Boron-driven energy technologies: borophene and its derivatives in supercapacitors

Abstract

Boron-based two-dimensional materials have emerged as promising candidates for high-performance supercapacitor electrodes owing to their intrinsic high conductivity, structural tunability, and excellent chemical stability. Within this family, borophene stands out for its metallic transport behavior, ultrahigh carrier mobility, pronounced anisotropic Dirac dispersion, and remarkable mechanical flexibility. Theoretical studies further reveal a significantly enhanced electronic density of states near the Fermi level, endowing borophene with high quantum capacitance (>1900 F g⁻¹), while experimentally fabricated CVD-grown borophene electrodes have demonstrated specific capacitances exceeding 350 F g⁻¹. In parallel, transition-metal borides and their derivatives, characterized by strong M–B covalent bonding, metallic conductivity, and multivalent redox activity, offer substantial pseudocapacitive contributions, with typical specific capacitances of 600–800 F g⁻¹ and competitive rate capability and cycling durability. Taken together, borophene and borides constitute a coherent “boron-driven” materials space that couples quantum electric-double-layer storage with faradaic charge transfer in a single compositional landscape, enabling simultaneous optimization of energy density, power density, and durability. This review systematically summarizes recent advances in the synthesis, structural engineering, electronic properties, and electrochemical performance of borophene and borides, establishing correlations between their intrinsic physicochemical characteristics and charge-storage mechanisms. Moreover, we critically examine the key challenges that remain, including the limited scalability of high-quality borophene synthesis, its susceptibility to oxidation, interfacial instability, compositional control in multimetal borides, and the still-incomplete understanding of underlying storage mechanisms. Finally, we outline future research directions including atomic-scale structural design, heterointerface engineering, multidimensional hybrid architectures and sustainable manufacturing strategies, which are expected to advance the practical deployment of boron-based materials in next-generation high-performance energy-storage systems.

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1. Introduction

The accelerating global energy transition, driven by the depletion of fossil fuels and the intensifying impacts of climate change, has compelled humanity to seek efficient, sustainable, and high-performance energy-storage systems.^1–6 Although renewable energy sources such as solar and wind power are abundant, their inherent intermittency and instability necessitate reliable storage technologies to ensure grid stability and continuous power supply. Consequently, developing storage devices that combine high power density, long cycle life, and environmental sustainability has become a central challenge in materials and energy science. Among various electrochemical energy-storage systems, supercapacitors (SCs) have attracted extensive attention owing to their rapid charge–discharge capability, high power density, and long-term cycling stability.^6–11 These devices effectively bridge the performance gap between conventional capacitors and rechargeable batteries. Nevertheless, the low energy density of commercial SCs remains a key limitation, hindering their large-scale implementation in smart grids and next-generation electronic technologies. To address this issue, the development of novel electrode materials with high intrinsic conductivity, large specific surface area, tunable pore structures, and abundant electroactive sites is imperative to achieve efficient and stable charge storage and transport. Against this backdrop, two-dimensional (2D) nanomaterials have emerged as a transformative class of functional materials for improving energy-storage efficiency.^12–16 Since the successful isolation of graphene, 2D materials have revolutionized our understanding of nanoscale physics and electrochemical energy conversion.¹⁷ Their atomic-scale thickness, high surface-to-volume ratio, and unique electronic configurations provide unprecedented opportunities for optimizing charge transport and ion accessibility. Over the past decade, the 2D materials landscape has expanded from graphene to transition-metal dichalcogenides (TMDs), layered double hydroxides (LDHs), and MXenes, among others.^18–24 Despite their excellent conductivity, chemical stability, and pseudocapacitive behavior, these systems still face intrinsic challenges. Graphene suffers from low electronic density of states (DOS) near the Fermi level, resulting in limited quantum capacitance, whereas MXenes are prone to surface oxidation and exhibit poor environmental stability.²⁵ These issues have motivated the search for new 2D materials that simultaneously combine high conductivity, structural stability, chemical versatility, and environmental sustainability.

Boron-based 2D materials, particularly borophene and metal borides (MBs, including their 2D derivatives known as MBenes), have rapidly gained prominence as next-generation electrode candidates.^26,27 Borophene refers to atomically thin 2D boron sheets. Different from graphene and most van der Waals layered 2D solids, borophene does not originate from a naturally layered bulk parent phase and is instead stabilized by boron's electron-deficient, multicenter bonding, which enables a rich polymorphic landscape and strongly anisotropic atomic arrangements.^26–33 As a result, borophene is often regarded as the boron analogue of graphene, yet it differs fundamentally in lattice symmetry, bonding, and electronic structure: while graphene features an isotropic honeycomb network with a low DOS near the Fermi level, borophene commonly exhibits metallic behavior with vacancy-modulated polymorphs (e.g., β₁₂, χ₃, 2-Pmmn, α′, and δ₆), leading to pronounced anisotropy and tunable electronic properties.^25,31–33 In comparison with semiconducting TMDs or other layered 2D materials where charge storage is frequently limited by conductivity and interlayer transport, borophene offers inherently high carrier mobility and surface-accessible active sites, relative to MXenes that rely on surface terminations but can suffer from environmental oxidation. Borophene provides an alternative 2D platform where electronic structure and surface chemistry can be engineered through vacancy regulation, functionalization, and heterostructure construction.^{18–25,31–33} Boron's electron deficiency enables extensive multicenter bonding and gives rise to diverse 2D frameworks with tunable properties.^28–30 Borophene, the monolayer allotrope of boron, is regarded as the boron analogue of graphene, yet it displays fundamentally distinct features.³¹ Unlike graphene's isotropic honeycomb lattice, borophene exhibits various polymorphs such as β₁₂, χ₃, 2-Pmmn, α′, and δ₆ whose stability depends on the density of hexagonal vacancies within the lattice.^32,33 Such structural diversity leads to pronounced anisotropy, excellent mechanical flexibility, and metallic conductivity. Since its pioneer synthesis on copper substrates via chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) in 2015, borophene has drawn enormous interest for its Dirac cone-like electronic dispersion, high Fermi velocity (∼6.6 × 10⁵ m s⁻¹), and strong electron–phonon coupling.²⁶ Recent advances by Tai's group have further expanded the controlled synthesis, structural stabilization, and energy-storage applications of borophene. In 2020, the team successfully synthesized crystalline, semiconducting hydrogenated borophene via a metal-substrate-free thermal decomposition-hydrogenation strategy.³⁴ The obtained hydrogenated borophene exhibited remarkable chemical stability, maintaining its integrity even under ambient conditions and in strong acidic or alkaline environments.³⁴ This achievement overcame the long-standing limitations of pristine borophene, including its tendency toward rapid oxidation and poor environmental durability, thereby providing a robust material foundation for its integration into energy devices. Building upon this foundation, the group later demonstrated stacked hydrogenated borophene synthesized via an in situ molten-salt co-synthesis route as a high-performance electrode for supercapacitors.³⁵ The stacked architecture significantly enhanced ion transport, charge accumulation, interfacial stability, and conductive-network continuity. These findings established a critical foundation for the practical implementation of 2D borophene materials in high-power-density energy-storage systems. Theoretical predictions further suggest that borophene exhibits superconductivity with a critical temperature (T_n) of 10–20 K and a quantum capacitance exceeding 1900 F g⁻¹, far surpassing that of graphene.^36–41 These unique characteristics position borophene as a promising material platform for energy storage, catalysis, and nanoelectronics. In parallel with borophene, the development of MBs has attracted increasing attention. MBenes are considered the boride counterparts of MXenes, derived from layered ternary MAB phases through selective etching of the A-layer (typically Al or Ga), leaving behind alternating metal (M) and boron (B) layers.⁴² The resulting MBenes inherit the metallic nature and structural robustness of their MAB parent phases, while their strong M–B covalency yields higher DOS at the Fermi level. Compared with carbides or nitrides, borides exhibit superior oxidation resistance, higher thermal stability, and wider electrochemical windows. These advantages enable MBenes to serve as highly conductive and stable electrodes that integrate electric double-layer capacitance (EDLC) and pseudocapacitive behavior. Transition-metal borides such as TiB₂, MoB₂, and Cr₂B₂ have been reported to possess multiple redox-active metal centers and robust layered frameworks, facilitating fast ion diffusion and reversible redox reactions, features highly desirable for high-performance supercapacitors.^43,44

The outstanding energy-storage performance of borophene and MB systems originates primarily from their dual charge-storage mechanisms. Both the delocalized π-electrons and the high DOS near the Fermi level in borophene contribute to strong quantum-capacitance effects, substantially enhancing the EDLC performance, while the abundant B–B and B–M bonds provide reversible redox-active sites that introduce pseudocapacitive contributions.^45–47 The synergy between quantum and faradaic processes endows boron-based materials with high fast-charging capability and high specific capacitance. For instance, first-principles calculations predict that the β₁₂ phase of borophene can achieve a specific capacitance of 1900–2000 F g⁻¹,⁴⁸ while experimental studies on CVD-grown borophene electrodes have confirmed values exceeding 350 F g⁻¹.⁴⁹ In MB systems, strong M–B bonding promotes rapid electron transport, and transition-metal sites provide reversible redox reactions, yielding specific capacitances of 600–800 F g⁻¹ with excellent rate capability and long-term cycling stability.⁵⁰ Collectively, these findings demonstrate that boron-based 2D materials can effectively bridge quantum and pseudocapacitive storage mechanisms, achieving simultaneous enhancement in both energy and power densities. Despite their theoretical and experimental promise, practical implementation of boron-based materials still faces significant challenges. First, borophene is extremely sensitive to oxidation in air, leading to rapid deterioration of its electrical conductivity. Second, synthesizing large-area, high-quality borophene remains difficult because existing MBE and high-temperature CVD processes are costly, energy-intensive, and substrate-dependent. Similarly, precise control of stoichiometry and surface chemistry in MBenes remains technically demanding. Overcoming these issues requires innovations in synthetic methodology, interface engineering, and surface passivation strategies to ensure stability and reproducibility.

This review aims to provide a systematic and comprehensive overview of the current research progress in boron-based supercapacitors, with emphasis on synthesis strategies, tunable properties, electrochemical mechanisms, and sustainable energy applications (Fig. 1). Here, we define “boron-driven energy storage” as a mechanism-oriented materials framework in which the unique electronic structure and bonding characteristics of boron play an essential role in electrochemical charge storage. Owing to boron's electron-deficient nature and multicenter bonding, boron-based materials can exhibit a high density of electronic states near the Fermi level, which is fundamentally favorable for interfacial charge accumulation and fast electron transport. This framework is conceptually distinct from conventional carbon-based electric double-layer capacitors, where the limited density of states near the Dirac point may impose a quantum-capacitance constraint, as well as from MXene- or transition-metal-oxide-based systems, which rely predominantly on surface terminations or redox activity but often face challenges related to environmental stability or electronic conductivity. In contrast, boron-derived materials constitute a boron-based material system extending from metal borides to borophene and Mbenes. Within this system, borophene predominantly supports surface-dominated capacitive charging facilitated by its high electronic density of states, whereas metal borides and MBenes more commonly contribute to metal-centered pseudocapacitive processes. This compositional continuity provides a boron-based platform for integrating capacitive and faradaic charge-storage mechanisms.

Fig. 1 Schematic showing the topics discussed in this review.

The discussion begins with the structural and electronic fundamentals underpinning their superior performance, followed by an examination of major synthesis approaches including top-down exfoliation, bottom-up vapor deposition, and chemical transformation methods. Subsequently, we summarize the electrochemical performance of these systems within different device configurations and elucidate the underlying structure property correlations. Finally, we discuss key challenges such as air stability, large-scale manufacturability, and environmental impact and propose potential strategies and future directions to accelerate the translation of boron-based materials into practical, high-performance and sustainable energy-storage systems.

2. Structural and electronic fundamentals of borophene and MBs

2.1. Structural and electronic configurations of borophene

Borophene is among the most structurally diverse and electronically complex members of the 2D materials family. Its pronounced polymorphism arises from the intrinsic electron deficiency of boron, which compels the formation of multicenter bonds to achieve structural stability.^27,31 Unlike graphene's ideal honeycomb lattice, dominated by conventional two-center two-electron (2c–2e) σ bonds, borophene accommodates three-center two-electron (3c–2e), and four-center two-electron (4c–2e) bonds.^27,31,51 This unconventional bonding topology endows borophene with notable structural adaptability, enabling a wide range of geometries from planar to buckled configurations—with tunable hexagonal vacancy densities (η), defined as the ratio of hexagonal holes to the total number of boron atoms in the lattice.

The boron atom [(He) 2 s² 2p¹], positioned between nonmetallic carbon and metallic beryllium in the periodic table, exhibits remarkable bonding diversity, allowing the formation of complex allotropes with distinct physical and chemical properties.⁵² At present, research on borophene structures has evolved from single-layer configurations to chemically stable hydrogenated borophene, and further to bilayer borophene, which combines both structural stability and functional versatility. According to composition and bonding motifs, single-layer borophene structures can be classified as distorted hexagonal (DH), buckled triangular (BT), and mixed triangular-hexagonal (MTH) types. Based on coordination number (CN), they can be further divided into α-type (CN = 5, 6), β-type (CN = 4, 5, 6), χ-type (CN = 4, 5), δ-type (CN = m, where m is a single number), and ψ-type (CN = 3, 4, 5).⁵³ The graphene-like honeycomb borophene with CN = 3 is referred to as δ₃, while the fully triangular configuration with CN = 6 is denoted as δ₆. Studies have shown that δ₆ borophene exhibits a buckled morphology due to σ–π orbital hybridization, whereas δ₃, δ₄, and δ₅ tend to remain planar. Representative low-energy borophene structures (δ, χ, α, and β types) are illustrated in Fig. 2a–d. Among them, α₁ and β₁ monolayers, with η = 1/8, possess high cohesive energies and exhibit excellent thermodynamic stability. Several borophene polymorphs have been experimentally and theoretically confirmed, including the β₃, χ₃, α′, and δ₆ phases. The β₃ phase (η = 1/6) consists of parallel rows of hexagonal vacancies embedded within a triangular boron network, while the χ₃ phase (η = 1/5) exhibits a denser vacancy arrangement, resulting in slightly higher flexibility but lower in-plane stiffness.²⁸ The δ₆ phase, predicted to be energetically competitive, features a periodically buckled layer composed of alternating ridges and furrows of boron atoms. Its anisotropic lattice constants (a ≠ b) give rise to direction-dependent mechanical and electronic properties, which is an intrinsic anisotropy absent in graphene. The δ₆ phase also displays metallic behavior and strong electron delocalization, making it an anisotropic conductor with unique transport characteristics.

Fig. 2 Surface views of different low-energy structures of boron monolayer sheets, recent hydrogenated borophene and bilayer borophene. (a) δ-, (b) χ-, (c) α-, and (d) β-type. Red and yellow balls denote boron atoms causing buckled boron sheets due to their outward or inward movement with respect to the plane. Reproduced from ref. 28 with permission from American Chemical Society, copyright 2016.²⁸ (e) Top and side views of the atomic structure of hydrogenated borophene. Reproduced from ref. 34 with permission from Wiley-VCH Verlag GmbH & Co. KGaA, Copyright 2020.³⁴ (f) Top view and side view of the calculated atomic structure of bilayer borophene. The detailed atomic structures of the first and second layers of bilayer borophene from the top view. Reproduced from ref. 122 with permission from Springer Nature Ltd, copyright 2022.¹²² (g) Bilayer α-borophene structures in plan view (top) and cross-sectional view (bottom). Reproduced from ref. 183 with permission under the terms of the Creative Commons Attribution License.¹⁸³

In addition, hydrogenated borophene (α′-4H-borophene) is a crystalline semiconducting form of borophene synthesized under metal-free conditions through a three-step thermal decomposition of NaBH₄. Its structural characteristics are determined by the α′-phase boron framework terminated with four hydrogen atoms. Both experimental observations and density functional theory (DFT) calculations reveal that hydrogenated borophene adopts a slightly buckled, nonplanar configuration with a stable B₈H₄ lattice (vacancy concentration of 1/9). The lattice forms a BH network stabilized by three-center two-electron (3c–2e) bonds, which markedly enhances its air and chemical stability (Fig. 2e). This material exhibits an optical bandgap in the range of 2.04–2.48 eV, indicating its semiconducting nature, and maintains structural integrity even under strong acidic and alkaline environments, making it one of the most chemically stable configurations within the borophene family.³⁴

Recently, bilayer borophene has been recognized as a promising structure that combines chemical robustness with functional versatility, offering potential applications in electronics and energy storage. The β₁₂-phase bilayer borophene, grown on Cu(111) via MBE, represents the first experimentally confirmed borophene system stabilized by interlayer covalent bonding. Structurally, it consists of two β₁₂ borophene layers featuring hexagonal vacancy patterns with vacancy concentrations of 1/6 in the top layer and 5/36 in the bottom layer, as shown in Fig. 2f. The two layers are strongly coupled through interlayer B–B covalent bonds, forming a short interlayer spacing of 1.82 Å and a slip-stacked arrangement. This configuration significantly enhances structural stability and mitigates the surface oxidation that typically affects monolayer borophene. The bilayer exhibits an energy 55 meV·atom⁻¹ lower than that of freestanding β₁₂ monolayer borophene, indicating superior thermodynamic stability. Unlike the complete oxidation observed in single-layer borophene, the bilayer structure experiences oxidation of only about 23% of surface atoms upon air exposure, while the underlying lattice remains intact, demonstrating its strong resistance to oxidation.¹²² Furthermore, the α-phase bilayer borophene has been successfully synthesized on an Ag(111) substrate via vapor deposition. It is composed of two identical v_1/9-type monolayers stacked in an AA configuration, interconnected by vertical interlayer B–B bonds to form a three-dimensional coupled lattice, representing another prototypical “non-van der Waals” bilayer borophene structure (Fig. 2g). DFT simulations further indicate that this bilayer exhibits higher oxygen adsorption energy than the monolayer counterpart, and the interlayer charge redistribution facilitates oxygen diffusion, thereby enhancing its overall oxidation resistance.¹⁸³

The structural anisotropy of borophene directly governs its macroscopic physical properties. Along the armchair direction, the periodic ridges modulate electron transport, while along the zigzag direction, delocalized π-electron networks ensure high carrier mobility.^54–57In situ scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) measurements have confirmed this directional dependence, revealing strongly anisotropic Fermi surfaces and work function variations between crystallographic directions. This distinct atomic-scale geometry not only facilitates high electrical conductivity but also provides abundant ion adsorption sites, rendering borophene particularly promising for electrochemical energy storage applications where surface accessibility is critical.

