2D borophene: An emerging material for supercapacitor applications

Abstract

The progress of high-performance supercapacitor electrodes based on emerging 2D materials has garnered tremendous attention due to their high power density (>10 kW kg⁻¹) and long charge–discharge cycle life (>10⁵ cycles). Having been discovered in 2015, 2D borophene has emerged as a unique material among the Xenes due to its excellent electron mobility, metallic behaviour, thermal conductivity, Dirac nature, strength, and flexibility, compared to graphene. Theoretical studies show that borophene possesses a high electron density near the Fermi level which contributes to enhanced charge storage capability and quantum capacitance. This review article aims to provide recent developments in supercapacitor applications of pristine 2D borophene and their hybrid nanostructures with other emerging suitable materials. Initially, the synthesis methods with structural aspects of borophene are introduced and then the progress of borophene in supercapacitors is thoroughly discussed. Finally, current challenges associated with borophene synthesis, energy storage performance, and device fabrication are highlighted. Furthermore, the possible solutions and future perspectives are summarized.

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Gopinath Sahoo

Gopinath Sahoo is a Postdoctoral Researcher at the Indian Institute of Technology, Bhubaneswar, India, funded by the NPDF, SERB, and DST and working with Prof. Saroj Kumar Nayak. Prior to the current position, Dr Sahoo was a Brain Pool Postdoctoral Fellow (2021–2023) at Chungbuk National University, Chungbuk, South Korea, and worked with Prof. Sang Mun Jeong. He received his Ph.D. in 2021 under the supervision of Prof. M. Kamruddin, from the Indira Gandhi Centre for Atomic Research, Kalpakkam, a constituent institution under HBNI affiliated to DAE, India. His current research work focuses on the development of electrode materials for energy storage applications.

Sang Mun Jeong

Sang Mun Jeong is a Professor in the Department of Chemical Engineering at Chungbuk National University, South Korea, a leader of the Regional Leading Research Center (RLRC) for developing next-generation battery materials funded by the National Research Foundation of Korea, and a director of the Korea Institute of Chemical Engineers. Also, he was the Dean of Research Affairs at Chungbuk National University (2021–2023). His research focuses on energy-related materials and processes based on chemical and electrochemical engineering to develop efficient, clean and renewable future energy.

Chandra Sekhar Rout

Chandra Sekhar Rout is a Full Professor at the Centre for Nano & Material Sciences (CNMS), Jain University. Before joining CNMS, he was a DST – Ramanujan Fellow at IIT Bhubaneswar, India (2013–2017). He obtained his Ph.D. from JNCASR, Bangalore (2008), under the supervision of Prof. C. N. R. Rao followed by postdoctoral research at the NUS, Purdue University, and UNIST. His research is focused on applications of 2D layered materials in different devices. He has authored more than 200 research papers and 8 books. His h-index is 64 with total citations >16 000. He was ranked among the top 2% of scientists by the Stanford study in 2020–2022.

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1. Introduction

Borophene, the 2D allotrope of boron, has gained notable attention in recent years because of its unique structure, diverse electronic characteristics, diverse chemical properties, polymorphism, massless Dirac fermions, inherent metallicity, high carrier mobility, and high-temperature superconductivity.^1–6 The two-dimensional nature of borophene favours its application for supercapacitor (SC) applications due to its high surface-to-volume ratio, larger active sites, planar diffusion channels for charge storage, and suitability for intercalation of ions into the layers during charging and discharging processes. Graphene is a widely explored 2D material for supercapacitor applications due to its several advantages but intrinsically it has a limited ability to achieve specific capacitance due to the quantum capacitance effect caused by low electron density near the Fermi level.^7–10 Due to this reason, alternative 2D materials such as borophene, with better electronic properties and a higher density of states near the Fermi level, are being explored for supercapacitor applications. Due to its lighter atomic weight than carbon, borophene-based electrodes are expected to show better gravimetric capacitance than graphene. Comparative theoretical studies of graphene and borophene interfaces with ionic liquid and lithium-based electrolytes have revealed that electrolyte cations reach closer to the borophene electrodes than to graphene due to the smaller diffusion barriers.^11,12 Theoretical calculations using density functional theory (DFT) have confirmed that Li atoms combine effectively with the borophene surface due to the low Li diffusion energy barrier of 0.007 eV and ultrahigh diffusivity of 10⁴ (to 10⁵) times higher than that of similar materials like MoS₂ and graphene.^11,13,14 These observations suggest that borophene is possibly a promising electrode material for energy storage applications with extremely high rate capability and specific capacity. The capacitive properties of borophene-related sheets have favourable or even higher Fermi velocity compared to graphene as indicated by theoretical studies. The important characteristics like high surface charge storage, larger differential quantum capacitance, and integrated quantum capacitance of borophene-based electrodes, compared to graphene, make them promising candidates for supercapacitor applications.¹⁵ Joint DFT calculations of six different types of boron sheets with different hole densities and arrangement of the holes revealed extremely high specific capacitance (∼400 F g⁻¹), about 3–4 times higher than the value for graphene because of their low weight and metallic nature.¹⁶ First-principles calculations by Kolavada et al. revealed that borophene with 1- and 4-monolayer thickness is preferred for cathode-type electrode materials, whereas 2-monolayer-thick borophene is suitable for anode-type electrode materials to achieve the highest quantum capacitance in borophene-based supercapacitors.¹⁷ Furthermore, composite materials provide the direction to achieve materials with suitable and tuned properties to complement their qualities and enhance overall performance. The composite structure can provide enhanced electrochemical activity and large interlayer voids for ion movement, and the synergistic effects facilitate improved structural and electrochemical stability. The flexibility of borophene, combined with the properties of other hybrid material components, can enable the creation of flexible and bendable supercapacitors for wearable electronics and other applications.¹⁸ Recently, borophene composites with carbon nanotubes, graphene, MXenes, and MOFs have been explored for supercapacitor applications.^19–21 In this frontier article, we provide a brief overview of synthesis methods, important electronic properties, and theoretical and experimental reports on borophene and its engineered hybrid materials for supercapacitor applications (Fig. 1). Future perspectives and challenges of this emerging material for supercapacitor applications are discussed.

