, ZrB₂, HfB₂, MoB₂, WB₂, NiB₂, RuB₂, MnB₂, NbB₂, and TaB₂, with specific surface areas ranging from 5–30 m² g^–1 (notably ∼ 70 m² g^–1 for ZrB₂ and HfB₂) via borothermal reduction in molten KCl/NaCl salts. The metal oxide precursors (ZrO₂, HfO₂, Cr₂O₃, MoO₃, WO₃, NiO, RuO₂, TiO₂, MnO₂, V₂O₅, Nb₂O₅, Ta₂O₅) were mixed with boron powder, ground thoroughly, and heated at 950–1000 °C for 1 h under Ar. Powder XRD confirmed the phase purity of MB₂ products, while TEM revealed distinct morphologies: TiB₂, ZrB₂, HfB₂, VB₂, and CrB₂ exhibited nanosheet structures; NbB₂, TaB₂, and MnB₂ formed needle-like structures; and MoB₂, WB₂, and RuB₂ were obtained as aggregated nanoparticles.

Exfoliation of layered materials using acids or alkalis is strongly influenced by etchant concentration, reaction time, and temperature. , Exfoliating 2D layered materials such as graphene, MXenes, and MBenes into nanosheets is a well-established route to achieve unique mechanical, electrical, and optoelectronic properties. For MBenes, selective breaking of Al–Al, Al–M, and Al–B bonds, while preserving other bonds, is essential; thus, the systems having weaker M–Al bonding can be exfoliated easily. Common strategies include liquid-phase, chemical, and mechanical exfoliation, with the bond-breaking efficiency largely determining process success. For example, Yousaf et al. synthesized quasi-2D nanosheets of several metal diborides (CrB₂, HfB₂, TaB₂, TiB₂, ZrB₂, etc.) using ultrasonication-assisted liquid exfoliation in DMF and NMP, highlighting the importance of surface energy and cleavage-plane stability.

The etching protocol for Al removal from the MAB phase varies with the nature of MBenes. The first experimental demonstration of a possible two-dimensional MBene, CrB, was achieved through selective chemical etching of its layered parent MAB phase, Cr₂AlB₂. In this process, mild acid etching, typically dilute hydrochloric acid (HCl), was employed at room temperature or under mild hydrothermal conditions to selectively remove the interleaved aluminum (Al) layers separating the chromium boride slabs. However, the etching reaction was often hindered by the formation of a passivating aluminum oxide or oxyhydroxide layer on the Cr₂AlB₂ surface, which limited the extent of Al dissolution. To overcome this challenge, a secondary mild alkaline treatment, such as LiOH-assisted hydrothermal etching, was introduced to dissolve these oxide barriers and expose fresh Al sites for continued acid attack. This two-step sequence, Al removal by acid, followed by mild alkaline dissolution, enabled controlled exfoliation of layered CrB nanosheets while minimizing overetching or surface degradation. Careful tuning of pH, etching time, and thermal conditions was required to achieve high structural integrity and low defect density. The resulting CrB nanosheets displayed a few-layer morphology with clean surfaces, and localized etching patterns observed under microscopy correlated with microstructural variations in the parent Cr₂AlB₂, suggesting a microstructure-dependent etching behavior. ,

The etching of MoAlB to synthesize two-dimensional MoB MBene is chemically more challenging due to the higher bond strength and thermodynamic stability of Mo–Al linkages compared to Cr–Al. The selective removal of Al from MoAlB generally requires a multistage or mixed-chemical approach. Reported strategies include alkaline etching with NaOH, or acidic etching using LiF + HCl systems and HCl + H₂O₂ mixtures under mild microwave-assisted hydrothermal conditions. The inclusion of hydrogen peroxide (H₂O₂) serves as a controlled oxidizing agent, promoting the dissolution of Al and facilitating partial delamination within shorter durations (∼4 h). Nevertheless, complete exfoliation of MoAlB remains difficult. The process often results in incomplete Al removal, accompanied by residual oxide phases (e.g., Al₂O₃, MoO _x ), vacancy defects, and layer dislocations, which together indicate the generation of structural strain during etching. Optimisation of acid concentration, reaction temperature, and time is therefore essential to balance between sufficient Al extraction and preservation of the MoB layered framework. Despite these complexities, the etched MoB products exhibit nanosheet-like morphologies confirmed by XRD, SEM, TEM, and XPS analyses, verifying successful though partial chemical exfoliation. However, the bulk synthesis of defect-free 2D MoB remains considerably more challenging than CrB, primarily due to its chemical inertness and stronger Mo–Al bonding. , Overall, the contrasting etching behaviors of Cr₂AlB₂ and MoAlB highlight the importance of tailoring etching chemistry according to the intrinsic bonding characteristics and thermodynamic stability of each MAB precursor. While CrB MBenes can be effectively obtained through mild acid and alkaline treatments, yielding relatively defect-free nanosheets, MoB MBenes require more complex, oxidizing-alkaline systems that often generate structural imperfections. These mechanistic differences provide valuable insight for designing optimized, element-specific etching protocols for the scalable synthesis of high-quality 2D MBenes with controlled morphology and surface properties.

Using microscopic techniques (ADF-STEM and HRTEM) Alameda et al. investigated the formation of MoB from MoAlB phase via etching of Al layers and proposed the possible growth mechanism (Figure ). Before etching, on the surface of MoAlB crystal (0k0) stacking faults were sporadically distributed. Then, regions of high stacking fault density, stacking fault bands (Al single layers) and normal stacking sites (Al double layers) appeared when MoAlB was treated with 10% NaOH for 2 h. Upon initial NaOH treatment, partial deintercalation of Al from MoAlB occurs unusually. The remaining Al atoms in MoAlB, after the partial removal of Al from the Al double layer, are redistributed uniformly between the adjacent MoB layers, and instead of forming vacant channels, the formation of a single Al layer that binds the boride layers together occurs. When NaOH etching exceeded 2 h, the stacking fault bands became continuous as Al deintercalates from the unperturbed regions of MoAlB, accompanied by a simultaneous increase in stacking fault density. Consequently, regions containing nanoscale domains with diverse MoB–Al stacking motifs emerged, giving rise to compositions with Mo _x Al_x–1B _x stoichiometry, ranging from Mo₆Al₅B₆ to Mo₂AlB_2, and these intergrowth structures were identified by ADFSTEM imaging. Formation of similar ordered regions was also observed when the MoAlB crystal was etched with LiF + HCl. Each ordered nanoscale domain was surrounded by random distributions of stacking faults. Formation of etched cavities was observed after treating MoAlB with 10% NaOH for 24 h, accompanied by an increased stacking fault density relative to that of the Al double layers. The cavities were ∼10 nm wide and periodically distributed at the crystal surface. Stacking faults propagate a few micrometers below the crystal surface before terminating, after which the structure reverts to unperturbed MoAlB. This behavior, nearly identical to that observed with the NaOH treatment, was also seen when the MoAlB crystals were etched in a solution of 2 M LiF in 6 M HCl. Etching with LiF + HCl for 2 h led to moderate stacking fault formation, while prolonged treatment for 24 h produced a high density of stacking faults along with etched cavities. Unlike NaOH, the LiF + HCl etchant caused significant corrosion of the boride layers in addition to Al deintercalation. Etched “branches” propagated along [010], corroding MoB layers and generating a network of cavities. These observations suggest that the MoB layer dissolves slowly in HF at room temperature, making LiF + HCl unsuitable for Al deintercalation from MoAlB as corrosion processes prevail. From these observations, Alameda et al. proposed a plausible multistep mechanism for partial etching of Al from MoAlB: (1) through topochemical deintercalation, Al atoms are selectively removed, converting Al double layers into stacking faults that contain single Al layers and extend as bands of a few micrometres into the crystal bulk. (2) The stacking fault bands eventually reach saturation (with a defect density >50%), resulting in a uniformly faulted surface region. At this stage, the surface composition more closely resembles Mo₂AlB₂. (3) Al single layers are subsequently removed from the stacking faults, eliminating Al between two adjacent MoB layers and producing an etched cavity that swells to approximately 10 nm in width. Alternatively, this step may occur in reverse: cavities initially form due to strain, followed by the dissolution of Al atoms lining the cavity interior. Generation of MBenes from the MAB phases necessitates full exfoliation, which was achieved by the removal of the entire Al layers separating the MoB layers. However, typically, only a single Al layer is removed from the double layer, leading to the formation of a stacking fault containing one Al layer rather than a complete cavity. The formation of certain stacking sequences may cease deintercalation, thereby preventing complete Al removal from the MAB phase. Microscopic studies revealed that one of the stacking variants, which are formed upon partial Al deintercalation of MoAlB, transforms into the Mo₂AlB₂ structure. As this stacking variant cannot be formed from Mo₂AlB₂, it could be a preferable precursor than MoAlB for the synthesis of MoB. Several analytical techniques, including X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF), etc., have been employed to detect residual Al following the etching of MAB phases. Alameda et al. used EDS to investigate Al removal from MoAlB single crystals and showed that the Mo/Al atomic ratio changed from 51:49 in the pristine crystal to 57:43 after etching with 10% NaOH, indicating partial removal of Al layers. Field-emission scanning electron microscopy (FESEM) and annular dark-field scanning transmission electron microscopy (ADF-STEM) are employed techniques that enable direct observation of the formation of delaminated and exfoliated layered morphologies during the etching of MAB phases. As an example, Figure shows an SEM micrograph illustrating the formation of MoAlB slabs after etching with 10% NaOH, along with periodic gaps of 10–100 nm in the partially etched MoAlB single crystal. , Anosov et al. performed XRD analysis on pristine and HF-etched MoAlB crystals and observed the formation of MoB MBene, accompanied by a minor AlF₃ impurity phase. Owing to the hydrolysis of AlF₃ in water, Al₂O₃ was also detected. To eliminate these Al-based impurities, the etched product was further treated with 25 wt % aqueous NaOH under boiling conditions for 1 h, leading to the formation of soluble sodium aluminate species (Na₂Al₂₂O₃₄) and improved MBene purity. Similarly, Yan et al. reported the formation of Al₂O₃ during HF etching of MoAlB and oxygen–aluminum compounds during NaOH etching. EDS compositional analysis revealed that the Mo/Al atomic ratio increased from 1:1 in the pristine MoAlB to approximately 2:1 after etching, further confirming partial Al extraction. Using XRF analysis, Chandel et al. demonstrated that Al removal from MoAlB is more extensive during acid etching than alkali etching. XPS analysis of alkali-etched samples identified Al₂O₃, Al–OH/AlO­(OH), and AlCl₃ species, whereas acid-etched samples exhibited Al 2p contributions primarily from Al₂O₃ and AlCl₃. Collectively, these studies highlight that residual Al, often present as oxide, hydroxide, or fluoride species, is difficult to completely eliminate and strongly depends on the etching chemistry and post-treatment conditions.