The electronic properties of borophene originate from its vacancy-modulated lattice and multicenter bonding configuration.^31,58 First-principles calculations indicate that most borophene polymorphs exhibit metallic character, with partially filled pz bands crossing the Fermi level.^27,58 Dirac cones near the Fermi energy—analogous to those in graphene—give rise to massless charge carriers (Dirac fermions) with ultrahigh mobility.^27,59 However, unlike graphene, where Dirac cones arise solely from pz orbitals, those in borophene result from the combined contribution of px, py, and pz orbitals, leading to anisotropic band dispersion and direction-dependent effective masses.^27,60 The high density of states (DOS) near the Fermi level, combined with linear band dispersion, endows borophene with quantum capacitance (C_q). Because C_q is proportional to DOS(E_F) (C_q = e²·DOS(E_F)), the partially filled metallic bands of borophene yield capacitance values significantly higher than those of graphene.²⁸ Theoretical predictions estimate C_q values of approximately 1930 and 1850 F g⁻¹ for the β₁₂ and χ₃ phases, respectively. This enhanced quantum capacitance directly improves its charge-storage capacity, particularly in electric double-layer capacitors (EDLCs), where total capacitance is determined by the series combination of C_q and the electrochemical double-layer capacitance (C_dl), thereby enabling superior electrochemical performance. In addition to its quantum capacitance, borophene exhibits strong electron–phonon coupling, conferring potential superconductivity and high thermal conductivity. Calculations suggest that the superconducting transition temperature (T_c) ranges between 10 and 20 K, depending on the concentration of hexagonal vacancies. The electron–phonon interaction is primarily mediated by out-of-plane acoustic phonons, which couple efficiently with delocalized π electrons.²⁸ The anisotropic phonon dispersion further results in direction-dependent thermal conductivity, with reported values of approximately 147 W m⁻¹ K⁻¹ along the armchair direction and 76 W m⁻¹ K⁻¹ along the zigzag direction. Such coupled thermal-electronic transport behavior helps maintain charge equilibrium and mitigates Joule heating during high-rate cycling, ensuring operational stability in high-performance energy-storage systems.

Collectively, borophene's unconventional multicenter bonding, tunable vacancy architecture, and pronounced anisotropy make it a multifunctional 2D material combining metallic conductivity, electrochemical activity, and high thermal transport capability offering a new structural and electronic paradigm for the design of high-energy and high-power density electrochemical devices.

2.2. Structural and electronic configurations of MBs

Metal borides and their 2D derivatives, MBenes, constitute an emerging class of materials that uniquely integrate metallic conductivity, structural robustness, and electrochemical activity. Although structurally distinct from borophene, MBs share profound similarities in bonding characteristics and electron delocalization mechanisms. Conventional metal borides typically consist of alternating metal and boron layers, forming either layered or three-dimensional frameworks. The MAB phases, precursors of Mbenes, crystallize in hexagonal or orthorhombic lattices analogous to MAX phases, except that boron atoms occupy the X sites. Their general formula, M_n+1AB_n, features early transition metals (e.g., Ti, Mo, Cr) as M, Group 13 to 16 elements (e.g., Al, Ga, Si) as A, and boron atoms forming covalently bonded layers that stabilize the lattice framework.^61,62

Since the discovery of the i-MAX phase and its derived MXenes, significant theoretical and experimental efforts have been devoted to expanding the chemistry and functionality of MBs (Fig. 3a–c and f–h). Alloying has been demonstrated to be an effective strategy for expanding the chemical composition of materials with MAX-phase structures.^63–65 Among them, the i-MAX phase, characterized by in-plane chemical ordering, serves as a representative example. A remarkable feature of this structure is that two-dimensional MXenes obtained through different etching techniques can exhibit in-plane chemical or vacancy ordering, thereby showing great potential for applications in catalysis and energy storage.^66–73

Fig. 3 The representative historical timeline of MBs. (a) Magnification image of the cavity containing the MoB sheets with idealized structure of the delaminated region of the MBene sheets. Reproduced from ref. 78 with permission from American Chemical Society, copyright 2018.⁷⁸ (b) Microstructure of 2D CrB nanosheets prepared by etching for 8 h in dilute HCl solution. Reproduced from ref. 79 with permission from Elsevier Ltd, copyright 2019.⁷⁹ (c) Crystal structure of RT-LiNiB. Reproduced from ref. 80 with permission from American Chemical Society, copyright 2021.⁸⁰ (d) Chemical ordering upon metal alloying of M₂AlB (M from groups 3 to 9) in orthorhombic and hexagonal symmetry with first principles study. 15 stable novel phases with in-plane chemical ordering are identified, coined i-MAB. Reproduced from ref. 74 with permission under the terms of the Creative Commons Attribution License.⁷⁴ (e) The single-layer 2D molybdenum boride sheets with ordered metal vacancies, Mo_4/3B_2−xT_z (where T_z is fluorine, oxygen, or hydroxide surface terminations). Reproduced from ref. 75 with permission from American Association for the Advancement of Science, copyright 2021.⁷⁵ (f) The synthesis of MoB nanosheets was experimentally achieved using a molten-salt etching method. Reproduced from ref. 184 with permission under the terms of the Creative Commons Attribution License.¹⁸⁴ (g) The first calculated charge density differences of Mo₂B₂ with one Li atom adsorbed. Reproduced from ref. 78 with permission from American Chemical Society, copyright 2018.⁷⁸ (h) Crystal structure of stable boron-containing ternary phase Ti₂InB. Reproduced from ref. 81 with permission under the terms of the Creative Commons Attribution 4.0 International License.⁸¹ (i) The synthesis of Mo₂AlB from MAB phase MoAlB by treatment with LiF/HCl. Reproduced from ref. 82 with permission under the terms of the Creative Commons Attribution 4.0 International License.⁸² (j) Top view of the Na diffusion path for V₂B₂ indicated by the black dotted arrows. Reproduced from ref. 76 with permission under the terms of the Creative Commons Attribution 4.0 International License.⁷⁶ (k) 2D MoB from the reaction between MoAlB and NaOH with a fluorine-free hydrothermal-assisted alkane solution etching method. Reproduced from ref. 77 with permission from Elsevier Ltd, copyright 2022.⁷⁷ (l) Different phases of Mo_xB_x were obtained through wet chemical exfoliation using NaOH/LiF–HCl, confirming the electrochemical advantages of the Immm and Cmcm phases in flexible all-solid-state supercapacitors. Reproduced from ref. 185 with permission under the terms of the Creative Commons Attribution 4.0 International License.¹⁸⁵

Inspired by the discovery of the i-MAX phase, Martin et al.⁷⁴ theoretically predicted fifteen novel MAB phases with in-plane chemical ordering, termed i-MAB phases (Fig. 3d), revealing that alloying is also an effective pathway to expand the MAB family. Subsequently, Zhou et al.⁷⁵ successfully synthesized Mo_4/3B_2−xT_z (Fig. 3e) by selectively etching a three-dimensional i-MAB phase in aqueous hydrofluoric acid (HF). Wei et al.⁷⁶ explored the feasibility of two-dimensional hexagonal V₂B₂ (Fig. 3i) as a promising anode material for sodium-ion batteries. More recently, Xiong and co-workers synthesized 2D MoB using MoAlB as the precursor via a fluorine-free hydrothermal-assisted alkane etching process (Fig. 3j) and systematically evaluated its electrochemical performance as an anode material for lithium-ion batteries.⁷⁷ Recently, the synthesis of MBs has been further optimized. A molten-salt etching process using ZnCl₂ was, for the first time, demonstrated to progressively extract Al atoms from MoAlB at relatively low temperatures (≤600 °C), leading initially to the formation of a stable Mo₂AlB₂ intermediate phase. Upon further heating to approximately 700 °C, this intermediate was completely converted into high-purity two-dimensional MoB nanosheets (Fig. 3f). Compared with conventional acid etching, this approach offers higher efficiency and improved operational safety.¹⁸⁴ Moreover, MoB nanosheets with high crystallinity and distinct structural phases were successfully synthesized via a NaOH and LiF–HCl chemical exfoliation strategy (Fig. 3l). When applied as electrodes in all-solid-state flexible micro-supercapacitors, these nanosheets delivered an areal capacitance of 20.3 mF cm⁻² and exhibited 92% capacitance retention after 1300 charge–discharge cycles. Furthermore, theoretical calculations confirmed their intrinsic metallic conductivity and structural robustness.¹⁸⁵

Initially, the accessible chemical space of MAB phases was primarily limited to combinations such as M = Fe, Mn, Mo, Cr, W, and A = Al. However, with the advancement of synthetic chemistry and phase engineering, researchers have successfully expanded the MAB family to include diverse compositions containing Hf, Ni, Zn, and In.⁸¹ To date, several orthorhombic MBs have been experimentally identified, while their hexagonal counterparts (h-MBs) remain relatively rare. As illustrated in Fig. 4a and b, six distinct orthorhombic MAB (ortho-MAB) structures, including M₂AB₂ (212), M₂A₂B₂ (222), M₃A₂B₂ (322), M₄AB₄ (414), M₃AB₄ (314), and M₄AB₆ (416) have been reported. These structures consist of alternating stacks of transition-metal boride layers and A (or M₂A) layers, forming an ordered lamellar framework. Theoretical calculations predict that more than fourteen ortho-MAB phases are potentially synthesizable, among which four have been experimentally confirmed as thermodynamically stable, while the remaining ten hexagonal MAB (h-MAB) phases are considered metastable.^83–93

In 2018, Bo et al. used molecular dynamics simulations to predict a series of hexagonal diborides, including Ti₂B₂, Sc₂B₂, V₂B, Y₂B₂, Cr₂B₂, MoB₂, and Zr₂B₂. Subsequently, Wang et al. achieved the first successful synthesis of a hexagonal MAB phase, Ti₂InB₂ (space group P6m2).⁸¹ Further theoretical studies have proposed the possible existence of two MAB₄-type and five M₂AB₂-type hexagonal MAB phases (shown in Fig. 4c). These continuing developments have not only expanded the compositional diversity of MAB and MB systems but also provided valuable theoretical and experimental foundations for their functional design in energy storage and electronic devices.

Fig. 4 Orthorhombic and hexagonal structures of MAB phases. (a and b) Known orthorhombic (ortho) and (c) hexagonal (h) structures of MAB phases. Reproduced from ref. 81 with permission under the terms of the Creative Commons Attribution 4.0 International License.⁸¹

The bonding framework of these boride systems is governed by the mixed covalent and metallic character of M–B interactions. Strong B–B intralayer covalent bonding imparts high mechanical strength and thermal stability, whereas metallic M–M interactions ensure high electrical conductivity. DFT calculations on representative TiB₂, MoB₂, and CrB₂ systems reveal broad d–p hybrid bands spanning −3 to +3 eV, yielding a high DOS at the Fermi level, a hallmark of metallic behavior.^94,95 This p–d hybridization not only enhances electron delocalization but also introduces multivalent redox-active centers capable of reversible faradaic reactions, a key contributor to pseudocapacitive energy storage.

Through selective chemical etching, typically using acid or molten-salt routes, the A layer in MAB compounds can be removed to yield MBenes, two-dimensional boron transition metal sheets (M_n+1B_n−x). This top-down transformation not only exposes abundant active sites but also introduces surface terminations such as –O, –OH, and –F, enabling precise modulation of surface hydrophilicity and electronic structure.⁶¹ The –O and –OH groups generally induce p-type doping and increase the work function, while –F terminations improve structural stability albeit with a minor reduction in conductivity. Moreover, surface vacancies and defects promote electron localization and facilitate ion adsorption, accelerating faradaic reaction kinetics. Distinct from MXenes, MBenes exhibit stronger p–d orbital hybridization between boron and transition-metal atoms, which significantly elevates the DOS near the Fermi level and enhances quantum capacitance.^96–98 The B–B network within the basal plane serves as an efficient electronic transport channel, giving rise to conductivities as high as 10⁵–10⁶ S m⁻¹ comparable to, or exceeding, those of most MXenes. In particular, TiB₂-derived MBs exhibit remarkable cycling durability owing to their rigid covalent M–B backbone, which resists structural degradation during repeated ion intercalation/deintercalation processes.^97–99 The synergy between metallic conductivity and structural resilience positions MBs as ideal electrode materials that combine EDLC with pseudocapacitive behavior for high-performance energy storage.

Overall, the MAB-MB-MBene material family establishes a multiscale material framework that bridges three-dimensional and two-dimensional regimes. Their p–d hybridization driven electronic structures, tunable surface chemistry, and mechanically robust architectures collectively endow them with high conductivity, chemical stability, and superior charge-storage capability. These attributes make boron-based materials a compelling platform for next-generation electrochemical energy devices, uniting high energy density, power density, and long-term durability within a single material design paradigm.

2.3. Fundamental physicochemical mechanisms underpinning electrochemical performance

The remarkable electrochemical performance of borophene and MBs arises from three fundamental physicochemical principles: high charge-carrier mobility, a high density of surface-active sites, and the synergistic yet non-identical coupling between quantum and faradaic charge-storage processes. These macroscopic electrochemical advantages ultimately originate from their intrinsic electronic structures and bonding characteristics, while the relative contribution of each mechanism differs between borophene and MBs, as widely recognized in recent studies on boron-derived electrodes.

In borophene, the linear band dispersion and metallic DOS allow ultrafast electron transport across the 2D plane, minimizing internal resistance. The β₁₂-borophene phases are taken as examples, and their energy band diagrams are shown in Fig. 5a and b, respectively.^100,101 The high DOS near the Fermi level favors a large quantum capacitance, which promotes rapid interfacial charge accumulation and surface-dominated capacitive charging, a feature repeatedly emphasized in theoretical and device-level studies of borophene-based supercapacitors.^{28,49,100,101} Vacancies, step edges, and functional groups in borophene provide accessible adsorption sites for electrolyte ions, enhancing electric double-layer formation.