Fig. 1 Schematic showing the topics discussed in this review of borophene materials.

2. Crystal structure and electronic features of borophene

Since boron [(He) 2s² 2p¹] is present between the non-metallic carbon and metallic beryllium, it possesses great diversity for bonding, which leads to the formation of complex allotropic structures with unique properties.¹ Depending on the composition patterns, structures of the borophene can be classified into several types. These structures include distorted hexagonal (DH) planes, buckled triangular (BT) planes, and mixed triangular-hexagonal (MTH) planes. According to the coordination number (CN), the structures are named α-type (CN = 5, 6), β-type (CN = 4, 5, 6), χ-type (CN = 4, 5), δ-type (CN = m, with m is a single number) and ψ type (CN = 3, 4, 5).²² The graphene-like honeycomb borophene with CN = 3 is named δ₃, and the triangular sheet with CN = 6 is named δ₆. Studies show that δ₆ borophene sheets are buckled because of σ–π mixing, whereas other borophene structures (δ₃, δ₄, and δ₅) prefer sheets in planar shape. Some of the low-energy structures of borophene (δ, χ, α, and β-type) are highlighted in Fig. 2a–d, respectively. The boron monolayer sheets, α₁ and β₁ with η = 1/8 (η is a parameter describing the area density of hexagon holes in the structure) are highly stable due to the high cohesive energies.

Fig. 2 Top surface views of different low-energy structures of boron monolayer sheets: (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 with permission. Copyright 2012, American Chemical Society.²²

Some of the allotropes of borophene that are widely studied are 2-Pmmm, β₁₂, χ₃, and honeycomb structures (Fig. 3a–d). The honeycomb structure of borophene is similar to that of graphene; the 2-Pmmm phase has a wave-like structure with a buckling height of 0.91 Å, whereas the β₁₂ and χ₃ phases maintain planarity.^23,24 The electronic properties of monolayer and bilayer borophene polymorphs have been vastly studied by first-principles calculations.^2,3,25,26 Due to their anisotropic atomic structure, ability to tune its band gap and the presence of Dirac fermions, borophene exhibits tunable electronic properties which can be further modulated by different engineering perspectives such as doping, vacancy engineering, hetero-structuring, and strain engineering. Due to the presence of Dirac fermions, pristine borophene exhibits metallic behaviour with high electronic velocity ∼6.6 × 10⁵ m s⁻¹.²⁷ For example, borophene with a close-packed rectangular unit cell (Fig. 3e–g) showed an anisotropic metallic character with highly dispersive bands in the K_x direction, and some bands cross the Fermi level parallel to the K_y direction in certain regions, confirmed from the electronic band structure (Fig. 3h). Furthermore, experiments confirmed the partial oxidation of borophene within several hours, which affects the electronic and optical properties. The DFT study revealed that oxidized borophene (Fig. 3i–k) still maintains its metallic character with lower electronic conductivity and carrier mobility due to the existence of localized states around the O-defects (Fig. 3l). The analysis confirmed that several new peaks appear in the density of states (DOS) curve, making the plot noisier compared to the pristine one but still non-zero at the Fermi level. In the charge-transfer process, the O-adatom loses 0.15–0.17e in the bridge position and loses 0.23–0.25e in the top position; consequently, the charge gained by borophene is spread over the sheet.

Fig. 3 Structures of different allotropic forms of borophene with unit cells and lattice vectors a₁ and a₂. (a) 2-Pmmn, (b) β₁₂, (c) χ₃ and (d) honeycomb structure. Reproduced with permission. Copyright 2016, American Chemical Society.²⁴ (e and f) Side and (g) top views of borophene (3 × 3 supercell). The 1 × 1 unit cell consists of two boron atoms (B1 and B2 are represented by green and blue spheres, respectively). (h) Electronic band structure and the corresponding DOS of pristine borophene. The inset shows the Brillouin zone. The highest group velocity for a band crossing the Fermi level is indicated by the red dashed line.²⁷ (i and j) Side and (k) top views of oxidized borophene (3 × 3 supercell), where O adatoms are represented by red spheres, and (l) electronic band structure and the corresponding DOS of oxidized borophene. Reproduced with permission. Copyright 2016, Institute of Physics Publishing.²⁷

3. A brief overview of the synthesis of borophene

Borophene, a 2D monolayer of boron, is synthesized using both top-down and bottom-up approaches. Top-down methods involve exfoliating bulk boron by utilizing energy from physical, chemical, and biological sources to split complex structures into smaller pieces utilizing approaches of mechanical cleavage, ultrasonication, ion intercalation exfoliation, and different types of etching, while bottom-up methods, like chemical vapor deposition (CVD) and molecular beam epitaxy (MBE), grow borophene on substrates, where borophene is produced from individual atoms or molecules.^28–30