5.

ADF-STEM images of MoAlB at various stages of etching with 10% NaOH. (a) As-synthesized MoAlB showing a few stacking faults as high-contrast lines vertically proceeding down from the crystal surface along the crystal lattice planes. The inset shows a schematic of the crystal and stacking faults. (b) Atomic-scale-resolution ADF-STEM image of pristine MoAlB. (c) Atomic-scale-resolution image of MoAlB showing a single stacking fault in the vertical direction indicated by an line below the image. (d) MoAlB after 2 h of etching showing the formation of several stacking fault bands. The inset shows a schematic of the crystal and stacking fault bands. (e) Higher-magnification image of the sample with a high density of stacking faults. (f) Higher-magnification image of two stacking fault bands separated by a region of mostly unperturbed MoAlB with no stacking faults. Reproduced from ref . Copyright 2018 American Chemical Society.

The intricate crystal structure and compositional complexity of the MBenes present significant challenges in their synthesis. Several issues are linked to the MBenes synthesis, including the degree of etching, presence of unreacted precursors, uniformity in morphological evolution, creation of defects and vacancies, lattice distortions, partial surface oxidation, and the formation of surface terminations, all of which can adversely affect the quality and reproducibility of the resulting materials. Consequently, all of the theoretically viable MBenes could not be realized experimentally. Advances in experimental studies are constrained by the difficulty of synthesizing MBenes with high-quality monolayer or few-layer structures. In many cases, the precursor MAB phases are synthesized by high-temperature solid-state route. As diffusion is one of the limiting factors that compromise the homogeneity and purity of the product, the synthesized MAB phases are contaminated with impurities. As an example, when MoAlB was synthesized by a combination of SHS and HP techniques, the formation of Al₂O₃ impurity was detected. In a review, Yang et al. have compared different synthesis methods of the synthesis of different MBenes and the effect of various precursors, reaction conditions, advantages, and disadvantages of the methods, which are presented in Table .

3. Summary of Recent Methods for Synthesizing MBenes .

precursor	2D transition metal borides (MBenes)	synthetic type	synthesis method	synthesis effects	component composition	surface groups	advantages of the method	disadvantages of the method	ref
(Mo2/3Y1/3)2AlB2(Mo2/3 Sc1/3)2AlB2	Mo4/3B2–x T z	chemical exfoliation (acid etching)	(1) 40 wt % HF, 33–35 °C, 3.5 h; (2) 20 wt % TBAOH, RT, 2 min	single-layer 2D molybdenum boride sheets with ordered metal vacancies	Mo4/3B2–x T z	–F, −O, −OH	The etching time is short, and the temperature is low	(1) HF is harmful to the human body and the environment, (2) vacancy defects are inevitable
(Mo2/3Y1/3)2AlB2	Mo4/3B2–x T z	chemical exfoliation (acid etching)	(1) HF, 35 °C, 18 h; (2) 1 M TMAOH, RT, 2 h	smooth several layers of 2D sheets	Mo4/3B2–x T z	–F, −O, −OH	ordered Y vacancies and plenty of unsaturated B sites	HF is highly corrosive
Cr2AlB2	CrB	chemical exfoliation (acid etching)	1.25 mol L–1 HCl, RT, 6 or 8 h	6 h: 2D CrB sheets 8 h: Curled 2D CrB sheets	CrB, Cr2AlB2	–OH	safe and environmentally benign	low yield
Cr2AlB2	CrB	chemical exfoliation (acid etching)	0.5 M HCl, RT, 7 days	pure phase 2D-CrB nanosheets	CrB (thickness varies from several to tens of nanometers)	no mention	Al was almost completely etched	take a long time
MoAlB	MoB	chemical exfoliation (acid/alkali etching)	10% NaOH, RT, 24 h/2 M LiF in 6 M HCl, RT, 24 h	(1) form four etching intermediates (2) Al was partially removed (3) form etched cavities	A large amount of MoAlB and a small amount of MoB	no mention (The guesses are −OH, −O, −F, and −Cl)	(1) etching with NaOH is a green etching method. (2) The topochemical deintercalation of the etching process is well-studied	etching is not complete
MoAlB	MoAl1–x B	chemical exfoliation (alkali etching)	(1) 10% NaOH, RT, 24 h; (2) undiluted TMAOH, ascorbic acid, H2O2, 140 °C, 24 h	MoAl1–x B with a rich exposition of basal planes and Mo-vacancy de- defects	MoAl1–x B	–O	the synthetic MBene has rich etched cavities	large amounts of Al
MnB/MnB2 bulks	MnB/MnB2	chemical exfoliation with microwave assisted	CH3COOH, H2O2, Bi(NO3)3·5H2O	monolayer flower-like MnB/MnB2 nanosheets with 1.04 ± 0.34 nm thickness	MnB/MnB2	B–OH, B–Bi	bianchored MBene displays superior NIR-photothermal conversion efficiency (η = 59.4%)	high energy provided by microwaves gives MBene a small transverse size (150 nm)
ZrB2	ZrB2	chemical exfoliation with microwave-assisted	(1) CH3COOH, H2O2, EG; (2) HA	ZrB2 nanosheets with 150 nm in diameter and 7.4 nm in height	ZrB2 with HA	B–OH, –HA	well disperse in water	MBene has a small transverse size
Ti2InB2	TiB	high-temperature	1050 °C, ∼10–4 Pa, 6 days	layered TiB compound with almost no In	TiB with associated TiB2 impurity phase	no mention	novel and green method	(1) phase structure was changed, (2) complex operation and take a long time
Hf2InB2	HfBO	molten salt etching	(1) CuCl2, KCl, NaCl, 700 °C, 24 h; (2) 0.2 m, (NH4)2S2O8	2D HfBO with excellent Li+ storage potential	HfBO with HfO2	–O	efficient and novel way	etching process with Cu generation
(Mo2/3Y1/3)2AlB2	MBene-Br	molten salt etching	CuBr2, 504 °C, 8 h	well-defined layered and conductive MBene with ordered metal vacancies and –Br RATGs	MBene-Br (all Al atoms and some, Y atoms were removed, while, Br groups were introduced) with generating Al2O3	–Br	MBene equipped, With redox-active terminal groups	obtained MBene was mixed with Al2O3
Mn powder and B powder	MnB	thermal fragmentation	1400 K, 5 GPa	grain size of MnB varies from 100 to 1400 nm	MnB	no mention	A fast and efficient, Physical method	MBene needs to be further stripped
FeSO4·7H2O and LiBH4	FeB2	chemical reduction	in Ar, 500°C, 2 h	porous nanosheet structure of FeB2	FeB2	no mention	take a short time	synthesized MBene has no regular morphology
MoAlB	MoB	magnetron sputtering	MoAlB target, Al2O3 substrate, in Ar, 700 °C, 0.4 Pa	MoB MBene and MoAlB are present in the vicinity of the synthesized AlO x	MoAlB–MoB–AlO x heterostructures	no mention	no etching reagent is required	no etching reagent is required

Though both MXenes and MBenes possess 2-D layered structures but they are different in many aspects. B atoms are the backbone in the structure of MBenes. Whereas Metal-C/Metal-N acts as the backbone of MXenes. Hence, they are electronic structures, mechanical, optical, optoelectronic, catalytic properties, etc. are different. Table lists a comparison between MXenes and MBenes.