Fig. 5 The electronic structures and bonding characteristics of typical borophene. (a) The electronic band structure and density of states for the β₁₂-B₅H₃ phase of borophene. The band structure is depicted, with the band dispersions color-coded according to the orbital characters, ranging from the p–x orbital (blue) to the p–z orbital (red) of boron. A van Hove singularity (vHS) is indicated by an arrow. The projected density of states (PDOS) of borophene is shown. Magnified views of the band structures are provided for the regions circled with different colors in the band structure, highlighting the intricate details of the electronic structure. The 3D band structure with colored dashed circles indicating the positions of the Dirac points. Reproduced from ref. 100 with permission under the terms of the Creative Commons Attribution 4.0 International License.¹⁰⁰ (b) ARPES intensity plots of the β₁₂-borophene sheet on Ag(111), measured with p polarized light along cut 1 to cut 3, respectively. The yellow dashed lines indicate the Dirac cones (DC). Reproduced from ref. 101 with permission from American Physical Society, copyright 2017.¹⁰¹

In contrast, MBs primarily derive their charge-storage capability from pseudocapacitive faradaic processes. In these systems, exposed metal sites act as redox centers that reversibly change oxidation states during charge/discharge cycles, contributing substantial pseudocapacitance, as summarized in multiple reviews on transition-metal-boride-based supercapacitors.^13,67–69 Meanwhile, the delocalized d–p electron networks in borides facilitate long-range conductivity through metallic channels, ensuring that faradaic reactions proceed with favorable kinetics. This intrinsic mobility underpins the high rate capability and power density of boron-based supercapacitors. The total capacitance (C_total) of boron-based electrodes can be expressed as a combination of quantum capacitance (C_q) and electrochemical capacitance (C_elec):

As for materials with high DOS near the Fermi level, such as borophene and MBs, C_q becomes comparable to or exceeds C_elec, leading to unprecedented overall capacitance values.²⁸ Under practical electrochemical conditions, however, the apparent contribution of quantum capacitance and pseudocapacitance is not fixed, but depends strongly on electrode chemistry, electrolyte environment, and operating rate. For graphene-based electric double-layer capacitors, extensive experimental and theoretical studies have shown that the low density of electronic states near the Dirac point renders the quantum capacitance comparable to or smaller than the electrochemical double-layer capacitance, such that the total interfacial capacitance is frequently limited by C_q even when a large surface area is available.^12–14

In contrast, for boron-derived conductors with a high density of states near the Fermi level, including borophene and many conductive metal borides, quantum capacitance is generally sufficiently large that it does not constitute the dominant limitation in the series capacitance. In these systems, the experimentally accessible capacitance is therefore more strongly governed by ion accessibility, interfacial charge-transfer resistance, and electrolyte compatibility than by electronic density-of-states constraints.^28,34

For redox-active electrodes such as transition-metal borides, numerous studies further indicate that pseudocapacitive faradaic processes contribute substantially to charge storage at moderate scan rates or current densities, whereas at higher rates the effective utilization of faradaic reactions becomes increasingly constrained by kinetic and transport limitations, leading to a progressively larger surface-controlled capacitive contribution.^67–69 These rate-dependent characteristics are commonly observed in device-level measurements and highlight that the relative dominance of quantum-capacitive and pseudocapacitive mechanisms evolves with operating conditions rather than being intrinsic constants of the material system. In this context, EDLC and pseudocapacitance together constitute the electrochemical interfacial contribution, whereas quantum capacitance reflects the electronic charge-accumulation capability of the electrode itself. The experimentally observed capacitance arises from their coupled response rather than from a single mechanism alone. Meanwhile, faradaic reactions associated with B–M bonds supplement this with reversible charge transfer, yielding the observed dual EDLC and pseudocapacitive behavior.¹³ This synergy explains why experimentally measured capacitances in borophene, MBene or boron-based composites significantly surpass those of traditional carbon electrodes.³⁴ Beyond charge-storage mechanisms, mechanical robustness and chemical resilience also play vital roles. The strong covalent B–B and M–B bonds resist volumetric changes during ion intercalation, ensuring high cycling stability.³⁰ Additionally, the inherent chemical inertness of boron reduces parasitic side reactions, extending device lifetime and enabling operation across a wide potential window, even in harsh electrolytic environments.

Taken together, borophene and MBs possess intrinsic structural and electronic advantages that make them ideal for next-generation supercapacitor electrodes. Borophene's vacancy-modulated polymorphism, high anisotropy, and Dirac-electron metallicity confer large quantum capacitance and fast surface charging, while metal borides combine robust covalent frameworks with multivalent redox functionality. Borophene and MBs should be regarded as complementary components within the boron-driven energy-storage framework, with borophene primarily enabling quantum-capacitance-enhanced surface charging, and metal borides predominantly contributing to metal-centered pseudocapacitive energy storage. Together, these properties result in synergistic quantum and pseudocapacitive energy storage, enabling high power and energy densities, long cycle life, and environmental stability. Understanding these fundamental structure–property relationships provides a foundation for rational design and optimization of boron-based materials for sustainable electrochemical energy storage.

2.4. Degradation mechanisms and durability-oriented material design

Stability remains a key bottleneck for boron-derived electrodes, and reported performance decay is generally governed by surface chemistry evolution and interfacial impedance growth. For borophene, ambient oxidation is frequently identified as the dominant degradation pathway, because oxygen exposure can rapidly alter the surface bonding environment and disrupt metallic charge-transport pathways; consequently, stable handling and device integration often require deliberate chemical or structural stabilization strategies.^34,122,183

Several design approaches have emerged as particularly effective for improving durability. Hydrogenation has been demonstrated to convert borophene into more chemically stable borophane polymorphs, providing a direct route to suppress oxidation while retaining favorable low-dimensional electronic characteristics.^34,112 Beyond chemical passivation, multilayer/stacked architectures can improve mechanical integrity and interfacial robustness in device electrodes; for instance, stacked borophene-based electric double-layer supercapacitors have been reported to exhibit improved electrode stability and cycling retention, consistent with the idea that structural reinforcement and interfacial contact optimization can mitigate degradation during repeated operation.³⁵ In addition, scalable preparation routes yielding few-layer borophene with enhanced air stability further suggest that thickness and stacking can function as a practical lever to improve long-term oxidation resistance.^122,183 More broadly, recent reviews emphasize that stability improvements can also be achieved through nanoarchitecting strategies such as controlled decoration/doping and protective integration, which regulate surface reactivity and limit oxygen/water-induced degradation.^158–161

For metal borides and related 2D MBenes, cycling degradation is more often associated with surface oxidation or hydroxylation, interfacial reconstruction of redox-active sites, and gradual accumulation of resistive surface layers, which collectively increase charge-transfer resistance and reduce effective utilization of redox centers over long-term cycling.^151,152,176 In this context, durability-oriented designs commonly rely on interfacial and structural engineering, including conductive scaffolds (carbon networks or polymers) to maintain percolation pathways, core–shell or heterointerface architectures to regulate surface chemistry, and electrolyte and voltage window optimization to suppress parasitic reactions.^{151,152,176,181} Recent reviews on MBenes further highlight that their chemical stability and corrosion resistance are closely tied to strong B-based bonding frameworks and the formation/tunability of protective surface layers, underscoring surface engineering as a decisive factor for long-term performance in harsh electrolytes.^96–98

Taken together, available evidence indicates that oxidation suppression for borophene and surface-layer or impedance control for borides or MBenes are the dominant durability levers, and the most promising pathways toward practical cycling stability involve passivation like hydrogenation, multilayer or stacked electrode architectures, and interface-engineered composites supported by electrolyte- and window-matched device design.

3. Brief review of the synthesis strategies of borophene and MBs

Unlike graphene or transition-metal dichalcogenides (TMDs), which possess intrinsic van der Waals interlayer interactions and can be mechanically exfoliated using simple “Scotch-tape” methods,^17,18 borophene and MBs lack naturally layered bulk counterparts.³¹ As a result, their fabrication into atomically thin forms relies heavily on precise atomic-scale control of chemical bonding, charge transfer, and reaction environment.²⁷ In general, current synthetic routes can be classified into two main categories: top-down and bottom-up approaches. The former typically employs mechanical, chemical, or electrochemical methods to “disassemble” bulk or complex precursors into 2D nanosheets, whereas the latter builds thin films or nanosheets from atomic or molecular precursors on solid substrates or in gas/liquid phases.^26,29

3.1. Top-down strategies for borophene

The synthesis of high-quality borophene is essential for fully realizing its potential in energy-storage applications. To address the challenges associated with its electrochemical utilization, a variety of synthesis strategies have been explored, each offering distinct advantages and limitations. Among them, top-down approaches focus on converting bulk boron into a two-dimensional form through physical or chemical exfoliation, offering notable scalability and cost efficiency. Common techniques include mechanical exfoliation, liquid-phase exfoliation (LPE) (employing solvents and ultrasonication), and electrochemical exfoliation (ECE). The following subsections provide a comprehensive overview of these synthesis methods, elucidating their underlying principles, strengths, and challenges. In terms of borophene purity and crystal structure, bottom-up epitaxial routes (MBE/CVD) generally yield the highest structural quality, whereas top-down exfoliation routes are more compatible with higher production rate but may suffer from broader thickness/phase distributions. Therefore, a practical comparison should explicitly distinguish high-purity and high-crystallinity production from high-yield and large-scale production, as these two goals are not always simultaneously achieved by the same methods.

3.1.1. Mechanical exfoliation for borophene. Mechanical exfoliation, originating from the early development of graphene-based two-dimensional materials, involves applying physical forces such as adhesion, friction, or shear to peel atomic layers from bulk boron. Chahal et al. first demonstrated the mechanical exfoliation of borophene using double-sided foam adhesive tape, where a combination of normal pressure and shear force enabled the extraction of mono-to few-layer borophene sheets from bulk boron (Fig. 6a).³⁰ This method overcame the long-standing limitation that non-layered bulk structures cannot yield corresponding two-dimensional materials. It offers several advantages, including operational simplicity, absence of chemical contamination, and preservation of high crystal purity. However, due to the strong interlayer binding energy of boron atoms, the exfoliation efficiency remains low, and the lateral dimensions of the obtained sheets are limited. Consequently, this technique is primarily suitable for the preparation of experimental-scale samples rather than large-scale production. Although it is facile and contamination-free, the inherently low yield and uncontrollable layer thickness restrict the applicability of mechanical exfoliation in the fabrication of high-performance electrode materials.

Fig. 6 Top-down strategies for borophene. (a) Various steps of micromechanical exfoliation of borophene nanosheets. Reproduced from ref. 30 with permission from Wiley-VCH GmbH, copyright 2021.³⁰ (b) Chemical-exfoliated borophene obtained through etching its precursor. Reproduced from ref. 108 with permission under the terms of the Creative Commons Attribution 4.0 International License.¹⁰⁸ (c) Description of the electrochemical exfoliation process for the synthesis of 2D boron nanosheets. Reproduced from ref. 109 with permission under the terms of the Creative Commons Attribution 4.0 International License.¹⁰⁹ (d) Schematic diagram of the growth process of the hydrogenated borophene. Reproduced from ref. 34 with permission from Wiley-VCH GmbH, copyright 2020.³⁴

3.1.2. LPE synthesis for borophene. Li et al. employed a sonication-assisted LPE strategy to produce borophene nanosheets under ambient conditions, using dimethylformamide (DMF) and isopropyl alcohol (IPA) as exfoliation media.¹⁰² The resulting sheets exhibited average thicknesses of approximately 1.8 nm (≈4 atomic layers) when exfoliated in DMF and about 4.7 nm (≈11 layers) in IPA. Building on this approach, Ranjan et al. broadened the solvent spectrum to include acetone, ethylene glycol, and water, and reported that boron monolayers could be formed within 12–20 hours in selected solvents.^103,104 Lin et al. subsequently introduced a low-temperature LPE method to synthesize freestanding few-layer β₁₂-borophene single crystals, achieving yields on the order of several tens of milligrams.¹⁰⁵ The obtained borophene sheets exhibited outstanding electrochemical activity, serving efficiently as polysulfide immobilizers and electrocatalysts in lithium–sulfur batteries. Guo et al. further prepared ultrathin two-dimensional borophene nanosheets (<2 nm) through a similar sonication-assisted LPE process, demonstrating their promise as lithium-ion battery anodes, with a remarkable reversible capacity of 782.9 mAh g⁻¹ over 500 charge–discharge cycles at a current density of 167 mA g⁻¹.¹⁰⁶

Expanding upon these developments, Zhang et al. proposed a solvothermal-assisted LPE route that combined ball-milling thinning, solvothermal swelling, and probe ultrasonication, yielding few-layer borophene flakes with large lateral dimensions (average ≈ 5.05 µm for four-layer borophene in acetone).¹⁰⁷ Moreover, Xie et al. reported a selective chemical etching method using hydrochloric acid to synthesize borophene from an AlB₂ precursor, effectively overcoming the challenges associated with direct exfoliation of strongly covalent bulk boron (Fig. 6b).¹⁰⁸ Despite these advantages, LPE of borophene still faces several methodological constraints that limit its reproducibility and scalability. The exfoliation outcome is highly sensitive to solvent polarity and sonication parameters, frequently leading to broad distributions in thickness, lateral size, and defect density, which complicates reliable structure–property correlations. In addition, prolonged ultrasonication can introduce edge fragmentation and amorphization, while polar aprotic solvents commonly used for dispersing boron sheets like DMF raise concerns regarding solvent removal, residual contamination, and environmental burden. More importantly, borophene is susceptible to oxidation during exfoliation and subsequent processing under ambient conditions, which may alter surface chemistry and electronic conductivity prior to device fabrication. These limitations highlight the need for standardized processing windows (solvent selection, oxygen/moisture control, and post-treatment protocols) to achieve reproducible borophene electrodes from LPE routes.

3.1.3. ECE synthesis for borophene. Electrochemical exfoliation has proven to be an efficient and scalable method for synthesizing few-layer borophene flakes.¹⁰⁹ In this process, bulk boron is embedded within a conductive metal mesh, which facilitates its electrochemical delamination in different electrolytic environments. The exfoliated borophene typically exhibits a thickness of 3–6 nm and presents diverse crystalline phases depending on the electrolyte employed. Mechanistic investigations into this process have offered valuable understanding of the exfoliation dynamics and its feasibility for large-scale fabrication (Fig. 6c).¹⁰⁹

Building on this concept, Mohammad et al. introduced an innovative electrochemical strategy for borophene synthesis, inspired by the exfoliation route widely adopted for graphene.¹¹⁰ In their design, borophene served as the cathodic material; however, its inherently low electrical conductivity—especially at reduced temperatures—necessitated engineering modifications distinct from those used in graphite-based systems. To address this limitation, the authors developed a hollow cylindrical boron electrode equipped with an internal heating coil, enabling precise temperature control and thereby improving the material's conductivity during exfoliation. Using platinum as the anode, borophene flakes were obtained through electrochemical exfoliation, and subsequently purified via vacuum filtration, sonication in acetone, and centrifugation to isolate thin borophene layers from residual boron. The resulting nanosheets demonstrated robust structural integrity and tunable performance across varying temperature regimes, providing valuable insights into process optimization. This study not only established a reproducible and scalable route for borophene fabrication but also underscored future possibilities for refining electrochemical synthesis toward higher efficiency and quality control.¹¹⁰

Nevertheless, electrochemical exfoliation of borophene remains constrained by several practical factors. The requirement of a sufficiently conductive boron electrode elevates temperatures to improve conductivity and introduces additional engineering complexity and energy input during synthesis. Moreover, electrolyte-dependent reactions can yield phase heterogeneity and non-uniform surface chemistry, while residual ions or byproducts may necessitate extensive purification steps (filtration/sonication/centrifugation), potentially reducing yield and introducing variability. Therefore, although ECE provides a promising pathway for scalable borophene production, controlling flake thickness, crystallinity, and surface state in a reproducible manner remains a key methodological challenge for translating ECE-derived borophene into consistent supercapacitor electrodes.

3.1.4. ECE synthesis for borophene. In addition to the aforementioned strategies, several other novel top-down approaches for borophene synthesis have been explored. Hou et al. achieved the substrate-free growth of crystalline hydrogenated borophene through a three-step thermal decomposition and hydrogenation process of NaBH₄, significantly enhancing its resistance to air, acids, and alkalis, thereby overcoming the intrinsic instability and oxidation issues of borophene (Fig. 6d).³⁴ Nishino et al. utilized MgB₂ as a precursor and synthesized two-dimensional hydrogen boride sheets via an ion-exchange-exfoliation route, which exhibited both scalability and high yield.¹¹¹ Similarly, atomic hydrogen treatment under ultra-high vacuum (UHV) conditions to obtain multiple borophane polymorphs has been employed, realizing atomic-level control over hydrogen-passivated borophene structures.¹¹² Collectively, these methods demonstrate the critical role of top-down hydrogenation strategies originating from bulk or preformed borophene in improving the chemical stability of boron-based 2D materials, thereby providing a robust and chemically stable platform for energy storage and device-oriented applications.¹¹²

Thus, top-down synthesis technologies offer a diverse and scalable set of approaches for borophene fabrication, each presenting distinct advantages and opportunities for tailoring material properties. Continued research in this area aims to enhance the yield, quality, and controllability of borophene, paving the way for its practical deployment in energy storage, catalysis, and nanoelectronic applications.

3.2. Bottom-up strategies for borophene

Bottom-up approaches, such as vapor-deposition techniques, enable the direct synthesis of borophene films by assembling individual atoms or molecules onto substrates, thereby allowing precise control over the material's structure and properties. Among these, chemical vapor deposition (CVD) and physical vapor deposition (PVD) have achieved remarkable success in producing high-quality, large-area borophene films and nanostructures. These vapor-phase deposition methods provide a versatile platform for exploring the polymorphism of boron, facilitating the integration of borophene into the energy-storage field through property-driven material design.