The top-down approaches involve physically breaking down larger boron structures into 2D layers which includes techniques like mechanical exfoliation, liquid-phase exfoliation (using solvents and sonication), and electrochemical exfoliation. A facile liquid-phase synthesis of freestanding borophene sheets via sonochemical exfoliation and the role of solvent have been investigated.³¹ Furthermore, a low-temperature liquid exfoliation method where the temperature range is between −20 and −25 °C was used for the synthesis of freestanding β₁₂ borophene.³² The main issue in the top-down methods is the large-scale production of borophene, and towards that, Chowdhury et al. introduced a temperature-subject electrochemical exfoliation method. In this method, a boron rod was linked to a temperature controller to maintain the desired level of boron temperature which acted like a cathode, and platinum acted as the anode.³³ Hou et al. developed ultrastable crystalline hydrogenated borophenes by the thermal decomposition of sodium borohydride (NaBH₄), where a three-step heating technique was used for the thermal decomposition of sodium borohydride under hydrogen as the carrier gas.³⁴ The bottom-up approaches build borophene from individual atoms or molecules. In the case of CVD boron-containing precursors are decomposed on a substrate, and the boron atoms arrange into a 2D structure, whereas in the case of MBE, the boron atoms are evaporated and deposited onto a heated substrate, allowing for precise control over the layer thickness and quality. Different substrates like Ag (111), Au (111), Cu (111), Ir (111), Al (111), Ru (0001), and quartz have been used as suitable substrates for borophene synthesis.^35–38 Atomically thin 2D γ-boron films on Cu foil via CVD were synthesized using a mixture of boron and boron oxide powder as the boron source and hydrogen gas as the carrier gas.³⁹ A two-zone CVD was used by Liu et al. to synthesize transferrable few-layer β₁₂ borophene, where bis (triphenylphosphine) copper tetrahydroborate ((Ph₃P)₂ Cu(BH₄)) was used as the boron source.⁴⁰ Sielicki et al. successfully synthesized borophene by a bottom-up electrochemical exfoliation of boron, where bulk boron attached to a metal mesh of nickel or copper was used as one electrode and a platinum wire was used as the counter electrode with two different kinds of electrolytes, LiCl and Na₂SO₄. An electric current (1 A, 0.5 A, or 0.1 A) is supplied between the cathode and anode throughout the electrochemical exfoliation method.⁴¹ As the main growth modes of borophenes by MBE, surface adsorption, surface segregation, layer-by-layer growth, cluster-assisted growth, and vapour–liquid–solid growth are specifically highlighted. In the growth of borophene on Ag and Cu substrates, boron atoms form stable monolayers by adsorption onto the surface of Ag and Cu substrates.⁴² At high temperatures (≥660 K), boron atoms tend to diffuse into the subsurface layers of the metal, and upon cooling the substrates, boron atoms segregate to the surface and spontaneously assemble into borophene.⁴³ By using MBE, Zhong et al. successfully synthesized borophene on an Ag (110) substrate at a substrate temperature of roughly 550 K.⁴⁴ However, by employing Ag (100) as the substrate rather than Ag (111) via the MBE process, regular mixed quasi-1D boron chains with long-range order have recently been realized.⁴⁵ Bilayer borophene was successfully synthesized by Liu et al. using MBE on an Ag (111) substrate, which comprises two α-phase layers that are covalently bonded.⁴⁶

Challenges in borophene synthesis include its inherent instability, the difficulty in achieving high-quality, large-area sheets, and the need for specific substrates for growth. The 2D materials, including borophene, are inherently unstable and prone to oxidation or degradation, making it difficult to handle and process them.^47,48 The synthesis of large, defect-free, and uniform borophene remains a significant challenge, especially for applications requiring high performance.⁴⁹ Many current synthesis methods for borophene are not easily scalable for industrial production, limiting their widespread use. Furthermore, materials like borophene require specific substrates for epitaxial growth. The interaction between the substrate and the 2D material can affect its properties and stability. As discussed, CVD and MBE often face limitations in terms of scalability and purity, which are required for better supercapacitor performance. More recent advancements are exploring techniques like physical vapor deposition (PVD), atomic layer deposition (ALD), and electrochemical exfoliation to address these challenges and achieve higher purity and larger-scale production of borophene.^3,50 Researchers are actively exploring combinations of these techniques and the development of new approaches to overcome the challenges of large-scale, high-purity borophene production. This includes investigations into optimizing the synthesis parameters, exploring the use of different substrates and catalysts, and developing novel purification methods.

The synthesis method significantly impacts borophene's performance in supercapacitors by influencing its crystallinity, phase purity, and overall structure, which in turn affect its electrical conductivity, surface area, and ion storage capabilities. CVD and MBE methods can produce high-quality, crystalline borophene with fewer defects, leading to enhanced electrical conductivity and improved ion storage capacity in supercapacitors. Liquid-phase exfoliation has more defects and imperfections, potentially impacting the material's overall performance. Furthermore, borophene's layered structures achieved by MBE or controlled exfoliation impact its surface area and charge storage ability. Thinner layers generally offer a larger surface area for electrolyte interaction. Moreover, the controlled interface between borophene and the substrate is crucial, as interfacial defects can impede performance. Optimizing the substrate material and deposition conditions is important for achieving high-quality borophene films.