4. Comparison between MXenes and MBenes.

aspect	MXenes	MBenes
synthesis	(i) Precursor: MAX phases (layered carbides/nitrides, e.g., Ti3AlC2). (ii) Etching: Requires strong acids, often HF or LiF/HCl mixtures. (iii) Strong metal–carbon/nitrogen (M–C/N) bonds; selective removal of weaker metal–aluminum (M–A) bonds.	(i) Precursor: MAB phases (layered transition metal borides, e.g., MoAlB). (ii) Uses milder etchants such as dilute alkali solutions or molten salts. (iii) Metal–boron (M–B) bonds generally weaker than M–C/N, facilitating easier Al layer removal.
conductivity	(i) High electronic conductivity (e.g., Ti3C2T x ∼ 2000–10,000 S/cm), metallic nature - Can decline with oxidation and restacking effects. (ii) Often metallic, but can transition to semiconducting states based on composition and surface termination. (iii) Prone to contact resistance at interfaces due to surface groups.	(i) Superior conductivity, often higher (e.g., Nb/Tc-based MBenes resistivity 2–5 × 10–8 Ω·m). (ii) Bulk and surface retain metallic character after functionalization. (iii) Less affected by oxidation; robust electron pathways remain active. (iv) Often higher intrinsic conductivity and lower resistivity than MXenes due to metallic band structure and better electron delocalization.
electronic structure	(i) Derive from transition metal carbide/nitride; electronic structure influenced by choice of transition metal and functionalization. (ii) Electronic properties: delocalized electrons confer high conductivity, yet prone to vacancy/defect-driven disorder, particularly after surface oxidation. (iii) Fermi level and band dispersion can be tuned via functional groups, but stability and control are ongoing challenges.	(i) Boron in structure introduces unique electronic states, leading to strong anisotropy and pronounced direction-dependent properties. (ii) Surface functional groups are more easily tailored, allowing broader band gap and Fermi level engineering. (iii) Some MBenes show linear scaling relationships in adsorption energies, enabling tailored catalytic selectivity (notably for CO2 and HER). (iv) Density of states (DOS) analyses reveal more robust electronic features resistant to oxygen or water induced disruption.
Stability	(i) Prone to oxidation under ambient conditions; surface functional groups (−OH, – O, – F) help interaction but also accelerate oxidation requiring vacuum storage or passivation for long-term use. (ii) Structural degradation in aqueous or humid environments, limiting application scope. (iii) Restacking due to van der Waals forces can reduce accessible surface area. (iv) Cycling stability in batteries and electrochemical cells is challenged by layer disruption and surface chemistry instability.	(i) Exhibit superior oxidation resistance, enabling stable performance in air and aqueous environments. (ii) Boron-containing backbone (M–B bonds) confers higher chemical stability and resilience under harsh conditions. (iii) Retain integrity over longer cycling in batteries, attributed to stronger bonds and robust layered structure. (iv) Lower restacking tendency and high Young’s modulus further enhance stability. (v) Self-healing-like behavior observed under mechanical stress, useful for flexible applications.

4. Hydrogen Evolution Reaction

The energy demand is steadily increasing, driven by rapid technological advancement and the growing needs of modern lifestyles. In this scenario, green hydrogen is emerging as the future fuel. Production of green hydrogen via water splitting offers a carbon-free solution, which could be one of the key technologies in achieving sustainable energy goals. ,, In 1789, Troostwijk et al. first reported that in the electrolysis of water (H₂O_(l) → H_2(g) + 1/2 O_2(g)), the two half–cell reactions are the hydrogen evolution reaction (HER: 2H⁺ _(aq) + 2e^–→ H_2(g)) and the oxygen evolution reaction (OER: 2H₂O_(l) → 4e^– + 4H⁺ _(aq) + O_2(g)). For this water electrolysis, water ΔG° = +237.2 kJ mol^–1 and ΔE° = 1.23 V vs normal hydrogen electrode (NHE). However, to overcome the limitations associated with the unfavorable thermodynamics, reduction of overpotential, and minimization of the cost of the required electric power for electrocatalysis HER, the need for efficient electrocatalysts is paramount. Hence, all over the world, researchers are working on developing HER catalysts with a variety of compositions, morphologies, hybrid structures, etc. − However, to date, noble metals platinum, palladium, iridium, and ruthenium are still the most efficient electrocatalysts. However, their limited availability and high cost pose significant constraints to industrial production. ,− Thus, there is a pressing need to develop and explore non-noble metal-based electrocatalysts that are highly efficient for HER. Though HER reaction pathways in acidic and alkaline media are similar, the reaction in alkaline medium is two to 3 orders of magnitude slower than that in acidic medium. To understand HER, several theories, such as water dissociation theory, hydrogen binding energy (HBE) theory, interface water and/or anion transfer theory, etc., have been proposed. Usually, the overall HER pathway can be described by a three-state diagram, composed of an initial state H⁺ + e^–, an intermediate adsorbed H*, and a final product 1/2 H₂. The sources of protons are the hydronium ion (H₃O⁺) in acidic electrolytes and the water molecule in alkaline electrolytes. The production of an adsorbed hydrogen atom (H*) occurs on the catalyst surface (M) by the Volmer reaction (eqs and ). After that, H₂ generation occurs via the Tafel reaction (eq ), the Heyrovsky reaction (eqs and ), or a combination of both. During the Heyrovsky stage, one proton attaches to the H*, and subsequently, an electron interacts, resulting in the production of H₂. Scheme depicts the different steps of HER reactions in acidic and alkaline media. The reactions which are involved in the HER process can be indicated as follows. ,−

• (1)
  Electrochemical hydrogen adsorption (Volmer reaction, eqs and ):

  $${\mathrm{H}}_3{\mathrm{O}}^{+}+\mathrm{M}+{\mathrm{e}}^{-}\rightleftarrows {\mathrm{M-H}}^{*}+{\mathrm{H}}_2\mathrm{O}(\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}\mathrm{i}\mathrm{c})$$ 11

  $${\mathrm{H}}_2\mathrm{O}+\mathrm{M}+{\mathrm{e}}^{-}\rightleftarrows {\mathrm{M-H}}^{*}+{\mathrm{O}\mathrm{H}}^{-}(\mathrm{a}\mathrm{l}\mathrm{k}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{e})$$ 12
• (2)
  Electrochemical desorption (Heyrovsky reaction, eqs and ):

  $${\mathrm{H}}^{+}+{\mathrm{e}}^{-}+{\mathrm{M-H}}^{*}\rightleftarrows {\mathrm{H}}_2+\mathrm{M}(\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}\mathrm{i}\mathrm{c})$$ 13

  $${\mathrm{H}}_2\mathrm{O}+\mathrm{M}+{\mathrm{e}}^{-}\rightleftarrows {\mathrm{H}}_2+{\mathrm{O}\mathrm{H}}^{-}+\mathrm{M}(\mathrm{a}\mathrm{l}\mathrm{k}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{e})$$ 14

• (3)
  Chemical desorption (Tafel reaction, eq ):

  $$2{\mathrm{M-H}}^{*}\rightleftarrows {\mathrm{H}}_2+2\mathrm{M}(\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}\mathrm{i}\mathrm{c}\mathrm{or}\mathrm{a}\mathrm{l}\mathrm{k}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{e})$$ 15

The aforementioned reaction steps appear very simple, but in reality, HER is not at all this simple.

4. Mechanism of Hydrogen Evolution on the Surface of an Electrode in (a) Alkaline and (b) Acidic Media .

^a Adapted from ref . Copyright 2019 American Chemical Society.

The HER activity of an electrocatalyst is generally tested by performing linear sweep voltammetry (LSV) and by determining the overpotential (η) produced at a current density of 10 mA cm^–2. Overpotential (η) is defined as the difference between the equilibrium potential for a given reaction (also called the thermodynamic potential) and the potential at which the catalyst operates at a specific current under specific conditions. The lower the overpotential generated, the better the electrocatalytic performance is. Electrocatalytic performance of an electrocatalyst is further influenced by factors such as iR compensation, electrochemically active surface area (ECSA), coverage effects, mass-transport effects, etc. The iR drop in electrochemical measurements primarily arises from the uncompensated solution resistance between the working and reference electrodes. This resistance is governed by multiple factors, including the ionic conductivity of the electrolyte, the distance between the working and reference electrodes, the electronic conductivity of the catalyst and electrode substrate, as well as the surface roughness of the working electrode and the thickness of the electrical double layer. Because polarization curves are inherently distorted by iR losses, appropriate correction is often necessary prior to meaningful interpretation and subsequent kinetic analyses, such as Tafel slope determination. Nevertheless, substantial ambiguity persists in the literature regarding iR compensation practices, particularly with respect to the necessity of correction under different experimental conditions and the appropriate degree of compensation to apply. Such inconsistencies frequently lead to divergent or ambiguous electrochemical metrics, thereby complicating reliable comparison across studies. electrochemically active surface area (ECSA) is defined as the true surface area of a catalyst that is actively involved in electrocatalytic reactions, rather than the total geometric area. , Higher ECSA enables increased surface coverage by reactant species, leading to enhanced reaction rates. Surface coverage effects reflect the extent of hydrogen adsorption on the catalyst surface and are governed by parameters such as adsorbate binding energy, adsorption configuration, and lateral interactions among adsorbed species. Hydrogen coverage plays a critical role in determining both the kinetics and mechanistic pathways of HER, with optimal coverage being essential for achieving high catalytic efficiency. In multicomponent, multisite, and heterostructured electrocatalysts, the hydrogen binding energy and adsorption extent are influenced by complex synergistic interactions among their constituent elements, surface sites, morphology, etc. Consequently, rational catalyst design strategies must carefully balance adsorption strength and surface coverage to maximize HER activity. The rate of the HER is further constrained by mass transport effects, which govern the flux of reactants (H⁺ or H₂O) to the electrode surface and the removal of the product (H₂) away from it. The pH of the reaction medium (acidic, neutral, or alkaline) significantly affects mass transport behavior by altering reactant availability and transport properties. Additionally, the formation and accumulation of H₂ bubbles on the electrode surface can obstruct catalytically active sites, leading to diminished reaction rates. Factors such as catalyst morphology, electrolyte flow conditions, stirring rate, or rotation speed in rotating disk electrode measurements, and the efficiency of bubble detachment collectively modulate mass transport and, thus, influence HER performance. ,