3.2.1. CVD synthesis for borophene. CVD is increasingly recognized as a viable approach for producing high-quality borophene and associated nanostructures over large areas. This bottom-up synthesis strategy relies on thermally decomposing boron-based precursors, which facilitates the deposition of atomic or molecular boron onto selected substrates and ensures accurate regulation of the material growth dynamics. Key benefits of CVD include scalable processing, adjustable operational parameters, and seamless integration with conventional semiconductor fabrication technologies.

There are two pioneering works. In the year 2015, Tai et al. reported a low-pressure chemical vapor deposition (LPCVD) approach for growing atomically thin γ-borophene films on copper foils via a vapor-solid growth process (Fig. 7a); optical absorption and PL measurements revealed a direct bandgap of approximately 2.25 eV, in good agreement with first-principles calculations predicting a bandgap of ∼2.07 eV.¹²⁴ In 2023, the tetragonal borophene synthesized by LPCVD was also proved to exhibit an electrical conductivity in the range of (4–5) × 10⁻⁴ S cm⁻¹ and a bandgap of approximately 2.1 eV, confirming its narrow semiconducting characteristics and potential as an electrode.¹²⁵ Meanwhile, Xu et al. achieved the synthesis of single-crystalline ultrathin boron nanosheets (UBNS, Fig. 7b) through an effective vapor-solid process via CVD.¹²⁶ Their method involved the thermal decomposition of diborane (B₂H₆) at 950 °C under a reduced pressure of 8 Pa, resulting in deposition onto a silicon wafer. The synthesized crystalline boron sheets displayed high structural order, with lateral dimensions spanning tens of nanometers up to 3 µm in width, lengths of 3–20 µm, and a consistent thickness close to 10 nm (Fig. 7c), identified as a multilayer α-boron phase. These works established a new direction for borophene fabrication via CVD.¹²⁶

Fig. 7 CVD strategies for borophene. (a) Schematic representation of the home-made two-zone furnace used to obtain atomically thin 2D γ-B₂₈ films by CVD. Reproduced from ref. 125 with permission from American Chemical Society, copyright 2023.¹²⁵ (b) A typical image of enlarged UBNS with a micro-Raman spectrum as an inset. (c) AFM picture (top) and the corresponding height profiles (bottom) of UBNS. Reproduced from ref. 126 with permission under the terms of the Creative Commons Attribution License.¹²⁶ (d) Schematic diagram of the growth of borophene on quartz by chemical vapor deposition, with the structure of an α′-2H-borophene. Reproduced from ref. 129 with permission from the Royal Society of Chemistry, copyright 2022.¹²⁹ (e) Borophene crystal structure and van der Waals epitaxy growth diagram of borophene on a mica substrate. Reproduced from ref. 123 with permission from American Chemical Society, copyright 2021.¹²³ (f) Schematic diagram of borophene nanosheet growth on Al foil via the CVD method. (g) The HRTEM images at the junction of nanosheets with the false-colored inverse FFT image as an inset. Reproduced from ref. 155 with permission from Wiley-VCH GmbH, copyright 2024.¹⁵⁵

CVD has further been adapted to produce borophene nanosheets with targeted catalytic functions. Tai et al. employed sodium borohydride as a boron source together with hydrogen as a carrier gas, depositing borophene nanosheets directly onto carbon cloth through CVD.¹²⁷ The resulting nanosheets conformed to the theoretical α′-borophene phase and showed outstanding electrocatalytic behavior for the hydrogen evolution reaction (HER), characterized by a Tafel slope of 69 mV dec⁻¹ and robust cycling durability in 0.5 M H₂SO₄ electrolyte. These performance advantages were linked to a high density of accessible active sites and reduced charge-transfer resistance.

Addressing the challenge of scalable borophene production, Mazaheri et al. adopted a diborane pyrolysis route to generate a pure boron vapor source for CVD growth of atomically thin borophene.¹²⁸ They conducted a systematic analysis of how temperature, deposition rate, and pressure affect the morphology and crystalline phase of the resulting 2D boron layers. The CVD-synthesized borophene featured an average thickness of 4.2 Å, crystallized in the χ₃ phase, and displayed metallic conduction behavior. Evidence of bilayer and trilayer stacking was also reported, contributing fundamental insights for subsequent experimental studies.

Most recently, Wu et al. reported the direct CVD growth of borophene on insulating quartz substrates, utilizing NaBH₄ as the boron source and hydrogen as the carrier gas (Fig. 7d).¹²⁹ The produced “borophene glass” closely aligned with the predicted α′-2H structural model. This transfer-free methodology allows direct fabrication of borophene on insulating platforms, simplifying its implementation in devices for energy conversion, storage, and sensing applications. van der Waals epitaxy via CVD has been proposed as a means to circumvent the restrictions of metallic substrates, allowing the growth of borophene on insulating or semiconducting surfaces. Wu et al. achieved borophene film growth on mica substrates using NaBH₄ as the boron precursor and hydrogen as the carrier gas (Fig. 7e). The structure and growth process has been shown in Fig. 7g.¹²³ The resulting α′-boron lattice exhibited remarkable photoresponse characteristics, including a photoresponsivity of 1.04 A W⁻¹ and a specific detectivity of 1.27 × 10¹¹ jones under 625 nm illumination at a reverse bias of 4 V.

Recently, Wu et al. reported for the first time the direct synthesis of multilayer borophene nanosheets on commercial Al foil using CVD, marking a crucial transition of borophene growth from the traditional UHV-MBE approach to a scalable CVD route (Fig. 7f). Specifically, NaBH₄ undergoes thermal decomposition in a hydrogen atmosphere to release boron atoms, which subsequently nucleate, expand, and coalesce on the aluminum surface to form a quasi-continuous film with an average thickness of approximately 9.6 nm. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) confirmed that the resulting film consists of coexisting β₁₂ and α′-4H borophene phases, and its growth kinetics follow the Avrami three-stage model. In contrast to the continuous single-crystalline films obtained by MBE, the CVD-grown borophene exhibits nanoscale crystalline domains separated by amorphous boundaries of about 5.9 nm (Fig. 7g), resulting in a more stable multilayer structure.

Overall, the CVD method represents one of the most versatile fabrication routes for borophene, borane, and related boron-based nanostructures, offering broad tunability in material composition, dimensional architecture, and heterostructure design. At the same time, CVD-based borophene synthesis is accompanied by intrinsic constraints that currently limit its translation toward large-area, electrode-grade films. Many reported routes rely on highly reactive boron precursors (e.g., diborane) and elevated growth temperatures, which increase safety requirements, energy consumption, and overall cost. In addition, borophene growth is often strongly substrate-dependent, and the resulting films frequently consist of nanoscale crystalline domains separated by grain boundaries rather than long-range single-crystalline sheets, potentially introducing electronic discontinuities and variability in electrochemical response.

Recent developments nevertheless indicate promising directions for improving the production rate and manufacturing compatibility of CVD-grown borophene. Examples include transfer-free growth on insulating substrates and direct growth on commercially relevant metal foils or current collectors, which reduce post-growth handling steps and improve integration potential. However, such advances also highlight a fundamental trade-off between production efficiency and crystalline continuity. Consequently, ongoing research efforts are increasingly focused on refining precursor chemistry, lowering growth temperatures, expanding substrate generality, and implementing post-growth stabilization strategies to balance material quality, process efficiency, and operational stability. These developments collectively position CVD as a leading, though still evolving, route toward scalable borophene electrodes for energy-storage applications.

3.2.2. PVD synthesis for borophene. PVD, which utilizes the vaporization and condensation of materials under vacuum conditions, has been pivotal for the controlled synthesis of borophene (Fig. 8a). Within this framework, epitaxial growth techniques such as molecular beam epitaxy (Fig. 8b) have demonstrated substantial capability in producing large-area, high-quality borophene films.¹¹³ By adjusting key deposition parameters, these approaches allow for precise manipulation of borophene's structural phases and the fabrication of multilayer configurations. Moreover, the recently proposed step-edge epitaxy on insulating substrates presents a promising route for the formation and in-depth analysis of borophene phases electronically isolated from metallic substrates.

Fig. 8 PVD strategies for borophene. (a) Schematic representation delineating a typical PVD technique. Reproduced from ref. 113 with permission under the terms of the Creative Commons Attribution 4.0 International License.¹¹³ (b) An illustration demonstrating the application of the beam epitaxy process for depositing boron atoms onto a substrate. (c) Schematics of a distorted B₇ cluster and growth setup with an atomic structure model and STM topography rendering.²⁶ (d) large-scale STM topography of borophene sheets. Regions of homogeneous-phase, striped-phase island, and striped-phase nanoribbon are indicated with red, white, and blue arrows, respectively.²⁶ (e) striped-phase atomic-scale structure. Inset shows a rectangular lattice with overlaid lattice vectors.²⁶ (f) Scanning tunneling spectroscopy of borophene. Reproduced from ref. 26 with permission from American Association for the Advancement of Science, copyright 2015.²⁶ (g) STM image of boron sheets after annealing at 650 K. The two different phases are labelled S1 and S2. Reproduced from ref. 27 with permission from Springer Nature, copyright 2016.²⁷ (h) High-resolution STM image of S1 phases. (i) High-resolution STM image of S2 phases. (j) Summary of borophene structure and properties. Reproduced from ref. 31 with permission from Springer Nature, copyright 2018.³¹

Epitaxial growth, which involves the deposition of a crystalline overlayer onto a crystalline substrate, has become an essential route for obtaining large-scale, high-crystallinity borophene.¹¹⁴ Various epitaxial strategies, including MBE and E-beam evaporation epitaxy, provide different advantages in modulating crystal orientation, film morphology, and electronic behavior. Among them, MBE is regarded as one of the most precise approaches, offering atomic-level control over nucleation dynamics and phase evolution.^114,115 From the perspective of producing high-purity borophene with well-defined polymorphs, UHV-assisted epitaxial growth, especially MBE on metallic substrates, remains the most established route, because it offers atomic-level regulation of nucleation dynamics, phase evolution, and layer formation. However, the practical deployment of MBE-grown borophene is constrained by ultra-high-vacuum requirements, substrate dependence, and limited throughput, which collectively lead to low production rates despite the excellent crystallinity and structural precision achievable in laboratory studies.

Mannix et al. first achieved the synthesis of atomically thin two-dimensional borophene using MBE, in which boron atoms were deposited onto an Ag(111) substrate under ultra-high vacuum (UHV) conditions via electron-beam evaporation (Fig. 8c). Scanning tunneling microscopy (STM) observations revealed two characteristic structural phases, homogeneous and striped, both consisting of chain-like atomic arrangements aligned along specific crystallographic directions, accompanied by pronounced anisotropic buckling (Fig. 8d and e). This structural diversity indicates that the flexibility of borophene originates from the competition among multicenter B–B bonds, enabling the formation of complex two-dimensional atomic configurations. More importantly, scanning tunneling spectroscopy (STS) measurements confirmed the highly anisotropic metallic nature of borophene, with electrical conduction preferentially occurring along the chain-growth direction, thus demonstrating its unique metallic behavior within the two-dimensional framework (Fig. 8f).

Subsequently, Feng et al. synthesized a structurally ordered borophene phase on the same Ag(111) substrate using the MBE method. They identified two distinct planar configurations, the β₁₂ (S1) and χ₃ (S2) phases, and elucidated their crystallographic characteristics through high-resolution STM imaging combined with density functional theory analysis (Fig. 8g). The β₁₂ phase exhibited a honeycomb-like vacancy-chain structure composed of triangular lattices with periodically arranged hexagonal holes (Fig. 8h), whereas the χ₃ phase featured a more compact, zigzag-like arrangement (Fig. 8i). Compared with the 2015 work of Mannix et al., this study provided the first precise and theory-consistent structural determination of borophene, further confirming that borophene grown on Ag(111) maintains an almost perfectly planar geometry with minimal out-of-plane distortion.

With a deeper understanding of borophene's atomic configurations, Mannix et al. later presented a comprehensive summary of MBE growth mechanisms for borophene in 2018 (Fig. 8j). Based on extensive experimental evidence, they concluded that the weak interaction between boron atoms and the Ag(111) surface is crucial for stabilizing borophene in its two-dimensional form. Furthermore, the nucleation of boron on the Ag substrate follows a defect-assisted mechanism, coupled with vacancy-chain modulation, giving rise to diverse polymorphs (such as v_1/5 and v_1/6). This mechanism allows borophene to flexibly transition between triangular lattices and periodic vacancy superlattices. In addition, by correlating theoretical predictions with experimental observations, the study clarified that the structure, electronic properties, and stability of borophene are jointly governed by substrate interactions rather than intrinsic layer-by-layer extension, underscoring the substrate's pivotal role in determining borophene's formation and properties.

The substrate choice exerts a decisive influence on the structural and physical characteristics of borophene.^114,116,118 Kiraly et al. reported borophene formation on Au substrates, where boron atoms diffused into the Au lattice at elevated temperatures and segregated to the surface during cooling to form borophene islands.¹¹⁷ Similarly, Karthikeyan et al. obtained a flat, non-buckled honeycomb monolayer on Al(111), confirmed by scanning tunneling microscopy (STM).^119,120 Density functional theory (DFT) simulations predicted the presence of a 9 R borophene configuration on Al(111), consisting of a hexagonal double-chain network built from boron nonagons. Although not yet verified experimentally, Shao et al. predicted a metallic and thermally stable δ₅ borophene phase on Ni(111).¹²¹ Additionally, MBE has enabled the fabrication of bilayer borophene on Cu(111), where interlayer covalent B–B bonding links two β₁₂-like sheets.¹²² The bilayer structure exhibits higher conductivity and oxidation resistance than the monolayer, making it a promising candidate for energy-storage electrodes.

Overall, epitaxial approaches are indispensable for fabricating high-quality borophene films with well-defined crystalline phases and tunable properties. MBE on metallic substrates remains the most widely investigated technique, enabling the synthesis of diverse borophene polymorphs and bilayer configurations, while van der Waals epitaxy extends the feasible substrate range, offering new directions for metal-free synthesis. Ongoing developments in epitaxial growth aim to enhance borophene's structural precision, scalability, and adaptability, fostering its future integration into electronics, optoelectronics, and energy-related technologies.

3.3. Synthesis strategies of metal borides

Metal borides can be synthesized from their precursor MAB phases through top-down approaches. Although research in this area remains relatively limited, only a few MAB compounds have so far been reported for the successful preparation of MBs. The top-down synthesis routes primarily include chemical etching and liquid-phase exfoliation. In addition, bottom-up techniques such as CVD have also been employed for MB fabrication. Studies have shown that this approach can yield materials with higher production efficiency, which can be directly applied in energy storage and catalytic systems. In contrast, while bottom-up synthesis generally produces lower yields, it provides unique advantages in controlling single-crystal quality, offering a potential source of high-standard materials for energy storage and electronic devices.