4. Recent developments on borophene for supercapacitor applications

Generally, 2D materials like graphene have been extensively studied for supercapacitors due to their interesting material properties. Furthermore, graphene has limitations in energy storage applications due to the quantum capacitance effect which causes a low electronic DOS at the Fermi level.⁸ Moreover, doping is also not that effective in improving the electronic properties, which opens up research for alternative 2D materials. The main advantage of borophene over other 2D materials is its exceptional flexibility while maintaining its structural integrity and strength, surpassing even graphene in certain configurations, along with its theoretical metallic properties.^51,52 Borophene's electronic structure can be tuned, making it versatile for applications like catalysis and sensing.⁵³ Borophene is a lightweight yet robust 2D material with high potential for various applications. Furthermore, the excellent properties of borophene such as high surface area, electronic conductivity, and carrier mobility make it a suitable electrode material compared to other 2D materials for supercapacitor applications.⁴⁷ For example, the specific capacitance of borophene is found to be higher than that of graphene, and rGO indicates its potential in energy storage applications. A DFT study on an electrode–electrolyte system calculated the 2D boron sheet showed specific capacitance of 400 F g⁻¹, which is four times that of graphene.¹⁶ A small energy barrier for Li diffusion in borophene is noticed that leads to an ultrahigh diffusion of ions for excellent energy storage performance.¹³ Therefore, here the energy storage performance of pristine borophene and borophene-based hybrid materials is reviewed. The theoretical studies that give insight into understanding the charge-storage mechanisms are highlighted. In supercapacitor energy storage, it is crucial to differentiate between capacitance measured at the device level (total capacitance) and at the electrode level (specific capacitance). Understanding this distinction is vital for optimizing supercapacitor performance, as specific capacitance guides material selection and device design, while total capacitance determines the overall energy storage capacity. Hence, the capacitance of borophene both at the device level and at the electrode level is discussed.

Recently, 2D materials like MXenes and GO/rGO have been well studied for supercapacitor applications. As borophene is a new 2D material and the material is less studied, highlighting the unique properties and potential applications of each material would make the review more comprehensive and appealing. Studies showed that MXenes can have structural variations and surface modifications, and GO/rGO research has focused on their tunable properties.^54,55 Similarly, borophene has advantages in structural variation due to its different buckling patterns, which alter the properties and hence the supercapacitance performance.⁵¹ MXenes excel in fast ion intercalation and high capacitance, while borophene offers advantages in flexibility, capacitance, and conductivity.⁵⁶ GO/rGO composites are also widely used, with rGO often showing better electrochemical stability.⁵⁷ Furthermore, combination of these materials to create layered hybrid structures to enhance their properties can be demonstrated.⁵⁸ Zhang et al. proposed a graphene/MXene-based system having a unique 3D-porous structure that facilitates superior permeability for the electrolyte and excellent transport for the charge carriers. The restacking problem of porous graphene layers is prevented by the MXene, and this helps in widening the interlayer spacing that can act as a transport channel for ions and electrons.⁵⁹

4.1. Theoretical investigations

Theoretical studies have revealed that borophene sheets with Dirac cones exhibited high gravimetric differential quantum capacitance of 1933.2–1133.4 F g⁻¹ at ±0.8 V with superior performance to graphene (375 F g⁻¹ at ±0.8 V).^15,17 Furthermore, the surface charge storage values are reported to be 895.0–346 C g⁻¹, whereas the value for graphene is ∼152 C g⁻¹. Theoretical calculations of the specific capacitance of 2D boron sheets exhibited three to four times higher values than that of graphene in a voltage window range from −0.6 V to +0.6 V due to its metallicity and low weight.¹⁶ Since borophene is a layered 2D material, it can undergo inherent layer restacking causing damage to the structure during repeated charge–discharge cycling when used as a supercapacitor electrode. Restocking and aggregation effects limit the performance of the device since they affect ion movement in the electrolyte and simultaneously reduce the accessible surface area for electrochemical reactions. Hence, the integration of borophene with other efficient electrode materials such as MXenes is found to serve as an ideal hybrid material for developing high-performance supercapacitors. Alikhani et al. reported theoretical studies on the electrochemical energy storage performance of a honeycomb borophene heterostructure with various MXenes such as Ti₃C₂O₂, Ti₃C₂F₂, and Ti₃C₂(OH)₂ in 1 M NaF.²¹ The findings demonstrated that the borophene heterostructure with Ti₃C₂(OH)₂ exhibited energy storage performance that was superior by more than 30%. Theoretically, Yong et al. demonstrated that the introduction of hydroxylated borophene not only inhibited the re-stacking of the MXene layers but also provided more active sites and allowed fast ion transport for charge storage without an obvious increase in the migration barrier.⁶⁰Fig. 4 shows the theoretically calculated thermodynamic stability and storage kinetics of the electrodes. The asymmetrical case (MXene–B) showed an improved binding strength of −3.26 eV compared to the weaker binding strength (−0.84 eV) of the symmetrical case (MXene–MXene) in Fig. 4a, confirming the MXene inclination for binding towards hydroxylated borophene. The difference in charge-density distribution at the interface of the hetero-structure confirms a strong electrostatic interaction with high charge transfer in the case of the MXene–B system with respect to the MXene–MXene system (Fig. 4b and c). The thermodynamic stability of the Ti–O bond in both cases was evaluated by crystal orbital Hamilton population (COHP) analysis and MXene–B showed a greater binding strength of 1.7 eV (Fig. 4d). The MXene–B system depicted higher Zn-ion absorption energy for calculation carried out on the MXene surface and the hetero-structure interfaces, compared to the MXene–MXene systems (Fig. 4e). Furthermore, the Zn-ion transport kinetics are studied for both the cases, and Fig. 4f and g depicts the top views of MXene–MXene and MXene–B, respectively. The corresponding diffusion energy barrier graphs presented in Fig. 4h and i confirm that hydroxylated borophene does not significantly block the transport of the Zn atoms on the MXene surface during the storage process.