The Gibbs free energy of the intermediate state, |ΔG _H*|, is considered a major indicator of the HER catalytic activity for various catalysts. ,− A “volcano-type” plot (exchange current density (j ₀) vs hydrogen binding energy (ΔG _H)) has been proposed by Parsons. Theoretically, as the standard hydrogen electrode potential is defined as zero, the ideal HER electrocatalyst should have ΔG _H* = 0, which is the peak of the volcano plot. The smaller the |ΔG _H*| value, the better the HER performance of the material. , For example, Pt, the most highly active and well-known HER catalyst, shows a value of ΔG _H* = −0.09 eV. The easy interaction between the partially filled d orbital of Pt and the 1s orbital of H creates highly effective active H adsorption sites. Hence, the quest is on for the HER electrocatalysts that can exhibit a very low value of ΔG _H*, as well as being very cost-effective, easily producible. The unique properties of MBenes include exceptional chemical stability, electrical conductivity, cost-effectiveness, and a distinctive electronic interaction between boron and metal atoms. Nanostructuring of MBenes can provide benefits like increased surface area, fast reaction kinetics, better access to catalytic centers that are exposed, decreased charge transfer resistance, and customizable electronic architectures. Hence, MBenes have the potential to be an efficient HER electrocatalyst.

4.1. DFT Studies on HER Activity of MBenes

The unique two-dimensional structure, superior electrical conductivity, mechanical integrity, and chemical stability of MBenes, combined with the ability to modulate their electronic structure and conductivity through compositional and surface termination tuning, render them highly promising electrocatalysts for a wide range of electrochemical reactions. Although MBenes are generally perceived to exhibit excellent electrocatalytic performance, reports indicate that not all members of the MBene family in their pristine form possess inherently high catalytic activity. Therefore, to enhance the catalytic performance of MBenes for specific reactions, ongoing efforts are devoted to the identification of their appropriate compositions and crystal phases, introduction of structural modifications such as defects, vacancy engineering, and heteroatom doping, controlling surface terminals, integrating them with suitable single-atom catalysts, etc. Given that the understanding of MBene chemistry remains in its infancy, the fundamental mechanisms governing their electrocatalytic activities are still not well established. The majority of reports in the literature utilize DFT calculations to propose the mechanisms governing the electrocatalytic behavior of MBenes.

In studies of HER electrocatalysis, ΔG _H* has been computed for different types of pristine MBenes and MBene-derived catalysts. According to the Sabatier principle, when hydrogen binds weakly to the catalyst surface, the ΔG _H* becomes overly positive, leading to sluggish hydrogen adsorption kinetics. In contrast, excessively strong hydrogen binding results in a highly negative ΔG _H*, which impedes the release of adsorbed hydrogen as H₂. Hence, the value of ΔG _H* equal to zero would be the most suitable for efficient HER. The ΔG _H* value near zero indicates an optimal balance between hydrogen adsorption and desorption. This suggests that the surface can efficiently adsorb H atoms without binding them too strongly, and release H₂ easily during the reaction. Therefore, materials exhibiting ΔG _H* values near zero are expected to achieve suitable hydrogen coverage and are considered excellent candidates for HER catalysis. Generally, for practical purposes, electrocatalysts with ΔG _H* < 0.2 eV can be considered appropriate for efficient HER. The HER activity of the catalyst is highly dependent on H coverage.

The free adsorption energy (ΔG _H*) can be expressed as eq

$${\mathrm{\Delta }G}_{{\mathrm{H}}^{*}}={\mathrm{\Delta }E}_{{\mathrm{H}}^{*}}+\mathrm{\Delta }E-T\mathrm{\Delta }S$$ 16

where ΔE _H* is the energy required to absorb the H on the catalytic surface, and it can be calculated from eq :

$${\mathrm{\Delta }E}_{{\mathrm{H}}^{*}}=E_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}+\mathrm{H}}-E_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}}-\frac{1}{2}{E}_{{\mathrm{H}}_2}$$ 17

ΔE is the Zero-point energy correction, and TΔS is the Entropy correction.

The ΔG _H* explores reaction pathways such as the Volmer–Tafel and Volmer–Heyrovsky routes, and evaluates how the catalyst’s structural and electronic features influence the rate-determining step (RDS). These simulations model the catalyst surface, considering factors such as defects, dopants, and alloying effects, to identify optimal compositions and reveal how such modifications enhance catalytic activity and overall performance.

In an attempt to study the variation of ΔG _H* with changing H*, Guo et al. have carried out DFT calculations for 2D Mo₂B₂, revealing a strong interaction between the Mo surface atoms and adsorbed hydrogen, indicating favorable H binding. They also studied Fe₂B₂, which exhibited a hydrogen adsorption free energy ΔG _H* very close to zero, which was found to be smaller than that of the benchmark Pt catalyst (0.09 eV) and also lower than typical values for MoS₂ or WS₂. This near-zero ΔG _H* suggests that 2D Fe₂B₂ possesses excellent thermodynamic characteristics for hydrogen adsorption and desorption, highlighting its potential as a highly active HER electrocatalyst. Zhang et al. explored a range of MBenes with various compositions (M₂B₂, M₃B₄, and M₄B₆ type MBenes (M = Cr, Mn, Fe, Co, and Ni)) for HER, by determining the most favorable adsorption sites for one H atom on their surfaces. To identify the sites with the lowest adsorption energy, they have considered four sites, i.e., the top site, bridge-1, bridge-2, and the hollow site and the site-1 was found to be the most stable for H adsorption (Figure ). The calculated ΔG _H* values for Mn₂B₂, Fe₂B₂, and Fe₃B₄ indicated their potential for excellent HER activity. In contrast, MBenes such as Cr₃B₄, Cr₄B₆, Mn₃B₄, Ni₃B₄, and Ni₄B₆ exhibited ΔG _H* values above 0.2 eV, suggesting that these monolayers are less favorable for hydrogen evolution catalysis. Mn₂B₂, Fe₂B₂, Co₂B₂, Co₃B₄, and Mn₄B₆ showed high HER activity not only in low H coverage but also with high H coverage. An increase in hydrogen coverage results in a slight weakening of the binding interaction between H atoms and the MBene surface. It was observed that, for Fe-based and Co-based MBenes, an increase in hydrogen coverage led to a slight elongation of the Fe–H/Co–H bond length, likely due to minor structural distortions of the neighboring Fe or Co atoms induced by hydrogen adsorption. According to Bader charge analysis, the charge accumulated on each adsorbed hydrogen atom decreases slightly with increasing hydrogen coverage, which can be ascribed to the Coulombic repulsion among adjacent hydrogen atoms. This diminished charge transfer reflected a gradual attenuation of the metal–hydrogen bonding strength, thereby leading to an increase in bond length. For Fe₂B₂, Co₂B₂, and Co₃B₄, with increasing H coverage, the variation trend of charge transfer was opposite to the bond length. High charge transfer and short bond length correspond to lower adsorption energy, which also suggests the increase of H coverage leading to slight weakening of ΔG _H*, being able to remain lower than 0.2 eV, exhibiting superior HER catalytic activity. In these systems, the variation in charge transfer with increasing hydrogen coverage exhibited an inverse relationship with bond length. Enhancement of charge transfer and shortening of M-H bonds might cause a lowering in adsorption energies. Although ΔG _H* slightly increased with higher H coverage, it remained below 0.2 eV, indicative of superior HER catalytic performances of Fe₂B₂, Co₂B₂, and Co₃B₄.

6.

(a) Top and side view of the MBenes at four possible H* adsorption sites: top, bridge1, hollow, and bridge2 sites, (b) the total energy of MBenes on the optimal adsorption H* sites, (c) the ΔG _H* of MBenes. Reproduced from ref . CC BY-NC 3.0.