3.3.1. Wet chemical etching synthesis. The wet chemical etching method utilizes acidic or alkaline solutions to remove the “A” layer from MAB phases in order to obtain MBs, and this process can also be achieved through a dealloying mechanism.⁸¹ However, due to the strong bonding between aluminum and its surrounding atoms, both chemical etching and interlayer exfoliation are relatively challenging. Taking MoAlB as an example, Alameda et al. investigated the use of NaOH and HF in the etching process and found that the boride layer is damaged in the HF etchant, while immersing MoAlB powder in a 10% alkaline solution for 24 hours yielded MoAlB plates with thickness ranging from 50 to 300 nm. Nevertheless, this method could not effectively remove the Al layer, and thus the formation of layered MoB was not achieved.⁷⁸ Kim et al. employed ab initio molecular dynamics (AIMD) simulations and demonstrated that MoAlB undergoes a phase transition from Mo₄Al₃B₄ to Mo₂AlB₂ during etching, consistent with Alameda's experimental findings.¹³⁰ In their study, metastable Mo₂AlB₂ was successfully synthesized using an acid etching process with 3 M LiF/10 M HCl at 40 °C. Similarly, Zhang et al. synthesized Cr-based MBs (CrBs) through acid etching. High-purity CrB powder was first prepared, followed by the synthesis of a high-purity MAX phase, Cr₂AlB₂. Subsequent immersion of Cr₂AlB₂ in 200 mL of dilute HCl solution at room temperature for seven days produced CrB nanoflakes.¹³¹

To address the harsh reaction conditions and long processing times typical of MB synthesis, Li et al. proposed a green, alkali-assisted hydrothermal etching method for MAX phases.¹³² For reference, xiong et al. achieved environmentally friendly and efficient MB synthesis, which was then applied as an anode material in lithium-ion batteries. Although approximately 26.8% of the resulting sample consisted of MoAl_1−xB impurities, XRD analysis confirmed the successful formation of MoB (Fig. 9a).⁷⁷

Fig. 9 Wet chemical etching strategy for MBs. (a) The schematic of the preparation of 2D MoB from the reaction between MoAlB and NaOH, with the XRD pattern as an inset. Reproduced from ref. 77 with permission from Elsevier Ltd, copyright 2022.⁷⁷ (b) Wet etching methods for Mo₂B₂ flake synthesis. Reproduced from ref. 134 with permission from Wiley-VCH GmbH, copyright 2023.¹³⁴ (c) Schematic illustration of the exfoliation process employed to disperse metal diborides in solvents via sonication. And a photograph of vials of metal diboride dispersions in various organic solvents: HfB₂, TiB₂, and TaB₂ in ACN, CrB₂ in ACT, MgB₂ and AlB₂ in EtOH, ZrB₂ in IPA, and NbB₂ in DMF. Reproduced from ref. 136 with permission from American Chemical Society, copyright 2021.¹³⁶ (d) Schematic of the proposed growth method of ultrathin TaB using tantalum and copper immiscible metal sheets, heated at elevated temperature in the presence of B powder. Reproduced from ref. 139 with permission from Wiley-VCH GmbH, copyright 2017.¹³⁹

In 2020, Miao et al. employed an evolutionary structure search combined with first-principles calculations to predict several stable layered ternary borides, including Zr₂PbB₂, Zr₂TlB₂, Hf₂SnB₂, Hf₂InB₂, and Zr₂InB₂, all of which adopt the Ti₂InB₂-type structure.¹³³ In addition, two new structural types, Zr₃CdB₄ and Hf₃PB₄, were discovered. Although they share the same space group as Ti₂InB₂, these compounds represent distinct crystal structures. This study further indicated that boron-based ternary compounds could serve as a new platform within the MAX phase family.¹³³ With continued research, the synthesis of MBenes through the etching of ternary MAB phases has expanded to quaternary M′M″AB systems, providing new possibilities for MBene design. Zhou and co-workers theoretically predicted 15 layered borides with in-plane chemical ordering (i-MABs), described by the general formula .⁷⁵ Among them, (Mo_2/3Sc_1/3)₂AlB₂ and (Mo_2/3Y_1/3)₂AlB₂ were successfully synthesized and experimentally verified through chemical exfoliation. HF aqueous solution was employed for selective etching, removing not only the aluminum layer but also partially eliminating yttrium and scandium, which resulted in the formation of ordered metal vacancies. A dilute tetrabutylammonium hydroxide solution was then used as an intercalating agent, expanding the interlayer spacing and inducing spontaneous exfoliation in water to form a stable two-dimensional colloidal suspension. The obtained Mo_4/3B_2−xT_z nanosheets (>50 nm) exhibited well-defined metal-site vacancies, with T_z denoting surface terminations such as F, O, and OH. The boron content in Mo_4/3B_2−xT_z was slightly lower than that in the original precursor, with x reaching up to 0.5.

More recently, Bury et al. employed an HCl/H₂O₂ aqueous solution for 24, 48, and 72 hours (Fig. 9b) to successfully obtain single-to few-layer 48-MBene flakes. Further simulations and XRD analyses revealed that the optimally exfoliated 48-MBene Mo₂B₂ exhibits an orthorhombic lattice structure.¹³⁴ In addition to conventional acid or alkaline etchants, liquid Lewis acid salts can also serve as alternative media for etching intermediate layers. Miao et al. synthesized Hf₂InB₂ precursors through solid-state reactions and subsequently etched them in a molten salt system composed of CuCl₂, NaCl, and KCl, yielding HfBO nanosheets with promising potential for electrochemical energy storage applications.⁸³

Based on the reported studies discussed above, wet chemical etching of MAB phases is accompanied by several method-intrinsic limitations that should be considered. For example, the use of HF-containing etchants, while effective in removing the A layer, has been shown to partially damage the boride layers, resulting in structural disruption and reduced crystallinity in the etched products. In contrast, alkali-based etchants such as NaOH often exhibit limited selectivity toward the A layer, leading to incomplete removal even after extended reaction times.

In addition, prolonged etching processes and repeated washing steps may introduce residual byproducts or secondary phases, which complicates precise control over phase purity. Another commonly observed outcome of wet etching is the formation of surface terminations such as –F, –O, and –OH, as documented in multiple reports. While such terminations can enhance surface wettability and ion accessibility, they also introduce variability in surface chemistry that depends on etchant composition and processing conditions. Taken together, these observations indicate that wet chemical etching provides a versatile route for producing boride-derived materials, but achieving consistent structural integrity and surface-state control remains a methodological challenge that requires further optimization.

3.3.2. Liquid intercalation and ultrasonic exfoliation synthesis. Liquid-phase exfoliation is one of the most widely used techniques for synthesizing 2D materials. The fundamental concept involves introducing suitable additives and applying external energy, such as ultrasonic waves, to facilitate interlayer separation. High-frequency ultrasonic vibrations generate cavitation bubbles within the liquid medium, producing localized high pressure that peels apart layered structures into thin nanosheets, which can then be stabilized using appropriate solvents. For instance, Saroj et al. dispersed MgB₂ powder in water and employed ultrasonication to disrupt interlayer ionic interactions, thereby obtaining few-layer nanosheets with micrometer-scale lateral dimensions.¹³⁵ Similarly, ZrB₂ was prepared via a microwave-assisted chemical etching route and subsequently functionalized using hyaluronic acid. Through a combination of CH₃COOH, H₂O₂, and microwave irradiation, layered ZrB₂ with nanoscale thickness was successfully achieved.

The ultrasonic-assisted exfoliation of various metal borides (MBs) has also been extensively studied. Yousaf et al. fabricated a series of quasi-2D nanosheet dispersions of ZrB₂, TiB₂, TaB₂, NbB₂, MgB₂, HfB₂, CrB₂, and AlB₂ using ultrasonic exfoliation (Fig. 9c).¹³⁶ Structural characterization confirmed that the nanosheets retained their original chemical constituents while exhibiting thicknesses ranging from 2 nm to several tens of nanometers.

Ion intercalation has emerged as another efficient approach for achieving delamination. Experimental studies have shown that introducing tetramethylammonium hydroxide (TMAOH) into sulfuric-acid-treated bulk MgB₂ induces interlayer expansion and subsequent exfoliation, yielding multilayer sheets with thicknesses of 300–400 nm. Additional ultrasonication further reduced the thickness of these sheets to 3–5 nm.¹³⁷ Furthermore, James et al. reported that ultrasonic treatment of bulk MgB₂ promotes interlayer separation due to the partial depletion of magnesium atoms.¹³⁸ Based on this finding, they employed disodium ethylenediaminetetraacetate (Na₂EDTA) to selectively remove magnesium atoms from MgB₂ crystals, successfully obtaining multilayer Mg-deficient MgB₂ nanosheets without the assistance of ultrasonication.

3.3.3. CVD synthesis for MBs. In the bottom-up CVD process, the choice of substrate plays a decisive role, with interfacial interactions between the precursor species and the growth substrate being particularly critical. Wang et al. employed a Cu–Ta bilayer structure as the substrate for CVD growth, where Ta clusters migrated through the copper bulk and along grain boundaries to the surface under high-temperature conditions, subsequently reacting with boron to form layered TaB (Fig. 9d).¹³⁹ However, due to non-equilibrium growth conditions, dendritic TaB structures with an average thickness of approximately 15 nm were ultimately formed on the copper surface.¹⁴⁰ In a subsequent study, the same group utilized a dual-zone tube furnace, using molybdenum foil combined with B/B₂O₃ as the reaction system, to synthesize Mo₃B films with a smaller thickness of only 6.48 nm. In this configuration, the molybdenum foil simultaneously served as both the substrate and the metal source.¹³⁹

Moreover, the synthesis of phase–pure transition metal borides (TMBs) remains challenging due to their structural diversity and competing crystal phases. To address this issue, Si et al. proposed a high-purity synthesis strategy based on controlled chemical potential. In this method, tin metal particles were placed on top of transition metal foils, whereupon molten tin formed a smooth liquid surface at high temperatures, acting as a template for epitaxial growth, while the metal foil provided the transition metal atoms through limited diffusion.¹⁴¹ By spatially separating the metal and boron sources, gas-phase reactions were effectively suppressed. It was found that the atomically smooth liquid tin surface significantly reduced nucleation density. Theoretical simulations indicated that MoB₂ clusters exhibited negligible diffusion barriers on the liquid tin surface, enabling rapid atomic migration and a homogeneous distribution of chemical potential. By adjusting the reaction duration, highly pure multilayer MoB₂ single crystals and ultrathin MoB single crystals were successfully synthesized. Extending this approach to tungsten and chromium systems, the researchers also achieved the growth of ultrathin W₂B and Cr₂B single crystals. This technique, offering precise chemical potential control with formation energy resolution down to 0.01 eV, provides an efficient route for synthesizing ultra-pure single-crystalline MBs.

Beyond metallic substrates, silicon carbide (SiC) has also been employed as a growth platform for MgB₂ thin films. Acharya et al. utilized a high-pressure chemical vapor deposition system with magnesium as the metal source and diborane (B₂H₆) as the boron precursor to achieve epitaxial growth of MgB₂ ultrathin films on SiC substrates. After argon ion etching, the film thickness was reduced to 1.8 nm, although intergranular connectivity was found to deteriorate in the ultrathin regime. In addition to CVD, MBE and co-evaporation techniques have also been applied to fabricate ultrathin MgB₂ films.^142,143

3.4. Synthesis-structure–performance synergistic regulation and summary

The synthesis process fundamentally determines the structural order, defect distribution, and interfacial characteristics of two-dimensional borophene and boride materials, thereby exerting a profound influence on their electrochemical kinetics and energy-storage mechanisms. Distinct synthesis methods give rise to pronounced variations in crystallinity, layer-thickness control, and surface chemical composition, establishing an intrinsic relationship among synthetic pathway, structural modulation, electrochemical performance, and manufacturability.

For bottom-up vapor-deposition approaches, molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) enable the preparation of highly crystalline boron-based materials with atomically ordered lattices and controllable layer numbers. Such structures typically exhibit superior lattice integrity, minimal defect density, and excellent electrical conductivity, making them particularly suitable for constructing high-power electrodes dominated by electric double-layer capacitance (EDLC). However, these high-precision techniques generally require stringent vacuum or high-temperature conditions and rely on reactive boron precursors, which are associated with low production rates, high energy consumption, safety concerns, and complex equipment dependence, thereby limiting their direct applicability to large-area electrode manufacturing.

In contrast, bottom-up liquid-phase exfoliation, chemical reduction, and other conversion-based routes more often yield porous, defect-containing, or amorphous structures with a higher density of electrochemically accessible sites. Although such materials typically exhibit reduced long-range crystallinity, they can provide substantial pseudocapacitive contributions, rendering them attractive for high-energy-density electrode designs. From a manufacturing perspective, these wet-chemical and molten-salt-assisted processes are generally more compatible with bulk powder production, slurry-based coating, and roll-to-roll fabrication, particularly for metal borides and boride-derived materials, even though precise control over phase purity and compositional homogeneity remains a challenge.

Beyond material synthesis, film-forming strategies that directly deposit boron-based active layers onto current collectors represent an additional pathway to bridge laboratory synthesis and device integration. Direct deposition can improve interfacial adhesion and reduce reliance on binders or conductive additives, thereby simplifying electrode fabrication and highlighting a practical route toward scalable and flexible supercapacitor architectures.

Within this synthesis–structure–performance coupling framework, future research should prioritize three key directions:

3.4.1. Green and scalable synthesis. Develop low-temperature, energy-efficient, and environmentally benign CVD or molten-salt-assisted processes that enable large-area, high-purity two-dimensional boron-based materials while reducing the energy consumption, safety risks, and equipment dependence associated with conventional MBE.

3.4.2. Stability and interface engineering. Employ surface passivation strategies such as hydrogenation, encapsulation, and heterostructure construction to suppress oxidation and degradation, while simultaneously optimizing electrode–electrolyte interfaces to enhance ion-transport kinetics and long-term cycling stability.

3.4.3. Mechanistic characterization and theoretical modeling. Combine in situ transmission electron microscopy (TEM), operando X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy to track structural evolution during material growth and electrochemical cycling, and integrate density functional theory (DFT) and molecular dynamics (MD) simulations to elucidate electronic reconstruction and bonding evolution under different synthesis conditions, thereby providing theoretical guidance for rational, design-driven synthesis.

Overall, synthesis routes not only dictate the microstructure and surface chemistry of borophene and borides but also fundamentally govern their electrochemical behavior and prospects for practical deployment. Through the synergistic optimization of process control, interfacial engineering, mechanistic understanding, and manufacturing considerations, boron-based supercapacitor electrodes may achieve a balanced combination of high power density, high energy density, long-term durability, and realistic scalability.

4. Progress of borophene and MBs in supercapacitors

2D materials such as graphene have been extensively investigated for supercapacitor applications due to their remarkable physicochemical properties.¹⁴⁴ However, graphene still faces intrinsic limitations in energy storage, primarily attributed to its quantum-capacitance effect, which results in a low electronic density of states (DOS) at the Fermi level.¹⁴⁵ Furthermore, conventional doping strategies have proven only marginally effective in enhancing its electronic conductivity, thereby motivating the exploration of alternative 2D materials with more favorable charge-storage characteristics.^146,187 Among these emerging candidates, borophene stands out for its remarkable mechanical flexibility while maintaining structural integrity and tensile strength—surpassing even graphene under certain configurations—along with its theoretically predicted metallic nature.²⁶ Its tunable electronic structure endows borophene with versatility across various domains, including catalysis, sensing, and electronic devices.³¹ As a lightweight yet robust 2D material, borophene exhibits a unique combination of mechanical strength, flexibility, and electronic adaptability that distinguishes it from other 2D counterparts.

Moreover, borophene's inherently large surface area, high electronic conductivity, and superior carrier mobility make it an ideal candidate for supercapacitor electrodes.¹⁴⁷ Studies have shown that the specific capacitance of borophene surpasses that of graphene and reduced graphene oxide (rGO), underscoring its immense potential in energy-storage technologies.¹⁴⁸ A density functional theory (DFT) investigation of a boron electrode–electrolyte system predicted a theoretical specific capacitance of approximately 400 Fg⁻¹, nearly four times greater than that of graphene.¹⁴⁹ In addition, borophene exhibits a low diffusion barrier for lithium ions, enabling ultrafast ion transport and superior charge–discharge kinetics, which collectively contribute to its outstanding energy-storage performance.¹⁵⁰

In parallel, MBs have recently emerged as highly promising electrode materials for next-generation supercapacitors, owing to their combination of metallic conductivity, structural robustness, and rich redox activity.¹³ Unlike conventional carbon-based materials that primarily store charge through EDLC, metal borides feature mixed covalent-metallic M–B bonding. This bonding nature not only ensures excellent electrical conductivity (10⁵–10⁶ S m⁻¹) but also exposes abundant transition-metal active sites that facilitate reversible redox reactions.¹³ The strong covalent B–B and M–B frameworks impart outstanding mechanical stability, allowing the electrodes to withstand repeated ion intercalation/deintercalation without structural degradation, an essential requirement for long-term cycling durability.^13,24 Additionally, the intrinsically high DOS near the Fermi level promotes rapid electronic transport, while multivalent transition-metal centers (e.g., Ti³⁺/Ti⁴⁺, Co²⁺/Co³⁺, Fe²⁺/Fe³⁺) provide additional pseudocapacitive contributions.^13,24,29 Together, these characteristics enable metal-boride-based electrodes to deliver high specific capacitances (typically 600–800 F g⁻¹) as reported under the corresponding electrolyte and voltage window, excellent rate performance, and superior cycling stability, with capacitance retention exceeding 90% after 5000 cycles.^151,152 Importantly, many metal borides can be synthesized via cost-effective and scalable methods such as molten-salt reactions or hydrothermal processing, offering strong potential for sustainable and industrially viable production.

Accordingly, this section provides an overview of the electrochemical performance of borophene and metal borides in supercapacitor applications, with a particular focus on the underlying charge-storage mechanisms that have been elucidated through both experimental and theoretical studies. It should be noted that the literature reports capacitance values at different evaluation levels. Electrode-level capacitance is typically obtained from single-electrode measurements (often in three-electrode configurations) and is commonly normalized by active mass (F g⁻¹) or electrode area (mF cm⁻²). In contrast, device-level capacitance is derived from assembled two-electrode configurations such as symmetric cells, asymmetric supercapacitors, and flexible or solid-state devices, and reflects the performance of the complete device, where metrics can be affected by electrode balancing, packaging, electrolyte/separator resistance, and mass loading. Capacitance values are discussed at the electrode level or device level and should be compared only within the same testing configuration.