Fig. 4 Theoretical investigation of thermodynamic stability and storage kinetics of the hydroxylated borophene–MXene heterostructure electrodes. (a) Binding energy and the corresponding differential charge density distribution for (b) MXene–MXene and (c) MXene–B electrodes, respectively. (d) COHP analysis of the Ti–O bonds of MXene in two kinds of electrodes. (e) Adsorption energy of Zn atoms on the MXene surface and at the MXene/B interface. Insets show the corresponding optimized structures of the MXene–B cases. Top views of the Zn diffusion path on the MXene surface for (f) MXene–MXene and (g) MXene–B electrodes, as well as their (h and i) corresponding energy barriers. Reproduced with permission. Copyright 2024, Wiley.⁶⁰

4.2. Pristine borophene for supercapacitors

Starting from 2018, experimental works on borophene-based materials for supercapacitor applications have evolved in recent years along with the supported theoretical studies (Fig. 5). The first experimental report is on the elemental boron-based materials, where Xue et al. demonstrated supercapacitor applications of single-crystalline boron nanowires prepared by chemical vapour deposition.⁶¹ The fabricated supercapacitors presented notably high capacitance and excellent rate capability in alkaline, neutral, and acidic electrolytes with a capacitance of up to 60.2 mF cm⁻². Similarly, in the same year, 2D borophene sheets prepared by sonication-assisted liquid phase exfoliation were reported to show impressive energy storage performance with a wide potential window of up to 3.0 V and an excellent energy density of 46.1 Wh kg⁻¹ at a power density of 478.5 W kg⁻¹ with 88.7% capacitance retention after 6000 cycles.⁶²

Fig. 5 Roadmap of borophene-based supercapacitor research in recent years.

In 2021, Joshi et al. reported that oxygen defects in borophene sheets suited the energy storage performance in both aqueous and non-aqueous electrolytes due to the improved electroactive surface area, stabilization of the structure, and high conductivity (96.12 S m⁻¹).⁶³ In 2022, Abdi et al. reported on nano-supercapacitors based on CVD-grown 2D borophene and the device exhibited a high specific capacitance of 350 mF cm⁻².⁷ In 2024, it was reported that stacked borophene electrodes prepared through a rolling process with an optimal borophene/additive (B/A) ratio of 1 : 1 show superior mechanical and electronic properties suitable for flexible supercapacitor applications.⁶⁴Fig. 6a shows a schematic of the flexible supercapacitor device. The cyclic voltammetry (CV) profiles of the stacked borophene device in flat and rolled conditions are shown in Fig. 6b and c. After rolling, the CV curves are nearly symmetrical rectangular shapes, confirming excellent reversibility and ideal capacitive characteristics. The specific areal capacitance of the rolled borophene was calculated from the CV profiles to be 374.6 mF cm⁻² at a 10 mV s⁻¹ scan rate, and the device showed a specific areal capacitance of 80.5 mF cm⁻² at 200 mV s⁻¹ (Fig. 6d). The rolled device presented a linear galvanostatic charge–discharge (GCD) profile (Fig. 6e), and the specific areal capacitance from GCD was calculated to be 417.3 mF cm⁻² (2-fold enhancement compared with non-stacked borophene). Fig. 6f shows that the capacitance of the non-stacked borophene-based capacitor is higher compared to the carbon-based capacitors. Furthermore, the EIS analysis in Fig. 6g confirms a decrease in internal resistance in the low-frequency region of the spectra for the rolled device compared to the planar state. The device achieved 89.3% capacitance retention after 5000 continuous cycles (Fig. 6h). Preliminary studies on “S” and “Fe” doped mono/few-layered β₁₂ borophene prepared under controlled microwave exposure conditions revealed its potential for supercapacitors.⁶⁵ In another work, CVD-grown borophene structures achieved a specific areal capacitance of 44.5 mF cm⁻² and a gravimetric capacitance of 4238 F g⁻¹ with a retention rate of 60% over 10 000 cycles.⁶⁶ Recently, Yong et al. constructed Zn-ion supercapacitors based on modified hydroxylated borophene and PVA gel electrolytes integrated with MXenes, which demonstrated superior capacitance, cycling stability, and rate performance.⁶⁷ The PVA–borophene gel porous 3D network structure facilitated Zn²⁺ ion diffusion due to the disruption of PVA chain ordering by hydroxylated borophene leading to increased inter-chain spacing. Table 1 summarizes the performance of supercapacitor electrodes based on borophene and its hybrid materials.