Zhang et al. also investigated the HER activities of a series of Cr-based MBenes (Cr _n+1B_2n (n = 1–3)) with different thickness. ΔG _H* for the different H atom adsorption sites were calculated. Hydrogen adsorption predominantly occurs at the top site of the Cr atom in the lower layer. DFT calculation of ΔG _H* with different H coverages showed that for Cr₂B₂, at high H coverage, the value of ΔG _H* was too negative. This indicates an excessively strong interaction between Cr atoms and H, which hinders the desorption of the adsorbed H* intermediates. In the case of Cr₃B₄, when the hydrogen coverage is 3/4 ML, ΔG _H* is reduced to 0.046 eV. For Cr₄B₆, the absolute ΔG _H* remains close to zero for all examined hydrogen coverages (e.g., 1/2, 3/4, and 1 ML). The enhanced HER catalytic activity for thicker Cr _n+1B_2n is due to the hydrogen adsorption becoming weaker with increasing the thickness of Cr _n+1B_2n , Therefore, the improved HER performance observed in thicker Cr _n+1B_2n can be attributed to the progressive weakening of hydrogen adsorption as the layer thickness increases. Bader charge analysis was conducted to study the interaction between Cr and H atoms. It was observed that at each H coverage, the thicker the Cr _n+1B_2n were, the fewer electrons H atoms gained from Cr atoms, corresponding to a weaker Cr–H interaction. Therefore, in Cr₄B₆, H atoms interact with Cr atoms more slightly than in Cr₂B₂ and Cr₃B₄. As a result, the hydrogen desorption was easier for Cr₄B₆ with a relatively less negative value. When fully covered with a monolayer of hydrogen, Cr₄B₆ exhibits exceptionally high HER catalytic activity. DFT calculation for the HER of three ortho-MBenes (Cr₂B₂, Cr₃B₄, and Cr₄B₆) was conducted by Zhang et al. The very low values of |ΔG _H*| (0.003–0.129 eV) for Cr₄B₆ indicated its highest HER activity among these three MBenes. Shukla et al., through DFT calculations, explored the HER activity of pristine Mo₂B with 1T and 2H Mo₂B monolayers having hexagonal crystal structures with space groups P3̅m1 and P6̅m2. They have considered three sites, TH (top of the hollow site), TB (top of the B atom), and TMo (top of the Mo atom) sites of pristine Mo₂B for H adsorption. DFT calculations showed H was more strongly interacting with the 1T phase than the 2H phase for Mo₂B, with an adsorption energy trend of TB < TH < TMo, and the computed ΔG _H* values indicated that these phases may not be suitable for practical hydrogen evolution reaction (HER) applications. Consequently, the pristine Mo₂B phase is deemed unsuitable for efficient hydrogen evolution reaction (HER) catalysis. In an effort to enhance the catalytic properties of Mo₂B, various 3d, 4d, and 5d transition metals (TM) have been explored as single-atom catalysts anchored on the Mo₂B substrate for efficient hydrogen evolution. Defective Mo₂B containing a single Mo vacancy was employed in the doping studies. Crystal orbital Hamilton population (COHP) and PDOS analyses were carried out to investigate the interactions between hydrogen and the TM centers in TM–Mo₂B. Comparison of the PDOS for 2H TM–Mo₂B (TM = Ir, Co, Ni, and Pt) with pristine 2H Mo₂B showed significant overlap between the TM d states and hydrogen orbitals, indicating strong interactions that enhance HER activity following TM doping. In conclusion, 1T Ni–Mo₂B emerged as the most promising HER catalyst among all TM-embedded MBenes. DFT calculations were performed by Li et al. on the single-atom supported Mo₂B₂O₂ and W₂B₂O₂ (Ti-, V-, Zn-, Cu-, Ag-, Au-, and Pt-single atom) HER catalysts. In these catalysts, owing to the charge transfer of a single-metal atom to the adjacent oxygen atom, the O–H bonds became weaker. The low ΔG _H (<0.2 eV) of single-atom-supported W₂B₂O₂ indicated the presence of more preferable adsorption sites than the single-atom-supported Mo₂B₂O₂.

By using DFT study, Zhang et al. investigated the HER activity of Cr _n+1B_2n . It showed that H atoms prefer to adsorb on the top site Cr atom of the lower layer. The calculated values of ΔG _H* for Cr₂B₂, Cr₃B₄ and Cr₄B₆ were −0.198, −0.178 and −0.078 eV, respectively, and Cr₄B₆ exhibited the superior HER performances. The Badar Charge analysis revealed that for each H coverage, the extent of gaining of electron gain by the H atom from the Cr atom reduces with increasing thickness of Cr _n+1B_2n . Hence, in Cr₄B₆, the interaction between H atoms with Cr is more than that of Cr₂B₂ and Cr₃B₄ and thus Cr₄B₆ showed easy H₂ desorption. Owing to the presence of a relatively lower number of valence electrons in B, it shows different covalent bonding with the different MBene lattices. To evaluate the HER activities of MBenes, their ΔG _H* values were calculated by the researchers. Liu et al. have reported the ΔG _H* values of Mn₂B₂, Mn₃B₄, Cr₂B₂, Fe₂B₂, and Mo₂B₂ with different H coverage. By DFT calculations, the work functions of these MBenes were calculated, which showed that the energy levels of the d-band center of Mn₂B₂ and Fe₂B₂ satisfied the target criteria and hence better HER electrocatalyst among these MBenes. The partially empty binding sites of Mn₂B₂ and Fe₂B₂ help in the adsorption and enhancement of HER. The presence of surface terminals (eg, –O, −OH) influences the electronic structure of MBenes, and thus affects the adsorption/desorption of H during HER.

By performing DFT calculations, Li et al. showed that when a TiB monolayer is fully covered with oxygen (TiB–O*), the surface O acts as a catalytically active site which favors H binding. ΔG _H* of this system was found to be close to zero. The Badar charge and DOS analysis were conducted on O–H interaction in TiB–O* with one hydrogen adsorption. The calculations revealed that prior to H-adsorption, 0.89 e was gained by O from TiB. After H adsorption, the bonded O gained an enhanced value of 1.21 e, where H contributed 0.7 e and Ti 0.49 e. PDOS showed the presence of a partially occupied O 2p and fully occupied H 1s around the Fermi level, because the orbital level interaction between bonding and antibonding states leads to a favorable H adsorption. The close to zero value of ΔG _H* and high exchange current density indicated the excellent HER activity of TiB with –O surface functional. By performing ab initio calculations, it was shown that as the ΔG _H* values of h-ZrBO, h-MoO, and h-Nb₂BO₂ are close to zero, and hence these MBenes could be excellent HER electrocatalysts. However, ΔG _H* of h-CrBO and h-VBO was less than 0.2 eV with a higher H* coverage, indicating their comparatively strong adsorption energy, which might limit the release of hydrogen. Development of a single-atom catalyst by embedding different transition metal atoms (eg, TM = I, V, Cr, Mn, Fe, C, Ni, and Cu) in the Mo vacancy (TM@Mo₂B₂) was reported by Zhang et al. The negative values of the binding energy of these catalysts confirmed their excellent stability. In these systems, charge accumulations occurred between TM and B atoms, showing the TM–B bond formation. ΔG _H* of Mo₂B₂ was 0.37 eV, whereas that of Ni@Mo₂B₂ was −0.09 and 0.12 eV for H coverage of 1/4 and 1/9, respectively. The lower ΔG _H* value of Ni@Mo₂B₂ indicated the superior HER activity of Ni@Mo₂O₂ to pure Mo₂B₂.

Chen et al. performed DFT calculations for understanding the mechanism of HER activity of the MoAl_1–x B sheet catalyst under alkaline medium. In this study, bulk MoAlB phase was compared with Al-rich MoAl_1–x B and Al-deficient MoAl_1–x B. For the Al-rich MoAl_1–x B catalyst, the MoAlB phase and −Al–O–Mo– structure acted as the substrate for charge transfer, and the active sites for water adsorption, activation, and deprotonation steps, respectively, during the HER process. A good charge transfer (value 6.54 e) was predicted at the interface between the MoAlB substrate and −Al–O–Mo– structure. The difference in the charge-density analysis showed that for the Al-rich MoAl_1–x B catalyst, the accumulation of electrons mainly occurred around the bridging oxygen atoms in −Al–O–Mo–, and this electron accumulation was higher than the bulk MoAlB and Al-deficient MoAl_1–x B. In the alkaline HER process, the active sites for the dissociated positive H* intermediates were these electron-rich oxygen atoms. ΔG _H* values of Al-rich MoAl_1–x B, bulk MoAlB phase, and Al-deficient MoAl_1–x B were −0.37, −0.54, and −0.41 eV, respectively.

The ΔG _H* value of −0.37 eV for Al-rich MoAl_1–x B suggested its superior HER performance among these three electrocatalysts. For the benchmarking catalyst Pt/C, ΔG _H* is found to be very close to zero. Although the Al-rich MoAl_1–x B exhibited a ΔG _H* value close to that of Pt/C, it showed a significantly higher overpotential of 130 mV at 10 mA cm^–2, compared to the much lower overpotential of ∼20–50 mV at 10 mA cm^–2 observed for Pt/C. This conflict between the results obtained from the experiment and predicted by DFT calculation could be for various reasons. In the synthesized MoAl_1–x B MBene, the presence of surface terminals, extent of Al etching, generation of stacking faults, and defects during etching of MAB phase, etc., severely affect its HER performances. However, these complex structural and chemical effects are largely beyond the reliable predictive capability of conventional DFT calculations, representing a major bottleneck in theoretical modeling.