4.1. Borophene and its hybrids for SCs

4.1.1. Pristine borophene electrodes. Since 2018, research on the application of borophene-based materials as electrodes in supercapacitors has steadily progressed, achieving notable advances in both theoretical understanding and experimental validation (Fig. 10). The first experimental breakthrough originated from elemental boron systems, in which Xue et al. synthesized single-crystalline boron nanowires via CVD and demonstrated, for the first time, their feasibility as supercapacitor electrode materials.¹⁵³ The assembled devices exhibited outstanding electrochemical performance across alkaline, neutral, and acidic electrolytes, delivering high specific capacitance and excellent rate capability, with a maximum areal capacitance of 60.2 mFcm⁻². In the same year, 2D borophene nanosheets prepared through ultrasonication-assisted liquid-phase exfoliation were also reported to possess remarkable energy-storage properties. The electrodes operated over a wide potential window of 3.0 V, achieving an energy density of 46.1 Whkg⁻¹ at a power density of 478.5 Wkg⁻¹, while retaining 88.7% of their initial capacitance after 6000 charge–discharge cycles, highlighting their excellent electrochemical stability.¹⁰²

Fig. 10 Roadmap of borophene-based supercapacitor research in recent years.

In 2021, Joshi et al. demonstrated that introducing oxygen vacancies into borophene sheets could significantly enhance their energy-storage capability in both aqueous and non-aqueous electrolytes.¹⁵⁴ This improvement was attributed to the enlarged electroactive surface area, enhanced structural stability, and increased electrical conductivity (96.12 Sm⁻¹).¹⁵⁴

In 2022, Abdi et al. fabricated transferable two-dimensional borophene nanosheets through aluminum-assisted CVD.⁴⁹ The obtained borophene sheets exhibited polygonal crystalline domains with thicknesses corresponding to monolayer, bilayer, and trilayer structures (4.2 Å, 7.4 Å, and 11.5 Å, respectively). Atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), and SAED jointly confirmed that the dominant structure was the χ₃ phase, accompanied by partial features of the β₁₂ phase (Fig. 11a–d). The synthesized borophene sheets were subsequently transferred via a wet-transfer process and deposited onto a graphite foil substrate to construct nanosupercapacitor electrodes. Electrochemical characterization revealed that borophene significantly enhanced the electrode performance. In a three-electrode system, the borophene-based electrode achieved a specific capacitance exceeding 350 F g⁻¹ at 5 mVs⁻¹, which is much higher than that of pristine graphite foil (only 0.25 Fg⁻¹). The cyclic voltammetry (CV) curves maintained ideal capacitive behavior across scan rates of 5–100 mVs⁻¹, while the galvanostatic charge–discharge (GCD) profiles exhibited nearly symmetric charge–discharge curves, indicating excellent rate capability and reversibility.

Fig. 11 Morphology and electrochemical characterization of a 2D borophene supercapacitor. (a) FESEM images of as-prepared borophene sheets on the initial substrate (gray parts, borophenes; bright regions, Al domains). (b) HRTEM lattice image of a borophene sheet showing the parallel strips with an interplanar distance of around 4.15 Å with an SAED pattern of a typical sheet as an inset. (c) Lattice structure. (d) Statistical distribution of the thickness of borophene sheets collected over 30 microscale sheets. (e) Area-normalized and sCap calculated from a GCD test. (f) Area-normalized and sCap calculated from CV measurements. (g) CV curves at different scan rates. (h) GCD test at different current densities. Reproduced from ref. 49 with permission from American Chemical Society, copyright 2022.⁴⁹

Furthermore, joint density functional theory (JDFT) calculations were conducted to evaluate the DOS, potential of zero charge (PZC), and specific capacitance of various borophene polymorphs, including β₁₂, χ₃, and the striped phase. Theoretical results predicted a specific capacitance in the range of 468–600 Fg⁻¹, which is in strong agreement with the experimental value (>350 Fg⁻¹), confirming that borophene possesses a substantial quantum capacitance contribution and outstanding surface charge-storage capability. Overall, this work provides the first experimental demonstration of transferable borophene sheets as supercapacitor electrodes, showing a specific capacitance far superior to those of conventional carbon-based materials and establishing an important experimental foundation for developing high-performance borophene-based energy storage devices.⁴⁹

Further advancements were reported in 2024, when stacked borophene electrodes fabricated through a mechanical rolling process, using an optimized borophene/additive (B/A) ratio of 1 : 1, exhibited outstanding mechanical flexibility and superior electronic properties, making them ideal for next-generation flexible supercapacitor devices.³⁵Fig. 12a presents a schematic of the flexible borophene-based supercapacitor. The cyclic voltammetry (CV) curves of the stacked device in both flat and rolled states (Fig. 12b and c) retain nearly symmetric rectangular profiles after rolling, indicating excellent reversibility and ideal capacitive behavior. From the CV analysis, the rolled SC device with a borophene electrode delivered a specific areal capacitance of 374.6 mFcm⁻² at a scan rate of 10 mVs⁻¹ and 80.5 mFcm⁻² at 200 mVs⁻¹ (Fig. 12d). Galvanostatic charge–discharge (GCD) curves (Fig. 12e) displayed a linear profile, yielding a calculated areal capacitance of 417.3 mFcm⁻², nearly twice that of non-stacked borophene. As shown in Fig. 12f, the non-stacked borophene electrode already surpasses typical carbon-based capacitors in capacitance. Electrochemical impedance spectroscopy (EIS) results (Fig. 12g) reveal that the rolled device possesses lower internal resistance at low frequencies compared to its planar counterpart, confirming improved charge transport. Remarkably, the rolled electrode retained 89.3% of its capacitance after 5000 continuous charge–discharge cycles (Fig. 12h).

Fig. 12 Electrochemical characterization of a stacked borophene-based dual-layer supercapacitor. (a) Schematic diagram of a borophene-based dual-layer supercapacitor. (b) CV curves of the supercapacitor in a flat state with B/A = 1 : 1 at different scan rates. (c) CV curves of the supercapacitor after being rolled with B/A = 1 : 1 at different scan rates. (d) Area capacitance calculated from CV measurements. (e) GCD curves of the supercapacitor after being rolled with B/A = 1 : 1 at different current densities. (f) Area capacitance calculated from the GCD test by multi-material comparison. (g) Nyquist plot of the borophene-based supercapacitor in different states. (h) Cycling performance of the rolled borophene-based device. Reproduced from ref. 35 with permission from Elsevier Ltd, copyright 2024.³⁵

Subsequent studies also explored element-doped borophene systems. For instance, mono- and few-layer β₁₂ borophene doped with sulfur and iron, synthesized under controlled microwave-assisted conditions, exhibited promising electrochemical properties for supercapacitor applications. In another investigation, CVD-grown borophene electrodes achieved a specific areal capacitance of 44.5 mFcm⁻² and a gravimetric capacitance of 4238 Fg⁻¹ at a scan rate of 5 mVs⁻¹, maintaining 60% capacitance retention after 10 000 cycles.¹⁵⁶ More recently, Yong et al. developed zinc-ion supercapacitors using modified hydroxylated borophene combined with MXene-integrated PVA gel electrolytes.¹⁵⁷ The PVA borophene hybrid formed a porous three-dimensional framework that facilitated Zn²⁺ ion diffusion, as hydroxylated borophene disrupted PVA chain ordering and expanded the inter-chain spacing, thereby enhancing ionic transport. To facilitate comparison across different two-dimensional materials, Table 1 compares borophene with representative 2D systems used in supercapacitor applications, focusing on their typical charge-storage mechanisms, key characteristics, advantages and limitations.

Table 1 Comparison of representative 2D materials for supercapacitor applications

2D material system	Typical charge-storage mechanism	Stability	Scalability of synthesis	Advantages	Limitations	Ref.
Borophene	EDLC + quantum-capacitance-assisted surface charging; pseudocapacitance in hybrids	Prone to oxidation; requires passivation or encapsulation	Currently limited; mainly vapor-deposition routes	High carrier mobility; strong quantum-capacitance contribution; rich polymorphism	Low production rate; environmental sensitivity	28, 35, 49, and 156
Graphene	EDLC-dominated	Excellent chemical stability	Highly scalable (CVD, exfoliation)	High surface area; mature processing	Low DOS near the Dirac point; limited quantum capacitance	12–14
MXenes	EDLC + pseudocapacitance (surface terminations)	Susceptible to oxidation in aqueous environments	Moderate; etching-based synthesis	High volumetric capacitance; tunable surface chemistry	Oxidation; termination-dependent performance	15–18
TMDs	Predominantly pseudocapacitance/intercalation	Generally good chemical stability	Scalable via CVD or solution routes	Rich redox chemistry	Low conductivity; sluggish kinetics	19–21
MOFs/COFs	Pseudocapacitance	Framework-dependent stability	Limited scalability	Tailorable porosity	Poor conductivity; structural instability	22–25

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4.1.2. Borophene-carbon hybrid electrodes. In 2020, Wu et al. reported the fabrication of 2D heterostructured electrodes composed of anisotropic boron-carbon hybrid nanosheets. These hybrid electrodes, featuring high interlayer conductivity, abundant ion transport channels, and chemically accessible active surfaces, greatly facilitated ion migration and storage within the electrolyte.¹⁵⁸ The anisotropic boron-carbon nanosheet (ABCN)-based assembled flexible supercapacitor device (Fig. 13a) exhibited a high volumetric capacitance of 534.5 F cm⁻³ and an energy density of 167.05 mWh cm⁻³, while maintaining 94% capacitance retention after 10 000 charge–discharge cycles (Fig. 13b and c). As illustrated in Fig. 13d, bulk boron possesses a limited surface area, restricting ion accumulation and consequently reducing energy-storage capability. Although boron nanosheets offer an enlarged surface area, their insufficient interlayer conductivity impedes charge transport and energy-storage efficiency. By incorporating carbon nanosheets between the boron layers, the ABCN hybrid structure achieved enhanced interfacial charge transfer and improved charge-transport kinetics, thereby boosting overall electrochemical performance. Owing to its superior electrochemical characteristics and mechanical flexibility, this device demonstrated significant potential for use in wearable physiological signal sensors.

Fig. 13 Supercapacitor application of borophene-carbon hybrid electrodes. (a) Illustration of a flexible supercapacitor. (b) Capacitances and energy densities of bulk boron, boron nanosheets, and ABCN FSCs. (c) Energy density of ABCN FSCs compared to other 2D material-based FSCs. (d) Electron transfer and ion storage in variously designed electrodes. Reproduced from ref. 158 with permission from Wiley-VCH GmbH, copyright 2020.¹⁵⁸

Considering the synergistic effects of electrical conductivity, mechanical robustness, and structural stability, Nanda et al. investigated a flexible all-solid-state symmetric supercapacitor based on a 2D borophene-graphene composite hydrogel.¹⁵⁹ The high electrical conductivity of borophene, combined with the large surface area of graphene, enabled the formation of a highly porous and interconnected three-dimensional framework, making the material highly suitable for flexible energy-storage devices. Furthermore, this study extended its findings to potential applications in wearable electronics, smart packaging, and low-power self-sustaining sensor systems.

In addition, borophene-based hybrid structures can be synthesized on various substrates such as Ag, Au, and Cu. Integration with carbon nanotubes or amorphous carbon can further increase the density of electrochemically active sites and improve electron-transport pathways.^160,161 Borophene grown on Ag(111) exhibits graphene-like anisotropic behavior, while on Au(111) it demonstrates metallic characteristics.^117,161 By optimizing CVD parameters, highly crystalline borophene films with controllable thickness have also been successfully grown on Cu substrates. The resulting borophene-based devices achieved a specific areal capacitance of 44.5 mF cm⁻² at a scan rate of 5 mV s⁻¹, and their low interfacial resistance further enhances their suitability for energy-storage applications.¹⁵⁶ Moreover, when intercalated with sulfur- and nitrogen-doped zero-dimensional carbon nanospheres, borophene exhibited a high specific surface area of 2100 m² g⁻¹ and an improved specific capacitance of 833 F g⁻¹ in 2 M KOH electrolyte at a low scan rate of 10 mV s⁻¹.¹⁶²

4.1.3. Borophene-polymer and hybrid composite electrodes. Conducting polymer composite electrodes based on borophene (such as PANI and PEDOT:PSS) enable a synergistic effect between electronic conduction and pseudocapacitive redox reactions, combining high stability with large specific capacitance.¹⁶³ A nanocomposite electrode composed of two-dimensional borophene and sulfur/nitrogen co-doped mesoporous carbon achieved a specific capacitance of 607 F g⁻¹, an energy density of 29.2 Wh kg⁻¹, and a power density of 3500 W kg⁻¹.¹⁶⁴ The enhanced electrochemical performance is primarily attributed to the strengthened charge coupling at the hybrid interfaces and the hierarchical porous architecture that facilitates superior ion accessibility.

4.1.4. Borophene-MXene hybrid electrodes. Borophene-MXene composite aerogels exhibit outstanding energy-storage performance owing to their highly porous architecture and abundance of chemically active sites, which facilitate rapid ion transport within the electrolyte.^165,186 Through electrostatic self-assembly, positively charged borophene sheets were combined with negatively charged MXene nanosheets, and electrophoretic deposition was employed to fabricate MXene-borophene (MxB) electrodes, effectively preventing undesirable sheet restacking.¹⁶⁶ The hybrid material, with an increased number of electrochemically active sites and enlarged interlayer spacing, significantly enhanced ion diffusion and energy-storage capacity. The CV and GCD curves of the MxB 50 : 50//MxB 50 : 50 cell are exhibited in Fig. 14a and b. The assembled device delivered a specific capacitance of 375 F g⁻¹ (187.5 F cm⁻² at 1 A g⁻¹) and retained 71% of its capacitance even at a high current density of 20 A g⁻¹ (Fig. 14c). After 10 000 charge–discharge cycles, the device maintained 93.6% of its capacitance with a coulombic efficiency of 100% (Fig. 14d). The MXB-based supercapacitor exhibited an energy density of 75 Wh kg⁻¹ at a power density of 600 W kg⁻¹ and maintained 53.3 Wh kg⁻¹ at 12 000 W kg⁻¹ (Fig. 14e), surpassing the performance of most previously reported MXene-based supercapacitors. The practical applicability of the device was further validated by powering a red LED (Fig. 14f).

Fig. 14 Supercapacitor application of borophene-Mxene hybrid electrodes. (a) CV and (b) GCD profiles of the cell (MxB 50 : 50//MxB 50 : 50) at various scan rates and current densities. (c) Specific capacitance and areal capacitance are calculated at various current densities. (d) Cycling stability and coulombic efficiency test for 9000 cycles at 10 A g⁻¹. (e) Ragone plot comparing the SC performance of the device with those of other reported materials. (f) Red LED illumination. (g) and (i) Photographs of the bending (1 cm) and twisting (40° axial) of a full cell device, respectively. (h) and (j) Comparison of the GCD profiles of the device in different cycles for mechanical bending and twisting cases, at 1 A g⁻¹, respectively. Reproduced from ref. 166 with permission from Elsevier Ltd, copyright 2024.¹⁶⁶

To evaluate its flexibility, the device was subjected to bending with a radius of 1 cm and torsion at 40° (Fig. 14g–i), with no observable structural damage. After 500 bending and twisting cycles, the device retained 78.48% and 90.39% of its initial capacitance, respectively (Fig. 14h–j). Yong et al. further reported zinc-ion capacitors based on self-assembled MXene/borophene film electrodes, where hydroxylated borophene served as a multifunctional mediator.⁵⁰ The mediator enabled synergistic interfacial modulation within the heterostructure, facilitating high-efficiency energy storage at the interface. Compared with pristine MXene, the symmetric zinc-ion capacitor demonstrated a fourfold improvement in cycling stability, achieving an areal capacitance of 245 mF cm⁻² with almost no performance degradation after 40 000 cycles.