Fig. 6 Stacked borophene-based supercapacitors with B/A = 1 : 1: (a) Schematic diagram of the device. CV curves for the (b) flat and (c) rolled-state SC at different scan rates. (d) Areal capacitance at different scan rates calculated from CV. (e) GCD curves of the supercapacitor in a rolled state at different current densities. (f) Areal capacitance was calculated from GCD and plotted with supercapacitor performance of a similar type of material. (g) EIS plot of the borophene-based supercapacitor in different states (inset compares the internal resistance). (h) Capacitance retention performance of the rolled borophene device (inset depicts some GCD cycles). Reproduced with permission. Copyright 2024, Elsevier.⁶⁴

Table 1 Recent developments in borophene-based electrodes for supercapacitor applications

Material	Borophene synthesis method	Type of supercapacitor (electrode, device)	Electrolyte	Energy storage performance (specific capacitance (Csp); energy density (ED); power density (PD))	Stability (%, cycles)	Ref.
Pristine borophene
Borophene sheets	Liquid-phase exfoliation	Gravimetric, symmetric	1-Butyl-3-methylimidazolium hexafluorophosphate	C sp = 142.6 F g^−1; ED = 46.1 Wh kg^−1; PD = 478.5 W kg^−1	88%, 6000	62
Borophene	CVD	Areal, symmetric	H2SO4	C sp = 350 mF cm^−2	—	7
Borophene	CVD	Areal, gravimetric	Na2SO4	C sp (areal) = 44.5 mF cm^−2; Csp (gravimetric) = 4238 F g^−1;	60%, 10 000	66
Stacked borophene	CVD	Areal, symmetric	Organic electrolyte (DLC302)	C sp = 417.3 mF cm^−2;	88%, 5000	64
Oxygen defective borophene	Liquid-phase exfoliation	Areal, symmetric	KOH, H2SO4	C sp (KOH) = 107 mF cm^−2; Csp (H2SO4) = 145 mF cm^−2; ED = 25.1 Wh kg^−1 at PD = 636.13 W kg^−1 (in ionic liquid electrolyte)	75.4%, 8000	63
S-,Fe-doped borophene	Exfoliation	3 electrode based	6 M KOH	C sp (S-doped) = 202 F g^−1; Csp (Fe-doped) = 120 F g^−1	100%, 5000 (S-doped); 61%, 5000 (Fe-doped)	65
Borophene-based hybrid materials
Borophene–carbon nanosheets	Gas-phase exfoliation	Volumetric, symmetric	PVDF–HFP/BMIBF4	C sp = 534.5 F cm^−3; ED = 167.05 mWh cm^−3@PD = 0.15 W cm^−3	94.1%, 10 000	69
Borophene–PANI	Ultrasonic exfoliation	3 electrode based	H2SO4	C sp = 960 F g^−1	95%, 1000	70
Borophene–PEDOT PSS	Physical exfoliation	3 electrode based	H3PO4–PVA	C sp = 835 F g^−1	95%, 1000	71
Borophene/S, N-doped carbon	Liquid-phase exfoliation	Gravimetric, symmetric	PVA–KOH	C sp = 607 F g^−1; ED = 29.2 Wh kg^−1; PD = 3500 W kg^−1	90.6%, 5000	72
Boron quantum dot/MXene	Sonication-assisted liquid-phase exfoliation	Areal, symmetric	PVA–H2SO4	C sp = 552 mF cm^−2; ED = 40.4 Wh cm^−3@PD = 416 W cm^−3	93%, 5000	81
Borophene–MXene	Ultrasonic exfoliation	Gravimetric, symmetric	PVA–H2SO4	C sp = 626.7 F g^−1; ED = 75.6 Wh kg^−1; PD = 12 000 W kg^−1	93.6%, 10 000	58
Borophene–graphene	Sonochemical synthesis	Gravimetric, symmetric	PVA–KOH	C sp = 193 F g^−1; ED = 36.77 Wh kg^−1@PD = 585.5 W kg^−1	80.8%, 10 000	74

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4.3. Borophene-based hybrids for supercapacitors

Supercapacitor applications of 2D borophene sheets are hindered due to the issues of π–π restacking and poor stability under ambient environmental conditions. To tackle these issues, borophene-based hybrid materials have been studied for supercapacitor fabrication and high-performance device applications.⁶⁸ In 2020, Wu et al. demonstrated that 2D hetero-nanosheets consisting of anisotropic boron–carbon nanosheet electrodes facilitate easy ion migration and sufficient ion storage in the electrolyte because of the high interlayer conductivity, ionic pathways, and chemically accessible surfaces.⁶⁹ The fabricated anisotropic boron–carbon nanosheet (ABCN) flexible supercapacitors (Fig. 7a) based on the hybrid nanosheets exhibited a high volumetric specific capacitance of 534.5 F cm⁻³) and an energy density of 167.05 mWh cm⁻³ with a cycling stability of 94% after 10 000 cycles (Fig. 7b and c). As shown in Fig. 7d, the bulk boron offers a smaller surface area, resulting in less ion accumulation and minimal energy storage. Furthermore, boron nanosheets offer an increased surface area, but poor interlayer conduction impedes the improvement in energy storage capacity. Additionally, by the incorporation of carbon nanosheets within the boron interlayers, the ABCN interfacial charge transfer and charge kinetics improve which enhances the charge-storage capacity. By considering the impressive electrochemical performance and high flexibility of the device, its application in practical use in wearable sensor systems for detecting physiological signals is demonstrated. Borophene-conducting polymer (PANI and PEDOT-PSS) based inorganic–organic hybrid electrodes have been reported as promising materials with good stability and high specific capacitance for supercapacitors.^70,71 Nanocomposites composed of 2D borophene and S,N-doped mesoporous carbon exhibited an impressive specific capacitance of 607 F g⁻¹ with an energy density and power density of 29.2 Wh kg⁻¹ and 3500 W kg⁻¹, respectively.⁷² MXene sheets modified with boron quantum dots and by boron vacancy doping are found to be beneficial for supercapacitor applications since they help optimize the structure for the proton intercalation and enable smooth migration of the ions.⁷³ By considering the synergistic effects of better conductivity and mechanical resistance with minimized material degradation, Nanda et al. studied flexible solid-state symmetric supercapacitor applications of a 2D borophene–graphene composite hydrogel.⁷⁴ Borophene's conductivity and graphene's large surface area contributed significantly to achieving a highly porous and interconnected hydrogel, ideal for flexible SC devices. Furthermore, the study is also extended for application in wearable electronics, smart packaging, and low-power self-sustained sensor systems, etc.