4.2. Experimental Perspectives on Kinetics and HER Mechanism on MBene

In 2012, Hu’s group first demonstrated that commercially available polycrystalline molybdenum boride, consisting mainly of α-MoB with minor β-MoB, could serve as an efficient and low-cost HER electrocatalyst. Despite this, borides have received little attention until recently due to the synthetic challenges associated with boride-based materials. Commercial α-MoB is rarely phase-pure and is usually contaminated with β-MoB. Such minor impurities generally do not significantly affect HER activity and may even enhance it. Fokwa’s group later synthesized four binary bulk molybdenum borides (Mo₂B, α-MoB, β-MoB, and MoB₂) by arc melting and investigated their HER properties. All borides exhibited activity in acidic media, with a boron-content-dependent trend of Mo₂B < α-MoB < β-MoB < MoB₂. Since then, numerous nanostructured transition-metal borides, including Mo₃B, MoB₂, Mo₂B₄, Co₂B, NiB, VB₂, FeB₂, MoAlB, Co–Ni–B, and Co–Mo–B, have been reported for HER applications. Park et al. have synthesized different Mo-borides (e.g., Mo₂B, α-MoB, β-MoB, and MoB₂). The superior HER activity of MoB₂ and β-MoB was observed compared to Mo-rich Mo₂B. This demonstrated the strong boron-dependency of HER activities of Mo-borides. Alameda et al. performed HER in 0.5 M H₂SO₄ medium using a nonetched MoAlB single crystal and a partially etched MoAlB as electrocatalysts. Etched MoAlB exhibited superior HER catalytic performance with a lower overpotential of 301 mV at a current density of 10 mA cm^–2 than nonetched MoAlB single crystal (a overpotential of 400 mV at the same current density). The enhanced HER activities of partially etched MoAlB could be attributed to the exposure of the catalytically active planes during etching. During NaOH etching of MoAlB for 2 h, (0k0) stacking fault density (stacking-fault bands) formed ubiquitously at the crystal edges, although they were absent in the pristine material. Upon extending the etching time beyond 2 h, owing to the progressively deintercalated Al, these bands became continuous as from previously unperturbed regions of MoAlB, resulting in a concomitant increase in stacking-fault density. For nonetched, as-synthesized MoAlB, both basal planes and edge sites are exposed during HER; however, the edge sites exhibit lower catalytic activity than the basal planes. Etching exposes additional basal planes that were previously buried, such that etched MoAlB single crystals present more basal planes with enhanced active sites alongside edge sites. Consequently, the electrochemically active surface area (ECSA) of etched MoAlB is greater than that of the nonetched MoAlB crystal, which exposes only the top basal plane. Partial etching further generates minor fractures and defect sites that facilitate charge transport and interfacial electron transfer. The combined increase in accessible basal-plane active sites and improved charge-transfer kinetics synergistically enhances the HER activity. , Yang et al. reported that as a HER electrocatalyst, Ru–MoO_3‑x/Mo₂AlB₂ possessed better catalytic efficiency than MoAlB, Mo₂AlB₂, and MoO_3‑x /Mo₂AlB₂ in 0.5 M H₂SO₄. In this catalysis reaction, the Volmer-Heyrovsky step was reported as the dominant kinetic step. The Ru doping and presence of O vacancy created microregions in the catalyst. The multiple electron-rich O atoms within these microregions acted as the HER active sites. Because of the presence of these O atoms, the ΔG _H* of this catalyst becomes ∼0. Electrochemical impedance spectroscopy revealed a low charge transfer resistance of 3.5 Ω for Ru–MoO₃/Mo₂ALB₂, confirming the efficient charge transport due to strong metal–support interactions. Additionally, its double-layer capacitance reached 27 mF/cm², indicating a high density of active sites. Its electrochemical surface area was 675 cm², which is significantly higher than MoAlB (13.5 cm²), Mo₂AlB₂ (197.5 cm²), and MoO_3‑x /Mo₂AlB₂ (395 cm²). Ru–MoO_3‑x /Mo₂AlB₂ also exhibited an excellent mass activity­(146.6 A g Ru^1– at 100 mV), which suppressed the commercial Pt/C by 1.6 times. This catalyst maintained high stability, retaining over 95% of its activity after 100 h at various applied potentials. The HER activities of these materials are shown in Figure . Park et al. investigated the HER activity of Pt-incorporated MoAlB-based catalysts, reporting a remarkable performance improvement via structural modification. The HER activities of pristine MoAlB, defect-rich MoAl_1–x B, and Pt–MoAl_1–x B were measured in 1.0 M KOH and 0.5 M H₂SO₄. Pt–MoAl_1–x B showed the highest activity, with an overpotential of 32 mV (alkaline) and 18 mV (acidic) at 10 mA cm^–2. Theoretical calculations revealed more favorable ΔG _H* for H₂O adsorption, dissociation, and H₂ desorption, accelerating HER in both media. Tafel slopes for Pt–MoAl_1–x B in both alkaline and acidic media were close to those of Pt/C and significantly lower than those of Pt–Mo₂C, Pt–MoS₂, and MoAl_1–x B, and also suggested a Volmer–Heyrovsky mechanism. Its exchange current density in both media was comparable to Pt/C and markedly higher than that of Pt–Mo₂C, Pt–MoS₂, and MoAl_1–x B. EIS Nyquist plots showed reduced charge-transfer resistance for Pt–MoAl_1–x B. Bode/Nyquist analyses at varying potentials also revealed the reduced relaxation time and resistance for Pt–MoAl_1–x B, highlighting faster interfacial charge transfer and improved kinetics over Pt–Mo₂C and Pt–MoS₂. Jothi et al. reported the synthesis of nanocrystalline MoB₂ via a facile, low-temperature method and evaluated its HER activity in acidic media. Compared with commercial 5 wt % Pt/C, Mo metal, amorphous boron, and carbon sheets, MoB₂ exhibited superior activity, by showing a low overpotential of ∼154 mV at 10 mA cm^–2 vs RHE, which is one of the lowest reported for boride catalysts, including bulk MoB. The results highlight the advantages of nanoscaling of MoB. This improvement was attributed to its markedly high electrochemically active surface area, double-layer capacitance, which are larger than that of bulk MoB₂ and comparable to active carbides and phosphides. A Tafel slope of 49 mV dec^–1 indicated the prominence of the Volmer–Heyrovsky mechanism on HER, with faster kinetics. DFT calculations identified the B–B bridge site on the B-terminated {001} facet as the most active site, where the value of ΔG for hydrogen adsorption is near zero at 75–100% coverage. Helmer et al. reported that 2D Mo_4/3B_2‑x T _z , composed of abundant elements, shows promising HER activity in acidic media. , The HER activities of different MBenes have been summarized in Table .

7.

(a) HER polarization curves for Ru–MoO_3–x /Mo₂AlB₂, MoO_3–x /Mo₂AlB₂, Mo₂AlB₂, and MoAlB at a scan rate of 1 mV s^–1 in 0.5 M H₂SO₄. (b) Corresponding Tafel diagrams. (c) Nyquist plots.(d) C _dl values of different samples. (e) Comparison of various properties of the Ru–MoO_3–x /Mo₂AlB₂, MoO_3–x /Mo₂AlB₂, Mo₂AlB₂, and MoAlB catalysts. (f) Reported overpotentials versus Tafel slopes for different catalysts. (g) Mass activity curve of Ru–MoO_3–x /Mo₂AlB₂ and the mass activity value of Ru atom in Ru–MoO_3–x /Mo₂AlB₂ and Pt atom in Pt/C at 100 mV. (h) 100 h long-term i–t tests of Ru–MoO_3–x /Mo₂AlB₂ in 0.5 M H₂SO₄ electrolyte. (i) LSV curves and Tafel slopes of Ru–MoO_3–x /Mo₂AlB₂ before and after 100 h long-term test. Reproduced with permission from ref . Copyright 2025 Wiley-VCH.

5. Summary of MBenes-Based HER Electrocatalyst.

sr. no	electrocatalyst	electrolyte	η (mV) (10 mA cm–2)	ref
1	MoB2	0.5 M H2SO4	154
2	(Al–R) MoAl1–x B	1 M KOH	130
3	α-MoB2	0.5 M H2SO4	141
4	α-MoB2	1 M KOH	124
5	β-MoB2	0.5 M H2SO4	187
6	MoB microparticles	0.5 M H2SO4, 1 M KOH	215, 220
7	MoB2 microparticles	0.5 M H2SO4	250
8	MoB/g-C3N4	1 M KOH	150
9	Ni2B/g-C3N4	1 M KOH	707
10	nonetched MoAlB	0.5 M H2SO4	400
11	MoAlB etched	0.5 M H2SO4	301
12	MoAlB etched	1 M KOH	328
13	Ni–B	1 M HClO4	132

5. Other Applications of MBenes

Considerable global efforts have been devoted to investigating the diverse applications of MBenes across a wide range of advanced and emerging technologies. Despite the growing interest in these materials, comprehensive experimental insights into their intrinsic properties, environmental stability, and reliable synthesis protocols remain limited. Consequently, extensive theoretical investigations have been undertaken to elucidate their potential applications and predict their functional performance. In this context, significant attention has been directed toward applications such as energy storage and conversion, electrocatalysis, and biomedicine.