Nevertheless, challenges persist in the synthesis of Mxene-borophene hybrid systems, including borophene oxidation and unfavorable interactions between MXene surface terminations (–O, –OH, –F) and borophene, which can disrupt the borophene lattice. To address these issues, optimization of the synthesis environment, modification of MXene surface terminations, adoption of appropriate assembly techniques, and selection of suitable solvents and additives are required.¹⁶⁷ Conducting synthesis under an inert atmosphere can effectively minimize borophene oxidation, while optimized temperature and pressure conditions help to prevent structural defects.¹⁶⁸ Moreover, substituting –OH groups on MXene can mitigate adverse interactions with borophene.²¹,⁷⁹ Techniques such as ligand exchange, surface coating, or heteroatom doping can further tune the surface chemistry of MXenes, enhancing compatibility with borophene and suppressing unwanted reactions.^169,170 Layer-by-layer deposition enables more controlled integration of hybrid structures, ensuring uniform interfaces and minimal interfacial interactions.¹⁷¹ Proper interface engineering between borophene and MXene can yield more stable and functional hybrid architectures. In addition, dispersing MXenes and borophene in nonpolar solvents helps reduce excessive surface interactions, while the introduction of surfactants or dispersants improves dispersion uniformity, prevents aggregation, and facilitates homogeneous mixing.¹⁶⁹ Post-synthesis annealing under an inert atmosphere can further stabilize the hybrid structure, heal structural defects, and strengthen the interfacial bonding between borophene and MXene.¹⁶¹

4.1.5. Borophene-MOF hybrid electrodes. Recently, researchers successfully synthesized a borophene-metal–organic framework (MOF, HKUST-1) composite (Borophene@HKUST-1) using a facile mixing strategy and applied it to the fabrication of flexible supercapacitor electrodes.¹⁷² This study highlights the synergistic integration of the high electrical conductivity of two-dimensional borophene with the large specific surface area and porous architecture of HKUST-1, offering a new design concept for flexible energy-storage electrodes. SEM analysis (Fig. 15a) revealed that the Borophene@HKUST-1 composite exhibits an octahedral HKUST-1 crystalline framework decorated with nanoscale borophene layers on the surface. This hybrid configuration maintains the structural integrity of both components while providing an enlarged interfacial contact area and hierarchical porosity, which facilitate rapid ion diffusion and efficient electron transport.

Fig. 15 Supercapacitor application of borophene-MOF hybrid electrodes. (a) SEM micrographs of the Borophene@HKUST-1 structure. (b) Cyclic voltammograms of various flexible electrodes recorded in a potential range of 0.0 V–0.4 V at a scan rate of 50 mV s⁻¹ in 1.0 M H₂SO₄ solution. (c) Nyquist diagrams of various flexible electrodes recorded in 1.0 M H₂SO₄ solution. (d) GCD curves of the CFE4 flexible electrode at 1–5 mA cm⁻² current densities. (e) Specific capacitance curves of various flexible electrodes recorded over 1000 cycles in 1.0 M H₂SO₄ solution. (f) Coulombic and energy efficiency curves of the CFE4 flexible electrode recorded over 1000 cycles in 1.0 M H₂SO₄. Reproduced from ref. 172 with permission from Elsevier Ltd, copyright 2025.¹⁷²

Electrochemical characterization demonstrated the outstanding energy-storage performance of the composite electrode in 1 M H₂SO₄ electrolyte. The CV profiles (Fig. 15b) display nearly rectangular shapes, indicative of ideal electric-double-layer capacitive behavior. The electrochemical impedance spectroscopy (EIS) spectrum (Fig. 15c) shows a low charge-transfer resistance (R_ct) of only 3.94 Ω, substantially smaller than that of pristine borophene or HKUST-1 electrodes, confirming a pronounced synergistic effect between the two materials. GCD curves (Fig. 15d) further validate the excellent electrochemical response, with the composite electrode delivering a high areal capacitance of 333 mF cm⁻² at a current density of 3 mA cm⁻² and retaining 63.4% of its initial capacitance after 1000 cycles (Fig. 15e). The corresponding coulombic- and energy–efficiency curves (Fig. 15f) indicate that the system maintains superior long-term electrochemical stability.

The superior performance of the Borophene@HKUST-1 electrode can be primarily attributed to the synergistic coupling between borophene and HKUST-1. The MOF component provides a highly porous network and large specific surface area, enhancing ion adsorption and storage, while the high carrier mobility of borophene significantly improves electrical conductivity and charge-transfer kinetics. Moreover, the reversible Cu²⁺/Cu⁺ redox couples within HKUST-1 contribute additional pseudocapacitance during cycling, imparting both electric-double-layer and faradaic characteristics to the composite. This dual-mechanism behavior effectively enhances the overall energy density and cycling durability. Collectively, this work demonstrates the potential of the Borophene@HKUST-1 composite system for constructing flexible supercapacitors with high energy density and long-term stability, establishing a promising strategy for the functional integration of two-dimensional borophene with MOF-based materials.

4.2. Metal-boride derived materials for supercapacitors

4.2.1. Monometallic metal boride electrodes. As energy-storage materials combining metallic conductivity with multivalent pseudocapacitive behavior, metal borides have been actively investigated in the field of SCs for nearly a decade. Early studies revealed that the strong mixed covalent-metallic M–B bonds endow these compounds with excellent electrical conductivity and chemical robustness, making them an attractive complement to conventional carbon-based materials. Initial investigations primarily focused on monometallic borides, such as CoB, NiB, and FeB. Representative studies demonstrated that even in their amorphous states, metal borides maintain high carrier mobility and distinct pseudocapacitive behavior. For instance, NiB nanoparticles were first employed as SC electrodes in 2017, delivering a high specific capacitance of 2230 F g⁻¹ at 1 A g⁻¹, with the charge-storage mechanism attributed to the reversible Ni²⁺/Ni³⁺ redox couple.¹⁷³ The assembled asymmetric supercapacitor (ASC) device was subsequently constructed using Ni_xB/G as the electrode material (Fig. 16a).¹⁷⁴ The device exhibited a typical GCD curve within a current density range of 1 to 20 A g⁻¹ (Fig. 16b). As depicted in Fig. 16c, both ASCs demonstrated excellent capacitive behavior, with the Ni_xB/G//AC electrode delivering a specific capacitance of 133 F g⁻¹ at 1 A g⁻¹ and maintaining 60 F g⁻¹ even at a higher current density of 20 A g⁻¹. Furthermore, the ASC achieved an energy density of 50.4 Wh kg⁻¹ at a power density of 200 W kg⁻¹ (Fig. 16d) and preserved nearly 96% of its initial capacitance after 2000 continuous charge–discharge cycles (Fig. 16e). In addition, several ASCs connected in series were able to light up an “XJTU” logo (Fig. 16f), illustrating the device's excellent stability and practical applicability in energy-storage systems. At approximately the same time, CoB and FeB systems were reported to possess strong ion-adsorption capability and efficient electron-transport characteristics, providing the earliest experimental evidence for the coexistence of pseudocapacitive and EDLC mechanisms.

Fig. 16 Supercapacitor application of monometallic MB electrodes. (a) The representation of the ASC cell, (b) GCD curves of the Ni_x B/G//AC ASC device, (c) the calculated specific capacitances, (d) Ragone plots of Ni_x B/G//AC, Ni_x B//AC, and other ASCs, (e) cycle retention of the Ni_x B/G//AC ASC after 20 000 cycles at 5 A g⁻¹, and (f) an XJTU logo applying three ASCs made using 27 red LED lamps that light up. Reproduced from ref. 174 with permission from American Chemical Society, copyright 2019.¹⁷⁴

These pioneering works established transition-metal borides as a new class of high-power electrode materials. Their intrinsically high electrical conductivity effectively minimizes polarization losses, while the multivalent metal centers offer favorable faradaic reaction kinetics. Nevertheless, monometallic borides generally suffer from limited surface area and a deficiency of electrochemically active sites, which makes it challenging to balance cycling stability and energy density. These inherent limitations have motivated subsequent research toward composite architectures and multimetallic synergistic designs to overcome the trade-off between power and energy performance.

4.2.2. Bimetallic metal boride electrodes. Between 2018 and 2021, the research focus gradually shifted from monometallic systems to multimetallic cooperative architectures. Researchers recognized that the differences in redox potentials and electronic configurations among various metal ions could induce a “multi-electron activation and synergistic pseudocapacitance” effect within the boride framework. Consequently, bimetallic borides such as Ni–Co, Co–Fe, and Ni–Fe emerged as major research hotspots. During this period, Chen et al. reported an amorphous Ni–Co–B alloy electrode that achieved an impressive specific capacitance of 2226.96 F g⁻¹ and an energy density of 66.4 Wh kg⁻¹, outperforming most conventional transition-metal oxides and hydroxides (Fig. 17).¹⁷⁵ Mechanistic analysis revealed that the Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ redox couples operate synergistically within overlapping potential regions, facilitating cooperative charge transfer and enhancing pseudocapacitive contributions. Subsequently, Meng et al. synthesized ultrathin Co–Fe–B nanosheets (∼10 nm thick) through a chemical reduction method, which delivered a specific capacitance of 981 F g⁻¹ at 1 A g⁻¹ and retained 73% of its capacity after 3000 cycles.¹⁷⁶ The nanosheet morphology effectively shortened ion-diffusion pathways, highlighting the kinetic advantages of two-dimensional architectures.

Fig. 17 Supercapacitor application of bimetallic MB electrodes. (a) Schematic for the preparation of Ni–Co–B. (b) The GCD curves of the ASC cell under different current densities, (c) the Ragone plot of the ASC cell (inset: the red, purple and green LEDs powered by two ASC cells connected in series). Reproduced from ref. 175 with permission from Elsevier Ltd, copyright 2017.¹⁷⁵

Another defining feature of this stage was the introduction of structural engineering. By constructing core–shell architectures, heterointerfaces, and porous networks, researchers successfully mitigated particle agglomeration and high interfacial resistance. For instance, a CoMoO₄/CoB heterostructured electrode achieved a specific capacitance of 436 F g⁻¹ and an energy density of 23.2 Wh kg⁻¹, while a Ni₃V₂O₈/CoB heterostructure reached nearly 1800 F g⁻¹, benefiting from synergistic redox activity and pore-structure compatibility.¹⁷⁷ Mechanistically, studies during this period elucidated three key aspects: (i) electronic conduction networks created by M–B mixed bonds enabling continuous charge pathways; (ii) multivalent coupling between dual-metal centers (e.g., Ni/Co, Fe/Co) modulating electron density to reinforce faradaic pseudocapacitance; and (iii) surface/interface activation, where nanosheet or hollow structures expose abundant active sites to enhance charge-storage efficiency.

4.2.3. Ternary and other composite metal boride electrodes. Since 2021, advancements in situ characterization techniques and density functional theory calculations have provided deeper insights into the energy-storage mechanisms of metal boride electrodes. Aydın et al. proposed that the charge-storage behavior of transition metal borides arises from a synergistic interplay between quantum capacitance and pseudocapacitance. Meanwhile, strategies involving interfacial synergy and hierarchical composite engineering have been extensively adopted. Integrating borides with conductive carbon materials or polymers effectively enhances electron and ion transport while improving mechanical flexibility. For instance, a hybrid supercapacitor (HSC) with a Ni₂B/reduced graphene oxide (RGO) composite electrode exhibited a remarkable specific capacitance of 1073 F g⁻¹ at 1 A g⁻¹, retaining 72.4% of its capacity after 2500 cycles (Fig. 18).¹⁷⁸ The high electrical conductivity of graphene significantly reduced interfacial resistance, whereas the interlayer porosity of RGO facilitated efficient electrolyte ion diffusion.

Fig. 18 HSC of composite electrodes. (a) The internal mechanism and structures for the capacitors fabricated with Ni₂B, Ni₂B/RGO and Ni₂B/RGO-200. (b) Comparison of specific capacitances of Ni₂B and Ni₂B/RGO hybrid electrodes prepared at different annealing temperatures and different current densities. (c) Cycling performance of the HSC at a constant current density of 6 A g⁻¹. Reproduced from ref. 178 with permission from Elsevier Ltd, copyright 2017.¹⁷⁸

Surface-chemistry modulation has also emerged as a critical design strategy. Through boron doping, controlled oxidation, or incorporation of nonmetallic elements, researchers have achieved precise regulation of surface charge distribution and electrolyte affinity. A representative example is the Ni₃B/Ni(BO₂)₂@NiCoMoO₄ hierarchical composite electrode, which exhibited a specific capacitance of 370.7 F g⁻¹ at 1 A g⁻¹ and an energy density of 131.8 Wh kg⁻¹.¹⁷⁹ In this configuration, the highly conductive Ni₃B inner layer and the pseudocapacitive oxide shell jointly establish dual pathways for rapid electron transport and ion-active interfacial reactions, resulting in a substantial enhancement in rate capability.

With the maturation of materials-design concepts, research entered a multicomponent synergy and composite optimization stage. Ternary and hybrid TMBs have become central research targets. Introducing a third metal species (e.g., Fe, Cu, or Mo) into Ni–Co–B frameworks yielded electronically tunable multimetal systems balancing conductivity and chemical stability. NiCoFe–B nanosheets displayed a specific capacitance of 192 Fg⁻¹, an energy density of 45 Whkg⁻¹, and retained 86% of their capacity after 10 000 cycles.¹⁸⁰ The inclusion of Fe broadened the redox potential window, enabling simultaneous high energy and high power performance. Building on this concept, an ASC device with NiSe₂–Fe₃Se₄@NiCoB core-hell composite electrodes exhibited combined electric double-layer and pseudocapacitive characteristics within a voltage window of 0–1.6 V. The CV curves maintained well-defined shapes as the scan rate increased from 5 to 100 mV s⁻¹, indicating excellent rate capability and efficient electron transport (Fig. 19a–c). The device delivered specific capacitances of 327.1, 313.9, 282.2, 250.1, and 214.6 C g⁻¹ at current densities of 1, 5, 10, 15, and 20 A g⁻¹, respectively (Fig. 19d and e). It achieved a high energy density of 72.7 Wh kg⁻¹ at a power density of 710.7 W kg⁻¹ (Fig. 19f) and retained 91.1% of its capacity along with 99.8% of its coulombic efficiency after 20 000 charge–discharge cycles (Fig. 19g), demonstrating outstanding cycling durability and structural integrity.¹⁸¹

Fig. 19 Supercapacitor application of ternary MB electrodes. (a) CV profiles of the NiSe₂–Fe₃Se₄@NiCoB/CC and CSC electrodes at 10 mV s⁻¹. (b) CV profiles of the ASC cell obtained in various potential windows at 20 mV s⁻¹, and (c) at various scan rates. (d) GCD profiles and (e) specific capacitance values of the ASC cell at several current densities. (f) Ragone plots of the ASC cell and related references. (g) Capacitance retention and coulombic efficiency of the ASC cell. Reproduced from ref. 181 with permission from Elsevier Ltd, copyright 2022.¹⁸¹

Moreover, rare-earth and refractory borides such as LaB₆, MoB, and TiB₂ have recently entered the supercapacitor arena. Their strong covalent bonding and high thermal stability not only extend cycle life but also ensure operational reliability across wide temperature ranges. For instance, MoB nanosheet electrodes exhibited a specific capacitance of 445 Fg⁻¹ and 98% retention in H₂SO₄ electrolyte, reaffirming the suitability of borides for harsh-environment energy storage.¹⁸² Collectively, these advances highlight an integrated mechanism combining electron–ion coupling, faradaic redox reactions, and quantum-capacitance effects, which together underpin the notable performance of transition-metal borides in modern supercapacitor technologies.

To provide an overview of the supercapacitor performance discussed above, Table 2 summarizes the reported electrochemical metrics of borophene-based electrodes, including pristine borophene, structurally engineered borophene, and borophene-based composites prepared via different synthesis routes. The comparison highlights the strong dependence of the measured capacitance on synthesis procedure, electrode architecture, device configurations, and performance.