Fig. 7 Supercapacitor application of 2D boron–carbon sheets. (a) Schematic of the SC device. (b) Comparison of capacitances and energy densities of bulk boron, boron nanosheets, and ABCNs. (c) Energy density of the ABCN device compared to devices fabricated from other 2D materials. (d) Schematic of the charge storage process and electron transfer in different electrode configurations. Reproduced with permission. Copyright 2020, Wiley.⁶⁹

Furthermore, hybrids of borophene can be synthesized on different substrates like Ag, Au, and Cu, which by integrating with carbon nanotubes or amorphous carbon, can enhance the active sites and electron transfer.^68,75 Borophene on Ag (111) exhibits anisotropic behaviour similar to that of graphene, and on Au (111), it demonstrates metallic characteristics.^36,75,76 Successful growth of borophene on copper substrates using optimized CVD parameters has also been achieved, resulting in controlled thickness and high crystallinity. These borophene-based devices exhibited a remarkable specific areal capacitance of 44.5 mF cm⁻², and a low interfacial resistance is advantageous for potential applications.⁶⁶ 0D carbon spheres doped with S and N heteroatoms were utilized to intercalate borophene, which showed a high specific surface area of 2100 m² g⁻¹ and a specific capacitance of 833 F g⁻¹.¹⁹ Recently, a borophene and MOF (HKUST-1) composite was synthesized for the first time by a simple mixing method, where conductive inks were prepared using the obtained borophene@HKUST-1 composite. The carbon-felt electrodes, which allowed the formation of flexible electrodes, were then coated with these conductive inks. The composite flexible electrode exhibited a high specific areal capacitance value of 333 mF cm⁻² at a current density of 3 mA cm⁻² with a 63.4% capacitance retention after 1000 cycles.²⁰

Similarly, aerogels composed of borophene and MXenes have demonstrated excellent energy storage performance due to their porous structure and chemically active sites which enabled the transport of ions in the electrolyte.⁷⁷ MXene–borophene (MxB) electrodes prepared by electrophoretic deposition involving the electrostatic self-assembly of borophene (positively charged) and MXene sheets (negatively charged) helped avoiding restacking.⁵⁸ More electrochemically active sites enhanced the ion diffusion and larger interlayer gap favoured improved energy storage performance with high measured values of specific capacitance, energy density, power density, and cycling stability. Fig. 8a and b show the CV and GCD profiles of the MxB 50 : 50//MxB 50 : 50 cell, respectively. The specific capacitance of the device is calculated to be 375 F g⁻¹ (187.5 F cm⁻²) at 1 A g⁻¹ and maintains a 71% capacitance at a high current density of 20 A g⁻¹ (Fig. 8c). The device maintained a 93.6% capacitance retention after 10 000 cycles with 100% coulombic efficiency (Fig. 8d). The device demonstrated a high energy density of 75 Wh kg⁻¹ at a power density of 600 W kg⁻¹ and maintained an energy density of 53.3 75 Wh kg⁻¹ at a power density of 12 000 W kg⁻¹ (Fig. 8e). Furthermore, the obtained energy density was higher than that of previously published MXene-based supercapacitors. The practicality of the device was proved by illuminating a red LED by using two cells connected in series (Fig. 8f). For flexible energy storage device applications, the cell was tested by bending with a 1.0 cm radius and axial twisting at 40 degrees (Fig. 8g and i). The bending and twisting of the device were performed without causing any structural damage to the material. The GCD curves are recorded for different cycles after bending and twisting and the cell retained of 78.48% and 90.39% of its initial capacitance after undergoing 500 cycles for bending and twisting, respectively (Fig. 8h and j). Yong et al. reported MXene/borophene self-assembled film electrodes for zinc-ion capacitors, where hydroxylated borophene acted as a trifunctional mediator.⁶⁰ The mediator within the heterostructure provided integrated modification effects and promoted high-efficiency interfacial energy storage. The symmetric zinc-ion capacitor showed a four-fold cycling stability performance compared to pristine MXene with an areal capacitance of 245 mF cm⁻² and almost no loss of capacitance over 40 000 cycles. Furthermore, the production of MXene–borophene hybrids has various limitations, such as the oxidation of borophene and the interaction of surface terminations of MXenes (–O, –OH, –F) with borophene, which unintentionally disrupts the structure of borophene. To overcome these problems, precautions need to be taken by controlling the synthesis environment, modifying the surface terminations of MXenes, employing suitable hybrid assembly methods, and utilizing suitable solvents and additives.⁷⁸ Synthesis can be carried out under an inert atmosphere to minimize the oxidation of borophene and control the synthesis process by optimized temperature and pressure conditions to avoid structural defects in the borophene.²⁵ Additionally, replacing the –OH groups present in MXene can minimize the interactions with borophene to disrupt its structure.^21,79 Techniques like ligand exchange, surface coating, or heteroatom doping can be used to alter the surface chemistry of MXenes, improving their compatibility with borophene and preventing unwanted reactions.^21,78 Employing methods like layer-by-layer deposition can facilitate better control over the integration of borophene and MXenes, allowing for more precise placement and minimal interaction.¹⁸ Engineering the interface between borophene and MXene by controlling the interfacial interactions can lead to more stable and functional hybrid structures.⁸⁰ When dispersing MXenes and borophene, using non-polar solvents can help prevent excessive interactions between surface terminations and borophene. Adding surfactants or dispersants to the synthesis process can improve the dispersion of MXenes and borophene, preventing aggregation and facilitating better mixing.²¹ Post-synthesis annealing under inert conditions can help stabilize the hybrid structure, reduce defects, and improve the interactions between borophene and MXene.⁷⁵