5.1. Energy Storage

The layered structure and low resistivity of MBenes make them promising candidates for electrochemical energy storage applications. Owing to the widespread applications of batteries (such as lithium-ion batteries (LIBs), Li-sulfur battery, Na/Mg ion batteries, etc.), considerable research efforts have been devoted to identifying suitable MBenes for use as electrode materials. Owing to their two-dimensional structure, MBenes exhibit low ion-diffusion barriers and high theoretical gravimetric capacities for Li⁺ ions, suggesting that they are promising candidates for negative electrode materials in lithium-ion batteries (LIBs). For instance, M₂B₂-type MBenes are predicted to possess capacities ranging from 265 to 969 mA·h·g^–1.

It has been reported that both orthorhombic and hexagonal Ti₂B₂ MBenes can accommodate only a single layer of Li atoms, whereas Cr₂B₂, Mn₂B₂, Fe₂B₂ orthorhombic MBenes, and Hf₂B₂ hexagonal MBenes can accommodate up to two layers of Li atoms. Remarkably, V₂B₂ orthorhombic and Y₂B₂ and Zr₂B₂ hexagonal MBenes can accommodate up to three layers of Li atoms without structural distortion, resulting in high theoretical gravimetric capacities. Several other MBenes, including V₂B₂ (orthorhombic), Cr₂B₂, Mo₂B₂, Mo₂B₂S₂, Mo₂B₂Se₂, and Mo₂B₂Te₂, have shown promising results as anode materials for sodium-ion batteries (NIBs).

The application of MBenes has been investigated to mitigate key challenges in Li–S batteries. One major issue that adversely affects Li–S battery performance is the shuttle effect. The deposition of nonconductive Li₂S₂/Li₂S species on the surfaces of both electrodes increases the internal resistance and polarization of the cell. Among various MBenes, Mo₂B₂O₂ and Mo₂B₂F₂ have been identified as promising candidates to address this issue. Hou et al. fabricated near-neutral Zn–air batteries (NNZABs) of both coin-type and pouch cell-type, in which Mo₄/₃B₂– _x T _z MBene with Y vacancies was used to prepare the cathodes. These vacancies preferentially adsorb O₂ and ZnO₂ species, thereby enhancing the formation and decomposition of ZnO₂ during the oxygen reduction and evolution reactions (ORR and OER). The NNZABs exhibited significantly improved rate capability and cycling stability. They operated under ambient air conditions with high reversibility for 380 h at a current density of 2 mA cm^–2 with 5 h cycling, and also performed well at higher current densities, demonstrating an appreciable lifespan.

Bay demonstrated the application of CVD-grown Mo₂B on a Cu substrate as electrodes for supercapacitors. A two-electrode test cell composed of stainless steel was assembled for electrochemical evaluation. The device exhibited a high specific capacitance of 1227.6 F g^–1, delivered a power density of 1704 W kg^–1, and retained 99% of its capacity after 20,000 cycles.

5.2. Electrocatalysis

The pivotal properties of a catalyst, including 2-D layered structure, exposed surface sites and excellent conductivity, are critical for its performance across a range of reactions, including the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), carbon dioxide reduction reaction (CO₂RR), nitrate reduction reaction (NO₃RR), nitric oxide reduction reaction (NORR), nitrogen reduction reaction (NRR), oxygen reduction reaction (ORR), etc. Several reviews have highlighted the potential of various MBenes for applications in electrocatalysis. ,,−

Since this review emphasizes the use of MBenes as HER electrocatalysts, the relevant background and developments have been addressed in detail in the preceding sections.

Zhu et al. employed density functional theory (DFT) calculations to investigate the catalytic activity of three experimentally accessible two-dimensional MBenes-Mo₂B₂, Ti₂B₂, and Cr₂B₂ for the simultaneous electrocatalytic coupling of N₂ and CO₂ to produce urea under ambient conditions. Among these, 2D Mo₂B₂ and Cr₂B₂ exhibited superior resistance to surface oxidation and self-corrosion under electrochemical conditions, highlighting their potential as promising electrocatalysts for urea synthesis.

By analyzing Gibbs free energies and electronic structures, Feng et al. attributed the catalytic activity of 2D h-MBenes to the energy redistribution of transition metal d_z2 orbitals during alloying, which enables efficient activation of N₂ and N-containing intermediates and breaks the scaling relationship. Using a volcano-shaped plot and the orbital averaging concept, they designed a series of bimetallic 2D borides with enhanced eNRR activity.

Yang et al. reported that FeB₂, RuB₂ and OsB₂ with surface B atoms as the active sites exhibit superior activity for N₂ fixation and reduction than those with metal active sites. Such excellency can be ascribed to the partially occupied p orbitals, advantageous adsorption properties, and large adaptability as electron reservoirs of the surface B atoms.

Through a theoretical study, Liu et al. investigated MBenes as CO₂RR catalysts and found that Mo₂B₂ and Cr₂B₂ display high selectivity, owing to their poor HER activity and low limiting potential for CO₂ reduction.

Xiao et al. provided theoretical insights to extend the application of 2D MBene systems as CO₂ electroreduction catalysts, thereby advancing the development of 2D materials for designing next-generation high-performance catalysts for CO₂RR. Among the studied systems, Au₂B and V₃B₄ were identified as promising candidates with high catalytic activity and selectivity for CH₄ formation, whereas Au₂B, Fe₂B₄, Ru₂B₄, and V₃B₄ exhibited potential for CH₃OH production. In addition, Fe₂B₄ and Ru₂B₄ were recognized as efficient catalysts for CO generation.

Chen et al. examined the OER performance of exfoliated MoAlB (EMAB). Etching-induced removal of Al-exposed Mo sites that adsorbed hydroxyl intermediates and optimized the reaction pathway, resulting in more favorable OER kinetics. The few-layer structure increased the active surface area, while a reduced Mo valence state improved structural stability. Consequently, EMAB demonstrated enhanced activity and durability toward OER.

Zhang et al., through first-principles calculations, explored the potential of TM@Mo₂B₂ as bifunctional electrocatalysts for the hydrogen evolution reaction/oxygen evolution reaction (HER/OER) and oxygen evolution reaction/oxygen reduction reaction (OER/ORR) processes. They predicted that the catalytic activity of TM@Mo₂B₂ can be tuned by embedding different transition metals. Among the investigated systems, Ni@Mo₂B₂ was identified as a promising bifunctional electrocatalyst for overall water splitting, exhibiting a low hydrogen adsorption free energy (|ΔG _H*| = −0.09 eV) under 1/4 H coverage for the HER and an overpotential of 0.52 V for the OER. In contrast, Cu@Mo₂B₂ was recognized as a potential OER/ORR bifunctional electrocatalyst, displaying low overpotentials of 0.31 and 0.34 V for the OER and ORR, respectively.

Zhang et al. conducted a systematic investigation of M₂B₂, M₃B₄, and M₄B₆-type MBenes (M = Cr, Mn, Fe, Co, and Ni) for HER, OER, and ORR electrocatalysis. Fe₃B₄, Co₃B₄, and Ni₂B₂ were found to resist O*/OH* poisoning of active sites, with Fe₃B₄ showing promise as a bifunctional HER/ORR electrocatalyst, and Co₃B₄ and Ni₂B₂ serving as trifunctional HER/OER/ORR catalysts.

Recently, in situ characterization techniques have been employed to investigate the electrocatalysis of MBenes. For example, in situ Raman spectroscopic analysis by Zheng et al. demonstrated that the structural integrity of Pt–MoAl₁– _x B is maintained during the HER. Zhang et al. monitored the electrocatalytic activity of FeB₂ for the nitrate reduction reaction to ammonia (NO₃RR) using operando Raman and Fourier transform infrared (FTIR) spectroscopy, together with quasi-operando electron paramagnetic resonance (EPR), and proposed the corresponding reaction mechanism. By employing in situ EPR spectroscopy, Liu et al. reported the photocatalytic performance of In₂S₃/Mo₄/₃B₂– _x T _z toward ^•O₂ ^– generation from real groundwater with a low dissolved oxygen (DO) concentration.

5.3. Biomedical Application

To harness the unique properties of MBenes, their applications in the advancement of biomedical technologies have been extensively explored in recent years. These studies highlight the versatile potential of MBenes in various fields, including biosensing, neurodegenerative disease treatment, near-infrared (NIR) photothermal applications, etc. The potential biomedical applications of MBenes have been comprehensively reviewed by Batool et al. and Iravani et al., underscoring their emerging significance in this field. ,

In the field of biosensing applications, their special features can be exploited to enhance the detection and quantification of biomolecules. The Cu nanocluster superlattice/MBene-based biosensor exhibited excellent selectivity, stability, and a low detection limit for clinical cancer detection, demonstrating high performance in actual tumor tissue analysis. ,

Taking advantage of the improved transducing surface of MBenes (e.g., MoB), Rhouati et al. explored their application in an aptasensor. An MBene/AuNPs/MWCNT-modified electrode functionalized with an aptamer specific to the hormone 11-deoxycortisol (11-DCL) exhibited excellent sensing performance, with a determination range of 0.01–100 pg mL^–1 and a limit of detection of 0.014 pg mL^–1 for 11-DCL. Density functional theory (DFT) calculations indicated that the 2D Mo₄/₃B₂ MBene-based platform possesses the potential to exhibit superior SERS activity, demonstrating enhanced sensitivity toward Raman signals. The MoB QDs/NPoM biosensor exhibited excellent potential for evaluating thyroid cancer progression and shows promise for future clinical diagnostic applications. The 2D ZrB nanosheets demonstrated outstanding NIR photothermal performance, achieving high photothermal conversion efficiency in the NIR-II region (1060 nm), thereby highlighting their promise as a multifunctional drug delivery platform. ,,

6. Challenges

The exploration of MBenes is still in its infant stage. A significant number of researchers are engaged in developing several members of MBenes and uncovering the applications of MBenes in myriad fields. However, the major challenges (Figure ) which are frequently encountered in establishing the credibility of MBenes in a plethora of applications can be categorized as (i) inadequacy of synthesis protocols, (ii) lack of standard characterization techniques, and (iii) scarcity of authentic data on the properties of various MBenes. Additionally, more accurate theoretical calculation techniques are also required.