Table 2 Supercapacitor performance of borophene-based electrodes prepared by different synthesis routes

Electrode system	Synthesis	Configuration	Performance	Cycling/durability	Ref.
Single-crystalline boron nanowires	CVD	Device tested in alkaline/neutral/acid electrolytes	Max areal capacitance of 60.2 mF cm^−2	—	153
2D borophene nanosheets	Ultrasonication-assisted LPE	Wide voltage window of 3.0 V	Energy density of 46.1 Wh kg^−1 at 478.5 W kg^−1	88.7% after 6000 cycles	102
Oxygen-vacancy borophene sheets	Defect engineering	Aqueous & non-aqueous electrolytes	Enhanced conductivity reported (96.12 S m^−1)	—	154
Transferable 2D borophene sheets on graphite foil	Al-assisted CVD; wet transfer	Three-electrode; graphite foil substrate	>350 F g^−1 (≈350 mF cm^−2)	—	49
Borophene polymorphs	JDFT for β12/χ3/striped	—	Predicted 468–600 F g^−1	—	49
Stacked borophene electrode	Melting salt-assisted CVD	Flexible full cell	Areal C = 374.6 mF cm^−2 (CV, 10 mV s^−1); 417.3 mF cm^−2 (GCD)	89.3% after 5000 cycles	35
Thickness-controlled borophene films	CVD	—	44.5 mF cm^−2 and 4238 F g^−1 (as reported)	60% after 10 000 cycles	156
	On Cu substrate
Anisotropic boron/carbon hybrid nanosheets	2D heterostructured B/C hybrid	Flexible SC	Volumetric C = 534.5 F cm^−3; E = 167.05 mWh cm^−3	94% after 10 000 cycles	158
Borophene/graphene composite hydrogel	3D porous interconnected hydrogel	All-solid-state symmetric SC	—	—	159
Borophene with S/N-doped 0D carbon nanospheres	Intercalation with doped carbon spheres	—	Specific surface area of 2100 m^2 g^−1; C = 833 F g^—1	—	162
Borophene with an S/N co-doped mesoporous carbon composite	Hierarchical porous hybrid	—	C = 607 F g^−1; E = 29.2 Wh kg^−1; P = 3500 W kg^−1	—	164
Mxene/borophene	Electrostatic self-assembly with electrophoretic deposition	Symmetric cell (MxB50 : 50//MxB50 : 50)	C = 375 F g^−1 (≈187.5 F cm^−2, 1 A g^−1); E = 75 Wh kg^−1 at 600 W kg^—1	93.6% after 10 000 cycles; bending/twisting retention also reported	166
Mxene/borophene film	Hydroxylated borophene as a mediator	Symmetric Zn ion capacitor	Areal C = 245 mF cm^−2	Almost no degradation after 40 000 cycles	50
Borophene@HKUST-1 (MOF hybrid)	Facile mixing; borophene-decorated HKUST-1	Flexible electrode; 1 M H2SO4	R ct = 3.94 Ω; areal C = 333 mF cm^−2 (3 mA cm^−2)	63.4% after 1000 cycles	172

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4.3. Device-level performance and configurations

While a substantial body of work has focused on elucidating the intrinsic electrochemical properties of boron-based materials at the electrode level, increasing attention has been directed toward their performance in device-relevant supercapacitor configurations, including full cells and flexible devices. Representative studies have demonstrated that the favorable conductivity and surface accessibility of borophene and metal borides can be retained beyond three-electrode testing when incorporated into assembled supercapacitor devices.^35,49,156

At the device level, boron-based materials have been implemented in a variety of architectures, such as symmetric supercapacitors, asymmetric supercapacitors, and hybrid configurations. In symmetric devices, borophene-based electrodes typically function as high-conductivity capacitive components, where fast surface charging enables high power output.^35,49 In contrast, in asymmetric or hybrid supercapacitors, metal borides are more frequently employed as pseudocapacitive electrodes paired with capacitive counter-electrodes, allowing for expanded operating voltage windows and enhanced energy density.^50,166

Flexible and solid-state supercapacitors further highlight the practical potential of boron-derived materials. Several studies have reported the integration of borophene- or boride-based electrodes with gel or solid electrolytes and flexible substrates, demonstrating mechanical compliance while maintaining electrochemical functionality under bending or deformation.^35,166,172 Such device formats are particularly relevant for wearable and miniaturized electronics, although their performance is often influenced by electrode thickness, interfacial resistance, and electrolyte compatibility.

Despite these encouraging demonstrations, it should be noted that most reported device-level studies remain at the proof-of-concept stage. Common limitations include relatively low areal mass loading, thin electrode architectures, and limited long-term cycling evaluation under practical operating conditions.^35,156,166 Consequently, further progress toward real-world applications will require systematic optimization at the device level, including electrode architecture design for higher loading, improved electrode–electrolyte interfaces, and standardized two-electrode testing protocols.

5. Conclusions, challenges, and future perspectives

In the past few years, boron-based materials particularly borophene and metal borides have emerged as promising next-generation electrode candidates for supercapacitors. Their remarkable combination of high electrical conductivity, metallic bonding character, and structural robustness provides distinct advantages over traditional carbon materials or transition-metal oxides/hydroxides. Two-dimensional borophene, with its lightweight nature, delocalized metallic electrons, and high density of states near the Fermi level, has demonstrated outstanding potential for fast and efficient charge storage. Meanwhile, transition-metal borides exhibit excellent electrochemical stability, multi-electron redox activity, and mechanical durability, offering a complementary route toward robust pseudocapacitive electrodes. Together, these materials represent a rapidly growing frontier for designing high-power, high-energy, and long-life supercapacitors.

5.1. Progress and advantages

Recent advances in both theoretical and experimental research have clarified how the intrinsic physicochemical features of boron-based materials translate into superior electrochemical behavior. In borophene, the coexistence of metallic bonding and two-dimensional morphology enables ultrafast electron transport and efficient ion adsorption. The tunability of its properties through heterostructure engineering, defect/vacancy creation, doping, and morphology control has been shown to significantly improve ionic diffusion, surface area, and interfacial conductivity. These design strategies not only optimize charge transfer but also enhance mechanical flexibility and chemical adaptability—attributes that are crucial for miniaturized and flexible energy-storage systems.

In the case of metal borides, the strong covalent-metallic M–B bonding framework ensures high electrical conductivity (10⁵–10⁶ Sm⁻¹), while transition-metal centers (such as Ni, Co, Fe, and Mo) serve as redox-active sites for reversible faradaic reactions. The integration of such features results in electrodes that exhibit both electric-double-layer and pseudocapacitive characteristics, leading to high specific capacitance (typically 600–1000 Fg⁻¹) and outstanding cycling stability (>90% retention after 5000 to 20 000 cycles) within the reported voltage window and scanning rate. Moreover, the structural stability of borides under repeated ion intercalation cycles allows them to maintain excellent rate performance even at high current densities. Overall, the combination of borophene's quantum capacitance and boride's pseudocapacitive redox activity offers a new pathway toward synergistic energy-storage mechanisms—where electronic and ionic processes are effectively coupled to deliver both high power density and enhanced energy density.

5.2. Current challenges

Although borophene and metal borides have demonstrated remarkable electrochemical performance both theoretically and experimentally, several critical challenges remain regarding their scalable synthesis, structural stability, mechanistic understanding, and device integration. These issues have significantly limited the practical application and industrial translation of boron-based materials in supercapacitors. Overall, the current challenges are mainly associated with synthetic controllability, environmental stability, electrochemical mechanisms, compositional homogeneity, electrode–electrolyte compatibility, and sustainable manufacturing.

While a wide range of research directions have been proposed for boron-based supercapacitors, progress toward practical applications is currently constrained by a limited number of critical challenges.

First, scalable and reproducible synthesis with controlled phase, thickness, and defect density remains a primary bottleneck, particularly for borophene. While significant advances have been achieved at the laboratory scale, many high-quality borophene growth routes still rely on stringent conditions and substrate-specific processes, highlighting the need for manufacturable synthesis strategies compatible with large-area electrode fabrication.

Second, long-term environmental and electrochemical stability is a decisive requirement for real-world deployment. Oxidation-driven degradation has been repeatedly identified as a dominant limitation for borophene, motivating stabilization strategies such as chemical passivation and structural engineering. For metal borides and related MBenes, surface chemistry evolution and interfacial degradation during extended cycling similarly underscore the importance of durability-by-design approaches.

Third, electrode and device-level engineering under practical operating conditions represents a universal challenge. Most reported boron-based supercapacitors remain at the proof-of-concept stage, often employing low mass loadings and thin electrodes. Achieving commercially relevant areal capacitance and long-term cycling stability will require electrode architectures that maintain fast ion/electron transport at high loading, together with systematic validation in two-electrode configurations. Taken together, addressing these challenges (manufacturable synthesis, durability-oriented material design, and device-level engineering) is expected to have the greatest impact on bridging the gap between the promising material-level properties of boron-derived electrodes and their translation into practical supercapacitor technologies.

From a synthesis and structural-control perspective, the low-cost and scalable fabrication of high-quality borophene remains the foremost bottleneck for practical applications. The growth of borophene requires strict regulation of temperature, vacuum level, and substrate type, and is currently dominated by MBE and high-temperature CVD. Although these vapor-deposition techniques can yield highly crystalline borophene with well-defined polymorphs, they inherently suffer from low production rates, high energy consumption, and complex operational requirements, which severely limit their suitability for large-scale production.

Accordingly, high-purity borophene has been most consistently achieved via epitaxial vapor-deposition routes, particularly MBE and carefully controlled CVD, owing to their precise regulation of nucleation, phase evolution, and layer thickness. However, these approaches are intrinsically constrained by low production rates and strong substrate dependence. In contrast, recent CVD-based trends—such as transfer-free growth on insulating substrates and direct growth on commercially relevant metal foils or current collectors—suggest more realistic pathways toward improving production rates, albeit often at the expense of long-range crystalline continuity. Meanwhile, emerging top-down and conversion-based strategies, including electrochemical deposition and solution precursor conversion, offer attractive scalability but currently face challenges in achieving the phase purity, thickness uniformity, and reproducibility characteristic of epitaxial growth.

Beyond production rate limitations, the polymorphic nature and vacancy sensitivity of borophene further complicate reproducibility, as slight variations in synthesis conditions can induce phase transitions among β₁₂, χ₃, and α′ configurations, leading to performance inconsistency. In addition, commonly used precursors such as B₂H₆ and NaBH₄ pose safety and environmental concerns. To address these challenges, current research is increasingly focused on developing low-temperature, green, and scalable synthesis routes such as molten-salt-assisted methods, electrochemical deposition, and solution-based precursor conversion, which are aimed at improving both structural controllability and production efficiency. Similar issues arise in metal boride systems, where phase segregation and compositional inhomogeneity during chemical reduction or co-deposition disrupt electrical continuity and undermine long-term electrode stability, underscoring the need for improved compositional control in scalable synthesis.

The environmental stability and structural degradation of boron-based materials present another major challenge. Due to its high surface reactivity, borophene readily oxidizes in air to form a B₂O₃ passivation layer, which compromises its metallic nature and two-dimensional conduction network, leading to the deterioration of electrochemical performance. Similar issues are observed in metal borides, particularly in amorphous or nanoparticulate electrodes, where high surface areas can induce volume expansion and interfacial side reactions, thereby reducing cycling stability. To mitigate these effects, several structural stabilization strategies have been proposed, such as surface hydrogenation or fluorination to passivate active sites, graphene or h-BN encapsulation to achieve gas isolation, and the construction of heterostructures such as borophene/MXene and borophene/graphene to provide electronic passivation and physical protection. While these approaches improve environmental tolerance and cycling durability to some extent, achieving a balance between the electrical conductivity and chemical inertness of protective layers remains a key research focus.

The fundamental understanding of the charge-storage mechanism in boron-based materials is still incomplete. Although the high quantum capacitance of borophene and the multielectron pseudocapacitive characteristics of metal borides have been experimentally confirmed, their synergistic relationship within real electrodes remains ambiguous. Most current studies emphasize enhancing overall capacitance, yet lack a systematic analysis of the coupling among quantum capacitance, interfacial charge transfer, and ion diffusion. Furthermore, in situ investigations into charge distribution, band structure regulation, and interfacial stress evolution are scarce, limiting precise correlations between structure and electrochemical performance. The limited specific surface area of borophene films restricts the utilization of active sites and energy density, whereas metal borides, despite having abundant redox centers, often suffer from large ion-diffusion barriers, impeding high-rate capability. Designing hierarchical porous architectures, two-dimensional layered networks, and core–shell heterostructures can effectively alleviate these issues, though their formation mechanisms and long-term stability require further exploration.

Challenges related to compositional control and material uniformity are particularly pronounced in multimetallic or amorphous boride systems. In multicomponent borides, different metal ions may undergo phase segregation or valence imbalance during reduction, resulting in the breakdown of conductive networks and irreversible electrochemical behavior. Although amorphous borides provide a higher density of active sites, their complex defect structures and poor synthesis reproducibility hinder scalability and industrial applicability. Future research should leverage high-throughput computational screening and in situ characterization to achieve precise control of elemental doping, phase structure, and electronic properties, thereby improving the controllability and stability of boron-based systems.

At the device-engineering level, electrode–electrolyte interfacial matching remains a critical factor affecting performance. Conventional aqueous electrolytes are limited by their narrow electrochemical window (<1.23 V), restricting achievable energy density. Organic electrolytes can expand the voltage window but suffer from low ionic conductivity and flammability. Recent efforts have focused on ionic-liquid, gel, and solid-state electrolytes, which offer wider potential windows and enhanced safety; however, their high cost, complex fabrication, and limited interfacial compatibility remain significant challenges. Additionally, the integration of borophene and borides into flexible or miniaturized energy-storage devices is still in its infancy. Achieving a balance between electrochemical performance, mechanical durability, and packaging stability is essential for enabling commercial implementation.

Finally, industrial scalability and sustainable manufacturing remain pressing challenges. Borophene production currently suffers from low yield and high cost, and most demonstrations remain at the laboratory scale with limited device-level evaluation. Although boron-based electrodes exhibit superior energy density compared to carbon-based materials, their cycle life, cost efficiency, and scalability still fall short of industrial standards. Moreover, the high energy consumption and potential environmental pollution associated with synthesis processes hinder green manufacturing. Future research should focus on developing eco-friendly fabrication routes and metal-recycling strategies such as synthesizing borides from reclaimed transition metals in spent batteries to achieve resource-efficient and low-carbon production.

Overall, the key challenges for borophene and metal borides in supercapacitor applications lie in controllable synthesis, environmental stability, electrochemical mechanisms, compositional regulation, electrode-interface optimization, and sustainable manufacturing. Future progress requires a synergistic effort bridging fundamental research and engineering practice.

5.3. Future perspectives

Looking forward, several strategies can be envisioned to accelerate the practical deployment of borophene and boride-based supercapacitors. Future efforts should emphasize the atomic-scale design of borophene and boride nanostructures. For borophene, heterostructure and defect engineering, layer-number control, and chemical doping can be employed to tailor its electronic structure and enhance quantum capacitance. For borides, introducing heteroatoms or forming multimetallic phases (e.g., Co–Ni–Fe–B or Mo–W–B) could expand the redox potential window and increase faradaic capacity. The creation of three-dimensional architectures or interconnected nanosheet networks will facilitate both electron percolation and ion diffusion. Building hybrid electrodes by integrating borophene or borides with other emerging materials such as MXenes (V₂CT_x, Nb₂CT_x, Nb₄C₃T_x), graphene, or conductive polymers can produce synergistic effects that enhance capacitance and durability. In particular, combining borophene with redox-active transition-metal oxides, sulfides, or selenides could merge high quantum capacitance with rich faradaic behavior. Such hybridization also offers structural stability and resistance to oxidation, which are essential for long-term operation. To rationally design high-performance electrodes, it is crucial to deepen our understanding of charge-storage mechanisms. In situ or operando spectroscopic techniques (e.g., XPS, Raman, and EIS mapping) should be utilized to probe dynamic oxidation states and interfacial ion adsorption during cycling. Meanwhile, theoretical tools such as molecular dynamics simulations and density-functional theory calculations can help visualize ion migration pathways, quantify diffusion barriers, and predict the influence of defects and doping on energy storage behavior. Developing low-cost, scalable synthesis technologies is vital for industrial translation. Molten-salt-assisted, electrochemical deposition, and low-temperature CVD routes are promising candidates for producing large-area, uniform films. Device integration strategies should also consider flexible, micro-, or hybrid supercapacitors to meet the requirements of wearable and integrated electronics. Exploring borophene-based micro-supercapacitors, metal-ion capacitors, or hybrid systems combining battery and capacitor functionalities could open new horizons in multifunctional energy devices. Additionally, the sustainability of boron-based materials should be considered in the near future. Recycling transition metals from industrial waste or spent batteries to synthesize borides could reduce resource consumption and environmental impact. Additionally, the toxicity and reactivity of boron precursors should be mitigated through safer synthetic chemistries, aligning these technologies with green manufacturing principles.

In summary, borophene and transition-metal borides offer an exciting platform for the next generation of high-performance supercapacitors. Their combination of metallic conductivity, tunable electronic structures, and chemical robustness bridges the gap between conventional carbon materials and redox-active transition-metal compounds. However, to translate their laboratory success into real-world devices, future research must resolve critical challenges related to stability, scalability, and mechanistic understanding. Through advances in synthesis, hybrid material engineering, in situ characterization, and sustainable processing, boron-based materials could soon evolve from fundamental scientific curiosity into practical energy-storage technologies that deliver high energy density, high power density, and high cycling durability. Their versatile chemistry and structural flexibility make them not only suitable for supercapacitors but also promising for broader applications such as in hybrid capacitors, micro-energy storage systems, and multifunctional flexible electronics. Continuous interdisciplinary collaboration among materials scientists, chemists, and engineers will be pivotal in realizing the full potential of these remarkable boron-derived materials in future sustainable energy landscapes.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included, and no new data were generated or analyzed as part of this review.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (61774085), the Research Fund of State Key Laboratory of Mechanics and Control for Aerospace Structures (MCAS-I-0425G01, MCAS-IS-0124K01), the Fundamental Research Funds for the Central Universities (YQR23095, YQR23094), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

Notes and references

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