Fig. 8 (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 on different cycles for mechanical bending and twisting cases, at 1 A g⁻¹, respectively. Reproduced with permission. Copyright 2024, Elsevier.⁵⁸

5. Conclusion, challenges and future perspectives

Due to its better electronic properties, lighter mass, metallic nature, and the presence of a higher density of states near the Fermi level, 2D borophene has emerged as an important material for supercapacitor applications. Furthermore, tuning its properties by different engineering approaches such as heterostructure, doping, vacancy/defect engineering, etc., can facilitate ionic migration, ionic conductivity, and surface area. In this frontier article, we discussed the advantages and recent developments in theoretical and experimental reports on borophene-based supercapacitors. The challenges and future perspectives are highlighted here, as shown in Fig. 9.

Fig. 9 Schematic depicting the challenges and future perspectives of borophene for SC applications.

There are several challenges in this research field, which need to be addressed in the future. One of the challenges is its large-scale production by cost-effective synthesis approaches and controlled growth of the large area and high-quality sheets without agglomeration. Low-temperature CVD growth for borophene is a challenge that needs to be addressed to improve the quality and quantity of borophene. Borophene possesses high reactivity and undergoes easy oxidation under real environmental conditions when exposed during synthesis and fabrication of energy storage devices, as borophene is synthesized under ultrahigh vacuum conditions. The oxidation significantly reduces its metallic conductivity and impacts its structural integrity, hence hindering its industrial scalability and posing a major challenge for its application. These issues need to be addressed by understanding the mechanisms of borophene oxidation to develop effective strategies for stabilization, preparing hybrids with more stable materials, optimizing synthesis methods like CVD, or developing encapsulation techniques/protective coatings (like surface hydrogenation) by surface engineering/modification to preserve its properties and simultaneously its resistance to oxidation during device fabrication. Borophene-based hybrid materials with other emerging 2D materials and nanocarbons have not yet been explored, which may solve some of the major issues associated with supercapacitor applications. Boron precursors used for the synthesis of borophene may pose significant environmental, health, and safety concerns that need to be addressed. Further research on the modification of the borophene-based electrode materials by different approaches such as heterostructure/hybrid engineering with other nanomaterials (0D, 1D, and 2D), morphology engineering, tuning the number of layers, defect/vacancy engineering, doping, integration strategies, and configuration design, etc., will be helpful to achieve better energy storage performance. The new class of MXene materials, V₂CT_x, Nb₂CT_x, and Nb₄C₃T_x, consists of different metal redox sites that are helpful for better capacitance and composites of these material with borophene need to be explored. Furthermore, heterostructure composites of mono-metallic and bimetallic transition metal oxides/sulphides/selenides with borophene can be studied. Electrolytes play an important role in supercapacitor performance since their appropriate selection leads to the achievement of higher working potential windows, energy density, and stability. Hence, further research involving the exploration of different higher working window electrolytes will be helpful in achieving better performance. Research on borophene-based electrodes is in its preliminary stage, and other related fields such as hybrid capacitors, metal-ion capacitors, micro-supercapacitors, self-healing supercapacitors, and integrated supercapacitors with other functionalities has not yet been explored. By considering the important properties of borophene, it can be investigated for these applications. Furthermore, a theoretical investigation by molecular dynamics simulations and density functional theory calculations will be helpful in understanding the charge storage mechanisms and ionic movements of the borophene-based electrodes, which will provide information for the design of high-performance supercapacitors. In situ/operando spectroscopic studies on the borophene-based supercapacitor electrodes should be carried out to gain insights into the charge storage mechanisms for the successful design and fabrication of more efficient energy storage devices.

Conflicts of interest

The authors declare no conflict of interest.

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

The authors extend their appreciation for the financial assistance provided by the ANRF Core Research Grant (Grant No. CRG/2022/000897) and JAIN University (JU/MRP/CNMS/118/2025). C. S. R. acknowledges backing from the National Research Foundation of Korea under the Brain Pool Program, funded by the Ministry of Science and ICT, South Korea (Grant No. RS-2023-00222186). The work is further supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (Grant No. RS-2024-345983). Gopinath Sahoo acknowledges the DST-SERB for the National Post-Doctoral Fellowship (Grant No. PDF/2023/000545).

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