8.

Pictorial representation of various applications and challenges based on MBenes.

Theoretical investigations predicted the possibility of a variety of MAB phases and MBenes, but in reality, so far, only a few MBenes have been experimentally synthesized. The preparation of the impurity-free MAB phases is one of the concerns, and also, quite often, the existing synthesis routes of MAB synthesis require high temperature (>1000 °C) processing. To date, etching the Al layer from MAB phases to obtain the corresponding MBenes by using either acidic (e.g., HF or LiF + HCl) or alkaline (aqueous solution of NaOH) etchants is the most popular protocol. However, the limitation associated with the acidic etching agent is that it sometimes adversely affects the layers of MBenes. As an example, during the synthesis of Mo₂B₂, corrosion by the acidic agent was noticed. On the other hand, the treatment of MoAlB by 10% NaOH solutions was not capable of producing fully etched Mo₂B₂. Furthermore, these methodologies are not environmentally friendly.

Moreover, owing to the van der Waals forces, the interlayer stacking between consecutive layers of the MBene introduces instability of its structure and deteriorates its properties. As the presence of impurity as well as the extent of formation of well-spaced layers in MBenes structures considerably affect their properties (e.g., electronic, electrical, electrochemical, mechanical, magnetic, etc.), the development of easy eco-benign preparation routes for the preparation of stable MBenes with precise composition and structure is crucial at this stage.

The availability of reliable instrumental techniques, which can precisely determine the compositions, structures, surface functional groups, etc., of MBenes is limited. Sometimes, the obtained results are contradictory and inclusive. For example, according to the theoretical predications, the layers of Cr₂B₂ were very much susceptible to being oxidized by OH, but EDS analysis did not detect the presence of OH or Cl on its surface. Moreover, techniques to identify and quantify the formation of defects and vacancies, which are created during the etching process, are inadequate. Hence, special attention needs to be paid to characterizing the structure of MBenes.

An in-depth knowledge of the electronic structures of MBenes is crucial to understanding their different properties, which is essential in predicting and assessing their efficacy in different applications. DFT calculations is one of the important tools to determine the electronic properties, band structures, magnetic properties, etc. of the materials. However, in many MBenes, the DFT calculations produce contradicting results, which could have been generated from different calculation methods. It is widely known that the electronic and magnetic properties of systems containing transition metals are closely related to the localized (strongly correlated) electrons in d and f orbitals, which cannot be well described with the GGA method. The most frequently used remedy is the DFT + U method, in which the value of U influences the total energy, electronic structures, and magnetic ground states significantly. In the DFT + U method. ABINIT, Vienna ab initio simulation package (VASP), QUANTUM ESPRESSO, etc., are generally used to calculate the value of U. However, the value of U is system-dependent rather than element-dependent. Hence, the value of U of other systems, such as MXenes or oxides, would not be appropriate for MBenes. The value of U is generally tested by fitting the experimentally obtained values of band gaps or magnetic moments. In the case of MBenes, the absence of adequate experimental data is a major challenge for obtaining reasonably accurate theoretical results. Though DFT calculations predicted several MAB phases and MBenes, but yet all of them have not been experimentally synthesized. For example, in the M₃AB₄ and M₄AB₆ compositions, only two phases are experimentally known, Cr₃AlB₄ and Cr₄AlB₆. Zhang et al discussed the theoretical studies of the magnetic properties of Orth-Cr₂B₂, Mn₂B₂ and Fe₂B₂. Ambiguous results were obtained when the magnetic ground states of these MBenes were predicted from the DFT calculations with varying U (Table ). Moreover, the presence of different types of surface terminals, which are dependent on the MBene synthesis route, makes the DFT calculations difficult and results in perplexing results.

6. Summary of the Magnetic Ground States of Cr₂B₂, Mn₂B₂ and Fe₂B₂ Orth-MBenes Using Different Calculation Methods .

systems	magnetic ground state	method	ref
Cr2B2	ferromagnetic	GGA + U (U = 4 eV)
	ferromagnetic	GGA + U (U = 4 eV)
	antiferromagnetic	GGA
Mn2B2	ferromagnetic	GGA + U (U = 2 eV)
	ferromagnetic	GGA
Fe2B2	ferromagnetic	GGA
	antiferromagnetic	GGA and HSE

7. Conclusion and Future Perspectives

This review presents a comprehensive overview of the advancements in an emerging class of two-dimensional (2D) materials known as MBenes, highlighting progress made through both theoretical and experimental studies. The fundamentals of the structures of MBenes, their synthesis methods, properties, and potential applications have been discussed here. A dedicated section of this review focuses on the application of MBenes as electrocatalysts in the hydrogen evolution reaction (HER) through water splitting, which is an increasingly vital technology in response to the global demand for clean and sustainable fuel sources.

MBenes embarks on a compelling journey, which is full of promise and excitement for future generation technologies. The distinctive layered structure and intriguing properties (such as high electrical conductivity, mechanical strength, presence of surface functional groups, abundant active sites on the surface, etc.) of MBenes have positioned it as a rising star in the realm of 2D materials. Owing to their wide-ranging potential applications, MBenes offer a plethora of opportunities for exploration, research, and innovation. However, it is crucial to recognize that the study of MBenes is still at an early stage of development. Intensive research is currently underway to uncover the potential applications of MBenes in various fields, including electrochemical energy storage systems, sensors, electromagnetic interference shielding, actuators, and a range of biomedical uses like antibacterial activity, antifouling surfaces, and drug delivery systems. The development of MBene-based electrocatalysts for applications such as the Hydrogen Evolution Reaction (HER), CO₂ reduction, nitrogen reduction reaction (NRR), and oxygen reduction reaction (ORR) has recently attracted significant attention from researchers, and considerable efforts are being devoted to unravelling the complex mechanistic details underlying these reactions.

MBenes have emerged as a promising active electrode material for advanced energy storage applications, including supercapacitors, batteries, and hybrid supercapbatteries. The electrochemical performance of MBenes can be significantly enhanced by forming MBenes-based nanocomposites. Nanocomposites exhibiting superior electrochemical performance can be engineered by integrating pseudocapacitive MBenes with electric double-layer capacitive (EDLC) materials such as activated carbon or porous carbon. These nanocomposites could be used to prepare electrodes for energy storage devices. This synergistic hybridization has led to its improved energy and power densities, superior rate performance, and enhanced cycling stability. Therefore, MBenes-based nanocomposites hold great potential as next-generation electrode materials for high-performance energy storage devices.

MBenes have demonstrated significant potential as efficient electrocatalysts for a variety of electrochemical reactions, including the hydrogen evolution reaction (HER). The catalytic performance of MBenes can be further enhanced by decorating their layers with suitable nanoparticles, which serve as highly active catalytic sites. Such MBenes-based catalysts can be effectively employed in the fabrication of electrodes for electrolyzers, enabling sustainable hydrogen (H₂) production through water-splitting reactions.

Owing to their exceptional electrical conductivity and tunable surface chemistry, MBenes can be explored for the development of highly efficient photovoltaic devices. Their excellent electrical conductivity, adjustable work function, and strong interfacial coupling capability make them ideal components for designing advanced heterojunction architectures. In particular, all-solid-state S-scheme heterojunctions can be constructed by incorporating MBenes at the interface between two semiconductors, where they act as efficient electron mediators. Such configurations promote effective charge separation and transfer, thereby enhancing the overall optoelectronic performance and power conversion efficiency of photovoltaic devices.

However, to realize the long-term potential of MBenes, researchers must address and overcome several associated challenges. There is a pressing need to develop cost-effective, environmentally friendly synthesis processes for producing high-quality MBenes with tailored structure, precise chemical composition, and consistent properties. Equally important is the advancement of reliable and accurate characterization techniques for MBenes. Emphasis should be directed toward the fabrication of MBene-based nanocomposites tailored for specific, targeted applications. This review is intended to encourage and stimulate further research into this budding class of materials.

For a review article, no primary research data are generated.

†.

Darshana Anand Upar and Sarifa Regina Fernandes have equally contributed. Darshana Anand Upar: conceptualization, visualization, writing and editing - original draft. Sarifa Regina Fernandes: conceptualization, visualization, writing and editing - original draft. Narendra Nath Ghosh: funding acquisition, resources, supervision, validation, writing - original draft. Darshana Anand Upar and Sarifa Regina Fernandes have equally contributed in this review article, and Narendra Nath Ghosh is the corresponding author.

The authors declare no competing financial interest.